Simple Lock-In Amplifier

Discussion in 'The Projects Forum' started by moot, Oct 30, 2009.

  1. moot

    Thread Starter Member

    Sep 20, 2009
    I'm working on assembling a lock-in amplifier for the purpose of locking a tunable diode laser to the peak of a spectroscopic feature. I feel pretty confident about it, and I've tested everything seperately except for the mixer (which is being shipped), but I thought I should have some experts check it over to see if there's anything that could go wrong when it's all put together. I've attached a PDF of the circuit.

    Sparing the gory details, the basic idea is that the laser light is sent through a vapor cell and onto a photodetector (PD). The laser is tuned so that its frequency corresponds to the peak of an absorption feature (i.e., atoms absorb the most light at that frequency, so the output of the PD is a minimum at that frequency). The output of the PD is sent into the lock-in amplifier.

    A waveform generator then sends a sinusoidal signal to both the laser and the lock-in amplifier. The effect on the laser is that its output frequency is modulated about the "peak", so the output of the PD rises and falls back and forth over the peak.

    The sine wave from the waveform generator is also sent to the lock-in amplifier, and combined with the PD signal. The product of these two is sent through either a low-pass filter or an integrator, which removes everything that is not at the exact modulation frequency. The resulting DC voltage acts as an error signal, and is sent back to the laser.

    It is important that the modulation of both the PD signal and the reference (straight from the wave generator) are in phase. The addition of the "manual scan" and "ramp" at the end of the circuit aid in searching for the spectroscopic feature. All op-amps are from TL074ACN ICs.

    Some questions:
    1) Are followers needed anywhere?
    2) What's the advantage (if any) of using an active low-pass filter at the end of the circuit over a series of passive low-pass filters?
    3) If I replaced the low-pass filter with an integrating op-amp, does it matter what I set the RC time-constant to?
    4) Any other wise advice for someone who is basically doing this sort of thing for the first time?