倾斜长周期光纤光栅薄膜传感器特性研究

为设计高灵敏光纤光栅薄膜传感器,基于耦合模理论研究了表面镀制高折射率敏感薄膜的倾斜长周期光纤光栅（TLPFG）耦合特性和薄膜折射率传感特性。TLPFG能激发高阶（角向序数l1）包层模耦合,高阶包层模耦合系数随倾角增大而增大,80时,高阶包层模耦合系数远小于1阶包层模耦合系数。研究表明,光栅倾角变化不影响包层模耦合谐振波长,但影响光谱透过率。进一步研究了TLPFG在80时1阶5次包层模耦合的光谱透过率对薄膜折射率变化的响应,分析了光栅结构参数（L、、）和薄膜参数（h3,n3）对薄膜折射率响应灵敏度的影响,结果表明,响应灵敏度相比于非倾斜LPFG可提高一个数量级,对薄膜折射率的分辨率可达10-9。     

Simultaneous measurement of temperature and strain based on arc-induced long-period fibre gratings

A study on arc-induced long-period fibre gratings (LPFGs) revealed that their strain sensitivity depends on the electric current of the arc discharge. Based on that property, a sensor scheme comprising two concatenated LPFGs was implemented for discrimination of temperature and strain effects. This sensor presented resolutions of /spl plusmn/0.1/spl deg/C//spl radic/Hz and /spl plusmn/35 /spl mu//spl epsiv///spl radic/Hz, respectively.     

A Highly Sensitive Fiber-Optic Refractive Index Sensor Based on an Edge-Written Long-Period Fiber Grating

A novel long-period fiber grating (LPFG) is edge-written without any destructive damage on the fiber based on the thermal shock and rapid cooling effects of the high-frequency CO₂ laser pulses exposure method in this letter. The refractive index disturbance induced by high-frequency CO₂ laser pulses mainly occurs in the edge region of the fiber cladding rather than in the fiber core. An edge-written LPFG with a resonant peak of ~18 dB is obtained using a standard telecommunication fiber. The experimental results show that such a novel LPFG structure has much higher refractive index sensitivity over conventional LPFGs side-written, which can be used as a refractive index sensor, or a tunable filter and modulator by controlling the refractive index sensitive film which is coated on the cladding of such an LPFG.     

Investigation of a periodically segmented waveguide Fabry-Pe/spl acute/rot interferometer for use as a chemical/biosensor

We present a periodically segmented waveguide Fabry-Pe/spl acute/rot interferometer (PSW-FPI) intended to be used for tagless real-time chemical/ biological sensing through bulk-material interaction. The differential sensor detects changes in the refractive index (RI) of a sample regardless of its absolute RI value. Experiments with a series of sucrose solutions of various concentrations are compared with theoretical results, and a very good match is found between the two. The theoretical sensitivity limit for a 50-dB-signal-to-noise-ratio (SNR) measurement system is estimated as /spl delta/n=3/spl middot/10/sup -7/. For a 29-dB-SNR measurement system, a measured sensitivity limit of /spl delta/n=4/spl middot/10/sup -5/ is comparable with the previously reported sensitivities of label-free real-time optical biosensors (2/spl middot/10/sup -5/-5/spl middot/10/sup -5/). However, the total required sensing length (720 /spl mu/m) of our PSW-FPI sensor is much shorter than that of the previously reported devices (9-20 mm).     

基于强度检测的包层模谐振特种光纤传感系统

提出了一种基于强度检测的包层模谐振特种光纤传感系统。包层模谐振特种光纤是一种新型的传感元件,对折射率和弯曲等物理量十分敏感。利用它的光谱滤波特性,结合分布式反馈激光器的窄带光谱,可将传感参量改变而引起的光谱移动,最终转化为光强度的变化。在该传感系统中设置了参考光路,抑制了光源波动对实验结果的影响。分析了传感系统中其他影响传感精度的因素。这种方法基于光强度的测量,采用全光设计,无需机械部件调谐,具有方法简单、成本低廉等优点。通过实验,实现了0.0005 R.I.U.(单位折射率)的解调精度。     

