Integrated optical circulator implementing polarization diversity

Abstract

Integrated optical circulators that enable polarization diversity are disclosed. A photonic integrated circulator (200) can be fabricated by including multiple polarization beam splitters (220, 222) and optical polarization rotators (230, 232, 234) such that two copies of an optical signal are output at a receiver in substantially aligned polarization states. The circulator (200) can be used to facilitate bidirectional communication between photonic integrated circuit devices that are inherently polarization sensitive while reducing signal loss.

Description

[0001] This application is a divisional application of the original application with the application number 201880033560.3 and the filing date of 28 November 2018, and the title "Integrated optical circulator enabling polarization diversity".

[0002] Cross Reference to Related Applications

[0003] This application claims the benefit of and priority to U.S. Patent Application No. 15 / 975,268, filed May 9, 2018, which claims the benefit of and priority to U.S. Provisional Application No. 62 / 595,539, filed December 6, 2017, entitled "Integrated Optical Circulator Enabling Polarization Diversity," the entire contents of both applications are incorporated herein by reference for all purposes. TECHNICAL FIELD

[0004] The present application relates to an integrated optical circulator enabling polarization diversity. BACKGROUND

[0005] Today's optical communication networks require managing a large number of fiber interconnections. In many of these interconnections, the transmitted and received signals are carried in two separate optical fibers. A effective method to reduce the number of optical fibers used in an optical fiber communication network is bidirectional communication, in which the upstream and downstream signals are carried in a single optical fiber. The key to enabling such bidirectional communication is to efficiently separate and combine the upstream and downstream signals. One method to separate and combine the upstream and downstream signals is to add an optical circulator at each end of the optical fiber. SUMMARY

[0006] At least one aspect is directed to an integrated optical circulator that implements polarization diversity. The integrated optical circulator includes a first port configured to receive a first optical signal. The integrated optical circulator includes a second port configured to transmit the first optical signal received at the first port and further configured to receive a second optical signal. The integrated optical circulator includes a first polarization beam splitter configured to: receive the second optical signal; split the second optical signal into a first optical signal component and a second optical signal component; direct the first optical signal component toward a first reflective surface, where the first reflective surface is configured to further direct the first optical signal component through a first optical polarization rotator; and direct the second optical signal component through a second optical polarization rotator. The integrated optical circulator includes a second polarization beam splitter configured to receive the second optical signal component after the second optical signal component passes through the second optical polarization rotator and direct the second optical signal component toward a second reflective surface. The second reflective surface directs the second optical signal component toward a third optical polarization rotator. The integrated optical circulator includes a third port configured to transmit the first optical signal component of the second optical signal after the first optical signal component of the second optical signal passes through the first optical polarization rotator, the third port transmitting the first component in a first polarization state. The integrated optical circulator includes a fourth port configured to receive the second component of the second optical signal after the second component of the second optical signal passes through the third optical polarization rotator, the fourth port transmitting the second optical signal component in a second polarization state that is substantially aligned with the first polarization state.

[0007] These and other aspects and implementations are discussed in detail below. The foregoing information and the following detailed description include illustrative examples of various aspects and implementations, and provide an overview or framework for understanding the nature and character of the claimed aspects and implementations. The drawings provide one explanation of aspects and implementations and are included to provide a thorough understanding of the claimed subject matter and its principles and sought disadvantages. The claims should not be limited to the features shown in the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0008] The accompanying drawings are not intended to be drawn to scale. The same reference numbers and designations in different drawings indicate the same elements. Not every component can be called out in every drawing. In the drawings:

[0009] Figure 1 A schematic diagram showing an example of a bidirectional communication system implemented with a circulator is shown in accordance with an illustrative implementation;

[0010] Figure 2 A schematic diagram showing an example of an integrated optical circulator is shown in accordance with an illustrative implementation;

[0011] Figure 3 A schematic diagram showing a further example of an integrated optical circulator is shown in accordance with an illustrative implementation, and

[0012] Figure 4 A diagram illustrating another example of an integrated optical circulator configured for use with multiple transceivers is shown, in accordance with an illustrative implementation. DETAILED DESCRIPTION

[0013] The various concepts introduced above and discussed in greater detail below can be implemented in any of numerous ways, as the described concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided for illustrative purposes only.

