CA2119330A1

CA2119330A1 - Methods to determine spatial angle of a light beam

Info

Publication number: CA2119330A1
Application number: CA 2119330
Authority: CA
Inventors: Ishiang Shih; Shaolin Shi; L. Ngo Phong; Philip Laou; Cindy Xing Qiu
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-03-17
Filing date: 1994-03-17
Publication date: 1995-09-18

Abstract

METHODS TO DETERMINE SPATIAL ANGLE
OF A LIGHT BEAM
ABSTRACT OF DISCLOSURE
Methods for determining the spatial angle of inci-dence of a light beam are disclosed. In one embodiment, incident light is focused by a stripe lens onto an array of detectors. The spatial angle of incidence is determined by the position of the light beam focused on the detector array. In another embodiment, incident light is focused by a stripe lens onto an optical fiber array which guides the light onto a detector array. The spatial angle of incidence of the light beam is determined from the combination of signals received by detectors in the detector array.

Description

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BACKGROUND OF THE INVENTION

1. Field of the invention This invention relates generally to devices to detect or monitor laser radiation. More specifically, the inven-tion relates to determination of spatial angle of incidence of a light beam.

2. Description of the prior art Laser radiation or radiation from light emitting diodes has found numerous applications in manufacturing, control, electronic and laboratory equipment. For these applications, it is important to determine the spatial angle of incidence of the radiation. In robotics, for instance, it is often required to determine the angular velocity of an object. This angular velocity can be ob-tained by measuring change of angular position of a light source attached to the object with the change of time. The laser beam is also very important in modern warfare. For example, a laser beam is often used by the enemy to target a tank, ship, aircraft or strategic site and to guide ,:,!, ,j~
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21193~ -projectiles in attacking the above targets. Once the pulsed laser beam has been sent, the enemy's detection system will detect the reflected pulsed laser beam and analyze the delay to determine the distance among other things. Pulsed laser beams thus constitute threats in the battlefield. In order to avoid reprisals, a jamming means or counter attack means must be exercised before or immediately after the attack.

Several methods have been proposed and used to deter-mine the spatial angle of incidence of laser beams. In U.S.
paten~ No. 4,946,277 entitled "Opto-electronic Device for Detecting and Locating a Radiant Source" to Marquet, Le-maire and Dunouvion, one method involves the use of a quadrant detector array (see Fig. 1) has been described. A
direction finding unit generally indicated as (1) is used in this method. The unit consists of a detector array (2) with four detector elements (3,4,5,6) disposed under a square or circular optical mask (7) with a square or circu-lar transparent window (8). The surface of the detector array is parallel to the mask so that selected symmetrical illumination may be obtained. Around the transparent win-dow, the mask is opaque to incident light (9). When the light beam is incident to the detector unit, an illuminated area (10) is projected on the detector array. Depending on the angle of incidence of the light with respect to the r~
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detector array, different portions of the detector elements will be illuminated. Photons in the light beam striking the elements through the square or circular window will be absorbed and produce electrical signals in these detector elements, in the form of photo currents. Since the value of electrical signal produced is proportional to the amount of incident photons, the relative values of the electrical signals from the four detector elements may be used to determine the spatial angle of incidence of the light beam.

This method, although simple, has several drawbacks.
When the intensity of the incident beam is too large, one or more of the detector elements may be saturated by the incident beam and the electrical signals thus obtained may not accurately indicate the angle of incidence of the light beam. In addition, the variation of the electrical i signals with the amount of incident photons falling on the detector elements may not be linear over a practically large range. A separate means of calibration could thus be necessary for ascertaining the true spatial angle of inci-dence of the beam. Another drawback of this method is that, in order to detect beams with large angles of incidence ~for example, +45), the dimensions of the detector ele-ments would have to be substantially greater than the width of the window in the optical mask. Due to the relatively small width of the window, only a portion of each detector ~,:, . . , . ~ ' ' " ' ` -- 2119330 element will be illuminated by the incident beam. Through the unilluminated portion of the detector element a dark current will flow, which may be much greater than the dark current flowing through the illuminated portion. The flow of the dark current in the unilluminated portion will reduce the sensitivity of the detector element.

Apart from the dark current problem, the capacitance or the unilluminated portion of each detector element may be much greater than the capacitance of the illuminated region. Capacitance charging and discharging associated with the unilluminated region of each detector element may retard the detector array operation speed. Another drawback of this method is that it is only the portion of the inci-dent light beam falling within the window of the optical mask that is effective for the angle of incidence determi-nation. Apart from the nonlinearity problem with the varia-tion of incident light intensity, the electrical signals from detector elements under low intensity illumination conditions ~such as when the laser beam source of the enemy is at a large distance from the detector) may not be useful to determine the angle of incidence of the laser threaten-ing source.
To overcome part of the above-mentioned drawbacks, there is proposed in U.S. patent No.4,769,546 by Kniffler and Legner a method to determine the angle of incidence of 2 1~

a light beam. In this method, the direction finding unit (11), shown in Fig. 2, is used. This unit consists of a detector array (12) with stripe elements (13,14,15,16). A
mask (17) with an elongated window (18) is mounted in front of and parallel to the detector array. The long axis of the elongated window (19) is aligned perpendicular to the direction of the stripe elements (20). Most of the inci-dent light (21) is blocked by the mask (17) and only the portion in the window region (18) is allowed to reach the detector array. The light passing through the window forms an elongated illuminated region (22) on the detector array.
Each stripe element o~' the detector array consists of active (23, transparent areas) and inactive regions (24, opaque areas). By measuring the electrical signal from each stripe element (13,14,15,16) and by knowing the distribu-tion of the active and inactive regions of the detector array, the angle of the light beam may be estimated.

