Optical transmission system, receiver and device, and method for receiving an optical signal

Abstract

An optical receiver includes at least one local oscillator that provides local oscillator light at a local oscillator center frequency that is offset by a frequency difference relative to a received optical signal. A combiner combines the optical signal and the local oscillator light and outputs at least one combined optical signal. An optoelectronic converter down-converts the combined optical signal to a corresponding RF electrical signal at the frequency of the frequency difference. A filter can be set to at least substantially remove one of an upper electrical signal sideband and a lower electrical signal sideband to output one of a VSB RF signal and an SSB RF signal before rectification. A local oscillator frequency controller can adjustably control the local oscillator frequency to provide a difference frequency that creates a specified group delay in the RF electrical signal, where the specified group delay can be specified via optical receiver performance parameters and / or user input and offset using a group delay filter.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit and priority of U.S. Provisional Patent Application Serial No. 62 / 883,846, filed on August 7, 2019, the entire content of which is incorporated herein by reference.

[0003] Background of the Invention Field of Technology

[0004] The present invention generally relates to communication systems and receivers having improved performance. More specifically, the systems and receivers of the present invention enable receiving information from a standard (i.e., double / dual - sideband) modulated optical signal using a single - sideband electrical signal generated from the optical signal, and / or provide adjustable / tunable electrical dispersion and other distortion compensation for single - sideband or double - sideband optical signals. Background Art

[0005] The efficient generation, transmission, and reception of information - carrying signals over networked communication systems supports modern society. The ability to extend the reach and information - carrying capacity of these communication systems is directly related to the cost of the systems. The continued expansion of this reach and capacity enables the delivery of additional services over the network, which in turn drives additional expansion. To increase the information transmission rate in these systems, a wide variety of transmission impairments, such as interference within and between information channels in the system, must be overcome.

[0006] As the transmission rate increases and the number of information channels in the system increases, transmission impairments become more significant. For example, in optical systems, the material and design of standard single - mode fiber (SSMF) cause light of different wavelengths to propagate through the fiber at different group velocities, which is known as dispersion. Figure 1 Illustrates how dispersion varies with optical wavelength in the most commonly used fiber in optical communication systems (i.e., standard single - mode fiber (SSMF)), which typically has a zero - dispersion region near a wavelength of 1310 nm and a dispersion value of approximately 17 ps / (nm km) in the C - band near a wavelength of 1550 nm. This dispersion causes the broadening of the transmitted pulses due to different optical components arriving at the receiver at different times, and thus results in inter - symbol interference (ISI). Higher symbol rates have a wider spectrum, leading to a higher velocity difference between the spectral components in the signal. Additionally, since each symbol is allotted a shorter time slot, the amount of pulse broadening that can be tolerated decreases with higher symbol rates before the pulse broadening amount becomes detrimental to the received signal quality.

[0007] Figure 2A Illustrates the pulse - broadening effect and ISI.Figure 2B A plot showing the frequency response of an intensity modulation / direct detection (IM / DD) link in the case of a single-mode fiber (SMF) at 10 km, 20 km, 30 km, 40 km, 50 km, and 100 km is shown, assuming D = 18 ps / (nm km) and λ = 1550 nm, as calculated by Neto et al. in "Simple Estimation of Fiber Dispersion and Laser Chirp Parameters Using the Downhill Simplex Fitting Algorithm", J. Lightwave Technol. 31, 334 - 342 (2013).

[0008] Dispersion is just one of the various transmission impairments that must be addressed to provide a robust communication system. Although transmitters, receivers, and other devices used in communication systems can be designed to at least partially compensate for the impairments, in many applications, the cost of such systems is uneconomical.

[0009] Therefore, there is a continuing need for communication systems with lower cost and higher performance so that systems with larger information transmission capacities can be deployed across the network. This need is particularly acute in metropolitan area networks and access networks where the deployment of high-cost, high-performance systems is economically infeasible. SUMMARY OF THE INVENTION

[0010] The present invention addresses the above-mentioned need by providing such communication systems, devices, and methods, i.e., the communication systems, devices, and methods achieve a lower-cost, higher-performance system by using a receiver that compensates for optical transmission impairments in the electrical domain.

[0011] In various embodiments, one or more optical receivers can be employed in an optical communication system, where a double-sideband optical signal is transmitted by one or more optical transmitters. The optical receiver can employ optical heterodyne detection to generate an electrical radio-frequency double-sideband signal. One or more sideband filters can be provided in the receiver to substantially reduce the presence of one sideband, thereby generating a vestigial sideband signal (VSB), or essentially eliminate the presence of one sideband to generate a single-sideband signal (SSB). Although essentially eliminating one of the sidebands removes a part of the signal carrying information, the inventors have found that essentially eliminating one sideband can reduce the negative impact of optical transmission impairments on receiver and system performance.

[0012] Various optical system and optical receiver embodiments can include:

[0013] · One or more local oscillators, each local oscillator providing light at a local oscillator frequency;

[0014] · at least one coupling device that optically couples an information-carrying optical signal having a center frequency with an upper sideband and a lower sideband to light from a local oscillator to provide a combined optical signal;

[0015] · one or more photoelectric converters (e.g., photodiodes) having a predefined frequency bandwidth, which receive the combined optical signal and output an RF electrical signal carrying information with an upper sideband and a lower sideband; and

[0016] · at least one RF sideband filter that substantially filters one of the upper RF sideband and the lower RF sideband to generate one of a vestigial sideband signal and a single-sideband RF signal carrying information that can be further processed.

