1 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

2 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.

3 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/I 0 We measure the absorption of light at different frequency or wavelength.

4 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)

5 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.

6 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 Westminster College in western Pennyslvania St. Mary's College in Notre Dame, Indiana Garland Collection of Classic Physics Apparatus at Vanderbilt University

7 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

8 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.

9 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.

10 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.

11 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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