Vahid Ataie

Overcoming Kerr-induced capacity limit in optical fiber transmission

Getting around the capacity crunch The growing appetite for an ever-faster Internet and enhanced long-haul communication requires the pumping of more light down optic fibers. However, light-induced nonlinearities limit how much light can be pumped into the fiber without compromising the signal. This limitation has led to the prospect of a “capacity crunch.” Temprana et al. eliminated the effects of nonlinearity by using digital back-propagation methods with mutually coherent laser pulses from a single frequency comb. Science, this issue p. 1445 Digital back-propagation is used to mitigate light-induced nonlinear effects in optic fiber. Nonlinear optical response of silica imposes a fundamental limit on the information transfer capacity in optical fibers. Communication beyond this limit requires higher signal power and suppression of nonlinear distortions to prevent irreversible information loss. The nonlinear interaction in silica is a deterministic phenomenon that can, in principle, be completely reversed. However, attempts to remove the effects of nonlinear propagation have led to only modest improvements, and the precise physical mechanism preventing nonlinear cancellation remains unknown. We demonstrate that optical carrier stability plays a critical role in canceling Kerr-induced distortions and that nonlinear wave interaction in silica can be substantially reverted if optical carriers possess a sufficient degree of mutual coherence. These measurements indicate that fiber information capacity can be notably increased over previous estimates.

Spectrally Equalized Frequency Comb Generation in Multistage Parametric Mixer With Nonlinear Pulse Shaping

A novel approach to improve the flatness of wide-band parametric frequency combs is presented. The method relies on nonlinear pulse shaping prior to parametric mixing in a comb-generating device. The parametric comb is generated in a cavity-less multistage shock-wave mixer, using continuous-wave (CW) laser as comb seed. The pulse shaping is accomplished in a regenerative stage incorporating nonlinear optical/amplifying loop mirror (NOLM/NALM) and provides near-ideal seeds for spectrally equalized frequency comb generation in the output mixing stage. The new technique was implemented separately on two comb generators with a coarse (100 GHz) and dense (10 GHz) tone spacing. A 4 dB flatness over 100 nm is achieved with wide-pitched comb, while the fine-pitched device yielded 1500 tones and possessed sub-2 dB spectral flatness over 120 nm bandwidth.

Wideband Parametric Frequency Comb as Coherent Optical Carrier

We report the first use of the CW-seeded parametric comb as a multiwavelength source for coherent channel transmitters. The new comb source relies on a multistage parametric mixer design seeded by a single kilohertz-linewidth master laser. The resultant frequency comb provides more than 120 tones in a 100-nm-wide continuous band, all possessing kilohertz-scale linewidth inherited from the master laser. The applicability of the new comb source as a coherent channel carrier was quantified by error-vector magnitude and bit-error rate metrics of individual spectral lines modulated by QPSK signaling. The performance of the new source exceeded that of the standard tunable coherent transmitter while providing a qualitatively lower frequency uncertainty and drift.

Ultrahigh Count Coherent WDM Channels Transmission Using Optical Parametric Comb-Based Frequency Synthesizer

A novel optical frequency synthesizer capable of generating a multitude of narrow linewidth oscillations, fully compatible with high-speed fiber-optic transmission requirements, is presented. The new device is based on an ultra-dense and wideband parametric comb capable of generating more than 1800 spectral lines with 6.25 GHz spacing within 100 nm (C + L band) of optical bandwidth and 3 dB spectral flatness. The parametric comb generation relies on the highly efficient nonlinear interaction of the ultra-short and high peak power pulse train in the precisely controlled dispersive medium. The generated comb source is used for coherent transmission of 1520 Nyquist ultra-dense wavelength division multiplexed channels in a flex-grid compatible optical access network architecture, offering up to 30 Gb/s per user over a 50 km of standard single mode fiber with estimated aggregate capacity of 32 Tb/s. The line spacing tunablity and one-to-multiple coherent cloning capability make the proposed multicarrier generator a strong contender fully capable of replacing thousands of discrete lasers in optical data communication systems.

Generation of 1500-tone, 120nm-wide ultraflat frequency comb by single CW source

Dense 1500-tone frequency comb is derived from a single CW laser. Sub-2dB flatness was measured over 120 nm, with 40 dB optical signal-to-noise-ratio. The new generator uses regenerative shock formation prior to parametric mixing process.