[0014] Today's optical communication networks require managing a large number of fiber interconnections. In many of these interconnections, the transmitted and received signals are carried in two separate optical fibers. Employing bidirectional communication, in which the transmitted and received signals are carried in a single optical fiber, can reduce the number of optical fibers used in an optical fiber communication network. A key to implementing such bidirectional communication is to efficiently separate and combine the upstream and downstream signals at each end of the optical fiber.

[0015] One method of establishing bidirectional communication is to add an optical circulator at each end of the optical fiber. An optical circulator is generally a three-port device with a ring connectivity. For example, one input optical signal enters port 1 of the circulator and is directed to port 2, while another input optical signal enters port 2 and is directed to port 3. To achieve this functionality, the circulator is equipped with a combination of polarizers, non-reciprocal / magneto-optic materials such as garnets, and phase retarders.

[0016] Figure 1 A bidirectional communication system 100 implemented with optical circulators is shown. The bidirectional communication system 100 includes a first transceiver 110, a second transceiver 120, a first circulator 130, a second circulator 140, and an optical fiber 150. The first transceiver 110 includes a first transmitter 112 and a first receiver 114. Similarly, the second transceiver 120 includes a second transmitter 122 and a second receiver 124. The first circulator 130 includes a first port 131, a second port 132, and a third port 133. The second circulator 140 includes a fourth port 141, a fifth port 142, and a sixth port 143. The first port 131 is optically coupled to the first transmitter 112, while the third port 133 is optically coupled to the first receiver 114. Similarly, the fourth port 141 is optically coupled to the second transmitter 122, while the sixth port 143 is optically coupled to the second receiver 124. The second port 132 and the fifth port 142 are connected by the optical fiber 150.

[0017] The first transmitter 112 sends an optical signal into the first port 131. The first circulator 130 directs the optical signal entering the first port 131 out of the second port 132. The optical signal then propagates through the optical fiber 150 and enters the fifth port 142 of the second circulator 140. Once the optical signal enters the fifth port 142, the second circulator 140 directs the optical signal out of the sixth port 143, where it is received by the second receiver 124. Similarly, the second transmitter 122 sends an optical signal to the fourth port 141 of the second circulator 140. The second circulator 140 directs the optical signal entering the fourth port 141 out of the fifth port 142. The optical signal then propagates through the optical fiber 150 and enters the second port 132 of the first circulator 130. Once the optical signal enters the second port 132, the first circulator 130 directs the optical signal out of the third port 133, where it is received by the first receiver 114. As shown in this example, attaching a circulator at each end of an optical fiber eliminates the need for two optical fibers to connect the transmitters and receivers of two transmitters. In addition to reducing the number of optical fibers required for an optical communication network, optical circulators can also improve the efficiency of a network employing optical circuit switching techniques.

[0018] One drawback to adding traditional circulators to an optical fiber network is that realizing their benefits requires adding the circulators outside of the optical transceiver, which can increase the cost, size, and insertion loss of the optical fiber link. However, integrated silicon photonics is an emerging technology in the field of optical communications that can enable low power and low cost photonic integrated circuits by fabricating integrated photonic devices in silicon using low cost, high yield, silicon complementary metal-oxide-semiconductor (CMOS) integrated circuits, where various transmitter and receiver devices of a transceiver will be combined in a single chip. These photonic integrated circuits make up the engine of an optical transceiver. Therefore, it is preferable to configure a circulator so that it can be integrated with a photonic integrated circuit in order to eliminate the need to add a circulator outside of an optical transceiver.

[0019] Integrated components have great potential to reduce the cost and size of fiber optic networks. However, many of the key devices utilized in silicon photonics, such as waveguide-based devices, are sensitive to the polarization state. This means that the characteristics of the devices vary based on the polarization of the light used within the device. For example, many types of Mach-Zehnder-based Rx demux devices are polarization sensitive because the effective index of the waveguides depends on the polarization. Thus, when receiving an optical signal propagating in one polarization state, the integrated devices perform more efficiently than when they receive an optical signal of the other polarization state. In the case of silicon photonics, the devices are more efficient when receiving an optical signal propagating in the p polarization state relative to the s polarization signal. Thus, in most standard CMOS processes, it is difficult to design integrated devices to be polarization diverse. Although silicon photonics IC transmitters can be configured to send optical signals in a particular polarization state, the optical signals typically do not arrive at the corresponding receivers in the same polarization state. This is because the optical signals become depolarized as they propagate through the fiber due to the inherent properties of the fiber.

