Optical wireless data communication system, and transmitter and receiver used therefor

Based on binary transmission data 12, a receiver 11 emits electromagnetic waves 13 with modulated plane of polarization into a free space 16. The data propagates along the free space 16 as the azimuth of the plane of polarization. A receiver 17 accepts the electromagnetic wave 13 with modulated plane of polarization discriminates the data based on the state of the plane of polarization, and outputs the binary received data 18. The data and the clock data are transmitted from a plurality of light-emitting units with different polarization states or wavelengths to omit PLL circuits.

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 .mu.m or more. However, the problem is the related cost.
 In order to utilize wavelengths of 1 .mu.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
 .mu.m or less. It has no sensitivity in wavelengh range exceeding this
 limit. With wavelengths exceeding 1 .mu.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 .mu.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 .mu.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 light beams, and a receiver
 having one or more light detecting parts for discriminating and accepting
 said two or more types of light, said receiver comprising a received data
 acquiring means for acquiring received data corresponding to said
 transmission data from the results of the calculation of the modulated
 signals obtained by accepting said modulated signal light which has
 propagated along free space.
 According to one embodiment of the present invention, the transmission data
 is used as said first transmission data and a data row of the reversion of
 the transmission data is used as said second transmission data.
 According to another embodiment, the exclusive OR of the divided clock by
 two and said transmission data is used as said second transmission data.
 Another embodiment uses a data row obtained by RZ modulating said
 transmission data as said first transmission data, and a data row obtained
 by RZ modulating the reversion of said transmission data as said second
 transmission data.
 A further embodiment uses a combination of linearly polarized light beams
 with crossing directions as said two or more light beams with mutually
 differing optical characters.
 Another embodiment uses clockwise and counterclockwise circularly polarized
 light beams as the two or more types of light beams with mutually
 differing optical characters.
 Another embodiment provides a combination of light beams with different
 wavelengths used as said two or more types of light beams with mutually
 differing optical characters.
 The present invention is characterized in having a transmitter and receiver
 used for this communication system.
 According to the present invention, an inexpensive optical wireless data
 communication system, and a transmitter and receiver used therefor, have
 been provided, which is little affected by the disturbing light, which
 does not affect existing optical wireless communication systems, and which
 is not affected by existing optical wireless communication systems.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Now, the embodiments of the present invention will be described with
 reference to the drawings.
 First Embodiment
 A first embodiment described below provides a system of optical wireless
 data communication utilizing two types of light beams with different
 polarization. Particularly, an example will be explained below for
 transmitting the transmission data using one polarization state, and
 transmitting the inverted data row of the transmission data by using
 another polarization state.
 FIG. 1 shows an example of the wireless data communication system according
 to the present invention. It shows the structure of transmitting data from
 a transmitter 11 and receiving such data with a receiver 17. The binary
 transmission data 12 is input into the input terminal 11a of the
 transmitter 11. The transmitter 11 emits the electromagnetic wave 13, in
 which the polarization state has been modulated based on the binary
 transmission data 12, into a free space 16. The electromagnetic wave 13
 travels along the direction of the z-axis. The electromagnetic wave 13 as
 used herein includes electric waves and light, but the embodiment below
 will be described mainly with regard to light of 100 .mu.m or less in
 wavelength. Furthermore, the free space 16 is not a common waveguide
 wherein the space is limited by dielectrics or metals, but shall mean a
 space in which the electromagnetic wave can freely propagate. In reality,
 in our living space, the ground, walls, ceilings, windows, constructions
 and human beings are factors which prevent the propagation of
 electromagnetic waves, but as long as they do not form a specific
 waveguide, such living space shall also be included in the free space 16.
 The electromagnetic wave 13 has a modulated state of polarization. The
 figure shows a state in which a linearly polarized light beams crossing
 perpendicularly each other are modulated in correspondence to the binary
 transmission data. The linearly polarized light beams 14a and 14b are
 linearly polarized light beams parallel to the x-axis. The linearly
 polarized light beam 15a is a light beam parallel to the y-axis. As an
 example, the polarization state is modulated so that the "1" value of the
 binary transmission data 12 corresponds to a linear polarization parallel
 to the x-axis, and the "0" value corresponds to a linear polarization
 parallel to the y-axis. As a result, the binary transmission data 12 will
 propagate along the free space 16 as changes in the polarization state.
 The electromagnetic data 13 reaches the receiver 17. The receiver 17
 outputs voltage corresponding to the polarization state at its output
 terminal 17a. For example, if it receives a linearly polarized light beam
 parallel to the x-axis, it outputs voltage corresponding to "1," and if it
 receives a linearly polarized light beam parallel to the y-axis, it
 outputs voltage corresponding to "0." This in other words is the binary
 transmission data 18. It is clear from the above descriptions that the
 contents of this binary transmission data 18 are equal to the contents of
 the binary transmission data 12. This modulation of the polarization state
 provides wireless data communication.
 In the example above, a combination of linearly polarized light beams
 crossing perpendicularly each other was used for modulation of the
 polarization state. There is a degree of freedom in this combination, and
 is not limited to linearly polarized light. This will be explained by
 using FIG. 2. The combination in FIG. 2(a) is made of linearly polarized
 light beams. The combination of a linearly polarized light beam 14 which
 is parallel to the x-axis and a linearly polarized light beam 15 which is
 parallel to the y-axis correspond to the positions "1" and "0" of the
 binary data. Here, the coordinate axes may be set arbitrarily, and there
 is no special meaning in making the coordinate axes parallel to the
 directions of polarization. It only functions to clearly show that the
 linearly polarized light beams are emitted perpendicularly each other. The
 combination shown in FIG. 2(b) is a combination of elliptically polarized
 light beams. With elliptically polarized light beams, a possible
 combination may be one in which the light beams have equal ellipticity,
 the major axes have crossing directions, and the rotating directions are
 opposite. In other words, a possible elliptically polarized light beam
 pair may have equal absolute values of ellipticity (if the ellipticity
 includes the rotation directions), and inverse signs. The combinations of
 these elliptically polarized light beams are at opposite positions on the
 Poincare sphere. These combinations are sometimes called "independent
 polarization states." An "independent polarization state" is characterized
 by enabling conversion into a perpendicular combination of linearly
 polarized light beams by addition of the same phase difference. The
 elliptically polarized light beams 21 and 22 shown in FIG. 2(b) are each
 independent polarization states, and may be utilized for communication by
 allocating these respectively to "0" or "1" of the binary data.
