FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

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4.3 INTEGRATED OPTICAL CIRCUITS (IOCS) The optical counterpart of the field of integrated electronics is the field of integrated optics. However, though integrated optical circuita have been made, they are for the moat part not commercially available. They are the subject of extensive research and development efforta at a large number of laboratories. The goal of these efforts is to commercially fabricate in large quantities these miniaturized devices with interconnected waveguides all on a single substrate. The generation, detection, propagation, modulation, switching, and coupling of light on such substrates have been accomplished. Techniques for the fabrication of substrates and the production of the high resolution material distribution patterns required for these integrated optical circuits exist. Most integrated optical circuits and associated devicea operate in a singlemode, therefore they are compatible with singlemode optical fibers. In essence most LEDs, diode lasers, and photodiodea are integrated optic devices. Integrated optical circuits are considered here because they are a part of second generation fiberoptic sensor technology. The waveguides used in integrated optical circuits are usually of two types namely planar films, that confine the light wavea in the vertical direction, and planar strips, (channels) that confine lightwaves in two dimensions. In both caaea, the waveguides are made of a higher refractive index than the surrounding material. Light is trapped in the layer or channel in much the same way that it is trapped in the core of an optical fiber. These planar waveguides can be formed by sputtering glass on a substrate of lower refractive index. A very uaeful type of channel waveguide can be formed by evaporating titanium (Ti) onto lithium niobate (LiNb03) or lithium tantalate in the desired waveguide patterns, and diffusing the Ti into the aubstrate thus forming a channel of higher refractive index. This is shown in Fig. 4.23. The attenuation in “’’”’””-r LiNb03 SUBSTRATE “1 OPTICAL n2 CHANNEL& nl>nz Si02_ ELECTRODES— Fig. 4.23 n1>n2>n3 (A) TITANIUM STRIP ON LiNb03 SUBSTRATE (B) TITANIUM DIFFUSEDAT HIGH T INTO SUBSTRATE FORMING CHANNEL WAVEGUIDE (C) PROTECTIVE LAYEROF Si02 SPUTTERED ON SURFACE (D) ELECTRODES APPLIED Steps in the fabrication of a channel waveguide. auch channels is typically 1 dB/cm. A protective layer of silicon dioxide (Si02) ia often sputtered on top in order to protect the surface. Electrodes may be deposited on the Si02. Coupling a fiber to an integrated optic channel has been accomplished as shown in Fig. 4.24. The 1 v “’SILICON SINGLE-MODE FIBEti IN ALIGNMENT V-GROOVES IN SILICON Fig. 4.24 An optical fiber pigtailed to a lithium niobate (LiNb03) fiberoptic chip. fiber is aligned and epoxied in an etched silicon (Si) V-groove. The fiber end and the Si edge are polished and butted against the polished edge of LiNb03. Indexmatching liquid is used between the fiber and substrate ends. Similar techniques are uaed to couple diode lasers and photodetectors to the substrate. The dimensions of channel waveguides are close to those of both ainglemode fiber corea and the radiating areas of stripe geometry diode lasers. For stripe geometry, the long dimension of the channel should be oriented perpendicular to the long dimension of the radiating area of the diode. Insertion leases as low as a few dB can be achieved. Channel-to-channel couplers can be formed on the integrated optical circuit aubstrate by paralleling two channels in cloae proximity to each other for a sufficient distance to achieve the desired coupling ratio. The coupling length is defined as the distance required for complete power transfer to occur. The length required to achieve a apecified coupling ratio depends on the separation between channels and the refractive indices of the channels and subatrate. The refractive index of LiNb03 ia a linear function of the applied electric field. This is known aa the Pockels effect. Thus, if electrodes are applied as shown in Fig. 4.25, the refractive index can be varied and the light can be awitched to the other of the two output ports of the coupler. The direction of the electric field is oriented oppositely in the two channels resulting in equal and opposite refractive index changes. When the proper value of voltage is applied, the coupling length is made ahorter and the light will exit from the same fiber it entered. When the voltage is not applied, the coupling length is longer and the light will emerge from both ports. The exit channels from this switch may be combined to form the Mach-Zehnder interferometer shown in Fig. 4.26. The result is an electrooptic intensity modulator. When the light wavea recombine they excite the fundamental mode of the exit channel. When they arrive out of phase they tend to excite the aecond mode but the second mode can’t propagate in the singlemode channel, therefore it radiatea. An acoustooptic modulator (Bragg Cell) is ahown in Fig. 4.27. The interdigited ultrasonic transducer is shown at the bottom. The surface acoustic wave 4-12