[0020] Traditional circulators utilize birefringent materials to separate the orthogonal polarizations of an incoming optical signal. However, as the optical signal propagates through the circulator, the orthogonal polarizations are recombined by additional birefringent materials. Thus, the optical signal carries a mixed polarization when it exits the circulator and is received by the transceiver. However, as noted above, integrated silicon devices can not perform efficiently with the s polarization component of the optical signal and an undesirable amount of optical signal loss can occur. Thus, it is not advantageous to integrate a traditional circulator with a silicon photonics IC. Therefore, it would be beneficial to design a device that enables bidirectional communication between a photonic IC-based transceiver while addressing the polarization sensitivity of the silicon-based devices.

[0021] One method of enabling bidirectional communication with silicon photonics integrated devices while addressing the polarization sensitivity of the silicon-based devices is to use a traditional external circulator and integrated grating couplers in the silicon photonics IC. Certain grating couplers have been shown to separate the orthogonal polarization modes. However, many grating couplers have high fundamental optical loss. Furthermore, as noted previously, the use of an external circulator can be undesirable due to the increased size, cost, and optical loss associated with the circulator. Thus, it is preferable to construct an integrated device that allows for a bidirectional link to be combined with silicon photonics technology to enable low cost, high density optical interconnects. The present disclosure proposes an integrated circulator that is configured to separate the polarization state of an input signal into two component signals and send the component signals to separate ports that are substantially aligned in polarization state.

[0022] Devices according to the present disclosure can address problems associated with implementing bidirectional communication between a photonic IC-based transceiver in primarily several ways. For example, in terms of size, the host device is designed to be small enough so that it can be integrated with a photonic IC-based transceiver. Further, to reduce optical loss, the polarization split / combiner assembly of the host device is configured in such a way to split a mixed polarization optical signal input into two separate optical signal components with orthogonal polarizations, and further rotate these separate optical signals to produce an output that accommodates the signal polarization requirements of a silicon photonic IC. It is certain that by separating a mixed polarization optical signal input into two signal components and providing the signal components to the integrated transceiver in substantially the same polarization state, signal loss is reduced and the two signal components are provided to the photonic chip in polarization states that allow for efficient demultiplexing and other functions to be implemented in the chip.

[0023] Figure 2 A schematic diagram showing an example of an integrated optical circulator 200 according to an illustrative implementation is shown. The integrated optical circulator 200 can be used to implement a bidirectional, polarization-diverse optical link using silicon photonics. The integrated optical circulator 200 includes a first port 210, a second port 211, a third port 212, and a fourth port 213. The integrated optical circulator also includes a first polarization beam splitter 220, a first reflective surface 221, a second polarization beam splitter 222, and a second reflective surface 223. The first polarization beam splitter 220 and the second polarization beam splitter 222 can be polarization beam splitters known in the art, such as a polarization cube beam splitter composed of a birefringent medium. In some embodiments, the first reflective surface 221 or the second reflective surface 223 is a polarization beam splitter. In other embodiments, both are polarization beam splitters. The integrated optical circulator also includes a first optical polarization rotator 230, a second optical polarization rotator 231, a third optical polarization rotator 232, a fourth optical polarization rotator 233, and a fifth optical polarization rotator 234.

