THORLABS MX10B Series, Руководство пользователя

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MX10B, MX40B Series of Optical Transmitters Chapter 4: Operating Instructions
Rev. D, March 19, 2019 Page 13
4.3.1 Quadrature Mode
Quadrature is the default mode and biases the MZM at the 50% point on the MZM transmission curve (see figure). This is the
recommended mode for digital signals such as On-Off-Keying (OOK) that require a high extinction ratio. The screen-shot below
shows the parameters that can be controlled in the Quadrature mode.
Figure 20 Buttons to Select Preset and Define Custom Dither Frequencies
The bias control circuit uses an AC dither tone to stabilize the bias point of the modulator. The graphic below shows how it works:
Figure 21 Illustration Showing How Dither is Used to Stabilize Modulator Bias Point
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 (Quad, Peak or
Null). The DC bias voltage is then continuously adjusted to maintain the correct set-point.
The frequency of the dither tone (under 10 kHz) is usually well below the low frequency cut-off for the RF signal of interest, so
it does not interfere. The dither tone can also be turned if other methods of bias control are desired (e.g. manual control, or
input/output ratio control as described later).
Select your own Dither
Frequency here.
Choose one of three preset
Dither Frequencies.