Unit7_WDM_Components_PPT.pdf

Introduction

• Conventional optical receiver is not capable of handling rapid change of signal amplitude and clock phase alignment.

• Burst mode receiver is designed to handle such busty nature of packets.

• The receiver can quickly extract the decision threshold and determine the signal phase from a set of overhead bits placed at the beginning of each packet burst.

• Two NW terminals have impact on Burst mode receiver.

• Passive optical network (PON)

Passive Optical Networks (PONs)

• A passive optical network (PON) uses CWDM over a single bidirectional optical fiber.

• Only passive optical components guide traffic from the central office to the customer premises and back to the central office.

• In the central office, combined data and digitized voice are sent downstream to customers by using a 1490-nm wavelength.

• The upstream (customer to central office) uses a 1310-nm wavelength.

• Video services are sent downstream using a 1550-nm wavelength.

Active PON Modules

• The optical line termination (OLT) is located in a central office and controls the bidirectional flow of information across the network.

• An optical network termination (ONT) is located directly at the customer premises.

• The ONT provides an optical connection to the PON on the upstream side and to interface electrically to the local customer equipment.

• An optical network unit (ONU) is similar to an ONT, but is located near the customer and is housed in an outdoor equipment shelter.

Burst mode receiver

• Burst nature of the arrived packet is represented by amplitude and phase variation of the arrived packet from different ONTs

Unit: 7

WDM OPTICAL NETWORKS

• Considerable increase in traffic became a driving force for WDM and its evolution into dense WDM (DWDM).

• WDM refers to the technology of combining multiple wavelengths onto the same optical fiber.

• Each wavelength is a different channel.

• At the transmitting end, there are W independent transmitters. Each transmitter Tx is a light source, such as a laser, and is independently modulated with a data stream. The output of each transmitter is an optical signal on a unique wavelength λ_i , i = 1, 2, . . . , W.

• WDM: ~200 GHz spacing

Basic Principle

• WDM components are nothing but star couplers.(Wavelength divider or combiner)

• Principle role is to combine the powers from N inputs and divide the equally among M output ports.

• Techniques: Fused fibers, gratings, micro optic techniques, Integrated optics schemes.

• Example: 7×7, 1×19, 4×4 star couplers.

• For a N×N star coupler,

• Splitting Loss =

• Excess Loss =

10log₁₀(𝑁

10log₁₀ 𝑃𝑖𝑛

𝑗=1 𝑀

Realization of 8×8 star coupler using 2×2 coupler

Simplified structure

Fig. Realization of 8×8 star coupler using 2×2 3-dB coupler 2×2 coupler

𝑁

Simple calculation!!!

• No of 3-dB coupler required for a N×N star coupler

• If the fraction of power traversing is 𝐹_𝑇 (0<𝐹_𝑇<1), then fraction of loss power = (1-𝐹_𝑇)

• Excess loss =

• Splitting loss = 10log(N)

• Total loss = −10log₁₀(𝐹_𝑇log2𝑁) + 10log₁₀𝑁

=

𝑁_𝑐 = 𝑁

2 . log2𝑁

−10log₁₀(𝐹_𝑇log2𝑁

−10log₁₀ 𝐹𝑇

log₂𝑁

Problem:

• Consider a commercially available 32×32 single mode coupler made from cascading 3 dB fused fiber 2×2 couplers, where 5% of the power is lost in each element. Calculate total loss.

Need for Optical Isolator

• Optical isolators are devices that transmit light only in one direction. • If back-reflected and scattered light enters into the laser, the lasing

process will fluctuate and the output power of the laser will fluctuate. • But in fiber optic networks, most of the reflections are harmful to the

stability of the system which is especially true for lasers and optical amplifiers.

• Optical isolators play a vital role in fiber optic systems by stopping back-reflection and scattered light from reaching sensitive

Limitations of Polarization dependent Isolator

• Polarization state of the output beam of the laser should be exactly aligned to the transmission axis of the input linear polarizer.

• Any mismatch in the polarization state gives insertion loss.

Polarization independent Isolator – How it works?

• Regardless of the polarization state of the input beam, the output beam will propagate through the fiber with zero insertion loss.

• The reflected beam will be isolated from the source.

• It replaces the two linear polarizers using Birefringent wedge or crystal.

• ‘o’ – Ordinary Ray, ‘e’- Extra-ordinary Ray

Polarization independent Isolator (Contd..)

• The first wedge crystal splits the input beam into ‘e’ and ‘o’ rays which are linearly polarized and orthogonal to each other.

• Both the rays will be rotated by +45°. Second wedge axis is at 45° relative to first wedge. So the light rays are focused and emerge out from the wedge as single ray.

• For reflected ray the second wedge splits the beam into ‘e’ and ‘o’ ray. Due to Faraday rotator of +45°, ‘e’ ray becomes ‘o’ ray and ‘o’ ray becomes ‘e’ ray.

• The first wedge again separate out the two beams and diverges the rays.

Summary:

Polarization dependent Isolator Polarization independent Isolator

 It is dependent on input signal polarization  It is independent on input signal polarization

 It uses fixed polarizers with specific rotation  It uses Birefringent Crystal with specific rotation

 Less flexible for usage  More flexible for usage

 More insertion loss  Less insertion loss

MEMS

Introduction

• Micro-Electro-Mechanical Systems or MEMS Technology is a

precision device technology that integrates mechanical elements, sensors, actuators, and electronics on a common silicon substrate through micro fabrication technology.

Micro : Small size, micro fabricated structures

Electro : Electrical Signal/ Control

Mechanical : Mechanical functionality

Systems : Structures, Devices, Systems

• MEMS is also referred to as MST (Microsystems Technology in Europe) and MM (Micromachines in Japan). MEMS with optics is called

Introduction (contd..)

• IC- “Brain” of a system, MEMS- “Sense and limbs” of the brain

• These systems can sense, control and actuate on microscale, and function individually or in arrays to generate effects on macroscale.

• In MEMS devices, a three-dimensional spatial structure is formed on the

substrate and mechanical blocks are formed within that structure, A system is then created by fabricating electrical circuits that drive those mechanical blocks on the same substrate.

• This technology is targeted at devices that range in size from mm down to

General Structure

• In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and microelectronics, all integrated onto the same silicon chip.

• Microsensors detect changes in the system’s environment by measuring mechanical, thermal, magnetic, chemical or electromagnetic information or phenomena.

• Microelectronics processes this information and signals the microactuators to react and create some form of changes to the environment.

References:

• WDM Operation: Keiser 4th _Ed.

• Isolator : Note is uploaded to website • MEMS technology:

• http://www.engineersgarage.com/articles/mems-technology

• http://www.egr.msu.edu/classes/ece410/mason/files/MEMS%20overview.pdf