Temperature measurements 1
Noise In a BLIP-limited detector: in(bg) = s NEP = s NA√AB/DBLIP = NA√[2 es r(l)Dl. AB] but signal is then i = s Dp = s NA 2 A Dl k r(l) DT/T S/N = k (DT/T) NA √[ s Ar(l)Dl/2 e. B] and the NEDT - noise equivalent differential temperature DT @ S/N=1: NEDT = (T/k. NA) √[2 e. B/s r(l)Dl. A] = 2 k. T 2 DBLIP (1/h. NA)√(B/A) 2
Theoretical NEDT h=0. 5, NA=0. 5 A=0. 01 cm 2 DBLIP =6. 1010 (W-1 cm√Hz) 3
NED in real detectors If detector is real (not BLIP-limited): in(bg) = s NEP = s NA√AB/D**, signal is the same S/N = Di/in(bg) = NA √(A/B) k r(l) Dl (DT/T) D** and NEDT = T [ NA k r(l) Dl D**]-1√(B/A) = 2 k. T 2 (D 2 BLIP/D**) (1/h. NA)√(B/A) 4
Thermovisions Spectral range of operation: MIR, l=3 -5 mm or FIR, l=8 -14 mm GENERATIONS: 0 -th: pioneer’s work: Spectracon (oil -film camera), 1950 01 st: LN-cooled In. Sb scan camera (AGA, Huges, etc. ) 1970 - general purpose 2 nd: LN-cooled CMT and LTT FPA, 1980 - military 3 rd: TEC-cooled Bolometer, 1985 - portable 4 th: uncooled Pt-Si and VOx bolometer 1990 - camcorder 5
Optical preamplification Optical power gain is G. At output, added to amplified signal GPs a dc power due to amplified spontaneous emission is found: ASEout = nsp(G-1)hn. Dn 0 6
OA noise equivalent circuit Pu Noise input: ASE shot-noise plus excess noise (factor F) Pu = GPs + GASEi s 2 Pu = 2 F G 2 hn Ps B + 2 hn G 2 ASEi B 7
AO performance best range of operation Performance is typical for EDFAs, 1480 -1540 nm 8
Requirements for OA preamplification • SIGNAL LEVELS: ASEi limits minimum signal amplitudes Pi=1÷ 10 m. W (or -30÷-20 d. Bm). Onset of saturationis at about 1 -10 m. W • WAVELENGTHS: a few available, in correspondence to laser lines (e. g. , 1500, 1300, 1060, 850 nm) • SIGNAL MODE: a single spatial mode is required, or the low coupling h to DFA fiber would frustrate any amplification • Large PIXEL #: extension theoretically feasible but not yet demonstrated: problem is that, in AO with N modes, ASE increases N times becoming very high for images with N =105. . 106 pixels 9
Il Fotomescolamento (rivisitazione di una vecchia tecnica !) LASER 1 Potenza ottica E 1=E 01 cos(2 f 1 t+ 1) fibra o guida ottica controll polarizz f 1 f I(t) E 1+E 2 2 = E 12 +E 22+2 E 1 E 2 cos[2 (f 1 -f 2)t + 1 - 2] Fotodiodo a larga banda LASER 2 Potenza ottica accoppiatore f 2 f out Potenza elettrica E 2=E 02 cos(2 f 2 t+ 1) f 1 - f 2 f 10
Il Fotomescolamento, II per rilassare il requisito di stabilità di frequenza dei due laser, é meglio usare un laser a 2 modi oppure in regime di mode-locking infine, e’ opportuno incidere sul fotodiodo con la potenza ottica più alta possibile, inserendo un amplificatore ottico al posto della guida 11
Limiting resolution A generic MTF diagram always starts from MTF=1 at k» 0 (low spatial frequency), and gradually decreases to zero at increasing spatial frequencies. The cutoff frequency is defined as that of eye perceptivity to contrast, Clim=0. 03, and the corresponding spatial frequency is calledlimit spatial frequency (or, limiting resolution). 12
Optical rule circuits RIFARE sin F cos F I I 0 0 p 2 p x By counting the sin F and cos F crossings of mean level I 0, (4 period) displacement is measured with p/4 resolution (and sign) 13
Infrared MSA 14
Classification near-infrared viewers 1 st, 2 nd, 3 rd generation types for radiology à in visible (IMAGE INTENSIFIERS or IIT) or infrared, UV (IMAGE CONVERTERS or ICT) and X-rays à ELECTROSTATIC or MAGNETIC focalization à electron ENERGY or electron NUMBER amplification à special functions can be included: gate, zoom, sweep hand-held night visors and LLLTV bulky, for astronomy with MCPs fast (ns) shutters streak cameras 15
Magnetic focussing ICT 16
ICT parameteres Spectral response : all transmission Ph. C available, with preference of S-20 and S-1 Display characteristics : phosphor P-20 is preferred, has medium persistence (0. 35 ms), good yield(k=80 ph /ke. V) Radiant gain G=Eu/Ei: reference source is the 2850 K lamp (as representative of artificial illuminance and of residual illuminance of natural scenes at dark Response dynamic range : ratio of max. to min. reproduced illuminance, is given by Esat/GICTEBI, where Esat=screen saturation level, GICT=gain and EBI=equivalent background illuminance (typ. 10 -7 to 10 -4 lux) Linearity : Eu=KEi can reach a conformity better than 1% for an individual pixel, but from pixel to pixel K may vary over ± 10%, because of Ph. C and PS disuniformity Spatial resolution : is described by the MTF; total limiting resolution is about 50 cycles/mm. 17
Photodetectors Devices, Circuits and Applications ' by: S. Donati, Prentice Hall, USA, 2000 XVIII+425 pages, bound, price: 75$, ISBN 013 020337 -8 see web: http: //www. phptr. com/booksrch/index. html or buy online from: http: //vig. pearsoned. com/store/product/ 18
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