Attenuation measurement of infrared optical fibers by use of a hollow-taper-based coupling method

An alternative method for attenuation measurement of infrared (IR) fibers is described. The method includes a simple technique for direct laser-to-fiber coupling with an uncoated glass hollow taper. The operating principle of the hollow taper is based on the grazing-incidence effect of light reflection. The hollow taper forms a smooth Gaussian-shaped profile of the output laser emission and provides the proper conditions for equilibrium-mode distribution of optical power within the test IR fibers. The experimental hollow-taper-based coupling method is used for measurement of attenuation and bending losses of various kinds of IR fiber, including solid-core (fluoride, chalcogenide, and germanium-doped) and hollow fibers.     

Thermal effects in a hollow waveguide beam launch for CO(2) laser power delivery

Thermal effects caused by launching conditions in a CO(2) laser beam delivery that uses metallic hollow waveguides is investigated. It is found that front-end clipping is the main cause of thermal loading and generates a steep temperature gradient at the fiber front end while the continuous beam attenuation produces a temperature distribution declining slowly along the waveguide.     

Refractive indices and extinction coefficients of polymers for the mid-infrared region

Refractive indices and extinction coefficients of several polymers have been measured for the wide infrared wavelength region. The polymers used as coating materials have also been found useful for low-loss hollow waveguides for the transmission of Er:YAG, CO, and CO(2) laser light, respectively.     

卤化银光纤式CO2激光手术刀

卤化银多晶光纤「ＡｇＣｌｘＢｒ（１－ｘ），０≤ｘ≤１」是一种性能优良的中红外传能光纤。光纤原料采用高真空熔炼、氯气气氛下区域融熔和高真厂长发瓿内单昌生长等特殊工艺方法提纯，光纤用热挤压法成型。已制成的直径Φ１．０（ｍｍ）光纤Ｌ＝１．６４（ｍ）输出ＣＯ２激光功率〉２０（Ｗ），损耗０．３￣０．５ｄＢ／ｍ，Ｆｏｒｕｒｉｅｒ红外光谱（ＦＴＩＲ）测量结果显示，光纤在４￣１６μｍ波段内有良好的透过率，用光纤制     

Low-loss deflection prism for a laser beam with Brewster angle

A low-loss deflection prism for a laser beam is proposed, and its various characteristics such as beam quality, transmittance, deflection angle, and polarization state are presented. The prism having a trapezoidal form is made from BK7 glass and is designed for a He-Ne laser beam. When a p-polarized beam is incident on the slant surface of the prism at the Brewster angle, the totally reflected and transmitted beam is deflected by 90 degrees, and the measured transmittance is nearly 98%. The theoretical transmittances of the proposed prism are compared with those of a Pellin-Broca prism.     

Aluminum-coated hollow glass fibers for ArF-excimer laser light fabricated by metallorganic chemical-vapor deposition

A hollow fiber composed of a glass capillary tube and a metal thin film upon the inside of the tube is proposed for the delivery of ArF-excimer laser light. From theoretical analysis, aluminum is chosen as the metal layer. A thin aluminum film is deposited by metallorganic chemical-vapor deposition, with dimethylethylamine alane employed as the source material. Measured loss spectra in vacuum-ultraviolet and ultraviolet regions and losses for ArF-excimer laser light show the low-loss property of the aluminum-coated fiber at the 193-nm wavelength of ArF-excimer laser light. The straight loss of the 1-m long, 1-mm-bore fiber is 1.0 dB.     

Cyclic olefin polymer-coated silver hollow glass waveguides for the infrared

Cyclic olefin polymer (COP) is newly used as the inner dielectric of infrared, hollow glass waveguides because of its low extinction coefficient in the mid-infrared region. A liquid-flow coating and dry-cure process are employed to form the COP layer on the inside of a silver-coated hollow glass tube. In the coating process, cyclohexane is chosen as the solvent of COP to form a smooth and uniform COP layer. It is shown that COP-coated silver hollow glass waveguides show low loss properties for CO(2), CO, and Er:YAG laser light when the thickness of the COP layer is properly chosen.     

Optical properties of hollow calcium aluminate glass waveguides

Calcium aluminate glass has a refractive index less than 1 at 10.6 μ, and therefore it is a good candidate for a hollow fiber for the transmission of CO(2) laser energy. We have drawn hollow calcium aluminate glass fibers with inner diameters ranging from 380 to 500 μ. The loss for our 500-μm inner-diameter hollow glass fibers measured at 10.6 μm is 8.6 dB/m.     

