A 3 X 2 optical fiber flat coupler can be made by fusion and elongation of three adjacent fibers. Two types of resonant structure can be formed by leading back one of the output ports to one of the input ports. They are called symmetrical and asymmetrical ring resonators. The study is limited to the symmetrical case assuming low coupling coefficients. The output signals are very sensitive to the input wavelength and the length of the fiber ring. In this work, two cases are studied. In case A, the light launched to one of the input ports results in two in-phase output signals, while the light launched to other input port results in two out of phase output signals. In case B, there is a complete switching of power from one output port to the other.
A fiber-optic frequency shifter utilizing torsional acoustic waves to produce a travelling perturbation which couples the polarization eigenmodes of a linearly birefringent optical fiber, is demonstrated. The frequency shifter configuration is such that the acoustic wave generator and optical fiber are co-axial. The device is thus more rugged than previous 'free fiber' frequency shifters which have incorporated a T-junction between acoustic generator and optical fiber. The shift in optical frequency was 3.204 MHz and an optical coupling efficiency of 12% was obtained with 4 W of electrical drive power applied to the acoustic transducer. The theory of operation of this device is presented using a coupled mode approach, and the coupling coefficient for a fundamental torsional mode is derived in terms of its peak angular displacement.
A new form of all-fiber frequency shifter is presented based on the use of a ring resonator as a high resolution filter illuminated by a phase modulated input beam. The input phase modulation results in the generation of frequency components which are then filtered by the ring resonator. The cavity length is 'locked' to one frequency component and a fraction of that signal is removed from the cavity using an optical tap. The configuration offers a frequency tunable output which can provide complete suppression of unshifted light and very high suppression of unwanted sidebands through judicious choice of modulation index. A maximum frequency conversion efficiency of 17% has been measured with sidebands at -40 dB.
We have designed a low-loss Polarization Independent All-Fiber Optical Circulator for use in bi-directional fiber optics communication systems which can effectively double the information carrying capacity of existing fiber optics communications connections. Previous designs based on bulk optics or planar waveguides have had limited utility because of their high losses (often higher than 2.5 dB) compared to a 3 dB directional coupler. While coupling between a bulk optics or a planar waveguide optical circulator and an optical fiber results in a large loss, our design reduces coupling losses by keeping the path of the signal within the fiber throughout the device. Because splice losses between fibers can be as low as 0.2 dB, elimination of large coupling losses give our design low overall losses. We are currently evaluating different implementation of our design.
Fiber undercut and protrusion have been shown to play a key role in back reflection performance of pc polished connectors over temperature. End users are placing tolerances of +/-0.10 microns or less on the height differential between the fiber and the ferrule after polishing to get their performance requirements. Two methods for measuring this submicron height are compared, a stylus profilometer and an interference microscope. The surface profilometer provides an accurate two dimensional profile of the connector end face. The interferometric technique uses a tilt phase to provide information on the relation of the fiber to the ferrule surface that normally would not be seen. Advantages are shown in the three dimensional nature of the interferometric data which is ideal for measuring spherical surfaces and the simplicity and speed in which measurements can be made.
This paper reports a novel reflection type plastic molded optical switch which comprises two spherical prisms for totally reflecting incident light and a shutter positioned between the prisms for controlling the transmission loss of the light. The switch shows insertion loss of 9.7 dB when the switch is used in conjunction with optical fibers with 1 mm core diameter and extinction ratio of 35.1 dB. The insertion loss is lower by 8 dB than the conventional reflection type switch that employs flat reflection type prisms. In designing the switch, we evaluated the switch performance by means of a newly developed ray tracing computer program that considers optical power for each ray trace, that has not been done by the conventional evaluation technique. Discrepancy between the experimental and calculated insertion losses is +/- 1 dB. This small discrepancy confirms usefulness of our computer program.
Optical attenuators and powermeters are very important parts of the test equipment for fiber optic systems. A simple way to reach attenuation is by bending the fiber. This method suffers under the problem of limited reproducibility. We have combined an optical fiber bending attenuator with in-line power couplers at the optical input and output of the device. This allows simultaneous measurement of optical power and controlled reproducible setting of attenuation. The instrument has less than 2 dB insertion loss and reaches attenuation down to -55 dBm limited by system noise.
