PHTN1400: Principles Of Laser Systems
High Power Laser Alignment (2018)

Introduction

This lab introduces students to the alignment procedures for a large-bore laser. Last term, you had already aligned a small-bore laser using both an autocollimator as well as rock-and-search techniques. While these techniques work for most lasers, large-bore lasers can use a HeNe beam for alignment (the technique is outlined in section 6.13 of Fundamentals of Light Sources and Lasers by Csele - see diagram 6.13.1). In the case of our IN-100 carbon-dioxide laser this is easily accomplished because the OC is made of zinc-selenide (ZnSe) which transmits red light ... some CO2 lasers use germanium or silicon optics which are completely opaque to all visible light and require a slightly different technique. Once alignment is complete, beam mode structure is examined using a pyrocam beam profiler.

In addition to alignment and beam profiling skills, the lab will also provide a review of concepts from last term including the use of the simple model to predict output power and will expand upon that model by considering longitudinal modes in a gas laser and how homogeneous or inhomogeneous models apply.

Pyrocam Output of a CO2 Laser Beam

A pyrocam view of the mode structure of a carbon-dioxide laser beam. Beam analysis is also a part of the lab.

MPB-CO2 Laser, Photo (C) Wiley 2004

In the high power laser lab (V15A), an MPB IN-100 carbon-dioxide laser is used for alignment practice. Being a large-bore laser (> 10 mm diameter), the laser is aligned using a HeNe laser beam aligned coaxially to the plasma tube.

BEFORE this lab ...

You must have successfully completed both the Laser Safety Quiz on Blackboard (certifying that you have seen the video and understand the basic safety procedures required in our lab) as well as the Safety Glasses Quiz in lab #0. Both quizzes should have been completed last week.

If a student shows-up to the lab WITHOUT successfully completed quizzes, he/she will be DENIED entrance to the lab, will be marked absent (with a 5% penalty on the total mark in the course), and placed on COURSE CONDITION (in which case you will be expelled if you fail to successfully complete both before the next lab period).

There is no negotiating safety requirements, period.


Prelab:

Safety

The carbon-dioxide is a class-IV laser producing, potentially, 200 Watts of power. Safety glasses for 10.6μm are required. Be careful with the position of the beam since it will start fires as well as burn flesh if carelessly positioned!

In a Class IV laser lab, use of safety glasses is not optional. Anyone found in the presence of an operating laser without safety glasses is subject to immediate expulsion!

Experiment

When you enter the lab the laser will be refilled with a working charge of carbon-dioxide gas mix however the optics (HR and OC) will purposely be misaligned inhibiting lasing action. This is a 'standard' design of industrial laser employing multiple plasma tubes so alignment will be made more difficult by the fact that total-internal reflections will occur with the alignment beam!

CO2 Lab Setup
The lab setup showing the front of the laser (with mirror adjustment micrometer heads), the Coherent 201 power meter, and the alignment HeNe.

Alignment reflections With the power supply OFF, begin by removing the cover of the laser (TWO people are required to do this) if not already done. Following 6.13 in the text, setup a HeNe laser about 1m away from the front of the laser and align the beam of a HeNe laser to be coaxial with the plasma tubes. The alignment beam must pass through the OC, through the first amplifier tube, bounce off the two mirrors at the end of the laser, through the second amplifier tube and strike the HR before making the return trip through the laser to exit the OC again Ask yourself: how does one know that the beam is a "direct path" through the tubes and not a total internal reflection?. It is easier, to begin, if the HeNe beam is already aligned such that the beam is parallel to the table (measure the height at the laser apeture and at the entry point to the CO2 laser).
Using business cards with small holes (as demonstrated at the beginning of the lab), find the alignment beam and begin to align the HR first followed by the OC. By blocking the HR one may ascertain which beams are attributed to the beam from the HR at the end of the long path.

DO NOT ALIGN THE REAR 'FOLDING' MIRRORS, only the OC and HR, both at the front of laser and aligned via four micrometer adjustments.

Optical Path

The shutter, shown here immediately before the HR, must be opened during the alignment procedure. To do this, turn the safety key (on the rear of the laser supply) ON and switch the shutter control to EXT. This will open the shutter while preventing the main high-voltage power supply from activating.

Step 1: Setup the Alignment Beam

Initial Alignment
Begin by aligning the external HeNe beam to enter the OC, travel through both tubes, and emerge, as shown, from the second tube just before the shutter. The beam should be as round as possible - crescent shaped images usually indicate total internal reflections from the walls of the tube. Inspection of the amplifier tubes will also reveal whether the alignment beam is striking the tube walls, producing a beam visible in the outer cooling water jacket, or passing through unfettered Ask the professor to demonstrate what total internal reflection looks-like.

