Patent Description:
A typical optical network, for example a dense wavelength division multiplex (DWDM) network, has many passive and active components. Each of these components may add noise to a propagating optical channel signal. Monitoring noise levels at each component separately may help to detect and analyze particular sources of the noise. The information obtained from such monitoring may be used to address or compensate for sources of noise and may allow for improving the performance of the optical network.

Optical signal-to-noise ratio (OSNR) is one of the most important signal quality parameters in the optical communication system. It may be advantageous to provide improved OSNR monitoring techniques that are any or all of low cost, accurate, easy to use and non-intrusive.

<CIT> describes a method for determining an OSNR of a dual polarization optical signal. The method includes: detecting, in the dual polarization optical signal, a modulation signal which modulates, at at least one low amplitude level that is approximately zero and at a high amplitude level, the dual polarization optical signal, and determining the OSNR from a measurement of the power of the modulation signal.

<CIT> describes methods for monitoring and measuring OSNRs in optical communications systems. One exemplary method involves intentionally inserting zero power symbols into an optical signal stream such that those periods of time in which only zero power symbols are transmitted may be detected and compared with periods of time in which signals modulated with information including both signal and noise are detected such that the OSNR may be determined.

The document "<NPL> describes an in-band OSNR monitor based on differential pilot sequence at transmitter side together with DSP algorithms after coherent detection.

<CIT> describes an OSNR monitoring device. The OSNR monitoring device includes: a signal processing device that performs non-linear processing on a pilot signal in a received signal, or on a pilot signal in the received signal and data signals within a predetermined range neighboring to the pilot signal; and a calculation device that calculates the OSNR in the received signal based on a result of the non-linear processing.

An object of the present disclosure is to provide a technique to monitor performance of an optical network and in particular of optical network equipment.

In accordance with this objective, an aspect of the present disclosure provides methods, systems and apparatuses for transmitting and receiving an optical channel signal having low-power gaps and a pilot tone, and performing optical network equipment monitoring based on such low-power gap and the pilot tone.

Advantageous embodiments of the invention are given in the dependent claims.

When propagating through optical network equipment, an optical channel signal may accumulate noise from equipment of the optical network.

Noise in optical signal transmitted through the optical network includes noise caused by optical amplifiers and nonlinear noise. The noise caused by the optical amplifiers is a so-called amplifier spontaneous emission (ASE) noise. Nonlinear noise is caused by fiber nonlinear interference (NLI) and is one of the most important impairments of coherent optical communication systems.

Optical signal-to-noise ratio (OSNR) of the optical channel signal is usually monitored in the optical networks. In order to measure OSNR, ASE noise needs to be detected and analyzed separately from the nonlinear noise.

The technology as disclosed herein permits monitoring ASE noise and nonlinear noise separately.

The technology as disclosed herein also permits monitoring OSNR. The technology as described herein may be low-cost, accurate, and fast and provide real-time measurement monitoring data of OSNR. The technology permits using the advantages of amplitude modulation pilot tone and coherent digital signal processing (DSP).

According to one aspect of the disclosed technology, there is provided a method for monitoring performance of optical network. The method comprises receiving an optical channel signal. The optical channel signal comprises low-power gaps, gap power in the low-power gaps being at least five times lower than a signal power of the optical channel signal. The low-power gaps may be applied and detected at a gap frequency. The optical channel signal also comprises an amplitude modulation pilot tone having a pilot tone modulation frequency. The pilot tone modulation frequency may be different from the gap frequency. The method further comprises monitoring performance of fiber optical network equipment based on the amplitude of the pilot tone detected from the gap power in the low-power gaps of the received optical channel signal.

In accordance with additional aspects of the present disclosure, there is provided another method for monitoring performance of optical networks. The method comprises receiving an optical channel signal. The optical channel signal comprises low-power gaps, gap power in low-power gaps being at least five times lower than a signal power of the optical channel signal. The optical channel signal also comprises an amplitude modulation pilot tone. The low-power gaps may have a gap frequency and the amplitude modulation pilot tone may have a pilot tone modulation frequency. The pilot tone modulation frequency may be different from the gap frequency. The method further comprises removing chromatic dispersion from the optical channel signal and monitoring an optical signal-noise-ratio of the optical channel signal based on the amplitude of the pilot tone detected in low-power gaps of the optical channel signal with removed chromatic dispersion.

