Photonic Analog-to-digital Converter Research Articles

Real-time jitter correction in a photonic analog-to-digital converter.

A real-time jitter meter is used to measure and digitally sample the pulse-to-pulse timing error in a laser pulse train. The jitter meter is self-referenced using a single-pulse delay line interferometer and measures timing jitter using optical heterodyne detection between two frequency channels of the pulse train. Jitter sensitivity down to 3×10-10fs2/Hz at 500 MHz has been demonstrated with a pulse-to-pulse noise floor of 1.6 fs. As a proof of principle, the digital correction of the output of a high-frequency photonic analog-to-digital converter (PADC) is demonstrated with an emulated jitter signal. Up to 23 dB of jitter correction, down to the noise floor of the PADC, is accomplished with radio-frequency modulation up to 40 GHz.

Ultra-wideband signal acquisition by use of channel-interleaved photonic analog-to-digital converter under the assistance of dilated fully convolutional network

We demonstrate a photonic architecture to enable the separation of ultra-wideband signals. The architecture consists of a channel-interleaved photonic analog-to-digital converter (PADC) and a dilated fully convolutional network (DFCN). The aim of the PADC is to perform ultra-wideband signal acquisition, which introduces the mixing of signals between different frequency bands. To alleviate the interference among wideband signals, the DFCN is applied to reconstruct the waveform of the target signal from the ultra-wideband mixed signals in the time domain. The channel-interleaved PADC provides a wide spectrum reception capability. Relying on the DFCN reconstruction algorithm, the ultra-wideband signals, which are originally mixed up, are effectively separated. Additionally, experimental results show that the DFCN reconstruction algorithm improves the average bit error rate by nearly three orders of magnitude compared with that without the algorithm.

Stability optimization of channel-interleaved photonic analog-to-digital converter by extracting of dual-output photonic demultiplexing

We investigate a channel-interleaved photonic analog-to-digital conversion (PADC) system’s ability to work stably over a long duration with an optimal driving voltage. The influence of optimum bias point drift of a Mach–Zehnder modulator (MZM)-based photonic switch on this system was analyzed theoretically and experimentally. The feasibility of extracting feedback signals from the PADC system was derived. A high-stability channel-interleaved PADC was constructed by extracting a feedback signal from a parallel demultiplexing module to control the MZM-based photonic switch’s driving voltage. Consequently, the amplitude mismatch between the channels was limited to within 0.3 dB over 12 hours of operation.

An Interleaved Broadband Photonic ADC Immune to Channel Mismatches Capable for High-Speed Radar Imaging

A dual channel interleaved broadband subsampling photonic analog-to-digital converter (ADC) immune to channel mismatches capable for high-speed radar imaging is presented. Through modeling as a combination of photonic subsampling front-end and interleaved Nyquist sampling electronic ADC back-end, the mechanism of photonic interleaved subsampling of broadband signal and the appearance of spurious components caused by channel mismatches are explained. The effect of these spurious components on radar detecting is revealed. Through fractional Fourier domain filtering, the spurious components are eliminated, making the SNR of received X-band chirp signal (8–11.9 GHz) improved from 13.26 to 23.88 dB. Thanks to its downsampling and interleaved architecture, the demands of bandwidth, sampling rate, and storage depth for post electronics are all lowered, making it capable for high-speed and mass data receiving. In the experiment, inverse synthetic aperture radar imaging is performed characterized with range and cross range resolution of 4.9 and 7.9 cm. The false target due to the spurious component is distinguished through band-pass filtering in matched fractional Fourier domain.

Deep-learning-powered photonic analog-to-digital conversion

Analog-to-digital converters (ADCs) must be high speed, broadband, and accurate for the development of modern information systems, such as radar, imaging, and communications systems; photonic technologies are regarded as promising technologies for realizing these advanced requirements. Here, we present a deep-learning-powered photonic ADC architecture that simultaneously exploits the advantages of electronics and photonics and overcomes the bottlenecks of the two technologies, thereby overcoming the ADC tradeoff among speed, bandwidth, and accuracy. Via supervised training, the adopted deep neural networks learn the patterns of photonic system defects and recover the distorted data, thereby maintaining the high quality of the electronic quantized data succinctly and adaptively. The numerical and experimental results demonstrate that the proposed architecture outperforms state-of-the-art ADCs with developable high throughput; hence, deep learning performs well in photonic ADC systems. We anticipate that the proposed architecture will inspire future high-performance photonic ADC design and provide opportunities for substantial performance enhancement for the next-generation information systems.

