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part A

Related Reading: SO p. 318, Sections 7.6.1, 7.6.2, 8.4

Reference: Projects in Fibers Optics Applications Handbook, Newport Corporation, Fountain Valley CA 1986, Section 5, p. 47-53.

Guide for Spectroscopy, ISA Jobin Yvon SPEX Instruments Group, Edison NJ, 1994.

NOTE: Please bring an IBM formatted disk to lab. You will save your data from the spectrometer in ASCII format on this disk. This data can be imported into your favorite graphing package after some editing.

In this lab, you will learn about different sources of light including white light sources, gas (HeNe) and semiconductor (diode) lasers, and LEDs. Specifically, you will focus on their optical intensity versus wavelength and for the diodes, their optical power versus input electrical current.

SAFETY NOTE: In this lab, we will use lasers which emit in both the visible and the infrared. NEVER look directly into a laser as severe damage to your eye could result. Laser safety goggles will be provided in lab.

EQUIPMENT

• Spex 500M Monochrometer with computer control
• Spex 1683 Tungsten Halogen 30W Light Source in Spex 1682AG Housing
• Spex PS200 Power Supply for Tungsten Halogen Lamp
• Edmunds Laser Pointer (670 nm)
• Melles Griot 2 mW HeNe Laser (633 nm)
• Newport ULM-Tilt and 340RC HeNe Mount
• Laser Safety Goggles
• Newport 1830C Optical Power Meter
• Newport FK-DRV LED and Laser Diode Driver
• Newport FK-ILD - Laser Diode at 780 nm
• Newport FK-LED - LED at 830 nm
• Newport FK-POL Polarizing Sheet
• Fluke Digital Multimeter

Funding for this equipment came from the Pennsylvania Equipment Grant Program, the Bucknell College of Engineering, and the National Science Foundation through Grant #DUE 9552260 of the Instrumentation and Laboratory Improvement Program.

PROCEDURE

Due to limited equipment, each section of the lab should be performed in a group of 2. Your group may start with either PART A or PART B.

PART A SPECTROMETER

BACKGROUND

An optical spectrometer is an instrument capable of measuring the intensity versus wavelength or "spectrum" of a source. The main components of a spectrometer system are shown in FIGURE 1 and include an entrance slit, a collimating element (to make rays parallel, usually a mirror, M1), a dispersing element (a diffraction grating), a focusing element (a mirror, M2), an exit slit, and a photodetector.

FIGURE 1 Schematic of Spectrometer System in Dana 13.

At Bucknell,. we have a SPEX 500M with variable exit and entrance slits and adjustable from 3 µm to 3 mm. (For your reference, this snazzy system cost about $22,000.) The smaller the slits, the higher the resolution, but the smaller the signal. A computer controls the stepper motors on the 500M which move the diffraction grating to change the wavelength of light at the exit slit i.e. perform a wavelength scan. To start the Autoscan software for the 500M, type "AS" from a DOS prompt.

When performing a measurement with the spectrometer, you must choose the widths of the slits, range of wavelengths to be scanned, step size, and length of time for the measurement.

Before starting a measurement, be sure that all of the following are ON

• IBM PC
• Power Strip
• DS1000 computer control for 500M
• MSD2 Controller Interface
• PS/TC-1 Temperature controller for the Detector

PART A1 WHITE LIGHT SOURCE

• Turn on the PS200 Power supply for Tungsten-Halogen "White Light" source. Set the Mode button to RUN to turn on the light. (Note: It will take a little while for the power to ramp up and the light to turn on.)
• OBTAIN spectrum of intensity vs. wavelength of the white light source. Choose an appropriate wavelength region for taking your data. Be sure that this covers the visible range. Use a step size of at least 10Å. Be sure to save your data in ASCII format on your disk.
• Turn off the white light by setting the Mode button to STOP. Turn off the PS200 Power Supply.
• Place the FK-POL polarizing sheet in front of the white light source. Is the output polarized? If so, determine the plane of polarization.

PART A2 HeNe LASER

• Remove the white light source from the spectrometer.
• Position the HeNe laser so that its beam goes through the entrance slit of the spectrometer. Maximize the signal using "Real Time Display" in Autoscan and the two adjustment knobs on the HeNe ULM-Tilt mount. . Begin with a large slit size and make the slits more narrow as your alignment improves. Obtain spectrum of intensity vs. wavelength of a HeNe laser.
• What is the full width at half maximum (FWHM) of the HeNe?
• What is the peak wavelength of the HeNe to the nearest 0.01 Å?
• Is the output of the HeNe polarized? If so, determine the plane of polarization.

PART A3 LASER DIODE (OPTIONAL)

• Position the laser pointer so that its light goes through the entrance slit. Use the "Real Time Display" in Autoscan to verify that the detector is receiving the laser light. Begin with a large slit size and make the slits more narrow as your alignment improves. Be sure laser pointer is stable before beginning your scan.
• Obtain spectrum of intensity vs. wavelength of the laser pointer.
• Is the laser diode's output polarized? If so, determine the plane of polarization.

PART A4 NEWPORT DEVICES

Spectra for the Newport FK-LED and FK-ILD used in Part B are provided in FIGURE 2 and FIGURE 3. Use this data to estimate the FWHM for each device. (Use main mode for the ILD.) How do they compare?

