PHTN1306: Lasers III
DPSS Lasers Part 1: Semiconductor Pump Lasers (2018F)

Introduction

The characteristics of a common 808nm Nd:YAG pump laser diode are examined including the wavelength coefficient of temperature and the rate of current tuning. These parameters will be used as a 'building block' when designing the DPSS laser used as a vehicle in this course.

Prelab:

Experiment:

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 small portion (4%) of the output from the laser is split allowing wavelength to be determined while the majority passes through the beamsplitter to a power meter. A 2.4A constant-current power supply powers the diode laser.

Part 1: Determining Wavelength Coefficient of Temperature

The most important parameter of a laser diode is the wavelength coefficient of temperature which relates how wavelength drifts as temperature increases.

Begin by donning SAFETY GLASSES suitable for this diode (see the parameters on the datasheet).

Turn on the ILX LDT-5910B Temperature Controller and set the temperature to 15C. Wait for the temperature to stabilize. Set the power supply as follows: VOLTAGE METER to 2.4V DC range, RANGE to x1, MODE to Power Supply, CURRENT RANGE to 2.4A DC. Turn the current control fully counter-clockwise (i.e. to zero) then turn the power supply on and increase the output current for a constant current of 2A. Ensure the power meter is set for a wavelength of 808nm. Record both the output power of the laser diode and the center emission wavelength. Now, increase the temperature in one degree increments and record the output parameters again. Continue until a temperature of 30C is reached. From this data the wavelength coefficient of temperature can be determined.

Part 2: Determining Characteristic Temp and rate of current tuning

In order to determine this parameter, the exact threshold current must be found at two different temperatures. A similar method to that employed in previous laser courses will be used here.

Set the temperature to 15C and allow it to stabilize. Set the current to 200mA and record the output power of the laser and the center wavelength. Increase the current in 200mA increments and record the output power, output wavelength, and voltage across the device at each step (optical power, wavelength, current, and device voltage data points) until a current of 2A is reached. Sketch a quick L/I curve to ensure the data is valid.

Increase the temperature of the diode to 35C and repeat the observations, again ensuring recorded points are valid.

For homework, plot proper L/I graphs for the diode in the same manner as the application note of the prelab and determine the threshold current at each temperature using lines of best-fit.

Homework: Knowing the typical voltage across the diode (from calculations of the bandgap voltage) and the current, plot electrical Power IN versus optical Power OUT and determine the slope efficiency of the diode at 15C (Power IN being electrical power, Power OUT being optical power).

Part 3: Miscellaneous

Observe the output spectrum at high resolution and determine the spacing of two longitudinal modes. This will allow determination of the length of the device and then a theoretical computation of threshold current.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2929129/

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 Fundamentals by 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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