JPH04107421A - Optical waveguide type phase modulator - Google Patents

Abstract

PURPOSE:To minimize bias variation due to reflected light on an end surface connected to an optical propagation path by providing a reflection coefficient equal to that of a conventional optical waveguide phase modulator and giving an optical path difference between reflected light beams on the end surface connected to the optical propagation path when a 1st and a 2nd light beam are projected on the optical propagation path. CONSTITUTION:In the optical propagation path coupled optically with the optical fiber coil, an incident light beam is distributed as the 1st and 2nd light beams which prescribe both directions and light beams which are propagated in the optical fiber coil in both directions are coupled with an optical distribution and coupling part. When the 1st and 2nd light beams are projected on the side of the optical propagation path, the optical path difference is given between the respective reflected light beams B1 on the end surface 11 connected to the optical propagation path. Consequently, the bias variation due to the reflected light beams on the end surface 11 connected to the optical propagation path is made narrow.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a phase modulator for an optical fiber gyro, and more specifically, the present invention relates to a phase modulator for an optical fiber gyro, and more particularly, the present invention relates to a phase modulator for an optical fiber gyro. Wavelength light is simultaneously propagated in the clockwise and counterclockwise directions and phase modulated, and the phase difference of the light based on the Sagnac effect is detected from the intensity of the interference light of the light propagated in both directions to determine the angular velocity or angle. The present invention relates to an optical waveguide type phase modulator that performs the above phase modulation in an optical fiber gyro configured to obtain an output indicating an increment.

[Conventional technology]

FIG. 5 schematically shows a part of the configuration of an optical fiber gyro using a conventional optical waveguide type phase modulator.

In the figure, 1 is a light source, 2 is an optical splitter/coupler, 3 is an optical waveguide type phase modulator, and 4 is a polarization plane-preserving device wound perpendicularly to the rotation axis.
An optical propagation path formed of a mode optical fiber, 5 a photoelectric conversion circuit, 201 a demosi j-rake, 202 a sine wave phase modulation circuit, 203 a sawtooth phase modulation circuit, 20
4, 205.206 are the output signals of each circuit, and 20
7 indicates a gyro output signal. In addition, the optical waveguide type phase modulator 3, for example, H, C, 1, EFEVRE, etc.
rlNTE(JATED 0PTIC3: A
PRACTICAL SOL[lTl0N FORTII
E FIBER-OPTICGYRO3COPEJ (
PROCEEDING OF 5PIE, Vol, 7
19. Fl, BER-OPTICGYRO3pp, 1
01.1.986).

In the configuration shown in FIG. 5, the first beam 11 emitted from the light source 1 enters the optical splitting/coupling device 2, is divided into two beams, and is split into two beams.
, becomes the third light beam. The second light beam travels in the direction of the solid arrow and enters the optical waveguide type phase modulator 3 made of a material having an electro-optic effect, such as LiNb0a. The second light beam incident on the optical waveguide type phase modulator 3 transmits only a certain polarized wave through the polarizer 3a formed in the phase modulator, and is split into two at the Y branch 3b. Fourth
, becomes the fifth light beam.

The fourth light beam travels from the Y branch 3b in the direction of the dashed arrow, and enters the sine wave phase modulator 3c with Φ□・sin(ω,
1) undergoes sine wave phase modulation. Here, Φ1 is the maximum phase deviation of sinusoidal phase modulation, ω is the angular frequency of sinusoidal phase modulation, and t represents time. The fourth light beam that has undergone sinusoidal phase modulation is a sixth light beam that enters the light propagation path 4 and a seventh light beam that is reflected at the connection end surface 3e between the light propagation path 4. This results in a foul beam Bl) in Figure 6. The sixth light beam propagates counterclockwise through the optical propagation path 4 and then enters the optical waveguide phase modulator 3 again. In an ideal case, the sixth light beam re-entering the optical waveguide type phase modulator 3 is qω-1(1-τ) due to the sawtooth wave phase modulator 3d formed within the phase modulator. [−π～+π[radl
It receives a sawtooth wave phase modulation of [periodic function]. here,
q represents the slope (polarity) of the sawtooth wave, ω5t represents the sawtooth wave phase modulation angular frequency, and τ represents the propagation delay time of the optical propagation path 4. The sixth light beam subjected to sawtooth phase modulation is directed to the Y branch 3b.
to be re-injected.

