PHTN1300: Lasers and Light Sources
Gas Laser Cavity Optics Lab (2016F)

Introduction

Students will construct a working helium-neon laser on an optical breadboard using a plasma tube with windows and external optics mounted in kinematic mounts. Students will learn resonator alignment techniques as well as study TE modes. Gain and loss in a gas laser will be examined.


Students are seen here aligning the resonator optics for a helium-neon laser

This is a multi-part lab which will require two to three consecutive lab periods (weeks) to complete. The first lab period will consist of determining the technique necessary to align a laser resonator (this is a learned skill, and more of an art than science). The second/third periods will consist of a second alignment (this should, of course, go faster that the first) and experiments with both the cavity and intra-cavity elements. Submit a lab report only after all sections are complete.

This lab is worth one-third of the total practical marks in the course, as such the write-up is intense and will require the dedication of much more time than any previous lab - start it early!

SAFETY WARNING

This lab uses a 'bare' plasma tube with exposed terminals. Be aware that these lasers operate at 2400 VOLTS and the starting pulse for such a tube can reach 10,000 VOLTS. Short-out the high voltage terminal to ground before touching the wiring since the output can store energy even after it is turned off.

Pre-Lab

Part A: Resonator Alignment

Mount a Melles-Griot WPR252 tube on the breadboard in the two mounts provided. This is a custom-built plasma tube with anti-reflective coated windows instead of mirrors. The tube should be mounted in the middle of the board leaving room for optics at each end. Be sure both optics are situated on the same optical table. The anode (electrically "live") should face the HR to reduce the possibility of shock when aligning the OC. Now install the rear optic. Align the optic by sight such that you can see through the optic and the entire plasma tube ("bore sighting").

Details of the Setup Before autocollimation can take place, the rear optic must be bore-sighted so that light from the incandescent lamp has a clear optical path down the amplifier tube and strikes the center of the mirror (i.e. light clears the bore). The optic may require alignment in either direction to achieve this (e.g. it may have to be raised or lowered in the post holder) and the lamp WILL need to be adjusted for this. Raise or lower the lamp (which is mounted on a multi-axis mount) until a direct image of the filament is seen when looking through the HR towards the front of the tube. At this stage, the filament MUST be visible in the center of the tube bore ("close" does not count here and if in doubt, ask, since failure to bore-sight will make alignment almost impossible).

Next, install a beamsplitter in a 3-point mount where the front optic (OC) should go. Align the beamsplitter at 45 degrees so that you can see down the tube. An incandescent lamp is now mounted so that light from the filament passes through the beamsplitter and down the tube reflecting from the rear optic. Reflected light passes back down the tube and can be observed via the beamsplitter. This arrangement is called an autocollimator and is one method of aligning laser mirrors. It is the best method for small-bore laser tubes like the HeNe. Refer to the photographs below (although the anode and cathode are reversed in the photos):

Setup for aligning the rear optic

Details of the Setup

The rear optic must now be precisely aligned so that it is perpendicular to the bore of the tube. Verify that you bore-sighted the lamp filament image correctly (i.e. by looking through the HR from the outside towards the tube, and can see a VERY VERY bright filament "line" when looking directly through the HR and through the bore of the amplifier tube). It is important that the actual filament be visible, not just a "bright spot". Now, look through the tube from the other end via the beamsplitter and adjust the HR so that light from the lamp is reflected into your eye. To do this, sight the image of the filament reflecting from the rear optic and adjust each screw until the image is centered. Be careful as you will see many images reflected from the walls of the bore itself (total internal reflection from the polished glass surfaces). When the rear optic is correctly adjusted you will see a small section of filament in the center with a concentric ring of light around it ... it is important that you see the bright filament image in the center of the reflected image not just a bright ring. The ring is perfectly centered and round when the optic is adjusted correctly. This line is included only to see if you are cutting and pasting the procedure directly from this lab like a lemming. Do not include this line or the previous line. The following photos show what you'd expect to see when looking into the beamsplitter.

Filament image as seen in the beamsplitter

Filament image as seen in the beamsplitter

Filament image as seen in the beamsplitter A drawing of the expected view through the beamsplitter. Looking into the beamsplitter, move your head until you can see an orange image from the HR. Align the HR such that the direct image of the filament is visible ... the actual, direct image must be seen not a vague "swirl" of orange light. If the lamp filament lies horizonal, the image of the filament will also be horizontal as seen here.

The image is TINY so you might need to really focus to see it.

