Optical transmitter

Abstract

The present invention provides an optical transmitter that suppresses an increase in chip size due to placement of a bending waveguide, and that suppresses beam clipping of signal light, thereby making it possible to output high-power signal light. An optical transmitter according to the present disclosure has a linear waveguide core for outputting laser light from an exit end face, said optical transmitter comprising: a first groove that is formed at a position deeper than the bottom surface of the waveguide core, that completely reflects, substantially perpendicularly to the optical axis direction of the laser light, the laser light that propagates in the waveguide core, and that outputs the laser light to the outside of the waveguide core; and a second groove that is formed at a position deeper than the bottom surface of the waveguide core and that reflects, to the exit end face side and in a direction having an angle with respect to a direction perpendicular to the exit end face, the laser light which has been completely reflected by the first groove.

Description

Technical Field

[0001] The present disclosure relates to an optical transmitter, and more specifically, to an optical transmitter using a laser.

Background Art

[0002] With the recent spread of video distribution services and the increase in mobile traffic demand, network traffic has been explosively increasing. For this reason, in the optical transmission path that bears the network, the requirements for higher transmission rates and lower power consumption are increasing. In addition, there is a growing trend to reduce the cost of the network by extending the transmission distance, and there is also an increasing demand for higher speed and higher output for semiconductor modulation light sources used in the extended network. And, as one of the optical transmitters suitable for such an extended network, a distributed feedback laser (hereinafter referred to as DFB laser) equipped with an electro-absorption (hereinafter referred to as EA) modulator has attracted attention.

[0003] FIG. 1 is a side view showing a schematic configuration of a general EADFB laser 10. In FIG. 1, the optical axis direction corresponds to the x direction. A general EADFB laser 10 has a structure in which a DFB laser 11 that oscillates laser light and an EA modulator 12 that modulates the intensity of the laser light are integrated in the same chip. The DFB laser 11 includes an active layer 111 having a multiple quantum well (hereinafter referred to as MQW) structure and a diffraction grating 112 formed in the resonator, and emits laser light of a single wavelength. On the other hand, the EA modulator 12 includes an optical absorption layer 121 having an MQW structure with a composition different from that of the DFB laser. By applying a high-frequency voltage to the optical absorption layer 121, the amount of optical absorption is changed, the intensity of the laser light emitted from the DFB laser 11 is modulated, and the light is emitted as signal light. The EADFB laser 10 having such a configuration has high extinction characteristics and excellent chirp characteristics as compared with other directly modulated lasers. Therefore, it has been used in a wide range of applications as an optical transmitter, including a light source for an access network.

[0004] On the other hand, as a problem of the EADFB laser 10, since the EA modulator 12 involves a large optical loss, it is difficult to increase the output power. As a prior art for solving this problem, an EADFB laser (hereinafter referred to as AXEL) in which a semiconductor optical amplifier (hereinafter referred to as SOA) is further integrated on the output end side of the EADFB laser 10 has been proposed (see, for example, Non-Patent Document 1).

[0005] FIG. 2 is a side view showing a schematic structure of AXEL20 according to the prior art. As shown in FIG. 2, AXEL20 according to the prior art has a structure further including an SOA21 on the output side of the EADFB laser 10. Then, the signal light intensity-modulated by the EA modulator 12 is introduced into the active layer 211 of the SOA21, and the signal light is amplified by the SOA21. As a result, the output of the finally emitted signal light is improved, and output characteristics about twice as high as those of a general EADFB laser 10 can be obtained. In addition, since AXEL20 can operate with high efficiency due to the SOA integration effect, when driven under operating conditions where the same optical output is obtained, power consumption can be reduced by about 40% compared to a general EADFB laser 10. Furthermore, for AXEL20, an active layer having the same MQW structure as the DFB laser 11 can be used for the active layer 211 of the SOA. Therefore, a process of separately regrowing the active layer 211 of the SOA21 is unnecessary, and it is possible to fabricate it in the same manufacturing process as a conventional EADFB laser.

