JP2024060573A - Photonic Integrated Circuits - Google Patents

Abstract

A photonic integrated circuit for reducing phase noise introduced by an optical channel is provided. In an optical system (11), a photonic integrated circuit (4) is formed on an integrated chip and includes a demultiplexer (14), first to fifth phase control elements (181 to 185), and a multiplexer (20). The demultiplexer receives a multiplexed optical signal comprising a first signal having a first wavelength and a second signal having a second wavelength. The first phase control element provides a first phase shift to the first signal and the second phase control element provides a second phase shift to the second signal. The multiplexer multiplexes the first phase shifted signal and the second phase shifted signal to produce a modified multiplexed signal. The third phase control element provides a third phase shift to the modified multiplexed signal to produce a further modified multiplexed signal, and outputs the further modified multiplexed signal. [Selected Figure]

Description

The embodiments described herein relate to photonic integrated circuits.

In optical systems, information can be stored in the phase of optical signals. Such optical signals are transmitted between distant nodes using optical channels, e.g., optical fibers. Optical channels can introduce phase drift, which contributes to phase noise. Compensation for such phase noise can be essential to the performance of these systems.

Many optical communication systems use wavelength division multiplexing, i.e., multiple information signals carried by separate wavelengths along the same optical channel. For example, wavelength division multiplexing can be used to increase the number of separate information signals that can be transmitted along a given optical channel. Phase noise introduced by the optical channel can affect each of the multiple information signals.

In a quantum communication system, information is sent between a transmitter and a receiver by a single encoded quantum, such as a single photon. A bit of information can be encoded according to a property of the photon, such as its polarization, phase, time, or energy.

Quantum communication systems can be used to implement quantum key distribution (QKD), which allows two parties to share a cryptographic key. Some QKD methods require precise measurement of the phase of photons received by the receiver.

Phase drift induced by the optical channel can degrade the performance of phase-based QKD methods. Typically, single photons carrying the transmitted information cannot be used to reliably determine the phase drift. Instead, using wavelength division multiplexing, a strong reference signal can be coupled into the optical channel and used to monitor changes in the optical channel.

1 is a schematic diagram of an example optical system according to an embodiment. 2 is a schematic diagram of a variation of the optical system of FIG. 1 according to an embodiment. 2 is a schematic diagram of a further variation of the optical system of FIG. 1 according to an embodiment. 1 is a schematic diagram of an example optical communication system according to an embodiment. 1 is a schematic diagram of a QKD network according to an embodiment; 4 is a further embodiment of an optical system according to an embodiment.

In an embodiment, a photonic integrated circuit is provided. The photonic integrated circuit is formed on an integrated chip and includes a demultiplexer, first, second, and third phase control elements, and a multiplexer. The demultiplexer is configured to receive a multiplexed optical signal including a first signal having a first wavelength and a second signal having a second wavelength different from the first wavelength. The demultiplexer is further configured to demultiplex the multiplexed optical signal to extract the first signal and the second signal. The first phase control element is configured to provide a first phase shift to the first signal to produce a first modified signal. The second phase control element is configured to provide a second phase shift to the second signal to produce a second modified signal. The multiplexer is configured to multiplex the first modified signal and the second modified signal to produce a modified multiplexed signal. The third phase control element is configured to provide a third phase shift to the modified multiplexed signal to produce a further modified multiplexed signal and output the further modified multiplexed signal.

Photonic integrated circuits can be used to shift the phase of wavelength multiplexed optical signals in a flexible manner. Photonic integrated circuits allow not only to shift the phase of all multiplexed signals collectively, but also to shift the phase of signals individually. This enables a variety of applications of photonic integrated circuits. In embodiments, the photonic integrated circuit may be part of an optical system or optical network and may be used to perform phase compensation such that phase noise introduced by the optical channel is corrected. In embodiments, the photonic integrated circuit may be used to reduce phase noise in optical communication systems.

Providing all components of a photonic integrated circuit on a single integrated chip leads to chip-based devices that can have a much smaller footprint, lower cost, and higher manufacturing yields compared to conventional systems consisting of discrete, off-chip components. Photonic integrated circuits enable small, inexpensive devices suitable for mass production. Furthermore, photonic integrated circuits can be monolithically integrated with other photonic integrated devices on a single chip. In contrast, conventional systems are typically not compatible with monolithic integration. By enabling scalability, mass production, and compatibility with monolithic integration, photonic integrated circuits enable more extended architectures than conventional technologies.

Furthermore, photonic integrated circuits can be polarization selective and polarization maintaining. In contrast, conventional systems typically suffer from polarization drift or polarization dispersion.

In an embodiment, the optical Mach-Zehnder interferometer includes a first arm and a second arm, and the first arm includes a photonic integrated circuit. In this case, the optical Mach-Zehnder interferometer having the first and second arms and the photonic integrated circuit can be formed on a single integrated chip.

In an embodiment, the optical ring interferometer comprises a photonic integrated circuit.

In embodiments, photonic integrated circuits may be used to stabilize optical phase in optical phased arrays.

The first and second phase control elements may be thermal phase shifters, and the third phase control element may be an electro-optical phase modulator.

Alternatively, the first, second and third phase control elements may be either thermal phase shifters or electro-optical phase modulators.

The demultiplexer and multiplexer can be integrated (de)multiplexers, such as arrayed waveguide gratings, angled multimode interferometers, or dispersive wavelength multiplexers.

In an embodiment, an optical system is provided that includes a transmitter and a receiver. The transmitter includes an encoding unit configured to encode information using a phase on a first optical information signal, the first optical information signal having a single first wavelength. The transmitter is configured to output a first reference signal. The first reference signal has a reference wavelength different from the first wavelength. The transmitter further includes a first multiplexer configured to multiplex the first optical information signal and the first reference signal to create a multiplexed signal and output the multiplexed signal to a communication channel. The receiver includes a photonic integrated circuit formed on an integrated chip. The photonic integrated circuit includes a first demultiplexer, first, second, and third phase control elements, and a second multiplexer. The first demultiplexer is configured to receive the multiplexed signal and demultiplex the multiplexed signal to extract the first reference signal and the first optical information signal. The first phase control element is configured to provide a first phase shift to the first reference signal to create a modified reference signal. The second phase control element is configured to provide a second phase shift to the first optical information signal to produce a modified optical information signal. The second multiplexer is configured to multiplex the modified reference signal and the modified optical information signal to produce a modified multiplexed signal. The third phase control element is configured to provide a third phase shift to the modified multiplexed signal to produce a further modified multiplexed signal and output the further modified multiplexed signal. The receiver further comprises a second demultiplexer configured to demultiplex the further modified multiplexed signal received from the transmitter to extract the first optical information signal and the first reference signal. The receiver further comprises a decoder configured to decode phase information in the first optical information signal and a phase compensation unit configured to estimate a phase change in the first optical information signal caused by the communication channel from the first reference signal and to compensate for the phase change in the first optical information signal caused by the communication channel using a photonic integrated circuit.

In the above, phase noise affecting the information signal can be compensated based on the reference signal. By using a reference signal that does not carry confidential information of the first information signal, the reference signal can be bright, i.e. of high enough intensity to provide a good signal-to-noise ratio, and can be used to provide feedback to correct phase drift in the information signal. Also, by using a reference signal with a different wavelength than the information signal, it is possible to wavelength multiplex the information signal and the reference signal, so that the rate of information transmitted to the receiver is not reduced by additionally sending a reference signal.

