# PHTN1400: Principles Of Laser Systems DPSS Lasers (2019W)

## Introduction

Purpose:

• 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 amplifier 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 PHTN1306 Lasers III 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

## Prelab

First, download the Absorption and Energy Level Data for Vanadate which is required for the lab and for the prelab.

Second, prepare a prelab submission in which you will calculate the expected output power of the laser employed in Part A of this lab (this will represent the maximum power of the DPSS laser). Do this as follows (outlining each step as "Part a", "Part b", etc):

1. Calculate the optical threshold gain (in m-1) using the usual formula
2. Calculate the re-absorption loss (also in m-1) assuming the amplifier is at room temperature (20C). Expect an answer less than the optical loss but greater than 1m-1
3. Combine the above two values to get a total value for gth which incorporates all losses.
4. Compute the saturation power for this specific laser
5. Calculate the expected output power assuming a homogeneous model

Submit a page (or two) with the above calculations upon entry to the lab.

This prelab assignment is worth 10% of the total lab mark and is due at the beginning of the lab period. Late marks are not assigned if the prelab is not received at the beginning of the lab: you lose 10% of the total lab marks immediately with no recourse if it is not received upon entering the lab (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).

This is prerequisite material from PHTN1300 so you might need to refer back to your notes from last term: see "Cross-Sections and Gain" for a complete solved example. In your submission 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).

## PART A: Absorption of Vanadate

### SAFETY WARNING:

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 quite powerful 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 450mW (or about 24mW 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 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 amplifier 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.

Setup Parameters for the LDT-5910-C (for tech use only): P/I/D parameters are 17.5/0.25/0.01; Steinhart-Hart parameters C1/C2/C3 are 0.5931/2.9881/0.0001

## Experiment Procedure: Part A

1. Turn on the OSA, LDC controller, Bipolar Power Amplifier, LDT controller, two power meters, and TEC cooling fan.
2. Ensure the room interlock is active, and safety glasses are worn, before powering the diode.
3. 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).
4. Set the DPSS temperature on the LDT-5910C to 20C and turn the unit ON.
5. Turn on the pump diode current and set the diode current on the LDC-3742 so that the DPSS laser produces an output power of 1mW on the 1064nm power meter. Record the pump power at 808nm required remembering that only a small portion of the pump diode output passes to the meter and so the meter reading must be multiplied by 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. The target diode current will be between 800mA and 840mA.
6. 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).

7. Determining the effect of pump wavelength on small-signal gain: in this part of the lab the amplifier temperature remains constant and the diode temperature is varied. As diode temperature changes the wavelength from this diode shifts, the amount of pump power absorbed by the amplifier changes, and the small-signal is affected accordingly. Ultimately, you will produce a graph of small-signal gain (g0) versus pump wavelength.

8. Record the exact diode temperature, pump diode optical power at 808nm, and center emission wavelength of the pump diode (as observed on the OSA: press SINGLE then PEAK WAVELENGTH on the marker buttons). These observations will show (i) how diode wavelength varies with temperature and (ii) how absorption of the Nd:YVO4 laser varies with pump diode wavelength.
9. 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 output of the DPSS laser at 1064nm, pump diode output power (which will also vary with temperature, albeit only a little), and pump diode output wavelength. Continue this process until the pump diode reaches 40C.

10. Determining the effect of amplifier temperature on re-absorption loss and hence output power: in this part of the lab diode temperature (and hence, ultimately, small-signal gain) remains constant and amplifier temperature is varied. As amplifier temperature changes, re-absorption loss changes and so threshold gain (gth) is affected accordingly.

11. Set the temperature of the diode to the optimal value (where maximum output power from the DPSS was found). Leave the pump diode current set to the previous value. 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 15C, 20C, 25C, and 30C. 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 PHTN1306 next year).

12. 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 is an unmodified is a BWN-532 Laser manufactured by B&W Tek. This cylinder encapsulates the entire laser in which radiation from a powerful 808nm laser diode pumps a tiny crystal of vanadate (Nd:YVO4) oscillating at 1064nm producing a powerful intra-cavity beam. A separate KTP frequency-doubling crystal then doubles this IR to 532nm. The diode, along with the amplifier, is mounted on a thermoelectric cooler and the KTP crystal is mounted on a second, separate, thermoelectric cooler.

The green output beam passes through a filter which removes residual IR (both 808nm pump radiation and 1064nm laser radiation). The 532nm 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 becomes 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.