[0006] In AXEL20 having such a configuration and characteristics, it often becomes a problem that a part of the emitted signal light is reflected at the output end face of the chip. This reflected signal light (reflected light) returns to the SOA21 and is re-incident, and the reflected light is also subjected to an amplification effect. For this reason, the amplified reflected light is fed back to the EADFB laser 10, and as a result, the output characteristics (particularly, waveform quality) of the chip deteriorate. Therefore, in AXEL20 according to the prior art, a configuration for suppressing reflection in the vicinity of the output end face was necessary.

[0007] FIG. 3 is a top view schematically showing the structure near the emission end face of AXEL20 according to the prior art, including a bent waveguide 31 and a window region 32 for suppressing reflection near the emission end face. In FIG. 3, the optical axis direction of the signal light is indicated by the arrow of the dashed line. As shown in FIG. 3, near the emission end face of AXEL20 according to the prior art, the bent waveguide 31 and the window region 32 are arranged to suppress the deterioration of the output characteristics due to the reflected light. Specifically, by arranging the bent waveguide 31, the optical axis direction of the emitted signal light is designed not to be perpendicular (90°) to the emission end face of the chip, and the waveguide core is removed in the window region 32 to expand the beam of the signal light, thereby suppressing the reflection of the emission end face of the chip.

[0008] However, when the bent waveguide 31 is a single-mode waveguide based on an InP-based semiconductor material, if the bent portion of the bent waveguide 31 is abrupt (the curvature radius R is small), output reduction due to radiation loss may occur, so the bent portion needs to have a gentle curvature. For example, when the angle θ formed between the optical axis after passing through the bent portion of the bent waveguide 31 and the perpendicular to the emission end face wg is about 5°, the curvature radius R of the bent portion needs to be about several tens of μm. In addition, in the bent waveguide 31, a straight section for stabilizing the single mode is required after the bent portion (on the emission end side). Implementing such a bent waveguide 31 with such a structure increases the chip size of AXEL20, resulting in a decrease in the chip yield per wafer.

[0009] Also, when the bent waveguide 31 is a single-mode waveguide based on an InP-based semiconductor material, the optical axis direction preferably follows the crystal orientation <0011> direction of the InP crystal. Deviating from this condition may reduce the flatness of the grown chip surface. When implementing the bent waveguide 31 having such a gentle curvature as described above, it is difficult to align the crystal orientation of such an InP crystal in the <0011> direction, which may reduce the reproducibility of the implementation process and lead to a deterioration in yield.

[0010] In addition, AXEL20 according to the prior art also has problems in the window region 32.

[0011] FIG. 4 is a side view schematically showing the state of beam divergence occurring in the vicinity of the window region 32 of AXEL20 according to the prior art. As shown in FIG. 4, in the window region 32 formed by implantation, the thickness (height in the y direction) of the window region 32 formed by regrowth is likely to be thinner than that of a general overclad. Also, in the window region 32, for the reason of spreading the beam of the signal light, as shown in FIGS. 3 and 4, the bending waveguide 31 needs to be removed. In such a case, there is a possibility that the beam spreading in the optical axis direction interferes (beam divergence) at the upper end surface of the portion where the thickness of the window region 32 is thin. To address such problems, it is possible to suppress the occurrence of beam divergence by setting the length of the window region 32 to a length at which beam divergence does not occur by design. However, in the cleavage process for chip formation in the manufacturing process, a variation of about 5 μm generally occurs in the cleavage position. Therefore, even in a design that avoids beam divergence, beam divergence may occur due to manufacturing variations, resulting in a structure where beam divergence occurs. The occurrence of this beam divergence is cited as an important issue in increasing the output power of AXEL20 because it reduces the coupling efficiency to the optical fiber.

Prior Art Documents

Non-Patent Documents

[0012]

Non-Patent Document 1

Summary of the Invention

[0013] The present disclosure has been made in view of the above problems, and an object thereof is to provide an optical transmitter capable of emitting high-power signal light by suppressing an increase in chip size due to the arrangement of a bent waveguide and suppressing beam divergence of the signal light.

