Patent Description:
<CIT> describes that an apparatus may include a drive circuit that selectively supplies a first plurality of electrical signals and a second plurality of electrical signals to a modulation circuit. It is described that based on the first plurality of electrical signals, the modulation circuit may output a first in-phase component being modulated in accordance with a first tone having a first frequency and a first quadrature component being modulated in accordance with a second tone having a second frequency different than the first frequency. It is described that based on the second plurality of electrical signals, the modulation circuit may output a second optical signal having a second in-phase component being modulated in accordance with the second tone and a second quadrature component being modulated in accordance with the first tone.

Coherent optical transmitters (also called coherent transmitters) are used to transmit optical signals by modulating the amplitude and phase of light transmitted through an optical channel, such as a fiber optic cable. For example, data may be encoded using quadrature amplitude modulation (QAM), and the transmission capacity may be doubled using polarization multiplexing (PM). One technique for coherent transmission involves using two separate electrical data channels - an in-phase (I) data channel and a quadrature (Q) data channel - to modulate the I and Q phase modulation components of the optical signal using an electro-optical modulator (EOM), such as a Mach-Zehnder Modulator (MZM). In a polarization-mutiplexing IQ modulator, I and Q channels may be used to modulate signals in each of two polarizations: an I channel for a horizontally (X-direction) polarized signal labelled XI, an I channel for a vertically (Y-direction) polarized signal labelled YI, a Q channel for the X-direction polarized signal labelled XQ, and a Q channel for the Y-direction polarized signal labelled YQ. (It will be appreciated that X need not be horizontal and Y need not be vertical, as long as X and Y are mutually orthogonal or substantially so.

Time skew between the I and Q data channels of a coherent transmitter (called IQ time skew or simply IQ skew) is an impairment that may degrade the performance of the coherent transmitter, especially for high-order modulation formats at high baud rates, resulting in signaling error at the receiver. Some optical receivers may include digital signal processors (DSP), also called DSP units, that implement IQ equalization algorithms to compensate for IQ skew to some extent. However, receiver-side compensation performance may be degraded by channel impairments such as polarization mode dispersion (PMD), polarization dependent loss (PDL), and amplified spontaneous emission (ASE) noise. The signaling error from IQ time skew may cause signal-to-noise ratio (SNR) degradation that cannot be compensated for by a conventional 2x2 multiple-in and multiple-out (MIMO) equalizer in the receiver's DSP. Moreover, an IQ skew value of larger than half baud, i.e. <NUM> unit intervals (<NUM> U. ), may cause acquisition failure by the receiver DSP because the signaling error may be beyond the capacity of the 2x2 MIMO equalizer. Furthermore, receiver-side DSP compensation for IQ skew may increase the implementation complexity of the receiver application-specific integrated circuit (ASIC) and the chip power consumption. Therefore, performing accurate calibration and compensation of the IQ skew at the transmitter side may present advantages over receiver-side IQ time skew compensation.

IQ skew in a coherent transmitter may also be compensated for via a one-time factory calibration of the transmitter. The gross IQ skew is estimated from length difference of the printed circuit board (PCB) wires between the I and Q data channels, which may achieve a precision at the scale of pico-seconds. Fine skew, at a precision of better than <NUM> picoseconds, may be obtained by manually adjusting the skew value until an optimally smallest bit error rate (BER) reading is obtained from the receiver DSP. However, such manual IQ skew calibration is relatively inefficient, and the calibration value obtained in the factory calibration may not be accurate due to a number of factors: receiver equalizer convergence status, component aging effects, changing operating environment, and so on. Therefore, a transmitter capable of self-calibration at a transmitter power-up stage may present further advantages.

Transmitter IQ conjugation (also called phase conjugation, or simply conjugation) is a further impairment of optical communication that may cause acquisition failure at the receiver DSP due to IQ flipping. It is difficult to identify IQ conjugation at the transmitter side because of the large number of factors that may cause transmitter conjugation. Therefore, a simple and reliable technique to identify and/or compensate for conjugation at the transmitter side may present advantages.

Appended claim <NUM> defines a device. Appended claim <NUM> defines a method. The invention and its scope of protection are defined by these independent claims.

In various embodiments described herein, methods and devices are disclosed that provide self-calibration of in-phase and quadrature (IQ) time skew and conjugation in a coherent transmitter. Various embodiments may enable an optical transmitter to combine a pilot tone with each of two frequency bands of the digital data signal used to modulate the optical signal. The optical signal output of the transmitter is monitored by a pilot tone detector component of the transmitter, which detects the power of each pilot tone and calculates power ratios between the detected pilot tones of the two signal frequency bands. A DSP of the transmitter applies an IQ time skew bias value to the digital data signal; by sweeping the IQ time skew bias across a range of values, a linear interpolation may be constructed identifying an IQ time skew bias value at which the power ratio of the two detected pilot tones is <NUM>:<NUM>, indicating a balance point at which the IQ time skew of the optical signal is equal to zero. The same linear interpolation, combined with phase bias information from the optical modulator, allows the transmitter to determine whether the optical signal is conjugated. Conjugation may be compensated for by either setting a bias point of the optical modulator or inverting the polarity of the digital data signal used to modulate the optical signal.

In some aspects, the present disclosure describes a device. The device has a pilot tone generator configured to combine a first pilot tone with a first frequency band of a digital data signal, and combine a second pilot tone with a second frequency band of the digital data signal, thereby generating a modified digital data signal. The device has an electro-optic modulator (EOM) configured to generate an optical signal based on the modified digital data signal. The device has a pilot tone detector configured to receive the optical signal, generate a detector digital signal based on the optical signal, and detect a first pilot tone power and a second pilot tone power based on the detector digital signal. The device has a control unit configured to determine an IQ time skew between an in-phase component and a quadrature component of the optical signal based on the first pilot tone power and second pilot tone power.

In some examples, the device further comprises a digital delay filter configured to receive the modified digital data signal, set an IQ time skew bias based on control information from the control unit, and apply the IQ time skew bias to the modified digital data signal. The control unit is further configured to generate the control information based on the IQ time skew, and provide the control information to the digital delay filter.

In some examples, the first frequency band is a portion of digital signal spectrum above a predetermined frequency, and the second frequency band is a portion of digital signal spectrum below the predetermined frequency.

In some examples, the control unit is further configured to perform IQ time-skew calibration by providing skew sweep control information to the digital delay filter such that the digital delay filter sets the IQ time-skew bias equal to each of a plurality of IQ time-skew bias values; for each of the plurality of IQ time-skew bias values, calculating a power ratio between the first pilot tone power and second pilot tone power for the respective IQ time-skew bias value; calculating the IQ time skew based on the plurality of IQ time-skew bias values and the respective plurality of power ratios; and providing IQ skew calibration control information to the digital delay filter such that the digital delay filter sets the IQ skew bias to compensate for the IQ time skew.

