Diode lasers use nearly microscopic chips of Gallium-Arsenide or other exotic semiconductors to generate coherent light in a very small package. The energy level differences between the conduction and valence band electrons in these semiconductors are what provide the mechanism for laser action. This is not the sort of laser you can build from scratch in your basement as the required fabrication technology costs megabucks or more to set up. You will have to be content with powering a commercial laser diode from a home-made driver circuit or using a pre-packaged module like a laser pointer. Fortunately, laser diodes are now quite inexpensive (with prices dropping as you read this) and widely available.
The active element is a solid state device not all that different from an LED. The first of these were developed quite early in the history of lasers but it wasn't until the early 1980s that they became widely available - and their price dropped accordingly. Now, there are a wide variety - some emitting many *watts* of optical power. The most common types found in popular devices like CD players and laser pointers have a maximum output in the 3 to 5 mW range.
A typical configuration for a common low power edge emitting laser diode is shown below:
+ +
o o
______________|______________ _______|_______
Laser | P type semiconductor | Laser | P type |
beam | | beam | |
<=======|:::::::::::::::::::::::::::::|=======> |ooooooooooooooo|
| Junction---^ | | |
End ->| N type semiconductor |<- End | N type |
facet |_____________________________| facet |_______________|
| |
o o
- -
(Side view) (End view)
|<----------------------- 1 mm ------------------------>|
This configuration above is called a 'homojunction' since there is only one P-N junction. It turns out there are benefits to using several closely spaced junctions formed by the use of layers of P and N type materials. These are called 'heterojunction' laser diodes. There are many many more advanced structures in use today and new ones are being developed as you read this! For example, see the section: Vertical Cavity Surface Emitting Laser Diodes (VCSELs) for a description of one type that has the potential to have a dramatic impact in many areas of technology.
The 'end facets' are the mirrors that form the diode laser's resonant cavity. These may just be the cleaved surfaces of the semiconductor crystal or may be optically ground, polished, and coated.
For these types of integrated laser diodes, everything takes place inside the chip. Therefore, the output wavelength is fixed and determined by the properties of the semiconductor material and the device's physical structure. Or, at least that's the way it is supposed to work though with some, reflection of the laser light back into the chip can cause stability problems or even be used to advantage to frequency stabilize the output. There are also tunable diode lasers using external cavity optics to provide a continuous and in some cases, quite wide range of wavelengths without mode hopping.
There are also pulsed laser diodes requiring many amps to to reach threshold and providing watts of output power but only for a short time - microseconds or less. Average power is perhaps a few mW. These are gallium arsenide (GaAs) heterojunction laser diodes. They are not that common today but some surplus places are selling diodes like these as part of the Chieftain tank rangefinder assembly. They mention the high peak power output but not the low average power. :( Modern devices with similar specifications are also available from manufacturers like OSRAM Opto Semiconductors. Go to "Products", "High Power Laser Diodes", "Product Catalog...", "Pulsed Laser Diodes in Plastic Packages".
Electrical input to the laser diode may be provided by a special current controlled DC power supply or from a driver which may modulate or pulse it at potentially very high data rates for use in fiber optic or free-space communications. Multi-GHz transmission bandwidth is possible using readily available integrated driver chips.
However, unlike LEDs, laser diodes require much greater care in their drive electronics or else they *will* die - instantly. There is a maximum current which must not be exceeded for even a microsecond - and this depends on the particular device as well as junction temperature. In other words, it is not sufficient in most cases to look up the specifications in a databook and just use a constant current power supply. This sensitivity to overcurrent is due to the very large amount of positive feedback which is present when the laser diode is lasing. Damage to the end facets (mirrors) can occur very nearly instantaneously from the concentrated E/M fields in the laser beam. Closed loop regulation using optical feedback to stabilize beam power is usually implemented to compensate for device and temperature variations. See the sections on CD and visible laser diodes later in this document before attempting to power or even handle them. Not all devices appear to be equally sensitive to minor abuse but it pays to err on the side of caution (from the points of view of both your pocketbook and ego!).
In their favor, laser diodes are very compact - the active element is about the size of a grain of sand, low power (and low voltage), relatively efficient (especially compared to the gas lasers they replaced), rugged, and long lived if treated properly.
In fact, high power laser diodes - those outputting WATTs of optical power - are without a doubt the most efficient light emitter - not just lasers - in existence. Some have electrical to optical efficiencies (DC W in to light W out) of greater than 50 percent! In other words, put 2 watts of DC power in and get out 1 W of light. And, research is in progress to improve this to 80 percent or beyond. The common incandescent lamp is only 5 percent, fluorescent lamps are 15 or 20 percent efficient, high intensity discharge lamps are somewhat better, but even the best can't match the laser diodes in existence now. Just think: If those super high efficiency high power laser diodes could be mass produced in visible wavelengths and were used to replace all light bulbs, the World's energy crisis would be over, not to mention hobbyist access to high power lasers! OK, back to reality. :)
Laser diodes do have some disadvantages in addition to the critical drive requirements. Optical performance is usually not equal to that of other laser types. In particular, the coherence length and monochromicity of some types are likely to be inferior. This is not surprising considering that the laser cavity is a fraction of a mm in length formed by the junction of the III-V semiconductor between cleaved faces. Compare this to even the smallest common HeNe laser tubes with about a 10 cm cavity. Thus, these laser diodes would not be suitable light sources for high quality holography or long baseline interferometry. But, apparently, even a $8.95 laser pointer may work well enough to experiment in these areas and some results can be surprisingly good despite the general opinion of laser diode performance.
Even if not as good as a helium-neon laser in the areas of coherence and stability, for many applications, laser diodes are perfectly adequate and their advantages - especially small size, low power, and low cost - far outweigh any faults. In fact, these 'faults' can prove to be advantageous where the laser diode is being used simply as an illumination source as unwanted speckle and interference effects are greatly reduced.
As noted, not all laser diodes have the same performance. See the section: Interferometers Using Inexpensive Laser Diodes for comments that suggest some common types may indeed have beam characteristics comparable to typical HeNe lasers. And, for short range applications, see: Can I Use the Pickup from a CD Player or CDROM Drive for Interferometry?. Also see the section: Holography Using Cheap Diode Lasers.
The following sites provide some relatively easy to follow discussions of the principles of operation, construction, characteristics, and other aspects of laser diode technology:
Here's a link to a historical look at the early days of laser diodes:
The closeups below were scanned at 600 dpi - laser diodes (at least the small ones we are dealing with) are really not this HUGE! These two laser diodes can also be found in the group photo, above.
The Closeup of laser diode from the Sony KSS361A Optical Pickup shows a type that is found in many CD players and CDROM drives manufactured by Sony. The actual laser diode is inside the brass barrel shown in the photo of the optical pickup. The front of the package is angled so that the exit window (anti-reflection coated) is also mounted at angle to prevent any remaining reflections from the window's surfaces - as small as the are - from feeding back into the laser diode's cavity or interfering with the detected signal. The output of these edge emitting laser diodes is polarized. See the section: What is a Brewster window?.)
