Patent Description:
Typical optical networks, such as, for example, dense wavelength division multiplex (DWDM) networks, transmit multiple optical channel signals. Each of these optical channel signals have to propagate through various optical network elements, such as, optical filters.

In order to fulfill current demands for increased capacities and higher data rate transmissions, signal baud rates need to be as high as possible. Higher baud rates may be achieved with narrower and more stable guard bands between the optical channel signals. However, optical network elements are susceptible to temperature changes, manufacturing defects, and other various factors that may result in relative frequency offsets between the transmittances of optical network elements and the spectrum of a transmitted optical channel signal. Examples of such relative frequency offsets include relative frequency offsets between optical filter transmittance and the optical channel spectrum of the transmitted optical channel signal. These relative frequency offsets may alter the guard bands between the optical channel signals and reduce the baud rate, thereby compromising the throughput performance of the optical network. <CIT> concerns feedback controlled locking of optical channel signals in optical receivers in a wavelength division multiplexing (WDM) communication systems, including ultra dense WDM systems. <CIT> addresses a technique for repelling carriers in a frequency division multiplexing (FDM) lightwave communication system to minimize mutual interference between signals, and <CIT> discusses the use of multiple shared wavelength lockers to stabilize transponders in a WDM network.

Particular embodiments are specified in the dependent claims. An object of the present disclosure is to further provide a technique for controlling optical channel signal in order to improve performance of an optical network. The apparatuses, methods and systems as disclosed herein permit reducing the relative frequency offset between optical filter transmittance and an optical channel spectrum of a transmitted optical channel signal in order to improve optical network performance.

In accordance with this objective, various aspects of the present disclosure provide methods, systems and apparatuses for transmitting and receiving an optical channel signal that is dithered with regards to frequency, and for controlling optical signal by adjusting the optical network equipment based on measured bit error rates.

Based on measured bit error rates, a receiver is configured to determine whether the optical channel spectrum needs to be adjusted. The optical channel spectrum may be shifted with regards to frequency by shifting towards higher or lower frequencies. An original signal reference frequency may be requested to be increased or decreased in order to reduce the relative frequency offset. A transmitter is instructed to either increase the original signal reference frequency of the optical channel signal or to decrease the original signal reference frequency.

At the same time, or alternatively, an optical filter may be instructed to shift an optical filter transmittance by increasing or decreasing the filter reference frequency in order to reduce the relative frequency offset.

In accordance with this objective, an aspect of the present disclosure provides a method for controlling an optical channel signal in an optical network, the optical channel signal having an optical channel spectrum and an original signal reference frequency. The method comprises transmitting a dithered optical channel signal obtained by alternately detuning of the optical channel spectrum with regards to frequency, a dithered signal reference frequency of the dithered optical channel signal being detuned to a first signal reference frequency during first time periods and to a second signal reference frequency during second time periods, the first signal reference frequency being lower than the second signal reference frequency. The method further comprises receiving a request to shift the optical channel spectrum of the optical channel signal with regards to frequency, the request comprising an indication of a direction of shifting of the optical channel spectrum with regards to frequency; and shifting the optical channel spectrum with regards to frequency based on the received request. The dithered optical signal spectrum is more aligned with a transmittance of the optical filter than the optical signal spectrum.

The request to shift the optical channel spectrum of the optical channel signal may comprise a request to increase the original signal reference frequency of the optical channel spectrum or a request to decrease the original signal reference frequency of the optical channel spectrum. The detuning of the optical channel spectrum with regards to frequency may be performed digitally by a digital signal processor or by detuning of a laser light source. The detuning of the optical channel spectrum with regards to frequency may be performed digitally by a digital signal processor and shifting the optical channel spectrum with regards to frequency based on the received request may be performed by a laser light source. The request to shift the optical channel spectrum may further comprise a frequency adjustment step.

The method may further comprise receiving a request to adjust the optical channel spectrum of the optical channel signal based on a bit error rate difference between a second bit error rate and a first bit error rate. The first bit error rate may be measured and averaged during the first time periods, and the second bit error rate may be measured and averaged during the second time periods. The method further comprises adjusting the optical channel spectrum of the optical channel signal based on the received request.

The request to adjust the optical channel spectrum of the optical channel signal may comprise a request to increase an original signal reference frequency of the optical channel spectrum or a request to decrease the original signal reference frequency of the optical channel spectrum. The request to increase the original signal reference frequency may be received in response to the bit error rate difference being negative, and the request to decrease the original signal reference frequency may be received in response to the bit error rate difference being positive. The request to adjust the optical channel spectrum may further comprise a frequency adjustment step. The first time periods and the second time periods may be repeated during a monitoring time period.

The optical channel signal may be a first carrier of a dual-carrier optical signal, the dual-carrier optical signal comprising the first carrier and a second carrier. The second carrier may have a second optical channel spectrum and a second original signal reference frequency. The method may further comprise: transmitting a second dithered carrier obtained from the second carrier by alternately detuning of the second optical channel spectrum with regards to frequency, the second dithered optical channel spectrum having a second dithered signal reference frequency being detuned to: a third signal reference frequency during third time periods, and a fourth signal reference frequency during fourth time periods; receiving a request to shift the second optical channel spectrum of the second carrier with regards to frequency, the request comprising an indication of a direction of shifting of the second optical channel spectrum with regards to frequency; and shifting the second optical channel spectrum of the second carrier with regards to frequency based on the received request. The third signal reference frequency may be lower than the second original signal reference frequency. The fourth signal reference frequency may be higher than the second original signal reference frequency. The fourth signal reference frequency may be higher than the third signal reference frequency.

In accordance with other aspects of the present disclosure, there is provided an apparatus for optical networks. The apparatus comprises a laser light source configured to generate an optical channel signal having an optical channel spectrum; and a processor. The processor is configured to dither optical channel spectrum with regards to frequency. The dithered optical channel signal has a dithered optical channel spectrum. A dithered signal reference frequency is detuned to: a first signal reference frequency during first time periods, and a second signal reference frequency during second time periods, the second signal reference frequency being higher than the first signal reference frequency. The processor is also configured to receive an indication of a direction of shifting of the optical channel spectrum with regards to frequency; shift, by a frequency adjustment step, the optical channel spectrum with regards to frequency based on the indication.

In accordance with additional aspects of the present disclosure, there is provided a method for controlling an optical network equipment in the optical network. The method comprises receiving a dithered optical channel signal. The dithered optical channel signal may be obtained from an optical channel signal by dithering an optical channel spectrum with regards to frequency. The dithered optical channel signal has a dithered signal reference frequency that is detuned to: a first signal reference frequency during first time periods, and a second signal reference frequency during second time periods, the second signal reference frequency being higher than the first signal reference frequency. The method further comprises measuring and averaging a first bit error rate of the dithered optical channel signal during the first time periods and measuring and averaging a second bit error rate of the dithered optical channel signal during the second time periods. The method further comprises transmitting a request to the optical network equipment to adjust operation of the optical network equipment based on a bit error rate difference between the second bit error rate and the first bit error rate.

The optical network equipment may be a transmitter. The request to adjust operation of the optical network equipment may further comprise a request to adjust the optical channel spectrum of the optical channel signal. The request to adjust operation may comprise an indication based on the bit error rate difference being positive or negative. The request to adjust operation of the optical network equipment may comprise a request to increase an original signal reference frequency, and the request to increase the original signal reference frequency may be transmitted in response to the bit error rate difference being negative. The request to adjust operation of the optical network equipment may comprise a request to decrease the original signal reference frequency, and the request to decrease the original signal reference frequency may be transmitted in response to the bit error rate difference being positive.

The dithered optical channel signal may be a first dithered carrier of a dual-carrier optical signal and the bit error rate difference may be a first carrier bit error rate difference. The dual-carrier optical signal may comprise the first dithered carrier and a second dithered carrier. The second dithered carrier may be dithered with regards to frequency. The second dithered carrier may have a second dithered signal reference frequency detuned to a third signal reference frequency during third time periods and a fourth signal reference frequency during fourth time periods.

The request to adjust the optical channel spectrum of the optical channel signal may be further based on a second bit error rate difference between a fourth bit error rate and a third bit error rate. The third bit error rate may be measured and averaged when the second dithered carrier reference frequency is detuned to the third signal reference frequency, and the fourth bit error rate may be measured and averaged when the second dithered carrier reference frequency is detuned to the fourth signal reference frequency.

The method may further comprise: measuring and averaging a third bit error rate of the dithered optical channel signal during the third time periods and a fourth bit error rate of the dithered optical channel signal during the fourth time periods; and transmitting a request to the optical network equipment to adjust operation of the optical network equipment. The request to adjust operation of the optical network equipment may be based on the first carrier bit error rate difference; a second carrier bit error rate difference between the fourth bit error rate and the third bit error rate; and a difference between the third bit error rate and the first bit error rate.

The dithered optical channel signal may be received after propagating through an optical filter. The optical network equipment may be the optical filter, and the request to adjust operation of the optical network equipment may further comprise a request to shift an optical filter transmittance of the optical filter by increasing or decreasing an optical filter reference frequency. The request to adjust operation of the optical network equipment may be based on the bit error rate difference being positive or negative. The request to adjust operation of the optical network equipment may comprise an indication of the bit error rate being positive or negative. In accordance with other aspects of the present disclosure, there is provided another apparatus for optical networks. The apparatus comprises: a photodetector configured to receive a dithered optical channel signal, and a processor. The dithered optical channel signal has a dithered signal reference frequency being detuned to: a first signal reference frequency during first time periods, and a second signal reference frequency during second time periods, the second signal reference frequency being higher than the first signal reference frequency. The processor is configured to: determine an averaged first bit error rate during the first time periods and an averaged second bit error rate during the second time periods; and generate and transmit a request to optical network equipment to adjust operation of the optical network equipment based on a bit error rate difference between the second bit error rate and the first bit error rate.

The optical network equipment may be a transmitter. The request to adjust operation of the optical network equipment may comprise a request to adjust an optical channel spectrum of the optical channel signal. The request to adjust an optical channel spectrum of the optical channel signal may be based on the bit error rate difference being positive or negative. The request to adjust the optical channel spectrum may comprise a request to increase an original signal reference frequency of the optical channel spectrum or a request to decrease the original signal reference frequency of the optical channel spectrum. The request to increase the original signal reference frequency may be generated in response to the bit error rate difference being negative. The request to decrease the original signal reference frequency may be generated in response to the bit error rate difference being positive. The request to adjust the optical channel spectrum may comprise an indication of the bit error rate difference being positive or negative. The request to adjust the optical channel spectrum may comprise an indication of a direction of a shift of the optical channel spectrum with regards to frequency.

The optical network equipment may be an optical filter, and the request to adjust operation may comprise a request to shift an optical filter transmittance with regards to a frequency by increasing or decreasing of optical filter reference frequency. The request to adjust operation of the optical network equipment may be based on the bit error rate difference being positive or negative. The optical channel signal may be a first carrier of a dual-carrier optical signal, the dithered optical channel signal may be the first dithered carrier, and the bit error rate difference may be the first carrier bit error rate difference. The photodetector may be further configured to receive the first dithered carrier and a second dithered carrier, the second dithered carrier may have a dithered signal reference frequency that is detuned to a third signal reference frequency during third time periods and a fourth signal reference frequency during fourth time periods, the fourth signal reference frequency being higher than the third signal reference frequency. The processor may be further configured to: determine an averaged third bit error rate during the third time periods and an averaged fourth bit error rate during the fourth time periods; and generate and transmit a request to the optical network equipment to adjust operation of the optical network equipment based on: the first carrier bit error rate difference; a second carrier bit error rate difference between the fourth bit error rate and the third bit error rate; and a difference between the third bit error rate and the first bit error rate.

In accordance with additional aspects of the present disclosure, there is provided a method for controlling an optical channel signal in an optical network, the optical channel signal having an optical channel spectrum and an original signal reference frequency. The method comprises transmitting a dithered optical channel signal obtained by detuning of the optical channel spectrum with regards to frequency, the dithered optical channel signal having a dithered signal reference frequency being detuned to: a first signal reference frequency during first time periods, the first signal reference frequency being lower than the original signal reference frequency, and a second signal reference frequency during second time periods, the second signal reference frequency being higher than the original signal reference frequency. The method further comprises receiving a request to shift the optical channel spectrum of the optical channel signal with regards to frequency, the request comprising an indication of a direction of shifting of the optical channel spectrum with regards to frequency; and shifting the optical channel spectrum with regards to frequency based on the received request.

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. Furthermore, it is also to be understood that the drawings and ensuing descriptions are intended for illustrative purposes only and that such disclosures are not intended to limit the scope of the claims.

The instant disclosure is directed to systems, methods and apparatuses to address the deficiencies of the current state of the art. To this end, the instant disclosure describes systems, apparatuses and methods directed to reducing relative frequency offset between an optical channel spectrum and an optical filter transmittance, allowing for higher signal baud rate, and therefore improving throughput performance of the optical network.

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.

The optical network equipment, as referred to herein, comprises one or more of at least one of active and passive optical network components. These components or modules are typically elements of an optical network, including, but not limited to, optical fiber, optical amplifiers, optical filters, WSSs, optical links, arrayed waveguide gratings, laser light sources, transmitters and receivers.

