Self-coherent optical transmission system and receiver

Abstract

An optical receiver (100) includes: a polarization controller (102) arranged to receive a first modulated optical signal having a first polarization and an unmodulated optical carrier signal polarized and aligned with the first modulated optical signal as its input, the first modulated optical signal having a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the unmodulated optical carrier signal; an optical filter device (104) having a primary polarization mode; and a coherent optical receiver device (106), wherein the polarization controller is arranged to apply a polarization rotation to the first modulated optical signal and the unmodulated optical carrier signal such that their polarizations are aligned with the primary polarization mode of the optical filter device, the optical filter device is arranged to receive and separate the unmodulated optical carrier signal from the first modulated optical signal, and the coherent optical receiver device is arranged to receive the separated signal and use the unmodulated optical carrier signal as a local oscillator (LO) signal to perform coherent detection of the first modulated optical signal.

Description

Technical Field

[0001] This invention relates to an optical receiver and a self-coherent optical transmission system comprising said optical receiver. The method further relates to a method for coherent optical detection and to a method for self-coherent optical transmission incorporating said method for coherent optical detection. Background Technology

[0002] In the vast majority of current optical transmission networks, 100 Gbit / s transmission is achieved using dual-polarized quadrature phase-shift keying (QPSK) coherent optical interfaces. Capacity enhancement to 400 Gbit / s is possible by using 16-level quadrature amplitude modulation (16QAM) instead of QPSK and doubling the device bandwidth from 25 GHz to 50 GHz. Standards for interoperable 100 and 400 Gbit / s optical interfaces operating over metropolitan areas are specified by ITU-T Recommendation G.6982 and the Optical Interconnect Networks Forum (OIF) Scheme 400ZR, respectively.

[0003] Compared to intensity-modulated direct detection systems where data is encoded into the amplitude level of the optical signal, coherent systems transmit and detect information encoded in both the signal amplitude and phase. This has the dual advantages of allowing the use of complex modulation schemes (such as QAM) to increase system capacity, and equalization techniques to correct propagation losses (such as fiber dispersion). The latter has led to significant savings in network costs, eliminating the need for dispersion-compensating fibers (and the two-stage optical amplifiers that host them) in the optical link. The absence of dispersion-compensating fibers has greatly simplified system design rules (in older systems, sizing the size and location of compensation modules was crucial for optimizing system performance) and has a positive impact on operating costs. Although coherent optical transmission was proposed decades ago, it was only after the integration and miniaturization of electronic circuitry made it possible to implement complex processing functions in small integrated circuits, accompanied by acceptable power consumption of tens of watts.

[0004] Photonic circuits have followed a similar path toward higher integration (albeit with considerable latency); the number of functions that can be integrated into a photonic chip is orders of magnitude less than that in an electronic chip of the same size. Furthermore, optical processing functions are mostly analog, reducing processing possibilities compared to what can be accomplished through electronic processing. However, combining photonic and electronic processing in the same system can enable new architectures with improved cost and power consumption.

[0005] The success of coherent optical transmission in metropolitan and long-range optical networks (spanning hundreds to thousands of km) cannot be replicated in the more cost-sensitive shorter-range radio and fixed access networks. Energy efficiency is another issue; the integrated circuits used for signal post-processing in coherent receivers consume tens of watts, while a few watts would be acceptable. On the other hand, the capacity required by modern radio access networks can easily reach hundreds of Gbit / s, making coherent transmission an attractive technology regardless of cost and power consumption.

[0006] Coherent optical systems use a laser at the receiver as a local oscillator. The local oscillator light is coupled to the received signal. If the local oscillator frequency equals the transmitted optical carrier frequency, the beating term generated by photodetection is orthogonal to the modulated signal. If they are not equal, there is a frequency offset (several GHz in practical systems) that will be compensated for at the receiver, increasing the complexity of digital signal processing (DSP). In both cases, local oscillator phase noise is transmitted to the signal, and mitigating this requires expensive, narrow-linewidth lasers to be used as the local oscillator.

[0007] In a coherent optical system, a laser at the receiver is used as a local oscillator. The local oscillator light is coupled to the received signal. If the local oscillator frequency equals the transmitted optical carrier frequency, the hopping term generated by photodetector is orthogonal to the modulated signal. If they are not equal, there is a frequency offset (several GHz in practical systems) that will be compensated for at the receiver, increasing DSP complexity. In both cases, local oscillator phase noise is transmitted to the signal: mitigating it requires using an expensive, narrow-linewidth laser as the local oscillator.

[0008] Self-coherent optical systems do not have a local oscillator at the receiver; instead, they transmit the optical carrier along with the modulated signal (the optical carrier is suppressed in conventional coherent optical systems), thus avoiding the problems of phase noise transmission and frequency shift. However, appropriate techniques must be used in self-coherent receivers to separate the unmodulated carrier and the modulated signal.

