JP2019121691A - Integrated laser light source and optical transceiver using the same - Google Patents

Abstract

To provide a light source which operates in a single mode over a wide temperature range and also has return-light resistance.SOLUTION: An integrated laser light source has: an optical waveguide substrate which has a silicon photonics waveguide and a mirror structure arranged on the silicon photonics waveguide or adjacently to the silicon photonics waveguide; and a distribution feedback type laser element which is mounted on a mounting surface formed on the optical waveguide substrate. An output end of the laser element faces an input end of the silicon photonics waveguide, and the mirror structure has a reflection band with a specific bandwidth including a Bragg wavelength of the laser element within a predetermined temperature range, and reflects part of light output from the laser element in the reflection band to feed it back to the laser element.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an integrated laser light source and an optical transceiver using the same.

  Silicon photonics devices are being investigated as key devices to realize optical transceivers for optical interconnects. In silicon photonics, light is introduced using light sources based on other materials instead of non-emitting silicon.

  When a light source is mounted on a silicon photonics substrate, a configuration in which a package incorporating optical elements such as a laser, an isolator, and a lens is mounted on the substrate with high accuracy is conventionally used. Placing the isolator in the package prevents reflected light from returning to the laser and stabilizes operation. However, the manufacturing cost of the laser package mounted with high accuracy is high, and the package including a plurality of optical elements is enlarged. As a result, the size and cost of silicon photonics devices also increases.

  On the other hand, a direct bonding structure in which a laser is directly mounted on a silicon photonics substrate and an optical waveguide of a laser light source is coupled to an optical waveguide of silicon is also implemented. Since this configuration mounts a small laser chip on a silicon photonics substrate, the area occupied is small, and size and cost reduction are expected. On the other hand, since the isolator can not be inserted in the direct junction type configuration, the return light from the silicon photonics circuit and the return light from the front thereof return to the laser light source. The disturbance due to the return light disturbs the phase of the light fed back in the laser resonator, generating noise and making the laser operation unstable.

  There is a quantum dot (QD: Quantum Dot) laser as a laser having high return light resistance. In the QD laser, since electrons and holes are confined in a minute dot structure, carrier movement is suppressed even at high temperatures, and the temperature dependence of current versus light output is smaller as compared with a quantum well laser. However, it may not be completely unaffected by temperature changes, and multimode oscillation may occur under the influence of changes in environmental temperature.

  A DFB (Distributed Feedback) laser having a diffraction grating inside a resonator emits in a single mode in order to reinforce specific wavelengths selected by the diffraction grating. There is known an optical integrated device in which a semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier) and a quartz optical circuit device provided with a grating are mounted on a silicon bench (see, for example, Patent Document 1). The grating of this optical integrated element constitutes an external resonator. There is also known a configuration in which part of light is fed back to a semiconductor laser by a grating (see, for example, Patent Document 2).

Unexamined-Japanese-Patent No. 2004-117706 JP-A-6-37386

  The output end face of the DFB laser is provided with an antireflective film (AR coating) for transmitting light of a specific wavelength. In order to reduce the influence of the return light, the reflectance at the laser emission facet is increased to some extent to strengthen the light intensity by the laser feedback, and the influence of the return light is made relatively small to be less susceptible to phase disturbance. Conceivable. However, when this measure is applied to a single mode DFB laser, as shown in FIG. 1, the temperature dependence of the gain spectrum of the laser and the Bragg wavelength causes a problem of insufficient oscillation in multimode oscillation or DFB mode.

  In FIG. 1, when the environmental temperature changes to 0 ° C., 35 ° C., and 70 ° C., the gain of the laser light source and the oscillation wavelength (Bragg wavelength) change. The oscillation wavelength is affected by the temperature dependence of the refractive index of the constituent material of the DFB laser, and the gain is determined by the band gap of the active layer. The oscillation wavelength and the gain peak diverge due to the influence of temperature change.

  If the gain peak and the Bragg wavelength are designed to match at low temperature, the gain is insufficient at high temperature and does not work (see 70 ° C. gain spectrum). Designing the gain peak and the Bragg wavelength to coincide at high temperature so as to operate even at high temperature reduces the gain near the Bragg wavelength on the low temperature side. As a result, the gain peak reaches the threshold gain in the Fabry-Perot (FP) mode and multimode oscillation occurs (the gain spectrum reference at 0 ° C.) before the threshold wavelength of the DFB mode is reached at the Bragg wavelength. ).

  As described above, in the configuration in which the laser light source is directly coupled to the silicon photonics waveguide, compatibility between return light resistance and single mode operation becomes an issue.

  An object of the present invention is to provide a light source which operates in a single mode in a wide temperature range and has return light resistance.

In one aspect, the integrated laser light source
An optical waveguide substrate comprising: a silicon photonics waveguide; and a mirror structure disposed on or adjacent to the silicon photonics waveguide.
A distributed feedback laser device mounted on a mounting surface formed on the optical waveguide substrate;
Have
The output end of the laser element faces the input end of the silicon photonics waveguide,
The mirror structure has a reflection band of a specific bandwidth including the Bragg wavelength of the laser element in a predetermined temperature range, and reflects part of the light output from the laser element at the reflection band, It returns to the laser element.