A fiber-optic delivery system consisting of a laser-to-fiber coupling unit and a fiber-to-fiber connector is described. Graded-index microlenses have been used as coupling optics. The realized system presents low intrinsic losses and good fiber misalignments tolerances, so it can be conveniently used in industrial and biomedical fields, where medium or high power lasers are required. Details of the design and of the experimental characterization are reported.
As work on wavelength multiplexed networks matures, it is becoming increasingly clear that there is a need for small, monolithic WDM elements for practical implementation. It is the purpose of this paper to discuss the design and fabrication of one such element, produced using silica waveguides on silicon. The design parameters for the device are for 16 channels of 1 nm bandwidth, with 2 nm channel separation, operating in the third telecoms wavelength region and coincident with the erbium fiber amplifier.
High resolution (1 nm), low loss (< 0.2 dB) optical fiber WDMs based on evanescent field coupling between a single mode fiber, side polished close to the core, and a planar high index overlay have been reported previously. By using electro-optic materials in the role of either the overlay or superstrate (above the overlay) the wavelength characteristics may be dynamically tuned under operational conditions. Here we report the results of a theoretical and experimental investigation of the performance limits of these devices. Specific aspects dealt with include the practical limits on the optimization of resolution and tunability (active and passive). Material investigations for use in the overlay structure include lithium niobate, liquid crystals, zinc selenide (vacuum deposited), and silicon. It is concluded that sub nanometer resolution and tuning ranges of several tons of nanometers are realistic for these systems.
Singlemode Fiber Couplers are key components in many communication applications and sensors. It has applications in optical filtering, optical reflecting, optical power, etc. This paper describes our experience in the development of directional fiber couplers, where we have used two techniques, FBT (Fused Biconical Taper Coupling) and Mechanical Polishing. This paper will focus the Fusion System, Package Methods and the practical results. Polarization Preserving Couplers are also investigated, using a fiber orientation system which permits fast and simple fabrication of these devices.
A miniature fused PM coupler which exhibits stable environmental performance from -55 degree(s)C to +100 degree(s)C and an overall package size of 25 mm long and 3 mm in diameter is presented. Experimental data on the wavelength dependence, extinction ratios, and thermal stability of these devices is shown and a low cost measurement system is discussed.
In this paper we propose and demonstrate experimentally a elliptical-core two-mode optical fiber device that is capable of tunably splitting, multiplexing and demultiplexing optical signals using. A crosstalk of 15 dB with an insertion loss of 1.2 dB has been obtained.
In the fused biconical taper process (FBT) the mode diameter of the optical fields within the fibers are spread to the extent that the surrounding medium acts as the fiber cladding. Usually this medium is air whose optical properties are benign resulting in stable devices. However, if the coupler is recladded with a material other than air, the optical fields spread further into the cladding region and the behavior of the coupler becomes substantially more sensitive to variations in the refractive index of the cladding material. In this paper we report the results of a study of recladded FBT couplers. Experimental results will be presented and compared to the predictions of a model previously used to describe the coupling characteristics of FBT couplers.
A fiber-optic intensity modulator was constructed based on coupling from a side polished single-mode fiber to a high index multi-moded electro-optic polymer overlay. Resonant coupling from the fiber to the film is controlled by applying an electric field to the polymer film causing modulation of fiber throughput intensity. The polymers used were cross linkable azo dye based species exhibiting a pockels co-efficient of up to 8 picometer/volt. Extinction ratios greater than 15 dB with insertion losses less than 0.6 dB were achieved. The effects of electrode thickness, overlay index and thickness on device performance are addressed. Wavelength selective filters were also fabricated using polymers giving linewidths of 7 nm (FWHM).
