Patent Application
20150256265
The present invention relates to the field of optical communications, and, in particular embodiments, to a system and method for chromatic dispersion tolerant direct optical detection.
Chromatic dispersion (CD) can result in frequency-dependent fading for double-side band (DSB) optical signal transmission when a direct detection receiver type is used. This fading effect can be circumvented by transmitting a single-side band (SSB) signal generated via digital signal processing (DSP). The SSB causes orthogonal frequency-division multiplexing (OFDM) subcarriers after direct detection to experience a frequency-dependent phase shift without CD. The frequency-dependent phase shift can then be compensated using a one-tap linear equalizer. To facilitate direct detection, the optical carrier is transmitted along with the SSB signal over the optical fiber. One method to generate this optical carrier is to use a DC bias voltage at the optical modulator at the transmitter. However, the presence of the DC bias causes the optical modulator to operate in a nonlinear region of its transfer function. This produces a non-ideal SSB signal and its transmission becomes no longer immune to the CD induced fading effect. Hence, the detection performance is severely degraded. There is a need for an improved transmission scheme that allows efficient CD tolerant direct optical detection.
In accordance with an embodiment of the disclosure, a method by a transmitter for a direct detection system includes driving, via a drive voltage, a single-side band (SSB) signal at an optical modulator on a first optical path of the transmitter. The SSB signal is sufficiently linear with respect to the drive voltage for allowing direct detection at a receiver. The method further includes generating a DC carrier signal on a second path of the transmitter. The SSB signal is combined with the DC carrier signal at an output of the transmitter.
In accordance with another embodiment of the disclosure, a method by a transmitter for a direct detection system includes generating, using digital signal processing (DSP), a digital signal for optical communications, and generating, according to the optical signal, a drive voltage for an optical modulator. The method further includes splitting a laser output into a first path coupled to an optical modulator, and a second path separate from the optical modulator, and driving, using the drive voltage, a SSB signal at the optical modulator and the first path. The SSB signal is sufficiently linear with respect to the drive voltage for allowing direct detection at a receiver. Further, a DC carrier signal is generated at the second path. The SSB signal is combined with the DC carrier signal into an output signal of the transmitter.
In accordance with yet another embodiment of the disclosure, a transmitter for a direct detection system comprises a laser source, a first path and a second path both coupled to the laser source, an optical modulator coupled to the first path, at least one processor coupled to the optical modulator, and a non-transitory computer readable storage medium storing programming for execution by the at least one processor. The programming includes instructions to generate, using DSP, a digital signal for optical communications, and generate, according to the optical signal, a drive voltage for the optical modulator. The programming includes further instructions to drive, using the drive voltage, a SSB signal at the optical modulator. The SSB signal is sufficiently linear with respect to the drive voltage for allowing direct detection at a receiver. The transmitter is further configured to generate a DC carrier signal on the second path, and combine the SSB signal with the DC carrier signal into an output signal of the transmitter.
The foregoing has outlined rather broadly the features of an embodiment of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of embodiments of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
Typically, OFDM transmissions with double-side band (DSB) from the transmitter 110 to the receiver 120 experience chromatic dispersion as they propagate via a fiber. This results in frequency-dependent fading and affects detection performance (increases signal errors). To overcome the fading effect due to CD, single-side band (SSB) signals are instead transmitted. As such, at the receiving DSP unit 128, OFDM subcarriers may only experience a frequency-dependent phase shift instead of the frequency-dependent fading. The frequency-dependent phase shift can then be compensated at the receiving DSP unit 128, for example, using a simple one-tap equalizer for instance.
To facilitate direct detection, an optical carrier frequency is transmitted along with the SSB signal over the optical fiber, and is not suppressed as in other coherent optical transmission systems. This optical carrier can be generated by applying DC bias to the optical modulator 118 away from the null point. However, this DC bias scheme would force the optical modulator 118 to operate in the nonlinear region of its transfer function. As such, the resultant optical SSB signal becomes non-ideal and its transmission is no longer immune to the CD induced fading effect, which can severely degrade detection.
Embodiments are provided herein to resolve this issue and improve direct detection. The embodiments include using a modified transmitter electro-optics (EO) architecture to avoid performance degradation in direct detection. The architecture comprises splitting the laser output in to two paths. A first path is used for modulating the SSB signal using an optical modulator without introducing DC bias, which reduces or eliminates the nonlinear behavior of the modulator transfer function, and thus eliminates the fading effect. A second path is used to provide the carrier frequency (DC carrier). The two paths combine at the output to send a single-side band (SSB) signal combined with the DC carrier. Using this architecture and operation scheme also avoids signal to noise ratio (SNR)/bit error ratio (BER) frequency-dependent fading and improves transmission performance. This architecture and scheme can be used to improve transmission capacity and/or error performance in the presence of residue and/or uncompensated CD from optical fiber transmission. Although the embodiments are described in context of OFDM signals, the embodiments herein can be applied and extended to other optical signals which can be digitally generated.
The transmitter 400 or DSP unit 412 further includes an arc-sin function 410. The arc-sine function 410 is preconfigured and coupled or added to the DSP unit to further improve modulator linearity. The arc-sine function 410 can be used in the transmitter 400 so that the nonlinear mapping is kept ideal or acceptable at the optical modulator 418. Alternatively, the EO architecture of the transmitter 400 can be effective even without using the arc-sin function.
The output of the laser 416 is split into two optical paths, for instance into two optical fibers or any other suitable optical waveguides. Additionally, two optical couplers 490 can be added between the output of the laser 416 and the two paths to control the signal splitting ratio between the two paths to reflect the desired carrier to signal ratio. The splitting is realized by a first coupler 490 at the output of the laser, and the two optical paths are then recombined by a second coupler 490 at the input to the fiber. One path is modulated by the optical modulator 418 to produce a SSB signal, e.g., as described above. The other path is DC biased and serves as the DC carrier after both paths recombine (at the output of the transmitter 400). This EO architecture enables the optical modulator 418 to be biased at null point, so the modulator linearity region can be extended. In this case, NLE and linear equalizer components are not needed, which simplifies the DSP design. For example, a conventional transmitter DSP design similar to the DSP unit 112 can be used.
The CPU 610 may comprise any type of electronic data processor. The memory 620 may comprise any type of system memory such as static random access memory (SRAM), dynamic random access memory (DRAM), synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof, or the like. In an embodiment, the memory 620 may include ROM for use at boot-up, and DRAM for program and data storage for use while executing programs. The mass storage device 630 may comprise any type of storage device configured to store data, programs, and other information and to make the data, programs, and other information accessible via the bus. The mass storage device 630 may comprise, for example, one or more of a solid state drive, hard disk drive, a magnetic disk drive, an optical disk drive, or the like.
The video adapter 640 and the I/O interface 690 provide interfaces to couple external input and output devices to the processing unit. As illustrated, examples of input and output devices include a display 660 coupled to the video adapter 640 and any combination of mouse/keyboard/printer 670 coupled to the I/O interface 690. Other devices may be coupled to the processing unit 601, and additional or fewer interface cards may be utilized. For example, a serial interface card (not shown) may be used to provide a serial interface for a printer.
The processing unit 601 also includes one or more network interfaces 650, which may comprise wired links, such as an Ethernet cable or the like, and/or wireless links to access nodes or one or more networks 680. The network interface 650 allows the processing unit 601 to communicate with remote units via the networks 680. For example, the network interface 650 may provide wireless communication via one or more transmitters/transmit antennas and one or more receivers/receive antennas. In an embodiment, the processing unit 601 is coupled to a local-area network or a wide-area network for data processing and communications with remote devices, such as other processing units, the Internet, remote storage facilities, or the like.
While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.
In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.
