Introduction

Many thanks to Mr. D. Turnbull of Central Supply who engineered the experiment. He is also the author of a key paper on the technique of photoacoustic spectroscopy

A brilliant 4.5W pump beam from an argon ion laser is seen here exciting a Titanium Sapphire rod.

Pre-Lab

• Read the article The Local mode Model and Recent Advances in Laser Based Photoacoustic Spectroscopy, Canadian Journal of Applied Spectroscopy, B.R.Henry, H.G. Kjaergaard, B. Niefer, B.J. Schattka, and D.M. Turnbull. This article will be made available in class for review.
• If using the DYE laser (n=6 spectrum), read the PROCEDURE for the experiment employing the Coherent model 599 DYE laser
• If using the Ti:Sapphire laser (n=5 spectrum), review the principles of operation of a Ti:Sapphire laser, including use of a birefringent filter.
• Research the operation of a lock-in amplifier
• Read the primer on the ICL-PAS technique



• Read the UPDATED NOTES for 2009 covering details of the setup procedure and calibration of the birefringent filter.

The Experiment

The laboratory setup using a Ti:Sapphire laser

The first seven parts of the experiment involve setup of the Ti:Sapphire or Dye laser. This can be a time-consuming and potentially dangerous operation and will be performed prior to your experiment by the professor

Ti:Sapphire Procedure
1. Put on correct laser goggles.
2. Turn on I-90 or I-200 argon laser as per SOP make sure beam is properly terminated in a beam dump.
3. The operational condition and alignment of the Ti: Sapph laser must be known to use this procedure. If the laser is improperly aligned and started serious damage to the Ti: Sapph crystal can occur. More thorough procedures for set-up and alignment can be found in the Ti: Sapph manual.
4. Turn on cooling water for Ti: Sapph - small valve "tee" from argon water. Water flow should be a slow trickle. Too much water flow can cause condensation on the Ti: Sapph crystal. Water flow can be checked by looking at the flow from the hose into the drain.
5. Start the mechanical light chopper. The chopper should be aligned so the argon beam passes through the f2 opening. The chopper should be run with a 50Hz TTL signal from a function generator (square wave)(BNC to Sync in). A BNC line from function generator should also be connected to the A/D trigger. The Sync out 2 should be connected to the lockin (reference or monitor connection).
6. With the argon laser power set low (~20 amps) close the shutter to the argon laser and remove the beam dump. Opening the shutter the beam should pass through the mechanical chopper and into the Ti:Sapph laser without obstructions. Set the micrometer dial on the birefringent tuning element to the setting that gives the highest power (check the laser log book for this info - approx 8.0). Turn the argon current up full (40 Amps @ full apeture ~ 5W). Lasing should be observed from the Ti: Sapph (as observed on power meter as beam is in NIR range). If lasing is not observed immediately close the shutter on the argon laser and replace beam dump - there may be alignment issues. Have everything checked by a qualified individual before proceeding.
7. If laser lasing, small adjustments to the HR adjustment screws and very small adjustments to the pump mirror adjustment screws can be made (rule of thumb - power should not decrease by more than 20% from a single adjustment). Do not adjust the periscope adjustment screws. If the laser stops lasing when turning the pump mirror adjustment screws do not continue adjusting - damage could occur if not done correctly. The HR adjustment screws can still be adjusted as damage to the laser cannot occur from adjusting these. (If the laser is not lasing a light sensitive detector can be used to maximize the fluorescence which will normally get the laser to lase if lasing stops due to adjusting the HR).

Dye Laser Procedure
1. Put on correct laser goggles. Remove any jewelry and watches
2. Turn on I-200 argon laser as per SOP make sure beam is properly terminated in a beam dump (Note that if chopper is in place that it is in open position).
3. The operational condition and alignment of the dye laser must be known to use this procedure. If the laser is improperly aligned and started serious damage can occur. More thorough procedures for set-up and alignment can be found in the dye laser manual.
4. Turn on cooling water for the dye laser, needle valve "tee" by argon water valve labeled dye. Water flow should be a slow trickle. Water flow can be checked by looking at the flow from the hose into the drain.

