Jose L. Cruz

Photonic microwave tunable single-bandpass filter based on a Mach-Zehnder interferometer

The authors present the theoretical analysis and experimental demonstration of a novel single-bandpass tunable microwave filter. The filter is based on a broadband optical source and a fiber Mach-Zehnder interferometer and shows a high Q factor over a tuning range of 5-17 GHz. A generalized analysis considering that the optical signal propagates along optical delay lines with a dispersion slope different from zero is presented.

Variable delay line for phased-array antenna based on a chirped fiber grating

We present a theoretical and experimental analysis of the performance of phased-array antennas steered by a single chirped fiber grating. Two approaches consisting of conventional and single-sideband (SSB) modulation techniques of the optical signal are presented in order to compare their performance and suitability for beamforming applications in microwave antennas. By using a 40-cm-long chirped grating, we measure the phase and amplitude response and calculate the corresponding radiation patterns to demonstrate wide-band operation and continuous spatial scanning properties of both configurations. SSB modulation Is presented as a real alternative to the first one offering broader operation band (4-18 GHz) for a given chirped grating and being less demanding on the fiber grating characteristics.

Ultrahigh birefringent nonlinear microstructured fiber

In this letter, we present the fabrication of a microstructured fiber with high birefringence suitable for nonlinear applications. The fiber has been fabricated adapting the stack and draw method for microstructured fibers. The birefringence of the fiber is achieved inducing ellipticity to the air-hole array by controlling the lateral tension between preform and jacketing tube. Values of the group index birefringence higher than 7/spl times/10/sup -3/ are demonstrated.

In-line highly sensitive hydrogen sensor based on palladium-coated single-mode tapered fibers

We report a fiber-optic sensor for detecting low concentrations of hydrogen. The sensor is based on the absorption change of the evanescent fields in a Pd-coated, single-mode tapered fiber. We fabricated a collection of polarization-sensitive and polarization-insensitive devices and we studied the effect of the different parameters on the sensitivity of the device. We found that the sensors response is wavelength dependent and it can be adjusted with the taper diameter. Dramatic transmission changes were observed when the sensors were exposed to hydrogen concentrations below 4%. We complement the experimental results with theoretical calculations of the attenuation constant of the fundamental fiber-mode.

Dual-Wavelength DFB Erbium-Doped Fiber Laser With Tunable Wavelength Spacing

A novel tunable dual-wavelength distributed-feedback fiber laser is presented. It is based on the optical resonances induced by two local phase shifts introduced in the periodic structure of an erbium-doped fiber Bragg grating. A dynamic control of the phase shifts, which are generated by piezoelectric transducers, permits the tunability of the wavelength spacing between the optical harmonics of the laser signal. The wavelength spacing is continuously tuned from 0.128 to 7 GHz. Moreover, a microwave carrier is created within such frequency tuning range by the heterodyne photodetection of the dual-wavelength laser signal.

Analysis of a microwave time delay line based on a perturbed uniform fiber Bragg grating operating at constant wavelength

This paper presents the modeling of a new microwave time delay line based on a uniform fiber Bragg grating (FBG) with a section which period has been perturbed. The time delay can be modified by changing the position of the perturbation along the grating. Theoretical results are presented for a 5-cm long uniform grating and optical signal is modulated up to 18 GHz. Experimental setup used magnetic fields to change the period locally along the grating and a 330 ps maximum time delay is shown. Comparison between calculations and measurements show a perfect agreement. Such a delay line offers many advantages for beamforming applications in phased array antennas where the operation at a fixed wavelength is required in order to simplify the architecture of these systems.

Simple fiber optic device to interrogate fiber optic Bragg gratings used as sensors

A simple and compact device for the monitoring and diagnostic of fiber optic Bragg gratings, employed in optical sensor applications, was developed. The deice consist of an optical fiber Bragg grating bound to a piezoelectric element. When a sinusoidal voltage is applied to the piezoelectric, the Bragg wavelength of the grating changes and the transmission spectrum swings in time. The device acts as a temporal window and can be used to interrogate a Bragg grating sensor, permitting to monitor the sensor state in real time and with very good resolution.

Hybrid surface plasma modes in circular metal-coated tapered fibers

The theory of hybrid surface plasma modes in metal-coated dielectric cylinders has been developed in recent years. We demonstrate that tapered fibers with a uniform waist and a circular metal coating can be designed for an efficient excitation of the fundamental hybrid surface plasma mode. Our experimental results are in good agreement with the theory and give the basis for the development of a novel type of all-fiber polarization-independent refractive-index sensor and tunable broadband wavelength filter.

Simple high-resolution wavelength monitor based on a fiber Bragg grating.

A compact and low-cost device for monitoring the peak wavelength of single-peak spectral distributions is presented. The system is based on the transmission properties of a fiber Bragg grating when its period is modulated. Different types of optical signal, such as the emission of distributed-feedback lasers and the reflection of a broadband optical source produced by fiber gratings used in sensor systems, can be measured with this device. We demonstrate that a high wavelength resolution of micro 1 pm can be achieved and that our proposal can be used for real-time monitoring.

Tunable chirp in Bragg gratings written in tapered core fibers

In this paper we present a technique to control the chirp of a fiber grating capable to produce positive or negative chirp in a single grating. The device is based on a uniform period grating written in a tapered core fiber, this grating has a positive chirp that can be reduced and removed by subjecting the fiber to mechanical stress, after some stress level the grating chirps to negative values. A chirp range of ±4 nm with respect to a central wavelength of 1546 nm has been experimentally achieved.