Patent Application
20090252502
Not Applicable
Not Applicable
Not Applicable
The present invention is directed to optical communication. More particularly, the invention provides methods and system for modulating optical signals, which can deliver increased system nonlinear tolerance and optical signal-to-noise ratio (OSNR) tolerance for use in optical communication systems. Merely by way of example, the invention has been applied to Differential Quadrature Phase Shift Keying (DQPSK) modulation for optical signal transmission. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be used in high-speed dense wavelength division multiplexing (DWDM) optical transmission systems.
Recent optical transport systems are built on wavelength division multiplexing (WDM) transmission techniques. WDM systems with line rates of 10 Gbit/s have been deployed, and 40 Gbit/s systems are being actively introduced to carriers at present. More over, 100 Gbit/s transport is being considered. Various modulation formats have been applied, including On-Off-Keying (OOK) formats such as Non-Return-to-Zero (NRZ), Return-to-Zero (RZ), Optical Duobinary (ODB), etc., and Phase Shift Keying (PSK) formats such as Differential Phase Shift Keying (DPSK), and Differential Quadrature Phase Shift Keying (DQPSK), etc.
DQPSK, in particular RZ-DQPSK, is widely considered as one of the promising techniques for next generation 40 Gbit/s (NG-40G) transmission systems. Since DQPSK is based on the transmission of 2 bits/symbol, it gives either a doubled capacity when the baud rate is chosen to be the same as the line rate, or a reduced line rate if the baud rate is half the targeted line rate. Therefore, DQPSK systems are expected to benefit, to some extent, from the reduced line rate characteristic, when compared with traditional time division multiplexed (TDM) systems. For example, costs of components required to build a DQPSK system are reduced, and system tolerances such as chromatic dispersion (CD) tolerance and polarization mode dispersion (PMD) tolerance could be improved, as a result of the reduced line rate characteristic.
DQPSK is also referred to as NRZ-DQPSK. Other DQPSK formats can be generated, for example, by applying a pulse carving. To generate NRZ-DQPSK signals, there are generally three types of approaches, depending on what types of modulators are used. The first type is based on an integrated DQPSK modulator (Dual-parallel Mach-Zehnder Modulator Approach); the second type is based on multiple discrete modulators (Multiple Discrete Modulator Approach); and the third type is based on a single dual-drive modulator (Single Dual-Drive Modulator Approach).
Even though conventional optical signal modulation systems have found wide use, there are still limitations that can restrict the scope and performance of optical communication systems. These limitations include signal nonlinearity and system noise as discussed further below.
From the above, it is seen that an improved technique for optical signal modulation is desired.
The present invention is directed to optical communication. More particularly, the invention provides methods and system for modulating optical signals, which can deliver increased system nonlinear tolerance and optical signal-to-noise ratio (OSNR) tolerance for use in optical communication systems. Merely by way of example, the invention has been applied to Differential Quadrature Phase Shift Keying (DQPSK) modulation in conjunction with Modified-Carrier-Suppressed Return-To-Zero (mCSRZ) modulation. This combination forms a Modified-Carrier-Suppressed Return-To-Zero Differential Quadrature Phase Shift Keying (mCSRZ-DQPSK) optical modulator that can provide enhanced nonlinearity tolerance and reduced distortions in the optical signal. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be used in high-speed DWDM optical transmission systems.
According to a specific embodiment, the present invention provides an optical communication system. The system includes an optical carrier signal source that produces an optical carrier signal and one or more optical modulators coupled to the optical carrier signal source. The optical modulators modulate the optical carrier signal to produce a continuous wave optical signal in response to one or more input electrical signals. Additionally, the system includes a pulse modulator coupled to the optical modulators to receive the continuous wave optical signal. The pulse modulator is selectively configured to modulate the continuous wave optical signal to cause carrier energy suppression and nonlinearity reduction. In a specific embodiment, the pulse modulator modulates the continuous wave optical signal in response to a pulse signal, which is characterized by an amplitude and a bias point. At least one of the amplitude and the bias point is adaptively selected to cause carrier energy suppression and nonlinearity reduction. In an embodiment, both of the amplitude and the bias point are adaptively determined. Additionally, in a specific embodiment, the system also includes an optical spectral monitor for maintaining modulator bias stabilization.
