OPTICAL BEAM SPLITTER

Abstract: An optical beam splitter which splits a single light beam (L o ) into a plurality of split beams (L i ) including a plurality of transparent plates (P i ) of uniform thickness arranged in a stack with fully reflective films coated on the transparent plates at either end of the stack and semi-reflective films (M i ) coated on the remaining transparent plates in the stack. The reflectivities of the individual semi-­ reflective films gradually increase from one end to the other in the stack and a light beam entering the stack is first entirely reflected by the rear-end fully reflective film (M o ), then passes through the semi-­ reflective films while being reflected partially by the semi-reflective films, and then finally entirely reflected by the front-end fully reflective film (M n ) so that a single beam is split into a plurality of parallel split beams.

Description: The present invention relates to an optical beam splitter used in recording halftone plate duplicate images. An optical beam splitter is known which is used in recording halftone plate duplicate images on a recording material by controlling a light-exposure device in accordance with image signals obtained by photoelectric manipulation of an original image so that a plurality of beams are independently modulated by the image signals. In such conventional method, a multiple number of light beams, which are lined up in a row, are independently modulated by image signals so that they are relatively scanned on the surface of recording materials. In most cases, the multiple number of light beams are obtained by reflecting a single light beam, which comes from an argon laser, on a plurality of fully reflective and semi-reflective mirrors so that the beam is split by these mirrors. The light beams thus obtained are independently modulated by a modulator, reduced in diameter by a crystal optical system and then directed onto the surface of the recording material. It is also prior art practice for a single light beam to be split into a multiple number of light beams lined up in a row by repeating internal reflections of the light beam using a single optical beam splitter. This method is disclosed, for example, in Japanese Patent Application Laid-Open Nos. 52-l22l35 and 58-l07l3. In this prior art, one surface of a single glass plate is coated with a fully reflective film, while the other surface of the glass plate is coated in different regions with semi-reflective films having different reflectivities. These semi-reflective film coatings are described in greater detail as follows: When a single light beam is to be split into ten (l0) light beams, for example, the first region is coated with a semi-reflective film having a reflectivity of 9/l0 (i.e., a transmissibility of l/l0): the next region is coated with a semi-reflective film having a reflectivity of 8/9 (i.e., a transmissibility of l/9). The next region is coated with a semi-reflective film having a reflectivity of 7/8 (i.e., a transmissibility of l/8, and the successive regions are coated with semi-reflective films having reflectivities gradually decreasing in a numerical series so that the last region is coated with a semi-reflective film having a reflectivity of l/2 (i.e., a transmissibility of l/2). The techniques in which a single laser beam generated by an argon laser is split into a multiple number of laser beams via mirrors and optical fibers as described above requires a large amount of space for installation of the apparatus. Also, adjustment of the mirrors and optical fibers requires a high degree of skill and considerable time and is therefore very inconvenient. The above-mentioned spatial drawbacks are eliminated by the optical beam splitters described in Japanese Patent Application Laid-Open Nos. 58-l07l3 and 52-122l35. In these optical beam splitters, however, in order for a single light beam to be split into ten (l0) light beams, semi-reflective coatings having different reflectivities as described above must be coated in nine extremely narrow regions, i.e. regions with a width of 2 mm or less. A flaw in the coating work, even in only one region, is not permissible, and therefore, this coating process is extremely difficult. The optical beam splitter disclosed in Japanese Patent Application Laid-Open No. 52-l22l35 is advantageous in that there is no need to increase the size of the crystal optical system even if the single beam is split into a large number of beams. However, since the split light beams are not parallel, the modulation efficiency of the modulator drops, and light leakage may occur. Furthermore, the formation of the acoustic electrodes of the modulator is very difficult. It is also difficult to maintain the thickness and angular position of the two surfaces of this optical beam splitter at prescribed values with an ultrahigh degree of precision. Thus, the manufacture of this optical beam splitter involves great difficulty. If the thickness and angular positions of the two surfaces of the optical beam splitter differ even slightly from the predetermined values, the focal distances of the various light beams become greatly different . Thus, the spacing between the optical beam splitter and modulator, as well as the spacing between the modulator and crystal optical system, will be unavoidably different in each individual apparatus. Accordingly, it is a primary object of the present invention to provide an optical beam splitter which allows the formation of large