News Column

Patent Application Titled "Tunable Coherent Optical Receiver and Method" Published Online

January 30, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventor Lee, Kun-Jing (Fremont, CA), filed on July 3, 2013, was made available online on January 16, 2014.

No assignee for this patent application has been made.

Reporters obtained the following quote from the background information supplied by the inventors: "The need to transmit ever increasing amounts of data necessitates the development of optical communication systems that combine dense wavelength division multiplexing (DWDM) of optical channels with advanced spectral-efficient modulation formats incorporating phase modulation. One of the data transmission technologies considered is the Dual-Polarization Quadrature Phase-Shift Keying, or 'DP-QPSK', in combination with the coherent detection at the receiver. The DP-QPSK modulation format includes transmitting two orthogonally polarized light signals, each phase-modulated with two quadrature 'I' and 'Q' signals according to the QPSK modulation format. The advantages of this technique, in addition to the spectral efficiency, includes robustness with respect to the chromatic dispersion (CD) and the polarization mode dispersion (PMD) in optical fibers.

"Besides having intrinsically large PMD and CD tolerance, the coherent detection of the phase-modulated light at the receiver can also be used to implement 'colorless receivers' and therefore 'colorless networks', that is networks without specific wavelengths assigned to individual add/drop (A/D) ports; see for example an article 'Combining 40G DP-QPSK with 10G OOK channels on metro/regional networks' by N. L. Swenson, Lightwave.RTM., December 2012. With a coherent detection, the receiver may be able to select a desired channel from multiple received channels simply by tuning a local oscillator (LO) laser without the need for an optical DEMUX, a wavelength selective switch (WSS), or a reconfigurable add-drop module (ROADM), thereby reducing optical loss in the network, greatly simplifying the network, an making it potentially less expensive. This can reduce the number of ROADM/WSS elements required in the network and dramatically simplify network topology. However, wavelength tuning in most commercially available tunable lasers that can be used as local oscillators in a tunable coherent receiver is relatively slow, so that tuning or switching the LO laser wavelength from one DWDM channel to another takes tens of seconds or even minutes. Furthermore, the slow channel switching times make the receiver auto-configuration procedure too long for many real-life network applications, as it may involve scanning the entire channel spectrum to identify all the received channels.

"An object of the present invention is to overcome the shortcomings of the prior art by providing a method for fast channel detection and/or auto-configuration at a coherent receiver, and a tunable optical receiver implementing the method."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventor's summary information for this patent application: "Accordingly, the present invention relates to a method for operating a coherent receiver in a WDM system comprising a plurality of optical transmitters for transmitting a plurality of optical WDM channels carrying high-speed data signals, wherein low-frequency trace tones are used to identify optical channels, wherein the receiver is provided with a trace detector for detecting the presence or absence of the trace tones, and wherein the trace tones detected at the receiver are used to configure the receiver for receiving a particular optical channel.

"In accordance with an aspect of the present invention, the method comprises i) modulating each of the optical WDM channels with a unique low-frequency trace tone, and ii) at the coherent receiver, performing the following operations: a) splitting a received optical signal comprising one or more of the optical WDM channels into first and second optical signal portions; b) directing a first optical signal portion to a coherent detection & demodulation circuit (CDC) comprising a tunable local oscillator (LO) source of an optical LO signal; c) directing a first optical signal portion to a trace tone detection circuit (TTDC) for detecting the presence of one or more low-frequency tones therein; d) using information about the one or more trace tones present in the received optical signal to identify the one or more received optical channels, and to select one of the received optical channels therefrom; e) tuning the LO source to the selected optical channel; and, f) mixing the LO optical signal with the second optical signal portion for coherently de-modulating the selected optical channel.

"An aspect of the present invention relates to a tunable coherent optical receiver for receiving an optical WDM signal which comprises one or more optical channels carrying high-speed data signals, wherein each of the one or more optical channels is modulated with a low-frequency trace tone uniquely associated with said optical channel. The tunable coherent optical receiver comprises the following elements: an optical splitter for splitting a received optical WDM signal into first and second optical signal portions, a coherent detection/demodulation circuit (CDC) coupled to the optical splitter for receiving the first optical signal portion and for selectively and coherently demodulating the high-speed data signal from one of the one or more optical channels, wherein the CDC comprises a tunable local oscillator (LO) source of an optical LO signal, and a trace tone detection circuit (TTDC) coupled to the optical splitter for receiving the second optical signal portion and adopted for detecting the presence of one or more low-frequency trace tones therein and for generating a trace detection signal identifying one or more low-frequency trace tones that are present in the received optical signal. The tunable coherent optical receiver further includes an LO control circuit that is coupled to the tunable source for tuning an optical frequency thereof to a selected optical channel that is selected among the one or more optical channels present in the received optical signal based on the trace presence information.

"According to feature of the present invention, the tunable coherent optical receiver may further include a micro-controller coupled to the LO control circuit for providing thereto a channel selection signal for tuning the LO frequency to the selected optical channel, wherein the microcontroller may comprise hardware or software logic for selecting the selected optical channel based on the trace presence signal received from the TTDC. Accordingly, in this aspect of the invention the tunable coherent receiver is able to auto-configure itself by automatically performing channel discovery and selection upon start-up or a change in the system configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

"The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, in which like elements are indicated with like reference numerals, and wherein:

"FIG. 1 is a schematic diagram illustrating an optical communication link utilizing trace tone modulation and a tunable coherent receiver;

"FIG. 2 is a general block diagram of an optical transmitter utilizing trace tone modulation;

"FIG. 3 is a schematic block diagram of a tunable coherent receiver utilizing trace tone detection for self-tuning or external tuning;

"FIGS. 4A and 4B are schematic block diagrams of two exemplary embodiments of an electronic trace detector;

"FIG. 5 is a schematic block diagram of a trace tone processing circuit including a trace tone decoder;

"FIG. 6 is a schematic diagram illustrating a BFSK decoder for service data decoding

"FIG. 7 is a schematic diagram illustrating a BPSK decoder for service data decoding;

"FIG. 8-11 are flowchart illustrating various embodiments of the method of operating the tunable coherent receiver of FIG. 3 ;

"FIG. 12 is a schematic block diagram of a coherent mixer/detector for DP-QPSK signals."

For more information, see this patent application: Lee, Kun-Jing. Tunable Coherent Optical Receiver and Method. Filed July 3, 2013 and posted January 16, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=2934&p=59&f=G&l=50&d=PG01&S1=20140109.PD.&OS=PD/20140109&RS=PD/20140109

Keywords for this news article include: Patents.

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Source: Politics & Government Week


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