LaserDiode Information

Contents:

[Document Version: 1.10] [Last Updated: 7/30/96]


1. About the Author & Copyright

LASERDIODE INFORMATION

Author: Samuel M. Goldwasser
E-Mail: sam@stdavids.picker.com
Corrections/suggestions: [Feedback Form] [mailto]

Copyright (c) 1994, 1995, 1996
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

  1. This notice is included in its entirety at the beginning.
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1.1) Related material

See the document: "Notes on the Troubleshooting and Repair of Compact Disc Players and CDROM Drives" for more info on how the CD players and CD ROM drives worked to begin with.


1.2) How do I use a visible laserdiode?

The quick answer - very carefully for two reasons:

I am assuming this is a typical 5 mW visible laserdiode.

  1. You can easily destroy the typical laserdiode through instantaneous overcurrent, static discharge, probing them with a VOM, or just looking at them the wrong way.
  2. Anytime 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 poassible instantaneous retinal damage.

The third lead is for an optical sensing photodiode used to regulate power output when used in a feedback circuit which controls your current.

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. This is one reason why many 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 laserdiode by exceeding the safe current even for an instant. It is critical to the life of the laserdiode that under no circumstances do you exceed the safe current limit even for a microsecond!

laserdiodes are also extremely static sensitive, so use appropriate precautions. Also, do not try to test them with a VOM which could on the low ohms scale supply too much current.

It is possible to drive laserdiodes with a DC supply and resistor, but unless you know the precise value needed, you can easily exceed the ratings.

You can identify the laserdiode and the photodiode because the photodiode's forward voltage drop will be in the approximately .7 V range rather than 1.7-2.5 V for the laserdiode. 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 laserdiodes have very low reverse voltage ratings and will be destroyed by modest reverse voltage. Check your spec sheet.

One approach that works for testing is to use a 0-10 VDC supply with a, say 100 ohm resistor in series with the diode, and slowly bring the current up until you get a beam. However, you still have no idea of when you are at the safe current limit without an optical power meter.

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 laserdiode mounted on one.

The raw beam from a typical laserdiode is wedge shaped - 10x30 degrees typical divergence. You will need a short focal length convex lens to produce anything approaching a collimated beam.

The circuit below is of a constant current regulated power supply for a CW visible laser light but it may not apply directly to your configuration of laserdiode and photodiode (polarities may differ). Three terminal regulators can be used in constant current mode as long as you guarantee that there will be no overshoot, etc. These things really are finicky.


1.3) CD Player Laserdiodes

Note that these are infrared (IR) emitters, usually 780 nm. There is a very slightly visible red emission. This may be a spurious very low power line in the red part of the spectrum or your eye's response to the IR appearing red and about 10,000 times weaker than the actual beam. The main beam is IR and invisible. Take care. A collimated 5 mW beam is potentially hazardous to your eyes.

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 will be useful in generating a collimated or focused beam.

The optics typically consist of a collimating lens, diffraction grating (to get the three beams in a three beam pickup), polarizer, prism or mirror, focussing (objective) lens. With the objective lens removed, you should get a more or less collimated main beam and two weaker side beams. Mix and match optics for your needs (if you can get it apart non-destructively). WARNING: A collimated 5 mW bean 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 lens to within less than a um of optimal based on the return beam.

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 output. For a diode rated at a typical current of 40 mA, the threshold current may be 30 mA. This is one reason why many applications of laserdiodes 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 laserdiode by exceeding the safe current even for an instant. It is critical to the life of the laserdiode that under no circumstances do you exceed the safe current limit even for a microsecond!

Laserdiodes are also supposed to be extremely static sensitive, so use appropriate precautions. Also, do not try to test them with a VOM which could on the low ohms scale supply too much current.

It is possible to drive laserdiodes with a DC supply and resistor, but unless you know the precise value needed, you can easily exceed the ratings.

You can identify the laserdiode and the photodiode because the photodiode's forward voltage drop will be in the approximately .7 V range rather than 1.7-2.5 V for the laserdiode. So, for the test below if you get a forward voltage drop of under a volt, you are on the photodiode leads.

One approach that works for testing is to use a 0-10 VDC supply with a, say 100 ohm resistor in series with the diode, and slowly bring the current up until you get a beam. Use an IR detector for this! If you get the polarity backwards, the voltage across the diode will go above 3 volts. This does not seem to damage the types of diodes found in CD players. Then, turn power off and reverse the leads. This will not damage either diode.

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 laserdiode mounted on one. CD laserdiodes are designed for continuous operation. You can use LM317 type three terminal regulators in constant current mode as long as you guarantee that the current never exceeds the set point.

