Patent Publication Number: US-2021175685-A1

Title: Semiconductor Optical Element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry of PCT Application No. PCT/JP2019/013591, filed on Mar. 28, 2019, which claims priority to Japanese Application No. 2018-077341, filed on Apr. 13, 2018, which applications are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a semiconductor optical element provided with a wavelength tunable laser with an external resonator. 
     BACKGROUND 
     An increase in communication traffic volume on the Internet and the like has resulted in demand for an increase in speed and an increase in capacity of optical fiber transmission. To meet these demands, development of digital coherent communication technology in which coherent optical communication technology and digital signal processing technology are utilized has advanced, and a 100G system has come into practical use. Such a communication system requires a high-power narrow-linewidth wavelength tunable laser as a local light source for transmission and reception. Also, there is demand for a reduction in size and cost of a device. 
     A wavelength tunable laser mainly includes a light emitting portion made of a semiconductor, and an external resonator provided with an external optical filter, and the oscillation wavelength of the laser is controlled by controlling the optical wavelength characteristics of this optical filter using heat, a carrier plasma effect, or the like. Wavelength tunable lasers in which a super-structure grating, a multi-ring filter, and a ladder filter are used have been reported (see Patent Literature 1). The main material of these wavelength tunable lasers is a direct-band-gap III-V compound semiconductor. Optical confinement is realized by adjusting the composition of the III-V compound semiconductor and imparting a difference in a refractive index between a core and a cladding. 
     In recent years, with demand for a reduction in the cost of optical transmitting-receiving devices in mind, silicon photonics technology has gained attention. With this technology, mature silicon processing technology is utilized to form an optical device with a high density in a large area on a circular silicon substrate (wafer) with a large diameter, thus making it possible to reduce the cost of the optical device and an optical integrated circuit. However, since silicon is an indirect-band-gap semiconductor, a highly efficient silicon laser has not been realized, and integration of a silicon laser and other optical devices on one silicon substrate has not been realized. Although a flip chip bonding method can be used to connect a semiconductor laser made of a III-V compound semiconductor to a silicon photonics circuit, the assembly cost increases, and thus a reduction in the cost of optical transmitting-receiving devices remains limited. 
     On the other hand, using wafer bonding technology that is currently under research and development enables hetero-integration of a silicon photonics circuit and a semiconductor laser on one silicon substrate. Accordingly, if a wavelength tunable laser can be provided on a silicon substrate using the wafer bonding technology, it is more likely that the potential for reducing the cost of an optical transmitting-receiving device can be increased. 
     CITATION LIST 
     Patent Literature 
     PTL 1—Japanese Patent No. 4402912 
     SUMMARY 
     A wavelength tunable laser mainly includes a light emitting portion (optical amplifier), an optical resonator, and a mechanism (e.g., heater) for adjusting a resonance spectrum of the optical resonator. In the case of forming a wavelength tunable laser on a silicon substrate, a III-V compound semiconductor formed through direct bonding has been used as the material for forming a light emitting portion, and a III-V compound semiconductor (e.g., InP) or silicon (Si) has been used as the material of an optical waveguide core included in an optical resonator. 
     However, InP and Si are materials with a high non-linear optical constant, and thus a non-linear effect cannot be ignored. For example, two-photon absorption loss is increased due to high power density inside the optical waveguide. This is a practical problem in a laser that requires high power. In addition, since a relative refractive index difference between InP or Si and a material of a cladding such as SiO2 is large, the properties of the optical resonator are remarkably impaired due to a dimensional error or variation in the width, thickness, and the like of the optical waveguide core. This impairs the properties and yield of a laser. 
     The present invention was achieved to solve the foregoing problems, and an object thereof is to make it possible to form a smaller wavelength tunable laser on a substrate made of silicon without impairing the properties and yield of the laser. 
     Means for Solving the Problem 
     A semiconductor optical element according to embodiments of the present invention includes a wavelength tunable laser formed on a substrate made of silicon, wherein the wavelength tunable laser includes a light emitting portion made of a III-V compound semiconductor, and an external resonator provided with an optical filter, the external resonator includes an optical waveguide including: a lower cladding layer that is made of an oxide and is formed on the substrate; a core that is made of SiN, SiON, or SiOn (n is smaller than 2) and is formed on the lower cladding layer; and an upper cladding layer that is made of an oxide and covers the core, and the light emitting portion and the external resonator are optically connected via a spot-size converting portion. 
