Improvement in signal-to-noise ratio of Rayleigh backscatteringmeasurement using optical low coherence reflectometry

I have calculated the signal-to-noise ratio (S/N) of fluctuations remaining in Rayleigh backscattered signal distributions obtained after performing moving and frequency averaging with an optical low coherence reflectometer (OLCR). From the calculation, I obtained the number of measurements needed for each averaging that achieves the required S/N with the minimum sampling data. Specifically, I derived the numbers of measurements numerically for Gaussian and Lorentzian light spectra, respectively. The result was successfully applied to diagnosis on long silica-based waveguides by using an OLCR with a high-power narrow-band light source     

Jaggedness-free millimeter-resolution low coherence reflectometry

We report a millimeter resolution optical low coherence reflectometer (OLCR) that exhibits reduced jaggedness in the Rayleigh backscatter signal. This OLCR was achieved by using an erbium-doped superfluorescent fiber source that can tune narrow-band superfluorescence (~0.2 nm full-width at half-maximum) over a 3-nm range with an output of -8 dBm, The signal averaging that resulted from wavelength tuning with this source efficiently reduced the jagged fluctuation to ±1.4 dB. The spatial resolution was 1.2 mm, and the hidden spaces defined by the full-widths at -20 dB and -60 dB maximum of a Fresnel response of the OLCR were 6 mm and 1.2 cm, respectively. These two hidden spaces are at least ten times narrower than those of a previously reported photon-counting OTDR, although their spatial resolutions are roughly the same. The OLCR enabled the weak Rayleigh backscattering near a connector joint to be measured with a dynamic range of 18 dB without any deconvolution. We confidently expect that this OLCR will be applied to short-haul optical fiber fault location     

Optical low coherence reflectometer with a 47-dB dynamic rangeachieved by using a fluoride-based erbium-doped fiber amplifier

We describe a jaggedness-free optical low coherence reflectometer (OLCR) with a high power tunable narrow-band light source which employs a fluoride-based erbium-doped fiber amplifier. The use of the amplifier simplifies the structure of the source and enables us to generate narrow-band light with a power of 18±0.2 dBm in the 1530-1560 nm wavelength range and with a reduced background level in each spectrum. The OLCR achieves a dynamic range of 47 dB and the signal fluctuations caused by residual speckle-like noise in the Rayleigh backscattering measurement are less than ±1 dB. This is the best data yet obtained with a diode-pumped fiber amplifier. The diagnosis of fiber-optic attenuators is successfully demonstrated using the OLCR     

Measurement of depolarization ratio and ultimate limit ofpolarization crosstalk in silica-based waveguides by using a POLCR

A polarization optical low coherence reflectometer (POLCR) is described that enables us to excite only the transverse electric (TE) mode of a test waveguide and to measure its Rayleigh backscatter signal distributions in the TE and transverse magnetic (TM) modes at a spatial resolution of 0.3 mm. The depolarization ratios of silica-based waveguides with relative refractive index differences of Δ=0.45 and 0.75% are obtained as 0.14 and 0.10, respectively, by measuring the bias in the ratio between the distributions in the TM and TE modes of each waveguide. By using the depolarization ratios and the Rayleigh backscatter signal levels, we calculate the ultimate polarization crosstalks to be -53 and -51 dB over 1 km, respectively. The actual polarization crosstalks of previously fabricated waveguides are about 50 dB higher than their ultimate limits for the same length of fiber     

Losses in tantalum pentoxide waveguides

Thin films of tantalum pentoxide for possible use as waveguides have been prepared by reactive sputtering and by thermal and anodic oxidation of sputtered tantalum films. Measurements have been made of the losses in these films for different guided modes. These results have been analyzed to determine the loss mechanism. The losses do not fit the Rayleigh scattering theory precisely and the deviation is attributed to absorption by impurities at the surface as well as in the bulk. The effects of bulk and surface loss mechanisms have been obtained and causes of these are discussed.     

Statistical distribution of polarization-dependent loss in thepresence of polarization-mode dispersion in single-mode fibers

The statistical distribution of polarization-dependent loss in single-mode fibers is studied via Monte-Carlo simulations. In the presence of polarization-mode dispersion, it is found that the probability density function of polarization-dependent loss is a combination of Rayleigh and Maxwell distributions     

Mode field radius definitions for arbitrary planar single-modeoptical waveguides and their relations to splice loss and propagationconstant

Root-mean-square and Laplacian mode field radius (MFR) definitions for planar single-mode optical waveguides (PSMOW) with arbitrary index profile are proposed based on the second moment of the near-field (NF) and far-field (FF) distributions. Expressions of these definitions in both NF and FF, as well as their simple relations to the propagation constant and splice loss between two identical waveguides, are derived     

Spectral Loss Characterization of Femtosecond Laser Written Waveguides in Glass With Application to Demultiplexing of 1300 and 1550 nm Wavelengths

