Multimode Fiber Bragg Grating Research Articles

Measuring Bragg gratings in multimode optical fibers.

Fiber Bragg gratings (FBG) in multimode optical fibers provide a means for cost-effictive devices resulting in simplified and robust optic sensor systems. Parasitic mode effects in optical components of the entire measurement system strongly influence the measured multi-resonance reflection spectrum. Using a mode transfer matrix formalism we can describe these complex mode coupling effects in multimode optical systems in more detail. We demonstrate the accordance of the theory by two experiments. With this formalism it is possible to understand and optimize mode effects in multimode fiber optic systems.

Reflective Refractometer Based on a Thin-Core Fiber Tailored Multimode Fiber Bragg Grating

A temperature-independent reflective refractometer configuration based on a core-diameter-mismatch is proposed and experimentally demonstrated. The configuration consists of a short section of thin-core fiber (TCF) with a multimode fiber (MMF) tip inscribed with a fiber Bragg grating (FBG). The TCF excites cladding modes into the downstream MMF via the mismatch-core splicing interface, and the parts of the core and cladding modes are reflected back to the lead-in fiber core by the MFBG. The recoupling efficiency of the core and cladding modes is highly dependent on the surrounding refractive index (SRI) of the TCF and MMF. Experimental results show that some recoupled modes show a high sensitivity to SRI. Power-referenced and temperature-independent SRI measurement with improved sensitivity for resolutions with SRI values near 1.33 is achieved through the wavelengths of selective mode monitoring.

A highly efficient 57 pm Tm-doped fiber laser with two multimode fiber Bragg gratings

An LD-clad pumped Tm-doped fiber laser is built with two multimode double-clad fiber Bragg gratings (FBGs) to generate narrow linewidth output around 2 μm. The slope efficiency of this laser system is 43.1% with respect to the launched pump power, corresponding to a quantum efficiency of 109%, which means that at least 9% of the doped thulium ions take part in the two-for-one cross relaxation process. Exploiting two multimode FBGs and wrapping the fiber onto an aluminum spool, a single peak narrow linewidth spectrum is generated. A 3 dB linewidth of 57 pm centering at 1996.2 nm is obtained at an output of 3.6 W.

Power-compensated displacement sensing based on single mode-multimode fiber Bragg grating structure

In this paper, power-compensated displacement sensing is proposed and investigated experimentally based on single mode-multimode fiber Bragg grating (FBG) structure, which is fabricated by a single mode fiber and an FBG written on 105/125μm graded-index multimode fiber (MMF). Experimental results verify that the reflected peak power of multiple wavelengths in single mode-multimode fiber Bragg grating structure shows different response to displacement induced bending of transmitting multimode fiber as the result of multimode interference (MMI). By employing different bending responses between multiple wavelengths of multimode FBG, ratiometric detection based high sensitive displacement measurement can be achieved, which provides a simple and practical method for displacement sensing and meanwhile a potential solution for multi-parameter measurement.

CW single transverse mode all-fiber Tm3+-doped silica fiber laser

The CW 25.6 W output power with a slope efficiency of 30.6% respected to the pump power from a CW single transverse mode all-fiber Tm3+-doped Silica Fiber Laser is reported. The all-fiber laser is made up by progressively splicing the pigtail fiber, matched FBG fiber and Tm fiber. The reflective FBG and Tm3+-doped fiber end Fresnel reflection build up the laser resonance cavity. Due to the multi-mode FBG as the reflective mirror, the output laser spectrum is multi-peaks at high output power, but the spectrum width is less than 2 nm at 1.94 μm. We estimate the beam quality to be M 2 = 2.39, clearly indicating nearly diffraction-limited beam propagation.

Multi-Wavelength Fiber Laser Based on a High Finesse Fiber Ring Filter and Multimode FBG

A multi-wavelength fiber laser with a narrow bandwidth based on an erbium-doped fiber ring filter and multimode fiber Bragg grating (MM-FBG) has been proposed, a high finesse fiber ring filter are introduced .The fiber ring filter consists of two optical couplers and a section of weak pumped erbium-doped fiber (EDF）. Thanks to the delay of cavity length and the gain generated by the EDF, the filter has spectral response with a high finesse. The incorporation of the fiber ring filter leads to the suppression of undesirable mode and makes the output laser narrower bandwidth. With 980 nm LD pumped, at the room temperature, the space of the two wavelengths is 0.07nm, which improved the effect of the filter.

Analysis of multimode POF gratings in stress and strain sensing applications

Polymer fiber Bragg gratings (FBGs) are made using the modified sagnac system with a 355 nm pulsed laser from a photosensitive polymer optical fiber (POF) with external and core diameters of 290.6 and 21.0 μm, respectively. Multimodes are reflected based on the reflection spectra of the gratings. The reflectivity spectra are also studied when such multimode polymer FBGs are subjected to axial static stress and strain. The respective effects of stress and strain on the sensor are decoupled and analyzed independently. Experiments and regression show that the effect of stress and the relaxation of stress in multimode FBGs (MM FBGs) in POF during loading and unloading have a more evident non-linear effect than that of strain. These non-linear properties make FBGs attractive for mechanical sensing applications.