In some examples, the control unit is further configured to perform conjugation calibration by determining a phase conjugation status of the optical signal based on the plurality of IQ time-skew bias values and the respective plurality of power ratios and phase bias information received from the EOM, and compensating for the phase conjugation status by setting a bias point of the EOM or inverting a polarity of the modified digital data signal.

In some examples, the control unit is further configured to perform IQ time-skew calibration and conjugation calibration during a power-up phase of the device.

In some examples, calculating the IQ time skew based on the plurality of IQ time-skew bias values and the respective plurality of power ratios comprises calculating a linear interpolation of the plurality of IQ time-skew bias values and the respective plurality of power ratios; identifying a balance point on the linear interpolation corresponding to a power ratio value of <NUM>:<NUM>; and setting the IQ time skew equal to an IQ time skew bias value corresponding to the balance point.

In some examples, determining a phase conjugation status of the optical signal comprises calculating a linear interpolation of the plurality of IQ time-skew bias values and the respective plurality of power ratios; identifying a positive or negative sign of a slope of the linear interpolation; and determining that the optical signal is conjugated based on the sign of the slope of the linear interpolation and the phase bias information.

In some examples, the first pilot tone has a modulation frequency that is different from a modulation frequency of the second pilot tone, and the pilot tone generator is configured to modulate the power of the first frequency band with the first pilot tone while modulating the power of the second frequency band with the second pilot tone.

In some examples, the first pilot tone and second pilot tone are both a single pilot tone having a single pilot tone modulation frequency. The pilot tone generator is configured to alternate over time between modulating the power of the first frequency band with the single pilot tone, and modulating the power of the second frequency band with the single pilot tone.

In some examples, the pilot tone generator combines the first pilot tone with the first frequency band of the digital data signal and combines the second pilot tone with the second frequency band of the digital data signal by applying a fast Fourier transform to the digital data signal to generate a frequency-domain digital data signal; applying a digital signal processing unit to the frequency-domain digital data signal to generate a frequency-domain first frequency band signal and a frequency-domain second frequency band signal; applying an inverse fast Fourier transform to the frequency-domain first frequency band signal and the frequency-domain second frequency band signal to generate a time-domain first frequency band signal and a time-domain second frequency band signal; modulating the amplitude of the time-domain first frequency band signal using the first pilot tone to generate a modified time-domain first frequency band signal; modulating the amplitude of the time-domain second frequency band signal using the second pilot tone to generate a modified time-domain second frequency band signal; and combining the modified time-domain first frequency band signal and modified time-domain second frequency band signal to generate the modified digital data signal.

In some examples, modulating the amplitude of the time-domain first frequency band signal and modulating the amplitude of the time-domain second frequency band signal is performed using a modulation index between <NUM>% and <NUM>%.

In some examples, the first pilot tone and second pilot tone each have a respective modulation frequency between <NUM> and <NUM>.

In some examples, the electro-optic modulator comprises a dual-polarization IQ Mach-Zehnder modulator.

In some aspects, the present disclosure describes a device. The device has a pilot tone detector configured to receive an optical signal, generate a detector digital signal based on the optical signal, and detect a first pilot tone power and a second pilot tone power based on the detector digital signal. The device has a control unit configured to determine an IQ time skew between an in-phase component and a quadrature component of the optical signal based on the first pilot tone power and second pilot tone power.

In some examples, the pilot tone detector comprises a low-speed photodetector for receiving the optical signal and generating a detector analog signal based on the optical signal, an analog-to-digital converter (ADC) for generating the detector digital signal based on the detector analog signal, and a pilot tone detector digital signal processing (DSP) unit for detecting the first pilot tone power and second pilot tone power and determining the IQ time skew.

In some aspects, the present disclosure describes a method. A first pilot tone is combined with a first frequency band of a digital data signal, and a second pilot tone is combined with a second frequency band of the digital data signal, thereby generating a modified digital data signal. An optical signal is generated based on the modified digital data signal. A detector digital signal is generated based on the optical signal. A first pilot tone power and a second pilot tone power are detected based on the detector digital signal. The first pilot tone power and second pilot tone power are used to determine an IQ time skew between an in-phase component and a quadrature component of the optical signal.

In some examples, the method further comprises performing IQ time-skew calibration by setting an IQ time-skew bias equal to each of a plurality of IQ time-skew bias values; for each of the plurality of IQ time-skew bias values, applying the IQ time-skew bias to the digital data signal and calculating a power ratio between the first pilot tone power and second pilot tone power for the respective IQ time-skew bias value; calculating the IQ time skew based on the plurality of IQ time-skew bias values and the respective plurality of power ratios; and applying the IQ time skew bias to the digital data signal to compensate for the IQ time skew.

In some examples, the method further comprises performing conjugation calibration by determining a phase conjugation status of the optical signal based on the plurality of IQ time-skew bias values and the respective plurality of power ratios and phase bias information of an optical modulator used to generate the optical signal, and compensating for the phase conjugation status by setting a bias point of the optical modulator or inverting a polarity of the modified digital data signal.

In some examples, IQ time-skew calibration and conjugation detection are performed during a power-up phase.

As used herein, a "modulation component" of a signal may refer to any characteristic of a signal that may be modulated to encode data, e.g., amplitude, frequency, and phase. As used herein, however, unless otherwise specified, the term "modulation component" refers specifically to either an in-phase (I) component or a quadrature (Q) component of a quadrature amplitude modulated (QAM) signal. In the case of a polarization-multiplexed signal (i.e. a first QAM signal at a first polarization direction and a second QAM signal at a second, typically orthogonal, polarization direction), a modulation component may refer to the I or Q component of one of the QAM signals.

As any of the above-noted modulation components of an optical signal may encode data from a data channel at the transmitter and may be decoded to yield a data channel at a receiver, a modulation component may occasionally be referred to herein as a "channel" or "modulation channel" of the optical signal. As a QAM signal consists of two phase orthogonal data channels (corresponding to the I and Q modulation components), a polarization-multiplexing quadrature-phase modulator generates a signal having four modulation channels: a first modulation component in a first polarization direction (e.g. an I modulation component in a horizontal X polarization direction), a second modulation component in the first polarization direction (e.g. a Q modulation component in the X polarization direction), a first modulation component in a second polarization direction (e.g. an I modulation component in a vertical Y polarization direction), and a second modulation component in a second polarization direction (e.g. a Q modulation component in the vertical Y polarization direction). Each of these channels may encode data independently of the other three channels. Furthermore, the various modulation components may occasionally be referred to herein by a shortened form thereof. An in-phase (real) phase modulation component may be referred to herein as an "in-phase modulation component", an "in-phase component", an "I modulation component", an "I component", or sometimes simply "I". It will be appreciated that the capital letters Q and I as used herein refer to the respective modulation components of a signal, the corresponding data channels used to modulate said signal components, or the corresponding data channels decoded or demodulated from said signal components.

