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optical Components

optical Components. 9/20/11. Applications. See notes. Optical Devices. Optical Devices Active Passive (reciprocal & non-reciprocal) Wavelength Selectivity Fixed Tunable Parameters Temperature dependency Insertion loss (input output loss) Inter-channel cross-talks

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optical Components

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  1. optical Components 9/20/11

  2. Applications • See notes

  3. Optical Devices • Optical Devices • Active • Passive (reciprocal & non-reciprocal) • Wavelength Selectivity • Fixed • Tunable • Parameters • Temperature dependency • Insertion loss (inputoutput loss) • Inter-channel cross-talks • Manufacturability • Fast tunability • Stability and polarization dependency • Impacting the system: • Error-free • Selectivity • # of channels that can be supported • Interferences

  4. Spectral Width Spectral content of a channel

  5. Passive Devices • Reciprocal (input/outputs act the same way) • Couplers • Half-wavelength plates • Non-reciprocal • Circulators • Rotators • Insulators

  6. Couplers • Structure • NxN (e.g., 2x2) • α is proportional to l (α is coupling ratio, l is coupling length) • Parameters of interest • Coupling ratio • Coupling length • Excess loss (beyond α) • Type • WL dependent (α has WL-dependency) • WL independent • Splitting ratio • 3dB (splitting the power evenly) - α=0.5 • Taps (e.g., α ∼ 1 – thus, a very small portion is dropped)

  7. Couplers • They can combine or separate different wavelengths • The lights (different wavelengths) are coupled together • Example: 8x8 3-dB couplers 1310 (signal) Amplified Signal 1550 nm (pump)

  8. Half-Wavelength Plates • Passive reciprocal devices • They maintain the polarization but rotate the orientation of polarization is rotated by by ΔΦ=2πR; (R=+/-0.25 for λ/4) • Note d= Rλ/Δn; d is the thickness of the birefringent plate – assuming mica or quartz plate

  9. Passive Non-Reciprocal Devices • Types • Isolators • Faraday Rotators • Circulators

  10. Isolators • Transmit in one direction only • Avoid reflection of laser – or any reflection • One input, one output or multiple ports • Key parameters are insertion loss and excess loss • Example of circulators:

  11. Operation of Isolators Only Ex exists State of polarization is fixed (SOP) Rotator rotates by 45 degree

  12. Operation of Isolators – more realistic Polarization Independent Isolator Half-wavelength plates are used to rotate 45 degree The Spatial-walk-off polarizer splits the signal into two orthogonally polarized signals

  13. Prism

  14. Spectral-Shape Parameters Cascaded filters  narrower passband We desire broad passband at the end of the cascade Thus, each filer must have a flat passband (accommodating for small changes in WL) The flatness of the filer is measure by 1-dB bandwidth

  15. Components

  16. Gratings • Describe a device involving interference among multiple optical signals coming from the same source but having difference phase shift • There are a number of gratings • Reflective • Transmission • Diffraction • Stimax (same as reflection but integrate with concave mirrors

  17. Gratings • --- Transmission • The incident light is transmitted through the slits • Due to diffraction (narrow slits) the light is transmitted in all direction • Each Slit becomes a secondary source of light • A constructive interference will be created on the image plane only for specific WLs that are in phase  high light intensity • Narrow slits are placed next to each other • The spacing determines the pitch of the gratings • Angles are due to phase shift

  18. Diffraction Gratings • It is an arrayed slit device • It reflects wavelengths in different directions

  19. Bragg Grating Structure (notes) • Arrangement of parallel semi-reflecting plates

  20. Fiber Bragg Gratings • Widely used in Fiber communication systems • Bragg gratings are written in wavelengths • As a result the index of refraction varies periodically along the length of the fiber • Variation of “n” constitutes discontinuities  Bragg structure • Periodic variation of “n” is occurred by exposing the core to an intense UV interference pattern • The periodicity of the pattern depends on the periodicity of the pattern

  21. Optical Add/Drop Using Fiber Bragg Grating FBG has very low loss (0.1 dB) Temperature dependent  change of fiber length The are very useful for WDM systems They can be used with 3-port Circulators

  22. Optical Add/Drop Using Fiber Bragg Grating

  23. Fiber Bragg Chirped Grading • Fiber Bragg grating with linear variable pitch compensates for chromatic dispersion • Known as chirped FBG • Due to chirps (pitches) wavelengths are reflected back • Each WL reflection has a different phase (depth of grating) •  compensating for time variation  compensating for chromatic dispersion

  24. Fabry-Perot Filters • A cavity with highly reflective mirrors parallel to each other (Bragg structure) • Acts like a resonator • Also called FP Interferometer • Also called etalon

  25. Fabry-Perot Filters (notes)

  26. Power Transfer Function • Periodic in terms of f • Peaks are called the passbands of the transfer function occurring at f (fτ=k/2) • R is the coefficient of reflection or reflectivity • A is the absorption loss

  27. FSR and Finesse • Free spectral range (FSR) is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima • An indication of how many wavelength (or frequency) channels can simultaneously pass without severe interference among them is known as the finesse Transfer function is half

  28. Tunability of Fabry-Perot • Changing the cavity length • Changing the refractive index within the cavity • Mechanical placement of mirrors • Not very reliable • Using piezoelectric material within the cavity • Thermal instability

  29. Multilayer Dielectric Thin Film • Dielectric thin-film (DTF) interference filters consist of alternating quarter-wavelength thick layers of high refractive index and low refractive index • each layer is a quarter-waveleng th thick. • The primary considerations in DTF design are: • Low-pass-band loss « 0.3 dB) • Good channel spacing (> 10 nm) • Low interchannel cross-talk (> -28 dB)

  30. Thin-Film Resonant Multicavity Filter • Two or more cavities separated by reflective dielectric thin-film layers • Higher number of cavities leads to a flatter passband • Lower number of cavities results in sharper stop band

  31. Thin-Film Resonant Multicavity Filter • A wavelength multiplexer/demultiplexer

  32. Mach-ZehnderInterferometer • Uses two couplers • The coupling ratio can be different • A phase difference between two optical paths may be artificially induced • Adjusting ΔL changes the phase of the received signal • Because of the path difference, the two waves arrive at coupler 2 with a phase difference • At coupler 2, the two waves recombine and are directed to two output ports • each output port supports the one of the two wavelengths that satisfies a certain phase condition • Note: • Δf=C/2nΔL • ΔΦ=2πf.ΔL.(n/c)

  33. Tunability • Can be achieved by altering n or L

  34. Absorption Filter • Using the Mach-Zehnder Interferometer • consist of a thin film made of a material (e.g., germanium) that exhibits high absorption at a specific wavelength region

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