Low-loss criterion for fused polarisation-maintaining fibre couplers

A simple low-loss criterion for polarisation-maintaining directional couplers made of fused tapered stress-induced birefringent fibres is proposed and used to explain the loss characteristics of some experimental couplers. It is shown that the very low excess loss of some experimental couplers is due to the combined use of index-matched stress-applying regions and a high core-cladding index difference.<>     

Use of highly overcoupled couplers to detect shifts in Braggwavelength [strain sensors]

The authors present a technique for detecting Bragg wavelength shifts using highly overcoupled couplers (HOCC). An HOCC of 60%/nm was fabricated with 626 cycles. With our HOCC, a 5 μW LED and a 95% reflective Bragg grating we were able to detect 10 μstrain (Δλ=0.01 nm)     

Single mode fused couplers insensitive to external refractive index

Wavelength multiplexers produced using the fused taper technique have been fabricated to be insensitive to variations in the external refractive index, allowing them to be potted in silicone for environmental protection. The technique exploits two opposing dependencies by fabrication at a particular coupler cross-section     

Variational analysis of fused tapered couplers

A variational technique is used to evaluate the coupling ratio of a fused coupler. The technique can be used for both strongly and lightly fused couplers. Simple formulas are presented for use in regions of high V-value and are shown to be in agreement with the more lengthy beam propagation calculations. The importance of the fibre core in determining the field shape is highlighted. A realistic coupler model containing a gradual taper is presented and the strong wavelength response of the coupling ratio is confirmed by numerical means.     

Low loss fused D fibre couplers

Low loss fused D fibre couplers are fabricated by a technique simpler than any previously reported. No pre-treatment of the D fibre is required and because of the simplicity of the coupler structure, heat treatment such as core diffusion and tapering, which increase coupling, and annealed twists, which decrease coupling, can be applied without compromising the coupler's excess loss.<>     

Polarisation analysis of strongly fused and weakly fused tapered couplers

The polarisation properties of both strongly fused and weakly fused single-mode tapered fibre couplers have been analysed. A comparison with experimental results shows excellent agreement with our analysis.     

Fused couplers: condition for insensitivity to external refractive index

The condition necessary for fused tapered couplers to be insensitive to change in the external refractive index is derived. This condition depends critically on the shape of the coupler cross-section, e.g. it cannot occur in either the rectangular or in the elliptical cross-section.     

Completely fused tapered couplers: comparison of theoretical and experimental results

Completely fused single-mode fibre couplers, characterised by an almost circular cross-section at the waist, have been fabricated. An analytic model which takes into consideration the taper and variable aspect ratio of the coupler has been developed. Good agreement is obtained between the theoretical and experimental values.     

Polarization properties of fused fiber couplers and polarizingbeamsplitters

The polarization properties of fused taped fiber couplers were investigated experimentally by changing cross-sectional shapes at the coupler waist. The coupling difference between x and y polarized light was studied for many test fused fiber couplers with various cross-sectional shapes. On the basis of the experimental results, two polarizing beamsplitters with the aspect ratio of 1.1 and 1.8 were produced, and their chromatic characteristics were measured. It was evident that the aspect ratio should be more than 1.7 to manufacture polarizing beamsplitters with few power transfer cycles, i.e. long spectral period. Splitting ratios of polarizing beamsplitters with the aspect ratio of 1.1 and 1.8 were 17.6 and -12.7 dB for x polarized light and -16.1 and 11.7 dB for γ polarized light, respectively. Their losses were less than 0.6 and 0.3 dB     

Analysis of fused couplers by the effective-index method

Fused couplers of various cross-sections are analysed by the effective-index method. Results show good agreement with those given by an asymptotic analysis. It is also shown that a stadium-shaped coupler which resembles the realistic one has the polarisation beam-splitting property similar to that of a rectangular one.     

Optical bandwidth and fabrication tolerances of multimodeinterference couplers

Analytical expressions are derived which relate the optical bandwidth and the fabrication tolerances of multimode interference (MMI) couplers to their most important design parameters. These expressions compare adequately with mode analysis simulations. For strong guided structures, the optical bandwidth is shown to he inversely proportional to the number of input and output ports N and to the length of the device. The fabrication tolerances are independent of N and proportional to the output channel separation D. Measurements on MMI couplers in InP/InGaAsP corroborate the theoretical predictions     

Reproducible fabrication method for polarization preservingsingle-mode fiber couplers

The adaptation of the biconically-tapered-fused (BTF) fiber coupler fabrication approach to polarization preserving fiber couplers is reported. Two practical methods for identifying and aligning the birefringent axes of the fibers forming the coupler are described. The fabrication approaches are highly reproducible and the 3-dB couplers fabricated exhibit less than 1.0-dB insertion loss, controllable splitting ratio, and an average polarization extinction ratio of 15 to 20 dB     

Wavelength-flattened fibre couplers produced by UV-trimming of fused hybrid fibres

A novel method is described for fabricating wavelength-flattened fibre couplers with the combination of one intrinsically photosensitive fibre and one standard singlemode fibre. The spectrum monitored in real time can be shifted and flattened to the desired wavelength band by exposing the entire coupler to intense ultraviolet light.     

