Femtosecond laser direct writing of multiwavelength Bragg grating waveguides in glass

Novel Bragg grating waveguide structures have been fabricated in bulk borosilicate glass through a type II photosensitivity mechanism driven by single femtosecond laser pulses. Low-loss single-mode waveguides and narrow-linewidth Bragg gratings were generated simultaneously by forming an array of refractive index voxels in a single laser scan. Laser pulse duration is shown to significantly affect the grating strength and waveguide loss. Bragg wavelengths, defined by the periodicity of laser-modified volumes, were fully controlled by the sample scan speed to provide resonances anywhere in the 1200-1620 nm telecommunication bands. Four linear Bragg filters with distinct resonant wavelengths are presented that define the first demonstration of laser writing of multiple-wavelength and cascaded Bragg grating waveguides in a single process step.     

An analysis of As_2S_3 chirped fiber grating formed by two-photon absorption effect

When femtosecond laser pulses interfere with chirped femtosecond laser pulses in As2S3 fiber, a chirped fiber grating is formed. An analytical expression is given to describe the chirped grating, and its Bragg reflectivity is calculated. Because of the high photosensitive effect of As2S3 material, the chirped fiber grating has a wide Bragg reflective spectrum and high reflectivity by choosing proper parameters. This indicates that the chirped fiber grating can be used as a stretcher in the femtosecond chirped pulse amplification (CPA) system.     

All-fiber actively Q-switched Yb-doped laser

An all-fiber actively Q-switched Yb-doped laser is presented. Q-switching is performed by modulating a fiber Bragg grating via a magnetostrictive rod which is fixed to the fiber at the position of the grating. By exposing the rod to a changing magnetic field, the rod is stretched and relaxed causing the Bragg wavelength of the grating to shift and thereby changes the Q-factor of the cavity. Using Yb-doped fiber, pulses at 1052 nm are obtained at repetition rates from 1 to 200 kHz. At 75 kHz, 0.5 μJ pulses with peak powers of 3 W can be produced when 180 mW of pump power is applied. To the knowledge of the authors, this is the first all-fiber actively Q-switched Yb-doped laser presented to date.     

Wavelength-tunable, passively mode-locked fiber laser based on graphene and chirped fiber Bragg grating

We demonstrate a wavelength-tunable, passively mode-locked erbium-doped fiber laser based on graphene and chirped fiber Bragg grating. The saturable absorber used to enable passive mode-locking in the fiber laser is a section of microfiber covered by graphene film, which allows light-graphene interaction via the evanescent field of the microfiber. The wavelength of the laser can be continuously tuned by adjusting the chirped fiber Bragg grating, while maintaining mode-locking stability. Such a system has high potential in tuning the mode-locked laser pulses across a wide wavelength range.     

Generation of high-density electrons based on plasma grating induced Bragg diffraction in air

Efficient nonlinear Bragg diffraction was observed as an intense infrared femtosecond pulse was focused on a plasma grating induced by interference between two ultraviolet femtosecond laser pulses in air. The preformed electrons inside the plasma grating were accelerated by subsequent intense infrared laser pulses, inducing further collisional ionization and significantly enhancing the local electron density.     

Q-switched all-fibre laser using a fibre-optic resonant acousto-optic modulator

We report an actively Q-switched all-fibre laser using a fibre-optic resonant acousto-optic modulator as Q-switching element. The modulator consists of a short-length fibre Bragg grating modulated by a standing longitudinal elastic wave, being the grating length much shorter than the acoustic wavelength. Q-switch pulses were successfully obtained from an Er3+-doped fibre laser at repetition rates of 18 and 37 kHz.     

A Novel Method for Eliminating Chirp and Compressing Pulses from Gain-Switched Distributed-Feedback Bragg Semiconductor Laser

A novel method is proposed that simultaneously eliminates chirp and compresses pulses from gain-switched distributed-feedback Bragg (G-S DFB) semiconductor laser employing chirped fiber Bragg grating (FBG) with narrow bandwidth. It is demonstrated that with this method, only the central frequency of the FBG is slightly shifted from the central frequency of the G-S DFB laser; the nearly transform-limited narrow pulse with symmetrical waveform can be obtained.     

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.     

A self-seeded fiber laser incorporated with a fiber Bragg grating external cavity semiconductor laser

A self-seeded fiber laser incorporated with a fiber Bragg grating external cavity semiconductor laser (FBG-ECL) and a Mach-Zehnder interferometer (MZI) were reported in this paper. The MZI provided a Q-switching with response time in the order of micro-seconds. The FBG-ECL provided narrow pulses as seeds to shorten the Q-switched pulses. Experimentally, pulse width of 0.8 μs was measured, which was one fifth of the pulse width without self-seeding.     

Broadband square-pulse operation of a passively mode-locked fiber laser for fiber Bragg grating interrogation

A long-cavity, passively mode-locked erbium fiber laser operated in the square-pulse regime is demonstrated as a useful light source for interrogating fiber Bragg grating arrays. Output pulses with 4-W peak-power, 10-ns pulse widths, and bandwidths greater than 60nm were used successfully to interrogate 2% fiber Bragg gratings.     

Tunable, passively Q-switched single-longitudinal-mode Nd:YVO4 laser using a chirped volume Bragg grating

A Nd:YVO₄ laser was locked with a chirped volume Bragg grating to achieve single-longitudinal-mode output. Tuning was performed from 1,063 to 1,065?nm by translating the grating and a maximum output power of 4?W was obtained. With a Cr⁴⁺:YAG saturable absorber, Q-switched pulses with 4?ns and a pulse energy of 5.7???J were achieved which could be frequency doubled in PPKTP with a conversion efficiency exceeding 50?%.     

Grating sensor array demodulation by use of a passively mode-locked fiber laser

A fiber Bragg grating sensor array is interrogated by use of a passively mode-locked fiber laser source. A novel demodulation scheme that uses highly dispersive fiber to convert the grating wavelength shift to a temporal shift in the arrival time of the reflected pulses is demonstrated. The source bandwidth of >85 nm permits interrogation of many-grating arrays, and the demodulation technique permits fast sensing of large strains.     

Generation of tunable multi-wavelength optical short pulses using self-seeded Fabry-Perot laser diode and tilted multimode fiber Bragg grating

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.Dualand 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 dualand 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.     

利用啁啾光纤光栅色散特性实现波长可调谐的主动锁模掺铒光纤环行腔激光器

在环形腔主动锁模光纤激光器中引入啁啾光纤光栅，利用啁啾光纤光栅的大色散特性，通过调节调制频率，实现波长调谐，调谐范围2nm。所得脉冲为重复频率2．5GHz，脉宽约60ps的正啁啾脉冲，。     

Actively mode-locked dual-wavelength fiber laser with close wavelength spacing using a fiber Bragg grating

A simple actively mode-locked fiber ring laser is proposed and successfully demonstrated to generate dual-wavelength picosecond pulses with close wavelength spacing using one Bragg grating in standard single-mode fiber. The proposed laser can be made to operate in stable dual-wavelength at room temperature, due to the birefringence characteristic of the FBG induced by transverse strain. Transverse strain loading on the FBG allows the wavelength spacing to be controlled. Generation of stable dual-wavelength pulses with a pulsewidth of 212–234 ps and a tunable wavelength separation from 0.2 to 0.44 nm at a pulse rate of 1.05 GHz was demonstrated.     

Sub-5-ps, multimegawatt peak-power pulses from a fiber-amplified and optically compressed passively Q-switched microchip laser

We report on high-energy picosecond pulse generation from a passively Q-switched and fiber-amplified microchip laser system. Initially, the utilized microchip lasers produce pulses with durations of around 100 ps at 1064 nm central wavelength. These pulses are amplified to energies exceeding 100 μJ, simultaneously chirped and spectrally broadened by self-phase modulation using a double stage amplifier based on single-mode LMA photonic crystal fibers at repetition rates of up to 1 MHz. Subsequently, the pulse duration of chirped pulses is reduced by means of nonlinear pulse compression to durations of 2.7 ps employing a conventional grating compressor and 4.7 ps using a compact compressor based on a chirped volume Bragg grating.     

全光纤调Q掺铒光纤激光器的脉冲研究

报道一种全光纤调Q掺铒光纤激光器,在其结构中采用带有光纤光栅的光纤环形镜作为调Q装置此调Q方案同时具有对腔内损耗进行调制和对激光输出波长选择的功能理论上分析了不同臂长差时的脉冲出现情况,与实验基本相符并对激光单脉冲产生及稳定性进行了分析讨论.     

High frequency fiber laser emission generated by pump spiking

A stable and short pulse train of ∼100 MHz repetition frequency was obtained from an erbium doped fiber laser excited by a “continuous” semiconductor laser and by using a linear cavity defined by a Bragg grating pair. The operation frequency of the fiber laser was greater (∼5-15 times) than the cavity round-trip frequency. Emission properties obtained from the erbium doped fiber laser were correlated with those taken from the pump laser, which presented a particular optical noise (very short pulses) added to the continuous emission. From the temporal and radio-frequency analysis of both systems, we conclude that the pump emission characteristics are the responsible of the fiber laser pulsed behaviour.     

Cascaded mode-locking of a spectrally controlled Yb:KYW laser

Self-starting and stable mode-locking of an Yb:KYW laser in the picoseconds pulse regime has been achieved by exploiting a positive cascaded Kerr lens in periodically poled KTP. The use of a volume Bragg grating (VBG), for locking the output spectrum of the laser, was essential to achieve a stable mode-locked operation in this wide gain bandwidth laser material. The laser emitted stable, nearly transform-limited pulses with a duration of 16 ps, at a repetition rate of 210 MHz, and with an energy of 3.2 nJ. The mode-locked spectrum was centred at 1,029.1 nm and featured a FWHM bandwidth of 85 pm, which was effectively determined by the VBG. Combination of a large cascaded Kerr nonlinearity with spectral limiting by a VBG represents so far the best opportunity for stable mode-locking of a broadband laser to produce near-transform-limited picosecond pulses.