Femtosecond laser written waveguides in glass were characterized across the full telecom spectrum to gain insight into waveguide loss mechanisms, and to aid in the design of a low-loss 1300/1550 nm wavelength demultiplexer. A lambda^-4 wavelength scaling of propagation loss confirms Rayleigh scattering as a principal loss mechanism. Laser exposure was optimized for generating low-loss directional couplers with high isolation between the 1300 and 1550 nm bands. Dispersive coupling in the straight and curved wavelength regions was balanced with a 1.5-fold difference in 1300 and 1550 nm beat lengths, leading to the first demonstration of 1300/1550 nm demultiplexer written with a laser. A minimum interaction length of 3.2 mm, ~2 dB insertion loss and channel isolations of 16.7 and 18.8 dB are reported.     

High sensitivity and submillimeter resolution optical time-domainreflectometry based on low-coherence interference

Characteristics of the low-coherence optical time-domain reflectometer (OTDR) are presented, and optical waveguide diagnosis using the OTDR is demonstrated. Ultrahigh sensitivity in the shot noise limited operation is discussed, and jagged-shaped Rayleigh-backscatter signals are predicted and observed experimentally. Signal smoothing by widening the resolution from 14 to 400 μm can drastically reduce the jagging. In the experiment, polarization-sensitive and insensitive OTDRs using a fiber-coiled piezoelectric phase modulator are proposed. Measurements of beat length and bending loss, quality characterization of low-loss waveguides by polarization-sensitive OTDR, and cancellations of polarization fading in discrete backreflection and polarization jagging in Rayleigh backscattering by polarization-insensitive OTDR are successfully demonstrated     

Computational modeling of diffused channel waveguides using adomain integral equation

A domain integral equation is proposed for the computational modeling of diffused channel waveguides. In the modeling the propagation properties and the field distributions of the lower order guided modes are computed. The method is used to design the channel waveguides that are realized by an ion-exchange process in glass substrates. In particular, the method is applied to the design of ion-exchanged waveguides with low fiber/chip coupling losses. The aim is to realize modal distributions in the channel waveguides that closely match the rotationally symmetric field distributions of the HE₁₁ fiber mode. Some technological aspects of the realization of such ion-exchange waveguides are indicated, and various numerical results relevant to the design process are presented     

弯曲光波导端面修饰的设计方法

光波导的弯曲损耗一直是集成光学中一个值得人们重视的问题,为有效地减小光波导弯曲损耗,弯曲光波导的设计成为集成光学波导设计中的一个重要内容.本文在弯曲光波导保形变换方法的基础上,应用几何光学分析方法给出了波导弯曲损耗产生的简洁的物理图像,从而深入地分析了弯曲损耗产生的根本原因,并由此提出了弯曲光波导端面修饰的设计方法.理论分析表明,与传统的弯曲的光波导设计相比,该种设计方法可以有效地减小导波的模式泄漏,从而减小波导弯曲损耗.尤其是在弯曲光波导曲率半径、材料折射率等参数被设定的情况下,该种设计方法成为一种有效的设计方法.     

DS-OCDMA encoder/decoder performance analysis using optical low-coherence reflectometry

Direct-sequence optical code-division multiple-access (DS-OCDMA) encoder/decoder based on sampled fiber Bragg gratings (S-FBGs) is characterized using phase-sensitive optical low-coherence reflectometry (OLCR). The OLCR technique allows localized measurements of FBG wavelength and physical length inside one S-FBG. This paper shows how the discrepancies between specifications and measurements of the different FBGs have some impact on spectral and temporal pulse responses of the OCDMA encoder/decoder. The FBG physical lengths lower than the specified ones are shown to affect the mean optical power reflected by the OCDMA encoder/decoder. The FBG wavelengths that are detuned from each other induce some modulations of S-FBG reflectivity resulting in encoder/decoder sensitivity to laser wavelength drift of the OCDMA system. Finally, highlighted by this OLCR study, some solutions to overcome limitations in performance with the S-FBG technology are suggested.     

Thermal Aging Reliability of Package-Level Polymer Optical Waveguides

Polymer optical waveguides are viewed as a potential interconnect solution in board-level optoelectronic systems. In this paper, the optical loss changes in siloxane polymer waveguides during thermal aging conditions are studied for the wavelengths of 850 and 1310 nm. The optical loss in waveguides during intended operation and temperature exposure can increase due to factors such as oxidation of waveguides, increased absorption, and scattering. In addition to these inherent changes in the optical properties of the waveguides, physical failures such as delamination and cracking of waveguides will also increase the optical loss. This paper focuses on the first set of parameters that affects the optical loss and as a first step; the optical absorption of the polymer material is characterized through spectroscopy experiments. The thermal-aging dependent optical loss is determined for waveguide samples at several different accelerated temperature conditions. The temperature contours in a polymer waveguide with an embedded laser are determined from experiments as well as finite-element modeling. Using experimental data, analytical models have been developed that relate the optical loss with temperature and time, and provide a practical way of determining the reliability of the optical waveguides during field-use conditions.     