In examples disclosed herein, methods and apparatuses are described that provide self-calibration of in-phase and quadrature (IQ) time skew and conjugation in a coherent transmitter.

As noted above, one technique for coherent transmission involves using an in-phase (I) data channel and a quadrature (Q) data channel to modulate the I and Q phase modulation components of the optical signal using an electro-optical modulator (EOM), such as an in-phase and quadrature (IQ) Mach-Zehnder Modulator (IQ-MZM). An example of IQ time skew will now be described with reference to an example IQ-MZM using I and Q data channels to modulate the I and Q components of the optical signal.

<FIG> is a schematic illustrating the channel paths of an electro-optical modulator (EOM) <NUM>, shown here as a IQ Mach-Zehnder Modulator (IQ-MZM) having an in-phase (I) channel path <NUM> modulated by an analog electrical in-phase (I) data channel <NUM> and a quadrature (Q) channel path <NUM> modulated by an analog electrical quadrature (Q) data channel <NUM>. The present described embodiments are not limited to an IQ-MZM but may use any optical modulator suitable to carry out IQ modulation.

The EOM <NUM> receives an optical input <NUM> in the form of a laser providing a light source at a particular frequency (single wavelength). The optical input <NUM> is split into the I channel path <NUM> and Q channel path <NUM> in accordance with optical interferometry techniques. The I path <NUM> receives an analog electrical signal from an I channel <NUM>, which modulates an I component of the optical signal propagated through the I channel path <NUM>. The Q path <NUM> receives an analog electrical signal from a Q channel <NUM>, which modulates a Q component of the optical signal propagated through the Q channel path <NUM>. The optical signal output of the I path <NUM> and the optical signal output of the Q path <NUM> are coupled or combined to form a phase modulated optical signal <NUM> at the output of the EOM <NUM>.

The IQ skew is a relative delay between the data path of the I channel <NUM> and the data path of the Q channel <NUM>, which is defined as <MAT> wherein ΔτIQ is the time skew, ΔτQ is the time delay of the I data path, and ΔτI is the time delay of the Q data path. The IQ skew is caused by a physical time delay difference between the I channel <NUM> and Q channel <NUM>. The time delay in these channels <NUM>, <NUM> may be caused by a combination of transmitter components: e.g., a digital-to-analog converter (DAC) used to generate the analog channels from digital inputs, a driver used to drive the EOM <NUM>, the EOM <NUM> itself, and/or the printed circuit board (PCB) wires used by the transmitter.

As noted above, the present described embodiments may use any optical modulator suitable to carry out IQ modulation. A second technique for coherent transmission involves using four separate electrical data channels to modulate four modulation components of the optical signal: the four orthogonal signals carried on the four channels are the XI, XQ, YI, and YQ signals described above, modulating the I and Q components of each of two polarization directions (X and Y) of the optical signal. This technique may be referred to as polarization-multiplexing optical modulation.

<FIG> is a schematic illustrating the channel paths of a second example EOM in the form of a dual-polarization IQ Mach-Zehnder Modulator (DP-IQMZM) <NUM>, having four channel paths <NUM>, <NUM>, <NUM>, <NUM>, each channel path being modulated by an analog electrical data channel input <NUM>, <NUM>, <NUM>, <NUM>.

In the DP-IQMZM <NUM>, the optical input <NUM> is split into two channel paths, each of which is split into a further two channel paths. This yields four channel paths: an XI path <NUM>, an XQ path <NUM>, a YI path <NUM>, and a YQ path <NUM>. The XI path <NUM> receives an analog electrical signal from an XI channel <NUM>, which modulates an I component of the signal propagated through the XI channel path <NUM> to form an XI modulated signal with X polarization. Each of the other three paths is similarly modulated by an analog electrical signal: the XQ path <NUM> by XQ channel <NUM> to form an XQ modulated signal with X polarization; the YI path <NUM> by YI channel <NUM> to form a YI modulated signal with Y polarization, and the YQ path <NUM> by YQ channel <NUM> to form a YQ modulated signal with Y polarization. The optical signal outputs of the XI path <NUM> and XQ path <NUM> are coupled or combined to form a modulated complex signal (XI + j*XQ) with X polarization, and the optical signal outputs of the YI path <NUM> and YQ path <NUM> are coupled or combined and fed through a polarization rotator <NUM> to form a modulated complex signal (YI+j*YQ) with Y polarization, thereby enabling polarization multiplexing. The X-polarized signal and Y-polarized signal are coupled or combined to form a polarization-multiplexed optical signal <NUM> at the output of the DP-IQMZM <NUM>. The four orthogonal data channels XI <NUM>, XQ <NUM>, YI <NUM>, and YQ <NUM> are analog electrical data channels of an amplified analog electrical data signal used to drive the PM-IQMZM <NUM>.

<FIG> is a block diagram of a coherent transmitter device <NUM> with IQ skew detection and compensation. The device <NUM> uses an EOM <NUM>, such as the IQ-MZM of <FIG>, to modulate an optical signal <NUM> for transmission across an optical communication link <NUM>. The optical signal <NUM> encodes one or more pilot tones generated by a pilot tone generator <NUM> and combined with the digital data signal. The optical signal <NUM> modulated by the EOM <NUM> is also propagated to a pilot tone detector <NUM>, which detects power ratios between the pilot tones present in the optical signal <NUM> and uses these power ratios to calculate transmission IQ time skew. It will be appreciated that the power of a signal, or a component of a signal (such as an amplitude-modulated pilot tone), is equal to the square of the amplitude of that signal or signal component. Therefore, any description of power herein may be understood to equivalently apply to amplitude as long as the mathematical relationship between amplitude and power is maintained. Thus, e.g., calculation of a power ratio between two signal components may be carried out as a calculation of a ratio of the square of the amplitude of each signal component.

In operation, a data signal is propagated through several stages of an optical transmitter before being used to modulate the optical signal <NUM> of the EOM <NUM>. The data signal exists in different formats at different stages: digital, analog, time-domain, frequency-domain, and pre- and post-modification or alteration by the various stages of the transmitter. At each such stage, the data signal may be referred to as the "data signal"; at any analog stage it may be referred to as an "analog data signal"; at any digital stage it may be referred to as a "digital data signal"; at any time-domain stage it may be referred to as a "time-domain data signal"; and at any frequency-domain stage it may be referred to as a "frequency-domain data signal".

The data signal is first received by the device <NUM> as an input digital data signal <NUM>. A transmitter DSP <NUM> generates a DSP digital data signal <NUM> based on the input digital data signal <NUM>. The DSP <NUM> may apply digital signal processing operations unrelated to IQ time skew calibration to the input digital data signal <NUM> in order to generate the DSP digital data signal <NUM>.

The DSP digital data signal <NUM> is received by a pilot tone generator <NUM>, which combines one or more pilot tones with the DSP digital data signal <NUM> by modulating the amplitude of a frequency band of the DSP digital data signal <NUM> with a respective pilot tone <NUM>,<NUM>. The operation of the pilot tone generator <NUM> will now be described in detail.

The pilot tone generator <NUM> first applies a Fourier transform operation, shown here as a fast Fourier transform (FFT) operation <NUM>, to the DSP digital data signal <NUM> to transform the DSP digital data signal <NUM> into a frequency-domain digital data signal <NUM>. A frequency splitter <NUM> (e.g., a further digital signal processing unit of the pilot tone generator <NUM>) is used to split the frequency-domain digital data signal <NUM> into two frequency bands or spectrum bands to generate a frequency-domain first frequency band signal <NUM> and a frequency-domain second frequency band signal <NUM>. The splitting operation performed by the frequency splitter <NUM> may in some embodiments generate a frequency-domain first frequency band signal <NUM> which occupies a frequency band that is higher in frequency relative to the band occupied by the frequency-domain second frequency band signal <NUM>. In such embodiments, the frequency-domain first frequency band signal <NUM> may be referred to as an "upper spectrum band signal", whereas the frequency-domain second frequency band signal <NUM> may be referred to as a "lower spectrum band signal". It will be appreciated that the frequency-domain first frequency band signal <NUM> and the frequency-domain second frequency band signal <NUM> may be transformed back into the time domain at various stages or operations of the device <NUM>; regardless of the domain in which these two signals are encoded, they may be referred to as a "first frequency band signal" and a "second frequency band signal", respectively.

<FIG> shows an example representation of the two frequency bands of a data signal in such an embodiment. A schematic <NUM> shows the upper spectrum band <NUM> and lower spectrum band <NUM> plotted along the frequency dimension (X axis). A center frequency fc <NUM> defines the upper bound of the lower frequency band <NUM> and the lower bound of the upper frequency band <NUM>. The first pilot tone modulation frequency fPT1 <NUM> modulates the upper frequency band <NUM>, whereas the second pilot tone modulation frequency fPT2 <NUM> modulates the lower frequency band <NUM> of the data signal.

In some embodiments, the frequency-domain first frequency band signal <NUM> and frequency-domain second frequency band signal <NUM> may be generated by applying the frequency splitter <NUM> to split the frequency-domain digital data signal <NUM> at a predetermined frequency. In some embodiments, the first frequency band occupied by the first frequency band signal is a portion of digital signal spectrum above the predetermined frequency, whereas the second frequency band occupied by the second frequency band signal is a portion of digital signal spectrum below the predetermined frequency. The predetermined frequency may be a zero frequency in some embodiments; in such embodiments, the first frequency band signal <NUM> may be referred to as a "positive frequency band signal", whereas the frequency-domain second frequency band signal <NUM> may be referred to as a "negative frequency band signal".

The frequency-domain first frequency band signal <NUM> and frequency-domain second frequency band signal <NUM> are each transformed back to the time domain by applying an inverse Fourier transform (shown as inverse fast Fourier transform operation <NUM>) to the frequency-domain first frequency band signal <NUM> to generate a time-domain first frequency band signal <NUM> and to the frequency-domain second frequency band signal to generate a time-domain second frequency band signal <NUM>.

A pilot tone is then combined with each of the two signals. A first pilot tone PT<NUM> <NUM> is used to modulate the amplitude of the time-domain first frequency band signal <NUM> to generate a modified time-domain first frequency band signal <NUM>, and a second pilot tone PT<NUM> <NUM> is used to modulate the amplitude of the time-domain second frequency band signal <NUM> to generate a modified time-domain second frequency band signal <NUM>. In some embodiments, the first pilot tone <NUM> has a different modulation frequency from that of the second pilot tone <NUM>. In some such embodiments, both pilot tones <NUM>, <NUM> may be applied simultaneously to modulate the amplitude of each of the two time-domain signals <NUM>, <NUM>. Other embodiments may use a single pilot tone having a single pilot tone modulation frequency as both the first pilot tone <NUM> and the second pilot tone <NUM>. In some such embodiments, the pilot tone generator <NUM> may be configured to alternate over time between, during a first time period, modulating the amplitude of the first frequency band (i.e. the time-domain first frequency band signal <NUM>) with the single pilot tone and, during a second time period, modulating the amplitude of the second frequency band (i.e. the time-domain second frequency band signal <NUM>) with the single pilot tone. Such time-alternating embodiments may require synchronization of the pilot tone generator <NUM> with the sampling operations of the pilot tone detector <NUM> described below.

In some embodiments, the pilot tone modulation is performed using a modulation index between <NUM>% and <NUM>%, thereby creating only a minor perturbance of each signal being modulated. Embodiments using a lower modulation index (e.g., <NUM>%) may require the use of a longer averaging window to detect the pilot tone than embodiments using a higher modulation index (e.g. <NUM>%).

In some embodiments, each pilot tone has a low modulation frequency: some embodiments may use pilot tones having respective modulation frequencies in the range of KHz to MHz, and some embodiments may specifically use pilot tones having respective modulation frequencies between <NUM> and <NUM>.

In some embodiments, the first pilot tone PT<NUM> <NUM> is used to apply an amplitude modulation to the first frequency band digital signal <NUM>, effectively multiplying the signal amplitude by (<NUM> + m × cos(2πfPT2t)), wherein fPT1 indicates the modulation frequency of the first pilot tone PT<NUM> <NUM> and m indicates the modulation index. The second pilot tone PT<NUM> <NUM> is used to apply an amplitude modulation to the second frequency band digital signal <NUM>, effectively multiplying the signal amplitude by (<NUM> + m × cos(<NUM>πfPT2t)) wherein fPT2 indicates the modulation frequency of the second pilot tone PT<NUM> <NUM> and m indicates the modulation index. Whereas this example shows the pilot tones as sinusoidal signals, it will be appreciated that other signal forms may be used for the pilot tones, such as square waves.

