JPH0964437A - Rare-earth element-doped optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a rare-earth element-doped optical fiber amplifier which is provided with a signal generation function and which reduces a noise accumulation effect in a multistage cascade connection at a time when light is amplified and relayed by a method wherein an original signal is self-extracted, the signal is processed in such a way that its waveform is shaped or that its timing is adjusted and exciting signal light is modulated so as to be amplified in synchronization with the injection of signal light to be amplified. SOLUTION: A part of a light signal is taken out by an optical branching filter 30, and it is received with high sensitivity by a light-signal reception means 4 so as to be photoelectrically converted. The received signal which has been converted into an electric signal is timing-regenerated or waveform-shaped through an input drive-type timing filter 33 so as to be returned to the original signal. Exciting light from an exciting light source is modulated by the original signal. The modulated exciting light is multiplexed with the light signal by an optical multiplexer 12, and their multiplexed light is amplified through a rare-earth element-doped optical fiber 15 in synchronization with the injection of the light signal. When the light signal is multiplexed by the optical multiplexer 12, the timing of both is different. As a result, an optical delay circuit 31 is inserted into a light signal line so as to make the timing agree.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rare earth element-doped optical fiber amplifier used in, for example, an optical fiber communication trunk system or an optical subscriber system in an optical communication network.

[0002]

2. Description of the Related Art As an optical communication network, an optical fiber communication network having a wide transmission band has been urgently laid as an information communication infrastructure for the multimedia age. FIG. 5 shows an example of a standard optical fiber communication trunk line. This is 10Gbit over a distance of 505km.
Fig. 5 is an example of using an optical amplifier in / s transmission.
5A is its configuration diagram, and FIG. 5B is its level diagram. In an optical communication network, an optical amplifier is indispensable in order to compensate for the loss of the transmission line in the optical fiber communication main line system and in the optical subscriber system to compensate for the reduction of the optical power due to the number of branches to the subscriber. Is. Therefore, an optical amplifier is installed every about 80 km, and each is about 20d.
B is amplified and transmitted.

Optical fiber transmission lines used in optical fiber communication networks are passive in nature and only attenuate signals. However, the advent of rare earth element-doped optical fiber amplifiers has made practical use of optical communication possible. It spread greatly.

FIG. 3 is an explanatory view of the operating principle of this rare earth element-doped optical fiber amplifier. And common rare earth elements such as erbium (Er ³⁺ ), neodymium (N
FIG. 4 shows a configuration example of an optical fiber amplifier doped with a rare earth element such as d ³⁺ ) or praseodymium (Pr ³⁺ ). The operation principle of the rare earth element-doped optical fiber amplifier is such that the energy of the added rare earth element in the laser medium is inverted due to optical pumping, that is, optical pumping, and the amplification operation is performed by stimulated emission.

First, the principle of amplification operation of the rare earth element-doped optical fiber amplifier will be described with reference to FIG. When the rare earth element-doped fiber is irradiated with excitation light, the laser medium added with the energy (hν _pump ) of the excitation light absorbs it,
An inversion distribution of energy is formed in the intermediate absorption level. After that, energy (hν _sig ) is released in the stimulated emission process of transition from the upper level to the lower level of the stimulated emission, and the amplifying action is performed. This energy level transition process includes a three-level process and a four-level process.

FIG. 3A is an explanatory diagram of the amplifying action of the three-level process of erbium and the like, and FIG. 3B is an explanatory diagram of the four-level process of neodymium and the like. In the figure, i is the intermediate absorption level, u is the upper level of stimulated emission, l is the lower level of stimulated emission, and g is the ground level. Here, when the energy level e is higher than the stimulated emission upper level u by a wavelength corresponding to the excitation light source wavelength, ESA (Excited State)
A phenomenon called "absorption" occurs and the energy of the excitation light is consumed, and the excitation efficiency decreases. In the case of an erbium-doped optical fiber amplifier (hereinafter referred to as "EDFA"), the pumping light wavelength at which this ESA does not occur is 0.98μ
m, 1.48 μm or 1.54 μm wavelength is used. In the case of a neodymium-doped optical fiber (hereinafter referred to as “NDFA”), a wavelength of 1.06 μm or 1.32 μm is used.

