* Image not to scale.

An acousto-optic modulator (AOM) to be analyzed is coupled with red, green, or blue laser light. An RF signal used to control the deflection of the AOM is swept over a range of frequencies of interest. A slit-masked optic-power meter mounted on a precision linear actuator is swept orthogonally across the deflected light’s linear sweep range. The measured optical power, linear actuator position, and input RF frequency are recorded at each sample. This data is used to build a two-dimensional intensity graph in order to map input RF frequency to approximate angular deflection of the deflected light.

The AOM is an anisotropic waveguide with adjacent transducers that create surface acoustic waves (SAWs). These SAWs interact with the guided light, causing it to couple into a leaky mode of polarization orthogonal to the direction of propagation of the SAWs.

Link to Jove article and video presentation here.

The characterization stage as of May 2017, on top of the small optics table in CB496:

AOM samples are placed on the glass wafer.

The host computer and linear controller are underneath the small optics table in CB 496.

The system was upgraded by Drew in June 2017 to use a rotational stage instead of a linear stage. This enables for more accurate measurements of the AOM angular response. The GUI also has been edited to use angles and degrees as opposed to translation and millimeters. With this setup, the sensor wand is moved to the center of the scan range. When the test is started, the wand will move left (clockwise looking from above) half of the angular range specified in the GUI, scanning right (counter-clockwise) until it reaches the other end of its scan range. It will return to the center after finishing. If the test is canceled during scanning, you'll need to move the wand back to the center.

Comprehensive pictures, .STL files, and LabVIEW files for the sensor wand are available on the Holography Group's Google Drive here.

Video of the rotational stage actuating:

Typical operation of the characterization is as follows:

1) The AOM of interest is placed in an alignment device, connected to an RF amplifier and signal generator, and is coupled with a monochromatic laser beam. Input RF power to the AOM is typically set to 30dBm.
2) The RF Signal Generator is manually swept over a wide range of frequencies, typically 10 to 1000 MHz, and the frequency range that causes the AOM to operate at that wavelength of light is recorded.
3) The rough range of lateral beam deflection (position range) for the frequencies of interest at the distance of the power meter is recorded.
4) The computer controlling the RF Signal Generator, Linear Actuator, and Power Meter is programmed to sweep the RF frequency and detector position in predefined step lengths across the frequency and position ranges found in 2) and 3).
5) The linear actuator is set to a position step by the computer.
6) The RF Signal Generator output is enabled*.
7) The RF frequency is set to a frequency step by the computer.
8) The measurement of the optical-power meter, the RF frequency, and the position of the optical-power meter / linear actuator are recorded simultaneously and appended to the dataset of the test.
9) Steps 7) and 8) are repeated until data spanning the entire frequency range of interest is recorded.
10) The RF Signal Generator output is disabled*, and step 8) is repeated to record background noise.
11) Steps 5) through 10) are repeated until data spanning the entire position range of interest is recorded.
12) A new dataset is created by subtracting the optical power background noise of each position step from the optical-power measurements made at that location.
13) The new dataset is parsed to plot output power as a function of input RF frequency and optic-power meter linear position.
14) The plot is analyzed, and the experiment is repeated with changes to parameters of interest.

* Note: Our RF Signal Generator may use a mechanical element to normally enable/disable the RF output, so we chose to vary the power level and frequency instead to reduce possible wear and decrease testing time. While in the ‘disabled’ mode, the output power was set to 0dBm and the frequency output was set to the minimum frequency of the signal generator (9kHz). While in the ‘enabled’ mode, the output power was set to +20dBm and the frequency set to the frequency of interest. It was verified that this alternative technique had an impact equal to the mechanical control.

* * Note: Do not supply more than 0 dBm input RF power to the HELA-10 amplifier if no load is attached. Do not exceed +20 dBm input RF power with a load attached. Ensure that there is a fan keeping the HELA-10 cool. source

The GUI is layed out in various groups of widgets (GUI elements):

Sample Number, Color, Transducer, Guided Mode, Averaging Rate, Additional notes
- these just get appended to the log and are used to tie the characterization files with the design parameters of the AOM.

Laser Voltage, Wand Offset from Zero, Distance to Detector, Output to File
- I don't know; these were added by Kamran and I haven't dug through the code yet. (DH)

Wand Angle Reading - I don't know what this is; this was added by Kamran and I haven't dug through the code yet. (DH)

Initial Frequency (MHz), Final Frequency (MHz), Frequency Step(MHz), Rough Initial Position (mm), Rough Final Position (mm), Position Step (mm), Number Frequency Steps, Number Position Steps
- these control the range and level of granularity (detail) of the scan and indicate how many frequency/position steps will be made based upon that granularity.

Save Directory - change this with the little folder icon next to the top-left corner of this box

Filename - Specify the beginning of the filename? (DH)

File Saved To - (DH)

Reset Home Position in Next Measurement -
This option should be enabled for one test if the Linear Actuator Controller has been power-cycled. Enabling this has the Linear Actuator find the lowest mechanical limit of the actuator stage, and resets the internal zero-marker in the Controller upon finding it. (There are internal buttons on the end of the actuator's limits so it doesn't damage itself.) It takes quite some time for the linear actuator to traverse to the limits, so this option is left disabled for most tests.

Notify once done if Estimated Duration is greater than… -
- You can enable this option and have LabVIEW send you an email or text message once the program is done, if the estimated test duration is longer than the specified amount. Emails are sent using the byuholography@gmail.com account. Instructions to send text messages are given below. The sent text messages look like this:

Absolute Power Meter - Indicates the power in Watts for the current (red) and background (gray) light.

Current Frequency - Indicates the current frequency with an overall progress bar.

Current/Target Position - Indicates the measured and commanded position with an overall progress bar.

Graph Original/Compensated - Disables/enables subtracting out the background noise in the 3D plot.

Interpolate Shading - Changes the shading method used in the 3D plot.

Estimated Time Remaining - The estimated amount of time remaining for the test, taking into account the start date/time and the amount of time needed for each row and measurement.

Estimated Completion Time - The approximate day and time the test should be finished, given current estimates.

Estimated Duration - The estimated amount of time for the test, given current estimates.

Elapsed Time - The amount of elapsed time since the test was started.

seconds for each freq step - The measured average amount of time needed for each frequency change and optical power measurement; affected by the Signal Generator GPIB transmit latency and the USB Optical Power Meter receive latency.

seconds for each position step - The measured average amount of time needed for each position change; affected by the speed of the linear stage and the Linear Actuator Controller GPIB transmit latency.

Log - Report - This is just a copy of the report written to the -config.csv. The actual measurements used to be displayed here.

Signal Generator GPIB Address - This needs to be set to the GPIB address used by the Signal Generator. The Signal Generator can also have its address changed to match this current value.

Motor Driver GPIB Address - This needs to be set to the GPIB address used by the Linear Actuator Controller. The Controller may be able to have its own GPIB address changed to match this current value.

Readout Config - The unit of measurement of the Power Meter. Usually left on 'POW' for Watts.

Timeout - The amount of time the host computer's GPIB controller should wait before issuing a timeout response.

IDN String, Sensor Name, Sensor SN - Parameters received from the Optical Power Meter on a successful connection.

Error log, Error Out - Error messages from the USB and GPIB interface code.

Three CSV files and a JPEG are written to make appending, parsing, and reviewing easier. For a given test name (ie, “AOM-A5-Blue-6.18.2015-5.31PM”), the following suffixes are added:

• -config.csv This just contains test information.
• -data.csv This contains all of the position, frequency and optical power measurements.
• -no_stim.csv This contains the position and optical power measurements when the RF source is disabled. It is used to provide the background noise to subtract out from the normal data.
• -graph.jpeg This is a copy of the 3D plot showing the frequency-position-power relationships.

The Comman-Separated Value (CSV) files internally have titles and units of measurement for each column on the first row, such as p (W),f (MHz),x (mm). Each subsequent row stores one sample, such as -1.45997738E-8,100,0.04233. The order of the rows comes from the order they were recorded.

The specific equipment used in our experiment was the following:

default GPIB address: 1
USB not "currently" supported

need to turn motors on first before commanding
1MO - turn on motor 1
1MF - turn off motor 1

1PA+3 - move the motor +3 units absolute scale
(eg, if 1sn? returns "2", then you'd move to +3.000 mm)
1PR-2 - move -2 units relative

1PA? - get current absolute position (doesn't work while moving)
1TP - get current absolute position (while moving; no question-mark)

1MT+ / 1MT- / 1MT? - move to hardware travel limit or ask if done moving (1:T; 0:F)

ST - stop motion without turning off the motors
AB - abort motion AND turns off the motors

MFN25PP range:
+09.69764 mm
-15.47221 mm
0.15 mm/s default, 0.30 mm/s max

VA - set velocity?

see
http://www.newport.com/images/webdocuments-en/images/12587.pdf
page 10 of 24 for more mechanical limits

MD? - move done?

OR - move to home position (see datasheet)
DH - define home

NOTE: HOME POSITION GETS RESET TO WHEREVER THE ACTUATOR IS EVERY POWERCYCLE

1sn? - get the motor units
some return values:
2  mm millimeter
3  um micrometer
4  " inch
5  mil milli-inches

POW:AMPL -4.9DBM - set the output power to -4.9 dBm

OUTP:STAT ON - turn the RF output ON

FREQ:CW 110.445MHZ - set the output frequency to 110.445 MHz (max 9 digits, 10Hz resolution)

SYST:LANG? returns "scpi"

The LabVIEW program was originally developed in stages around each main driver/sensor component (the linear actuator, signal generator, and the power meter). Both the linear actuator and the signal generator are controlled over GPIB, which is basically serial communication, so only the GPIB manuals and a GPIB adapter were needed to get this part working. The power meter communicates over USB, which is much less forward than GPIB. Fortunately, the manufacturer, Thorlabs, had example LabVIEW code, and it was trimmed down to the minimal functionality we needed.

Because there are thousands of measurements taken, there were three things that had to be considered as the program was written:

1. Frequency sweeps should be taken between between position steps. It's much faster to change frequency than it is to change position. Also, the position repeatability isn't that great with the linear actuator we're using (we have high measurement precision, but only relatively decent control precision). Revisiting a previous position doesn't reproduce the same position or results.
2. The program has to be tailored for speed. Current tests could take much longer than they do now if this was not an objective. This was done by writing small, individual LabVIEW programs to develop the fastest possible routines for the GPIB and USB instruments. GPIB, which is basically serial, has a low baud rate by default (I'm not sure if it can be increased easily), thus, even the number of characters transmitted has an impact. Also, not reading a terminating character can cause an instrument to wait before receiving-timeout (in one example, reading 22 characters of a command string took 40ms, whereas reading 23 characters took 2ms).
3. Data collected was initially appended to a string, but this is an O(n) operation, meaning that the larger the string, the longer it took to append new measurements, and the program eventually slowed down to an unacceptable speed. This was fixed by appending the string to a file and clearing the string per frequency sweep.

The VI program “development VIs/LabVIEW - Signal Generator GPIB testing.vi” was used to develop and measure fast GPIB routines.

The ESP7000 Controller front panel is nonfunctional ever since the ElectroHolography group obtained it from Dr. Schultz's lab in early 2014. The LCD fluorescent tube backlight driver arced and shorted out in late 2014. Drew Henrie replaced the backlight driver and screen (the fluorescent tube was bad) in Summer 2017.

The screen's normal working appearance.

The backlight driver that burned out. It is an LXM1611-01B module.

If there are GPIB communication errors, these can sometimes be resolved by power-cycling the system a couple of times. I'm not sure about the power-up order. (DH)

A word of caution, there may be a bug (found Mon 12 Jan 2015) still in the LabVIEW program when the frequency passes through 256 MHz. When this bug occurs, the Signal Generator switches rapidly between LSN and TLK modes. A solution may be to ensure that the frequency step is less than 250 kHz (0.25 MHz).

Minor edits to the LabVIEW program were made by Kamran Qaderi throughout 2016.

The apparatus was originally set up, and the LabVIEW program was originally written by Drew Henrie during Summer 2014.

The small optics table in Aug 2014.