FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

More documents

Recommendations

Info

$Course Material\Efficiently Coding Communication Protocols in C++ ...$

shown in Fig. 2.39. This state is known as the population inversion state. It corresponds to the condition in which holes exist in the valence band and electrons exist in the conduction band simultaneously. This leads to the production of photons. The energy gap is indicated by E in Fig. 2.39. When electrons from the conduction ban~ lose some of their energy and drop down Fig. 2.39 T CONDUCTION BAND + BAND-GAP,EG Q—Fc hv Fc= CONDUCTION SANO ENERGY LEVEL SPONTANEOUS: h.=EG FERMI STIMULATED: tWCEFc ‘EFV PHOTONS AMPLIFY THEMSELVES Electrons (cross-hatch) from the valence band are stimulated to the energy level of the conduction band in a population inversion situation. Their return to the valence band causes the emission of photons. into the valence band they recombine with holes and photons are produced. This process is known as recombination. In the desired case, the energy is given up entirely in the form of photons. If this process occurs spontaneously, the energy of the emitted photon iS approximately equal to the band gap energy, Eg. The photons produced travel in random directions. On the other hand, if sufficient density of photons exist in the recombination region both spontaneous emission (or recombination) and stimulated recombination occur. The stimulated photons that are produced travel in the same direction as the primary photons. In the latter case the photon energy is less than the difference between the Fermi energy level in the conduction band, EFc, and the Fermi level in the valence band, EFV. These conditions, spontaneous and stimulated emission, are necessary for the proper operation of light emitting diodes (LED’S) and diode lasers, respectively. In order to understand how LED’s or diode lasers can be produced and operate in practice, consider the condition at a p-n junction as shown in Fig. 2.40. In this case, GaAs } & 1%z w P - CONDUCTION BAND m I 1 , I , 1 I N 4 P-N JUNCTION Fig. 2.40 The I 1:~~ energy levels in a p-n junction. is doped with an acceptor material on one side of the junction, resulting in a p-type semiconductor region, and with a donor material on the other side, resulting in an n-type semiconductor region. The spatial separation between the p- and n-type regions is known as the p-n junction. The electron energy levels in the conduction and valence bands are as shown in the upper part of Fig. 2.40. Notice that when a bias voltage is not applied, as on the left, a population inversion does not occur. Electrons from the valence band of the p-type region flow into the conduction band of the n- type region until the electron energy levels on each side of the p-n junction are equal, then essentially no more electrons flow across the junction. The energy level difference across the junction constitutes a barrier to further current flow. If a forward bias voltage is applied, as shown on the right of Fig. 2.40, electrons are forced or injected into the n-type region and holes are formed in the p-type region. When the energy level of a sufficient number of electrons are raised to the energy level of the conduction band, their electron energy level exceeds the barrier energy and electrons flow across the junction into the p-region. More detail is shown in Fig. 2.41. In the population L cc 1,1 4, hv rFv Fig. 2.41 I RECOMBINATION l-REGlON*HOMOJuNcTlON ! .1 ‘ I ‘ I ~ VG= EG/e FORWARD BIAS The various regions and energy levels at a forward-biased p-n junction of a semiconductor diode. inversion region, just on the left of the p-n junction, electrons can spontaneously recombine with holes producing photons. Since in this region there is a finite lifetime for the electrons depending upon the average time it takes for such recombination to occur (typically 3 to 5 ns), the inversion region is restricted in spatial extent as shown in Fig. 2.41. In this case the junction is known as a homojunction and under proper conditions may be used to fabricate a homojunction LED or diode laser. Current density is directly proportional to the thickness of the recombination layer. Therefore, in order to reduce the current it is important to reduce the thickness of the recombination layer. This can be accomplished in a gallium arsenide (GaAs) crystal by the use of layers alloyed with varying amounts of aluminum (Al). The substitution of aluminum (Al. ) for gallium (Ga) occurs with little or no distortion of the crystal lattice. The energy gap plotted against the fraction of aluminum that replaces an equal fraction of gallium ( Ga) , forming gallium aluminum arsenide (GaAIAs) is shown in Fig. 2.42. For up to 37% Al substituted for Ga, the energy gap increaaes from 1.43 electron-volts to 1.92 electron-volts. This is approxi- 2-18