Further in this embodiment, the optical system may also include a second transmitter, the second transmitter including an encoding unit configured to encode information using a phase on a second optical information signal, the second optical information signal having a first wavelength, and the transmitter configured to output a second reference signal having a reference wavelength. The transmitter further includes a multiplexer configured to multiplex the second optical information signal and the second reference signal to create a multiplexed second signal and output the multiplexed second signal to a communication channel. The decoder is configured to decode phase information in the first and second optical information signals. The phase compensation unit is configured to estimate a phase change in the first and second optical information signals caused by the communication channel from the interference of the first and second reference signals, and to control the third phase shift such that the phase change in the first and second optical information signals caused by the communication channel is reduced. The phase compensation unit is further configured to estimate a remaining phase change of the first and second optical information signals caused by the communication channel from an interference of the first and second optical information signals, and to control the second phase shift such that the phase change of the first and second optical information signals caused by the communication channel is further reduced.

The above is an example of a three-node network, where the first transmitter is the first node, the second transmitter is the second node, and the receiver is the third node. Such a network can be used in twin-field QKD "TF-QKD", where both the first and second transmitters send signals to the receiver, and the receiver performs first order optical interference between the signal from the first transmitter and the signal from the second transmitter. By disclosing the results of the interference, it is possible for the two transmitters to establish a secret key.

In the above, the first and second phase control elements may be thermal phase shifters and the third phase control element may be an electro-optical phase modulator. The third phase control element may be used to implement fast feedback, i.e., to stabilize the fast drift introduced by the optical channel. For example, the fast feedback system may be implemented by monitoring the interference of a bright reference. From the interference of the bright pulse, an error signal is derived and used to adjust the phase shift applied by the third phase control element. The fast feedback system may provide a nearly instantaneous correction of the phase drift. In some cases, the fast feedback may not completely correct the phase drift relative to the information signal. Any such remaining phase drift may be compensated for by the first and second phase control elements.

The optical system may be configured as a quantum communication system configured to distribute a key between a first transmitting unit and a second transmitting unit.

In a broad sense, the optical system 1 illustrated in FIG. 1 may be used to control the phase of wavelength division multiplexed optical signals exchanged between two nodes of an optical network. In one application, the optical system 1 may be used to control the phase of the multiplexed optical signals such that phase noise introduced by the optical channels is compensated.

The optical system 1 includes a transmitter 2, a photonic integrated circuit (PIC) 4, and a receiver 6.

The transmitter 2 comprises a first multiplexer 10 configured to provide a multiplexed optical signal and having first to fifth input channels 8 ₁ to 8 _5. The first to fifth input channels 8 ₁ to 8 ₅ of the multiplexer 10 are configured to receive first to fifth input signals, respectively. The first to fifth input signals have first to fifth wavelengths, respectively, that are different from one another. The first multiplexer 10 is configured, for example, to perform wavelength division multiplexing to multiplex the first to fifth input signals onto a single optical channel 12. In this embodiment, the first multiplexer 10 is configured to group the first to fifth input signals together such that they may be transmitted over the same optical channel 12. The optical channel 12 may be an optical fiber.

PIC 4 is configured to receive the multiplexed optical signals transmitted from transceiver 2 over optical channel 12 and to provide controlled phase shifts to the first through fifth input signals before transmitting the phase shifted signals to receiver 6, as described below.

The PIC 4 comprises a demultiplexer 14 and first to fifth photonic waveguides 16 ₁ to 16 ₅ comprising first to fifth phase control elements 18 ₁ to 18 ₅ , respectively. The PIC 4 also includes a second multiplexer 20 and a further phase control element 22. The demultiplexer 14 is configured to demultiplex the received multiplexed optical signal transmitted from the transceiver 2, for example, to extract first to fifth input signals, and to provide the first to fifth input signals to the first to fifth waveguides 16 ₁ to 16 ₅ , respectively.

The first to fifth waveguides 16 ₁ to 16 ₅ are configured to connect the output of the demultiplexer 14 to the input of the second multiplexer 20 and to propagate the first to fifth input signals extracted by the demultiplexer 14 to the second multiplexer 20, respectively. The first to fifth waveguides 16 ₁ to 16 ₅ include first to fifth phase control elements 18 ₁ to 18 ₅ configured to provide first to fifth phase shifts to the optical signals propagating in the waveguides 16 ₁ to 16 ₅ , respectively. Each of the first to fifth phase shifts may be different from each other. Thus, the first to fifth phase control elements 18 ₁ to 18 ₅ are configured to produce first to fifth phase shifted signals. In this example, the phase control elements 18 ₁ - 18 ₅ are configured to provide an individual controllable phase shift to each of the input signals extracted by the demultiplexer 14 before they propagate to a second multiplexer 20 .

The second multiplexer 20 is configured, for example, to perform wavelength division multiplexing to multiplex the signals received from the phase control elements 18 ₁ -18 ₅ and to provide the multiplexed output to a further phase control element 22. In other words, the second multiplexer 20 is configured to combine the first through fifth phase shifted signals together so that they can be provided as inputs to the further phase control element 22.

The further phase control element 22 is configured to receive the output of the second multiplexer 20 and to provide a further phase shift to the received signal. The further phase control element 22 is configured to output the phase shifted signal to an output port of the PIC 4 that is connected to an optical channel 24. The optical channel 24 may be an optical fiber.

The receiver 6 includes a second demultiplexer 26 having first through fifth output channels 28 ₁ - 28 ₅ and is configured to receive the phase-shifted multiplexed optical signals transmitted by the PIC 4 through the optical channel 24. The second demultiplexer 26 is configured to demultiplex the received phase-shifted multiplexed optical signals, for example to extract first through fifth input signals, and to provide the first through fifth input signals to the first through fifth output channels 28 ₁ - 28 ₅ , respectively.

In an embodiment, all components of the PIC 4 are integrated onto a single photonic integrated circuit chip. This allows the PIC 4 to have a smaller footprint and be more suitable for mass production than a corresponding system made up of discrete off-chip optical components. Furthermore, providing all components of the PIC 4 on a single photonic integrated circuit chip reduces assembly time and manufacturing costs. Another advantage over a system made up of discrete off-chip optical components is that additional photonic integrated devices can be monolithically integrated into the PIC 4, which is not generally possible with off-chip components.

PIC4 may be formed of any suitable semiconductor material system, including InP, Si, SOI, SiN, SiO2, SiON, or GaAs. Alternatively, PIC4 may be formed of glass or polymer. Hybrid integration and heterogeneous integration techniques can be used to form PIC4 using more than one of the material systems.

The choice of material for forming the PIC4 can be application specific. For example, forming the PIC4 on SiN can provide a device with low optical loss, which can be used in applications with weak optical signals. Furthermore, forming the PIC4 on Si can enable a device with a particularly small footprint, which can be used in applications requiring small devices. Furthermore, forming the PIC4 on SOI or InP can enable the use of both thermal phase shifters and electro-optic phase modulators as phase control elements on the same chip, without the need to integrate different material platforms.

In an example, an application may require polarization preservation of wavelength division multiplexed optical signals sent from the transmitter 2 to the receiver 6. To this end, the waveguide and (de)multiplexer structure may be designed such that the PIC 4 is polarization preserving, i.e. the polarization of the optical signals propagating through the PIC 4 is well preserved. In this case, the PIC 4 overcomes the problem of polarization dispersion that is commonly present in conventional off-chip systems.

The photonic waveguides 16 ₁ - 16 ₅ of the PIC 4 may be formed using (optical and/or electronic) lithography, (plasma and/or chemical) etching, direct laser writing, ion exchange, nanoimprinting, and the like.

