JP2007214430A - Multimode optical communication system and multiwavelength surface emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low cost multimode optical communication system capable of transmitting at high speed, and to provide a multiwavelength surface emitting element. <P>SOLUTION: Two VCSELs (vertical cavity surface emitting laser) 2, 3 having an aperture 9a with a diameter of not more than 8 μm for emitting single mode light having different wavelengths are provided to a substrate 1. The diameter D of a circle circumscribed to the aperture 9a is smaller than that of a core 21 of a multimode optical fiber 20. This can directly introduce the laser beam emitted from the VCSELs 2, 3 to the core 21 without the optical system. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a multimode optical communication system using a plurality of vertical cavity surface emitting lasers (VCSELs) and multimode optical fibers, and a multi-surface optical laser including a plurality of surface emitting lasers having different oscillation wavelengths. The present invention relates to a wavelength surface light emitting device.

  In recent years, optical communication represented by FTTH (Fiber To The Home) has penetrated into ordinary homes at a very high speed and has become very familiar. In addition, it is inevitable that the access system will shift from 1 Gbps to 100 Gbps by taking advantage of the super-high speed that is the merit of optical communication.

  For optical communication, a single mode optical fiber suitable for high-speed transmission and long-distance communication, or a multimode optical fiber that can be used at a short distance, is relatively easy to handle, and is inexpensive. A single mode optical fiber has a very small core diameter, which is a light propagation region, of 10 μm or less, and it is difficult to accurately guide light from the outside to this portion. For this reason, there is a problem that adjustment of the optical system is difficult and costly. Since the multimode optical fiber has a large core diameter of generally 50 μm or more, the optical system can be easily adjusted. In addition, since a multimode optical fiber propagates light in a plurality of modes in the core and the propagation speed of the light is different for each mode, the optical signal spreads in time. Used in the following short distances.

  In order to spread widely to general users, it is important that the price is low, and the use of a multi-mode optical fiber can reduce the cost.

  As a conventional optical communication system using such a multimode optical fiber, for example, one using a multimode optical fiber and a single surface emitting laser having an aperture diameter larger than 8 μm is known ( For example, see Patent Document 1.) This system actively uses higher order modes for signal transmission.

  In addition, a method of guiding light to a single multimode optical fiber using a plurality of surface emitting lasers (VCSEL) having different wavelengths has been studied.

  FIG. 6 shows an example of the multimode optical communication system. The multimode optical communication system 200 transmits laser beams from the first and second VCSELs 201 and 202 having different wavelengths from the core 206a and the clad 206b via the first and second optical waveguides 203 and 204 and the lens 205. Is guided to a multimode optical fiber 206. According to this configuration, light can be guided to the core 206a of the multimode optical fiber 206 even when the interval between the VCSELs is large.

  Further, as a conventional multimode optical fiber, for example, one in which a plurality of VCSELs having different wavelengths are attached to a mounter and guided to the multimode optical fiber without passing through an optical system such as a lens is known (for example, a patent) (See Document 2.) Further, by appropriately adjusting the material of the quantum well active layer, for example, the ratio of Al and Ga, oscillation wavelengths having different wavelengths by 20 nm are obtained.

As a conventional method of changing the oscillation wavelength between a plurality of VCSELs formed on the same substrate, the volume velocity of MOCVD (Metal Organic Chemical Vapor Deposition) is affected by changing the mesa diameter between VCSELs. Given, the entire film thickness of DBR or MQW (multi quantum well structure) is different, and accordingly, a method of changing the oscillation wavelength by changing the cavity length (see, for example, Non-Patent Document 1), a resonator. There is a method of changing the oscillation wavelength by changing the cross-sectional area of the portion or the waveguide portion (see Patent Document 3).
Japanese Patent No. 3480899 Japanese Patent Laid-Open No. 2005-3847 IEEE Photonics Technology Letters Vol. 7, no. 1 p10 (1995) JP-A-6-97578

  However, according to the configuration of Patent Document 1, when many high-order modes are incident on the optical fiber, a group delay occurs between the modes, which is not suitable for high-speed transmission.

