THORLABS MX40G Series User manual

MX40G Series
40 GHz E-O Converters
MX70G Series
70 GHz E-O Converters
User Guide

MX40G, MX70G Series of E-O Converters
Table of Contents
Chapter 1Introduction .............................................................................................................................................................. 1
1.1Description ..................................................................................................................................................... 1
1.2Parts List ........................................................................................................................................................ 1
1.3Block Diagram................................................................................................................................................ 2
1.4Front and Back Panel Overview ................................................................................................................... 3
Chapter 2Safety......................................................................................................................................................................... 5
Chapter 3Quick Start Guide ..................................................................................................................................................... 7
3.1Hardware Set Up ............................................................................................................................................ 7
3.2Controls on the Home Page .......................................................................................................................... 8
3.3System Wavelength Setting .......................................................................................................................... 9
3.4Controls on the Settings Pages .................................................................................................................. 10
3.5Quick Start ................................................................................................................................................... 11
Chapter 4Operating Instructions ........................................................................................................................................... 12
4.1The Modulator Transmission Function ...................................................................................................... 12
4.2De-Embedding Procedures ......................................................................................................................... 12
4.3Control Loop Diagram ................................................................................................................................. 13
4.4Bias Settings Page ...................................................................................................................................... 14
4.4.1Quadrature Mode .............................................................................................................................. 15
4.4.2Peak Mode ........................................................................................................................................ 17
4.4.3Null Mode .......................................................................................................................................... 17
4.4.4Manual Mode .................................................................................................................................... 18
4.5Variable Optical Attenuator Settings Page ................................................................................................ 20
4.6Laser Settings Page .................................................................................................................................... 22
4.7Load Page .................................................................................................................................................... 24
4.8Menu Page .................................................................................................................................................... 25
4.8.1System Wavelength .......................................................................................................................... 25
4.8.2Display and Sound Settings Page ..................................................................................................... 26
4.8.3System Information Page .................................................................................................................. 26
4.8.4Accent LED Settings Page ................................................................................................................ 27
4.8.5Thorlabs Help Page .......................................................................................................................... 27
Chapter 5Specifications ......................................................................................................................................................... 28
5.1General System Specifications .................................................................................................................. 28
5.2Power and Environmental Specifications ................................................................................................. 29
5.3Internal Control Specifications ................................................................................................................... 29
5.4Internal Modulator Specifications .............................................................................................................. 29
5.5Laser Specifications .................................................................................................................................... 30
Chapter 6Control and PC Connections ................................................................................................................................. 31
6.1General Purpose I/O, RS-232, and USB Connections ............................................................................... 31
6.2The Laser Safety Interlock .......................................................................................................................... 32
Chapter 7Mechanical Drawings ............................................................................................................................................. 33
Chapter 8Maintenance, Repair, and Fuses ........................................................................................................................... 34
8.1Maintenance and Repair ............................................................................................................................. 34

MX40G, MX70G Series of E-O Converters
8.2Replacement Parts ...................................................................................................................................... 34
8.3Replacing the Main Fuse ............................................................................................................................. 35
Chapter 9Troubleshooting ..................................................................................................................................................... 36
Chapter 10Declaration of Conformity ........................................................................................................................... 37
Chapter 11Thorlabs Worldwide Contacts .................................................................................................................... 38