 Moreover, particularly useful are combinations where the ellipticity of the
 elliptically polarized light beams above is .+-.1, namely combinations of
 circularly polarized light beams. FIG. 2(c) shows a combination of a
 counterclockwise circularly polarized light beam 23 and a clockwise
 circularly polarized light beam 24. This combination is formed of opposite
 points at the top and bottom poles of the Poincare sphere, and form
 "independent polarization states." In other words, the counterclockwise
 circularly polarized light beam 23 and the clockwise circularly polarized
 light beam 24 correspond to "0" or "1" of the binary data, respectively,
 to provide a system of communication. This is illustrated in FIG. 3. When
 the binary transmission data 12 is input into the input terminal 31a of
 the transmitter 31, the transmitter 31 emits either the counterclockwise
 circularly polarized light beams 23a or 23b, or the clockwise circularly
 polarized light beam 24a, in correspondence with "1" or "0" of the binary
 transmission data 12. The electromagnetic wave 32 which has been modulated
 in the rotating direction of the circularly polarized light beams above
 propagates along the free space 16 in the direction of the z-axis to reach
 a receiver 33. The receiver 33 generates the binary received data 18 by
 outputting at its output terminal 13a voltage equal to "1" or "0" of the
 binary data in correspondence with the rotating direction of the
 circularly polarized light beam. In this way, wireless data communication
 is also possible by assigning the rotating directions of the circular
 polarization to correspond to "1" or "0" of the binary data. Further
 relevant is that it is particularly useful to select the combination of
 circularly polarized light beams as the "independent polarization states."
 The reason is that even if the transmitter 31 and the receiver 33 both
 rotate around the z-axis, the communication state does not depend on this
 angle of rotation. Circularly polarized light does not have a particular
 azimuth, so it does not change with the rotation around the z-axis. In
 case of a linear polarization as shown in FIG. 1, the transmitter 11 and
 the receiver 17 need to be laid at a particular angle. This requires
 providing a mechanism for rotating the receiver 17 to the most preferable
 receiving state at the inside or outside of the receiver. The combination
 of the circularly polarized light beams shown in FIG. 3 is advantageous in
 that this mechanism is not necessary.
 Next, the characteristics of the communication system according to the
 present invention which utilize a modulation of the polarization will be
 described below. FIG. 4 shows the time variations of the optical output in
 the communication using polarized light beam modulation according to the
 present invention. By assuming the independent polarization states as
 state A and state B, the optical output from the transmitters 11 and 31 at
 state A shall be referred as the polarized light component A, and the
 optical output at state B shall be referred to as the polarized light
 component B. Based on the binary transmission data 12, the time variation
 41 of polarized light component A and the time variation 42 of the
 polarized light component B of the output electromagnetic wave 13 undergo
 complementary changes. In other words, if the output of the polarized
 light component A rises to a high level, the polarized light component B
 concurrently drops to a low level. The relation between the polarized
 light components A and B may be called a differential output. If the
 amplitudes of the polarized light components A and B are equal, it can be
 seen from FIG. 4 that the total quantity of light 43 obtained by adding
 these two components becomes a constant, DC light. In other words, when
 observing only the light intensity and not dividing the light into the
 electromagnetic waves 13 and 32 output from the transmitters 11 and 31,
 there is only a DC light, as shown by 43. This is important in that
 influences to existing optical wireless communication equipment can be
 ignored. Existing optical wireless communication equipment perform
 communication by modulating the optical output, so that if two optical
 wireless communication equipment performs light intensity modulated
 communication on the same wavelength, both optical outputs will be
 superimposed, which will become an obstacle in the communication. However,
 as the optical wireless data communication system according to the present
 invention only outputs unchanging DC light as the optical output, the
 electromagnetic waves 13 and 32 output from the transmitters 11 and 31
 according to the present invention will reach and give a very small
 influences to the receivers of existing optical wireless communication
 devices. DC light which enters the receiver of an existing optical
 wireless communication device will offset all of the received light, but
 such offset can be easily removed during the reproduction of the data. In
 this way, electromagnetic waves output under the optical wireless data
 communication system according to the present invention will not affect
 existing optical wireless data communication equipment.
 Furthermore, the optical wireless data communication system according to
 the present invention is characterized by being little influenced by
 electromagnetic waves output from the transmitters of existing optical
 wireless data communication equipment. This will be explained by using the
 time variations of the polarized light components in FIG. 5. Assuming that
 the polarized light component A modulated like the waveform 51, and the
 polarized light component B modulated like the waveform 54 enter the
 receiving window of the receivers 17 and 33 used for the optical wireless
 data communication system according to the present invention. In reality,
 illuminations and electromagnetic waves output from the transmitters of
 existing optical wireless data communication equipment enter the receiver.
