Optical detection apparatus and free-space optics communication apparatus

There is provided an optical detection apparatus, comprising a light-receiving element, an optical system which forms a spot of light flux on a light-receiving surface of the light-receiving element by externally incident light flux, and an information generating section which generates information with respect to a position of the spot based on the output from the light-receiving element. The optical system includes an optical element array having a plurality of optical element portions, and a plurality of spots formed by the plurality of optical element portions substantially overlap to each other on the light-receiving surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a free-space optics communication apparatus and an optical detection apparatus, which allow apparatuses, which are arranged opposite to each other and spaced with a predetermined interval, to perform bidirectional information communication.

2. Description of Related Art

In Japanese Patent Application Laid-Open No. H05-133716, the free-space optics communication apparatus comprising an optical axis correction unit which detects an incident direction of light flux emitted from a counterpart apparatus and emits its light flux toward the incident direction is disclosed. A characteristic structure of the conventional free-space optics communication apparatus is shown inFIG. 12. The free-space optics communication apparatus has a light-transmitting optical system and a light condensing optical system as shown inFIG. 12, and these two apparatuses having substantially the same structure are arranged opposite to each other and spaced from each other to perform bidirectional communication.

Laser lights emitted from a laser diode1, which are linearly polarized in the direction perpendicular to the paper surface of the same figure, are converted to substantially parallel light flux by a lens unit2having a positive power and are reflected at a boundary surface (a polarized light separating surface) of a polarization beam splitter3. And the reflected light is reflected again by a mirror4of an optical axis direction varying section10, and then transmitted as sent light LA from an apparatus A to an apparatus B which is not shown.

The received light LB which is emitted from the counterpart apparatus and is incident on the main apparatus is reflected by the mirror4and transmitted through the polarization beam splitter3to reach a received light splitting mirror5. In this case, about 90% of the received light LB is transmitted through the received light splitting mirror5and is condensed at a main signal detection light-receiving element6by a lens unit7having a positive power. And the rest of about 10% is reflected by the received light splitting mirror5and is condensed at a position detection light-receiving element8by a lens unit109having a positive power.

An optical element on whose attached surface a multi-layered thin film is deposited is used as the polarized beam splitter3. This multi-layered thin film is configured so that S polarized light is reflected and P polarized light is transmitted. In order to attain the most efficient light transmission and reception using the polarized beam splitter3, it is preferable to have the received light LB being P polarized light when the sent light LA is S polarized light.

Moreover, in order to perform the most efficient light transmission and reception by arranging the light-transmitting apparatus and the light-receiving apparatus having the same structure arranged opposite to each other, it is preferable to have an “optical axis on the beam splitter side”13disposed to be inclined behind the paper surface of the same figure which is a common optical axis for transmission and reception, such that the polarization direction of the sent light LA and that of the received light LB are perpendicular to each other when these two apparatuses are arranged to face each other.

In addition, in order to perform communication having a large amount of transmitting information, small elements such as an avalanche photodiode, which has a diameter of an effective light-receiving area less than 1 mm, should be employed for a main signal detection light-receiving element6. Accordingly, in order to dispose the received light LB within the effective light-receiving area of the main signal detection light-receiving element6, an angle of the mirror4is adjusted so that the position of the main signal detection light-receiving element6is aligned with that of a position detection light-receiving element8. Thus, the optical axis of the received light LB is arranged to be substantially at the center of the position detection light-receiving element8.

In this case, in order to effectively transmit the sent light LA toward the counterpart apparatus, the optical axis of the sent light LA, namely, the laser diode1preferably coincides with the center of the position detection light-receiving element8.

Position deviation information of a spot SP, formed on the light-receiving surface of the position detection light-receiving element8by the received light LB, is sent by a signal processing section11to an optical axis direction control section12as an optical axis deviation correction signal, and the optical axis direction control section12sends an optical axis direction changing signal to an optical axis direction varying section10.

Further, based on this optical axis direction polarized light signal, the angle of the mirror4is adjusted so that the optical axis of the sent light LA coincides with that of the received light LB.

Such control is continued during communication, and the bidirectional communication apparatuses, which are disposed opposite to each other and spaced with a predetermined interval, are corrected mutually such that the optical axis of the received light LB transmitted from the counterpart apparatus coincides with the center of the position detection light-receiving element8. Thus, the optical axis of the transmitted light LB can be arranged to coincide with that of the received light LA.

