PHTN1400: Principles Of Laser Systems
Q-Switched Lasers (2019)

Introduction

This lab introduces the principles and properties of both common types of Q-switches (acousto- and electro-optic types), alignment techniques for these switches, and characteristics of lasers employing these devices. This is a two week lab, one lab report is required for both parts.

AO Modulator

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

The class will be split into two groups with each group performing parts A and B alternately on two consecutive weeks.

PART A: AOM, EOM, and PCAOM Devices | PART B: Q-Switch Alignment

Pre-Lab (to be done BEFORE the lab)

PART A: Acousto-Optic and Electro-Optic Modulators

The most common Q-Switch in use is the AOM (Acousto-Optic Modulator). AOMs can operate in one of two modes, Raman-Nath and Bragg, with the later mode providing higher 'blocking' of intra-cavity radiation. Alignment of an AOM is critical for optimal performance and in this part of the lab you will be able to align an AOM which is not part of an active laser - it is separate, and in a mount allowing alignment in any axis. This will demonstrate angular alignment and the AOM will also be characterized in terms of performance. In addition, the frequency characteristics of an AOM will be investigated with an AOM used as a PCAOM (or AOTF) and driven by a variable frequency RF source.

PreLab (Part A)

Data Sheets

For the AOM / Q-Switch part of the lab (A-1):
For the PCAOM part of the lab (A-3):

Lab Experiment (Part A)

Before starting the first part of the lab, outlined below, start the WhiteKnight HeCd laser and allow it to preheat (if not already done). Switch the Ready switch on the rear of the unit upwards (which lights the green indicator on the front), and insert the key to start the laser. The shutter will be closed so no beam will appear (leave the laser in this state for preheating to avoid dangers of this beam when it finally appears). Both the Helium and Cadmium heater indicators will light and, in about 20 minutes, the laser will become ready (a laser beam will be present, thereafter, when the shutter is opened).

Part A-1: 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.

Introduction

The majority of AOM Q-switches are configured as "pulse off" such that with RF energy applied, a large loss occurs inside the laser cavity and when the RF energy is switched off, a pulse appears. In this case, the switch must be aligned so that the inserted loss on the zeroth order, when on, is more than sufficient to inhibit oscillation of the laser. The switch is aligned for the Bragg angle where maximum loss occurs. It is also possible (although rare) to configure the switch as "pulse on" in which case the mirror is aligned for the first-order diffracted beam - the laser then oscillates only when RF energy is applied and only if the alignment of the switch is sufficient so that the loss in this first order is small. Once again, the Bragg angle is used. The point of this part of the lab is to align an AOM device for Bragg diffraction and measure the performance of the device since this affects suitability for application.

AO Modulator Setup

AO Modulator Mount

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:

AOM Output FIRST, align the modulator position for best transmission (and hence lowest IL). Measure the power of the central beam before the AOM as well as the output from the AOM when the AO is OFF (i.e. DRIVE power is zero). The formula for IL is found in the Isomet application note you read for the prelab - optimize for this.

Now, turn on the RF drive power (to full) to the AOM and align the modulator carefully for Raman-Nath diffraction. Measure the power of each of the diffracted beams of various orders - draw a diagram of the resulting output with the RF on to keep the orders straight. Raman-Nath will be achieved when the power of the first-order beams is approximately equal (unlike that shown in the diagram to the left in which such symmetry is not evident). Take power readings needed to 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 of the cavity in order to switch it).

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 since you really don't have time to read the prelab readings in the lab). 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.

You will now have recorded sufficient data to calculate:

Obviously, you found this in the prelab and so know what power readings are required.

When measuring the powers of each diffracted order, measure the distance to the screen and the separation of the first (Bragg) order so that the angle of separation may be determined.

Finally, record the effect of RF drive power (sometimes called "Psat" but NOT to be confused with optical power of any kind since this is the RF drive power). This was required to be determined during the alignment procedure and is done after Bragg angle adjustment - the ISOMET application note on Maximizing AO DE (AN1106) from the prelab readings cover this. Record the zeroth power with the RF turned off and then the zeroth order with the RF on at 10% interals between 0% and 100%. The control is set so that "10.0" is 100% RF output (one watt) and it scales linearly. There are two reasons for determining how loss is affected by RF drive power: (i) In order to optimize DE the "optimal" drive power is required and too much drive power actually results in lower DE and (ii) by lowering RF drive power the loss inside a cavity can be lowered allowing low-power oscillation which will serve to depopulate a huge initial ULL population and so can be used to correct for a "massive first pulse" (this is covered in a future lab).

