JPH03218946A - Light amplifying fiber - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention is directed to a quartz fiber having a waveguide function by incorporating Ge or the like into the core to make the refractive index of the core larger than the refractive index of the cladding. E optically excites dispersed Er ions and uses the stimulated emission light to amplify the signal light propagating within the fiber.
This invention relates to an r-doped quartz fiber for optical amplification.

(5. Prior art and its problems) Silica-based glass fiber, which has a waveguide function due to the difference in refractive index between the core and cladding, can guide light with a low loss of about 0.2 dB/km, so it is suitable for the 1μ band. It has already been put into practical use as an optical transmission medium. As a means for making the refractive index of the core of a quartz fiber relatively larger than the refractive index of the cladding, there is a method of doping germanium (Ge) or the like into the core of 1S to make the refractive index of the core relatively larger. There is a method of doping the cladding with fluorine (F) or the like to make the refractive index of the cladding relatively small. However, since it is easier to create a refractive index difference between the core and the cladding in the former case, here we will explain a silica-based glass fiber whose core is doped with germanium (G e ) etc. to relatively increase the refractive index of the core. .

Optical communication systems that use light in the 1.5μ band, which is the lowest loss wavelength band of quartz fibers, are the most suitable systems for long-distance communication because the attenuation of light is small. However, even in a 1.5 μ band communication system, the non-repeater optical transmission distance is at most about 200 kn+, so in order to transmit over a longer distance, it is necessary to use a repeater to regenerate the attenuated optical signal. The repeaters used in current systems convert attenuated optical signals into electrical signals.
This method performs regenerative amplification using an electrical circuit, but in the future it will be possible to use optical signals without opto-electrical conversion due to the need for smaller and more economical repeaters and the possibility of flexible system design without fixed transmission speeds. It is desired to realize an optical amplification method that performs regenerative amplification as is.

Currently, methods for performing such optical amplification have been proposed, such as a method using a semiconductor amplifier, a method using the nonlinear optical effect of a fiber, and a method using the optical amplification effect of an Er-doped fiber. The method using Er-doped fiber has high coupling efficiency with the transmission fiber,
It is considered the most promising because it has low noise and is independent of polarization.

The principle of optical amplifiers using Er-doped fibers is to amplify weak signal light using stimulated emission light of Er ions excited with pump light with a wavelength different from that of the signal light. The following problems exist when applied to cable systems. That is, current Er-doped fibers have low efficiency in converting pumping light into stimulated emission light, and therefore a pumping light source as strong as Loom is required to obtain the gain necessary for a long-distance optical submarine cable system. However, as such high-output, highly reliable semiconductor lasers do not currently exist, it is difficult to apply optical amplifiers to long-distance optical submarine cable systems, and the development of optical amplification fibers with high conversion efficiency is difficult. Highly desired.

In general, methods to increase the efficiency of glass lasers and amplifiers that use rare earth elements such as Nd as active ions include adding small amounts of other rare earth ions or transition metal ions as sensitizers along with the active ions, or changing the composition of the host glass. There are known methods for changing this.

Regarding the former, it is known that yb has a somewhat sensitizing effect on Er ions, but an ion that has a large exacerbating effect is not yet known. On the other hand, regarding the latter, N
The d-ion has been investigated in detail, and it is well known that phosphate glass is more efficient than silicate glass. By analogy with the example of Nd ions, it cannot be said that silica glass is optimal in terms of luminous efficiency in the case of Er ions as well, and there is naturally a glass with a composition that increases luminous efficiency. Therefore, if an Er-doped optical amplification fiber is made using glass with such a composition, the E
This should result in a more efficient fiber than current r-doped fibers. However, fibers made of glass that has a large composition difference from silica glass have problems when used in optical communication systems, even if they have excellent amplification characteristics.

In the case of optical amplification fibers used in optical communication systems,
It is necessary to have consistency with the quartz fiber for signal transmission, for example, in order to fusion splice the two, it is necessary to have almost the same viscosity-temperature characteristics, and to have fiber parameters such as core diameter and mode field diameter close to each other. Therefore, they are required to have refractive indexes similar to each other, and to have reliability comparable to quartz fiber in terms of weather resistance, chemical resistance, etc.

