JP3513432B2 - Optical frequency domain reflection measuring device and optical frequency domain reflection measuring method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a distance or multiple position measurement (FMCW method: Frequency M) based on a measurement principle using a light source capable of continuous optical frequency sweep.
Odulations Continous Wav
e, OFDR method: Optical Frequency
Domain Reflectometry, hereinafter collectively referred to as "optical frequency domain reflectometry").

[0002]

2. Description of the Related Art In the field of optical measurement, an optical frequency domain reflectometry method is applied to a range finder, an optical fiber sensor, an optical fiber flaw detector, and the like. This optical frequency domain reflectometry is a method of measuring a distance or multiple positions using a light source capable of continuous optical frequency sweep. Specifically, the light wave from this light source is frequency-modulated (FM: FrequencyMeasurement).
The light wave of the FM is emitted from the measurement location toward the measurement target object, the reflected light reflected from the target object is received again, and the reflected light is interfered with the reference light on the measurement side, thereby Frequency difference corresponding to the optical delay time (optical path difference, distance), that is, beat (beat) frequency (b)
Eat frequency) is measured. From this, the desired measurand (in this case the distance,
Pressure is also possible).

That is, in this measuring method, an optical FM wave repeatedly output from a light source capable of linear sweeping in time is heterodyne-detected by an interference system, and a desired measurement amount is measured from frequency information of a beat signal at that time. The beat frequency has a characteristic that it changes depending on the optical path difference time (distance) between the reference light and the measurement target. Here, "linearly sweeping in time" means changing the frequency at regular intervals with respect to the time axis.

FIG. 13 shows the basic device configuration of this measuring method, and FIGS. 14 and 15 show the principle from the generation of the optical FM to the detection of the beat signal. An example in which the semiconductor laser 2 is used as the light source in FIG. 13 will be described. However, the light source is not limited to the semiconductor laser, and is not limited thereto as long as it is a wavelength tunable light source. The semiconductor laser 2 outputs an optical FM wave 3 whose frequency is swept linearly with time by the oscillator 1. The optical frequency of this output light 3 is shown in FIG.
It behaves like the waveforms 21 and 22 in (a). 14
Δν in (a) is an optical modulation bandwidth (optical frequency sweep width). In this case, a sawtooth wave is taken as an example, but the same can be said for a triangular wave.

The optical FM output 3 is split by the beam splitter 4 into reference light 5 and object light (measurement light) 7. The reference light 5 is reflected by the reference mirror 6 separated from the beam splitter 4 by a distance L ₁ and is reflected again. Then, the object light 7 is similarly reflected by the measurement target 8 which is separated by the distance L ₂ and is reflected again. In this case, the distance difference ΔL is | L ₁ −L ₂ |
Becomes As a result, the respective reflected lights 5 and 7 are superposed by the beam splitter 4 to become interference light 9.

FIG. 14A shows the relationship between the optical FM waves of the reflected lights 5 and 7 at this time. FIG. 14A shows an optical delay time τ corresponding to the round-trip amount 2ΔL of the optical path difference between the reference FM 6 and the measurement target 8 of the optical FM waves 21 and 22 due to the interference light 9 of FIG.
It shows the relationship where they are overlapped with each other. From this relationship, it can be seen that the frequency difference f _b occurs over almost the entire region of the modulation cycle 1 / f _m (f _m is the modulation frequency). This is the beat frequency.

FIG. 14 shows the time variation of f _b .
It is (b). Two beat the frequency is generated does not occur only at time intervals of the beat frequency 28 of the high-frequency side tau, yet a very short time compared with the 1 / f _m is tau low frequency beats 27 and a high frequency beat 28 from FIG. Since it is an interval, it can be ignored. Therefore, the beat frequency generally called here is the beat frequency 27 on the low frequency side.
Is.

Now, the interference light 9 in FIG.
Squared detection is performed by the
It The waveform is the beat signal strength waveform of FIG. 14 (c).
Showed. The signal waveform is analyzed by a spectrum analyzer.
Or an arithmetic unit 12 such as a computer through the A / D converter 11.
Frequency analysis algorithm such as FFT
The beat frequency spectrum of FIG.
The frame 31 is measured. Beat frequency f obtained from this_b
And the distance ΔL is calculated using known parameters
It can be calculated from (1).   ΔL = (c / 2Δν) × (f_b/ F_m). (1) Where c is the speed of light and Δν is the optical modulation bandwidth (optical frequency
Several sweep widths) and f _mIs the modulation frequency.

An object of the present invention is to expand the range of measurement distance of optical frequency domain reflectometry. The problems that prevent it will be explained.

There are two problems that limit the measuring distance range of the "optical frequency domain reflectometry", one is the disappearance of the beat signal due to the coherence of the light source, and the other is the time. On the other hand, the beat signal disappears due to the influence of some nonlinearity on the optical frequency sweep. The former is the limit of this principle method itself, while the latter limits the range-finding range because the beat peculiar to the measurement distance becomes unclear and the ranging accuracy deteriorates before the limitation by the former. ing.

The former is determined by the coherence peculiar to the light source, and the half of the coherence length L _c determined by the line width Δν _f (Hz) of the light source (because of the reflection round trip) is the limit L _limit of the measurement distance range. The relationship is shown in formula (2). L _limit = L _c / 2 = c / (2πΔν _f ). (2) where c is the speed of light.

For example, in a commercially available inexpensive semiconductor laser, its Δν _f is 100 MHz, and when it is substituted into the equation (2) and its measurement range limit L _limit is calculated, it is 0.48 m.
For a DBR or DFB type semiconductor laser with a built-in diffraction grating for optical communication, Δν _f is several hundred kHz to several MHz.
Therefore, L _limit is about 50 m or more, and the measurement range can be expanded.

However, even if such a light source having good performance is used, the measurement range is limited due to the coherence peculiar to the light source. Moreover, these descriptions are only for the "principle measurable distance range" of the former, it does not guarantee the measurement accuracy (resolution) in the measurement range limit L _{lim it.} At the same time, in order to guarantee the measurement accuracy, the "non-linearity of the optical frequency sweep" explained in the latter must be corrected to a straight line. The longer the measurement distance range (the longer), the more strict the required accuracy of the correction accuracy becomes. After all, it is unavoidable that the measurement accuracy becomes worse as the measurement distance becomes longer (Details of the improvement of the distance measurement accuracy by correcting the nonlinearity of the optical frequency sweep are described in Japanese Patent Application No. Hei 10-220879 by the same inventor. ing).

[0014]

The present invention was devised in view of the problems as described above, and the measurement range can be expanded without being limited by the coherence peculiar to any light source, and at the same time, the conventional measurement can be performed. An object of the present invention is to provide an optical frequency domain reflectometry apparatus and method which eliminates deterioration of measurement accuracy due to expansion of the range and does not deteriorate measurement accuracy even when the measurement range is expanded.

[0015]

Numbers with () appearing in the following description in which the means for solving the problem are expressed in correspondence with the claims are those in which the items described in the claims are described in detail later. It shows that it corresponds to a member, a process, and an operation of at least one form among a plurality of forms of the above, but the solution means of the present invention is limited to the member of the embodiment indicated by those numbers. It is not for the purpose of clarifying the correspondence.

The optical frequency domain reflectometry apparatus of the present invention comprises a light emitting section (61) capable of varying the wavelength of light at a required period (1 / f _m ) and a light output from the light emitting section (61) ( 6
2) is distributed and the first reference light (90) and the measurement light (9) are distributed.
1), a light distribution unit (63), a reference optical unit (6c) that generates a second reference light (69) based on the first reference light (90), and the measurement light (91). An optical combining unit (70) that combines the measurement reflected light (73) reflected by the measurement target (72) and the second reference light (69) to generate interference light (74), and the interference. The measurement reflected light (73) and the second reference light (69) based on the light (74).
Of the measurement light frequency (f _b1 ) is detected as a measurement light frequency difference (f _{b 1} ), and the detected measurement light frequency difference (f _b1 ) is detected.
Based on the measured quantity (L
_a ) and a detection unit (76) for obtaining the a), and the reference optical unit (6c) allows the first reference light (90) to variably set a distance (L _r ′, L _r ′ + ΔL _r ′). Is N (N ≧ 2)
The light (69) as a result of a round trip or a round trip can be generated as the second reference light (69).

In the above description, N ≧ 2 is set in FIG.
Since the reference light 5 of (1) is reciprocated once for the distance L ₁ (N = 1), this means that N = 1 is excluded.