  In one aspect, a light source operating in a single mode over a wide temperature range and with return light tolerance is realized.

It is a figure explaining the problem at the time of applying the measure which raises return light tolerance to DFB laser. It is a figure explaining the principle of the present invention. It is a figure which shows the basic composition of the integrated laser light source of embodiment. It is a figure which shows the example of mounting of the laser element to an optical waveguide board | substrate. It is a structural example of the integrated laser light source of 1st Embodiment. It is a figure which shows the structural example of the diffraction grating as an example of a mirror structure. It is a figure which shows the relationship between the detuning amount of DFB wavelength of a laser element, and the reflectance of a diffraction grating. It is a structural example of the integrated laser light source of 2nd Embodiment. It is a structural example of the integrated laser light source of 3rd Embodiment. It is a structural example of the integrated laser light source of 4th Embodiment. It is a structural example in the case of using a heater as a phase shifter. It is a flow chart which shows operation of a controller at the time of actual operation of an integrated laser light source. FIG. 6 is a diagram showing the relationship between the operating temperature of the integrated laser light source 10 and single mode operation. It is a figure which shows the relationship of the gain peak of a laser element in each temperature, a Bragg wavelength, the center wavelength of the mirror structure by the side of an optical waveguide board | substrate, and heater electric power. It is a figure which shows the application example of the integrated laser light source of embodiment.

  FIG. 2 is a diagram for explaining the principle of the present invention. In order to cause single mode oscillation at the Bragg wavelength and / or prevent multimode oscillation, the threshold gain of the DFB mode is lowered at a predetermined wavelength band including the fluctuation range of the Bragg wavelength of the laser element. In FIG. 2, the DFB mode threshold gain is partially lowered in the range including the variation of Bragg wavelength at 0 ° C, 35 ° C and 70 ° C, but the threshold gain in FP mode remains high. Be done. In order to realize this, a mirror structure having a reflection band in a predetermined wavelength band including the Bragg wavelength of the laser element is provided on the silicon photonics substrate.

  The mirror structure provided on the silicon photonics substrate strengthens the resonance of the laser element by a certain degree to relatively reduce the influence of the return light from the outside. By providing the mirror structure on the silicon photonics substrate, a configuration equivalent to providing a low reflection film on the emission end face of the laser element is realized.

  If a low reflection film is directly attached to the emission end face of the laser element, multimode oscillation is facilitated and light loss increases. By realizing the same function as the low reflection film using silicon photonics technology, the necessary amount of light is returned to the laser element in the necessary band to prevent multimode oscillation and return light from the outside To mitigate the effects of By appropriately controlling the reflection bandwidth and reflectance of the mirror structure, it is possible to prevent multimode oscillation and reduce the influence of light returned from the outside.

  In FIG. 2, if the reflection bandwidth near the Bragg wavelength is too wide, the region where the threshold gain of the FP mode decreases is broadened, and the gain peak at low temperature (for example, 0 ° C.) may reach the PF mode threshold gain. . In order to prevent multimode oscillation at the low temperature side, the upper limit of the reflection bandwidth is set to 30 nm, although it depends on the reflectance. If it exceeds 30 nm, multimode oscillation may occur in a band in which oscillation is not desired.

  The Bragg wavelength fluctuates due to temperature change, and also fluctuates due to manufacturing variations of the DFB laser diffraction grating. Therefore, the lower limit of the reflection band is set to 10 nm which covers the fluctuation range of the Bragg wavelength depending on at least one of temperature change and manufacturing variation. If it is narrower than 10 nm, it can not sufficiently contribute to the oscillation of the Bragg wavelength having temperature dependency.

  As an example, when the temperature change range of the environment is 80 ° C., the fluctuation range of the DFB wavelength due to the temperature change is about 8 nm, and the fluctuation range of the DFB wavelength due to manufacturing variations is 10 nm, the reflection bandwidth of the mirror structure is about 20 nm Designed.

  The reflectivity of the mirror structure determines the reduction in DFB mode threshold gain near the Bragg wavelength. If the reflectivity is too high, the threshold gain is reduced but high power can not be obtained. If the reflectance is too small, the influence of external return light can not be sufficiently mitigated, and the return light resistance becomes insufficient. Therefore, the reflectance and reflection bandwidth of the mirror structure are optimally designed by a method described later.

  When oscillating in the DFB mode, an adequate amount of feedback is obtained from the mirror structure to enhance the resistance to return light and realize stable operation in a single mode. By controlling the bandwidth of the reflected light, the reflected light is fed back only to the DFB mode, and feedback to the FP mode is avoided to stably obtain single mode operation at the Bragg wavelength.

  In the mirror structure, a phase shifter may be disposed between the mirror structure and the laser element to control phase change. By arranging the phase shifter, even when the laser element itself is not in the phase state operating in the single mode, it is possible to control to the operating point operating in the single mode as a whole of the integrated laser light source. The control amount of the phase shifter may be controlled according to the temperature.

  Below, the specific structural example of an integrated laser light source is demonstrated.

