Methods of Bi-Directional Optical Communication and Optical Transceiver

Abstract

A method (10) of bi-directional optical communication, the method comprising: generating (12) a first optical communication signal for transmission in one direction through an optical fibre, generating the first optical communication signal comprising: receiving information for transmission and generating (14) a baseband signal comprising a representation of the information; performing digital upconversion (16) of the baseband signal to form an upconverted baseband signal; performing optical modulation (18) of an optical carrier signal with the upconverted baseband signal; and restricting an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot by performing one of digital filtering (16) in addition to digital upconversion and optical filtering (36) after optical modulation; and receiving (20) a second optical communication signal transmitted in an opposite direction through the optical fibre, the second optical communication signal having an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

Claims

1-16. (canceled)

17. A method of bi-directional optical communication, the method comprising: generating a first optical communication signal for transmission in one direction through an optical fiber, wherein generating the first optical communication signal comprises: receiving information for transmission and generating a baseband signal comprising a representation of the information; performing digital upconversion of the baseband signal to form an upconverted baseband signal; performing optical modulation of an optical carrier signal with the upconverted baseband signal; and restricting an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot by performing one of digital filtering in addition to digital upconversion and optical filtering after optical modulation; and receiving a second optical communication signal transmitted in an opposite direction through the optical fiber, the second optical communication signal having an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

18. The method of claim 17, wherein the baseband signal is a real baseband signal and digital filtering is performed in addition to digital upconversion to restrict the optical spectrum of the first optical communication signal to the first portion of the optical channel frequency slot.

19. The method of claim 18, wherein the digital filtering comprises upper-single sideband modulation.

20. The method of claim 17, wherein the baseband signal is a complex baseband signal and optical filtering is performed after the optical modulation to restrict the optical spectrum of the first optical communication signal to the first portion of the optical channel frequency slot.

21. The method of claim 20, wherein the second optical communication signal is generated using the same method as for generating the first optical communication signal and wherein one of optical high-pass filtering and optical low-pass filtering is performed after the optical modulation for generating the first optical communication signal and the other of optical high-pass filtering and optical low-pass filtering is performed after the optical modulation for generating the second optical communication signal.

22. The method of claim 17, wherein the optical modulation is performed by an optical modulator having an operating frequency and the baseband signal has a bandwidth that is substantially half the operating frequency of the optical modulator.

23. The method of claim 22, wherein the baseband signal has a higher-order modulation format.

24. The method of claim 17, wherein the receiving comprises receiving an optical signal comprising the second optical communication signal and a reflected portion of the first optical communication signal; performing coherent optical demodulation of the optical signal to obtain a digital signal; performing digital filtering on the digital signal to remove a part of the digital signal representing the reflected portion of the first optical communication signal; and downconverting the filtered digital signal to obtain a baseband signal.

25. An optical transceiver comprising: a digital signal processor configured to: receive information for transmission and generate a baseband signal comprising a representation of the information, and perform digital upconversion of the baseband signal to form an upconverted baseband signal; a digital to analogue converter configured to convert the upconverted baseband signal into a modulation signal; optical signal generation apparatus configured to receive the modulation signal and to modulate an optical carrier signal with the modulation signal to generate a first optical communication signal for transmission, wherein the digital signal processor is additionally configured to perform digital filtering in addition to digital upconversion to restrict an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot or the optical signal generation apparatus additionally comprises an optical filter to restrict an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot; and coherent optical demodulation apparatus configured to receive a second optical communication signal and configured to perform coherent detection of the second optical communication signal, wherein the second optical communication signal has an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

26. The optical transceiver of claim 25, wherein the baseband signal is a real baseband signal and the digital signal processor is configured to perform digital filtering in addition to digital upconversion to restrict the optical spectrum of the first optical communication signal to the first portion of the optical channel frequency slot.

27. The optical transceiver of claim 26, wherein digital filtering comprises upper-single sideband modulation.

28. The optical transceiver of claim 25, wherein the coherent optical demodulation apparatus is configured to receive an optical signal comprising the second optical communication signal and a reflected portion of the first optical communication signal and the coherent optical demodulation apparatus is configured to perform coherent detection of the received optical signal to obtain a digital signal, and wherein the digital signal processor is additionally configured to: perform digital filtering on the digital signal to remove a part of the digital signal representing the reflected portion of the first optical communication signal; and downconvert the filtered digital signal to obtain a real baseband signal.

29. The optical transceiver of claim 25, wherein the baseband signal is a complex baseband signal and the optical transceiver further comprises an optical filter configured to filter the optical carrier signal after modulation with the modulation signal.

