Design of low-loss single-polarization single-mode photonic-crystal fiber based on polymer

A new structure for single-polarization single-mode photonic-crystal fiber is proposed and numerically analyzed by using a full vector finite element method with anisotropic perfectly matched layers. The cutoff wavelength of two linearly polarized states can be controlled artificially by varying the structure parameters of photonic crystal fiber. The confinement loss are also numerically calculated and optimized at 650 nm communication wavelength of polymer optical fiber. From the numerical results it is confirmed that the proposed fiber is low-loss single-polarization single-mode photonic-crystal fiber within the wavelength range from 0.63 to 0.73 µm, where only the slow-axis mode exists and the confinement loss is less than 0.05 dB m^?1.     

Single-mode regime of square-lattice photonic crystal fibers

The modal cutoff of square-lattice photonic crystal fibers with a finite number of air-hole rings has been accurately investigated to our knowledge for the first time. By analyzing the leaky behavior of the second-order mode, we have obtained a phase diagram that describes the regions of single-mode and multimode operation as well as the endlessly single-mode regime. Furthermore, starting from these results, we have obtained the cutoff normalized frequency according to two different formulations of the V parameter previously adopted for fibers with a triangular lattice. A final comparison of the cutoff properties of fibers characterized by a square lattice and a triangular lattice has been carried out.     

Comparison of cutoff wavelength measurements for single-mode waveguides

The intrinsic cutoff wavelength of the LP11 mode is investigated using three different types of measurement for an ITT single-mode fiber. Characterization of the far-field pattern of the LP01 mode gives a cutoff value approximately 660 nm, a near-field transmission experiment gives approximately 690 nm, and a refracted power measurement gives approximately 830 nm. We conclude that the refracted power technique is the best experimental method for the determination of the intrinsic cutoff wavelength of a fiber sample. The effect of the loss of coupling efficiency into the LP11 mode as cutoff is approached on the transmission and refracted power experiments is noted.     

Measuring cutoff wavelength of HE21, TE01, and TM01 modes: a novel method using single-polarization launching

A new method for measuring the cutoff wavelength of HE21, TE01, and TM01 modes in single-mode fibers is reported. The method is based on the difference in polarization between the HE11 and second-order modes launched in the offset condition. A single-polarization fiber several meters in length is used as a launching fiber to eliminate cladding mode disturbance and realize linear polarization launching into a test fiber. Good agreement between theoretical and measured cutoff wavelengths for fibers with various refractive-index profiles confirms the high accuracy of the method. A resolution of +/-0.005 microm is attained with the method.     

Design of a single-polarization single-mode photonic crystal fiber with a near-Gaussian mode field and wide bandwidth

Single-polarization single-mode (SPSM) fiber can efficiently eliminate polarization mode coupling, polarization mode dispersion, and polarization-dependent loss. Up to now, most single-polarization fibers have been designed based on form birefringence, which would result in a non-Gaussian field distribution and a small effective mode field area. In this paper, a novel structure of SPSM photonic crystal fibers based on the resonant coupling phenomena is proposed and analyzed by using a full-vector finite-element method with a second-order transparent boundary condition. From the numerical results it is confirmed that this fiber has a near-Gaussian mode field within the wavelength range from 1.46 to 2.2 μm, where only one polarized mode exists effectively, and the mode field area is about 79 μm(2) at the wavelength of 1.55 μm, matching that of the conventional single-mode fiber.     

Accurate Measurements of the Zero-Dispersion Wavelength in Optical Fibers

We have developed a frequency-domain phase shift system for measuring the zero-dispersion wavelength and the dispersion slope of single-mode optical fibers. A differential phase shift method and nonlinear four-wave mixing technique were also investigated. The frequency-domain phase shift method is used to produce Standard Reference Materials that have their zero-dispersion wavelengths characterized with an expanded uncertainty (k = 2) of ± 0.060 nm.     

Mode-selective dynamic light scattering: theory versus experimental realization

We present a quantitative experimental comparison of fiber-based, single- and few-mode dynamic light scattering with the classical pinhole-detection optics. The recently presented theory of mode-selective dynamic light scattering [Appl. Opt. 32, 2860 (1993)] predicts a collection efficiency and a signal-tobaseline ratio superior to that of a classical pinhole setup. These predictions are confirmed by our experiments. Using single-mode optical fibers with different cutoff wavelengths and commercially available mechanical components, we have constructed a mode-selective detection optics in a simple and compact dynamic light-scattering spectrometer that permits an optimal compromise between signal intensity and dynamical resolution.     

