Apparatus and method for mitigating multipath interference in optical fiber transmission systems

Abstract

An optical receiver comprises: an input port for receiving an intensity modulated optical signal with multipath interference (MPI); a detector for converting the intensity modulated optical signal with multipath interference into an electrical signal; a high pass filter (HPF) for filtering the electrical signal and suppressing low-frequency components caused by the MPI to generate a filtered electrical signal; and an output port for sending the filtered electrical signal.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] none Technical Field

[0003] The present application relates to network communications, and more particularly, to systems and methods for mitigating multipath interference in fiber optic transmission systems. Background Art

[0004] The rapid growth of data center business is driving operators, enterprises and service providers to rely on Ethernet solutions to provide high-speed network communications. Previously deployed 100G Ethernet (100G Ethernet, 100GE) uses a 4-channel 25 Gigabit baud (Gbaud) non-return-to-zero (NRZ) modulation format, using parallel single-mode 4-channel fiber (PSM4) through space division multiplexing for 500 meters (m) range, or wavelength division multiplexing (WDM) for 2 kilometers (km), 10km and 40km range. Utilize 4-channel local area network (LAN) WDM (LWDM-4) or coarse WDM (CWDM-4). For 400GE currently being deployed, the optical modulation format uses 4-level pulse amplitude modulation (PAM4). In the Institute of Electrical and Electronics Engineers (IEEE) standard, 500m uses 50Gbaud PAM4PSM4, and 2km and 10km use 8-channel 25Gbaud (LWDM-8). In the 100G Lambda multi-source agreement (MSA) industry alliance, 2km and 10km also use 4-channel 50Gbaud CWDM-4PAM4 to reduce costs. Summary of the invention

[0005] A first aspect relates to an optical receiver, comprising: an input port for receiving an intensity modulated optical signal with multipath interference (MPI); a detector for converting the intensity modulated optical signal with multipath interference into an electrical signal; a high pass filter (HPF) for filtering the electrical signal and suppressing a low frequency component caused by the MPI to generate a filtered electrical signal; and an output port for sending the filtered electrical signal.

[0006] A second aspect relates to a polarization tracking optical receiver, comprising: a polarization tracker, for receiving the dual polarization signal, and demultiplexing the dual polarization signal into an X-channel optical signal and a Y-channel optical signal; a first detector, for receiving the X-channel optical signal and converting the X-channel optical signal into a first electrical signal; a second detector, for receiving the Y-channel optical signal and converting the Y-channel optical signal into a second electrical signal; a first high-pass filter (HPF), for receiving the first electrical signal and filtering the first electrical signal to suppress a low-frequency component caused by MPI, thereby generating a first filtered electrical signal; a second HPF, for receiving the second electrical signal and filtering the second electrical signal to suppress a low-frequency component caused by MPI, thereby generating a second filtered electrical signal; a first output port, for sending the first filtered electrical signal; and a second output port, for sending the second filtered electrical signal.

[0007] A third aspect relates to a polarization tracking self-homodyne detection (SHD) receiver, comprising: a polarization tracker, for receiving the SP-SHD channel with MPI from the MPI impaired optical fiber link, and separating the SP-SHD channel into the signal and the pilot signal, wherein the pilot signal is used as a local oscillator (LO); a 90-degree optical hybrid mixer, for receiving the signal and the LO, the 90-degree optical hybrid mixer is used to generate an in-phase (I) positive output signal, an I negative output signal, a quadrature (Q) positive output signal and a Q negative output signal; a first balanced detector (balanced The invention relates to a first filter electric signal comprising a first output port and a second output port for transmitting the first filtered electric signal and a second output port for transmitting the second filtered electric signal. The first filter electric signal comprises a first detector, a BD, a Q positive output signal, a Q negative output signal, and a Q data signal. The first HPF is used to receive the I data signal and filter the I data signal to suppress the low-frequency component caused by the MPI, thereby generating a first filtered electric signal. The second HPF is used to receive the Q data signal and filter the Q data signal to suppress the low-frequency component caused by the MPI, thereby generating a second filtered electric signal. The first output port is used to send the first filtered electric signal. The second output port is used to send the second filtered electric signal.

[0008] The fourth aspect relates to a polarization-division multiplexing (PDM) optical fiber system, comprising: a dual-polarization optical transmitter; a polarization tracking optical receiver having multiple high pass filters (HPF) for mitigating multipath interference (MPI); and an optical fiber link that undergoes the MPI, the optical fiber link coupling the dual-polarization optical transmitter and the polarization tracking optical receiver.

[0009] The fifth aspect relates to a single polarization (SP) self-homodyne detection (SHD) transmission system, comprising: an SP-SHD transmitter; a multipath interference (MPI) damaged optical fiber link; and a polarization tracking SHD receiver having a first high pass filter (HPF) and a second HPF for mitigating MPI.

[0010] The above aspects advantageously remove undesirable carrier-carrier beat noise from an optical signal with multipath interference. The beat noise is undesirable crosstalk caused by the beat frequency between the optical signal carrier and the MPI carrier. Therefore, the above aspects can reduce the impact of MPI in intensity modulation direct detection (IMDD) systems, such as but not limited to PAMn systems (e.g., PAM4 systems), single polarization IMDD systems, polarization division multiplexing IMDD systems, and SP-self-homodyne detection (SHD) systems. The above aspects can be very beneficial in short-distance networks and client-side optical networks (such as within and between data centers).

[0011] Optionally, in any of the above aspects, the HPF has a cutoff frequency in the range of 1 megahertz (MHz) to about 100 MHz.

[0012] Optionally, in any of the above aspects, the HPF has a cutoff frequency in the range of 1 MHz to about 200 MHz.

[0013] Optionally, in any of the above aspects, the ratio of the HPF cutoff frequency to the transmitter laser line width is in the range of 3 to 200.

[0014] Optionally, in any of the above aspects, the HPF has a cutoff frequency of approximately 20 MHz.

[0015] Optionally, in any of the above aspects, a transimpedance amplifier (TIA) is used to amplify the electrical signal.