 These influences shall be called disturbing light. Here, the disturbing
 light is unpolarized light, and shall not be modulated with a particular
 polarized light component, nor change. In fact, existing optical wireless
 data communication equipment use unpolarized light, and many light sources
 such as illuminations output unpolarized light, so these premises are
 practical. The disturbing light can then be divided in an disturbing light
 A component 52 and an disturbing light B component 55, which both have the
 same intensity. As shown by the waves 52 and 55, the disturbing light
 changes with time. The components A and B, which are the intensity of the
 electromagnetic waves on the receiver, become waveforms 53 and 56 by
 adding 51 and 52, and 54 and 55, respectively. As the disturbing light
 also changes with time, the waveforms 53 and 56 change in a very complex
 manner. However, as the receivers 17 and 33 will detect only the component
 wherein the polarization state is modulated as the signal component, it
 can reproduce the signal obtained by subtracting the waveform 56 from the
 waveform 53, namely the modulated polarized light component 57. In this
 way, even if unpolarized disturbing light enters under the optical
 wireless data communication system according to the present invention,
 such light can be cancelled as common phase components, so that such
 disturbing light will have little effect. In other words, even if the
 source of disturbing light is the transmitter of an existing optical
 wireless data communication device, the optical wireless data
 communication system according to the present invention will be able to
 conduct communication without being affected thereby.
 Concluding from the above, the optical wireless data communication system
 according to the present invention will little affect, and little be
 affected by, existing optical wireless data communication equipment, so
 that a concurrent use is possible even when the same wavelength. For
 example, with the existing optical wireless data communication system
 IrDA, the peak wavelength of the communication light is prescribed to be
 within the range from 850 nm to 900 nm. When using the optical wireless
 data communication system according to the present invention, wireless
 data communication will be possible which does not interfere with the IrDA
 system when using the same wavelength range of 850 nm to 900 nm. This
 wavelength band is a range in which GaAs or AlGaAs semiconductor lasers
 can be used as the light source of the transmitter, and PIN photodiodes
 made of silicon can be used as the light detecting element of the
 receiver. These materials are most advantageous for providing an
 inexpensive communication device. In other words, the optical wireless
 data communication system according to the present invention can provide
 optical wireless data communication which does not interfere with existing
 optical wireless data communication equipment and which uses the most
 cost-effective wavelength band.
 As explained with FIG. 5, the optical wireless data communication system
 according to the present invention can provide stable received signals
 regardless of disturbing light entering the receiver, as the influences of
 such light can be removed. The free space 16 is full of light which has
 far larger intensity than the electromagnetic waves used for communication
 due to sunlight, glow lamps and fluorescent lamps. However, as all of
 these types of light are unpolarized, the optical wireless data
 communications system according to the present invention can remove the
 effects of this unpolarized light effectively. Therefore, it is able to
 extract communication light buried among these strong disturbing light,
 and the signal has a high SN ratio.
 As the optical wireless data communication system of the present invention
 emit the DC light in stead of AC light from the transmitter, the average
 output obtained is approximately twice that of conventional light
 communication which has been modulated in intensity. As a result, the
 number of photons per pulse can be increased to twice the number, thereby
 increasing the SN ratio by approximately 3 dB.
 Next, the transmitter used for the optical wireless data communication
 system according to the present invention will be described in detail.
 FIG. 6 shows an example of the transmitter according to the present
 invention. It shows a transmitter 31 used for counterclockwise and
 clockwise circularly polarized light. The modulated linearly polarized
 light-emitting element 61 is an element which, in correspondence with the
 input, can output linearly polarized light beams 14c and 15b crossing
 perpendicular directions. If this linearly polarized light itself is
 emitted into the free space 16, it will be a transmitter 11 for linearly
 polarized light. When using a set of counterclockwise and clockwise
 circularly polarized light beams, a quarter-wave plate 62 is used for
 converting the linearly polarized light beams into circularly polarized
 light beams 24b and 23c. If the quarter-wave plate 62 is arranged so that
 its optical axis 62a is inclined by an angle of 45 degrees against the
 linearly polarized light beams 14c and 15b, it is possible to convert
 linearly polarized light into circularly polarized light. In this process,
 the relation between the azimuth of the linearly polarized light and the
 rotating direction of the circularly polarized light differ depending on
 whether the optical axis 62a is slow or fast. Assuming the linearly
 polarized light beam 14c which is parallel to the x-axis is to be
 converted into a clockwise circularly polarized light beam 24b, then the
 linearly polarized light beam 15b which is parallel to the y-axis will be
 converted into a counterclockwise circularly polarized light beam 23c.
 As the circularly polarized light beams 24b, 23c with modulated directions
 of rotation have been generated as above, they may be emitted are into the
 free space 16, but in many cases, it is effective to adjust the angle of
 dispersion of the light by passing it through a lens system 63.
 Now, the modulated linearly polarized light-emitting element 61 will be
 described in detail. A light-emitting element which can modulate its
 optical output is generally known, but elements which can modulate its
 polarization state is not so generally known. However, such kind of
 modulated polarized light-emitting element can be realized with a single
 element or a combination of a plurality of elements. For example, when
 combining a plurality of elements, it is possible to modulate the
 direction of the plane of polarization of the light emitted from a laser
 light source by using an optically rotating element which utilizes a
 nematic liquid crystal. However, in this case, there is the disadvantage
 that the speed of response of the liquid crystal is not fast enough to be
 used for communication. As a single element, a semiconductor laser may be
 used. Normal semiconductor lasers emit a linearly polarized light beam
 with a fixed plane of polarization, but by designing the structure. it is
 possible to modulate the plane of polarization. As an example, a vertical
 cavity surface-emitting semiconductor laser will be described below.