mately a 0.5 electron-volt increase. For fractional parts of Al greater than 0.37, i.e., x > 0.37, mechanisms in addition to simple photon production occur during recombination with the result that not all of the energy goes into producing photons, part of it goes into thermal energy with the possibility of crystal damage and a reduced tendency for lasing. The wavelength can be obtained from the photon energy relation E t = hf and from the wavelength-frequency-velocity relation ~f= = c/n, from which the relation 1 = hc/nEt is obtained, where h is Planck’s constant, c is the velocity of light in a vacuum, n is the refractive index taken as unity, and E t is the energy lost by a particle. For a particle with a charge of one electron that loses energy equal to the gap energy, A = 1.24/Eg, where h is the wavelength in microns and Eg is the gap energy in electron-volts. Thus for GaAs, 1 = 0.90 micron and for 37% Al, 1 = 0.64 micron. Longer wavelength lasers (1.1 micron to 1.6 micron) can be produced by using the quarternary alloy iridium-gallium-arsenic-phosphorous (InGaAsP). 1 P, A —P —~ qi - N— I I I I 1 1 ++++++++++++++++++++++++++ I 1 Gal.xA~As:Ge ,~Ga l-y A’yAs ~ Gal_xAlxAs:Sn/Te I Ge ‘ S;;Te Fig. 2.43 The energy levels of a semiconductor forward-biased double heterostructure laser in a junction of lower concentration alumlnum surrounded by higher concentration aluminum. ‘“’~ “’:;’v:;:N:;GAp0.37COMPETING 9 /. ~ ,- PROCESSES OCCUR MAKING /- m2.O u ...” ONSET OF LASING LESS ..0” PROBABLE n- a > 0 AIXGal.XAs ● FOR X INCREASING FROM OT00.37 THE REFRACTIVE % 300”K z INDEX DECREASES BY 5% u.1 15 [1111111111 o 0.5 1.0 GaAs x AIAs Fig. 2.42 The band-gap energy level versus aluminum galium arsenide composition (AIXGA(l-X)AS). Another important effect is that as the fractional part of Al, x, increases from zero to 0.37 the refractive index decreases by 5%. Thus, as x increases, the energy gap increases and the refractive index decreases. The energy gap increases by almost 30% and the refractive index by about 5%. The energy band structure for a crystal in which a higher concentration of aluminum in two regions sandwich a third region of lower aluminum content between them is shown in Fig. 2.43. The corresponding crystal structure can be formed by a number of processes one of which is the liquid-phase epitaxial growth process. Epitaxial growth is the growth of a crystal from the surface. For the case of interest the following is a highly simplified description. The process begins with a crystal of gallium arsenide (GaAs), one surface of which is put in contact with a high temperature solution of gallium aluminum arsenide (GaAIAs). The crystal is maintained at a slightly lower temperature than the liquid and crystal growth occurs from the surface. Once the proper thickness of this particular composition has been achieved, the crystal is removed from the bath and put in contact with another liquid having the composition corresponding to that of the next layer. A crystal results with a p-type and an n- type layer, each of which have a higher aluminum content, larger energy gap, and lower refractive index, 2-19 and between which is a recombination layer with lower aluminum content, smaller energy gap, and higher refractive index. The amount of aluminum in the recombination layer determines the wavelength of the light emitted. In this manner the structure corresponding to the energy diagram shown in Fig. 2.43 can be formed. By this process the recombination layer can be made thin, often as small as a few tenths of a micron. The longer-wavelength quarternary InGaAsP alloys are produced by liquid-phase epitaxial growth on an iridium phosphorus (InP) substrate. The recombination layer has lower aluminum content and therefore, a smaller energy gap, while the layers on each side have greater aluminum content and a resulting larger energy gap. In this case, when an electrical bias is applied, electrons are introduced from the n-type layer into the recombination layer. Recombination occur overwhelmingly more often in the layer with the lowest energy gap. 2.3.2 Light Emitting Diodes (LEDs) and Diode Lasers The use of crystal structures to fabricate either an LED or a diode laser is shown in Fig. 2.44. Electrons are introduced into the bottom of the crystal and holes are introduced into the top. In the recombination layer, holes and electrons recombine to form photons that tend to move outward in all directions as shown on the left of Fig. 2.44. In this case, the device behaves as an LED. The light, emitted in all directions, results from spontaneous emission. In order to produce a laser it is necessary to confine and guide the emitted light. This increases the light intensity to the level where stimulated emission occurs. This is accomplished in the following way. The recombination layer has less aluminum therefore it has the lower energy gap and recombination occurs here. The use of some aluminum in the recombination layer allows the wavelength to be adjusted but in addition it reduces the probability of crystal damage. Furthermore, the layer with the smallest energy gap also has the highest refractive index. Thus, a higher refractive index layer is sandwiched between two layers of lower refractive index. This is exactly the situation that leads to lightwave trapping in optical fibers. Similarly for the structure shown on the right in Fig. 2.44, photons tend to be reflected from the lower refractive index surface back into the higher-refractiveindex recombination layer. Photons are retained in the
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: MAXIMUM TENSILE STRENGTH (GN/m2 = l
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 and 52: the aingle-sideband phaae noiae int
Page 53 and 54: sets up periodic spatial fluctuatio
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
show all

FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?