JP7172182B2 - Light emitting device, optical signal transmitter and optical transmission system - Google Patents

Description

The present invention relates to a light emitting device, an optical signal transmission device and an optical transmission system.

Patent Document 1 discloses a semiconductor laser fitted in a semiconductor laser holder, a lens held by a lens holder arranged in the semiconductor laser holder, and an optical fiber holder attached to the semiconductor laser holder. An optical fiber coupling optical system comprising an optical fiber held by a tool, wherein the light emitting portion of the semiconductor laser, the optical axis of the lens, and the core portion of the optical fiber are arranged on the same axis, wherein the light An optical fiber coupling optical system is disclosed in which the fiber holder is fixed on the semiconductor laser holder by penetrating a thin plate and laser welding.

JP 2013-171112 A

By the way, in an optical transmission system that transmits information using a light emitting element as a light source, a lens couples light emitted from the light emitting element to an optical transmission line such as a fiber. At this time, if the light emitted from the light-emitting element is reflected at the end of the fiber, etc., and returned to the light-emitting point of the light-emitting element, the operation of the light-emitting element becomes unstable, resulting in high-speed information transmission. may be hindered.
An object of the present invention is to provide a light emitting device or the like in which the amount of light returned to the light emitting point is reduced compared to the case where the optical axis of the lens and the center of the light emitting point are aligned.

The invention according to claim 1 comprises: a light emitting unit having a plurality of light emitting points for emitting light; and a lens coupled with an externally provided optical transmission unit, wherein the plurality of light emitting points are odd in number and arranged on the optical axis in a direction perpendicular to the optical axis of the lens. The light-emitting device is a light-emitting device in which return light generated at symmetrical positions with respect to the optical axis with respect to each of the plurality of light-emitting points does not overlap other light-emitting points. .
In the invention according to claim 2, the plurality of light emitting points are positioned on a straight line parallel to the optical axis deviated from the optical axis of the lens so that the maximum coupling efficiency can be obtained with respect to the optical transmission section. 2. The light-emitting device according to claim 1, wherein the light-emitting device is provided at a position closer to the front focal point of the lens.
In the invention according to claim 3, the plurality of light emitting points are provided at positions where the amount of return light decreases by 10 dB or more with respect to the amount of light returning to the light emitting points at the position where the maximum coupling efficiency is obtained. 3. The light-emitting device according to claim 2.
The invention according to claim 4 is the light-emitting device according to claim 2 or 3, wherein the plurality of light-emitting points are provided at positions where a decrease in coupling efficiency is 1 dB or less with respect to the maximum coupling efficiency. be.
The invention according to claim 5 is the light emission according to claim 1, wherein the plurality of light emitting points are provided at a distance of 0.9 times or more and less than 1.3 times the front focal length of the lens. It is a device.
The invention according to claim 6 is the light-emitting device according to claim 2, wherein the lens has an end at a rear focal point of the lens.
The invention according to claim 7 is the light-emitting device according to claim 1, wherein the plurality of light-emitting points are arranged along a circle centered on the optical axis of the lens.
The invention according to claim 8 is the light-emitting device according to claim 7, wherein the plurality of light-emitting points are arranged at regular intervals along the circle.
The invention according to claim 9 is the light-emitting device according to claim 1, wherein the plurality of light-emitting points in the light-emitting portion are surface-emitting laser elements.
The invention according to claim 10 is the light-emitting device according to claim 9, wherein the surface emitting laser element oscillates in a single mode.
<11> The light emitting device according to any one of <1> to <10>, the light emitting device is driven, and the obtained electric signal is converted into an optical signal by a light emitting point provided in the light emitting device. and a driver for converting.
According to a twelfth aspect of the present invention, there is provided an optical transmission system comprising: the optical signal transmission device according to the eleventh aspect; and an optical transmission section provided at a position where a principal ray output from the optical signal transmission device is coupled. System.

According to the first aspect of the invention, the amount of light returning to the light emitting point is reduced compared to the case where the optical axis of the lens is aligned with the center of the light emitting point.
According to the second and third aspects of the invention, the amount of light returning to the light emitting point is suppressed compared to the case where the lens is provided at a position where the maximum coupling efficiency can be obtained on a straight line parallel to the optical axis of the lens. .
According to the fourth aspect of the invention, the amount of light returning to the light emitting point is suppressed while the decrease in coupling efficiency relative to the maximum coupling efficiency is suppressed to 1 dB or less.
According to the fifth aspect of the invention, the coupling efficiency is improved compared to the case where the lens is provided at a distance of less than 0.9 times the front focal length, and the lens is provided at a distance of 1.3 times or more the front focal length. The amount of returned light is suppressed as compared with the case where
According to the sixth aspect of the present invention, light parallel to the optical axis can be condensed on the optical axis at the end of the lens, as compared with the case where the end of the lens is not the rear focal point.
According to the seventh aspect of the invention, it is easier to arrange the light-emitting points than when they are not arranged along a circle.
According to the eighth aspect of the invention, heat can be dispersed more uniformly than when they are not arranged at regular intervals.
According to the ninth aspect of the invention, it is easier to combine with a lens than with an edge-emitting element.
According to the tenth aspect of the invention, the transmission distance can be increased compared to multimode oscillation.
According to the eleventh aspect of the invention, compared with the case where the optical axis of the lens is aligned with the center of the light emitting point, optical transmission can be performed with a reduced amount of light returning to the light emitting point.
According to the invention of claims 1 and 2 , compared to the case where the optical axis of the lens coincides with the center of the light emitting point, optical transmission can be performed with a reduced amount of light returning to the light emitting point.

1 illustrates an example of an optical transmission system; FIG. It is a figure explaining a light source part. (a) is a front view of a light emitting part, and (b) is a cross-sectional view of a light source part taken along line IIB-IIB in (a). FIG. 3 is a diagram showing light emission points, return light, and the light intensity distribution of the return light on the surface of the light emission section; FIG. 4 is a diagram showing a light intensity distribution on an end face of a core in an optical fiber; FIG. 4 is a diagram for explaining the relationship between the amount of return light that irradiates a light-emitting point and the distance from a coupling lens to a light-emitting section; 4 is a diagram for explaining the relationship between the amount of light incident on the core of the optical fiber and the distance from the coupling lens to the light emitting section; FIG. FIG. 7 is a diagram for explaining the relationship between the amount of light incident on the core of the optical fiber and the distance from the coupling lens to the light emitting section, in order to explain the results of FIG. 6; FIG. 10 is a diagram showing the amount of returned light when the center of the light emitting portion is deviated from the optical axis; (a) is a diagram for explaining the relationship between the center of the light emitting portion and the optical axis, and (b) is the amount of return light. FIG. 4 is an enlarged view of a light-emitting part having three light-emitting points; (a) is a top view of a light-emitting portion, and (b) is a cross-sectional view of the light-emitting portion taken along line IXB-IXB of (a). It is a figure explaining the manufacturing method of a vertical cavity surface emitting laser. (a) is a semiconductor layer laminate forming step, (b) is a mesa etching step, (c) is an oxidized region forming step, (d) is a p-side electrode forming step and an interlayer insulating layer forming step, (e) ) is an output surface protective layer forming step and an n-side electrode forming step, and (f) is a wiring layer forming step.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
In addition, below, it may describe using an element symbol, such as aluminum being Al.

(Optical transmission system 1)
FIG. 1 is a diagram showing an example of an optical transmission system 1. As shown in FIG. The optical transmission system 1 is, for example, a system that uses optical communication technology to transmit digital signals such as data between electronic devices such as computers and servers. In a system with multiple computers and servers, such as a data center, multiple computers, servers, and other electronic devices are mounted on racks, and these electronic devices operate in parallel while transmitting digital signals at high speed between them. is done. By performing optical communication using light for the transmission of digital signals between electronic devices, it is possible to achieve higher speeds and to be less susceptible to noise than in the case of using electrical signals.

The optical transmission system 1 includes an optical signal transmitter 10 , an optical fiber 20 and a signal processor 30 .
The optical signal transmission device 10 includes a light source section 11 and a signal modulation section 12 . The light source unit 11 is an example of a light emitting device, and the signal modulation unit 12 is an example of a driving unit. Also, the optical fiber 20 is an example of an optical transmission section.

(Optical signal transmitter 10)
The light source unit 11 included in the optical signal transmitting device 10 includes a light emitting point (light emitting point 110 in FIG. 2, which will be described later).
The signal modulation unit 12 acquires the electrical signal processed by the signal processing device 30, drives the light source unit 11, and converts it into an optical signal corresponding to a digital signal. The light source section 11 is driven by the signal modulation section 12 and emits an optical signal from the light emitting point toward the core 21 of the optical fiber 20 .

(optical fiber 20)
The optical fiber 20 has a core 21 and a clad 22 . The core 21 is made of a material having a smaller refractive index than the clad 22 . An optical signal that has entered the core 21 propagates through the core 21 while being repeatedly reflected at the boundary between the core 21 and the clad 22 .
Here, as an example, the optical fiber 20 is assumed to be multimode. In the multimode optical fiber 20, the diameter of the core 21 is, for example, 50 μm. Note that the optical fiber 20 may be of a single mode, in which the diameter of the core 21 is smaller than that of a multimode.
It should be noted that instead of the optical fiber 20, a light guiding device provided with a light guiding region having a refractive index different from that of other regions may be used.

(Signal processing device 30)
The signal processing device 30 has a function of processing digital signals such as data. The signal processing device 30 is, for example, a computer or a server. These include a CPU, ROM, RAM, input/output ports, etc., and process digital signals such as data by a program developed in an executable state on the RAM. The digital signal in this case is an electrical signal. Then, the signal processing device 30 outputs the processed digital signal as an electrical signal to the optical signal transmitting device 10 connected to the input/output port so that it can be processed by another signal processing device.

Although not shown in FIG. 1, the optical signal transmitted through the optical fiber 20 is received by a signal receiver having a light receiving section and a signal demodulating section. The light receiving unit receives an optical signal and converts it into an electrical signal corresponding to the optical signal. The signal demodulator acquires an electrical signal corresponding to the optical signal from the light receiver and converts it into a digital signal that can be processed by another signal processing device.
Then, the digital signal is processed as data or the like by another signal processing device connected to the signal receiving device.

(Light source unit 11)
The light source section 11 includes a light emitting section 100 and a coupling lens 200 .
FIG. 2 is a diagram for explaining the light source unit 11. As shown in FIG. 2(a) is a front view of the light emitting unit 100, and FIG. 2(b) is a sectional view of the light source unit 11 taken along line IIB-IIB in FIG. 2(a).
As shown in FIG. 2A, the light emitting section 100 of the light source section 11 has three light emitting points 110 as an example. Here, when distinguishing between the three light emitting points 110, they are denoted as light emitting points 110-1, 110-2, and 110-3. The light emitting points 110-1, 110-2, and 110-3 are arranged at regular intervals on a circle 111 having a center O and a radius r. The center O is referred to as the center of the light emitting section 100 .
Here, the light emitting point 110 is referred to as a light emitting point 110. However, the light emitting point 110 is not a point, but a portion of a light emitting element that emits light having a predetermined area from which light is emitted (an emission surface shown in FIG. 9 to be described later). 112). Note that the center of the light emitting point 110 is defined as the center C of the light emitting point 110 . That is, the center C of two adjacent light emitting points 110 is arranged at a central angle of 120° with respect to the center O. FIG.

The coupling lens 200 comprises a plano-convex lens portion 210 and a cylinder portion 220 . The plano-convex lens portion 210 is a lens in which one surface is a convex lens-like convex surface and the other surface is a flat surface. The plano-convex lens portion 210 has the function of condensing light. Both end portions of the cylindrical portion 220 are flat surfaces. A plane of the plano-convex lens portion 210 is configured to be in contact with one plane of the cylindrical portion 220 . The central axis of the cylindrical portion 220 coincides with the optical axis of the plano-convex lens portion 210 . Here, the plano-convex lens portion 210 and the cylindrical portion 220 are integrally formed of optical glass that transmits the light emitted by the light emitting point 110 of the light emitting portion 100 . Here, the central axis of the cylindrical portion 220 is referred to as the optical axis OA of the coupling lens 200. FIG. The diameters of the plano-convex lens portion 210 and the cylindrical portion 220 of the coupling lens 200 are each set larger than the diameter (2×r) of the circle in which the light emitting points 110 are arranged.

The optical axis OA of the coupling lens 200 passes through the center O of the light emitting section 100 . Further, the optical fiber 20 is provided so that the optical axis OA and the optical axis of the core 21 of the optical fiber 20 are aligned. That is, the optical axis of the core 21 of the optical fiber 20 is on the optical axis OA of the coupling lens 200, and the center O of the light emitting section 100 having the three light emitting points 110 is present. In other words, none of the three light emitting points 110 are arranged on the optical axis OA, and are arranged to be shifted from the optical axis OA in the direction perpendicular to the optical axis OA.

Here, it is assumed that the light emitting point 110 is configured by a vertical cavity surface emitting laser VCSEL (Vertical Cavity Surface Emitting Laser) as an example of a surface emitting laser element. The size of the emission surface 112 (see FIGS. 9 and 10, which will be described later) that functions as the light emitting point 110 is set small so that the laser oscillates in a single mode. The diameter of the exit surface 112 is set to, for example, 5 μm or 7 μm. Therefore, the diameters of the light emitting surface of 5 μm and 7 μm are regarded as a point compared to the diameter of the core 21 of 50 μm exemplified above. Note that the diameter of the emission surface 112, which is set for single-mode laser oscillation of the vertical cavity surface emitting laser VCSEL, varies depending on the wavelength of emitted laser light. Therefore, the structure of the vertical cavity surface emitting laser VCSEL, such as the emission surface 112, should be set so that laser light of a predetermined wavelength oscillates in a single mode. Here, the center of the emission surface 112 is the center C of the light emitting point 110 .

The vertical cavity surface emitting laser VCSEL is an example, and the light emitting point 110 may be configured by a light emitting element that emits light, such as a laser element other than the vertical cavity surface emitting laser VCSEL or a light emitting diode (LED). . Also, although the vertical cavity surface emitting laser VCSEL is oscillated in a single mode, it may be oscillated in a multimode. Laser elements are more susceptible to return light than light emitting diodes (LEDs).

As shown in the cross-sectional view of FIG. 2B , a coupling lens 200 is provided on the light emitting point 110 side of the light emitting section 100 so that the convex surface of the plano-convex lens portion 210 of the coupling lens 200 faces the light emitting point 110 . there is A core 21 of the optical fiber 20 is provided so as to face the plane of the cylindrical portion 220 of the coupling lens 200 . A light emitting point 110-1 is shown in the light emitting section 100. As shown in FIG.

As an example, the light emitting point 110 is provided on the front focal plane FFP provided perpendicular to the optical axis OA from the front focal point FF on the optical axis OA of the coupling lens 200 . Here, the intersection of the convex surface of the plano-convex lens portion 210 of the coupling lens 200 with the optical axis OA is the front principal point of the coupling lens 200 . Further, as an example, the plane of the coupling lens 200 on the core 21 side is provided on the rear focal plane RFP provided perpendicularly to the optical axis OA from the rear focal point RF on the optical axis OA. The end face 23 of the core 21 of the optical fiber 20 on which the light is incident is provided with a gap Zf in the z direction away from the rear focal plane RFP of the coupling lens 200 .
Note that the gap Zf may be set to "0". However, when the gap Zf is set to "0", the amount of light incident on the core 21 is not necessarily the highest. Therefore, the gap Zf should be set to a value that increases the amount of light incident on the core 21, that is, increases the coupling efficiency.

With the above arrangement, the light emitted from the light emitting point 110 - 1 travels along the optical path indicated by the solid line, becomes parallel light by the coupling lens 200 , and irradiates the end face 23 of the core 21 in the optical fiber 20 . That is, light rays traveling parallel to the optical axis OA indicated by the thick solid line travel to the rear focal point RF. In other words, the light beam traveling parallel to the optical axis OA is irradiated to the center of the end surface 23 of the core 21 in the optical fiber 20 . Here, let the ray passing through the back focal point RF be the principal ray. The principal ray from the optical signal transmitting device 10 and the optical fiber 20 are coupled so that the principal ray is irradiated to the center of the end surface 23 of the core 21 in the optical fiber 20 .

Of the light irradiated to the end surface 23 of the core 21 , the light input to the core 21 propagates through the core 21 of the optical fiber 20 . However, part of the light irradiated to the end surface 23 of the core 21 does not enter the core 21 and is reflected by the end surface 23 to return to the light emitting section 100 . This reflected light travels along the optical path indicated by the dashed line, is converged by the coupling lens 200 , and illuminates the light emitting section 100 . At this time, the converged reflected light is applied to symmetrical positions with respect to the center O of the light emitting unit 100 . Here, the reflected light that is reflected by the end face 23 of the core 21 of the optical fiber 20 and returns to the surface of the light emitting section 100 is referred to as return light 120 . The return light 120 is a position where the surface of the light emitting unit 100 is irradiated with the return light. The return light 120 shown in FIG. 2 is referred to as return light 120-1 because it is light emitted from the light emitting point 110-1. The return light emitted from the light emitting point 110-2 is referred to as return light 120-2, and the return light emitted from the light emission point 110-3 is referred to as return light 120-3.

The positional relationship between the light emitting point 110 and the front focal plane FFP is not limited to the above, as will be described later. Also, the gap Zf is not limited to a value described later.
Here, although the coupling lens 200 is composed of the plano-convex lens portion 210 and the cylindrical portion 220, it may be a lens of another shape. For example, the coupling lens 200 may be a plano-convex lens, a biconvex lens, or the like without the cylindrical portion 220 .

FIG. 3 is a diagram showing a light emitting point 110, return light 120, and light intensity distribution of the return light 120 on the surface of the light emitting section 100. As shown in FIG. The light intensity distribution was obtained by simulation. Here, the light intensity distribution is shown by black and white gradation. In other words, white portions are portions with high light intensity, and black portions are portions with low light intensity. The light intensity is strong in the central portion, and the light intensity decreases toward the peripheral portion. Also, the radius r of the circle 111, which is the distance between the center O of the light emitting section 100 and the light emitting point 110, is 0.020 mm.

The light-emitting points 110-1, 110-2, and 110-3 are evenly spaced on a circle 111 with a radius r from the center O. FIG. The returned lights 120-1, 120-2, and 120-3 are generated at positions symmetrical about the center O with respect to the light-emitting points 110-1, 110-2, and 110-3. -1, 120-2, and 120-3 are also arranged at regular intervals on a circle 111 with a radius r from the center O. In other words, adjacent return beams 120 are arranged at a central angle of 120° with respect to the center O. FIG. The adjacent light emitting point 110 and return light 120 are positioned at a central angle of 60° with respect to the center O. FIG.

As shown in FIG. 3, the return light 120-1 is generated at positions adjacent to the light emitting points 110-2 and 110-3, but does not overlap the light emitting points 110-2 and 110-3. Moreover, as shown in the light intensity distribution, the return light 120-1 is located in the middle of the circle 111 between the light emitting points 110-2 and 110-3, so the portion with the high light intensity is the light emitting point 110-2. , 110-3. Furthermore, as can be seen from the light intensity distribution, portions with weak light intensity are less likely to overlap the light emitting points 110-2 and 110-3. This is because the odd number of light emitting points 110 are arranged on a circle 111 centered at O at regular intervals. As a result, reflected light from the end face 23 of the core 21 in the optical fiber 20 is generated at a symmetrical position with respect to the center O of the light emitting point 110, that is, at a position where the light emitting point 110 is not provided. This suppresses reflected light from overlapping the light emitting point 110 . Moreover, since the light-emitting point 110 of the light-emitting unit 100 is arranged on the front focal plane FFP of the coupling lens 200, the return light 120 is focused on the surface of the light-emitting unit 100 in a point shape similar to that of the light-emitting point 110 or in a small area. be illuminated. As a result, the return light 120 is prevented from being condensed on the surface of the light emitting section 100 and spreads, so that a portion with high light intensity spreads so as to overlap the light emitting point 110 .

When the light emitting point 110 is irradiated with the return light 120 , the light emitting element such as the vertical cavity surface emitting laser VCSEL constituting the light emitting point 110 is irradiated with the return light 120 . The light-emitting element has a light-emitting layer (see light-emitting layer 412 in FIG. 9 to be described later) having a bandgap set by the wavelength of emitted light. The light emitted from the light emitting point 110 and the return light 120 have the same wavelength. Therefore, when the light-emitting element is irradiated with the return light 120, electron-hole pairs are excited, which may adversely affect the operation of the light-emitting element, such as unexpected light emission. That is, there is a risk that high-speed information transmission will be hindered.

As described above, in the present embodiment, by shifting the center of the light emitting point 110 in the direction perpendicular to the optical axis OA of the coupling lens 200, the light amount of the return light 120 to the light emitting point 110 is reduced. At this time, by arranging the light emitting point 110 on the front focal plane FFP, the return light 120 is suppressed from converging and spreading. This reduces the light amount of the return light 120 to the light emitting point 110 . Note that the light amount of the return light 120 may be referred to as the return light amount.

Note that the number of light emitting points 110 may be one. In this case, the light emitting point 110 may be located not at the center O but at a position perpendicular to the optical axis OA, for example, on the circle 111 having the radius r of the center O described above. As in the case where there are three light emitting points 110 , the return light 120 is suppressed from overlapping the light emitting points 110 . In addition, since the return light 120 is condensed on the surface of the light emitting unit 100 , the portion of the return light 120 with high light intensity is prevented from spreading so as to overlap the light emitting point 110 .

Also, the number of light emitting points 110 may be an odd number other than three. Also in this case, the light-emitting points 110 are preferably arranged at regular intervals on a circle 111 having a center O and a radius r. The returned light 120 is converged and irradiated between adjacent light emitting points 110 on the circle 111 . Note that the light emitting points 110 may be arranged on circles having a plurality of radii around the center O, that is, on concentric circles around the center O. FIG. By arranging the odd number of light emitting points 110 on the circle 111 at regular intervals, the heat generated by the light emitting section 100 is uniformly distributed.
It should be noted that convergence means that light rays converge on a narrow area such as one point. In addition, it may be described as condensing.

Furthermore, the number of light emitting points 110 may be an even number. When the light-emitting points 110 are arranged on the circle 111 having the center O and the radius r, each light-emitting point 110 is preferably not provided at a symmetrical position with respect to the center O of the other light-emitting points 110 . Then, the light-emitting point 110 may be arranged at a position where it is suppressed that a portion of the return light 120 having a high light intensity overlaps with the light-emitting point 110 .
Further, the light emitting point 110 does not have to be provided on the circle of the center O and the radius r.
As described above, the light-emitting point 110 is arranged away from the center O, and the return light 120 on the surface of the light-emitting section 100 is set at a position where it is unlikely to adversely affect the operation of the light-emitting elements that constitute the light-emitting point 110. Just do it.

FIG. 4 is a diagram showing the light intensity distribution on the end face 23 of the core 21 in the optical fiber 20. As shown in FIG. The light intensity distribution was obtained by simulation. The diameter of the core 21 is 50 μm as described above. Here, similarly to FIG. 3, the light intensity distribution is shown by contour lines. The light intensity is high in the central portion indicated by the contour lines, and the light intensity decreases toward the peripheral portion. Here, the three light emitting points 110-1, 110-2 and 110-3 shown in FIG. 3 emit light simultaneously.

As mentioned above, the end face 23 of the core 21 in the optical fiber 20 is located near the back focal plane RFP of the coupling lens 200 . Therefore, the light emitted from the light emitting point 110 is collimated by the coupling lens 200 and radiated onto the end surface 23 of the core 21 .
However, as shown in FIG. 4, the light irradiated to the end face 23 of the core 21 includes a portion with high light intensity inside the core 21, and the amount of light irradiated to the outside of the core 21 is suppressed. Therefore, it is not necessary to focus the light on the end surface 23 of the core 21 in order to make it incident on the core 21 . That is, the light emitting unit 100 is placed on the front focal plane FFP of the coupling lens 200, and the end face 23 of the core 21 is placed on the rear focal plane RFP, so that the end face 23 of the core 21 can be irradiated with parallel light. .

As described above, by driving the plurality of light emitting points 110 in parallel, the amount of light transmitted through the optical fiber 20 is increased. The plurality of light emitting points 110 may be configured to be driven in parallel by connecting in parallel, or may be configured to be driven in parallel by being connected in series. Moreover, the structure which drives the several light emission point 110 independently may be sufficient.

In the above description, it is assumed that the light emitting unit 100 is installed on the front focal plane FFP of the coupling lens 200 . However, the light emitting unit 100 does not necessarily have to be placed on the front focal plane FFP of the coupling lens 200 .
FIG. 5 is a diagram for explaining the relationship between the amount of return light applied to the light emitting point 110 and the distance D from the coupling lens 200 to the light emitting section 100. As shown in FIG. The horizontal axis in FIG. 5 is the distance D (mm) from the end of the coupling lens 200 to the light emitting section 100 . In addition, in FIG. 2, since the right direction is the +z direction, the distance D is expressed as a negative value. Here, the front focal plane FFP of the coupling lens 200 is at a distance D from the coupling lens 200 of −0.26 mm. The vertical axis represents the amount of return light (dBm). In FIG. 5, the amount of return light is expressed as the amount of return light (dBm). A gap Zf between the coupling lens 200 and the end face 23 of the core 21 in the optical fiber 20 is set to 0.6 mm. Three cases of 0.020 mm, 0.022 mm, and 0.024 mm are shown as the radius r of the circle 111 on which the light emitting points 110 are arranged.
The relationship shown in FIG. 5 between the amount of return light applied to the light emitting point 110 and the distance D from the coupling lens 200 to the light emitting section 100 was obtained by simulation.

As shown in FIG. 5, at any radius r, the amount of returned light is -60 dBm or less in the range of the distance D from -0.307 mm to -0.235 mm. The distance D is -0.5 mm and the return light amount is -30 dBm when the light emitting point 110 is irradiated with the return light 120 . In other words, if the distance D from the coupling lens 200 to the light emitting unit 100 is in the range of -0.307 mm to -0.235 mm, the return light 120 irradiated to the light emitting point 110 is -30 dB or less.

In other words, if the front focal plane FFP is 0 mm, the light emitting unit 100 may be provided offset from the front focal plane FFP within a range of -0.047 mm to 0.024 mm from the front focal plane FFP.

As described above, it is assumed that the light emitting unit 100 is installed on the front focal plane FFP of the coupling lens 200 . As shown in FIG. 4, it is known that when the light emitting unit 100 is installed on the front focal plane FFP of the coupling lens 200, there is light irradiated outside the core 21 of the optical fiber 20. FIG. Therefore, the relationship between the amount of light incident on the core 21 of the optical fiber 20 (hereinafter referred to as the amount of incident light) and the distance D from the coupling lens 200 to the light emitting section 100 will be described.

FIG. 6 is a diagram for explaining the relationship between the amount of light incident on the core 21 of the optical fiber 20 and the distance D from the coupling lens 200 to the light emitting section 100. As shown in FIG. The horizontal axis in FIG. 6 is the distance D (mm) from the end of the coupling lens 200 to the light emitting section 100, as in FIG. The vertical axis represents the amount of light incident on the core 21 of the optical fiber 20 (dBm). A gap Zf between the coupling lens 200 and the end face 23 of the core 21 in the optical fiber 20 is set to 0.6 mm.
FIG. 6 also shows three cases of 0.020 mm, 0.022 mm, and 0.024 mm as the radius r of the circle 111 on which the light emitting points 110 are arranged.
The relationship between the amount of light incident on the core 21 of the optical fiber 20 and the distance D from the coupling lens 200 to the light emitting section 100 shown in FIG. 6 was obtained by simulation.

Assuming that all the lights emitted from the three light emitting points 110 are incident on the core 21 of the optical fiber 20, the amount of light incident on the optical fiber 20 is calculated to be 4.7 dBm. That is, the amount of light incident on the optical fiber 20 is 4.5 dBm to 4.7 dBm when the distance D is -0.5 mm, and almost all the amount of light emitted from the three light emitting points 110 enters the optical fiber 20. It shows that it will be in a state of incidence.

On the other hand, as shown in FIG. 6, the amount of light incident on the optical fiber 20 is 3.6 to 3.9 dBm when the distance D is in the range of -0.307 mm to -0.235 mm. In other words, it can be seen that there is light irradiated outside the core 21 of the optical fiber 20 . However, this decrease in the amount of incident light is 0.8 dB to 1.1 dB compared to the state in which all the amounts of light emitted from the three light emitting points 110 are incident on the optical fiber 20 . This means that about 80% of the light amount of 4.7 dBm when all the three light emitting points 110 are incident on the optical fiber 20 is incident on the core 21 of the optical fiber 20 . The ratio of the light emitted from the light emitting point 110 that enters the core 21 of the optical fiber 20 is called coupling efficiency. In the above case, the coupling efficiency is approximately 80%.

FIG. 7 is a diagram for explaining the relationship between the amount of light incident on the core 21 of the optical fiber 20 and the distance from the coupling lens 200 to the light emitting section 100, in order to explain the results of FIG.
In FIG. 7, in addition to the front focal point FF, the front focal plane FFP, the rear focal point RF, and the rear focal plane RFP, a 2× front focal point FF2 and a 2× front focal plane FFP2 are arranged from the coupling lens 200 toward the front focal point FF. It is written. Further, in FIG. 7 , a 2× back focus RF2 and a 2× back focal plane RFP2 are indicated from the coupling lens 200 to the back focus RF side. That is, the 2× front focal point FF2 is located on the optical axis OA of the coupling lens 200 at a position that is twice the front focal length of the front focal point FF. The 2x front focal plane FFP2 is a plane perpendicular to the optical axis OA at the 2x front focus FF2. Similarly, the 2x back focus RF2 is located on the optical axis OA of the coupling lens 200 at twice the back focal length relative to the back focus RF. Also, the 2x rear focal plane RFP2 is a plane perpendicular to the optical axis OA at the 2x rear focus RF2.

On the front focal plane FFP corresponding to the present embodiment, light emitted from a point P1 at a radius r away from the center O is emitted from the coupling lens 200 as parallel light. On the other hand, on the 2x front focal plane FFP2, the light emitted from the point P2, which is a radius r away from the center O, is emitted from the coupling lens 200 as converged light toward the 2x rear focal plane RFP2. Therefore, light converging toward the double rear focal plane RFP2 is incident on the end face 23 of the core 21 in the optical fiber 20 .

Therefore, the light emitted from the point P2 has a larger amount of light incident on the core 21 of the optical fiber 20 than the light emitted from the point P1. As shown in FIG. 6, if the distance D corresponding to the 2× front focal plane FFP2 is around −0.5 mm, the distance D is in the range of −0.307 mm to −0.235 mm. big in comparison. That is, the distance D near -0.5 mm is the position where the maximum coupling efficiency is obtained.

However, if the distance D is −0.5 mm, that is, if the light emitting point 110 is placed on the double front focal plane FFP2 in FIG. The light 120 is not converged on the surface of the light emitting unit 100 but spreads out. As a result, the return light 120 irradiates the light emitting point 110, which may destabilize the operation of the light emitting element such as the vertical cavity surface emitting laser VCSEL.

However, if the operation of the light emitting element does not become unstable, the light emitting point 110 may be provided at a position closer to the front focal plane FFP than the double front focal plane FFP2. For example, if the decrease in the amount of return light is 10 dB or more compared to the amount of return light at the position where the maximum coupling efficiency is obtained, the operation of a light-emitting element such as a vertical cavity surface emitting laser VCSEL is less likely to become unstable. In other words, the light emitting point 110 may be provided at a position where the return light amount is reduced by 10 dB or more compared to the return light amount at the position where the maximum coupling efficiency is obtained.

Further, the smaller the decrease in coupling efficiency, the better. is obtained, the coupling efficiency decreases, that is, the coupling loss becomes 1 dB or less.

According to FIGS. 5 and 6, the above range is a range in which the distance D is -0.235 mm or more and less than -0.338 mm. That is, the distance D should be 0.9 times or more and less than 1.3 times the front focal length of the front focal point FF of −0.26 mm.

When the light-emitting point 110 is placed on the 2x front focal plane FFP2 and the end face 23 of the core 21 in the optical fiber 20 is placed on the 2x back focal plane RFP2, the end face 23 of the core 21 in the optical fiber 20 is , where the focused light is incident. Therefore, it is considered that the coupling efficiency between the light emitting point 110 and the optical fiber 20 is good. However, the return light reflected by the end face 23 of the core 21 in the optical fiber 20 returns to the light emitting point 110 and illuminates the light emitting point 110 . In other words, the operation of a light emitting device such as a vertical cavity surface emitting laser VCSEL may become unstable.

So far, the explanation has been given on the assumption that the center O of the three light emitting points 110 is on the optical axis OA. However, the center O of the light emitting point 110 may deviate from the optical axis OA.
FIG. 8 is a diagram showing the amount of returned light when the center O of the light emitting unit 100 is deviated from the optical axis OA. FIG. 8(a) is a diagram for explaining the relationship between the center O of the light emitting unit 100 and the optical axis OA, and FIG. 8(b) is the amount of return light. In FIG. 8B, the horizontal axis is the y-direction offset S (μm), which is the amount of deviation of the center O from the optical axis OA, and the vertical axis is the amount of return light (dBm).

As shown in FIG. 8(a), similarly to FIG. 2(a), three light emitting points 110-1, 110-2, 110-3 arranged on a circle 111 centered at O are arranged at equal intervals. It is Here, as shown in FIG. 8A, since the direction passing between the light emitting point 110-1 and the light emitting points 110-2 and 110-3 is the y direction, the distance between the optical axis OA and the center O The offset S (μm), which is the amount of deviation, is the amount of deviation in the y direction. Therefore, the y-direction offset is S (μm). In FIG. 8B, the radius r of the circle 111 from the center O of the light emitting point 110 in the light emitting section 100 is 0.020 mm. Also shown are cases where the distance D from the coupling lens 200 of the light emitting unit 100 is -0.25 mm, 0.26 mm, and 0.27 mm. Then, as shown in FIG. 8B, regardless of the distance D between the light emitting unit 100 and the coupling lens 200, the increase in the return light amount is within 10 dB when the offset S in the y direction is in the range of -3 μm to 3 μm. suppressed. Therefore, in the range of distance D (-0.307 mm to -0.235 mm) where the amount of returned light is 30 dB or less than the amount of returned light at the position where the maximum coupling efficiency is obtained, as shown in FIG. Even with an increase of 10 dB, the effect on the operation of a light-emitting element such as a vertical cavity surface emitting laser VCSEL is suppressed.

As described above, in the optical signal transmission device 10 according to the present embodiment, the amount of light returned to the light emitting point 110 is 30 dB or less, and the reduction in coupling efficiency is 1 dB or less.

In the following, the structure and manufacturing method of a vertical cavity surface emitting laser VCSEL will be described on the assumption that the light emitting point 110 is composed of the vertical cavity surface emitting laser VCSEL.
(Structure of Vertical Cavity Surface Emitting Laser VCSEL)
FIG. 9 is an enlarged view of the light emitting section 100 having three light emitting points 110. FIG. 9(a) is a top view of the light emitting section 100, and FIG. 9(b) is a sectional view of the light emitting section 100 along line IXB-IXB of FIG. 9(a). Although the light emitting unit 100 is configured as a chip with a rectangular outer shape, it may have another shape.

Here, it is assumed that the emission point 110 is composed of a vertical cavity surface emitting laser VCSEL with reference to the top view of FIG. 9A. Therefore, hereinafter, the vertical cavity surface emitting laser VCSEL is referred to instead of the light emitting point 110 . Here, as an example, there are three light-emitting points 110, and three vertical cavity surface emitting lasers VCSEL-1, VCSEL-2, and VCSEL-3 are used to distinguish between the three vertical cavity surface emitting lasers. write.

As shown in FIG. 9A, the three vertical cavity surface emitting lasers VCSELs are arranged along a circle 111 around the center O at equal intervals. FIG. 9B shows cross sections of the vertical cavity surface emitting lasers VCSEL-2 and VCSEL-3.
Since the three vertical cavity surface emitting lasers VCSEL have the same structure, one vertical cavity surface emitting laser VCSEL, here, the vertical cavity surface emitting laser VCSEL-2 is taken as an example. to explain. The vertical cavity surface emitting laser VCSEL-2 is referred to as a vertical cavity surface emitting laser VCSEL.

As shown in FIG. 9A, the vertical cavity surface emitting laser VCSEL has a circular outer shape when viewed from above. The central portion of the circle is an emission surface 112 from which the vertical cavity surface emitting laser VCSEL emits light to the outside. Note that the emission surface 112 may be the light emitting point 110 . The upper surface of the light emitting section 100 is covered with a wiring layer 419 except for the emission surface 112 .
The outer shape of the vertical cavity surface emitting laser VCSEL as viewed from above does not have to be circular, and may be elliptical, rectangular, polygonal, or any other shape.

Next, the cross-sectional structure of the vertical cavity surface emitting laser VCSEL will be described with reference to the cross-sectional view shown in FIG. 9(b).
The vertical cavity surface emitting laser VCSEL has an n-type lower distributed Bragg reflection layer 411, an active layer 412, and an n-type lower distributed Bragg reflection layer 411 on one surface (this is referred to as the surface) of an n-type substrate 410 made of, for example, GaAs. It comprises an oxidized constricting layer 413 , a p-type upper distributed Bragg reflection layer 414 , a p-type contact layer 415 , a p-side electrode 416 , an interlayer insulating layer 417 , an emission surface protective layer 418 , and a wiring layer 419 . . Since the distributed Bragg reflector layer is called a DBR (Distributed Bragg Reflector) layer, hereinafter, the n-type lower distributed Bragg reflector layer 411 is referred to as the lower DBR layer 411, and the p-type upper distributed Bragg reflector layer 414 is referred to as the upper DBR layer 411. It is written as a DBR layer 414 .

The three vertical cavity surface emitting lasers VCSELs in the light emitting section 100 are separated by a mesa structure, that is, a mesa 113 created by so-called mesa etching. That is, the vertical cavity surface emitting laser VCSEL has an n-type lower DBR layer 411, an active layer 412, an oxidized constriction layer 413, a p-type upper DBR layer 414, and a contact layer 415 on the surface of an n-type substrate 410 in this order. It has a laminated construction. An interlayer insulating layer 417 is provided on the surface (including the side surface of the mesa 113) excluding the emission surface 112 and the p-side electrode 416. FIG. When viewed from above, a p-side electrode 416 is provided so as to surround the emission surface 112, and an emission surface protective layer 418 is provided in the central portion. Furthermore, a wiring layer 419 is provided on the interlayer insulating layer 417 except for the output surface 112 . Note that the wiring layer 419 is connected to the p-side electrodes 416 of the plurality of vertical cavity surface emitting lasers VCSELs.

Also, an n-side electrode 420 that makes ohmic contact with the n-type substrate 410 is provided on the other surface (rear surface) of the n-type substrate 410 . This configuration is for the case of using an n-type substrate, and when using a p-type substrate, the polarities of p and n are reversed, and the directions of voltage and current are also reversed.

Then, when a reference potential (such as GND) is applied to the n-side electrode 420 provided on the back surface of the substrate 410, and a positive drive potential is applied to the p-side electrode 416 via the wiring layer 419, the vertical resonator plane The light emitting laser VCSEL performs laser oscillation and emits laser light from the emission surface 112 . The laser light is emitted in a direction perpendicular to the emission surface 112 , that is, in a direction perpendicular to the substrate 410 . Note that the center of the emission surface 112 is the center C of the light emitting point 110 . Note that the exit surface 112 does not necessarily need to be circular. When the shape of the output surface 112 is an ellipse, a triangle, a square, a polygon exceeding a square, or the like, the center C of the light emitting point 110 is the center of gravity when the output surface 112 is plate-shaped.

Below, the structure of each layer shown in FIG.9(b) is demonstrated.
The substrate 410 is described using n-type GaAs as an example, but may be intrinsic (i) GaAs to which no impurity is added. Also, semiconductor substrates made of InP, GaN, InAs, other III-V group, II-VI materials, sapphire, Si, Ge, etc. may be used. If the substrate is modified, the material that is monolithically deposited on the substrate uses materials that approximately match the lattice constant of the substrate (including strained structures, strain-relaxed layers, and metamorphic growth). However, if the substrate 410 is electrically insulating, it is necessary to separately provide wiring for supplying a reference potential (such as GND). In addition, when the semiconductor layer laminate other than the substrate 410 is adhered to another support substrate and the semiconductor layer laminate is provided on the other support substrate, it is not necessary to match the lattice constant with the support substrate. A detailed description of these will be omitted.

The lower DBR layer 411 is formed by alternately stacking two semiconductor layers having different refractive indices. In other words, two semiconductor layers having different refractive indices have a film thickness of 0.25λ/n, where λ is the oscillation wavelength of the vertical cavity surface emitting laser VCSEL, and n is the refractive index of the semiconductor layer as a medium. is set to be By doing so, the lower DBR layer 411 functions as a reflective layer that selectively reflects light of wavelength λ, that is, as a dielectric mirror.

The active layer 412 includes, from the lower DBR layer 411 side, a lower spacer layer 412a, a quantum well active layer 412b, and an upper spacer layer 412c. The quantum well active layer 412b is composed of, for example, four barrier layers made of GaAs and three quantum well layers made of InGaAs each provided between the barrier layers. The lower spacer layer 412 a is between the quantum well active layer 412 b and the lower DBR layer 411 , and the upper spacer layer 412 c is between the quantum well active layer 412 b and the oxide confinement layer 413 . Note that the oxidized constricting layer 413 may be regarded as part of the upper DBR layer 414 and may be provided inside the upper DBR layer 414 . The lower spacer layer 412a and the upper spacer layer 412c are located between the lower DBR layer 411 and the upper DBR layer 414, adjust the length of the resonator, and function as clads for confining carriers.

The oxidized constricting layer 413 provided on the active layer 412 comprises an unoxidized region 413a and an oxidized region 413b. As will be described later, the oxidized region 413b is formed from the side surface of the mesa 113, so that when viewed from above, it is formed from the periphery to the center of the vertical cavity surface emitting laser VCSEL. The central portion becomes the non-oxidized region 413a. As will be described later, the oxidized region 413b is a region where current does not easily flow, and the non-oxidized region 413a is a region where current easily flows. In other words, the oxidized constricting layer 413 constricts the current flow path in the vertical cavity surface emitting laser VCSEL.

Since the periphery of the mesa 113 has many defects due to mesa etching, non-radiative recombination is likely to occur. Therefore, if the oxidized region 413b is not provided, oscillation of the vertical cavity surface emitting laser VCSEL tends to become unstable, and power is consumed for non-radiative recombination. Therefore, by providing the oxidized region 413b, the current can be easily concentrated in the central non-oxidized region 413a, and the power consumed for non-radiative recombination can be suppressed, thereby reducing the power consumption and improving the light extraction efficiency. . Furthermore, laser oscillation of the vertical cavity surface emitting laser VCSEL is stabilized.

That is, in the vertical cavity surface emitting laser VCSEL, laser oscillation occurs in the non-oxidized region 413a. Therefore, by reducing the diameter of the non-oxidized region 413a to 5 μm, 7 μm, or the like, the vertical cavity surface emitting laser VCSEL oscillates in a single mode.

Like the lower DBR layer 411, the upper DBR layer 414 is formed by alternately stacking two semiconductor layers having different refractive indices. In other words, the two semiconductor layers having different refractive indices are each set to have a film thickness of 0.25λ/n. By doing so, the upper DBR layer 414 functions as a reflective layer that selectively reflects light of wavelength λ, that is, as a dielectric mirror. A resonator is formed with the lower DBR layer 411 to confine light of wavelength λ.

The contact layer 415 is a layer for facilitating ohmic contact with the p-side electrode 416 .
The p-side electrode 416 is provided so as to surround the center of the mesa when viewed from above. The p-side electrode 416 is made of a metal material that facilitates ohmic contact with the p-type contact layer 415 .

The interlayer insulating layer 417 is provided so as to cover the surface of the vertical cavity surface emitting laser VCSEL (including the side surface of the mesa 113) except for the portion where the emission surface 112 and the p-side electrode 416 are formed. . The interlayer insulating layer 417 is composed of, for example, a silicon nitride film. Note that the interlayer insulating layer 417 may be a silicon oxide (SiO ₂ ) film, a silicon oxynitride (SiON) film, or the like, in addition to the silicon nitride film.

The output surface protective layer 418 has a transmittance of light having a wavelength λ emitted from the active layer 412 to the output surface 112, which is a central region of the mesa surrounded by the p-side electrode 416 when viewed from above. is made up of high quality materials. The output surface protective layer 418 is composed of, for example, a silicon nitride (SiN) film. Note that the interlayer insulating layer 417 and the output surface protective layer 418 may be formed at the same time. In this case, the interlayer insulating layer 417 and the output surface protective layer 418 are made of the same material.

A wiring layer 419 provided on the interlayer insulating layer 417 is provided so as to cover the surface of the vertical cavity surface emitting laser VCSEL except for the emission surface 112 of the vertical cavity surface emitting laser VCSEL. A wiring layer 419 is provided to cover the surface of the light emitting section 100 so as to connect the p-side electrodes 416 of the vertical cavity surface emitting lasers VCSELs in parallel.

(Manufacturing method of vertical cavity surface emitting laser VCSEL)
Next, a method for manufacturing a vertical cavity surface emitting laser VCSEL will be described.
FIG. 10 is a diagram for explaining a method of manufacturing a vertical cavity surface emitting laser VCSEL. 10(a) is a semiconductor layer laminate forming step, FIG. 10(b) is a mesa etching step, FIG. 10(c) is an oxidized region forming step, FIG. 10(d) is a p-side electrode forming step and FIG. 10(e) is an output surface protective layer forming step and an n-side electrode forming step, and FIG. 10(f) is a wiring layer forming step.

In the step of forming a semiconductor layer stack shown in FIG. 10A, an n-type lower DBR layer 411, an active layer 412, an oxidized constricting layer 413, and a p-type upper DBR layer 414 are formed on a substrate 410 of n-type GaAs. , p-type contact layers 415 are successively stacked to form a semiconductor layer stack.

The n-type lower DBR layer 411 is formed of AlAs and GaAs with a carrier concentration of 1×10 ¹⁸ cm ⁻³ by, for example, a metal organic chemical vapor deposition (MOCVD) method so that each film thickness is 0.25λ/n. are alternately stacked for 30 cycles. Note that unlike FIG. 9B, FIG. 10A does not show a multi-layer structure.

The active layer 412 includes an undoped Al _0.22 Ga _0.78 As lower spacer layer 412a, an undoped quantum well active layer 412b, and an undoped Al _0.22 Ga _0.78 As upper spacer layer 412a. It is formed by stacking layers 412c. The quantum well active layer 412b is composed of three InGaAs quantum well layers with a thickness of 80 nm and four GaAs barrier layers with a thickness of 150 nm. The thickness of the active layer 412 is set to λ/n.

The oxidized constricting layer 413 is AlAs with a carrier concentration of 1×10 ¹⁸ cm ⁻³ . The thickness of the oxidized constricting layer 413 is set to 0.25λ/n.

The upper DBR layer 414 is formed by alternately laminating Al _0.9 Ga _0.1 As having a carrier concentration of 1×10 ¹⁸ cm ⁻³ and GaAs so that each layer has a thickness of 0.25λ/n for 22 cycles. It is formed by The total film thickness is set to be about 2 μm.
As previously mentioned, the oxidized constricting layer 413 may be part of the upper DBR layer 414 .

The p-type contact layer 415 is GaAs with a carrier concentration of 1×10 ¹⁹ cm ⁻³ . The thickness of the p-type contact layer 415 is set to λ/n.

Trimethylgallium, trimethylaluminum, trimethylindium, and arsine were used as raw material gases in the MOCVD method. As impurity (dopant) materials, cyclopentazinium magnesium was used for p-type and silane was used for n-type. The substrate temperature was set to 750.degree.

In the mesa etching step shown in FIG. 10B, the semiconductor layer stack is etched halfway down the lower DBR layer 411 to form a mesa 113 . Here, the oxidized constricting layer 413, which is AlAs, is exposed. That is, an etching mask (resist mask) R is formed by photolithography in a portion that will become the vertical cavity surface emitting laser VCSEL, and the mesa 113 is formed by reactive ion etching using carbon tetrachloride (CCl ₄ ) as an etching gas. formed.

In the oxidized region forming step of FIG. 10C, the oxidized constricting layer 413 made of AlAs is oxidized to form an oxidized region 413b. That is, after removing the resist mask R, steam oxidation is performed in a furnace at about 400° C. to oxidize the oxidized constricting layer 413 made of AlAs from the side surface of the mesa 113 . The peripheral portion of the oxidized mesa 113 becomes an oxidized region 413b, and the central portion of the mesa 113 that is not oxidized becomes an unoxidized region 413a. In other words, in the oxidized region 413b, AlAs is oxidized to include insulating Al ₂ O ₃ , so that current becomes difficult to flow. On the other hand, the non-oxidized region 413a remains as a region where current flows easily, that is, as a current injection region.

In the p-side electrode forming step in the p-side electrode forming step and interlayer insulating layer forming step of FIG. That is, a resist mask (not shown) is formed except for the portion where the p-side electrode 416 is to be formed, and a metal material that will become the p-side electrode 416 is deposited by vacuum deposition or the like. Then, the p-side electrode 416 is formed by the so-called lift-off method of removing the metal material on the resist mask together with the resist mask. As a result, the metal material that forms the p-side electrode 416 remains in the portion where the p-side electrode 416 is to be formed. The p-side electrode 416 is made of Ti/Au, which easily forms an ohmic contact with p-type GaAs.

In the interlayer insulating layer forming step in the p-side electrode forming step and the interlayer insulating layer forming step of FIG. An interlayer insulating layer 417 is deposited. That is, in the same manner as the formation of the p-side electrode 416, a resist mask (not shown) is formed in a region surrounded by the p-side electrode 416 and the emission surface 112, and a nitriding layer that becomes the interlayer insulating layer 417 is formed. Silicon (SiN) is deposited by vacuum deposition or the like. Then, along with the resist mask, the silicon nitride on the resist mask is removed (lift-off method). As a result, an interlayer insulating layer 417 is formed on the surface including the side surfaces of the mesa 113 of the vertical cavity surface emitting laser VCSEL.

In the output surface protective layer forming step in the output surface protective layer forming step and wiring layer forming step of FIG. In other words, a resist mask (not shown) is formed except for a region surrounded by the p-side electrode 416 and serving as the emission surface 112, and silicon nitride (SiN) serving as the emission surface protection layer 418 is deposited by vacuum deposition or the like. Then, along with the resist mask, the silicon nitride on the resist mask is removed (lift-off method). As a result, the emission surface protective layer 418 is formed in the region surrounded by the p-side electrode 416. FIG.

In the n-side electrode forming step in the output surface protective layer forming step and the n-side electrode forming step of FIG. That is, a metal material to be the n-side electrode 420 is deposited on the rear surface side of the substrate 410 by vacuum deposition or the like. For the n-side electrode 420, Au/Ge, which easily forms an ohmic contact with n-type GaAs, is used.

In the wiring layer forming step in the wiring layer forming step of FIG. The wiring layer 419 is formed to cover the surface of the vertical cavity surface emitting laser VCSEL, excluding the emission surface 112, as described above.

Various modifications may be made as long as they do not contradict the gist of the present invention.

DESCRIPTION OF SYMBOLS 1... Optical transmission system 10... Optical signal transmission apparatus 11... Light source part 12... Signal modulation part 20... Optical fiber 21... Core 22... Clad 23... End surface 30... Signal processing apparatus 100... Light emission Part 110 Light-emitting point 111 Circle 112 Output surface 113 Mesa 120 Return light 200 Coupling lens 210 Plano-convex lens 220 Cylindrical part 410 Substrate 411 Lower distribution Bragg Reflective layer (lower DBR layer) 412 Active layer 413 Oxidized constricting layer 413a Non-oxidized region 413b Oxidized region 414 Upper distributed Bragg reflector layer (upper DBR layer) 415 Contact layer 416 p-side electrode 417 interlayer insulating layer 418 output surface protective layer 419 wiring layer 420 n-side electrode λ center wavelength D distance between light source and coupling lens FF front focus , FFP... front focal plane, RF... rear focal plane, RFP... rear focal plane, VCSEL... vertical cavity surface emitting laser, n... refractive index, r... radius

Claims (12)

a light emitting unit having a plurality of light emitting points for emitting light;
a lens provided on the optical path of the light emitted from the plurality of light emitting points and coupling the light emitted from the plurality of light emitting points with an externally provided optical transmission unit;
the plurality of light-emitting points are odd in number, none of the light-emitting points are arranged on the optical axis in a direction perpendicular to the optical axis of the lens, and they are not provided at symmetrical positions with respect to the optical axis;
A light-emitting device in which return light generated at symmetrical positions with respect to the optical axis for each of the plurality of light-emitting points does not overlap other light-emitting points . The plurality of light-emitting points are positioned closer to the front focal point of the lens than the position at which the maximum coupling efficiency is obtained with respect to the light transmission section on a straight line parallel to the optical axis of the lens that is displaced from the optical axis. 2. The light emitting device according to claim 1, which is provided at a close position. 3. The light emission according to claim 2, wherein the plurality of light emitting points are provided at positions where the amount of return light decreases by 10 dB or more with respect to the amount of light returning to the light emitting points at the position where the maximum coupling efficiency is obtained. Device. 4. The light-emitting device according to claim 2, wherein the plurality of light-emitting points are provided at positions where a decrease in coupling efficiency is 1 dB or less with respect to the maximum coupling efficiency. 2. The light emitting device according to claim 1, wherein the plurality of light emitting points are provided at a distance of 0.9 times or more and less than 1.3 times the front focal length of the lens. 3. A light emitting device according to claim 2, wherein the lens has an end at a rear focal point of the lens. The light-emitting device according to claim 1, wherein the plurality of light-emitting points are arranged along a circle centered on the optical axis of the lens. 8. The light-emitting device according to claim 7, wherein the plurality of light-emitting points are arranged at regular intervals along the circle. 2. The light-emitting device according to claim 1, wherein the plurality of light-emitting points in the light-emitting portion are surface-emitting laser elements. 10. The light emitting device according to claim 9, wherein said surface emitting laser element oscillates in a single mode. A light emitting device according to any one of claims 1 to 10;
a drive unit that drives the light emitting device and converts the acquired electrical signal into an optical signal by a light emitting point provided in the light emitting device;
An optical signal transmitter comprising: An optical signal transmission device according to claim 11;
and an optical transmission unit provided at a position where a principal ray output from the optical signal transmission device is combined.

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