JPH06167421A - Measurement of brillouin gain spectrum of optical fiber - Google Patents

Abstract

(57) [Summary] [Structure] A white probe signal is oscillated from a white probe signal light source 2 and is incident on the optical fiber 1 from one end thereof. The output light from the excitation light source 7 is amplified by the EDFA 8 and then branched by the optical coupler 10. One of the branched lights is further amplified by the EDFA 11 and is incident on the optical fiber 1 from the other end side. The other branched light and the optical signal emitted from the other end of the optical fiber 1 are combined by the optical coupler 4, and the spectrum of the obtained optical signal is measured by the high frequency spectrum analyzer 6. [Effect] The Brillouin gain spectrum can be measured easily and stably. In addition, it is possible to perform measurement with excellent resolution and sensitivity.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for measuring the Brillouin gain spectrum of an optical fiber.

[0002]

2. Description of the Related Art In recent years, due to the rapid progress of erbium-doped optical fiber amplifiers (hereinafter referred to as EDFAs),
It has become easy to obtain high-power optical signals in the 1.55 μm communication wavelength range. When the intensity of the optical signal incident on the optical fiber is increased, stimulated Brillouin scattering (Sti
muted Brillouin Scatter
ing: hereinafter, abbreviated as SBS) occurs, and it is known that this has a great influence on the transmission of light. For this reason, SBS has become one of the major concerns in handling high power light in optical fibers. The Brillouin gain spectrum obtained by spontaneous emission light contains important information such as the Brillouin frequency shift ν _B and its line width Δν _B , which are very important in system design. Furthermore, the effect of SBS can be applied to an optical fiber amplifier, and even in the design of an optical fiber amplifier using this,
The Brillouin gain factor and Brillouin gain frequency shift are important.

[0003]

There are various conventional methods for measuring the Brillouin gain spectrum. For example, a method using two lasers, a pumping light oscillation laser and a probe signal light oscillation laser, is used. There is (Shib
ata, N., Azuma, Y., Horiguchi, T., Tateda, T., and Tateda,
M .: "Identification of longitudinal acoustic modes
guided in the core region of a signal-mode optical
fiber by Brillouin gain spectra mesurments ", OPTI
CS LETTERS / Vol.13, No.7 / July 1988 / 595-597), (B
olle, A., Grosso, G., Cosentino, A., and Daino, B.,: Influ
ence of phase modulation on the Brillouin gain curv
e ", ECOC-89 / 1989 / Pages 119 to 122). Fig. 4 is a schematic configuration diagram showing an example of a conventional Brillouin gain spectrum measurement system. The probe signal oscillated from the probe laser 22 is made to enter in the direction from one end to the other end, and the excitation light oscillated from the excitation light oscillation laser 23 is made to enter in the direction from the other end to the one end. That is,
In the measured optical fiber 21, the excitation light is propagated in the opposite direction to the probe signal light, and the optical signal output from the other end of the measured optical fiber 21 is measured by the power meter 24 or the lock-in amplifier 25. To do.

However, this method has a problem that it involves some strict conditions. That is, the above-mentioned two lasers used must be very stable in both power and frequency, and stable measurement cannot be performed with a commonly used single-wavelength laser. Further, it is necessary to consider the polarization of the two laser signal lights propagating in the optical fiber, and therefore, it is necessary to take measures such as using the polarization control devices 26 and 27. In addition, the line widths of the probe lasers must be clearly separated, the power of the probe lasers must be small enough not to cause a decrease in the pump light power, and the frequency of the probe lasers must be Must be fully variable within the Brillouin gain band,
There were such conditions as described above, and it was difficult in practical use. Furthermore, the resolution by this conventional measurement method affects the accuracy of the result, but Δν _B reported so far is between 33 and 55 MHz. However, for fused silica glass, 16 MHz has been reported as a theoretical value of Δν _{B at} a wavelength of 1.55 μm. As described above, the measured value obtained by the conventional measuring method is larger than the theoretical value, and there has been some dissatisfaction with respect to measurement resolution and sensitivity.

The present invention has been made in view of the above circumstances, and provides a novel Brillouin gain spectrum measuring method capable of easily measuring a Brillouin gain spectrum and improving its resolution and sensitivity. The purpose is to

[0006]

In order to solve the above-mentioned problems, the Brillouin gain spectrum measuring method for an optical fiber according to the present invention makes a white probe signal light which propagates in the optical fiber in a direction from one end to the other end thereof. Then, the excitation light propagating in the direction from the other end to the one end is made incident on the optical fiber, and the emitted light from the other end of the optical fiber is measured. Moreover, the oscillation light from the excitation light source is branched,
One of the branched lights is made to enter the optical fiber as the excitation light, the other branched light is combined with the light emitted from the other end of the optical fiber, and the spectrum of the optical signal obtained by the combination is measured. Is.

