MX10B, MX40B Series of Optical Transmitters
Chapter 4: Operating Instructions
Page 16
TTN048897-D02
4.3.4 Manual Mode
The Manual mode allows the user to bias the modulator at any point of the transmission function desired. Manual mode offers
two modes of operation: 1) Constant Bias, and 2) Constant Ratio. In both of these modes, the dither function is not active, and
the controller uses different techniques to hold the bias steady.
Figure 26
Constant Bias Manual Mode Settings
Constant Bias is the most basic mode of operation and will apply a user selected DC voltage to the MZM. This can be useful
for performing brief measurements that only take a few minutes. During longer duration experiments, the MZM is more likely
to drift.
The Constant Ratio mode employs active control of the MZM bias, but without using a dither tone. As a result, it can achieve
superior SNR compared to the Quadrature mode. This mode is recommended for analog signals or higher level Pulse-
Amplitude Modulation (PAM) formats.
Constant Ratio works by holding the MZM ratio of input light (at Mon-1) to output light (at Mon-2) at a constant value (typically
at or close to Quadrature). Note that it does NOT take into account the insertion loss of the MZM. Therefore, the user must
have some knowledge of the IL between the Monitors. For example. If the modulator has an IL of 3 dB, then the I/O ratio at
maximum transmission is already 2:1. To bias the modulator at the 50% point, the ratio must be doubled to 4:1. The MZM
insertion loss is listed in the specifications.
Figure 27
Constant Ratio Manual Modes Settings
Bias control in Manual mode;
Tap here to change mode.
Tap here to adjust Bias voltage.
Tap here to select Hold mode:
Constant Bias or Constant Ratio
Tap here to manually adjust the
modulator transmission ratio in
Constant Ratio mode.
This is the ratio of modulator input
power to output power.
