Photonic Analog-to-digital Converter Research Articles

Time-Frequency Uncertainty in the Photonic A/D Converters Based on Spectral Encoding

In recent years, some approaches to realize photonic analog-to-digital conversion based on spectral encoding have been proposed. In these approaches, the amplitude information of input analog signals is first converted into the frequency information of the optical carrier via a nonlinear process, which is equivalent to optical frequency modulation; then, the spectral information of the optical carrier is quantized and encoded with the help of an optical filter group. In this letter, we propose, for the first time to our knowledge, that the time-frequency uncertainty principle puts a fundamental limit on this type of photonic analog-to-digital converters (ADCs). The condition for achieving acceptable performance in terms of effective number of bits is discussed. The presented theoretical results are verified by numerical simulations and a proof-of-concept experiment in electrical domain. We believe that the presented result is important to the design of spectral-encoding-based photonic ADCs.

Comparison of the Jitter Performance of different Photonic Sampling Techniques

The jitter performance of different photonic sampling techniques are investigated. Two basic sampling mechanisms are identified. Depending on analog-to-digital converter (ADC) architectures, the sampling instant is defined either by a time instant at the rising edge of the detected pulses or by their center of mass. A generalized noise model is introduced and used for performance analysis. It is shown that using the center of mass of optical pulses for the sampling process is about one order of magnitude more precise in time than using a time instant at the rising edge. But the incThe jitter performance of different photonic sampling techniques are investigated. Two basic sampling mechanisms are identified. Depending on analog-to-digital converter (ADC) architectures, the sampling instant is defined either by a time instant at the rising edge of the detected pulses or by their center of mass. A generalized noise model is introduced and used for performance analysis. It is shown that using the center of mass of optical pulses for the sampling process is about one order of magnitude more precise in time than using a time instant at the rising edge. But the increased timing precision of down to few fs is accompanied by a higher amplitude uncertainty decreasing the photonic ADC resolution to ≈ 9 effective number of bits.reased timing precision of down to few fs is accompanied by a higher amplitude uncertainty decreasing the photonic ADC resolution to ≈ 9 effective number of bits.

Dual-output modulation in time-wavelength interleaved photonic analog-to-digital converter based on actively mode-locked laser

This Letter demonstrates the application of dual-output modulation in a photonic analog-to-digital converter (PADC) with a high sampling rate and resolution. The PADC is time-wavelength interleaved and based on an actively mode-locked laser. According to theoretical analysis, the dual-output PADC system shows a better linearity for achieving a higher dynamic range. In the experiment, third-order distortion is significantly suppressed by ～40 dB when the dual-output modulator is used and the effective number of bits of the PADC has reached 9.0 bits below 0.2 GHz and 6.4 bits at 6.1 GHz in our PADC with a sampling rate of 20 GS/s.

Simplified 2-bit photonic digital-to-analog conversion unit based on polarization multiplexing

A 2-bit photonic digital-to-analog conversion unit is proposed and demonstrated based on polarization multiplexing. The proposed 2-bit digital-to-analog converter (DAC) unit is realized by optical intensity weighting and summing, and its complexity is greatly reduced compared with the traditional 2-bit photonic DACs. Performance of the proposed 2-bit DAC unit is experimentally investigated. The established 2-bit DAC unit achieves a good linear transfer function, and the effective number of bits is calculated to be 1.3. Based on the proposed 2-bit DAC unit, two DAC structures with higher (>2) bit resolutions are proposed and discussed, and the system complexity is expected to be reduced by half by using the proposed technique.