semi-reflective films so as to facilitate the formation of semi-reflective films with highly accurate reflectivities. It is a further object of the present invention to provide an optical beam splitter which does not require a large amount of space. It is a still further object of the present invention to provide a beam splitter which can split a light beam so that the light quantities of the resulting split beams are equal. The above-mentioned objects of the present invention are accomplished by a unique structure for optical beam splitters wherein a plurality of transparent plates (for example n [n=integer]) of uniform thickness (to a high degree) are stacked together in parallel. First and second fully reflective films are respectively applied on the two transparent plates which are at either end of the stack, and a plurality of semi-reflective films are respectively provided on the surfaces of the remaining transparent parallel plates of the stack. The semi-reflective films have respective reflectivities gradually increasing from the one at the rear-or back-end of the stack toward the front-end of the stack in numerical series such as l/n, l/(n-l), l/(n-2),..l/4, l/3 and l/2. The fully reflective films and semi-reflective films are arranged so that a light beam entering the stack of the transparent parallel plates is first entirely reflected by the fully reflective first film which is at the back-end of the stack and then passes through the semi-reflective films until it reaches the fully reflective second film so that the light beam is totally reflected by the second fully reflective film. Split light beams into which the original light beam is split when it passes through the stack of transparent parallel plates having semi-reflective films are reflected out of the stack of the transparent parallel plates. As to the semi-reflective films, since the reflectivity of those through which the reflective light beam passes is l/n, l/nth of the quantity of light of the light beam is reflected and split off toward the back end of the stack, while the remaining (n-l)/nths of the quantity of light of the light beam passes through the semi-reflective film and travels toward the next semi-reflective film. Since the reflectivity of the next semi-reflective film is l/(n-l), l/nth of the quantity of light of the light beam which has passed through the previous semi-reflective film is reflected and split off toward the back end of the stack, while the remaining (n-2)/nths of the quantity of light of the light beam passes through the next semi-reflective film, and so on. Thus, beam splitting is repeated as the remaining light passes through each successive semi-reflective film until the light reaches the fully reflective second film of the plate at the front end of the stack and is reflected toward the back end of the stack, thus producing a plurality of, for example in the number n , split light beams. In particular, since the reflectivities of the semi-reflective films increase gradually in a numerical series such as l/n, l/(n-l), l/(n-2),..l/4, l/3, l/2 as described above, each of the split light beams has l/nth of the quantity of light of the light beam, and the split light beams are radiated from the back end of the stack parallel to each other. Furthermore, the optical beam splitter of this invention can also be constructed so that transparent parallel plates are sandwiched between two additional transparent parallel plates to fully protect the reflective films from dust and allow both surfaces to be wiped. The present invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which: Fig. l (a) is a side view of a first embodiment of the optical beam splitter of the present invention; Fig. l (b) is a top view of the surface of a transparent plate which is used to manufacture the optical beam splitter of Fig. l(a); Fig. 2 (a) is a side view of a second embodiment of the optical beam splitter of the present invention; and Fig. 2 (b) is a top view of the surface of a transparent plate which is used to manufacture the optical beam splitter of Fig. 2 (a). In Fig. l (a), n + 2 (n being an integer) transparent plates are stacked together. These n + 2 transparent plates P o , P l , P₂, P₃,...P n-l , P n , and P n+l are obtained by cutting out a single oblong transparent plate P having a uniform thickness into n + 2 plates along the single-dot chain lines shown in Fig. l(b). Thus, the thicknesses of the transparent plates P o through P n+l are exactly the same to a high degree of precision. Before the transparent plate P, shown in Fig. l(b), is cut into n + 2 pieces of individual plates P o to P n+l , a fully reflective first film M o is applied to the plate P o , semi-reflective films M l , M₂, M₃,..M n-2 , and M n-l are respectively applied to the plates P l to P n-l , and another fully reflective second film M n is applied to the plate P n . These films have different sizes and are applied to the plates in different prescribed positions. The application of the films on each plate can be achieved in any coating methods. In this embodiment, the beam splitter is constructed so that ten split light beams are obtained (n = l0), and the transparent plate P is cut into l2 pieces. The reflectivity of the semi-reflective film M l is set at l/l0 (i.e., a transmissibility of 9/l0), the reflectivity of the semi-reflective film M₂ is set at l/9 (i.e., a transmissibility of 8/9), the reflectivity of the semi-reflective film M₃ is set at l/8 ( i.e., a transmissibility of 