JP2014167971A - Wavelength-variable laser element and optical module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength-variable laser element in which characteristics of an FFP are improved, and to provide an optical module.SOLUTION: The wavelength-variable laser element includes: a first filter composed of an emission side reflection film formed on an emission side end face and a lateral direction coupling type co-directional optical coupler type wavelength-variable filter arranged in contact with the emission side end face; a second filter provided with filter characteristics having a plurality of periodical peaks; and a gain part. The first filter includes a first mesa stripe including a first optical waveguide and a second mesa stripe including a second optical waveguide extended in parallel with the first optical waveguide, the second mesa stripe is extended to the emission side end face and arrives at the emission side end face, the first mesa stripe is extended in parallel with the second mesa stripe and is curved to the opposite side of the first mesa stripe side on its end part, and a reflection factor of the emission side reflection film is 50% or less.

Description

  The present invention relates to a wavelength tunable laser element, and more particularly, to improvement of characteristics of a far-field image (FFP: far field pattern) of emitted output light.

  2. Description of the Related Art In recent years, a wavelength tunable laser element including a laterally coupled co-directional optical coupler type tunable filter has been used. Such a wavelength tunable laser element is disclosed in Example 2 described in Patent Document 1, for example.

JP 2010-153826 A

  A wavelength tunable laser element including a laterally coupled co-directional optical coupler type tunable filter disclosed in Example 2 of Patent Document 1 will be described. As shown in FIG. 7A of Patent Document 1, the wavelength tunable laser element includes a diffraction grating type reflecting mirror 134, a gain region 133, a phase adjustment region 132, a laterally coupled co-directional optical coupler type wavelength tunable filter 131 in order. It is a semiconductor laser device that is monolithically integrated side by side. The diffraction grating type reflecting mirror 134 is, for example, a distributed Bragg reflector (hereinafter referred to as DBR) filter. On the other hand, a highly reflective film is formed on the end face on the side where the laterally coupled co-directional optical coupler type tunable filter 131 is disposed. Whereas the laterally coupled co-directional optical coupler type tunable filter 131 has transmission characteristics and it is necessary to form a highly reflective film on the end face, the diffraction grating type reflecting mirror 134 has reflective characteristics. ing. Therefore, in the wavelength tunable laser element, the end face on the diffraction grating type reflecting mirror 134 side is the end face on the emission side, and the side coupling type concentric optical coupler type wavelength tunable filter 131 side on which the high reflection film is formed is provided. In general, the end surface is the end surface opposite to the end surface on the emission side.

  In the laser element, the FFP of the output light output from the laser element is one of the important characteristics for the element characteristics. In particular, the wavelength tunable laser element has an external wavelength monitor for controlling the wavelength of the output light. Need to be placed. When the wavelength locker is arranged on the emission side of the wavelength tunable laser element, if the disturbance of the FFP of the output light of the wavelength tunable laser element becomes large, the wavelength controllability of the wavelength locker is deteriorated.

  The inventors produced a semiconductor laser element using a DBR filter as the diffraction grating type reflecting mirror 134 and emitting light from the end face on the DBR filter side, and observed the FFP of the emitted output light. . As a result, the inventors have obtained the knowledge that FFP disturbance has occurred to such an extent that it becomes a problem for the wavelength controllability of the wavelength locker. This is considered that scattered light is generated in the diffraction grating of the DBR filter, and the scattered light is included in the output light as stray light, so that the FFP of the output light is disturbed.

  The present invention has been made in view of the above problems, and an object of the present invention is a wavelength tunable laser element including a laterally coupled co-directional optical coupler type tunable filter, which improves FFP characteristics. Provided are a wavelength tunable laser device and an optical module.

  (1) In order to solve the above-mentioned problem, a wavelength tunable laser device according to the present invention is disposed on an exit-side reflection film formed on the exit-side end face, and in contact with the exit-side end face. A first filter composed of a directional optical coupler type tunable filter, and a filter characteristic having a plurality of periodic peaks within a wavelength range in which the peak wavelength of the first filter is shifted, A wavelength tunable laser element comprising: a second filter; and a gain unit that is disposed between the first filter and the second filter and emits light when supplied with a current. The first filter includes {a first mesa stripe including a first optical waveguide extending from the gain portion} and {extends to optically couple in parallel with the first optical waveguide and First optical waveguide A second mesa stripe including a second optical waveguide having a refractive index lower than the refractive index, and the second mesa stripe extends to the end face on the exit side and the end face on the exit side The first mesa stripe extends in parallel with the second mesa stripe, and bends at the end of the first mesa stripe opposite to the first mesa stripe side. The reflectance is 50% or less.

  (2) In the wavelength tunable laser element according to (1) above, the reflectance of the emission-side reflection film may be 6.5% or more and 50% or less.

  (3) The wavelength tunable laser element according to (1) or (2), wherein a tip end of the first mesa stripe reaches an end face on the emission side, and the end face on the emission side The angle formed by the extending direction of one mesa stripe and the normal line of the end face on the emission side may be a critical angle or more.

  (4) The wavelength tunable laser element according to (1) or (2), wherein a tip end of the first mesa stripe is located inside an end face on the emission side, and the first mesa stripe A groove or a stripe may be formed between the tip and the end face on the emission side.

  (5) In the wavelength tunable laser device according to (3), a groove or a stripe may be formed between the first mesa stripe and the second mesa stripe.

  (6) The wavelength tunable laser element according to any one of (1) to (5), wherein the second mesa stripe is bent toward the first mesa stripe at an end thereof, and The first mesa stripe side may be bent opposite to the first mesa stripe side and reach the end face on the exit side so as to follow the normal line of the end face on the exit side.