The Closeup of Typical Laser Diode shows one that is from a laser printer. It was mounted in a massive module (relative to the size of this laser diode, at least) which included the objective lens and provided the very important heat sink. In some high performance laser printers, a solid state Peltier cooler is used to stabilize the temperature of the laser diode. The low power laser diodes in CD and LD players, and CDROM and other optical drives (at least read-only types) get away with at most, the heat sink provided by the casting of the optical block - and many don't even need this being of all plastic construction.
One can think of an LED as a laser without a feedback cavity. The LED emits photons from recombining electrons. It has a very broad spectrum.
When we add a high Q cavity to it, the feedback can be high enough to trigger true laser action. Most laser diodes have the cavity built right into the device but there are such things as external cavity diode lasers.
The addition of the high Q cavity cuts down drastically the number of modes operating (in fact, it is almost improper to speak of mode structure with an LED. The result is that the emission line narrows drastically (more monochromatic) and the beam narrows somewhat spatially. One can still not easily get true single mode lasing with normal diode lasers, however, so the line will not be as sharp as a gas laser, nor the beam as narrow.
For more info, see the section: How LEDs Compare to Laser Diodes - Wavelengths, Spectrum, Power, Focus, Safety.
(From: Don Stauffer (stauffer@htc.honeywell.com).)
Yes indeed, a diode laser is a true laser. That being said, looking at matters quantitatively, it is harder to make a diode laser with a very narrow line emission than a gas laser or large crystal laser. Adding cavity length to a laser in general acts to narrow the line (in spectral space, though a higher Q cavity does tend to narrow beam in space also). It is possible to use a larger, high Q external cavity with a laser diode to increase its coherence.
(From: David Schaafsma (drdave@jnpcs.com) and Rajiv Agarwal (agarca@giascl01.vsnl.net.in).)
A couple of minor points:
High Q cavities narrow the spatial profile only if they are confocal - planar high Q cavities (as in diode lasers, and especially vertical-cavity diode lasers) are prone to problems with walk-off and the mode must be confined physically.
In a gas laser, you also start with a much narrower fluorescence line and thus the gain spectrum is limited spectrally. Diode lasers (being band-to-band or excitonic semiconductor transitions) have much broader fluorescence spectra.
The typical edge-emitting diode laser actually lases in quite a few fundamental modes (especially when operated using its own facets as the cavity) and though each lasing mode is "monochromatic", the overall spectrum really isn't. External cavities are really the only way to obtain approximately single mode operation from an edge-emitting diode laser.
VCSELs are usually true single mode devices. The reason you can get away with lengthening the cavity in a gas laser is that you don't need to worry about lowering the free spectral range because the gain bandwidth is small.
DFB or DBR lasers achieve very similar results and have Side mode suppression ratios better than 30 db. These lasers have been the mainstay of Optical fiber base telecom for a while now.
DFB Lasers are use for long haul telecommunications network - the kind used by say Sprint (>1GB for up to 25 miles) for their phone networks between cities. These have been for Trans-Atlantic cables (TAT) between US and Europe. LEDs are used more for FDDI type application between computers (~100Mb and less than 1 mile).
(From: Vishwa Narayan (vishwa.narayan@ericsson.com).)
While LEDs are quite popular in Datacom applications (read short distances), Telecom applications typically use DFBs, either directly modulated for low speeds (e.g., OC-3 155 Mb/sec) or externally modulated for high speeds (e.g., OC-48 2.5 Gb/sec). Distances can typically range over tens of kilometers, to hundreds of kilometers with optical amplification, sans repeaters.
One should never look into the beam of any laser - especially if it is collimated. Use an indirect means of determining proper operation such as projecting the beam onto a white card, using an IR detector card or tester (where needed), or laser power meter.
Currently, green laser pointers are not simple diode lasers but are Diode Pumped Solid State Frequency Doubled (DPSSFD) lasers (this may change in the future, however). For a given power, green appears substantially brighter than red wavelengths but are also limited a maximum power of 5 mW. However, since there is a high power IR laser diode inside a green pointer and not all include an adequate IR-blocking filter, there could be other dangers lurking even if the green output is weak or dead.
(From: Gregory Makhov (lsdi@gate.net).)
According to a recent report by Dr. David Sliney, who is one of the leading "gurus" of laser safety, there are no confirmed accidents or injuries caused by laser pointer of 5 milliwatts radiant power or less. There is an awful lot of nonsense and false claims about this. Pointers are extremely bright, can cause visual distraction, afterimages, and other effects, such as headaches, but under most any typical usage condition, DO NOT cause eye injury. Dr. Sliney works for US Army, and has published papers and books on laser safety for over 20 years.
With both of these, the beam from the bare laser diode is highly divergent and therefore less of a hazard since the lens of the eye cannot focus it to a small spot. However, there is still no reason to look into the beam.
With these high power laser diodes, even the divergent beam from the bare device is a definite hazard at close range. Where there are collimating optics (even an almost invisible microlens), the result is a mostly or totally invisible beam that can be dangerous to vision from direct exposure and specular reflection at distances of several feet. These are particularly scary especially for people who have become complacent about diode laser safety due to their expectation of a widely diverging beam.
For IR laser diodes in particular, especially if you are considering selling a product:
(Portions from: Steve Roberts (osteven@akrobiz.com).)
You need to take a close look at the CDRH rules, because there is no blink reflex in the IR. IR diode lasers are considered much more dangerous and therefore are in a higher class. CDRH has a curve of power versus wavelength that is used for determining safety classes. The only way a IR laser gets less then a IIIb rating (read: dangerous) is if the beam is totally enclosed or of very low power. Go to CDRH, call them and request a manufacturers' packet by mail. It's huge and confusing, but covers the requirements for products using IR laser diodes such as 3-D scanners, perimeter sensors, and so forth.
Visible laser diodes have replaced helium-neon lasers in supermarket checkout UPC scanners and other bar code scanners, laser pointers, patient positioning devices in medicine (i.e., CT and MRI scanners, radiation treatment planning systems), and many other applications. The first visible laser diodes emitted at a wavelength of around 670 nm in the deep red part of the spectrum. More recently, 650 nm and 635 nm red laser diodes have dropped in price.
Due to the nonuniformity of the human eye's response, light at 635 nm appears more than 4 times brighter than the same power at 670 nm. Thus, the newest laser pointers and other devices benefitting from visibility are using these newer technology devices. Currently, they are substantially more expensive than those emitting at 670 nm but that will change as DVDs become popular:
Laser diodes in the 635 to 650 nm range will be used in the much hyped DVD (Digital Video - or Versatile - Disc) technology, destined to replace CDs and CDROMs in the next few years. The shorter wavelength compared to 780 nm is one of several improvements that enable DVDs to store about 8 times (or more - 4 to 5 GB per layer, the specifications allow up to 2 layers on each side of a CD-size disc!) the amount of information or video/audio as CDs (650 MB). A side benefit is that dead DVD players and DVDROM drives (I cannot wait) will yield very nice visible laser diodes for the experimenter. :-)
Like their IR cousins, the typical maximum power from these devices is around 3 to 5 mW. Cost is in the $10 to $50 for the basic laser diode device - more with optics and drive electronics. Higher power types (10s of mW) are also available but expect to spend several hundred dollars for something like a 20 mW module. Very high power diode lasers using arrays of laser diodes or laser diode bars with power output of WATTs or greater may cost 10s of thousands of dollars!
