María R. Fernández-Ruiz

Time-domain holograms for generation and processing of temporal complex information by intensity-only modulation processes.

The time-domain counterpart of traditional spatial holography is formalized and experimentally demonstrated. This concept involves the recording, generation and/or processing of complex (amplitude and phase) optical time-domain signals using intensity-only temporal detection and/or modulation optical devices. The resulting procedures greatly simplify present approaches aimed to similar generation and processing tasks. As a proof-of-concept, we successfully demonstrate a time-domain computer holography scheme. This scheme is used for experimental generation of user-defined complex optical temporal signals, in particular, a sequence of arbitrarily chirped Gaussian-like optical pulses and complex-modulation (16-QAM) optical telecommunication data streams, by CW-light intensity-only modulation.

Design of Ultrafast All-Optical Signal Processing Devices Based on Fiber Bragg Gratings in Transmission

A general and practical approach for designing ultra-fast all-optical (all-fiber) signal processing devices based on chirped fiber Bragg gratings (C-FBGs) working in transmission is presented. The approach can be used to design any minimum-phase linear optical device, e.g., first and high-order all-optical time differentiators and integrators and a large variety of optical pulse shapers, significantly overcoming the bandwidth limitations of previous FBG-based designs. Processing speeds in the THz range, corresponding to sub-picosecond time features, can be achieved using readily feasible grating apodization profiles. The approach is successfully proved through the design of two relevant functionalities, namely a THz-bandwidth optical differentiator and a 1-ps flat-top optical pulse shaper.

SNR enhancement in high-resolution phase-sensitive OTDR systems using chirped pulse amplification concepts

Phase-sensitive optical time-domain reflectometry (φOTDR) is widely used for the distributed detection of mechanical or environmental variations with resolutions of typically a few meters. The spatial resolution of these distributed sensors is related to the temporal width of the input probe pulses. However, the input pulse width cannot be arbitrarily reduced (to improve the resolution), since a minimum pulse energy is required to achieve a good level of signal-to-noise ratio (SNR), and the pulse peak power is limited by the advent of nonlinear effects. In this Letter, inspired by chirped pulse amplification concepts, we present a novel technique that allows us to increase the SNR by several orders of magnitude in φOTDR-based sensors while reaching spatial resolutions in the centimeter range. In particular, we report an SNR increase of 20 dB over the traditional architecture, which is able to detect strain events with a spatial resolution of 1.8 cm.

Phase-sensitive OTDR probe pulse shapes robust against modulation-instability fading

Typical phase-sensitive optical time-domain reflectometry (ϕOTDR) schemes rely on the use of coherent rectangular-shaped probe pulses. In these systems, there is a trade-off between the signal-to-noise ratio (SNR), spatial resolution, and operating range of the ϕOTDR system. To increase any of these parameters, an increase in the pulse peak power is usually indispensable. However, as it is well known, there is a limit in the allowable increase in probe power due to the onset of undesired nonlinear effects such as modulation instability. In this Letter, we perform an analysis of the effect of the probe pulse shape on the visibility fading due to modulation instability. In particular, four different temporal profiles are chosen: rectangular, Gaussian, triangular, and super-Gaussian (order 2). Our numerical and experimental analyses reveal that the use of triangular or Gaussian-like pulses can significantly inhibit the visibility fading issues. As such, an increase in the range up to twofold for the same pulse energy (i.e., SNR) and nominal spatial resolution can be achieved, as compared with the results obtained when using rectangular pulses. This is due to a more robust behavior of the Gaussian and triangular pulses against the Fermi-Pasta-Ulam recurrence occurring in modulation instability.

Tunable, nondispersive optical filter using photonic Hilbert transformation

We propose and numerically demonstrate a new design concept for implementing nondispersive complementary (band-pass/band-reject) optical filters with a wide range of bandwidth tunability. The device consists of two photonic Hilbert transformers (PHTs) incorporated into a Michelson interferometer (MI). By controlling the central frequency of PHTs with respect to each other, both the central frequency and the spectral width of the rejection/pass bands of the filter are proved to be tunable. Bandwidth tuning from 260 MHz to 60 GHz is numerically demonstrated using two readily feasible fiber Bragg grating-based PHTs. The designed filter offers a high extinction ratio between the pass band and rejection band (>20  dB in the narrow-band filtering case) with a very sharp transition with a slope of 170  dB/GHz from rejection to pass band.

Arbitrary Time-Limited Optical Pulse Processors Based on Transmission Bragg Gratings

We propose a new, general approach to implement an arbitrary linear optical pulse processing operation, generally requiring a nonminimum phase spectral response, using a minimum-phase optical filter. This approach is valid for implementing any desired operation, as long as it can be limited over a prescribed time window, such as for arbitrary short pulse processors or reshapers. The proposed concept is particularly interesting for the design of optical pulse processors based on fiber/waveguide Bragg gratings (BGs) operating in transmission, enabling us to overcome the processing bandwidth limitations of their reflective counterparts. In particular, processing speeds in the THz range can be achieved using readily feasible grating apodization and chirp profiles. The approach is numerically demonstrated through the design of a relevant nonminimum-phase linear pulse processor, a real-time photonic Hilbert transformer, with an unprecedented 1.5-THz bandwidth, based on a feasible linearly chirped fiber BG structure.

High-contrast linear optical pulse compression using a temporal hologram

Temporal holograms can be realized by temporal amplitude-only modulation devices and used for generation and processing of complex (amplitude and phase) time-domain signals. Based on the temporal hologram concept, we numerically and experimentally demonstrate a novel design for linear optical pulse compression using temporal modulation of continuous-wave light combined with dispersion. The newly introduced scheme overcomes the undesired background problem that is intrinsic to designs based on temporal zone plates, while also offering an energy efficiency of ~25%. This pulse compression scheme can ideally provide an arbitrarily high time-bandwidth product using a low peak-power modulation driving signal, though in practice it is limited by the achievable modulation bandwidth and dispersion amount.

All-optical wavelength conversion based on time-domain holography

All-optical wavelength conversion of a complex (amplitude and phase) optical signal is proposed based on an all-optical implementation of time-domain holography. The temporal holograms are generated through a cross-phase modulation (XPM) process in a highly-nonlinear optical fiber, avoiding the necessity of accomplish the phase matching condition between the involved pump and probe signals, and reducing the power requirements compared to those of the traditional wavelength conversion implementations using four wave mixing (FWM). The proposed scheme also achieves symmetric conversion efficiency for up- and down-conversion. As a proof-of-concept, wavelength conversion of a train of 10 GHz chirped Gaussian-like pulses and their conjugated is experimentally demonstrated.

Design of picosecond flat-top optical pulse generator using a linearly-chirped fiber Bragg grating in Transmission

A picosecond rectangular pulse-shaper based on a linearly-chirped fiber Bragg grating in transmission is presented. The design exploits the space-to-frequency mapping and the degree of freedom in the reflection spectral phase specifications.

Protecting fiber-optic links from third party intrusion using distributed acoustic sensors

A major cause of faults in optical communication links is related to unintentional third party intrusions (normally related to civil/agricultural works) causing fiber breaks or cable damage. Distributed acoustic sensors can be used to detect these threats to the fiber-optic infrastructure before they cause damage to the infrastructure and proactively re-route the traffic towards links were no threat is detected. In this talk we will review our recent progress on distributed acoustic sensing and will provide some key considerations for the deployment of these systems in connection with their use in the protection of optical networks.