30. The optical transceiver of claim 25, wherein the optical signal generation apparatus comprises an optical modulator configured to modulate the optical carrier signal with the modulation signal and configured to operate at a modulation frequency and the baseband signal has a bandwidth that is substantially half the modulation frequency.

31. The optical transceiver of claim 30, wherein the baseband signal has a higher-order modulation format.

32. The optical transceiver of claim 25, wherein the optical transceiver comprises a laser configured to generate an optical signal used to provide both the optical carrier signal and a local oscillator signal of the coherent optical demodulation apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIGS. 1 to 3 show the steps of methods according to embodiments of the invention of bi-directional optical communication;

[0038] FIGS. 4 to 6 illustrate generation of optical communication signals for bi-directional optical communication, according to an embodiment of the invention;

[0039] FIG. 7 shows constellation diagrams for 4-QAM modulation format (grey circles) and for an 8-ASK modulation format for use in the method illustrated by FIGS. 4 to 6;

[0040] FIG. 8 shows the steps of a method according to an embodiment of the invention of bi-directional optical communication;

[0041] FIGS. 9 to 11 illustrate generation of optical communication signals for bi-directional optical communication, according to an embodiment of the invention;

[0042] FIG. 12 shows constellation diagrams for a 4-QAM modulation format (grey circles) and for a 16-QAM modulation format for use in the method illustrated by FIGS. 9 to 11;

[0043] FIG. 13 shows steps of receiving the second optical communication signal;

[0044] FIG. 14 illustrates A) low-pass digital filtering and B) high-pass digital filtering of a digital signal received in a method according to an embodiment of the invention of bi-directional optical communication; and

[0045] FIGS. 15 to 18 are schematic representations of optical transceivers according to embodiments of the invention.

DETAILED DESCRIPTION

[0046] The same reference numbers will used for corresponding features in different embodiments.

[0047] An embodiment of the invention provides a method 10 of bi-directional optical communication, as illustrated in FIG. 1.

[0048] The method comprises generating 12 a first optical communication signal for transmission in one direction through an optical fibre and receiving 20 a second optical communication signal transmitted in an opposite direction through the optical fibre.

[0049] Generating the first optical communication signal comprises receiving information for transmission and generating 14 a respective baseband signal comprising a representation of the information. Digital upconversion 16 of the baseband signal is then performed to form a respective upconverted baseband signal and digital filtering 16 is performed to restrict an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot. Optical modulation 18 of a respective optical carrier signal with the upconverted and digitally filtered baseband signal is performed.

[0050] The second optical communication signal has an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

[0051] An optical channel frequency slot is the frequency range allocated to a slot, and unavailable to other slots, in a WDM frequency grid, i.e. the grid of frequencies allocated to the various optical channels in a WDM system. A frequency slot is defined by its nominal central frequency and its width; see for example ITU-T Recommendation G.694.1 Spectral grids for WDM applications: DWDM frequency grid.

[0052] A baseband signal will be understood to mean the original signal, carrying the information for transmission, including any overhead, generated by an information source. For example, the baseband signal may comprise a client signal wrapped into an optical transport unit, OTU, frame, such as OTU4, and then forward error correction, FEC, coded, with training sequences.

[0053] It will be understood that the baseband signal is in the electrical domain and will be upconverted into an optical signal as a result of optical modulation.

[0054] Another embodiment, illustrated in FIG. 2, provides a method 30 of bi-directional optical communication which is similar to the method 10 of FIG. 1. In this embodiment, digital upconversion 34 of the baseband signal is performed to form a respective upconverted baseband signal, without any digital filtering. The optical spectrum of the first optical communication signal is instead restricted to a first portion of an optical channel frequency slot by performing optical filtering 36 after optical modulation.

[0055] FIG. 3 illustrates a method 30 of bi-directional optical communication according to an embodiment of the invention, which is similar to the method 10 of FIG. 1. In this embodiment, the baseband signal is a real baseband signal, that is to say a baseband signal having only real components.

[0056] A method of bi-directional optical communication according to a further embodiment of the invention, substantially the same as the method 30 of FIG. 3, is illustrated in FIGS. 4 to 7. In this embodiment, the digital filtering comprises upper-single sideband, SSB, modulation.

[0057] In this example, the method is applied to implement bi-directional optical communication in an optical channel frequency slot of a single-fibre optical link; the frequency slot is a standard 50 GHz frequency slot. The symbol rate for such an arrangement is typically 28-32 GBaud and enables single-wavelength transmission of 100 DP-QPSK. The method 30 uses real baseband signals only and requires only extra digital signal processing, DSP, to implement the digital filtering.

