JP2010192929A - Optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide at a low cost an optical amplifier, detecting noise optical power, signal optical power or the like, generated upon amplifying a signal light, through a simple optical circuit structure with a high accuracy, and stably acquiring WDM (Wavelength Division Multiplex) light having flat wavelength characteristic. <P>SOLUTION: The optical amplifier is equipped with: a light amplifying circuit 1 which amplifies signal light; a light reflecting medium arranged on an optical fiber F connected to the light amplifying circuit 1 and employing a tilt chirp type fiber grating having a transmission wavelength characteristic for flattening the gain wavelength characteristic of the optical amplifying circuit 1; a light receiver 3 which receives light of signal band reflected by the light reflecting medium to detect power; an operation circuit 4 which obtians the wavelength characteristic of the WDM light transmitted through the light reflecting medium based on the detecting result of the light receiver 3; and a control circuit 7 which controls the light amplifying circuit 1, based on the wavelength characteristic of the WDM light. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical amplifier mainly used for optical communication, and more particularly to an optical amplifier having a monitor function for detecting the power of noise light generated when signal light is amplified.

  An optical amplifier is one of key components for realizing a long distance and large capacity of an optical communication system. Optical amplifiers are classified into laser amplifiers using stimulated emission from inversion distributed media and amplifiers based on nonlinear optical effects such as Raman scattering and Brillouin scattering. Furthermore, laser amplifiers include semiconductor laser amplifiers that use rare-earth doped fiber amplifiers and semiconductor amplification media. The former operates as an optical amplifier by optical excitation, and the latter operates as an optical amplifier by injection current excitation. Among these optical amplifiers, rare earth doped optical fiber amplifiers have great advantages in terms of performance such as bit rate free, high gain, low noise, broadband, low coupling loss, low polarization dependence, and high efficiency. ing. Among rare earth-doped optical fiber amplifiers, erbium-doped fiber amplifiers (hereinafter referred to as EDFAs) are common and are currently in practical use in optical fiber communication systems. For such an optical amplifier, in order to improve the performance and cost performance of the applied optical communication system, it is required to achieve higher performance while having a simple optical circuit configuration. Demand for such optical amplifiers is increasing.

  By the way, in a WDM optical communication system that performs relay transmission of wavelength division multiplexing (WDM) light including a plurality of optical signals having different wavelengths, the wavelength characteristics of signal light power are as follows in order to satisfy predetermined transmission characteristics: It is desired to be flat. However, due to various factors such as the conditions of the optical transmission line and the accumulation of gain wavelength characteristics (for example, tilt, ripple, etc.) in optical amplification repeaters using rare earth doped optical fiber amplifiers, Raman amplifiers, etc., the signal light on the receiving side Deterioration of wavelength flatness of power is regarded as a problem. For this reason, as one problem related to the operation control of the optical amplifier, it is possible to establish a technique for controlling the output wavelength characteristic of the optical amplification repeater station (first problem).

  In the optical amplifier, spontaneous emission light (ASE), which is a noise component, is generated with optical amplification. This ASE occurs over a wide wavelength band although the optical power level is overwhelmingly smaller than that of signal light. For this reason, when performing control of the optical amplifier such as constant output control and constant gain control using a general output monitoring technique that branches the output light of the optical amplifier and is received by the light receiver, ASE that is a noise component together with the signal light. Therefore, the influence of ASE appears in the output monitor result and the control accuracy of the optical amplifier is deteriorated. In addition, the ASE of the optical amplifier arranged upstream also deteriorates the input shutdown function of the optical amplifier arranged downstream (the function of detecting the input signal light power input interruption and turning off the pumping light power of the optical amplifier). End up. Such a problem caused by ASE becomes serious particularly in a system or the like in which the number of signal wavelengths dynamically changes because the amount of ASE generated also greatly changes. For this reason, as another problem relating to the operation control of the optical amplifier, a technique for accurately estimating (monitoring) the ASE power generated in the optical amplifier, correcting the control target value of the optical amplifier, and correcting the input disconnection detection threshold. Establish (second problem).

  Furthermore, the optical amplifier also has a problem that in a wavelength region having a larger gain coefficient, energy concentrates and oscillates in a high inversion distribution state, and noise components increase to deteriorate transmission characteristics. The oscillation phenomenon of this optical amplifier has a threshold corresponding to the relationship between the gain of the optical amplification medium and the reflection attenuation amount on the input / output side of the optical amplification medium, and the gain conditions of the optical amplifier and the reflection attenuation amount of the constituent optical components The above problems become obvious due to factors such as deterioration. Specifically, for example, when the number of signals input to the optical amplifier is small, or when the amount of isolation in the optical isolator on the optical path connected to the optical amplifying medium deteriorates, the oscillation operation (increase in noise component) occurs. Become prominent. For this reason, another problem relating to the operation control of the optical amplifier is to establish a control technique for preventing the output light level from exceeding the oscillation threshold in a wavelength region having a large gain coefficient that is known in advance (third). Issue).

  In order to improve the performance and reliability of the WDM optical communication system, it is important to simultaneously solve the first to third problems related to the operation control of the optical amplifier as described above. Specifically, the wavelength characteristic of the WDM signal light power can be monitored with high accuracy for the first problem, and the power of noise light such as ASE can be monitored with high precision for the second and third problems. It must be possible.

  As a configuration of an optical power monitor in a conventional optical amplifier, for example, as shown in FIG. 12, an optical branching device 101 is inserted on the optical path of the main signal system through which WDM light propagates, and the optical branching device 101 has a branching port. A configuration is known in which an optical spectrum analyzer (OSA) 102 is arranged, and the measurement result of an optical spectrum monitored by the optical spectrum analyzer 102 is transmitted to a variable gain equalizer (VGEQ) 103 or the like to perform control. . Further, in place of the optical spectrum analyzer 102, a wavelength separation device (for example, a grating or an optical filter) and a light receiver are provided at the branch port of the optical branch device 101, and the light demultiplexed by the wavelength separation device is received by the light receiver. There is also a configuration for monitoring power (see, for example, Patent Document 1).

JP 2001-168841 A

  However, in the configuration of the conventional optical power monitor as described above, it is necessary to newly add the optical splitter 101 and the expensive optical spectrum analyzer 102 or the wavelength separation device to the optical amplifier. There is a disadvantage that the cost increases.

  In addition, as illustrated in the lower left of FIG. 12, noise light such as ASE whose optical power per unit minute wavelength region is overwhelmingly smaller than the signal light is branched at the same branching ratio as the signal light in the optical branching device 101. The The branching ratio of the optical branching unit 101 is set so that the ratio on the monitor light side is low because it is necessary to suppress the decrease in the power of the main signal light as much as possible (for example, 95 to 99% on the main signal light side, 1-5% on the light side). For this reason, the noise light included in the monitor light is extremely small, the level of the noise light received by the optical spectrum analyzer 102 is low, and therefore the light receiving sensitivity is poor, and it is difficult to monitor the noise light power with a desired accuracy. There is also the problem of becoming.

  As one method for solving the problems of the conventional configuration as described above, for example, a typical value of the power of noise light generated in an optical amplifier is obtained in advance by experiments, simulations, etc. It is conceivable to control the amplifier. However, with such a technique, the amount of noise light generated dynamically depends on the differences in individual components of the optical amplification medium, changes in the environment (for example, temperature, humidity, etc.), and the number of wavelengths of signal light included in the WDM light. Therefore, it is difficult to accurately estimate the power value of the noise light generated in the optical amplifier. For this reason, it becomes impossible to correct the noise light component accurately for the optical output power actually monitored, so that the control accuracy of the optical amplifier is deteriorated and the performance and reliability of the WDM optical communication system are deteriorated. .

  The present invention has been made paying attention to the above points, and has a monitor function capable of detecting noise light power, signal light power, etc. generated when amplifying signal light with high accuracy by a simple optical circuit configuration. An object of the present invention is to provide an optical amplifier that can stably obtain WDM light having a flat wavelength characteristic.

  In order to achieve the above object, an optical amplifier according to the present invention is disposed on an optical amplifying unit that amplifies WDM light in which a plurality of signal lights are wavelength-multiplexed, and an optical fiber connected to an output side of the optical amplifying unit. A tilt-chirped fiber grating in which a Bragg diffraction grating is formed obliquely with respect to the axial direction of the optical fiber, and the lattice spacing of the Bragg diffraction grating is gradually changed along the axial direction of the optical fiber. And transmitting a part of the WDM light according to the transmission wavelength characteristic capable of flattening the gain wavelength characteristic of the optical amplification unit with respect to the signal band of the WDM light, and according to the transmission wavelength characteristic A light reflecting medium capable of reflecting a part of the WDM light outside the core of the optical fiber; and the plurality of light beams included in the WDM light reflected from the light reflecting medium outside the core of the optical fiber. A light receiving unit that receives signal light and detects power; and the light output from the light amplification unit based on the power of the plurality of signal lights detected by the light receiving unit and the transmission wavelength characteristics of the light reflecting medium. A calculation unit that obtains the wavelength characteristic of the WDM light that has passed through the reflection medium; and a control unit that controls the optical amplification unit based on the wavelength characteristic of the WDM light obtained by the calculation unit.

  In the optical amplifier having such a configuration, the light output from the optical amplifying unit and propagating through the optical fiber is given to the light reflection medium using the tilted chirped fiber grating. In this light reflection medium, part of the WDM light in the signal band is transmitted and the remaining part is emitted outside the core of the optical fiber according to the transmission wavelength characteristic that can flatten the gain wavelength characteristic of the optical amplification unit. Is done. The WDM light emitted outside the core of the optical fiber is received by the light receiving unit, the power of the plurality of signal lights is detected, and the detection results are transmitted to the calculation unit. In the arithmetic unit, the wavelength characteristic of the WDM light output from the optical amplifying unit and transmitted through the light reflecting medium is obtained based on the detection result of the light receiving unit, and the optical amplifying unit is controlled by the control unit based on the arithmetic result. .

  The optical amplifier of the present invention forms a light reflection medium using a tilted chirped fiber grating having a gain equalizing function of an optical amplifying unit on an optical fiber through which output light from the optical amplifying unit propagates. By determining the wavelength characteristics of the WDM light transmitted through the light reflection medium based on the optical power of the signal band radiated out of the core of the optical fiber according to the transmission wavelength characteristics of the reflection medium, the optical amplification unit is controlled. It is possible to stably obtain an amplified output of WDM light having flat wavelength characteristics in which the signal light power of each wavelength is made uniform.

  Other objects, features, and effects of the present invention will become apparent from the following detailed description and the accompanying drawings.

1 is a block diagram showing a configuration of an optical amplifier according to a first embodiment of the present invention. It is a figure which shows an example of the wavelength characteristic of the light output from the optical amplifier circuit in the said 1st Embodiment. It is a figure which shows an example of the transmission wavelength characteristic of the light reflection medium used for the said 1st Embodiment. It is a block diagram which shows the structure of the optical amplifier by 2nd Embodiment of this invention. It is a figure which shows an example of the transmission wavelength characteristic of GEQ used for the said 2nd Embodiment. It is a figure which shows an example of the reflective wavelength characteristic of GEQ used for the said 2nd Embodiment. It is a figure explaining the radiation angle of the reflected light in the tilt type FBG, and the inclination of a grating surface. It is a figure for demonstrating the condensing position of the reflected light in a tilt chirp type FBG. It is a figure which shows the wavelength characteristic of the output light in the said 2nd Embodiment. It is a block diagram which shows the structure of the optical amplifier by 3rd Embodiment of this invention. It is a block diagram which shows an example of the optical monitor circuit to which this invention is applied. It is a figure which shows the structural example of the optical power monitor in the conventional optical amplifier.

The best mode for carrying out the present invention will be described below with reference to the accompanying drawings. Note that the same reference numerals denote the same or corresponding parts throughout the drawings.
FIG. 1 is a block diagram showing the configuration of the optical amplifier according to the first embodiment of the present invention.

In FIG. 1, the optical amplifier according to the first embodiment includes, for example, an optical amplification circuit 1 as an optical amplification unit that amplifies input signal light _LIN, and an optical fiber formed on an optical fiber F connected to the optical amplification circuit 1. Based on the reflection medium 2, the light receiver 3 that receives the light reflected by the light reflection medium 2 and emitted outside the core of the optical fiber F and detects the power, and the detection result of the light receiver 3. And an arithmetic circuit 4 as an arithmetic unit for calculating the total power of the noise light generated in the optical amplifier circuit 1. Further, this optical amplifier detects the power by receiving an optical branching device 5 that branches a part of the light output from the optical amplifier circuit 1 and propagating through the optical fiber F, and the light branched by the optical branching device 5. It has a light receiver 6 and a control circuit 7 that controls the optical amplifier circuit 1 in accordance with output signals from the arithmetic circuit 4 and the light receiver 6.

The optical amplifying circuit 1 is, for example, rare earth-doped optical fiber amplifier or a Raman amplifier is configured using a known optical amplifier, such as a semiconductor optical amplifier, the signal light L _IN input via an optical fiber F to the required level Amplified and output to the optical fiber F. In this optical amplifier circuit 1, noise light such as spontaneous emission light (ASE) is generated when the input signal light _LIN is amplified. Therefore, the output light L ₁ including the amplified signal light L _S and noise light L _N is output from the optical amplification circuit 1 to the optical fiber F as shown in FIG. Although an example in which WDM light including a plurality of signal lights having different wavelengths is amplified by the optical amplifier circuit 1 is shown here, the present invention is applicable even when single wavelength signal light is amplified by the optical amplifier circuit 1. Is valid.

The light reflecting medium 2 is a reflection in which noise light (see FIG. 2) existing in a predetermined wavelength region Δλ _N outside the signal band Δλ _S among noise light L _N such as ASE generated in the optical amplifier circuit 1 is set in advance. was radiated to the outside of the core of the optical fiber F is reflected in accordance with the rate, it has a structure capable of transmitting light other than the wavelength region [Delta] [lambda] _N. FIG. 3 shows an example of the transmission wavelength characteristic of the light reflecting medium 2. Specific examples of the light reflecting medium 2 include a fiber grating, a photonic crystal, a dielectric multilayer film, a Mach-Zehnder type device, and the like. Note that a preferable configuration example of the light reflecting medium 2 will be described in detail in other embodiments described later.

Light receiver 3 is arranged to noise light L _R that is reflected by the light reflecting medium 2 is radiated to the outside of the core of the optical fiber F to the light receiving position capable, changes in level according to the power of the noise light L _R An electric signal is output to the arithmetic circuit 4.

Arithmetic circuit 4, based on the reflectance to the noise light L _R of the power and the light reflecting medium 2 in the wavelength region [Delta] [lambda] _N indicated by the output signal from the light receiver 3, occurs over a wide wavelength band in the optical amplifying circuit 1 The total power of the noise light _LN to be calculated is calculated, and a signal indicating the calculation result is output to the control circuit 7. In the above arithmetic processing, for example, the relationship between the noise light power in the wavelength region Δλ _N monitored by the light receiver 3 and the total power of the noise light L _N generated in the optical amplifier circuit 1 is obtained in advance through experiments or simulations. It shall be done with reference to the table. Also, in order for this table it is to have a good accuracy, as the setting of the wavelength region [Delta] [lambda] _N, to such band high gain coefficient in the optical amplifying circuit 1 is selected is desirable. As a specific example, when an erbium-doped optical fiber amplifier (EDFA) is used for the optical amplifier circuit 1 and the signal band Δλ _S is C-band (1.55 μm band), the wavelength region Δλ _N It is preferable to select the vicinity of the 1.53 μm band. When the signal band Δλ _S is an L-band (1.58 μm band), it is preferable to select the vicinity of the 1.57 μm band as the wavelength region Δλ _N.

Optical splitter 5, here, for example, is disposed on the light reflecting medium second-stage on the optical fiber F, 2 branches according to a preset branching ratio of light L _T which has passed through the light reflecting medium 2, one branched light and outputs to the outside of the optical amplifier as the output light L _OUT, and it outputs the light receiver 6 the other branched light as a monitor light L _M. This optical branching device 5 is set so that the ratio on the monitor light side is low in order to suppress the decrease in the power of the main signal light, similarly to the optical branching device 101 in the conventional configuration shown in FIG. Yes. Monitor light L _M which is branched by the optical branching device 5 is a light including a branch signal light L _{S 'and} a noise light L _N' in the same branching ratio.

Photodetector 6 receives the monitor light L _M from the optical branching device 5, and outputs an electrical signal changes in level according to the power of the monitor light L _M to the control circuit 7.
The control circuit 7 receives the output signal from the light receiver 6 and obtains the total power of the output light L _OUT , and then uses the total power of the noise light L _N indicated by the output signal from the arithmetic circuit 4 to generate a noise component. Correction is performed, the output power for only the signal light is calculated, and the operation of the optical amplifier circuit 1 is controlled according to the result.

In the optical amplifier configured as described above, in addition to the total output power (signal light + noise light) monitored by the optical branching device 5 and the light receiver 6 already provided in a general optical amplifier, an optical amplifier circuit 1 total power of the noise light L _N generated in the optical reflection medium 2, will be monitored by the light receiver 3 and the computation circuit 4 corrects the total output power by using a total power of the noise light L _N Thus, it becomes possible to monitor the output power of only the signal light with high accuracy. Thereby, by controlling the optical amplifier circuit 1 using the output power of only the signal light (for example, constant output control, constant gain control, etc.), the signal light can be stably amplified with high accuracy. become able to.

Further, since the light reflecting medium 2 formed on the optical fiber F for monitoring the total power of the noise light L _N adds reflection characteristics outside the signal band, the original performance of the optical amplifier (for example, , Gain, noise figure (NF), etc.) are not deteriorated. Thus, the reflection amount with respect to the noise light in the wavelength range [Delta] [lambda] _N of the optical reflection medium 2, can be arbitrarily designed depending on the light-receiving sensitivity of the light receiver 3. Specifically, the power of the noise light reflected by the light reflecting medium 2 falls within a light receiving level range (for example, −30 to 0 dBm / ch) in which good light receiving sensitivity is obtained by the light receiver 3. The reflection characteristics of the light reflecting medium 2 may be designed. Therefore, the problem that the noise light power cannot be monitored with sufficient accuracy due to the branching of the noise light with the same branching ratio as the signal light in the optical branching device as in the prior art is achieved at a low cost by adding a simple optical circuit configuration. It becomes possible to solve.

Further, in the optical communication system configured by using a plurality of optical amplifiers of the present embodiment, the total power of the noise light _LN calculated by the arithmetic circuit 4 of the optical amplifier arranged upstream is used as the optical amplifier arranged downstream. It is also possible to transmit and perform noise light correction in detection of signal light input interruption in the downstream optical amplifier. Specifically, in the downstream optical amplifier, for example, the optical power of only the signal light is calculated by subtracting the total power of the noise light _LN transmitted from the upstream optical amplifier from the total output optical power. Based on this, it is possible to detect the input interruption of the signal light. In this way, it is possible to reliably perform the input shutdown control in the downstream optical amplifier with almost no influence of noise light generated in the upstream optical amplifier.

In addition, by utilizing the noise light power in the wavelength range [Delta] [lambda] _N monitored by the light receiver 3, it is also possible to perform control to avoid oscillation of the optical amplifier. That is, if the wavelength region Δλ _N is set near the peak wavelength of the gain coefficient in the optical amplifier circuit 1, the noise light power in this wavelength region Δλ _N is monitored, and the monitored value exceeds the preset oscillation threshold value. By controlling the amplification operation of the optical amplifier circuit 1 so as not to occur, the occurrence of an oscillation phenomenon can be prevented in advance.

Next, a second embodiment of the present invention will be described.
FIG. 4 is a block diagram illustrating a configuration of the optical amplifier according to the second embodiment. The optical amplifier according to the second embodiment is characterized by the light reflection in the first embodiment described above with respect to the gain equalizing optical filter (GEQ) 8 that is generally provided in a known optical amplifier that collectively amplifies WDM light. The function as the medium 2 is also provided.

The GEQ 8 is, for example, a tilted configuration in which a Bragg diffraction grating is formed obliquely with respect to the axial direction of the optical fiber F, and the grating spacing of the Bragg diffraction grating is gradually changed along the axial direction of the optical fiber F. A fiber grating (hereinafter referred to as a tilted chirped FBG) is combined with the chirped configuration. This tilt-chirp type FBG has a transmission wavelength characteristic that can flatten the gain wavelength characteristic of the optical amplifier circuit 1 with respect to the signal band Δλ _S , and noise light that exists in the wavelength region Δλ _N outside the signal band. It is designed to have a characteristic capable of reflecting _LN and radiating it out of the core of the optical fiber F. In the tilt and chirp type FBG designed as described above, part of the light in the signal band Δλ _S is also emitted outside the core of the optical fiber F.

5 and 6 are diagrams showing an example of the transmission wavelength characteristic and the reflection wavelength characteristic of the GEQ 8 using the tilt-chirp type FBG. As shown in each figure, the light of the signal band Δλ _S is designed so that the transmittance has a wavelength dependency opposite to the gain wavelength characteristic of the optical amplifier circuit 1 (see FIG. 2). reflection amount of light in the signal band [Delta] [lambda] _S, which is radiated to the outside of the core of the optical fiber F in correspondence with the transmission wavelength characteristics of _S also changes. As for the wavelength region [Delta] [lambda] _N outside the signal band is designed so that the reflectance is higher than the average reflectance of the signal band [Delta] [lambda] _S, the optical power per unit micro-wavelength region as compared with the signal light overwhelmed Even if the noise light is extremely small, the light receiver 3 can reliably monitor the light.

The light receiver 3 _N receives the noise light L _RN in the wavelength region Δλ _N that is reflected by the GEQ 8 and emitted outside the core of the optical fiber F, and measures the power, and corresponds to the focus of the noise light L _RN . Placed in position. Each of the light receivers 3 _{1 to} 3 _M measures light by receiving light L _{R1 to} L _RM corresponding to each signal light wavelength reflected by the GEQ 8 and radiated outside the core of the optical fiber F. And are arranged at positions corresponding to the focal points of the respective lights. Each wavelength band indicated by a thin line in FIG. 6 described above conceptually represents the light receiving range of each of the light receivers 3 _N and 3 _{1 to} 3 _M.

ALU 4, as in the case of the first embodiment described above, based on the reflection characteristics of the power and GEQ8 of the noise light L _RN in the wavelength range [Delta] [lambda] _N measured by the light receiver 3 _N, the optical amplifying circuit 1 The total power of the noise light _LN generated in is calculated. Further, based on the power of the light L _{R1 to} L _RM corresponding to each signal light wavelength measured by each of the light receivers 3 _{1 to} 3 _M and the reflection characteristic of the GEQ 8, the output light L _OUT output from the present optical amplifier The wavelength characteristic is obtained, and the total output power (signal light + noise light) is also calculated. The calculation result in the calculation circuit 4 is transmitted to the control circuit 7. In the control circuit 7, in the same manner as in the first embodiment, the total output power is corrected using the total power of the noise light _LN to obtain the output power for only the signal light. Control is performed.

Here, the tilt / chirp FBG applied to the GEQ 8 will be described in detail.
First, in general, a fiber grating is one in which a Bragg diffraction grating (grating) is formed on an optical fiber using a refractive index change induced by ultraviolet light at the core of the optical fiber, and reflects only the light of the Bragg wavelength (or Functions as a reflective filter. In addition, the fiber grating can realize sharp spectral characteristics in which the reflectance (or transmittance) changes sharply with respect to the wavelength by forming a lattice of tens of thousands of layers in the longitudinal direction of the optical fiber.

Specifically, the Bragg reflection wavelength λ _B of the fiber grating is expressed by the following equation (1) using the actual refractive index n and the grating interval (grating pitch) P for the propagation mode of the optical fiber.

λ _B = 2nP (1)
The bandwidth Δλ _B of the reflection spectrum is expressed by equation (2) using the grating length L and the refractive index modulation amplitude Δn.

Δλ _B = {λ _B ² / (πnL)} × {π ² + (πΔnL / λ _B ) ² } ^1/2 (2)
Furthermore, the grating reflectivity R _B, using the ratio of the propagation light energy contained in the core region gamma, represented by equation (3).

R _B = tanh ² (πLΔnγ / λ _B ) (3)
In addition to reflecting light of a specific wavelength in the incident direction, the reflected light can be radiated to the cladding region by creating a grating oblique to the axial direction of the optical fiber. Since the light coupled to the optical fiber is emitted outside the optical fiber, application examples to an optical spectrum monitor, a gain equalizer, etc. have been reported (for example, Non-Patent Document 1: CK Madsen et al., “Planar Waveguide Optical Spectrum Analyzer Using a UV-Induced Grating ”, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL.4, No.6, NOVEMBER / DECEMBER 1998,925-929. ., “Fiber Grating Optical Spectrum Analyzer Tap”, ECOC, 1997, 65-68, postdeadline paper V.5., Non-Patent Document 3: Tetsuro Komukai, 1 other, “Recent development of optical fiber grating technology”, Shin Academic Report OPE95-114 (1995-12) etc. reference).

The reflection wavelength λ _B ′ of the tilt type FBG in vacuum is more clad than the Bragg reflection wavelength λ _B (when the grating direction is perpendicular to the axial direction of the optical fiber) shown in the above equation (1). Since it shifts to the short wavelength side by the mode effective refractive index difference, it is expressed by the following equation (4) using the effective refractive index n _core of the core of the optical fiber and the effective refractive index n _clad of the cladding.

λ _B '= P (2n _core -n _clad ) (4)
Further, the relationship between the reflection wavelength λ _B ′ of the tilt type FBG and the radiation angle θ ₀ of the reflected light and the inclination angle θ _{T of the} grating surface as shown in FIG. 7 is expressed by the following equation (5). It is also known (see Non-Patent Document 1 above).

λ _B ′ = λ _B (1 + cos θ ₀ ) / 2 cos θ _T (5)
From the relationship of the above equations (4) and (5), it is understood that the radiation angle θ ₀ of the reflected light is determined according to the reflection wavelength λ _B ′ and the grating pitch P. When the reflection wavelength λ _B ′ is fixed, the radiation angle θ ₀ becomes a large value when the grating pitch P is long.

Further, it is known that the tilted FBG has a chirped configuration by gradually changing the grating pitch P in the longitudinal direction of the optical fiber, so that the reflected light is condensed at different positions for each wavelength. (For example, see US Pat. No. 5,061,032). That is, the tilt-chirp FBG can collect reflected light of a desired wavelength at a predetermined position along the longitudinal direction of the optical fiber by appropriately performing chirp design. Therefore, GEQ 8 in the present embodiment, by utilizing the above characteristics of the tilted and chirped FBG, the noise light and the light corresponding to each signal light wavelength of the wavelength region [Delta] [lambda] _N to the core out of different locations of the optical fibers F Collect light.

The reflection wavelength λ _B (z) in the vacuum of the tilt-chirp type FBG is, as shown in FIG. 8, when the longitudinal direction of the optical fiber in which the tilt-chirp type FBG is formed is the Z axis. It is known that the grating pitch is P (z) and is expressed by the following equation (6) (see Non-Patent Document 1 above).

λ _B (z) = n {P (z) / cos θ _T } (1 + cos θ ₀ ) (6)
Further, the reflection wavelength λ _B (z) is defined as c (z) where the chirp amount of the tilted chirped FBG (grating pitch change amount per unit length in the longitudinal direction of the optical fiber) is the center position of the chirped FBG. The wavelength of light reflected from Z ₀ can be expressed by the following equation (6) ′, where λ _B0 is used.

λ _B (z) = λ _B0 + c (z) (z−z ₀ ) (6) ′
Focusing position x ₀ and the focal length f of the X-axis direction of the light of the reflection wavelength lambda _{B (z)} (the direction perpendicular to the longitudinal direction of the optical fiber) is expressed by the following equation (7) and (8) be able to.

x ₀ = {z · tan [θ ₀ (z)]} | _{z =} z ₀ (7)
f = z ₀ / cos θ ₀ (8)
The design parameters of the tilt / chirp FBG as described above (grating pitch P (z), effective refractive index n, refractive index modulation amount Δn, grating surface inclination θ _T , chirp amount c (z), etc.) are appropriately set. By setting the value, noise light in the wavelength region Δλ _N outside the signal band and a function as a GEQ (transmission wavelength characteristic that can flatten the gain wavelength characteristic of the optical amplifier circuit 1 for the signal band Δλ _S ) and A demultiplexing optical system that reflects light corresponding to each signal light wavelength and collects it outside the core of the optical fiber F can be formed.

Specifically, in order to achieve a desired transmission wavelength characteristic for the signal band Δλ _S , it is possible to optimize the transmittance for each wavelength by adjusting the refractive index modulation amount Δn (for example, non-patent) (Ref. 4: Niko Aba et al., “Gain equalizer using slant fiber grating”, Fujikura Technical Report, October 2002).

Then, in order not to impair the original gain equalization function of GEQ, the wavelength λ _B (z) of the light reflected by the tilt-chirp FBG and the reflection thereof using design parameters other than the refractive index modulation amount Δn. The light emission angle θ ₀ and the focal length f are optimized, and the arrangement of the light receivers 3 _N and 3 _{1 to} 3 _M is determined according to the position where the reflected light in the desired wavelength band is collected.

  As a manufacturing method of the tilt-chirp type FBG designed in this way, the reflectance (transmittance) with respect to each wavelength light is changed by controlling the irradiation time and light quantity of ultraviolet rays at each position in the longitudinal direction of the FBG. It is good. For example, when the reflectance is increased, the refractive index modulation is performed by increasing the ultraviolet irradiation time or the amount of ultraviolet light at the time of forming the FBG at a predetermined position in the longitudinal direction of the FBG where light in the target wavelength band is reflected. The amount Δn is increased.

The focal length f of the reflected light in the tilted and chirped FBG, the effective refractive index n, the chirp amount c (z), is known to vary depending on the slope angle theta _T and the refractive index modulation Δn of the grating surface (For example, see Non-Patent Document 2 described above). Shortening the focal length f is equivalent to being able to arrange a light receiver in the vicinity of the tilt-chirp type FBG while having a high wavelength resolution, and is therefore effective in improving monitor accuracy and downsizing the optical amplifier. Even when the focal length f is shortened, it is preferable to use design parameters other than the refractive index modulation amount Δn so as not to impair the original gain equalization function of GEQ. Desirably, by selecting and increasing the chirp amount c (z), the focal length f can be shortened with little influence on the function as the GEQ. For example, when a focal length of about 18 cm is required to obtain a wavelength resolution of 0.1 nm, it is calculated that the focal length can be reduced to half 9 cm by changing the chirp amount from 547 nm / cm to 273 nm / cm. Is required. Note that this example does not mean that the design parameter selected for shortening the focal length f is limited to the chirp amount c (z).

  In the above example, the basic idea is to arrange the photoreceiver according to the focal length of the reflected light. On the other hand, the monitor is placed by deliberately placing the photoreceiver at a position shorter than the focal length. It is also possible to reduce the cost of the optical amplifier by reducing the size of the optical amplifier and reducing the number of light receivers. Specifically, with respect to the focal length f of the reflected light of each wavelength determined by the design parameter of the tilt / chirp type FBG, the position of the light receiver is brought closer to a point where the required wavelength resolution can be obtained than the focal length f. Thus, the wavelength resolution may be intentionally roughened to reduce the size of the optical amplifier and reduce the number of light receivers. Thus, the wavelength resolution of the monitor and the arrangement and number of light receivers can be arbitrarily designed according to the required performance of the optical amplifier. In addition, as an application example to simplify the assembly of the light receiving section, use an arrayed light receiver (PD array) that is arranged so that a plurality of light receivers corresponding to each wavelength band are aligned. It is also effective to do. By applying such a configuration, it is possible to realize an optical amplifier having a simple configuration at a lower cost.

According to the second embodiment in which the GEQ 8 is designed using the tilt-chirp FBG as described above, the main signal light is propagated using the GEQ provided in a general WDM optical amplifier. A monitoring system having a simple optical circuit configuration can be realized without inserting a monitoring optical branching device or the like on the optical fiber F to be transmitted. As a result, the total amount of noise light L _N generated in the optical amplifier circuit 1 can be realized. The power can be monitored with high accuracy, and the wavelength characteristics of the output power of the light corresponding to each signal light wavelength can be monitored. By controlling the optical amplifier circuit 1 using these monitoring results, for example, as shown in FIG. 9, the optical output L _OUT having a flat wavelength characteristic in which the signal light power of each wavelength is made uniform at a desired level. Can be obtained stably. Further, the monitor system using the GEQ 8 basically corresponds to a general GEQ having a reflection characteristic added outside the signal band, so that the original gain equalization function of the GEQ is not impaired. . Furthermore, since the reflectance of the GEQ 8 for the wavelength region Δλ _N outside the signal band can be arbitrarily set according to the performance of the light receiver 3 _N , etc., without depending on the reflectance of the signal band Δλ _S. It is possible to monitor noise light with good accuracy by receiving noise light having a large power as compared with a monitor system using the optical branching device. In addition, since the wavelength resolution of the monitor can be changed in accordance with the arrangement of each of the light receivers 3 _N and 3 _{1 to} 3 _M with respect to the GEQ 8, the flexibility that a monitor function can be realized without excess or deficiency with respect to the required performance. Also have.

Also in the optical amplifier of the second embodiment, the total power of the noise light _LN monitored by the upstream optical amplifier is transmitted to the downstream optical amplifier in the same way as in the first embodiment described above, and the downstream It is of course possible to perform noise light correction in the detection of signal light input interruption in the optical amplifier, and control to avoid the oscillation operation of the optical amplifier using the noise light power monitored by the light receiver.

  In the second embodiment, the case where the GEQ 8 is configured using a tilt / chirp FBG has been described in detail. However, the present invention is not limited to this, and for example, Non-Patent Document 5: Two outside Shusen Izumi, "Wavelength-selective devices using multidimensional periodic structures", wavelength integration and manipulation photonics-Toward the ultimate use of optical spectrum resources-2003 Open Symposium, Proceedings, p.85-88, 20041 It is also possible to form a tilt-chirped diffraction grating by using photonic crystals described on Monday 21-22, etc. instead of FBG.

  In addition, a function as a light reflection medium is added to a GEQ provided in a general WDM optical amplifier to monitor noise light power, etc., but, for example, a single wavelength signal light is amplified. In the optical amplifier, a function as a light reflection medium may be added by applying a tilt type FBG to an optical filter for removing noise light provided instead of the GEQ.

Further, in the second embodiment, the monitoring of the noise light existing in the predetermined wavelength region Δλ _N outside the signal band and the monitoring of the wavelength characteristic of the output light corresponding to each signal light wavelength are simultaneously performed by a common GEQ 8. However, the present invention is effective even when a tilt-chirp FBQ in which the noise light monitoring function is omitted is applied to a general GEQ. In this case, it is possible to monitor the wavelength characteristic of the output light with a simple optical circuit configuration as compared with the output light monitoring system using a conventional optical branching device or the like.

In addition, in the second embodiment, the configuration example in which the light reflected by the GEQ 8 and emitted outside the core of the optical fiber F is directly received by each of the light receivers 3 _N and 3 _{1 to} 3 _M is shown. In addition, a lens medium (for example, a ball lens, an aspherical lens, a cylindrical lens or a prism, or a combination of these lenses) is provided between the GEQ 8 and each of the light receivers 3 _N and 3 _{1 to} 3 _M. The light collecting property of the reflected light reaching each of the light receivers 3 _N and 3 _{1 to} 3 _M may be improved. Further, if the wavelength resolution of the monitor is within an allowable range, it is possible to suppress the attenuation of reflected light even if each light receiver is arranged close to the outer surface of the clad of the optical fiber F on which the GEQ 8 is formed. .

Next, a third embodiment of the present invention will be described.
FIG. 10 is a block diagram showing the configuration of the optical amplifier according to the third embodiment.
In FIG. 10, the optical amplifier of the third embodiment is a specific example when the present invention is applied to a configuration in which a distributed Raman amplifier (DRA) 100 and an erbium-doped optical fiber amplifier (EDFA) 200 are connected in cascade.

DRA100 previous stage, by supplying the excitation light source (LD) transmission line 110 Raman amplification pumping light L _P1 that generates through an optical multiplexer 111 on the input side (optical amplification medium), propagated through said transmission path Signal light to be amplified by the Raman effect. Noise light is generated along with the Raman amplification of the signal light in this transmission path, and the noise light passes through the optical multiplexer 111 together with the Raman amplified signal light to the DRA gain equalizer (GEQ) 120. Entered. Although an example of a distributed Raman amplifier having a transmission line as an optical amplification medium is shown here, the present invention is not limited to this. For example, a dispersion compensating fiber fiber (DCF) or the like is used as an optical amplification medium, and pumping light is injected therein. It is also possible to use DCFRA that Raman-amplifies the signal light.

The DRA GEQ 120 is similar to the GEQ 8 in the second embodiment described above, in addition to the function of flattening the gain wavelength characteristic of Raman amplification in the transmission path, the noise light existing in a predetermined wavelength region outside the signal band, and each It has the function of reflecting light corresponding to the signal light wavelength and radiating it outside the core of the optical fiber F. The light of each wavelength reflected by the DRA GEQ 120 and radiated out of the core of the optical fiber F is received by the light receiver (PD) 130, and each power is measured, and generated by Raman amplification based on the measurement result. The calculation circuit 140 calculates the total power of the noise light and the wavelength characteristics of the signal light output power of each wavelength amplified by Raman amplification. The calculation result of the calculation circuit 140 is transmitted to the DRA control circuit 170 that controls the excitation light source 110 and is also transmitted to the EDFA control circuit 270 at the subsequent stage. A part of the light input to the subsequent EDFA200 through the DRA for GEQ120 is branched as a monitor light _{L M1} in the optical divider 150, the power of the monitor light _{L M1} is measured by the light receiver (PD) 160 Is done. The measurement result of the light receiver 160 is transmitted to the DRA control circuit 170 and the EDFA control circuit 270, respectively.

In the EDFA 200, the pump light L _P2 generated by the pump light source (LD) 210 is supplied via the optical multiplexer 211, and the EDF 212 on the front stage side is supplied, and the pump light L _P3 generated by the pump light source (LD) 215 is the optical multiplexer. The EDF 217 on the rear stage side supplied via the H.216 has a two-stage configuration, the variable optical attenuator (VOA) 280 for output level control is provided between the stages of the EDFs 212 and 217, and the rear stage of the optical multiplexer 216. On the optical fiber F, an EDFA GEQ 220 and an optical branching device 250 are provided. The EDFA GEQ 220 is similar to the GEQ 8 in the second embodiment described above, in addition to the function of flattening the gain wavelength characteristic of the entire EDF 212, 217, noise light existing in a predetermined wavelength region outside the signal band, and each It has the function of reflecting light corresponding to the signal light wavelength and radiating it outside the core of the optical fiber F. The light of each wavelength reflected by the EDFA GEQ 220 and radiated out of the core of the optical fiber F is received by the light receiver (PD) 230, and each power is measured, and each EDF 212, 217 is measured based on the measurement result. The calculation circuit 240 calculates the total power of the noise light generated in step S1 and the wavelength characteristics of the signal light output power of each wavelength amplified by the EDFs 212 and 217. The calculation result of the calculation circuit 240 is transmitted to the EDFA control circuit 270. Further, a portion of the light passing through the EDFA for GEQ220 is branched as a monitor light _{L M2} in the optical branching device 250, the power of the monitor light _{L M2} is measured by the light receiver (PD) 260. The measurement result at the light receiver 260 is transmitted to the EDFA control circuit 270. The EDFA control circuit 270 controls the excitation light sources 210 and 215 and the variable optical attenuator 280 based on the output signals from the arithmetic circuit 240 and the light receivers 160 and 260, respectively. Note that the system monitoring control circuit 300 in the figure sends signal light information relating to the number of wavelengths and wavelength arrangement of signal light included in the WDM light amplified by the optical amplifier to the arithmetic circuits 140 and 240 of the DRA 100 and the EDFA 200. To give. The signal light information from the system supervisory control circuit 300 includes, for example, each of the optical signals in order to prevent the monitor accuracy from deteriorating due to changes in the operation status (number of signals, signal band, etc.) of the optical communication system in which the present optical amplifier is used. Reference is made when noise light correction is performed by the arithmetic circuits 140 and 240.

In the optical amplifier of the present embodiment having the above-described configuration, in the previous DRA 100, the Raman characteristics are determined based on the wavelength characteristics of the DRA output power in the signal band monitored by the DRA GEQ 120, the light receiver 130, and the arithmetic circuit 140. The supply state of the amplifying pumping light L _P1 (for example, the ratio of the pumping light power of each wavelength when pumping light of a plurality of wavelengths is supplied to the transmission line) is controlled by the DRA control circuit 170 to gain the DRA Wavelength characteristics are optimized. The total power of the DRA output thus changed is controlled to a desired level by adjusting the supply power of the Raman amplification pumping light L _{P1 and} the like based on the monitor values by the optical branching device 150 and the light receiving device 160. . In addition, information regarding the total power of noise light generated by Raman amplification obtained by the arithmetic circuit 140 is transmitted from the arithmetic circuit 140 to the EDFA control circuit 270.

In the EDFA 200 at the subsequent stage, the wavelength characteristics of the total power of noise light generated by the EDFs 212 and 217 and the output power of the signal band monitored by the EDFA GEQ 220, the light receiver 230, and the arithmetic circuit 240 are transmitted to the EDFA control circuit 270. At the same time, the total power of the output light L _OUT monitored by the optical splitter 250 and the optical receiver 260 is transmitted to the EDFA control circuit 270. Then, in the EDFA control circuit 270, not only the noise light generated in the EDFs 212 and 217 but also the noise light generated in the previous DRA 100 is added to perform the noise light correction on the output light L _OUT , and based on the result, The gain wavelength characteristic of the EDFA is controlled by adjusting the supply state of the pumping lights L _P2 and L _P3 , and the output level is controlled by adjusting the attenuation amount of the VOA 280. Further, in the EDFA control circuit 270, the noise light correction of the threshold for detecting the input interruption is performed using the total power of the noise light transmitted from the arithmetic circuit 140 of the DRA 100, and the monitor value of the light receiver 160 is more than the threshold. Is detected, the shutdown of the excitation light sources 210 and 215 is controlled. Furthermore, when the EDFA 200 has a large gain coefficient, the noise light power in the wavelength region near the gain peak is monitored by the EDFA GEQ 220, the light receiver 230, and the arithmetic circuit 240, and the monitor value is set in advance as a reference. The power of each of the pumping lights L _P2 and L _P3 is controlled so as not to exceed the previously set oscillation threshold, and the oscillation operation of the EDFA 200 is avoided.

  As described above, according to the optical amplifier of the third embodiment, noise light generated in the DRA 100 and the EDFA 200 and the wavelength characteristics of the output powers of the DRA 100 and the EDFA 200 can be monitored with high accuracy by a simple optical circuit configuration. By performing various controls of the DRA 100 and the EDFA 200 based on the monitoring results, a flat signal light output having a wavelength characteristic controlled to a desired output level can be stably obtained, and at the same time, the influence of noise light can be reduced. The input shutdown control can be surely performed without receiving it.

  In the third embodiment, the configuration example in which the DRA 100 and the EDFA 200 are connected in cascade is shown. However, the configuration of the optical amplifier of the present invention is not limited to the above example, and a known configuration is used. It is possible to apply the monitoring technique of the present invention to this optical amplifier.

In the first to third embodiments described above, an optical filter such as a light reflection medium or a GEQ to which the function is added is provided inside the optical amplifier. However, the present invention is not limited to this, for example, FIG. As shown in FIG. 2, the light reflection medium 2 is arranged on the transmission line connecting the upstream optical amplifier A1 and the downstream optical amplifier A2, and the optical receiver 3 and the arithmetic circuit 4 are provided in the vicinity thereof to constitute an optical monitor circuit. Then, the power of the noise light _LN generated in the upstream optical amplifier A1 and the wavelength characteristics of the signal light output may be monitored. Even in such an optical monitor circuit, it is possible to obtain the same effect as in the above-described embodiments.

  The main inventions disclosed in this specification are summarized as follows.

(Appendix 1) an optical amplifying unit for amplifying signal light;
Arranged on the optical fiber connected to the optical amplifying unit, out of the noise light generated in the optical amplifying unit, the noise light existing in a predetermined wavelength region outside the signal band is reflected to the outside of the core of the optical fiber A light reflecting medium capable of radiating;
A light receiving unit that receives noise light reflected by the light reflecting medium and radiated out of the core of the optical fiber, and detects power;
An arithmetic unit that calculates the total power of noise light generated in the optical amplification unit based on the detection result of the light receiving unit;
An optical amplifier characterized by comprising:

(Appendix 2) The optical amplifier according to appendix 1,
When an optical filter device that transmits signal light according to a preset transmission wavelength characteristic is provided on an optical fiber connected to the optical amplification unit, the optical filter device is provided with a function as the light reflecting medium. An optical amplifier.

(Appendix 3) The optical amplifier according to appendix 2,
The optical amplification unit amplifies wavelength multiplexed light including a plurality of signal lights having different wavelengths,
The optical filter device has a transmission wavelength characteristic capable of flattening the gain wavelength characteristic of the optical amplifying unit with respect to the signal band of the wavelength multiplexed light, and exists in a predetermined wavelength region outside the signal band. An optical amplifier comprising a gain equalizing optical filter having a reflection characteristic capable of reflecting noise light and radiating it out of the core of the optical fiber.

(Supplementary note 4) The optical amplifier according to supplementary note 3, wherein
The gain equalizing optical filter has a reflection characteristic capable of reflecting the light corresponding to the wavelength of each signal light and radiating it out of the core of the optical fiber,
The light receiving unit receives a noise light reflected from the gain equalizing optical filter and radiated out of the core of the optical fiber and detects power, and is reflected by the gain equalizing optical filter. And at least one second light receiver that detects light by receiving light corresponding to the wavelength of each signal light emitted outside the core of the optical fiber,
The calculation unit calculates the total power of noise light generated in the optical amplification unit based on the detection result of the first light receiver, and calculates the optical power of the signal band based on the detection result of the second light receiver. An optical amplifier characterized by determining wavelength characteristics.

(Appendix 5) The optical amplifier according to Appendix 2,
The optical amplification unit amplifies signal light having a single wavelength,
The optical filter device has a transmission band centered on the wavelength of the signal light, and reflects and emits noise light existing in a predetermined wavelength region outside the transmission band to the outside of the core of the optical fiber. An optical amplifier characterized by being an optical filter for removing noise light having a reflection characteristic capable of being reflected.

(Appendix 6) The optical amplifier according to appendix 1,
The light reflecting medium has a diffraction grating arranged with the normal direction of the grating plane inclined with respect to the axial direction of the optical fiber, and a part of the light propagating through the optical fiber is reflected by the diffraction grating. An optical amplifier that radiates out of the core of the optical fiber.

(Appendix 7) The optical amplifier according to appendix 6,
The optical amplifier includes a tilt type fiber grating in which a Bragg diffraction grating is formed obliquely with respect to the axial direction of the optical fiber.

(Supplementary note 8) The optical amplifier according to supplementary note 7,
The optical amplifier includes a tilt-chirped fiber grating in which a grating interval of the Bragg diffraction grating is gradually changed along an axial direction of the optical fiber.

(Supplementary note 9) The optical amplifier according to supplementary note 8,
The optical amplifier, wherein the light receiving unit is arranged at a position corresponding to a focal length of light reflected by the light reflecting medium and emitted outside the core of the optical fiber.

(Supplementary note 10) The optical amplifier according to supplementary note 8,
The optical amplifier according to claim 1, wherein the light receiving unit is disposed closer to the light reflecting medium than a focal position of light reflected by the light reflecting medium and emitted outside the core of the optical fiber.

(Supplementary note 11) The optical amplifier according to supplementary note 6,
In the optical amplifier, the diffraction grating is formed by disposing a photonic crystal on the optical fiber.

(Supplementary note 12) The optical amplifier according to supplementary note 1, wherein
Based on the total power of the noise light calculated by the calculation unit, the optical output power of only the signal light component is calculated by correcting the noise light power included in the optical output power of the optical amplification unit, and the calculation result An optical amplifier comprising a control unit that controls the optical amplification unit according to

(Supplementary note 13) The optical amplifier according to supplementary note 1, wherein
Only the signal light calculated by subtracting the noise light power generated in the upstream optical amplifier from the total output optical power in the downstream optical amplifier is transmitted to the downstream optical amplifier, and the total power of the noise light calculated in the calculation unit is transmitted. An optical amplifier characterized in that an input interruption of signal light is detected based on the optical power of.

(Supplementary note 14) The optical amplifier according to supplementary note 1, wherein
The light reflection medium is capable of reflecting noise light in the vicinity of the gain peak wavelength of the optical amplification unit and radiating it out of the core of the optical fiber,
An optical amplifier comprising: a control unit that controls the optical amplifying unit so that noise light power near the gain peak wavelength detected by the light receiving unit is smaller than a preset oscillation threshold value.

(Supplementary note 15) The optical amplifier according to supplementary note 1, wherein
The optical amplifier includes a rare earth doped fiber amplifier.

(Supplementary note 16) The optical amplifier according to supplementary note 1, wherein
The optical amplifier includes a Raman amplifier that injects excitation light into an optical amplification medium to Raman-amplify signal light.

(Supplementary note 17) The optical amplifier according to supplementary note 1, wherein
The optical amplifier includes a semiconductor optical amplifier.

(Supplementary note 18) A diffraction grating is formed on an optical fiber through which a plurality of light beams having different wavelengths propagates, and is arranged with the normal direction of the grating plane inclined with respect to the axial direction of the optical fiber. The reflection characteristic of the diffraction grating is such that the reflectance of the propagating light with respect to the light within the first wavelength band differs from the reflectance with respect to the light in the second wavelength band different from the first wavelength band. A light reflection medium designed and capable of emitting the reflected light out of the core of the optical fiber;
A first light receiving unit that receives light in the first wavelength band reflected by the light reflecting medium and radiated out of the core of the optical fiber to detect power;
A second light receiving unit that receives light in the second wavelength band reflected by the light reflecting medium and radiated out of the core of the optical fiber to detect power;
An optical monitor circuit comprising:

(Supplementary note 19) The optical monitor circuit according to supplementary note 18,
The light propagating through the optical fiber includes signal light in the first wavelength band, and only noise light exists in the second wavelength band,
The reflection characteristic of the diffraction grating is designed so that the light reflection medium has a higher reflectivity for noise light in the second wavelength band than a reflectivity for light in the first wavelength band. Optical monitor circuit.

(Supplementary note 20) The optical monitor circuit according to supplementary note 18,
The optical monitor circuit, wherein the light reflecting medium includes a tilt type fiber grating in which a Bragg diffraction grating is formed obliquely with respect to the axial direction of the optical fiber.

(Supplementary note 21) The optical monitor circuit according to supplementary note 20,
2. The optical monitor circuit according to claim 1, wherein the light reflecting medium includes a tilted and chirped fiber grating in which a grating interval of the Bragg diffraction grating is gradually changed along an axial direction of the optical fiber.

1 ... Optical amplifier circuit (AMP)
2 ... Light reflection medium 3, 3 _N , 3 _{1 to} 3 _M , 6 ... Light receiver (PD)
DESCRIPTION OF SYMBOLS 4 ... Arithmetic circuit 5 ... Optical branching device 7 ... Control circuit 8 ... Gain equalization optical filter (GEQ)
F ... Optical fiber L _S ... Signal light L _N ... Noise light L _R , L _RN , L _{R1 to} L _RM ... Reflected light Δλ _S ... Signal band Δλ _N ... Wavelength region outside signal band 100 ... Distributed Raman amplifier (DRA)
200: Erbium-doped fiber amplifier (EDFA)

Claims (1)

An optical amplification unit that amplifies WDM light in which a plurality of signal lights are wavelength-multiplexed;
The optical fiber is disposed on an optical fiber connected to the output side of the optical amplifier, and forms a Bragg diffraction grating obliquely with respect to the axial direction of the optical fiber. The WDM light has a tilt-chirped fiber grating that is gradually changed along the axial direction, and the WDM light according to a transmission wavelength characteristic capable of flattening the gain wavelength characteristic of the optical amplification unit for the signal band of the WDM light. A light reflection medium capable of reflecting a part of the WDM light to the outside of the core of the optical fiber according to the transmission wavelength characteristic while transmitting a part of
A light receiving unit that detects the power by receiving the plurality of signal lights included in the WDM light reflected from the light reflection medium to the outside of the core of the optical fiber;
An arithmetic unit that obtains the wavelength characteristics of the WDM light output from the optical amplifying unit and transmitted through the light reflecting medium based on the power of the plurality of signal lights detected by the light receiving unit and the transmission wavelength characteristic of the light reflecting medium When,
An optical amplifier comprising: a control unit that controls the optical amplification unit based on a wavelength characteristic of WDM light obtained by the arithmetic unit.

Images