JPH0496027A - Second harmonic wave generating element - Google Patents

Abstract

PURPOSE:To obtain second harmonic waves of a high output by forming a clad layer which satisfies specific equation on a thin-film waveguide layer formed on a substrate. CONSTITUTION:Various optical materials are usable for the substrate, the thin- film waveguide layer and the clad layer. For example, LiNbO3, alpha-quartz, etc., are usable as the thin-film waveguide layer; for example, LiTaO3, SiO2 as the substrate and ZnO, MgO, etc., as the clad layer. The refractive indices of the materials for the above-mentioned substrate and thin-film waveguide layer can be adjusted by incorporating different elements, such as Na, Cr, Mg, Nd, Ti, and V, therein. Above all, the combination using the LiTaO3 single crystal as the substrate, the LiNbO3 as the thin-film waveguide layer and the ZnO as the clad layer is more adequate.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a second harmonic generation element (SHC) that is easy to manufacture and has high conversion efficiency.

(Prior art) An SHC element is an element that converts the wavelength of incident light to 1/2 and outputs it by utilizing the nonlinear optical effect of an optical crystal material that has a nonlinear optical effect, and the wavelength of the output light is is 1/2
Because it is converted to be able to.

Conventionally, a bulk single crystal nonlinear optical crystal using a high-power gas laser as a light source has been used as a SHO element. However, in recent years, there has been a strong demand to downsize entire devices such as optical disk devices and laser printers, and gas lasers have
Optical modulation requires an external modulator, which is not suitable for miniaturization, allows direct modulation, and is cheaper than gas lasersT: SHG elements that can use easily handled semiconductor lasers are required. ing.

By the way, when a semiconductor laser is used as a light source, the output of the semiconductor laser is generally low, ranging from several mW to several tens of mW.
HG elements are required. (Miyama, Miyazaki; Institute of Electronics and Communication Engineers Technical Research Report, ○QE75- (1975), Miyazaki,
Hoshino, Akao.

Electromagnetic Field Theory Study Group Materials, EMT-78-5 (1978)
).

In other words, ordering second harmonic light using a thin film waveguide is as follows:
■ Being able to utilize the energy of light concentrated in a thin film;
2. Because light waves are confined within a thin film and cannot spread out,
3. The interaction can take place over long distances; 3. This is because even materials that cannot be phase-matched in bulk can be phase-matched by utilizing the mode dispersion of a thin film.

However, in order to achieve high conversion efficiency in a SHO device with a thin film waveguide structure, it is necessary to match the thickness of the waveguide layer to the theoretical phase matching film thickness, and the allowable range of the phase matching film thickness is 100%. Since it is less than 1/1 μm, high precision lI
! Thickness opening control technology was essential.

The reason why the allowable range of the phase matching film thickness is narrow is explained as follows.

When considering the electric field distribution in the substrate, WI film waveguide layer, and cladding layer when fundamental wave laser light or second harmonic light propagates through the thin film waveguide layer, if the cladding layer is air, air refraction Since the refractive index is 1, which is smaller than the refractive index of the substrate, the electric field distribution of the fundamental laser beam or the second harmonic beam is not symmetrical.

Further, the output of the second harmonic light emitted from the thin film waveguide layer is proportional to the overlapping integral of the electric field distribution of the fundamental wave laser light and the electric field distribution of the second harmonic light.

However, if the thickness of the thin film waveguide layer does not completely match the theoretical phase matching # thickness, the electric field distribution of the second harmonic light becomes thick (shifts toward the substrate side, and the electric field distribution becomes asymmetrical). As a result, the overlap of the electric field distributions is significantly reduced, and the output of the second harmonic light is reduced. Therefore, the allowable range of the phase matching film thickness of the thin film waveguide is significantly narrowed.

As a result of intensive research, the present inventors have newly discovered that the allowable range of phase matching film thickness can be expanded by forming a cladding layer having a specific refractive index condition on a thin film waveguide layer. completed.

