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.     

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.     

Femtosecond Bessel-beam-assisted high-aspect-ratio microgroove fabrication in fused silica

A simple and repeatable method to fabricate high-aspect-ratio(HAR) and high-quality microgrooves in silica is reported. The method consists of two steps:(1) formation of laser-modified regions by femtosecond Bessel beam irradiation, and(2) removing laser-modified regions through HF etching. Uniform, straight microgrooves can be fabricated and the highest aspect ratio that can be reached is ～52. The phenomenon is attributed to the uniform energy distribution in the long propagation distance, which leads to the long and uniform laser-modified regions and subsequent HF acid etching of laser-modified regions with high selectivity. This method will have potential applications in fabrication of HAR microgrooves in transparent materials.     

Femtosecond laser fabrication of nanostructures in silica glass

A femtosecond laser beam focused inside fused silica and other glasses can modify the refractive index of the glass. Chemical etching and atomic-force microscope studies show that the modified region can have a sharp-tipped cone-shaped structure with a tip diameter as small as 100 nm. Placing the structure near the bottom surface of a silica glass sample and applying a selective chemical etch to the bottom surface produces clean, circular, submicrometer-diameter holes. Holes spaced as close to one another as 1.4 microm are demonstrated.     

Direct laser fabrication of blaze gratings in fused silica

Results of our investigations on direct laser fabrication of blaze gratings in fused silica using a special fluorine-laser mask projection technology allowing to produce plane and smooth reflecting areas and optimum blaze geometry will be presented. In particular, it will be shown that gratings with nearly optimum blaze geometry and relatively low surface roughness of the reflecting areas can be produced. The technique is a one-step method and has a high variability with regard to grating geometry and also substrate materials. We use masks made of tantalum foil, projected onto a fused silica substrate surface with a demagnification ratio of 26:1 and also novel scattering masks made of calcium fluoride. In these masks, up to 25 times smaller transmission apertures can be manufactured allowing thus grating constants in the range of a few μm. The average surface roughness of the reflecting areas along a blaze grating line was measured to be R _a=38 nm without any surface smoothing. The maximum variance of the blaze angle within one grating is about 0.5 degree.     

Femtosecond laser writing of porous capillaries inside fused silica glass

We demonstrate that within a restricted optical pulse duration-pulse energy parameter space tightly focused femtosecond laser radiation can be used to fabricate porous capillaries in bulk fused silica glass by simply moving the laser focus through the material. We show that the rate of penetration of liquids into the porous capillaries can be controlled by the laser polarization, which determines their morphology. The fluid propagation is measured using the form birefringence of nanocrack/nanovoid structures produced inside the capillaries. We also demonstrate the nanofiltration capabilities of the capillaries by separating the relatively small molecules of Rhodamine 6G dye from their solvent.     

Femtosecond laser fabrication of tubular waveguides in poly(methyl methacrylate)

Femtosecond laser direct writing is employed for the fabrication of buried tubular waveguides in bulk poly(methyl methacrylate). A novel technique using selective chemical etching is presented to resolve the two-dimensional refractive-index profile of the fabrication structures. End-to-end coupling in the waveguides reveals a near-field intensity distribution that results from the superimposition of several propagating modes with different azimuthal symmetries. Mode analysis of the tubular waveguides is performed using the finite-difference method, and the possible propagating mode profiles are compared with the experimental data.     

Tunable phase-shifted fiber Bragg grating based on a microchannel fabricated by a femtosecond laser

A phase-shifted fiber Bragg grating(PS-FBG) based on a microchannel was proposed and realized by combining the pointby-point scanning method with chemical etching. The PS-FBG is composed of a fiber Bragg grating(FBG) and a microchannel through the fiber core. The microchannel can introduce phase shift into the FBG. What is more important is that it exposes the fiber core to the external environment. The phase shift peak is sensitive to the liquid refractive index, and it shows a linear refractive index response wavelength and intensity sensitivity of 2.526 nm/RIU and-111 d B/RIU, respectively.Therefore, such gratings can be used as sensors or tunable filters.     

Control of light transmission in laser-written phase-shifted Bragg grating couplers

We demonstrate the fabrication by direct laser writing and the operation of a directional coupler containing Bragg gratings in each waveguide. We achieve high-precision control over the longitudinal shift between the gratings, which feature first-order Bragg resonance at telecommunication wavelengths. We observe fundamental differences between light transmission characteristics in couplers with unshifted and shifted gratings in agreement with theoretical predictions.     

104 nm period grating fabrication in fused silica by immersion two-beam interferometric laser induced backside wet etching technique

A substantial extension of the method of two-beam interferometric laser induced backside wet etching (TWIN-LIBWE), the immersion TWIN-LIBWE, is used to fabricate fused silica gratings with a 104 nm period. The spatially filtered fourth harmonic of Nd:YAG laser (λ=266 nm, τ_FWHM=8 ns) pulses were split into two parts which then interfered at the backside of the fused silica target in contact with a liquid absorber (naphthalene methyl methacrylate saturated solution with a concentration of 1.85 mol/dm³). The hypotenuse of a rectangular fused silica prism is attached to the fused silica target with the use of distilled water as the immersion liquid. On steering the beams through the sides of the prisms, the angle between the two laser beams has been substantially increased. The resulting period of 104 nm is the minimal grating constant achievable under such experimental conditions and, to our knowledge, the smallest laser generated grating period in fused silica at present. PACS 42.62; 42.79; 81.65     

飞秒激光精密微纳加工的研究进展

飞秒激光由于其超快时间特性和超高峰值功率特性在精密微纳加工领域引起了人们广泛的重视．在与物质的相互作用中它能快速、准确地将能量作用在特定的区域内，从而可以获得极高的分辨率和加工精度。文章综述了飞秒激光精密微纳加工的最新研究进展，分别就飞秒激光烧蚀微加工和飞秒激光双光子聚合产生三维微纳结构进行了介绍，阐述了各自的物理机制．最后对飞秒激光微纳加工的研究前景做了初步探讨。     

Stress in femtosecond-laser-written waveguides in fused silica

We identify two states of stress induced in waveguides fabricated by femtosecond lasers in fused silica and show how they can be relieved by annealing. In-plane stress and stress concentration are revealed through birefringence and loss measurements. Another kind of laser-induced stress appears in the form of swelling of the glass surface when waveguides are written near the surface and is a manifestation of confined rapid material quenching. By annealing the sample we reduce the losses by approximately 30% (at 633 nm) and decrease the birefringence by a factor of 4 in fused silica.     

Femtosecond laser erasing and rewriting of self-organized planar nanocracks in fused silica glass

Tightly focused, linearly polarized, femtosecond laser radiation can produce highly birefringent nanograting structures inside fused silica glass. Here we report that when the polarization direction of the femtosecond light is changed, old nanogratings are erased and simultaneously replaced with new ones whose orientation is solely determined by the polarization of the rewrite beam. We also show that these volume nanogratings can be rewritten 1000 times with little degradation in their quality.     

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.     

A phase-shifted linearly chirped fiber Bragg grating with tunable bandwidth

We demonstrate a new method to broaden and adjust the transmission bandwidth of a phase-shifted linearly chirped fiber Bragg grating (PS-LCFBG) after its fabrication. A movable thermal head is used to heat the PS-LCFBG at a small contact point. The original ultra-narrow transmission bandwidth of the PS-LCFBG can be broadened due to both the presence of the passband peak caused by the thermally-induced temporary phase shift and the passband caused by the thermally-induced local Bragg wavelength shift. The transmission bandwidth of the new passband peak can be adjusted by changing the position of the thermal head. The transmission bandwidth of the PS-LCFBG can be broadened from <10 pm to 0.16 nm, and can be tuned from 0.16 to 1.05 nm.     

Inscription of high contrast volume Bragg gratings in fused silica with femtosecond laser pulses

We present volume Bragg gratings (VBGs) with a period of 1.075 μm inscribed in fused silica using a femtosecond laser and a phase mask. The femtosecond-inscribed VBGs can be used as reflecting elements with reflectivities of about 80% for a 1-mm-long grating. Due to the non-sinusoidal refractive-index shape, higher order Bragg resonances up to the 7th reflection order could be measured. Therefore, the Bragg gratings also reflect light in the visible-wavelength range.     

Solgel grating waveguides for distributed Bragg reflector lasers

Solgel grating waveguides and their application to the fabrication of external-cavity distributed Bragg reflector (DBR) lasers are demonstrated. A new composition of aluminosilicate material is developed for the fabrication of single-mode waveguides and Bragg reflectors. An average loss of <0.2 dB/cm is measured in the single-mode waveguides at 1550 nm. The reflectors show filtering greater than 97% near 1530 nm, with a bandwidth of ~0.6 nm . The Bragg reflectors are used as feedback resonators for DBR lasers. Single-mode lasing with a sidemode suppression of better than 25 dB is demonstrated.     

单点相移光纤光栅光谱特性的研究与应用

可调谐窄线宽掺铒光纤激光器具有线宽窄、噪声低等优点,在光纤通信、光纤传感、相干探测和合成等方面有广泛的应用。根据传输矩阵法, 利用Matlab软件仿真出单点相移光纤光栅的透射谱,通过对不同相移量、不同相移点位置、不同光栅的长度以及不同纤芯有效折射率的单点相移光纤光栅透射谱进行分析,得到了影响其相移峰的透射率、线宽、位置的主要因素,并提出了一种利用相移量为π且相移点在中点的相移光纤光栅作为滤波窗口的可调谐窄线宽掺铒光纤激光器的方案。     

Femtosecond laser-induced damage and filamentary propagation in fused silica

Bulk damage induced by fs IR laser pulses in silica is investigated both experimentally and numerically. In a strong focusing geometry, a first damage zone is followed by a narrow track with submicron width, indicating a filamentary propagation. The shape and size of the damage tracks are shown to correspond to the zone where the electron density created by optical field ionization and avalanche is close to 10(20) cm(-3). The relative role of avalanche and photoionization is studied. The plasma density produced in the wake of the pulse is shown to saturate around 2-4x10(20) cm(-3).