Method and apparatus for dispersion mitigation in optical links

An optical communications system includes an optical transmitter that generates a modulated optical signal at an output. The modulated optical signal propagates through an optical link where the dispersion of the optical link is imprinted onto an optical spectrum of the modulated optical signal. A demodulator receives the modulated optical signal and filters at least a portion of the optical spectrum with the imprinted dispersion of the optical link, thereby mitigating effects of dispersion in the modulated optical signal and generating a demodulated optical signal at an output. An optical detector generates an electrical data signal from the demodulated optical signal.

The section headings used herein are for organizational purposes only and should not to be construed as limiting the subject matter described in the present application.

BACKGROUND OF THE INVENTION

The increasing need for high bit-rate data transmissions through optical fibers presents significant challenges to the fiber-optic telecommunications industry. For example, high bit-rate systems are highly susceptible to optical fiber nonlinearities. One option to confront these challenges is to transmit the data in the Differential-Phase Shift Keying (DPSK) modulation format. The DPSK modulation format is a well known digital modulation format that conveys data by modulating the phase of the carrier wave. The DPSK modulation format is compatible with both RZ and NRZ data formats at high data rates.

The DPSK modulation format has numerous advantages over the more standard On-Of-Keying (OOK) modulation format. One advantage of using the DPSK modulation format is that, when DPSK signals are detected using a balanced receiver, the OSNR sensitivity is improved by up to 3 dB. See, for example, A. H. Gnauck and P. J. Winzer, “Optical Phase-Shift-Keyed Transmission,” IEEE Journal of Lightwave Technology, vol. 23, pp. 115-30, 2005. Another advantage of using the DPSK modulation format over the OOK modulation format is that the DPSK modulation format is more tolerant to fiber optic non-linearities, such as self-phase modulation.

However, the DPSK modulation format is more complicated than the OOK modulation format. In DPSK systems, the digital information is written in the optical phase of the signal and, therefore, the digital information cannot be detected by ordinary intensity detectors. Consequently, DPSK receivers typically include optical demodulators, which convert the phase modulated signal to an amplitude modulated signal. The resulting amplitude modulated signal can then be detected by ordinary optical power detectors.

For most optical networks, chromatic dispersion is a major limitation to the distance of the fiber optical links and determines the complexity of network. Chromatic dispersion is a well known effect in all fiber optic systems that causes inter-symbol interference. In most cases, dispersion effects are a direct function of the transmitted signal bandwidth. The higher the bandwidth, the higher the link penalty due to chromatic dispersion. DPSK modulation, in particular, has a larger bandwidth compared to most conventional data formats and, therefore, has a stringent tolerance to fiber chromatic dispersion.

DETAILED DESCRIPTION

The present teachings will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein. For example, although aspects of the present invention are described in connection with DPSK modulation, one skilled in the art will appreciate that the methods and apparatus of the present invention can be used with any type of modulated optical signal having second-order and higher-order dispersion characteristics, such as DPSK, RZ, NRZ, and DuoBinary modulated optical signals.

There is currently strong interest in DPSK optical communication systems because these systems can transmit and receive data at data rates that are higher than 40 Gbps and these high date rates can be achieved in existing optical network infrastructures that have been optimized for lower data rates. In particular, there is currently a desire to transmit 43 Gbps data rate signals in spectrally narrow channels through commercial 10 Gbps rate data channels. Conventional OOK data modulation formats can not be used to transmit 43 Gbps data rate signals through commercial 10 Gbps rate data channels because the quality of these high data rate signals will be severely deteriorated in the narrow spectrum filter used in add/drop filters, multiplexers, and demultiplexers along the optical link.

One disadvantage of DPSK optical communications systems is that they are particularly sensitivity to the chromatic dispersion in the underlying optical link. Chromatic dispersion occurs because the phase velocity of optical signal propagating in an optical fiber is frequency dependent. The different wavelengths of the optical signal, therefore, travel at different speeds along the optical link. Chromatic dispersion causes optical pulses to spread while they propagate in an optical fiber. The spreading or dispersion in the optical pulses degrades the optical signal and, therefore, reduces the optical signal-to-noise ratio over long distances.

