Patent Description:
This section introduces aspects that may help facilitate a better understanding of the disclosure.

Protection switching in optical networks is a subject of great importance to service providers and equipment manufacturers.

As used in the art of protection switching, the term "<NUM>+<NUM> protection" generally refers to a protection-switching architecture in which: (i) an optical splitter is used to bridge the optical signal generated by the transmitter at the head-end of the optical link and then dual feed it on differently routed working and protection optical paths, and (ii) an optical switch is used at the tail-end of the optical link to select one of the two optical paths for the receiver. In the event of the working optical-path failure, the optical switch can be reconfigured to connect the receiver to the protection optical path, thereby restoring the optical signal to the receiver.

A <NUM>:<NUM> protection-switching architecture typically uses a second optical switch instead of the optical splitter at the head-end of the optical link.

In general, it is desirable to restore the optical signal to the receiver with the least amount of service downtime possible.

<CIT> discloses an all optical <NUM>+<NUM> protection unit.

Disclosed herein are various embodiments of an optical end terminal in which protection switching is implemented by using (i) the optical data receiver thereof for detecting a path failure and (ii) the optical data transmitter thereof for signaling the detected path failure to the protection-switching circuit. In an example embodiment, the optical data receiver is configured to detect a path failure based on the presence of certain data-recovery errors. The optical data transmitter is operatively connected to the optical data receiver and configured to generate an in-band dither tone of a predetermined frequency in response to such failure detection. The protection-switching circuit is configured to (i) detect in-band dither tones in the optical signals passing therethrough and (ii) connect the optical data receiver to the protection path instead of the working path in response to detecting the in-band dither tone of the predetermined frequency.

An example embodiment of an optical end terminal configured to operate in this manner is advantageously capable of restoring the optical signal to the receiver within approximately <NUM> or faster.

In some embodiments, the protection-switching circuit is further configured to use some in-band dither tones to monitor the status of various wavelength channels and to use the monitoring results to make protection-switching decisions.

According to an example embodiment, provided is an apparatus according to claim.

Other aspects, features, and benefits of various disclosed embodiments will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which:.

For illustration purposes and without any implied limitations, various example embodiments are described herein in reference to the <NUM>+<NUM> protection-switching architecture. Based on the provided description, a person of ordinary skill in the art will understand, without undue experimentation, how to modify the disclosed circuits and control methods to arrive at embodiments suitable for use in communication systems designed using the <NUM>:<NUM> protection-switching architecture.

<FIG> shows a conceptual view of a network-protection system <NUM> in which various embodiments can be practiced. System to provide protection switching for bidirectional links, wherein an individual optical fiber is configured to carry optical signals in one direction. For example, as indicated in <FIG>, each of optical fibers <NUM>A and <NUM>B is configured to carry Eastward-propagating optical signals, whereas each of optical fibers <NUM>A and <NUM>B is configured to carry Westward-propagating optical signals.

In some embodiments, optical fibers <NUM>A and <NUM>A may be parts of a fiber bundle, e.g., a fiber-optic cable <NUM>A. Optical fibers <NUM>B and <NUM>B may similarly be parts of a fiber-optic cable <NUM>B. Fiber-optic cables <NUM>A and <NUM>B may be laid along different respective physical conduits that may have significant lateral separation from one another in at least some locations.

In an example embodiment, optical fibers <NUM>A,B and <NUM>A,B can be connected between protection circuits <NUM><NUM> and <NUM><NUM> as indicated in <FIG>. In some embodiments, a protection circuit <NUM>i (where i=<NUM>, <NUM>) can be implemented as a circuit pack or a functional (e.g., pluggable) module.

Protection circuit <NUM>i (where i=<NUM>, <NUM>) comprises an optical (e.g., <NUM>-dB) splitter <NUM>i, a <NUM>×<NUM> optical switch <NUM>i, and six optical ports <NUM>i, <NUM>i, <NUM>i, <NUM>i, <NUM>i, and <NUM>i. Optical ports <NUM>i, <NUM>i, and <NUM>i are input ports. Optical ports <NUM>i, <NUM>i, and <NUM>i are output ports. Optical ports <NUM>i, <NUM>i, and <NUM>i are internally connected to optical splitter <NUM>i. Optical ports <NUM>i, <NUM>i, and <NUM>i are internally connected to optical switch <NUM>i. The configuration of optical switch <NUM>i can be changed using a control signal <NUM>i.

The designation of two redundant optical paths between protection circuits <NUM><NUM> and <NUM><NUM> as the "working path" and the "protection path" is not absolute and may depend on the configuration of optical switches <NUM><NUM> and <NUM><NUM>. For example, in the shown configuration of optical switches <NUM><NUM> and <NUM><NUM>, optical fiber <NUM>A is configured to provide the "working path" for the Eastward-propagating signals, while optical fiber <NUM>A is configured to provide the "working path" for the Westward-propagating signals. Optical fibers <NUM>B and <NUM>B are configured to provide "protection paths. " If optical switch <NUM><NUM> is flipped, then optical fiber <NUM>B may be referred to as being configured to provide the "working path" for the Westward-propagating signals. If optical switch <NUM><NUM> is flipped, then optical fiber <NUM>B may be referred to as being configured to provide the "working path" for the Eastward-propagating signals.