Beam homogenizer for hollow-fiber delivery system of excimer laser light

A beam homogenizer for a hollow-fiber-based, UV laser delivery system is proposed. A rectangular glass waveguide with an inner aluminum coating that has a 1-mm square cross section is attached at the output end of the circular-core hollow fiber with a 1-mm inner diameter. The rectangular waveguide generates a number of higher-order modes and results in a uniform top-hat profile. The configuration of the waveguide is designed by a ray-tracing technique so that both the low transmission loss and the high uniformity in the output beam are obtained. The fabricated waveguide shows a low loss of 0.4 dB, and the intensity variation coefficient is 7%. The output beam from the rectangular waveguide is expanded by a lens to the size larger than 10-mm square. It is also shown that the profile does not change with the bending condition.     

介质(Ag/AgI)镀层空芯玻璃光纤的制备及红外传输性能的研究

介质（Ａｇ／ＡｇＩ）镀层空芯玻璃光纤是一种较好的传输红外激光的柔性光纤材料．采用液相化学沉积法成功地制备了内径分别为１．０、０．５３、０．３２ｍｍ,长度为１．５ｍ的介质（Ａｇ／ＡｇＩ）镀层玻璃空芯光纤．利用傅里叶红外光谱仪和 ＬＪＬ－３５Ａ ＣＯ_２激光器分别测试了介质（Ａｇ／ＡｇＩ）镀层玻璃空芯光纤的传输损耗,测试结果表明：介质（Ａｇ／ＡｇＩ）镀层空芯玻璃光纤在２．５～２０μｍ波长范围内有较低的损耗值;随着光纤内径（α）的增大,空芯光纤的传输损耗（α）降低,这与Ｍｉｇａｇｉ理论（α　∝１／α^３）相符合．另外,由于光纤的注入端头发热致使介质（Ａｇ／ＡｇＩ）镀层空芯玻璃光纤的传输损耗随注入功率的增加而增大．     

Two-photon resonance-enhanced refractive-index change and self-focusing in a dye-solution-filled hollow fiber system

The general behavior of two-photon absorption-enhanced refractive-index change in a third-order nonlinear optical medium is briefly described. The nonlinear medium was the solution of a new dye, trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methyl pyridinium iodide (ASPI) in dimethyl sulfoxide as the solvent, that was poured into a 20-cm-long quartz hollow fiber of 100-mum internal diameter. This dye solution has a strong two-photon absorption and subsequent upconversion fluorescence emission when excited with 1064-nm laser radiation. When the input peak intensity reached 500-1500-MW/cm(2) levels, obvious changes in beam profiles of the output IR laser beam were observed due to a self-focusing or self-trapping process occurring inside the fiber system. As a result of this process, highly directional frequency-upconverted superradiant lasing output was obtained with a beam size ~5 times smaller than that of a linearly transmitted He-Ne probe laser beam. The demonstrated mechanism can be useful for fiber laser-amplifier and fiber-integrated optics devices.     

Use of hollow core fibers, fiber lasers, and photonic crystal fibers for spark delivery and laser ignition in gases

The fiber-optic delivery of sparks in gases is challenging as the output beam must be refocused to high intensity (approximately 200 GW/cm(2) for nanosecond pulses). Analysis suggests the use of coated hollow core fibers, fiber lasers, and photonic crystal fibers (PCFs). We study the effects of launch conditions and bending for 2 m long coated hollow fibers and find an optimum launch f# of approximately 55 allowing spark formation with approximately 98% reliability for bends up to a radius of curvature of 1.5 m in atmospheric pressure air. Spark formation using the output of a pulsed fiber laser is described, and delivery of 0.55 mJ pulses through PCFs is shown.     

High-peak-power, pulsed CO(2) laser light delivery by hollow glass waveguides

Flexible hollow glass waveguides with internal metallic and dielectric coatings have been used to deliver high-peak-power transversely excited atmosphere CO(2) laser energy. The straight guide loss is as low as 0.17 dB/m for 1000-mum-bore guides and 0.46 dB/m for 530-mum-bore guides propagating the HE(11) mode. The loss increases to 0.93 and 1.36 dB/m, respectively, when guides are bent to a radius of 0.25 m. The hollow glass waveguides have been used to deliver pulsed CO(2) laser energy successfully with a peak power of 0.7 MW and an energy of 350 mJ per pulse with a gas purge through the hollow core. The delivered average power is as high as 27 W. It is concluded that these waveguides are promising candidates for pulsed CO(2) laser delivery in medical and surgical applications.     

