JPH0715074A - Wavelength multiplexing optical amplifier - Google Patents

Abstract

PURPOSE:To provide a wavelength multiplexing optical fiber employing a rare earth optical fiber in which the gain is increased in the amplification of signal light by suppressing the spontaneous emission in the optical fiber. CONSTITUTION:Optical fibers, e.g. optical fiber type optical couplers 411 exhibiting periodic transmission characteristics for the variation of wavelength, are inserted between rare earth optical fibers 408, 409 and 409, 410 in order to match the wavelength of multiplexed signal light with that of the light transmitted through the optical fiber thus transmitting the signal light while attenuating the spontaneously emitted light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical direct amplifier used for optical fiber communication, and more particularly to an optical amplifier capable of obtaining a high signal light gain by a wavelength multiplexing system.

[0002]

2. Description of the Related Art An optical fiber doped with a rare earth element (referred to as a rare earth optical fiber in the present specification) has characteristics as a laser active medium, and an optical fiber amplifier using this is realized. For example, when 1 microwatt (μW) signal light and 100 milliwatt (mW) pump light are incident on the same rare earth optical fiber and propagated for about 10 meters (m), the signal light is easily amplified to about 1 mW. .

FIG. 1 shows the basic configuration of a known optical fiber amplifier. Reference numeral 101 is an optical connector code for inputting signal light, and 102 is an optical multiplexer for multiplexing signal light and pumping light. Reference numeral 103 is a rare earth optical fiber, 104 is an optical filter for removing noise, 105 is an optical connector code for outputting signal light, 106 and 107 are optical isolators for blocking return light, and 108 is an excitation light source. . An erbium optical fiber doped with erbium as a rare earth element is generally used. In this case, a semiconductor laser having an oscillation wavelength of 1.48 μm (μm) is used as the excitation light source 108, and a signal light wavelength is 1 0.55 μm is suitable.

Signal light is amplified in the rare earth optical fiber 103 of FIG. 1, but at the same time, spontaneous emission light is generated in 103 and amplified in the same manner as the signal light. Amplified spontaneous emission is usually an amplified spontan
It is called ASE for abbreviation of Eous Emission. Since ASE is noise and harmful to optical communication, most of it is removed by an optical filter 104 for noise removal. However, since it is still amplified in the rare earth optical fiber 103, there remains a problem that the pumping light is wasted. In order to solve this, an optical filter may be inserted in the middle of the rare earth optical fiber 103 (H. Masuda an).
d T. Takada: High gain two
stageamplification withe
rbium-doped fibre amplifi
er, Electronics Letters, Vo
l. 2, No. 10, pp. 661-662,199
0) is proposed. In the wavelength division multiplexing method, a plurality of signal lights having different wavelengths propagate through the rare earth optical fiber 103, but no method for suppressing ASE in that case has been proposed.

FIG. 2 shows a configuration of an optical filter for wavelength-multiplexed signal light, which is an example of 4-wavelength multiplexing. Reference numeral 201 is an optical connector code for inputting signal light, 202 is an optical demultiplexer for separating signal light by wavelength, 203, 204, 205 and 206 are narrow band optical filters, and 207 is multiplexing signal light of different wavelengths. A multiplexer / demultiplexer 208 is an optical connector cord for outputting signal light. The function of this optical filter is clear. A separate optical filter is used for the four signal lights to attenuate wavelength components other than the signals. Japanese Patent Laid-Open No. 62-245740 proposes a method of inserting this filter after an optical amplifier, that is, in an optical transmission line following the optical amplifier. Further, in this publication, it is proposed to insert an interference filter having a periodic transmission characteristic of wavelength in the same manner. The intent of those methods is
The purpose is to eliminate the ASE generated in the optical amplifier, but it does not contribute to the high gain of the optical amplifier.

FIG. 3 is a schematic diagram of the structure of an optical coupler for explaining the operation of a conventionally known optical fiber type optical coupler, in which two optical fibers are partly melt-bonded. It is shown that. The part where the cores are close to each other will be referred to as a connecting part below. Reference numerals 301, 302 and 303 denote an input end and an output end of one optical fiber, a core, and 3 respectively.
04, 305 and 306 are the input end and the output end and the core of the other optical fiber, respectively. Reference numeral 307 indicates a connecting portion. When the structures of the two optical fibers forming the optical coupler are the same, the amplitudes A ₁ (z) and A ₂ (z) of the light wave at the distance z from the starting end of the coupling part are the amplitudes at the starting end of the coupling part. It has been clarified by the optical coupling theory that the following formula can be expressed using A ₁ (o) and A ₂ (o).