 FIG. 7 shows an exterior view of a vertical cavity surface-emitting
 semiconductor laser (hereinafter called surface-emitting laser). A
 surface-emitting laser is characterized by a laser beam 73 being emitted
 vertically to the surface of a substrate 71 from an aperture 72. The
 surface-emitting laser is made by forming a semiconductor layer on the
 substrate 71 through the epitaxial technique, and processing this layer
 through the photolithography technique. A cross section of this
 surface-emitting laser is shown in FIG. 8. When carriers of electrons or
 positive holes are injected from a bottom electrode 86 and a top electrode
 82, these carriers continue to diffuse to reach an activated layer 84. An
 even more desirable structure is that the carriers injected from the top
 electrode 82 are squeezed via a layer for squeezing the width of the
 current path 83, and then collected at the activated layer 84 in a center
 are directly under the aperture 72. The electrons and positive holes
 reaching the activated layer 84 are recombined and emit light. This light
 reciprocates inside the resonator formed by a bottom semiconductor mirror
 85 and a top mirror 81. When the reciprocating light passes the activated
 layer 84, it is amplified by inducing a stimulated emission, and light
 with large intensity is bound inside the resonator. A portion thereof
 passes through the top mirror 81 and is emitted to the exterior as the
 laser beam 73.
 A surface-emitting laser operates in the way described above, but as it is
 also clear from FIG. 7, the shape of the resonator 74 can be designed
 freely through photolithography processing, so controlling the polarized
 light is made possible. For example, by making the cross section of the
 resonator 74 the shape of a circle, the azimuth is not specified, so there
 is a large degree of freedom in the plane of polarization of the laser
 beam 73. Therefore, by changing the injected current as shown in FIG. 9,
 it is possible to switch the plane of polarization. FIG. 9 shows the
 relation of dependency between a polarized light component 91 in the
 parallel direction (parallel shall mean any convenient direction) and a
 polarized light component 92 in the vertical direction (vertical to the
 parallel direction), and the injected current. If the injected current is
 less than Ith, mainly linearly polarized light in the parallel direction
 is emitted, but if the injected current exceeds Ith, it is clear that the
 emission switches to the linearly polarized light in the vertical
 direction. In other word, by modulating the injected current in the
 vicinity of Ith, it is possible to modulate the plane of polarization of
 the linearly polarized light. The description above relates to an example
 of modulating the injected current, but it is also possible to apply
 electric or magnetic fields, causing strain. injecting polarized light,
 etc., to modulate the plane of polarization. The use of these
 surface-emitting lasers as the modulated linearly polarized light-emitting
 element 61 can realize a transmitter according to the present invention.
 It is also possible to use a surface-emitting laser wherein there is no
 switching of the polarization plane, namely wherein the polarization plane
 is fixed, a the modulated linearly polarized light-emitting element 61.
 This can be realized by providing a surface-emitting laser which emits
 different polarized light beams on the same substrate, as shown in FIG.
 10. By forming the resonators 74a and 74b with rectangular cross sections
 as shown in FIG. 10, it is possible to control the plane of polarization
 to fix in one fixed direction. When the resonators 74a and 74b are formed
 rectangular, the plane of polarization becomes fixed in a direction
 parallel to the short side of the rectangle, by forming a rectangular
 resonator 74a with its short side parallel to the x-axis and a rectangular
 resonator 74b with its short side parallel to the y-axis neighboring each
 other on one same substrate 71, a linearly polarized light beam 14d
 parallel to the x-axis and a linearly polarized light beam 15c parallel to
 the y-axis, respectively, can be obtained. This structure does not provide
 a modulation and emission of linearly polarized light beams
 perpendicularly polarized each other from a single aperture, but the
 emission of linearly polarized light beams perpendicularly polarized each
 other from very closely placed apertures 72a and 72b, respectively. When a
 linearly polarized light beam parallel to the x-axis is to be emitted,
 current is injected from the top electrode 82a, and when a linearly
 polarized light beam parallel to the y-axis is to be emitted, current is
 injected from the top electrode 82b. In order to operate this multiple
 aperture type surface-emitting laser 101 as the modulated linearly
 polarized light-emitting element 61, it is practical to use the circuits
 shown in FIG. 11.
 In FIG. 11, OP1, TR1, and R1 form a constant current source for supplying
 the current corresponding to the voltage set by VR from the emitter of
 TR1. When one terminal (minus terminal) of OP1 is set to V1, by TR1
 supplies a current Ie=(V-V1)/R1. TR2 and TR3 are switching transistors,
 and function to switch the current Ie to either surface-emitting lasers
 LDX or LDY. The bases of TR2 and TR3 are driven by the binary transmission
 data 12 or the inverted signals passing through an inverter INV. In other
 words, TR2 and TR3 are complimentarily switched to their on/off states,
 and operate so that when one is in an on state, the other is in an off
 state. This leads to current Ie flowing in only either one of the lasers
 LDX and LDY. These lasers LDX and LDY are the multiple aperture type
 surface-emitting lasers 101 formed on the same substrate 71. Assuming LDX
 is the surface-emitting laser which emits linearly polarized light
 parallel to the x-axis and LDY is the surface-emitting laser which emits
 linearly polarized light parallel to the y-axis, the multiple aperture
 type surface-emitting laser 101 will emit linearly polarized light
 parallel to the x-axis if the binary transmission data 12 is "1," and
 linearly polarized light parallel to the y-axis when "0." The transmitter
 according to the present invention can be realized by utilizing this kind
 of multiple aperture type surface-emitting laser 101 as the modulated
 linearly polarized light-emitting element 61 driven by the circuit shown
 in FIG. 11. This type of circuit structure in which TR1 provides a
 constant current and TR2 and TR3 change the path of this current is
 characterized in that is can perform high-speed modulations.
 As shown in FIG. 10, surface-emitting lasers in which the plane of
 polarization is fixed can be manufactured en masse via the
 photolithography technique. The direction of polarization could be
 controlled to be fixed effectively by designing the laser so that the long
 side is at least 1.2 times the length of the short side of the rectangle.