In accordance with other aspects of the present disclosure, there is provided an optical network monitoring apparatus. The apparatus comprises a receiver and a processor. The receiver is configured to receive an optical channel signal comprising: low-power gaps, a gap power in the low-power gaps being at least five times lower than a signal power of the optical channel signal, and an amplitude modulation pilot tone applied to the optical channel signal at a pilot tone modulation frequency. The low-power gaps may have gap frequency. The pilot tone modulation frequency may be different from the gap frequency. The processor is configured to monitor performance of optical network equipment based on the amplitude of the pilot tone detected from the gap power of the received optical channel signal. The receiver may be a coherent receiver.

In accordance with other aspects of the present disclosure, there is provided a non-transitory computer readable medium with computer executable instructions stored thereon that, when executed by a processor, cause the processor to determine gap power in low-power gaps of an optical channel signal; obtain the amplitude of the pilot tone based on the power in the gap (also referred to as gap power); and monitor performance of optical network equipment based on the amplitude of the pilot tone and the power in low-power gaps of the optical channel signal. The optical channel signal comprises low-power gaps, power in the low-power gaps being at least five times lower than a signal power of the optical channel signal. The optical channel signal also comprises an amplitude modulation pilot tone applied to the optical channel signal at a pilot tone modulation frequency. The low-power gaps may have gap frequency. The pilot tone modulation frequency may be different from the gap frequency.

In at least one embodiment, the pilot tone modulation frequency is lower than the gap frequency.

The amplitude modulation pilot tone and pilot tone modulation amplitude may be detected using fast Fourier transform.

In at least one embodiment, chromatic dispersion is removed from the optical channel signal before detecting the pilot tone modulation amplitude. The processor may be configured to remove chromatic dispersion from the optical channel signal.

In at least one embodiment, the gap power is between about <NUM>% and about <NUM>% of the signal power of the optical channel signal. The gap power may be about <NUM>% or lower than <NUM>% of the signal power of the optical channel signal.

Monitoring performance of the fiber optical network equipment may comprise monitoring an optical signal-to-noise ratio. In some embodiments, in addition to making use of information associated with the power in the low power gaps, the optical signal-noise-ratio may be calculated by using data of received optical channel signal outside of the low-power gaps.

Monitoring performance of the fiber optical network equipment may comprise monitoring of amplifier spontaneous emission (also referred to herein as ASE noise). Monitoring performance of the fiber optical network equipment may comprise monitoring of fiber nonlinear noise.

The features and advantages of the present disclosure will become apparent from the following detailed description, taken in combination with the appended drawings, in which:.

It is to be understood that throughout the appended drawings and corresponding descriptions, like features are identified by like reference characters.

As used herein, the term "about" or "approximately" refers to a +/-<NUM>% variation from the nominal value. It is to be understood that such a variation is always included in a given value provided herein, whether or not it is specifically referred to.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the described embodiments appertain to.

The instant disclosure is directed to a method and apparatus to address the deficiencies of the current state of the art. In particular, the instant disclosure describes systems, apparatuses and methods for monitoring performance of fiber optical networks.

Generally, embodiments of the present disclosure provide methods and apparatuses for transmitting, receiving and using an optical channel signal to monitor performance of an optical network based on low-power gaps and pilot tone both applied to the optical channel signal.

Referring now to the drawings, <FIG> depicts a block diagram of an optical network <NUM>. Such optical network <NUM> typically has a plurality of nodes, each node may include an optical add-drop multiplexer, such as a reconfigurable optical add-drop multiplexer (ROADM) <NUM>, having at least one wavelength selective switch (WSS). The optical network <NUM> may also have one or more laser light sources.

The optical network <NUM> typically includes a plurality of optical amplifiers, e.g. erbium-doped fiber amplifiers (EDFAs), for amplifying optical signals. The light sources and optical amplifiers are omitted in <FIG> for simplicity.

The optical network equipment, as referred to herein, comprises one or more passive and/or active optical network components and/or modules of the optical network <NUM>, including, but not limited to, optical fiber, optical amplifiers, optical filters, optical links, WSSs, arrayed waveguide gratings, and laser light sources.