A ₁ (z) = A ₁ (o) cosCz + jA ₂ (o) sinCz ...... (1) A ₂ (z) = A ₂ (o) cosCz + jA ₁ (o) sinCz ...... (2) where j Represents an imaginary number. C is called a coupling coefficient and is a function of the shape of the coupling portion and the wavelength of the propagating light wave.
Equations (1) and (2) show that A ₁ (z) and A ₂ (z) change periodically with respect to the change of Cz. Such an optical fiber type optical coupler has been conventionally used as a multiplexer or demultiplexer for two light waves. Letting the wavelength of one light wave to be combined be λ ₁ and the wavelength of the other light wave be λ ₂ , the light wave of λ ₁ is incident on 301 and the light wave of λ ₂ is incident on 304.
As the light wave of λ ₁ propagates through the coupling part, 303 → 306 →
The propagation paths are switched in the order of 303 → 306. λ ₂
Light wave of 306 → 303 → 30 as it propagates through the coupling part
Propagation paths are exchanged as 6 → 303. Since the wavelength of the light wave of λ ₁ is different from that of the light wave of λ ₂ , the coupling coefficient is different.
Since the propagation cycles are different, the light wave of λ ₁ and λ ₁
At the same time, there is a coupling portion length in which _two light waves are output from 302. A multiplexer is selected by selecting such a coupling length. If a coupler with the same structure is used as the input end and the output end and the output end is used as the input end, it becomes an optical demultiplexer. Conventionally, an optical fiber type optical coupler has been used as an optical multiplexer / demultiplexer for light as described above, but there is no example used as an optical filter for multiple wavelengths, and it is inserted in the middle of a rare earth optical fiber. Thus, suppressing the amplification of spontaneous emission light has never been implemented.

[0008]

In an optical amplifier using a rare earth optical fiber, light having a wavelength other than that of the signal light is naturally generated and further amplified. Therefore, in order to increase the amplification gain of the signal light, the rare earth light is used. It is effective to insert an optical filter that transmits only the signal light and the pump light in the middle of the fiber, but a spontaneous emission light suppression method for amplifying the wavelength-multiplexed signal light has not been proposed. The conventional method is to extract each wavelength component by an optical demultiplexer used for optical wavelength division multiplexing, connect a narrow band wavelength tunable filter to the output end of each wavelength, and then multiplex with an optical multiplexer to extract an optical signal. By analogy with technology, the insertion loss is 10 decibels (d
It is as large as B), and there is a disadvantage that even if it is inserted in the middle of a rare earth optical fiber, it does not contribute to a high gain.

Therefore, it is an object of the present invention to provide an optical amplifier for wavelength-multiplexed optical signals, in which the gain is increased for the wavelength-multiplexed optical signal by suppressing the spontaneous emission of the optical fiber optical amplifier. .

[0010]

As an optical filter having a periodic transmission characteristic with respect to a wavelength change and having a small optical loss at a transmission wavelength, a wavelength change is generated between two adjacent optical filter cores. An optical fiber type optical coupler having periodic light coupling, or an optical fiber Fabry-Perot filter which is an optical resonator, or an interference filter by the action of a multilayer thin film is provided at one or more locations in the middle of a rare earth optical fiber. It is configured to be inserted into. In the case of an optical fiber type optical coupler, the shape of the coupling portion is designed, in the case of an interference filter, by the type and thickness of the film, and in the case of an optical fiber Fabry-Perot filter, the design of the resonator portion A desired transmission wavelength can be obtained. Further, the wavelength of the signal light is variable to some extent. Therefore, it is possible to exactly match the wavelength of the wavelength-multiplexed signal with the periodic transmission wavelength.

[0011]

When an optical filter having a periodic transmission characteristic with respect to a change in wavelength is inserted in the middle of a rare earth optical fiber, wavelengths other than the transmission wavelength are greatly attenuated by this optical filter, so spontaneous emission light Amplification is suppressed, the pumping light of the optical amplifier effectively acts on the signal light, and it becomes a wavelength-multiplexed optical amplifier with high gain.