<Basic configuration>
FIG. 3 shows the basic configuration of the integrated laser light source 10 of the embodiment. FIG. 3A is a cross-sectional view, and FIG. 3B is a top view. The integrated laser light source 10 includes an optical waveguide substrate 30 having an optical waveguide 32 manufactured by silicon photonics technology, and a laser element 20 mounted on the optical waveguide substrate 30.

  The laser element 20 is a quantum dot laser which has a quantum dot which makes 1.29 micrometers a gain peak at room temperature (25 ° C), for example in an active layer. The element length of the laser element 20 in the optical axis direction is, for example, 600 μm. An antireflective film (AR) is formed on the end face on the output side of the laser element 20 connected to the optical waveguide substrate 30. A high reflection film (HR) is formed on the end face (rear end face) opposite to the output end face.

  The optical waveguide 21 of the laser device 20 is an optical confinement layer including an active layer, and is vertically sandwiched by cladding. The band gap of the optical waveguide 21 is smaller than that of the cladding, and the refractive index is higher than that of the cladding. The optical waveguide 21 may be a refractive index waveguide surrounded by a low refractive index cladding not only in the vertical direction but also in the horizontal direction. Alternatively, it may be a gain waveguide defined by stripe-shaped current injection regions. Also, instead of the embedded waveguide as shown in FIG. 3, a ridge waveguide may be used.

  In the optical waveguide 21, a diffraction grating 22 having a uniform period is formed overall, except for the central portion. The diffraction grating 22 is formed, for example, at the boundary between the active layer and the cladding. A phase shifter (PS) without a periodic structure may be formed at the center of the element. The period of the diffraction grating 22 is designed to have a Bragg wavelength of 1.309 μm at room temperature (25 ° C.).

  The laser element 20 is mounted on the mounting surface 35 of the optical waveguide substrate 30. The optical waveguide substrate 30 is made of, for example, an SOI (Silicon on Insulator) wafer, and has an optical waveguide 32 having a silicon layer as a waveguide core on a silicon substrate 31. The silicon core optical waveguide 32 is surrounded by a silicon dioxide cladding 39.

  In the optical waveguide 32, at least an end region optically connected to the laser element 20 is an input waveguide to the optical waveguide substrate 30. The tip of the input waveguide is a spot size conversion waveguide 33 for efficiently inputting the output light from the laser element 20 into the optical waveguide 32. The spot size conversion waveguide 33 has a tapered shape so that the mode diameter at the output end of the laser element 20 and the mode diameter at the input end of the optical waveguide 32 substantially match.

  As an example, the height of the optical waveguide 32 is about 250 nm and the width is about 400 nm. The spot size conversion waveguide 33 is a silicon waveguide whose waveguide width gradually narrows toward the tip, and the width of the tip is about 180 nm. The optical waveguide 32 and the spot size conversion waveguide 33 are formed in one processing step by silicon photonics technology. The optical waveguide 32 and the spot size conversion waveguide 33 may be collectively referred to as a “silicon photonics waveguide”.

  In the region of the optical waveguide substrate 30 on which the laser element 20 is mounted, a mounting surface 35 on which the laser element 20 is disposed is formed. The depth of the mounting surface is designed so that the height of the optical waveguide 21 of the laser element 20 and the height of the silicon photonics optical waveguide (more specifically, the height of the end face of the spot size conversion waveguide 33) match. There is.

  The laser element 20 is connected to the mounting surface 35 by the metal solder layer 38. The metal solder layer 38 is, for example, an AuSn layer. The metal solder layer 38 is heated and melted when the laser element 20 is mounted, and fuses the laser element 20 to the optical waveguide substrate 30.

  The laser output light is efficiently transmitted through the output end of the optical waveguide 21 of the laser element 20 and the input end of the optical waveguide 32 of the optical waveguide substrate 30 (more specifically, the tip of the spot size conversion waveguide 33). Are aligned horizontally in a straight line with a gap 34 of less than 5 .mu.m. The coupling loss between the laser element 20 and the optical waveguide 32 in this arrangement is about 1.5 dB.

  As a feature of FIG. 3, a mirror structure 40 for returning a part of light to the laser element 20 is provided on the optical waveguide 32 of the optical waveguide substrate 30. The mirror structure 40 is a reflecting mirror of a diffraction grating formed on a silicon optical waveguide 32 as described later, or a reflecting mirror combining a diffraction grating and a splitter such as a directional coupler.

  A part of light is fed back from the mirror structure 40 to the laser element 20 in the range of 4% to 10%, preferably 5% to 9% of the emitted light of the laser element. By providing a phase adjuster in at least one of the laser element 20 and the mirror structure 40, optical feedback can be obtained in an optimal phase state. When it is less than 4%, it becomes difficult to sufficiently reduce the influence of the external return light. If it exceeds 10%, the output power of the integrated laser light source 10 is affected.

  FIG. 4 shows an example of mounting the laser element 20 on the optical waveguide substrate 30. The laser element 20 is flip chip bonded to the mounting surface 35 of the optical waveguide substrate 30. The laser element 20 is aligned with the optical waveguide substrate 30 using the alignment mark 37 formed on the mounting surface 35 and the alignment mark 37 formed on the bottom surface of the laser element 20, and the electrode 23 of the laser element 20 , Melt-join the electrode 36 of the mounting surface 35.

