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From page 255... ... 5 Fundamental and Engineering Limits of Active Electro-Optical Sensing ILLUMINATION SOURCES There are a small number of what might be called truly "fundamental" limits to the performance of sources, and these involve the basic physics concept of energy conservation. While physics is involved in all other types of limits, the concepts here are more in the engineering category, involving, say, properties of the materials involved and thermal management of the devices. Read the entire page →
From page 256... ... 256 LASER RADAR cavity, the finite spectral response of mirrors, and nonlinear effects. The calculated pulsewidth for the mode-locked, eye-safe Cr:ZnSe laser at 2,500 nm is 7 fs, while experiments report much longer pulsewidths (60 fs) Read the entire page →
From page 257... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 257 Optical parametric oscillators OPOs) , in principle, have a fundamental limit for overall conversion efficiency from pump to signal plus idler of 100 percent. Read the entire page →
From page 258... ... 258 LASER RADAR wavelength of the incident photons nears the detector cutoff. The ideal is when the photons have just enough energy to create carriers by exciting them over the bandgap and there is no lost excess energy. Read the entire page →
From page 259... ... FUNDAME MENTAL AND ENGINEERING LIMITS OF ACTIVE ELEC E G A CTRO-OPTICA SENSING AL 259 Noi Sources4 ise Incident Flux Shot No F oise Usually photon are random in nature,5 described by P U ns m d Poisson statis stics, and have arrival varia e ations called sho noise. This noise in not related to any flaws in a de ot r y etector and is unavoidable. Read the entire page →
From page 260... ... 260 LASER RADAR Dark Current Noise In semiconductor detectors, additional noise signal that comes from thermally generated electrons called dark current. It appears even when there are no impinging photons. Read the entire page →
From page 261... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 261 FIGURE 5-2 SNR gain is limited after about a dozen reads in MCDS. SOURCE: Massimo Robberto. Read the entire page →
From page 262... ... 262 LASER RADAR Johnson noise are generally more important in thermal detectors than in photodetectors. 1/f noise is only important at lower frequencies. Read the entire page →
From page 263... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 263 FIGURE 5-4 SNCR model for missile detection that includes many factors in addition to the detector performance. Read the entire page →
From page 264... ... 264 LASER RADAR TABLE 5-1 Infrared Detector Parameter Limits with Comments on the Major Implications Parameter Limit State of the art Implications -3 a Dark current at 140 K, 10 electrons/s/pixel 0.01 e/sec/pixel at 90 K Improves SNR in low HgCdTe arrays, 2.5 µm background cutoff kTC noise 0 electrons 3 electrons/64 reads Reduces SNR Johnson noise 0 electrons Few electrons/s Important mostly for thermal detectors 1/f noise Generally a theoretical Few electrons/s Important for low mystery but well frequencies understood in particular casesb Quantum efficiency 100 percent 70-80 percent Improves SNR NOTE: See also Table 4-3, "Performance Parameters Comparison of Linear-, Geiger-, and Coherent-Mode Detectors" for more details on state-of-the-art APDs. Read the entire page →
From page 265... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 265 FIGURE 5-5 FPA variation within each chip and on the wafer. The pixel variations in dark count rate (DCR) Read the entire page →
From page 266... ... 266 LASER RADAR FIGURE 5-6 Pixel pitch for HgCdTe photodiodes (left) and amorphous silicon micro-bolometers (right) Read the entire page →
From page 267... ... FUNDAME MENTAL AND ENGINEERING LIMITS OF ACTIVE ELEC E G A CTRO-OPTICA SENSING AL 267 FIGURE 5-8 HgCdTe detector spectr range exten ral nded by remova of substrate. Read the entire page →
From page 268... ... 268 LASER RADAR "Type II InAs/GaInSb superlattice structure is a relatively new alternative IR material system and has application potential for LWIR/VLWIR spectral ranges with performance comparable to HgCdTe, but it does not compete at the shorter wavelengths" 25. Sb-based superlattices are fabricated using standard IIIV technology that may be more competitive due to lower costs of series production. Read the entire page →
From page 269... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 269 FIGURE 5-9 Components of a signal processing algorithm for an FPA. SOURCE: Figure 4-10 from National Research Council, 2010, Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays, The National Academies Press, Washington, D.C. Read the entire page →
From page 270... ... 270 LASER RADAR TABLE 5-2 Limitations on Various Signal Processing Parameters Parameter Limit State-of-the-Art Implications 4 Digital process energy kTLn(2) = 0.0178 eV 1 × 10 eV Battery life, system size, and Moore's Law seems to prevent improvement beyond 100 ×. Read the entire page →
From page 271... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 271 of the circuitry goes up. This aspect of adding functionality on the ROIC is common to passive detectors as well and has been extensively addressed in a recent NRC report.38 In summary, there are a few unique efforts to develop more efficient image processing algorithms and specialized IR image processing hardware. Read the entire page →
From page 272... ... 272 LASER RADAR FIGURE 5-10 Atmospheric absorption. Figure created by R Read the entire page →
From page 273... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 273 FIGURE 5-11 Thin turbulence is shown on the left, and distributed turbulence -- sometimes called deep turbulence -- is shown on the right. where D is the diameter of the illuminator aperture. Read the entire page →
From page 274... ... 274 LASER RADAR TABLE 5-3 Atmospheric Loss as a Function of Absorption Lengths Number of Absorption Lengths Transmission τ Loss (db) 1 0.367879 4.3 2 0.135335 8.7 3 0.049787 13 4 0.018316 17.4 5 0.006738 21.7 6 0.002479 26.1 7 0.000912 30.4 8 0.000335 34.7 Another limit is the turbulence on receive, which will limit angular resolution. Read the entire page →
From page 275... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 275 The degree to which seawater is transparent is a function of the combined effects of scattering and absorption of light by the water. Both absorption and scattering cause the amount of light to decrease exponentially with distance z: I = I o exp(αz) Read the entire page →
From page 276... ... 276 LASER RADAR FIGURE 5-12 Spectral absorption coefficient α abs and total scattering coefficient α scat for pure water. The two losses at wavelength 488 nm sum to a total minimum attenuation. Read the entire page →
From page 277... ... FUNDAMENTAL AND ENGINEERING LIMITS OF ACTIVE ELECTRO-OPTICAL SENSING 277 sensing has consequently risen in other nations, putting U.S. leadership at risk, to the degree that in some instances the United States no longer leads. Read the entire page →
From page 279... ... Appendixes Read the entire page →

From page 255...

... 5 Fundamental and Engineering Limits of Active Electro-Optical Sensing ILLUMINATION SOURCES There are a small number of what might be called truly "fundamental" limits to the performance of sources, and these involve the basic physics concept of energy conservation. While physics is involved in all other types of limits, the concepts here are more in the engineering category, involving, say, properties of the materials involved and thermal management of the devices.