01.B Diode Lasers

Introduction:
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Note: throughout this document, we will use the terms 'laser diode' and 'diode
laser' somewhat interchangeably.

Diode lasers use nearly microscopic chips of Gallium-Arsenide or other exotic
semiconductors to generate coherent light in a very small package.

Laser diodes are solid state devices not all that different from LEDs. The first laser diodes 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 *watts* of optical power. The most common types found in common devices like CD players and laser pointers have an output in the 3 to 5 mW range.

However, unlike LEDs, laser diodes require much greater care in their drive electronics or else they *will* die - instantly. See the sections on CD and visible laser diodes, below, before attempting to power or even handle them.

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), efficient (especially compared to the gas lasers they replaced), rugged, and long lived if treated properly.

They 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 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, most laser diodes would not be suitable light sources for holography or interferometry, for example.

However, 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 as a light source as unwanted speckle and interference effects are greatly reduced.

The most common types on the planet by far are those used in CD players and CDROM drives. These produce a (mostly) invisible beam in the near infrared part of the spectrum at a wavelength of 780 nm. The optical power output from the raw laser diodes may be up to 5 mW but once it passes through the optics, what hits the CD is typically in the .3 to 1 mW range.

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), 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-orange 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 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 :-).

How do I use a Visible Laser Diode?

--------------------------------------------

The quick answer is *very carefully* for two reasons: I am assuming this is a typical 3 to 5 mW visible laser diode probably emitting at a wavelength in the 635 to 670 nm range.

1. You can easily destroy the typical laser diode through instantaneous overcurrent, static discharge, probing them with a VOM, or just looking at them the wrong way :-).

By far the easiest way to experiment with these devices is to obtain complete laser diode modules. Versions are available with both the drive circuitry and (adjustable) collimating optics. They are more expensive than raw laser diodes but are also virtually foolproof. Inexpensive laser pointers are one source for similar devices which may be adequate for your needs but modifying them is probably difficult. See the chapter: "Parts Sources" for suppliers of both raw laser diodes and laser diode modules.

2. Any time you are working with laser light you need to be careful with respect to exposure of a beam to your eyes. This is particularly true if you collimate the beam as this will result in the lens of your eye bringing it to a sharp focus with possible instantaneous retinal damage.

Typical currents are in the 30-100 mA range at 1.7-2.5 V. However, the power curve is extremely non-linear. There is a lasing threshold below which there will be no coherent output (though there may be LED type emission). For a diode rated at a typical current of 85 mA, the threshold current may be 75 mA. That 10 mA range is all you have to play with. Go to 86 mA (in this example) and your laser diode may be history in the blink of an eye.

This is one reason why most applications of laser diodes include optical sensing to regulate beam power. As the temperature of the laser diode changes (heats with use), the current requirements change as well.

The third lead is for an optical sensing photodiode used to regulate power output when used in a feedback circuit which controls your current. This is very important to achieve any sort of stable long term operation.

You can easily destroy a laser diode by exceeding the safe current even for an instant. It is critical to the life of the laser diode that under no circumstances do you exceed the safe current limit even for a microsecond!

Laser diodes are also extremely static sensitive, so take appropriate precautions when handling and soldering. Also, do not try to test them with an analog VOM which could on the low ohms scale supply too much current.

It is possible to drive laser diodes with a DC supply and resistor, but unless you know the precise value needed or have a laser power meter at your disposal, you can easily exceed the ratings before you realize it.

You might hear someone bragging "I have driven thousands of laser diodes by just connecting them to a battery and resistor and never have blown any". Sure, right. While it is quite possible that the susceptibility to instant damage due to overcurrent varies with the type of laser diode, unless you know the precise behavior, you must err on the side of caution. Some designers have gone to extremes, however.

For an actual application, you should use the optical feedback to regulate beam power. You should also use a heatsink if you do not already have the laser diode mounted on one.