  Further, in the configuration shown in FIG. 6, since the light is guided to the multimode optical fiber through an optical system such as a lens, it takes time to align the multimode optical fiber, the optical system, and the VCSEL, resulting in high cost. .

  When an optical communication system using a multimode optical fiber is configured at low cost, the oscillation wavelength of the two-wavelength VCSEL is 15 nm or more so that a low-cost edge filter can be used for wavelength separation on the receiving side. Desirably, it should be 20 nm or more away.

  However, according to the configuration of Patent Document 2, it is possible to obtain a wavelength difference of 20 nm, but it is necessary to mount the mounter with the VCSELs accurately maintained, which takes time and effort. Further, according to the configuration of Non-Patent Document 1, since it is directly affected by the gas distribution in the MOCVD apparatus, the reproducibility is poor. In the configuration of Patent Document 3, the range of wavelengths that can be changed is as narrow as several nm or less, and can only be used as a light source for DWDM (Dense Wavelength Division Multiplexing), and is not suitable for large wavelength separation of 15 nm or more.

  Accordingly, an object of the present invention is to provide a multi-mode optical communication system and a multi-wavelength surface light emitting device that are capable of high-speed transmission at low cost.

  In order to achieve the above object, one embodiment of the present invention provides the following multimode optical communication system and multiwavelength surface light emitting device.

[1] A multi-mode optical fiber and a plurality of surface-emitting lasers having a plurality of single-mode light beams having different diameters and having a diameter of an aperture that emits light of 8 μm or less. A multimode optical communication system.

  According to the multimode optical communication system configured as described above, the cost of the system can be reduced by using a relatively inexpensive multimode optical fiber as the optical fiber. Further, by setting the diameter of the apertures of the surface emitting lasers having different wavelengths to 8 μm or less, single mode light can be emitted, and high-speed transmission is possible. In order to obtain a more complete single mode, the diameter of the aperture is preferably 6 μm or less. Further, by using a surface emitting laser as the laser, the two-dimensional arrangement of light emitting points is facilitated. Here, in the claims and the specification, the “aperture” means an aperture of the oxidized constriction layer.

[2] The multi-mode optical communication system according to [1], wherein the plurality of surface-emitting lasers make the plurality of single-mode light incident on the multi-mode optical fiber without passing through an optical system. . By not using an optical system, the number of parts can be reduced and alignment becomes easy. The optical system includes a collimator lens and a condenser lens. An optical system may be used as necessary.

[3] The multimode light according to [1], wherein the plurality of surface emitting lasers have a diameter of a circle circumscribing the plurality of apertures smaller than a diameter of a core of the multimode optical fiber. Communications system. According to this configuration, laser light from each surface emitting laser can be guided to the multimode optical fiber without passing through the optical system.

[4] The multimode optical communication system according to [3], wherein a diameter of a circle circumscribing the plurality of apertures is 50 μm or less. Since the core diameter of a multimode optical fiber is generally 50 μm or more, the diameter of a circle circumscribing a plurality of apertures is set to 50 μm or less so that laser light from each surface emitting laser can be transmitted without passing through an optical system. It can enter into an optical fiber.

[5] The multimode optical communication system according to [1], wherein the plurality of surface emitting lasers are formed on a common substrate. According to this configuration, alignment work between the surface emitting lasers is not necessary.

[6] The multi-mode multi-mode according to [1], wherein the plurality of surface-emitting lasers have different wavelengths of the plurality of single-mode lights due to different thicknesses of the phase adjustment layers. Mode optical communication system. According to this configuration, the light emitting point interval (or pitch) can be shortened, and the oscillation wavelength difference can be increased. The phase adjustment layer can be composed of a semiconductor such as a TiO ₂ film or a dielectric such as a SiO ₂ film or a Si ₃ N ₄ film.

[7] The multimode optical communication system according to [1], wherein the plurality of surface emitting lasers have oscillation wavelength peaks separated from each other by 15 nm or more. With this configuration, a low-cost edge filter can be used to perform wavelength separation on the receiving side, and the cost of the system can be reduced. Note that the peaks of the oscillation wavelengths are preferably separated from each other by 20 nm or more.

