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Some more laser diode questions

Posted: Tue Jun 08, 2004 11:51 pm
by Ted Park
I've obtained a 50 mw laser diode, Hitachi HL6504FM. I am going to try driving it to 35 mw using an IC voltage regulator and a resistor. Has anyone tried something similar to this, or do you really need a constant current system?

Thanks for your help,


Some more laser diode questions

Posted: Wed Jun 09, 2004 1:42 am
by Tom B.
Yes - it's not generally recommended, but it works for me.

*** !!!! CAUTION !!! ****


I use a very quiet old HP variable lab power supply with 0.1 uf ceramic cap and 2200 uF electrolytic on the output feeding a 2 pair shielded cable, shield tied to supply ground. One pair is used for diode anode (+) drive and ground, and the other for the photodiode pin and ground. At the diode end, a 5V Tranzorb suppresser, 0.1 uF ceramic cap, 50 ohm 1 watt current limiting resistor, and then right at the laser diode socket anode, a couple of ferrite beads, and a 1000 pF NPO cap and a reverse biased Schottky diode between anode and cathode to protect the diode against reverse transients (very important!). The photodiode monitor wire goes back to a 9V bias battery and current meter (0-200 microamps) near the power supply for output monitoring.

The power up ritual is:
1. Visual check of all connections
2. Make CERTAIN that the power supply voltage knob is set to minimum output.
3. Turn on power supply switch
4. Gradually increase the output voltage, checking for the first signs of visible light from the diode.
5. Check that the photodiode monitor is showing some photocurrent as the output increases. If not, gradually power down and recheck connections.
6. Increase power output to desired level. The first time, you will need a laser power meter to directly measure the output power right at the diode window and determine the photodiode response. Afterward, you can just use the photodiode current, checking only occasionally with the power meter. Note that back reflections from optics in front of the diode will, with some diodes, tend to produce an over-estimate of the power. It's fortunate that it's not the other way around. To avoid going on too long, I will skip the topic of finding a stable operating point and temperature stabilization - a forum archive search should turn up some discussions.
7. Don't try to run too near the maximum rating - about the most I'm willing to risk is 40 mW on a 50 mW diode.

Power down procedure:
1. Don't just turn off the power switch - turn the output voltage down to minimum.
2. Then turn off the power switch

I haven't looked into the possibility of using a variable IC regulator instead of the lab supply, (since I didn't need to) but I expect with some care it can be made to work. The common LM317 regulator I think only goes down to 1.3V without additional circuitry, but that should be OK since it's below the laser diode turn-on voltage. Better regulators with lower output noise and better line rejection are available nowadays - but this could be a later improvement you could make. It would be best to feed the regulator with a quiet, regulated supply. (NOT a switching regulator!)

Best of luck!

Some more laser diode questions

Posted: Wed Jun 09, 2004 2:28 am
by Tom B.
Just took a look at the HL6504 datasheet and realized that with no internal photodiode on this part, some of what I said would not apply. Since the diode output power versus current is dependent on temperature, you would need to be more creative to ensure that you never exceeded the maximum output power. The most straightforward approach would be to check the output with a laser power meter whenever you set the drive current, and be aware that the current needed to blow the diode goes down with temperature.

Some more laser diode questions

Posted: Wed Jun 09, 2004 3:38 pm
by Ted Park
Thanks for the info. I suppose (as always) that some experimentation is in order. I might try some experiments with the cheap laser pointer diodes before powering up my "good" devices. It appears as though it's important
to have electronics in there to protect the laser diode from transients and
other bad things.

Having said that, do you have a good source of laser power meters?


Some more laser diode questions

Posted: Thu Jun 10, 2004 3:02 am
by Tom B.
Good idea re starting with pointers - I started out building a battery box for my sawed off laser pointer. Those things can be very forgiving - I think they must use highly derated laser chips (capable of much higher power) so that they can get away with ultra cheap and simple resistor-battery operation over a wide temperature range. It would be interesting to put a pointer in the freezer and check the output power when it's really cold...

For diode work, I ended up buying the Coherent Lasercheck (also sold by Edmund Optics under their own label)

Pros: wide power and wavelength range, 5% accuracy, compact

Cons: non-continuous readout, sensor and filter not AR coated, hard to use in the dark, mushy rubber bottons (no tactile feedback), Lithium battery is not user-replaceable.

Cost is about $300 U.S.

Data here: ... 2&PDID=250
What's inside:

Some more laser diode questions

Posted: Thu Jun 10, 2004 6:51 am
by Colin Kaminski
I got to use one of these at PCGG: ... eName=WDVW

It worked very well.

Here is a link to his ebay store: ... &fp=0&st=2

Some more laser diode questions

Posted: Thu Jun 10, 2004 12:09 pm
by Ted Park
Thanks - I checked those out - seems highly useful. I guess it only costs as much
as a few blown diodes