THORLABS MX40G Series, User Manual

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MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Rev. E, March 19, 2019 Page 15
4.4.1 Quadrature Mode
Quadrature mode is the default mode and biases the MZM at the 50% point on the MZM transmission curve shown in Figure
23. Quadrature mode is essential for VNA applications in which linear response is critical. The screen shot below shows the
parameters that can be controlled in the Quadrature mode.
Figure 22 Dither Frequency Selection while Quadrature Mode Enabled
The bias control circuit uses an AC dither tone to stabilize the bias point of the modulator, as illustrated below. A small AC
voltage is applied to the DC bias so that the optical output is also slightly modulated. Both the amplitude and the frequency of
AC dither tone can be selected by the user. The modulated optical output is detected by a frequency and phase sensitive
detector, which can then interpret whether the DC bias is at the correct level for the chosen set-point (Quadrature, Peak or Null).
The DC bias voltage is then continuously adjusted to maintain the correct set-point.
The frequency of the dither tone will not exceed 10 kHz. This frequency limit, which is usually well below the low frequency cut-
off for the RF signal of interest, is chosen to prevent interference. As described in the following, the dither tone can also be
turned if other methods of bias control are desired (e.g. manual or input/output ratio control).
Figure 23 AC dither tone stabilizes the bias point of the modulator.
Another option: specify custom dither
frequency between 1 and 10 kHz.
One option: choose one of three
preset dither frequencies.