Design of high-sensitivity metal-coated LPFG sensor based on material dispersion

A high-sensitivity metal-coated long-period fiber grating(LPFG) sensor based on material dispersion is designed.Based on the coupled mode theory,the influence of the material dispersion on the dual-peak characteristics of the metal-coated LPFG is studied.After considering the material dispersion,the jumping region of the dual-resonant-wavelength shifts toward the thinner film thickness,and the sensitivity of the dual-peak metal-coated LPFG sensor to liquid refractive index(RI) can be obtained to supply accurate parameter combinations.Experimentally,two kinds of silver-coated LPFGs with different film thicknesses and grating periods are fabricated to monitor the salt solution,and the sensitivities of these two sensors are compared.The experimental results are consistent with the theoretical analyses.     

Microfluidic integration of porous photonic crystal nanolasers for chemical sensing

We have recently developed planar photonic crystal nanolasers based on porous cavity designs. High-quality factor cavities confine light within the pores of the photonic crystal and, thus, our lasers are ideally suited for the investigation of nanoscale interactions between light and matter. We have demonstrated the operation of photonic crystal lasers within different chemical solutions, embedded them into silicone microfluidic flow channels, and were able to detect refractive index changes as small as /spl Delta/n=0.005. We predict that our porous nanolasers can detect refractive index changes as small as /spl Delta/n=8.23/spl middot/10/sup -4/.     

Suppression of cladding-mode coupling loss in fiber Bragg gratings by independent control of refractive index and photosensitive profiles in a single-mode optical fiber

A new method to suppress the cladding-mode coupling loss in fiber Bragg gratings, by independent control of the core refractive index profile and the photosensitive profile, is proposed and experimentally demonstrated. Across the core and the inner cladding, a uniform step photosensitive profile was introduced by co-doping GeO/sub 2/ and B/sub 2/O/sub 3/. The core refractive index was selectively raised by further doping Al/sub 2/O/sub 3/ in the core that has negligible photosensitivity at 244 nm. For strong Bragg gratings inscribed on the fiber, the cladding-mode coupling loss was suppressed below 0.3 dB.     

Strain Characteristics of CO2-Laser-Carved Long Period Fiber Gratings

Unique strain characteristics of long period fiber gratings (LPFG) fabricated by a focused CO₂ laser beam carving periodic grooves on the fiber are investigated for the first time to our knowledge. Resonant wavelength, transmission attenuation, and polarization dependent loss (PDL) of the CO₂-laser-carved LPFG are found to depend strongly on the tensile strain applied, and their strain sensitivities are dependent on the depth of grooves and/or the initial refractive index modulation. The average strain sensitivity of resonant wavelength for LPFG is increased by 229 times and is up to -102.89 nm/mepsiv by means of carving periodic grooves on the fiber. When a stretching force is applied to the LPFG, the resonant wavelength can "blue" shift by -11.84 nm, the absolute value of peak transmission attenuation and the maximum PDL can be increased by 25.913 and 26.535 dB, respectively. The CO₂-laser-carved LPFG combines the features of the three types of LPFGs reported previously, i.e, the CO ₂-laser-induced LPFGs without physical deformation, the corrugated LPFGs fabricated by hydrofluoric acid etching, and the microbend-induced LPFGs. The mechanisms of refractive index modulation in the CO₂-laser-carved LPFGs under tensile strain are rather complicated and may be regarded as a combination of the stress-relaxation-, the groove-, the strain-, and the microbend-induced refractive index perturbations     

镀膜长周期光纤光栅双峰谐振折射率传感特性

基于严格的相位匹配公式,在充分考虑材料色散的基础上,详细研究了镀膜长周期光纤光栅的双峰谐振效应.研究发现,随着环境折射率的变化,发生谐振的双峰也发生变化,且变化趋势相反.给出了镀膜长周期光纤光栅双峰谐振波长之间的距离随环境折射率变化的关系,并与普通长周期光纤光栅的同等情形进行了比较,发现镀膜长周期光纤光栅的双峰谐振能够极大提高长周期光纤光栅的折射率灵敏度.但由于发生双峰谐振需要的包层模式次数较高,故其探测范围比应用低次耦合的单峰探测范围要窄.     

Novel fiber-optic sensors based on long-period fiber gratings written by high-frequency CO/sub 2/ laser pulses

In this paper, we report a novel long-period fiber grating (LPFG) fabricated by using a new writing technique that is mainly based on the thermal shock effect of focused high-frequency CO/sub 2/ laser pulses at several kilohertz. A number of unique characteristics of such a LPFG, such as bend, torsion, and transverse load, are observed by experiments, for the first time, to our knowledge. Based on these unique features, a novel bend-insensitive LPFG sensor that could solve the problem of cross-sensitivity between bend and other measurands, a novel torsion sensor that can realize absolute measurement of twist rate, and a load sensor that can achieve simultaneous measurement of transverse load and temperature using a single LPFG element are proposed and demonstrated. These unique features of the LPFGs are mainly due to the asymmetrical distribution of the refractive index on the cross section of the LPFG induced by high-frequency CO/sub 2/ laser pulses.     

基于双开口环型的太赫兹超材料传感器设计

设计了一款可用于检测材料折射率及厚度的双开口环型太赫兹超材料传感器,其结构由双开口方环与圆环嵌套的超材料结构和聚酰亚胺衬底两部分构成.当太赫兹波垂直入射超材料表面时,该传感器结构在0.8～1.8 THz范围内形成三个高Q值谐振峰(中心频率分别为f1,f2和f3).通过探讨超材料结构表面电流分布与三个谐振峰形成的关系,观察到超材料结构对入射太赫兹波的不同响应特性导致产生不同的表面电流分布.此外,还对该传感器在折射率传感和厚度传感方面的应用进行了探究.在待测物厚度一定的情况下,该传感器在谐振频率f1,f2和f3处的传感灵敏度分别可达170,103和119 GHz/RIU,均具有优越的传感特性,可利用其多谐振峰进行高灵敏度折射率传感.这种高灵敏度的多谐振峰折射率传感器可以检测到待测分析物的微小变化,在生物化学检测领域具有广阔的应用前景.     

Transmission spectra of coated phase shifted long-period fiber gratings

The transmission spectrum of the coated phase-shifted long-pcriod fiber gratings （LPFCrs） with single and multiple phase shifts is analyzed by the coupled-mode theory and the transfer matrix method, and the influences of the film parameters on the spectral characteristics are also studied. It is shown that these parameters will affect the LPFG filtering characteristics. The loss peak of Wansmission spectrum decreases with the increase of film thickness, and the peak position shifts with the film refractive index. Compared with the non-coated phase-shifted LPFG, the coated one has the similar desirable filtering characteristics, and it has a flexible ability to adjust the transmission properties.     

Controllable in-line UV sensor using a side-polished fiber coupler with photofunctional polymer

A novel ultraviolet (UV) sensor using evanescent field coupling between single-mode-polished fiber and photochromic dye-dispersed polymer waveguide was demonstrated. It was found that resonance wavelength shifts occurred due to variations in the refractive index of the photochromic dye-dispersed polymer waveguide relative to UV intensities. A spiroxazine dye was chosen as the overlay material because of its photochromic isomerization induced by UV irradiation. The wavelength responses of these sensors by UV exposure power were measured 0.18 and 0.25 nm/mW when UV was exposed for 5 and 10 s, respectively. The recovery time for the sensor was independent of UV exposure power, and 80% recovery time was shown 90 s.     

