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I am using a function generator to generate a sine input to a modulated laser. The function generator has output settings of 50 Ω and High-Z, while the laser for some reason has a 1 kΩ input impedance. What would be the best way to connect these to modulate the laser as close to the input signal as possible? When just connecting both and looking at the laser output I am seeing fluctuations (which may or may not be due to the mismatch).

Is an impedance matching circuit required? Will a simple hand-soldered L-pad of just resistors do the job? Maximum frequency is around 50 kHz.

Bonus question: Why does the laser have 1 kΩ input impedance? Why not any other value, specifically why not 50 Ω or some large value like 1 or 10 MΩ?

The datasheet of the laser can be found here (DPL laser, analog modulation, page 14. Ignore the maximum frequency, that's a typo).

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    \$\begingroup\$ At 50 kHz, impedance matching in the traditional HF sense with reflections is not a concern. Please provide oscillograms of your ”I am seeing fluctuations”. \$\endgroup\$
    – winny
    Commented Apr 20, 2023 at 11:59
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    \$\begingroup\$ Bandwidth is low enough that impedance matching isn't necessary, so 1k is a reasonable value. You can hook the 50 ohm output directly to 1k and it will just work like you'd expect. \$\endgroup\$ Commented Apr 20, 2023 at 13:05

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The function generator settings affect only the function generator output amplitude at such low frequencies. You can consult your function generator manual, but below explains how it typically works.

When you set the function generator to High-Z you'll get almost the output amplitude you set with a 1K input Z. (Actually 1000/1050 = 0.952 x the set amplitude).

If you set the function generator to 50Ω you'll get almost double the output amplitude you set (Actually 2000/1050 = 1.90 x the set amplitude).

The function generator output amplifier has 50Ω in series and you are telling it what output load to expect when it determines the output voltage ahead of the 50Ω resistance.

So, aside from (perhaps) a much larger or slightly smaller than expected output amplitude there should be no great effect on the output waveform provided the function generator is not running out of headroom.

If you were to parallel a 1kΩ input impedance with 52.6Ω and set the function generator to 50Ω you'd get the set output amplitude (within whatever tolerances), but that's pretty much unnecessary and wasteful (and also limits the output voltage maximum to about half).

schematic

simulate this circuit – Schematic created using CircuitLab


As to why the designers of the laser driver chose 1kΩ as the input Z, it could be because they wanted minimum noise pickup if the input was to be left open or for some other design or system related reason.

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