<FIG> shows a block diagram of an optical end terminal <NUM> according to an embodiment. Terminal <NUM> comprises a protection circuit <NUM> connected to an optical transceiver <NUM>. For illustration purposes, protection circuit <NUM> is shown in <FIG> as being connected to optical fibers <NUM>A,B and <NUM>A,B in a manner corresponding to that of protection circuit <NUM><NUM> (<FIG>). Based on the provided description, a person of ordinary skill in the art will understand how to connect protection circuit <NUM> of terminal <NUM> for other possible uses and/or in alternative network configurations.

Transceiver <NUM> comprises an optical receiver <NUM>, an optical transmitter <NUM>, a photodetector (e.g., photodiode) PD4, and an electronic controller <NUM>, all operatively connected, e.g., as explained below. Receiver <NUM> is optically connected to an optical input port <NUM> of transceiver <NUM>, which is externally optically connected, e.g., by way of an optical fiber <NUM>, to receive optical signals from optical output port <NUM> of protection circuit <NUM>. Transmitter <NUM> is optically connected to an optical output port <NUM> of transceiver <NUM>, which is externally optically connected, e.g., by way of an optical fiber <NUM>, to transmit optical signals to optical input port <NUM> of protection circuit <NUM>. In an example embodiment, the distance between transceiver <NUM> and protection circuit <NUM> is relatively short, e.g., smaller than ca. <NUM>, and can typically be on the order of <NUM>.

Optical receiver <NUM> is a tunable optical receiver that is capable of: (i) selecting for detection any wavelength channel of a wavelength-division-multiplexed (WDM) signal applied to optical input port <NUM> while rejecting other wavelength channels thereof; (ii) demodulating and decoding the selected wavelength channel to recover the payload data transmitted thereby; (iii) detecting certain signal-failure (SF) conditions; and (iv) communicating to transmitter <NUM> instances of such detection.

In an example embodiment, a WDM signal applied to optical input port <NUM> has N wavelength channels, each associated with the corresponding optical carrier wavelength λi, where i=<NUM>, <NUM>,. This is indicated in <FIG> by the mathematical expression ∑λi(focm,i). The parenthetical notation (focm,i) used therein indicates that, in addition to payload-data modulation, the carrier wavelength λi is modulated with a dither tone (sometimes referred to as a pilot subcarrier) having the frequency focm,i that uniquely identifies the corresponding wavelength channel. The number N can be, e.g., on the order of one hundred.

For example, a modulated carrier wavelength λi that carries payload (e.g., user) data at a bit rate greater than about <NUM> Gbit/s can additionally be modulated using one or more relatively low-frequency (e.g., <<NUM>) dither tones. Some of the dither tones can be configured to provide wavelength-channel identification, e.g., as described in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, and <CIT>. Some other dither tones can be configured to carry control messages and/or some service, operation, and administration messages and information (i.e., non-payload data), e.g., as described in <CIT> and <CIT>. The depth of amplitude modulation corresponding to a dither tone can be relatively small, e.g., < <NUM>%, in order not to have a noticeable adverse effect on the transmission of payload data. In some embodiments, the non-payload data and/or control messages may be encoded, e.g., in the phase of the dither tone. In some embodiments, binary signaling can be implemented using the presence and absence of the corresponding dither tone.

A person of ordinary skill in the art will understand that dither tones generated in the above-indicated manner are transmitted in band and are generally non-intrusive. As used herein, the term "non-intrusive" should be construed as meaning without interrupting the in-band optical flow of payload data. The term "in-band" means using one or more carrier wavelengths allocated for payload-data transport in system <NUM>. Dither tones that are spectrally located within the bandwidth allocated to a wavelength channel are considered to be in-band. The term "out-of-band" should be construed as indicating the use of communications means that do not rely on or utilize any of the optical carrier wavelengths λi or any of their subcarriers. An example of such out-of-band communications may rely on the Internet Protocol (IP) implemented over a wireline, wireless, or optical-supervisory-channel (OSC) connection. Note that, despite being "optical," an OSC connection is not considered to be an in-band wavelength channel because an optical supervisory channel does not typically transport user data.

In an example embodiment, optical receiver <NUM> is configured to detect one or more SF conditions, e.g., from the following non-exclusive list: (i) bit-error rate (BER) is greater than a fixed BER threshold value; (ii) a number of frames that the employed forward-error-correction (FEC) code is unable to error-correct is greater than a fixed threshold number; (iii) an out-of-frame (OOF) event; (iv) duration of an OOF event is greater than a fixed threshold time; and (v) complete loss of signal. An OOF event occurs, e.g., when the receiver is unable to recognize the organization of the data stream. Complete loss of signal occurs, e.g., when substantially no light or insufficient optical power is being received by the receiver due to a physical disconnection, fiber break, or component failure.

Example optoelectronic devices that can be used to implement optical receiver <NUM> are described below in reference to <FIG>. Some embodiments of optical receiver <NUM> may benefit from the use of at least some circuits and devices disclosed in <CIT> and <CIT>.

Optical transmitter <NUM> is a tunable optical transmitter that is capable of: (i) generating a modulated optical output signal having a carrier wavelength λo corresponding to any selected wavelength channel used for payload-data transport in the relevant direction or degree of system <NUM>; (ii) performing optical modulation in a manner that causes the selected wavelength channel to carry payload data and at least two in-band dither tones; (iii) generating one of the in-band dither tones in a manner that provides identification of the selected wavelength channel; (iv) receiving from receiver <NUM> an alert when an instance of an SF condition is detected; and (iv) generating another one of the in-band dither tones to carry a protection-switching control message generated in response to the received alert.