High-resolution fused silica mold fabrication for UV-nanoimprint

This paper describes the fabrication steps developed to pattern nano scale features on thin silica wafers. The optimization of e-beam exposure dose is presented. The use of a chrome layer on top of the silica wafer implies higher doses to crosslink the negative NEB22 resist, but the results show a very large window process. Specific etching processes have been developed. It is demonstrated how the micro-trenching and CD bias are reduced. Thanks to the optimization of both the exposure dose during e-beam lithography and the plasma dry etch steps, features with a resolution as low as 30 nm have been achieved.     

Large wavelength shifts in UV-exposed 3 dB and WDM fused taperfibre couplers

The spectral response of 3 dB and 1300/1550 nm wavelength division multiplexing fused taper fibre couplers can be shifted by over 100 nm by exposing the entire coupler with intense UV light at a wavelength of 193 nm and without prior hydrogenation. There is no discernable increase in excess loss. The major contribution to these shifts for these couplers originates in the down- and up-taper regions     

激光诱导等离子体加工石英微通道的损伤模型

利用调Q的Nd∶YAG激光器输出的纳秒激光脉冲诱导等离子体加工石英微通道,加工深度可达4.2 mm,通道内壁光滑。本文对激光脉冲与物质相互作用的移动损伤模型进行了研究,建立了适用于纳秒激光脉冲诱导等离子体加工石英微通道的损伤模型,利用损伤模型分析了通道特点,模型结果与实验结果基本相符。     

Directional couplers made of nonidentical asymmetric slabs. Part II: Grating-assisted couplers

In this second part of the paper we discuss TE and TM mode directional couplers made of nonsynchronous, nonidentical asymmetric slab waveguides. Coupling is accomplished with the help of a diffraction grating that is placed on either of the two guides on the side facing the opposite slab. The grating couples the compound modes of the complete slab structure to each other. However, since the two slabs are nonsynchronous, the compound modes resemble closely the modes of each slab in isolation. We also provide approximate expressions that result from approximating the compound modes with the modes of the individual slabs. The accuracy of these approximations is estimated by comparison with the exact results.     

Low-loss coupling of multiple fiber arrays to single-mode waveguides

A method for obtaining permanent low-loss couping between arrys of single-mode fiber and Ti:LiNbO3waveguides is described. The technique, based on the use of silicon chip V-grooves, simplifies the coupling problem by simultaneously aligning the entire array and by providing a large surface area for a higher integrity adhesive bond. Atlambda = 1.3 mum, we measure an average 1.9-dB coupling loss (exclusive of propagation loss) for the assembled array. The average excess loss due to the fiber array is 0.8 dB. We present an analysis of the effect of various types of array misalignment on coupling efficiency. Angular alignment and array periodicity are found to be critical. If the fiber and waveguide periodicities are matched exactly, the fibers need only be placed withinpm 1.3mum of their optimum position to maintain coupling efficiencies greater than 90 percent.     

Effects of external pressure on arteries distal to the cuff during sphygmomanometry

The aim of this study was to examine the effect on distal arteries of external pressure, applied by upper arm sphygmomanometer cuff. Photoplethysmographic (PPG) signals were measured on the index fingers of 44 healthy male subjects, during the slow decrease of cuff air pressure. For each pulse the ratio of PPG amplitude to its baseline (AM/BL) and its time delay (deltaTD) relative to the contralateral hand were determined as a function of cuff pressure. At cuff pressures equal to systolic blood pressure, pulses reappeared with the pulse time delay in the cuffed arm significantly greater than in the noncuffed arm, with (deltaTD) (mean +/- SD) 150 +/- 31 ms (p < 0.001). At cuff pressures equal to diastolic blood pressure (81 +/- 12 mmHg), deltaTD was 42 +/- 19 ms (p < 0.001), and at 50 mmHg, which is below diastolic blood pressure, (deltaTD) was still significantly positive at 6 +/- 9 ms (p < 0.001). AM/BL relative to its initial value rose at cuff pressures between systolic and diastolic blood pressure, then deceased to 0.6 +/- 0.41 (p < 0.001) at diastolic blood pressure and 0.54 +/- 0.24 (p < 0.001) at 50 mmHg. The changes in (deltaTD) and AM/BL can be interpreted as originating from changes in the compliance of conduit arteries and small arteries with cuff inflation and deflation.