Photonic crystalline IR fibers for the spectral range of 2-40 μm

Monocrystals of Ag(1-x)Tl(x)Br(1-x)I(x) and Ag(1-x)Tl(x)Cl(y)I(z)Br(1-y-z) for the spectral range from 2.0 to 40.0 μm with improved photostability were developed and grown. The grown crystals were used for fabrication of single-mode IR fibers. Experimental studies of optical properties of these fibers have confirmed their single-mode operation at CO2 laser wavelength and demonstrated wider mode field for microstructured fiber compared to fibers with conventional double-layered structure.     

Proposal for optical fiber designs with ultrahigh effective area and small bending loss applicable to long haul communications

A proposal for the multiclad MII optical fiber structure with ultralarge effective area and small bending loss is presented. For the proposed structure small dispersion and dispersion slope are obtained thanks to what we believe to be a novel design method. The suggested design method is based on a weighted fitness function, which is applied to the genetic algorithm optimization technique. In the meantime, the foregoing structure introduces a special fiber whose mode field diameter is small and approximately insensitive to the variation of the effective area. Compared to the work reported previously, our method can precisely set the zero dispersion wavelength. The designed dispersion-shifted single-mode fibers have effective area, mode field diameter, and quality factor respectively within [150-194.79] microm(2), [6.82-7.95] microm, and [3.04-3.85] at lambda(0)=1.55 microm. An analytical method is used for the calculation of the dispersion and its slope. These calculations give dispersion and dispersion slope of [(-2.57 x 10(-4))-(-0.085)] ps/km/nm and approximately 0.064 ps/km x nm(2), respectively.     

Single-mode fiber measurements

The author discusses the various techniques used to characterize the following transmission parameters of single-mode fibers: attenuation, cutoff wavelength, mode-field diameter, and chromatic dispersion. The cutback method, the insertion-loss method, and optical time-domain reflectometry are considered for attenuation measurements. The single-band and power step methods are considered for cutoff-wavelength measurements. The transverse-offset, and near- and far-field techniques are considered for the measurement of mode-field diameter, and time- and frequency-domain methods and interferometry are considered for chromatic-dispersion measurements. The single-bend and power step methods are considered for cutoff-wavelength measurements     

Leaky-mode loss of the second propagating mode in single-mode fibers with index well profiles

Radiation loss characteristics are calculated for the first two LP modes of a double-clad lightguide structure in which the inner cladding forms a low refractive-index well between the core and the outer cladding. The higher-order mode is defined to be effectively cut off at wavelengths where the LP(11) mode power is sufficiently attenuated to ensure negligible modal noise due to interference between LP(01) and LP(11) modes. Calculations apply for various deposited cladding thicknesses and findings can be easily extrapolated if specifications are changed for system requirements on the minimum unspliced fiber length or the extinction ratio between P(11) and P(01) mode powers, Re = 10 log P(11)/P(01). The latter is specified at points such as splice joints where modal noise is generated. Theoretical results are correlated with measurements in order to determine the effective cutoff wavelength for fibers in which leaky-mode loss mechanisms dominate. The procedure should be extremely useful for determining the shortest usable wavelength for single-mode lightwave systems.     

Fiber designs with significantly reduced nonlinearity for very long distance transmission

A class of low-nonlinearity dispersion-shifted fibers based on depressed-core multistep index profiles is investigated. A systematic approach for designing these fibers in which a reference W-index profile is used to initiate the design ispresented. Transmission properties, including effective area, mode-field diameter, dispersion, dispersion slope, and cutoff wavelength, are evaluated for several design examples. The effects of varying fiber dimensions and indices on effective area and mode-field diameter are assessed. It is shown that there is a trade-off between these two properties and, generally, larger effective areas are associated with larger mode-field diameters. Dispersion-shifted single-mode fiber designs with effective areas of from 78 to 210 mum(2) and the corresponding mode-field diameter of from 8.94 to 14.94 mum, dispersion less than 0.07 ps/nm km, and dispersion slope of approximately 0.05 ps/nm(2) km are presented.     

Analysis of cladding-mode couplings for a lensed fiber integrated with a long-period fiber grating by use of the beam-propagation method

The beam-propagation method (BPM) is employed to analyze the coupling behavior of our scheme proposed previously, which combines a lensed fiber and a long-period fiber grating (LPFG) [Chen and Wang, Appl. Opt. 39,4490-4500 (2000)]. The influences of a core within the fiber lens are investigated. As for the fiber dependence of our coupling scheme, two typical fibers are studied: dispersion-shifted and single-mode, step-index fibers. With the BPM, the optimal coupling efficiencies for various source waists with corresponding lens radii and working distances are determined. We also compare the results with those obtained by use of the ABCD method and found that the BPM gives better agreement with experimental results.     