Nodes <NUM> within the optical network <NUM> typically transmit signals on one of a plurality of optical wavelength channels. Throughout the present disclosure, the term "wavelength channels" denotes modulated optical signals at particular wavelengths. Wavelength channels are also referred to herein as "channels". Each channel is characterized by a channel bandwidth and a channel central frequency, typically defined by a frequency grid.

As referred to herein, the term "transmitted optical channel signal" refers to an optical channel signal before propagating through the optical network equipment. As referred to herein, the term "received optical channel signal" refers to an optical channel signal after propagating through the optical network equipment.

In the optical network <NUM>, an amplitude modulation pilot tone signal is used to monitor the channel power. The "amplitude modulation pilot tone" (also referred to herein as "pilot tone") is a low-frequency (e.g. kHz to MHz) modulation of the intensity of the pilot tone applied to a high-speed optical channel.

The modulation depth of the pilot tone is usually small (e.g. a few percent of the channel power). The pilot tone provides an in-band ancillary channel for performance monitoring.

Each channel can be modulated with a different pilot tone. For example, different modulation frequencies may be applied to different channels. Different spreading sequences may also be applied to spectra-spread pilot tones. Accordingly, a power of a particular pilot tone may be used for indicating the power of the high-speed optical channel in a wavelength division multiplexed (WDM) system. The pilot tone may be further modulated to carry channel characterizing information.

<FIG> illustrates more details of a link <NUM> between two ROADMs <NUM> of <FIG>. One ROADM <NUM> of <FIG> can receive one or more DWDM signals <NUM>, <NUM>, drop one or more channels from DWDM signals <NUM>, <NUM> at a receiver Rx, add one or more channels generated by a transmitter Tx, and pass through other channels. The signals may be added or dropped using a WSS (not shown). A transmitter Tx includes a laser light source (not shown). In some cases, dropped channels are converted from optical to electrical domain, and added channels are converted from electrical to optical domain. Otherwise, channels are switched or passed through in the optical domain. It should be noted that a coherent transceiver 227a, 227b may include transmitter Tx 220a, 220b and receiver Rx 225a, 225b.

The link <NUM> also includes a plurality of optical amplifiers <NUM>, e.g. EDFAs. Each optical amplifier <NUM> introduces noise to the optical channel signal. Such noise is related to amplifier spontaneous emission (ASE) and is referred to as "ASE noise". The ASE noise contributes to a total link noise. Monitoring of the total link noise may be performed to obtain a metric of a health condition in the network.

In addition to the ASE noise, fiber nonlinear noise is also a contribution to the total link noise. Fiber nonlinear noise is related to two major fiber nonlinearities in coherent fiber optical communication systems. Such fiber nonlinearities are self-phase modulation (SPM) and cross-phase modulation (XPM). SPM is the nonlinear effect caused by the signal itself as it propagates within the channel. XPM is the nonlinear effect on a signal carried within a channel caused by signals carried within the neighboring channels. The NLI caused by SPM/XPM behaves like additional noise and is often referred to as fiber nonlinear noise (also referred to herein as "nonlinear noise").

Nonlinear noise is an important impairment in coherent optical communication systems. Monitoring of the nonlinear noise in optical network operation is important for channel power optimization, fault localization, etc..

Existing methods and systems for monitoring fiber nonlinear noise have various limitations and drawbacks.

Optical signal-to-noise ratio (OSNR) qualifies the degree of optical noise interference on optical signals. OSNR is determined as a ratio of the power of optical channel signal to the power of the noise affecting the signal within a given bandwidth: <MAT> where Pi is signal power in the i-th channel, Bm is equivalent noise bandwidth, Ni is the noise power introduced within the Bm, Br is the reference optical bandwidth, which is usually <NUM>. It should be understood that in many implementations, Ni is treated as only the ASE noise, as the OSNR value is determined over a length of <NUM>.

OSNR is one of the most important parameters in transmission of the optical signal via the optical link and the optical network in general. Optical amplifiers are used in optical networks to compensate for various losses, such as fiber loss, and other component losses. OSNR, however, is degraded due to amplifications. While OSNR measurement is carried out for a number of reasons, including fault cause assessment and health monitoring. This information may be used to improve network performance. OSNR should take into account ASE noise and not the nonlinear noise. However, total noise, that is usually measured, includes both ASE noise and nonlinear noise.