[0058] FIG. 4A) and the constellation diagram 60 in FIG. 7 illustrate a QPSK (4-QAM) baseband signal, as would normally be used for single-wavelength transmission at a 28-34.5 GBaud symbol rate. The arrow-head symbol 72 indicates the sense of transmission of the optical signal in the fibre.

[0059] FIG. 4B) and the constellation diagram 70 in FIG. 7 illustrate an 8-ASK real baseband signal generated using the method 30. It can be seen that using a higher-order modulation format results in the required bandwidth being halved. Higher-order modulation will be understood to mean a digital modulation format with an order of 4 or higher, such that it encodes at least 2 bits per symbol.

[0060] Using 8-ASK modulation format instead of QPSK modulation format halves the bandwidth of the baseband signal but results in the bit rate being reduced to 75% of the bit rate available using QPSK. Alternatively, the same order modulation format may be used for the baseband signal, i.e. 4-ASK, which would reduce the bit rate by 50%.

[0061] In order to maintain the bit rate as compared to QPSK, 16-ASK modulation format must be used. In general terms, using the method of the present invention, changing from using M-QAM (M-PSK) modulation format, as in the known dual-fibre bi-directional transmission systems, to using N-ASK will halve the required bandwidth of the baseband signal and: if N=4M the bit rate will remain the same; if N=2M the bit rate will be reduced by 25%; and if N=M the bit rate will be reduced by 50%.

[0062] The optical modulation is performed by an optical modulator having an operating frequency and the bandwidth of the baseband signal is substantially half the operating frequency of the optical modulator.

[0063] FIG. 4C) illustrates the upconverted baseband signal, following digital upconversion of the real baseband signal. The process of upconversion takes the baseband signal and shifts is frequency spectrum up to a carrier frequency, f.sub.IF. In a baseband signal, the negative frequency components overlie the positive frequency components. When the baseband signal is upconverted, the positive and negative components are unfolded, with the negative components below the carrier frequency and the positive components above the carrier frequency.

[0064] FIG. 4D) illustrates the upconverted signal after digital filtering, i.e. after upper-SSB modulation. The resulting signal occupies the higher-frequency portion of the electrical bandwidth. The upper-SSB upconversion is performed in the digital domain by DSP.

[0065] As indicated by the parentheses in FIG. 4, the digital upconversion and digital upper-SSB modulation are performed together.

[0066] FIG. 4E) shows the resulting first optical communication signal 74, generated by upconversion at the optical frequency f.sub.C of the RF-SSB signal generated at the previous step i.e. after optical modulation. The optical frequency range in between is available for transmission in the opposite sense.

[0067] FIG. 5 illustrates generation of the second optical communication signal 78, for transmission in the opposite sense 76 (indicated by the arrow tails). The second optical communication signal is simply generated from an 8-ASK real baseband signal 70, as illustrated in FIG. 5B), which is used to perform optical modulation of a further optical carrier signal, having the same optical frequency f.sub.C as for the first optical communication signal, to form the second optical communication signal shown in FIG. 5E). The same letters are used to label FIG. 5 as are used in FIG. 4 to indicate equivalent signals.

[0068] As can be seen from FIGS. 4 and 5, both the first and second baseband signals are contained within the bandwidth of the currently used, QPSK baseband signal.

[0069] FIG. 6 illustrates both the first optical communication signal 74 and the second optical communication signal 78 within the optical channel frequency slot. As can be seen, the first optical communication signal occupies a first portion of the optical channel frequency slot and the second optical communication signal occupies a second portion of the optical channel frequency slot. The optical spectra of the first and second optical communications signals therefore do not overlap, therefore avoiding in-band crosstalk between them.

[0070] FIG. 8 illustrates a method 90, 100 of bi-directional optical communication. The method comprises generating 90 a first optical communication signal for transmission in one direction through an optical fibre and receiving a second optical communication signal transmitted in an opposite direction through the optical fibre.

[0071] Generating 90 the first optical communication signal comprises receiving information for transmission and generating 92 a respective complex baseband signal comprising a representation of the information. Digital upconversion 94 of the complex baseband signal is then performed to form a respective upconverted baseband signal. Optical modulation 18 of a respective optical carrier signal with the upconverted and digitally filtered baseband signal is performed. Optical high-pass filtering 96 is then performed on the modulated optical carrier signal to restrict the optical spectrum of the first optical communication signal to the first portion of the optical channel frequency slot.

