JP3195237B2 - Optical transmission device, optical communication system, and method for amplifying input optical signal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical communication system having an optical fiber amplifier, and more particularly, to an optical communication system having an optical fiber amplifier using a rare earth doped fiber doped with a rare earth element.

An optical amplifier that directly amplifies an optical signal without using an electric circuit is a bit-rate-free optical memory, is easy to increase the capacity, and is capable of multi-channel collective amplification. It is actively researched by various research institutions as a key device of the system.

As an optical communication system having an optical amplifier of this kind, an optical amplifier is used as an optical power booster, and a branching / combining method is used.
Use of an optical amplifier as an optical preamplifier to improve reception sensitivity, and use of an optical amplifier as an optical repeater to compensate for insertion loss and increase transmission power, and reduce the size and reliability of the repeater. What has been planned has been proposed, and optimization of the method is being sought.

[0004]

2. Description of the Related Art Optical amplifiers which have been studied in the past are roughly classified into rare earth elements (Er, Nd, Yb, etc.).
Using an optical fiber doped with (a rare earth-doped fiber in this specification, and this term is used as a wide range including a waveguide structure such as a waveguide doped with a rare earth element), and a semiconductor. The laser type is a type utilizing a nonlinear effect in an optical fiber.

[0005] Among them, the optical amplifier using the rare-earth-doped fiber has excellent characteristics such as no dependency on polarization, low noise, and small coupling loss with the transmission line.

When an optical amplifier is used as an optical repeater, its monitoring control function is indispensable. Conventional monitoring control methods applicable to semiconductor laser type optical amplifiers include, for example, Ellis, AD et al .: Supe
rvisory system for cascaded semiconductor laser am
plifier repeaters, Electron. Lett., Vol. 25, No.
5, pp. 309-311 (2nd March 1989).

However, since this method detects the injection current of a semiconductor laser type optical amplifier, it cannot be directly applied to an optical communication system having an optical fiber amplifier. That is, there is no conventional technology for monitoring and control methods suitable for optical fiber amplifiers.

[0008]

In an optical communication system having an optical fiber amplifier, pumping light is used to perform optical amplification. Therefore, if information transmission is performed by pumping light in addition to information transmission by signal light. Can be monitored and controlled in an optical communication system having an optical fiber amplifier as an optical repeater.

Further, since the optical amplification is performed not only when the pumping light introduced into the rare earth doped fiber is in the same direction as the signal light but also in the opposite direction, if information transmission by the pumping light is possible, , Thereby enabling bidirectional transmission.

The present invention has been made in view of the above points, and provides a configuration for detecting a monitoring signal of an optical fiber amplifier in order to realize a monitoring control method suitable for an optical fiber amplifier. It is an object. For example, in addition to the information transmission using the signal light, the above-described monitoring information can be transmitted using the excitation light.

[0011]

FIG. 1 shows the principle of optical amplification using a rare earth doped fiber. 2 is a rare-earth doped fiber composed of a core 2a and a clad 2b,
The core 2a is doped with a rare earth element such as erbium (Er).

When excitation light (pumping light) is incident on such a rare-earth doped fiber 2, rare-earth atoms are excited to a high energy level. When signal light enters the rare earth atoms in the optical fiber 2 excited to a high energy level, stimulated emission of light occurs and the rare earth atoms transition to a low energy level. The signal light gradually increases along the fiber, and the signal light is amplified.

The doped rare earth element is erbium (E
In the case of r), when amplifying the signal light having the wavelength of 1.55 μm, for example, laser light having the wavelength of 1.49 μm can be used as the excitation light.

In the case where the doped rare earth element is neodymium (Nd), when amplifying signal light having a wavelength of 1.3 μm, laser light having a wavelength of 0.8 μm may be used as excitation light. it can. In the following description, the present invention will be described assuming that the doped rare earth element is erbium.

When pumping light of wavelength λ _P having a predetermined wavelength relationship with signal light of wavelength λ _S is made incident on a rare earth doped fiber, the spectrum of the rare earth doped fiber is indicated by F in FIG. Such fluorescence occurs near the spectrum of the signal light.

[0016] The change in the intensity of fluorescence is not able to match completely change in intensity of the excitation light and time, as shown in FIG. 3, for example, assuming that stops the incident excitation light at time t ₀ The intensity of the fluorescence does not instantaneously become zero, but gradually decreases with an appropriate time constant.

If the time τ required for the intensity a of the fluorescence to decrease to 1 / e (e is the base of the natural logarithm) of the intensity b before stopping the incidence of the excitation light is defined as the fluorescence lifetime, It is known that, within a range in which the fluorescence lifetime τ elapses from t ₀ , the amplification action continues without unstable gain fluctuation with respect to the signal light even though the injection of the excitation light is stopped. (Laming, RI et al .: Mu
ltichannel crosstalkand pump noise characterizatio
n of Er ³⁺ -doped fiber amplifier pumped at980 nm, E
lectron. Lett., Vol. 25, No. 7, pp. 455-456 (30th M
arch 1989).).

