Open, modular and scalable optical transmission system

Abstract

Reconfigurable optical add / drop multiplexer (ROADM) (100) comprising: several interconnected ROADM blocks (110), each ROADM block (110) comprising an input amplifier (245) with switchable gain, an output power detector (365, 465) coupled to an output of the input amplifier (245) with switchable gain, and a wavelength-selective switch (225) coupled to the output of the input amplifier (245) with switchable gain; several add / drop blocks (120) coupled to the wavelength-selective switches (225) of the several ROADM blocks (110); and a control unit (140) configured to receive a specification of an output signal power of the output power detector (365, 465) and to adjust gain and equalization parameters of the input amplifier (245) with switchable gain based on the received specification of the output signal power, wherein The input amplifier (245) with switchable gain includes a low-gain fiber amplifier and a high-gain fiber amplifier; and This includes adjusting the gain and equalization parameters and selectively directing an optical signal through the low-gain fiber amplifier or the high-gain fiber amplifier.

Description

background

[0001] Reconfigurable optical add / drop multiplexers (ROADMs) perform the routing, addition, equalization, and dropping of the optical signal in an optical line system. ROADMs may include amplifiers to boost the power of optical signals. However, many conventional optical signal amplifiers have uneven gain characteristics. For example, an optical signal amplifier amplifying a broadband optical signal, such as a wavelength-division multiplexed signal, may not amplify all wavelengths of the signal equally. For an optical signal passing through multiple amplifiers in a ROADM, through two or more ROADMs, or through one or more line amplifiers between two ROADMs, the cumulative effect of uneven gain can result in some wavelengths of the signal having too much power while others have too little power.Equalization can be applied to level the optical signal based on feedback of the signal's power spectral density. Traditionally, however, this has required a closed-loop control system between terminal devices communicating with each other and sharing a common communication protocol, which typically meant that terminal equipment had to be consistent with a single vendor to implement feedback-based control.

[0002] US 2009 / 0 162 067 A1 discloses an optical device including a first and a second optical amplifier, an optical signal processor and a controller.

[0003] US 2014 / 0 334 814 A1 discloses an optical amplifier using software-defined optical network (SDON) technology with a centralized controller and flexible physical hardware.

[0004] US 2009 / 0 116 841 A1 discloses systems and methods for adaptive control of amplifier target power.

[0005] US 2015 / 0 055 952 A1 discloses an intranodal reconfigurable optical add / drop multiplexer (ROADM) fiber management device and a system using this device.

[0006] US 2016 / 0 315 708 A1 discloses an optical amplifier having an amplification section, a detection section, an input side detector, a variable optical attenuator, an output side detector, a controller and an adjusting device.

[0007] US 2016 / 0 094 304 A1 discloses a device including an optical noise generator and a noise combiner. Summary

[0008] The invention is defined by the independent claims. The dependent claims specify embodiments thereof.

[0009] At least one aspect is directed to a reconfigurable optical add / drop multiplexer (ROADM). The ROADM includes a plurality of interconnected ROADM blocks. Each ROADM block includes a switchable gain input amplifier, an output power detector coupled to an output of the switchable gain input amplifier, and a wavelength-selective switch coupled to the output of the switchable gain input amplifier. The ROADM includes a plurality of add / drop blocks coupled to the wavelength-selective switches of the plurality of ROADM blocks. The ROADM includes a controller configured to receive an indication of output signal power from the output power detector and to adjust gain and equalization parameters of the switchable gain input amplifier based on the received indication of output signal power.

[0010] At least one aspect is directed to a method for operating a reconfigurable optical add / drop multiplexer (ROADM). The method includes receiving a wavelength division multiplexed (WDM) optical signal over a fiber optic link at a first ROADM block of a plurality of ROADM blocks. The method includes amplifying the WDM optical signal using the switchable gain input amplifier. The method includes detecting an output signal power of the amplified WDM optical signal using an output power detector coupled to an output of the switchable gain input amplifier. The method includes adjusting, by a controller coupled to the first ROADM block, gain and equalization parameters of the switchable gain input amplifier based on the detected output signal power.The method includes transmitting, using a wavelength-selective switch, the first ROADM block of an optical channel signal from the amplified WDM optical signal to a second ROADM block from the plurality of ROADM blocks.

[0011] These and other aspects and implementations are discussed in detail below. The foregoing information and the following detailed description contain illustrative examples of various aspects and implementations and provide an overview or framework for understanding the nature and essence of the claimed aspects and implementations. The drawings provide illustration and further understanding of the various aspects and implementations and are incorporated into and constitute a part of this specification. Brief description of the drawings

[0012] The accompanying drawings are not intended to be drawn to scale. Like reference numerals and designations in the various drawings indicate like elements. For clarity, not every component may be identified in every drawing. In the drawings: Fig. 1 is a block diagram of a reconfigurable optical add / drop multiplexer (ROADM) according to an illustrative implementation; Fig. 2 is a block diagram of a ROADM block of a ROADM according to an illustrative implementation; Fig. 3 is a circuit diagram of an exemplary switchable gain amplifier for use in a ROADM according to an illustrative implementation; Fig. 4 is a circuit diagram of an exemplary switchable gain amplifier for use in a ROADM according to an illustrative implementation; Fig. 5 is a block diagram of a Raman module for use in a ROADM according to an illustrative implementation; Fig. 6 is a block diagram of a wavelength-selective switch for use in a ROADM according to an illustrative implementation; Fig. 7 is a block diagram of an add / drop block for use in a ROADM according to an illustrative implementation; Fig. 8 is a flowchart of an example method for operating a ROADM according to an illustrative implementation; and Fig. 9 is a block diagram illustrating a general architecture for a computer system that may be employed to implement elements of the systems and methods described and illustrated herein, according to an illustrative implementation. Detailed description

[0013] Systems and methods of this disclosure generally relate to a reconfigurable optical add / drop multiplexer (ROADM) with one or more local feedback control loops. A ROADM can switch traffic of a wavelength division multiplexing (WDM) system at the wavelength layer. For example, the ROADM can add, drop, or steer individual or multiple wavelengths carrying data without the need to convert the signals on all WDM optical signal channels from the optical domain to the electronic domain and back.

[0014] The addition of local feedback loops within the ROADM enables the ROADM to function as a modular, interchangeable component of an optical transport line system. Traditionally, optical transport line systems contain proprietary end-to-end software control, requiring each component to communicate with the others, generally limiting the optical transport line system to equipment from a single vendor. Such end-to-end dependencies make it difficult or impossible to replace or upgrade an individual component unless the replacement either implements the same control scheme (i.e., is from the same vendor and designed to work with the legacy system) or all other components in the dedicated control loop are replaced simultaneously.

[0015] The local ROADM feedback control loops can maintain the overall power and / or spectral power density flatness across the bandwidth of the WDM signals within a specified range. This enables interoperability at the optical component level because the ROADM controls the output at each interface based on analog specifications for the optical signal itself. Several features, including but not limited to those outlined below, enable local control of the ROADM. Not all features are required in every embodiment.

[0016] The ROADM can employ amplifiers with switchable gain. The amplifiers can be of various types, including laser amplifier blocks such as erbium-doped fiber amplifiers, fiber amplifiers, and Raman amplifiers. Each amplifier can have a switchable gain for controlling the gain and output power of the optical signal. Switchable gain can be achieved by using switches to route the optical signal to either a low-gain or high-gain amplifier path. Switchable gain can additionally or alternatively include a variable-gain amplifier.

[0017] The ROADM can apply noise loading to selected nodes to add a "dummy" or "null" optical signal to unused channels. This creates a network condition similar to full channel loading. Noise loading improves transient performance, simplifies the implementation of (local) control at the node level, and enables easier monitoring of the edge of the link without the need to perform optical signal-to-noise measurements. In cases where the ROADM does not provide sufficient isolation between the noise loading channel and the used channels, an additional wavelength blocker can be added between the noise loading and ROADM ports.

[0018] The ROADM may include equalizers or signal levelers, such as a passive equalizer or a dynamic gain equalizer. Because some optical amplifiers have uneven gain profiles across their operating bandwidth (and amplify some wavelengths more than others), equalization or "leveling" of the signal may occasionally be required to keep all channels of wavelength-division multiplexed optical signals within a specified amplitude range.