(Means for Solving the Invention) The present invention provides a second harmonic generation element in which a thin film waveguide layer is formed on a substrate, and a cladding layer is formed on the thin film waveguide layer. Become,
The cladding layer is characterized in that it satisfies relational expressions 1) and 2).

no, -0,50≦noc≦nos-o, os・H・Formula 1) nes-0,70≦nec≦no os 0.15
...Formula 2) n, s: Ordinary refractive index of the substrate at the fundamental laser light wavelength (λμm) noc: Ordinary refractive index of the cladding layer at the fundamental laser light wavelength (λμm) net: Second harmonic wavelength (λμm) /2) Extraordinary optical refractive index of the substrate n□: Extraordinary optical refractive index of the cladding layer at the second harmonic wavelength (λμm/2) (effect) The SHG element of the present invention has a thin film waveguide layer formed on the substrate. It is essential that a cladding layer is further formed on the thin film waveguide layer.

The reason for this is that by providing the cladding layer on the thin film waveguide layer, the substrate, the thin film waveguide layer, and the cladding layer become nearly symmetrical with respect to the refractive index. The distribution can be made symmetrical, and even if the thickness of the thin film waveguide layer does not completely match the theoretical phase matching film thickness, the decrease in the output of the second harmonic light can be alleviated. This is because an SHG element with a wide tolerance range and high conversion efficiency can be obtained.

In addition, the cladding layer acts as a protective layer and can prevent damage to the waveguide layer and light scattering due to adhesion of dust and dirt, and completely prevent chipping (pi-notching) of the waveguide layer, which is a problem when polishing the end face. can.

Furthermore, the cladding layer needs to satisfy relational expressions 1) and 2).

n1le' Extraordinary refractive index of the cladding layer at the second harmonic wavelength (λμm/2) This is because the cladding layer satisfies the above equations 1) and 2), so that the second harmonic light and the fundamental wave laser This is because the electric field distribution of light can be minimized, the allowable range of phase matching film thickness is wide, and an SHG element with high conversion efficiency can be obtained.

In particular, in order to expand the allowable range of phase matching errors in film thickness, it is preferable to satisfy equations 3) and 4).

nos 0.50≦00ゎ≦nos-0,05...Formula 1) nes-0,70≦nec≦n's-0,15...
・Formula 2) nos-0,25≦n6c≦n1lf O,
10” 'Equation 3) nils 0.55≦n-≦nes
--0.20...Formula 4) nos: Ordinary refractive index of the substrate at the fundamental laser light wavelength (λμm) noc: Ordinary refractive index of the cladding layer at the fundamental laser light wavelength (λμm) n-: Second harmonic Wave wavelength (λμm/2) 4 Extraordinary refractive index of the substrate n03/Ordinary refractive index of the substrate at the fundamental laser light wavelength (λμm) nOc: Ordinary refractive index n of the cladding layer at the fundamental laser light wavelength (λμm) , Extraordinary optical refractive index of the substrate at the second harmonic wavelength (λμm/2) n1 IC'Extraordinary optical refractive index of the cladding layer at the second harmonic wavelength (λμm/2) The SHO element of the present invention has a thin film waveguide layer. The incident angle (θ) of the fundamental laser beam with respect to the optical axis (Z axis) of is 0±15@
Alternatively, it is preferably within the range of 90±15°.

The reason is that the incident angle (θ) of the fundamental laser beam is
This is because within the above range, the conversion efficiency to the second harmonic is extremely high. The angle of incidence of the fundamental laser beam is preferably within the range of 0±5° or 90±5°.

The wavelength (λ) of the fundamental laser light incident on the SHO element of the present invention is preferably 0.4 to 16 μm.

The reason for this is that although it is advantageous for the fundamental wave laser beam (λ) to have a wavelength as short as possible, it is practically impossible to generate a laser beam with a wavelength shorter than 0.4 μm using a semiconductor laser. This is because it is difficult to
On the other hand, when a fundamental laser beam with a wavelength longer than 1.6 μm is used, the wavelength of the second harmonic obtained is 1/2 of the fundamental laser beam, so it can be generated relatively easily by directly using a semiconductor laser. This is because there is no advantage in using the SHG element in the wavelength range where it can be used. The wavelength (λ) of the fundamental wave laser light is advantageously 0.6 to 1.3 μm since semiconductor laser light sources are relatively easily available, and 0.68 to 0.94 μm is particularly preferred in practice.

The thickness of the thin film waveguide layer of the SHG element of the present invention is 0°1 to 2
Preferably, it is 0 μm.

