Optical Sources

• Optical transmitter coverts electrical input signal into corresponding optical signal. The optical signal is then launched into the fiber. Optical source is the major component in an optical
• Popularly used optical transmitters are Light Emitting Diode (LED) and semiconductor Laser Diodes (LD).

Characteristics of Light Source of Communication

 

• To be useful in an optical link, a light source needs the following characteristics:

1. It must be possible to operate the device continuously at a variety of temperatures for many
2. It must be possible to modulate the light output over a wide range of modulating

• For fiber links, the wavelength of the output should coincide with one of transmission windows for the fiber type

1. To couple large amount of power into an optical fiber, the emitting area should be
2. To reduce material dispersion in an optical fiber link, the output spectrum should be
3. The power requirement for its operation must be

• The light source must be compatible with the modern solid state devices.
• The optical output power must be directly modulated by varying the input current to the

1. Better linearity of prevent harmonics and intermodulation distortion.
2. High coupling
3. High optical output

• High
• Low weight and low

 

Two types of light sources used in fiber optics are light emitting diodes (LEDs) and laser diodes (LDs).

Light Emitting Diodes(LEDs) p-n Junction

• Conventional p-n junction is called as homojunction as same semiconductor material is sued on both sides The electron-hole recombination occurs in relatively wide layer = 10 µm. As the carriers are not confined to the immediate vicinity of junction, hence high current densities can not be realized.

• The carrier confinement problem can be resolved by sandwiching a thin layer ( = 0.1 µm) between p-type and n-type layers. The middle layer may or may not be doped. The carrier confinement occurs due to bandgap discontinuity of the junction. Such a junction is call heterojunction and the device is called double
• In any optical communication system when the requirements is –

1. Bit rate f 100-2—Mb/sec.
2. Optical power in tens of micro watts. LEDs are best suitable optical

LED Structures Heterojuncitons

• A heterojunction is an interface between two adjoining single crystal semiconductors with different bandgap.
• Heterojuctions are of two types, Isotype (n-n or p-p) or Antisotype (p-n).

Double Heterojunctions (DH)

 

In order to achieve efficient confinement of emitted radiation double heterojunctions are used in LED structure. A heterojunciton is a junction formed by dissimilar semiconductors. Double heterojunction (DH) is formed by two different semiconductors on each side of active region. Fig. 3.1.1 shows double heterojunction (DH) light emitter.

 

 

 

 

• The crosshatched regions represent the energy levels of Recombination occurs only in active InGaAsP layer. The two materials have different bandgap energies and different refractive indices. The changes in bandgap energies create potential barrier for both holes and electrons. The free charges can recombine only in narrow, well defined active layer side.
• A double heterjuction (DH) structure will confine both hole and electrons to a narrow active layer. Under forward bias, there will be a large number of carriers injected into active region where they are efficiently confined. Carrier recombination occurs in small active region so leading to an efficient device. Antoer advantage DH structure is that the active region has a higher refractive index than the materials on either side, hence light emission occurs in an optical waveguide, which serves to narrow the output beam.

LED configurations

 

• At present there are two main types of LED used in optical fiber links –

1. Surface emitting
2. Edge emitting

Both devices used a DH structure to constrain the carriers and the light to an active layer.

Surface Emitting LEDs

 

• In surface emitting LEDs the plane of active light emitting region is oriented perpendicularly to the axis of the A DH diode is grown on an N-type substrate at the top of the diode as shown in Fig. 3.1.2. A circular well is etched through the substrate of the device. A fiber is then connected to accept the emitted light.

 

• At the back of device is a gold heat The current flows through the p-type material and forms the small circular active region resulting in the intense beam of light.

Diameter of circular active area = 50 µm Thickness of circular active area = 2.5 µm Current density = 2000 A/cm² half-power Emission pattern = Isotropic, 120^o beamwidth.

• The isotropic emission pattern from surface emitting LED is of Lambartian pattern. In Lambartian pattern, the emitting surface is uniformly bright, but its projected area diminishes as cos θ, where θ is the angle between the viewing direction and the normal to the surface as shown in 3.1.3. The beam intensity is maximum along the normal.

• The power is reduced to 50% of its peak when θ = 60^o, therefore the total half-power beamwidth is 120^o. The radiation pattern decides the coupling efficiency of

Edge Emitting LEDS (ELEDs)

 

• In order to reduce the losses caused by absorption in the active layer and to make the beam more directional, the light is collected from the edge of the LED. Such a device is known as edge emitting LED or ELED.
• It consists of an active junction region which is the source of incoherent light and two guiding layers. The refractive index of guiding layers is lower than active region but higher than outer surrounding material. Thus a waveguide channel is form and optical radiation is directed into the fiber. 3.1.4 shows structure of ELED.

