Abstract:
Provided are: an optical waveguide that relatively easily expands a spot size and that can suppress an increase in optical coupling loss with another optical waveguide element; and an optical component and variable-wavelength laser that use the optical waveguide. The optical waveguide is provided with: a cladding member; and a core layer that is disposed within the cladding member and that is formed as an elongated body having a rectangular cross-sectional shape from a material having a higher refractive index than the material configuring the cladding member. Here, the cross-sectional shape of the core layer is characterized in having a rectangular shape in which the length in the lateral direction is at least 10 times the length in the vertical direction.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical waveguide, and an optical component and a variable wavelength laser which use the same, and particularly relates to an optical waveguide provided with a thin-wire optical waveguide, and an optical component and a variable wavelength laser which use the same. 
       BACKGROUND ART 
       [0002]    Developed and application of Planar Optical Circuits (PLC: Planar Lightwave Circuit) have been advanced mainly in quartz-based types, and they have played an important role as a key component underpinning recent optical communication markets in Array Waveguide Gratings (AWG) or splitters among other things. Recently, development of a new functional element has also been advanced, such as a wavelength variable light source having a compound semiconductor amplifier (SOA: Semiconductor Optical Amplifier) hybrid-mounted on a quartz-based PLC. It has been actively explored to mount an active element and a passive element together on a common PLC substrate, to implement a small-sized inexpensive system on one chip. 
         [0003]    However, with required functions being increasingly complicated and advanced, the size of elements and driving electrical-power-consumption in PLCs has increased, and limitations have emerged in functions and performances achievable by using quartz-based types. For this reason, research and development of SOI (Silicon on Insulator) waveguides utilizing silicon microfabrication technologies such as silicon thin-wires and Photonic Crystals (PCs) has attracted attention, and their feasibility is examined for key components having small-sized, low electrical-power-consuming, and inexpensive characteristics. 
         [0004]    In SOI waveguides, silicon was used as a core material to increase the difference of specific refractive index from that of cladding materials (SiO 2  and its dielectrics), to achieve miniaturization. Quartz-based waveguides have a difference of specific refractive index A of about 5% and a bending radius of about 500 μm, while A of silicon thin-wire optical waveguides is 40% or more and their bending radius can be decreased down to a few μm. For this reason, silicon thin-wires enable significant reduction in the PLC size. 
         [0005]    However, when the difference of specific refractive index is increased, the core diameter must be decreased to satisfy a single mode condition for propagating light, and this causes a difference of spot size from that of other waveguide elements such as optical fiber, resulting in increase in loss of optical coupling. In view of this, PTL1 and PTL2, for example, disclose a technology for enlarging the spot size. In PTL2, the disclosed technology enlarges the spot size by forming an output/input region of a silicon thin-wire optical waveguide into a tapered shape such that the width and thickness of the core layer are each decreased in a direction of light propagation. 
       CITATION LIST 
       [0006]    [PTL1] Japanese Patent Application Laid-open No. 2002-162528 
         [0007]    [PTL2] International Publication No. 2008/111447 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    However, since silicon thin-wire optical waveguides intrinsically have a small core diameter and a high light-confinement rate, a sufficiently long taper must be secured in order to enlarge the spot size. In addition, the fabrication process of the taper becomes complicated when reducing each of the width and thickness of the core layer in a direction of light propagation. 
         [0009]    The present invention has been carried out in view of the above-mentioned problems, and an object thereof is to provide an optical waveguide which enables relatively easy enlargement of the spot size to inhibit increase in the loss of optical coupling with other waveguide elements, and an optical component and a variable wavelength laser which use the same. 
       Solution to Problem 
       [0010]    To achieve the above-mentioned object, an optical wave guide according to the present invention includes: a cladding member; and a core layer which is disposed in the cladding member and is formed into an elongated body having a rectangular cross-sectional shape from a material having a refractive index higher than that of a material constituting the cladding member, wherein the cross-sectional shape of the core layer is a rectangle having a length in a lateral direction 10 times or more as long as a length in a vertical direction. 
