After a long break I found time to check a few diodes for single longitudinal mode operation.
1) Alleged new, cheap 635/638nm diodes on ebay. These were actually 658nm diodes and also not new. I returned them and so far am still waiting for a refund…..
2) Mitsubishi ML520G71 300mW/635nm laser diode. Unsuitable both in free-running and ECDL mode.
3) HL63133DG 170mW/638nm laser diode. Excellent in ECDL mode, so far the best diode! Clean stable output beyond 100mW can be achieved. See the pic at 635nm and around 100mW:
4) Update: The diode currently sold as Opnext HL45023TG on ebay turned out to be the PL450 from Osram. It has quite similar data, so most wouldn’t complain, but for purposes of single longitudinal mode operation there may be a major difference, potentially. Let’s hope for it, because the diode seems pretty useless for holography. I found it completely unsuitable in free running mode, and (after a lot of tinkering) quite OK in ECDL mode. Initial attempts produced less than than 20mW in single mode operation, but with some optimizations more than 40mW was achieved. And the beam quality is good, especially compared to the common 445nm/1W diodes which do not provide much more power in stable single mode operation.
Further Update: Now after all worked well I built everything “ready to go” into a nice case (see below), and for unknown reasons I can’t get any more stable SLM operation to more than 25-30mW. The only thing that was changed was the collimator, and it seems that the adjustment is extremely critical for stability. Indeed it turns out that the stability depends crucially on how far the focus point, or beam waist, is from the laser; changing it between 1m, 2m, and 5m makes a huge difference, and this amounts to very minor rotations of the collimator.
In order to more systematically investigate the effects of feedback, I made it variable by using a waveplate and a polarizing beam splitter cube; preliminary results show that indeed the feedback must be in a relatively narrow window, but this alone does not guarantee SLM operation…. more work is necessary! Here a pic of the first test setup:
From left to right: grating, beamsplitter, waveplate in rotation mount, diode mount.