Wavelength switching of picosecond pulses generated from a self-seeded Fabry–Perot laser diode with a tilted fiber Bragg grating formed in a graded-index multimode fiber

We demonstrated the generation of wavelength-switchable picosecond pulses from a self-seeded Fabry-Perot laser diode that used a tilted fiber Bragg grating (FBG) formed in a graded-index multimode fiber as an external optical feedback element, where wavelength switching was achieved by controlling the modal distribution in the FBG. We measured the reflection spectra of multimode FBGs fabricated with different tilt angles and discussed the effects of the tilt angle on wavelength selection. By using a 20 mm long 1.65° tilted FBG and a fiber deformer to control the modal distribution in the FBG, we generated 2 GHz pulses with a wavelength switchable over 14 wavelengths at a spacing of ∼0.8 nm.

基于自注入式法布里—珀罗激光二极管与倾斜多模光纤光栅的多波长可调谐短脉冲激光器

We experimentally demonstrate the simultaneous generation of tunable multi-wavelength picosecond laser pulses using a self-seeding configuration that consists of a gain-switched Fabry-Perot laser diode (FPLD) with an external cavity formed by a tilted multimode fiber Bragg grating. Dual- and triple-wavelength pulses are obtained and tuned in a flexible manner by changing the temperature of the FPLD. The side mode suppression ratio larger than 25 dB is achieved at different dual- and triple-wavelengths and the typical pulsewidth of the output pulses is ~70 ps. In the experiment, the wavelength separation can be narrowed to 0.57 nm.

Top-hat beam Tm3+-doped fiber laser using an intracavity abrupt taper

The top-hat beam clad-pumped Tm3+-doped fiber laser was realized simply using an intracavity multi-mode abrupt taper. The ratio of the flat-top diameter to the spot diameter reaches 53%, with a small intensity variation less than 6%, and the top-hat beam’s half-divergence angle is only 5.3°. The fiber laser has a maximal output power of 5 W with slope efficiency of 39.7%, pumped by the 792 nm diode laser (LD). The abrupt taper is directly made on the multi-mode double-clad Tm3+-doped fiber near the fiber laser output end with the 0.45 ratio of taper waist diameter to fiber clad diameter, and this fiber end 4% Fresnel reflection is used to be the output coupler. The fiber laser’s high reflective coupler is an intracore multi-mode FBG, which is directly written into the multi-mode Tm3+-doped fiber core using femtosecond laser and phase mask, at the other fiber end. The abrupt taper has no obviously influence on the fiber laser output power, and the output laser spectrum.

Nonpigtail optical coupling to embedded fiber Bragg grating sensors

In recent decades, optical fiber has proven useful for many sensor applications. Specifically, fiber Bragg grating (FBG) sensors have shown great utility for integrity management and environmental sensing of composite structures. One major drawback of FBG sensors, however, is the lack of a robust, nonpigtail technique for coupling to the embedded FBG sensor. In this paper, a novel method of free-space passive coupling of light into FBG sensors is described. An angled 45-deg mirror integrated directly into the fiber was used as an input coupling technique. We investigated the application of this approach to both single- and multimode glass fibers containing FBGs. For multimode FBGs, we studied the grating's uniformity across the fiber diameter and its effect on normal free-space coupling. In single-mode investigations, a novel method of coupling to the sensor via splicing a multimode fiber to a single-mode FBG (SMFBG) was developed. Finally, free-space coupling to an embedded SMFBG was employed to measure the tensile strain. Excellent agreement was found between the FBG and conventional electrical resistance strain gauges. We conclude that this coupling method might eliminate the need for pigtailing by providing a more robust coupling method for FBG sensors.

Ld-Clad-Pumped All-Fiber Tm3+-Doped Silica Fiber Laser

The CW 39.4 W all-fiber LD-clad-pumped Tm3+-doped fiber laser output is reported with a slope efficiency of 34% in respect to the pump power. The all-fiber laser is made up by progressively splicing the pigtail fiber, matched FBG fiber and Tm-doped fiber. The reflective FBG and Tm-doped fiber end fresnel reflection build up the laser resonance cavity. Due to the multi-mode FBG as the reflective mirror, the output laser spectrum is multi-peaks at high power output, whereas the total spectrum width is less than 2nm at nearly 1.94 μm.

Multi-peak output Tm-doped silica fiber laser based on femtosecond-written multi-mode fiber Bragg grating

The all-fiber Tm-doped double-clad laser was reported, incorporating a phase mask scanning technology writing FBG directly into the Tm-doped multi-mode fiber core as cavity mirror, using 800 nm femtosecond laser sources. A fiber grating of 12 mm length was realized with a period of 1.35 μm. Pumped by the 793 nm pigtail fiber output LD, the continuous wave (CW) power could scale to 25.4 W at 1.96 μm with the slope efficiency of 38% respected to the LD output power. The output laser spectrum exhibited multi-peak, due to the multi-mode FBG reflective characteristic.