In the optical frequency domain reflectometer of the present invention
And the detection unit (76) relates to the measurement target (72).
Measured amount (L_a) To the coarse part (| N | L_r) And fine
Part (l_a) Is calculated by adding
(L_a) To the first said distance (L_r') Corresponding to the above
Measuring light frequency difference (f_b1) Based on the first data
Calculate the rough part (| N | L_r) From the second to the fifth
And the second data is calculated based on the first data.
Distance of (L_r′) And the third data is the first data
Distance (L_r′) Corresponding to the measured optical frequency difference
(F_b1) And the first distance (L_r2) different from ')
The distance (L_r’＋ ΔL_r') Corresponding to the measuring light
Frequency difference (f '_b1) Changed amount (Δf_b1’)
However, the fourth data indicates the direction of the change and the fifth data
The data is the reference light frequency difference (f _br) And the first
Reference light frequency difference (f ′) for two distances_br) Change amount
Then, the reference light frequency difference (f_br) Is
The first reference light (90) is moved to the first distance (L_r’)
related to k as a result of k (k ≧ 0) round trips or rounds
The second reference light (45) and the first reference light (90)
Is the first distance (L_r′) The above k + 1 round trips or
The second reference light (4
6) frequency difference (f_br) And relates to the second distance
Reference light frequency difference (f '_{b r}) Is the first reference light (9
0) is the second distance (L_r’＋ ΔL_r′) Is the k
The second reference concerning k as a result of making a round trip or a round trip
Illumination and the first reference light (90) are between the second distance (L
_r’＋ ΔL_r′) Was reciprocated or circulated k + 1 times
Resulting frequency difference of the second reference light with respect to k + 1
(F '_br).

The optical frequency domain reflectometer of the present invention comprises a light emitting part (61) capable of varying the wavelength of light at a required period (1 / f _m ) and an output light output from the light emitting part (61). An input light generation unit (68) that generates an input light (62a) based on (62), and a light distribution that distributes the input light (62a) to generate a reference light (69) and a measurement light (91). The part (63a) and the measuring light (91) are measured (7
2) the reflected light (73) reflected by the measurement light and the reference light (69) are combined to generate an interference light (74); Based on this, the frequency difference (f _b1 ) between the measurement reflected light (73) and the reference light (69) is detected as the measurement light frequency difference (f _b1 ), and the detected measurement light frequency difference (f _b1 ). A detection unit (76) for obtaining _a measurement amount (L _a ) for the measurement target (72) based on
8) shows the light (62a) as a result of the output light (62) reciprocating or circling a variably settable distance (L _r ′, L _r ′ + ΔL _r ′) N (N ≧ 2) times. , Can be generated as the input light (62a).

The optical frequency domain reflectometer of the present invention has a frequency (ν0, ν0 +) at a set period (1 / f _m ).
Light emitting unit (61) that outputs light (62) with a change in Δν)
And a first to generate a first reference light (90) and a measurement light (91) by distributing the light output from the light emitting unit (61).
Beam splitter (63) and the first reference light (90)
A reference optical section (6c) for generating a second reference light (69), a measurement reflected light (73) obtained by reflecting the measurement light (91) on a measurement target (72), and a second reference. Light (6
9) and a second beam splitter (70) for generating interference light (74); and based on the interference light (74), the measurement reflected light (73) and the second reference light (6).
9) the frequency difference (f _b1 ) is detected, and the frequency difference (f _b1 ) is detected.
_b1 ), and a detection unit (76) for obtaining _a measurement amount (L _a ) for the measurement target (72), and the reference optical unit (6c) includes first and second reflecting mirrors (64, 6
6) and the first and second reflecting mirrors (64, 66) (L _r ′), the reflecting mirror interval (L _r ′) is variable (L _r ′ + ΔL _r ′) interval variable unit. (67), the first reflecting mirror (64) transmits and inputs the first reference light (90), and the first and second reflecting mirrors (64, 66).
Of the first reference light (90) in cooperation with each other.
Between the first and second reflecting mirrors (64, 66) (L
_r ') is reflected multiple times, and the reflector spacing (L _r ')
Orbiting light (65) that makes multiple rounds or rounds
The second reflecting mirror (66) partially transmits the orbiting light (65) every time the orbiting light (65) makes one turn or reciprocates the reflecting mirror interval (L _r ') once. The second
The reference light (69) is output.

The optical frequency domain reflectometer according to the present invention has its frequency (ν0, ν0 +) at a set period (1 / f _m ).
Light emitting unit (61) that outputs light (62) with a change in Δν)
A first optical fiber coupler (100) that distributes the light output from the light emitting unit (61) to generate a first reference light (108) and a measurement light (101); and the first reference light (108). ), A reference optical unit (115) for generating a second reference light (112), and a measurement reflected light (106) obtained by reflecting the measurement light (101) on a measurement target (105).
And the second reference light (112) are overlapped with each other, and the interference light (1
A second optical fiber coupler (113) for generating 14),
Based on the interference light (114), the measurement reflected light (1
06) and the frequency difference (f _b1 ) between the second reference light (112) and a detection unit (L _a ) for the measurement target (105) based on the frequency difference (f _b1 ). (76), and the reference optical section (11
5) is a third optical fiber coupler (109) having first and second input ports (,) and first and second output ports (,), an input section (116a) and an output section (116b). And the input unit (116a)
Is connected to the second output port () of the third optical fiber coupler (109), and the output section (116b) is connected to the second output port () of the third optical fiber coupler (109). Optical fiber (116) and the input / output unit (116) in the optical fiber (116).
a, 116b) and the input / output unit (116
a, 116b) to change the optical path length between the optical path changing unit (1
11), the optical fiber (116) generates orbiting light (110) that orbits the input / output units (116a, 116b) a plurality of times, and the third optical fiber coupler (10).
9) splits the first reference light (108) input from the first input port () to generate first and second split lights, and outputs the first split light to the first split light. Output from the output port () as the second reference light (112),
The second branched light is output from the second output port () to the optical fiber (116) as the nth (n ≧ 1) -th circulating light (110), and the second input port () is output. The nth circulating light (110) input from the above is branched to generate third and fourth branched lights, and the third and fourth branched lights are generated.
Of the second branched light (112) from the first output port (), and the fourth branched light from the second output port () to the optical fiber (116).
And outputs it as the (+1) th orbital light (110),
The n + 1th orbiting light (110) from the second input port () is changed to the nth orbiting light (110).
Enter as.

The optical frequency domain reflectometry apparatus of the present invention further comprises optical amplifiers (120, 123) for amplifying the intensity of the circulating light (65, 110).

The optical frequency domain reflectometer according to the present invention has a frequency (ν0, ν0 +) at a set period (1 / f _m ).
Light emitting unit (61) that outputs light (62) with a change in Δν)
And a first to generate a first reference light (80) and a measurement light (91) by distributing the light output from the light emitting unit (61).
Beam splitter (63) and the first reference light (80)
The reference optical section (68) for generating the second reference light (134) and the measurement light (91) based on the measurement target (72).
Measurement reflected light (73) reflected by the second reference light (1)
34) to produce an interference light (74)
A beam splitter (70) and the measurement reflected light (73) and the second reference light (1) based on the interference light (74).
Frequency difference 34) detects the _{(f b1),} based on the frequency difference _{(f b1),} provided with a measured amount and (detecting section for obtaining the _{L a)} (76) relating to the measurement object (72), the reference The optical section (68) includes a first and a second reflection film surface (8
2, 85) and the first and second reflection film surfaces (82,
85) Birefringent optical fiber (130) provided between
And the first stage of the first reflective film surface (82) and the second stage.
First and second polarizing sections (131, 133), which are provided after the reflecting film surface (85) and determine the polarization direction of the light input to and output from the birefringent optical fiber (130); And a rotation unit that rotates the second polarization unit (131, 133) to change the polarization direction determined by the first and second polarization units (133). The film surface (82) has the first polarizing part (13).
1) through which the first reference light (80) is transmitted and input, and the first and second reflective film surfaces (82, 85) cooperate with each other to obtain the first reference light (80). The first
The first and second reflective film surfaces (82, 85) are reflected a plurality of times between the first and second reflective film surfaces (82, 85).
The orbiting light (132) that makes a plurality of turns or reciprocates between the two is generated, and the second reflection film surface (85) is turned by the orbiting light (1
32) is the first and second reflection film surfaces (82, 85)
The orbiting light (13
The second reference light (134) formed by partially transmitting 2) and passing through the second polarization unit (133) is output.

The optical frequency domain reflectometry method of the present invention comprises:
And to provide (a) the time interval set (1 / _{f m)} at the frequency (.nu.0, .nu.0 + .DELTA..nu) light (62) which varies, as the input light (62), (b) group endpoints (63) To the terminal point (70) through the first intermediate point (68), the optical path length of which is known (63 → 68 →
70) and (c) providing a second optical path (63 → 72 → 70) from the base end point (63) to the end point (70) via the second intermediate point (72). What to do
(D) The input light (62) is output from the base end point (63) to the first optical path (63 → 68 → 70), and the first optical path (63 → 68 →) at the termination point (70). 7
0) to the reference light (6)
9), and (e) the input light (62) from the base end point (63) to the second optical path (63 → 72 →
70) and outputs the second signal at the termination point (70).
Providing the input light (62) output to the optical path (63 → 72 → 70) as the measurement light (73),
(F) The first and second optical paths (63 → 68 → 70,
Detecting the frequency difference between the reference light (69) and the measurement light (73) corresponding to the optical path difference (63 → 72 → 70) as a beat frequency (f _b1 ), and (g) the beat frequency (f _b1 ). f _b1 ), determining the measured quantity for the second intermediate point (72), and (h) the first
The optical path length of the optical path (63 → 68 → 70) is variable, and (f) is the first optical path (63 → 68 → 70) in which the optical path length is variable. And the second
The frequency difference between the reference light (69) and the measurement light (73) corresponding to the optical path difference of the optical path (63 → 72 → 70) of
The beat frequency (f _{b 1} ) is detected.

The measured quantity of the second intermediate point (72) in (g) is the distance from the base end point (63) to the second intermediate point (72), the second intermediate point ( 72)
Temperature, pressure can be included. Further, it includes measuring (defect detection) that there is a defect (non-uniform portion such as a defect in the material to be inspected) at the second intermediate point (72).

Of the components of the present invention described above, FIG.
To those that can be assigned each component shown in 3.
The allocation is as follows. The technique shown in FIG. 13 provides (a) light (3) whose frequency (ν0, ν0 + Δν) changes at a set time interval (1 / f _m ) as input light (3), b) providing a first optical path (4 → 6 → 4) from the base point (4) to the terminal point (4) via the first intermediate point (6), and (c) the base point ( 4) to provide a second optical path (4 → 8 → 4) from the intermediate point (8) to the terminal point (4), and (d) the input light (3) The input light (3) output from the proximal end point (4) to the first optical path (4 → 6 → 4) and output to the first optical path (4 → 6 → 4) at the termination point (4). ) Is provided as reference light (5), and (e) the input light (3) is output from the base end point (4) to the second optical path (4 → 8 → 4), and the end point is output. Before in (4) The input light (3) output to the second optical path (4 → 8 → 4) is provided as measurement light (7), and (f) the first and second optical paths (4 → 6). → 4,
The frequency difference between the reference light (5) and the measurement light (7) corresponding to the optical path difference of 4 → 8 → 4) is defined as the beat frequency (f
b), and (g) the beat frequency (f)
The optical frequency domain reflectometry method comprising: determining a measurement amount for the second intermediate point (8) based on b).

In the optical frequency domain reflectometry method of the present invention, the step (h) includes generating a plurality of the first optical paths (63 → 68 → 70) having different optical path lengths, and the step (f) ) Is the plurality of first optical paths (63 → 68).
→ 70), the second optical path (63 → 72 → 70)
The optical path length closest to the optical path length of the first optical path (6
3 → 68 → 70) and the second optical path (63 → 72 → 7)
The frequency difference between the reference light (69) and the measurement light (73) corresponding to the optical path difference of 0) is detected as the beat frequency (f _b1 ).

According to the present invention, the beat frequency (f _b1 ) is determined by the plurality of first optical paths (44, k = N).
Detected as corresponding to an optical path difference (τ _a ) between the first optical path (45, k = 2) closest to the second optical path (43) and the second optical path (43). It Therefore, in comparison with the closest first optical path (45, k = 2), the measured quantity relating to the second intermediate point (72) (for example, a distance, a relative distance l _a particularly _here) can be determined . With respect to the distance of the first optical path set in the k = 2, the second in the case of detecting a shift of the distance of the middle point (72) (position shift) may be sufficient detection of the relative distance l _a There, it is not necessary to measure the absolute distance is determined by determining the N of the k = N (L _a).

[0029] The relative distance _{l a} is obtained by the following equation (11). _{l a = (cf b1 / α} ). Here, c is the speed of light. α = 2Δνf _m . Δν is the optical FM bandwidth, which is the change in the frequency of the input light (62). fm is the modulation frequency of the input light (62). …… (11)

In FIG. 1, the interval length between the reflecting mirrors 64 and 66 is appropriately adjusted so that there is a correlation only between arbitrary adjacent reference lights. As a result, the first optical path 4 which is the closest to the second optical path 43 is automatically generated.
5, the beat frequency 41 corresponding to the optical path difference τ _a from
Only the beat frequency 42 corresponding to the optical path difference from the first optical path 46 which is the second closest is generated.

In the optical frequency domain reflection measuring method of the present invention, the above (h) is the plurality of first optical paths (63 → 68).
→ 70) is generated at the second set time interval (τ _r ), and the plurality of first optical paths (63 → 68 → 7) are generated.
0) The plurality of first optical paths (63 → 68 → 70) are generated so that the optical path length differences (L _r ) between them are the same.

The second set time interval and the optical path length difference between the plurality of first optical paths (63 → 68 → 70) correspond to the distance (L _r ') between the reflecting mirrors 64 and 67. is doing.

The optical frequency domain reflection measuring method of the present invention comprises:
(Aa) Set time interval (1 / f _m) At that frequency
The light (62) whose (ν0, ν0 + Δν) changes is input light
(62), and (ab) base point (63)
To the end point (70) through the first intermediate point (68)
, Whose first optical path is known (63 → 68 →
70) and (ac) whether the base end point (63)
To the end point (70) via the second intermediate point (72).
To provide the second optical path (63 → 72 → 70) in
(Ad) The input light (62) from the base end point (63)
Output to the first optical path (63 → 68 → 70), and
The first optical path (63 → 68 → 7) at the end point (70)
0) to the reference light (6)
9) and (ae) the input light (62)
From the base point (63) to the second optical path (63 → 72
→ 70), and outputs to the end point (70)
The input light output to the second optical path (63 → 72 → 70)
(62) is provided as measurement light (73), and (a)
f) The first and second optical paths (63 → 68 → 70, 6
The reference light (6) corresponding to the optical path difference of 3 → 72 → 70)
9) and the frequency difference between the measuring light (73) and the beat frequency.
Wave number (f_b1), And (ag) the bee
Frequency (f_b1Based on the second intermediate point (7
2) determining a measurement quantity relating to
The first optical path (63 → 68 → 70) corresponds to the first optical path (6
The light (65) passing through 3 → 68 → 70) is set to the third
Distance (L_r′) Goes around more than once or goes back and forth
Is included.

The optical frequency domain reflectometry method of the present invention comprises:
(M) the time interval set (1 / _{f m)} at the frequency (.nu.0, .nu.0 + .DELTA..nu) and to provide a light (62) which varies, as the input light (62), (n) group endpoints (63) To the terminal point (70) through the first intermediate point (68), the optical path length of which is known (63 → 68 →
70) and (o) provide a second optical path (63 → 72 → 70) from the base end point (63) to the end point (70) via the second intermediate point (72). What to do
(P) The input light (62) is output from the proximal end point (63) to the first optical path (63 → 68 → 70), and the first optical path (63 → 68 →) at the termination point (70). 7
0) to the reference light (6)
9) and (q) the input light (62) from the base end point (63) to the second optical path (63 → 72 →
70) and outputs the second signal at the termination point (70).
Providing the input light (62) output to the optical path (63 → 72 → 70) as the measurement light (73),
(R) The first and second optical paths (63 → 68 → 70,
Detecting the frequency difference between the reference light (69) and the measurement light (73) corresponding to the optical path difference (63 → 72 → 70) as the beat frequency (f _b1 ), and (s) the beat frequency (f _b1 ). f _b1 ), determining a measured quantity with respect to the second intermediate point (72), and (t) the first
The optical path length of the optical path (63 → 68 → 70) is variable, and (r) is the first optical path (63 → 68 → 70) in which the optical path length is variable. And the second
The frequency difference between the reference light (69) and the measurement light (73) corresponding to the optical path difference of the optical path (63 → 72 → 70) of
The first beat frequency (f _b1 ) is detected, and (t) is the optical path length (k = 2) of the first optical path (63 → 68 → 70). _b1 ) is set to a re-change amount (k = 2 ′) different from the optical path length of the first optical path (63 → 68 → 70) corresponding to ( _b )
Corresponds to the first optical path (63 → 68 → 70) in which the optical path length is set to the re-change amount (k = 2 ′) and the second optical path (63 → 72 → 70). Reference light (6
9) and the frequency difference between the measurement light (73) are detected as the second beat frequency (f ′ _b1 ), and (s)
Is the first and second beat frequencies (f _b1 , f ′).
Based on _b1 ), the measured quantity for the second midpoint (72) is determined.

In the optical frequency domain reflection measuring method of the present invention, the (t) includes generating a plurality of the first optical paths (63 → 68 → 70) having different optical path lengths from each other, The plurality of first optical paths (63 → 68 → 70) are generated so that the optical path length differences between the first optical paths (63 → 68 → 70) are the same, and the (r) is the plurality of optical paths. Of the first optical paths (63 → 68 → 70), the first optical path (63 → 68 → 70) having an optical path length closest to the optical path length of the second optical path (63 → 72 → 70), The frequency difference between the reference light (69) and the measurement light (73) corresponding to the optical path difference of the second optical path (63 → 72 → 70) is detected as the first beat frequency (f _b1 ),
The difference (t) is the difference between the optical path length of the first optical path (63 → 68 → 70) corresponding to the first beat frequency (f _b1 ) and the re-change amount (k = 2 ′). , The plurality of first optical paths that are the same as each other (63 → 68 → 70)
The re-change amount (k = 2 ′) is set so that the difference does not exceed the optical path length difference (L _r ).

The device configuration of the present invention is such that a light source section (light emitting means) (61) capable of varying the wavelength (frequency) of light in a required cycle.
A wavelength tunable control unit (wavelength tunable control means) (60) composed of a control circuit and a signal generator required to perform the wavelength tunable control from the outside, and a light source unit (6) from the outside.
The optical element (59) for preventing the instability of the optical frequency due to the returning light to 1) and the output light (62) from the light source (61) are measured with the reference optical system (first optical means) (68). A light distributor (63) that distributes light (91) in two directions,
A measuring optical system (70) for combining the reflected / transmitted light (69) from the reference optical system (68) and the measured reflected light (73) from the object to be measured (72) ( Second
(Optical means), a photodetector (75) for detecting the interference signal light (74) obtained by the second optical means as an electric signal, and the frequency component of the electric signal (beat signal) obtained thereby is analyzed. Detection analysis processing unit (7
In the optical frequency domain reflectometry device configured from 6), the configuration of the reference optical system (first optical means) (68) on the optical path undergoes multiple round trips and orbits at a certain cycle length interval (τ _r ) again. measuring optical system Yes so as to return to the (second optical means), the optical path length of the fundamental period of the multiple round-trip-orbiting (L _r) variable slightly or optical path length of slightly different fundamental period, (L _r) By selecting or generating
The reference optical path length can be arbitrarily delayed.

The means of the present invention comprises a basic optical path length L _{r of} a reference optical system (first optical means) of an optical frequency domain reflectometry device,
A number of turns N, and variation of the beat frequency at the time of changing the delay variation amount [Delta] L _r from the basic optical path length L _r, the direction information of the change amount ultimately desired to be measured measurement values (L _a)
Coarse portion seeking (| | N L _r), fine portions (l _a) is obtained from the beat frequency value detected before taking delay variation, coarse-fine measuring the combined sum of both of them (| N | L _r ±
l _a ) makes it possible to widen the dynamic range while maintaining accuracy.

The first optical means (reference optical system) (6
8) is arranged immediately after the light source section (61) (the distributor (6
3) The effect similar to that of the above-described invention (before it is divided into the reference light (90) and the measurement light (91)) (the dynamic range is widened while maintaining the accuracy) is obtained.

[0039]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of an optical frequency domain reflectometry method according to the present invention will be described in detail below with reference to the accompanying drawings.

(First Embodiment) Here, a first embodiment and an actual measurement example will be described with reference to FIGS. 1 to 8. FIG. 1 shows the configuration of this embodiment, FIGS. 2 to 7 show the operation of this embodiment, and FIG. 8 shows an example of actual measurement. In the first embodiment, the orbiting reference optical system is a Fabry-Perot type. Figure 1
Describes that part with an example of a space type of a mirror pair, but if the part is replaced with a fiber Fabry-Perot type, the same effect is obtained.

The operation of the first embodiment will be described using the configuration of FIG. 1 and with reference to FIGS. 2 to 7. The light source 61 of FIG. 1 has an optical frequency modulation (FM) controlled by an optical frequency modulation controller 60.
It is received and is output as an optical FM output 62 through the isolator 59. The output light 62 is split by the beam splitter 63 into a reference light 90 and a measurement signal light 91. Among them, the reference light 90 is the Fabry-Perot type circular reference optical system 68.
By being repeatedly reflected by the reflecting mirror 64 and the reflecting mirror 66, the orbiting reflected light 65 that goes around the reflecting mirror interval L _r 'is created.

As a result, the orbiting reflected light 65 is reflected by the optical system 68.
Each round trip, the partially transmitted light from the reflecting mirror 66 makes one round trip time (τ _r = L _{r /} cc: speed of light, L _r = 2
It is output regularly for each L _r '). This contributes to the expansion of the measurement range in the present embodiment as the circular reference light 69. The principle of the operation up to this point will be described with reference to FIGS.

FIG. 2 shows changes in the optical frequency sweep time when the circulating reference light is used. The dotted line in the figure shows the optical frequency sweep of the reference light in each round, and the solid line is the measurement reflected light 43. Many beat frequencies are generated due to the combination of these optical frequency differences, but since they are theoretically regular combinations and occur in a limited range, only the beat frequency (or within the frequency band) that exists in advance is focused on. In this case, the distance can be obtained by observing.

The process will be specifically described with reference to FIGS. 2 to 4. At the same time, a general case (in the case of N revolutions) will be described.

The circulating reference beam, as shown in FIG. 2, k = 0, 1, 2, 3 for each lap time interval tau _r, ...... N lap time corresponding to each _{t = 0, τ r, 2τ} r , 3τ _r ,…
A plurality of dots are formed by time multiplexing (dotted line) like Nτ _r .
At present, even at this stage, the beat signal is generated due to the frequency difference between the orbiting reference lights even without the measurement signal light 43. The basic beat frequency is the frequency difference f between adjacent reference lights.
_br 40, 2f on both sides and 3f on each side
_br , 3f _br , which are discretely generated in the form of harmonic components of f _br .

However, even if it makes N revolutions, it does not mean that harmonic components of all beat frequencies up to Nf _br are generated. Its harmonic region is determined by the magnitude of the degree of correlation (coherence) between the circular reference lights. If the correlation is infinite, all harmonic components can exist, but in reality, the correlation is finite, so the number of beat frequencies between the orbiting reference lights also exists only within the correlation time range.

Here, consider a case where there is a correlation only between arbitrary adjacent orbiting reference lights (the actual operation is as shown in FIG.
The degree of correlation can be freely adjusted only by changing the distance between the reflecting mirrors 64 and 66 using the moving stage 67). Then, only the basic reference beat frequency f _br 40 exists as the beat frequency between the circular reference lights. F _{b r} at this time
Is expressed by the following equation (3). f _br = ατ _r . α = 2Δνf _m . ...... (3) However, .DELTA..nu: Light FM bandwidth, is _{f m} is the modulation frequency.

The operation thereafter will be described again with reference to FIG. 1, and at the same time, the present embodiment will be described with reference to the principle diagrams of FIGS. 2 to 4.

Now, under such conditions of the orbiting reference light, the measurement reflected light 73 is reflected from the measurement object 72 of FIG. 1 (corresponding to the optical path difference time 2τ _r + τ _a with the reference light for k = 0). Will come again. The measurement reflected light 73 and the reference light 69 are combined via the beam splitter 70 and are combined as interference light 74 in the light receiver 7
Square-law detection is performed at 5. Then, the signal processing unit 76 analyzes the frequency component of the beat signal. From these results, the measurement reflected light 43 is newly added to the relationship between the orbiting reference lights when viewed from the relationship of the principle diagram of FIG.

Under the condition that there is a correlation only between arbitrary adjacent orbiting reference lights, these orbiting reference lights and the measurement signal light 4
The correlation with 3 is that the reference light 45 for k = 2 and k
= 3 and the frequency difference (beat frequency) from the measurement signal light 43 is f _b1 41 and f 2.
_b2 42.

The magnitude relationship between these beat frequencies corresponds to the beat frequency spectra 48 and 49 shown in FIG. 3, and at the same time, the basic beat frequency spectrum 47 between the orbiting reference lights is also generated.

These beat frequency spectra f _br
The relation between f _b1 and f _b2 has a feature that always satisfies the relation of the following expression (4) even if the optical path difference of the measurement reflected light 43 from the reference light changes. f _br = f _b1 + f _b2 . (4) Here, (i) f _b1 = ατ _a . (0 ≦ f _b1 ≦ (f _br / 2)). (Ii) f _b2 = α (τ _r −τ _a ). ((F _br / 2) ≦ f _b2 ≦ f _br ). (5)

If the two beat frequency spectra f _b1 48 and f _b2 49 of the equation (5) always satisfy the equation (4) even if the measurement distance L (see FIG. 4) changes, then with respect to f _b1 The locus indicates a zigzag turn back locus with the cycle optical path length L _r as one cycle, as indicated by reference numeral 51 in FIG. The number attached to each trajectory is the orbital number of the orbiting reference light that contributes to the beat in the measurement distance area, and the ± sign is + for the reference light far from the measurement signal. When they are close to each other, they are indicated by-.

That is, the measurement standard necessary for expanding the measurement range (in this case, expanding the distance measuring range) is automatically expanded by N times the reference optical path length at equal intervals every time the reference light circulates N times. In the measurement reflected light 43 which is the example of this time, the second reference light (2τ
beat frequency (4) corresponding to the optical path difference (τ _a ) from _r )
1) occurs.

At this stage, a relative distance l _a (a distance corresponding to the optical path difference τ _a ) from which an arbitrary distance (in this case, the optical path difference 2τ _r of the second orbiting reference light) is used as an offset is obtained. However, at this stage, since the number N of turns of the orbiting reference light (N = 2 in this case) is not known, the absolute distance measured with k = 0 as an absolute reference is not yet known.
However, this step is sufficient if only to know the relative distance l _a .

Described next is a procedure for further obtaining the absolute distance. This procedure will be described with reference to the “N determination method” shown in FIGS. By combining this method with the method for obtaining the relative distance described above, the number of laps (N laps) of the reference light was obtained by interfering with the beat signal (determination of the absolute value of N), and the reference It is judged whether the light is far or nearer than the measurement signal light (judgment of the sign of N), and these are combined to obtain the absolute distance L _a = | N | L _r ± l _a .
Can be asked.

The procedure and principle of the "determination of N and code" will be described with reference to FIGS. 1 and 5 to 7. First, as an operation procedure for determining N, one reflecting mirror 66 of the Fabry-Perot interference system of the orbiting reference optical system 68 of FIG. The optical path interval length L _r and the orbiting time τ _r, which are the basis of the reflecting mirror 64 and the reflecting mirror 66, are L _r → L _r + Δ, respectively.
L _r . , Τ _r → τ _r + Δτ _r . Change like.

This change is ΔL _r , Δτ every N revolutions.
As a result of the cumulative addition of _r (N times), the changes in the Nth turn are NΔL _r and NΔ, respectively, as compared with before the interval change.
It will change by τ _r . The change amount is reflected in the amount of change in the beat frequency detected as shown in FIG. 7 (frequency difference between the dotted line and the solid line in FIG. 7). The relationship of each light before and after changing the circulating optical path difference is shown in FIG. The dotted line in FIG. 5 is in a state before the distance between the reflecting mirrors 64 and 66 is changed, and the one-dot chain line is after the distance is changed. The solid line is the measurement reflected light. Incidentally, since the measurement signal light is not related to the change in the circulating optical path length, it is the same as that shown by the reference numeral 43 in FIG.

In order to obtain the absolute distance L _a from the beat frequency of an arbitrary N-turn reference light, these operations for slightly changing the basic optical path length L _r of the reference light described above are necessary. The absolute distance L _a obtained from these pieces of information can be obtained using the following equation. _{(I) N> 0 L a} = | N | L r -cf b1 / α. (Ii) N <0 L _a = | N | L _r + cf _b1 / α. Here, c is the speed of light. (6)

Here, N is important, and the others are already known.
It is a parameter of knowledge. N can be calculated as follows.
Wear. By the above operation, the basic optical path length L_rIs L_rTo L
_r+ ΔL_rChange to the beat frequency spectrum of Fig. 6
Mu f_b148 (dotted line) is f ' _b1Like 54 (solid line)
Change. The amount of change is Δf_b1’, The line in FIG.
Under the condition (k = 2, k = 2 '), the change amount Δf_b1’Is
2Δf_br(See k = 2 in FIG. 7).

What is more important is the direction of this amount of change. That is, if the direction in which the basic optical path difference of the orbiting reference light becomes longer is defined as +, and the direction in which it becomes shorter is defined as −, naturally, Δ
The change amount of f _b1 ′ has a sign. The N and the sign can be discriminated by these preparations and the equation (7), and the absolute distance L _a is calculated by substituting the result into any of the equations (6). N = Δf _b1 ′ / Δf _br . (7)

With respect to the specific example assumed in the first embodiment (principle FIGS. 2 to 7) using these relational expressions, N and its sign are obtained to find the absolute distance. First, paying attention to the beat frequency spectrum f _b1 48 detected in FIG.
As described above, the basic optical path length L _r of the orbiting reference optical system is changed so as to be longer by ΔL _r (optical path time Δτ _r ) (that is, in the description of FIG. 45 changes to the circular reference light 53 related to k ′ = 2), and the frequency f _b1 48 of the obtained beat signal is f ′.
It changes to _b154 (FIG. 6).

The respective relationships are shown in equation (8). f _b1 = ατ _a . f ′ _b1 = f _b1 + Δf _b1 ′. Δf _b1 ′ = −2αΔτ _r . (8)

Similarly, the amount of change Δf _{br in} beat frequency between the circular reference lights is as follows. Δf _br = αΔτ _r . (9)

Substituting equations (8) and (9) into equation (7) and rearranging them, N is obtained. The result is as shown in Expression (10). N = Δf _b1 ′ / Δf _br = (− 2αΔτ _r ) / (αΔτ _r ) =-2. (10) The value of N and the sign can be determined from the equation (10), and the absolute distance L _a is obtained from the equation (6) (ii) from this.

Now, FIG. 8 shows an actual measurement example in which a beat signal is actually detected with the configuration of FIG. As described above, f _b1 , f
_It can be seen that the beat frequency spectra 94, 95, and 96 appear in the relationship of _b2 and f _{br in} the form of the relationship of Expression (4). Therefore, the effect of actually expanding the measurement range can be expected by the principle of using the circular reference light as in the present invention.

Reference numerals 6a and 6b in FIG. 1 are reflecting mirrors. Reference numeral 6c is a reference optical unit. The reference optical unit 6c includes an orbiting reference optical system 68 and reflecting mirrors 6a and 6b.

FIG. 9 shows a modification of the first embodiment. In this modification, the Fabry-Perot type orbiting reference optical system 68 shown in FIG. 1 is arranged immediately after the light source unit 61. Unlike FIG. 1, the circulating reference light 65 is generated before the FM light 62 is divided into the reference light 90 and the measurement signal light 91 by the beam splitter 63a. Other configurations are basically the same as in FIG. 1 and are as follows.

[0069] Modification of the first embodiment shown in FIG. 9, the light emitting portion 60 which can the wavelength of the light variable at a required period 1 / f _m,
61, an input light generation unit (circulating reference optical system) 68 that generates an input light 62a based on the output light 62 output from the light emitting units 60, 61, and the reference light 6 by dividing the input light 62a.
9 and the light distributor (beam splitter) 63a for generating the measurement light 91, the measurement reflected light 73 formed by the measurement light 91 reflected by the measurement target 72, and the reference light 69 are combined to generate the interference light 74. Optical multiplexer (beam splitter) 63
Based on a and the interference light 74, the frequency difference f _b1 between the measurement reflected light 73 and the reference light 69 is detected as the measurement light frequency difference f _b1 , and based on the detected measurement light frequency difference f _{b 1} .
And a detection unit 76 that obtains _a measurement amount L _a for the measurement target 72. Reference numeral 6d is a reference mirror.

The input light generating section 68 determines that the output light 62 has variably settable distances L _r ′ and L _r ′ + ΔL _r ′ for N (N ≧
2) It is possible to generate the light 62a as a result of the round trip or the round trip, as the input light 62a. Of the input light 62a that is periodically output at each time of making one round trip in the interval L _r 'between the reflecting mirrors 64 and 66, the input light 62a having the number of turns N of 1 is the measurement light 9
Set to 1. The reference light 69 is the same as in FIG. 2 regardless of the number of circulations N (K).

According to this modification, as in FIG. 1, it is possible to perform measurement with a wide dynamic range while maintaining measurement accuracy.

(Second Embodiment) A second embodiment will be described with reference to FIG. The difference between the second embodiment and the first embodiment is the configuration of the orbiting reference optical system, and the procedure from the beat signal detection to the absolute distance calculation is the same as in the first embodiment.

The orbiting reference optical system of the second embodiment orbits the reference light in the form of a fiber loop, and when the basic optical path length L _{r of} the orbiting reference light becomes long, the spatial type of the first embodiment is used. In the optical system described above, there is a problem that the space is large and the alignment is difficult. However, in this embodiment, since that portion has a fiber loop structure, even if the basic optical path length becomes long, multiple layers are formed in the same space. Winding the fiber has the advantage that these problems do not occur.

The second embodiment is a circular reference optical system 115.
Since the configuration and operation of the other parts are almost the same as those of the first embodiment, the orbiting reference optical system 115 will be focused and described here to avoid duplication.

The orbiting reference optical system 115 of the second embodiment shown in FIG. 10 will be described. The orbiting reference optical system 115 has a 2 × 2 optical fiber coupler (or X-branch optical waveguide).
109, and an optical fiber 116 for circulating, and an optical path delay / selector 111 (for example, a moving stage, an optical fiber, a waveguide, a birefringent crystal, etc.) inserted in the fiber for making an Nth order determination. Other than that, if the purpose is to change or select at least two kinds of optical paths, it is also included).

First, the reference light 108 is finally the circulating reference light 1
It is introduced into the orbit reference optical system 115 to be output as 12. The reference light 108 is a 2 × 2 optical fiber coupler 10.
9 (branching ratio x: 1-x, x is arbitrary) is input to the input end port, and according to the branching ratio of the 2 × 2 optical fiber coupler 109,
Part of the light is output from the port as is as output light 112, and the other is output from the port as circulating light 110.

The circulating light 110 is input to the input end port of the 2 × 2 optical fiber coupler 109 again via the optical path delay / selector 111, part of which is output from the output end port as it is as output light 112, and the rest is output end again. It is output from the port as the circulating light 110 for the second round. The behavior of the circulating light 110 after that is the above-described repetition. By repeating these operations, the orbiting reference optical system 115 always outputs the orbiting reference light 1 from the output end port of the 2 × 2 optical fiber coupler 109.
12 can be output.

The N-th order determination method necessary for the absolute length measurement in this embodiment is the same as that in the first embodiment, and therefore it is omitted here to avoid duplication. This embodiment uses a fiber for the orbiting reference optical system, and by constructing it in a loop shape, even if the basic optical path length becomes long, it is possible to cope with miniaturization by winding the loop portion in multiple layers. .
As a result, the present embodiment is effective in expanding the length measurement range of the first embodiment and simultaneously reducing the size of the optical system.

In FIG. 10, reference numeral 100 is 1
A × 2 fiber directional coupler (coupler) 101 is a measurement light, 102 is a collimator lens, and 10
3 is a measurement light, 104 is a beam splitter,
105 is a measurement target, 106 is a measurement reflected light,
107 is a coupling lens, 113 is an interference 1 × 2 optical fiber coupler, and 114 is interference light.

(Third Embodiment) FIG. 11 (a),
The third embodiment will be described with reference to (b). The difference between this embodiment and the other embodiments (first and second embodiments) is that the optical amplifiers 120 and 123 are inserted in the respective loop optical paths of FIGS. 11A and 11B. is there.
Thus, the intensity attenuation of the circular reference light due to the transmittance and the branching ratio, which is unavoidable due to its optical configuration as in the first and second embodiments, is always guaranteed by the present embodiment, and by eliminating these constraints. , It is possible to dramatically increase the number of revolutions of the orbiting reference light, that is, to further extend the measurement range.

FIG. 11 (a) shows a case where the orbiting reference optical system of the first embodiment is replaced with an optical fiber 83 and an optical amplifier 120 is added (the same effect can be obtained by inserting a semiconductor optical amplifier even in the spatial optical system). However, the use of fiber makes these configurations easier and more compact). This action will be described.

The reference light 80 undergoes multiple reflection between the end faces 82 and 85 of the high reflection film to generate a circulating light 121, which is amplified by the optical amplifier 120, passes through the optical path delay / selector 84, and is amplified from the end face 85. It is output as the reference light 122. Further, FIG. 9B shows a case where the optical amplifier 123 is added to the optical configuration of the second embodiment. Since the operation until the circular reference light is obtained in the configuration is the same as that in the second embodiment, the description thereof is omitted here.

In the description of this embodiment, the process of determining the orbiting operation of the reference light and its N-th order and calculating the absolute distance is the same as that described in the first and second embodiments. Therefore, the description thereof is omitted. 1st from this embodiment
Also, the limitation of the measurement range due to the limitation of the number of turns due to the intensity attenuation of the orbiting reference light that is unavoidable in the configuration of the second embodiment due to the optical configuration of the orbiting reference light due to the transmittance and the branching ratio is limited to the number of turns of each embodiment. It is always guaranteed by inserting an optical amplifier in the optical path length, and by eliminating these constraints, the number of rounds of the round reference light is dramatically increased, that is, the length measurement range is further expanded.

In FIG. 10A, reference numeral 80 is reference light, which corresponds to reference numeral 90 in FIG. In FIG. 10B, reference numeral 124 is a circulating light, and reference numeral 124 in FIG.
The reference numeral 125 is the orbiting reference light and corresponds to the reference numeral 112 in FIG.

(Fourth Embodiment) FIGS. 12A and 12B.
The fourth embodiment will be described using. The present embodiment is different from the other first, second and third embodiments in the mechanical part that makes the circulating optical path length necessary for the Nth order determination variable.
In any of the first, second, and third embodiments, this part has a mechanism in which a moving stage and optical paths having different lengths can be selected due to a physical change in optical path length. Since all of these depend on the mechanical mechanism, it takes time for the Nth order determination, and at the same time, the size of the apparatus is increased and the loop loss is caused, which becomes a factor of limiting the length measurement range.

Therefore, in this embodiment, the variable mechanism of the circulating optical path length required for the Nth order determination is equivalent to the effect of changing the optical path length by changing the polarization direction of the light by utilizing the birefringence effect of the light. It has a feature that
As there is no need to insert an optical path delay / selector, as a result, it is possible to reduce the factors that hinder the extension of the measurement range, and to extend the measurement range, and also because the device configuration is simple and low cost, and the Nth order determination. It contributes to shortening the time.

In this embodiment, there is a big difference in the mechanism for changing the optical path length from the other embodiments, and the N-th order determination method after changing the optical path length is exactly the same as the other embodiments. is there. Therefore, except the matters peculiar to the present embodiment, the description of the method of absolute length measurement by the N-th order determination is redundant here, and the description is omitted.

The configuration and operation of this embodiment (a) will be described. A Fabry-Perot interference system in which reference light 80 having a certain linearly polarized light passes through a λ / 2 plate 131, and is formed of a birefringent optical fiber (polarization maintaining fiber) 130 and both end surfaces of high-reflectance mirror films 82 and 85. Entered in. Further, here, even if the reference light 80 is circularly polarized light, the λ / 2 plate 13
1 may be replaced with a deflector.

Then, the round-trip reference light 132 is generated by reciprocally reflecting the fiber type Fabry-Perot interference system, and the reference light for each round is output from the fiber end 85 every round trip of the fiber. The output light passes through the λ / 2 plate (or the polarizer) 133 according to the polarization state, and thus the circular reference light 134 having the same polarization direction as the input reference light 80 is output. One circular optical path length in the polarization state before determining this time N following (a p-polarized light) ^{_{L r p = n p L r}} .
(L _r ^p : physical fiber length viewed from p-polarized light, n _p : fiber refractive index viewed from p-polarized light).

When the beat frequency is detected and the Nth order is determined, each λ / 2 plate (or polarizer) 131 on the input / output side,
133 is rotated and the polarization direction is changed by 90 degrees (orthogonal), and the polarization state is changed to s-polarized light. Then, the birefringence difference in the birefringent optical fiber 130 caused by the difference in the polarization direction due to the birefringence effect can produce an effect equivalent to changing the physical length of the fiber.

That is, ΔL _r = L _r ^p −L _r ^s = (n _p
-N _s ) L _r . ( _Lr ^s : fiber physical length viewed from s-polarized light, n _s : fiber refractive index viewed from s-polarized light), and specifically, the refractive index difference (n _p −n _s ) is 10 ⁻² to 10 ^−3.
Even if the fiber length is 1 m, the optical path difference is 1 to 10 mm. If the optical path difference of this degree can be generated, the Nth order can be sufficiently determined, and the absolute distance can be calculated from this.

Similarly, the configuration and operation of FIG. 9B will be described. The configuration is a 2 × 2 polarization-maintaining fiber coupler 143.
Λ / 2 plate (or polarizer) 1 at the input and output ends
It is composed of 40 and 145. Reference light 108, 14
1 enters the orbiting reference optical system, and finally the orbiting reference light 1
The processes up to 44 and 146 are the same as those described in the second embodiment (FIG. 10), and therefore the description thereof is omitted here to avoid duplication. The method of obtaining the optical path difference by the polarization plane manipulation necessary for the Nth order determination is also the same as the above-described description of FIG. 12A, and therefore the description thereof is omitted here. Note that reference numeral 142 is a circulating light.

In FIG. 12B, the circulation style of the circulation reference light is different, but the effect is basically the above-mentioned, there is no need to separately insert an optical path delay / selector in the circulation optical path, and as a result, It is possible to reduce the factors that hinder the extension of the length measurement range, and to extend the length measurement range, simplify the device configuration to reduce cost and size, and shorten the Nth order determination time.

According to the present embodiment, even if the coherence of the light source is low, the measurement range can be extended to a long distance, and at the same time, the measurement accuracy can be maintained. Can be supplied at cost.

According to this embodiment, not only the relative displacement amount but also the Nth order of the rough measurement can be obtained, and by combining these, the absolute distance can be calculated at the same time.

[0096]

According to the optical frequency domain reflectometry apparatus of the present invention, the measurement range can be expanded without being limited by the coherence peculiar to any light source, and at the same time, the measurement accuracy can be improved with the expansion of the measurement range up to now. Even if the deterioration is eliminated and the measurement range is expanded, the measurement accuracy does not deteriorate.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a first embodiment of an optical frequency domain reflectometry device of the present invention.

FIG. 2 is a diagram showing a change over time in optical frequency sweep regarding the principle of detecting a beat signal by the orbiting reference light in the first embodiment.

FIG. 3 is a diagram showing a beat frequency spectrum regarding a principle of detecting a beat signal by the orbiting reference light in the first embodiment.

FIG. 4 is a diagram showing a locus of beat frequency values according to a measurement distance L, regarding the principle of detecting a beat signal by the orbiting reference light in the first embodiment.

FIG. 5 is a diagram showing a change over time in optical frequency sweeping with a change in a circulating optical path length in a method of determining the number of rounds N in the first embodiment.

FIG. 6 is a diagram showing a change in a beat frequency spectrum associated with a change in a circulating optical path length in a method of determining the number of rounds N in the first embodiment.

FIG. 7 is a diagram showing the determination of the number of laps N in the first embodiment.
Circulating optical path difference ΔL _rBeat frequency due to change
It is a figure which shows the locus | trajectory of a numerical value.

FIG. 8 is a diagram showing an example of actual measurement of a beat frequency spectrum by the circular reference light in the first embodiment.

FIG. 9 is a diagram showing a configuration of a modified example of the first embodiment.

FIG. 10 is a diagram showing a configuration of a second exemplary embodiment.

FIG. 11 is a diagram showing a main part configuration of a third embodiment,
(A) is a figure applied to 1st Embodiment, (b) is a figure applied to 2nd Embodiment.

FIG. 12 shows a main part configuration of a fourth embodiment,
(A) is a figure applied to 1st Embodiment, (b) is a figure applied to 2nd Embodiment.

FIG. 13 is a diagram showing a configuration of a conventional general optical frequency domain reflectometry device.

FIG. 14 is a diagram showing a beat signal detection principle in the apparatus of FIG. 13, (a) shows a change in optical FM wave, (b) shows a change in beat frequency, and (c) shows a beat signal intensity waveform. Is.

15 is a diagram showing beat frequency components regarding the beat signal detection principle in the apparatus of FIG. 13;

[Explanation of symbols]

1 oscillator 2 Semiconductor laser 3 optical FM output 4 beam splitter 5 Reference light 6 reference mirror 6a reflector 6b reflector 6c Reference optical unit 6d reference mirror 7 Object light 8 measurement target 9 Interference light 10 Photodetector 11 A / D converter 12 arithmetic unit 40 Interference beat frequency f between adjacent circular reference lights
_br 41 k = beat between the orbiting reference light and the measurement reflected light on the second lap
Frequency f_b1 42 k = beat of the reference light and the reflected light on the second orbit
Frequency f_b2 43 Measured reflected light 44 k = Orbiting reference light on the Nth lap 45 k = Orbiting reference light on the second lap 46 k = Orbiting reference light on the 3rd lap 47 Interference beat frequency spectrum between adjacent orbiting reference beams
Tram 48 k = beat between the reference light and the reflected light on the second orbit
Frequency spectrum 49 k = beat between the orbiting reference light and the measured reflected light on the third lap
Frequency spectrum 500 ≦ f_b1≤ f_br/ 2 range beat frequency f
_b1Relationship with absolute distance 53 Orbiting reference light on the 2nd orbit after the change of optical path difference 54 Reference light and measurement signal light for the second round after the change of the optical path difference
Beat frequency spectrum 59 Optical Isolator 60 Optical frequency modulation controller 61 light source 62 optical FM output 63 beam splitter 63a Beam splitter 64 Reflector 1 65 Orbiting light 66 Reflector 2 67 Moving stage 68 Fabry-Perot type orbiting reference optical system 69 orbit reference light 70 Beam splitter 71 Measurement signal light 72 measurement target 73 Measured reflected light 74 Interfering light 75 Light receiver 76 Signal processing unit 80 reference light 82 Input side high reflection film end face 83 optical fiber 84 Optical Path Delay / Selector 85 Output side high reflection film end face 94 f in actual measurement example_b1Beat frequency spectrum
Mu 95 f in actual measurement example_b2Beat frequency spectrum
Mu 96 f in actual measurement example_brBeat frequency spectrum
Mu 100 1 × 2 fiber directional coupler (coupler) 101 Measuring light output 102 collimating lens 103 measuring light 104 beam splitter 105 measurement target 106 Measurement reflected light 107 coupling lens 108 Reference light output 109 2 × 2 optical fiber coupler 110 orbiting light 111 Optical path delay / selector 112 orbit reference light 113 1 × 2 optical fiber coupler for interference 114 interference light 115 Orbiting reference optical system 116 optical fiber 116a Input part of optical fiber 116b Output part of optical fiber 120 optical amplifier 121 Circulating light in fiber Fabry-Perot interferometer 122 Orbiting reference light 123 Optical amplifier 124 Orbiting light 125 orbit reference light 130 Polarization maintaining fiber 131 Input side polarizing element 132 Orbiting light in fiber Fabry-Perot interferometer 133 Output side polarizing element 134 orbit reference light 140 Polarizer on input side 141 reference light 142 Orbiting light 143 2 × 2 polarization-maintaining optical fiber coupler 144 orbit reference light output 145 Polarizer on output side 146 Orbiting reference light whose polarization plane is adjusted

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Claims (13)

(57) [Claims] 1. A light emitting unit capable of varying the wavelength of light in a required cycle; a light distributing unit that distributes light output from the light emitting unit to generate first reference light and measurement light; A reference optical unit that generates a second reference light based on the reference light, a measurement reflected light obtained by reflecting the measurement light on a measurement target, and the second reference light are combined to generate an interference light. An optical multiplexer and, based on the interference light, a frequency difference between the measurement reflected light and the second reference light is detected as a measurement light frequency difference, and the measurement target is based on the detected measurement light frequency difference. A detection unit that obtains a measurement amount regarding the reference optical unit, wherein the reference optical unit outputs the light as a result of the first reference light reciprocating or circling a variably settable distance N (N ≧ 2) times. 2. An optical frequency domain reflectometer capable of generating as reference light. 2. The optical frequency domain reflectometry device according to claim 1, wherein the detection unit obtains a measurement amount of the measurement target by adding a rough portion and a fine portion, and the fine portion. Is calculated based on first data indicating the measured light frequency difference corresponding to a first distance, the rough portion is calculated based on second to fifth data, and the second data is A change in the measurement optical frequency difference corresponding to the second distance different from the first distance and the measurement optical frequency difference corresponding to the first distance, the third data indicating the first distance. The fourth data indicates the direction of the change, and the fifth data indicates the reference light frequency difference and the second distance according to the first distance.
The change amount of the reference light frequency difference related to the distance is indicated, and the reference light frequency difference related to the first distance is obtained as a result of the first reference light reciprocating or circling the first distance k (k ≧ 0) times. Is the frequency difference between the second reference light associated with k and the second reference light associated with k + 1 as a result of the first reference light reciprocating or orbiting the first distance k + 1 times; The reference light frequency difference related to the distance is the second reference light related to k as a result of the first reference light reciprocating or orbiting the second distance k times, and the first reference light is the second reference light. The optical frequency domain reflectometry device which is the frequency difference of the second reference light with respect to k + 1 as a result of reciprocating or orbiting the distance of k + 1 times. 3. A light emitting section capable of changing the wavelength of light at a required cycle, an input light generating section for generating input light based on the output light output from the light emitting section, and distributing the input light. A light distributor for generating reference light and measurement light, a measurement reflection light in which the measurement light is reflected on a measurement target, and a light multiplexer for generating interference light by multiplexing the reference light, Based on the interference light, the frequency difference between the measurement reflected light and the reference light is detected as a measurement light frequency difference, based on the detected measurement light frequency difference, the detection unit for determining the measurement amount for the measurement object, The input light generation unit is capable of generating, as the input light, a light as a result of the output light reciprocating or circling a variably settable distance N (N ≧ 2) times. Reflection measuring device. 4. A light emitting section that outputs light whose frequency changes at a set cycle, and a first beam splitter that distributes the light output from the light emitting section to generate first reference light and measurement light. A reference optical unit that generates a second reference light based on the first reference light, a measurement reflected light obtained by reflecting the measurement light on a measurement target, and a second reference light are overlapped to generate interference light. A second beam splitter to generate, a detection unit that detects a frequency difference between the measurement reflected light and the second reference light based on the interference light, and determines a measurement amount regarding the measurement target based on the frequency difference, Wherein the reference optical unit has a first and second reflecting mirror, and an interval changing unit that changes a reflecting mirror interval between the first and second reflecting mirrors, The reflecting mirror transmits and inputs the first reference light, And the second reflecting mirror cooperate with each other to reflect the first reference light a plurality of times between the first and second reflecting mirrors, and circulate or reciprocate the reflecting mirror interval a plurality of times. Circulating light is generated, and the second reflecting mirror causes the circulating light to have an interval of the reflecting mirror of 1
An optical frequency domain reflectometry device that outputs the second reference light that partially transmits the orbiting light every time the light orbits or turns. 5. A light emitting unit that outputs light whose frequency changes at a set cycle, and a first optical fiber coupler that distributes the light output from the light emitting unit to generate a first reference light and a measurement light. A reference optical section that generates a second reference light based on the first reference light; a measurement reflected light obtained by reflecting the measurement light on a measurement target; Detecting a frequency difference between the measurement reflected light and the second reference light based on the interference light, and obtaining a measurement amount related to the measurement target based on the frequency difference. And a third optical fiber coupler having first and second input ports and first and second output ports, and an input unit and an output unit. The third of the third optical fiber couplers An optical fiber connected to an output port of the optical fiber, the output section being connected to the second output port of the third optical fiber coupler, and an input / output section provided in the middle of the input / output section of the optical fiber. And an optical path changing unit that changes the optical path length of the optical fiber, the optical fiber generates circular light that circulates a plurality of times between the input and output units, and the third optical fiber coupler includes the first input port. The first reference light inputted from the first reference light is generated to generate first and second branched lights, the first branched light is output from the first output port as the second reference light, and the second reference light is output. Is output from the second output port to the optical fiber as the nth (n ≧ 1) th circulating light, and the nth circulating light input from the second input port is branched. To generate third and fourth split lights, The third branched light is output from the first output port as the second reference light, and the fourth branched light is output from the second output port to the optical fiber as the (n + 1) th circulating light. An optical frequency domain reflectometry device for inputting the (n + 1) th orbiting light as the nth orbiting light from the second input port. 6. The optical frequency domain reflectometry device according to claim 4 or 5, further comprising an optical amplifier for amplifying the intensity of the circulating light. 7. A light emitting section that outputs light whose frequency changes at a set cycle, and a first beam splitter that distributes the light output from the light emitting section to generate first reference light and measurement light. A reference optical unit that generates a second reference light based on the first reference light, a measurement reflected light obtained by reflecting the measurement light on a measurement target, and a second reference light are overlapped to generate interference light. A second beam splitter to generate, a detection unit that detects a frequency difference between the measurement reflected light and the second reference light based on the interference light, and determines a measurement amount regarding the measurement target based on the frequency difference, The reference optical unit includes first and second reflective film surfaces, a birefringent optical fiber provided between the first and second reflective film surfaces, and a first reflective film surface of the birefringent optical fiber. It is provided in the front stage and the rear stage of the second reflective film surface, First and second polarizing sections that determine the polarization directions of light input to and output from the birefringent optical fiber, and the first and second polarizing sections are rotated and determined by the first and second polarizing sections. And a rotation unit that changes the polarization direction, the first reflection film surface transmits and inputs the first reference light through the first polarization unit, and the first and second Each of the reflective film surfaces cooperates with each other to reflect the first reference light a plurality of times between the first and second reflective film surfaces, and thereby between the first and second reflective film surfaces. It generates orbiting light that makes multiple turns or reciprocates, and the second reflecting film surface causes the orbiting light to rotate each time the orbiting light makes one revolution or reciprocation between the first and second reflecting film faces. Frequency domain reflection that partially transmits light and outputs the second reference light that has passed through the second polarization section. Constant apparatus. 8. A (a) light whose frequency change at a set time interval, and to provide as an input light, (b) comprises a revolving portion, and the first from the proximal end point to a final end point and to provide an optical path, and to provide a second optical path to the end point through the measurement object from the (c) the group endpoints, the branched at the base end points the input light (d)
Outputting to the first optical path and the second optical path, and (e) the light output to the first optical path,
Reciprocating or circling 0 to N times each minute
Generates a plurality of orbiting reference beams, and
Providing a reference light composed of illumination light to the terminal point, and (f) measuring the light output to the second optical path as the measurement target.
Generating measurement light by reflecting the measurement light and providing the measurement light to the terminal point, and (g) the reference light and the measurement light at the terminal point.
To generate an interference light; (h) to detect a beat frequency of the interference light; and (i) to measure a measurement amount related to the measurement target based on the beat frequency. and Ru optical frequency domain reflectometry method name comprises a seeking. 9. The optical frequency domain reflectometry method of claim 8, wherein in the (h), among the plurality of orbiting reference light closest circulating reference light optical path length between the measurement light which it propagates detection frequency difference between the light and <br/> the measurement light as the beat frequency
Optical frequency domain reflectometry methods. 10. The optical frequency domain reflectometry method according to claim 9, wherein the plurality of orbiting reference lights are generated at a second set time interval, and the optical path length differences between the optical paths through which they are propagated. A method for measuring optical frequency domain reflectivity that is generated so that the two are the same. 11. A (m) light whose frequency change at a set time interval, and to provide as a first input light, wherein the (n) winding portions, and, first from the base end point to a final end point 1) to provide a second optical path from the base end point to the terminal point via a measurement target , and (p) split the first input light at the base point . do it
Outputting to the first optical path and the second optical path, and (q) outputting to the first optical path in (p).
Light is reciprocated 0 to N times in the orbiting portion.
Or a plurality of first orbiting reference beams
To generate a first reference light composed of the plurality of first orbiting reference lights.
And to provide to the end point, (r) the second of the measurement target light output to the optical path
Generate a first measurement light by reflecting
Providing the first measurement light to the termination point, and (s) at the termination point, the first reference light and the first reference light.
Generating a first interference light by combining the measurement beam, and detect a first beat frequency from (t) said first interference light, for changing the optical path length of (u) the winding portions things and, (v) after the optical path length of the winding portions is changed, the time
Light whose frequency changes at intervals is used as the second input light
And (w) splitting the second input light at the base end point.
Outputting to the first optical path and the second optical path, and (x) outputting to the first optical path in (w).
Light is reciprocated 0 to N times in the orbiting portion.
Or two orbiting reference lights
The second reference light composed of the plurality of second orbiting reference lights.
And (y) is output to the second optical path in (w).
Reflected light by the measuring object
2 measuring light is generated and the second measuring light is provided to the terminal point.
To that and, (z) at the end point and the second reference beam and the second
Generating a second interference light by combining the measurement light, and possible to leave <br/> detects the second beat frequency from (aa) the second interference light, (ab) the first and based on the second beat frequency, <br/> optical frequency domain reflectometry method in which a possible to find the measured quantity relating to the measurement target. 12. The optical frequency domain reflection according to claim 11.
In the measurement method, the measurement amount is an optical frequency that is an absolute distance of the measurement target.
Area reflection measurement method. 13. The optical frequency domain reflectometry method according to claim 11 , wherein the plurality of first orbiting reference lights are the first orbiting reference beams.
The optical path length differences between the light paths that propagate light are the same.
And the first beat frequency is generated by the plurality of first orbiting reference lights.
Of the optical path length of the first measurement light
The orbiting reference light closest to the optical path length and the first measurement light
Is detected as a frequency difference between the second beat frequency and the second beat frequency.
Of the optical path length through which the second measurement light propagates
The orbiting reference light closest to the optical path length, and the second measurement light
Is detected as the frequency difference of, and in (m), the optical path length of the circulating portion is
The first orbit reference used to calculate the beat frequency of 1
Calculation of optical path length through which light propagates and the second beat frequency
And the optical path length of the second orbiting reference light used for
The optical path difference is the optical path through which the plurality of first orbiting reference lights propagate.
An optical frequency domain reflectometry method that is modified so that the optical path length difference between them is not exceeded .