Ultra High Precision MZM modulator Bias Controller Automatic Bias Controller

Short Description:

Rofea’ modulator bias controller is specially designed for Mach- Zehnder modulators to ensure a stable operation state in various operating environments. Based on its fully digitized signal processing method, the controller can provide ultra stable performance.

The controller injects a low frequency, low amplitude dither signal together with bias voltage into the modulator. It keeps reading the output from the modulator and determines the condition of the bias voltage and the related error. A new bias voltage will be applied afterwords according to the previous measurement. In this way, the modulator is ensured to work under proper bias voltage.

Product Detail

Rofea Optoelectronics offer Optical and photonics Electro-optic modulators products

Product Tags


• Bias voltage control on Peak/Null/Q+/Q−
• Bias voltage control on arbitrary point
• Ultra precise control: 50dB maximum extinction ratio on Null mode;
±0.5◦ accuracy on Q+ and Q− modes
• Low dither amplitude:
0.1% Vπ at NULL mode and PEAK mode
2% Vπ at Q+ mode and Q− mode
• High stability: with fully digital implementation
• Low profile: 40mm(W) × 30mm(D) × 10mm(H)
• Easy to use: Manual operation with mini jumper;
Flexible OEM operations through MCU UART2
• Two different modes to provide bias voltage: a.Automatic bias control
b. User defined bias voltage

Electro-optic modulator Electro-optical modulator Modulator Bias Controller Bias point controller IQ Modulator DP-IQ Modulator MZM Bias Controller Automatic Bias Controller


• LiNbO3 and other MZ modulators
• Digital NRZ, RZ
• Pulse applications
• Brillouin scattering system and other optical sensors
• CATV Transmitter



Figure 1. Carrier Supression


Figure 2. Pulse Generation


Figure 3. Modulator max power


Figure 4. Modulator minimum power

Maxim DC extinction ratio

In this experiment, no RF signals were applied to the system. Pure DC extinciton has been measured.
1. Figure 5 demonstrates the optical power of modulator output, when modulator controlled at Peak point. It shows 3.71dBm in the diagram.
2. Figure 6 shows the optical power of modulator output, when modulator controlled at Null point. It shows -46.73dBm in the diagram. In real experiment, the value varies around -47dBm; and -46.73 is a stable value.
3. Therefore, the stable DC extinction ratio measured is 50.4dB.

Requirements for high extinction ratio

1. System modulator must have high extinction ratio. Characteristic of system modulator decides the maximum extinction ratio can be achieved.
2. Polarization of modulator input light shall be taken care of. Modulators are sensitive to polarization. Proper polarization can improve extinction ratio over 10dB. In lab experiments, usually a polarization controller is needed.
3. Proper bias controllers. In our DC extinction ratio experiment, 50.4dB extinction ratio has been achieved. While the datasheet of the modulator manufacture only lists 40dB. The reason of this improvement is that some modulators drift very fast. Rofea R-BC-ANY bias controllers update the bias voltage every 1 second to ensure fast track response.








Control Performance
Extinction ratio









Dither amplitude: 2%Vπ
Stablization time



Tracking points: Null & Peak


Tracking points: Q+ & Q-
Positive power voltage





Positive power current




Negative power voltage





Negative power current




Output voltage range




Output voltage precision



Dither frequency





Version: 1kHz dither signal
Dither amplitude



Tracking points: Null & Peak
2%Vπ Tracking points: Q+ & Q-
Input optical power3




Input wavelength




1. MER refers to Modulator Extinction Ratio. The extinction ratio achieved is typically the extinction ratio of modulator specified in modulator datasheet.
2. CSO refers to composite second order. To measure CSO correctly, the linear quality of RF signal, modulators and receivers shall be ensured. In addition, the system CSO readings may vary when running at different RF frequencies.
3. Please be noted that input optical power does not correspond to the optical power at selected bias point. It refers to the maximum optical power that the modulator can export to controller when bias voltage ranges from −Vπ to +Vπ .

User Interface


Figure5. Assembly




Photodiode 1 PD: Connect MZM photodiode’s Cathode Provide photocurrent feedback
GND: Connect MZM photodiode’s Anode
Power Power source for bias controller V-: connects the negative electrode
V+: connects the positive electrode
Middle probe: connects the ground electrode
Reset Insert jumper and pull out after 1 second Reset the controller
Mode Select Insert or pull out the jumper no jumper: Null mode; with jumper: Quad mode
Polar Select2 Insert or pull out the jumper no jumper: Positive Polar; with jumper: Negative Polar
Bias Voltage Connect with the MZM bias voltage port OUT and GND provide bias voltages for modulator
LED Constrantly on Working under stable state
On-off or off-on every 0.2s Processing data and searching for controlling point
On-off or off-on every 1s Input optical power is too weak
On-off or off-on every 3s Input optical power is too strong
UART Operate controller via UART 3.3: 3.3V reference voltage
GND: Ground
RX: Receive of controller
TX: Transmit of controller
Control Select Insert or pull out the jumper no jumper: jumper control;with jumper:UART control

1. Some MZ modulators have internal photodiodes. Controller setup should be chosen between using controller’s photodiode or using modulator’s internal photodiode. It is recommended to use controller’s photodiode for Lab experiments for two reasons. Firstly, controller photodiode has ensured quality. Secondly, it is easier to adjust the input light intentsity. Note: If using modulator’s internal photodiode, please make sure that the output current of photodiode is strictly proportional to input power.
2. Polar pin is used to switch the control point between Peak and Null in Null control mode(determined by Mode Select pin) or Quad+
and Quad- in Quad control mode. If jumper of polar pin is not inserted, the control point will be Null in Null mode or Quad+ in Quad mode. Amplitude of RF system will also affect the control point. When there is no RF signal or RF signal amplitude is small, controller is able to lock the work point to correct point as selected by MS and PLR jumper. When the RF signal amplitude exceeds certain threshold, polar of the system will be changed, in this case, the PLR header should be in the opposite state, i.e. the jumper should be inserted if it is not or pulled out if it is inserted.

Typical Application


The controller is easy to use.

Step1. Connect 1% port of the coupler to the photodiode of the controller.
Step2. Connect bias voltage output of the controller(through SMA or 2.54mm 2-pin header) to bias port of the modulator.
Step3. Provide controller with +15V and -15V DC voltages.
Step4. Reset the controller and it will start to work.
NOTE. Please be ensured that RF signal of the whole system is on before resetting the controller.

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  • Rofea Optoelectronics offers a product line of commercial Electro-optic modulators, Phase modulators, Intensity modulator, Photodetectors, Laser light sources, DFB lasers,Optical amplifiers, EDFA, SLD laser, QPSK modulation, Pulse laser, Light detector, Balanced photodetector, Laser driver, Fiber optic amplifier, Optical power meter, Broadband laser, Tunable laser, Optical detector, Laser diode driver, Fiber amplifier. We also provide many particular modulators for customization, such as 1*4 array phase modulators, ultra-low Vpi, and ultra-high extinction ratio modulators, primarily used in universities and institutes.
    Hope our products will be helpful to you and your research.

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