The two dither tones transmitted in-band by transmitter <NUM> using carrier wavelength λo are indicated in <FIG> using the parenthetical notation (focm,fps) placed after the symbol λo, wherein focm denotes the frequency of the in-band dither tone that uniquely identifies the wavelength channel corresponding to carrier wavelength λo, and fps denotes the frequency of the in-band dither tone that is used to carry the protection-switching control message.

In an example embodiment, the protection-switching signaling can be implemented such that (i) the presence of the fps dither tone indicates the presence of an SF condition at receiver <NUM>, and (ii) the absence of the fps dither tone indicates that the payload data are being received by receiver <NUM> in a normal manner, e.g., without any SF conditions being in effect.

Example optoelectronic devices that can be used to implement optical transmitter <NUM> are described below in reference to <FIG>.

In an example embodiment, photodetector PD4 enables monitoring of the modulated optical output signal λo(focm, fps) generated by transmitter <NUM>, based on which monitoring transceiver <NUM> can provide a corresponding input to controller <NUM>. Controller <NUM> carries out signal processing, performs certain control and communication functions, and executes certain logic steps that enable transceiver <NUM> to generate and transmit a protection-switching control message on the fps dither tone of carrier wavelength λo. Example circuits and signal processing associated with controller <NUM> are described below in reference to <FIG>.

Protection circuit <NUM> of terminal <NUM> comprises optical splitter <NUM>, optical switch <NUM>, and optical ports <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, all of which have been described above in reference to <FIG>.

Protection circuit <NUM> further comprises photodetectors (e.g., photodiodes) PD1, PD2, and PD3 and an electronic controller <NUM>. Each of photodetectors PD1 and PD2 enables monitoring of the in-band dither tones focm,i of the corresponding optical signal ∑λi(focm,i) received at the corresponding one of optical input ports <NUM> and <NUM>, based on which monitoring photodetectors PD1 and PD2 can provide corresponding electrical inputs to controller <NUM>. Photodetector PD3 enables reception of protection-switching control messages transmitted on the fps dither tone of the optical signal λo(focm, fps) received at optical input port <NUM>. Controller <NUM> carries out signal processing, performs certain control and communication functions, and executes certain logic steps in response to the inputs received from photodetectors (e.g., photodiodes) PD1, PD2, and PD3 to generate and apply to optical switch <NUM> an appropriate control signal <NUM> (also see <FIG>). Example circuits and signal processing that can be used to implement protection circuit <NUM> are described in more detail below in reference to <FIG>.

<FIG> shows a block diagram of optical receiver <NUM> according to an embodiment. As shown, receiver <NUM> is a coherent optical receiver configured to: (i) receive an optical WDM signal <NUM>, such as the optical signal ∑λi(focm,i) applied to optical input port <NUM> by protection circuit <NUM>; (ii) select for detection one wavelength channel of signal <NUM> while rejecting other wavelength channels thereof; (iii) convert the light of the selected wavelength channel into one or more electrical signals <NUM>; (iv) process a digital form of electrical signals <NUM> to recover payload data <NUM> carried by the selected wavelength channel of signal <NUM>; and (v) assert an SF control signal <NUM> if an SF condition is detected, e.g., during the processing of electrical signals <NUM>.

The shown embodiment of receiver <NUM> is configured to receive and process an optical input signal <NUM> that is not polarization-division multiplexed. However, a person of ordinary skill in the art will understand, without undue experimentation, how to modify receiver <NUM> for handling polarization-division multiplexed signals.

Receiver <NUM> comprises a tunable optical local-oscillator (OLO) source <NUM> configured to generate an OLO signal <NUM> whose carrier wavelength λLO can be changed in response to a control signal <NUM>. In an example embodiment, OLO source <NUM> can be implemented using a tunable laser. Control signal <NUM> can be generated such as to cause the output wavelength λLO generated by the tunable laser to be approximately the same as the carrier wavelength λi of the selected wavelength channel of signal <NUM>.

Receiver <NUM> further comprises an optical hybrid <NUM> configured to receive optical signals <NUM> and <NUM>.

As used herein, the term "optical hybrid" refers to an optical mixer designed to mix a first optical input signal having a carrier frequency and a second optical input signal having approximately the same (e.g., to within ±<NUM>) carrier frequency to generate a plurality of mixed optical signals corresponding to different relative phase shifts between the two optical input signals. An optical <NUM>-degree hybrid is a particular type of an optical hybrid that is designed to produce at least four mixed optical signals corresponding to the relative phase shifts between the two optical input signals of approximately <NUM>, <NUM>, <NUM>, and <NUM> degrees, respectively (e.g., to within an acceptable tolerance). Depending on the intended application, the acceptable relative phase-shift tolerances can be, e.g., to within ±<NUM> degrees or ±<NUM> degrees, etc. A person of ordinary skill in the art will understand that each of the relative phase shifts is defined without accounting for a possible additional phase shift that is an integer multiple of <NUM> degrees. A dual-polarization optical hybrid operates to perform the above-indicated optical signal mixing on a per-polarization basis.

