Optical Polarization Diversity Receiver

Abstract

A tri-mask optical polarization diversity receiver with a single input terminal and three output terminals prevents polarization induced signal fade, and may be used in an optical interferometry system for coherent detection. The device is composed of optical collimators, non-polarizing beam splitters, linear polarizers and photodetectors. In addition, the structural design incorporates two mechanically identical modulets, as well as a beam displacement compensation mechanism for ease of alignment and assembly. Compared to fiber-based design, the free-space configuration gets rid of inevitable birefringence in fused fiber couplers which detrimentally alter the polarization state received by the polarizers. As a result, it facilitates effective and precise measurements of optical interference with optimized visibility.

Claims

1. An optical polarization diversity receiver assembly, comprising: an optical collimator; a first non-polarizing beam splitter; a second non-polarizing beam splitter; a first linear polarizer; a first photodetector; a second linear polarizer; a second photodetector; a third linear polarizer; and a third photodetector, wherein, during operation: with an input of an optical dual-beam interference signal launched into the optical collimator, a collimated beam emerges and propagates in a free space, the collimated beam is divided into three beams with lowered power and directed along different paths upon encountering the first non-polarizing beam splitter followed by the second non-polarizing beam splitter, towards the first, second and third linear polarizers, permissible polarization transmission axes of the first linear polarizer, the second linear polarizer and the third linear polarizer are set apart from each other by a predetermined angle, with a respective transmitted beam manifesting interference across each corresponding axis, three separate beams generated from the first linear polarizer, the second linear polarizer and the third linear polarizer are coupled correspondingly into the first photodetector, the second photodetector and the third photodetector to be converted into electrical signals, and a maximum electrical signal among the electrical signals is selected for analysis, and each of the three separate beams in the optical polarization diversity receiver assembly maintains a same polarization state as that of the input when entering the respective linear polarizer by propagating through the free space or a polarization-independent media.

2. The optical polarization diversity receiver assembly of claim 1, wherein the optical collimator comprises a convex lens.

3. The optical polarization diversity receiver assembly of claim 1, wherein the optical collimator comprises a gradient-index lens.

4. The optical polarization diversity receiver assembly of claim 1, wherein each of the first non-polarizing beam splitter and the second non-polarizing beam splitter comprises a plate beam splitter.

5. The optical polarization diversity receiver assembly of claim 1, wherein each of the first non-polarizing beam splitter and the second non-polarizing beam splitter comprises a cube beam splitter.

6. The optical polarization diversity receiver assembly of claim 1, wherein each of the first linear polarizer, the second linear polarizer and the third linear polarizer comprises a birefringent crystal.

7. The optical polarization diversity receiver assembly of claim 1, wherein each of the first linear polarizer, the second linear polarizer and the third linear polarizer comprises a dichroic filter.

8. The optical polarization diversity receiver assembly of claim 1, wherein each of the first linear polarizer, the second linear polarizer and the third linear polarizer comprises a Brewster polarizer.

9. The optical polarization diversity receiver assembly of claim 1, wherein each of the first linear polarizer, the second linear polarizer and the third linear polarizer comprises a wire grid polarizer.

10. The optical polarization diversity receiver assembly of claim 1, wherein two mechanically identical modulets are incorporated.

11. An optical polarization diversity receiver assembly, comprising: a first optical collimator; a first non-polarizing beam splitter; a second non-polarizing beam splitter; a first linear polarizer; a second optical collimator; a second linear polarizer; a third optical collimator; a third linear polarizer; and a fourth optical collimator; wherein, during operations: with an input of an optical dual-beam interference signal launched into the first optical collimator, a collimated beam emerges and propagates in a free space, the collimated beam is divided into three beams with lowered power and directed along different paths upon encountering the first non-polarizing beam splitter followed by the second non-polarizing beam splitter, towards the first, second and third linear polarizers, permissible polarization transmission axes of the first linear polarizer, the second linear polarizer and the third linear polarizer are set apart from each other by a predetermined angle, with a respective transmitted beam manifesting interference across each corresponding axis, three separate beams generated from the first linear polarizer, the second linear polarizer and the third linear polarizer are coupled correspondingly into the second optical collimator, the third optical collimator and the fourth optical collimator, the second optical collimator, the third optical collimator and the fourth optical collimator send optical signals through optical fibers correspondingly to a first external photodetector, a second external photodetector and a third external photodetector located remotely, to convert optical signals into electrical signals with a maximum electrical signal among the electrical signals selected for analysis, and each of the three separate beams in the assembly maintains a same polarization state as that of the input when entering the respective linear polarizer by propagating through the free space or a polarization-independent media.

