More diode tests

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.

Here a brief movie (2MB) showing single mode operation from the CCD spectrum analyzer at around 40mW (popping sounds indicate mode jumps):

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.


3 Responses to “More diode tests”

  1. Tonio Says:

    A bit off-topic: I worked now for almost five years in a small laser diode company. In the past year I focused on developing systems to analyze diode lasers with respect to their linewindth and tunabilty and their amplification. For this purpose I built ECDLs and MOPA setups. The wavelengths were between 650…1100. The company I am working at is not interested in selling whole systems and is focusing its buisness on just the component. I am quitting now and plan to build and sell these kind of devices by myself. Your descriptions of the FPI are very very useful. It’s exactly what I have been looking for.

    I’m wondering if we could do together some small projects. What I can offer is my expierience in the IR and some equipment. Your interesest is mainly holography and you need lasers in the visible domain. Let’s see

  2. rxlaser Says:

    Hi Tonio,

    thanks for your interest! Well, I am doing this for hobby purposes and only when I have time, which is sporadic; so I can’t really put pressure on me with deadlines etc, which would be required for any more serious project. But if I can be of help in some questions, eg. electronics wise, I’d be happy to help.

  3. Tonio Says:

    thank you for the offer. In the meantime I will follow your homepage and this blog. Keep on writing 😉

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