Second-harmonic wave generating element

Abstract: An element for generating second-harmonic waves of a short wavelength from a fundamental wave emitting from the semiconductor laser and the like. The element comprises a substrate (111;) made of a ferroelectric crystal, a second-harmonic wave generator (119;) for generating second-harmonic wave from fundamental wave entered therein, and periodically arranged portions formed in a front stage region (135;) and a rear stage region (137;) of the element. The periodic portions or structures, each has pairs of parts or regions (131, 133;) of a high optical refractive index and of a low optical refractive index so as to confine the fundamental waves in the second-harmonic wave generating portion. These regions have various kinds of material diffused or introduced therein and a thickness. According to other example of the element, the sign of a nonlinear optical coefficient of the region is periodically reversed and the region satisfies a quasi-phase-matching condition.

Description: The present invention relates to an element for generating a second-harmonic wave of the fundamental wave entered to the element. Semiconductor lasers, solid lasers and gas lasers have been known as light sources outputting coherent light beams and they are being employed in various fields such as a measurement field, a communication field, and the like. Due to a restriction of the material used, these lasers above have another restriction in oscillation wavelength generated by the lasers. In particular, it has been difficult to obtain light of short wavelength. Consequently, nowadays elements enabling to generate a second-harmonic wave of the fundamental wave which are generated from the semiconductor laser and the like have been studied. The elements above hereinafter are referred to as a second-harmonic wave generating element or generator. One example of such second-harmonic wave generating element is disclosed in the literature (a) of Electronics Letters, Vol.25 (1989) pp 174, 175. The conventional second-harmonic wave generating element disclosed in the literature (a) above has a region formed in a LiNbO₃ substrate so as to function as a second-harmonic wave generator. The region has a nonlinear optical coefficient a sign of which is periodically reversed and, its period satisfying a quasi-phase-matching condition. The principle of a second-harmonic wave generation according to the quasi-phase-matching method has been theoretically foretold by Armstrong in the literature (Physical Review, Vol. 127, p 1918 (1962) prior to the literature (a) above. However, it is quite recently that the element is practically formed and a generation of the second-harmonic wave is confirmed as described typically in the literature (a) disclosing the second-harmonic wave generating element. Here, the construction of this second-harmonic wave generating element will be explained in detail with reference to Fig. 1 which is a perspective view depicting schematically the construction of the second-harmonic wave generating element. The second-harmonic wave generating element uses a changing phenomenon of the sign of a nonlinear optical coefficient of LiNbO₃ according to the direction of spontaneous ferroelectric polarization of the crystal. It is noted that the fact of a reversing of the spontaneous ferroelectric polarization of LiNbO₃ crystal can make happen when titanium (Ti) is diffused on a high temperature atmosphere (for example, about 1000°C) in the predetermined part of the LiNbO₃ crystal has been disclosed in, for example, a literature "Japanese Journal of Applied Physics, Vol.6, p 318 (1967)". In the second-harmonic wave generating element disclosed in the literature (a), the direction of the spontaneous ferroelectric polarization is reversed due to a titanium diffusion. Consequently, as shown in Fig. 1. at a +C surface of the LiNbO₃ substrate 11 of a thickness 0.5 mm, a titanium diffused region 13 in a shape of stripes is ferroelectric optionally formed. The direction of spontaneous ferroelectric polarization in the striped region 13, in which titanium is diffused, is reversed by the diffusion of titanium into the downward direction as shown in Fig. 1. On the contrary, another direction of spontaneous ferroelectric polarization in the region 15, in which titanium is not diffused, is remained in its original upward direction. Thereby, spontaneous ferroelectric polarization periodic inverted (domain inverted) structures 13 and 15 are formed on the LiNbO₃ substrate, resulting in a construction of a region 19 (hereinafter it may be referred to as a domain inverted region 19) in which the sign of a nonlinear optical coefficient is periodically reversed. The width of the region 13 having titanium diffused and the width of another region 15 provided with no titanium dispersed or diffused are shown respectively by ℓ n and ℓ p . The coherence length of the fundamental waves is shown by ℓ c . The quasi-phase-matching condition will be satisfied when the total periods ℓ n and ℓ p of the periodic structure 17 satisfied the following equation (3). ℓ n = ℓ p = (2m +1) ℓ c Consequently, it is preferable to form the periodic structure so as