The raw beam from a laser diode is generally wedge shaped - 10 x 30 degrees is a typical divergence. You will need a short focal length convex lens to produce anything approaching a collimated beam. The optics from a dead CD player (even though CD players and CDROM drives use infra-red laser diodes, the optics can likely still be used with visible laser diodes), a low to medium power microscope objective, or even an old disc camera can provide a lens that may be entirely suitable for your needs.

CD Player Laser Diodes

------------------------------

The major difference between these and the visible laser diodes discussed in the section: "How do I use a visible laser diode?" is that the output is near IR - usually at 780 nm (wavelengths from 400 to 700 nm are generally considered the visible portion of the electromagnetic spectrum). Therefore, you must use an IR detector device to even confirm laser emission.

Thus, they make truly lousy laser pointers or laser light shows as the emission is just barely visible in subdued light. If you hoped for a Star Wars type laser beam, better go hunting for a 25 W argon laser :-). However, for data or voice communications, various kinds of scanning or sensing, and electro-optic applications where visibility is not needed or not desirable, these low cost sources of coherent light are ideal.

Similar types are found in CDROM drives and CD-R recorders, Minidisc equipment, newer laserdisc players, magneto-optical drives. Other optical storage technology uses laser diodes as well. WORM drives, in particular, may use devices with higher power output - 30 mW or more. High resolution laser imagers, typesetters, and plotters may use laser diodes producing 150 mW or more. Take additional precautions if you have a laser diode from one of these (or don't really know where yours spent its earlier life). There are laser diodes with optical output measured in watts, though these will not be what you would call tiny and probably require buss bars for electrical power and plumbing for cooling!

CD laser diodes are infrared (IR) emitters, usually 780 nm, with a maximum power output of around 5 mW. There is also a very slightly visible deep red emission from all those I have seen. This may be a spurious very low power line in the red part of the spectrum or your eye's response to the near IR appearing red and about 10,000 times weaker than the actual beam. Despite what the EM spectrum charts show, the eye's response does not drop off to zero at exactly 700 nm so there decreasing sensitivity out to 800 nm or beyond depending on the individual. The main beam is IR and invisible. Take care. A collimated 5 mW beam is potentially hazardous to your eyes. Don't be misled into thinking the laser is weak due to the weak appearance of the beam. It is not supposed to be visible at all!

Typical CD laser optics put out about .3-1 mW at the objective lens though the diodes themselves may be capable of up to 4 or 5 mW depending on type. If you saved the optical components, these may be useful in generating a collimated or focused beam. The aspheric objective lens will be optimized for producing a diffraction limited spot about 1 to 3 mm from its front surface when the optical system is used intact.

The optics may include a collimating lens, diffraction grating (to produce the three beams in a three beam pickup), beam splitter prism or mirror, turning mirror (for horizontally mounted optics), and focusing (objective) lens. Older pickups tend to have larger and more substantial sets of optics. Despite their small size and low cost, these are very high quality optical components.

However, depending on design, some of the parts may be missing or combined into one component. For example, many Sony pickups do not appear to use a collimating lens. For pickups with a collimating lens, if the objective lens is removed, you should get a more or less parallel main beam and two weaker side beams. Mix and match optics for your needs (if you can get it apart non-destructively). Where there is no collimating lens, the objective lens may be used for this purpose if positioned closer to the laser diode.

WARNING: A collimated 5 mW beam is hazardous especially since it is mostly invisible. By the time you realize you have a problem it will be too late.

The coils around the pickup are used for servo control of focus and tracking by positioning the objective lens to within less than a um (1/25,400 of an inch) of optimal based on the return beam reflected from the CD. See the document: "Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives" for more information on optical pickup organization and operation.

