JPS598098B2 - Optical transmission method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical transmission system using optical fibers.

As an optical transmission system using optical fiber, graded multimode optical fiber is considered to be a promising transmission line.

This is due to the relatively thick core diameter of 60 μm to 80 μm, which makes connection easy, and the fact that mode dispersion is small and the transmission band is wide. On the other hand, candidates for the light source include a laser diode LD or a light emitting diode LED, but either light source has a wide range of emission wavelengths. For example, a wavelength spread of about 30 λ has been observed for an LD and about 800 λ for an LED.

Therefore, an optical signal propagated through a multimode optical fiber is subjected to distortion due to mode dispersion and material dispersion, and the transmission band is determined. For example, regarding mode dispersion, a graded optical fiber can provide a group delay time difference Δτm between the highest order mode and the lowest order mode of less than lnsec/km. On the other hand, material dispersion is approximately 10 ps/λ at a wavelength of 800 nm.
A value of As it becomes possible to manufacture fibers, the proportion of material dispersion increases.As for light sources, light sources with narrow emission wavelength widths are being researched, but as the emission wavelength width becomes narrower, the optical power decreases or the lifetime becomes shorter. There are disadvantages such as the increase in the temperature and the high manufacturing cost.

Furthermore, the loss of optical fibers is lowest at wavelengths around 1.6 μm, and optical fibers with a loss of 5 dB/km are also manufactured.

In such a low-loss wavelength band, transmission band limitation due to chromatic dispersion of optical fibers becomes an important factor in optical transmission systems. Therefore, an object of the present invention is to provide an optical transmission system that alleviates the limitation of the transmission band due to chromatic dispersion, and its basic idea is to construct the system so that material dispersion and mode dispersion cancel each other out.

The optical transmission system according to the present invention selects the wavelength width of the light source and the refractive index distribution of the optical fiber material so that the material dispersion and mode dispersion of the optical fiber cancel each other out, and transmits the light beam from the light source with wavelength spread. The wavelengths are spatially separated by a demultiplexer, and then incident on the multimode fiber by a concentrator so as to have a desired relationship between each separated wavelength and the distance from the center of the core of the optical fiber to be excited. Features. This will be explained in detail below with reference to the drawings. As a specific example, a light source with a center wavelength of 0
．． Consider a 81μm LED and a silica-based graded multimode fiber as the optical fiber. Figure 1 shows the emission spectrum of the LED, with a spectral half-width of approximately 750λ,
The full spectrum width is about 1500λ.

The o mark in the figure represents a measured value, and the curve represents a Gaussian distribution. FIG. 2 shows the material dispersion of quartz-based fibers.

For example, at a wavelength of 0.81 μm, the material dispersion value is 10.1 ps.
/λ-. As can be seen from this figure, the group delay time at longer wavelengths is smaller than at shorter wavelengths. The group delay time difference Δτl due to material dispersion in the light source is approximately 15.2 ns/K
Becomes In.

Therefore, considering the time axis, the longer wavelength light of the LED emission wavelength is approximately 15.2 ns/I faster than the shorter wavelength light.
<In arrive early. This time difference is one of the factors that limits the transmission band. Next, we will discuss mode dispersion.

The refractive index distribution in the core of a graded multimode fiber can be expressed as shown in equation (1). Here, NO is the refractive index at the core center, Δ is the ratio of the refractive index difference between the core center and the cladding, a is the core radius, and α is the refractive index distribution shape.

The group delay time △τ due to the mode dispersion of an optical fiber having the distribution form of equation (1). is given by equation (2). However, L
is the propagation length, and C is the speed of light in direct air.

FIG. 3 shows equation (2) when Δ is 0.01 (1%).

This figure shows that when d is larger than 2, the group delay time of the higher-order mode is larger than that of the lower-order mode.
It is shown that when α is smaller than 2, the relationship is reversed. Therefore, the group delay difference △τ1 due to the material dispersion mentioned above is expressed as Δτ. In order to cancel each other out at
=4, it is sufficient to excite the light on the long wavelength side to a high-order mode and the light on the short wavelength side to a low-order mode. Note that α
The relationship is the opposite for those with a distribution of =1. Next, a method for selectively exciting modes with respect to wavelength will be described.

FIG. 4 schematically shows the trajectory of a light ray in a graded optical fiber, where 1 represents the core and 2 represents the cladding.

In this figure, A, b, and c each indicate how to excite from a low-order mode to a high-order mode. In other words, it can be seen that for higher-order modes, it is sufficient to excite the outer peripheral portion of the core. FIG. 5 shows an embodiment of the present invention based on the above principle, in which 1 is a core, 2 is a cladding, 3 and 4 are condensing lenses, and 5
indicates a light source, and 6 indicates a splitting prism.

When the light from the light source is focused by the lens 4 and enters the prism, the output angle θ differs by δθ depending on the wavelength. For a fiber with an aperture of Ct=4, the dispersion can be canceled by confining the light on the long wavelength side to the outer periphery as shown in FIG. 5 using the lens 3. Assuming that the apex angle of the prism is d, the angle between incidence and exit is θ, the focal length of the prism is f, and the distance at the focused point is δX, the following relationship holds true. Let us now consider the light source and the actual fiber.

△λ = 0.15 μm 1 δx = 30 μm 1 When ordinary optical glass is used as a prism, n = 1.5, Dn/dλ
20.02μm-1 (λ-0.81μm), (4
), α=451 and f=10.7n. Therefore, it can be seen that the configuration shown in FIG. 5 can be easily realized. Also, in FIG. 5, a diffraction grating can be used instead of the prism.

In this case, the angular dispersion dθ/dλ is expressed by the following equation. where m is the order of diffraction, and d is the grating pitch interval 4. ．． Book】−
＋? 'P'-4$h★koφ. 1hL] To obtain angular dispersion of 1st order, d~50.4μm from equation (5),
The grid pitch is 20 lines/Mm.

This is a value that can be easily achieved with current technology. Note that even when an LD is used as a light source, the above configuration becomes an achievable parameter. The present invention is very effective when the distance dependence of material dispersion and mode dispersion is appropriate.

In other words, the effect of material dispersion is proportional to distance. Therefore, if the modal dispersion effect is proportional to distance, the total dispersion can be canceled at any propagation length. At present, the loss of optical fibers has been reduced, and it has been confirmed that the mode dispersion effect is almost proportional to the first power of the distance. As explained above, since it is possible to create a configuration that cancels material dispersion and mode dispersion, it is possible to realize a transmission line with a wide transmission band using a light source with a wavelength spread, and a long-distance high-capacity transmission method. This is very advantageous in configuring.

[Brief explanation of the drawing]

Fig. 1 shows the emission spectrum of the LED, Fig. 2 shows the group delay difference due to the material dispersion of the silica fiber, Fig. 3 shows the group delay difference due to the mode dispersion of the α-th power optical fiber, and Fig. 4 shows the group delay difference due to the mode dispersion of the α-th power optical fiber. The figure is a schematic diagram of light rays in a graded optical fiber, and FIG. 5 is a block diagram of an embodiment of the present invention. 1...Core, 2...Kratsding,
3, 4... Concentrating lens, 5... Light source, 6... Prism.

Claims (1)

[Claims] 1 The wavelength width of the light source and the refractive index distribution of the optical fiber material are selected so that the material dispersion and mode dispersion of the optical fiber cancel each other out, and the light beam from the wavelength-spread light source is spatially split using a demultiplexer. An optical transmission system characterized by separating each wavelength and inputting the light into a multimode fiber using a condenser so as to have a desired relationship between each separated wavelength and the distance from the center of the core of the optical fiber to be excited.