FI83708B - Alloy optical form field transforming fibre - Google Patents

Abstract

The invention concerns an elongated optical form field transformer comprising a core part 50, 52, 53 and a shell part 54 which directly and essentially coaxially surrounds the core part, the diffraction coefficient n0 of which shell part is lower than the diffraction coefficient n3 of the material in the core part 50, 52, 53 where it is in contact with the shell part 6. According to the invention the core part 50, 52, 53 comprises, for example, a central part 1 which is an erbium alloy and consists of a material which has the highest average diffraction coefficient in the transformer, an intermediate part 53 directly surrounding the central part 50 consisting of a material with the lowest diffraction coefficient in the transformer and an outer part 52 directly surrounding the intermediate part, consisting of a material with a diffraction coefficient n3 which is lower than the diffraction coefficient n1 of the material of the central part 50 but higher than the diffraction coefficient n2 of the material of the intermediate part 53. By means of a construction according to the invention for example a semiconductor laser or some other light channel which differs from an ordinary single form fibre can easily be connected to an ordinary single form fibre with good optical efficiency. By means of a fibre which is an alloy according to the invention both the pumping effect can be concentrated on a small cross-section area of the active area and a good level of connection efficiency can be achieved in relation to an ordinary single form fibre by thinning of the fibre. <IMAGE>

Description

83708

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

Doped fibers are generally understood to mean quartz fibers doped with rare earth metals. The term doped fibers comes from the English term "doped fibers". The doping amounts are typically 10-1000 ppm, so these substances have no significant effect on the refractive index distribution of the fiber. The refractive index structure of these fibers is determined in the same way as that of ordinary fibers, i.e. their core consists mainly of quartz glass doped with germanium. Of the rare earth metals, erbium is by far the most interesting because its emission wavelength range is 1.53 to 1.56 μm, at which the attenuation of a conventional telecommunication fiber is at a minimum.

Erbium-doped fiber is likely to be used in optical amplifiers in various telecommunications applications. The use of fiber is limited, at least at present, by the low power of semiconductor lasers suitable for optical pumping of the dopant. There is a solution to this problem: reducing the size of the fiber core. As the size of the core decreases, the peak value of the optical field in the fiber is significantly increased. This reduces the need for pumping power. Unfortunately, at the same time, the coupling between the ordinary communication fiber and the doped fiber is degraded (the coupling of the pumping power is degraded) and this factor eliminates the benefit of the smaller core in the doped fiber.

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 thinning field field transducers, tapers, for example, heating and drawing have reduced the diameter of the transducer 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 ash breakers 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 distribution of the optical shape field after thinning the fiber is not Gaussian, but closer to the exponential distribution. This causes coupling losses from the thinned end of the taper to the communication fiber, because the distribution of the shape fields of the communication fiber 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 stepwise coefficient shape field transducer does not allow much alignment of the non-circularly symmetric and circularly symmetrical waveguides, since the thinning of the core does not significantly affect the shape field asymmetry.

The object of the present invention is to eliminate 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 tapered shape field transducer consists of a layer with three different refractive indices so that the core of the shape field transducer has the highest refractive index, the shell layer has the second highest refractive index and the intermediate layer between the core and the shell layer has the lowest refractive index.

More specifically, the doped 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 connection with the thinned part and the ordinary single-mode fiber is good. The attenuation is not substantially different from the attenuation of two single-mode fiber joints. In addition, the field distribution in the undiluted portion of the converter is limited to an area having only 1/5 of the field distribution area of a conventional communication fiber. It follows that the required optical pumping power is only 1/5 compared to ordinary doped fiber. Because the taper is made of the doped fiber itself, there is no need to attach a separate optical component to convert the field distribution. Likewise, the taper does not cause any extra reflections.

In addition, the size of the shape field and the shape of the distribution at different ends of the taper; they can be modified almost independently of each other, thus achieving a good switching efficiency 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 one three-part shape field converter.

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

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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.

Figure 5 shows a cross-section of a doped shape field converter according to the invention.

Figure 6 shows in cross-section one application of a fiber according to the invention.

Figure 7 shows the electron shell structure of erbium.