On the other hand, the seventh light beam re-enters the sinusoidal phase modulator 3c and undergoes a sinusoidal phase change of Φ, ·sin [(ω1, n(t-tl)]. Here, tl is Indicates a time that is twice the propagation delay time from the sine wave phase modulator 3c to the connection end surface 3e between the optical propagation path 4 (ω.tl<1).The seventh light beam subjected to sine wave phase modulation re-enters the Y branch 3b.

On the other hand, the fifth light beam travels from the Y branch 3b in the direction of the dashed-dotted line arrow, and in the sawtooth wave phase modulator 3d, qωs
It receives sawtooth wave phase modulation of Lj. The fifth light beam that has undergone sawtooth phase modulation is converted into a ninth light beam (a ninth light beam) that is reflected at the connection end surface 3e between the eighth light beam incident on the light propagation path 4 and the light propagation path. This becomes the reflected beam B2) in Figure 6.

The eighth light beam propagates in the clockwise direction through the optical propagation path 4 and then enters the optical waveguide phase modulator 3 again. The eighth light beam re-entering the optical waveguide phase modulator 3 undergoes sinusoidal phase modulation of Φ, ·sin [(ωm(moτ)) 1 in the sinusoidal phase modulator 3c, and then passes through the Y branch 3b. On the other hand, the ninth light beam re-enters the sawtooth phase modulator 3d and undergoes sawtooth phase modulation of qω5t(t-tz), where t2 is the sawtooth phase modulator 3d
to the connection end surface 3e with the optical propagation path 4 (indicates a time twice the general delay time (ωt, +< 1.). The ninth optical beam subjected to sawtooth wave phase modulation If, Y branch 3b
to be re-injected.

The sixth and eighth light beams and the seventh light beam re-entered the Y branch 3b.
, the ninth light beam is recombined into the tenth and eleventh light beams, respectively. The tenth and eleventh light beams are incident on the polarizer 3a, and only certain polarized components are transmitted therethrough, and the light beams are incident on the optical splitting coupler 2. The tenth and eleventh light beams incident on the optical splitting coupler 2 are divided into two to become the eleventh, twelfth, thirteenth, and fourteenth light beams, respectively.
Of these, the 12th and 14th light beams are connected to the photoelectric conversion circuit 5.
incident on . Here, the 12th light beam is a light beam having phase difference information due to the Sagnac effect, and the 14th light beam is a disturbance beam caused by the end face reflected light of the optical waveguide type phase modulator 3.

A photoelectric conversion output signal 20 which is an output signal of the photoelectric conversion circuit 5
4, it is input to the demosimulator 201 together with the sine wave phase modulation signal 205 which is the output of the sine wave phase modulation circuit 202, and the scalar product of both signals is obtained there. The output signal 206 of the demosimulator 201 is expressed by the following equation.

Vo knee (2/π) PG (VS 1,000 Vn)・・・・・・(
1)■3−γ[φS]Jl(η1) Xsin(φ3”−qω1τ)・・・・・・・・・・・・
・(2) VR=to γ[φR]Jl(η2) Xsin[φ. +θR1qω□(2t-tz)]...
...(3) η1-2Φi5in(ω□τ/2)...
・・・・・・・・・・・・・・・(4) η2−2Φ,
cos(ωmjl/2)ζ2Φ,...(
5) T[φs]-exp[-(λφs, /2/a/I
−C) 2] −(6)T[φR1=exp[(λφR
/2/JV;J'E/L c) 2]...(7
)φs−4πRLCΩ/λC・・・・・・・・・・・・
・・・・・・・・・(8)φ□=2πLR/λ・・・・
・・・・・・・・・・・・・・・・・・・・・・・・
・(9) LC-λ2/δλ・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・(10) Here, ■o is the demosimulator output signal, and P is the clockwise or counterclockwise direction of input to the photoelectric conversion circuit. , T[φ1 is the coherence degree of the 12th light beam, T[φR] is the coherence degree of the 14th light beam, J
, is a linear Bessel function, η is the phase modulation degree of the 12th light beam, η2 is the phase modulation degree of the 14th light beam, K(<
1) is the reflection coefficient at the connection end face with the optical propagation path 4, φ3
where R is the optical propagation path radius, LF is the optical propagation path length, Ω is the manual rotation angular velocity, λ is the wavelength of light in vacuum, C is the speed of light in vacuum, φ7 is the seventh and The phase difference between the ninth light beam, LR is the optical path length difference between the seventh and ninth light beams, θ is the additional phase due to end face reflection at the connection end face with the optical propagation path 4, r -C is the coherence length, and δλ is the half-width of the spectrum of the light emitted from the light source 1 in vacuum.