  The optical waveguide substrate 30 has a silicon waveguide array 32 a including a plurality of optical waveguides 32, and a spot size conversion waveguide 33 is formed at the tip of each optical waveguide 32. A mirror structure 40 is disposed in each of the optical waveguides 32. By providing the optical waveguide array 32a, optical communication in a plurality of channels is realized.

  Such an integrated laser light source 10 is used, for example, for short distance optical communication in a data center. In data center applications, the integrated laser light source 10 is often placed on the same blade as an LSI device or the like that handles a huge amount of data. Even when a cooling mechanism such as a heat sink is used in combination, the range of temperature change in the environment where the integrated laser light source 10 is placed may reach 80 ° C. or more. In the integrated laser light source 10, since the ratio of the feedback light to the laser element 20 and the reflection bandwidth are properly designed, the laser oscillation is stably performed in a single DFB mode regardless of the temperature change of the environment and the manufacturing variation. And resistance to return light.

  In the following example, for simplicity of illustration, attention is focused on one optical waveguide 32 and the corresponding mirror structure 40.

First Embodiment
FIG. 5 is a schematic top view showing a configuration example of the integrated laser light source 10A of the first embodiment. In the first embodiment, as the mirror structure 40, a reflecting mirror 40A of the diffraction grating 41 formed on the optical waveguide 32 is used. The laser element 20 is a DFB quantum dot laser having a diffraction grating 22 as described with reference to FIG. At the center of the diffraction grating 22, a phase shifter without a periodic structure may be formed as shown in FIG. The active layer of the laser element 20 has a quantum dot layer having a gain peak of 1.29 μm at room temperature (25 ° C.), and the Bragg wavelength of the diffraction grating 22 is set in the vicinity of 1.3 μm.

  As an example of such a laser device 20, for example, a quantum dot laser with a multilayer structure in which InAs quantum dots are formed on GaAs and crystal growth of InAs and GaAs is alternately repeated 10 to 20 times is used. A diffraction grating 22 is formed by forming a periodic trench structure of GaAs in the uppermost layer of the active layer and embedding it with InGaP.

  The reflection center wavelength of the diffraction grating 41 formed on the optical waveguide 32 of the optical waveguide substrate 30 is 1.3 μm, and the reflection bandwidth near the center wavelength is about 20 nm. The diffraction grating 41 reflects light in part of the light output from the laser element 20, preferably in the range of 4% to 10% of the output light power. The reflectivity can be designed appropriately according to the reflection bandwidth.

  FIG. 6 is a configuration example of the diffraction grating 41. As shown in FIG. The diffraction grating 41 is formed at the interface 42 between the silicon optical waveguide 32 and the cladding 39. The height h of the optical waveguide 32 is, for example, 220 nm, and the line width is 400 nm. The thickness of the lower cladding layer 39a of the optical waveguide 32 is 2 μm or more, and the thickness of the upper cladding layer 39b is 2 μm or more. The lower cladding layer 39a may be a buried oxide layer of the SOI wafer. The upper cladding layer 39 b may be a silicon oxide layer formed after the formation of the silicon optical waveguide 32 by etching.

  In the example of FIG. 6, the diffraction grating 41 is formed at the interface 42 between the upper surface of the optical waveguide 32 and the cladding 39. The diffraction grating 41 has, for example, a line width W of 450 nm, a length L of the periodically arranged grid 47 of 500 nm, a length of one cycle of about 200 nm, and a repetition number (period number) of 160. It is not limited to this example. The number of periods or the like may be changed in a range in which the Bragg wavelength of the laser element 20 substantially coincides with the central wavelength of the diffraction grating 41 forming the reflecting mirror 40A.

  FIG. 7 shows the relationship between the gain wavelength of the laser element 20 and the detuning amount of the Bragg wavelength (DFB wavelength), and the reflectance from the diffraction grating 41 to the laser element 20. As described above, the Bragg wavelength and the gain peak wavelength may be separated due to the temperature dependency of the oscillation wavelength and the temperature dependency of the gain. The detuning amount (horizontal axis) from the gain peak wavelength of the Bragg wavelength of the laser element 20 can be obtained by measuring the temperature characteristic (see FIG. 1) of the laser element 20 in advance. The vertical axis is the maximum reflectance when oscillation in the FP mode occurs.

  In the example of FIG. 7, the FP mode operates when the reflectance to the laser exceeds 5% (the value of the vertical axis is 0.05) when the oscillation center wavelength of the laser element 20 is 17 nm or more away from the gain peak wavelength. . When the light of 5% of the output light power of the laser element 20 is to be fed back, the diffraction grating 41 having a reflection bandwidth of 30 nm (15 nm × 2) or less is formed.

  Referring to FIGS. 7 and 2, 5% to 10% of the emitted light is fed back to the DFB oscillation mode within a predetermined range including the range in which the Bragg wavelength fluctuates due to temperature change, manufacturing variation and the like. By setting the reflectance to 1% or less in the range other than the oscillation wavelength, it is possible to obtain a threshold profile in which the threshold gain level of the DFB mode is partially lowered only near the Bragg wavelength as shown in FIG. . By providing the diffraction grating 41 in the optical waveguide 32 outside the laser element 20 and appropriately designing the profile of the threshold gain of the DFB mode, light is fed back only to the DFB mode and is not fed back to the FP mode. Thereby, single mode operation at the Bragg wavelength can be stably obtained, and the influence of light returned from the outside can be reduced.

Further, the reflectance profile of FIG. 7 is interpolated in the direction of decreasing the detuning amount, and the fifth-order diffraction of period number 7 is performed so that the reflectance at the central wavelength is 9% and the reflection bandwidth is 20 nm or less. A grid may be formed. If the DFB (Bragg) wavelength of the laser element 20 substantially matches the center wavelength of the diffraction grating 41 of the optical waveguide substrate 30 and the diffraction grating synchronized with the Bragg wavelength is a first-order Bragg reflector, five times (5 A similar effect can be obtained by forming a diffraction grating with a repetition number of 7 and a period length of x λ _B ).

Second Embodiment
FIG. 8 is a schematic top view showing a configuration example of an integrated laser light source 10B of the second embodiment. In the second embodiment, as the mirror structure 40, a reflecting mirror 40BA disposed adjacent to the optical waveguide 32 is used. The reflecting mirror 40B has a directional coupler (or branching device) 42 formed of a silicon waveguide and a diffraction grating (Bragg reflector) 41 formed of a silicon waveguide.

  The branching ratio of the directional coupler 42 is about 30%. The diffraction grating 41 has a period number of 160, a bandwidth of about 20 nm with a reflectance of 96% or more, and a central wavelength of 1.311 μm at room temperature (25 ° C.). Most (96% or more) of the light branched by the directional coupler 42 is reflected by the diffraction grating 41, and 30% of the light is coupled again into the optical waveguide 32 by the directional coupler 42, and the spot size conversion waveguide It returns to the laser element 20 through 33. The ratio of the reflected light to the output light of the laser element 20 is about 9%.

  The reflecting mirror 40B is advantageous in that the reflectance and the reflection bandwidth can be individually controlled. When the diffraction grating 41 as a mirror structure is formed on the silicon optical waveguide 32 as shown in FIG. 5, both the reflectance and the reflection bandwidth are controlled only by the diffraction grating 41, so that the design and manufacture become complicated There is. On the other hand, in the configuration of FIG. 8, the reflection bandwidth can be controlled by the diffraction grating 41, and the reflectance can be designed relatively easily by the branching ratio of the directional coupler 42.

  The light returned from the reflecting mirror 40B to the laser element 20 causes the resonance at the DFB wavelength of the laser element 20 to be high, and the ratio of the return light from the outside is relatively reduced. When the laser element 20 has a phase shifter (PS) at the center as shown in FIG. 3, it is possible to adjust the phase rotation of the returned light to align the phases.

  Even in this configuration, stable single mode oscillation can be maintained in an environment with temperature change, and the influence of light returned from the outside can be reduced.

Third Embodiment
FIG. 9 is a schematic top view showing a configuration example of an integrated laser light source 10C of the third embodiment. In the third embodiment, a reflecting mirror 40C disposed adjacent to the light guide 32 is used as the mirror structure 40. The reflecting mirror 40C has a ring resonator 43a formed of a silicon waveguide, a ring resonator 43b, a directional coupler 42, and a diffraction grating 41.

  The resonant frequency of the ring resonators 43a and 43b is determined by the refractive index and the circumferential length of each ring waveguide. The ring resonators 43 a and 43 b are designed such that their resonant frequency is equal to the resonant frequency of the laser element 20.

  The gap and coupling length between the optical waveguide 32 and the ring resonator 43a, the gap and coupling length between the ring resonator 43a and the ring resonator 43b, the gap and coupling length between the ring resonator 43b and the directional coupler 42 The coupling efficiency is determined by Therefore, by appropriately designing these parameters, the coupling ratio and the passband can be controlled. As an example, as in the second embodiment, 30% of the light emitted from the laser element 20 is coupled from the optical waveguide 32 to the diffraction grating 41 side, and the reflectance of the diffraction grating 41 is 96% or more. . The reflection bandwidth of the diffraction grating 41 may be set to, for example, 30 nm, and the bandwidth may be further narrowed by the ring resonators 43a and 43b.

  Since the reflectance and the reflection bandwidth can be finely adjusted by the ring resonators 43a and 43b, the reflectance and the reflection bandwidth of the diffraction grating 41 can be designed wide or roughly. Also in this configuration, as in the second embodiment, stable single mode operation can be maintained in an environment with temperature change, and the influence of light returned from the outside can be reduced.

  The number of ring resonators 43 used in the multiple ring resonator is not limited to two, and an appropriate number of ring resonators may be cascaded. The shape of the ring resonator 43 is not limited to a circle or an ellipse, and a suitable shape may be selected, such as a racetrack shape or a square shape having corners curved with a predetermined curvature radius.

Fourth Embodiment
FIG. 10 is a schematic top view showing a configuration example of an integrated laser light source 10D of the fourth embodiment. In the fourth embodiment, as the mirror structure 40, a reflecting mirror 40D disposed adjacent to the light guide 32 is used. In the reflecting mirror 40D, a phase shifter 44 is disposed between the directional coupler 42 and the diffraction grating 41.