In general, the selection of the wavelengths and bandwidths of the waveguides 16 ₁ - 16 ₅ of the PIC 4 and the wavelength spacing between the different photonic waveguides 16 ₁ - 16 ₅ will depend on the specific requirements of the application in which the PIC 4 will be used.

In an embodiment, the demultiplexer 14 and the second multiplexer 20 are arrayed waveguide gratings (AWGs). This is particularly advantageous for applications requiring dense wavelength division multiplexing (DWDM) because AWGs allow for narrow waveguide bandwidth and narrow wavelength spacing between different waveguides.

In another embodiment, the demultiplexer 14 and the second multiplexer 20 are angled multimode interferometers (AMMIs). This is particularly advantageous for applications requiring coarse wavelength division multiplexing (CWDM) because AMMIs are typically easier to design and less sensitive to fabrication tolerances.

In another embodiment, the demultiplexer 14 and the second multiplexer 20 are distributed wavelength multiplexers.

In an embodiment, the phase control element of PIC4 is a thermal phase shifter. Thermal phase shifters can provide a large phase shift, for example a phase shift of several π. Thermal phase shifters can typically provide a larger phase shift and exhibit lower optical loss than electro-optic phase modulators. A thermal phase shifter may comprise a waveguide section, a filament, contact pads, and a cladding material as a spacer between the waveguide and the filament. The phase shift may be based on the thermo-optic effect and may be achieved by applying a drive current to the filament.

In another embodiment, the phase control element of PIC 4 is an electro-optic phase modulator. This is advantageous because electro-optic phase modulators are typically high speed phase modulators, i.e., they allow for high speed phase control. In many cases, electro-optic phase modulators can provide faster phase shifting than thermal phase shifters.

In a further embodiment, the PIC 4 includes thermal phase shifters in the individual waveguides 16 ₁ -16 ₅ and an electro-optic phase modulator at the output of the multiplexer 20. This is advantageous because it allows providing small phase shifts at high speeds as well as large phase shifts at low speeds. This is particularly advantageous for applications in quantum communications, where the phase noise introduced by optical fibers exhibits fast fluctuations and slow drifts that need to be compensated for.

In an embodiment, the first to fifth phase control elements 18 ₁ to 18 ₅ are thermal phase shifters, hi another embodiment, the first to fifth phase control elements 18 ₁ to 18 ₅ are electro-optic phase modulators.

In one embodiment, the further phase control element 22 is a thermal phase shifter. In another embodiment, the further phase control element 22 is an electro-optical phase modulator.

In an embodiment, the first to fifth phase control elements 18 ₁ to 18 ₅ are thermal phase shifters and the further phase control element 22 is an electro-optical phase modulator.

In an embodiment, PIC4 includes a grating coupler or edge coupler to receive the multiplexed optical signal from optical channel 12.

In this embodiment, the second to fifth phase control elements 18 ₂ to 18 ₅ are thermal phase shifters, and the phase control element 18 ₁ is an electro-optical phase modulator.

In the embodiment, at least one of the first to fifth phase control elements 18 ₁ to 18 ₅ is a thermal phase shifter, and at least one of the other first to fifth phase control elements 18 ₁ to 18 ₅ is an electro-optic phase modulator. For example, the first and second phase control elements 18 ₁ and 18 ₂ may be thermal phase shifters, and the third to fifth phase control elements 18 ₃ to 18 ₅ may be electro-optic phase modulators.

In an embodiment, PIC4 includes a grating coupler or edge coupler to provide the multiplexed optical signal to optical channel 24.

1 has five photonic waveguides 16 ₁ -16 ₅ , embodiments of the photonic integrated circuit are not limited to this example. For example, the photonic integrated circuit may have more than five photonic waveguides. In other embodiments, the photonic integrated circuit may have at least two photonic waveguides. In an embodiment, the photonic integrated circuit has two photonic waveguides.

In an embodiment, instead of being configured to receive the output of the multiplexer 20, the further phase control element 22 may be configured to receive the multiplexed optical signals transmitted from the transceiver and to provide a phase shift to the received multiplexed signals before transmitting the phase shifted signals to the demultiplexer 14.

Although the illustrative photonic integrated circuit (PIC) 4 shown in FIG. 1 is described as being configured to receive multiplexed optical signals from the transmitter 2 and transmit those signals (after providing an appropriate phase shift) to the receiver 6, those skilled in the art will recognize that the photonic integrated circuit (PIC) 4 is bidirectional. For example, the receiver 6 can use the optical system 1 shown in FIG. 1 to provide multiplexed optical signals and send them to the transmitter 2 via the PIC 4.

In an embodiment, the PIC 4 may be formed on an SOI platform. The photonic waveguide, the (de)multiplexer and any ports for receiving or providing optical signals may be formed together. For this purpose, patterns may be defined by photolithography and/or E-beam lithography and corresponding structures may be formed by plasma dry etching methods. For example, RIE methods such as inductively coupled plasma (ICP) reactive ion etching (RIE) and deep RIE (DRIE) may be used to form the structures.

The thermal phase shifter can be included in the waveguide by forming a filament and providing a cladding material as a spacer between the photonic waveguide and the filament, a metal line, and a metal contact pad. The cladding material can be SiO2 or SiN, which can be deposited using (PE)CVD. The filament can be TiN, which can be deposited using a sputtering system. Those skilled in the art will appreciate that the filament can be formed of a wide range of materials. The metal line and contact pad can be Au, which can be deposited using an evaporator. An additional layer between the Au and the cladding can be included to act as a seed layer. The additional layer can be formed of Cr/Ti/Ni.

An electro-optic phase modulator can be included in a photonic waveguide by incorporating a built-in PN junction into the waveguide and by forming metal tracks and vias to connect the metal tracks and the PN junction. The metal tracks and the waveguide can be spaced using a cladding material such as SiO2 or SiN. The PN junction can be formed using ion implantation. The metal tracks, vias, and contact pads can be a stack of different metals configured to carry an RF signal. The metal tracks, vias, and contact pads can be formed using a sputtering system and/or a deposition system.

Those skilled in the art will recognize that various modifications, for example using other known micro/nano fabrication methods, may be made to the embodiments described above without departing from the scope of the present invention.

1, the transmitter 2 prepares a multiplexed optical signal by providing first through fifth input signals in first through fifth input channels 8 ₁ through 8 ₅ to a first multiplexer 10. The first multiplexer 10 may, for example, perform wavelength division multiplexing to multiplex the first through fifth input signals onto optical channel 12.

The PIC 4 receives the multiplexed optical signals transmitted from the transceivers 2 through the optical channel 12 and provides a controlled phase shift to the first through fifth input signals before transmitting the phase shifted signals to the receiver 6. The demultiplexer 14 demultiplexes the received multiplexed optical signals transmitted from the transceivers 2, for example, to extract the first through fifth input signals and provides the first through fifth input signals to the first through fifth waveguides 16 ₁ through 16 ₅ , respectively. The first through fifth phase control elements 18 ₁ through 18 ₅ provide first through fifth phase shifts to the demultiplexed optical signals propagating in the waveguides 16 ₁ through 16 ₅ , respectively. From this, the first through fifth phase control elements 18 ₁ through 18 ₅ produce the first through fifth phase shifted signals.

The second multiplexer 20 may, for example, perform wavelength division multiplexing to multiplex the signals received from the phase control elements 18 ₁ -18 ₅ and provide the multiplexed output to a further phase control element 22. The further phase control element 22 receives the output of the second multiplexer 20 and provides a further phase shift to the received signals. The further phase control element 22 outputs the phase shifted signal to an output port of the PIC 4 which is connected to an optical channel 24.