Photonic preprocessor for analog-to-digital-converter using a cavity-less pulse source

A photonic preprocessor for analog to digital conversion is demonstrated and characterized using a cavity-less optical pulse source. The pulse source generates high fidelity pulses at 2 GHz repetition rate with temporal width of 3 ps. Chirped pulses are formed by cascaded amplitude and phase modulators, and subsequently compressed in dispersion compensating fiber. Sampling operation is performed with a dual-output Mach-Zehnder modulator, where the complimentary output enables a reduction of noise by 3 dB. Phase noise characterization shows that the phase noise of the generated pulses is fully dictated by the RF source. The high quality of the pulse source used in a sampling preprocessor experiment was verified by measuring 8 effective number of bits at 10 GHz and 7.0 effective number of bits at 40 GHz.

Two-fold transmission reach enhancement enabled by transmitter-side digital backpropagation and optical frequency comb-derived information carriers

We demonstrate a two-fold reach extension of 16 GBaud 16-Quadrature Amplitude Modulation (QAM) wavelength division multiplexed (WDM) system based on erbium doped fiber amplifier (EDFA)-only amplified standard and single mode fiber -based link. The result is enabled by transmitter-side digital backpropagation and frequency referenced carriers drawn from a parametric comb.

Coherent Filterless Wideband Microwave/Millimeter-Wave Channelizer Based on Broadband Parametric Mixers

An essential capability in many applications, ranging from commercial, surveillance and defense, is to analyze the spectral content of intercepted microwave and millimeter-wave signals over a very wide bandwidth in real-time and with high resolution. A range of photonic schemes have been introduced for the real-time processing of wideband signals to overcome limitations of current conventional electronic frequency measurement approaches. Here, a novel microwave/millimeter-wave channelizer is presented based on a RF photonic front-end employing parametric wavelength multicasting and comb generation. This new technology enables a contiguous bank of channelized coherent I/Q IF signals covering extremely wide RF instantaneous bandwidth. High channel counts and wide RF instantaneous bandwidth are enabled by use of parametrically generated frequency-locked optical combs spanning >4 THz. Full field analysis capabilities of the coherent detection system are demonstrated by frequency domain analysis of 18 contiguous 1.2 GHz IF channels covering 15.5 GHz to 37.1 GHz input frequency range, and time and spectral domain analysis of a 75 GHz harmonically generated input signal. Sensitivity and dynamic range of the system are analyzed and discussed.

Demonstration of 40 GHz analog-to-digital conversion using copy-and-sample-all parametric processing

We present the result of an optical analog-to-digital conversion front-end based on the copy-and sample-all principle for the first time. The new architecture uses self-seeded multicasting and polychromatic sampling. Operation at 40 GHz and 6 ENOBs is experimentally demonstrated.

Subnoise detection of a fast random event

Detecting a transient needle in a haystack Discriminating signals within a noisy environment is an issue crucial to many disciplines, from observational astronomy to secure communication and imaging. If the signal is periodic, then averaging over many measurements can help enhance the signal-to-noise ratio. However, for signals that present as a single transient event, the detection capability has been limited. Ataie et al. developed a detector that can lift that limitation by combining signal cloning with frequency combs and signal-processing techniques (see the Perspective by Vasilyev). Their detector could detect signals buried within noise that would otherwise be undetectable. Science, this issue p. 1343; see also p. 1314 Single transient signals can be detected even when buried within a noisy environment. [Also see Perspective by Vasilyev] Observation of random, nonrepetitive phenomena is of critical importance in astronomy, spectroscopy, biology, and remote sensing. Heralded by weak signals, hidden in noise, they pose basic detection challenges. In contrast to repetitive waveforms, a single-instance signal cannot be separated from noise through averaging. Here, we show that a fast, randomly occurring event can be detected and extracted from a noisy background without conventional averaging. An isolated 80-picosecond pulse was received with confidence level exceeding 99%, even when accompanied by noise. Our detector relies on instantaneous spectral cloning and a single-step, coherent field processor. The ability to extract fast, subnoise events is expected to increase detection sensitivity in multiple disciplines. Additionally, the new spectral-cloning receiver can potentially intercept communication signals that are presently considered secure.