         [0011]    To achieve the above-mentioned object, an optical component according to the present invention includes the optical wave guide. 
         [0012]    To achieve the above-mentioned object, a variable wavelength laser according to the present invention includes: a ring oscillator comprising the ring oscillator and a compound semiconductor amplifier, wherein a connection side end of the core layer of the ring oscillator connected to the compound semiconductor amplifier is formed into a tapered shape having a decreasing a length in a lateral direction. 
       ADVANTAGEOUS EFFECT OF INVENTION 
       [0013]    The above-described aspect of the present invention enables relatively easy enlargement of the spot size to inhibit increase in the loss of optical coupling with other waveguide elements. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a transparent perspective view of an optical waveguide  100  according to a first exemplary embodiment. 
           [0015]      FIG. 2A  is a cross-sectional view of a conventional optical waveguide  900 . 
           [0016]      FIG. 2B  is a cross-sectional view of an optical waveguide  100  according to the exemplary embodiment. 
           [0017]      FIG. 3  is a graph indicating a light propagation constant for each cross-section of a silicon waveguide. 
           [0018]      FIG. 4A  is a graph indicating a relationship between the structure of a waveguide and a beam diameter, in the lateral direction, of light propagating through the waveguide, along with lateral direction. 
           [0019]      FIG. 4B  is a graph indicating a relationship between the structure of the waveguide and the beam diameter, in the vertical direction, of light propagating through the waveguide. 
           [0020]      FIG. 5  is a transparent perspective view of a variable wavelength laser  200  according to a second exemplary embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
       [0021]    A first exemplary embodiment according to the present invention will be explained. An optical waveguide according to this exemplary embodiment is depicted as a transparent perspective view in  FIG. 1 . In  FIG. 1 , the optical waveguide  100  is constituted by a cladding member  110  and a silicon flattened optical waveguide  120 . 
         [0022]    The cladding member  110  is made of a material such as SiO 2  and its dielectrics, and the silicon flattened optical waveguide  120  is disposed inside the cladding member  110 . The cladding member  110  is formed on a non-illustrated substrate such as a silicon substrate by depositing SiO 2 , by using, for example, silicon microfabrication technologies. 
         [0023]    The silicon flattened optical waveguide  120  is made of a material having a higher refractive index than that of the cladding member  110 , and light signals propagate through the silicon flattened optical waveguide  120 . The silicon flattened optical waveguide  120  according to this exemplary embodiment is formed into a rectangular parallelepiped shape having a cross-section the aspect ratio of which is about 1:10. For example, the silicon flattened optical waveguide  120  is formed into a shape of 1.0 μm in optical waveguide width and 0.05 μm in optical waveguide thickness (aspect ratio is 1:20) by silicon microfabrication technologies such as CMOS (Complementary Metal Oxide Semiconductor) processes. The dimension of the optical waveguide width and the optical waveguide thickness of the silicon flattened optical waveguide  120  are not limited to the above-mentioned values so long as the ratio of the optical waveguide width to the optical waveguide thickness is 10 or more. 
         [0024]    Operation of the optical waveguide  100  according to this exemplary embodiment will be explained in comparison with a conventional waveguide  900 .  FIG. 2A  depicts a cross-sectional view of the conventional waveguide  900 , and  FIG. 2B  depicts that of the optical waveguide  100  according to this exemplary embodiment. 
         [0025]    As shown in  FIG. 2A , the conventional waveguide  900  is constituted by a cladding member  910  and a silicon thin-wire optical waveguide  920 . The cladding member  910  is configured in a similar manner to that of the cladding member  110  of the optical waveguide  100  according to this exemplary embodiment. On the other hand, the silicon thin-wire optical waveguide  920  is formed into a rectangular parallelepiped shape of nearly 0.4 μm in waveguide width and nearly 0.2 μm in waveguide thickness, with the aspect ratio being about 1:2. In other words, the aspect ratio of the silicon flattened optical waveguide  120  of the optical waveguide  100  according to this exemplary embodiment is about ten times as large as that of the thin-wire optical waveguide  920  of the conventional waveguide  900 . 
