A device for the changing of the wavelength of light

Abstract: A device for the changing of the wavelength of light comprising a non-linear optical crystal, first and second optical waveguides that are formed in the non-linear optical crystal, and an introducing means that is positioned adjacent to the second optical waveguide, the light being incident upon the first optical waveguide and propagated within the first optical waveguide, resulting in harmonic light that satisfies the phase-matching conditions, and the harmonic light being introduced, by the introducing means, into the second optical waveguide from which the harmonic light is output.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to a device for the changing of the wavelength of light that is suitable for use in, for example, information processing devices that make use of laser light, apparatus for measurement with the use of light, and the like. More particularly, this invention relates to a device for shortening the wavelength of light. This shortening of the wavelength of the laser light used in, for example, optical disk systems, laser beam printers, and other such devices that make use of laser light, can result in higher performance. 2. Description of the prior art: In recent years, devices for information processing and the like that make use of semiconductor lasers, such as optical disk systems and laser beam printers, have been put into practical use. The laser light that is used in such systems as a light source has an oscillating wavelength of, for example, 780 µm or 830 µm. There is a proportional relationship between the wavelength of the laser light and the diameter of the light-focusing spot (the minimum light-focusing diameter), and the shorter the wavelength of laser light, the smaller the diameter of the light-focusing spot. The smaller the diameter of the light-focusing spot, the more possible becomes the increase in the recording density of, for example, an optical disk of an optical disk system, so that a laser beam printer can achieve greater resolution. Also, in the field of measurement with the use of light such as interference measurement and the like, the use of short wavelengths of laser light can increase the accuracy of the results of measurement. In such ways, the performance of devices that make use of laser light can be improved by the use of a device for the shortening of the oscillation wavelength of this light. By the use of an InGaAlP system for the material of the semiconductor laser, it has been found experimentally that it is possible to obtain laser light with a wavelength in the region of 600 µm, but there are still a number of problems that remain concerning reliability and the like that must be solved before this is put into practical use. Also, at the present, materials for semiconductors that will give laser light of yet shorter wavelengths are not known. Devices for the changing of the wavelength of light, i.e., the shortening of the wavelength of laser light, in general, produce a harmonic component with a wavelength of λ/n, wherein λ is the wavelength of laser light that is incident upon the said devices and n is an integer (e.g., 2, 4, 6, ... or 3, 6, 9, ...). At present, with the use of a YAG (yttrium-aluminium-garnet) laser with a wavelength of 1.06 µm, it is possible to produce laser light with a wavelength of 0.53 µm for green laser light, and with the use of a semiconductor laser with a wavelength of 0.83 µm, it is possible to produce laser light with a wavelength of 0.415 µm for blue laser light. Figure 6 shows a conventional device 10 for the changing of the wavelength of light, which has a non-linear optical crystal 1 such as the Z plate of crystals of LiNbO₃ in which the direction of arrow A in Figure 6 is in the direction of the crystal axis Z. In the upper portion of the non-linear optical crystal 1 shown in Figure 6, an optical waveguide 2 with a width, for example, of 2 µm and a depth of 0.55 µm is formed from one side surface of the non-linear optical crystal 1 to the other side surface thereof by the proton exchange method or the like so as to be parallel with, for example, crystal axis Y. In device 10 for the changing of the wavelength of light, for example, laser light 3 from a YAG laser or the like with a wavelength of 1.06 µm is fed into one facet of the optical waveguide 2 and proceeds along the inside of the optical waveguide 2. At this time, because the non-linear optical coefficient of the non-linear optical crystal 1 is large, a harmonic light 4 with a wavelength that is 1/2 of the wavelength of the laser light 3 (0.53 µm) is radiated inside of the non-linear optical crystal 1 at an angle ϑ that satisfies the phase matching conditions. When YAG laser light (with a wavelength of 1.06 µm) is fed into LiNbO₃ crystals, the harmonic light 4 is radiated at a 12.5° angle from the direction in which the YAG laser light is proceeding. This harmonic light 4, as shown in Figure 6, is radiated along the entire region of the optical waveguide 2, and radiated out from the device 10 for the changing of the wavelength of light. The harmonic component 4 is produced with an intensity in proportion to the second power of the intensity of the laser light that is fed in the device 10. For this reason, at the vicinity of the surface of the optical waveguide 2 into which the laser light is fed, the efficiency of the change from the long wavelength light into the harmonic light 4 is high and the laser light decays as it proceeds along the optical waveguide 2, resulting in a decrease in the efficiency of the change and a decrease in the intensity of the harmonic light 4. Moreover, because the harmonic light 4 is output from the narrow optical waveguide 2, diffraction causes the harmonic light 4 to expand in the direction of the width of the optical waveguide 2. Therefore, the harmonic light 4 that is output takes on a narrow, long oval shape, and its intensity also is not uniform. This kind of light beam in an oval shape of non-uniform intensity is not suitable for use as a light source in optical disk apparatuses or laser printers. Moreover, because there is a fractional angle (of 12.5°) between the direction of the feeding in of laser light and the output of the harmonic light 4, the construction of a system with this kind of device 10 for the changing of the wavelength of light is complex. In the conventional device 10 for the changing of the wavelength of light, the problems mentioned above limit the field of applications. US-A-3 624 406 discloses a device for changing the wavelength of light, said device comprising: - a non-linear optical crystal; and a first optical waveguide formed in a first face of said non-linear optical crystal, said first optical waveguide being non-linear; said light being incident upon said first optical waveguide, and said light being propagated within said first optical waveguide thereby producing harmonic light, said harmonic light satisfying phase-matching conditions. SUMMARY OF THE INVENTION According to the present invention there is provided a device for changing the wavelength of light as defined in claim 1. In a preferred embodiment, the introducing means is a grating layer with a diffraction grating period. In a preferred embodiment, the introducing means is a tapered coupler. In a preferred embodiment the device further comprises a cladding layer position on said grating layer. In a preferred embodiment, the diffraction grating period of said grating is layer is selected so as to satisfy the following equation; n S sin ϑ₁ = N + λ Λ q (1) where Λ is the diffraction grating period of said grating layer, ϑ₁ is an angle at which said harmonic light is incident upon said grating layer, λ is the wavelength of said harmonic light, n S is the refraction index of the cladding layer if said cladding layer is provided and n S is the refractive index of the atmosphere if said cladding layer is not provided N is the refraction index of said grating layer, and q is an integer In another embodiment, the refraction index N of said grating layer and the diffraction grating period Λ of the grating layer further satisfy the following relationship: N + n S > λ/Λ > N (2) Thus, the invention described herein makes possible the objective of providing a device for the changing of the wavelength of light by which the light that is fed into the said device is changed to harmonic light with a desired optical-intensity distribution and the harmonic light is emitted in a desired direction. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a perspective view showing a device for the changing of the wavelength of light of this invention. Figure 2 is a sectional view showing the device of Figure 1. Figure 3 is a sectional view showing another device for the changing of the wavelength of light of this invention. Figure 4 is a sectional view showing another device for the changing of the wavelength of light of this invention. Figure 5 is a sectional view showing another device for the changing of the wavelength of light of this invention. Figure 6 is a perspective view showing a conventional device for the changing of the wavelength of light. DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention provides a device in which light comes into a first optical waveguide, formed in a non-linear optical crystal, which gives rise to harmonic light at an angle that satisfies the phase-matching conditions within the non-linear optical crystal. The said harmonic light is introduced into a second optical waveguide by means of an introducing means, and in this second optical waveguide, the light is propagated, collected and output from the said second optical waveguide. Example 1 Figure 1 shows a device 20 for the changing of the wave-length of light of this invention, which comprises a non-linear optical crystal 11 that is made of a crystal Z-board of LiNbO₃ or the like, a first optical waveguide 12 that is formed in the non-linear optical crystal 11, a second optical waveguide 15 that is also formed in the non-linear optical crystal 11, and a grating layer 16 that has a surface adjacent to the second optical waveguide 15. In the device 20, for example, YAG laser light 13 with a wavelength of 1.06 µm, of the