JP2015103716A - Direct modulation laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a direct modulation laser array capable of directly coupling with a 7-cores multiple core fiber.SOLUTION: A direct modulation laser comprises: a first mirror which has an n-InP layer 402 formed on an InP substrate, an active layer 403 formed on the n-InP layer, and a p-InP layer 405 formed on the active layer, and is formed on the InP substrate, and which emits a first laser light horizontally emitted from a first edge of a resonator to the substrate, to a first surface side where the direct modulation laser is formed; a second mirror which emits a second laser light horizontally emitted from a second edge of the resonator to the substrate, to a second surface side at a side opposite to the first surface of the InP substrate; a lens which is formed by processing the InP layer formed on the surface of the direct modulation laser and in which a P lens is formed at a part where the first laser reflected by the first mirror reaches; and an AR coat formed on the surface of the lens and the second surface of the InP substrate.

Description

  The present invention relates to a direct modulation laser array that can be directly coupled to a multicore fiber, and more particularly to a laser element portion of a laser array that couples laser light to a seven-core multicore fiber with a small loss.

  In general, when laser light emitted from a semiconductor laser is coupled to an optical fiber, it is necessary to condense the electric field intensity distribution of the laser light onto the optical fiber using a coupling lens. The same applies to multicore fibers and 7-core multicore fibers. Since an ordinary semiconductor laser has a NA (numerical aperture) that is greatly different from that of an optical fiber, in order to efficiently couple the laser beam to the optical fiber, the emitted light is collected into a multi-core fiber using a coupling lens having an NA of about 0.6. I have to shine. However, the NA0.6 coupling lens diameter is much larger than the diameter of the multi-core fiber (125 μm), and even larger for the core pitch (40 · m). For this reason, it is impossible to arrange lenses corresponding to the number of cores of a multicore fiber on the fiber side, and it is not possible to optically couple light emitted from a semiconductor laser to all seven cores of a multicore fiber using a coupling lens. It was possible.

  As a report of an element in which a lens is integrated on the same substrate as the semiconductor laser and the optical fiber can be coupled without the above-described coupling lens, there is a report in which an InP lens is integrated on the back surface of the EA / DFB laser. (Non-Patent Document 1).

  FIG. 1 is a diagram showing an EA / DFB laser described in Non-Patent Document 1. FIG. 1A is a cross-sectional view of a laser element of the EA / DFB laser, and FIG. 1B is formed by an EA / DFB laser. It is a top view of the manufactured substrate. In the EA / DFB laser 100 shown in FIG. 1, the rear end face of the laser element is formed by cleavage, and the cleavage face 110 is coated with HR. The emitted light from the DFB laser is extracted to the substrate 113 side using the 45 ° mirror 112 and is collected by the InP lens 111 formed on the back surface (substrate side) of the resonator 114.

S. Makino el al, OFC2012, OTh3F2, “A 40-Gbit / s MMF Transmission with 1.3-μm Lens-integrated EA / DBA Lasers for Optical Interconnect.

  The EA / DFB laser 100 of FIG. 1 has a laser pitch of 250 μm and the InP lens 111 has a diameter of about 100 μm.

  When the laser beam propagates, the electric field distribution of the laser beam spreads according to the propagation distance, but the InP lens collects this spread light, so it is set equal to or larger than the diameter of the electric field distribution of the laser beam. There must be. Therefore, when the propagation distance of the laser beam is increased, the electric field distribution is widened, and the diameter of the InP lens necessary for condensing is also increased. Here, in the configuration of the EA / DFB laser 100, after the light emitted from the DFB laser is reflected by the 45 ° mirror 112, light is emitted by the thickness of the substrate 113 in order to reach the InP lens 111. Since it must be further propagated, the electric field distribution is further widened before reaching the InP lens 111. In calculation, light having an electric field distribution with a diameter of 2 μm spreads to a diameter of 45 to 50 μm when propagated by 150 μm, which is equal to the thickness of a general polished InP substrate. Therefore, the diameter of the InP lens 111 of the EA / DFB laser 100 must be designed to be larger than this value.

  Since the core interval of the 7-core multicore fiber is generally 40 μm, considering that the laser beam emitted from the EA / DFB laser is coupled to the multicore fiber, the diameter of the InP lens needs to be 40 μm or less. There is. However, in the configuration of the EA / DFB laser 100 described in Non-Patent Document 1, the diameter of the InP lens 111 cannot be made 40 μm or less. Therefore, even if the laser elements are arranged in an array, the core of the 7-core multicore fiber is used. InP lenses cannot be integrated at an interval of 40 μm.

  In order to achieve such an object, the direct modulation laser of the present invention includes an n-InP layer formed on an InP substrate, an active layer formed on the n-InP layer, A p-InP layer formed on the active layer, and the first laser beam emitted in a horizontal direction with respect to the substrate from the first end of the resonator of the direct modulation laser is reflected by the p A first mirror that emits to the InP layer, and a second laser beam that is emitted from the second end of the resonator in the horizontal direction with respect to the substrate to the InP substrate side by reflection. And a lens formed by processing the p-InP layer, wherein the InP lens is formed at a portion where the first laser reflected by the first mirror reaches The surface of the lens and the front of the InP substrate. Characterized in that it comprises a AR coat applied to the second surface.

  Further, the direct modulation laser of the present invention has a horizontal distance of 30 μm between the first end of the resonator and the first mirror on the InP substrate, and the first mirror and the lens. The distance from the surface in the direction perpendicular to the InP substrate is 2 μm to 8 μm, the diameter of the lens is 7 μm to 15 μm, and the sag amount of the lens is 0.5 μm to 1.6 μm. .

  Further, the direct modulation laser of the present invention is characterized in that it is formed at seven locations on the InP substrate.

  Further, the lens of the direct modulation laser of the present invention is characterized in that it is arranged at the same interval as the interval of the cores of the multi-core fiber to which the first laser beam is coupled.

  The multi-core fiber to which the first laser beam of the direct modulation laser according to the present invention is coupled has seven cores.

  As described above, according to the present invention, the propagation distance of laser light can be shortened and the electric field distribution can be adjusted, so that the diameter of the InP lens can be made smaller than the core interval of the 7-core multicore fiber. Seven InP lenses can be integrated on the same semiconductor substrate. Therefore, it becomes possible to couple the laser beam to all the cores of the seven-core multicore fiber with a small loss.

It is a figure which shows the structure of the EADFB laser of a nonpatent literature 1. It is a block diagram which shows the structure of the direct modulation laser array concerning one Embodiment of this invention. FIG. 3 is a cross-sectional view of a seven-core multicore fiber coupled with the direct modulation laser array described in FIG. 2. It is a figure which shows the structure of one laser element of the direct modulation laser array described in FIG. It is a chart showing the relationship between the propagation distance of a laser beam and the diameter of the electric field strength distribution after propagation. FIG. 5 is a diagram showing the spread and reflection of the electric field intensity distribution of the first laser beam at the first 45 ° mirror position of the laser element shown in FIG. 4. It is a graph which shows the lens sag amount with respect to each lens diameter, and the spread angle of the electric field strength distribution at that time. It is a graph which shows the relationship between the lens diameter of an InP lens and lens sag amount for obtaining the divergence angle of 8.05 degrees. FIG. 5 is a schematic diagram showing the relationship among the allowable lens radius r of the InP lens, the lens sag amount t, and the longitudinal propagation distance y of the first laser light in the laser element of FIG. 4. It is sectional drawing of one laser element of a direct modulation laser array. It is a top view of one laser element of a direct modulation laser array.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 2 is a top perspective view showing the configuration of the direct modulation laser array 200 according to one embodiment of the present invention. In the direct modulation laser array 200 of FIG. 2, seven laser elements 210 are integrated on an SI substrate 201. The laser element 210 includes a resonator 211, a first 45 ° mirror 212 that reflects the first laser light emitted from the first end 211-1 of the resonator 211, and a first 45 ° mirror 212. An InP lens 213 that condenses the first laser light reflected by the second mirror 21, and a second 45 ° mirror 214 that reflects the second laser light emitted from the second end 211-2 of the resonator 211. Consists of. The resonator length of the resonator 211 is 70 μm or less, and the front and back surfaces of the direct modulation laser array 200 are AR coated.

  The first 45 ° mirror 212 is configured so that the first laser beam can be extracted to the surface (first surface) side of the SI substrate 201 where the InP lens 213 is formed with a reflection angle of 90 °. It is formed so as to form an angle of 45 ° with respect to the horizontal plane of the substrate 201. Further, the second 45 ° mirror 214 propagates the second laser light to a surface (second surface) opposite to the surface on which the resonator 211 of the SI substrate 201 is formed with a reflection angle of 90 °. The first 45 ° mirror 212 is formed in a direction opposite to the 180 ° so that the first 45 ° mirror 212 can be used. The seven first 45 ° mirrors 212 are all formed in the same direction, and the seven second 45 ° mirrors 214 are also formed in the same direction. Therefore, the first mirror 213 and the second mirror 214 can be manufactured by a single mirror process.

  The InP lens 212 of the direct modulation laser array 200 is integrated at the same pitch as the core pitch of the multicore fiber in order to obtain good coupling with the multicore fiber. Here, FIG. 3 is a cross-sectional view of a seven-core multicore fiber coupled to the direct modulation laser array 200 in a direction perpendicular to the waveguide direction. The seven-core multi-core fiber 300 in FIG. 3 has a diameter of 125 μm, and seven cores 301 (one at the center and six at the periphery) are arranged at intervals of 40 μm. The diameter of the core is 9 μm. 3 is merely an example, and in the present invention, the interval between the cores of the 7-core multicore fiber is not limited to 40 μm.

  In the direct modulation laser array 200 of FIG. 2, the InP lenses 213 are arranged so as to coincide with the intervals of the cores 301 of the multicore fiber shown in FIG. 3, and arranged so that all the seven cores 301 of the multicore fiber 300 can be connected. Is done.

  FIG. 4 is a diagram showing the configuration of one laser element of the direct modulation laser array 200 shown in FIG. 4 is formed on an SI substrate 401, an n-InP layer 402 on the SI substrate 401, an active layer 403 serving as a resonator formed on the n-InP layer 402, and n A first waveguide 404 formed on the InP layer 402 from which the first laser light is emitted from the first end face 403-1 of the active layer 403; and a second end face 403-2 of the active layer 403 And a p-InP layer 405 formed on the active layer 403, the first waveguide 404, and the second waveguide 406. . Here, although the substrate shown in FIG. 4 has a buried hetero (BH) structure, a laser element can be formed on a substrate having a ridge structure.

  Here, at one end of the first waveguide 404 (on the side opposite to the portion to which the first end 403-1 of the active layer 403 is connected), the first 45 ° mirror 411 is disposed on the horizontal plane of the SI substrate 401. The surface of the SI substrate 401 on which the p-InP layer 405 is formed (first surface) side is formed by forming an angle of 45 ° with the first 45 ° mirror 411. Can be taken out. Further, an InP lens 412 is formed by processing InP on a portion of the surface of the p-InP layer 405 where the first laser beam reflected by the first 45 ° mirror 411 reaches, and the first surface A first laser beam can be coupled to one core of multi-core fibers (not shown) arranged on the side.

  Also, one end of the second waveguide 406 of the laser element 400 (on the side opposite to the portion to which the second end 403-2 of the active layer 403 is connected) forms an angle of 45 ° with respect to the horizontal direction. Thus, the second 45 ° mirror 413 is formed. Since the mirror 411 and the mirror 413 have the same angle with respect to the substrate, they can be formed by a single mirror process. Further, the AR coating 414 is applied to the surface (second surface) opposite to the surface on which the laser element 400 of the SI substrate 401 is formed, and the AR coating 415 is also applied to the surface of the InP lens 412. . In an ordinary DFB laser, AR coating is applied to both end faces of the resonator to maintain a stable oscillation state. However, in the present invention, a 45 ° mirror in a different direction is created and the first laser beam is reflected by reflection. The second laser beam is guided to the InP lens 412 to the second surface, and the InP lens 412 to which the laser beam reaches and the AR coating are applied to the second surface, thereby maintaining a stable oscillation state.

  In the laser element 400 shown in FIG. 4, the distance from the first end 403-1 of the active layer 403 to the center of the first 45 ° mirror 411 on the center line of the first waveguide 404 is x. To do. x is the propagation distance of the first laser beam in the horizontal direction of the SI substrate 401. Further, the distance from the center of the first 45 ° mirror 411 to the upper surface of the InP lens 412 is y. y is the propagation distance of the first laser beam in the vertical direction of the SI substrate 401. Further, the sag amount of the lens when processing the InP lens 412 on the p-InP layer 405 is t.

  Next, in the laser element 400, the values of x, y and t and the shape of the InP lens 412 for the present invention to exert an effect will be examined.

  First, consider the values of x and y in FIG.

First, the calculation result of the spread of the electric field distribution after light propagates inside the InP material is shown. FIG. 5 is a chart showing the relationship between the propagation distance of the laser beam and the diameter of the electric field intensity distribution after propagation. In FIG. 5, the diameter of the electric field distribution before propagation was 2 μm, and the wavelength of the propagated light was calculated as 1.3 μm (value divided by the refractive index in the semiconductor). The core diameter of the multi-core fiber is 8-9 μm. (The electric field distribution is naturally wider than the core diameter.)
In the case of a semiconductor laser, the NA is significantly different from that of a glass material device such as an optical fiber. Therefore, high coupling can be obtained if the electric field distribution inside the semiconductor is expanded until it approaches the core diameter of the multi-core.

  Since the core diameter of the core 301 of the multicore fiber 300 shown in FIG. 3 is 9 μm, it is necessary to set the diameter of the electric field distribution after propagation in the semiconductor element 400 to 8 to 9 μm. Needs to be set to about 25 μm. In the configuration of the EA / DFB laser 110 described in Non-Patent Document 1 that extracts light to the substrate side, the substrate thickness alone is expected to be 150 μm, and the distance (propagation distance) from the resonator to the InP lens is 25 μm. difficult.

  Next, the range of the propagation distance of the first laser light that can be realized in the laser element 400 will be described. Here, if the diameter of the electric field distribution after propagation is about 10 μm, the propagation distance of the first laser light is assumed to be 30 μm or less.

  First, the optimum value of the distance y is examined. In FIG. 4, y is determined by the layer thickness of the p-InP layer 405 epitaxially grown on the laser active layer 403. When InP is grown by MOCVD, the InP layer thickness is about 8 μm at most. In the present embodiment, the p-InP layer on the laser active layer 403 is preferably in the range of 2 to 8 μm.

  Next, the optimum value of the distance x is examined. In FIG. 4, x determines the diameter of the electric field distribution of the first laser beam at the position of the first 45 ° mirror 411. FIG. 6 is a diagram showing a state of reflection of the first laser beam by the first 45 ° mirror 411. The upper limit value of x is set so that the first laser beam at the mirror position does not reach the upper surface of the cladding layer in the waveguide 404 as shown in FIG. In FIG. 6A, assuming that the value of the width a in the vertical direction of the waveguide is 10 μm, the diameter a ′ of the optical electric field distribution can be allowed up to 10√2 μm = about 14 μm. Assuming that the error in manufacturing the laser element is 2 μm above and below the waveguide, the diameter a ′ of the electric field distribution on the etching surface forming the mirror can be allowed to about 10 μm. From the chart of FIG. 5, the propagation distance of the laser beam with an electric field distribution of 10 μm is 30 μm. If x becomes a smaller value of 30 μm or less, as shown in FIG. 6B, the diameter a ′ of the electric field distribution is further reduced, so that the etching surface can be made shallower and the fabrication of the laser device is facilitated.

  Next, the value of t and the InP lens shape in the laser element 400 will be examined.

  FIG. 7 is a chart showing the lens sag amount t for each lens diameter and the beam divergence angle at that time. The laser beam is condensed at a beam divergence angle> 0, and the laser beam is emitted at a beam divergence angle <0. At the time of lens production, as the lens diameter is reduced, the production tolerance (allowable range of production error) with respect to the lens sag amount t is reduced (excessive variation with respect to a deviation of 0.1 μm is large). In actual production, the lens sag amount of the seven lenses has a variation of about ± 0.1 μm. Therefore, it is necessary to increase the lens tolerance as much as possible to increase the manufacturing tolerance.

  In order to couple the direct modulation laser array 200 with the multi-core fiber 300, the beam divergence angle of the InP lens 213 (412) is set to 8.05 °. This corresponds to the NA of a general optical fiber. FIG. 8 is a chart showing the relationship between the lens diameter of the InP lens and the lens sag amount for obtaining the divergence angle 8.05. The larger the lens diameter, the higher the required lens sag amount.

  Here, a method for determining the upper limit of the diameter of the lens will be described. The upper limit of the lens diameter is determined by the value of y (FIG. 4). For example, consider a case where y of 7 μm (waveguide radius 5 μm + InP layer thickness 2 μm) is assumed.

  FIG. 9 is a schematic diagram showing the relationship among the allowable lens radius r of the InP lens 412, the lens sag amount t, and the longitudinal propagation distance y of the first laser light in the laser element 400 of FIG. 4. From FIG. 10, r cannot be larger than yt.

  If a lens with a diameter of 12 μm (radius r = 6 μm) is made, the required sag amount is about 1.1 μm from the chart of FIG. Since 7 μm−1.1 μm = 5.9 μm <6 μm, if the y value is set to 7 μm, a lens having a diameter of 12 μm cannot be made. In the present invention, y <8 μm, r <7.5 μm (diameter <15 μm), and t <1.6 μm are assumed as possible ranges.

[Example]
An embodiment of the present invention will be described with reference to FIG. 10 is a cross-sectional view of one laser element of the direct modulation laser array, FIG. 10A is a cross-sectional view of the laser element 1000 in the resonator and waveguide directions, and FIG. 10B is a cross-sectional view of FIG. It is sectional drawing in AA 'of the laser element 1000 in FIG. FIG. 11 is a top view of a direct modulation laser array 1100 in which seven laser elements 1000 are integrated.

  The laser element 1000 of the present invention uses an SI (Semi insulating) -InP substrate 1001 as a substrate, an n-InP layer 1002 is formed on the InP substrate 1001 by 2 μm, and the InGaAlAs is formed on the n-InP layer 1002. An active layer 1003 of material is formed. Next, an undoped i-InP layer 1015 is butt-jointed on the n-InP layer 1002 and before and after the active layer 1003. Further, a diffraction grating 1008 is formed on the active layer 1003, and a p-InP layer 1005 is formed on the entire upper surface of the active layer 1003 and the i-InP layer 1015 by 2 μm. Thereafter, other portions are removed by dry etching while leaving the waveguide portion, and an Fe-added InP layer 1004 is formed at the removed portion. Thereafter, an i-InP layer 1009 to be a lens portion is formed to 3 μm. Thereafter, the n-InP layer 1002 is dug, and a p-electrode 1006 and an n-electrode 1007 are formed. Thereafter, the first 45 ° mirror 1010 and the second 45 ° mirror 1011 are formed by one milling. The propagation distance (x) of the i-InP layer in the horizontal direction of the substrate was 8 μm. Thereafter, an InP lens 1012 is formed on the i-InP layer 1009. The diameter of the lens was 8 μm. Finally, AR coatings 1114 and 1115 were applied to the front and back surfaces.

  In the direct modulation laser array of this example, as shown in FIG. 11, the length of the resonator was 45 μm, and the InP lens 1012 was formed at a pitch of 40 μm so as to match the core pitch of the 7-core multicore. When a coupling experiment with a multi-core fiber was performed using the directly modulated laser array 1100 produced, good characteristics with a coupling loss of 4 dB or less were realized for all the cores. Further, 56 Gb / s operation was performed in one laser element 1000 of the direct modulation laser array, and a large capacity transmission of 56 × 7 = 392 Gb / s was made possible as a whole.

  The present invention can be used for a large capacity and low power consumption large capacity optical transmitter used in a data center.

100 EA / DFB lasers 111, 213, 412, 1012 InP lenses 112, 212, 214, 411, 413, 1010, 1011 45 ° mirror 114, 211 resonator 200 direct modulation laser array 210, 400 laser element 300 multi-core fiber 311 core 401 SI substrate 402, 1002 n-InP layer 403 active layer 404, 406 waveguide 405, 1005 p-InP layer 414, 415, 1013, 1014 AR coat 1001 SI-InP substrate 1003 InGaAls active layer 1004 Fe-doped InP layer 1006 p Electrode 1007 n-electrode 1008 Diffraction grating 1009, 1015 i-InP layer

Claims (5)

An n-InP layer formed on an InP substrate;
An active layer formed on the n-InP layer;
A direct modulation laser comprising a p-InP layer formed on the active layer,
A first mirror that emits a first laser beam emitted from a first end of the resonator of the direct modulation laser in a horizontal direction with respect to the substrate to the p-InP layer by reflection;
A second mirror that emits a second laser beam emitted from the second end of the resonator in a horizontal direction with respect to the substrate toward the InP substrate by reflection;
A lens formed by processing the p-InP layer, wherein the InP lens is formed at a portion where the first laser reflected by the first mirror reaches;
A direct modulation laser comprising: the surface of the lens; and an AR coat applied to the second surface of the InP substrate. The horizontal distance between the first end of the resonator and the first mirror in the InP substrate is 30 μm,
A distance in a direction perpendicular to the InP substrate between the first mirror and the lens surface is 2 μm to 8 μm;
The diameter of the lens is 7 μm to 15 μm;
The direct modulation laser according to claim 1, wherein a sag amount of the lens is 0.5 μm to 1.6 μm.   The direct modulation laser according to claim 1, wherein the direct modulation laser is formed at seven locations on the InP substrate.   4. The direct modulation laser according to claim 3, wherein the lenses are arranged at the same interval as the interval between the cores of the multi-core fiber to which the first laser beam is coupled.   The direct modulation laser according to claim 4, wherein the multi-core fiber to which the first laser beam is coupled has seven cores.

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