___
| | Metal case
| |_______________________________
| \
| _____________________________ |
| | | |
LD -------:===:------------------+ | |
| |__ | |__|
| | |___ ______|______ : :
| | | | | | : :
PD -------:===:----+ |<---|:::::::::::::|============> Main beam
| | |___|____|_____________|_ : : (divergent)
| | Photodiode Laser diode | :__:
| |\__________________________| | | Protective window
Com -------+ | Heat sink | |
| |_____________________________| |
| |
| _______________________________/
| |
|___|
The main beam as it emerges from the laser diode is wedge shaped and highly
divergent (unlike a helium-neon laser) with a typical spread of 10 by 30
degrees. External optics are required to produce anything approaching a
parallel (collimated) beam. A simple (spherical) short focal length convex
lens will work reasonably well for this purpose but diode laser modules and
laser pointers might use a lens where at least one surface is aspheric (not
ground to a spherical shape as are with most common lenses).
In the case of a sample I removed from a dead diode laser module, the surface facing the laser diode was slightly curved and aspheric while the other surface was highly curved and spherical. The effective focal length of the lens was about 5 mm. It appeared similar to the objective lens of a CD player - which was perhaps its original intended application and thus a low cost source for such optics.
Due to the nature of the emitting junction which results in a wedge shaped beam and unequal divergence (10 x 30 degrees typical), a laser diode is somewhat astigmatic. In effect, the focal length required to collimate the beam in X and Y differs very slightly. Thus, an additional cylindrical lens or a single lens with an astigmatic curvature is required to fully compensate for this characteristic. However, the amount of astigmatism is usually small and can often be ignored. The general beam shape is elliptical or rectangular but this can be circularized by a pair of prisms.
The light from these edge emitting laser diodes is generally linearly polarized. You can easily confirm this even with a simple laser pointer by reflecting at about a 45 degree angle from a piece of glass (not a metal coated mirror). Rotate the pointer and watch the reflection - there will be a very distinct minimum and maximum with the elongated shape of the beam at close range being aligned with the glass and perpendicular, respectively. For the advanced course, determine the Brewster angle. :)
For addition information, see the section: Beam Characteristics of Laser Diodes.
The beam from the back end of the laser diode chip hits a built-in photodiode which is normally used in an opto-lectronic feedback loop to regulate current and thus beam power. Note that the photodiode is likely mounted at an angle (not possible to show in ASCII) so that the reflection does not interfere with the operation of the laser diode.
CAUTION: Some complete modules may use the reflection from external optics along with an external photodiode for power stabilization as it is more accurate since the actual output beam is sampled. For these, one should never attempt to clean or even focus the lens when operating near full power as this may disturb the feedback loop and damage the laser diode.
Note: Some of the symbols below are not exactly what is found in the datasheet so they can be represented in ASCII. However, the meaning should be obvious.
Parameter Symbol Conditions Min Typ. Max Unit
------------------------------------------------------------------------------
Threshold current Ith 30 40 mA
Operating current Iop Po = 5mW 35 45 mA
Operating voltage Vop Po = 5mW 2.2 2.4 V
Wavelength lambdap Po = 5mW 650 660 nm
Radiation angle
Perpendicular theta_|_ Po = 5mW 22 30 40 Deg.
Parallel theta|| Po = 5mW 5 7 12 Deg.
Positional accuracy dx,dy,dz Po = 5mW +/-150 um
Angular accuracy
Perpendicular phi_|_ Po = 5mW +/-3 Deg.
Parallel phi|| Po = 5mW +/-3 Deg.
Differential eff. nD Po = 5mW 0.3 0.6 0.9 mW/mA
Astigmatism As Po = 5mW 7 15 um
Monitor PD current Imon Po = 5mW, Vr = 5V 0.05 0.1 0.25 mA
Descriptions of the parameters are provided below:
"I was just browsing Meredith Instrument's site, and noticed that they have 635 nm diodes rated at 500 mW. Has anyone ever dealt with these things? Looking around on the site, it appears I could put together a half watt red diode laser for under $600, or a 250 mW one for under $400. Is there some catch to using these? The whole setup would be cheaper than a 25 mW HeNe laser".
Yes. Aside from the ease with which one of those pricey diodes can be blown out due to improper drive, the beam quality is no where near that of even a cheap HeNe laser. It is multimode and very non-circular and astigmatic. The latter can probably be dealt with using some (expensive) optics. However, multimode operation means that these are unsuitable for applications like holograpy or interferometry.
(From: Frank DeFreitas (director@holoworld.com).)
I have a 500 mW laser diode from Polaroid. 660nm I believe. It needs the heftier driver that Meredith offers - the one that can put out 1000 mA or so. The laser diode is gain guided/multi-mode, rather than index guided/single (mono) mode -- so you can pretty much forget any application that would call for any type of coherency or high contrast fringes.
The output beam profile is basically a line. It is very similar to taking a standard HeNe beam and sending it through a cylindrical lens. (However, on the other hand, I'm wondering if a cylindrical lens would actually help it when used in the other dimension. Or at least bring it to a spot which could be collimated utilizing secondary optics in the path.)
I'd also like to point out that it's not a diode to play around with. The optical output at 500 mW is not going to knock any missles out of the sky, but will certainly warrant caution when working with the beam. The beam is much more powerful than it appears at 660 nm due to the eye's reduced sensitivity at that wavelength compared to HeNe 632.8 nm.
About those laser diode bars:
(From: Walter Skrlac (Walter.Skrlac@t-online.de).)
"Bars are a 10 mm wide chip with typically 16 to 24 emitters, each emitter being about 150 microns wide and emitting up to 2 watts of power per emitter. The highest power for solid state laser pumping is 40 watts from a 19 emitter bar. Almost all bars are a single chip, multiple emitter device. I do know that in the beginning days of bars, Siemens produced a 5 watt device consisting of 5 separate 1 watt laser diodes mounted in a row 10 mm long. The individual laser diodes are connected in parallel so you can't switch them individually."
The good news is that this technology is developing very rapidly.
The bad news from our perspective is that there are no really low cost sources, new or surplus, for these diode lasers as far as I know at the present time. However, prices have been dropping rapidly since this was first written. The cost of 1 W 808 nm laser diodes has dropped below $100 new, and with luck, much cheaper from surplus sources and on eBay.
Actually, it isn't necessarily the diode itself that is so expensive. A 1.5 W 800 nm diode chip goes for about $10 when they are purchased in reasonably large quantities. However, these are only about 0.5 mm on a side and maybe 0.1 mm thick. Mounting means using low temperature solder and flux to bond the chip to a large heat sink and copper strip (for the two connections - no monitor photodiode, that function must be performed externally). The soldering is best done on a hot plate (to raise the temperature of the heat sink and chip to almost the melting point of the solder), with a fine tip iron for the last few degrees. They have an HR and OC side, and a top and bottom, and thus orientation matters. So, if you have access to a surface mount rework station with a stereo microscope, a steady hand, infinite patience, and don't sneeze much (which will blow your chips away to never be found again), you could try your hand at the mounting. I have a couple of these diode chips so once I get up the nerve to try this, I will report on success or failure.