to make the period times of an odd number of the coherence length ℓ c . The symbol m in the equation (3) above is zero or positive integer. It is known that the coherence length ℓ c is determined by the following equation (4) in, for example, the literature of Applied Physics Letters, Vol.47 (1985) pp 1125 - 1127. ℓ c = λ 4(n(2ω) - n(ω)) Wherein, λ is a wavelength of the fundamental wave in vacuum, n(2ω) is a refractive index of LiNbO₃ substrate relating to the second-harmonic wave, and n( ω ) is a refractive index of the LiNbO₃ substrate with reference to the fundamental wave. Some examples of the concrete values of the coherence length ℓ c are shown in the literature of Applied Physics Letters Vol.37 (1980) pp 607 - 679; when LiNbO₃ is used and λ = 1.06 µm, ℓ c = 3.4 µm. According to the literature (a) above, on the LiNbO₃ substrate 11 of the second-harmonic wave generating element disclosed in the literature (a), an optical waveguide or wave-guiding route 21 (shown in Fig. 3 by a dash-line) having a thickness substantially equal to that of the domain inverted region 19 is formed by a proton exchange method after the domain inverted region 19 is provided. When fundamental wave L₁ is entered to the second-harmonic wave generating element according to the prior art through an end 11a, for example, shown in Fig. 1 along a direction perpendicular to the stripe direction of the striped region 13 of the domain inverted region 19, the fundamental wave L₁ and the second-harmonic wave L₂ of the fundamental wave emit through another end 11b (end of emitting side of the element). In addition, it was possible to enter fundamental wave of wavelength 1.06 µm of 1mW power through the optical waveguide 21, resulting in obtaining of a second-harmonic wave of a wavelength 532 nm of blue at 0.5 nW power. However, according to the conventional second-harmonic wave generating element described and shown in Fig. 1, it is apparent that despite the incident fundamental wave of 1 mW power, the resulting second-harmonic wave has only 0.5 nW power as described above. That is, the conversion efficiency (power of second-harmonic wave/power of incident fundamental wave) was merely or so low as 0.5 x 10⁻⁶ = 0.5 nW/1 mW. Further, considering a coupling efficiency between the second-harmonic wave generating element and a light source (for example, laser means) for giving a fundamental wave to the element, the light power must emit a fundamental wave of a power higher than the power to be given to the emitting second-harmonic wave generating element. Consequently, the conversion efficiency between the power of the fundamental wave when emitting from the light source and another power of the second-harmonic wave emitting from the second-harmonic wave generating element becomes smaller than the conversion efficiency previously mentioned. As a result, the practical value of the second-harmonic wave generating element of the prior art decreases when it used as a blue light emitting element. Apparently the present invention has been done considering the shortcomings of the prior art set forth above, so that it is an object of the present invention as readily understood from the above description to provide a second-harmonic wave generating element or device enabling to exceedingly improve the conversion efficiency of a power of the fundamental wave to a power of the second-harmonic wave. Further examples of second harmonic generation are given in "12th European Conference on Optical Communication, 22-25 September 1986, Barcelona, Spain, Technical Digest Vol. III, pp49-52, R. Regener & W. Sohler, Efficient Second Harmonic Generation in Matched Waveguide Resonators." and " I.P. Kaminov and R.S. Tucker, Mode-Controlled Semiconductor Lasers, Section 5.4, in T. Tamir (ed.), Guided-Wave Opto-Electronics, Springer-Verlag, Berlin, 1988". According to the present invention, there is provided a harmonic wave generating element comprising: a ferroelectric crystal substrate formed with a region of spatially periodic polarization for generating a harmonic of an incident fundamental wave; and confining means in the substrate for confining the fundamental to said harmonic generating structure, wherein the confining means includes first and second structures of spatially periodically varying refractive index, the refractive index of said first structure varying such that said first structure is reflective of said fundamental wave and said harmonic and the refractive index of said second structure varying such that said second structure is reflective of said fundamental wave and transmissive of said harmonic. In embodying the present invention, it is preferable that the high optical refractive index portion is constituted by a region formed by introduction of a different material into the ferro-electric crystal, a period Λ i of the periodic structure of the refractive index, which structure is put at the front stage of the second-harmonic generating portion has a value satisfying equation (1). A period Λ o of the periodic structure of the refractive index, which structure is placed at the rear stage of the second-harmonic wave generating portion, has a value satisfying equation (2), Wherein, λ means a wavelength of the fundamental wave in vacuum; k₁, k₂, m₁ and m₂, respectively mean zero or positive integer; n( ω ) means a refractive index of the periodic structure having the ferroelectric crystal refractive index with reference to the fundamental wave; and n(2 ω ) means a refractive index of the periodic structure having the ferroelectric crystal refractive index with reference to the second-harmonic wave. Also, it is preferable that the periodic structure of the particular refractive index is constituted by an optical waveguide provided with regions each formed by an introduction of different kind material of a thickness d₁ onto the ferroelectric crystal and other regions each formed by an introduction of the different kind material of a thickness d₂ (d₁>d₂) onto the ferroelectric crystal, the regions and other regions are repeatedly and periodically arranged, the period Λ i of the periodic structure of the refractive index at the front stage side of the second-harmonic wave generating portion has a value satisfying the following equation (A), and the period Λ o of the periodic structure of the refractive index at the rear stage side of the second-harmonic wave generating portion has a value satisfying the following equation (B), Wherein, λ is a wavelength of the fundamental waves in vacuum; K₁, k₂, m₁ and m₂, respectively are zero or positive integer; n( ω )eff(d₁) is an effective refractive index, with reference to the fundamental wave, of a region of a thickness d₁ of the periodic structure having a ferroelectric crystal; n( ω )eff(d₂) is an effective refractive index, with reference to the fundamental wave, of a region of a thickness d₂ of the periodic structure of the ferroelectric crystal; n(2 ω )eff(d₁) is an effective refractive index, concerning the second-harmonic wave, of a region of a thickness d₁ of the periodic structure of the ferroelectric crystal; and n(2 ω)eff(d₂) is an effective refractive index, with reference to the second-harmonic wave of a region of a thickness d₂ of the periodic structure of the ferroelectric crystal. Some examples of introducing foreign and different kind material into the ferroelectric crystal are thermal dispersive introduction of titanium of the different kind material into LiNbO₃ and a proton-exchange introduction of proton (H⁺) of the foreign material into LiNbO₃. Furthermore, in an embodiment of the present invention, the second-harmonic wave generating portion is preferably constituted by a region formed on the ferroelectric crystal, which region having a periodically reversed sign of a nonlinear optical coefficient and satisfying a quasi-phase-matching condition. According to the second-harmonic wave generating element, the front stage and the rear stage of the second-harmonic wave generating element, respectively have periodic structure of the refractive index thereon for confining the fundamental wave in the second-harmonic wave generator, so it is possible to confine the fundamental wave within the second-harmonic wave generating portion. It is known from, for example, literature "Optics Communications, Vol.6 (1972) pp 301 - 304", that the conversion efficiency from the fundamental wave to the second-harmonic wave is proportioned to the energy condensation of the fundamental wave emitting from the second-harmonic wave generating portion. Consequently, when the fundamental wave can be confined to the second-harmonic wave generating portion as described above, it is possible to improve or raise the energy density of the fundamental wave at the second-harmonic wave generating portion, resulting in an improvement of the conversion efficiency from the fundamental wave to the second-harmonic wave. For example, in case that the period Λ i of the periodic structure of the refractive index at the front stage of the second-harmonic wave generating portion has its value satisfying the equation (1) or (A) above, the periodic structure of the refractive index becomes to have periods of times of an integer of a half of the fundamental wavelength and of times of an integer of a half of the wavelength of second-harmonic wave. Resultantly, the periodic structure of the front stage refractive index depicts a high refractive condition of both the fundamental wave and the second-harmonic wave. Consequently, it is possible to prevent the fundamental wave after it enters the second-harmonic wave generating portion from emitting from the front stage side, and also to prevent the second-harmonic wave after it is generated in the second-harmonic wave generating portion from emitting from the front stage side. In other words, it is possible to effectively confine the fundamental wave and the second-harmonic wave within the front stage of the second-harmonic wave generating portion of the element according to the present invention. Further, when the period Λ o of the periodic structure of the refractive index previously mentioned provided at the rear stage of the second-harmonic wave generating portion satisfies the equation (2) or (B) above, the periodic structure of the particular refractive index is said that it has periods of times of an integer of a half of the wavelength of the fundamental wave