Application Note for LN Modulators

P3/P5

4, How the Phase Modulator Works

The Phase modulator is a device which changes the "phase" of optical signals by applying voltage. Let's assume the following case. When voltage is not applied to the RF-electrode, n number of waves exist in the certain length. When voltage is applied to the RF-electrode, one more wave is added , which now means n+1 waves exist in the same length. In this case, the phase has been changed by 2π (360degree) and the half voltage of this is called the driving voltage. In case of long distance optical transmission, waveform is susceptible to degradation due to non-linear effect such as self-phase modulation etc. The phase modulator is applied to compensate for this degradation and makes it possible to transmit long distance.

5, Bias Control Configuration

In the case of the LN modulator, the bias point control is vital as the bias point will shift long term. To compensate for the drift, it is necessary to monitor the output signals and feed it back into the bias control circuits to adjust the DC voltage so that operating points stay at the same point (e.g.quadrature point). It is the manufacturer fs responsibility to reduce DC drift so that DC voltage is not beyond the limit throughout the life time of device.

P3/P5