JPH06334238A - Method and apparatus for monitoring noise factor of optical amplifier - Google Patents

Abstract

PURPOSE:To realize high resolution and high response by detecting the light discharged from an optical waveguide of amplifying medium to the outside and passed through a slit and then determining a noise factor F from a value PSE thus detected. CONSTITUTION:A slit 2 having a width wide for the signal light input end of an amplifying medium 1 but narrow for the signal light output end thereof is provided in the vicinity of the amplifying medium 1. The light emitted spontaneously from the amplifying medium 1 is weighted and the light passed through the slit 2 is detected and then a noise factor F is operated according to a formula; F=2RN2eff/[(1+R)N2eff-1], where R=Oemi/Oabs, N2effidenticalPSE/PSEO, Oemi represents an inductive emission cross-sectional area at the wavelength of signal light, Oabs represents an absorptive cross-sectional area at the wavelength of signal light, N2eff represents an effective occupancy at higher level, PSE represents a measurement of spontaneously emitted light, PSEO represents a saturable value of the spontaneously emitted light at the time of N2eff=1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is used in optical fiber transmission systems. In particular, it relates to monitoring the noise figure of optical amplifiers used in optical fiber transmission systems.

[0002]

2. Description of the Related Art FIG. 11 is a block diagram showing a conventional noise figure monitoring apparatus for monitoring the noise figure of an optical amplifier.

This conventional example device has a gain detector 21 for detecting an optical amplifier gain, a demultiplexer 22 for extracting amplified spontaneous emission light (ASE light) output from the amplification medium 20, and a polarization controller. 23, an analyzer 24, an optical bandpass filter 25, and a photodetector 26. The demultiplexer 22 splits the output light of the amplification medium 20, and the polarization controller 23 adjusts the polarization, so that the signal light component included in the split light is removed by the analyzer 24.
From the measured gain G and output ASE optical power P _ASE ,
The noise figure F of this optical amplifier is calculated by the following equation.

[0004]

[Equation 1] Where ν is the frequency of the signal light, h is Planck's constant, and Δν
Is the frequency width of the optical bandpass filter 25 for cutting out the output ASE optical power.

For the formula shown in the equation 1, PR Morkel a
nd RILaming, Opt.Lett., Vol.14, pp.1062-1064, 19
Details on 89. Further, regarding the configuration and operation shown in FIG. 11, J. Aspell et al., OFC'92, ThA4, pp.189-161,
For details, see 1992 Japanese Patent Application No. 4-228890.

[0006]

However, in the conventional monitoring device, (1) the resolution of F is limited mainly by the resolutions of G and P _ASE , and these resolutions are about 0.1 dB at most. The resolution is as low as 0.1 dB at most,
(2) The monitoring response time of F is mainly limited by the operation speed of signal polarization detection and signal light removal, and is slow at most about 1 second.
(3) There are problems that the number of components is large, the device is large and expensive.

An object of the present invention is to solve the above problems and provide a noise figure monitoring method and apparatus which are capable of high resolution and high speed response and which are simple and inexpensive.

[0008]

According to a first aspect of the present invention, noise figure monitoring of an optical amplifier for detecting spontaneous emission light emitted from the amplifying medium of the optical amplifier and monitoring the noise figure of the amplifying medium. In the method, a slit having a wide width for the signal light input end of the amplification medium and a narrow width for the signal light output end is provided in the vicinity of the amplification medium, and the slit is emitted to the outside from the optical waveguide of the amplification medium and passes through the slit. There is provided a noise figure monitoring method for an optical amplifier, characterized in that the detected light is detected and the noise figure F is obtained from the detected value P _SE by the following equation.

[0009]

[Equation 2] Where R≡σ _emi / σ _abs N _2eff ≡P _SE / P _SE0 σ _emi : stimulated emission cross section at the signal light wavelength σ _abs : absorption cross section at the signal light wavelength N _2eff : upper level (of erbium-doped fiber) In this case, the effective occupation ratio of ⁴ I _13/2 ) P _SE : Measured value of spontaneous emission light P _SE0 : Saturation value of spontaneous emission light and the value when N _2eff = 1. [sigma] _emi and [sigma] _abs are basic constants determined by the medium, and can be obtained in advance by measurement together with _PSE0 .

