Daniel J. Esman

Highly Linear Broadband Photonic-Assisted Q-Band ADC

A highly linear broadband photonic-assisted analog-to-digital converter (ADC) based on high-frequency optical sampling utilizing a dual output Mach-Zehnder modulator operating with signal frequencies up to 50 GHz is presented. The pulses employed in the optical sampling were generated by a cavity-less pulse source operated at 10 GHz in preference to conventional mode-locked lasers. The optical sampling front-end greatly extends the operational frequency range of the Nyquist limited electronic digitization back-end. The performance of the sampling system is characterized with 7.1 effective number of bits (ENOBs) at 40 with 5 GHz fully accessible bandwidth, and greater than 99 dB·Hz2/3 spurious free dynamic range for the 30-40 GHz frequency range. Furthermore, more than 8 ENOB was achieved by reducing the effective bandwidth to 1 GHz with a digital filter, demonstrating the additional advantage of using a higher sampling rate compared to previous demonstrations. A new figure of merit of photonic-assisted sub-sampled ADCs is also presented accompanied with a comparison to previous implementations.

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.

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.

A fully frequency referenced parametric polychromatically sampled analog-to-digital conversion

We present a novel scalable photonically-sampled analog-to-digital-converter based on parametric multicasting, polychromatic sampling and frequency referenced lasers. A sampling rate of 30-GS/s is achieved with three subrate-channels with a 6.2-ENOB performance of a 19-GHz signal.

Photonic parametric sampled analog-to-digital conversion at 100 GHz and 6 ENOBs

A broadband photonic parametric sampling gate capable of capturing high frequency signals is demonstrated. The parametric-sampled ADC performance is characterized with a record high resolution of 6.0 ENOBs at a signal frequency in excess of 100 GHz.

Subnoise Signal Detection and Communication

Radio frequency spectrum is one of the scarcest commodities in existence, with progressively increasing value. As a physical foundation of an untethered society, it now carries the majority of social, defense, and commercial interactions. All of these must reside within narrow, strictly regulated spectral windows allocated for cellular, military, navigation, and broadcast services. Band localization minimizes interference but also mandates that the entire cellular traffic be confined in less than one percent of the physical radio-frequency range. To defy this restriction and emit freely in any band, the signal power must be small to avoid interference with existing traffic. By spreading the signal over a sufficiently wide spectral range, the emission in any band can be maintained below naturally occurring noise. Unfortunately, the reception of a spectrally broadened, subnoise data channel poses a fundamental challenge: a fast, bursty waveform must be detected, separated from noise and reconstructed at rates exceeding gigahertz. Here, we show that a 20-MHz-wide signal can be spread by 300-fold, detected and reconstructed by a physical Fourier transform even when it is much weaker than the received noise. Rather than quantizing the 6-GHz-wide signal and computing its correlation with the decoding waveform, the signal was physically detected and reconstructed by coherently coupled frequency combs. By eliminating high-speed electronics from the receiver, it is now possible to access the entire radio-frequency range that extends beyond 100 GHz. We anticipate that new, band-unrestricted wireless services will emerge to maximize throughput, mitigate interference, and achieve a high level of physical security.

High-speed, rate-scalable photonic-assisted digitizer equalization by frequency comb referencing

A scalable analog-to-digital converter based on polychromatic sampling and optical-domain frequency referencing is described. The new architecture relies on low-distortion replication of an optical signal to spectrally distinct copies and subsequent polychromatic parametric sampling. Frequency comb referencing of parametric replication and sampling was used to convert processor distortions into quasi-stationary impairments and enable a practical equalization implementation. The operation of the new digitizer was demonstrated at 30 GS/s, achieving 6.5 effective number of bits in the first Nyquist zone. In contrast to conventional analog-to-digital converters, the new preprocessor sampling bandwidth is not restricted to the first Nyquist zone, and can operate in the second and third Nyquist zones beyond 40 GHz.

Comb-Assisted Cyclostationary Analysis of Wideband RF Signals

Signals arising in nearly all disciplines, including telecommunications, mechanics, biology, astronomy, and nature are generally modulated, carrying corresponding signatures in both the temporal and spectral domains. This fact was long recognized by cyclostationary and cumulant analysis, providing qualitatively better means to separate stochastic from deterministically modulated radiation. In contrast to simple spectral analysis, the cyclostationary technique provides a high level of spectral discrimination, allowing for considerable signal selectivity even in the presence of high levels of background noise and interference. When performed with sufficient resolution, cyclostationary analysis also provides the ability for signal analysis and classification. Unfortunately, these advantages come at a cost of large computational complexity posing fundamental detection challenges. In the case of modern ultrawideband signals, the requirements for persistent cyclostationary analysis are considerably beyond the processing complexity of conventional electronics. Recognizing this limit, we report a new photonically assisted cyclostationary analyzer that eliminates the need for high-bandwidth digitization and real-time Fourier processors. The new receiver relies on mutually coherent frequency combs used to generate a Fourier representation of the received signal in a computation-free manner. With the advent of practical, cavity-free optical frequency combs, the complexity for cyclostationary analysis can be greatly reduced, paving a path toward persistent wideband cyclostationary analysis in an ultrawideband operating regime.

Frequency-Hopping Pulse Position Modulation Ultrawideband Receiver

Pulse position modulation (PPM) has been used in the radio-frequency (RF) domain to achieve both low-dissipation requirements and provide precision ranging. In ultrawideband (UWB) architectures, it underpins an asynchronous receiver, multiple access environments, and interference-resistant transmission. When combined with frequency hopping (FH), it allows for an additional level of immunity to jamming and low probability of intercept. Realization of a FH-PPM transceiver poses a practical challenge, particularly in the UWB RF range. With UWB pulses reaching the multi-gigahertz range, FH adds to the effective bandwidth at which the receiver must be operated, exceeding the performance of a modern quantizer and digital demodulation backplane. This study describes a new photonics-assisted FH-PPM receiver architecture that rests on mutually coherent frequency combs. The performance of the new receiver was characterized by receiving and decoding an 80–Mb/s rate FH-PPM UWB signal.

Detection of Fast Transient Events in a Noisy Background

Transient signals accompanied by a high noise level pose both basic and practical detection challenges. Disciplines that range from communication and astronomy to molecular physics face a very similar detection problem. When phenomena of interest are repetitive, different averaging techniques can be applied in order to elevate the signal above the detection threshold. In contrast, nonrepetitive signals, commonly occurring in communication and astronomy, cannot be processed using averaging techniques. With the advent of near-noiseless replication techniques, single-instance signal separation from noise has become possible. Here, we demonstrate that a single-instance signal can be mapped to 300 optical carrier frequencies and detected with low-bandwidth receivers to generate detection gains of 24 dB with respect to a single integrating receiver with the same bandwidth. The spectral-replicating receiver was used to detect a single-instance signal with power that was four times lower than the accompanying noise level.