 When the length of the long side was set to a least 1.4 times the length
 of the short side, the control could be secured at an even higher rate. As
 the surface-emitting laser is principally able to conduct modulation up to
 at least 1 GHz, the system shown in FIG. 10 guarantees a modulation of the
 plane of polarization up to 1 GHz. As the circuit structure shown in FIG.
 11 is suited for high-speed modulation, high-speed and large-capacity
 optical wireless data communication can be realized by utilizing the
 above-described multiple aperture type surface-emitting laser 101 as the
 modulated linearly polarized light-emitting element 61.
 Operation is possible when the diameter of the aperture of the
 surface-emitting laser is set within a range of 1.mu.m to 100 .mu.m, and
 the distances between the apertures of the multiple aperture type
 surface-emitting laser 101 may be within the range of 2 .mu.m to 400
 .mu.m. As the distance between the apertures are very small compared to
 the total size of the transmitter or the size of the free space 16, it is
 the same as if the light were emitted from one single aperture. If the
 optical output of each one of the surface-emitting lasers is too small to
 provide optical wireless data communication it is possible to form a large
 number of surface-emitting lasers 74a or 74b which emit linearly polarized
 light having the same direction of polarization on the same substrate to
 improve the output. This emission of communication light from a plurality
 of surface-emitting lasers not only increases the optical output as
 largely as possible but also has the effect of enhancing safety to the
 naked eye. When a laser beam is emitted from a single aperture, and a
 portion of this laser beam enters the naked eye, it is possible that the
 entering energy will be focused in one point of the retina due to the lens
 function of the naked eye. Laser light focuses in one point has a great
 energy density, which will sometimes damage the retina. On the other hand,
 when laser light emitted from a plurality of apertures enters the naked
 eye, the energy density is low even if the same energy as above has
 reached the retina, because this laser light is not focused in one point.
 Therefore, when considering that the output is the same, it can be said
 that the laser light emitted from a plurality of apertures is safer.
 The transmitter example above explains mainly the use of a surface-emitting
 laser, but other light-emitting elements may be used too. Conventional
 edge-emitting semiconductor lasers output polarized light (TE waves)
 parallel to the activated layer, but it is possible to manufacture a
 semiconductor laser which can increase the gain of the TM waves and switch
 between TE waves and TM waves by utilizing a strained superlattice
 structure. The transmitter may be structured by using this improved
 edge-emitting laser as the modulated linearly polarized light-emitting
 element 61. By perpendicularly crossing their directions of polarization
 and mounting conventional edge-emitting semiconductor lasers which can
 only output TE waves, it is possible to manufacture an element which can
 seemingly output two types of polarized planes, so it is possible to
 manufacture the transmitter by using this structure. However, mounting
 edge-emitting semiconductor lasers which have perpendicularly crossing
 directions of polarization require an adjustment of the alignment of the
 optical axes and the angle of dispersion, so the cost, which does not
 become lower than with surface-emitting lasers, is a problem.
 Next, an example of the receivers 17 and 33 will be described. Assuming the
 electromagnetic wave 32 with modulated, circularly polarized direction of
 rotation is to be received. The electromagnetic wave 32 which has
 propagated along the free space 16 is collected through a lens-filter
 system 131. As electromagnetic waves of wavelengths other than those used
 for communication cause disturbance, they are desirably removed through
 reflection or absorption by the lens-filter system 131 from the passed
 light. The circularly polarized light beams 23d and 24c which have passed
 through the lens-filter system 131 are converted into linearly polarized
 light through a quarter-wave plate 132. This is the reverse operation of
 the conversion of linearly polarized light to circularly polarized light
 as described with the transmitter. For example, the counterclockwise
 circularly polarized light beam 23d is converted into linearly polarized
 light parallel to the x-axis, and the clockwise circularly polarized light
 beam is converted into linearly polarized light parallel to the y-axis. At
 this time, it is possible to set the x-axis and the y-axis to the azimuth
 of 45 degrees to the optical axis 132a of the quarter-wave plate 132. The
 analyzer 133 is arranged to pass, reflect or absorb polarized light
 components which are parallel or vertical to such x-axis or y-axis. The
 illustrated example shows one case in which a polarized beam splitter is
 used as the analyzer 133. The polarized beam splitter 133 is arranged to
 reflect polarized light components parallel to the x-axis, and pass
 polarized light components parallel to the y-axis. Accordingly, light
 which as been converted into linearly polarized light parallel to the
 x-axis will be guided to a photodetector 134a. On the other hand, linearly
 polarized light parallel to the y-axis will be guided to a photodetector
 134b. The photodetectors generate current in correspondence with the light
 intensity entering the photodetector. Current waveforms corresponding to
 53 and 56 in FIG. 5 will be obtained from photodetectors 134a and 134b.
 The circuit shown in FIG. 14 will be used for obtaining a modulated
 polarized light component of the waveform corresponding to the binary data
 in 57 from the waveforms 53 and 56 in FIG. 5. Here, a PIN photodiode has
 been used as the photodetectors 134a and 134b. PINX and PINY correspond to
 the photodetectors 134a and 134b. A reverse bias voltage Vr is applied to
 the PIN photodiodes PINX and PINY. A current Is corresponding to the
 optical output flows from the anodes of the PIN photodiodes PINX and PINY,
 and then to the current-voltage conversion circuit formed by OP2 and R2,
 and OP3 and R3. These current-voltage conversion circuits convert Is to
 the voltages Is.multidot.R2, and Is.multidot.R3, respectively. Normally,
 it is desirable to set R2=R3. After their conversion into voltage signals,
 the two signals are input into a differential amplifier formed by OP4, R4,
 R5, R6, and R7, the differential components are amplified, and the common
 phase components are removed. Normally, it is designed so that R4=R6 and
 R5=R7 to obtain the amplification rate R5/R4. A waveform 57 equal to the
 modulated polarized light component can be obtained thereby. At this
 stage, the signal is an analog signal, so for the conversion into the
 binary received signal, it is inputted into a data discriminator 141 and
 converted into binary data, thereby obtaining the binary received data 18
 is output terminals 17a and 33a.
