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This application claims the benefit of U. Provisional Patent Application No. This application relates to optical communications based on optical wavelength division multiplexing WDM and sub-carrier multiplexed SCM optical communication. Optical communications use broad bandwidths in optical carriers to carry large amounts of data or information that are difficult to achieve by using microwave or RF carriers.
Optical wavelength division multiplexing WDM further and optical sub-carrier multiplexing may be used to enhance the capacity of optical communication links and systems. The bandwidth occupied by a data channel is as a valuable asset in optical communications. Ideally, the signal bandwidth for a given data channel should be as narrow or small as possible so that more data channels may be transmitted within a given spectral bandwidth, especially in dense and ultra dense WDM optical links and networks.
Notably, optical sub-carrier multiplexing may place two or more optical sub carriers at different optical wavelengths within one ITU window to achieve high density WDM. In various optical communication applications, the bandwidth occupied by a data channel in optical communications should be as small as possible also because different spectral components within the bandwidth of the data channel may experience different dispersion effects during transmission, e.
Due to dispersion, the data in the channel may degrade to cause an increase in the bit error rate and even loss of the data in some circumstances.
Many deployed optical communication systems use non-return-to-zero NRZ binary modulation. As an alternative modulation approach, baseband optical duobinary modulation, has some advantages over the NRZ modulation, including increased chromatic dispersion tolerance and improved spectral efficiency with a pre-filtered pulse shape. The increased chromatic dispersion tolerance allows for data transmission over a longer distance without the need for dispersion compensation. The improved spectral efficiency can be used to implement a denser wavelength spacing in a dense WDM DWDM system to increase the capacity of the system.
This application describes techniques, devices and systems for combining duobinary modulation and optical subcarrier multiplexing in optical communication applications.
In some implementations, a single optical carrier beam from a single laser may be used to generate multiple optical subcarriers to respectively carry different data channels. Different optical subcarriers can remain stabilized relative to one another in frequency even if the optical carrier frequency of the laser fluctuates or drifts since all such optical subcarriers experience the same change in frequency. This implementation avoids the need for locking different lasers in frequency relative to one another when the lasers are used to produce different optical carrier signals for carrying different data channels.
In addition, such subcarrier multiplexing allows for dense channel spacing. Various examples are described. In one example, a first analog signal mixer is used to mix a first duobinary signal which represents a first data channel signal and a first local oscillator signal at a first local oscillator frequency to produce a first modulation control signal. A second analog signal mixer is used to mix a second duobinary signal which represents a second data channel signal and a second local oscillator signal at a second local oscillator frequency different from the first local oscillator frequency to produce a second modulation control signal.
The first and second modulation control signals are then applied to modulate a CW laser beam at an optical carrier frequency to produce an optical output beam which comprises optical subcarriers at optical subcarrier frequencies different from the optical carrier frequency to carry to carry the first and the second data channels. In another example, a device is described to include analog signal mixers to respectively produce a plurality of analog modulation control signals that respectively carry a plurality of data channels.
Each analog signal mixer is configured to receive and mix a data channel encoded as a duobinary encoded signal and a local oscillator signal at a local oscillator frequency different from local oscillator frequencies received by other analog signal mixers to produce a corresponding analog modulation control signal.
This device also includes an optical modulator to receive an input CW laser beam at an optical carrier frequency and to modulate the input CW laser beam in response to the analog modulation control signals to produce an optical output beam which comprises a plurality of different optical subcarriers at optical subcarrier frequencies different from the optical carrier frequency and respectively related to the local oscillator frequencies of the local oscillator signals.
Each optical subcarrier carries a baseband signal comprising information of a corresponding data channel of the data channels so that the different optical subcarriers carry baseband signals corresponding to the plurality of data channels, respectively. The optical modulator may be implemented in various configurations, including optical double sideband modulators and optical single side band modulators.
In another example, at least two binary electronic signals are modulated to produce duobinary encoded signals. A CW laser beam at an optical carrier frequency are also modulated in response to the duobinary encoded signals to produce two optical single sideband subcarriers at optical frequencies different from the optical carrier frequency as an optical output.
The optical output is then transmitted through an optical transmission link or network. In yet another example, a device is described to include a plurality of electronic duobinary signal modulators to respectively receive and modulate input binary signals and to output duobinary encoded signals, and a plurality of local oscillators to produce a plurality of local oscillator signals corresponding to the electronic duobinary signal modulators, respectively.
This device also includes a plurality of electronic signal mixers each of which is coupled to mix a duobinary encoded signal with a local oscillator signal from a corresponding local oscillator to produce a modulation control signal. An optical single sideband modulator is further included to receive an input CW beam at an optical carrier frequency and to modulate the beam in response to the modulation control signals from the electronic signal mixers to produce an optical output comprising the optical carrier, optical single sideband subcarriers at frequencies different from the optical carrier.
These and other examples, implementations, and their applications and operations are described in greater detail in the attached drawings, the detailed description and the claims. The techniques, devices and systems described in this application use duobinary modulation to compress the bandwidth of each data channel and use optical modulation to multiplex optical sub carriers modulated with such compressed data channels onto an optical carrier.
Other optical modulation techniques for optical SCM modulations may also be used such as the optical double side band modulation and various optical amplitude modulation techniques. These and other SCM modulators allow for SCM demodulation using optical filters, traditional heterodyne technique such as the technique described by W. The system includes a transmitter , an optical link or network , and a receiver The transmitter produces an optical output that includes an optical carrier and data-carrying optical subcarriers at different optical subcarrier wavelengths modulated onto the optical carrier.
The optical output is transmitted through the optical link or network to the receiver The optical link or network may be a point-to-point fiber link, a part of one or more optical networks in various configurations including, e. The transmitter includes two or more duobinary modulators A and 11 B for modulating input binary data channels to produce duobinary encoded signals. Each duobinary encodes signal is then sent into a respective analog signal mixer e.
In other implementations, the design in FIG. An example for four channels is described later in this application. Each duobinary modulator A or B modulates the phase of each optical binary pulse in a data channel to produce the corresponding duobinary signal. Yonenaga and Kuwano show the reduced bandwidth of the duobinary signal in comparison with the original binary signal and illustrate the improved tolerance to chromatic dispersion.
In the current system, each duobinary signal has three digital levels and is mixed with an analog local oscillator signal at an RF or microwave frequency to produce a modulation control signal that represents the corresponding input binary data channel. In the example in FIG. The three-level duobinary signal received by the mixer is shown in FIG. The operation of the transfer function in FIG.
This mixer output signal can be used to modulate a CW optical carrier beam. The amplitude and phase of marks and spaces at the output of the microwave mixer are shown in FIG. A typical microwave spectrum at the output of a duobinary subcarrier is shown in FIG. This microwave subcarrier is then applied to amplitude-modulate an optical Mach-Zehnder modulator. Various duobinary encoders or modulators may be used to implement the duobinary modulators A and B in FIG.
The modulator in FIG. Two separate optical paths are provided and an input splitter is used to split the input into two signals for the two optical paths and an optical combiner is used to combine the two modulated optical signals from the two paths into a single output signal. Four RF or microwave MW signal connectors are provided for each arm of the optical modulator.
RF or microwave phase modulators or shifters are used in the signal paths to provide the desired phase shifts as shown in FIG. A corresponding analog signal mixer is used to supply the corresponding modulation control signal. Only the mixer for the channel f 1 is shown and the mixers for other channels are omitted.
At the output of the mixer, a signal splitter is used to split the modulation control signal into two parts, one for the AC electrode of the upper optical arm and another for the aC electrode of the lower optical arm.
Four separate signals f 1 , f 2 , f 3 , and f 4 are multiplexed onto the optical carrier, each producing both an upper side band and a lower side band. Adjacent channels in each optical arm are 90 degrees out of phase with each other. Hence, assuming f 1 , f 2 , f 3 and f 4 are in ascending order in frequency, the channels f 1 and f 2 are phase shifted by 90 degrees with each other; channels f 2 and f 3 are phase shifted by 90 degrees with each other; and channels f 3 and f 4 are phase shifted by 90 degrees with each other.
The optical phase modulation also produces two identical sidebands symmetrically on opposite sides of the optical carrier. As such, 8 side bands are generated for the four channels and each channel is duplicated in the optical signal. The channels in the lower optical arm are similarly phase shifted as shown in FIG.
Each of the signals, f 1 , f 2 , f 3 and f 4 is applied to the lower arm in quadrature with the corresponding signal f 1 , f 2 , f 3 and f 4 in the upper arm. In addition, one optical arm is then placed in quadrature with the other optical arm by the DC bias on the DC electrode.
As a result, upper sidebands for channels f 1 and f 3 in the upper optical arm are phase shifted by degrees with respect to upper side bands for channels f 1 and f 3 in the lower optical arm, respectively. Upper sidebands for channels f 2 and f 4 in the upper optical arm are in phase with respect to upper side bands for channels f 2 and f 4 in the lower optical arm, respectively. The lower sidebands for channels f 1 and f 3 in the upper optical arm are in phase with respect to lower side bands for channels f 1 and f 3 in the lower optical arm, respectively.
The upper sidebands for channels f 2 and f 4 in the upper optical arm are phase shifted by degrees with respect to lower side bands for channels f 2 and f 4 in the lower optical arm, respectively.
Likewise, in the lower side band, f2 and f2 signals are cancelled, leaving only f 1 and f 3. The system can be easily modified to reverse the order such that the lower side band will carry f 2 and f 4 and the upper will carry f 1 and f 3.
In this particular example, a Mach-Zehnder modulator is used to perform the optical modulation of a CW optical carrier from a laser. The two modulation controls signals f 1 m 1 and f 2 m 2 produced from the two different duobinary encoded signals No. The RF or microwave frequencies of the two signals f 1 m 1 and f 2 m 2 may be different so that the side band modulations on both sides of the optical carrier from the two modulation controls signals f 1 m 1 and f 2 m 2 do not overlap and are spaced apart, e.
This asymmetric channel arrangement can also be used for more than two channels such as the 4-channel example shown in FIGS. The optical output signal in FIG. In the above OSSB, the optical carrier can be suppressed by optical filtering to reject the optical carrier.
The lower left spectrum chart shows the optical spectrum of the optical output from the OSSB modulator for the baseband data channel No. The two subcarrier frequencies are different. The eye diagrams for the signals at three different stages in the system are also shown in FIG. Referring back to FIG. The optical filter may be a fixed bandpass filter to select a particular predetermined optical carrier frequency for detection or processing.
The optical filter may also be a tunable optical bandpass filter to tunably select a desired optical carrier frequency and to select different signals to detect at different times if desired.
The optical subcarriers that are rejected by the optical filter may be directed to other optical receivers designed to detect signals at different optical subcarriers in some implementations or discarded in other implementations. A fiber Bragg grating filter, tunable or fixed, may be used as the optical filter and may be combined with an optical circulator to direct the filtered and rejected light signals. Alternatively, an optical WDM demultiplexer may be used to replace the optical filter and to separate different optical signals at different subcarrier frequencies to different optical paths for detection or processing.
The signal equalizer may be optional and can be used to equalize the signal amplitudes of different frequency components in an input signal. Various electronic components in the transmitter , such as the signal mixers A and B, may have limited bandwidths in their device transfer functions and thus may undesirably attenuate certain frequency components of the signals, e.
The signal equalizer may be designed to exhibit different signal gains at different frequency components e. One example is optical double side band ODSB modulation. The optical double-sideband modulation technique can be used to achieve even higher spectral efficiency than optical single-sideband modulation techniques.
The bias voltages on the DC electrodes of the two optical arms differ in phase by degrees, and the phases of the modulating signals on the AC electrodes of the two arms also differ by degrees.