The better way to deal with these laser diodes is to have them already mounted on a heat sink. But now we're talking about $100s for a single unit. But, for a number of reasons, the best type of high power laser diode to get is probably a fiber-coupled module. Then you don't have to mess with beam shape issues, the diode is safely tucked away out of harm, and the fiber output can easily be adapted to your favorite crystal shape. Some power is lost in the coupling but it appears as though the specs I've seen are similar for the bare diode assembly and fiber-coupled module. Of course, the cost for such a module now appoaches that of a nicely equipped PC. :) For more info, see the section: Anatomy of Fiber-Coupled Laser Diodes.
Laser diode bars/assemblies of much higher power are available - up to the kW range and beyond. Of course, the prices go up as well. Check out CEO Laser as one possible supplier. They have a wide variety of really interesting items but unfortunately without any prices. Bars can be connected in series to ease the power supply requirements enabling them to be driven with lower current at higher voltage (e.g., a 4 bar configuration would use 8 V at 50 A instead of 2 V at 200 A). With individual chips on a common heat sink, this really isn't an option.
Note that most high power diode lasers are near IR - often around 800 nm for pumping DPSS lasers or 830 to 870 nm for thermal platesetters. High power visible laser diodes are much less common and usually limited to less than a watt at 670 nm. Not that this is terrible. :)
If you have your heart set on one of these for your birthday, all I can suggest at the present time is to keep track of what is available surplus and to check with the manufacturers listed in the chapter: Laser and Parts Sources. They do show up on eBay but accuracy of the description and operating conditoin may be unknown. If this is for some sort of academic project with a legitimate research objective, you may be able to obtain a cosmetic reject or one that doesn't quite meet specs by persistent pleading with one of the laser diode manufacturers. Or, if you can deal with the bare chips, it may be possible to beg a few from one of the companies that produces DPSS laser systems since they use them by the carload, and when purchased by the carload, the cost goes way down.
Keep in mind that obtaining the diode is only a small part of the problem. To drive them reliably, particularly near their maximum power rating, will require a suitable constant current laser diode driver and proper cooling. However, if reasonable precautions are taken and they aren't run near their maximum ratings, actually blowing them out totally isn't nearly as easy as with their low power counterparts.
And, needless to say, at these power levels, your eyes (and flammable objects) don't get a second chance - laser safety must be at the top of your list of priorities.
These laser diodes come in plastic packages that look much like LEDs and thus there is no real possibility of decent cooling. Therefore, power dissipation is one of the major limiting factors. It may be possible to use a lower peak current with a longer pulse width than what's specified in the datasheet as long as the average power dissipation rating isn't exceeded. However, with the high threshold current, this probably doesn't provide much benefit. And, no guarantees of any kind with laser diodes!
There is some info on driver circuits for pulsed laser diodes in the section: Pulsed Laser Diode Drivers.
The following assumes a device rated at 16 W peak power, 100 ns max pulse width, 0.1% max duty cycle:
(From: Roithner Lasertechnik" (office@roithner-laser.com).)
The absolute limit is the heat stress of the LD chip inside. Under normal conditions, the laser will emit a pulse of the rated 16 W, 100 ns at 10 kHz (200 ns at 5 kHz is the absolute limit) - which is highly recommended for an expected long lifetime of several khours with usual chip degradation. Take this integrated V x I (voltage x current) thermal heat stress as a final constant. If you run with a higher frequency than the rated, but with a shorter pulse width, still never go higher than this constant. If you go higher, the laser pulse power will go down rapidly due to overheating of the LD chip (still reversible, LD is not yet blown) but overall lifetime is shortened. Keep in mind, that the rise and fall time of this LD is typically 1 ns, so you will get the next limit soon.
VCSELs, on the other hand, emit their beam from their top surface (and potentially bottom surface as well). The cavity is formed of a hundred or more layers consisting of mirrors and active laser semiconductor all formed epitaxially on a bulk (inactive) substrate.
This approach provides several very significant technical advantages:
The beam from a typical VCSEL exits from a circular region 5 to 25 um in diameter. Since this is much larger than for the FP laser diode, the divergence of the resulting beam is much lower. And, because it is also circular, no corrections for asymmetry and astigmatism are required - a simple lens should be able to provide excellent collimation.
There are also numerous manufacturing and cost advantages:
On the other hand, an entire wafer of VCSELs can be tested as a unit with each device evaluated for lasing threshold and power, and beam shape, quality, and stability, It is possible to form millions of VCSELs on a single wafer as a batch process and then test and evaluate the performance of each one automatically. The entire wafer can be burned in to eliminate infant mortalities and assure higher reliability of the final product. Each device can then be packaged or thrown away based on these findings.
For a general review article, see: "The Ideal Light source for Datanets", K.S. Giboney, L.B. Aronson, B.E. Lemoff, IEEE Spectrum V.35 (2) p. 43, Feb 1998.
If you want to play with VCSELs, bare chips, packaged chips, and even VCSEL arrays are available from various laser suppliers and prices aren't totally rediculous. For example, see Roithner Lasertechnik's VCSEL Page. Available wavelengths are currently 780, 850, 980 nm, but wavelengths beyond 1,300 nm are available from other suppliers and the range is being extended in both directions.
If you suspect that one of your laser diodes might be a VCSEL without admitting it, just check the raw beam pattern. The output of a VCSEL will be fairly symmetric while that of an edge emitting laser will typically have a 4:1 angular spread as discussed above.
There is also something called a "Resonant Cavity LED", which in essence places an LED junction between mirrors. Some of these efforts result in stimulated emission with the appearance of a longitudinal mode structure, but not enough gain to reach lasing threshold. However, I'm not sure if these structures differ from VCSELs in any fundamental way. See, for example: Stimulated Emission from InGaN-Based Resonant Cavity Light Emitting Diodes.
The first commercial OPSL was the Coherent, Inc. "Sapphire", a replacement for low power argon ion lasers at 488 nm. (I think the use of Sapphire is unfortunate as this has absolutely nothing to do with the Ti:Sapphire laser with which it may be confused.) The Sapphire is a Vertical External Cavity Surface Emitting Laser (VECSEL), but one that is optically pumped. (Also see the next section.) The resonator is in many ways similar to that of a frequency doubled Diode Pumped Solid State (DPSS) laser but with an InGaAs quantum-well semiconductor instead of a laser crystal as the gain medium. It is pumped by a high power 808 nm laser diode and lasing at the fundamental IR wavelength of 946 nm. This is intracavity doubled to 488 nm.
Go toe Coherent, then "Lasers and Systems", "OPSL" for more information.
One beauty of the OPSL approach is that with an appropriate choice of material and doping, the basic gain medium - the semiconductor disk - can be designed to lase at most or all of the range from 635 nm to 1,500 nm and beyond. (The UV/blue area is probably not viable yet). Thus, this entire range of wavelengths as well as one half the wavelength (frequency doubled or SHG), and possibly higher harmonics are available with the beam characteristics of a solid state laser. This technology may also be called a "Semiconductor Disk Laser".