[0009] To double the bit rate of the transmitted signal, data can be transmitted in two linearly orthogonal polarization states. During propagation in the fiber optic link, these two polarization states remain orthogonal but rotate so that their angle with the local oscillator's polarization state is random. At the front end of a conventional coherent receiver, the local oscillator and the modulated signal are mixed in a so-called 90° hybrid device, and the output is detected by a photodiode (dual-polarization receiver). Alternatively, a polarization controller can be used to rotate the polarization of the local oscillator, aligning it with the received modulated signal. For example, a polarization controller based on a phase shifter in an optical interferometer was reported by V. Sorianello et al., “Polarization Controller for Si photonic integrated circuits with an active closed loop control” (42nd European Conference on Optical Communications, Düsseldorf, September 18-22, 2016). After optical detection, the data transmitted on the two polarizations is restored using an electrical equalizer.

[0010] In a self-coherent system, the transmitted optical carrier is used as a local oscillator so that its angle with the received signal is known and polarization correction is not required. Summary of the Invention

[0011] The objective is to provide an improved optical receiver. A further objective is to provide an improved self-coherent optical transmission system. A further objective is to provide an improved method for coherent optical detection. A further objective is to provide an improved method for self-coherent optical transmission.

[0012] One aspect of the present invention provides an optical receiver comprising a polarization controller, an optical filter device, and a coherent optical receiver device. The polarization controller is arranged to receive a first modulated optical signal having a first polarization and an unmodulated optical carrier signal polarized-aligned with the first modulated optical signal as its input. The first modulated optical signal has a negligible spectral power density within a predetermined bandwidth (BW) around the spectrum of the unmodulated optical carrier signal. The optical filter device has a primary polarization mode. The polarization controller is arranged to apply a polarization rotation to the first modulated optical signal and the unmodulated optical carrier signal such that their polarizations are aligned with the primary polarization mode of the optical filter device. The optical filter device is arranged to receive the unmodulated optical carrier signal and the first modulated optical signal from the polarization controller and is arranged to separate the unmodulated optical carrier signal from the first modulated optical signal. The coherent optical receiver device is arranged to receive the separated signal from the optical filter device and is arranged to use the unmodulated optical carrier signal as a local oscillator (LO) signal to perform coherent detection of the first modulated optical signal.

[0013] The optical receiver mitigates phase noise transmitted from the local oscillator signal (the unmodulated optical carrier signal) to the received modulated optical signal without requiring the use of an expensive narrow-linewidth laser as the local oscillator source. The optical receiver does not require any correction of the frequency offset between the local oscillator signal and the transmitted optical carrier signal. The polarization alignment of the first modulated optical signal and the unmodulated optical carrier signal to the primary polarization mode of the optical filter device enables the optical front end (particularly the optical filter device) to be cost-effectively implemented in silicon photonics.

[0014] In one embodiment, the polarization controller is arranged to additionally receive a second modulated optical signal having a second polarization orthogonal to the first polarization. The polarization controller is arranged to separate the second modulated optical signal from the first modulated optical signal and the unmodulated optical carrier signal. The optical receiver further includes a second coherent optical receiver device arranged to use a portion of the unmodulated optical carrier signal output from the optical filter device as a local oscillator (LO) signal to perform coherent detection of the second modulated optical signal.

[0015] In one embodiment, the polarization controller is a polarization rotator-splitter.

[0016] In one embodiment, the optical filter device includes an optical band splitter having a first output, a second output, and an input, the input being arranged to receive the unmodulated optical carrier signal and the first modulated optical signal from the polarization controller, wherein the optical band splitter is arranged to send an optical signal in a first spectral band containing the spectrum of the unmodulated optical carrier signal to the first output and to send an optical signal in a second spectral band containing the spectrum of the first modulated optical carrier signal to the second output.

[0017] In one embodiment, the optical band splitter is one of a Bragg grating and a ring resonator. The Bragg grating is configured to reflect one of the first and second spectral bands and to transmit the other of the first and second spectral bands. The ring resonator has a resonant peak and is configured to drop the optical signal in the first spectral band to the first output and transmit the optical signal in the second spectral band to the second output.

[0018] In one embodiment, the first spectral band has a bandwidth that is more than twice the linewidth of the spectrum of the unmodulated optical carrier signal and less than twice the predetermined bandwidth BW. This ensures effective separation between the unmodulated optical carrier signal and the first modulated optical signal.

[0019] In one embodiment, the optical band splitter is a tunable optical band splitter. The optical receiver further includes a controller. The controller is configured to generate a control signal for the polarization controller. The control signal is configured to control the applied polarization rotation. The controller is further configured to generate a control signal for the tunable optical band splitter. The control signal is configured to tune at least one of the first and second spectral bands. The controller is arranged to modify the control signal until the optical power of the unmodulated optical carrier signal output from the optical filter device is maximized.

[0020] The use of a tunable optical band splitter can enable compensation for frequency drift of the unmodulated optical carrier signal (e.g., due to thermal drift). A common controller for controlling the applied polarization rotation and the tuning of the tunable optical band splitter can enable rapid convergence and mitigate outage problems, such as the insufficient dynamic range faced by existing polarization controllers.