Part A-2: EOMs:

The characteristics of an electro-optic modulator are investigated including alignment of the analyzer and transmission of the switch as a function of applied voltage.

EOM Lab Setup
The complete EOM setup in V115B. Light from a HeCd laser on the right passes through an EOM then through a polarizer consisting of a stack of six Brewster plates orthogonal to the polarization of the HeCd laser. The HV power supply for the EOM is on the overhead.

EOM Brewster Plates The polarizer consists of six quartz plates aligned at Brewster's angle yielding twelve air-to-glass or glass-to-air interfaces where reflections occur (in the perpendicular only). This polarizer stack is aligned orthogonally to the linearly polarized HeCd laser beam. Three plates are shown here.

Rotating the analyzer The HeCd laser must have already been preheated for a half-hour (this should have already been done at the start of the lab). The beam of the HeCd laser is highly polarized and the analyzer, in this case a stack of six glass plates at Brewster's angle, must be aligned completely orthogonal to this polarization. Procedure to do this: with the power supply off and no applied voltage to the EOM, monitor the optical power of the 441.6nm (blue) beam after passing through the EOM and the analyzer and rotate the entire analyzer (the black tube) so that a minimum power is transmitted - be careful not to loosen the threaded ring holding the assembly together.

Power Supply for the EOM Record the optical power on the blue, green, and red beams. Now set the voltage control on the high voltage supply to 100V (the "0-100V" control is set for 10.0) and switch it on. Record the optical power transmitted by the EOM now for both the green and blue beams (wavelengths may be found in the prelab documents provided) with 100V applied then increase the high voltage output in 100V increments by setting various combinations of the two left switches (in the example shown the voltage is 2200V: 2000V on the first knob, 100V on the second knob, and 100V on the final control). Continue until 3kV is reached (so about thirty readings will be taken).


Part A-3: 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. Alternately, one may also employ such techniques to use a PCAOM as a beam deflector/scanner.


DANGER: This is a class-IIIb laser with >20mW of output in the blue wavelength (and typically under 5mW at both the red and green wavelengths). Laser safety glasses suitable for the blue wavelength (441.6nm) are required - these are usually the same ones used with an argon-ion laser.

Before this part of the lab, use the Easy-Haz Online Calculator to CALCULATE the OD required at 441.6nm for safety glasses.


White Knight HeCd laser output

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 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.

PCAOM Setup
The AA Optronics PCAOM passing the beam from a HeCd laser. Evident in this photo is refraction of the central beam (discussed below)

Adjust the VCO for 50MHz, 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 by eye - measuring the power of the red beam (or any beam) is unnecessary for this part of the experiment. Place a paper target at the output (on the opposite side of the optical bench) and note the position of both the zeroth order beams (there are three), the position of the diffracted (first order) beams (again there are three), and the distance from the AOM to the screen. Later, you will calculate the ANGLE of each 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 80MHz and record the same positions for each beam (again, measure the locations relative to the zeroth orders).

White Knight HeCd laser output

The "reference point" for the angle of the diffracted beams is taken as the zeroth order beam for the same wavelength (e.g. use the zeroth order green beam as the reference for the diffraction of the green beam, the zeroth order red beam as the reference for the diffracted red beam, etc). For this reason, record the relative positions of each beam of the zeroth order as these will serve as references for the angles to be determined.

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 frequency f (this is fixed for this AOM) and acoustic velocity 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 very 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 since you are using a HeCd laser. You will need to research the optical parameters of TeO2 and the wavelengths of the laser used.

PART B: Q-Switch Alignment and Gain Parameters

In this portion of the lab, a commercial Q-switch will be optimally aligned and the sensitivity of the alignment (insertion loss vs. angle) will be determined. In addition, gain of the laser will be determined in order to determine the loss which the Q-switch has inserted into the cavity.

Introduction

In this part of the lab you will apply what you learned in Part A to aligning a Q-Switch on a real laser. From lectures, you have calculated the holdoff required of a Q-Switch to properly switch a laser ... better alignment of the switch means more induced intra-cavity loss in the "closed" state. In this lab you will see for yourself what happens when a switch is not properly aligned.