Note: with the configuration on the optical bench, care must be taken to not lean across the bench to open or close the aperture or adjust birefringent filter tuning element (see later) as lasers are typically designed to direct reflected beams upwards.
5. Close the I-200 aperture (open/close slide not the aperture dial). Start the mechanical light chopper. The chopper should be aligned so the argon beam passes through the f2 opening. The chopper should be run with a 50Hz TTL signal from a function generator (generator should read 500) (square wave)(BNC to Sync in). A BNC line from function generator should also be connected to the A/D trigger. The Sync out 2 should be connected to the lockin reference.
6. With the argon laser power set at 42 amps and the shutter closed, remove the beam dump. Opening the shutter, the beam should pass through the mechanical chopper and into the dye laser without obstructions. Set the micrometer dial on the birefringent tuning element to the setting that gives the highest power (check the laser log book for this info). Lasing should be observed from the dye laser. If lasing is not observed immediately close the shutter on the argon laser and replace beam dump - there may be alignment issues. Have everything checked by a qualified individual before proceeding.
7. If the laser is lasing, small adjustments to the HR adjustment screws and very small adjustments to the pump mirror adjustment screws can be made (rule of thumb - power should not decrease by more than 20% from a single adjustment). If the laser stops lasing when turning the pump mirror adjustment screws do not continue adjusting - damage could occur if not done correctly. The HR adjustment screws can still be adjusted as damage to the laser cannot occur from adjusting these.

Components of the Ti:Sapphire Laser including the intra-cavity gas cell. Shown with safety cover removed.

The actual experiment begins here ...

8. Insert the photoacoustic cell - Access to the photoacoustic cell mount can be obtained by removing the gray plastic cover on the metal Ti:Sapph cover or the dye laser cover. This cell must be handled very carefully as it can be easily broken. Close shutter on argon laser, insert cell into mounts, and open shutter. Once the cell is installed it can be aligned in the cavity by adjusting the cell mount screws until the residual blue light from the argon laser can be seen passing through the cell. Once lasing is detected on the power meter slight adjustments to the screws can be made to optimize the power (Cell mounting screws, HR, and pump mirror). The cell can also be slightly rotated side to side (make sure clamps are not too tight). Make sure windows are clean otherwise this will lead to a lot of window noise in the spectrum (Window noise can sometimes be seen as a constant background signal seen on the lock-in). A good PAS cell should have < ~20% power loss (you did note the original power?). Connect the microphone jack to the cell and a good 9V battery. Replace the gray plastic cover.
9. Turn on the power for the lock-in and A/D converter (on back). The two cables from the A/D (as labeled) should be connected to the channel 1 output signal from the lock in and to the power meter (Located on the rear of Melles Griot power/energy meter - 1V output connection). A/D channel 1 is for the power meter and Channel 2 the lock-in (cables are marked on the BNC connectors).
10. Set the lock-in- (For Parr 129: A-B input, TC=10ms, post 0.1 sec, filters set min and max respectively)(For SR530 lock-in: A-B input; TC=10ms, post 0.1 sec; filters set as follows BP,L1,L2 = in dyn res=high). Observing the lock-in signal while turning the birefringent filter (BF) micrometer screw manually through the tuning range one should see some kind of signal increase due to an absorption. If no signal is observed try turning up the lock-in sensitivity and/or switching the lock-in phase by 90 . Try turning the BF again. Once the largest signal is observed adjust the lock-in phase and sensitivity to maximize the signal.

The Princeton Applied Research (PAR) 129A Lock-In amplifier. Alternately, this experiment may utilize a Stanford Research SR530 Lock-In.
11. While observing the signal on the lock-in and the power on the power meter, manually rotate the birefringent filter micrometer screw through the tuning range and determine the region to scan (where a signal is detected). Once determined set the micrometer screw to the lower micrometer screw setting (always turning the screw up to the point to remove slack) and make sure the slide between the micrometer screw and the stepper motor is centred.
12. You are now ready to record a spectrum. (Presently, the experiment is controlled by an old DOS based computer with the programs written in Pascal). The program, V12scn.exe, is triggered to take readings with the chopper opening and will collect X number of reads/point from the lock-in and power meter (program reads 125 reads/chopper opening and user enters #reads (125 reads x user entered #)then turns the stepper motor by the defined increment (an increment =1 corresponds to 1600 steps per 1 full rotation of the micrometer screw). Turn on the computer if not already on. The program used is located in f:\niagara directory and is called V12scan.exe. Once in the directory, the IEEE card must be initialized by typing "Init" (runs Init.bat) which loads the IEEE card drivers. Typing "v12scan" will run the scan program.
13. A typical spectrum can be recorded with good signal to noise using a step=1, reads/point=10, and 8 full rotations of the micrometer screw (i.e. 7.0 to 9.0 on micrometer) in about 10 minutes. A quick first scan can be done to determine region of interest followed by a longer scan in that region ( 1st incr=2, 3 reads/point, entire tuning range. 2nd incr=1, 15 reads/point, ~4 rotations on the micrometer screw.).
14. When complete run the program fa.exe( type: "fa <filename> filename.fa" with arrows (< and >) around the argument filename). This will convert the data file recorded (laser power and lock-in signal) to micrometer setting on the BF versus the normalized signal (lock-in/power). This file may need edited to remove runtime error at the end of the file and correct the total # data points at top = #lines in file-2. This is a complete data file with data pairs representing micrometer screw reading and output signal. It may be saved to a floppy disc and graphed using a spreadsheet package.
15. For further analysis, this file can now be brought into spectra calc d:\sc (then type "sc"). In spectra calc go to arithmetic, programs, IDL-PAS. This will import the spectrum. Watch the data "count in" if it stops near the end there is most likely an error with the total # of data points in the file.
16. Once in SC the program can be open and scaled by pressing F4 or using environment and setting the x and y limits.
17. The original data file can also be converted to wavelenghts using the wn.exe program in a similar way as described above (This converts the micrometer screw setting to wavelengths vs. lock-in signal/laser power). Further corrections can be made correcting for variations in output coupler transmission and power meter frequency response (silicon detectors) , but these changes are usually small in magnitude and won't be done in this experiment.

Run at least two spectra, one covering a wide range of wavelengths (for the dye laser, the entire region over which lasing was observed) and a second covering the 'propane' peaks. Overall, the output should look like this ...

... well, not exactly, because this particular sample of propane was contaminated. As well, the experiment was disturbed when a curious faculty member leaned on the optical table holding the argon laser :)

Ti:Sapphire Laser (NIR/IR region): Convert the micrometer readings to wavelengths (in nm) and submit a graph showing the spectrum of propane similar to that above - with WAVELENGTH on the x-axis. The conversion formula for micrometer reading to wavelength is a third-order polynomial as follows:

y = -0.2108x³+3.1468x²+9.8135x+598.7 where x is the micrometer reading and y is wavelength in nm

DYE Laser (Visible region): Convert the micrometer readings to wavelengths (in nm) and submit a graph showing the spectrum of propane similar to that above - with WAVELENGTH on the x-axis. The conversion formula for micrometer reading to wavelength is a third-order polynomial as follows:

y = 0.2963x³-4.254x²+38.27x+476.25 where x is the micrometer reading and y is wavelength in nm

Useful Links

Coherent 599 SOP and Data Sheet showing optical configuration of the Dye laser.
Coherent 890 Data Sheet showing optical configuration of the Ti:Sapphire laser.
Princeton Applied Research 129A Lock-In a brief overview
Spectroscopy Primer from Mr. D. Turnbull
Photoacoustic Spectroscopy Applications an article from Spectroscopy Europe, 2002
Chapter 1 of Photothermal Spectroscopy Methods for Chemical Analysis Photospectroscopy Applications from the author's web site

Lab Report

In addition to the spectrum of propane as determined from the experiment, hand-in a WORD PROCESSED (not handwritten) lab assignment as follows (to be done individually):

Technology ...

1. If the Ti:Saph laser was used, describe the optical path of the tunable Ti:Sapph laser (a Coherent 890) showing all mirrors and optical elements involved. Show a diagram of the laser, including the sample cell. Alternately, if the Dye laser was used, describe the optical path of the tunable Dye laser (a Coherent 599) showing all mirrors and optical elements involved. Show a diagram of the laser, including the sample cell.
2. Describe how a birefringent filter works
3. Describe the energy levels in the tunable laser (i.e. a tunable solid state laser such as the Ti:Sapph laser or dye laser) and why an argon laser can effectively be used as a pump source.
4. Detail how a lock-in amplifier works. Be specific when mentioning signal-to-noise concerns and identify the purpse of the reference signal.
5. Why is a lock-in required for an experiment such as this? Detail precisely what is expected as a signal, and what is potentially a source of noise
6. Draw an overall block diagram of the experiment (the one in the primer is _not_ adequate and the one in the article uses a different laser). Be sure to identify the reference, input, and output for the lock-in as well as details of the chopper used.

Spectroscopy Technques ...

7. What would the addition of an inert buffer gas into the photoacoustic cell accomplish? (Hint: how does sound travel in a vacuum compared to at atmospheric pressure).
8. Describe the harmonic vs. anharmonic oscillator models and which would be most suited to describe a spectrum recorded in the NIR region?
9. In ICL-PAS only relative absorbance, not absolute, can be determined - Explain why. Is there a method to be able to deduce the absolute absorbance? (internal references). How can internal references be used to calibrate spectrum in x and y-axis?
10. Can you think of a laser technique as sensitive as ICL-PAS that can obtain absolute absorbance? (Ring down Cavity). Describe the technique.
11. Can you explain window noise? Brewster Angle on Windows?
12. Why does power have to be recorded and the spectrum normalized to it?