JPH03278034A - Fiber type wavelength conversion element - Google Patents

Abstract

PURPOSE:To improve the output of an SH wave be providing the fiber type wavelength conversion element with at least a pair of electrodes arranged through a clad. CONSTITUTION:The optical fiber type second harmonic generating element (SHG) consists of a cylindrical core 10 constituted of non-linear crystal, a cylindrical clad layer 20 cylindrically surrounding the core 10 and at least a pair of electrodes 30 arranged through the clad layer 30 in their diameter direction. Both the electrodes 30 are connected to a prescribed power supply 40 and a prescribed voltage is impressed between the electrodes 30 so that the equivalent refractive index of a clad mode coincides with that of primary light. Thus, the refractive index of a core material is slightly changed by changing the impressed voltage and utilizing the electro-optical effect of the non-linear material charged into the core 10. Thus, the output of the second harmonic wave (SH wave) matching the phases of light can be automatically improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an optical fiber type wavelength conversion element using this Cerenkov radiation type phase matching.

BACKGROUND ART An optical wavelength conversion element is known that is formed in the shape of an optical fiber (hereinafter referred to as an eyebar) consisting of a nonlinear crystal core and a cladding surrounding the core and uses Cerenkov radiation type phase matching. This optical wavelength conversion element is an optical fiber type second harmonic generation element (Second 1lars).
onlcs Generator) (hereinafter referred to as SRG). In the Cerenkov radiation method, it is possible to generate second harmonic waves (hereinafter referred to as SH waves) that are automatically phase-matched.
G is applied to short wavelength light sources.

As shown in FIG. 4, the short wavelength light source includes a semiconductor laser 1, a coupling lens 2 for condensing the light emitted from the semiconductor laser and injecting the light into the end face of the SHG, and a core made of a nonlinear crystal. It is composed of the SHG 3 and an axicon 4 for shaping the wavefront of the SH wave converted and radiated by the wavelength conversion element and converting the SH wave into a parallel light beam.

FIG. 5 is a conceptual diagram of such an SHG, and this SHG consists of a cylindrical core 10 and a cylindrical cladding layer 20 concentrically surrounding the core 10.

In FIG. 5(a), the fundamental mode has an effective refractive index N(
When propagating from left to right in the figure through the core 10 with ω),
The nonlinear polarization wave that generates the SH wave also has the same phase velocity C/N.
(ω) (C: speed of light). This nonlinear polarization wave is SH at point A in the figure in a direction that makes an angle θ with the waveguide direction
Assume that a SH wave is generated, and after a unit time, an SH wave is generated again in the θ direction at point B as before. For example, if the SH wave generated at point A propagates through the cladding layer 20 and reaches point C after a unit time, and θ is such an angle that AC and BC are perpendicular to each other, then the SH wave generated between A and B is a nonlinear polarization wave. The wave front of the wave becomes BC, and as a result, a coherent SH wave is generated.

The SH wave generated in this way propagates as a cladding mode that repeats total reflection at the boundary between the cladding layer 20 and air, as shown in FIG.
The light is emitted in a conical direction in the direction determined by . 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.

As shown in FIG. 5, SHG was considered to have a fiber base in which the generated SH waves are reflected at the interface between the cladding and air and returned to the core. This is because it was thought that the returning SH waves would interfere with the SH waves that were generated there and would disappear as a result. Since conversion efficiency was thought to be proportional to the fiber group, fibers with large cladding diameters were used to improve conversion efficiency. Also,
In order to improve the conversion efficiency, it was thought that the conversion efficiency could be improved if the nonlinear material filled in the core had a large nonlinear constant, so no careful attention was paid to the selection of the fiber's cladding material, etc. .

However, it is difficult to produce fibers with such a large rad diameter, and in the end, if the diameter is small, only short fibers can be used, which hinders efficiency improvement. Furthermore, when a thin and long SHG is manufactured, it is difficult to achieve phase matching due to manufacturing errors in the core diameter or cladding diameter, errors in the refractive index of the core or cladding, and the like. Therefore, S
The disadvantage of the wavelength conversion efficiency of HG is that when it is actually manufactured, it cannot be made with high efficiency.

