JP2006295496A - Optical transmission system and light source thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission system capable of extending transmission distance by reducing the influence of mode distribution noise, and to provide a light source for the optical transmission system at low costs. <P>SOLUTION: The diameter of a luminous surface in a face light-emitting laser (VCSEL) is set to approximately 10μm for operation in a single mode. The output light is transmitted via an optical fiber F in the single mode, thus reducing the mode distribution noise, minimizing deterioration in a transmission waveform, and hence extending the transmission distance and reducing the costs of a system by utilizing the advantage of the costs in the face light-emitting laser. Additionally, a wavelength of at least 3 is used in a short waveband of 600nm or higher and 1μm or smaller, and each wavelength light is modulated at the modulation speed of 1-13GHz, thus constructing a wavelength division multiplex optical transmission system capable of coping with the Ethernet (R) having a transmission speed of 1-10Gbps. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission system that includes a plurality of light sources and performs wavelength division multiplexing transmission, and a light source used in this type of system. In particular, the present invention can be suitably used for medium / short-distance networks such as access networks and user networks.

  In optical transmission systems for medium and short-distance networks such as access networks and user networks, the combination of a laser light source with an oscillation wavelength of 800 nm to 900 nm and a multimode optical fiber with a core diameter of 50 μm to 62.5 μm is economical. I am trying. In particular, by using a surface emitting laser that oscillates at a wavelength of 850 nm, the cost of the system can be reduced as compared with a conventional Fabry-Perot laser (FP-LD) or a distributed feedback laser (DFB-LD).

  By the way, in the recent standardization of Ethernet (Ethernet: (registered trademark)), a transmission rate of 10 Gbps is realized, and an optical transmission method is exclusively used. The wavelengths of the light source being studied for this type of application are the 850 nm band, the 1310 nm band, and the 1550 nm band. Among these, the existing optical transmission system using the 850 nm band, which is a short wavelength band, has a relatively short transmission distance because a multimode optical fiber is used. For example, when a multimode optical fiber having a core diameter of 50 μm is used, the maximum transmission distance is about 80 m.

The reason why the transmission distance does not increase is mainly due to band degradation due to mode dispersion occurring in the fiber. Furthermore, the existing refractive index distribution structure of the multimode optical fiber is mainly used to reduce the modal dispersion around the wavelength of 850 nm. Therefore, when using wavelengths of 800 nm or less and 900 nm or more, modal dispersion is used. Band degradation becomes more prominent. In the same short wavelength band, when a single mode optical fiber (SMF) having a core diameter of about 10 μm or a photonic crystal structure fiber (PCF) used in the long wavelength band is used as a transmission medium, any of the fibers has several modes. It is known that single mode transmission can be expected and the influence of mode dispersion is small (see Patent Document 1 and Non-Patent Document 1).
JP 2002-277648 A "Short Wavelength Band Transmission Characteristics in 1.3μm Single Mode Fiber" IEICE Transactions BI Vol. J74-BI No.4 pp.360-368 April 1991

As described above, if a single mode optical fiber can be used, there is a possibility that the transmission distance can be extended particularly in an 850 nm band called a short wavelength band. However, a surface emitting laser (VCSEL) light source used in an existing wavelength division multiplexing optical transmission system operates in a multimode in a short wavelength band. Therefore, when the output light is incident on the single mode optical fiber, there is a problem that the transmission distance is limited due to the influence of mode distribution noise.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical transmission system capable of extending the transmission distance by reducing the influence of mode distribution noise and its light source at low cost.

  In order to achieve the above object, according to one aspect of the present invention, a plurality of light sources having different output light wavelengths and a combination of the output lights of the plurality of light sources to output wavelength multiplexed light. , An optical transmission path on which the wavelength multiplexed light is incident, a duplexer that demultiplexes the wavelength multiplexed light transmitted through the optical transmission path for each wavelength, and each output from the duplexer A plurality of light receivers for receiving wavelength light, wherein each of the plurality of light sources is a surface emitting laser having a light emitting surface diameter of 4 μm to 12 μm, and the optical transmission path is a single mode optical fiber. An optical transmission system is provided.