         [0026]      FIG. 2A ,  FIG. 2B  depict a spread of a light signal for communication of 1.3 to 1.55 μm in wavelength injected into the conventional silicon thin-wire optical waveguide  920  of  FIG. 2A  and into the silicon flattened optical waveguide  120  according to this exemplary embodiment of  FIG. 2B , respectively. 
         [0027]    As shown in  FIG. 2A , most of light intensity of the light signal injected into the silicon thin-wire optical waveguide  920  of the conventional waveguide  900  is distributed within the cross-section of the silicon thin-wire optical waveguide  920 . The reason for this is that the width and the thickness of the silicon thin-wire optical waveguide  920  are formed into dimensions such that light signals are confined easily. In the conventional waveguide  900 , in order to minimize the size of optical devices, light confinement rate is designed to be as high as possible so that radiation loss of light is in a negligible level even for bent waveguides. For example, in the conventional waveguide  900 , the light loss becomes approximately 0 dB even for a very small bent waveguide having a radius of curvature of 10 μm. 
         [0028]    On the other hand, as shown in  FIG. 2B , most of the light intensity of the light signal injected into the silicon flattened optical waveguide  120  of the optical waveguide  100  according to this exemplary embodiment is distributed outside the cross-section of the silicon flattened optical waveguide  120 . The reason for this is that the thickness of the silicon flattened optical waveguide  120  is thin, and therefore, the light signal cannot be perfectly confined in the thickness direction, leading to a distribution where a large part of the light intensity distribution is outside the cross-section of the silicon flattened optical waveguide  120 . 
         [0029]    As described above, since the optical waveguide  100  according to this exemplary embodiment has a small light confinement rate, the radius of curvature is difficult to be reduced. However, in the optical waveguide  100  according to this exemplary embodiment, the following effects can be actualized by the aspect ratio, greater than or equal to 10, of the cross-sectional shape of the silicon flattened optical waveguide  120 . 
         [0030]    The first effect is that the optical waveguide  100  according to this exemplary embodiment can reduce a scattering loss down to about one tenth of that of the conventional waveguide  900 , which scattering loss is caused by side-surface roughness (roughness) of the waveguide. As for the propagation loss of light, most of the loss comes from scattering loss in the sidewall roughness of a waveguide in the case of a material, such as silicon, which does not absorb light in itself. In particular, when the rate of light confinement into a core is large as in the case of silicon thin-wire optical waveguide  920 , light on the sidewall of the core is intense and scattering loss significantly appears in comparison to waveguides in a different structure or of a different material. 
         [0031]      FIG. 3  depicts a change in a light propagation constant for each cross-sectional structure of the silicon waveguide. In  FIG. 3 , the slope of the propagation constant is equivalent to a magnitude of the influence of the sidewall roughness of the waveguides. For example, in the case of the waveguide width of 0.1 to 0.5 μm, increase in the waveguide thickness up to 0.1 μm or more leads to a great variation in the propagation constant in response to the change in the waveguide width. In other words, in the case of the aspect ratio of about 1:1 to 2 of the cross-section of the silicon waveguide, the variation of the waveguide width caused by the roughness among other things leads to a variation of the propagation constant, sensitive to the variation of the waveguide width. 
         [0032]    In  FIG. 3 , Point A and Point B indicate a position corresponding to the waveguide  900  having the silicon thin-wire optical waveguide  920  and a position corresponding to the optical waveguide  100  according to this exemplary embodiment having the silicon flattened optical waveguide  120 , respectively. The conventional waveguide  900  (Point A) is significantly influenced by roughness because the propagation constant greatly varies in response to the change in the waveguide width. On the other hand, the slope of the propagation constant of the optical waveguide  100  according to this exemplary embodiment (Point B) is about one tenth of that of the waveguide  900  (Point A), and the influence of the sidewall roughness is reduced down to about one tenth of that of the waveguide  900  in the optical waveguide  100 . 
         [0033]    The second effect is that the optical waveguide  100  according to this exemplary embodiment can provide a satisfactory optical coupling with optical waveguides based on a different material, only by narrowing the width of the silicon flattened optical waveguide  120 . The silicon thin-wire optical waveguide  920  of the conventional waveguide  900  has a smaller cross-section than other optical waveguides, causing a large difference of light beam diameter in a connection portion of optical waveguides. For example, the light beam diameter in an optical fiber is about 10 μm, which is quite different from a submicron beam diameter of the silicon thin-wire optical waveguide  920 . Therefore, it is proposed to enlarge the beam diameter of the silicon thin-wire optical waveguide  920  to conform the beam diameter to that of other optical waveguides. 
         [0034]      FIG. 4A  and  FIG. 4B  depict a relation between a waveguide structure and a beam diameter of light propagating through the waveguide. For example, when a laser is constituted by a combination of the conventional waveguide  900  and an SOA, in order to conform the beam diameter of the optical waveguide based on InP, which is a raw material of the SOA, to the beam diameter of the silicon thin-wire optical waveguide  920 , the SOA waveguide and the silicon thin-wire optical waveguide  920  are each processed to be formed into a tapered shape so that their beam shape becomes a circle of 3 μm in diameter. 
         [0035]    In  FIG. 4A  and  FIG. 4B , Point T, Point A, and Point B indicate a position where the beam diameter is 3 μm or more, a position corresponding to the waveguide  900  having the silicon thin-wire optical waveguide  920 , and a position corresponding to the optical waveguide  100  according to this exemplary embodiment having the silicon flattened optical waveguide  120 , respectively. As shown in  FIG. 4A ,  FIG. 4B , Point T is 0.3 μm in waveguide width and nearly 0.05 μm in waveguide thickness; Point A is nearly 0.4 μmin waveguide width and nearly 0.2 μm in waveguide thickness; Point B is nearly 1.0 μm in waveguide width and nearly 0.05 μm in waveguide thickness. 
         [0036]    Further, when the waveguide  900  is connected to the SOA, a structure such as a two-dimensional tapered structure in which both of the waveguide width and the waveguide thickness are both changed or a two-step cladding structure is used to reduce the waveguide size down to a point where the beam diameter of the silicon thin-wire optical waveguide  920  (Point A) becomes 3 μm (Point T). However, when the two-dimensional tapered structure or the two-step cladding structure is applied, its process becomes complicated. Further, since the silicon thin-wire optical waveguide  920  strongly confines light signal therein, the light beam diameter cannot be readily enlarged only by changing the waveguide size. 
         [0037]    On the other hand, when a laser is constituted by a combination of the optical waveguide  100  according to this exemplary embodiment and the SOA, the waveguide thickness does not have to be changed, as shown in  FIG. 4A ,  FIG. 4B , and a desirable beam diameter can be obtained by a structure laterally tapered in one direction in which the waveguide width varies from about 1.0 μm to nearly 0.3 μm. Further, the silicon flattened optical waveguide  120  according to this exemplary embodiment intrinsically has small light confinement rate, and therefore, the size of the light beam diameter can be readily adjusted. Accordingly, the loss of optical coupling to other optical elements, such as SOAs, based on a different material can be readily reduced. 
         [0038]    The third effect is that when a laser is constituted by a combination of the optical waveguide  100  according to this exemplary embodiment and the SOA, the laser can be operated more stably than in the case of a combination of the conventional waveguide  900  and the SOA. 
         [0039]    As described above, the conventional waveguide  900  exhibits a large roughness scattering, and in this case, reflection of light becomes larger at scattering points. A laser using such an optical waveguide  900  as an external resonator leads to unstable laser operation owing to unnecessary internal reflection. 
         [0040]    In contrast to this, when a laser is constituted by a combination of the optical waveguide  100  according to this exemplary embodiment and the SOA, the influence of the sidewall roughness of the waveguide is small, and the amount of the internal reflection within the waveguide is also small. Accordingly, it is possible to constitute a stably operating laser by using the optical waveguide  100  according to this exemplary embodiment as an external resonator. 
