JPH0493091A - Optical amplifier - Google Patents

Abstract

PURPOSE:To simply combine many excitation lights without combination of polarized waves by using a plurality if excitation light sources having different oscillation wavelengths. CONSTITUTION:An incident signal light is combined with an excitation light having 1.48mum wavelength emitted from a semiconductor laser 31 by an optical fiber coupler 21. The combination light is further combined with an excitation light having 0.98mum wavelength emitted from a semiconductor laser 32 by an optical fiber coupler 22, and incident on an Er-added optical fiber 11. An amplification gain of 35dB and a saturated output of +4dBm are obtained by a semiconductor laser 20mW having 1.48mum wavelength and a semiconductor laser 10mW having 0.98mum wavelength. If the source has only the laser having 1.48mum of wavelength, a gain of 20dB and a saturated output of -3dBm are obtained by 15mW of excitation power. If the source has only the laser having 0.98mum of wavelength, a gain of 20dB and a saturated output of -3dBm are obtained by 10mW of excitation power. Accordingly, the amplification gain and the saturated output can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical amplifier that optically amplifies signal light in an optical amplification medium doped with rare earth elements.

(Prior art) In recent years, research has been actively conducted on optical amplifiers that directly amplify signal light as light without photoelectric conversion, with the aim of making optical communication repeaters more compact and economical, and compensating for losses due to optical branching. It is being done. Up to now, optical amplification methods have been: ■ Those that use a semiconductor laser medium ■ Those that use an optical fiber doped with a rare earth element such as Er in the tip portion ■ The use of nonlinear optical effects such as stimulated Raman scattering and stimulated Brillouin scattering of optical fibers ■Items using crystals or glass having waveguides doped with rare earth elements have been reported (Optics, Vol. 18, No. 6, p. 282).

Among them, optical amplifiers using optical fibers doped with rare earth element Er have the lowest optical fiber loss of 1.5/im.
It has many advantages over other optical amplification methods, such as high gain in the band, no polarization dependence of gain, and the ability to couple to a transmission line with low loss. Therefore,
Research on Er-doped optical fiber amplification is being actively conducted at many research institutions (Optics, Vol. 19, No. 5, 27
(page 6).

Optical amplification using a rare earth doped optical fiber is performed by inputting excitation light having a wavelength corresponding to the absorption band of the rare earth ion into the rare earth doped optical fiber. The absorption wavelength of Er ions is mainly in the 0.5 μm band and 0.6 μm band. Zm band, 0.
8. Zm band, 0.98/, Im band, 1.48. Zm band etc. are known, but among them, 0.98 μm band and 1.48 μm band.
Since semiconductor lasers with high absorption efficiency and high power can be used in the μm band, research on Er-doped optical fiber amplifiers excited at these wavelengths is being actively conducted.

The direction of incidence of the excitation light is forward excitation, which is incident in the same direction as the signal light from the signal light input side, backward excitation, which is incident in the opposite direction to the propagation direction of the signal light from the signal light output side, and further forward. There is bidirectional excitation, which is a combination of excitation and backward excitation. Bidirectional pumping is effective in obtaining high pumping light power. The amplification gain of the optical amplifier and the output amplified signal optical power (
The higher the saturation output), the more desirable it is, but this requires greater pumping light power. The methods known so far for injecting high pumping light power into an optical amplification medium are: - bidirectional pumping using two pumping light sources that oscillate at the same wavelength;
■There is a method in which two excitation light sources that oscillate at the same wavelength are polarized and combined (Electronics Letters (El
electronics Letters), Volume 25, No. 24, Page 1656).

When using optical amplifiers in optical communication systems, the main
Possible methods include using it as a booster amplifier for transmitted light, using it as an optical direct amplification repeater, and using it as a receiver preamplifier. In particular, when used as a preamplifier of a receiver, in order to prevent pumping light from entering the photodetector, it is necessary to provide an optical filter at the output of the optical amplifier to block pumping light except for backward pumping. .

(Problem to be Solved by the Invention) In order to combine excitation lights that oscillate at the same wavelength, there is a problem that the polarization planes of the two excitation lights must be orthogonal to each other when they are combined. Furthermore, polarization synthesis can only combine two excitation lights. In order to make these pump lights incident on the optical amplification medium without polarization synthesis, it is possible to use bidirectional pumping, but only bidirectional pumping can only use up to two pumping light sources.

A first object of the present invention is to provide an optical amplifier that can easily combine the powers of two or more pumping lights without polarization combining, and thereby has a large amplification gain and saturation output.

A second object of the present invention is to provide an optical amplifier with a small number of parts, which eliminates the need to newly provide an optical filter for blocking pumping light even when the optical amplifier is used as a preamplifier.

(Means for Solving the Problems) The optical amplifier of the present invention is characterized in that by using a plurality of pumping light sources with different oscillation wavelengths, a large number of pumping lights can be easily synthesized without performing polarization synthesis.

Furthermore, at least one of the pumping light sources is arranged for backward pumping, and an optical multiplexer/demultiplexer for inputting the pumping light into the optical amplification medium is provided with a function of blocking the forward pumping light. shall be.

