JPH01250909A - Elliptical jacket type polarization plane maintaining optical fiber - Google Patents

Abstract

PURPOSE:To optimize the structure as a polarizer and to improve characteristics by forming a clad and support to the same refractive index as the refractive index value of silica and specifying the specific refractive index difference of the core with respect to the clad, the specific refractive index difference of an elliptical jacket with respect to the clad and the mode double refractive indices. CONSTITUTION:The clad 2 and support 4 of the polarization plane maintaining type optical fiber having a 4-layered structure are formed to the same refractive index as the refractive index value of silica. The specific refractive index difference of the core 1 with respect to the clad specified to <=0.3%, the specific refractive index difference of the elliptical jacket 3 with respect to the clad 2 to -0.15--0.25% and the mode double refractive indices to >=4.52X10<-4>. The structure as the polarizer is thereby optimized. The polarizer which is excellent in characteristics together with extinction ratio and insertion loss is obtd. by winding the optical fiber at a certain radius.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an ellipsoidal, get-type, wavefront preserving optical fiber suitable for use as an optical fiber polarizer.

[-Prior art 1] In order to construct a polarizer using a polarization-maintaining optical fiber, it is necessary to use a polarizer that does not exhibit absolute single polarization operation in which one mode of two orthogonal eigenharmonic modes is blocked in the 1υ1 state. (absolutely single polarization maintaining optical fiber) is preferably used. In this regard, the applicant! 1. +i Gansho 61-12
/1663 specification, Japanese Patent Application No. 61-124G (C4-ri specification and Japanese Patent Application No. 61-203751 specification S, respectively, to No. 6-1-1(a) (C).

717I(a)-1c) and Figures 8(a)-(C)
We have proposed an absolute J-toe polarization maintaining optical fiber as shown in the figure below.

The optical fiber 61 shown in FIGS. 6(a) to 6(C) has an MC (mudded crat) type refractive index distribution on the short axis side of the elliptical jacket 62, and a W type refractive index distribution on the long axis side. Absolutely single polarization operation is possible by utilizing the fact that the cutoff wavelength of the fundamental mode does not exist in the MC type but exists in the W type.

On the other hand, the optical fiber 71 shown in FIGS. 7(a) to (C) forms a W-shaped refraction/45 distribution on both the short axis side and the long sleeve side of the elliptical jacket 72, but the extinction depths are different from each other. It has the following. In a W-type fiber, the single mode operating range is determined by the depth of the fiber, so by making a difference in the depth of the pot, the single mode operating range of modes polarized to the short axis side and the long axis side can be increased. There is an operating range in which one is a low-loss guided mode and the other is a high-loss leaky mode, and this is utilized to make it possible to convert the absolute Ji'-(Q wave.

Furthermore, the optical fiber 81 shown in FIGS. 8(a) to (C) is
The optical fiber shown in the figure is difficult to manufacture because it is necessary to divide the refractive index distribution of the elliptical jackets 62 and 72 in the circumferential direction so that the refractive index in the short axis direction and the long sleeve direction are different. This is an improvement on the poor yield of 4S of optical fibers 7 and 7, which is as low as the price, and the cladding 2. In a polarization-maintaining optical fiber of 4 ellipses I/1 structure provided with an elliptical jacket 3 and a sabot 4, the elliptical jacket 3 has a refractive index lower than that of the cladding 2, and the support 4 has a refractive index equal to that of the cladding 2. , the elliptic jacket 1-3 has an ellipticity of 25 to 40% and a relative refractive index of -0.05 to -0.03% with respect to the cladding 2. Since the elliptical jacket 3 has the same refractive index in its minor axis direction and major axis direction, it has the advantage that it can be manufactured easily.

However, the elliptical jacket type polarization maintaining optical fibers shown in FIGS. It is not optimal for the intended use, i.e., it is not optimized as a polarizer because the effect of mode birefringence was not taken into account, and therefore its properties, such as extinction The ratio, insertion loss, etc. were also not the best.