12. The optical polarization diversity receiver assembly of claim 11, wherein each of the second optical collimator, the third optical collimator and the fourth optical collimator comprises a convex lens.

13. The optical polarization diversity receiver assembly of claim 11, wherein each of the second optical collimator, the third optical collimator and the fourth optical collimator comprises a gradient-index lens.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings are included to aid further understanding of the present disclosure, and are incorporated in and constitute a part of the present disclosure. The drawings illustrate a select number of embodiments of the present disclosure and, together with the detailed description below, serve to explain the principles of the present disclosure. It is appreciable that the drawings are not necessarily to scale, as some components may be shown to be out of proportion to size in actual implementation in order to clearly illustrate the concept of the present disclosure.

[0015] FIG. 1 is a diagram showing the structure of an optical polarization diversity receiver assembly and the associated beam propagation, in accordance with one embodiment of the present disclosure.

[0016] FIG. 2 is a diagram showing the components comprising a modulet in the design, in accordance with one embodiment of the present disclosure.

[0017] FIG. 3 is a diagram showing the compensation of beam displacement in the design, in accordance with one embodiment of the present disclosure.

[0018] FIG. 4 is a diagram showing the structure of an optical polarization diversity receiver assembly and the associated beam propagation, in accordance with the other embodiment of the present disclosure.

[0019] FIG. 5 is a diagram showing the components comprising a modulet in the design, in accordance with the other embodiment of the present disclosure.

[0020] FIG. 6 is a diagram showing the compensation of beam displacement in the design, in accordance with the other embodiment of the present disclosure.

[0021] FIG. 7 is a diagram showing the orientation of the transmission axes of the three polarizers of the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Overview

[0022] Various implementations of the present disclosure and related inventive concepts are described below. It should be acknowledged, however, that the present disclosure is not limited to any particular manner of implementation, and that the various embodiments discussed explicitly herein are primarily for purposes of illustration.

[0023] Various proposed designs, schemes and embodiments in accordance with the present disclosure of a compact optical polarization diversity receiver, which generates separate interference signals across some distinct polarization axes, are described in detail below. This is achieved via a free-space beam propagation design to avoid birefringence in optical waveguides and ensure that the state of polarization arriving at all the polarizers is identical.

[0024] The following provides a description of the working principle of the designed optical tri-mask polarization diversity receiver, which has an input port and three output ports. Initially, an optical dual-beam interference signal is collimated by an optical collimator at the input port, which propagates within non-birefringent media and is partitioned into three distinct beams by non-polarizing beam splitters. Each beam keeps the same polarization state and is redirected towards its respective linear polarizer, where the interference signal across the polarization transmission axis is extracted and then coupled into either a photodetector or an optical fiber. The three polarizers are strategically set with their axes 60 degrees apart, allowing for the selection of the most substantial output for post-processing.

[0025] One embodiment of the optical polarization diversity receiver is depicted in FIG. 1. The assembly 1000 comprises an optical collimator 1120 having an optical fiber pigtail 1121, a first non-polarizing beam splitter 1130, a second non-polarizing beam splitter 1140, a first linear polarizer 1150, a second linear polarizer 1160, a third linear polarizer 1170, a first photodetector 1180, a second photodetector 1190, and a third photodetector 1200.

