WO2006117778A2

WO2006117778A2 - Compact apparatus and method for high angular resolution imaging

Info

Publication number: WO2006117778A2
Application number: PCT/IL2006/000515
Authority: WO
Inventors: Vladimir Shamis
Original assignee: Israel Aerospace Industries Ltd.
Priority date: 2005-05-05
Filing date: 2006-04-27
Publication date: 2006-11-09
Also published as: WO2006117778A3

Abstract

An apparatus (10) includes an input fiber optic taper (12) having a narrow aperture (13) and a wide aperture (14) and having a bundle of component tapered optical fibers. An objective (11) is disposed for focusing incident electromagnetic radiation on to the narrow aperture of the input fiber optic taper, and an electro-optical imaging device (15) having an input fiber optic faceplate (16) is attached to the wide aperture of the input fiber optic taper for imaging the electromagnetic radiation. The electro-optical imaging device (15) has an effective pixel size that is larger than a narrow aperture of each component optical fiber in the input fiber optic taper.

Description

Compact apparatus and method for high angular resolution imaging

FIELD OF THE INVENTION

This invention relates generally to electro-optical imaging systems and in particular to laser gated night vision systems.

BACKGROUND OF THE INVENTION Electro-optical imaging systems such as intensified charged coupled devices

(ICCD), daytime imaging cameras on the basis of CCD or CMOS technology or, thermal imaging cameras require large focal length lenses to image distant objects with a high angular (spatial) resolution. Current electro-optical systems of the like are bulky, heavy and occupy a large volume and are not suitable for carrying platforms which have comparatively light size and slight dimensions. Additionally, for environments which have low background illumination, the addition of an external light source such as an illuminator or laser is needed in addition to the camera to observe the image further militating against the compactness of the imaging system.

It is well known to the imaging community that fiber optic tapers constructed from a bundle of tapered optic fibers can be used with ordinary lenses to focus light onto an imaging sensor. These tapers are optimized for different chip sizes. Additionally, fiber optic tapers are used to couple the intensifier from an ICCD to the imaging CCD imager. Examples of such fiber optic tapers are to be found in US Patent No. 5,448,671 (Wimmer et al.) entitled "Replacement assembly for an image intensifier device including fiber optic inverter expander". WO 97/38329 to Xitec, Inc. entitled "Optical Imaging System", which describes an optical imaging system that includes a scintillating screen for converting incoming radiation to visible light, an image intensifier for increasing the intensity of the visible light. A fiber optic taper forms a viewable image based on the visible light and a lens directs the visible light to the fiber optic taper. US Patent No. 4,641,931 (Loy) entitled "Dual magnification telescope" discloses a dual magnification telescope such as, for example, an intensifier telescope, comprising an objective, an intensifier tube with an optical fiber input, and an eyepiece. Dual magnification is obtained by an optical fiber array comprising a straight fiber having unity magnification and tapered fibers having a magnification less than unity.

U.S. Patent Nos. 6,392,241 and 6,646,272 both in the name of Rushbrooke et. al. and entitled "Fiber optic coupling device for detecting fluorescence samples" teach the use of fiber optics couplings for detecting fluorescence samples and detecting luminescent samples. The smaller aperture of the fiber optics tapers faces the incoming light thus achieving less than unity magnification.

U.S. Patent No. 6,730,913 (Remillard et al.) entitled "Active night vision system or vehicles employing short-pulse laser illumination and a gated camera for image capture" describes a night vision system and method related thereto for detecting objects at relatively low visibility light levels. The system so described is for night and no mention of compactness and high angular resolution is presented.