In an example embodiment, optical hybrid <NUM> is an optical <NUM>-degree hybrid having input ports S and R and output ports <NUM>-<NUM>. Input port S is configured to receive optical input signal <NUM>. Input port R is configured to receive an OLO signal <NUM>. Optical hybrid <NUM> operates in a conventional manner to mix signals <NUM> and <NUM> to generate four mixed (e.g., optical interference) signals <NUM><NUM>-<NUM><NUM> at output ports <NUM>-<NUM>, respectively. Optical signals <NUM><NUM>-<NUM><NUM> are then detected by four photodetectors (e.g., photodiodes) <NUM><NUM>-<NUM><NUM>. The resulting electrical signals generated by photodiodes <NUM><NUM>-<NUM><NUM> are electrical signals <NUM><NUM>-<NUM><NUM> that are converted into digital form using an analog-to-digital converter (ADC) <NUM> and processed using a digital signal processor (DSP) <NUM> to generate data stream <NUM> and assert or de-assert SF control signal <NUM>.

In some embodiments, photodiodes <NUM><NUM>-<NUM><NUM> may be configured to operate, e.g., as two balanced detectors, each of the balanced detectors having a respective pair of the photodiodes.

In some embodiments, optical hybrid <NUM> can be replaced by any suitable optical mixer, e.g., an optical coupler. Such an optical mixer may have fewer or more than four optical output ports and/or more than two optical input ports.

<FIG> shows a block diagram of optical receiver <NUM> according to an alternative embodiment. As shown in <FIG>, receiver <NUM> is a tunable optical receiver designed for receiving intensity-modulated optical signals that is configured to: (i) receive an optical WDM signal <NUM>, e.g., the optical signal ∑λi(focm,i) applied to optical input port <NUM> by protection circuit <NUM>; (ii) select for detection one wavelength channel of signal <NUM> while rejecting other wavelength channels thereof; (iii) convert the light of the selected wavelength channel into an electrical signal <NUM>; (iv) process a digital form of electrical signal <NUM> to recover payload data <NUM> carried by the selected wavelength channel of signal <NUM>; and (v) assert an SF control signal <NUM> if an SF condition is detected, e.g., during the processing of electrical signal <NUM>.

The embodiment of receiver <NUM> shown in <FIG> comprises a tunable optical filter <NUM> having an optical pass band whose spectral position can be changed in response to a control signal <NUM>. Control signal <NUM> can be generated such as to cause the center wavelength of the optical pass band to be approximately aligned with the carrier wavelength λi of the selected wavelength channel of signal <NUM>. The spectral width of the optical pass band can be such as to cause the light of the other wavelength channels of signal <NUM> to be substantially blocked from being present in a filtered optical signal <NUM> produced by filter <NUM>.

The embodiment of receiver <NUM> shown in <FIG> further comprises a photodetector (e.g., photodiode) <NUM> configured to convert filtered optical signal <NUM> into electrical signal <NUM>. Electrical signal <NUM> is then converted into digital form by an ADC <NUM> and processed using a DSP <NUM> to generate data stream <NUM> and assert or de-assert SF control signal <NUM>.

<FIG> shows a block diagram of optical transmitter <NUM> according to an embodiment. As shown, optical transmitter <NUM> comprises a tunable laser <NUM> configured to generate an optical output beam <NUM> whose carrier wavelength λo can be changed in response to a control signal <NUM>. Control signal <NUM> can be generated such as to cause the carrier wavelength λo to be nominally the same as the carrier wavelength of the selected wavelength channel for which an optical output signal <NUM> is intended.

Optical transmitter <NUM> further comprises an optical modulator <NUM> configured to generate optical output signal <NUM> by modulating optical beam <NUM> in response to an electrical drive signal <NUM>. Depending on the embodiment, optical modulator <NUM> can be an IQ modulator, an amplitude (e.g., intensity) modulator, or a phase modulator. Electrical drive signal <NUM> is generated by a digital-to-analog converter (DAC) <NUM> configured to convert into analog form a digital drive signal <NUM>.

In an example embodiment, the digital circuitry configured to apply digital drive signal <NUM> to DAC <NUM> comprises digital drivers <NUM> and <NUM> and a digital signal combiner <NUM>.

Digital driver <NUM> operates to generate a digital drive signal <NUM> configured to cause optical output signal <NUM> to carry payload data <NUM>. The corresponding modulation speed may be relatively high, e.g., ><NUM>.

Digital driver <NUM> operates to generate a digital drive signal <NUM> configured to cause optical output signal <NUM> to carry one or more in-band dither tones in addition to the payload data. For example, digital driver <NUM> may operate to generate a first in-band dither tone in response to a control signal <NUM> in a manner that causes the first dither tone to provide appropriate identification of the wavelength channel corresponding to the carrier wavelength λo. A person of ordinary skill in the art will understand that control signals <NUM> and <NUM> may be generated in a coordinated manner. Digital driver <NUM> may further operate to generate a second in-band dither tone in response to a control signal <NUM> in a manner that causes the second dither tone to carry a protection-switching control message. The modulation speeds corresponding to the dither tones may be relatively low, e.g., <<NUM>.

Digital signal combiner <NUM> operates to combine digital drive signals <NUM> and <NUM>, thereby generating digital drive signal <NUM>.

Digital and analog drivers, at least some of which can be used in alternative embodiments of transmitter <NUM> for imprinting in-band dither tones onto optical output signal <NUM>, are disclosed, e.g., in the above-cited <CIT>, <CIT>, <CIT>, <CIT>, <CIT>,<CIT>,<CIT>, and <CIT>.

