Patent Application
20090310906
This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-158090, filed on Jun. 17, 2008, the entire contents of which are incorporated herein by reference.
The present invention relates to an optical device having an optical waveguide integrated on a substrate.
In optical communication technology, the demand for miniaturization of transmission systems has been increasing more and more recently. In order to respond to this demand, it is necessary to integrate functions which have heretofore been realized by connecting a plurality of individual optical devices on one substrate (one chip), and to further miniaturize the size of each optical device. As an important technique for such miniaturization and integration, adoption of a curved waveguide for the optical waveguide in the optical device can be mentioned. Recently as the curved waveguide, a three-dimensional curved waveguide (that is to say, one that is also bent in the depth direction of the device) is being studied.
When designing a curved waveguide in a substrate, it is obvious that reduction of the radius of curvature as much as possible is effective for miniaturization. On the other hand however, as the radius of curvature decreases, the optical loss (radiation loss) increases, leading to performance deterioration of the optical device having the optical waveguide, and hence care may be taken. Techniques for reducing such an optical loss are disclosed in Japanese Laid-Open Patent Publication No. 2007-094440 (reference document 1), Japanese Patent Publication No. 2847844 (reference document 2), and Japanese Patent Publication No. 2855676 (reference document 3).
Reference document 1 discloses a technique for arranging an air layer on a side of the waveguide by making a curved waveguide into a ridge structure on the substrate, and using the air layer as a cladding. Moreover reference documents 2 and 3 disclose a technique for forming a cladding area for the optical waveguide in a substrate surface layer on a side of the waveguide by using external diffusion or ion exchange.
All of the techniques disclosed in reference documents 1, 2 and 3 have a point to be solved in that they may not be applied to a three-dimensional curved waveguide passing through the inside of the substrate away from the substrate surface. That is to say, the curved waveguide buried inside the substrate may not have the ridge structure in reference document 1, and the cladding area may not be formed inside the substrate away from the substrate surface, by the external diffusion or ion exchange used in reference documents 2 and 3.
Focusing on the above point, it is required that an optical device structure and a producing method thereof that, even for an optical waveguide formed inside a substrate, is capable of providing a cladding for the curved waveguide.
An optical device proposed herein includes: a crystal substrate; an optical waveguide formed in the crystal substrate; and an amorphous area formed in the crystal substrate, adjacent to at least a part of the optical waveguide.
Moreover as a method for producing the optical device, there is proposed herein a method including a process of irradiating an ultrashort pulse laser onto a portion adjacent to an optical waveguide in a crystal substrate in which the optical waveguide is formed, to form an amorphous area in the adjacent portion.
Additional objects and advantage of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The optical device according to the present proposal is provided with an amorphous area adjacent to an optical waveguide. It is known that when an amorphized part is formed in a crystal substrate, a refractive index of the amorphized part decreases more than for the other parts. On the other hand, the optical waveguide in the crystal substrate is a part with a refractive index higher than for the other parts, and hence, if the amorphous area is formed adjacent to the optical waveguide, a difference in the refractive index between these parts increases. That is to say, the amorphous area can be used as the cladding of the optical waveguide.
Such an amorphous area can be formed inside a crystal substrate by irradiation of an ultrashort pulse laser. Therefore even when an optical waveguide is formed inside the substrate away from the substrate surface, the amorphous area can be formed along the optical waveguide inside the substrate by adjusting a focal position of the laser. That is to say, the amorphous area can be formed in a required position even inside the substrate away from the substrate surface. Therefore even for a three-dimensional curved waveguide, a structure of an optical device capable of having a cladding, and a producing method thereof can be provided.
An optical device 1 according to the first embodiment employs a lithium niobate (LiNbO3) substrate (hereinafter, referred to as an LN substrate) 2 as the crystal substrate, and an optical waveguide 3 is buried and formed inside the substrate away from a substrate surface 2a. The optical waveguide 3 is formed in an S-shaped curved planar shape having curved waveguides 3a and 3b at two places along the way.
For such an optical waveguide 3, in the first embodiment, amorphous areas 4 and 5 are formed adjacent to both of an internal circumference side and an external circumference side of the curved waveguides 3a and 3b, so as to sandwich the curved waveguides 3a and 3b therebetween. That is to say, one amorphous area 4 is continuously formed from the external circumference side of the curved waveguide 3a toward the internal circumference side of the curved waveguide 3b, and the other amorphous area 5 is continuously formed from the internal circumference side of the curved waveguide 3a toward the external circumference side of the curved waveguide 3b. However, if the cladding for the curved waveguides 3a and 3b is at least on the external circumference side of the curve, a necessary minimum function can be obtained. Therefore, as illustrated in
If the optical waveguide 3 in the LN substrate 2 is one that is formed in the substrate surface layer, it can be formed by thermal diffusion of titanium (Ti) as illustrated in the aforementioned reference document 1. However in the case where this is one that is formed inside the substrate as in the first embodiment, it can be formed by irradiation of an ultrashort pulse laser. For example, a technique for forming an optical waveguide by irradiating a femtosecond laser onto the crystal substrate to induce a physical change at a focal position thereof, and increase the refractive index at the portion where the physical change is induced, is disclosed in the following papers 1, 2, 3 and 4. Such an optical waveguide 3 formed by irradiation of a femtosecond laser is a part where the refractive index is higher than in other parts of the LN substrate 2.