 In this way, it is possible to receive the electromagnetic wave 32 with
 modulated direction of rotation of the circularly polarized light, and
 demodulate the binary data. When receiving the electromagnetic wave 13
 with a modulated plane of polarization of the linearly polarized light,
 there is no need for converting circularly polarized light to linearly
 polarized light, so receiving is possible by removing the quarter-waver
 plate 132 from the structure in FIG. 13. Regarding the other elements and
 circuits, receiving is possible through the same principle as with the
 circularly polarized light above.
 The photodetectors 143a and 134b of the receiver above have been explained
 by using PIN photodiodes, but it is also possible to use other optical
 energy conversion elements. When using semiconductors, it is advantageous
 to use avalanche photodiodes regarding the sensitivity and the speed. It
 is also possible to use optical transistors which control carriers through
 light. With semiconductors, it is possible to provide an inexpensive
 receiver if the wavelength is 1 .mu.m or less, as silicon can be used. As
 described above, the optical, wireless data communication system according
 to the present invention will cause no interference with conventional
 optical wireless communication systems in using wavelengths from 850 nm to
 900 nm, so this wavelength band can be used. Among elements other than
 semiconductors, it is possible to use photoelectric amplifier tubes. The
 above examples have explained the analyzer with the polarized beam
 splitter 133, but there is no problem in using various types of
 polarization elements. It is possible to use birefringent polarizors such
 as a Glan-Taylor polarization prism, a Glan-Thompson polarization prism, a
 Wollaston prism, and a Thompson prism, a dichroic resin film, a metal
 whisker film, or a multilayered refractive resin film.
 Second Embodiment
 A second embodiment explained below provides an optical wireless data
 communication system using two types of light with differing polarization
 states. The difference to the first embodiment is that two types of
 polarized light with differing polarization states are used for
 transmitting the transmission data and the clock data.
 The two polarization states used here are "independent polarization states"
 as described with the first embodiment which are positioned at opposite
 points on the Poincare sphere, Special combinations are a combination of
 crossing linear directions, or a combination of counterclockwise and
 clockwise circularly polarized light. These combinations of polarization
 states shall be referred to as combinations of a first polarization state
 and a second polarization state.
 FIG. 15 is a view showing the binary transmission data 12 to be
 transmitted, a light intensity change 151 of the first polarization state
 and a light intensity change 152 of the second polarization state which
 correspond to the data. FIG. 15 also shows a clock signal 153 which will
 be the base for the time axis of the binary transmission data 12, and a
 divided clock signal 154 which is devided the clock signal by two. It is
 clear that the first polarization state is modulated in intensity based on
 the binary transmission data 12. On the other hand, the second
 polarization state is an exclusive OR of the divided clock signal 154 and
 the binary transmission data 12.
 The structure in FIG. 16 is used for emitting the intensity modulated light
 of such first polarization state and second polarization state. FIG. 16 is
 a circuit diagram for intensity modulating the surface-emitting lasers LDA
 and LDB based on the binary transmission data 12 and the clock signal 153.
 Here, the surface-emitting laser LDA emits light of a first polarization
 state, and the surface-emitting laser LDB emits light of a second
 polarization state. The binary transmission data 12 is inputted into
 inverter INV2, and the output of INV2 is connected with the gate of a
 transistor TR4. In other words, when the binary transmission data 12 is
 "1," the voltage at the gate of TR4 becomes lower than the voltage of the
 emitter, so TR4 is in an ON state, and current flows to the
 surface-emitting laser LDA. As a result, the surface-emitting laser LDA
 emits a laser beam of a first polarization state. The level of the current
 is determined by the resistance R2. If the binary transmission data 12 is
 "0," TR4 is in an OFF state, so no current flows and no laser beam is
 emitted.
 Furthermore, the clock signal 153 is input into the 1/2 devider FF, and the
 output obtained is the divided clock signal 154. This signal and the
 binary transmission data 12 are input into the exclusive OR gate EXOR, and
 the output thereof rives the inverter INV3 and the transistor TR5, and is
 emitted as the laser beam output from the surface-emitting laser LDB.
 As described with the first embodiment in FIG. 13, the receiver separates
 light to the first polarization state and the second polarization state,
 and includes an optical system for converting their respective optical
 intensity values into electric signals. As a result, a first received
 signal 171 corresponding to the first polarization state is obtained as
 the output of the photodiode PDA, and a second received signal 172 which
 is the electric signal corresponding to the second polarization state is
 obtained as the output of the photodiode PDB, as shown in the waveform
 chart in FIG. 17 and the circuit diagram in FIG. 18. As the outputs of
 both photodiodes PDA and PDB are currents, each is converted into a
 voltage value through the current/voltage conversion circuits IVA and IVB,
 which is then input into the comparators CPA and CPN to obtain the binary
 signal. As the first received signal 171 corresponds to the transmission
 signal, the binary signal above need only be input into the data
 discriminator 181 to obtain digital data, but actually, clock data is
 needed for sampling the received signal 171 inside the data discriminator
 181. Therefore, when the output of the comparators CPA and CPB are input
 into the exclusive OR gate EXOR for calculating the exclusive OR of the
 first received signal 171 and the second received signal 172, the original
 received clock signal 173 corresponding to the divided clock signal can be
 obtained. By delaying this signal by a fixed duration to generate delayed
 received clock signal 174 which is then input into the data discriminator
 181, the output of the comparator CPA can be sampled at the rising edges
 or falling edges of this delayed clock signal, thereby obtaining the
 target binary received data 18.
