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.
A pyrocam view of the mode structure of a carbon-dioxide laser beam. Beam analysis is also a part of the lab.
In the high power laser lab (V115B), 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.
The prelab assignment is worth 20% of the total lab mark and is due at the beginning of the lab period. Late marks are not assigned if the prelab is not received at the beginning of the lab: you lose 20% of the total lab marks immediately with no recourse if it is not received upon entering the lab (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).
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!
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 DC-excited 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!
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.
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
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
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).
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.
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.
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.
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:
A few references as required to answer questions from the lab:
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:
Cavity Stability and Spot Size:
Longitudinal Modes in a Laser: