Infrared radiation modulating device

Details Infrared radiation modulating device Infrared radiation modulating device

1998-03-19

2001-01-23

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S343000, C250S332000

Reexamination Certificate

active

06177674

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non mechanical modulating device for use in infrared detection and has particular, although not exclusive, use in thermal imaging cameras.
2. Discussion of Prior Art
In such cameras it is desirable to modulate the intensity, that is introduce a time variation of the intensity, of the incident beam of infrared radiation from the image scene, in particular in the 8-14 &mgr;m wavelength region. Where the camera uses an array of pyroelectric detectors, the modulation is an essential part of the camera operation. For thermal imaging cameras based on other types of detectors, for example resistive bolometric detectors, capacitive bolometric detectors and ferroelectric detectors, the modulation may not be essential but is a preferred feature. The modulating device may have additional applications in other systems incorporating pyroelectric or other infrared detectors or detector arrays, for example gas sensing or measuring systems where the modulation of infrared radiation from a thermal source is required.
The present technique used to achieve this modulation in both thermal imaging and gas sensing systems makes use of a mechanical chopping device in the form of a rotating disc. Such devices consume substantial power which reduces the battery life when used on portable equipment. In addition, any non-uniformity in the emissivity of the disc can give rise to non-uniformity in the image and which leads to reduced camera performance.
There has been a long felt need for a more convenient, non-mechanical means of modulating the intensity of infrared radiation in thermal imaging cameras. However, the present non-mechanical methods of modulating infrared light in the spectral region of interest have proved to be unsatisfactory. Those based on the electro-optic or acoustic optic effects are bulky, costly and typically only function for one wavelength and one polarization. This is not effective for the wide band of wavelengths and random polarization encountered in a typical infrared image. These techniques normally only function satisfactorily with well collimated beams, in contrast to the low F-number optics used in thermal imaging systems. Attempts to use liquid crystal devices for modulation purposes have also met with very limited success.
Previously, weak modulation effects have been observed in the infrared region by using the injection of free carriers into semiconductor materials such as germanium (Ge) or silicon (Si) [Yamada,
Elec. Lett.
19 No. 22 (922-944), McQuistan,
J. App. Phys.
35 No. 4 (1243-1248)]. However, modulating devices for use in thermal imaging cameras have never reached practical operation in the 8-14 &mgr;m spectral region. Similarly, devices relying on etalon effects in Si have been described for use in the far infrared but are not suitable for use with thermal imaging cameras as they operate over a narrow band of wavelengths outside the region of interest [H. Alius and G. Dodel,
Infrared Phys. and Technol.,
Vol. 35, No. 1, pp 73-78 (1994), H. Alius and G. Dodel,
Appl. Phys. Lett.,
57 16 (1990), H. Alius and G. Dodel,
Infrared Physics,
32 pp 1-11 (1981)].
Stronger modulation effects have been observed in the infrared region by using the injection of free carriers into germanium and utilising interband transitions between the split valence bands in germanium [Umeno et al, “High-Efficiency Ge Modulator for Infrared (Laser) Beams”
Japanese Journal of Applied Physics, Supplements,
vol. 40, 1971, Tokyo]. However, the device proposed provides fast modulation (typically 10-20 kHz) unsuitable for use with infrared imaging systems. For example, pyroelectric detector arrays typically operate with readout frequencies of between 50-150 Hz (i.e. around an order of magnitude slower). Furthermore, the device cannot provide the uniformity of modulation depth required for a modulator device for an imaging system.
SUMMARY OF THE INVENTION
The present invention relates to a non-mechanical modulating device for use in infrared detection and operates successfully in the 8-14 &mgr;m spectral region. It has particular use in thermal imaging cameras where it offers a convenient alternative to the know mechanical infrared modulators used in existing cameras, in particular those comprising an array of pyroelectric detectors. A significant advantage of the device is its insensitivity to shock and vibration compared to the mechanical choppers used in existing systems. The present device is also has the ability to operate reliably after long storage. The device may also be used in gas sensing or measuring systems where it is preferable, and in some cases essential, to modulate an infrared radiation source prior to detection.
In this specification, the use of the word “detector” should be taken to relate to the detector or array of detectors of which the infrared detection system is comprised.
According to the present invention, a device for modulating radiation input to an infrared detector comprises;
an element of germanium, having a region doped with acceptor atoms and a region doped with donor atoms wherein the element of germanium has facing input and output surfaces and a valence band comprising light hole and heavy hole sub-bands and
means for electrically injecting carriers into the germanium so as to vary the carrier concentration across the germanium, whereby the variation in carrier concentration gives rise to modulation of the intensity of infrared radiation transmitted through said germanium element, wherein said modulation is substantially due to carrier transitions between the light hole and heavy hole sub-bands,
characterised by a central region (
23
) between the region (
20
) doped with acceptor atoms and the region (
21
) doped with donor atoms, wherein the central region (
23
) has a carrier concentration sufficient to substantially prevent absorption of infrared radiation (
8
) in the absence of electrically injected carriers.
The infrared detector may be part of a thermal imaging camera or a gas sensing system.
The region doped with acceptor atoms may be any one of a p-type region or a p
+
-type region, and the region doped with donor atoms may be any one of an n-type region or an n
+
-type region. The central region may be an intrinsic region or a weakly doped n-type region. For example, the germanium may have a structure of any one of the form p-i-n, p-&ugr;-n, p
+
-i-n, p
+
-&ugr;-n, p
+
-i-n
+
, p
+
-&ugr;-n
+
.
The germanium may be divided into areas which can be individually energised and which may be spatially separated from each other. Preferably, the input surface of the element of germanium has an effective diameter of at least 0.5 mm and, more preferably, an effective diameter of at least 1.0 mm. The germanium element may be a single crystal and, preferably, may have a long minority carrier lifetime.
The means for electrically injecting carriers may be spatially non-uniform so as to enable the spatial modulation of the incident infrared radiation.
The surface of the germanium element may be treated so as to reduce the surface recombination velocity. The surface treatment may include any one of chemical etching, electrochemical etching, reactive ion etching, plasma etching or ion beam milling. Furthermore, the surface of the germanium element may be polished.
The germanium element may have an anti reflection coating applied to one or more of its surfaces, for example this may be a multiple layer coating.
Alternatively, a layer of semiconductor material having a wider bandgap than germanium, grown epitaxially on one or more surfaces of the germanium element. For example, the semiconductor material may be any one of gallium phosphide, gallium arsenide, gallium-arsenide-phosphide alloys or silicon-germanium-alloys. An anti reflection coating, for example a multiple layer coating, may then be applied to one or more surfaces of the semiconductor material.


REFERENCES:
patent: 2692950 (1954-10-01),

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