Various embodiments of the methods and apparatus of the present invention use optical filtering and/or optical demodulation to shape the optical bandwidth of modulated optical signal to reduce or eliminate the portion of the optical spectrum of the modulated optical signal that includes the dispersion imprint of the optical link. Using such optical filtering and optical demodulation can significantly reduce dispersion effects in the received modulated optical signal. In particular, optical filtering according to the present invention can be used in the transmitter, receiver, and/or the optical link to shape the optical bandwidth of the modulated optical signal to reduce or eliminate the portion of the optical spectrum of the modulated optical signal that includes the dispersion imprint of the optical link.

FIG. 1illustrates a block diagram of a first embodiment of an optical communications system100which mitigates the effects of dispersion by reducing or eliminating the portion of the optical spectrum that includes the dispersion imprint of the optical link by narrow optical filtering according to the present invention. The communications system100includes a transmitter102that includes a data source104that generates data at an output. The data source104can be a pulse pattern generator which generates predetermined data patterns. However, in a commercial communications system, the data source104is payload data and voice signals for transmission through the communications system100.

An input of a differential pre-coder106is electrically connected to the output of the data source104. The differential pre-coder106encodes the data and generates a pre-coded data modulation signal at an output. For example, the pre-coder106can include an XOR pre-coder with a 1-bit delay. Such an optical transmission system does not require a decoder on the receiver side to recover the original data that was transmitted.

An input of an RF amplifier108is electrically connected to the output of the differential pre-coder106. The RF amplifier108generates an amplified pre-coded data modulation signal at an output. The RF amplifier108amplifies the pre-coded data modulation signal to signal levels that are desirable for modulation with electro-optic modulators. For example, the RF amplifier108can amplify the pre-coded data modulation signal to an amplitude that is equal to twice the Vpi of an electro-optic modulator. The Vpi of an electro-optic modulator is the switching voltage or the voltage that is required to switch the electro-optic modulator from a minimum intensity to a maximum intensity.

A modulation input of an electro-optic modulator110is electrically connected to the output of the RF amplifier108. In many embodiments, the electro-optic modulator110is a Mach-Zehnder type interferometric (MZI) modulator, such as a Lithium Niobate (LiNbO3) MZI modulator. Such modulators are well known in the industry and have well defined characteristics.

An optical input of the electro-optic modulator110is electrically connected to an output of an optical source112. The optical source112can be a continuous wave (CW) laser that generates a CW optical beam. A bias voltage power supply114is electrically connected to a bias input of the electro-optic modulator110. The bias voltage power supply114biases the electro-optic modulator110at the desired operating point of the electro-optic modulator110. The electro-optic modulator110then generates a modulated optical signal that is modulated by the data modulation signal at an output.

In some embodiments of the present invention the transmitter100also includes a narrow band optical filter116. The narrow band optical filter116can be optically coupled to the output of the electro-optic modulator110. The characteristics of the narrow band optical filter116are chosen to reduce the spectral broadening caused by second-order and higher-order non-linearities in the modulated optical signal.

An optical fiber link118is coupled to the output of the optical filter116or to the output of the electro-optic modulator110if a narrow band optical filter is not used in the transmitter100. The optical fiber link118can be any type of optical fiber transmission link, such as a terrestrial or undersea optical fiber link. The optical fiber link118can also be a fiber spool, which is commonly used for testing.

The optical communications system100also includes a receiver150that is used to demodulate the received phase-encoded optical data and to detect the demodulated data. The receiver150includes a narrow-band optical filter demodulator152that demodulates the encoded optical signal to recover the data. It is well-known that a single optical filter can demodulate phase-encoded data. See, for example, F. Jacobsson, “DPSK Modulation Format for Optical Communication Using FBG Demodulator”, Msc. Thesis, Depart. Science and Technology Linköpings University. Also, see I. Lyubomirsky and B. Pitchumani, “Impact of Optical Filtering on Duobinary Transmission”, IEEE Photon. Technol. Lett. 16, 1969 (2004). Using a single optical filter is a relatively simple and inexpensive way to demodulate signals.