It is impossible for fibers with large differences in glass composition to satisfy such requirements, and even if they have excellent amplification characteristics, they cannot be used in optical communication systems.

As explained above, current Er-doped optical amplification fibers have low amplification efficiency for use in long-distance communication systems. No method was disclosed.

(Object of the Invention) The present invention has been made in view of the problems of the prior art described above, and has high conversion efficiency between excitation light and stimulated emission light, and satisfies various requirements necessary for use in an optical communication system. The purpose of this invention is to provide a fiber for optical amplification.

(Means for solving the problem) In order to achieve this object, the optical amplification fiber of the present invention optically excites Er ions dispersed in a silica-based glass fiber and propagates the stimulated emission light within the fiber. In a quartz fiber for Er-doped optical amplification that amplifies signal light, E
The O ions existing near the r ions are locally substituted with anions that increase the asymmetry of the Er ion's ligand field, and the concentration of the substituted ions is low enough to not interfere with the compatibility with the transmission fiber. It has a configuration characterized by the following.

(Example 1) FIG. 1 is a diagram showing a state in which 0 ions existing in the vicinity of Er ions according to the present invention are locally replaced with other anions.

First, the reason why it is possible to improve the conversion efficiency of excitation light and stimulated emission light by replacing only the 0 ions near the Er ions without changing the composition of the entire glass will be explained.

Generally, when a small signal is amplified using a solid-state laser amplifier doped with rare earth ions, the gain per unit length is given by the following equation.

G-exp(σ,・ΔN) 1...song...(
1) Here, σ is the stimulated emission cross section and ΔN is the population inversion density. In order to increase ΔN, it is necessary to increase the pumping light intensity, so in order to obtain a high gain while keeping the pumping light intensity constant, it is necessary to increase σ.

Further, the stimulated emission cross section σ of the rare earth ion transition from state J to J is given by the following equation.

Here, A,・1 is the radiative transition probability from state J to J,
Δλeff is the effective half-width of the emission spectrum, λ is the emission wavelength, n is the refractive index, and C is the speed of light. As can be seen from equation (2), in order to obtain a large σ2, the probability of radiative transition between states can be increased, or the effective half-width of the emission spectrum can be decreased.

A method for increasing the radiative transition probability will be described below.

What determines the radiative transition probability is the transition intensity parameter Ω, and the larger Ω, the greater the radiative transition probability. Also, Ω is expressed by the following formula.

Ωt-ΣlAq'l32(k − t) --
-- (3) Here, Aqk is an odd parity term when the crystal field is expanded, and it depends on the symmetry and strength of the ligand field of the rare earth ion. On the other hand, three (k,
t) is a term that includes the overlap integral of the wave function and the energy difference between the 4f orbital and the orbital with a different parity mixed in the 41st orbital, and it depends on the nature of the bond between the rare earth ion and the ligand. That is, equation (3) shows that an increase in the asymmetry of the ligand field of the rare earth ion, an increase in the overlap between the orbits of the rare earth ion and the ligand, etc. have the effect of increasing Ω1. Therefore, in order to increase the probability of radiative transition, it is effective to replace the 0 ion of the ligand or the Si ion bonded to the 0 ion with another ion so as to increase the asymmetry of the ligand field of the rare earth ion. it is conceivable that.

The simplest way to perform such substitution is to change the overall composition of the glass, but as can be seen from the above discussion, it is the O coordinating to Er that is directly involved in Ω. Since it is a Si ion bonded to an ion or a 0 ion, E
Even if only the ions close to r are partially replaced, a sufficient effect should be obtained. It is said that the concentration of Er ions actually added is preferably about 50 to 500 ppm in order to avoid concentration quenching. Therefore, assuming that the coordination number of Er ions is 7 to 8, the amount of ions coordinated to Er ions is at most 400 to 4000 ppm, and if these coordination ions are efficiently replaced, silica glass A sufficient effect can be obtained with a slight change in composition within a few percent that does not significantly change the physical properties of the material. That is,
The concentration of the replacement ions is a low concentration within a range that does not interfere with compatibility with the transmission fiber.