and of time of an odd number of a quarter of the wavelength of the second-harmonic wave. Due to the fact or characteristic above of the second-harmonic wave generating element of the present invention, the periodic structure of the rear stage refractive index shows a high refractive condition of the fundamental wave and a low refractive condition of the second-harmonic wave. As a result, it is possible to prevent the fundamental wave entered into the second-harmonic wave generating portion from emitting from the rear stage side of the portion and to make the second-harmonic wave generated in the second-harmonic wave generating portion effectively emit to the rear stage of the portion. As a result, it is possible to confine the fundamental wave in the second-harmonic wave generating portion and effectively take the second-harmonic wave out of the rear stage. The foregoing and other objects, features and advantages of the present invention will be better understood from the following description taken in connection with the accompanying drawings, in which: Fig. 1 is a schematic perspective view showing a conventional second-harmonic wave generating element; Fig. 2 is a diagrammatic perspective view illustrating a second-harmonic wave generating element according to the present invention; and Figs. 3A and 3B are schematic perspective view and a section of the second-harmonic wave generating element of the present invention, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be explained with reference to the accompanying drawings. In the figures, the shapes and sizes of constituents as well as the positional relationships therebetween are drawn so schematically as to merely facilitate the understanding of the present invention. The following examples are merely suitable cases, and hence should not be constructed as limiting the scope of the present invention. FIRST EMBODIMENT First, the second-harmonic wave generating element according to the present invention will be described with reference to Fig. 2. Fig. 2 is a diagram showing briefly and perspectively the second-harmonic wave generating element above. The second-harmonic wave generating element of the first embodiment uses a LiNbO₃ substrate 111 of a ferroelectric crystal. The LiNbO₃ substrate 111 provides with a region 119 similar to the region (domain inverted region 19 in Fig. 1) with the sign of nonlinear optical coefficient, the sign being reversed periodically as described above, and functioning as a second-harmonic wave generator which generates second-harmonic wave of incident fundamental wave. Further, the substrate 111 has periodic or repeated structures 135 and 137, respectively having alternately and periodically arranged portions 131 of a high optical refractive index and other portions 133 of a low optical refractive index. The portion 135 is formed in a front stage region to which region the fundamental wave of the domain inverted region 119 incidents and the portion 137 in a rear stage region from which region the second-harmonic wave is output. The periodic structure 135 of a particular refractive index to be formed in the front region of the domain inverted region 119 may be conveniently called as a periodic structure of a front stage refractive index or a front-periodic structure, and other periodic structure 137 of another refractive index situated on the rear region of the domain inverted region 119 may be conveniently called as a periodic structure of a rear stage refractive index or a rear-periodic structure. In addition, according to the second-harmonic wave generating element of this embodiment, the optical waveguide 121 similar to the conventional optical waveguide 21 (Fig. 1) is formed not only in the domain inverted region 119, but also in the surface regions in which the front-and rear-periodic structures 135 and 137 are formed in the front stage and the rear stage of LiNbO₃ substrate 111, respectively. It is noted that the high optical refractive index portion 131 and the low optical refractive index portions 133 may be formed by using the suitable conventional method. For example, titanium (Ti) of foreign material of different kind is introduced by a thermal diffusion into such region of the LiNbO₃ substrate 111 that a high optical refractive index is expected to be formed, thereby making a difference in refractive indexes between a Ti-diffused region and a Ti-non-diffused region. The condition of the formation of above-mentioned titanium diffusion concentration is different from that of the formation of the domain inverted region 119. It is preferable that both the high optical refractive index portion 131 and the low optical refractive index portion 133 are identical with each other in width and change or inclination of the refractive index between the portions 131 and 133 is gentle. Next, one period Λ i consists of a single high optical refractive index portion 131 and a single low optical refractive index portion 133, respectively of the front-periodic structure 135. Here, a value of the period Λ i satisfies the following equation (1). Also, other period Λ o consists of a single high optical refractive index portion 131 and a single low optical