[8] A plurality of surface-emitting lasers that emit a plurality of single-mode light beams having different diameters and having a diameter of an aperture that emits light of 8 μm or less on a common substrate. The multi-wavelength surface emitting device is characterized in that the thicknesses of the phase adjustment layers are different from each other, so that the wavelengths of the plurality of single mode lights are different from each other.

  According to the multi-wavelength surface light emitting device having the above-described configuration, by setting the diameter of the aperture of each surface emitting laser having different wavelengths to 8 μm or less, single mode light can be emitted and high-speed transmission is possible. In order to obtain a more complete single mode, the diameter of the aperture is preferably 6 μm or less. By changing the oscillation wavelength when the thicknesses of the phase adjustment layers are different from each other, the oscillation wavelength difference can be increased. Further, by using a surface emitting laser as the laser, the two-dimensional arrangement of light emitting points is facilitated. In particular, when three or more wavelengths are used, the light emitting points can be arranged on concentric circles, and the light can be easily guided directly to the core of the multimode optical fiber.

  According to the present invention, high-speed transmission is possible at low cost.

[First Embodiment]
(Configuration of multi-wavelength surface light emitting device)
FIG. 1 shows a multi-wavelength surface light emitting device according to a first embodiment of the present invention. The multi-wavelength surface light emitting device 100 is formed by monolithically forming first and second VCSELs 2 and 3 that emit two single-mode lights having different wavelengths on a common substrate 1 made of GaAs or the like. The mode optical fiber 20 is used in close proximity to the end face.

  The first and second VCSELs 2 and 3 include cylindrical mesa portions 4A and 4B, and electrodes 5A and 5B in which an emission window 5a is formed on the top surfaces of the mesa portions 4A and 4B. The diameter D of the circle circumscribing each aperture 9a of the oxidized constriction layer, which will be described later, of the first and second VCSELs 2 and 3 is set to be equal to or smaller than the diameter of the core 21 of the multimode optical fiber 20. Note that the outer shape of the mesa portions 4A and 4B may be prismatic.

  Further, it is desirable that the first and second VCSELs 2 and 3 have an oscillation wavelength peak separated by 15 nm or more. Specifically, the first VCSEL 2 has an oscillation wavelength of 840 nm, for example, and the second VCSEL 3 has an oscillation wavelength of 860 nm, for example. In addition, the aperture diameter d of the first and second VCSELs 2 and 3 is, for example, 8 μm or less, and 6 μm or less is preferable to obtain a more complete single mode.

  The multimode optical fiber 20 includes a core 21 and a clad 22 provided outside the core 21. For example, the multimode optical fiber 20 has a core diameter of 62.5 μm, 50 μm, etc., depending on applications and standards. Accordingly, if the interval (light emitting point interval) between the apertures 9a of the first and second VCSELs 2 and 3 is 50 μm or less, the multimode optical fiber 20 having a core diameter of 62.5 μm can be formed without using an optical system such as a lens. It can be guided. Further, if the distance between the apertures 9a is 40 μm, the light can be guided to the multimode optical fiber 20 having a core diameter of 50 μm without using an optical system.

  In this embodiment, a configuration without an optical system that does not require alignment is used, but an optical system can be used as necessary. For example, when the distance between the apertures 5a between two VCSELs is 50 μm and the core diameter of the multimode optical fiber 20 is 50 μm, a simple reduction optical system can be used to make a multi-core having a core diameter substantially equal to the aperture distance. The light can be guided to the mode optical fiber 20. Further, by using a resin or the like between the VCSEL exit port and the fiber core, the difference in refractive index between the VCSEL exit port and the fiber core may be reduced to reduce the reflected light.

(Manufacturing method of multi-wavelength surface light emitting device)
FIG. 2 shows a manufacturing process of the multi-wavelength surface light emitting device 100. First, as shown in FIG. 2A, a lower mirror 6, an active layer 7, and a protective layer 8 are sequentially formed on a substrate 1 made of GaAs by MOCVD.

Lower mirror _6, Al _{x Ga 1-x} As a DBR structure formed by laminating altered so by 40 cycles of x, and the thickness of each layer is (850 nm in lambda ₀ = vacuum) λ _0/4. The active layer 7 is deposited on the lower mirror 6 with a quantum well structure of i-GaAs.

