JPS6153711B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an element that converts the wavelength of laser light using a fiber for an optical communication transmission line, and more specifically, the present invention relates to an element that converts the wavelength of a laser beam using a fiber for an optical communication transmission line. By using a so-called polarization-maintaining birefringent fiber, which propagates by breaking the degeneracy of two HE ₁₁ modes of light whose polarization planes are orthogonal to each other, we can suppress the third-order nonlinear polarization effect of the fiber. The present invention relates to an optical wavelength conversion element that converts the wavelength of light.

Conventionally, it has been known that when high-energy density light is incident on an optical fiber, a portion of the incident light energy is converted into light of a different wavelength due to the nonlinear effect of the optical fiber material. An optical wavelength conversion device shown in FIG. 1 is known as an optical wavelength conversion device that utilizes the principle of optical wavelength conversion based on such a third-order nonlinear effect. To explain the figure, reference numeral 1 is a laser light source, 2 is an optical fiber as an optical wavelength conversion element, 3 and 4 are objective lenses, and 5 is a demultiplexer.
As the laser light source 1, one having a Q-switch function is usually used because it is necessary to increase the power incident on the optical fiber 2 in order to obtain a strong nonlinear effect. In addition, the length of the optical fiber 2 is from several meters to several meters.
It is 10m.

In the above device, a part of the laser beam of wavelength λ _P emitted from the laser light source 1 is caused by the third-order nonlinear effect inside the optical fiber 2, causing the incident light (λ
_P ) is converted into light of different wavelengths (λ _S , λ _A ), and is separated into each wavelength λ _P , λ _S , λ _A (however, λ _A < λ _P < λ _S ) by the demultiplexer 5. be done. Therefore, according to the apparatus shown in FIG. 1, the light of wavelength λ _P emitted by the laser light source 1 can be converted into light of wavelengths λ _S and λ _A.

By the way, the wavelength conversion of light by the apparatus shown in FIG. 1 is performed under the following conditions:
_S and λ _A are determined.

The wavelength of the incident light is λ _P , and the wavelengths of the light converted by the third-order nonlinear effect are λ _S , λ _A (λ _A < λ _P
<λ _S ), then 1/λ _A -1/λ _P = 1/λ _P -1/λ _S = Δ...
(1) holds true. Note that Δ is the normalized frequency shift amount. Furthermore, if the propagation constants of the light waves λ _P , λ _S , and λ _A in the optical fiber are k _P , k _S , k _A , then the following phase matching condition holds: k _S +k _A −2k _P =0 ……(2) . This phase matching condition is a term Δk (Δ
) and the term f(Δ) depending on the structure of the optical fiber, etc., and Δk(Δ)+f(Δ)=0...(3) Δk(Δ)=2π(n ^A ₂ + n ^S ₂ −2n ^P ₂ ) /λ _P +2π(n ^A ₂ −n ^S ₂ )Δ (4) where n ⁱ ₂ (i is any one of A, S, or P) is the refractive index of the cladding. (1) to (4) above
Based on the relationship of the formula, the incident light (λ _P ) has wavelengths λ _S ,
It is converted into converted light having a wavelength of _λA .

Therefore, the conventional optical wavelength conversion device shown in _FIG . The amount of shift (Δ) was uniquely determined, and only light with fixed wavelengths λ _S and λ _A could be obtained. Therefore, conventionally, in order to change the converted lights λ _S and λ _A while keeping the wavelength λ _P of the incident light constant,
In fact, there is no other way than to use optical fibers with different fiber structure parameters such as relative refractive index difference and core diameter, and it is extremely inconvenient in practice to use the device shown in Figure 1 as a wavelength tunable light source. .

In view of the above circumstances, the present invention aims to provide an optical wavelength conversion element that can continuously vary the wavelength of converted light, and is characterized in that a stress applying device is provided on a birefringent fiber. It is.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a principle diagram of an optical wavelength conversion device to which the optical wavelength conversion element of the present invention is applied. In this figure, the same components as in FIG. 1 are given the same reference numerals and their explanations will be omitted. The optical wavelength conversion element 10 shown in this figure is composed of a birefringent fiber 11 and a stress applying device 12 for applying stress to the birefringent fiber 11. Birefringent fiber 1
1, due to birefringence caused by residual stress inside the fiber 11, the light incident on the fiber 11 is divided into two polarization planes whose polarization planes are orthogonal to each other.
It has the property of propagating by solving the degeneracy of HE ₁₁ mode. In this case, the short axis and long axis of the fiber 11 are set in the x and y directions, respectively. Further, the stress applying device 12 applies stress to the birefringent fiber 11, and the magnitude of the force can be adjusted.

In addition, the code|symbol 13 is an analyzer arrange|positioned on the optical path between the objective lens 4 and the branching filter 5.

The optical wavelength conversion element 10 converts a part of the light of wavelength λ _P emitted from the laser light source 1 into wavelengths λ _S , λ _A
In this case, the stress applying device 1
By adjusting the stress applied to the fiber 11 by 2, the wavelengths λ _S and λ _A of the converted light can be continuously changed.

Therefore, according to the optical wavelength converter shown in FIG. 2, converted light whose wavelengths λ _S and λ _A are continuously variable can be obtained.

Next, the principle of the wavelength conversion action of the above optical wavelength conversion element 10 will be explained.

Now, if the wavelength λ _P of the incident light is set to 1.3 μm or less, Δk (Δ) in equation (4) mentioned above becomes positive, so
As an example of satisfying the phase matching condition of equation (2),
A case can be considered in which the polarization plane of the incident light is set in the y direction and the polarization plane of the converted light is set in the x direction. Note that various combinations of the polarization planes of the incident light and converted light can be set in addition to the above.
In addition, when λ _P > 1.3 μm, Δk (Δ) becomes negative, but if the combination of the polarization planes of the incident light and the converted light is considered, the present invention can be implemented in the same way based on the principle described below. can.

Under the above conditions, f(Δ) in equation (3) is the birefringence Bg due to the structure of the optical fiber and the birefringence due to stress remaining or applied to the optical fiber.
Using Bs, it can be expressed as f(Δ)=−4π(Bg+Bs)/λ _P (5), where Bg is approximately given by equation (6) and Bs is given by equation (7).

Bg＝(n ₁ −n ₂ )(bx−by) ……(6) Bs＝(C ₁ −C ₂ )(σ _x −σ _y ) ……(7) However, n ₁ and n ₂ are each core , the refractive index of the cladding, bx and by are the short axes at the wavelength λ _P , respectively,
It represents the normalized propagation constant of the long axis, C ₁ and C ₂ are the photoelastic constants, and σ _x and σ _y are the stress in the x and y directions, respectively, which remain or are applied to the optical fiber.

Also, consider a side bit fiber as an example of a birefringent fiber. FIG. 3 is a cross-sectional view of a side bit fiber, in which reference numeral 15 is a core, 16 is a side bit, and 17 is a cladding. It is assumed that there is x to this side bit fiber
How the wavelength of the converted light changes when a stress of W is applied from the direction or the y direction will be explained. The relative refractive index difference Δ of the side bit fibers is
0.2~0.4%, P ₂ O ₅ -SiO ₂ for the core, side bits
When B ₂ O ₃ -SiO ₂ and the cladding is SiO ₂ , the contribution due to residual stress in equation (7) is σ _x -σ _y = 2 (Kg/
mm ² ). Further, Bg in equation (6) has a negative sign, and in the wavelength band of 1 μm, the size is 10 ^-5 or less, and normally Bs is about one order of magnitude larger than Bg. Therefore, it can be said that the wavelength dependence of the total birefringence B (=Bg+Bs) is extremely small. Figures 4 and 5 show W and total birefringence B (=Bg+Bs) and (σ _x
−σ _y ), and from both figures, B=4.8×10 ^-5 −5.6×10 ^-5 W (x direction) B=4.8×10 ^-5 +5.3×10 ^-5 W ( y direction)...(8) The following relationship is obtained.

On the other hand, the relationship between Δk(Δ) and Δ in equation (4) is as shown by the solid line in FIG. 6 when the wavelength λP of the incident light is 1.06 μm. Therefore, if f(Δ) given by equations (5) and (8) and k(Δ) shown in FIG. 6 are applied to the phase matching condition of equation (3), the applied stress W The relationship between and the optical wavelength shift amount Δ can be determined as follows. For example, for λ _P = 1.06 μm, Δ
= 200 cm ^-1 , that is, λs = 1.08 μm, λ _A = 1.04 μm, then -f(Δ) is 0.22
(cm ^-1 ), and the required total birefringence B is from equation (5),
0.2×10 ^-5 , and the stress is 0.8 (Kg/cm) in the x direction
It turns out that all you have to do is apply it. FIG. 7 shows the relationship between W applied in the y direction and the optical wavelength shift amount Δ. From the figure, when W = 0 to 1.5 (Kg/cm), Δ = 430 to 700 (cm ^-1 ), and the wavelengths λ _S and λ _A of the converted light are 1.11 to 1.14 μm and 0.99 to 0.99 μm, respectively.
Converted to a range of 1.01 μm. Similarly, when stress is applied in the x direction, the wavelength of the converted light can be made variable depending on W. Furthermore, if estimated from the elastic limit of quartz used as a birefringent fiber material, the maximum value of applied stress is about 10 Kg/cm, and therefore the maximum value of Δ is about 1500 cm ^-1 , which is extremely wide. Wavelength conversion can be performed within the range.

Note that the nonlinear effect utilized in the optical wavelength conversion element described above maintains the coherency of the incident light, so that converted light with extremely high coherency can be obtained. In addition, by setting the polarization direction of the incident light to the x direction or both x and y directions, even if the stress strength and applied direction are the same, the amount of optical wavelength shift differs depending on the polarization direction of the incident light. Δ can be obtained, and therefore it is possible to increase the degree of freedom in wavelength selection.

According to the optical wavelength conversion element 10 described above, the wavelength of incident light can be converted continuously and over a relatively wide range by changing the stress applied to the birefringent fiber. Therefore, by applying the optical wavelength conversion element 10 described above to the optical wavelength conversion device shown in FIG. 2, for example, a simple wavelength tunable light source can be obtained.

In addition, in the above embodiment, an example was explained in which a side bit fiber was used as the birefringent fiber, but other birefringent fibers may be used, such as a non-circular core fiber or a fiber in which the side bit is sufficiently separated from the core. You can also do it.

As explained above, the optical wavelength conversion element of the present invention includes a birefringent fiber and an application device for applying stress to the birefringence fiber, and the stress application device generates stress. This has the advantage that the wavelength of the converted light obtained through the birefringent fiber can be made continuously variable by changing the force applied to the birefringent fiber. Therefore, it can be used as a wavelength tunable light source for, for example, measuring optical fiber transmission characteristics, and it can also be used to select structural parameters such as relative refractive index difference and dopant materials of the birefringent fiber (polarization preserving fiber) used. By making various preparations and further changing the wavelength of the incident light, it is possible to realize a wavelength tunable light source that can continuously cover almost the entire wavelength band of 1 μm, which is necessary for measuring optical fiber transmission characteristics. . Furthermore, since the nonlinear effect utilized by the optical wavelength conversion element of the present invention maintains the coherency of the incident light, it has the advantage that converted light with extremely high coherency can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a principle diagram of an optical wavelength conversion device using a conventional optical fiber, FIG. 2 is a principle diagram of an optical wavelength conversion device constructed using an optical wavelength conversion element of the present invention, and FIG.
The figure is a cross-sectional view of the side bit fiber, and Figure 4 is
Figure 5 shows the relationship between the stress W applied in the y direction and B, (σ _{x -σ y )} . 6 is a diagram showing the relationship between Δk (Δ) and Δ, and FIG. 7 is a diagram showing the relationship between Δ and the stress W applied in the y direction. DESCRIPTION OF SYMBOLS 1... Laser light source, 5... Demultiplexer, 10... Optical wavelength conversion element, 11... Birefringent fiber, 12
... Stress application device, 13 ... Analyzer.

Claims (1)

[Claims] 1 comprises a birefringent fiber and a stress applying device for applying stress to the birefringent fiber, and by changing the stress applied to the birefringent fiber by the stress applying device, 1. An optical wavelength conversion element characterized in that the wavelength of converted light obtained by being guided through a birefringent fiber can be changed.