Although the problem of signal saturation encountered in the method involving the quadrant detector array is minimized, the dark current and capacitance effects associated with the unilluminated regions of each stripe element still severely impair detection sensitivity. In addition, the prior art method shown in Fig. 2 has several other drawbacks. One of the drawbacks is that the amount of photons allowed to strike the stripe elements is very ,; ;, , , , ~ ~, f. f . , ., ~
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small. This is understood by considering the width of the window (25) in the mask (17) re~uired to achieve a given resolving power. The width (25) of the window (18) would have to be equal to or less than the minimum width (26) of the active regions in the stripe element with the finest pitch. For instance, to resolve an angle of incidence of one degree for an overall angle of view of 100, a minimum active region width of 50 micrometers would be used. The corresponding width of the window (25) would be 50 microme-ters or less and the total length of each stripe element would be 50xlO0 = 5000 micrometers (or 5 mm). ~ecause of this small window width, the amount of photons in the incident beam allowed to reach the detector array would be very small. Thus, the detection efficiency of the stripe detector array in the prior art method is very small. This is especially true when the intensity of incident light is low, producing electrical signals too small for unequivo-cal incident light direction determination. ~-Even when the intensity of the incident light beam is high enough so that reasonable photo signals can be ob-tained from the detector elements, the resolution of the angle of incidence detection cannot be increased further by decreasing the width of slit. When the slit width is de-creased, diffraction effects of light around the edges of l~ the slit become more important to the sharpness of the 7 `

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light stripe formed on the detector array by the slit. This is especially true when considering that the practical distance between the slit and the detector array is in the order of 1 mm or greater.

Another drawback of the prior art method described in Fig. 2 is that the unit (11) is only effective for identi-fying the angle of incidence (e) of a light beam (21) falling on the plane defined by the direction (20-1) of the long axis of the stripe elements and the normal line (27) of the surface of the mask (17). When the beam is incident at a different direction (21-1), with an angle ~' between 21 and 21-1 (the angle between projections of the two beams on the plane defined by 19 and 27 is ~), the resulting illuminated region (22) will fall in the same position on the stripe elements as long as the projection of (21-1) on the plane defined by 20-1 and 27 coincides with (21). Under these conditions, all of the stripe elements will generate the same electrical signals as when the light beam was in the original direction (21). Therefore, the stripe detector array proposed in the prior art method is not capable of indicating simultaneously the values of e and ~. Both e and are required to determine the spatial direction of a light beam.

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SUMMARY OF THE INVENTION

It is an object of this invention to provide a device which allows more light to be coupled to detector arrays for the determination of angle of incidence of a light beam.

Another object of the invention is provide a more efficient method to determine the angle of incidence of a light beam using miniature detector elements. In this method, photons in the incident light beam are focused by a stripe lens and allowed to incident on a coded optical mask containing at least one coded stripe. Under the mask is disposed an integrated optical fiber array. All fibers within a coded stripe are merged together to a single fiber. Photons incident on one end of each fiber are thus guided to the single fiber. At the other end of the single fiber is a sensitive detector element in the detector array. By measuring the electrical signals from the detec-tor elements and by knowing the coding sequence of the fiber array, the angle of incidence of the light beam can be determined.

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BRIEF DESCRIPTION OF THE DRAWINGS
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Fig. 1 is a schematic top view of the quadrant detector array from the prior art.

Fig. 2 shows the cross-sectional view of the prior art stripe detector array with an optical mask for the determi-nation of angle of incidence.

Fig. 3 is a schematic view of a unit consisting of a mask with two stripe windows and two stripe detector segments for the determination of spatial direction of an incident light beam.

Fig. 4 is a schematic view of a stripe lens structure that illuminates a rectangular region so the spatial angle of an incident light beam may be determined.

Fig. 5 is a schematic diagram of the unit consisting of a stripe lens and a coded detector array for the determina-tion of the angle of an incident light beam.

Fig. 6 is a schematic view of a unit consisting of a mask with two stripe lenses and two stripe detector segments for the determination of spatial angle of an incident light beam.

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Fig. 7 is a schematic view of a unit consisting of a mask with one stripe lens and fiber arrays for the determination of the spatial angle of an incident light beam.

Fig. 8 is a schematic view of a unit consisting of a mask with an array of fibers mounted to face the center of the -stripe lens.

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DETAILED DESCRIPTION

In order to determine the spatial angle of incidence of a light beam, a detector array (28) containing two stripe detector segments (12,29) is fabricated with the long axis (20) of the first segment (12) perpendicular to the long axis (30) of the second segment (29). For clarity of description, four stripe elements (13,14,15,16) are shown in each detector segment. It is understood that the number of stripe elements in each segment can be less than or greater than 4, depending on the resolution required.
The length of segment (12) is L1 and the width is W1, whereas the length of segment (29) is L2 and the width is W2. A mask (17) containing two optical slits (18,31), preferably perpendicular to each other, is mounted on a plane parallel to the detector array. The length of slit (19) is LWl and the width is W~1, whereas the length of slit (31) is LW2 and the width is Ww2.

, The orientation of the mask is adjusted so that the ~'long axis (19) of one slit (18) is perpendicular to the long axis of the stripe detector segment (12). The long axis (20-1) of the other slit (31) is perpendicular to the 12 ` ;

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long axis of the stripe detector segment (29). In order to cover a specific range of angles of incidence, for instance ~max=+45 or ~max=+45~ the lengths of the slits, LWl and LW2, must be selected so that W1+2d is less than or equal to LWl whereas W2+2d is less than or equal to LW2 (~max and ~max are the maximum angles of the light beam to be detected). Thus when a parallel light beam (21-1) is incident on the detector array, an illuminated rectangular area (22) will be formed on the detector segment (12) and the other illuminated rectangular area (32) will fall on the other detector segment (29). The position of (22), which is determined by e, is measured by taking electrical signals from the stripe elements in (12~. The position of (32), which is determined by ~, is measured by taking electrical signals from stripe elements in the other detec-tor segment (29). Consequently, incident light beam direc-tion (21-1) can be determined from the angles e and For angles of emax and ~max which are different from +45, the length of the first slit, LWl, will have to adjusted so that W1 + (2)(d)tan(emax) is less than or equal to LWl whereas the length of the second slit, LW2, will have to be adjusted so that W2~(2)(d)tan(~max) is less than or equal to LW2. Furthermore, dimensions of the detector array must be selected so that the length of stripe detec-tor elements in first detector segment (12), L1, is not 21~93~

less than (2)(d)tan(~max) whereas the length of stripe detector elements in secon~ detector segment (29), L2, is not less than (2)(d)tan(emax).

