Patent Description:
Long-haul wavelength division multiplexing (WDM) optical networks are configured to transmit optical channel signals over a few hundred kilometers. However, the capacity of conventional optical channel in WDM networks is limited to about <NUM> Gigabits/s (Gbit/s). To increase the channel capacity beyond this limit and to take advantage of WDM interface, optical super-channel signals may be used.

An optical super-channel signal transmits data in optical super-channels. A super-channel is an optical channel that binds several subcarriers for unified transmission. Each subcarrier is a signal with a single-wavelength spectrum. The subcarriers of one super-channel can be processed simultaneously by optical networks hardware. Using subcarriers in the optical super-channel may considerably increase the transmission capacity and spectral efficiency of the WDM networks by reducing a guard band between the subcarriers within the super-channel.

In the conventional optical channel signal, a guard band is required between any two optical channels in order to mitigate an optical filtering effect and neighboring channel crosstalk. There is no optical filtering between subcarriers in the super-channel. As such, the guard band between subcarriers in an optical super channel can be allocated based on consideration of only the mitigation of cross talk between channels. If the crosstalk is low and subcarrier frequencies are stable, the bandwidth allocated to the guard band between the subcarriers can be reduced. However, each subcarrier is generated by a laser that may manifest an optical frequency drift. Such an optical frequency drift for a particular laser may cause a drift of the corresponding subcarrier towards a neighboring subcarrier in the optical super-channel signal.

A pilot processing method is known from <CIT>. Further prior art useful for understanding the background of the present invention is described in <CIT>, in <CIT>, and in <NPL>.

Therefore, there is a need for improvements in laser frequency control techniques that compensate for problems related to laser drift issues that may cause channel crosstalk in the optical super-channel signal.

In order to overcome these and other problems, the present invention provides the subject-matter defined by the independent claims. Particular embodiments are specified in the dependent claims. A further objective of the present disclosure is to provide a technique for controlling subcarrier frequencies in an optical super-channel signal in order to improve performance of an optical network. The apparatuses, methods and systems as disclosed herein permit controlling a difference between a first center frequency of a first optical subcarrier and a second center frequency of a second optical subcarrier of a super-channel optical signal.

In accordance with this objective, a first aspect of the present disclosure provides a method for controlling a difference between a first center frequency of a first optical subcarrier and a second center frequency of a second optical subcarrier of an optical super-channel signal in an optical network. The method comprises: modulating the first optical subcarrier at a first optical side component frequency with a first side modulation frequency; modulating the second optical subcarrier at a second optical side component frequency with a second side modulation frequency, a difference between the first optical side component frequency and the second optical side component frequency being smaller than a difference between the first center frequency and the second center frequency. The method further comprises coupling together the modulated first and second optical subcarriers to obtain a modified optical signal; and monitoring a variation of the difference between the first center frequency and the second center frequency by detecting a radio-frequency (RF) power at a modulated beat frequency tone in the modified optical signal. The modulated beat frequency tone bears information of the first side modulation frequency and the second side modulation frequency. The first and the second side component frequency are in the range between <NUM> and <NUM> THz. The first and the second modulation component frequency are in the range of tens of MHz. A spacing between the first and the second side component frequency is in the range of hundreds of MHz.

The monitoring a variation of the difference between the center frequencies of the first and the second optical subcarriers comprises: converting the modified optical signal to an electrical signal; filtering the electrical signal by a first electronic filter to obtain a filtered electrical signal within a first bandwidth corresponding to the difference between the first optical side component frequency and the second optical side component frequency; squaring the filtered electrical signal; filtering again the filtered and squared electrical signal by a second electronic filter to obtain an RF signal at the modulated beat frequency tone; and measuring the RF power of the RF signal at the modulated beat frequency tone.

In at least one embodiment, a difference between the first optical side component frequency and the first center frequency of the first subcarrier may be maintained constant. In at least one embodiment, a difference between the second optical side component frequency and the second center frequency of the second subcarrier may be maintained constant.

The method may further comprise detuning at least one of the first center frequency and the second center frequency to maximize the RF power of the RF signal at the modulated beat frequency tone.

In some embodiments, a sum of a power of a first side optical component at the first side optical component frequency and a power of a second optical component at the second side optical component frequency may be less than <NUM>% of the total power of the optical signal.

In some embodiments, the modulated beat frequency tone may be a difference between the first side modulation frequency and the second side modulation frequency. In some embodiments, the modulated beat frequency tone is a sum of the first side modulation frequency and the second side modulation frequency.