[0024] The integrated optical circulator 200 is a free-space optical assembly. The second port 211 is positioned to receive the second optical signal 241. In some implementations, the second optical signal 241 includes a first optical signal component 241a and a second optical signal component 241b. The first polarization beam splitter 220 is optically coupled with the second port 211 and positioned to receive the second optical signal 241 after the second optical signal 241 propagates through the second port 211. The third optical polarization rotator 232 is optically coupled with the first polarization beam splitter 220 and positioned to receive the second optical signal component 241b after the second optical signal component 241b transmits through the first polarization beam splitter 220. The fourth optical polarization rotator 233 is optically coupled with the third optical polarization rotator 232 and positioned to receive the second optical signal component 241b after the second optical signal component 241b propagates through the third optical polarization rotator 232. The second polarization beam splitter 222 is optically coupled with the fourth optical polarization rotator 233 and positioned to receive the second optical signal component 241b after the second optical signal component 241b propagates through the fourth optical polarization rotator 233. The second reflective surface 223 is optically coupled to the second polarization beam splitter 222 and positioned to receive the second optical signal component 241b after the second polarization beam splitter 222 reflects the second optical signal component 241b. The fifth optical polarization rotator 234 is optically coupled with the second reflective surface 223 and positioned to receive the second optical signal component 241b after the second optical signal component 241b propagates through the fifth optical polarization rotator 234. The first port 210 is optically coupled with the second polarization beam splitter 222 and positioned to receive the first optical signal 240. The first reflective surface 221 is optically coupled with the first polarization beam splitter 220 and positioned to receive the first optical signal component 221a after the first optical signal component 221a is reflected by the first polarization beam splitter 220. The first optical polarization rotator 230 is optically coupled with the first reflective surface 221 and positioned to receive the first optical signal component 241a after the first optical signal component 241a is reflected by the first reflective surface 221. The second optical polarization rotator 231 is optically coupled with the first optical polarization rotator 230 and positioned to receive the first optical signal component 241a after the first optical signal component 241a propagates through the first optical polarization rotator 230. The third port 212 is optically coupled with the second polarization rotator 231 and positioned to receive the first optical signal component 241a after the first optical signal component 241a propagates through the second optical polarization rotator 231.

[0025] The first port 210 can receive the first optical signal 240 from the transmitter for coupling into the bidirectional optical link. The second port 211 can be coupled to the bidirectional optical link. The second port 211 can transmit the first optical signal 240 to the bidirectional optical link to which the second port 211 is coupled. The second port 211 can also receive a second optical signal 241 from the bidirectional optical link. The first polarization beam splitter 220 can be configured to receive the second optical signal 241. The first polarization beam splitter 220 can also be configured to split the second optical signal 241 into a first optical signal component 241a and a second optical signal component 241b. The second optical signal 241 has a mixed polarization when it is initially received at the second port 211, as shown by the solid and dashed lines. When the first polarization beam splitter 220 splits the second optical signal 241, it separates the signal into separate optical signal components having different linear polarizations. Ideally, these separate optical signals are completely polarized in orthogonal polarizations after being split.

[0026] The first polarization beam splitter 220 can be further configured to direct the first optical signal component 241a to the first reflective surface 221. The first reflective surface 221 can be configured to further direct the first optical signal component 241a through the first optical polarization rotator 230 and the second optical polarization rotator 231. In some implementations, the first optical polarization rotator 230 can be a half-wave plate and the second optical polarization rotator 231 can be a Faraday rotator. The third port can be configured to transmit the first optical signal component 241a from the integrated optical circulator 200 after the first optical signal component 241a passes through the second optical polarization rotator 231. When the first optical signal component 241a is transmitted from the third port 212, the first optical signal component 241a can have a first state of polarization.

[0027] The first polarization beam splitter 220 can also be configured to direct the second optical signal component 241b through a third optical polarization rotator 232 and a fourth optical polarization rotator 233. In some implementations, the third optical polarization rotator 232 can be a half-wave plate and the fourth optical polarization rotator 233 can be a Faraday rotator. The second polarization beam splitter 222 can be coupled to the fourth optical polarization rotator 233 and configured to receive the second optical signal component 241b after it passes through the fourth optical polarization rotator. The second polarization beam splitter 222 can also be configured to direct the second optical signal component 241b toward a second reflective surface 223. The second reflective surface 223 can be configured to direct the second optical signal component 241b toward a fifth optical polarization rotator 234. The fourth port 213 can be configured to transmit the second optical signal component 241b after it passes through the fifth optical polarization rotator 234. The fourth port 213 can be coupled to the second receiver port and configured to transmit the received second optical signal component 241b to the second receiver port. When the second optical signal component 241b is transmitted from the fourth port 213, it can have a second polarization state that is substantially aligned with the first polarization state. Note that the second optical polarization rotator 231 and the fourth optical polarization rotator 233, which are Faraday rotators in this example, are optional and provide additional performance benefits if the corresponding receiver port produces high optical reflections. By ensuring that both optical signal components are substantially aligned when transmitted at the third port 212 and the fourth port 213, the circulator can reduce signal loss.

[0028] When the first optical signal 240 is received by the first port 210, the first optical signal 240 is p-polarized. This is because most semiconductor lasers are p-polarized. Thus, the first optical signal 240 is fully polarized and can propagate through various polarization beam splitters and rotators to be received by the second port.

