JPS6153708B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical amplification element that amplifies optical power using the third-order nonlinear polarization effect of an optical fiber.

<Prior art> When high-energy density light is input as pump light and light with minute power is input as signal light into an optical fiber, if the phase matching conditions described below are satisfied, it is possible to It is known that four-photon mixing converts a part of the energy of pump light into signal light having a wavelength different from that of the pump light, and as a result, the power of the signal light is amplified. First, the principle of optical amplification using a conventional third-order nonlinear effect using an optical fiber will be explained. FIG. 1 shows the amplification device, in which pump light from a pump light source 11 is reflected by a mirror 12 and enters a multiplexer 13. On the other hand, the signal light from the signal light source 14 passes through the objective lens 15 and mirrors 16 and 17 and enters the multiplexer 13. The pump light and signal light combined by the multiplexer 13 are connected to one end of the circular optical fiber 18 through the objective lens 1.
9. Output light 22 is emitted from the other end of optical fiber 18 via objective lens 21 . In order to obtain a large optical amplification factor, the pump light source 11 is usually equipped with a Q-switch function, and the optical fiber 18 has a length of several tens of centimeters to several tens of meters. The wavelength of the pump light is λ _P , the wavelengths of the signal light and idler light are λ _S and λ _A , respectively, and △
Assuming that the normalized frequency shift amount is the amount of normalized frequency shift, the following relational expression is established between these three light waves.

1/λ _P −1/λ _S =1/λ _A −1/λ _P =Δ (1) Note that the idler light is light with a different wavelength that is generated simultaneously with the amplification of the signal light. At this time, the wavelength λ _P ,
Letting the phase constants of the light waves λ _S and λ _AS in the optical fiber 18 be k _P , k _S , and k _A , respectively, the phase matching condition k _S +k _A −2k _P =0 (2) must be satisfied. When this phase matching condition does not hold, the power of the signal light and idler light decreases in proportion to sin(δk ² )/δk ² (δk=k _S +k _A −2k _P ), resulting in efficient optical amplification. In order to do so, it is an essential condition that formula (2) holds true. This phase matching condition can be separated into a term △k (△) due to the refractive index dispersion of the optical fiber material and a term f (△) due to the structure of the optical fiber, etc., and △k (△) + f (△) = 0 (3) becomes. FIG. 2 shows the relationship between Δk(Δ) and Δ for a silica optical fiber for various pump wavelengths. Moreover, the dashed-dotted line schematically shows -f(△). From this figure, the wavelength λ _P is 1μ
The change in △k (△) relative to △ is rapid near m, and becomes gentle near a wavelength of 1.25 μm. When trying to satisfy the phase matching condition of equation (3) for signal light that differs from the pump light by △ ₀ , conventionally the wavelength is 1 μm or shorter wavelength side and
On the wavelength side longer than 1.25 μm, for example, in order to set the pump light and signal light to the LP ₀₁ mode, which is the fundamental mode, phase matching has been achieved by selecting the idler light as a higher-order mode. In addition, in the so-called zero dispersion wavelength band near the wavelength of 1.25 μm, pump light, signal light,
All three light waves of the idler light can be set to the LP ₀₁ mode, and there is no need for complicated mode selection to obtain phase matching. The optical amplification factor can be obtained in the range of 10 ³ to 10 ⁴ when using a silica optical fiber.
Confirmed experimentally. However, in all of the conventional methods described above, in order to achieve phase matching for an optical signal of a certain wavelength, fiber parameters such as the core diameter of the optical fiber and the relative refractive index difference must be uniquely determined. For optical signals of different wavelengths, there is no other way than to use optical fibers with different fiber parameters.
It was extremely inconvenient in practice to use the device shown in the figure for optical amplification.

<Summary of the invention> The purpose of the invention is to provide an optical amplification element that can efficiently amplify light over a wide band without precisely controlling parameters such as the core diameter of an optical fiber and the relative refractive index difference according to the wavelength of the light. It's about doing.

According to this invention, a birefringent optical fiber is used as an optical fiber for optical amplification, and the stress in the optical fiber is reduced by applying stress to the optical fiber from the outside or by bending it for a given wavelength of signal light. By changing the phase constant of the optical fiber, it is possible to achieve highly efficient optical amplification by changing the phase constant of the optical fiber from the outside.

<Embodiment> FIG. 3 shows an embodiment of the present invention, and parts corresponding to those in FIG. 1 are given the same reference numerals. A birefringent optical fiber 23 is used as the optical fiber, and it is made possible to apply stress to the optical fiber 23 by means of a stress applying device 24, such as a press, for example. Alternatively, as shown in FIG. 4, the optical fiber 23 is bent by a bobbin 25. Here, the short axis and long axis of the birefringent optical fiber 23 are set in the x and y directions, respectively. Now, if the wavelength λ _P of the pump light is 1.25 μm or less, △k (△) is positive from Fig. 2, so the polarization plane of the pump light is set in the y direction, and the wavelength to be amplified is λ _S (1/λ _P −1/λ _S =Δ ₀ ), assuming that the plane of polarization of the signal light is in the x direction, f(Δ) in equation (3) is given by the following equation.

f(△)=−4π(B _g +B _S )/λ _P (4) B _g is HE ₁₁ ^x and HE ₁₁ ^y due to the structure of the optical fiber.
It is the mode birefringence between modes, and B _S =(B _S ^x −B
_S ^y ) is the modal birefringence resulting from stress remaining in the optical fiber or stress applied to the optical fiber. Therefore, as shown in FIG. 5, by changing the stress applied to the optical fiber 23 in some way, B _S , f(△) is changed, and as a result, when △=△ ₀ , △ It can be seen that a phase matching of k(Δ ₀ )+f(Δ ₀ )=0 (5) can be obtained. If B _S0 is the contribution of residual stress generated in the optical fiber manufacturing process to B _S , then in the stress application type shown in Figure 3, the relationship between stress W (Kg/cm) and B _S is given by the following equation. .

In addition, in the case of the bending shown in Figure 4, where the outer radius of the optical fiber is r and the bending radius is R, B _S is B _S = B _S0 ±0.9×10 ⁶ 1/λ _P (r/R) ² (7). However, the unit of λ _P is μm, and the symbols + and − in the formula indicate the case where the optical fiber is bent in the yz plane and the xz plane, respectively. FIG. 6 shows the wavelength region _Δ0 of the signal light in which phase matching can be achieved with the applied stress for various pump light wavelengths when B _S0 =5×10 ^-5 . The figure shows that a wide wavelength range of Δ ₀ to 2000 cm ⁻¹ can be obtained with a relatively small applied stress of W<2 Kg. For example, λ _P =
At 1.25 μm, △ ₀ = 2000 cm ^-1 corresponds to λ _S = 1.67 μm, and signal light in the range of 1.25 μm < λ _S < 1.67 μm can be amplified with a single optical fiber by only slightly changing the externally applied stress. .

Next, FIG. 7 shows a numerical example when bending is applied to the optical fiber 23, and _Δ0 is shown with respect to the bending diameter 2R. It is assumed that the fiber outer diameter 2r is the usual 125 μm and that the optical fiber is bent in the yz plane. It can be seen that when 2R>1 cm, which is permissible for ordinary optical fibers, _Δ0 can be obtained at 500 cm ^{-1 or} more even when λ _P =1.0 μm, and the same effect as when stress is applied can be obtained. The above is an example of an embodiment, but by appropriately selecting the birefringence B _S0 due to residual stress, the center value of _the allowable wavelength range of signal light can be freely selected. , 1 μm to 2 μm if you prepare 3 wires
The signal light wavelengths in the band can be easily covered. Furthermore, regarding the fiber parameters of the optical fiber used,
It is only necessary to set the cutoff wavelength λc ₁ of the LP ₁₁ mode to λ _P >λc ₁ , and there is no need to precisely control the core diameter and relative refractive index difference as in the conventional case.

<Effects> As described above, if a birefringent optical fiber is used as an optical amplifying element, stress can be applied to the optical fiber from the outside or the stress of the optical fiber can be changed by stress changing means such as bending. This has the advantage that signal light can be amplified with high efficiency over a wide wavelength range simply by changing refraction. Further, the present invention has the advantage that there is no need to precisely control the core diameter and relative refractive index difference of the optical fiber used in accordance with the wavelength of the signal light.
Furthermore, by simply preparing several optical fibers with different birefringence B _S0 caused by residual stress, it is possible to easily cover the range of 1 μm to 2 μm required for measuring the transmission characteristics of optical fibers, making it suitable as a practical optical amplification device. Useful.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram showing a conventional optical amplification device;
The figure shows the relationship between △k (△) and △ of a quartz optical fiber, FIGS. ), -f(△) and △. Figure 6 shows the relationship between applied stress and signal light wavelength range △ ₀
FIG. 7 is a diagram showing the relationship between the bending diameter 2R and the wavelength range _Δ0 of the signal light. 11: Light source for pump, 12, 16, 17: Mirror,
13: Multiplexer, 14: Signal light source, 15, 19,
22: Compatible lens, 22: Output light, 23: Birefringent optical fiber, 24: Stress application device, 25: Bending bobbin.

Claims (1)

[Claims] 1 Due to the third-order nonlinear polarization effect of optical fiber 4
In an optical fiber type optical amplification device that utilizes photon mixing, a birefringent optical fiber is used as the optical fiber, and the stress applied to the optical fiber from the outside is changed by a stress changing means according to the wavelength of the signal light to be amplified. Therefore, the phase constants of the pump light, signal light, and idler light within the optical fiber are k _P ,
An optical amplification element characterized in that it optically amplifies signal light by establishing a phase matching condition of 2k _P =k _S +k _AS , where k _S and k _AS .