FI85195C - Optical form field transformer - Google Patents

Description

85195

The present invention relates to a shape field converter according to claim 1.

Taperization, i.e. the thinning of a fiber, is a well-known method of changing the transverse distribution of the shape (light) propagating in the core of a single-mode fiber. In conventional thinned shape field transducers, tapers, for example, the diameter of the transducer has been reduced by pulling so that the transducer forms a truncated cone. In the same context, the geometry of the converter parts at different ends of the converter can also be changed. Fiber splitters are an example of components based on this phenomenon. Known converters are the so-called stepwise refractive index transducers, i.e., transducers having only one region of refractive index different from the base material.

If a single step fiber with a standard step refractive index is thinned, the core of the fiber becomes "too small" for the light to be limited only to the core area. This spreading of the distribution is easily a more powerful phenomenon than the reduction in the size of the core, so as the single-mode fiber is thinned, the size of the shape advancing in the fiber increases.

When using only a step refractive index profile in the taper, the shape field distribution is not Gaussian after thinning the fiber, but is closer to the exponential distribution. This causes coupling losses from the thinned end of the taper to the waveguide, because the distribution of the shape fields of the waveguides is usually Gaussian. The width of the field also increases strongly with thinning, so that the tolerances for thinning are strict. Similarly, the manufacturing method of the step refractive index shape field transducer does not allow much non-circularly symmetrical and circularly symmetrical waveguide to be matched, because the thinning of the core does not significantly affect the shape field asymmetry.

It is an object of the present invention to obviate the disadvantages of the technique described above and to provide a completely new type of shape field converter.

The invention is based on the fact that the core of a tapered shape field transducer consists of a layer with three different refractive indices so that the central part of the core has the highest refractive index, the shell layer has the second highest refractive index and the intermediate layer between the middle part and the shell layer has the lowest refractive index.

More specifically, the shape field converter according to the invention is characterized by what is set forth in the characterizing part of claim 1.

The invention provides considerable advantages.

The size and shape of the shape field distribution at different ends of the taper can be modified almost independently of each other, and thus a good switching efficiency can be obtained between two very different light channels (both in shape and size).

The invention will now be examined in more detail with the aid of application examples according to the accompanying drawings.

Figure 1 shows the undiluted end of a shape field converter according to the invention.

Figure 2 shows the calculated value of the field distribution at the undiluted end of the transducer of Figure 1.

Figure 3 shows the calculated value of the field distribution at the thinned end of the transducer of Figure 1.

Fig. 4 is a perspective view of the shape field converter of Fig. 1.

As shown in Figure 1, in the solution according to the invention, the refractive index profile of the core consists of three substantially coaxial parts. The refractive index 3 85195 of the innermost, central part 1 is the highest. Next, the intermediate region 2 has the lowest refractive index value of the profile, i.e. it is a tomb, and the third, the shell part 3 has a lower refractive index value than the central part 1, but higher than the intermediate region 2. When the fiber is thick, the field of the basic shape is almost exclusively confined to the area of the central part 1, which thus determines the size and distribution of the shape field. When the fiber is thinned to e.g. half of its original diameter, the size of the central part 1 is no longer enough to hold the field inside (which is also the case in the usual step refractive index case), but the field "escapes" to other areas of the converter. Since the shell part 3 of the core has a positive refractive index difference with respect to the intermediate part 2, it starts to act as a limiting part of the field. Thus, at the thin end of the transducer, the size and shape of the shape field distribution is mainly determined by the shape and size of the outer part 3 of the core. Thus, the shape of the outer part 3 can be varied at different ends of the transducer depending on the fiber to be connected to the transducer.

In the solution according to Figure 1, the refractive index differences with respect to the refractive index no (Δηη = nn - no) of the surrounding quartz glass shell structure are as follows: Δη ^ = 29.5 * 10 “3, Δη £ = -4.83 * 10-3 and Δη 3 * 6, 43 * 10-3. A converter according to the invention suitable for LiNbC> 3 light channels can be prepared, for example, by doping quartz with GeC> 2 and fluorine. A positive refractive index difference (Δη ^, Δη3) is achieved by doping quartz with, for example, germanium oxide and a negative refractive index difference (Δη2) in turn by doping quartz with, for example, fluorine. In the solution according to Figure 1, the cross section of the central part 1 is elliptical. The ratio of the axes of the ellipse is about 3 and the maximum dimension at the undiluted end is about 6, etc. The outer surface of the intermediate part 2 immediately surrounding the central part 1 is circular and 15 ym.

Fig. 4 shows a transducer bent in the viewing direction so that the undiluted end 4 is on the left side and the 85195 thinned end 5 is on the right side. The diameter of the thinned head 5 is about 50% of the diameter of the undiluted head 4. The diameter of the quartz glass shell structure 6 surrounding the core has been reduced in the figure for drawing technical reasons. The outer diameter 6 of the substrate drop tube is typically about 100.

In this context, the possibilities of the profile according to the invention have been mathematically modeled. This modeling was performed with a computer program based on the Beam Propagation method developed by A. Tervonen. The shape field sizes of the example case according to Figures 1 and 4 are shown in Figures 2 and 3 when the tapering ratio is 2 (= ratio of the diameters of the transducer ends). As can be seen from Fig. 2, the field remains at the undiluted end 4 of the transducer according to Fig. 4 in the region of the approximately elliptical central part 1. At the thinned end 4, according to Fig. 3, the field has spread over the entire area of the core with an outer diameter of about 7.5 μm and at the same time the field has changed from elliptical to circular.

With the solution according to the invention, the shape of the field can be defined at the undiluted end of the transducer by the shape of the central part 1 and at the thinned end by the shape of the outermost part 3. In addition to the elliptic-circular transformation shown, the solution according to the invention in principle enables field-shape transformations between all known optical components. The refractive indices used as well as the profile shapes are practically always determined by the application.

Claims (5)

A tapered, elongate optical shape field transducer comprising a first, non-tapered end (4) and a second, tapered end (5), wherein the shape field transducer tapers substantially uniformly from the non-tapered end (4) to the tapered end (5), and the transducer comprises - a core portion ( 1, 2, 3) and - a shell structure (6) immediately and substantially coaxially surrounding the core part (1, 2, 3), the refractive index (no) of the material of which is less than the refractive index of the material adjoining the shell structure (6) of the core part (1, 2, 3) (n3), characterized in that the core part (1, 2, 3) comprises - a central part (1) with the highest refractive index (ni) in the converter, - an intermediate part (2) immediately surrounding the central part (1), the refractive index (n2) of the material ) the core part has the smallest, and - the shell part (3) immediately surrounding the intermediate part (2), the refractive index (n3) of the material of which is less than the refractive index (ni) of the material of the central part (1) and greater than the intermediate part n (2) refractive index of the material (n2). Shape field transducer according to Claim 1, characterized in that the refractive index (n2) of the material of the intermediate part (2) is smaller than the refractive index (ng) of the material of the shell structure (6). Shape field converter according to Claim 1 or 2, characterized in that the cross-sectional shape of the central part (1) is an ellipse. Shape field converter according to one of the preceding claims, characterized in that the cross-sectional shape of the shell part (3) is circular. Shape field transducer according to one of the preceding claims, characterized in that the material of the central part (1) is quartz glass doped with germanium oxide, the intermediate part (2) is quartz glass doped with fluorine and the shell part (3) is quartz glass doped with germanium oxide. 7 85195