MX40G, MX70G Series of E-O Converters Chapter 1: Introduction
Rev. E, March 19, 2019 Page 1
Chapter 1 Introduction
1.1 Description
Thorlabs' MX40G series (MX40G, MX40G-LB, and MX40G-1310) and MX70G series (MX70G, MX70G-LB, and MX70G-1310)
of Electrical to Optical (E-O) Converters offer frequency response to 40 GHz and 70 GHz, respectively. Each converter
incorporates a telecommunication grade lithium niobate (LiNbO3) modulator that is stabilized by a fully automatic bias controller.
Select an internal tunable laser source or fixed-wavelength laser source or couple an external laser operating from 1250 nm to
1610 nm to the PM FC/PC fiber optic connector on the input panel. The instrument is fully driven using the intuitive graphical
user interface (GUI) touchscreen, and the RS-232 connection on the back panel enables remote control of many instrument
functions. Please contact Thorlabs technical support for up-to-date information on the firmware revisions and control functions
available.
These converters are particularly well suited for use as accessories that provide optical test capability for an all-electrical vector
network analyzer (VNA). This pairing enables an optical to electrical (O-E) device under test (DUT) to be accurately measured.
To support this use, each E-O converter ships with an *.s2p file characterizing its frequency response. This frequency response
can be used to de-embed the response of the O-E DUT by separating its response from the overall measurement response.
Please see Section 4.2 for more information.
These instruments can be controlled in two ways. The simplest method is directly via the built-in graphical user interface (GUI)
and touchscreen. The instrument can also be operated remotely via the RS-232 or USB ports on the back panel. Remote
control is enabled using simple SCPI-type serial commands from a PC. See the remote control user guide (RCUG), which can
be downloaded from https://www.thorlabs.com/manuals.cfm, for details.
The most recent firmware and remote control software tool can be downloaded by visiting
https://www.thorlabs.com/software_pages/viewsoftwarepage.cfm?code=MXG. Thorlabs’ technical support can provide up-to-
date information on available firmware revisions and control functions.
1.2 Parts List
Inspect the shipping container for damage. If the shipping container seems to be damaged, keep it until you have inspected the
contents and tested the unit mechanically and electrically. Verify that you have received the following items within the package:
1. E-O Converter Main Unit
2. Power Cord According to Local Power Supply
3. PM Loopback Fiber Optic Cable
4. Interlock Keys
5. 2.5 mm Interlock Pin (in Back Panel)
6. 1.25 A 250 VAC Fuse
7. USB Type A to Type B Cable, 6' Long
8. USB Storage Device with *.s2p File Characterizing Instrument


MX40G, MX70G Series of E-O Converters Chapter 1: Introduction
Page 2 TTN116413-D02
1.3 Block Diagram
Each E-O converter is fully integrated and contains the laser source, lithium niobate (LiNbO3) Mach-Zehnder intensity modulator
(MZM), and automatic bias controller; the only required external input is the signal source to the Modulator RF In port.
The MX40G/MX70G, MX40G-LB/MX70G-LB, and MX40G-1310/MX70G-1310 include a C-band tunable laser, an L-band
tunable laser, and a 1310 nm fixed-wavelength laser, respectively. Both the C- and L-band laser sources are tunable on the ITU
50 GHz grid and include a dither feature for wavelength stabilization. An external laser source, operating from 1250 nm to
1610 nm, can also be used to provide the optical input.
Either the internal laser or an external laser source may be coupled to the Laser In port, which uses PM fiber with light linearly
polarized along the slow axis, as shown on the front panel. The maximum input power to the Laser In port is 20 dBm (100 mW).
Optical power is monitored in three places (Mon-1,-2,-3) for the purpose of enabling bias and power control. These power values
are also available at the I/O port. Mon-1 is at the Laser Input, Mon-2 is at the MZM Output, and Mon-3 is at the final Optical
Output.
Figure 1 Key Components of the E-O Converter


MX40G, MX70G Series of E-O Converters Chapter 1: Introduction
Rev. E, March 19, 2019 Page 3
1.4 Front and Back Panel Overview
Figure 2 Front Panel
Callout Description
F1 Touchscreen Display
F2 Adjustment Knob
F3 Key Switch and Status Indicator
Lasing Disabled; Lasing Enabled
F4 Grounding Jack (Banana Connector)
Earth Ground
F5 Laser Output (PM FC/PC Connector)
F6 Laser Input (PM FC/PC Connector)
F7 Optical Output (FC/PC Connector)
F8
Modulator RF Input
(MX40G Series: 2.92 mm Connector;
MX70G Series: 1.85 mm Connector)
F9 Standby Button

F3
F4
F1 F2
F9
F8
F7
F6
F5

MX40G, MX70G Series of E-O Converters Chapter 1: Introduction
Page 4 TTN116413-D02
Figure 3 Back Panel
Callout Description
B1 I/O Port (DB15 Connector)
B2 Laser Interlock (2.5 mm Connector)
B3 Option Label
B4 RS-232 Port (DB9 Connector)
B5 USB Port (USB Type B Connector)
B6 Power Connector
B7 Fuse Tray
B8 Power Switch
Supply On; Supply Off
B5
B4
B3
B1
B8
B2
B7
B6