FIG. 13shows a structure of a position detecting element in accordance with the related art. A four-division sensor13which is divided into four elements14is generally employed as the position detection light-receiving element8. However, when such a light-receiving element is used for the position detection light-receiving element8, it is preferable to allocate a proper area to the spot SP of the received light LB so as to repress a rapid change of a sensor output when the light crosses a separation zone15between the separated elements. Accordingly, the position of the light-receiving surface is generally set at a position defocused from the condensing point.

However, in the free-space optics communication apparatus for performing light transmission and reception in the atmosphere, the transmission is affected by fluctuation phenomena of transmitted beams caused by vibrations of an installed position of the apparatus or fluctuations of the atmosphere. These atmospheric fluctuations may be classified into a macro-fluctuation in which the whole transmitted light fluctuates and a micro-fluctuation in which the intensity distribution of the transmitted light fluctuates. In this case, although the macro-fluctuation of the atmosphere may be overcome along with the vibration associated with the installed position, another method should be taken into consideration to deal with the micro-fluctuation.

FIG. 14is a conceptual diagram in which the micro-fluctuation of the atmosphere is modeled. Reference character W denotes a spread when the received light LB emitted from the counterpart apparatus reaches the main apparatus. The atmosphere is a non-uniform medium which has a convection current caused by pressure or temperature differences and whose refractive index varies not only in space but also in time. As a result, the received light LB is diffused, and a portion W1having a strong intensity and a portion W2having a weak intensity appear in the spread W.

This intensity distribution varies as a function of time, so that W2is seen as fluctuating in the spread W where the sent light LA is diffused. This is referred to as the micro-fluctuation of the atmosphere, and this fluctuation occurs randomly. In the free-space optics communication apparatus of the related art, the light-receiving surface of the position detection light-receiving element8is disposed at a position defocused from the condensing point, so that, in a state of the micro-fluctuation of the atmosphere as described above, the spot SP having a proper area on the light-receiving surface does not have a uniform intensity distribution and the distribution of the light intensity at a beam taking inlet M into an apparatus corresponding to an entrance pupil is transmitted as it is. (SeeFIG. 15.)

FIG. 16shows a feature of the spot SP formed with light flux collected from the beam taking inlet M. The spot SP of a diameter T has a portion P1with a strong intensity (unshaded) and a portion P2with a weak intensity shown with oblique lines. The center of gravity of spot light PC, which is different from the center of the light flux BC, is judged to be an optical axis, and there occurs a deviation toward the optical axis direction of the sent light LA by an angle associated with the amount of position deviation S thereof. As a result, the sent light LA is deviated from the counterpart apparatus, which causes the problem in that communication cannot be performed.

In addition, the four-division sensor has been described up to now, but the defocusing problem may be avoided by repressing a rapid output change by employing a sensor referred to as a position sensitive detector (PSD) such as a semiconductor image position detecting element, which does not have the above-mentioned separation zone. However, in an apparatus whose communication distance ranges from several tens meters to several kilometers, it is difficult to dispose the position detection light-receiving element8at a place nearest to the best position.

Moreover, such apparatuses should be adjusted to have the following position relationship between the laser diode1and the lens unit2. In case of a short distance, the beam is made broad so that the sent light LA does not adversely affect on human eyes. In case of a long distance, the beam is made narrow so that the beam energy securely reaches the counterpart apparatus. Therefore, the defocusing method cannot be avoided on the position detection light-receiving element8.

Accordingly, the defocusing method should be taken into consideration in the case of PSD. The PSD should detect the center of gravity position of the spot light even in the defocused state, and there is no difference from the case of the four-division sensor.

The present invention is made to overcome the above-mentioned problems, and it is an object of the present invention to provide a free-space optics communication apparatus and an optical detection apparatus, which are capable of performing stable communication by reducing an optical axis deviation correction error regardless the non-uniform intensity distribution of the received light due to the micro-fluctuation of the atmosphere.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided an optical detection apparatus, comprising a light-receiving element, an optical element array which form a plurality of spots of light flux on a light-receiving surface of the light-receiving element with externally incident light flux, and an information generator which generates information with respect to each of positions of the plurality of spots based on an output from the light-receiving element. Here, the plurality of spots are formed on the light-receiving surface so as to substantially overlap to each other.

According to another aspect of the present invention, there is provided free-space optics communication apparatus for performing communication with a counterpart apparatus by light flux transmitted through space, comprising a light-receiving element, an optical element array which form a plurality of spots of light flux on a light-receiving surface of the light receiving element with the light flux incident from the counterpart apparatus, and an information generator which generates information with respect to each of positions of the plurality of spots based on an output from the light receiving element. Here, the plurality of spots are formed on the light-receiving surface so as to substantially overlap to each other.

Features of the optical detection apparatus and the free-space optics communication apparatus of the present invention will become apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows a block diagram of a first free-space optics communication apparatus101which is an embodiment of the present invention. In addition, elements having the same functions are denoted by the same reference numerals as those of the conventional invention throughout the specification as much as possible. Laser lights emitted from a laser diode1, which is linearly polarized in a direction perpendicular to the paper surface of the same figure, is converted to substantially a parallel light flux by a lens unit2having a positive power. This substantially parallel light flux is reflected at a boundary surface of a polarized beam splitter3, further reflected by a mirror4of an optical axis direction varying section10, and then transmitted as sent light LA from the first free-space optics communication apparatus101to a second free-space optics communication apparatus102.

A received light LB, which is substantially parallel to the paper surface from the second free-space optics communication apparatus and substantially linearly polarized, is incident on the first free-space optics communication apparatus101, reflected by the mirror4, then transmitted through the polarization beam splitter3, and reaches to the received light splitting mirror5.

In this case, most of the received light LB is transmitted through the received light splitting mirror5to be condensed on a light-receiving surface of a main signal detection light-receiving element6by a lens unit7having a positive power. The rest of the received light LBb reflected by the received light splitting mirror5is condensed by a lens unit9having a positive power, then transmitted through the lens array21, and reaches to a light-receiving surface of a position detection light-receiving element8.

Next, a function of the lens array21will be described with reference toFIG. 2.FIG. 2shows a detailed diagram of a portion of the first free-space optics communication apparatus101which is surrounded by the dotted line A inFIG. 1. In addition,FIG. 2is reversed left-side to right on the paper surface ofFIG. 1. The received light LBb guided to the lens unit9is condensed on the lens unit9. The condensed light flux is incident on the lens array21disposed at a defocused position where focusing is not made yet.

The lens array21is configured on a flat glass plate by forming several micro lenses with various radii of curvature shaped as hexagon or quadrangle shown inFIG. 3Aor3B. This lens array21is fabricated by a molding method in which a glass material softened by heating is poured into a metallic mold having a lens array shape, a method for forming micro lenses with a high molecular material such as a transparent plastic on a surface of glass, a method employing integral molding only using plastic, or the like. Each micro lens constituting the lens array21may have a light diffusion function or a light condensing function.

In addition, the lens array21may be formed on a substrate having a lens shape. Furthermore, a circle or a triangle shape may be possible in addition to the hexagon or quadrangle. In addition, the lens array having only two micro lenses may be possible, however, it is preferable to dispose at least nine lenses within an effective light flux so as to repress the problem in that communication cannot be performed.

However, in order to avoid the reduction of light efficiency due to the diffusion resulted from refraction, the size of the lens may become too small and it is difficult to fabricate a fine lens with a large curvature radius. And, when the distance between the lens array21and the position detection light-receiving element8cannot be properly maintained, the number of micro lenses needs to be limited.

In this case, the light fluxes emitted from the micro lenses of the lens array21are numbered as L1 to L5in order from the upper side of the figure. The light flux L3, which includes the optical axis of the lens9, is once condensed on a focal plane F, and then spread until it reaches to the position detection light-receiving element8.

L1, L2, L4, and L5 are light fluxes emitted from the fine lenses, which are off-axial with respect to the lens9. In a manner similar to the case of the light flux L3, these light fluxes are once condensed near the focal plane F, and then spread until they reach the position detection light-receiving element8.

The position detection light-receiving element8is disposed near a position where the light fluxes L1 to L5reached to the position detection light-receiving element8are overlapped most on the light-receiving surface. The reason to specify being “the near” is as follows. There is a case where the aberration relation between the lens9and the lens array21cannot be clearly defined, or a case where a position deviation may be acceptable at the level of not causing any operational problems. Next, a function of the lens array21will be described with reference toFIG. 4.

FIG. 4shows a diagram in which the lens array21is overlaid on the intensity distribution of the light flux LBc (which is the same as the intensity distribution of the spot SP shown inFIG. 16). It can be seen that the light flux LBc is divided into areas by each micro lens (hereinafter, referred to as a cell lens) of the lens array21. Five cell lenses shown is the arrow direction indicated by Ce include a cross section shown inFIG. 2. That is to say, they are lenses through which the light fluxes L1 to L5 are emitted.

FIG. 5shows simplified diagrams of the intensity distributions of the light fluxes L1 to L5 right after being emitted from each cell lenses. There still remains the intensity distribution in each light flux, but, after they are overlapped on the light-receiving surface of the position detection light-receiving element8, the intensity distributions are averaged as shown inFIG. 6.

In the above explanation, only one row in the arrow direction indicated by Ce is considered. However, since the light flux and the lens array are distributed in the two-dimensional direction, all the cell lenses shown inFIG. 4are used for averaging. In addition, the more the cell lenses are used, the better the average is obtained.

The size of the spot SP6shown inFIG. 6should be arranged to be smaller than the diameter of the light-receiving portion81which is divided into four areas in the position detection light-receiving element8. When it is larger than the diameter of the light-receiving portion81, the light-receiving portion81is included just inside the averaged light flux as shown inFIG. 7, so that the position change of the spot SP6cannot be detected. In addition, when the number of cell lenses is small and when the average effect is not sufficient, the size of the spot SP6is better to be small.

However, when it is too small, the effect of the dead zone of the four-division sensor increases as described in the related art. Accordingly, in order to reduce the effect of the dead zone, the size of the spot SP6needs to be limited to a range from the size, in which the fraction of the dead zone area occupied in the area of the spot SP6is 50% or less, to the size, in which the diameter does not exceed the size of the four-division sensor in addition to areas including an installation condition and a tolerance, namely, the size which does not exceed about 90% of the minimum length of the light-receiving surface81.

Next, drive control of the mirror4will be described with reference to a flow chart shown inFIG. 11. When the received light LB is incident on the position detection light-receiving element8and a spot is formed on the light-receiving portion81, a signal is outputted from the light-receiving portion81to a signal processing section11in response to the intensity distribution of the spot light (Step1). The signal processing section11evaluates whether the spot position is located at the center based on the signal outputted from the light-receiving portion81(Step2). When the spot position is away from the center, it calculates the incident direction of the received light LB and concurrently outputs an optical axis deviation correction signal generated based on the calculation result to an optical axis direction control section12(Step3).

Further, the optical axis direction control section12calculates the driving direction and the driving angle of the mirror4based on the optical axis deviation correction signal inputted from the signal processing section11and concurrently outputs a driving signal generated by the calculation result to an actuator, which is not shown, as a driving source for the mirror4(Step4).

When the actuator is driven based on the driving signal inputted from the optical axis direction control section12, the mirror4is moved to a predetermined position so that the optical axis of the received light LB coincides with that of the sent light LA (Step5). Accordingly, the problem can be repressed in that the communication between the first free-space optics communication apparatus101and the second free-space optics communication apparatus102cannot be performed.

In the embodiment of the present invention, it is described that the direction of the mirror4is adjusted so that the optical axis of the received light LB coincides with that of the transmitted light LA, but the direction of the whole apparatus may be changed by a pan head.

In addition, the optical system of the present embodiment may be applied to various apparatuses in addition to the free-space optics communication apparatus and the optical detection apparatus.

Furthermore, as shown inFIG. 8, a lens41may be disposed between the lens array21and the position detection light-receiving element8so that the light flux emitted from the lens array21may be guided toward the position detection light-receiving element8. As shown in the same figure, the lens41is a convex lens, but a concave lens or a meniscus lens may be employed. In addition, the substrate which constitutes the lens array21may be a lens-shaped substrate as well as the flat panel substrate.

In the embodiment of the present invention, the lens array is used as an optical element array having a plurality of optical element portions, however, a refractive index distributed-type lens as shown inFIG. 9may be employed, or a concave mirror array as shown inFIG. 10may be employed.

In addition, in the received light splitting mirror5of the present invention, the amount of light toward the main signal detection light-receiving element is larger than that toward the position detection light-receiving element, but it is obvious that the reversed arrangement is also possible due to the effect of the sensitivity of the light-receiving element or the like.

While preferred embodiments have been described, it is to be understood that modification and variation of the present invention may be made without departing from the scope of the following claims.

“This application claims priority from Japanese Patent Application No. 2003-406175 filed on Dec. 4, 2003, which is hereby incorporated by reference herein.”