JPS6155662B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to an apparatus for converting the wavelength of a laser beam used for measuring the transmission characteristics of an optical fiber for optical communications or as a light source for various spectroscopic measurements. Conventionally, a bulk crystal of a nonlinear optical crystal such as LiNbO ₃ or an optical waveguide has been used as an element that converts optical wavelength by a second-order nonlinear polarization effect. In order to obtain the second harmonic wave, sum frequency, and difference frequency with high conversion efficiency, it is necessary to increase the energy density of the incident laser beam in the medium, and also to increase the energy density of the incident laser beam and the generated second harmonic wave, sum frequency, and difference frequency. It is necessary to satisfy phase matching conditions in the frequency medium. In the case of bulk crystals, the disadvantage is that it is not possible to increase the energy density of incident light within the crystal due to the influence of diffraction, but with the latter optical waveguide type,
Since light is guided by confining it in a micro cross section (core) with a high refractive index, it is possible to increase energy density and obtain high conversion efficiency. however,
Both require the use of the birefringence of the crystal in order to satisfy the phase matching condition, and have the disadvantage that even if the nonlinearity is large, materials with no or small birefringence cannot be used. The present invention aims to improve the above-mentioned drawbacks of the conventional technology.The purpose of the present invention is to use an optical fiber as a nonlinear medium to satisfy the phase matching condition using an optical waveguide type and without using the birefringence of the crystal. The object of the present invention is to provide an optical wavelength conversion element that obtains a second harmonic, sum frequency, or difference frequency with high conversion efficiency. One of the features of the present invention is that it is composed of an optical fiber, and converts an incident laser beam with an angular frequency ω ₁ into an angular frequency ω ₃ (=2ω ₁ ) by a second-order nonlinear polarization effect.
is an optical wavelength conversion element that converts into the second harmonic of the optical fiber, the core diameter and relative refractive index difference (Δ
= (n ₁ ² - n ₂ ² )/2n ₁ ² ) is the formula δβ = β ₃ - 2β ₁ = 0 (phase matching condition) ...(a) β ₁ ; Phase constant β of the incident laser beam in the optical fiber ₃ ; phase constant n ₁ of the second harmonic in the optical fiber; refractive index n ₂ of the core of the optical fiber; refractive index of the cladding of the optical fiber. Another feature of the invention is that the optical fiber is configured to change the angular frequency of the two incident laser beams to ω.
₁ , ω ₂ (ω ₁ ≠ ω ₂ ) Due to the nonlinear polarization effect, the sum frequency (ω ₁ +ω ₂ ) or the difference frequency (ω ₁ −ω
₂ ) is an optical wavelength conversion element that converts the optical fiber into a core diameter and a relative refractive index difference (Δ=(n ₁ ² −
n ₂ ² )/2n ₁ ² ) is the formula

【table】

[Table] The optical wavelength conversion element is selected to satisfy the following. Examples will be described below with reference to the drawings. Of the two conditions mentioned above to obtain the second harmonic, sum frequency, and difference frequency with high conversion efficiency, energy density can be essentially increased by using optical fiber, since it is a type of optical waveguide. be able to. Here, regarding another condition, a phase matching condition, a method of performing phase matching without using birefringence will be explained. The normalized propagation constant b and V value of the guided mode of the optical fiber are b=(β/k−n ₂ )/(n ₁ −n ₂ ) (1) V=2πn ₁ a√2/λ (2) When defining, the relationship between the two is shown in FIG. However, β, k, and λ are the propagation constant in the longitudinal direction of the optical fiber, the wave number in vacuum, and the wavelength, respectively. The left and right integers ν and μ in the circle in FIG. 1 represent the mode orders in the circumferential direction and radial direction, respectively, in the LP〓〓 mode display. Now, as an example, a case of conversion to a second harmonic will be described. As shown in Fig. 2, the wavelength λ ₁ (=
The incident laser has a wavelength of 2πc/ω ₁ , C: the speed of light in vacuum), and the second harmonic V has a wavelength of λ ₃ (=λ ₁ /2).
The values and normalized propagation constants are V ₁ , b ₁ and
Considering the case where V ₃ and b ₃ and the incident laser beam is carried in the LP ₀₁ mode and the second harmonic is carried in the LP ₀₂ mode, the phase matching condition of equation (a) can be expressed as the following equation. δβ=4π{(n ₂ (3)−n ₂ (1))+Δ/1−Δ(n ₂ (3)b ₃ −n ₂ (1)b ₁ )}/λ ₁ (3) However, n ₂ (1) and n ₂ (3) are wavelengths λ ₁ and λ ₃ respectively
is the refractive index of the cladding. In Figure 3, λ ₁ is 1.064μm, Δ=0.02,
In the case of 0.03, the change in δβ with respect to V ₁ is shown. Note that the cutoff V value in the LP ₀₂ mode is V ₃ =4 as shown in the figure, and V ₁ must satisfy V ₁ >2. From the figure, in order to set δβ=0, for example, Δ=
It can be seen that for 0.02, V=2.18, that is, the core diameter should be 2.53 μm. Figure 4 shows the second harmonic.
This is the case when it is carried by LP ₃ mode, and λ ₁ is similarly
It is 1.064 μm. In addition, the cutoff V value of LP ₀₃ mode is
V ₃ =7.8, and V ₁ must be V ₁ >3.9.
From the figure, it can be seen that for Δ=0.014, the core diameter at which δβ=0 is 6.24 μm. As mentioned above, relative refractive index difference Δ
It is possible to satisfy the phase matching condition by appropriately selecting the core diameter and the core diameter, and given the current optical fiber manufacturing technology, it is possible to manufacture an optical fiber with the fiber parameters of this example. Therefore, it can be said that this is an effective phase matching method. Next, regarding the sum frequency and the difference frequency, similarly, based on the dispersion curve of the optical fiber waveguide mode shown in Figure 1, the mode that carries the light wave is appropriately selected, and the Δ or core diameter is adjusted relative to the wavelength of the incident laser light. It is possible to satisfy the phase matching condition by selection. For example, in the case of sum frequency, the incident laser light wavelength in LP ₀₁ mode is λ ₁ = 1.152 μm, λ ₂ = 1.064 μm.
By setting Δ=0.03 for m and 2.9 μm for the core diameter, δβ in formula (b) can be set to 0,
A sum frequency of λ ₃ =0.553 μm in LP ₀₂ mode can be obtained with high conversion efficiency. In addition, in the case of difference frequency, the incident laser light wavelength λ ₁ = 1.152 μm, λ ₂ = 0.553 μm
If m is LP ₀₁ and LP ₀₂ mode respectively, Δ=0.03, and core diameter is 2.9 μm, λ ₃ of LP ₀₁ mode =
A difference frequency of 1.064 μm is obtained. Next, when the phase matching condition is satisfied (Δβ=
0) conversion efficiency η will be explained. Since η is first proportional to the superposition integral of the field distribution of the incident laser beam and the mode carrying the generated light wave, a constant value can be obtained by selecting modes having the same order in the circumferential direction as shown in the embodiment. In addition, η is proportional to the square of the optical fiber length, and if it is about 1 m, an efficiency of about 10% can be obtained, compared to when using conventional crystals, the crystal length is limited to the cm order. An advantage of using optical fibers is that they can provide a sufficient nonlinear interaction length. However, in optical fibers, there is a slight fluctuation in fiber parameters in the longitudinal direction with a period of several meters, and if the fiber length is several tens of meters, the phase matching condition may shift locally in the longitudinal direction, and the effective optical The fiber length is slightly reduced by this shift. As described above, if an optical fiber is used as an optical wavelength conversion element for generating second harmonic, sum frequency, and difference frequency,
By appropriately selecting the core diameter and relative refractive index difference with respect to the wavelength of the incident laser light, the phase matching condition can be satisfied and the optical energy density within the optical fiber can be sufficiently increased. , it has the advantage of being able to convert a light wavelength different from the wavelength of the incident laser light with high conversion efficiency. Therefore, it has a wide range of applications as a light source for measuring the transmission characteristics of optical fibers or for spectroscopic measurements.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing dispersion curves of optical mode waveguide modes, and Figures 3 and 4 are diagrams numerically showing phase matching conditions in the case of second harmonic generation in the present invention. .

Claims (1)

[Claims] 1. An optical wavelength conversion element that is constructed of an optical fiber and converts an incident laser beam of angular frequency ω ₁ into a second harmonic of angular frequency ω ₃ (=2ω ₁ ) by a second-order nonlinear polarization effect. The core diameter and relative refractive index difference (Δ=(n ₁ ² −n ₂ ² )/2n ₁ ² ) of the optical fiber are expressed by the formula δ β=β ₃ −2β ₁ =0 (phase matching condition) β ₁ ; Phase constant of the incident laser beam in the optical fiber β ₃ ; Phase constant of the second harmonic in the optical fiber n ₁ ; Refractive index of the core of the optical fiber n ₂ ; Satisfy the refractive index of the cladding of the optical fiber. An optical wavelength conversion element characterized by being selected. It is composed of two optical fibers and converts the angular frequency of the two _incident laser beams into the sum frequency (ω ₁ +ω ₂ ) or the difference frequency (ω ₁ -ω ₂ ) using the nonlinear polarization effect (ω ₁ ≠ω ₂ ) _. An optical wavelength conversion element that converts the optical fiber into a core diameter and relative refractive index difference (Δ
(n ₁ ² −n ₂ ² )/2n ₁ ² ) is the formula [Table] Constant
An optical wavelength conversion element characterized by being selected so as to satisfy [Table].