Optical module supporting wide-band operation and its frequency offset compensation method

Abstract

The present invention discloses an optical module supporting wide-band operation: a first laser and a second laser are two tunable lasers forming a light source, a 2x2 optical switch is used to control the output direction of the light source, and an electro-optical modulator is used to modulate the output signal of the light source; a coherent receiver is used to receive an external optical signal; and a digital signal processor is used to complete frequency deviation compensation and output a fine-tuning value for the laser. The present invention can pre-compensate most of the frequency deviations before performing digital frequency deviation estimation by using a preset frequency deviation fixed by dual lasers. The residual frequency deviation after frequency deviation pre-compensation is consistent with the internal difference detection situation. On this basis, the residual frequency deviation can be further reduced by fine-tuning the laser frequency, and the difficulty of compensating the residual frequency deviation can be lower than the internal difference detection of a single light source. The present invention also provides a corresponding frequency deviation compensation method for an optical module supporting wide-band operation.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber communication, and more specifically, relates to an optical module supporting wide-band operation and its frequency offset compensation method. Background Art

[0002] With the continuous rise of application scenarios such as 5G mobile networks and data center networks, new service demands such as artificial intelligence and machine learning, AR (Augmented Reality) / VR (Virtual Reality), and high-definition video are growing continuously, and the capacity demand of optical communication networks is also correspondingly increasing continuously. Expanding the working spectrum of optical transmission networks is one of the important ways to increase the capacity of optical networks. The currently popular OTN (Optical Transport Network) system usually uses the C-band with a spectral width of about 4.8 THz to 6 THz as the main working frequency band. In the foreseeable future, the working frequency band of optical transmission networks will be expanded to the C+L band of 12 THz, and even further expanded to the wider S+C+L band.

[0003] With the expansion of the wavelength band of the entire optical transmission network, optical transceiver modules also need to support the corresponding working wavelength bands. The working wavelength band of an optical module is determined by the laser light source it uses. Commonly, there are optical modules in the O-band, C-band, and L-band. In a wide-spectrum optical network, usually more than two types of optical modules are required to meet the network capacity requirements. Since the signals of optical modules with different working wavelength bands cannot communicate with each other, it will increase the corresponding costs and limitations in network design and operation and maintenance. An optical module of the same model that can cover the entire expanded wavelength band will be an important part of future wide-spectrum optical transmission networks.

[0004] Traditional coherent optical transceiver modules use one or two identical tunable lasers as light sources and perform coherent detection of received signals based on the principle of heterodyne detection. Since the tuning frequency range of tunable lasers usually does not exceed 6 Thz, traditional optical modules cannot completely cover the expanded wide-spectrum optical network and need to use multiple optical modules with different working frequency bands. Summary of the Invention

[0005] In view of the above deficiencies or improvement requirements of the prior art, the present invention provides an optical module supporting wide-band operation. Based on two lasers with different working frequency bands, the optical module is designed to work in a coherent heterodyne detection mode. The present invention reduces the difficulty of heterodyne detection and realizes an optical module with continuously tunable operation in a wide frequency band.

[0006] To achieve the above object, according to one aspect of the present invention, an optical module supporting wide - band operation is provided, which includes a first laser, a second laser, a 2x2 optical switch, an electro - optic modulator, a coherent receiver, and a digital signal processor, wherein:

[0007] The first laser and the second laser, two tunable lasers, form a light source. The 2x2 optical switch is used to control the output direction of the light source, and the electro - optic modulator is used to modulate the output signal of the light source;

[0008] The coherent receiver is used to receive external optical signals;

[0009] The digital signal processor is used to complete frequency offset compensation and output fine - tuning values for the lasers.

[0010] In an embodiment of the present invention, the first laser and the second laser are a low - frequency - band tunable laser and a high - frequency - band tunable laser respectively. The operating frequency bands of the two lasers are continuous and have the same frequency - band width, and the tuning step sizes of the two lasers are the same;

[0011] Among them, the wavelengths emitted by the two lasers are λ 1 and λ 2 , and the frequency interval between λ 1 and λ 2 is fixedly controlled to be ∆ω.

[0012] In an embodiment of the present invention, the two beams of laser light with wavelengths of λ 1 and λ 2 respectively pass through the 2x2 optical switch and are introduced into two optical interfaces, TX and LO;

[0013] Among them, the light input to the TX optical interface is the operating wavelength of the current optical module. When it is necessary to switch the operating wavelength of the optical module from the low - frequency band to the high - frequency band, or from the high - frequency band to the low - frequency band, it is achieved by switching which of the first laser and the second laser is conducted to the TX optical interface by the optical switch; the light input to the LO optical interface comes from the other laser and serves as the local oscillator light source at the receiving end.

