I. Torres-Gómez

Compact optical fiber curvature sensor based on concatenating two tapers.

A low-loss, compact, and highly sensitive optical fiber curvature sensor is presented. The device consists of two identical low-loss fused fiber tapers in tandem separated by a distance L. When the optical fiber is kept straight and fixed, no interference pattern appears in the transmitted spectrum. However, when the device is bent, the symmetry of the straight taper is lost and the first taper couples light into the cladding modes. In the second taper, a fraction of the total light guided by the cladding modes will be coupled back to the fundamental mode, producing an interference pattern in the transmitted spectrum. As the fiber device is bent, visibility of the interference fringes grows, reaching values close to 1. The dynamic range of the device can be tailored by the proper selection of taper diameter and separation between tapers. The effects of temperature and refractive index of the external medium on the response of the curvature sensor is also discussed.

Wavelength tuning of fiber lasers using multimode interference effects

We report on a novel scheme to fabricate a simple, cheap, and compact tunable fiber laser. The tuning is realized by splicing a piece of single-mode fiber to one end of an active double-clad fiber, while the other end of the single-mode fiber is spliced to a 15 mm long section of 105/125 multimode fiber. The fluorescence signal entering into the multimode fiber will be reproduced as single images at periodic intervals along the propagation direction of the fiber. The length of the multimode fiber is chosen to be slightly shorter than the first re-imaging point, such that the signal coming out from the single mode fiber is obtained in free space, where a broadband mirror retroreflects the fluorescence signal. Since the position of the re-imaging point is wavelength dependent, different wavelengths will be imaged at different positions. Therefore, wavelength tuning is easily obtained by adjusting the distance between the broadband mirror and the multimode fiber facet end. Using this principle, the tunable fiber laser revealed a tunability of 8 nm, ranging from 1088-1097 nm, and an output power of 500 mW. The simplicity of the setup makes this a very cost-effective tunable fiber laser.

Ultra-widely tunable long-period holey-fiber grating by the use of mechanical pressure

We report an ultra-widely tunable long-period holey-fiber grating, which combines the wide-range single-mode behavior and transverse strain sensitivity of the holey fibers with the advantages of mechanically induced long-period fiber gratings. We obtain a versatile widely tunable long-period holey-fiber grating with attractive transmission spectral characteristics for optical communications, fiber-based amplifiers, and lasers. The mechanically induced long-period holey-fiber grating shows a continuous tuning range over 500 nm, more than 12 dB depth notches with less than 0.75 dB out-of-band losses, and bandwidth control from 10 to 40 nm.

Continuous-wave measurement of the fiber nonlinear refractive index

We propose a technique for measuring the nonlinear refractive index of fiber based on transmission-coefficient measurements in a fiber Sagnac interferometer. In contrast with traditional methods, the proposed method uses a single optical source operating in cw mode and direct intensity measurement, enabling one to avoid the errors caused by fiber dispersion and uncertainty of spectral peak difference measurements that occur with pulse-based methods. The nonlinear refractive index in 20-mol. % GeO(2) fiber was measured to be (3.1 +/- 0.2) x 10(-16) cm(2) W(-1) at 1064 nm.

A reconfigurable multimode interference splitter for sensing applications

A reconfigurable multimode interference (MMI) coupler is demonstrated. The device operates by modifying the phase of the multiple images that are formed around the midpoint of the MMI section. This modifies the properties of the following set of images, and light can be directed to a specific output waveguide, which is therefore ideal to develop a reconfigurable MMI coupler. In our device the phase change is achieved by current injection, and therefore minimizing current spreading is crucial for optimal operation. A zinc in-diffusion process has been implemented to selectively define p–n regions and effectively regulate current spreading by controlling the depth of the zinc doping. Using this process, a reconfigurable 3 dB MMI coupler has been fabricated. Our experimental results revealed that the device can be easily set to a perfect 3 dB splitter using only 0.7 mA of current injection. In addition, the device can be adjusted all the way from a 90:10 to a 30:70 splitting ratio. The results are very encouraging since, to our knowledge, this degree of tuning of the optical power has never been experimentally demonstrated in MMI devices. Furthermore, this concept can easily be applied to a wide variety of semiconductor photonic switches that operate on MMI effects.

An intrinsic fiber-optic single loop micro-displacement sensor.

A micro-displacement sensor consisting of a fiber-loop made with a tapered fiber is reported. The sensor operation is based on the interaction between the fundamental cladding mode propagating through the taper waist and higher order cladding modes excited when the taper is deformed to form a loop. As a result, a transmission spectrum with several notches is observed, where the notch wavelength resonances shift as a function of the loop diameter. The loop diameter is varied by the spatial displacement of one end of the fiber-loop attached to a linear translation stage. In a displacement range of 3.125 mm the maximum wavelength shift is 360.93 nm, with 0.116 nm/μm sensitivity. By using a 1,280 nm broadband low-power LED source and a single Ge-photodetector in a power transmission sensor setup, a sensitivity in the order of 2.7 nW/μm is obtained in ∼1 mm range. The proposed sensor is easy to implement and has a plenty of room to improve its performance.

Supercontinuum optimization for dual-soliton based light sources using genetic algorithms in a grid platform

We present a numerical strategy to design fiber based dual pulse light sources exhibiting two predefined spectral peaks in the anomalous group velocity dispersion regime. The frequency conversion is based on the soliton fission and soliton self-frequency shift occurring during supercontinuum generation. The optimization process is carried out by a genetic algorithm that provides the optimum input pulse parameters: wavelength, temporal width and peak power. This algorithm is implemented in a Grid platform in order to take advantage of distributed computing. These results are useful for optical coherence tomography applications where bell-shaped pulses located in the second near-infrared window are needed.

Linear cavity fiber laser with 100 nm wavelength tuning range

We report a 100 nm widely tunable double-clad Ytterbium doped fiber laser. The fiber laser has a linear cavity configuration, where the feedback is provided by a Sagnac fiber mirror on one side, and an external bulk grating on the other side. On the bulk grating side, the fiber has a balled curved-core-termination that suppress backreflection by roughly 50 dB, and does not affect the pump coupling efficiency. The tuning range goes from 1055 nm through 1155 nm, with a maximum output power of 360 mW at 1080 nm, and 0.3 nm linewidth.

Background loss minimization in arc-induced long-period fiber gratings

We demonstrate the minimization of background loss for arc-induced long-period fiber gratings in standard fiber by Taguchis optimization method. We use Taguchis method to determine the optimum values for parameters like electric-arc power, arc duration, and tensile strain applied over the fiber during the inscription process. With these optimal parameters, we minimize the background loss resulting from the geometrical deformations of the fiber. The experimental results show that background loss can be reduced from more than 1 dB to less than 0.3 dB at rejection bands with isolation >15 dB.

Determination of the Mueller matrix of UV-inscribed long-period fiber grating.

An explicit method for determination of the Mueller matrix elements of a commercial long-period fiber grating inscribed with ultraviolet CW laser irradiation (UV-LPFG) is presented. From the Mueller matrix obtained for such UV-LPFG, the full polarimetric response of the grating was found. Our polarimetric analysis was focused mainly on the polarization-dependent loss and other polarimetric properties, such as the polarizance, the depolarization index, and the diattenuation parameters. The full polarimetric analysis allows us to obtain more complete information than the usually reported ones, in which only two orthogonal linear polarizations are considered; for example, with our analysis, we prove that a small depolarization effect is inherent in UV-LPFG and that attenuation depends on the polarization state. This additional polarimetric information could be useful to control the output LPFG signal, for instance, for the realization of wavelength switchable or Q-switched fiber lasers, among other applications.