The reason is that when the thickness of the thin film waveguide layer is thinner than 0.1 μm, it is difficult to make the fundamental laser beam enter.
This is because it is difficult to obtain a substantially high SHO conversion efficiency due to the low incident efficiency.On the other hand, if the thickness is more than 20 μm, the optical power density is low and the SHC conversion efficiency is low.
This is because in either case, it is difficult to use it as an SHG element. The thickness of the thin film waveguide layer is preferably 0.5 to 10 μm, and particularly preferably 1 to 8 μm for practical purposes.

Further, the thickness of the cladding layer of the SHO element of the present invention is 0.
A thickness of 2 to 30 μm is desirable. The reason for this is that if the thickness is less than 0.2 μm, the guided light cannot be confined;
This is because if it is thicker than 0 μm, the crystallinity of the cladding layer decreases, resulting in a decrease in optical characteristics.

The cladding layer preferably has a thickness of 0.5 to 10 μm, and has a thickness of 1 to 10 μm.
8 μm is suitable.

Various optical materials can be used for the substrate, thin film waveguide layer, and cladding layer in the present invention, and examples of the thin film waveguide layer include LiNbO3, α-quartz, KTiOPO, (KTP
), β-BaBz O-(BBO), KB, Os ・
4H,O (KBS), KH,PO, (KDP),
KD2 PO, (KD''P), NH,'
Hz POn (ADP), C1H2ASO.

(CDA), C6D, Ash, (CD'''A), Rb
H, PO, (RDP), RbH2ASO, (RDA)
, B e SO-・4 Hz 03LiCI0゜3H
20, Li1O:+, α-LiCdB○3, LiB, 0
5 (LBO), urea, polyparanitroaniline (p
-PNA), polydiacetylene (DCH), 4-
(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN), 4-nitrobenzaldehyde
Examples of substrates include hydrazine (NBAH), 3-methoxy-4-nitrobenzaldehyde hydrazine, and 2-methyl-4-nitroaniline (MNA); for example, LiTaO3, SiO! , alumina, KTP, BBO
lLBOlKDP, and @image compound, soda glass,
Pyrex glass, polymethyl methacrylate (PMM)
A) etc. can be used, and as the cladding layer,
ZnO2MgO, AlzOx, PMMA, SiO
□, Pyrex glass, soda glass, etc. can be used.

The material for the substrate and thin film waveguide layer is NaCrM
By containing different elements such as g, Nd, Ti, and V, the refractive index can be adjusted.

As a method for incorporating different elements such as Na, Cr, Mg, Nd, Ti, and V, the raw material and the different elements or different element compounds are mixed in advance, and the mixture is deposited on the substrate by liquid phase epitaxial growth. ``Method for forming a film waveguide layer, or the substrate or TRW waveguide layer may be coated with Na,
It is desirable to use a diffusion method for diffusing different elements such as Mg, Nd, and Ti.

Among the combinations suitable for the SHG element, LiTa0. LiNb as single crystal, thin film waveguide layer
A combination using O5 and Zn○ as the cladding layer is suitable.

The reason is that the LiNb0. L i T aO3 has a large nonlinear optical constant, low optical loss, and can form a uniform film.
The crystal structure is similar to that of iNbos, and the above Li
This is because it is easy to form a thin film of NbOz, and it is easy to obtain high-quality and inexpensive crystals, and Zn○ can be easily formed into a homogeneous film by the sputtering method or the like.

Furthermore, the SHO element of the present invention is advantageously of a channel type with a width of 1 to 10 μm.

The reason why a channel type SHG element is advantageous is that the optical power density can be higher than that of a slab type, and the reason why a width of 1 to 10 μm is advantageous is as follows.
This is because if the width is smaller than 1 μm, it is difficult to introduce the incident light into the waveguide, and the incidence efficiency is low, resulting in a low SHG conversion efficiency. This is because if it is large, the optical power density decreases, resulting in a decrease in SHG conversion efficiency.

The SHG element of the present invention has a transmittance of 100 for second harmonic light.
%, a wavelength-selective film (filter) that does not transmit the fundamental laser beam is formed behind or directly on the light output end face, or the transmittance of the second harmonic light is low. The polarization plane of the polarizing plate or polarizing film that is 100% or nearly 100% is 9% the polarization plane of the fundamental laser beam.
It is preferable that they be formed directly behind the output end surface or directly on the output surface so as to differ by 0°.