Below 100-fs Timing Jitter Seamless Operations in 10-GSample/s 3-bit Photonic Analog-to-Digital Conversion

We experimentally realize seamless operations with below 100-fs timing jitter in a 10-GSample/s 3-bit photonic analog-to-digital converter (ADC) with an input 2.5-GHz sinusoidal electrical signal. To address the energy efficiency, it is necessary to explore some serial approaches to get most operations in a photonic ADC done before serial-to-parallel conversion to save the number of devices. To press forward with the work on subsequent operations after optical sampling in a photonic ADC, we have investigated optical quantization and coding and demonstrated their performances. The experimental results successfully demonstrated seamless operations in a photonic ADC, i.e., sampling, quantization, and coding, while keeping its parallel-configuration-free characteristics and low timing jitter below 100 fs. This demonstration could address the energy efficiency by reduction of the number of devices, including electrical ADCs for subsequent operations after optical sampling in existing high-performance photonic ADCs.

Spurious-free dynamic range improvement in a photonic time-stretched analog-to-digital converter based on third-order predistortion

Spurious-free dynamic range (SFDR) limited by intermodulation distortions is a usually accepted measure for dynamic performance of a photonic time-stretched analog-to-digital converter (ADC). In this paper, SFDR improvement in a photonic time-stretched ADC based on third-order predistortion is proposed. The third-order predistortion is achieved optically within an integrated dual-parallel Mach–Zehnder modulator (DPMZM). The mechanism of SFDR improvement with third-order predistortion in the DPMZM is theoretically analyzed. Compared with a conventional scheme without predistortion, the experimental results show that the SFDR improvement of ∼26 dB in the proposed scheme is proved. [Publisher's Note: This paper is part of the Microwave Photonics special issue published in Photonics Research, Vol. 2, No. 4. It was mistakenly published in Issue No. 5.]

Analog-to-digital converters using photonic technology

This paper presents a brief review of photonic analog-to-digital converters (ADCs). Some typical schemes and operation principles of photonic ADCs, including photonic sampling ADCs, photonic quantizing ADCs, photonic sampling & quantizing ADCs and photonic assisted ADCs, are summarized and analyzed. In addition, some recent breakthrough results in the photonic ADCs are introduced.

Ultrahigh sampling rate photonic time stretch analog-to-digital converter employing phase modulation

The performance of analog-to-digital converter (ADC) can be greatly enhanced by preprocessing the ultrafast analog signal using the photonic time-stretch technique. In the previous time-stretch ADC, the amplitude modulation is adopted. The amplitude of stretched signal has nonuniformity after the time-stretch preprocessing and the power of stretched signal is very weak, this will influence on recovering distortionless stretched signal, increase the difficult of detector and sampling. In this paper, we propose the photonic time-stretch ADC employing the phase modulation and the optical coherent detection to eliminate the nonuniformity phenomenon on the amplitude and increase power of stretched signal by adjusting the local oscillation (LO) power. The sampling rate of the ADC can reach 1.56TSa/s by using large dispersion photonic crystal fiber (PCF).

Proposal for an all optical analog-to-digital converter based on modal birefringence in a polarization maintaining fiber

A photonic analog-to-digital converter (ADC) utilizing the modal birefringence of the polarization maintaining fiber (PMF) is proposed. In the PMF, sampling pulses with different wavelengths have different phase shifted transfer functions, which could be used for quantizing and encoding in a photonic ADC. A proof-of-concept experiment is performed and a 2.5 GHz sinusoidal electrical analog signal is quantized by using 16 different wavelengths. The experimental result with an effective number of bits of 4.5 is achieved.

An 8-bit Integrate-and-Sample Receiver for Rate-Scalable Photonic Analog-to-Digital Conversion

Jitter limitations pose significant challenges for high-resolution and sampling-rate analog-to-digital converters (ADCs). This paper describes an integrate-and-sample (IAS) receiver suitable for use in an optical parametric photonic ADC. Rate-scalable photonic-sampling techniques provide low-jitter optical sampling and analog-to-digital conversion of the wideband signal up to 10 GHz and beyond. An 8-bit 2-GS/s IAS receive channel is described for a rate-scalable photonic ADC. Electronic measurements are shown for an RF tone and a photonic Gaussian pulse source and compared to simulations. A two-channel IAS array is fabricated in a 120-nm SiGe BiCMOS process and packaged onto a printed circuit board for integration into the photonic-sampling setup. A single 2-GS/s channel achieves a measured performance higher than 8.1 ENOB. The two-channel integrated circuit consumes 890 mA per channel from 5- and 2.5-V supplies and occupies an area of 1.6 × 2.0 mm2.

Novel 2N bit bipolar photonic digital-to-analog converter based on optical DQPSK modulation coupled with differential detection

A novel 2N bit bipolar photonic digital-to-analog converter (PDAC) scenario based on the optical differential quadrature phase shift keying (ODQPSK) modulation coupled with differential detection is proposed. Compared with other proposed schemes, this bipolar PDAC has a greater dynamic range and a larger noise margin with good scalabilities both in speed and resolution. We demonstrate a 4 bit PDAC in a proof-of-principle experiment at a sampling rate of 2.5 GS/s.

Photonic ADC: overcoming the bottleneck of electronic jitter

Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated for many years as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs - a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated.

SNR approach for performance evaluation of time-stretching photonic analogue to digital converter system

A semi-analytical simulation method (SASM) is proposed to evaluate the signal-to-noise ratio (SNR) of time stretched signals at the output of photonic analogue-to-digital converter (Ph-ADC) system. Analytical expressions of the signal at Ph-ADC output considering generic electrical signals applied to the electro-optic modulators of the Ph-ADC are derived. The contribution to the total variance of the received signal from the noise introduced by the electrical transmitter and receiver, and by the optical amplifier are derived analytically taking into account the pulsed nature of the optical signal. The proposed SASM shows excellent agreement of SNR estimates with the estimates provided by Monte Carlo simulation. This result is confirmed for variance dominantly imposed by the noise introduced by the electrical transmitter, by the optical amplifier and by the electrical receiver. A simplified approach is also proposed and compared with previous work. It is shown that mean power estimates obtained from this simplified approach are valid while the modulator is operating in the linear region and the signal is not affected by the frequency response of the electrical receiver filter. Additionally, it is concluded that the estimates of the noise variance due to the electrical transmitter are acceptable when a small signal analysis of noise along the Ph-ADC is valid.

Demonstration of optical sample parallelisation for high-speed photonic assisted ADCs

An experimental demonstration of an optical parallelisation scheme for a high-sampling-rate photonic-assisted analogue-to-digital converter (ADC) is presented. By exploiting four-wave mixing in high nonlinear fibre, a tenfold parallelisation of a 9.952GS/s signal is realised with signal-to-noise ratio ranging from 24.94 to 27.53 dB, corresponding to effective number of bits (ENOB) from 3.86 to 4.26 bits. Capability for ENOB of 6 bits is also permitted, and potentials for parallelisation of larger scale are also discussed.

Localization and Fingerprint of Radio Signals Employing a Multichannel Photonic Analog-to-Digital Converter

The fingerprint and localization of radio signals employing a multichannel photonic analog-to-digital converter (ADC) is proposed, analyzed, and demonstrated in a laboratory experiment. The photonic ADC detects the radio signals with high sensitivity in a large bandwidth without down-conversion stages. This is of special interest when processing emerging low-power wireless standards like ultra-wideband (UWB) radio. The optical processing in the multichannel photonic ADC is tailored for the localization and fingerprint of generic radio transmitters when orthogonal-frequency division multiplexing (OFDM) modulation is employed in the transmission. The photonic ADC includes engineered optical and electrical amplification. The experimental work demonstrates that detection of radio signals with -65 dBm power with signal-to-noise ratio better than 20 dB is feasible, which is in good accordance with the theoretical analysis. The multichannel photonic ADC comprises five optical channels which are precisely time-aligned in optical domain achieving 0.23-m spatial resolution (median) in the localization of radio transmitters. The experimental work also demonstrates that photonic-ADC processing is adequate for OFDM-based UWB radio-signal fingerprint including estimation of the average power, frequency band of operation, and time-frequency hopping pattern if applicable. UWB transmitter localization has been experimentally demonstrated with 0.4-m error.

