aka "The Laser-Pumped-Laser-Pumped-Laser" Lab

A tunable Ti:sapphire laser producing tunable near-IR radiation pumps a frequency-doubled vanadate crystal, in turn emitting 532nm radiation. Fog, which makes the beam visible, also reveals a great deal of scattering of IR pump radiation, seen here as white haze.

The class will be split into two groups with each group performing parts A and B alternately on two consecutive weeks.

Purposes:

• To investigate the wavelength sensitivity of vanadate in order to determine the effect of pump-wavelength drift
• To use a tunable Ti:Sapphire laser system (also a solid-state laser)

Prelab (Part A):

• Download the data sheet for a Laser Components ADL-80Y04TL diode of the type which would normally be used for pumping this DPSS laser. Specifically, examine how output wavelength is a function of temperature.
• Review the principles of operation of a Ti:Sapphire laser, including use of a birefringent filter.
• Read chapter 12 of Csele on solid-state lasers, especially DPSS.
• Read this reference on the design of DPSS lasers.

For this experiment, the pump (diode) laser and beam-shaping optics are removed from a "standard" DPSS system and replaced with a Coherent model 890 tunable Ti:Sapphire laser (itself pumped by a large-frame Coherent Innova-200 argon-ion laser) which allows the pump wavelength to be swept across absorption bands of the vanadate material. In a real DPSS system, pump diode wavelength may drift as the diode operates causing it to sweep across absorption bands.

The entire experiment is seen here mounted on a large optical bench which holds two tunable lasers (one a Ti:Saph operating in the IR and the other a dye laser operating in the visible region). In the front, to the left, is the Ti:Sapphire laser, the output of which drives the vanadate laser in the foreground. On the shelf (left-to-right) is the ANDO OSA used to accurately determine the wavelength of the pump radiation, a meter used strictly for alignment of the laser, a Coherent Fieldmaster meter used to measure the actual output power of the Ti:Sapphire laser, and the thermoelectric controller which keeps the temperature of the vanadate/KTP laser constant.

This laboratory setup utilizes many wavelengths of laser radiation making the use of broadband laser safety glasses unfeasible. Safety glasses suitable for the near-IR band (780-1064nm) will be used but these fail to protect against the intense pump radiation from the argon laser. Be sure that all beam blocks are in place around the pump beam and be careful to avoid that area of the setup since the safety glasses will not protect against these wavelengths.

The Experiment

The experiment is configured as follows: The beam from an ion-laser pumped tunable Ti:sapphire laser with shortwave (NIR) optics enters from the left. The beam is split by two beamsplitters: one beam is pulled-off to be fed to an ANDO AQ6312B OSA (via the yellow fiber in the above photo) used to determine the wavelength of the pump beam, the second beam is fed to a Melles-Griot Broadband power meter (or the Coherent Fieldmaster in the above photo) to allow monitoring of pump power (the beamsplitter allows only a small portion of the tunable laser output to hit the power meter - the power read must be multiplied by a factor, given in the lab, to represent the true power incident on the vanadate laser).

The beam then passes to the vanadate laser which consists of a vanadate crystal coupled to an integrated KTP frequency-doubler. Green light (532nm) then exits the laser passing through a filter to remove excess IR radiation (both pump radiation around 800nm and cavity radiation at 1064nm). The output can then be metered by the OPHIR meter shown or simply observed by eye to determine if threshold has been reached.

Turn the thermocooler controller for the vanadate crystal ON (Unlike the one in the photo above, an ILX 5910 on the shelf is used now). Set the temperature of the laser for 20C and press the OUTPUT button. The Ti:Sapphire laser, with associated pump laser, is already aligned on the optical bench. Ensure water flow through the Ti:Saph laser (by observing the flowmeter on the bench or the output line as it dumps into the drain). Place a beamstop between the second beamsplitter and the vanadate laser and turn the argon pump laser on. Set the birefringent filter on the Ti:Sapphire laser to 0.3285 on the micrometer screw (around 809nm) and increase tube current on the argon pump laser to 40.0A. Ensure the argon laser has an output of about 7.1 Watts at this point (enough to ensure the Ti:Sapphire reaches threshold - if it does not, the argon laser requires alignment). The Pump power should read about "0.046 Watts" ... the actual pump power may be found by multiplying by 2.17 for an actual pump power to the vanadate of 100mW (this multiplier is subject to change year-to-year as the setup is adjusted - it will be provided in the lab).