The receiver 6 receives the phase-shifted multiplexed optical signals transmitted by the PIC 4 through the optical channel 24. The second demultiplexer 26 demultiplexes the received phase-shifted multiplexed optical signals, for example to extract first through fifth input signals, and provides the first through fifth input signals to first through fifth output channels 28 ₁ through 28 ₅ , respectively.

The phase noise introduced by the optical channels 12, 24 is compensated for, ie corrected, by controlling the first to fifth phase control elements 18 ₁ to 18 ₅ and the further phase control elements accordingly.

During operation in an illustrative application, the receiver 6 determines the optical phase of each of the optical signals provided to the first through fifth output channels 28 ₁ -28 _5. The receiver may use an interferometer to determine the phase. Then, for each of the five output channels 28 ₁ -28 ₅ , the receiver determines the difference between the determined phase and a respective predetermined phase and controls a corresponding phase control element, i.e., one of the phase control elements 18 ₁ -18 _5, such that the provided phase shift reduces the difference between the phase of the optical signal provided to the corresponding output channel and the predetermined phase. In this example, the phase of each signal included in the multiplexed signal can be precisely controlled and corrected.

In a further example, one of the first to fifth input signals, for example the first input signal, is used as a reference signal for correcting the phase of all input signals. In this example, the receiver 6 determines the optical phase of the optical signal provided to the first output channel 28 _1. The receiver may use an interferometer to determine the phase. The receiver 6 then determines the difference between the determined phase and the respective predetermined phase and controls the further phase control element 22 such that the provided phase shift reduces the difference between the phase of the optical signal provided to the first output channel 28 ₁ and the predetermined phase. The further phase control element 22 provides a phase shift to the multiplexed signal received from the multiplexer 20, so that each of the optical signals contained in this multiplexed signal is phase shifted. In this example, the phase of the signal provided to the second to fifth output channels 28 ₂ to 28 ₅ is corrected based on the phase of the optical signal provided to the first output channel 28 _1. This is particularly advantageous in applications where the reference signal is a strong optical signal and the second to fifth input signals are weak optical signals, for example at the level of a single photon.

The further phase control element 22 may provide a phase shift that depends on the wavelength of the optical signal. This means that, as explained above, if one input signal is used as a reference signal, the further phase control element 22 may not be able to accurately correct the phase of all input signals. In this case, the wavelength dependence of the further phase control element 22 is generally known, and the additional phase shift provided by the phase control elements 18 ₁ to 18 ₅ can further improve the phase compensation so that all signals provided to the first to fifth output channels 28 ₁ to 28 ₅ are accurately compensated.

In a further illustrative application in which the first to fifth phase control elements 18 ₁ -18 ₅ are thermal phase shifters and the further phase control element 22 is an electro-optic phase modulator, the receiver 6 determines either the phase of each of the signals in the output channels 28 ₁ -28 ₅ or the phase of the reference signal only, as described above, and controls the further phase control element 22 to provide compensation for fast phase variations and controls the phase control elements 18 ₁ -18 ₅ to provide compensation for slow phase drift. In this case, the further phase control element 22 may provide phase correction at high frequencies, e.g., faster than 100 kHz. The phase control elements 18 ₁ -18 ₅ may provide phase correction at low frequencies, e.g., slower than 100 kHz.

In a further illustrative application, when one of the input signals, for example the first input signal, is known to be more affected by fast phase noise than the other four input signals, the PIC 4 may comprise thermal phase shifters as the second to fifth phase control elements 18 ₂ to 18 ₅ , and an electro-optic phase modulator as the first phase control element 18 _1. This allows fast feedback for the first input signal affected by fast noise while using the thermal phase shifters with low optical loss for the signal affected mainly by slow phase drift.

In a further illustrative application in which one input signal is used as the reference signal, as described above, the reference signal may be provided by receiver 6 instead of transmitter 2. In this case, the reference signal propagates in the opposite direction to the optical input signal multiplexed and sent by transmitter 2. This is advantageous because crosstalk and interference between the co-propagating signals is reduced.

The above examples illustrate that PIC4 enables phase control of multiplexed signals for a variety of applications.

Figure 2 shows a variation of the optical system of Figure 1. Optical system 50 is an example of a system that uses a PIC 4 to stabilize the optical phase difference between the two arms of a Mach-Zehnder interferometer.

Transmitter 2 is configured to provide a multiplexed optical signal as described with reference to FIG. 1. In contrast to optical system 1 shown in FIG. 1, transmitter 2 in optical system 50 does not send the multiplexed signal directly to PIC 4, but into a first input port of the Mach-Zehnder interferometer.

The Mach-Zehnder interferometer is formed by the first and second optical beam splitters 52, 56. The first arm of the Mach-Zehnder interferometer is formed by the optical channel 54 connecting a first output of the first beam splitter 52 with a first input of the second beam splitter 56. The second arm of the Mach-Zehnder interferometer is formed by the optical channel 58, the PIC4, and the optical channel 60. The optical channel 58 connects a second output of the first beam splitter 52 with an input port of the PIC4. The optical channel 60 connects an output port of the PIC4 with a second input port of the second beam splitter 56.

In an embodiment, the overall optical path length of the first arm of the Mach-Zehnder interferometer is substantially the same as the overall optical path length of the second arm. In other words, the two arms form a symmetric Mach-Zehnder interferometer. Alternatively, the overall optical path length of the first arm of the Mach-Zehnder interferometer is different from the overall optical path length of the second arm. In this case, the two arms form an asymmetric Mach-Zehnder interferometer.

The first output of the second beam splitter 56 is connected to the receiver 6 described with reference to FIG. 1.

During operation of the optical system 50, the transmitter 2 sends the multiplexed signal to the Mach-Zehnder interferometer. The receiver 6 may determine the optical power level in any of the first to fifth output channels 28 ₁ to 28 ₅ and may determine from the detected power level that the optical path length difference between the first arm and the second arm has changed. In response, a feedback signal may control the first to fifth phase control elements 18 ₁ to 18 ₅ and the further phase control element 22 such that the change in the optical path length difference is corrected. Depending on the application, the feedback signal may be one in which the optical path length difference is corrected for all optical signals in the multiplexed signal. Alternatively, the feedback signal may be one in which the optical path length difference is corrected only for the signal whose power is to be determined.

Figure 3 shows a further variation of the optical system of Figure 1. Optical system 70 is an example of an optical ring system that uses a PIC 4 to stabilize the optical path length of the optical ring.

The transmitter 2 is configured to provide a multiplexed optical signal as described with reference to FIG. 1. As in the optical system 50 shown in FIG. 2, the transmitter 2 sends the multiplexed signal to a first input port of a beam splitter 52. As shown in FIG. 3, the first and second output ports of the first beam splitter 52 are connected to each other via optical channels 72, 74 and a PIC 4. The second output port of the beam splitter is connected to a receiver 6 as described with reference to FIG. 2.

During operation of the optical system 70, the transmitter 2 sends the multiplexed signal to the first input port of the beam splitter 52. The beam splitter 52 may be configured to have a 99 to 1 transmission to reflection ratio. The multiplexed signal then circulates inside the optical ring, i.e. the beam path formed by the first beam splitter 52, the optical channels 72, 74 and the PIC 4. Due to the interference of the circulating optical signals in the first beam splitter, the relative strength of the optical signals leaving the ring via the first or second input port depends on the optical path length of the ring. Thus, by controlling the phase shift provided by the first to fifth phase control elements 18 ₁ to 18 ₅ and the further phase control element 22, it is possible to control the exit port of the optical signal. The optical signals leaving the ring via the second input port of the beam splitter 52 are received and demultiplexed by the receiver 6.