In accordance with another aspect of the present disclosure, there is provided an apparatus for controlling an optical super-channel signal in an optical network, the optical super-channel signal having a first optical subcarrier with a first center frequency and a second optical subcarrier with a second center frequency. The apparatus comprises: a first electro-optic modulator configured to: modulate the first optical subcarrier at a first optical side component frequency with a first side modulation frequency; a second electro-optic modulator configured to: modulate the second optical subcarrier at a second optical side component frequency with a second side modulation frequency, a difference between the first optical side component frequency and the second optical side component frequency being smaller than a difference between the first center frequency and the second center frequency. The apparatus also comprises a coupler configured to couple together the modulated first and second optical subcarriers to obtain a modified optical signal; and an RF power meter configured to measure an RF power of an RF signal generated from the modified optical signal, the RF power being measured at a modulated beat frequency tone. The modulated beat frequency tone bears information of the first side modulation frequency and the second side modulation frequency. The first and the second side component frequency are in the range between <NUM> and <NUM> THz. The first and the second modulation component frequency are in the range of tens of MHz. A spacing between the first and the second side component frequency is in the range of hundreds of MHz.

The apparatus further comprises: a photodetector configured to receive the modified optical signal and to generate an electrical signal; a first electronic filter configured to filter the electrical signal to obtain a filtered electrical signal within a first bandwidth corresponding to a difference between the first optical side component frequency and the second optical side component frequency, the first electronic filter configured to significantly attenuate the electrical signal around the first side modulation frequency and the second side modulation frequency; and a squaring device configured to square the filtered electrical signal.

The apparatus may further comprise an amplifier configured to amplify the electrical signal after it was generated by the photodetector. The apparatus may further comprise a second electronic filter located between the squaring device and the RF power meter and configured to filter again the filtered and squared electrical signal within a second bandwidth having a center corresponding to the modulated beat frequency tone to obtain the RF signal. The squaring device may comprise a splitter and a multiplier.

In accordance with another aspect 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: perform the method according to the first aspect.

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 controlling a difference between a first center frequency of a first optical subcarrier and a second center frequency of a second optical subcarrier of an optical super-channel signal in an optical network.

Referring now to the drawings, <FIG> depicts a block diagram of a WDM transmitter <NUM> for generating an optical super-channel signal <NUM>, in accordance with at least one non-limiting embodiment of the present disclosure. <FIG> illustrates a portion <NUM> of the optical spectrum of optical super-channel signal <NUM> generated by transmitter <NUM>.

It should be understood that each subcarrier of optical super-channel signal <NUM> can be used to carry an independent signal. Each of these subcarriers is associated with a control branch within transmitter <NUM>. Each of these branches comprises a transmitter digital signal processors (DSPs) <NUM>, a digital to analog converter (DAC) <NUM>, an electrical driver <NUM>, a laser <NUM>, and an electro-optical (EO) modulator <NUM>. As shown in <FIG>, a plurality of branches are present, each of which corresponds to a respective super-channel subcarrier <NUM>, <NUM>, <NUM>, <NUM>. As depicted in <FIG>, in each branch, DSP <NUM> transmits a digital signal to a DAC <NUM>, which converts the digital signal to an analog signal. The digital signal is the data signal to be encoded for transmission. The EO modulator <NUM> may be modulated electrically by electrical drivers <NUM> in accordance with the analog signal output by DAC <NUM>. The output of laser <NUM> is optically modulated by EO modulator <NUM>. A coupler <NUM> may be used to combine separate subcarriers (e.g. <NUM>, <NUM>, <NUM>, <NUM>, etc.) generated by each of the branches into one optical super-channel signal <NUM>. If each of the lasers is tuned to a different frequency, the coupling of the generated subcarriers can be a rather simple process.

It is contemplated that transmitter <NUM> may also comprise other devices, and the technology described herein is not limited to the embodiment illustrated in <FIG>.

<FIG> depicts two super-channels <NUM>, <NUM>. Each super-channel <NUM>, <NUM> comprises four subcarriers: four super-channel subcarriers <NUM>, <NUM>, <NUM>, and <NUM> in the first super-channel <NUM> and four super-channel subcarriers <NUM>, <NUM>, <NUM>, <NUM> in the second super-channel <NUM>.

A guard band is typically required between neighboring super-channels <NUM>, <NUM> and between neighboring subcarriers (<NUM>, <NUM>, <NUM>, <NUM> and <NUM>, <NUM>, <NUM>, <NUM>) in order to mitigate the effects of crosstalk. In order to improve spectral efficiency, a subcarrier guard band <NUM> between neighboring subcarriers in the same super channel (for example, between neighboring subcarriers <NUM> and <NUM>) may be narrower than a super-channel guard band <NUM> located between neighboring super-channels <NUM>, <NUM>. Typically, subcarrier guard band <NUM> between neighboring subcarriers may be on the order of a few GHz.

Referring again to <FIG>, each subcarrier <NUM>, <NUM>, <NUM>, <NUM> of super-channel <NUM> is generated by a different laser <NUM>. Each laser <NUM> has a laser optical frequency which corresponds to a center frequency of the corresponding subcarrier. For example, in <FIG>, subcarrier <NUM> has a center frequency f<NUM> (illustrated by an arrow). Also in <FIG>, subcarrier <NUM> has a center frequency f<NUM> (also illustrated by an arrow). Thus, the laser associated with subcarrier <NUM> would have a laser optical frequency of f<NUM> and the laser associated with subcarrier <NUM> would have a laser optical frequency of f<NUM>.

