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So it appears that the mirror reflectivity for the invar cavities is so high it is making them hard to work with. Ordered a broadband (lower reflectivity) mirror from thorlabs, we'll see if that helps.
In the meantime, i've been working with the other "new" laser i have been working on, i would like to get it in the bullpen. There was really no reason it wasn't working, i just needed to keep playing with it until i found some feedback. Interestingly, i think the tilt of the grating
Initially i noticed feedback with the polarization of the laser diode oriented 90 deg to what I expected, this lead me to compare to the other diodes but they are in fact the same. I then changed the grating angle to get the correct output wavelength. Maybe i was just unable to get the grating angle close enough before to observe any feedback.
Initially the feedback was not very good, it was not a single peak (in output power vs tilt space), but a hill with many sharp peaks that have different output wavelengths, each local peak about a 20%increase in output power. Eventually i was able to get better performance by messing with tilt and collimation as well as grating angle. I adjusted the asphere position to try and get better feedback but that suffered from the fact that just pushing on the lens and changing its tilt in the mount changed how well the feedback worked; this is a pretty significant problem with our lasers, relying on adjusting collimation by turning the lens in its mount. I followed up by carefully measured the grating angle and tried to get it closer to what it's supposed to be. At some point, while moving the grating to a closer angle to what i expect, the feedback performance greatly improved. Now see about a factor of 6 due to optical feedback, .25mW to >1.5mW output power at about 22mA input current.
For a 1200 rulings/mm grating and 1055nm output, i'd expect about 39.3deg off the normal, but ended up at about 37.7. This is not unreasonable as i'm not able to measure the angle super accurately. Looking at the other laser design, that turns about the grating face, that one appears to have an angle of about 41 deg. Interestingly, these seem a lot more angled than our other 1055nm laser.
Lesson learned re: tuning up lasers, if the grating angle isn't right to get feedback, you're not going to see anything. Useful to make sure you're at the right angle (or close) if you want to have hope of finding feedback.
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