JP3243774B2 - Optical frequency domain reflection measurement method and measurement circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and circuit for measuring reflection in an optical frequency domain for an optical component or an optical transmission line.

[0002]

2. Description of the Related Art As one of conventional measurement techniques for optical components and optical transmission lines, there is an optical frequency domain reflection measurement method. In the optical frequency domain reflection measurement method, the frequency modulation light is incident on an optical component to be measured, and the frequency analysis of the beat signal between the reflected light from the optical component to be measured and the reference light that has been branched beforehand is performed to obtain the target. This technology measures the intensity of reflected light from each position in a measurement component to measure loss and failure of optical components and the like.

FIG. 4 shows a basic configuration of an optical frequency domain reflection measuring device. For the measurement, coherent light whose optical frequency is swept linearly with time is used. The coherent light whose frequency is swept from the frequency-swept coherent light source 1 is split by the optical directional coupler A, one of which is used as reference light, and the other is incident on the optical component 2 to be measured. The light reflected inside the optical component under test 2 is extracted by the optical directional coupler B and used as signal light, and is heterodyne-received by the heterodyne receiver 3 together with the reference light extracted in advance, and the beat of the obtained two light waves is obtained. The signal is analyzed by the frequency analyzer 4. As shown in FIG. 5, the frequency of the coherent light is set to a sweep time T and a frequency sweep width Δ
Considering the case of sweeping at F, the frequency Fb of the beat signal obtained due to the signal light reflected at a certain point in the measured optical component 2 is proportional to the optical path length difference ΔL between the reference light and the signal light,
Further, depending on the optical frequency sweep speed γ = ΔF / T, Fb = γ · ΔL / v. Here, v is the group velocity of light. Therefore, the beat spectrum obtained by the frequency analysis of the received signal corresponds to the reflected light intensity distribution from each point in the measured optical component 2. The optical frequency domain reflection measurement method measures the reflected light intensity distribution in the measured optical component 2 based on the above principle,
This is a technique for performing loss measurement and failure diagnosis of optical components and the like.

As one of means for sweeping the optical frequency of the coherent light for performing the optical frequency domain reflection measurement, there is a method of modulating the phase or intensity of the coherent light using an external light modulator. Considering the case where the external light modulator is driven by a sine wave having a modulation frequency fm and phase modulation is applied to the output light of the laser light source having high coherence, the output light spectrum from the external light modulator is shown by a solid line in FIG. As described above, the carrier component having the same frequency as the output light frequency f0 of the laser light source, the fundamental wave component whose frequency is shifted by fm from the frequency of the carrier component, and the frequency difference from the carrier component are 2fm.
And higher harmonic components such as the second harmonic component. Generally N
Is an integer, the frequency difference between the modulation sideband component and the carrier component is Nfm. Here, by sweeping the modulation frequency fm, the optical frequency of each of the modulation sideband components is swept, so that a frequency sweep of coherent light having high coherence is realized, and the frequency is a multiple of the sweep speed of the modulation frequency. A light wave having a sweep speed is obtained from each modulation sideband component.

[0005]

However, in the optical frequency domain reflection measurement, since the frequency of the obtained beat signal depends on the optical frequency sweep speed, beat spectra caused by the modulation sideband components overlap on the frequency axis. As a result, there is a problem that a correct reflected light intensity distribution cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and in order to obtain an accurate reflected light intensity distribution in an optical frequency domain reflection measurement using coherent light whose frequency has been swept by an external optical modulator, different sweep speeds are used. It is an object of the present invention to provide an optical frequency domain reflection measurement method and a measurement circuit having means for separating a beat spectrum caused by each modulation sideband component having on the frequency axis.

[0007]

In order to achieve the above object, according to the present invention, in an optical frequency domain reflection measurement, a reference light directly from an optical frequency swept light source to an optical receiver and a measured light from an optical frequency swept light source are measured. By inserting a delay optical fiber into the reference optical path or the signal optical path so that the optical path length difference from the signal light reaching the optical receiver after being reflected inside the optical component becomes N times or more the length of the optical component under test. The frequency bands occupied by beat signals generated due to the (N-1) or less-order modulation sideband components included in the output light of the optical frequency sweep light source are separated.

