JP2002277648A - Light transmission system and single mode optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light transmission system capable of realizing a transmission distance of >=1 km by using a light source of a short wavelength band of a wavelength from 0.75 μm to 1 μm and a light receiving circuit and a single mode optical fiber to be used for the short wavelength band. SOLUTION: The light transmission system uses a photonic crystal structure fiber setting up the wavelength of the light source to 0.75 μm to 1 μm and forming a grating hole (a hole of grating structure) having the same diameter on a clad part surrounding a core part as an optical fiber to be a transmission line and setting up a zero dispersion wavelength determined by the interval (grating interval) Λ of the grating hole and the diameter d of the grating hole to a value close to the wavelength of the light source. When the interval (grating interval) of the grating hole of the photonic crystal structure fiber is Λand the diameter of the grating hole is d in the single mode optical fiber, the diameter (2nΛ-d) of the core part is set up to 2 to 20 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system for transmitting a high-speed signal at a relatively short distance such as a LAN (local area network) or a local communication network. The present invention also relates to a single mode optical fiber used in a short wavelength band in the optical transmission system at a relatively short distance.

[0002]

2. Description of the Related Art In a relatively short-distance optical transmission system (a short-distance optical transmission system) such as a LAN or a local communication network, a multi-mode optical fiber is used as a transmission line for economic reasons, and a wavelength used is 800 to 800 nm. A configuration using a short wavelength band of about 900 nm has been developed. The short wavelength band is economically superior from the viewpoint of the light source and the light receiving element for the following reason.

(1) With the recent increase in demand for compact discs (CDs) and the like, wavelengths for reading information from CDs have been increased.
Light sources of 700 to 800 nm are mass-produced, and light sources (Fabry-Perot laser (FP-LD) and distributed feedback laser (DFB-LD)) in the wavelength region are provided at low cost. Also, a vertical surface emitting laser (VCSEL) having a wavelength of 850 nm, which has been used in existing LANs, is provided at low cost. (2) In this wavelength region, an avalanche photodiode (APD) using silicon has conventionally been used, and a device excellent in price and noise characteristics has been provided.

FIG. 5 shows a configuration example of a conventional short-distance optical transmission system. In the figure, an electric signal output from a signal generator 51 is input to a light source 52, and is converted into an optical signal. This optical signal is transmitted to the light receiving circuit 54 via the multi-mode optical fiber 53, received by the light receiving circuit 54, and converted into an original electric signal.

Here, the light source 52 has a wavelength of 800 to 900 nm.
It is a light source in a short wavelength band of the order, and a semiconductor laser (FP-LD, DFB-LD, VCSEL) is mainly used. When a bias current needs to be applied to this LD, a bias current circuit is inserted between the signal generator 51 and the light source 52. In this configuration, the bias current of the light source 52 is modulated by the electric signal output from the signal generator 51, and the output light of the light source 52 is directly modulated.

In recent years, Ethernet (registered trademark) (Eth
The speed of LANs represented by IEEE (Enet (registered trademark)) is increasing, and the Ethernet (Institute of Electrical and Electronics) is aiming at a transmission speed of 10 Gbit / s in Ethernet.
ronics Engineers). This 10Gb
It / s Ethernet is to use optical communication instead of electricity. The wavelength used is 850 nm,
1310 nm and 1550 nm are being studied. In the short wavelength transmission method with a wavelength of 850 nm, the transmission distance is relatively short because a multimode optical fiber is used (approximately 70 m with an existing optical fiber with a core diameter of 50 μm). I have. The standardization of 10 Gbit / s Ethernet specifications is described in Nikkei Electronics September 2000.
The 25th issue (No. 779), p. 29, "Physical layer specification of 10 Gbit Ethernet is determined" is described in detail.

[0007]

[Problems to be Solved by the Invention] By the way, 10 Gbit / s
If the transmission distance can be extended to several kilometers in the short-wavelength transmission system with a wavelength of 850 nm of Ethernet, the above-mentioned inexpensive short-wavelength light source and light-receiving circuit can be used, so that there is a great economic advantage. However, when an optical fiber having a conventional structure is used as a transmission line at a wavelength of 1 μm or less, there are the following problems.

(1) It is extremely difficult to reduce the zero-dispersion wavelength of an optical fiber to 1.3 μm or less. (2) Due to the influence of Rayleigh scattering of the glass material forming the optical fiber, it is difficult to reduce the transmission loss at a wavelength of 1 μm or less.

Due to such characteristics of the optical fiber, for example, a wavelength 8 used in a LAN represented by Ethernet is used.
In the transmission path for optical signals in the 50 nm band, the transmission distance is 100
m or less, and the application of the short wavelength transmission system has been extremely limited, such as connection between devices.

The present invention relates to an optical transmission system for realizing a transmission distance of 1 km or more using a light source and a light receiving circuit in a short wavelength band of 0.75 μm or more and 1 μm or less, and a single mode optical fiber used in the short wavelength band. The purpose is to provide.