Q-Switch Output vs. Alignment The image to the right shows the pulse output of a YAG laser as captured on an oscilloscope. The trace is triggered at the same time as the flashlamp (labelled as "0μs") at which point the RF driver turns ON to inhibit lasing. 120μs later, the RF driver turns OFF at which point the laser should fire. With the Q-Switch properly aligned, this is indeed what happens, as seen in the red trace, however improper Q-Switch alignment leads to premature lasing which occurs before the switch actually opens ... in this case around 80μs as seen in the blue trace.

In this part of the lab you will see the effects of alignment and why it is so important to align the AOM properly, as per Part A, to achieve a high enough intra-cavity loss to prevent lasing.

The expected behaviour:

  • When the Q-switch is properly aligned, the output pulse coincides with the output of the pulse generator (i.e. when the RF drive turns off, the pulse is generated).
  • When the Q-switch is improperly aligned (or the RF turned off completely), the output pulse is emitted before the output of the pulse generator since the Q-switch cannot induce enough loss to prevent oscillation. The output will not be in the form of a "short pulse" but rather a "long pulse" and so not what was desired at all.
  • When the Q-switch is properly aligned and turned on continually (i.e. constant RF signal), no output pulse is seen at all since the switch never "opens".

The Q-switch and driver have been upgraded since the images to the right were taken (the "RF ON" pulse is now upside-down to accomodate the new driver and the detector is different), however the basic principles and outcomes are the same.

PreLab (Part B)

Lab Experiment (Part B)


DANGER: This is a class-IV laser with EXTREMELY high peak powers capable of ocular damage with only one pulse. The particular danger here is the Q-switched infrared output at 1064nm since hazards presented by specular reflections are not obvious. Ensure the beam is intercepted as close as possible to the laser and pay attention to spurious refections from optical elements in the beam path.

SAFETY GLASSES MANDATORY - DO NOT REMOVE THEM WHEN THE LASER IS OPERATING ... Q-Switched YAG lasers are responsible for more eye injuries than any other types of laser combined!

(See the anecdote later on this page - this laser can vapourize optical coatings so imagine what it can do to your retina!)

Q-Switch Alignment Setup
The complete Q-Switch alignment setup on an optical table. The basic laser controls are on the right side in the 19-inch rack cabinet including the main capacitor supply, trigger supply, and firing/interlock control. The overhead contains the Q-Switch driver, pulse generator, and oscilloscope used to monitor the timing of the laser output.

Setup: Set the flashlamp anode voltage to 230V, insert the 8% static loss between the OC and the amplifier rod, set the pulse width to 700μs.

Part B-1: AOM Alignment:

So, how critical is alignment of a real AOM Q-switch (the most common type employed)? In this part of the lab a commercial Q-switch will be aligned into the cavity of a laser. This flashlamp-pumped laser has higher gain than the more common diode-pumped version and so alignment will be all that much more critical.

The Q-Switch is installed onto the CL-5 optical rail along with an 8% (per pass) intra-cavity optical attenuator to increase threshold gain (since small-signal gain is excessively high as this is a flashlamp-pumped laser). As per the SOP, set the trigger voltage (lower supply) to 320V and the main capacitor voltage (upper supply) to 230V. With the oscilloscope set to capture a single pulse, turn the Q-switch driver OFF and fire the laser once (via the FIRE button on the rack-mounted control) to verify operation and determine where (the time elapsed after the trigger pulse) the laser oscillates (reaches threshold) in "normal" long-pulse mode - this should be around 300μs.

At any point required, you may insert a USB key into the scope and press "PRINT" to save an image of the oscilloscope screen for the lab report. Since you have read through the lab already as part of the prelab, you will know which observations you need to take.

Q-Switch Perpendicular Alignment Pattern With the Q-Switch driver OFF, fire the laser several times and note where the pulse appears (relative to the trigger point). Now, turn cooling water to the Q-switch on, and power the Q-Switch Temperature Controller and Q-Switch driver both ON, and set the Modulation control to OFF - this will apply a constant RF signal to the switch (i.e. the Q-switch is constantly ON and is inducing a loss into the laser). Rotate the Bragg alignment knob CW, then slowly CCW until the AOM it is perpendicular to the optical axis of the laser. Place a business card in front of the laser (in front of the detector) to view the red HeNe alignment beam passing through the entire laser. Align the switch to produce Raman-Nath diffraction (i.e. with diffracted beams visible on both sides of the central beam) ... the pattern will resemble that to the left but will also include two small reflections above and below the central spot (those small reflections make it easy to find true perpendicular). This is the "zero" point. Fire the laser at this point - the output pulse should be observed to be premature and will appear before expected time where the RF signal is turned off (if not, the "zero" point is incorrect).