     The above-mentioned semiconductor optical element includes a semiconductor core that is made of a III-V compound semiconductor, is optically connected to the light emitting portion through butt coupling, and is tapered away from the light emitting portion, wherein a tapered region of the semiconductor core is covered by the core, and the spot-size converting portion is constituted by the tapered region of the semiconductor core covered by the core. 
     In the above-mentioned semiconductor optical elements, the light emitting portion includes an active layer and current injection portions that are arranged sandwiching the active layer, and the light emitting portion is formed on the lower cladding layer. 
     In the above-mentioned semiconductor optical elements, a heater for controlling wavelength characteristics of guided light is provided on a portion of the optical waveguide included in the optical filter. 
     In the above-mentioned semiconductor optical elements, the optical filter includes a lattice filter, a ring filter, an another-mode interferometer, and a loop mirror. 
     In the above-mentioned semiconductor optical element, the lattice filter includes a plurality of delay interference filters that are optically connected in a multi-stage manner, and delay lengths of the stages formed of the plurality of delay interference filters are different from each other. 
     The above-mentioned semiconductor optical elements include a core portion made of a semiconductor that is provided on a port of any of the lattice filter, the ring filter, and the another-mode interferometer, or an anti-reflection structure formed of a metal layer. 
     The above-mentioned semiconductor optical elements include a structure for a power consumption reduction including a core portion that is made of a semiconductor and is provided in a region of the optical filter. 
     Effects of Embodiments of the Invention 
     As described above, with embodiments of the present invention, the core included in the external resonator is made of one of SiN, SiON, and SiOn (n is smaller than 2), and thus an excellent effect that a smaller wavelength tunable laser can be formed on a substrate made of silicon without impairing the properties and yield of the laser is obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional view showing a configuration of a semiconductor optical element according to an embodiment of the present invention. 
         FIG. 1B  is a plan view showing a portion of the configuration of the semiconductor optical element according to the embodiment of the present invention. 
         FIG. 1C  is a cross-sectional view showing a portion of the configuration of the semiconductor optical element according to the embodiment of the present invention. 
         FIG. 1D  is a cross-sectional view showing a portion of the configuration of the semiconductor optical element according to the embodiment of the present invention. 
         FIG. 1E  is a cross-sectional view showing a portion of the configuration of the semiconductor optical element according to the embodiment of the present invention. 
         FIG. 2  is a plan view showing the configuration of the semiconductor optical element according to the embodiment of the present invention. 
         FIG. 3  shows characteristic diagrams showing calculation results of a transmission spectrum of a lattice filter  131 . 
         FIG. 4  shows characteristic diagrams showing calculation results of a transmission spectrum of a ring filter  133 . 
         FIG. 5A  is a plan view showing a portion of a configuration of a terminal portion. 
         FIG. 5B  is a plan view showing a portion of the configuration of the terminal portion. 
         FIG. 6A  is a cross-sectional view showing a portion of a configuration of an optical filter provided in an external resonator. 
         FIG. 6B  is a cross-sectional view showing a portion of a configuration of the optical filter provided in the external resonator. 
         FIG. 7  is a characteristic diagram showing a relationship between the coupling coefficient of a directional coupler included in a loop mirror  135  with the reflectivity (solid line) and the transmissivity (broken line) of the loop mirror  135 . 
         FIG. 8A  is a cross-sectional view showing a portion of a configuration of another semiconductor optical element according to the embodiment of the present invention. 
         FIG. 8B  is a cross-sectional view showing a portion of the configuration of the other semiconductor optical element according to the embodiment of the present invention. 
         FIG. 9  is a plan view showing the configuration of the other semiconductor optical element according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, a semiconductor optical element according to an embodiment of the present invention will be described with reference to  FIGS. 1A to 1E . It should be noted that  FIG. 1A  shows a cross section that is parallel to a direction in which light is guided.  FIG. 1B  is a plan view.  FIGS. 1C to 1E  show cross sections that are orthogonal to the direction in which light is guided. This semiconductor optical element includes a wavelength tunable laser  102  formed on a substrate  101  made of single-crystal silicon. The wavelength tunable laser  102  includes a light emitting portion  103  made of a III-V compound semiconductor, and external resonators  103   a  and  103   b,  provided with an optical filter. 