Therefore, for monitoring optical networks performance, it is important to have systems and methods for monitoring of the nonlinear noise separately from ASE noise. Such methods and systems are also important when monitoring OSNR of the optical networks.

<FIG> depicts optical power of a transmitted waveform <NUM>. The optical channel signal with waveform <NUM> may be transmitted for example, by transmitter Tx 220a depicted in <FIG>.

In <FIG>, transmitted optical signal <NUM> has transmitted signal power <NUM> over a monitoring time period <NUM>. The transmitted waveform <NUM> has low-power gaps <NUM> during gap time periods <NUM>. Transmitted gap power <NUM> is much lower than transmitted signal power <NUM> outside of low-power gaps <NUM>.

<FIG> depicts optical power of a received waveform <NUM>. The optical channel signal with waveform <NUM> may be, for example, received by receiver Rx 225b depicted in <FIG>.

In <FIG>, received waveform <NUM> has received signal power <NUM> over monitoring time period <NUM>. Low-power gaps <NUM> received during gap time periods <NUM> have received gap power <NUM>. The received gap power <NUM> is much lower than received signal power <NUM> outside of low-power gaps <NUM>.

As discussed above, ASE noise and nonlinear noise are added to the signal during propagation through the link.

It should be noted that the nonlinear noise is present in low-power gaps <NUM> of the received waveform <NUM> due to spreading of signal symbols into neighboring signal symbols during propagation. The low-power gap <NUM> may disappear after propagation in link <NUM> due to chromatic dispersion. This may happen if low-power gap <NUM> is much narrower than the symbol spreading scale. After chromatic dispersion is removed digitally in the coherent receiver, low-power gaps <NUM> reappear as depicted in <FIG>.

<FIG> illustrates that received waveform <NUM> has ASE noise <NUM> and fiber nonlinear noise <NUM> introduced by fiber nonlinearity. These noise levels are generally additive to the signals. As such, received signal power <NUM> includes contributions from ASE noise <NUM> and fiber nonlinear noise <NUM>. The contribution of ASE noise <NUM> and fiber nonlinear noise <NUM> also forms a power level in the lower power gaps <NUM>. If corresponding transmitted gap power <NUM> is approximately <NUM>, the received gap power <NUM> represents total link noise.

Measuring the received gap power <NUM> in the low-power gap <NUM> provides an estimate or measure of the total noise power added to the optical signal during propagation through link <NUM>.

In order to monitor ASE noise and nonlinear noise separately, an amplitude modulation pilot tone can be applied to the optical channel signal, in addition to applying a low-power gap. Application of the pilot tone will now be described.

<FIG> depicts optical power of an alternative transmitted waveform <NUM>, in accordance with various embodiments of the present disclosure. The optical channel signal with waveform <NUM> may be transmitted, for example, by transmitter Tx 220a of coherent transceiver 227a in <FIG>. The transmitted optical channel signal with waveform <NUM> has, in addition to low-power gaps <NUM>, amplitude modulation pilot tone <NUM>, only one pilot tone period is shown here.

As depicted in <FIG>, the pilot tone modulation frequency is different from the frequency at which low-power gaps <NUM> occur (also referred to herein as "gap frequency"). In some embodiments, the pilot tone modulation frequency is lower than frequency of low-power gaps <NUM>. In some other embodiments, the pilot tone modulation frequency may be higher than frequency of low-power gaps <NUM>.

In some embodiments, the frequency of low-power gaps <NUM> may be variable with time. In some embodiments, low-power gaps <NUM> may be applied irregularly such that a time interval between neighboring low-power gaps <NUM> (also referred to herein as a "gap interval") may vary with time. In some embodiments, low-power gaps <NUM> may be configured to coincide in time and/or be compatible with a signal frame structure. When referring to low power gaps, it should be understood that the power in a low power gap should be substantially below the power outside of the gap. In some embodiments, the powers may differ by a factor of five. Thus, the power in the gap may be one-fifth the power of the other parts of the baseline signal.

