PHTN1300: Lasers and Light Sources
Gas Laser Cavity Optics Lab (2018F)

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 in Part A (this is a learned skill, and more of an art than science) as well as observations of TE Modes for Part B.

The second period will consist of a second alignment (Part A again, although this should go much faster than in the first period) and Part C and D (determining gain and optics parameters).

The third period will be available for completion of the experiment for those who have not yet completed all parts of the lab.

Submit a lab report only after all sections are complete (i.e. all three weeks).

This lab is worth one-third of the total practical marks in the course, as such the report 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 (Do this at the beginning of each lab period)

Since you have already read section 6.13 in Fundamentals of Light Sources and Lasers (FLL) by Csele outlining the alignment procedure used in this lab (there is certainly no time available in the lab to complete this and failure to come prepared will inevitably result in failure to complete the lab in the time allotted), follow that procedure to align the laser optics as follows ...

The alignment process will be demonstrated at the beginning of the first lab period. Be sure you are in attendance and ON TIME since this demonstration will not be repeated.

Assembly of the Components

Details of the Setup - Mounts FIRST, prepare by obtaining two tube mounts and two mirror posts. These are labeled "MELLES GRIOT" and are NOT the same as the ones in the lab benches! Be sure to obtain two metric screws for the posts plus two washers as well: while the rest of the bench is imperial, the Melles-Griot parts for this lasre are metric so do NOT mix the two!

Do NOT, for now, obtain a tube nor power supply (wait until the mounts are assembled).


Details of the Setup - Tube Mounts SECOND, prepare by screwing the two tube mounts to the breadboard. Since the holes are one inch apart, place the mounts about seven or eight inches apart in the center of a single two-by-two breadboard leaving room on either side for optics. Remove the top clamps so the tube can be placed in the cradles. Screw the posts into the breadboard as well as shown: while the lab benches consist of two small breadboards side-by-side, ALL components for this laser including the tube and mirrors must be situated on the same optical table as the tube for stability.


Details of the Setup - Tube THIRD, with the tube mounts installed securely, obtain and mount a Melles-Griot WPR252 tube on the breadboard in the two mounts. This is a custom-built plasma tube with anti-reflective coated windows instead of mirrors. The anode (which is electrically "live") should face the HR (right side of this photo) to reduce the possibility of shock when aligning the OC. Assemble the top clamps and gently tighten the tube into the mount. Do not connect a power supply yet. Now install the rear HR optic using a metric screw and washer to the mirror post which is already fixed to the table. For now, install only the HR - the OC will be installed only after the HR is aligned by autocollimation.


Aligning the HR (Procedure)

FOURTH: Before autocollimation can take place, the rear optic must be bore-sighted so that a clear path exists down the tube axis to the center of the center of the mirror (i.e. light clears the bore hence the name "bore sighting"). 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 potentially moved side-to-side by loosening the screw and washer) and the lamp WILL need to be adjusted for this. Use the regular CFL desk lamp for this procedure. Raise or lower the lamp until light from the lamp is seen when looking through the HR towards the front of the tube. At this stage, you must be able to see down the bore of the tube to sight the lamp ("close" does not count here and if in doubt, ask, since failure to bore-sight will make alignment impossible). The previous photograph (Third step, above) shows the desk lamp aligned for this purpose.

Details of the Setup - Autocollimator FIFTH: install a beamsplitter (from the lab bench supplies) in a 3-point mount with a post (also from the lab bench) in a location between the tube and where the front optic (OC) should go (but has not yet been installed). Align the beamsplitter at 45 degrees so that you can see a reflection down the tube. The CFL desk lamp is now aligned so that light from the lamp passes through the beamsplitter and down the tube reflecting from the rear HR 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 procedure from the prelab (in the Fundamentals text


Details of the Setup - Autocollimator SIXTH: The rear optic must now be precisely aligned so that it is absolutely perpendicular to the bore of the tube. Verify that you bore-sighted the lamp image correctly (i.e. by looking through the HR from the outside towards the tube, and can see down the bore of the amplifier tube to see light from the desk lamp. It is important that you can see straight down the actual bore and not just a "bright spot" composed of total internal reflections. 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 light from the lamp reflecting from the rear optic and adjust each screw until the image is centered - it will be a bright ORANGE dot. 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 orange dot in the center of the tube bore - it appears as a dot with a concentric dark ring around it ... it is important that you see the entire dot and that it is centered (not just "bright"). The ring is perfectly centered and round when the optic is adjusted correctly.