[0072] The second optical communication signal has an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

[0073] The second optical communication signal is generated 102 by receiving information for transmission and generating 102 a respective complex baseband signal comprising a representation of the information. Digital upconversion 94 of the complex baseband signal is then performed to form a respective upconverted baseband signal. Optical modulation 18 of a respective optical carrier signal with the upconverted and digitally filtered baseband signal is performed. Optical low-pass filtering 104 is then performed on the modulated optical carrier signal to restrict the optical spectrum of the second optical communication signal to the second portion of the optical channel frequency slot.

[0074] FIG. 9A) and the constellation diagram 110 in FIG. 12 illustrate a QPSK (4-QAM) baseband signal, as would normally be used for single-wavelength transmission at a 28-34.5 GBaud symbol rate. The arrow-head symbol 122 indicates the sense of transmission of the optical signal in the fibre.

[0075] FIG. 9B) and the constellation diagram 120 in FIG. 12 illustrate a 16-QAM real baseband signal generated using the method 90. It can again be seen that using a higher-order modulation format results in the required bandwidth being halved.

[0076] In general terms, using the method of the present invention, changing from using M-QAM modulation format, as in the known dual-fibre bi-directional transmission systems, to using N-QAM will halve the required bandwidth of the baseband signal and: if N=4M the bit rate will remain the same; if N=2M the bit rate will be reduced by 25%; and if N=M the bit rate will be reduced by 50%.

[0077] The optical modulation is performed by an optical modulator having an operating frequency and the bandwidth of the baseband signal is substantially half the operating frequency of the optical modulator.

[0078] FIG. 9C) illustrates the upconverted baseband signal, following digital upconversion of the complex baseband signal. The process of upconversion takes the baseband signal and shifts is frequency spectrum up to a carrier frequency, f.sub.IF. In a baseband signal, the negative frequency components overlie the positive frequency components. When the baseband signal is upconverted, the positive and negative components are unfolded, with the negative components below the carrier frequency and the positive components above the carrier frequency.

[0079] FIG. 9D) illustrates an optical communication signal 123 generated by performing optical modulation of an optical carrier signal with the upconverted baseband signal.

[0080] FIG. 9E) shows the resulting first optical communication signal 124, produced by optical high-pass filtering the optical communication signal 123.

[0081] FIG. 10 similarly illustrates generation of the second optical communication signal 128, for transmission in the opposite sense 126 (indicated by the arrow tails). The second optical communication signal is generated using the same method as for generating the first optical communication signal but uses optical low-pass filtering, as illustrated in FIG. 10D).

[0082] As can be seen from FIGS. 9 and 10, both the first and second baseband signals are contained within the bandwidth of the currently used, QPSK baseband signal.

[0083] FIG. 11 illustrates both the first optical communication signal 124 and the second optical communication signal 128 within the optical channel frequency slot. As can be seen, the first optical communication signal occupies a first portion of the optical channel frequency slot and the second optical communication signal occupies a second portion of the optical channel frequency slot. The optical spectra of the first and second optical communications signals therefore do not overlap, therefore avoiding in-band crosstalk between them.

[0084] FIG. 13 illustrates receiving 20 a second optical communication signal as part of a method according to a further embodiment of bi-directional optical communication. The first optical signal is generated according to the method described in any of the previous embodiments relating to a real baseband signal.

[0085] In this embodiment, receiving 20 the second optical signal comprises receiving 22 an optical signal comprising the second optical communication signal and a reflected portion of the first optical communication signal. Coherent optical demodulation 24 of the optical signal is then performed to obtain a digital signal. The digital signal is digitally filtered 26 to remove a part of the digital signal representing the reflected portion of the first optical communication signal. The filtered digital signal is then downconverted 28 to obtain a baseband signal.

[0086] As illustrated in FIG. 14 A), if the second optical communication signal 210 occupies the inner part of the frequency range, the digital signal is filtered with a digital lowpass filter 214 to reject any reflected power of the first optical communication signal 212.

[0087] As illustrated in FIG. 14 B), if the second optical communication signal 220 occupies the outer part of the frequency range, the digital signal is filtered with a digital highpass filter 224 to reject any reflected power of the first optical communication signal 222.

[0088] FIGS. 14 A) and B) both show a slight offset between the frequency of the optical carrier signal, f.sub.c, and the frequency of the local oscillator, f.sub.LO, used in the coherent optical demodulation 24. This difference may be corrected for digitally by the DSP.

[0089] An embodiment of the invention provides an optical transceiver 300 as illustrated in FIG. 15. The optical transceiver 300 comprises a digital signal processor, DSP, 302, a digital to analogue converter, DAC, 306, optical signal generation apparatus 308 and coherent optical demodulation apparatus 322.