[0021] In one embodiment, the primary polarization mode is a linearly polarized primary propagation mode. The polarization alignment of the first modulated optical signal and the unmodulated optical carrier signal to the linearly polarized primary propagation mode of the optical filter device enables the optical filter device to be cost-effectively implemented in silicon photonics.

[0022] The corresponding embodiments and advantages apply to the self-coherent optical transmission system described below.

[0023] One aspect of the present invention provides a self-coherent optical transmission system including an optical receiver and an optical transmitter.

[0024] The optical receiver includes a polarization controller, an optical filter device, and a coherent optical receiver device. The polarization controller is arranged to receive a first modulated optical signal having a first polarization and an unmodulated optical carrier signal polarized and aligned with the first modulated optical signal as its input. The first modulated optical signal has a negligible spectral power density within a predetermined bandwidth (BW) around the spectrum of the unmodulated optical carrier signal. The optical filter device has a primary polarization mode. The polarization controller is arranged to apply a polarization rotation to the first modulated optical signal and the unmodulated optical carrier signal such that their polarizations are aligned with the primary polarization mode of the optical filter device. The optical filter device is arranged to receive the unmodulated optical carrier signal and the first modulated optical signal from the polarization controller and is arranged to separate the unmodulated optical carrier signal from the first modulated optical signal. The coherent optical receiver device is arranged to receive the separated signal from the optical filter device and is arranged to use the unmodulated optical carrier signal as a local oscillator (LO) signal to perform coherent detection of the first modulated optical signal.

[0025] The optical transmitter includes a light source, an optical modulator, and an optical splitter arranged to generate an optical carrier signal. The optical splitter is arranged to power split the optical carrier signal and to route a first portion of the optical carrier signal to the optical modulator and a second portion of the optical carrier signal to bypass the optical modulator. The optical modulator is arranged to apply signal encoding to the first portion of the optical carrier signal to form a first modulated optical signal, the signal encoding being configured such that the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth (BW) around the spectrum of the optical carrier signal. The first modulated optical signal has a first polarization, and the unmodulated optical carrier signal is polarized aligned with the first modulated optical signal. The optical transmitter is configured to combine the first modulated optical signal and the unmodulated optical carrier signal for transmission.

[0026] The signal encoding ensures that the unmodulated optical carrier signal and the first modulated optical signal are spectrally separated, creating a band gap between them. This allows the optical filter at the optical receiver to effectively separate the unmodulated optical carrier signal from the first modulated optical signal. Forming the unmodulated optical carrier signal and the first modulated optical signal from the same optical carrier signal reduces phase noise transmitted from the local oscillator signal (unmodulated optical carrier signal) to the first modulated optical signal at the optical receiver and ensures that the local oscillator signal and the first modulated optical signal maintain polarization alignment during transmission.

[0027] In one embodiment, the optical modulator includes a dual-polarization modulator arranged to separate the first portion of the optical carrier signal into a first optical signal having the first polarization and a second optical signal having a second orthogonal polarization. The dual-polarization modulator is arranged to apply corresponding signal encoding to the first and second optical signals to form a first modulated optical signal having the first polarization and a second modulated optical signal having the second orthogonal polarization. The dual-polarization modulator is configured to combine the first modulated optical signal and the unmodulated optical carrier signal for transmission. The dual-polarized modulated optical signal can thus be transmitted along with the unmodulated optical carrier signal, which serves as a common local oscillator signal at the optical receiver, the polarization of which is aligned with one of the modulated optical signals (i.e., the first modulated optical signal).

[0028] In one embodiment, the optical modulator is arranged to apply signal encoding with a cutoff frequency equal to or higher than the predetermined bandwidth. This ensures that the unmodulated optical carrier signal and the first modulated optical signal are spectrally separated.

[0029] One aspect of the present invention provides a method for coherent optical detection comprising the following steps. A step is to receive a first modulated optical signal having a first polarization and an unmodulated optical carrier signal polarized and aligned with the first modulated optical signal. The first modulated optical signal has a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the optical carrier signal. A next step is to apply a polarization rotation to the first modulated optical signal and the unmodulated optical carrier signal such that their polarizations are aligned with a dominant polarization mode. A next step is to act on the signal aligned with the dominant polarization mode to separate the unmodulated optical carrier signal from the first modulated optical signal. A next step is to use the separated signal, using the unmodulated optical carrier signal as a local oscillator signal, to perform coherent detection of the first modulated optical signal.

[0030] The method reduces phase noise transmitted from the local oscillator signal (the unmodulated optical carrier signal) to the received modulated optical signal without requiring the use of an expensive narrow-linewidth laser as the local oscillator source. The method does not require any correction of the frequency offset between the local oscillator signal and the transmitted optical carrier signal.

[0031] In one embodiment, the method further includes the following steps: A step is to receive a second modulated optical signal having a second polarization orthogonal to the first polarization. A step is to separate the second modulated optical signal from the first modulated optical signal and the unmodulated optical carrier signal. A step is to use the separated signal, with the unmodulated optical carrier signal used as a local oscillator signal, to perform coherent detection of the second modulated optical signal.

[0032] In one embodiment, separating the unmodulated optical carrier signal from the first modulated optical signal includes band splitting the unmodulated optical carrier signal and the first modulated optical signal. The band splitting is arranged to send an optical signal in a first spectral band containing the spectrum of the unmodulated optical carrier signal to a first output, and to send an optical signal in a second spectral band containing the spectrum of the first modulated optical carrier signal to a second output.

[0033] In one embodiment, the first spectral band has a bandwidth that is more than twice the linewidth of the spectrum of the unmodulated optical carrier signal and less than twice the predetermined bandwidth BW. This ensures effective separation between the unmodulated optical carrier signal and the first modulated optical signal.

[0034] In one embodiment, the method further includes varying the applied polarization rotation and tuning at least one of the first and second spectral bands until the optical power of the unmodulated optical carrier signal is maximized after the separation. Any frequency drift in the unmodulated optical carrier signal (e.g., due to thermal drift) can thus be compensated for during the separation.

[0035] The corresponding embodiments and advantages apply to the self-coherent optical transmission method described below.

[0036] One aspect of the present invention provides a method for self-coherent optical transmission including the following transmission steps and receiving and detection steps. The transmission steps include a step of generating an optical carrier signal having a first polarization. The next step is power separation of the optical carrier signal. A step involves applying signal encoding to a first portion of the optical carrier signal to form a first modulated optical signal. The signal encoding is configured such that the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth (BW) around the spectrum of the optical carrier signal. The next step is combining the first modulated optical signal with a second unmodulated portion of the optical carrier signal. The first modulated optical signal has a first polarization, and the unmodulated optical carrier signal is polarization aligned with the first modulated optical signal. The next step is transmitting the combined signal.

[0037] The method further includes the following receiving and detection steps after transmission. One step is to receive a first modulated optical signal having a first polarization and an unmodulated optical carrier signal polarized and aligned with the first modulated optical signal. The first modulated optical signal has a negligible spectral power density within a predetermined bandwidth (BW) around the spectrum of the optical carrier signal. The next step is to apply a polarization rotation to the first modulated optical signal and the unmodulated optical carrier signal such that their polarizations are aligned with a primary polarization mode. The next step is to separate the unmodulated optical carrier signal from the first modulated optical signal by acting on the signal aligned with the primary polarization mode. The next step is to use the separated signal, using the unmodulated optical carrier signal as a local oscillator signal, to perform coherent detection of the first modulated optical signal.

[0038] The signal encoding ensures that the unmodulated optical carrier signal and the first modulated optical signal are spectrally separated, creating a band gap between them, thus achieving effective separation. Forming the unmodulated optical carrier signal and the first modulated optical signal from the same optical carrier signal reduces phase noise transmitted from the local oscillator signal (unmodulated optical carrier signal) to the first modulated optical signal and ensures that the local oscillator signal and the first modulated optical signal maintain polarization alignment during transmission.

[0039] In one embodiment, the modulation includes polarization splitting the first portion of the optical carrier signal into a first optical signal having the first polarization and a second optical signal having a second orthogonal polarization, and applying corresponding signal encoding to the first and second optical signals to form a first modulated optical signal having the first polarization and a second modulated optical signal having the second orthogonal polarization. The dual-polarized modulated optical signal can thus be transmitted along with an unmodulated optical carrier signal used as a common local oscillator signal at the optical receiver, the polarization of which is aligned with one of the modulated optical signals (i.e., the first modulated optical signal).

[0040] In one embodiment, the signal encoding applied to the first optical signal has a cutoff frequency equal to or higher than the predetermined bandwidth BW. This ensures that the unmodulated optical carrier signal and the first modulated optical signal are spectrally separated.

[0041] Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings. Attached Figure Description

[0042] Figure 1 and 2 This is a block diagram illustrating an embodiment of the optical receiver;

[0043] Figure 3 This is a block diagram illustrating an embodiment of an optical filter device for an optical receiver;

[0044] Figure 4 This is a block diagram illustrating one embodiment of a light receiver;

[0045] Figure 5 and 6 This is a block diagram illustrating an embodiment of an optical transmission system; and

[0046] Figure 7 and 8 This is a flowchart illustrating an embodiment of the method steps. Detailed Implementation

[0047] The same reference number will be used for the corresponding feature in different embodiments.

[0048] refer to Figure 1 One embodiment provides an optical receiver 100, which includes a polarization controller 102, an optical filter device 104 having a primary polarization mode, and a coherent optical receiver device 106. The polarization controller is arranged to receive a first modulated optical signal S. xThe first modulated optical signal is input to the unmodulated optical carrier signal (LO). The first modulated optical signal has a first polarization, and the unmodulated optical carrier signal is polarized aligned with the first modulated optical signal. The first modulated optical signal has a negligible spectral power density within a predetermined bandwidth (BW) around the spectrum of the unmodulated optical carrier signal.

[0049] The polarization controller 102 is arranged to apply polarization rotation to the first modulated optical signal and the unmodulated optical carrier signal such that their polarization is aligned with the main polarization mode of the optical filter device.

[0050] Optical filter device 104 may be arranged to receive the unmodulated optical carrier signal and the first modulated optical signal from the polarization controller; the unmodulated optical carrier signal and the first modulated optical signal may therefore be received by the optical filter device, wherein their polarization is aligned with the main polarization mode of the optical filter device. The optical filter device may be arranged to separate the unmodulated optical carrier signal from the first modulated optical signal.

[0051] Coherent optical receiver device 106 may be arranged to receive, from the optical filter device, the separated unmodulated optical carrier signal LO and the first modulated optical signal S. x The coherent receiver device may be configured to use the unmodulated optical carrier signal as a local oscillator (LO) signal to perform coherent detection of the first modulated optical signal.

[0052] In one embodiment, the main polarization mode of the optical filter is the main propagation mode of the linear polarization of the optical filter.

[0053] exist Figure 2 In the optical receiver 200 of the embodiment shown, the polarization controller 202 can be arranged to additionally receive a second modulated optical signal S. y The second modulated optical signal has a second polarization orthogonal to the first polarization. The polarization controller 202 is arranged to separate the second modulated optical signal from the first modulated optical signal and the unmodulated optical carrier signal.

[0054] The optical receiver 200 of this embodiment may include a first coherent optical receiver device 204x and a second coherent optical receiver device 204y, respectively arranged to perform coherent detection of the first and second modulated optical signals. Both the first and second coherent optical receiver devices use a portion of the unmodulated optical carrier signal output from the optical filter device 104 as a local oscillator (LO) signal.

[0055] In one embodiment, the polarization controller 202 may be a polarization rotator splitter, such as that described, for example, in Wesley D. Sacher et al., “Polarization rotator-splitters in standard active siliconphotonics platforms” (Optics Express, Vol. 22, No. 4, pp. 3777-3786, February 10, 2014).

[0056] In one embodiment, the optical filter device 300 may include an optical band splitter 302, such as in Figure 3 As shown in the diagram. The optical band splitter can be used in any of the optical receivers 100 and 200 described above.

[0057] An optical band splitter is a device that separates the spectrum of an input signal into two parts with separate frequency bands, sending each band to a different output. The signal encoding applied here ensures that a band gap exists between these parts.

[0058] The optical band splitter has a first output 304, a second output 306, and an input 308, the input being arranged to receive an unmodulated optical carrier signal 320 and a first modulated optical signal 322 from polarization controllers 102, 202. The optical band splitter is arranged to send an optical signal within a first spectral band 310 to the first output and an optical signal within a second spectral band 312 to the second output. The first spectral band contains the spectrum of the unmodulated optical carrier signal 320, and the second spectral band contains the spectrum of the first modulated optical carrier signal 322. The optical band splitter effectively acts as a bandpass filter (with respect to the unmodulated optical carrier signal) and a notch filter (with respect to the first modulated optical signal).

[0059] The optical band splitter may be, for example, a Bragg grating formed in a planar waveguide or optical fiber, the Bragg grating being configured to reflect one of the first and second spectral bands and to transmit the other of the first and second spectral bands. Figure 3 In the example shown, the Bragg grating is configured to reflect the first spectral band and transmit the second spectral band.

[0060] The optical band splitter may alternatively be, for example, a ring resonator formed in a planar waveguide or optical fiber, the ring resonator having a resonant peak and configured to place an optical signal in the first spectral band to the first output and to transmit an optical signal in the second spectral band to the second output.

[0061] The optical band splitter may alternatively be, for example, an optical power splitter followed by a band-stop filter and a band-pass filter on its two output arms.

[0062] In one embodiment, the first spectral band has a bandwidth that is twice as large as the linewidth of the spectrum of the unmodulated optical carrier signal and less than twice the predetermined bandwidth BW around the spectrum of the unmodulated optical carrier signal.

[0063] exist Figure 4 In the optical receiver 400 of the illustrated embodiment, the optical band splitter may be a tunable optical band splitter 402. The optical receiver further includes a photodetector 406 and a controller 404.

[0064] The controller includes a control circuit module and an interface circuit module configured as a polarization controller 102 (or 202) to generate a control signal X1. The control signal is configured to control the applied polarization rotation such that the polarization of the unmodulated optical carrier signal and the first modulated optical signal can be rotated to align with the primary polarization mode of the optical filter device; if the signal is received along with its polarization aligned with the primary polarization mode of the optical filter device, a zero polarization rotation is applied.

[0065] The polarization controller may, for example, comprise a series of Mach Zehnder interferometers, as described in V. Sorianello et al., “Polarization Controller for Si photonic integrated circuits with anactive closed loop control” (42nd European Conference on Optical Communications, Düsseldorf, September 18-22, 2016). The control signals comprise three voltage levels controlling the phase shift in the arms of three corresponding Mach Zehnder interferometers.