[0029] Another advantage of the present circulator is that it eliminates the need for an optical isolator. Today, most transceivers use an optical isolator, which is placed in front of or downstream of the transmitter, in order to redirect reflected signals outside of the transceiver away from the transmitted laser light. This is because unwanted incoming external optical reflected signals can affect the operation and stability of the laser light. However, with the present circulator, the transmitter port Tx of the silicon photonic chip is isolated from the optical signals, and thus an optical isolator is not needed. The elimination of the isolator reduces optical loss and cost and increases transceiver density.

[0030] The specific arrangement of the various circulator components is not limited to the configuration shown Figure 2 only. For example, Figure 3A diagram illustrating an additional example of an integrated optical circulator 300 according to an illustrative implementation is shown. The integrated optical circulator 300 can be used to implement a bidirectional, polarization-diverse optical link using silicon photonics. The integrated optical circulator 300 is similar to the integrated optical circulator 200, except that the first optical polarization rotator 230 and the third optical polarization rotator 232 are one contiguous optical polarization rotator, and the second optical polarization rotator 231 and the fourth optical polarization rotator 233 are also one contiguous optical polarization rotator. The integrated optical circulator 300 includes a first port 310, a second port 311, a third port 312, and a fourth port 313. The integrated optical circulator also includes a first polarization beam splitter 320, a first reflective surface 321, a second polarization beam splitter 322, and a second reflective surface 323. The integrated optical circulator also includes a first optical polarization rotator 330, a second optical polarization rotator 331, and a third optical polarization rotator 332.

[0031] The first port 310 can receive a first optical signal 340 from a transmitter for coupling into a bidirectional optical link. The second port 311 can be coupled to the bidirectional optical link. The second port 311 can transmit the first optical signal 340 to the bidirectional optical link to which the second port 311 is coupled. The second port 311 can also receive a second optical signal 341 from the bidirectional optical link. The first polarization beam splitter 320 can be optically coupled to the second port 311. The first polarization beam splitter 320 can be configured to receive the second optical signal 341. The first polarization beam splitter 320 can also be configured to split the second optical signal 341 into a first optical signal component 341a and a second optical signal component 341b.

[0032] The first polarization beam splitter 320 can also be configured to direct the first optical signal component 341a toward the first reflective surface 321, where the first reflective surface 321 is configured to further direct the first optical signal component 341a through the first optical polarization rotator 330 and the second optical polarization rotator 331. In this example, the first optical polarization rotator 330 is a half-wave plate and the second optical polarization rotator 331 is a Faraday rotator. The third port 312 can be configured to transmit the first optical signal component 341a after it passes through the second optical polarization rotator 331. The third port can also be coupled to a first receiver port and configured to transmit the received first optical signal component 341a to the first receiver port. When the first optical signal component 341a is transmitted from the third port 312, the first optical signal component 341a can have a first polarization state.

[0033] The first polarization beam splitter 320 can also be configured to direct the second optical signal component 341b through the first optical polarization rotator 330 and the second optical polarization rotator 331. The second polarization beam splitter 322 can be configured to receive the second optical signal component 341b after the second optical signal component 341b passes through the second optical polarization rotator 331. The second polarization beam splitter 322 can also be configured to direct the second optical signal component 341b toward the second reflective surface 323. The second reflective surface 323 can be configured to direct the second optical signal component 341b toward the third optical polarization rotator 332. The fourth port 313 can be configured to transmit the second optical signal component 341b after the second optical signal component 341b passes through the third optical polarization rotator 332. The fourth port 313 can also be coupled to the second receiver port and configured to transmit the received second optical signal component 341b to the second receiver port. When the second optical signal component 341b is transmitted from the fourth port 313, the second optical signal component 341b can have a second polarization state that is substantially aligned with the first polarization state. Note that the second optical polarization rotator 331, which is a Faraday rotator in this example, is optional and provides additional performance benefits if the corresponding Rx1 and Rx2 ports produce high optical reflections.

[0034] Note that the integrated optical circulator 300 is similar to the integrated optical circulator 200, except that the first optical polarization rotator 230 and the third optical polarization rotator 232 are now a continuous first optical polarization rotator 330. In addition, the second optical polarization rotator 231 and the fourth optical polarization rotator 233 are now a continuous second optical polarization rotator 331. This design allows for efficient fabrication of the integrated optical circulator, as all components can now be easily stacked and divided into multiple integrated optical circulators.