Several other companies are developing lasers using a similar approach and systems at many wavelengths - including those in the yellow/orange "no laser land zone" - are now available or will be in the near future. And, apparently, some companies call their OPS lasers "DPSS" even though strictly speaking, they aren't solid state in the traditional sense. There are a couple of ways of telling if a DPSS laser is really solid state:
The first is the extended cavity semiconductor laser like the optically pumped Coherent Sapphire, but they may also be electrically driven like a normal laser diode (so only one laser instead - no pump diodes), and mentioned in the next section. However, many companies are now using a directly doubled diode approach - a laser diode feeding a doubler crystal outside the laser cavity, possibly periodically poled lithium niobate (PPLN) or KTP (PPKTP). This is probably not efficient enough to be practical for high power lasers, but for a 10s of mW, it's much simpler.
Now, if all the light bulbs in the World were replaced with these high efficiency laser diodes mass produced in visible wavelengths, the energy crisis would be over since it's been estimated that half the World's energy usage goes into lighting and most of this is presently highly inefficient. Incandescent lamps are only about 5 percent efficient; halogen lamps around 7 to 10 percent; and fluorescents, about 15 to 20 percent. High brightness LEDS suitable for lighting applications are advancing but are currently somewhere around halogen lamps in efficiency (though under some conditions, LEDs at low power may exceed 25 percent efficiency). But, it's unlikely that the LED could even match the laser diode due to the basic physics.
A side benefit of mass produced laser light bulbs might be that hobbyists' access to high power lasers would be greatly improved! :)
Before you say that it would be too dangerous to have every table lamp using a high power laser, it would be a relatively simple matter to mold a diffuser onto the laser diode in such a way that it would be virtually impossible to disassemble (sorry hobbyists but maybe if we lobby hard enough, a special tool could be made available!) and then wouldn't be any more dangerous than a common light bulb.
Aside from reducing the cost of high power laser diodes by about 3 or 4 orders of magnitude, wavelength is a definite stumbling block that still needs to be overcome before any of this could be practical. Either red, green, and blue laser diodes will need to be combined in a single lamp assembly to produce something approaching white light or a combination of high efficiency phosphors will be needed to convert near-UV to visible light. One can envision a lighting panel in standard sizes like 2x2 or 2x4 feet that replaced fluorescent ceiling fixtures but used less than 25 percent of their power. Or, CLLs (Compact Laser Lamps) that replaced incandescent or compact fluorescent lamps. Needless to say, high power multicolor or UV laser diodes do not presently exist but a market measured in billions of units compared to current usage of 10s of thousands could provide a lot of incentive to develop them! :)
In particular:
See the section: On-Line Introduction to Lasers for the current status and on-line links to these courses, and additional CORD LEOT modules and other courses relevant to the theory, construction, and power supplies for these and other types of lasers.
Several modules would be of particular interest for diode lasers. Unfortunately, the on-line manuals (in PDF format) have disappeared from the MEOS Web site. But I have found and archived most of them:
If MEOS should complain, these will have to be removed. So, get them while you can! But I doubt they'll complain. And most are also archived at the Wayback Machine Web Site.
Some very good basic information about laser diodes is provided in of all places, manufacturer's catalogs! :) Try companies like Mitsubishi, Fujitsu, Hitachi, Sharp, Sony, NEC, etc. They have introductory sections at the front or the back of their laser diode catalogs. Just call the and ask for a laser diode catalog. Much of this is now on-line.
The divergence angle (half of total), Theta, (in degrees) is given by:
Wavelength * 720
Theta = -------------------------
pi * pi * Beam Diameter
At a wavelength of 670 nm, this works out to about 48 x 16 degrees for a 1 um x 3 um emitter and 48 x 0.48 degrees for a 1 x 100 um emitter (compared to around 0.05 degrees for a 1 mm diameter beam from a 632.8 nm helium-neon laser). However, since laser diodes with 100 um emitters are always multiple spatial mode, this low value of divergence will never be seen in practice. Such diodes tend to have similar divergence to that of single mode emitters.
Note that since at least one of the dimensions of the end-facet is close to the wavelength that the laser diode emits - it may even be smaller - this simple equation is not very precise but typical low power laser diodes do produce beams with a divergence of around 10 x 30 degrees.
Laser diode divergence will generally be given in terms of the Full Width Half Maximum (FWHM) in terms of output power, or "T full width". At the 10% level, this may be more like 70 or 80 degrees than the 30 degrees in the specifications.
For more information (and some medium weight math) on the beam characteristics of common laser diodes, check the Power Technology, Inc. Go to "Resource Library", "White Papers".
There are ways of correcting for all of these artifacts with a single special lens close to the laser diode itself. For example, Blue Sky Research offers combined laser diodes and microlenses which they claim perform as well as larger more expensive diode laser modules using various discrete lenses and prisms to implement the beam correction.
Note that VCSEL (Vertical Cavity Surface Emitting Laser diodes) need not suffer from astigmatism and/or an elliptical beam profile since their emitting aperture can be made to be perfectly symmetrical. I would also expect them not to need to be polarized for this reason as well. See the section: Vertical Cavity Surface Emitting Laser Diodes (VCSELs).
At Philips we used three difference techniques to measure astigmatism in laser diodes:
Without any type of correction, the output of a bare laser diode is more like that from a mediocre flashlight than what is normally thought of as a laser source. Some optics are needed to produce a reasonably well collimated beam (like the one from a cheap laser pointer) and more sophisticated optics are needed to provide optimal beam quality (which can be very good indeed). Of course, depending on the particular application, one or more of these so called 'defects' may actually be considered desirable.
An alternative technique, apparently used in many optical pickups, is to pass the beam through a thick optical plate having parallel sides at an angle (actually combined with the 45 degree beamsplitter mirror when used for this application). This component has a very significant astigmatic effect whose magnitude is easily controlled by selecting the thickness or adjusting the angle of the plate. In the optical pickup, it is used to add astigmatism for the focusing servo but can just as easily be used to eliminate it. See the document: Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives for more information on optical pickup characteristics.
Still another approach which will correct for the elliptical beam profile and astigmatism all at once is to couple the beam into a single mode optical fiber using two short focal length lenses. With a sufficiently long fiber (well, relative to the wavelength), the output beam characteristics will be entirely determined by the quality of the output face of the fiber. Then, a simple collimating lens can be used.
Whatever type of external optics are added, take care that significant power isn't reflected back into the laser diode itself. This can destabilize the lasing process as well as fooling the built-in photodiode into thinking the output power is higher than it really is causing the optical feedback circuit to reduce it.
Some additional comments are provided below:
(Portions from: Mark W. Lund (lundm@physc1.byu.edu).)
A simple short focal length lens will collimate the beam. However, laser diodes tend to be astigmatic which means that you will have one axis collimated at a different focus than the other. A typical value for this astigmatism is 40 microns. A cylindrical lens in addition to the spherical collimating lens or a special lens designed for this purpose can correct this but may not be needed for non-critical applications.
Any camera lens will be able to produce a reasonably well collimated beam (subject to the astigmatism mentioned above). Put the laser diode at the focal point of the lens. If you want the type of narrow beam produced by a HeNe laser, you need a short focal length lens, such as a microscope objective. A good compromise between cheap and short focal length would be an old disk camera lens. These cameras can be found at thrift shops, garage or yard sales, and flea markets for a couple dollars or less.