[0014] In an embodiment of the present invention, the electro - optic modulator modulates the service signal onto the optical carrier of the light source TX. This electro - optic modulator integrates polarization multiplexing and quadrature phase modulation functions. The high - speed modulation signal is generated by the transmitting end of the digital signal processor to modulate and encode the service signal to be transmitted, and is decomposed into four high - speed signals according to X / Y polarization and I / Q quadrature phase and input into the electro - optic modulator to form a phase - intensity - modulated signal light for transmission.

[0015] In an embodiment of the present invention, coherent heterodyne detection is used to detect optical signals at the receiving end. The detection process includes using a coherent receiver to detect optical signals and using digital signal processing for data recovery.

[0016] In one embodiment of the present invention, a coherent receiver is used for optical signal detection. Among them, the coherent receiver inputs the received signal light at the receiving end and the LO local oscillator light. After processes such as polarization beam splitting and coherent detection of the signal light and the local oscillator light, the optical signal is converted into four high-speed electrical signals with X / Y polarization and I / Q quadrature phases.

[0017] In one embodiment of the present invention, the four high-speed electrical signals generated by the coherent receiver are input into the digital signal processing system at the receiving end. In the digital signal processing flow, analog-to-digital conversion, chromatic dispersion compensation, polarization recovery, and phase and frequency offset recovery processes are performed, and finally the service signal at the receiving end is restored through modulation decoding.

[0018] In one embodiment of the present invention, during the process of frequency offset recovery, pre-compensation is performed according to the frequency interval ∆ω, and then two rounds of residual frequency offset compensation are performed for the residual frequency offset that has not been fully compensated.

[0019] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the following beneficial effects are obtained:

[0020] (1) The present invention can pre-compensate most of the frequency offsets before digital frequency offset estimation through the preset frequency deviation of the dual lasers. The residual frequency offset after frequency offset pre-compensation is consistent with the situation of heterodyne detection. On this basis, the residual frequency offset can be further reduced through fine-tuning of the laser frequency. Finally, the compensation difficulty of the residual frequency offset can be lower than that of the heterodyne detection of a single light source;

[0021] (2) After solving the frequency difference compensation problem of heterodyne detection, compared with the optical module using a single-band laser as the light source, the present invention can double the working bandwidth of the optical module. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic structural diagram of an optical module supporting wide-band operation in an embodiment of the present invention;

[0023] Figure 2 It is a schematic diagram of the laser wavelength frequency setting and frequency offset compensation process in an embodiment of the present invention;

[0024] Figure 3 It is a schematic structural diagram of a coherent receiver in an embodiment of the present invention;

[0025] Figure 4 It is a schematic diagram of the laser frequency band and wavelength frequency setting in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0027] The present invention provides an optical module supporting wide-band operation, including a first laser, a second laser, a 2x2 optical switch, an electro-optic modulator, a coherent receiver and a digital signal processor, wherein:

[0028] The first laser and the second laser, two tunable lasers, form a light source. The 2x2 optical switch is used to control the output direction of the light source, and the electro-optic modulator is used to modulate the output signal of the light source;

[0029] Among them, the first laser and the second laser are a low-frequency band tunable laser and a high-frequency band tunable laser respectively. The operating frequency bands of the two lasers are continuous and have the same frequency band width, and the tuning step sizes of the two lasers are the same; the wavelengths emitted by the two lasers are λ 1 and λ 2 , and the frequency interval between λ 1 and λ 2 is fixedly controlled to be ∆ω;

[0030] The two lasers emit two beams of laser light with wavelengths of λ 1 and λ 2 , which are respectively introduced into two optical interfaces, TX and LO, through the 2x2 optical switch; the light input to the TX optical interface is the working wavelength of the current optical module. When it is necessary to switch the working wavelength of the optical module from the low-frequency band to the high-frequency band, or from the high-frequency band to the low-frequency band, it is achieved by switching which of the first laser and the second laser is conducted to the TX optical interface by the optical switch; the light input to the LO optical interface comes from the other laser and serves as the local oscillator light source at the receiving end.

[0031] The coherent receiver is used to receive external optical signals;

[0032] The digital signal processor is used to complete frequency offset compensation and output the fine-tuning value for the laser.

