THORLABS MX40G Series, Руководство пользователя

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MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Page 18 TTN116413-D02
4.4.4 Manual Mode
The Manual mode allows the user to bias the modulator at any desired point of the transmission function. Manual mode offers
two operation options: 1) Constant Bias, or 2) Constant Ratio. In both of these options, the dither function is not active, and the
controller uses different techniques to hold the bias steady. The desired option is chosen by selecting the HOLD field.
Figure 28 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), see Figure 1 for reference, at
a constant value (typically at or close to Quadrature). Note that Constant Ratio does NOT take into account the insertion loss
(IL) of the MZM. Therefore, the user must have some knowledge of the IL between Mon-1 and Mon-2, which is equivalent to the
IL of the modulator. 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 29 Constant Ratio Manual Modes Settings
Manual mode is active, tap this field
to change.
Tap here to adjust Bias voltage.
Tap here to select: between Constant
Bias or Constant Ratio options.
Tap here to manually adjust the
modulator transmission ratio in
Constant Ratio mode.
This is the ratio of modulator input
power to output power.