[0014] In view of the above problems, the present disclosure provides an optical transmitter having a linear waveguide core for outputting laser light from an output end face, wherein the laser light propagating in the waveguide core is totally reflected substantially perpendicular to the optical axis direction of the laser light, and the laser light is emitted outside the waveguide core. The optical transmitter includes a first groove formed to a position deeper than the bottom surface of the waveguide core for totally reflecting the laser light, and a second groove formed to a position deeper than the bottom surface of the waveguide core for reflecting the laser light totally reflected by the first groove in a direction having an angle with respect to the output end face side and in a direction perpendicular to the output end face.

Brief Description of Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5A

Figure 5B

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0016] Hereinafter, various embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant descriptions may be omitted. The materials and numerical values are for illustrative purposes and are not intended to limit the technical scope of the present disclosure. The following description is an example, and unless departing from the gist of an embodiment of the present disclosure, some configurations can be omitted, modified, or implemented with additional configurations.

[0017] The optical transmitter according to the present disclosure has a form of an EADFB laser equipped with an SOA, similar to AXEL according to the prior art. However, as an alternative to the bent waveguide, by including a plurality of grooves that reflect the signal light in a desired direction, the bent waveguide is made unnecessary (it is possible to apply a straight waveguide without curvature). This makes it possible to suppress the increase in the chip size described above.

[0018] Furthermore, in the optical transmitter according to the present disclosure, the arrangement of the reflection surfaces of each of the plurality of grooves is set so that reflection does not occur at the emission end face of the chip. This makes it possible to suppress a decrease in output due to reflected light, similar to AXEL having a bent waveguide or a window region according to the prior art.

[0019] In addition, in the optical transmitter according to the present disclosure, after a clad in which a waveguide core through which signal light propagates is embedded, the plurality of grooves described above are processed by etching or the like. Therefore, unlike the window region according to the prior art formed by separate embedding, the generation of a partially thin region is suppressed. That is, the generation of beam squint is suppressed, and an optical transmitter with higher output than the prior art can be obtained.

[0020] (First Embodiment) Hereinafter, a first embodiment of the optical transmitter according to the present disclosure will be described in detail with reference to the drawings. The optical transmitter in this embodiment relates to a form in which two grooves are provided near the emission end face as an alternative to the bending waveguide 31 of AXEL20 according to the prior art.

[0021] FIG. 5A is a top view schematically showing the structure near the emission end of the optical transmitter 50 according to the first embodiment of the present disclosure. In FIG. 5A, the propagation direction of the signal light is indicated by a one-dot chain line. The optical transmitter 50 in this embodiment has a basic configuration similar to that of the EADFB laser 20 equipped with the SOA 21, and is manufactured by the same manufacturing technology as the EADFB lasers 10 and 20 according to the prior art. However, the structure near the emission end face of the chip does not include the bending waveguide 31 and the window region 32 as shown in FIG. 5A. Instead, it includes a waveguide core 51, a first groove 52 that reflects the signal light on the second groove 53 side and in a direction (z direction) substantially perpendicular to the optical axis direction of the signal light guiding the waveguide core 51, and the reflected signal light is reflected on the emission end face side and in a direction that is not perpendicular to the emission end face of the chip (has an angle with respect to the perpendicular direction of the emission end face of the chip).

[0022] Unlike the bending waveguide 31 of AXEL20 according to the prior art, the waveguide core 51 has a straight shape without curvature and is connected flush with the reflecting surface of the first groove 52.

[0023] The first groove 52 and the second groove 53 are hollow and contain air inside. These first groove 52 and second groove 53 can be formed by etching, for example, after the embedding (regrowth) of the cladding in the mounting process. Also, the depth in the y - direction of each of the first groove 52 and the second groove 53 is formed deeper than the bottom surface of the waveguide core 51.

[0024] The angle θ of the inclination of the reflecting surface of the first groove 52 (the angle formed by the reflecting surface and the optical axis direction of the signal light propagating in the waveguide core 51) e is set from the wavelength of the signal light, the refractive index of the waveguide core and air so that the signal light guiding the waveguide core 51 is totally reflected substantially vertically.

[0025] The angle of the inclination of the reflecting surface of the second groove 53 (the angle formed by the reflecting surface and the optical axis direction of the signal light propagating in the waveguide core 51) θ a is smaller than the angle θ of the inclination of the reflecting surface of the first groove 52 e and is set so that the optical axis direction of the signal light totally reflected by the reflecting surface of the second groove 53 is not perpendicular (90°) to the emission end surface of the chip as described above.

