Automatic Bias Controllers for Lithium Niobate (LiNbO3) Modulators

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Figure 1.1  Block Diagram of the Internal Setup of MBX Series Modulator Bias Controllers
See the Operation tab for details.

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Features

• Designed for Fiber-Coupled LiNbO₃ Intensity Modulators
• Three Versions for Different Wavelength Ranges:
  • MBX2 Controller: 770 - 980 nm
  • MBX3 Controller: 980 - 1310 nm
  • MBX Controller: 1250 - 1610 nm
• Fully Automated or Fixed-Voltage Bias Control for Intensity Modulators
  • Dithered Operation in Automated Quadrature, Peak, and Null Set Point Modes
  • Ditherless Operation in Automated Constant Ratio Mode
  • Ditherless Operation in Fixed-Voltage (Constant Bias) Mode
• Built-In Variable Optical Attenuator (VOA) for Automatic or Manual Power Control
• Three Calibration Wavelengths per Controller:
  • MBX2 Controller: 785 nm, 852 nm, and 940 nm
  • MBX3 Controller: 980 nm, 1064 nm, and 1310 nm
  • MBX Controller: 1310 nm, 1550 nm, and 1590 nm
• Control via Intuitive Touchscreen Interface or Remotely using USB or RS-232 Connections

Thorlabs' MBX Series Bias Controllers provide complete and precise control of DC bias and optical output power for fiber-coupled lithium niobate (LiNbO₃) electro-optic intensity modulators, regardless of signal speed. Three models are available with three different wavelength ranges spanning 770 nm to 1610 nm. Each model has three selectable power calibration points (See the Features tab or Specs tab for more details). Whether operated in an R&D laboratory or manufacturing environment, these fully-featured modulator bias controllers are ideal for use within a customized setup that uses an external laser, LiNbO₃ intensity modulator, signal source, and RF amplifier.

Several operation modes are provided. Automated bias control operation continuously adjusts the DC bias voltage to maintain the correct set point, and there are dithered and ditherless options. When operating in a dithered control mode, an adjustable dither tone with a user-selectable frequency between 1 and 10 kHz is used as a pilot tone. Each automated dithered mode biases the modulator at a different operating point (Peak, Null, or Quadrature); all operating points are referenced to the modulator transmission function described in the Operation tab. If the application requires automated, but ditherless, bias control of the modulator, Constant Ratio mode may be chosen. It is also possible to operate each controller in the fixed-voltage and ditherless Constant Bias mode, in which a user-selected DC bias voltage is applied to the modulator. This mode can be useful for brief measurements, but its inability to track modulator drift renders it impractical for measurements longer than a few minutes.

MBX series controllers can be controlled in two ways. The simplest method is using the intuitive touchscreen interface, which gives the user complete control over all instrument functionality. These instruments can also be operated remotely via the RS-232 or USB ports on the back panel. The Operation tab describes graphical user interface (GUI) and user-customizable features, and we provide a remote control user guide and a remote control software tool (see the Software tab) for download.

Closely Related Items
These controllers are part of a range of fiber-coupled LiNbO₃ electro-optic modulator driver and control instruments developed by Thorlabs to meet a variety of application requirements. The key features of each instrument in the range are listed in the Selection Guide tab. Please contact the Ultrafast Optoelectronics Team if your application requires a driver or controller with additional customization. We also offer a full range of LiNbO₃ modulators for use with these instruments.

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Figure 3.1  Block Diagram of the Internal Setup of MBX Series Modulator Bias Controllers
See the Operation tab for details.

System Overview

Figure 3.1 illustrates the internal setup of MBX series modulator bias controllers. The fiber-coupled light path is traced by the red arrows, and the blue arrows show the electrical connections. The optical laser power is coupled to the Laser Input port and read by Monitor 1 before exiting to the external modulator. The optical output of the modulator is coupled to the Return from Modulator port and routed through the variable optical attenuator (VOA)-based power control system before exiting as the Final Optical Output. Automatic and Manual bias control is performed by referencing intensity readings from Monitor 1 and Monitor 2, and the power controller uses intensity readings from Monitor 2 and Monitor 3 to control and stabilize the output optical power. The rear panel features output ports for several additional monitor and control functions.

