JPH0158484B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a single mode optical fiber that constitutes a long transmission line with little crosstalk degradation in a transmission line used in coherent optical communication. .

(Prior Art) As a birefringent fiber (or polarization maintaining fiber) that maintains the polarization state of linearly polarized light incident on an optical fiber, a fiber having a cross-sectional structure as shown in FIG. 1 is known ( T.Hosaka et al. “Low−
loss single polarization fibers with
asymmetric strain birefringence”, Electron.
Lett., vol.17, p.530, 1981). In Figure 1, 1
2 is a core, 2 is a cladding, and 3 is a stress applying portion made of glass having a coefficient of thermal expansion larger than that of the cladding. In such an optical fiber, different stresses are applied in the x-axis direction and y-axis direction in the core due to contraction of the stress-applying part during the cooling process after drawing, and the light is polarized in the x-axis direction due to the photoelastic effect. The propagation constant βx of the light polarized in the y-axis direction is different from the propagation constant βy of the light polarized in the y-axis direction. As a result of the propagation constants of the two polarization modes being different, mode coupling between the x and y polarizations due to disturbance is reduced, making it possible to stabilize the polarization state.

Conventionally, such optical fibers have been designed so that the cutoff wavelength λc of the higher-order mode is λ/1.3λcλ (1) with respect to the wavelength λ of the laser beam. Therefore, even when only the x-polarized wave is excited at the input end, the mode-coupled component to the y-polarized wave accumulates little by little, and as shown in Figure 2, the crosstalk deteriorates as the fiber length L increases (Y.Sasaki Others, “8−km long
polarization−maintaining fiber with highly
stable polarization state”, Electron.Lett.,
vol.19, p.792, 1983). In addition, crosstalk CT
is given by the following equation.

CT=10log ₁₀ Py(L)/Px(L) (dB) (2) However, Px(L) and Py(L) are the optical intensities of x and y polarized waves at distance L, respectively.

As described above, conventional polarization-maintaining fibers have the disadvantage that crosstalk deteriorates with distance, and as the fiber becomes longer, the polarization state becomes unstable.

(Problems to be solved by the invention) An object of the present invention is to eliminate the deterioration of crosstalk due to distance by utilizing a region where the loss of one polarization mode is larger than that of the other polarization mode, in order to eliminate such drawbacks of the conventional technology. The object of the present invention is to provide a transmission path suitable for long coherent optical communications.

(Structure and operation of the invention) The mode coupling equation of both x and y polarized waves in a birefringent fiber is given by the following equation.

dPx (Z)/dZ=-2α _x P _x (Z)+h
[Py (Z) - Px (Z)] (3) dPy (Z) / dZ = -2α _y P _y (Z) + h
[Px (Z) - Py (Z)] (4) Here, h is the mode coupling coefficient, α _x and α _y are each x
and the loss coefficient of y polarization. When equations (3) and (4) are solved under the initial conditions Px(O)=Po and Py(O)=O, the following equation is obtained.

Px (Z) = Poe ^- 〓 ^z cosh (qz) {1+
(α _y − α _x ) / qtanh (qz)} (5) Py (Z) = h / qPoe ^- 〓 ^z sinh (qz) (6) where γ = (α _x + α _y + h) (7) q = √ ( _y − _x ) ² + ² (8). The crosstalk at Z=L is expressed by formula (2),
From (5) and (6), CT=10log ₁₀ {htanh(qL)/q+(α
_y − α _x ) tanh (qL)} (9).

When α _x = α _y , that is, the losses of both polarized waves are equal, q = h from equation (8), so the crosstalk becomes CT = 10log ₁₀ [tanh (hL)] (10). When h=1×10 ^-7 (m ^-1 ), the distance dependence of crosstalk expressed by equation (10) is expressed as follows:
It will look like a in Figure 3. As is clear from curve a in FIG. 3, when α _x =α _y , the crosstalk degrades with distance. Now, α _x = 1.15×10 ^-4 m ^-1 (corresponding to a loss of 1 dB/Km in x polarization), and α _y = 5.75×10 ^-3 m ^-1
(corresponding to a loss of 50 dB/Km of y polarization), then formula (9)
The distance characteristic of crosstalk expressed as b in Fig. 3 is
It can be seen that the value approaches a constant value. The distance Lo at which the crosstalk becomes a constant value is expressed as Lo1/q. Therefore, it can be seen that in order to suppress the deterioration of crosstalk due to distance, it is sufficient to make the loss of one polarization (here, y polarization) larger than that of the other polarization.

FIG. 4 shows the loss spectroscopy of the birefringent fiber having the structure shown in FIG. 1 for both x and y polarizations. The structural parameters of the fiber are as follows. Core relative refractive index difference Δ = 0.28%, core diameter 2a = 5.4 μm, fiber outer diameter 2b = 150 μm, inner radius of stress applying part r ₁ = 3.1a, boron concentration in stress applying part 15 mol%, higher mode Cutoff wavelength λc = 0.81 μm. From Figure 4, the wavelength λ1.12μm
It can be seen that the loss of y-polarized waves is increasing.
The measurements were conducted with the fiber wound around a drum with a diameter of 30 cm, and the difference in loss characteristics shown in Figure 4 is due to the difference in bending loss for both polarized waves.

The refractive index distribution felt by both x and y polarized waves in a birefringent optical fiber is expressed by the following equation.

n _x (r, θ)=n(r, θ)−c ₁ σ _x (r
, θ) − c ₂ [σ _y (r, θ) + σ _z (r, θ)] (11) n _y (r, θ) = n(r, θ) − c ₁ σ _y (r
, θ) − c ₂ [σ _z (r, θ) + σ _x (r, θ)] (12) where n(r, θ) is the refractive index distribution in the absence of stress,
c ₁ and c ₂ are photoelastic constants, and σ _x , σ _y , and σ _z are principal stresses. FIG. 5 shows the refractive index distribution on the y-axis of the fiber used in the measurements in FIG. 4. In the figure, βx/k and βy/k are calculated from each refractive index distribution.
This is the propagation constant at 1.15 μm. From Figure 5, y
Since the propagation constant βy/k of the polarized wave is close to the refractive index of the cladding, it can be seen that for the same bending, the polarized wave suffers a larger bending loss than the x-polarized wave. FIG. 6 shows the results of measuring the loss characteristics due to bending of various birefringent fibers with different core refractive index difference Δ, inner radius r ₁ of the stress-applying part, cutoff wavelength λ _c , etc. The area surrounded by diagonal lines is the range in which data of various fibers used for measurement are distributed. From FIG. 6, the following can be concluded.

(1) The wavelength range in which the loss of y-polarized waves is greater than the loss of x-polarized waves is λ14.λ _c .

(2) The excess loss suffered by the x-polarized wave, which is the propagation mode, is
To be less than 1 dB/Km, the wavelength of light must be λ1.65λ _c .

Therefore, in order to have low loss and good crosstalk characteristics in a long length as a single polarization single mode optical fiber, the cutoff wavelength λ _c of the higher order mode should be set to λ/1.65 with respect to the wavelength λ of the light source. It must be designed to satisfy the following condition: λ _c λ/1.4 (13). The cutoff wavelength λ _c of the higher-order mode of the optical fiber is given by λ _c =2π/v _c n ₁ a√2 (14). Here, n ₁ is the refractive index of the core, a is the core radius, Δ (= (n ₁ ² − n ₂ ² )/2n ₁ ² , n ₂ is the refractive index of the cladding) is the relative refractive index difference, and v _c is the relative refractive index difference. When the core has a step-like refractive index distribution, v _c is a constant given by =2.405. As an example, when n ₁ = 1.463 and Δ = 0.3%, the following relationship is obtained: λ _c = 0.296a (15). That is, core radius a = 2.77μ
m, the cutoff wavelength can be set to λ _c =0.82 μm. In this way, by changing the core radius a or the relative refractive index difference Δ, the cutoff wavelength λ _c can be determined to a desired value. Therefore, it can be seen that the core diameter or relative refractive index difference should be designed so that λ _c satisfies equation (13) with respect to the wavelength λ of light propagating in the optical fiber.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, there is no crosstalk deterioration due to distance, and a long transmission line with stable polarization characteristics can be realized. Therefore, it has great advantages as a transmission line for coherent optical communication or as a fiber polarizer.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a birefringent fiber, Figure 2 is the crosstalk distance characteristic of a conventional polarization maintaining fiber, and Figure 3a is the crosstalk distance when α _x = α _y and when α _y > α _x . Figure 4 is a diagram showing the wavelength characteristics of loss for each polarized wave, Figure 5 is a diagram showing the refractive index distribution of each polarized wave of birefringent fiber, and Figure 6 is a diagram showing excess loss due to bending. It is. 1: core, 2: cladding, 3: stress applying part.

Claims (1)

[Claims] 1. In a birefringent optical fiber with different propagation constants for two orthogonal linearly polarized lights, when the wavelength of the laser light from the light source is λ, a high-order fiber having an electric field distribution other than a Gaussian waveform in the fiber A single polarization single mode optical fiber characterized in that the longest cutoff wavelength λ _c of the cutoff wavelengths of a mode group satisfies the condition λ/1.65λcλ/1.4.