PHTN1400: Principles Of Laser Systems
Q-Switched Lasers (2017)

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 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-2):

Lab Experiment (Part A)

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 (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 apllied 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

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).

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 IL. 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%) to do this (the formula for IL is found in the Isomet application note you read for the prelab). Now, align the modulator carefully for Raman-Nath diffraction. 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). 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). 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). 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 effect of drive power Psat also (this was required to be determined during the alignment procedure, done after Bragg angle adjustment) - the ISOMET application note on Maximizing AO DE (AN1106) from the prelab readings cover this.

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

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 V15A. 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 Turn on the HeCd laser and preheat it 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. 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 and green 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 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.


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.

Use the Easy-Haz Online Calculator to CALCULATE the OD required at 441.6nm for safety glases and the NHZ (which you need to know for diffused observation of the beam). You might have to guess at several parameters: substitute various values to see how this affects both NHZ and OD.


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 - 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) red beam at 50MHz. 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 first order diffracted green and blue beams to where the red beam was: record the frequency for each.

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
  • 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".
  • When the Q-switch is properly aligned and turned on continually, no output pulse is seen at all

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.

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% intra-cavity optical attenuator to decrease laser gain (which 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 260V. 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 200μs.

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 indusing 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 300μs - 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). 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 300μs since it will occasionally misfire and the pulse will shorten in which case repeat firing at the same setting.

2016 NOTE: Set the flashlamp anode voltage for 240V and the pulse length for 500μs. This will ensure that the Q-Switch will have enough holdoff to completely suppress pulses (it has higher gain than expected owing to the fact it is flashlamp pumped). By switching the modulation control on the Q-Switch driver to "OFF" the RF supply will be left on continually maintaining a large cavity loss an completely suppressing oscillation (if, and only if, the Q-switch is properly aligned).

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 (producing a constant RF signal so the switch is always "closed"). Fire the laser and you should see the output pulse appear well after the intended firing point (and perhaps even non existent if aligned particularly well).

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. In this part of the lab we determine the gain of the laser and the loss that that AOM must have inserted into the cavity 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.

Now, the gain of the laser is determined using the standard method of inserted-loss. Leave the voltage the same as the previous experiment and TURN THE Q-SWITCH OFF (this is extremely important). Remove the 8% inserted loss and install the circular filter wheel. Install the Melles-Griot broadband meter in front of the laser and set it to ENERGY mode.

The energy meter, pictured here, will capture the energy of a single pulse.

To measure the output of the laser:

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. Repeat again with a higher loss until the laser finally ceases to oscillate.

Plot the observed output energy (y-axis) vs. wheel transmission (in % on the x-axis) and plot a best-fit line. Where the best-fit line intercepts the x-axis (at y=0), the inserted loss for threshold is found.

In the simplest analysis, at the point where lasing ceases (i.e. at the minimum transmission found), subtract 8% (to compensate for the original 8% loss in the cavity) and you will have determined the minimum loss which the Q-switch must have inserted into the laser cavity in the first part of this experiment to cause lasing to holdoff. Ensure this makes sense: if you determined that a total inserted loss of 40% causes the laser to threshold then with the laser in the experimental configuration with an 8% static loss the AOM must have inserted a loss of 32% minimum to allow the laser to Q-switch properly. 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 notes.

Now, compute the GAIN of the laser amplifier in the usual manner (i.e. by formulating a threshold gain equation incorporating all losses in the cavity ... 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.

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 samepage 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.

