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Tapered optical fiber components and sensors.

The development of low-loss single-mode fibers and the associated fiber-optic components has been driven largely by the needs of the telecommunications industry. The need for optical components has grown significantly due to the expanding role of fiber-optic communications in local-area networks (LANs) and cable television. Some examples of these optical fiber components are directional couplers, wavelength division multiplexers, filters, polarizers, polarization controllers, isolators, phase modulators, optical fiber amplifiers and sources. Fiber sensor technology also has requirements for many optical fiber components. in this paper, only all-fiber components and sensors are considered as opposed to bulk optical devices that are connected via optical fibers. And, more particularly, those components and sensors that arise from tapering the optical fibers are examined. It is expected that optical circuits built with all-fiber components, where the optical signal is contained entirely within the fiber, will have increased mechanical component stability over either bulk optical or integrated optic counterparts. It also is expected that all-fiber components should have very low intrinsic loss, which is a characteristic of single-mode optical fibers.

Single-mode optical fiber components that arise from tapering the optical fibers are used in couplers, wavelength filters, coaxial couplers and mode transformers. The fiber sensors that arise from tapering the single-mode optical fibers are coupler sensors, biconically tapered bend sensors and evanescent wave sensors where the transmission through the tapered region depends on the index refraction of the external medium. Additionally, there are tapered multimode optical fiber devices.

TAPERING

Optical fibers are tapered by heating to the softening point while tension is applied. The heat source may be a flame, an electric arc or resistively heated element. When the optical fiber is tapered, the core radius decreases. The V-number of the fiber [V.sub.core], is given by:

[Mathematical Expression Omitted]

where

[n.sub.c] = the core index

[n.sub.d] = the cladding index a(z) = the core radius

[lambda] = the operating wavelength

In a conventional fiber, the radius a(z) will not be a function of the length. When the V-number of the fiber falls below one, the optical field, which was originally contained in the core, spreads into the cladding. As a result of the optical field spreading into the cladding, the cladding and the medium surrounding the cladding play a significant role in the transmission of light through the tapered region.

The taper is mechanically protected usually by embedding it in a medium of lower index of refraction than the silica. The surrounding medium also protects the taper from environmental conditions, such as high humidity.

There are additional ways of getting at the field that is in the core of the fiber other than the tapering process. For example, the cladding may be polished away until the evanescent field of the core is reached, or the cladding may be etched away. Additionally, bending the optical fiber causes light to be lost from the core, and cleaving the optical fiber provides access to the field of the light that is in the core.

FUSED TAPERED COUPLERS

Fused tapered couplers are fabricated from two lengths of optical fiber that have their jackets removed over a short length. The two bare sections of fiber are brought into contact, heated, fused together and then drawn out into a taper. Light from a laser source is launched into the input end and both outputs are monitored during the tapering process. As tapering proceeds the light couples back and forth between the input fiber and the adjacent fiber in a periodic way. After the desired number of cycles and the desired splitting ratio is obtained, the tapering process is halted. The tapered region behaves as a multimode fiber, supporting antisymmetric and symmetric modes. At the end of the fused region, light from these modes will be transferred to the different ports. The fibers are removed and potted in a lower refractive index material that helps to stabilize the tapered region. Both four-port and six-port couplers have been fabricated. Couplers have been manufactured with an excess loss of 0.1 dB and very good temperature stability. Also couplers can be fabricated to any desired splitting ratio. Couplers typically are fabricated so that they are polarization insensitive. However, they may have some polarization properties; in fact, they can be fabricated from polarization-preserving fibers. The required and the achieved characteristics for optical fiber couplers are precise splitting ratio at a given wavelength, input polarization insensitivity, low excess loss, environmental insensitivity, small size and low cost.

Couplers, where significant power fluctuations occur during tapering, also may show polarization sensitivity, even when the fibers are not the polarization-maintaining type. This polarization sensitivity arises from the asymmetry in the fused region and the resulting birefringence. Unpolarized input light thus can be split into vertical and horizontal polarizations that appear at the two output ports. The coupling in the fused region and the ensuing polarization splitting will be strongly wavelength dependent.

WAVELENGTH DEPENDENT

COUPLERS

The coupling of modes in the tapered region depends on wavelength. When the coupler is made longer and the power transfers back and forth more times between the two fibers, the wavelength dependence of the coupler becomes narrower. In fact, 100% coupling may be obtained at one wavelength and 0% at another wavelength provided that these two wavelengths are sufficiently separated. A difference of 100 nm in wavelength is sufficient to achieve very good isolation between the two output arms of the coupler. These couplers may be used for wavelength division multiplexing.

Optical fiber couplers may also be connected (concatenated) in series. in this way the wavelength transmission properties of the couplers become multiplicative. Therefore, a narrow passband may be obtained by this concatenation of couplers. By using 2 X 2 couplers as building blocks, star couplers with as many input and output ports as is desired may be fabricated. Since 2 X 2 couplers are much more easily fabricated than any other N by N combination, 2 X 2 is favored as the building block for the star coupler. For example, an 8 X 8 coupler could be fabricated by using 12 2 X 2 couplers.

SINGLE-FIBER COAXIAL

COUPLERS

In a single-mode optical fiber in which there are two cladding layers, it is possible to have coupling between the core and the outer cladding. This process is usually very inefficient because of the large difference between the mode propagation constants in the two regions. As the fiber is tapered the propagation constants become equal, and a fairly efficient exchange of energy can take place between the core and the cladding. Therefore, it is possible in a single-mode fiber to get the energy to couple periodically between the core and the cladding. Very strong core oscillations have been observed during taper fabrication with extinction ratios in excess of 30 dB.

These tapered single-mode fibers also behave as filters just as in the single-mode coupler case. The longer taper with a large number of core oscillations has a stronger wavelength dependence.

The taper coaxial coupler filters can be arranged in series just as in the case of four-port couplers. The overall transmission response of the series of filters is equal to the product of the individual responses of each taper in the series. The coaxial coupler filters are somewhat easier to fabricate than the fused biconically tapered couplers.

TAPERS FOR CONTROLLING

MODE FIELD DIAMETER

An alternative means for creating tapers in the core without the external diameter of the fiber changing is to heat the fiber locally for some period of time so that the core diffuses from its original location out into the fiber.when this happens, that is, when the index of refraction of the core is decreased and the size is increased, the V-number stays roughly the same. However, the mode field diameter increases in size. This expanded mode field is particularly useful for embedding miniature optical devices, such as polarizers and isolators, between the fiber ends. This technique of enlarging the mode field is very effective in reducing splice loss arising from lateral misalignments. However, these devices are very sensitive to angular misalignments due to the reduced value of the numerical aperture. This technique has been used in both germanium doped core and in fluorine doped cladding fibers. The annealing time depends on the temperature. Heating times of a few hours are required at 1200 [degrees] C and a few minutes at 1700 [degrees] C to observe changes in the core profile.

TAPERED OPTICAL FIBER

SENSORS

In general, optical fiber sensors can be categorized by the characteristics of the light that is modulated in the sensor. These characteristics are the amplitude, phase, polarization and wavelength. In many of the phase sensors, optical fiber couplers are used to split the light into two branches of an interferometer. Both the Mach Zehnder and the Michelson interferometers are formed this way. in the Sagnac interferometer, the coupler acts to couple light traveling in opposite directions into a loop. This interferometer is sensitive to rotation and is used as a gyroscope. There is also a loop interferometer that uses a single directional coupler.

When a single-mode fiber is tapered so that there are two biconically tapered regions and a waist in between, two single-mode fibers with a multimode fiber section in between are obtained. When the waist of this biconically tapered fiber is bent, the single mode that is propagating through the waist couples its energy into other modes. These modes continue to exchange energy until the light gets to the region where the taper of increasing diameter occurs, and then only the power that remains in the fundamental mode couples back into the core. The power that is observed in the core oscillates with a bend angle. The bend curve may be seen to be quite steep with a maximum slope occurring at 2 [degrees]. If the fiber is kept at this bend angle very small displacements of the waist may be determined. Displacements of about 0.1 nm are observable. Therefore, this device may be used to sense various parameters that will displace the waist of the fiber. Based on this tapered fiber sensor, an altimeter has been fabricated as well as an alternating gradient field magnetometer and magnetic field sensors. The biconically tapered single-mode fiber sensor has been used to measure magnetization curves on 25 [mu]m diameter spheres of metallic glass with a signal-to-noise ratio in excess of 1000. This implies that this sensor would be able to sense magnetization in volumes of material as small as [10.sup.-11] [cm.sup.3]

It also has been observed that when two of these tapers are used in series, as they have been in some of the filter devices, the region in between the two tapers is also sensitive to perturbation. This interference arises from an interaction between the light propagating in the cladding and the light propagating in the core when mixing at the second taper.

Two-mode elliptical core fibers have been used as vibration sensors. This fiber sensor can be made to have variable sensitivity along its length. The differential propagation constant in a twomode fiber is directly dependent on the V-number. Tapering the E-core fiber changes the V-number as well as the sensitivity of the sensor along its length. The sensors are fiber-optic analogs of shaped-piezoelectric model sensors that have been used recently in the area of structural control.

Biconically tapered fused couplers have been used as strain sensors. In these sensors, the coupler is usually encapsulated in a flexible material, such as silicone rubber. The evanescent field of the tapered fiber extends into the silicone potting material. Changes in the index of refraction due to stress birefringence and microbending in the coupling region Gause changes in the output coupling ratio. When this device was employed as a differential pressure sensor, it had a linear dynamic range of 54 dB.

TAPERED MULTIMODE

FIBERS

In the same fashion as couplers are made from single-mode fibers, couplers can also be made from multimode fibers. The major difference is that when a star coupler is made from multimode fibers, many fibers can be fused at one time. Thermal fusion has been used to make a star coupler with as many as 100 fibers joined together. Fibers are first twisted together to form a rope-like joint. Tension then is applied to the fibers before they are fused together. For better results it is necessary to form a biconical taper at the joint. Using a star coupler, optical power can be fed into N fibers from any one of the fibers in the bundle on the opposite side of the fiber joint. Insertion losses of less than 0.6 dB have been achieved in such a coupler.

CONCLUSION

Tapered single-mode fibers are being used extensively in fiber-optic components. They also play an, important role in fiber-optic sensors.

L.C. Bobb is with the Naval Air Warfare Center in Warminster, PA. P.M. Shankar is with Drexel University, Philadelphia, PA.
COPYRIGHT 1993 Horizon House Publications, Inc.
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:EW Design Engineers' Handbook & Manufacturers Directory
Author:Bobb, L.C.; Shankar, P.M.
Publication:Journal of Electronic Defense
Date:Jan 1, 1993
Words:2157
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