Alejandro Martinez-Rios

Optical microfiber mode interferometer for temperature-independent refractometric sensing.

We report on a functional optical microfiber mode interferometer and its applications for absolute, temperature-insensitive refractive index sensing. A standard optical fiber was tapered down to 10 μm. The central part of the taper, i.e., the microfiber, is connected to the untapered regions with two identical abrupt transitions. The transmission spectrum of our device exhibited a sinusoidal pattern due to the beating between modes. In our interferometer the period of the pattern-an absolute parameter-depends strongly on the surrounding refractive index but it is insensitive to temperature changes. The period, hence the external index, can be accurately measured by taking the fast Fourier transform (FFT) of the detected interference pattern. The measuring refractive index range of the device here proposed goes from 1.33 to 1.428 and the maximum resolution is on the order of 3.7×10(-6).

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.

Compact fiber-optic curvature sensor based on super-mode interference in a seven-core fiber

A compact, low loss, and highly sensitive optical fiber curvature sensor is presented. The device consists of a few-millimeter-long piece of seven-core fiber spliced between two single-mode fibers. When the optical fiber device is kept straight, a pronounced interference pattern appears in the transmission spectrum. However, when the device is bent, a spectral shift of the interference pattern is produced, and the visibility of the interference notches changes. This allows for using either visibility or spectral shift for sensor interrogation. The dynamic range of the device can be tailored through the proper selection of the length of the seven-core fiber. The effects of temperature and refractive index of the external medium on the response of the curvature sensor are also discussed. Linear sensitivity of about 3000  nm/mm(-1) for bending was observed experimentally.

Analysis of a Q-switched ytterbium-doped double-clad fiber laser with simultaneous mode locking

The characteristics of a Q-switched ytterbium-doped double-clad (YDDC) fiber laser are presented. Due to the self-mode-locking effect in the Q-switching process, the Q-switched pulses split up. Their initiation and evolution are experimentally analyzed, and the mechanism is detailed. Based on the proposed mechanism, an effective model is developed to simulate this Q-switched YDDC fiber laser and to reconstruct simultaneously Q-switched and mode-locked pulses. The simulated results agree well with the experimental results.

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.

Tailoring Mach–Zehnder Comb-Filters Based on Concatenated Tapers

An all-fiber comb-filter using a Mach-Zehnder interferometer based on concatenated tapers is presented. The central wavelength, the amplitude, the period, and the spectral width of the transmission interference pattern can be tailored by varying the diameter and length of the waist, the length of the up- and down-transition of the fiber tapers, and the separation between the two tapers. Depending on the geometrical parameters of the tapers, the central position and span of the spectral interference pattern were tailored in the range of 1200-1650 and 50-300 nm, respectively. By varying the interferometer length, the fringe period and bandwidth were tailored between 3-36 and 2.5-11 nm, respectively. The physical length of the devices fabricated ranges from 15 to 30 mm. The proposed device is simple and suitable for applications in optical communications systems, fiber lasers, and optical fiber sensors for the monitoring of strain and displacement.

Efficient operation of double-clad Yb3+-doped fiber lasers with a novel circular cladding geometry.

A highly efficient double-clad Yb3+-doped fiber laser with a novel cladding geometry is described. A round double-clad fiber with a small D-shaped hole for breaking a circular symmetry in a cladding combines the advantages of ease of manufacture and handling round fibers with efficient absorption of pump light. Fiber lasers with a double-D cladding shape and a D-hole cladding are compared. We report what are to our knowledge the highest slope efficiencies of 73% and 69%, respectively, pumping at 915 nm. Output powers in excess of 13 W are demonstrated.

Three wavelength Raman fiber laser with reliable dynamic control

We present a novel concept of multiwavelength Raman fiber laser based on a chain of Raman cavities. Developed Raman laser has efficient power control of all generated wavelengths in a large dynamic range.

Long-Range Interferometric Displacement Sensing With Tapered Optical Fiber Tips

A Fabry-Perot interferometer built with a properly selected tapered optical fiber tip and a flat reflecting target is proposed for long range displacement sensing. By scaling down the diameter of the core and cladding of the optical fiber that forms the interferometer, the divergence of the output beam is reduced. The tapered fiber tip also couples the reflected light from the target more efficiently. As a result, well-defined interference patterns are observed even for long cavities which allows to sense displacements as long as 80 mm. The fabrication of the fiber tips is simple and reproducible and the interrogation of the interference patterns is fast as a conventional fiber Bragg grating interrogator can be used. The possibility of multiaxis displacement sensing is discussed.