Optical transmitter and optical communication system

Abstract

To provide an optical transmitter that can minimize the amount of reflected light coupling and suppress the effects of noise.SOLUTION: An optical transmitter 100 includes a first optical waveguide 110, one end of which is connected to a light-emitting element 200, and which propagates a first optical signal incident from the light-emitting element 200 in a first direction. The optical transmitter 100 also includes a second optical waveguide 120, one end of which is connected to an optical fiber, and which includes a coupling path that runs parallel to the first optical waveguide 110 for a predetermined length in the first direction, and which propagates a second optical signal coupled from the first optical waveguide 110 in the coupling path and outputs the second optical signal to the optical fiber. Furthermore, the second optical waveguide 120 has an optical path length difference from the first optical waveguide 110 that is equal to half the wavelength of the first optical signal or an integer multiple of half the wavelength.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical transmitter and an optical communication system. [Background technology]

[0002] Conventionally, techniques have been proposed for suppressing noise caused by signal reflection in optical communications. Patent Document 1 discloses an optical transmission system for suppressing intensity noise. The optical transmission system disclosed in Patent Document 1 suppresses reflected light from the output end face from returning to the optical waveguide by obliquely polishing the output end face of the optical waveguide at an angle of 8 degrees, thereby suppressing intensity noise caused by phase noise. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-187662 Summary of the Invention [Problem to be solved by the invention]

[0004] In the optical transmission system disclosed in Patent Document 1, the output end of the optical waveguide is polished at an angle of 8 degrees to suppress light reflection and reduce the effects of relative intensity noise. However, even when the output end of the optical waveguide is polished at an angle of 8 degrees, it may still be affected by noise due to returned light, etc.

[0005] The present invention has been made in view of the problems inherent in the conventional technology, and an object of the present invention is to provide an optical transmitter that can reduce the amount of coupled return light and suppress the effects of noise. [Means for solving the problem]

[0006] An optical transmitter according to an aspect of the present invention comprises a first optical waveguide having one end connected to a light-emitting element and propagating a first optical signal incident from the light-emitting element in a first direction, and a second optical waveguide having one end connected to an optical fiber and comprising a coupling path running parallel to the first optical waveguide for a predetermined length in the first direction, propagating a second optical signal coupled from the first optical waveguide in the coupling path and emitting the second optical signal to the optical fiber, wherein the second optical waveguide has an optical path length difference from the first optical waveguide that is half the wavelength of the first optical signal or an integer multiple of the half wavelength.

[0007] An optical communication system according to another aspect of the present invention includes the optical transmitter described above and a light emitting element that emits an optical signal. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an optical transmitter that can reduce the amount of coupled return light and suppress the influence of noise. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating a configuration of an optical communication system according to an embodiment of the present invention. [Figure 2] 5A and 5B are diagrams for explaining fluctuations in optical power in the optical transmitter according to the embodiment. [Figure 3A] 3A and 3B are diagrams for explaining propagation of an optical signal in the optical transmitter according to the embodiment. [Figure 3B] 3A and 3B are diagrams for explaining propagation of an optical signal in the optical transmitter according to the embodiment. [Figure 3C] 3A and 3B are diagrams for explaining propagation of an optical signal in the optical transmitter according to the embodiment. [Figure 4] FIG. 10 is a diagram illustrating a configuration of an optical transmitter according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] An optical communication system 10 and an optical transmitter 100 according to this embodiment will be described in detail below with reference to the drawings. Note that the dimensional proportions in the drawings are exaggerated for the sake of explanation and may differ from the actual proportions. In addition, in the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0011] (Optical Communication System 10) 1 is a diagram showing the configuration of an optical communication system 10 according to this embodiment. The optical communication system 10 includes a light emitting element 200 and an optical transmitter 100.

[0012] The light emitting element 200 is configured, for example, by a quantum dot laser, which is a type of semiconductor laser in which quantum dots are spread all over the active layer. A quantum dot laser is configured by quantum dots, which are particles of compound semiconductor or oxide semiconductor, each having a diameter of 2 to 10 nm or less.