Typical drive currents are in the 30 to 100 mA range at 1.7 to 2.5 V. However, the power curve is quite non-linear (though perhaps not as extreme as the typical visible laser diode). There is a lasing threshold below which there will be no coherent output (just IR LED emission). For a diode rated at a nominal current of 50 mA (typical for Sony pickups, for example), the threshold current may be 30 mA. This is one reason why most applications of laser diodes include optical sensing (there is a built in photodiode in the same case as the laser emitter) to regulate beam power. You can easily destroy a laser diode by exceeding the safe current even for an instant. It is critical to the life of the laser diode that under no circumstances do you exceed the safe current limit even for a microsecond!

Laser diodes are also supposed to be extremely static sensitive, so use appropriate precautions. Also, do not try to test them with an analog VOM which in particular could on the low ohms scale supply too much current. It is possible to drive laser diodes with a DC supply and resistor, but unless you know the precise value needed, you can easily exceed the ratings.

For an actual application, you should use the optical feedback to regulate beam power. You should also use a heatsink if you do not already have the laser diode mounted on one. CD laser diodes are designed for continuous operation.

Testing of Low Power Laser Diodes

---------------------------------------------

If you have pinouts and specifications for your laser diode, these procedures can be greatly simplified. The following assumes you know nothing about your device other than that it is a 3 to 5 mW laser diode.

The first step is to identify which pair of terminals are the laser diode and photodiode. Your laser diode assembly will be configured like one of the following:

The photodiode's forward voltage drop will be in the approximately .7 V range compared to 1.7-2.5 V for the laser diode. So, for the test below if you get a forward voltage drop of under a volt, you are on the photodiode leads. If your voltage goes above 3 V, you have the polarity backwards. Warning: Some laser diodes have very low reverse voltage ratings and will be destroyed by modest reverse voltage. Check your spec sheet. However, the laser diodes found inn CD players seem to be happy with 4 or 5 volts applied in reverse. Of course, a shorted or open reading could indicate a defective laser diode or photodiode. The metal case is often one of the terminals, probably C but not always.

If the laser diode is still connected to its circuitry (probably a printed flex cable), it is likely that the laser diode will have a small capacitor directly across its terminals and the optical sensing photodiode will be connected to a resistor or potentiometer. In particular, this is true of Sony pickups and may help to identify the correct hookup.

Either of the circuits below can be used to identify the proper connections and polarity and then to drive the laser diode for testing purposes.

* One approach that works for testing is to use a 0 to 10 VDC supply with a current limiting resistor in series with the diode:

If your power supply has a current limiter, set it at 50 or 60 mA to start.
You can always increase it later.

* Alternatively, a fixed supply with a potentiometer can be used:

R2 limits the maximum current. If you know the specs for your diode, this
is a good idea (and to protect your power supply as well). You can always
reduce its value if your laser diode requires more than about 85 mA (with
R2 = 100 ohms).

The two capacitors provide some filtering to reduce the risk of a transient
blowing the laser diode. C2 should be mounted close to the laser diode.

Before attempting to obtain lasing action with either of these circuits,
monitor the voltage across what you think is the laser diode as you slowly
increase the power supply or potentiometer.

* If you guessed correctly (or have the pinout diagram from the spec sheet
or determined from its former life), the voltage will increase until around
1.5 to 2 V and then climb more slowly. Don't push your luck unless you are
also monitoring the laser diode current and optical output.

* If you are across the laser diode or photodiode in the reverse biased
direction, the voltage will continue to climb above 2 V without slowing.
Don't push your luck here - the breakdown voltage of the laser diode may
be only a little more than this and - you guessed it - exceeding this is
not healthy for the laser diode either.

* If you are on the photodiode in the forward direction, the voltage will get
stuck around .7 V.

Once you have identified the correct connections, monitor the current through
the laser diode as you check for a laser beam.

* For IR laser diodes, you *must* use an IR detector circuit, card, video
camera or camcorder (with the requisite 3 hands) to monitor for an actual
IR laser beam.

Note: If you are trying to use a video camera or camcorder as an IR detector,
confirm its sensitivity to near IR by looking at an active IR remote control
through its viewfinder. It may have a built in IR blocking filter which will
prevent it from being sensitive to IR. This may be removable.

