Fabrication of gradient refractive index ball lenses using the method of combination of ion exchanging and sagging

Based on the Fick’s diffusion equations, the distribution function of refractive index of a gradient refractive index ball lens (GRIN ball lens/GBL) is derived. Lithium containing silicate glass is fabricated and GRIN ball lenses (GBLs) which diameters are from 0.3 mm to 3.0 mm are made by the method of combination of Ion exchanging and sagging in sodium nitrate. Refractive index profiles of these GBLs are measured by interferometer, and the performances such as effective focal length (EFL), back focal length (BFL) and numerical aperture (NA) between GBLs and homogeneous ball lenses (HBLs) are compared. Results show that the distribution of the index of refraction is parabolic curve and its Δn is about 0.0002, the performances of the former are super to the latter.     

1 × 4 coupler based on all solid five-core photonic crystal fibers

An improved 1 × 4 coupler based on all solid multi-core photonic crystal fiber is proposed and analyzed. The expressions to calculate the coupling length and the coupling efficiency are deduced based on the coupled-mode equations firstly. Then a full-vector finite element method (FEM) is used to calculate the coupling length and the coupling efficiency. Next, the propagation characteristics and the performances of the coupler are analyzed through using a full vector beam propagation method (BPM). Research shows that the results derived by FEM agree with that by BPM. The coupling length of the coupler is 4.1 mm at λ = 1.55 μm. A maximum coupling efficiency of 24.96% can be obtained. The coupling ratio is more than 22.5% over a wavelength range of 100 nm. The polarization-dependent loss at λ = 1.55 μm is equal to 0.73 dB. Finally, the influences of the micro-variation of structure parameters and the material refractive index on the working performances of the coupler are investigated.     

1 W tunable near diffraction limited light from a broad area laser diode in an external cavity with a line width of 1.7 MHz

A novel unstable external cavity for a broad area laser diode is presented. The cavity is based on a V-shaped setup that improves the slow axis beam quality by coupling the internal modes of a gain guided laser diode. The novelty here is the compact unstable resonator design without lenses in direction of the slow axis. For frequency stabilisation and to narrow the line width of the laser diode emission a diffraction grating in a Littrow configuration is used. With this setup up to 1 W of near diffraction limited light with a beam quality of M² ? 1.3 and a line width of 1.7 MHz could be achieved. The external cavity laser was tunable over a range of 35 nm (FWHM) around the center wavelength of 976 nm.     

980 nm泵浦激光二极管与柱形透镜光纤耦合的角向容忍度

给出了一个计算980 nm泵浦激光二极管与柱形透镜光纤耦合角向容忍度的理论模型,并选择圆形的柱透镜光纤进行数值计算,得出了在水平和垂直方向上偏转加平移时的相对耦合效率及绕光纤轴向转动的相对耦合效率的数值计算结果并加以分析,同时也研究了光纤透镜的形状和激光二极管与光纤的间距对相对耦合效率的影响.     

High efficiency Tm fiber laser of seed amplification

We use all-fiber single-mode lasers as seed source, amplifiers and pump laser coupling amplifier has been experimentally investigated, respectively. The maximum output power is 10.4 W, with the slope efficiency of 54.4% when the seed laser power is 1 W, and the slope efficiency is 48.7% when the seed laser power is 2 W. The slope efficiency of this scheme is higher than that of the all-fiber amplifier. This can be explained by the loss of pump power at the splice dots. The wavelength of the fiber amplifier is 1947.6 nm, the same as the seed laser's, with a linewidth of 2 nm. We estimate the beam quality to be M² = 2.39, clearly indicating nearly diffraction-limited beam propagation.     

Dual-wavelength high-power Yb-doped double-clad fiber laser based on a few-mode fiber Bragg grating

We report a high-power dual-wavelength Yb-doped double-clad fiber laser based on a few-mode fiber Bragg grating (FMFBG). The FMFBG was fabricated by using a piece of fiber in a length of fiber with a cutoff wavelength of 1225 nm, which supported two modes at 1060 nm. The laser was pumped by a fiber pigtailed laser diode working at 915 nm. Single-wavelength, dual-wavelength and triple-wavelength laser oscillations were observed when the fiber laser operated under different low pump powers. However, stable dual-wavelength operation was achieved at higher pump power of 3.9 W and remained unchanged until the output power reached 5.67 W under the maximum available pump power of 10.7 W. The laser wavelengths were centered at 1059.12 and 1060.80 nm with a full-width at half-maximum of 37 and 43 pm, respectively. The signal-to-noise-ratio was greater than 50 dB and the beam quality factor (M²) was about 1.9.     