[0019] The ROADM can incorporate power detectors, including pick-offs and photodiodes, to monitor the power of the optical signals in the analog domain. The power detectors can be equipped to detect the power spectral density (power at each wavelength) or just the total power. A control unit or monitoring unit of the ROADM's optical channel can use the optical signal power measurements to adjust the equalizer and gain settings of the switchable gain amplifiers to achieve a specified total power and flatness for a ROADM interface.

[0020] The ROADM can consist of multiple blocks. Each block can contain, without limitation, one or more amplifiers, switches, multiplexers, and / or power detectors. Each block of the ROADM can be controlled locally with respect to overall power and flatness, without software dependencies between blocks. Each block can therefore be equipped with monitoring capabilities for control purposes, such as the power detectors described above. A control unit—either a single central control unit or an individual control unit in each block—can receive power measurements from the power detectors and adjust amplifier and equalizer settings as appropriate.

[0021] Fig. 1 is a block diagram of a reconfigurable optical add / drop multiplexer (ROADM) 100 according to an illustrative implementation. The ROADM 100 includes three ROADM blocks 110a-110c (collectively, "ROADM blocks 110"). In some implementations, the ROADM 100 may include more or fewer ROADM blocks 110. The ROADM 100 includes three add / drop blocks 120a-120c (collectively, "add / drop blocks 120"). In some implementations, the ROADM 100 may include more or fewer add / drop blocks 120. The ROADM 100 includes a controller 140 for coordinating the operation of the ROADM 100 and its components. In some implementations, the ROADM 100 may optionally include a fiber patch panel 130. The fiber patch panel 130 may include passive optical interconnects to simplify the cabling of the ROADM 100; however, it is possible to route cables among the ROADM blocks 110 and add / drop blocks 120 directly without using the fiber patch panel.

[0022] Each ROADM block 110 connects to a next-hop element via an optical transmission link (OTS) connection over fiber optic links 115a-115c (collectively, "fiber optic links 115"). Each ROADM block 110 can transmit and receive wavelength-division multiplexed optical (WDM) signals over fiber optic links 115. Each ROADM block 100 can demultiplex the WDM optical signal received at its fiber optic link 115 and transmit each individual optical channel signal to the appropriate destination; e.g., another ROADM block 110 or an add / drop block 120a. Similarly, each ROADM block 110 may receive individual optical channel signals from other ROADM blocks 110 and add / drop blocks 120, multiplex them into an optical WDM signal, and transmit the optical WDM signal over fiber optic links 115.

[0023] Each add / drop block 120 can transmit and receive individual optical add / drop channel signals over fiber optic links 125a-125c (collectively, "fiber optic links 125"). ROADM 100 can add an individual optical channel signal received at fiber optic link 125 to a WDM optical signal transmitted by one of fiber optic links 115. Similarly, ROADM 100 can extract an individual optical channel signal from a WDM optical signal received at one of fiber optic links 115 and transmit the individual optical channel signal over fiber optic link 125.

[0024] The ROADM 100 can therefore perform four basic functions without limitation: (1) a same-direction express function (e.g., transporting an optical channel signal from fiber optic link 115a to fiber optic link 115c); (2) a different-direction express function (e.g., transporting an optical channel signal from fiber optic link 115a to fiber optic link 115b); (3) adding an optical channel signal to a WDM optical signal (adding function); and (4) dropping an individual optical channel signal from a WDM optical signal (dropping function). The ROADM 100 can perform additional functions such as optical signal power detection, optical signal amplification, and optical signal equalization, and can additionally include closed-loop control of the total optical signal power and power spectral density.These additional features and the structures that enable them are described in more detail below with reference to the . Fig. 2-9 described.

[0025] The ROADM 100 may be a colorless-directionless-contentionless architecture. Colorless means that the ROADM 100 can transmit or receive any wavelength on any port. Directionless means that the ROADM 100 can steer a wavelength over any connected fiber path (also referred to as a degree). Contentionless means that the ROADM 100 can add or drop two optical signals that have the same wavelength (i.e., one optical signal will not block another optical signal that has the same wavelength) if they are destined for different fiber paths.

[0026] The control unit 140 may coordinate the functions of the ROADM 100 and its components. In some implementations, the control unit 140 may include a single central processing unit that may monitor and control each module or block of the ROADM 100, including the ROADM blocks 110 and the add / drop blocks 120. In some implementations, the control unit 140 may include distributed processors local to each block or module for performing local monitoring and control of the block or module. In some implementations, the control unit 140 may include a combination of centralized and distributed components. The control unit 140 may be implemented in one or more computer systems, such as the computer system 900 described below with reference to Fig. 9 as described in more detail.

[0027] Fig. Figure 2 is a block diagram of a ROADM block 110 of a ROADM 100 according to an illustrative implementation. The ROADM block 110 includes a ROADM module 205 and a line module 210. The ROADM block 110 may optionally include a Raman amplifier module 215 for amplifying an incoming WDM optical signal received over the fiber optic links 115. The ROADM block 110 may demultiplex the WDM optical signal into individual optical channel signals and transmit them to another ROADM block 110 or an add / drop block 120 over the fiber optic links 220a and 220b (collectively, "fiber optic links 220"). The ROADM block 110 may multiplex the optical channel signals received via the fiber optic links 220 from other ROADM blocks 110 or the add / drop blocks 120 into a second optical WDM signal and transmit the second optical WDM signal via the fiber optic link 115.

[0028] The ROADM module 205 includes one or more wavelength-selective switch (WSS) blocks 225a and 225b (collectively, "WSS blocks 225"). The WSS block 225 can demultiplex a WDM optical signal into individual optical channel signals. The WSS block 225 can route any individual optical channel signals to any port. The WSS block 225 can also multiplex individual optical channel signals into one WDM optical signal. In some implementations, the ROADM 100 can handle optical signals in more than one frequency or wavelength band. In some implementations, the ROADM 100 can handle optical signals in both the C-band and the L-band. The ROADM module 205 may include a WSS block 225a for multiplexing and demultiplexing C-band optical channel signals and a WSS block 225b for multiplexing and demultiplexing L-band optical channel signals.In implementations of the ROADM module 205 that include multiple WSS blocks 225, the ROADM module 205 may include a multiplexer / demultiplexer (MUX) (also referred to as a C / L-band splitter / combiner) 230. The MUX 230 may multiplex and demultiplex two or more optical WDM signals carrying optical channel signals in different frequency bands.

[0029] In some implementations, the ROADM module 205 may optionally include one or more noise amplifiers 235a and 235b (collectively, "noise amplifiers 235"). The noise amplifiers 235 may add a "dummy" or "null" optical signal to one or more unused ports of the WSS block 225. In implementations where the ROADM 100 can handle optical signals in two frequency bands, each WSS block 225 may be associated with the noise amplifiers 235 that can produce an optical signal in the frequency band handled by the WSS block 225. The WSS blocks 225 and the noise amplifiers 235 are described in more detail below with reference to Fig. 6 discussed.

[0030] The line module 210 includes one or more output amplifiers 240a and 240b (collectively, "output amplifiers 240") and one or more input amplifiers 245a and 245b (collectively, "input amplifiers 245"). The output amplifiers 240 amplify an outgoing optical WDM signal. The input amplifiers 245 amplify an incoming optical WDM signal. In some implementations, the ROADM 100 can handle optical signals in more than one frequency or wavelength band. In some implementations, the ROADM 100 can handle optical signals in both C-band and L-band. The line module 210 can include an output amplifier 240a for amplifying an outgoing C-band WDM optical signal and an output amplifier 240b for amplifying an outgoing L-band WDM optical signal.Similarly, the line module 210 may include an input amplifier 245a for amplifying an incoming C-band WDM optical signal and an input amplifier 245b for amplifying an incoming L-band WDM optical signal. In implementations of the line module 210 that include multiple output amplifiers 240 and input amplifiers 245, the line module 120 may include a MUX 250 and a MUX 255 for multiplexing and demultiplexing two or more WDM optical signals carrying optical channel signals in different frequency bands. The output amplifiers 240 and the input amplifiers 245 may be switchable gain amplifiers, such as those described below with reference to FIG. Fig. 3 and Fig. 4 are described in more detail.

