JPS6161661B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a device for amplifying signal light using the third-order nonlinear effect of an optical fiber.

(Prior art) When a high-energy density light as a pump light and a micro-power light as a signal light are incident on an optical fiber, if the phase matching conditions described below are satisfied, the nonlinear effect of the optical fiber material is applied. 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. Figure 6 shows
The amplification device is shown.

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 is transmitted through an objective lens 15, a mirror 16,
17 and enters the multiplexer 13. The pump light and signal light combined by the multiplexer are transferred to the circular optical fiber 1.
The light is incident on one end of the lens 8 through the objective lens 19 . 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 length of the optical fiber 18 is from several tens of centimeters to several tens of meters. If 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 the amount of normalized frequency shift is △, then the following relational expression holds between these three light waves. .

1/λ _P -1/λ _S = 1/λ _A -1/λ _P =△ (1) Note that the idler light is light with a different wavelength that is generated simultaneously with the amplification of the signal light, and the wavelength λ at this time is
If the phase constants of the light waves _P , λ _S , and λ _A in the optical fiber 18 are k _P , k _S , and k _{A ,} respectively, then the phase matching condition must be satisfied as k _S +k _A −2k _P =0 (2) . This phase matching condition is due to the term △k(△) due to the refractive index dispersion of the optical fiber material.
It can be separated into a term f(△) depending on the structure of the optical fiber, etc. △k(△)+f(△)=0 (3). FIG. 7 shows the relationship between Δk(Δ) and Δ for quartz fibers for various pump wavelengths. Moreover, the dashed-dotted line schematically shows -f(Δ). This figure shows that the change in Δk (Δ) relative to Δ is rapid when the wavelength λ _P is around 1 μm, and becomes gentle when the wavelength is around 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, for wavelengths of 1 μm or shorter wavelengths and longer wavelengths than 1.25 μm, for example, pump light and signal light are basically To obtain the LP ₀₁ mode, phase matching was achieved by selecting the idler light as a higher-order mode. In addition, in the wavelength vicinity of 1.25 μm, all three light waves of pump light, signal light, and idler light are
LP ₀₁ mode, and there is no need for complex mode selection to obtain phase matching. It has been experimentally confirmed that an optical amplification factor of about 10 ³ to 10 ⁴ can be obtained when a quartz optical fiber is used.

(Problems to be Solved by the Invention) However, in all of the conventional methods described above, in order to achieve phase matching for an optical signal of a certain wavelength, the core diameter of the optical fiber, the relative refractive index difference, etc. The parameters must be determined uniquely, and for optical signals of different wavelengths, there is no other way than to use optical fibers with different fiber parameters, and the device shown in Figure 6 can be used for optical amplification. It was extremely inconvenient in practice.

An object of the present invention is to provide a device that can efficiently amplify light over a wide band without precisely controlling fiber parameters such as the refractive index difference and core diameter of an optical fiber according to the wavelength of signal light. .

(Means for Solving the Problems) The present invention is characterized by an optical amplification device that inputs signal light and pump light into an optical fiber and amplifies the signal light by four-photon mixing due to the third-order nonlinear polarization effect of the optical fiber. In the optical amplification device, the optical fiber is a birefringent optical fiber, a heating device is provided to control the temperature of the optical fiber, and stress birefringence is controlled by controlling the temperature.

(Function) Since the characteristics of the optical fiber are controlled by the heating device, the optical signal can be efficiently amplified over a wide band, and the above-mentioned objective is achieved.

(Embodiment) FIG. 1 shows an embodiment of the present invention, and parts corresponding to those in FIG. 6 are given the same reference numerals. A birefringent fiber 23 is used as the optical fiber, and a heating device 2 applies temperature to the birefringent fiber 23.
4 is provided. Here, the short axis and long axis of the birefringent 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 Figure 7, so the polarization plane of the pump light is set in the y direction, and the polarization of the signal light with the wavelength λ _S to be amplified is determined. Assuming that the wavefront is in the x direction, f(△) in equation (3) is given by the following equation.

f(△)=-4π(B _g +B _s )/λ _P (4) B _g is the birefringence caused by the structure of the optical fiber, and B _s is the birefringence caused by the stress remaining in the optical fiber. be. Therefore, as shown in FIG. 2, by changing the stress applied to the optical fiber 23 in some way, B _s , f (△) is changed, and as a result, when △=△ ₀ , △k It can be seen that the following phase matching can be obtained: (Δ ₀ )+f(Δ ₀ )=0 (5).

For example, considering an elliptical core fiber,
The stress birefringence B _s of the elliptical core fiber is B _s = B _s0 H(V) (6) B _s0 = −0.5n ³ (p ₁₁ −p ₁₂ )(α ₀
−α ₁ )q△T (7) Here, B _s0 is the stress birefringence at the core, and H
(V) is a quantity related to the spread of mode field distribution and stress distribution, n is the refractive index of the core, p ₁₁ and p ₁₂ are Pockels constants, and α ₀ and α ₁ are SiO ₂ and GeO _{2 ,} respectively.
The linear expansion coefficient q of the doped core is the molar concentration of GeO ₂ ,
ΔT is the difference between room temperature and solidification temperature. Therefore, when the elliptical core fiber is heated, the stress birefringence B _s0 changes because ΔT changes. When using an elliptic core fiber with an ellipticity of 40% and a relative refractive index difference of 0.34%,
The relationship between the frequency shift amount (Δ) and the heating temperature T is given in FIG.

In general, the stress birefringence B _s of a birefringent fiber changes with temperature, so B _s
=F(n, p ₁₁ , p ₁₂ , α ₀ , α ₁ , ..., q)△
It can be expressed as T (8).

FIG. 4 shows the relationship between the total birefringence and the heating temperature T of the elliptic clad fiber when the pump light is 1.064 μm. Birefringence changes significantly with temperature changes. FIG. 5 shows the relationship of _Δ0 to the heating temperature T of an elliptic clad fiber. From this figure, when the pump wavelength is 1.064μm, the temperature is approximately 700℃.
The signal light can be amplified over a wide wavelength range of Δν ₀ to 2000 cm ⁻¹ with respect to the change in . The above is an example, but the allowable wavelength range of signal light can be freely selected by appropriately selecting the birefringence B _s0 due to residual stress, so if several optical fibers with different B _s0 are prepared, Signal light wavelengths in the ~2 μm band can be completely covered. Furthermore, for the fiber used, it is sufficient to set the cutoff wavelength λ _C1 of the LP ₁₁ mode so that λ _P >λ _C1 , and there is no need to precisely control fiber parameters such as core diameter and relative refractive index difference as in the conventional case.

(Effects of the Invention) As described above, this optical amplification device uses a birefringent fiber and applies heat to the optical fiber from the outside to change the stress within the optical fiber, thereby eliminating birefringence. It has the advantage that the signal light can be amplified over a wide band and with high efficiency just by changing it. Furthermore, the present invention has the advantage that there is no need to precisely control parameters such as the core diameter and relative refractive index difference of the optical fiber used in accordance with the wavelength of the signal light. Furthermore, it is a practical optical amplifying device because it can completely cover signal light wavelengths of 1 μm to 2 μm by simply preparing several fibers with different birefringence B _s0 caused by residual stress.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the optical amplification device according to the present invention, and Fig. 2 shows △k (△), -f (△).
Figure 3 is a diagram showing the relationship between temperature T applied to the elliptical core fiber and _Δ0 , Figure 4 is a diagram showing the relationship between Δ0 and
The figure shows the relationship between the temperature T applied to the elliptic clad fiber and the total birefringence B _s . Figure 5 shows the relationship between the temperature T applied to the elliptic clad fiber and _Δ0 . Figure 6 shows the relationship between the temperature T applied to the elliptic clad fiber and Δ0. A configuration diagram of the optical amplification device,
FIG. 7 is a diagram showing the relationship between Δk (Δ) and Δ of a quartz optical fiber. 11: Light source for pump, 12, 16, 17: Mirror,
13: Multiplexer, 14: Signal light source, 15, 19,
21: Objective lens, 22: Output light, 23: Birefringent fiber, 24: Heating device.

Claims (1)

[Claims] 1 Inject the signal light and pump light into the optical fiber,
An optical amplification device that amplifies signal light by four-photon mixing due to the third-order nonlinear polarization effect of an optical fiber, wherein the optical fiber is a birefringent optical fiber, and a heating device is provided to control the temperature of the optical fiber, An optical amplification device characterized in that stress birefringence is controlled by controlling temperature.