     Each of the external resonators  103   a  and  103   b  includes an optical waveguide including a lower cladding layer  104  that is made of an oxide and is formed on the substrate  101 , a core  105  that is formed on the lower cladding layer  104 , and an upper cladding layer  106  that is made of an oxide and covers the core  105 . In the embodiment, the core  105  is made of one of SiN, SiON, and SiO n  (n is smaller than 2). For example, the core  105  is made of SiN. 
     The lower cladding layer  104  is made of silicon oxide (SiO 2 ) and has a thickness of about 3 μm, for example. It is sufficient that the lower cladding layer  104  is formed by performing thermal oxidation on the surface of the substrate  101  made of silicon, for example. The upper cladding layer  106  is made of silicon oxide and has a thickness of about 3 μm, for example. 
     The light emitting portion  103  is optically connected to the external resonators  103   a  and  103   b  via spot-size converting portions  115 . As shown in  FIGS. 1B and 1C , a semiconductor core  107   a  including a tapered portion  107   b  that is tapered away from the light emitting portion  103  is optically connected to the light emitting portion  103  through butt coupling. The semiconductor core  107   a  is made of InP, for example. The semiconductor core  107   a  and the tapered portion  107   b  are covered by the core  105  ( FIG. 1C ). The spot-size converting portion  115  is constituted by a region including the tapered portion  107   b  covered by the core  105 .  FIG. 1C  shows a cross section taken along line aa′ in  FIG. 1B . 
     It is preferable that the width of the leading end of the tapered portion  107   b  is small because the smaller the width of the leading end is, the more optical coupling loss is reduced, however the aspect ratio between the width and thickness of the leading end is increased, thus making it difficult to process this portion. Accordingly, in order to reduce the aspect ratio, it is preferable that the semiconductor core  107   a  and the tapered portion  107   b  are thin. For example, it is sufficient that the thicknesses of the semiconductor core  107   a  and the tapered portion  107   b  are 200 to 300 nm. A lateral-current-injection-type embedded active layer thin film structure of the light emitting portion  103 , which will be described later, can be joined to the semiconductor core  107   a  via a butt joint, and is thus excellent from the viewpoint of optical connection to the optical waveguide including the core  105  made of SiN, which is a different material. 
     The light emitting portion  103  is formed on the lower cladding layer  104  and includes an active layer  107 , and current injection portions  108  and  109  that are arranged to sandwich the active layer  107 , as shown in  FIG. 1D . In the embodiment, the active layer  107  and the current injection portions  108  and  109  are covered by a core extension portion  105   a  that is made of SiN and is formed to be continuous with the core  105 , and the upper cladding layer  106  is formed on the core extension portion  105   a . The core extension portion  105   a  serves as a passivation layer in the light emitting portion  103 . The active layer  107  and the current injection portions  108  and  109  are made of InP, for example. The active layer  107  is made of bulk InP, for example. The active layer  107  may also have a multiple quantum-well structure that is made of InGaAsP or InGaAlAs and has a thickness of about 100 nm.  FIG. 1D  shows a cross section taken along line bb′ in  FIG. 1B . 
     The current injection portion  108  is doped with a p-type dopant and is thus of a p-type, for example, and the current injection portion  109  is doped with an n-type dopant and is thus of an n-type, for example. The current injection portions  108  and  109  are, for example, respectively connected to electrodes in and  112  via contact layers  113  and  114  made of a compound semiconductor that is doped with a dopant at a higher concentration. The electrodes in and  112  are formed passing through the core extension portion  105   a  and the upper cladding layer  106 . It should be noted that, in the example shown in  FIG. 1D , a so-called lateral-current-injection-type embedded active layer structure is employed, but there is no limitation thereto. A configuration may also be employed in which the active layer is sandwiched between two current injection portions in the normal direction of the surface of the substrate  101 . 
     As shown in  FIG. 1E , a heater  117  formed of a metal layer is formed on a portion of the upper cladding layer  106  of the external resonator  103   b,  and thus an optical filter for controlling the wavelength characteristics of guided (transmitted) light is formed. 