In a specific embodiment, the pulse signal is an electrical pulse signal. Depending on the embodiment, various bias conditions can be used. In one example, the bias point of the electrical pulse signal is between a null point and a maximum point, but does not include the null point or the maximum point, of the continuous wave optical signal. In another example, the amplitude of the electrical pulse signal is selected such that the electrical pulse signal drives the pulse modulator through the null point. In yet another example, the electrical pulse signal is biased at a quadrature point and drives through the null point of the phase modulators to achieve enhanced carrier suppression. In an alternative example, the electrical pulse signal is biased at a non-quadrature point and has a predetermined amplitude, and an output optical signal is characterized by a flat top spectral profile. In another example, the electrical pulse signal is biased at a non-quadrature point and has a predetermined amplitude, and an output optical signal is characterized by a central dip spectral profile. In yet another example, the electrical pulse signal is biased at a non-quadrature point and has a predetermined amplitude, and an output optical signal is characterized by a broad bell-shaped spectral profile.
In a specific embodiment, the one or more phase modulators includes a first and a second phase modulators and produce an NRZ-DQPSK continuous wave optical signal, and the pulse modulator adaptively produces a mCSRZ-DQPSK. In another embodiment, the one or more phase modulators includes a first and a second phase modulators and produce an NRZ-DQPSK continuous wave optical signal. The pulse modulator adaptively produces a mCSRZ-DQPSK in response to an RF electrical pulse signal having a bias point at a quadrature point and an amplitude greater than Vπ, thereby causing enhanced carrier suppression by means of raising the RF power levels.
According to another specific embodiment, the invention provides an optical modulation system. The system includes a laser that produces an optical carrier signal, and a first and a second Mach-Zehnder modulators coupled to the laser. The Mach-Zehnder modulators modulate the optical carrier signal to produce a NRZ-DQPSK optical signal in response to one or more input electrical signals. The system also includes a pulse modulator coupled to the Mach-Zehnder modulators to receive the continuous wave optical signal. The pulse modulator adaptively modulates the NRZ-DQPSK optical signal in response to an electrical pulse signal. The electrical pulse signal is biased at a shifted non-quadrature biasing point and has a predetermined amplitude. In the system, the non-quadrature biasing point is adaptively determined to cause carrier energy suppression and nonlinearity reduction. In a specific embodiment, the system also includes a mini optical spectral analyzer (OSA) for maintaining modulator bias stabilization.
According to an alternative embodiment, the invention provides another optical modulation system. The system includes a laser that produces an optical carrier signal, and a first and a second Mach-Zehnder modulators coupled to the laser. The Mach-Zehnder modulators modulate the optical carrier signal to produce a NRZ-DQPSK optical signal in response to one or more input electrical signals. The system also includes a pulse modulator coupled to the Mach-Zehnder modulators to receive the continuous wave optical signal. The pulse modulator adaptively modulates the NRZ-DQPSK optical signal in response to an RF electrical pulse signal. In the system, the electrical pulse signal is biased at a quadrature biasing point and has an amplitude greater than Vπ of the Mach-Zehnder modulators. The amplitude is adaptively determined to cause carrier energy suppression and nonlinearity reduction.
According to an embodiment of the present invention, a method is provided for optical signal modulation. The method includes providing an optical carrier signal and modulating the optical carrier signal in response to one or more input electrical signals using one or more optical modulators to produce a continuous wave optical signal in response to one or more input electrical signals. The method also includes adaptively modulating the continuous wave optical signal to cause carrier energy suppression and nonlinearity reduction. In a specific embodiment, the method further includes providing an electrical pulse signal characterized by an amplitude and a bias point. At least one of the amplitude and the bias point is adaptively determined. Moreover, the method includes modulating the continuous wave optical signal in response to the electrical pulse signal to cause carrier energy suppression and nonlinearity reduction.
In a specific embodiment, the bias point of the electrical pulse signal is between a null point and a maximum point, but does not include the null point or the maximum point, of the continuous wave optical signal. In another embodiment, the amplitude of the electrical pulse signal is selected such that the electrical pulse signal drives the pulse modulator through the null point. In yet another embodiment, the electrical pulse signal is biased at a quadrature point and drives through the null point of the phase modulators to achieve enhanced carrier suppression.
