JP2801359B2 - Erbium-doped fiber optical amplifier - Google Patents

Description

The present invention relates to an erbium-doped fiber optical amplifier, and aims to provide an erbium-doped fiber optical amplifier with good amplification efficiency, in which pumping light in a wavelength band of 0.8 μm is injected into an Er-doped optical fiber. Thus, in an erbium-doped fiber optical amplifier for directly amplifying signal light in the 1.55 μm band, a 1.25 μm band reflection filter is provided at both ends of the optical fiber so that the distance between them is the resonator length for light in the 1.25 μm band. , In the 1.25 μm band to lower the energy level of Er in the excited state.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier that directly amplifies a signal light by injecting a signal light into a erbium-doped fiber doped with an erbium (Er) and pumping light (pumping light).

In an optical fiber communication system currently in practical use, repeaters are inserted at regular intervals in order to compensate for optical signal attenuation due to optical fiber loss. The repeater is configured to convert an optical signal into an electric signal using a photodiode, amplify the signal using an electronic amplifier, convert the signal into an optical signal using a semiconductor laser or the like, and send the signal again to an optical fiber transmission line. If this optical signal can be directly amplified with low noise as it is, the optical repeater can be reduced in size and economical.

Therefore, research on optical amplifiers capable of directly amplifying optical signals is being actively pursued, and the optical amplifiers to be studied are roughly classified into optical fibers doped with rare earth elements (Er, Nb, Yb, etc.). There are three types: a combination of pumping light, a semiconductor laser doped with a rare earth element, an induced Raman amplifier utilizing nonlinear effects in an optical fiber, and an induced Brillouin amplifier.

Among them, an optical amplifier combining a rare earth doped optical fiber (hereinafter abbreviated as “doped optical fiber”) and pumping light has no polarization dependence, low noise,
It has excellent features such as a small coupling loss with the transmission line, and is expected to enable a dramatic increase in transmission relay distance in an optical fiber transmission system and distribution of optical signals to a large number.

2. Description of the Related Art FIG. 3 shows the principle of optical amplification using an Er-doped optical fiber. Reference numeral 2 denotes an optical fiber composed of a core 4 and a clad 6, and the core 4 is doped with erbium (Er). When pumping light (excitation light) is incident on such Er-doped optical fiber 2, Er atoms are excited to a high energy level. When the signal light enters the Er atoms in the optical fiber 2 excited to the high energy level, the Er atoms transition to the low energy level. At this time, stimulated emission of light occurs, and the signal light is emitted. Is gradually increased along the optical fiber, and signal light is amplified.

FIG. 4 shows an example of a conventional optical fiber amplifier using such a principle. Numeral 10 denotes an Er-doped fiber doped with Er. A signal light in a 1.55 μm band is incident on the Er-doped fiber 10 from a signal light input end 12 via an optical isolator 14 and a pumping light source (pumping light source) 16. Pump light (excitation light) emitted from the optical isolator 18
And through the multiplexer / demultiplexer 20. By increasing the optical power of the pumping light sufficiently, the Er-doped fiber
10 can be excited to a high energy level, and the light of the same wavelength is stimulated and emitted by the incidence of signal light in the 1.55 μm band, and the amplified signal light is split into the multiplexer / demultiplexer 20 and the optical isolator. The signal light is emitted from the signal light emitting end 24 via the light 22. As the pumping light source 16, a semiconductor laser emitting a laser of 1.48 μm or a semiconductor laser emitting a laser of 0.8 μm, which is easily available, has been used.

Problems to be Solved by the Invention A wavelength of 0.8 as a pumping light source for the Er-doped fiber 10
The operation when a semiconductor laser of μm is used will be described with reference to the energy level diagram of FIG.

By the ground level ⁽⁴ I _15/2) Er atoms wavelength 0.8μm pumping light is incident, is excited in the wavelength 0.8μm energy level ⁴ I _9/2, immediately wavelength 1.55μm energy level Transition to ⁴ I _13/2 . When signal light having a wavelength of 1.55 μm is incident on such an excited state, stimulated emission of light having a wavelength of 1.55 μm occurs as shown by an arrow A, and the signal light is amplified. However, part of the Er atoms that have transitioned from the 0.8 μm level to the 1.55 μm level are further excited to the energy level ⁴ S _3/2 at the wavelength of 0.51 μm by the energy of the pumping light having the wavelength of 0.8 μm. Then, a transition occurs from the ⁴ S _3/2 level to the ground level. Due to this phenomenon, wavelength 1.55μm
However, there is a problem that the population (number of distributions) of the ⁴ I _13/2 level corresponding to the energy of the _GaAs decreases and the amplification characteristics are adversely affected.

The present invention has been made in view of such a point,
An object of the present invention is to provide an erbium-doped fiber optical amplifier having high amplification efficiency.

