Laser noise

I've been measuring the beat-note (wikipedia talks about sound-waves, but it works for light-waves too) between two HeNe lasers. It jumps around maybe +/- 5 MHz quite rapidly which is not nice at all:

One laser is a commercial stabilized laser (I've tried both a HP5501A and a Mark-Tech 7900), and the other laser is a tunable one which I want to use for my experiment. But with this much jumping around the tunable laser is no good for the experiment I want to do 🙁

4 thoughts on “Laser noise”

  1. Hello, Anders! Could you please drop a hint on electrical equipment you used for photodetection? I'm from Russian ITMO University ( and we want to do similar experiment, so it would be really appreciated if you share your experience about photodetection, laser beating, ampification etc.

    Best regards, Anton.

  2. If you have two stable HeNe lasers it should be quite easy to get a beat-signal just by overlapping the two beams well.
    You will need Faraday Isolators (40dB or more isolation) in front of both lasers to avoid optical feedback.
    If you feed the beat-signal to a frequency counter you want maybe >40dB of SNR to quickly get accurate readings on the counter.

    It is possible to build photodiode amplifiers with a bandwidth of a few hundred MHz quite easily. 1 GHz or more is harder.
    The book "building electro optical systems" by Robert Hobbs has some useful discussion on transimpedance amplifiers. You can google for the authors website also. Usually one wants a design where the shot-noise from the light dominates over the Johnson noise in the transimpedance amplifier. The easy rule-of-thumb is 51mV or more over the feedback(transimpedance) resistor.

    The detector I used here is based on an APD with a high (1kV ?) bias voltage, bandwidth about 1.5 GHz. Sorry I don't have the schematic for it or a link to a paper.


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