[0016] Optionally, in any of the above aspects, the TIA is located after the HPF and is used to amplify the filtered electrical signal.

[0017] Optionally, in any of the above aspects, the HPF is a resistor-capacitor (RC) HPF with surface mounted capacitance on a microstrip transmission line.

[0018] Optionally, in any of the above aspects, the frequency transfer function of the HPF is in, is the cut-off frequency.

[0019] Optionally, in any of the above aspects, the HPF is an existing direct component (DC) block with a replacement capacitor having the cut-off frequency

[0020] Optionally, in any of the above aspects, the intensity modulated optical signal with MPI uses pulse amplitude modulation n (PAMn) as baseband modulation, where n is an integer equal to or greater than 2.

[0021] Optionally, in any of the above aspects, the intensity modulated optical signal with MPI uses one of carrierless amplitude phase (CAP) modulation, quadrature amplitude modulation (QAM) or discrete multiple tone (DMT) as subcarrier modulation.

[0022] Optionally, in any of the above aspects, the optimal cutoff frequency of the HPF increases with an increase in the line width of a laser used to transmit an intensity modulated optical signal to the optical receiver.

[0023] Optionally, in any of the above aspects, the MPI is generated in an optical fiber link coupled to the input port of the optical receiver.

[0024] Optionally, in any of the above aspects, the HPF introduces a filtering penalty, the filtering penalty being dependent on a baud rate used when modulating data in the intensity modulated optical signal received by the optical receiver.

[0025] For clarity, any of the above-described embodiments may be combined with any one or more of the other embodiments described above to create new embodiments within the scope of the invention.

[0026] These and other features, as well as advantages thereof, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] For a more complete understanding of the present invention, reference is made to the following brief description taken in conjunction with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

[0028] Figure 1 is a schematic diagram showing an optical receiver with a high pass filter (HPF) according to an embodiment of the present invention.

[0029] Figure 2 In Figure 2 A to Figure 2 C is a schematic diagram illustrating the principle of a provided method for mitigating the impact of multipath interference (MPI) using a HPF in an optical receiver according to an embodiment of the present invention.

[0030] Figure 3A is a schematic diagram of a configuration of a HPF according to an embodiment of the present invention.

[0031] Figure 3B is a graph of frequency response of a resistor-capacitor (RC) HPF having a cutoff frequency of 20 megahertz (MHz) according to an embodiment of the present invention.

[0032] Figure 4 is a schematic diagram of an optical fiber transmission system according to an embodiment of the present invention.

[0033] Figure 5A is a graph of a radio frequency (RF) spectrum of a detection signal having an equivalent MPI of -26.5 decibels (dB) according to an embodiment of the present invention.

[0034] Figure 5B is a graph of an RF spectrum after an RC HPF having a cutoff frequency of 20 MHz is applied to an MPI distorted signal according to an embodiment of the present invention.

[0035] Fig. 6A and Figure 6Bis an eye diagram of a 4-level pulse amplitude modulation (PAM4) signal with -26.5 dB MPI according to an embodiment of the present invention.

[0036] Fig. 7A and Figure 7B is a simulation graph of simulated bit error rate (BER) versus received optical power (ROP) at different HPF cutoff frequencies at -26.5 dB MPI according to an embodiment of the present invention.

[0037] Fig. 8A and Figure 8B is a simulated graph comparing the MPI power penalty (ie, power penalty relative to MPI) between scenarios without HPF and with HPF at an optimal cutoff frequency according to an embodiment of the present invention.

[0038] 9A to 9C is a simulated graph of the effect of laser line width evaluated at -26.5 dB MPI according to an embodiment of the present invention.

[0039] Fig.10 is a graph showing power penalty caused by HPF filtering at different baud rates relative to cutoff frequency according to an embodiment of the present invention.

[0040] Fig.11A and Fig. 11B is a graph of BER versus ROP for 26.6 Gbaud PAM4 with different HPF cutoff frequencies according to an embodiment of the present invention.

[0041] Fig.12 is a schematic diagram of a polarization-division multiplexing (PDM) optical fiber system according to an embodiment of the present invention.

[0042] Fig.13 is a schematic diagram of a single polarization (SP) self-homodyne detection (SHD) transmission system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0043] First, it should be understood that although a description of one or more embodiments is provided below, the disclosed systems and / or methods can be implemented using any number of techniques, whether currently known or in existence. The present invention is in no way limited to the illustrative implementations, drawings, and techniques described below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims and the full scope of their equivalents.

[0044] Intensity modulated optical signals have a strong optical carrier in addition to broadband data. The carrier spectrum width is determined by the laser linewidth and is very narrow compared to the data spectrum width. MPI is the sum of double reflections of the transmitted signal in the optical fiber link. When MPI and the signal are detected at the optical receiver, the MPI and the signal are combined by superposition in the optical field. After optical detection, the beat frequency of the optical signal carrier and the MPI carrier generates a low-frequency component, which is an undesirable crosstalk, hereinafter referred to as carrier-carrier beat noise. The spectrum of the carrier-carrier beat noise is the convolution between the signal carrier and the MPI carrier. Its spectral distribution is determined by the laser linewidth and the delay between the signal and MPI. For a typical electro-absorption modulated laser (EML) with a linewidth of several MHz, the spectrum width of the fully decorrelated carrier-carrier beat noise is approximately twice the laser linewidth, which is still very narrow compared to the data spectrum of up to tens of GHz.

[0045] For the next generation 800GE, PAM4 (single polarization or polarization multiplexing) or self-homodyne detection (SHD) is a promising solution because of their low complexity and easier upgrade from 400GE. However, PAM4 is susceptible to MPI caused by the reflectivity of fiber connectors. For example, the customer-side optical network involves multiple reflection points from fiber connectors, transmitters, and receivers. These reflection points generate MPI, especially when the connectors are dirty. In actual deployments, low MPI tolerance is the main limitation of direct-detection PAM4 applications. To accommodate PAM4 applications, standard bodies such as IEEE802.3 and 100GLambda MSA tightened the fiber connector return loss (RL) specification from -26 decibels (dB) to -35dB in 2016. But in actual systems, the return loss of "dirty connectors" (e.g., standard / subscriber connectors (SC) or ferrule connectors (FC)) may be much higher than -26dB, making it difficult to use PAM4 formats. Specifically, in short-distance fiber transmission systems such as data centers, MPI has always been the main limiting factor of performance.