Spectral Encoded Photonic Analog-to-Digital Converter Based on Cascaded Unbalanced MZMs

We present a novel scheme of a spectral encoded photonic analog-to-digital converter (ADC). Several cascaded unbalanced lithium niobate Mach-Zehnder modulators with doubling path length imbalances are used to convert the amplitude of the electrical analog signal to the shift of the transmission characteristic. Compared with former spectral encoded photonic ADCs, a particular benefit of our presented scheme is no requirement of any nonlinear effect to realize optical quantization.

Photonic Digital-to-Analog Converter Based on Summing of Serial Weighted Multiwavelength Pulses

A novel photonic digital-to-analog converter (PDAC) scheme based on summing of serial weighted multiwavelength pulses is proposed and experimentally demonstrated. The weighted multiwavelength pulses are delayed and summed in time domain through dispersion, and each weighted pulse with specific wavelength corresponds to a bit of input digital data. Regardless of the resolution, this scheme only requires one electrooptic modulator (EOM). In the experiment, a 3-bit PDAC with a sample rate of 2.5 GS/s is realized, and the input digital signal is mapped to an analog output linearly by a transfer function. This simple and compact setup could reduce the system complexity and avoid the synchronization of multiple EOMs.

Photonic subsampling analog-to-digital conversion of microwave signals at 40-GHz with higher than 7-ENOB resolution

Conversion of analog signals into digital signals is one of the most important functionalities in modern signal processing systems. As the signal frequency increases beyond 10 GHz, the timing jitter from electronic clocks, currently limited at approximately 100 fs, compromises the achievable resolution of analog-to-digital converters (ADCs). Owing to their ultralow timing jitter, the use of optical pulse trains from passively mode-locked lasers has been considered to be a promising way for sampling electronic signals. In this paper, based on sub-10 fs jitter optical sampling pulse trains, we demonstrate a photonic subsampling ADC that downconverts and digitizes a narrowband microwave signal at 40 GHz carrier frequency with higher than 7 effective-number-of-bit (ENOB) resolution.

Coded output photonic A/D converter based on photonic crystal slow-light structures

A photonic analog-to-digital converter (PADC) utilizing a slow-light photonic crystal Mach-Zehnder interferometer (MZI) is proposed, to enable the optically coded output of a PADC with reduced device size and power consumption. Assuming an index modulation for the MZI on the Taylor's PADC structure, limiting factors in device size, speed, and effective number of bits are derived considering the signal transition time of the light and the slow light dispersion effects. Details of the device design and results of a time domain assessment of the device performance is described with discussions on the feasibility of sub-mm size, 20GS/s operation of the device having the ENOB (effective number of bits) > 5.

EXtreme Chirped Pulse Oscillator Operating in the Nanosecond Stretched Pulse Regime

An eXtreme Chirped Pulse Oscillator (XCPO) implemented with a Theta cavity and based on a semiconductor optical amplifier (SOA) is presented for generating 10 ns frequency-swept pulses and 3.6 ps compressed pulses directly from the oscillator. In this experiment, we show the two distinct characteristics of the XCPO which are the scalability of the output energy and the mode-locked spectrum. By using these characteristics, we obtain a pulse energy of 58.4 pJ from the stretched pulse and a mode-locked optical bandwidth of 14.6 nm (10 dB) directly from the oscillator. The laser cavity design allows for low repetition rate operation <100 MHz, as well. The cavity, significantly, reduces nonlinear carrier dynamics, integrated self phase modulation (SPM), and fast gain recovery in an SOA. Due to the laser's ability to generate directly frequency-swept pulses from the oscillator, this oscillator can be used for high speed frequency-swept optical coherence tomography (OCT) and time-stretched photonic analog to digital converters (P-ADC).