 

 

 

 

Edge emitter’s emission pattern is more concentrated (directional) providing improved coupling efficiency. The beam is Lambartian in the plane parallel to the junction but diverges more slowly in the plane perpendicular to the junction. In this plane, the beam divergence is limited. In the parallel plane, there is no beam confinement and the radiation is Lambartian. To maximize the useful output power, a reflector may be placed at the end of the diode opposite the emitting edge. Fig. 3.1.5 shows radiation from ELED.

Features of ELED:

 

1. Linear relationship between optical output and
2. Spectral width is 25 to 400 nm for λ = 8 – 0.9 µm.
3. Modulation bandwidth is much
4. Not affected by catastrophic gradation mechanisms hence are more
5. ELEDs have better coupling efficiency than surface
6. ELEDs are temperature

 

Usage :

 

1. LEDs are suited for short range narrow and medium bandwidth
2. Suitable for digital systems up to 140 Mb/sec.
3. Long distance analog

 

 

 

 

Light Source Materials

 

• The spontaneous emission due to carrier recombination is called electro luminescence. To encourage electroluminescence it is necessary to select as appropriate semiconductor The semiconductors depending on energy bandgap can be categorized into,

1. Direct bandgap
2. Indirect bandgap
   • Some commonly used bandgap semiconductors are shown in following table 1.1

 

Semiconductor	Energy bandgap (eV)	Recombination Br (cm3 / sec)
GaAs	Direct : 1.43	7.21 x 10-10
GaSb	Direct : 0.73	2.39 x 10-10
InAs	Direct : 0.35	8.5 x 10-11
InSb	Direct : 0.18	4.58 x 10-11
Si	Indirect : 1.12	1.79 x 10-15
Ge	Indirect : 0.67	5.25 x 10-14
GaP	Indirect : 2.26	5.37 x 10-14

 

Table 3.1.1 Semiconductor material for optical sources

 

• Direct bandgap semiconductors are most useful for this In direct bandgap semiconductors the electrons and holes on either side of bandgap have same value of crystal momentum. Hence direct recombination is possible. The recombination occurs within 10^-8 to 10^-10 sec.
• In indirect bandgap semiconductors, the maximum and minimum energies occur at different values of crystal The recombination in these semiconductors is quite slow i.e. 10^-2 and 10^-3 sec.
• The active layer semiconductor material must have a direct bandgap. In direct bandgap semiconductor, electrons and holes can recombine directly without need of third particle to conserve In these materials the optical radiation is sufficiently high. These

materials are compounds of group III elements (Al, Ga, In) and group V element (P, As, Sb). Some tertiary allos Ga_1-x Al_x As are also used.

• Emission spectrum of G_a1-x Al_xAs LED is shown in Fig. 1.6.

• The peak output power is obtained at 810 nm. The width of emission spectrum at half power (0.5) is referred as full width half maximum (FWHM) spectral For the given LED FWHM is 36 nm.
• The fundamental quantum mechanical relationship between gap energy E and frequency v is given as –

where, energy (E) is in joules and wavelength (λ) is in meters. Expressing the gap energy (E_g) in electron volts and wavelength (λ) in micrometers for this application.

Different materials and alloys have different bandgap energies.

• The bandgap energy (E_g) can be controlled by two compositional parameters x and y, within direct bandgap region. The quartenary alloy In_1-x Ga_x As_y P_1-y is the principal material sued in such Two expression relating E_g and x,y are –

                                                                                        

                                                                                                                                                          … 3.1.3

                                                                                             … 3.1.4

Example 3.1.1 : Compute the emitted wavelength from an optical source having x = 0.07.

 

Solution :                     x = 0.07

E_g = 1.513 eV

Now

 

 

                                                                                                                      …Ans.

Example 3.1.2 : For an alloy In0.74 Ga_0.26 As_0.57 P_0.43 to be sued in Led. Find the wavelength emitted by this source.

Solution : Comparing the alloy with the quartenary alloy composition.

In_1-x Ga_x As P_1-y it is found that

x = 0.26 and y = 0.57

Using

E_g = 1.35-(0.72 x 0.57) + 0.12 x 0.57²

E_g = 0.978 eV

Now

                                                                                                                 … Ans.