[0026] The optical collimator 1120 comprises a lens to collimate an input divergent light beam emerging from the fiber pigtail 1121. Typically, the fiber 1121 is secured inside a ferrule. Examples of lenses include convex lens and gradient-index lens.

[0027] Each of the first non-polarizing beam splitter 1130 and the second non-polarizing beam splitter 1140 incorporates a coating layer to split an incident light beam into two spatially separate beams at a designated ratio irrespective of the polarization state. A specific portion of the incoming light power is reflected by the coating while the remainder transmits through it. For example, a tap thin-film filter is a plate of substrate with partially reflecting coating on one surface facing towards the incident beam. Alternatively, a cube beam splitter can be employed, which is constructed by cementing two prisms together where the coating is at the internal interface. Ideally, to divide the input power evenly so the three outputs each obtains of the total power, the reflection ratio of the first beam splitter 1130 should be , and the second beam splitter 1140 should have a reflection ratio of . The coating layers of 1130 and 1140 need to be orientated properly to align the reflected beams with the respective output photodetectors 1180 and 1190.

[0028] The first linear polarizer 1150, the second linear polarizer 1160 and the third linear polarizer 1170 positioned perpendicularly to the optical axis only allow a specific linear polarization component of light waves to traverse while the orthogonal component is blocked by absorption or reflection. Various types of linear polarizers can be used including, but not limited to, birefringent crystals, dichroic filters, Brewster polarizers and wire grid polarizers. When two beams are coherently combined and pass through the linear polarizer, the output is essentially the interference projected along the polarizer's transmission axis. FIG. 7 illustrates the orientations (in any sequence) of the three polarizers, with the polarization axes 3150, 3160 and 3170 separated equally by 60 degrees.

[0029] The first photodetector 1180, the second photodetector 1190 and the third photodetector 1200 function as the output receivers that convert optical signals into electrical signals. The intensity of the received beam is monitored by each photodetector for subsequent evaluation.

[0030] A feature of the design of polarization diversity receiver assembly 1000 is that it is constructed with two mechanically identical modulets consisting of a non-polarizing beam splitter, a linear polarizer and a photodetector. FIG. 2 illustrates the components comprising a modulet within assembly 1000. One of the modulets includes 1130, 1150 and 1180, while the other modulet contains 1140, 1160 and 1190. The structure of the modulets is the same except the split ratio of the beam splitter. During the assembling process, each modulet can be individually fabricated and subsequently integrated into the overall assembly 1000. Therefore, the design provides simplified mechanical structure for ease of manufacturing.

[0031] Referring to FIG. 1, an input two-beam interference signal enters the fiber pigtail 1121 and is collimated to a light beam 1111 via the optical collimator 1120. This collimated beam 1111 is then split into two paths, i.e., 1112 and 1113, by the first non-polarizing beam splitter 1130. The reflected beam 1112 contains of the initial power while retaining the original state of polarization, and subsequently passes through the first linear polarizer 1150 towards the first photodetector 1180. Meanwhile, the transmitted beam 1113 holds of the input power which gets further divided by the second non-polarizing beam splitter 1140, with 50% split ratio, into two beams 1114 and 1115. Therefore, same as beam 1112, the reflected beam 1114 and the transmitted beam 1115 each possesses of the input power as well as consistent polarization. Ultimately, beam 1114 illuminates the second linear polarizer 1160 and is received by the second photodetector 1190, while beam 1115 is incident on the third linear polarizer 1170 and captured by the third photodetector 1200.

[0032] In the design described above, the light beams predominantly travel in free space rather than being guided through dielectric waveguides or other media such as optical fibers. Additionally, the optical collimator 1120, the first beam splitter 1130 and the second beam splitter 1140 can be fabricated from isotropic and polarization-independent materials with optimized compactness to limit internal optical path length. Therefore, the input optical signal undergoes minimal birefringence effects, and the polarization state of the beams received by the polarizers 1150, 1160 and 1170 are identical to that of the input. Furthermore, for the input signal which is a coherent combination of two beams, with the configuration shown in FIG. 7, it is ensured that at least one of the three polarizer axes acquires non-zero projections from both of the original interfered beams. Consequently, by continuously selecting the largest output, such tri-mask polarization diversity setup prohibits signal fade, enhancing visibility while preserving structural simplicity and compactness.