Once you are satisfied that the rear optic is aligned, remove the beamsplitter and lamp and install the front optic (the OC). The NOTCH on the kinematic mirror mount must face DOWNWARD to allow the mirror to be rocked. If the notch is at the top of the mount, reverse it now.

Again, observe that you can see directly through both optics and the bore of the tube (i.e. bore-sight the OC). To do this, place a lamp outside the cavity facing the HR and look (from the front end) through the OC, tube, and HR and ensure you can see the lamp filament unimpeded. This is critical: you must be able to see through the tube and both mirrors DIRECTLY ... if you cannot do this, it might be required to adjust the OC laterally (sideways) so that a clear path exists between the mirrors and the optics (done using parts from the bench). Do not continue if this is not possible.

Install the power supply. The POSITIVE terminal connects to the 'small' end of the tube (the anode) which is hopefully pointed towards the HR. Switch ON the supply and the tube will glow.

Laser with both optics installed

Front optic alignment is done by 'rocking' the mirror. Begin by purposely aligning the mirror so that the reflected beam is to the top and left of the tube (i.e. the white light from the tube and reflected from the OC appears higher than expected and to one side). As you gently turn the X-axis (horizontal) screw 1/16th of a turn at a time, rock the mirror downward. Continue this until a 'blink' is seen in the laser output. When the laser blinks, STOP. The X-axis is now close so adjust only the Y-axis screw (vertical) until the beam appears continuously.

Rocking the Mirror

The final process is 'tweaking' the mirrors for maximum output using a procedure known as 'walking' the mirrors. This is done by adjusting one mirror at a time while monitoring the output power on a meter. VERY GENTLE adjustments are required here. When complete get a final reading of laser power.

Note that NONE of the photos or diagrams above are sufficient for the background section of the lab since they do not show the optical path of the autocollimator in use in the lab.

Reference: FLL by Csele page 187

This is a multi-part lab. After 100 minutes, disassemble the laser components and begin again next lab period. ALL EQUIPMENT must be DISASSEMBLED and SAFELY PACKED by 115 minutes - NO RUSHING the lab at the end since this is extremely expensive and sensitive equipment and must be stored carefully! The laser will be reassembled, aligned, and Parts B and C will be completed during the next lab period(s).

For the last lab period, we will spend 50 minutes attempting to align as many lasers as possible. It is unlikely that every group will have a working, aligned laser by this point so we will then regroup into larger groups centered around each of these working lasers and complete parts B and C below. Both parts B and C should take about 40 minutes to complete.

Part B: Observing Modes

TEM01 Mode output, Copyright John Wiley & Sons, 2004 By aligning the mirrors carefully several TE modes may be made to oscillate in the laser, for example TEM01 (also called 'donut' 01 mode) as seen in the photograph to the left


Align the mirrors to create several different modes. For each unique mode, draw the beam shape (you may want to expand the beam using a lens onto a paper on the wall), and measure the power of each mode. At a minimum be sure to obtain TEM01, TEM11, and at least two other unique modes (e.g. TEM03). Now align the mirrors for TEM00 and measure the power output on that mode as well. This is trickier than it sounds and requires precise adjustments of both rear optic and output coupler. Do not vibrate the table when aligning the optics. Does TEM00 mode mean maximum power for a laser ?? Include observations of power output vs. mode selected in your report.

Reference: FLL by Csele page 185

Part C: Brewster's Angle and Cavity Loss

Mount a clean glass slide on a rotating stage so that it intercepts the intra-cavity beam at an angle varying from 0 to 90 degrees. Rotate the slide until the laser oscillates and set the angle for maximum power output (this is Brewster's angle). At this stage, adjust the horizontal adjustments only of the cavity mirrors to maximize output power. Now read this angle which _is_ Brewster's angle (the stage will, of course, indicate some arbitrary angle). Including this bogus line into your lab report will result in a big loss of marks. Now tabluate data for output power vs. angle in both directions until the loss imposed by the slide extinguishes oscillation (every two degrees is sufficient). You will now have many readings of output power vs. angle theta that can be plotted, especially important though are the angles at which the laser ceases oscillation and the angle of maximum power output.

An alternate way of finding zero angle The actual angle read from the rotary stage must be calibrated by determining an offset as per the method of lab #1. There are two methods to do this. First, try aligning the glass slide perpendicular with the optical axis of the intra-cavity beam. TO do this, gently wobble the slide observing the reflection from the surface which is projected back onto the surface of the plasma tube cathode. When the reflection is aligned in the same vertical plane as the axis, the zero angle is found (record it). Now, knowing Brewster's angle (i.e. the angle at which maximum output power occurs), compute the index of refraction of the slide and use this value in further calculations.