[0035] Further, the integrated optical circulator can be used in multi-channel devices, where multiple transmitters and receivers are integrated on a single silicon photonic chip. Since the integrated optical circulator is a free-space optical component, the vertical displacement of the incoming and outgoing optical signals allows the circulator components and their functionality to be reused and replicated for multiple channels. Thus, a single integrated optical circulator can be used for two or more transmitter / receiver ports of a silicon photonic chip.

[0036] For example, Figure 4A diagram showing an additional example of an integrated optical circulator 400 according to an illustrative implementation is shown. The integrated optical circulator 400 can be used to implement a bidirectional, polarization-diverse optical link using a silicon photonic device with multiple transceivers. The integrated optical circulator 400 is similar to the integrated optical circulator 200, except that the integrated optical circulator 400 is configured to receive and separate additional optical signals. The integrated optical circulator 400 includes a first port 410, a second port 411, a third port 412, a fourth port 413, a fifth port 410a, a sixth port 411a, a seventh port 412a, and an eighth port 413a. The integrated optical circulator also includes a first polarization beam splitter 420, a first reflective surface 421, a second polarization beam splitter 422, and a second reflective surface 423. The integrated optical circulator also includes a first optical polarization rotator 430, a second optical polarization rotator 431, a third optical polarization rotator 432, a fourth optical polarization rotator 433, and a fifth optical polarization rotator 434.

[0037] The first port 410 and the fifth port 410a can receive a first optical signal 440 and a third optical signal 442, respectively, where the two optical signals are received from a transmitter for coupling to a bidirectional optical link. The second port 411 and the sixth port 411a can be coupled to the bidirectional optical link. The second port 411 and the sixth port 411a can transmit the first optical signal 440 and the third optical signal 442, respectively. The second port 411 and the sixth port 411a can transmit received optical signals to the bidirectional optical link to which they are coupled. The second port 411 and the sixth port 411a can also receive a second optical signal 441 and a fourth optical signal 443 from the bidirectional optical link to which they are coupled. The first polarization beam splitter 420 can be optically coupled to the second port 411 and the sixth port 411a. The first polarization beam splitter 420 can be configured to receive the second optical signal 441 and the fourth optical signal 443. The first polarization beam splitter 420 can also be configured to split the second optical signal 441 into a first optical signal component 441a and a second optical signal component 441b. The first polarization beam splitter 420 can also be configured to split the fourth optical signal 443 into a third optical signal component 443a and a fourth optical signal component 443b.

[0038] The first polarization beam splitter 420 can also be configured to direct the first optical signal component 441a and the third optical signal component 443a toward the first reflective surface 421, where the first reflective surface 421 is configured to further direct the first optical signal component 441a and the third optical signal component 443a through the first optical polarization rotator 430 and the second optical polarization rotator 431. In this example, the first optical polarization rotator 430 is a half-wave plate and the second optical polarization rotator 431 is a Faraday rotator. The third port 412 and the seventh port 412a can be configured to transmit the first optical signal component 441a and the third optical signal component 443a, respectively, after the first optical signal component 441a and the third optical signal component 443a pass through the second optical polarization rotator 431. The third port 412 and the seventh port 412a can also be coupled to the first receiver port and the second receiver port, respectively. The third port 412 can be configured to transmit the first optical signal component 441a to the first receiver port. The seventh port 412a can be configured to transmit the third optical signal 443a to the second receiver port. The first optical signal component 441a and the third optical signal component 443a can have a first polarization state when the first optical signal component 441a and the third optical signal component 443a are transmitted from the third port 412 and the seventh port 412a, respectively.