7/8), and so on, so that the reflectivity of the semi-reflective film M n-2 is set at l/3 (i.e., a transmissibility of 2/3) and the reflectivity of the semi-reflective film M n-l is set at l/2 (i.e., a transmissibility of l/2). Thus, each of the ten split light beams L l , L₂, L₃... L n-l , and L n shown in Fig. l(a) has l/l0 of the quantity of light of the light beam L o . The width of the semi-reflective film M l is set equal to the small dimension h shown in Fig. l (a). The width of the semi-reflective film M₂ is 3/2h, the width of the semi-reflective film M₃ is 2h, and so on, so that the width of the semi-reflective film M n-2 is 4h + l/2h, and the width of the semi-reflective film M n-l is 5h. These semi-reflective films M l to M n-l are all applied with their respective left ends precisely aligned. The fully reflective first film M o is formed as a large film with the right end thereof pulled in by a distance of l/2h from the right end of the semi-reflective film M l , and with the left end of the film M o extending to the vicinity of the left edge of the plate P. The fully reflective second film M n is also formed as a large film with the left end thereof aligned with the left end of the semi-reflective film M n-l and the right end extending to the vicinity of the right edge of the transparent plate P. The transparent plate P coated with the reflective films M o to M n is cut along the single-dot chain lines shown in Fig. l(b) and split into l2 pieces of plates, and these plates are stacked together as shown in Fig. l (a). Plates having reflective films M o through M n are arranged in a 3-dimensional stack such that a light beam L o entering the stack of transparent parallel plates at a prescribed oblique angle from one end (front-end) of the stack is not interrupted by the fully reflective second film M n or the semi-reflective films M l to M n-l . With the above-described structure, the light beam L o reaches the back-end fully reflective first film M o and is entirely reflected toward the front-end of the stack so that the light beam L o passes through all of the semi-reflective films M l , M₂, M₃, ...M n-2 . At each of the semi-reflective films M l , M₂, M₃...M n-2 , a split light beam L l , L₂,L₃, ... and L n-2 is reflected toward the back of the stack, in other words towards the back-end transparent plate P o . The light reaching the semi-reflective film M n-l is reflected and split into a split light beam L n-l which travels back toward the back-end of the stack and a split light beam L n travels toward the front-end of the stack, in other words towards the transparent plate P n+1 . Finally, the split light beam L n is entirely reflected by the fully reflective second film M n so that beam L n travels back toward the back-end of the stack. More specifically, the light beam which enters the stack of the transparent parallel plates at a prescribed oblique angle X from the front-end of the stack is first entirely reflected by the fully reflected first film M o at the back-end of the stack and directed toward the semi-reflective films located in front of the fully reflective second film M n . Since the reflectivity of the semi-reflective film M l through which the reflected light beam passes is l/n, l/nth of the quantity of light of the light beam is reflected and split off toward the back-end of the stack, while the remaining (n- l)/nths of the quantity of light of the light beam passes through the semi-reflective film M l and travels toward the next semi-reflective film M₂. Since the reflectivity of the next semi-reflective film M₂ is l/n - l), l/nth of the quantity of light of the light beam which has passed through the previous semi-reflective film M₂ is reflected and split off toward the back end of the stack, while the remaining (n - 2)/nths of the quantity of light of the light beam passes through the semi-reflective film M₂, and so on. Thus, similar beam splitting is repeated as the remaining light passes through the successive semi-reflective films until the light reaches the fully reflective second film M n of the plate P n at the front-end of the stack. The fully reflective second film M n then reflects the light beam which has passed through the semi-reflective films toward the back-end of the stack, thus producing n split lights. In particular, since the reflectivities of the semi-reflective films increase gradually in numerical series such as l/n, l/ (n - l), l/(n - 2),..l/4, l/3, and l/2 as described above, each of the split light beams has l/nth of the quantity of light of the light beam. Furthermore, since the reflectivity of the semi-reflective film M n-2 is l/3, a split light beam L n-2 with l/nth of the quantity of light beam L o is split off toward the back-end of the stack by this semi-reflective film M n-2 , while a light beam with the remaining 2/nths of the quantity of light of the light beam L o passes through the semi-reflective film M n-2 and travels toward the semi-reflective film M n-l . Since the reflectivity of the semi-reflective film M n-l is l/2, a split light beam L n-l with l/nth of the quantity of light of the light beam L o is split off toward the back end of the stack by the semi-reflective film M n-l , while the remaining l/nth of the quantity of light of the light beam L o passed through the semi-reflective film M n-l and travels toward the front-end fully reflective film M n . Further, since the semi-reflective film M l to M n-l consist of a group of semi-reflective films whose reflectivities