     With the embodiment, in the external resonators  103   a  and  103   b  provided with an optical filter, the lower cladding layer  104  and the upper cladding layer  106  are made of an oxide such as silicon oxide, and are thus have a very different refractive index from the core  105  and active layer  107  made of a compound semiconductor such as InP. Accordingly, high optical confinement can be achieved in the core  105  and the active layer  107 . In the external resonators  103   a  and  103   b , abrupt bending of the optical waveguide and high integration can be achieved, and thus a reduction in the size of the optical filter can also be achieved. In addition, the gain per unit length is increased, thus making it possible to reduce the length of the light emitting portion  103  in the guiding direction. 
     As described above, the feature of the embodiment is that the cores  105  included in the external resonators  103   a  and  103   b  are made of one of SiN, SiON, and SiO n  (n is smaller than 2). This feature will be described below. 
     First, Si and InP, which have been conventionally used as materials of the core, are materials having a high non-linear optical constant, and two-photon absorption loss is caused due to high power density inside the optical waveguide. Thus, a non-linear effect cannot be ignored. The value of the effective non-linear parameter y determined in consideration of the non-linear constant of a material and the mode field diameter of an optical waveguide is about 300 (W −1 m −1 ) in the case of an optical waveguide in which the core is made of Si (Si optical waveguide). In the case where the core is made of InP, γ is substantially the same. A problem that a non-linear optical effect is exhibited inside the Si optical waveguide becomes evident particularly when light with several tens dBm of power is incident thereon. 
     This is a practical problem in a laser that requires high power. On the other hand, in the case of an optical waveguide in which the core is made of SiN (SiN optical waveguide), γ is about  1 . 4  (W −1 m −1 ), and the non-linear optical effect can be sufficiently suppressed compared with the Si optical waveguide, thus making it possible to increase power resistance compared with the case where Si is used. In other words, the non-linear optical effect is less likely to be exhibited in the SiN optical waveguide compared with the Si optical waveguide, and thus the SiN optical waveguide is suitable as a material of an external resonator for a high-power narrow-linewidth wavelength tunable laser. 
     A relative refractive index difference (Δ) between the Si core and the SiO 2  cladding is about 41% (substantially the same value is obtained in the case of the InP optical waveguide), high optical confinement is achieved, and the minimum bending radius is about 3 μm, which is very small. Therefore, a high degree of integration can be expected. However, in the case of the Si optical waveguide, a large phase error is caused by a processing error due to high optical confinement, resulting in the impairment of the external resonator properties. It is difficult to realize uniform in-plane distribution due to an increase in the area of a substrate, resulting in a large processing error. Therefore, in order to ensure a sufficient processing tolerance, a material of an optical waveguide needs to be selected such that a relative refractive index difference is smaller compared with the Si optical waveguide, and SiN whose refractive index is between those of SiO x  and Si is suitable as a material of the core from the viewpoint of the integration degree and the processing tolerance. For these reasons, a material such as SiN, SiON, or SiO n  (n is smaller than 2) is used as the material of the cores  105  included in the external resonators  103   a  and  103   b.    
     When the core  105  is formed of SiN, it is sufficient that an SiN film is formed using a known ECR plasma CVD method, for example. In the ECR plasma CVD method, ions with high electron energy are used to advance a film forming reaction, and therefore, there is no need to heat the substrate, thus making it possible to form the film at a low temperature. With this film formation method, the cores  105  included in the external resonators  103   a  and  103   b , the core extension portion  105   a,  and the like can be formed without breaking active optical devices formed on the Si substrate, such as the light emitting portion. 
     Incidentally, when a material gas such as SiH 4  or Si 2 H 6  is used to form a film made of SiN, SiON, or SiO n  (n is smaller than 2), an N—H group is formed in the deposited film. Light at a wavelength of about 1.510 nm is absorbed due to the stretching vibration of the N—H group. This wavelength is used in optical communication, and therefore, if an optical waveguide is formed using a film containing an N—H group therein, the absorption of light becomes problematic. To address this, the formation of an N—H group is suppressed by using a material gas such as deuterium silane gas in which deuterium is used instead of hydrogen, thus making it possible to solve the foregoing problems. 
