S. Torres-Peiró

Waveguiding properties of a photonic crystal fiber with a solid core surrounded by four large air holes

The polarization-dependent guiding properties of a hexagonal-lattice photonic crystal fiber with a solid-core surrounded by four large air holes are investigated. The appearance of a polarization dependent cutoff frequency, together with several parameters as the birefringence, the modal effective area, the group velocity dispersion and the polarization dependent loss are analyzed. A collection of fibers with different structural parameters were fabricated and characterized. An effective anti-guide structure from at least 450 nm to 1750 nm, a polarizing fiber with a polarization dependent loss of 16 dB/m at 1550 nm, and an endlessly singlemode polarization-maintaining fiber with group birefringence of 2.1x10(-3) at 1550 nm are reported. Experimental results are compared with accurate numerical modeling of the fibers.

Fundamental-mode cutoff in liquid-filled Y-shaped microstructured fibers with Ge-doped core

We report on the cutoff characteristics of a Ge-doped Y-shaped microstructured fiber in which the holes are filled with a liquid of refractive index higher than silica but lower than the Ge-doped core. It is found that the cutoff wavelength was very sensitive to temperature variations as a result of the refractive index changes of the liquid. The basic properties of such a fiber permit the fabrication of wideband tunable short-pass filters, as well as temperature sensors with high sensitivity. A temperature sensitivity of 25 nm/degrees C is reported.

Simultaneous Switching of the

We report a pulsed erbium-doped fiber laser with simultaneous switching of the laser cavity Q-value and generation wavelength. The laser generates a train of short pulses with adjacent pulses alternately being on one of two fixed wavelengths. This regime is realized at periodical modulation of physical length of overwritten fiber Bragg gratings composing the laser cavity

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The cutoff properties of the fundamental mode of liquid filled Ge-doped microstructured fiber can be exploited in a novel set of wavelength encoded sensor applications. A number of temperature, strain and vibration sensors have been implemented using a Ge-doped core Y-shaped microstructured fiber. The temperature sensitivity of the cutoff wavelength is 25 nm/°C, more than two orders of magnitude higher than the values reported for fiber Bragg gratings. Similarly, the strain sensitivity is 16.8 pm/μ∈, about one order of magnitude higher than a fiber Bragg grating. Amplitude interrogation of the sensor heads can be implemented in rather easy configurations using a light emitting diode or a laser diode, without requiring an accurate selection of the laser wavelength. This permits, for example, the development of vibration sensors with fast time response, at least better than 0.1 ms.

-Value and Operation Wavelength in an Erbium-Doped Fiber Laser

The fundamental mode cutoff properties of Ge-doped microstructured fibers, filled with a liquid, permit the implementation of wavelength- and amplitude-encoded temperature sensors with an ultra-high sensitivity. The cutoff wavelength changes with temperature, and the thermo-optic coefficient of the liquid determines the sensitivity of the sensor. Sensitivity as high as 25  is reported. In addition, simple amplitude interrogation techniques can be implemented using the same sensor heads.

Sensor Applications Based on the Cutoff Properties of Liquid-Filled Ge-Doped Microstructured Fibers

The stack and draw technique provides an enormous flexibility and permits to obtain rather complex photonic crystal fibers and other nanoweb structures. It is possible to fabricate Ge‐doped microstructured fibers combining silica capillaries and doped wires of silica glass. Here, we will present the fabrication and postprocessing of Ge‐doped Y‐shaped fibers. These structures may exhibit strong evanescent fields in the air holes, while the presence of a Ge‐doped core increases the versatility when designing new fiber‐optic components.

Temperature Sensor Based on Ge-Doped Microstructured Fibers

Several alternatives have been investigated to achieve enhanced supercontinuum generation in the nanosecond pump regime using specialty silica-based microstructured optical fibers (MOFs). Among these alternatives we can point out the use of (a) MOF tapers, (b) highly Ge-doped core Y-shaped MOF and (c) Er-doped MOF. These specialty MOFs provide either an increased material nonlinear response or a rather small effective area, at the same time that the dispersion characteristics are adjusted to work at different excitation conditions. In our experiments, we have been using either a Q-switched Nd:YAG laser (10 Hz repetition rate) or an Yb-doped fiber laser (2 kHz repetition rate), both emitting at 1064 nm.

Fabrication and Postprocessing of Ge‐Doped Nanoweb Fibers

Supercontinuum generation in a highly Ge-doped core Y-shaped microstructured optical fiber using long pump pulses of 9 ns duration at 1064 nm is reported. The generation of nonlinear effects takes advantage of the large nonlinear refractive index and Raman gain of the Ge-doped core, as well as the air hole structure that surrounds the core. The fiber is easy to fabricate due to its simple structure and shows good compatibility with standard fibers. Although the fiber was pumped in normal dispersion far from the zero dispersion wavelength, flat and smooth supercontinuum in the fundamental mode from 550 nm to beyond 1750 nm was generated with a value of fiber length and pump peak power product of 11.7 kW•m.

Enhanced supercontinuum generation in the nanosecond pump regime using specialty microstructured fibers

The guidance of the fundamental mode of microstructured fibers with a Ge-doped core can be cut off by filling the holes with a material, liquid in our case, that increases the refractive index in the air-holes up to a value between the refractive index values of the Ge-doped region and the silica. A section of such a liquid-filled fiber defines a short-pass filter widely tunable by adjusting slightly the refractive index values. Thus, a small change of temperature or a small strain of the fiber can be used to adjust the transmittance of the filter. Alternatively, temperature changes and strain could be determined by measuring the cutoff wavelength. A simple measurement of the power transmitted through a short section of liquid-filled fiber can be used to monitor temperature and strain, enabling the measurement of fast transients such as mechanical vibrations. Several Y-shaped fibers with a Ge-doped core were fabricated. This microstructure with only three big holes surrounding the core makes straightforward the filling of the fiber with liquids. The experimental characterization includes the measurement of the cutoff wavelength of a number of devices with different lengths, different microstructure geometries and different refractive index liquids. The cutoff wavelength and the transmission were measured as a function of temperature and strain. A simple theoretical simulation permits to explain the experimental results.

Ge-doped Y-shaped microstructured fiber for supercontinuum generation

We present experimental results on supercontinuum (SC) generation in Er-doped microstructured optical fibers (MOF) pumped with nanosecond pulses at 1064 nm. The Er doping increases the effective nonlinear refractive index [1]. As a result, nonlinear interactions are enhanced and broad supercontinuum spectra are generated with relatively low pump power. Supercontinuum spectra generated in two fibers with the zero dispersion wavelengths (ZDW) close to the pump wavelength are presented. The first MOF is an endlessly singlemode fiber with normal dispersion at the pump wavelength, while the second fiber is a large air-filling fraction MOF with anomalous dispersion at 1064 nm.