[0026] In the optical transmitter 50 according to this embodiment having such a configuration, the signal light amplified by the SOA 21 is totally reflected substantially vertically in the y - direction at the reflecting surface of the first groove 52. At this time, the reflected signal light is radiated to the outside (cladding side) of the waveguide core 51. For this reason, the optical confinement effect disappears and the beam spreads, so that the same effect as the window region 32 in the prior art can be obtained. If the cladding is made to have a sufficient thickness in advance and the upper surface is formed smoothly in the regrowth process, it is possible to suppress the occurrence of beam kink as in the prior art.

[0027] Next, the signal light is totally reflected in the x direction at the reflecting surface of the second groove 53. As described above, the optical axis direction of the signal light totally reflected by this second groove 53 is set not to be perpendicular to the emission end face of the chip. Therefore, similar to the bending waveguide 31 in the prior art, an effect of suppressing reflection at the emission end face of the chip is achieved. Also, with such a configuration, it becomes unnecessary to arrange a bending waveguide having a gentle curvature as in the prior art, so it is possible to suppress an increase in the chip size. Note that since the signal light emitted from the emission end face of the chip is refracted even at the chip end face, it is emitted at an angle of θ o with respect to the chip end face.

[0028] (Embodiment) Hereinafter, the setting of the arrangement in the first and second grooves will be described in detail with reference to embodiments. Here, as an example, the optical transmitter 50 is an embedded InGaAsP / InP semiconductor laser in which the active layers of the DFB laser and the SOA corresponding to the waveguide core 51 have an MQW structure and are made of InGaAsP (refractive index: 3.4), and the substrate and the cladding are made of InP (refractive index: 3.2). Also, the propagating signal light is laser light having a single wavelength of 1310 nm.

[0029] Also, here, the thickness of the active layer corresponding to the waveguide core 51 is 300 nm, the thickness of the cladding (overcladding: p-type InP) formed on the upper surface of the active layer is 2 μm, and the width in the z direction of the region (mesa region) where the signal light propagates is 1 μm. Further, the optical axis direction of the signal light incident on the emission end face of the chip is set to have an angle of 5° with respect to the optical axis direction of the signal light propagating through the waveguide core 51.

[0030] In such an optical transmitter 50, the above-described first groove 52 and second groove 53 are formed by etching. The inclination angle θ e of the first groove 52 is 45°, and the inclination angle θ aIt was set to 42.5°. Also, the transmission distance of the signal light (the length between A and B in FIG. 5A) from the time it is totally reflected by the reflecting surface of the first groove 52 until it is incident on the reflecting surface of the second groove 53 is 5 μm, and the transmission distance of the signal light (the length between B and C in FIG. 5A) from the time it is totally reflected by the second groove 53 until it is incident on the exit end surface of the chip is 5 μm.

[0031] Now, since the refractive index of the waveguide core 51 is 3.4 and the first groove 52 is hollow, the refractive index at its reflecting surface corresponds to the refractive index of air, which is 1.0. Therefore, the total reflection angle in the first groove 52 is calculated to be approximately 17°. From this result, as a setting for totally reflecting the signal light substantially vertically, the inclination angle θ e of the first groove 52 was set to 45°. Along with this, the inclination angle θ a of the second groove 53 was set to 42.5° from the viewpoint that the optical axis direction of the signal light incident on the exit end surface of the chip has an angle of 5° with respect to the optical axis direction of the signal light propagating through the waveguide core 51.

[0032] Next, the setting of the width (area) of each reflecting surface will be described.