Measure the length of the optical path through the discharge tubes since this parameter is required to answer a question at the end of the lab. As well, estimate the diameter of the inner-bore of the laser plasma tube (Careful: the inner bore is the most innermost tube).

Step 2: Aligning the HR

HR Spot Open the shutter. Now, align the HR (adjustments M1/M2) to reflect the alignment beam off the HR and back through the amplifier tubes until an image is seen on the OC like that in the photo to the left. In this photo, taken of the OC from the inside of the laser, the central, bright spot is that of the HeNe beam entering the laser from the outside. Also visible is a weak reflection from the HR - having made a round trip through the dual amplifier tubes, reflected off the HR, and back through the tubes again. Align this reflection to be on top of the initial alignment beam (the beam and the HR are now perpendicular).

Step 3: Aligning the OC

Align the OC (adjustments M3/M4) so that the first-surface reflection from the alignment HeNe (on the outside of the laser) is on top of the incoming HeNe beam. This spot will be seen on the front of the HeNe laser itself. Assuming the OC has parallel surfaces, this mirror is also perpendicular to the alignment beam. Now, inspect the OC ... you will see several reflections. 'Tweak' the mirrors until all are as close as possible to being on top of each other.

Step 4: Test Firing and Fine Mirror Adjustments

When the mirrors are fully aligned, remove all intra-cavity cards. Ensure you are wearing safety glasses (suitable for 10.6μm), and that the Coherent 201 power meter sensor is placed into the path of the beam (i.e. DO NOT allow the output to strike the HeNe). Install a beam dump into the beam path as well. Turn on the water cooling supply, ensure the laser is properly filled with gas (consult the instructor), and turn the laser ON (the shutter must be set to INT first). The laser incorporates the usual ten-second safety delay (required for class-IV lasers) as well as interlocks for the cover and water flow. The cover interlock must be bypassed as required when performing maintenance. Be sure to keep clear of the area marked 'DANGER HIGH VOLTAGE': electrodes here reach a potential of almost 25kV at 60mA - enough to ensure certain DEATH!. This area is encased in a plexi shield, do not insert fingers or any other object into or near the safety cover.

When the discharge is observed, open the shutter to enable laser output. Assuming output is observed (as a deflection of the power meter) this means mirror alignment is at least 'close' but probably nowhere near 'optimal. If no laser output is observed, attempt to gently move one adjustment screw at a time until lasing occurs. Note the position of the adjustment screws.

NOTE the position of the micrometer heads M1-M4 so that you might return to this original alignment if required. If you cannot read the micrometer, draw a picture in your lab book for now (and determine HOW to read it properly later).

Now, optimize the output using the standard WALKING procedure from last term (adjusting the horizontal and vertical micrometer adjustments alternately).

When satisfied that maximum power was achieved, note the micrometer settings again, close the shutter (via the CLOSE button on the front panel of the power supply), place the pyrocam in front of the laser output, open the shutter and observe (and capture) the mode structure of the beam. To operate the pyrocam first apply power to the camera itself then start the "Pyrocam 3 Control Console" on the PC. Ensure the system is set for 48Hz CHOPPED and select View. From the view screen select a 4-times scale (or higher) and select a 3-D view from the settings menu. You can save a bitmap from the file menu.

Close the shutter, place the power meter back into the beam path, and open the shutter again to measure the power of this mode.

Repeat the procedure, 'Tweak'ing the mirrors to generate different modes. Capture these modes using the pyrocam and accompanying powers using the meter.

With the fully optimized setting, observe (and capture) the mode structure using the pyrocam. As well, place a burn paper into the beam path and open then close the shutter QUICKLY to produce a burn pattern from which the beam diameter may be experimentally measured.

Oops, Burn Paper Ignites! The laser can usually produce outputs of over ten watts and has been seen to produce up to twenty. Careful as the beam may well start the burn paper on fire!

With the laser turned OFF, measure the length of the optical path (i.e. the cavity length) and other lengths as required to answer questions in this lab.


CO2 laser optimal pressure Note that during extened operation it may be necessary to repump the laser and fill it with fresh gas. The photos to the right show the laser operating at too high (left photo) and optimal (right photo) pressures. With a high operating pressure the discharge appears thin and filamentary with a dull violet colour.

CO2 laser gas recharge module The gas recharge module (permanently attached to the laser), is used to evacuate the tube to below 100mTorr then refill it to the optimal gas pressure of 2.0 Torr to 2.4 Torr. The valves are shown to the left.

To evacuate and repump the tube:

  • Turn on the main power switch (left) and open both tube valves (1 and 2)
  • Turn on the vacuum pump switch (beside valve 3)
  • Press the evacuate button (valve 3) to lower the tube pressure. Assuming the tube is still powered, do not let the tube pressure fall below 100mTorr.
  • Now, to refill the tube, press the gas inlet button (valve 4) very briefly. The pressure will likely rise too high, so simply open the evacuate button (valve 3) to lower it to operating pressure (between 2.0 and 2.4 Torr).

