Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1997-02-11
1998-09-22
Negash, Kinfe-Michael
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359145, 359164, 359167, 359179, 455517, 375254, H04B 1012
Patent
active
058122966
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a fiber optics transmission system which transmits radio signals over optical fiber lines and, more particularly, to a fiber optics transmission system which is used to build mobile communication systems in underground shopping malls, tunnels and similar radio dead zones.
PRIOR ART
As a radio signal transmission system for the use of pocket telephones or similar mobile radios in underground shopping malls, tunnels and like radio blind areas, there has been proposed in Japanese Patent Application Laid-open No. 6-153255 such a fiber optics transmission system as shown in FIG. 1. In this fiber optics transmission system, radio access units 200.sub.1 to 200.sub.N are connected via optical couplers 26.sub.1 to 26.sub.N to an optical fiber line 300, which is, in turn, connected to a base unit 100. Radio frequency signals (hereinafter referred to as RF signals) received by antennas 21 of the radio access units 200.sub.1 to 200.sub.N are fed to semiconductor laser devices 25, each formed by a laser diode, in the access units, wherein the received RF signals are used to intensity-modulate laser light of wavelengths .lambda..sub.1 to .lambda..sub.N emitted from the laser diodes. The optical signals of the wavelengths .lambda..sub.1 to .lambda..sub.N from the access units 200.sub.1 to 200.sub.N are each provided via an optical coupler onto the optical fiber line 300 and combined with other optical signals one after another, and the combined light is fed to a photodiode 11 of the base unit 100. The photodiode 11 converts the combined light into radio-frequency signals corresponding to the intensity-modulated components of the respective wavelengths, and the base unit 100 sends the radio-frequency signals to base stations not shown.
The oscillation frequencies of the laser diodes of the access units 200.sub.1 to 200.sub.N are chosen so that they are spaced apart to prevent the generation of beat noise by wavelength differences in the working radio-frequency band. By installing such radio access units 200.sub.1 to 200.sub.N in an underground shopping mall or tunnel at suitable intervals so that no blind zones are formed, it is possible to send an up-link RF signal, for example, from a portable telephone 40 to the base station via the nearest access unit 200.sub.1 the optical fiber line 300 and the base unit 100. In this way, the conventional fiber optics transmission system permits the use of portable telephones or like mobile radios in underground shopping malls.
As described above, in the fiber optics transmission system the optical signals of the wavelengths .lambda..sub.1 to .lambda..sub.N, emitted from the semiconductor laser devices 25 of the radio access units 200.sub.1 to 200.sub.N, are provided onto the optical fiber line 300 and combined with one another and the combined optical signal is fed to the photodiode 11 of the base unit 100, so that the beat noise is generated which results from wavelength differences among the optical signals sent from the radio access units 200.sub.1 to 200.sub.N. Assuming, for example, that the semiconductor laser devices 25 of the access units 200.sub.1 to 200.sub.N each oscillate laser light of a 1.3-.mu.m band and that the wavelength difference between two optical signals is about 0.005 nm, the beat frequency is around 1 GHz, which falls in the using radio frequency band. It is necessary, therefore, in the prior art to set, as shown in FIG. 2, the oscillation wavelengths of the semiconductor laser devices of the radio access units at predetermined wavelength intervals .DELTA..lambda. or more so that the wavelengths do not overlap lest the beat noise should fall in the radio signal transmission band. To perform this, it is required that N laser diodes, which satisfy the predetermined wavelength spacing, be selected from a large number of purchased laser diodes.
To build such a fiber optics transmission system at low cost, mass-produced commercially available laser diodes need to be used as laser diodes forming the semiconducto
REFERENCES:
patent: 4916460 (1990-04-01), Powell
patent: 5301353 (1994-04-01), Barras et al.
patent: 5493436 (1996-02-01), Karasawa et al.
Chu et al; "Fiber Optic Microcellular Radio", IEEE Trans-actions on Vehicular Technology, vol. 40, No. 3, Aug. 1991. pp. 559-606.
Ito Yasushi
Nojima Toshio
Tarusawa Yoshiaki
Negash Kinfe-Michael
NTT Mobile Communications Networks Inc.
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