<FIG> depicts optical power of a received waveform <NUM> corresponding to the transmission of waveform <NUM> through a link such as link <NUM>, in accordance with various embodiments of the present disclosure. Waveform <NUM> may be received, for example, at receiver Rx 225b in <FIG>. It should be noted that received waveform <NUM> is representative of the received signal after equalizations have been applied to mitigate or remove the chromatic dispersion and other linear effects, such as polarization mode dispersion. For example, the chromatic dispersion may be removed at a digital signal processor (DSP) with dispersion equalizer. The chromatic dispersion may be removed using any number of different techniques including frequency domain equalization.

Transmitted gap power <NUM> is much lower than transmitted signal power <NUM> outside of low-power gaps <NUM>. For example, transmitted gap power <NUM> may be about or lower than one third of the transmitted signal power <NUM>. In the illustrated embodiments, transmitted gap power <NUM> is approximately <NUM>. Gap power <NUM> may be about or lower than one fifth of the signal power <NUM>. Gap power <NUM> may be about or lower than <NUM>% of the signal power <NUM>, about or lower than <NUM>% of the signal power <NUM>, between about <NUM>% and about <NUM>% of the signal power <NUM>.

In some embodiments, low-power gaps <NUM> may be applied periodically at equal gap periods.

The duration of low-power gaps <NUM>, <NUM> - gap time periods <NUM>, <NUM> - may be chosen such that the gap time periods <NUM>, <NUM> are not too long in order to reduce overhead. If the gap time periods <NUM>, <NUM> are too long, it may need to take larger dispersion to fill the low-power gap with power at the receiver. For example, the duration of low-power gap <NUM>, <NUM> may be of the order of a few symbols to minimize overhead.

The received waveform <NUM> has both pilot tone <NUM> and low-power gaps <NUM>. When propagating though link <NUM>, the optical signal accumulates both ASE noise <NUM> and fiber nonlinear noise <NUM>. Therefore, received gap power <NUM> and received signal power <NUM> both include ASE noise <NUM> and nonlinear noise <NUM>. When summed, ASE noise <NUM> and nonlinear noise <NUM> represent at least a portion of total noise <NUM>. In the illustrated example, where transmitted gap power <NUM> is approximately <NUM>, received gap power <NUM> is largely representative of the summation of ASE noise <NUM> and nonlinear noise <NUM>.

<FIG> depicts received gap power <NUM> detected in low-power gaps <NUM> of received waveform <NUM>. <FIG> also depicts levels of ASE noise <NUM> and nonlinear noise <NUM> in received gap power <NUM>.

Because ASE noise is generated by optical amplifiers in the link, it is independent from the transmitted signal. Accordingly, ASE noise is not modulated by the pilot tone and remains constant. Nonlinear noise <NUM> is modulated by the pilot tone. This difference can be used to separately measure the two types of noise.

ASE noise is not modulated by the pilot tone. Accordingly, nonlinear noise <NUM> may be measured by detecting the amplitude of the pilot tone. To measure the nonlinear noise, the amplitude of the pilot tone is determined from measured received gap power <NUM>.

Detecting power <NUM> in the low-power gaps <NUM> provides a measurement of the amplitude of the pilot tone in the received optical signal (waveform <NUM>). The pilot tone modulation depth is fixed by the transmitter. However, the amplitude of the pilot tone in the received optical signal can be measured (as noted above this can be done in the low power gaps). The measured pilot tone amplitude then be used to assist in determining a nonlinear noise level. Total link noise includes both nonlinear noise and ASE noise. Therefore, ASE noise may be determined from total link noise and nonlinear noise. Thus ASE noise and nonlinear noise may be monitored separately. Furthermore, OSNR may be obtained from the ASE noise.

Methods and systems for optical network monitoring and for determining and monitoring of fiber nonlinear noise, ASE noise and OSNR will now be described in further detail.

