An AO modulator, of the type used for Q-Switching or light gating, seen here producing Raman-Nath diffraction.

This lab introduces students to the concept of Q-switched lasers and Acousto-Optic modulators. This is a two week lab in which students in small groups will complete both parts alternately. One lab report is required for both parts.

PART A: Acousto-Optic Modulators

See Application Notes from Isomet outlining the procedure (employed in this lab) for aligning an AOM for true Bragg angle. See "AOTF Notes", "Bragg Angle Errors", and "Maximizing Efficiency"
Application Note from Crystal Technology on AOMs and proper alignment (with figures)
Data Sheet for the Crystal Technology 3080-120 AO Modulator

Data Sheet for the AA Optoelectronics AA.MTS.110/A3-VIS AOM used as a PCAOM in this experiment
White Knight HeCd Laser brochure
PCAOM basics as applied to laser light shows, from Laser FX

Lab Experiment

Before starting the first part of the lab, outlined below, start the WhiteKnight HeCd laser and allow it to preheat. Switch the Ready switch on the rear of the unit upwards, and insert the key. Both the Helium and Cadmium heater indicators will light and, in about 20 minutes, the laser will become ready and a laser beam will be evident.

AOMs as Q-Switches:

This part of the lab examines the use and alignment of an AOM in Bragg mode - suitable for use as a Q-switch in, for example, a YAG laser.

On a breadboard, mount a small HeNe laser and the Crystal Technologies AO modulator like the setup above so that the beam passes through the AO modulator and strikes a target at least 1m away. Connect the RF supply to the modulator using the push-on coaxial connector. Energize the modulator, set the RF supply to INTERNAL and set the DRIVE control set to full power (10.0 on the control which represents 100% drive power). Vary the angle of incidence using the mount until diffraction is seen. Verify the modulator is working by changing the DRIVE control from zero to full power (this may easily be done by switching the INT/EXT switch to EXT with no external signal applied).

The AO modulator is mounted on a three-axis adjustable mount which attaches to the breadboard using two screws. The mount allows adjustments to be made to the angle of the modulator relative to the beam axis as well as to translate the modulator both vertically and horizontally using the mount. Loosen slightly the two translate lock screws as well as the two angular lock screws and use an allen key to adjust the translation (sideways motion) of the modulator to intercept the beam properly. The angle of the modulator may also be adjusted using the angular adjust screws on the mount.

In order to align the AOM you will require the following tools:

• 0.050" Allen Key for the rotate adjustment
• 5/64" Allen Key for the rotate lock, horizontal adjustment, and horizontal lock
• 3/32" Allen Key for the height adjustment
• 9/64" Allen Key for the height lock

Align the modulator carefully for Raman-Nath diffraction. Measure the power of the central beam when the AO is OFF (i.e. DRIVE is zero) and when the DRIVE is set to full power (100%). Measure the power of each of the diffracted beams (of various orders) as well at full drive power (draw a diagram of the resulting output with the RF on). Calculate the extinction ratio for the AO operating in the Raman-Nath regime - extinction ratio is the ratio of the power of the transmitted (central) beam with the modulator off to the power of the transmitted beam with the modulator on:

E = Zero order (RF on) / Zero Order (RF off)

This is a measure of the ability of the AO in this mode to reduce intensity of the zeroth order ("straight through") beam. Since the "through beam" is usually the main optical path of the laser, this is a measure of how much loss the AOM can induce into the laser cavity (and hence reduce Q).

Now set the mode of operation for Bragg diffraction using the procedure described in the Isomet application note on maximizing DE and the Crystal Technologies application note showing figures of correct alignment (hopefully you examined these while doing the prelab). Once again, measure the extinction ratio as well as DE and IL as outlined in the application note, and once again draw a diagram of the output with the RF on and measure the optical power of each diffracted beam component at optimum. Record the drive power Psat also (this was required to be determined during the alignment procedure).

Finally, measure the distance to the screen and the separation of the first (Bragg) order so that the angle of separation may be determined.

PCAOMs:

This part of the lab examines the use of an AOM element as a tunable filter (called an AOTF or a PCAOM: PolyChromatic AOM) - this device can be used to select a single laser line from a multi-wavelength laser.

In this part of the lab the beam from a White Knight helium-cadmium laser, with emission wavelengths in the red, green, and blue, is separated using a PCAOM. The beam, as it exits the laser, appears white (hence the name "white knight"). The beam is seen here after passing through the AOM from this lab experiment - the first order is to the left and the zeroth order to the right. The zeroth-order beam is actually refracted (which occurs regardless of whether the AOM is energized or not).

The complete PCAOM lab setup. The HeCd laser is in the foreground along with the AOM (and, in this photo, a beamstop). The overhead shelf contains the HeCd laser power supply, power meter, frequency counter, power supply, and VCO.