The narrow band optical filter demodulator152converts the phase information encoded on the modulated optical signal into amplitude changes. The characteristics of the narrow band filter demodulator152are chosen to shape the optical bandwidth of the received optical signal to reduce or eliminate the portion of the optical spectrum of the received optical signal that includes the dispersion imprint of the optical link118. The receiver150also includes an optical detector154that is optically coupled to the output of the narrow band optical filter demodulator152. The optical detector154converts the demodulated optical signal to an electrical demodulation signal.

In operation, the differential pre-coder106pre-codes the data from the original data source104. The RF amplifier108amplifies the pre-coded data signal to an amplitude that is equal to the twice the Vpi of the electro-optic modulator110(or some other suitable amplitude). The bias voltage power supply114biases the electro-optic modulator110at the desired operating point. The CW laser112transmits an optical signal through the electro-optic modulator110. The CW optical signal is then modulated by the electro-optic modulator110so that the modulated optical signal is phase pre-coded such that for every minimum-to-maximum transition in the RF modulation signal, a phase inversion occurs in the optical phase of the modulated optical signal.

In some embodiments, the modulated optical signal is then filtered by the narrow band optical filter116to reduce spectral broadening caused by second-order and higher-order non-linearities in the modulated optical signal that will cause pulse spreading due to dispersion during transmission. In these embodiments, the transmitted phase-encoded data from the electro-optic modulator110is the same as the original data, so there is no need for any electrical decoding at the receiver.

The modulated optical signal is then transmitted through the optical fiber link118. The transmitted optical signal is received by the receiver150and is then demodulated by the narrow band optical filter demodulator152. The narrow band optical filter demodulator152converts the phase information modulated on the optical signal into amplitude changes. In addition, the narrow band optical filter demodulator152reduces or eliminates the portion of the optical spectrum of the received optical signal that includes the dispersion imprint of the optical link118. The optical detector154then converts the demodulated optical signal to an electrical demodulation signal.

FIG. 2illustrates a block diagram of a second embodiment of an optical communications system200which mitigates the effects of dispersion by reducing or eliminating the portion of the optical spectrum that includes the dispersion imprint of the optical link by optimizing demodulation according to the present invention. The optical communications system200is similar to the optical communications system100described in connection withFIG. 1. However, the receiver250includes an interferometric demodulator252that is optimized to reduce the effects of dispersion along the optical link118.

In some embodiments, the receiver250includes a narrow band optical filter254having an input that is optically coupled to the output of the optical fiber link118. The characteristics of the narrow band optical filter254are chosen to shape the optical bandwidth of the received modulated optical signal to reduce the portion of the optical spectrum of the received modulated optical signal that includes the dispersion imprint of the optical link118.

The output of the narrow band optical filter254is optically coupled to the optical input of the interferometric demodulator252. The interferometric demodulator252is a delay-interferometer (DI) type demodulator, which can be constructed from an asymmetric Mach-Zehnder- or Michelson-type interferometer having a differential delay. The differential delay is caused by an optical delay253in one arm of the interferometer that is different from the optical delay in the other arm of the interferometer. The delay-interferometric demodulator252regenerates the amplitude information from the encoded phase information, thereby recovering the transmitted original data and providing the data at a constructive256and a destructive output port258of the delay-interferometric demodulator252.

The optical delay253in the delay-interferometric demodulator252is chosen or optimized to provide some optical filtering of the received optical modulation signal. The optical filtering is provided by a mismatch in the optical paths of the optical signals propagating in the two arms of the delay-interferometer demodulator252that is provided by the delay generator253. In another embodiment, an interferometric modulator can be used that provides a mismatch in the amplitudes of the optical signals propagating in the two arms of the interferometer demodulator. This type of optical filtering can provide the same optimized filter shape as the optical filter demodulator152described in connection withFIG. 1.

The receiver250also includes a first260and second optical detector262that are optically coupled to respective ones of the constructive optical output port256and the destructive optical output port258of the delay-interferometer demodulator252. The first and second optical detectors260,262detect the demodulated data and generate an electrical data signal. A combiner264subtracts the signals from the constructive output port256and the destructive output port258.