[0039] The first polarization beam splitter 420 can also be configured to direct the second optical signal component 441b and the fourth optical signal component 443b through a third optical polarization rotator 432 and a fourth optical polarization rotator 433. In this example, the third optical polarization rotator 432 is a half-wave plate and the fourth optical polarization rotator 433 is a Faraday rotator. The second polarization beam splitter 422 can be configured to receive the second optical signal component 241b and the fourth optical signal component 443b after the second optical signal component 241b and the fourth optical signal component 443b pass through the fourth optical polarization rotator 433. The second polarization beam splitter 422 can be configured to direct the second optical signal component 441b and the fourth optical signal component 443b toward a second reflective surface 423. The second reflective surface 423 can be configured to direct the second optical signal component 441b and the fourth optical signal component 443b toward a fifth optical polarization rotator 434. The fourth port 413 can be configured to transmit the second optical signal component 441b after the second optical signal component 441b passes through the fifth optical polarization rotator 434. The fourth port 413 can also be coupled to the third receiver port and configured to transmit the second optical signal component 441b to the third receiver port. An eighth port 413a can be configured to transmit the fourth optical signal component 443b after the fourth optical signal component 443b passes through the fifth optical polarization rotator 434. The eighth port 413a can also be coupled to the fourth receiver port and configured to transmit the received fourth optical signal component 443b to the fourth receiver port. The second optical signal component 441b and the fourth optical signal component 443b can have a second polarization state that is substantially aligned with the first polarization state when the second optical signal component 441b and the fourth optical signal component 443b are transmitted from the fourth port 413 and the eighth port 413a, respectively.

[0040] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what can be claimed, but as descriptions of features that are particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented in combinations with each other. Conversely, various features that are described in the context of a single implementation can also be implemented separately from that single implementation or in any other suitable combination. Moreover, although features can be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination and the claimed combination can be directed to a subcombination or variation of a subcombination.

[0041] Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring such order nor that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing can be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated in a single software product or packaged into multiple software products.

[0042] References to "or" can be construed as inclusive so that any terms described using "or" can indicate any of a single, more than one, and all of the described terms. The labels "first," "second," "third," and the like can not necessarily indicate an order or precedence and are generally used to distinguish between different items or elements.

[0043] Various modifications to the implementations described in this disclosure can be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but is to be accorded the widest scope consistent with this disclosure, the principles and new features disclosed herein.

[0044] The claims are intended to cover all such modifications that are within the scope of this disclosure, along with their equivalents.

[0045] The claims are intended to cover all such modifications that are within the scope of this disclosure, along with their equivalents.

Claims

1. An integrated optical circulator that implements polarization diversity, the integrated optical circulator comprising: a first port configured to receive a first optical signal; a second port configured to transmit the first optical signal received at the first port and further configured to receive a second optical signal; a first polarization beam splitter configured to: receive the second optical signal; split the second optical signal into a first optical signal component and a second optical signal component; direct the first optical signal component toward a first reflective surface; and direct the second optical signal component through a first optical polarization rotator; a second polarization beam splitter configured to receive the second optical signal component after the second optical signal component passes through the first optical polarization rotator, wherein the second polarization beam splitter does not receive the first optical signal component; a third port configured to transmit the first optical signal component of the second optical signal in a first polarization state after the first optical signal component of the second optical signal passes through a second optical polarization rotator, and a fourth port configured to transmit the second optical signal component in a second polarization state aligned with the first polarization state after the second optical signal component passes through the first optical polarization rotator. The first reflective surface is configured to further direct the first optical signal component through the first optical polarization rotator.

3. The integrated optical circulator of claim 1, wherein the first optical polarization rotator is a half-wave plate.

2. The integrated optical circulator of claim 1, wherein, 4. The integrated optical circulator of claim 1, wherein the first optical polarization rotator is a continuous optical polarization rotator.

5. The integrated optical circulator of claim 1, wherein: a fifth port is configured to receive a third optical signal; a sixth port is configured to transmit the third optical signal received at the fifth port and further configured to receive a fourth optical signal; the first polarization beam splitter is further configured to: receive the fourth optical signal; split the fourth optical signal into a third optical signal component and a fourth optical signal component; direct the third optical signal component toward the first reflective surface, wherein the first reflective surface is configured to further direct the third optical signal component through the first optical polarization rotator; and direct the fourth optical signal component through the first optical polarization rotator; the second polarization beam splitter is configured to receive the fourth optical signal component after the fourth optical signal component passes through the first optical polarization rotator and direct the fourth optical signal component toward a second reflective surface that directs the fourth optical signal component toward a third optical polarization rotator; a seventh port is configured to transmit the third optical signal component after the third optical signal component passes through the first optical polarization rotator, the seventh port transmitting the third optical signal component in a first polarization state; and a fourth optical polarization rotator is configured to rotate the polarization of the fourth optical signal component to a second polarization state. ​ ​ ​ The eighth port is configured to transmit the fourth optical signal component after the fourth optical signal component passes through the third optical polarization rotator, the eighth port transmitting the fourth optical signal component in a second polarization state.