  Next, as shown in FIG. 2A (b), cylindrical mesa portions 4A and 4B having a diameter of 25 μm were formed at intervals of 30 μm by dry etching or the like in order to enable oxidation constriction. The mesa units 4A and 4B correspond to the first and second VCSELs 2 and 3, respectively.

  Next, as shown in FIG. 2A (c), the protective layer 8 on the mesa portions 4A and 4B is removed, the mesa portions 4A and 4B are oxidized and constricted, and an oxidized constricting layer 9 having an aperture 9a is formed. Form a pathway. Here, the diameter d of the aperture 9a formed by the oxidized constricting layer 9 is set to 8 μm or less so that a higher mode is difficult to enter. Higher-order modes need to be reduced as much as possible because the fundamental wave and oscillation wavelength are different.

Next, a dielectric film made of SiO ₂ is deposited to a thickness of t1 on each surface of the mesa portions 4A and 4B by a photolithography process. Next, as shown in FIG. 2A (d), the dielectric film of the mesa unit 4B is removed, and a process of leaving the dielectric film of the mesa unit 4A is performed. The phase adjustment layer 10 is used.

  The thickness of the first phase adjustment layer 10 is obtained in advance by grasping the relationship between the wavelength of the phase adjustment layer (QWDT: quarter-wave optical-thickness) and the wavelength in correspondence with the reflection spectrum and the transmission spectrum. Alternatively, the film thickness is determined while measuring the transmittance as needed.

  For example, if an optical system for measuring reflectance (or transmittance) is arranged in a film deposition apparatus (chamber) and a deviation from a desired oscillation wavelength is measured in the film deposition apparatus, it is intermediate during the production. It is possible to control the oscillation wavelength without removing the product from the film deposition apparatus.

Next, as shown in FIG. 2B (e), the second phase adjustment layers 11A and 11B made of SiO ₂ are deposited on the surfaces of the first phase adjustment layer 10 and the mesa portion 4B to a thickness of t2. As a result, it is possible to oscillate two wavelengths λ ₁ and λ ₂ that greatly change the oscillation wavelengths of the first and second VCSELs 2 and 3.

Next, as shown in FIG. 2B (f), after a lift-off resist is formed, a film made of TiO ₂ / SiO _{2 is} laminated for 6.5 periods to form a dielectric multilayer. The thickness of the laminate is a λ _0/4. Next, the upper mirror 12 made of TiO ₂ / SiO ₂ is formed by lifting off.

  Further, after a gold (Au) lift-off resist is formed, gold is deposited. Next, lift-off is performed to form electrodes 5A and 5B as shown in FIG. 2B (e). A plan view of the two-wavelength VCSEL thus fabricated is shown in FIG.

  In the multi-wavelength surface light emitting device 100 fabricated as described above, the oscillation wavelengths of the first and second VCSELs 2 and 3 at room temperature were 840 nm and 860 nm, the emission point interval was 30 μm, and the oscillation wavelength interval was 20 nm. .

(Effects of the first embodiment)
According to the first embodiment, the following effects are obtained.
(A) Even if the first and second VCSELs 2 and 3 are arranged close to each other, the thickness (t1 + t2) of the phase adjustment layer of the first VCSEL2 and the thickness t2 of the phase adjustment layer of the second VCSEL3 are different from each other. By providing, the oscillation wavelength can be largely changed to about 20 nm, and the multi-wavelength surface light emitting device 100 of the two-wavelength VCSEL can be easily configured.
(B) If the light from the multi-wavelength surface emitting device 100 by the two-wavelength VCSEL is guided to the multimode optical fiber 20, the light can be separated using an inexpensive filter.
(C) When the distance between the first and second VCSELs 2 and 3 is 30 μm, optical coupling with a general multi-mode optical fiber 20 having a diameter of 50 μm can be performed without using an optical system. When used, alignment can be eliminated.

[Second Embodiment]
FIG. 3 shows a multi-wavelength surface light emitting device according to the second embodiment of the present invention. In the present embodiment, the number of VCSELs mounted on the multi-wavelength surface light emitting device 100 is three in the first embodiment, and other configurations are the same as those in the first embodiment. In FIG. 3, the electrodes are not shown.

  In the multi-wavelength surface light emitting device 100, three first to third VCSELs 31, 32, and 33 having different oscillation wavelengths are arranged on a common substrate 1 in an equilateral triangle. The diameter D of the circle circumscribing each aperture 9a of the first to third VCSELs 33 is made smaller than the diameter of the core 21 of the multimode optical fiber 20.

In addition, the multi-wavelength surface light emitting device 100 is formed by, for example, depositing a dielectric film made of Si ₃ N ₄ or the like on the phase adjustment layers 11, 10 A, and 10 B shown in the first embodiment. It can be manufactured by forming the third phase adjustment layer.

  According to the second embodiment having the above configuration, the number of VCSELs that can be arranged on the inner side of the diameter of the core 21 of the multimode optical fiber 20 can be used. can do.

[Third Embodiment]
FIG. 4 shows a multi-wavelength surface light emitting device according to the third embodiment of the present invention. In this embodiment, the number of VCSELs mounted in the multi-wavelength surface light emitting device 100 is four in the first embodiment, and the other configurations are the same as those in the first embodiment. In FIG. 4, illustration of electrodes is omitted.

  In this multi-wavelength surface light emitting device 100, four first to fourth VCSELs 41, 42, 43, and 44 having different oscillation wavelengths are arranged on a common substrate 1 in a square shape. The diameter D of the circle circumscribing each aperture 9a of the first to fourth VCSELs 41 to 44 is made smaller than the diameter of the core 21 of the multimode optical fiber 20.

In addition, the multi-wavelength surface light emitting device 100 is formed by, for example, depositing a dielectric film made of Si ₃ N ₄ or the like on the phase adjustment layers 11, 10 A, and 10 B shown in the first embodiment. It can be produced by forming a third phase adjusting layer and further a fourth phase adjusting layer.

  According to the third embodiment having the above-described configuration, the number of VCSELs that can be arranged inside the diameter of the core 21 of the multimode optical fiber 20 can be used. can do.

[Fourth Embodiment]
(Configuration of multi-mode optical communication system)
FIG. 5 shows a multimode optical communication system according to the fourth embodiment of the present invention. The multimode optical communication system 50 includes a multiwavelength surface light emitting device 100 and a multimode optical fiber 20 having the two-wavelength VCSEL configuration shown in FIG. 1, and a wavelength disposed on the optical axis at the other end of the multimode optical fiber 20. A wavelength separation filter 51 as separation means, a first photodiode 52 disposed on the transmission optical path of the wavelength separation filter 51, and a second photodiode 53 disposed on the reflection optical path of the wavelength separation filter 51. And is configured.

  An optical modulator (not shown) is connected to the first and second VCSELs 2 and 3 of the multi-wavelength surface light emitting device 100, and the first and second photodiodes 52 and 53 are not shown. A demodulator is connected.

  As the wavelength separation filter 51, for example, an etch filter that reflects a predetermined wavelength or less and transmits a predetermined wavelength or more can be used. In addition to the wavelength separation filter, a diffraction grating, a wavelength selective optical fiber coupler, an optical fiber Bragg grating (FBG), or the like can be used as the wavelength separation means.

  As the multimode optical fiber 20, for example, a glass optical fiber whose core and clad are made of fluoride glass, or a plastic optical fiber made of a core made of PMMA (acrylic) and a clad made of fluorine-based resin, for example, can be used. .

(Operation of multi-mode optical communication system)
Here, a multimode optical fiber 20 having a core diameter of 200 μm or less is used. The two laser beams from the first and second VCSELs 2 and 3 of the multi-wavelength surface light emitting device 100 are simultaneously incident on the core 21 at one end of the multimode optical fiber 20 and propagate through the core 21 to transmit the multimode optical fiber. 20 reaches the other end, and further reaches the wavelength separation filter 51.

  The 840 nm laser light from the first VCSEL 2 is reflected by the wavelength separation filter 51 and reaches the first photodiode 52, and is converted into an electric signal by the first photodiode 52. On the other hand, the 860 nm laser light from the second VCSEL 3 is reflected by the wavelength separation filter 51 and reaches the second photodiode 53, and is converted into an electric signal by the second photodiode 53.

