Lab VII Semiconductor Laser Properties ECE 476 I. Purpose The electrical and optical properties of a semiconductor laser will be investigated. II. Background Laser diode basics A semiconductor laser is a p-n junction diode constructed from a direct band-gap semiconductor material. The typical characteristic of a semiconductor laser is shown in Fig. 1. Below the threshold current the device operates as an LED with an output of light Figure 1: LED laser Current characteristics. across a wide spectral region (20-30 nm). In the laser region the spectral output narrows to a single emission wavelength. The transition of the output from LED characteristics to laser characteristics is shown in Figure 2.Figure 2: Radiant output as a function of frequency for a p-n junction laser. The light output for spontaneous emission in the LED operating region is not polarized and the light output for stimulated emission in the laser operating region is often polarized. Figure 3: Laser LED vector board. LED laser protection Semiconductor lasers operate at very high current densities to get stimulated emission. To help dissipate the heat a small heat sink is attached to each laser diode (the metal ‘star’ in figure 3). The laser diodes are more expensive (25-30 dollars) than LEDs (few cents), so use caution to not damage the lasers. (Too much current is the most common damage mechanism, so limit your current to the rated operating current.) The specification for the semiconductor laser you will use is given below. The maximum threshold current is specified as 40 mA. Often the threshold current is lower in the range of 27-35 mA. It is recommended you not exceed a current rating of 45 mA. The laser is already connected in series with a resistor of value 470 ohms. This resistor is used to help protect the laser diode from overcurrent damage.LED laser datasheet LN9705 Optical output: 5 mW Threshold current: 40 mA Operation current: 50 mA Wavelength: 788 nm Radiation Angle (parallel): 10 degrees Radiation Angle (vertical): 25 degrees Dark Current: 0.1 uA Operation Voltage: 1.75 V III. Procedure Part A: Power Measurement Set-up 1. Re-read the background section on LED laser protection. These particular LEDs are no longer manufactured and would be very difficult to replace if damaged. Experiment 1. Measure the output light power versus input current. Plot this data and determine the threshold current value. Note: Be sure to calibrate your power meter. Note: You will need to measure optical power output for LED current between 0mA to no more than 45mA. You may measure the optical power for every milliamp, however it is not necessary. Since the power does not change much in the lower range of currents (0-15mA), you could make readings every 5mA. From 15-25mA, you might take readings every 2mA. From 25-40mA, I recommend taking readings at every milliamp, since this is where you will find your critical threshold current. 2. Above the threshold current condition, where the device is operating as a laser, calculate the external differential quantum efficiency ηex, which is the ratio of the increase in photon output to the increase in the current input (How many photons are emitted for each electron input.) The expression for the efficiency is IhfPqexΔΔ=ηwhere q is the charge of an electron, hf is the photon energy, ΔP is the change in light output, and ΔI is the change in current input. Note: The best way to judge the value of threshold current is to examine the rate of change among your measurements in the upper range (25-40mA). The greatest change among the rates of change (i.e. maximum of the second derivative) is approximately where the threshold is. For instance, if 25 to 26mA changes by 20uW, 26 to 27mA changes by 100uW, and 27 to 28mA 120mW, you can approximate your threshold current to be 26mA. 3. How does the efficiency of the laser LED compare to the efficiency of the LEDs from the previous lab? 4. Use your infrared detector card to look at the shape of the output laser beam. Describe this shape in your lab notebook. Part B: Polarization Experiment 1. You will measure the polarization percentage of the laser diode. Measure the power output from the laser for different polarizer angles with a polarizer placed between the laser and the optical power meter. (Definition: Polarization percentage is the ratio of (minimum power)/(maximum power) where maximum power and minimum are for different rotations of a linear polarizing filter.) Perform this measurement for two cases: (1) a current level 4 mA above the threshold current and (2) a current level of 5 or 10 mA below the threshold current value. Discuss your results. Note: You need to report 6 measurements: minimum power and maximum power for below threshold, minimum and maximum power for above threshold, polarization percentages for both cases. C. Spectroscopy Set-up 1. You will measure the spectral output of your laser using the monochromator you used for the LED experiment. Follow the operating instructions for the monochromator found in the beginning of the previous lab. Experiment 1. Take a spectral measurement at three current levels including 5 mA below threshold current, at the threshold current, and 4 mA above the threshold current. For each of these plots determine the FWHM (full width at half maximum) in nm for the light output. Discuss your results. Note: Make sure the monochromator is calibrated to the correct wavelength. Note: You need three plots. Compare them to figure 2 to make sure you’ve captured them correctly. IV. Conclusion Draw conclusions on this experiment. Comment on the performance of the laser LED at different current