OPTICAL BEAM SPLITTER PRISM

Abstract: An improved geometry for an optical beam splitter prism (59) permits all input and output beams (56, 57, 58) to be parallel and to remain parallel while the prism (59) is misaligned. It is particularly useful in con­ junction with fiber optic couplers and is applicable to splitting with respect to light intensity, polarization or wavelength. This can be achieved through the construction of a beam splitting prism having an addi­ tional reflecting surface on one input beam and on one output beam, whereby each of these surfaces are parallel to the beam splitting surface and with two parallel input and output surfaces through which all beams pass so as to make all input and output beams parallel and remain parallel as the prism is translationally or rotationally misaligned about any axis.

Description: This invention relates to a birefringent filter type wavelength multiplexer/demultiplexer. The object of the invention is to provide a birefringent filter type wavelength multiplexer/demultiplexer in which all input and output beams are parallel and remain parallel even if the optical component of said multiplexer are rotationally misaligned about any axis. The object of the invention is solved with a multiplexer having the features specified in claim 1. According to the invention as claimed, a birefringent filter type wavelength multiplexer/demultiplexer includes a housing for retaining various optical components such as beam splitter prisms and a birefringent element. The housing has a pair of opposed walls whose external surfaces are parallel to each other, each wall having a pair of orifices therethrough. First and second expanded beam lens connectors, each having a lens for projecting light from a suitable source (such as optical fiber), are coupled to the external surface of one housing wall and aligned therewith via respective flat reference surfaces on the connectors, which reference surfaces are perpendicular to the optical axes thereof. Third and fourth expanded beam connectors are coupled to the external surface of the other housing wall so as to provide outputs to optical fiber coupled thereto. Reference surfaces of the connectors engage the flat external surface of the aforesaid other housing wall. The flat reference surfaces are perpendicular to the axes of the third and fourth connectors. All four connectors are coupled with their reference surfaces in engagement with the external surfaces of the housing at the various orifices thereof. A first beam splitter prism receives a first, applied, input expanded beam of light along a first optical axis; the second expanded beam of light is applied along the second optical axis as a second input beam thereto, thereby providing a pair of parallel split beams each having different pairs of polarization states from the first expanded and the second expanded beam. A birefringent element, oriented to receive the parallel split beams, issues a pair of parallel beams. One of the issued parallel beams is applied as one input beam to a second beam splitter prism; the other of the issued parallel beams is applied as the other input of the second beam splitter prism, thereby passing energy at one wavelength from the first connector to the third connector and from the second connector to the fourth connector. Another wavelength is applied from the first connector to the fourth connector and from the second connector to the third connector. Thus, the first expanded beam and the second expanded beam entering the first beam splitter prism, and the third expanded beam and the fourth expanded beam exiting from the second beam splitter prism, are parallel and remain parallel even if the prisms are translationally or rotationally misaligned. Each of said beam splitter prism includes a pair of prism sections joined together at an optical interface within which a partially transmitting optical coating is present. Both prisms sections having a respective, planar, full reflective surface; and where each of the prism sections has a respective, planar, optical interface surface joined together within which the coating is present, wherein each of the planar fully reflective surfaces is parallel to each of the planar, optical interface surfaces and to each other. The prism has planar input and output surfaces that are parallel to each other. Thus, an input beam entering the input surface of the prism and an output beam exiting from the output surface are parallel, and remain parallel, as the prism is translationally or rotationally misaligned. In accordance with a preferred embodiment, the foregoing partially transmitting optical coating can be selected from the group consisting of a thin metallic layer, a multilayer thin film stack of dielectric coatings, and a combination thereof. According to the function of the birefringent filter type wavelength multiplexer/demultiplexer two input beams and two output beams of the said beam splitter prisms have components having a first polarization characteristic and a second polarization characteristic. As the input beams reach the optical interface, those components having the first characteristic are transmitted therethrough, and those having the second characteristic are reflected by the optical interface. One output beam contains the first characteristic of the first input beam and the second characteristic of the second output beam. The other output beam contains the first characteristic of the second input beam and the second characteristic of the first input beam. A beam splitter prism is known from FR-A-2 516 254 consisting of two prisms (PA,PB) which are joined at an interface having a partially transparent coating and one of the prisms (PB) have a reflective surface (R-2) parallel to the interface, whereby said prism has a planar input surface (T-1) and