  (7) The wavelength tunable laser element according to any one of (1) to (6), wherein the first mesa stripe extends in parallel with the second mesa stripe. A diffraction grating may be formed only on the side surface opposite to the second stripe side of the both side surfaces of the mesa stripe.

  (8) In the wavelength tunable laser element according to any one of (1) to (7), the second filter may have a sample gourd structure having 10 cycles or more.

  (9) The optical module according to the present invention may be an optical module including the wavelength tunable laser element according to any one of (1) to (8).

  According to the present invention, there are provided a wavelength tunable laser element including a laterally coupled co-directional optical coupler type wavelength tunable filter, and a wavelength tunable laser element with improved FFP characteristics, and an optical module.

1 is a top view of a wavelength tunable laser device according to a first embodiment of the present invention. 1 is a cross-sectional view of a wavelength tunable laser device according to a first embodiment of the present invention. It is a schematic diagram which shows the structure of the LGLC filter part of the wavelength tunable laser element which concerns on the 1st Embodiment of this invention. It is a schematic diagram which shows the structure of the LGLC filter part of the wavelength tunable laser element which concerns on the 1st Embodiment of this invention. It is a figure which shows the observation result of the horizontal direction FFP of the output light from the wavelength variable laser element which concerns on the 1st Embodiment of this invention. It is a figure which shows the optical output characteristic of the wavelength tunable laser element which concerns on the 1st Embodiment of this invention. It is a schematic diagram which shows the structure of the LGLC filter part of the wavelength tunable laser element which concerns on the 2nd Embodiment of this invention. It is a figure which shows the observation result of the horizontal direction FFP of the output light from the wavelength variable laser element which concerns on the 2nd Embodiment of this invention. It is a schematic diagram which shows the structure of the LGLC filter part of the wavelength tunable laser element which concerns on the 3rd Embodiment of this invention. It is a schematic diagram which shows the structure of the LGLC filter part of the wavelength tunable laser element which concerns on the 4th Embodiment of this invention. It is a schematic diagram which shows the structure of the LGLC filter part of the wavelength tunable laser element which concerns on the 5th Embodiment of this invention. It is a schematic diagram which shows the structure of the LGLC filter part of the wavelength tunable laser element concerning the 6th Embodiment of this invention. It is a schematic diagram which shows the structure of the optical module which concerns on the 7th Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described specifically and in detail based on the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted. In addition, the drawings shown below are merely examples of the embodiment, and the size of the drawings and the scales described in this example do not necessarily match.

[First Embodiment]
FIG. 1 is a top view of a wavelength tunable laser device according to the first embodiment of the present invention. The wavelength tunable laser device according to this embodiment includes a laterally coupled type unidirectional optical coupler type (Lateral Grating).
Assisted Lateral Co-directional coupler: a wavelength tunable laser element (hereinafter referred to as LGLC wavelength tunable laser element) having a wavelength tunable filter. As shown in FIG. 1, the wavelength tunable laser element according to this embodiment includes a DBR filter unit 1 (second filter), a gain unit 2, a phase adjustment unit 3, and an LGLC filter along the light emission direction. The unit 4 (first filter) is an integrated semiconductor laser element integrated on the same semiconductor substrate. In FIG. 1, the upper electrode of the DBR filter unit 1 is used as the DBR filter electrode 5, the upper electrode of the gain unit 2 is used as the gain electrode 6, and the upper electrode of the phase adjustment unit 3 is used as the phase adjustment electrode 7. The upper electrodes of the part 4 are respectively shown as LGLC filter electrodes 8. The gain unit 2 is disposed between the LGLC filter unit 4 and the DBR filter unit 1 and emits light when supplied with current. The LGLC filter unit 4 includes a low refractive index optical waveguide 9 (second optical waveguide) and a high refractive index optical waveguide 10 (first optical waveguide) that extend in parallel with each other. A portion where the high refractive index optical waveguide 10 and the low refractive index optical waveguide 9 extend in a straight line parallel to each other functions as an optical coupler that optically couples. The end face shown on the right side of FIG. 1 is the end face on the exit side, and the LGLC filter unit 4 is disposed in contact with the end face on the exit side, and light is emitted from the tip of the low refractive index optical waveguide 9. The end face shown on the left side of FIG. 1 is the end face opposite to the emission side, and a reflection film 11 (low reflection film: reflection suppression film) is formed on the end face. Further, a reflection film 12 (an emission side reflection film) is formed on the end face on the emission side. Here, the reflectance of the reflective film 11 is 0.1%, and the reflectance of the reflective film 12 is 50%. Note that the LGLC filter unit 4 is a first filter constituted by an LGLC wavelength variable filter, and the DBR filter unit 1 is a second filter.

  The high refractive index optical waveguide 10 is an optical waveguide extending to the DBR filter unit 1, the gain unit 2, the phase adjustment unit 3, and the LGLC filter unit 4, and the semiconductor multilayer including the high refractive index optical waveguide 10 is the first mesa. It is a stripe. That is, the high refractive index optical waveguide 10 extends from the gain unit 2, passes through the phase adjustment unit, and reaches the LGLC filter unit 4. The low refractive index optical waveguide 9 is an optical waveguide having a refractive index lower than that of the high refractive index optical waveguide 10, and the semiconductor multilayer including the low refractive index optical waveguide 9 is a second mesa stripe. The high refractive index optical waveguide 10 (first mesa stripe) has a shape extending linearly in the left-right direction of FIG. 1 and is bent at both ends. The end of the high refractive index optical waveguide 10 (first mesa stripe) on the LGLC filter section 4 side (end on the exit side) is bent downward in FIG. 1, and its tip is on the end face on the exit side. The window structure is not reached. The end portion of the high refractive index optical waveguide 10 (first mesa stripe) on the DBR filter portion 1 side is bent upward in FIG. 1 and its tip does not reach the end surface opposite to the emission side. The low refractive index optical waveguide 9 (second mesa stripe) has a shape that extends linearly in parallel with the high refractive index optical waveguide 10 (first mesa stripe) in the main part of the LGLC filter section 4. It extends straight on the exit side and reaches the end face on the exit side. At the left end of FIG. 1 (end opposite to the emission side), the phase adjustment unit 3 is bent upward in FIG.