         [0041]    The fourth effect is that the optical waveguide  100  according to this exemplary embodiment can reduce absorption loss of light within the silicon flattened optical waveguide  120  in comparison to the conventional waveguide  900 . The light absorption of silicon is in a negligible level for a single material, but as light density becomes high, two-photon absorption occurs significantly, leading to larger light loss. When two-photon absorption occurs, the amount of the light absorption also increases with increase in output power of laser light, and as a result, the light output power saturates. 
         [0042]    In contrast to this, since the optical waveguide  100  according to this exemplary embodiment has the flattened optical waveguide  120  which has a smaller light confinement rate than that of the silicon thin-wire optical waveguide  920 , it is not greatly influenced by light absorption in the silicon flattened optical waveguide  120 . In other words, since light density is low in the silicon flattened optical waveguide  120 , it is, in particular, scarcely influenced by absorption, such as two-photon absorption, which is proportional to the square of light intensity. 
         [0043]    As described above, there is an advantage described below to applying the silicon flattened optical waveguide  120  having a cross-sectional aspect ratio of about 1:10, comparing with applying the conventional silicon thin-wire optical waveguide  920  having a cross-sectional aspect ratio of about 1:1 to 2. In other words, an optical waveguide can be constituted, in spite of difficulty in reducing the radius of curvature thereof, which optical waveguide exhibits small scattering loss caused by the influence of the side surface roughness thereof, a small amount of the internal reflection therein, and small two-photon absorption. 
         [0044]    Not that the above-mentioned effects appear within a range where the aspect ratio of the cross-sectional shape of the silicon flattened optical waveguide  120  is 10 or more. For example, in the case of  FIG. 3 ,  FIG. 4A , and  FIG. 4B , this is within the rage where the waveguide width of the silicon flattened optical waveguide  120  is 0.8 to 1.4 μm and the waveguide thickness thereof is 0.02 to 0.08 μm. Further, more significant effects appear within the range where the waveguide width of the silicon flattened optical waveguide  120  is 0.9 to 1.2 μm and the waveguide thickness thereof is 0.04 to 0.06 μm. 
       Second Exemplary Embodiment 
       [0045]    A second exemplary embodiment will be explained. This exemplary embodiment applies a variable wavelength laser, as an external resonator, which uses a Si photonics element provided with the silicon flattened optical waveguide  120  explained in the first exemplary embodiment. The variable wavelength laser according to this exemplary embodiment is constituted by not only an optical resonator constituted by planar optical waveguides such as PLCs, but also a laser diode (LD) or an SOA directly implemented on the PLCs. 
         [0046]      FIG. 5  depicts a transparent perspective view of the variable wavelength laser according to this exemplary embodiment. In  FIG. 5 , the variable wavelength laser  200  is constituted by a Si photonics element  300  and an SOA  400 . The Si photonics element  300  is constituted by a silicon substrate  310 , a cladding member  320 , a first silicon flattened optical waveguide  330 , a second silicon flattened optical waveguide  340 , a third silicon flattened optical waveguide  350 , a first silicon flattened ring waveguide  360 , and a second silicon flattened ring waveguide  370 . 
         [0047]    The cladding member  320  is formed on the silicon substrate  310 . The cladding member  320  is the same as the cladding member  110  explained in the first exemplary embodiment. 
         [0048]    In the cladding member  320 , the silicon flattened optical waveguides  330 ,  340 , and  350 , and the silicon flattened ring waveguides  360  and  370  are aligned so as to constitute a loop mirror. 
         [0049]    The silicon flattened optical waveguides  330 ,  340 , and  350  are each formed in the same manner as in the case of the silicon flattened optical waveguide  120  explained in the first exemplary embodiment. In other words, the silicon flattened optical waveguides  330 ,  340 , and  350  are each formed into a shape of 1.0 μm in optical waveguide width, 0.05 μm in optical waveguide thickness, and about 1:20 in cross-sectional aspect ratio. 
         [0050]    The silicon flattened ring waveguides  360  and  370  constitute a ring oscillator which utilizes a slight difference of the circumferential length between each of the waveguides, to generate the vernier effect, in which both peaks match to each other to provide a desirable wavelength within a variable wavelength range. Thereby, a stable single-mode oscillation can be achieved in the variable wavelength laser  200 . The silicon flattened ring waveguides  360 ,  370  only have to be able to constitute a ring oscillator, and the number of the silicon flattened ring waveguides is not limited to two. 