[0012]

FIG. 4 is a schematic diagram of the configuration of an embodiment of the present invention. Reference numeral 401 is an optical connector code for inputting signals to the optical amplifier, 402 and 403 are optical multiplexers for pumping light and signal light, 4
Reference numerals 04 and 405 are excitation light sources, 406 and 407 are optical isolators, 408, 409 and 410 are rare earth optical fibers, and 411, 412 and 413 are optical fiber type optical couplers. FIG. 4 shows a well-known bidirectional pumping type optical amplifier, except that 411, 412 and 413 are inserted.

FIG. 5 is a schematic view of the configuration of the optical fiber type optical coupler section. 501 is an input end for signal light and pumping light, 5
Reference numeral 02 is an optical demultiplexer for signal light and pumping light, 503 is an optical fiber type optical coupler, 504 is an optical fiber for bypassing pumping light, 505 is an optical multiplexer for signal light and pumping light, and 506 is signal light This is the output end of the excitation light. The function of the optical fiber type optical coupler section of FIG. 5 will be described. Signal light composed of excitation light and light waves of a plurality of wavelengths enters from 501. Excitation light is 5
It passes through 02, 504, and 505 and outputs from 506. The signal light passes through 501, 502, 503, 505 and is output from 506. The structure of the optical fiber type optical coupler is shown in FIG.
Incident from 302 and output from 302,
5 is an open end. In the formula (1), if A ₂ (o) = 0 and the coupling portion length = L, then A ₁ (z) = A ₁ (o) cosCL (3), but the coupling coefficient C changes when the wavelength changes. Therefore, when the wavelength is continuously changed, the light intensity observed at 303 changes sinusoidally. That is, the light wave of the specific wavelength is output from the 303 with almost no attenuation, but the light wave of the intermediate wavelength is greatly attenuated. Hereinafter, a wavelength that is output with almost no attenuation is referred to as a perfect transmission wavelength. By matching the wavelengths of multiple signal lights with the perfect transmission wavelength, the signal lights propagate with small optical loss,
Light having a wavelength other than the signal light, that is, spontaneous emission light is greatly attenuated. By such an action, the consumption of the pumping light is prevented by the amplification of the spontaneous emission light, and the optical amplifier has a high gain for the signal light.

FIG. 6 shows an example of measurement of characteristics of an optical fiber type optical coupler capable of two-wavelength multiplex amplification in an erbium-doped optical fiber amplifier which is a typical optical amplifier. The horizontal axis represents wavelength and the vertical axis represents transmitted light loss. The perfect transmission wavelength is two wavelengths of 1.53 μm and 1.57 μm.

The larger the wavelength interval between the completely transmitted wavelengths, the easier the production of the optical fiber type optical coupler, but the interval can be reduced by increasing the length of the coupler.

As spontaneous emission light propagates through the rare earth optical fiber, its intensity becomes exponentially large.
It is preferable to insert a plurality of optical fiber type optical coupling portions into the rare earth optical fiber at appropriate intervals. However, since there is a loss of transmitted light even at the perfect transmission wavelength, there is an upper limit to the number.

In FIG. 5, the attenuation of the excitation light is avoided by providing a bypass for the excitation light,
In some cases, the optical fiber type optical coupler can be designed so that the pumping light has a perfect transmission wavelength. Like the Erbium-doped optical fiber amplifier, the pumping light wavelength (= 1.48μ
is possible when m) is close to the signal light wavelength,
In this case, the optical multiplexer / demultiplexer for the signal light and the pump light is not required, and the cost of the wavelength division multiplexing optical amplifier can be reduced.

The optical fiber type optical coupler is usually made of quartz optical fiber, but it is effective to make the same optical fiber as the rare earth optical fiber constituting the optical amplifier in order to reduce the transmitted light loss. is there.

As an optical component having a periodic transmission wavelength, a kind of resonator called an optical fiber Fabry-Perot filter can be used. This is a physical phenomenon in which, when multiple reflections of light are caused between two adjacent planes, a sharp sharp wavelength selectivity is exhibited in the transmitted wavelength, and a commercially available product can be used. Since the wavelength selectivity is good, the ASE can be effectively suppressed, but it has the drawbacks of being expensive and having a larger insertion loss than the optical fiber type optical coupler.