Q-Switch Alignment Setup
The LEE Laser 32027-50-5 Q-Switch mounted on the CL-5 laser rail. The Bragg Alignment knob allows fine adjustment of the angle - optimally the Bragg angle is used for maximum inserted loss.

Now, set the Q-switch driver Modulation control for Internal GATED operation and 50KHz modulation frequency. The CSC 4001 pulse generator must be set for SINGLE-SHOT mode and COMPLementary output (both buttons must be depressed) and with a pulse width of 700μs - pressing the white "ONE SHOT" button on the generator will produce a pulse to verify this width on the scope. Fire the laser and, owing to misalignment of the Q-Switch, you should see laser output before the expected point in time (i.e. before the end of the trigger pulse, and likely at the same spot as when the RF driver was OFF completely meaning the Q-switch is doing nothing). Note the output as "premature" or "on time". Since the Q-Switch driver is modulated at 50KHz, the output pulse can be delayed up to 20μs after the expected time (i.e. after the end of the pulse from the 4001 pulse generator). Rotate the alignment knob one-eigth turn counter-clockwise and fire the laser again. IMPORTANT: Do NOT turn the screw backwards (CW) since it has some amount of "play" - move the screw in one direction only for this part of the experiment ... if this is done, turn the knob several turns CW, then several turns CCW back again. Continue rotating the alignment knob and produce a table of ANGLE (radians) versus OUTPUT (premature, on-time, missing) to determine how critical the alignment of the Q-switch really is. After each pulse, verify the point at which output appears and ensure the trigger pulse width is still 700μs since it will occasionally misfire and the pulse will shorten in which case repeat firing at the same setting. Verify, also, that the slowly-rising low-level radiation from the flashlamp pumping pulse can be seen in the trace indicating that the lamp has indeed fired.

Now, set the Q-Switch angle to the optimal found (i.e. the Bragg angle) by turning the knob back to the zero position and then CCW to the position where the Q-switch was aligned properly. Fire the laser to verify the Q-switch is still aligned properly, then set the driver modulation to OFF via the rotary switch on the Q-switch driver (producing a constant RF signal so the switch is always "closed"). Fire the laser and you should see NO output pulse appear at all (just low-level residual light from the flashlamp).

AOM Bragg Angle Adjustment The relationship between knob position and angle: The alignment knob has a 48TPI thread and is 55mm from the center pivot so trigonometry can be used to calculate the angle of the AOM for each turn of the knob. You, of course, did this in the prelab.

When complete, you should have the following observations ...

Ensure the pulses are as expected - if not, be sure to ask in the lab

An anecodote from the professor on this lab:

When prototyping the experiment it was found that the gain of the laser was too high - the obvious solution would be to lower the anode voltage of the lamp but too low and the lamp will not fire - the solution, then, was to insert a loss intra-cavity. A variable-density wheel was already installed on the system so that wheel was used. The experiment always worked: ONCE that is. The first time the Q-switch was aligned, it always produced the pulse where expected however subsequent shots were often premature. This problem persisted for days until the wheel was removed to reveal that the neutral-density coating was literally vapourized from the wheel where the beam passes through it! When fired for the first time, the attenuator works as expected but on subsequent shots, it was completely gone so the resulting gain was much larger than expected: large enough to cause oscillation even with the Q-switch "closed".

Replacement of the neutral-density wheel with a piece of uncoated optical-grade glass (with the resulting loss of 4% per surface predicted by the Fresnel equation) solved that problem and the results are reproducible.

Can you say "DAMAGE THRESHOLD" ... I knew you could!

More on damage thresholds in this course when we discuss second-harmonic generators ....

Part B-2: Laser Gain:

Alignment of the Bragg angle in part B-1 was likely found to be critical to induce the most loss possible (and certainly enough to allow proper Q-switching). In this part of the lab we determine the small-signal gain of the laser and the loss that that AOM must have inserted into the cavity in that part of the experiment in order to properly Q-switch the laser.

The laser, a modified Control Laser CL-5, was originally a krypton lamp-pumped CW laser. The rod is 50mm in length by 4mm diameter, the HR is essentially 100% reflecting, and the OC is 90% reflecting at 1064nm.

Examine the laser and sketch a diagram outlining the configuration of the optical components used in "Q-switched" mode (since it will now be modified).