<FIG> shows a block diagram of an optoelectronic circuit <NUM> that can be used to implement a portion of transceiver <NUM> (<FIG>) according to an embodiment. Circuit <NUM> includes controller <NUM> operatively connected to: (i) an embodiment of receiver <NUM> shown in <FIG> or <FIG>, and (ii) transmitter <NUM> shown in <FIG>. Controller <NUM> is also operatively connected to a memory <NUM> configured to store therein and/or provide configuration information that is relevant to properly executing the pertinent control functions of the controller. In an example embodiment, controller <NUM> can be programmed to implement a control method <NUM> that is described below in reference to <FIG>.

In circuit <NUM>, controller <NUM> is configured to receive three input signals and generate five output signals.

The three input signals include digital signals <NUM>, <NUM> (or <NUM>), and <NUM> (or <NUM>). If controller <NUM> is connected to an embodiment of receiver <NUM> shown in <FIG>, then the last two input signals are digital signals <NUM> and <NUM> (see <FIG>). Alternatively, if controller <NUM> is connected to an embodiment of receiver <NUM> shown in <FIG>, then those two input signals are digital signals <NUM> and <NUM> (see <FIG>).

The five output signals include control signals <NUM> (or <NUM>), <NUM>, <NUM>, and <NUM> and the data stream <NUM>. If controller <NUM> is connected to an embodiment of receiver <NUM> shown in <FIG>, then the first control signal is signal <NUM> (see <FIG>). Alternatively, if controller <NUM> is connected to an embodiment of receiver <NUM> shown in <FIG>, then the first control signal is signal <NUM> (see <FIG>).

Circuit <NUM> generates digital signal <NUM> to enable controller <NUM> to monitor optical output signal <NUM> (see <FIG>), e.g., for implementing feedback-based control of transmitter <NUM>. The corresponding circuit chain comprises photodetector PD4 (also see <FIG>), a transimpedance amplifier (TIA) <NUM>, a band-pass filter (BPF) <NUM>, and an ADC <NUM>. Photodetector PD4 is connected to receive light through an optical tap <NUM> coupled to optical output port <NUM> of transceiver <NUM> (also see <FIG>). A resulting electrical signal <NUM> generated by photodetector PD4 is amplified using TIA <NUM> and filtered using BPF <NUM>. ADC <NUM> then converts a resulting filtered electrical signal <NUM> into digital form, thereby generating digital signal <NUM>.

The frequency characteristics of BPF <NUM> and the bandwidth and sampling speed of ADC <NUM> are selected such as to enable controller <NUM> to use digital signal <NUM> for extracting and monitoring the relevant characteristics of the in-band dither tones imprinted onto optical output signal <NUM> by transmitter <NUM> in response to control signals <NUM> and <NUM> (also see <FIG>). For example, the modulation depths and frequencies of individual dither tones can be monitored in this manner. Based on the monitoring results, controller <NUM> can adjust control signals <NUM> and <NUM> applied to transmitter <NUM> as appropriate or necessary.

<FIG> shows a flowchart of a control method <NUM> that can be used in controller <NUM> according to an embodiment.

At step <NUM> of method <NUM>, controller <NUM> retrieves from memory <NUM> the relevant configuration parameters that can be used to configure transceiver <NUM> to: (i) select for detection a desired wavelength channel of the optical WDM signal <NUM> (<FIG>) or <NUM> (<FIG>) applied to optical input port <NUM> of the transceiver; and (ii) select a desired wavelength channel for optical output signal <NUM>.

At step <NUM>, controller <NUM> uses the information retrieved at step <NUM> to generate control signals <NUM> (or <NUM>), <NUM>, and <NUM>. Control signal <NUM> (or <NUM>) configures receiver <NUM> to select for detection the desired wavelength channel while rejecting other wavelength channels, e.g., as explained above in reference to <FIG>. Control signal <NUM> configures transmitter <NUM> to generate optical output signal <NUM> for the desired wavelength channel, e.g., as explained above in reference to <FIG> and <FIG>. Control signal <NUM> configures transmitter <NUM> to imprint onto optical output signal <NUM> one or more in-band dither tones that uniquely identify that optical output signal and the corresponding wavelength channel for downstream receivers.

At step <NUM>, controller <NUM> checks the state of the SF control signal <NUM> (or <NUM>). If the SF control signal is asserted, then the processing of method <NUM> is directed to step <NUM>. If the SF control signal is de-asserted, then the processing of method <NUM> is directed to step <NUM>.

At step <NUM>, controller <NUM> generates control signal <NUM> in a manner that configures transmitter <NUM> to imprint onto optical output signal <NUM> the fps dither tone used for protection-switching (PS) signaling. As already indicated above, in an example embodiment, the presence of the fps dither tone in signal <NUM> indicates the presence of an SF condition at receiver <NUM>.

At step <NUM>, controller <NUM> generates control signal <NUM> in a manner that configures transmitter <NUM> not to imprint onto optical output signal <NUM> the fps dither tone. For example, if prior to step <NUM> the fps dither tone was present, then it is removed at step <NUM>. If prior to step <NUM> the fps dither tone was absent, then no changes are made at step <NUM><NUM><NUM>, and thefps dither tone remains absent. As already indicated above, in an example embodiment, the absence of the fps dither tone in signal <NUM> indicates that the payload data are being received by receiver <NUM> in a normal manner, e.g., without any SF conditions being in effect.