[0031] In some implementations, the line module 210 may optionally include an optical time-domain reflectometer (OTDR) 250 and an optical monitoring channel (OSC) 262. The OTDR 260 may use optical time-domain reflectometry to detect and locate a discontinuity along an optical channel, including on a fiber optic link external to the ROADM 100. For example, a damaged or severed fiber optic cable may have a refractive index discontinuity at the break point that will reflect a portion of an optical signal traveling in the fiber. The OTDR 260 may detect the reflection and thus detect the discontinuity. The OTDR 260 may measure the time elapsed between the transmission of a test signal and the reception of the resulting reflection and calculate the distance to the discontinuity based on the speed of light through the fiber optic cable.

[0032] The OSC 262 is a device in the ROADM 100 that can use an additional wavelength, possibly outside the bandwidth of the ROADM 100 amplifiers, to send and receive information via optical signals on the line, and to exchange states of the equipment at each port of the fiber optic link.

[0033] In some implementations, the line module 210 may optionally include one or more optical channel monitoring units (OCMs) 265a and 265b (collectively, "OCMs 265"). The OCM 265 may measure wavelength, power, and optical signal-to-noise ratio (OSNR) of an optical channel without directly considering the bit sequence carried by the channel. In some implementations, the OCM 265 may receive measurements of the optical signal power from optical signal power detectors, such as the power detector 305, described below with reference to Fig. 3. The OMCs 265 and the control unit 140 may use the measured optical signal power to determine whether equalization should be applied to an optical signal passing through the ROADM block 110. In some implementations, the line module 210 may include an OCM 265 for each signal band passing through the line module 210. In some implementations, the OCM 265a may monitor C-band optical signals, and the OCM 265b may monitor L-band optical signals.

[0034] In some implementations, the line module 210 may optionally include a Raman amplifier module 215. The Raman amplifier module 215 may amplify signals received or transmitted over the fiber optic link 115. The Raman amplifier module 215 is described below with reference to Fig. 5 described in more detail.

[0035] Fig. 3 is a circuit diagram of an exemplary switchable gain amplifier 300 for use in a ROADM 100 according to an illustrative implementation. The switchable gain amplifier 300 may be implemented as an output amplifier 240, an input amplifier 245, or as an amplifier of the amplifier group 710 of the add / drop block 120, described below with reference to Fig. 7. The switchable gain amplifier 300 can amplify an optical signal with various gain levels and, in some implementations, also employ equalization. The switchable gain amplifier 300 includes a low-gain path and a high-gain path. The switchable gain amplifier 300 additionally includes a uniform gain path and a dynamic gain equalization path. The switchable gain amplifier 300 can switch between the various amplification modes.

[0036] The switchable gain amplifier 300 includes an input power detector 310 at the input 305. The input power detector 310 may include a collector and a photodetector. The collector may split a small portion of the optical input signal at the input 305 and direct it to the photodetector. The photodetector may include a photodiode or other light-sensitive semiconductor, sensor, or device. In some implementations, the input power detector 305 may detect the power spectral density. That is, the input power detector 310 may determine the power level at different wavelengths of an optical WDM signal. In some implementations, the input power detector 310 may only detect the total power density.

[0037] In some implementations, the switchable gain amplifier 300 may be at the edge of the ROADM 100; i.e., the input 305 of the switchable gain amplifier 300 may be connected directly to the fiber optic link 115. In such implementations, the input power detector 310 is effectively at the input of the ROADM 100 and may be used to ensure that an incoming signal at the fiber optic link 115 is within appropriate amplitude and flatness parameters. If the incoming optical signal is not within these parameters, the control unit 140 and / or the optical channel monitoring unit 265 may determine that something external to the ROADM 100—e.g., the fiber optic link 115 or remote equipment sharing the fiber optic link 115—is faulty.The OSC / OTDR 260 may be capable of providing additional information about the fault and its location. The optical channel control unit 140 and / or monitoring unit 265 may issue an alarm indicating a potential problem affecting the fiber optic link 115 or a neighboring network element on the fiber optic link 115.

[0038] Switchable gain amplifier 300 includes switches 315, 320, 325, and 330. Switches 315, 320, 325, and 330 can route the optical signal through low- or high-gain amplifier blocks and through an equalizing or non-equalizing attenuator. The first switch 315 can route the optical signal to the first-stage low-gain amplifier block 335 or high-gain amplifier block 340. The second switch 320 can receive the optical signal from the selected first-stage amplifier block and route it to either the wavelength blocker 345 or the variable optical attenuator (VOA) 350.

[0039] The wavelength blocker 345 can equalize the optical signal by selectively attenuating one or more wavelengths. Placing the wavelength blocker 345 between the first and second amplification stages can maintain the output power relative to its placement after the second amplification stage. Placing the wavelength blocker 345 between the first and second amplification stages can also minimize the optical signal-to-noise penalty by combining the spacing loss with the insertion loss of the wavelength blocker 345. However, in some implementations, the wavelength blocker 345 can be placed before or after the first and second amplification stages. The wavelength blocker 345 is a spectrum-selective device capable of providing a given attenuation (or even complete blocking) over a range of frequencies, with a specified frequency resolution.

[0040] The third switch 325 can receive the optical signal from the selected attenuator and direct it to the low-gain amplifier block 355 or the high-gain second-stage amplifier block 360. The fourth switch 330 can receive the optical signal from the selected second-stage amplifier block and direct it to the output 370. The switchable-gain amplifier 300 includes an output power detector 365. The output power detector 365 is similar to the input power detector 310. The output power detector 365 includes a collector and a photodetector. The collector can split off a small portion of the optical input signal from the output 370 and direct it to the photodetector. The photodetector can include a photodiode or other light-sensitive semiconductor, sensor, or device.In some implementations, the output power detector 365 can detect the power spectral density. That is, the output power detector 365 can determine the power level at different frequency components of an optical WDM signal. In some implementations, the output power detector 365 can only detect the total power density.

[0041] In some implementations, one or more of amplifiers 335, 340, 355, and 360 may be an optical laser amplifier, such as an erbium-doped fiber amplifier (EDFA). In some implementations, the amplifier may include a medium doped with a transition metal or other rare-earth metal. Switchable gain amplifier 300 may include EDFAs that have different gains in the low-gain path and the high-gain path, respectively. EDFAs and similar amplifiers may not have a perfectly linear gain profile. In other words, when amplifying an optical signal that has a finite bandwidth, the amplifier may amplify some wavelengths of the optical signal more than others. Amplifying an optical signal with multiple similar amplifiers in series without equalization may result in a large cumulative disparity in channel power.Therefore, it may be useful to perform equalization of the signal in the amplifier. In some implementations, the output power detector 365 can detect the power spectral density. Using the power spectral density information, the controller 140 and / or the OCM 265 can determine whether it would be desirable to flatten or equalize the optical signal. The decision to include the wavelength blocker 345 in the circuit involves a trade-off between improving the flatness of the optical signal and the attenuation of the optical signal caused by the wavelength blocker. If the controller 140 and / or the OCM 265 determines that flattening the optical signal is desirable, they can set the switches 320 and 325 to route the optical signal through the wavelength blocker 345.The control unit 140 and / or the OCM 265 can then control the wavelength blocker 345 to apply the desired wavelength-selective attenuation to the optical signal. In some implementations, the equalization can be controlled to maintain gain flatness at 1 dB across all channels of the WDM optical signal. If the control unit 140 and / or the OCM 265 determine that equalization is not necessary, the control unit 140 and / or the OCM 265 can route the optical signal through the variable optical attenuator 350 or bypass both the wavelength blocker 345 and the VOA 350. The control unit 140 and / or the OCM 265 can then adjust the variable optical attenuator 350 to attenuate the optical signal by the desired amount, including possible zero attenuation.