Below are circuits for a laserdiode power supply and IR detector.


1.4) CW Laser Light (reverse engineered from commercial unit)

This circuit was traced from a commercial CW laser light. Errors may have been made in the transcription. The type and specifications for the laserdiode assembly (LD and PD) are unknown. The available output power is unknown but the circuit should be suitable for the typical 3-5 mW visible laserdiode (assuming the same polarity of LD and PD or with suitable modifications for different polarity units.)

If you do build this or any other circuit for driving a laserdiode, I suggest testing it first with an LED and discrete photodiode to verify current limited operation. Them with the laserdiode in place, start with a low voltage supply rather than 9V until you have determined optimal settings and work up gradually. Laserdiodes are very unforgiving.

Note the heavy capacitive filtering. Change would be needed to enable this circuit to be modulated at any reasonable rate.

+9    D1
>-----|>|-------+---------+---------------+-----+--------+
     1N4001     |         |               |     |        |
    Rev. Prot.  |         |    Pwr Adj    |    _|_     __|__
                |         /    R3 10K (2) | PD /_\  LD _\_/_
                |     R2  \   +----+      |     |        |
                |    560  /   |    |      +-----|---||---+
                |         \   +---/\/\--+-------+   C4   |
                |         |   |         |          .1 uF |
                |+        |   |         +----||----+     |
              __|__       |   |       __|__  C2 (1)|     /
           C1 -----       |   |     E /   \  100 pF|     \
          10 uF | -       +---|------' Q1  '-------+     / R4
                |         |   |    BC328-25 (5)    |     \ 3.9
                |         |   |       (PNP)        |     |
                |         |   |                    |     |
                |     +---+   |                    |   |/ Q2
                |     |_ _|_i |                    +---|  BD139 (NPN)
                | VR1  _'/_\_ |                   +|   |\ (5)
                | LM431   |   |              C3  __|__  E|
                | 2.5 V   |   |           10 uF  -----   |
                | (3)     |   |                   -|     |
     R1 3.9     |         |   |                    |     |
>----/\/\/\-----+---------+---+--------------------+-----+
GND
Notes:

  1. Capacitor C4 value estimated.
  2. Potentiometer R3 measured at 6K.
  3. LM431 shunt regulator setup as 2.5 V zener.
  4. Supply current measured at 150 mA (includes power on LED not shown).
  5. Transistor types do not appear to be critical.


1.5) IR detector circuit

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

Component values are not critical. Purchase photodiode sensitive to near IR (750-900 um) or salvage from optocoupler 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.

      Vcc (+9 V)
      >-------+---------+
              |         |
              |         \
              /         /  R3
              \ R1      \  500
              / 3.3K    /
              \       __|__
              |       _\_/_  LED1 Visible LED
            __|__       |
  IR ---->  _/_\_ PD1   +--------> Scope monitor point
    Sensor    |         |
  Photodiode  |     B |/ C
              +-------|    Q1 2N3904
              |       |\ E
              \         |
              / R2      +--------> GND
              \ 27K     |
              /         |
              |         |
 GND >--------+---------+
             _|_
              -

1.6) LaserDiode power supply

(From: Brian Mork (bmork@comtch.iea.com)).

Best circuit I've found:

          In        Out    18ohm*
(+) ---+---- LM317 --------/\/\/\----+-----+------- LD anode
       |       | Adj                 |     |        |
 22uf ===      +---------------------+    === 1uf   V
       |                                   |        |
(-) ---+-----------------------------------+------- LD cathode

Power is 5.5 to 9 VDC. I use a 9 volt battery.

Watch the pin arrangement on the LM317. On the LM317L (the TO-92 plastic transistor type case) and the LM317T (TO-220 7805-type case), the pins are, left to right, Adjust-Output-Input.

For the resistor, I use a small carbon 10 ohm in series with a precision 10-turn 20 ohm adjustable. The combo was empirically set to about 17 ohms.

On initial power on, use three garden variety diodes stacked in series instead of the laserdiode. Put a current meter in series with the diode stack and adjust the precision resistor for 50-60 mA. Disconnect power and replace the diode stack with the laserdiode. Connect up power again, still watching on the current meter. The diode will probably initially glow dimly. I use a diode that lases at about 72 mA, and has a max rating of 100 mA. I use about 85 mA for normal ops.

Turn up the current, never exceeding your diode's max limit. The dim glow will increase in intensity, but at some point, a distinctive step in intensity will occur. Your diode is lasing. Remove the current meter as desired. Enjoy!


Written by Samuel M. Goldwasser. [Feedback Form] [mailto]. The most recent version is available on the WWW server http://www.paranoia.com/~filipg/ [Copyright] [Disclaimer]