The distance between the mask (17) and the detector array (28), d, is ad~usted so that the illuminated rectan-gular region (22) is within the detector array (12) and the other illuminated region (32) is within the detector array (29) when the incident beam has the maximum or mini-mum angle of incidence, e and ~. For instance, in order to cover the range of angles -45<e~45 and -45<~<45, the length of slit (18), LWl, should be greater than or equal to W1+2d whereas the length of slit (31), LW2, should be greater than or equal to W2+2d. It should be noted that the length of slit (19), LWl may be chosen so as to be equal the length of slit (31), LW2. The above length requirements thus ensure that all of the elements in detector array (12) are illuminated by (22) and all of the elements in the detector array (29) are illuminated by (32).

The mask will also have to be large enough to pre-vent unwanted light from reaching array areas around the two slits (12,29). To obtain coverage of angles -45<e<45 and -45<~<45, the lengths of stripe detector elements in both segments (12) and (29) must not be less than 2d.
Furthermore, the separation between the detector regions (12,29) must be sufficiently large or else a light barrier t~

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must be provided between the two regions in order to pre-vent light passing through the first slit (18) from reach-ing the other detector region (29). Light passing through the second slit (31) must also be prevented from reaching detector region (12) as this will affect the operation of the detector array.

In applications for the detection and determination of the angle of incidence of a laser beam, it is preferable to add a narrow optical filter in front of the mask (17).
The band pass value of this narrow optical filter is equal to the wavelength of the laser beam to be detected so that only the laser beam may penetrate the mask and strike the stripe detector elements. All other stray light from the background is blocked by the narrow optical filter so that unwanted electrical signals from the stripe detector ele-ments are minimized. It should be noted that the narrow optical filter may be deposited directly on the mask.

Although the detector array with a mask having two slits shown in Fig. 3 may be used to determine the spatial ~ . .
direction of a light beam, this may be effective only for light beams with sufflciently high intensity. This is because only a small amount of light is allowed to reach the detector arrays and because the length of detector elements (L1, L2) with respect to the width of the illumi-"" ., ~ .~, . .,, ~ , " , . . .
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nated rectangular region (22,29) is large. Due to the largelength of detector elements, the capacitance or leakage current for each detector element will be large. To in-crease the amount of photons striking the detector ele-ments, an improved method for capturing photons in the incident beam for the determination of the spatial angle of a light beam is given.

Fig. 4 (a) shows a structure (33) to form an illumi-nated rectangular region (34). The structure consists of a substrate (35) which is transparent to light to be detect-ed (36). In the central region of the substrate, a stripe lens (or called cylindrical lens) (37) is fabricated using micro fabrication technology. The material used to form the stripe lens must be transparent to the light beam. The substrate outside the stripe lens is covered with a layer of material (38) opaque to incident light. This material may be a layer of Al with a thickness of 0.5 micro meter or greater. The length of the stripe lens (39) is ~1 and-the width ~40) is Wwl. The stripe lens (37) is convex and transparent to the incident light (36) so that an illumi~
nated stripe region (34) formed near the focus position of the stripe lens. The distance between the stripe lens and the focus position, d, is controlled by the curvature of the cross section of the stripe lens. Exterior to the illuminated stripe region (34), no light originating from ~;
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the incident beam will be present.

The structure in Fig. 4(a) has several advantages compared to the prior art structure which uses a narrow slit to define an illuminated rectangular region (shown in Fig. 4(b)). For the prior art structure, the width (42) of the slit (43) would have to be small to obtain an illumi-nated rectangular region (44) with a small width (42-1).
This small width would be required to cover a substantial-ly large range e and ~, for instance, -45<e<45 and -45<~<45. To obtain the above coverage, while maintaining a reasonable resolution of the angle of the incident beam of say 2, the width of the slit in the prior art structure (42) should be less than 1/45 of the length of the detector elements (L1, L2) shown in Fig. ~. A practical value for the slit width would be 20 ,um. For this small width of slit, the amount of photons which form the illuminated rectangular region (44) is rather small. The resulting electrical signals from the stripe detector elements for the determination of the spatial angle of the incident beam ~., ~ would also be small.

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On the other hand, the width of the slit in the prior method can not be made too small even when intensity of the incident light is large enough and one wants to increase resolution of the angle of incidence using a detector array with a small length. This limit is due to diffraction of '7 ~ 1 7 ~' :
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light through the slit, which becomes severe when the slit is separated from the detector array. The main advantage of the structure shown in Fig. 4 (a) is thus obvious. Using the stripe lens or cylindrical lens, the amount of photons allowed to reach the rectangular region near the focus position can be much greater than the amount using the prior art method. For instance, for a width of stripe lens (40) of 1 mm (1000 ~m) and a rectangular region width (41) of 10 ~m, the light intensity at (34) will be 100 times that of the prior art method shown in Fig. 4 (b). The intensity of the illuminated rectangular region (34) can be increased by increasing the width of the stripe lens (40) or by decreasing the width of the illuminated rectangular region (41).

