Single-shot photonic time-intensity integration based on a time-spectrum convolution system

High-speed all-optical processing for spectrum

Data-processing techniques in spectroscopy are fundamental and powerful analytical tools for lots of practical applications. In the age of big data, high-speed data-processing in spectroscopy is in urgent need, especially for the real-time analysis/feedback of data stream of spectroscopy or the capture of non-repetitive/rare phenomena in fast dynamic process. So far, intensive researches focus on high-speed processing of light signal in time/spatial domain but few people find a way to do it in spectral domain. Here, we report an optical computing technology for high-speed optical spectrum processing with features of real time, multiple functions, all-fiber configuration and immunity to electromagnetic interference. The software-controlled system could perform as, but not limited to, the first-order (or arbitrary fractional-order) differentiator/integrator/Hilbert transformer and tunable band-pass filter, respectively, to handle spectral data rapidly. High-speed processing of optical spectrum at a rate of 10,000,000 times per second is demonstrated.

Reconfigurable microwave signal processor with a phase shift of π

We propose and experimentally demonstrate a reconfigurable microwave signal processor, with a bandwidth up to tens of gigahertz. In this technique, any microwave signal processing function with a phase shift of π could be performed by shaping the input optical intensity spectrum. The phase shift of π is implemented by using a differential detection. Thanks to the broad bandwidth provided by the incoherent optical source and the high resolution of the user-defined optical filter, the frequency response of our approach could be in a great agreement with that of an ideal signal processing function. In the experiment, temporal intensity Hilbert transformations and temporal intensity differentiations of Gaussian-like pulses with widths of 125ps, 85ps and 68ps are accurately achieved.

In-fiber high-speed recognition of incoherent-light broadband energy spectrum patterns

A fiber-optic system is proposed and experimentally demonstrated for real-time, on-the-fly identification of an incoherent-light energy spectrum pattern based on dispersion-induced time-spectrum convolution. In the proposed system, the incoming frequency-spectrum patterns to be identified are modulated by a time-mapped version of the target intensity profile. Following propagation through a suitable fiber-optic dispersive medium, the measured output temporal waveform provides a correlation of the incoming spectra with the programmed target pattern. This enables direct, real-time detection of the matching energy spectra, without any further numerical post-processing. We experimentally demonstrate successful recognition of a target infrared spectral pattern, extending over a bandwidth of 1.5 THz with a resolution of ∼12  GHz, with sub-megahertz update rates. A path for further performance improvements is also suggested.

Reconfigurable single-shot incoherent optical signal processing system for chirped microwave signal compression

We propose and demonstrate a reconfigurable and single-shot incoherent optical signal processing system for chirped microwave signal compression, using a programmable optical filter and a multi-wavelength laser (MWL). The system is implemented by temporally modulating a specially shaped MWL followed by a suitable linear dispersive medium. A microwave dispersion value up to 1.33ns/GHz over several GHz bandwidth is achieved based on this approach. Here we demonstrate a single-shot compression for different linearly chirped microwave signals over several GHz bandwidth. In addition, the robustness of the proposed system when input RF signals are largely distorted is also discussed.

Microwave photonic integrator based on a multichannel fiber Bragg grating

We propose and experimentally demonstrate a microwave photonic integrator based on a multichannel fiber Bragg grating (FBG) working in conjunction with a dispersion compensating fiber (DCF) to provide a step group delay response with no in-channel dispersion-related distortion. The multichannel FBG is designed based on the spectral Talbot effect, which provides a large group delay dispersion (GDD) within each channel. A step group delay response can then be achieved by cascading the multichannel FBG with a DCF having a GDD opposite the in-channel GDD. An optical comb, with each comb line located at the center of each channel of the FBG, is modulated by a microwave signal to be integrated. At the output of the DCF, multiple time-delayed replicas of the optical signal, with equal time delay spacing are obtained and are detected and summed at a photodetector (PD). The entire operation is equivalent to the integration of the input microwave signal. For a multichannel FBG with an in-channel GDD of 730 ps/nm and a DCF with an opposite GDD, an integrator with a bandwidth of 2.9 GHz and an integration time of 7 ns is demonstrated.

Real-time Fourier transformation of lightwave spectra and application in optical reflectometry

We propose and experimentally demonstrate a fiber-optics scheme for real-time analog Fourier transform (FT) of a lightwave energy spectrum, such that the output signal maps the FT of the spectrum of interest along the time axis. This scheme avoids the need for analog-to-digital conversion and subsequent digital signal post-processing of the photo-detected spectrum, thus being capable of providing the desired FT processing directly in the optical domain at megahertz update rates. The proposed concept is particularly attractive for applications requiring FT analysis of optical spectra, such as in many optical Fourier-domain reflectrometry (OFDR), interferometry, spectroscopy and sensing systems. Examples are reported to illustrate the use of the method for real-time OFDR, where the target axial-line profile is directly observed in a single-shot oscilloscope trace, similarly to a time-of-flight measurement, but with a resolution and depth of range dictated by the underlying interferometry scheme.

