JPH02306677A - Photoamplifier - Google Patents

Abstract

PURPOSE:To reduce the length of a rare earth element or transition metal-added optical fiber by applying a reflecting mirror for reflecting only an exciting light to the output terminal of a rare earth element or transition metal-added optical fiber, and again introducing the light to the optical fiber by using the mirror. CONSTITUTION:An exciting light IP is reflected to the output end face of a rare earth element or transition metal-added optical fiber 1 of an active medium, and a reflecting mirror 4 for transmitting a signal light IS is added thereto. Thus, if the reflecting mirror 4 does not exist, the light IP to be externally discharged is again introduced into the fiber 1. Accordingly, the light is effectively absorbed into the fiber 1. In this manner, the fiber 1 can be shortened, and a photoamplifier can be reduced in size.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical amplifier required in the fields of optical communication and optical measurement.

[Conventional technology and its issues]

Conventionally, the mainstream of optical amplifiers has been to use semiconductor LDs, and semiconductor amplifiers are usually used as traveling wave amplifiers in which an antireflection film is applied to a semiconductor FP (Fabry-Perot) type LD.

However, semiconductor amplifiers have the disadvantage that not only the degree of amplification is polarization-dependent, but also the degree of amplification between bits changes in high-hit rate signal light due to pattern effects.

On the other hand, optical fiber amplifiers that use optical fibers doped with rare earth elements or transition metals and utilize the laser return of the rare earth elements or transition metals have polarization independence by using a fiber with a perfectly circular core. It can be easily realized and has no pattern effect, which makes it superior to optical amplifiers using semiconductor LDs.

FIG. 5 shows the basic configuration of a conventional optical amplifier using an optical fiber doped with rare earth or transition metal.

Symbol l is a rare earth element or transition metal doped optical fiber having a length of several tens of meters, symbol 2 is an excitation light source consisting of a semiconductor LD that generates excitation light ip to excite the rare earth element or transition metal doped optical fiber l, and symbol 3 is a signal. A guichroic mirror having the characteristic of transmitting light Is and reflecting excitation light 1p is shown.

Since this optical fiber amplifier uses an optical fiber doped with rare earth elements or transition metals with a length of up to several tens of meters, it has the disadvantage that it is larger than a semiconductor optical amplifier. Furthermore, the cost of optical fiber doped with rare earth elements or transition metals is extremely high. Therefore, shortening the length of the optical fiber 1 doped with rare earth elements and transition metals is an important issue in order to reduce the size and cost of optical amplifiers.

[Means to solve the problem]

The present invention was made in view of the above circumstances, and its purpose is to shorten the length of rare earth element-doped or transition metal-doped optical fiber used in optical amplifiers in order to reduce the size and cost of optical amplifiers. It's about doing.

The optical amplifier of the present invention includes an active medium consisting of an optical fiber or an optical waveguide whose core and/or cladding is doped with a rare earth element or transition metal having a laser transition level;
An optical amplifier consisting of an excitation light source that excites this active medium, and a reflecting mirror that transmits the signal light and reflects only the excitation light to prevent it from entering the active medium again, at or near the output end of the active medium. The solution was to exist.

[Effect]

Since the excitation light is reflected at the output end of the active medium and the reflected light is returned to the active medium, the optical fiber serving as the active medium can be shortened.

The present invention will be explained in more detail below.

An example of the basic configuration of the present invention is shown in FIG.

The biggest feature of the optical amplifier of the present invention shown in FIG. 1 compared to the conventional configuration is that the pumping light tp
A reflecting mirror 4 that reflects the signal light Is and transmits the signal light Is has just been inserted.

FIG. 2 is a diagram explaining the principle of the optical amplifier of the present invention.

In principle, the optical amplifier shown in FIG. 1 has a reflector 4 added to the output end face of an optical fiber l doped with a rare earth element or a transition metal as an active medium, which reflects the pumping light 1p and transmits the signal light Is. (Figure 2, top). Further, the intensity of the excitation light (p) in the rare earth element or transition metal doped optical fiber I can be expressed as shown in the lower diagram of FIG. However, IP+ indicates traveling light, and 1p- indicates reflected light.

The amplification degree of the optical fiber 1 doped with rare earth elements or excitation metals is the integral value of the excitation light in the optical fiber S 'o I p''
The larger (z)+I p-(z)dz is, the larger it is. When the length of the optical fiber l doped with rare earth elements or transition metals is shortened in a conventional optical amplifier, the integral value of the light intensity absorbed by the optical fiber for pumping S 'o l p''(z)dz
The remaining amount is emitted to the outside of the optical fiber 1. On the other hand, in the optical amplifier of the present invention having the reflecting mirror 4, when the reflecting mirror 4 is not provided, the excitation light 1p is emitted to the outside.
- is introduced into the optical fiber I again, so the excitation light is effectively absorbed into the optical fiber I.

Theoretically, rare earth element or transition metal doped optical fiber 1'
t<l f) Amplification degree 150ut/I sint, optical fiber! The relationship between the intensity of the traveling excitation light tp+ and the intensity of the reflected excitation light ip- is expressed by the following equation (1).

Is”t/Is”=exp(k(lp”(z)+IP
-(z))dz+αs4) (1) where k is a proportionality constant, αS is a loss constant of signal light, and L is the length of the rare earth element or transition metal doped optical fiber l. Also, [p”(z) and I p−(z) are lp+=
Lp"exp(-α p-2)...
...(2) Ip-=R1p”exp(αp(z-2L
)) ・-” (2°), R is the reflectance of the excitation light on the reflecting mirror 4, and αp is the loss constant of the excitation light.