  FIG. 2 is a cross-sectional view of the wavelength tunable laser element according to this embodiment. The cross section shown in FIG. 2 is a cross section taken along the line II-II in FIG. 1, and the structure of the first mesa stripe is shown in FIG. In the DBR filter unit 1, the phase adjustment unit 3, and the LGLC filter unit 4, a semiconductor multilayer including an n-type InP substrate 20, a lower core layer 25 made of InGaAsP, an n-type InP layer 24, and an upper core layer 23 made of InGaAsP. Are stacked. In contrast, in the gain section 2, a semiconductor multilayer composed of a lower core layer 25, an n-type InP layer 24, and a gain layer 26 (multiple quantum well layer) is stacked on the n-type InP substrate 20. In the first mesa stripe, the high refractive index optical waveguide 10 is formed in the upper core layer 23, and the first mesa stripe has regions on both sides of the region where the high refractive index optical waveguide 10 is formed in the semiconductor multilayer. This is a mesa stripe structure to be removed. On the other hand, in the second mesa stripe (not shown), a semiconductor multilayer composed of the lower core layer 25 and the n-type InP layer 24 is laminated on the n-type InP substrate 20. In the second mesa stripe, the low refractive index optical waveguide 9 is formed in the lower core layer 25, and the second mesa stripe has a region on both sides of a region in the semiconductor multilayer where the low refractive index optical waveguide 9 is formed. This is a mesa stripe structure to be removed. By embedding the first mesa stripe and the second mesa stripe with a semi-insulating InP buried layer (not shown), a semi-insulation is provided on both sides of the first mesa stripe and on both sides and the upper side of the second mesa stripe. A conductive InP buried layer is formed. A p-type InP layer 21 is further stacked on the upper surface of the first mesa stripe and the upper surface of the semi-insulating InP buried layer extending on both sides thereof. In the DBR filter unit 1, a diffraction grating 22 is formed inside the p-type InP layer 21. As described above, the upper electrode is formed on the upper surface of the p-type InP 21, and the common electrode 29 is formed on the lower surface (back surface) of the n-type InP substrate 20 as the lower electrode.

  3 and 4 are schematic views showing the structure of the LGLC filter unit 4 of the wavelength tunable laser element according to this embodiment. The rectangle shown in FIGS. 3 and 4 schematically represents the LGLC filter unit 4, and the right end of the rectangle represents the end face on the emission side. As described above, the low-refractive-index optical waveguide 9 extends in the horizontal direction in the figure, and the tip of the end reaches the end surface on the emission side. The center line of the low refractive index optical waveguide 9 extends in a straight line and intersects the end face on the emission side perpendicularly. Light is emitted from the tip of the low refractive index optical waveguide 9. Here, the light emitted from the low refractive index optical waveguide 9 out of the light output from the wavelength tunable laser element is referred to as main light 101.

  The high refractive index optical waveguide 10 is bent at the end thereof on the side opposite to the low refractive index optical waveguide 9 side (downward in the drawing). The center line of the high refractive index optical waveguide 10 extends linearly in parallel with the center line of the low refractive index optical waveguide 9 and is bent at the end. The tip of the end does not reach the end surface on the emission side, and a p-type InP layer 21 is formed between the tip of the end and the end surface on the emission side. The leading edge of the high-refractive index optical waveguide 10 does not reach the end face on the emission side, and has a window structure. Light propagating through the high-refractive index optical waveguide 10 is reflected at the end of the high-refractive index optical waveguide 10 and returned. This is to prevent the light from propagating again through the high refractive index optical waveguide 10 as light. Although omitted in FIG. 3 for simplicity of explanation, as shown in FIG. 4, long-period diffraction gratings 41 are formed on both side surfaces of the high refractive index optical waveguide 10. A long period diffraction grating 41 is formed in a portion where the high refractive index optical waveguide 10 extends linearly in parallel with the low refractive index optical waveguide 9. Details of FIGS. 3 and 4 will be described later.

  FIG. 5 is a diagram showing the observation result of the horizontal FFP of the output light from the wavelength tunable laser element according to the embodiment. As will be described later, FFP disturbance 50 is observed in the area surrounded by the broken line, but compared to the case where the end face on the DBR filter section 1 side is the end face on the output side, the FFP disturbance of the output light to be output is It is suppressed. In the wavelength tunable laser device according to the embodiment, when the end surface on the LGLC filter unit 4 side is the end surface on the output side, the reflectivity of the reflective film 12 (the output side reflective film) from the viewpoint of the filter characteristics of the LGLC filter unit 4 However, an increase in the reflectance of the reflective film 12 causes a reduction in the intensity of the output light to be output. The inventors have intensively studied the reflectance of the reflective film 12 and found that the reflectance of the reflective film 12 is desirably 50% or less. More preferably, the reflectance is in the range of 6.5% to 50%.