Throughout the present disclosure, the term "optical channel signal" refers to modulated optical signals at particular carrier frequencies, that is a signal that is carried in an optical link. Similarly, the term "transmitted optical channel signal" refers to an optical channel signal that is transmitted into the optical link by an optical transmitter. The term "received optical channel signal" refers to an optical channel signal, after having been propagated through the optical link, as received by an optical receiver.

In addition, a signal reference frequency, as disclosed below, refers to at least one of a signal carrier frequency, a signal central frequency and a frequency of a maximum of optical channel spectrum, which may coincide or may be different from each other. It should be understood that when an optical channel spectrum is shifted with regard to frequency locations, the signal reference frequency commensurately shifts. Furthermore, a filter reference frequency, as discussed below, refers to at least one of an optical filter central frequency and a frequency of optical filter transmittance' maximum (peak), which may coincide or may be different from each other. It should be understood that when an optical filter transmittance is shifted with regards to frequencies, the filter reference frequency shifts with regards to frequencies.

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.

Referring now to the drawings, <FIG> (Prior Art) depicts a block diagram of a representative optical network <NUM>. Generally, optical network <NUM> comprises multiple nodes/elements, in which each node may include an optical add-drop multiplexer, such as, for example, a reconfigurable optical add-drop multiplexer (ROADM) <NUM>. ROADM <NUM> may include at least one wavelength selective switch (WSS). The optical network <NUM> may also include one or more laser light sources as well as amplification nodes (which have been omitted for simplicity).

Optical network <NUM> is generally designed to transmit a plurality of optical channel signals, in which each optical channel signal is characterized by a channel bandwidth and a signal central frequency, in accordance with frequency grid guidelines, such as, for example, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) frequency grid.

Each optical channel signal has an optical channel spectrum. Channel central frequencies of two neighbouring optical channel signals are separated by a frequency spacing. A guard band separating the two neighbouring optical channel signals is defined by the optical channel spectra.

Returning back to <FIG>, ROADMs <NUM> may comprise one or more optical filters. By way of a non-limiting example, the WSS of ROADMs <NUM> may act as an optical filter for the optical channel signals routed by the WSS. Each of these optical filters may be characterized by an optical filter transmittance and a filter bandwidth.

<FIG> depicts a block diagram of a three-degree ROADM node <NUM>. ROADM node <NUM> comprises WSSs <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> (also referred to herein collectively as WSS <NUM>), receivers Rx <NUM>, <NUM>, <NUM>, and transmitters Tx <NUM>, <NUM>, <NUM>. The transmitters <NUM>, <NUM>, <NUM> may include, for example, laser light sources (not shown).

The ROADM node <NUM> is configured to receive one or more DWDM signals <NUM>, <NUM>, <NUM> having a plurality of optical channel signals. One or more optical channel signals may be dropped from DWDM signals <NUM>, <NUM>, <NUM> at receivers <NUM>, <NUM>, <NUM>. WSS <NUM> may act as an optical filter in order to select one or more optical channel signals from DWDM signals <NUM>, <NUM>, <NUM>. ROADM node <NUM> may be also configured to add one or more optical channel signals generated by transmitters <NUM>, <NUM>, <NUM>, as well as allow the passing through of optical channel signals. WSS <NUM>, acting as an optical filter, may also filter the optical channel signals after they have been generated by transmitters <NUM>, <NUM>, <NUM>.

Dropped optical channel signals may be converted from the optical domain to the electrical domain, and added optical channel signals may be converted from the electrical domain to the optical domain. Otherwise, optical channel signals are switched or passed through in the optical domain.

As noted above, optical channel signals and signal reference frequencies are generally defined by frequency grid guidelines, such as, for example, the ITU-T frequency grid specifications. However, various factors, such as any one or more of changes in temperature, manufacturing errors, and transmitter control errors may result in the detuning of the optical channel spectra as well as the detuning of signal frequencies, emitted by transmitters <NUM>, <NUM>, <NUM>, from ITU-T grid frequencies.

Furthermore, any one of more of temperature changes, manufacturing errors and control errors may also compromise performance of optical filters. These factors may also result in the detuning of the optical filter transmittances and detuning of filter frequencies from ITU-T grid frequencies.

It will be appreciated that the detuning of at least one of the optical channel spectrum and the optical filter transmittance may result in the relative frequency offsets. These relative frequency offsets may occur due to one or both of the optical filters and the transmitters that emit optical channel signals. For example, the accuracy of a laser central frequency (or of a laser peak frequency) of a laser light source used in transmitters <NUM>, <NUM>, <NUM> is usually specified with precision of +/-<NUM>. A filter central frequency of WSSs <NUM> is also specified with precision of +/-<NUM>. However, the laser central frequency and the filter central frequency may both be detuned (i.e., drift) in different frequency directions. In other words, the laser central frequency may increase while the filter central frequency may decrease, or vice-versa. These detuning issues may result in at least a few-GHz relative frequency offsets between the signal central frequency and the filter central frequency.

Such relative frequency offset may cause significant impairment to the performance of the optical network. As noted above, the relative frequency offset may require larger guard bands between the optical channel signals which, in turn, results in lower signal baud rates. In prior art optical networks, additional guard bands may be implemented to account for the unwanted effects of relative frequency offsets. However, the use of such guard bands results in the inefficient use of spectral bandwidth, as the bandwidth allocated to the guard bands cannot be used for data transmission.

The disclosed embodiments provide methods and systems directed to implementations configured to mitigate relative frequency offsets that may occur between the optical channel spectrum and the optical filter transmittance during optical signal transmission.

It should be understood that the relative frequency offset between the optical channel spectrum and the optical filter transmittance corresponds to the relative frequency offset between the signal reference frequency and the filter reference frequency. Both are referred to herein as the relative frequency offset.

It should also be understood that, although reference frequencies and the relative frequency offset are discussed herein, the embodiments discussed herein may be equally applied to central wavelengths and relative wavelength offset, by using frequency to wavelength conversion known in the art.

<FIG> depicts an optical link <NUM> having a transmitter <NUM> and a receiver <NUM>, in accordance with embodiments of the present disclosure, as described herein. As, shown, the transmitted optical channel signal is generated by transmitter <NUM>. The received optical channel signal, after having propagated through an optical filter <NUM>, is received at receiver <NUM>, which in some embodiments may be a coherent receiver.

The transmitter <NUM> may have a laser light source <NUM> configured to emit an optical channel signal, and a processor such as a digital signal processor (DSP) <NUM>. The receiver <NUM> may have a photodetector <NUM> configured to receive the optical channel signal, and a receiver processor <NUM>. Transmitter <NUM> and receiver <NUM> may also have other components such as, for example, digital-to-analog converters (DAC), drivers, and electro-optic (EO) modulators, not depicted in <FIG>.