FIG. 4 shows a configuration example of an optical amplifier using the rare earth element-doped optical fiber amplifier 10. Here, EDFA is used. FIG. 4A shows the case of the forward excitation type. The forward pumping type EDFA _{10 1} inputs the signal light from the optical input terminal 11, and the optical multiplexer 12 drives the pumping light source 1
It is multiplexed with the excitation light from 3. The optical frequencies of the signal light and the excitation light are different, and the optical wavelength of the excitation light is shorter. That is, the optical frequency is high. This combined light is an optical isolator 14
After passing through, the signal light frequency is amplified by the amplification effect by the stimulated emission by the excitation light in the erbium-doped optical fiber 15.

In the rare earth element-doped optical fiber amplifier 10, since the signal light and the pumping light are confined in a small core over a long length, a large gain can be obtained by increasing the length L of the doped optical fiber even with a relatively small gain coefficient. Obtainable. The amplified signal light passes through the optical isolator 16 and the optical bandpass filter 17 and is guided to the optical output terminal 20. FIG. 4A shows a forward pumping type in which a pumping light source 13 is provided on the input side, but there is also a backward pumping type in which a pumping light source is provided on the output side. There is also a bidirectional excitation type provided on the side.

Further, there is also a reflection type excitation type EDFA 10 ₂ as shown in FIG. 4 (B). This is an optical circulator 1
8 and a reflecting mirror 19, the signal light from the optical input terminal 11 is guided to the amplification side by the optical circulator 18, and the signal light and the pumping light from the pumping light source 13 are combined by the optical multiplexer 12. Amplifying with erbium-doped optical fiber 15 and reflecting mirror 1
The signal light is totally reflected at 9 and again amplified by the erbium-doped optical fiber 15, and the amplified signal light is guided to the optical output terminal 20 by the optical circulator 18.

[0010]

As described above, in any of the configurations, the signal light is amplified, but there is no function of regenerating and repeating the signal waveform. Therefore, noise is accumulated by connecting the optical amplifiers in multiple stages, and this noise has been a constraint condition for communication. In other words, there is a problem that there is no waveform or timing reproduction action, although there is an optical amplification action.

Even in the same communication network, in long-distance transmission of a telecommunication network, in order to eliminate the fluctuation of the phase of the pulse waveform and the disturbance of the timing due to the noise of the transmission line, the original signal is restored and amplified. There is a signal regeneration relay technology for long-distance transmission, which prevents deterioration of signal transmission quality.

Similar to the telecommunication system, the present invention is to provide the optical amplifier with a signal reproducing function to reduce the noise accumulation effect at the multistage cascade connection in the optical amplification relay.

[0013]

In order to achieve the above object, the present invention is a rare earth element-doped optical fiber amplifier, in which a part of an optical carrier modulated with an original signal is demultiplexed, and the original signal is extracted from it. After self-extracting and performing signal processing such as waveform shaping and timing adjustment, the pumped signal light that is the pumping light is modulated by the processed signal, and this modulated pumping light is injection-locked to the signal light to be amplified. Then, it is amplified with an optical fiber doped with a rare earth element. That is, it is a rare earth element-doped optical fiber amplifier having an amplification function and a signal reproduction function.

According to the first aspect of the present invention, a part of the optical signal is taken out by the optical demultiplexer, and the optical signal is received by the optical signal receiving means with high sensitivity and photoelectrically converted. Direct photoelectric conversion may be performed when the optical signal level is high, but a heterodyne system or a homodyne system is used when the optical signal level is low. The received signal converted into an electric signal is passed through an input drive type timing filter such as a Nyquist filter or a Gaussian filter to perform timing reproduction and waveform shaping to restore the original signal. This original signal modulates the excitation light from the excitation light source. The modulated pumping light is multiplexed with an optical signal by an optical multiplexer, injection-locked, and the multiplexed light is passed through a rare earth element-doped optical fiber to be amplified.

Here, the electrical signal processing for returning a part of the input signals to the original signals requires some time. Therefore, when the optical signal is multiplexed with the optical multiplexer, the timings of the two differ from each other. Therefore, an optical delay circuit is inserted in the optical signal line so as to match the timing. Since the electrical signal processing time can be constant, the delay time can be constant.