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Figure 3.3  System Wavelength Selection Screen

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Figure 3.2  Home Screen

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Figure 3.4  Bias Controller Settings Screen to Adjust Dither
Amplitude

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Figure 3.6  Bias Controller Settings Screen to Adjust
Operating Mode

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Figure 3.5  Bias Points for an Intensity Modulator
V_π is the half-wave voltage.

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Figure 3.7  VOA Settings Screen to Adjust Output Power

Instrument Control

The graphical user interface (GUI) gives the user complete control over all instrument functionality. Each modulator controller uses a resistive touchscreen display sensitive to both finger pressure and taps from a plastic stylus. The knob on the front panel of the housing can be used in place of the on-screen arrow buttons to quickly change setpoint values. Pressing (clicking) the knob will confirm a new setpoint value. Additionally, the instruments can be driven using serial commands delivered via connectors on the rear panel. This is described in the remote control user guide, and a Windows program that demonstrates remote control of the converter is available in the Software tab.

The home screen of the controller, shown in Figure 3.2, is organized into three main sections. The green dot that appears in the upper-right of the center column panels indicates that those functions are stable. The dot will blink if that function is still stabilizing.

• Left Column:
  • Buttons show the on/off status of the different instrument functions.
  • Tap a button to toggle the function on/off.
• Middle Column:
  • Current operating parameters of each control function are shown.
  • Tap in this column to access the Settings page for each function.
• Right Column:
  • Buttons provide access to various utility and help functions.
  • Tap to review and customize system settings.

Functions and controls enabled by the GUI are further discussed in the following sections.

System Wavelength Settings

The system wavelength may be selected on the Menu page, which is shown in Figure 3.3. The choice of system wavelength specifies which calibration settings to apply to the intensity monitors in the controller. Change the power monitor calibration wavelength to the value closest to the wavelength of the laser source being used.

The MBX2 controller can be used at wavelengths anywhere between 780 nm and 980 nm and calibration settings are supplied for 785 nm, 852 nm, and 940 nm. The MBX3 controller can be used at wavelength anywhere between 980 nm and 1310 nm and calibration settings are supplied for 980 nm, 1064 nm, and 1310 nm. The MBX modulator controller can be used at wavelengths anywhere between 1250 nm and 1610 nm and calibration settings are supplied for three wavelengths: 1310 nm, 1550 nm, and 1590 nm. These wavelengths represent the centers of the O-Band, C-Band, and L-Band.

Modulator Bias Controller

The bias controller and VOA settings can be adjusted using pages accessed by touching the center region of the Home page. A setting screen for the Quadrature mode of the bias controller is shown in Figure 3.4. The parameters written in white font are modified using the function buttons, which are customized for each parameter and stacked in the column to the right of the screen. The parameters in orange font may be monitored here, but not set. Toggle Dither on and off by tapping its value in the center column of the screen. Dither is discussed in more detail in the following section. In this screen shot, Dither Amplitude is selected and its set point value can be adjusted by tapping the arrows on the right of the touchscreen. Tap the check mark to confirm a value, and the X to cancel any changes. Set point values may also be quickly changed by dialing the knob on the front panel of the housing. Pressing (clicking) the knob will confirm a new set point value.

Bias Settings
The Bias Settings screens allow the operator to select among and customize the different operating modes, which are chosen by touching the Mode value and then navigating using the function buttons on the right of the screen. Figure 3.6 shows these function buttons, which are labeled PEAK, QUAD, NULL, and MAN.

Three of the four automated operating modes, Peak, Quadrature, and Null, reference the regions labeled on the modulation transmission function, which is illustrated in Figure 3.5. When one of these modes is selected, a dither tone is used to hold the modulator at the respective bias point. The dither tone is part of a lock-in approach that maintains a stable bias point by compensating for modulator drift, which occurs over time due to the temperature sensitivity of the modulator. The dither tone can be set to a frequency between 1 and 10 kHz, and the amplitude of the tone can also be selected. Quadrature mode gives the option of operating on either the positive or negative slope of the function.