It has been widely accepted that the best performances can be achieved in delay-interferometric demodulators with differential delays equal to ΔT=nB−1, where n is an integer number and B is the symbol rate. However, it has been recently demonstrated, both experimentally and theoretically (with numerical simulations), that the undesirable effects of the spectrally narrow optical filters in the communication system can be partially compensated by using a delay interferometer with a differential delay that is smaller than the symbol time slot, ΔT<B−1.

Using a delay-interferometer demodulator with a differential delay that is smaller than the symbol time slot, ΔT<B−1can also optimize the demodulation to reduce the effects of dispersion in the received signal. Simulations have been performed to determine delays that best reduce the effects of dispersion in the received signal.FIGS. 3A-3Cpresent simulated eye diagrams that compare the narrow band optical filter demodulator152described in connection withFIG. 1with the delay-interferometer demodulator252described in connection withFIG. 2.

FIG. 3Apresents a simulated eye diagram300of a 40 Gbps DPSK signal after propagating through an 8 km optical link with a +140 psec/nm dispersion and being demodulated with a 32 GHz narrow band optical filter. The simulated eye diagram300shows minimal inter-symbol interference due to dispersion and clearly distinguished eyes that are open enough to distinguish the received digital signal with most receivers.

FIG. 3Bpresents a simulated eye diagram310of a 40 Gbps DPSK signal after propagating through an 8 km optical link with a +140 psec/nm dispersion and being demodulated with a delay-interferometric demodulator having a standard 1-bit delay. The simulated eye diagram310shows substantial inter-symbol interference due to dispersion. However, the simulated eye diagram310has center eye patterns that are open enough to distinguish the transmitted data but with a significant penalty due to chromatic dispersion.

FIG. 3Cpresents a simulated eye diagram320of a 40 Gbps DPSK signal after propagating through an 8 km optical link with a +140 psec/nm dispersion and being demodulated with a delay-interferometric demodulator having a 0.6-bit delay. The 0.6-bit delay has been determined through extensive simulations to significantly reduce the effects of dispersion in the received signal. In one embodiment of the present invention, a delay-interferometric demodulator having a bit delay in the range of 0.55 to 0.75 is used. The simulated eye diagram330shows minimal inter-symbol interference due to dispersion and clearly distinguished eyes that are open enough to distinguish the received digital signal with most receivers. The simulated eye diagram330is similar to the simulated eye diagram300obtain by simulating the demodulation with a 32 GHz narrow band optical filter.

FIG. 3Dpresents a simulated eye diagram330of a 40 Gbps DPSK signal after propagating through a 14 km optical link with a +240 psec/nm dispersion and being demodulated with a 32 GHz narrow band optical filter. The simulated eye diagram330shows some inter-symbol interference due to dispersion, but the eyes are open enough to clearly distinguish the transmitted digital signal with most receivers.

FIG. 3Epresents a simulated eye diagram340of a 40 Gbps DPSK signal after propagating through a 14 km optical link with a +240 psec/nm dispersion and being demodulated with a delay-interferometric demodulator having a standard 1-bit delay. The simulated eye diagram340shows eyes that are completely closed in the center region indicating an unacceptably high inter-symbol interference due to dispersion. Most of the information in such a demodulated signal is lost. The open areas in the bottom of the eye diagram do not contain any useful information.

FIG. 3Fpresents a simulated eye diagram350of a 40 Gbps DPSK signal after propagating through a 14 km optical link and being demodulated with a delay-interferometric demodulator having a 0.6-bit delay, which has been determined to significantly reduce the effects of dispersion in the received signal. The simulated eye diagram350shows some inter-symbol interference due to dispersion, but there are clearly distinguished eyes that are open enough to distinguish the received digital signal with most receivers. The simulated eye diagram350is similar to the simulated eye diagram330for the demodulation with the 32 GHz narrow band optical filter that was described in connection withFIG. 3D.