In an exemplary application, the optical power level of the optical signal exiting either the first or second input ports of the beam splitter 52 may be determined and a feedback signal may be generated based on the detected power such that noise in the optical path length of the ring is corrected when the feedback signal is applied to the first to fifth phase control elements 18 ₁ to 18 ₅ and the further phase control element 22.

In an embodiment, the optical system 70 of FIG. 3 is part of an optical gyroscope.

In another embodiment, the optical channel 72 of the optical system 70 of FIG. 3 includes an optical gain medium configured to increase the intensity of at least one of the circulating optical signals. For example, in this case, the optical system 70 may be part of a fiber laser system.

Figure 4 is a schematic diagram of an illustrative optical system. Figure 4 shows a transmitter 30 connected to a receiver 32 by a communication channel 33.

In this embodiment, the transmitter 30 comprises a light source 34 that emits an optical signal of a single wavelength λ ₁ to an encoder 35. In this embodiment, the light source 34 is provided within the transmitter 30. However, the light source 34 could be provided external to the transmitter 30 and a conduit provided within the transmitter to direct light from the light source 34 to the encoder 35. However, the encoder provides a phase-variation modulation to the signal from the light source 34 to produce a first information signal having a single wavelength λ ₁ .

The transmitter also comprises a second light source 37 emitting an optical signal of a single wavelength λ _REF , i.e. the first reference signal. λ ₁ and λ _REF are different from each other, however in an embodiment they will be of similar wavelength.

The first information signal and the first reference signal are then multiplexed together by wavelength division multiplexer 39 to produce a first multiplexed signal. The first multiplexed signal is then output to communication channel 33, which carries the first multiplexed signal to receiver 32.

The receiver 32 comprises a photonic integrated circuit (PIC) 4' and a wavelength division demultiplexer 36. The PIC 4' is identical to the PIC 4 described with reference to Fig. 1, but the PIC 4' comprises only the first and second photonic waveguides ₁₆₁ , ₁₆₂ with the first and second phase control elements ₁₈₁ , _182. Like the PIC 4 described with reference to Fig. 1, the PIC 4' allows phase corrections to be applied i) individually to the signals contained in the multiplexed signal via the first and second phase control elements ₁₈₁ , ₁₈₂ , and ii) collectively to all signals contained in the multiplexed signal via the further phase control element 22. The PIC 4' receives the first multiplexed signals and the demultiplexer 14 demultiplexes them to provide a first reference signal to the first photonic waveguide ₁₆₁ and a first information signal to the second photonic waveguide ₁₆₂ . The first and second phase control elements 18 ₁ , 18 ₂ provide a phase shift to the first reference signal and the first information signal, respectively, before the multiplexer 20 multiplexes these signals and provides them to the further phase control element 22. The further phase control element 22 provides a further phase shift to the multiplexed signal and transmits the phase shifted first multiplexed signal to the wavelength division demultiplexer 36.

The wavelength division demultiplexer 36 receives the phase-shifted first multiplexed signals and demultiplexes them to recover the first information signal and the first reference signal. The recovered first information signal is then directed to the decoder 38, which decodes the phase information from the first information signal. The recovered first reference signal is then directed to the phase controller 40.

The information carried by the first information signal is encoded in phase. When the information signal is passed from the transmitter 30 to the receiver 32, phase noise will be introduced. For this reason, if it is desired to decode the phase in the decoder 38, correction must be made for phase drift.

This is achieved, for example, by processing the first reference signal via a phase controller 40 to determine the phase drift experienced by the first reference signal. The phase controller 40 is then used to provide feedback to the PIC 4' such that the first and second phase control elements ₁₈₁ and ₁₈₂ and the further phase control element 22 cooperate to provide a phase shift to the first information signal and the first reference signal such that the phase noise is corrected.

In some communication networks, such as quantum communication networks, it is necessary to keep the strength of the first information signal very low, and to maintain the security of the network, it is not possible to amplify the first information signal and use it to correct phase drift.

A specific example of the arrangement of FIG. 4 applied to a Twin Field Quantum Key Distribution (TF-QKD) system will now be described with reference to FIG. 5.

Figure 5 shows a TF-QKD scheme in which Alice and Bob each encode information in the phase of the optical pulses and phase noise reduction is applied.

In Figure 5, the term "WDM" is used to refer to wavelength division multiplexing and demultiplexing.

Alice 82 and Bob 84 use a local continuous wave (CW) laser LS1 to generate light of wavelength λ ₁ , shown in the diagram by a solid line (-). Alice's LS1 86 serves as a phase reference. The light is split into two at beam splitter BS 88. One part is sent to Bob 84 through a service fiber 90, shown by a dashed-dotted line (- -), and is used to lock Bob's LS1 92 via a heterodyne optical phase-locked loop (OPLL).

Both Alice 82 and Bob 84 send some of the light from their LS1 to an encoder 94. The encoder 94 performs phase and intensity modulation and outputs phase-encoded pulses, allowing different TF-QKD protocols to be implemented.

Alice 82 further comprises a second laser LS2 which generates a bright reference signal λ _REF . Light from LS2 at a wavelength of λ _REF (indicated in the diagram by a dotted line (...)) is directed to a beam splitter where it is split into two parts. One part of the bright reference is directed to a multiplexer where it is multiplexed with pulses from LS1 at a wavelength of λ _1. The multiplexed light, indicated in the diagram by a dashed-dotted line (-...-), is then sent to Charlie 98 via an optical channel (also called a quantum channel). The other part of the bright reference is directed from beam splitter 88 to a multiplexer which combines it with the light from LS1. The multiplexed light from LS1 and LS2 is directed to Bob via a service fiber 90.

At Bob 84, light from service fiber 90 is directed to a demultiplexer where light from LS1 at λ ₁ is separated from the bright reference at λ _REF . Light at λ ₁ from Alice is directed to an OPLL, which locks Bob's LS1 to Alice's LS1. The bright reference light at λ _REF is directed to another multiplexer where it is multiplexed with pulses from Bob's encoder 94 and then sent over the quantum channel to Charlie.

At Charlie 98, the light received from Bob is sent to beam splitter 100.

At Charlie 98, the light received from Alice is passed through PIC 4', which was described above with reference to FIG. 4. Demultiplexer 14 separates the quantum signal from the bright reference signal. The bright reference signal is passed through a waveguide that includes phase control element 18 ₁ , while the quantum signal is passed through a waveguide that includes phase control element 18 ₂ , which may adjust the phase of the quantum signal. Phase control element 18 ₂ implements a slow feedback system _, which is described below.

The quantum signal is then recombined with the reference signal in multiplexer 20, passed through further phase control element 22, and then directed to beam splitter 100. Further phase control element 22 may act on both the quantum signal and the bright reference signal to adjust the phase of the signals. Further phase control element 22 implements the fast feedback system described.

At the beam splitter 100, the light from Alice and Bob interfere. The interference output is directed to two demultiplexers where the interference result of the quantum signal (at λ ₁ ) is separated from the interference result of the reference signal (at λ _REF ). In one demultiplexer, the interference result at λ ₁ is monitored by detector D0 and the interference result at λ _REF is monitored by detector D2. The output of D2 is directed to a further phase control element 22 to implement fast feedback. D0 provides Charlie's output. In the other demultiplexer, the interference result at λ ₁ is separated and then directed to detector D1 for monitoring. The output of D1 is directed to phase control element 18 ₂ to implement slow feedback.