[0011]

According to the optical transmission system of the present invention, a photonic crystal in which lattice holes having the same diameter (holes having a lattice structure) are provided in a clad portion surrounding a core portion as an optical fiber serving as a transmission path. It is characterized by using a structural fiber. The photonic crystal structure fiber can set the zero-dispersion wavelength in a wavelength region that cannot be realized by an existing optical fiber by confining light by a unique cladding structure.

In this photonic crystal structure fiber, the zero-dispersion wavelength is determined by the spacing (lattice spacing) 格子 of the lattice holes provided in the clag portion and the diameter d of the lattice holes, and the signal in the short wavelength band of 0.75 μm or more and 1 μm or less is determined. Low dispersion and low loss characteristics for light can be realized. As a result, an optical transmission line having a zero-dispersion wavelength in a short wavelength band of 800 to 900 nm, which is rich in inexpensive existing optical components, is realized.
As an optical transmission system, long-distance transmission of 1 km or more, which has been impossible in the past, can be performed using inexpensive optical components in a short wavelength band.

The diameter of the core (2n） -d) is 2 to
The photonic crystal structure fiber set to 20 μm can function as a single mode optical fiber in a short wavelength band. As a result, long-distance transmission in a short wavelength band becomes possible.

[0014]

FIG. 1 shows an embodiment of an optical transmission system according to the present invention. In the figure, an electric signal output from a signal generator 11 is input to a light source 12, and is converted into an optical signal. This optical signal is transmitted to the light receiving circuit 14 via the photonic crystal structure fiber 13, is received by the light receiving circuit 14, and is converted into an original electric signal.

Here, the electric signal output from the signal generator 11 may be either a voice system or a data system represented by the Internet Protocol (IP). Further, either an analog signal or a digital signal may be used. The light source 12 is a light source in a short wavelength band of about 800 to 900 nm, and mainly includes a semiconductor laser (FP-LD, D
FB-LD, VCSEL) are used. When a bias current needs to be applied to this LD, a bias current circuit is inserted between the signal generator 11 and the light source 12.
In this configuration, the bias current of the light source 12 is
1 is modulated by the electric signal output from the
Is directly modulated.

FIG. 2 shows a photonic crystal structure fiber 1.
3 shows a cross-sectional configuration. In the figure, the photonic crystal structure fiber 13 has a structure in which lattice holes (holes having a lattice structure) 23 are provided in a cladding portion 22 surrounding a core portion 21 and the lattice holes 23 are covered with a jacket portion 24. The light confinement mechanism of the present optical fiber includes (1) a core 21 and a clad 22
And (2) Bragg reflection by the lattice holes 23 provided in the clad portion 22. Core part 21
The former is dominant when is glass. On the other hand, even when the refractive index of the core portion 21 is lower than that of the cladding portion 22, light can be confined in the core portion 21 by Bragg reflection by the lattice holes 23 provided in the cladding portion 22.
That is, it is possible to guide light even when the core portion 21 is, for example, air.

The zero-dispersion wavelength of the photonic crystal structure fiber 13 is determined by the spacing (lattice spacing) 格子 of the lattice holes 23 provided in the cladding portion 22 and the diameter d of the lattice holes 23. Reference (JRRanka et al., "Optical properties of
high-delta air-silica microstructure optical fiber
s ", Optics Letters, vol.25, No.11, pp.796-798, 2000)
Shows an example in which a photonic crystal structure fiber having a zero-dispersion wavelength of 780 nm is realized when Λ = 1.55 μm and d = 1.4 μm. FIG. 3 shows the dispersion characteristics of the photonic crystal structure fiber.

The zero-dispersion wavelength determined by the lattice structure (Λ and d) of the photonic crystal structure fiber 13 is as follows:
It turned out that it changes according to the design value. For example,
For a photonic crystal structure fiber having a zero dispersion wavelength of 850 nm, Λ = 2.1 μm and d = 1.9 μm, and FIG. 4 shows the dispersion characteristics. As in these two examples, the photonic crystal structure fiber 13 can set the zero-dispersion wavelength over a wide range within a short wavelength band by appropriately selecting its structural parameters (Λ and d).

As described above, a conventional optical fiber represented by a single-mode optical fiber or a dispersion-shifted fiber cannot reduce the zero-dispersion wavelength to 1.3 μm or less in principle. The nick crystal structure fiber 13 can set the zero dispersion wavelength to a short wavelength band having a wavelength of 0.75 μm or more and 1 μm or less.

(Example of Converting Photonic Crystal Structure Fiber to Single Mode Optical Fiber) The core portion 21 of the photonic crystal structure fiber 13 is obtained by replacing one lattice hole 23 provided in the cladding portion 22 with glass. Then, its diameter can be regarded as 2Λ-d. The size of this core diameter (2Λ-
When d) is set to about 2 to 3 μm, the photonic crystal structure fiber 13 functions as a single mode optical fiber having a zero dispersion wavelength in a short wavelength band. Generally, when n is an integer of 1 or more and the size of the core diameter (2nΛ-d) is set to about 2 to 20 μm, the photonic crystal structure fiber 13 has a single wavelength having a zero dispersion wavelength in a short wavelength band. Functions as a mode optical fiber. If this is used as a transmission line, waveform deterioration due to multi-mode dispersion does not occur, and waveform deterioration due to chromatic dispersion does not occur.