  FIG. 6 is a diagram showing the light output characteristics of the wavelength tunable laser element according to this embodiment, and shows results actually measured by experiments. The light output of the wavelength tunable laser element according to this embodiment, in which the reflectance of the output side reflection film (reflection film 12) is 6.5%, 27%, and 50%, respectively, is a curve. Shown as S1, S2, S3. For comparison, the optical output of the wavelength tunable laser element according to the prior art is shown as a curve P1. Here, the wavelength tunable laser element according to the prior art is an LGLC wavelength tunable laser element that emits light from the end face on the DBR filter side, and a reflective film having a reflectance of 95% is formed on the end face on the LGLC filter side. This is a wavelength tunable laser element. The reflectivity of the end face on the DBR filter side is 0.1%. The vertical axis in the figure is the optical output Po [mW] of the wavelength tunable laser element, and the horizontal axis in the figure is the current Igain [mA] injected into the gain unit 2.

  As described above, the LGLC wavelength tunable laser element generally uses the end face on the DBR filter side as the end face on the output side. Like the wavelength tunable laser element according to the prior art shown by the curve P1, a high reflectivity film with a reflectivity of 95%, for example, is used for the reflect film formed on the end face on the LGLC filter side, thereby achieving high output. Realized. In contrast, when the end face on the LGLC filter side is the end face on the output side as in the wavelength tunable laser element according to the embodiment, the inventors reflect the reflection film formed on the end face on the output side. The relationship between the rate and the optical output of the laser element was studied earnestly. As a result, as shown in FIG. 6, when the reflectance of the reflective film 12 is lowered, the light output increases. When the reflectance is 50% (curve S3), the wavelength tunable laser element according to the prior art ( The inventors have found that a higher light output can be obtained than the curve P1). That is, by setting the reflectance of the reflective film 12 to 50% or less, a light output equal to or higher than the light output of the wavelength tunable laser element according to the prior art can be obtained. Furthermore, the inventors have found that when the reflectance is lowered below 50%, as the curve S2 in the figure shows, the light output further increases as the reflectance decreases. The inventors have found that when the reflectance of the reflective film 12 is 6.5%, a very high light output can be obtained as shown by the curve S1 in the figure. The inventors have also experimentally measured the case where the reflectance of the reflective film 12 is further reduced below 6.5%, and the reflectance of the reflective film 12 is further reduced below 6.5%. In some cases, the inventors have also found that the threshold increases, rather the light output decreases, and in some cases it does not oscillate. Therefore, the inventors have found that the reflectance of the reflective film 12 is preferably 6.5% or more and 50% or less.

  The LGLC filter unit 4 is long-period uniform grading, and the DBR filter unit 1 is sampled grading. The LGLC filter unit 4 has a filter characteristic in which the transmission spectrum changes gently with respect to the wavelength. The DBR filter unit 1 has a filter characteristic having a plurality of peaks having a periodic reflection spectrum within a wavelength range in which the peak wavelength of the LGLC filter unit 4 is shifted. The DBR filter unit 1 and the LGLC filter unit 4 can each control the peak wavelength according to the supplied current, and the peak of the transmission spectrum of the LGLC filter unit 4 and the peak of the reflection spectrum of the DBR filter unit 1 Laser oscillation is performed in the longitudinal mode near the matching wavelength.

  In the wavelength tunable laser device according to the present invention, light emitted from the DBR filter unit 1 to the left side in FIG. 1 is unnecessary, and the half width of the reflection spectrum becomes narrower as the sampled grading period increases in the DBR filter unit 1. The side mode suppression ratio (SMSR) is improved. From the viewpoint of improving the SMSR, it is desirable that the filter characteristic of the DBR filter unit 1 is a filter characteristic having a periodic peak of 10 or more within the wavelength range in which the peak wavelength of the LGLC filter unit 4 is shifted. It is desirable that the filter unit 1 has a sampled grading structure having 10 cycles or more.

  The present invention relates to an LGLC wavelength tunable laser element, and the main feature thereof is a reflecting film (outgoing side reflecting film) formed on the end face on the exit side, with the end face on the LGLC wavelength tunable filter side being the end face on the exit side. The reflectance is 50% or less. The reflectance is more preferably 6.5% or more and 50% or less. The FFP disturbance of the output light output from the wavelength tunable laser element according to the first embodiment is suppressed more than when the end face on the DBR filter side is the end face on the output side. Furthermore, a high-power LGLC wavelength tunable laser element can be provided.

  However, as shown in FIG. 5, a disturbance 50 is generated in the FFP of the output light output from the wavelength tunable laser element according to the first embodiment. The inventors examined the cause of the occurrence of the disturbance 50 in the FFP, and as shown in FIG. 3, the light emitted from the tip of the high refractive index optical waveguide 10 is included in the light output as the stray light 102. It has been found that the fact that the interference pattern of the main light and stray light appears in the FFP is the main cause of the disturbance 50 in the FFP. And the structure which suppresses that the stray light 102 is discharge | released outside the element was investigated, suppressing the return light from the front-end | tip of a high refractive index optical waveguide. Hereinafter, a wavelength tunable laser element that further improves the FFP characteristics will be described.

[Second Embodiment]
The wavelength tunable laser element according to the second embodiment of the present invention is an LGLC wavelength tunable laser element, and is different from the first embodiment except that the structure of the output side end of the high refractive index optical waveguide 10 is different. It has the same structure as the tunable laser element.

FIG. 7 is a schematic diagram showing the structure of the LGLC filter unit 4 of the wavelength tunable laser element according to the embodiment. The low-refractive index optical waveguide 9 extends in the horizontal direction of the drawing, similarly to the low-refractive index optical waveguide 9 according to the first embodiment shown in FIG. 3, and the tip of the end portion reaches the end face on the emission side. On the other hand, the high refractive index optical waveguide 10 according to the embodiment is opposite to the low refractive index optical waveguide 9 side at the end thereof (as shown in the lower side of the figure), as in the first embodiment. ) Although bent, unlike the first embodiment, the tip of the high refractive index optical waveguide 10 reaches the end face on the emission side. The center line of the high-refractive index optical waveguide 10 extends linearly in parallel with the center line of the low-refractive index optical waveguide 9, is bent at the end, and reaches the end surface on the emission side. An angle formed by the extending direction of the high-refractive index optical waveguide 10 (first mesa stripe) and the normal line of the end surface on the exit side at the exit end surface is an angle θ _i . That is, an angle θ _i is an angle formed by the center line of the high refractive index optical waveguide 10 and the normal line of the end face on the emission side. The angle θ _{i according} to this embodiment is equal to or greater than the critical angle θ _o , and the light that propagates through the high refractive index optical waveguide 10 and reaches the exit-side end face is totally reflected at the exit-side end face, so that the light is Release to the outside is suppressed. Further, the light reflected on the end face on the exit side is shown as stray light 102 in FIG. 3 and is reflected on the end face on the exit side, so that it is also prevented from propagating again through the high refractive index optical waveguide 10 as return light. Is done.

Incidentally, the critical angle theta _o is the refractive index n of the material constituting the regions high refractive index waveguide 10 is formed, (often air) of the device exterior gas between the refractive index n _o of it is an angle defined by (equation _{1) sinθ o = n o /} n. Since the refractive index of the gas outside the element can be approximated to 1 (n _o ≈1), (Equation 1) can be expressed as (Equation 2) sin θ _o = 1 / n. The high refractive index optical waveguide 10 is formed in the upper core layer 23. The material constituting the upper core layer 23 is InGaAsP, and the refractive index of the material is 3.3. In this case, the critical angle θ _o is θ _o = 17.6 °. That is, in the embodiment, the angle θ _i is 17.6 ° or more.

  FIG. 8 is a diagram showing the observation result of the horizontal FFP of the output light from the wavelength tunable laser element according to the embodiment. As shown in the figure, the FFP according to this embodiment is further prevented from being disturbed than the FFP according to the first embodiment shown in FIG. 5, and further improvement of the FFP characteristics is realized. That is, in FIG. 8, the FFP disturbance 50 observed in the region surrounded by the broken line in FIG. 5 is not observed. It is considered that light is suppressed from being emitted from the tip of the high refractive index optical waveguide 10 to the outside of the element, and an interference pattern between main light and stray light hardly appears in the FFP.

In this embodiment, in order to set the angle θ _i to 17.6 ° or more, it can be considered that the radius of curvature of the bent portion of the high refractive index optical waveguide 10 is reduced, but the radius of curvature is reduced. As a result, the propagation loss of light increases. As a result of simulations conducted by the inventors, it has been found that the radius of curvature needs to be 200 μm or more in order to suppress the loss below a desired value. A larger radius of curvature accordingly, in order to achieve the desired angular theta _i becomes necessary high refractive index waveguide 10 is to increase the length of the portion to bend, when the radius of curvature 200 [mu] m, angle theta _In order to set _i = 17.6 °, the distance from the position where the high refractive index optical waveguide 10 starts to bend from a straight line to the end face on the emission side is required to be 60 μm or more. As this distance increases, the length of the wavelength tunable laser resonator increases, the wavelength interval between Fabry-Perot modes decreases, and the side mode suppression ratio (SMSR) decreases. In order not to lower the SMSR, it is desirable that the length of the bent portion of the high refractive index optical waveguide 10 is as short as possible within the range of the above-described restrictions. As described above, the SMSR can be increased by setting the filter characteristic of the DBR filter unit 1 to a reflection spectrum having a narrower half width. That is, by making the DBR filter unit 1 a sampled grading structure having 10 cycles or more, the length of the portion where the high refractive index optical waveguide 10 bends can be increased while the SMSR is set to a higher value.

[Third Embodiment]
The wavelength tunable laser device according to the third embodiment of the present invention is an LGLC wavelength tunable laser device, and a groove is formed between the end surface on the emission side and the tip of the high refractive index optical waveguide 10 (first mesa stripe). Except for the provision, it has the same structure as the wavelength tunable laser device according to the first embodiment.

  FIG. 9 is a schematic diagram showing the structure of the LGLC filter unit 4 of the wavelength tunable laser element according to the embodiment. The low refractive index optical waveguide 9 and the high refractive index optical waveguide 10 have the same shape as the low refractive index optical waveguide 9 and the high refractive index optical waveguide 10 according to the first embodiment shown in FIG. In the LGLC filter unit 4 according to this embodiment, unlike the first embodiment, a groove 31 is formed between the tip of the high refractive index optical waveguide 10 (first mesa stripe) and the end face on the emission side. . The shape of the groove 31 according to this embodiment viewed from above is in the vicinity of the end surface on the emission side, is located at a predetermined distance from the end surface on the emission side, and is shaped like a band along the end surface on the emission side. Stretched. One end of the belt-like shape reaches the vicinity of the low-refractive-index optical waveguide 9, and the other end of the belt-like shape extends further to the opposite side of the low-refractive-index optical waveguide 9 than the tip of the high-refractive-index optical waveguide 10. ing. The groove 31 is arranged to shield the stray light 102 emitted from the tip of the high refractive index optical waveguide 10, and the stray light 102 is suppressed from being emitted to the outside of the element. Therefore, the groove 31 intersects a virtual optical waveguide that extends further from the front end of the high refractive index optical waveguide 10 along the extending direction at the front end of the high refractive index optical waveguide 10. That is, the shape of the groove 31 viewed from above intersects with a line that extends the center line of the high refractive index optical waveguide 10 further than the tip of the high refractive index optical waveguide 10. Further, as described above, the p-type InP layer 21 is formed between the tip of the high refractive index optical waveguide 10 (first mesa stripe) and the end face on the emission side. The groove 31 is formed by removing the p-type InP layer 21, but the bottom of the groove 31 is at least at a location where it intersects with a virtual optical waveguide that extends further than the tip of the high refractive index optical waveguide 10. It is desirable to be deeper than the optical waveguide (upper core layer 23).