The invention of claim 2 is a form of modulation when the pumping light source is a semiconductor laser. That is, the original signal current is injected into the semiconductor laser to modulate the excitation light. According to a third aspect of the present invention, in the rare earth element-doped optical fiber amplifier according to the first or second aspect, an optical heterodyne detecting means is used to enhance the sensitivity of the optical signal receiving means. The invention of claim 4 also uses the optical homodyne detecting means in the rare earth element-doped optical fiber amplifier according to claim 1 or 2 in order to enhance the sensitivity of the optical signal receiving means. Examples will be described below.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a block diagram of one embodiment of the present invention, and FIG. 2 shows a block diagram of another embodiment. Here, parts corresponding to those of FIG. 4 are designated by the same reference numerals. The present invention can be applied to any of the types shown in FIG. That is, it can be applied to any of the forward pumping type, the backward pumping type, the bidirectional pumping type, and the reflective pumping type, but in this embodiment, the forward pumping type will be described. Further, although any kind of rare earth element-doped optical fiber amplifier of the rare earth element-doped optical fiber can be applied, the EDFA will be described here.

The whole drawing of FIG. 1 is a forward excitation type EDFA1.
FIG. The optical signal is input from the optical input terminal 11, passes through the optical delay circuit 31, the optical multiplexer 12 that combines with the pumping light, and the optical isolator 14, is amplified by the erbium-doped optical fiber 15, and then the optical isolator 16 and the optical signal. Light output terminal 2 after passing through bandpass filter 17
Reaches 0. The optical isolators 14 and 16 are used here to prevent the optical signal from moving backward, and the optical bandpass filter 17 is inserted to remove optical frequencies such as ESA.

With respect to the above-mentioned main line, by a circuit separately provided according to the present invention, the pumping light is modulated with the original signal, multiplexed by the optical multiplexer 12 with the optical signal of the main line, and injection-locked to the optical signal. Let Therefore, as a separate circuit, a part of the optical signal on the main line is taken out by the optical demultiplexer 30 and received by the optical signal receiving means 40 with high sensitivity. In FIG. 1, the heterodyne detection means is shown for high sensitivity detection. When the optical signal level is high, it may be directly photoelectrically converted into an electric signal.

The heterodyne detecting means splits the optical signal into two beams by the beam splitter 41, and one light wave is slightly shifted in frequency by the optical frequency shifter 45 and guided to the beam splitter 42 through the light reflecting plate 44. The optical frequency shifter 45 can be easily constructed by using an acoustooptic device. The other light wave is guided to the beam splitter 42 through the light reflection plate 43, and is combined with the light wave shifted here. The beat signal of the combined light is taken out and amplified by an amplifier with a bandpass filter, and converted by a converter 46 into an electric signal.

Since the electric signal extracted from the optical signal is a signal containing jitter, the input drive type timing filter 3 such as Nyquist filter or Gaussian filter is used.
3, the timing is reproduced, the waveform is shaped, and the original signal is reproduced. The pumping light from the pumping signal source 13 ₁ is modulated by the optical modulator 36 with the electric signal reproduced as the original signal.
The modulated pumping light is multiplexed by the optical multiplexer 12 with the optical signal on the main line. The modulation timing of the modulated pumping light is slightly different from the signal timing of the main line optical signal. This is because the electric signal processing time requires a certain time and is delayed. Therefore, the optical delay circuit 31 is provided so as to match this delay time, and the delay time is adjusted so as to match.

The pumping light modulated into the original signal is supplied to the optical multiplexer 1
At 2, it combines with the optical signal of the main line. This combined light is amplified by the erbium-doped optical fiber 15 through the optical isolator 14. The amplified optical signal is guided to the optical output terminal 20 through the optical isolator 16 and the optical bandpass filter 17, and is output.

(Other Embodiments) FIG. 2 shows another embodiment.
This is an example when the excitation light source 13 ₂ is a semiconductor laser. A semiconductor laser can be injected with a current to oscillate and can be modulated by the injected current. Then, the reproduced current of the original signal is injected into the pumping light source 13 ₂ of the semiconductor laser and oscillated to obtain modulated pumping light. The modulated pumping light is multiplexed by the optical multiplexer 12 with the optical signal to be amplified. Other configurations are the same as those in FIG.

