BATP9401 Laser Systems
Lab - Wavelength Sensitivity of DPSS Lasers
aka "The Laser-Pumped-Laser-Pumped-Laser" Lab
Purposes:
Prelab:
Seen here before modification is the Crystal Laser Green DPSS system. Radiation from a powerful 808nm laser diode, mounted on a thermoelectric cooler, is shaped and focussed by a two lens and two prisms. The focussed beam pumps a tiny crystal of vanadate (Nd:YVO4) with an attached KTP frequency-doubling crystal. The entire vanadate/KTP crystal is mounted on a second thermoelectric cooler.
The green output beam then passes through a beamsplitter which diverts a small portion of the output towards a photodiode used to monitor the output power - the rest passes through a filter which removes residual IR (both 808nm pump radiation and 1064nm laser radiation) to become the output beam.
The pump laser is mounted on a thermoelectric cooler for two reasons: one is to simply sink excess heat produced during the operation of the laser itself and the second is to thermally-stabilize the diode to maintain wavelength stability ... if the diode wavelength is allowed to drift during operation, output power will also drift (one major point of this experiment).
For this experiment, the pump (diode) laser and beam-shaping optics are removed 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.
The entire experiment is seen here mounted on a large optical bench. Visible to the right, on the cart, is the ANDO OSA used to determine, accurately, the wavelength of the pump radiation.
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 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 to allow monitoring of pump power. 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 - the "cool" light will extinguish (then blink slowly) when the crystal has been cooled to operating temperature. The Ti:Sapphire laser, with associated pump laser, is already aligned on the optical bench. Place a beamstop in front of the Ti:Sapphire 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). 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.
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.
Now, while observing the output of the vanadate (by eye), decrease the current of the pump laser until it just stops lasing (i.e. threshold point). Read the pump power from the broadband power meter (remember to multiply by 2.17). 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.
Plot (using a spreadhseet) Pump Wavelength (nm, on the X axis) vs. Threshold Power (mW, on the Y axis).
Hand-in a WORD PROCESSED (not handwritten) lab assignment as follows (to be done individually):
Results ...
Technology ...
Coherent 890 Laser Data Sheet showing optical configuration
Copyright (C) Niagara College, Canada, 2007-2009
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