Image intensifiers include an objective for focusing a distant object on to an electro-optical image sensor. In order to increase the acquisition range for such an image intensifier and allow more distant objects to be imaged at a resolution that is discernible to the user, the focal length of the objective must be increased. This increases the magnification but also necessitates an increase in objective aperture (input pupil diameter) since, in order to maintain the same F-number for the same intensifier photocathode illumination and to avoid diffraction aberration. This means that the desire to increase the far distance at which a discernible image can be obtained clashes with the desire to minimize the size of the objective aperture and thereby render apparatus more portable. Put another way, it would be desirable to reduce the size of the apparatus without reducing the magnification.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an optical imaging system of reduced dimension without reducing the magnification determined by the focal length of an objective of the optical imaging system. This object is realized in accordance with the invention by an apparatus, comprising: an input fiber optic taper having a narrow aperture and a wide aperture and comprising a bundle of component tapered optical fibers, an objective disposed for focusing incident electromagnetic radiation on to the narrow aperture of the input fiber optic taper, and an electro-optical imaging device having an input fiber optic faceplate attached to the wide aperture of the input fiber optic taper for imaging said electromagnetic radiation, said input fiber imaging device having an effective pixel size that is larger than a narrow aperture of each component optical fiber in the input fiber optic taper.

In such an arrangement the spatial resolution of the apparatus is given by:

S = P- f where:

S = spatial resolution; p = the effective pixel size of the electro-optical imaging sensor; and

/= focal length of objective.

Thus comparing a conventional arrangement having an objective of focal length

/; and an electro-optical imaging sensor of pixel size pi with an arrangement according to the invention wherein an objective of focal length^ is coupled to the electro-optical imaging sensor via an fiber optic taper, having pixel size p₂ in order for the spatial resolution to remain the same,

whereby it follows that:

Thus, if Pi< P_\ , then fi < f_\ . In other words, providing that the pupil diameter (aperture) of the objective is reduced, so too can its focal length be reduced without derogating from the spatial resolution of the final image. The invention achieves this by coupling the objective to the optical image sensor assembly via a fiber optic taper whose narrow aperture (i.e. small pupil diameter) is disposed toward the objective and whose wide aperture (i.e. large pupil diameter) is attached to the image sensor assembly fiber faceplate. Coupling the objective to the optical image sensor assembly in such manner allows an objective of lower focal length to be used, thus reducing the size of objective aperture for the same F-number and angular resolution, allowing reduce apparatus size.

According to another aspect of the invention, there is provided a method for focusing incident electromagnetic radiation on to an electro-optical imaging device via an objective, the method comprising: disposing between the objective and the electro-optical imaging device an input fiber optic taper having a narrow aperture and a wide aperture and comprising a bundle of component tapered optical fibers each having a narrow aperture that is smaller than an effective pixel size of the electro-optical imaging device; attaching an input fiber optic faceplate of the electro-optical imaging device to the wide aperture of the input fiber optic taper; and focusing the incident electromagnetic radiation on to the narrow aperture of the input fiber optic taper.

According to yet another aspect of the invention, there is provided a method for manufacturing a compact apparatus for focusing incident electromagnetic radiation on to an electro-optical imaging device via an objective, the method comprising: disposing between the objective and the electro-optical imaging device an input fiber optic taper having a narrow aperture and a wide aperture and comprising a bundle of component tapered optical fibers each having a narrow aperture "g" that is smaller than an effective pixel size "p" of the electro-optical imaging device; and attaching an input fiber optic faceplate of the electro-optical imaging device to the wide aperture of the input fiber optic taper; whereby a focal length of the objective is reduced by a factor of "g/p" compared to the focal length of an objective that would be required in an equivalent apparatus lacking the input fiber optic taper. In a first embodiment, the large face of a first fiber optic taper (constituting an input fiber optic taper) is attached to the light input fiber optic faceplate of an ICCD having an effective pixel size that is larger than a narrow aperture of each component optical fiber in the input fiber optic taper. The second intensifier facing (phosphor anode) of the ICCD is optically attached to an image sensor such as a CCD or a CMOS camera by a second fiber optic taper (constituting an output fiber optic taper). The angular resolution of the apparatus is determined by the geometrical opening i.e. aperture of the fiber optics (g) which defines the small face of the first fiber optic taper and the focal length (fn) of the aforementioned objective. For an ICCD without the attached first fiber optic taper, the effective pixel size of the so described ICCD is (p). When g is less than p, the focal length of the apparatus objective is reduced according to the ratio "g/p". With the use of a reduced objective focal length, the objective aperture is reduced for the same F-number and the overall apparatus size is diminished. In the case that an illuminator, such as laser is used, its emission power may be reduced thus also allowing the size of the apparatus to be reduced.