[0042] The control unit 140 and / or the OCM 265 may support various control modes. In some implementations, the ROADM 100 may have a set-and-forget control mode in which gain and equalization parameters are set at the time of installation or initialization and maintained constant throughout the lifetime of the ROADM 100 or until a modification, upgrade, or repair changes the hardware configuration or behavior of the ROADM 100. The set-and-forget control mode may benefit from noise loading, as detailed below with reference to the noise loading amplifiers 235 in Fig. 6. In some implementations, the ROADM 100 may have a continuous or real-time control mode of operation. In continuous control modes, the control unit 140 and / or the OCM 265 may receive two or more real-time inputs; e.g., from each of the input power detector 305 and the output power detector 365. The control unit 140 and / or the OCM 265 may monitor the input power detector 305 and transmit a warning or alarm if the incoming optical WDM signal is not within expected parameters; e.g., the total power of the optical signal is too low, or the power spectral density is too uneven across its bandwidth.The controller 140 and / or the OCM 265 may monitor the output power detector 365 and apply feedback to limit the output signal power to a user-defined function by changing the gain and equalization of the switchable gain amplifier 300. The user-defined function may specify the desired optical output signal power distribution. In some implementations, the user function may be specified as power versus wavelength, power versus the inverse of wavelength, power versus wavelength squared, or any other one-to-one mapping between power and wavelength / frequency. The continuous control modes may also benefit from noise loading, as detailed below with reference to the noise loading amplifiers 235 in FIG. Fig. 6, which can improve the transient behavior and linearity of the switchable gain amplifier 300. In a configuration where the link is operated by interacting with a software-defined network control unit, the control scheme would be as follows. The SDN control unit would configure the links (with components such as amplifiers, ROADMs, MCSs) for parameters such as gain, spectral power density targets, and spectral slots to be opened. The links would then be monitored using telemetry. Any dramatic change in the link condition (due to a fiber break or a significant change in fiber loss) would cause the traffic traversing the link to be rerouted along a different path. The control unit would then reoptimize the link's parameter settings and, after reconfiguration, switch the previous traffic back onto the link.The reoptimization would be based on fiber loss estimation derived from both the amplifiers' input and output power monitoring units and Raman gain signals (in the case where Raman gain is used on a subset of distances). These, in turn, would lead to the derivation of the new optimal starting powers for the link.

[0043] Amplifiers 335, 340, 355, and 330 can be fixed-gain or variable-gain amplifiers. Using a variable-gain amplifier can provide more precise gain adjustment; however, fixed-gain amplifiers can be simpler, cheaper, and more efficient. In some implementations, fixed-gain low-gain amplifiers 335 and 355 can provide approximately 15 dB of gain. In some implementations, fixed-gain high-gain amplifiers 340 and 360 can provide approximately 22 dB of gain. In some implementations, variable-gain low-gain amplifiers 335 and 355 can provide approximately 10-18 dB of gain. In some implementations, variable-gain high-gain amplifiers 340 and 360 can provide approximately 16-24 dB of gain.Other gain values ​​may be selected as needed for the particular design without departing from the scope of the disclosure. Implementations of switchable gain amplifier 300 that include variable gain amplifiers may not require variable optical attenuator 350. In such implementations, the switchable gain amplifier may have a simplified design, such as the design of switchable gain amplifier 400 described below with reference to FIG. Fig. 4 is described.

[0044] Fig. 4 is a circuit diagram of an exemplary switchable gain amplifier 400 for use in a ROADM 100 according to an illustrative implementation. The switchable gain amplifier 400 is similar to the switchable gain amplifier 300, but differs in that it has only a wavelength blocker 445 and no variable optical attenuator. The switchable gain amplifier 400 may be implemented as an output amplifier 240, an input amplifier 245, or as an amplifier of the amplifier group 710, described below with reference to Fig. 7. The switchable gain amplifier 400 can amplify an optical signal at various gain levels and, in some implementations, also apply equalization. The switchable gain amplifier 400 can switch between a low-gain path and a high-gain path and includes a wavelength blocker 445 for performing wavelength-selective attenuation on the optical signal.

[0045] The switchable gain amplifier 400 includes an input power detector 410. The input power detector 410 may include a collector and a photodetector. The collector may split a small portion of the optical input signal from the input 405 and direct it to the photodetector. The photodetector may include a photodiode or other light-sensitive semiconductor, sensor, or device. In some implementations, the input power detector 410 may detect the power spectral density. That is, the input power detector 410 may determine the power level at different frequency components of an optical WDM signal. In some implementations, the input power detector 410 may only detect the total power density.

[0046] The switchable gain amplifier 400 includes switches 415, 420, 425, and 430. The switches 415, 420, 425, and 430 can route the optical signal through the low-gain or high-gain amplifier blocks and the wavelength blocker 445. The first switch 415 can route the optical signal to the low-gain amplifier block 435 or the high-gain amplifier block 440 of the first stage. The second switch 420 can receive the optical signal from the selected first-stage amplifier block and route it to the wavelength blocker 445. The wavelength blocker 445 can be similar to the wavelength blocker 345 described above with reference to Fig. 3 is discussed.

[0047] The third switch 425 can receive the optical signal from the wavelength blocker 445 and direct it to the low-gain amplifier block 455 or the high-gain amplifier block 460 of the second stage. The fourth switch 430 can receive the optical signal from the selected second-stage amplifier block and direct it to the output 470. The switchable-gain amplifier 400 includes an output power detector 465. The output power detector 465 can be similar to the output power detector 410. The output power detector 465 can include a collector and a photodetector. The collector can split off a small portion of the optical input signal from the output 470 and direct it to the photodetector. The photodetector can include a photodiode or other light-sensitive semiconductor, sensor, or device.In some implementations, output power detector 465 may detect the power spectral density. That is, output power detector 465 may determine the power level at different frequency components of an optical WDM signal. Control of the components of switchable gain amplifier 400 may be similar to that described above with respect to switchable gain amplifier 300, including gain path selection, control of wavelength blocker 445, and detection of errors external to ROADM 100.

[0048] Fig. 5 is a block diagram of a Raman amplifier module 215 for use in a ROADM 100 according to an illustrative implementation. The Raman amplifier module 215 can amplify incoming and / or outgoing signals on the fiber optic link 115. The Raman amplifier module 215 includes an input power detector 505, a light source 510 coupled to the fiber optic line via a coupler 515, and an output power detector 520. The Raman amplifier module 215 can send the amplified optical signal to the line module 210 via the fiber optic link 525.

[0049] The Raman amplifier module 215 can use stimulated Raman scattering in an optical fiber or other Raman-active medium to amplify an optical signal. The light source 510 can emit light at a specific wavelength slightly offset from the wavelength of the signal to be amplified. The light source 510 can be a monochromatic light source or a narrow-bandwidth light source, such as a laser. In some implementations, the Raman amplifier module 215 can include a backward pumping Raman amplification block. Light from the light source 510 can be pumped into the light guide of the fiber optic link 115 using the coupler 515. In some implementations, the coupler 515 can direct the pumped light in a direction toward the fiber optic link 115, where stimulated Raman scattering can occur.In some implementations, coupler 515 may direct the pumped light in a direction toward fiber optic interconnect 525, where stimulated Raman scattering may occur. In some implementations, Raman amplifier module 215 may include a dedicated length of fiber optic or other medium separate and distinguishable from fiber optic interconnects 115 and 525 for performing Raman amplification. Gain of Raman amplifier module 215 may be controlled by adjusting the wavelength and / or intensity of light source 510.

[0050] The input power detector 505 can be used to monitor whether the incoming signal over the fiber optic link 115 is within expected parameters for amplitude and flatness. In implementations of the ROADM 100 that include the Raman amplifier module 215, the input power detector 505 can monitor the input and / or output of the ROADM 100. If the input signal, as measured by the input power detector 505, deviates sufficiently from expected parameters, the optical channel control unit 140 and / or the optical channel monitoring unit 265 can determine that something external to the ROADM 100—e.g., the fiber optic link 115 or remote equipment sharing the fiber optic link 115—is faulty. The OSC / OTDR 260 may be capable of providing additional information about the fault and its location.The optical channel control unit 140 and monitoring unit 265 may issue an alarm indicating a potential problem affecting the fiber optic link 115 or an adjacent network element on the fiber optic link 115.