A simple reflection-type two-dimensional refractive index profile measurement technique for optical waveguides

We report a new configuration of a reflection-type confocal scanning optical microscope system for measuring the refractive index profile of an optical waveguide. Several improvements on the earlier design are proposed; a light emitting diode at 650 nm wavelength instead of a laser diode or He-Ne laser is used as a light source for better index precision, and a simple longitudinal linear scanning and a curve fitting techniques are adapted instead of a servo control for maintaining an optical confocal arrangement. We have obtained spatial resolution of 800 nm and an index precision of 2 × 10⁻⁴. To verify the system’s capability, the refractive index profiles of a conventional multimode fiber and a home-made four-mode fiber were examined with our proposed measurement method.     

Performance of a 80 W copper vapor laser with “alignment free” unstable CAT-EYE resonator and other configurations using intra-cavity apertures

Performance of a kinetically enhanced copper vapor laser (KE-CVL) with various stable/unstable “alignment free” CAT-EYE resonator configurations are presented here in this paper. The laser used in the experiment was a 45 mm bore (∼2 l discharge volume) kinetically enhanced copper vapor laser developed in our laboratory and capable of generating maximum power of ∼80 W (at ∼9.8 kHz). The efficiency of the laser was ∼1.4% and beam divergence of ∼3.5 mrad in a plane-plane standard multimode cavity. For the first time performance of unstable CAT-EYE resonator is demonstrated with a CVL/KE-CVL. On using unstable CAT-EYE resonator the divergence of the laser beam reduced to ∼0.22 mrad (∼20-fold reduction as compared to conventional plane-plane cavity), ∼40 W output power and with excellent misalignment tolerance. The laser output power was found to be within ∼5% drift/decline with misalignment angle of about 4 mrad between the mirrors. This is a significant improvement in comparison to standard conventional unstable resonator (M ∼ 50) CVL where ∼0.5 mrad divergence is achieved with power drift/decline of about 45% at ∼4 mrad misalignment angle.Off-axis unstable CAT-EYE unstable resonator was also demonstrated for the first time with further reduction in beam divergence to ∼0.13 mrad and with output power of ∼28 W. The misalignment tolerance was found to be highest in case of off-axis unstable CAT-EYE resonator with decline/drift in laser power of only ∼10% for misalignment angle as high as ∼8 mrad. Performance with intra-cavity apertures in plane-plane type CAT-EYE resonator for transverse mode control is also presented for the first time in CVLs. It is observed that the laser beam divergence reduces significantly to 1.25 mrad (a factor of 2) on using an aperture of ∼3.5 mm at the CAT-EYE reflector as compared to its normal (R = F = d) configuration without aperture. In case of stable CAT-EYE resonator the average beam divergence reduces from 8 mrad to 4 mrad (factor of 2) on using intra-cavity aperture of about 3 mm. It was also observed that high misalignment tolerance was retained on using intra-cavity apertures in almost all the CAT-EYE resonators. Use of intra-cavity mesh was also demonstrated for the first time with stable CAT-EYE resonator for improving the beam focus-ability. Average beam divergence was reduced by a factor of 2.5 (from 8 mrad to 3 mrad) on using intra-cavity mesh. These new configurations in CAT-EYE resonators in KE-CVLs are found to be effective in improving and controlling the laser beam divergence significantly with additional characteristic of high misalignment tolerance.     

Fabrication of gradient refractive index ball lenses

The theoretical model of the Maxwell fish eye sphere lens has long been regarded as an absolute optical instrument without practical application since it was proposed. But the study on the theory of Maxwell fish eye micro-spherical lens shows that at certain condition it has fine image formation ability and coupling efficiency. Based on the Fick's diffusion equations, the distribution function of refractive index of gradient refractive index (GRIN) ball lenses is derived. Lithium-containing silicate glass is fabricated and GRIN ball lenses whose diameters are from 0.3 to 3.0 mm are made by ion exchange and the sagging method in sodium nitrate. Radial refractive index profiles of these GRIN ball lens are measured by interferometer. Results show that the distribution of the index of refraction is a parabolic curve and in concordance with the computational consequence and its Δn is 0.0002.     

Lasing of a single crystal Nd fluoride fiber

We present laser action in a single crystal fluoride fiber obtained by the micro-pulling-down technique. The 700 μm diameter and 1 cm long Nd:LiYF₄ fiber was pumped by a beam shaped diode bar emitting at 806 nm. A peak power of 300 mW was achieved with a slope efficiency of 12%. When considering the pump power fraction absorbed by the laser mode, a slope efficiency of 37% was achieved. To the best of our knowledge, this is the first report of a Nd: LiYF₄ fiber laser.     

Highly efficient lens couplers for laser-diodes based on long period grating in special graded-index fiber

A theoretical scheme of a new device consisting of a lensed fiber and a long period grating (LPG) imprinted in a special single mode gradient-index fiber (SSMGIF) is presented. The SSMGIF consists of the single mode uniform core and the graded-index cladding. The performance of the proposed device is based on specific properties of SSMGIF. For high efficiency coupling between number of cladding modes and co-propagating core mode in this special type of the fiber we can use a number of weak LPGs with the same amplitudes of the refractive index modulation and with almost the same lengths. This scheme provides higher coupling efficiency, relaxed longitudinal alignment tolerance, and smaller device size, than an identical scheme with the LPG imprinted in a step-index (SI) fiber. Two different types of fiber lenses (hemispherical and hyperbolic) are presented for comparison.     

Passive Q-switching of short-length Tm-doped silica fiber lasers by polycrystalline Cr:ZnSe microchips

We demonstrate passive Q-switching of short-length double-clad Tm³⁺-doped silica fiber lasers near 2 μm pumped by a laser diode array (LDA) at 790 nm. Polycrystalline Cr²⁺:ZnSe microchips with thickness from 0.3 to 1 mm are adopted as the Q-switching elements. Pulse duration of 120 ns, pulse energy over 14 μJ and repetition rate of 53 kHz are obtained from a 5-cm long fiber laser. As high as 530 kHz repetition rate is achieved from a 50-cm long fiber laser at ∼10-W pump power. The performance of the Q-switched fiber lasers as a function of fiber length is also analyzed.     

Designing a coupler for the intersatellite optical communication system

Coupling a single-mode laser diode with 200 mW to a single-mode fiber (SMF) through an orthonormal aspherical cylindrical lens and a GRIN lens for the intersatellite optical communication system is proposed and demonstrated. We experimentally studied how the coupling efficiency changes with the SMF's position displacement and axial angle variation, and obtained 80 mW output power at the end of the SMF, which shows that the coupling units have satisfied the designed request.     

Optical performance of a versatile illumination system for high divergence LED sources

An efficient flat-top illuminating optical system optimized for an extended light source is presented. The source is a high-brightness high divergence light emitting diode (LED), sized 1 mm × 1 mm, producing monochromatic emission (525 ± 5 nm) with viewing angle of 130°. The design is based on a rotationally symmetrical catadioptric system, developed on a geometrical optics basis, and modelled with ZEMAX^® software. The device consists of two optical systems: (i) a collimating system which, in turn, is formed by an aspheric lenses system (low numerical apertures, NA < 0.26) and two-mirror system (0.26 < NA < 0.86), and (ii) an external mirror (NA > 0.86) designed and optimized for each purpose. By itself, the collimating system works with a residual divergence of θ_C = 1.46°. The external mirror can be adequately designed to produce some given conditions. For instance, a flat-top profile is obtained in the selected focusing plane, with a maximum transversal intensity variation of 2.5% over 18 mm. In addition, when the focusing mirror is allowed to move along the optical axis in a ±1 mm range, other interesting profiles can be reached for a given working distance, therefore increasing the versatility of the system.     

