Transcript No Slide Title

Temperature measurements
ther mal
baffle
fi lter
objective
T
fov

targ et
TD
detector
radiant power
blackbody
T =310
T =300 K
gray
filter to TD
3
8
non-gray
deviation
20
(m)
1
Noise
In a BLIP-limited detector:
in(bg) = s NEP = sNA√AB/DBLIP
= NA√[2pes r()DAB]
but signal is
then
i = sDp = s pNA2 A Dkr() DT/T
S/N = k(DT/T) NA √[psAr()D/2eB]
and the NEDT - noise equivalent differential temperature DT @ S/N=1:
NEDT = (T/kNA) √[2eB/spr()DA]
= 2kT2 DBLIP (1/hNA)√(B/A)
2
Theoretical NEDT
-1
10
6 107
8
6 10 8
MINIMUM DETECTABLE TEMPERATURE
NEDT (°C)
5
3
2
-2
10
9
8
6 10
5
3
h=0.5, NA=0.5
A=0.01 cm2
DBLIP =6.1010
2
-3
10
8
5
3
10
D** = 6 10
2
-4
10
8
DB
5
LIP
(W-1cm√Hz)
it
lim
3
2
-5
10
1
10
100
1k
10k
BANDWIDTH B - ( 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) = pNA √(A/B) k r() D(DT/T) D**
and
NEDT = T [pNA k r() D D**]-1√(B/A)
= 2kT2 (D2BLIP/D**) (1/hNA)√(B/A)
4
Thermovisions
Spectral range of operation: MIR, =3-5m or FIR, =8-14m
GENERATIONS:
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 VOx bolometer 1990 - camcorder
5
Optical preamplification
Ps
I ph
G
G Ps
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)hnDn0
6
OA noise equivalent circuit
Ps
I ph
G
Pu
ASE i +
+ 2 h n ASE i B
2(F-1) hn Ps B
Noise input: ASE shot-noise plus
excess noise (factor F)
Pu = GPs + GASEi
s2Pu = 2F G2 hnPs B + 2 hnG2ASEi B
7
AO performance
30
9
G
20
6
G(dB)
F(dB)
10
3
F
0
best range of operation
-40
-30
-20
-10
0
+10
0
P i (dBm)
Performance is typical for EDFAs, 1480-1540 nm
8
Requirements for OA preamplification
• SIGNAL LEVELS:
ASEi limits minimum signal amplitudes
Pi=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 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
E1=E01cos(2pf1t+1)
fibra o guida
ottica
controll
polarizz
f1
f
I(t)E1+E22 = E12
+E22+2E1E2cos[2p(f1-f2)t +1-2]
Fotodiodo
a
larga banda
LASER 2
Potenza
ottica
accoppiatore
f2
f
out
Potenza
elettrica
E2=E02cos(2pf2t+1)
f1- f2
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
laser a 2 o piu' modi,
meglio se mode-locked
con spaziatura Dn
uscita elettrica
a f requenza Dn
(typ. 60 GHz)
f ibra o guida
f otodiodo
ultrarapido
infine, e’ opportuno incidere sul fotodiodo con la potenza ottica più
alta possibile, inserendo un amplificatore ottico al posto della guida
11
Limiting resolution
1
MTF
0.5
limiting
resolution
0.03
0
0
k - spatial frequency
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
cos  signal
sin  signal
sin 
cos 
I
I0
0
p
2p
x
By counting the sin 
and cos crossings of
mean level I0, (4 per
period) displacement is
measured with p/4
resolution (and sign)
13
Infrared MSA
k
N rows
M columns
+V bb
j
k-column
j-row
v ideo
out
Y
metal
Y
Y'
VO X
bolometer
X
X
B
E
Y
readout
metal
14
Classification
near-infrared
viewers
1st, 2nd, 3rd
generation types
for
radiology
 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
hand-held
night visors
and LLLTV
bulky, for
astronomy
with
MCPs
fast (ns)
shutters
streak
cameras
15
Magnetic focussing ICT
FOCU SING COIL
PHOTOCATHODE
EL EC TR ON TR AJ ECTOR IES
PHOSPHOR
GLASS
WIND OW
FACEPL ATE
AXIAL FIELD RINGS
GLASS
WIND OW
FACEPL ATE
+V
0V
bb
(ty p. 1 5 k V)
16
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(k=80 ph /keV)
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 PhC 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