* For visible laser diodes, you can use your eyeballs or any more sophisticated
detector as desired. Look from an oblique angle or better yet, place a white
card a couple of inches in front of the laser diode. Even a 1 mW laser diode
is an intense source of light - there will be no doubt when lasing begins.

Some typical operating currents for laser diodes of various wavelengths are
listed below. THESE ARE JUST EXAMPLES. Your laser diode may have a lower
operating current than the ones listed here! The lasing threshold may be as
little as 5 or 10 mA below the operating current and the operating current may
be 5 mA or less below the maximum current.

 

Wavelength	Operating Current
808 nm	60 - 70 mA
780 nm	45 - 55 mA
670 nm	30 - 35 mA
660 nm	55 - 65 mA
650 nm	65 - 85 mA
640 nm	70 - 90 mA

Of course, if you inherited a bag of identical laser diodes and can afford to
blow one: (1) I could use a few before you do this :-) and (2) you probably
could fairly accurately characterize them by testing one to destruction.

For a current below the lasing threshold for your laser diode, there will be
some emission due to simple LED action. As you slowly increase the current,
at some point (if the laser diode is good) as you exceed the threshold current,
the character of the emission will change dramatically and a very slight
increase in laser diode current will result in a significant increase in
intensity. Congratulations! The laser diode is lasing.

CAUTION: unless you have a laser power meter, don't push your luck. The
maximum safe current may be as little as 5% above the lasing threshold. Go
over by 6% and your diode may be history. The exponential power curve seems
to be steeper with visible laser diodes but there is no way to be sure without
specifications. It is all too easy to convert laser diodes into extremely
useless DELDs - Dark Emitting Laser Diodes - or very expensive LEDs.

I have used this approach with laser diodes from dead CD players without
difficulty. In the case of many of these, the operating current is printed
on a sticker on the optical block, often as a 3 digit number representing
the current in 10ths of mAs. Typical values are 35 to 60 mA (350 to 600).
Sony pickups typically average around 50 mA. Without this information, the
best you can do is to estimate when it is lasing at the proper intensity by
comparing the brightness of the 'red dot' one sees by looking into the lens
from a safe distance at an oblique angle. However, this is not very reliable
as the optical power at the objective lens depends on the particular CD player.

Reasons to leave the CD laser diode in the optical block:
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There are several good reasons to leave your CD laser diode installed in the
optical block assembly even if you are not going to use it with the objective
lens and focus and tracking actuators:

1. The pickup block provides the very important heat sink which is necessary for continuous operation.

2. There is less risk of damaging it through careless handling and ESD.

3. There may be a collimator lens in there - probably the first or second optical element in front of the laser diode. It may be combined with the laser diode in its metal barrel. If there is a collimator, you should be able to get a nice nearly parallel beam without much work. At most, a small lens will be needed to optimize it.

Remove the objective (front) lens and its associated coils unless you require them for a short range application. They will likely come off as a unit without too much effort. However, try not to destroy this assembly as you never can tell what might be needed in the future.

4. The multisegment photodiode sensor and focus and tracking actuators may be useful for a variety of applications.

While there are many variations on the construction of optical pickups even from the same manufacturer, they all need to perform the same functions so the internal components are usually quite similar.

Here is the connection diagram for a typical Sony pickup:

The laser diode assembly and photodiode chip connections are typically all on a single flex cable with 10 to 12 conductors. The actuator connections may also be included or on a separate 4 conductor flex cable. The signals may be identified on the circuit board to which they attach with designations similar to those shown above. The signals A,C and B,D are usually shorted together near the connector as they are always used in pairs. The laser current test point, if present, will be near the connections for the laser diode assembly.

It is usually possible to identify most of these connections with a strong light and magnifying glass - an patience - by tracing back from the components on the optical block. The locations of the laser diode assembly and photodiode array chip are usually easily identified. Some regulation and/or protection components may also be present.