In another embodiment, the smaller face of the first fiber optic taper is juxtaposed to the optical objective which faces the incoming radiation. The larger face of the first fiber optic taper is attached directly to the imaging sensor input fiber faceplate. If the effective pixel size of the imaging sensor is denoted as (pi) and the narrow aperture of each component tapered optical fiber in the first fiber optic taper is denoted by (gl), then when gl is less than pi, the focal length of the apparatus objective is reduced according to the ratio "gl/pl" from the original apparatus without the first fiber optic taper.

The apparatus thus described will be more compact than an apparatus lacking the first fiber optic taper. A judicial choice of the narrow aperture of the fiber optic taper will enable the apparatus to be used on a small platform not ordinarily suitable for the apparatus without the first fiber optics taper.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a compact electro-optical imaging system in accordance with the preferred embodiment in which the imager is an ICCD; FIG. 2 is a schematic diagram indicating a critical dimension of the optical fiber shown in FIG. 1 ; and

FIG. 3 is a schematic diagram of a compact electro-optical imaging system in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the following description of some illustrative embodiments, identical reference numerals will be used to denote identical or equivalent components in different figures.

Referring to FIG. 1, there is shown an ICCD apparatus 10, comprising an objective 11 for focusing incident electromagnetic radiation on to the narrow aperture 13 of an input fiber optic taper 12 having a wide aperture 14 and comprising a bundle of tapered optic fibers (not shown). An electro-optical imaging device 15 has an input fiber optic faceplate 16 that is attached to the wide aperture 14 of the input fiber optic taper 12 for imaging the electromagnetic radiation. FIG. 2 shows schematically an end elevation of the input fiber optic taper 12 having a narrow aperture equal to (g).

The spatial resolution of the apparatus 10 satisfies the equality:

Sn ∞ — Jh where: Sn = spatial resolution; g = the narrow aperture of the input fiber optic taper 12; and focal length of objective 11.

By comparison, the spatial resolution of the same apparatus but without the fiber optic taper 12, satisfies the equality:

So ∞ — fo where:

So = spatial resolution; p = the effective pixel size of the electro-optical imaging device 15; and fo= focal length of objective 11. If the angular resolution is unchanged, such that Sn is equal to So, then:

fa fo In other words, the focal length of the objective 11 may be reduced by a factor:

Jh_ ₌ 8_ Jo P The resulting apparatus as described in FIG 1 is consequently reduced in size when "g" is less than "p" since the focal length of the objective is similarly reduced. For example, the size may be halved if "g" is "p/2,

FIG. 3 shows schematically a preferred embodiment of the apparatus 10, wherein the electro-optical imaging device 15 is an intensified charge coupled device (ICCD) camera comprising an intensifier luminescent plate 17 displaced from the input fiber optic faceplate 16 via a cascade element 18 that can include a vacuum or micro- channel plate. The cascade element 18 includes a photocathode (not shown) that converts incoming radiation to electrons that are directed under the influence of a high voltage field to a phosphor (also not shown) and promotes electron gain so as to produce charge amplification. The wide aperture 19 of an output fiber optic taper 20 adjoins the intensifier luminescent plate 17 and its narrow aperture 21 is mounted proximate an electro-optical imaging sensor 22 comprising a plurality of pixels (not shown). The input fiber optic faceplate 16 is responsive to incident light (or other electromagnetic radiation depending on the sensor material) for producing electrons that are accelerated toward the intensifier luminescent plate 17 by an electric field applied across the cascade element 18 between the photocathode and the phosphor so as to produce at the intensifier luminescent plate 17 light of higher intensity than the incident light. The output fiber optic taper 20 is adapted to convey the higher intensity light to corresponding pixels of the electro-optical imaging sensor 22. For background lighting which is insufficient to see an object, it is required to illuminate the background scene. For a substantially reduced apparatus, there will be room to place an object illuminator 25. When the illuminator 25 is a laser, to suppress near region back reflection, gating is added to the ICCD. The drawings and description disclose some typical preferred embodiments of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. It will be apparent to those skilled in the art that modifications can be made to the invention without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS:

