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1. Optical Sources and detectors MODULE2

2. Light Emitting Diode (LEDs) To achieve a high radiance and a high quantum efficiency, the LED structure must provide a means of confining the charge carriers and the stimulated optical emission to the active region of the pn junction where radiative recombination takes place . Carrier confinement enhances radiative recombination in this region, leading to high quantum efficiency optical confinement prevents surrounding material from absorbing the emitted light.

3. CONT…, Various LED structures :- homojunctions , single heterojunctions , and double heterojunctions A s in fig b,c Double- heterostructure (DH) LEDs , which have two different alloy layers around the active region, are the most effective. The bandgap differences between layers trap charge carriers, and the refractive index differences confine the optical field. This dual confinement improves efficiency and radiance. Factors like optical absorption, recombination at interfaces, doping levels, carrier density, and active layer thickness also affect performance .

4. CONT…, Two main LED configurations for fiber optics are : Refer fig 4.9 Surface emitters (or Burrus /front emitters ): The light-emitting region is perpendicular to the fiber axis, with a well etched into the substrate to hold the fiber, which accepts the emitted light. The active area is typically 50 µm in diameter and up to 2.5 µm thick , with an isotropic emission pattern and a 120° half-power beam width .

5. CONT…, 2. Edge emitters : Emit light parallel to the fiber axis .as shown in fig 4.10 It has an active junction region that emits incoherent light , surrounded by two guiding layers with lower refractive indices than the active region but higher than the surrounding material.

6. CONT…, This structure forms a waveguide that directs light to the fiber core. The contact stripes are 50–70 µm wide to match fiber-core diameters (50–100 µm), with active region lengths of 100–150 µm In the plane parallel to the junction, the beam is lambertian ( varying as cos Ɵ) with a Ɵ | | = 120 ° half-power width , while in the perpendicular plane, the half-power beam Ɵ ⊥ can be reduced to 25–35° with an appropriate waveguide thickness.

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8. Light Source Materials The active layer of an optical source must use a direct- bandgap semiconductor for efficient radiative recombination and adequate optical emission. In direct- bandgap semiconductors , electrons and holes recombine directly without needing a third particle to conserve momentum. Single-element semiconductors are not direct-gap materials, but many binary compounds . The most important direct-gap materials come from III-V compounds , combining group III elements (e.g., Al, Ga , In) with group V elements (e.g., P, As, Sb ). Ternary and quaternary combinations of these binary compounds are also direct-gap materials and suitable for optical sources.

9. CONT…, For operation in the 800–900 nm range , the ternary alloy Ga ₁–ₓ AlₓAs is commonly used. The value of x determines the bandgap and emission wavelength . Typically , x is selected to emit wavelengths between 800–850 nm . An example LED with x = 0.08 emits light with a peak at 810 nm and a FWHM spectral width of 36 nm . For longer wavelengths (1.0–1.7 µm), the quaternary alloy In₁–ₓ GaₓAsᵧP ₁–ᵧ is used. For simplicity, notations like GaAlAs and InGaAsP are used

10. CONT…, GaAlAs and InGaAsP alloys are chosen for semiconductor light sources because their lattice parameters can be matched at heterostructure interfaces using a suitable combination of binary, ternary, and quaternary materials . Bandgap equation is the peak emission wavelength

11. CONT…, Spectral emission pattern of a representative Ga 1– x Al x As LED with x = 0.08. The width of the spectral pattern at its half-power point is 36 nm. Bandgap energy and output wavelength as a function of aluminum mole fraction x for Al x Ga 1–x As at room temperature .

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13. Quantum efficiency and LED power Quantum efficiency refers to how effectively an LED converts electrical energy into light energy . It is an important factor in determining the performance and efficiency of an LED . An excess of electrons and holes in p- and n-type material, respectively (referred to as minority carriers) is created in a semiconductor light source by carrier injection at the device contacts When carrier injection stops, the carrier density returns to the equilibrium value . In general, the excess carrier density decays exponentially with time according to the relation

14. CONT…, where n is the initial injected excess electron density and the time τ constant is the carrier lifetime. This lifetime is one of the most important operating parameters of an electro-optic device Radiative recombination : Produces photons with energy close to the bandgap ( hv ). Non- radiative recombination : Includes processes like self-absorption, interface recombination , where energy is transferred to another carrier.