Due to changes in temperature during operation, differences between an operating temperature and the temperature at which a calibration was performed, and other such effects, the laser optical frequency may drift with time. The drift of the laser optical frequency over time may be as high as +/-<NUM>. Such frequency drift may significantly contribute to an undesired variation of frequency spacing between the subcarriers. An additional guard band between subcarriers is usually introduced to take into account such laser frequency drift. This guard band may be defined on the basis of a worst case drift from two different lasers (e.g. each of the lasers drifting towards the other). This increased guard band reduces the spectral efficiency of the system. The technique described herein may help to reduce the bandwidth allocated to guard bands between the subcarriers attributable to accommodating the laser frequency draft. The technique described herein permits reducing the variation of spacing between frequencies of the lasers that generate the subcarriers and to improve the relative stability of subcarriers in the super-channel.

The technique described herein permits reduce variation of the frequency difference |f<NUM> - f<NUM>| between two neighboring subcarriers (also referred to herein as a "subcarrier spacing").

<FIG> illustrates consequences of the drift of the optical frequency of one of lasers <NUM> in transmitter <NUM>.

A portion <NUM> of the spectrum of optical signal <NUM> contains two neighboring subcarriers <NUM> and <NUM> of super-channel <NUM>. First subcarrier <NUM> has a subcarrier center frequency f<NUM>, and second subcarrier <NUM> has a subcarrier center frequency f<NUM>.

In <FIG>, scenario <NUM> illustrates first and second subcarriers <NUM>, <NUM> when they have optimal desired subcarrier spacing. The difference between optical frequencies f<NUM> and f<NUM> is approximately equal to the desired frequency difference Δfd. The desired variation of a subcarrier spacing (f<NUM>-f<NUM>) may be, for example, a few MHz.

Scenario <NUM> illustrates first and second subcarriers <NUM>, <NUM> when they are located too far from each other. In such case, the difference Δfl between optical frequencies f<NUM> and f<NUM> is larger than Δfd. Therefore, a portion of the spectrum is wasted.

Scenario <NUM> illustrates first and second subcarriers <NUM>, <NUM> when they are located too close to each other, so that optical frequency difference Δfs between first and second subcarriers <NUM>, <NUM> is less than Δfd. In this case, the crosstalk between first and second subcarriers <NUM>, <NUM> is higher than desired.

In accordance with at least one embodiment of the present disclosure, a modified optical signal is generated in order to control the subcarrier spacing.

<FIG> illustrates a portion <NUM> of a spectrum of the modified optical signal, generated in accordance with at least one embodiment of the present disclosure. The modified optical signal contains first and second side frequency components <NUM>, <NUM>. The first side frequency component <NUM> has frequency fA. The second side frequency component <NUM> has frequency fB. In some embodiments, first and second side frequency components <NUM>, <NUM> may be generated on neighboring edges of first and second subcarriers <NUM>, <NUM>. As is described herein below, these first and second side frequency components <NUM>, <NUM> are used to monitor the subcarrier spacing (f<NUM>-f<NUM>) between first and second subcarriers <NUM>, <NUM>.

In at least one embodiment, a frequency difference Δf<NUM>A between optical center frequency f<NUM> of first subcarrier <NUM> and frequency fA of first side frequency component <NUM> is maintained as constant. The optical center frequency f<NUM> of first subcarrier <NUM> may drift away from the desired frequency due to the drift of the laser <NUM> that generates first subcarrier <NUM>. The frequency fA of first side frequency component <NUM> also follows the drift of the subcarrier frequency f<NUM> of first subcarrier <NUM>. Therefore, by monitoring the difference between frequencies fA, fB of side frequency components <NUM>, <NUM> of neighboring subcarriers <NUM>, <NUM>, it is possible to monitor the subcarrier spacing.

<FIG> illustrates the drift of subcarrier <NUM>, in accordance with at least one embodiment of the present disclosure. For simplicity of the illustration, center frequency f<NUM> of first subcarrier <NUM> is maintained constant in <FIG>. It is contemplated that center frequencies of any subcarrier of one super-channel may increase or decrease.

Scenario <NUM> illustrates first and second subcarriers <NUM>, <NUM> when they have optimal desired subcarrier spacing. Scenario <NUM> illustrates first and second subcarriers <NUM>, <NUM> when they are located too far from each other. In scenario <NUM>, when subcarrier center frequency f<NUM> decreases, the frequency difference between subcarrier center frequencies f<NUM>,f<NUM> of first subcarrier <NUM> and second subcarrier <NUM>, respectively (i.e. subcarrier spacing) increases. The frequency difference ΔfAB between frequencies of first and second side frequency components <NUM>, <NUM> also increases.

When the center frequency f<NUM> increases (scenario <NUM>), the subcarrier spacing (f<NUM>-f<NUM>) decreases. The frequency difference ΔfAB between frequencies of first and second side frequency components <NUM>, <NUM> also decreases.

Referring now to <FIG>, which depicts a block diagram of logical blocks of DSP <NUM>, DAC module <NUM> of transmitter <NUM>, and electrical driver <NUM> of <FIG>, in accordance with at least one non-limiting embodiment of the present disclosure. There are four data streams for dual-polarization QAM transmitter <NUM>. The logical blocks of DSP <NUM> may comprise: encoding block <NUM>, bit-to-symbol mapping block <NUM>, header insertion block <NUM>, pulse shaping block <NUM>, and pre-distortion block <NUM>. It should be noted that DSP <NUM> may have other configuration and structure. DAC module <NUM> has four DAC units <NUM>.

As illustrated in <FIG>, DSP <NUM> transmits four V(t) signals to DAC module <NUM>, one V(t) signal for each DAC unit <NUM>. These four V(t) signals are: two signals in X-polarization VXI(t), VXQ (t), and two signals in Y-polarization VYI (t), VYQ (t). I and Q correspond to in-phase and quadrature phase signals, respectively.

The desired optical output with optical subcarrier may be obtained when Vxi(t), VXQ (t), VYI (t), VYQ (t) are applied to EO modulator <NUM> and EO modulator <NUM> is an in-phase quadrature phase MZM (IQ-MZM).

The resulting field may be described as: <MAT>.

The equation (<NUM>) is for X polarization of the field. The field in Y polarization can be described with a similar equation.

The transmitter <NUM> with DSP <NUM> and DAC module <NUM> of <FIG> transmits optical super-channel signal illustrated in <FIG>.

In at least one embodiment, in order to generate the modified optical signal containing subcarriers <NUM>, <NUM> with side frequency components <NUM>, <NUM>, a side frequency component generation logical block is added in DSP <NUM> of transmitter <NUM>.

<FIG> depicts a block diagram of logical blocks of a modified DSP <NUM> and DAC module <NUM> for generation of optical super-channel signal with side frequency components, in accordance with at least one non-limiting embodiment of the present disclosure. To generate modified optical signals <NUM>, <NUM> with side frequency components <NUM>, <NUM>, DSP <NUM> in transmitter <NUM> is replaced with modified DSP <NUM> of <FIG>.

<FIG> illustrates how the side frequency component with amplitude modulation is applied in X polarization. Mathematical operations of adding are applied in digital domain. The following term (corresponds to block 491a in <FIG>) is added to VXI(t) signal in X-polarization at adder 492a: <MAT>.

Therefore, DSP <NUM> transmits to DAC module <NUM> the I-component in X polarization of V(t) signal that can be written as follows: <MAT>.

The following term (corresponds to block 491b in <FIG>) is added at to Vxo(t) signal in X-polarization at adder 492b: <MAT>.

DSP <NUM> transmits to DAC module <NUM> the Q-component in X polarization of V(t) signal that can be written as follows: <MAT>.

Adder 492b adds term (<NUM>), which corresponds to block 491b, to VXI(t) signal in X polarization, the Q-component in X-polarization of V(t).

Optionally, similar terms may be added to corresponding V(t) components in Y polarization.

As a result of using DSP <NUM> in transmitter <NUM>, the optical signal has an optical side component frequency fA with a side frequency component modulation frequency <MAT>.

Referring also to <FIG>, the frequency difference f<NUM>A between subcarrier center frequency f<NUM> of first subcarrier <NUM> and optical side component frequency fA of first side frequency component <NUM> may be maintained very accurately, because the side frequency component is generated digitally in DSP <NUM>. An error may be due to a clock error in transmitter <NUM>. However, such clock error may be less than <NUM> parts per million (ppm). For example, for frequency difference f<NUM>A = <NUM>, the maximum error may be <NUM>.

The first side frequency component <NUM> having optical side component frequency fA is modulated at first modulation frequency <MAT>. A second side frequency component <NUM> with optical side component frequency fB is modulated at second modulation frequency <MAT>. It is contemplated that other subcarriers may have side frequency components that are modulated with corresponding side frequency component modulation frequencies <MAT>.

Each side optical frequency component <NUM>, <NUM> has a very narrow spectrum, which is determined by the modulation frequency <MAT>. The modulation frequency <MAT> may be between a few MHz and a few tens MHz. In at least one embodiment, the power of the optical frequency components <NUM>, <NUM> is much smaller compared to the total power of the optical super-channel signal. In some embodiments, the power of the optical frequency components <NUM>, <NUM> may be, for example, less than <NUM>% of the total power.

The side component frequencies fA and fB are in the range of C-band which is between <NUM> and <NUM> terahertz (THz). A side component spacing fA - fB may be between <NUM> and a few GHz. The modulation frequencies <MAT> and <MAT> of first and second side frequency components <NUM>, <NUM> are in the range of tens of MHz, and the side component spacing fA - fB is in the range of hundreds of MHz.

<FIG> is an illustration of a spectral portion <NUM> with three neighboring subcarriers <NUM>, <NUM>, <NUM> having side frequency components <NUM>, <NUM>, <NUM>, <NUM>, in accordance with various embodiments of the present disclosure. In some embodiments, one subcarrier <NUM> may have two side frequency components <NUM>, <NUM>.

Referring back to <FIG>, to transmit information over the fiber link, each of the two subcarriers <NUM>, <NUM> is modulated with data (using MZMs 471a, 472b depicted in <FIG>). After data modulation, the electrical fields of first and second subcarriers <NUM>, <NUM> can be written as follows: <MAT> <MAT> where E<NUM>(t) and E<NUM>(t) are the baseband complex electrical fields.

One can assume that the frequency difference between subcarrier optical center frequency f<NUM> of first subcarrier <NUM> and optical side component frequency fA of side frequency component <NUM> is positive: Δf<NUM>A = f<NUM> - fA > <NUM>. One can also assume that a difference between subcarrier optical center frequency f<NUM> of second subcarrier <NUM> and optical side component frequency fB of second side frequency component <NUM> is negative: Δf<NUM>B = f<NUM> - fB < <NUM>. The electrical fields of first and second subcarriers <NUM>, <NUM> may be derived as follows: <MAT> <MAT>.

In some embodiments, side frequency component <NUM> (with optical side component frequency fA) is located in a vicinity of an edge of the subcarrier spectrum of first subcarrier <NUM>. However, side frequency component <NUM> may have any frequency within the bandwidth of the corresponding subcarrier <NUM> (e.g., within the bandwidth Δf<NUM> of first subcarrier <NUM>).

In some embodiments, for convenience, the differences between the central subcarrier frequencies and side component frequencies may be equal: |Δf<NUM>A| = |Δf<NUM>B|. Referring again to <FIG>, one may derive that a controlling difference between side component frequencies of two neighboring side frequency components <NUM>, <NUM> is: <MAT>.

In equation (<NUM>), f<NUM> - f<NUM> is a target subcarrier spacing between two neighboring subcarriers <NUM>, <NUM>. The target subcarrier spacing f<NUM> - f<NUM> may be pre-determined, for example, from the design of the optical link. If (|Δf<NUM>A|+|Δf<NUM>B|) is known, a difference between two neighbouring side frequency components fA-fB is maintained constant at a desired value in order to maintain the target subcarrier spacing f<NUM> - f<NUM> constant.

<FIG> depicts an apparatus <NUM> configured to detect the difference between side component frequencies fA-fB of two neighbouring side frequency components, in accordance with various embodiments of the present disclosure.

It should be noted that, although the apparatus is depicted for two subcarriers, it may be configured to detect subcarrier spacings for pairs defined between any number of subcarriers. In some embodiments, one subcarrier may be selected to be a master subcarrier and the other subcarriers may be slaves. The slave subcarriers may track (follow) the master subcarrier.

The apparatus <NUM> comprises a signal generator <NUM> and a controller <NUM>. The signal generator <NUM> comprises DAC converters <NUM>, electrical drivers <NUM>, lasers <NUM> and EO modulators <NUM>, which have been described above.

The DSPs <NUM> were discussed and depicted in further details in <FIG>. The DSPs <NUM> cause the optical signal with subcarriers <NUM>, <NUM> to have side frequency components <NUM>, <NUM> (illustrated in <FIG>). The side frequency components <NUM>, <NUM> at fA and fB are modulated at side modulation frequencies <MAT>. Two subcarriers <NUM>, <NUM> with side frequency components <NUM>, <NUM> are coupled by a coupler <NUM>. The output optical signal of coupler <NUM> comprises a first subcarrier at frequency f<NUM> with a first side frequency component at fA and a second subcarrier at frequency f<NUM> with a second side frequency component at fB.

At the output of coupler <NUM>, one portion of light <NUM> in the output optical signal may be transmitted to the optical link (not depicted), while another portion of light <NUM> may be tapped to controller <NUM> in order to determine the side component spacing fA - fB, as described below.

The tapped light <NUM> is converted to photocurrent by a photodetector (PD) <NUM>. The PD <NUM> is a low-pass PD and may have a bandwidth that is larger than the designed spacing |fA - fB|. In some embodiments, the photocurrent may be amplified by a transimpedance amplifier (not depicted).

In at least one embodiment, frequencies of each of side frequency components <NUM>, <NUM> relative to subcarrier frequencies f<NUM> and f<NUM>, |Δf<NUM>A| = |Δf<NUM>B|, may be chosen such that a side component spacing fA - fB is equal to the filter center frequency of a first electronic filter <NUM> in <FIG>. In at least one embodiment, the first electronic filter <NUM> is a bandpass filter with a narrow passband, as described herein.