梯度折射率球透镜的制备及其光线轨迹测试

自麦克斯韦鱼眼透镜理论模型提出后,长期以来学术界一直以为它只是一种绝对光学仪器,在实践中并没有应用的可能。但通过对麦克斯韦鱼眼微球透镜的理论研究表明,在一定条件下,它具有良好的成像性能和耦合效率。根据菲克定律推导出梯度折射率球透镜的折射率分布函数。并且熔制了含Li+硅酸盐玻璃和采用槽沉法以及Na+/Li+离子交换法制备直径为0.3—3.0mm的梯度折射率球透镜。通过干涉法测得了梯度折射率球透镜的折射率分布曲线,并研究了光线通过梯度折射率球透镜的轨迹。研究结果表明:在590℃,离子扩散系数为3.07×10-6mm2/s;所制备的梯度折射率球透镜的折射率分布是抛物线分布,并且和理论分析相吻合;梯度折射率球透镜的光线轨迹满足椭圆方程。     

1097 nm Nd:YVO4 self-Raman laser

An acousto-optically Q-switched self-Raman laser emitting at 1097 nm is demonstrated with a c-cut Nd:YVO₄ crystal, using a fiber-coupled 880 nm diode laser as the pumping source. Raman laser performances in concave-plane and plane-plane oscillating cavities are studied and compared. With an absorbed diode power of 12.4 W and a pulse repetition rate of 50 kHz, the highest output power of 1.45 W is obtained from the plane-plane cavity, corresponding to an optical-to-optical conversion efficiency of 11.7%.     

Waveguide structures written in SF57 glass with fs-laser pulses above the critical self-focusing threshold

Waveguide writing with femtosecond (fs) laser pulses in glasses is compromised by uncontrollable non-linear propagation phenomena for powers exceeding a certain threshold, P_cr, which is inversely proportional to the material linear and non-linear refractive indices, n₀ and n₂, respectively. We have studied the behaviour of a commercial glass, Schott SF57, with high values of n₀ and n₂ when it is processed above P_cr with 800 and 1260 nm, 100 fs laser pulses to produce waveguide structures. Two types of structures, longitudinal (l-) and transversal (t-) ones depending on the sample translation, were obtained. In both of them, material damage surrounded by guiding regions was generated. We attribute the formation of the guiding regions of increased refractive index to the high local pressure reached in the focal volume where catastrophic damage occurs. This pressure can be high enough to compress the material nearby thus generating regions with a permanent refractive index increase up to 5 × 10⁻⁴. The efficiency of the guiding region is conditioned by the pulse energy. Mode profile analyses reveal double-sided exponential guided modes in the l-waveguides. As an alternative route to obtain suitable mode shapes and, simultaneously, to increase the coupling efficiency, the use of multiple structures has been investigated. Various l-structures have been written close to each other generating a region of increased refractive index large enough to effectively support a Lorentzian mode at 633 nm.     

The approximate ABCD matrix for a parabolic lens of revolution and its application in calculating the coupling efficiency

Using ray optics, we present an explicit formulation of the ABCD matrix for reflection and refraction of light beams at a parabolic interface separating media of different refractive indices under paraxial approximation. Based on the formulated ABCD matrix for refraction by a parabolic lens tip, we present a simple theoretical investigation of the coupling efficiency between a laser diode and a single-mode fiber with a parabolic lens formed on the fiber tip. The results show that this technique is effective and will be of benefit to designing suitable microlenses applying the laser-coupling technique.     

Three-level operation of a diode-bar-pumped Yb:S-FAP laser

We present an all solid-state Yb:S-FAP laser system running on the three-level laser transition at 985 nm. The pump source was a high fill-factor laser diode bar, with the output reformatted using a two-mirror beamshaping system to produce a rectangular pump beam that focused to a square spot. A nearly on-axis multipassing system was used to obtain four pump passes through a 1.6 mm Yb:S-FAP laser crystal. Gain-switched three-level laser output was achieved with an efficiency of 4.3% with respect to incident pump power. Electro-optic Q-switching produced 0.12 mJ pulses for a pump pulse energy of 11 mJ. Intra-cavity second-harmonic generation yielded a maximum pulse energy at 492.5 nm of 12 μJ.     

Wavelength tunability of L-band fiber ring lasers using mechanically induced long-period fiber gratings

We report on oscillation wavelength control in erbium-doped fiber ring lasers by adjusting the period of a mechanically induced long-period fiber grating (LPFG) inserted into the fiber ring resonator. Pump light is provided by a 974 nm laser diode (LD), the emission of which is coupled into the fiber ring resonator through a wavelength-division multiplexing coupler. Laser oscillation occurs with a threshold pump LD current of 40 mA, corresponding to a threshold pump power of 5 mW. When a periodic pressure of 0.81 N/mm is applied to form the LPFG, the fiber ring laser exhibits the tunable range of 40.9 nm, i.e., from 1563.1 to 1604 nm, by changing the grating period.