EEE440

Semester 1 2011-2012

EEE440Modern Communication Systems

Optical Fibre Communication Systems

En. Mohd Nazri MahmudMPhil (Cambridge, UK)BEng (Essex, UK)[email protected] 2.14

mailto:[email protected]

Announcement

EEE440Test on mobile and wireless

Tuesday 6/12/2011;11am to 1pmDK1



System elements


System elements


Light sources• Semiconductor light-emitting diodes (LEDs) and laser diodes are

suitable• Major differences between LED and Laser

– LED has an incoherent optical output whereas Laser produces highly coherent, monochromatic and directional output because a cavity exist for wavelength selectivity

• LED– Generally used for multimode fibre– For optical communications requiring bit rates less than 100-200 Mb/s– Best for high-speed local applications which needs many wavelengths

on the same fibre• Laser diodes

– the best light source for long-hauled fibre-optic links due to brightness, narrow spectral width and coherence


Light source - LASER• LASER stands for Light Amplification by Stimulated Emission of

Radiation• Principle of operation

– Semiconductor material can generate light when current is injected directly into it due to the stimulated emission of photons in the material

– The stimulated emission of photons occur when an external photon impinges on an excited laser material

– The direct injection of current causes the particles of the laser materials to undergo the process of excitation whereby the particles move from a lower energy level (or ground state) to a higher energy level (or excited state)

– To initiate the lasing action, the number of particles in the excited state must be made greater than the number of particles in the ground state (ie. Population inversion)


Light source - LASER

• Principle of operation– The excited particles in the population inversion state are

unstable and can return to the stable ground state again and spontaneously emit photon

– The photons from the spontaneous emission trigger stimulated emission of other photons resulting in a cascade of stimulated emission (ie lasing action that generate optical signal)

LASER


Laser diode: principle of operation: (a) Stimulated emission; (b) light amplification and positive feedback; (c) pumping to create population inversion

http://zone.ni.com/devzone/cda/ph/p/id/125

LASER


Lasing effect: (a) Gain and loss; (b) input-output characteristic; (c) setup to measure input-output characteristic



Light source - LASER

• There are many semiconductor laser types– Fabry-Perot laser– Distributed Feedback (DFB) laser– Distributed Bragg Reflector (DBR) laser– Distributed Reflector (DR) Laser


Fabry-Perot Laser

• Consists of a heterojunction-structured semiconductor laser: 2 adjoining semiconductor materials with different band-gap energies

• A pair of flat, partially reflecting mirrors are directed toward each other to enclose the cavity

• When the junction is forward bias, electrons and holes are injected into the p and n regions

• These can recombine and release a photon energy, hv


Fabry-Perot Laser

• The two mirrors and the active medium between them form a laser • Mirrors provide positive feedback: the return of stimulated photons to an

active medium to stimulate more photons• The two mirrors form a resonator with length L• Let an arbitrary wave travel from the left-hand mirror to the right-hand one

– At the right-hand mirror, the wave experiences a 180° phase shift and continues to propagate. At the left-hand mirror, this wave again has the same phase shift and continues to travel yielding a stable pattern called a standing wave

Fabry-Perot Laser


• The only difference between the two waves shown in Figures 9.13(b) and 9.13(c) is their wavelengths. Thus, a resonator can support only a wave with a certain wavelength, the wave that forms a standing-wave pattern

• The resonator supports a wavelength where 2L/N = 1300.8 nm. • But this resonator also supports wavelengths equal to 2L/(N ± 1), 2L/(N ± 2), 2L/(N ±

3), and so forth. • Many wavelengths may exist. Wavelengths selected by a resonator are called

longitudinal modes. • When the length of a resonator increases or decreases, the laser switches from one

longitudinal mode to another. This is called mode hop.

Fabry-Perot Laser


• However, the active medium provides gain within only a small range of wavelengths. • Since a laser is formed by a resonator and an active medium and since radiation is

the result of their interaction, only several resonant wavelengths that fall within the gain curve might be radiated.

• Light generation starts only when gain exceeds loss. Thus, eventually only those resonant wavelengths that are within the gain-over-loss curve will actually be radiated.

• Waves With N, N±1, and N±2 might be radiated, but only waves with N and N±1 will be the actual laser output. Modes N±2, depicted in black, are not generated.

DFB Laser• To reduce the spectral width, we need to make a laser diode merely radiate

only one longitudinal mode with distributed-feedback (DFB) laser diodes• Has the Bragg grating incorporated into its heterostructure in the vicinity of

an active region. • The Bragg grating works like a mirror, selectively reflecting only one

wavelength, B



EEE440

Documents

laser mirrors

laser materials

cascade of stimulated

light source laserprinciple

light source laserthere

light source laserlaser

return of stimulated

number of particles