15. CONT…, Equilibrium Condition in LEDs: Under a constant current, the injected electrons and holes reach equilibrium. The rate of carrier generation is the sum of the externally supplied rate J/ qd ( where J is current density, q is electron charge, and d is the recombination region thickness) and the thermal generation rate n/τ Carrier Recombination Rate Equation:

16. CONT…, Internal Quantum Efficiency ( ηint ): Internal quantum efficiency measures the efficiency of converting electrons and holes into photons (light) inside the LED . Quantum efficiency is the ratio of radiative recombination rate R r ​ to the total recombination rate R nr When:- a nd then ηint is

17. CONT…, Simple homojunction LEDs have quantum efficiency around 50 %. Double- heterojunction LEDs can achieve 60–80% efficiency due to reduced non- radiative recombination . R r is the total number of photons generated per second and that each photon has an energy hv , then the optical power generated internally to the LED is

18. CONT…, This is defined as the ratio of the photons emitted from the LED to the number of internally generated photons It considers factors like light extraction efficiency , as many photons generated inside the LED may be lost due to reflection or absorption . The external quantum efficiency , approximately given by optical power emitted from the LED is

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21. Laser Diodes: Modes and threshold conditions For optical fiber systems needing bandwidths over 200 MHz , semiconductor laser diodes are preferred due to their fast response times (under 1 ns), narrow spectral widths (2 nm or less), and ability to couple high power into small-core fibers . Most laser diodes are multilayered heterojunction devices . While LEDs evolved faster due to simpler construction, better temperature stability , and no catastrophic degradation , laser diodes require more complex designs , particularly for current confinement in the lasing cavity.

22. CONT…, The radiation in one type of laser diode configuration is generated within a Fabry -Perot resonator cavity Here the cavity is approximately 250–500 µ m long, 5–15 µ m wide, and 0.1–0.2 µ m thick . These dimensions commonly are referred to as the longitudinal, lateral , and transverse dimensions of the cavity, respectively

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24. CONT…, Fabry -Perot Resonator Construction : partially reflecting mirrors are used to form the Fabry -Perot resonator cavity . These mirrors are created by cleaving the semiconductor crystal along natural planes . The mirrors provide strong optical feedback in the longitudinal direction , turning the device into an oscillator and light emitter . The sides of the cavity are roughened to minimize unwanted emissions in lateral directions.

25. CONT…, As the light reflects back and forth within the Fabry -Perot cavity, the electric fields of the light interfere on successive round trips . Those wavelengths that are integer multiples of the cavity length interfere c onstructively so that their amplitudes add All other wavelengths interfere destructively and thus cancel themselves out . The optical frequencies at which constructive interference occurs are the resonant frequencies of the cavity The resonant wavelengths are called the longitudinal modes of the cavity because they resonate along the length of the cavity

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27. D istributed-feedback (DFB) laser, The fabrication of this device is similar to the Fabry -Perot types , except that the lasing action is obtained from Bragg reflectors (gratings) or periodic variations of the refractive index (called distributedfeedback corrugations), which are incorporated into the multilayer structure along the length of the diode.

28. Different laser diode modes and threshold value The optical radiation within the resonance cavity of a laser diode sets up a pattern of electric and magnetic field lines called the modes of the cavity These can conveniently be separated into two independent sets of transverse electric (TE) and transverse magnetic (TM) modes . Each set of modes can be described in terms of the longitudinal, lateral, and transverse half-sinusoidal variations of the electromagnetic fields along the major axes of the cavity

29. CONT…, longitudinal modes are related to the length L of the cavity and determine the principal structure of the frequency spectrum of the emitted optical radiation Lateral modes lie in the plane of the pn junction . These modes depend on the side wall preparation and the width of the cavity , and determine the shape of the lateral profile of the laser beam . Transverse modes are associated with the electromagnetic field and beam profile in the direction perpendicular to the plane of the pn junction

30. CONT…, To determine the lasing conditions and the resonant frequencies , we express the electromagnetic wave propagating in the longitudinal direction in terms of the electric field phasor Lasing is the condition at which light amplification becomes possible in the laser diode Lasing occurs when the gain of one or several guided modes is sufficient to exceed the optical loss during one roundtrip through the cavity ; that is , z = 2L.