<FIG> illustrates a relative frequency offset <NUM>, in accordance with various embodiments of the present disclosure. <FIG> depicts an optical channel spectrum <NUM> of the transmitted optical channel signal transmitted by transmitter <NUM>, and an optical filter transmittance <NUM> of optical filter <NUM>. The optical channel spectrum <NUM> and optical filter transmittance <NUM> are detuned from each other by relative frequency offset <NUM>. <FIG> also illustrates a signal reference frequency <NUM> of the optical channel signal and filter reference frequency <NUM>. In the illustrated embodiment, signal reference frequency <NUM> is a signal central frequency, and filter reference frequency <NUM> is a filter central frequency. It should be noted that amplitudes of optical channel spectrum <NUM> and of optical filter transmittance <NUM> are provided in <FIG> for illustration purposes only.

Asymmetric filtering occurs when optical channel spectrum <NUM> is shifted compared to optical filter transmittance <NUM>. The amount by which the spectrum <NUM> and transmittance <NUM> are offset is referred to as the relative frequency offset <NUM> (also referred to herein as a frequency offset <NUM>). Such asymmetric filtering may result in a higher penalty compared to transmission without the asymmetric filtering, such as, for example, when the frequency offset <NUM> is zero. In exemplary <FIG>, the width of optical channel spectrum <NUM> is approximately the same as the width of optical filter transmittance <NUM>. Accordingly, the relative frequency offset <NUM> may cause impairment of the signal transmission in the optical network. Therefore, relative frequency offset <NUM> needs to be reduced.

In the embodiments presented by the instant disclosures, a receiver, such as receiver <NUM>, can be configured to measure bit error rates (BERs) of the received optical channel signals.

<FIG> depicts an example of BER <NUM> measured as a function of a transmitter frequency offset. The transmitter frequency offset, as referred to herein, is a frequency difference between a central frequency of the transmitted optical channel signal, and its nominal value. The nominal value of the central frequency of the transmitted optical channel signal, as provided in <FIG>, was the central frequency of the spectrum of the optical channel signal emitted initially by the laser light source, in accordance with manufacturer's settings.

In the BER measurement depicted in <FIG>, the optical signal was a <NUM> gigabaud (Gbaud) <NUM> quadrature amplitude modulation (QAM) coherent signal. An optical filter implemented through the use of a WSS was used. During the measurements, the optical filter transmittance was stable with regards to frequency, and therefore <FIG> illustrates also BER <NUM> as a function of frequency offset <NUM>.

<FIG> illustrates that BER <NUM> generally increases with an increase of absolute value of frequency offset <NUM>. BER <NUM> has a minimum BER <NUM> when frequency offset <NUM> is approximately <NUM>. In this particular measurement, the optical channel spectrum <NUM> was initially (at the nominal value of the central frequency of the transmitted optical channel signal) detuned from the optical filter transmittance <NUM>. This initial detuning of optical channel spectrum <NUM> resulted in higher BER at zero transmitter frequency offset, compared to minimum BER <NUM>.

For example, if filter reference frequency <NUM> is stable and frequency offset <NUM> is negative, BER decreases <NUM> with increase in signal reference frequency <NUM>. However, if filter reference frequency <NUM> is stable and frequency offset <NUM> is positive, BER increases <NUM> with increase in signal reference frequency <NUM>.

The systems and methods as described herein use BER <NUM> behaviour with regards to frequency offset <NUM> (as illustrated in <FIG>) in order to minimize BER, or to reduce it to an acceptable level. The disclosed embodiments permit detuning of optical channel spectrum <NUM> and its signal reference frequency <NUM> towards a signal target reference frequency, thus reducing frequency offset <NUM> to approximately <NUM> (or at least below a threshold).

As referred to herein, the signal target reference frequency is approximately equal to the value of signal reference frequency <NUM> when BER <NUM> is the lowest. With reference to <FIG>, the signal target reference frequency is signal reference frequency <NUM> that provides BER minimum <NUM>. With reference also to <FIG>, the signal target reference frequency corresponds to filter reference frequency <NUM>, and therefore provides zero frequency offset <NUM>, if both optical filter transmittance <NUM> and optical channel spectrum <NUM> are symmetric.

In order to reduce impairment of signal transmission in the optical network, embodiments of the present invention can aid in reducing the asymmetric filtering. Asymmetric filtering can be reduced by shifting optical channel spectrum <NUM> such that its signal reference frequency <NUM> (for example, the signal central frequency) is shifted towards the signal target reference frequency. When the signal reference frequency <NUM> aligns with the signal target reference frequency, the BER should reach its minimum <NUM>.

The receiver <NUM> is configured to determine whether optical channel spectrum <NUM>, and therefore signal reference frequency <NUM>, needs to be detuned towards the lower or the higher frequencies. In other words, receiver <NUM> is configured to determine whether signal reference frequency <NUM> needs to be increased or decreased. The receiver <NUM> then transmits to transmitter <NUM> this information and a request to shift optical channel spectrum <NUM>.

In addition to the request to shift optical channel spectrum <NUM>, transmitter <NUM> may also receive from receiver <NUM> a value of frequency adjustment step. Alternatively, transmitter <NUM> may have a pre-determined frequency adjustment step, as described in detail below. By shifting signal reference frequency <NUM> step-by-step with instructions received from receiver <NUM>, transmitter <NUM> is configured to reduce BER and to achieve transmission of the original signal reference frequency being approximately equal to the signal target reference frequency.

In order to determine whether signal reference frequency <NUM> needs to be increased or decreased, receiver <NUM> is configured to measure BER at frequencies that are above and below signal reference frequency <NUM>.

Referring again to <FIG> and <FIG> and assuming that signal optical spectrum <NUM> originally has an original signal reference frequency <NUM>, receiver <NUM> is configured to measure BER<NUM> (also referred to herein as a "first BER") when the received optical channel signal is detuned to a first signal reference frequency <NUM> and to measure BER<NUM> (also referred to herein as a "second BER") when the received optical channel signal is detuned to a second signal reference frequency <NUM>. The receiver <NUM> is then configured to determine whether optical channel spectrum <NUM>, and therefore original signal reference frequency <NUM>, needs to be shifted towards a higher or the lower frequency based on the determined difference between BER<NUM> and BER<NUM> or vice versa. It should be noted that original signal reference frequency <NUM>, first signal reference frequency <NUM>, and second signal reference frequency <NUM> are illustrated in <FIG> with reference to their corresponding points of the graphical dependence of BER on the transmitter frequency offset.

It should be noted that a drift of filter reference frequency <NUM> is usually a slow process (for example, of the order of minutes). The receiver <NUM> may measure BER such that decrease and increase in signal reference frequency <NUM> towards first signal reference frequency <NUM> or second signal reference frequency <NUM>, respectively, correspond to increase or decrease of relative frequency offset <NUM>. The filter reference frequency <NUM> is assumed to be stable during BER measurements.

If the original signal reference frequency is such that BER increases with the increase of the original signal reference frequency, then BER<NUM>-BER<NUM> is positive. If BER<NUM>-BER<NUM> is positive, relative frequency offset <NUM> may be reduced by reducing of the original signal reference frequency. If the original signal reference frequency is such that BER decreases with the increase of the original signal reference frequency, then BER<NUM>-BER<NUM> is negative. If BER<NUM>-BER<NUM> is negative, the relative frequency offset <NUM> may be reduced by increasing the original signal reference frequency.

In the embodiment illustrated in <FIG> for original signal reference frequency <NUM>, receiver <NUM> would determine that original signal reference frequency <NUM> of the transmitted optical channel signal needs to be shifted towards the lower frequencies to reduce the frequency offset.

In accordance with present technology, in order to determine BER<NUM> and BER<NUM> at receiver <NUM>, transmitter <NUM> is configured to transmit a dithered optical channel signal.

<FIG> depicts dithering of the optical channel signal, in accordance with embodiments of technology described herein. <FIG> also depicts optical filter transmittance <NUM> and filter reference frequency <NUM>.

As referred to herein, the dithering of the optical channel signal is configured to alternately detune, in a repetitive manner, the optical channel spectrum from the original optical channel spectrum <NUM> between lower frequencies and higher frequencies.

In order to obtain a dithered optical channel signal <NUM>, optical channel spectrum <NUM> is dithered with regards to frequency by transmitter <NUM>. In other words, in order to obtain dithered optical channel signal <NUM>, and therefore dithered signal reference frequency <NUM>, the optical channel spectrum <NUM> is sequentially and repetitively detuned to the lower frequencies by dithering amplitude δf (e.g., the signal reference frequency reaching first signal reference frequency <NUM>) and to the higher frequencies by dithering amplitude δf (e.g., the signal reference frequency reaching second signal reference frequency <NUM>).

As illustrated in <FIG>, the dithering amplitude δf is the amplitude of dithering applied to optical channel spectrum <NUM> and corresponds to a frequency difference between second signal reference frequency <NUM> and original signal reference frequency <NUM>. A difference between second reference frequency f<NUM> <NUM> and first signal reference frequency f<NUM> <NUM> may be approximately <NUM>*δf. For example, a representative dithering amplitude δf may be approximately <NUM>.

With reference to <FIG>, when optical channel spectrum <NUM> is detuned by dithering amplitude δf to lower frequencies, optical channel spectrum <NUM> becomes a negatively detuned optical channel spectrum <NUM>. When optical channel spectrum <NUM> is detuned by dithering amplitude δf to higher frequencies, optical channel spectrum <NUM> becomes a positively detuned optical channel spectrum <NUM>.

<FIG> depicts dithered signal reference frequency <NUM> of dithered optical channel signal <NUM> as a function of time, in accordance with embodiments of technology described herein.

The dithered signal reference frequency <NUM> dithers (alternates) between first reference frequency f<NUM> <NUM> and second signal reference frequency f<NUM> <NUM>.

As described above, the difference between second reference frequency f<NUM> <NUM> and first signal reference frequency f<NUM> <NUM> is approximately <NUM>*δf, and: <MAT> <MAT> where fc is original signal reference frequency <NUM>.

It should be understood that a corresponding function may be used for signal central wavelength λc varying between shorter wavelength λS and longer wavelength λL.

It should also be understood that the whole optical channel spectrum <NUM> is dithered and therefore detuned along with signal reference frequency fc <NUM>. Dithering of optical channel spectrum <NUM> is illustrated in <FIG> by negatively detuned optical channel spectrum <NUM> and positively detuned optical channel spectrum <NUM>.

Referring again to <FIG>, during a first time period <NUM>, signal reference frequency is tuned to first signal reference frequency <NUM>. During a second time period <NUM>, signal reference frequency is tuned to second signal reference frequency <NUM>. In illustrated embodiment, first signal reference frequency <NUM> is lower than second signal reference frequency <NUM>. As noted above, first signal reference frequency <NUM> may be lower than original signal reference frequency <NUM> by dithering amplitude δf and second signal reference frequency <NUM> may be higher than original signal reference frequency <NUM> by dithering amplitude δf.

During a dithering period <NUM>, dithered signal reference frequency <NUM> is detuned to first reference frequency f<NUM> <NUM> during first time period <NUM> and to second signal reference frequency f<NUM> <NUM> during second time period <NUM>. As depicted in <FIG>, dithering period <NUM> comprises both first time period <NUM> and second time period <NUM>. The dithering period <NUM> may be, for example, <NUM> seconds (corresponding to <NUM>/(<NUM>)). The dithering period <NUM> may repeat every <NUM> seconds during a monitoring time period. First time period <NUM> may be approximately equal to second time period <NUM>.

In order to avoid penalty induced by frequency dithering, a maximum slew rate between first signal reference frequency <NUM> and second signal reference frequency <NUM> may be defined, for example, by characteristics of receiver <NUM>. The duration of transition time period <NUM> may be chosen so that receiver <NUM> may track the frequency change without additional penalty. As a non-limiting example, first time period <NUM> may be <NUM> milliseconds, and transition time period <NUM> may be <NUM> millisecond. It should be understood that different lengths of first time period <NUM> and transition time period <NUM> may be used in order to provide high periodicity of detuning of signal reference frequency <NUM> (for example, the dithering period <NUM> being <NUM> seconds) and at the same time to reduce additional penalty.

The frequency dithering may continue during the monitoring time period. The monitoring time period is longer than several dithering periods <NUM>. The monitoring time period may be, for example, several seconds or minutes.

In order to improve accuracy of measurements and to reduce impact of noise on BER measurements, BER is measured multiple times during the monitoring time period and the result is averaged. For example, the first BER (BER<NUM>) is measured multiple times during first time periods <NUM> and then averaged. The second BER (BER<NUM>) may be measured multiple times during second time periods <NUM> and then averaged.

<FIG> depicts a flowchart that illustrates a method <NUM> for controlling an optical channel signal in an optical network, in accordance with embodiments of technology described herein.

The transmitter <NUM> generates <NUM> an original optical channel signal and then generates <NUM> the dithered optical channel signal <NUM>. The transmitter <NUM> applies the frequency dithering to the optical channel signal such that dithered optical channel signal <NUM> has first signal reference frequency <NUM> during first time period <NUM> and second signal reference frequency <NUM> during second time period <NUM>. Such frequency detuning pattern repeats and the frequency dithering continues during the monitoring time period. During this monitoring time period, transmitter <NUM> transmits <NUM> dithered optical channel signal <NUM> to the optical link.