Summary of the Invention [Object of the Invention] The present invention has been made to improve these drawbacks, and provides a method for designing a fiber-type SRG with high conversion efficiency, making full use of the capabilities of the nonlinear material filled in the core. The present invention provides means for equalizing the propagation constants of a nonlinear polarization wave propagating in the core and a cladding mode propagating in the cladding, that is, a method of phase matching.

[Structure of the Invention] The SHG of the present invention includes a core and a cladding surrounding the core, and is characterized by being provided with at least one pair of electrodes sandwiching the cladding.

Further, in the SHG of the present invention, the refractive index of the primary light guided through the core is ηg, and the refractive index of the core with respect to the converted wavelength is n. It is characterized in that a material with na<ηg is used, and n5≧nc, where ηs is the refractive index of the fundamental wave of the cladding material.

Furthermore, in the SHG of the present invention, the refractive index of the primary light guided through the core is ηg, the refractive index of the core with respect to the converted wavelength is n6, and the LP01 mode propagating through the core is made of a material with na<ηg. The equipolarized refractive index n e t t is naI
A feature is that the cladding material and core diameter are selected so that I>na.

[Operation of the Invention] According to the present invention, the cladding material is selected according to the refractive index of the core material, and the refraction of the core material is achieved by applying voltage to a pair of electrodes and utilizing the electro-optic effect of the nonlinear material filled in the core. Phase matching can be achieved by slightly changing the index so that the equipolarized refractive index of the cladding mode and the equipolarized refractive index of the guided primary light are made equal.

Example Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 1 is a perspective view of a SHG according to the present invention.

This SHG includes a cylindrical core 10 made of nonlinear crystal and a cylindrical cladding layer 2 concentrically surrounding the core 10.
0 and at least one pair of electrodes 30 sandwiching the cladding layer 20 in the diametrical direction thereof. As shown in FIG. 2, the electrodes 30 are connected to a predetermined power source 40, and a predetermined voltage can be applied between the pair of electrodes 30. In this way, by changing the applied voltage, the refractive index of the core material is slightly changed by utilizing the electro-optic effect of the nonlinear material filled in the core 10.

Considering the existence of the boundary between the cladding and the core, making the cladding diameter finite like in an actual fiber, and analyzing the electromagnetic field, it was found that the SH wave is represented by a superposition of discrete modes propagating through the cladding. understood. There are a large number of equipolar refractive indices of such modes, which differ by only small amounts.

Among these modes, energy is transferred to a mode that is very close to the propagation speed of the nonlinear polarization wave, and the SH wave propagates.

This is very similar to SHG in the case of mode-mode phase matching, in which the SH wave propagates in the core and performs phase matching with the guided primary light. In the case of mode-mode phase matching, in order to create a mode of secondary light guided through a core that has an equipolarized refractive index equal to that of the guided primary light, the core diameter and refractive index must be adjusted extremely. need to be strictly controlled.

On the other hand, the mode guided through the cladding exists in the container even without strict manufacturing error control, and moreover, it exists extremely densely. Therefore, by slightly changing the refractive index by an electro-optic effect or the like, the equipolarized refractive index of the primary light can be changed to match the equipolarized refractive index of these cladding modes. In this way, the length of the fiber can be increased without making it thicker, and efficiency can be improved.

The SH wave output was analyzed using the following method, taking into account the effects of the boundary between the core and cladding and the boundary between cladding and air.

It is assumed that the waveguide direction is two axes, the core radius is a1, the length of the crystal is L, and an LP mode propagating here with a propagation constant β is guided. The nonlinear polarization pNL excited by this is defined below.

pNL-godcJo(ur)2Cxp(-12β2)
C: Constant, [2-ω2μ0ε1-β2ε,: Permittivity of the core SR wave E2''(r) in the core uses the dielectric constant ε1 of the crystal, and uses the Green's function CD(r, r') considering the boundary. , E”(r) −12ω2μ0εodC. GD(r, r') is the Green's function C in the entire space; using (r, r') and an arbitrary function A(λ), C
D(r, r')-G(r, r゛)+A(λ)Jo(rζ
)Jo(r'ζ).