31. CONT…, During this roundtrip , only the fractions R1 and R2 of the optical radiation are reflected from the two laser ends 1 and 2, respectively, where R1 and R2 are the mirror reflectivities or Fresnel reflection coefficients , which are given by At the lasing threshold , a steady-state oscillation takes place , and the magnitude and phase of the returned wave must be equal to those of the original wave . This gives the conditions for the amplitude and for the phase

32. CONT…, The condition to just reach the lasing threshold is the point at which the optical gain is equal to the total loss α t , in the cavity

33. Laser Diode Rate Equations R ate equations that govern the interaction of photons and electrons in the active region rate equations are given by The first term in Eq. (4.30) is a source of photons resulting from stimulated emission. The second term, describing the number of photons produced by spontaneous emission, is relatively small compared with the first term . The third term in Eq. (4.30) indicates the decay in the number of photons caused by loss mechanisms in the lasing cavity.

34. CONT…, In Eq. (4.31), the first term represents the increase in the electron concentration in the conduction band as current fl ows into the device. The second and third terms give the number of electrons lost from the conduction band owing to spontaneous and stimulated transitions Solving these two equations for a steady-state condition will yield an expression for the output power. condition is

35. CONT…, consider the photon and electron rate equations in the steady-state condition at the lasing threshold . After solving above two Eqn number of photons per unit volume is The first term in above is the number of photons resulting from stimulated emission The second term gives the spontaneously generated photons

36. External Quantum Efficiency The external differential quantum efficiency η ext is defined as the number of photons emitted per radiative electron–hole pair recombination above threshold. Under the assumption that above threshold the gain coefficient remains fixed at g th , η ext is given by η i is the internal quantum efficiency . This is not a well-defined quantity in laser diodes, but most measurements show that η i 0.6–0.7 at room temperature.

37. CONT…, Experimentally, η ext is calculated from the straight-line portion of the curve for the emitted optical power P versus drive current I , which gives

38. Resonant Frequencies resonant frequencies of the laser Thus some lasers are single-mode and some are multimode. The relationship between gain and frequency can be assumed to have the gaussian form

39. CONT…, frequency, or wavelength, spacing between the modes of a multimode laser consider only the longitudinal modes To find the frequency spacing, consider two successive modes of frequencies V m-1 and V m represented by the integers m – 1 and m

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41. Operational Principles of WDM Wavelength Division Multiplexing (WDM) uses discrete wavelengths as separate channels (carriers) that can be routed, switched, and separated without interference . This channel isolation is maintained if the optical power is low enough to avoid nonlinear effects like stimulated Brillouin scattering and four-wave mixing , which can degrade performance. WDM networks need both passive and active devices to manage optical power across wavelengths

42. CONT…, Passive devices don't need external control and are mainly used for combining, splitting, or tapping optical signals. Active devices , such as tunable optical filters , tunable sources , and optical amplifiers , can be controlled electronically or optically, providing more flexibility for network management . Figure 10.1 shows the implementation of passive and active components in a typical WDM link containing various types of optical amplifiers

43. CONT…, At the transmitting end there are several independently modulated light sources , each emitting signals at a unique wavelength. Here a multiplexer is needed to combine these optical outputs into a continuous spectrum of signals and couple them onto a single fiber. At the receiving end a demultiplexer is required to separate the optical signals into appropriate detection channels for signal processing

44. CONT…, there are many independent operating regions across the fig To find the optical bandwidth corresponding to a particular spectral width in these regions, we use the fundamental relationship C= λ V , which relates the wavelength λ to the carrier frequency V , where c is the speed of light.

45. Isolators and Circulators Optical Isolators : Devices that allow light to pass in only one direction, preventing backward-traveling light . Purpose : Prevent reflected or scattered light from entering components like laser diodes, avoiding output instabilities . Design Types : Some designs depend on the polarization state of the input light . Power Loss : Polarization-dependent isolators can cause a 3-dB power loss (50%) when used with unpolarized light . Practical Requirement : In optical communication systems, isolators should operate independently of polarization , as optical signals are typically unpolarized .

