PHTN1400: Principles Of Laser Systems
DPSS Lasers (2017W)
- To investigate the thermal characteristics of semiconductor lasers used as pump sources for DPSS lasers
- To investigate the dependence of vanadate absorption on wavelength in order to determine the effect of pump-wavelength drift
- To investigate the nature of phase-matching of SHG materials (which will be covered later in lectures)
- To understand how commercial DPSS lasers work and be able to apply temperature-tuning to optimize performance
- To calculate re-absorption loss in a real laser material
Thermal effects are one of the largest problems with solid-state lasers. The emission wavelength of diode lasers used to pump these materials is temperature dependent, the thermal re-absorption loss within the solid-state material itself is temperature dependent, and second-harmonic generators used to produce visible laser radiation (e.g. 532nm) from infrared solid state materials are also temperature dependent. In this lab, you will observe all three effects. If you continue to PHTN1500 Advanced Laser Theory next year, you will produce mathematical models to predict such effects as well.
The class will be split into two groups with each group performing parts A and B alternately on two consecutive weeks.
PART A: Pump Diode and Solid-State Materials Characteristics |
PART B: SHG Temperature Chsracteristics
PART A: Absorption of Vanadate
The laser used in this part of the lab is a DPSS system with multiple wavelengths present. The pump diode operates at 808nm with an output power of 800mW while the DPSS operates at 1064nm with a power output of up to 100mW. Given the number of optical elements in use, and the fact that these are infrared wavelengths, safety glasses covering both wavelengths are essential.
NOTE #1: The temperature controllers have 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.
NOTE #3: The laser diode driver has preset current limits. Do NOT attempt to change this limit under any circumstances!
Prelab (Part A):
Before the lab, calculate the minimum OD required at each wavelength. MPE tables can be found by searching for "OSHA technical manual laser mpe" on the web. Use your best judgement and ask on entry if in question about any assumptions made.
Experiment Setup: Part A
The complete experiment for PART A. A 1064nm DPSS laser, mounted on a TEC, is pumped from a high-powered 808nm diode laser also on a TEC. By varying the temperature of the diode, the wavelength can be shifted to match the absorption curve of vanadate.
The Peltier cooler module for the pump diode is downright huge (at a Q of 51W) and the basic LDC controller lacks the current to drive it so an external bipolar amplifier (the Kepco BOP 20-5M) is used to drive the Peltier module to it's maximum current of 6A. Being a bipolar amplifier, it can drive the module in either polarity (meaning it can cool, as well as heat, the mount on which the diode is mounted).
The optical components and paths of the experiment. Shown in orange is the path of the 808nm pump beam.
As shown in the above diagram, pump radiation originates from the 800mW diode on the left (which sits on top of a powerful TEC controlled by the ILX LDC-3742), is split by a small beamsplitter before proceeding to the DPSS laser (which has it's own TEC regulated by the ILX LDC-5910B). The portion of the beam which is split is then split again by a second beamsplitter - the portion passing through allows monitoring of the output power of the diode and the weakest (reflected) portion enters a fiber allowing wavelength monitoring via an ANDO OSA. In this manner, both the wavelength and output power from the diode can be determined for any value of diode drive current or diode case temperature.
It was found that when the diode output (measured at the diode apeture) was 79.5mW the pump power meter reading was 4.23mW. This implies 75.3mW of actual pump power at the DPSS and so the measurement ratio is 4.23/75.3 = 0.0562. Keep the actual DPSS pump power below 375mW (or about 21mW as read on the pump power meter) since the DPSS is not designed for very large optical pump powers.
The output of this DPSS laser occurs at 1064nm, almost the same wavelength as Nd:YVO4, and may be detected by an infrared detector card. In this experiment the output is normally detected by a power meter - as pump intensity is increased, eventually reaching the minimum threshold for the device, the DPSS will begin to oscillate and output power will be observed to climb.
Note, too how the DPSS is mounted on a thermoelectric cooler module (complete with integral fan) which is on a multi-axis mount allowing precision adjustment of the orientation of the device.
Experiment Procedure: Part A
- Turn on the OSA, LDC controller, Bipolar Power Amplifier, LDT controller, two power meters, and TEC cooling fan.