FIG. 4illustrates a block diagram of a third embodiment of an optical communications system400which mitigates the effects of dispersion by reducing or eliminating the portion of the optical spectrum that includes the dispersion imprint of the optical link by using a Reconfigurable Optical Add Drop Multiplexer (ROADM). The optical communications system400is similar to the optical communications system200described in connection withFIG. 2. However, the optical communications system400includes at least one ROADM402that is operated to reduce the effects of dispersion.

Reconfigurable optical add drop multiplexers are optical elements that can be deployed throughout an optical network as a way of rerouting optical signals. These devices are well known in the art. Some particular ROADMs, such as those sold by Optium Corporation of Horsham, Pa., the assignee of the present invention, include optical filtering functionality in addition to optical switching functionality. These particular ROADM devices use Liquid Crystal on Silicon (LCOS) technology. By precisely controlling the voltage on the LCOS devices in these ROADMs, the user can generate a diffraction pattern that can perform optical filtering for bandwidth optimization according to the present invention. In addition, these ROADM devices can incorporate a linear phase across the band pass filter response, which can be used to add a predetermined amount of chromatic dispersion to the optical spectrum to compensate for dispersion in the modulated optical signal that occurs during transmission through the optical link118.

The methods and apparatus of mitigating the effects of chromatic dispersion in the optical fiber link according to the present invention can work with various modulation formats and at various data rates. Data is presented for the DPSK modulation format. However, one skilled in the art will appreciate that the methods and apparatus of the present invention are not limited to using only the DPSK modulation format and that other modulation formats, such as RZ, NRZ, and DuoBinary can be used. In addition, one skilled in the art will appreciate that the methods and apparatus of the present invention are not limited to application where data is transmitted at 10 and 40 Gbps.

FIG. 5presents an optical spectrum500that compares a 10 Gbps NRZ data spectrum502with a DPSK data spectrum504before propagation through the optical network. The optical spectrum500indicates that the DPSK modulation format has a relatively large optical bandwidth, which results in a relatively high tolerance to optical fiber non-linearities. This relatively high tolerance of optical fiber non-linearity allows the use of high power optical signals. Using high power optical signals results in transmitted optical signals that have relatively high signal-to-noise ratio at the receiver. However, the relatively large optical bandwidth of DPSK modulated signals also causes a significant chromatic dispersion penalty in known optical communications systems.

FIGS. 6A-6Cpresent DPSK spectrum that illustrates how filtering and demodulation according to the present invention reduce or eliminate the portion of the optical spectrum carrying the dispersion imprint of the optical link.FIG. 6Apresents a 10 Gbps DPSK data optical pulse spectrum after generation by a DPSK transmitter and transmission through the optical link, but before optical filtering. The DPSK optical spectrum600illustrates regions602that exhibit second-order non-linearities that will cause pulse broadening due to dispersion in the optical link. Most direct detection receivers use square law detectors that square the signal. Any imbalance in the received optical spectrum will degrade performance and will cause the received electrical eye to have significant distortions. Filtering regions of the spectrum with higher order effects that cause the degraded performance can be used to reduce or eliminate these distortions in the optical spectrum. Filtering these spectral regions602will greatly improve the optical system tolerance to system non-linearity, in particular, chromatic dispersion.

FIG. 6Bpresents an optical filter spectrum620having a passband chosen to remove the second-order part of the DPSK data pulse spectrum600shown inFIG. 6A. The particular filter used is an 8 GHz passband filter.FIG. 6Cpresents the filtered DPSK data pulse spectrum640that was filtered by an optical filter with the optical filter spectrum620presented inFIG. 6B.FIGS. 6A-6Cshow that optical filtering and optical demodulation according to the present invention can reduce or eliminate the portion of the optical spectrum in the DPSK data pulse that carries the dispersion imprint of the optical link.

FIG. 7illustrates a comparison between narrow optical filtering on the transmitter side and narrow optical filtering on the receiver side of an optical communications system according to the present invention.FIG. 7Apresents a simulated eye diagram700of a 40 Gbps DPSK signal that was filtered by a 32 GHz narrow band optical filter according to the present invention on the transmitter side and then transmitted through a 12 km optical link with a +210 psec/nm dispersion.FIG. 7Bpresents a simulated eye diagram710of a 40 Gbps DPSK signal after propagating through a 12 km optical link with a +210 psec/nm dispersion and being demodulated with a 32 GHz narrow band optical filter according to the present invention. The eye diagrams700and710inFIGS. 7A and 7Bindicate that optical filtering according to the present invention, which mitigates the effects of dispersion of the modulated optical signal, can be performed on either the transmitter side or the receiver side of the optical communications system.

