Patent Description:
The present disclosure relates to analog-to-digital converters (ADCs). More specifically, the present disclosure relates to photonic monobit ADCs, such as photonic monobit differential ADCs.

As communication systems evolve over time, digital data rates tend to increase. As a result, there is an ongoing effort to increase the speed and accuracy of analog-to-digital conversion to support the increase in communication rates.

High bandwidth and high spur-free dynamic range analog-to-digital conversion is a common desire across multiple domains but is difficult to achieve. With the advent of photonics, the analog-to-digital conversion can be improved beyond the electronic conversion by harvesting the photonics bandwidth and balancing the functional partition between electronics and photonics.

<CIT> discloses optical communication apparatus, methods, and systems based on balanced-detection phase-shaped binary transmission (BD-PSBT).

<NPL>, discloses the influence of the differential phase-shift keying decoder free spectral range (FSR) when strong optical filtering is considered for the nonreturn-to-zero and return-to-zero modulation formats and shows that larger FSR can improve performance.

<NPL>, presents a structure to realize photonics-assisted compressive sensing (CS) with optical integration. In the system, a spectrally sparse signal modulates a multiwavelength continuous-wave light and then ismixed with a random sequence in optical domain.

In a first aspect, the present disclosure provides a photonic monobit analog-to-digital converter, comprising: an incoherent optical source configured to generate an optical noise signal, the optical noise signal comprising random signal phases that are uniformly distributed over a range of the optical noise signal; an optical modulator configured to modulate an analog input electrical signal onto an input optical signal to generate an optical modulated signal; a coupler configured to couple the optical modulated signal with the optical noise signal to generate at least one coupled signal; a photodetector configured to generate a phase difference between the optical modulated signal and the optical noise signal using the at least one coupled signal; a limiter configured to output a decision signal based on the phase difference; and a digital signal processing 'DSP' circuit configured to generate a digital signal representative of the analog input electrical signal based on the decision signal.

In a second aspect, the present disclosure provides a method for generating a digital signal representative of an analog input electrical signal, the method comprising: phase-modulating an input optical signal using an analog input electrical signal to generate an optical phase-modulated signal; coupling the optical phase-modulated signal with an optical noise signal to generate a first coupled signal and a second coupled signal, the optical noise signal comprising random signal phases that are uniformly distributed over a range of the optical noise signal; generating a phase difference between the optical phase-modulated signal and the optical noise signal based on the first coupled signal and the second coupled signal; generating a decision signal based on limiting the generated phase difference; and generating a digital signal representative of the analog input electrical signal based on the decision signal.

Corresponding reference characters indicate corresponding parts throughout the several views. Elements in the drawings are not necessarily drawn to scale. The configurations shown in the drawings are merely examples and should not be construed as limiting the scope of the invention in any manner.

The following description and the drawings sufficiently illustrate aspects to enable those skilled in the art to practice them. Other aspects may incorporate structural, logical, electrical, process, and other changes. Portions and features of some aspects may be included in, or substituted for, those of other aspects. Aspects set forth in the claims encompass all available equivalents of those claims.

Techniques disclosed herein can be used to realize a photonic ADC with high spur free dynamic range (SFDR) based on a monobit concept, based on dithering an input signal with uniform noise. More specifically, a photonic monobit ADC can include an optical incoherent noise source to phase dither an optical waveform that is representative of a radio frequency (RF) waveform of interest. By mapping an incoming RF waveform into an optical phase and combining with an optical high bandwidth incoherent noise source, the photonic interferometry can be utilized to dither and detect the phase difference using wideband detectors (e.g., dither the modulated phase with an incoherent source). The phase difference is then detected and compared for signal processing and recovery. In this regard, by using photonic capabilities in monobit ADC technology, lower power consumption and better performance at the higher sampling rates can be achieved.

<FIG> illustrates a conceptual block diagram of a monobit ADC <NUM>, in accordance with some aspects. Electronic monobit ADCs convert an analog signal to its digital representation based on dithering an input signal with uniform noise. Referring to <FIG>, the monobit ADC <NUM> can include a comparator <NUM> and a limiter <NUM>. The comparator <NUM> is configured to receive an analog input signal (S) <NUM> and a uniform noise signal (U) <NUM>. The analog input signal <NUM> can have a signal profile as illustrated in graph <NUM>, and the uniform noise signal <NUM> can have a noise distribution as illustrated in graph <NUM>.

