measuring output power

John K · Post by **John K** » Sat Jul 06, 2002 12:35 am

In bulding a working laser diode and adjusting the output, can a person get by with out measuring the output power, and just measure the input current? I am trying to keep the cost down by not buying a optical power meter.
One tip I have in working with developer, maby others do this? is to use quart plastic zip-lock baggies for mixing, developing and storage. I mix the chemicals in the bag, put the film in the bag and do my developing in it, and store the baggie in the frezzer when done. I find I am able to use the developer several times, several weeks apart. I think it is less messy than trays.

Tom B. · Post by **Tom B.** » Sat Jul 06, 2002 5:28 pm

Unfortunately I know of no way to safely adjust
laser diode current without a power meter. Most laser
diodes have a very small safe operating zone
(5-10%) between lasing threshold current and
maximum output beyond which the diode dies.
This is also strongly temperature dependant.
The manufacturer specs on diodes are so loose
as to be useless in determining a safe operating
current. Dam's Laser FAQ has some useful info:
http://www.misty.com/people/don/laserdps.htm
More general info on laser diodes is in the main FAQ.

As for putting bags of toxic chemicals in the freezer
beside the the ice cream and frozen peas, I think my
wife might object Another reason to get a little
fridge freezer for the lab - good for storing film, too.

John K · Post by **John K** » Sun Jul 07, 2002 11:23 pm

Tom, thanks for the information. I was hoping there was some way around buying a optical power meter. Is there someone that sells a cheap meter?
My wife would also take a dim view of developer in the family fridge, I use my small old dorm fridge for film and developer.

Colin Kaminski · Post by **Colin Kaminski** » Mon Jul 08, 2002 3:08 pm

$300 seems to be the buy in price for a Laser Check. Coherent, Edmund Industrial Optics and Roithner sell them on-line. If you find a better price let me know.

If you have a laser of known output you could do an exposure test on film to adjust the laser diode power. I haven't tried it but it should work. Both lasers would have to be close to the same frequency.

So far I have been lucky and not blown my diode by setting it just by measuring the output power of the driver. I have not exceeded 80mw of drive current for the ML1016R.

I wonder how the D&S circuit deals with the wide range of specs on the Mitsubishi ML1016R 35mw diode?

Here is a link to the IR3C07 data sheet that was graciously sent to me by a D&S laser owner. I don't know how long I will be able to serve it as it is large so download it if you need it.

http://home.pacbell.net/colinsk/holo/IR3C07.pdf

John K · Post by **John K** » Mon Jul 08, 2002 11:19 pm

Colin, I have wondered the same thing about how the D&S laser driver kits,can drive a laser diode with out adjusting the output power to the diode if there is such a wide variation between parts.
One option would be to see if someone at a university physics dept could help a person out. I would think they would have a power meter.

Colin Kaminski · Post by **Colin Kaminski** » Mon Jul 08, 2002 11:48 pm

I don't fully understand the D&S circuit as my electronics is rather weak. If had to make a guess I would think they aimed for the middle of the specs. Assumed normal photo-diode current and threshold operation then supplied enough current to run the diode half way between lasing threshold and max output power. This would give them the largest margin.

My diode has gone through a large number of mishaps and I have not been able to destroy it. I learned to solder small parts with it, hooked it up wrong and tried a driver that reverse powered the laser diode. All without apparent damage. I have a spare but I have never decided to hook it up.

The temperature range the diode is expected to run in is not very large and as it overheats it becomes less efficient. So this should work to protect the diode from over output power. I wonder how well the driver would deal with cooling the diode down. I have taken my D&S driven diode to 15.0C without and adverse effect. I have also run it up to 30.0C.

I have found that for a given output power there is a temperature that is far from a mod-hop zone and most of the time lowering the output power a little will get the diode out of a mode-hop zone.

I have been told by laser designers experienced with DPSS lasers and diodes that as a diode ages it will change where it's mode-hop zones occur. (Remember they work in 808nm diodes and their experience is not necessarily valid for us.) Equally important to us is the fact that frequency drifts as the temperature fluctuates even when the diode is operating in between mode-hop zones.

I worked very hard to build a heatsink for the D&S laser and now wonder if it was a waste of time. The better the heatsink design the more sensitive the laser becomes to draft induced frequency shift. I wonder if a better solution would be to insulate the diode! It would shorten the diode life but I would guess it would be more stable. And at $50 per diode I could afford to replace mine once a year if needed.

Tom B. · Post by **Tom B.** » Tue Jul 09, 2002 2:51 am

I think it would be better to keep the heatsink, but put insulation around it to isolate it from drafts. The thermal mass of the heatsink helps to filter out and slow down temperature changes, and enough heat will slowly leak through the insulation to keep the diode temperature down. On the downside, the more thermal mass, the longer it takes for the diode to reach its equilibrium temperature after turning it on.

Colin wrote:

I worked very hard to build a heatsink for the D&S laser and now wonder if it was a waste of time. The better the heatsink design the more sensitive the laser becomes to draft induced frequency shift. I wonder if a better solution would be to insulate the diode! It would shorten the diode life but I would guess it would be more stable. And at $50 per diode I could afford to replace mine once a year if needed.

Tom B. · Post by **Tom B.** » Tue Jul 09, 2002 3:06 am

I wound up getting a Coherent LaserCheck from Edmund Optics for about $300. It works, but I'm not all that thrilled with it. It's very hard to hold steady enough to get repeatable measurements, which seem very sensitive to the angle of incidence. Maybe because the sensing photodiode is not AR coated, so back-reflections can be a problem when trying to measure an APC diode. The non-user-replaceable battery is another sore point. I wonder what it will cost to have the factory replace it? And that mushy little rubber button - it's hard to tell in the dark if you have pressed it or not. My plan is to build a simple power meter that is easier to use, use the LaserCheck to calibrate it and then use the Lasercheck more rarely.

John K · Post by **John K** » Tue Jul 09, 2002 8:33 pm

Manufactures bulding crystal controled oscillators that need a high degree of stability will employ a small enclosed heater to control temperture to minimize frequency drift. I wouldn't do that to a laser doide, but using an enclosure to keep away drafts around a heat sink, should help stabilize temperature.

Sergio Oliveira · Post by **Sergio Oliveira** » Wed Jul 10, 2002 6:45 pm

Any device to control the laser crystal temperature will not work because rise time crystal temperature change is high, almost immediately the diode reaches the operating temperature and then the thermal energy is transferred to the base and then to the mount, the solution is treat the laser as a power IC and attach a small heat sink to dissipate this low power, low thermal resistance device, this worked well on my system, physically I guess the unique solution..