The modified time-domain first frequency band signal <NUM> is combined with the modified time-domain second frequency band signal <NUM> (e.g., by a simple signal addition operation) to generate a modified digital data signal <NUM>, which can be represented as a modified I component I' and a modified Q component Q' multiplied by j, i.e. I' + jQ', to form a complex signal. The Q' component passes through a digital delay filter <NUM>, shown as a finite impulse response (FIR) filter, in order to apply any I/Q skew digitally to compensate for the physical IQ skew between the I and Q data path. It will be appreciated that the I' component of the modified digital data signal <NUM> may include XI' and YI' channels, and that the Q' component may include XQ' and YQ' channels, in embodiments using polarization multiplexing. In such embodiments, the digital delay filter <NUM> would apply time skew bias to the XQ' and YQ' channels.

The modified digital data signal <NUM> is transformed into an analog data signal, shown in <FIG> as a pair of analog data signal channels <NUM>, by a digital-to-analog converter (DAC) <NUM>. The analog data signal channels <NUM> are amplified by a set of amplifiers <NUM>, thereby generating a pair of amplified data signal channels <NUM>, <NUM> corresponding to the I channel <NUM> and Q channel <NUM> of <FIG>. It will be appreciated that the encoding of the data signal into I and Q channels could be performed by any of a number of components other than the DAC <NUM>, at either a digital or analog stage.

The I channel <NUM> and Q channel <NUM> are used as inputs to the EOM <NUM> to modulate the optical input <NUM> to generate the optical signal <NUM>, as described above with reference to <FIG>. The optical field of the optical signal <NUM> carrying the amplitude modulated pilot tones is described by the equation: <MAT>
wherein fPT1,<NUM> are the pilot-tone frequencies and m is the modulation index. The EOM <NUM> is thus used to generate the optical signal <NUM> based on the modified digital data signal <NUM>. It will be appreciated that other embodiments may be used, such as embodiments using a polarization-multiplexing EOM such as the EOM shown in <FIG>. Such embodiments may use four orthogonal digital data channels to drive the EOM.

It will be appreciated that the data encoded in a given data signal or data channel may be equivalently encoded in the same signal or channel at a different stage (e.g., digital vs. analog, amplified vs. pre-amplified, optical vs. electrical). Statements herein regarding the modulation of a signal or channel by another signal or channel, or the detection of characteristics of a first signal or channel in a second signal or channel, may refer to either direct or indirect modulation or detection. For example, the DSP digital data signal <NUM> may be said to encode the input digital data signal <NUM>, whereas the analog data channels <NUM> or the amplified analog data channels <NUM>, <NUM> may be said to encode the modified digital data signal <NUM>.

Returning to <FIG>, a pilot tone detector <NUM> receives the optical signal <NUM> at a photo detector, shown here as a low-speed photodiode <NUM>. The photodiode <NUM> generates a pilot tone detector analog signal <NUM> based on the optical signal <NUM>, which is provided to an amplifier <NUM> to generate an amplified pilot tone detector analog signal <NUM>. The amplified pilot tone detector analog signal <NUM> is provided to an analog-to-digital converter (ADC) <NUM>, which generates a pilot tone detector digital signal <NUM>.

The pilot tone detector digital signal <NUM> is provided to a pilot tone amplitude ratio detection unit <NUM>, which detects the first pilot tone <NUM> and second pilot tone <NUM> decoded from the optical signal <NUM>, detects a first pilot tone amplitude and a second pilot tone amplitude, and calculates a power ratio between the amplitudes of the two pilot tones <NUM>, <NUM>. The ratio between the two detected pilot-tone amplitudes may be determined by the equation: <MAT>
wherein VI,q are the in-phase and quadrature fields of the optical signal <NUM>, respectively, and HI,q is the corresponding Hilbert transform. Δθ is the quadrature phase deviation from <NUM> degrees (i.e., the quadrature error), Δτ is a delay applied by the digital delay filter <NUM>, and ΔτIQ is the physical delay between the I channel <NUM> and Q channel <NUM>. The cross-correlation between the received signal and its Hilbert transform has a strong dependence on the timing skew between the I channel <NUM> and Q channel <NUM>. The cross-correlation value thus equals zero only if the skew between V and H is canceled, as shown in the transfer function calculated for a 65Gbaud non-return to zero (NRZ) signal in <FIG>.

<FIG> shows a transfer function <NUM> for a 65Gbaud quadrature-phase-shifted-keying (QPSK) optical signal <NUM>. The normalized CV,H cross-correlation value between the signal and its Hilbert transform is mapped on the Y axis <NUM>. The IQ time skew of the signal is mapped on the X axis <NUM>. The curve <NUM> showing the relationship between these two properties has a zero point <NUM> at which the cross-correlation value is zero only if the IQ time skew is zero.

Thus, the device <NUM> applies an amplitude modulated pilot-tone to the data channels, and the power ratios between the in-phase and quadrature data channels can be accurately measured by the pilot-tone detector <NUM> to determine IQ time skew. The pilot tone power can be extracted from the power spectrum density obtained by performing a Fourier transform (e.g., a fast Fourier transform) of the pilot tone detector digital signal <NUM> with a digital signal processor (DSP) of the pilot tone power ratio detection unit <NUM>. In some embodiments, the pilot tone power in each channel is monitored by the pilot tone detector <NUM> tapping a small portion of power to the pilot tone detector <NUM>. The pilot tone detector <NUM> may measure the power of each pilot tone at a single frequency to cancel any measurement error caused by a receiver frequency response ripple.

In some embodiments, the pilot tone detector <NUM> may be relatively low-cost. The pilot tone detector <NUM> may consist of a low-speed photodetector such as low-speed photodiode <NUM>, an analog-to-digital convertor <NUM>, and a pilot tone detector digital signal processor (DSP) implementing the pilot tone power ratio detection unit <NUM>.

IQ time skew calibration is carried out by the device <NUM> based on feedback from the pilot tone power ratio detection unit <NUM>. The digital delay filter <NUM> is used to apply an IQ time skew bias to the DSP digital data signal <NUM> (shown in <FIG> as a FIR filter applying time skew bias to the Q' component of the modified digital data signal <NUM>), thereby biasing the IQ time skew of the data signal to potentially correct for IQ time skew introduced by various operations in the data path of the I channel <NUM> and/or Q channel <NUM>. The amount of IQ time skew bias applied to the data signal by the digital delay filter <NUM> is determined by control information (e.g., control signals) received from a control unit <NUM>. In some embodiments, the control unit <NUM> may be a processor or controller configured to perform calculations, make decisions, and store and retrieve data in registers or a memory in order to carry out the various methods and operations described herein.