 In this way, a PLL circuit is not used for obtaining the clock data
 necessary for the data discriminator 181. Therefore, by omitting the
 circuit with high precision required as the PLL circuit and the stable
 power source needed for the PLL circuit, cost reduction can be achieved.
 As there is no need to extract the clock from the received signal, the
 process of encoding and decoding vias a special modulation code can be
 omitted, thereby enabling the provision of inexpensive optical wireless
 data communication.
 The system described below is also effective as a system which does not us
 a PLL circuit, namely the RZ (Return to Zero) code system. As shown in the
 waveform chart in FIG. 19, the RZ signal 191, which is the RZ encoded
 binary transmission data 12, temporarily transits to "1" when the binary
 transmission data "1," and thereafter always returns from the "1" state to
 the "0" state. The reversed RZ signal 192, which is the RZ encoded
 inversion of the binary transmission data 12, temporarily holds a "1"
 state when the binary transmission data is "0," and thereafter returns to
 "0." This RZ signal 191 and the inverse RZ signal 192 are respectively
 transmitted as changes in light intensity of the first polarization state
 and the second polarization state, respectively. For this process, a
 surface-emitting laser made to emit light of each of such polarization
 states need only be modulated based on the RZ signal 191 and the reversed
 RZ signal 192.
 The receiver shown in FIG. 20 has a detecting part PDC for converting the
 intensity of the light passing through the polarization element 201 for
 separating the first polarization state into an electric signal, and a
 detecting part PDD for converting the light intensity into an electric
 signal regardless of the polarization state. In other words, the detecting
 part PDC generates electric signals corresponding to the RZ signal 191,
 and the detecting portion PDD generates electric signals corresponding to
 the sum signal 193 obtained by adding the RZ signal 191 and the reversed
 RZ signal 193. This sum signal 193 can be used as the clock necessary for
 sampling data. Therefore, the output of the detecting parts PPC and PPD
 pass through the current/voltage converters IVC and IVD, are binarized by
 comparators CMC and CMD, then input into the data discriminator 202 as the
 data and the clock, thereby obtaining a binary received data 13
 synchronized with the clock.
 In the example above, the sum signal 193 was obtained as the sum of light
 intensity values through a polarization-independent photodetector PDD.
 However, it is also possible to realize a similar receiver by performing
 photoelectric conversion of the light with an element for separation of
 polarized light, such as the photodetector PDC, then converting such
 converted light into an electric signal to calculate the sum signal 193
 electrically.
 Third Embodiment
 A third embodiment provides a system for performing optical wireless
 communication using two or more types of light with different wavelengths.
 Particularly, one example will be explained in which the transmission data
 is transmitted as is by using one wavelength, and the data row of the
 reversion of the transmission data is transmitted by using the other
 wavelength.
 FIG. 21 is an abstract view showing the structure of a surface-emitting
 laser module 214 which can emit two types of wavelengths. Semiconductor
 chips 211 and 212 which have surface-emitting lasers 211a and 212a mounted
 on the same plane as the pedestal 213. As the surface-emitting laser emits
 laser beams vertical to the substrate, the semiconductor chips need only
 be mounted closely to each other on the same plane to easily align the
 directions of the emitted laser beams the same. The surface-emitting
 lasers 211a and 212a emit different wavelengths .lambda.a and .lambda.b.
 By modulating the current which flows through the surface-emitting lasers
 211a and 212a above, the transmitter performs modulation of the intensity
 of the light of wavelength .lambda.a based on the binary transmission data
 12, and modulation of the intensity of the light of wavelength .lambda.b
 based on the data row of the reversion of the binary transmission data 12,
 and emits two types of wavelengths .lambda.a and .lambda.b into the free
 space.
 The receiver shown in FIG. 22 includes a band pass filter 221, and a
 dichroic mirror 222 for separating the wavelengths .lambda.a and
 .lambda.b. As a result, the wavelengths .lambda.a and .lambda.b are
 respectively guided to photodiodes PDE and PDF, and generate electric
 signals corresponding to their respective light intensity values. Not only
 he light emitted from the transmitter, but also the components of the
 disturbance light generated from the environment which pass through the
 band pass filter enter into the photodiodes PDE and PDF. In many cases,
 this disturbance light is white light, or colored light with a broad
 spectrum. In other words, these disturbance light beams have a light
 intensity spectrum which is independent of the wavelength within a range
 of 100 nm. Therefore, by setting the difference of the wavelengths
 .lambda.a and .lambda.b emitted from the surface-emitting lasers 211a and
 212a, namely .vertline..lambda.a-.lambda.b.vertline., to 100 nm or less,
 the intensity of the disturbance light entering the photodiodes PDE and
 PDF can be approximately the same, and by obtaining the difference between
 their output values with a differential amplifier DEF, the disturbance
 light can be canceled. On the other hand, as both light beams of
 wavelengths .lambda.a and .lambda.b are modulated in mutually inverse
 phases via the transmitter, inputting the output of the photodiodes PDE
 and PDF into the differential amplifier will cause the modulated component
 to be amplified to an output with an amplitude of twice the gain. As a
 result, it is possible to obtain a received signal with an effective S/N
 rate by minimizing the influences of the disturbance light.
 Fourth Embodiment
 The fourth embodiment explained below provides a system for performing
 optical wireless data communication which uses two types of light with
 different wavelengths. The example below transmits transmission data and
 clock data by using two types of light with different wavelengths.
 The transmitter used is a surface-emitting laser module 214 for emitting
 laser beams of the wavelengths .lambda.a and .lambda.b as shown in FIG.