 

Quantum Efficiency and Power

 

• The internal quantum efficiency (η_int) is defined as the ratio of radiative recombination rate to the total recombination rate.

 

                                                                                                                  … 3.1.5

Where,

 

R_r is radiative recombination rate.

R_nr is non-radiative recombination rate.

 

If n are the excess carriers, then radiative life time,  and non-radiative life time,

• The internal quantum efficiency is given as –

                                              … 3.1.6

 

 

• The recombination time of carriers in active region is τ. It is also known as bulk recombination life

 

                                                           … 3.1.7

 

 

 

Therefore internal quantum efficiency is given as –

 

                                                                             … 3.1.8

• If the current injected into the LED is I and q is electron charge then total number of recombinations per second is –

 

                                                                                        From equation 3.1.5

                                                                                                                                                  ... 3.1.9

• Optical power generated internally in LED is given as –

                                                                                                                  ... 3.1.10

                                                                                                             

• Not all internally generated photons will available from output of The external quantum efficiency is used to calculate the emitted power. The external quantum

 

efficiency is defined as the ratio of photons emitted from LED to the number of photons generated internally. It is given by equation

                                                                                                            … 3.1.11

• The optical output power emitted from LED is given as –

 

 

Example 3.1.3 : The radiative and non radiative recombination life times of minority carriers in the active region of a double heterojunction LED are 60 nsec and 90 nsec respectively. Determine the total carrier recombination life time and optical power generated internally if the peak emission wavelength si 870 nm and the drive currect is 40 mA. [July/Aug.-2006, 6 Marks]

Solutions : Given :           λ = 870 nm 0.87 x 10^-6 m

τ_r = 60 nsec. τ_nr = 90 nsec.

I = 40 mA = 0.04 Amp.

 

1. Total carrier recombination life time:

 

    τ = 36 nsec.                                                                                       … Ans.

1. Internal optical power:

 

 

 

 

                                                                                                        … Ans.

 

Example 3.1.4 : A double heterjunciton InGaAsP LED operating at 1310 nm has radiative and non-radiative recombination times of 30 and 100 ns respectively. The current injected is 40 Ma. Calculate –

1. Bulk recombination life
2. Internal quantum

• Internal power

 

Solution :                  λ = 1310 nm = (1.31 x 10^-6 m) τ_r = 30 ns

τ_nr = 100 ns

I = 40 MA – 0.04 Amp.

 

i)        Bulk Recombination Life time (τ) :

 

 

\                               τ = 23.07 nsec.                                                                                    … Ans.

ii)      Internal euqntum efficienty (η_int) :

 

                                                                                                                … Ans.

 

iii)    Internal pwer level (P_int) :

 

 

                                                                                                      … Ans.

 

Advantages and Disadvantages of LED Advantages of LED

1. Simple
2. Ease of
3. Simple system
4. Low
5. High

 

Disadvantages of LED

 

1. Refraction of light at semiconductor/air
2. The average life time of a radiative recombination is only a few nanoseconds, therefore nodulation BW is limited to only few hundred megahertz.
3. Low coupling
4. Large chromatic

 

Comparison of Surface and Edge Emitting LED

 

LED type	Maximum modulation frequency (MHz)	Output power (mW)	Fiber coupled power (mW)
Surface emitting	60	< 4	< 0.2
Edge emitting	200	< 7	< 1.0

 

 

 

 

Injection Laser Diode (ILD)

 

• The laser is a device which amplifies the light, hence the LASER is an acronym for light amplification by stimulated emission of

The operation of the device may be described by the formation of an electromagnetic standing wave within a cavity (optical resonator) which provides an output of monochromatic highly coherent radiation.

Principle :

 

• Material absorb light than emitting. Three different fundamental process occurs between the two energy states of an atom.

• Absorption 2) Spontaneous emission 3) Stimulated
  • Laser action is the result of three process absorption of energy packets (photons) spontaneous emission, and stimulated emission. (These processes are represented by the simple two-energy-level diagrams).

Where E₁ is the lower state energy level. E₂ is the higher state energy level.

• Quantum theory states that any atom exists only in certain discrete energy state, absorption or emission of light causes them to make a transition from one state to The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the two states.

If E₁ is lower state energy level. and E₂ is higher state energy level. E = (E₂ – E₁) = h.f.

Where, h = 6.626 x 10^-34 J/s (Plank’s constant).

• An atom is initially in the lower energy state, when the photon with energy (E₂ – E₁) is incident on the atom it will be excited into the higher energy state E₂ through the absorption of the absorption of 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 E = f. The emission process can occur in two ways.