Another advantage of the structure shown in Fig. 4(a) over the prior art structure in Fig. 4(b) is that the resolution of the spatial angle of a light beam (36) can be increased even if detector arrays with a small element length are used. This is achieved by making the width of the illuminated rectangular region smaller than that can be obtained, practically, using the prior art slit method. By carefully forming the stripe lens, an illuminated rectangu-lar region (34) with a width of less than 5 ~m may be obtained for a parallel incident laser beam at 0.63 ~m.
With this small a width, the total length of the detector ''''" .

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elements may be as small as 250 ~m. A resolution of 2 may thus be conveniently achieved for a range of -45<e<45 and -45<~<45.

Although the stripe lenses shown in Fig. 4 and subse-quent figures are facing the light source, it is understood that the lenses can be inverted so that the transparent substrate is facing the light source. Furthermore, in order to minimize effects of noises from light sources other than the light beam to be detected, narrow band pass optical filters may be placed in front of the stripe lenses or between the lenses and the detector arrays. The narrow band pass filter may also be deposited directly on the transpar-ent substrate. To minimize the reflection loss of the light beam striking the unit, an anti-reflection layer may be deposited directly on the stripe lenses or substrates.
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: -To determine ~ of an incident light beam, the struc-ture shown in Fig. 4(a~ can be positioned over a stripe detector array segment (12) having the total active region width W1 and length L1 shown in Fig. 5. The stripe lens (33) is positioned so that it is parallel to the detector array segment (12). The length of the stripe lens (37) is LWl and the width is Wwl. The separation between the stripe lens (37) and the detector segment (12), d, is ~ adjusted so that a nearly focused illuminated rectangular ;, ~ 19 "

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region (22) forms on the detector segment (12). The orien-tation of the stripe lens is selected so that the long axis of the lens (45) is perpendicular to the long axis of the stripe detector elements (20). For clarity, the four stripe elements (13,14,15,16) are shown in the detector segment. It is understood that the number of stripe ele-ments in the segment can be less than or greater than 4, depending on the resolution required.

When a light beam (36) is incident to the detector assembly (46), light shines on the rectangular region (22) of the detector segment. In order to increase intensity of light in the illuminated rectangular region (22), it is preferable to adjust the distance between the stripe lens and detector segment, d, so that it is approximately equal to the focus length of the stripe lens, f. An electrical signal will be produced in a detector element (13-16) when there is an active area within (22). 8y monitoring simulta-neously or sequentially the electrical signals from all elements of the detector se~ment, the angle e of the inci~
dent light beam may be determined. To ensure that a beam with a different ~, for instance -45<~<45, can form an illuminated rectangular region covering all of the detector elements, the length of the stripe lens (39) t LWl, would have to be greater than or equal to W1+2d (here d is the separation between the stripe Iens and the detector array).

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21~93~0 To determine spatial direction, both e and ~ must be measured simultaneously. This is accomplished using the unit shown in Fig. 6. Here, a detector array (28) contain-ing two stripe detector segments (12,29) is fabricated with the long axis (20) of the first detector segment (12) perpendicular to the long axis (30) of the second segment (29). A substrate (47) containing two stripe lenses (37,48) is mounted on a plane parallel to the detector array. The length of stripe elements in the first detector segment is L1 and the total active region width is Wl, the length of stripe elements in the second detector segment is L2 and the total active region width is W2. The mask (47) contain-.~ - . .
ing two stripe lenses (37,48) is positioned so that it is parallel to the detector array (28). The length of the first stripe lens (37) is LWl and the width is Wwl, the length of the second stripe lens (48) is LW2 and the width is Ww2. The separation between the long axis (45) of the first stripe lens ~37) is perpendicular preferably to the long ~xis (49) of the second stripe lens (48~.

The orientation of the substrate (47) is adjusted so that the long axis (45) of the first stripe lens (37) is perpendicular to the long axis of the first stripe detector segment (12). The long axis (49) of the second stripe lens (48) is perpendicular to the long axis of the second stripe detector segment (30). The distance between the lenses -, 21i932~

(37,48) and the detector array, d, is selected to be equal to the focus length of the stripe lenses, f, so that the detector array is located at the focus position. When a parallel light beam (36) is incident to the detector array, an illuminated rectangular area (22) will be formed on the first detector segment (12). The other illuminated rectan-gular area (32) will also be formed on the second detector segment (29). The position of (22), which is determined by ~, is measured by recording the electrical signals from the stripe elements in detector segment (12). The position of (32), which is determined by ~, is measured by taking electrical signals from stripe elements in detector se~ment (29). Thus the angle of the incident light beam (36) can be determined from the two angles e and ~
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To ensure that a beam with different e and ~ for :

instance -45<e<45 and -45<~<45, can produce an illumi-~,~ , "
~nated rectangular region covering all of the detector -~elements in the first detector segment (12) and another ;~- :
illuminated rectangular region covering all of the detector :-;elements in the second detector segment (29), the length of the stripe lens (37), ~ 1~ will have to be greater than or -equal to W1+2d the length of the stripe lens (37). The ~:length of the second stripe lens ~48), ~2~ will have to be : ;~
F~ ~greater than or equal to W2+2d (here d is the separation between the stripe lens and the detector array).

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gles of ~max and ~max which are different from +45, the length of the first stripe lens, LWl, will have to be adjusted so that Wl + (2)(d)tan(~max) is less than or equal to ~1 whereas the length of the second stripe lens, LW2, will have to be adjusted so that W2+(2)(d)tan(~max) is less than or equal to LW2. Furthermore, dimensions of the detector array must be selected so that the length of stripe detector elements in the first detector segment (12), Ll, is not less than (2)(d)tan(~max) whereas the length of stripe detector elements in second detector segment (29), L2, is not less than (2)(d)tan(~max).