[0033] Furthermore, the electro-optic modulator modulates the service signal onto the light carrier of the light source TX. This electro-optic modulator integrates polarization multiplexing and quadrature phase modulation functions. The high-speed modulation signal is generated by modulating and encoding the service signal to be transmitted by the transmitting end of the digital signal processor, and is decomposed into four high-speed signals according to X / Y polarization and I / Q quadrature phase and input into the electro-optic modulator to form a phase-intensity modulated optical signal for transmission.

[0034] Further, at the receiving end, an optical signal is detected by means of coherent heterodyne detection. The detection process includes using a coherent receiver to detect the optical signal and using digital signal processing for data recovery.

[0035] Use a coherent receiver to detect the optical signal; wherein, the coherent receiver inputs the received signal light and the LO local oscillator light of the receiving end. After processes such as polarization beam splitting and coherent detection of the signal light and the local oscillator light, the optical signal is converted into four high-speed electrical signals with X / Y polarization and I / Q quadrature phase.

[0036] Further, input the four high-speed electrical signals generated by the coherent receiver into the digital signal processing system at the receiving end. In the digital signal processing flow, analog-to-digital conversion, chromatic dispersion compensation, polarization recovery, and phase and frequency offset recovery processes are carried out, and finally the service signal at the receiving end is recovered through modulation decoding.

[0037] During the process of frequency offset recovery, pre-compensation is carried out according to the frequency interval ∆ω, and then two rounds of residual frequency offset compensation are carried out for the residual frequency offset that has not been fully compensated.

[0038] Specifically, the structure of the optical module supporting wide-band operation adopted by the present invention is as Figure 1 shown:

[0039] The first laser (laser 1 in the figure) and the second laser (laser 2 in the figure) are respectively a low-frequency band laser and a high-frequency band laser. The operating frequency band widths of the two lasers are equal and continuous, and at the same time, the tuning step sizes of the two lasers are the same. That is, the operating frequency band widths of both lasers are ∆ω, and they form a continuous wide-spectrum frequency band with a width of 2∆ω. Therefore, a fixed frequency difference of Δω can always be maintained when the wavelengths are configured with the dual lasers.

[0040] The 2x2 optical switch respectively guides the two laser beams with wavelengths of λ 1 and λ 2 emitted by the two lasers into the two optical interfaces of TX and LO. The light input to the TX optical interface is the operating wavelength of the current optical module. When it is necessary to switch the operating wavelength of the optical module from the low-frequency band to the high-frequency band, or from the high-frequency band to the low-frequency band, the output direction of the switching optical switch can be realized. For the convenience of subsequent description, first assume that the TX operating wavelength is set to λ 1 , and the LO local oscillator light wavelength is set to λ 2 .

[0041] The electro-optic modulator supporting wide-spectrum operation is the main optical device at the transmitting end of the optical module. This wide-spectrum modulator integrates polarization multiplexing and quadrature phase modulation functions, and modulates the TX light through the corresponding drive and control circuits to form a signal light for phase-intensity modulation and transmit.

[0042] When the optical module of the present invention is working, like a traditional optical module, two optical modules with the same configured working wavelength can form a pair of transceivers. Therefore, at the receiving end of the optical module, the received RX optical signal wavelength is λ 1 . Due to the configuration of the optical switch, the LO optical signal wavelength at the receiving end is λ 2 .

[0043] The received RX optical signal at the receiving end is first input into the coherent receiver together with the LO optical signal. The coherent receiver performs polarization beam splitting on the RX optical signal and uses the LO optical signal for mixing. The balanced detector generates an optical current containing phase modulation information, and the optical current passes through a transimpedance amplifier (TIA) and the voltage signal is output from the coherent receiver.

[0044] The voltage signal is input into the digital signal processor DSP (Digital Signal Processing) at the receiving end to complete the compensation for signal transmission impairments and the decoding of the modulated signal. When performing frequency offset compensation during the DSP processing, according to the preset frequency difference ∆ω, frequency offset pre-compensation is first performed to complete the compensation for most of the total frequency difference in heterodyne detection under known conditions. The remaining frequency difference is further compensated by fine-tuning the frequency of the laser emitting the LO optical signal and the frequency offset estimation algorithm in the DSP algorithm.