- Ensure the room interlock is active, and safety glasses are worn, before powering the diode.
- Set the diode temperature on the LDC-3742 to 20C (select the "Adjust TEC" function under the control knob, set the target temperature via the control knob, and turn the TEC on via the button in the "TEC MODE" function selector).
- Set the DPSS temperature on the LDT-5910C to 20C and turn the unit ON.
- Set the diode current on the LDC-3742 so that the DPSS laser is just brought to threshold and an output of 10μW is observed (on the 1064nm power meter). Record the power at 808nm required to bring the laser to threshold - remember that only a small portion of the pump diode output passes to the meter and so the meter reading must be multiplied by the a constant. To set laser diode current, select the "Adjust LASER" function under the control knob, set the target current via the control knob, and turn the laser on via the button in the "LASER MODE" function selector.
- Set the OSA center wavelength to 805nm, and the span to 10nm. Press SINGLE to record a trace and ensure the diode output can be seen on the OSA (and hence center wavelength can be determined).
- Record the exact diode temperature, pump diode optical power (to bring the DPSS to threshold), and center wavelength (as observed on the OSA: press SINGLE then PEAK WAVELENGTH on the marker buttons). These observations show (i) how diode wavelength varies with temperature and (ii) how absorption of the Nd:YVO4 laser varies with wavelength.
- Now, increase the temperature of the diode in approximately 1 degree C increments. When the temperature has stabilized after each adjustment (as seen on the LDC controller), record the exact temperature, pump diode output power required to bring the DPSS to threshold, and pump diode output wavelength (these will be graphed later). Continue this process until the pump diode reaches 40C.
- Set the temperature of the diode to the optimal value (where lowest pump power to threshold the DPSS was found). Set the pump diode current for a 1064nm output of about 10mW. Now, without varying the DIODE CURRENT or the DIODE TEMPERATURE, record the output power with the DPSS Amplifier temperature (controlled by the LDT-5910C controller) at 20C, 15C, and 25C. This should demonstrate the effect of thermalization of the LLL of a solid-state amplifier on output power (A full mathematical model for this will be developed in PHTN1500 next year).
Shutdown the laser diode first (by pressing the LASER ON button to toggle it off), then all other equipment.
Part B: Second Harmonic Generators
Introduction to DPSS Systems
As discussed in class, building a stable DPSS laser requires a lot more than simply shining a large 808nm diode pump laser at a small vanadate crystal and hoping for green light :). Consider the Crystal Laser DPSS system, a typical small DPSS:
Seen here unmodified is the Crystal Laser Green DPSS system employed in this lab. 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 - this is a major point of this experiment.
As well as the pump diode, the vanadate/KTP hybrid crystal is also mounted on a thermoelectric crystal for stability - the necessity for accurate temperature control of a Second Harmonic generator (SHG) crystal will become obvious during this experiment.
In order to tune a system such as this, the temperature must be known. One must add some sort of temperature sensor to each thermoelectrically-cooled block - the diode and the crystal - in order to monitor each during tuning. In the case of this lab, the factory-installed thermistors are used and 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
- Power the ILX LDT-5910 temperature controller for the laser diode (it will be marked as "Diode Temp"). Ensure it is set for 20 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 - press the output button to toggle it on).
- Power the second ILX LDT-5910C temperature controller for the vanadate/SHG crystal (it will be marked as "Vanadate/SHG Temp" - it is an updated version of the older controller shown in the photo to the right). Set the temperature for 20 degrees C and turn the output ON (as seen by the green indicator). Press the 'enable' button to allow setting of the temperature via the control knob.
- Turn on the Newport 505 Laser Driver. Set the PRESET current to 490mA (if the PRESET current is not displayed, select it using the DISPLAY button then set using the knob). Ensure the diode temperature controller is ON as well as the room interlock and turn the unit ON via the button labelled OUTPUT. Green output from the DPSS should be evident.
- 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.
- Wait a few minutes for the system to fully stabilize before proceeding - this will be evident when the temperature of the SHG and the output power are both stable.
- Record the actual temperature and the output power (at 532nm) at 20C.