  By adopting such means, the surface emitting laser can be driven in a single mode, and the output light can be transmitted through the single mode optical fiber. By transmitting the output light of an inexpensive surface emitting laser (that is, a VCSEL light source) that performs single mode operation through the single mode optical fiber in this way, mode distribution noise in the optical fiber can be reduced, and therefore the transmission distance. Can be extended.

  According to the present invention, it is possible to provide an optical transmission system capable of extending the transmission distance by reducing the influence of mode distribution noise and its light source at low cost.

  FIG. 1 is a cross-sectional view showing a laser module using a surface emitting laser (VCSEL light source) according to the present invention. In FIG. 1, an n-type Bragg reflection film 2 is laminated on a substrate 1, and a p-type Bragg reflection film 4 is further laminated via a quantum well structure 3. Insulating films 5 and 6 are formed so as to cover these reflective films 2 and 4 and quantum well structure 3 from both sides. An electrode plate 7 is formed outside one insulating film 6, and an insulating film 8 is further formed outside the electrode plate 7. Among these, an n-type electrode 9 is connected to the electrode plate 7. A p-type electrode 10 is formed on the p-type Bragg reflection film 4. The p-type electrode 10 has a light emitting surface, and laser light is output from the light emitting surface.

  In this embodiment, the diameter of the light emitting surface is 10 μm, and the output laser light is guided to an optical fiber F having a core diameter of 10 μm. The optical fiber F is a single mode optical fiber that transmits single mode signal light in a wavelength band of 1.2 μm or more.

  FIG. 2 is a system diagram showing an example of a wavelength division multiplexing optical transmission system using the light source of FIG. In FIG. 2, the output lights of the laser light sources 21 to 23 having different output wavelengths are wavelength-multiplexed by the multiplexer 30, and the wavelength-multiplexed light is output. The wavelength multiplexed light is incident on the single mode optical fiber F and transmitted to the receiving end. The transmitted wavelength multiplexed light is demultiplexed into light of each wavelength by the demultiplexer 40, and received by the light receivers 51 to 53, respectively, to regenerate the signal. The laser light sources 21 to 23 are all VCSEL light sources having the configuration shown in FIG. 1, and the output light wavelengths thereof are 650 nm, 780 nm, and 850 nm, respectively. As the light receiver, an avalanche photodiode (APD) using silicon having excellent light receiving sensitivity at a wavelength of 800 nm to 900 nm can be used.

  FIG. 3 is a schematic diagram showing an existing optical transmission system for comparison. FIG. 3 shows a system that has been used especially for existing medium and short range networks. In the system of FIG. 3, for example, a VCSEL light source having a wavelength of 850 nm is driven in multimode, and the output light is transmitted to a light receiver through a multimode optical fiber. In order to realize this system at low cost, it is preferable to use a surface emitting laser (VCSEL) light source instead of an expensive light source such as FP-LD or DFB-LD. However, in the existing system, since the surface emitting laser is driven in multimode, there is a problem that the transmission waveform is easily disturbed and the transmission distance is limited.

  In contrast, in this embodiment, since the diameter of the light emitting surface of the surface emitting laser is very narrow (10 μm) as shown in FIG. 1, the light source of FIG. 1 operates in a single mode. By transmitting the output light through the single-mode optical fiber F, the mode distribution noise can be reduced, and therefore the deterioration of the transmission waveform can be minimized. Therefore, the transmission distance can be extended. Further, as shown in FIG. 2, by using three wavelengths in a short wavelength band of 600 nm or more and 1 μm or less and modulating the output light of each light source 21 to 23 at a modulation speed of 1 GHz to 13 GHz, a transmission speed of 1 Gbps to 10 Gbps. It is possible to construct a wavelength division multiplexing optical transmission system compatible with Ethernet (registered trademark). Even in such a network, according to this embodiment, since it is a single mode operation, mode distribution noise in the optical fiber can be reduced, and the transmission distance can be extended. In addition, since an inexpensive surface emitting laser can be used as the light source, cost reduction of the system can be promoted. From these facts, according to the present embodiment, it is possible to obtain a more remarkable merit particularly in a medium / short distance network such as an access network or a user network. In summary, in this embodiment, an inexpensive VCSEL light source is operated in a single mode, and the output light is incident on the single mode optical fiber F. Since it did in this way, it becomes possible to provide the optical transmission system which can reduce the influence of mode distribution noise, and can extend a transmission distance, and its light source at low cost.