The effective cutoff wavelength is known to be dependent on fiber length, bending radius and fiber connections. In this paper we show that the effective cutoff wavelength is also temperature dependent in depressed clad single mode fibers. The annealing condition which affects the results is discussed. A theoretical model based on residual stress remaining in the fiber during the drawing process is proposed to explain the experimental observation. The temperature dependence of the effective cutoff wavelength in matched clad fiber is also presented for comparison.
The design of single-mode fibers for wideband communication networks is of considerable current interest. The triangular-index fiber with a capability of shifting the zero-dispersion wavelength from 1.3 micrometers to 1.55 micrometers has been attracting much attention. In this work, we direct our attention to the bending loss property of the triangular-index fiber with a depressed cladding ring and present some near-optimal designs. The numerical method used to obtain the propagation constants and fields is a vector mode multi-point power series expansion method which has been tested to be very accurate and efficient for calculations with a triangular-index profile. The bending loss results are computed using a vectorial volume current method. Results show that the depressed-ring can be added either to reduce the bending sensitivity of the usual triangular-index dispersion-shifted fiber, or to create a dispersion-flattening effect--which is advantageously combined to the dispersion-shifting feature of the triangular-index core. It is also found that for both of these design cases, the best core and when it is made as deep as possible. In the case where a reduction of bending loss is wanted, a comparison made with the elevated-ring design--known for its very low sensitivity to bending--shows that the depressed-ring design is more sensitive to bending than the elevated-ring design. However it offers a lower dispersion slope. As for the dispersion- flattened design, it is found that in terms of macrobending loss, the triangular-index W-fiber competes well with the step-index W-fiber.
We present a study of the dispersion curves of a four-layer metal-loaded waveguide (FLMW) which is bent across the direction of propagation. The bending of the waveguide referred to may arise naturally owing to manufacturing defects and the effect of environment. The curvature of the waveguide is so small that the asymptotic field solutions can be used to study the propagation characteristics. Considering a step-index profile and making use of the scalar wave equation, we obtain the dispersion curves of an FLMW for a large number of modes and an interesting mode-proliferation property of this waveguide is noticed. Using the same phenomenon, the dispersion curves for a four-layer metal-loaded planar waveguide (FLPW) are also presented. A comparative study of the modal characteristics of FLMW and FLPW yields that the FLMW shows the mode-proliferation aspect whereas the FLPW lacks this interesting feature. Therefore, this proliferation may be attributed to the bending of the waveguide.
We present a unified analytical method to study mode fields and other characteristics in the ellipsoid shaped fibers. The model takes account of both elliptical and cylindrical coordinate systems, simultaneously. We obtain spot size equations, field distribution and propagation constants of several lower modes, and birefringence of the fundamental modes by using the variational method and Hermite-Gaussian approximation. As an example, we discuss the solutions of the double elliptical-clad fiber and give some numerical results. It is shown that the method and the expressions discussed in universal form can be applied to analyze arbitrary E-shaped fiber.
New materials for fiber optics based on KBF4-BPO4-LiF and KBF4-BPO4 glasses are present. Transition of fluor-ion from BF4 groups to phosphorous were investigated. New glasses have refractive index lower than 1.430. With these glasses construction of fiber tip was offered. This new tip has a mechanical resistance 2 - 3 larger than polymer analogs, is efficient at LNT and may collect a set of 500 W laser without being destroyed. Corrosion degree of SiO2-glass on several fluor-containing melts was determined under some conditions. These melts appear corrosion activity as under as above the melt.
An experimental investigation is conducted to study the effect of alternate winding techniques in reducing fiber optic gyroscope sensitivity to environmental perturbations caused by time- varying temperature and stress. A stress-induced birefringent polarization-maintaining fiber and an elliptical core fiber are wound under two different winding configurations to experimentally study thermally-induced nonreciprocity in small interferometric fiber optic gyroscope coils. The spools, designed to permit temperature control throughout the fiber pack, are constructed with a thermally-insulating material. Fiber extinction ratio and radial temperature gradient data for the coils are obtained in an environmental control chamber. The experimental data are discussed in this paper. Our results indicate that alternating of the winding configuration has no notable effect on the extinction ratio as expected, however proper winding techniques and efficient coil designs can improve the overall performance of the coil over temperature.