  The phase shifter 44 controls the amount of phase rotation of light propagating through the silicon optical waveguide forming the reflecting mirror 40D. The phase shifter 44 may be configured to change the refractive index by heating the silicon waveguide extending between the directional coupler 42 and the diffraction grating 41 with a heater or applying a voltage. Alternatively, a photonic crystal waveguide may be connected to the silicon waveguide to change the group velocity of light by the slow light effect. A combination of a Mach-Zehnder interferometer formed of a silicon waveguide and a photonic crystal waveguide may be used. The photonic crystal waveguide can be manufactured by forming a regular fine pattern in a silicon waveguide using a CMOS process and embedding it with a different material (silicon oxide film).

  By providing the phase shifter 44 in the reflecting mirror 40D, the integrated laser light source 10D can be controlled to an operation point in which the integrated laser light source 10D operates in a single mode as a whole, even when the laser element 20 is not in a phase state in a single mode.

  FIG. 11 shows a configuration example of an integrated laser light source 10E in the case where the heater 44a is used as the phase shifter 44. The integrated laser light source 10E has a reflecting mirror 40E as a mirror structure 40. The heater 44a used in the reflecting mirror 40E is formed as a metal thin film on the silicon waveguide extending from the directional coupler 42, and generates heat by energization. The metal thin film is formed of titanium (Ti), tantalum (Ta), platinum (Pt), a laminate of these, or the like. As an example, a metal film of Ti is disposed over a length of 150 μm on a silicon waveguide. At this time, the heat capacity of the silicon waveguide heated by the heater 44a is about 1 mW / K.

  A temperature monitor 48 is disposed on the optical waveguide substrate 30. The output of temperature monitor 48 is connected to the input of controller 70. The temperature monitor 48 is, for example, a pn diode sensor. A constant forward current is allowed to flow through the pn diode, and the temperature change is monitored by measuring the change in voltage across the pn diode. The controller 70 controls the amount of current applied to the heater 44a based on the monitoring result. The controller 70 is realized by a combination of a memory and a microprocessor, or a microprocessor incorporating a memory.

  The control by the controller 70 is, for example, as follows. After production of the integrated laser light source 10E, the heater control power at the optimum point for single mode operation at, for example, 80 ° C. is measured using a measuring instrument. This temperature is a reference temperature for temperature control. Furthermore, the relationship between the heater power and each temperature is obtained in advance, and the temperature monitor 48 is calibrated with respect to the temperature.

  FIG. 12 is a flow chart showing the operation of the controller 70 during the operation of the integrated laser light source 10E. At the time of actual operation, in a state where a predetermined current is injected into the laser element 20, the operating temperature of the integrated laser light source 10E is measured by the temperature monitor 48 (S1). From the difference between the measured temperature and the reference temperature (for example, 80 ° C.), the drive power (input value to the phase shifter) of the heater 44a is determined (S2). The heater 44a is driven with the determined power value (input value) (S3). Since the operations of steps S1 to S3 are repeated while the integrated laser light source 10E operates, the flow of FIG. 12 is a loop.

  FIG. 13 is a diagram showing the relationship between the operating temperature of the integrated laser light source 10E and the single mode operation. The horizontal axis is heater power (mW), and the vertical axis is operating temperature (° C.). The gray area is an area operating in the multi mode. The white area diagonally crossing from the lower left to the upper right of the graph is the single mode operation area. The straight line at the center of the single mode operation area is the control value of the heater 44a with respect to the temperature.

  The specifications of the integrated laser light source 10E used for the measurement of FIG. 13 are as follows.

Device length of the laser device 20 (quantum dot DFB laser): 600 μm
Diffraction efficiency: of the laser element 20: 30 cm ^-1
Output end face and rear end face of laser element: AR and HR
Distance from the laser output end to the input end of the reflector 40E: 200 μm
Heater length: 150 μm
By controlling the driving power of the heater 44a based on the measurement result of the temperature monitor 48, the integrated laser light source 10E can be operated at an optimum phase as a whole to achieve single mode operation. In addition, since an appropriate amount of reflected light is fed back to the DFB mode, single mode oscillation can be maintained and the influence of light returned from the outside can be reduced.

  FIG. 14 shows the gain peak wavelength of the active layer of the laser element 20, the Bragg wavelength, the center wavelength of the mirror structure 40 on the optical waveguide substrate 30, and the heater driving power at each operating temperature. At 80 ° C., which is the optimum condition, in the laser element 20, the gain peak and the Bragg wavelength substantially match, and the central wavelength of the mirror structure 40 also matches. At this time, a feedback equivalent to a reflectance of about 5% from the mirror structure 40 to the laser element 20 is obtained in an optimal phase state. As a result, laser oscillation in a single DFB mode and resistance to return light can be obtained.

  The Bragg wavelength fluctuates by 8 nm in the temperature range of 0 ° C. to 80 ° C. The fluctuation of the Bragg wavelength due to the manufacturing variation of the diffraction grating 22 in the laser element 20 is added to this to make the reflection bandwidth of the mirror structure 40 20 nm.

