Hydrogen loading for fiber grating writing with a femtosecond laser and a phase mask

The threshold for the fabrication of fiber Bragg gratings with ultrafast 800-nm radiation and a phase mask was studied in SMF-28 and all-silica core fiber by use of 125-fs pulses. High-pressure molecular hydrogen loading (H2 loading) was observed to significantly lower the grating writing threshold in standard Ge-doped telecommunication fiber. No reduction was observed with all-silica core fiber. The index change appeared to be confined to the Ge-doped core region of the fiber. Gratings in H2-loaded SMF-28 had thermal annealing behavior similar to UV-induced gratings. Unlike UV-induced H2-loaded gratings, no absorption associated with Ge-OH defect formation was observed.     

Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask

We compare the cladding patterns present in grating structures fabricated with an ultrafast laser and a phase mask with a cw beam interference model. We find that the observed patterns agree well with the model results for picosecond pulses; however, for femtosecond pulses, we show that the full bandwidth and the pulsed nature of the sources must be considered because the pattern can be affected by group-velocity walk-off. An interesting consequence of order walk-off is the possibility of pure two-beam interference generation with a phase mask in the femtosecond pulse regime.     

Phase-only sampled 45 channel fiber Bragg grating written with a diffraction-compensated phase mask

A novel phase-only sampled 45 channel fiber Bragg grating (channel spacing, 100 GHz) with high interchannel and intrachannel reflection uniformity and precise dispersion matching capability to an 80 km SMF-28 fiber is demonstrated. This grating is fabricated by the side-writing technique, in which phase information of continuous phase-only sampling is completely incorporated into a diffraction-compensated phase mask.     

Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique

We report on the fabrication of highly efficient fiber Bragg gratings (FBG) in non-photosensitive fibers based on nonlinear absorption of fs laser light. Up to 40 mm long gratings with a transmission of T=-25 dB at the Bragg reflection wavelength were obtained and their coupling constant determined by spectral analysis. Therefore, a phase mask scanning technique with appropriate control of the focus was established. PACS  42.60.-v; 42.62.-b; 42.65.Re; 42.79.Dj; 42.81.-i     

Bragg grating recording in low-defect optical fibers using ultraviolet femtosecond radiation and a double-phase mask interferometer

The fabrication of Bragg reflectors in fluorine depressed cladding silicate glass and SMF-28 fibers by employing a double-phase mask interferometer and 248 nm, 500 fs laser radiation is demonstrated here. The maximum refractive index changes obtained were of the order of 6 x 10(-4) for pulse intensities of 220 GW/cm(2) and accumulated energy densities of 3.5 kJ/cm(2). The Bragg gratings fabricated in the F-doped fiber endured temperatures greater than 700 degrees C, while those inscribed in the standard telecom fiber demarcated at 900 degrees C. The experimental results presented depict that the combination of the two phase mask interferometer and the 248 nm photon at sub-TW/cm(2) intensities constitute an efficient route in the fabrication of Bragg gratings in low-defect silicate glass optical fibers.     

Development of an optical fiber hydrophone with fiber Bragg grating

A new type of optical fiber hydrophone is constructed with a fiber Bragg grating (FBG) based on the intensity modulation of laser light in an FBG under the influence of sound pressure. The FBG hydrophone shows linearity, with dynamic range about 70 dB. It can measure amplitude and phase of an acoustic field in real time, and operates in a wide range of acoustic frequency, at least from 1 kHz to 3 MHz. No signal distortion is observed in the detected signal. Because of the simplicity in its operating principle and geometry, an FBG hydrophone is expected to be an acoustic sensor of high practicality compared to a conventional optical fiber hydrophone.     

Fabrication and enhanced optical absorption of submicrometer pits array on 6H-SiC via two-beam interference of femtosecond laser

We report the fabrication of submicrometer pits array (SP-array) on 6H-SiC surface by the interference of two femtosecond laser beams. Formation mechanisms and optical absorption of SP-array are studied. The relative reflectivity and transmissivity of white light decrease to 10% of the values of SiC crystal, and the optical absorption is enhanced to 97%. The relative reflectivity and transmissivity of incident angles within the range of 20°-60° are kept below 25%. The enhancement mechanism of optical absorption of the SP-array is also discussed.     

Generation of millimeter-wave in optical pulse carrier by using an apodized Moiré fiber grating

A novel scheme is proposed to transform a Gaussian pulse to a millimeter-wave frequency modulation pulse by using an apodized Moiré fiber Bragg grating in radio-over-fiber system. The relation between the input and output pulses is analyzed theoretically by Fourier transformation method and the requirements for the proposed fiber grating are presented. An apodized Moiré fiber Bragg grating is designed and its characteristics are studied. It is shown that the proposed device is feasible, and the new scheme is believed to be an effective solution for the generation of millimeter-wave sub-carrier in future radio-over-fiber systems.     

Generation of optical domain-wall structures from modulational instability in a bimodal fiber

We study experimentally modulational instability in a normally dispersive bimodal fiber under modal group-velocity-matching conditions. In the strong pump depletion regime, higher order sideband harmonics detected in the output spectra as well as autocorrelation measurements reveal the formation of subpicosecond domain-wall structures. Across these temporal structures the electromagnetic field distribution switches abruptly between the two transverse modes of the fiber. These structures are reminiscent of the so-called domain-wall soliton. Our results constitute therefore an experimental indication of the existence of this fundamental soliton.     

Bragg modulational instability induced by a dynamic grating in an optical fiber

By means of a dynamic Bragg grating obtained through cross-phase modulation with a beating wave in a highly birefringent fiber, we perform a detailed experimental study of Bragg modulational instability.     

Fiber bragg gratings made with a phase mask and 800-nm femtosecond radiation

High-quality retroreflecting fiber Bragg gratings were written in standard Ge-doped telecom fiber (Corning SMF-28) after a few minutes exposure with pulsed 800-nm, 120-fs laser radiation by use of a deep-etch silica zero-order nulled phase mask optimized for 800 nm. Induced index modulations of 1.9 x 10(-3) were achieved with peak power intensities of 1.2 x 10(13) W/cm2 without any fiber sensitization. The fiber gratings are stable and did not erase after 2 weeks at 300 degrees C. The primary mechanism of induced index change results from a structural modification to the fiber core.     

Tunable writing Tm-doped intracore fiber Bragg gratings using 800 nm femtosecond laser and phase mask

The intracore fiber Bragg gratings (FBGs) were successfully written into the non-photosensitive multi-mode Tm-doped fiber core by the 800 nm fs laser and a phase mask. Using the varied focal length lens, the tunable writing was realized, and the tunable wavelength band was over 84 nm. A geometrical optics imaging model and numerical calculation were described to explain the tunable writing mechanism. The interference modulation intensity characteristics were also theoretically investigated considering the beam incident angle variation. Based on the intracore FBGs, the all-fiber Tm-doped fiber laser had a maximal output power of 15.8 W with a slope efficiency of 30% and threshold of 5.1 W.     

Generation of near-field hexagonal array illumination with a phase grating

A hexagonal array not only is a nature-preferred pattern but also is widely used in optoelectronical materials and devices. We report a simple method of hexagonal array illumination based on the Talbot effect that has a theoretical efficiency of 100%. An experimental efficiency of 90.6% with a binary phase (0, pi) hexagonal grating is given. This method should be highly interesting for applications of hexagonal array illumination in optical devices as well as in other hexagonal cells.     

Single-step writing of Bragg grating waveguides in fused silica with an externally modulated femtosecond fiber laser

For the first time to our knowledge, high-strength (>30 dB) first-order Bragg grating waveguides were fabricated in bulk fused silica glass in a single-scanning step by modulating a high-repetition-rate femtosecond fiber laser with an external acousto-optic modulator. The modulation induced a waveguide segmentation by delivering controlled bursts of laser pulses to define an array of partially overlapped refractive index voxels. With appropriate choice of modulation frequency and sample scanning speed, low loss waveguides could be formed at high writing speeds to yield sharp Bragg spectral resonances tunable over the 1300 to 1550 nm telecom band. Effects of acousto-optic modulation duty cycle on propagation loss and grating strength are characterized. This modulation method offers facile control and integration of multiwavelength Bragg grating devices to enhance overall functionality of optical circuits in three-dimensional geometries.     