Reduce the pump power (via the argon laser, by reducing tube current until the IR output power is observed to be less than 5mW), Remove the beamstop and, using an IR detector card, locate the IR beam (keep your IR glasses on ... what would you EXPECT to see anyway ? IR ?). Align the vanadate laser in the beam path so that the IR pump beam is focussed on the face of the vanadate crystal - output power of the vanadate may be monitored to optimize alignment (a green output _will_ be visible now). The laser may require adjustment in the vertical and horizontal directions - begin by locating the beam through the center of the lens then, using the smaller IR detector card, position the vanadate laser so that the focussed pump radiation is incident on the center of the crystal. Increase the tube current of the argon pump laser (which will increase IR output as well) until green output is observed from the vanadate laser.

Now, while observing the output of the vanadate (by eye), decrease the tube current of the pump laser until it just stops lasing (i.e. threshold point). Read the IR pump power from the broadband power meter (and multiply by 2.17 or 3.26 depending on the specific configuration: check this while in the lab). Press "AUTO" on the ANDO OSA then read the center wavelength as well. Now, adjust the micrometer screw on the birefringent filter to a wavelength 0.2nm below the current wavelength. Again, adjust the current of the ion laser to threshold the vanadate output and record the wavelength and power of the pump beam. It is necessary to press 'Auto' on the OSA after every few readings to bring the peak back onto the screen. When complete you will have a chart of Pump Wavelength (in nm) vs. Pump Power (in mW).

While continuing the experiment, keep the pump power (on the meter) below "0.050" (about 110mW) to avoid damaging the vanadate crystal. Sweep through the range from 805.5nm to 810nm and from about 815nm to 817nm, both in increments of no larger than 0.25nm.

Analysis

Plot (using a spreadhseet) Pump Wavelength (nm, on the X axis) vs. Threshold Power (mW, on the Y axis).

Part B: Second Harmonic Generators

Visible DPSS laser systems use a powerful laser diode to pump the vanadate or YAG rod optically and are quite sensitive to the wavelength of pump radiation (so even small shifts of the wavelength of the pump diode will cause large variations in the output power of the laser) - this is covered in part A of this lab. This portion of the lab will demonstrate how sensitive the second-harmonic generator is to variations in temperature which affect phase-matching. Using separate temperature controllers, vanadate temperature can be independently varied and the effect on DPSS output determined.

Prelab (Part B):

• Review chapter 8 from Csele on Non-Linear Optics, specifically on phase-matching

NOTE #1: The temperature controller has preset parameters including temperature limits, gain, and thermistor constants. Do NOT attempt to reprogram the controllers (i.e. don't press buttons wildly unless you are sure of what they do, especially avoid the SELECT and SET buttons on the temperature controllers).
NOTE #2: The temperature controllers MUST be switched-on before the laser diode driver or an interlock will open causing the diode driver to fail to operate.
NOTE #3: The laser diode driver has preset current limits. Do NOT attempt to change this limit under any circumstances!

The Experiment: SHG Characteristics

The DPSS experimental setup. Twin thermoelectric controllers allow independent control of pump diode and Vanadate/KTP assembly temperature and a laser diode driver allows control over pump diode power.

