Miguel V. Andrés

Designing the properties of dispersion-flattened photonic crystal fibers.

We present a systematic study of group-velocity-dispersion properties in photonic crystal fibers (PCFs). This analysis includes a thorough description of the dependence of the fiber geometrical dispersion on the structural parameters of a PCF. The interplay between material dispersion and geometrical dispersion allows us to established a well-defined procedure to design specific predetermined dispersion profiles. We focus on flattened, or even ultraflattened, dispersion behaviors both in the telecommunication window (around 1.55 microm) and in the Ti-Za laser wavelength range (around 0.8 microm}. We show the different possibilities of obtaining normal, anomalous, and zero dispersion curves in the above frequency domains and discuss the limits for the existence of the above dispersion profiles.

Full-vector analysis of a realistic photonic crystal fiber

We analyze the guiding problem in a realistic photonic crystal fiber, using a novel full-vector modal technique. This is a biorthogonal modal method based on the non-self-adjoint character of the electromagnetic propagation in a fiber. Dispersion curves of guided modes for different fiber structural paremeters are calculated, along with the two-dimensional transverse intensity distribution of the fundamental mode. Our results match those achieved in recent experiments in which the feasibility of this type of fiber was shown.

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.

Refractometric sensor based on whispering-gallery modes of thin capillarie.

Whispering-gallery modes resonances of submicron wall thickness capillaries exhibit very large wavelength shifts as a function of the refractive index of the medium that fills the inside. The sensitivity to refractive index changes is larger than in other optical microcavities as microspheres, microdisks and microrings. The outer surface where total internal reflection takes place remains always in air, enabling the measure of refractive index values higher than the refractive index of the capillary material. The fabrication of capillaries with submicron wall thickness has required the development of a specific technique. A refractometer with a response higher than 390 nm per refractive index unit is demonstrated. These sensors are readily compatible with microfluidic systems.

Q-switching of an all-fiber laser by acousto-optic modulation of a fiber Bragg grating

We report active Q-switching of an all-fiber laser using a Bragg grating based acousto-optic modulator. Q-switching is performed by modulating a fiber Bragg grating with an extensional acoustic wave. The acoustic wave modulates periodically the effective index profile of the FBG and changes its reflection features. This allows controlling the Q-factor of the cavity. Using 1 m of 300 ppm erbium-doped fiber and a maximum pump power of 180 mW, Q-switch pulses of 10 W of peak power and 82 ns wide were generated. The pulse repetition rate of the laser can be continuously varied from few Hz up to 62.5 kHz.

Fiber-optic 40-GHz mm-wave link with 2.5-Gb/s data transmission

A broad-band millimeter-wave (mm-wave) transmission link for downstream transmission is investigated. The mm-wave carrier is created by a dual-frequency optical source based on suppressed-carrier double-sideband modulation of a narrow linewidth laser. The optical sidebands generated in the modulation are divided into two separate fibers by filtering with fiber Bragg gratings. One of the waves is modulated with baseband data up to 2.5 Gb/s. The fibers are combined and connected with the base station (BS) via a single-mode fiber. At the BS, the mm-wave modulated signal with data is created with a wide bandwidth photodetector through heterodyne mixing of the two optical waves. This mm-wave signal is amplified and transmitted to the mobile unit, where the reception is performed using self-homodyne mixing, in order to ensure carrier frequency independence. Error-free data transmission was demonstrated for the downlink after 44 km of single-mode optical fiber.

Microwave-photonic frequency multiplication utilizing optical four-wave mixing and fiber Bragg gratings

A novel technique for optical multiplication of a millimeter-wave carrier is presented. It utilizes optical four-wave mixing (FWM) in a highly nonlinear fiber (HNLF) and the filtering properties of matched fiber Bragg gratings (FBGs). The technique includes a sixfold electrical frequency multiplication in the optical domain. In this experiment, the multiplicator is driven electronically at 6.67 GHz, and the created millimeter wave has a frequency of 40 GHz. The generated carrier has a linewidth lower than 3 Hz and a carrier to noise ratio exceeding 50 dB. Furthermore, successful data transmission over the optical fiber of 2.5 Gb/s on the generated millimeter-wave carrier was performed.

Vector description of higher-order modes in photonic crystal fibers

We extensively study the propagation features of higher-order modes in a photonic crystal fiber (PCF). Our analysis is based on a full-vector modal technique specially adapted to accurately describe light propagation in PCFs. Unlike conventional fibers, PCFs exhibit a somewhat unusual mechanism for the generation of higher-order modes. Accordingly, PCFs are characterized by the constancy of the number of modes below a wavelength threshold. An explicit verification of this property is given through a complete analysis of the dispersion relations of higher-order modes in terms of the structural parameters of this kind of fiber. The transverse irradiance distributions for some of these higher-order modes are also presented, showing an excellent agreement with recent experimental measurements. In the same way, the full-vector nature of our approach allows us to analyze the rich polarization structure of the PCF mode spectrum.

A magnetostrictive sensor interrogated by fiber gratings for DC-current and temperature discrimination

A magnetostrictive sensor head with temperature compensation has been developed for measurement of static magnetic fields. The device consists on two different alloys with similar thermal expansion coefficient one of which has a giant magnetostriction, the expansion of both materials produced by heat and magnetism is detected by two fiber gratings. One of the gratings measures the temperature of the sensor and the difference between the wavelengths reflected by the gratings is a measurement of the magnetically induced strain.

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.