Lecture Note on Optical Components Optical Couplers Combines

Lecture Note on Optical Components

Optical Couplers • • Combines & splits signals Light couples from one waveguide to a closely placed waveguide because the propagation mode overlaps the two waveguides Wavelength independent or selective Fabricated using waveguides in integrated optics = coupling ratio Power (Output 1) = Power (Input 1) Power (Output 2) = (1 - ) Power (Input 1) – – – • Power splitter if =1/2: 3 -d. B coupler Tap if close to 1 -selective if depends upon Lossless combining is not possible

8 -Port Splitter by Cascading Y-Couplers

8 x 8 Star Coupler Power from all inputs equally split among outputs

Isolators and Circulators • Extension of coupler concept • Non-reciprocal type will not work same way if inputs and outputs reversed • Isolator allows transmission in one direction, but blocks all transmission (e. g. , reflection) in the other. • Circulator is similar to isolator, but with multiple ports.

Filters and Multiplexers

Filters • • Low insertion (input-to-output) loss Filter passband independent of temperature Flat passband Sharp skirts on the passband & crosstalk rejection

Gratings • Device using interference among optical signals from same source, but with different relative phase shifts (i. e. different path lengths)

Transmission versus Reflection Grating

$Diffraction Gratings$

Diffraction Gratings

Mux/Demux Using Cascaded Filters • Each filter passes one and reflects the other s • Very flat top and sharp skirts

Amplification

Optical Amplifiers versus Regenerators 40 -80 km Terminal Regenerator - 3 R (Reamplify, Reshape and Retime) 120 km Terminal EDFA - 1 R (Reamplify) Terminal Terminal EDFA amplifies all ls EDFA is a common optical amplifier. Terminal

OEO Regenerator

1 R, 2 R and 3 R Regenerations 1 R = Reamplify 2 R = Reamplify, Reshape 3 R = Reamplify, Reshape, Retime

Regenerators versus Optical Amplifiers • Regenerators specific to bit rate and modulation format used; optical amplifiers are insensitive. • A system with optical amplifiers can be more easily upgraded to higher bit rate without replacing the amplifiers. • Optical amplifiers have large gain bandwidths => key enabler of DWDM • Issues – Amplifiers introduce additional noise that accumulates. – Spectral shape of gain (flatness), output power, transient behavior need to be carefully designed.

Optical Regenerator

Optical Receivers

Electronic versus Photonic Coding

Optical Modulation Methods

Eye Diagrams

WDM Multiplexing • TDM: 10 Gb/s upper limit B b/s 1 NB b/s 2 N • WDM: Use multiple carrier frequencies to transmit data simultaneously 1 2 N B b/s . . .

Multiplexers, Filters, Routers • Filter selects one wavelength and rejects all others • Multiplexer combines different wavelengths • Router exchanges wavelengths from one input to a different output

Considerations in Switch • Number of switch elements: complexity of switch • Loss uniformity: different losses to different outputs (especially for large switches) • Number of crossovers: waveguide crossovers introduce power loss and crosstalk • Blocking: any unused input port can be connected to any unused output port