sets up periodic spatial fluctuations in the refractive index of the waveguide. These act as phase gratings deflecting a portion of the lightwave incident perpendicular to the resulting grating. The lightwave that passes straight through the grating exits with its frequency unchanged, while the diffracted ray has its frequency shifted by an amount equal to the acoustic frequency, e.g., 500 MHz. Such Bragg cells can be used for optical heterodyning. nl—An n, +dn + l-l+ ACOUSTIC ABSORBER I FREQUENCY SHIFTED BEAM ACOUSTIC SURFACE WAVE —–—= -— LIGHT . INCIDENT—— UNDIFFRACTED /’ / BEAM OPTICAL FIBER /’ INTERDIGITED CHANNEL / ‘ E3-- TRANSDUCER WAVEGUIDE ACOUSTIC ABSORBER ELECTRODES ( n. > LiNb03 +h - , , /s.BsTRATE Fig. 4.27 A Bragg cell integrated acoustic modulator in which an interdigital transducer is used for frequency shifting in accordance with an electrical input signal that develops an ultrasonic wave in a bifurcated optical waveguide. Fig. 4.25 INCIDENT BEAMJ WAVEGUIDE A Pockels effect electrooptic binary switch mounted on a lithium miobate substrate. Technology exists for integrating an array of channel couplers connected to an array of photodiodes with charge-coupled device readout all on a single substrate. Using flip-chip techniques (mounting the chip face down) multiple fiber pigtails can be attached to a single chip. Such integrated devices promise to significantly reduce the size and cost of arrays of optical sensors. A MODULATED LIGHT OUT ,- OP POLARIZ Fig. 4.26 A Mach-Zehnder interferornetric electrooptic intensity modulator mounted on a lithium niobate (LiNb03) substrate chip. 4-13
Page 1 and 2: FIBEROPTIC SENSOR TECHNOLOGY HANDBO
Page 3 and 4: FOREWORD THE FIBEROPTIC SENSOR TECH
Page 5 and 6: 4.2 Phase 4.2.1 4.2.2 4.2.3 4.2.4 4
Page 7 and 8: CHAPTER 1 INTRODUCTION 1.1 BACKGROU
Page 9 and 10: CHAPTER 2 FIBEROPTIC SENSOR COMPONE
Page 11 and 12: fleeted each time it is incident on
Page 13 and 14: 6=0 REFERENCE PLANE that propagate
Page 15 and 16: in the right center of Fig. 2.13. T
Page 17 and 18: The vertical height of the various
Page 19 and 20: 60 40 20 10.C ~8 x z 6 n u 4 % c 62
Page 21 and 22: : 1.5 z o GeOz 0 ~ =1.49 u w > ~1.4
Page 23 and 24: tension it collapses to eliminate t
Page 25 and 26: MAXIMUM TENSILE STRENGTH (GN/m2 = l
Page 27 and 28: mately a 0.5 electron-volt increase
Page 29 and 30: I in one portion of the recombinati
Page 31 and 32: crated electron-hole paira that are
Page 33 and 34: sink fiber. Likewise angular misali
Page 35 and 36: fused together. The ends are bent a
Page 37 and 38: A laboratory beam splitter set-up u
Page 39 and 40: aged multichannel unit. High-streng
Page 41 and 42: CHAPTER 4 LIGHTWAVES IN FIBEROPTIC
Page 43 and 44: tor addition indefinitely, as indic
Page 45 and 46: modes. Since at point (a) the two e
Page 47 and 48: as a number of wavelengths. An expr
Page 49 and 50: 100 meters of fiber, length changes
Page 51: the aingle-sideband phaae noiae int
Page 55 and 56: ient configuration in the lower rig
Page 57 and 58: sors or a pressure gradient sensor.
Page 59 and 60: nickel H. = 3.6 oersteds and for a
Page 61 and 62: optical source of constant intensit
Page 63 and 64: in a straight section of the fiber
Page 65 and 66: A simplified design of the cladding
Page 67 and 68: Referring once more to Fig. 5.35, i
Page 69 and 70: The path length AL traveled by ligh
Page 71 and 72: 5.4.9 Fiberoptic Rotation-Rate Sens
Page 73 and 74: a modulation scheme, the output of
Page 75 and 76: Fig. 5.60 lists several sources of
Page 77 and 78: CHAPTER 6 FIBEROPTIC SENSOR ARRAYS
Page 79 and 80: method of sensor energization will
Page 81 and 82: 1 The preceding discussion applied
Page 83 and 84: SIMPLEX FIBEROPTIC TRANSMITTER FIBE
Page 85 and 86: MAXIMUM ALLOWABLE RISETIME = 0.7/RN
Page 87 and 88: PROCESSING STATION TRANSMISSION wAV
Page 89 and 90: fiberoptic cable (optical cable), a
Page 91 and 92: angstrom. A unit of length equal to
Page 93 and 94: core. The central primary light-con
Page 95 and 96: diffraction. 1. The procesa by whic
Page 97 and 98: fiberoptic. Pertaining to optical f
Page 99 and 100: methods of coupling power outside t
Page 101 and 102: M Mach-Zehnder interferometer. An i
Page 103 and 104:
N nanometer. One thousandth of a mi
Page 105 and 106:
velocity is also the velocity at wh
Page 107 and 108:
where nl and n2 are the reciprocals
Page 109 and 110:
slab dielectric optical waveguide.
Page 111 and 112:
v vacancy defect. In the somewhat o
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