[0013] By controlling the size of the quantum dots, quantum dot lasers can control the band gap of the semiconductor, making it possible to set the emission wavelength as desired. Quantum dot lasers also consume less power and have better temperature stability than general light-emitting elements. Furthermore, quantum dot lasers can operate stably even at temperatures above 150 degrees, meeting the in-vehicle temperature requirements when installed in a vehicle.

[0014] The optical transmitter 100 is a small and lightweight optical integrated device that is constructed by densely mounting the functions of an optical transceiver on a silicon substrate using semiconductor manufacturing technology. This optical integrated device is configured as an evanescent coupler that includes a first optical waveguide 110 and a second optical waveguide 120.

[0015] In this embodiment, one end of the first optical waveguide 110 is connected to the light emitting element 200, and the other end is connected to a monitoring device that monitors the light emission power. The light emission power monitor may be a general device that can measure the optical power of an optical signal. The first optical waveguide 110 propagates the optical signal incident from the light emitting element 200 in a first direction. The first direction is the direction indicated by the X direction in FIG. 1, which is the longitudinal direction of the first optical waveguide 110, and is the direction from one end connected to the light emitting element 200 to the other end connected to the monitoring device. The optical signal propagating in the first direction in the first optical waveguide 110 corresponds to the first optical signal.

[0016] One end of the second optical waveguide 120 is connected to an optical fiber, and an optical signal emitted from the light emitting element 200 propagates through the first optical waveguide 110, is coupled to the second optical waveguide 120 at the coupling path, and is output to the optical fiber. In the example shown in Fig. 1, the coupling path corresponds to a section where the first optical waveguide 110 and the second optical waveguide 120 run side by side in close proximity.

[0017] The other end of the second optical waveguide 120 is connected to a monitoring device that monitors the optical power of the returning light. The power of the returning light may be monitored by a general device that can measure the optical power of an optical signal. That is, the second optical waveguide 120 includes a coupling path that runs parallel to the first optical waveguide 110 for a predetermined length in the first direction. The optical signal propagating in the second optical waveguide 120 in the first direction corresponds to the second optical signal.

[0018] An evanescent coupler is a waveguide made of silicon or polymer, and is configured with a waveguide structure that couples the signal emitted from a laser light source in the optical waveguide with high efficiency and low loss. For example, the ratio of the optical intensity in the coupled state to that in the uncoupled state (extinction ratio) varies depending on the design and manufacturing process, but is generally 20 dB or more.

[0019] The evanescent coupler can be manufactured by known manufacturing methods such as photolithography and laser drawing. Silicon and polymer materials are materials with excellent high-temperature resistance, and for example, organic-inorganic hybrid resins can be used for polymer materials. When the evanescent coupler is manufactured by laser drawing, quartz glass material is used.

[0020] The optical transmitter 100 according to this embodiment includes a first optical waveguide 110 and a second optical waveguide 120. As shown in Fig. 1, the first optical waveguide 110 and the second optical waveguide 120 have a section having a coupling path length L1 adjacent to each other as part of the coupling path, and in this section of coupling path length L1, an optical signal incident on the first optical waveguide 110 travels to the second optical waveguide 120. This utilizes the seepage of an electric field due to an evanescent wave.

[0021] If the penetration depth of this evanescent wave (the distance between the first optical waveguide 110 and the second optical waveguide 120) is defined as a penetration distance d, which indicates how much the electric field caused by the evanescent wave penetrates into the second optical waveguide 120, then the penetration distance d is determined by the following equation (1). Here, λ represents the wavelength. Furthermore, n1 represents the refractive index of the medium on the incident side, and n2 represents the refractive index of the medium on the coupling side. Furthermore, θ1 represents the angle of incidence.

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[0022] According to the above formula (1), by setting the refractive index to n1>n2, light leaks out only to one side, and the returning light does not return to the quantum dot laser side of the first optical waveguide 110.

[0023] As shown in FIG. 1 , the optical path lengths of the first optical waveguide 110 and the second optical waveguide 120 have an optical path length difference L2 that is half the wavelength of the optical signal emitted from the quantum dot laser or an integer multiple of that half wavelength. By providing the optical path length difference L2 between the first optical waveguide 110 and the second optical waveguide 120, a phase difference of 180 degrees is imparted to the incident light, minimizing the amount of coupled returned light. That is, by providing a phase difference of 180 degrees to the incident light, the incident light and the coupled returned light are in opposite phases, the wavelengths of the light cancel each other out, minimizing the amount of coupled returned light. Therefore, the optical transmitter 100 can suppress the effects of noise, such as relative intensity noise and phase noise.

[0024] 2 is a diagram illustrating fluctuations in optical power in the optical transmitter 100 according to this embodiment. As shown in Fig. 2, in a section having a coupling path length L1, the optical power of the optical signal propagating through the first optical waveguide 110 decreases to a point where it is minimum (hereinafter referred to as minimum point), and then increases to a point where it is maximum (hereinafter referred to as maximum point) where it is maximum. Thereafter, the optical signal leaks from the second optical waveguide 120 to the first optical waveguide 110, and the optical signal of the first optical waveguide 110 whose optical power has increased from the minimum point and the optical signal of the second optical waveguide 120 whose optical power has decreased from the maximum point are propagated.

[0025] 3A to 3C are diagrams for explaining propagation of an optical signal in the optical transmitter 100 according to this embodiment. As shown in Fig. 3A, an optical signal input from the light emitting element 200 propagates through the first optical waveguide 110, and is coupled to the second optical waveguide 120 at a coupling path with the second optical waveguide 120. The optical signal coupled to the second optical waveguide 120 is output to a subsequent optical fiber.

[0026] 3B, the optical signal propagated to the first optical waveguide 110 via the coupling path is output to the optical power monitoring device at the subsequent stage, thereby enabling the optical communication system 10 to constantly monitor the optical power of the quantum dot laser.

[0027] 3C, the returning light from the second optical waveguide 120 is output for returning light monitoring. This allows the optical communication system 10 to constantly monitor the returning light and return loss returning to the light emitting element 200. Furthermore, as described above, the returning light has an opposite phase to the incident light, so that the wavelengths of the light cancel each other out, minimizing the amount of coupling of the returning light.

[0028] As described above, the optical transmitter 100 according to this embodiment includes a first optical waveguide 110 having one end connected to the light-emitting element 200 and propagating a first optical signal input from the light-emitting element 200 in a first direction. The optical transmitter 100 also includes a second optical waveguide 120 having one end connected to an optical fiber and a coupling path running parallel to the first optical waveguide 110 for a predetermined length in the first direction, propagating a second optical signal coupled from the first optical waveguide 110 in the coupling path and emitting the second optical signal to the optical fiber. The second optical waveguide 120 has an optical path length difference from the first optical waveguide 110 that is half the wavelength of the first optical signal or an integer multiple of the half wavelength.

[0029] As a result, the optical transmitter 100 provides a 180-degree phase difference with respect to the incident light, so that the incident light and the returning light are in opposite phase, the wavelengths of the light cancel each other out, and the amount of coupling of the returning light is minimized. As a result, the optical transmitter 100 can suppress the effects of noise such as relative intensity noise and phase noise.

[0030] The light-emitting element 200 may also be a quantum dot laser. This allows the optical communication system 10 to control the band gap of the semiconductor by controlling the size of the quantum dots, thereby making it possible to arbitrarily set the emission wavelength. Furthermore, quantum dot lasers have lower power consumption and superior temperature stability compared to general light-emitting elements. Furthermore, quantum dot lasers can operate stably even at temperatures of 150 degrees or higher, and can meet the in-vehicle temperature requirements when installed in a vehicle.

[0031] (Other embodiments) Although the embodiments have been described in detail with reference to the drawings, the present embodiments are not limited to the contents described in the above embodiments. Furthermore, the components described above include those that can be easily imagined by a person skilled in the art and those that are substantially the same. Furthermore, the configurations described above can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the configurations can be made without departing from the spirit of the embodiments.

[0032] 4 is a diagram showing the configuration of an optical communication system 11 including an optical transmitter 100 according to another embodiment. The optical waveguide used in the optical transmitter 100 is a single-mode or multi-mode optical waveguide. When the input side of the optical transmitter 100 is single-mode, a configuration may be applied in which a spot size converter or the like is used to convert the single-mode to a multi-mode waveguide near the output end of the second optical waveguide 121. This makes it possible for the optical transmitter 100 shown in FIG. 4 to be coupled to a multi-mode optical fiber connected to the output end.

[0033] 4, the output end face (reflection surface) of the second optical waveguide 121 may be inclined at a predetermined angle. For example, the predetermined angle is 8 degrees. That is, the output end face of the second optical waveguide 121 may have an inclination angle of 8 degrees with respect to the second direction, which is perpendicular to the first direction. In FIG. 4, the first direction is indicated by the X direction, and the second direction is indicated by the Y direction. The predetermined angle is indicated by θ2.

[0034] Here, the reflectance with respect to the angle of the output end is determined by the Fresnel reflectance shown in the following equation (2): where m1 is the refractive index of the incident medium, m2 is the refractive index of the reflecting medium, and R is the reflectance.

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[0035] For example, if the end face of an optical fiber is cut at an 8-degree angle, the light enters at an angle, causing the reflected light to deviate from the central axis of the fiber and travel through the cladding, which is the portion away from the core. Furthermore, because the refractive index of the cladding is lower than that of the core, the reflected light undergoes repeated total reflections within the cladding and eventually leaks out of the fiber, significantly reducing the amount of reflected light returning to the fiber core. The same effect can be achieved with an optical waveguide when the end face is cut at an 8-degree angle, making it possible for the optical transmitter 100 to have an anti-reflection effect.

[0036] Furthermore, an AR coating (Anti-Reflection Coating) for absorbing reflection may be applied to the output end face (reflection surface) of the second optical waveguide 120. This enables the optical transmitter 100 to significantly prevent light from returning to the laser light source.

[0037] In the above-described embodiment, the phase difference is adjusted by providing the first optical waveguide 110 and the second optical waveguide 120 with an optical path length difference L2. Furthermore, the optical transmitter 100 may be configured to include a phase shifter, a temperature adjustment function, or the like, to provide a phase difference between the first optical waveguide 110 and the second optical waveguide 120, and to monitor the optical intensity and perform feedback control. This enables the optical transmitter 100 to adjust the phase difference with high precision.

[0038] The features of the optical transmitter 100 and the optical communication system 10 are described below.

[0039] The optical transmitter 100 according to the first aspect includes a first optical waveguide 110 having one end connected to a light-emitting element 200 and propagating a first optical signal input from the light-emitting element 200 in a first direction. The optical transmitter 100 also includes a second optical waveguide 120 having one end connected to an optical fiber and a coupling path running parallel to the first optical waveguide 110 for a predetermined length in the first direction, propagating a second optical signal coupled from the first optical waveguide 110 in the coupling path and emitting the second optical signal to the optical fiber. The second optical waveguide 120 has an optical path length difference from the first optical waveguide 110 that is half the wavelength of the first optical signal or an integer multiple of the half wavelength.

[0040] According to the above configuration, the optical transmitter 100 has a phase difference of 180 degrees with respect to the incident light, so that the incident light and the returning light have opposite phases, the wavelengths of the light cancel each other out, and the amount of coupling of the returning light is minimized. Therefore, the optical transmitter 100 can suppress the influence of noise such as relative intensity noise and phase noise.

[0041] The output end face of the second optical waveguide 121 of the optical transmitter 100 according to the second embodiment may have an inclination angle of 8 degrees with respect to the second direction, which is perpendicular to the first direction.