a parallel output surface (TA-1). FIG. 1A is a design of an optical beam splitter used in the present invention; FIG. 1B is a view depicting the beam splitter prism of Fig. 1A slightly rotated; FIG. 2 is a diagram of a birefringent filter type wavelength multiplexer in accordance with the present invention and incorporating the optical beam splitter. A beam splitter as used in the multiplexer according to the invention is depicted in Fig. 1A. It consists of two prism sections 21 and 22 joined together, as by cement, at an interface 23 within which is present a partially transmitting coating. This coating may be a thin semitransparent metallic film or a multilayer thin film stack of dielectric coatings or a combination thereof to unselectively reflect part of the light energy or to selectively reflect one polarization state or one band of wavelengths and transmit the remaining light energy. Prism facets 24 and 26 are precisely ground and polished to be parallel to each other and to the interface 23. Prism facets 27 and 28, which extend across the interface onto the section 22, are also ground and polished to be parallel to each other. An input light beam 29 enters the prism section 21 and is reflected by the facet 24. It is partially reflected at the interface 23 and exits as an output beam 31 through the facet 28. The remaining light energy is transmitted into the prism section 22, reflected at the facet 26 and exits as an output beam 32, also through the facet 28. The reflecting surfaces 24 and 26 may be given mirror coatings or the reflection may be Via total internal reflection if a suitably large refractive index material is chosen to construct the prism sections 21 and 22. An alternate input light beam 33 can enter the prism section 22, wherein it is partially transmitted at the interface 23 into the prism section 21 and exits as an output 31 through the facet 28. The remaining light energy is reflected at the interfaces 23 and 26 and exits from the prism section 22 as an output beam 32. Parallel prism facets 23, 24, and 26 are depicted at a nominal 45° orientation to the parallel prism facets 27, 28. However, other angles are also suitable. Along each path through the beam splitter, depicted in Fig. 1, light undergoes two refractions at the parallel facets 27, 28 and undergoes an even number (0 or 2) of reflections at the parallel facets 23, 24, or 26. When an entire beam splitter is rotationally misoriented about any axis, including the axis normal to the drawing, as depicted in Fig. 1B, the dual refractive and dual reflective beam deviations cancel each other and the output beams 31 and 32 remain parallel to the input beams 29 and 33. Thus, the parallelism of all the input and output beams is dependent only on the degree of parallelism between the facets 27 and 28 and among the facets 23, 24, and 26. Optical finishing techniques for achieving such parallelism are well known in the art. Lateral deviations of the output beams as the prism is rotated or translated can be readily calculated with the help of a geometric construction (not shown). There, the optical path, from where the input beam 29 enters the facet 27 to where the output beam 32 exits the facet 28, is equivalent to a straight passage of a beam through a parallel plate having a thickness t equal to the path length through the original beam splitting prism. The lateral beam shift d resulting from moving i degrees off normal incidence can be shown to be d = t(tan i - tan r) cos i where r is the angle of refraction that may be computed using the law of refraction. From this parallel plate model it is also apparent that small lateral misalignments of the beam splitting prism are insignificant on the output beams. The birefringent filter type wavelength multiplexer or demultiplexer, according to the invention is depicted in Fig. 2. As shown therein, expanded beam lens connectors 61, 62, 63, and 64 attached to the ends of optical fibers 66, 67, 68, and 69 are pressed into contact with a metal housing 71 such that their forward flat reference surfaces 72, 73, 74, and 76 coincide with either of the flat and parallel alignment surfaces 77 and 78 of the housing 71. As above, this insures that beams 79, 81, 82 and 83 are precisely parallel. Beam splitter prisms 84 and 86, constructed as in Fig. 2, intercept these beams and connect their paths which also pass through a birefringent element 87 that is designed with precisely parallel optical facets 88 and 89. Because of this parallelism and that of the beam splitter facets, these three optical elements 84, 86, and 87 may be installed into the housing 71 with considerable tolerance to both translational and rotational misorientation. When used as a part of a birefringent filter type optical multiplexer/demultiplexer assembly, as shown in Fig. 2 , the beam splitters 84 and 86 selectively reflect one polarization state and transmit the other at the interface 23. The design of such multilayer films for the interface is well known in the art. The birefringent element 87 may be a single crystal of a crystalline material such as quartz or calcite oriented with its principal axes of birefringence at an angle of 45 degrees relative to the principal polarization axes of beam splitters 84 and 86. This birefringent multiplexer/demultiplexer passes energy at one wavelength from the fiber 67 to the fiber 66 and from the fiber 68 to the fiber 69 and at another wavelength from the fiber 67 to the fiber 69 and from the fiber 68 to the fiber 66. These inputs do not need to have a specific polarization state as both polarization components are processed simultaneously by this device. The foregoing are but two examples of how the instant invention can be incorporated into fiber optic devices. Various modifications can be performed without departing from the scope of this invention.