After the completion of step <NUM> or <NUM>, the processing of method <NUM> is directed back to step <NUM>.

Method <NUM> is typically re-executed starting from step <NUM>, e.g., when either the input channel selection or the output channel selection is changed in transceiver <NUM>; and receiver <NUM> and/or transmitter <NUM> have been retuned accordingly.

<FIG> shows a block diagram of protection circuit <NUM> (<FIG>) according to an embodiment. Some of the circuit elements of protection circuit <NUM> shown in <FIG> have already been described above, e.g., in reference to <FIG>. The description of those circuit elements is not repeated here. Rather, the description of <FIG> focuses on the circuit elements of protection circuit <NUM> first shown in <FIG>.

Each of photodetectors PD1-PD3 is connected to receive light through a respective one of optical taps <NUM><NUM>-<NUM><NUM>, each coupled to a respective optical port of circuit <NUM> (also see <FIG>). More specifically, optical taps <NUM><NUM>-<NUM><NUM> are connected to optical ports <NUM>, <NUM>, and <NUM>, respectively. The electrical output signals generated by photodetectors PD1-PD3 in response to the light received through optical taps <NUM><NUM>-<NUM><NUM> are amplified using TIAs <NUM><NUM>-<NUM><NUM> and filtered using BPFs <NUM><NUM>-<NUM><NUM>, respectively. A copy of the electrical output signal <NUM> generated by TIA <NUM><NUM> is filtered using a BPF <NUM>. An ADC <NUM> then operates to convert a filtered electrical signal <NUM> generated by BPF <NUM> into digital form, thereby generating a digital input signal <NUM> for controller <NUM>.

An ADC <NUM> is configured to receive filtered electrical signals <NUM><NUM>-<NUM><NUM> generated by BPFs <NUM><NUM>-<NUM><NUM>, respectively. ADC <NUM> operates to sample signals <NUM><NUM>-<NUM><NUM> on a rotating schedule to generate a digital input signal <NUM> for controller <NUM> in a manner that enables the controller to use that input signal for extracting and monitoring the relevant characteristics of the corresponding optical signals. More specifically, the frequency characteristics of BPFs <NUM><NUM>-<NUM><NUM> and the bandwidth and sampling speed of ADC <NUM> are selected such as to enable controller <NUM> to use digital signal <NUM> for extracting and monitoring (i) the dither tones focm,i of the optical signals received at optical input ports <NUM> and <NUM> and (ii) the dither tone focm of the optical signal received at optical input port <NUM>. In an example embodiment, controller <NUM> can be configured to use digital signal <NUM> to update, e.g., every second, the information about the spectral content of (e.g., the list of carrier wavelengths and/or wavelength channels corresponding to) each of those optical signals.

The frequency characteristics of BPF <NUM> and the bandwidth and sampling speed of ADC <NUM> are selected such as to enable controller <NUM> to use digital signal <NUM> for determining the presence or absence of the fps dither tone in the optical signal received at optical input port <NUM>. In an example embodiment, controller <NUM> can be configured to use digital signal <NUM> to detect the events of appearance and disappearance of the fps dither tone in the optical signal received at optical input port <NUM> with a relatively short response time, e.g., faster than about <NUM>.

Controller <NUM> is further configured to use the information derived from digital input signals <NUM> and <NUM> and the relevant information stored in a memory <NUM> to generate control signal <NUM> for switch <NUM>. In an example embodiment, controller <NUM> can be programmed to execute for this purpose a control method <NUM> that is described below in reference to <FIG>.

At step <NUM> of method <NUM>, controller <NUM> stores in memory <NUM> the reference information about the optical signals expected to be received at optical input ports <NUM>, <NUM>, and <NUM> of protection circuit <NUM>. The reference information may include but is not limited to: (i) a list of carrier wavelengths and/or wavelength channels for each optical signal; (ii) a list of the dither tone frequencies focm,i and focm assigned to different wavelength channels; and (iii) the dither tone frequency fps assigned to PS signaling. This reference information can be loaded into memory <NUM> at the initial system setup and/or using appropriate control and/or OAM channels, where OAM stands for operations, administration, and management.

At step <NUM>, controller <NUM> uses digital input signal <NUM> to determine the wavelength channels currently present at optical input ports <NUM>, <NUM>, and <NUM>. This determination can be done, e.g., by detecting the presence of the corresponding dither tones focm,i and focm as already indicated above and using the reference information stored in memory <NUM>.

At step <NUM>, controller <NUM> compares the lists of the wavelength channels present at optical input ports <NUM> and <NUM> and stores the comparison result in memory <NUM>. In an example embodiment, the comparison result can be reduced to a form of a binary "ports status" value. This binary value can be "TRUE" or "FALSE," wherein: (i) the "TRUE" value indicates that optical input ports <NUM> and <NUM> receive the same set of protected wavelength channels, and (ii) the "FALSE" value indicates that optical input ports <NUM> and <NUM> receive the sets of protected wavelength channels that differ in at least one wavelength channel.

In an example embodiment, steps <NUM> and <NUM> can be executed in a loop and repeated, e.g., every second or with any other selected frequency.