<FIG> is a flowchart illustrating a method <NUM> for optical network monitoring, in accordance with various embodiments of the present disclosure. Optical channel signal <NUM> is generated <NUM> by transmitter Tx 220a (depicted in <FIG>). During gap time periods <NUM>, low-power gaps <NUM>, <NUM> are applied <NUM> to optical channel signal <NUM>. It should be noted that these steps are not necessarily performed sequentially. Low power gaps may be applied in the overhead portion of the frame structure to avoid delaying the delivery of payload information. Thus, the generation of the optical signal may be done in a manner that already includes low power gaps. The modulation of this signal using an AM pilot tone may be performed by the transmitter as a function of the transmission of a signal into a channel.

Pilot tone <NUM> is then applied to optical channel signal <NUM> with low-power gaps <NUM>, <NUM> to obtain transmitted optical channel signal <NUM>. The applied pilot tone <NUM> has a pilot tone frequency and a pilot tone modulation depth.

Those skilled in the art will appreciate that from the perspective of an outside observer of a transmitter, the transmitter is generating an optical signal having low-power gaps that is modulated in accordance with a pilot tone. This generated signal will have a characteristic modulation but will also have gaps in the signal representative of the lower power gaps.

<FIG> depicts an apparatus <NUM> for monitoring the performance of fiber optical network, in accordance with various embodiments of present disclosure.

The apparatus <NUM> is a so-called "coherent receiver" and uses an optical local oscillator (LO). The apparatus <NUM> may be included at least partially in receiver Rx 225a, 225b, and/or may be at least partially included in coherent transceiver 227a, 227b.

The optical signal <NUM> and local oscillator's signal LO <NUM> are received at polarization and phase diversity hybrid <NUM>, then the signals are detected by balanced photodetectors (BPD) <NUM>. The output of BPDs <NUM> is then transmitted to analog-to-digital converters (ADC) <NUM>.

After the signal passes through a digital signal processor (DSP) <NUM>, the signal is transmitted to a gating block <NUM> and to a symbol decision module <NUM>. In some embodiments an equalizer may serve the function of DSP <NUM>. Those skilled in the art will appreciate that the equalizer can be used to remove linear impairments caused by the propagation of the signal through the link. Such an equalizer may be embodied within a coherent receiver DSP.

The gating block <NUM> is configured to determine power in low-power gaps <NUM> of the signal and collect data related to power in low-power gaps <NUM>. The gating block <NUM> then transmits the data related to gap power in low-power gaps to a monitoring module <NUM>. It should be noted that power, amplitude or both of the signal may be determined and transmitted at any stage in apparatus <NUM>. It should be understood that gating block <NUM> can be implemented in any of a number of ways, including as a controlled switch.

The monitoring module <NUM> is configured to monitor performance of fiber optical network. In particular, the monitoring module <NUM> is configured to determine and monitor ASE noise, nonlinear noise, and OSNR as described herein.

It should be understood that receiver's DSP <NUM> may include gating block <NUM> and monitoring module <NUM>.

<FIG> is a flowchart illustrating a method <NUM> for optical network monitoring, in accordance with various embodiments of the present disclosure.

The receiver, for example receiver Rx 225b, receives <NUM> optical channel signal <NUM>. The received gap power <NUM> in low-power gaps <NUM> is collected and determined. This may be done, for example, at gating block <NUM>. The gating block <NUM> then transmits gap power <NUM> in low-power gaps <NUM>. The effects of chromatic dispersion can be removed from the received signal by use of an equalizer or at a DSP as discussed above. The removal of the effects of chromatic dispersion from the overall signal allows the determination <NUM> of gap power <NUM> to reflect total noise <NUM> in low-power gaps <NUM>. The removal of chromatic dispersion effects may also be performed at the same time as correction of linear impairment caused Inter-Symbol Interference (ISI). ISI may also obscure the measurement of total noise <NUM>, so its removal along with the chromatic dispersion effects in advance of noise measurement is beneficial.

To improve detection of low-power gaps <NUM>, chromatic dispersion and other impairments may be mitigated or removed <NUM>. The corrected signal can then be analysed to determine <NUM> the received gap power <NUM> in low-power gaps <NUM>, as it is illustrated in <FIG>. Alternatively, chromatic dispersion may be removed from gap power <NUM> after determining of received gap power <NUM> in low-power gaps <NUM>.

Although nonlinear noise may be present in received signal <NUM>, it may be hard to distinguish nonlinear noise without removing chromatic dispersion from the received signal. The removal of other linear impairments is also of value. In order to determine nonlinear noise level in low-power gaps <NUM>, dispersion equalization should be applied.

For example, if signal spectral width is <NUM>, after propagation through <NUM> of standard single mode (SSMF) fiber with about 17ps/nm/km chromatic dispersion coefficient, a symbol spread may be about <NUM> ps. This corresponds to more than <NUM> symbol duration. If the gap time period is <NUM> symbols long, then, in dispersion uncompensated links, low-power gap may only show in the waveform in the first span. For the remaining of the link, low-power gap may become undistinguishable in the optical signal. However, nonlinear interferences in the link happen all the time. This explains why the low power gaps can be used to monitor fiber nonlinear noise: with chromatic dispersion, the zero power gaps are actually filled with power spread from the neighboring symbols, there is no gaps in the signal for the majority of the propagation.

Therefore, the accumulated chromatic dispersion needs to be removed at the receiver in order to recover the signal with the low-power gaps, along with nonlinear noise. The impact of removing the chromatic dispersion from the signal is illustrated in <FIG>.

<FIG> depicts simulation results for <NUM> Gbaud Quadrature Phase Shift Keying (QPSK) signal with a gap of <NUM> symbols. In the simulation, the signal propagated through <NUM> spans of SSMF with <NUM> dBm launch power. The signal experienced dispersion, as well as SPM.

In <FIG>, a thick solid line <NUM> represents an initial amplitude waveform with no power in gap <NUM>. A narrower line <NUM> represents a waveform after single channel nonlinear propagation (<NUM> spans of <NUM> SSMF, <NUM> dBm launch power) without removing dispersion. There is no low-power gap in the waveform if chromatic dispersion is not removed. A dashed line <NUM> represents a waveform after single channel nonlinear propagation with removed chromatic dispersion. After the chromatic dispersion is removed, the low-power gap reappears, and the power in the low-power gap is not zero due to SPM noise. No ASE noise was added in the simulation.

Referring again to <FIG>, after total noise <NUM> has been determined <NUM> in low-power gaps <NUM>, the amplitude of the pilot tone can be determined <NUM> in accordance with gap power <NUM>. In some embodiments, fast Fourier transform (FFT) algorithms may be used to detect the pilot tone and its modulation depth. If the pilot tone has a known frequency, other algorithms may be used. For example, Goertzel algorithm may be used.

To determine the amplitude of the pilot tone modulation from gap power <NUM>, low-power gaps <NUM>, <NUM> are applied at a gap frequency that may be different from the pilot tone modulation frequency. This permits to have sufficient data in order to determine the pilot tone amplitude. The gap frequency may be lower or higher than the pilot tone modulation frequency. As noted above, low-power gaps <NUM> may be irregular such that a time interval between neighboring low-power gaps <NUM> may vary with time.

The nonlinear noise power may then be determined by converting the pilot tone amplitude/amplitude to power, using known modulation depth.

A scaling factor k may be applied to pilot tone modulation depth to determine <NUM> nonlinear noise according to the following formula : <MAT> where PnonASE is nonlinear noise power <NUM>, PPT - is gap power <NUM>.

The scaling factor k may be inversely proportional to pilot tone modulation depth.

ASE noise <NUM> may then be determined <NUM> based on the following formula: <MAT> where Pt is total noise in low-power gaps. If the transmitted gap power <NUM> was approximately <NUM>, then Pt is equal to received gap power <NUM> measured in low-power gaps <NUM>.

The monitoring module <NUM> may monitor performance of fiber optical network and its equipment by monitoring <NUM> ASE noise <NUM> and by monitoring <NUM> nonlinear noise. Data may be collected during monitoring time period <NUM>.

It should be noted that monitoring time period <NUM> may depend on the application. Monitoring time period <NUM> may be long enough so that pilot tone amplitude may be determined accurately. Because nonlinearity is generally static or slowing changing (often as a result of loading and channel power) monitoring time period <NUM> can be set to a value that allows for treating the nonlinearity as static. Hence, the monitoring of the nonlinear noise does not have to be very fast. For example, monitoring time period <NUM> of <NUM> second may provide enough statistics to calculate noises using data collected for low-power gap.