  At a temperature of 25 ° C., the gain peak wavelength of the laser element 20 is a short wavelength of about 18 nm with respect to the Bragg wavelength of the laser element 20. The central wavelength of the reflecting mirror 40E (or mirror structure) of the optical waveguide substrate 30 is about 1.7 nm longer than the Bragg wavelength. At this time, since the FP mode is outside the reflection band of the mirror structure 40, it does not oscillate in the FP mode. On the other hand, since the Bragg wavelength of the laser element 20 is within the reflection band of the mirror structure 40, feedback to the DFB mode of the laser element 20 can be obtained, and resistance to external return light can be obtained. When the monitor temperature is 25 ° C., by setting the heater power to 22.5 mW, the same phase state as in the case of 80 ° C. is obtained, and it operates stably in a single mode.

  At a temperature of 0 ° C., the gain peak wavelength of the laser element 20 is a short wavelength of about 26 nm with respect to the Bragg wavelength of the laser element 20. The central wavelength of the mirror structure 40E of the optical waveguide substrate 30 is about 2.4 nm longer than the Bragg wavelength. Also in this case, feedback to the DFB mode of the laser element 20 is obtained. When the monitor temperature is 25 ° C., by setting the heater power to 7.5 mW, the same phase state as at 80 ° C. is obtained, and stable single mode operation is obtained.

  In the above embodiment, the laser device 20 has the quantum dot active layer, but the same effect as the above embodiment can be obtained as long as it emits light at a wavelength transparent to silicon. When a quantum dot is used, the effect of the present invention is large because carrier confinement is strong and the range of gain wavelength is narrow.

  Further, in the above-described embodiment, the gain wavelength of the laser element 20, the Bragg wavelength, and the center wavelength of the mirror structure are 1.3 μm bands, but a mirror structure in which the reflection band and the reflectance are appropriately designed as in the embodiment. By using the above, similar effects can be obtained within the range of wavelengths generally used in optical communication.

  In the configurations of the second to fourth embodiments, although the reflection bandwidth of the diffraction grating 41 used in the mirror structure 40 is 20 nm and the number of periods is 160, the central wavelength reflectance of the diffraction grating 41 is approximately 100%. The same effect can be obtained even if the number of cycles is appropriately changed within the following range.

  As in the first embodiment, when the diffraction grating 41 as a mirror structure is directly formed on a silicon waveguide, the central wavelength reflectance of the diffraction grating 41 is 10% or less, and the reflection bandwidth is 20 nm or less. , Suitable reflectors may be formed.

<Optical transceiver>
FIG. 15 is a schematic view of an optical transceiver 1 as an application example of the integrated laser light source of the embodiment. The optical transceiver 1 is used, for example, in an optical interconnect such as a data center.

  The optical transceiver 1 includes an optical-to-electrical conversion circuit 50 that converts between an electrical signal and an optical signal at an optical transmission / reception front end, and drive electronic circuit 60 that drives the optical-to-electrical conversion circuit 50.

  The photoelectric conversion circuit 50 includes a laser light source 110, a light modulator 51, and a light receiver 52. The laser light source 110 may use any of the integrated laser light sources 10 and 10A to 10E of the above-described embodiment.

  On the transmission side, the light output from the laser light source 110 enters the light modulation unit 51. The light modulation unit 51 is, for example, a Mach-Zehnder (MZ) interferometer type light modulator, and is used to perform multi-value modulation of incident light (carrier wave) of a predetermined wavelength. That is, the light incident from the laser light source 110 is modulated by a high-speed drive electrical signal input from the drive electronic circuit 60 and output to an external optical wiring such as an optical fiber. When the light modulator is a semiconductor modulator formed of a silicon waveguide, the light modulator can be formed on the same substrate as the substrate 31 of the integrated laser light source 10.

  The drive electronics 60 comprise a driver which generates from the transmitted electrical signal a high speed drive signal according to its logic value. The light modulation unit 51 is driven by the drive electrical signal output from the driver.

  On the reception side, an optical signal input from an external optical fiber is detected by the photodetector of the light reception unit 52. The photodetector outputs a photocurrent corresponding to the detected light amount to the drive electronic circuit 60.

  The drive electronic circuit 60 has a transimpedance amplifier that converts the photocurrent into a voltage, and outputs the received electrical signal to, for example, an LSI chip on a server blade.

  The optical transceiver 1 may be formed as a module in which the drive electronic circuit 60 and the photoelectric conversion circuit 50 are mounted on the same package substrate. In the laser light source 100 of the optical transceiver 1, an optimum amount of light is fed back to the DEB mode of the laser element 20 with the optimum reflection bandwidth by the mirror structure 40. Therefore, the single mode operation is maintained, and the influence of the return light from the light modulation unit, the external optical fiber, etc. is reduced, and a high gain, low noise light source is realized.

  For example, by arranging the optical transceiver 1 on the same board as the LSI chip, high-speed optical communication can be performed between servers, between boards, etc. by minimizing the transmission path of electric signals.