     Here, as shown in  FIG. 2 , a configuration can be applied in which the external resonator  103   b  includes a lattice filter  131 , a multimode interferometer (MMI)  132 , a ring filter  133 , a loop mirror  135 , and the like. It should be noted that  FIG. 2  shows an example in which an optical modulator/amplifier  151  is connected to the leading end side of the loop mirror  135 . These members are optically connected. The lattice filter  131  has a configuration in which delay interference filters are connected in a multi-stage manner, and the free spectral range (FSR) can be determined based on the delay lengths thereof. It should be noted that, normally, a resonator has a plurality of resonance frequencies, and an interval between these resonance frequencies is called an FSR. 
     The transmission peak wavelength can be changed by heating the arms of the lattice filter  131  using a heater  117 , and thus the lattice filter  131  can be used as an optical filter with a wide variable wavelength range. The transmissivity of the ring filter  133  varies at a constant frequency interval, and thus the ring filter  133  is used as an optical filter for wavelength locking. The loop mirror  135  is constituted by a directional coupler and a loop-type optical waveguide, and the transmissivity and the reflectivity can be adjusted as desired depending on the coupling coefficient of the directional coupler. The optical filter of the wavelength tunable laser configured as described above has a configuration in which a ring filter  133  for wavelength locking that has an FSR corresponding to the interval of an oscillating frequency to be output, and a lattice filter  131  having a large variable wavelength range are combined. 
       FIG. 3  shows calculation results of a transmission spectrum of the lattice filter  131 , and  FIG. 4  shows calculation results of a transmission spectrum of a ring filter  133 . In the ring filter  133 , light at a non-resonance wavelength is output to a port  133   a . If the core end face of the optical waveguide that forms such a port  133   a  located at the terminus is cut at a right angle, for example, light is reflected off this end face and becomes stray light, and thus the wavelength characteristics of the optical filter are impaired. This causes instability in the oscillation mode of the laser. Even if the poll  133   a  is tapered and light is emitted thereto from the optical waveguide, the characteristics of the laser may be unexpectedly impaired due to cross talk caused by the emitted light being incident on the external resonator  103   b  included in the optical filter or the light emitting portion  103 . 
     Here, when Si or InP is used as the material of the optical filter, reflection on the port end face is suppressed by doping Si or InP for forming the port with a dopant when active optical devices such as an optical modulator are formed and thus allowing the port to absorb light. However, in the embodiment, the core  105  is made of SiN, and therefore, it is difficult to suppress reflection on the end face by doping the core  105  with a dopant in a subsequent step because the core  105  has been already formed. 
     Accordingly, the following configuration (anti-reflection measure) is applied to the embodiment. 
     As shown in  FIG. 5A , as an anti-reflection structure, a semiconductor core portion  121  made of InP is provided at the leading end of the core  105  made of SiN and is optically connected thereto. The semiconductor core portion  121  is doped with the p-type dopant that was used to form the light emitting portion  103 . A p-type InP has a high optical absorption coefficient, thus making it possible to allow quenching in the semiconductor core portion  121 . Since the dopant concentration of 1×10 18  cm −3  corresponds to 20 cm −1 , the doping concentration is set to about 5×10 18  cm −3 , and the length of the core of the semiconductor core portion  121  is set to about 500 μm. This enables quenching of about 20 dB. It is preferable that the semiconductor core portion  121  is formed in a meandering or spiral folded shape instead of a linear shape in order to reduce the area thereof. The same measure is also applied to the lattice filter  131  and an unnecessary port of the MMI  132 . 
     Anti-Reflection Measure  2   
     As shown in  FIG. 5B , as an anti-reflection structure, a surface plasmon optical waveguide made of a metal layer  122  is provided on the leading end side of the core  105  made of SiN and is optically connected thereto. With a surface plasmon mode in the surface plasmon optical waveguide, light can be attenuated over a short distance due to the electron scattering mechanism in the metal layer  122 . In order to excite the surface plasmon mode from the mode of the optical waveguide including the core  105 , the leading end of the core  105  is formed in an asymmetrical shape, and thus a polarized wave is rotated to a polarized wave for exciting the surface plasmon. In addition, the leading end of the core  105  is formed in a tapered shape to extend the optical mode. The same measure is also applied to the lattice filter  131  and an unnecessary port of the MMI  132 . 