Therefore, when the excitation light is modulated by a high-frequency modulation signal having a period shorter than the fluorescence lifetime in the excited state, the effect of this modulation does not appear in the amplified signal light.

4 and 5 are diagrams for explaining the principle of the present invention. FIG. 4 shows a case where signal light and pumping light propagate in the same direction in a rare-earth doped fiber, and FIG. Are propagated in opposite directions.

The system of the present invention is an optical communication system including an optical fiber amplifier for amplifying the signal light 4 by propagating the signal light 4 together with the pumping light 6 to the rare earth doped fiber 2 doped with the rare earth element. The present invention relates to an optical transmission device.

The present invention focuses on monitor information relating to pump light of an optical fiber amplifier as monitor information of an optical fiber amplifier. In an optical transmission apparatus which amplifies an input optical signal in order to detect such monitor information, Having an end and an output end, an optical fiber doped with a rare earth element, and means for inputting the input optical signal to an input end of the optical fiber,
Having a light source that generates pump light, inputting the pump light to an input end or output end of the optical fiber, and a unit that outputs a monitoring signal of the light source; amplifying the input optical signal; It is configured to output from the output end of the optical fiber.

The monitor information of the light source (excitation light source) that generates the excitation light includes information on the optical output of the excitation light source or
Information on the excitation current of the excitation light source (in the case of a semiconductor laser, its bias current) can be considered.

The monitor information thus obtained is included in, for example, a high frequency modulation signal 8 having a period shorter than the fluorescence life of the excitation state, and the excitation light is modulated by the modulation signal 8. Thus, in addition to the information transmission by the signal light 4, the information transmission (monitoring signal transmission) by the pumping light 6 can be performed.

Here, the rare earth-doped fiber doped with a rare earth element means a broad concept including a waveguide structure such as a waveguide doped with a rare earth element, as described above. An optical fiber amplifier that amplifies the signal light by propagating the signal light together with the pumping light to the rare-earth doped fiber,
It includes not only an optical amplifier using an optical fiber as a light propagation medium, but also an optical amplifier using a waveguide structure such as an optical waveguide as a light propagation medium.

As shown in FIG. 4, when the signal light and the pump light are made to propagate in the same direction through the rare-earth doped fiber, the transmission of the monitoring signal of the optical repeater using the pump light as the carrier is performed. It can be carried out.

On the other hand, as shown in FIG. 5, when the signal light and the pump light are made to propagate in the opposite directions through the rare-earth-doped fiber, information transmission by the signal light and information using the pump light as a carrier are performed. Transmission enables bidirectional transmission.

[0027]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 6 is an explanatory diagram of an optical repeater to which the present invention is applied. FIG. 1 shows a system including a single or a plurality (three in the figure) of optical repeaters 16 in the middle of a reciprocating optical transmission line including an upstream optical transmission line 12 and a downstream optical transmission line 14. ing.

The optical repeater 16 includes an upstream repeater 18 connected to the upstream optical transmission line 12 and a downstream repeater 20 connected to the downstream optical transmission line 14. Reference numeral 20 exchanges monitoring information that controls the monitoring control function of the optical repeater in addition to the general relay function.

The transmission of the monitoring information between the upstream and downstream repeaters 18 and 20 is performed by electric signals. The transmission of the monitoring information in the optical transmission lines 12 and 14 is performed by exciting the rare earth doped fiber included in the optical repeater 16. It is done by light.

FIG. 7 shows a block diagram of the uplink repeater 18. Downlink repeater 20 has the same block configuration as uplink repeater 18. In FIG. 7, the wavelength of the signal light propagating in the upstream optical transmission line 12 is, for example, 1.536 μm or 1.552 μm, and the wavelength of the pump light is, for example, 1.49.
μm.

The pump light that has not contributed to the optical amplification reaches the upstream repeater. The signal light and the pump light are distributed, for example, at a ratio of 1: 100 by the optical coupler 21, and the smaller one of the distributed lights is input to the input signal level detector 22 to detect the level of the signal light. The more distributed light is the optical fiber amplifier 24
To enter.

The optical fiber amplifier 24 includes a multiplexer / demultiplexer 26 and a rare earth doped fiber 28 doped with erbium in the core.
It is comprised including. The multiplexer / demultiplexer 26 is provided in the optical coupler 21.
The signal light and the pump light are combined and demultiplexed, and the signal light is guided to the rare-earth doped fiber 28, the pump light is guided to the receiver 32, and the pump light from the pump light source 30 is guided to the rare earth-doped fiber 28. The multiplexer / demultiplexer 26 may be composed of a multiplexer and a demultiplexer.

The signal light amplified by the optical fiber amplifier 24 and the pump light not consumed by the amplification are input to the optical coupler 36 via the optical isolator 34. The optical isolator 34 is provided with the rare earth doped fiber 2.
This is to prevent the occurrence of oscillation based on the gain of the rare-earth-doped fiber 28 due to the formation of the resonator structure in the optical path including 8.

The optical coupler 36 distributes the input signal light and pump light to, for example, 1: 100, the larger distributed light is led into the upstream optical transmission line 12 again, and the smaller distributed light is the output signal level. Input to the detector 38. The output signal level detector 38 removes the excitation light by a built-in optical filter and detects the level of the amplified signal light.

In this embodiment, the pumping light source 30 is formed of a semiconductor laser, and the power of the pumping light or the average value of the pumping light outputted from the pumping light source 30 is controlled by an automatic output level control circuit (APC: Autom
Atic Power Control) 40 controls the output signal level to be constant. With this control, the uplink repeater can always output a signal light of a constant level regardless of the level of the signal light input to the uplink repeater.

The input signal level from the input signal level detector 22, the output signal level from the output signal level detector 38, the excitation current (bias current of the semiconductor laser) in the excitation light source 30, and the excitation light output are monitored as upstream monitoring information. And transmitted to the downstream optical transmission line 14 by modulating the pump light there (see also FIG. 6).

On the other hand, the downstream monitoring information is transmitted to the signal processing circuit 4.
4 and is taken into the up-link repeater 18 and the pump light source 30
Is modulated based on the downstream monitoring information, so that the downstream monitoring information is transmitted via the upstream optical transmission line 12.

Hereinafter, this operation will be described. When the receiver 32 receives the instruction to send the monitoring information to the uplink repeater 18, the receiver 32 decodes the instruction and notifies the controller 46. This decoding can be performed by detecting whether or not the address information received by the receiver 32 matches the address information stored in advance by the match detection circuit 42.

The controller 46 receives the monitoring information transmission command and controls the modulation circuit 48 based on the downstream monitoring information from the signal processing circuit 44.
Is intensity-modulated, for example.

If the modulation speed at this time is made sufficiently larger than the reciprocal of the fluorescence lifetime of the rare-earth doped fiber 28,
Even if the pumping light from the pumping light source 30 is modulated, this modulated component hardly appears in the signal light that is amplified and transmitted from the upstream repeater 18. Therefore, in addition to information transmission by direct optical amplification of signal light, information transmission of monitoring information using pump light as a carrier (carrier) is possible.

It should be noted that there is no coincidence of the address information in the coincidence detecting circuit 42 and the downstream monitoring information is transmitted to the upstream optical transmission line 12.
When it is not necessary to transmit the transmission light to the receiver 32, the transmission information based on the excitation light received by the receiver 32 is reproduced and amplified by the controller 46 to modulate the excitation light source 30.

FIG. 8 is a block diagram of a bidirectional transmission system configured by applying the present invention. In this system, one optical fiber 5 is connected between the first terminal 50 and the second terminal 52.
4 to enable bidirectional transmission.

The first terminal station 50 includes a transmitting section 56 for transmitting signal light in the 1.55 μm band, a receiving section 60 for receiving modulated pumping light in the 1.49 μm band, and a duplexer 58. It is configured.

The second terminal station 52 includes a preamplifier 62 and
A receiver 64 for receiving the signal light in the 55 μm band;
and a transmission unit 66 for modulating and transmitting the μm-band excitation light.

The transmitting section 66 includes an excitation light source 68 and an excitation light source 68.
And a modulation circuit 70 for intensity-modulating the semiconductor laser. The preamplifier 62 includes a rare earth-doped fiber 72 connected to the optical fiber 54 and a multiplexer / demultiplexer 74 that sends the modulated pump light to the rare earth fiber 72 and sends the signal light amplified by the rare earth doped fiber 72 to the receiving unit 64. And

The signal light from the transmitting section 56 of the first terminal station 50 is sent out to the optical fiber 54 via the demultiplexer 58, amplified by the preamplifier 62 of the second terminal station 52, and received by the receiving section 6.
4. In this case, the signal light is amplified by the operation of the preamplifier 62 (optical fiber amplifier), so that the receiving sensitivity increases.

On the other hand, the modulated pump light transmitted from the transmitting section 66 of the second terminal station 52 contributes to the amplification of the signal light from the first terminal station 50 without affecting the modulation, and then the The signal is transmitted to the first terminal station 50 via the fiber 54,
Is transmitted to the receiving unit 60 via the. And the transmission information is reproduced.

As described above, since the signal light and the pumping light propagate through the rare-earth doped fiber 72 in mutually opposite directions, bidirectional transmission can be achieved by information transmission using the signal light and information transmission using the pumping light as a carrier. Achieved.

The modulation of the pumping light source 68 in the transmitting section 66 of the second terminal station 52 has a period shorter than the fluorescence life of the excited state of the rare-earth doped fiber, as in the embodiment described with reference to FIGS. Is performed by the high-frequency modulation signal.

When the rare earth-doped fiber is a rare earth-doped fiber doped with erbium as a rare earth element, the fluorescence lifetime in the excited state is about 14 ms, for example.
A practically sufficient transmission capacity can be obtained.

[0051]

As described above, since the excitation light is modulated by the modulation signal of a high frequency having a period shorter than the fluorescence lifetime of the excitation state, the amplification of the signal light by the excitation light is adversely affected. , It is possible to transmit information (monitoring signal transmission) using pump light in addition to information transmission using signal light. Therefore, as in the present invention, when monitoring information on the excitation light source of the optical fiber amplifier,
For example, such monitoring information can be transmitted.

As an application example, when the signal light and the pumping light are made to propagate in the same direction through the rare-earth doped fiber, the monitoring signal of the optical repeater is easily transmitted using the pumping light as a carrier. If the signal light and the pumping light are made to propagate through the rare-earth-doped fiber in opposite directions to each other, the information transmission by the signal light and the information transmission by the pumping light can be bidirectionally transmitted. Becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the principle of optical amplification by a rare earth doped fiber.

FIG. 2 is an explanatory diagram of fluorescence.

FIG. 3 is an explanatory diagram of a fluorescence lifetime.

FIG. 4 is a diagram illustrating the principle of the present invention when the signal light and the pump light are in the same direction.

FIG. 5 is a diagram illustrating the principle of the present invention when the signal light and the pump light are in opposite directions.

FIG. 6 is an explanatory diagram of an optical repeater according to an embodiment of the present invention.

FIG. 7 is a block diagram of an uplink repeater according to an embodiment of the present invention.

FIG. 8 is a block diagram of a bidirectional transmission system according to an embodiment of the present invention.

[Explanation of symbols]

 2,28,72 rare earth doped fiber 4 signal light 6 pumping light 8 modulation signal 16 optical repeater 18 uplink repeater 20 downlink repeater

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-1-94329 (JP, A) JP-A-62-229783 (JP, A) JP-A-1-1455881 (JP, A) JP-A-1- 130638 (JP, A) JP-A-58-48513 (JP, A) JP-A-63-229783 (JP, A) JP-A 62-177143 (JP, U) JP-A 62-168668 (JP, U) (58) Field surveyed (Int.Cl. ⁷ , DB name) H04B 10/08-10/17 G02F 1/35 501

Claims (4)

(57) [Claims] 1. A light transmission apparatus for amplifying an optical signal, having an input terminal and an output terminal, the optical signal is input from the input end, an optical fiber doped with a rare earth element, an excitation light generation to have a semiconductor laser, inputs the excitation light to the input end or the output end of the optical fiber, separately from the pumping light input to the optical fiber, and means for outputting the monitoring information of the semiconductor laser, the Optical signal amplified and output from the output end of the optical fiber
Means for detecting the output level of the signal. A first device for transmitting an optical signal to an input end of the first optical fiber; and an optical signal from an output end of the second optical fiber. A second device for receiving, optically connected to an output end of the first optical fiber and an input end of the second optical fiber, and amplifies an optical signal from the output end of the first optical fiber. An optical transmission device, wherein the optical transmission device has an input end and an output end, a third optical fiber doped with a rare earth element, and an optical signal from the output end of the first optical fiber. Means for inputting light to the input end of the third optical fiber, and a semiconductor laser for generating excitation light, and inputting the excitation light to the input end or output end of the third optical fiber, Apart from the pump light input to the optical fiber,
A means for outputting the monitoring information of the semiconductor laser, and an amplified output from the output end of the third optical fiber.
Means for detecting the output level of the optical signal, amplifying the optical signal from the output end of the first optical fiber, outputting from the output end of the third optical fiber, An optical communication system for inputting to an input end of an optical fiber. 3. A method for amplifying an optical signal using an optical fiber having an input end and an output end and to which a rare earth element is added, comprising: inputting the optical signal to an input end of the optical fiber; inputs the more output excitation light to the input end or the output end of the optical fiber, separately from the excitation light input to the optical fiber, and outputs the monitoring information of the semiconductor laser, the output end of said optical fiber Optical signal amplified and output
A method for amplifying an optical signal , comprising detecting an output level of a signal . 4. The monitoring information according to claim 1, wherein a bias current of the semiconductor laser is used as the monitoring information.
The optical transmission device, the optical communication system, or the optical signal amplification method according to any one of the above items.