The control unit <NUM> is configured to determine an IQ time skew between the in-phase component and quadrature component of the optical signal <NUM> based on the first pilot tone power and second pilot tone power as determined by the pilot tone power ratio detection unit <NUM>. After the pilot tone power ratio detection unit <NUM> calculates the power ratio between the two detected pilot tones, power ratio information <NUM> is passed from the pilot tone power ratio detection unit <NUM> to the control unit <NUM>. The control unit <NUM> uses the power ratio information <NUM> to determine the IQ skew of the optical signal <NUM>, as described in greater detail below. The control unit <NUM> may then send control information to the digital delay filter <NUM> to apply an amount of IQ time skew bias to the DSP digital data signal <NUM> based on the calculated IQ time skew of the optical signal <NUM>.

In the context of IQ time skew calibration or compensation, the input digital data signal <NUM> may be considered a non-biased digital data signal, and the digital delay filter <NUM> is configured to receive the non-biased digital data signal (i.e. input digital data signal <NUM>), set an IQ time skew bias based on control information from the control unit <NUM>, and generate the digital data signal by applying the IQ time skew bias to the non-biased digital data signal. The control unit <NUM> is configured to calculate the IQ time skew as described above, generate the control information based on the calculated IQ time skew, and provide the control information to the digital delay filter <NUM>.

In some embodiments, an IQ detection and calibration step is carried out during a power-up phase of the device <NUM>, or otherwise outside of a service mode of the device <NUM> in which the device <NUM> is actively transmitting data to a receiver. The IQ detection and calibration step may involve identifying the IQ time skew of the optical signal <NUM> and applying compensatory IQ time skew bias using the digital delay filter <NUM>. In some embodiments, IQ skew detection and/or calibration could be performed during the service mode, i.e. in real time during data transmission, provided a small amount of offset is applied to the phase bias point of the EOM <NUM>.

In order to accurately determine the IQ time skew of the optical signal <NUM> and potentially compensate therefor, the device may include additional components to carry out IQ skew detection and calibration. These components are described below as components or operations of the transmitter DSP <NUM>, but in some embodiments they may be included in the device <NUM> somewhere other than the transmitter DSP <NUM>.

<FIG> shows a block diagram of an alternative embodiment of a transmitter DSP <NUM>, wherein the pilot tone generator <NUM>, control unit <NUM>, and digital delay filter <NUM> are considered to be components of the DSP <NUM>. The transmitter DSP <NUM> includes the digital delay filter <NUM>, control unit <NUM>, and digital delay filter <NUM> in addition to a conventional set of DSP components that perform other DSP operations <NUM>. It will be appreciated that the other DSP operations <NUM> may correspond in some embodiments to the operations carried out by the transmitter DSP <NUM> of <FIG>. The input digital data signal <NUM> is received by the components performing the other DSP operations <NUM>, which are performed prior to the pilot tone generation and digital delay to correct for IQ time skew. <FIG> is a plot of simulation results <NUM> for detecting the pilot tone amplitude ratio as a function of the IQ time skew of the optical signal at different phase bias points. The IQ time skew <NUM> (in picosends) is plotted on the X axis, and the pilot tone amplitude ratio R <NUM> (in decibels) as detected by the pilot tone detector <NUM> is plotted on the Y axis. The pilot tone amplitude ratio <NUM> for five different phase bias points (set by, e.g., the transmitter DSP <NUM>) is plotted against the IQ time skew <NUM> as the IQ time skew is varied by sweeping the IQ time skew bias value of the digital delay filter <NUM> across a range of values from -<NUM> picoseconds to <NUM> picoseconds. The five different phase bias points shown in the plot <NUM> are: ΔΦ = <NUM> (<NUM>), ΔΦ = π/<NUM> (<NUM>), ΔΦ = π/<NUM> (<NUM>), ΔΦ = π/<NUM> (<NUM>), and ΔΦ = π/<NUM> (<NUM>).

As expected from equation (<NUM>) above, the pilot tone power ratio <NUM> has no dependence on the IQ time skew <NUM> when the phase is biased at π/<NUM> (<NUM>). By applying a phase bias of ΔΦ = <NUM> (<NUM>), a strong dependence of R <NUM> on the IQ skew <NUM> detuning can be observed. For all plotted phase bias points <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, the pilot tone power imbalance (i.e. power ratio between the two pilot tones is not equal to <NUM>:<NUM> or <NUM> dB) is canceled only when the IQ skew <NUM> is zero, at balance point <NUM>. As shown in the simulation results <NUM>, for a phase bias at ΔΦ = π/<NUM> (<NUM>), the skew measurement sensitivity of the pilot tone detector <NUM> may be better than <NUM> picoseconds, or <NUM> dB accuracy for pilot tone power ratio <NUM> measurement. In some embodiments, the measurement sensitivity increases with the signal baud rate.

An example method will now be described for performing an IQ skew detection and calibration step using the device <NUM> or another coherent transmitter having pilot tone generation and detection capabilities.

<FIG> is a flowchart showing an example method <NUM> for performing IQ skew detection and calibration in a coherent transmitter in accordance with device <NUM> described above. The method <NUM> operates by sweeping the IQ time-skew bias of the digital delay filter <NUM> across a range of values and recording the power ratios of the detected pilot tones at each bias value, then plotting a linear interpolation of the ratios to determine the IQ skew bias point for calibration of IQ time skew. The control unit <NUM> provides skew sweep control information to the digital delay filter <NUM> such that the digital delay filter <NUM> is instructed to set the IQ time-skew bias equal to each of a plurality of IQ time-skew bias values. The skew sweep control information may comprise instructions or control signals causing the digital delay filter <NUM> to set the IQ time skew bias value to a predetermined number of values between a minimum value and a maximum value defining an IQ time skew bias range. In some embodiments, the minimum value and a maximum value of the IQ time skew bias range may be -<NUM> picoseconds and <NUM> picoseconds, respectively, as in the plot of <FIG>. In some embodiments, the predetermined number of IQ time skew bias values is <NUM>, as in the plot of <FIG>. It will be appreciated that different embodiments may vary any of these values; a wider IQ time skew bias range and a larger predetermined number of IQ time skew bias values may yield greater accuracy, range and resolution in calculating the IQ time skew of the optical signal <NUM>. The sweeping range and predetermined number of values can also be represented as a sweeping range and step size.

First, at <NUM>, a first such IQ time skew bias value of the digital delay filter <NUM> is set in response to skew sweep control information provided by the control unit <NUM>. In some embodiments, as phase bias offset of the EOM <NUM> may also be set at this step <NUM>.