Filament image as seen in the beamsplitter This is what you can expect to see via the beamsplitter (although the orange dot is very small). You will need to move your head, or the beamsplitter, or both to see the reflection from the HR as an orange dot.


Aligning the OC (Procedure)

Details of the Setup - OC

SEVENTH: Once you are satisfied that the rear optic (HR) is aligned, remove the beamsplitter and lamp and install the front optic (the OC) as shown onto the mirror post using a metric screw and washer. 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 this time). 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 unimpeded through the amplifier bore. 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, while the NEGATIVE terminal connects to the cathode (the large aluminum cylinder inside the tube).


STOP!!!

Before proceeding have the lab instructor do a safety check of the wiring, check the polarity of the tube (anode to positive), and clean the tube windows with an optics alcohol wipe.

Rocking the Mirror EIGTH: Ensure the beam from the OC is terminated onto a white paper or book. Switch on the power supply and the tube will glow. Front optic alignment is done by 'rocking' the mirror as per the procedure in the text you researched as part of the prelab. 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.


NINTH: 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.

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 In this part of the lab, you will align the mirrors carefully to produce several TE modes, for example TEM01 (also called 'donut' 01 mode) as seen in the photograph to the left.

Optimize the laser, first, for maximum output power then align the mirrors to create several different modes. For each unique mode, draw or photograph 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, you need to produce, identify, and record the output power at least four unique modes (including TEM00). Normally, you can obtain TEM01, TEM11, and at least one other unique mode (e.g. TEM03). Be sure to align the mirrors for TEM00 as well and measure the power output on that mode. The process of adjusting the laser to produce modes is 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

    When this part of the lab is complete, you should have:
  1. Drawings or photographs of at least four unique TE modes including 00
  2. Power measurements from the OC of the laser when produding each mode above

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). 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.

Brewsters Angle Experiment
The glass slide in a filter holder mounted on a rotary stage. Note the stray beam produced by reflection from the slide. BLOCK STRAY BEAMS using textbooks and cardboard pieces as necessary.

    When this part of the lab is complete, you should have:
  1. Power measurements from the OC at divisions of every two degrees of the glass slide between the angles where the laser is seen to oscillate

Analysis of the observed data:

Now, determine the gain of the laser. This is done by equating gain to total loss at the lasing threshold (i.e. the point where the total loss caused by both the inserted loss of the glass slide as well as losses in the mirrors and tube windows is equal).

Analysis may now proceed by equating gain and losses in the laser. You need the following to do this:

  1. 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)
  2. 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
  3. Calculate 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)
  4. Research antireflective coatings on the CVI Laser Optics 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). Check under "Shop Optical Products" then "Windows and Optical Flats". They are two-sided coatings (W2) designed for one specific wavelength (they are not broadband) - specifications will be listed on the right side
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

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 the previous lab 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. Both measurements must be made at the same time on the same laser (and without adjustments made between measurements of the OC and HR). From these measurements you can (a) calculate the transmission then reflectivity of the OC, (b) calculate the intra-cavity power then (c) calculate the transmission and then the reflectivity of the HR. These measured values will then be used with the gain threshold equation to determine the gain of the laser.

    When this part of the lab is complete, you should have:
  1. Power measurements from the OC and the HR of a working laser
  2. Transmission measurements for the OC of the laser (in a similar manner to the previous lab

Summarizing the Laser Parameters (Homework)

Modify the gain threshold equation to include the windows and calculate the threshold gain of the system. This represents the normal threshold gain of the laser under normal operating conditions (i.e. when producing full output power). 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.

Assignment

This is a three week lab and so the lab writeup is more intense than many of the simpler labs in this course. It is also worth more marks than most labs in this course. The lab 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!

The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, the student's name and ID number, and the names of your lab partner(s).

