mode stabilizing diode laser

[climbak]

So at work, I have a diode laser with a tunable external cavity in a Thorlabs TEC diode driver. The cavity can single out a single longitudinal mode and the TEC helps to stabilize it, but so far I still can't get it stable in single mode operation for more than a few minutes. Tomorrow and Friday I will be recording various temperature and current combinations and their corresponding stability. Anyone have any ideas on how I might make it stay in a single mode for longer? I pretty much need to get stability on the order of an hour or so, longer if possible.

 

[nikokapo]

Sadly this escapes my knowledge. Althugh I've tried TEC Cooling and didn't get very good results. Have you tried with the simple option of aluminum/copper & fans?

 

[climbak]

well the reason we are using a tec unit is because we need the temp to be as stable as possible. we have the whole thing isolated from any air currents and the tec controller can get the temp of the diode itself stable to 0.01 degree C. I figured this would be enough, but so far no dice. It slowly drifts out of the mode. I think I may need to let the entire system stabilize as well. I imagine the external cavity is shifting due to temperature too which would change the length and thus the mode.

 

[nikokapo]

Oh, I see then. You NEED the TEC cooling. I don't know a lot about this but is there any way to cool something else with the TEC that maintains the temperature it's given? (Instead of going straight into the object you need to cool)

 

[MarioMaster]

maybe you should try a HeNe - they're known for being very stable

 

[FrothyChimp]

...I imagine the external cavity is shifting due to temperature too which would change the length and thus the mode.

That's exactly what is happening. Even though you are TEC cooling the diode doesn't mean there are not temperature gradients within the system. It doesn't take much change to pass additional modes. Are you taking advantage of a feedback diode to stabilize output to minimize power fluctuations so the TEC doesn't have to adjust to slight changes all the time? I've never done it with a diode laser but it may be possible to apply an external etalon to pass a single frequency from a narrow band.

 

[climbak]

maybe you should try a HeNe - they're known for being very stable

Can't use a HeNe. We need a specific wavelength that HeNes don't lase at and we need around 100mW. We have a bunch of really expensive DPSS rated at 300mW, but we are exploring diode options for cost reduction.

That's exactly what is happening. Even though you are TEC cooling the diode doesn't mean there are not temperature gradients within the system. It doesn't take much change to pass additional modes. Are you taking advantage of a feedback diode to stabilize output to minimize power fluctuations so the TEC doesn't have to adjust to slight changes all the time? I've never done it with a diode laser but it may be possible to apply an external etalon to pass a single frequency from a narrow band.

Thats what I figured. I forgot about an etalon. We may have to try that if I can't get the current cavity to stablize after a period of time. Unfortunately the diodes we are using now and most likely will continue using for a while don't have onboard photodiodes. I may be able to wire one in externally using a low reflection beam splitter if it becomes necassary.

 

[MarioMaster]

maybe you should temperature regulate the entire laser module, lens/casing/whatever all included to prevent temperature variants from affecting it, TECs are available in the 500W area now so capacity shouldn't be a huge issue

 

[climbak]

That may work. I was also thinking that it may be possible to get very fine temperature measurements along the cavity and have that drive a signal to the piezos controlling the cavity length. It may end up being stable enough as it is though once I let the system come to as close to equilibrium as it can.

 

[likewhat]

I have used these many times. We just built our own laser housings and then mounted gratings on optical mounts inside of them and then bought current supplies for the diodes. We also built our own temperature control circuits.

So if you have a noisy current supply then it will mode hope or if the temperature drifts it will mode hope.

If you want to know if the cavity length change is causing problems then you can just calculate how far the cavity would move and from there you can determine if that is your problem. I would suspect that it isnt however and it is just caused by the temperature of the actual diode drifting.

When i used these (depending on the free spectral range of the laser, ours were typically a few GHz) after adjusting the temperature of the diode it would take maybe at least 30 min for the temp to settle near the set point such that it was within maybe 10 mK of the set point. once the temperature was that stable (which you claim yours to be) the diode would not mode hop assuming the frequency of interest was in the middle of the mode. If you try running these near a mode hop they wont stay put for long. I assume you are using an atomic reference such as a saturated absorption spectroscopy system to determine the frequency of the laser. Sometimes the cavity modes are just unstable which you can fix by making a gross change in the cavity length. Other times the diode modes just seemed to suck so by changing the current by usually around 20 mA I would be able to find another set of modes at the correct frequency that would be stable.

sometimes you have to make a giant change in temperature to make the diode happy then change the grating to get back to the correct frequency.

We locked our lasers to atomic transitions (I dont know if you need to do that) and we tied our piezo voltage control to the current mod on the diode current supply and could tune mode-hope free for about 20 GHz.

Basically it is a pain but the 3 knobs you have are temp, current, and cavity length. If it doesnt work at 1 setting of the 3 make a big change and see if you can find a place that is happy.

one last note on stability. Some of our lasers would stay fixed in frequency on the MHz level for up to several days. Other times we would be unable to every get it locked in first place. It isnt too bad when you only have 1 of them but with 6 (all at different frequencies) it can be a real nightmere.

(im not going to proofread this so sorry about the mistakes)

 

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