The various bias conditions can lead to different output power spectrum characteristics. In one example, the electrical pulse signal is biased at a non-quadrature point and has a predetermined amplitude, and an output optical signal is characterized by a flat top spectral profile. In another example, the electrical pulse signal is biased at a non-quadrature point and has a predetermined amplitude, and an output optical signal is characterized by a central dip spectral profile. In yet another example, the electrical pulse signal is biased at a non-quadrature point and has a predetermined amplitude, and an output optical signal is characterized by a broad bell-shaped spectral profile. In an embodiment, the method also includes maintaining modulator bias stabilization using an optical spectral monitor.
Many benefits are achieved by way of the present invention over conventional techniques. For example, in an embodiment, the invention provides an optical modulation system and associated methods including an adaptive clock modulator and an optical signal modulator for enhanced nonlinear tolerances and improved OSNR performance. In a specific embodiment, the optical modulation system and method includes a modified CSRZ clock modulator and a DQPSK modulator.
In embodiments of the invention, techniques are provided for adaptive modulation for improved nonlinear tolerance and improved OSNR performance. In a specific embodiment, a method is provided for biasing the clock pulse in the clock modulator such that the clock pulse drives through null but is not biased at either the null or the peak for carrier energy suppression. In another embodiment, adaptive biasing method for the clock modulator uses a optical spectral analyzer (OSA). In an alternative embodiment, a clock modulator, e.g. a pulse carver, has fixed biasing and adaptive clock amplitude setting for enhanced nonlinear tolerances and improved OSNR performance.
Additionally, the various embodiments are compatible with conventional product technology without substantial modifications to conventional equipment and processes. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits will be described in more detail throughout the present specification and more particularly below.
Various additional objects, features and advantages of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow.
The present invention is directed to optical communication. More particularly, the invention provides methods and system for modulating optical signals, which can deliver increased system nonlinear tolerance and optical signal-to-noise ratio (OSNR) tolerance for use in optical communication systems. Merely by way of example, the invention has been applied to Differential Quadrature Phase Shift Keying (DQPSK) modulation in conjunction with Modified-Carrier-Suppressed Return-To-Zero (mCSRZ) modulation. This combination forms a Modified-Carrier-Suppressed Return-To-Zero Differential Quadrature Phase Shift Keying (mCSRZ-DQPSK) optical modulator for optical communication systems. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be used in high-speed DWDM optical transmission systems.
As discussed above, conventional DQPSK modulation, though successful in certain applications, suffers from many limitations. Because there is a carrier phase difference of 90 degree between I-component and Q-component in DQPSK, strong nonlinear interactions (in particular, cross phase modulation) between the two exist. This aspect is easily noticeable in DQPSK transmission experiments.
Certain improvements may be achieved by using Carrier Suppressed Return-to-Zero DQPSK (CSRZ-DQPSK). CSRZ-DQPSK can be generated by using DQPSK with a CSRZ pulse carver. In an example, to generate a CSRZ pulse, a MZ modulator is driven with a 2Vπ sinusoidal RF signal. In conventional CSRZ-DQPSK systems, the RF signal is biased exactly at the null.
A conventional CSRZ-DQPSK was used by Y. Zhu et al in ‘1.6 bit/s/Hz orthogonally polarized CSRZ-DQPSK transmission of 8×40 Gbit/s over 320 km NDSF’, presented at Optical Fiber Communication Conference (OFC'2004), paper TuF1. The nonlinear advantage of CSRZ-DQPSK over NRZ-DQPSK for applications to high spectral efficiency system was shown by Y. Zhu et al, in the paper entitled ‘Highly spectrally efficient transmission using CSRZ-DQPSK’, presented at IEEE Workshop on Advanced Modulation Formats, San Francisco, June 2004.