The longer the focal length the larger your beam will be, but the less effect the astigmatism will have. The diameter of the beam will be the size of the aperture of the lens (in which case you are throwing away light) or the size of the beam at the distance of one focal length, whichever is less.
(From: Steve Nosko (q10706@email.mot.com).)
One thing to note is that the laser diode actually has two apparent point sources. One for the wide axis of the beam and another for the narrow axis. This means that the lens must be more like two crossed cylindrical lenses with different focal lengths. There are different types of laser diodes with varying degrees of this so that some are easier to to design lenses for. There probably are types, by now, where there aren't two.
I think of it like this (right or wrong). The astigmatism has two components. One is the difference in divergence between the two axes. I think this can be even if there is ONLY one apparent point source. It is just a point source with an oval aperture letting light through. The other is the different apparent point sources for the two axes.
I have tested a Blueskyresearch PS106 (now discontinued but similar to the VPSL-0655-007) which is a 650 nm, 7 mW Circulaser(tm). The beam is indeed nearly perfectly circular with a divergence of about 8 degrees FWHM - less than that of the lower divergence (slow) axis of the typical bare laser diode. For datasheets, go to Blueskyresearch, then "Semiconductor Laser Products".
Aside from the convenience of not having to worry about their funny beam shape, putting a microlens next to the laser diode itself results in much more of the light being used compared with what gets through inexpensive external optics. With the typical collimating lens used in laser pointers and diode laser modules, as much as 40% or more of the light from the diode may be wasted largely due to its high divergence in the fast axis (30 or 40 degrees total at the half power point, perhaps twice this at the 10% point) - a very significant fraction gets blocked by the small aperture of the collimating lens.
One supplier is Creative Technology Lasers. They even have a super miniature collimated diode laser module only 4 mm (less than 1/6th of an inch) in diameter which connects directly to a 3.3 VDC power source. Check out their "LS" series of diode laser modules.
Given the many advantages of this approach, I wouldn't be surprised if it becomes most common for visible laser diodes used for applications like laser pointers and barcode scanners.
So for the fast axis, two lenses will produce a diffraction limited collimated beam. A very short focal length cylindrical lens is placed almost touching the diode to reduce the typical 40 degree divergence of the raw diode to a few degrees. This is usually a very thin rod lens or piece of fiber core. A second conventional lens is then used to control the beam diameter and collimation. Note that to only affect the fast axis, this would also be a cylindrical lens.
For the slow axis, an anamorphic prism pair may be used to expand the beam followed by a lens to collimate it. With care in the design, that second lens can be the same spherical positive lens for both axes. But you can also do it with separate cylindrical lenses. A pair of cylindrical lenses can be used in place of the anamorphic prisms
There are many other ways of doing this. For example, the output of the can have just the initial fast axis correction using a fiber lens and then be coupled directly into a multimode fiber. The output of the fiber core is then used as the source for a projection lens. But there may be annoying variations in granularity or speckle with any bending of the fiber, temperature changes, vibration, etc.
However, this general rule appears not to apply for all laser diodes including those in many common diode laser modules and even cheap ($9.95) laser pointers. These are now being used routinely for experiments in interferometry and even holography. While their stability over time (e.g., wavelength drift and susceptibility to mode hopping) - is probably less than stellar, over the short term, coherence lengths of 20 cm or more are not unusual. This is similar to that of a typical helium-neon laser.
For more on applications that may benefit from long coherence length diode lasers, see the sections: Interferometers Using Inexpensive Laser DiodesCan I Use the Pickup from a CD Player or CDROM Drive for Interferometry?. Also see the section: Holography Using Cheap Diode Lasers.
(From: Mark W. Lund (mlund@powerstream.com).)
The 1970's grade pulsed laser diodes have coherence lengths of 500 microns or so. Modern CW single mode diodes have coherence lengths of meters. I once asked Don Scifries why they had such long coherence lengths compared to gas lasers with much larger cavities and he referred me some papers. The impression that remains after 13 years is not that laser diodes are so good, but that HeNe Lasers are so bad. Line width of a typical 780 nm CD laser can be 10s of kHz.
(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)
**Crudely**, a CW laser will go SLM (Single Longitudinal Mode) spontaneously if the mode separation exceeds the *inhomogeneous* linewidth. The homogeneous linewidth can exceed the mode separation because inter mode competition suppresses the other modes CW. But if mode than mode falls within an inhomogeneous width, and is above threshold, all may oscillate as they do not compete.
The coherence length of a HeNe laser is a simple matter: inhomogeneous linewidth set by Doppler broadening, mode separation set by length, usually a few modes run (or it would power cycle badly) so coherence length is approximately the cavity length divided by number of modes. When it goes single mode (but, unless stabilized, very unstable power output) the coherence length is typically huge. *AND* the absolute frequency is then pretty stable, within half a mode spacing of the atomic line. Simple HeNes are so 'bad' to get reasonable power stability as the cavity length drifts; less than 3 modes->poor.
Most diodes have a pretty broad spontaneous linewidth and how much it is homogeneous or inhomogeneous I'm not clear; possibly as manufacturing has improved the inhomogeneous component has tended to reduce to below the mode spacing? Cavity length is way sub-mm, so as soon as it does twin mode the coherence length is awful.
I have *directly* measured the output spectrum of many near IR diodes, and all bar one set were severely multimode. One set (normal FP lasers) were all single, which surprised me. I think I've only looked at one visible (a while back) and it was heavily multi mode.
When a simple diode does go SLM, surely one might expect it still to have pretty severe wavelength drift with chip temperature? This can certainly wreck holography.
Obviously people have found pragmatically you can get away without an expensive DFB laser; that crude diodes can be SLM; it opens up the interesting qn of just why it seems modern diodes are tending to go SLM spontaneously, & how stable the output wavelength is when they do go SLM (order nm/degree from memory?)
(From: Bret Cannon (bdcannon@owt.com).)
There are two temperature tuning rates for a diode laser, one is the tuning of a given longitudinal mode with temperature and the other is the tuning over larger temperature changes where the lasing mode hope from longitudinal mode to longitudinal mode to be close to the peak of the gain curve. The average tuning rate for this later rate is typically 0.3 nm/°C while for small enough temperature changes the tuning of longitudinal mode is much smaller. For a temperature stability of 1 mK a diode laser frequency is stable to better than 0.001 cm-1, perhaps even a good as 0.0001 cm-1 as determined by tuning onto a Doppler-free atomic transition. Thus at 780 nm the temperature tuning of a longitudinal mode is less than 0.06 nm/°C. With a temperature tuning of less than 1 cm-1/C, a temperature stability of 0.1 °C during an exposure would give a coherence length longer than 10 cm.