(Function) In order to combine two laser beams that oscillate at the same wavelength, the polarization planes must be orthogonal to each other. Otherwise, the components with the same polarization directions will cancel each other out, and the sum of the powers before combination will increase. decrease more. However, a plurality of laser beams having different oscillation wavelengths can be simply combined without polarization combining and with almost no power loss by simply using an appropriate optical multiplexer/demultiplexer.

The optical amplifier of the present invention focuses on the fact that rare earth ions have a plurality of absorption wavelength bands, and combines a plurality of excitation light sources that excite different absorption wavelength bands among the plurality of absorption wavelength bands that rare earth ions have. waves to simultaneously excite multiple absorption wavelength bands of rare earth ions. As a result, it is possible to obtain a pumping light power greater than that of the conventional method without polarization combining, and optical amplification with a large amplification gain and saturation output is possible.

Furthermore, by adding a function of blocking forward pumping light to the optical multiplexer/demultiplexer for backward pumping, it is possible to prevent the pumping light from being output from the optical amplifier together with the amplified signal light. Therefore, even when this optical amplifier is used as a preamplifier, high quality reception characteristics can be obtained without providing an optical filter.

(Example) The optical amplifier of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of an optical amplifier according to the present invention. In this embodiment, the optical amplification medium is an Er-doped optical fiber 11, and the excitation light sources are a semiconductor laser 31 with an oscillation wavelength of 1.48□m band and a semiconductor laser 32 with an oscillation wavelength of 0.98□m band.
The optical multiplexer/demultiplexer combines light with a wavelength of 1.48 μm and light with a wavelength of 1.54 μm.
Single mode optical fiber coupler 21 for wavelength multiplexing capable of multiplexing m-band light, and wavelengths from 1.48 pm to 1.54 □
This is a wavelength multiplexing single mode optical fiber coupler 22 capable of multiplexing m-band light and 0.98 μm-band light.

An optical fiber coupler is manufactured by bringing two optical fibers close together, heating and melting them, and stretching them.

The transmission wavelength dependence can be changed by changing the length of the melted portion.

In the optical amplifier of this embodiment, the incident signal light is first combined with the 1.48 μm excitation light emitted from the semiconductor laser 31 by the optical fiber coupler 21 . This combined light is further combined with the excitation light of 0.98 μm emitted from the semiconductor laser 32 by the optical fiber coupler 22, and the Er
The light is incident on the doped optical fiber 11.

In this example, the excitation power of 20 mW of a 1.48 μm semiconductor laser and 10 mW of a 0.98 μm semiconductor laser was used to
An amplification gain of 5 dB and a saturated output of +4 dBm were obtained.

When the pump light source is only a semiconductor laser with a wavelength of 1.48 μm, a gain of 20 dB and a saturation output of -3 dBm can be obtained with a pump power of 15 mW, and when the pump light source is only a semiconductor laser with a wavelength of 0.98 □m, the pump power 20d at 10mW
A gain of B and a saturation output of -3 dBm were obtained. Therefore, according to the present invention, the amplification gain and saturation output can be increased.

FIG. 2 is a block diagram of another embodiment of the optical amplifier according to the present invention. In this embodiment, the optical amplification medium is an Er-doped optical fiber 11, and the excitation light sources are a semiconductor laser 33 with an oscillation wavelength of 1.48 μm band and a semiconductor laser 3 with an oscillation wavelength of 0.98A1m band.
4. The optical multiplexer/demultiplexer separates light with a wavelength of 1.48μm% and wavelength 1.5%.
A wavelength multiplexing single mode optical fiber coupler 23 capable of combining light in the 4 μm band and light in the 1.54 μm band and 0.5 μm in wavelength.
It is possible to combine light in the 98/im band, and the wavelength is 1.48/, m.
This is a single mode optical fiber coupler 41 for wavelength multiplexing with a filter function that has a characteristic of blocking light.

The manufacturing method of this optical fiber coupler 41 is the same as that described above, and the transmission wavelength characteristics are particularly controlled during manufacturing. The input and output terminals of the optical fiber coupler 41 are shown in FIG. 3(a). The transmission wavelength characteristics from the terminal 411 to the terminal 413 are shown in FIG. 3(b). In this optical fiber coupler, the 1.54 pm light incident from the terminal 411 is
% is passed through the terminal 413, but 1, which is incident from the terminal 411,
90% of the 48 μm light passes through the terminal 414. terminal 4
The light of 0, 98/, m incident from 14 is sent to terminal 411 at 9
0% pass.