- Now, increase the temperature of the vanadate crystal in 1 degree increments. The SET temperature will be displayed in the lower right corner while the large display will show the actual temperature. When the actual temp has stabilized (this is quite quick given that the controller uses a tuned PID loop) read the power output again at this new temperature.
- Complete the process and plot output power vs. temperature for the range 20C to 40C. Twenty readings will be required. Be sure to increase temperature only, do not vary temperatures up and down.
- Return the temperature to 20C.
- Now, increase the temperature in 0.1 degree increments only over the range where output changes quickly (this will be a range of under ten degrees).
- Shutdown the laser diode driver BEFORE turning the temperature controllers off.
Hand In a WORD PROCESSED (not handwritten) lab report with contents as outlined below.
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 3) as a single paragraph.
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
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.
- A table of data from PART A with columns: Temperature (°C), Pump diode output power (mW) to threshold the DPSS, and Pump wavelength (nm).
- A graph of diode wavelength vs. diode temperature (in the same manner as it would appear in a typical datasheet like the kind we have examined in lectures). As with ALL graphs, be sure to add a title, axis labels with units, and x/y gridlines (one per degree and one per nm minimum). Add a linear trendline and report the slope in units of nm/°C
- A graph of threshold pump power (in mW) vs. diode temperature. In reality, this indicates absorption of the Nd:YVO4 material at various wavelengths (since lower pump power means higher absorption of pump radiation).
- For Part B, a graph of DPSS output power at 532nm (y-axis) vs. SHG crystal temperature in degrees (x-axis). Data, at various intervals of temperature, can be incorporated as a single graph or displayed as two graphs.
- After adjusting the temperature setting of either the diode or the crystal in PART B, or the pump diode temperature in PART A, you may have noticed the resulting temperature oscillates: too high, too low ... the cycle repeats each time getting closer to the final target value. Find, on the web, a tutorial on "PID tuning" which explains this behaviour (search, for example, for "PID tuning ControlsWiki"). Outline the causes of the oscillation (what parameter in the system is set too low or too high?), show a graph of a P/PI/PID controller producing such oscillations in the output, and finally find a graph of an "optimally" tuned controller (which does not oscillate).
- Predict the maximum output power of the DPSS used in Part A in the same manner as you did in PHTN1300 (this is prerequisite material so you might need to refer back to your notes from last term: see "Cross-Sections and Gain" for a complete solved example. See, too, the corrected prelab #1 and lab #1 solutions since they cover the same type of calculations which will surely be on the midterm as well.). The specific parameters of this DPSS are:
To compute threshold gain: (thermal effects may be neglected for this question)
Show ALL calculations and ALL intermediate steps here: numerical answers without work shown will receive a mark of ZERO! For example, when the threshold gain is computed show the original formula, the formula with all substitutions, and the final answer. Do this for ALL calculations required for this question (including gth, Psat, and Poutput). Finally, remember that this is a solid-state laser and is totally homogeneous (so use the correct saturation formula).
To compute saturation power:
Other parameters may be found in Laser Modeling, chapter 8 (see table 8.8 for vandate)
- The above calculations do not take thermalization of the LLL of the solid-state material into account. For a more realistic value calculate the expected output power from the OC again, this time with re-absorption loss (i.e. γthermal) factored in as follows:
- Calculate (using values from table 8.8 of Laser Modeling) re-absorption loss, then gth, then expected output power of the same laser at both 15C and 25C (again, use a homogeneous solution). Show work for both temperatures (although many parameters from the above question are recycled here so just use them)
- What was the expected theroretical output power decrease (as a percentage) when going from 15C to 25C using the above numbers? Simply report the powers and the ratio of P25C over P15C.
- What was the observed power decrease as seen in the lab when going from 15C to 25C? Again report the two observed powers and the same ratio as above.
Remember, this is vanadate with a larger cross-section than YAG so effects of thermalization will be greater than those shown in lecture examples. The exact energy levels of Nd:Vanadate, which vary slightly from those of Nd:YAG, may be found here: use the actual terminal level of the transition identified as "laser" (it is the "Y1" level). All energy levels are shown in cm-1 and must be converted to eV before use. Remember, as well, that this is a rough estimate that does not take Stark splitting into account.