[0051] The output power detector 520 can measure the optical output signal transmitted to the line module 210 via the fiber optic link 525. Based on the power measured by the output power detector 520, the optical channel control unit 140 and / or the optical channel monitoring unit 265 can adjust the light source 510 to adjust the power of the optical signal transmitted via the fiber optic links 515.

[0052] Fig. 6 is a block diagram of a wavelength-selective switch (WSS) block for use in a ROADM 100 according to an illustrative implementation. The WSS block 225 may demultiplex a WDM optical signal received from the line module 210 over the fiber optic link 605 into individual optical channel signals transmitted over the fiber optic links 220. The WSS block 225 may route any individual optical channel signals to any port. The WSS block 225 may also multiplex optical channel signals received over the fiber optic links 220 into an optical WDM signal transmitted over the fiber optic link 605. The WSS block 225 includes an input power detector 610, a WSS 615, and output power detectors 620. In the Fig. In the implementation shown in Figure 6, the power detectors 610 and 620 are incorporated into the WSS block 225. However, in some implementations, the power detectors 610 and 620 are discrete units external to the WSS block 225. In some implementations, the WSS 615 may be a 1x20, 1x24, or 1x32 optical switch.

[0053] The WSS block 225 may include one or more optical attenuators 625 for attenuating the optical signals on one or both of the fiber optic link 605 and the fiber optic link 220. The optical attenuator 615 may have a fixed or variable attenuation and may be switched into or out of the circuit by the optical channel control unit 140 and / or the optical channel monitoring unit 265. The optical channel control unit 140 and / or the optical channel monitoring unit 265 may switch or adjust the attenuation of the optical attenuator 625 based on the optical power levels indicated by the power detectors 610 and 620.The control unit 140 may also provide a warning if one of the power detectors 610 and 620 measures an optical power level of a signal arriving at the WSS block 225 that has a power level above or below an expected level. Such an aberrant measurement could indicate a fault in another part of the ROADM 100.

[0054] In some implementations, each WSS block 225 may include noise loading amplifiers 235 that may be connected to any unused port of the WSS 615. The noise loading amplifiers 235 may inject a "null" or "dummy" signal into any unused port of the WSS 615. In some implementations, the noise loading amplifiers 235 may be implemented in groups of two, four, eight, or any other number of noise loading amplifiers 235 per noise loading amplifier module. Each noise loading amplifier 235 may be coupled to a port of the WSS 615. Noise loading may improve the linearity and transient behavior of the ROADM 100 and may help support set-and-forget control of the ROADM 100 components.

[0055] The gain of an optical amplifier can vary depending on the amplitude and bandwidth of the input signal. For example, the response of an optical amplifier amplifying a WDM optical signal can change depending on whether all or only some of the possible channels of the WDM optical signal are occupied. Noise loading using amplified spontaneous emission (ASE) can occupy the empty spectrum in a transport link. This can produce an accurate estimate of the link's nonlinear penalty under full load (as long as the noise matches the spectral power density that channels would have achieved, despite the difference in spectrum shape). This noise source could be implemented with an open-loop erbium-doped fiber amplifier (EDFA) at the transmission node for each ROADM block 110.At each ROADM block 110 (where traffic is added and removed), the ASE noise can be filtered out and added back at the next WSS block 225 facing the line.

[0056] By applying noise loading to fill the unused channels and maintaining a WDM optical signal that appears full to the amplifiers, the amplifier performance can be kept constant and predictable both over the lifetime of the ROADM 100 as the optical channels fill up and the ROADM 100 reaches full capacity, and in response to the transient optical channel signals appearing and disappearing.

[0057] The noise loading of unused ROADM 100 optical channels at the time of installation allows amplifier parameters to be adjusted so that the desired optical output signal power amplitude and flatness can remain relatively constant over the lifetime of the ROADM 100 despite increased channel loading over time. In some implementations, this may enable set-and-forget operation, where amplifier parameters are set at the time of ROADM 100 installation and remain constant unless a hardware failure or other issue necessitates recalibration of the ROADM 100.

[0058] Noise exposure can also help the ROADM maintain consistent and predictable performance in response to transient optical channel signals appearing and disappearing. Regarding (b), in the absence of noise exposure, adding (or removing) an optical channel signal to a WDM optical signal can change the apparent amplitude or flatness of the WDM optical signal. The response of an optical amplifier amplifying the WDM optical signal can shift as a result of these transients, so that the output of the ROADM 100 can deviate from the desired optical power amplitude and flatness. However, with noise exposure, a new optical channel signal simply replaces the dummy signal previously held by the noise exposure amplifier 235, and the WDM optical signal maintains the same amplitude and flatness despite the added channel.This can mitigate the effects of transients and keeps the amplifier gain and thus the ROADM 100 output relatively constant despite changes in channel loading.

[0059] Noise loading also enables improved monitoring of the optical signal-to-noise ratio (OSNR) by the optical channel monitoring units 265. The variation in the figure of merit (Q) of the WDM optical signal can depend on both the proportion of occupied channels and the OSNR. With noise loading, all channels of the WDM optical signal appear full, and thus any change in Q will primarily result from a change in the OSNR. Thus, noise loading can support immediate OSNR monitoring in a non-intrusive manner. Monitoring the OSNR allows the ROADM 100 to track the aging of its individual components, including the line modules 210.

[0060] Fig. 7 is a block diagram of an add / drop block 120 for use in a ROADM 100 according to an illustrative implementation. The add / drop block 120 may receive optical channel signals over the optical fiber links 220, amplify them, and route them to the optical fiber links 125. Extracting an individual optical channel signal from the WDM optical signal may be referred to as drop functionality. The add / drop block 120 may receive optical channel signals over the optical fiber links 125, amplify them, and route them to the optical fiber links 220. Adding an individual optical channel signal to a WDM optical signal may be referred to as add functionality. The add / drop block 120 may include one or more amplifier groups 710a and 710b (collectively, "amplifier groups 710") and one or more multicast switches 720.In some implementations, the amplifier groups 710 may have a modular design with two, four, or eight amplifiers in each direction on an amplifier group module. Each amplifier of the amplifier groups 710 can achieve a gain of up to 23 dB, which can support a large number of add / drop ports and higher baud rate terminal optics. Each amplifier group 710 can include optical power detectors 715a or 715b (collectively, "optical power detectors 715") at the input and output of each amplifier. The multicast switch 720 can include an optical power detector 715 at each port.

[0061] Each amplifier of the amplifier groups 710 may be a switchable gain amplifier 300 or 400 as described above with reference to Fig. 3 or Fig. 4. In some implementations, each amplifier may be a simplified version of switchable gain amplifier 300 or 400, having low-gain and high-gain paths, but not including a wavelength blocker such as wavelength blocker 345. In some implementations, each amplifier may include a variable optical attenuator such as VOA 350. In some implementations, each amplifier may include the variable optical attenuator and only a single gain path. Optical power detectors 715 may measure the power of the optical signals before and after each amplifier. Controller 140 may adjust the gain of individual amplifiers to maintain the add / drop signals transmitted and received over fiber optic links 220 at the desired amplitude.In some implementations, the controller 140 may use measurements of incoming optical signal power over the fiber optic links 220 to determine whether a fault exists elsewhere in the ROADM 100.

[0062] The amplifiers of amplifier groups 710 may not require equalization because, unlike the output and input amplifiers of ROADM block 110, add / drop block 120 does not handle WDM optical signals but instead handles the relatively narrower-bandwidth fiber optic links 125. Furthermore, the optical signals passing through add / drop block 120 cannot experience cascaded optical amplifiers that would combine the wavelength-dependent effects of the amplifiers. Instead, an optical signal passing through add / drop block 120 may only pass through a single amplifier stage, either before leaving the optical link system or after entering the optical link system. In contrast, when an optical WDM signal passes through a ROADM 100, it may pass through two or more optical amplifiers (e.g., in ROADM block 110a and ROADM block 110c).If the optical WDM signal passes through multiple ROADMs 100 without equalization, the power difference between different wavelengths of the optical WDM signal may result in an individual optical channel signal having a power that is too high or too low.