As an optical component having a periodic transmission wavelength, an interference filter utilizing the interference of light by a dielectric multilayer film can be used. Interference filters with various characteristics have been developed depending on the combination of the material of the thin film, the film thickness, and the number of layers. FIG. 7 is an example of actually measured values of the characteristics of the interference filter. In the figure, the horizontal axis represents wavelength and the vertical axis represents transmittance. The light loss at the wavelength of maximum transmittance is 0.6 dB, and the light loss at the second largest transmission wavelength is 1.2 dB. These two wavelengths are not suitable for amplification of an erbium-doped optical fiber, but the wavelength difference that maximizes the transmittance is 2
Since it is 0 nm, it can be inferred that a transmission wavelength of two or more wavelengths can be obtained with respect to the amplification wavelength range of about 50 nm of the erbium-doped optical fiber by changing the design of the film thickness.

[0021]

As described above, according to the present invention, an optical fiber type optical coupler or an optical fiber Fabry-Perot filter that selectively transmits light waves of a plurality of wavelengths in the middle of a rare earth optical fiber of an optical fiber amplifier. Or because an interference filter is inserted, when amplifying a wavelength-multiplexed optical signal,
Amplification of light other than the signal light is suppressed, and high gain can be obtained in a plurality of signal lights.

[Brief description of drawings]

FIG. 1 is a basic structural diagram of a known optical fiber amplifier.

FIG. 2 is a configuration diagram of an optical filter of a wavelength multiplexing optical amplifier that has been conventionally proposed.

FIG. 3 is a schematic view of the structure of a coupler for explaining the operation of a conventionally known optical fiber type optical coupler.

FIG. 4 is a schematic diagram of a configuration of an embodiment of the present invention.

FIG. 5 is a schematic diagram of a configuration of an optical fiber type optical coupler unit.

FIG. 6 is an actual measurement value of characteristics of an optical fiber type optical coupler for performing dual wavelength multiplex amplification in an erbium-doped optical fiber amplifier.

FIG. 7: Measurement example of light transmission characteristics of an interference filter

[Explanation of symbols]

 101 Optical Connector Code 102 Optical Multiplexer 103 Rare Earth Optical Fiber 104 Optical Filter 105 Optical Connector Code 106 Optical Isolator 107 Optical Isolator 108 Excitation Light Source 201 Optical Connector Code 202 Optical Demultiplexer 203 Narrowband Optical Filter 204 Narrowband Optical Filter 205 Narrowband Optical filter 206 Narrow band optical filter 207 Optical multiplexer 208 Optical connector code 301 Optical input end 302 Optical output end 303 Optical fiber core 304 Optical input end 305 Optical output end 306 Optical fiber core 307 Coupling 401 Optical connector code 402 Optical coupling Waver 403 Optical multiplexer 404 Pumping light source 405 Pumping light source 406 Optical isolator 407 Optical isolator 408 Rare earth optical fiber 409 Rare earth optical fiber 410 Rare earth optical fiber 411 Optical fiber Optical coupler section 412 optical fiber type optical coupler section 413 optical fiber type optical coupler section 501 optical input terminal 502 optical demultiplexer 503 optical fiber type optical coupler 504 optical fiber 505 optical multiplexer 506 optical output terminal

Claims (4)

[Claims] 1. A rare earth optical fiber, a pumping light source for pumping the rare earth optical fiber, a multiplexer for multiplexing the pumping light and the signal light and entering the rare earth optical fiber.
In an optical fiber optical amplifier whose main component is an optical isolator that is a one-way element of light, an optical filter having a periodic transmission characteristic with respect to a wavelength change is a predetermined one part or a plurality of parts of the rare earth optical fiber. An optical filter having a periodic transmission characteristic with respect to the wavelength change, which is inserted at a location, has an optical fiber having a periodic coupling with respect to the wavelength change between cores of two adjacent optical fibers. Optical amplifier for wavelength division multiplexing, which is a type optical coupler. 2. An optical amplifier for wavelength division multiplexing, wherein one of the wavelengths transmitted by the optical filter having a periodic transmission characteristic with respect to the wavelength change according to claim 1 is a wavelength of a pumping light source. . 3. An optical amplifier for wavelength division multiplexing, wherein the optical filter having a periodic transmission characteristic with respect to wavelength change according to claim 1 is an optical fiber Fabry-Perot filter which is an optical resonator. . 4. An optical amplifier for wavelength division multiplexing, wherein the optical filter having a periodic transmission characteristic with respect to a wavelength change according to claim 1 is an interference filter.