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 (with the title "Question 2") and question 3 starts at the top of a different page (with the title "Question 3"), etc. You'll have, therefore, at _least_ eleven pages in this report (but probably a lot more than that).

The lab must be submitted in a report cover (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 outline, used for all condensed labs in this course, will result in deduction of marks

PLAGIARISM WARNING: All labs are to be done individually so while the data will (and must) be the same as your lab partners, each lab and each analysis presented must be your own unique work. Data presented must be your own as well, not "borrowed" from another group. Where two or more labs are copied, each student involved will receive a mark of zero and will be required to meet with the associate dean who will assess further penalties (including expulsion for repeat offenders).

    Observations:
  1. During the experiment you observed many different transverse (TE) modes. Show several drawings/photos of various modes observed on your laser (including TEM00) along with accompanying observed output power for each. Identify the mode subscript where possible (e.g. 00, 01).
  2. Show a graph of observed output power (in mW) versus glass slide angle (in degrees). As always, include a proper title, axes labels, grid lines, and numeric values on each axis.
  3. Describe the procedure for calculation of the index of refraction (not assumed) from the data and show complete calculations to find this value.
  4. Detail the calculation of the transmission of the glass slide as follows:
    1. Present a complete set of example calculations to calculate the optical transmission of the slide at one angle where lasing extinguishes (you observed two angles where this happens, just show calculations for one). All steps and all substituted values (such as the index of refraction) must be shown. Report the transmission of the glass slide at both angles where lasing ceases.
    2. Graph output power vs. transmission of the glass slide (not just ANGLE, but actual TRANSMISSION in percent as calculated every two degrees where the laser was seen to oscillate).
  5. Knowing the transmission of the slide at the point where lasing extinguishes from the previous question, report the transmission at this point where lasing extinguishes as "xxx +/- yyy %" where "yyy" is the error determined. There must be two transmission figures here: one for each angle where lasing was determined to cease on each side of the center (Brewster's) angle. Report both with uncertanties - the uncertainty will be different for each.
  6. Show observations and calculations of the reflectivities of both the OC and HR. This was done in the same manner as in the previous lab (but using the optics on this specific laser) where the transmission of the OC was measured directly and the transmission of the HR calculated based on power measurements from both ends of the laser.
  7. Summarize miscellaneous parameters required for threshold gain calculations including:
    1. State, from research on the web, the estimated loss (numerically) at the windows (cite a source explicitly including a full URL - it must be a full URL and not a "google search string")
    2. The estimated attenuation in the amplifier tube (cite the source)
    3. The length of both the GAIN element and the attenuation length as measured in the lab (hopefully you realize the gain length is not simply the distance between the mirrors nor the length of the tube)
  8. Show the development of the formula for small-signal gain of the laser (determined when the inserted loss of the glass slide as lasing extinguishes is used). This is the usual gth equation with all losses included.
  9. Using the above formula, calculate GAIN (in m-1) showing all formulae used as well as one complete set of mathematical calculations at one angle. Now, summarize in a table the gain calculated at the other angle at which lasing ceases as well as angles required to compute the uncertainty figure (i.e. at a small offset from the two angles at which lasing ceases). Express each gain (two figures) as "xxx +/- yyy m-1". The error figure should be computed in a similar manner to that employed in the prelab (and will relate to gain figures outlined in the table).

  10. Discussions:
  11. Provide a description of TE modes including:
    1. Where they originate (Chapter 9 of the FLL text covers this well under RESONATORS)
    2. How rectangular mode numbers are assigned (i.e. the "xx" in TEMxx)
    3. What TEM00 mode is and the intensity (side) profile of this mode (A diagram is required here)
    4. What modes represent highest output power
    5. When the use of TEM00 mode is desirable (give an example application, not simply "when purity is desired").
  12. Our laser tubes use flat windows. Is there anything special about these windows from an optical perspective? Can any gas laser (e.g. an argon ion) use flat windows like this or is a Brewster window required? If a Brewster window is used, what effect does this have on the output beam (as opposed to a flat window like ours)?.

Note that the calculation of gain, use of the Fresnel equations, and the concepts of gth and g0 are all "fair game" for exam questions! Be sure to do them here as they will be on the final test.