Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-10-23
2001-10-30
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06310707
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wireless communication system for performing wireless communication in free space by using light, and a transmitter and receiver used for this system.
2. Related Art
In order to realize movable communication with multimedia functions, radio communication with higher transfer rates is required, and there is a need to develop new frequencies. In the field of radio waves, development is proceeding which aims at realizing radio communication with submillimeter and millimeter waves. On the other hand, there is also an expanded utilization of infrared rays in the field of radio communication, although they are not regulated by the law as radio waves.
Using infrared rays, namely a wide band which is not under restriction as radio waves in the field of optical wireless data communication, the provision of high speed data communication may be possible. As light is characterized by not penetrating non-transparent objects such as walls, it is suitable for use in wireless LANs of individual room or for short distance data communication. At present, the most typical wireless communication system using infrared rays is the IrDA (Infrared Data Association) system. This system is composed of an infrared light-emitting diode and a light-acceptor, and realize data exchange at high speeds from 115.2 kbps to 4 Mbps. The communication distance is short, namely 1 m or less, but the greatest characteristic is that it can provide wireless data communication at low costs.
In the future, the demand will grow for an optical wireless data communication system with a larger transmission capacity and greater communication distance. However, when using light-emitting diodes as the light source, the light emitted from the light-emitting diode has a wavelength width of 100 nm or more, so the effective utilization of the band is ineffective. Furthermore, due to the long life time of carriers in LEDs, a modulation exceeding 100 MHz is difficult. In order to solve these problems, it is effective to use semiconductor lasers as the light source. By using a semiconductor laser, a wavelength width of 1 nm or less is easily obtained, and modulation to 1 GHz or more is principally also possible. However, there is the problem that errors may be caused by crosstalk. As light is not subject to legal restriction as radio carrier waves, it can be used without restriction, but the disadvantage may arise in that optical wireless equipment which utilize the same wavelength will mutually interfere. For example, existing optical wireless data communication systems such as the IrDA system utilize wavelengths from 850 nm to 900 nm, as their peak wavelengths. Even if a communication device using semiconductor lasers which provides high speed transmission and long communication distances were realized, using any of the wavelengths from 850 nm to 900 nm would lead to interference with the IrDA system. As the IrDA system is widely used with existing computers, the interference must be avoided in practice, even though it may not be a legal problem.
In order to prevent the interference, the wavelengths need to be selected so as not to overlap with the wavelengths already in use. For example, in order to avoid interference with the IrDA, it is effective to use a wavelength of 1 &mgr;m or more. However, the problem is the related cost. In order to utilize wavelengths of 1 &mgr;m or more, there is the need to use an InGaAsP mixed crystal formed on an InP substrate as the transmitting semiconductor laser. This substrate is more expensive than a GaAs substrate and increasing the diameter is difficult. Therefore, this system is less cost effective than those systems which use wavelengths of 900 nm or less which can use the GaAs substrate. Furthermore, PIN photodiodes for receiver can be cost problems as well. The reason is that PIN photodiodes made of silicon are limited to using only wavelengths of 1 &mgr;m or less. It has no sensitivity in wavelengh range exceeding this limit. With wavelengths exceeding 1 &mgr;m, a PIN photodiode made of InGaAs formed on an InP substrate is necessary. As material and production costs for this element highly exceed those made of silicon, a light detector whose area is comparatively large will cause a large cost difference.
Accordingly, the realization of high-speed optical wireless data communication utilizing semiconductor lasers is difficult due to the increased cost in making an approach of using long wavelengths of 1 &mgr;m or more to prevent interference with existing optical wireless data communication systems.
There is also an additional problem in providing an optical wireless data communication system at low costs. A clock synchronized with the received data is necessary to reproduce the received data from the received light. Conventional optical wireless data communication systems extract the clock via a PLL (Phase Locked Loop) circuit from an electric signal obtained by photoelectrically converting the received light. The stable operation of this PLL circuit required high precision circuits and stable power sources, which became factors for raising the cost. It also required the modulation into RLL-type (Run Length Limited: limited number of bits in which “1” and “0” follow successively) modulated codes during the transmission so as to allow easy extraction of the clock from the received light. The modulation and demodulation circuits were other factors for raising the cost.
SUMMARY OF THE INVENTION
The present invention aims at providing an inexpensive optical wireless data communication system which is little affected by disturbing light, which does not affect existing optical wireless communication systems, and which is not affected by existing optical wireless communication systems. Another object of the present invention is to provide an optical wireless data communication system which does not interfere with the IrDA system even when performing light modulation communication using a wavelength band of 850 nm to 900 nm, which is used by the IrDA system.
A further object of the present invention is to provide a stable wireless data communication system which does not interfere with conventional systems at a low cost. The present invention can also provide an optical wireless data communication system which can reproduce stable modulated signals under complementary modulation of light with two different wavelengths, as same phase components can be removed through differential detection.
Another object of the present invention is to provide a communication system in which data discrimination is possible without using a PLL circuit, as the data to be transmitted and the clock data have been propagated through the free space by using two types of light with differing polarization states or wavelengths. Another object is to provide an inexpensive optical wireless data communication system by omitting the process of encoding and decoding via special modulation codes, as there is no need for extracting the clock from the received signal. Another object is to provide a transmitter which can be made compact by using a surface-emitting laser which emits two types of light with differing polarization states or wavelengths, and wherein the optical axis is easily adjustable, so that as result, the cost is reduced.
In order to achieve the above objectives, the present invention provides an optical wireless data communication system for performing communication by emitting light of a wavelength of 100 &mgr;m or less into free space, including a transmitter having one or more light sources for emitting two or more types of light with mutually differing optical characteristics, said transmitter comprising an emitting means for individually modulating said two or more types of light based on first transmission data obtained by modulating the transmission data and second transmission data obtained by calculating and converting the transmission data, and emitting two or more mutually differing modulated signal ligh
Kaneko Takeo
Kawase Takeo
Kitamura Shojiro
Kondo Takayuki
Oliff & Berridg,e PLC
Pascal Leslie
Phan Hanh
Seiko Epson Corporation
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