Spectral broadening of femtosecond pulses in an nonlinear optical fiber amplifier

A promising method for the generation of a supercontinuum with a high spectral power density based on the spectral broadening of ultrashort pulses in a fiber amplifier is considered. The advantage of the method, as compared to the conventional way of the supercontinuum generation in a microstructure fiber, is a lower pulse spectral broadening rate, which allows one to achieve higher SC spectral power densities. The initial stage of the supercontinuum generation in an ytterbium fiber amplifier (a fiber core diameter of 7 μm) with side pumping from an array of laser diodes with a total power of 8 W at a wavelength of 976 nm is experimentally studied. Yb:KYW laser pulses with a duration of 250 fs, a central wavelength of 1046 nm, and an average power of 150 mW have been supplied to the input of the amplifier. In this case spectrally broadened radiation with an average spectral power density of higher than 65 mW/nm and a spectrum width of 50 nm has been obtained.     

用相位掩模法制作光纤光栅的技术

准分子激光照射相位掩模后,在近场形成周期的条纹分布,利用近场光强对光敏光纤进行曝光,可以在纤芯中写人光栅。对光纤光栅折射率分布进行合理假设,运用耦合模理论计算了光纤光栅的反射率,对其反射率与相关参数的关系进行了讨论,并且分析了用相位掩模技术制造光纤光栅的影响因素,得出要获得窄带宽高反射率光栅需有适当的折射率调制和大的光纤光栅长度的结论。另外还导出了掩模板后表面的近场光强分布和光纤光栅的周期公式。     

High-energy femtosecond stretched-pulse fiber laser with a nonlinear optical loop mirror

A stretched-pulse fiber laser with a nonlinear optical loop mirror (NOLM) that produces 100-fs pulses with 1-nJ energy is demonstrated. These results constitute a 30-fold increase in pulse energy over previously reported femtosecond fiber lasers with a NOLM. Compared with previous stretched-pulse lasers, this laser offers a cleaner spectrum and improved stability, with comparable pulse duration and energy. Implications for the construction of truly environmentally stable lasers are discussed.     

Soliton formation in the propagation of phase-modulated femtosecond pulses through an optical fiber with a cubic nonlinearity

A way of soliton self-formation upon propagation of femtosecond light pulses through an optical fiber with a cubic nonlinearity is described with allowance for the dispersion of the nonlinear response of the medium. For soliton formation to occur, a low-frequency phase modulation of the initial pulse is necessary. Several solitons formed in this way differ in both maximal intensities and group velocities. The duration of an individual soliton may be several (up to ten) times smaller than the initial duration of the input pulse.     

Demonstration of an all-optical switching operation using an optical fiber grating coupler

An optical fiber grating coupler (FGC) is a fused optical fiber coupler with a tapered region in which refractive index-modulated gratings are written. In the FGC, the light with specific wavelength satisfying the Bragg condition of the grating can be dropped to one output port and other lights are transmitted to another output port when lights with various wavelengths are launched into the input port. The FGC can operate as an all-optical switch by controlling the Bragg wavelength of the grating using a third order nonlinear optical effect caused by a control light that are launched with a signal light. In this paper, an all-optical switching operation due to a third order nonlinear optical effect in an FGC is first demonstrated for a signal light with 1.55 μm-wavelength to be changed from one port of the FGC to another one by the 720 W peak of a control light from a Nd:YAG laser with 1.06 μm-wavelength. The switching efficiency obtained was 7%. It was clarified that a longer pulse length of the control light compared to the grating length is required to obtain a large Bragg wavelength shift for the switching. It was also clarified that the Bragg wavelength shift is caused by a third order nonlinear effect and a photothermal effect. A contribution of the photothermal effect was estimated. We also estimated the switching efficiency for pump power in the FGC switch.