PHTN1306: Lasers III
Mathematical Model of a Semiconductor Laser and Thermal Issues (2017F)

Introduction

The threshold of pump power for a semiconductor is determined using a mathematical model and then compared to experimental results from a real diode laser. Using results of this first experiment a mathematical model predicting the buildup of laser power (based on gain saturation) is developed using a spreadsheet and compared to the Rigrod approach. Finally, the effect of temperature on diode performance is investigated.

Prelab:

Experiment:

Part 1: Determining Laser Diode Parameters

Diode Laser Setup
The complete laser diode setup. The diode itself is mounted inside an ILX LDM-4412 test fixture which features twin powerful Peltier-effect thermoelectric cooler modules as well as a TS-510 calibrated thermistor.

A US-Lasers 808-5 diode is installed in the test fixture. Turn on the ILX LDT-5910B Temperature Controller and set the temperature to 15C. Install a fiber such that radiation from the laser diode can be analyzed using the Agilent 86142B OSA (the fiber should be blue - swing it upwards such that the apeture intercepts the infrared beam from the diode).

Agilent 86142B OSA With the room interlock system enabled, turn ON the ILX LDX-3412 precision current source and increase the current to approximately 30mA. Using an infrared viewer card, align the fiber so that it intercepts the diode output and then align manually such that output appears on the OSA. The laser diode output should appear on the OSA around 808nm and multiple modes may well be visible.

The OSA is seen in this photo along with a sample output from the laser diode. See chapter 13 of Csele to get an idea of where the modes originate. This will allow you to compute the length of the cavity and hence the gain medium, a parameter not found on the data sheet for the diode.

Part 2: Determining Characteristic Temp

Part 3: Determining Wavelength coefficient of Temperature

Analysis:

You might need a few numbers to get started:
Cross section σ0 = 1 * 10-19 m2
ULL Lifetime τ = 1 * 10-9 s
Index of refraction of AlGaInP = 3.7
Attenuation γ = 25 cm-1

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 ...

    Threshold:
  1. Hand-in a graph of Poptical vs. drive current for the laser diode as observed in the lab for various temperatures.
  2. Show, on the graphs above, the threshold current for the device and explain in a paragraph how it was determined (i.e. summarize the method from the ILX application note that you used)
  3. Calculate the slope efficiency of the diode (you might have to research this). Include a Power IN:OUT plot and show how slope efficiency was computed. Remember that electrical power is calculated as current times voltage (outline where you obtained any constants required such as device voltage).
  4. Calculate the threshold gain of the diode (In the same manner as problem 5, chapter 5 of Csele, but using the device parameters found experimentally in this lab).
  5. Calculate the theoretical threshold current for the diode (show all calculations and formulae used) and compare to the experimentally-determined value. This question is worth considerably more marks than others in the lab and a large degree of detail is required to show how you developed the model. Show all intermediate steps and equations.
  6. Calculate the characteristic temperature (T0) of the diode using the method outlined in the prelab reading (from ILX). Show all calculations required (e.g. ln(Jth)) as well as the graph (for which the slope of that graph is T0).
  7. Calculate the wavelength coefficient of temperature showing all work.
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