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 device - the diode and the crystal - in order to monitor each during tuning. In the case of this lab, factory-installed thermistors are used and using separate temperature controllers, amplifier and doubler temperature can be independently varied and the effect on DPSS output determined. The BWN-532 laser is suited well for this experiment as the KTP doubler crystal has its own TEC cooler separate from the amplifier (eliminating one more variable) - a separate TEC (the original controller) controls the diode and amplifier temperature while the external LDT-5910-C controller controls only the KTP temperature to observe the effect on output power.

Setup Parameters for the LDT-5910-C (for tech use only): Limits are +0.5A, -0.5A; P/I/D parameters are 4.5/0.2/0.23; Steinhart-Hart parameters C1/C2/C3 are 1.0100/2.5363/0

### SAFETY WARNING:

2018: Due to a reconfiguration of lab spaces, this experiment is located in V115A on an optical bench in close proximity with PART A of this experiment. The green output beam need not be accessed since it passes directly into the power meter, and so poses little danger, however the adjacent experiment is "open beam" and does pose a hazard. As such, Safety glasses suitable for PART A are required when performing this part of the lab.

## 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 1. Power the ILX LDT-5910C temperature controller for the SHG crystal. 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.
2. Once the temperature of the SHG crystal has stabilized at 20C, turn on the BWN-532 Laser Driver. Green output from the DPSS should be evident.
3. 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.
4. 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.
5. Record the actual temperature and the output power (at 532nm) at 20C.
6. Now, increase the temperature of the SHG 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.
7. 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.
8. 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), for example in the range 27C to 32C.
9. Shutdown the laser driver BEFORE turning the temperature controller off.

## Laser Parameters

To calculate threshold gain:
RHR=100%
ROC=99%
xg=xa=1mm
To compute saturation power:
Φ(beam diameter)=0.5mm
Other parameters may be found in Laser Modeling, chapter 8 (see table 8.8 for vandate)

This is vanadate with a larger cross-section than YAG so effects of thermalization will be greater than those shown in some lecture and text 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.

## Lab Report

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.

Observations ...

1. Present the observed data from Part A (first part) as follows:
1. A table of data from PART A with columns as follows: Pump diode temperature (°C), Pump diode output power (mW) at 808nm, Pump diode wavelength (nm), and Output power of the DPSS (mW) at 1064nm.
2. 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).
3. Add a linear trendline to the above graph and report both the line equation (on the graph) and the slope in units of nm/°C (this slope is called the "wavelength coefficient of temperature" of the pump diode) - the slope must be reported OUTSIDE the graph as a single numerical value with proper units (do not just show the equation again).
4. A graph of DPSS output power (at 1064nm, in mW) vs. diode temperature. In reality, this indicates absorption of the Nd:YVO4 material at various wavelengths (since higher output power means higher absorption of pump radiation) and will be converted to small-signal gain in the analysis that follows.

2. In the second part of Part A of the experiment, you varied the amplifier temperature while keeping the pump diode temperature (and hence pump diode wavelength) constant at an optimal value. This assumes that g0 stays constant (since pump temp and current were untouched) and only gth (actually γ-thermal) changes. Present the observed data as:
1. A table (with columns for amplifier temperature and the corresponding observed DPSS output power)
2. A graph the output of the DPSS as a function of temperature.

Analysis: Absorption of Pump Power and Small-signal gain as a function of temperature ...

3. Given the observed output power of the DPSS at various pump wavelengths, calculate the small-signal gain (g0) at each diode temperature both in table form (with columns for diode temperature, DPSS output power, small-signal gain) and then graph it. In the first part of the experiment, in which the amplifier temperature was kept constant at 20C, the value of gth (which incorporated both optical and re-absorption losses) is constant and so any variations seen in output power of the DPSS are due solely to variations in small-signal gain which is a function of absorption of pump radiation (which is also, in turn, a function of temperature of the pump diode). An example calculation showing how one small-signal gain value was derived is required. It must also detail how you calculated any required parameters such as saturation power and re-absorption loss.
4. Analysis: Re-aborption Loss and threshold gain as a function of temperature ...

5. Calculate re-absorption loss at 15C, 20C (already done in the prelab), 25C, and 30C. Show one complete example calculation. Now, compute the expected output power at each temperature in the same manner as the prelab showing an example calculation and providing a table of all values you calculated (with columns for temperature, γ-thermal, g0, and expected output power). For g0 use the value you calculated in the first part of this experiment (at the correct pump diode wavelength used, which remained constant for this whole part of the experiment).
6. Second-Harmonic Generation ...

7. For Part B, a graph of DPSS output power at 532nm (y-axis) vs. SHG crystal temperature in degrees (x-axis). Incorporate data into a single graph.