[0066] The tunable optical band splitter 402 can be a Bragg grating or a ring resonator, as described above, both of which can be configured to be tunable. As those skilled in the art will appreciate, a Bragg grating can be tuned by varying the temperature or strain of the application, and a ring resonator can be tuned by varying the temperature of the application or the injected current.

[0067] Alternatively, the tunable optical band splitter may include an acousto-optic tunable filter (AOTF), an electro-optic tunable filter (EOTF), a tunable Fabry-Perot etalon, one or more arrayed waveguide gratings (AWG), one or more Mach Zhender interferometers (MZI), an active optical filter, and micromechanical optical devices, as described in D. Sadot and E. Bolmovich's "Tunable optical filter for Dense WDM networks" (IEEE Communications Magazine, December 1998, pp. 50-55).

[0068] The controller 404 is further configured as a tunable optical band splitter 402 to generate a control signal X2. This control signal is configured to tune at least one of the first and second spectral bands. For example, the tunable optical band splitter may be a Bragg grating; changing the applied temperature or strain will tune the first spectral band (the reflection band of the grating) and correspondingly change the transmission band, i.e., the second spectral band. Alternatively, the tunable optical band splitter may be a ring resonator; changing the applied temperature or injected current will tune the first spectral band (the resonant peak of the ring resonator), which will correspondingly change the second spectral band.

[0069] Controller 404 is configured to modify control signals X1 and X2 until the optical power of the unmodulated optical carrier signal LO output from the optical filter device, as measured by photodetector 406, is maximized. This may include iteratively varying the applied polarization rotation and tuning the first and / or second spectral bands until they converge at values ​​that maximize the optical power of the unmodulated optical carrier signal. Known algorithms (such as steepest descent or gradient algorithms) may be used to perform this maximization.

[0070] For example, if the polarization controller 202 is a polarization rotator separator as described in the article by Wesley D. Sacher et al. (ibid.), then the controller 404 is configured to iteratively adjust the three voltages v1, v2, and v3 that control the phase shift in the arm of the continuous Mach Zehnder interferometer.

[0071] refer to Figure 5 One embodiment provides a self-coherent optical transmission system 500 including an optical transmitter 510 and an optical receiver 530 connected by an optical link 540.

[0072] The optical transmitter 510 includes a light source 512, an optical modulator 514, and an optical splitter 516.

[0073] The light source 512 is typically a laser, which is arranged to generate an optical carrier signal with a spectrum and a first polarization.

[0074] Optical splitter 516 is configured to efficiently split the optical carrier signal and to route a first portion of the optical carrier signal to the optical modulator and a second portion of the optical carrier signal to bypass the optical modulator, thereby becoming an unmodulated optical carrier signal.

[0075] Optical modulator 514 is arranged to apply signal encoding to a first portion of the optical carrier signal to form a first modulated optical signal. The optical modulator shown in this embodiment may be a dual-polarized optical modulator; however, it will be understood that a single-polarized optical modulator may alternatively be used as described above. Figure 1 Or, as described in 4, the transmission to the light receiver.

[0076] The dual-polarized optical modulator is arranged to separate a first portion of the optical carrier signal into a first optical signal having the first polarization and a second optical signal having a second orthogonal polarization. The dual-polarized optical modulator is arranged to apply corresponding signal encoding to the first and second optical signals to form a first modulated optical signal having the first polarization and a second modulated optical signal having the second orthogonal polarization. The signal encoding is configured such that the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth (BW) around the spectrum of the optical carrier signal and therefore around the spectrum of the unmodulated optical carrier signal.

[0077] The first modulated optical signal has the first polarization, and the unmodulated optical carrier signal is polarized aligned with the first modulated optical signal because both originate from the optical carrier signal.

[0078] The optical transmitter is configured to combine the first modulated optical signal, the second modulated optical signal, and the unmodulated optical carrier signal for transmission.

[0079] In one embodiment, the optical modulator may be arranged to apply signal coding having a cutoff frequency equal to or higher than a predetermined bandwidth BW. This signal coding is configured to deplete the signal spectrum around the DC component (the optical carrier signal) and may be one of Manchester coding, Alternate Mark Inversion (AMI) coding, or 64B66B coding.

[0080] In one embodiment, the optical transmitter 510 further includes an optical dispersion pre-compensation device 520 and / or an electrical dispersion pre-compensation device 518 configured to apply pre-compensation for the dispersion accumulated during transmission across the optical link 540 by the first modulated optical signal, the second modulated optical signal, and the unmodulated optical carrier signal.

[0081] The optical receiver 530 is as described above. Figure 2 The described device includes a polarization controller 202, an optical filter device 104, a first coherent optical receiver 204x, and a second coherent optical receiver 204y, and also includes a controller 404, as referred to above. Figure 4 As described.