[0033] FIG. 6 is a diagram showing the result of simulating the beam spread in the window region when incident from a waveguide with a mesa region width of 1 μm into the window region as in this embodiment by the EME method (EngenMode Expansion Method). Here, the beam diameter on the vertical axis is the 1 / e 2 beam radius. When the width of the mesa region is 1 μm, in the waveguide core 51, due to the refractive index relationship with InP, which is the cladding (= 3.2), the signal light is confined within the waveguide core 51 and propagated with a certain beam diameter. Based on the simulation results shown in FIG. 6, it can be seen that the beam diameter of the signal light propagating through the waveguide core 51 in the xy plane is 1.23 μm. When this signal light is incident on the first groove 52, the inclination angle θ eSince it is 45°, the beam diameter on the reflecting surface of the signal light totally reflected by the reflecting surface of the first groove 52 becomes 1.23×(1 / cos(π / 4)), which is about 1.74 μm, and when converted to diameter, it is about 3.5 μm. On the other hand, 1 / e 2 It is known that if the reflecting surface has a vertical and horizontal size 1.5 times that of the beam diameter defined by the 1 / e beam diameter, more than 99% of the light can be reflected by that surface. Therefore, the reflecting surface of the first groove 52 only needs to have an area of 5.3 μm × 5.3 μm or more.

[0034] The signal light totally reflected by the reflecting surface of the first groove 52 is incident on the second groove 53. Since this totally reflected signal light does not propagate in the waveguide core 51, according to the tendency shown in FIG. 5B, the beam propagates while spreading as shown in FIG. 6. Therefore, the width (area) of the reflecting surface of the second groove 53 needs to be set in consideration of the beam diameter in FIG. 6.

[0035] As described above, the transmission distance of the signal light from the reflecting surface of the first groove 52 to the reflecting surface of the second groove 53 is set to 5 μm. The beam diameter at the position where the signal light has traveled 5 μm is about 2.55 μm, about 5.1 μm in diameter, as shown in FIG. 6. Furthermore, the inclination angle θ a of the reflecting surface of the second groove 53 is 42.5°, so the beam diameter of the signal light incident on the second groove 53 becomes 5.1×(1 / cos(42.5π / 180))×1.5, which is calculated to be about 11 μm. Therefore, it only needs to have an area of 11 μm × 11 μm or more.

[0036] In this way, the first groove 52 and the second groove 53 of the optical transmitter 50 are set in terms of arrangement (angle, area, distance, etc.) according to the total reflection angle and the spread of the beam diameter.

[0037] Actually, when the optical loss of the reflected light from the emission end face of the chip in the optical transmitter 50 designed in this way was simulated by the EME method, it was -39.6 dB. Furthermore, when an AR (Anti Reflection) coating was applied to the emission end face of the chip, it was confirmed that the optical loss became -60 dB. This is equal to or higher than the numerical value required for a conventional optical transmitter (for example, AXEL20). That is, the optical transmitter 50 according to the present disclosure can achieve an output equal to or higher than that of the prior art while suppressing an increase in chip size due to the installation of the bent waveguide.

[0038] (Second Embodiment) Hereinafter, a second embodiment of the present disclosure will be described in detail with reference to the drawings. The optical transmitter according to this embodiment relates to a form in which, in addition to the configuration of the optical transmitter 50 described in the first embodiment, a third groove is further included to suppress the influence of errors in cleavage during the manufacturing process.

[0039] FIG. 7 is a top view schematically showing the configuration near the emission end face of the optical transmitter 70 according to the second embodiment of the present disclosure. The optical transmitter 70 according to this embodiment further includes a third groove 71 between the first groove 52 and the second groove 53 and the emission end face of the chip, in addition to the configuration of the optical transmitter 50 in the first embodiment.

[0040] As shown in FIG. 7, the inner end face of the third groove 71 has a plane parallel to the emission end face of the chip, and the width in the z direction is formed so as to include the width of the range from the first groove 52 to the second groove 53. Also, the third groove 71 can be formed by etching in the same manner as the first groove 52 and the second groove 53. In FIG. 7, as an example, the third groove 71 is shown in a form connected to the first groove 52, but they do not necessarily have to be connected.

[0041] In the optical transmitter 70 according to this embodiment having such a configuration, the signal light exits from the end face inside the third groove 71 as the exit end face. Therefore, the influence of errors in the cleavage during the manufacturing process is alleviated, and it becomes possible to control the length of the window region with submicron accuracy determined by the exposure accuracy.