Implementation of the photonic time-stretch concept using an incoherent pulsed light source

We propose a new technique to realize photonic time stretching of radio-frequency (RF) signals by using a time-gated (pulsed) incoherent-light source. The proposed system provides similar performance specifications (stretch factor, temporal aperture, and resolution) to those of a conventional coherent system but using a temporal gating process that is significantly longer than the transform-limited pulse duration of the equivalent coherent configuration. We experimentally demonstrate temporal magnification and compression of high-speed RF signals, with time-stretch factors ranging from 0.65 to 8.66, using a broadband (11.6 nm) incoherent-light source temporally gated over ∼163  ps. In one of the reported experiments, we achieve a resolution of ∼67.5  ps over a temporal aperture of ∼23  ns, representing a time-bandwidth product of >340.

Analysis of all-optical temporal integrator employing phased-shifted DFB-SOA

All-optical temporal integrator using phase-shifted distributed-feedback semiconductor optical amplifier (DFB-SOA) is investigated. The influences of system parameters on its energy transmittance and integration error are explored in detail. The numerical analysis shows that, enhanced energy transmittance and integration time window can be simultaneously achieved by increased injected current in the vicinity of lasing threshold. We find that the range of input pulse-width with lower integration error is highly sensitive to the injected optical power, due to gain saturation and induced detuning deviation mechanism. The initial frequency detuning should also be carefully chosen to suppress the integration deviation with ideal waveform output.

Incoherent-light temporal stretching of high-speed intensity waveforms

We propose and demonstrate experimentally the first incoherent-light scheme for temporal imaging (magnification) of intensity waveforms. The scheme is based on a time-domain equivalent of a pinhole camera under incoherent illumination, involving two dispersive lines and temporal intensity modulation with a short gate. We report incoherent-light temporal stretching of radiofrequency waveforms by a magnification factor of 2.86, with a time-bandwidth product exceeding 160, i.e., a resolution of ~50 ps over a temporal aperture of ~8 ns, totally avoiding the use of chirp-controlled pulsed lasers. This work opens up new perspectives for realization of many critical high-speed signal-processing modules using practical incoherent light-wave schemes.

Coupled-resonator optical waveguides for temporal integration of optical signals

In this paper, we propose and numerically investigate an all-optical temporal integrator based on a photonic crystal cavity. We show that an array of photonic crystal cavities enables high-order temporal integration. The effect of the value of the cavity's free spectral range on the accuracy of the integration is considered. The influence of the coupling coefficients in the resonator array on the integration accuracy is demonstrated. A compact integrator based on a photonic crystal nanobeam cavity is designed, which allows high-precision integration of optical pulses of subpicosecond duration.

Active Fabry-Perot cavity for photonic temporal integrator with ultra-long operation time window

In this paper, a photonic temporal integrator based on an active Fabry-Perot (F-P) cavity is proposed and theoretically investigated. The gain medium in the F-P cavity is a semiconductor optical amplifier (SOA) with high gain coefficient. Key feature of the proposed photonic integrator is that the length of integration time window is widely tunable and could be ideally extended to infinitely long when the injection current is approaching lasing condition. Based on an F-P cavity with practically feasible parameters, a photonic temporal integrator with an integration time window of 160 ns and an operation bandwidth of 180 GHz is achieved. The time-bandwidth product of this photonic temporal integrator is 28,800, which is about two-orders of magnitude higher than any previously reported results. Gain recovery effect has been also considered and analyzed for the impact on performance of the photonic integrator, followed by the simulation results of the impact of gain recovery.

Discretely tunable comb spacing of a frequency comb by multilevel phase modulation of a periodic pulse train

We experimentally demonstrate tunable comb spacing of an original 10-GHz periodic frequency comb by spectral Talbot effect over an unprecedented range of even and odd comb spacing division factors, from 2 to 9. The implementation has been achieved by periodic electro-optic (EO) temporal phase modulation of the original comb (conventional mode-locked optical pulse train) with multilevel modulation functions, produced by an arbitrary waveform generator (AWG). These comb spacing division processes have been observed through the use of a high-resolution (20-MHz) optical spectrum analyzer. Comb spacing tuning is achieved without essentially affecting the spectral bandwidth and total energy of the original comb signal. Our results also confirm that the spectral Talbot method does not require carrier-envelope phase stabilization in the input frequency comb. Numerical studies on the impact of deviations in the applied phase modulation functions confirm the robustness of the technique, in agreement with the experimental results.