increase gradually in numerical series l/n, l/(n-l), l/(n- 2),...l/4. l/3, and l/2 as described above, each of the abovementioned split light beams L l to L n has l/nth of the quantity of light of the light beam L o , and the split light beams L l to L n are radiated from the back end of the stack parallel to each other. The small dimension h shown in Fig. l(a) is determined by the angle of travel α of the light beam L o through the stack and the thickness t of the transparent plates. Specifically, the dimension h is determined by the following equation: h = 2t x tan α Furthermore, the travelling angle X of the light beam L o travelling through the stack is determined by the angle of incidence B of the light beam L o and the refractive index of the transparent plate P. Accordingly, the space C of the split light beams is determined by the angle of incidence of the light beam L o , the refractive index β of the transparent plate P and the thickness t of the transparent plate P. Thus, the thickness t of the transparent plate P can be fixed by determining the space C of the split light beams, the angle of incidence β of the light beam L o and the material of the transparent plate P. In this way, the manufacturing conditions of the multi-coated plate shown in Fig. l(b) can all be determined. Figs. 2(a) and 2(b) illustrate a second embodiment of the optical beam splitter of this invention. The optical beam splitter of Fig. 2(a) is constructed in the same manner as the one shown in Fig. l (a) except for the size of the reflective films. In particular, twelve transparent plates P o , P l , P₂, P₃, ... P n-l , P n , and P n+l are stacked together, and the reflective films M o to M n on the plates P o to P n all have a width h and are successively shifted by a distance l/2h. The thickness of the transparent plates P o to P n+1 are uniform to a high degree. The transparent plate P of Fig. 2(b) which is oblong in shape has a uniform thickness, is cut along the single-dot chain lines and is thus split into twelve plates. These twelves plates are then stacked together, and before the transparent plate P is split, the plate P is coated with the fully reflective film M o , semi-reflective films M l , M₂, M₃, ...M n-2 , and M n-l fully reflective film M n . The fully reflective film M o entirely reflects the light beam L o entering from the front of the stack back toward the front-end of the stack. Each of the semi-reflective films M₂, M₃,...M n-2 splits off a split light beam L l through L n-2 (which has l/nth of the quantity of light of the light beam L o ) toward the back-end of the stack. The semi-reflective film M n-l splits off a split light beam L n-l which has l/nth of the quantity of light of the light beam L o toward the back-end of the stack and allows another split light beam L n which has l/nth of the quantity of light of the light beam L o to travel toward the front-end of the stack. Then finally, the fully reflective second film M n entirely reflects the split light beam L n toward the back-end of the stack. The optical beam splitters described above can also be constructed so that stacked transparent parallel plates are sandwiched between two additional transparent parallel plates to fully protect the reflective films from dust and allow both surfaces to be wiped. Accordingly, the optical beam splitter according to the present invention has the following advantageous features: it can split a beam into split beams having an equal quantity of light; the semi-reflective films can be formed as large films so that accurate reflectivities can be obtained; and a large amount of space is not necessary to install the splitter.

Claims: An optical beam splitter comprising a plurality of transparent plates (P o , P l ....P n ) of uniform thickness to a high degree of precision assembled in a stack; first and second fully reflective films (M o , M n ) respectively provided on the surfaces of said transparent plates (P o , P n ) which are at either end of said stack; and a plurality of semi-reflective films M l , ....M n-l ) respectively provided on the surfaces of adjacent transparent plates located between the end plates in the stack, the respective reflectivities of said semi-reflective films (M l ,...M n-1 ) gradually increasing from the plate (P l ) closest to the rear end plate of said stack to the plate (P n ) closest to the front end plate of said stack in a numerical series l/n, l/(n-l), l/(n-2), ...l/4, l/3 and l/2, said fully reflective films (M o , M n ) and said semi-reflective films (M l ...M n-l ) having prescribed sizes and being applied to the plates in prescribed positions such that a light beam (L o ) entering said stack of transparent plates at a prescribed angle from the front end of said stack is first entirely reflected by said rear-end fully reflective film (M o ) and then passes through said semi-reflective films (M l ...M n-l ) to be entirely reflected by said front-end fully reflective film (M n ) so that the light beams (L l ....L n ) into which said entering light beam (L o ) is split by the reflection at said semi-reflective films (M l ....M n-l ) and said fully reflective films (M o , M n ) are reflected out of said stack. An optical beam splitter according to claim l, characterised in that said stack of transparent plates are sandwiched between two additional transparent plates.

Inventors: TANIURA, HIROSHI
Applicants: THINK LABORATORY CO., LTD.
Application year: 1989
Main classification: G02B27
IPC codes: G02B27