PHTN1306 Lasers III - Lab
DPSS Laser Design - Part 1 (2017F)

Introduction

The reabsorption loss for the quasi-three-level 946nm transition of Nd:YAG will be both predicted as a function of temperature as well as measured in the lab. This is one part of the lab and the model will be applied to overall DPSS design.

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

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

The following prelab assignment (worth 30% 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 30% 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).

This prelab is MANDATORY - arriving without a complete prelab indicates unpreparedness and will result in a zero on this section as well as the student being placed on COURSE CONDITION.

Assuming the amplifier is operating on the 946nm transition of Nd:YAG (not Nd:YVO4), calculate, using the methodology in Laser Modeling, the expected re-absorption loss of the amplifier. Begin by computing the exact Stark fraction for the LLL. Some parameters may be found in chapter 8 of Laser Modeling.

Outline, explicitly, the calculation of re-absorption loss at 10C showing all mathematical steps and parameter substitutions. The complete steps required to compute the Stark fraction must be outlined here as well for full marks.

Next, calculate the re-absorption loss at 15C, 20C, 25C, and 30C and outline these values in a table (Explicit steps need not be shown here since they will be the same as for the 10C value).

Experiment:

946nm Laser Setup

The basic experimental setup: The PSU-III power supply unit powers the pump diode (maintaining constant diode current) and two LDC-5910C temperature controllers set the laser diode temperature and Nd:YAG amplifier temperature separately.

A Laserglow R94050XSX 946nm DPSS laser is connected such that the original power supply operates the pump diode at a constant current and two external ILX LDC-5910C temperature controllers controls the temperature of the pump diode and Nd:YAG amplifier separately. This allows determination of reabsorption loss as a function of temperature alone.

Program the LDC-5910C controllers with the correct Steinhart-Hart parameters (determined experimentally) for the laser as follows:

Set the pump diode temperature to 27C, enable the controller output, and leave it constant for the duration of this experiment. Set the Nd:YAG amplifier temperature for 300K (27C) and enable the controller. Next, switch the "PSU-III" power supply on which will operate the diode at a constant pump current (the R94050XSX laser features a separate diode and amplifier TEC so the diode temp is independent). Place an optical power meter in the path of the output beam and set the wavelength of the meter appropriately - an external IR filter is not required as it is built into the laser to exclude 808nm radiation from the output.

When the temperature of the TEC has stabilized, measure the output power of the laser.

Now, vary the amplifier temperature to 10C and measure the output power again. Repeat at amplifier temperatures of 15C, 20C, 25C,and 30C - as temperature increases we expect to see output power decrease.

Assignment

Hand In a WORD PROCESSED (not handwritten) lab assignment 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).

To be done individually ...

  1. Assuming the optical parameters of the laser are (Nd:YAG amplifier is 1mm in length, the HR is essentially 100%, the OC 99% reflecting, and the beam diameter 0.5mm), calculate the small-signal gain (g0) of the laser at 10C. This small-signal gain remains constant and is independent of amplifier temperature (re-absorption loss, however, increases with temperature). Outline, explicitly, all calculations required including saturation power.
  2. Using the small-signal gain from the above question, and the re-absorption loss calculated at various temperatures from the prelab, calculate the expected output power of the laser at 15C, 20C, 25C, and 30C. Present the results as a table with the columns Temperature, re-absorption loss, predicted output power (theoretical), observed output power(from this lab)
  3. One could argue that re-absorption loss as measured in this experiment is actually saturated loss. Knowing the output power, and hence the intra-cavity power, calculate the saturated re-absorption loss at 10C using the relations in Laser Modeling section 5.5 (specifically equation 5.15). Formulate re-absorption loss as a function of intra-cavity power "P" - it is expected that re-absorption loss will decrease from the "small signal" value calculated above as intra-cavity power increases. Assuming the saturated value is not significantly lower than the small-signal value, our calculated values are valid.