Presentation on theme: "1 Temperature measurements. 2 Noise In a BLIP-limited detector: i n(bg) = NEP = NA√AB/D BLIP = NA√[2 e r( ) AB] but signal is i = p = "— Presentation transcript:
2 Noise In a BLIP-limited detector: i n(bg) = NEP = NA√AB/D BLIP = NA√[2 e r( ) AB] but signal is i = p = NA 2 A r( ) T/T then S/N = T/T) NA √[ Ar( ) /2eB] and the NEDT - noise equivalent differential temperature - T @ S/N=1: NEDT = (T/ NA) √[2eB/ r( ) A] = 2kT 2 D BLIP (1/ NA)√(B/A)
3 Theoretical NEDT =0.5, NA=0.5 A=0.01 cm 2 D BLIP =6. 10 10 (W -1 cm√Hz)
4 NED in real detectors If detector is real (not BLIP-limited): i n(bg) = NEP = NA√AB/D**, signal is the same S/N = i/i n(bg) = NA √(A/B) r( ) T/T) D** and NEDT = T [ NA r( ) D**] -1 √(B/A) = 2kT 2 (D 2 BLIP /D**) (1/ NA)√(B/A)
5 Thermovisions Spectral range of operation: MIR, =3-5 m or FIR, =8-14 mGENERATIONS: 0- 0-th: pioneer’s work: Spectracon (oil -film camera), 1950 1st: LN-cooled InSb scan camera (AGA, Huges, etc.) 1970 - - general purpose 2nd: LN-cooled CMT and LTT FPA, 1980 - military 3rd: TEC-cooled Bolometer, 1985 - portable 4th: uncooled Pt-Si and VO x bolometer 1990 - camcorder
6 Optical preamplification Optical power gain is G. At output, added to amplified signal GP s a dc power due to amplified spontaneous emission is found: ASE out = n sp (G-1)h 0
7 OA noise equivalent circuit Noise input: ASE shot-noise plus excess noise (factor F) P u = GP s + GASE i 2 Pu = 2F G 2 h P s B + 2 h G 2 ASE i B PuPu
8 AO performance Performance is typical for EDFAs, 1480-1540 nm best range of operation
9 Requirements for OA preamplification SIGNAL LEVELS: ASE i limits minimum signal amplitudes P i =1÷10 W (or -30÷-20 dBm). Onset of saturationis at about 1-10 mW 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 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 =10 5..10 6 pixels
10 Il Fotomescolamento (rivisitazione di una vecchia tecnica !) Potenza ottica ff2f2 ff1f1 LASER 2 LASER 1 E 1 =E 01 cos(2 f 1 t+ 1 ) E 2 =E 02 cos(2 f 2 t+ 1 ) Fotodiodo a larga banda accoppiatore fibra o guida ottica I(t) E 1 +E 2 2 = E 1 2 +E 2 2 +2E 1 E 2 cos[2 (f 1 -f 2 )t + 1 - 2 ] Potenza elettrica f f 1 - f 2 out controll polarizz
11 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
12 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, C lim =0.03, and the cor- responding spatial frequency is calledlimit spatial frequency (or, limiting resolution).
13 Optical rule circuits x 0 I 0 I p 2p sin cos By counting the sin and cos crossings of mean level I 0, (4 per period) displacement is measured with p/4 resolution (and sign) RIFARE
15 Classification in visible ( IMAGE INTENSIFIERS or IIT) or infrared, UV ( IMAGE CONVERTERS or ICT) and X-rays ELECTROSTATIC or MAGNETIC focalization electron ENERGY or electron NUMBER amplification special functions can be included: gate, zoom, sweep 1st, 2nd, 3rd generation types hand-held night visors and LLLTV bulky, for astronomy with MCPs fast (ns) shutters streak cameras for radiology near-infrared viewers
17 ICT parameteres Spectral response : all transmission PhC available, with preference of S-20 and S-1 Display characteristics : phosphor P-20 is preferred, has medium persistence (0.35 ms), good yield( =80 ph/keV) Radiant gain G=E u /E i : reference source is the 2850 K lamp (as repre- sentative of artificial illuminance and of residual illuminance of natural scenes at dark Response dynamic range : ratio of max. to min. reproduced illuminan- ce, is given by E sat /G ICT EBI, where E sat =screen saturation level, G ICT =gain and EBI=equivalent background illuminance (typ. 10 -7 to 10 -4 lux) Linearity : E u =KE i can reach a conformity better than 1% for an indivi- dual pixel, but from pixel to pixel K may vary over ±10%, because of PhC and PS disuniformity Spatial resolution : is described by the MTF; total limiting resolution is about 50 cycles/mm.
18 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/