On the other hand, for example in the case of lithium niobate, a situation where the refractive index is decreased by about 0.1 as compared to when in a crystalline state due to amorphization, is reported in the following Paper 5.
Consequently, for example in the LN substrate 2 having a refractive index of 2.2, the refractive index of the amorphous areas 4, 5, 14, and 15 becomes 2.1 (=2.2-0.1), being lower than the refractive index of the LN substrate 2 by 4.5%, so that the refractive index difference with the optical waveguide 3 is increased. That is to say, the amorphous areas 4, 5, 14, and 15 function as the cladding, so that the optical loss in the curved waveguides 3a and 3b can be reduced. As a result, the radius of curvature of the curved waveguide can be reduced further than in a conventional case, contributing to miniaturization of the optical device.
An optical device 21 according to the third embodiment in
An optical device 31 according to the fourth embodiment in
The one optical waveguide 33 is the same as that in the second embodiment, in which amorphous areas 35 and 36 are formed as the cladding, adjacent to the external circumference side of two curved waveguides 33a and 33b. Such an optical waveguide 33 can be formed by thermal diffusion of titanium, or by femtosecond laser irradiation as described above.
The other optical waveguide 34 has three-dimensional curved waveguides 34a and 34b which curve and bend into the inside of the substrate for crossing. That is to say, the curved waveguides 34a and 34b curve in a horizontal direction along the LN substrate 32, and also curve in a perpendicular direction (thickness direction) to the LN substrate 32. Such an optical waveguide 34 is preferably formed by the above-mentioned femtosecond laser irradiation. Amorphous areas 37, 38, and 39 are formed for the curved waveguides 34a and 34b, adjacent to horizontal and perpendicular external circumferences. The amorphous areas 37 and 38 are formed in portions above and on the side, which become the external circumferential sides, of the curved waveguides 34a and 34b where these start to bent into the inside of the substrate. Moreover the amorphous area 39 is formed below the arch shape portion, which is the external circumferential side thereof, the arch shape portion being arranged to spanning under the one optical waveguide 33 along the curved waveguides 34a and 34b.
The optical devices according to the above respective embodiments can be used for example as a Mach-Zehnder optical modulator, or an optical switch.
The amorphous area in the above respective embodiments can be formed by irradiation of an ultrashort pulse laser, for example, a femtosecond laser. That is to say, as shown in
As described above, the femtosecond laser is also used for forming the optical waveguide. However, the irradiation power in this case is within a range from about 0.1 to 0.2 mW and works so as to increase the refractive index of the LN substrate. On the other hand, a technique for forming a void (depletion) along a light propagating layer by the femtosecond laser is disclosed in Japanese Laid-Open Patent Publication No. 2004-029286. This is for inducing ablation in the LN substrate as the physical change, and the irradiation power of the femtosecond laser in this case takes a value exceeding 2 mW. In contrast to these cases, when the irradiation power of the femtosecond laser is within a range from 0.4 to 2.0 mW, amorphization is induced in the LN substrate, to decrease the refractive index of the LN substrate, however ablation is not caused. In other words, when the irradiation power of the femtosecond laser irradiated onto the LN substrate is increased, the physical properties of the LN substrate change in the order of: an increase in refractive index; then amorphization (a decrease in refractive index); and then ablation. By using the power range of amorphization within these power ranges, the amorphous area can be formed by irradiation of the femtosecond laser. If ablation is caused in the LN substrate, there is concern that scattering of light might increase due to the void. Therefore, the amorphous area may be used.
As a specific example, in the LN substrate, an irradiation condition of the femtosecond laser for forming the amorphous area at a position of 40 to 70 μm depth from the substrate surface is such that the pulse width is 80 to 100 femtoseconds, the irradiation power is 0.5 to 2.0 mW, and the pulse repetition frequency is 100 to 1000 Hz.
In this way, the amorphous area in the LN substrate can be formed by irradiation of the femtosecond laser. Therefore the amorphous area can be formed in an adjacent portion even for a three-dimensional curved waveguide, thereby contributing to miniaturization of the optical device by adopting the three-dimensional curved waveguide. Besides, the LN substrate has been explained specifically, however, the present invention can be applied similarly to a crystal which is amorphized by laser irradiation to decrease the refractive index of the amorphous area.
The results of optical waveguiding simulation obtained by a beam propagation method (BPM) are illustrated in
As illustrated in the figures, in the optical waveguide in
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-158090 | Jun 2008 | JP | national |