Unless there is external optical feedback or a very sophisticated electronic feedback there is no way that a 780 nm CD laser would have a linewidth of 10s of kHz. With a sufficiently low noise current supply (less than 1 microamp RMS in a 1 MHz bandwidth) and temperature stabilization to about 1 mK, the intrinsic linewidth of diode lasers can be measured and they are proportional to the inverse of the output power. Linewidths of about 50 MHz for a 3 mW laser and 5 MHz for a 30 mW laser are typical. These linewidths are 5 to 50 times the Shawlow-Townes linewidth for these lasers and results from the coupling of the refractive index and the population inversion. Moradian (sp?) who was at MIT at the time published experimental measurements in the late 1970s and early 1980s. Henry published an analysis of this line broadening mechanism but I don't remember exactly when.
The linewidth decreases with the square of the cavity length and with external cavities a few cm long people have achieved linewidths of less than 1 kHz. An example of this is work by Leo Holberg and colleagues at NIST in Boulder for an optical clock based on an inter-combination line in optically cooled and trapped atomic calcium.
It depends on the laser diode, the power supply that is used, and the external optical feedback into the diode laser. With a single longitudinal mode diode, without external optical feedback, and a current noise of less than 1 uA RMS in a 1 MHz bandwidth, you can get linewidths of 10 MHz for a coherence time of nanoseconds. With optical feedback the linewidth can collapse to a few Hz or explode to several terahertz, depending on its intensity and the delay time between the light leaving the diode and returning to it.
The wavelength shift for 808 nm diodes is generally around 2.5 nm (+/- 0.2 or 0.3 nm) per 10 °C (or just say 0.3 nm/°C)(, with the wavelength shift to the red (longer) with increasing temperature.
For the violet/blue Nichia laser diodes, it's typically 0.04 nm per °C.
Note that diode current also affect wavelength, partially due to temperature. So, as a diode ages and requires more current for the same output, its wavelength will also change.
(From: Lynn Strickland (stricks760@earthlink.net).)
It really depends on the laser (i.e., manufacturer) and temperature range you are talking about. A good rule of thumb is 0.3 nm per °C over the operating temperature range of the device (About 30 GHz per °C). That's the average slope of the curve though - it includes mode hops. If you're operating at a mode hop, you can get a lot more change than 30 GHz with a 1 °C temperature change. If you are between mode hops, it can be much less.
Mode hops can be a moving target too. Optical feedback can cause them (even minute amounts). Or, you can operate at a specific temperature where there are no mode hops today, but next week it might mode hop at that temperature.
Note that you can only go so far if you want to use temperature to reduce the wavelength. Even if you got the electronics to work under frigid conditions, there is a minimum laser wavelength you can get from a particular diode laser chip. I'm not a physicist, but it has to do with the bandgap of the materials used. What you would get, as you cooled the thing, is lower and lower threshold current, lower operating current, and longer lifetime.
(From: Richard Alexander (pooua@aol.com).)
Back in the old days, about 15 years ago, the only way to get visible light from a laser diode was by using cryogenic cooling. My textbooks from my laser degree program only knows of this type of visible laser diode (they were written in the early '80s). The first room temperature visible laser diode was invented about 1991; I still have a "Radio-Electronics" issue mentioning it.
(From: Flavio Spedalieri (fspedalieri@nightlase.com.au).)
All laser diodes have a tolerance when it comes to wavelength, these tolerances can be as high as +/- 10 nm.
The wavelength tolerances are due to thermal effects, and current. As the diode heats up, the wavelength will change 0.3 nm/°C. and results in mode-hopping.
There are several types of noise in laser diodes: mode hopping as a temperature effect; intensity noise related to spontaneous emission; optical feedback due to reflection in the optics; speckle noise. What you try to control is mode hopping and optical feedback noises.
As temperature varies, shift between modes is an issue but is intrinsic to the LD. By using cooling elements, temperature is roughly regulated.
Optical feedback is part of the light emitted which returns to the laser cavity after reflection on the mirrors but mainly after reflection on the disk surface. The optical feedback varies from one system to another, and from one disk to another, and even according to the area of the disk. A maximum is about 5 to 8%.
Anyway, it has about the same effect as temperature, with mode hopping that appears. The hops are kind of random with the optical feedback. Globally, the RIN increases. The phenomenon is the most apparent with monomode lasers. Measures show that multimode lasers have a much more constant RIN with optical feedback. Adding a HF modulation makes the LD to be driven multimode. That's why DVD manufacturers use a modulator. They usually use single mode lasers as they have better characteristics (lower noise, lower lasing threshold).
What I still don't entirely get is how the modulation works and its effects. The system works fairly well only if the amplitude and the frequency are high enough. The signal amplitude is such as you are alternately in the linear curve and below the threshold (where the laser is always multimode), and the frequency is well above the speed of transmission (somewhere between 300 - 800 MHz, according to the application and the LD).
However, neither of these devices is designed to be modulated at any more than a couple of Hz (if that) due to the heavy internal filtering to protect the laser diode from power spikes. Therefore, they are generally unsuitable for laser communications applications (though some laser pointers are so cheaply designed that such protection may be absent entirely). See the section: The Benefits of Cheap Laser Pointers for Modulation.
Common visible laser diodes have a maximum optical output power of 3 to 5 mW. Due to the sensitivity curve of the human eye, a wavelength of 635 nm appears at least 4 times brighter than an equivalent power level at 670 nm. Thus, shorter wavelength laser diodes will be best where maximum visibility is important.
Where the use of a diode laser module or laser pointer is suitable for your application, I would highly recommend this over attempting to cobble together something from a bare laser diode and homemade power supply - or even a commercial driver if it isn't explicitly designed for your particular laser diode. It really is all too easy to fry expensive laser diodes through improper drive or handling. Once blown, laser diodes don't even work very well as visible LEDs!
See the chapter: Laser Parts Sources for a number of suppliers of both diode laser modules and laser pointers. In additiona, Don's Klipstein (don@misty.com) maintains a Web page with a List of Suppliers of Inexpensive Lasers. While not exhaustive, it does include some popular distributors and he does strive to keep it reasonably up to date. Some of these companies now sell laser pointers for under $6! Pretty soon, you will be able to find free laser pointers in cereal boxes. :)
However, there is no way to know how reliable or robust an inexpensive laser pointer will be - or if the beam quality is acceptable before purchase. Diode laser modules are generally more expensive and of higher quality (though not always) so they may be a better bet for serious applications. Also consider a helium-neon laser since even the cheapest type is likely to generate a beam with better beam quality than the typical diode laser module or laser pointer. While any Tom, Dick, or Harry, can put together a laser pointer of questionable design from readily available parts and sell it on the Internet, only a handful of companies manufacturer HeNe tubes and their quality is all very high. With a HeNe laser, the tube alone determines most of its characteristics requiring at most a simple lens to collimate or focus the beam. See the chapter: Helium-Neon Lasers for more information.
The best source for inexpensive medium power (above 5 mW to approximately 150 mW) visible red (~650 nm) laser diodes are DVD burners. Some high performance units have diodes of up to 100 mW or more and they are dirt cheap - much cheaper than trying to buy the laser diodes individually from the manufacturer. In fact, dead DVD burners may have perfectly good laser diodes as the drive circuits for these are probably quite will designed and the diodes are high quality. But if your Dad's DVD-RW drive suddenly stopped working just before your laser projector was completed, you dind't hear this from me. :)
(From: Dr. Bob (stanwax@hotmail.com).)