Fabrication of copper oxide-coated hollow waveguides for CO2 laser radiation

Hollow fibers for transmitting CO(2) laser light were fabricated by the chemical vapor deposition (CVD) method. A dielectric film of copper oxide (Cu(2)O) was deposited upon the inside of a Ag-coated glass capillary by use of a metal acetylacetonate as the precursor. The waveguide, which was coated with Cu(2)O and had a bore diameter of 700 mum, showed a loss of 0.9 dB/m for CO(2) laser light. The Cu(2)O film deposited by CVD had high chemical and heat resistivity. Therefore a hollow fiber coated with copper oxide is suitable for high-power laser applications in a severe environment.     

Laser-fiber system for ablation of intraocular tissue using the fourth harmonic of a pulsed Nd:YAG laser

We report on a method for delivering high fluence pulsed 266 nm laser radiation to the target tissue via an optical fiber. The fourth harmonic of a Nd:YAG laser was concentrated using a hollow glass taper and launched into an optical fiber. Fluences of up to 2 J/cm(2) were routinely output at the tapered optical fiber tip. The maximum fluence generated before failure of the optical fiber was between 3.5 and 8 J/cm(2). Ablation of ocular tissue was demonstrated using fluences of 1.0 and 0.4 J/cm(2). The delivery system has the potential for use in intraocular surgical procedures.     

Optical properties of end-sealed hollow fibers

We propose sealing techniques for medical hollow fibers to protect the inner surface of fibers from debris or water that scatters from targets. First, hollow fibers are sealed with a film of polymer that is easily formed by use of a dipping technique. The transmission loss of 20-microm-thick sealing film was 0.2 dB for Er:YAG laser light, and the maximum energy that is available for the film was 180 mJ. Second, a sealed glass cap was applied to the output end of hollow fiber. The silica-glass cap with a wall thickness of 400 microm shows a transmission loss of 0.5 dB and was not damaged by radiation of 400-mJ energy pulses.     

Fabrication of hollow waveguides for CO2 lasers

Germanium-coated metal (silver, gold, and copper) hollow waveguides for CO(2) laser energy delivery have been fabricated by electron-beam evaporation and plating techniques in which an acid-soluble glass mandrel with small surface roughness was used. The transmission characteristics of Ge-coated metal hollow waveguides were studied. Ge-coated Ag hollow waveguides showed smaller loss, 0.2 dBm, for CO(2) laser light than Ge-coated Au and Cu waveguides. The transmission characteristics of Ge-coated Ag hollow waveguides were measured in relation to a core diameter and a bending radius.     

Comparative study of mid-infrared fibers for modal filtering

We compare the filtering capabilities of two infrared fibers developed to achieve a high rejection ratio of the higher order modes in order to obtain compact modal filters devoted to stellar interferometry. Two types of double-clad fibers are studied: a fiber with a second thin absorbing cladding and a fiber with a second thick absorbing cladding closer to the fiber core; both are single mode around the CO(2) band (10.6 μm). We present the single-mode spectral domain and the nulling capabilities of both fibers for different fiber lengths, comparing simulations with experimental results. We show that the filtering capabilities are improved when the absorbing clad is closer to the fiber core, as the propagation distance needed to filter out these modes is shorter. Thus, to obtain high rejection ratios in compact devices, an absorbing cladding close to the core of the fiber is compulsory in order to suppress cladding modes that could eventually recouple into the waveguide. We present an empirical model that allows determining the minimum filter length, considering only one effective leaky mode with low attenuation, which considerably simplifies the theoretical studies.     

二氧化锗空芯光纤的研制

建立了纯ＧｅＯ＿２空芯光纤的制作方法，分析了该种光纤的传光原理，计算了几种氧化物空芯光纤的模式损耗。测试了我们制作的纯ＧｅＯ＿２空芯光纤的传输损耗和传输功率，得到了较好的结果。     

Processing and characterization of silver films used to fabricate hollow glass waveguides

Hollow glass waveguides are an increasingly popular fiber for the delivery of high-power IR laser radiation. At CO(2) laser wavelengths the measured and theoretical losses agree, but at the 3-microm Er:YAG laser wavelength the losses remain higher than expected. The reason for this is the surface roughness of the silver film used to form the first layer of the Ag/AgI thin-film structure. We found that the roughness of the silver film increases fivefold as silvering times increase from 5 to 80 min. This increased surface roughness produces a concomitant linear increase in the attenuation coefficient for the silver-only guides for wavelengths shorter than approximately 5 microm.