An object of the present invention is to provide an elliptical jacket type polarization-maintaining optical fiber that exhibits absolute single IIrM wave operation and is suitable as a polarizer by also considering mode birefringence.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a polarization preserving four-layer structure in which a cladding, an elliptical jacket, and a support are sequentially provided around a core around its outer periphery. In an optical fiber, the refractive index of the cladding and support is the same as that of silica, the relative refractive index difference Δ0 of the core to the cladding is 0.3% or less, and the relative refractive index difference Δ− of the elliptical jacket to the cladding is −0, 15% to -0.25%, and the mode birefringence B is 4.52X10' or more, and an elliptical jacket type surface wavefront preserving optical fiber is constructed.

In addition, useful items for downsizing polarizers include:
: 4 layers Is with cladding, elliptical jaggery 1~, and support arranged in order around the outer periphery of 1A.
In a 3fj polarization maintaining optical fiber, the refractive index of the cladding and support is the same as that of silica, the relative refractive index difference Δ of the core to the cladding is 0.3 to 0.4%, and the relative refractive index difference of the elliptical jacket to the cladding is 0.3 to 0.4%. An elliptical jacket type polarization-maintaining optical fiber is constructed with -0,2 to -0,3χ and a mode birefringence B of 6.3X10-4 or more.

[Effect 1: The relative refractive index difference Δ0 of the core with respect to the cladding is 0.3% or less, the relative refractive index difference Δ− of the elliptical jacket with respect to the cladding
is -0.15% to -0.25%, and mode birefringence B is 4.
Although the elliptical jacket type polarization-maintaining optical fiber with a parameter of 52x10-4 or more is obtained by theoretical analysis using a computer that takes into account the modal birefringence, it can be used as a polarizer within these numerical ranges. By winding the optical fiber with optimized winding and a certain radius, one-phase photons with excellent extinction ratio and insertion loss characteristics can be obtained.

Also, the relative refractive index difference Δ→ of the core with respect to the cladding is 0.3
~0.4″%, i of elliptical jack 1~ to cladding
The parameter that the t-refractive index difference - is -0,2 to -0,3X-mode birefringence B is 6.3 x 10-4 or more is 1.5.
Similarly, for the elliptic Jaken 1 type surface wavefront preserving optical fiber,
The structure of the polarizer can be optimized by taking into account the mode birefringence and performing a theoretical analysis to increase the difference in bending loss between the X-polarized wave and the X-polarized wave. Moreover, this optical fiber is smaller than the winding radius or bend made to accommodate the polarizer, and the small polarizer is broken.

``Example'' Hereinafter, an example of the present invention will be described with reference to the accompanying drawings.

FIG. 1 (aHbHc) shows 4y! of an absolutely single polarization maintaining optical fiber according to an embodiment of the present invention. Cross section, X
They are a refractive index distribution diagram in the axial direction and a refractive index distribution diagram in the Y-axis direction. In the figure, 1 is a core made of SiO□ glass doped with Ge, and a cladding 2 made of pure SiO2 glass is provided around the outer periphery of the core 1. Furthermore, an elliptical jaggery 1-3 is provided on the outer peripheral portion of the cladding 2 so as to surround the ridge 11.

This elliptical jacket 1-3 is 5 inch with B2O3 added.
It consists of two glasses and has a refractive index lower than the refractive index of cladding 2. Further, sabots 1 to 4 made of pure 5IO2 glass and having the same refractive index as the cladding 2 are provided on the outer circumference of the elliptical jacket 3 so as to surround it.

1.11 (aH and (C)), a is the radius of core 1, bc is the major axis radius of elliptical jacket 3, b is the minor axis radius of elliptical jacket 1 to 3, and the radius of cladding 2 is the radius of cladding 2. The ratio of the thickness to the core'V diameter is expressed as (1+δ)a. Δ- is the relative refractive index difference of the core 1 with respect to the cladding 2, and Δ− is the relative refractive index difference of the elliptical jacks 1-3 with respect to the cladding 2.

The extinction depth of refraction 4-(distribution n(r), that is, 8-) is the same on the short axis and long axis sides of the elliptical Jaegera 1-3, and the relative refractive index difference Δ- for the same silica is - 0,15~-0,2
Set within a range of 5%. In addition, the width of the refractive index distribution n (r) is 25 to 40'', with an ellipticity ε of elliptic shag 1 to 3.
Defined within the range of A.

On the premise of such a rod configuration, the optimal structure as a polarizer is theoretically analyzed by computer using the finite element method while taking into account the effect of the mode birefringence B.

In this theoretical analysis, for convenience, the area up to the elliptical jacket 3 is treated as the core, and the normalized phase constant U in this effective core part and the support l around it are treated as C as the effective cladding part. Let us examine the normalized lateral damping constant W.

To explain in detail, first, from the refractive index distribution of the long axis (X-axis) and short axis (y-axis) of the ellipse as the effective core part, there is an X-polarized wave with an electric field in the X-axis direction and an electric field in the Regarding the y-polarized waves, the propagation constant β8 in the traveling direction, respectively. Find β2. The difference between the two, Δβ−β8−β, is the propagation constant difference between the (It is desirable that it be larger than 1.) There is the following relationship between 1llJ, 1-birefringence 2B, the propagation constant difference Δβ, and the coupling length C1, and it is possible to reduce the coupling between both modes. Because it can be done.

However, k, (mi2π/′λ) is the wave number of light in free space,
λ is the wavelength of light σ) in free space.

Next, these values β1. Using βt and Δβ, the positive ′jJ in the effective core of the X polarization,! The normalized phase constant U, and the normalized phase constant Uy in the effective core portion of the X-polarized wave are determined by the following equation.

Then, rlo is the refractive index at the center (r-) within the effective core.

On the other hand, the normalized lateral attenuation constant W in the effective cladding part (support part) of the X polarization, y (frequency) is the above (1
1, (Using the normalized phase constant U1.Uy and the normalized frequency Vyy of Equation 21, it is determined by the following equations, respectively.

W・-5 votes 70 intoxicants ・・・・・・(3) lol,
=, f' to buy 1 ...... (4
)1Q L, normalized frequency V, , V, is the refractive index rto in the effective core - the wave number k in free space. , long sleeve radius bc
, the minor axis radius 0, and the relative refractive index difference Δ between the effective core and the effective cladding, and V x ''' 16
k. b c E mouth A −-+5]V,
= n, kob
t, L-m=7- and [defined in LSI.

Therefore, the bending loss characteristic that becomes a problem in 1F, which is aimed at optimization as a polarizer, is the various quantity Ut of the above equations (1) to [F]).
.

Based on WL, Vmu (t = x, y), it can be calculated by knowing the electromagnetic field distribution using the finite element method, and the refractive index distribution at that time can be calculated using RNF (RefraCie
dNear Field), from which the configuration parameters can be determined.

The solid line in Figure 2 shows the bending loss characteristics obtained by calculation using the above theoretical analysis method, the black circle plot shows the actual measured value of the bending loss for X polarization, and the white circle plot shows the actual measured loss for the y polarization song. shows. It can be seen from this that the bending loss characteristics obtained by the above theoretical analysis method are extremely similar to the actually measured bending loss characteristics. Therefore, by the above-mentioned theoretical analysis method, it is possible to calculate an elliptical jacket type plane-preserving optical fiber having desired bending loss characteristics, and it is also possible to obtain a refractive index distribution that is optimal as a polarizer.

Using the above analysis method, λ is 1.55.1.30.0.8
The theoretical bending loss characteristics designed for each wavelength of 5 μm are shown in Figure 3, and the waveguide structure at that time is shown in Table 1.6Table 1 However, the normalized mode birefringence B is G, 34xlO-'. , Also, the normalized frequencies of the optical fibers for wavelengths of 1.55 μm, shown by ■, for wavelengths of 1.30 μm, shown by ■, and for wavelengths of 0.85 μm, not shown by ■, are as follows: 9<V, <11 ．． It is in the range of 3.5<V<5.55.