In other words, the dithered signal reference frequency alternates (oscillates) between first signal reference frequency <NUM> and a second signal reference frequency <NUM>.

The frequency dithering may be performed by the laser light source <NUM> located in transmitter <NUM>. The optical channel spectrum and therefore the laser reference frequency (for example, either or both of the laser central frequency and the laser peak frequency) may be detuned, for example, by changing the electrical current applied to the laser light source <NUM>, by changing the temperature, or using other methods known in the art. The laser reference frequency may be detuned sequentially and repetitively such that it is dithered between first signal reference frequency <NUM> and second signal reference frequency <NUM>. Alternatively, the frequency dithering may be performed digitally by frequency detuning in transmitter's DSP <NUM>.

After the monitoring time period is over <NUM>, transmitter <NUM> is configured to receive <NUM> a request <NUM> from receiver <NUM> to adjust the original optical channel spectrum of the transmitted optical channel signal based on a BER difference. The BER difference is determined as a difference between the second BER (BER<NUM>) and the first BER (BER<NUM>), where the first BER was measured and averaged during the first time periods <NUM>, and the second BER was measured and averaged during second time periods <NUM>.

The request to adjust the optical channel spectrum may be a request to shift the optical channel spectrum. The request to shift the optical channel spectrum may include an indication of a direction of a shift of the optical channel spectrum with regards to frequency, or, similarly, an indication of a direction of a shift of the original signal reference frequency. It should be understood that shifting of the optical channel spectrum with regards to frequency and shifting of the original signal reference frequency with regards to frequency in the same direction provide the same effect on BER.

The direction of the shift of the optical channel spectrum (or the shift of the original signal reference frequency) with regards to frequency may be positive, corresponding to a shift of the optical channel spectrum (or that of the original signal reference frequency) towards higher frequencies. The direction of the shift of the optical channel spectrum (or the shift of the original signal reference frequency) with regards to frequency may be negative, corresponding to the shift of the optical channel spectrum (or that of the original signal reference frequency) towards lower frequencies.

For example, the indication of the direction of the shift may be any indication permitting transmitter <NUM> to determine whether the shift should be positive or negative, or whether the optical channel spectrum (or the original signal reference frequency) should not be shifted. The request to adjust the optical channel spectrum may include an indication whether the BER difference, determined by receiver <NUM>, is positive or negative.

The request to adjust the optical channel spectrum may include a request to shift original signal reference frequency <NUM> to the higher or to the lower frequencies. With reference to <FIG> and to BER of original signal reference frequency <NUM> depicted therein, transmitter <NUM> would receive a request to decrease original signal reference frequency <NUM>.

In response to the received request, transmitter <NUM> shifts <NUM> original signal reference frequency <NUM>, by frequency adjustment step Δf, according to instructions <NUM> received from receiver <NUM>.

The frequency adjustment step Δf may be pre-determined at transmitter <NUM>, transmitted from receiver <NUM> (e.g. along with the request to tune the original signal reference frequency), or determined at transmitter <NUM> based on dithering amplitude δf.

For example, the frequency adjustment step may be approximately equal to dithering amplitude δf or may be longer or shorter than dithering amplitude δf. The frequency adjustment step Δf may be approximately equal to two dithering amplitudes <NUM>*δf. The frequency adjustment step Δf may be, for example, approximately <NUM> or approximately <NUM>.

After receiving instructions from receiver <NUM>, transmitter <NUM> shifts optical channel spectrum <NUM>. The transmitter <NUM> shifts original signal reference frequency <NUM> by the frequency adjustment step Δf to an adjusted signal reference frequency <NUM>.

It should be noted that the term "original signal reference frequency" is used herein to refer to a signal reference frequency of the optical channel signal (also referred to herein as the "original optical channel signal") generated by transmitter <NUM> without or before frequency dithering. Based on the request received from receiver <NUM>, transmitter <NUM> may shift the original signal reference frequency by the frequency adjustment step towards the adjusted signal reference frequency. This adjusted signal reference frequency becomes a new original signal reference frequency for the next frequency adjustment. The frequency adjustment with the frequency adjustment steps, based on the BER difference, may be repeated until the original signal reference frequency becomes approximately equal to the signal target reference frequency.

It should also be noted that the terms "detuning", "detuned", "detune" are used herein with regards to dithering of the signal reference frequency of the optical channel signal. The terms "shifting", "shifted", "shift" are used herein with regards to applying of a frequency adjustment step by the transmitter after receiving the instructions from the receiver. It should be understood that increasing or decreasing the original signal reference frequency by shifting the original signal reference frequency and detuning when dithering of the optical channel signal may be performed using the same techniques known in the art.

For example, one or more of shifting and detuning of original signal reference frequency <NUM> may be performed by the laser located in transmitter <NUM>. The optical channel spectrum and therefore the laser reference frequency may be shifted, detuned or both, for example, using changes in current, temperature or using other methods known in the art for shifting the laser reference frequency. Alternatively, the optical channel spectrum and therefore original signal reference frequency <NUM> may be one of shifted and detuned digitally by using DSP <NUM> located in transmitter <NUM>.

For example, the frequency dithering of the optical channel signal may be performed digitally, while original signal reference frequency <NUM> may be shifted by laser light source <NUM> located in transmitter <NUM>, after receiving the request from receiver <NUM>.

<FIG> depicts a flowchart that illustrates a method <NUM> for controlling optical network equipment, in accordance with embodiments of technology described herein. The receiver <NUM>, e.g. at photodetector <NUM>, receives <NUM> dithered optical channel signal <NUM>. The receiver <NUM> then measures <NUM> BER. For example, BER may be determined by receiver processor <NUM>.

The BER is measured <NUM> each time signal reference frequency <NUM> is detuned to first signal reference frequency <NUM> (BER<NUM>) or second signal reference frequency <NUM> (BER<NUM>). These measurements are performed synchronously with frequency dithering, so that receiver <NUM> may measure and collect values of BER<NUM> and BER<NUM> separately from each other.

Different techniques may be used to inform receiver <NUM> of signal reference frequency change. For example, transmitter <NUM> may transmit to receiver <NUM> information in overhead bits about the reference frequency detuning from first signal reference frequency <NUM> to second signal reference frequency <NUM> and vice versa.

In order to detect that the signal reference frequency has been detuned from first signal reference frequency <NUM> to second signal reference frequency <NUM> and vice versa, local oscillator frequency offset (LOFO) coherent detection at coherent receiver <NUM> may also be used.

The receiver <NUM> separately averages <NUM> BER<NUM> and BER<NUM> values, each measured multiple times during the monitoring time period, to improve the signal to noise ratio. Averaging BER values may help to smooth out natural fluctuations in BER measurements.