[0082] At the optical receiver, the first modulated optical signal, the second modulated optical signal, and the unmodulated optical carrier signal arrive with their polarizations rotated by random angles relative to their first and second polarizations for transmission. The polarization controller rotates the polarization of the dual-polarized signals.

[0083] In one embodiment, the polarization controller may be a polarization rotator splitter configured to both apply polarization rotation and separate the orthogonally polarized first and second modulated optical signals, which are then sent to two outputs. A polarization rotator splitter as described in Wesley D. Sacher et al., “Polarization rotator-splitters in standard active silicon photonics platforms” (Optics Express Vol. 22, No. 4, February 24, 2014, pp. 3777-3786), may be used.

[0084] refer to Figure 6 One embodiment provides a self-coherent optical transmission system 600 including an optical transmitter 610 and an optical receiver 630 connected by an optical link 540.

[0085] In this embodiment, the optical modulator may include a first IQ modulator 514x and a second IQ modulator 514y. The first IQ modulator is configured to encode a signal applied to the first optical signal having the first polarization, and the second IQ modulator is configured to encode a signal applied to the second optical signal having the second orthogonal polarization.

[0086] Each IQ modulator 514x, 514y is provided with a corresponding digital-to-analog converter DAC 518x, 518y (configured to apply electro-dispersion pre-compensation).

[0087] In the optical receiver 630, the first and second coherent optical receivers include a corresponding 90° light mixer, a balanced photodetector, an Rx 634, and a digital signal processing (DSP) module 636.

[0088] The embodiments described above provide a self-coherent optical transmission system, wherein:

[0089] In the transmitter, the unmodulated optical carrier is transmitted together with the modulated signal, and the modulated signal is encoded such that its spectral power density is negligible within a predetermined bandwidth BW around the carrier.

[0090] In a dual-polarization system, the optical carrier signal polarization is aligned with one of the two modulated optical signals transmitted in two orthogonal polarization states;

[0091] The receiver includes a polarization controller followed by an optical filter device for extracting the unmodulated optical carrier signal for use as a local oscillator signal;

[0092] The optical filter device separates the optical carrier from the modulated optical signal by means of a passband narrowband tunable optical filter having a bandwidth equal to or narrower than a predetermined bandwidth BW.

[0093] The polarization controller and the optical filter device can be cost-effectively implemented in silicon photonics and can be monolithically integrated;

[0094] The optical filter device can be tuned to compensate for frequency drift (e.g., due to thermal drift) of the optical carrier signal generated by the optical transmitter; and

[0095] The optical filter device and the polarization controller share a common controller that sets their input variables to maximize the optical power of the unmodulated optical carrier signal output from the optical filter device.

[0096] The self-coherent optical transmission system advantageously mitigates phase noise transmitted from the local oscillator to the received modulated optical signal without requiring the use of expensive narrow-linewidth lasers. It does not require any correction of the frequency offset between the local oscillator and the transmitted optical carrier. The optical front end of the receiver can be cost-effectively implemented in silicon photonics. Concurrent control of optical carrier extraction and received signal polarization using the optical filter device alleviates the problems of current polarization controllers in terms of dynamic range and interruption probability.

[0097] refer to Figure 7 One embodiment provides a method 700 for coherent light detection.

[0098] The method includes the following steps:

[0099] The receiver 702 receives a first modulated optical signal having a first polarization and an unmodulated optical carrier signal polarized to the first modulated optical signal, wherein the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the optical carrier signal.

[0100] Then, polarization rotation 704 is applied to the first modulated optical signal and the unmodulated optical carrier signal so that their polarizations are aligned with the dominant polarization mode.

[0101] Then, acting on the signal aligned with the primary polarization mode, the unmodulated optical carrier signal is separated from the first modulated optical signal 706; and

[0102] Then, using the separated signals, the unmodulated optical carrier signal is used as a local oscillator signal to perform coherent detection of the first modulated optical signal 708.

[0103] In one embodiment, the method further includes the following steps:

[0104] Receive a second modulated optical signal having a second polarization orthogonal to the first polarization;

[0105] Separate the second modulated optical signal from the first modulated optical signal and the unmodulated optical carrier signal; and

[0106] Using the separated signals, the unmodulated optical carrier signal is used as a local oscillator signal to perform coherent detection of the second modulated optical signal.

[0107] In one embodiment, the step of separating the unmodulated optical carrier signal from the first modulated optical signal includes band splitting the unmodulated optical carrier signal and the first modulated optical signal. The band splitting is arranged to send an optical signal within a first spectral band containing the spectrum of the unmodulated optical carrier signal to a first output and an optical signal within a second spectral band containing the spectrum of the first modulated optical carrier signal to a second output.

[0108] In one embodiment, the first spectral band has a bandwidth that is twice as large as the linewidth of the spectrum of the unmodulated optical carrier signal and less than twice the predetermined bandwidth BW.

[0109] In one embodiment, the method further includes varying the applied polarization rotation and tuning at least one of the first and second spectral bands until the optical power of the unmodulated optical carrier signal is maximized after the separate steps.