At step <NUM>, controller <NUM> uses digital input signal <NUM> to detect the presence/absence of the fps dither tone at optical input port <NUM>. If the fps dither tone is not present, then step <NUM> is repeated after a predetermined fixed time delay. If the fps dither tone is detected, then the processing of method <NUM> is directed to step <NUM>.

At step <NUM>, controller <NUM> reads out the current "ports status" value from memory <NUM>. If the read value is "TRUE," then the processing of method <NUM> is directed to step <NUM>. If the read value is "FALSE," then the processing of method <NUM> is directed to step <NUM>.

At step <NUM>, controller <NUM> generates control signal <NUM> that causes switch <NUM> to flip, thereby changing the optical input port to which optical output port <NUM> is connected. The processing of method <NUM> is then directed back to step <NUM>. In an example embodiment, the redirection to step <NUM> can be performed after an appropriate time delay.

At step <NUM>, controller <NUM> generates a system-failure alert and transmits it to the competent network entity.

In an example embodiment, end terminal <NUM> configured to operate in the above-described manner is advantageously capable of restoring the optical signal to receiver <NUM> within approximately <NUM> or even faster.

Although various example embodiments have been described above in reference to the <NUM>+<NUM> protection-switching architecture, alternative embodiments adapted for use under the <NUM>:<NUM> protection-switching architecture are also possible. As known in the art, the <NUM>:<NUM> protection-switching architecture can be obtained by replacing splitter <NUM> (<FIG>) by an appropriately controlled <NUM>×<NUM> optical switch. A person of ordinary skill in the art will understand, without undue experimentation, how to modify the disclosed circuits and control methods to arrive at an embodiment suitable for use under the <NUM>:<NUM> protection-switching architecture.

According to an example embodiment disclosed above, e.g., in the summary section and/or in reference to any one or any combination of some or all of <FIG>, provided is an apparatus (e.g., <NUM>, <FIG>) comprising: a protection circuit (e.g., <NUM>, <FIG>); an optical data receiver (e.g., <NUM>, <FIG>, <FIG>, <FIG>) connected to receive, by way of the protection circuit, an optical input signal (e.g., <NUM>, <FIG>; <NUM>, <FIG>) modulated with first data (e.g., <NUM>, <FIG>; <NUM>, <FIG>); and an optical data transmitter (e.g., <NUM>, <FIG>, <NUM>) connected to transmit, by way of the protection circuit, an optical output signal (e.g., <NUM>, <FIG>) modulated with second data (e.g., <NUM>, <FIG>); wherein the optical data transmitter is configured to generate the optical output signal to carry a first dither tone (e.g., fps, <FIG>) in response to being signaled by the optical data receiver of a path failure; and wherein the protection circuit is configured to change (e.g., using <NUM>, <FIG>) an optical path through which the optical data receiver receives the optical input signal in response to detecting the first dither tone in the optical output signal.

In some embodiments of the above apparatus, the optical data receiver is configured to detect a presence of the path failure based on the optical input signal.

In some embodiments of any of the above apparatus, the protection circuit comprises: a first optical input fiber port (e.g., <NUM>, <FIG>); a second optical input fiber port (e.g., <NUM>, <FIG>); and an optical switch (e.g., <NUM>, <FIG>) configured to selectively connect the optical data receiver to the first optical input fiber port or to the second optical input fiber port.

In some embodiments of any of the above apparatus, the protection circuit is further configured to: (i) detect one or more dither tones (e.g., focm,i, <FIG>) in an optical signal at the first optical input fiber port; and (ii) detect one or more dither tones (e.g., focm,i, <FIG>) in an optical signal at the second optical input fiber port.

In some embodiments of any of the above apparatus, the protection circuit is further configured to: (i) identify wavelength channels of the optical signal at the first optical input fiber port using the one or more dither tones detected therein; and (ii) identify wavelength channels of the optical signal at the second optical input fiber port using the one or more dither tones detected therein.

In some embodiments of any of the above apparatus, the protection circuit further comprises an electronic controller (e.g., <NUM>, <FIG>, <FIG>) configured to generate a control signal (e.g., <NUM>, <FIG>) in response to the first dither tone being detected in the optical output signal; and wherein the optical switch is configured to change an optical input fiber port through which the optical data receiver receives the optical input signal in response to the control signal.

In some embodiments of any of the above apparatus, the electronic controller is further configured to generate the control signal (e.g., <NUM>, <FIG>) if a set of dither tones detected in an optical signal at the first optical input fiber port matches a set of dither tones detected in an optical signal at the second optical input fiber port.

In some embodiments of any of the above apparatus, the apparatus further comprises an optical splitter (e.g., <NUM>, <FIG>) configured to: (i) split the optical output signal into a first portion and a second portion; (ii) direct the first portion to a first optical output fiber port (e.g., <NUM>, <FIG>); and (iii) direct the second portion to a second optical output fiber port (e.g., <NUM>, <FIG>).

In some embodiments of any of the above apparatus, the optical data receiver is a tunable optical receiver configured to: (i) receive data transmitted using a selected wavelength channel of the optical input signal; and (ii) reject signals corresponding to one or more other wavelength channels of the optical input signal.

In some embodiments of any of the above apparatus, the optical data receiver comprises a tunable optical local-oscillator source (e.g., <NUM>, <FIG>).

In some embodiments of any of the above apparatus, the optical data receiver comprises a tunable optical band-pass filter (e.g., <NUM>, <FIG>).