Note: There are often a pair of solder pads on two adjacent traces. These can be shorted with a glob of solder (use a grounded soldering iron!) which will protect the laser diode from ESD or other damage during handling and testing. This added precaution probably isn't needed but will not hurt. If these pads are shorted, then there is little risk of damaging the laser diode and a multimeter (but do not use a VOM on the X1 ohms range if it has one) can be safely used to identify component connections and polarity.

Laser diode life:
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For all intents and purposes, laser diodes in properly designed circuits do not degrade significantly during use or when powered on or off. However, it doesn't take much to blow them (see the sections: "How do I use a visible laser diode?" and "CD player laser diodes"). I have seen CD players go more than 10,000 hours with no noticeable change in performance. This doesn't necessarily mean that the laser diode itself isn't gradually degrading in some way - just that the automatic power control is still able to compensate fully. However, this is a lower bound on possible laser diode life span.

Some datasheets list expected lifetimes for laser diodes exceeding 100,000 hours - over 12 years of continuous operation. Of course, I trust these about as much as the latest disk drive MTBFs of 1 million hours :-).

Laser diodes that fail prematurely were either defective to begin with or, their driver circuitry was inadequate, or they experience some 'event' resultling in momentary (greater than a few microseconds) overcurrent.

As noted elsewhere, a weak laser diode is well down on the list of likely causes for CD player problem

Of course, in the grand scheme of things, even LEDs gradually lose brightness with use.

How sensitive are laser diodes, really?:
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Not all laser diodes are created equal and their susceptibility to damage through improper handling or improper drive likely varies widely. Here is a discussion of some of the issues:

From: Eric Rechner (erechner@jetstream.net)).

"Does anyone have any experience with Hitachi laser diode HL7843MG 5 mW 780nm? I find this diode to be possibly extremely sensitive (ESD??), more so than any other 780nm laser diode. Does anyone know if there are problems with Hitachi MQW type diodes? Are MQW diodes more sensitive to ESD than Double Heterojunction diodes? Does anyone have info on possibly 'bad' or defective lasers out there?"

(From: Jon Elson (jmelson@artsci.wustl.edu)).

Strange. I think I've used some of these. I hear everybody babbling about extreme static sensitivity on these devices, yet I've never had a failure, and I've been using just the usual minimum precautions with any semiconductor device. I suspect that people may be exceeding the optical power MAXIMUMS on the devices. I've been very conservative on that, since the devices only carry an optical maximum, and don't have that correlated to forward diode current (difficult, because it varies strongly with temperature). I try to run them at a good bit less than rated power, maybe 2-3 mW optical output. I'm using a diode sold by Digi-Key for $19.00, just because it is cheaper than the Panasonic in the 5.4 mm case. I think the manufacturer is NVG or something like that. I've got 10 of them I am working with, designing a closed-loop driver for a photoplotter, which pulses the lasers on and off as fast as 10 uS on, 10 uS off. It is working pretty well now. I included a series resistor (as well as the control transistor), so that if the loop becomes unstable or the sensing diode gets disconnected, it won't fry the laser diode.

(From: Dr. Mark W. Lund (lundm@xray.byu.edu)).

The babbling starts here: You don't have to be a total idiot to blow these things, in fact I have blown a few myself. Identifying the source of the trouble is extremely costly and difficult because it only takes a spike of a few nS to to the damage. I would say that 99.9999% of the time it is the power supply. Either it spikes on turn-on, turn-off, or at random. We used to toast lasers with a $5,000 laser diode power supply that would spike every time you sent certain signals on the IEEE 488 control line. This was a tough one to figure out, I can tell you. In the process we tried to damage one using static to try to get a handle on the sensitivity, but were not able to get a catastrophic failure this way (we may have induced some latent failures, however). Other laser diodes may vary.

(From: Jon Elson (jmelson@artsci.wustl.edu)).