Note that equations (6) and (7) are based on J, 1. r DEGREEOF POLARIZATION of SAKΔ■ etc.
TN ANISOTROPICSINGLE-M
ODEOPTICAL FIBER5: THEO
RYJ (IEEE J, QUANTUMEL
ECTRON1, Vol, QE-18, pp, 488
-495.1982), this holds true for light with a Gaussian distribution spectrum, such as the light emitted from a super luminescent diode, which is often used as a light source for optical fiber gyros.

Now, equation (3) can be rewritten as follows.

VR=KT[φIIIJI(η2)
)] ... (11) φ8. −φto1−θR+(1ω,
t(2t-tz)φsclωstτ・・・・・・・・・
(12) From equation (11), equation (1) can be rewritten as follows.

V, CC-(2/π)P.

XAs1n(φs +q, 6) st T: ”−φ
No...(13) A2-[T[φs], r+(η
, ) 10 KT [φRIJI (772) C03 (φR1) 12+
[Kγ[φR]Jl(772) C03(φR1)]2
...-...(14) φEl = tan-'(K 7
[φR]Jl(772) C03(φR1)/[T[
φS]Jl(η,)”− to γ[φIIIJI(η),) cos(φR,)])
(15) Here, ■ γ[φ3]-1 in the normal input angular velocity range, ■ ω to reduce bias fluctuation. −π/τ, that is, η1=η2, ■From the definition of coherence, γ[φR1≦1, ■Reflection coefficient K<
1, Equations (14) and (15) can be expressed as follows.

A':r[φS]Jl(η,)・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・(16)φEl
';tan-' l[K r [φRI CO3(φ
, 1) 1≦Kr[φR1・・・・・・・・・・・・・・・
(17) [Problems to be Solved by the Invention] In the optical fiber gyro using the conventional optical waveguide type phase modulator described above, the sawtooth wave phase modulation circuit 203 is Demosimulator output signal ■ expressed by equation (13). (
206) is zero, the sawtooth wave phase modulation signal 207
The angular frequency ω and polarity q are changed. Therefore, even when the input angular velocity is zero, in other words, even when the phase difference φ5 due to the Sagnac effect is zero, the angular frequency ωst and polarity q corresponding to the error phase φ become the error bias. Furthermore, since the additional phase θ8 due to end face reflection changes randomly, bias fluctuation occurs.

In addition, the conventional optical waveguide type phase modulator 3 is shown in FIG. 6(a).
, as shown in (b), the modulator and the optical propagation path 4
The connection end surface 3e between the two is perpendicular to the light propagation direction (
(see figure (a)) and is inclined by about 10 degrees in the thickness direction of the modulator (see figure (b)), and is formed by polishing. Therefore, the reflection coefficient is reduced to about 10-5, but since there is no difference in optical path length between the seventh light beam and the ninth light beam (LIl=O), equations (7) and (9 ), T
[φR]=1. Therefore, from equation (17), l
φl:1 ≦104 [rad], and the optical scale factor 5F (=4πRLF/λC) is 1 (d
c4/('/5ec)], ±2 ['/hrl
This will result in a bias variation of .

In other words, in an optical fiber gyro using a conventional optical waveguide type phase modulator, the optical path length difference LR between the reflected lights B+ and Bz at the connection end surface 3e between the phase modulator and the optical propagation path 4 is zero. Therefore, there was a drawback that bias fluctuations caused by reflected light were large.

The present invention was created in view of the problems in the prior art described above, and is an optical waveguide type phase modulator that can minimize bias fluctuations caused by repulsed light at the connection end face with the optical propagation path. is intended to provide.

[Means to solve the problem]

In order to solve the above problems, according to the present invention, light is simultaneously propagated from both directions of an optical fiber coil cooperating with a rotating shaft, phase modulated, and the phase difference of the light due to the Sagnac effect is detected to increase the angular velocity or angular increment. An optical waveguide type phase modulator that performs the phase modulation in an optical fiber gyro configured to obtain an output indicating the direction of The optical fiber coil is provided in an optical distribution/coupling unit that divides the light into a defined first and second light beam and combines the light from both directions propagated through the optical fiber coil, and the first and second light beams are connected to the optical propagation path side. An optical waveguide type phase modulator is provided, characterized in that the optical waveguide type phase modulator is formed so that the optical path lengths of the respective reflected lights at the connection end face with the optical propagation path are different when the light is emitted to the optical propagation path.

[Effect]

According to the above-mentioned configuration, it has a reflection coefficient (K) equivalent to that of a conventional optical waveguide type phase modulator, and has a reflection coefficient (K) equivalent to that of a conventional optical waveguide type phase modulator, and An optical path length difference is given between the respective reflected lights at the connection end face with the optical propagation path. Thereby, bias fluctuations caused by reflected light at the connection end face between the present optical waveguide type phase modulator and the optical propagation path can be minimized.

Note that other structural features and details of the operation of the present invention will be explained using the embodiments described below with reference to the accompanying drawings.

〔Example〕

FIGS. 1(a) and 1(b) schematically show the shape of an optical waveguide type phase modulator as an embodiment of the present invention.

The optical waveguide type phase modulator 10 of this embodiment is applied in place of, for example, the optical waveguide type phase modulator 3 of the optical fiber gyro shown in FIG. Connection end face 11 of modulator 10
, 12 is a plane containing the fourth and fifth light beams (see explanation in FIG. 5) divided into two at the Y branch 3b with respect to the light propagation direction as shown in FIG. 1(a). It is polished and formed with an inclination (angle of inclination α) inside. In addition, in the thickness direction of the modulator, as shown in FIG. 3(b), the connection end surface LL12 is polished and formed without being inclined (vertically).

The operation of the optical fiber gyro using the optical waveguide phase modulator of this embodiment will be described below.

According to the configuration of this embodiment, the seventh light beam (B1) and the ninth light beam (B2) are reflected light at the connection end surface 11 between the optical waveguide type phase modulator 10 and the optical propagation path 4. Between,
An optical path length difference LR (≠0), represented by 2 below, occurs.

Le+ -2n+, +' I-c -2nW HHw ・tan(90°-a) ---
・=−(18) Here, n is the optical waveguide type phase modulator 1
0, B6 is the geometric propagation length difference between reflected lights, Hw is the optical waveguide branch width of the Y branch 3b, and α is the end face inclination angle.

Furthermore, in the optical waveguide type phase modulator of this example,
Bias variation width β required for optical fiber gyro
For [rad/sec or-r], exp [-(
πL R/good m/L c ) 2] ≦β・SF・
It is characterized in that the following relationship (1, 9) holds true. Note that SF is the optical scale factor (-4πRL
F/λC).

Usually, the wavelength λ of a light source used in an optical fiber gyro is 0.84 [μm], the coherence length is ~50 [μm1], and the refractive index n9 of the phase modulator of the optical waveguide type phase modulator is ~2.
2 (in the case of LiNb03), optical waveguide branch width H9b1
25 [μm1. Here, if the end face inclination angle α is set to approximately 80° in order to have a reflection coefficient equivalent to that of a conventional optical waveguide phase modulator, then from equation (7), the coherence T [φ becomes T[φR1#5.45X10-”.

Therefore, in the optical fiber gyro using the optical waveguide type phase modulator of this embodiment, the error phase φ becomes φ, ≦5.45×10−29 [rad] since the reflection coefficient is ~10″5. , optical scale factor 5F (-4KRLF/λC)
When is 1 [deg/(0/5ec)], the bias fluctuation due to the end face reflected light of the optical waveguide phase modulator is ±10-23 ['/hr], which is a value that can be practically ignored. .

In this way, according to this embodiment, when the two light beams that have the same reflection coefficient K as the conventional optical waveguide type phase modulator and are split into two at the Y branch 3b are emitted to the optical propagation path side, An optical path length difference is provided between the respective reflected light beams B1 and B2 at the connection end face 11 between the light propagation path and the light propagation path. Therefore, bias fluctuations caused by reflected light at the connection end face 11 with the optical propagation path can be minimized.

In the embodiment described above, the connection end faces 11 and 12 of the modulator 10 are inclined at an inclination angle α with respect to the light propagation direction (the first
(see figure (a)) and perpendicular to the thickness direction (see figure (b)), but the shape of the modulator is not limited to this.

For example, as shown in FIG. 2(a), one of the connecting end surfaces (12a) of the modulator may be formed perpendicular to the light propagation direction, or as shown in FIG. 3(a). One of the connection end faces (12b) of the modulator may be formed to be inclined in the direction opposite to that shown in FIG. ), the other connecting end surface (11, a) of the modulator may be formed to be inclined in the thickness direction.

Further, in the above embodiment, the polarizer 3a and the sawtooth wave phase modulator 3d are formed within the optical waveguide type phase modulator, but
Similar effects can be obtained even if this is not formed within the modulator. Furthermore, the optical waveguide type phase modulator of the present invention includes a sawtooth wave phase modulator 3d and a sawtooth wave phase modulation circuit 2.
The present invention can also be applied to a simple phase modulation type optical fiber gyro that is not equipped with 03.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain an optical fiber gyro in which bias fluctuations caused by reflected light at the connection end face with the optical propagation path are minimized.

[Brief explanation of drawings]

FIGS. 1(a) and (b) are diagrams schematically showing the shape of an optical waveguide type phase modulator as an embodiment of the present invention, and FIGS. 2(a), (b) to 4( a) and (b) are diagrams showing a modification of the embodiment of FIG. 1; FIG. 5 is a diagram partially schematically showing the configuration of an optical fiber gyro using a conventional optical waveguide type phase modulator; 6(a) and (b) are diagrams schematically showing the shape of the phase modulator in FIG. 5. (Explanation of symbols) ■... Light source, 2... Optical distribution coupler, 4... Optical propagation path, 5... Photoelectric conversion circuit, 10.10a, 10b,
10c... Optical waveguide type phase modulator, ILlla...
・Connection end surface (between the optical propagation path) of the phase modulator, Bl,
B2. B+... Edge reflected light, 201... Demosimulator, 202... Sine wave phase modulation circuit, 203...
Sawtooth phase modulation circuit.

Claims (1)

[Claims] 1. Light is simultaneously propagated from both directions of an optical fiber coil that co-moves with the rotation axis, phase modulated, and the phase difference of the light due to the Sagnac effect is detected to produce an output that indicates the angular velocity or angular increment. An optical waveguide type phase modulator that performs the phase modulation in an optical fiber gyro according to the present invention, the optical waveguide type phase modulator performing the phase modulation in the optical fiber gyro, the first and second beams defining the two directions of the incident light beam in the optical propagation path optically coupled to the optical fiber coil. Provided in an optical distribution/coupling unit that divides the light into a second light beam and combines the light from both directions propagated through the optical fiber coil, and when the first and second light beams are emitted to the light propagation path side. An optical waveguide type phase modulator characterized in that the optical waveguide type phase modulator is formed so as to have a difference in the optical path length of each reflected light beam (B_1, B_2) at a connection end surface (11, 11a) between the optical propagation path. 2. The connecting end surface with the light propagation path is inclined with respect to the light propagation direction within a plane that includes the incident light beam and the first and second light beams, and is formed by polishing. The optical waveguide type phase modulator according to claim 1, characterized in that: 3. The connection end surface with the optical propagation path is expressed by the following formula: L_R=2n_w・H_w・tan(90°−α), and exp[−(πL_R/√(1n2)/L_C)^2
]≦β・SF, where L_R is the optical path length difference between the reflected light beams at the connection end surface, n_w is the refractive index of the optical waveguide part of the phase modulator, and H_w is the difference between the first and second light beams in the optical waveguide. The beam spacing, L_C is the coherence length of the light beam with phase difference information due to the Sagnac effect, β is the bias variation width required for an optical fiber gyro, and SF is the optical scale factor. The optical waveguide type phase modulator according to claim 2, characterized in that: