JPH03218945A - Light amplifying fiber - Google Patents

Abstract

PURPOSE:To obtain the light amplifying fiber having high efficiency in converting the excitation light into induced-emission light and satisfying the requirements for use in an optical communication system by substituting Si ion close to Er ion with a specified ion in the Er-addition light amplifying quartz glass fiber. CONSTITUTION:In the Er-addition light amplifying quartz glass fiber, the Si ion close to Er ion is locally substituted with an ion capable of increasing the asymmetry in the ligand field of the Er ion, a cation forming a network forming oxide in the glass, e.g. P<3+>, B<3+>, As<3+> and Sb<3+> or a cation forming an intermediate oxide in the glass, e.g. Al<3+>, Ga<3+>, Ti<4+>, Sn<4+>, Ta<3+>, Nb<3+> and Bi<3+>, to obtain a light amplifying fiber for optically exciting Er ion and amplifying the signal light propagating in the fiber with the induced-emission light. The concn. of the substituent ion is kept low so that the consistency with the transmission fiber is not hindered.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to an optical amplification fiber used in a 1.5 μ band optical communication system using an optical amplification repeater.

(Prior Art and its Problems) Optical communication systems that utilize 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 about 200 km at most, 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 the attenuated optical signal into an electrical signal, and perform regenerative amplification using an electrical circuit. In the future, it is desired to realize an optical amplification method that regenerates and amplifies the optical signal without performing photoelectric conversion, since it allows for flexible system design.

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 strong pumping light source of about 100 mW 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 development of optical amplification fibers with high conversion efficiency is required. is strongly 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 exacerbating effect on Er ions, but no ion has yet been known to have a large exacerbating effect. 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 an Er-doped quartz glass fiber for optical amplification that amplifies signal light, Si ions existing near Er ions are locally replaced with ions that increase the asymmetry of the Er ion's ligand field, and It has a configuration characterized by a low concentration that does not impede compatibility with the transmission fiber.

(Example 1) FIG. 1 is a diagram showing a state in which Si 2 ions existing in the vicinity of Er ions of the present invention are locally replaced with other ions.

First, the reason why it is possible to improve the conversion efficiency of excitation light and stimulated emission light by replacing only Si 2 ions in the vicinity of 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=ex directσ.・ΔN) ・1・− (1) Here, σ2 is the stimulated emission cross section, and ΔN is the population inversion density.

In order to increase ΔN, the pumping light intensity must be increased, so in order to obtain high gain while keeping the pumping light intensity constant, σ
, needs to be increased.

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

Here, A,·, is the radiative transition probability from state J to J,
Δλ. ff 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 σp, the probability of radiative transition between states is increased, but 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 Ωt, and the larger Ω, the greater the radiative transition probability. Moreover, Ω1 is expressed by the following formula.

Ω1=Σl A. "l three" (k-t) ・---
--- (3) Here, A♂ 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 into the 4f 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 Ω. Therefore, in order to increase the probability of radiative transition, it is effective to replace the O ion of the ligand or the Si ion bonded to the O 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, it is E as shown in the schematic diagram of the glass structure in Figure 2.
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 only about 400 to 4000 ppm, and if these coordination ions are efficiently replaced, the physical properties of silica glass can be improved. It becomes possible to obtain a sufficient effect with a slight compositional change within a few percent that does not cause a significant change in the composition. 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, it is considered effective to increase the asymmetry of the ligand field in order to increase the probability of radiative transition. 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 some of the O ions coordinating to Er ions with other anions; 《However, S bonded with the coordinated 〃0 ion
It is also possible to increase the asymmetry of the ligand field by replacing the i ions. The symmetry of the ligand field is Er and 0
Determined by the distance and angle between the ions, these are strongly dominated by the cations bound to the 0 ion.

Therefore, 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.

In order to replace Si ions with other cations for this purpose, the added cations must be ions that can occupy similar sites to Si ions in the glass, but alkaline and alkaline earth Ions that become network-modifying oxides in glass, such as similar ions, coordinate with Er.
Since it is not directly involved in bonding with ions, it has little effect.

On the other hand, pS+B becomes a network-forming oxide in the glass.
A It!, which becomes ions such as S-As", Sb5'", or intermediate oxides! +, ca”, T i”,
Since ions such as S n"Ta"Nb"+ Bt3+ directly combine with the 0 ions coordinating to Er, a remarkable effect can be obtained. Next, a manufacturing method for realizing the optical amplification fiber of the present invention will be described. Let's discuss an example.

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, in the above explanation, there was no mention of a means of imparting a waveguide function to the quartz fiber by the refractive index difference between the core and the cladding, but as a means of making the refractive index of the core relatively larger than the refractive index of the cladding. In general, there is a method of doping the core with germanium (Gel) to relatively increase the refractive index of the core, or a method of doping the cladding with fluorine (F) or the like to relatively decrease the refractive index of the cladding. The present invention is applicable to any type of quartz fiber as long as the quartz fiber is doped with Er.

(Effects of the Invention) As explained in detail above, the present invention provides 0
By replacing the Si ions that bond with ions with ions 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. By locally replacing only Si ions, it is possible to have viscosity-temperature characteristics, refractive index, and reliability similar to those of a transmission quartz 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]

Fig. 1 is a schematic diagram of the present invention showing a state in which St ions near Er ions in an Er-doped optical amplification fiber are locally replaced, and Fig. 2 shows the present invention when Si ions near Er ions are replaced. FIG. 1 is a schematic diagram of a glass structure according to

Claims (3)

[Claims] (1) In a quartz glass fiber for Er-doped optical amplification, which optically excites Er ions dispersed in the silica-based glass fiber and amplifies the signal light propagating within the fiber using stimulated emission light, Si present near the Er ions is used. The ion is locally substituted with an ion that increases the asymmetry of the ligand field of the Er ion, and the concentration of the substituted ion is configured to be a low concentration that does not interfere with compatibility with the transmission fiber. An optical amplification fiber characterized by: (2) The substitution ion is P^5^+, B^5^+, As
The fiber for optical amplification according to claim 1, characterized in that the fiber is a cation such as ^5^+, Sb^5^+, which forms a network-forming oxide in the glass. (3) The substitution ion is Al^3^+, Ga^3^+,
Ti^4^+, Sn^4^+, Ta^5^+, Nb^5
The fiber for optical amplification according to claim 1, characterized in that the fiber is a cation which becomes an intermediate oxide in the glass, such as ^+, Bi^3^+, etc.