DFB module: Frequency change via laser diode current

Joerg,

good description, pulling against an oak tree.

the laser diode is a highly tuned optical cavity, Q's > 1000. you are not pulling any ppm but rather ppb! Perhaps an LED wouldbe able to pull, but mutliplexing two lasers and fsk'ing them would absolutley work..good luck.

Marc Popek

Folks out here do that. But only if they have a monster truck with a monster winch :-)

We need to vary over a range, not just FSK. It's more like a specialized interferometer application. DFBs can pull about 4-5 picometers per milliamp in current change. I just need one that will do it fast enough. Or another kind of laser diode that puts out a nice and narrow spectral line where we'd mount our own TEC to it for the wide range tuning part. The TEC tuning speed isn't so critical although it was a bear to make that work at any reasonable tuning speed without too much overshoot.

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Regards, Joerg

http://www.analogconsultants.com

You're seeing millisecond delays in _current_tuning_? Wild. Ought to be much, much faster than that if it's the usual plasma-optic effect--you must be doing temperature tuning instead. Is there a smaller, much faster modulation you can measure?

Cheers,

Phil Hobbs

Well, I was able to make the inner loop (which regulates the current) oscillate at over 100Hz but only at really tiny excursions. Less than a picometer. When I disabled that loop and its connection to the other loop (the one for the TEC) and then stepped the current between 35 and

40mA the reaction was rather sluggish. It did scoot the expected 20pm or so but that took hundreds of milliseconds. Sure looks like temperature tuning. Now I am wondering if DFB modules can really do that or whether we may have to try out ECDL.

Laser stuff is not my home turf. One of the frustrations is the rapid model changing and corporate takeovers. I talked to an engineer at Emcore and he said their 1751 can be tuned fast via current. But when the prototype was done that part was already gone from their site. Have emailed him, maybe they have something that can work.

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Regards, Joerg

http://www.analogconsultants.com

I do not know what your application is but I have worked on similar problems in the past. The difficulty in what you are doing comes most in proportion to the accuracy with which you want to hit any given frequency in on any given time scale and how much amplitude modulation you willing to accept in the process. Diodes like DFB lasers are designed to resist frequency shift current modulation. This is so they will stay within their allotted channel as they are modulated on and off. Fabry-Perot diodes have much larger FM verse current modulation tuning coefficient. EC diode lasers are the real champs of large fast tuning applications but have all the problems associated with mechanical (usually PZT) devices like long term drift mechanical resonances hysterieses etc.

FM modulation by current tuning is often troublesome do to the fact that large and small modulation effects take place on different time scales from 100s of microseconds to a few nanoseconds.

Knowing exactly how much frequency you can tolerate on what timescale and if you can tolerate significant AM on your laser will help you converge to the right solution with the least pain.

Good luck

Steve

Drift and non-linearities can be handled here because we have a closed loop system, no need to steer based on a current versus wavelength calibration table. This loop works quite well but the laser behaves like a drag chute, very sluggish in response.

Yep, I bet that's part of the problem here.

We can regulate out AM quite well, even if it was fast. This is partly due to the advent of blazingly fast opamps from TI and others. I have a

1GHz GBW opamp from Texas in there. Costs only $2-$3 and it truly impressed me. I was mentally prepared to do it all with RF transistors like usual but didn't have to.

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Regards, Joerg

http://www.analogconsultants.com

I do not know about DFB lasers, but with Fabry-Perot diodes a modulation of the injection current has two effects:

a) The temperature of the diode junction changes. This leads to an increase/decrease of the optical resonator length due to the temperature dependance of the refractive index and due to thermal expansion. b) The charge carrier density in the diode also influences the refractive index.

For modulation frequencies 10MHz b) dominates.

Unfortunately a) and b) cause frequency shifts in opposite directions so that the transfer function has a 180° phase shift between 1 and 10 MHz making a high speed frequency control challenging.

I can imagine that the a) effect is much less pronounced in DFB lasers so that the a) b) transition frequency is much lower.

Cheers, Jürgen

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Juergen, that's what I suspected as well. What puzzled me though was that this cut-off seems to be really low. Somewhere in the sub-10Hz range. DFB module have a long chamber buried inside, maybe that's why.

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Regards, Joerg

http://www.analogconsultants.com