like is fed into the device, and harmonic light 14 that is green and has a wavelength of 0.53 µm is output therefrom. The optical waveguide 12 is formed in an upper portion of the non-linear optical crystal 11 from the light-input facet 21 to the light-output facet 22 by using changes in the diffraction ratio caused by the exchange of protons by the proton-exchange method or the like in the non-linear optical crystal 11. This optical waveguide 12 has a width, for example, of 2 µm and a depth of 0.55 µm. The optical waveguide 15 is formed by a proton-exchange method or the like in a lower portion of the non-linear optical crystal 11. In a bottom portion of the optical waveguide 15, there are layered a grating layer 16 and a cladding layer 17. The grating layer 16 that makes up the diffraction grating is formed in the bottom portion of the optical waveguide 15 by plasma assisted chemical vapor deposition as follows: In this example, SiN is formed on the bottom portion of the optical waveguide 15 and a photo-resist is coated on the SiN. Then, the photo-resist is exposed to light by the holographic exposing method, followed by development, etching, and removal of the photo-resist in a series of steps, in that order. The cladding layer 17 is formed by sputtering of, for example, SiO₂. Below, the functioning of the device 20 of Figure 1 will be explained by reference to Figure 2. Laser light 13, which is YAG laser light or the like, is fed into the light-input facet 21 of the optical waveguide 12 so that the laser light 13 becomes parallel to the optical waveguide 12, and proceeds along inside of the optical waveguide 12. Because the non-linear optical coefficient of the non-linear optical crystal 11 is large, harmonic light 18 with a wavelength that is 1/2 times the wavelength of the laser light 13 is radiated inside the non-linear optical crystal 11 at an angle ϑ that satisfies the phase-matching conditions. This harmonic light 18 is introduced into the optical waveguide 15 by the grating layer 16. This grating period Λ of the grating layer 16, the incident angle ϑ₁ at which the harmonic light 18 is incident upon the grating layer 16, the wavelength λ of the harmonic light 18, the refraction index n S of the cladding layer 17, and the refraction index N of the grating layer 16 can be selected by the following formula so as to satisfy the relationship shown here. n S sin ϑ₁ = N + λ Λ q (1) wherein q is an integer that can be set freely. By the grating layer 16 that is formed to have a grating period Λ that satisfies the above-mentioned relationship, the harmonic light 18 is efficiently introduced into the optical waveguide 15. In this way, the harmonic light 18 that is produced in all regions of the optical waveguide 12 is introduced into the optical waveguide 15 and output from the optical waveguide 15 to the outside, so that the degree of ovality is small, and the output light 14, which has a satisfactory intensity distribution, is output in the same direction as it was fed into the device. In this example, when the light-input facet 21 where laser light 13 is fed into the device, is coated with a substance by which the laser light 13 with a wavelength of 2λ is reflected at a low reflectivity, but the harmonic light 18 with a wavelength of λ is reflected at a high reflectivity and when the light-output facet 22 is coated with a substance by which the laser light 13 with a wavelength of 2λ is reflected at a high reflectivity, but the harmonic light 18 with a wavelength of λ is reflected at a low reflectivity, the exchange-efficiency from the incident light 13 into the harmonic light 18 is increased, and leakage of the incident light 13 is prevented. Example 2 Figure 3 shows another device 20a of the changing of the wavelength of light of this invention, which has the same structure as mentioned in Example 1, except that not only Equation 1 mentioned above is fulfilled, but also the relationship between the refraction index N of the grating layer 16 and the grating period Λ is represented by the following equation: N + n S > λ/Λ > N (2) The grating layer 16 functions as a backward-type grating connector, which gives rise to an output light 14 that proceeds in the opposite direction but parallel to the laser light 13 that is fed into the device 20a for the changing of the wavelength of light. Example 3 Figure 4 shows another device 20b for the changing of the wavelength of light, which comprises a non-linear optical crystal 11, first and second optical waveguides 12 and 15, a grating layer 16, and a cladding layer 17. The first optical waveguide 12 is formed in the non-linear optical crystal 11 in the same way as that of the device 20 of Example 1. The second optical waveguide 15 is formed by the proton-exchange method mentioned above at the facet 22 that is opposite to the light-input facet 21 on which the laser light is incident. The grating layer 16 is formed by the method described above on the surface of the optical waveguide 15 so that the grating period Λ fulfills the relationship shown in Equation 1. The cladding layer 17 is formed by being layered on the grating