High-Speed, Fiber-Optic, Linear Reference Transmitters

Key System Specificationsa
Item #		MX35E Series	MX65E Series
Amplifier Type		Linear (Analog)
Bandwidth		35 GHz (Typical)	65 GHz (Typical)
Typical Frequency Response (Click Icon to View Graph)
Internal Laser Wavelength		C-Band, L-Band, or 1310 nmb	C-Band, L-Band, or 1310 nm
Internal Laser Output Power		13.5 dBm (Typical)
External Input Laser Wavelength Rangec		1250 nm - 1610 nm
RF Amplifier Gain		10 dB to 23 dB	11 dB
Low Frequency Cutoff		200 kHz	70 kHz
Optical Extinction Ratio		13 dB (Typical)
Modulator Type		Intensity

• For additional specifications, see the Specs tab and the manuals.
• Please contact Tech Support to order an MX35E transmitter with an 850 nm fixed-wavelength internal laser.
• User-Provided

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20 Gb/s NRZ Eye Diagram from MX35E Series

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25 GBaud/s PAM4 Eye Diagram from MX35E Series

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40 Gb/s NRZ Eye Diagram from MX35E Series

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56 GBaud/s PAM4 Eye Diagram from MX65E Series

Features

• Turnkey Reference Transmitter Solution
• Linear RF Amplifier
• Integrated Laser, Intensity Modulator, and Optical Power Control
• External Loop-Back Cables Support Custom Configurations
• Control via Intuitive Touchscreen Interface or Remotely Using USB or RS-232 Connections
• Custom Systems Available by Contacting the Ultrafast Optoelectronics Team

Thorlabs' all-in-one linear reference transmitters are based on proven lithium niobate (LiNbO₃) modulator technology driven by high-fidelity RF amplifiers. They are designed for high-speed fiber optic test and measurement applications. These user-configurable systems integrate a Mach-Zehnder intensity modulator (MZM) with fully featured bias control, a tunable or fixed-wavelength laser source, RF amplifier, and variable optical attenuator. These instruments are ideal for use in either an R&D laboratory or in a manufacturing environment for linear applications that require frequency responses up to 35 GHz (MX35E series) or 65 GHz (MX65E series), including high bit-rate modulation schemes that rely on multi-level encoding, such as PAM4.

The transmitters on this page are based on the use of an RF amplifier that provides a linear relationship between the input and output voltages. Note, however, that the linear range of the whole system is still limited by the non-linear response of the lithium niobate (LiNbO₃) modulator. As a result, large RF output signals will experience some compression, which can be advantageous in some applications.

Internal Laser
Each reference transmitter has a built-in telecom-grade laser. Internal C- and L-band lasers are tunable on the ITU 50 GHz grid. A dither feature for wavelength stabilization is available in both tunable internal laser options (see the Operation tab for more information). An internal 1310 nm fixed-wavelength laser is also available as an option for all reference transmitters. MX35E series reference transmitters with an integrated 850 nm fixed-wavelength laser are also available upon request (contact Tech Support). An external laser source between 1250 and 1610 nm can also be used to provide the optical input.

System Connections
The laser input port uses PM fiber with light linearly polarized along the slow axis (aligned to the connector key) and can accept a maximum input power of 20 dBm (100 mW). Each fiber bulkhead accepts FC/PC connectors. The amplifier RF port connector type is either SMA or 1.85 mm (V), depending on the frequency response of the device. See the Front & Back Panel tab for more information.

We offer a selection of microwave cables and adapters for incorporating these transmitters into a system.

Control
These reference transmitters 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 the 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.  

We also offer digital reference transmitters with speeds up to 40 Gb/s and optical transmitters based on phase modulators. Please see the Selection Guide tab above for all of our transmitter instruments.

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Block diagram Showing Internal Setup of the High-Speed, Linear Reference Transmitters.

System Overview

These Linear Reference Transmitters are fully integrated and contain both the laser source and the lithium niobate (LiNbO₃) Mach-Zehnder intensity modulator; the only required external input is the signal source to the Amplifier RF In port. Either the internal laser or an external laser source may be coupled to the Laser In port, which can be seen at the bottom-left corner of the diagram below. This port uses polarization maintaining (PM) fiber with light linearly polarized along the slow axis, as shown on the front panel of the instrument. Optical power is monitored in three places (Monitor 1, Monitor 2, and Monitor 3) for the purpose of enabling bias and power control. These power values are also available at the I/O port. Monitor 1 is at the laser input, Monitor 2 is at the output of the modulator, and Monitor 3 is at the final optical output.

A Block Diagram of the Internal Setup of the High-Speed Reference Transmitters

Instrument Control

The graphical user interface (GUI) gives the user complete control over all instrument functionality. It is 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 for quickly changing set-point values. Pressing (clicking) the knob will confirm a new set-point value. Additionally, the instruments can be driven using serial commands delivered via connectors on the rear panel.