refractive index portion 133, respectively of the rear-periodic structure 137. A value of the period Λ o above satisfies the following equation (2). Wherein, in both the equations, λ is wavelength of the fundamental wave in vacuum; k₁, k₂, m₁ and m₂, respectively zero or positive integers; n( ω ) is a refractive index of the respect to the fundamental wave; and n(2 ω ) is a refractive index of the periodic structure of the refractive index of LiNbO₃ concerning second-harmonic wave. n( ω ) and n(2 ω ) in this embodiment are supposed to be as shown below. n(ω) = 2.1520 n(2ω) = 2.2340 The respective values of the period Λ i appropriate to the fundamental wave and second-harmonic wave were studied according to the equation (1) above while using λ = 1.06 µm, n( ω ) = 2.1520, and n(2 ω) = 2.2340 and k₁ and k₂ of parameters. The resultant values of the study are shown in Table 1 below. It is noted that harmonic waves in Table 1 means the second-harmonic waves. As apparent from Table 1, it is the case of (k₁, k₂) = (0, 1), (1, 3), (2, 5), (3, 7) and etc. that these values of the period Λ i for fundamental waves and other period Λ i for second-harmonic waves are made substantially equal to each other (becoming the different between both these values to several %). Accordingly, when one of the pairs of (k₁, k₂) above is selected in order to set a particular period Λ i , the front-periodic structure 135 becomes to have a period multipled by an integer, which period is a half of respective wavelengths of the fundamental wave and the second-harmonic waves. In consequence, the front-periodic structure 135 has a characteristic or property of a high reflection to the fundamental waves and the second-harmonic waves. As a result, it is difficult that the second-harmonic waves generated in the domain inverted region 119 and the fundamental waves incident to the domain inverted region 119 to come out through the incident side of the fundamental waves of the domain inverted region 119. Anyway, the selection of the pairs of (k₁, k₂) above is done according to the design such as dimensions of the second-harmonic wave generating element. Further, the number of the periods each consisting of the portion 131 of a high refractive index and another portion 133 of a low refractive index may preferably be determined according to the design of the generating element of the present invention. While, supposing that λ is 1.06µm, n( ω ) and n(2ω) are these figures mentioned above, the period Λ o appropriate to the fundamental waves and the second-harmonic waves is examined with m₁ and m₂ of parameters. The resultant of the calculation will be shown in Table 2 shown below. It is noted that harmonic waves described in Table 2 is the second-harmonic waves. It is apparent from Table 2 that it is the case of (m₁, m₂) = (2, 6), (3, 8) ..... that the values of the period Λ o for fundamental waves and of their period Λ o for the second-harmonic waves are made substantially equal to each other (becoming the difference between both these values several %). Consequently, when anyone of the pairs of (m₁, m₂) above is selected and the period Λ o is set, the periodic structure 137 of the rear stage refractive index becomes to have a period multipled by an integer and a half of the wavelength of the fundamental waves and multipled by an odd number and a quarter of the wavelength of the second-harmonic waves. Consequently, the rear-periodic structure 137 becomes to have a high reflective characteristic or property for the fundamental waves and a low reflective characteristic or property for the second-harmonic waves. As a result, the fundamental waves incident to the domain inverted region 119 is apt to be difficult to emit through the emitting side of the region 119. The second-harmonic waves generated in the domain inverted region 119 is made easy to emit through the emitting side of the region 119. Considering, for example the dimensions of the second-harmonic waves generating element, it is preferable to select one of the pairs of (m₁, m₂) above according to the particular design of the element. In addition, the number of the periods, respectively consisting of the high refractive index portion 131 and the low refractive index portion 133 may be appropriately determined according to the design. When the periods Λ i and Λ o are set as mentioned above, the fundamental waves L₁ is restricted or confined in the domain inverted region 119 after passing through the periodic structure 135 having the front stage refractive index. The second-harmonic waves L₂ generated in the domain inverted region 119 can be taken out effectively of other end (end of emitting side) 111b (Fig. 2) of the second-harmonic wave generating element. SECOND EMBODIMENT Another second-harmonic wave generating element according to the second embodiment of the present invention will be explained with reference to Figs. 3A and 3B. Fig. 3A is a perspective view schematically showing the second-harmonic wave generating element of the second embodiment and Fig. 3B is a diagram showing a section taken along the line I - I in Fig. 3A to show the second-harmonic wave generating element above. The second-harmonic wave generating element