Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-07-10
2003-03-18
Allen, Stephone B (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C257S014000
Reexamination Certificate
active
06534758
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a self-compensation device for subtractive detectors.
The aim of the invention is to improve thermal imaging systems using subtractive architecture to eliminate the continuous component of the integrated current. The principle of a subtractive detector is described in the French patent No 2 756 666 and is recalled in
FIGS. 1
a
and
1
b.
2. Description of the Prior Art
As can be seen in a simplified view in
FIG. 1
a,
a detector according to the French patent No 2 756 666 comprises the following elements stacked on a substrate:
a contact layer C
2
,
an active photoconductive layer D
2
,
a common contact layer Cc
an active photoconductive layer D
1
,
and a contact layer C
1
.
The active photoconductive layers D
1
, D
2
may be layers made of a photoconductive semiconductor material such as silicon. They may also be made in the form of stacks of layers constituting quantum well detectors. The two active layers D
1
, D
2
are photoconductive in the same range of wavelengths &lgr;. One of the active layers is designed to be highly absorbent in the range of wavelengths &lgr; while the other layer is designed to absorb very little or be practically non-absorbent. This can be designed by having different thicknesses for the active layers, or by a greater doping of the quantum well layers of the more absorbent active layer. It is possible for the contact layers C
1
, C
2
, Cc not to cover the entire surface of the photoconductive layers.
Since the detector is illuminated by the radiation to be detected as can be seen in
FIG. 1
a,
the active layer D
2
first receives the radiation RZ.
Should the layer D
1
be more absorbent than the layer D
2
, a diffraction grating is preferably provided. The diffraction grating is associated with the face of the layer D
1
bearing the contact layer C
1
. This grating receives the light that had not been absorbed during the first crossing of the layer D
1
and diffracts it towards the layer D
1
. The diffracted light will be absorbed or almost absorbed by the layer D
1
.
The contact layers C
1
and C
2
are used to apply control potentials. The contact layer Cc is common with the two detector elements comprising the active photoconductive layers.
It is set at a reference potential and enables the detection of the photo currents generated by the detector D
1
, D
2
.
The substrate is transparent to the range of wavelengths to be measured. The detector therefore receives the radiation RZ through the substrate.
When a radiation RZ is received by the detector, to detect the wavelength (or range of wavelengths) &lgr;, the following are applied:
a potential V
1
to the contact layer C
1
,
a potential V
2
to the contact layer C
2
:
a floating potential Vc (or ground), between V
1
and V
2
, to the common contact layer Cc.
In the structure D
1
, the following current flows:
I
1
=I
1
d+I
1
opt
And in the structure D
2
, the following current flows
I
2
=I
2
d+I
2
opt
The currents I
1
d
and I
2
d
are the dark currents in D
1
and D
2
. The currents I
1
d
and I
2
d
may also represent the sum of a dark current and a current corresponding to the surroundings. The currents I
1
opt and I
2
opt are the currents due to the wavelength &lgr; to be detected in D
1
and D
2
.
In
FIG. 1
b,
the current i collected by the read circuit has the following value:
I=I
1
−I
2
By adjusting the voltage V
1
or V
2
, it is possible to adjust I
1
d=I
2
d. The value of the detected current is therefore:
I=I
1
opt
−I
2
opt
By planning the structure so that one of the two active layers absorbs very little energy from the wave &lgr;, the current I is the one generated by the active layer that has the strongest response.
The total current of a thermal imaging device is the sum of (a) an offset current, constituted by a dark current thermally activated according to a law of the Arrhenius type, I=I
0
exp(−hc/&lgr;)kT), and (b) the current of the optical signal generated by the variations in emissivity and temperature of the scene. The architecture of a subtractive focal plane is used to subtract the continuous component before integration and therefore make full use of the frame time available to integrate the signal without saturating the individual storage capacity of each pixel. This improves the signal-to-noise ratio of the detectors. The two stages QWIP
1
and QWIP
2
are identical structures. The stage QWIP
1
, biased at −V
s
, is the detection stage and the stage QWIP
2
, reverse biased at +V
ref
, is the reference mirror stage, enabling the total or partial subtraction of the current. The intermediate contact is connected to the corresponding storage capacitor of the multiplexer and thus enables the collection of the resulting current, namely the difference in the currents flowing through the two stages.
A thermal imaging device comprises a cooling unit (Stirling machine, Joule-Thomson pressure-reducing device, liquid nitrogen bath etc) and a regulation system capable of stabilizing the temperature of the focal plane T
0
to within ±&Dgr;T. The slow fluctuation of the temperature, which has a variation in amplitude of 2&Dgr;T, will generate a variation of the thermal current of each of the stages.
The invention can be used to resolve this problem.
SUMMARY OF THE INVENTION
The invention therefore relates to a device for the detection of electromagnetic waves comprising at least two photoconductor-based electromagnetic wave detectors, each comprising:
at least two separate, flat-shaped, stacked photoconductor-based active detector elements, comprising a common reading contact, the unit being held between two control contact layers;
means to apply control voltages to each control contact layer, a voltage applied to the common reading contact layer having a value ranging between the voltages applied to the control contact layers;
means connected to the common contact to detect the difference between the photoconduction currents of the detector elements;
wherein at least one detector is provided, on one of its plane faces, with a diffraction grating and wherein a subtraction circuit is used to subtract the read signal of a detector not provided with a diffraction grating from the read signal of a detector provided with a diffraction grating.
REFERENCES:
patent: 5086327 (1992-02-01), Rosencher et al.
patent: 5187715 (1993-02-01), Weisbuch et al.
patent: 5228777 (1993-07-01), Rosencher et al.
patent: 5326984 (1994-07-01), Rosencher et al.
patent: 5506418 (1996-04-01), Bois et al.
patent: 5677544 (1997-10-01), Duboz et al.
patent: 5712499 (1998-01-01), Duboz et al.
patent: 5719670 (1998-02-01), Duboz et al.
patent: 5726500 (1998-03-01), Duboz et al.
patent: 5812190 (1998-09-01), Audier et al.
patent: 5869844 (1999-02-01), Rosencher et al.
patent: 5998777 (1999-12-01), Aurdier et al.
patent: 6091126 (2000-07-01), Costard et al.
patent: 6157020 (2000-12-01), Krapf et al.
patent: 6184538 (2001-02-01), Bandara et al.
patent: 6414298 (2002-07-01), Nakajima
patent: 0 664 554 (1995-07-01), None
patent: 0 849 798 (1998-06-01), None
patent: 26 16 992 (1988-12-01), None
patent: 27 61 813 (1998-10-01), None
Audier Marcel-Francis
Bois Philippe
Costard Eric
Allen Stephone B
Hill Bradford
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Thales
LandOfFree
Electromagnetic wave detector using quantum wells and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electromagnetic wave detector using quantum wells and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electromagnetic wave detector using quantum wells and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3053700