UNIT 4 Optical components Integrated optics 4 1

UNIT 4 Optical components & Integrated optics

4. 1 Optical couplers and isolators • Couplers are needed for signal distribution where the permanent splices and demountable connectors are used for connecting the optical fiber. • Main advantage of these coupling components are that these do not require optical element such as reflectors or lenses.

The charactering parameter of couplers • The coupling ratio: it is the ratio of simple output divided by total output power. • Excess loss: it is the ratio of output power divided by total input power.

Types according to function wise • 1) Directional • 2) Distributive • 3) Wave length dependent couplers

Types according to mechanism • 1)Diffusion type • 2)Area Splitting type • 3)Beam Splitting type

4. 2. 1. 1 Diffusion Couplers In this two types of coupling used. 1)Evanescent coupling 2)Radiative coupling --in these method coupler must be placed parallel to each other over a finite distance called coupling length Lc. --in evanescent field of primary builts up a propagation field in the secondary and provide two outputs.

In radiative coupling bent fibers are coupled to each other via the radiated field.

Distributive coupler • It is also known as a star coupler. • Star couplers distribute an optical signal from a single input fiber to multiple output fibers. • Main wo methods used for producing multimode fiber star coupler. • 1)mixed rod method • 2)fused biconical taper

Transmission star and reflective star couplers

Transmission star and reflective star couplers

4. 2. 2 Optical isolator • This prevents scattered or reflected light from traveling in the reverse direction. • E. g. , can keep backward-traveling light from entering a laser diode and possibly causing instabilities in the optical output.

4. 3 Optical switch • Directional coupler switches • The switch below constructed on a lithium niobate waveguide. An electrical voltage applied to the electrodes changes the substrate’s index of refraction. The change in the index of refraction manipulates the light through the appropriate waveguide path to the desired port.

Directional coupler switch

Y junction switch Y – Junction Configuration l a t gi Di PH 1 Ii PH 2 I 2

4. 3 Beam Splitter • Beam splitters are a classs of diffractive optic that are the optical version of a copying machine. • Application • 1)DWDM • 2)Color separation filters • 3)sensors and detectors • 4)grids and targets

1)Transmission grating beamsplitters • Transmission grating beamsplitter are commonly used for laser beam division and multiple laser line separation in visible wavelengths. • Grating equation • θn=sin-1 (nʎ/d) • θ=diffraction angle for nth order • n=diffracted order • ʎ=Wavelength of light • d = grating period

Reflecting /transmitting “polka dot” beamsplitter • The aluminum coating is applied in apertures giving the beamsplitter a “polka dot” appearance.

4. 4 Optical multiplexer and demultiplexer • In order to implement WDM based system multiplexer is require at the transmitting side and demultiplexer is require at the receiver side. • Commonly used WDM may be classified into two categories. • 1)interference filter based devices • 2)angular dispersion based devices

Basic configuration of two wavelength interference filter demultiplex

Mech zehnder interferometer • By splitting the input beam and introducing a phase shift in one of the paths, the recombined signals will interfere constructively at one output and destructively at the other. • In the central region, when the signals in the two arms come from the same light source, the outputs from these two guides have a phase difference

WDM using 2 x 2 mach zehnder interferometer

4. 5 Optical wavelength converter • An optical wavelength converter is a device that can directly translate information on an incoming wavelength to a new wavelength without entering the electrical domain. • Thid is an important component in WDM networks.

Optical gating wavelength converters

4. 6 Bragg grating • Operating Principle: Incident optical wave at l 0 will be reflected back if the following grating condition is met: l 0 = 2 neff. L, where neff is average weighting of n 1 and n 2 and L = grating period (periodicity of index variation) •

Formation of Bragg grating in a fiber core

4. 7 Optical Amplifiers • • General application of optical amplifiers 1)In line optical amplifiers 2)Preamplifiers 3)Postamplifiers

Possible application of optical amplifiers

Basic operation of amplifier

4. 8 Concept of Integrated optics • Integrated optics involves minimization of optical device and components on planar substrate. • It requires less power and insensitive to vibration. • These have wide application in optical signal processing and communication.