For those applications that require an automated, but ditherless, approach to maintaining a stable bias set point, MBX series controllers include the Constant Ratio mode. It is enabled by tapping the MAN function on the right of the screen shown in Figure 3.6 and configuring the Ratio Set Point and Slope values. This mode adjusts the bias voltage to maintain a chosen ratio between the intensity values reported by Monitor 1 and Monitor 2. The Slope setting allows the user to choose whether increasing voltage on the modulator increases or decreases the optical output power.

It can be useful to operate for brief periods of time at a bias fixed voltage and without a dither tone. The controller allows a fixed bias voltage to be applied in one of two ways. When operating in Quadrature, Peak, or Null modes, tapping the value of Dither will toggle it between on and off states. When dither is toggled off, the value of the fixed bias voltage is held at the most recent automated bias voltage. This enables the user to make quick measurements, without the dither tone present, while the modulator is biased at one of the common modulator transmission function set points. The Constant Bias mode can also be accessed by tapping the Mode value and then the MAN function on the right of the screen. This page allows either the automated Constant Ratio or fixed-voltage Constant Bias modes to be enabled and configured.

Variable Optical Attenuator

The VOA provides the means for adjusting and stabilizing the modulated optical output power. The VOA settings screen, which is shown in Figure 3.7, allows the user to choose between and adjust the parameters of the two operational modes. In Constant Attenuation mode, the attenuation level between the Return from Modulator input port and the Optical Output port is fixed, which allows power fluctuations at the input of the power controller to be transferred to the output. In Constant Output Power Mode, the final optical output power is held constant independent of the input fluctuations. In this mode, the VOA is effectively used as a power stabilizer. Tap the Step function button at the right of the screen to change the step size by which the arrows increment or decrement the set point values.

The VOA settings screen also allows the user to select the units used to report the power readings and parameters on all pages. Use the Power Units field to choose whether power values are reported as mW or dBm.

Rear Panel

The rear panel provides additional utility functions such as the power monitor output, RS-232, and USB ports. Both the USB and RS-232 interfaces can be used for remote control operation of the modulator bias and power controller. The serial commands and control features available are fully described in the remote control user guide. The USB interface is also used for installing firmware upgrades as they become available.

Each Modulator Bias Controller Includes:

• Modulator Bias Controller Main Unit
• Region-Specific Power Cord (Determined by Ordering Location)
• 1.25 A, 250 VAC Fuse
• USB Type A to Type B Cable, 6' Long

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Figure 7.1  The GUI of the Remote Control Software Tool

Software for MBX Series Automated Bias Controllers

Control the Bias Controller Remotely via Serial Commands
Serial commands sent to the bias controller can control the functionality of the internal EO modulator bias controller and variable optical attenuator (VOA), as well as general system parameter settings. The commands can be sent from a computer running any operating system to the RS-232 port on the back panel of the controller. Computers running Windows^® 7, or later versions of the operating system, can send serial commands to the USB port on the back panel of the controller. The touchscreen interface remains active while the controller is controlled remotely. Descriptions of how to connect a controlling computer to the controller, the serial command set, and descriptions of each command are included in the Remote Control User Guide.

Application Demonstrating GUI-Based Remote Control of MBX Series Bias Controllers
The Remote Control Software Tool, which is available for download, is an example graphical user interface (GUI) provided for testing, demonstrating, and exploring the use of the different serial commands. This program is not required to operate the controller remotely. It opens a connection to the controller and sends commands in response to buttons clicked by users. Commands sent to the controller, responses from it, and status information messages are logged to the three rectangular fields located beneath the buttons. Please see the Remote Control User Guide for more information. This program can be used as a basis for the development of custom applications. Please contact us to request the source code.

Software

Version 1.8.9 (August 2, 2024)

Click on the Software button to download the Remote Control Software Tool.

Firmware Update

MBX2 - Version 2.1.2 (August 2, 2024)
MBX3 - Version 2.1.2 (August 2, 2024)
MBX - Version 1.8.3 (January 13, 2020)

Click on the Firmware Updates button to download the latest firmware.