Owing to the significant cross phase modulation effect between the I- and Q-component, the nonlinear impairments of a DQPSK signal can be much stronger than that of a TDM signal, even for a single optical wavelength. As a result, the application of DQPSK could be only suitable to very limited systems, where dispersion maps are carefully chosen and signal launch power tends to be very low, in order to avoid the nonlinear penalties. As a result, network deployments based on DQPSK and system operating margins can be limited. Hence there is a need to develop more nonlinear tolerant DQPSK modulation technique.
The nonlinearity tolerances of conventional NRZ-DQPSK, RZ-DQPSK and CSRZ-DQPSK are often not strong enough to ensure significant system margins, especially in systems strongly limited by XPM impairments, such as in systems with mixed transmission of 10G OOK and 40G DQPSK signals.
The frequency chirping in CRZ-DQPSK can help to improve the nonlinear tolerances of CRZ-DQPSK beyond those achievable by NRZ-DQPSK, RZ-DQPSK and CSRZ-DQPSK. However, the amount of chirp that can be applied is eventually limited if a very tight optical spectral occupancy of the signal is required by the system. On the other hand, in quadrature modulation, the OSNR performance of a DQPSK signal is degraded, as compared to that of a DPSK signal.
Hence techniques to enhance nonlinear tolerances and improve the OSNR performances of DQPSK systems are highly desirable.
Depending upon the embodiment, the present invention includes various features, which may be used. These features include the following:
As shown, the above features may be in one or more of the embodiments to follow. These features are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.
In a specific embodiment, the pulse modulator 130 receives an electrical pulse signal 133. The electrical pulse signal is biased by a bias signal 135. The pulse modulator 130 modulates the continuous wave optical signal 124 in response to the electrical pulse signal 133. In a specific example, the electrical pulse signal is a radio frequency (RF) signal, e.g. at 20 GHz. The electrical pulse signal 133 is characterized by an amplitude and a bias point. In an embodiment, either the amplitude or the bias point is adaptively selected. Under these conditions, the pulse modulator 130 is capable of producing an On-Off-Keying (OOK) or Phase-Shift-Keying (PSK) optical signal that is characterized by enhanced carrier energy suppression and nonlinearity reduction. Further details of the optical communication system are illustrated in the examples below.
Next, the NRZ-DQPSK signal 224 is sent through the mCSRZ pulse carver 230. The mCSRZ pulse carver 230 modulates signal 224 and provide the output mCSRZ-DQPSK signal. According to a specific embodiment, the invention provides a method for enhanced carrier energy suppression and nonlinearity reduction.
In various embodiments of mCSRZ-DQPSK, signal generation is achieved by the modulators that drive through null. Apart from null and maximum point of MZ, the bias points can be anywhere on the modulator characteristic curve such as shown in
Other than the embodiment of
In system applications of mCSRZ-DQPSK, a stabilization of modulator bias is desirable.
According to a specific embodiment, the present invention provides a method for adaptively selecting a bias point for the clock modulator to enhance carrier energy suppression. The method can be briefly summarized below.
Although the systems in
According to an embodiment of the present invention, it provides a method for optical signal modulation. The method includes providing an optical carrier signal and modulating the optical carrier signal in response to one or more input electrical signals using one or more optical modulators to produce a continuous wave optical signal in response to one or more input electrical signals. The method also includes adaptively modulating the continuous wave optical signal to cause carrier energy suppression and nonlinearity reduction. In a specific embodiment, the method further includes providing an electrical pulse signal characterized by an amplitude and a bias point. At least one of the amplitude and the bias point is adaptively determined. In a specific embodiment, both the amplitude and the bias point are adaptively determined. Moreover, the method includes modulating the continuous wave optical signal in response to the electrical pulse signal to cause carrier energy suppression and nonlinearity reduction.
In a specific embodiment, the bias point of the electrical pulse signal is between a null point and a maximum point, but does not include the null point or the maximum point, of the continuous wave optical signal. In another embodiment, the amplitude of the electrical pulse signal is selected such that the electrical pulse signal drives the pulse modulator through the null point. In yet another embodiment, the electrical pulse signal is biased at a quadrature point and drives through the null point of the phase modulators to achieve enhanced carrier suppression.