[0042] (Third Embodiment) Hereinafter, the third embodiment of the present disclosure will be described in detail with reference to the drawings. The optical transmitter according to this embodiment relates to a form that further includes a raised portion for further suppressing the generation of beam divergence in the vicinity of the exit end face, in addition to the configuration of the optical transmitter 50 described in the first embodiment or the optical transmitter 70 described in the second embodiment.

[0043] FIG. 8 is a diagram schematically showing the configuration in the vicinity of the exit end face of the optical transmitter 80 according to the third embodiment of the present disclosure. (a) is a top view, and (b) is a cross-sectional observation image taken along the VIIIb-VIIIb cross-section line of the actually fabricated optical transmitter 80. The cross-sectional observation image in FIG. 8(b) was obtained by a scanning electron microscope (SEM). The optical transmitter 80 according to this embodiment further includes raised portions 81a and 81b made of the same material as the cladding on the upper surfaces of the claddings installed on both side surfaces of the waveguide core 51, in addition to the configuration of the optical transmitter 50 in the first embodiment or the optical transmitter 70 in the second embodiment.

[0044] The raised portions 81a and 81b can be formed, for example, by depositing an extra cladding (InP) in a regrowth process of re-embedding the cladding in the portion removed in the processing of the mesa region in the mounting process.

[0045] In the optical transmitter 80 according to this embodiment having such a configuration, since the raised portion 81a is formed in advance, the height in the y direction of the cladding (substantially corresponding to the window region) in the vicinity of the exit end face is set higher than that of the prior art and the optical transmitter 50 or the optical transmitter 70. Therefore, it becomes possible to more efficiently suppress the generation of beam divergence on the upper surface of the cladding.

[0046] In FIG. 8, the raised portions are shown in a form formed on both sides of the mesa region. However, the same effect can be achieved even with only the raised portion 81a. In other words, if a raised portion is formed in the region where the signal light propagates, it becomes possible to more efficiently suppress the occurrence of beam spreading on the upper surface of the cladding.

[0047] As described above, the optical transmitter according to the present disclosure can mitigate reflection at the emission end face of the signal light while suppressing an increase in chip size compared to an optical transmitter according to the prior art (for example, AXEL20). In addition, since there is no portion where the thickness generated during the embedding of the window region becomes thin, it is also possible to suppress the occurrence of beam spreading.

[0048] In this specification, the optical transmitters according to the present disclosure have all been described in the form of an EADFB laser integrating an SOA. However, the present disclosure is not limited thereto, and any optical transmitter using a laser can achieve the same effect.

Industrial Applicability

[0049] As described above, the optical transmitter according to the present disclosure can mitigate reflection at the emission end face of the signal light while suppressing an increase in chip size by applying a bent waveguide. In addition, since there is no portion where the thickness generated during the embedding of the window region becomes thin, it is also possible to suppress the occurrence of beam spreading. The optical transmitter according to the present disclosure having such characteristics is expected to be applied to optical communication traffic and the like as an optical transmitter having a smaller chip size and higher output than before.

Claims

1. An optical transmitter having a linear waveguide core for outputting laser light from an output end face, a first groove formed to a position deeper than the bottom face of the waveguide core, which totally reflects the laser light propagating in the waveguide core substantially perpendicular to the optical axis direction of the laser light and outputs the laser light outside the waveguide core; a second groove formed to a position deeper than the bottom face of the waveguide core, which reflects the laser light totally reflected by the first groove in a direction having an angle with respect to the output end face side and perpendicular to the output end face; and an optical transmitter comprising the same.

2. The optical transmitter according to claim 1, further comprising a third groove installed between the first groove and the second groove and the output end face, having an inner end face parallel to the output end face, and formed so as to include the width of the range from the first groove to the second groove.

3. The optical transmitter according to claim 1 or 2, further comprising a raised portion installed on the upper face of a cladding covering the periphery of the waveguide core and to which the same material as the cladding is applied.

4. a DFB laser oscillating at a single wavelength; an EA modulator for modulating the output of the DFB laser to output signal light; and an optical transmitter according to claim 1, further comprising an SOA for amplifying the signal light from the EA modulator and outputting the same to the waveguide core as the laser light.

5. The optical transmitter according to claim 1, wherein an AR coat is formed on the output end face.

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