PHTN1306 Lasers III - Lab
DPSS Laser Design - Part 4 (2018F)

Introduction

While convolution (covered in the last lab) is used to predict the effect of pump diode temperature on laser output, in this lab we will experimentally determine actual small-signal gain of the laser (which embodies absorption) as a function of pump diode temperature.

Experiment:

PART A

In this part of the lab you will prove that small-signal gain (g0) is linearly proportional to pump power. For this lab the same 1064nm DPSS as employed in PHTN1400 will be used.

Begin by setting the diode to a constant temperature of 34C and the DPSS laser to a constant temperature of 20C. Monitor both the pump power (at 808nm) and the output power from the DPSS (at 1064nm) and sweep through a range of pump powers from the onset of DPSS lasing to about 12mW output (in steps of about 2mW of output).

PART A (Analysis for Homework)

The DPSS is a VANADATE laser. Compute the re-absorption loss at 20C (which will be small, regardless), calculate the saturation power, and use the homogeneous formula to compute the small-signal gain. Do this for each observed output power. Now, plot small-signal gain (y-axis) against pump power (x-axis) remembering that actual pump power is many times larger than the meter reading.

Add a trendline, and hopefully it is linear and a good fit.

946nm Laser Setup

The basic experimental setup: The PSU-III power supply unit controls the pump diode current (keeping it constant at 2.000 A) while the Nd:YAG amplifier temperature is kept constant by an ILX-5910C temperature controller (keeping re-absorption loss constant for this experiment). A second ILX temperature controller allows the temperature of the pump diode (and hence the pump diode wavelength) to be varied.

PART B

A Laserglow R94050XSX 946nm DPSS laser is connected such that the external ILX LDC-5910C temperature controller controls the temperature of the pump diode. A second ILX LDC-5910B temperature controller keeps the amplifier at a constant optimal temp (the R94050XSX laser features a separate diode and amplifier TEC so the amplifier temp is independent of pump diode temp). This allows determination of absorption and then small-signal gain as a function of pump diode temperature (and hence emission wavelength) alone.

Program the LDC-5910C controller with the correct Steinhart-Hart parameters for the pump diode TEC, and the second controller with the correct parameters for the amplifier TEC. Set the amplifier temperature to 10C and leave it constant for the experiment. Set the diode temperature for 27C and enable the controller. Next, switch the "PSU-III" power supply on which will operate the diode at a constant pump current. 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.

Now, decrease the diode temperature alone to 10C and increase in 1C increments and measure the output power at each temperature after allowing it to stabilize. Continue until a temperature of 42C is reached.

Part B: Analysis (for Homework)

#1: Knowing the temperature of the amplifier, calculate the re-absorption loss of the amplifier (which remains constant for the experiment).

#2: Knowing the observed output power, and parameters of the laser, use a simple homogeneous model to compute small-signal gain of the laser as a function of output power.

#3: Convert each temperature reading for the diode into emission wavelength by knowing the peak wavelength was observed to be 808.67 nm at a temperature of 24.00 C and that the wavelength temperature coefficient is 0.208 nm/C (these values were measured on the actual laser, not taken from a datasheet).

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. For part A, outline a complete set of calculations to determine the small-signal gain for one observed output power value. Include calculations of saturation power and re-absorption loss
  2. For part A, show a graph of small-signal gain (of Vanadate) vs. pump power. Include the trendline and show the equation right on the graph.
  3. For part B, show a plot of observed data in this lab including laser output power vs pump diode temperature and a second plot of laser output power vs pump diode wavelength.
  4. 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), and knowing the observed output power, calculate re-absorption loss (which remains constant throughout the experiment) and then using a homogeneous model the actual small-signal gain (g0) of the laser at 10C. Outline, explicitly, all calculations required including re-absorption loss, saturation power, and show how small-signal gain was calculated.
  5. Calculate the small-signal gain as a function of pump diode temperature as well as pump diode wavelength. Submit a table of pump wavelength vs. small signal gain. Include, as well, a plot (graph) of the same data (pump wavelength vs laser gain) as well as the predicted laser output from the last lab (convolution) on the same graph with the same x-axis for comparison (this graph must be a second series on the same graph).