JPH0336530A - Optical shaping device - Google Patents

Abstract

PURPOSE:To shape light into light having superior convergence characteristics by returning radiating light emitted from the linear light source of a waveguide, etc., to the waveguide again. CONSTITUTION:Light emitting from a semiconductor laser 10 is converged by a lens 20 and made incident as (z)-axial polarized light (TM polarized light) on a 1st (y)-propagation waveguide 32 on a 1st substrate 30 and a higher harmonic (TM polarized light) which has its axis of polarization on a zy plane is radiated in the substrate 30 and projected from a projection-side end surface 35. A beam A is made incident on a lens 40 to become a parallel beam, which has its polarizing direction rotated by 90 deg. by passing through a lambda/4 plate 44 to become (x)-axial TE polarized light, which is projected as a beam B by passing through a 2nd conical plane lens 48. The beam B is made incident on the end surface 55 of a 2nd substrate 50 and converged on a 2nd waveguide 52, and the light is propagated in the 2nd waveguide 52 as (z)-axial polarized light (TM polarized light) by polarization mode conversion coupling and projected as the shaped light from its end surface. Consequently, the light is converted to a shape which has excellent light convergence characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical shaping device suitable for shaping light emitted with directionality from a linear light source such as a waveguide so that it can be narrowly focused. More specifically, the present invention relates to an optical shaping device that can be used to narrowly focus short wavelength light in order to increase the recording density of an optical disc.

<Prior Art> In recent years, optical wavelength conversion has been attempted as a method of obtaining short wavelength laser light using a small device. Typical methods of optical wavelength conversion include sum frequency generation, in which the frequencies of multiple lights are added together, second harmonic generation, in particular, in which two or three frequencies of light of the same frequency are added together; There is a method called third harmonic generation. Currently, the second harmonic can be generated using a YAG (YAG) laser with a wavelength of 1.06 μm, for example, with a wavelength of 0.0 μm.
Generation of blue laser light of 0.415 to 0.42 μ9 has been realized using a green laser beam of 53 μ9 and a semiconductor laser having a wavelength of 0.83 to 0.84 μ.

As one such example, an optical waveguide was fabricated by proton exchange on a LiNbO3 substrate, and a 0.84 μyrb
*0.0 with 1% conversion efficiency from 40mW semiconductor laser light.
A report on the generation of a 4 mW second harmonic was published in Applied Physics, Vol. 56, No. 12, pp. 1637-1641 (1987).
) is described in J. When a semiconductor laser beam is incident on a waveguide with a door width of 2μ and a depth of 0.4μ, harmonics are emitted approximately 16.2° below the substrate surface. In this case, the phase matching condition between the fundamental wave and the harmonics is almost automatically satisfied, and there are no restrictions on the angle between the light beam and the crystal or the temperature of the crystal.

<Problems to be Solved by the Invention> The first reason for using short wavelength light sources in optical discs is to improve recording density, and it is essential to realize good light focusing characteristics. However, in the case of harmonic generation according to this example, there is a problem in that it is difficult to focus the beam. Ordinary lenses are designed on the premise that the light source is point-like, and cannot be used as is. A cylindrical lens that corrects light from a linear light source as if it were emitted from a point light source is effective when the linear light source is perpendicular to the optical axis, but as in this case, the direction of the linear light source is If it is close to , the correction will be insufficient.

Means for Solving the Problems> The present invention is directed to ``waveguide mode/radiation mode coupling by polarization mode conversion,'' in which light emitted with directionality from a line light source such as a waveguide-type optical wavelength conversion element is applied to a birefringent substrate. This phenomenon is used to propagate through a waveguide, and the light is emitted from its end face or grating coupler, converting it into a light shape with good light focusing characteristics.

<Function> Of the two refractive indexes for orthogonal polarized light components on a birefringent material, the smaller one is n (a) and the larger one is n
(b). For example, in a negative uniaxial crystal, the extraordinary refractive index is (a) and the ordinary refractive index is n(b).

Waveguide mode/radiation mode coupling by polarization mode conversion is
When the isoretic refractive index of the guided mode is Neff (a) and the refractive index of the substrate for polarized light perpendicular to the guided mode is n - (b), the following equation occurs when N eff (al < no(b)). At this time, the angle of the direction of propagation of the emitted light with respect to the waveguide is given by cos' (Neff (a)/n-(b)).

Note that this mode conversion is written on ■LiNbO3 and the y board,
Using an X plate, shift the propagation direction from the optical axis, ■L 1Nb
03 * Utilizing the electro-optic effect in L 1Tao3, ■Increasing the amount of incident light in LiNbO3 beyond the threshold value at which the photo-induced refractive index effect occurs (if the wavelength is short, the photo-induced refractive index effect can occur with a small amount of light), ■When fabricating a proton-exchanged LiNbO3 waveguide, rapid heating and cooling are performed to generate crystal distortion in the waveguide, polycrystalization of the waveguide, and variation in particle orientation, which can be achieved efficiently. can.