[0017] [[ID=IO]]In various embodiments, one or more optical receivers may be employed in an optical communication system, wherein a single-sideband or double-sideband optical signal is transmitted by one or more optical transmitters. The optical receiver may employ optical heterodyne detection and control the frequency difference between the center wavelength of the optical signal and the optical frequency of the local oscillator to generate a corresponding electrical radio frequency signal in a specific frequency band, wherein a group delay variation has been purposefully introduced into the transfer function of the receiver to compensate for optical transmission losses. Although introducing signal distortion into the heterodyne signal is contrary to the general intention of downconversion, the inventors have found that if the method according to the present invention and the apparatus using the present invention are employed, the signal distortion can improve the performance of the optical receiver and the system.

[0018] Various optical systems and optical receiver embodiments may include:

[0019] · one or more local oscillators, each local oscillator providing light at a local oscillator frequency;

[0020] · at least one combiner that combines an information-carrying optical signal having a center frequency with light from a local oscillator having a local oscillator center frequency to provide a combined optical signal; and

[0021] · one or more photoelectric converters (e.g., photodiodes) having a predefined frequency bandwidth, which receive the combined optical signal and output an RF electrical signal carrying information;

[0022] · Among them, the local oscillator frequency is adjusted by a local oscillator controller to generate a frequency difference between the local oscillator frequency and the center wavelength of the optical signal, and to control the frequency of the RF signal generated by down-conversion, which in turn affects the performance of RF components (such as band-pass filters, transimpedance amplifiers, and group delay filters) in the receiver. For example, the frequency can be controlled to introduce a group delay in the converted RF signal, so as to compensate for the dispersion in the optical fiber link by using a group delay filter or other means.

[0023] Various system and receiver implementations may include one or both of VSB / SSB filtering and electrical signal distortion compensation. The optical receiver may include other components present in various optical heterodyne receivers, such as components that can be used in coherent and quasi-coherent optical transmission systems.

[0024] Additionally, although various aspects of the present invention provide low-cost, high-performance systems, the teachings of the present invention can be used in other systems to improve the performance of such systems. For example, the present invention can be used in optical systems including various optical dispersion compensation devices (such as dispersion compensation fiber (DCF), Bragg gratings, etc.).

[0025] Accordingly, the present disclosure addresses the ongoing need for systems and receivers with improved cost and performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings are included to exemplify aspects of the present invention to assist in explanation and understanding, and are not intended to limit the present invention, wherein:

[0027] Figure 1 is an exemplary plot showing the dispersion as a function of wavelength in a standard single-mode fiber.

[0028] Figure 2A depicts the effect of dispersion on the shape of the optical signal.

[0029] Figure 2B shows plots of the frequency response of intensity modulation / direct detection (IM / DD) links in the case of SSMF at 10 km, 20 km, 30 km, 40 km, 50 km, and 100 km, assuming D = 18 ps / (nm km) and λ = 1550 nm.

[0030] Figures 3 to 5 shows an exemplary optical system implementation.

[0031] Figures 6 to 12 shows an exemplary optical receiver implementation.

[0032] Figure 13A and Figure 13BExemplary all-pass filters and frequency responses are shown respectively.

[0033] Figure 14A and Figure 14B An exemplary receiver eye diagram is shown.

[0034] Figure 15 The relationship between the exemplary group delay and frequency measurement results of an exemplary optical receiver is shown.

[0035] Figures 16A to 16C The relationships between the exemplary bit error rates and the receiver input power for the difference frequencies of 27.5 GHz, 30 GHz, and 32.5 GHz are shown respectively.

[0036] In the accompanying drawings and the detailed description, the same or similar reference numerals may identify the same or similar elements. It should be appreciated that, unless explicitly stated or inherently infeasible, the implementations, features, etc. described with reference to the embodiments in a particular figure may be implemented with reference to other embodiments in other figures. Detailed Embodiments

[0037] The optical system 10 of the present invention can be used in various known configurations in unidirectional or bidirectional systems, which can be point-to-point or multi-point-to-point or multi-point configurations, where the nodes are deployed in linear, ring, mesh, and other network topologies. Generally, free space and / or optical fiber can be used to deploy the system 10, but it should be appreciated that many applications can involve fiber-based systems.

[0038] In addition, the optical system 10 can generally support one or more wavelength channels, which can be arranged in a channel grid over various ranges in the spectrum. For example, a single-channel system can operate using wavelength channels centered at 1310 nm and / or 1550 nm. A dense wavelength division multiplexing (DWDM) system, for example, can divide the spectrum in the nominal range from 1490 nm to 1625 nm (S band, C band, L band) into dozens of wavelength channels with fixed or variable bandwidths (such as 50 GHz, 100 GHz, etc.), depending on the design and application of the system 10. For example, wavelength channels based on the ITU grid can be used to define the system, https: / / www.itu.int / itu-t / recommendations / rec.aspx?rec=11482. An optical signal can be transmitted through the system 10 at a wavelength falling within one of the wavelength channels. Although the channel grid can be continuous with adjacent channels sharing channel edges, the system 10 can provide a guard band near the channel edges. The guard band is a wavelength range adjacent to the channel edge where optical signals should not be transmitted, which is used to reduce the amount of interference between signals in adjacent channels by maintaining a minimum separation.

[0039] Figure 3 and Figure 4 depicts an exemplary embodiment of an optical system 10 employing point-to-multipoint links (1) and point-to-point links (2) between nodes. The links may be independent optical communication links or may be part of a larger network as described in the preceding paragraph, where the nodes 11 may include optical line terminals or regenerators, optical network units, optical switches, add / drop multiplexers (OADMs), optical amplifiers (OAs), etc. employing various physical and management network architectures such as Figure 5 those in.

[0040] In Figure 3 an exemplary optical system 10 embodiment may include an optical line terminal or regenerator (OLT) 12. The OLT 12 may communicate with one or more optical network units (ONUs) 16 in a unidirectional or bidirectional manner via one or more optical fibers 14. The OLT 12 and the ONU 16 may be connected to one or more input / output lines 18, which may be optical and / or electrical depending on the network implementation.

[0041] Figure 4 shows an exemplary optical system 10 embodiment including a point-to-point link between two OLTs 12. Depending on the network configuration, Figures 3 to 5 the embodiment may or may not include an optical amplifier 20.

[0042] The Figures 3 to 5 embodiment may be deployed in various layers of the network, including the metro and access layers of the network. In an access network including fronthaul, backhaul, and aggregation, the system 10 may operate as a passive optical network (“PON”) or may include line optical amplifiers 20 to provide amplification between nodes and / or the nodes may include distributed amplifiers (such as Raman amplifiers).

[0043] Figure 5 shows an exemplary OLT 12 and ONU 16 node embodiment, which may include an optical combiner / splitter 22 that may combine and / or separate optical signals when more than one transmitter or receiver (OTRx) 24 is used in the system 10.

[0044] Depending on whether the optical system is deployed as a single - wavelength and / or wavelength - division multiplexing system, the optical combiner / splitter 22 can include a passive combiner and wavelength - specific multiplexers and demultiplexers. For example, the optical system 10 can be deployed as a time - division multiplexing (“TDM”), wavelength - division multiplexing (“WDM”), or time - and - wavelength - division multiplexing (“TWDM”) system, where each ONU 16 communicating with the OLT 12 can use the same or different wavelengths, as will be further described herein. It should be appreciated that if the nodes in the system only transmit and / or receive one channel and there is only one channel present on the optical fiber link 14 connecting the nodes, the optical combiner / splitter 22 can be used in the nodes.

[0045] Depending on the system configuration, the optical transmitter or receiver (OTRx) 24 can include only a transmitter or a receiver, separate transmitters and receivers, or a transceiver. In various embodiments, it may be cost - effective to use an integrated transceiver to reduce costs, but in other embodiments, it may be more desirable to use separate transmitters and receivers and provide only one - way communication.

[0046] The optical transmitter in the OTRx 24 typically includes one or more light sources with fixed or adjustable wavelengths, such as lasers with narrow or wide line widths. Various methods can be used to impart the information in one or more information streams to the light (i.e., the optical carrier) emitted by the source, such as directly modulating the source, modulating the light using an external modulator, and / or up - converting the electrical carrier carrying the information to generate an optical signal carrying the information at one or more wavelengths / frequencies.

[0047] One or more modulation techniques can be used to impart the information, including amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), or any combination of AM, FM, and PM. Additionally, the information can be imparted in analog or digital formats using various modulation formats that support two or more modulation levels (e.g., “0” state and “1” state, RZ, NRZ, etc.). Advanced / higher - order / multi - level modulation formats (such as duobinary and other higher - order constellations) can be used so that more bits of information can be transmitted per symbol, or components with a bandwidth less than that of an equivalent binary signal can be used. For example, a system with four amplitude levels will be able to encode two bits per symbol, a system with four frequency levels will be able to encode two bits per symbol, a system with four amplitude levels and four frequency levels independently will be able to encode four bits per symbol, and duobinary or other partial - response systems of higher order will be able to encode one or more bits per symbol using a reduced spectrum. In addition to amplitude and frequency, information can also be encoded in the phase of the carrier, in the polarization of the carrier, as a change in pulse width, or as a change in pulse position, etc.

[0048] It should also be appreciated that additional signal processing such as forward error correction (FEC) can be performed on the information before transmitting the information as an optical signal. In various embodiments, error correction and / or testers can be used to provide feedback to control the various transmitters and receivers in system 10.

[0049] In various embodiments, the signal can be encoded by one or more simultaneous AM and / or FM devices such as frequency chirp lasers, directly modulated lasers (DMLs), externally modulated lasers (EMLs), vertical cavity surface emitting lasers (VCSELs), etc. Both DMLs and VCSELs have a wide linewidth and are generally low cost. In various embodiments, pure AM can be used for signal modulation by using an external modulator with a wide variety of lasers, as is known in the art.

[0050] Regardless of how the AM and / or FM signals are generated, frequency modulation is responsible for converting different states into different frequencies, and amplitude modulation is responsible for separating different states in terms of amplitude, thus conveniently providing further information about different states, as not included in conventional systems.

[0051] The different frequencies (i.e., different states) are separated by frequency separation (also known as FM shift). Thus, FM shift is defined as the frequency separation between two states of a frequency modulation (FM) signal. As an example, the FM shift is the difference between the "0" state and the "1" state of a combined AM - FM signal (i.e., an optical signal).

[0052] Figure 6 An exemplary embodiment of an optical receiver 30 is shown, which can be used separately from the optical transmitter in OTRx 24 or as part of a transceiver to receive an optical signal (OS). Additionally, it should be appreciated that other optical receivers used in optical system 10 can be Figure 6 different from the embodiment shown and can be used with or without the embodiments of the present invention.

[0053] Optical receiver 30 generally may include one or more fixed or tunable local oscillators (“LOs”) 32, such as lasers of various linewidths, to provide LO light at one or more LO center frequencies that may be offset relative to the center frequency of the optical signal, i.e., LO frequency offset. The optical local oscillator laser (LO) emits light at an optical frequency (Flo) that is offset from the optical signal center frequency (Fc) by a frequency offset or frequency difference (dF), which may also be referred to as an intermediate frequency IF. The frequency difference dF is the frequency of the RF electrical signal that has been down-converted from the optical signal. Relative to the optical signal carrying information through system 10, the LO light typically occupies a narrow spectral band, and the information will be down-converted onto the RF electrical signal.

[0054] In various embodiments, the local oscillator 32 may be a cooled or uncooled laser, such as a VCSEL, DFB, DBR, ECL, or other type of laser. The local oscillator 32 may be tuned to the frequency or wavelength of the signal. This may be an in-band configuration or an out-of-band configuration. In an in-band configuration, the LO is tuned to a frequency or wavelength within the spectral band of the signal. In an out-of-band configuration, the local oscillator 32 is tuned to a frequency or wavelength outside the spectral band of the signal. In this way, wavelength selectivity may be achieved using the local oscillator 32. Using the local oscillator 32 as a wavelength selector enables the system to operate with or without an optical filter.

[0055] The combiner combines the incoming optical signal OS with the LO light and outputs one or more combined optical signals to a corresponding number of optoelectronic (OE) converters 36, such as photodiodes. The OE converter 36 down-converts the optical signal, which includes information and other signal characteristics (e.g., bandwidth, distortion, etc.), into such an RF electrical signal that has a center frequency equal to the center frequency difference between the optical signal and the local oscillator light. The RF electrical signal is electrically processed by an electrical processing unit 38 that recovers the information, as will be further described herein.

[0056] In various embodiments, a local oscillator controller 39 may be employed to control the local oscillator center frequency based on various receiver performance parameters and user input described herein. The local oscillator center frequency may be controlled by monitoring the frequency of the local oscillator light output by the local oscillator 32 and various receivers and the signal parameters / metrics at various points in the receiver (such as in the electrical processing unit 38), by calculating the bit error rate, signal power, signal frequency, etc.

[0057] The local oscillator controller 39 can be integrated with or separated from the local oscillator 32 and employs one or more processors and associated storage devices / memory to execute instructions to vary the local oscillator center frequency as described herein. These instructions can be stored in one or more transient and / or non-transient computer-readable media, which are located locally within or near the receiver and / or remotely within the system 10 that communicates with the controller.

[0058] The combination of the optical signal and the local oscillator, and the splitting of the combined optical signal (if more than one combined signal is desired in the design), can be provided as an integrated or discrete combiner 40 and splitter 42 in the receiver 30, recognizing that discrete devices may involve higher losses. The combiner 40 and splitter 42 can be implemented as polarization or non-polarization maintaining devices using free space or optical fiber or a combination thereof. Further, the combiner can be provided as a passive coupler, such as a 50 / 50 coupler or other combination ratio, or as a wavelength-dependent device. The splitter 42 can be a polarization beam splitter (PBS) to split the combined optical signal provided by the combiner 40 into combined optical signals of orthogonal polarization. Thus, in each axis, the optical data signal and the LO signal are polarization-aligned. Given the unknown polarization of the incoming optical signal, in designs employing more than one OE converter 36, the detected signals may need to be combined after photodetection to fully recover the information carried by the incoming optical signal. In embodiments employing only one OE converter 36, it may be desirable to provide the local oscillator light in a form different from linear polarization (e.g., depolarized, orthogonal polarization, etc.) to ensure good mixing with the optical signal.

[0059] Figure 7 and Figure 8 An exemplary receiver embodiment that employs more than one OE converter 36 to receive an optical signal is shown. Figure 7 An optical receiver embodiment that employs a 2×2 combiner / splitter 34 is shown to provide two combined optical signals COS1 and COS2 to two OE converters 36, which output corresponding RF signals, e.g., ES1 and ES2, at a frequency offset from the LO frequency. These corresponding electrical signals can be provided to an electrical processing unit 38, which can rectify the information as an electrical signal and output it via output line 18 for further signal processing within the receiver and / or further transmission within or outside the system 10.

[0060] Exemplary optical receivers 30 that employ two or more photoelectric converters 36 and various electrical signal processing functions to process an optical signal (OS) are described in U.S. Patent Application No. 15 / 927,792, filed Mar. 21, 2018 (now U.S. Patent No. 10,367,588), U.S. Patent Application No. 16 / 459,604, filed Jul. 1, 2019 (now U.S. Patent No. 10,608,747), and U.S. Provisional Patent Application Serial No. 62 / 474,599, filed Mar. 21, 2017, and in other references such as “Signal processing in an optical polarization diversity receiver for 560-Mbit / s ASK heterodyne detection” by ENNING B et al., JOURNAL OF LIGHTWAVE TECHNOLOGY, IEEE SERVICE CENTER, NEW YORK, NY, US, vol. 7, no. 3, 1 Mar. 1989 (1989-03-01), pages 459-464, XP011479323, ISSN: 0733-8724, DOI: 10.1109 / 50.16881, all of which are incorporated herein by reference in their entirety.

[0061] Figure 8 Embodiments of various optical receivers 30 are shown in which two OE converters 36 and an electrical processing unit 38 can be implemented for a 10 Gbps (NRZ) bit rate using a 40 GHz bandwidth. The bandwidth of the OE converters 36 can be referred to as the receiver's channel bandwidth. The optical signal frequency can be anywhere within the allocated wavelength channel, which allows for variations due to signal bit rate and bandwidth, chirp, and drift due to aging and temperature variations, as well as transmitter-to-transmitter variations in TDM, WDM, and TWDM systems.

[0062] In various embodiments, the electrical processing unit 38 can include a rectifier 44 and a combiner 46. Rectification can be applied either digitally or analogously. Using a rectifier can provide reduced computational complexity and / or hardware, and thus reduce the total cost. For example, a rectifier can be used without an analog / digital (A / D) converter. Rectification can be performed as half-wave rectification, such that the positive or negative part of the signal is removed. A half-wave rectification can be performed using a gate with a non-linear transfer function. The gate can be biased such that, for example, the negative part of the signal is below the threshold of the gate. Rectification can also be performed as full-wave rectification (such as a squaring element), where all negative values are converted to positive values, and can be implemented using hardware or software. When implemented in software, an analog / digital converter can be implemented before processing in a digital signal processor (DSP). However, various other solutions are possible. Examples of analog rectifiers include an XOR gate and a diode bridge. Both the XOR gate and the diode bridge allow real-time signal processing without a DSP and can thus be preferred over a DSP.

[0063] As Figure 8 shown, the rectifier can be implemented as an envelope detector. A first envelope detector / rectifier converts a first electrical signal from one of the photodiodes into a non-inverted non-return-to-zero (NRZ) data signal. A second envelope detector / rectifier converts a second electrical signal from the other photodiode into an inverted NRZ data signal. The inverted electrical signal and the non-inverted electrical signal are combined to provide an electrical data signal. The combiner 46 can be a differential amplifier that performs subtraction to recombine the signals or other subtractors known in the art. The combined electrical signal can then be further processed by the optical receiver 30 and / or further transmitted within or outside the system 10.

[0064] Figure 9 Exemplary aspects of an optical receiver 30 that can be employed in an optical communication system 10 are shown, where a double-sideband optical signal is transmitted by one or more optical transmitters. The optical receiver can employ optical heterodyne detection to generate a radio-frequency double-sideband electrical signal having an upper sideband and a lower sideband.

[0065] A combined optical signal (an information-carrying optical signal optically combined with a local oscillator optical signal) can be received by a photoelectric converter 36 such as a photodiode to generate an information-carrying RF signal having a center frequency (IF) at the optical signal - LO frequency difference. The electrical processing unit 38 can include a transimpedance amplifier (TIA) 50 to shape the electrical signal. For example, the TIA 50 can exhibit a relatively high cut-off frequency at which the gain will drop from the gain at low frequencies. The specific cut-off frequency and slope of the rolling-off transfer function will depend on the particular TIA implementation (topology, number of stages, IC technology, packaging, parasitics, etc.). By placing the RF signal center frequency (IF) near the cut-off frequency of the TIA, the roll-off of the TIA 50 can be used to at least partially suppress the upper sideband of the RF signal. Thus, VSB / SSB filtering can be provided by the roll-off of the TIA 50 without an additional VSB / SSB filter. The frequency of the optical signal emitted from the local oscillator 32 can be controlled to maintain and / or adjust the RF signal center frequency relative to the cut-off frequency of the TIA 50.

[0066] An intermediate electrical filter 52 having one or more passbands or stopbands (such as a high-pass filter, a band-pass filter, or a low-pass filter (HPF / BPF / LPF)) can be used to substantially remove or remove one of the two sidebands of the electrical signal to generate a vestigial sideband signal (VSB) or a single sideband signal (SSB), respectively. The VSB / SSB electrical signal output from the intermediate electrical filter 52 is provided to the additional electrical processing section 54 as discussed above. Although substantially removing one of the sidebands removes a portion of the information carrying the signal, the inventors have found that substantially removing one of the sidebands in a receiver can reduce the negative impact of optical transmission impairments on receiver and system performance.

[0067] Figure 9 Also depicted is the general shape of the signal as it passes through the components of the receiver. The different signal shapes shown below the intermediate filter 52 depict different signals generated using different filter types. It should be appreciated that one or more filters can be used to implement the intermediate filter 52 to achieve the desired signal shape. Together with the transfer function of the TIA 50, the intermediate filter 52 can achieve USB / LSB / VSB operation.

[0068] Figure 10Shows an embodiment in which the electrical processing section may include additional filters (such as one or more input filters 56 and / or output filters 58). Since the input photocurrent has a bandpass characteristic (near the difference frequency dF), a high-pass filter (HPF) or a bandpass filter (BPF) can be implemented at the input of the TIA 50. This also allows for the sinking of the generated DC photocurrent (DC offset compensation). The low cut-off frequency and filter shape can be optimized for USB / VSB operation. By selecting the IF close to the filter cut-off point, the lower sideband is effectively suppressed. In practice, the input HPF can be integrated with the TIA on the same component.

[0069] The output filter 58 can be used to select the demodulated baseband signal. The filter 58 is typically implemented as one or more LPFs, which can also be customized to further shape the signal being output.

[0070] Figure 11 Depicts various options that can be implemented in an embodiment such as Figure 10 shown. Options 1, 2, 4 support USB / LSB / VSB operation. Option 3 only supports upper sideband suppression.

[0071] The remaining portion of the spectrum after VSB filtering still contains the phase information of the received signal, so although the impairment of dispersion is significantly reduced, it is not completely removed. Since dispersion is a well-known phenomenon, knowing the distance of the transmitter allows for a fairly accurate calculation of the phase distortion introduced by the optical fiber. For example, in most practical applications, at a working wavelength of approximately 1550 nm, the dispersion coefficient is D≈17 ps / (nm·km), which converts to D≈-0.2 ps / (GHz·km) in the frequency domain.

[0072] In an optical heterodyne receiver, the latter definition of the dispersion coefficient is particularly useful for quantifying the phase distortion of the IF signal due to dispersion. This also provides a specification for the design of an electrical dispersion compensator (ECDC) to compensate for dispersion and other signal distortion effects in the electrical domain after heterodyning.

[0073] For example, an analog all-pass filter 60 can be employed, which acts only on the phase of the IF signal without affecting the amplitude of the IF signal and has a specified phase response opposite to the dispersion coefficient. As Figure 12 shown, the ECDC 60 can be placed just after the intermediate filter 52 and before the signal rectification as Figure 8 shown. Alternatively, the ECDC can be placed before the intermediate filter 52, or the intermediate filter 52 can be designed to have the desired group delay characteristics so as to perform the functions of the intermediate filter 52 and the ECDC 60 in a single component.

[0074] Figure 13A Shows an exemplary embodiment of an all-pass filter 60 that can be employed, the all-pass filter including an LC network that can be tuned to have a group delay (GD) variation such as Figure 13B shown. The group delay slope as a function of frequency can be designed to be GD(f) = 2 ps / GHz in the range of 20 GHz to 38 GHz for use with an SSMF. Those skilled in the art can employ other all-pass filter designs, for example, see "Design of cascaded all pass network with monotonous group delay response for broadband radio frequency applications", IET IET Microwaves, Antennas & Propagation, 2016, Vol. 10, Issue 7, pp. 808 - 815, which is incorporated herein by reference.

[0075] Based on the known value of the dispersion coefficient D at 1550 nm, this GD curve is well-suited for compensating for the dispersion that occurs in a 10 km fiber optic link. Additionally, simulation results show that the filter 60 can also be tuned to compensate for other distances. The tuning of the filter 60 can be performed by changing the impedance of one or more components in the filter, as is known in the art. The control of the filter tuning can be performed by a separate controller based on various receiver performance parameters / metrics and user input, and / or can be controlled via the local oscillator controller 39.

[0076] Figure 14A and Figure 14B Shows the eye diagrams of a heterodyne receiver operating at 25 Gbps without and with Figure 13A the all-pass filter ECDC 60, with a 30 GHz offset between the signal and the LO reported for a 20 km transmission. It can be seen that the ECDC 60 provides additional eye opening, which generally translates into improved system performance.

[0077] Unfortunately, all-pass filters are typically limited because they must be designed to compensate for a specific amount of dispersion (e.g., a specific length of a specific optical fiber) and be tunable over a range of dispersion amounts, so these filters are not widely deployable but can be designed for a specific range of applications. While multiple sets of all-pass filters or adjustable filters can be considered, these solutions may be too costly for some applications. However, another advantage of the VSB / SSB implementation of the present invention using ECDC is that removing some or all of one sideband essentially reduces the signal bandwidth by approximately 50%. Thus, the dynamic range of the ECDC 60, such as an all-pass filter, is extended because the ECDC only needs to compensate for approximately 50% of the dispersion of the signal bandwidth compared to a double-sideband signal.

[0078] In various embodiments, one or more optical receivers 30 can be employed in the optical communication system 10, where a single-sideband optical signal or a double-sideband optical signal is transmitted by one or more optical transmitters. The optical receiver 30 can employ optical heterodyne detection and employ a local oscillator controller located in or communicating with a local oscillator to adjustably control the frequency difference dF or intermediate frequency IF between the center wavelength of the optical signal and the local oscillator optical frequency to generate a corresponding electrical RF signal in which a specified group delay variation has been introduced across the bandwidth of the electrical RF signal for the purpose of compensating for optical transmission impairments (such as dispersion), which is referred to herein as heterodyne dispersion compensation. Although introducing signal distortion into the heterodyne signal is contrary to the general intention of an optical receiver, the inventors have found that if performed according to the methods and using the apparatus of the present invention, the signal distortion via optical heterodyning can improve the performance of the optical receiver 30 and the system 10.

[0079] The local oscillator controller can adjust the IF and thus the frequency of the RF electrical signal to match the specified group delay variation characteristics based on the optical receiver performance data and / or user input. For example, the user can input a target specified group delay based on the length and fiber type of the transmission link on which the optical receiver is being used. The optical receiver can or cannot adjust the specified group delay during operation. For example, the optical receiver can provide performance data (such as bit error rate (BER)) to the local oscillator controller via one or more processors that can be used to adjust the specified group delay.

[0080] In various optical receiver embodiments employing optical heterodyne (such as Figures 6 to 12 embodiments), the center frequency of the local oscillator 32 can be controlled relative to the center frequency of the optical signal such that the difference frequency dF causes signal distortion to be introduced in the frequency domain of the down-converted RF signal.

[0081] Frequency distortion takes the form of group delay, which is typically opposite to the group delay introduced by dispersion in an optical fiber link. The frequency dependence of the group delay variation can be non-linear. Thus, the dispersion compensation can be tuned by tuning the frequency difference between the received optical signal and the local oscillator.