OSNR may be determined <NUM> as follows. First, ASE noise <NUM> for the whole monitoring time period <NUM> may be determined <NUM>. Signal power <NUM> outside of the low-power gaps <NUM> may also be determined <NUM>. It should be understood that in some embodiments, the signal power of the signal inclusive of the gaps may be determined in place of the signal power <NUM> outside of the low power gaps. ASE noise power needs to be scaled <NUM> to power per <NUM> according to standard OSNR definition. OSNR may be then determined from ASE noise <NUM> for the optical channel signal.

A portion of gap power <NUM>, induced by fiber nonlinearity, is proportional to the signal power around the low-power gap. Therefore, if the signal power is modulated by pilot tone <NUM>, then the nonlinear noise power is also modulated by pilot tone <NUM>. In order to verify that nonlinear noise <NUM> is part of transmitted gap power <NUM> and that nonlinear noise <NUM> is modulated by pilot tone <NUM>, the following computer-based simulations have been performed.

<FIG> depicts simulated nonlinear noise power <NUM> in low-power gap as a function of low-power gaps in a waveform with self-phase modulation (SPM). Only the signal channel was simulated in the nonlinear propagation through <NUM> spans of <NUM> SSMF fiber. Random data symbols were divided into blocks, each block had <NUM> symbols, and there were <NUM> blocks. In each block, the power of <NUM> symbols was set to zero at Tx. Gap periods <NUM> were <NUM> symbols, and the gap interval (time interval between the low-power gaps) was <NUM> symbols. For the pilot tone, a modulation of <NUM>% was used, which was significantly higher than what would be used in real systems. The pilot tone frequency was chosen so that there is one period in the entire simulation length (<NUM> blocks of <NUM> symbols). There was no ASE noise in the simulation, so the noise power in the low-power gap was nonlinear noise only.

<FIG> also depicts a fitting sinusoid curve <NUM> with <NUM>% modulation depth. <FIG> shows that the nonlinear noise power in the low-power gaps was modulated by the pilot tone, and the modulation depth of the received pilot tone was the same as modulation depth of the transmitted pilot tone.

<FIG> depicts simulated nonlinear noise power <NUM> in low-power gaps as a function of low-power gaps in a waveform with SPM and XPM. The simulation parameters were similar to simulations in <FIG>, except that there were several adjacent channels to introduce cross-phase modulation (XPM). Gap periods <NUM> were <NUM> symbols, and the gap interval (time interval between the low-power gaps) was <NUM> symbols. The nonlinear noise (SPM and XPM) in the low-power gap was also modulated by the pilot tone with the same modulation depth as in the transmitted signal. <FIG> also depicts a fitting sinusoid curve <NUM> with <NUM>% modulation depth.

The simulations depicted in <FIG> and <FIG> show that it is possible to monitor the nonlinear noise by detecting the pilot tone amplitude using the power measured in low-power gaps of the optical channel signal.

In at least one embodiment, there is a non-transitory computer readable medium with computer executable instructions stored thereon that, when executed by a processor, cause the processor to perform methods described herein, such as method <NUM> and method <NUM>. In some embodiments, the instructions, executed by a processor, may cause the processor to: determine a gap power in low-power gaps of an optical channel signal, determine a pilot tone amplitude based on the gap power; and monitor performance of optical network equipment based on the pilot tone determined in low-power gaps of the optical channel signal.

In some embodiments, the processor may be configured to determine at least one of a fiber nonlinear noise and an amplifier spontaneous emission separately. In some embodiments, the processor may be configured to determine an optical signal-noise-ratio based on the pilot tone modulation depth determined from the gap power detected in the low-power gaps of the optical channel signal.

Claim 1:
A method (<NUM>) for monitoring performance of optical networks comprising:
receiving (<NUM>) an optical channel signal, the optical channel signal comprising:
low-power gaps (<NUM>) having a power (<NUM>) no greater than one fifth of a signal power (<NUM>) of the optical channel signal outside the low-power gaps (<NUM>), and
an amplitude modulation pilot tone having a pilot tone modulation frequency; and
monitoring performance of fiber optical network equipment based on the power (<NUM>) in the low-power gaps (<NUM>) and the amplitude of the pilot tone detected from the gap power in the low-power gaps of the received optical channel signal.