  In the embodiment, the groove 31 is disposed to block stray light. However, the groove 31 is not limited to the groove, and any structure having a function of blocking stray light may be used. For example, stray light may be shielded by arranging a semiconductor stripe structure that suppresses light propagation instead of the groove 31. In this case, the upper end of the stripe structure is higher than the optical waveguide (upper core layer 23) at least at a location where it intersects with a virtual optical waveguide extending further than the tip of the high refractive index optical waveguide 10. desirable.

  The portion where the low refractive index optical waveguide 9 and the high refractive index optical waveguide 10 extend in a straight line parallel to each other functions as an optical coupler, and light propagating through one of the optical waveguides in this portion. Transition to the other optical waveguide. As described above, the end portion of the high refractive index optical waveguide 10 is bent to the side opposite to the low refractive index optical waveguide 9 side. As a result of examination by the inventors, stray light is a region between the low-refractive index optical waveguide 9 and the high-refractive index optical waveguide 10 and in the vicinity of the portion where the high-refractive index optical waveguide 10 starts bending from a straight line. The knowledge that 103 occurs is obtained. Since the groove 31 reaches the vicinity of the low refractive index optical waveguide 9, the stray light 103 can be blocked in addition to the stray light 102, and the stray light 103 is suppressed from being emitted to the outside of the element. Therefore, in the wavelength tunable laser element according to the embodiment, further improvement in FFP characteristics is realized. It is desirable that the bottom of the groove 31 is deeper than the low refractive index optical waveguide 9 (lower core layer 25) at least at a portion where the main part of the stray light 103 propagates. Here, the low refractive index optical waveguide 9 is the low refractive index optical waveguide 9 (lower core layer 25) formed in a lower layer than the high refractive index optical waveguide 10 (upper core layer 23) in this embodiment. Therefore, it is desirable that the depth is lower than the lower one of the two optical waveguides. Also, a location where the center line of the low refractive index optical waveguide 9 intersects the end surface on the emission side, and a location where a line extending the center line of the high refractive index optical waveguide 10 in the extending direction at the tip intersects the end surface on the emission side. The point where the normal part of the stray light 103 propagates is defined by a point where the normal of the end face on the emission side passing through the middle point intersects the groove 31. Further, when a stripe structure is arranged instead of the groove 31, the stripe structure has a high refractive index optical waveguide 10 (upper core layer 23), that is, two lines at least in a portion where the main part of the stray light 103 propagates. It is desirable that the height of the optical waveguide is higher than that of the high optical waveguide.

[Fourth Embodiment]
The wavelength tunable laser element according to the fourth embodiment of the present invention is an LGLC wavelength tunable laser element, and includes a high refractive index optical waveguide 10 (first mesa stripe) and a low refractive index optical waveguide 9 (second mesa stripe). ), The same structure as that of the wavelength tunable laser device according to the second embodiment is provided.

  FIG. 10 is a schematic diagram showing the structure of the LGLC filter unit 4 of the wavelength tunable laser element according to the embodiment. The low refractive index optical waveguide 9 and the high refractive index optical waveguide 10 have the same shape as the low refractive index optical waveguide 9 and the high refractive index optical waveguide 10 according to the second embodiment shown in FIG. In the LGLC filter unit 4 according to this embodiment, unlike the second embodiment, the high refractive index optical waveguide 10 (first mesa stripe) and the low refractive index optical waveguide 9 (second mesa stripe) are provided. A groove 32 is formed. The shape of the groove 32 according to this embodiment viewed from above is in the vicinity of the exit-side end surface, is located at a predetermined distance from the exit-side end surface, and extends in a strip shape along the exit-side end surface. doing. One end of the belt-like shape reaches the vicinity of the low refractive index optical waveguide 9, and the other end of the belt-like shape reaches the vicinity of the high refractive index optical waveguide 10. The groove 32 is arranged to shield the stray light 103 generated in the vicinity of the portion where the high refractive index optical waveguide 10 starts to bend from a straight line, and the stray light 103 is suppressed from being emitted to the outside of the element. Yes. Therefore, in the wavelength tunable laser element according to this embodiment, the FFP characteristics are further improved as compared with the wavelength tunable laser element according to the second embodiment. It is desirable that the bottom of the groove 32 be deeper than the low refractive index optical waveguide 9 (lower core layer 25) at least at a portion where the main part of the stray light 103 propagates. That is, it is desirable that it is deeper than the lower one of the two optical waveguides. Further, the middle point of the location where the center line of the low refractive index optical waveguide 9 intersects the end surface on the emission side and the location where the center line of the high refractive index optical waveguide 10 intersects the end surface on the emission side is defined, A portion where the normal line of the end surface on the emission side passing through the midpoint intersects the groove 32 is a portion where the main part of the stray light 103 propagates.

  In the present embodiment, the grooves 32 are arranged to block stray light. However, like the third embodiment, the grooves 32 are not limited to the grooves, and a semiconductor stripe structure is used instead of the grooves 32. It may be arranged. In this case, the upper end of the stripe structure is higher than the high optical waveguide of the high refractive index optical waveguide 10 (upper core layer 23), that is, the two optical waveguides, at least at the portion where the main part of the stray light 103 propagates. It is desirable.

