# 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 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:

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 =   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

14 Infrared MSA

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

16 Magnetic focussing ICT

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/

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