[0063] The multicast switch 720 may include one or more optical switches 730a and 730b (collectively, "optical switches 730") and optical power detectors 725a or 725b (collectively, "optical power detectors 725") at each port of the optical switches 730. In some implementations, the optical switches 730 may be 4x32 optical switches. Multiple optical switches 730 may be combined for a total of 16x32 switches. In some implementations, a different number of optical switches 730, having different numbers of inputs and outputs, may be combined to form different configurations of multicast switches 720.

[0064] In some implementations, the multicast switch 720 may include a variable optical attenuator. The optical power detectors 725 may measure the power of the optical signals before and after each amplifier. The controller 140 may adjust the gain of individual amplifiers to maintain the add / drop signals over the fiber optic links 125 at the desired amplitude. In some implementations, the controller 140 may use measurements of incoming optical signal power over the fiber optic links 125 to determine whether a fault has occurred in the equipment downstream of the ROADM 100 along the fiber optic links 125.

[0065] In some implementations, it may only be necessary to have optical power detectors at the edges of the add / drop block 120. That is, it may only be necessary to include optical power detectors 715 and 725 at or adjacent to the interfaces with the fiber optic links 220 and the fiber optic links 125. In this configuration, the control unit 140 can adjust gain and attenuation parameters for each channel based on the measurements from the optical power detectors 715 and 725 and the control unit 140's knowledge of the switching state of the optical switches 730.

[0066] Fig. 8 is a flowchart of an example method 800 for operating a ROADM 100 according to an illustrative implementation. The method 800 includes receiving a wavelength division multiplexed (WDM) optical signal at a first ROADM block 100 (stage 810). The method 800 may optionally include detecting an input signal power of the WDM optical signal using an input power detector coupled to an input of a switchable gain input amplifier of the first ROADM block (stage 820). The method 800 may optionally include determining, by a controller of the ROADM, that the input signal power indication is outside a specified optical power range (decision block 830). If the input signal power is outside the specified optical power range, the method 800 includes issuing a warning by the controller (stage 840).If the input signal power is within the specified power range, the method 800 proceeds to stage 850. The method 800 includes amplifying the optical WDM signal using the switchable gain input amplifier (stage 850). The method 800 includes detecting an output signal power of the amplified optical WDM signal using an output power detector coupled to an output of the switchable gain input amplifier (stage 860). The method 800 includes adjusting, by a control unit connected to the first ROADM block, gain and equalization parameters of the switchable gain input amplifier based on the detected output signal power (stage 870). Based on the selection in stage 870, the method 800 includes adjusting the gain and equalization parameters of the switchable gain input amplifier in stage 850.The method 800 includes transmitting, using a wavelength-selective switch, the first ROADM block of an optical channel signal from the amplified WDM optical signal to a second ROADM block from the plurality of ROADM blocks (stage 880).

[0067] The method 800 includes receiving a wavelength division multiplexed (WDM) optical signal at a first ROADM block 100 (stage 810). The ROADM block 110 may receive the WDM optical signal over a fiber optic link 115. In some implementations, the method 800 may include amplifying the WDM optical signal with a Raman amplifier coupled between the ROADM block 110 and the fiber optic link 115.

[0068] Method 800 may optionally include detecting an input signal power of the optical WDM signal using an input power detector coupled to an input of a switchable gain input amplifier of the first ROADM block (stage 820). In some implementations, the optical WDM signal may be received at a switchable gain amplifier of the ROADM block 110, such as switchable gain amplifier 300 or 400. Input power detector 310 or 410 may detect the optical signal power of the optical WDM signal. In some implementations, method 800 may include amplifying the optical WDM signal with a Raman amplifier coupled between the ROADM block 110 and the fiber optic link 115.In ROADM implementations that include a Raman amplifier, the Raman amplifier module 215 may include an input power detector 505 for detecting the input signal power of the optical WDM signal.

[0069] The method 800 may include determining, by a control unit of the ROADM, that the input signal power indication is outside a specified optical power range (decision block 830). In some implementations, a control unit 140 or an optical channel monitoring unit 265 may receive the measured optical power level and determine whether the WDM optical signal is within acceptable parameters for absolute power amplitude and power spectral density. If the input signal power is outside the specified optical power range, the method 800 includes issuing a warning by the control unit 140 or the optical channel monitoring unit 265 (step 840). In some implementations, the second ROADM block 110 may monitor a condition of the fiber optic link 115 using an optical time domain reflectometer (OTDR) 260.The OTDR may monitor a condition of the fiber optic link 115 and the remote equipment along the link, and in the event of a link fault, provide a message including the location of the fault. If the optical channel control unit 140 or monitoring unit 265 issues a warning due to the out-of-spec optical input power, as determined at decision block 830, the warning may include information related to the condition of the fiber optic link as determined by the OTDR 260. If the input signal power is within the specified power range, the method 800 proceeds to step 850.

[0070] The method 800 includes amplifying the optical WDM signal using the input gain amplifier (stage 850). The ROADM block 110 may amplify the optical WDM signal using a switchable gain amplifier, such as the switchable gain amplifier 300 or 400. Based on an optical output signal power, as detected at stage 860 below, the optical channel controller 140 or the optical channel monitor 265 may adjust gain parameters (including selecting the low gain path or the high gain path) of the switchable gain amplifier 300 or 400 at stage 870 below. In some implementations, the optical channel controller 140 or the optical channel monitor 265 may adjust the equalization parameters of the switchable gain amplifier 300 or 400.

[0071] The method 800 includes detecting an output signal power of the amplified WDM optical signal using an output power detector coupled to an output of the input switchable gain amplifier (stage 860). The switchable gain amplifier 300 or 400 of the ROADM block 110 may include an output power detector 365 or 465. In some implementations, the output power detector 365 or 465 may detect the total power amplitude or power spectral density (i.e., power at each wavelength). The output power detector 365 or 465 may send an indication of the output signal power to the optical channel controller 140 or monitoring unit 265. In some implementations, the indication of the output signal power may include information related to the power spectral density.

[0072] The method 800 includes adjusting, by a control unit connected to the first ROADM block, gain and equalization parameters of the input amplifier with switchable gain based on the detected output signal power (stage 870). The control unit 140 or the optical channel monitoring unit 265 may receive the indication of the output signal power from the output power detector 365 or 465. Based on the selection in stage 870, the method 800 includes adjusting the gain and equalization parameters of the input amplifier with switchable gain in stage 850. The control unit 140 or the optical channel monitoring unit 265 may use the information to select either the low-gain path or the high-gain path for amplification in stage 850.In some implementations, the switchable gain amplifier 300 or 400 may include a wavelength blocker 345 to compensate for wavelength-based nonlinearities of the optical amplifiers. The optical channel controller 140 or the optical channel monitor 265 may adjust the equalization parameters of the wavelength blocker so that it selectively attenuates specific wavelengths of the WDM optical signal. In one implementation, the optical channel controller 140 or the optical channel monitor 265 may use the power spectral density information to adjust the wavelength blocker 345 to apply flattening or equalization to the optical signal to achieve a desired flatness of the amplified signal; for example, the optical channel controller 140 or the optical channel monitor 265 may adjust the wavelength blocker 345 to maintain an output signal flatness of 1 dB across the signal bandwidth.In some implementations, the switchable gain amplifier 300 or 400 may also include a variable optical attenuator 350. The optical channel controller 140 or monitoring unit 265 may adjust the variable optical attenuator 350 to fine-tune the overall gain of the switchable gain input amplifier 300.

[0073] The method 800 includes transmitting, using a wavelength-selective switch of the first ROADM block, an optical channel signal from the amplified WDM optical signal to a second ROADM block of the plurality of ROADM blocks (stage 880). The switchable gain amplifier 300 or 400 may transmit an amplified but spectrally relatively flat WDM optical signal to a wavelength-selective switch (WSS) 225 of the first ROADM block 110. The WSS block 225 may demultiplex the amplified WDM optical signal and transmit an optical channel signal from the amplified WDM optical signal to a second ROADM block 110. In some implementations, a second WSS block 225 of the second ROADM block 110 may multiplex the optical channel signal into a second WDM optical signal.The second ROADM block 110 may also include amplifier stages employing switchable gain amplifiers of a technology similar to the amplifiers of the first ROADM block 110. In some implementations, the equalization performed by the wavelength blocker 345 may mitigate the effect of cumulative wavelength-dependent gain nonlinearity of the multiple amplifiers in series.