[Fifth Embodiment]
The wavelength tunable laser element according to the fifth embodiment of the present invention is an LGLC wavelength tunable laser element, except that the structure of the output side end portion of the low refractive index optical waveguide 9 (second mesa stripe) is different. The same structure as the wavelength tunable laser element according to any one of the first to fourth embodiments is provided.

  FIG. 11 is a schematic diagram showing the structure of the LGLC filter unit 4 of the wavelength tunable laser element according to the present embodiment. The high refractive index optical waveguide 10 is bent at the end thereof and reaches the end surface on the emission side, similarly to the high refractive index optical waveguide 10 according to the second embodiment shown in FIG. On the other hand, the structure of the low refractive index optical waveguide 9 according to this embodiment is different from that of the second embodiment at the end. The low refractive index optical waveguide 9 (second mesa stripe) is bent at the end thereof toward the high refractive index optical waveguide 10 (first mesa stripe), and further on the side opposite to the high refractive index optical waveguide 10. It bends and reaches the end face on the exit side so as to follow the normal line of the end face on the exit side. It is desirable that the center line of the low refractive index optical waveguide 9 intersects the end face on the emission side perpendicularly. The part where the low refractive index optical waveguide 9 starts to bend from a straight line is preferably located closer to the end face on the emission side than the part from which the high refractive index optical waveguide 10 starts to bend from a straight line.

  As described above, stray light 103 is generated from the vicinity of the portion where the high refractive index optical waveguide 10 starts to bend from a straight line. The stray light 103 propagates to the emission side end face side, but at least a part of the stray light 103 is on the side surface (side surface on the high refractive index optical waveguide 10 side) of the low refractive index optical waveguide 9 (second mesa stripe). Since it reaches and is scattered on the side surface, the stray light 103 is suppressed from being emitted to the outside of the element. Further, since the low refractive index optical waveguide 9 is further bent to the side opposite to the high refractive index optical waveguide 10 side, the low refractive index optical waveguide 9 is arranged on the output side so as to follow the normal line of the end surface on the output side. Can reach the end face. Therefore, it is realized that the emission direction of the light emitted from the low refractive index optical waveguide 9 is along the normal line of the end surface on the emission side.

  The LGLC filter unit 4 shown in FIG. 11 is obtained by applying the structure of the low refractive index optical waveguide 9 to the second embodiment, but is not limited to this, and the first, third, or It may be applied to any of the fourth embodiments.

[Sixth Embodiment]
The wavelength tunable laser element according to the sixth embodiment of the present invention is an LGLC wavelength tunable laser element, except that the structure of the LGLC filter unit 4 of the high refractive index optical waveguide 10 (first mesa stripe) is different. The same structure as that of the wavelength tunable laser device according to any one of the first to fifth embodiments is provided.

  As shown in FIG. 4, long-period diffraction gratings 41 are formed on both side surfaces of the high refractive index optical waveguide 10 according to the first embodiment. Scattered light is generated in the long-period diffraction grating 41, and the scattered light propagates as stray light 104 and 105 to the exit-side end face. As a result of studies by the inventors, among the stray light generated from the long-period diffraction grating 41 of the high-refractive-index optical waveguide 10, the stray light 104 generated from the long-period diffraction grating 41 formed on the side surface on the low-refractive-index optical waveguide 9 side. It has been found that (a part of) propagates and reaches the end face on the emission side and is emitted to the outside of the device, and this stray light affects the FFP characteristics. On the other hand, stray light 105 generated from the long-period diffraction grating 41 formed on the side surface opposite to the low refractive index optical waveguide 9 side is bent in the high refractive index optical waveguide 10 (first stripe). It is considered that the influence of the stray light on the FFP characteristics is very limited due to the scattering at the side surface of the portion to be scattered and the stray light 105 propagating away from the low refractive index optical waveguide 9.

  FIG. 12 is a schematic diagram showing the structure of the LGLC filter unit 4 of the wavelength tunable laser element according to the present embodiment. Of the both side surfaces of the high refractive index optical waveguide 10 (first mesa stripe) according to the present embodiment, no diffraction grating is formed on the side surface on the low refractive index optical waveguide 9 (second stripe) side. A diffraction grating (long-period diffraction grating 41) is formed on the side surface opposite to. That is, the diffraction grating is formed only on the side surface opposite to the low refractive index optical waveguide 9 side. Of the two side surfaces of the second mesa stripe, on the side opposite to the low-refractive-index optical waveguide 9 side, in the portion where the upper core layer 23 where the high-refractive-index optical waveguide 10 is formed is located, the diffraction grating Is formed. Note that the portion where the long-period diffraction grating 41 is formed in the high refractive index optical waveguide 10 is a portion extending in parallel with the low refractive index optical waveguide 9. Are connected.

  Unlike the first embodiment shown in FIG. 4, the long period diffraction grating 41 is not formed on the side surface on the low refractive index optical waveguide 9 side among the both side surfaces of the high refractive index optical waveguide 10. The stray light 104 is not generated in the LGLC filter unit 4 of the wavelength tunable laser element according to the embodiment. Therefore, in the first embodiment, the FFP may be disturbed due to the stray light 104. In this embodiment, further improvement of the FFP characteristics is realized. The LGLC filter unit 4 shown in FIG. 12 is obtained by applying the structure of the high refractive index optical waveguide 10 to the first embodiment, but is not limited to this, and any of the second to fifth It may be applied to such an embodiment.

[Seventh Embodiment]
An optical module according to the seventh embodiment of the present invention is an optical module including the wavelength tunable laser element according to any one of the first to sixth embodiments. FIG. 13 is a schematic diagram illustrating a configuration of an optical module according to the present embodiment. As shown in the figure, the optical module according to this embodiment includes an LGLC wavelength tunable laser element 61, a wavelength locker 62, and a half mirror 63. The LGLC wavelength tunable laser element is a wavelength tunable laser element according to any one of the first to sixth embodiments, and the wavelength locker 62 includes a lens, an etalon filter, and the like. The half mirror 63 is disposed on the optical axis of the light emitted from the LGLC wavelength tunable laser element 61, and is split into light 110 and light 111 by the half mirror. The light 110 is normal output light output from the LGLC wavelength tunable laser element 61, and a modulator or the like is disposed at the propagation destination of the light 110. On the other hand, the light 111 is light for monitoring the wavelength of the LGLC wavelength tunable laser element 61. A wavelength locker 62 is disposed at the propagation destination of the light 111, the wavelength locker 62 detects the wavelength output by the LGLC wavelength tunable laser element 61, and outputs information on the wavelength to a control circuit (not shown). The control circuit controls the LGLC wavelength tunable laser element based on the wavelength information, so that the LGLC wavelength tunable laser element 61 can stably output light having a desired wavelength. The LGLC wavelength tunable laser element 61 is a wavelength tunable laser element with improved FFP characteristics, and the FFP characteristic of the output light output from the wavelength tunable laser element is sufficient to use (part of) the output light as a wavelength locker. Has been improved. Therefore, the optical module according to this embodiment can control the wavelength of light output from the LGLC wavelength tunable laser element 61 using the wavelength locker 62.

  The wavelength tunable laser element and the optical module according to the present invention have been described above. In the above embodiment, the DBR filter is an example of the second filter, but the present invention is not limited to this as long as the filter uses a diffraction grating type reflecting mirror. Further, the second filter may be, for example, a (liquid crystal) etalon filter or a ring resonator. Moreover, in the said embodiment, although the phase adjustment part 3 is arrange | positioned between the gain part 2 and the LGLC filter part 4, it is not limited to this, For example, the gain part 2 and the DBR filter part 1 It may be arranged between. The present invention can be widely applied to a wavelength tunable laser element including a laterally coupled type unidirectional optical coupler type wavelength tunable filter.

  DESCRIPTION OF SYMBOLS 1 DBR filter part, 2 gain part, 3 phase adjustment part, 4 LGLC filter part, 5 DBR filter electrode, 6 gain electrode, 7 phase adjustment electrode, 8 LGLC filter electrode, 9 low refractive index optical waveguide, 10 High refractive index optical waveguide, 11, 12 reflective film, 20 n-type InP substrate, 21 p-type InP layer, 22 diffraction grating, 23 upper core layer, 24 n-type InP layer, 25 lower core layer, 26 gain layer, 29 common Electrode, 31, 32 groove, 41 long-period diffraction grating, 50 disorder, 61 LGLC tunable laser element, 62 wavelength locker, 63 half mirror, 101 main light, 102, 103, 104, 105 stray light, 110, 111 light.

Claims (9)

An exit-side reflective film formed on an end face on the exit side;
A first filter that is disposed in contact with the end face on the emission side and is composed of a laterally coupled co-directional optical coupler type tunable filter;
A second filter comprising a filter characteristic having a plurality of periodic peaks within a wavelength range in which a peak wavelength of the first filter shifts;
A gain unit that is disposed between the first filter and the second filter and emits light when supplied with a current;
A tunable laser element comprising:
The first filter is:
A first mesa stripe including a first optical waveguide extending from the gain portion;
A second mesa stripe including a second optical waveguide extending to optically couple in parallel with the first optical waveguide and having a refractive index lower than that of the first optical waveguide;
With
The second mesa stripe extends to the exit-side end surface and reaches the exit-side end surface,
The first mesa stripe extends in parallel with the second mesa stripe and bends at the end thereof to the side opposite to the first mesa stripe side,
The reflectance of the exit side reflective film is 50% or less,
A tunable laser element characterized by the above. The tunable laser device according to claim 1,
The reflectance of the output side reflective film is 6.5% or more and 50% or less.
A tunable laser element characterized by the above. The wavelength tunable laser element according to claim 1 or 2,
The front end of the first mesa stripe reaches the end face on the exit side, and the angle formed by the extending direction of the first mesa stripe and the normal line of the end face on the exit side is a critical angle on the end face on the exit side. That's it,
A tunable laser element characterized by the above. The wavelength tunable laser element according to claim 1 or 2,
The tip of the first mesa stripe is located inside the end surface on the emission side,
A groove or stripe is formed between the tip end of the first mesa stripe and the end face on the emission side.
A tunable laser element characterized by the above. The tunable laser device according to claim 3, wherein
A groove or stripe is formed between the first mesa stripe and the second mesa stripe,
A tunable laser element characterized by the above. The wavelength tunable laser device according to any one of claims 1 to 5,
The second mesa stripe is bent at the end toward the first mesa stripe, further bent toward the side opposite to the first mesa stripe, and follows the normal of the end face on the exit side. To the end face on the exit side as
A tunable laser element characterized by the above. The wavelength tunable laser device according to any one of claims 1 to 6,
In a portion where the first mesa stripe extends in parallel with the second mesa stripe, a diffraction grating is formed only on a side surface opposite to the second stripe side among both side surfaces of the first mesa stripe. Is formed,
A tunable laser element characterized by the above. The wavelength tunable laser device according to any one of claims 1 to 7,
The second filter has a sample gourd structure having 10 cycles or more.
A tunable laser element characterized by the above.   An optical module comprising the wavelength tunable laser element according to claim 1.

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