The home screen is shown in Figure 1. It is organized into three main sections:

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Figure 1: Home Screen of the MX35E Series

• 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 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. 

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

Laser and System Wavelength Settings

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Figure 3: System Wavelength Selection Screen

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Figure 2: Laser Settings Screen

The laser setting screen shown in Figure 2 is accessed directly from the home screen. Each instrument includes a C- or L-band telecom-style laser that is tunable on the ITU 50 GHz grid or a 1310 nm fixed-wavelength laser. An 850 nm fixed-wavelength laser can be substituted by contacting Tech Support. ITU Channel wavelengths are indexed for convenience; use the arrow buttons to step through the indices to select the desired wavelength. This screen also allows the user to control whether or not the dither feature (available for tunable C- and L-band lasers only) is used to stabilize the wavelength. Turning dither off results in lower phase and intensity noise (see the Specs tab for a representative plot), but doing so may also result in the wavelength drifting slightly over time. If an external laser is used, the internal laser can be turned off by tapping the laser button on the home screen.

If an external laser is used, it may be necessary to change the power monitor calibration settings. These instruments 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. The default calibration setting corresponds to the wavelength range of the internal laser. If operating outside that wavelength range, change the power monitor calibration settings by tapping the Menu button on the home page. Select the System Wavelength setting, shown in Figure 3, to change the power monitor calibration wavelength to the value closest to the wavelength of the laser source being used.

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Figure 5: Linear amplifiers provide a linear relationship between voltage input and output. Different line slopes indicate different gain settings in the MX35E series. Gain is user-adjustable in 1 dB increments.

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Figure 4: Linear RF Amplifier Settings Screen for the MX35E Series

Linear RF Amplifier

The amplifier used in these reference transmittes is a linear amplifier that accurately reproduces the input signal with minimal distortion. In addition, the MX35E series has adjustable gain that can be controlled from the Amplifier Settings page shown in Figure 4. This allows for linear amplification for a wide range of input signal levels before being applied to the RF input port of the modulator. Figure 5 shows how the gain setting changes the slope of the output vs. input signal relationship. Note, however, that the linear range of the whole system is still limited by the non-linear response of the lithium niobate modulator. See the specifications in the Specs tab and manual for signal limitations in this regard.

Modulator Bias Controller

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Figure 7: Bias Settings Screen

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Figure 6: Bias Points for an Intensity Modulator, in which V_pi is the Half-Wave Voltage

There are four bias control modes, which provide the option of operating with fully automatic bias control or under manual control. 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 6. 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, select the Constant Ratio mode. It is enabled by tapping the MAN function on the right of the screen shown in Figure 7 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. A fixed bias voltage can 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

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Figure 8: VOA Settings Screen

The VOA provides the means for adjusting and stabilizing the modulated optical output power. The VOA settings screen, which is shown in Figure 11, 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 safety and utility functions such as the laser safety interlock and the power monitor output, RS-232, and USB ports. The USB interface is currently used only for firmware upgrades that are made available on Thorlabs website. Future revisions of the firmware will provide for remote control of the instrument's functions.

All units are shipped from Thorlabs with a shorting device that is already installed in the interlock connector, thus allowing the instrument to be operated normally right out of the box. To make use of the interlock feature, a 2.5 mm plug can be wired to the remote interlock switch and plugged into the back-panel interlock jack in place of the shorting plug. Electrical specifications for this function are provided in the manuals, which can be accessed by clicking on the red document icons () below.

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Each reference transmitter offers the option to customize the color of the underbody accent lights or turn them off.

Included with Thorlabs' High-Speed Reference Transmitters:

• Optical Transmitter Main Unit
• Power Cord According to Local Supply (Determined by Ordering Location, Not Shown)
• PM Loop-Back Fiber Optic Cable
• Interlock Keys for Front Panel
• 2.5 mm Interlock Pin (Pre-installed in Back Panel, Not Shown)
• 1.25 A, 250 VAC Fuse (Not Shown)
• USB Type A to Type B Cable, 6' Long (Not Shown)

Software for the Linear Reference Transmitters

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The GUI of the Remote Control Software Tool 

Control Our Reference Transmitters Remotely via Serial Commands
Serial commands sent to the MX35E and MX65E series of reference transmitters can control the functionality of the internal laser, RF amplifier, bias controller for the built-in modulator, 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 reference transmitter. Computers running Windows^® 7, or later versions of the operating system, can send serial commands to the USB port on the back panel of reference transmitter. The touchscreen interface remains active while the reference transmitter is controlled remotely. Descriptions of how to connect a controlling computer to the reference transmitter, the serial command set, and descriptions of each command are included in the Remote Control User Guide.

Application Demonstrating GUI-Based Remote Control of Our Reference Transmitters
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 reference transmitter remotely. It opens a connection to the laser source and sends commands in response to buttons clicked by users. Commands sent to the reference transmitter, 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 Downloads

Software Version 1.6.1 (September 26, 2017)

Firmware Update Version 1.6.2 (November 15, 2017)

The Remote Control Software Tool and firmware updates are available by clicking on the links below.

Transmitter Instruments and Tunable Lasers
Item #	Speed	Internal Laser	Internal Modulator (Type)	RF Amplifier (Type)	Bias Controller	Variable Optical Attenuator (VOA)	Block Diagram
Automatic Bias Controller
MBX	N/A	-	-	-
Tunable Telecom-Grade Laser Sources
TLX1	N/A	C-Band, Tunable	-	-	-
TLX2	N/A	L-Band, Tunable
High-Speed Modulator Drivers
MX10A	12.5 Gb/sa	-	-	Digital
MX40A	40 Gb/sa
High-Speed Optical Transmitters
MX10B	12.5 Gb/sa	C-Band, Tunable	Intensity	Digital
MX10B-LB	12.5 Gb/sa	L-Band, Tunable
MX10B-1310	12.5 Gb/sa	1310 nm, Fixed
MX10C	12.5 Gb/sa	C-Band, Tunable	Phase	Digital	-
MX10C-LB	12.5 Gb/sa	L-Band, Tunable
MX10C-1310	12.5 Gb/sa	1310 nm, Fixed
MX35E	35 GHzb	C-Band, Tunable	Intensity	Linear
MX35E-LB	35 GHzb	L-Band, Tunable
MX35E-1310	35 GHzb	1310 nm, Fixed
MX40B	40 Gb/sa	C-Band, Tunable	Intensity	Digital
MX40B-LB	40 Gb/sa	L-Band, Tunable
MX40B-1310	40 Gb/sa	1310 nm, Fixed
MX40C	40 Gb/sa	C-Band, Tunable	Phase	Digital	-
MX40C-LB	40 Gb/sa	L-Band, Tunable
MX40C-1310	40 Gb/sa	1310 nm, Fixed
MX65E	65 GHzb	C-Band, Tunable	Intensity	Linear
MX65E-LB	65 GHzb	L-Band, Tunable
MX65E-1310	65 GHzb	1310 nm, Fixed
E-O Converters for VNA Applications
MX40G	40 GHzb	C-Band, Tunable	Intensity	-
MX40G-LB	40 GHzb	L-Band, Tunable
MX40G-1310	40 GHzb	1310 nm, Fixed
MX70G	70 GHzb	C-Band, Tunable	Intensity	-
MX70G-LB	70 GHzb	L-Band, Tunable
MX70G-1310	70 GHzb	1310 nm, Fixed

• Maximum Supported Digital Bit Rate
• Maximum Analog Bandwidth

Selection Guide for Transmitter Instruments

The capabilities of Thorlabs' extensive range of transmitter instruments are summarized in the text and table below. All members of this product series share a similar interface, as well as a common remote control command set.

Automatic Bias Controller
Thorlabs' fully-featured automatic bias controller provides complete and precise control of DC bias and optical output power for any fiber-coupled LiNbO₃ EO intensity modulator, regardless of signal speed. Automatic bias controllers are ideal for use within a customized setup that uses an external laser, intensity modulator, signal source, and RF amplifier.

Tunable Telecom-Grade Laser Sources
Emitting in the C-band or the L-band, these lasers have narrow typical linewidths of 10 kHz. A frequency dither option aids in stabilizing the laser wavelength, and the integrated variable optical attenuator (VOA) provides optical output power control. These lasers are tunable in 50 GHz steps across the ITU frequency grid, and feature a 1 MHz step size fine-tune capability, as well.

High-Speed Modulator Drivers
With an operational wavelength range of 1250 nm to 1610 nm, each modulator driver provides control for an external fiber-coupled LiNbO₃ EO modulator. Each modulator driver includes an RF amplifier with amplitude and eye-crossing controls and accepts an external drive signal source. Models with integrated automatic bias controllers are offered for use with intensity EO modulators.

High-Speed Optical Transmitters
Designed to provide fully-integrated solutions for high-speed light modulation, these systems are built around a LiNbO₃ intensity or phase modulator. The MX10B, MX40B, MX10C, and MX40C series of systems include a digital (limiting) RF amplifier, which offers fixed gain and an adjustable output voltage swing. The MX35E and MX65E series include a high-bandwidth linear (analog) RF amplifier with adjustable gain, making it well suited for pulse amplitude modulation and related applications.

E-O Converters for VNA Applications
With our MX40G and MX70G series of E-O converters, any E-E vector network analyzer can be used to perform optical testing up to 40 GHz or 70 GHz respectively. The E-O converter is a fully-integrated solution that includes a laser, a modulator, and bias control.