[0082] Figure 15 The measured group delay of an optical heterodyne receiver is shown. By selecting a frequency offset of about 20 GHz, a very flat group delay is observed, which would be suitable for short links or back-to-back situations. Instead, selecting a frequency offset of about 35 GHz results in a steeper group delay variation across the signal bandwidth, which can be tailored to compensate for longer links.

[0083] Figures 16A to 16C A plot showing the relationship between the bit error rate (BER) and the receiver input power is shown, where, for the case of 16 ps / (nm*km) dispersion at the center wavelength of the optical signal, the difference frequency is set at 27.5 GHz, 30 GHz, and 32.5 GHz respectively for the reception of a 25 Gbps signal for back-to-back (B2B) transmission and for transmission through 20 km of optical fiber. In Figure 16A , the LO is tuned such that the frequency offset is 27.5 GHz. In this case, the back-to-back performance is significantly better than that of the 20 km transmission case. In Figure 16B , a 30 GHz offset is selected, which results in similar performance between B2B and 20 km transmission. In Figure 16C , a 32.5 GHz offset is selected, and better performance is obtained for the case of 20 km transmission. It can be seen that heterodyne dispersion compensation can be deployed as a tunable dispersion compensator in a heterodyne optical receiver. This compensation can be applied to longer transmission distances, optical fibers with different dispersion values, and different bit rates.

[0084] Various system and receiver implementations can include one or both of VSB / SSB filtering and heterodyne dispersion compensation. The optical receiver can include other components present in various optical heterodyne receivers, such as components that can be used in coherent and quasi-coherent optical transmission systems.

[0085] In addition, while various aspects of the present invention provide low-cost, high-performance systems, the teachings of the present invention can be used in higher-cost systems to improve the performance of such systems. For example, the present invention can be used in optical systems that include various optical dispersion compensation devices (such as dispersion compensation fiber (DCF), Bragg gratings, etc.).

[0086] The foregoing disclosure provides examples, illustrations, and descriptions of the present invention, but is not intended to be exclusive or to limit these implementations to the exact forms disclosed. Modifications and variations can be made in light of the above disclosure, or can be obtained from practice of the implementations. These and other variations and modifications of the present invention are possible and contemplated, and the foregoing specification and the appended claims are intended to cover such modifications and variations.

[0087] As used herein, the term "component" is intended to be broadly construed as hardware, firmware, and / or a combination of hardware and software. It is evident that the systems and / or methods described herein can be implemented in different forms of hardware, firmware, or a combination of hardware and software. The actual specific control hardware or software code used to implement these systems and / or methods does not limit the implementation. Accordingly, the operation and behavior of the systems and / or methods are described herein without reference to specific software code - it should be understood that software and hardware can be designed based on the description herein to implement the systems and / or methods.

[0088] The various elements of the system can be implemented at various levels of photonics, electrical, and mechanical integration. Multiple functions can be integrated on one or more modules or line cards housed in one or more shelves or racks contained within system 10.

[0089] The scope of hardware processor modules can range, for example, from general - purpose processors and CPUs to field - programmable gate arrays (FPGAs) to application - specific integrated circuits (ASICs). Software modules (executed on hardware) can be expressed in a variety of software languages (e.g., computer code), including: C, C++, Java TM , Javascript, Rust, Go, Scala, Ruby, Visual Basic TM , FORTRAN, Haskell, Erlang, and / or other object - oriented, procedural, or other programming languages and development tools. Computer code can include microcode or microinstructions, machine instructions (such as those generated by a compiler), code used to generate web services, and files containing high - level instructions that are executed by a computer using an interpreter and that employ control signals, encrypted code, and compressed code.

[0090] Some implementations are described herein in connection with thresholds. As used herein, meeting a threshold can refer to a value that is greater than the threshold, more than the threshold, higher than the threshold, greater than or equal to the threshold, less than the threshold, fewer than the threshold, lower than the threshold, less than or equal to the threshold, equal to the threshold, etc.

[0091] Certain user interfaces have been described and / or illustrated in the accompanying drawings. A user interface can include a graphical user interface, a non-graphical user interface, a text-based user interface, etc. The user interface can provide information for display. In some implementations, a user can interact with the information, such as by providing an input via an input component of the device, which provides the user interface for display. In some implementations, the user interface is configurable by the device and / or the user (e.g., a user can change the size of the user interface, the information provided via the user interface, the location of the information provided via the user interface, etc.). Additionally or alternatively, the user interface can be pre-configured into a standard configuration, a specific configuration based on the type of device on which the user interface is displayed, and / or a set of configurations based on the capabilities and / or specifications associated with the device on which the user interface is displayed).

[0092] Even if a specific combination of features is recited in the claims and / or disclosed in the specification, such combinations are not intended to limit the disclosure of possible implementations. In fact, many of these features can be combined in ways not specifically recited in the claims and / or not disclosed in the specification. Although each of the dependent claims listed below may directly depend on only one claim, the disclosure of possible implementations includes combinations of each of the dependent claims with every other claim in the claim set.

[0093] Any element, act, or instruction used herein should not be construed as critical or essential unless expressly described as such. Also, as used herein, the articles "a" and "an" are intended to include one or more items and may be used interchangeably with "one or more." Additionally, as used herein, the term "set" is intended to include one or more items and may be used interchangeably with "one or more." The term "one" or similar language is used when referring to only a single item. Also, as used herein, the term "has," "have," "having," etc. are intended as open-ended terms. Additionally, unless expressly stated otherwise, the phrase "based on" is intended to mean "at least partially based on."