The output at detector D0 can be output by Charlie according to a TF-QKD scheme. In the Twin Field QKD (TF-QKD) scheme, information is encoded in the electromagnetic phase of the photons. It is desirable for Alice and Bob to exchange keys. Alice 82 and Bob 84 transmit to a trusted quantum receiver 98. TF-QKD has been developed for situations where the security of the measurement device owned by Bob may be questionable. In TF-QKD, user Bob 84 is configured as an optical transmitter, as is another user Alice 807. The two optical transmitters Alice 82 and Bob 84 send optical pulses to a repeater, usually called "Charlie", which optically combines and measures the optical pulses. Alice and Bob can distil the private key from the published result of Charlie's count. In TF-QKD, users Alice and Bob are both configured as optical transmitters, and thus security is not compromised by the vulnerability of the optical receiver. Securing an optical transmitter is much easier than securing an optical receiver. In the former, the optical pulses are prepared locally by a trusted user, whereas in the latter, the optical pulses are received from the outside and prepared by someone who is untrusted and potentially interested in breaking the security of the system. It is worth noting that if Charlie is malicious and does not follow the correct implementation of the TF-QKD protocol, two honest users, Alice and Bob, can always detect his cheating attempts with a very high probability due to the laws of quantum mechanics.

In this simplified scenario, a common fixed phase reference φ _R is always available to all users. Since the phase reference is common and constant for everyone, it can be assumed without loss of generality that φ _R =0. Alice 82 has a phase-locked light source 86 and a phase modulator 94. The phase-locked light source 86 generates optical pulses with a constant phase and outputs them to the phase modulator 94. The output of the phase modulator 94 is then multiplexed with the reference signal by WDM, as described above.

Bob's transmitter 84 is configured in the same manner as Alice's transmitter 82 and to avoid any unnecessary repetition. Like reference numbers are used to indicate like features.

Alice prepares a first light pulse using her light source 86 to produce the pulse, and then encodes her secret information in the electromagnetic phase difference between the light pulse and a phase reference φ _R using phase modulator 94. In this particular example, the encoding of the BB84 protocol [CH Bennett and G. Brassard, Proc. of IEEE Int. Conf. on Comp. Sys. Sign. Process. (IEEE, New York, 1984), pp. 175-179] is considered, where Alice encodes a random "base" of either Z or X by choosing a phase value α _A =0 or α _A =π/2, respectively, and encodes a random "bit" of either 0 or 1 by choosing a phase value β _A =0 or β _A =π, respectively.

The optical pulse prepared by Alice will then carry a total electromagnetic phase α _A + β _A. Alice then moves on to the next pulse and repeats the procedure. Bob performs similar steps with phases α _B and β _B. The total electromagnetic phases of the pulses leaving Alice's and Bob's modules are denoted by φ _A and φ _B , respectively:
Alice: φ _A ＝α _A ＋β _A (1)
Bob: φ _B =α _B +β _B (2)
Because all phases are stable, the phase values of Alice 82 and Bob 84 remain constant during propagation through the communication channel. When the light pulse reaches Charlie's non-polarizing beam splitter, it undergoes so-called "first order interference", which is the same kind of interference seen in double-slit interference experiments and standard QKD. This means that in order to interfere deterministically, the phases of Alice's and Bob's pulses should satisfy the following interference condition:
φ _B -φ _A = 0 mod π (3)
Here, "mod π" means "addition modulo π." Since the phase values associated with the bits are either 0 or π, equation (3) reduces in this case to the following condition on the basis coincidence condition:
α _B -α _A = 0 (4)
If this condition is met,
β _B -β _A = 0 (5)
light emerges from the port connected to detector 0, whereas
β _B - β _A = π (6)
When , light emerges from the port connected to the detector. Thus, after Charlie publishes his count, and after Alice and Bob publish their bases, Alice and Bob can reconstruct the bit values prepared by the other user for all cases where the bases match. If the bases do not match, the users discard the data, as in the standard BB84 protocol. In an embodiment, Charlie publishes all instances where exactly one of his detectors clicked. For these instances, he also publishes which detector clicked.

Another possibility is that Charlie also announces when both his detectors click. These double clicks are useless to the final key and can be handled in two ways:
1) Alice and Bob abandon execution if Charlie announces a double-click.
2) Alice and Bob convert the double click into a single click by randomly deciding which of Charlie's detectors did the click.

The security is the same in both cases. In the example, D0 and D1 are single photon detectors.

By way of example, D0 and D1 are superconducting nanowire single photon detectors (SNSPDs). By way of further example, the SNSPDs are Single Quantum EOS 410 CS cooled at 2.9 K.

In the example, detector D2 is the same as D1 or D0. Alternatively, D2 is a photodiode detector.

Note that to compensate for different stretching rates and changes in the length of the quantum channel connecting Charlie to the two users, Alice and Bob, the pattern encoding of one user to the other can be delayed by an empirically determined amount so that the pulses arriving at Charlie's BS821 are time aligned. The amount of delay can be adjusted at regular intervals to maintain time alignment. In an example, the amount of delay is adjusted between once every 4 minutes up to once every 30 minutes.

Phase stabilization may use the count rate of the dark reference detector D1 or the bright reference detector D2 as a feedback signal.

At short integration intervals, the counts detected by D1 or D2 are

where C ₀ represents the count floor, while C ₁ is the amplitude of the interference between the reference pulses,

is the phase difference that drifts rapidly due to phase noise from the channel. For the feedback provided by D2,

where C ₁ is the count rate of the reference signal (at λ _REF ) provided by Alice 82 and Bob 84. For the feedback provided by D1, C ₀ is the count rate associated with the phase-encoded pulse sent over λ ₁ , while C ₁ is the count rate of the dark reference signal (unmodulated phase) sent by Alice 82 and Bob 84 over λ _1. For the feedback provided by D1, when the strengths and probabilities (of occurrence) of the phase-encoded pulse and the reference pulse are equal,

The lock point for stabilization is the orthogonal point.

can be selected in.

Near the lock point, the count rate is approximately a linear function of the phase drift, thus allowing for improved phase compensation.

The lock point is obtained by varying the phase shift provided by the phase control element 18 ₂ or the further phase control element 22 such that the phase drift can be countered and a constant count rate of C ₀ +C ₁ can be maintained. The phase compensation is provided by a PID controller which receives as input the number of photons collected by the photon counter connected to the detector D2 or D1 and modifies a control voltage arranged to control the phase provided by the phase control element 18 ₂ or the further phase control element 22, respectively, through its amplified 12-bit DAC. In an example, the phase provided by the further phase control element 22 is corrected every 5 μs, while the phase provided by the phase control element 18 ₂ is corrected every 10-100 ms.

The fast feedback strategy described above allows the quantum channel to be stabilized without affecting the encoding at the wavelength reserved for the quantum signal (λ ₁ ) or the clock rate at which the quantum signal pulses can be encoded. In the example, the clock rate is set to 500 MHz.