A second aspect of the present invention is an apparatus for carrying out the above-mentioned method, which comprises a slit having a wide width with respect to the signal light input end of the amplification medium and a narrow width with respect to the signal light output end, and an amplification device. And a photodetector for detecting the light emitted from the optical waveguide of the medium to the outside and passing through the slit.

The difference in the width of the slit with respect to the optical waveguide is for weighting the measurement, and it is sufficient if the shape is tapered in the optical waveguide direction. It is preferable that the width h (z) at the position of z along the optical waveguide from the input end substantially conforms to the following equation.

H (z) = h (0) exp {a [1- (1 + R) N _2ch ] z} where, a: absorption coefficient of amplification medium N _2ch : characteristic occupancy of upper level (0 <N _2ch <1), which is a constant determined for the operating range of the optical amplifier R≡σ _emi / σ _abs σ _emi : stimulated emission cross section at the signal light wavelength σ _abs : absorption cross section at the signal light wavelength. a, σ _emi, and σ _abs are constants that can be obtained in advance, and N _2ch is a constant that is determined by determining what operating range the optical amplifier is used in. For the amplifier, the above equation is a function of only the variable z.

[0013]

The noise figure of the optical amplifier is mainly determined by the population inversion of the amplification medium, and particularly the population inversion at the signal light input end has a great influence. Therefore, the spontaneous emission light (SE light) emitted in the lateral direction from the optical waveguide of the amplification medium is measured by weighting the signal light input end side. From this measurement, the state of excitation, that is, the state of population inversion can be known. When the state of population inversion is known, the noise figure F can be obtained using the basic constant peculiar to the amplification medium.

To monitor the noise figure, the noise figure F
It is not always necessary to obtain the value of
In some cases it may be sufficient to know the measured value corresponding to. That is, the light passing through the slit may be detected by the photodetector, and the detected value may be monitored.

As the slit, a slit is used that allows the spontaneous emission light emitted laterally from the amplification medium to pass therethrough. The slits need not be continuous along the amplification medium. For example, when a rare earth-doped fiber is used as the amplification medium, it may be used in a wound state and a slit may be provided in the vicinity thereof. In that case, the spontaneous emission light is measured at each position of each wound length.

[0016]

1 is a block diagram showing the basic structure of the present invention.

This noise monitoring device is provided with a slit 2 having a wide width with respect to the signal light input end of the amplification medium 1 and a narrow width with respect to the signal light emission end, and a slit emitted from the optical waveguide of the amplification medium 1 to the outside. And a photodetector 3 that detects light that has passed through 2. The amplification medium 1 to be monitored may be a rare earth-doped fiber such as an erbium-doped optical fiber (EDF), a praseodymium-doped fiber, a neodymium-doped fiber, or a laser diode.

When the signal light is input to the amplification medium 1, the signal light is amplified by the amplification medium 1. At this time, SE light is generated in the amplification medium 1. A part of this SE light propagates in the amplification medium 1 and is amplified together with the signal light, and the other is emitted from the optical waveguide of the amplification medium 1 to the outside. Conventionally, the ASE light propagated in the amplification medium 1 and amplified was measured, but in the present invention, the SE light emitted from the optical waveguide to the outside is measured by the photodetector 3 through the slit 2.

FIG. 2 is a view showing an example of the shape of the slit, and the slit width h with respect to the axial distance z in the amplification medium.
(z) is shown normalized by the width h (z) at z = 0. z =
0 is the signal light input end of the amplification medium, and z = L is the signal light output end of the amplification medium. L is the length of the amplification medium. h
(z) is set to be large near z = 0 and small near z = L. In particular, as shown in FIG. 2, it is preferable to set h (z) = h (0) exp {a [1- (1 + R) N _2ch ] z}. Here, a is the absorption coefficient of the amplification medium, and N _2ch is the upper level of the amplification medium ( ^{4 in} the case of EDF).
I _13/2 ) characteristic occupancy (0 <N _2ch <1), which is a constant determined for the operating range of the optical amplifier. N _2ch is a value determined by determining the operating range of the optical amplifier.