G(r, r') is It is.

Here, Ho1l) and Jo are the zero-order Hankel function and Bessel function of the first kind, and ζ2−4ω2μ0ε1−λ
It was set as 2. The power P2'' of the SH wave can be determined by the following formula.

P2'-8ru3BOε02d2C2 (i: imaginary unit) The unit G (λ) can be determined from the boundary conditions, where the dielectric constant of the cladding is ε2 and the dielectric constant of air is ε. given that,
η, ξ are defined below, η2−4ω2μ0ε2−λ2 ξ2−λ2−4ω2uo E.

Functions A (λ), A' (λ), B (λ), C (λ),
C' (1), D(λ) as A(λ)ah(i[)J
+ (sl)(N+ (*ζ)JO(11)A'(λ)
=tJo(!r)J+ (*q)−ζJ1(1ζ)J
o(n)B(λ)-rNo(bthought)K1(bF)-calyx N
+(b)Ko(bF)C(λ) -rNo (B)J
l(![)-sN+ (tl)Jo i[)C'(λ)
-rNo(IW)N+ (tr)-IN+ (It)
No(1ζ)D(λ)−ξJo(bz)II (bF)
-1JI (bl) KO (bF), it becomes. The function G(λ) is a function having a first-order pole λ1 on the real axis, and the power of the SH wave expressed by equation (1) can be expressed by the orders of many of these poles.

That is, G(λ) is λ-λH(J-1, 2,...
), and if the order at this singularity is Re5G(λ, ), then it can be expressed as P2・−8π2ω3μ0ε02 d 2 (2. This discrete pole is the SH wave propagating through the cladding. In other words, the SH wave propagates in the cladding as a cladding mode with a propagation constant λ, and its power can be expressed as the sum of the powers of these modes.

The power of the SH wave is maximized when the propagation constants of the nonlinear polarization wave propagating in the core and the cladding mode propagating in the cladding are equal. That is, when the phase matching condition is satisfied. In other words, if 2β-λj is achieved, large power can be obtained.

In order to further improve the conversion efficiency, it is necessary to increase the value of the integral, which has a large effect on the power of harmonics. This is the overlap integral of the nonlinear polarization Jo(Ur)2 and the harmonic field distribution J0(rζ), and ζ−(2k) nc ” −n−x ′U−kηg
'-n, -1- ηg: refractive index of the core for the second harmonic n9: refractive index of the core for the fundamental wave 77att' is the equipolarized refractive index of the LP, mode of the fundamental wave propagating through the core.

In order to increase the chain of this integral, it is necessary to set the chain of n, . . . as close to the value of na as possible.

However, n, , is smaller than ηg, so for materials with large refractive index dispersion, 77 a t t will not be very close to nc, and the overlap integral will not become large. In order to improve efficiency, a material with small refractive index dispersion or a type such as d 23 + d 3 + where the polarization of the fundamental wave and the polarization of the secondary light are different is used, and ηg and n
6, a crystal light distribution with a small difference must be selected.

The core is made of nonlinear material DAN (-4-(N, N-dlm
etbylamlno)-3-acetamldonl
trobenzene), and the core diameter was 1.8 μm with SHG using SFI glass as the cladding glass.
1 fiber diameter is 1-m, fiber length is 6
Let it be mm. This fiber has a YA wavelength of 11084n.
A G laser is incident, and the incident light excites the LP01 mode. A configuration was considered in which the polarization direction is within the xz plane. Although the two axes of the crystal are inclined by 55'' with respect to the fiber axis, this configuration allows effective use of d23 of the nonlinear susceptibility tensor, and the nonlinear constants d, 1.
becomes del -40pm/V. Further, it is assumed that the power of the guided LP01 mode is 40 mW.

The refractive index of the core material DAN is for a wavelength of 11064n,
1.738, which is 1.7274 for the wavelength of the converted SH wave 532 nm, and the refractive index of SFI glass as the cladding material is 1°693 for the wavelength of 1164 nm, and the wavelength of the converted SH wave 532 nm. 1.72 against
It is 54.