The following appendices are presented to the above explanation.
(Supplementary Note 1)
An optical waveguide substrate comprising: a silicon photonics waveguide; and a mirror structure disposed on or adjacent to the silicon photonics waveguide.
A distributed feedback laser device mounted on a mounting surface formed on the optical waveguide substrate;
Have
The output end of the laser element faces the input end of the silicon photonics waveguide,
The mirror structure has a reflection band of a specific bandwidth including the Bragg wavelength of the laser element in a predetermined temperature range, and reflects part of the light output from the laser element at the reflection band, An integrated laser light source characterized by returning to the laser element.
(Supplementary Note 2)
The integrated laser light source according to claim 1, wherein the reflection band includes a fluctuation range of the Bragg wavelength corresponding to at least one of a temperature change within the predetermined temperature range and a manufacturing variation.
(Supplementary Note 3)
The integrated laser light source according to claim 2, wherein the bandwidth of the reflection band is 10 nm to 30 nm.
(Supplementary Note 4)
The integrated laser light source according to any one of appendices 1 to 3, wherein the mirror structure feeds back 4% to 10% of the output light of the laser element to a single oscillation mode of the laser element.
(Supplementary Note 5)
The integrated laser light source according to claim 1, wherein the mirror structure is a reflecting mirror of a diffraction grating formed in the silicon photonics waveguide.
(Supplementary Note 6)
The mirror structure includes a directional coupler provided adjacent to the silicon photonics waveguide, and a diffraction grating connected to the directional coupler, and the reflection according to the branching ratio of the directional coupler The integrated laser light source according to claim 1, characterized by having a ratio and a reflection bandwidth determined by the diffraction grating.
(Appendix 7)
The mirror structure includes a ring resonator provided adjacent to the silicon photonics waveguide, a directional coupler adjacent to the ring resonator, and a diffraction grating connected to the directional coupler. The integrated laser light source according to claim 1, wherein a band of the reflection band is limited by the ring resonator.
(Supplementary Note 8)
The integrated laser light source according to claim 1, wherein the mirror structure includes a phase shifter that adjusts a phase change amount of the light propagating through a silicon waveguide forming the mirror structure.
(Appendix 9)
The phase shifter is a heater provided in the silicon waveguide,
A temperature monitor for monitoring the temperature of the optical circuit board;
A controller that controls the power of the heater based on the output of the temperature monitor;
The integrated laser light source as set forth in appendix 8, further comprising:
(Supplementary Note 10)
The integrated laser light source according to any one of appendices 1 to 9, wherein an antireflective film is formed on the emission end face of the laser element, and a high reflection film is formed on the rear end face opposite to the emission end face. .
(Supplementary Note 11)
The integrated laser light source according to any of the preceding claims, wherein the silicon photonics waveguide comprises a spot size conversion waveguide provided on the incident side facing the laser element.
(Supplementary Note 12)
The integrated laser light source according to any one of appendices 1 to 11, wherein the laser element has a quantum dot active layer.
(Supplementary Note 13)
The integrated laser light source according to any one of claims 1 to 12, wherein a gain peak wavelength of the laser element and a Bragg wavelength coincide with each other on the high temperature side of the predetermined temperature range.
(Supplementary Note 14)
The integrated laser light source according to any one of appendices 1 to 13,
An optical modulator that modulates output light from the integrated laser light source;
An electronic circuit for driving the light modulator;
An optical transceiver characterized by having:

Reference Signs List 1 optical transceiver 10, 10A to 10F integrated laser light source 20 laser element 30 optical waveguide substrate 31 substrate 32 optical waveguide (silicon photonics waveguide)
33 spot size conversion waveguide 35 mounting surface 40 mirror structure 40A to 40E reflection mirror 41 diffraction grating 42 directional coupler 43a, 43b ring resonator 44 phase shifter 44a heater 48 temperature monitor 50 photoelectric conversion circuit 60 drive electronic circuit 70 controller AR Antireflection film HR High reflection film 110 Laser light source

Claims (7)

An optical waveguide substrate comprising: a silicon photonics waveguide; and a mirror structure disposed on or adjacent to the silicon photonics waveguide.
A distributed feedback laser device mounted on a mounting surface formed on the optical waveguide substrate;
Have
The output end of the laser element faces the input end of the silicon photonics waveguide,
The mirror structure has a reflection band of a specific bandwidth including the Bragg wavelength of the laser element in a predetermined temperature range, and reflects part of the light output from the laser element at the reflection band, An integrated laser light source characterized by returning to the laser element.   The integrated laser light source according to claim 1, wherein the reflection band includes a variation range of the Bragg wavelength corresponding to at least one of a temperature change within the predetermined temperature range and a manufacturing variation.   The integrated laser light source according to claim 2, wherein the bandwidth of the reflection band is 10 nm to 30 nm.   The integrated laser according to any one of claims 1 to 3, wherein the mirror structure feeds back 4% to 10% of the output light of the laser element to a single oscillation mode of the laser element. light source.   The integrated laser light source according to any one of claims 1 to 4, wherein the laser element has a quantum dot active layer.   The integrated laser light source according to any one of claims 1 to 5, wherein the gain peak wavelength and the Bragg wavelength of the laser element coincide with each other on the high temperature side of the predetermined temperature range. An integrated laser light source according to any one of claims 1 to 6,
An optical modulator that modulates output light from the integrated laser light source;
An electronic circuit for driving the light modulator;
An optical transceiver characterized by having: