We demonstrate a record high performance of frequency-interleaved analog-to-digital conversion using a phase-noise-engineered dual frequency comb photonic technique, enabling 7 effective number of bits (ENOB) for signals up to 40 GHz

Deakin, Callum

Liu, Zhixin

UCL Discovery

Frequency interleaving dual comb photonic ADC with7 bits ENOB up to 40 GHzCallum Deakin and Zhixin LiuOptical Networks Group, Department of Electronic and Electrical Engineering, University College London,London, U.K.callum.deakin.17@ucl.ac.ukAbstract: We demonstrate a record high performance of frequency-interleaved analog-to-digital conversion using a phase-noise-engineered dual frequency comb photonic tech-nique, enabling 7 effective number of bits (ENOB) for signals up to 40 GHz. © 2022 TheAuthor(s)1. IntroductionAnalog to digital converters (ADC) that enable digital processing of optical signals has transformed lightwavecommunication systems in the last decade. Modern communications systems demand high resolution ADCs inorder to detect higher order modulation formats and achieve a more effective compensation of nonlinear distor-tions [1]. The resolution of high speed (i.e. > 1 GHz) electronic ADCs is typically limited by clock jitter and, atespecially high frequencies, the speed of the component transistors that results in comparator ambiguity [2]. Thispresents a trade-off between the frequency of the detected signal and accuracy, defined by the signal-to-noise-and-distortion-ratio (SINAD) or effective number of bits (ENOB). In a jitter limited ADC, the SINAD decreasesquadratically with increasing frequency, giving a 6 dB SINAD penalty for every doubling of the input frequency.Many have investigated photonic techniques for improving the performance of high speed ADCs [3]. Anextensively-studied approach is to optically sample the incoming signal with an ultra low jitter mode-locked laser(MLL), where the sampling pulses are de-interleaved and the signal reconstructed digitally in a time interleavingarchitecture. While this approach has achieved a superior performance of 44 dB SINAD at a single frequency of40 GHz [4], the impressive results are difficult to scale to detect broadband signals and translate to an integratedplatform due to low power and jitter performance of high repetition integrated MLLs, which is fundamentallylimited by the semiconductor material gain and the cavity length.As an alternative to the time-interleaving approach, frequency-interleaved ADCs were proposed to divide abroadband signal into a number of sub-band signals that can be detected in parallel by a bank of low speed, highresolution ADCs. The frequency interleaving approach can relax jitter requirements and reduce channel mismatcherrors compared to time interleaving, but has not seen widespread adoption in electronic ADCs due to the difficultyof designing the required analog electronic filter banks and synthesizing many low noise local oscillators [5].In [6], we proposed a photonic frequency-interleaving ADC architecture based on dual optical frequency combs,potentially overcoming the aforementioned design challenges in the electronic frequency interleaving architecture.Nevertheless, the demonstrated performance so far was limited to 33 dB SINAD due to the un-optimized relativephase noise in the dual-comb system.In this paper, we significantly improve the relative phase noise, optical power and receiver front end of ourdual comb system and present a record high frequency-interleaved ADC performance that can detect signals up to40 GHz at 7 bits ENOB with 12.5 GHz bandwidth.2. Experimental setup and resultsThe dual frequency comb ADC concept has been described previously [6] and the implementation for this experi-ment is shown in Fig 1. Two frequency combs of spacing fsig = 26 GHz (‘signal comb’) and fLO = 25 GHz (‘LOcomb’) are generated from a low linewidth (< 1 kHz) laser at 1555 nm through electo-optic modulation. The RFsignals for the combs are generated from a common reference through ultra low phase noise frequency synthe-sizers. Furthermore, both electrical and optical paths on each comb are matched to ensure maximum coherenceLO combfsig = 26 GHzSignal comb Modulator AWGADCSignal under test t LO combModulated signal combChn+ChnCh1f ffsig~fLO = 25 GHz~fLOAWGChn-TIA1kHz CW laser(a) (b) Ref. clockFig. 