layer 16. By this kind of device 20b, output light 14 can be obtained that proceeds in the downward direction in Figure 4, at a 90° angle with respect to the laser light 13 that is incident upon the device 20b. Example 4 Figure 5 shows another device 20c for the changing of the wavelength of light of this invention, which comprises optical waveguides 12 and 15, a grating layer 16, and a cladding layer 17 in the same way as for the device 20b of Example 3. The grating layer 16 is formed so as to satisfy the relationship shown above in Equation 1 and Inequality 2 for the grating period Λ. The device 20c for the changing of the wavelength of light that is made in this way, produces an output light 14 that proceeds in the upward direction at a 90° angle to the laser light 13 that is incident upon the device 20c. In the examples given above, LiNbO₃ crystals were used for non-linear optical crystals 11, but it is also possible to use LiTaO₃ crystals or the like instead. Moreover, an optical waveguide 15 that is a flat optical waveguide was disclosed as a second optical waveguide, but it can be formed to be three-dimensional as a second optical waveguide. Moreover, a cladding layer 17 made of the material SiO₂ was formed to protect that grating layer 16, but this cladding layer 17 is not essential to this invention, and it may be omitted. When the cladding layer 17 is omitted, the refraction index of the atmosphere can be used for the refraction index n s for cladding layer 17 that appears in Equation 1 and Inequality 2. In order to obtain a yet higher introduction efficiency with which the harmonic light 18 is introduced into the second optical waveguide, a grating layer is formed that has been blazed, whereby an output light 14 with greater intensity can be output. In these examples, a grating coupler that is constituted by a grating layer was used as an introducing means, but it is also possible to use other kinds of introducing means such as a tapered coupler and the like. Moreover, laser light with a wavelength of 1.06 µm from an YAG laser was used as an input laser light, but other kinds of lasers such as a gas laser, semiconductor laser, etc., can be also used as the light source. As mentioned above, harmonic light that is produced is introduced into the second optical waveguide by an introducing means and condensed inside the said second optical waveguide, and then the condensed harmonic light is output in the desired direction, resulting in a harmonic light with the desired intensity distribution.

Claims: A device for converting the wavelength of light (13), said device comprising: - a non-linear optical crystal (11); and a first optical waveguide (12) formed in a first face of said non-linear optical crystal (11), said first optical waveguide (12) having non-linear optical properties; said first optical waveguide (12) being arranged such that said light (13) being incident upon said first optical waveguide is propagated within said first optical waveguide (12) thereby producing harmonic light (18) i.e. light of higher harmonic frequency, said harmonic light (18) satisfying phase-matching conditions; said device being characterised in that it further comprises a second optical waveguide (15) formed in a second face of said non-linear optical crystal (11), a longitudinal axis of said first optical waveguide (12) being located in a same plane as a longitudinal axis of said second optical waveguide (15); in that an introducing means (16) is positioned adjacent to said second optical waveguide (15) to introduce said harmonic light (18) into said second optical waveguide; and in that said harmonic light (18) is output from an output end of said second optical waveguide (15). A device according to claim 1, wherein said introducing means (16) is a grating layer on said second face of said non-linear optical crystal (11), said grating layer having a diffraction grating period. A device according to claim 1, wherein said introducing means (16) is a tapered coupler. A device according to claim 2, which further comprises a cladding layer (17) that is positioned on said grating layer. A device according to claim 2 or 4, wherein the diffraction grating period of said grating layer is selected so as to satisfy the following equation: n S sin ϑ 1' = N + λ Λ q (1) wherein Λ is the diffraction grating period of said grating layer, ϑ₁ is an angle at which said harmonic light (18) is incident upon said grating layer, λ is the wavelength of said harmonic light (18), n S is the refraction index of said cladding layer (17) if said cladding layer (17) is provided and n S is the refraction index of the atmosphere if said cladding layer is not provided, N is the refraction index of said grating layer, and q is an integer. A device according to claim 5, wherein the refraction index N of said grating layer and the diffraction grating period Λ of the grating layer further satisfy the following relationship: N + n S > λ/Λ > N (2)

Inventors: YAMAMOTO OSAMU, YOSHIDA TOSHIHIKO, YAMAMOTO, OSAMU, YOSHIDA, TOSHIHIKO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G02F1
IPC codes: G02F1, G02B6