    Introduction (Concepts):
  1. One-half page describing the concept of Q-switching (in general) as it applies to solid-state lasers
  2. DO NOT "cut and paste" a description from a web page or other reference but rather put this in YOUR OWN WORDS. Specifically, reference (a) the expected output with no Q-switch (or the Q-switch turned off) and (b) the difference between Q-switching and GAIN switching.
  3. One-half page describing, in general, how an A-O modulator works
  4. Again, desribe this IN YOUR OWN WORDS and be sure to describe the difference between REfraction and DIFfraction as it applies to the operation of the device.
  5. One-half page describing the differences between Bragg and Raman-Nath diffraction


  6. AOM / PCAOM:
  7. 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 (i.e. show what the beam should resemble for an aligned AOM).
  8. Power Measurements:
    1. Tables of raw data (power measurements for all observable orders (including zeroth, first, and second) for both modes of operation of the Crystal Technologies AOM (in both Raman-Nath and Bragg modes).
    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. Show calculations.
    3. Report Psat (the RF power at which saturation of the switch occurs as defined in the Crystal Technology notes) for Bragg mode (where "10.0" on the control represents full RF power from the Crystal Technologies 1080-25 driver - all control settings are linear). SHOW your data as a table.
    4. Calculate, from experimental results, DE and IL for the AOM operating in Bragg mode. Show calculations.
  9. Deflection angles:
    1. Determine the expected Bragg angle of the Crystal Technologies AOM for the HeNe beam (in radians). 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. Determine the expected angle of the red beam (of the HeCd laser) for the AA PCAOM at 50MHz. 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 calculations.
    4. Using trigonometry, compute the observed separation angle of the red beam (of the HeCd laser). Show calculations.
    5. Show a diagram with the relative positions of the three zeroth order beams and the red beam position from above. Include measurements of these positions and show calculations of the expected angle for each beam at the same spot (it will be different for each of the three wavelengths due to this refraction of the zeroth order beam.
    6. Make a chart showing (i) the wavelength, (ii) the actual expected separation position on the screen, (iii) the expected deflection angle in radians, (iv) the predicted frequency for alignment, and (v) 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 50MHz at the "known" (both observed and calculated) position. Show all calculations for the Blue beam as an example of how the computations were done.

    EOM:
  10. Plot the observed optical transmission of the EOM (as a percentage of maximum) as a function of applied voltage then, from observations, determine the half-wave voltage at 441.6nm
  11. Report the observed half-wave voltage of the EOM at the blue 441.6nm wavelength. From this, calculate the half-wave voltage at the HeCd green wavelength and then the expected transmission of the EOM to the green wavelength when optimized for the "blue" half-wave voltage of the EOM (it will, of course, not be 100% since the half-wave voltage is a function of wavelength). Show all calculations.
  12. In theory, the minimum transmission of the switch is determined by the transmission of the analyzer which consists of six glass plates at Brewster's angle. Calculate the transmission of the analyzer stack by (a) first calculating the reflection, then transmission in the perpendicular then (b) treating the stack in the usual matter (by multiplying the transmission at each interface). Report the (minimum) transmission of the analyzer to parallel polarized incident light. Show calculations.

  13. YAG Q-Switch alignment and Gain:
  14. Q-Switch Alignment Results:
    1. A screen capture from the oscilloscope showing firing with the Q-switch OFF
    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. For each of the three preceding screen captures, include a line or two for each describing the expected output(e.g. premature or on time) and why.
    5. A chart showing angle (radians) vs output (premature, or Q-switched)
  15. Hand-in a graph of Eoptical (Joules) vs. inserted loss (%).
    1. Draw a "best fit" trendline and identify the threshold insertion loss on the graph.
    2. Calculate the DE of the Q-switch when optimally aligned (careful: be sure to check the definition of DE as it must be inferred from observations and deduced from what you know about the distribution of power in thee orders when operating in Bragg mode). Show calculations.
    3. Since the configuration with the Q-switch included an 8% static loss (which was not present when the wheel was used to determine actual gain of the laser so that the actual loss of the Q-switch was 8% less than the observed loss using the wheel), factor this loss in when computing DE.
    4. Calculate g0 for the laser using the standard method of inserted loss. Be sure to include all "static" losses in the laser cavity (as observed to be in the cavity of the laser when gain was determined by adding the loss of the filter wheel). Show calculations.