Now, the gain of the laser is determined using the standard method of inserted intra-cavity loss. Leave the voltage the same as the previous experiment and TURN THE Q-SWITCH OFF by turning the driver power off completely (this is extremely important !). Remove the 8% inserted loss from the rail and install the circular filter wheel in that spot. Note the new optical configuration.

To measure the output of the laser select the minimum inserted loss on the wheel (position 1) then fire the laser. The output pulse will be displayed on the scope. Replace the scope with a Melles-Griot ENERGY METER and measure the actual output energy in Joules.

Now, set the filter wheel for a higher density (the wheel is inside the optical cavity of the laser between the rod and the HR). Repeat the experiment with the new (higher) inserted loss - obviously the output energy will decrease (if not, be sure to zero the meter after each shot). Repeat again with a higher loss until the laser finally ceases to oscillate and no output pulse is seen.

Analysis: Develop a threshold gain equation for the laser which incorporates the inserted loss as well as anything else still in the cavity (such as the Q-switch which is OFF but still there and so represents a small optical loss). At the point where lasing ceases (i.e. at the minimum transmission found), calculate the value for threshold gain: this is actually small-signal gain of the amplifier. Now, develop a second gain threshold equation to describe the laser in the form used in the first part of the experiment (i.e. with the Q-switch and 8% loss in the cavity). Set this threshold equal to the small-signal gain you have determined then solve to Q-switch transmission. This will give an idea of how well aligned the AOM was when the laser operated properly. From this data, compute the DE of the AOM (You know the transmission of the zeroth order already, you now need to consider the proportion of the beam that would optimally be dumped to the first order to make that occur). This was covered in lectures including how to deal with the insertion loss - review your lecture notes.

You will need to start with two simple diagrams of the laser configuration to determine the proper threshold equations for each ... start with a simple diagram and ensure you include all losses and the number of times each is encountered during a round-trip. The AOM transmission is now known, as is the reflectivity of the optics. The insertion loss of the AOM switch may be found by researching the 32027-50-x Q-switch (Sintec offers a similar replacement switch, the 32037-50-4, which may be used as an example).

The parameters for the CL-5 laser are as follows: ROC=90%, RHR=100%, Rod Length = 50mm, Rod Diameter = 4mm. All other parameters can be assumed from chapter 8 of Laser Modeling by Csele for an Nd:YAG laser.

Lab Report

Hand In a WORD PROCESSED (not handwritten) lab report with contents as outlined below.

Lab Report The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, and the student's name and ID number

Answer each question as "1", "2", etc with each new question starting on a NEW PAGE so that question 2 starts on the top of a new page and question 3 starts at the top of a different page, etc. Where a question has multiple parts (e.g. 3a, 3b, 3c ...) you may answer those on the same page however each part must begin in a separate paragraph with a title identifying the question in the form "3a., 3b., 3c. ...". Do NOT answer an entire question (e.g. question 3) as a single paragraph with nothing to denote section (a) from section (b), etc.

This format will assist you in ensuring EACH and EVERY question is answered since marks cannot be given for work not completed, nor would it be expected that you could complete the TEST QUESTIONS which will most certainly be similar to those you see here! (Hint !)

The lab must be submitted in a report cover (preferably either a three-hole punched cover or one with a clamp on the left side, not a binder), and NEVER as a stapled mass of loose papers!

Failure to follow this simple format, used for all condensed labs in this course, will result in deduction of marks

For ALL CALCULATIONS, work must be shown! Answers without calculations will receive a mark of ZERO. Where a calculation is repeated many times (e.g. to complete a table of values) show ONE complete set of example calculations.

