Patent Application
20250198850
The present invention generally relates to optical sensors, and more specifically, to an optical polarization diversity receiver used in a fiber-optic interferometer for coherent detection.
In fiber-optic sensors, optical fibers serve as the sensing elements to monitor environmental physical quantities such as temperature, velocity and pressure as they alter the properties of fibers, thereby affecting the light traveling through them. For example, a common configuration for optical sensing is the Mach-Zehnder interferometer which consists of a sensing arm and a reference arm. As the sensing arm is exposed to and interacts with the environment, the interference signal generated by the mixed optical beams from both arms can be sent to a receiver for demodulation to obtain the desired measurement.
A challenge of such two-beam interferometry is signal fade due to polarization mismatch. Interference occurs perfectly for collinear polarizations with maximized visibility. Conversely, if the polarization directions of the two beams are orthogonal to each other, interference is negated and the visibility drops to zero. A demodulator is required to have a high visibility in order to operate correctly. However, in practice, the polarization state of the beam in each interferometer arm is usually unpredictable, and consequently, there's a possibility that the interferometer enters a faded state, rendering visibility too low to make a measurement.
To combat polarization sensitivity, an approach known as polarization diversity can be employed, where the interference signal is divided and projected across multiple polarization axes, and the strongest output is selected for further processing. Theoretically, a tri-mask or tri-cell polarization diversity receiver (PDR) equipped with three polarizers set 60 degrees apart from each other has been proved to effectively limit the minimum normalized visibility to around 40%. While signal fades are still possible with two polarizers, using more than three polarizers reduces signal fade at the price of increased complexity. With different input polarization states, the maximum interference signal will change from one polarizer output to another, and the demodulator must automatically track the best interference signal for demodulation.
A critical requirement for the PDR to work as desired is that the polarization state received by all the polarizers should be the same. Traditionally, a 1×3 fused fiber-optic coupler is used to distribute an input signal into three outputs. But owing to factors like manufacturing imperfections and mechanical stress that disrupt circular symmetry, optical fibers always exhibit certain degree of birefringence in practice, which leads to an uncontrolled shift in the output polarization state. As a consequence, the three beams directed from the coupler to the corresponding polarizers have different polarization, compromising the accuracy of measurements. Therefore, it is an objective of the present invention to provide an optical PDR without the influence of birefringence.
A tri-mask optical polarization diversity receiver with an input port and three output ports is constructed with non-polarizing beam splitters and linear polarizers. In addition, optical collimators and photodetectors are used at the four terminals to interface with optical signals.
In one aspect, the configuration of the optical elements is arranged such that an incoming two-beam interference signal is collimated and split into three beams with equal power. Each of these beams travels towards a linear polarizer allowing the transmission of a specific polarization axis, wherein the interference projected along that axis exits at the corresponding output.
In the second aspect, the risk of beam propagation through materials with potential birefringence effects is mitigated in the design, so the beam's polarization remains unchanged until interacting with the linear polarizers. Therefore, each polarizer accurately processes the same state of polarization.
In the third aspect, the polarization diversity receiver contains two mechanically identical modulets formed by a non-polarizing beam splitter, a linear polarizer and a photodetector or optical collimator. Each modulet can be made independently and integrated into the overall assembly thereafter, thus the manufacturing is streamlined.
In the fourth aspect, the configuration of the components is arranged such that beam displacement caused by refraction in one beam splitter is compensated by the other beam splitter.
In one embodiment, the polarization diversity receiver device includes a photodetector at each output port to convert optical signals into electrical signals.
In the other embodiment, the polarization diversity receiver device includes an optical collimator with a fiber pigtail at each output port, which sends the optical signal to an external photodetector properly located at a suitable remote position.
Various embodiments are disclosed below, with reference to the attached figures to provide better understanding of the principles and benefits of present invention.
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.
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.
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.
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.
One embodiment of the optical polarization diversity receiver is depicted in
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.
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 ⅓ of the total power, the reflection ratio of the first beam splitter 1130 should be ⅓, and the second beam splitter 1140 should have a reflection ratio of ½. 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.
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.
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.
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.
Referring to
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
Note that the design described above also benefits from compensation of beam displacement. As shown in
A second embodiment of the optical polarization diversity receiver is depicted in
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.
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 ⅓ of the total power, the reflection ratio of the first beam splitter 2130 should be ⅓, and the second beam splitter 2140 should have a reflection ratio of ½. 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.
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.
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.
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.
Referring to
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
Note that the design described above also benefits from compensation of beam displacement. As shown in
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.
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.