[0110] refer to Figure 8 One embodiment provides a method 800 for coherent optical transmission.

[0111] The method includes the following transmission steps:

[0112] Generate an 802 optical carrier signal with the first polarization;

[0113] Power separation of the optical carrier signal described in 804;

[0114] The signal encoding 806 is applied to a first portion of the optical carrier signal to form a first modulated optical signal, the signal encoding being configured such that the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the optical carrier signal.

[0115] The first modulated optical signal is combined with the second unmodulated portion of the optical carrier signal 808, wherein the first modulated optical signal has a first polarization, and the unmodulated optical carrier and the first modulated optical signal are polarization aligned;

[0116] Transmit the combined signal.

[0117] The method further includes the step of receiving and detecting the first modulated optical signal according to the coherent optical detection method 700 described above after transmission.

[0118] In one embodiment, the step of applying signal encoding includes polarizing the first portion of the optical carrier signal into a first optical signal having the first polarization and a second optical signal having a second orthogonal polarization. Corresponding signal encoding is then applied to the first and second optical signals to form a first modulated optical signal having the first polarization and a second modulated optical signal having the second orthogonal polarization.

[0119] In one embodiment, the signal encoding applied to the first optical signal has a cutoff frequency equal to or higher than a predetermined bandwidth BW.

Claims

1. A self-coherent optical transmission system, comprising: - An optical receiver includes a polarization controller, the polarization controller being arranged to receive a first modulated optical signal having a first polarization and an unmodulated optical carrier signal polarized to the first modulated optical signal as inputs, the first modulated optical signal having a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the unmodulated optical carrier signal. - Optical filter device with a primary polarization mode; as well as - Coherent optical receiver equipment; as well as - Optical transmitter, including: The light source is arranged to generate an optical carrier signal; ο optical modulator; and An optical splitter is configured to efficiently separate the optical carrier signal and to route a first portion of the optical carrier signal to the optical modulator and a second portion of the optical carrier signal to bypass the optical modulator, wherein the optical modulator is configured to apply signal encoding to the first portion of the optical carrier signal to form a first modulated optical signal, the signal encoding being configured such that the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the optical carrier signal; and Wherein, the first modulated optical signal has a first polarization, and the unmodulated optical carrier signal is polarized aligned with the first modulated optical signal, and wherein the optical transmitter is configured to combine the first modulated optical signal and the unmodulated optical carrier signal for transmission.

2. The self-coherent optical transmission system as described in claim 1, wherein: The optical modulator includes a dual-polarization modulator, which is arranged to separate the first portion of the optical carrier signal into a first optical signal having the first polarization and a second optical signal having a second orthogonal polarization. The dual-polarization modulator is arranged to encode corresponding signals and apply them to the first and second optical signals to form a first modulated optical signal having the first polarization and a second modulated optical signal having the second orthogonal polarization; as well as The dual-polarization modulator is configured to combine the first modulated optical signal and the unmodulated optical carrier signal for transmission.

3. The self-coherent optical transmission system as described in claim 1, wherein, The optical modulator is arranged to apply signal encoding with a cutoff frequency equal to or higher than the predetermined bandwidth.

4. A method for self-coherent optical transmission, comprising the following steps: - Generate an optical carrier signal with first polarization; -Power separation of the optical carrier signal; - Apply signal encoding to a first portion of the optical carrier signal to form a first modulated optical signal, the signal encoding being configured such that the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the optical carrier signal; - The first modulated optical signal is combined with the second part of the optical carrier signal to form an unmodulated optical carrier signal, wherein the first modulated optical signal has the first polarization, and the unmodulated optical carrier signal and the first modulated optical signal are polarized aligned; - Transmit the combined signal; and -After transmission, the first modulated optical signal is received and detected through the following steps: - Receive the first modulated optical signal having the first polarization and the unmodulated optical carrier signal aligned with the polarization of the first modulated optical signal, wherein the first modulated optical signal has a negligible spectral power density within a predetermined bandwidth BW around the spectrum of the optical carrier signal. Then, polarization rotation is applied to the first modulated optical signal and the unmodulated optical carrier signal so that their polarizations are aligned with the dominant polarization mode. Then, acting on the signal aligned with the primary polarization mode, the unmodulated optical carrier signal is separated from the first modulated optical signal; and Then, using the separated signals, the unmodulated optical carrier signal is used as a local oscillator signal to perform coherent detection of the first modulated optical signal.

5. The method of claim 4, wherein, The application signal encoding includes: separating the first portion polarization of the optical carrier signal into a first optical signal having the first polarization and a second optical signal having a second orthogonal polarization, and applying corresponding signal encoding to the first and second optical signals to form a first modulated optical signal having the first polarization and a second modulated optical signal having the second orthogonal polarization.

6. The method of claim 5, wherein, The signal encoding applied to the first optical signal has a cutoff frequency equal to or higher than the predetermined bandwidth BW.

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