I have recently destroyed a couple of Liteon X16 DL DVD-RW drives. I bought them new (retail boxed) for $32 just to rape them for the diode (they are even cheaper now). Unfortunately I don't have the manufacturer or specs for the diode but I have driven one at 200 mA, and with a laser check set to 658 nm I measured 150 mW. Now that is a good deal - though ultimately wasteful, it's a good price for a 150 mW diode. I have combined one of these into a projector with a DPSS green laser and modified the diode driver to provide analogue blanking. The results are pretty good. I did turn the green laser output down (it kicks out 90 to 100 mW normally) so that the red isn't swamped out with the red laser operating at about 120 mW.
The first laser-based laser pointers used helium-neon (HeNe) lasers with their high voltage power supplies packaged as compactly as possible but still required a separate power pack or bulky case which included heavy batteries. Being true lasers, the beam was very clean and well collimated. Both red and green HeNe laser pointers were produced (yes, HeNe lasers come in green).
But the real laser pointer revolution came about as a result of the development of inexpensive visible laser diodes. Laser diodes are only slightly larger than a grain of sand, run on low voltage low current, and can be mass produced - originally driven by the CD player/CDROM revolution, barcode scanners, and other applications where a compact low cost laser source is needed. Now manufactured by the millions, these laser diodes cost well under $1.
Most green laser pointers have in the past used Automatic Current Control (ACC) - a constant current driver. The result is generally fluctuations in output power as the pointer heats up. These may be quite large and result in either a very dim spot or an excessive and illegal super bright beam. The trend now is to use an APC driver to eliminate variability and also make it harder to "boost" the output to an illegal and dangerous power level.
The shape and size of a pointer is determined largely by the type of battery used. The tiniest red pointers use button cells like the one shown in Components of Simplest Red Laser Pointer. Long thin pointers (red or green) use a pair of AAA Alkaline cells like the one shown in Components of Typical Green DPSS Laser Pointer. The fat squat type shown in Components of Edmunds Scientific L54-101 Green DPSS Laser Pointer uses a CR2 lithium cell. Whether aesthetics determines the battery design choice or vice-versa is anyone's guess.
In general, it is best to remove the batteries if the pointer won't be used for even a short time. Batteries have been known to leak and/or swell, usually once they go dead. This is probably most likely to happen with the cheap carbon-zinc cells provided as original equipment. It's virtually impossible to salvage a pointer once such damage occurs because the cells essentially wedge themselves in place as they expand. :(
By now, you're probably totally confused. My advice: Use the specs for guidance but if you really care about the quality of your laser pointer, try a few out which come with money back no-questions-asked warranties and keep the one you like. If, on the other hand, you just want to use the pointer for presentations (what a concept!) and not to stroke your ego, the cheapest red one will probably be just fine. :)
Wavelength Relative Factor Color Type
----------------------------------------------------------------
555 nm 1.000 33 Green Reference peak
543.5 nm .974 30 " Green HeNe laser
532 nm .885 28 " Green DPSS laser
632.8 nm .237 8 Orange-red Red HeNe laser
635 nm .217 7 " Red diode laser
640 nm .175 5 " "
650 nm .107 3 Red "
660 nm .061 2 " "
670 nm .032 1 " "
The term "Relative" refers to the visibility compared to the 555 nm peak of human vision; the "factor" compares the brightness to that of an older 670 nm pointer. Note that visual perception of brightness is not linear. Thus, a 1 mW 532 nm green laser pointer isn't actually going to appear 28 times brighter than a 1 mW 670 nm red model. What it means is that a 1 mW green pointer will appear similar in brightness to a 28 mW 670 nm red one (if such a thing existed).
As far as I know, CDRH approval will not be granted for any device of this type over 5 mW actual beam power since their classification would then need to be IIIb. So, don't expect to find a laser diode with an actual output power of 30 mW in anything like a laser pointer! Frankly, I don't understand how laser pointers with an output above 1 mW gain approval in any case. The 670 nm pointers especially (since they APPEAR less bright) represent a definite hazard to vision at close range. Do not underestimate the stupidity of some people who totally ignore all the safety warnings - "Wow, look at these cool afterimages." - and then wonder why their vision never quite returns to normal (though I do not know of any confirmed cases of irreversible damage to vision even from this sort of abuse).
Another popular 'specification' is how far away the laser pointer is visible. What the seller is probably actually referring to is the distance that their Marketing department *thinks* the beam should be visible so long as this value is greater than that of their competition. :-)
Seriously, who knows? There is no standards organization overseeing these ratings. It could be the maximum distance to the screen that the beam is visible:
Another consideration, of course, is whether this requires a moonless night!
Laser pointer marketers don't appear to have discovered (3) as yet (most likely due to liability issues) since the number would be extremely impressive - being in the many miles range! Apparently the Space Shuttle astronauts were able to see a 5 mW red HeNe laser (632.8 nm, similar to the best red laser pointers) from orbit, about 250 miles or 1.3 million feet. Claims could be even more impressive for a green DPSS laser pointer (532 nm), being about 5 times brighter for the same output power. Any marketing types reading this? :)
A common red laser pointer contains the following components as shown in Typical Red Laser Pointer:
See the section: Basic Characteristics, Structure, Safety, Common Types
However, The Far East imports now flooding the market use only a resistor to limit current - driving the laser diode just like an LED. The circuitry consists of only 4 parts: laser diode, resistor, switch, battery. Apparently, the type of laser diode they use has a wider operating range and can be driven safely this way, though the output brightness will decrease as the batteries are drained. See Components of Simplest Red Laser Pointer and Closeup of Laser and Mount from Simplest Laser Pointer. The inset in the first photo shows the laser diode chip itself attached to a tiny metal block which is soldered directly to the cast metal which acts as a heatsink. The top contact is a 1 mil gold bonding wire.
Without the schematic there is no way to know how much protection is provided by the driver. With some, the diode which can easily be destroyed in an instant by using the wrong type of batteries, an external power source (even one that you would think should work), or even putting the batteries in backwards. The best designs will use a circuit that regulates optical output based on feedback from the laser diode's built-in monitor photodiode with respect to a fixed reference (voltage) and maintain output power nearly constant under the battery is almost totally drained.
On most pointers and diode laser modules, the laser diode driver is on a tiny printed circuit board soldered directly to the leads of the laser diode package. However, on some, the driver may be right next to the diode, sealed in metal and look like part of the diode can, but isn't (possibly glued or press-fit). This is likely the case if what appears to be the laser diode only has two leads - all the visible laser diodes I know of come in 3 (or possibly 4) lead packages to accommodate the monitor photodiode connections.
See the sections: Power Regulators in Laser Pointers and: Laser Diode Driver from Cheap Laser Pointer (LP-LD1).
Note that with the typical optics used in laser pointers, as much as 40% or more of the light from the diode may be wasted largely due to its high divergence in the fast axis (30 or 40 degrees total at the half power point, perhaps twice this angle at the 10% point) - a very significant fraction gets blocked by the small aperture of the collimating lens. I found that an NVG D660-5 laser diode with an NVG collimating lens resulted in just about a 50% loss between what was measured with the sensor of the laser power mete