[0024]

As described above in detail, according to the present invention, in the rare earth element-doped optical fiber amplifier used in the optical fiber communication network, not only the amplification action but also the reproduction of the signal waveform and the reproduction of the timing can be performed. That is, since it is a rare earth element-doped optical fiber amplifier having a signal reproducing function, it is possible to remove noise and disturbance due to the signal path and the like. Therefore, it becomes possible to carry out optical signal transmission over an extremely long distance, and its practical value is great.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 is a configuration diagram of another embodiment of the present invention.

FIG. 3 is an explanatory diagram of an operating principle of amplification action of a rare earth element-doped optical fiber amplifier. FIG. 3A shows the amplifying action of the three-level process, and FIG. 3B shows the amplifying action of the four-level process.

FIG. 4 is a block diagram of an example of a conventional optical fiber amplifier. FIG. 4A is a forward excitation EDFA, and FIG. 4B is a reflection excitation EDFA.

FIG. 5 is a configuration diagram of a conventional standard optical fiber communication trunk line. FIG. 5 (A) is its configuration diagram, and FIG. 5 (B).
Is the level diagram.

[Explanation of symbols]

10 rare earth element-doped optical fiber amplifier 10 ₁ forward pumping EDFA 10 ₂ reflection pumping EDFA 11 optical input terminal 12 optical demultiplexer 13, 13 ₁ , 13 ₂ pumping light source 14, 16 optical isolator 15 rare earth element doping Optical fiber 17 Optical bandpass filter 18 Optical circulator 19 Reflector 20 Optical output terminal 30 Optical demultiplexer 31 Optical delay circuit 32 Photoelectric converter 33 Input drive timing filter 34 Electric signal amplifier 35 Optical modulator 40 Optical signal receiving means 41 , 42 Beam splitter 42, 43 Optical reflector 45 Optical frequency shifter 46 Selective amplifier

Claims (4)

[Claims] 1. In a rare earth element-doped optical fiber amplifier for amplifying an optical signal by passing it through a rare earth element-doped optical fiber together with pumping light, an optical component for extracting a part of the optical signal input to an optical input terminal (11). Wave device (30), optical signal receiving means (40) for receiving the optical signal from the optical demultiplexer (30), and photoelectric converter for converting the optical signal from the optical signal receiving means (40) into an electrical signal A converter (32), an input drive type timing filter (33) for inputting an electric signal from the photoelectric converter (32) and shaping the waveform, and a waveform shaped electric signal from an excitation light source (13 ₁ ). An optical modulator (35) for modulating pumping light, an optical delay circuit (31) for delaying an optical signal input from the optical input terminal (11) for a certain time, and an optical signal from the optical delay circuit (31). Change with the above optical modulator (35) Optical multiplexer for multiplexing the excitation light (1
2) and an optical output terminal (20) for amplifying and outputting the optical multiplexed signal multiplexed by the optical multiplexer (12) with the rare earth element-doped optical fiber (15). Rare earth element doped optical fiber amplifier. 2. An optical demultiplexer (30) for extracting a part of an optical signal input to an optical input terminal (11) in an optical fiber amplifier for amplifying an optical signal by passing it through a rare earth element-doped optical fiber together with pumping light. ), An optical signal receiving means (40) for receiving the optical signal from the optical demultiplexer (30), and a photoelectric converter (32 for converting the optical signal from the optical signal receiving means (40) into an electric signal. ), An input drive type timing filter (33) for inputting an electric signal from the photoelectric converter (32) to shape the waveform, and an excitation light modulated by injecting a current into the excitation light source. A pumping light source (13 ₂ ), an optical delay circuit (31) that delays an optical signal input from the optical input terminal (11) for a certain time, an optical signal from the optical delay circuit (31) and the optical signal. Modulated by modulator (35) Optical multiplexer for multiplexing the excitation light (1
2) and an optical output terminal (20) for amplifying and outputting the optical multiplexed signal multiplexed by the optical multiplexer (12) with the rare earth element-doped optical fiber (15). Rare earth element doped optical fiber amplifier. 3. A rare earth element-doped optical fiber amplifier according to claim 1, wherein the optical signal receiving means (40) is an optical heterodyne detecting means. 4. The rare earth element-doped optical fiber amplifier according to claim 1 or 2, wherein the optical signal receiving means (40) is an optical homodyne detecting means.