<Examples> Hereinafter, the present invention will be described in detail based on Examples, but the present invention is not limited thereto.

(Example 1) FIG. 1 is a sectional view of an optical shaping device that is a first example of the present invention. Light (fundamental wave) emitted from the semiconductor laser 10 with a wavelength of 0.83 μ9 and an output of 100 mW is focused by a lens 20, and is transmitted through a y-propagating first guide on a first substrate 30 made of a two-cut MgO-doped LiNbO3 substrate. The light enters the wave path 32 as biaxially polarized light (7M polarized light). A harmonic wave (7M polarized light) having a polarization axis in the zy plane is emitted from each part of the first waveguide 32 into the substrate, and is emitted from the emission side end face 35 of the first substrate 30 (beam A).

The beam A enters a lens 40 having a conical surface on one side and becomes a parallel beam. Subsequently, the polarization direction is rotated by 90' by passing through the Ge4 plate 44, and the polarization in the X-axis direction (substrate 30.5) is rotated by 90'.
TE polarization). This light passes through the second conical lens 48 and is emitted as a beam B. Note that since the exit angle of the beam A from the first waveguide 32 and the coupling angle of the beam B to the second waveguide 52 are generally different, the surface shapes of the lenses 40.48 are different.

The beam B is incident on the end surface 55 of the second substrate 50, is focused on the second waveguide 52, and is converted into 2 by polarization mode conversion coupling.
The light propagates through the second waveguide 52 as axially polarized light (TM polarized light) and exits from the end face as shaped light. Note that LiNbO3 is a negative uniaxial crystal with the Z-axis as the optical axis, and light polarized in the Z-axis direction has an extraordinary refractive index, while light polarized in the X-axis direction has an ordinary refractive index, and the ordinary refractive index is larger. , a coupling condition exists between the X-axis polarized radiation mode and the biaxially polarized propagation mode. However, since the waveguide mode senses an isolateral refractive index that is slightly higher than the refractive index of the substrate, there are cases where the extraordinary light equivalent refractive index is larger than the ordinary light refractive index and does not satisfy the coupling condition.

Here, the second substrate 50 was manufactured using the same material and orientation as the first substrate 10, and the waveguide 52 was manufactured in the same process as the waveguide 32 except for the width and depth. This happens to be L i N b 03 1
This is because the material was excellent in both harmonic generation efficiency and polarization mode coupling efficiency, so there is no problem in using another material.

The waveguides 32 and 52 were manufactured as follows. 2
It was immersed in birophosphoric acid at 20° C. for 25 minutes to form a proton exchange layer with a depth of about 64 μm. The waveguide width is 2μ for the waveguide 32 and 1.45μ for the waveguide 52. The waveguide 52 was fabricated with several line widths in consideration of variations in isolateral refractive index, and the waveguide 52 with the highest coupling efficiency was selected as -V/. Although hyrolic acid was used for proton exchange, the present invention is not limited to this, and other methods such as phosphoric acid,
Benzoic acid may also be used.

The input and output surfaces of the substrates 30 and 50 are optically polished.
Antireflection coating was applied to each wavelength. Button lens 40.48. The surface of the λ/4 plate 44 was also coated with an antireflection coating.

Here, the beam shape is completely axially symmetrical for the beam B with respect to the waveguide, whereas the beam shape for the beam A is slightly different from the symmetrical shape. This is because the LiNbO3 substrate 30.50 is a uniaxial crystal with two optical axes, and the beam B is polarized light (TE
However, the beam A has a polarization axis in the zy plane (TM polarization), and the downward light and the sideways light have two polarization directions.
This is because the angles with respect to the axis are different. However, in reality, most of the light intensity of beam A is concentrated on the lower side.

Therefore, the lens 40 was selected depending on the angle of the downward emitted light.

According to experiments, the amount of fundamental wave light was 100 mW, including 32 waveguides.
When the amount of light coupled to is about 35mW, the harmonic wave is about 0.5m
W was generated, and 40% of it was emitted from the waveguide 52.

(Example 2) A second embodiment f of the invention is shown in FIG.

The main points different from the first embodiment are as follows.

The substrate 50 is an X-cut LiN b that is not doped with MgO.
An Os substrate was used, and the waveguide 52 was fabricated by a Ti diffusion method. The line width of the waveguide 52 was 3 μm. The estimated waveguide depth is 2.
It is a 7μ door. In the Ti diffused waveguide, since the difference in the refractive index of debris between the substrate and the waveguide is small, single mode conditions can be maintained even if the waveguide depth is increased in this way. Waveguide 1il? By setting the aspect ratio of the surface to approximately 1, the shape of the emitted light from the waveguide 52 becomes suitable for condensing light. Furthermore, since the refractive index difference is small, the polarization mode conversion radiation angle of the Ti diffused waveguide mode is almost constant (mostly determined by the birefringence of the substrate), which facilitates manufacturing.

Since the X-cut substrate is used as the substrate 50, the TM radiation mode and the TE propagation mode are combined in the substrate 50. Therefore, the TM emitted light from the waveguide 32 can enter the substrate 50 with the same polarization, and the λA plate 44 becomes unnecessary. Along with this, lenses 40 and 48 were bonded together.

