JP5287886B2 - Optical device in which light propagating in optical waveguide and diffraction grating are coupled - Google Patents

Description

  The present invention relates to an optical element, and more particularly to an optical element in which light propagating through an optical waveguide and a diffraction grating are coupled to narrow the spectrum of the light propagating through the optical waveguide.

  Along with the explosive increase in Internet demand, efforts to increase the speed and capacity of optical communication and optical transmission have become active. In particular, there is a demand for an inexpensive semiconductor laser element that can be directly modulated at 10 Gb / s or more without cooling for Ethernet (registered trademark) having a gigabit transmission band. As a semiconductor laser element that can meet this requirement, a distributed feedback (DFB) type laser element can be cited.

  In order to manufacture the DFB laser device at low cost, a ridge type laser device that can be manufactured by one crystal growth, that is, does not need to perform the second crystal growth after the etching process is promising. In the case of forming a distributed feedback diffraction grating in the ridge type laser element, it is more advantageous in terms of manufacturing cost to form the diffraction grating on both sides of the ridge than to make the diffraction grating inside the crystal.

  FIG. 24 is a perspective view of a conventional ridge type DFB laser device. On the semiconductor substrate 500, an active layer 501 and a clad layer 502 are sequentially laminated. A ridge 503 extending in one direction is formed on the cladding layer 502. A diffraction grating 504 is formed on the side surface of the ridge 503. The active layer 501 below the ridge 503 functions as an optical waveguide.

  FIG. 25 shows another example of a conventional ridge type DFB laser element. In the ridge type DFB laser device shown in FIG. 24, the diffraction grating 504 is formed on the side surface of the ridge 503. However, in the example shown in FIG. 25, the diffraction grating 504 is replaced with a flat surface on both sides of the ridge 503. A diffraction grating 504A is formed. Other configurations are the same as those of the laser element shown in FIG.

  FIG. 26 shows the positional relationship between the guided light propagating through the optical waveguide and the diffraction grating. Diffraction gratings 504 or 504A are arranged on both sides of the ridge 503. The light intensity distribution of the fundamental transverse mode of the guided light becomes maximum at the center in the width direction of the ridge 503 as shown by the solid line 510, and decreases as the distance from the center increases. The light intensity distribution of the first higher-order transverse mode (hereinafter abbreviated as “secondary transverse mode”) shows a local minimum value at the center in the width direction of the ridge 503 and moves away from the center, as indicated by a solid line 511. As the light intensity increases, the maximum value is exhibited on both sides. In a region outside the position showing the maximum value, the light intensity monotonously decreases as the distance from the center of the ridge 503 increases.

  Since no diffraction grating is disposed near the center of the ridge 503 and diffraction gratings are disposed on both sides of the ridge 503, the light intensity of the second-order transverse mode in the region where the diffraction grating is disposed Stronger than the light intensity of the mode. For this reason, the coupling coefficient between the secondary transverse mode and the diffraction grating is about 1.5 to 2 times the coupling coefficient between the fundamental transverse mode and the diffraction grating. As a result, secondary transverse mode oscillation is likely to occur.

  In order to reduce the coupling coefficient between the second-order transverse mode and the diffraction grating, the ridge 503 is narrowed and the diffraction grating is brought closer to the center of the ridge 503. However, when the ridge 503 is made thinner, the electrical resistance of the laser element increases. For this reason, narrowing the ridge 503 causes an increase in power consumption and a decrease in light output due to heat generation during large current injection.

  Patent Document 1 below discloses a semiconductor laser element capable of suppressing higher-order transverse mode oscillation.

  FIG. 27 is a plan sectional view of the ridge portion of the semiconductor laser device disclosed in Patent Document 1. In FIG. A diffraction grating 521 is formed on the side surface of the ridge 520. A light absorption layer 522 made of InGaAs that absorbs oscillation light is formed on the outer surface of the diffraction grating 521 that is uneven. Since the light absorption layer 522 absorbs a higher order transverse mode more than the fundamental transverse mode, the oscillation of the higher order transverse mode can be suppressed.

  Next, a transverse mode of guided light in a laser element having an embedded waveguide (hereinafter referred to as an embedded laser element) will be described.

  FIG. 28 is a perspective view of a conventional embedded laser element. A mesa portion 551 extending in one direction is formed on the surface of the semiconductor substrate 550. A diffraction grating 552 is formed on the upper surface of the mesa portion 551, and an active layer 553 is formed thereon. A buried layer 555 is formed on the flat surfaces on both sides of the mesa portion 551. A current confinement layer 556 is formed on the buried layer 555. An upper cladding layer 557 covers the active layer 553 and the current confinement layer 556.

  FIG. 29 shows an example of the relationship between the coupling coefficient between the guided light and the diffraction grating of the buried laser element shown in FIG. 28 and the width of the active layer. Curve a shows the coupling coefficient of the fundamental mode, and curve b shows the coupling coefficient of the secondary transverse mode.

  Since the diffraction grating 552 is disposed over the entire width direction of the active layer 553, in principle, the coupling coefficient a of the fundamental transverse mode is larger than the coupling coefficient b of the secondary transverse mode. In particular, if the width of the active layer 553 is 1.4 μm (cutoff width) or less, the oscillation in the secondary transverse mode does not occur.

JP 2003-152273 A

  In the laser element disclosed in Patent Document 1, the waveguide loss in the fundamental transverse mode also increases due to the arrangement of the light absorption layer 522 shown in FIG. As a result, the oscillation threshold value increases by about 50 to 200%.

  In the buried laser element shown in FIG. 28, if the width of the active layer 553 is equal to or smaller than the cutoff width, the oscillation in the secondary transverse mode does not occur. However, as shown in FIG. 29, when the width of the active layer 553 is increased, the coupling coefficient of the secondary transverse mode also increases. As the active layer 553 becomes wider, the coupling coefficient of the fundamental transverse mode and the coupling of the secondary transverse mode are increased. The difference from the coefficient becomes smaller. In the example shown in FIG. 29, when the width of the active layer 553 is 2.5 μm or more, the coupling coefficient of the secondary transverse mode reaches a size necessary for oscillation. For this reason, it is not preferable to make the width of the active layer 553 wider than 2.5 μm.

  Also in the ridge type laser element in which the diffraction grating is disposed inside the substrate under the ridge, as in the embedded type laser element shown in FIG. 28, when the width of the ridge is increased, secondary transverse mode oscillation is likely to occur.

  In the DFB laser element, it is effective to increase the width of the ridge and the active layer in order to improve the light output characteristics. A technique for suppressing secondary transverse mode oscillation even when the width of the ridge or the active layer is widened is desired.

  In addition, the space on both sides of the ridge 503 of the ridge type laser element shown in FIGS. 24 and 25 is usually filled with air or with a dielectric material such as SiO 2 or benzocyclobutene (BCB). . The refractive index of these materials is about 1 to 1.7, which is significantly lower than the general refractive index of semiconductor materials of 3.2 to 3.5. Due to this difference in refractive index, the light intensity distribution of the guided light propagating in the active layer 501 along the ridge 503 greatly spreads toward the substrate side, and the spread to the space on both sides of the ridge 503 is extremely small. Become. For this reason, in the DFB laser element in which the diffraction grating is disposed on the side surface of the ridge 503 or the flat surfaces on both sides of the ridge 503, the coupling coefficient between the guided light and the diffraction grating is larger than that in the element in which the diffraction grating is disposed in the substrate. κ becomes smaller.

In the DFB laser element in which the diffraction grating is arranged in the substrate, the coupling coefficient κ is about 250 cm ⁻¹ . On the other hand, in a DFB laser element in which a diffraction grating is arranged on the substrate surface, the coupling coefficient κ is about 90 cm ⁻¹ (Watanabe et al., “Laterally Coupled Strained MQW Ridge Waveguide Distributed-Feedback Laser Diode Fabricated by Wet- Dry Hybrid Etching process ", IEEE Photonics Technology Letters, Vol. 10, No. 12, December 1998).

In general, it is effective to reduce the parasitic capacitance of a laser element in order to improve high-speed direct modulation characteristics. In order to reduce the parasitic capacitance, as an example, the optical resonator length L of the laser element is preferably set to 250 μm or less. For example, the normalized coupling coefficient κL of a laser element having an optical resonator length L of 250 μm and a coupling coefficient κ of 90 cm ⁻¹ is 2.25. However, the optimum normalized coupling coefficient κL of a 1/4 wavelength shifted DFB laser element capable of high-speed modulation of 10 Gb / s or more is 2.4 to 2.7. When the normalized coupling coefficient κL is smaller than 2.4, the relaxation oscillation frequency that is an index of the modulation speed of the direct modulation operation is lowered, and as a result, high-speed modulation characteristics of 10 Gb / s or more cannot be obtained. When the normalized coupling coefficient κL is larger than 2.7, the light distribution is concentrated in the vicinity of the portion where the diffraction grating is phase-shifted with respect to the axial direction of the optical resonator. As a result, the secondary mode is likely to oscillate due to axial hole burning, and single longitudinal mode oscillation is hindered. In addition, since light is more strongly confined in the optical resonator, the light output from the end face is reduced.

