I have built a Mach Zehnder interferometer in which a laser beam is first split. After each splitted beam is passed through a AOM driven at a fixed RF in my case 40 MHz and 40.04 MHz they are recombined. The envelop of laser rep rate is modulated at the difference frequency, 40 kHz in my case. Its a pulsed laser with 80 MHz rep rate. The attached image 1 shows the laser rep rate profile on a scope and after changing the horizontal scale (keeping everything same) you can see the modulated envelop(image 2). How can we calculate the modulation index from this?

The second query is related to the same modulation scheme in which I monitor the waveform after modulation on the FFT display of my lock-in (SR 865a). Image 3 shows when the signal to lock in is provided by splitting the photodiode signal with a BNC t and supplying signal to scope and lock in simultaneously. The fourth image shows when the photodiode output is dedicately fed to the lock in. They are many side bands each equally spaced at 40kHz. Also, if we see the dB value of the signal they in the range of 30-40. Is it satisfactory as my lock in is not able to "lock-in" when i feed the photodiode signal as a reference. However if I use a photodiode amplifier it locks in readily. How can I remove the sidebands ?Clockwise 1. Laser pulse train (horizontal scale 20 ns) 2. Modulated (horizontal scale 10 microsec) 3. lock in FFT when signal split with scope 4. lock in FFT with total signal

  • \$\begingroup\$ - 1)2) m = 100% ignoring -ve envelope from overshoot which appears to be 20% - so 3) is locked and 4) is not and when/where was TIA amp used? \$\endgroup\$ – Sunnyskyguy EE75 Jan 6 at 0:48
  • \$\begingroup\$ How to get rid of them? a 40kHz BPF? \$\endgroup\$ – Sunnyskyguy EE75 Jan 6 at 1:03
  • \$\begingroup\$ @TonyEErocketscientist The amplifier was added after the photodiode and the amplifier output was split between scope and lock-in. The side bands appear only when the signal is not split and directly fed from the photodiode in to lock in amplifier will the negative overshoot compromise my measurements. If yes, any suggestions for getting rid of it ? Also, are the electric band pass filter a commercially available ? \$\endgroup\$ – user188062 Jan 6 at 2:11

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