Ah! This is good anecdotal evidence! I've often suspected that there might be more of this going on, and instead of examining the drivers, people just attribute problems to an invisible gremlin! I sure can see how a closed circuit driver can oscillate or overshoot on transients, and there could be a situation where some percentage of drivers will be less stable due to component tolerances. Unless you rigorously test a good batch of your drivers, you could have this sort of thing and not know it. (Of course, any time you put a computer in the loop, especially one that is canned inside an instrument, then the probability of unanticipated gremlins increases dramatically!).

Of course, I was designing a fixed-purpose driver to be used in a specific application, inside an instrument, so I had it easier than the guys designing a lab-quality pulser for who knows what application. So, I could put in a resistor, which will limit current to some 'safe' level, even if the loop is unstable, which it certainly was when I was tuning up my driver.

I DO use generally sound anti-static precautions, almost subconsciously, to protect all semiconductor devices. But, I am aware that I have occasionally, by accident, touched a cable going to the laser diode before I was grounded, and I have never noted a catastrophic failure. I will have to go through some rigorous life-testing to make sure I'm not causing latent failures, but I've run these diodes for quite a few hours while testing things, and nothing of note has turned up yet.

By babbling, I meant some items in print media, as well as a lot on this and other newsgroups, indicating that if you even touch one lead of a diode laser, it is ABSOLUTELY destroyed, with a probability of 1.000! Obviously not true! Your comments are well reasoned, and indicate real experience. Others have also written that only a huge corporation, with millions in test equipment, could ever make their own laser diode driver. Now, clearly, the nanosecond multi-watt pulsers ARE much more difficult to do right, fast risetimes without overshoot is tricky. But, I did it in my basement with just over $1,000 in test equipment, mostly a decent oscilloscope. I also had the confidence that if I DID blow a few diodes, it wasn't so painful at $19 each.

So, now, I'm babbling!

IR detector circuit:
-------------------

This IR Detector may be used for testing of IR remote controls, CD player laser diodes, and other low level near IR emitters.

Component values are not critical. Purchase photodiode sensitive to near IR (750-900 um) or salvage from opto-coupler or photosensor. Dead computer mice, not the furry kind, usually contain IR sensitive photodiodes. For convenience, use a 9V battery for power. Even a weak one will work fine. Construct so that LED does not illuminate the photodiode!

The detected signal may be monitored across the transistor with an oscilloscope.

Divergence of Laser Diode:
-----------------------------------

(Portions from: Mark W. Lund (lundm@physc1.byu.edu)).

The divergence specification for laser diodes is measured to the half power points. T full width at the 10% level may be more like 70 or 80 degrees than the 30 degrees in the specifications.

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 it 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.

Why can't an LED be focused like a laser diode?:
------------------------------------------------------------------

The cheap laser diode from a CD player can be focused to a spot less than 2 um in diameter. Why is this not possible with an LED?

The quick answer is that an LED does not appear as a point source and has as effective emitting area which is huge compared to a laser diode. Even though the emitting area of a laser diode is not a point, due to the way the laser beam is generated - collimation wise - it appears as a point source.

And, a point source can be focused to another point.

The effective emitting area of an LED is perhaps .25 x .25 mm. To focus an incoherent source like this to a 2 um spot with imaging optics would require a ratio of distances of roughly 125:1 for the LED-to-lens compared to the lens-to-image plane.

With any kind of real world optics, you will get a vanishingly small amount of power at the image plane.

Similarly, an LED beam cannot be cleaned up with a spatial filter (pinhole) as very little of the beam will make it through.

The laser diode is coherent and monochromatic (enough) that relatively simple optics can be used to focus it to a spot smaller than 2 um. While the dimensions of the laser diode chip are not all that much different from the LED, the characteristics of the laser emission makes such focusing a relatively easy task.

Consider that the beam from a HeNe or ruby laser doesn't come from point source either. The beam can be sharply focussed because it is very well collimated.

The availability of relatively cheap laser diodes really was the enabling technology for the CD revolution.

(From: Steve Nosko (q10706@email.mot.com)).