Claims: A birefringent filter type wavelength multiplexer/demultiplexer comprising: a housing (71) for retaining optical components therewithin, said housing including a pair of opposed walls having internal and external surfaces, said external surfaces (77) and (78) being parallel to each other, each of said walls having a pair of orifices therethrough; a first expanded beam lens connector (62) having a lens for projecting a first expanded beam of light along a first optical axis, and a flat reference surface (73) perpendicular to said first axis; a second expanded beam lens connector (63) having a lens for projecting a second expanded beam of light along a second optical axis, and a flat reference surface (74) perpendicular to said second axis; a third expanded beams lens connector (61) having a lens for receiving a third expanded beam of light from along a third optical axis, and a flat reference surface (72) perpendicular to said third axis; a fourth expanded beam lens connector (64) having a lens for receiving a fourth expanded beam of light from along a fourth optical axis, and a flat reference surface (76) perpendicular to said fourth axis, said first connector (62) being coupled to said housing (71) with said first connector reference surface (73) in engagement with the external surface of one of said walls (77) at one of said orifices thereof, said second connector (63) being coupled to said housing (71 ) with said second connector reference surface (74) in engagement with the external surface of said one of said walls (77) at the other of said orifices thereof, said third connector (61) being coupled to said housing (71) with said third connector reference surface (72) in engagement with the external surface of the other of said walls (78) at one of said orifices thereof, and said fourth connector (64) being coupled to said housing (71) with said fourth connector reference surface (76) in engagement with the external surface of the other of said walls (78) at the other of said orifices thereof, whereby said first optical axis is parallel to said second optical axis, said third optical axis is parallel to said fourth optical axis, and said first optical axis is parallel to said third and said fourth optical axes; said optical components comprising a first and a second beam splitter prism (84,86) each comprising a partially transmitting optical coating (23) that divides incident light into beams having different polarization states, and each of said prisms (84,86) comprises two prism sections (21,22) joined together at an optical interface (23) where said coating is present, whereby both of said prism sections (21,22) of each of said prisms have a respective planar fully reflective surface (24,26) and each of said prism sections (21,22) has a respective planar optical interface surface (23) the prism sections being joined together to form the interface where said coating is present, wherein each of said planar fully reflective surfaces of each prism (24,26) is parallel to its corresponding planar optical interface surface (23) and to the other planar fully reflective surface; and each prism has a planar input surface (27 or 28) and a planar output surface (28 or 27), wherein said input surface (27 or 28) is parallel to said output surface (28 or 27), said first expanded beam of light (81) propagating along said first optical axis being applied as a first input beam and said second expanded beam (82) of light propagating along said second optical axis being applied as a second input beam to said first beam splitter prism (84), thereby providing a pair of parallel split beams each having different pairs of polarization states from said first expanded beam (81) and second expanded beam (82); a birefringent element (87), oriented to receive said parallel split beams, and to issue a pair of parallel output beams, one of the issued parallel beams being applied as one input beam and the other of the issued parallel beams being applied as an other input beam to said second beam splitter prism (86), the birefringent element being such that it passes energy at one wavelength from said first connector (62) to said third connector (61) and from said second connector (63) to said fourth connector (64), and at another wavelength from said first connector (62) to said fourth connector (64) and from said second connector (63) to said third connector (61), whereby said first expanded beam (81) and said second expanded beam (82) entering said first beam splitter prism (84), and said third expanded beam (79) and said fourth expanded beam (83) exiting said second beam splitter prism (86) all being parallel, and remaining parallel, even if said prisms (84) and (86) are translationally or rotationally misaligned about any axis. The birefringent filter type wavelength multiplexer/demultiplexer as claimed in claim 1, wherein said partially transmitting optical coating is selected from the group consisting of a thin metallic layer, a multilayer thin film stack of dielectric coatings, and any combination thereof. The birefringent fitter type wavelength multiplexer/demultiplexer as claimed in claim 1 or 2, wherein the first input light beam (29) applied to said planar input surface (27) is reflected by said reflective surface (24) of one of said prism sections to said optical interface (23), wherein light having a first polarization characteristic is transmitted through said optical interface (23) to said reflective surface (26) of the other of said prism sections (22), and reflected outward through said output surface (28) as a first component of a first output light beam (32) having said first characteristic, and light having a second polarization characteristic is reflected by said coating (23) through said output surface (28) as a first component of a second output light beam (31) having said second characteristic; and the second input light beam (33) applied to said planar input surface (27) is directed to said optical interface (23), wherein light from said second input beam (33) having said second characteristic is reflected by said optical coating to said reflective surface (26) of the other of said prism sections (22), and reflected outward through said output surface (28) as a second component of said first output light beam (32) having said second characteristic, and light from said second input beam (33) having said first characteristic is transmitted through said optical coating, through said output surface (28) as a second component of said second output light beam (31), having said first characteristic.

Inventors: BUHRER, CARL F.
Applicants: GTE LABORATORIES INCORPORATED
Application year: 1986
Main classification: G02B6
IPC codes: G02B6, G02B27, G02B5