FIG. 3 shows an example of theoretical calculation of the change in noise figure with respect to the input signal light power. In this figure, "true value"
Is obtained by a known theoretical calculation, and the "present invention" is obtained by the above-mentioned equation (2). From this calculation result, it can be seen that the difference between the approximate value and the true value of the noise figure obtained by Equation 2 is sufficiently small. Further, since the noise figure monitoring response time in the present invention is mainly limited only by the response time of the photodetector, a response time of 1 μs or less is possible.

4 to 6 show a concrete embodiment of the present invention, FIG. 4 shows the overall structure, and FIGS. 5 and 6 show the positional relationship between the amplification medium and the slits and photodetectors. Here, an example of monitoring the noise figure of an erbium-doped fiber amplifier (EDFA) will be described.

The erbium-doped fiber amplifier includes an erbium-doped fiber 10 as an amplification medium, and further,
An input side isolator 11, a fiber coupler 12, a pumping light source 13, an output side isolator 14 and a narrow band optical filter 15 are provided. Signal light having a wavelength of 1.552 μm enters the optical amplifier from the upper transmission line. When the linear repeaters are connected in multiple stages, amplified spontaneous emission light (ASE light) enters in addition to the signal light. This signal light is
The light enters the erbium-doped fiber 10 through the isolator 11 and the fiber coupler 12. The erbium-doped fiber 10 also receives the excitation light from the excitation light source 13.
The light enters through the fiber coupler 12. As the excitation light source 13, for example, a semiconductor laser having a wavelength of 0.98 μm is used. The output light of the erbium-doped fiber 10 is output through the isolator 14 and the narrow band optical filter 15. The narrow band optical filter 15 is for removing beat noise between spontaneous emission lights.

To monitor the noise figure of this erbium-doped fiber amplifier, a slit 2 and a photodetector 3 are provided near the erbium-doped fiber 10.

In this embodiment, an erbium-doped fiber 10 having a length of 6.3 m is used, which is wound into a coil and fixed to a black plate 16 at eight positions as shown in FIG.
It was arranged at 2 mm. This black plate 16 is S from other parts
It is for removing the E light. The slit 2 is shown in FIG.
As shown in, it has a shape according to the above-mentioned formula of h (z). However, in this case, the erbium-doped fiber 10
Since z is wound in a coil shape, the value of z is a discrete value, and the slit 2 has a shape in which the value of h (z) for each value of z is connected. The parameter value of the above equation of h (z) is a =
It is 3.3 dB / m and R = 1.6. As the photodetector 3, a large light receiving area InGaAs photodiode having a diameter of 5 mm was used.

FIG. 7 shows the calculation results of the true value and the approximate value by the equation 1 when N _2ch = 0.75. This noise figure is for an erbium-doped fiber (EDF), and the noise figure for the operating range of an erbium-doped fiber amplifier (EDFA) adds to it the insertion loss of the optical components preceding the EDF. The difference between the two in the operating range of the EDFA shown in FIG. 7 is as small as 0.08 dB, which shows that the present embodiment is sufficiently accurate in a wide operating range of the EDFA.

FIG. 8 shows a measurement example of the noise figure according to this embodiment. The inset is a partial enlargement. As shown in this inset, the noise figure has a resolution of 3 ...
It is about 0.01 dB at 4 dB. Since the resolution according to the conventional technique is 0.1 dB at most, the resolution according to the present embodiment is dramatically improved.

FIG. 9 shows the time response characteristic of noise figure. The input pumping light power is 20 mW. FIG. 9A shows the input signal light power, and FIG. 9B shows the time fluctuation of the noise figure caused by the fluctuation of the input signal light power. The time resolution in this measurement was 50 μs. Since the time resolution of the prior art is at most 1 second, the time resolution is dramatically improved by this embodiment.

As shown in FIGS. 4 to 6, the structure of this embodiment has a smaller number of parts than the prior art, and the structure is simple and inexpensive.

FIG. 10 shows a modified example of the specific example shown in FIG. This configuration example is different from the configuration shown in FIG. 4 in that the fiber coupler 12 and the pumping light source 13 are arranged on the output side of the erbium-doped fiber 10 and pumping light is input in the opposite direction to the signal light instead of the same direction. The excitation method of FIG. 4 is called forward excitation, and the excitation method of FIG. 10 is called backward excitation. In the case of backward pumping, an N _2ch value different from that in the case of forward pumping is used. When the same components as those described with reference to FIG. 4 are used, N _2ch = 0.7 for the same input pumping light power and input signal light power.

In the above embodiments, an example in which an erbium-doped optical fiber is used as an amplification medium has been described, but the present invention can be similarly applied to other amplification media. When a laser diode is used as the amplification medium, the spatial size is different, but there is no difference in principle.

[0031]

As described above, the method and apparatus for monitoring the noise figure of the optical amplifier according to the present invention can monitor the noise figure of the optical amplifier with high resolution and high speed response, and with a simple and inexpensive structure. be able to. The present invention is particularly effective when used to monitor the operation of an optical amplifier provided in an optical repeater or the like.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing a basic configuration of the present invention.

FIG. 2 is a diagram showing an example of the shape of a slit.

FIG. 3 is a diagram showing an example of theoretical calculation of changes in noise figure with respect to input signal light power.

FIG. 4 is an overall configuration diagram showing a specific embodiment of the present invention.

FIG. 5 is a diagram showing a positional relationship between an amplification medium, a slit, and a photodetector, which is a diagram of the slit and the photodetector viewed from a lateral direction.

FIG. 6 is a diagram showing a positional relationship between an amplification medium, a slit, and a photodetector, as viewed from the photodetector side.

FIG. 7 is a diagram showing calculation results of true values and approximate values according to the present invention when N _2ch = 0.75.

FIG. 8 is a diagram showing a measurement example of a noise figure.

FIG. 9 is a diagram showing a time response characteristic of noise figure,
(A) shows the input signal light power, and (b) shows the time fluctuation of the noise figure caused by the fluctuation of the input signal light power.

10 is a diagram showing a configuration example in which the specific example shown in FIG. 4 is modified.

FIG. 11 is a block configuration diagram showing a conventional noise figure monitoring device for monitoring the noise figure of an optical amplifier.

[Description of Reference Signs] 1, 20 Amplifying medium 2 Slit 3 Photodetector 10 Erbium-doped fiber 11, 14 Isolator 12 Fiber coupler 13 Excitation light source 15 Narrowband optical filter 21 Gain detector 22 Demultiplexer 23 Polarization controller 24 Analyzer 25 Optical bandpass filter 26 Photodetector

Claims (3)

[Claims] 1. A noise figure monitoring method for an optical amplifier for detecting spontaneous emission light emitted from an amplification medium of an optical amplifier to monitor the noise figure of the amplification medium, wherein a signal of the amplification medium is provided in the vicinity of the amplification medium. A slit having a wide width with respect to the light input end and a narrow width with respect to the signal light emission end is provided, and the light emitted from the optical waveguide of the amplification medium to the outside and passing through the slit is detected, and the detected value P _The noise figure F from _SE is F = 2RN _2eff / [(1 + R) N _2eff −1] R≡σ _emi / σ _abs N _2eff ≡P _SE / P _SE0 σ _emi : Stimulated emission cross section at the signal light wavelength σ _abs : Signal Absorption cross section at light wavelength N _2eff : Effective occupation ratio of upper level P _SE : Measured value of spontaneous emission light P _SE0 : Saturation value of spontaneous emission light, which is _obtained by the value when N _2eff = 1 Noise figure monitoring method for optical amplifiers. 2. A noise figure monitoring device for an optical amplifier, which detects spontaneous emission light emitted from an amplification medium of an optical amplifier and monitors the noise figure of the amplification medium, wherein: A light characterized by comprising a slit having a wide width and a narrow width with respect to the signal light emitting end, and a photodetector for detecting the light emitted from the optical waveguide of the amplification medium to the outside and passing through the slit. Amplifier noise figure monitor. 3. The slit has a width h at a position z along the optical waveguide from the signal light input end of the amplification medium.
(z) is substantially h (z) = h (0) exp {a [1- (1 + R) N _2ch ] z} a: absorption coefficient of amplification medium N _2ch : characteristics of upper level of amplification medium Occupancy rate (0 <N _2ch
<1), which is a constant determined for the operating range of the optical amplifier R≡σ _emi / σ _abs σ _emi : stimulated emission cross section at the signal light wavelength σ _abs : absorption cross section at the signal light wavelength The noise figure monitoring device for an optical amplifier according to claim 2.