sistec notes optical source

Sagar Institute of Science & Technology, SISTecGandhi Nagar, Bhopal

Made By:

Prof. Manish Soni

(Associate Prof. ECE Deptt )

Optical Fibre CommunicationSystems

Sagar Institute of Science & Technology, SISTec

What is optical source

• The optical source is often considered to be the active component in an optical fiber communication system.

• Its fundamental function is to convert electrical energy in the form of a current into optical energy (light) in an efficient manner which allows the light output to be effectively launched or coupled into the optical fiber.


Types of optical source

• Three main types of optical light source are available. These are:

(i) wideband ‘continuous spectra’ sources (incandescent lamps);

(ii) monochromatic incoherent sources (light-emitting diodes, LEDs);

(iii) monochromatic coherent sources (lasers).


Major requirements for an optical sourceThese two sources fulfill the major requirements for an optical

fiber emitter which are outlined below:1. Highly directional:- A size and configuration compatible with

launching light into an optical fiber.2. Linear:- Must accurately track the electrical input signal to

minimize distortion and noise. 3. Should emit light at wavelengths where the fiber has low losses

and low dispersion and where the detectors are efficient.4. Wide bandwidth:- Preferably capable of simple signal

modulation over a wide bandwidth extending from audio frequencies to beyond the gigahertz range.


Major requirements....

5. Must couple sufficient optical power to overcome attenuation in the fiber plus additional connector losses and leave adequate power to drive the detector.

6. Very narrow spectral bandwidth (linewidth):- in order to minimize dispersion in the fiber.

7. stable optical output:- Must be capable of maintaining a stable optical output which is largely unaffected by changes in ambient conditions (e.g. temperature).

8. It is essential that the source is comparatively cheap and highly reliable in order to compete with conventional transmission techniques.


Absorption and emission of radiation

• Different energy states for the atom correspond to different electron configurations, and a single electron transition between two energy levels within the atom will provide a change in energy suitable for the absorption or emission of a photon.


• when the atom is initially in the higher energy state E2 it can make a transition to the lower energy state E1 providing the emission of a photon at a frequency corresponding to Eq.

E = (E2 − E1) = hf, where h = 6.626 × 10−34 J s is Planck’s constant• This emission process can occur in two ways:(a) by spontaneous emission in which the atom returns to the lower energy

state in an entirely random manner;(b) by stimulated emission when a photon having an energy equal to the energy

difference between the two states (E2 − E1) interacts with the atom in the upper energy state causing it to return to the lower state with the creation of a second photon.

• The random nature of the spontaneous emission process gives incoherent radiation. A similar emission process in semiconductors provides the basic mechanism for light generation within the LED.


• This spontaneous emission of light from within the diode structure is known as electroluminescence. The light is emitted at the site of carrier recombination which is primarily close to the junction, although recombination may take place through the hole diode structure as carriers diffuse away from the junction region (see Figure). However, the amount of radiative, recombination and the emission area within the structure is dependent upon the semiconductor materials used and the fabrication of the device.


Figure An illustration of carrier recombination giving spontaneous emission of light in a p–n junction diode


Direct and indirect bandgap semiconductors• Direct bandgap:- The most useful materials for

electroluminescence purpose are direct bandgap semiconductors in which electrons and holes on either side of the forbidden energy gap have the same value of crystal momentum and thus direct recombination is possible. This process is illustrated in Figure (a) when electron–hole recombination occurs the momentum of the electron remains virtually constant and the energy released, which corresponds to the bandgap energy Eg, may be emitted as light. This direct transition of an electron across the energy gap provides an efficient mechanism for photon emission.

• Indirect bandgap:- In indirect bandgap semiconductors, however, the maximum and minimum energies occur at different values of crystal momentum (Figure (b)).


Direct and indirect bandgap semiconductors

Figure: Energy–momentum diagrams showing the types of transition: (a) direct bandgap semiconductor; (b) indirect bandgap semiconductor


Table:- Some direct and indirect bandgap semiconductors with calculated recombination coefficients

Direct and indirect bandgap semiconductors


• The semiconductor materials used for optical sources must broadly fulfill several criteria. These are as follows:

1. p–n junction formation. The materials must lend themselves to the formation of p–n junctions with suitable characteristics for carrier injection.

2. Efficient electroluminescence. The devices fabricated must have a high probability of radiative transitions and therefore a high internal quantum efficiency. Hence the materials utilized must be either direct bandgap semiconductors or indirect bandgap semiconductors with appropriate impurity centers.

3. Useful emission wavelength. The materials must emit light at a suitable wavelength to be utilized with current optical fibers and detectors (0.8 to 1.7 μm). Ideally, they should allow bandgap variation with appropriate doping and fabrication in order that emission at a desired specific wavelength may be achieved.

Sagar Institute of Science & Technology, SISTec Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000


Spectral width of LED types

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000


Rate equations, Quantum Efficiency & Power of LEDs

• When there is no external carrier injection, the excess density decays exponentially due to electron-hole recombination.

• n is the excess carrier density,

• Bulk recombination rate R:

• Bulk recombination rate (R)=Radiative recombination rate + nonradiative recombination rate

/0)( tentn [4-4]

lifetime.carrier :densityelectron excess injected initial :0

n

n

dtdnR [4-5]


)1rate(ion recombinat venonradiati )1( rateion recombinat radiative)1( rateion recombinatbulk

r nrnrr /τR/τR/τR

With an external supplied current density of J the rate equation for the electron-hole recombination is:

regionion recombinat of thickness: electron; theof charge :

)(

dq

nqdJ

dttdn

[4-6]

In equilibrium condition: dn/dt=0

qdJn

[4-7]


rnrr

nr

nrr

r

RRR

int

Internal Quantum Efficiency & Optical Power

[4-8]

region active in the efficiency quantum internal :int

Optical power generated internally in the active region in the LED is:

qhcIh

qIP intintint [4-9]

region active current to Injected :power, optical Internal :int

IP


External Quantum Eficiency

• In order to calculate the external quantum efficiency, we need to consider the reflection effects at the surface of the LED. If we consider the LED structure as a simple 2D slab waveguide, only light falling within a cone defined by critical angle will be emitted from an LED.

photons generated internally LED of No.LED from emitted photons of No.

ext


dTc

)sin2()(41

0ext [4-11]

221

21

)(4)0(tCoefficienon Transmissi Fresnel :)(

nnnnTT

[4-12]

211

ext2 )1(11 If

nn

n [4-13]

211

intintext )1(

power, optical emitted LED

nnPPP [4-14]


Modulation of LED• The frequency response of an LED depends on: 1- Doping level in the active region 2- Injected carrier lifetime in the recombination region, . 3- Parasitic capacitance of the LED• If the drive current of an LED is modulated at a frequency of the

output optical power of the device will vary as:

• Electrical current is directly proportional to the optical power, thus we can define electrical bandwidth and optical bandwidth, separately.

20

)(1)(

i

PP

[4-15]

i

current electrical : power, electrical:)0(

log20)0(

10log BW Electrical

IpI

)I(p

)p(

[4-16]

)0()(log10

)0()(log10 BW Optical

II

PP

[4-17]




Light Source Material• Most of the light sources contain III-V ternary & quaternary

compounds.• by varying x it is possible to control the band-gap energy

and thereby the emission wavelength over the range of 800 nm to 900 nm. The spectral width is around 20 to 40 nm.

• by changing 0<x<0.47; y is approximately 2.2x, the emission wavelength can be controlled over the range of 920 nm to 1600 nm. The spectral width varies from 70 nm to 180 nm when the wavelength changes from 1300 nm to 1600 nm. These materials are lattice matched.

AsAlGa xx1

y1yxx1 PAsGaIn

Table : Some common material systems used in the fabrication of electroluminescent sources for optical fiber communications



Light-Emitting Diodes (LEDs)

• For photonic communications requiring data rate 100-200 Mb/s with multimode fiber with tens of microwatts, LEDs are usually the best choice.

• LED configurations being used in photonic communications: 1- Surface Emitters (Front Emitters) 2- Edge Emitters


Cross-section drawing of a typical GaAlAs double heterostructure light emitter. In this structure, x>y to provide for both carrier confinement and optical guiding. b) Energy-band diagram showing the active region, the electron & hole barriers which confine the charge carriers to the active layer. c) Variations in the refractive index; the lower refractive index of the material in regions 1 and 5 creates an optical barrier around the waveguide because of the higher band-gap energy of this material.

)eV(240.1m)(

gE [4-3]



Planar LED• The planar LED is the simplest of the structures that are available

and is fabricated by either liquid- or vapor-phase epitaxial processes over the whole surface of a GaAs substrate.

• This involves a p-type diffusion into the n-type substrate in order to create the junction Forward current flow through the junction gives spontaneous emission and the device emits light from all surfaces. However, only a limited amount of light escapes the structure due to total internal reflection and therefore the radiance is low.


Dome LED

• A hemisphere of n-type GaAs is formed around a diffused p-type region. The diameter of the dome is chosen to maximize the amount of internal emission reaching the surface within the critical angle of the GaAs–air interface. Hence this device has a higher external power efficiency than the planar LED. However, the geometry of the structure is such that the dome must be far larger than the active recombination area, which gives a greater effective emission area and thus reduces the radiance.


DH surface-emitting LED (SLED)

Figure: The structure of an AlGaAs DH surface-emitting LED (SLED)

• This type of surface emitter LED (SLED) has been widely employed in OFC.

• These structures have a low thermal impedance in the active region allowing high current densities and giving high-radiance emission into the optical fiber. Furthermore, considerable advantage may be obtained by employing DH structures giving increased efficiency as well as less absorption of the emitted radiation.

• The internal absorption in this device is very low due to the larger bandgap-confining layers, and the reflection coefficient at the back crystal face is high giving good forward radiance.

• The power coupled Pc into a multimode step index fiber may be estimated from the relationship : Pc = π (1 − r)ARD(NA)2

• r = Fresnel reflection coefficient at the fiber surface, A = smaller of the fiber core cross-section or the emission area of the source & RD = radiance of the source. Sagar Institute of Science & Technology,

SISTec


Figure: Small-area InGaAsP mesa-etched surface-emitting LED structure.


• Previous structure allows significant lateral current spreading (for contact diameters less than 25 μm) which results in a reduced current density as well as an effective emission area substantially greater than the contact area.

• mesa structure is a technique which has been used to reduce the current spreading in very small devices as illustrated in Figure.

• In this case mesas with diameters in the range 20 to 25 μm at the active layer were formed by chemical etching.

• These InGaAsP/InP devices which emitted at a wavelength of 1.3 μm had an integral lens formed at the exit face of the InP substrate in order to improve the coupling efficiency particularly to single-mode fiber.


Figure : Structure of a stripe geometry DH AlGaAs Edge-emitting LED

• It is another basic high-radiance structure currently used in optical communication.

• It takes advantage of transparent guiding layers with a very thin active layer (50 to 100 μm) in order that the light produced in the active layer spreads into the transparent guiding layers, reducing self-absorption in the active layer.

• Most of the propagating light is emitted at one end face only due to a reflector on the other end face and an antireflection coating on the emitting end face. The effective radiance at the emitting end face can be very high giving an increased coupling efficiency into small NA fiber.

• edge emitters couple more optical power into low NA (less than 0.3) than surface emitters, whereas the opposite is true for large NA (greater than 0.3). Sagar Institute of Science & Technology,

SISTec


• The stripe geometry allows very high carrier injection densities for given drive currents. Thus it is possible to couple a milliwatt of optical power into low-NA (0.14) multimode step index fiber operating at high drive currents (500 mA)

• Edge emitters have a substantially better modulation bandwidth of the order of hundreds of megahertz than comparable surface-emitting structures with the same drive level.

• In general it is possible to construct edge-emitting LEDs with a narrower linewidth than surface emitters.

• a number of ELED structures have been developed using the InGaAsP/ InP material system for operation at a wavelength of 1.3 μm. A common device geometry which has also been utilized for AlGaAs/GaAs ELEDs is shown in next figure . This DH edge-emitting device is realized in the form of a restricted length, stripe geometry referred to as truncated stripe ELEDs.

• This short stripe structure (around 100 μm long) improves the external efficiency of the ELED by reducing its internal absorption of carriers.


Figure : Truncated stripe InGaAsP edge-emitting LED

• To provide both high-speed transmission and significant launch powers into single-mode fiber the two device structures shown in the next slide.

• The ELED in fig.(a) comprises a mesa structure with a width of 8 μm and a length of 150 μm for current confinement. The tilted back facet of the device was formed by chemical etching in order to suppress laser oscillation.

• The ELED active layer was heavily doped with Zn to reduce the minority carrier lifetime and thus improve the device modulation bandwidth. In this way a 3 dB modulation bandwidth of 600 MHz was obtained.(a) mesa structure ELED



• Fig.(b) displays another advanced InGaAsP ELED which was fabricated as a V-grooved substrate BH device. In this case the front facet was antireflection coated and the rear facet was also etched at a slat to prevent laser action. This device, which again emitted at a center wavelength of 1.3 μm, was reported to have a 3 dB modulation bandwidth around 350 MHz, with the possibility of launching 30 μW of optical power into single-mode fiber

• Another device geometry which is providing significant benefits over both SLEDs and ELEDs for communication applications is the superluminescent diode or SLD.

• This device type offers advantages of: (i) High output power (around four to five times higher than ELED(ii) Directional output beam; and (iii) Narrow spectral linewidth.• The SLD has optical properties that are bounded by the ELED and the

injection laser.• The output of the SLD is spectrally broad (i.e. 20 to 150 nm) and therefore

exhibit sufficient output signal power & can be used as broadband optical power sources.

• Potential drawbacks associated with the SLD in comparison with conventional LEDs are:-

(i) the nonlinear output characteristic.(ii) The increased temperature dependence of the output power.(iii) The required current density is substantially higher (three times),

necessitating high drive currents.Sagar Institute of Science & Technology,

SISTec


• For operation the injected current is increased until stimulated emission, and hence amplification, occurs (i.e. the initial step towards laser action), but because there is high loss at one end of the device, no optical feedback takes place. Therefore, although there is amplification of the spontaneous emission, no laser oscillation builds up. However, operation in the current region for stimulated emission provides gain causing the device output to increase rapidly with increases in drive current. High optical output power can therefore be obtained, together with a narrowing of the spectral width.

• An early SLD is shown in fig.(a) which employs a contact stripe together with an absorbing region at one end to suppress laser action. Such devices have provided peak output power of 60 mW at a wavelength of 0.87 μm in pulsed mode. Antireflection coatings can be applied to the cleaved facets of SLDs in order to suppress resonance. Such devices have launched 550 μW of optical power in multimode graded index fiber of 50 μm diameter at drive currents of 250 mA and 250 μW into single-mode fiber using drive currents of 100 mA.


Figure: Superluminescent LED structures: (a) AlGaAs contact stripe SLD (b) high output power InGaAsP SLD


• The structure of an InGaAsP/InP SLD is illustrated in Fig.(b). The device which emits at 1.3 μm comprises a buried active layer within a V-shaped groove on the p-type InP substrate. This technique provides an appropriate structure for high-power operation because of its low leakage current.

• a light diffusion surface is placed within this device. It is applied diagonally on the active layer of length 350 μm. It serves to scatter the backward light emitted from the active layer and thus decreases feedback into this layer. In addition, an AR coating is provided on the output facet.


The LED has a following number of distinct advantages

1. Simpler fabrication. There are no mirror facets and in some structures no striped geometry.

2. Cost. The simpler construction of the LED leads to much reduced cost which is always likely to be maintained.

3. Reliability. The LED does not exhibit catastrophic degradation and has proved far less sensitive to gradual degradation than the injection laser. It is also immune to self-pulsation and modal noise problems.

4. Generally less temperature dependence. The light output against current characteristic is less affected by temperature than the corresponding characteristic for the injection laser. Furthermore, the LED is not a threshold device and therefore raising the temperature does not increase the threshold current above the operating point and hence halt operation.


5. Simpler drive circuitry. This is due to the generally lower drive currents and reduced temperature dependence which makes temperature compensation circuits unnecessary.

6. Linearity. Ideally, the LED has a linear light output against current characteristic unlike the injection laser. This can prove advantageous where analog modulation is concerned.


Drawbacks of LED• An incoherent light source as the emitted photons have

random phases.• Much wider spectral linewidth (30-40 nm) (app. 100 times

more than the injection laser)• Low output power.• Lower optical power coupled into a fiber (in μW).• Usually lower modulation bandwidth.• Low E/O conversion efficiency

sistec notes optical source

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