Accordingly, in an optical transmission system using a photonic crystal structure fiber having a zero dispersion wavelength in a short wavelength band and a core diameter (2nΛ-d) of about 2 to 20 μm as a transmission line, 10 Gbit Even a high-speed signal of / s or more can be transmitted over a long distance of 1 km or more without waveform deterioration. The limit of the transmission distance is mainly the loss of optical fiber in the short wavelength band (about 3 dB in the case of a glass core).
/ Km), and if a transmission loss of 30 dB is allowed, the transmission distance is 10 km. In addition, since the loss is considered to be further reduced in the photonic crystal structure fiber having an air core, the transmittable distance can be 10 km or more. This is a value far exceeding the transmission distance of the conventional multimode optical fiber.

[0022]

As described above, the optical transmission system of the present invention is economical by using a photonic crystal structure fiber having a zero-dispersion wavelength in a short wavelength band of 0.75 μm to 1 μm as a transmission line. It is possible to use a light source in a short wavelength band excellent in noise and a light receiving circuit excellent in economy and noise characteristics.

In a photonic crystal structure fiber having a zero-dispersion wavelength in a short wavelength band, the diameter (2
By setting nΛ−d) to 2 to 20 μm, it is possible to function as a single mode optical fiber, so that long-distance transmission in a short wavelength band is possible.

If a relatively short-distance optical transmission system such as a LAN is constructed by using such a photonic crystal structure fiber as a transmission line, a high-speed signal can be transmitted over a long distance using existing inexpensive optical components. The merit is extremely large.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an optical transmission system according to the present invention.

FIG. 2 is a diagram showing a cross-sectional configuration of a photonic crystal structure fiber 13.

FIG. 3 is a view showing dispersion characteristics (zero dispersion wavelength: 780 nm) of the photonic crystal structure fiber described in the literature.

FIG. 4 is a diagram showing dispersion characteristics of a photonic crystal structure fiber designed to have a zero dispersion wavelength of 850 nm.

FIG. 5 is a block diagram showing a configuration example of a conventional short-distance optical transmission system.

[Explanation of symbols]

 REFERENCE SIGNS LIST 11 signal generator 12 light source 13 photonic crystal structure fiber 14 light receiving circuit 21 core part 22 clad part 23 lattice hole 24 jacket part 51 signal generator 52 light source 53 multimode optical fiber 54 light receiving circuit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masataka Nakazawa 2-3-1 Otemachi, Chiyoda-ku, Tokyo Inside Nippon Telegraph and Telephone Corporation (72) Inventor Hirokazu Kubota 2-3-3, Otemachi, Chiyoda-ku, Tokyo No. 1 Within Nippon Telegraph and Telephone Corporation (72) Kazunori Suzuki 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Moriyuki Fujita 4-chome Ikejiri, Itami-shi, Hyogo No. 3 Mitsubishi Electric Cable Industry Co., Ltd. Itami Works (72) Inventor Masatoshi Tanaka 4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable Co., Ltd. Itami Works F term (reference) 2H050 AC00 AC01 AC09 AC64 AC71 AD01 5K002 AA01 AA03 BA13 CA01 FA01 FA02

Claims (4)

[Claims] A light source that converts an electrical signal into an optical signal and outputs the optical signal; an optical fiber that transmits the optical signal output from the light source; and an optical fiber that receives the optical signal transmitted through the optical fiber, An optical transmission system comprising: a light receiving circuit for converting a signal into a signal; wherein the optical fiber is a photonic crystal structure fiber in which a lattice hole (a hole having a lattice structure) having the same diameter is provided in a clad part surrounding a core part. An optical transmission system characterized by the above. A light source that converts an electric signal into an optical signal and outputs the optical signal; an optical fiber that transmits the optical signal output from the light source; and an optical fiber that receives the optical signal transmitted through the optical fiber, and A light receiving circuit for converting the light into a signal, wherein the wavelength of the light source is not less than 0.75 μm and not more than 1 μm; An optical transmission system comprising: a photonic crystal structure fiber in which a hole is provided and a zero-dispersion wavelength determined by a lattice hole interval (lattice interval) Λ and a lattice hole diameter d is set near the wavelength of the light source. 3. The optical transmission system according to claim 2, wherein the photonic crystal structure fiber has an interval (lattice interval) of the lattice holes and a diameter d of the lattice holes.
An optical transmission system, wherein the diameter (2nΛ-d) of the core portion is set to 2 to 20 μm (n is an integer of 1 or more). 4. A photonic crystal structure fiber in which lattice holes (vacancies having a lattice structure) having the same diameter are provided in a cladding portion surrounding a core portion, wherein the lattice holes have a spacing (lattice spacing) Λ and the lattice holes. The zero-dispersion wavelength determined by the diameter d is 0.75 μm
Is set to 1 μm or less, and the diameter of the core portion (2nΛ)
A single mode optical fiber, wherein (d) is set to 2 to 20 μm (n is an integer of 1 or more).