Unit3_LED_LASER_PPT.pdf

Unit:3

10EC72

ECE Department ,CMRIT

Desirable characteristics of optical sources

 Emission with low loss window of the fiber

 Narrow spectral width

 Capacity to couple sufficient power into fiber

 Ease of coupling to fiber

 Linearity of modulation

 High modulation speed

 High reliability

What are the main sources for OFC?

 LED

_{Suitable for Multimode fiber.} Coupling efficiency is less

Low BW applications, Sensor applications

 LASER

Suitable for single as well as multimode fiber

More coupling efficiency

Light Emitting diode - LED

 Forward biased p-n junction, operates in low current densities than LASER

 Incoherent light, emitted photons are having random phases.

 Emitted photons energy is not exactly same as bandgap energy

 Advantages:

 Simpler fabrication

 Lower cost for construction

 Greater reliability. Immune to self pulsation and modal noise problems

 Less temperature dependence

 Simple drive circuit

 Linearity in output current characteristics suitable for analog modulations

 Disadvantages:

• Low coupling with SMF

• Lower modulation bandwidth

Semiconductor material band gap

Example of materials

LED operation

 Probability of photon generation:

 No of photon generated:

Photons will be emitted when >

𝑃_𝑝ℎ∞𝑒−(𝐸𝐾𝑇2−𝐸1

𝑁_𝑝ℎ∞ 𝐸_𝑝ℎ − 𝐸_𝑔 . 𝑒

−𝐸_𝑝ℎ 𝐾𝑇

𝐸_𝑝ℎ 𝐸_𝑔

𝜆 = 1.24

LED with different wavelengths

 GaAs has Eg=1.4 eV, λ=1.24/1.4=885 nm

 For , E(eV)= , 0<x<0.37

 For , E(eV)= Where, and

𝐺𝑎_𝑥𝐴𝑙_1−𝑥𝐴𝑠 1.424 + 1.266𝑥 + 0.266𝑥2

𝐼𝑛_1−𝑥𝐺𝑎_𝑥𝐴𝑠_𝑦𝑃_{1−𝑦 1.35 − 0.72𝑦 + 0.12𝑦}2

Spectral pattern of practical LED

 Effective spectral width = 2KT

Due to impurities, practically ∆λ=1.5KT to 3.5KT

𝛥𝜆

𝜆 = −

𝛥𝐸_𝑝ℎ

𝐸_𝑝ℎ = −

2𝐾𝑇 𝐸_𝑝ℎ

λ

∆λ

Temperature dependency on output spectrum

Radiative and non radiative recombination

 All the recombination processes does not emit photon.

Quantum efficiency

 It is conversion of electrical energy to photon and guiding by optical fiber

Quantum Efficiency

Surface emitting LED

Edge emitting LED

Injection LASER

Coherency of optical sources

 Temporal Coherency

Defines how signal is time correlated.

Power spectral density Correlation function S(ω) R(τ)

𝑅 𝜏 =

−∞ ∞

𝐴(𝑡 . 𝐴∗(𝑡 − 𝜏 𝑑𝑡 𝐹𝑇

𝐹𝑇

𝐹𝑇 𝜏_𝑐𝑜ℎ = 1

Spatial coherency

 Defines the directivity of the signal

 If R(λ) uniform, then radiation pattern is highly directional

 Power radiation pattern R(λ)

𝑅(𝜆 =

−∞ ∞

𝐴(𝑥 . 𝐴(𝑥 − 𝜆 𝑑𝑥

Non spatial coherence

Spatial Coherence

Absorption and emission of radiation

Important relations:

 According to Boltzmann's distribution

 Spontaneous emission rate =

Where, = Average time that the electron exists in the excited state = Einstein coefficient of spontaneous emission

 Rate of absorption =

Where, = Density of atoms in energy level E1

= Spectral density of the radiation energy at the transition frequency = Einstein coefficient of absorption

 Rate of stimulated emission =

Where, = Density of atoms in energy level E1

= Einstein coefficient of stimulated emission

𝑁₁ 𝑁₂ =

𝑔₁. exp(−𝐸₁ 𝐾 𝑇 𝑔₂. exp(−𝐸₂ 𝐾 𝑇 =

𝑔₁ 𝑔₂ . 𝑒

𝐸₂−𝐸₁ 𝐾𝑇 _{= 𝑒}

ℎ𝜈 𝐾𝑇

𝑁₂. 1 𝜏₂₁ 𝜏₂₁

= 𝑁₂. 𝐴₂₁ 𝐴₂₁

𝑁₁. 𝜌_𝑓. 𝐵₁₂ 𝑁₁

𝜌_𝑓 𝐵₁₂

𝑁₂. 𝜌_𝑓. 𝐵₂₁ 𝑁₂

Important relations (contd..)

 Total downward transition (R21)= +

 Total upward transition (R12) =

 Conservation of energy: R21=R12

 Flux density for a black body radiation

𝜌_𝑓 =

𝐴₂₁ 𝐵₂₁ 𝐵₁₂

𝐵₂₁ . 𝑒

ℎ𝑓 𝐾𝑇 _{− 1}

𝑁₂. 𝐴₂₁ 𝑁₁. 𝜌_𝑓. 𝐵₁₂

𝑁₂. 𝜌_𝑓. 𝐵₂₁

𝜌_𝑓 = 8𝜋ℎ𝑓

3

𝑐3

1

exp(ℎ 𝑓 𝐾 𝑇 − 1 𝑅_{𝑠𝑡𝑖𝑚𝑢𝑙𝑎𝑡𝑒𝑑}

𝑅_{𝑠𝑝𝑜𝑛tan𝑒𝑜𝑢𝑠} =

𝑁₂𝜌_𝑓𝐵₂₁ 𝑁₂𝐴₂₁ =

𝜌_𝑓𝐵₂₁ 𝐴₂₁ =

1

Population inversion

Three-Four level energy state

Ground state Metastable

Threshold condition for laser oscillation

 Mirror reflectivity r1 and r2

Where, =Loss coefficient inside amplifying medium per unit length (/cm) L = Length of the amplifying medium

g= Gain coefficient per unit length

Direct bandgap suitable for laser operation

 Relative minority carrier lifetime is given by

Where, N,P are respective minority carrier concentration in n and p type regions, =Recombination coefficient

Example:

 For GaAs (Direct bandgap), N=P= cm¯³, cm³/sec So,

= 0.69 nsec

 For Silicon (Indirect bandgap), N=P= cm¯³, m³/sec So,

= 0.28 msec

So direct bandgap GaAs has radiative carrier lifetime times less than indirect bandgap silicon.

𝜏

𝜏_𝑟 = 𝐵_𝑟(𝑁 + 𝑃 −1

1018 𝐵_𝑟 = 7.21 × 10−10 𝐵_𝑟

𝜏_𝑟 = 7.21 × 10−10 × 2 × 1018 −1

1018 𝐵_𝑟 = 1.79 × 10−15 𝜏_𝑟 = 1.79 × 10−15 × 2 × 1018 −1

Threshold current density

 Threshold gain coefficient =

 Threshold current density for stimulated emission is given by,

Now,

 Threshold current

𝑔_𝑡ℎ

𝐽_𝑡ℎ 𝑔_𝑡ℎ = 𝛽. 𝐽_𝑡ℎ

𝑔_𝑡ℎ = 𝛼 + 1 2𝐿 ln

1 𝑟₁𝑟₂ 𝐽_𝑡ℎ = 1

𝛽 𝛼 + 1 2𝐿ln

1 𝑟₁𝑟₂

Semiconductor Injection LASER

Efficiency

 External quantum efficiency

where, = Optical power emitted from the device I= Incident current

e= Electron charge, hf= Photon energy

 Hence defines the slope of the output characteristics

 Typical value of is 40-60%

𝜂_𝐷 = 𝑑𝑃𝑒 ℎ 𝑓 𝑑 𝐼 𝑒 =

𝑑𝑃_𝑒

𝑑𝐼(𝐸_𝑔 𝐸𝑔 = ℎ𝑓

𝑒 𝑃_𝑒

𝜂_𝐷

Internal quantum efficiency

 Typical value is 50-100%.

 are related using the expression:

 Now, and So,

Single frequency injection laser

 Produces single longitudinal mode unlike fabry-parot cavity.

 Mainly two types:

i) Distributed feedback laser (DFB laser)

DFB Laser

 Use of distributed resonator into LASER structure gives wavelength selectivity.

Period of corrugation

 Single mode of operation possible when, Period of corrugation

Where,

=Integer order of the grating =Bragg wavelength

=Effective refractive index of the wavelength

=1 gives highest coupling, =2 is easier to fabricate

𝑇_𝑝 = 𝑙. 𝜆𝐵 2𝑛_𝑒 𝑙

𝜆_𝐵 𝑛_𝑒

Types of DFB laser

 Mainly three types:

i. Distributed feedback laser (DFB), ii. Distributed Bragg reflector (DBR),

iii. Distributed reflector(DR) laser.

DR laser

Vertical cavity surface emitting laser (VCSEL)

 Emits coherent light perpendicular to the device substrate

 Unlike surface emitting laser, a short vertical cavity is formed.

 Reflectivity of the bottom mirror(n-type) is more than top mirror(p-type).

Active cavity

Temperature effect

 Threshold current is dependent on temperature.

Where, T0 = Relative temperature (120°-165°C)

= Constant [when T=T0, = ]

 With T0=135°C , Variation of threshold current is 0.8%

Light source linearity

 Analog signal s(t) directly used to modulate optical source about bias point

m= Modulation Index,

Introduction:

 Optical detectors converts the received optical signal into electrical signal

 System performance is determined at the detector side.

(Due to dispersion and attenuation, receiver design is challenging)

 Desirable characteristics:

 _{High sensitivity at operating wavelength}  High linearity

 Large electrical response to the received optical power  Minimum noise

 Short response time

 Stability of performance characteristic  _{Small size}