         [0051]    Then, by the loop mirror constituted by the silicon flattened optical waveguides  330 ,  340 ,  350 , and the silicon flattened ring waveguides  360 , and  370 , light emitted from the SOA  400  makes round-trips, and oscillates at the wavelength selected by the ring oscillator (silicon flattened ring waveguides  360  and  370 ) between the mirror and the emitting end of the SOA 400 , to achieve laser-resonance. 
         [0052]    The first silicon flattened optical waveguide  330  has a connection side formed into a laterally tapered structure  331 , which side is connected to the optical waveguide of the SOA  400 . In other words, the connection interface of the first silicon flattened optical waveguide  320  with the optical waveguide of the SOA  400  is adjusted so that the beam shape becomes a circle of 3 μm in diameter, by applying a laterally tapered structure  331  in which the waveguide width changes from about 1.0 μm to nearly 0.3 μm, with the waveguide thickness kept constant. Thereby, connection loss upon connecting the first silicon flattened optical waveguide  330  to the SOA  400  is reduced down to 0.1 dB or less. 
         [0053]    As the laterally tapered structure  331  may be applied not only a linear taper where the waveguide width changes linearly, but also an exponential taper where the waveguide width changes exponentially. Note that the exponential taper can reduce the loss than the linear taper. 
         [0054]    In addition, it is desirable to apply non-reflective coating to the connection interface between the cladding member  320  and the SOA  400 . The applied non-reflective coating can prevent light reflection at an emitting end from generating larger connection loss. When the connection interface of the SOA  400  is constituted by a material, such as silica, different from semiconductors, the connection portion of the SOA  400  may be coated with a material such as gel for adjusting refractive index, and in addition, a coating which is non-reflective against the gel for adjusting refractive index may be formed on the connection interface of the SOA 400 . 
         [0055]    The above-mentioned variable wavelength laser  200  as-is exhibits the four effects explained in the first exemplary embodiment because, in the Si photonics element  300  constituting an external resonator, the silicon flattened optical waveguides  330 ,  340 , and  350  are used which are constituted in the same manner as in the case of the silicon flattened optical waveguide  120  explained in the first exemplary embodiment. In other words, since the variable wavelength laser  200  according to this exemplary embodiment is characterized in that the silicon flattened optical waveguides  330 ,  340  and  350  have low propagation loss, low internal reflection, and low coupling loss, the laser can oscillate at low electrical power and achieve stable laser resonance in a desirable wavelength. 
         [0056]    Note that the above-mentioned effects are significantly exhibited in DBR-LDs (Distributed Bragg reflector laser diodes) which have a long resonator length. In other words, since DBR-LDs have many resonance points in the device, RIN (Relative Intensity Noise) becomes large when scattering loss caused by the influence of the roughness is large. Therefore, when the silicon flattened optical waveguides  330 ,  340 , and  350  according to this exemplary embodiment is applied for an external resonator, a DBR-LD having small RIN can be provided. 
         [0057]    The present application invention is not limited to the above-mentioned exemplary embodiments, and the invention encompasses any design change without departing from the spirit of the invention. 
       INDUSTRIAL APPLICABILITY 
       [0058]    The invention in the present application can be widely applied for optical waveguides connected to silicon thin-wire optical waveguides. 
         [0059]    This application is based upon and claims the benefit of priority from Japanese patent application No. 2014-044935, filed on Mar. 7, 2014, the disclosure of which is incorporated herein in its entirety by reference. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Optical waveguide 
           110  Cladding member 
           120  Silicon flattened optical waveguide 
           200  Variable wavelength laser 
           300  Si photonics element 
           310  Silicon substrate 
           320  Cladding member 
           330 ,  340 ,  350  Silicon flattened optical waveguide 
           360 ,  370  Silicon flattened ring waveguide 
           400  SOA 
           900  Optical waveguide 
           910  Cladding member 
           920  Silicon thin-wire optical waveguide