FIGS. 8A-8Eillustrate simulated and measured eye diagrams for 10 Gbps data streams generated and transmitted under different conditions.FIG. 8Aillustrates a simulated eye diagram800of a 10 Gbps data stream generated by a transmitter before filtering and transmission through the optical link. The simulated optical spectrum800of the 10 Gbps data stream has no dispersion imprint from the optical link.FIG. 8Billustrates a corresponding measured eye diagram810of a 10 Gbps data stream generated by a transmitter before filtering and transmission through the optical link. The corresponding measured spectrum810of the 10 Gbps data stream also has no dispersion imprint from the optical link. The simulated eye diagram800closely matches the corresponding measured eye diagram810.

FIG. 8Cillustrates a simulated eye diagram820of a 10 Gbps data stream transmitted through a 96 km optical fiber link with a +1,600 psec/nm dispersion and demodulated with an 8 GHz optical filter according to the present invention.FIG. 8Dillustrates a corresponding measured eye diagram830of a 10 Gbps data stream transmitted through a 96 km optical fiber link with a +1,600 psec/nm dispersion and demodulated with an 8 GHz optical filter according to the present invention. The simulated eye diagram820closely matches the corresponding measured eye diagram830. In addition, both the simulated eye diagram820and the corresponding measuring eye diagram830show minimal inter-symbol interference due to dispersion.

FIG. 8Eillustrates a simulated eye diagram840of a 10 Gbps data stream transmitted through a 175 km optical fiber link with a +3,000 psec/nm dispersion and demodulated with an 8 GHz optical filter according to the present invention.FIG. 8Fillustrates a corresponding measured eye diagram850of a 10 Gbps data stream transmitted through a 175 km optical fiber link with a +3,000 psec/nm dispersion and demodulated with an 8 GHz optical filter according to the present invention. The simulated eye diagram840closely matches the corresponding measured eye diagram850. In addition, both the simulated eye diagram840and the corresponding measuring eye diagram850also show minimal inter-symbol interference due to dispersion.

Thus,FIGS. 8A-8Eillustrate a close correlation between simulated and measured eye diagrams for 10 Gbps data streams transmitted and received according to the present methods of the invention. Similar correlations between simulated and measured eye diagrams for 40 Gbps data streams transmitted and received according to the present methods of the invention can be obtained.

Therefore, the methods and apparatus of the present invention mitigate the effects of dispersion in the modulated optical signal by using narrow band optical filtering and/or optimizing the optical demodulation. The methods and apparatus of the present invention can be used with any type of modulated optical signal having second-order and higher-order dispersion characteristics, such as DPSK, RZ, NRZ, and DuoBinary modulated optical signals. Receivers according to the present invention optimize the demodulated optical signals and generate normal binary-valued signals that can be detected with a conventional optical detector, while also providing high dispersion tolerance. The improvement in the dispersion tolerance comes from eliminating the portion of the optical spectrum carrying a substantial fraction of the dispersion imprint of the optical link.

In addition, one skilled in the art will appreciate that the various methods of mitigating the effects of dispersion according to the present invention are not exclusive methods that can only work independently. Instead, the various methods of mitigating the effects of dispersion in the modulated optical signal according to the present invention can be used separately or in any combination. For example, both narrow optical filtering and optimized demodulation can be used together to mitigate the effects of dispersion in the modulated optical signal. In addition, the optical filtering can be performed using optical filters anywhere in the optical communications systems, such as in the transmitter end, optical link or transmission system, and in the receiver end. Furthermore, one or more reconfigurable optical add drop multiplexer positioned in the optical link can be positioned anywhere in the optical link to provide optical filtering.

EQUIVALENTS

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art, may be made therein without departing from the spirit and scope of the invention.