Then, by substituting equations (2) and (2°) into equation (1), we get G (=ls0ut/Is") = elp((k"IP"/ff P) (1-ex
p(-α PL)+R-exp(-2αp-L) (
exp(αp-L-1)-α5-L)--(3) is obtained. That is, in the optical amplifier of the present invention, the amplification can be increased by two terms on the right side of equation (3) compared to the optical amplifier of the conventional structure (R=0).

Therefore, compared to the conventional structure, the optical amplifier of the present invention reflects the pumping light at the output end of the optical fiber 1 doped with rare earth elements or transition metals, and returns the reflected light once again into the optical fiber i, which is the active medium. Elemental or transition metal doped optical fiber! can be shortened, and the optical amplifier can be miniaturized.

Note that in the examples shown in FIGS. [FIG. The medium is not limited to this, and an optical waveguide doped with a rare earth element or a transition metal that does not hit the first laser transition position can be used, and there is no difference in its operation and effect.

〔Example〕

The present invention will be described in more detail below with reference to the drawings.
The embodiments disclosed below are merely illustrative of the present invention;
They are not intended to similarly limit the scope of the invention.

FIG. 3 is a diagram illustrating the present invention in detail, in which the symbol l represents Er as a rare earth element having a laser transition level of about 10
0 ppm+ doped optical fiber, signal light 1.53μ
False and unimodal. Reference numeral 2 denotes a pumping light source consisting of a semiconductor LD that supplies pumping light, and has an oscillation wavelength of 0.98 μm and an output power of 96 mW.

Reference numeral 3 denotes a gicroic mirror that transmits the signal light of 1.53 μm without loss and reflects 100% of the excitation light with a wavelength of 098 μm. Reference numeral 4 denotes a reflecting mirror made of a dielectric multilayer film that transmits signal light of 1.53 μm without loss and reflects 100% of excitation light with a wavelength of 0.98 μm. Reference numeral 5 indicates a single mode destination fiber for guiding the signal light, and reference numeral 6 indicates a single mode optical fiber for taking out the amplified light. All lenses are aspheric lenses, and the coupling loss of the signal light from the optical fiber 5 to the Er-doped optical fiber L is 1 clB, the coupling loss of the excitation light from the excitation light source 2 is 3 dB, and the E
r-doped optical fiber! The coupling loss of the signal light from the Er-doped optical fiber 1 to the optical fiber 6 is 1 dB, and the coupling loss of the pumping light reflected from the Er-doped optical fiber 1 by the reflecting mirror 4 and entering the Er-doped optical fiber 1 again is ~OdB.

Fig. 4 shows the experimental values of the change in signal light 1+1 gain 1s out /1 sl n when the length of the Er-doped optical fiber l is changed, and the solid line in the figure shows the case where there is a reflector 4. The case where there is no reflecting mirror 4 is shown by a dotted line.

According to this characteristic, when there is no reflector 4, the optimum Er-doped optical fiber length is 35 ffl and a gain of 49 dB, whereas by inserting the reflector 4, the optimum length is 23 m.
The gain increases to 68 dB. Furthermore, the length of the Er-doped optical fiber is 9 m to obtain a gain of 49 dB without the reflecting mirror 4.

Therefore, from this result, it was confirmed that by using the reflecting mirror 4, the length of the rare earth element-doped optical fiber 1 used in the optical fiber amplifier can be made shorter than when the conventional reflecting mirror 4 is not provided.

In this example, an Er-doped optical fiber was used as a rare-earth element or transition metal-doped optical fiber. is not limited to this, and the wavelengths of the excitation light and signal light are not limited to those of this embodiment. Furthermore, although a semiconductor LD is used to generate excitation light, a solid-state laser may also be used.

〔Effect of the invention〕

As explained above, in the optical amplifier of the present invention, a reflector that reflects only the excitation light is added to the output end of the rare-earth element or transition metal-doped optical fiber, and the excitation light is redirected to the rare-earth element using this reflector. Because it is incident on the optical fiber doped with rare earth elements or transition metals, it is possible to pump the optical fibers doped with rare earth elements or transition metals with high efficiency. There are 111 points in which the length of the optical fiber can be reduced, which makes it possible to realize a smaller optical amplifier and also to reduce the cost.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of an optical amplifier according to the present invention, FIG. 2 is a diagram explaining the principle of the optical amplifier according to the present invention, and FIG. 3 is a schematic configuration diagram showing an embodiment of the optical amplifier according to the present invention. FIG. 4 is a graph showing changes in optical amplification gain depending on the presence or absence of a reflecting mirror, and FIG. 5 is a schematic diagram of a conventional optical amplifier. 1... Rare earth element or transition metal doped optical fiber (active medium), 2... Excitation light source, 4...
··Reflector. Applicant: Nippon Telegraph and Telephone Corporation Figure 1 Figure 2 Base Figure 3 Erno) Length of 770 Oaf Eye Bar (m) Figure 5

Claims (1)

[Claims] An active medium consisting of an optical fiber or optical waveguide whose core and/or cladding is doped with a rare earth element or transition metal having a laser transition level, and an excitation light source that excites this active medium. What is claimed is: 1. An optical amplifier comprising: a reflecting mirror at or near the output end of the active medium that transmits the signal light and reflects only the excitation light so that it enters the active medium again.