For each monitoring time period, an averaged BER difference ΔBER is obtained from averaged BER<NUM> and averaged BER<NUM>: <MAT> where BER<NUM> is the averaged BER calculated during second time periods <NUM>, and BER<NUM> is the averaged BER calculated during first time periods <NUM>.

The BER difference ΔBER is then analyzed. If receiver <NUM> determines <NUM> that the BER difference ΔBER is positive, i.e. ΔBER><NUM>, the signal reference frequency <NUM> is higher than the signal target reference frequency. Therefore, if ΔBER is positive (ΔBER><NUM>), receiver <NUM> generates <NUM> a request to shift optical channel spectrum <NUM>, and therefore original signal reference frequency <NUM>, to lower frequencies. The receiver <NUM> then transmits <NUM> to transmitter <NUM> the generated request (instructions) in order to reduce BER.

If receiver <NUM> determines <NUM> that BER difference ΔBER is negative, i.e. ΔBER<<NUM>, the relative frequency offset is negative. The signal reference frequency is lower than the signal target reference frequency. Therefore, if ΔBER<<NUM>, receiver <NUM> generates <NUM> a request to shift optical channel spectrum <NUM>, and therefore original signal reference frequency <NUM>, to higher frequencies. The receiver then transmits <NUM> to transmitter <NUM> the generated request in order to reduce BER.

As mentioned above, the request received by transmitter <NUM> from receiver <NUM> may include the value of frequency adjustment step Δf.

The instructions (the generated request) <NUM> may be sent from receiver <NUM> to transmitter <NUM> and signal reference frequency <NUM> is shifted by one frequency adjustment step Δf after another, until BER is minimized.

In some embodiments, receiver <NUM> may be configured to collect and store <NUM> at least one value of ΔBER. Collection and storage of ΔBER values may permit receiver <NUM> to compare <NUM> the current value of BER difference ΔBER(current) with the previous value of BER difference ΔBER(previous), i.e. with BER difference measured during previous monitoring time period. For example, if ΔBER(current) has different sign than ΔBER(previous) and an absolute value of ΔBER(current) is less than an absolute value of ΔBER(previous), then receiver <NUM> may abstain from sending any request to transmitter <NUM>.

In some embodiments, receiver <NUM> may also compare ΔBER(current) to a pre-determined minimum BER in order to determine whether to send the instructions to receiver <NUM> or to abstain.

In some embodiments, receiver <NUM> may be configured to determine a relative BER change γ: <MAT>.

For example, receiver <NUM> may compare the relative BER change with a pre-determined threshold relative BER change TH. For example, if γ >= TH, receiver <NUM> may instruct transmitter <NUM> to decrease signal reference frequency <NUM>. If γ <= -TH receiver <NUM> may instruct transmitter <NUM> to increase signal reference frequency <NUM>. If -TH < γ < TH, receiver <NUM> may abstain from instructing transmitter <NUM> to change signal reference frequency <NUM>.

In at least one embodiment, receiver <NUM> may also instruct a controller of an optical filter to adjust operation of the optical filter. The optical filter may be requested to shift optical filter transmittance <NUM>, <NUM> by increasing or decreasing optical filter reference frequency <NUM>, <NUM> in order to reduce BER. The request transmitted to the controller of the optical filter may indicate the direction of the shift of the optical filter transmittance <NUM>, <NUM> with regards to frequency. The request may comprise an indication of a desired increase or decrease of the optical filter reference frequency <NUM>. The request may be an indication of whether the determined BER difference is positive or negative. Such technique may be practical if there is one optical filter in optical link <NUM>. Instructing the controller of the optical filter may be also done in addition to instructing transmitter <NUM> to adjust the transmitted optical channel signal.

<FIG> depicts experimental results of an adjustment of signal reference frequency <NUM> to minimize BER, obtained in accordance with embodiments of technology described herein. <FIG> depicts BER measured in a first experiment <NUM> and BER measured in a second experiment <NUM>. The signal reference frequency <NUM> was initially intentionally offset to about -<NUM> in first experiment <NUM>. In second experiment <NUM>, signal reference frequency <NUM> was initially intentionally offset to about +<NUM>.

BER was measured each time original signal reference frequency <NUM> was shifted by frequency adjustment step Δf at transmitter <NUM>, in response to the request received from receiver <NUM>. The embodiments disclosed herein permitted to successfully reduce BER of the received optical channel signal.

The disclosed embodiments may be used when the optical signal has to pass through several optical filters. It is possible to reduce BER and the frequency offset of signal reference frequency with regards to an average of filter reference frequencies.

<FIG> depicts experimental results of adjusting of the signal reference frequency of the optical channel signal propagated through several cascaded WSSs, in accordance with embodiments of technology described herein. The dithered optical channel signal was transmitted from transmitter <NUM>, through several cascaded WSS, to receiver <NUM>. Based on measured BER difference, as described herein, receiver <NUM> instructed transmitter <NUM> to shift signal reference frequency <NUM>.

<FIG> depicts a Q-factor <NUM> measured as a function of the transmitter frequency offset. The Q-factor <NUM> was measured each time original signal reference frequency <NUM> has been shifted. The dithered optical channel signal propagated through <NUM> WSS (curve <NUM>), <NUM> WSS (curve <NUM>), <NUM> WSS (curve <NUM>), <NUM> WSS (curve <NUM>), and <NUM> WSS (curve <NUM>). <FIG> illustrates that the embodiments disclosed herein may permit to adjust optical channel spectrum <NUM> and therefore signal reference frequency <NUM> when optical link <NUM> has several optical filters.

<FIG> depicts OSNR penalty measured for received optical channel signal after it has propagated through several WSSs and adjusted in accordance with embodiments of technology described herein. OSNR penalty is depicted with regards to a quantity of cascaded WSS (<NUM>, <NUM>, and <NUM>), through which the dithered optical channel signal propagated.

The signal reference frequency of the transmitted optical channel signal was initially offset by -<NUM> (curve <NUM>) and +<NUM> (curve <NUM>) from the nominal transmitter frequency. Curve <NUM> was measured with zero frequency offset between signal reference frequency <NUM> and the average of filter reference frequencies of the cascaded WSSs. Dashed curve <NUM> represents OSNR penalty for the received optical channel signal after the adjustment of signal reference frequency of the transmitted optical channel signal in accordance with technology described herein.

<FIG> illustrates that initially detuned signal reference frequency, and therefore initially detuned optical channel spectrum, may be adjusted in accordance with embodiments of technology described herein to reduce OSNR penalty. The level of OSNR penalty <NUM> measured for the received optical channel signal after the adjustment approximately coincides with OSNR penalty <NUM> measured when the frequency offset of the transmitted optical channel signal was approximately zero.

The disclosed embodiments may be applied to dual-carrier optical signal transmission. Dual-carrier transmission is achieved by transmitting two carriers bundled as one channel. The two carriers have a reduced spacing between them.