[0045] The following describes the working method of the optical module of the present invention that supports wide-band operation:

[0046] (1) Wavelength setting. When configuring the required working wavelength, first determine whether the target wavelength is in the low-frequency band or the high-frequency band. For the sake of concise subsequent narration, assume that the target wavelength is λ in the low-frequency band 1 , and the corresponding frequency is ω 1 . After clarifying the target wavelength band, set the working frequency of laser 1 to ω 1 . Based on the characteristics of laser 1 and laser 2, the two lasers have continuous and the same frequency band width ∆ω, and the frequency of laser 2 can be configured to ω 2 to ensure that for any ω 1 , the following formula can be satisfied:

[0047]

[0048] Generally, the frequency tuning widths of the C and L dual lasers are kept consistent, and each of the C band and the L band supports N channels to form a C+L wide-spectrum optical module with a total of 2N continuous channels.

[0049] When setting the working wavelengths ω1 and ω2, it can be ensured that the frequency difference between the two frequencies is always

[0050]

[0051] is the channel spacing.

[0052] (2)Wavelength allocation. According to the requirements of the target wavelength, configure the conduction direction of the 2x2 optical switch, and input the light with the working wavelength λ 1 in the TX direction as the signal light, and input the light with another wavelength λ 2 in the LO direction as the local oscillator light.

[0053] (3)Coherent modulation transmission. The processing method of the optical module of the present invention at the transmitting end is the same as that of the traditional optical module. As Figure 1 shown, use an electro-optic phase modulator that supports wide-spectrum operation to perform polarization multiplexing and phase-intensity modulation on the light in the TX direction to form a TX signal light for transmission.

[0054] (4)Coherent heterodyne detection. The optical module of the present invention uses coherent heterodyne detection to detect optical signals at the receiving end. As Figure 2 shown, the transmitting-end optical module uses the same wavelength setting and allocation method as the receiving-end optical module. Therefore, the frequency of the RX signal light received by the receiving-end optical module is ω 1 ′. Due to the individual differences of the lasers at the transmitting and receiving ends, there is a certain frequency deviation between ω 1 and ω 1 ′, usually not exceeding 1.5 GHz. In the coherent receiver, first use a polarization beam splitter to split the two polarizations of the RX and LO lights, and each polarization is independently mixed by 90° and balanced detected. Taking any polarization as an example, after 90° mixing, two optical signals E1, E2 and E3, E4 are generated on the in-phase I and quadrature phase Q respectively, as Figure 3 shown. After the two optical signals on each phase are balanced detected, the signal optical current can be expressed as shown in the following formula:

[0055]

[0056] where R is the responsivity of the photodiode in the balanced detector, 𝑃 𝑅𝑋 and 𝑃 𝐿𝑂 are the optical powers of the RX signal light and the LO local oscillator light respectively, θs is the modulated phase signal, and θn is the phase noise. ∆ω′ is the frequency difference between the RX signal light and the LO local oscillator light, which can be expressed as shown in the following formula:

[0057]

[0058] After the photocurrent signal I is amplified by the transimpedance amplifier TIA, it is converted into a voltage signal and received and processed by the receiving-end DSP.

[0059] (5) Frequency offset pre - compensation. In the digital signal processing flow of heterodyne detection, the compensation of the frequency difference between the signal light and the local oscillator light is an important part. In the present invention, it is to compensate ∆ω′. The processes of frequency offset pre - compensation and residual frequency offset compensation are as Figure 4 shown. Due to the frequency setting and distribution of the dual - lasers, it can be known that for any signal light frequency ω 1 a local oscillator frequency ω with a frequency difference of ∆ω can be correspondingly set 2 . Therefore, before performing the usual frequency offset estimation and frequency offset compensation algorithms, the optical module DSP of the present invention will directly perform a frequency offset pre - compensation of ∆ω. This process is relatively fixed, without the process of searching, estimating, and convergence, and most of the frequency offsets can be compensated under the condition of extremely low algorithm complexity.

[0060] (6) Residual frequency offset compensation. After the frequency offset pre - compensation, the main part ∆ω of the frequency difference ∆ω′ has been compensated. The residual frequency offset mainly comes from the frequency difference caused by the individual differences between the lasers at the transmitting and receiving ends. Since the light emitted by laser 2 with a frequency of ω 2 is only used as the local oscillator light at the receiving end, it can be finely adjusted in real - time without affecting the functions of other parts of the optical module. Therefore, after the DSP estimates the residual frequency offset, first, the output frequency of laser 2 is finely adjusted. Relying on the frequency fine - tuning accuracy of laser 2, the amount of the residual frequency offset is further reduced. After the frequency fine - tuning of the laser is completed, finally, the remaining frequency offset and phase noise are compensated by the digital algorithm. Combining with other functions of the coherent DSP, the phase information modulated in the RX signal light can be obtained, and the signal reception and recovery are completed.