     Next, the optical filters in the external resonators  103   a  and  103   b  will be described. Since SiN exhibits a thermooptic effect, the wavelength characteristics of the optical filter can be controlled using heat. Accordingly, as described with reference to  FIG. 1E , the heater  117  for heating is arranged above the core  105  with the upper cladding layer  106  made of SiO 2  being located therebetween, and the refractive index of the core  105  is changed by heat to perform wavelength control of the optical filter. It is necessary to apply electric power to the heater  117  from the outside to heat the heater  117  in order to cause a desired wavelength change. Needless to say, it is desirable that the amount of electric power is small. Here, a structure for causing a wavelength change with a smaller amount of electric power will be described. Since Si and InP have a higher thermooptic coefficient than that of SiN, a wavelength change can be caused with a smaller amount of electric power by using semiconductor materials such as Si and InP together. 
     A power consumption reduction measure will be described below. 
     Power Consumption Reduction Measure  1   
     As shown in  FIG. 6A , in the optical filter region, a semiconductor core portion  123  made of InP, which has a high thermooptic coefficient, is embedded in the core  105  as a structure for a power consumption reduction. The semiconductor core portion  123  is arranged at a position overlapping the optical mode of the optical waveguide including the core  105 . The semiconductor core portion  123  is tapered in order to suppress light reflection at the end portion of the semiconductor core portion  123 . Since InP has a higher thermooptic coefficient than that of SiN, a change in an effective refractive index caused by heat is greater in the optical waveguide structure including the core  105  in which the semiconductor core portion  123  is embedded. As a result, the amount of electric power required to cause a desired wavelength change can be reduced. 
     Power Consumption Reduction Measure  2   
     As shown in  FIG. 6B , in the optical filter region, the core portion is replaced with a semiconductor core portion  124  made of InP. In this example, the semiconductor core portion  124  serves as a structure for a power consumption reduction. At a portion where the optical waveguide including the core  105  and the optical waveguide including the semiconductor core portion  124  are optically connected to each other, a spot-size converting structure such as a tapered leading end of the semiconductor core portion  124  is employed. The amount of electric power to be applied thereto can be reduced for the same reason as described in the power consumption reduction measure  1 . 
     It should be noted that the above-described optical filter region (in which a wavelength change is to be caused) is a region located directly under the heater  117  in the lattice filter  131 , which was described with reference to  FIG. 2 , for example. However, semiconductor cores as described above are not used in the directional coupler, which is likely to be affected by a processing error, and the ring filter  133 , which is likely to be affected by a non-linear optical effect. 
     Examples of a light reflection mechanism include a mechanism in which a difference in a refractive index between an optical waveguide and a gap provided in a portion of the optical waveguide is utilized, and a mechanism in which a difference in a refractive index between air and an end face of an optical waveguide routed to the end face of a chip is utilized. In the former case, an external quantum efficiency is reduced due to scattering of light at the gap of the optical waveguide. In the latter case, there is a problem that an optical modulator or a booster optical amplifier cannot be integrated in the chip. 
     On the other hand, with the above-described loop mirror  135 , the transmissivity and the reflectivity can be adjusted as desired depending on the coupling coefficient of the directional coupler theoretically without loss. The reflectivity and the transmissivity are less dependent on a wavelength compared with a diffract grating and the like.  FIG. 7  is a graph in which the horizontal axis indicates the coupling coefficient of the directional coupler included in the loop mirror  135 , and the vertical axis indicates the reflectivity (solid line) and the transmissivity (broken line) of the loop mirror  135 . The reflectivity and the transmissivity can be controlled theoretically without loss (the sum of the reflectivity and the transmissivity is 100%). As described above, with the reflection mechanism in which the loop mirror  135  is used, the wavelength dependency is smaller compared with a diffract grating, and the reflectivity and the transmissivity can be determined as desired. For example, integration of a wavelength tunable light source and an optical modulator or a booster optical amplifier in a chip can be realized by connecting the wavelength tunable light source to a port on a reflection side and the optical modulator or booster optical amplifier to a port on a transmission side. 