<FIG> depicts dual-carrier optical signal <NUM> with two carriers bundled as one channel, in accordance with embodiments of technology described herein. A first carrier <NUM> and a second carrier <NUM> are transmitted in one channel, and are filtered by one optical filter having optical filter transmittance <NUM>. For example, first carrier <NUM> and second carrier <NUM> may be two <NUM> Gbps signals to form one <NUM> Gbps channel.

<FIG> depicts a first carrier reference frequency <NUM> as a function of time in a first dithered carrier and a second carrier reference frequency <NUM> as a function of time in a second dithered carrier, in accordance with embodiments of technology described herein. Two carriers <NUM>, <NUM> may have different dithering periods: a first dithering period <NUM> of first carrier <NUM> may be different from a second dithering period <NUM> of second carrier <NUM>. For example, dithering frequency step may be applied more frequently to the second carrier <NUM>, as illustrated in <FIG>. For example, one carrier signal may have first dithering period <NUM> of <NUM> seconds and another carrier signal may have second dithering period <NUM> of <NUM> seconds.

The frequency dithering may be applied to first carrier <NUM> and second carrier <NUM> with different time periods <NUM>, <NUM>, <NUM>, <NUM>. For example, first time period <NUM> and second time periods <NUM> may be longer for dithering applied to first carrier <NUM>, compared to third time period <NUM> and fourth time period <NUM> for dithering applied to second carrier <NUM>, as illustrated in <FIG>.

Frequency dithering as described herein may be used to balance crosstalk and filtering in dual-carrier optical signal transmission. The disclosed embodiments permit to reduce the transmission impairment for each carrier <NUM>, <NUM> separately and for dual-carrier optical signal <NUM>.

The receiver <NUM> may measure separately BER of first carrier <NUM> (the first BER and the second BER) and BER of second carrier <NUM> (a third BER and a fourth BER) multiple times during the monitoring time period. The third BER may be measured and averaged during third time periods <NUM> and the fourth BER may be measured and averaged during the fourth time periods <NUM>.

The receiver <NUM> may then determine whether to shift a first optical channel spectrum of first carrier <NUM> and a second optical channel spectrum of second carrier <NUM> separately for each carrier <NUM>, <NUM>, in accordance with technology described above. The receiver <NUM> may then send to transmitter <NUM> a request to shift the first optical channel spectrum of first carrier <NUM> and the second optical channel spectrum of second carrier <NUM> with regards to frequency. The request may comprise at least one of an indication of a direction of shifting of the first optical channel spectrum of first carrier <NUM> and a direction of shifting of the second optical channel spectrum of second carrier <NUM> with regards to frequency.

In addition to determination whether to shift and how to shift the first optical channel spectrum of first carrier <NUM> and whether to shift and how to shift the second optical channel spectrum of second carrier <NUM>, values of BER measured for first carrier <NUM> may be compared with values of BER measured for second carrier <NUM>. Such comparison of BER for two carriers may help to balance both crosstalk and filtering in dual-carrier optical signal transmission.

The receiver <NUM> may determine a difference between values of BER of first carrier <NUM> (for example, the first BER, the second BER, or their average) and values of BER of second carrier <NUM> (for example, the third BER, the fourth BER, or their average) and compare them. For example, receiver <NUM> may determine a difference between the first BER and the third BER. Alternatively, receiver <NUM> may determine a difference between averaged values of BER of first carrier <NUM> and averaged values of BER of second carrier <NUM> and may use it when generating the request to adjust operation of the optical network equipment.

If the difference, determined between averaged values of BER of first carrier <NUM> and averaged values of BER of second carrier <NUM>, is higher than a threshold carrier BER difference, and BER of first carrier <NUM> and BER of second carrier <NUM> are each higher than a threshold carrier BER, then receiver <NUM> may request transmitter <NUM> to adjust first carrier <NUM> and second carrier <NUM>. If such difference is about or less than a threshold carrier BER difference, and BER of first carrier <NUM> and BER of second carrier <NUM> are each about or less than a threshold carrier BER, then receiver <NUM> may abstain from sending a request to transmitter <NUM>.

One skilled in the art will appreciate that in the above described embodiments, the effects of asymmetric filtering are mitigated. Asymmetric filtering may occur when an optical signal has a spectrum that is offset (by a relative frequency offset) from the optical filter transmittance. Even if the optical signal spectrum and the optical transmittance are aligned at the time of transmission, there may be a shift in the signal spectrum during transmission. Because these shifts cannot be completely modeled during deployment of the system, a dynamic method of mitigating the asymmetric filtering is provided herein. The misalignment between the optical filter transmittance and the signal spectrum results in a BER that can be measured at a receiver. The receiver can measure the BER and can then request that the transmitter begin dithering the optical signal. Dithering of the signal results in the signal being transmitted with a series of different signal reference frequencies. This has the effect of slightly shifting the location of the optical signal spectrum. As the dithering progresses, the optical signal spectrum may become more aligned with the filter transmittance. During this process, the receiver is able to observe a change in the BER. The receiver can notify the transmitter when the received signal has a minimal BER, or at least a BER below a threshold. In some embodiments, the receiver can also indicate to the transmitter a direction in which to dither the signal. The signal transmitted during the dithering process may be referred to as a dithered signal, and it may have a dithered signal reference frequency.

The technology as described herein may be applied in a similar manner to optical signal transmission with any number of carriers in one optical channel.

It should be understood, that methods as described herein may be implemented fully or partially using a non-transitory computer readable medium with computer executable instructions stored thereon that. When executed by a processor, the computer executable instructions cause the processor to perform the methods fully or partially.

Claim 1:
A method for controlling an optical channel signal in an optical network having an optical filter, the optical channel signal having an optical channel spectrum and an original signal reference frequency, the method comprising:
alternately detuning of the optical channel spectrum with regards to frequency to obtain a dithered optical channel signal (<NUM>),
a signal reference frequency of the dithered optical channel signal being detuned to a first signal reference frequency during first time periods and to a second signal reference frequency during second time periods, the second signal reference frequency being higher than the first signal reference frequency, wherein at least one of the dithered optical signal spectrum is more aligned with a transmittance of the optical filter than the optical signal spectrum;
transmitting through the optical filter, the dithered optical channel signal (<NUM>);
receiving a request (<NUM>) to shift the optical channel spectrum of the optical channel signal with regards tc frequency, the request comprising an indication of a direction of shifting of the optical channel spectrum with regards to frequency; and
shifting the optical channel spectrum (<NUM>) with regards to frequency based on the received request to reduce a relative frequency offset between an optical filter transmittance of the optical filter and the optical channel spectrum, wherein a signal reference frequency refers to at least one of a signal carrier frequency, a signal central frequency and a frequency of a maximum of optical channel spectrum