The waveguide was fabricated in a direction inclined by about 20' with respect to the two axes of the crystal. This is because the coupling coefficient between the radiation mode and the propagation mode becomes maximum at this time.

A YAG laser was used as the light source 11. Since the wavelength has changed, the depth of the waveguide 32 was set to approximately 0.55μ in order to maintain optimum harmonic generation efficiency.

The fundamental wave enters the waveguide 32 and the harmonic wave exits from the waveguide 52 not from the end face but from the grating coupler 31.5.
3 was used. This method is suitable for mass production at low cost because it does not require end face polishing.

According to experiments, the amount of light generated from the waveguide 32 was 0.3 m
When W was used, the amount of light emitted from the waveguide 52 was 70% of the amount of generated light.

(Embodiment 3) FIG. 3 shows an embodiment in which the present invention is applied to an optical fiber.

This optical fiber was produced as follows.

The diameter of the LiNbO5 crystal is 1 so that the optical axis direction is the y-axis.
Cut out into a cylindrical shape with a diameter of 00μ. A thin MgO film layer is formed on the surface. Thereafter, MgO is diffused at high temperature to make the diffusion region a cladding layer. By appropriately adjusting this diffusion condition, the core diameter can be controlled. Next, the shape of the crystal is processed and polished into a tapered shape so that the size of the circle differs between the tip and end surfaces, the second cladding 120 made of acrylic resin is coated, and finally the tip and end surfaces are Optically polished.

Light from the semiconductor laser light source 10 with a wavelength of 0.83 μm was focused onto the core 110 by the lens system 20 and entered as polarized light in the X-axis direction of the crystal. In this case, the second harmonic of the X-axis polarization is radiated to the cladding 100 at a constant angle due to the nonlinear optical coefficient dll of the core 110. This light is totally reflected at the interface between the cladding 100 and the second cladding 120. The interface forms a taper angle with respect to the optical axis direction, and the button 9
, it is possible to match the harmonic radiation angle at the front of the core with the radiation/waveguide mode coupling angle at the rear of the core. Therefore, the harmonics reflected at the interface became a propagation mode polarized in the biaxial directions of the core 110, and were emitted from the end of the core as light from a good point light source.

In this embodiment, the optical wavelength conversion section at the front of the core and the optical shaping section at the rear of the core are integrated, but optical fibers having respective functions may be combined. At that time, optical systems 40 to 48 used in Examples 1 and 2 may be used between the optical wavelength conversion fiber and the optical shaping fiber.

Among the above embodiments, various materials can be used for the substrate 50 or the cladding 100 as long as they have birefringence and are easy to fabricate a waveguide. In addition to LiNbO3, typical examples include:
L 1Ta03 , KNbO3+ β-BaB20a
, BNN (Ba2NaNbs015), KDP (KH2
PO,), KTP (KT iOP 04), KT
A (K T t OA s 04 ), B
Inorganic nonlinear optical materials such as a T r 03, 5iC)
+ (crystal).

ZnS, Zn5e, CaCO3, PLZT(Pb-La
-Zn-Ti.0 compound), T i02. Al2
O3* Fe2O3+ Fe203. NaNO3,c
as.

CaAs, GaP, GaAlAs, InGaAIP.

Inorganic materials such as I nGaAsP, and MNA.

M A P (3-methyl-2,4-dini
trophenylaminopropanatc),
mNA (metanitroanil), LAP (L-
Arginine Phosphate Monohydr
organic nonlinear optical materials such as DLAP (deuterated LAP), urea, polyvinyl alcohol, acrylic, and PMMA that have been oriented by stretching to give them birefringence.
(polymethyl methacrylate), PVDF (polyvinylidene fluoride), etc. may be used.

<Effects of the Invention> The light shaping device of the present invention can return the emitted light emitted from a linear light source such as a waveguide to the waveguide, and can shape the light into light with excellent focusing characteristics. Therefore, it becomes possible to realize a high-density optical disk using short wavelength light emitted from the waveguide.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the structure of the first embodiment of the present invention;
The figure is a schematic diagram showing a second embodiment of the invention, and FIG. 3 is a sectional view showing a third embodiment of the invention. 10... Sea 4 light source 20... Lens 30.
... Optical wavelength conversion waveguide substrate 32 ... Optical wavelength conversion waveguide 40.48 ... Conical lens 44 ...
λ/4 plate 50... Optical shaping waveguide substrate 52...
・Optical shaping waveguide

Claims (1)

[Scope of Claims] 1. A first optical waveguide formed on a first substrate or an optical fiber; a second optical waveguide formed on a second substrate or optical fiber and made of a birefringent material; The incident angle and polarization direction of light emitted from the first optical waveguide with directionality toward the first substrate or optical fiber to the second optical waveguide are set, and the polarization mode conversion coupling is performed to An optical shaping device comprising: a converging optical system that propagates light to a second optical waveguide, and emits shaped light from the second optical waveguide.