All-fiber Tm-doped double-clad fiber laser with multi-mode FBG as cavity

An all-fiber LD-clad-pumped Tm-doped fiber laser was reported, and the CW maximal output power reached 24 W at nearly 1.94 μm. The double-clad Tm-doped fiber had a demission of 25/250 μm with the core NA 0.1 and inner-clad NA 0.46. A matched passive multi-mode FBG acted as the front cavity. Cooling by the water, the 56% high slope efficiency was achieved and threshold was 6.4 W, respected to the launched pump power. At the low power pump, the fiber laser spectrum had only one peak at 1.936 μm. Increasing the launched pump power, the output laser wavelength grew to 3–4 peaks. Because the multi-mode FBG reflectivity was not very high, both ends of the fiber laser had laser output power, and the ratio was nearly 10:1.

Wavelength switchable multiwavelength actively mode-locked fiber-ring laser based on highly nonlinear photonic crystal fiber and multimode fiber Bragg grating

A wavelength switchable multiwavelength actively mode-locked fiber-ring laser based on highly nonlinear photonic crystal fiber (PCF) and polarization-dependent mode coupling to multimode-fiber Bragg grating (MMFBG) is proposed. Twenty meters of highly nonlinear dispersion-shifted PCF (DS-PCF) is inserted into the fiber ring cavity to suppress gain competition through the four-wave-mixing (FWM) effect. A mode scrambler is attached to the multimode fiber (MMF) to introduce a polarization-dependent mode coupling to the MMFBG. Through changing polarization in the cavity, stable and switchable one-wavelength, two-wavelength (wavelength spacing of 0.8 and 2.4 nm), and three-wavelength simultaneously actively mode-locked optical pulse trains at a repetition rate of 5 GHz are obtained. We show that high nonlinearity introduced by the nonlinear PCF in the cavity can improve significantly supermode suppression ratio (SMSR) of the multiwavelength pulses generated and a sidemode suppression ratio (SMSR) of 58 dB is realized.

LD-cladding-pumped 50 pm linewidth Tm^3+-doped silica fiber laser

We report on a Tm(3+)-doped fiber laser source operating at 1936.4 nm with a very narrow linewidth (50 pm) laser output. Up to 2.4 W cw laser power was obtained from an 82 cm long Tm(3+)-doped multimode-core fiber cladding pumped by a 792 nm laser diode (LD). The fiber laser cavity included a high-reflective dichroic and a low-reflective FBG output coupler. The multimode fiber Bragg grating (FBG) transmission spectrum and output laser spectrum were measured. By adjusting the distance between the dichroic and the Tm(3+)-doped fiber end, the multipeak laser spectrum changed to a single-peak laser spectrum.

Multimode Fiber Bragg Grating Wavelength Filter in a 10-Gb/s System

A 10-Gb/s transmission experiment has been performed on multishell multimode fiber Bragg gratings. The device can simultaneously serve as wavelength filter and mode selector and is compatible with conventional graded refractive index multimode fiber (MMF) components. This could be a promising technology for wavelength-division multiplexing and mode diversity in practical MMF-based optical access networks.

All-Fiber Full-Duplex Multimode Wavelength-Division-Multiplexing Network for Radio-Over-Multimode-Fiber Distribution of Broadband Wireless Services

We present a full-duplex multimode wavelength-division-multiplexing ring network for the distribution of broadband wireless services. It successfully combines the use of all-fiber wavelength selecting multimode add/drop multiplexing nodes with a radio-over-multimode-fiber technique that is robust against modal dispersion impairments in multimode fiber links, and allows a reliable delivery of wireless services beyond 5 GHz. The multimode add/drop multiplexing nodes are built with commercially available narrowband multimode fiber Bragg gratings. The feasibility of the proposal is experimentally investigated with the distribution of 120-Mb/s 64 quadrature amplitude modulation radio signals at 24 GHz over a 4.4-km-long multimode fiber ring with multimode add/drop multiplexing nodes, achieving error vector magnitude values below 4.5%.

Switchable multiwavelength ytterbium-doped double-clad fiber laser based on a multimode fiber grating

A simple switchable multiwavelength double-clad fiber laser using a multimode fiber Bragg grating (MMFG) as a wavelength selective component is demonstrated. The MMFG is fabricated directly in the active double-clad fiber, resulting in low splicing loss and compact configuration. By properly adjusting the polarization controller in the cavity, the double-clad fiber laser can stably operate at different multiwavelength states at room temperature. At lower pump levels, the laser can be switched between single- and dual-wavelength operations, while at higher pump levels the laser can switch between dual- and triple-wavelength operations. The slope efficiency of the system is 41% with the maximum output power of 6.2 W.

Reconfigurable multiwavelength erbium‐doped fiber laser using two multimode fiber Bragg gratings

AbstractA reconfigurable multiwavelength erbium‐doped fiber laser, using two multimode fiber Bragg gratings (MFBGs), is presented. Two MFBGs in L‐band are cascaded to form two overlapping resonant cavities. Various configurations of four‐, three‐, or two‐wavelength lasing are demonstrated experimentally by appropriately controlling the two polarization controllers inside the cavities. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1509–1511, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22493