I am trying to lock laser at half transmission point of scanning fabry perot etalon (SA200 ) using electrical feedback to laser driver current source.How can I achieve it. my experiment consists of a DFB-LD , input to circulator port 1, at port 2 etalon is connected and observing laser reflected spectrum.the transmitted signal from etalon is photodetected and feedback to laser driver current source.
Thanks in advance.
The FP etalon will convert frequency modulation (FM) to amplitude modulation (AM) -- but you already know that. You can use thermal tuning for frequency modulation up to several tens of kHz. For high frequency tuning, you can use the intrinsic FM response of the DFB laser which is highly variable from laser to laser depending on the details of the DFB construction and facet phases. But because thermal and carrier tuning are of opposite signs, you will have to design a cross-over for middle frequency response. Best to measure the amplitude and phase response of your DFB laser first -- you can do that using your FP etalon and a network analyzer. Then, you can use the FP reflection as input to an analog electrical network or a digital network to adjust the error signal to the DFB laser to stay locked to the midpoint of the etalon response curve. I've always considered one of the best references for locking DFB lasers to be Sanford and Antill, "Laser frequency control using an optical resonator locked to an electronic oscillator," IEEE Journal of Quantum Electronics ( Volume: 33, Issue: 11, Nov 1997 ).
Abbas:
I suggest you use a PID algorithm to control your laser. You can implement this as an analog circuit (more difficult but faster performance), or as a software code. You can possibly even re-purpose an Omega Engineering PID module if it's properly configured.
If you have access to LabVIEW, or even Matlab, PID algorithms are built in. In this case, though, the loop cycle time is probably no faster than 1 to 10 Hz. That is to say, the laser frequency is adjusted once every 1/10th of a second. It all depends on how fast your detection circuitry and laser current setting circuitry is. And that combined with the PC.
You could use a National Instruments multi-function DAQ to both read the detected intensity (A to D conversion) and then output an an analog control voltage (D to A conversion) to your laser current control. Optionally, you can output a digital value from the DAQ. You could alternatively send a command to a programmable DC supply (that would drive the laser) directly via USB or RS232 and use a programmable DMM to perform the A-D conversion function.
It's not clear from your question what kind of performance you are looking for. If your intent is to simply frequency stabilize the laser a simple PID implemented in software should do the trick. But if you want to perform FM or AM modulation or you want to stabilize in the very short time domain, you'll probably need the PID implemented as an analog circuit. You might also consider the methods recommended by Mr. Ackerman.
On the other hand, if you have one of these SBC's out there (like a Raspberry Pi for example), you can use that running as a 'real time' controller with loop cycle times exceeding 1kHz (possibly faster). LabVIEW directly supports real time programs running on a Raspberry Pi.
Abbas, I hope this helps!
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