[0074] In some implementations, multiple noise loading amplifiers 235 may introduce a noise loading signal into each unused port of wavelength-selective switch 225. Wavelength-selective switch 225 may send a second optical WDM signal to a switchable gain output amplifier of ROADM block 110, such as switchable gain amplifier 300 or 400. Switchable gain input amplifier 300 or 400 may amplify the second optical WDM signal. Output power detector 365 or 465 of switchable gain output amplifier 300 or 400 may detect a second output signal power of the amplified second optical WDM signal. Optical channel control unit 140 or monitoring unit 265 may adjust gain and equalization parameters of the switchable gain output amplifier based on the detected second output signal power.The method 800 may perform additional steps, fewer steps, or perform the steps in a different order without departing from the scope of the disclosure.

[0075] Fig. Figure 9 is a block diagram illustrating a general architecture for a computer system 900 that may be employed to implement elements of the systems and methods described and illustrated herein, according to an illustrative implementation. The computing system 900 may be configured to implement the methods for operating, for example, a Fig. 1-8 or any component thereof containing the control unit 140 and other blocks / modules.

[0076] In broad terms, computing system 910 includes at least one processor 950 for performing actions in accordance with instructions and one or more memory devices 970 or 975 for storing instructions and data. The illustrated example computing system 910 includes one or more processors 950 in communication, via a bus 915, with at least one network interface controller 920 having one or more network interface ports 922 connecting to one or more network devices 924, memory 970, and any other devices 980, e.g., an I / O interface. Generally, processor 950 will execute instructions received from memory. The illustrated processor 950 includes or is directly coupled to cache memory 975.

[0077] More specifically, the processor 950 may be any logic circuitry that processes instructions, e.g., instructions fetched from the memory 950 or the cache 975. In many embodiments, the processor 950 is a microprocessor unit or a special-purpose processor. The computing device 900 may be based on any processor or group of processors capable of operating as described herein. In some implementations, the processor 950 may be configured to perform the methods for maintaining a user session during an access network handover, as described in the Fig. 2-5. The processor 950 may be a single-core or multi-core processor. The processor 950 may be multiple processors. In some implementations, the processor 950 may be configured to run multi-threaded operations. In some implementations, the processor 950 may host one or more virtual machines or containers, along with a hypervisor or container manager for managing the operation of the virtual machines or containers. In such implementations, the Fig. 8 may be implemented within the virtualized or containerized environments provided on the processor 950.

[0078] The memory 970 may be any device suitable for storing computer-readable data. The memory 970 may be a fixed storage device or a device for reading removable storage media. Examples include all forms of non-volatile memory, media, and storage devices, semiconductor storage devices (e.g., EPROM, EEPROM, SDRAM, and flash memory devices), magnetic disks, magneto-optical disks, and optical disks (e.g., CD-ROM, DVD-ROM, and Blu-ray® disks). A computing system 900 may include any number of memory devices 970. In some implementations, the memory 970 may include instructions corresponding to the method for operating a reconfigurable optical add / drop multiplexer (ROADM) described in Fig.8. In some implementations, storage 970 supports virtualized or containerized storage accessible by virtual machine or container execution environments provided by compute system 910.

[0079] Cache memory 975 is generally a form of computer memory placed in close proximity to processor 950 for fast read times. In some implementations, cache memory 975 is part of or on the same chip as processor 950. In some implementations, multiple levels of cache 975, such as L2 and L3 cache layers, are present.

[0080] The network interface controller 920 manages data exchange over the network interfaces 922 (also referred to as network interface ports). The network interface controller 920 handles the physical and data link layers of the OSI model for network communications. In some implementations, some of the network interface controller's tasks may be handled by the processor 950. In some implementations, the network interface controller 920 is part of the processor 950. In some implementations, the computing system 910 includes multiple network interface controllers 920. The network interfaces 922 are connection points for physical network links. In some implementations, the network interface controller 920 supports wireless network connections, and an interface port 922 is a wireless transceiver.In general, a computing device 910 exchanges data with other network devices 924 over physical or wireless links to a network interface 922. In some implementations, the network interface controller 920 implements a network protocol such as Ethernet.

[0081] The other network devices 924 are connected to the computing device 910 via a network interface port 922. The other network devices 924 may be peer computing devices, network devices, or any other computing device with network functionality. For example, a first network device 924 may be a network device such as a hub, bridge, switch, or router that connects the computing device 910 to a data network such as the Internet.

[0082] The other devices 980 may be an I / O interface, external serial ports for devices, or any additional coprocessors. For example, a computing system 910 may include an interface (e.g., a Universal Serial Bus (USB) interface) for connecting input devices (e.g., a keyboard, microphone, mouse, or other pointing device), output devices (e.g., a video display device, speakers, or printer), or additional storage devices (e.g., a portable flash drive or external media drive). In some implementations, a computing device 900 includes an additional device 980 such as a coprocessor. For example, a math coprocessor may assist the processor 950 in high-precision or complex calculations.

[0083] Implementations of the subject matter and operations described in this specification may be implemented in digital electronic circuitry or in computer software embodied on a tangible medium, firmware, or hardware incorporating the structures disclosed in this specification and their structural equivalents, or combinations of one or more thereof. Implementations of the subject matter described in this specification may be implemented as one or more computer programs embodied on a tangible medium, that is, one or more modules of computer program instructions encoded on one or more computer storage media for execution by or for controlling the operation of a data processing device.A computer storage medium may be or be included in a computer-readable storage device, a computer-readable storage substrate, a storage array, a random or serial access storage device, or a combination of one or more of these. The computer storage medium may also be or be included in one or more separate components or media (e.g., multiple CDs, disks, or other storage devices). The computer storage medium may be tangible and non-transitory.

[0084] The operations described in this specification may be implemented as operations performed by a computing device on data stored in one or more computer-readable storage devices or received from other sources. The operations may be performed within the native environment of the computing device or within one or more virtual machines or containers hosted by the computing device.

[0085] A computer program (also referred to as a program, software, software application, script, or code) may be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it may be distributed in any form, including as a stand-alone program or as a module, component, subroutine, object, or any unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program may be contained in a section of a file containing other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in several coordinated files (e.g.,A computer program may be stored in a program file (e.g., files that store one or more modules, subroutines, or sections of code). A computer program may be distributed to run on one computer or on multiple computers, or on one or more virtual machines or containers located at one location or distributed across multiple locations and interconnected by a communications network. Examples of communications networks include a local area network (“LAN”) and a wide area network (“WAN”), an internetwork (e.g., the Internet), and peer-to-peer networks (e.g., ad hoc peer-to-peer networks).

[0086] The processes and logic sequences described in this specification may be executed by one or more programmable processors running one or more computer programs to perform actions by operating on input data and generating outputs. The processes and logic sequences may also be executed by special-purpose logic circuitry, such as a field-programmable gate array (FGPA) or an application-specific integrated circuit (ASIC), and the device may be implemented thereby.

[0087] Although this specification contains many specific implementation details, these should not be interpreted as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular implementations of the claimed subject matter. Specific features described in this specification in the context of separate implementations may also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation may also be implemented separately or in any suitable subcombination in multiple implementations.Furthermore, although features are described above as operating in specific combinations and are even initially claimed as such, in some cases one or more features from a claimed combination may be deleted from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination.

[0088] Similarly, although operations are depicted in the drawings in a particular order, this should not be understood to require that such operations be performed in the particular order shown or in sequential order, or that all of the illustrated operations be performed to achieve desirable results. Under special circumstances, multitasking and parallel processing may be advantageous. Furthermore, the separation of various system components in the implementations described above should not be understood to require such separation in all implementations, and it should be understood that the described program components and systems may generally be integrated together in a single software product or packaged into multiple software products.

[0089] References to "or" may be interpreted as inclusive, so that any terms described using "or" may indicate any one, more than one, or all of the described terms. The terms "first," "second," "third," and so on are not necessarily intended to indicate an order and are generally used merely to distinguish between identical or similar units or elements.

[0090] Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but are intended to correspond to the principles and novel features disclosed herein to the greatest extent consistent with this disclosure.