Wavelength-division demultiplexing using graded-index planar structures

In this paper, the authors propose a novel technique for wavelength-division demultiplexing using a graded-index planar structure. The device consists of three layers of the same bulk material: The two outer layers are homogeneous media with refractive indices n/sub 1/ and n/sub 2/, while the inner layer is an inhomogeneous medium where its refractive index is graded according to a certain profile. The proposed technique exploits the spatial shift that results from the material dispersion found in dispersive media such as silicon dioxide (silica). It is found that the graded-index structure produces a spatial shift that is much higher than that encountered in conventional prisms, provided a certain refractive index profile is chosen. Unlike graded-index fibers, it is found that the value of /spl alpha/ of the refractive index profile (/spl alpha/-profile) for the proposed device must be < 1 to get large spatial dispersion. A mathematical expression for the spatial shift between adjacent wavelengths is found by determining the path profiles followed by the different wavelengths as they propagate through the graded-index layer. Theoretically, it is found that any spatial shift can be obtained by either reducing the value of /spl alpha/ far below 1 for a fixed size of the structure or increasing the size of the structure for a fixed value of /spl alpha/.     

Optical and Interferometric Lithography - Nanotechnology Enablers

Interferometric lithography (IL), the interference of a small number of coherent optical beams, is a powerful technique for the fabrication of a wide array of samples of interest for nanoscience and nanotechnology. The techniques and limits of IL are discussed with particular attention to the smallest scales achievable. With immersion techniques, the smallest pattern size for a single exposure is a half-pitch of /spl lambda//4n where /spl lambda/ is the optical wavelength and n is the refractive index of the immersion material. Currently with a 193-nm excimer laser source and H/sub 2/O immersion, this limiting dimension is /spl sim/34 nm. With nonlinear spatial frequency multiplication techniques, this limit is extended by factors of 1/2, 1/3, etc.-extending well into the nanoscale regime. IL provides an inexpensive, large-area capability as a result of its parallelism. Multiple exposures, multiple beams, and mix-and-match with other lithographies extend the range of applicability. Imaging IL provides an approach to arbitrary structures with comparable resolution. Numerous application areas, including nanoscale epitaxial growth for semiconductor heterostructures; nanofluidics for biological separations; nanomagnetics for increased storage density; nanophotonics including distributed feedback and distributed Bragg reflectors, two- and three-dimensional photonic crystals, metamaterials, and negative refractive index materials for enhanced optical interactions are briefly reviewed.     

Correlation study between VAD preform deposition surface and Germanium doping profiles

The refractive index profile of germanium doped preforms for optical fibers is determined by the radial distribution of germanium concentration. Knowing that there is a correlation between the germanium doping profile and the deposition surface profile of vapor-phase axial deposition (VAD) preforms, the study of this correlation has been carried out in order to estimate, indirectly, the refractive index profile of VAD preforms for optical fibers during the deposition stage. This correlation was studied through the parameterization of the preform deposition surface using two parameters: the power law index profile that best fits the preform bottom profile (/spl alpha/) and the axial distance from the bottom tip to a reference height (h). A range of values of these parameters to produce VAD preforms with standard and special doping profiles has been presented. Preforms with triangular index profile can be fabricated with /spl alpha/ and h values of about 2.0 and 5.0 mm, respectively, and preforms with parabolic index profiles can be produced with /spl alpha/ and h values of about 2.0 and 4.0 mm, respectively.     

薄包层长周期光纤光栅的研究

报道了在薄包层光纤上用高频CO2激光脉冲写入的长周期光纤光栅(LPFG),并分析了该类薄包层LPFG横截面折射率的分布.结合LPFG的腐蚀实验,通过传输谱图的比较发现:较先写入光栅后腐蚀光纤包层的方法,先腐蚀光纤包层后写入光栅的方法更能有效地调整谐振波长.进一步研究发现薄包层LPFG对环境折射率的敏感度更高.     

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.     

Liquid core modal interferometer integrated with silica waveguides

An integrated structure is demonstrated as a refractive index sensor. The structure consists of a liquid-filled elliptical microchannel embedded in silica glass and integrated with waveguides. The microchannel features entry points that are open to the top surface of the device and distinct from the optical input. The structure allows light to couple from a solid-core input waveguide to the liquid-core waveguide formed by the microchannel, and back to a solid-core output waveguide. Bimodal interference allows the structure to be sensitive to the refractive index of the liquid, with a full beat corresponding to a refractive index change of /spl sim/10/sup -4/. The structure allows the direct integration of optical fluids with silica waveguides for sensing and optical processing applications.