[0046] Therefore, the disclosed embodiments include a new optical receiver configuration having at least one high pass filter (HPF) to remove low frequency carrier-carrier beat noise, thereby reducing the impact of MPI in the optical fiber link. Therefore, the disclosed embodiments can reduce the impact of MPI in IMDD systems, such as but not limited to PAMn systems, single polarization IMDD systems, polarization division multiplexing IMDD systems, and SP-SHD systems. Various embodiments include polarization tracking optical receivers and polarization tracking SHD optical receivers having one or more HPFs. The disclosed embodiments can be very beneficial in short-distance networks and client-side optical networks, such as within and between data centers. The disclosed embodiments provide a simple passive solution that does not require additional power consumption. As will be described herein, theoretical analysis, simulations, and experiments are performed to verify the effectiveness of the disclosed embodiments.

[0047] Figure 1 1 is a schematic diagram showing an optical receiver 100 with a high pass filter according to an embodiment of the present invention. The optical receiver 100 includes an input port 102, a detector 104, an HPF 106, and an output port 110. The input port 102 receives an intensity modulated optical signal that may be combined with an MPI. In one embodiment, the detector 104 is a photodetector (PD), and its output can be serially connected to an optional transimpedance amplifier (TIA) 108. In one embodiment, the detector 104 receives or detects an optical input signal, converts it into an electrical signal by direct detection (envelope detection), and amplifies the electrical signal using the TIA 108. The HPF 106 can be used to remove low-frequency carrier-carrier beat noise to mitigate the carrier-carrier beat noise caused by the beat frequency between the signal carrier and the MPI carrier. Specifically, the HPF 106 filters the electrical signal and suppresses the low-frequency noise generated by the beat frequency between the signal carrier and the MPI carrier. In one embodiment, the HPF 106 has a cutoff frequency in the range of 1 MHz to 100 MHz or 1 MHz to 200 MHz. In one embodiment, the electrical output signal of the optical receiver is fed from the output port 110 to a digital signal processor (DSP after analog-to-digital conversion) or a decision circuit.

[0048] In another embodiment, the TIA 108 can be separate from the detector 104. The TIA 108 can be in multiple stages, where the first stage converts the photocurrent from the detector 104 into a voltage source. In some embodiments, the HPF 106 can be located between the detector 104 and the TIA 108.

[0049] The optical signal may be in a baseband format, such as multi-level pulse amplitude modulation PAMn, including PAM2 (NRZ), PAM4, or higher level PAM. Here, n in PAMn is an integer equal to or greater than 2. The optical signal may also be in a subcarrier-based format, such as carrier-less amplitude phase (CAP) modulation, quadrature amplitude modulation (QAM), or discrete multiple tone (DMT). In a conventional optical receiver, a direct component (DC) block may have been placed after the TIA 108. The DC block may exhibit high-pass filtering aspects, but its cutoff frequency is much lower than 1MHz, with typical values ​​ranging from tens of kHz to 100kHz. Here, the HPF cutoff frequency is defined as the frequency at which the output amplitude is equal to 70.7% of the input amplitude (or the response is equal to 50% of the peak response in the HPF transfer function). The frequency range below this cutoff point is generally referred to as the stopband, while the frequency range above this cutoff point is generally referred to as the passband. As shown below, this DC block has little or no effect on mitigating the effects of MPI. In one embodiment, the HPF 106 has a cutoff frequency in the range of 1 MHz to about 100 MHz or 1 MHz to about 200 MHz, which is different from a conventional DC block.

[0050] Figure 2 In Figure 2 A to Figure 2 C is a schematic diagram illustrating the principle of the provided method for mitigating the impact of MPI using a HPF in an optical receiver according to an embodiment of the present invention. Figure 2 A is the electrical spectrum of the HPF input, which is an electrical signal with carrier-carrier beat noise (i.e., Figure 1 The signal entering the HPF 106 is as follows. Figure 2 As shown in A, the signal includes broadband data and low-frequency narrowband carrier-carrier beat noise. Figure 2 B is the HPF frequency response, where the intensity in the vertical axis is the square of the amplitude. The frequency response is a quantitative measure of the frequency spectrum output by the HPF in response to a stimulus. It is a measure of the amplitude and phase of the output as a function of frequency compared to the input. Figure 2 C is the spectrum output by the HPF, which is the electrical signal with the carrier-carrier beat noise removed by the HPF (ie, broadband data with the low-frequency narrow-band carrier-carrier beat noise removed). Figure 2 It is shown that, using the HPF, the low-frequency narrowband carrier-carrier beat noise caused by the beat frequency between the signal carrier and the MPI carrier is removed from the broadband data.

[0051] Figure 3A3 is a schematic diagram of a configuration of an HPF 300 according to an embodiment of the present invention. In one embodiment, the HPF 300 is a resistor-capacitor (RC) high pass filter. A surface mount capacitor 302 may be introduced to a microstrip transmission line 304. A resistor (R) 308 is a load impedance. In one embodiment, the frequency transfer function of the HPF 300 is

[0052]

[0053] Where f is the frequency, j is a unit imaginary number,

[0054] is the cutoff frequency, which occurs in the response |H(f c )| 2 3dB below its peak value.

[0055] In one embodiment, according to formula (2), HPF 300 can also be implemented by replacing the existing DC block with a capacitor of much smaller value (i.e., with a cutoff frequency For example, for a cutoff frequency of 20MHz, the required capacitance is approximately 160pF. Figure 3B The cut-off frequency according to the embodiment of the present invention is 20 MHz (f c =20MHz) is a graph of the frequency response of an RC HPF.