The second method isn't as good and relies on assumptions. Assuming the first method does NOT work (it usually does so this is a last resort): You know the angle at which maximum output occurs (this is Brewster's angle - remember that this is the offset angle from where the beam would be perpendicular to the slide) and so assuming the index of refraction is n=1.54 (Brewster's angle is hence 57 degrees), the actual angle may be found and all angles calibrated from there. Obviously the first method is preferable.

Now, determine the gain of the laser. You can do this by equating gain to total loss at the lasing threshold. Total loss caused by both the inserted loss of the glass slide (which depends on angle and n of the glass) as well as losses in the mirrors and tube windows.

Analysis may now proceed by equating gain and losses in the laser. MEASURE the length of the plasma tube inside the HeNe tube (actual gain element) and the attenuation length (as was done in lectures and the previous lab). Measure loss at the OC as well using the same methodology as lab #4 (i.e. using a lab bench HeNe laser to measure transmission directly) ... this is outlined as Part D below but can be completed at any time when the laser is oscillating. This is another bogus line to detect cut and paste operations. Estimate the loss at the HR (measure the output from the OC as well as the HR at a given power output to do this in a similar manner to that used in the previous lab). Research antireflective coatings on the Melles-Griot website to determine an estimated value for loss at each window. These are plane (zero degree) glass windows with an antireflective coating on each side. They are among the very best coatings available (often "V" type coatings) so assume a low value if you find a range given. Now equate all losses (including the off-angle slide) and determine gain from this ... this will require formulation of a new threshold gain equation.

You MUST use the modified gain threshold equation to compute the gain here, NOT the simplified "sum of losses" technique which is not nearly as accurate. Derive the equation using separate attenuation and gain lengths as done in lectures. Assume an attenuation of 0.005m-1 in the gain medium. Like the prelab, estimate the uncertainty on the gain (i.e. the "+/-" figure) by computing the gain at three angles for each angular "side" of the slide where oscillation ceases (hence six separate computations of loss, and six computations of small-signal gain, are required).

Several lecture periods will be devoted to this lab and to explain how to derive the threshold gain equation for use in this part of the experiment: it is important that you attend these classes.

Reference: FLL by Csele section 4.9, page 106 as well as LM by Csele section 2.4, page 45

Brewsters Angle Experiment

Note the stray beam produced by reflection from the slide. BLOCK STRAY BEAMS using textbooks and cardboard pieces as necessary.

Part D: Determining Optics Parameters

In order to accurately describe losses in the laser, the exact reflectivity of the cavity optics is required. In the same manner as lab #4 measure the reflectivity of an OC optic (be sure to preheat the lab bench HeNe before measuring this parameter). Next, at some point during the lab, measure the power emitted from both the OC and HR of the laser you have built in this lab. From these measurements you can (a) calculate the transmission then reflectivity of the OC, (b) calculate the intra-cavity power then (c) calculate the trasnsmission and then the reflectivity of the HR. These measured values will then be used with the model to determine the gain of the laser.

Part E: Summarizing the Laser Parameters

Modify the gain threshold equation to include the windows and calculate the threshold gain of the system. This is the normal threshold gain of the laser under normal operating conditions. In an operating, CW laser, gain "burns down" to that level.

From Part (C) above, you will have determined the small-signal gain of the laser. This is the maximum gain that the amplification medium can deliver and is delivered only when intra-cavity power levels are very small (hence the term "small signal" gain). Think about the experiment in part (C) - gain was measured with the laser barely oscillating, the smallest signal for intra-cavity radiation possible.

Report both in the lab, and for g0 report it with a TOLERANCE (+/- figure). SAVE THESE RESULTS as they will be required for the next lab.

Lab Report

This is a three week lab and so the lab writeup is FAR more intense than many of the simpler labs in this course. It is also worth FAR more than most labs in this course. The lab will be quite large compared to others you have written this term and will consist of a good deal of mathematical analysis. Ensure you understand the concepts involved here and make an honest attempt to perform the analysis: it WILL be on the final test!

Refer back to the first lab for advice on how to write a full lab report.

Hand In a WORD PROCESSED (not handwritten) lab report (bound in a folder) with the following contents: