JPH03107929A - Fiber type optical wavelength conversion element - Google Patents

Abstract

PURPOSE:To attain miniaturization as improving conversion efficiency by forming a fiber type optical wavelength conversion element in which a specific condition can be satisfied among the radius of the element, a Cerenkov angle, and the wavelength of an incident beam. CONSTITUTION:The element is comprised of a cylindrical core 1 and a clad layer 2 enclosing the core by coming in contact with the outside of the core 1. And it is formed so as to satisfy the condition of equation I among the radius (a) of the fiber type optical wavelength conversion element, the Cerenkov angle theta, and the wavelength lambda of the incident beam assuming (m) as integer. In other words, it is comprised so that the radius of a fiber type SHG (secondary higher harmonic generating element) and the Cerenkov angle can be selected so as to set the optical path in the neighborhood of the reflection of an SH wave (secondary higher harmonic) reflecting on the interface of the clad layer 2 and the air at a value equivalent to the odd times the half of the wavelength of SH light. In such a way, it is possible to heighten the output of the SH light and to attain the miniaturization as improving the conversion efficiency.

Description

[Detailed description of the invention] Technical field The present invention relates to a fiber type optical wavelength conversion element.

BACKGROUND ART An optical pickup is known in which the wavelength of a laser beam emitted from a laser light source is halved using an optical wavelength conversion element, thereby making it possible to write and read information to and from a disk at higher density. (Refer to Japanese Unexamined Patent Publication No. 61-50122).

This optical wavelength conversion element is an optical fiber-type second harmonic generation element (Second) that uses a second-order nonlinear optical effect.
d Harmonies Generator) (
(hereinafter referred to as SHG). The optical fiber type SHG employs phase matching based on the Cerenkov radiation method. In this method, it is possible to generate a second harmonic wave (hereinafter referred to as an SH wave) whose phase is almost automatically matched. FIG. 2 is a conceptual diagram of an optical fiber type SHG, and this SHG consists of a cylindrical core 1 and a cylindrical cladding layer 2 concentrically surrounding the core 1.

In Fig. 2(a), the fundamental wave mode has an effective refractive index N(
When propagating from left to right in the figure through core 1 with ω), S
The nonlinear polarization wave that generates the H wave also has the same phase velocity C/N.
(ω) (C: speed of light). Assume that this nonlinear polarization wave generates an SH wave at point A in the figure in a direction that makes an angle of θ with the waveguide direction, and after a unit time, it generates an SH wave again in the θ direction at point B as before. . For example, the SH wave generated at point A propagates through the cladding layer 2 and reaches point C after a unit time,
If θ is such an angle that AC and BC are orthogonal, the wavefront of the SH wave generated between AB by the nonlinear polarization wave will be BC,
In the end, a coherent SH wave is generated.

The refractive index of the cladding layer 2 with respect to the wavelength of the SH wave is ns+i
(2ω), this phase matching condition is N (ω) −nc164 (2(IJ) coma
−・・−(1). In other words, N(ω) × n61.4(2ω) ・・・・・・(
2), SH waves are automatically generated in the θ direction with phase matching. Generally, the refractive index of the cladding layer 2 and the core 1 with respect to the fundamental wave is nglad (
ω) and n(ω), assuming the overlayer is air,
The conditions for the fundamental wave to propagate in core 1 as a mode are n, +, a (ω) < N ((IJ) < n (ω)
-(3). Also, considering the wavelength dispersion of the refractive index of the cladding layer 2, nglad(ω)<nc+□(2ω
), so n clad (ω) < n ((JJ) < n
coma (2ω)...If the condition (4) is satisfied, then equation (2) is satisfied for all fundamental wave modes for any core diameter. Also,
If n g, am (”) < n clad (2ω)
< n ((4)), but within a certain range of film thickness (2)
There exists a fundamental mode that satisfies the equation.

The SH waves generated in this way propagate as a cladding mode that undergoes repeated total reflection at the boundary between the cladding layer 2 and air, as shown in Figure 2(b), and form a conical shape from one end of the fiber in the direction determined by θ. It is emitted to Further, the equiphase front of the output wavefront of the SH wave outputted in this manner has a conical shape with the central axis of the fiber as its axis.

In this fiber type SHG, the generated SH waves are reflected at the interface between the cladding layer 2 and the air, and the cladding layer 2
Guide the wave inside. However, the reflected SH wave whose phase has been reversed by being reflected at the boundary surface returns to the core 1, causing interference with the SH wave newly generated in the core 1. This interference reduces the output of the fiber type SHG. To prevent this, fiber type SH
The action length of G, that is, the fiber length must be set to a length that does not allow the reflected SH wave to return to the core 1.

Furthermore, since the conversion efficiency of SHG using Cerenkov phase matching is proportional to L, in order to increase the fiber length, the radius of the cladding layer 2 may be increased. However, since the fiber type SHG is incorporated into an optical pickup device, it is difficult to make it too large due to the demand for miniaturization of the optical pickup device.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a fiber-type SHG that is downsized while improving conversion efficiency.

The fiber-type SHG according to the present invention is a fiber-type optical wavelength conversion element that includes a cylindrical core and a cylindrical cladding layer surrounding and in contact with the cylindrical core, and converts the wavelength of an incident beam and outputs it as a beam of a predetermined wavelength. When a is the radius of the fiber type optical wavelength conversion element, θ is the Cerenkov angle, λ is the wavelength of the incident beam, and m is an integer, 2・a−51nθ−(λ/2) (2m+1
) is characterized by a shape that satisfies the following conditions.

EXAMPLE Hereinafter, an example of the present invention will be explained in detail based on the drawings.

In FIG. 1, the fiber type SHG according to the present invention is
It is composed of a cylindrical core 1 and a cladding layer 2 surrounding and in contact with the outside of the core 1.

In FIG. 1, when the incident beam propagates through the core 1 from left to right in the figure, a nonlinear polarization wave that generates an SH wave also propagates. This nonlinear polarization wave generates an SH wave at point D in the figure in a direction that makes an angle of Cerenkov angle θ with the waveguide direction, and the SH wave is reflected at point E on the interface between the cladding layer 2 and air and has a half wavelength. It becomes a shifted secondary SH light, which returns to core 1 again,
Interference occurs with SH light generated separately in the cladding layer 2. If the phases of these SH destinations are shifted by half a wavelength, the SH
The lights weaken each other, and when the phases match, the SH lights strengthen each other.

As shown in the figure, if the optical paths before and after reflection of the SH wave reflected at point E on the interface between the cladding layer 2 and air are respectively Ω, then the SH wave reflected at point E is shifted by half a wavelength, so 2J-( When λ/2) (2+a+1) (where λ is the wavelength of the incident beam and m is an integer) is satisfied, the phases of the SH waves before and after reflection match.

From FIG. 1, the relationship between the optical path Ω and the Cerenkov angle θ is (1/a-sinθ), so when the radius of the fiber-type SHG consisting of the cylindrical core 1 and the cladding layer 2 is a, 2・a -8I
When the dimensions of the fiber-type SHG are set to satisfy nθ-(λ/2) (2+a+1), the phases before and after reflection of the SH wave reflected at point E on the interface between cladding layer 2 and air match. do.

In this way, by selecting the radius a of the fiber-type SHG, the secondary SH light reflected at the interface between the cladding layer 2 and the air returns to the core 1 and matches the phase of the SH light that is about to be generated there. It is possible to cause such interference and increase the output of the SH light. This is because, as shown in FIG. 1, this constructive interference occurs when the optical path 2II of <SH light is equal to an odd multiple of half the wavelength λ.

Effects of the Invention As described above, in the fiber-type SHG according to the present invention, the optical path before and after reflection of the SH wave reflected at the interface between the cladding layer and the air is made equal to an odd multiple of half the wavelength of the SH light. Since the radius and Cherenkov angle of the fiber type SHG are selected, it is possible to increase the output of the SH light and improve the conversion efficiency while reducing the size.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention, and FIG. 2 is a conceptual diagram of a fiber type SHG using Cerenkov radiation phase matching. Explanation of symbols of main parts 1... Core 2... Clad layer Figure 1 Door Figure 2

Claims (1)

[Claims] A fiber-type optical wavelength conversion element consisting of a cylindrical core and a cylindrical cladding layer surrounding and in contact with the cylindrical core, converting the wavelength of an incident beam and emitting it as a beam of a predetermined wavelength, wherein a is When it is the radius of the fiber-type optical wavelength conversion element, θ is the Cerenkov angle, λ is the wavelength of the incident beam, and m is an integer, 2・a・sinθ=(λ/2)(2m+1) A fiber-type optical wavelength conversion element characterized by having a shape that satisfies conditions.