In applications for the detection and determination of angle of incidence of a laser beam, it is preferable to add a narrow optical filter (not shown in Fig. 6) in front of the mask (47). The band pass value of this narrow opti-cal f~lter is equal to the wavelength of the laser beam to be detected so that only the laser beam may penetrate the mask and strike the stripe detector elements. All other stray light from the background will be blocked by the narrow optical filter so that unwanted electrical signals from the stripe detector elements are minimized. Further-more, an anti-reflection layer may be deposited directly on the surfaces of the stripe lenses (37,48) in order to minimize loss of incident light due to reflection. The , ~ 23 .~ ~

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anti-reflection coating layer may be deposited on the substrate when the mask (47) is inverted so that the lenses (37,48) are facing the detector array (28).

It is thus evident that the unit shown in Fig. 6 can be advantageously used to determine spatial angle of inci-dence of a light beam. However, this unit requires stripe detector elements which have a large inherent leakage current and large capacitance due to the large area of the detector elements. To improve further performance of the unit for the determination of spatial angle of the light beam, it is preferable to detect the incident light beam using detector elements with minimum leakage current and minimum capacitance. In one embodiment, this is achieved using the unit shown in Fig. 7.

In Fig. 7, arrays of optical fibers (50) are posi~
tioned in a linear fashion to form light acceptance stripes (51,52,53,54). General knowledge about use of optical fibers for collecting and transmitting light can be found in various literature, for instance, in "Understand-ing Fiber Optics" by Jeff Hecht (ISBN No. 0-672-27066-8), published in 1987 by Howard W. Sams & Company. An ordinary optical fiber consists of a core with one refractive index nO and a surrounding cladding with another refractive index n1. When nO is carefully selected so that it is slightly greater than n1, incident light within a specific ; ~ ~

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angle (called an acceptance angle) can be coupled into the core at one end of the fiber and guided to the other end.
In the stripe (51), light will not be collected in opaque areas but will be collected in transparent areas. There-fore, by introducing at least one optical fiber in each transparent area of stripes (51,52,53,54), photons striking the illuminated rectangular region (22) formed by the stripe lens (37) can be collected. For clarity of de-scription, four stripes (51,52,53,54) are shown in the fiber array. It should be understood that the number of stripes in the segment can be less than or greater than 4, depending on the resolution required.

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In the actual applications, it is also preferable to adjust the distance between the stripe lens (37) and plane where the tips of all the fibers are located, d, to be approximately equal to the focus length of the stripe lens, f. This will ensure high resolution and efficient photon collection in the device. All of the fibers (50) in each stripe (51,52,53,54) are merged into separate single fibers (56,57,58,59) through devices (55). The assembly including multiple fibers (50), a merging device (55) and fiber (56) is known and is commonly called a lxN Fiber-optic Tree Coupler (see, for example, Single-Mode Achromat-ic lxN Fiber-Optic Tree Couplers, Product Information, Corning Incorporated, Telecommunications Products Division, --` 211933~ ~
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Corning, New York). Photons collected by one or more of the fibers in a stripe (51,52,53,54) are thus guided through single fibers (56,57,58,59). The guided photons can be coupled to miniature photo detectors (60,61,62,63) for determination of spatial direction of the incident light beam. The main advantage of the unit shown in Fig. 7 is that miniature detector elements with minimum leakage current and capacitance can be used to replace the stripe detector elements required in the units shown in Fig. 3 and Fig. 6. The usage of the miniature detector elements will reduce the response time of the unit.

In order to collect most of photons within the rec-tangular region (22) of each stripe (51,52,53,54), it is preferable to position fibers in the transparent areas of each stripe so that there is substantial overlap of cores i :~
at least in the direction (45-1) for the adjacent fibers. ;--~
Therefore, photons within the rectangular region (22) striking each stripe (51,52,53,54) will be coupled to at least one fiber (50) and will be guided to (56,57,58,59).
In addition, in order to ensure that a maximum amount of photons can be coupled to cores of the fibers, the direc~
tion of each fiber accepting the photons will have to be aligned (see Fig. 8). In one embodiment, the fibers in a stripe are mounted on a plane (64) perpendicular to the substrate (38) supporting the stripe lens (37). The plane ~` 26 -~

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(64) intersects the long axis of the stripe lens (45) at point (65). All of fibers (66-73) are aligned so that the surfaces of cores are facing the point (65). This arrange-ment will ensure that a maximum amount of photons from the incident beam (36) will be collected by the fibers because the incident light beam will be normal to the tip of the fiber accepting the light.

In applications for detection and determination of angle of incidence of a laser beam, it is preferable to add a narrow optical filter in front of the mask (38). The band pass value of this narrow optical filter should be equal to the wavelength of the laser beam to be detected so that only the laser beam may penetrate the mask and strike the stripe detector elements. All other stray light from the background will be blocked by the narrow optical filter so that unwanted electrical signals from the stripe detector elements are minimized. Furthermore, an anti-reflection layer may be deposited directly on the surfaces of the stripe lenses (37) in order to minimize loss of incident light due to reflection. The anti-reflection coating may be deposited on the substrate when the mask (38) is inverted so that the lens (37) is facing the detector fibers in the fiber array (66-73).

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For applications which require a large spatial angle of operation, fibers prepared by cleavage or polishing of ~ ~:
end tips, may not provide a suf f iciently large accept-~' ance angle. To improve this, a micro-lens may be formed on or af f ixed to the tip of each f iber to increase the accept-ance angle. Turning to the actual alignment of fibers so that they all face point (65) and maintaining proper spac-ing to produce the maximum signals for light direction determination, a micro fabrication technology may be used to form supports and guides for the fibers. The technology involves the etching of a silicon substrate to form grooves of different directions using microelectronic processes.

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Fibers are positioned in the grooves and fastened.

Given below are some illustrative examples for the invention. It is understood they are nonlimitative as far as the spirit and principle of this invention are con-cerned.

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Example 1 Process to Form a Stripe Lens This example gives a process for the fabrication of stripe lenses in the unit for angle of incidence determina-tion of a light beam. A microscope glass slide with dimen-sions of 1 mm x 25 mm x 25 mm is used as the substrate. A
layer of Al with a thickness of 1 micrometer is evaporated on one side of the glass slide by a conventional method.
After the evaporation, a layer of positive photoresist such as Shipley 1470 is spun coated at 5000 rpm. After a baking at 90C for 15 minutes in air, the photoresist is dried and the thickness, about 2.3 micrometers, is measured. A first opaque mask with two transparent stripes to define the two stripe lenses is then placed on the substrate with the ,~, photoresist. The photoresist is exposed to ultraviolet light and then developed to remove the exposed photoresist within the two stripes which have a width of 500 ~m and a length of 1 cm. After a brief baking, the Al layer in the two stripe regions is removed by ~tching in a solution of H3P04 (80% by volume), HN03 (5%) and H20 (15%) at 60C.
After the etching, the photoresist is removed by immersing the substrate in acetone. On the glass substrate, the above process yields an Al layer with two stripe windows, one perpendicular to the other.

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A new layer of Shipley 1470 photoresist is then spun on the Al-side of the substrate at 5000 rpm and baked at 90C for 10 minutes. The thickness of the photoresist is 2.3 micro- meters after the baking. The above process is repeated another 9 times to produce a thick photoresist layer with a total thickness of 23 micrometers. After this, a new photomask is aligned on the substrate, exposed to the ultraviolet light and developed to define two photoresist stripes which coincide with but slightly greater than the stripe Al windows. The substrate with the two photoresist stripes is then heated to 180C for 3 minutes and finally 1'~
cooled to room temperature. During the heating, the pho-toresist melts and flows to form convex stripe lenses. The focus length of the lenses is determined by placing the substrate under an optical microscope, illuminating the substrate from underneath and observing the focusing posi-tion. For stripe lenses with a width of 500 ~m, the focus length at 0.6 ~m wavelength is about 1.2 mm. When an inci-dent light beam at 0.67 micrometer is directed at the focus position, the width of the focused light stripe is about 3 ~m. Therefore, the intensity of the focused light stripe is at least 100 times greater than the intensity of the inci-dent light beam. Since the Shipley 1470 photoresist is transparent to light, the stripe lens may be used in de-vices for finding the angle of incidence of light beams in in the wavelength range from 0.6 to 2 micrometers.

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Example 2 Device with Two Stripe Lenses for Determination of Angle of Incidence of a Light Beam A detector array containing two segments is fabricat-ed on a Si substrate using well established p-i-n micro fabrication technology. Each segment contains six stripe elements with progressively finer pitch. The length of the stripe elements is 2.4 mm and the width 200 ~m. The separa-tion between adjacent stripes is 200 ~m. An Al electrical contact is fabricated for each stripe element. The two segments are perpendicular to each other with a separation of 5 mm. In all these elements, the odd number areas are active (or transparent) to the incident light and the even number areas are non-active (opaque, covered by Al~. For example, in the first stripes of both segments, the first half is active to the incident light and the second half is non-active. For the second stripes, the first quarter is active, the second quarter is non-active, the third quarter is active and the last quarter is non-active. The length of -active areas in the finest pitch element (the sixth ele-ment) is about 40 ~m. Four alignment markers are also fabricated on the same Si substrate at positions outside ;, . , f,l.'.,", .; ,~ : : :

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the two segments. After the detector fabrication, the Si substrate is scribed and mounted onto a package of elec-tronic circuits. Conducting wires are attached between the electrical contacts on the Si substrate and the package.

A glass substrate containing two stripe lenses is fabricated using the process given in Example 1. The two stripes lenses are perpendicular to each other with a width of 500 ~m, a length of 5 mm and a focus length of 1.2 mm.
There are also four alignment markers on the glass sub-strate which match the four markers on the Si detector array substrate. When the four alignment markers on the glass substrate are aligned with the four alignment markers on the detector array, the projection of the center of the first stripe lens will fall at the center of the first segment of the detector array and the projection of the center of the second stripe lens will fall at the center of the second segment of the detector array. Furthermore, the long axis of the first stripe lens is perpendicular to the long axis of the elements in the first detector segment and the long axis of the second stripe lens is perpendicular to the long axis of the elements in the second detector seg-ment. The glass substrate with the stripe lenses is aligned over the detector array contained in the package under an optical microscope using a micro manipulator. A distance of 1.2 mm (the same as the focus length of the stripe lenses) -32 ;
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is maintained by placing thin spacers between lenses.
Once the lenses are properly aligned, the position of the glass substrate is fixed on the package by applying epoxy.

The above described unit may be used to determine the angle of incidence of a light beam in the wavelength range from 0.6 to 1.2 micrometer. For operation in other wave-length regions, detector arrays fabricated using semicon-ductor orther than Si may be used.

The foregoing description is illustrative of the principles of the present invention. Numerous extensions and modifications thereof would be apparent to the person skilled in the art. Therefore, all such extensions and modifications are considered to be within the scope and spirit of the present invention.