of the second embodiment provides with a periodic structure 235 having the refractive index of the front stage second-harmonic wave generating portion 219, and another periodic structure 237 of particular refractive index of the rear stage second-harmonic wave generating portion 119. These periodic structures 235 and 237, respectively provide with optical waveguides 221a and 221b (these waveguides may be identified together by 221). These optical waveguides 221a and 221b, respectively have regions of a thickness d₁ formed in the LiNbO₃ substrate 211 and other regions of a thickness d₂ formed in the substrate 211. The former regions are formed by an introduction of a different kind of material into a region with a depth d₁ and the latter regions are formed by an introduction of a different kind of material into a region with a depth d₂ (which is smaller than d₁). These regions of two types are arranged alternately or periodically along the light transmitting or propagating direction. According to the construction of such optical waveguide 221, periodic distribution of the effective refractive index is obtained in the regions due to a difference between these regions of different type in their depths d₁ and d₂. The optical waveguide 221 constructs a periodic structure of a refractive index, which structure provides with periodically arranged high optical refractive index portions and low optical refractive index portions according to the present invention. Between the optical waveguides 221a and 221b, respectively constituting periodic structures 235 and 237 of the front stage refractive index and of the rear stage refractive index, there is the domain inverted region 219. In the domain inverted region 219, there is formed an optical waveguide 223 of a thickness d₁ arranged in contact with the former-optical waveguide 221a and the rear-optical waveguide 221b through their respective facing ends. The widths of the optical waveguides 221 and 223 are identified for as "W". It is noted that the optical waveguide 221a is made such that the period Λ i satisfies the equation (A) above, the period Λ i being contained in the waveguide 221a and decided by the sum of the length ℓ₁ of the region having a thickness d₁ and the length ℓ₂ of the region having other thickness d₂ . Also, the period Λ o determined by the sum of the length ℓ A of the region having a thickness d₁ and the length ℓ B of the region having a thickness d₂ satisfies the equation (B) above. Here, an example of designing these optical waveguides 221 (221a and 221b) will be described. It is supposed that the wavelength λ of the fundamental waves is 830 nm, and the wavelength λ (2 ω ) of the second-harmonic waves is 415 nm, and the optical waveguide 221 is made by Li⁺ - H⁺ exchange (hereinafter referred to as a proton-exchange) method. An effective refractive index is determined by conditions of a thickness of the optical waveguide 221, a refractive index of the waveguide 221 and media (LiNbO₃ and air) surrounding the optical waveguide 221. Here, a refractive index of a proton-exchange portion (a part of the optical waveguide 221) with reference to the fundamental waves λ is n₁( ω ), another refractive index of a portion-exchange portion with reference to the second-harmonic waves is n₁(2 ω ), still another refractive index of the LiNbO₃ substrate 211 concerning the fundamental waves λ is n₂( ω ), and still another refractive index of the LiNbO₃ substrate 211 with reference to the second-harmonic waves is n₂(2 ω ). According to experiments carried out on the element of the present invention, these refractive indexes have their values as shown below. n₁(ω) = 2.30 n₁(2w) = 2.41 n₂(ω) = 2.18 n₂(2ω) = 2.27 Further, when d₁ = 1 µm and d₂ = 0.7 µm, respective refractive indexes of the region of the thickness d₁ and then region of the thickness d₂, respectively with reference to the fundamental waves ( λ = 830 nm) and the second-harmonic waves are determined as shown below by a calculation using an effective refractive index method. An effective refractive index of the region of a thickness d₁ with reference to the fundamental waves is n(ω)eff(d₁) = 2.295. An effective refractive index of the region of a thickness d₂ concerning the fundamental waves is n( ω ) eff(d₂) = 2.291. An effective refractive index of the region of a thickness d₁ with reference to the second-harmonic waves is n(2ω)eff(d₁) = 2.409. An effective refractive index of the region of a thickness d₂ with reference to the second-harmonic waves is n(2ω)eff(d₂) = 2.408. The value of the period Λ i given from the equation (A) above with the conditions described above, which value being suitable to effectively satisfy both the fundamental waves and the second-harmonic waves has been determined with the parameters of k₁ and k₂. According to the result, when for example (K₁, k₂) = (8, 18), it is found that both the period Λ i for the fundamental waves and another period Λ i for the second-harmonic waves have the substantial same value of about 1.63 µm. Consequently, the periodic structure 235 of the front stage side refractive index is constructed with the optical waveguide 221a having the period Λ i of 1.63 µm, by which the second-harmonic