1. By spontaneous emission in which the atom returns to the lower energy state in random
2. By stimulated emission when a photon having equal energy to the difference between the two states (E₂ – E₁) interacts with the atom causing it to the lower state with the creation of the second photon.

• Spontaneous emission gives incoherent radiation while stimulated emission gives coherent Hence the light associated with emitted photon is of same frequency of incident photon, and in same phase with same polarization.
• It means that when an atom is stimulated to emit light energy by an incident wave, the liberated energy can add to the wave in constructive The emitted light is bounced back and forth internally between two reflecting surface. The bouncing back and forth of light wave cause their intensity to reinforce and build-up. The result in a high brilliance, single frequency light beam providing amplification.

Emission and Absorption Rates

 

• It N₁ and N₂ are the atomic densities in the ground and excited

Rate of spontaneous emission

 

R_spon = AN₂                                                                                              … 3.1.13

Rate of stimulated emission

 

 

 

R_stim = BN₂ ρ_em                                                                                         … 3.1.14

Rate of absorption

 

R_abs = B’ N₁ ρ_em                                                                                          … 3.1.15

 

where,

 

A, B and B’ are constants. ρ_em is spectral density.

• Under equilibrium condition the atomic densities N₁ and N₂ are given by Boltzmann

 

                                                                                                      … 3.1.16

                                                                                                    … 3.1.17

where,

 

K_B is Boltzmann constant. T is absolute temperature.

• Under equilibrium the upward and downward transition rates are

AN₂ + BN₂ ρ_em = B’ N₁ ρ_em                                                                                   … 3.1.18

Spectral density ρ_em

 

                                                                                     … 3.1.19

Comparing spectral density of black body radiation given by Plank’s formula,

 

                                                                                                 … 3.1.20

Therefore,                                                                                                                                     … 3.1.21

… 3.1.22

 

• A and B are called Einstein’s coefficient.

 

 

 

 

Fabry – Perot Resonator

 

• Lasers are oscillators operating at frequency. The oscillator is formed by a resonant cavity providing a selective feedback. The cavity is normally a Fabry-Perot resonator i.e. two parallel plane mirrors separated by distance L,

Light propagating along the axis of the interferometer is reflected by the mirrors back to the amplifying medium providing optical gain. The dimensions of cavity are 25-500 µm longitudinal 5-15 µm lateral and 0.1-0.2 µm transverse. Fig. 3.1.10 shows Fabry-Perot resonator cavity for a laser diode.

• The two heterojunctions provide carrier and optical confinement in a direction normal to the junction. The current at which lasing starts is the threshold current. Above this current the output power increases sharply.

Distributed Feedback (DFB) Laser

 

• In DFB laster the lasing action is obtained by periodic variations of refractive index along the longitudinal dimension of the diode. Fig. 3.1.11 shows the structure of DFB laser

 

 

 

 

Lasing conditions and resonant Frequencies

 

• The electromagnetic wave propagating in longitudinal direction is expressed as –

E(z, t) = I(z) e^{j(ω t-β z)}                                                                           …3.1.23

 

where,

 

I(z) is optical field intensity. Ω is optical radian frequency. β is propagation constant.

• The fundamental expression for lasing in Fabry-Perot cavity is –

 

… 3.1.24

 

where,

 

Γ is optical field confinement factor or the fraction of optical power in the active layer. α is effective absorption coefficient of material.

g is gain coefficient. h v is photon energy.

z is distance traverses along the lasing cavity.

 

• Lasing (light amplification) occurs when gain of modes exceeds above optical loss during one round trip through the cavity i.e.z = 2L. If R₁ and R₂ are the mirror reflectivities of the two ends of laser diode. Now the expression for lasing expressing is modified as,

 

 

 

 

                                                                       … 3.1.25

 

The condition of lasing threshold is given as –

 

1. For amplitude : I (2L) = I (0)
2. For phase : e-j^{2β L} = 1

• Optical gain at threshold = Total loss in the

 

i.e.              Γ g_th = α_t

• Now the lasing expression is reduced to –

… 3.1.26

 

 

 

                                                                                              … 3.1.27

 

where,

 

Α_end is mirror loss in lasing cavity.

• An important condition for lasing to occur is that gain, g ≥ g th i.e. threshold

 

 

Example 3.1.5 : Find the optical gain at threshold of a laser diode having following parametric values – R₁ = R₂ = 0.32, α = 10cm^-1 and L = 500 µm.

Solution : Optical gain in laser diode is given by –

                                                                                                         … Ans.

 

Power Current Characteristics

 

• The output optic power versus forward input current characteristics is plotted in

3.1.12 for a typical laser diode. Below the threshold current (I_th) only spontaneous emission is emitted hence there is small increase in optic power with drive current. At

 

 

 

 

threshold when lasing conditions are satisfied. The optical power increases sharply after the lasing threshold because of stimulated emission.

• The lasing threshold optical gain (g_th) is related by threshold current density (J_th) for stimulated emission by expression –

g _th = β J_th                                                                                     … 3.1.28

where, β is constant for device structure.

 

Fig. 3.1.12 Power current characteristics External Quantum Efficiency

• The external quantum efficiency is defined as the number of photons emitted per electron hole pair recombination above threshold point. The external quantum efficiency η_ext is given by –

 

                                           … 3.1.29

 

where,

 

η_i = Internal quantum efficiency (0.6-0.7). g_th = Threshold gain.

α = Absorption coefficient.

 

 

 

• Typical value of η_ext for standard semiconductor laser is ranging between 15-20 %.

Resonant Frequencies

 

• At threshold lasing

2β L = 2π m

 

where,                                         (propagation constant)

m is an integer.

 

\                                m                                                                                           ... 3.1.30

Since                            c = vλ

 

\                                

Substituting λ in 3.1.30

 

… 3.1.31

 

• Gain in any laser is a function of For a Gaussian output the gain and frequency are related by expression –

 

… 3.1.32

 

where,

 

g(0) is maximum gain.

 

λ₀ is center wavelength in spectrum. σ is spectral width of the gain.

• The frequency spacing between the two successive modes is –

                                               … 3.1.33

 

 

 

The wavelength Spacing is given as –

 

                                           … 3.1.34

 

 

Optical Characteristics of LED and Laser

 

• The output of laser diode depends on the drive current passing through it. At low drive current, the laser operates as an inefficient Led, When drive current crosses threshold value, lasing action Fig. 3.1.13 illustrates graph comparing optical powers of LED operation (due to spontaneous emission) and laser operation (due to stimulated emission).

Spectral and Spatial Distribution of Led and Laser

 

• At low current laser diode acts like normal LED above threshold current, stimulated emission i.e. narrowing of light ray to a few spectral lines instead of broad spectral distribution, exist. This enables the laser to easily couple to single mode fiber and reduces the amount of uncoupled light (i.e. spatial radiation distribution). 3.1.14 shows spectral and spatial distribution difference between two diodes.

 

 

 

 

Advantages and Disadvantages of Laser Diode Advantages of Laser Diode

1. Simple economic
2. High optical
3. Production of light can be precisely
4. Can be used at high
5. Better modulation
6. High coupling
7. Low spectral width (3.5 nm)
8. Ability to transmit optical output powers between 5 and 10 mW.
9. Ability to maintain the intrinsic layer characteristics over long

 

Disadvantages of Laser Diode

 

1. At the end of fiber, a speckle pattern appears as two coherent light beams add or subtract their electric field depending upon their relative phases.
2. Laser diode is extremely sensitive to overload currents and at high transmission rates, when laser is required to operate continuously the use of large drive current produces unfavourable thermal characteristics and necessitates the use of cooling and power

Comparison of LED and Laser Diode

 

Sr. No.	Parameter	LED	LD (Laser Diode)
1.	Principle of operation	Spontaneous emission.	Stimulated emission.
2.	Output beam	Non – coherent.	Coherent.
3.	Spectral width	Board spectrum (20 nm – 100 nm)	Much narrower (1-5 nm).
4.	Data rate	Low.	Very high.
5.	Transmission distance	Smaller.	Greater.
6.	Temperature sensitivity	Less sensitive.	More temperature sensitive.
7.	Coupling efficiency	Very low.	High.

 

 

 

 

8.	Compatible fibers	Multimode step index multimode GRIN.	Single mode Sl Multimode GRIN.
9.	Circuit complexity	Simple	Complex
10.	Life time	105 hours.	104 hours.
11.	Cost	Low.	High.
12.	Output power	Linearly proportional to drive current.	Proportional to current above threshold.
13.	Current required	Drive current 50 to 100 mA peak.	Threshold current 5 to 40 mA.
14.	Wavelengths available	0.66 to 1.65 µm.	0.78 to 1.65 µm.
15.	Applications	Moderate distance low data rate.	Long distance high data rates.

 

 

Important Formulae for LED and Laser LED

1. 
2. 
3. 
4. 

 

 

LASER

 

1. 
2. 

3.