In the optical amplifier of this embodiment, the incident signal light is first combined with the 1.48 μm excitation light emitted from the semiconductor laser 33 by the optical fiber coupler 23, and is input into the Er-doped optical fiber 11. The light emitted from the Er-doped optical fiber 11 passes through the optical fiber coupler 41 and becomes the output of the optical amplifier. This optical fiber coupler 41 makes the excitation light of 0.98 □m emitted from the semiconductor laser 34 enter the Er-doped optical fiber 11 from behind. The amplified signal large side end of this optical fiber coupler 41 is a terminal 411, and the output end is a terminal 411.
3, not all of the light from the semiconductor laser 33 is absorbed by the Er-doped optical fiber 11, so not only the signal light but also the light of 1.48 □m enters the terminal 411. However, since the single mode optical fiber coupler 41 for wavelength multiplexing with filter function has a characteristic that prevents the 1.48 μm light propagated together with the signal light from exiting the output terminal, this optical amplifier is used as a preamplifier. In this case, only the signal light reaches the light receiving element.

According to this embodiment, the 164
The power of the excitation light at 8 μm was 117 dBm or less, and the 1.48 μm light could be blocked to a greater extent than at 17 dBm when this light was not blocked.

Although the optical amplifier according to the present invention has been described above using embodiments, the present invention is not limited to these embodiments, and several modifications can be made.

The optical amplification medium is not limited to optical fiber. For example, glass or crystal provided with a waveguide doped with rare earth elements may be used. It goes without saying that the rare earth element to be added is not limited to Er. Therefore, the wavelength of the excitation light and the wavelength of the signal light to be amplified need only be matched to the absorption wavelength band and fluorescence wavelength band of the rare earth element to be added.
This is not limited to the examples. For example, even when using an Er-doped optical amplification medium as discussed in the example, the excitation light wavelength is not limited to 1.48 μm and 0.98 μm, but is 0.5 μm.
A μm band, 0.6 pm band, 0.8 □m band, etc. may be used, and the excitation light wavelength may match any absorption wavelength of the rare earth metal used.

Furthermore, many absorption wavelength bands may be excited simultaneously, and the method of combining the pumping light may be combined with a conventional method of combining polarized waves. For example, the output lights of two 1.48 μm band semiconductor lasers may be polarized and combined, then wavelength-combined with signal light, and further wavelength-combined with pump light of 0.98/J, m.

The method of inputting the excitation light into the optical amplification medium is not limited to forward excitation in which the excitation light is input in the same direction as the signal light as in the embodiment shown in FIG.
It is also possible to use backward pumping or bidirectional pumping as in the embodiment of FIG.

The laser used is not limited to a semiconductor laser, and any laser may be used.

A dichroic mirror or the like may be used as the optical multiplexer/demultiplexer, and any element may be used as long as it has the performance. The same applies to an optical multiplexer/demultiplexer with a filter function. Second
Although the illustrated embodiment utilizes the characteristics of an optical fiber coupler, the function of an optical filter may be added, for example, by applying a coating to a dichroic mirror. An example of the transmission characteristics of the coating is shown in FIG. This coating serves as a long-wavelength transmission filter, which allows signal light in the 1.54 μm band to pass through well, but reflects light at wavelengths shorter than 1.48 □m. Therefore, the 1 that propagated with the signal light
,48/,m excitation light is blocked, but 0.98/,m
The excitation light is reflected and incident on the optical amplification medium.

Furthermore, the optical amplifier of the present invention may include an optical isolator or the like among the components.

(Effects of the Invention) As explained above, by applying the present invention, multiple excitation light sources that excite different absorption wavelength bands among the plurality of absorption wavelength bands possessed by rare earth elements can be combined without polarization synthesis. As a result, it is possible to obtain a pumping light power greater than that of conventional optical amplifiers, and to achieve an optical amplifier with a large amplification gain and saturation output.

Furthermore, by preventing the optical amplifier from outputting pump light together with the amplified signal light, there is no need to provide an optical filter, and the advantage is that the optical amplifier can be used as a preamplifier without increasing the number of parts. be.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing another embodiment of the present invention, and Fig. 3 (aXb) each has a filter function in the embodiment of the present invention. FIG. 4 is a diagram and a transmission characteristic diagram for explaining the characteristics of the optical fiber coupler, and FIG. 4 is a characteristic diagram of a dichroic mirror optical multiplexer/demultiplexer having a filter function. In the figure, 11... Optical amplification medium, 21.22.23... Optical multiplexer/demultiplexer, 31, 32.33.34... Pumping light source, 41.
...Optical multiplexer/demultiplexer with filter function, 411, 412.4
13.414: Input/output terminal of optical fiber coupler with filter function.

Claims (2)

[Claims] (1) An optical amplification medium doped with a rare earth element, a plurality of excitation light sources that excite different absorption wavelength bands among the plurality of absorption wavelength bands possessed by the rare earth element, and excitation light and signal light output from the excitation light source. and an optical multiplexing means for making each of the rare earth-doped optical amplification medium incident on the rare earth-doped optical amplification medium. (2) An optical amplifier in which excitation light from at least one of the excitation light sources is input to the optical amplification medium from an output end of the amplified signal light of the optical amplification medium by an optical multiplexer/demultiplexer, the optical multiplexer/demultiplexer 2. The optical amplifier according to claim 1, wherein the wave filter has a function of blocking at least a portion of the pumping light incident from the input end of the signal light.