The comparator <NUM> compares the analog input signal <NUM> with the uniform noise signal <NUM> to generate a comparison result <NUM>. The limiter <NUM> is configured to receive a clock signal <NUM> and the comparison result <NUM>, and hard limit the comparison result to +<NUM> (if the signal is greater than the noise) or -<NUM> (if the noise is greater than the signal). The limiter <NUM> outputs a decision signal (D) <NUM>, with the expected value (or average) of the limiter output signal D <NUM> being a digital signal representation <NUM> of the analog input signal <NUM>, after processing with a filter (e.g., in a digital signal processing block or a Fourier frequency transform (FFT) block such as FFT block <NUM>).

One of the main limitations of analog-to-digital conversion at higher rates is the introduced spurs of undesired tones resulting from realization imperfections. A significant advantage of the monobit ADC architecture is the high SFDR resulting from the dithering (or applying uniform noise to) the input signal.

<FIG> is a graphical representation <NUM> illustrating a frequency Fourier transform (FFT) magnitude of a tone based on the monobit ADC <NUM> of <FIG>. <FIG> is illustrative of the spur free range of monobit conversion. More specifically, <FIG> illustrates the frequency and power profile of a tone at <NUM> that is sampled at <NUM> with an acquisition time of <NUM>. The spurs appear relatively at the same power level as illustrated by the FFT of the limiter output.

Some techniques for implementing electronic monobit conversion can rely on generating digital pseudo-random noise, which can consume a large portion of the ASIC power and can be a limiting factor is the sampling rates increase. One of the advantages of photonics is its bandwidth and relative efficiency. In this regard, techniques disclosed herein can be used to realize a photonic monobit ADC, based on a modulator that modulates the electrical signal onto an optical carrier to be compared with an incoherent wide bandwidth noise source, as discussed herein below.

<FIG> is a block diagram of a photonic monobit ADC <NUM>, in accordance with some aspects. Referring to <FIG>, the photonic monobit ADC <NUM> can include a phase modulator <NUM>, an incoherent optical source <NUM>, a first coupler <NUM>, a second coupler <NUM>, a filter <NUM>, a delay circuit <NUM>, a balanced photodetector (BPD) <NUM>, a limiter <NUM>, and a DSP <NUM>.

The optical source <NUM> can be an incoherent signal source generating an optical or photonic noise signal <NUM>, where the signal phases are random and uniformly distributed over the range of the signal, from sample to sample, with low correlation existing between any two samples. <FIG> is a graphical spectral representation <NUM> of a noise signal (e.g., <NUM>) from the incoherent optical source <NUM> used in connection with the photonic monobit ADC <NUM>. In some aspects, the optical source <NUM> can be an incoherent white light emitting diode (LED) source with a high bandwidth, such as a bandwidth exceeding <NUM> THz, an amplified spontaneous emissions (ASE) light source, or another type of optical noise source. As illustrated in <FIG>, the noise signal <NUM> can be filtered so that a limited slice (e.g., <NUM>) can be selected for dithering with another optical signal within the photonic monobit ADC <NUM>.

The filter <NUM> can be configured to filter the optical noise signal <NUM> generated by the incoherent optical source <NUM>, to obtain an optical filtered noise signal <NUM>. In some aspects, the filter <NUM> can be a <NUM> filter that can be configured to generate a <NUM> optical noise signal slice with a random phase samples. In some aspects, the filtered noise signal <NUM> can be centered at <NUM> wavelength as shown in <FIG>.

The phase modulator <NUM> may comprise suitable circuitry, logic, interfaces and/or code and is configured to receive an input optical signal <NUM> and an analog input electrical signal <NUM>, to generate an optical modulated signal <NUM>. The input optical signal <NUM> can be a laser signal generated by laser <NUM>. In some aspects, the laser <NUM> can be a <NUM> laser or another wavelength laser. The phase modulator <NUM> is configured to phase modulate the analog input signal <NUM> onto the optical signal <NUM> to generate the optical modulated signal <NUM> (i.e., the phase of the optical modulated signal <NUM> corresponds to the signal amplitude of the analog input signal <NUM>).

The first coupler <NUM> is configured to couple the optical modulated signal <NUM> and the filtered noise signal <NUM> from the incoherent optical source <NUM>, to generate first and second optical coupled signals <NUM> and <NUM>. In some aspects, the lower arm coupled signal <NUM> can be offset (e.g., by <NUM>°) from the coupled signal <NUM> in the upper arm.

The delay circuit <NUM> is configured to delay the lower arm coupled signal <NUM> and generate an optical delayed coupled signal <NUM>. In some aspects, the delay circuit <NUM> can be a programmable delay circuit. In some aspects, the delay circuit <NUM> can provide a <NUM>-bit time delay in order to assist the BPD <NUM> to obtain phase difference.

The second coupler <NUM> is configured to couple the upper arm coupled signal <NUM> and the lower arm delayed coupled signal <NUM> to generate third and fourth optical coupled signals <NUM> and <NUM>. By using the second coupler <NUM>, DC-coupling between the coupled signals <NUM> and <NUM> can be removed.

The BPD <NUM> may comprise suitable circuitry, logic, interfaces and/or code and is configured to generate an electrical output signal <NUM> indicative of a phase difference between the optical modulated signal <NUM> and the filtered noise signal <NUM>. The limiter <NUM> is configured to receive the output signal <NUM> and an electrical clock signal <NUM>, and generate a decision signal <NUM> corresponding to the analog input signal <NUM> based on the output signal <NUM> from the BPD <NUM>. The decision signal <NUM> can be further processed (e.g., by filtering, signal reconstruction, and/or other signal processing) performed by the DSP module <NUM>, to generate an output digital signal <NUM>.

In some aspects, techniques disclosed herein can be used to implement a Σ-Δ ADC as the difference between a current and previous signal sample can be computed based on the delay provided by the delay circuit <NUM>.

In some aspects, signal phase modulation can introduce harmonics due to the phase modulation which can be expanded using the Jacobi-Anger Bessel expansion.

<FIG> is a block diagram of a photonic monobit ADC <NUM> using a Mach Zehnder modulator, in accordance with some aspects. Referring to <FIG>, the photonic monobit ADC <NUM> can include an MZM <NUM>, and incoherent optical source <NUM>, first and second couplers <NUM> and <NUM>, an optical signal filter <NUM>, a delay circuit <NUM>, a BPD <NUM>, a limiter <NUM>, and a DSP <NUM>. The MZM <NUM> is configured to receive the input analog signal <NUM> and an optical signal generated by the laser light source <NUM>.

The functionalities of the couplers <NUM> and <NUM>, the optical signal filter <NUM>, the delay circuit <NUM>, the BPD <NUM>, the limiter <NUM>, and the DSP <NUM> can be similar to the corresponding functionalities of the couplers <NUM> and <NUM>, the optical signal filter <NUM>, the delay circuit <NUM>, the BPD <NUM>, the limiter <NUM>, and the DSP <NUM> of <FIG>. The decision signal <NUM>, which corresponds to the analog input signal <NUM>, can be further processed by the DSP module <NUM> (e.g., by filtering and signal reconstruction) to generate the digital output signal <NUM>.

<FIG> illustrates generally a flowchart of example functionalities which can be performed in connection with analog-to-digital conversion, in accordance with some aspects. Referring to <FIG>, the method <NUM> includes operations <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. By way of example and not limitation, the method <NUM> is described as being performed by one or more of the components of the photonic monobit ADC <NUM> of <FIG>. At operation <NUM>, an analog input electrical signal is phase-modulated onto an input optical signal to generate an optical phase-modulated signal. For example, the input analog signal <NUM> is phase-modulated onto the input optical signal <NUM> by the modulator <NUM> to generate the optical phase-modulated signal <NUM>.

At operation <NUM>, the optical phase-modulated signal is coupled with an optical noise signal to generate a first coupled signal and a second coupled signal. For example, coupler <NUM> can couple the optical phase-modulated signal <NUM> and the filtered optical noise signal <NUM> to generate a first coupled signal <NUM> and a second coupled signal <NUM>.

At operation <NUM>, a phase difference is generated between the optical phase-modulated signal and the optical noise signal based on the first coupled signal and the second coupled signal. For example, the BPD <NUM> can generate the output signal <NUM> indicative of the phase difference between the optical phase-modulated signal <NUM> and the filtered optical noise signal <NUM>. Prior to communicating the coupled signals generated by first coupler <NUM>, the lower arm of the coupled signal outputs, i.e., the second coupled signal <NUM>, can be delayed by delay circuit <NUM> and additional coupling can be performed by a second coupler <NUM> to remove DC bias.

At operation <NUM>, a decision signal representative of the analog input electrical signal is generated based on the determined phase difference. For example, the limiter <NUM> generates the decision signal <NUM> based on the output signal <NUM> indicative of the phase difference between the optical phase-modulated signal <NUM> and the filtered optical noise signal <NUM>. At operation <NUM>, a digital signal representative of the analog input electrical signal is generated based on the decision signal. For example, the DSP <NUM> generates the output digital signal <NUM> representative of the input analog signal <NUM> based on the decision signal <NUM> generated by the limiter <NUM>.