The optical electric field for side frequency components <NUM>, <NUM>, EA and EB, respectively, may be written as follows: <MAT> <MAT> where AA and AB are the amplitudes, and t is time.

When side frequency components <NUM>, <NUM> are transmitted together, the optical intensity is: <MAT>.

Taking into account equations for optical electric field for side frequency components <NUM>, <NUM> of equations (<NUM>)-(<NUM>), one may re-write the intensity as follows: <MAT>.

The photocurrent and voltage VPD(t) generated by PD <NUM> are proportional to optical intensity I(t) of equation (<NUM>): <MAT>.

It should be noted that the equations provided herein assume that side frequency components <NUM>, <NUM> have the same polarization, which can be provided by optical design of MZMs 472a, 472b.

The first two terms of equation (<NUM>) separately contain information about side frequency component <NUM> and side frequency component <NUM>. The last term of equation (<NUM>) contains the side component spacing (fA - fB). However, the information cannot be extracted by detecting side frequency components <NUM>, <NUM>, the tone corresponding to their sum <MAT> or to their difference <MAT>, because the fast changing beat factor cos(<NUM>π(fA - fB)t) averages these tones out.

In order to detect side frequency components <NUM>, <NUM>, or the tones corresponding to their sum or difference, it is necessary to have a non-zero factor cos(<NUM>π(fA - fB)t) after averaging. In some embodiments, this may be achieved by a squaring operation. It should be noted that other operations may also be used to obtain a non-zero factor cos(<NUM>π(fA - fB)t) after averaging.

In at least one embodiment, the first two terms of equation (<NUM>) accounting for base-band beating can be neglected if a bandpass filter is applied to the signal. The bandpass filter may be designed such that the only term of equation (<NUM>) that may pass the bandwidth of the bandpass filter is the last term of equation (<NUM>).

Referring again to <FIG>, the electrical signal generated by the PD <NUM> is filtered by first electronic filter <NUM>. The first electronic filter <NUM> is a narrow bandpass analog electronic filter.

<FIG> is an illustration of the transfer function H of first electronic filter <NUM> of <FIG>, in accordance with at least one embodiment of the present technology.

A filter center frequency ff of first electronic filter <NUM> may be set to a specific frequency chosen such that it corresponds to a desired difference between fA and fB. The center frequency ff of first electronic filter <NUM> is chosen to be significantly higher than <MAT> and <MAT>, so that only the beat term fA - fB can pass through the first electronic filter <NUM>.

For example, the center frequency ff of first electronic filter <NUM> may be <NUM>. The passband of first electronic filter <NUM> may be a few MHz.

When the optical signal passes through first electronic filter <NUM>, the base-band <MAT> and <MAT> tones are significantly attenuated. Referring to equation (<NUM>), the first electronic filter <NUM> is configured to pass through only the last (third) term of equation (<NUM>), which is: <MAT>.

The difference between side component frequencies, i.e. side component spacing fA - fB, determines the beat term. The beat term corresponding to side component spacing fA - fB passes through first electronic filter <NUM>.

The locking range is determined by filter parameters of first electronic filter <NUM> such as, for example, center frequency, bandwidth, roll-off, etc. In at least one embodiment, the transfer function H of first electronic filter <NUM> acts as an etalon for optical wavelength locker with a much higher finesse.

As depicted in <FIG>, after passing through first electronic filter <NUM>, the intensity is then squared by a squaring device <NUM>. The squaring device <NUM> is configured to perform a square operation on the photocurrent (intensity of the optical signal). For example, the squaring device <NUM> may comprise a multiplier. In some embodiments, the square operation may be performed by a splitter and the multiplier.

After squaring the last term of the equation (<NUM>), the squared voltage that is obtained at the output of the squaring device <NUM> is: <MAT>.

Equation (<NUM>) may be rewritten as follows: <MAT>.

In at least one embodiment, the term corresponding to the difference frequency <MAT> may be detected. It should be noted that in some embodiments, a sum frequency term <MAT> may also be detected.

The power is therefore proportional to the following term, which one may detect <MAT> where H(f) is an amplitude transfer function of first electronic filter <NUM>.

The detected power of the difference frequency tone measured at frequency <MAT> <MAT> is: <MAT>.

Referring again to <FIG>, after the squaring operation performed by squaring device <NUM>, the optical signal passes through a second electronic filter <NUM>. The second electronic filter <NUM> is configured to filter radio-frequency (RF) power of the RF signal about a modulated beat frequency tone.

It should be noted that, in at least one embodiment, both first electronic filter <NUM> and second electronic filter <NUM> are narrow bandpass filters. The second electronic filter <NUM> may have a very narrow pass band, such as several kilohertz (kHz). The first electronic filter <NUM> is configured to pass through the beat term fA - fB, while the second electronic filter <NUM> is configured to pass through the modulated beat frequency tone. In some embodiments, the modulated beat frequency tone is a difference between the first side modulation frequency and the second side modulation frequency <MAT> (also referred to herein as a "difference frequency"). In some other embodiments, the modulated beat frequency tone is a sum of the first side modulation frequency and the second side modulation frequency <MAT>.

Referring to equation (<NUM>), in some embodiments, correlation may be used to detect the beat term. The controller <NUM> may have other devices that can generate another cosine function <MAT> and multiply the power expressing by equation (<NUM>). This additional multiplication operation may permit detecting the power of the beat term fA - fB, and therefore monitor the beat term fA - fB.

In some embodiments, an RF peak power detector may be used to detect the modulated frequency difference <MAT>.

As depicted in <FIG>, an RF power meter <NUM> measures the power of the modulated frequency difference <MAT>.

From equation (<NUM>), one can understand that the RF power received by the RF power meter <NUM> is maximized when the difference between side component frequencies of side frequency components <NUM>, <NUM> is lined up to the center of the second bandpass filter <NUM>. By maximizing the RF power at modulated frequency difference <MAT>, one may maintain constant the difference between the side component frequencies, i.e. side component spacing, fA - fB. It should be noted that by maximizing the RF power at the modulated frequency difference <MAT>, one may lock the side component spacing, fA - fB to the center frequency of first bandpass filter <NUM>.

A control algorithm may be used to control the optical frequencies of lasers 456a, 456b, so that the RF power detected by RF power meter <NUM> is maximal. In some embodiments, dithering of the laser frequency may be used to provide tuning direction information for controlling the optical frequencies of lasers 456a, 456b based on the power detected by RF power meter <NUM>.

In some embodiments, the modulated beat frequency tone is a sum of the modulation frequencies of side frequency components, <MAT>. The RF peak of the sum <MAT>, may be detected by RF power meter <NUM> and used to control the optical frequencies 456a, 456b in a similar manner. In such embodiments, first electronic filter <NUM> is configured to filter a sum of the modulation frequencies of side frequency components, <MAT>, and second electronic filter <NUM> is configured to filter RF peak of the sum of first side modulation frequency and second side modulation frequency, <MAT>.

In the embodiment depicted in <FIG>, analog components are used to monitor the modulated beat frequency tone. In other embodiments, the monitoring of the modulated beat frequency tone may be performed in a digital domain. An analog-to-digital convertor (ADC) (not depicted) may convert the signal generated by PD <NUM> to the digital domain using. Then, filtering and other operations as discussed herein may be performed by digital processing in order to extract the RF power of modulated beat frequency tone.

<FIG> depicts a flowchart illustrating a method for controlling a difference between a first center frequency f<NUM> of a first optical subcarrier and a second center frequency f<NUM> of a second optical subcarrier of the optical super-channel signal in the optical network, in accordance with various embodiments of the present disclosure.

At step <NUM>, the first optical subcarrier is modulated at a first optical side component frequency fA with a first side modulation frequency <MAT>. The difference between the first optical side component frequency and the first center frequency of the first subcarrier, Δf<NUM>A = f<NUM>-fA, is maintained constant.

At step <NUM>, the second optical subcarrier is modulated at a second optical side component frequency fB with a second side modulation frequency <MAT>. The difference between the second optical side component frequency and the second center frequency of the first subcarrier, Δf<NUM>B = f<NUM> - fB, is maintained constant. In at least one embodiment, the difference between the first optical side component frequency and the second optical side component frequency is smaller than a difference between the first center frequency and the second center frequency.

At step <NUM>, the modulated first and second optical subcarriers are coupled together to obtain a modified optical signal.

At step <NUM>, the modified optical signal may be converted to electrical signal.

At step <NUM>, the electrical signal may then be filtered by a first bandpass electronic filter. The first electronic filter has a bandwidth corresponding to a difference between the first optical side component frequency and the second optical side component frequency, i.e. side component spacing, fA - fB.

At step <NUM>, the filtered electrical signal may be squared.

At step <NUM>, the squared electrical signal may be filtered by the second electronic filter to obtain the RF signal at the modulated beat frequency tone <MAT> or <MAT>. The modulated beat frequency tone bears information of the first side modulation frequency and the second side modulation frequency by being either a sum <MAT> or a difference <MAT> of the first side modulation frequency and the second side modulation frequency.

At step <NUM>, the RF power is detected at the modulated beat frequency tone in the modified optical signal.

At step <NUM>, a variation of the difference between the first center frequency f<NUM> of the first optical subcarrier and the second center frequency f<NUM> of the second optical subcarrier is monitored. As discussed above, the RF power, monitored at step <NUM>, is related to the side component spacing fA - fB. The monitored RF power is smaller if the side component spacing fA - fB is away from the desired value. To reduce deviation of the difference between the first center frequency f<NUM> of the first optical subcarrier and the second center frequency f<NUM> of the second optical subcarrier (i.e. f<NUM> - f<NUM>) from the desired value, at least one of the first center frequency f<NUM> and the second center frequency f<NUM> may be tuned.

At step <NUM>, at least one of the first center frequency f<NUM> and the second center frequency f<NUM> may be detuned to maximize the RF power of the RF signal at the modulated beat frequency tone.

<FIG> depicts a block diagram of an experimental set-up <NUM> for confirmation of detectability of the side component spacing fA - fB. One laser <NUM> was used to generate two laser lines with known frequency spacing. Laser spectrum <NUM> of laser <NUM> is schematically illustrated in <FIG>.

The output of a continuous wave (CW) laser <NUM> passes through in-phase quadrature phase MZM (IQ-MZM) <NUM>. The IQ-MZM <NUM> is driven by a driver <NUM> and a DAC <NUM> to provide a frequency shift with envelope modulation. The IQ-MZM <NUM> modulates the laser output at <NUM> and <NUM>. Two laser lines <NUM> at frequencies fA, fB are schematically illustrated in <FIG>. The optical signal is converted to photocurrent at PD <NUM> which has a low pass bandwidth of about <NUM>.

The photocurrent then passes through a bandpass filter <NUM> with a bandwidth of about <NUM> centered at about <NUM>. The filtered signal is then squared by squaring device <NUM> and received by a scope <NUM>. The power of the difference frequency tone, <MAT>, which is <NUM> - <NUM> = <NUM> is then measured as a function of the spacing between <NUM> laser lines fA and fB.

<FIG> depicts a power of the modulated frequency difference tone (beat tone), <MAT>, measured at the scope <NUM> of the set-up of <FIG>. The frequency offset between two laser tones fA, fB is swept between <NUM> and <NUM>. The power of the modulated frequency difference tone at <NUM> depends on the separation between frequencies fA, fB and has a maximum when the laser frequency separation is around <NUM>. The modulated frequency difference tone <MAT> was measured to be <NUM> for each laser frequency spacing fA - fB.

In <FIG>, curve <NUM> depicts a squared transfer function of the passband filter <NUM>. Dots depict measured power of difference frequency tone <MAT>.

The set-up and measurements of <FIG> illustrate that the measured power of difference frequency tone at <MAT> may be indeed expressed by equation (<NUM>). The results illustrate that monitoring of <MAT> may help to monitor the deviation of the subcarrier spacing f<NUM> - f<NUM> from a desired value.

The described herein technique for controlling of the difference between center frequencies of optical subcarriers of an optical super-channel signal does not require an optical etalon. Furthermore, the second electronic filter acts as an RF frequency discriminator and provides a high resolution on the order of MHz. The RF components used in the technique as described herein may be low-speed, which can help to reduce the cost of the transmitter. The technique as described herein may help to lock the center frequencies of the subcarriers at a desired frequency separation. Moreover, such technique may allow for decreasing and increasing, on demand, of the frequency separation between the subcarriers.

It is to be understood that the operations and functionality of the disclosed methods and apparatuses may be achieved by hardware-based, software-based, firmware-based elements and/or combinations thereof. Such operational alternatives do not, in any way, limit the scope of the present disclosure.

Claim 1:
A method for controlling a difference between a first center frequency of a first optical subcarrier and a second center frequency of a second optical subcarrier of an optical super-channel signal in an optical network, the method comprising:
modulating (<NUM>) the first optical subcarrier at a first optical side component frequency with a first side modulation frequency;
modulating (<NUM>) the second optical subcarrier at a second optical side component frequency with a second side modulation frequency, a difference between the first optical side component frequency and the second optical side component frequency being smaller than a difference between the first center frequency and the second center frequency;
wherein the first and the second side component frequency are in the range between <NUM> and <NUM> THz, wherein the first and the second modulation component frequency are in the range of tens of MHz, and wherein a spacing between the first and the second side component frequency is in the range of hundreds of MHz;
coupling (<NUM>) together the modulated first and second optical subcarriers to obtain a modified optical signal; and
monitoring (<NUM>) a variation of the difference between the first center frequency and the second center frequency by detecting a radio-frequency (RF) power at a modulated beat frequency tone in the modified optical signal,
wherein the monitoring (<NUM>) a variation of the difference between the center frequencies of the first and the second optical subcarriers further comprises:
converting (<NUM>) the modified optical signal to an electrical signal;
filtering (<NUM>) the electrical signal by a first electronic filter to obtain a filtered electrical signal within a first bandwidth corresponding to the difference between the first optical side component frequency and the second optical side component frequency;
squaring (<NUM>) the filtered electrical signal;
filtering (<NUM>) again the filtered and squared electrical signal by a second electronic filter to obtain an RF signal at the modulated beat frequency tone; and
measuring the RF power of the RF signal at the modulated beat frequency tone.