In this case, ie, in a conventional SHG without an electrode pair, the SH wave is radiated into the air at 11.8 degrees. As a result of calculating the conversion efficiency of SH waves using this parameter,
The conventional one was 0.2796.

In order to check whether the phase matching condition is satisfied in such a conventional SHG, we calculated the position of the pole of Although it falls within the lobe, it does not match 2β. The spacing between the poles is the spacing of the propagation constant of the cladding mode, and if the propagation constant is divided by the wave number of the SH wave in vacuum and converted into a refractive index, it becomes about 0.5 Xl0-'. Phase matching can be expressed in terms of refractive index, in other words, the equipolarized refractive index N, 1 of the guided primary light.
The purpose is to match the equipolarized refractive index N'all of the cladding mode, and in this case, N a r r 1.7132O N'*f, -......,! , 71315.1.71
3219, . . . , which are close to each other but do not match, and it has been found that this is an impediment to improving efficiency.

Here, the difference between the equipolarized refractive index of the cladding mode and the equipolarized refractive index of the guided primary light is very small, and there is a possibility that they will match even if the refractive index changes due to temperature change.

Therefore, in the present invention, in order to actively match these, we focused on the electro-optic effect of the core material, and the first TI
! As shown in J, a pair of electrodes was provided around the cladding and a voltage was applied to the electrode pair to slightly change the refractive index.

Using the above parameters in the same way, we calculated the SH wave conversion efficiency for the SHG of this example, and found that the refractive index was only 0.
．． 12 When changing Xl0-', conversion efficiency is 0.54
I was able to get %.

Furthermore, in DAN, the polarization of the incident-order light is n g-1, since the polarization of the Z-direction secondary light uses the y-direction polarization.
7381: On the other hand, when na-1, 728, the refractive index for the secondary light is lower than the refractive index for the fundamental wave. In this case, SF4 glass is used for the cladding, and the core has a radius of 1.5 μ-
Then, it becomes n-x-1,729, and nc 2βmzt
'< 0. At this time, ζ becomes an imaginary number and becomes J. (ζ"
)−1°(lrlr).

10 (lrlr) is an increasing function and has a value of 1 or more. Therefore, in this case, the integral becomes very large, and a conversion efficiency of 3% was obtained under the same conditions.

Effects of the Invention As described above, according to the present invention, since the SHG is provided with at least one pair of electrodes sandwiching the cladding, the equirefractive index of the cladding mode and the equipolarized refractive index of the guided primary light can be By applying an electric field to the cladding and the core so that they match, the output of the SH wave can be improved.

[Brief explanation of the drawing]

Figure 1 is a perspective view of the SHG according to the present invention, Figure 2 is a sectional view of Figure 1, and Figure 3 is the main pole of 6 (λ) and 5 INK (λ) when the phase matching condition is not satisfied. A graph showing the relationship between lobes, FIG. 4 is a schematic diagram of a short wavelength light source using SHG, and FIG. 5 is a schematic cross-sectional view of the SHG. Explanation of symbols of main parts 3...SHG 10... Core 20... Clad 30... Electrode 40... Power supply

Claims (3)

[Claims] (1) A fiber-type wavelength conversion element comprising a core and a cladding surrounding the core, characterized in that at least a pair of electrodes sandwiching the cladding are provided. (2) The refractive index of the primary light guided through the core is η_g, the refractive index of the core for the converted wavelength is η_G, and η_
A fiber-type wavelength conversion element characterized in that, using a material where G<η_g, η_s≧η_G, where η_s is the refractive index of the fundamental wave of the cladding material. (3) The refractive index of the primary light guided through the core is η_g, the refractive index of the core for the converted wavelength is η_G, and η_
LP_0_1 using material with G<η_g and propagating through the core
The equivalent refractive index of the mode η_e_f_f is η_e_f_f>
A fiber-type wavelength conversion element characterized in that the cladding material and core diameter are selected so that η_G.