Investigation of electronic aperture jitter effect in channel-interleaved photonic analog-to-digital converter.

The electronic aperture jitter effect originating from the electronic digitization is investigated in the channel-interleaved photonic analog-to-digital converter (PADC) system. The influence of the electronic aperture jitter to the effective number of bits (ENOB) of the PADC system is extracted and evaluated. According to the theoretical analysis, the electronic aperture jitter can be significantly suppressed by the channel-interleaving scheme. In the experiment, the effect of electronic aperture jitter is measured under different optical-electronic conversion (OEC) bandwidths and channel numbers. It is eventually found that the condition of the OEC bandwidth equaling to the Nyquist frequency of one single channel is critical to optimize the electronic aperture jitter and ENOB.

Impact of optical–electrical conversion responsivity in sub-sampled photonic analog-to-digital converter

This Letter investigates the impact of the photodiode (PD) saturation in a sub-sampled photonic analog-to-digital converter (PADC) with two individual pulse lasers. It is essentially proved that when the optical power to the saturated PD increases, the optical–electrical conversion (OEC) responsivity and digitized output power of the PADC decrease. If femtosecond pulses are employed for the PADC sampling clock, the time-stretching process in a dispersive medium is necessary to decrease the impact of the PD saturation. In contrast, when the sampling clock with picosecond pulses is utilized, the PD saturation is more tolerable, and thus, the OEC responsivity can be improved by an increase of the optical power to the PD no matter if the time-stretching process is employed.

Performance Evaluation of Wavelength Division Multiplexing Photonic Analogue-to-Digital Converters for High-Resolution Radar Systems

The performance of the wavelength division multiplexing (WDM) photonic analogue-to-digital converter (ADC) used for digitization of high-resolution radar systems is evaluated numerically by using the peak signal-to-noise ratio (SNR) metric. Two different WDM photonic ADC architectures are considered for the digitization of radar signals with 5 GHz of bandwidth (spatial resolution of 3 cm), in order to provide a comprehensive study of the compromises present when deploying radar signals with high-resolution: 1) a four-channel architecture with each channel employing an ADC with 5 GSamples/s, and 2) an eight-channel architecture with each channel employing an ADC with 2.5 GSamples/s. For peak powers of the pulsed source between 10 and 20 dBm and a distance between the radar antenna and the sensing object of 2.4 meters, peak SNR levels between 29 and 39 dB are achieved with the eight-channel architecture, which shows higher peak SNR levels when compared with the four-channel architecture. For the eight-channel architecture and for the same peak powers of the pulsed source, peak SNR levels between 11 and 16 dB are obtained when the distance increases to 13.5 meters. With this evaluation using the peak SNR, it is possible to assess the performance limits when choosing a specific radar range, while keeping the same resolution.

Pipelined photonic analog-to-digital converter

Electronic analog to digital converters (EADCs) face serious challenges when the root mean square timing jitter of the sampling pulse is less than a femtosecond. This restriction limits the maximum allowable sampling frequency of an EADC. In photonic analog-to-digital conversion (PADC), using a mode locked laser as a sampling source limits the sampling frequency timing jitter only at sub-femtosecond levels. The current architectures for PADC use photonic techniques either for sampling or for quantization. Consequently, current PADC architectures are not suitable for higher frequency applications because of the limitations of their electronic components. In this paper, the feasibility of implementing concept architecture for a fully photonic pipelined ADC is analyzed and evaluated to provide a design for an 8-bit pipelined PADC, the performance of which is investigated through modeling and simulation. The 8-bit pipelined PADC’s effective number of bits is shown to be 4.34 bits at 200 gigasample per second.

Influence of the sampling clock pulse shape mismatch on channel-interleaved photonic analog-to-digital conversion.

We investigate the pulse shape mismatch of the sampling clock in channel-interleaved photonic analog-to-digital converters (PADCs) and its influence on the frequency response of the PADC system. Two schemes to generate sampling clock pulses are experimentally implemented to study the influence of the pulse shape mismatch. The scheme with the well-managed pulse shape in all channels is successful in eliminating the influence. Subsequently, a flat 35GHz frequency response of a four-channel 40GSa/s PADC is demonstrated, and the effective number of bits reaches 7.7bits at 11GHz and 7bits at 31GHz.