In a DFB laser element having a coupling coefficient κ of about 90 cm ⁻¹ , the optical resonator length L must be longer than 250 μm in order to realize a sufficient normalized coupling coefficient κL. If the optical resonator length L is longer than 250 μm, the parasitic capacitance increases and the high-speed modulation characteristics are deteriorated.

  As described above, in the conventional DFB laser element in which the diffraction grating is arranged on the surface, it is difficult to make both the normalized coupling coefficient κL a suitable value and obtain good high-speed modulation characteristics. .

According to one aspect of the invention,
An optical waveguide for propagating the laser beam;
A main diffraction grating coupled with guided light propagating through the optical waveguide;
A sub-diffraction grating coupled to the guided light propagating through the optical waveguide, and when only the main diffraction grating is arranged, compared to the propagation of the second-order transverse mode of the guided light propagating through the optical waveguide, When both the main diffraction grating and the sub diffraction grating are arranged, the main diffraction grating and the sub diffraction grating are guided light so that propagation of the second-order transverse mode of the guided light propagating through the optical waveguide is suppressed. And
The coupling coefficient between the fundamental transverse mode of the guided light and the sub-diffraction grating is smaller than the coupling coefficient between the secondary transverse mode and the sub-diffraction grating;
The periods of the main diffraction grating and the sub diffraction grating are the same, and the main diffraction grating and the sub diffraction grating have a periodic refractive index distribution with respect to the propagation direction of the guided light, and the refractive index distribution of the sub diffraction grating is An optical element whose phase is shifted by 180 ° with respect to the refractive index distribution of the main diffraction grating is provided.

  Since the propagation of the secondary transverse mode is suppressed, it becomes possible to propagate single mode guided light including only the fundamental transverse mode.

FIG. 1 is a perspective view of a ridge type laser device according to the first embodiment. FIG. 2 is a diagram showing the positional relationship between the main diffraction grating and the sub-diffraction grating of the ridge type laser device according to the first embodiment and the light intensity distributions of the fundamental transverse mode and the secondary transverse mode. FIG. 3 is a graph showing the relationship between the duty ratio of the diffraction grating and the diffraction intensity. A curve c indicates the diffraction intensity in a direction orthogonal to the propagation direction of the guided light, and a curve d indicates the propagation direction of the guided light. The diffraction intensity is shown. 4A and 4B are perspective views (part 1) of the ridge type laser device according to the first embodiment in the course of manufacturing. 4C and 4D are perspective views (part 2) of the ridge type laser device according to the first embodiment in the course of manufacturing. FIG. 5 is a perspective view of a ridge type laser device according to a modification of the first embodiment. FIG. 6A is a diagram showing a light intensity distribution of a ridge type laser device according to a modification of the first embodiment, and FIG. 6B is a diagram showing a light intensity distribution of a ridge type laser device according to a comparative example. FIG. 7 is a perspective view of a ridge type laser device according to the second embodiment. FIG. 8 is a diagram schematically showing the main diffraction grating and the sub-diffraction grating of the ridge type laser device according to the second embodiment and the guided light propagating in combination with this diffraction grating. FIG. 9 is a perspective view of a ridge type laser device according to the third embodiment. FIG. 10 is a diagram schematically showing the main diffraction grating and the sub-diffraction grating of the ridge type laser device according to the third embodiment and the guided light propagating by being coupled to this diffraction grating. FIG. 4 is a perspective view of a ridge type laser device according to the fourth embodiment. 12A and 12B are perspective views (part 1) of the ridge type laser device according to the fourth embodiment in the course of manufacturing. 12C and 12D are perspective views (part 2) of the ridge type laser device according to the fourth embodiment in the middle of manufacture. FIG. 12E is a perspective view (No. 3) of the ridge type laser element according to the fourth embodiment in the middle of manufacturing. FIG. 13 is a perspective view of a ridge type laser device according to the fifth embodiment. FIG. 14 is a perspective view of a ridge type laser device according to the sixth embodiment. FIG. 15 is a perspective view of a ridge type laser device according to the seventh embodiment. 16A and 16B are perspective views (part 1) in the middle of manufacturing the ridge type laser device according to the seventh embodiment. 16C and 16D are perspective views (part 2) in the middle of manufacturing the ridge type laser device according to the seventh embodiment. FIG. 16E is a perspective view (No. 3) of the ridge type laser device according to the seventh embodiment in the middle of manufacture. FIG. 17 is a perspective view of a ridge type laser device according to the eighth embodiment. FIG. 18 is a perspective view of a ridge type laser device according to the ninth embodiment. 19A and 19B are perspective views (part 1) in the middle of manufacturing the embedded laser device according to the tenth embodiment. 19C and 19D are perspective views (part 2) in the middle of manufacturing the embedded laser device according to the tenth embodiment. FIG. 19E is a perspective view (part 3) of the embedded laser device according to the tenth embodiment in the middle of manufacturing, and FIG. 19F is a perspective view of the embedded laser device according to the tenth embodiment. FIG. 20 is a perspective view of the embedded laser device according to the eleventh embodiment in the middle of manufacturing. FIG. 21 is a perspective view of an embedded laser device according to the twelfth embodiment in the course of manufacturing. FIG. 22 is a perspective view of a ridge type laser device according to the thirteenth embodiment in the middle of manufacturing. FIG. 23 is a sectional view of a DBR laser device according to the fourteenth embodiment. FIG. 24 is a perspective view of a conventional ridge type laser element. FIG. 25 is a perspective view of a conventional ridge type laser device. It is a figure which shows the positional relationship of the diffraction grating of the conventional ridge type laser element, and the light intensity distribution of the fundamental transverse mode and secondary transverse mode of guided light. It is a plane sectional view of a ridge portion of a conventional ridge type laser device. It is a perspective view of the conventional embedded type laser element. It is a graph which shows the relationship between the active layer width | variety of a conventional buried type laser element, and a coupling coefficient, the curve a shows the coupling coefficient of a fundamental transverse mode, and the curve b shows the coupling coefficient of a secondary transverse mode.

FIG. 1 is a perspective view of a ridge type laser device according to the first embodiment. On the main surface of the substrate 1 made of n-type GaAs, a lower cladding layer 2 made of n-type Al _0.5 Ga _0.5 As and having a thickness of 1.5 μm, and n-type Al _0.3 Ga _0.7 As A lower light guide layer 3 having a thickness of 0.15 μm, a quantum dot active layer (optical waveguide layer) 4, and an upper light guide layer 5 having a thickness of 0.15 μm made of p-type Al _0.3 Ga _0.7 As. They are stacked in this order.

  The quantum dot active layer 4 has a laminated structure in which a structure in which an InGaAs layer including a large number of quantum dots made of InAs is sandwiched between GaAs layers is repeated 10 times in the thickness direction.

  On the upper light guide layer 5, a ridge 10 having a height of 1.4 μm and a width of 2 μm that is long in one direction is disposed. Main diffraction gratings 11 having periodicity in the length direction of the ridge 10 are formed on the side surfaces on both sides of the ridge 10. The main diffraction grating 11 has a structure in which convex portions and concave portions extending in the height direction of the ridge 10 are alternately arranged in the length direction of the ridge 10. The period of the main diffraction grating 11 is 198 nm, and the height from the bottom of the concave portion to the tip of the convex portion is 500 nm.

The ridge 10 includes a 1.2 μm thick upper cladding layer 6 made of p-type Al _0.3 Ga _0.7 As, and a 0.2 μm thick contact layer 7 made of p-type GaAs formed thereon. It has a two-layer structure.

  Sub-diffraction gratings 12 having periodicity in the length direction of the ridge 10 are formed on the flat surfaces of the upper light guide layer 5 on both sides of the ridge 10. The sub-diffraction grating 12 includes a large number of convex portions arranged in the length direction of the ridge 10 at equal intervals. The period of the sub-diffraction grating 12 is 396 nm, that is, twice the period of the main diffraction grating 11. The convex portion constituting the sub-diffraction grating 12 has the same two-layer structure as the ridge 10, and its height is equal to the height of the ridge 10. The dimension of the convex portion in the length direction of the ridge 10 is 198 nm. Moreover, the dimension of the convex part regarding the direction orthogonal to the length direction of the ridge 10 is 0.5 μm.

  The convex portion of the sub-diffraction grating 12 may be arranged so that its end surface is in contact with the tip of the convex portion of the main diffraction grating 11, or a minute amount between the end surface and the tip of the convex portion of the main diffraction grating 11. You may arrange | position so that a clearance gap may be ensured.

  A p-side electrode 14 is formed on the ridge 10. An n-side electrode 15 is formed on the bottom surface of the substrate 1. The upper electrode 14 and the lower electrode 15 are made of, for example, AuZn / Au. Usually, the semiconductor surface of the laser element is covered with a protective film such as silicon oxide, silicon nitride, or benzocyclobutene (BCB).

  The quantum dot active layer 4 has an effective refractive index higher than any of the refractive indexes of the lower cladding layer 2, the lower light guide layer 3, the upper light guide layer 5, and the upper cladding layer 6. A region below the ridge 10 in the quantum dot active layer 4 is an optical waveguide that guides light in the length direction of the ridge 10. The guided light propagating through the optical waveguide is coupled to the main diffraction grating 11 and the sub diffraction grating 12.