The fast feedback system is configured to stabilize the fast drift introduced by the optical channel. The fast feedback system is configured to provide a fast error signal that can then be used to stabilize the fast drift. The stabilization method comprises monitoring the interference of a bright reference of wavelength λ _REF . From the interference of the bright pulse, a signal (i.e. an error signal) is derived and used to adjust the phase shift applied by the further phase control element 22. The fast feedback system may provide an almost instantaneous correction of the phase drift. For example, in an embodiment, the fast feedback is fast enough to be able to correct the phase drift occurring on the communication channel. For example, in the case of optical fiber, already over short distances (tens of kilometers), the phase drift is of the order of tens of radians per millisecond. In this example, the further phase control element 22 may be an electro-optical phase modulator.

The bright reference signal provides a bright light signal that allows the feedback system to operate at a fast rate. The bright reference signal (λ _REF ) can be brighter than the signal of interest (λ ₁ ) because the signal of interest is transmitted at a different wavelength.

The fast feedback system comprises a closed loop cycle that locks the interference between Alice's bright reference beam and Bob's bright reference beam to a given intensity level. This in turn locks the phase offset between these signals to a fixed value. The bright reference interference is monitored by detector D2.

In the example, the bright reference comprises a single photon that is detected by D2 and integrated over a period of time. In the example, the period of time is 5 μs. The intensity value of the bright signal can take on values within a range defined by a maximum and minimum level. The minimum level is the minimum intensity that the reference must have to provide an error signal fast enough to correct channel phase drift. The maximum level is the maximum intensity that the reference signal can have before it introduces excessive error to the signal of interest (or quantum signal). If the intensity of the reference signal is too high, effects such as inelastic scattering along the communication channel (Raman scattering), or limited optical isolation in WDM, will leak noise photons into the wavelength of the signal of interest, which is a problem.

As mentioned above, detector D2 does not have to be a single photon detector. It could be a photodiode. In that case, the operator would need to test that the reference signal does not introduce excessive noise into the signal wavelength, since a photodiode would require more light to provide useful information.

In the case of integration times, it will depend on the application, mostly on the amount of phase noise that has to be corrected. But since the phase drift introduced by long communication channels is on the order of tens of radians per second, integration times for information used by quantum feedback will need to be in the range of a few (tens) of microseconds to hundreds of nanoseconds.

The difference between the integrated counts and the set point constitutes the error signal of the PID controller. In the example, the PID controller is an FPGA clocked at 200 kHz. The FPGA controls the interference between the light references by adjusting the control voltage to a further phase control element 22 acting on the light coming from Alice.

Note that the phase shift applied by the further phase control element 22 affects both wavelengths λ _REF and λ _1. By way of example, feedback based on λ _REF completely stabilizes the bright reference light but only partially stabilizes the quantum signal at λ ₁ .

The remaining (slow) phase drift on λ ₁ is related to two factors: (i) λ ₁ and λ _REF may move separately in certain sections of the network, and (ii) the fast feedback introduces phase drift across λ ₁ as the optical path length difference seen by λ ₁ and λ _REF changes over time. The former component of the slow phase drift can be seen as phase noise picked up by the asymmetric Mach-Zehnder interferometer with the dimensions of those sections of the network where the two wavelengths move separately. The latter component can be explained as a result of the finite range of the further phase control element 22 and the phase locking of the fast feedback across λ _REF but not across λ ₁ .

An additional phase control element 22 in the fast feedback actively compensates for the fast phase drift. However, its finite adjustment range may not compensate for the entirety of the phase drift caused by fiber length variations. It relies on multiple (M) resets to maintain the λ _REF phase difference at φ=2πM+φ _t , where φ _t is the target phase. Due to the λ _REF -λ ₁ wavelength difference, this compensation will result in a residual phase drift (Δφ) over λ ₁ equal to Δφ=2πM×(λ _REF -λ ₁ )/λ ₁ .

The residual drift introduced by λ _REF -stabilization over λ ₁ is approximately 10 times larger than the original fiber length phase drift, assuming unidirectional fiber length drift.

is estimated to be small. In reality, the fiber length drift direction is random. Canceling the positive and negative 2π resets can result in substantially higher reduction factors.

A phase compensation scheme acting on the residual phase drift on λ ₁ is also called a slow feedback system. An error signal is obtained from the interference of the quantum signal at λ ₁ and is obtained from detector D1. The difference between this value and a set value provides the error signal to a PID controller implemented in a microcontroller. The microcontroller corrects the phase offset by controlling the phase shift provided by the phase control element 18 ₂ acting on the quantum signal coming from Alice. Unlike the stabilization based on the further phase control element 22 acting on both λ _REF and λ ₁ , the slow feedback acts only on the quantum signal (λ ₁ ) and is therefore able to correct its residual phase drift. In this example, a thermal phase shifter can be used. Unlike the fast feedback, the slow feedback does not require a high operating bandwidth. For this reason, a thermal phase shifter that changes the optical path length is suitable for the slow feedback. However, other types of phase modulators can be used for this task, for example electro-optical modulators.

Note that for the slow feedback system, the quantum signal at λ ₁ comprises an information-encoded pulse and a dark reference pulse. The dark reference pulse may be interleaved with the information-encoded pulse. In an example, the dark reference pulse has the same intensity as the brightest information carrying pulse. The presence of the dark reference pulse provides an interference output related to the residual phase offset during λ _1. The interference output is retrieved by integrating the single photons detected by D1 over a period of time. In an example, the period of time is 50 ms or 100 ms, depending on the distance of the optical channel. The difference between this value and a set value provides an error signal to a PID controller implemented in a microcontroller operating at a frequency of 20 Hz or 10 Hz, depending on the distance of the optical channel.

According to an example, the further phase control element 22 is an electro-optic modulator, where the refractive index of the material is a function of the applied electric field. A change in the refractive index results in a change in the optical path length, which results in a change in the phase shift applied by the phase modulator. Different voltages are applied to the phase modulator to give different phase shifts. A phase modulator as described may comprise a crystal, such as a lithium niobate (LiNbO3) crystal, where the refractive index is a function of the electric field strength, and the electric field may be applied by applying a voltage to electrodes positioned around the LiNbO3 crystal. In an embodiment, the further phase control element 22 has a high operating bandwidth. For example, the further phase control element 22 uses the electro-optic effect (maximum bandwidth of the order of GHz).

The relative phase shift imparted by the further phase control element is set by the PID controller described above, which may be configured to apply a control voltage signal to the further phase control element 22.

Note that the phase modulator provided in the encoder 94 may be similar to the PIC described above. Alternatively, adjustment of the phase shift may be achieved either through adjusting the phase modulator in the encoder 94 or by adding a control voltage signal to the drive signal applied to the phase modulator in the encoder 94.

By using a reference signal that does not carry sensitive information of the first information signal, it is possible to amplify the reference signal and use it to provide feedback to correct phase drift in the first information signal. Also, by using a reference signal that has a different wavelength than the information signal, it is possible to wavelength multiplex the information signal and the reference signal, so that the rate of information transmitted to the receiver is not reduced by sending an additional reference signal.

6 is a schematic diagram of an illustrative optical system 200 comprising a transmitter 202 configured to transmit an optical signal to a receiver 206 via an optical channel, a photonic integrated circuit 204, and an optical channel 214. The photonic integrated circuit 204 comprises a photonic waveguide 208 comprising first and second phase control elements 210, 212. The photonic integrated circuit 204 is configured to receive the optical signal transmitted by the transmitter 202 and provide the optical signal to the waveguide 208. The first phase control element 210 is configured to provide a first phase shift to the received optical signal. The second phase control element 212 is configured to provide a second phase shift to the received optical signal. The first phase control element 210 is a thermal phase shifter and the second phase control element 212 is an electro-optical phase modulator, or vice versa. The photonic integrated circuit 204 may be fabricated similarly to the photonic integrated circuit 4 described with reference to FIG. 1. In an embodiment, the transmitter 202 of the optical system 200 may encode information in the phase of the transmitted optical signal such that the receiver 206 can decode the optical signal to retrieve the information.

