JPH0348225A - Fiber for optical amplification - 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 (Field of Industrial Application) The present invention relates to an optical amplification fiber that can be used in an optical amplifier. (Prior Art) Conventionally, quartz-based optical fibers have been doped with ErNo ions that exhibit an optical amplification effect. 1.53-1.56
Research is being conducted on optical amplification fibers that can amplify optical signals in the μm band.As a result of this research, for example, when a dye laser beam with a center wavelength of 0.98 μm is used as an excitation light source. lOmW
1.535μm signal light is 20dB with input power of
The 1.49um semiconductor laser is amplified to 36mW.
It has been reported that a gain of about 14 dB was obtained with signal light of the same wavelength when input. Recently, in order to further improve optical amplification characteristics, E
It is predicted that it would be better to add r'' ions only to the center of the core instead of adding them to the entire core.The fiber structure in this case is called the limited type here. Results similar to this prediction have been obtained through numerical calculations.Recently, a method for manufacturing this limited fiber has also been considered.This method is one of the conventional methods for manufacturing optical fibers. It is based on a certain MCVD method, in which the core surrounding area (cladding) is fabricated inside a silica-based glass tube by chemical vapor deposition, and then the core area is fabricated in two steps. Without adding Er,
Ge. to increase the refractive index of the core portion. A.I. A layer is created by adding P etc. to quartz glass, and then E is added inside the layer.
This is to create a layer containing r. After that, a fiber base material was created by collapsing according to the normal MCVD procedure, and when it was drawn, an Er-doped part 3 was formed at the center of the internal core l of the cladding 2, as shown in Figure 1. This makes it possible to obtain a horizontally-shaped limited fiber. Defined fibers can be produced by the VAD method. In this case, contrary to the MCVD method, Ge. AI2,
A glass base material having a glass composition with a high refractive index doped with P etc. is prepared in a state where Er is added, and then Er is not added to the outer periphery of this glass base material (the other glass base materials are (Similar to )) form the core part by forming the glass base material.
(Problems to be solved by the invention) However, the above manufacturing method had the following problems. a. Both the MCVD method and the VAD method require a two-step core manufacturing process, which is one more step than the conventional core manufacturing method, resulting in longer manufacturing times and higher costs. b. Er is generally introduced in the form of ErCI2x, but this E r C 12 s may reduce G e O z that should be co-doped into the core glass and induce volatilization of Ge. The refractive index distribution may be disturbed. (Object of the Invention) The object of the present invention is to provide an optical amplification fiber that can obtain high gain with lower pumping light intensity and is easy to manufacture. (Means for Solving the Problems) The optical amplification fiber of the present invention is an optical amplification optical fiber in which a substance that exhibits an optical amplification effect is added to the core l of the optical fiber l, as shown in FIG. , the diameter of the core 1 is set equal to or smaller than the core diameter a0 that minimizes the mode field diameter. (Function) The optical amplification fiber of the present invention is easy to manufacture because the entire core is doped with Er.It is also possible that the diameter of the core l is equal to or larger than the core diameter a0 that minimizes the mode field diameter. Since it is set small, high gain can be obtained with low pumping light pth. (Example) Er"゜ added to the core emits light in a three-level system, so when the excitation light is weak or no excitation light is input, it functions as an attenuator rather than an amplifier. ．In order for the amplification characteristics to be exhibited, it is necessary to input pump light of a certain threshold value (P th) or more into the fiber.
h can be made smaller as the mode field diameter ω of the excitation light guided through the fiber is smaller. Therefore, in order to obtain high gain with low pumping light intensity, a fiber structure with a mode field diameter ωP as small as possible is desirable. In order to reduce the mode field diameter ω2, the refractive index n of the core l, which is the central part of the optical fiber and the leading wave part, is
and the refractive index n2 of the cladding 2 on its outer periphery △
It is desirable to increase Ia of core l by increasing Δmi(n+n*)/n1. When the diameter a of the cutting core l is made smaller than a certain core diameter a6 (a (1 is uniquely determined from the above △ and means the core diameter that minimizes the mode field diameter)), the mode field diameter ω becomes On the contrary, it becomes larger as shown in Fig. 3. The following definition formula was used to calculate the mode field diameter ω2: (ω/a) = 0.65 + 1.619/V'' + 2.
879/V'...■ Here, V is a fiber parameter called the normalized frequency, and the core diameter a and... Once the NA (numerical aperture) of the fiber and the wavelength of the input light (here, the wavelength of the excitation light) L are determined, it is given by the following equation. V=2π/λ・a-NA The equation (2) is an approximate equation, and there are other defining equations, but if the refractive index distribution is step-shaped, there is no big difference between them, and they are more approximate. In reality, it is a value that can be obtained by optical measurement. Therefore, it is considered that the most desirable setting is to set the core diameter a to the above-mentioned a0. Therefore, as shown in FIG. 4, from one fiber base material having a uniform Δ value and Er concentration, the core diameter a is adjusted in three steps a. ,
Obtain the fire bar changed to a b + a c (a
m is almost the same as a0 above)), and its efficiency (obtained gain/input pump power) and the above pth (
When we calculated the minimum excitation power that produces a positive gain, we obtained the results shown in Table 1. That is, the core diameter is a. The fiber set at pth started to show gain at the lowest minimum pump power pth and showed the highest value of efficiency. However, in this experiment, the length of each fiber was selected to give the best efficiency (the length was changed little by little, and the values at the time of the best efficiency are shown in Table 1). (Leaving space below) Table 1 As is clear from this table, high efficiency and low pth are achieved by setting the core diameter a smaller than the above a0, and rather widening the optical effective mode field diameter ω. Better results were obtained when This result was obtained because the predicted limited fiber was effectively obtained as described above. In other words, it was found that even if Er is added to the entire core, the same effectiveness as the limited fiber can be obtained by setting the diameter a of the core l 6 smaller than the core diameter a0 shown in Fig. 3. ．． (Effects of the Invention) The optical amplification fiber of the present invention has the following effects. ■． Because Er is added to the entire core. There is no need to go through a two-step process, not only when the core part is manufactured by the MCVD method, but also when it is manufactured by the VAD method.
It is easy to manufacture, shortens manufacturing time, and reduces costs. ■． Since the diameter of the core l is set to be equal to or smaller than the core diameter a0 that minimizes the mode field diameter, a high gain can be obtained with a low minimum excitation power pth.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of a limited optical amplification fiber,
Figure 2 shows the core diameter a. The mode field of the fiber and E
Fig. 3 is an explanatory diagram showing the relationship between the core diameter and mode field diameter, and Fig. 4 is an explanatory diagram showing the relationship between three types of core diameters and mode field diameter. l is the core 2 is the cladding 3 is the Er-added part Fig. 1

Claims (1)

[Claims] In an optical amplification optical fiber in which a substance that exhibits an optical amplification effect is added to the core 1 of the optical fiber, the diameter a of the core 1 is the core diameter a_ that minimizes the mode field diameter.
An optical amplification fiber characterized in that the fiber is set equal to or smaller than 0.