Modeling and simulation of macrobending losses in metal-coated single-mode fibers

A formula for calculating macrobending losses in single-mode fibers with metal coating is derived. The macrobending losses in a standard single-mode fiber coated with different metal coatings are simulated using this formula. Simulation results indicate that metal coating will induce strong bend loss oscillations as a function of bend radius and the stronger bend loss oscillations of the metal-coated fiber occur with the decrease of bend radius; the wavelength and coating refractive index also have significant impact on the fiber bend losses.     

Compact spot-size converters with fiber-matched antiresonant reflecting optical waveguides

We propose a concept for InGaAsP-InP 1.55-microm lasers integrated with spot-size converters based on modal interference between the modes of the structure formed by an active waveguide and an underlying fiber-matched antiresonant reflecting optical waveguide. Simulation results show that the spot-size converters exhibit low transformation loss, and narrowed far-field emission patterns (10 degrees x 20 degrees) and reduce the coupling loss to standard single-mode fibers from 8 to 2.6 dB over lengths approximately 200 microm shorter than the adiabatic concept. A tolerant design to fabrication variations is also proposed, which could be realized by standard processing techniques.     

Single-nanowire single-mode laser

We demonstrate single-mode laser emission in single nanowires. By folding a 200 nm diameter CdSe nanowire to form loop mirrors, single-mode laser emission around 738 nm wavelength is obtained with line width of 0.12 nm and low threshold. The mode selection is realized by the vernier effect of coupled cavities in the folded nanowire. In addition, the loop structure makes it possible to tune the nanowire cavity, opening an opportunity to realize a tunable single-mode nanowire laser.     

Photonic generation of tunable microwave signal using Brillouin fiber laser

A simple approach to generate two bands of tunable microwave signal is proposed and demonstrated. In this scheme, two single-mode fibers with optimized Brillouin frequency shift spacing have been chosen as the scattering medium in two cascaded ring cavities. Two bands of tunable microwave signal from 390 to 453 MHz and 10.863 to 11.076 GHz can be obtained through adjusting the temperature of the fiber and the pump wavelength. The tunable frequency range can be further expanded by using a temperature controller with a wider adjustment range. The generated microwave signal exhibits high stability on frequency.     

Eigenvalue equation and core-mode cutoff of weakly guiding tapered fiber as three layer optical waveguide and used as biochemical sensor

The core-mode cutoff plays a major role in evanescent field absorption based sensors. A method has been proposed to calculate the core-mode cutoff by solving the eigenvalue equations of a weakly guiding three layer optical waveguide graphically. The variation of normalized waveguide parameter (V) is also calculated with different wavelengths at core-mode cutoff. At the first step, theoretical analysis of tapered fiber parameters has been performed for core-mode cutoff. The taper angle of an adiabatic tapered fiber is also analyzed using the length-scale criterion. Secondly, single-mode tapered fiber has been developed to make a precision sensor element suitable for chemical detection. Finally, the sensor element has been used to detect absorption peak of ethylenediamine. Results are presented in which an absorption peak at 1540 nm is observed.     

Adaptive beam steering implemented in a ferroelectric liquid-crystal spatial-light-modulator free-space,fiber-optic switch

Active alignment of a 1 x 8 free-space optical switch was studied experimentally. Optical signals, carried on single-mode fibers, were switched by a ferroelectric liquid-crystal-on-silicon spatial light modulator. Continuous measurement of the in-coupled power to the fibers provided feedback for the switch control. The switch automatically located and locked to the output fibers. An advantage with adaptive switches of a similar kind is relaxed geometrical tolerances in the switch assembly. Further, such switches can adapt to possible geometrical changes and light wavelength drift during operation.     

Temperature-insensitive polarimetric vibration sensor based on HiBi microstructured optical fiber

A new type of highly birefringent microstructured optical fiber has been tested for vibration measurements using a polarimetric technique. This technique takes advantage of the stress-induced phase shift between the two orthogonally polarized fiber eigenmodes. Comparison of three different fiber types shows that standard single-mode fibers do not provide stable measurements and that conventional polarization-maintaining fibers lead to a significant cross-sensitivity to temperature. However, for highly birefringent microstructured fibers specifically designed to provide a temperature-independent birefringence, our experiments show repeatable vibration measurements over a frequency range extending from 50?Hz to 1?kHz that are unaffected by temperature variations (up to 120?°C).