How do you generate a filter narrower than the 10 GHz nominal bandwidth of the WaveShaper?

While we don’t have a WaveShaper with a sharper transfer function, it is possible to increase the filter roll-off in a WaveShaper 4000 (and also in a 16000) by double passing the light through the WaveShaper.

To do this in a 4000, set the WaveShaper output port to be port 3 – this also connects port 2 and 4. Now connect the input as normal, but connect port 3 to port 2. The light coming out port 4 will have done a double pass through the WaveShaper and so the filter roll-off should be twice as sharp and you should see a 3 dB bandwidth of around 7 GHz.   The downside of this is that the insertion loss seen by the signal is doubled.

You can also create a sharp notch filter by setting  a π phase change at the wavelength where you want to create the notch (the perfect notch won’t be exactly π, you’ll need to tune the phase slightly to get the deepest notch). In this case you’re creating destructive interference at the point where you swap the phase – hence creating a sharp dip.

See also “How do I setup a 16000 WaveShaper for multi-pass operation?