6. The integrated optical circulator of claim 1, wherein at least one additional optical polarization rotator is optically coupled to one or both of the first optical polarization rotator, the second optical polarization rotator.

7. The integrated optical circulator of claim 6, wherein the at least one additional optical polarization rotator is a Faraday rotator.

8. The integrated optical circulator of claim 1, wherein the first reflective surface is a polarization beam splitter.

9. The integrated optical circulator of claim 1, wherein the first polarization state and the second polarization state are p-polarization.

10. The integrated optical circulator of claim 1, wherein the first optical signal is p- polarized.

11. The integrated optical circulator of claim 1, wherein the first polarization beam splitter splits the second optical signal into the first optical signal component and the second optical signal component carrying orthogonal polarizations.

12. An integrated optical circulator implementing polarization diversity, the integrated optical circulator comprising: a first port configured to receive a first optical signal; a second port configured to transmit the first optical signal received at the first port and further configured to receive a second optical signal; a first polarization beam splitter configured to: receive the second optical signal; split the second optical signal into a first optical signal component and a second optical signal component; direct the first optical signal component toward a first reflective surface that directs the first optical signal component through a first optical polarization rotator; and direct the second optical signal component through the first optical polarization rotator; a second polarization beam splitter configured to receive the second optical signal component after the second optical signal component passes through the first optical polarization rotator, wherein the second polarization beam splitter does not receive the first optical signal component, and wherein the second polarization beam splitter is further configured to direct the second optical signal component toward a second reflective surface that directs the second optical signal component toward a second optical polarization rotator; a third port configured to transmit the first optical signal component after the first optical signal component passes through the first optical polarization rotator, the third port transmitting the first optical signal component in a first polarization state; and a fourth port configured to transmit the second optical signal component after the second optical signal component passes through the second optical polarization rotator, the fourth port transmitting the second optical signal component in a second polarization state aligned with the first polarization state.

13. The integrated optical circulator of claim 12, wherein the first optical polarization rotator is a half-wave plate.

14. The integrated optical circulator of claim 12, wherein the first optical polarization rotator is a continuous optical polarization rotator. ​ ​ 15. The integrated optical circulator of claim 12, wherein: the fifth port is configured to receive a third optical signal; the sixth port is configured to transmit the third optical signal received at the fifth port and is further configured to receive a fourth optical signal; the first polarization beam splitter is further configured to: receive the fourth optical signal; split the fourth optical signal into a third optical signal component and a fourth optical signal component; direct the third optical signal component toward the first reflective surface, wherein the first reflective surface is configured to further direct the third optical signal component through the first optical polarization rotator; and direct the fourth optical signal component through the first optical polarization rotator; the second polarization beam splitter is configured to receive the fourth optical signal component after it passes through the first optical polarization rotator and direct the fourth optical signal component toward a second reflective surface that directs the fourth optical signal component toward the second optical polarization rotator; the seventh port is configured to transmit the third optical signal component after it passes through the first optical polarization rotator, the seventh port transmitting the third optical signal component in a first polarization state; and the eighth port is configured to transmit the fourth optical signal component after it passes through the second optical polarization rotator, the eighth port transmitting the fourth optical signal component in a second polarization state.

16. The integrated optical circulator of claim 12, wherein at least one additional optical polarization rotator is optically coupled to one or both of the first optical polarization rotator, the second optical polarization rotator.

17. The integrated optical circulator of claim 16, wherein the at least one additional optical polarization rotator is a Faraday rotator.

18. The integrated optical circulator of claim 12, wherein the first reflective surface is a polarization beam splitter.

19. The integrated optical circulator of claim 12, wherein the second reflective surface is a polarization beam splitter.

20. The integrated optical circulator of claim 12, wherein the first polarization state and the second polarization state are p-polarization.

21. The integrated optical circulator of claim 12, wherein the first optical signal is p-polarized.

22. The integrated optical circulator of claim 12, wherein the first polarization beam splitter splits the second optical signal into the first optical signal component and the second optical signal component that carry orthogonal polarizations. ​

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