The effective refractive index of the optical waveguide and n _e, the period of the main diffraction grating 11 when the p _1, the wavelength λ of the guided light to be selected by the main diffraction grating 11,
λ = 2 × p ₁ × n _e
It becomes. When a voltage is applied between the upper electrode 14 and the lower electrode 15 to inject carriers into the quantum dot active layer 4, the ridge type laser element oscillates at the wavelength λ expressed by the above formula.

  In FIG. 1, the period of the main diffraction grating 11 and the sub diffraction grating 12 is expressed relatively large compared to the overall dimensions of the element. Also, in other drawings attached to this specification, the period of the diffraction grating is expressed relatively large compared to the dimensions of the entire element.

  FIG. 2 shows an example of the positional relationship between the main diffraction grating 11, the sub-diffraction grating 12, and the lateral light intensity distribution of the guided light in the ridge type laser device according to the first embodiment. A main diffraction grating 11 having a periodic structure in which high refractive index regions (convex portions) and low regions (concave portions) are alternately arranged is disposed on both sides of the ridge 10. Sub-diffraction gratings 12 each having a periodic structure in which high refractive index regions (convex portions) and low regions are alternately arranged are disposed outside the main diffraction grating 11.

  The light intensity distribution of the fundamental transverse mode of the guided light is indicated by a solid line 20, and the light intensity distribution of the secondary transverse mode is indicated by a solid line 21. The light intensity distribution 20 in the fundamental transverse mode shows a maximum value at the center in the width direction of the ridge 10, and the light intensity decreases as the distance from the center increases. The light intensity distribution 21 of the secondary transverse mode shows a minimum value at the center in the width direction of the ridge 10 and shows a maximum value near the side surface of the ridge 10.

  The secondary transverse mode has a relatively high light intensity in both the region A where the main diffraction grating 11 is disposed and the region B where the sub-diffraction grating 12 is disposed. The fundamental transverse mode shows a certain high light intensity in the region A where the main diffraction grating 11 is arranged. However, the light intensity of the fundamental transverse mode in the region B where the sub-diffraction grating 12 is arranged is significantly lower than the light intensity of the fundamental transverse mode in the region A where the main diffraction grating 11 is arranged.

For this reason, the coupling coefficient between the fundamental transverse mode and the sub-diffraction grating 12 is smaller than the coupling coefficient between the fundamental transverse mode and the main diffraction grating 11. On the other hand, the second-order transverse mode has a high coupling coefficient for both the main diffraction grating 11 and the sub-diffraction grating 12. The coupling coefficient between the fundamental transverse mode and the main diffraction grating 11 is k11, the coupling coefficient between the fundamental transverse mode and the sub-diffraction grating 12 is k12, the coupling coefficient between the second-order transverse mode and the main diffraction grating is k21, and the second-order transverse mode. If the coupling coefficient with the sub-diffraction grating 12 is k22, the following inequality is established.
(K11-k12)> (k21-k22)
That is, the fundamental transverse mode is strongly influenced by the main diffraction grating 11, but is hardly affected by the sub-diffraction grating 12. The second-order transverse mode is strongly influenced by the main diffraction grating 11 and also strongly influenced by the sub-diffraction grating 12.

  Next, with reference to FIG. 3, the influence of the guided light having the wavelength selected by the main diffraction grating 11 from the sub-diffraction grating 12 will be described.

  FIG. 3 shows the relationship between the geometric shape of the sub-diffraction grating 12 and the intensity of diffraction. The horizontal axis in FIG. 3 represents the ratio (duty ratio) of the convex portions in one period of the sub-diffraction grating 12 in the unit “%”, and the vertical axis represents the intensity of diffraction in an arbitrary scale. A curve c indicates the intensity of diffraction in the direction orthogonal to the substrate surface, and a curve d indicates the intensity of diffraction in the propagation direction of the guided light.

  When the duty ratio is 50%, the intensity of diffraction in the substrate normal direction is maximized. As the duty ratio goes away from 50%, the intensity of diffraction in the normal direction becomes weaker and becomes 0 when the duty ratio is 0% and 100%. The intensity of diffraction in the propagation direction of the guided light is maximized when the duty ratio is 25% and 75%. As the duty ratio deviates from 25% and 75%, the diffraction intensity decreases, and when the duty ratio is 0%, 50%, and 100%, the diffraction intensity becomes zero.

  For this reason, when the duty ratio of the sub-diffraction grating 12 is near 50%, the propagation loss of guided light becomes large. However, since the fundamental transverse mode is hardly affected by the sub-diffraction grating 12, the propagation loss of the fundamental transverse mode does not increase. On the other hand, the second-order transverse mode is strongly coupled to the sub-diffraction grating 12 and is diffracted in the direction perpendicular to the waveguide, so that the propagation loss increases.

  The “secondary transverse mode recombination strength by the sub-diffraction grating” is coupled to the main diffraction grating 11 and the second-order transverse mode light of the guided light propagating through the optical waveguide is diffracted by the sub-diffraction grating 12, The recombination intensity of the fundamental transverse mode by the sub-diffraction grating is defined as the intensity at which the fundamental transverse mode light of the guided light is diffracted by the sub-diffraction grating 12 and re-coupled to the optical waveguide. Define. In the first embodiment, the recombination strength of the secondary transverse mode by the sub-diffraction grating is weaker than the recombination strength of the fundamental transverse mode by the sub-diffraction grating. Thereby, the oscillation of the secondary transverse mode can be suppressed.

  In order to obtain a sufficient effect of suppressing the oscillation of the secondary transverse mode, the duty ratio of the sub-diffraction grating 12 is preferably set to 35% to 65%. In the first embodiment, the period of the sub-diffraction grating 12 is twice that of the main diffraction grating 11, but the period of the sub-diffraction grating 12 may be 1.2 times or more of the period of the main diffraction grating 11. For example, since the diffracted light is diffracted at an angle that does not recombine with the waveguide, it is possible to sufficiently suppress the oscillation of the secondary transverse mode.

  In the first embodiment, the basic transverse mode and the second-order transverse mode are compared, but the second-order or higher-order transverse mode is also strongly coupled to the sub-diffraction grating 12 as compared with the fundamental transverse mode. . Accordingly, the ridge type laser device according to the first embodiment can suppress the oscillation of the second-order or higher order transverse mode.

  Next, with reference to FIGS. 4A to 4D, a method of manufacturing the ridge type laser device according to the first embodiment will be described.

  As shown in FIG. 4A, a lower cladding layer 2, a lower light guide layer 3, a quantum dot active layer 4, an upper light guide layer 5, an upper cladding layer 6, and a contact layer 7 are formed on a substrate 1 with molecular beams. Growing by epitaxial growth (MBE).

  As shown in FIG. 4B, a resist film 8 for electron beam exposure is formed on the contact layer 7.

  As shown in FIG. 4C, the resist film 8 is exposed with an electron beam and developed to form resist patterns 8a and 8b. The resist pattern 8 a matches the planar shape of the ridge 10 (FIG. 1) having the main diffraction grating 11 formed on the side surface, and the resist pattern 8 b matches the planar shape of the convex portion of the sub-diffraction grating 12.

Using the resist patterns 8 a and 8 b as a mask, the contact layer 7 and the upper cladding layer 6 are etched, and the etching is stopped on the upper surface of the upper light guide layer 5. This etching can be performed, for example, by dry etching using Cl ₂ as an etching gas.

  As shown in FIG. 4D, a ridge 10, a main diffraction grating 11, and a sub diffraction grating 12 having the same planar shape as the resist patterns 8a and 8b are formed. After the etching, the resist patterns 8a and 8b are removed.

  As shown in FIG. 1, the p-side electrode 14 is formed on the top surface of the ridge 10, and the n-side electrode 15 is formed on the bottom surface of the substrate 15. These electrodes are formed by, for example, a vacuum deposition method. The planar shape of the p-side electrode 14 can be defined by, for example, a lift-off method.

  FIG. 5 is a perspective view of a ridge type laser device according to a modification of the first embodiment. The following description will be made focusing on differences from the ridge type laser device according to the first embodiment shown in FIG. The description of the same configuration as that of the ridge type laser device according to the first embodiment is omitted.

  In the first embodiment, the lower light guide layer 3 is disposed between the lower cladding layer 2 and the active layer 4, but in the modification, the lower light guide layer 3 is not disposed. The lower cladding layer 2 and the active layer 4 are in contact with each other. In the first embodiment, the upper light guide layer 5 has a thickness of 0.15 μm. However, in the modification, the thickness is 0.1 μm.

An example of the light intensity distribution in a cross section perpendicular to the optical waveguide is shown on the left side of FIG. 6A, and the refractive index distribution in the thickness direction is shown on the right side. The upper light guide layer 5 and the upper cladding layer 6 are made of Al _0.3 Ga _0.7 As, and the lower cladding layer 2 is made of Al _0.5 Ga _0.5 As. For this reason, the refractive index of the lower cladding layer 2 is lower than the refractive indexes of the upper light guide layer 5 and the upper cladding layer 6. For comparison, FIG. 6B shows a light intensity distribution and a refractive index distribution when the lower clad layer 2a is formed of AlGaAs having the same composition ratio as the upper light guide layer 5 and the upper clad layer 6.

  Light tends to spread into a higher medium than a medium with a low refractive index. Therefore, in the modification of the first embodiment shown in FIG. 6A, the light intensity above the active layer 4 is higher than that in the comparative example shown in FIG. 6B. The main diffraction grating 11 is formed on the side surface of the ridge 10, and the sub-diffraction grating 12 is formed on the flat surfaces on both sides of the ridge 10. Thus, any diffraction grating is disposed above the active layer 4. For this reason, as shown in the modification of the first embodiment, the refractive index of the medium disposed on the substrate side relative to the active layer 4, specifically, the refractive index of the lower cladding layer 2 is set higher than that of the active layer 4. The coupling coefficient κ can be increased by making the refractive index smaller than the refractive index of the medium disposed in the above, specifically the upper light guide layer 5 and the upper cladding layer 6. For example, the coupling coefficient κ of the ridge type laser device of the modification of the first embodiment is about 1.3 to 1.6 times the coupling coefficient κ of the laser device of the comparative example shown in FIG. 6B. The upper light guide layer 5 may be omitted, and the ridge-shaped upper cladding layer 6 may be directly disposed on the active layer 4.

  By adopting the structure of the modified example of the first embodiment, the optical resonator length L can be changed while keeping the normalized coupling coefficient κL defined by the product of the coupling coefficient κ and the optical resonator length L constant. It can be shortened. When the optical resonator length L is shortened, the parasitic capacitance is reduced, and an improvement in high-speed modulation characteristics is expected. As an example, in the laser element shown in FIG. 6B, the optical resonator length L must be 250 μm or more in order to ensure a sufficiently large normalized coupling coefficient κL. In contrast, in the laser element according to the modification of the first embodiment shown in FIG. 6A, the optical resonator length can be shortened to about 200 μm.

  In the first embodiment shown in FIG. 1, the lower light made of AlGaAs having the same composition ratio as that of the upper light guide layer 5 and the upper cladding layer 6 is interposed between the active layer 4 and the lower cladding layer 2. A guide layer 3 is disposed. However, since the thickness of the lower light guide layer 3 is at most 0.15 μm, the light intensity distribution extends into the lower cladding layer 2. Since the refractive index of the lower clad layer 2 is smaller than the refractive indexes of the upper light guide layer 5 and the upper clad layer 6, the same effect as the modification of the first embodiment shown in FIG. 5 can be obtained. Let's go.

  The coupling coefficient κ can be increased by disposing a medium having a refractive index smaller than that of the upper optical cladding layer 6 constituting a part of the ridge 10 on the substrate side within a range in which the light intensity distribution extends toward the substrate 1 side. it can.

  In the first embodiment and its modifications, the medium on the substrate side of the active layer 4 is n-type conductive and the medium on the ridge 10 side is p-type conductive. May be p-type conductivity, and the medium on the ridge 10 side may be n-type conductivity. The resistance of the laser element can be lowered by making the ridge 10 an n-type conductivity that can be easily reduced in resistance. Thereby, the light output can be improved.

  In the first embodiment and its modification, a GaAs substrate is used, the cladding layer is formed of AlGaAs, and the active layer is formed of an InGaAs / GaAs quantum dot layer. However, other compound semiconductors may be used. is there.

  As an example, in the modification of the first embodiment shown in FIG. 5, the substrate 1 and the lower clad layer 2 are formed of p-type InP. In the active layer 4, a 6-nm thick quantum well layer made of undoped AlGaInAs and a 10-nm thick barrier layer made of undoped AlGaInAs are alternately laminated 10 times, and this alternate laminated structure is further made from undoped AlGaInAs. And a structure sandwiched between 20 nm thick guide layers. The composition wavelength of AlGaInAs constituting the barrier layer and the guide layer is 1050 nm. The upper light guide layer 5 and the upper cladding layer 6 are formed of n-type GaInAsP or AlGaInAs having a composition wavelength of 950 nm, and the contact layer 7 is formed of n-type GaInAs. The electrode 14 in contact with the n-type contact layer is an AuGe / Au laminate, and the electrode 15 in contact with the p-type substrate 1 is an AuZn / Au laminate. In this case, the period of the main diffraction grating 11 is 198 nm.

  FIG. 7 is a perspective view of a ridge type laser device according to the second embodiment. When the ridge type laser device according to the second embodiment is compared with the ridge type laser device according to the first embodiment, the configuration of the sub-diffraction grating is different and the other configurations are the same.

  In the first embodiment, the phases of the two sub-diffraction gratings 12 arranged on both sides of the ridge 10 are the same, but in the second embodiment, the sub-diffraction grating 12A arranged on one side of the ridge 10 and the other The phase with respect to the sub-diffraction grating 12 </ b> A arranged at is shifted by ½ of the period of the main diffraction grating 11. The period of each of the two sub-diffraction gratings 12 </ b> A is twice the period of the main diffraction grating 11. Further, the ratio (duty ratio) occupied by the convex portions within one period of the sub-diffraction grating 12A is 25%.

  FIG. 7 shows an example in which the convex portion constituting the sub-diffraction grating 12A is lower than the ridge 10, but the sub-diffraction grating 12A is constructed in the same manner as in the first embodiment shown in FIG. The convex portion to be formed may be the same height as the ridge 10.

  FIG. 8 schematically shows the phase relationship between the main diffraction grating 11 and the sub-diffraction grating 12A. A main diffraction grating 11 is disposed on the side surface of the ridge 10, and a sub-diffraction grating 12A is disposed on the outer side thereof. The hatched region in FIG. 8 corresponds to a region having a relatively high refractive index.

The wavelength of the guided light propagating through the optical waveguide is twice the period p ₁ of the main diffraction grating 11. The guided light LA diffracted by one sub-diffraction grating 12A and propagating in the reverse direction and the guided light LB diffracted by the other sub-diffraction grating 12A and propagating in the reverse direction are of the phase of the two sub-diffraction gratings 12A. The phase is shifted by twice the shift amount, that is, by the period p ₁ of the main diffraction grating 11. This phase shift amount is equal to ½ of the wavelength of the guided light. Therefore, the phase of the diffracted light from one sub-diffraction grating 12A and the diffracted light from the other sub-diffraction grating 12A are 180 ° out of phase. For this reason, the diffracted lights LA and LB weaken each other.

  The fundamental transverse mode of the guided light propagating through the optical waveguide is strongly coupled to the main diffraction grating 11, but hardly coupled to the sub diffraction grating 12A, and therefore hardly weakened by the sub diffraction grating 12A. On the other hand, the higher-order transverse mode is strongly coupled to the sub-diffraction grating 12A as compared with the basic transverse mode, so that the loss due to the sub-diffraction grating 12A increases.

  Accordingly, it is possible to suppress the oscillation in the high-order transverse mode and preferentially oscillate the basic transverse mode.

  The second embodiment utilizes the fact that the phases of the two diffracted lights that are diffracted by the two sub-diffraction gratings 12A and propagate in the propagation direction of the guided light in the optical waveguide are shifted from each other. For this reason, it is preferable to design the sub-diffraction grating 12A so that the diffraction intensity in the propagation direction of the guided light is increased. As indicated by the curve d in FIG. 3, when the duty ratio is 25% and 75%, the diffraction intensity in the propagation direction of the guided light becomes maximum. In order to increase the diffraction intensity in the propagation direction of the guided light, it is preferable to set the duty ratio of the sub-diffraction grating within the range of 15 to 35% or within the range of 65 to 85%.

  Also in the second embodiment, unlike the modification of the first embodiment shown in FIG. 5, the lower light guide layer 3 is not disposed, and the lower cladding layer 2 and the active layer 4 are directly in contact with each other. May be. Further, the conductivity type of the medium on the substrate side with respect to the active layer 4 and the conductivity type of the medium on the ridge side may be reversed. In the case of the second embodiment, the coupling coefficient κ can be increased as in the modification of the first embodiment.

  FIG. 9 is a perspective view of a ridge type laser device according to the third embodiment. When the ridge type laser device according to the third embodiment is compared with the ridge type laser device according to the first embodiment, the configuration of the sub-diffraction grating is different, and the other configurations are the same.

  The sub-diffraction grating 12B of the ridge type laser element according to the third embodiment has the same period as the main diffraction grating 11, and the phase of the main diffraction grating 11 and the phase of the sub-diffraction grating 12B are shifted by 180 °. 9 shows a configuration in which the convex portion constituting the sub-diffraction grating 12B is lower than the ridge 10, but, similarly to the case of the first embodiment shown in FIG. You may make it the same height.

  FIG. 10 schematically shows the phase relationship between the main diffraction grating 11 and the sub-diffraction grating 12B of the ridge type laser device according to the third embodiment. The main diffraction grating 11 is disposed on the side surface of the ridge 10, and the sub-diffraction grating 12B is disposed on the outer side thereof. The hatched region in FIG. 10 corresponds to a region having a relatively high refractive index.

  The guided light propagating through the optical waveguide is diffracted by the main diffraction grating 11 and the sub-diffraction grating 12B. The diffracted light L1 from the main diffraction grating 11 and the diffracted light L2 from the sub-diffraction grating 12B are out of phase by 180 °. For this reason, the sub-diffraction grating 12 </ b> B has a function of weakening diffracted light synchronized with the main diffraction grating 11.

  The fundamental transverse mode of the guided light propagating through the optical waveguide is strongly coupled to the main diffraction grating 11 and hardly coupled to the sub diffraction grating 12B, and is hardly weakened by the sub diffraction grating 12B. On the other hand, the high-order transverse mode is strongly coupled to both the main diffraction grating 11 and the sub-diffraction grating 12B, and thus is weakened by the sub-diffraction grating 12B.

  For this reason, it is possible to suppress oscillation of the high-order transverse mode and preferentially oscillate the basic transverse mode.

  Also in the third embodiment, unlike the modification of the first embodiment shown in FIG. 5, the lower light guide layer 3 is not disposed, and the lower cladding layer 2 and the active layer 4 are directly contacted. May be. Further, the conductivity type of the medium on the substrate side with respect to the active layer 4 and the conductivity type of the medium on the ridge side may be reversed. Also in the case of the third embodiment, the coupling coefficient κ can be increased as in the modification of the first embodiment.

  FIG. 11 is a perspective view of a ridge type laser device according to the fourth embodiment. On the main surface of the substrate 51 made of n-type InP, a main diffraction grating 60 composed of irregularities periodically arranged is formed. On the surface on which the main diffraction grating 60 is formed, an optical guide layer 52 made of n-type GaInAsP and having a thickness of 0.1 μm is formed.

  A quantum well active layer 53 is formed on the light guide layer 52. In the quantum well active layer 53, a 6 nm thick quantum well layer made of undoped AlGaInAs and a 10 nm thick barrier layer made of undoped AlGaInAs are alternately laminated 10 times, and this alternate laminated structure is further undoped. It has a structure sandwiched between 20 nm thick guide layers made of AlGaInAs. The composition wavelength of AlGaInAs constituting the barrier layer and the guide layer is 1050 nm.

  An upper cladding layer 54 made of p-type InP is formed on the quantum well active layer 53. The upper clad layer 54 includes a film-like portion covering the entire main surface of the substrate, a ridge 61 extending in the first direction, and a sub-diffraction grating 62 including a plurality of convex portions arranged on both sides of the ridge 61. Is done. The thickness of the film-like portion is 0.1 μm. The ridge 61 has a height of 1.2 μm and a width of 2 μm. The convex portions constituting the sub-diffraction grating 62 are periodically arranged in a direction parallel to the direction in which the main diffraction grating 60 has periodicity.

  A contact layer 55 made of p-type GaInAs is formed on the ridge 61. A p-side electrode 56 is formed on the contact layer 55, and an n-side electrode 57 is formed on the bottom surface of the substrate 51.

  The main diffraction grating 60 and the sub diffraction grating 62 have a periodic structure in which the refractive index periodically changes in the extending direction of the ridge 61. The period of the main diffraction grating 60 is 200 nm, and the height from the bottom surface of the concave portion to the top surface of the convex portion is 50 nm. The size of each recess in the direction having periodicity is 100 nm. That is, the interval between two recesses adjacent to each other is also 100 nm, and the duty ratio is 50%.

  The sub-diffraction grating 62 has a period of 400 nm, the height of each convex part is 100 nm, and the dimension of each convex part in a direction having periodicity is 100 nm. That is, the duty ratio of the sub-diffraction grating 62 is 25%. The duty ratio may be set in the range of 15 to 35% or in the range of 65 to 85%. The periodic structure of the sub-diffraction grating 62 disposed on one side of the ridge 61 and the periodic structure of the sub-diffraction grating 62 disposed on the other side have a phase of 100 nm, that is, the period of the main diffraction grating 60. It is shifted by 1/2.

  A region below the ridge 61 in the quantum well active layer 53 constitutes an optical waveguide that propagates light in the length direction of the ridge 61. The main diffraction grating 60 is disposed in the entire width direction of the optical waveguide. On the other hand, the sub-diffraction grating 62 is not disposed in a region overlapping the optical waveguide in the width direction of the optical waveguide, and is disposed only outside the optical waveguide. For this reason, the fundamental transverse mode of the guided light propagating through the optical waveguide is strongly coupled to the main diffraction grating 60, but hardly coupled to the sub-diffraction grating 62. On the other hand, the higher-order transverse mode is coupled to the main diffraction grating 60 and is also coupled to the sub-diffraction grating 62 more strongly than the fundamental transverse mode.

  Therefore, the sub-diffraction grating 62 of the ridge type laser element according to the fourth embodiment is similar to the sub-diffraction grating 12A of the ridge type laser element according to the second embodiment shown in FIGS. Increases mode propagation loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  Next, with reference to FIGS. 12A to 12E, a method for manufacturing a ridge type laser element according to the fourth embodiment will be described.

  As shown in FIG. 12A, a resist pattern 70 is formed on the main surface of a substrate 51 made of n-type InP by interference exposure or electron beam exposure. The resist pattern 70 corresponds to the periodic structure of the main diffraction grating 60. Using the resist pattern 70 as a mask, the surface layer portion of the substrate 51 is dry-etched. After the etching, the resist pattern 70 is peeled off.

  FIG. 12B shows the substrate 51 after the resist pattern 70 is removed. A main diffraction grating 60 is formed which is constituted by a concave portion formed in a region not covered with the resist pattern 70 and a convex portion left between the concave portions.

  As shown in FIG. 12C, on the main surface of the substrate 51 on which the main diffraction grating 60 is formed, the light guide layer 52 made of n-type GaInAsP, the quantum well active layer 53, the upper cladding layer 54 made of p-type InP, p A contact layer 55 made of type GaInAs is grown by, for example, metal organic chemical vapor deposition (MOVPD). A mask pattern 71 that matches the ridge 61 shown in FIG. 11 is formed of silicon oxide on the contact layer 55.

  As shown in FIG. 12D, using the mask pattern 71 as a mask, the contact layer 55 is etched by dry etching, and further etched to the middle of the upper cladding layer 54.

  As shown in FIG. 12E, on the flat surfaces on both sides of the mask pattern 71, a resist pattern 72 that matches the convex portions constituting the sub-diffraction grating 62 shown in FIG. 11 is formed using electron beam exposure. Using the mask pattern 71 and the resist pattern 72 as a mask, the upper cladding layer 54 is etched halfway by dry etching. After the upper cladding layer 54 is etched, the resist pattern 72 and the mask pattern 71 are removed.

  As shown in FIG. 11, a ridge 61 and a sub-diffraction grating 62 are formed. A p-side electrode 56 is formed on the contact layer 55 using a lift-off method. An n-side electrode 57 is formed on the bottom surface of the substrate 51.

  FIG. 13 is a perspective view of a ridge type laser device according to the fifth embodiment. When the ridge type laser device according to the fifth embodiment is compared with the ridge type laser device according to the fourth embodiment, the configuration of the sub-diffraction grating is different and the other configurations are the same.

  In the fifth embodiment, the phases of the two sub-diffraction gratings 62A arranged on both sides of the ridge 61 are aligned. The period of the sub-diffraction grating 62A is 400 nm, the height of each convex part constituting the sub-diffraction grating 62A is 100 nm, and the dimension of each convex part in the extending direction of the ridge 61 is 200 nm. That is, the duty ratio of the sub-diffraction grating 62A is 50%. Note that the duty ratio may be set within a range of 35 to 65%.

  Similar to the sub-diffraction grating 12 of the ridge type laser device according to the first embodiment shown in FIGS. 1 to 3, the sub-diffraction grating 62A of the ridge type laser device according to the fifth example propagates higher-order transverse modes. Increase loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  FIG. 14 is a perspective view of a ridge type laser device according to the sixth embodiment. When the ridge type laser device according to the sixth embodiment is compared with the ridge type laser device according to the fourth embodiment, the configuration of the sub-diffraction grating is different and the other configurations are the same.

  In the sixth embodiment, the phases of the two sub-diffraction gratings 62B arranged on both sides of the ridge 61 are aligned. The period of the sub-diffraction grating 62B is 200 nm, the height of each convex part constituting the sub-diffraction grating 62B is 100 nm, and the dimension of each convex part in the extending direction of the ridge 61 is 100 nm. That is, the duty ratio of the sub-diffraction grating 62B is 50%. In the sub-diffraction grating 62B, the periodic structure of the refractive index distribution is shifted by ½ period, that is, 100 nm, compared to the main diffraction grating 60.

  The sub-diffraction grating 62B of the ridge type laser device according to the sixth embodiment is similar to the sub-diffraction grating 12B of the ridge type laser device according to the third embodiment shown in FIGS. Increase loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  FIG. 15 is a perspective view of a ridge type laser device according to the seventh embodiment. A main diffraction grating 109 is formed on the main surface of the substrate 101 made of n-type InP. On the main surface of the substrate 101 on which the main diffraction grating 109 is formed, an optical guide layer 102 made of n-type GaInAsP and having a thickness of 0.1 μm is formed. A quantum well active layer 103 is formed on the light guide layer 102. The quantum well active layer 103 has the same structure as the quantum well active layer 53 of the ridge type laser device according to the fourth embodiment shown in FIG.

  On the quantum well active layer 103, an upper light guide layer 104 made of p-type GaInAsP and having a thickness of 0.1 μm is formed. A sub-diffraction grating 110 is formed on the upper surface of the upper light guide layer 104. An upper cladding layer 105 made of p-type InP is formed thereon. The upper clad layer 105 includes a film-like portion that covers the entire surface of the substrate, and a ridge 111 that is formed thereon and extends in one direction. The thickness of the film-like portion is 0.1 μm, the height of the ridge 111 is 1.2 μm, and the width is 2 μm.

  A contact layer 106 made of p-type GaInAs is formed on the ridge 111. A p-side electrode 107 is formed on the contact layer 106, and an n-side electrode 108 is formed on the bottom surface of the substrate 101.

  The main diffraction grating 109 and the sub diffraction grating 110 have a periodic structure in which the refractive index periodically changes in the direction in which the ridge 111 extends. The main diffraction grating 109 is disposed immediately below the ridge 111 and in regions on both sides thereof. The sub-diffraction grating 110 is disposed in the regions on both sides of the ridge 111, but is not disposed directly under the ridge 111.

  The period of the main diffraction grating 109 is 200 nm, the height from the bottom surface of the concave portion constituting the main diffraction grating 109 to the upper surface of the convex portion is 50 nm, and the duty ratio is 50%. The period of the sub-diffraction grating 110 is 400 nm, the height of each convex portion constituting the sub-diffraction grating 110 is 100 nm, and the dimension in the extending direction of the ridge 111 is 200 nm. That is, the duty ratio is 50%. Note that the duty ratio of the sub-diffraction grating 110 may be set within a range of 35 to 65%.

  The quantum well active layer 103 below the ridge 111 acts as an optical waveguide for propagating guided light in the direction in which the ridge 111 extends. Since the main diffraction grating 109 is disposed immediately below the ridge 111, the fundamental transverse mode of the guided light propagating through the optical waveguide is strongly coupled to the main diffraction grating 109. However, since the sub-diffraction grating 110 is not disposed immediately below the ridge 111, the coupling between the fundamental transverse mode and the sub-diffraction grating 110 is weak. The higher-order transverse mode of guided light is strongly coupled to both the main diffraction grating 109 and the sub-diffraction grating 110.

  The sub-diffraction grating 110 of the ridge type laser device according to the seventh embodiment is similar to the sub-diffraction grating 12 of the ridge type laser device according to the first embodiment shown in FIGS. Increase loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  A method for manufacturing a ridge type laser device according to the seventh embodiment will be described with reference to FIGS. 16A to 16E.

  As shown in FIG. 16A, a resist pattern 120 is formed on the main surface of a substrate 101 made of n-type InP by interference exposure or electron beam exposure. The resist pattern 120 has a planar shape that matches the convex portions constituting the main diffraction grating 109 shown in FIG. Using the resist pattern 120 as a mask, the surface layer portion of the substrate 101 is etched by dry etching. After the etching, the resist pattern 120 is peeled off.

  As shown in FIG. 16B, a main diffraction grating 109 is formed on the main surface of the substrate 101 in which concave portions formed in regions not covered with the resist pattern 120 and convex portions left between the concave portions are alternately arranged. Is done.

  As shown in FIG. 16C, on the main surface of the substrate 101 on which the main diffraction grating 109 is formed, the lower light guide layer 102 made of n-type GaInAsP, the quantum well active layer 103, and the upper light guide made of p-type GaInAsP. Layer 104 is grown sequentially by MOVPE. On the upper light guide layer 104, a resist pattern 121 having a planar shape aligned with the convex portions constituting the sub-diffraction grating 110 shown in FIG. 15 is formed. The resist pattern 121 can be formed by combining interference exposure or electron beam exposure and normal ultraviolet exposure. Using the resist pattern 121 as a mask, the surface layer portion of the upper light guide layer 104 is etched. After the etching, the resist pattern 121 is peeled off.

  As shown in FIG. 16D, the sub-diffraction grating 110 is formed on the surface of the upper light guide layer 104.

  As shown in FIG. 16E, an upper cladding layer 105 made of p-type InP and a contact layer 106 made of p-type GaInAs are grown by MOVPE on the surface of the upper light guide layer 104 on which the sub-diffraction grating 110 is formed. A mask pattern 122 made of silicon oxide is formed on the contact layer 106. The mask pattern 122 has a planar shape that matches the ridge 111 shown in FIG.

  Using the mask pattern 122 as a mask, the contact layer 106 is etched by dry etching, and the upper cladding layer 105 is further etched halfway. After the etching, the mask pattern 122 is removed.

  As shown in FIG. 15, the ridge 111 is formed, and the contact layer 106 remains on it. A p-side electrode 107 is formed on the contact layer 106 using a lift-off method, and an n-side electrode 108 is formed on the bottom surface of the substrate 101.

  FIG. 17 is a perspective view of a ridge type laser device according to the eighth embodiment. When the ridge type laser device according to the eighth embodiment is compared with the ridge type laser device according to the seventh embodiment, the configuration of the sub-diffraction grating is different and the other configurations are the same.

  In the eighth embodiment, the period of the two sub-diffraction gratings 110A arranged on both sides of the ridge 111 is 400 nm, and the duty ratio is 25%. The duty ratio may be set in the range of 15 to 35% or in the range of 65 to 85%. Further, the phases of the two sub-diffraction gratings 110A are shifted from each other by ½ of the period of the main diffraction grating 109, that is, 100 nm.

  Similarly to the sub-diffraction grating 12A of the ridge type laser device according to the second embodiment shown in FIGS. 7 and 8, the sub-diffraction grating 110A of the ridge type laser device according to the eighth example propagates higher-order transverse modes. Increase loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  FIG. 18 is a perspective view of a ridge type laser device according to the ninth embodiment. When the ridge type laser device according to the ninth embodiment is compared with the ridge type laser device according to the seventh embodiment, the configuration of the sub-diffraction grating is different and the other configurations are the same.

  In the ninth embodiment, the period of the two sub-diffraction gratings 110B arranged on both sides of the ridge 111 is 200 nm, and the duty ratio is 50%. Further, in the sub-diffraction grating 110B, the periodic structure of the refractive index distribution is shifted from the main diffraction grating 109 by a half period, that is, 100 nm.

  Similarly to the sub-diffraction grating 12B of the ridge type laser device according to the third embodiment shown in FIGS. 9 and 10, the sub-diffraction grating 110B of the ridge type laser device according to the ninth example propagates higher-order transverse modes. Increase loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  In the seventh to ninth embodiments, the main diffraction grating 109, the quantum well active layer 103, and the sub-diffraction grating 110 are arranged in this order from the substrate side, but these three components are arranged in the other order. You may arrange in. For example, the sub-diffraction grating, the quantum well active layer, and the main diffraction grating may be arranged in this order, or both the main diffraction grating and the sub-diffraction grating may be disposed on the substrate side of the quantum well active layer, Both the main diffraction grating and the sub-diffraction grating may be disposed above the quantum well active layer.

  Next, with reference to FIGS. 19A to 19F, description will be given of a method for manufacturing an embedded laser device according to the tenth embodiment.

  As shown in FIG. 19A, a quantum well active layer 152 is formed on the main surface of a substrate 151 made of p-type InP. The quantum well active layer 152 has the same structure as the quantum well active layer 53 of the ridge type laser device according to the fourth embodiment shown in FIG. On the quantum well active layer 152, an upper optical guide layer 153 made of p-type GaInAsP and having a thickness of 0.1 μm is formed by MOVPE. A resist pattern 180 for forming a main diffraction grating is formed on the upper light guide layer 153 by interference exposure or electron beam exposure. The resist pattern 180 has a striped planar shape in which a plurality of band-like patterns having a width of 100 nm are arranged at equal intervals of 100 nm.

  Using the resist pattern 180 as a mask, the surface layer portion of the upper light guide layer 153 is etched. After the etching, the resist pattern 180 is peeled off.

  As shown in FIG. 19B, the main diffraction grating 170 is formed on the surface of the upper light guide layer 153. The main diffraction grating 170 is constituted by a recess having a depth of 50 nm formed in a region not covered with the resist pattern 180 and a protrusion remaining therebetween. The period of the main diffraction grating 170 is 200 nm, and the duty ratio is 50%.

  As shown in FIG. 19C, an upper cladding layer 154 made of p-type InP and having a thickness of 0.25 μm is formed by MOVPE on the upper light guide layer 153 on which the main diffraction grating 170 is formed.

  A mask pattern 181 made of silicon oxide is formed on the upper cladding layer 154. The mask pattern 181 extends in a direction parallel to the direction having the periodicity of the main diffraction grating 170, and a periodic uneven pattern for forming a sub-diffraction grating is provided on both sides of the mask pattern 181. The period of the concavo-convex pattern is 400 nm, the height from the bottom of the concave portion to the tip of the convex portion is 250 nm, and the duty ratio is 50%. Note that the duty ratio may be set within a range of 35 to 65%. In addition, a convex portion on the other edge is disposed at a position corresponding to the convex portion on one edge.

  Using the mask pattern 181 as a mask, the upper cladding layer 154, the upper light guide layer 153, and the quantum well active layer 152 are etched by dry etching, and the surface layer portion of the substrate 151 is further etched.

  As shown in FIG. 19D, a ridge 172 in which the quantum well active layer 152, the upper light guide layer 153, and the upper cladding layer 154 are stacked remains on the substrate 151. A sub-diffraction grating 171 having a periodic structure in which convex portions and concave portions extending in the height direction of the ridge 172 are alternately arranged in the length direction of the ridge 172 is formed on the side surface of the ridge 172.

  As shown in FIG. 19E, on the flat surfaces on both sides of the ridge 172, a first current confinement layer 160 made of p-side InP, a second current confinement layer 161 made of n-type InP, and a third current made of p-type InP. The constriction layer 162 is grown sequentially and selectively by MOVPE. Thereafter, the mask pattern 181 is removed.

  As shown in FIG. 19F, an upper clad layer 165 made of p-type InP and a contact layer 166 made of p-type GaInAs are formed on the upper clad layer 154 and the third current confinement layer 162 which are the uppermost layers of the ridge 172 by MOVPE. To grow in order. A p-side electrode 168 is formed on the contact layer 166 by a lift-off method, and an n-side electrode 169 is formed on the bottom surface of the substrate 151.

  The quantum well active layer 152 disposed in the ridge 172 functions as an optical waveguide that propagates guided light in the length direction of the ridge 172.

  In the embedded laser device according to the tenth embodiment, as shown in FIG. 19D, the main diffraction grating 170 is arranged over the entire region in the width direction of the quantum well active layer 152 serving as an optical waveguide. On the other hand, the sub-diffraction grating 171 is not disposed at the center portion in the width direction of the optical waveguide, but is disposed only on both sides of the optical waveguide. Therefore, the fundamental transverse mode of the guided light propagating through the optical waveguide is strongly coupled to the main diffraction grating 170, and the coupling between the fundamental transverse mode and the sub-diffraction grating 171 is weak. In contrast, the high-order transverse mode is strongly coupled to both the main diffraction grating 170 and the sub-diffraction grating 171. The main diffraction grating 170 does not need to be disposed in the entire region in the width direction of the optical waveguide, and may be disposed in a region including the center in the width direction.

  The sub-diffraction grating 171 of the embedded laser element according to the tenth embodiment is similar to the sub-diffraction grating 12 of the ridge laser element according to the first embodiment shown in FIGS. Increase loss. In the tenth embodiment, the high-order transverse mode is diffracted in the transverse direction parallel to the substrate surface. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  FIG. 20 is a perspective view of an embedded laser device according to the eleventh embodiment in the middle of manufacturing. The buried laser device according to the eleventh embodiment is obtained by changing the structure of the sub-diffraction grating 171 of the buried laser device according to the tenth embodiment shown in FIG. 19D.

  In the eleventh embodiment, the phases of the two sub-diffraction gratings 171A formed on both sides of the ridge 172 are shifted from each other by ½ of the period of the main diffraction grating 170, that is, 100 nm. The period of the sub-diffraction grating 171A is 400 nm, and the duty ratio is in the range of 15 to 35%, or in the range of 65 to 85%.

  The sub-diffraction grating 171A of the buried laser element according to the eleventh embodiment is similar to the sub-diffraction grating 12A of the ridge laser element according to the second embodiment shown in FIGS. Increase mode loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  FIG. 21 is a perspective view of an embedded laser device according to the twelfth embodiment in the middle of manufacturing. The buried laser device according to the twelfth embodiment is obtained by changing the structure of the sub-diffraction grating 171 of the buried laser device according to the tenth embodiment shown in FIG. 19D.

  The sub-diffraction grating 171B of the twelfth embodiment has the same period as the main diffraction grating 170, and the periodic structure of the refractive index distribution is shifted from the main diffraction grating 170 by a half period, that is, 100 nm. The duty ratio of the sub-diffraction grating 171B is in the range of 35 to 65%.

  The sub-diffraction grating 171B of the embedded laser element according to the twelfth embodiment is similar to the sub-diffraction grating 12C of the ridge laser element according to the third embodiment shown in FIGS. Increase mode loss. For this reason, it is possible to suppress oscillation in the high-order transverse mode.

  In the tenth to twelfth embodiments, the main diffraction grating is disposed on the quantum well active layer 152. However, it is also possible to dispose the main diffraction grating below the quantum well active layer.

  In any of the first to twelfth embodiments, the main diffraction grating and the sub-diffraction grating are compared with the propagation of the second-order transverse mode of the guided light propagating through the optical waveguide when only the main diffraction grating is arranged. When both are arranged, the main diffraction grating and the sub-diffraction grating are coupled with the guided light so that the propagation of the second-order transverse mode of the guided light propagating through the optical waveguide is suppressed.

  FIG. 22 is a perspective view of a ridge type laser device according to the thirteenth embodiment. In the following, description will be made focusing on differences from the ridge type laser device according to the modification of the first embodiment shown in FIG.

  In the modification of the first embodiment, the sub-diffraction grating 12 is formed on the flat surfaces on both sides of the ridge 10, but in the thirteenth embodiment, the sub-diffraction grating 12 is not formed. Other configurations are the same as those of the ridge type laser device according to the modification of the first embodiment. The ridge type laser device according to the thirteenth embodiment can be manufactured by forming only the resist pattern 8a for the main diffraction grating without forming the resist pattern 8b for the sub-diffraction grating shown in FIG. 4C.

  Also in the thirteenth embodiment, the coupling coefficient κ between the guided light and the main diffraction grating 11 can be increased as in the case of the modification of the first embodiment. The upper light guide layer 5 may be omitted, and the upper cladding layer 6 may be directly disposed on the active layer 4.

  In the thirteenth embodiment, since the sub-diffraction grating is not disposed, higher-order transverse mode oscillation is more likely to occur than in the modification of the first embodiment. When high-order transverse mode oscillation becomes a problem in operation, it is possible to suppress high-order transverse mode oscillation by narrowing the width of the ridge 10.

  FIG. 23 shows a schematic sectional view of a laser device according to the fourteenth embodiment. The laser elements according to the first to twelfth embodiments are distributed feedback (DFB) laser elements, whereas the laser elements according to the fourteenth embodiment are distributed reflection (DBR) laser elements.

  The DBR laser device according to the fourteenth embodiment has a structure in which Bragg reflection regions 201 are arranged on both sides of the amplification region 200 in the propagation direction of the guided light. Carriers are injected into the amplification region 200 from the electrodes 202 and 203. In the Bragg reflection area 201, a diffraction grating 205 for reflecting the guided light is formed. The diffraction grating 205 includes a main diffraction grating and a sub diffraction grating of the laser element according to the first to twelfth embodiments.

  Since the diffraction grating 205 in the Bragg reflection region 201 increases the loss of the high-order transverse mode of the guided light, oscillation of the high-order transverse mode can be suppressed.

  In the fourteenth embodiment, similarly to the diffraction gratings of the ridge-type laser elements according to the first to third and thirteenth embodiments, when the diffraction grating is disposed above the waveguide layer, the Bragg reflection region The refractive index of the medium disposed closer to the substrate than the waveguide layer 201 (the same layer as the active layer of the amplification region 200) is greater than the refractive index of the medium forming the ridge disposed above the active layer 4. It is preferable to make it small. With such a refractive index distribution, the coupling coefficient κ between the guided light and the diffraction grating is similar to the modification of the first embodiment shown in FIG. 5 and the thirteenth embodiment shown in FIG. Can be increased.

  In the laser device according to the above embodiment, GaAs, AlGaAs, InGaAs, InAs, InP, AlGaInAs, GaInAsP, etc. are used as materials for the substrate, cladding layer, light guide layer, active layer, etc., but other compound semiconductors are used. Is also possible. For example, in the first to third embodiments, an active layer including InAs quantum dots is formed on a GaAs substrate. In the fourth to twelfth embodiments, the quantum well active layer made of AlGaInAs is formed on the InP substrate. A quantum well active layer may be formed on a GaAs substrate, or a quantum dot active layer may be formed on an InP substrate.

  Moreover, in the said Example, although the sub-diffraction grating was formed with the semiconductor, you may form with another material. For example, you may form with metals, such as Cr.

  In the embodiment using the n-type substrate, a p-type substrate may be used instead of the n-type substrate. When a p-type substrate is used, the light guide layer, the clad layer and the like disposed on the active layer may be n-type. Further, a semi-insulating substrate may be used, or a substrate obtained by bonding a base substrate made of a desired material on a silicon substrate may be used.

  Further, the field of application of the main diffraction grating and the sub-diffraction grating employed in the elements according to the first to twelfth embodiments is not limited to laser elements. An optical waveguide having a main diffraction grating and a sub-diffraction grating has a function of increasing the loss of the higher-order transverse mode of guided light coupled to the main diffraction grating and preferentially propagating the fundamental transverse mode.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

  The invention shown in the following supplementary notes is derived from the above embodiments.

(Appendix 1)
An optical waveguide for propagating the laser beam;
A main diffraction grating coupled with guided light propagating through the optical waveguide;
A sub-diffraction grating coupled to the guided light propagating through the optical waveguide, and when only the main diffraction grating is arranged, compared to the propagation of the second-order transverse mode of the guided light propagating through the optical waveguide, When both the main diffraction grating and the sub diffraction grating are arranged, the main diffraction grating and the sub diffraction grating are guided light so that propagation of the second-order transverse mode of the guided light propagating through the optical waveguide is suppressed. Optical element combined with

(Appendix 2)
The intensity of the second-order transverse mode light of the guided light that is coupled to the main diffraction grating and propagates through the optical waveguide is diffracted by the sub-diffraction grating and is recombined with the optical waveguide. The optical element according to supplementary note 1, wherein the light is diffracted by the sub-diffraction grating and is weaker than the intensity of recombination with the optical waveguide.

(Appendix 3)
The coupling coefficient between the fundamental transverse mode of the guided light and the sub-diffraction grating is smaller than the coupling coefficient between the second-order transverse mode of the guided light and the sub-diffraction grating, and the period of the sub-diffraction grating is the main diffraction grating. The optical element according to supplementary note 2, which is 1.2 times or more of the period of

(Appendix 4)
The sub-diffraction grating has a structure in which first regions having a first refractive index and second regions having a second refractive index are alternately arranged in the propagation direction of guided light. The optical element according to appendix 3, wherein a ratio of the first region in one period of the diffraction grating is 35% or more and 65% or less.

(Appendix 5)
The coupling coefficient between the fundamental transverse mode of the guided light and the sub-diffraction grating is smaller than the coupling coefficient between the secondary transverse mode and the sub-diffraction grating;
The period of the sub-diffraction grating is twice the period of the main diffraction grating, and the sub-diffraction grating has a first periodic structure portion having a periodic refractive index distribution with respect to the propagation direction of the guided light and the second The optical element according to appendix 1, wherein the phase of the first periodic structure portion and the second periodic structure portion is shifted from each other by a half of the period of the main diffraction grating. .

(Appendix 6)
The first periodic structure portion and the second periodic structure portion of the sub-diffraction grating have a structure in which two types of regions having different refractive indexes are alternately arranged in the propagation direction of the guided light. The optical element according to appendix 5, wherein a ratio of one region in one period of the second periodic structure is 15% to 35% or 65% to 85%.

(Appendix 7)
The coupling coefficient between the fundamental transverse mode of the guided light and the sub-diffraction grating is smaller than the coupling coefficient between the secondary transverse mode and the sub-diffraction grating;
The periods of the main diffraction grating and the sub diffraction grating are the same, and the main diffraction grating and the sub diffraction grating have a periodic refractive index distribution with respect to the propagation direction of the guided light, and the refractive index distribution of the sub diffraction grating is The optical element according to appendix 1, wherein the phase is shifted by 180 ° with respect to the refractive index distribution of the main diffraction grating.

(Appendix 8)
The optical waveguide is formed of a semiconductor;
Furthermore, the optical element in any one of appendix 1-7 which has an electrode which supplies an electron and a hole to the said optical waveguide.

(Appendix 9)
further,
A substrate in which an active layer made of a semiconductor is embedded in a surface layer portion;
A ridge provided on a partial region of the surface of the substrate,
A region below the ridge of the active layer is the optical waveguide, the main diffraction grating is formed on a side surface of the ridge, and the sub-diffraction grating is formed on the substrate surface on both sides of the ridge. The optical element according to appendix 8.

(Appendix 10)
further,
A substrate in which an active layer made of a semiconductor is embedded in a surface layer portion;
A ridge provided on a partial region of the surface of the substrate,
The region below the ridge of the active layer is the optical waveguide, the main diffraction grating is embedded in the entire surface of the substrate, and the sub-diffraction grating is formed on the substrate surface on both sides of the ridge. The optical element according to appendix 8.

(Appendix 11)
further,
A substrate in which an active layer made of a semiconductor is embedded in a surface layer portion;
A ridge provided on a partial region of the surface of the substrate,
The region below the ridge of the active layer becomes the optical waveguide, the main diffraction grating is embedded in the entire surface of the substrate, and the sub-diffraction grating is not disposed directly under the ridge, The optical element according to appendix 8, embedded in regions on both sides of the ridge of the substrate.

(Appendix 12)
further,
A semiconductor substrate having a ridge formed on the surface;
A flat surface of the semiconductor substrate on both sides of the ridge, and a buried layer covering the side surface of the ridge,
Item 8. The supplementary note 8, wherein the optical waveguide is disposed in the ridge, the main diffraction grating is disposed in a region including a center in the width direction of the ridge, and the sub-diffraction grating is formed on a side surface of the ridge. Optical element.

(Appendix 13)
A semiconductor substrate;
A lower cladding layer formed on the semiconductor substrate;
An optical waveguide layer made of a medium having a refractive index higher than that of the lower cladding layer;
A ridge-shaped upper cladding layer formed in a medium having a refractive index higher than that of the lower cladding layer and extending in one direction on a partial region of the optical waveguide layer;
An optical element having a diffraction grating disposed above the optical waveguide layer and having periodicity in a direction in which a ridge formed of the upper cladding layer extends.

(Appendix 14)
14. The optical element according to appendix 13, wherein the diffraction grating is formed on at least one of the side surfaces on both sides and the flat surfaces on both sides of the upper cladding layer.

(Appendix 15)
The optical waveguide layer is formed of a semiconductor;
Furthermore, the optical element of Additional remark 13 or 14 which has an electrode which supplies an electron and a hole to the area | region below the said upper cladding layer among the said optical waveguide layers.

(Appendix 16)
The optical element according to supplementary note 15, wherein the semiconductor substrate and the lower cladding layer are formed of a p-type compound semiconductor, and the upper cladding layer is formed of an n-type compound semiconductor.

(Appendix 17)
The diffraction grating includes a main diffraction grating and a sub-diffraction grating coupled to the guided light propagating through the optical waveguide, and when only the main diffraction grating is arranged, the secondary transverse of the guided light propagating through the optical waveguide is disposed. Compared with mode propagation, when both the main diffraction grating and the sub-diffraction grating are disposed, the propagation of the secondary transverse mode of the guided light propagating through the optical waveguide is suppressed. And the sub-diffraction grating is coupled to the guided light.

1, 51, 101, 151 substrate
2 Lower cladding layer
3, 102 Lower light guide layer
4 Quantum dot active layer (optical waveguide layer)
5, 104, 153 Upper light guide layer
6, 105, 154, 165 Upper cladding layer
7, 55, 106, 166 Contact layer
8 resist film
10, 61, 111, 172 Ridge
11, 60, 109, 170 Main diffraction grating
12, 12A, 12B, 62, 62A, 62B, 110, 110A, 110B, 171, 171A, 171B Sub-diffraction grating
14, 56, 107, 168 Upper electrode
15, 57, 108, 169 Lower electrode
20 Light intensity distribution in the fundamental transverse mode
21 Light intensity distribution of secondary transverse mode
52 Light guide layer
53, 103, 152 Quantum well active layer
54 Upper cladding layer
70, 72, 120, 121, 180 resist pattern
71, 122, 181, 181A, 181B Mask pattern
160 First current confinement layer
161 Second current confinement layer
162 Third current confinement layer
200 amplification region
201 Bragg reflection area
202, 203 electrodes
205 diffraction grating

Claims (3)

An optical waveguide for propagating the laser beam;
A main diffraction grating coupled with guided light propagating through the optical waveguide;
A sub-diffraction grating coupled to the guided light propagating through the optical waveguide, and when only the main diffraction grating is arranged, compared to the propagation of the second-order transverse mode of the guided light propagating through the optical waveguide, When both the main diffraction grating and the sub diffraction grating are arranged, the main diffraction grating and the sub diffraction grating are guided light so that propagation of the second-order transverse mode of the guided light propagating through the optical waveguide is suppressed. And
The coupling coefficient between the fundamental transverse mode of the guided light and the sub-diffraction grating is smaller than the coupling coefficient between the secondary transverse mode and the sub-diffraction grating;
The periods of the main diffraction grating and the sub diffraction grating are the same, and the main diffraction grating and the sub diffraction grating have a periodic refractive index distribution with respect to the propagation direction of the guided light, and the refractive index distribution of the sub diffraction grating is An optical element whose phase is shifted by 180 ° with respect to the refractive index distribution of the main diffraction grating. The optical waveguide is formed of a semiconductor;
The optical device according to claim 1, further comprising an electrode for supplying electrons and holes to the optical waveguide. further,
A semiconductor substrate;
A lower cladding layer formed on the semiconductor substrate;
An optical waveguide layer made of a medium having a refractive index higher than that of the lower cladding layer;
A ridge-shaped upper cladding layer formed in a medium having a refractive index higher than that of the lower cladding layer and extending in one direction on a partial region of the optical waveguide layer;
The optical waveguide is defined in a region below the upper cladding layer in the optical waveguide layer,
The optical element according to claim 1, wherein the main diffraction grating and the sub diffraction grating are disposed above the optical waveguide layer.

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