Claims

1. An optical receiver, the optical receiver comprising: An optical signal input terminal; At least one local oscillator that provides local oscillator light having a center frequency; An optical combiner having a first optical input terminal connected to the optical signal input terminal, a second optical input terminal connected to the at least one local oscillator, and at least one output terminal for outputting a combined optical signal, the combined optical signal including an optical signal having the center frequency of the optical signal from the first optical input terminal and local oscillator light from the second optical input terminal; A photoelectric converter having an optical signal input terminal optically connected to the at least one output terminal of the optical combiner and an RF electrical signal output terminal; A filter electrically connected to the RF electrical signal output terminal and configured to introduce a group delay variation in the RF electrical signal passing through the filter; A local oscillator controller configured to adjust the center frequency of the local oscillator light to change the group delay variation introduced by the filter into the RF electrical signal; and A rectifier electrically connected to receive the RF electrical signal output from the filter and output a baseband electrical signal.

2. The optical receiver according to claim 1, wherein, The group delay variation corresponds to the amount of dispersion compensation in the optical fiber link.

3. The optical receiver according to claim 1, wherein, The introduced group delay variation is controlled based on at least one of receiver performance and user input.

4. The optical receiver according to claim 1, wherein The filter is an all-pass filter.

5. The optical receiver according to claim 4, wherein: The local oscillator controller controllably adjusts the center frequency of the local oscillator light relative to the center frequency of the optical signal, and the local oscillator controller controllably adjusts the frequency of the RF electrical signal to introduce a group delay variation in the RF electrical signal, the group delay variation being at least partially offset by the group delay variation of the all-pass filter.

6. The optical receiver according to claim 3, wherein The center frequency of the local oscillator light is adjusted to reduce the bit error rate performance of the receiver.

7. The optical receiver according to claim 1, wherein, The filter is at least partially implemented via a transimpedance amplifier, and the center frequency of the local oscillator light is controlled to filter at least a portion of one of the upper sideband and the lower sideband of the RF electrical signal.

8. The optical receiver according to claim 7, wherein, The filter is at least one of a high-pass filter, a band-pass filter, and a low-pass filter.

9. The optical receiver according to claim 1, wherein, The group delay variation introduced by the filter is monotonic in at least a partial frequency range of the RF electrical signal.

10. The optical receiver according to claim 1, wherein, The group delay variation introduced by the filter is non-linear in at least a partial frequency range of the RF electrical signal.

11. A method for receiving an optical signal having a center frequency and carrying information, the method comprising the following steps: Providing local oscillator light at a local oscillator light center frequency via at least one local oscillator; Combining the optical signal and the local oscillator light via an optical combiner to output a combined optical signal; Converting the combined optical signal into a corresponding RF electrical signal carrying the information via a photoelectric converter; Filtering the RF electrical signal via a filter to introduce a group delay variation into the RF electrical signal; Controlling the optical center frequency of the local oscillator via a local oscillator controller to change the group delay variation introduced into the RF electrical signal by the filter; and Rectifying the filtered RF electrical signal into a baseband electrical signal carrying the information via a rectifier.

12. The method according to claim 11, the method further comprising the steps of: Adjusting the optical center frequency of the local oscillator via a local oscillator controller to control the frequency of the RF electrical signal.

13. The method according to claim 11, wherein The optical center frequency of the local oscillator is adjusted to reduce the bit error rate measurement result of the information.

14. An optical transmission system, the optical transmission system comprising: An optical transmitter that transmits an optical signal having a center frequency and carrying information; and An optical receiver that receives the optical signal, the optical receiver comprising: An optical signal input terminal that receives the optical signal from the transmitter; At least one local oscillator that provides local oscillator light having a center frequency; An optical combiner that combines the optical signal provided to the first optical input terminal from the optical signal input terminal with the local oscillator light provided to the second input terminal from the at least one local oscillator, and outputs a combined optical signal including the optical signal and the local oscillator light; An optoelectronic converter having an optical signal input terminal that receives the combined optical signal from the output terminal of the optical combiner, and outputs an RF electrical signal having a frequency equal to the frequency difference between the center frequency of the optical signal and the center frequency of the local oscillator light; A filter that is electrically connected to the RF electrical signal from the optoelectronic converter to introduce a group delay variation into the RF electrical signal according to the frequency of the RF electrical signal; A local oscillator controller that adjusts the center frequency of the local oscillator light to control the frequency of the RF electrical signal and the group delay variation introduced by the filter; and A rectifier that rectifies the RF electrical signal output from the filter into a baseband electrical signal carrying the information.

15. The system according to claim 14, wherein The filter at least substantially removes one of the upper sideband and the lower sideband of the RF electrical signal, and outputs one RF signal of a VSB RF signal and an SSB RF signal, the filter is at least one of a high-pass filter, a band-pass filter, or a low-pass filter; and The local oscillator controller adjusts the center frequency of the local oscillator light to control the frequency of the RF electrical signal relative to the passband of the filter.

16. The system according to claim 14, wherein The filter is an all-pass filter that introduces at least some of the group delay variations into the RF electrical signal; and The local oscillator controller adjusts the center frequency of the local oscillator light to control the frequency of the RF electrical signal and the group delay variation introduced by the filter.

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