THORLABS MX40G Series, User Manual

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MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Page 16 TTN116413-D02
The dither tone frequency may be changed by tapping the Dither Frequency field, which is highlighted in Figure 22. Standard
frequencies are selected using the blue buttons, or a custom frequency may be chosen by pressing User Define. The dither
tone frequency usually has very little effect on the accuracy of the bias control, but in some cases a different frequency may
work better or be desirable depending on the RF signal applied or on the specific application. The User Define button allows
for the selection of an arbitrary frequency between 1 kHz and 10 kHz with 10 Hz resolution.
Figure 24 Dither Amplitude Adjustment
The dither tone amplitude can also be adjusted to any amplitude between 20 mV and 2 V with 1 mV resolution by tapping the
Dither Amplitude field highlighted in Figure 24. Higher amplitudes will typically be more stable in the presence of MZM drift and
broadband RF signals, but larger dither tones also decrease SNR. If the amplitude is too low, the MZM bias may not stay
locked. Typically, a value from 300 mV to 500 mV is a good starting point.
The dither tone can be optionally disabled to accommodate applications requiring the highest SNR. The user may quickly turn
the dither off, perform a measurement requiring low noise, and then turn the dither back on. When the dither is turned off, the
bias is simply held at the previous bias voltage. Manual Mode should be used for longer-term measurements without dither, so
that a constant ratio method can be used to stabilize the bias point, as described in Section 4.4.4.
Figure 25 Choose the Positive or Negative Slope of the Transmission Function
Quadrature mode also allows the user to select between two operating points by selecting the Slope field. Positive slope is the
non-inverting operating point where increasing voltage on the MZM results in increasing optical output power. Negative slope is
the inverting operating point where increasing voltage on the MZM results in decreasing optical output power. This effectively
changes the phase of the response function.
Tap here to turn dither On/Off.
Adjust the dither amplitude by
selecting this field.
Tap to select positive slope.
Tap to select negative slope.
Tap to change slope selection.