Wavelength converting device

Abstract: In a wavelength converting device including a light waveguide made of nonlinear optical material and outputting Cherenkov light, a reflecting mirror for reflecting guided light is provided at the end of the light waveguide. The light waveguide may include a light introducing portion and a curved light waveguide smoothly coupled with the light introducing portion. The light waveguide may have a Y-shaped structure including two light introducing portions partially parallel to each other, a junction part at which the two light introducing portions are smoothly joined together, and a linear light generating portion coupled with the junction part.

Description: The present invention relates to a wavelength converting device, and more particularly to a wavelength converting device which includes a light waveguide made of nonlinear material and generates SHG light or sum and difference frequency light. There has been known a wavelength converting device in which Cerenkov radiation light is produced from a light waveguide formed on a LiNbO₃ crystal by a proton exchange method. In this device, second harmonic generation (SHG) light in the form of Cerenkov radiation can be obtained with a converting efficiency of 1% when a semiconductor laser beam is incident on the light waveguide. The wavelength converting device with a similar light waveguide can produce sum and difference (sum/difference) frequency light in the form of Cherenkov radiation. For example, it is known that from two incident light beams having wavelengths λ₁ of 1.06µm and λ₂ of 0.824 µm, the device produces sum frequency light having wavelength λ₃ of 0.463 µm, where 1/λ₁ + 1/λ₂ = 1/λ₃. In such wavelength converting devices, much of the light introduced into the light waveguide is output with the wavelength remaining unconverted, so that the conversion efficiency is low. To improve the conversion efficiency, it is necessary to elongate an optical path of the light waveguide. However, this leads to an elongation of the device itself, making the device bulky. Further, in the above wavelength converting devices, the output light is obtained only in one direction, and not obtained in the other directions. Furthermore, in the sum/difference frequency generating device, a wave combing means such as a dichroic mirror must be provided prior to the light waveguide in order to introduce two input light beams of wavelengths λ₁ and λ₂, since the light waveguide is rectilinear in shape. In the case where two light beams of λ₁ and λ₂ are led through optical fibres to the device, the light beams emanating from the optical fibres are first collimated by objectives, the collimated light beams are combined and then converged by an objective, and are finally led to the light waveguide. Practically, it is very difficult to make the optical axis of the λ₁ light coincident with that of the λ₂ light. Further, the device becomes complicated and bulky. European Patent application EP-A-0 287 880 discloses a wavelength converting device of the type defined in the first part of claim 1. In the case of this prior art device, the waveguide is linear. The document EP-A-0,390,524 forms part of the state of the art under Article 54 (3) EPC and discloses a loop shaped light wave guide coupled to a light introducing portion. According to the present invention a wavelength converting device including a light waveguide formed in a substrate made of non-linear optical material, is characterised in that said light waveguide includes a linear light introducing portion and a curved light waveguide smoothly coupled with said light introducing portion wherein the curved light waveguide has a wavelength converting function and outputs wavelength converted light in a plurality of directions. The present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a first embodiment of a wavelength converting device; Figure 2 shows a second embodiment; Figure 3 shows a third embodiment; Figure 4 shows a Y shaped light introducing portion; Figure 5 shows a fourth embodiment; and Figure 6 shows a fifth embodiment. A first embodiment of the present invention shown in Figure 1 will be described. In the first embodiment, a light waveguide 28 includes a linear light introducing portion 28A and a circular light waveguide portion 28B smoothly connected to the former. An optical fibre 29 is coupled with the end of the light introducing portion 28A. Incident light is led from the optical fibre 29 to the light introducing portion 28A. In this embodiment, the substantial interactive length is infinite, thereby providing wavelength conversion of high efficiency. In a second embodiment shown in Figure 2, a light waveguide 30 is shaped in a spiral fashion so that the light waveguide 30 can be elongated without increasing the device area. Therefore, the interactive length is increased and the wavelength conversion efficiency is improved. A total reflection mirror 32 may be provided at the end of the spiral light waveguide, as in a third embodiment shown in Figure 3. Provision of the mirror 32 further elongates the interactive length, and therefore further improves the wavelength conversion efficiency. In the first to third embodiments, to obtain converted light along one direction, the combination of a total reflection mirror and a condensing lens is used. It is necessary to use a total reflection mirror shaped like a circular cone so as to reflect all of the output light beams. A method of producing sum/difference frequency light in response to the λ₁ light and the λ₂ light incident on the light waveguide, without using a dichroic mirror will now be described with reference to Figure 4. In Figure 4, a light waveguide 34 shaped like Y consists of two light introducing portions 34A and 34A which are partially parallel to each other, a junction part 34B at which the two introducing portions 34A are smoothly joined together and a single linear light generating portion 34C connected to the junction part 34B. Optical fibres 36A and 36B are coaxially coupled with the light introducing portions 34A and 34B, respectively, and lead the λ₁ light and the λ₂ light to those light introducing portions, respectively. The incident light beams of wavelengths λ₁ and λ₂ are input through the optical fibres 36A and 36B, travel as guided light through the introducing portions 34A and 34B, and are combined into a single beam of light at the junctions part 34B. Then, the light generating portion 34C generates output light of λ₃ as Cherenkov light. The two light introducing portions 34A and 34A join together at the junction part 34B. Since those two light introducing portions are combined to form a small angle there between, the light beams are little attenuated or scattered when those meet. Therefore, the light beams of wavelengths λ₁ and λ₂ can effectively be introduced to the light generating portion 34C, producing the intense output light of λ₃. There is no need to use a wave combining means such as a dichroic mirror before introduction of the light beams of wavelengths λ₁ and λ₂ into the light waveguide 34. This feature realizes the reduction of the device size. In the conventional device, to input the light beams of wavelengths λ₁ and λ₂ by the optical fibres, output light beams from the optical fibres are first collimated by objectives, combined together, and then again converged by an objective. In the example shown in Figure 4, there is no need for such an intricate process. A fourth embodiment of the present invention shown in Figure 5 will be described. In this embodiment, a circular light generating portion 34E is used in place of the linear light generating portion 34C shown in Figure 4. Since, in this embodiment, the substantial interactive length of the light generating portion 34E is infinite, high efficiency wavelength conversion is realized. Furthermore, since the light generating portion 34E is circular, the λ₃ light as output light can be obtained in every direction. A fifth embodiment of the present invention shown in Figure 6 will be described. The fifth embodiment is arranged such that a pair of linear light introducing portions 34F and 34G, which are parallel to each other, are tangentially coupled with a circular light generating portion 34E at the opposite positions. Optical fibres 36A and 36B for introducing the λ₁ and λ₂ light beams are respectively coupled with the linear light introducing portions 34F and 34G. In the fifth embodiment, there is no need to combine the two introducing portions. In this respect, the construction of the light waveguide is simpler than that of the fourth embodiment. Further, the linear light introducing portions 34F and 34G may be coupled with the circular light generating portion 34E tangentially in any desired direction. Accordingly, a freedom in coupling the optical fibres 36A and 36B with the device is improved. The light generating portion, which is circular or spirally shaped in each embodiment as mentioned above, may take any form, as long as it allows the substantial interactive length to become longer and does not need an increased device area. As seen from the foregoing description, the wavelength converting device according to the present invention can increase the substantial interactive length without increasing the device size, thereby improving the conversion efficiency of the wavelength of light. The light waveguide can be constructed with the light introducing portion and a curved light waveguide which is smoothly coupled with the light introducing portion. With this construction, the interactive length is further elongated, in spite of the small device size. This results in further improvement of the conversion efficiency. When the wavelength converting device is used as a sum/difference frequency generating device, light beams may be led to a light generating portion through two light introducing portions. Therefore, there is no need for using a wave combining means such as a dichroic mirror. Further, the light beams can be introduced directly from optical fibres. The above description is devoted to the Cerenkov radiation type wavelength converting device. The present invention is not limited to such a case, but applicable to another type of wavelength converting device which performs phase matching and can produce output light in a direction different from an input light direction.

Claims: A wavelength converting device including a light waveguide formed in a substrate (10) made of non-linear optical material, characterised in that said light waveguide includes a linear light introducing portion (28A; 34) and a curved light waveguide (28B; 30) smoothly coupled with said light introducing portion wherein the curved light waveguide has a wavelength converting function and outputs wavelength converted light in a plurality of directions. A wavelength converting device according to claim 1, wherein said curved light waveguide (28B) has the shape of a closed circle. A wavelength converting device according to claim 1, wherein said curved light waveguide (30) is spiral in shape. A wavelength converting device according to claim 3, further comprising reflecting means (32) provided at an end face of said spiral light waveguide (30) located opposite to said light introducing portion, for reflecting a guided light beam back to said spiral light waveguide. A wavelength converting device according to claim 1 or 2, wherein said device comprises a plurality of said light introducing portions (34), each smoothly coupled with said curved light waveguide. A wavelength converting device according to any one of claims 1 to 5, wherein said light introducing portion (34) has a generally Y-shaped structure including two light introducing sub-portions (34A) partially parallel to each other, a junction part (34B) at which said two light introducing sub-portions are smoothly joined together, and a connecting portion one end of which is coupled with said junction part and the other end of which is tangentially connected to said curved light waveguide. A wavelength converting device according to claim 5, wherein said light introducing portion (34F) is tangentially connected to said curved light waveguide at least at each connection point of a curved portion of said curved light waveguide. A wavelength converting device according to any one of claims 1 to 7, further comprising an optical fibre (29) coupled with an input end of said light introducing portion, for introducing an input light beam into said light introducing portion.

Inventors: AOSHIMA SHINICHIRO C O HAMAMAT, TAKAHASHI HIRONORI C O HAMAMAT, TSUCHIYA YUTAKA C O HAMMAMATSU, AOSHIMA, SHINICHIRO, C/O HAMAMATSU PHOTONICS K.K., TAKAHASHI, HIRONORI, C/O HAMAMATSU PHOTONICS K.K., TSUCHIYA, YUTAKA, C/O HAMMAMATSU PHOTONICS K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1990
Main classification: G02F1
IPC codes: G02F1, H01S3