waves generated in the domain inverted region 219 and the fundamental waves entered into the region 219 become difficult to emit from the incident side of the fundamental waves on the domain inverted region 219. While, the suitable values of the period Λ o given from the equation (B) above substantially satisfying both the fundamental waves and the second-harmonic waves have been tried to determine with parameters of m₁ and m₂. As a result, for example, supposing that (m₁, m₂ ) = ( 4, 10 ), it is found that the period Λ o for the fundamental waves and another period Λ o for the second-harmonic waves are values of around 0.904 which are substantially equal to each other. Consequently, constructing the rear periodic structure 237 with the optical waveguide 221b having a period of 0.904 µm makes the fundamental waves entered to the domain inverted region 219 difficult to emit from the emitting side of the region 219 and the second-harmonic waves generated from the domain inverted region 219 easy to emit from the emitting side of the region 219. When the periods Λ i and Λ o are set as described above, the fundamental waves L₁ is confined in the domain inverted region 219 after passing through the periodic structure 235 of the front stage refractive index. The second-harmonic waves L₂ generated in the domain inverted region 219 can be efficiently taken out of the corresponding second-harmonic waves generating element through its other end (emitting side end, Fig. 2) 211b. It is noted that , in the second embodiment, respective (k₁, k₂) and (m₁, m₂) have values other than those shown above, which is similar to that of the first embodiment. In consequence, what pair is selected from the paired values of (k₁, k₂) and what pair is selected from the paired values of (m₁, m₂) are solved according to the design considering, for example, the dimensions and sizes of the second-harmonic wave generating element. Also, the number of the combinations of the portions 221a having a thickness d₁ and the portions 221b having a thickness d₂ may appropriately determined according to the design of the element of the present invention. While preferred embodiments of the second-harmonic wave generating element according to the present invention have been explained, it is to be understood that other forms might be adopted. Although the second-harmonic wave generating portions in respective embodiments were constructed by the domain inverted regions according to a quasi-phase-matching method, the present invention can be adopted to the element for generating the second-harmonic wave according to the principle other than the quasi-phase-matching method. For example, when the second-harmonic wave generating portion of the element uses refractive index distribution of such optical waveguide as disclosed in the literature, on p 26 (Shingaku Giho MW-87-113, pp 23-30), it is possible to adopt the invention to the particular second-harmonic wave generating portion. Also, according to the first embodiment described above in detail, the second-harmonic wave generating element of a particular refractive index has periodic structures at both ends of the incident side of the fundamental waves of the element and of the emitting side of the element, however it is possible to mount optical waveguide(s) at one or both of the outsides of the refractive index periodic structures along a propagation direction of the laser beam. As apparent from the description above, according to the second-harmonic wave generating element of the present invention, it provides with refractive index periodic structures having a second-harmonic wave generating portion, as well as portions of a high optical refractive index and other portions of a low optical refractive index, respectively arranged periodically or repeatedly at the front stage and the rear stage of the second-harmonic wave generating portion, by which it is possible to confine the fundamental waves entered or incident to the second-harmonic wave generating element within the second-harmonic wave generating portion. As a result, energy concentration of the fundamental waves in the second-harmonic wave generating portion is improved (see literature : IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 24, No. 6, 1988, pp 913 - 919), so that the conversion efficiency between the fundamental waves and the second-harmonic waves is raised comparing to that of the conventional generating element provided with no periodic structure. In addition, according to the present invention, the second-harmonic waves can be generated or taken out of the second-harmonic wave generating portion through the periodic structure of a refractive index provided in the portion, by which it is possible to make the second-harmonic waves emit from the emitting side of the element in an efficiency higher than that of the conventional construction in which the second-harmonic wave generating portion comes in directly contact with air. In conclusion, the present invention provides the second-harmonic wave generating element through which the fundamental waves can be converted to the second-harmonic waves at a higher efficiency than that of the conventional one.