In some aspects, due to the fact that the phase modulator in the incoherent optical source output signals are photonic, signal parallelization can be used within a photonic monobit ADC by splitting the optical signals, while maintaining the uniformity of the signal and allowing use of multiple signal samples. In this regard, signal averages can be obtained quicker and more accurately within a photonic monobit ADC. An example photonic monobit ADC with photonic signal splitting is illustrated in <FIG>.

<FIG> is a block diagram of a photonic monobit ADC <NUM> using signal splitting, in accordance with some aspects. Referring to <FIG>, the photonic monobit ADC <NUM> can include a phase modulator <NUM>, a laser light source <NUM>, an incoherent optical source <NUM>, a plurality of BPDs (e.g., 708A, 708B, 708C, and 708D), a plurality of limiters (e.g., 710A, 710B, 710C, and 710D), DSP circuits <NUM> and <NUM>, photonic signal splitters <NUM> and <NUM>, and a plurality of delay circuits (e.g., 740A, 740B, 740C, and 740D). The functionality of the circuits illustrated in <FIG> in connection with photonic monobit ADC <NUM> can be similar to corresponding functionalities of the same circuits illustrated in connection with the photonic monobit ADC <NUM> in <FIG>.

In operation, an input analog signal <NUM> and an optical signal <NUM> generated by laser light source <NUM> can be communicated to the phase modulator <NUM>. The phase modulator <NUM> can generate a modulated optical signal <NUM> which can be split by splitters <NUM> into modulated optical signals 726A, 726B, 726C, and 726D for processing by the corresponding BPDs 708A, 708B, 708C, and 708D. The incoherent optical source <NUM> can generate an optical noise signal <NUM> which can be split by splitters <NUM> into optical noise signals 730A, 730B, 730C, and 730D that are delayed by delay circuits 740A, 740B, 740C, and 740D prior to processing by the BPDs 708A, 708B, 708C, and 708D. The limiters 710A, 710B, 710C, and 710D can use corresponding clock signals 732A, 732B, 732C, and 732D together with the detected phase signal output from the BPDs 708A-708D, to generate digital signals 734A, 734B, 734C, and 734D corresponding to the input analog signal <NUM>. The digital signals 734A, 734B, 734C, and 734D can be further processed by the DSP circuits <NUM> and <NUM>.

Even though <FIG> illustrates a photonic monobit ADC <NUM> that includes splitters <NUM> and <NUM> splitting a modulated optical signal into four separate modulated optical signals for processing by four separate BPD/limiter processing chains, the disclosure is not limited in this regard and the modulated optical signals <NUM> and <NUM> can be split into a different number of signals (e.g., a multiple of <NUM>) for processing by a corresponding number of BPD/limiter/DSP processing chains.

In some aspects, the photonic monobit ADC <NUM> can implement Σ-Δ processing functionalities. In this regard, the DSP circuits <NUM> and <NUM> can generate feedback <NUM> which can be combined with the input analog signal <NUM> prior to communication to the phase modulator <NUM> in connection with the Σ-Δ processing functionalities.

Although an aspect has been described with reference to specific example aspects, it will be evident that various modifications and changes may be made to these aspects without departing from the broader scope of the present disclosure. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. The accompanying drawings that form a part hereof show, by way of illustration, and not of limitation, specific aspects in which the subject matter may be practiced. The aspects illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other aspects may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various aspects is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled.

Such aspects of the inventive subject matter may be referred to herein, individually or collectively, merely for convenience and without intending to voluntarily limit the scope of this application to any single aspect or inventive concept if more than one is in fact disclosed.

Claim 1:
A photonic monobit analog-to-digital converter (<NUM>, <NUM>), comprising:
an incoherent optical source (<NUM>, <NUM>) configured to generate an optical noise signal (<NUM>), the optical noise signal comprising random signal phases that are uniformly distributed over a range of the optical noise signal;
an optical modulator (<NUM>, <NUM>) configured to modulate an analog input electrical signal (<NUM>) onto an input optical signal (<NUM>) to generate an optical modulated signal (<NUM>);
a coupler (<NUM>, <NUM>) configured to couple the optical modulated signal with the optical noise signal to generate at least one coupled signal (<NUM>, <NUM>);
a photodetector (<NUM>, <NUM>) configured to generate a phase difference (<NUM>) between the optical modulated signal and the optical noise signal using the at least one coupled signal;
a limiter (<NUM>, <NUM>) configured to output a decision signal (<NUM>, <NUM>) based on the phase difference; and
a digital signal processing, DSP, circuit (<NUM>, <NUM>) configured to generate a digital signal (<NUM>, <NUM>) representative of the analog input electrical signal based on the decision signal.