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Claims

1. A method for the detection and determination of spatial direction of a light beam comprising;

-- installing on a package, a detector array consisting of at least a first photo detector segment and a second photo detector segment, said first photo detector segment con-tains at least one stripe detector element with a length of L1 and a total width of W1, said second photo detector segment contains at least one stripe detector element with a length of L2 and a total width of W2, long axis of stripe detector elements in said first photo detector segment is substantially perpendicular to the long axis of stripe detector elements of said second photo detector segment, -- installing on said package and over said detector array a stripe mask containing at least a first stripe window with a length of Lw1 and a width of Ww1 and a second stripe window with a length of Lw2 and a width Ww2, long axis of said first stripe window is substantially perpendicular to long axis of said second stripe window, -- adjusting the mask so that a distance, d, is maintained between said mask and detector array, said mask is parallel to the plane of said detector array, with projection of said first stripe window perpendicular to the long axis of the stripe detector elements of said first photo detector segment, projection of said second stripe window is per-pendicular to the long axis of stripe detector elements of said second photo detector segment, -- measuring electrical signals from detector stripe ele-ments in said first photo detector segment due to illumi-nation of a stripe light region defined by said first stripe window to determine a first angle, .THETA.. Said angle, .THETA., is defined by the normal line of said mask and projection of said light beam on a plane parallel to said normal line and long axis of elements in said first detector segment, -- measuring electrical signals from detector stripe ele-ments in said second photo detector segment due to illumi-nation of a stripe light region defined by said second stripe window to determine a second angle, ?. Said angle, ?, is defined by the normal line of said mask and projec-tion of said light beam on a plane parallel to said normal line and long axis of elements in said second detector segment, -- determining the presence of the light beam by taking the sum of all the electrical signals from said stripe detector elements in said first detector segment and said second detector segment, -- determining spatial direction of said light beam from said first angle, .THETA., and said second angle, ?, when said sum of all electrical signals exceeds a value, -- indicating said spatial direction of the light beam.

2. A method for the detection and determination of spatial direction of a light beam in Claim 1, wherein said first and second stripe windows in said mask are transparent to said incident light and are opaque in areas around said first and second stripe windows in said mask .

3. A method for the detection and determination of spatial direction of a light beam in Claim 1, wherein each of said stripe detector elements in first stripe detector segment contains areas of equal length, alternately active and non-active to said light beam, said length decreases from said first stripe detector element to second and subsequent stripe detector elements.

4. A method for the detection and determination of spatial direction of a light beam in Claim 1, wherein each of said stripe detector elements in second stripe detector segment contains areas of equal length, alternately active and non-active to said light beam, said length decreases from said first stripe detector element to second and subsequent stripe detector elements.

5. A method for the detection and determination of spatial direction of a light beam in Claim 1, wherein length L1 is not less than (2)(d)tan(.THETA.max), .THETA.max being the maximum allowable angle of e for said light beam and length L2 is not less than (2)(d)tan(?max), ?max being the maximum allowable angle of ? for said light beam.

6. A method for the detection and determination of spatial direction of a light beam in Claim 1, wherein length of said first stripe window, Lw1, is not less than W1+(2)(d) tan(?maX), ?max being the maximum allowable angle, ?, for said light beam.

7. A method for the detection and determination of spatial direction of a light beam in Claim 1, wherein length of said second stripe window, Lw2, is not less than W2+(2)(d) tan(.THETA.maX), .THETA.max being the maximum allowable angle, .THETA., for said light beam.

8. A method for the detection and determination of spatial direction of a light beam in Claim 1, further comprising a step of adding a narrow optical filter in front of said mask, band pass value of said narrow optical filter being equal to wavelength of said light beam.

9. A method for the detection and determination of spatial direction of a light beam comprising;

-- installing on a package a detector array consisting of at least a first photo detector segment and a second photo detector segment, said first photo detector segment con-tains at least one stripe detector element of length L1 and a total width W1, said second photo detector segment contains at least one stripe detector element of length of L2 and a total width W2, long axis of stripe detector elements in said first photo detector segment being sub-stantially perpendicular to long axis of stripe detector elements of said second photo detector segment, -- installing on said package and over said detector array a stripe mask containing at least a first stripe lens of length of Lw1, width Ww1, focus length f, and a second stripe lens of length of Lw2, width Ww2 and focus length, f, long axis of said first stripe lens is substantially perpendicular to long axis of said second stripe lens, -- adjusting the mask so that a distance, d, is maintained between said stripe lenses and detector array, said mask is parallel to plane of said detector array with projection of said first stripe lens being perpendicular to long axis of stripe detector elements of said first photo detector segment, projection of said second stripe lens being perpendicular to long axis of stripe detector elements of said second photo detector segment, -- measuring electrical signals from detector stripe ele-ments in said first photo detector segment due to illumi-nation of a stripe light region defined by said first stripe lens, to determine a first angle, .THETA.. Said angle, .THETA., is defined by the normal line of said mask and projection of said light beam on a plane, parallel to said normal line and long axis of elements in said first detector segment, -- measuring electrical signals from detector stripe ele-ments in said second photo detector segment due to illumi-nation of a stripe light region defined by said second stripe lens to determine a second angle, ?, said angle, ?, is defined by the normal line of said mask and projection of said light beam on a plane parallel to said normal line and long axis of elements in said second detector segment, -- determining the presence of light beam by taking the sum of all electrical signals from stripe detector elements in said first detector segment and said second detector segment, -- determining spatial direction of said light beam from said first angle, .THETA., and said second angle, ?, when said sum of all electrical signals exceeds a value, -- indicating said spatial direction of the light beam.

10. A method for the detection and determination of spa-tial direction of a light beam in Claim 9, wherein said first and second lenses in said mask are transparent to said incident light and are opaque in areas in said mask around said first and second lenses, said first and second lenses focus said light beam onto said detector array.

11. A method for the detection and determination of spa-tial direction of a light beam in Claim 9, wherein distance between said stripe lenses and said detector array, d, is substantially equal to focus length, f.

12. A method for the detection and determination of spa-tial direction of a light beam in Claim 9, wherein each of said stripe detector elements in first stripe detector segment contains areas of equal length, alternately active and non-active to said light beam, said length decreasing from said first stripe detector element to second and subsequent stripe detector elements.

13. A method for the detection and determination of spa-tial direction of a light beam in Claim 9, wherein each of said stripe detector elements in second stripe detector segment contains areas of equal length, alternately active and non-active to said light beam, said length decreasing from said first stripe detector element to second and subsequent stripe detector elements.