     The lattice filter  131  narrows the bandwidth of this transmission spectrum and thus enables the wavelength tunable laser  102  of the embodiment to stably vary a wavelength. It is sufficient that the delay length is increased (the diffraction order is increased) in order to narrow the bandwidth of the transmission spectrum, but the FSR is reduced at the same time, and thus a problem arises in that oscillations occur at two or more wavelengths. Accordingly, in order to realize a stable oscillation in a single mode, transmissivities need to be reduced at diffraction orders other than a desired diffraction order without increasing the transmission bandwidth. This can be realized by setting the delay lengths of the stages of a delay interferometer, namely the diffraction orders of the stages thereof, to different values instead of the same value. 
     It should be noted that, in the description above, an example was described in which the materials of both the lattice filter  131  included in the external resonators  103   a  and  103   b  and the optical waveguide core of the ring filter  133  are made of SiN, but there is no limitation to this example. For example, a hybrid-type configuration may also be employed in which the optical waveguide core of the ring filter  133 , which is likely to be affected by a non-linear effect, is made of SiN, whereas the optical waveguide core of the lattice filter  131  is made of InP, which has a high thermooptic coefficient. 
     In the above-described embodiment, the structure of the spot-size converting portion  115  in which the optical waveguide including the semiconductor core  107   a  is optically connected to the optical waveguide including the core  105  has a configuration in which the semiconductor core  107   a  is covered by the core extension portion  105   a,  but there is no limitation to this configuration. For example, as shown in  FIGS. 8A and 8B , a configuration may also be employed in which a core extension portion  105 ′ made of SiN that is formed to be continuous with the core  105  (not shown) is arranged above the semiconductor core  107   a.    
     In the embodiment, the case where a core made of p-type InP is inserted or a surface plasmon optical waveguide formed of a metal layer is used was described as an example of the termination process for the port of the filter, but there is no limitation to this example. For example, a core made of a semiconductor such as n-type InP or Ge may also be used. 
     In the embodiment, the structure in which the semiconductor core portion  123  made of InP is embedded in the core  105  in the region (optical filter) in which a wavelength change is to be caused was described as an example, but there is no limitation to this example. For example, the semiconductor core portion  123  may also be made of amorphous silicon. 
     In the description above, the case where the core included in the external resonator is made of SiN was described, but there is no limitation to this case. For example, the core included in the external resonator may also be made of SiON. In this case, in consideration of the integration degree and the processing tolerance, the refractive index can be changed as desired by changing the ratio of N in SiON. Similarly, the core included in the external resonator may also be made of SiO n  (n is smaller than 2). 
     An example was described in which a lattice filter and a ring filter are used as the optical filter of the external resonator, but there is no limitation to this example. For example, as shown in  FIG. 9 , a configuration may also be employed in which ring filters  133  are connected to one end of the light emitting portion  103  in series, an end face mirror  134  is provided at an optical output end, and a loop mirror  135  is connected to the other end of the light emitting portion  103 . In this case, it is sufficient that heaters are provided on the ring filters  133 . 
     In the description above, the structure in which a wavelength change is caused by heat generated by the heater was described, but there is no limitation to this structure, and a mechanism in which a carrier plasma effect of a semiconductor such as InP is utilized may also be employed. 
     As described above, with embodiments of the present invention, the core included in the external resonator is made of one of SiN, SiON, and SiO n  (n is smaller than 2), and thus a smaller wavelength tunable laser can be formed on a substrate made of silicon without impairing the properties and yield of the laser. 
     It should be noted that the present invention is not limited to the embodiment described above, and it is obvious that many modifications and combinations can be made by those ordinarily skilled in the art within the scope of the technical idea of the present invention. 
     REFERENCE SIGNS LIST 
       101  Substrate 
       102  Wavelength tunable laser 
       103  Light emitting portion 
       103   a  External resonator 
       103   b  External resonator 
       104  Lower cladding layer 
       105  Core 
       105   a  Core extension portion 
       106  Upper cladding layer 
       107  Active layer 
       107   a  Semiconductor core 
       107   b  Tapered portion 
       108  Current injection portion 
       109  Current injection portion 
       111  Electrode 
       112  Electrode 
       113  Contact layer 
       114  Contact layer 
       115  Spot-size converting portion 
       117  Heater.