[0091] A reconfigurable optical add / drop multiplexer (ROADM) includes multiple interconnected ROADM blocks. Each ROADM block includes a switchable gain input amplifier, an output power detector coupled to an output of the switchable gain input amplifier, and a wavelength-selective switch coupled to the output of the switchable gain input amplifier. Each ROADM block includes multiple add / drop blocks coupled to the wavelength-selective switches of the multiple ROADM blocks. The ROADM includes a controller configured to receive an indication of output signal power from the output power detector and to adjust gain and equalization parameters of the switchable gain input amplifier based on the received indication of output signal power.

Claims

[1] Reconfigurable optical add / drop multiplexer (ROADM) (100), comprising: a plurality of interconnected ROADM blocks (110), each ROADM block (110) including a switchable gain input amplifier (245), an output power detector (365, 465) coupled to an output of the switchable gain input amplifier (245), and a wavelength-selective switch (225) coupled to the output of the switchable gain input amplifier (245); a plurality of add / drop blocks (120) coupled to the wavelength-selective switches (225) of the plurality of ROADM blocks (110); and a control unit (140) configured to receive an indication of an output signal power of the output power detector (365, 465) and to adjust gain and equalization parameters of the switchable gain input amplifier (245) based on the received indication of the output signal power, wherein the switchable gain input amplifier (245) includes a low gain fiber amplifier and a high gain fiber amplifier; and adjusting the gain and equalization parameters includes selectively routing an optical signal through the low-gain fiber amplifier or the high-gain fiber amplifier. [2] ROADM (100) according to claim 1, wherein: the switchable gain input amplifier (245) includes a variable gain fiber amplifier; and adjusting the gain and equalization parameters adjusting a gain of the variable gain fiber amplifier. [3] ROADM (100) according to one of claims 1 to 2, wherein each ROADM block (110) comprises: a switchable gain output amplifier (240), an input of the switchable gain output amplifier (240) being coupled to the wavelength-selective switch (225); a second output power detector (365, 465) coupled to an output of the switchable gain output amplifier (240); and a plurality of noise load amplifiers (235), each noise load amplifier (235) coupled to a terminal of the wavelength-selective switch (225), wherein the control unit (140) is configured: activating each noise load amplifier (235) coupled to an unused terminal of the wavelength selective switch (225); receive an indication of a second output signal power from the second output power detector (365, 465), and Adjusting gain and equalization parameters of the switchable gain output amplifier (240) based on the indication of the second output signal power. [4] ROADM (100) according to one of claims 1 to 3, wherein: the output power detector (365, 465) can detect the power spectral density; and the switchable gain input amplifier (245) includes an equalizer for flattening the output of the amplifier over a frequency range of the amplifier. [5] The ROADM (100) of claim 4, wherein the equalizer comprises a wavelength blocker (345); and adjusting the gain and equalization parameters includes selectively attenuating one or more wavelengths of an optical signal passing through the switchable gain input amplifier (245) based on the power spectral density detected by the output power detector (365, 465). [6] ROADM (100) according to one of claims 1 to 5, wherein: the wavelength-selective switch (225) is a C-band wavelength-selective switch (225a); the switchable gain input amplifier (245) is a switchable gain C-band input amplifier (245a); and each ROADM block (110) comprises an L-band wavelength-selective switch (225b) coupled to an L-band input amplifier (245b) with switchable gain. [7] ROADM (100) according to one of claims 1 to 6, wherein each ROADM block (110) comprises: an input power detector (310, 410) coupled to an input of the switchable gain input amplifier (245), wherein the control unit (140) is configured to receive an indication of an input signal power from the input power detector (310, 410), determine that the indication of the input signal power is outside a specified optical power range, and issue a warning based on the determination. [8] ROADM (100) according to claim 7, wherein each ROADM block (110) comprises: an optical time domain reflectometer for monitoring a condition of a fiber optic link (115) coupled to the ROADM block (110), wherein the alert includes information related to the condition of the fiber optic link (115). [9] ROADM (100) according to one of claims 1 to 8, wherein each ROADM block (110) comprises: a Raman amplifier (215) coupled between a fiber optic link (115) and the ROADM block (110); a second input power detector (505) coupled to an input of the Raman amplifier (215); and a second output power detector (520) coupled to an output of the Raman amplifier (215). [10] ROADM (100) according to one of claims 1 to 9, wherein each add / drop block (120) comprises: a multicast switch (720) coupled to a plurality of fiber optic links (125); and a group of amplifiers with switchable gain coupled between the multicast switch (720) and the wavelength-selective switches (225), wherein: each amplifier of the group includes a second output power detector (365, 465), and the control unit (140) is configured to receive indications of a second output signal power from the second output power detectors (365, 465) and to adjust the respective gain parameters of each amplifier of the group based on the received indications of the second output signal power. [11] ROADM (100) according to one of claims 1 to 10, wherein: the plurality of ROADM blocks (110) and the plurality of add / drop blocks (120) are interconnected via a fiber mix switchboard (130). [12] A method of operating a reconfigurable optical add / drop multiplexer (ROADM) (100), comprising: Receiving an optical wavelength division multiplexed (WDM) signal via a fiber optic link (115) at a first ROADM block (110) of a plurality of ROADM blocks (110); Amplifying the optical WDM signal using an input amplifier (300, 400) with switchable gain; Detecting an output signal power of the amplified optical WDM signal using an output power detector (365, 465) coupled to an output (370, 470) of the switchable gain input amplifier (300, 400); Adjusting, by a control unit (140) connected to the first ROADM block (110), gain and equalization parameters of the input amplifier (300, 400) with switchable gain based on the detected output signal power; and Transmitting, using a wavelength-selective switch (225) of the first ROADM block (110), an optical channel signal from the amplified optical WDM signal to a second ROADM block (110) of the plurality of ROADM blocks (110), wherein the switchable gain input amplifier (300, 400) includes a low gain fiber amplifier and a high gain fiber amplifier; and adjusting the gain and equalization parameters includes selectively routing the optical signal through the low-gain fiber amplifier or the high-gain fiber amplifier. [13] The method of claim 12, wherein: the switchable gain input amplifier (300, 400) includes a variable gain fiber amplifier; and Adjusting the gain and equalization parameters Adjusting the gain of the variable gain fiber amplifier. [14] A method according to any one of claims 12 to 13, comprising: Introducing a noise loading signal into each unused port of the wavelength selective switch (225) using a plurality of noise loading amplifiers (235); Sending a second optical WDM signal from the wavelength-selective switch (225) to a switchable gain output amplifier (300, 400) of the ROADM block (110); Amplifying the second optical WDM signal using the switchable gain output amplifier (300, 400); Detecting a second output signal power of the amplified second optical WDM signal using a second output power detector (365, 465) coupled to an output (370, 470) of the switchable gain output amplifier (300, 400); and Adjusting gain and equalization parameters of the switchable gain output amplifier (300, 400) based on the detected second output signal power. [15] A method according to any one of claims 12 to 14, wherein: detecting the output signal power includes detecting the power spectral density of the amplified second optical WDM signal, and adjusting the gain and equalization parameters includes selectively attenuating one or more wavelengths of the second optical WDM signal passing through the switchable gain input amplifier (300, 400) based on the detected power spectral density. [16] A method according to any one of claims 12 to 15, comprising: Detecting an input signal power of the optical WDM signal using an input power detector (310, 410) coupled to an input (305) of the input amplifier (300, 400) with switchable gain; Determining by the control unit (140) that the input signal power indication is outside a specified optical power range; and Outputting by the control unit (140) a warning based on the determination. [17] A method according to claim 16, comprising: Monitoring by an optical time domain reflectometer a state of a fiber optic link (115) coupled to the ROADM block (110); and Including information concerning the status of the fiber optic connection path (115) in the warning. [18] A method according to any one of claims 12 to 17, comprising: Detecting a plurality of second output signal powers using a plurality of second output power detectors (365, 465), each second output power detector (365, 465) being coupled to an amplifier from a group of switchable gain amplifiers coupled between a multicast switch (720) and the wavelength-selective switch (225); and Adjusting respective gain parameters of each amplifier of the group based on the detected plurality of second output signal powers.

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