From the bending loss characteristics shown in FIG. 3, for any of the optical fibers, if the bending radius is 40+ntll or less, the loss α for the X-polarized wave becomes α, > 1 dB#n. Therefore,
By winding the optical fiber with a radius of 40 mm or less, the X-polarized light is effectively generated, and the desired optimum optical fiber surface photons can be obtained.

Actually, the radius of bending a 0.85 μm optical fiber is 401
As a result of winding 11+11 15 times (length 3.8n+), the extinction ratio is -47 dB (circularly polarized wave incidence) and the insertion loss is 0.
．． An optical fiber surface photon having a 1, ν property of 3 (IB (x 14j J wave incident) was obtained.

The optical fibers in the design example above each have a bending radius of 4
It becomes a polarizer when it is 0 mn1 or less, but as a result of theoretical analysis, it was found that the structure as a polarizer can be optimized under the following conditions even in cases other than this.

That is, in the case where the refractive index of the cladding and the support is the same as that of silica, the relative refractive index difference Δ0 of :1A1 with respect to the cladding 2 is more than 0.3 thousand, and the relative refractive index difference of the elliptical jacket 3 with respect to the cladding 2 is Δ- is 10, 15 to -0.25%,
It is an elliptical jacket type surface wavefront preserving optical fiber with a mode birefringence B of 4.52xlO-4 or more.

The optical fiber 7 in this number fli range also becomes a photon below the value of the bending radius. Therefore, there are cases where the bending radius of the polarizer is larger than 40IIIIIIl.

By the way, from the curve ζ・no' loss characteristics in Figure 3, if the bending radius is less than 40 m + n, the bending loss of the X polarization is 1 dB#n
Because of the above, a polarizer of several 1119H is preferred, but in order to reduce the insertion loss to 0.5 (IC or less),
X 141j wave bending (・)゛Due to loss, the bending radius is approximately 1
In order to miniaturize the polarizer and make the entire system convex I-, which is limited to 5Illn+ or more, it is desirable to realize absolute Jii~-1 dog wave operation in a region with a small bending radius (10u or less). Ru.

Therefore, using the above-mentioned finite element method design method that takes into account the modal birefringence, an elliptical jacket type 111
1 Let jπ be the structural parameter of the wavefront preserving optical fiber.

First, let's start with the elliptical jacket polarization preserving (S) shown in Figure 1.
P) Set the sufficient refraction IJ of the optical fiber as follows. Core 1 is silica crow doped with Germani Uno,
The cladding 2 is made of pure silica glass, and the elliptical shagellas 1-3 are made of silica glass doped with poron and phosphorus.The outer periphery is surrounded by a support made of pure silica glass and having a refractive index equal to that of the cladding. Set 4?
-Jru. The relative refractive index difference Δ+ of the core 1 with respect to the cladding 2 is 0.3・-0.4%, the relative refractive index difference 8- with respect to the elliptical jacket 1-30 with respect to the cladding 2 is -0.2 to -03%, and the elliptical jacket 1- The ellipticity ε of * is set in the range of 30 to 45%, and the mode birefringence B is set in the range of 6.3 x 10' or more.

Assuming such a configuration, considering the effect of the mode birefringence B, the curve C-) operates as a single beam in a region with a small radius. The L-suitable side road is analyzed by the finite element method described in -L.

It has been confirmed that the loss characteristics and cutoff wavelength of the songs obtained using this design method match those of the actual measurement locations.

Using the above analysis method, S designed for a wavelength of 0.85 μm
Table 2 shows the structural parameters of the P optical fiber.

However, in the calculation, B = 6.3 x 10-'.

Table 2 The bending loss characteristics of this optical fiber are shown in FIGS. 4 and 5. Figure 4 shows the relative refractive index difference Δ between core 1 and cladding 2.
FIG. 5 shows an SP optical fiber (aHc) with a relative refractive index difference Δ--0.2% between the elliptical jacket 3 and the cladding 2. For all SP optical fibers shown in (a), (b), and (C) in Table 2, the normalized frequency is 9<V<15. It is in the φi range of 4<Vaku6.

Now, Figure 71 and Figure 5! In the curve of y-polarized wave, the bending radius is R1-x where the loss is 10 FI B / +n, and the bending radius where the polarization loss is 0.1[lB#n is R2
Then, from the bending loss characteristics shown in Figures X1 and 5, y
When the bending loss of 10 waves is 10 dB/n1 or more,
It can be seen that the bending f diameter R at which the polarization bending loss is 0°1 dB/IN or less exists at 10111 m or less. Therefore, in that range (R1 × R × 1 hair 2), this S
F) Several 111 turns of optical fiber are used to create a compact optical fiber for polarization.

Actually 0.81JJJ, lll SI) light 7y iba (
Bending a) to R- which was 2.5m8 with a radius of 6111111, the result is insertion loss 0.2dB - extinction ratio (at the time of circular wave incidence) -4
An optical fiber polarizer having a 'Lν property of 2 dB was obtained.

[Effects of the Invention] As described above, the elliptical Jagen 1 wavefront preserving optical fiber of the present invention has a refractive index distribution that takes into account the mode birefringence, and a 1=photon with enhanced characteristics can be obtained. Rano'shiru. This will lead to improvements in the functionality of optical measurement systems, optical fiber gyros, transmission systems 13 to Coheren 1, etc. that use these photons.

In particular, in the case of the elliptical jacket type polarization maintaining optical fiber of claim 2, the optical fiber polarizer is small and has excellent characteristics.
7, it is possible to downsize and improve the functionality of an optical measurement system using a polarizer, an optical fiber gyro, a transmission system to the coheren 1, etc.

[Brief explanation of the drawing]

Figure 1 faHb) and (c) are the cross-sectional view of the fiber, the refractive index distribution diagram in the X-axis power direction, the refractive index distribution diagram in the y-axis direction, and the second The figure shows the calculation of X polarization and Y polarization and the actual J! Bending loss 1 due to 11
, ν・rl, 3rd [n1] is a diagram showing the loss characteristics of the three types of optical fibers determined by R1 calculation, and 11th, 1, and 5 are diagrams showing the loss characteristics of the three types of optical fibers obtained by calculating R1. (') bending loss;
Fig. 7(a) FC) and Fig. 8(a) FC) are respectively 4i! of the optical fiber already proposed by JIl! ! 1
FIG. 3 is an explanatory diagram showing an EIi plane and a refractive index distribution. In the figure, 1 is a core, 2 is an oval jacket, 4 is a support.

Claims (1)

[Claims] 1. In a polarization-maintaining optical fiber with a four-layer structure in which a cladding, an elliptical jacket, and a support are sequentially provided on the outer periphery of the core, the refractive index of the cladding and the support is the same as that of silica. Similarly, the relative refractive index difference of the core to the cladding is 0.3% or less, the relative refractive index difference of the elliptical jacket to the cladding is -0.15 to -0.25%, and the mode birefringence is 4.52 × 10^ An elliptical jacket-type polarization-maintaining optical fiber characterized by having a polarization-maintaining optical fiber of −^4 or more. 2. In a polarization-maintaining optical fiber with a four-layer structure in which a cladding, an elliptical jacket, and a support are sequentially provided around the core, the refractive index of the cladding and the support is the same as that of silica, and the core relative to the cladding is The relative refractive index difference of 0.3 to 0.4%, the relative refractive index difference of the elliptical jacket to the cladding is -0.2 to -0.3%, and the mode birefringence is 6.3 × 10^-^4 An elliptical jacket type polarization maintaining optical fiber characterized by the above.