: Lasers and Light Sources
Characteristics of Laser Diodes (2016F)

Introduction

Laser diodes are by far the most common type of laser. In this lab the electrical and optical characteristics of a laser diode will be investigated including the effects of temperature. Concepts which will be introduced in this lab include the threshold of lasing (the minimum pump power required for a laser to produce an output), temperature control of optoelectronic devices (including the use of a thermistor as a temperature sensing device), and industry-standard characterizations.

The laser diode used in this experiment is seen here on a gold-plated mount. The diode sits atop a Peltier effect thermoelectric cooler module which can cool and heat the diode. On top of this assembly sits a thermistor used to accurately monitor the temperature of the diode.

Prelab (You need some key parameters to complete this lab)

WARNING: Arrival to the lab without your parts kit including meter and jumper cables will result in being marked ABSENT with the accompanying penalty (including a deduction in marks and being placed on course condition) - you will NOT be permitted time to "run home" to obtain your kit.

Experiment

Connect a laser diode module and determine the characteristics as follows:

  1. Mount the laser diode to the optical bench since the bench is used as a heatsink for the TEC.
  2. With no diode connected to the ILX supply, set the current limit to the maximum for the diode (from the datasheet) by selecting the "LIM I" parameter and holding the "SET" button while rotating the control knob until the current display reads the required value. Verify this with the professor if required.
  3. VERIFY the maximum current on the ILX supply is set to that specified in the datasheet BEFORE applying power to the diode!

  4. Connect a DMM set to read resistance across the thermistor allowing monitoring of temperature (these are the white wires on the terminal strip). The thermistor should read around 10KΩ at room temperature.
  5. Align a power meter so that it intercepts ALL of the resulting laser beam (it must be close to the device). Mount the power meter on the bench and use two posts to align the optical sensor directly in front of the diode.
  6. Set the wavelength of the meter to match that specified in the datasheet (use the "typical" value). With the laser diode off, zero the power meter.
  7. Plug the diode laser module into the DB-9 connector from the ILX supply.
  8. Rotate the current control knob counter-clock wise until it stops to set the output current to zero
  9. Enable the output (MODE, Output ON)
  10. Read the resistance of the thermistor from which temperature will later be computed
  11. Vary current from zero to the maximum current specified for the diode in 1mA to 2mA increments (start at 2mA and use 1mA where the output power is seen to change rapidly - it will make the graphs much smoother). At each step for current, record the optical output power in mW. (The current is kept lower than maximum since temperature will be elevated).

Laser Diode Setup

The complete setup. The laser diode and optical power meter sensor are mounted on the optical breadboard. Connections between the power supply and the multimeter are shown as wires but in reality jumper clips should be used.

Laser Diode Setup

The power meter mounted to intercept the beam from the diode laser. Be sure the entire assembly is stable. Finally, before beginning the experiment turn the current to 22mA (at room temp) and ensure the laser is oscillating and the output is over 500μW.

Now, complete the experiment again with the diode at a cool temperature. To cool the laser diode, connect the TEC black wire to the Agilent bench supply negative output terminal (V1) and the red wire to the postive output terminal. Increase the voltage very slowly until a current of 1A flows through the TEC. The resistance will be observed to increase as the device cools. Wait five to ten minutes until the lowest possible temperature is reached and is relatively stable (this may require the voltage to be reduced slightly) then repeat the measurements of optical output power vs. current. The TEC current is kept applied during this time to continually cool the device. After the run is complete, record the resistance of the thermistor so that the temperature of the diode can be calculated.

Now reverse the current through the TEC (reverse the positive and negative terminals), allow the diode to heat to a maximum point (again, stabilizing), and repeat measurements again (including thermistor resistance).

Upon leaving the lab, you should have three sets of Power-vs-Current data for the diode at three different temperatures. You will also have three resistance readings.

Analysis

To convert the resistance of the thermistor to a temperature an equation is used. The thermistor employed is a Vishay 238164063103 thermistor with a resistance of 10KΩ at 25C. Examine the data sheet for the thermistor - determine the β value for the specific thermistor, then the Steinhart-Hart coefficients. Report these coefficients (there are four: A1, B1, C1, and D1 which are all positive and all less than one which are the most commonly used) and calculate the temperature of the device from this (i.e. by substituting the coefficients and the measured resistance of the thermistor into the equation given in the datasheet on page 4). Use the formula from the datasheet, not from "just anywhere on the web" since the formula sheet is right from the manufacturer. From resistance measurements, you need to find the room temperature as well as the minimum (cool) and maximum (hot) temperature employed in this lab.

Before submitting values, be sure the room temperature reading is logical: it should be about 20C as the equation is very, very accurate.

Tutorial: Two-Slope Analysis using ExcelTM

Excel Two-Slope Analysis Using Excel to create a two-slope analysis: (This example assumes Excel 2013 ... actual procedure may vary slightly if a different version is used)

  1. Graph data in the usual manner: enter all experimental data into to columns (representing X and Y), highlight all the data points, and plot a graph. This is done for the example data given for the entire range x=1 through x=6.
  2. Determine the two ranges required for the two linear lines. In the example graph given, the upper range is defined by the x=4 through x=6 values and the lower range by the x=1 through x=3 values. This is done by inspection so that each range covers only the appropriate linear region (read the application note for more on these regions).
  3. Right click on the existing graph, click "Select Data", then "ADD" a series. Select the data for the new series using the upper range you determined, in this case the x=4 through x=-6 values.
  4. Add a trendline though the new series. Be sure to forecast backwards one period (or more, as required), and display the line equation on the graph.
  5. Repeat the same process with the lower range of data (add series, add trendline, forecast forward one period, display equation).
  6. Finally, to find the intercept of the two lines set the first equation (y=) equal to the second equation (y=) and solve for x algebraically. The answer will be in terms of x= which is precisely what is desired. In the example given, (X-3)=(0.175X-0.0833) and so solving for X gives 3.535.

Lab Report

Hand In a WORD PROCESSED (not handwritten) lab report with contents as outlined below.

Lab Report The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, the student's name and ID number, and the names of your lab partner(s).

Answer each question as "1", "2", etc with each new question starting on a NEW PAGE so that question 2 starts on the top of a new page (with the title "Question 2") and question 3 starts at the top of a different page (with the title "Question 3"), etc. You'll have, therefore, at _least_ seven pages in this report.

The lab must be submitted in a report cover (either a three-hole punched cover or one with a clamp on the left side, not a binder), and NEVER as a stapled mass of loose papers

Failure to follow this simple outline, used for all condensed labs in this course, will result in deduction of marks

LAB SUBMISSION:

    Observations
  1. Three tables of "raw" data, and corresponding graphs, of optical power-vs-current data (with current, in mA, on the x-axis). Be sure to label the graph axes and graph titles properly.
  2. In the last lab, you used the simple linear fit (single line) method to find the bandgap voltage from an I/V graph. While the same method could be used here to find the threshold current (i.e. the minimum current at which laser output begins) it has some shortcomings as outlined in the application note you read. To improve results, we will use the two-segment method.
    Using the method described in the ILX application note determine the threshold current of the laser diode at each temperature using the two-segment line-fit method. Show the graph of each set of I/P data with TWO lines superimposed on each graph (so three graphs are required, one at each operating temperature, and each graph will have the I/P data shown plus two linear lines). The equations for each line must be shown on the graph (to at least three significant figures) and an example algebraic calculation of threshold current shown for one graph. Show the numerical value of threshold current determined by this line-fit method DIRECTLY on each graph (i.e. add a text box as required to show it directly on the graph near the intersection of the two lines). NOTHING can be handwritten here - use the spreadsheet to make professional-looking graphs only.
  3. Steinhart-Hart parameters are used by every decent thermoelectric controller including many used at the college - you will use these type of controllers next year and will find they need to be programmed first with these same parameters you found in the analysis. Here, you will perform the same set of calculations as a controller does to determine exact temperature.
    Calculate the temperatures of the diode for each of the three runs from thermistor resistance using the Steinhart-Hart equation (more on this under "Analysis" above in this lab). Include one set of complete example calculations for the room-temperature resistance reading (these must show the Steinhart-Hart equation from the thermistor datasheet - it is on page 4 - and all numerical parameters must be shown). Report all three thermistor resistances and all three corresponding calculated temperatures.
  4. Research, in section 5.8 of Laser Modeling, the concept of characteristic temperature of a laser diode. Use the highest and lowest temperature values recorded (and corresponding values of threshold current determined in this experiment) to compute a value for the characteristic temperature of this laser diode. Show the complete set of calculations.
  5. Now that you have calculated the characteristic temperature of this device, use that value to compute the expected threshold current at the minimum and maximum operating temperatures for the device (as found in the datasheet). Show a complete set of calculations.
  6. Research what "slope efficiency" is (in section 3.5 of Laser Modeling) and then compute it from the power-vs-current graphs for each temperature. Show one set of calculations detailing how you obtained this value. The resulting slope of each graph will be in units of rise/run or mW/mA. Compare this to the corresponding values reported in the datasheet for the laser diode.

  7. Technology
  8. Peltier effect coolers are used extensively in the photonics industry to control the temperature of devices.
    Explain how a Peltier-effect thermoelectric element works. Include a diagram showing the composition of the device (including the semiconductor "pellets" which constitute the device). Be specific and explain, in a paragraph or two, how heat flows in the device and how the direction of current dictates the flow of heat (so that the same device can heat or cool).