The PCAOM is driven via a tunable RF source (a VCO for Voltage Controlled Oscillator), the frequency of which is measured by a frequency counter. Begin by switching on the power supply and ensuring the voltage is set to 24V. Set the VCO frequency control to midpoint and turn on the Leader frequency counter - set it to measure a 520MHz signal, 50 ohm input, 0.1 second timebase.

Adjust the VCO for 68MHz, and rotate the PCAOM to optimize the Bragg angle for the red wavelength in the first order (you did this in the previous part, do it again!). Optimization may be done simply by observing the output - measuring the power of the red beam is unnecessary. Place a paper target at the output (on the opposite side of the optical bench) and note the position of the diffracted (first order) beams (all three wavelengths). Noting where the zeroth beam was as well, measure the ANGLE of the red beam by measuring the horizontal deflection and the distance from the PCAOM to the target (and using a little trigonometry as well!). Now, vary the VCO frequency to move the green and blue beams to where the red beam was: record the frequency for each.

The "reference point" for the angle of the diffracted beam could be taken as the center of all of the refracted zeroth order beams or each individual beam (e.g. use the zeroth order green beam as the reference for the diffraction of the green beam) ... you must determine (by fitting the data) which is the correct approach.

Analysis:

Begin by reading several notes on the Isomet site, specifically the application note on "Bragg Angle Errors" (Pages 3 & 4) which describe the operation of an AOM and useful formulae. Use the formula for Bragg angle in air and be advised that the result is in RADIANS which can be converted to degrees by multiplying by (180/π).

For the Crystal Technologies AOM, check the datasheet for f (this is fixed for this AOM) and v (convert it to units of m/s). Calculate the Bragg angle, and separation angle for the wavelength used. Compare these figures, too, to those quoted in the datasheet (they should be close, if not, you have a calculation error).

For the AA Optoelectronics AOM (used as an AOTF), f is variable (set by the VCO) and v is different (it is in shear mode). Of course the wavelengths are different as well.

PART B: Q-Switched YAG Lasers

• Read chapter 12 in Fundamentals of Light and Lasers by Csele on Solid-state lasers
• Read the SOP for the Lee YAG laser on this site. MANDATORY
• Read through the lab procedure

Lab Experiment

The Lee LDP-20 Laser is a modern, diode-pumped, Q-switched YAG laser capable of outputs of 50W at 1064nm or 10W at 532nm. For this experiment the laser is setup for 1064nm operation. The laser will be operated in both CW and Q-switched modes.

The lab setup is seen here. The Lee Laser is mounted on the bench with the fast external detector in front. On the overhead shelf (left to right) is the LDM-50 internal power meter, a power supply, signal generator, oscilloscope, and the amplifier for the fast detector.

The output of the laser is monitored by an internal power meter (the LDM-50) and by a fast external p-i-n detector. The internal LDM-50 meter works by intercepting the output beam (when the meter is on no output beam will be present) and measures average output power. The p-i-n detector is fast and can measure the amplitude of individual laser pulses down to 10ns. Turn the alignment HeNe on and ensure the output beam will strike the external p-i-n detector before proceeding.

The laser is configured for 1064nm. Start the laser according to the SOP by starting the cooling pump. Set the Q-switch to Internal modulation at 5KHz, close the shutter on the laser, turn the LDM-50 power meter ON, turn the main power on and set the diode current for 10.0A, and open the shutter. The laser should be seen to operate at a power of about 1.5W on the LDM-50 meter.

Set the diode current to 10.0A, turn the LDM-50 power meter on, and turn the Q-switch driver off so that CW output is produced. Measure the average power.

Now, turn the Q-switch driver on and measure the average power (on the LDM-50) and the individual pulse amplitude (using the p-i-n detector) while operating the Q-switch at rates of 5 to 50KHz in increments of 5KHz. Capture the output of the laser (on the scope) at low (10KHz) and high (50KHz) repetition rates and save to a USB key for analysis and inclusion into the report. To measure the amplitude on the scope, set the trigger level on the oscilloscope to obtain a stable display.

In summary, make a chart of Q-Switch rate (Hz), Average Power (W), and Pulse Amplitude (mV, arbitrary units, from the scope). As well, two screen captures from the scope are required.

Now, investigate the "giant first pulse" effect which will be covered in lectures - this effect is evident when the Q-switch is closed for a long duration (for example between cuts on a materials-processing laser) and the Q-switch is suddenly opened ... the first pulse in a string of repetitive pulses that follow will be much larger than the rest.

Set the Q-switch driver to GATED (Internal Modulation, 5KHz) which will keep the switch closed and allow ΔN to build. Now, set the scope to trigger on the first pulse, single sequence, arm the trace, and open the Q-switch by manually setting the driver switch to FREE-RUN. Set the timebase to show the first (giant) pulse and subsequent (tapering-off) pulses - until pulses reach a constant amplitude - all on the same screen (a 500uS timebase should be sufficient). Determine the height of the first pulse and compare to the amplitude of 'regular' pulses which eventually follow when they reach a constant amplitude. Save this scope trace (to the USB key) showing the first pulse effect and the tapering-off of pulse energy.