According to a second aspect of the present invention, there is provided a laser light source for generating light having good coherence, an external light modulator for modulating the phase or intensity of output light of the laser light source in accordance with the frequency of a drive signal,
An optical frequency sweep light source constituted by a drive circuit for providing a frequency-swept drive signal to an external optical modulator, an output light of the optical frequency sweep light source as an input of port a, and an output light of port c as measurement light, A first port having input ports a and b and output ports c and d using output light of port d as reference light;
Optical directional coupler and port c of the first optical directional coupler
Input ports e and f, an output port g, and an output light of the port e, an output light of the port g is guided to the optical component to be measured, and a reflected signal light from the optical component to be measured is output from the port f. h, the output light of the port d of the first optical directional coupler and the output light of the port f of the second optical directional coupler are respectively connected to the ports i and j. A third optical directional coupler having input ports i and j and output ports k and l as inputs, and a frequency occupied by a beat signal caused by a different modulation sideband component contained in the output light of the optical frequency sweep light source An optical path from the first optical directional coupler to the second optical directional coupler, or an optical path from the first optical directional coupler to the third optical directional coupler to separate bands; Alternatively, the light from the second optical directional coupler to the third optical directional coupler A first delay optical fiber inserted into any one of the above, an optical receiver for receiving output light of the third optical directional coupler, and a frequency analyzer for performing frequency analysis of an output signal of the optical receiver. .

According to a third aspect of the present invention, there is provided a laser light source for generating light having good coherence, an external light modulator for modulating the phase or intensity of output light of the laser light source in accordance with the frequency of a drive signal,
An optical frequency sweep light source constituted by a drive circuit for providing a frequency-swept drive signal to an external optical modulator, and an output light of the optical frequency sweep light source as an input of port m, and an output light of port o to an optical component to be measured. A fourth optical directional coupler having input ports m, n and output ports o, p for guiding and using the reflected signal light from the optical component to be measured as an output of port n; and a fourth optical directional coupler. A fifth optical directional coupler having input ports q and r and output ports s and t having output lights of ports p and n as inputs of ports q and r, respectively, and output light of an optical frequency light source One of the two optical paths from the fourth optical directional coupler to the fifth optical directional coupler in order to separate the frequency band occupied by the beat signal caused by different modulation sideband components included in Second delay optical fiber inserted into the Comprising a bar, and an optical receiver for receiving the output light of the fifth optical directional coupler, a frequency analyzer for performing frequency analysis of the output signal of the optical receiver.

[0010]

In FIG. 4, the optical path length difference between the signal light reflected at the point P and the reference light is Ld, and the distance from the point P to the terminal end Q of the optical component 2 is Lt. Here, the optical path length difference ΔL for the signal light reflected on the optical path from the point P to the point Q changes within the range of Ld ≦ ΔL ≦ Ld + 2Lt, and the modulation sideband included in the output light of the optical frequency sweep light source. The area occupied by the beat frequency generated by the component is limited. For example, when sweeping the modulation frequency of the external modulator at the sweep speed γ, the region occupied by the frequency FbN of the beat signal generated due to the modulation sideband component having the frequency sweep speed of Nγ is Ld · Nγ / v ≦ FbN ≦ (Ld + 2Lt) ·
It is expressed as Nγ / v.

Consider a beat signal generated due to a modulation sideband component having a frequency sweep speed of γ, 2γ.
Assuming that each beat frequency is Fb1 and Fb2, the range occupied by Fb1 and Fb2 is limited by Ld and Lt as shown in FIG. According to the first aspect, Ld can be adjusted by inserting a delay optical fiber into the reference optical path or the signal optical path, and by inserting the delay optical fiber so that Ld> 2Lt, as shown in FIG.
The frequency regions occupied by b1 and Fb2 can be separated. That is, the beat spectrum of the fundamental wave component having the sweep speed of γ does not receive the modulation sideband component having the sweep speed of 2γ.

FIG. 8 shows how the measured beat spectrum changes due to the insertion of the delay optical fiber.
Considering the case where measurement is performed on an optical component having the reflectance distribution shown in FIG. 8A, when Ld <2Lt, the range occupied by the beat frequency Fb1 and the range occupied by the beat frequency Fb2 are as shown in FIG. The reflected light intensity distribution is not correctly reflected on the measurement signal because the shaded portions shown in ()) overlap. On the other hand, L
When d> 2Lt, as shown in FIG. 8C, the range occupied by the beat frequency Fb1 and the range occupied by the beat frequency Fb2 are separated. As described above, by adjusting Ld by using the delay optical fiber, it is possible to separate only the signal based on the fundamental wave component on the frequency axis and measure the reflected light intensity correctly even when there is a second harmonic component. Become.

On the basis of the same principle, by further increasing the length of the delay optical fiber so as to satisfy Ld> NLt, all signals originating from the (N-1) th and lower-order modulation sideband components are all on the frequency axis. Separated. That is, the beat spectrum caused by each of the modulation sideband components from the first order to the (N-1) th order is separated on the frequency axis without being affected by the Nth or higher order modulation sideband components. It is possible to know the correct reflected light intensity from the beat spectrum.

FIGS. 9 and 10 show optical frequency domain reflection measurement systems for explaining the operation of the second and third aspects, respectively. In the figure, 101 is an optical frequency sweep light source, 2 is an optical component to be measured, and 301 is an optical receiver. Assuming that L1 to L5 are the optical path lengths of the respective paths in the drawing, in claim 2, Ld = | L1-L2-L3 | In claim 3, Ld = | L4-L5 | Therefore, based on the measurement principle of claim 1, by inserting a delay optical fiber in any one of the optical paths L1 to L3 in claim 2, and by inserting a delay optical fiber in any one of L4 or L5 in claim 3, In optical frequency domain reflection measurement using coherent light frequency-swept by the modulator, the beat spectrum caused by the modulation sideband component having different sweep speeds is separated on the frequency axis,
Correct reflectance distribution measurement can be performed.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram for explaining a first embodiment of the present invention. 11 is an oscillation frequency spectrum line width of 100 kHz.
These are the following laser light sources. Specifically, a solid-state laser or a semiconductor laser whose spectral line width is narrowed by a known method is used. Reference numeral 12 denotes an external light modulator for modulating the output light of the laser light source 11. Specifically, several GHz to several tens GHz
The phase modulation is performed using an electro-optic modulator having the following frequency band. As a modulation method, intensity modulation or the like can be used. Reference numeral 13 denotes a drive circuit of the external optical modulator 12, and when sweeping the frequency of the output signal of the drive circuit 13,
The frequency of the modulation sideband component of the output light from the external light modulator 12 is swept. The modulation frequency of the external optical modulator 12 is swept linearly with respect to time, and the frequency sweep width ΔF = 5G
Hz, and the sweep time T = 100 ms. At this time, since the sweep speed γ = 50 GHz / s, the proportional relationship between the received beat signal and the position of the reflection point is represented by a beat frequency of 1 kHz.
The hit distance is 2 m. Reference numeral 14 denotes a first optical directional coupler having input ports a and b and output ports c and d. The port a is connected to the external optical modulator 12, and the output light from the port c is used as test light. And the output light from port d is used as reference light. Reference numeral 15 denotes a first delay optical fiber for separating harmonic components. Reference numeral 16 denotes a second optical directional coupler having input ports e and f and output ports g and h. Reference numeral 17 denotes an optical fiber to be measured. The port e of the second optical directional coupler 16 is connected to the first delay optical fiber 15 and the output light from the port g is transmitted to the optical fiber under test 17.
And the reflected light from the measured optical fiber 17 is output from the port f. Reference numeral 18 denotes a third optical directional coupler having input ports i, j and output ports k, 1 for inputting the reference light and the reflected light from the measured optical fiber 17 to the ports i and j, respectively. Reference numeral 19 denotes an optical receiver for receiving the output light of the third optical directional coupler 18;
Is a frequency analyzer for performing frequency analysis of the received beat signal.

FIG. 2 shows an optical fiber 1 to be measured having a length of 12 km.
7 is an example in which measurement was performed while changing the length of the first delay optical fiber 15. FIG. 2A shows a case where the first delay optical fiber 15 is not inserted, and FIG. 2B shows a case where the length of the first delay optical fiber 15 is 10 km. The beat spectrum is measured so as to overlap the beat spectrum of the second harmonic component, and the distribution of the reflected light intensity from the measured optical fiber 17 cannot be known accurately. Assuming that the length of the first delay optical fiber 15 is 30 km, which is twice or more the length of the measured optical fiber 17, FIG.
As shown in (c), the beat spectrum of the fundamental wave component is observed separately from the beat spectrum of the second harmonic component. Therefore, a correct reflected light intensity distribution can be known from the beat spectrum of the fundamental wave component. As described above, in the first embodiment, the beat signal caused by the fundamental wave component is separated on the frequency axis due to the effect of the first delay optical fiber 15, and coherent light whose frequency is swept by the external optical modulator 12 is used. In optical frequency domain reflection measurement,
The reflected light from the measured optical component can be correctly measured.

FIG. 3 is a diagram for explaining a second embodiment of the present invention. The laser light source 11, the external light modulator 12, and the drive circuit 13 have the same configuration as in the first embodiment. 21 is a fourth optical directional coupler having input ports m and n and output ports o and p, the port m being connected to the external optical modulator 12,
The output light from the port o is incident on the measured optical fiber 17, and the reflected light from the measured optical fiber 17 is output from the port n. Output light from port p is used as reference light. Reference numeral 22 denotes a second delay optical fiber connected to the port n of the fourth optical directional coupler 21 for separating harmonic components. 23 is a reference light and a second delay optical fiber 2
A fifth optical directional coupler having input ports q and r and output ports s and t, each of which has the output light of No. 2 as an input of a port q and a port r. Reference numeral 19 denotes an optical receiver for receiving the output light of the fifth optical directional coupler 23, and reference numeral 20 denotes a frequency analysis device for analyzing the frequency of the received beat signal.

With the above configuration, based on the same principle as that of the first embodiment, the beat spectrum of the second embodiment is also separated on the frequency axis by the beat spectrum of the fundamental wave component and the harmonic component. Due to the effect of the delay optical fiber 22, the reflected light measurement of the measured optical component can be performed correctly.

[0019]

As described above, according to the present invention, in an optical frequency domain reflection measurement using coherent light whose frequency has been swept by an external optical modulator, the beat caused by each modulation sideband component having a different sweep speed. The spectrum is separated on the frequency axis by a delay optical fiber and measured.
The reflected light intensity distribution in the measured optical component can be correctly measured.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory view showing a first embodiment of the present invention.

FIG. 2 shows the results obtained in the first embodiment of the present invention,
(A) is a characteristic diagram showing a beat frequency-signal light intensity characteristic when the first delay optical fiber is not inserted, and (b) is a characteristic diagram showing the first characteristic.
FIG. 4 is a characteristic diagram showing beat frequency-signal light intensity characteristics when the length of the delay optical fiber is set to 10 km, and FIG. 7C shows beat frequency-signal light intensity when the length of the first delay optical fiber is set to 30 km. It is a characteristic view showing a characteristic.

FIG. 3 is a configuration explanatory view showing a second embodiment of the present invention.

FIG. 4 is a configuration explanatory diagram showing a basic configuration example of an optical frequency domain reflection measuring device.

FIG. 5 is a characteristic diagram showing an example of a time characteristic of the frequency of the reference light and the frequency of the signal light.

FIG. 6 is a characteristic diagram showing an example of an output optical frequency spectrum of a modulated light source and a change of a spectrum at the time of optical frequency sweep.

FIG. 7 is a characteristic diagram illustrating an example of a range occupied by a beat spectrum by a fundamental wave component and a second harmonic component for explaining the operation of the present invention.

FIG. 8 is a characteristic diagram for explaining the operation of the present invention;
(A) is a characteristic diagram showing an example of a reflectance distribution in the optical component to be measured, (b) is a characteristic diagram showing an example when a beat spectrum of a fundamental wave component and a second harmonic component overlap, and (c).
FIG. 4 is a characteristic diagram showing an example when a beat spectrum is separated by a fundamental wave component and a second harmonic component.

FIG. 9 is a structural explanatory view for explaining the operation of claim 2 of the present invention.

FIG. 10 is a structural explanatory view for explaining the operation of claim 3 of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Laser light source 12 ... External light modulator 13 ... Drive circuit 14,16,18,21,23 ... Optical directional coupler 15,22 ... Delay optical fiber 17 ... Measurement optical fiber 19 ... Optical receiver 20 ... Frequency Analysis device

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-118954 (JP, A) JP-A-4-248434 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11/00-11/08

Claims (3)

(57) [Claims] In an optical frequency domain reflection measurement, a reference light directly reaching an optical receiver from an optical frequency swept light source and a signal light reaching an optical receiver after being reflected from an optical frequency swept light source inside an optical component to be measured. By inserting a delay optical fiber in the reference optical path or the signal optical path so that the optical path length difference is N times or more the length of the optical component to be measured, the output light of the optical frequency sweep light source can be a higher-order modulation sideband component. A frequency band occupied by a beat signal generated due to a modulation sideband component of order (N-1) or less. 2. A laser light source for generating light having good coherence in an optical frequency domain reflection measurement, and an external light modulator for modulating the phase or intensity of output light of the laser light source according to the frequency of a drive signal. An optical frequency sweep light source configured by a drive circuit that provides a frequency-swept drive signal to the external optical modulator; and an output light of the optical frequency sweep light source as an input of a port a.
Input ports a and b, output port c, using output light of port c as measurement light and output light of port d as reference light
a first optical directional coupler having an output port d, an output light from a port c of the first optical directional coupler as an input to a port e, and an output light from a port g to an optical component to be measured. A second optical directional coupler having input ports e and f and output ports g and h for making the reflected signal light from the measuring optical component an output of the port f; and a port of the first optical directional coupler d and the output light of port f of the second optical directional coupler are output to port i, respectively.
And a third optical directional coupler having input ports i and j and output ports k and l as inputs of port j, and different modulation sideband components included in output light of the optical frequency sweep light source. To separate the frequency band occupied by the beat signal, an optical path from the first optical directional coupler to the second optical directional coupler, or a third optical directional coupler from the first optical directional coupler. A first delay optical fiber inserted into an optical path leading to an optical device or an optical path leading from a second optical directional coupler to a third optical directional coupler; and the third optical directional coupler. 1. An optical frequency domain reflection measurement circuit, comprising: an optical receiver for receiving the output light of (1), and a frequency analyzer for performing frequency analysis of an output signal of the optical receiver. 3. A laser light source for generating light having good coherence in an optical frequency domain reflection measurement, and an external light modulator for modulating the phase or intensity of output light of the laser light source according to the frequency of a drive signal. An optical frequency sweep light source configured by a drive circuit that provides a frequency-swept drive signal to the external optical modulator; and an output light of the optical frequency sweep light source as an input of a port m;
A fourth light directivity having input ports m and n and output ports o and p for guiding the output light of the port o to the optical component to be measured and using the reflected signal light from the optical component to be measured as the output of the port n A fifth coupler having input ports q and r, and output ports s and t having output lights of ports p and n of the fourth optical directional coupler as inputs of ports q and r, respectively;
An optical directional coupler, and a fifth optical directional coupler for separating a frequency band occupied by a beat signal caused by a different modulation sideband component included in the output light of the optical frequency sweep light source. A second delay optical fiber inserted into one of two optical paths leading to the optical directional coupler, an optical receiver for receiving output light of the fifth optical directional coupler, An optical frequency domain reflection measurement circuit, comprising: a frequency analysis device that performs frequency analysis of an output signal of a receiver.