A friend had pulled in a pile of nice externel cavity diode lasers that run at 780nm, made for spectroscopical purposes, and hat given me one - thanks geso! Of course I had to convert it to a visible light, single longitudinal mode ECDL on the same day I got it. Here are a few remarks and observations that should help others to convert and operate this type of lasers.
The manufacturer appears to be "Thermo".
The laser head comes in a sealed plastic housing mounted on a metal plate.There is a diode mount which is temperature controlled by a small TEC and a 10K thermistor. It faces a grating and the beam is then turned back by an ordinary FS mirror and exits through a coated window. Both optics are mounted on small adjustable mounts.
There is a high quality coated aspheric collimator lens in the diode housing. It is a Thorlabs lens C230TME-B whose coating is still good until 650nm. It has the highest efficiency of all lenses I tested so far. However the mount did not excite me, it is quite loose and thus the lens must by secured by a set screw.
The grating is gold plated, UV blazed at 1800 lines per mm. I determined the following (approximate) efficiency data of the grating at 658nm:
horizontal s-polarization: back reflection to diode: 33%, output reflection: 57%
vertical p-tpolarization: back reflection to diode: 7.3%, output reflection: 87%
658nm is close enough to 780nm such as to expect that the grating, the coating of the collimator and of the exit window should work well for 658nm. One thing that needs to be changed is the angle of the grating, which must me changed from 45 to 54 degrees (ie, 9 degrees tilted towards the diode). This is more than the adjustable mount can do but fortunately one can turn the whole mount by using the screw underneath.
On the other hand, the beam then misses the mirror and the exit window.
So either one needs to drill a hole in the plastic case, or remount the mirror by drilling a new mounting hole 1cm away from the old hole (as close to the edge of the mounting bracket as possible). A straight exit beam is certainly more elegant, however the mirror and exit window cost a few percent of precious power. See the pic after modification:
I used an open can high power Mitsubushi ML101U29 diode of the same kind that worked well in my own ECDL setup.
Mounting was a little awkward because these $12 diodes are removed from DVD burners so their legs are cut short. Extra inconvenience comes from the fact that there is no ESD diode protection on the laser head. I added, as always, a 470nF ceramic cap, a 10K resistor, and a Schottky diode parallel to the laser diode.
Adjustment is slighly
nasty and it helps to measure the angle of the grating to be in the right ballpark in the first place. Very critical is the adjustment of the collimator lens in the diode mount, it needs translated out by a few turns. I adjusted it to get a small spot in a few meters distance.
Then adjustment is done near lasing threshold where the sweet spot can be easily recognized. This is best done with the diode/grating
removed from the housing, subsequent fine tuning is still cumbersome but possible when everything is back in place.
I obtained almost immediately more than 100mW at 200mA current, probably 50% more is possible but I didn't want to risk the diode.
However, at currents above 160mA I found sporadic multimode behavior, and by scanning over the temperature
I found a good single mode spot, see the display of the scanning interferometer:
One thing that bothers me is that the diode is mounted on a TEC separately of the grating. This means that the distance between them is not temperature controlled, unlike my my own ECDL setup. And indeed some preliminary investigations indicate a susceptibility to mode hops when the ambient temperature is changed. I will need to further investigate this. In the worst case, both grating and diode will need to be remounted on a common temperature controlled plate.
So far I was mainly using my own drivers to operate the unit. As for the driver that came with it, I reverse engineered the circuit with the following results:
There are three status indicator LEDs, one for "temperature too low/heat" (LED 1) , one for "temperature too high/cool" (LED 2) and one for "temperature ok/laser on" (LED3). It turns out that the diode is shut down if the temperature is outside a small window, so one gets easily mislead into thinking that the driver or the diode does not work. That is, whenever one of the temperature LEDs is on, the diode is off, and vice versa. Easiest for quick checking is to change the set temperature by adjusting tle leftmost POT 1. There is a small window where both temperature LEDs go off and the laser LED goes on.
I found that the board has provisions to boost the LD current capabilty (to like 2A) by adding a Wavelength Electronics WLD3343 LD driver module.
Probably other components on the board need to be changed for this modification as well. Otherwise it uses a bank of tiny mosfets to drive both TEC and LD. It is unclear what the limitations are; at least some stickers tell that the LD currents were in the order of 100mA. The LD current monitor (center-top above) yields 1V per 100mA.
My board turned out to have problems. The LD current was fixed at full throttle and likely had destroyed the original 780nm diode. I found that the current jumped when tapping at the board; after resoldering the rightmost LT1079 precision opamp the LD driver worked flawlessly and was adjustable by POT 3 from 70 to 150mA. There are however still problems with the cooling mode.
Note added: I got another of such lasers, and the driver board of that one seems 100% intact. It worked out of the box, however the max diode current it can supply was just 84mA. Another such board was adjustable to about 150mA.
Finally I have put together a complete working unit, it doesnt look particularly nice but it works well:
I added a copy of my proven driver board, and at the inside, I replaced the aluminum turning mirror by a high-efficiency dielectric mirror, and added a noise detector circuit including a Brewster beam sampler at the output window:
Things are quite tight and one needs to take away the plastic shell for adjusting the grating.
A few more pictures of this laser and modifications of it can be found here.
