Optoelectronic Modulator and Optical Sensors: In the early optical systems where the data rate was low, the lasers were modulated directly by combining the data signal with the bias of the laser diode. The advantage of the direct modulation system was is simplicity.

Indirect or External Modulation Beyond the data rates of 1-2 GHz, the complex dynamics of the coupling of electrons and photons within the laser cavity cause undesirable frequency variations, known as chirps. In presence of chirp, the signal gets distorted as propagates on the optical fiber. For high data rates a better option would be the external modulation. In this scheme, the laser operates on a constant bias producing a narrow line width continuous optical carrier. The data is imposed on the optical carrier by the modulator, external to the laser cavity.

The external modulation has following attractive features: Broad modulation bandwidth, Excellent spectral purity, High optical power handling capability Low modulation distortion

Electro-Optic Effect There are certain materials, called the electro-optic material whose dielectric constant changes when they are subjected to an electric field. Change in dielectric constant due to electric field is called the electro-optic effect or the Pockel effect. The parameter which characterizes the electro-optic material is called the electro-optic coefficient. The most widely used electro-optic material is the Lithium Niobate (LiNbO3). since it has a large electro-optic coefficient. The change in the dielectric constant therefore depends upon the magnitude of the electric field, and orientation of the electric field with respect to the crystal axis.

Optical Phase Modulator A binary data voltage is applied to the electrodes. The data voltage changes the electric field between the electrodes changing the refractive index of the waveguide. The change in refractive index changes the phase of the light.

Optical Amplitude Modulator: Mach-Zehnder Interferometer It consists of two integrated optical phase modulators, and two Y branch couples, one at the input for splitting the signals and other at the output for combining the optical signals. The input Y-junction splits the power equally into two branches to give fields E1 and E2 in the two arms of the interferometer.

Mach-Zehnder Interferometer II For binary data, the phase changes between  , and the output intensity changes from 2E0to 0

Directional Coupler Directional coupler is one of the very important devices which can be used for various applications like, optical amplitude modulator, power tapping, power divider, wavelength filter, optical switch, optical multiplexer, optical cross-connect and so on. The directional coupler consists of two channel optical waveguides placed close to each other so that their fields can interact with each other.

Important Observations: There is exchange of power between two waveguides due to overlapping of the field outside the waveguide. This is called evanescent mode coupling. Both waveguides have traveling waves with average phase constant The amplitude of both waves vary in amplitude as they travel along the waveguides.

Making use of dissimilar waveguides, L=Lc

LASER

Regenerators

Optical Amplifier

EDFA

EDFA

Advantages & Disadvantages of EDFA EDFA has high pump power utilization (>50%) Directly and simultaneously amplify a wide wavelength band (>80nm) in the 1550nm region, with a relatively flat gain Flatness can be improved by gain-flattening optical filters Gain in excess of 50 dB Low noise figure suitable for long haul applications Disadvantages Size of EDFA is not small It can not be integrated with other semiconductor devices

Raman Amplifier

Distributed Amplification

Raman Amplifier

Semiconductor optical amplifiers (SOA) Characteristics: Polarization dependent – require polarization maintaining fiber Relatively high gain ~20 dB Output saturation power 5-10 dBm Large BW Can operate at 800, 1300, and 1500 nm wavelength regions. Compact and easily integrated with other devices

Semiconductor optical amplifiers (SOA)

SOA (Working Principle) Stimulated emission to amplify an optical signal. Active region of the semiconductor. Injection current to pump electrons at the conduction band. The input signal stimulates the transition of electrons down to the valence band to acquire an amplification.

Advantages & Disadvantages of SOA The semiconductor optical amplifier is of small size and electrically pumped. It can be potentially less expensive than the EDFA and can be integrated with semiconductor lasers, modulators, etc. All four types of nonlinear operations (cross gain modulation, cross phase modulation, wavelength conversion and four wave mixing) can be conducted. SOA can be run with a low power laser. This originates from the short nanosecond or less upper state lifetime, so that the gain reacts rapidly to changes of pump or signal power and the changes of gain also cause phase changes which can distort the signals. Disadvantages The performance of SOA is still not comparable with the EDFA. The SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time.

Comparison with Erbium-doped Fiber Amplifiers The setup is much more compact, containing only a small semiconductor chip with electrical and fiber connections. The output powers are significantly smaller. The gain bandwidth is smaller, but devices operating in different wavelength regions can be made. The noise figure is typically higher. The amplification is normally polarization-sensitive.

Comparison of EDFA, Raman and EDFA

Fiber Optic Interferometer A fiber optic interferometer uses the interference between two beams that have propagated through different optical paths of a single fiber or two different fibers. So, they require beam splitting and beam combining components in any configurations The current trend of fiber optic interferometers is to miniaturize them for micro-scale applications.

Fabry-Perot Interferometer Sensor Generally composed of two parallel reflecting surfaces separated by a certain distance. Interference occurs due to the multiple superpositions of both reflected and transmitted beams at two parallel surfaces. the FPI can be simply formed by intentionally building up reflectors inside (intrinsic) or outside (extrinsic) of fibers.

The reflection or transmission spectrum of an FPI can be described as the wavelength dependent intensity modulation of the input light spectrum, which is mainly caused by the optical phase difference between two reflected or transmitted beams. The maximum and the minimum peaks of the modulated spectrum mean that both beams, at that particular wavelength, are in phase and out-of-phase, respectively, in the modulus of 2.