In some embodiments of any of the above apparatus, the optical data transmitter comprises a tunable laser (e.g., <NUM>, <FIG>) configured to change a carrier wavelength (e.g., λo, <FIG>) of the optical output signal.

In some embodiments of any of the above apparatus, the optical data transmitter is further configured to generate the optical output signal to carry a second dither tone (e.g., focm, <FIG>) different from the first dither tone; and wherein the optical data transmitter is configured to change a frequency of the second dither tone if the tunable laser changes the carrier wavelength.

In some embodiments of any of the above apparatus, the optical data transmitter is further configured to generate the optical output signal to carry a second dither tone (e.g., focm, <FIG>) different from the first dither tone.

In some embodiments of any of the above apparatus, the optical data receiver is configured to: (i) process the optical input signal to recover the first data; and (ii) signal (e.g., using SF, <FIG>; <NUM>, <FIG>; <NUM>, <FIG>) the path failure to the optical data transmitter in an event of a data-recovery error.

In some embodiments of any of the above apparatus, the optical data receiver is configured to signal the path failure if one or more of the following data-recovery errors occur: (i) a bit-error rate (BER) that is greater than a fixed BER threshold value; (ii) a number of frames that a used forward-error-correction code is unable to error-correct is greater than a fixed threshold number; (iii) unrecognizable organization of a received data stream; (iv) duration of time during which the optical data receiver is unable to recognize the organization of the received data stream is greater than a fixed threshold time; and (v) a loss of signal.

In some embodiments of any of the above apparatus, the protection circuit is configured to detect dither tones in two or more different optical signals passing therethrough (e.g., using PD1-PD3, <FIG>, <FIG>).

According to another example embodiment disclosed above, e.g., in the summary section and/or in reference to any one or any combination of some or all of <FIG>, provided is an apparatus (e.g., <NUM>, <FIG>) comprising: an optical data receiver (e.g., <NUM>, <FIG>, <FIG>, <FIG>) connected to receive, by way of an optical switch (e.g., <NUM>, <FIG>), an optical input signal (e.g., <NUM>, <FIG>; <NUM>, <FIG>) modulated with first data (e.g., <NUM>, <FIG>; <NUM>, <FIG>), the optical switch being configured to selectively connect the optical data receiver to a first optical input fiber port (e.g., <NUM>, <FIG>) or to a second optical input fiber port (e.g., <NUM>, <FIG>); an optical data transmitter (e.g., <NUM>, <FIG>, <FIG>) configured to generate an optical output signal (e.g., <NUM>, <FIG>) modulated with second data (e.g., <NUM>, <FIG>) to carry a first dither tone (e.g., fps, <FIG>) in response to being signaled by the optical data receiver of a path failure; and an electronic controller (e.g., <NUM>, <FIG>, <FIG>) configured to generate a control signal (e.g., <NUM>, <FIG>) in response to the first dither tone being detected in the optical output signal; and wherein the optical switch is configured to change an optical input fiber port through which the optical data receiver receives the optical input signal in response to the control signal.

In some embodiments of the above apparatus, the optical data receiver is configured to: (i) process the optical input signal to recover the first data; and (ii) signal (e.g., using SF, <FIG>; <NUM>, <FIG>; <NUM>, <FIG>) the path failure to the optical data transmitter in an event of a data-recovery error.

In some embodiments of any of the above apparatus, the electronic controller is connected to a plurality of photodetectors (e.g., PD1-PD3, <FIG>, <FIG>) to detect one or more dither tones in the optical output signal, an optical signal at the first optical input fiber port, and an optical signal at the second optical input fiber port.

While this disclosure includes references to illustrative embodiments, this specification is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments within the scope of the disclosure, which are apparent to persons skilled in the art to which the disclosure pertains are deemed to lie within the principle and scope of the disclosure, e.g., as expressed in the following claims.

Unless otherwise specified herein, the use of the ordinal adjectives "first," "second," "third," etc., to refer to an object of a plurality of like objects merely indicates that different instances of such like objects are being referred to, and is not intended to imply that the like objects so referred-to have to be in a corresponding order or sequence, either temporally, spatially, in ranking, or in any other manner.

Also for purposes of this description, the terms "couple," "coupling," "coupled," "connect," "connecting," or "connected" refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms "directly coupled," "directly connected," etc., imply the absence of such additional elements.

The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the disclosure is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

The functions of the various elements shown in the figures, including any functional blocks labeled as "processors" and/or "controllers," may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. Moreover, explicit use of the term "processor" or "controller" should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage.

Claim 1:
An apparatus comprising:
a protection circuit (<NUM>) to selectively optically connect one of a first optical path and a second optical path;
an optical data receiver connected to receive through the protection circuit selectively from one of the first and second optical paths (140B, <NUM> A)
an optical input signal modulated with first data, the protection circuit (<NUM>) being optically connected between the first and second optical paths and the optical data receiver; and
an optical data transmitter connected to transmit through the protection circuit to third and fourth optical paths (140A, 150B) an optical output signal modulated with second data, the protection circuit being optically connected between the third and fourth optical paths and the optical data transmitter;
wherein the optical data transmitter is configured to generate the optical output signal to carry a first dither tone in response to being signaled by the optical data receiver of a path failure; and
wherein the protection circuit (<NUM>) is configured to change the selection of the one of optical paths from which the optical data receiver receives the optical input signal in response to detecting the first dither tone in the optical output signal.