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 televison. Some examples of these optical fiber components[10,11] 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,[7,8] 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.[9,25] Additionally, there are tapered multi-mode optical fiber devices, which also are discussed.
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, as shown in Figure 1, 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.cl] = 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. As the fiber is being tapered, the optical transmission may go through a series of maxima and minima, as shown in Figure 2. The transmission property is attained, the heat and the tension are removed and the tapering process is halted. The strength of the fiber does not need to be affected by the tapering process. The taper is mechanically protected usually by imbedding 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.
Optical Fiber Taper Theory
The behavior of light inside a tapered single-mode fiber may be explained in terms of the change in the core radius taking place during tapering. Under normal conditions, the single-mode fiber is considered as an infinite cladding type where the thickness of the cladding is much larger than the core radius, and with the core radius having a sufficiently large value to maintain a V- value, given in Equation 1 as 2.405. The fiber under these conditions is considered to support the fundamental core mode. As the tapering proceeds, the core radius a(z) will decrease, reducing the [V.sub.core] value of the fiber. As the [V.sub.core] value goes below 1, light is no longer confined to the core and it spreads into the cladding. When this happens, the propagation of light in the tapered region will be controlled [1,31,32] by the original cladding acting as the core and the external medium (air) acting as the cladding. The core is now negligibly small and has very little effect on the light guidance. The V value, which is now identified as [V.sub.clad], will be very high. The tapered region can support a number of modes and these modes are referred to as cladding modes. [V.sub.clad] can be expressed as
[MATHEMATICAL EXPRESSION OMITTED]
b(z) = the radius of the fiber, core plus cladding
[n.sub.ext] = the index of the external
medium surrounding the fiber
One can now say that in the region bounded by [V.sub.core] < 1, the fiber behaves as a multimode fiber and the tapering acts as the physical mechanism that induces coupling among the cladding modes. On the expanding side of the taper, where [V.sub.core] > 1, the fiber acts as a conventional single-mode fiber. In the contracting region, light from the fundamental core mode moves to the fundamental cladding mode at the point where [V.sub.core] < 1. Coupling among the cladding modes takes place in the tapered region where [V.sub.core] < 1 and on the expanding side of the taper where [V.sub.core] > 1. Light in the fundamental cladding mode moves to the fundamental core mode.[16,18,23] The power that is measured at the output fluctuates due to the coupling in the region bounded by [V.sub.core] <1.
Fused Tapered Couplers
Fused tapered couplers, shown in Figure 3, 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,[26,33,34] 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.[14,27] 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 percent coupling may be obtained at one wavelength and 0 percent 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. The spectral response of a wavelength dependent coupler can be expressed in terms of the transmitted power T([lambda]) through the taper,
T([lambda]) = [cos.sup.] [[pi ([lambda - lambda.sub.0])/2[DELTA]] (3) where
[lambda].sub.o] = a reference wavelength
of the filter [DELTA] = the reference wavelength
response of the taper
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. The overall transmission [T.sub.N]([lambda]) then is given by a product of the individual transmission expressions
[Mathematical Expression Omitted]
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. A 4 X 4 coupler that is composed of four 2 X 2 couplers is shown in Figure 4.
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 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 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.[6,20] 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 imbedding miniature optical devices, such as polarizers and isolators, between the fiber ends, as shown in Figure 5. 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.[19,22] 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, shown in Figure 6, 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 interforemeter, 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,[5,29] 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, as shown in Figure 7. 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 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 two-mode 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 cause 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. The predicted sensitivity of these pressure sensors is about 10 [mu]Pa/ [Mathematical Expression Omitted].
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 tapper 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.
An optical fiber refractometer has been fabricated from a tapered multimode optical fiber, as shown in Figure 8. This step indexed optical fiber had a core index of refraction of 1.6 and a cladding index of refraction of 1.48. This fiber was tapered so that its final diameter waist region was one-third of the original diameter of the fiber. The cladding in the waist region then was etched away. The tapering modifies the modal distribution in the fiber and allows lower indices of refraction to be measured. This refractometer was used to determine indices of refraction over the range from 1.33 to 1.6. Without tapering, indices below the value of 1.48 could not have been determined.
Tapered single mode fibers are being used extensively in fiber-optic components. They also play an important role in fiber-optic sensors. A review of the different devices and sensors that have been fabricated using tapered fibers has been presented.
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|Title Annotation:||Technical Feature: Lightwave Series|
|Author:||Bobb, L.C.; Shankar, P.M.|
|Date:||May 1, 1992|
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