Means for Solving the Problems In order to solve the problems described above, the present invention directly amplifies signal light of the wavelength of 1.55 μm by injecting pumping light of the wavelength 0.8 μm into an Er-doped optical fiber. In an erbium-doped fiber optical amplifier, a 1.25 μm band reflection filter is provided at both ends of the optical fiber so that the distance between the two ends becomes a resonator length for light in the 1.25 μm band, and laser oscillation is performed in the 1.25 μm band, so that pumping is performed. The energy level of Er in the state is reduced.

A pair of 1.25 μm reflection filters provided so that the distance between them is the length of the resonator for light in the 1.25 μm band,
Laser oscillation in the 1.25 μm band occurs in the Er-doped fiber. As a result, stimulated emission of Er atoms excited to a high energy level is caused, a transition due to spontaneous emission from a high energy level to a ground level is reduced, and a wavelength of 1.55 μm
Increase the population of the ⁴ I _13/2 level corresponding to the band. Due to this population inversion, a wavelength of 1.55 μm with good amplification efficiency
A band optical amplifier can be provided.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description of the embodiment, the same components as those of the conventional example shown in FIG. 4 are denoted by the same reference numerals, and the description thereof will be omitted to avoid duplication.

FIG. 1 shows an overall configuration diagram of an embodiment of the present invention.
A pair of 1.25 μm band reflection filters 26 and 28 are inserted into both ends of the doped fiber 10 so as to have a cavity length for light having a distance of 1.25 μm. Other configurations are the same as those of the conventional example shown in FIG.

Pumping light having a wavelength of 0.8 μm emitted from a pumping light source 16 such as a semiconductor laser is incident on the Er dove fiber 10 via an optical isolator 18 and a multiplexer / demultiplexer 20, and a pair of 1.25 μm Band reflection filters 26, 28
Causes laser oscillation at a wavelength of 1.25 μm.

This will be described in more detail with reference to the energy level diagram of FIG. That is, by entering the pump light of wavelength 0.8μm in Er-doped fiber 10, the ground level ⁽⁴ I
_{The 15/2} ) Er atom is excited to the energy level ⁴ I _9/2 corresponding to the wavelength of 0.8 μm, but immediately transitions to the ⁴ I _13/2 level corresponding to the energy of the wavelength 1.55 μm. The Er atom at this level has an additional wavelength of 0.51μ due to the energy of the pumping light.
It is excited to a ⁴ S _3/2 level corresponding to an energy of m. In this embodiment, by laser oscillation light having a wavelength 1.25μm in the Er-doped fiber 10, to cause a stimulated emission of light, the energy level from ⁴ S _3/2 level to ⁴ I _11/2 level To lower. Therefore Er atoms energy level the transition to ⁴ I _13/2 level corresponding to the energy wavelength 1.55μm by spontaneous emission, to reduce the transition from ⁴ S _3/2 level to ground level, ⁴ The population (number of distributions) of the I _13/2 level can be increased.

Thus, ⁴ I corresponding to energy at a wavelength of 1.55 μm
_When the population of the _13/2 level increases, the wavelength is 1.55 μm from the signal light input terminal 12 through the optical isolator 14.
When the signal light is incident, a wavelength of 1.55
Stimulated emission of light of μm occurs, the signal light is efficiently amplified, and the signal light output terminal 24 is emitted through the multiplexer / demultiplexer 20 and the optical isolator 22.

In this embodiment, the pumping light is transmitted from the opposite direction to the signal light.
Since the light enters the Er-doped fiber 10, the pumping light is cut by the optical isolator 14 and the signal light input end 12
There is no adverse effect on the light source such as the semiconductor laser on the side.

Advantageous Effects of the Invention As described in detail above, the present invention provides a light having a wavelength of 1.25 μm to Er.
By lasing in doped fiber, possible to suppress the transition by over spontaneous emission from the level of ⁴ decreases to become ⁴ S _3/2 level of the population of I _13/2 to ground level effectively as an amplifier Thus, the effect of increasing the population of the ⁴ I _13/2 level and obtaining an efficient optical amplifier is obtained.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an embodiment of the present invention, FIG. 2 is an explanatory diagram for explaining an energy level of the embodiment, FIG. 3 is a schematic diagram showing a principle of optical amplification by an Er-doped fiber, and FIG. FIG. 5 is an explanatory diagram of an energy level of a conventional example. 10… Er doped fiber, 14,18,22… Optical isolator, 16… Pumping light source, 20… Multiplexer, 26,28… 1.25μm band reflection filter.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/00-3/30 JOIS

Claims (2)

(57) [Claims] An optical fiber (10) doped with Er has a wavelength of 0.5.
By injecting 8μm band pump light, wavelength 1.5
An erbium-doped fiber optical amplifier for directly amplifying a 5 μm band signal light, comprising: a pair of 1.25 μm reflection filters at both ends of the optical fiber (10) such that the distance between the two ends is a resonator length for 1.25 μm band light. 26, 28), an erbium-doped fiber optical amplifier. 2. The optical fiber according to claim 1, wherein the signal light and the pumping light are incident from opposite directions.
2. An erbium-doped fiber optical amplifier according to claim 1.