The alignment procedure of two types of lasers using two different techniques is covered in this lab: large-bore lasers using a coaxial HeNe beam and a dye laser using the fluorescence of the amplifier itself. This is a two week lab in which students in small groups will complete both parts alternately. One lab report is required for both parts, which have separate questions to be answered (the submission requirements are found at the end of this lab outline).

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

PART A: Alignment of Large Bore Lasers

In the high power laser lab (V15), an MPB IN-100 carbon-dioxide laser is used for alignment practice. Being a large-bore laser (> 10 mm), the laser is aligned using a HeNe laser beam aligned coaxially to the plasma tube.

Introduction

This lab introduces students to the alignment procedures for a large-bore laser. In PHTN1300 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. In the case of our carbon-dioxide laser this is easily possible because the OC is made of zinc-selenide which transmits red light ... some CO₂ lasers use germanium or silicon optics where are quite opaque to all visible light.

PreLab

• Read the Standard Operating Procedure (SOP) for the MPB IN-100 laser
• Research alignment procedures in Csele Section 6.13 and in the Amateur Scientist September 1971
• Take a look at the alignment procedures in CO2Alignment-SolidMirrors.pdf from the old CORD curriculum. This procedure is for a laser with an OC which is opaque to visible light.
• Read the basics of beam profiling on the Spiricon laser instruments site, specifically the article on the Benefits of Beam Profiling.

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

• KEEP THE DOOR CLOSED when the laser is operating
• SAFETY GLASSES are mandatory
• DO NOT run any other laser in the lab at the same time

Setup

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!

With the power supply OFF, begin by removing the cover of the laser (TWO people are required to do this) if not already done. 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. 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, 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.

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.

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.

When the mirrors are fully aligned, remove all intra-cavity cards. Ensure you are wearing safety glasses (suitable for 10.6mm), 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 laser incorporates the usual ten-second safety delay 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.

Close the shutter, 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 and select View. From the view screen select a 4-times scale 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. 'Tweak' all mirrors using the standard 'walking' procedure in each direction (horizontal and vertical) for optimal power output. The full-scale range of the meter may be increased as the laser power increases. When optimized, 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 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!

PART B: Alignment of Dye Laser Optics

An Innova-90 argon-ion laser is seen here pumping a Coherent 599 Dye Laser. A similar, shorter, version of this laser is used in this lab in which students align the complete cavity optics (HR, OC, and Fold mirrors).

Introduction

This lab introduces students to the alignment procedures for a dye laser. By following the procedure in the Coherent user's manual, a complete optics alignment will be performed. As well as the user's manual, a set of notes are provided in the lab outline to guide the student through the procedure safely.

PreLab

• Read the Standard Operating Procedure (SOP) for the Coherent 599 Dye Laser. This document is specific to the longer 599 laser, this lab employs a shorter version of the same laser (with the same optics setup).
• Print the Alignment Procedure extracted from the User's Manual
• Print the Lab Outline with the entire lab procedure

There are several optical dangers involved in this lab however the most dangerous is the argon-ion pump beam. Safety glasses suitable for 488nm - 514nm at 5 Watts are required. Be careful not to intercept the pump beam with any object since it will start fires as well as burn flesh if carelessly positioned!

• KEEP THE DOOR CLOSED when the laser is operating
• SAFETY GLASSES are mandatory

Procedure

Follow the procedure in the Lab Outline document to remove, then re-install and align, optics in the laser.

In general, you will be removing the cavity optics (HR, OC, and Fold Mirrors) from a working laser and re-aligning the entire cavity. The pump mirrors, which focus pump radiation from the argon-ion laser onto the dye jet, are not to be aligned in this lab due to time constraints ... you will begin at step 14 of the alignment procedure provided in the user's manual.

Knowing the dye in use (and hence the radius of each mirror used), and identifying the location of each mirror in the laser, identify, before the lab, the order in which mirrors will be adjusted.

Lab Submission:

For this experiment, an abbreviated lab report is required (word processed, never hand-written). Answer each question separately (e.g. Q1., Q2, ...., each with it's own paragraph), do not combine all questions into a single answer. Submit the lab report in a folder or binder NOT simply a pile of loose, stapled papers!

Large-Bore (CO₂ Laser Alignment):
1. Prepare a lab report of the entire alignment procedure of the laser. Include sketches (at least two) as required to illustrate steps in the procedure and appearance of reflections and beams as seen on the cards and what each means (i.e. the source 'surface' of each reflection).
2. Include at least two pyrocam outputs to show the mode structure of (a) an optimized laser output and (b) an unoptimized output.
3. Report the final positions of the micrometer heads for each mirror. These are labelled M1 through M4. (You may need to look at a tutorial on the web on how to read a micrometer properly)
4. Outline, in point form, the modified procedure required to align an OC which, unlike ours in the lab, is opaque to visible light (e.g. germanium). See the document on alignment and assume that we have already aligned the HR (by the same procedure used in our lab with our MPB laser). The opaque OC, removed to align the HR (since we cannot simply pass the HeNe beam through it as we do in this lab) is then to be aligned and re-installed. As well, explain WHY we do not need to be concerned about misalignment caused by vacuum/pressure in our laser.
5. Research (on SAM's - under "Homebuilt CO2 Lasers") how spot size is affected by optics and how to compute spot size. 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. Define the optical path as computed for this question (how it was determined). Does the spot size match what was observed in the lab (i.e. the burn spot)? If there is a discrepancy, explain it based on the criteria used to define "spot size" itself.
6. Calculate the Fresnel number in order to predict the efficiency of the optics for this laser


Dye Laser Optics Alignment:
7. Prepare a lab report of the entire alignment procedure of the dye laser. Include sketches as required ( at least two) to illustrate steps in the procedure and appearance of reflections as seen on the cards and what each means (i.e. the source of each reflection). It may be required to show the relative position of the reference mark (below) on the diagram(s).
8. How did we originally get the 'reference mark' on the far screen? Outline the procedure to get this mark in the first place during setup of the laser (before aligning or installing any of the other optics in the laser).
9. In order to make the laser tunable, a two- or three-plate birefringent filter may be used (this same filter is used in our tunable Ti:Sapphire laser as well). Describe how a birefringent filter works (being VERY specific to describe how light interacts in the filter elements) and describe how such a filter is tuned (i.e. how you use such a filter as a wavelength selector and how wavelength is set).
10. Some dye lasers are in the form of a 'ring' laser (see Coherent.Com as well as other sites on the web for a look at newer ring lasers available) with four cavity mirrors. Unlike standing-wave lasers, these lasers do not have discrete transverse modes and so are truly 'continuously tunable' for spectroscopy. Include a diagram detailing the optics of a ring dye laser.

Copyright (C) Niagara College, Canada, 2005-2010
Photo Copyright (C) John Wiley & Sons, 2004
This page is part of the PHTN1400 Course Page