1. Power the ILX LDT-5910B temperature controller for the laser diode (this is the lower TC unit of the two in the photo). Ensure it is set for 19 degrees C and that the output is ON (as seen by the green or red indicator in the lower right of the unit - it will be green, normally, to indicate cooling function).
2. Power the second ILX LDT-5910B temperature controller for the vanadate/SHG crystal (this is the top TC in the photo). Set the temperature for 35 degrees C and turn the output ON (as seen by the green or red indicator - it will be be red to indicate heating function or green to indicate cooling function).
3. Turn on the Newport 505 Laser Driver (second unit from the top). Set the PRESET current to 490mA (if the PRESET current is not displayed, select it using the DISPLAY button then set using the knob). Turn the unit ON via the button labelled OUTPUT. Green output from the DPSS should be evident.
4. Direct the output from the DPSS towards the power meter. Read the power output from the DPSS unit (in mW) on the optical power meter. A range of 2mW will ensure maximum precision in readings.
5. Now, increase the temperature of the vanadate crystal in 0.1 degree increments (as indicated by the ACTUAL TEMP display - press the appropriate button to select this function as required). Leave the laser diode temperature alone. After adjustment of the vanadate temperature, allow the temperature to stabilize for ten seconds (until the "actual temperature" display shows that the temperature is now stable) and read the power output again at this new temperature.
6. Repeat the process and plot output power vs. temperature for the range 35C to 40C.
7. Shutdown the laser diode driver BEFORE turning the temperature controllers off.

Lab Report

Hand-in a WORD PROCESSED (not handwritten) lab assignment as follows (to be done individually):

Results ...

1. A graph showing the threshold power required (Y axis) versus wavelength (X axis) for vanadate. Be sure to include units & titles on the graph axes.
2. Obviously, using a Ti:Sapphire laser is impractical for a DPSS (or is it now a LPLPSS ?) and so a small laser diode is used. The Sony SLD302V diode (808nm, 200mW maximum) is a good match for our vanadate laser (which can take a maximum drive power of 150mW). Knowing the wavelength stability of the diode laser from the datasheet ("Oscillation wavelength vs. temperature characteristics"), if the temperature of the laser diode was allowed to drift over a range of 10C to 40C (and hence the wavelength would drift), describe in the form of a graph of laser diode temperature (x axis) vs. vanadate output (y axis) the expected output of a DPSS pumped by this specific diode (you can assume that the gain will be more-or-less proportional to the inverse of threshold power as measured in this lab ... high threshold powers indicate lower gain while low threshold powers indicate higher gain). The "gain vs wavelength" graph would then be the opposite of the "threshold vs wavelength" graph you produced in this lab.
   
   To produce this graph, determine the wavelength of emission of the Sony diode for a given temperature then determine the threshold power of the vanadate at that wavelength. Now, deduce the gain at this wavelength by subtracting the threshold power recorded from a constant (say, the maximum threshold value recorded) - this inverts the entire graph. A spreadsheet will help here. Use a small 'granularity' in wavelength (0.25nm maximum) when producing the graph.
   This should illustrate the necessity of accurate temperature control for the pump diode (i.e. a small drift in diode temperature results in a large wavelength drift which in turn results in a large change in output power).
   For the final graph, show both the temperature as well as the wavelength on the x-axis. Your graph, then, should have two scales on the x-axis (temperature and diode wavelength, which must correspond) and one scale on the y-axis (predicted output power/gain). Two separate graphs may be used if desired.
   
3. Show a plot of DPSS output power vs. crystal temperature for the range 35C to 40C.

Technology ...

4. Describe the optical path of the tunable Ti:Sapph laser (a Coherent 890) showing all mirrors and optical elements involved. A diagram of the optics and optical path of this laser are required.
5. Describe the energy levels in a tunable solid state laser such as the Ti:Sapph laser (include a diagram showing all energy levels/bands) - include why an argon laser can effectively be used as a pump source and why this laser is tunable at all.
   On the required diagram of the energy levels/bands, add four labels showing PUMP, ULL, LLL, and GROUND levels. Ensure, also, that the absorption transition for pump radiation is shown (as an upwards arrow) and the lasing transition is shown (as a downwards arrow).

Useful Links

Coherent 890 Laser Data Sheet showing optical configuration

Copyright (C) Niagara College, Canada, 2007-2012
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