[0042] According to the above configuration, the optical transmitter 100 can have an anti-reflection effect.

[0043] In the optical transmitter 100 according to the third embodiment, the output end face of the second optical waveguide 121 may be provided with an AR coating to absorb reflections.

[0044] According to the above configuration, the optical transmitter 100 can significantly prevent light from returning to the laser light source.

[0045] The second optical waveguide 121 of the optical transmitter 100 according to the fourth embodiment may be provided with a spot size converter at the output end for converting from a single mode to a multimode.

[0046] According to the above configuration, the optical transmitter 100 can also be coupled to a multimode optical fiber connected to the output end.

[0047] An optical communication system 10 according to the fifth aspect includes the optical transmitter 100 described above and a light emitting element 200 that emits an optical signal.

[0048] According to the above configuration, the optical communication system 10 has a 180-degree phase difference with respect to the incident light, so that the incident light and the returning light have opposite phases, the wavelengths of the light cancel each other out, and the amount of coupling of the returning light is minimized. Therefore, the optical transmitter 100 can suppress the influence of noise such as relative intensity noise and phase noise.

[0049] The light emitting device 200 of the optical communication system 10 according to the sixth embodiment may be a quantum dot laser.

[0050] According to the above configuration, the optical communication system 10 can control the band gap of the semiconductor by controlling the size of the quantum dots, making it possible to arbitrarily set the emission wavelength. Furthermore, quantum dot lasers have lower power consumption and superior temperature stability compared to general light-emitting elements. Furthermore, quantum dot lasers can operate stably even at temperatures above 150°C, meeting the in-vehicle temperature requirements when installed in a vehicle.

[0051] A monitoring device that monitors the optical power of the first optical signal may be connected to the other end of the first optical waveguide 110 of the optical communication system 10 according to the seventh embodiment.

[0052] According to the above configuration, the optical communication system 10 can constantly monitor the optical power of the optical signal input from the light emitting element 200.

[0053] A monitoring device for monitoring the optical power of the returning light may be connected to the other end of the second optical waveguide 120 of the optical communication system 10 according to the eighth embodiment.

[0054] According to the above configuration, the optical communication system 10 can constantly monitor the return light and return loss. [Explanation of symbols]

[0055] 10, 11 Optical communication systems 100 Optical Transmitter 110 First optical waveguide 120, 121 Second optical waveguide 200 light-emitting elements

Claims

1. a first optical waveguide having one end connected to a light emitting element and configured to propagate a first optical signal incident from the light emitting element in a first direction; a second optical waveguide having one end connected to an optical fiber, the second optical waveguide including a coupling path running parallel to the first optical waveguide for a predetermined length in the first direction, the second optical waveguide propagating a second optical signal coupled from the first optical waveguide in the coupling path, and emitting the second optical signal to the optical fiber; an optical transmitter in which the second optical waveguide has an optical path length difference from the first optical waveguide that is half the wavelength of the first optical signal or an integral multiple of the half wavelength;

2. 2. The optical transmitter according to claim 1, wherein the output end face of the second optical waveguide has an inclination angle of 8 degrees with respect to a second direction that is perpendicular to the first direction.

3. 3. The optical transmitter according to claim 2, wherein an output end face of the second optical waveguide is provided with an AR coating for absorbing reflection.

4. 2. The optical transmitter according to claim 1, wherein the second optical waveguide has a spot size converter at an output end thereof for converting from a single mode to a multimode.

5. an optical transmitter according to claim 1; an optical communication system comprising the light-emitting element that emits an optical signal.

6. The optical communication system according to claim 5 , wherein the light emitting element is a quantum dot laser.

7. 7. The optical communication system according to claim 6, wherein a monitoring device that monitors the optical power of the first optical signal is connected to the other end of the first optical waveguide.

8. 7. The optical communication system according to claim 6, wherein a monitoring device for monitoring the optical power of the returning light is connected to the other end of the second optical waveguide.

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