In operation, the photonic integrated circuit 204 can be used to compensate for phase noise introduced by an optical channel. Broadly speaking, the transmitter 202 alternately sends signals carrying information and a reference signal. To compensate for the phase noise, the receiver 206 detects the reference signal, determines a feedback signal to compensate for the phase noise, and uses a first phase control element 210 to compensate for slow phase drift and a second phase control element 212 to compensate for fast phase variations.

While certain embodiments have been described, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. Indeed, the novel devices and methods described herein may be embodied in a variety of other forms, and further, various omissions, substitutions, and changes in the forms of the devices, methods, and products described herein may be made without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover such forms and modifications within the scope and spirit of the invention.

Claims (20)

1. A photonic integrated circuit formed on an integration chip, comprising:
A demultiplexer;
a first phase control element, a second phase control element, and a third phase control element;
a multiplexer; and a demultiplexer comprising:
receiving a multiplexed optical signal comprising a first signal having a first wavelength and a second signal having a second wavelength different from the first wavelength;
demultiplexing the multiplexed optical signal to extract the first signal and the second signal.
the first phase control element is configured to provide a first phase shift to the first signal to produce a first modified signal;
the second phase control element is configured to provide a second phase shift to the second signal to produce a second modified signal;
the multiplexer is configured to multiplex the first modified signal and the second modified signal to produce a modified multiplexed signal;
the third phase control element is configured to provide a third phase shift to the modified multiplexed signal to produce a further modified multiplexed signal, and output the further modified multiplexed signal. The photonic integrated circuit of claim 1, wherein the first phase control element and the second phase control element are thermal phase shifters, and the third phase control element is an electro-optic phase modulator. The photonic integrated circuit of claim 1, wherein the first phase control element, the second phase control element, and the third phase control element are either a thermal phase shifter or an electro-optic phase modulator. The photonic integrated circuit of claim 1, wherein at least one of the demultiplexer and the multiplexer is an arrayed waveguide grating. The photonic integrated circuit of claim 1, wherein at least one of the demultiplexer and the multiplexer is an angled multimode interferometer. The photonic integrated circuit of claim 1, wherein at least one of the demultiplexer and the multiplexer is a distributed wavelength multiplexer. 1. A method of operating a photonic integrated circuit formed on an integrated chip, the method comprising:
receiving a multiplexed optical signal comprising a first signal having a first wavelength and a second signal having a second wavelength different from the first wavelength;
demultiplexing the multiplexed optical signal using a demultiplexer to extract the first signal and the second signal;
providing a first phase shift to the first signal using a first phase control element to produce a first modified signal;
providing a second phase shift to the second signal using a second phase control element to produce a second modified signal;
multiplexing, using a multiplexer, the first modified signal and the second modified signal to produce a modified multiplexed signal;
providing a third phase shift to the modified multiplexed signal using a third phase control element to produce a further modified multiplexed signal;
and outputting the further modified multiplexed signal, wherein the demultiplexer, the first phase control element, the second phase control element, the third phase control element, and the multiplexer are included in the photonic integrated circuit. The method of claim 7, wherein the first phase control element and the second phase control element are thermal phase shifters, and the third phase control element is an electro-optic phase modulator. The method of claim 7, wherein the first phase control element, the second phase control element, and the third phase control element are either a thermal phase shifter or an electro-optic phase modulator. The method of claim 7, wherein at least one of the demultiplexer and the multiplexer is an arrayed waveguide grating. The method of claim 7, wherein at least one of the demultiplexer and the multiplexer is an angled multimode interferometer. The method of claim 7, wherein at least one of the demultiplexer and the multiplexer is a distributed wavelength multiplexer. 1. An optical Mach-Zehnder interferometer having a first arm and a second arm,
The optical Mach-Zehnder interferometer, wherein the first arm comprises the photonic integrated circuit of claim 1 . An optical ring interferometer comprising the photonic integrated circuit of claim 1. The optical ring interferometer of claim 14, further comprising an optical gain medium connected to the photonic integrated circuit via an optical channel and configured to increase the intensity of at least one optical signal circulating in the optical ring. 1. An optical system comprising a transmitter and a receiver,
the transmitter comprises an encoding unit configured to encode information using a phase on a first optical information signal, the first optical information signal having a single first wavelength; the transmitter configured to output a first reference signal, the first reference signal having a reference wavelength different from the first wavelength; the transmitter further comprises a first multiplexer configured to multiplex the first optical information signal and the first reference signal to produce a multiplexed signal and output the multiplexed signal to a communication channel;
The receiver includes:
a first demultiplexer;
a first phase control element, a second phase control element, and a third phase control element;
a second multiplexer;
1. A photonic integrated circuit formed on an integration chip, comprising:
receiving the multiplexed signal;
demultiplexing the multiplexed signal to extract the first reference signal and the first optical information signal.
the first phase control element is configured to provide a first phase shift to the first reference signal to produce a modified reference signal;
the second phase control element is configured to provide a second phase shift to the first optical information signal to produce a modified optical information signal;
the second multiplexer is configured to multiplex the modified reference signal and the modified optical information signal to produce a modified multiplexed signal;
the third phase control element is configured to provide a third phase shift to the modified multiplexed signal to produce a further modified multiplexed signal, and output the further modified multiplexed signal; and
a second demultiplexer configured to demultiplex the further modified multiplexed signal received from the transmitter to extract the first optical information signal and the first reference signal;
a decoder configured to decode phase information in the first optical information signal;
a phase compensation unit configured to estimate a phase change in the first optical information signal caused by the communication channel from the first reference signal, and to compensate for the phase change in the first optical information signal caused by the communication channel using the photonic integrated circuit. The optical system of claim 16, wherein the first phase control element and the second phase control element are thermal phase shifters, and the third phase control element is an electro-optic phase modulator. a second transmitter, the second transmitter comprising an encoding unit configured to encode information using a phase on a second optical information signal, the second optical information signal having the first wavelength, the transmitter configured to output a second reference signal having the reference wavelength, the transmitter further comprising a multiplexer configured to multiplex the second optical information signal and the second reference signal to produce a multiplexed second signal and to output the multiplexed second signal to a communication channel;
the decoder is configured to decode phase information in the first optical information signal and the second optical information signal;
The phase compensation unit includes:
estimating a phase change of the first optical information signal and the second optical information signal caused by the communication channel from interference of the first reference signal and the second reference signal;
controlling the third phase shift such that phase changes of the first optical information signal and the second optical information signal caused by the communication channel are reduced;
estimating residual phase changes of the first and second optical information signals caused by the communication channel from interference of the first and second optical information signals;
and controlling the second phase shift such that phase changes of the first optical information signal and the second optical information signal caused by the communication channel are further reduced. A quantum communication system comprising the optical system of claim 18, the quantum communication system being configured to distribute a key between a first transmitting unit and a second transmitting unit. 1. A photonic integrated circuit formed on an integration chip, comprising:
a first phase control element and a second phase control element, the first phase control element configured to receive a first signal and provide a first phase shift to the first signal to produce a first modified signal;
the second phase control element is configured to provide a second phase shift to the first modified signal to produce a second modified signal and output the second modified signal;
A photonic integrated circuit, wherein the first phase control element is a thermal phase shifter and the second phase control element is an electro-optic phase modulator.