Download Chapter 03a - Optical Spectrum Analysis

Chapter Contents
1. Basics of Optical Spectrum Analysers
2. Fabry-Perot Interferometers
3. Interferometers-based Optical Spectrum Analysers
4. Diffraction Grating-based Optical Spectrum Analysers
5. Anatomy of a Diffraction Grating-based Optical Spectrum Analysers
6. Spectral Measurements on Modulated Signals
7. Application Examples of Optical Spectrum Analysers
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Introduction
Spectroscopy - studying the properties of matter/device through its interaction
with different frequency components of the electromagnetic spectrum.
Immediate questions:
What does light do to sample?
How do you produce a spectrum?
Interaction of light with a sample can influence the sample and/or the light.
Processes involved: (1) excitation and (2) detection.
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Introduction
In most spectroscopies, we characterize how a sample modifies light entering it.
Absorption:
Change in intensity I of incident light
Sample attenuates light
Transmission T = I/I0
We measure the absorption of light at different frequency or wavelength.
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Introduction
Emission:
Excitation induces emission of light from the sample (usually of different
frequency).
Includes: Fluorescence (emission from excited electronic singlet states),
Phosphorescence (emission from excited electronic triplet states), Raman
Scattering (light scattering involving vibrational transition)
Optical Rotation:
Change of phase of light incident on sample (rotation of polarization)
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Introduction
Optical spectral analysis - measurement of optical power as a function of
wavelength
There are many kinds of spectral measurement devices, for example,
spectroscopes for human eye observation of the spectrum,
spectrometer to record spectral reflectance,
monochro meter to read a single narrow band,
spectro photometer for photometry,
spectro radiometer for measurement of spectral radiation etc.
However, in this section only optical spectrometers are of interest.
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Introduction
Many faces of classical spectrometers
Two-arm spectrometer
Three-arm spectrometer
One-arm spectrometer
Multiple-prism spectrometer
Hobart and William
Smith Colleges in
Geneva, New York
Garland
Collection
of
Classic Physics Apparatus
at Vanderbilt University
Westminster College in
western Pennyslvania
St. Mary's College in
Notre Dame, Indiana
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Introduction
Two types of spectrometers:
1) Dispersive
2) Fourier transform
Dispersive spectrometer:
Separate different frequency components
Fourier transform spectrometer:
A way of processing all wavelength/frequencies simultaneously
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Introduction
Dispersing spectrometers
Utilizes prisms or diffraction gratings.
A spectrum is obtained at the focal plane after a light ray passes through a slit
and dispersing element.
Typical dispersing spectrometers: Littrow spectrometer and Czerny - Turner
spectrometer.
8
Introduction
Interference spectrometers
Utilizes the interference of light.
Twin beam interference spectrometer - A distribution of the spectrum is
obtained by cosine Fourier transformation of the interferogram which is
produced by the inference between two split rays.
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Introduction
Interference spectrometers
Multi-beam interference spectrometer - The interference of light will occur if
oblique light is incident on two parallel semi-transparent plane mirrors. A
different spectrum is obtained depending on incident angle, interval of the two
mirrors and the refraction coefficient.
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Introduction
By analyzing the characteristics of the signal once its gone through the device/system, you can determine
the performance, find problems, troubleshoot, etc.
To measure the characteristics of the signal once its gone through the device/system, we need a passive
receiver (it doesn't do anything to the signal, it just displays it in a way that makes it easy to analyze the
signal). This is called a spectrum analyzer.
Spectrum analyzers usually display raw, unprocessed signal information such as voltage, power, period,
waveshape, sidebands, and frequency.
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Introduction
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