Now, wire the FPS input (a BNC connector) to a low-frequency signal generator set to 100Hz, 5V p-p, Square wave. Again, capture the output from the laser triggering on the first pulse (the easiest way to do this is to send the low-frequency square wave to channel 2 of the oscilloscope and trigger on the rising edge of that channel). Capture a scope output that shows the same type of trace as previously seen (with the same vertical and horizontal settings as the previous trace using manual gating) except with FPS enabled (as it would be during a cutting operation or when a SHG is used).

Lab Report

For this experiment, a condensed lab report is required (word processed, never hand-written) in question/answer format. Answer each question in the form "5a., 5b., 5c. ...". Do NOT answer an entire question (e.g. question 5) as a single paragraph. Submit the lab report in a folder or binder NOT simply a pile of loose, stapled papers! (a penalty will incur for lack of a folder)

Introduction:
1. One page describing the concept of Q-switching (in general) as it applies to solid-state lasers
2. One page describing, in general, how an A-O modulator works
3. One page describing the differences between Bragg and Raman-Nath diffraction


AOM / PCAOM:
4. Outline the full alignment procedure for an AO modulator to be used (in Bragg mode) as a Q-switch with optimal DE. Include diagrams/figures as required to show expected alignment (and misalignment) patterns expected.
5. Power Measurements:
   1. Tables of raw data (power measurements for each order) for both modes of the Crystal Technologies AOM
   2. Calculated extinction ratio (E) for both types of diffraction using an AO modulator. In this case, extinction ratio represents how good a 'switch' this device will make inside a laser cavity.
   3. Report Psat for Bragg mode (where "10.0" on the control represents full power from the Crystal Technologies 1080-25 driver - all control settings are linear).
   4. Calculate, from experimental results, DE and IL for the AOM operating in Bragg mode.
6. Deflection angles:
   1. Determine the expected Bragg angle of the Crystal Technologies AOM for the HeNe beam
   2. Using trigonometry, compute the observed separation angle (and then the Bragg angle) of the HeNe beam
   3. Determine the expected angle of the red beam (of the HeCd laser) for the AA PCAOM at 68MHz. Knowing the wavelength and other parameters as in the datasheets compute the expected (separation) angle for the beam (showing all work and assumed parameters).
   4. Using trigonometry, compute the actual separation angle of the red beam (of the HeCd laser)
   5. Make a chart showing (i) the wavelength, (ii) the predicted frequency for alignment, and (ii) the observed frequency for alignment when used as a AOTF to select R/G/B output to a single spot. The first entry in the table will be for the red beam at 68MHz at the "known" (both observed and calculated) position.
   
   YAG:
7. Q-Switched Results:
   1. Report the power available in CW and Q-switched modes.
   2. Does Q-switching imply that higher AVERAGE powers are available over CW operation?
   3. Include the chart of both power and amplitude vs. repetition rate
   4. Include output traces (from the scope) at low and high repetition rates.
8. You will notice that as the rate of the Q-switch increases, the pulse energy decreases. Explain this (in a paragraph or two) in terms of the build-up of inversion (ΔN) and the pump time between pulses.
9. At high repetition rates, the pulses may be seen to be inconsistent. Research (on the Lee Laser site) the concept of "Ding Dong". Determine if and when it occurs in our system (and at what repetition rate ?), and determine if this is reasonable (i.e. under what conditions are we operating this laser that are an invitation for this according to the tech notes)? Include a screen shot from the oscilloscope showing "ding dong" occuring in our laser.
10. "Giant" First Pulse:
    1. Show the giant first pulse effect as seen in the output of our laser: include a screen shot from the scope showing the first "giant" pulse occuring and subsequent pulses tapering to a nominal, constant, amplitude when manual gating is employed.
    2. These giant pulses can easily destroy a second-harmonic (SHG) generator so the Lee laser incorporates an option called FPS (First Pulse Supression) which can be found on their Tech Bulletins section. Describe (in a paragraph) the physics behind this method of correcting the first pulse so that it is of equal amplitude to all subsequent pulses - look at US Patent # 4,675,872 for a description of the technique as well as Sintec Optronics, a manufacturer of Q-Switch drivers which incorporate FPS. Describe physical parameters such as the population of specific YAG energy levels.
    3. From the description above (and the PDF files), deduce how RF power is applied to the AOM to cause FPS to occur. Include a sketch of the expected RF drive pulse (a graph of RF power on the y-axis vs. time on the x-axis) prior to the first pulse in order to suppress it: specifically, describe the expected RF signal driving the Q-switch during both "normal" operation and during "FPS" operation to contrast the two.
    4. Show a scope shot showing the output similar to (a) above except with FPS enabled. Calculate the amplitude of the first pulse as a percentage of the nominal pulse amplitude for both the FPS condition and the normal (manually gated) condition.