If a beam of light has nothing but *precisely* parallel rays, it can be focused to a point. Also, if the beam originated from a point, a lens will focus it to a point.

An LED has neither of these. First, it is an area source and light coming from that surface is not parallel. It would also be called a diffuse source, meaning light from all places on the surface travels in many directions. This kind of source can not be focused to anything but a smaller image of itself. The shorter the focal length of the lens, the smaller the image - but it is still an image of the source, not a spot. It is because of these rays, traveling in different directions, that a lens can't focus them all to the same point. If you draw the side view of a lens and trace rays this all should be obvious.

The gas laser, on the other hand, has rays which are much much closer to being parallel. The diode laser has rays which appear to come from an apparent point inside the diode.

There are two more subtle effects. One effect is the relatively wide range of wavelengths in the LED versus the narrow range of a laser. Simple optics don't focus all wavelengths at the same focal length. So the wide bandwidth of the LED causes a little trouble. There is another effect having to do with the size of the lens (diffraction limit) and the wavelength, but this is also secondary to an understanding of the *primary* reason why an LED can't be focused. I'll only talk about the largest effect due to the extended, non collimated source.

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.

Comments on driving laser diodes without optical feedback:
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(From: Dwight Elvey (elvey@civic.hal.com)).

If you intend to use the laser without the feedback, one has to realize that there are a number of problems. One is that as the temperature goes down, the laser efficiency goes up. This tends to cause the laser diode to destroy itself at lower temperatures while running that same current that was OK at some higher temperature. Generally, if the temperature doesn't vary to much, one can use something as simple as a limiting resistor and not run the laser at its highest output. I once made a burn-in driver for some power lasers that used constant current sources that had no feed back but I had to preheat the diodes to 100 degrees C before using that high a level of current. The level of current used would have wiped the diodes out at room temperatures.

The hardest part of the whole thing was making the circuit to have controlled levels of current during power on and power off. Most things like op-amps are not specified under these conditions. My first attempt wiped out 10 diodes :-( when I turned the power on.

To run the diodes at there maximum light out safely, requires using the feed back photo diode.

Visibility of Near-IR (NIR) laser diodes:
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The following describes an interesting and convincing experiment. I would tend to believe these results concluding that the visible light from a CD laser diode is probably a spurious emission rather than the human eye's weak sensitivity to 780 nm radiation. The fact that the red emission was undiminished even after the laser diodes were damaged by overcurrent is further confirmation of these conclusions. If the red is a spurious (LED-like) emission, it should appear below the laser threshold suggesting another test.

(From: Kjell Kraakenes (kkraaken@telepost.no)).

I once used 780 nm laser diodes similar to the types used in CD players, and something that puzzled me was that I was able to see some red radiation from these diodes. I used a microscope objective to focus the light on a wall a few meters away, and when properly focused, a red spot was visible to the naked eye. I had a piece of black card board on the wall, and there was no specular reflection. I used an IR viewer of the type sold by Edmund Scientific (Find-R-Scope), and if I looked at the spot with this IR viewer the beam appeared defocused. By adjusting the distance between the laser diode and the microscope objective, the spot (as it appeared through the IR viewer) could be brought to a better focus. The red, visible light was then so much defocused that it was no longer visible to the naked eye. From these observations, I assumed that the spot I saw through the IR viewer was the laser emission at 780 nm, and that the visible light was some weak emission at a shorter wavelength. Because of the chromatic aberrations in the microscope objective these two wavelength could not be expected to be in focus simultaneously. I did not notice whether the distance between the laser diode and the microscope objective was increased or decreased when shifting between the focus of the visible and the IR light, but since I did not know the chromatic aberrations of the microscope objective this information would not help me.

I damaged a few of these laser diodes. Probably by burning one of the facets such that the lasing threshold was increased. Electrically they were OK, and the visible output appeared as intense as before, but the total output was only a few microwatts.

I therefore believe that the light people see from NIR laser diodes is spurious emission within the visible band, and not intense NIR radiation.