46. CONT…, This design for a polarization-independent optical isolator consists of three key components : a 45° Faraday rotator placed between two wedge-shaped birefringent plates made of materials like YVO4 or TiO2. Forward Light Path : As light moves in the forward direction, the first birefringent plate separates the light into two types of rays—ordinary and extraordinary . These rays pass through the Faraday rotator, which rotates their polarization by 45°. The second birefringent plate is aligned to maintain the relationship between these two rays, allowing them to continue forward.

47. CONT…, Reverse Light Path : When light travels in the reverse direction (from right to left), the Faraday rotator reverses the relationship between the ordinary and extraordinary rays , rotating their polarization again by 45°. This causes the rays to no longer align properly when they exit, effectively blocking the reverse light and preventing it from returning. This design ensures the isolator works regardless of light polarization, making it effective in fiber optic systems .

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49. Optical Circulators An optical circulator is a nonreciprocal device with multiple ports that directs light in one direction from port to port , without allowing it to travel backward. It is commonly used in optical amplifiers, add/drop multiplexers, and dispersion compensation modules . Similar to an isolator but with more complex construction. Typically built with walk-off polarizers, half-wave plates, and Faraday rotators. Can have 3 or 4 ports. Three-Port Circulator : Input at port 1 goes to port 2. Input at port 2 goes to port 3. Input at port 3 goes to port 1.

50. CONT…, Four-Port Circulator : Ideally, it could have four inputs and four outputs , but this is difficult to fabricate. Usually configured with 3 input ports and 3 output ports: Port 1: input-only. Ports 2 and 3: input/output. Port 4: output-only . Circulators are available commercially with features like low insertion loss, high isolation across wide wavelengths, minimal polarization-dependent loss (PDL), and low polarization-mode dispersion (PMD).

51. Fiber Grating Filters A grating is an important element in WDM systems for combining and separating individual wavelengths. A grating is a periodic structure in a material . This variation in the material has the property of reflecting or transmitting light in a certain direction depending on the wavelength. Thus , gratings can be categorized as either reflecting or transmitting gratings.

52. CONT…, D efines various parameters for a reflection grating. Here, Ɵ i is the incident angle of the light, Ɵ d is the diffracted angle , and is the period of the grating In a transmission grating consisting of a series of equally spaced slits, the spacing between two adjacent slits is called the pitch of the grating. Constructive interference at a wavelength λ occurs in the imaging plane when the rays diffracted at the angle Ɵ d satisfy the grating equation given by

53. CONT…, Here, m is called the order of the grating . In general, only the first-order diffraction condition m = 1 is considered.

54. Dielectric Thin-Film Filters Dielectric Thin-Film Filter (TFF ) : TFF acts as an optical bandpass filter , allowing only a narrow wavelength band to pass through and reflecting others. TFF is based on a Fabry -Perot filter structure, which is a cavity formed by two highly reflective parallel mirror surfaces. This structure is also called a Fabry -Perot interferometer or a thin-film resonant cavity filter. When light enters the filter, it passes through the cavity and reflects off the inside surface on the opposite side . Some light exits the cavity, while some is reflected back.

55. CONT…, If the roundtrip distance between the two mirrors is a whole multiple of the wavelength (e.g., λ, 2λ, 3λ), the light wavelengths passing through interfere constructively . This constructive interference increases the intensity of the light at these specific wavelengths . These specific wavelengths are called the resonant wavelengths of the cavity and are transmitted through the filter.All other wavelengths are rejected .

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57. Diffraction Gratings A diffraction grating is an optical device that separates different wavelengths in a light beam . It consists of parallel slits or grooves spaced similarly to the wavelength of light . Types : Reflection Gratings : Light reflects off a surface with etched lines. Transmission Gratings : Light passes through the slits or grooves.

58. CONT…, Diffraction gratings separate wavelengths in parallel, unlike fiber-based Bragg gratings, which work in series . Reflection Grating Functionality : Light reflects off at varying angles based on its wavelength, producing a spectrum. For DWDM (Dense Wavelength Division Multiplexing), the grating reflects individual wavelengths at different angles.

59. CONT…, As shown in figure Fiber Reception fibers are positioned where the reflected light focuses, allowing each wavelength to be directed to separate fibers. The reflective diffraction grating works reciprocally; that is , if different wavelengths come into the device on the individual input fibers , all of the wavelengths will be focused back into one fiber after traveling through the device A photodiode array can be used instead of receiving fibers for monitoring power per wavelength .

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