 21. The intensity of one of the surface-emitting lasers 211a is modulated
 based on the binary transmission data 12, an the intensity of the other
 surface-emitting laser 212a is modulated based on the exclusive OR of the
 binary transmission data 12 and the divided clock signal 154. As a result,
 the transmitter emits light signals obtained by individually modulating
 wavelengths .lambda.a and .lambda.b into the free space.
 The receiver uses an optical system similar to that shown in FIG. 22.
 However, after the photoelectric conversion at the photodetectors PDE and
 PDF and the conversion into voltage values at the current/voltage
 converters IVE and IVF, the system differs from that in FIG. 22. The
 current/voltage converter IVE outputs voltage proportional to the light
 intensity of .lambda.a. This output is converted into a binary signal by a
 comparator, and then input into a data discriminator 181. Furthermore,
 after the output of current/voltage converters IVE and IVF are converted
 into binary signals by the comparator, the exclusive OR of both signals is
 calculated by the exclusive OR gate to obtain the received clock signal
 173. When the received clock signal 173 is delayed by a fixed duration to
 generate a delayed received clock signal 174 which is then input into the
 data discriminator 181, the target binary received data 18 can be obtained
 by sampling the original comparator output with the rise and the timing of
 the rise of this signal.
 Furthermore, it is clear from the above description that another possible
 similar system for optical wireless data communication modulates the
 intensity of the surface-emitting lasers 211a and 211b based on the RZ
 signal 191, which is the RZ modulated binary transmission data 12, and the
 inverse RZ signal 192, respectively.
 As described above, the present invention realizes optical wireless data
 communication between a transmitter for modulating the polarization state
 based on binary data, and a receiver for detecting changes in the
 polarization state, so that it can provide an inexpensive optical wireless
 data communication system which is little influenced by disturbing light,
 which does not influence existing optical wireless communication systems,
 and which is not influenced by existing optical wireless communication
 systems. Particularly, the optical wireless data communication system
 according to the present invention does not interfere with the IrDA system
 even if it performs polarized light modulation communication using the
 wavelength band from 850 nm to 900 nm used by the IrDA system. Therefore,
 it is possible to use an AlGaAs semiconductor laser growing on a GaAs
 substrate as the light-emitting element used for the transmitter, and
 thereby provide an inexpensive transmitter. It is also possible to use a
 PIN photodiode made of silicon which can use wavelengths of 1 .mu.m or
 less as the light-accepting element of the receiver, thereby providing an
 inexpensive receiver. In this way, the optical wireless data communication
 system according to the present invention is superior regarding the cost
 when using a wavelength of 1 .mu.m or less, but the present invention also
 provides many superior effects regarding all wavelength bands, including
 wavelengths of 1 .mu.m or more. As disturbing light which causes
 disturbance is mostly unpolarized, the present invention can provide
 stable communication by removing these influences. Furthermore, according
 to the optical wireless data communication system of the present
 invention, the optical output from the transmitter is DC light of the peak
 values of the output optical components are the same, so that in
 comparison to light with modulated intensity, twice the output can be
 obtained on an average level. As a result, the number of photons can be
 increased to twice the number, so that it is possible to increase the S/N
 ratio by approximately 3dB. The polarization states used for the present
 invention may be an arbitrary combination of "independent polarization
 states." Among thee combinations, the pair formed by counterclockwise and
 clockwise circularly polarized light beams is particularly effective as
 there is no change in the transmission and receiving states during the
 rotation around the communication azimuths of the transmitter and
 receiver. A transmitter for modulating a circularly polarized light can be
 realized easily through the combination of a modulated linearly polarized
 light-emitting element and a quarter-wave plate. Particularly the use of
 surface-emitting lasers as the modulated linearly polarized light-emitting
 element is effective for simplifying the structure of the device. A
 surface-emitting laser has a large degree of freedom regarding the
 direction of polarization, and the direct modulation of the polarization
 direction is possible, so the transmitter can be provided at a low cost.
 Furthermore, a high-speed modulation of the plane of polarization is
 easily possible with the system of performing modulation by using a
 plurality of surface-emitting lasers with fixed direction of polarization,
 so it is possible to provide a high speed and large capacity optical
 wireless data communication. By emitting communication light from a
 plurality of surface-emitting lasers, the optical output can be increased,
 and the safety to the naked eye can be enhanced. The receiver for
 receiving the electromagnetic wave with a modulated direction of rotation
 of the circularly polarized light can be realized through the combination
 of a quarter-wave plate, an analyzer and a photodetecting element. This
 receiver can remove same phase components corresponding to the entering
 unpolarized light through differential detection, so that the reproduction
 of a stable polarized modulated signal is possible.
 As described above, the optical wireless data communication system and the
 transmitter and receiver used therefor according to the present invention
 provides stable wireless data communication which does not interfere with
 conventional systems at low costs.
 When complementarily modulating light with two different wavelengths, same
 phase components can be removed through differential detection, so that
 reproduction of a stable modulated signal is possible. In other words, an
 inexpensive optical wireless data communication system can be provided
 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.
 By using light with two different polarization states or wavelengths, the
 data to be transmitted and the clock data are propagated along space, so
 that data discrimination is possible without the use of a PLL circuit.
 Therefore, the cost can be reduced by omitting the high precision PLL
 circuit and the stable power source required for the PLL circuit. As there
 is no need for extracting a clock from the received signal, the encoding
 and decoding via a special modulation code can be omitted, thereby
 providing inexpensive optical wireless data communication.
 Particularly, the use of surface-emitting lasers which emit two types of
 light which differ in the polarization state or the wavelength provides a
 compact device, and the optical axis can be adjusted easily, thereby
 providing a transmitter according to the present invention at low costs.