[0056] Figure 4 FIG. 4 is a schematic diagram of an optical fiber transmission system 400 according to an embodiment of the present invention. The optical fiber transmission system 400 includes an optical transmitter 402, an optical fiber link 404 that experiences multipath interference, and an optical receiver 406 having a HPF 408. In one embodiment, the optical receiver 406 is similar to Figure 1 The optical receiver 100 in FIG.

[0057] In one embodiment, the optical transmitter 402 generates the intensity modulated optical signal 414 from an electro-absorption modulated laser (EML) or a directly modulated laser (DML). In other embodiments, the optical transmitter 402 generates the intensity modulated optical signal 414 from a continuous wave (CW) laser followed by an intensity modulator such as a Mach-Zehnder modulator (MZM), where the CW light may come from a distributed feedback laser (DFB) or an external cavity laser (ECL). In one embodiment, the intensity modulated optical signal 414 may use PAMn as baseband modulation and CAP, QAM, or DMT as subcarrier modulation. In embodiments with subcarrier modulation, an analog HPF is not required if a matched filter is used in the receiver DSP. The optical transmitter 402 may employ various generation schemes and laser linewidths, such as, but not limited to, EML for laser linewidths of a few MHz, DML for laser linewidths up to 10 MHz or more, DFB+MZM for laser linewidths of a few MHz, and ECL+MZM for laser linewidths of tens of kHz.

[0058] Fiber link 404 receives intensity modulated optical signal 414 from optical transmitter 402 and sends the signal to optical receiver 406. Fiber link 404 may be composed of one or more sections of optical fiber. Fiber link 404 may have at least two reflection points, which may come from a transmitter, a receiver, or a fiber connector. Double reflection of the forward-traveling signal generates forward-traveling reflected light. MPI is the accumulation of all forward-traveling reflected light. The strength of MPI depends on the return loss of the connector, transmitter, and receiver, as well as polarization alignment, and the delay and phase shift between the signal and each double-reflected light.

[0059] like Figure 1 As described in , the optical receiver 406 receives and detects the optical signal with MPI, converts the optical signal into an electrical signal using the detector 410, amplifies the electrical signal using the TIA 412, suppresses the carrier-carrier beat noise of the electrical signal using the HPF 408, and outputs the data to the next stage for digital signal processing or decision making.

[0060] For a fiber link with MPI events, the worst case occurs when all MPIs have aligned polarization and amplitude, but have phase decorrelated with the signal. The worst case of cumulative MPI can be modeled by a single equivalent MPI event. The optical field with signal and MPI can be expressed as

[0061]

[0062] where ε is the equivalent MPI or reflectivity, φ(t) is the laser phase noise, and τ is the delay between the signal and the MPI.

[0063] To understand this problem analytically, the optical signal field and the MPI field can be simplified to the carrier and broadband data of the intensity modulation system. So equation (3) can be written as

[0064]

[0065] Where P0 is the average optical power, V S (t) is the modulated broadband data, V MPI (t) is the data part of MPI. After light detection, the photocurrent I PD It can be expressed as

[0066]

[0067] Here, η is the detector's responsivity. On the right side of equation (6), the first term "1" is the DC (0 Hz frequency) of the signal, and the second term V S (t) is the useful signal; the third term “ε” is the DC (0Hz frequency) caused by MPI, which can be removed by the DC block without affecting the performance; the fourth term is the carrier-carrier beat noise (caused by the beat frequency between the signal carrier and the MPI carrier), whose spectrum is the convolution of φ(t) and φ(t-τ), determined by the laser linewidth and delay between the signal and MPI. For the widely used optical transmitters composed of DFB+MZM or EML, the typical laser linewidth is several MHz. For a sufficiently decorrelated phase between the signal and MPI, the spectrum width of the carrier-carrier beat noise can be about twice the laser linewidth, still in the MHz range. The built-in DC block inside the receiver cannot suppress this carrier-carrier beat noise, so the disclosed embodiment uses a HPF filter.

[0068] Simulations have been performed to verify the scheme of the disclosed embodiments, and the simulation conditions are listed in Table 1.

[0069] Table 1

[0070]

[0071] To verify Figure 2 A to Figure 2 Schematic diagram shown in C, analyzing the received RF spectrum. Figure 5A is a graph showing the RF spectrum of a detection signal having an equivalent MPI of -26.5 dB according to an embodiment of the present invention. Figure 5A As shown, a strong low-frequency component 502 with a spectral distribution close to a Lorentzian shape is observed, which is caused by carrier-carrier beat noise. Figure 5B FIG. 4 is a graph of an RF spectrum after an RC HPF with a cutoff frequency of 20 MHz is applied to an MPI distorted signal according to an embodiment of the present invention. Figure 5B As shown, after the HPF is applied to the MPI distorted signal, the low frequency component / noise 502 (eg Figure 5A shown) is removed.

[0072] Fig. 6A and Figure 6B 601 to 606, where the surrounding (shaded) area 610 is caused by the inter-symbol interference caused by MPI and finite signal rise / fall times. For example, Fig. 6A The eye diagram of a PAM4 signal with -26.5dB MPI and no HPF is shown. It can be seen that the high-level eyes 605 and 606 are almost completely closed due to the crosstalk caused by MPI, which depends on the signal level and in which the high-level signal suffers more damage than the low-level signal. Figure 6B The results show that the MPI with -26.5dB and the HPF (f C =20MHz) of the PAM4 signal. Figure 6B In Figure 1, once a HPF with a cutoff frequency of 20MHz is applied to the distorted signal, the eye has been significantly opened due to the removal of the carrier-carrier beat noise.

[0073] Fig. 7A and Figure 7B is a simulation graph of simulated bit error rate (BER) versus received optical power (ROP) at different HPF cutoff frequencies at -26.5 dB MPI according to an embodiment of the present invention. Fig. 7Ais the result for an EML or DFB+MZM transmitter with a typical laser linewidth of 1 MHz. It can be seen that there is little improvement in performance with a HPF cutoff frequency of 100 kHz, so a typical DC block has no effect in removing carrier-carrier beat noise with a typical laser linewidth of 1 MHz. The optimal cutoff frequency is about 20 MHz. HPFs with cutoff frequencies below the optimal value do not adequately suppress the MPI effect, while HPFs with cutoff frequencies above the optimal value cause too much distortion due to the useful signal being cut off. These simulation results clearly show that by using the disclosed embodiments, the BER performance has been significantly improved. Figure 7B is a result for a DML transmitter with a typical laser linewidth of 10 MHz. The optimum cutoff frequency in this case is approximately 50 MHz. Figure 7B It can be seen that the BER performance is also improved by using HPF for 10MHz line width, but it is different from Fig. 7A The effectiveness is reduced compared to the 1MHz linewidth used in .

[0074] Fig. 8A and Figure 8B 1 is a simulated chart comparing the MPI power penalty (i.e., power penalty relative to MPI) between scenarios with HPF and without HPF at optimal cutoff frequency according to an embodiment of the present invention. In the illustrated embodiment, the power penalty is quoted by KP4FEC as the no-MPI and no-HPF cases @ 2e-4BER. Fig. 8A It is for lasers with 1MHz linewidth, without HPF and with HPF (f C =20MHz). At very low MPI (e.g. -40dB), the performance with HPF is slightly worse than without HPF due to small filtering penalty. When MPI is above -39dB, HPF improves performance. For 1dB power penalty, HPF provides about 7dB gain in MPI tolerance, correspondingly, connector return loss requirement is reduced by 3.5dB. Figure 8B It is for lasers with 10MHz linewidth, without HPF and with HPF (f C =50MHz). As the laser linewidth is significantly wider, the MPI tolerance gain at a 1dB power penalty drops to about 1.0dB, and the HPF improves the performance only when the MPI is above -32.5dB.

[0075] 9A to 9C is a simulated graph of the effect of laser line width evaluated at -26.5 dB MPI according to an embodiment of the present invention. 9A to 9C As shown, the operation and performance of MPI mitigation using HPF depends strongly on the laser linewidth. For example, Fig. 9Ais a graph of the optimal HPF cutoff frequency as a function of the laser linewidth according to an embodiment of the present invention, with an MPI of -26.5 dB. The optimal HPF cutoff frequency increases with increasing laser linewidth. For a typical EML or DFB+MZM optical transmitter with a laser linewidth of 1 MHz, the optimal HPF cutoff frequency is approximately 20 MHz. For an optical transmitter using an ECL with a linewidth of 50 kHz, the optimal HPF cutoff frequency is approximately 4 MHz. For a DML optical transmitter with a linewidth of 10 MHz, the optimal HPF cutoff frequency is approximately 50 MHz. In the simulation shown, the HPF cutoff frequency is limited to a range of 1 MHz to 200 MHz.

[0076] Fig. 9B is a graph of the ratio of the optimal HPF cutoff frequency to the laser line width as a function of the laser line width according to an embodiment of the present invention. As shown in the figure, the ratio decreases as the laser line width increases. For an optical transmitter using an ECL with a line width of 50kHz, the ratio is about 80; for a typical EML or DFB+MZM optical transmitter using a line width of 1MHz, the ratio is about 20; for a DML optical transmitter using a line width of 10MHz, the ratio is about 5. In the simulation shown, the ratio of the HPF cutoff frequency to the laser line width is limited to between 3 and 200.

[0077] Fig. 9C is a graph of the dependence of sensitivity gain on line width according to an embodiment of the present invention. In the illustrated embodiment, the gain is defined as the sensitivity difference between the sensitivity without HPF and the sensitivity with HPF at the optimal cutoff frequency, @2e-4BER. Fig. 9C As shown, the sensitivity gain decreases with increasing line width.

[0078] Fig.10 is a graph of the power cost caused by HPF filtering at different baud rates versus cutoff frequency according to an embodiment of the present invention. As mentioned above, the optimal HPF cutoff frequency is mainly determined by the laser line width. In addition to removing carrier-carrier beat noise, HPF also introduces filtering costs, such as Fig. 8A and Figure 8B As shown in Figure 2, the filtering cost depends on the baud rate. Therefore, the optimal cutoff frequency is also related to the baud rate. Fig.10 The power penalty caused by HPF filtering at 26.6Gbuad and 53.2Gbuad PAM4 is shown as a function of the cutoff frequency. The filtering penalty is defined relative to the filtering penalty without HPF at BER@2e-4. Fig.10As shown in Figure 1, the HPF filtering penalty increases with the increase of the HPF cutoff frequency. For example, for a cutoff frequency of 20MHz, for 53.2Gbuad PAM4, the penalty caused by filtering is about 0.2dB. The HPF filtering penalty is higher for low baud rates than for high baud rates.

[0079] Fig.11A and Fig. 11B is a graph of BER versus ROP for 26.6 Gbaud PAM4 with different HPF cutoff frequencies according to an embodiment of the present invention. Specifically, Fig.11A The BER vs. received optical power (ROP) for 26.6 Gbaud PAM4 is shown for -26.5 dB MPI results using experiments (i.e., physical builds rather than software simulations) for three HPF cutoff frequencies of 6 MHz, 23 MHz, and 37 MHz. The experiments were run at 26.6 Gbaud because only 25 Gbaud (rather than 50 Gbaud) optical receivers were available for evaluation. All HPFs provided improved BER performance compared to no HPF, demonstrating the effectiveness of the disclosed embodiments. In the illustrated embodiment, the 23 MHz HPF provided the best performance. Simulations (i.e., software models) were also run at 26.6 Gbaud PAM4 for comparison, as shown in Figure 2. Fig. 11B When HPF is used, the simulation results show that Fig.11A The experimental results in the effective MPI suppression. Fig.11A Similar to the experimental results in Fig. 11B The simulation results in show that 23MHz HPF filtering provides better performance than filtering with 6MHz and 37MHz cutoff frequencies.

[0080] Figure 12 is a schematic diagram of a polarization-division multiplexing (PDM) fiber system 1200 according to an embodiment of the present invention. The PDM fiber system 1200 includes a dual-polarization optical transmitter 1210, a fiber link 1220 that experiences multipath interference, and a polarization tracking optical receiver 1230 with two HPFs for mitigating MPI. In the illustrated embodiment, the dual-polarization optical transmitter 1210 uses CW light 1212 as input to an optical coupler (OC), which separates the light into two parts. Each part is sent to an intensity modulator (IM) 1214 (e.g., in an MZM) for data modulation. The two modulated lights (one of which is polarized 90 degrees by a polarization rotator (PR) 1216) are combined by a polarization beam combiner (PBC) 1218. The output is a polarization multiplexed channel and is sent to the fiber link 1220, which is similar to Figure 4 The same. After the fiber link 1220, the dual polarization channel with MPI is sent to the polarization tracking optical receiver 1230 with HPF. In the illustrated embodiment, the polarization tracking optical receiver 1230 includes a polarization tracker 1232, two detectors 1234, and two HPFs 1236. Each detector 1234 may include an optional TIA 1238. The polarization tracker 1232 receives the dual polarization signal and demultiplexes it into two channels (X and Y) in the optical domain. In one embodiment, the polarization tracker 1232 may be an endless polarization controller that can be automatically controlled and can be integrated in a silicon photonics platform. Each demultiplexed channel is input into the detector 1234 for optical to electrical conversion, followed by electrical signal amplification. The electrical signal then passes through the HPF 1236 for mitigating MPI. The electrical output signal of the polarization tracking optical receiver 1230 is fed to the DSP or decision circuit after analog-to-digital conversion.

[0081] Fig.131 is a schematic diagram of a single polarization (SP) self-homodyne detection (SHD) transmission system 1300 according to an embodiment of the present invention. The SP-SHD 1300 includes an SP-SHD transmitter 1310, an MPI impaired fiber link 1320, and a polarization tracking SHD receiver 1330 with two HPFs for mitigating MPI. In the SP-SHD transmitter 1310, CW light 1306 is input to the OC 1308 of the SP-SHD transmitter 1310, which separates the light into two parts. The first part is sent to the modulator 1314, such as a single polarization IQ modulator (SP-IQM), for data modulation. The second part passes through the waveguide without modulation, acts as a pilot or carrier, and is combined with the modulated light by the PBC 1318, one of which is rotated 90 degrees in polarization by the PR 1316. The output consists of a signal and a carrier with polarizations orthogonal to each other. The SP-SHD channel is sent into the optical fiber link 1320, which is Figure 4 same.

[0082] After transmission in the fiber link 1320, the SP-SHD channel undergoes MPI and is sent to a polarization tracking SHD receiver with HPF 1330. In the illustrated embodiment, the polarization tracking SHD receiver 1330 includes a polarization tracker 1332, a 90-degree optical hybrid mixer 1334, two balanced detectors (BD) 1336, and two HPFs 1338. In one embodiment, each BD 1336 includes an optional TIA 1340. The polarization tracker 1332 receives the SP-SHD channel with MPI and separates the signal and the pilot, wherein the pilot is used as a local oscillator (LO). The LO and signal are input to a 90-degree optical hybrid mixer 1334 to generate four outputs, of which two outputs (I positive and I negative) are sent to the first BD 1336 to generate and amplify the recovered in-phase (I) data, and the other two outputs (Q positive and Q negative) are sent to the second BD 1336 to generate and amplify the recovered quadrature (Q) data. The I and Q channels are input to their corresponding HPF 1338 to mitigate the MPI effect. The electrical signal from each HPF 1338 is then passed to a coupled TIA 1340, which amplifies the electrical signal. The electrical output signal of the polarization tracking optical receiver 1330 is fed to a DSP or decision circuit after analog-to-digital conversion.

[0083] In all the embodiments discussed above, the HPF is implemented in the analog domain. If there is no receiver DSP, the HPF can be located after the detector and before the decision circuit. If there is a DSP on the receiving side, the HPF can be located after the detector and before the analog-to-digital converter. For scenarios with a DSP on the receiving side, the HPF can also be implemented in the digital domain, where the digital HPF is located after the analog-to-digital converter. The number of HPF filter taps can be long because the cutoff frequency is in MHz, which is very low compared to the DSP sampling rate. In some embodiments, a multi-stage moving average filter with sampling rate conversion can be introduced to reduce the number of filter taps.

[0084] Although the present invention provides several embodiments, it should be understood that the disclosed systems and methods may also be embodied in a variety of other specific forms without departing from the spirit or scope of the present invention. The present examples should be considered illustrative rather than restrictive, and the present invention is not limited to the details given herein. For example, various elements or components may be combined or integrated in another system, or some features may be omitted or not implemented.

[0085] In addition, without departing from the scope of the present invention, the techniques, systems, subsystems and methods described and illustrated as discrete or separate in various embodiments may be combined or integrated with other systems, modules, techniques or methods. Other items shown or described as coupled to each other, or directly coupled, or communicating with each other may be indirectly coupled or communicated with each other electrically, mechanically or otherwise through some interface, device or intermediate component. Other examples of changes, substitutions and modifications may be determined by those skilled in the art, and changes, substitutions and modifications may be made without departing from the spirit and scope of the present invention.

Claims

1. An optical receiver, characterized in that: include: an input port for receiving an intensity modulated optical signal with multipath interference (MPI); the MPI being the sum of double reflections of the intensity modulated optical signal in an optical fiber link, the MPI and the intensity modulated optical signal being combined by superposition in an optical field when the MPI and the intensity modulated optical signal are detected at the optical receiver; A detector, used for converting the intensity modulated optical signal with multipath interference into an electrical signal; a high pass filter (HPF) for filtering the electrical signal and suppressing low frequency components caused by the MPI to generate a filtered electrical signal; wherein the HPF has a cutoff frequency in the range of 20 MHz to 200 MHz; The HPF is a resistor-capacitor (RC) HPF with surface-mount capacitors on a microstrip transmission line, or the HPF is an existing direct component (DC) block with a replacement capacitor; a transimpedance amplifier (TIA), located downstream of the HPF and used to amplify the filtered electrical signal; The output port is used to send the filtered electrical signal.

2. The optical receiver according to claim 1, characterized in that The HPF has a cutoff frequency in the range of 20 MHz to 100 MHz.

3. The optical receiver according to claim 1, characterized in that The ratio of the HPF cutoff frequency to the transmitter laser line width is in the range of 3 to 200.

4. The optical receiver according to any one of claims 1 to 3, characterized in that The HPF is a resistor-capacitor (RC) HPF with a surface-mount capacitor on a microstrip transmission line. The frequency transfer function of the HPF is Where f is the frequency, j is a unit imaginary number, is the cut-off frequency, R is the resistance in the HPF, and C is the capacitance in the HPF.

5. The optical receiver according to any one of claims 1 to 3, characterized in that The replacement capacitor has the cutoff frequency 6. The optical receiver according to any one of claims 1 to 3, characterized in that The intensity modulated optical signal with MPI uses pulse amplitude modulation n (PAMn) as baseband modulation, where n is an integer equal to or greater than 2.

7. The optical receiver according to any one of claims 1 to 3, characterized in that The intensity modulated optical signal with MPI uses one of carrierless amplitude phase (CAP) modulation, quadrature amplitude modulation (QAM) or discrete multiple tone (DMT) as subcarrier modulation.

8. The optical receiver according to any one of claims 1 to 3, characterized in that: The optimal cutoff frequency of the HPF increases as the line width of the laser used to transmit the intensity modulated optical signal to the optical receiver increases.

9. The optical receiver according to any one of claims 1 to 3, characterized in that: The MPI is generated in a fiber optic link coupled to the input port of the optical receiver.

10. The optical receiver according to any one of claims 1 to 3, characterized in that: The HPF introduces a filtering penalty that depends on the baud rate used when modulating data in the intensity modulated optical signal received by the optical receiver.

11. A polarization-division multiplexing (PDM) optical fiber system, characterized in that: include: Dual polarization transmitter; A polarization tracking optical receiver having a plurality of high pass filters (HPFs) for mitigating multipath interference (MPI) and a plurality of transimpedance amplifiers (TIA), wherein the TIA is located downstream of the HPF; the MPI is a double reflection sum of an optical signal transmitted in an optical fiber link, and when the MPI and the optical signal are detected at the polarization tracking optical receiver, the MPI and the optical signal are combined by superposition in an optical field; the HPF has a cutoff frequency in the range of 20 MHz to 200 MHz; The HPF is a resistor-capacitor (RC) HPF with surface-mount capacitors on a microstrip transmission line, or the HPF is an existing direct component (DC) block with a replacement capacitor; An optical fiber link couples the dual-polarization optical transmitter and the polarization tracking optical receiver.

12. The PDM optical fiber system according to claim 11, characterized in that: The dual-polarization optical transmitter comprises: an optical coupler (OC), configured to receive continuous wave (CW) light and separate the CW light into a first part and a second part; a first intensity modulator (IM) for receiving the first portion of the CW light and modulating the first portion with first data to generate a first modulated light; a second IM for receiving the second portion of the CW light and modulating the second portion with second data to generate a second modulated light; a polarization rotator (PR), configured to rotate the polarization of one of the first modulated light and the second modulated light by 90 degrees; a polarization beam combiner (PBC), configured to combine the first modulated light and the second modulated light after the polarization of one of the first modulated light and the second modulated light is rotated by 90 degrees to generate a dual-polarization signal; An output port is used to send the dual-polarization signal to the optical fiber link.

13. The PDM optical fiber system according to claim 12, characterized in that: The polarization tracking optical receiver comprises: A polarization tracker, configured to receive the dual polarization signal and demultiplex the dual polarization signal into an X channel optical signal and a Y channel optical signal; A first detector, configured to receive the X-channel optical signal and convert the X-channel optical signal into a first electrical signal; A second detector, used for receiving the Y channel optical signal and converting the Y channel optical signal into a second electrical signal; a first high pass filter (HPF), configured to receive the first electrical signal and filter the first electrical signal to suppress a low frequency component caused by the MPI, thereby generating a first filtered electrical signal; a second HPF, configured to receive the second electrical signal and filter the second electrical signal to suppress a low-frequency component caused by the MPI, thereby generating a second filtered electrical signal; a first transimpedance amplifier (TIA), configured to amplify the first filtered electrical signal; a second TIA, configured to amplify the second filtered electrical signal; a first output port, used for sending the first filtered electrical signal; The second output port is used to send the second filtered electrical signal.

14. The PDM optical fiber system according to claim 13, characterized in that: The polarization tracker is an endless polarization controller.

15. A polarization tracking optical receiver, characterized in that: include: A polarization tracker, configured to receive a dual polarization signal with multipath interference (MPI) and demultiplex the dual polarization signal into an X-channel optical signal and a Y-channel optical signal; wherein the MPI is a sum of dual reflections of the dual polarization signal in an optical fiber link, and when the MPI and the dual polarization signal are detected at the polarization tracking optical receiver, the MPI and the dual polarization signal are combined by superposition in an optical field; A first detector, configured to receive the X-channel optical signal and convert the X-channel optical signal into a first electrical signal; A second detector, used for receiving the Y channel optical signal and converting the Y channel optical signal into a second electrical signal; a first high pass filter (HPF) for receiving the first electrical signal and filtering the first electrical signal to suppress low frequency components caused by multipath interference (MPI) to generate a first filtered electrical signal; the first HPF having a cutoff frequency in the range of 20 MHz to 200 MHz; the first HPF being a resistor-capacitor (RC) HPF with a surface mount capacitor on a microstrip transmission line, or the first HPF being an existing direct component (DC) block with a replacement capacitor; a second HPF for receiving the second electrical signal and filtering the second electrical signal to suppress low-frequency components caused by the MPI, thereby generating a second filtered electrical signal; the second HPF having a cutoff frequency in the range of 20 MHz to 200 MHz; the second HPF being a resistor-capacitor (RC) HPF with a surface-mounted capacitor on a microstrip transmission line, or the second HPF being an existing direct component (DC) block with a replacement capacitor; a first transimpedance amplifier (TIA), located downstream of the first HPF and used to amplify the first filtered electrical signal; a second TIA, located downstream of the second HPF and configured to amplify the second filtered electrical signal; a first output port, used for sending the first filtered electrical signal; The second output port is used to send the second filtered electrical signal.

16. The polarization tracking optical receiver according to claim 15, characterized in that: The HPF has a cutoff frequency in the range of 20 MHz to 100 MHz.

17. A single polarization (SP) self-homodyne detection (SHD) transmission system, characterized in that: include: SP-SHD transmitter; Multipath interference (MPI) impaired fiber links; A polarization tracking SHD receiver having a first high pass filter (HPF), a second HPF, a first transimpedance amplifier (TIA), and a second TIA for mitigating MPI, wherein the first TIA is located downstream of the first HPF, and the second TIA is located downstream of the second HPF, wherein the MPI is a double reflection sum of an optical signal transmitted in the optical fiber link, and when the MPI and the optical signal are detected at the polarization tracking SHD receiver, the MPI and the optical signal are combined by superposition in an optical field; the first HPF has a cutoff frequency in the range of 20 MHz to 200 MHz; the second HPF has a cutoff frequency in the range of 20 MHz to 200 MHz; the first HPF and the second HPF are resistor-capacitor (RC) HPFs with surface-mounted capacitors on a microstrip transmission line, or the first HPF and the second HPF are existing direct component (DC) blocks with replaced capacitors.

18. The SP-SHD transmission system according to claim 17, characterized in that: The SP-SHD transmitter comprises: an optical coupler (OC), configured to receive continuous wave (CW) light and separate the CW light into a first part and a second part, wherein the second part of the CW light is used as a pilot signal; a first intensity modulator (IM) for receiving the first portion of the CW light and modulating the first portion with data to generate a first modulated light; a polarization rotator (PR), configured to rotate the polarization of one of the first modulated light and the second portion of CW light by 90 degrees; a polarization beam combiner (PBC) for combining the first modulated light and the second portion of CW light after the polarization of one of the first modulated light and the second portion of CW light is rotated by 90 degrees to generate an SP-SHD channel including a signal and a pilot signal having orthogonal polarizations to each other; An output port is used to send the SP-SHD channel to the MPI damaged optical fiber link.

19. The SP-SHD transmission system according to claim 18, characterized in that: The polarization tracking SHD receiver comprises: a polarization tracker for receiving the SP-SHD channel with MPI from the MPI impaired optical fiber link and separating the SP-SHD channel into the signal and the pilot signal, wherein the pilot signal is used as a local oscillator (LO); a 90-degree optical hybrid mixer, configured to receive the signal and the LO, the 90-degree optical hybrid mixer being configured to generate an in-phase (I) positive output signal, an I negative output signal, a quadrature (Q) positive output signal and a Q negative output signal; a first balanced detector (BD), configured to receive the I positive output signal and the I negative output signal, and restore an I data signal; A second BD, used for receiving the Q positive output signal and the Q negative output signal, and restoring a Q data signal; The first HPF is used to receive the I data signal and filter the I data signal to suppress the low frequency component caused by MPI, thereby generating a first filtered electrical signal; The second HPF is used to receive the Q data signal and filter the Q data signal to suppress the low frequency component caused by the MPI, thereby generating a second filtered electrical signal; The first TIA is used to amplify the first filtered electrical signal; The second TIA is used to amplify the second filtered electrical signal; a first output port, used for sending the first filtered electrical signal; The second output port is used to send the second filtered electrical signal.

20. The SP-SHD transmission system according to any one of claims 17 to 19, characterized in that: The cutoff frequencies of the first HPF and the second HPF are in the range of 20 MHz to 100 MHz.

21. A polarization tracking self-homodyne detection (SHD) receiver, characterized in that: include: A polarization tracker for receiving a single polarization (SP)-SHD channel with multipath interference (MPI) from an optical fiber link and separating the SP-SHD channel into a signal and a pilot signal, wherein the pilot signal is used as a local oscillator (LO); the MPI is a sum of double reflections of the SP-SHD channel transmitted in the optical fiber link, and when the MPI and the SP-SHD channel are detected at the polarization tracking SHD receiver, the MPI and the SP-SHD channel are combined by superposition in an optical field; a 90-degree optical hybrid mixer, configured to receive the signal and the LO, the 90-degree optical hybrid mixer being configured to generate an in-phase (I) positive output signal, an I negative output signal, a quadrature (Q) positive output signal and a Q negative output signal; a first balanced detector (BD), configured to receive the I positive output signal and the I negative output signal, and restore an I data signal; A second BD, used for receiving the Q positive output signal and the Q negative output signal, and restoring a Q data signal; a first high pass filter (HPF) for receiving the I data signal and filtering the I data signal to suppress low frequency components caused by MPI, thereby generating a first filtered electrical signal; the first HPF having a cutoff frequency in the range of 20 MHz to 200 MHz; the first HPF being a resistor-capacitor (RC) HPF with a surface mounted capacitor on a microstrip transmission line, or the first HPF being an existing direct component (DC) block with a replacement capacitor; a second HPF for receiving the Q data signal and filtering the Q data signal to suppress the low frequency component caused by the MPI, thereby generating a second filtered electrical signal; wherein the second HPF has a cutoff frequency in the range of 20 MHz to 200 MHz; the second HPF is a resistor-capacitor (RC) HPF with a surface mount capacitor on a microstrip transmission line, or the second HPF is an existing direct component (DC) block with a replacement capacitor; a first transimpedance amplifier (TIA), located downstream of the first HPF and used to amplify the first filtered electrical signal; a second TIA, located downstream of the second HPF and configured to amplify the second filtered electrical signal; a first output port, used for sending the first filtered electrical signal; The second output port is used to send the second filtered electrical signal.

22. The polarization tracking SHD receiver of claim 21, wherein: The first HPF and the second HPF have cutoff frequencies in the range of 20 MHz to 100 MHz.

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