The reason for this is that unnecessary fundamental laser light can be removed from the emitted light and only the necessary second harmonic light can be efficiently extracted.

Note that the filter and the polarizing plate or polarizing film may be formed at the same time.

In addition, by forming the wavelength-selective thin film directly on the output end face and adjusting it to satisfy the anti-reflection condition for the second harmonic light, it is possible to reduce the refraction between the thin film waveguide layer such as a lithium niobate single crystal thin film and the air. Since there is a large difference in the ratio, it is possible to reduce the loss due to reflection that occurs at the output end face, and it is possible to improve the SHG output.

The wavelength-selective thin film may be formed at a position remote from the output end face behind the output end face, or may be fixed on the output end face using a suitable adhesive.

When fixing on the output end face using the adhesive, the SHG output can be improved by adjusting the refractive index and thickness of the adhesive layer to meet the anti-reflection conditions for the second harmonic light ↓2. desirable.

The wavelength selective thin film may include a colored glass filter,
A glass substrate coated with wavelength-selective ttS can be used.

Materials for the wavelength selective thin film include 5iOt, Mg
O, ZnO, Aj! , oxides such as ○3, LiNbos
, LiTaOx, Y3 Gas○It-.

Composite oxides such as Gd, Ga, O□, or PMMA,
Organic substances such as MNA can be used, and a multilayer thin film made by stacking these can also be used.

Methods for producing the wavelength selective thin film include sputtering method, liquid phase epitaxial method, vapor deposition method, and MBE (Molecular Beam Epitaxial method).
M Epitaxial) method, MOCVD (Meta
l Organic Chemical Vap
or DeposiLi.

n) method, ion blating method, LB method, spin code method, dipping method, etc. can be used.

Next, as a manufacturing method of the SHe element according to the present invention, it can be manufactured by forming a thin film waveguide layer on a substrate by a method such as sputtering or liquid phase epitaxial growth method, and further, A Ti waveguide pattern is formed on the layer by photolithography and RF sputtering, and using this as an etching mask, ion beam etching is performed to form a channel-type SHe.
A method such as creating an element can be used.

Examples of the present invention will be described in detail below.

Example 1 (1) Ordinary refractive index (n, ,) at the fundamental laser light wavelength when the fundamental laser light wavelength (input) is 0.83 μm
is 2.151, and the extraordinary refractive index (n, ,
) is 2.261 and the thickness is 0.5 strokes.
The ordinary refractive index (n
) is 2264, and the extraordinary refractive index at the second harmonic (n
e,) is 2.263. After growing the LiNb○3 top film, the surface was mirror-polished to create a slab-type waveguide using this LiNb○ thin film as a waveguide layer.

(2) The film thickness of the slab type waveguide was changed by ion beam etching to a phase matching film thickness of 2.50 μm and 0.05 μm.
Adjusted to.

(3) The slab waveguide obtained in (1) and (2) above was photolithographically processed to have a width of 10 μm and a film thickness of 2.0 μm.
A channel waveguide of 50 μm±0.05 μm with a step of 1 μm was created.

(4) On the channel type waveguide obtained in (3), the ordinary refractive index (n, c) at the fundamental laser wavelength, 900, and the extraordinary refractive index (n, , c) is 1.900. A ZnO thin film was formed to a thickness of 5 μm to obtain a chamber waveguide with a three-layer structure in which the ZnO thin film was used as a cladding layer and a cladding layer.

(5) (4) Both end faces of the obtained channel-type waveguide were mirror-polished by Hough polishing to enable light to enter and exit from the end faces, thereby forming a second harmonic generation element.

The second harmonic generating element (SHC,
A polarizing plate was directly formed on the output end face of the element) so that the plane of polarization differed by 90° from the plane of polarization of the fundamental laser beam. This SHe
Using the device, the SHG output was measured using a 40 mW semiconductor laser with a wavelength of 0.83 μm as a light source, and the results are shown in Table 1.

Example 2 (]) Width 10 am, film thickness 2.70 II m ± 0.08 μm, step 1 by the same method as in Example 1 (1) to (3)
A ridge-type channel waveguide with a diameter of 4 μm was formed.

(2) The channel type waveguide obtained in (1) is coated with an RF sputtering method to give an ordinary refractive index (noc) of 1.710 at the fundamental laser beam wavelength and an extraordinary optical refractive index (noc) at the second harmonic. A thin MgO film with a value of 1.730 is
Formed to a thickness of μm, MgO! The channel-type waveguide has a three-layer structure with a film as the cladding layer.

(3) Both end faces of the channel waveguide obtained in (2) were polished in the same manner as in (5) of Example 1 to obtain a second harmonic generation element.

The second harmonic generation element (SHG) obtained in this way
A wavelength selective filter that does not transmit the fundamental laser beam and a polarizing plate whose polarization plane differs by 90° from the polarization plane of the fundamental laser beam were formed behind the output end face of the device.

Using this SHG element, SHC and output were measured using a 40 mW semiconductor laser with a wavelength of 0.83 μm as a light source, and the results are shown in Table 1.

Example 3 (1) A slab-type channel waveguide with a film r ¥2°20 μm±0.03 μm was prepared by the same method as in Example 1 (1) and (2).

(2) RF on top of the slab waveguide obtained in (1)
By the spanter method, the ordinary refractive index (noc) at the wavelength of the fundamental laser light is 1.710, and the extraordinary refractive index (n, c) at the second harmonic is 730.

A1□03 thin film was formed to a thickness of 4 μm, and Al2O,
The slab waveguide has a three-layer structure with a thin film as the upper layer.

(3) Both end faces of the slab waveguide obtained in (2) were polished in the same manner as in (5) of Example 1 to obtain a second harmonic generation element.

A polarizing plate was formed behind the output end face of the thus obtained second harmonic generation element (SHO element) so that the plane of polarization differed by 90 degrees from the plane of polarization of the fundamental laser beam.

Using this SHG element, the SHG output was measured using a 40 mW semiconductor laser with a wavelength of 0.83 am as a light source, and the results are shown in Table 1.

Comparative Example 1 Width 1 under the same conditions and method as (1) to (5) of Example 1.
0pm, lI! Thickness 2.50μm + 0.05μm, step 1
A second harmonic generation element (SHG element) consisting of a μm ridge channel waveguide was fabricated. Using the SHG element obtained in this way, SH
The G output was measured and the results are shown in Table 1.

Comparative Example 2 Same as Comparative Example 1, convergence 10 μm, opening 2. '70gm
±0.08. cam, a step of 1.4 μm, a SHG element consisting of a di-shaped channel waveguide, 5) (G output was measured, and the results are shown in Table 1.8 Comparative Example 3 ) to (2), the film thickness is 2.
．． We created an SHG element consisting of a slab-type channel waveguide with a thickness of 20μm±0.03am, and measured the SHG output.
The results are shown in Table 1.

Example SHC, output (mW) 2.3 2.8 2, 1 The second harmonic generation element of the present invention, which is provided as a cladding layer on top, can achieve phase matching even when using waveguides with the same film thickness accuracy. As a result of the wider tolerance range for film thickness, an SHG output approximately 8 times higher than that without a cladding layer was obtained.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide a second harmonic generation element having a thin film waveguide structure and having extremely high SHG output.

Comparative example 5H (1, output (mW))

Claims (1)

[Claims] 1. A second harmonic generation element comprising a thin film waveguide layer formed on a substrate, a cladding layer being formed on the thin film waveguide layer,
A second harmonic generation element, wherein the cladding layer satisfies relational expressions 1) and 2). n_o_s-0.50≦n_o_c≦n_o_s-0.
05...Formula 1) n_e_s-0.70≦n_e_c≦
n_e_s-0.15...Formula 2) n_o_s: Ordinary refractive index of the substrate at the fundamental laser light wavelength (λμm) n_o_c: Ordinary refractive index of the cladding layer at the fundamental laser light wavelength (λμm) n_e_s: Second harmonic The extraordinary refractive index n_e_c of the substrate at the wavelength (λμm/2): the extraordinary refractive index of the cladding layer at the second harmonic wavelength (λμm/2) is 2, and the substrate is a lithium tantalate single crystal. The second harmonic generating element described above. 3. The second harmonic generation element according to claim 1, wherein the thin film waveguide layer is a lithium niobate single crystal.