1. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11 波長 (Wavelength) 的精準量測 The amplitude versus wavelength, can be measured by wavelength meters or grating- based OSA’s. A wavelength meter can make the wavelength measurement with better than 0.001 nm accuracy. A grating-based OSA measures 0.1nm ( from “Fiber Optic Test and Measurement”, edited by D. Derickson)

2. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.1 Wavelength Definition The relationship between the wavelength in air and vacuum wavelength is : where is the vacuum wavelength, is the air wavelength,and is the index of refraction of air.

3. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.2 Accuracy and Resolution The major factor affecting relative wavelength resolution is the length of scan in the Michelson interferometer. The resolution capability of Fourier-transform Michelson wavelength-meter is given as Resolution(Hz)= Speed of light Total distance of the interferogram

4. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.3 Spectral Width The distance over which fringes are visible is a measurement of the coherence length of a source. If the wavelength meter doesn’t take into account the fact that fringes don’t exist away from zero path-length differences, incorrect wavelength measurement values will be displayed.

5. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.4 Methods of Wavelength Measurement Several approaches can be used to accurately measure wavelength: 1.Fabry-Perot(FP) or grating-based filters. 2.Interferometric fringe-counting techniques. 3.A sloping insertion loss versus wavelength can be used to determine optical wavelength.(wavelength discriminators) 4.Absorption cell for wavelength calibration.

6. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.5 Absorption-cell Technique The absorption cell technique can be used to calibration wavelength. The light from a broadband source such as an edging- emitting LED is passed through a glass tube containing a molecular gas (Gas Absorption Cell) Gas cells absorb radiation near the vibrational and rotational resonances of molecules. Broadband Light source Gas Absorption Cell OSA Fig 11.1 Wavelength Calibration using Natural absorption Lines.

7. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.6 Filter Techniques: Grating- Based Filter,Fabry-Perot Filter  Grating-Based Filter [from “Fiber Optic Test and Measurement”, edited by D. Derickson] Parameter Michelson Interferometer Grating-Based Optical Wavelength Meter Spectrum Analyzer Absolute wavelength 150ppm Accuracy 1ppm <30ppm with calibrator Wavelength 0.1 nm(12GHz at 1550 nm) 0.1 nm(12GHz at 1550 nm) Resolution depends on scan length depends on number of illuminated grating lines Dynamic Range 40dB limited by shot noise 70dB limited by filter stopband rejection Wavelength range All fiber optic wavelengths All fiber optic wavelengths

8. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.6 Filter Techniques: Grating- Based Filter,Fabry-Perot Filter  Fabry-Perot Filter FP filters are also referred to as etalons. In an FP interferometer, light is incident on an optical component with two reflecting surfaces. The light that exits the interferometer consists of a direct path added to a large number of reflected paths through the interferometer. The FP interferometer is most commonly used as a tunable filter.

9. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.6 Filter Techniques: Grating- Based Filter,Fabry-Perot Filter The FP interferometer is shown in Fig. The transmission loss through the filter is wavelength selective. The FP configuration can also be used as a fringe-counting wavelength meter. Dielectric Mirror stack Interference Between output Forms a bandpass filter Interference Between output Forms a bandpass filter T R T R Signal Input Cavity Length Fig 11.2 FP interferometer

10. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.7 Interferometric Fringe-Counting Techniques:Michelson Interferometer A diagram of a Michelson interferometer is shown in Fig.11.3 Moving Mirror Variable Arm Fixed Arm Fixed Mirror Signal Input Beam splitter Photodetector Fig 11.3 Block diagram of a Michelson interferometer.

11. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.7 Interferometric Fringe-Counting Techniques:Michelson Interferometer In order for interference to occur,the light from the two arms must overlap in space and be identical in polarization. The photocurrent generated from the interferometer detector is: I= the photodetector photocurrent, = the optical path length difference between the two interferometer arms, = the unknown wavelength of the light in the medium of the interferometer,and = a phase-shift difference for equal path length delays between the two arms.

12. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.8 Wavelength Discriminators A wavelength discriminator uses a filter that has its insertion-loss versus wavelength. An input signal is applied to the beam splitter, Fig shows the transmission and reflection characteristics versus wavelength for an example discriminator element.

13. 教育部顧問室光通訊系統教育改進計畫台科大 師大 淡江 東南 萬能 教育部顧問室光通訊系統教育改進計畫 台科大 師大 淡江 東南 萬能 11.8 Wavelength Discriminators If most of the photocurrent is found on detector 1 and very little on detector 2, then the signal wavelength must be on the short end of the wavelength range. Det 2 Det 1 Input Wavelength Dependent beamsplitter Det 1Det 2 Wavelength Photocurent Fig 11.4 Wavelength discriminator