[0033] Note that the design described above also benefits from compensation of beam displacement. As shown in FIG. 3, due to refraction, when light passes through the first non-polarizing beam splitter 1130, it's possible that the output beam 1113 is displaced by a certain distance s from the input beam 1111. The beam displacement s depends on the angle of incidence as well as the thickness and refractive index of the beam splitter. However, since the two modulets in the assembly 1000 are mechanically identical, the displacement induced in the first beam splitter 1130 is compensated by the displacement created in the second beam splitter 1140. As a result, after integrating the two modulets into the assembly, no displacement is introduced between the last beam 1115 and the input beam 1111, which facilitates easier optical alignment.

[0034] A second embodiment of the optical polarization diversity receiver is depicted in FIG. 4. The assembly 2000 comprises a first optical collimator 2120 having a first optical fiber pigtail 2121, a second optical collimator 2180 having a second optical fiber pigtail 2181, a third optical collimator 2190 having a third optical fiber pigtail 2191, a fourth optical collimator 2200 having a fourth optical fiber pigtail 2201, a first non-polarizing beam splitter 2130, a second non-polarizing beam splitter 2140, a first linear polarizer 2150, a second linear polarizer 2160, and a third linear polarizer 2170.

[0035] The first optical collimator 2120 comprises a lens to collimate an input divergent light beam emerging from the first fiber pigtail 2121. The second optical collimator 2180, the third optical collimator 2190 and the fourth optical collimator 2200 have similar construction to couple collimated light beams into the second fiber 2181, the third fiber 2191 and the fourth fiber 2201 respectively. Typically, the fibers 2121, 2181, 2191 and 2201 are secured inside ferrules. Examples of lenses include convex lens and gradient-index lens.

[0036] Each of the first non-polarizing beam splitter 2130 and the second non-polarizing beam splitter 2140 incorporates a coating layer to split an incident light beam into two spatially separate beams at a designated ratio irrespective of the polarization state. A specific portion of the incoming light power is reflected by the coating while the remainder transmits through it. For example, a tap thin-film filter is a plate of substrate with partially reflecting coating on one surface facing towards the incident beam. Alternatively, a cube beam splitter can be employed, which is constructed by cementing two prisms together where the coating is at the internal interface. Ideally, to divide the input power evenly so the three outputs each obtains of the total power, the reflection ratio of the first beam splitter 2130 should be , and the second beam splitter 2140 should have a reflection ratio of . The coating layers of 2130 and 2140 need to be orientated properly to align the reflected beams with the respective output collimators 2180 and 2190.

[0037] The first linear polarizer 2150, the second linear polarizer 2160 and the third linear polarizer 2170 positioned perpendicularly to the optical axis only allow a specific linear polarization component of light waves to traverse while the orthogonal component is blocked by absorption or reflection. Various types of linear polarizers can be used including, but not limited to, birefringent crystals, dichroic filters, Brewster polarizers and wire grid polarizers. When two beams are coherently combined and pass through the linear polarizer, the output is essentially the interference projected along the polarizer's transmission axis. FIG. 7 illustrates the orientations (in any sequence) of the three polarizers, with the polarization axes 3150, 3160 and 3170 separated equally by 60 degrees.

[0038] The first external photodetector 2182, the second external photodetector 2192 and the third external photodetector 2202 are positioned in any remote locations that are deemed suitable (e.g., within a low-noise environment), and used to collect light signals transmitted by the second optical fiber 2181, the third optical fiber 2191 and the fourth optical fiber 2201 respectively. They function as the output receivers that convert optical signals into electrical signals. The intensity of the received beam is monitored by each external photodetector for subsequent evaluation.