Next, we will discuss the types of ions that are effective in increasing the radiative transition probability. As mentioned above, in order to increase the probability of radiative transition, it is considered effective to increase the asymmetry of the ligand field. It is necessary to select ionic species that are not present. A direct method to increase the asymmetry of the ligand field is to replace a portion of the 0 ions coordinating to Er ions with other anions, and such replacement naturally results in differences in ion species. This should give rise to asymmetry in the ligand field due to In addition, in order to increase the asymmetry of the ligand field, as described in the "Light Amplification Fiber" filed by the same inventors on the same day, it is possible to increase the asymmetry of the ligand field without directly replacing the coordination ions. This is also possible by replacing the Si ions bonded with the 0 ions. The symmetry of the ligand field is E
Determined by the distance and angle between r and O ions, these are strongly dominated by the cations bound to the 0 ions. Therefore, the "light amplification fiber" filed on the same day focuses on the fact that even if some of the Si ions bonded to the 0 ion are replaced with other ions, the asymmetry of the ligand field can be increased. .

On the other hand, in the present invention, a part of the 0 ion coordinated to the Er ion is directly replaced with another anion, and a part of the 0 ion is substituted with other anions such as F-, Cf-, S'', etc. By replacing the 0 ions with ions, the asymmetry of the ligand field can be efficiently increased.In addition, even in the optical amplification fiber of the present invention in which some of the 0 ions are replaced, the Er ions naturally remain in the quartz fiber. It is most preferable to have it exist uniformly.

Next, an example of a manufacturing method for realizing the optical amplification fiber of the present invention will be described.

In order to add desired ions in the vicinity of Er ions, for example, the core soot can be immersed in a solution in which Er ions and added ions coexist using a liquid phase immersion method, and then dried and sintered to form a base material. . In this way, it is also possible to adjust the concentration of added ions near Er by adjusting the concentrations of Er ions and added ions in the solution.

In addition, as another method, for example, the manufacturing method of ``Rare earth element-doped quartz glass optical fiber base material and manufacturing method thereof'' (filed on June 6, 1999) has already been filed by the same applicant. Just apply it. Specifically, before the sintering step, core glass containing germanium may be heat treated in the presence of a fluorine compound to fix the rare earth element compound as a fluorine compound and dope with fluorine. If manufactured in this way, the movement and volatilization of the rare earth element Er will be eliminated, so it will be possible to produce highly concentrated and uniformly doped E.
It is also possible to obtain the effect that an r-doped quartz fiber can be easily produced.

In addition, since the core of the present invention is doped with Ge or the like to increase the refractive index of the core, the refractive index of the core and cladding is higher than that of a quartz fiber that is doped with fluorine or the like and relatively lowers the refractive index of the cladding. There is also the effect of being able to provide a large difference.

(Effects of the Invention) As explained in detail above, the present invention provides O coordinating to Er.
By replacing ions with anions that increase the asymmetric characteristics of the Er ligand field, it is possible to increase the conversion efficiency (amplification efficiency) of the Er-doped optical amplification fiber.

Therefore, the optical amplification fiber according to the present invention can be applied to long-distance optical communication systems, and its effects are extremely large.

[Brief explanation of drawings]

Figure 1 is a schematic diagram for explaining the principle of the present invention, showing a state in which Si or 0 ions near Er ions in an Er-doped optical amplification fiber are locally replaced. FIG. 2 is a schematic diagram of a glass structure according to the invention when ions are replaced;

Claims (2)

[Claims] (1) Optically excites Er ions dispersed in a silica-based glass fiber whose core contains Ge, etc. to make the refractive index of the core larger than the refractive index of the cladding and has a waveguide function, and the stimulated emission light is emitted. In an Er-doped quartz fiber for optical amplification that amplifies signal light propagating within the fiber, O ions existing near Er ions are locally replaced with anions that increase the asymmetry of the Er ion's ligand field. 1. A fiber for optical amplification, characterized in that the concentration of substituted ions is a low concentration that does not impede compatibility with a transmission fiber. (2) The substitution ion is F^-, Cl^-, S^2^-
The fiber for optical amplification according to claim 1, characterized in that the fiber is an anion that serves as a ligand for Er, such as Er.