LAB 2. SPECTROSCOPY AND INTRODUCTION TO OPTICS
FALL 2010 – ONE WEEK LAB
Objective
To become familiar with the optical part of the electromagnetic spectrum. After this lab, the
student should understand the frequency and wavelength ranges of the optical spectrum. This lab
will also place the visible spectrum within the context of the overall EM spectrum. Furthermore,
it is of fundamental importance that the students gain an understanding of the fundamental
differences between the RF and optical portions of the E-M spectrum.
PRELAB
1) What range of frequencies (Hz) make up the RF portion of the E-M spectrum? Optical
portion? List one (any) wavelength that would fall within the following color groups: A) Red;
B) Green; C) Blue. For example, if yellow were required, we could answer with 589nm.
2) What are the ranges of wavelengths for Question 1?
PART I
INTRODUCTION TO THE PHOTONICS LAB
Spend time exploring the lab and becoming familiar with the location of the optical components if
you did not already do this last week. The Instructor/TA will provide a tour and will explain the
proper care and handling of equipment. Equipment is scattered all around the room, so you should
not be afraid to look around for things as you may need them throughout the semester.
PART II
ELECTROMAGNETIC SIGNALS IN THE RF SPECTRUM
Discussion
Several common signals are transmitted via RF. This includes television and commercial radio
station broadcasts. We will use this part of the lab to investigate the frequency ranges of some
common signals. We will do this by looking at signals picked up by common TV rabbit ears on
an RF spectrum analyzer. This will help us understand the major difference between RF and
optical in terms of frequency orders-of-magnitude.
Experiment
1)
The output of the TV antenna has an F-type connector. Using the F-to-BNC adapter provided,
attach the TV signal to the appropriate input of the RF spectrum analyzer (this is also called an
electrical spectrum analyzer or ESA for short).
2)
Investigate the signals that are produced. Adjust the gain and direction of the antenna. What do
you observe? Are there any preferred directions for specific ranges of signal (i.e. UHF vs. VHF)?
3)
Record the frequencies of the strongest signal you observe. Zoom in on one and describe what
you see (for example, the college station at 107.1MHz). Are there modulation sidebands? Is the
signal static or dynamic? Explain. If you are confused about this step, see your TA.
4)
If the signal is very noisy, you might try to set up the ESA to average the signal. If you are
confused about how to do this, the TA can show you.
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PART III
OPTICAL DETECTORS
Discussion
The detection of optical signals, owing to the very small wavelength, is done in a way very
different from the traditional ‘rabbit ears’ you used in Part II. Usually, a semiconductor is
employed where a photon is absorbed and creates an electron-hole pair and subsequently a photocurrent is produced. This is very different from inducing a current on a physical antenna. Since
the detector absorbs a photon, the detector turns power (and not magnetic field) into current.
Normally, a specification, known as the responsivity, is provided for a detector. This spec usually
has units of Amps per Watt, and is has a strong wavelength (or color) dependence based on the
material that it is composed from. We will gain basic familiarity with detectors in this brief
exercise.
Experiment
1)
Find an optical detector head inside one of the drawers if it is not already on the table. If it has an
attenuator over it, remove it. Why might there be an attenuator on the detector? Inspect the
detector.
2)
Connect a silicon detector to an oscilloscope (model 818-SL printed on the back). What happens
when you hold the detector up to the light? Does the fluorescent light have a continuous-wave
(CW is the optical analog of DC) or modulated output? If modulated, what is the modulation
frequency? Explain.
3)
Try again with the germanium detector (model 818-IR). What do you observe? Why? Hint:
Take a look at the responsivity curve.
PART IV
SPECTROSCOPY OF BROADBAND OPTICAL SOURCES
Discussion
In this experiment we will use a commercial spectrometer with a CCD array to measure the
spectrum of some incoherent (or broadband) sources of light. An example of such a source would
be a white incandescent light bulb. The spectrometer displays the optical spectrum of the source
in wavelength units (nm), in contrast to the RF spectrum analyzer, which displays the spectrum in
frequency units (Hz).