[0090] The DSP 302 is configured to receive information 304 for transmission and to generate a real baseband signal comprising a representation of the information. The DSP is also configured to perform digital upconversion of the real baseband signal to form an upconverted baseband signal. The DAC 306 is configured to convert the upconverted baseband signal into a modulation signal. The optical signal generation apparatus 308 is configured to receive the modulation signal and to modulate an optical carrier signal with the modulation signal to generate a first optical communication signal 310 for transmission. The DSP is additionally configured to perform digital filtering in addition to digital upconversion to restrict an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot. The coherent optical demodulation apparatus 322 is configured to receive a second optical communication signal 324 and is configured to perform coherent detection of the second optical communication signal. The second optical communication signal has an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

[0091] In an embodiment, the real baseband signal has a higher-order modulation format.

[0092] In an embodiment, the digital filtering comprises upper-SSB.

[0093] In an embodiment, the coherent optical demodulation apparatus 322 is configured to receive an optical signal comprising the second optical communication signal 324 and a reflected portion of the first optical communication signal 310. The coherent optical demodulation apparatus is configured to perform coherent detection of the received optical signal to obtain a digital signal 326.

[0094] The DSP 302 in this embodiment is additionally configured to perform digital filtering on the digital signal to remove a part of the digital signal representing the reflected portion of the first optical communication signal. The DSP is also configured to downconvert the filtered digital signal to obtain a real baseband signal 328.

[0095] The optical transceiver 300 may be configured to implement the method 30 of bi-directional communication described above with reference to FIGS. 3 to 7.

[0096] An embodiment of the invention provides an optical transceiver 330 as illustrated in FIG. 16. The optical transceiver 330 comprises a digital signal processor, DSP, 302, a digital to analogue converter, DAC, 306, optical signal generation apparatus 308, an optical filter 332 and coherent optical demodulation apparatus 322.

[0097] The DSP 302 is configured to receive information 304 for transmission and to generate a complex baseband signal comprising a representation of the information. The DSP is also configured to perform digital upconversion of the complex baseband signal to form an upconverted baseband signal. The DAC 306 is configured to convert the upconverted baseband signal into a modulation signal. The optical signal generation apparatus 308 is configured to receive the modulation signal and to modulate an optical carrier signal with the modulation signal to generate a first optical communication signal 310 for transmission.

[0098] The optical filter 332 is configured to restrict an optical spectrum of the first optical communication signal to a first portion of an optical channel frequency slot.

[0099] The coherent optical demodulation apparatus 322 is configured to receive a second optical communication signal 324 and is configured to perform coherent detection of the second optical communication signal. The second optical communication signal has an optical spectrum occupying a second portion of the optical channel frequency slot, separate to the first portion.

[0100] In an embodiment, the complex baseband signal has a higher-order modulation format.

[0101] In an embodiment, the coherent optical demodulation apparatus is configured to perform detection of the second optical communication signal according to the method of detection of DP-16-QAM signals employing RF upconversion, optical modulation and SSB via optical filtering reported in G. Bruno et al, Performance of 112 Gbit/s RF-assisted multi-carrier DP-16-QAM in a transparent optical domain, European Conference on Optical Communication 2011, Technical Digest, Th.11.A.2.pdf.

[0102] The optical transceiver 330 may be configured to implement the method 90, 100 of bi-directional communication described above with reference to FIGS. 8 to 11.

[0103] A further embodiment of the invention provides an optical transceiver 340 as illustrated in FIG. 17. The optical transceiver 340 is similar to the optical transceiver 300 of FIG. 15, with the following modifications.

[0104] In this embodiment, the optical signal generation apparatus comprises an optical modulator 344 configured to modulate the optical carrier signal with the modulation signal. The optical modulator is configured to operate at a modulation frequency and the baseband signal has a bandwidth that is substantially half the modulation frequency. The baseband signal has a higher-order modulation format.

[0105] The optical transceiver 340 additionally comprises a laser 342 configured to generate an optical signal used to provide both the optical carrier signal and a local oscillator signal for the coherent optical demodulation apparatus 322.

[0106] FIG. 18 A) illustrates the transceiver 340 of FIG. 17 implemented as a pluggable module 350 having an external circulator 354 to couple the optical filter 310 and the coherent optical demodulation apparatus 322 to a single optical fibre.

[0107] FIG. 18 B) illustrates the transceiver 340 of FIG. 17 implemented as a pluggable module 360 having an internal circulator 354 to couple the optical filter 310 and the coherent optical demodulation apparatus 322 to a single optical fibre.