Note that although the Newport FK-ILD has a dominant or main mode, it also has many side modes. The spacing between these longitudinal modes Æl is given by

(LAB EQ1)

where lo is the wavelength of the main mode, n r is the refractive index of the laser, and L is the cavity length. If n r is about 3.5 for this laser, can you estimate L in µm?

FIGURE 2 Spectrum of intensity versus wavelength for the Newport LED operating at 1.5 mW. Data was taken on the Spex 500M with 20 µm exit and entrance slits, 5 Å/step, 0.3 s/step.

FIGURE 3 Spectrum of intensity versus wavelength for the Newport ILD operating at 1.3 mW. Data was taken on the Spex 500M with 20 µm exit and entrance slits, 1 Å/step, 0.3 s/step.

PART B LED vs. Laser Diode

BACKGROUND

Use the Newport FK-ILD laser diode, FK-LED LED, FK-DRV driver circuit, and 1830 Optical Power meter for these measurements. Note that the LED operates around 830 nm while the injection laser diode (ILD) operates at 780 nm. The optical power meter measures the optical power incident on its detector. The output of the source you want to measure should be placed directly in front of the center of the detector. You should also set the measurement wavelength appropriately. Use the Newport 818-SL detector with the attenuation filter (Newport 883-SL). Be sure to tell the power meter that you are using the attenuator!

In Electronics, we measured the current-voltage (I-V) characteristics of various diodes to learn about their behavior and properties. For light emitting devices, the power-current (PI) curve is often used to characterize device performance. While the I-V curves of LEDs and Laser Diodes look quite similar since both are diodes, the PI curves do not since their emission mechanisms are different.

FIGURE 4 Sample Power versus Current (PI) curves for a Laser Diode and an LED

For part B see also this page

PART B1 LED

• Gather optical power versus current (PI) data for the LED at room temperature. To measure the current through the LED, use the LED monitor output and the fact that the monitor voltage to current calibration is 50 mV/mA. The current is limited to 120 mA. Be sure to increase the current slowly to avoid damaging the device.
• Use the Newport F-IRC1 IR Phosphor card to observe the output from the LED. This LED has a lens in front of the actual diode. This makes the output appear to be more focused that it is coming from the semiconductor. Note what the shape looks like for comparison with the ILD.
• Place the FK-POL polarizing sheet in front of the LED. Is the LED output polarized? If so, determine the plane of polarization.

PART B2 LASER DIODE

• Gather optical power versus current (PI) data for the ILD at room temperature. To measure the current through the ILD, use the Laser Diode monitor output and the fact that the monitor voltage to current calibration is 70 mV/mA. The current is limited to 100 mA. Be sure to increase the current slowly to avoid damaging the device. A typical threshold current is 50 mA.
• Be sure to turn the laser current down to zero before disconnecting the power to the ILD.
• After plotting Power versus Current, you may estimate the threshold current by drawing a straight line through the curve after the power has increased (while the diode is lasing) and extrapolating back to the current axis. What is the threshold current that you measure?
• Use the Newport F-IRC1 IR Phosphor card to observe the output from the ILD. This ILD has a lens in front of the actual diode. How does the output compare with that of the LED?
• Place the polarizing sheet in front of the ILD. Is the ILD output polarized? If so, determine the plane of polarization.

Your write-up should focus on using the data obtained in lab to compare a white light source with a laser, a HeNe laser with an laser diode, an LED with a laser diode. You may submit one typed report per lab group. Be sure to include all data.

Please include your comments on the lab for future improvements. These may be sent via email to lord@bucknell.edu or written up anonymously. Please describe your favorite part of the lab, the part of the lab that you found the most difficult, and the most important concept that you learned.

*be sure to have back-up data for everything in case people can't get it.

BE SURE TO KEEP TRACK OF TIME

QUESTION: What sort of detector do we have on the spectrometer?

NEXT TIME

• IV data?
• For diode lasers, we will learn about the dependence of the output power and wavelength on temperature and operating current.
• Gather data for PI curve of Laser diode at 17°C and 35 °C Need ILX box
• Gather PI data for visible LED. Discuss differences between LEDs and Lasers
• Calculate mode spacings for ILD.
• Compare PI data on laser diode and LED to data sheet. (DON"T HAVE SPEC SHEET!!!)
• Ramesh 3 (polarization, edmunds?), 6 (time constants)
• LED mount from circuit specialists

The divergence of this laser is specified to be 15° x 30°. You may measure this by measuring the widths of the beam parallel and perpendicular to the diode junction and using the distance from the diode. (CHECK THIS, Newport p.20, 48, 52))

NOTES

Spex Guide book. P. 15 White Light Source Spectra

p. 33 intro to monochrometers

p. 41 instrument profile from real instrument showing broadening

Pollock p. 306 polarization

radiation pattern varies from Lambertian (uniformly scattered in all directions) to single spatial mode with angular divergence of about a mradiab.

surface emitting LED Lambertian (gives math) while edge emitting LED and laser more complicated, less disperse

nice table comparing white light, LED, diode laser, HeNe

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last updated 7/9/97