  As described above, in the multimode optical communication system 50, the two optical signals from the multimode optical fiber 20 are demultiplexed by the wavelength separation filter 51, and each is converted into an electric signal.

  According to the above-described fourth embodiment, the distance between the first and second VCSELs 2 and 3 of the multi-wavelength surface light emitting device 100 can be made smaller than the core diameter of the multimode optical fiber 20 without using an optical system. Light from the first and second VCSELs 2 and 3 can be easily guided to the multimode optical fiber 20, and short-range communication can be performed in parallel at high speed. For example, if transmission is performed at 10 Gbps or more per wavelength, signal transmission of a total of 20 Gbps or more can be realized. Further, since a wavelength separated by 15 nm or more is received, an inexpensive edge filter can be used as the wavelength separation filter 51.

[Other embodiments]
The present invention is not limited to the above embodiments, and various modifications can be made without departing from or changing the technical idea of the present invention.

1 is a front view of a multiwavelength surface light emitting device according to a first embodiment of the present invention. (A)-(d) is a figure which shows the manufacturing process of the multiwavelength surface emitting element which concerns on 1st Embodiment. (E)-(g) is a figure which shows the manufacturing process of the multiwavelength surface emitting element which concerns on 1st Embodiment. It is a front view of the multiwavelength surface light emitting element which concerns on the 2nd Embodiment of this invention. It is a front view of the multiwavelength surface light emitting element which concerns on the 3rd Embodiment of this invention. It is a block diagram which shows the multimode optical communication system which concerns on the 4th Embodiment of this invention. It is a block diagram which shows the communication system using the conventional multimode optical fiber.

Explanation of symbols

1 Substrate 2 First VCSEL
3 Second VCSEL
4A, 4B Mesa portion 5A, 5B Electrode 5a Exit window 6 Lower mirror 7 Active layer 8 Protective layer 9 Oxide constriction layer 9a Aperture 10, 11A, 11B Phase adjustment layer 12 Upper mirror 14 Insulating film 20 Multimode optical fiber 21 Core 22 Clad 31 First VCSEL
32 Second VCSEL
33 Third VCSEL
41 First VCSEL
42 Second VCSEL
43 Third VCSEL
44 Fourth VCSEL
50 Multimode Optical Communication System 51 Wavelength Separation Filter 52 First Photodiode 53 Second Photodiode 100 Multiwavelength Surface Light Emitting Element 200 Multimode Optical Communication System 201 First VCSEL
202 second VCSEL
203 First optical waveguide 204 Second optical waveguide 205 Lens 206 Multimode optical fiber 206a Core 206b Clad

Claims (8)

A multimode optical fiber;
A multi-mode optical communication system comprising: a plurality of surface-emitting lasers having a plurality of single-mode light beams having different diameters and having a diameter of an aperture for emitting light of 8 μm or less; .   2. The multimode optical communication system according to claim 1, wherein the plurality of surface emitting lasers make the plurality of single mode lights incident on the multimode optical fiber without passing through an optical system. 3.   2. The multimode optical communication system according to claim 1, wherein each of the plurality of surface emitting lasers has a diameter of a circle circumscribing the plurality of apertures smaller than a diameter of a core of the multimode optical fiber.   The multimode optical communication system according to claim 3, wherein a diameter of a circle circumscribing the plurality of apertures is 50 μm or less.   The multimode optical communication system according to claim 1, wherein the plurality of surface emitting lasers are formed on a common substrate.   2. The multimode optical communication system according to claim 1, wherein the plurality of surface emitting lasers have different wavelengths of the plurality of single mode light due to different thicknesses of the phase adjustment layers.   2. The multimode optical communication system according to claim 1, wherein the plurality of surface emitting lasers have oscillation wavelength peaks separated from each other by 15 nm or more. A plurality of surface emitting lasers that emit a plurality of single-mode light beams having different diameters and having a diameter of 8 μm or less on a common substrate;
The plurality of surface-emitting lasers are characterized in that the wavelengths of the plurality of single-mode lights are different from each other due to the thicknesses of the phase adjustment layers being different from each other.

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