14. A method for the detection and determination of spa-tial direction of a light beam in Claim 9, wherein length L1 is not less than (2)(d)tan(.THETA.max), .THETA.max being the maximum allowable angle of .THETA. for said light beam and length L2 is not less than (2)(d)tan(?max), ?max being the maximum allowable angle, ?, for said light beam.

15. A method for the detection and determination of spatial direction of a light beam in Claim 9, wherein length of said first stripe lens, Lw1 is not less than W1+(2)(d) tan(?maX), ?max being the maximum allowable angle, ?, for said light beam.

16. A method for the detection and determination of spatial direction of a light beam in Claim 9, wherein length of said second stripe lens, Lw2, is not less than W2+(2)(d) tan(.THETA.max), .THETA.max being the maximum allowable .THETA. angle for said light beam.

17. A method for the detection and determination of spatial direction of a light beam in Claim 9, further comprising a step of adding a narrow optical filter in front of said mask, band pass value of said optical filter is substan-tially equal to wavelength of said light beam.

18. A method for the detection and determination of spa-tial direction of a light beam in Claim 9, further compris-ing a step of adding an anti-reflection coating layer on said stripe lenses to reduce reflection losses.

19. A method for the detection and determination of spa-tial direction of a light beam comprising;

-- installing a fiber array consisting of at least a first segment and a second segment, said first segment having a length of L1 and a total width of W1 contains at least one fiber element, said second segment having a length of L2 and a total width of W2 contains at least one fiber ele-ment, long axis of fiber elements in said first segment being substantially perpendicular to the long axis of fiber elements in said second segment, -- installing over said fiber array a stripe mask contain-ing at least a first stripe lens with a length of Lw1, a width Ww1, focus length f, and a second stripe lens with a length of Lw2, a width Ww2 and focus length f, the long axis of said first stripe lens being substantially perpen-dicular to long axis of said second stripe lens, -- adjusting the mask to maintain a distance, d, between said stripe lenses and fiber array, said mask is parallel to the plane of said fiber array with projection of said first stripe lens is perpendicular to long axis of fiber elements in said first fiber array segment, projection of said second stripe lens is perpendicular to long axis of fiber elements in said second fiber array segment, -- merging fibers in each fiber element into a single fiber and aligning said single fiber to a miniature optical detector, -- measuring electrical signals from miniature optical detectors in said first fiber array segment due to illumi-nation of a stripe light region defined by said first stripe lens to determine a first angle, .THETA.. Said angle, .THETA., is defined by the normal line of said mask and projection of said light beam on a plane parallel to said normal line and long axis of elements in said first detector segment, -- measuring electrical signals from miniature optical detectors in said second fiber array segment due to illu-mination of a stripe light region defined by said second stripe lens to determine a second angle, ?. Said angle, ?, is defined by the normal line of said mask and projection of said light beam on a plane parallel to said normal line and long axis of elements in said first detector segment, -- determining the presence of the light beam by taking the sum of all electrical signals from miniature optical detec-tors in said first detector segment and said second detec-tor segment, -- determining spatial direction of said light beam from said first angle, .THETA., and said second angle, ?, when said sum of all electrical signals exceeds a value, -- indicating said spatial direction of the light beam.

20. A method for the detection and determination of spa-tial direction of a light beam in Claim 19, wherein said first and second lenses in said mask are transparent to the incident light and are opaque in areas in said mask around said first and second lenses, said first and second lenses focus said light beam onto said detector array.

21. A method for the detection and determination of spa-tial direction of a light beam in Claim 19, wherein dis-tance between said stripe lenses and said fiber array is substantially equal to focus length, f.

22. A method for the detection and determination of spa-tial direction of a light beam in Claim 19, wherein each of said fiber elements in first fiber segment contains areas of equal length, alternately active and non-active to said light beam, said length decreases from said first fiber element to second and subsequent fiber elements.

23. A method for the detection and determination of spa-tial direction of a light beam in Claim 19, wherein each of said fiber elements in second fiber segment contains areas of equal length, alternately active and non-active to said light beam, said length decreases from said first fiber element to second and subsequent fiber elements.

24. A method for the detection and determination of spa-tial direction of a light beam in Claim 19, wherein length L1 is not less than (2)(d)tan(.THETA.max), .THETA.max being the maximum allowable angle, .THETA., for said light beam and length L2 is not less than (2)(d)tan(?max), ?max being the maximum allowable angle, ?, for said light beam.

25. A method for the detection and determination of spatial direction of a light beam in Claim 19, wherein length of said first stripe lens, Lw1 is not less than W1+(2)(d) tan(?max), ?max being the maximum allowable angle, ?, for said light beam.

26. A method for the detection and determination of spatial direction of a light beam in Claim 19, wherein length of said second stripe lens, Lw2, is not less than W2+(2)(d) tan(.THETA.max), .THETA.max being the maximum allowable angle, .THETA., for said light beam.

27. A method for the detection and determination of spatial direction of a light beam in Claim 19, further comprising a step of adding a narrow optical filter in front of said mask, band pass value of said narrow optical filter being substantially equal to wavelength of said light beam.

28. A method for the detection and determination of spatial direction of a light beam in Claim 19, further comprising a step of adding an anti-reflection coating layer on said stripe lenses to reduce reflection loss of said light beam.

29. A method for the detection and determination of spatial direction of a light beam in Claim 19, further comprising a step of adding a micro lenslet on the tip of each fiber to increase angle of acceptance of said fiber.

30. A method for the detection and determination of spatial direction of a light beam in Claim 19, further comprising a step of aligning fibers in said fiber segments, fibers in first segment are facing center of said first stripe lens and fibers in second segment are facing center of second stripe lens to increase coupling efficiency of said light beam.