(From: Don Klipstein (Don@Misty.com)).

Some nominally IR wavelengths are indeed very slightly visible. In favorable conditions (mainly isolating from more visible wavelengths) I have seen with my own eyes:

1. The 766.49/769.9 nM potassium lines, as a contaminant in high pressure sodium lamps.

2. The 818.3/819.5 nM sodium lines in the spectra of high pressure sodium lamps.

3. The 762.1, 759.4, and 822.85 nM earth atmospheric absorption lines in the solar spectrum. (Usually with the sun somewhat low.)

4. The output of a laser diode in my CD player is visible at eye-safe intensities (half a meter from a source with a beam covering nearly a steradian for a few seconds). I have seen the spectrum of this along with that of a neon lamp placed next to it, and verified that what I saw was the laser line, with a wavelength around 800 nM. It could be as low as around 780 nM.

According to the C.I.E. "Y" or visibility function (or extrapolation thereof), the visibility of these lines is impressively low. However, considering the wide dynamic range of the human eye, these wavelengths are visible at eye-safe levels.

CAUTION: there is no advance warning of having exceeded eye-safe exposure to slightly visible wavelengths normally considered IR. You may permanently toast part of your retinas duplicating the above unless you verify retinal exposure below the Class I laser exposure limit.

I recently got a laser pointer with a wavelength of 660-661 nm or so and (guesstimated) 2 mW of output power.

I discovered that if I shine the beam through one of those dielectric interference bandpass filters, I got some weak beam output at other wavelengths. So, I investigated further.

About (very roughly estimated from standard issue eyeballs) .2 percent of the beam is spurious radiation with a continuous spectrum. I don't yet know well what it does at longer wavelengths, but a majority of the short wavelength side of this is in the few tens of nm below 660 nm. Slight traces exist down to 540 nm. With two 532 nm filters, I could stare into the beam and see a dim point of light. With a 570 nm filter, it was slightly bright to stare into and I could see the beam VERY DIMLY on a wall in a dark room. With a filter around 630 nm, I could easily see the beam on a wall in a dark room. I used my diffraction grating to verify that most of this was continuous spectrum in the passband of the filter.

The spurious radiation takes the same path that the laser radiation does.

With no filter, I could not see any continuous spectrum with my diffraction grating. The laser line was so much stronger.

As for IR lasers? If the spectrum is just a long-shifted version of what my visible laser does, the most visible part of the laser output would be the laser line. Having a wavelength 100 nm closer to visible increases its visibility only by about a factor of 1,000 and the total spurious output was (roughly) 1/1,000 of the laser line output. The wavelength of the bulk of this was nowhere near 100 nm shorter.

Although I can't be sure this would always be the case, the only spectrum components I could see using a diffraction grating with my CD player laser was the laser line at about 800 nm.

I suspect different IR laser diodes may have greatly different ratios of laser and LED output. If the LED output is only a fraction of a percent of the laser output, the visible output would be mainly the slightly visible laser line. If the LED output is equal to a few percent or more of the laser output, then it may be more visible than the laser line.

When will we see green and blue diode lasers?:
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(Portions from: Adam Cohen (adc20@eng.cam.ac.uk)).

Blue and green has been widely demonstrated by SHG (second harmonic generation a.k.a. frequency doubling) in nonlinear crystals (lithium niobate, KTP et al.), organic nonlinear materials, etc. etc.

The direct emission from a semiconductor has been the Holy Grail for several years. The semiconductor materials available with a sufficiently wide band-gap are notoriously difficult to deposit and cleave....But several groups are close to a commercial device now. In Japan, Nichia Chemicals, Sony, Pioneer and Toshiba (see p26 of Laser Focus World, March 1997) are all working on GaN-based devices (active layer in the Toshiba device is actually InGaN). I think 3M and some other US firms were concentrating on ZnSe, which emits at a slightly longer wavelength (more blue-green than blue)....