  The present invention is not limited to the above embodiment. For example, the number of light sources is not limited to three, and it is obvious that a wavelength division multiplexing optical transmission system can be constructed by using at least two light sources. Conversely, the number of wavelengths can be further increased. For example, as shown in FIG. 4, it is possible to arrange ten wavelengths on the frequency axis. As shown in FIG. 4, assuming that the band of each wavelength light is 30 nm and the wavelength interval, that is, the unassigned wavelength range is 10 nm, ten wavelengths can be arranged in a wavelength band of 600 nm to 1 μm or less. Even in this way, it is possible to realize wavelength division multiplex transmission that achieves a transmission rate required in Ethernet (registered trademark) of 1 Gbps to 10 Gbps. More generally, if each wavelength light band is set to 20 nm to 50 nm and the non-assigned wavelength range is set to 10 nm to 20 nm, wavelength division multiplexed optical transmission can be realized with sufficient separation between the channels. it can.

  In FIG. 2, the oscillation wavelengths of the laser light sources 21 to 23 are not limited to the 650 nm band, 780 nm band, and 850 nm band, and other wavelength bands may be used. In addition, if the diameter of the light emitting surface of the laser light source is about 4 μm to 12 μm, single mode operation is possible, so that the object of the present invention can be sufficiently achieved.

  Further, the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements within a range not departing from the gist thereof, such as the core diameter of the optical fiber F. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

Sectional drawing which shows the laser module using the surface emitting laser (VCSEL light source) concerning this invention. The system diagram which shows an example of the wavelength division multiplexing optical transmission system using the light source of FIG. FIG. 3 is a schematic diagram showing an existing optical transmission system for comparison. The figure which shows the other example of wavelength arrangement | positioning in embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... N-type Bragg reflection film, 3 ... Quantum well structure, 4 ... P-type Bragg reflection film, 5, 6, 8 ... Insulating film, 7 ... Electrode plate, 9 ... N-type electrode, 10 ... P-type Electrode, F ... optical fiber, 21-23 ... laser light source, 30 ... multiplexer, 40 ... demultiplexer, 51-53 ... light receiver

Claims (9)

A plurality of light sources having different output light wavelengths,
A multiplexer that multiplexes the output light of each of the plurality of light sources and outputs wavelength multiplexed light;
An optical transmission line on which the wavelength multiplexed light is incident;
A demultiplexer for demultiplexing the wavelength division multiplexed light transmitted through this optical transmission line for each wavelength;
A plurality of light receivers for receiving each wavelength light output from the duplexer,
Each of the plurality of light sources is a surface emitting laser having a light emitting surface diameter of 4 μm to 12 μm,
The optical transmission system is characterized in that the optical transmission line is a single mode optical fiber. The optical transmission system according to claim 1, wherein output light wavelengths of the plurality of light sources are 600 nm to 1 μm. The optical transmission system according to claim 1, wherein output light of the plurality of light sources is modulated at a speed of 1 GHz to 13 GHz. The optical transmission system according to claim 1, wherein the number of the plurality of light sources is three or more. 2. The optical transmission system according to claim 1, wherein output light wavelengths of the plurality of light sources are arranged on a frequency axis with respective bands of 20 nm to 50 nm and unassigned wavelength ranges of 10 nm to 20 nm. 6. The optical transmission system according to claim 5, wherein the number of wavelengths arranged on the frequency axis is 3 to 10. The light source used in an optical transmission system that combines output light from a plurality of light sources having different output light wavelengths and transmits the light through a single mode optical fiber,
A light source characterized in that the light emitting surface is a surface emitting laser having a diameter of 4 μm to 12 μm. The light source according to claim 7, wherein the output light wavelength is 600 nm to 1 μm. 8. The light source according to claim 7, wherein the output light is modulated at a speed of 1 GHz to 13 GHz.

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