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.
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.
In addition to the above prelab, there are specific prelabs for each part of the lab required before you perform that part of 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.
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.
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.
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:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
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% 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 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.
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).
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). 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 (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 flashlsamp).
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 small-signasl 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.
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
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). 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 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.
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.