For each of the plurality of IQ time-skew bias values, steps <NUM> through <NUM> are performed.

At step <NUM>, the pilot tone generator <NUM> is used to combine the first pilot tone <NUM> with a first frequency band of the digital data signal (e.g., time-domain first frequency band signal <NUM>) and combine the second pilot tone <NUM> with a second frequency band of the digital data signal (e.g., time-domain second frequency band signal <NUM>), thereby generating a modified digital data signal (e.g., modified digital data signal <NUM>). In device <NUM>, the modified digital data signal <NUM> is generated by adding together the modified time-domain first frequency band signal <NUM> and modified time-domain second frequency band signal <NUM>. It will be appreciated that the pilot tone generation and combination with the data signal takes place continuously and therefore simultaneously with other steps of the method <NUM> in some embodiments.

At <NUM>, the optical signal <NUM> is generated by the electro-optical modulator <NUM> based on the modified digital data signal <NUM>. In device <NUM>, the EOM <NUM> is directly modulated by the I channel <NUM> and Q channel <NUM>, which are amplified versions of the analog data channels <NUM>, which are in turn analog encodings of the modified digital data signal <NUM>.

At <NUM>, the detector digital signal <NUM> is generated based on the optical signal <NUM>. In device <NUM>, the detector digital signal <NUM> is a digital encoding of the amplified pilot tone detector analog signal <NUM>, which is an amplified version of the pilot tone detector analog signal <NUM>, which is an electrical encoding of the optical signal <NUM> received by the photodiode <NUM>.

At <NUM>, the pilot tone power ratio detection unit <NUM> is used to detect a first pilot tone power and a second pilot tone power based on the detector digital signal <NUM>, as described in detail above.

At <NUM>, the pilot tone power ratio detection unit <NUM> calculates a power ratio between the first pilot tone power and second pilot tone power for the respective IQ time-skew bias value set by the digital delay filter <NUM>. In device <NUM>, the pilot tone power ratio detection unit <NUM> calculates an power ratio between the first pilot tone power and second pilot tone power, thereby generating power ratio information <NUM>, which is passed to the control unit <NUM>.

Steps <NUM> through <NUM> are repeated for each IQ time skew bias value set by the digital delay filter <NUM> in its sweep. If a further bias value remains within the range, the IQ time skew bias value of the digital delay filter <NUM> is set to the next value in the range at step <NUM>, and the method <NUM> returns to step <NUM>.

Once the IQ time skew bias value sweep has been completed, the control unit <NUM> uses the power ratio information <NUM> to determine an IQ time skew between an in-phase component and a quadrature component of the optical signal <NUM>, as described above. The IQ time skew is calculated based on the plurality of IQ time-skew bias values and the respective plurality of power ratios received from the pilot tone power ratio detection unit <NUM> as power ratio information <NUM>. In device <NUM>, the control unit <NUM> may store the plurality of power ratio information <NUM> transmissions, e.g., in registers or a memory for calculation of the IQ time skew once the sweep is complete. The IQ time skew calculation is performed by the control unit <NUM> in steps <NUM> through <NUM> described below.

At <NUM>, the control unit <NUM> calculates a linear interpolation of the plurality of IQ time-skew bias values and the respective plurality of power ratios. The linear interpolation may be calculated or represented in any manner or format that preserves the information defining the linear interpolation, i.e. slope and the position of at least one point on the line of the linear interpolation. Examples of linear interpolations of plotted points on a graph of power ratio R against IQ time skew are shown as the linear interpolations <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> in <FIG>. Whereas the example liner interpolations in <FIG> are able to be plotted as a line passing through each data point, in some embodiments inaccuracies in measurement may require the linear interpolation to be plotted using, e.g., a least-mean-squares linear regression to fit the data points. It will be appreciated that a linear interpolation or approximation or estimation thereof may be generated using any of a number of mathematical techniques.

At <NUM>, the control unit <NUM> identifies a balance point on the linear interpolation corresponding to a power ratio value of <NUM>:<NUM> (i.e. <NUM> dB). In the example linear interpolations of <FIG>, the balance point is balance point <NUM>.

At <NUM>, the control unit calculates the IQ time skew as equal to an IQ time skew bias value corresponding to the balance point <NUM>, i.e., the IQ time skew is calculated to be the X value of a point on the linear interpolation corresponding to a hypothetical IQ time skew bias value that corresponds to a power ratio of <NUM>:<NUM> (i.e. <NUM> dB). The x-axis location of the balance point <NUM> provides this value for the IQ time skew of the optical signal <NUM>.

At <NUM>, the control unit <NUM> provides IQ skew calibration control information to the digital delay filter <NUM> such that the digital delay filter <NUM> sets the IQ skew bias to compensate for the calculated IQ time skew. The control information <NUM> at this step <NUM> comprises IQ skew calibration control information, and this step <NUM> may be referred to as the step of calibrating the device <NUM> to compensate for IQ time skew.

In some embodiments the above-described method <NUM> may calibrate a time skew with a precision (or accuracy) of better than <NUM> picosecond using a low-speed pilot detector <NUM> and detector ADC <NUM>.

In some embodiments, the device <NUM> may be further configured to perform phase conjugation detection and/or calibration. Phase conjugation (also referred to herein as IQ conjugation or transmitter IQ conjugation) may be caused by a number of factors, including transmitter hardware layouts, digital to analog channel mappings, and/or bias operating points for each EOM. Therefore, it may be difficult to detect conjugation using only a single parameter. Transmitter IQ conjugation may cause a number of problems: for example, it may cause the incorrect (i.e. opposite) sign to be used for chromatic dispersion applied in the transmitter DSP, and/or it may cause acquisition failure at the receiver DSP due to IQ flipping. Therefore, the identification and/or correction of transmitter conjugation may present advantages.

<FIG> shows transmitter conjugation causing a frequency band swap for two pilot tones applied in the digital domain. In the non-conjugated optical signal <NUM>, the first pilot tone modulation frequency fPT1 <NUM> modulates the first frequency band of the data signal (shown as upper spectrum band <NUM>), and the second pilot tone modulation frequency fPT2 <NUM> modulates the second frequency band of the data signal (shown as lower spectrum band <NUM>). For an optical signal spectrum, the predetermined frequency (the frequency to which the two frequency bands references to) is the center frequency <NUM> of the light source (also called the carrier frequency). In the conjugated optical signal <NUM>, the upper and lower frequency bands are swapped as compared the unconjugated one; the first pilot tone modulation frequency fPT1 <NUM> instead modulates the lower spectrum band <NUM>, and the second pilot tone modulation frequency fPT2 <NUM> modulates the upper spectrum band <NUM>. As shown in equation (<NUM>) above, the slope of the pilot tone power ratio versus IQ skew tuning is determined at a given sign of the quadrature phase offset. The phase offset may be determined in some embodiments by the voltage offset from the bias point at π/<NUM> and the slope sign of the EOM <NUM> power transfer function: it is at positive or negative slope of the power transfer function. The bias point information may be acquired in some embodiments during a bias control initialization stage of the EOM <NUM>.

<FIG> shows a plot <NUM> of data points captured for a non-conjugated signal <NUM> and a conjugated signal <NUM>. The data points are plotted with IQ time skew <NUM>, applied by the digital delay filter <NUM> as the X axis and the detected pilot tone power ratio R <NUM> as the Y axis, as in <FIG>. It can be observed that the slope of the linear regression of the data points for the non-conjugated signal <NUM> is negative, whereas the slope of the linear regression of the data points for the conjugated signal <NUM> is positive and approximately the inverse of the negative slope of the linear regression of the data points for the non-conjugated signal <NUM>.

<FIG> is a flowchart showing an example method <NUM> for detecting and calibrating conjugation of a coherent transmitter. In some embodiments, the transmitter phase conjugation calibration method <NUM> may be performed at a start-up phase of the transmitter, such as device <NUM>. The steps of the example method <NUM> will be described with reference to device <NUM>.

Method <NUM> may be carried out in some embodiments using the same IQ skew and pilot tone power ratio data gathered and calculated using the steps of IQ skew method <NUM>. The steps of method <NUM> described below assume that method <NUM> is being carried out concurrently or in addition to the steps of method <NUM>. In particular, method <NUM> relies on the data generated and calculated by steps <NUM> through <NUM> of method <NUM>.

At <NUM>, the control unit <NUM> receives the plurality of IQ time-skew bias values and the respective plurality of power ratios from steps <NUM> and <NUM> of method <NUM>, and receives phase bias information from the EOM <NUM>. In some embodiments, the EOM <NUM> may provide the control unit <NUM> with access to and control over a phase bias setting of the EOM <NUM>.

At steps <NUM> through <NUM>, the control unit calculates the phase conjugation status of the optical signal <NUM> (i.e., whether the optical signal <NUM> is conjugated or not) based on the plurality of IQ time-skew bias values, the respective plurality of power ratios, and the phase bias information.

At step <NUM>, the control unit calculates a linear interpolation of the plurality of IQ time-skew bias values and the respective plurality of power ratios. This step <NUM> may be omitted if the corresponding step <NUM> of method <NUM> is performed.

At step <NUM>, the control unit <NUM> identifies a positive or negative sign of a slope of the linear interpolation. This determination may be explicit or implicit: for example, in some embodiments steps <NUM> and <NUM> may be carried out by simply determining the relative R and IQ time skew values of two data points and determining whether they indicate a positive or negative trend line.

At step <NUM>, the control unit <NUM> determines that the optical signal <NUM> is conjugated based on the sign of the slope of the linear interpolation and the phase bias information. For example, in a linear interpolation such as those of <FIG>, a negative slope may indicate a negative conjugation status, whereas a positive slope may indicate a positive conjugation status.

At step <NUM>, the transmitter compensates for the phase conjugation status. This may be accomplished in various embodiments by, e.g., setting a bias point of the EOM <NUM>, or inverting a polarity of the modified digital data signal <NUM>. In some embodiments, the data signal may have its polarity inverted at a different stage. The bias point of the EOM <NUM> may be set by the control unit <NUM>, as described above. Typically, a negative conjugation status may be compensated for by doing nothing, whereas a positive status may be compensated for by setting a bias point of the EOM <NUM> or inverting the polarity of the data signal. In some embodiments, the control unit <NUM> may be further configured to control one or more components of the device <NUM> to invert the polarity of either the I' or Q' digital data channel of modified digital data signal <NUM> provided as input to the DAC.

Although the present disclosure describes methods and processes with steps in a certain order, one or more steps of the methods and processes may be omitted or altered as appropriate. One or more steps may take place in an order other than that in which they are described, as appropriate.

Although the present disclosure is described, at least in part, in terms of methods, a person of ordinary skill in the art will understand that the present disclosure is also directed to the various components for performing at least some of the aspects and features of the described methods, be it by way of hardware components, software or any combination of the two. Accordingly, the technical solution of the present disclosure may be embodied in the form of a software product. A suitable software product may be stored in a pre-recorded storage device or other similar non-volatile or non-transitory computer readable medium, including DVDs, CD-ROMs, USB flash disk, a removable hard disk, or other storage media, for example. The software product includes instructions tangibly stored thereon that enable a processor device (e.g., a personal computer, a server, or a network device) to execute examples of the methods disclosed herein.

The present disclosure may be embodied in other specific forms without departing from the subject matter of the claims. The described example embodiments are to be considered in all respects as being only illustrative and not restrictive. Selected features from one or more of the above-described embodiments may be combined to create alternative embodiments not explicitly described, features suitable for such combinations being understood within the scope of this disclosure.

Claim 1:
A device for compensating IQ time skew in a coherent transmitter, the device comprising:
a digital delay filter configured to: receive a digital data signal comprising in-phase and quadrature, <NUM>, components, and
set (<NUM>) an IQ time skew bias based on skew sweep control information from a control unit, such that the digital delay filter sets the IQ time-skew bias equal to each of a plurality of IQ time-skew bias values;
for each of the plurality of IQ time-skew bias values, the device is configured to: by a pilot tone generator:
combine (<NUM>) a first pilot tone with a first frequency band of the digital data signal, and
combine (<NUM>) a second pilot tone with a second frequency band of the digital data signal,
thereby generating (<NUM>) a modified digital data signal;
by an electro-optic modulator, EOM, generate (<NUM>) an optical signal based on the modified digital data signal;
by a pilot tone detector:
receive the optical signal;
generate (<NUM>) a detector digital signal based on the optical signal; and
detect (<NUM>) a first pilot tone power and a second pilot tone power based on the detector digital signal;
calculate (<NUM>) a power ratio between the first pilot tone power and the second pilot tone power, thereby generating power ratio information, which is passed to the control unit; and
once the IQ time skew bias value sweep has been performed for each of the plurality of IQ time-skew bias values, the control unit is configured to determine an IQ time skew between an in-phase component and a quadrature component of the optical signal based on the plurality of IQ time-skew bias values and the respective plurality of power ratios received from the pilot tone detector as power ratio information.