PHTN1500: Advanced Laser Theory
Thermal Effects on DPSS Lasers (2015F)

Introduction

In PHTN1400 you saw the effect of temperature on the output power of a 1064nm. The effect was quite modest and an increase from 10C to 30C generally results in a power decrease of 10%. This modest decrease is due to re-absorption loss (simplified, it is the decrease in inversion resulting from the increase in LLL population due to thermalization of that level).

In this lab, the effects of re-absorption loss (as introduced in chapter 5 of Laser Modeling) on the threshold pump power of a 946nm quasi-three-level DPSS laser system are investigated. A quasi-three level amplifier, however, features a much larger re-absorption loss than a four-level with the loss usually on the same order as that of the optical losses (i.e. re-absorption loss is about the same as the optical threshold loss). The exact solution requires calculation of the Stark levels of the LLL manifold.

Prelab (To be done prior to the lab, due upon entry)

Read chapter 5 of Laser Modeling by Csele on ther thermal concepts and models used in this lab

The following prelab assignment (worth 25% of the total lab mark) is due at the beginning of the lab period (the exact date of your lab period is on the main course home page). Late marks are not assigned if the prelab it is not received at the beginning of the class: you lose 33% of the total lab marks immediately with no recourse if it is not received upon entering the period in which it is due (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).

It was determined that the pump diode is similar to a type "AL0808F2000" 2 Watt 808nm laser diode in a "C" mount. The threshold current for the diode was determined experimentally to be 0.466A and the diode produced 2W of output at a current of 2.1A.

  1. Develop an equation relating diode current to output power. It must show a power of zero at 0.466A and 2W at 2.1A.
  2. Now, calculate the saturation intensity (in W/cm2) at 946nm. Assume a beam diameter of 0.5mm and calculate the saturation power in Watts.
  3. Assuming the Nd:YAG amplifier is 1mm in length, the HR is essentially 100%, and the OC 99% reflecting, calculate the gain threshold for the optical configuration of the laser. Now, using the same methodology in Laser Modeling (pp. 129), calculate the thermal re-absorption loss at 20C (Calculation of the Stark fractions will be required for this). Add this thermal loss to the optical loss for an accurate gth for the laser (i.e. the gain the laser must exhibit for lasing to ensue). This question is worth nothing if the Stark approach is not used.
  4. Using the methodology outlined in Laser Modeling (pp. 82), calculate the expected minimum pump power and then the expected pump diode current at which threshold should occur.
  5. Calculate, using the methodology in Laser Modeling, the re-absorption loss of the laser used last term in lab #4 of PHTN1400. This laser is Nd:YVO4 operating at 1064nm, the LLL of which exists in a different manifold than that of the 946nm transition. The exact levels for vanadate can be found on Blackboard under "Course Documents" and you must,of course, compute the exact Stark fraction for the LLL. Calculate the threshold gain of the laser at 10C and 30C and then the expected output power at each temperature. Parameters of a typical vanadate laser may be found in table 8.8 of Laser Modeling. Ratio these output powers to determine the expected decrease in output power as the amplifier temperature increases from 10C to 30C (it will be much smaller than the 946nm transition). This question is worth nothing if the Stark approach is not used.

SAFETY WARNING

The laser used in this part of the lab is a DPSS with 946nm output. Safety glasses covering 946nm with a maximum power of 100mW are required.

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.

Experiment Setup:

946nm Laser Setup

Turn on the PSU-III unit to start the diode TEC ... the diode operates at a constant temperature in this experiment and so wavelength will be stable. Set the Nd:YAG temperature to 10C using the LDC-5910C temperature controller. Now, increase the current from a value below the threshold (determined in the prelab and easily found experimentally) to 2.0A in 100mA increments recording the output power at each interval. Plot to determine the threshold. Repeat the experiment at 20C and 30C.

Lab Report

Hand In a WORD PROCESSED (not handwritten) lab assignment, done individually, as follows. Put each question on a new page and ensure each page has a title "Question 1", "Question 2", etc. Also, please ensure the lab report is in a folder for submission (no loose pages or binders).

  1. Hand in three graphs (one at each temperature: 10C, 20C, and 30C) of output power (y-axis) vs. input pump power (x-axis) and identify the pump threshold power at which lasing occurs by using a linear best-fit line. Show the lines on the graphs and report the threshold in mW.
  2. Calculate unsaturated re-absorption loss (i.e. with zero intra-cavity power) for the 946nm Nd:YAG laser at 10C, 20C, and 30C. Outline one complete set of calculations for one temperature and report the values in m-1.
  3. Calculate the theoretical minimum pump power at 10C, 20C,and 30C using the expression in section 2.7 of Laser Modeling.
  4. Calculate the saturated re-absorption loss for the 946nm laser as a function of output power (since it can be measured and intra-cavity power cannot) - produce a table, then a graph, of re-absorption loss vs. output power at 20C (where, at very low output powers the re-absorption loss is the same as the that predicted in the first question but this will decrease as power increases).