1. An apparatus (10), comprising: an input fiber optic taper (12 ) having a narrow aperture (13) and a wide aperture (14) and comprising a bundle of component tapered optical fibers, an objective (11) disposed for focusing incident electromagnetic radiation on to the narrow aperture of the input fiber optic taper, and an electro-optical imaging device (15) having an input fiber optic faceplate (16) attached to the wide aperture of the input fiber optic taper for imaging said electromagnetic radiation, said electro-optical imaging device (15) having an effective pixel size that is larger than a narrow aperture of each component optical fiber in the input fiber optic taper.

2. The apparatus according to claim 1, wherein the electro-optical imaging device (15) is an intensified charge coupled device (ICCD) camera.

3. The apparatus according to claim 2, wherein the ICCD camera comprises: an intensifier luminescent plate (17) displaced from the input fiber optic faceplate (16) via a cascade element (18), an output fiber optic taper (20) having a wide aperture (19) adjoining the intensifier luminescent plate (17) and having a narrow aperture (21), and an electro-optical imaging sensor (22) comprising a plurality of pixels and mounted proximate the narrow aperture (21) of the output fiber optic taper (20); the input fiber optic faceplate (16) being responsive to incident electromagnetic radiation for producing electrons that are accelerated toward the intensifier luminescent plate (17) by an electric field applied across the cascade element (18) so as to produce at the intensifier luminescent plate (17) electromagnetic radiation of higher intensity than the incident electromagnetic radiation; and the output fiber optic taper (20) being adapted to convey said higher intensity electromagnetic radiation to corresponding pixels of the electro-optical imaging sensor (22).

4. The apparatus as claimed in claim 3, wherein the cascade element (18) includes a micro-channel plate.

5. The apparatus as claimed in claim 3, wherein the cascade element (18) includes a vacuum.

6. The apparatus according to any one of claims 1 to 5, further comprising an object illuminator (25) disposed proximate the objective (11) for increasing the intensity

5 of light conveyed to the input fiber optic taper (12).

7. The apparatus according to claim 6 wherein the object illuminator (25) provides light having a wavelength between 0.2 microns to 2 microns.

8. The apparatus according to claim 7, wherein the object illuminator (25) is a laser.

10 9. The apparatus according to any one of claims 3 to 8, when dependent on claim

3, wherein said intensifier luminescent plate is fiber optically attached to the electro- optical imaging sensor.

10. The apparatus according to claim 9, wherein gating is added to the ICCD to suppress near region back reflection.

15 11. The apparatus according to any one of claims 1 to 10 wherein said electro- optical imaging device is adapted to sense radiation having a wavelength between 0.2 microns to 20 microns.

12. A method for focusing incident electromagnetic radiation on to an electro- optical imaging device (15) via an objective (11), the method comprising:

20 disposing between the objective and the electro-optical imaging device an input fiber optic taper (12) having a narrow aperture (13) and a wide aperture (14) and comprising a bundle of component tapered optical fibers each having a narrow aperture that is smaller than an effective pixel size of the electro -optical imaging device;; attaching an input fiber optic faceplate (16) of the electro-optical imaging device

25 (15) to the wide aperture of the input fiber optic taper; and focusing the incident electromagnetic radiation on to the narrow aperture of the input fiber optic taper. lS. A method for manufacturing a compact apparatus (10) for focusing incident electromagnetic radiation on to an electro-optical imaging device (15) via an objective (11), the method comprising: disposing between the objective and the electro-optical imaging device an input fiber optic taper (12) having a narrow aperture (13) and a wide aperture (14) and comprising a bundle of component tapered optical fibers each having a narrow aperture

"g" that is smaller than an effective pixel size "p" of the electro-optical imaging device;

(15); and attaching an input fiber optic faceplate (16) of the electro-optical imaging device (15) to the wide aperture of the input fiber optic taper; whereby a focal length of the objective (11) is reduced by a factor of "g/p" compared to the focal length of an objective that would be required in an equivalent apparatus lacking the input fiber optic taper (12).