In this introduction to Optisystem video we will be looking at the Gaussian optical filter and will be using it to filter out a signal from 2 signals that are multiplexed together. This video describes how to setup and modify the Gaussian optical filter in OptiSystem.

### Using The Component

The Gaussian optical filter component acts as an optical filter with a Gaussian frequency transfer function. In this setup we have two modulated signals multiplexed together and we will be experimenting with the Gaussian optical filter to observe the results of changing it’s various properties. Calculating the program with the Gaussian filter having default values we can have a look at the results. In the visualizers we see at the input of the Gaussian filter there are 2 signals visible in the spectrum analyzer but after filtering only 1 signal is present. Double clicking on the Gaussian optical filter we can change the value of frequency to 193.3 THZ. This will have the effect of moving the filter’s center frequency to 193.3 THZ. The result we should expect from this is for the 193.1 THZ signal to be filtered out and the 193.3 THZ signal should remain. Calculating the project and looking at the visualizer we can see that the signal at the output of the filter has a different wavelength and corresponds to a frequency of 193.3 THZ as we expected.

Going back into the properties of the filter we can try changing bandwidth as well.  Increasing the value of bandwidth increases the 3 db filter bandwidth. With a bandwidth of 100 we will start to see new frequencies get through the filter since our bandwidth is wider. Calculating the program and clicking the visualizer we see that the 193.3 thz signal has a wider bandwidth because the parts farther from the center are no longer filtered out. The 193.1 THZ signal is also appearing in the spectrum analyzer, but still being reduced by the filter since it is outside of the 3db bandwidth. Changing the bandwidth to an even larger value and then calculating the project gives us a filter output with neither signal filtered out because both signals are within the filter’s bandwidth. Attaching an optical power meter to the output of the filter and then going back into the properties we can also have a look at the effects of changing the value of insertion loss. Increasing insertion loss increases the loss applied to the signal as it enters the filter. Calculating the project with and insertion loss of 5 db we can see a slight decrease in the optical power at the output of the filter. If we change the insertion loss to an even larger value, we can see a more significant loss in the optical power meter. Resetting some of the values we will now have a look at varying the depth of the filter.

Changing the depth of the filter changes the maximum attenuation value for the filter. increasing the depth of the filter means that frequencies outside of the filter bandwidth get attenuated more. Calculating the project, we can see that this was indeed the case. Reducing the depth causes frequencies outside of the filter bandwidth to be attenuated less. Calculating the project, we can see the frequency at 193.1 was attenuated very little as we expected. Resetting the value of depth, we will now have a look at the effect of changing the order of the Gaussian filter.

Increasing the order of the filter increases the slope of the roll off of the filter. Higher order filters are often useful trying to separate two close frequencies.  In this case increasing the order of the filter further attenuates the unwanted channel. Going into the simulation window of the filter and checking the re-sample box will cause the filter to down sample the signal bandwidth to the filter sample rate. With a sample rate of 500 GHZ we will not notice much difference in the signal in the spectrum analyzer or optical time domain visualizer because it is a high sample rate. If we further reduce the sample rate, the signal bandwidth will be down sampled and we will start to lose information. Calculating the project, we can see that this is indeed the case. In the simulation window we can also select whether the optical Gaussian filter should be simulated as a digital filter. When digital filter is selected, we will also have the option to change the sample rate of the digital filter