Automated Bias Controller for Lithium Niobate (LiNbO3) Modulators

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Block Diagram of the Internal Setup of the MBX Modulator Bias Controller
See the Operation tab for details.

Janis Valdmanis, Ph.D. Optics
Ultrafast Optoelectronics
General Manager

We Design, Develop, and Manufacture
Equipment up to 70 GHz

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Features

• Compatible with Any Fiber-Coupled LiNbO₃ Intensity Modulator Operating from 1250 to 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: 1310 nm, 1550 nm, and 1590 nm
• Control via Intuitive Touchscreen Interface or Remotely using USB or RS-232 Connections

Thorlabs' MBX Bias Controller provides complete and precise control of DC bias and optical output power for any fiber-coupled lithium niobate (LiNbO₃) electro-optic intensity modulator, regardless of signal speed. The operational wavelength range of the MBX extends from 1250 nm to 1610 nm (O- through L-band), and the user may select from power calibration points at 1310 nm, 1550 nm, and 1590 nm. Whether operated in an R&D laboratory or manufacturing environment, this fully-featured modulator bias controller is 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 the MBX 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.

The MBX 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

The MBX is one of a range of LiNbO₃ electro-optic modulator driver and control instruments developed by Thorlabs to meet a variety of application requirements. Our fully integrated solutions include a tunable laser and modulator in addition to the driver electronics. The key features of each instrument in the range are listed in the table at lower-left. Please contact the Ultrafast Optoelectronics Team if your application requires a driver or controller with additional customization.

The lithium niobate intensity modulators we offer are listed in the table at lower-right. Our full range of LiNbO₃ modulators includes these and our phase modulators.

System Overview

The block diagram shown below illustrates the internal setup of the modulator bias controller. 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.

Block Diagram of the Internal Setup of the MBX Modulator Bias Controller

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Figure 2: System Wavelength Selection Screen

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Figure 1: Home Screen of the MBX

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Figure 3: Bias Controller Settings Screen to Adjust Dither
Amplitude

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Figure 4: Bias Controller Settings Screen to Adjust
Operating Mode

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Figure 5: Bias Points for an Intensity Modulator
V_π is the half-wave voltage. 

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Figure 6: VOA Settings Screen to Adjust Output Power

Touchscreen Interface

Full access to all MBX modulator bias controller functions is available using the resistive touchscreen display, which is sensitive to finger pressure or the tap of a plastic stylus. Figure 1 shows the Home screen, at a time when the bias controller was disabled and the VOA enabled. 

The Home screen is organized in three main sections:

• 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.

The system wavelength may be selected on the Menu page, which is shown in Figure 2. The choice of system wavelength specifies which calibration settings to apply to the intensity monitors in the MBX.

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. 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 Controller 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 4 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 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, the MBX includes the Constant Ratio mode. It is enabled by tapping the MAN function on the right of the screen shown in Figure 4 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 MBX 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.  

VOA Settings
The VOA provides the means for adjusting and stabilizing the modulated optical output power. The VOA settings screen, which is shown in Figure 6, 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 Final 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. The USB interface is currently used only for firmware upgrades that are made available on the Thorlabs website. Future revisions of the firmware will provide for remote control of the instrument’s functions.

Front Panel

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MBX Modulator Bias Controller Front Panel

Back Panel

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MBX Modulator Bias Controller Back Panel

Each Modulator Bias Controller Includes:

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

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The GUI of the Remote Control Software Tool

Software for the MBX Automated Bias Controller

Control the Bias Controller Remotely via Serial Commands
Serial commands sent to the MBX 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 MBX 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 MBX controller. The touchscreen interface remains active while the MBX controller is controlled remotely. Descriptions of how to connect a controlling computer to the MBX 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 the MBX Bias Controller
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 laser source and sends commands in response to buttons clicked by users. Commands sent to the MBX 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 1.8.1 (January 13, 2020)

Click on the link below to download the Remote Control Software Tool.

Firmware Update

Version 1.8.3 (January 13, 2020)

Click on the link below to download the latest firmware.