弯曲光波导模拟优化研究

用时域有限元差分（FDTD）方法弯曲光波导的优化仿真，减小弯曲光波导损耗。采用OPTIFDTD软件在曲率半径增大到一定的情况下，弯曲损耗在各种损耗中不占优势。在半径一定的条件下，适当选择波导的各个参数进行优化，可以使弯曲损耗达到最低，模拟了弯曲光波导中光的传播，得到了弯曲损耗的变化规律，波导参数不同，弯曲损耗不同，其中有一最佳参数点，由此得到弯曲光波导的参数优化设计值。     

Scattering loss of antiresonant reflecting optical waveguides

The scattering loss of two-dimensional antiresonant reflecting optical waveguides (ARROW) and of ARROW-B, which has a similar structure to ARROW and less polarization dependence, are analyzed by the first-order perturbation theory. Calculated results are compared with those of conventional three layer waveguides. Optimum design for the reduction of scattering loss of these ARROW-type waveguides is discussed. It was found that the scattering loss of ARROW-type waveguides is no larger than that of a conventional waveguide having a relative refractive-index difference, Δ of 2.5%, despite each interface of ARROW-type waveguides having a large Δ, normally larger than 20%. The optimum design for the reduction of essential radiation loss of ARROW is also optimum for the reduction of scattering loss     

Measurements of Dispersion Characteristics and Field Distributions in Rectangular-Dielectric Waveguide and its Modifications

The author has previously numerically analyzed transmission characteristics of several dielectric waveguides by applying generalized telegraphist's equations. The technique may be applicable to a wide class of dielectric waveguides with arbitrary dielectric profile and cross section. In this paper, the dispersion characteristics and the field distributions in rectangular-dielectric waveguides and in single material fibers have been measured at X-band frequencies to verify the validity of the analysis. Good agreement between measured and calculated results shows that the analysis is useful to study the transmission characteristics of dielectric waveguides.     

利用光子晶体波导定向耦合差频产生毫米波

文章利用耦合波理论研究了光子晶体两平行直波导的定向耦合差频特性,得出以频率相近的两单频波入射两波导输入端口时,波导中的光场分布的解析表达式,并采用时域有限差分法计算了耦合波导中的模场分布.计算结果表明:通过波导的耦合作用可以在两平行波导中检测到入射波的差频包络,其时域特性与理论分析相符.提出了利用光子晶体定向耦合波导实现光波-毫米波的转换.     

Leaky Waveguide: TE/sup cir//sub 01/ Circular Waveguide Periodic Array of Circular Apertures

This paper describes propagation characteristics of periodic leaky waveguides for helix waveguides and solid metal waveguides carrying a TE/sup cir//sub 01/ mode based primarily on experimental studies. For low-loss leaky waveguides, this type has not been considered feasible, because of the critical problem encountered in mode conversion-reconversion at the circular apertures. Additional loss due to the periodic array of the circular apertures and TE/sup cir//sub 01/ loss/frequency characteristics of the periodic leaky waveguide using helix waveguides and solid metal waveguides are measured by the shuffle-pulse method in the X-band. The periodic leaky waveguides using solid metal waveguides are seen to be acceptable for practical use as a low-loss leaky waveguide system, according to these measurements.     

Modeling of optical low coherence reflectometry recorded Braggreflectograms: evidence to a decisive role of Bragg spectral selectivity

This paper draws attention to the basic principles governing reflections in uniform Bragg reflectors (BR) when measured employing optical low coherence reflectometry (OLCR) technique. Using computations based on transfer matrix method (TMM), we first showed a strong spectral dependence of Bragg reflectograms on an OLCR probe spectrum. Later, this dependence is exploited to evaluate, for the first time, the coupling coefficient κ of a Bragg grating in a distributed Bragg reflector (DBR) laser on InP     

The κ-μ distribution and the η-μ distribution

This paper presents two general fading distributions, the κ-μ and the η-μ distribution, for which fading models are proposed. These distributions are fully characterized in terms of measurable physical parameters. The κ-μ distribution includes the Rice (Nakagami-n), the Nakagami-m, the Rayleigh, and the one-sided Gaussian distributions as special cases. The eta-mu distribution includes the Hoyt (Nakagami-q), the Nakagami-m, the Rayleigh, and the one-sided Gaussian distributions as special cases. Field measurement campaigns were used to validate these distributions. It was observed that their fit to experimental data outperformed that provided by the widely known fading distributions, such as the Rayleigh, Rice, and Nakagami-m. In particular, the κ-μ distribution is better suited for line-of-sight applications, whereas the η-μ distribution gives better results for non-line-of-sight applications