1: (a) Dual comb channelization concept. (b) Experimental setup. AWG, arrayed waveguide grating; TIA, transimpedanceamplifier; CW, continuous wave; ADC, analog to digital converter.of both the laser and RF phase noise, which ensures the lowest relative phase noise for high SINAD digitizationusing the dual comb technique [7].The signal comb is modulated with the input signal of interest by a 40 GHz bandwidth Mach-Zehnder modulator(MZM) biased at transmission null and is amplified by an erbium-doped fiber amplifier (EDFA). The modulatedsignal comb is then combined with the LO comb through a 50/50 coupler and split by two fLO = 25 GHz spacingarray waveguide gratings (AWGs) for parallel balanced detection. In this way, each channel contains a opticalsignal consisting of a single modulated comb line and a local oscillator comb line offset by n∆ f , where ∆ f =fsig − fLO = 1 GHz. After detection by a balanced detector, the beating between the LO and signal comb ineach channel simultaneously down-converts 1 GHz spectral slices of the original signal to baseband where theycan be detected by 4-GSa/s high resolution ADCs with 10 bits ENOB. The full bandwidth signal can then bereconstructed digitally, resulting in signal digitization at much higher resolution than would be achievable by asingle, high speed ADC.In this experiment, we performed sequential detection of each channel in a 12.5 GHz bandwidth receiver inorder to test the performance. The signal under test was a sine wave as per the the IEEE ADC testing standard [8],which we tested with various frequencies up to 40 GHz (4th Nyquist zone). The SINAD for the receiver across arange of frequencies is shown in Fig. 2(a). The SINAD is relatively flat with respective to frequency, indicatingthat the system is not fundamentally limited by clock jitter: in this case the system is limited by the amplifiedspontaneous emission noise of the EDFA following the modulator, which may be improved by using lower noisefigure amplifiers or further increasing the power per comb line. For comparison, other high performing photonicADCs are also plotted, along with the state-of-the-art electrical ADCs [9]. Our results outperform the reportedhigh-speed electronic ADCs and show the SINAD performance across a range of frequencies up to the 4th Nyquistzone, which has not been shown in prior photonic ADC demonstrations [4, 10, 11].An example fast Fourier transform of the detected 39.87 GHz signal is shown in Fig. 2(b). A low pass digitalfilter and Kaiser window is applied to signal after detection to prevent spectral leakage. This achieves 44 dBSINAD (equivalent to 7.0 bits ENOB) and a spurious free dynamic range of 63 dB. This result is equivalent to atiming jitter of 25 fs: i.e. an ADC with 25 fs jitter as its only source of noise would be able to detect the signalwith the same fidelity. This equivalent jitter performance is highlighted as a dashed line in Fig. 2(a).10 10020304050(a)jitter = 25 fsTest frequency (GHz)SINAD(dB)Electronic [9] This paperTI photonic [4, 11] FI photonic [10]0 100 200 300 400 500−80−60−40−200SFDR = 63 dBSINAD = 44 dBENOB = 7.0 bits8192-point FFT(b)Frequency (MHz)Power(dBc)Fig. 2: (a) Measured SINAD up to 40 GHz, with the prior art of the best time interleaving (TI) photonic ADCs (purpletriangles), frequency interleaving (FI) photonic ADCs (pink squares) and electronic ADCs (green dots). (b) An example fastfourier transform showing a 39.87 GHz signal detected in a single channel.3. ConclusionWe demonstrate, to the best of our knowledge, the lowest effective jitter for a frequency interleaving photonicADC, achieving 7 bits ENOB at 40 GHz. This outperforms all electronic ADC results and is equivalent to thebest reported results of time interleaving photonic ADCs, with the potential for further improvements due to therelaxed jitter requirements of the frequency interleaving design.References[1] L. Galdino et al., “The trade-off between transceiver capacity and symbol rate,” in 2018 Optical Fiber Communication Conference,(Optical Society of America, 2018), p. W1B.4.[2] R. H. Walden, “Analog-to-digital converter survey and analysis,” IEEE J. on selected areas communications 17, 539–550 (1999).[3] G. C. Valley, “Photonic analog-to-digital converters,” Opt. express 15, 1955–1982 (2007).[4] A. Khilo et al., “Photonic ADC: overcoming the bottleneck of electronic jitter,” Opt. Express 20, 4454–4469 (2012).[5] S. W. Callender, Wideband signal acquisition via frequency-interleaved sampling (University of California, Berkeley, 2015).[6] C. Deakin and Z. Liu, “Dual frequency comb assisted analog-to-digital conversion,” Opt. Lett. 45, 173–176 (2020).[7] C. Deakin and Z. Liu, “Noise and distortion analysis of dual frequency comb photonic RF channelizers,” Opt. Express 28, 39750–39769(2020).[8] IEEE, “IEEE standard for terminology and test methods for analog-to-digital converters,” IEEE Std 1241-2010 pp. 1–139 (2011).[9] B. Murmann, “ADC performance survey 1997-2021,” http://web.stanford.edu/˜murmann/adcsurvey.html. [Online].[10] D. Fang et al., “320 GHz analog-to-digital converter exploiting kerr soliton combs and photonic-electronic spectral stitching,” in 2021European Conference on Optical Communication (ECOC), (IEEE, 2021), pp. Th3C1–PD2.2.[11] S. Gupta and B. Jalali, “Time-warp correction and calibration in photonic time-stretch analog-to-digital converter,” Opt. letters 33, 2674–2676 (2008).

Frequency interleaving dual comb photonic ADC with 7 bits ENOB up to 40 GHz

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