[0039] A feature of the design of polarization diversity receiver assembly 2000 is that it is constructed with two mechanically identical modulets consisting of a non-polarizing beam splitter, a linear polarizer and an optical collimator. FIG. 5 illustrates the components comprising a modulet within assembly 2000. One of the modulets includes 2130, 2150 and 2180, while the other modulet contains 2140, 2160 and 2190. The structure of the modulets is the same except the split ratio of the beam splitter. During the assembling process, each modulet can be individually fabricated and subsequently integrated into the overall assembly 2000. Therefore, the design provides simplified mechanical structure for ease of manufacturing.

[0040] Referring to FIG. 4, an input two-beam interference signal enters the first fiber pigtail 2121 and is collimated to a light beam 2111 via the first optical collimator 2120. This collimated beam 2111 is then split into two paths, i.e., 2112 and 2113, by the first non-polarizing beam splitter 2130. The reflected beam 2112 contains of the initial power while retaining the original state of polarization, and subsequently passes through the first linear polarizer 2150 towards the second optical collimator 2180 and the first external photodetector 2182. Meanwhile, the transmitted beam 2113 holds of the input power which gets further divided by the second non-polarizing beam splitter 2140, with 50% split ratio, into two beams 2114 and 2115. Therefore, same as beam 2112, the reflected beam 2114 and the transmitted beam 2115 each possesses of the input power as well as consistent polarization. Ultimately, beam 2114 illuminates the second linear polarizer 2160 and is received by the third optical collimator 2190 and the second external photodetector 2192, while beam 2115 is incident on the third linear polarizer 2170 and captured by the fourth optical collimator 2200 and the third external photodetector 2202.

[0041] In the design described above, the light beams predominantly travel in free space rather than being guided through dielectric waveguides or other media such as optical fibers. Additionally, the first optical collimator 2120, the first beam splitter 2130 and the second beam splitter 2140 can be fabricated from isotropic and polarization-independent materials with optimized compactness to limit internal optical path length. Therefore, the input optical signal undergoes minimal birefringence effects, and the polarization state of the beams received by the polarizers 2150, 2160 and 2170 are identical to that of the input. Furthermore, for the input signal which is a coherent combination of two beams, with the configuration shown in FIG. 7, it is ensured that at least one of the three polarizer axes acquires non-zero projections from both of the original interfered beams. It should be noted that after the linear polarizer, it is only the intensity variation of the light that is of concern, so any polarization alteration in the fibers 2181, 2191 and 2201 has no impact on the application. Consequently, by continuously selecting the largest output, such tri-mask polarization diversity setup prohibits signal fade, enhancing visibility while preserving structural simplicity and compactness.

[0042] Note that the design described above also benefits from compensation of beam displacement. As shown in FIG. 6, due to refraction, when light passes through the first non-polarizing beam splitter 2130, it's possible that the output beam 2113 is displaced by a certain distance s from the input beam 2111. The beam displacement s depends on the angle of incidence as well as the thickness and refractive index of the beam splitter. However, since the two modulets in the assembly 2000 are mechanically identical, the displacement induced in the first beam splitter 2130 is compensated by the displacement created in the second beam splitter 2140. As a result, after integrating the two modulets into the assembly, no displacement is introduced between the last beam 2115 and the input beam 2111, which facilitates easier optical alignment.

Additional and Alternative Implementation Notes

[0043] Although the techniques have been described in language specific to certain applications, it is to be understood that the appended claims are not necessarily limited to the specific features or applications described herein. Rather, the specific features and examples are disclosed as non-limiting exemplary forms of implementing such techniques.

[0044] As used in this application, the term or is intended to mean an inclusive or rather than an exclusive or. That is, unless specified otherwise or clear from context, X employs A or B is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then X employs A or B is satisfied under any of the foregoing instances. In addition, the articles a and an as used in this application and the appended claims should generally be construed to mean one or more, unless specified otherwise or clear from context to be directed to a singular form.