Claims: A harmonic wave generating element comprising: a ferroelectric crystal substrate (111,211) formed with a region of spatially periodic polarization for generating a harmonic of an incident fundamental wave; and confining means (135,137;235,237) in the substrate for confining the fundamental to said harmonic generating structure, wherein the confining means (135,137;235,237) includes first (135;235) and second (137;237) structures of spatially periodically varying refractive index, the refractive index of said first structure (135;235) varying such that said first structure (135;235) is reflective of said fundamental wave and said harmonic and the refractive index of said second structure (137;237) varying such that said second structure (137;237) is reflective of said fundamental wave and transmissive of said harmonic. A harmonic wave generating element according to claim 1, wherein said first and second structures (135,137;235,237) are formed from alternating regions of high and low refractive index, said regions of high refractive index being formed by doping the ferroelectric crystal, the period Λ i of said first structure (135;235) satisfies: Λ i = 1 2 λ n (ω) ( k ₁+1) = 1 2 λ 2 n (2ω) ( k ₂+1) and the period Λ o of the second structure (137;237) satisfies: Λ o = 1 2 λ n (ω) ( m ₁+1) = 1 2 λ 4 n (2ω) (2 m ₂+1), where λ is the wavelength of said fundamental wave in free space; k₁, k₂, m₁ and m₂ are zero or positive integers; n(ω) is the refractive index of said regions of high refractive index at the frequency of the fundamental wave; and n(2ω) is the refractive index of said regions of high refractive index at said harmonic frequency. A harmonic wave generating element according to claim 1, wherein the confining means (135,137;235,237) comprise optical wave guides, each of which being provided with alternating regions, said regions being formed by doping to respectively a first depth d₁ and a second depth d₂ (d₁>d₂), the period Λ i of said first structure (135;235) satisfies: and the period Λ o of said second structure satisfies: where λ is the wavelength of said fundamental wave in free space; k₁, k₂, m₁ and m₂ are zero or positive integers; n(ω) is the refractive index of said regions of said high refractive index at the frequency of said fundamental wave; and n(2ω) is the refractive index of said regions of high refractive index at said harmonic frequency; n(ω)eff(d₁) is the effective refractive index at the fundamental wave frequency of the regions of depth d₁; n(ω)eff(d₂) is the effective refractive index at the fundamental wave frequency of the region of depth d₂; n(2ω)eff(d₁) is the effective refractive index at said harmonic frequency of the regions of depth d₁; and n(2ω)eff(d₂) is the effective refractive index at said harmonic frequency of the regions of depth d₂. A harmonic wave generating element according to claim 1, 2 or 3, wherein said regions of spatially periodic polarization conform to a quasi phase-matching condition. A harmonic wave generating element according to any preceding claim, wherein said harmonic is twice the frequency of said fundamental wave.

Inventors: NONAKA TOSHIO C O OKI ELECTRIC, SHINOZAKI KEISUKE C O OKI ELEC, NONAKA, TOSHIO, C/O OKI ELECTRIC IND. CO. LTD., SHINOZAKI, KEISUKE, C/O OKI ELECTRIC IND. CO. LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1991
Main classification: G02F1
IPC codes: G02F1