MX40G, MX70G Series of E-O Converters Chapter 2: Safety
Rev. E, March 19, 2019 Page 5
Chapter 2 Safety
All statements regarding safety of operation and technical data in this instruction manual will only apply when the unit is operated
correctly. If equipment is used in a manner not specified by the manufacturer, the protection provided by the equipment may be
impaired. Only with written consent from Thorlabs may changes to single components be carried out or components not supplied
by Thorlabs be used.
Warning: Risk of Electrical Shock
Before applying power to the instrument, make sure that the protective conductor of the 3 conductor mains power
cord is correctly connected to the protective earth contact of the socket outlet. Improper grounding can cause electric
shock with damage to your health or even death. Only use mains cable with sufficient current and voltage ratings for
this instrument. The local supply voltage must be in the range specified on the rear panel, and the correct fuse must
be installed in the fuse holder. If not, please replace the main fuse (see Section 8.3). Do not position equipment in a
way that makes it difficult for the user to operate the disconnecting device. Do not remove covers. Refer servicing to
qualified personnel.
Warning: Risk of Explosion
The instrument must not be operated in explosion endangered environments.
Warning: Laser Radiation
Avoid Exposure – Radiation emitted from apertures. Do not look into the laser
aperture while the laser is on. Injury to the eye may result. Laser should not be turned
on unless there is an optical fiber connected to the laser output port.
Caution – Use of controls or adjustments or performance of procedures other than
those specified herein may result in hazardous radiation exposure.
Caution: ESD Sensitive Component
The components inside this instrument are ESD sensitive. Take all appropriate precautions to discharge personnel
and equipment before making any connections to the unit. A front panel grounding jack is provided for connection to
a wrist strap.
Caution: Components not Water Resistant
This instrument should be kept clear of environments where liquid spills or condensing moisture are likely. It is not
water resistant. To avoid damage to the instrument, do not expose it to spray, liquids, or solvents.
Caution: Follow Intended Usage Guidelines
This product is not suitable for household room illumination.
Inputs and outputs must only be connected with shielded connection cables.
Do not obstruct the air ventilation slots in housing.
Mobile telephones, cellular phones or other radio transmitters are not to be used within the range of three meters of
this unit since the electromagnetic field intensity may then exceed the maximum allowed disturbance values according
to IEC 61326-1.

LASER RADIATION
DO NOT VIEW DIRECTLY WITH
OPTICAL INSTRUMENTS
CLASS 1M LASER PRODUCT

MX40G, MX70G Series of E-O Converters Chapter 2: Safety
Page 6 TTN116413-D02
The following statement applies to the products covered in this manual, unless otherwise specified herein. The statement for
other products will appear in the accompanying documentation.
Note: This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to Part 15 of the
FCC Rules and meets all requirements of the Canadian Interference Causing Equipment Standard ICES-003 for digital
apparatus. These limits are designed to provide reasonable protection against harmful interference in a residential installation.
This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the
instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not
occur in a particular installation.
This product has been tested and found to comply with the limits according to IEC 61326-1 for using connection cables shorter
than 3 meters (9.8 feet).
Thorlabs is not responsible for any radio television interference caused by modifications of this equipment or the substitution or
attachment of connecting cables and equipment other than those specified by Thorlabs. The correction of interference caused
by such unauthorized modification, substitution or attachment will be the responsibility of the user. The use of shielded I/O cables
is required when connecting this equipment to any and all optional peripheral or host devices. Failure to do so may violate FCC
and ICES rules.
This precision device should only be shipped if packed into the complete original packaging including the custom cut foam
padding. If necessary, ask for replacement packing material.

MX40G, MX70G Series of E-O Converters Chapter 3: Quick Start Guide
Rev. E, March 19, 2019 Page 7
Chapter 3 Quick Start Guide
3.1 Hardware Set Up
For first use, plug the main power cable into the rear panel connector, and then plug the other end into an AC wall
receptacle. Flip the power switch on the rear panel to the ON (I) position. The unit will now be in the “Standby” mode,
and the front panel standby button should glow amber.
Figure 4 Power Cable Port, Fuse, and Power
Switch
Figure 5 Indicator Glows Amber when Converter is
In Standby
Attach the PM loop-back fiber optic cable between the Laser Out and Laser In FC connectors on the front panel. Be sure to
clean both ends of the fiber as described in the Maintenance Section of this manual.
Figure 6 PM Loopback Fiber Cable Installed
Insert the key into the interlock switch and turn it towards the unlock symbol ( ). This allows the laser to be turned on, but the
LASER ON indicator will not glow green until the laser is actually turned on by the touchscreen button. Turn on the unit by
pressing the amber standby button on the front panel which will then turn green to indicate the unit is fully on. The touchscreen
display will come up with a boot screen for about 5 seconds and then go to the home page. The unit will initialize in the factory
default state with all functions OFF.
Figure 7 Interlock Key Switch
Figure 8 Indicator Glows Green when Converter is
Fully Enabled

MX40G, MX70G Series of E-O Converters Chapter 3: Quick Start Guide
Page 8 TTN116413-D02
3.2 Controls on the Home Page
Full access to all of the converter's controls and functions is available using the resistive touchscreen display, which is sensitive
to finger pressure or the tap of a plastic stylus. In addition, the knob on the front panel 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.
Figure 9 Home Screen Features
The Home screen (or dashboard) is organized into three main sections.
The left side contains the ON/OFF buttons for each of the main instrument functions. Tapping any of these buttons will toggle
the function on and off. The same ON/OFF functionality is also available on the individual Settings pages. The power buttons
turn green to indicate the function is ON, and they turn red to indicate the function is disabled.
The central section is the main dashboard for reporting operational values of each section. Tapping the screen in this middle
area will take the user to the corresponding Settings page for each section. Note that the green dot in each of these sections
glows steadily to indicate the function is stable, and it blinks to indicate the function is still stabilizing.
The right side of the screen provides access to the main utility functions of the box.
The screen shot below shows some of the common warning indicators on the Home page. Some functions can be disabled
when the laser power is low. In this case, buttons may be disabled and warnings indicators may appear.
Figure 10 Home Screen Warnings and Indicators

Tap these buttons
to turn functions
ON or OFF.
Tap in these areas
to go to the related
Settings pages.
Blue Button:
Utility Functions
Green dot flashes or is
steady to indicate
stability state.
Power units (dBm or
mW) are set in the
VOA Settings page.
Disabled
Functions
Low Power
Warnings
Interlock
Warning

MX40G, MX70G Series of E-O Converters Chapter 3: Quick Start Guide
Rev. E, March 19, 2019 Page 9
3.3 System Wavelength Setting
The operational wavelength range of each converter extends from 1250 nm to 1610 nm. The three calibration wavelengths of
the power monitors, 1310 nm, 1550 nm, and 1590 nm, represent the centers of the O-band, C-band, and L-band and provide
the user with accurate power readings at or near those wavelengths.
The system wavelength should always be set to the wavelength closest to that of the laser source coupled to the Laser In
bulkhead. The converter's system wavelength is factory-preset to correspond to the wavelength band of the integrated laser
source. If an external laser is to be used with the converter, the system wavelength may need to be changed. This function exists
in the Utility Menu.
From the Home screen, tap the MENU button to bring up the Utility Menu as shown.

Figure 11 Accessing the Utility Menu from the Home Screen
Then tap the System Wavelength bar as shown below to bring up the three wavelength choices. Tap the desired wavelength to
set the System Wavelength.
Figure 12 System Wavelength is Selected from the Utility Menu

Tap the MENU button to
access the Utility Menu.
Tap here to activate
wavelength selection.
Choose one of these
wavelengths.

MX40G, MX70G Series of E-O Converters Chapter 3: Quick Start Guide
Page 10 TTN116413-D02
3.4 Controls on the Settings Pages
The Settings pages all possess the same general design and functionality as shown in the example screen shot below. The
upper section with white letters displays the parameters that can be changed. Simply tap on the parameter of interest. A yellow
border highlights the choice and controls for that parameter are presented.
The lower section with amber letters displays selected related values convenient to monitor from the page.
The right-hand column provides the controls for changing the values for the selected parameters. The main control knob on the
front panel can also be used to adjust and confirm selected values. The screen shots below show examples of the touch-screen
controls.
Figure 13 Controls used to Adjust and Save Values of Selected Parameters
Fields that have adjustable values will show a flag if the minimum or maximum values have been reached. These are set by
firmware at the factory.
Figure 14 MIN and MAX Flags Indicate Bounds of Adjustable Value Ranges
Tap parameter to select it.
Yellow border highlights choice.
Increase value by tapping.
Decrease value by tapping.
Change step size by tapping.
Confirm new value by tapping.
Cancel any changes by tapping.
Monitor Area
MIN / MAX warnings
indicate that the setting
limit for a particular
parameter has been
reached.