[0007]

According to the measuring method of the present invention, the white probe signal light is made to enter the optical fiber to be measured, and the excitation light is made to enter in the opposite direction to this signal light, so that the white light affected by Brillouin amplification is obtained. The probe signal is obtained,
It is possible to perform measurement with excellent resolution and sensitivity. By using white signal light whose power spectrum is constant and whose polarization is completely random as the probe signal, stable measurement can be performed. Also, by combining such a white probe signal and the branched light from the pumping light source, a beat signal spectrum that gives a continuous gain spectrum can be obtained on a high frequency spectrum analyzer, which allows easy observation of the Brillouin gain spectrum. You can

[0008]

EXAMPLES FIG. 1 schematically shows an example of a measuring system suitably used in the measuring method of the present invention. Reference numeral 1 in the figure
Is an optical fiber to be measured, 2 is a white probe signal light source, 3 is a first optical coupler, 4 is a second optical coupler, 5 is a photodetection system including a photodiode and an amplifier, 6 is a high frequency spectrum analyzer, and 7 is Excitation light source, 8 is a first EDFA, 9 is an optical filter, 10 is a third optical coupler,
Reference numeral 11 is a second EDFA, 12 is an optical power meter, and I is an isolator.

A white probe signal light source 2 is connected to one end of the optical fiber 1 to be measured via an isolator I. The white signal light oscillated from the white probe signal light source 2 is incident as a probe signal from one end side of the measured optical fiber 1 and propagates in the measured optical fiber 1,
It is emitted from the other end. Then, it passes through the first optical coupler 3 and the isolator I and is guided to the second optical coupler 4.

At the other end of the optical fiber 1 to be measured, a first optical coupler 3, an isolator I, and two EDFs are provided.
The excitation light source 7 is connected via A11, 8 and the like.
The output light oscillated from this excitation light source 7 is the first EDF.
After being amplified by A8 and preferably passing through the optical filter 9, it is branched into two by the third optical coupler 10. One of the branched lights is amplified by the second EDFA 11, then passes through the isolator I, and enters the measured optical fiber 1 as excitation light from the other end side. Also, this excitation light is
Before being incident on the optical fiber 1 to be measured, it is branched into, for example, 8: 2 by the first optical coupler 3.
0% branched light is incident on the measured optical fiber 1 as excitation light. Further, the other 20% branched light is guided to the power meter 12 via the isolator I, and the excitation light incident on the optical fiber to be measured is monitored here.

The other split light split by the third optical coupler 10 is guided to the second optical coupler 4. In the second optical coupler 4, the white probe signal emitted from the other end of the optical fiber 1 to be measured as described above and the branched light oscillated from the excitation light source 7 and branched by the third optical coupler 10 are combined. Be waved. Then, a beat signal is obtained by self-heterodyne using the output light from the pumping light source 7 as a local oscillation signal, which is detected by the photodetection system 5. This beat signal spectrum is displayed on the high frequency spectrum analyzer 6. The Brillouin gain spectrum can be directly observed by the beat signal spectrum thus obtained.

As the white probe signal light source 2 used here, a light source which oscillates a white signal whose power spectrum is constant at least in the wavelength band for measuring the Brillouin gain spectrum and whose polarization is completely random is used. . For example, EDFA can be used as the white probe signal light source 2, and the white noise obtained thereby can be used as the white probe signal in this measurement system.
The pumping light source 7 oscillates light having appropriate power as pumping light in the fiber under measurement, and
A light source that oscillates light with excellent monochromaticity that can act as a high-power local oscillation signal in the heterodyne is used. For example, a 1.55 μm single mode semiconductor laser can be used. Further, the output of this excitation light source 7 is the first E
The power is set to be large enough to suppress the noise generated by the DFA 8 and the second EDFA 11.

Here, the fiber length of the measured optical fiber is L, the transmission loss of the measured optical fiber with respect to the probe signal light and the pumping light is α, the incident white probe signal light power on the measured optical fiber is P _S , When the pumping light power incident on the measurement optical fiber is S _p , the output power from the other end of the measured optical fiber is P ₀ , and the Brillouin peak gain coefficient is g ₀ , amplification is performed by Brillouin amplification at the Brillouin frequency shift ν _B. The gain G (ν _B ) of the amplified signal with respect to the non-amplified signal is represented by the following mathematical expression (I).

[0014]

[Equation 1]

In the above formula (I), K is a polarization coefficient,
It becomes 1/2 when the white probe signal is used.

Further, for the Brillouin line width Δν _B , ν
= Gain at _{ν B ± Δν B / 2 G} (ν B ± Δν B / 2)
Is represented by the following mathematical formula (II).

[Equation 2]

In this way, the Brillouin peak gain coefficient g ₀ and the Brillouin line width Δν _B , which are important parameters of SBS, are the gain G by the Brillouin amplification, the incident pumping light power S _p , the optical fiber transmission loss α, and If the fiber length L is known, it can be easily calculated.

(Example) Using the measurement system shown in FIG.
The Brillouin gain spectrum of the optical fiber was measured. The measured optical fiber 1 has a mode field diameter of 10
A pure silica core optical fiber having a μm and a transmission loss of about 0.18 dB / km was used. The line width of the excitation light source is Q73321.
It was smaller than 200 kHz as measured by (Advantest). The pumping light power incident on the measured optical fiber was 3.1 dBm at a wavelength of 1.55 μm. Further, as the light detection system 5, HP11982
A was used. FIG. 2 shows the beat signal spectrum observed on the high-frequency spectrum analyzer 6 when the excitation light and the white probe signal are made incident on the optical fiber to be measured. From the result of FIG. 2, a peak is obtained at a frequency of 11177 MHz, and this peak is theoretically calculated in a pure silica core optical fiber.
It was almost equal to 1.18 GHz. Also, the baseline due to unamplified signals in this spectrum is 31
μV, the peak due to the amplified signal was 90 μV. The Brillouin peak gain coefficient g ₀ obtained by the calculation was about 2.3 × 10 ⁻¹¹ m / W, and the Brillouin line width Δν _B was about 30 MHz.

(Comparative Example) In the above example, the white probe signal was not incident on the optical fiber to be measured, but only the excitation light was incident. The result is shown in FIG. From the results of FIG. 3, a peak was obtained at the same frequency of 11177 MHz as in the above example, but in this spectrum, the baseline due to the unamplified signal was 24 μV, and the peak due to the amplified signal was 27 μV. And it was very small.

The measuring method of the present invention is not limited to the measurement of the Brillouin gain spectrum. For example, in FIG. 1, the white probe signal emitted from the other end of the optical fiber 1 to be measured is the first coupler 3, After passing the isolator I,
It is also possible to measure the light intensity of this white probe signal by being configured so as to be guided to the power meter 13 (shown by the broken line in FIG. 1). Alternatively, by changing the excitation intensity in the second EDFA, the intensity of the excitation light incident on the optical fiber under measurement can be changed.
Then, by setting the pumping light intensity to a level at which SBS is generated in the fiber under measurement, it is possible to observe the nonlinearity of the optical fiber related to SBS.

[0021]

As described above, according to the Brillouin gain spectrum measuring method of the present invention, the Brillouin gain spectrum can be measured easily and stably, and its resolution and sensitivity can be improved.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of a measurement system suitably used in the measurement method of the present invention.

FIG. 2 is a graph showing spectra observed in Examples.

FIG. 3 is a graph showing a spectrum observed in a comparative example.

FIG. 4 is a schematic configuration diagram showing an example of a measurement system used in a conventional measurement method.

[Explanation of symbols]

1 ... Optical fiber to be measured, 2 ... White probe signal light source, 4
... second optical coupler, 5 ... photodetection system, 6 ... high-frequency spectrum analyzer, 7 ... excitation light source, 10 ... third optical coupler

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryozo Yamauchi 1440 Rokuzaki, Sakura City, Chiba Prefecture Fujikura Electric Cable Co., Ltd. Sakura Factory

Claims (2)

[Claims] 1. A white probe signal light propagating in a direction from one end to the other end of the optical fiber is made incident, and an excitation light propagating in a direction of the other end is made to enter the optical fiber, A method for measuring a Brillouin gain spectrum of an optical fiber, which comprises measuring light emitted from the other end of the optical fiber. 2. The oscillated light from the excitation light source is branched, one of the branched lights is made incident into the optical fiber as the excitation light, and the other branched light is combined with the light emitted from the other end of the optical fiber. The Brillouin gain spectrum measuring method for an optical fiber according to claim 1, wherein the spectrum of the optical signal obtained by the multiplexing is measured.