  • References:

    A few references as required to answer questions from the lab:

    Lab Submission:

    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 ...) answer each 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 5) as a single paragraph but be sure to separate with titles.

    Questions must be identified at the top of the page as QUESTION 1, QUESTION 2, etc. as shown to the left

    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 (no less than 10% but up to 25% for complete failure to follow the format given).

    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.

      Observations from the lab:

    1. Prepare a description of the entire alignment procedure of the laser. Include sketches (at least two) as required to illustrate steps in the procedure and sketches of the appearance of reflections and beams as seen on the cards and what each means (i.e. the 'surface' from which each reflection seen originates). These sketches will illustrate the beams as they appear on the alignment cards - both at the HR end and the beam reflected back towards the OC end. Pictures are unsuitable here unless actual reflections from various optical elements are identified directly on them (i.e. do NOT use any photos from this lab - they are unsuitable and will result in a big loss of marks - since you read through the lab ahead of time you will have taken any required photos of drawn any required sketches while in the lab).

    2. Include at least two pyrocam outputs to show the mode structure of (a) an optimized laser output with maximum output power and (b) an unoptimized output. Show the measured power of each (in Watts) and report the final positions of the micrometer heads for each of the four mirrors labelled M1 through M4. (You may need to look at a tutorial on the web on how to read a micrometer properly - report the micrometer positions in actual decimal units such as '0.123')
    3. Cavity Stability and Spot Size:

    4. Read "Resonator Stability" (section 6.9 in FLL by Csele) which describes how to compute g-parameters for optics - as a secondary reference you might also read the section "Useful Formulas for CO2 Laser Optics" on the page entitled "Homebuilt CO2 Lasers" on SAM's Laser FAQ detailing how spot size is affected by optics. Given the optic specifications from the SOP for our MPB laser, compute (showing all formulae used and all mathematical work) the predicted spot size at the OC and at the HR - you will need to compute the g-parameters first, be sure to outline this as well (these are not gain parameters, but rather optics parameters in this case with values between 0 and 1). If you chose the HR to be mirror #1, then it will have parameter g1 and the spot size at that optic will be w1. Use the formulae below since they are "generic" and work with all cavity arrangements:
      Spot Size Formulae

      Outline all calculations involved including calculation of g-parameters for each optic (show the formulas used and all substitutions made). Expect an answer between 1mm and 8mm and expect the OC spot to be smaller than the HR spot for this "long radius" cavity (ref LM pp.22 ... this is one of the most popular cavity configurations).

      As well as the calculations of spot size above, answer the following:
      1. The beam radii above (w) are computed based on the "1/e2" criteria. Describe what this is and how it is defined with respect to beam profile (i.e. explain in terms of how the edges of the diameter are even defined in the first place: again, this is discussed in SAM's laser faq).
      2. What was the measured spot size? Does the spot size calculated at the OC using the formulae above match what was observed in the lab (i.e. the burn spot)?

    5. Applying the above formulae to laser design:
      1. Calculate both spot sizes at the output assuming that the same IN-100 CO2 laser uses a 10m radius of curvature HR and a flat OC. Outline all calculkations (formuale used,and all numbers substituted including all used to calculate the g-parameters). Does this seem reasonable given that a TEM00beam should fill about a quarter of the amplifier volume?
      2. Calculate the spot size for the WPR-252 HeNe lasers used last term in PHTN1300. These lasers featured a flat OC and a 60cm radius-of-curvature HR. Estinate the cavity length and show all formulae, susbtitutions,and calculations.
      3. Using those same optics for the WPR-252, calculate explicitly and mathematically the range of distances possible between the mirrors for a stable cavity configuration. This will show why it was required to keep the entire laser on one bench.
    6. Longitudinal Modes in a Laser:

    7. You should have discovered that the output power calculated in the prelab varies radically based on which assumption of the medium (homogeneous or inhomogeneous) is used. As discussed in the text,one factor affecting the assumption of medium behaviour is is the number of longitudinal modes oscillating in the cavity. Calculate, showing all work for all parts below:
      1. the FSR of the cavity using the length measured in the lab
      2. the Doppler-broadened spectral width of the CO2 gas laser (expect an answer under 500MHz)
      3. the number of modes which will oscillate in this particular laser
      4. Refer to Fundamentals of Light and Lasers, sections 4.7 and 6.4, then explain, in your own words, the logic outlined in Laser Modeling section 3.4 which explains why sometimes a gas laser (normally considered a purely inhomogeneous medium) can be best described with a homogeneous solution.