An improved photonic analog-to-digital conversion scheme using Mach–Zehnder modulators with identical half-wave voltages

An improved photonic analog-to-digital conversion (ADC) scheme using Mach–Zehnder modulators (MZMs) is proposed and demonstrated. In the approach, all the MZMs have the same electrode length, which eliminates the need of MZMs with very low half-wave voltages as in the scheme using MZM array with geometrically-scaled half-wave voltages. An electric circuit is employed to realize linear combinations of the detected signals (including subtraction and addition). By properly biasing the MZMs and setting the thresholding voltages in the comparators, a digital Gray-code output is obtained. The proposed ADC scheme can effectively improve the bit resolution compared with the previous photonic ADCs based on the phase-shifting technique. Theoretical analysis and proof-of-concept experiments are presented to verify the feasibility of the proposed approach.

Photonic analog-to-digital conversion using a red frequency chirp in a semiconductor optical amplifier.

A photonic analog-to-digital conversion (PADC) based on intensity-to-frequency conversion using a frequency chirp in a semiconductor optical amplifier (SOA) is proposed. The presented PADC has a simple scheme whereby optical quantization is achieved using a single SOA with multiple rectangular bandpass filters placed in parallel. In this Letter, we successfully achieve 10 GSamples/s, eight-level optical quantization using a quantum-dot SOA. The PADC also has much lower input signal pulse power requirements for optical quantization, compared with conventional PADCs.

Simultaneous Microwave Photonic Analog-to-Digital Conversion and Digital Filtering

We proposed a scheme which can simultaneously realize photonic analog-to-digital conversion (PADC) and digital photonic filtering for microwave signals. A multi-tap optical pulse shaper is adopted in a photonic sampling and electrical quantizing PADC to change its equivalent channel response, which can be designed and flexibly reconfigured as a finite impulse response digital filter. The principle and operation conditions of the proposed scheme are theoretically analyzed. A system employing an optical time division multiplexing (OTDM)-based multi-tap shaper is demonstrated. Filtering features agreeing well with the theoretical and simulation results are experimentally measured by configuring the path attenuations and delays in the OTDM based shaper.

A novel time and wavelength interleaved optical pulsed signal for a high resolution photonic analogue to digital converter

This paper presents a parallel architecture for a photonic Analog to Digital Converter (ADC) in a direct conversion receiver. A novel technique for the generation of time and wavelength interleaved laser source having a broad spectrum and a repetition rate of 8 GHz is proposed for the sampling of the photonic ADC. The quantization is realized in electrical domain by employing four state of the art ADCs , each having a sampling rate of 2 GHz and a resolution of 10 bit. We investigate the performance of our novel architecture by computing the Signal to Noise Ratio and comparing it with the conventional electronic ADC. Additionally we have also observed the effect of laser jitter on the sensitivity of a direct conversion receiver.

Wideband signal detection based on high-speed photonic analog-to-digital converter

This Letter demonstrates the effectiveness of a high-speed high-resolution photonic analog-to-digital converter (PADC) for wideband signal detection. The PADC system is seeded by a high-speed actively mode locked laser, and the sampling rate is multiplied via a time-wavelength interleaving scheme. According to the laboratory test, an X-band linear frequency modulation signal is detected and digitized by the PADC system. The channel mismatch effect in wideband signal detection is compensated via an algorithm based on a short-time Fourier transform. Consequently, the signal-to-distortion ratio (SDR) of the wideband signal detection is enhanced to the comparable SDR of the single-tone signal detection.

Switching response of dual-output Mach–Zehnder modulator in channel-interleaved photonic analog-to-digital converter

This Letter theoretically and experimentally studies the response of photonic switching in a channel-interleaved photonic analog-to-digital converter (PADC) with high sampling rate and wide input frequency range. A figure of merit (FoM) is introduced to evaluate the switching response of the PADC when a dual-output Mach–Zehnder modulator (MZM) serves as the photonic switch to parallelize the sampled pulse train into two channels. After the optimization of the FoM and utilization of the channel-mismatch compensation algorithm, the system bandwidth of PADC is expanded and the signal-to-distortion ratio is enhanced.