Computer
USB
Ocean Optics
Spectrometer
Light
Fiber Input
Tunsten
Halogen
Lamp
Optional Lens
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Experiment
1)
Obtain a spectrum of a tungsten-halogen lamp (mag-lite) using the Ocean Optics spectrometer. Be
sure that the blue spectrometer is connected to the left front USB port, and open the program
called OOIBASE32. The spectrum will be displayed on the computer. To obtain a spectrum,
simply shine the light source directly into the input fiber as shown in the figure below. Record the
peak wavelength and width of the spectrum at half of the peak value. This is known as the
spectrum Full Width at Half Maximum (FWHM) and is the most common way used to describe
the spectral width of an optical signal. You may use a focusing lens if necessary.
2)
A current source and LED are provided. Repeat 1) for the LED source. Hint: You should
provide current to the LED. Do not exceed 75mA.
3)
Repeat for the tunable Bausch and Lomb monochromator. How do these spectra differ from that
of the LED and halogen light sources? What could be a possible reason for this? You may open
the monchromator to help you answer this question. Adjust the various settings on the
monochromator and comment on the results.
4)
Investigate other sources of light you may be interested in.
PART V
SPECTROSCOPY OF NARROWBAND OPTICAL SOURCES
Discussion
In this part of the lab, we will investigate the spectroscopy of narrowband optical sources, such as
lasers. In this part of the course, it is very important to gain a qualitative understanding of the
difference between narrowband and broadband optical spectra. For now, we may consider that the
laser spectrum is a ‘delta function’ or singular in frequency. Later in the course we will learn that
this is generally not the case, which can lead to some very interesting consequences in beam
propagation and image formation. At this point, we will not assign arbitrary numerical bounds on
whether a signal falls within the narrow or broad-band category, since this generally depends
largely on the application.
Experiment
1)
Obtain a spectrum of the various lasers on your bench using the Ocean Optics spectrometer. The
operating instructions will be found on the course website. To do this, simply shine the light
source directly into the fiber as shown in Figure 1. Record the peak wavelength and FWHM.
You may use a focusing lens if necessary.
PART VI
INTRODUCTION TO OPTICS
Discussion
The purpose of this exercise is to become familiar with setting up optical experiments using the
parts found in your optical station. By using the imaging equation, we will determine the focal
length of a lens.
Procedure
Mount a lens of unknown focal length. Using a white light source (Hint: Use a Mini MagLite
mounted in a cross-tie), form an image of a slide on a screen using your lens (see figure). If the
image of the MagLite filament is causing you problems, you may place a piece of frosted glass
just beyond the MagLite output. The TA can show you what this looks like. Measure the object
and image distances from the center of the lens. Measure the object and image heights using a
meter stick to determine the magnification M. Is the image upright or inverted? Move the lens
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relative to the object and move the screen to find the new image plane. Measure several
object/image distance pairs for the lens. Plot 1/z1 vs. 1/z2 and fit a line to the data.
Image in Focus
Light Source
Object
Lens
A
z1
z2
WRITEUP QUESTIONS
1)
Convert the frequencies you recorded in Part II, Procedure 3, to wavelength units.
2)
Can you use the detector and ESA to determine the modulation frequency of an amplitudemodulated optical signal? How would you do so, and explain why this would work.
3)
Estimate the temperature of a tungsten-halogen black body radiator using the data from Part IV.
How would the emission spectrum of a hot, glowing, electric stove top differ from the tungstenhalogen lamp? HINT: Think about the color of the glow and where the peak wavelength probably
appears. Might there be any invisible portions of the spectrum?
4)
Convert the spectral widths (nm) recorded in Parts IV and V to frequency units. Hint: You will
have to take the differential of the following equation: c   .
5)
What is the typical peak frequency of a red laser? How does this frequency compare to the
frequencies recorded in Part II? By how many orders of magnitude do they differ? Compare the
information carrying capacity of RF and optical sources in communications systems.
6)
Determine the focal length of your lens from your plot in Part VI by using the imaging equation.
Discuss the significance of the intersection of your data with the axes. Are these intersections at
the same values on the abscissa and ordinate axes?
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