MX40G, MX70G Series of E-O Converters Chapter 3: Quick Start Guide
Rev. E, March 19, 2019 Page 11
3.5 Quick Start
The following steps summarize the setup procedure required to operate the converter in the standard mode. Please refer to
previous sections in this chapter and the expanded operating instructions in Chapter 4 for additional information.
1. Turn on power to the converter via the power switch on the back panel (Figure 4).
2. Press and hold the button on the front panel until the indicator light changes from amber to green (Figure 5 and Figure 8).
3. Turn the key switch to unlock (Figure 7).
4. Turn the laser on by tapping the lower-left touchscreen button on the Home page (Figure 9).
5. Enable the bias controller by tapping the top-left touchscreen button on the Home page (Figure 9). Wait for completion of
the calibration process. The instrument will enable the default Quadrature mode with positive slope bias point (Section
4.4.1).
6. Take note of the output power, which is shown in the center right section of the Home page (Figure 9).
7. Turn on the VOA controller to adjust output power, if desired, by tapping the center-left touchscreen button on the Home
page (Figure 9).
8. Apply the input RF signal to the female RF IN connector on the front panel (Figure 6); the maximum modulator RF input for
linear response is specified in Chapter 5.
Figure 15 The Front Panel of the MX40G Converter


MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Page 12 TTN116413-D02
Chapter 4 Operating Instructions
4.1 The Modulator Transmission Function
The Mach-Zehnder modulator (MZM) has a repetitive transmission function with applied voltage as can be seen in the diagram
below. In order for the MZM to work correctly, a DC bias voltage must be applied and maintained at the desired set point. The
high-frequency AC signal can then be applied to the modulator to enable the correct optical modulation of the laser beam. The
most common operating points are the peak, null, and quadrature points as shown below.
The purpose of the Bias settings is to hold the modulator at one of these chosen points. Note that a real transmission function
does not go perfectly from 0% to 100%. This is characterized by the Extinction Ratio of the modulator (Peak power / Null power).
The efficiency of the modulator is also characterized by Vπ, which is defined as the voltage necessary to change the transmission
from Null to Peak. The most linear response of the modulator is achieved by biasing it at one of the Quadrature points where
the transmission is closest to 50%. Some non-linear, frequency doubling, and phase modulation applications require biasing at
the Null or Peak.
Figure 16 Illustration of the Modulator Transmission Function
4.2 De-Embedding Procedures
One of the primary applications of a calibrated electrical to optical (E-O) converter is enabling electrical to electrical (E-E) test
equipment, like a VNA, to characterize optical to electrical (O-E) devices. As illustrated in Figure 17, the device converts the
electrical signal from Port 1 of the E-E VNA to an optical signal that is input into the O-E DUT, and the electrical signal resulting
from the O-E DUT is then provided to Port 2 of the E-E test equipment. The response of the entire E-E system, which consists
of the converter and the O-E DUT together, is measured. Accurately recovering the response of the O-E DUT requires removing
the response of the converter from the measured E-E system response. This is referred to as de-embedding.
Figure 17 An E-O Converter Provides Optical Test Capability for an All-Electrical VNA

Electrical Vector
Network Analyzer
Optical
MX40G E-O Converter
Electrical
Port 2Port 1
Electrical
O-E Device
Under Test

MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Rev. E, March 19, 2019 Page 13
Thorlabs provides a file (.s2p format) with each E-O converter that contains the S
21
magnitude and phase response of the
modulator across its frequency range of operation. These data are used to perform the de-embedding process. Note that most
modern E-E VNA systems provide a built in function for doing this type of de-embedding based on the user-supplied .s2p file.
This is the easiest method. However, the de-embedding process can also be performed manually. These methods are
described in detail in the "De-Embed Application Note" PDF document, which is accessible by visiting
www.thorlabs.com/manuals and entering the item # of your E-O Converter.
A typical frequency response for each converter is plotted below in Figure 18. The phase response is not shown.
Figure 18 Typical Frequency Response of the MX40G Series and MX70G Series
4.3 Control Loop Diagram
The following diagram shows the control loops added to the block diagram. From this picture, the user can see how the power
monitors and VOA are used to provide stability and control to the whole system. It will be helpful to refer to this diagram to gain
a better understanding of the functionality of the unit as described in the upcoming sections of the manual.
Figure 19 Block Diagram of the Converter Showing Control Loops
0 5 10 15 20 25 30 35 40
-4
-3
-2
-1
0
1
MX40G Typical Frequency Response
Power (dB)
Frequency (GHz)
0 10203040506070
-7
-6
-5
-4
-3
-2
-1
0
1
2
MX70G Typical Frequency Response
Power (dB)
Frequency (GHz)

MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Page 14 TTN116413-D02
4.4 Bias Settings Page
The Bias Settings page is accessed by tapping the center bias monitors pane on the Home page. When the MZM Bias control
is first turned on, the converter performs a calibration routine to determine the approximate bias voltages required for the various
MZM operating points. This allows the instrument to quickly and effectively switch between bias modes. The user may perform
this calibration anytime by pressing the Reset Auto Bias button on the right side of the Bias Settings page. This button is not
available if the Bias is off or in Manual mode.
Figure 20 Bias Settings Home Screen
The Bias Settings page contains the settings for controlling the modulator bias and operating modes. There are four modes for
MZM bias control: 1) Quadrature, 2) Peak, 3) Null, and 4) Manual. The screen shot below is an example of the information
presented and the controls available when the Mode field is selected while the controller is operating in Quadrature mode. The
blue buttons enable switching between modes.
Figure 21 Bias Settings Screen when Mode Field is Selected while Quadrature Mode is Enabled
Active control of the MZM bias point is essential, as the modulator is temperature sensitive and will drift over time. The
Quadrature, Peak, and Null modes use a dither tone as part of a lock-in control scheme to keep the MZM bias stable. The dither
tone allows the control algorithm to track the drifting, but at the cost of decreased signal-to-noise ratio (SNR) due to the injection
of a single frequency tone into the MZM bias.

Tap to turn Bias On/Off.
Tap to Reset Modulator Bias.
New set points will be stored.
Tap to select Peak Mode.
Tap to select Quadrature Mode.
Tap to select Null Mode.
Tap to select Manual Mode.

MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Rev. E, March 19, 2019 Page 15
4.4.1 Quadrature Mode
Quadrature mode is the default mode and biases the MZM at the 50% point on the MZM transmission curve shown in Figure
23. Quadrature mode is essential for VNA applications in which linear response is critical. The screen shot below shows the
parameters that can be controlled in the Quadrature mode.
Figure 22 Dither Frequency Selection while Quadrature Mode Enabled
The bias control circuit uses an AC dither tone to stabilize the bias point of the modulator, as illustrated below. 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 (Quadrature, Peak or Null).
The DC bias voltage is then continuously adjusted to maintain the correct set-point.
The frequency of the dither tone will not exceed 10 kHz. This frequency limit, which is usually well below the low frequency cut-
off for the RF signal of interest, is chosen to prevent interference. As described in the following, the dither tone can also be
turned if other methods of bias control are desired (e.g. manual or input/output ratio control).
Figure 23 AC dither tone stabilizes the bias point of the modulator.
A
nother option: specify custom dither
frequency between 1 and 10 kHz.
One option: choose one of three
preset dither frequencies.

MX40G, MX70G Series of E-O Converters Chapter 4: Operating Instructions
Page 16 TTN116413-D02
The dither tone frequency may be changed by tapping the Dither Frequency field, which is highlighted in Figure 22. Standard
frequencies are selected using the blue buttons, or a custom frequency may be chosen by pressing User Define. The dither
tone frequency usually has very little effect on the accuracy of the bias control, but in some cases a different frequency may
work better or be desirable depending on the RF signal applied or on the specific application. The User Define button allows
for the selection of an arbitrary frequency between 1 kHz and 10 kHz with 10 Hz resolution.
Figure 24 Dither Amplitude Adjustment
The dither tone amplitude can also be adjusted to any amplitude between 20 mV and 2 V with 1 mV resolution by tapping the
Dither Amplitude field highlighted in Figure 24. Higher amplitudes will typically be more stable in the presence of MZM drift and
broadband RF signals, but larger dither tones also decrease SNR. If the amplitude is too low, the MZM bias may not stay
locked. Typically, a value from 300 mV to 500 mV is a good starting point.
The dither tone can be optionally disabled to accommodate applications requiring the highest SNR. The user may quickly turn
the dither off, perform a measurement requiring low noise, and then turn the dither back on. When the dither is turned off, the
bias is simply held at the previous bias voltage. Manual Mode should be used for longer-term measurements without dither, so
that a constant ratio method can be used to stabilize the bias point, as described in Section 4.4.4.
Figure 25 Choose the Positive or Negative Slope of the Transmission Function
Quadrature mode also allows the user to select between two operating points by selecting the Slope field. Positive slope is the
non-inverting operating point where increasing voltage on the MZM results in increasing optical output power. Negative slope is
the inverting operating point where increasing voltage on the MZM results in decreasing optical output power. This effectively
changes the phase of the response function.

Tap here to turn dither On/Off.
A
djust the dither amplitude by
selecting this field.
Tap to select positive slope.
Tap to select negative slope.
Tap to change slope selection.