In the solution shown in Figure 1, the refractive index profile of the core consists of three substantially coaxial parts. The refractive index 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, outer part 3 has a lower refractive index value than the central part l 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 sufficient 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 outer part 3 of the core has a positive refractive index difference with respect to the intermediate part 2, it begins 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 ng s 83/08 (Δηη = nn - no) of the surrounding quartz glass shell structure 6 are as follows: Δηχ = 29.5 * 10-3, Δη2 = -4.83 * 10-3 and Δη3 = 6.43 * 10-3. A converter according to the invention suitable for LiNbO 3 light channels can be prepared, for example, by doping quartz with GeO 2 and fluorine. A positive or coefficient difference (Δηχ, 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 about 15 etc. The outer diameter of the shell part 6 is about 200 etc.

Figure 4 shows a transducer bent in the viewing direction so that the undiluted end 4 is on the left side and the 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 of the shell part 6 is typically about 100 at the thinned end. ' ! et al.

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, the outer diameter of which is about 7.5 μm, and at the same time the field has changed from elliptical to circular.

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With a three-part converter, 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 ellipse-circle transformation shown, the proposed solution basically 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.

Figure 5 shows a cross-section of a doped converter according to the invention. The central portion 50 of circular cross-section is doped with erbium from its central portion 51. The diameter of the central portion 50 is about 2.5 μm, of which the diameter of the doped region 51 is about 2 μm. The refractive index differences with respect to the refractive index ng (Δηη = nn ~ n0) of the surrounding quartz-shell shell structure 54 are the same as in the solution of Fig. 1. The dimensions of the outer part 52 and the intermediate part 53 are also approximately the same as in Fig. 1. The diameter of the shell structure 54 is about 200 μm.

In the solution according to Figure 6, the converter 12 according to the invention is connected between two communication fibers 10 and 17 as an optical amplifier. The amplifier 12 comprises a central part 13 which encloses an erbium doping region (not shown). Around the central part 13 there is an intermediate part 29 and an outer part 14, which in turn is surrounded by a quartz glass shell part 25. Both the optical pumping power 23 and the actual signal 24 are fed to the first communication fiber 17 via an optical splitter (not shown). 47 μm, 0.98 μm or 0.53 μm, depending on the pump-laser used. The wavelength of the signal 24 is typically 1.55 and so on. The communication fibers 10 and 17 are normal two-part, step refractive index fibers. The diameters (about 10 μm) of the cores 11 and 18 of the communication fibers are approximately equal to the diameter of the outer portion 14 (outer portion 52 in Fig. 5) of the thinned ends 26 and 27 of the amplifier 12.

The length L of the amplifier 12 can vary between 0.5 and 50 m and depending on the erbium doping. The stronger the doping, the shorter the fiber required. The axial length 1 of the taper regions 15 and 16 of the amplifier 12 is about 10 mm and the diameter of the tapered end is about 50% of the diameter of the unapaptized part. The power distribution 21 in the first communication fiber 10 is in accordance with the standard and the diameter of the distribution is about 9. The distribution also spreads slightly over the area of the quartz glass shell structure 20. A similar phenomenon also occurs in the second communication fiber 17 in the shell region 19. In the amplifier 12, the distribution 22 narrows substantially to the region of the central part 13 (area 50 in Fig. 5), whereby the diameter of the power distribution also decreases to about 3.7. The distribution 22 also spreads slightly in the region of the intermediate part 29. The total diameter D of the amplifier part 12 in the unapapulated area 28 is about 200.

As shown in Figure 7, it is considered what happens when an erbium atom encounters pumping radiation 70. An electron in the atom's ground state 72 is excited to state 74. The probability of excitation depends on the power of the pumping light hitting the atom. Since the size of the atom is always small compared to the "size" (wavelength) of the photon, it can be thought that the transition probability is proportional to the power density of the optical field at that point. This probability corresponds to the fact that a certain default time elapses before tuning occurs. Based on the above, this time is inversely proportional to the power density of the pumping value at that point. The excited electron drops almost immediately from state 74 to state 76, which is a so-called metastable state. This means an excited state with a long lifespan, in the case of erbium about 14 ms. As the electron moves from state 76 to ground state 72, a photon 78 is generated with a wavelength close to 1.55. Next, consider the situation in terms of signal light (wavelength 1.55 pm). First, it is conceivable that • the erbium atom is either in the ground state (electron state 72) or in the excited state (state 76 occupied). If in the ground state the signal light 80 is absorbed and the erbium atom is excited (the electron moves from the state 72 to the state 76) with some probability a which is proportional to the power density of the signal light at the erbium atom. If, on the other hand, the erbium atom is in the excited state in which the state 76 is occupied, the signal light is amplified with the same probability a, which is still proportional to the power density of the light at the erbium atom. In order for the atom under consideration to act as an amplifier at the signal wavelength, the power density of the pumping light must be such that the expected value for the duration of the excitation 72 -> 74 is less than 14 ms (time constant of state 76). It can be easily concluded that increasing the pumping power density in the active range (er-doped range) is absolutely essential for the operation of the amplifier, because if the pumping power density does not exceed a threshold value (about 2.5 kW / cm1), the fiber amplifier acts as an attenuator. When using a conventional single-mode fiber structure with a core diameter of about 10 μm, this threshold power is about 2 mW. If a smaller core can be used, e.g. 4 μm in diameter, which is feasible with a normal quartz fiber manufacturing process, the required threshold power is reduced to 0.3 mW in the exemplary case. Practical amplifiers need a pumping power that is - 10 * threshold power. Minimizing pumping power is important for the amplifier. At present, for example, the power of all semiconductor lasers used in telecommunications is less than 5 mW, because the lifetimes of high-power lasers are clearly shorter than those of the above-mentioned high-power lasers. The use of a small nucleus is favored by yet another mechanism, the probability of stimulated emission of an excited atom is similarly proportional to the field density of the signal light. There is therefore only one obstacle to the use of a small-core fiber: the fiber amplifier must be connected to a standard data transmission fiber, which substantially reduces the switching efficiency.

• This applies to both the signal light and (more importantly) the pump light.

Claims (8)

A tapered elongated optical field transformer comprising a non-tapered end (28) and at least one tapered end (26, 27), wherein the tapered transformer tapers substantially uniformly from the non-tapered end (28) to the tapered end (26, 27). , and the transformer comprises - a core part (50, 52, 53) and - a shell structure (54) which immediately and substantially coaxially surrounds the core part (50, 52, 53) and which consists of a material whose coefficient of refraction (nQ) is lower than the coefficient of refraction (n3) of the material in the core portion (50, 52, 53) adjacent to the shell structure (54), characterized in that the core portion (50, 52, 53) comprises - a center portion (50) alloyed with a rare earth metal and exhibiting the highest refractive coefficient (n,) in the transformer, - an intermediate portion (53), which immediately surrounds the center portion (50) and whose material has the lowest refractive coefficient (n2) in the core portion, and - an outer portion (52), which immediately surrounds the intermediate part (52) and whose material has a refractive coefficient f (n3) which is lower than the refractive coefficient (n) of the material of the center part (50) and higher than the coefficient of refraction (n2) of the intermediate (53) ) material. 2. Form field transformer according to claim 1, characterized in that the center part (50) is alloyed with erbium. Form field transformer according to claim 2, characterized in that the erbium content in the alloyed zone (51) of the intermediate part (50) is about 800 - 1200 ppm. Form field transformer according to any of the preceding claims, characterized in that the form field transformer (12) tapers at both ends (26, 27), whereby the transformer (12) is used as an optical amplifier mounted between a data transfer fiber. Form field transformer according to claim 1, characterized in that the refractive coefficient (n2) of the material of the intermediate part (53) is lower than the refractive coefficient (n ") of the material of the shell structure (54). Form field transformer according to any one of the preceding claims, characterized in that the cross-section of the intermediate part (50) is in the form of an ellipse. Form field transformer according to any one of the preceding claims, characterized in that the cross-section of the shell part (52) has the shape of a circle. Form field transformer according to any one of the preceding claims, characterized in that the material of the center part (50) is made of quartz glass alloyed with germanium oxide, the intermediate part (53) consists of quartz glass alloyed with fluorine, and the shell part (52) consists of quartz glass alloyed with germanium oxide.