Broadband photonic ADC for microwave photonics-based radar receiver

A broadband photonic analog-to-digital converter (ADC) for X-band radar applications is proposed and experimentally demonstrated. An X-band signal with arbitrary waveform and a bandwidth up to 2 GHz can be synchronously sampled and processed due to the optical sampling structure. In the experiment, the chirp signal centered at 9 GHz with a bandwidth of 1.6 GHz is sampled and down-converted with a signal-to-noise ratio of 7.20 dB and an improved noise figure. Adopting the photonic ADC in the radar receiver and the above signal as the transmitted radar signal, an X-band inverse synthetic aperture radar system is set up, and the range and cross-range resolutions of 9.4 and 8.3 cm are obtained, respectively.

All-fiber-photonics-based ultralow-noise agile frequency synthesizer for X-band radars

We propose and demonstrate an agile X-band signal synthesizer with ultralow phase noise based on all-fiber-photonic techniques for radar applications. It shows phase noise of −145 dBc/Hz (−152 dBc/Hz) at 10 kHz (100 kHz) offset frequency for 10 GHz carrier frequency with integrated RMS timing jitter between 7.6 and 9.1 fs (integration bandwidth: 10 Hz–10 MHz) for frequencies from 9 to 11 GHz. Its frequency switching time is evaluated to be 135 ns with a 135 pHz frequency tuning resolution. In addition, the X-band linear-frequency-modulated signal generated by the proposed synthesizer shows a good pulse compression ratio approximating the theoretical value. In addition to the ultrastable X-band signals, the proposed synthesizer can also provide 0–1 GHz ultralow-jitter clocks for analog-to-digital converters (ADC) and digital-to-analog converters (DAC) in radar systems and ultralow-jitter optical pulse trains for photonic ADC in photonic radar systems. The proposed X-band synthesizer shows great performance in phase stability, switching speed, and modulation capability with robustness and potential low cost, which is enabled by an all-fiber-photonics platform and can be a compelling technology suitable for future X-band radars.

Broadband high-resolution microwave frequency measurement based on low-speed photonic analog-to-digital converters.

An approach to microwave frequency measurement with a high resolution and a broad bandwidth is proposed based on three parallel low-speed photonic analog-to-digital converters (ADCs) architecture. Through simultaneously bandpass sampling the input microwave signal with the three photonic ADCs, the input frequency can be calculated from the Fourier frequencies of the photonic ADCs using the proposed frequency recovery algorithm. Theoretical analysis and simulation results indicate that the proposed method is applicable for both single-tone and multi-tone microwave signals. By employing three ~1 GS/s@8 bits photonic ADCs, 0-100 GHz frequency measurement with an error of ± 0.5 MHz and a spur-free dynamic range of 94 dB-Hz2/3 over the full band has been numerically demonstrated. Additionally, a proof-of-concept experiment is carried out to demonstrate the effectiveness of the proposed method, where a frequency measurement range of 0-20 GHz with a measurement error of ± 8 kHz is realized by utilizing three photonic ADCs with sampling rates of 27.690 MS/s, 27.710 MS/s, and 27.730 MS/s. Larger frequency measurement range can be achieved by using an optical modulator with a larger bandwidth.

Compensation of multi-channel mismatches in high-speed high-resolution photonic analog-to-digital converter.

We demonstrate a method to compensate multi-channel mismatches that intrinsically exist in a photonic analog-to-digital converter (ADC) system. This system, nominated time-wavelength interleaved photonic ADC (TWI-PADC), is time-interleaved via wavelength demultiplexing/multiplexing before photonic sampling, wavelength demultiplexing channelization, and electronic quantization. Mismatches among multiple channels are estimated in frequency domain and hardware adjustment are used to approach the device-limited accuracy. A multi-channel mismatch compensation algorithm, inspired from the time-interleaved electronic ADC, is developed to effectively improve the performance of TWI-PADC. In the experiment, we configure out a 4-channel TWI-PADC system with 40 GS/s sampling rate based on a 10-GHz actively mode-locked fiber laser. After multi-channel mismatch compensation, the effective number of bit (ENOB) of the 40-GS/s TWI-PADC system is enhanced from ~6 bits to >8.5 bits when the RF frequency is within 0.1-3.1 GHz and from ~6 bits to >7.5 bits within 3.1-12.1 GHz. The enhanced performance of the TWI-PADC system approaches the limitation determined by the timing jitter and noise.