The various bias conditions can lead to different output power spectrum characteristics. Some examples are shown in
In a specific embodiment, the one or more phase modulators include a first and a second phase modulators and produce an NRZ-DQPSK continuous wave optical signal, and the pulse modulator adaptively produces a mCSRZ-DQPSK. In another embodiment, the one or more phase modulators include a first and a second phase modulators and produce an NRZ-DQPSK continuous wave optical signal, and the pulse modulator adaptively produces a mCSRZ-DQPSK in response to an RF electrical pulse signal having a bias point at a quadrature point and an amplitude greater than Vπ, thereby causing enhanced carrier suppression by means of raising the RF power levels.
In an alternative embodiment, the invention provides another method for optical signal modulation. The method includes providing an optical carrier signal and modulating the optical carrier signal using a first and a second Mach-Zehnder modulators. The method also includes producing an NRZ-DQPSK optical signal in response to one or more input electrical signals, and then modulating the NRZ-DQPSK optical signal in response to an electrical pulse signal. The electrical pulse signal is biased at a shifted non-quadrature biasing point and has a predetermined amplitude. The non-quadrature biasing point is adaptively determined to cause carrier energy suppression and nonlinearity reduction. In a specific embodiment, the method also includes maintaining modulator bias stabilization using a mini optical spectral analyzer (OSA).
According to yet another embodiment, the invention provides a method for optical signal modulation. The method includes providing an optical carrier signal and modulating the optical carrier signal using a first and a second Mach-Zehnder modulators to produce an NRZ-DQPSK optical signal in response to one or more input electrical signals. Additionally, the method includes modulating the NRZ-DQPSK optical signal in response to a pulse signal. The pulse signal is biased at a shifted non-quadrature biasing point and has a predetermined amplitude. The predetermined amplitude is adaptively determined to cause carrier energy suppression and nonlinearity reduction. In a specific embodiment, the amplitude of the pulse signal is greater than Vπ of the Mach-Zehnder modulators.
The methods discussed above include sequences of processes that include adaptively biasing a pulse modulator to provide signals having enhanced nonlinearity tolerance and reduced distortion according to embodiments of the present invention. It is understood that other alternatives can also be provided where steps are added, one or more steps are removed, or one or more steps are provided in a different sequence without departing from the scope of the claims herein.
In order to evaluate the nonlinear tolerance performance, we performed experimental studies. The mCSRZ-DQPSK and conventional RZ-DQPSK signals were propagated through 20 km single mode fiber (SMF), followed by a dispersion compensating module (DCM) with a dispersion of −340 ps/nm.
In extensive transmission comparisons, mCSRZ-DQPSK exhibited unique performance advantages over conventional DQPSK modulation techniques. For example, an experimental investigation was carried out to compare the system performances of mCSRZ-DQPSK vs. RZ-DQPSK in a 1600 km (20×80 km) single mode fiber (SMF) system with two in-line optical equalization (OEQ) stations and EDFA-only amplifications. The two in-line OEQs had totally four 100 GHz wide arrayed waveguides (AWGs) and four 50 GHz-spaced interleavers (ITLs). In addition to the effect of gain equalization, these OEQs also caused strong optical filtering effects. Thus the system is also a good test of the resilience of DQPSK different formats to cascaded optical filtering.
In another comparative transmission experiment, a 40G DQPSK signal and 11 OOK channels at 10G were mixed with 100 GHz spacing with the 40G DQPSK channel in the middle, and transmitted to over 1600 km, while dispersion per span is nearly completely compensated. Both RZ-DQPSK and mCSRZ-DQPSK were tested. The measured OSNR penalty after 1600 km transmission vs. signal power is shown in
The experimental results discussed above demonstrate the significant advantages of mCSRZ-DQPSK over conventional RZ-DQPSK modulation. Many benefits are achieved by way of the present invention over conventional techniques. For example, in an embodiment, the invention provides an optical modulation system and associated methods including an adaptive clock modulator and an optical signal modulator for enhanced nonlinear tolerance and improved OSNR performance. In a specific embodiment, the optical modulation system and method includes a modified CSRZ clock modulator and a DQPSK modulator. Additionally, in various embodiments of the invention, techniques are provided for adaptive modulation for improved nonlinear tolerance and improved OSNR performance.
While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.