    AOM / PCAOM Devices:
  1. Power Measurements: Basic AOM usage
    1. Raw data power measurements for all observable orders (including zeroth, first, and second) plus the zeroth order with RF off for both modes of operation of the Crystal Technologies AOM (in both Raman-Nath and Bragg modes). Be sure to identify the mode clearly on each set of measurements presented.
    2. Calculated extinction ratio (E) and diffraction efficiency (DE) 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. Show calculations and substituted values.
    3. A graph of E vs. applied RF power for a perfectly aligned AOM in Bragg mode. "10.0" on the control represents the full RF power of 1 Watt from the Crystal Technologies 1080-25 driver and all control settings are linear so "2.0" is 0.2W). SHOW your data as a table detailing zeroth-order power (RF off), first order power (RF on), and calculated extinction ratio E at each point (every 10% of full RF power).
  2. Deflection angles: Basic AOM
    1. Determine the expected Bragg angle of the Crystal Technologies AOM for the HeNe beam (in radians) using data from the datasheet. Show calculations.
    2. Using trigonometry, compute the observed separation angle (and then the Bragg angle) of the HeNe beam using the Crystal Technologies AOM. Show calculations.
  3. Deflection angles: PCAOM
    1. Determine the expected angle of each wavelength of the HeCd laser for the AA PCAOM at 50MHz and 80MHz. Knowing the wavelength and other parameters as in the datasheets compute the expected (separation) angle for the beam (showing all work and assumed parameters). Show one example calculation for the blue beam, show the angle for all three beams.
    2. Using trigonometry, compute the observed separation angles of each beam at 50MHz and 80MHz. Show a diagram of each beam which shows the positions of the zeroth and first orders and show an example calculation for the angle of the blue beam.

    EOM Devices:
  4. Half-wave voltage
    1. Plot the observed optical power of the EOM as a function of applied voltage for both the green and blue wavelengths then, from observations, determine the half-wave voltage at 441.6nm (the blue beam). State this half-wave voltage right on the graph in a text box.
    2. From the half-wave voltage of the blue beam (above), calculate the half-wave voltage at the HeCd green wavelength and then the expected transmission of the EOM to both the green and red wavelengths when operated at exactly the "blue" half-wave voltage of the EOM (the blue transmission will be 100% but the other two will both, of course, not be 100% since the half-wave voltage is a function of wavelength). Show all calculations and substitutions.
  5. Transmission:
      In theory, the minimum transmission of the switch is determined by the transmission of the analyzer which consists of six glass (quartz) plates at Brewster's angle separated by air (and so, twelve air-to-glass interfaces).
    1. Calculate the reflection of a single surface to polarized light (Rs), then transmission (Ts = 1-Rs) in the perpendicular
    2. Treating the stack in the usual matter (by multiplying the transmission at each interface in series), calculate the expected MINIMUM transmission of the EOM device with zero volts applied (and hence incoming light is polarized at 90-degrees to the analyzer which consists of those six glass plates). The polarized HeNe example from last term might help here. Report the (minimum) transmission of the analyzer to parallel polarized incident light (i.e. where polarizers are crossed and the EOM is not energized). Show calculations.

  6. YAG Q-Switch alignment and Gain:
  7. Q-Switch Alignment Results:
    1. A screen capture from the oscilloscope showing firing with the Q-switch OFF (i.e. zero RF power applied)
    2. A screen capture from the oscilloscope showing firing with a misaligned Q-switch
    3. A screen capture from the oscilloscope showing firing with an aligned Q-switch
    4. A screen capture from the oscilloscope showing firing with an aligned Q-switch which is constantly turned ON (i.e. modulation is OFF so the RF power is applied continually)
    5. For each of the four preceding screen captures, include a line or two for each describing the expected output (e.g. premature or on time) and why.
    6. A chart showing angle (radians) of the Q-switch vs observed output (either "premature" or "on-time" meaning properly Q-switched).
  8. Calculations of gain and required Q-switch loss:
    1. Plot observed output energy vs. wheel transmission of the Nd:YAG laser and add a trendline to identify the threshold transmission of the wheel (in %). Display that value directly on the graph as a textbox.
    2. Calculate the small-signal gain of the laser. Begin by showing a diagram of the configuration used when determining threshold gain equations (similar to any of those in chapter 2 of the LM text which identify all optical elements in the cavity, including the inserted loss of the wheel plus all other losses). Develop the gain threshold equation for this configuration (algebraically) which includes losses identified in the diagram plus re-absorption loss and finally, show the calculations for small-signal gain (in m-1) which includes all substituted values.
    3. Calculate the required maximum transmission of the Q-switch in the "on" state to operate properly as a Q-switched laser (i.e. to cease oscillation with the switch "on"). Show a new diagram of the optical configuration of the laser used (which identifies all optical elements in the cavity including the static loss used in the experiment and the Q-Switch itself - of course the inserted loss of the wheel will NOT be present now), show the gain threshold equation for this configuration (algebraically) again including re-absorption loss, and show the calculations for Q-switch transmission which includes all substituted values. Use the small-signal gain determined above in this calculation. This number represents the maximum transmission of the Q-switch in the "on" state as you had aligned it in this experiment to allow proper Q-switching operation.