Mach-Zehnder Interferometer Sensors

Configuration of various types of MZIs; the methods of using (a) a pair of LPGs, (b) core mismatch, (c) air-hole collapsing of PCF, (d) MMF segment, (e) small core SMF, and (f) fiber tapering.

Michelson Interferometer Sensors

Sagnac Interferometer Sensor

Applications Interferometric fiber optic sensors have great potential in practical applications such as real time deformation monitoring of aircrafts, ships, bridges, and constructions. Environmental sensors including explosive hydrogen sensor and biomedical sensors for health monitoring sensor systems are emerging fields.

Magneto-optic effect A magneto-optic effect is a phenomena in which an electromagnetic wave propagates through a medium that has been altered by the presence of a quasistatic magnetic field. In such a material, which is also called gyrotropic or gyromagnetic, left- and right-rotating elliptical polarizations can propagate at different speeds, leading to a number of important phenomena When light is transmitted through a layer of magneto-optic material, the result is called the Faraday effect: the plane of polarization can be rotated, forming a Faraday rotator The results of reflection from a magneto-optic material are known as the magneto-optic Kerr effect.

The rotation of the plane of polarization is proportional to the intensity of the component of the magnetic field in the direction of the beam of light.

Isolator Device through which light passes in one direction but not the other.

Acousto-optical effect Modification of the refractive index by the oscillating mechanical pressure of a sound wave. An acousto-optic modulator (AOM), also called a Bragg cell, uses the acousto-optic effect to diffract and shift the frequency of light using sound waves (usually at radio-frequency). They are used in telecommunications for signal modulation, and in spectroscopy for frequency control.

A piezoelectric transducer is attached to a material such as glass. An oscillating electric signal drives the transducer to vibrate, which creates sound waves in the glass. These can be thought of as moving periodic planes of expansion and compression that change the refractive index. Incoming light scatters (Brillouin scattering) off the resulting periodic index modulation and interference occurs. The interaction can be thought of as four-wave mixing between phonons and photons.

The properties of the light exiting the AOM can be controlled in five ways: Deflection:A diffracted beam emerges at an angle θ that depends on the wavelength of the light λ relative to the wavelength of the sound Λ sin θ = (mλ/2Λ) where m = ...-2,-1,0,1,2,... is the order of diffraction. As the AOM get thicker only phase-matched orders are diffracted, this is called Bragg diffraction

Intensity The amount of light diffracted by the sound wave depends on the intensity of the sound. Hence, the intensity of the sound can be used to modulate the intensity of the light in the diffracted beam. Typically, the intensity that is diffracted into m=0 order can be varied between 15% to 99% of the input light intensity. Likewise, the intensity of the m=1 order can be varied between 0% and 80%.

Frequency One difference from Bragg diffraction is that the light is scattering from moving planes. A consequence of this is the frequency of the diffracted beam f in order m will be Doppler-shifted by an amount equal to the frequency of the sound wave F. f → f + mF This frequency shift is also required by the fact that energy and momentum (of the photons and phonons) are conserved in the process.

Phase In addition, the phase of the diffracted beam will also be shifted by the phase of the sound wave. The phase can be changed by an arbitrary amount.

Polarization Collinear transversal acoustic waves or perpendicular longitudinal waves can change the polarization. The acoustic waves induce a birefringent phase-shift, much like in a Pockels cell. The acousto-optic tunable filter, especially the dazzler, which can generate variable pulse shapes, is based on this principle.

Realization Acousto-optic modulators are much faster than typical mechanical devices such as tiltable mirrors. The time it takes an AOM to shift the exiting beam in is roughly limited to the transit time of the sound wave across the beam (typically 5 to 100 microseconds). This is fast enough to create active modelocking in an ultrafast laser. When faster control is necessary electro-optic modulators are used. However, these require very high voltages (e.g. 10 kilovolts), whereas AOMs offer more deflection range, simple design, and low power consumption (<3 watts). The acoustic wave may be absorbed at the other end of the crystal.

Applications of Acousto-Optic Modulators

Q Switching: Q switching is a technique for obtaining energetic short pulses from a laser by modulating the intracavity losses and thus the Q factor of the laser resonator. The technique is mainly applied for the generation of nanosecond pulses of high energy and peak power with solid-state bulk lasers.

Cavity Dumping Cavity dumping is a technique for pulse generation which can be combined either with Q switching or with mode locking, or sometimes even with both techniques at the same time. In any case, the basic idea is to keep the optical losses of the laser resonator as low as possible for some time, so that an intense pulse builds up in the resonator, and then to extract this pulse within about one cavity round-trip time using a kind of optical switch (“cavity dumper”), such as an acousto-optic modulator or a Pockels cell.

Mode Locking Mode locking (sometimes written as modelocking) is a method (or actually a group of methods) to obtain ultrashort pulses from lasers, which are then called mode-locked lasers.

Optical Circulators An optical circulator is a nonreciprocal multiport passive device that directs light sequentially from port to port in only one direction. In the 3–port example, an input on port 1 is sent out on port 2, an input on port 2 is sent out on port 3, and an input on port 3 is sent out on port 1.