[0061] (7) Frequency switching. The above 6 implementation steps are all based on ω with a working frequency in the low - frequency band 1 . On this basis, when the working frequency is switched to the high - frequency band, the output frequency of laser 2 is configured as the target working frequency. While maintaining ∆ω, the frequency of laser 1 is synchronously adjusted. At the same time, the working state of the optical switch is switched, and the new λ 2 light is input to the TX side, and the new λ 1 light is input to the LO side. The above three processes are carried out synchronously, and the total time length is determined by the speed of the laser thermal switching, usually on the order of seconds.

[0062] Based on the above 7 implementation steps, the present invention can realize a continuously tunable wide - spectrum optical module.

[0063] It is easy for those skilled in the art to understand that the above - mentioned is only the preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An optical module supporting wide - band operation, characterized in that, it includes a first laser, a second laser, a 2x2 optical switch, an electro - optic modulator, a coherent receiver, and a digital signal processor, where: The first laser and the second laser form a light source. The 2x2 optical switch is used to control the output direction of the light source. The emitted lights of the first laser and the second laser are respectively input into the electro-optic modulator as the transmitted light source or input into the coherent receiver as the local oscillator light source through the 2x2 optical switch; the first laser and the second laser are respectively a low-frequency band tunable laser and a high-frequency band tunable laser. The operating frequency bands of the two lasers are continuous and have the same frequency band width, and the tuning step sizes of the two lasers are consistent; wherein, the wavelengths emitted by the two lasers are λ 1 and λ 2 , λ 1 and λ 2 The frequency interval between them is fixedly controlled to be ∆ω; The electro - optic modulator is used to modulate the output signal of the light source; At the receiving end, the optical signal is detected by means of coherent heterodyne detection; the coherent receiver is used to receive the external optical signal; the digital signal processor is used to complete frequency offset compensation and output the fine - tuning value for the laser.

2. The optical module supporting wide - band operation according to claim 1, characterized in that, Two lasers emit laser beams with wavelengths of λ 1 and λ 2 respectively pass through a 2x2 optical switch and are introduced into two optical interfaces, TX and LO; wherein, the light input through the TX optical interface is the working wavelength of the current optical module. When it is necessary to switch the working wavelength of the optical module from the low - frequency band to the high - frequency band, or from the high - frequency band to the low - frequency band, it is achieved by switching whether the first laser or the second laser is conducted with the TX optical interface of the optical switch; the light input through the LO optical interface comes from another laser and serves as the local oscillator light source at the receiving end.

3. The optical module supporting wide - band operation according to claim 1, characterized in that, The electro - optic modulator modulates the service signal onto the light source TX optical carrier. This electro - optic modulator integrates polarization multiplexing and quadrature phase modulation functions. The high - speed modulation signal is generated by modulating and encoding the service signal to be transmitted by the transmitting end of the digital signal processor, and is decomposed into four - way high - speed signals according to X / Y polarization and I / Q quadrature phase and input into the electro - optic modulator to form a phase - intensity - modulated signal light for transmission.

4. The optical module supporting wide - band operation according to claim 1, characterized in that, The detection process includes using a coherent receiver to detect the optical signal and using digital signal processing for data recovery.

5. The optical module supporting wide - band operation according to claim 4, characterized in that, Use a coherent receiver to detect the optical signal; wherein, the coherent receiver inputs the received - end signal light and the LO local oscillator light. After polarization beam splitting and the coherent detection process of the signal light and the local oscillator light, the optical signal is converted into four - way high - speed electrical signals of X / Y polarization and I / Q quadrature phase.

6. The optical module supporting wide - band operation according to claim 4, characterized in that, Perform receiving - end digital signal processing on the four - way high - speed electrical signals generated by the coherent receiver. In the digital signal processing flow, perform analog - to - digital conversion, chromatic dispersion compensation, polarization recovery, and phase and frequency offset recovery processes, and finally recover the service signal at the receiving end through modulation decoding.

7. The optical module supporting wide - band operation according to claim 6, characterized in that, During the process of frequency offset recovery, pre - compensation is performed according to the frequency interval ∆ω, and then two rounds of residual frequency offset compensation are performed for the residual frequency offset that has not been fully compensated.

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