JP2012060499A - Optical wireless communication apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem that a tracking control system for controlling an optical axis becomes unstable due to interception of a beacon light and provide an optical wireless communication device capable of maintaining a communication line even when the beacon light is intercepted, when adopting a convergence/divergence tracking light system in the optical wireless communication device for transmitting/receiving a signal light incident from an optical fiber and a beacon light having a short wavelength compared to the signal light in a common refractive index optical system.SOLUTION: An optical wireless communication apparatus includes an estimation device, in addition to a tracking device for correcting a directivity error of a transmission light based on an arrival direction of a beacon light. The estimation device for estimating a most probable arrival angle of a tracking light in the future based on a fluctuation of the arrival angle of the beacon light during a normal tracking operation is operated independently from the tracking device and correctly directs a directive direction of the transmission light incident from the optical wireless communication apparatus to a direction of a receiver by using the estimation value during an interception of the tracking light.

Description

  The present invention relates to an optical wireless communication apparatus having a tracking function. More specifically, by utilizing the wavelength dispersion characteristics of optical glass, the direction of the partner station can be accurately determined by transmitting signal light and laser light having a shorter wavelength than the signal light for tracking control from the same optical system. The present invention relates to an optical wireless communication apparatus having a tracking function to detect.

  In a conventional optical wireless communication device, a partner device (also called a partner station) is used by using spatial beam light that radiates a substantially parallel laser beam having an appropriate spread while utilizing the sharp directivity of laser light. However, if the optical axis shifts due to disturbance, the light does not reach the other station and the communication is interrupted. On the other hand, in order to maintain the communication line, the light from the other station (referred to as beacon light in the following description) is used as a reference to correct the optical axis deviation (tracking), and the light emission of the local station Measures are taken to match the direction (directivity angle) with the direction of the counterpart station (see Patent Document 1).

  The optical wireless communication apparatus can be positioned as an alternative to the optical fiber, but unlike the optical fiber, since it does not occupy space, there can be blocking of the spatial beam light by flying objects such as birds and insects. In an optical wireless communication device having a tracking function, not only the direct line is blocked by blocking the spatial beam light, but also a secondary communication line resulting from the unstable tracking control of the optical axis due to the blocking of the beacon light. There is a new problem of blocking.

  Therefore, as a conventional technology, in order to eliminate the unstable operation of tracking control when the beacon light is interrupted, when the received power of the beacon light falls below the reference value, the tracking control process is interrupted to control A configuration for fixing the command value is disclosed (see Patent Document 2).

  Here, considering the actual operating environment of the optical wireless communication device, the reason why the communication line is interrupted because the amount of light necessary for the operation of the angle-of-arrival detector that detects the beacon light cannot be obtained and the tracking is interrupted. (1) When the directivity error of the device receiving the beacon light becomes large, (2) When the directivity error of the device of the other party transmitting the beacon light becomes large and the beacon light is not irradiated to the reception side Or (3) when the beacon light is shielded by obstructions in the transmission path.

  Among these, in the case of (1) and (2), the line disconnection cannot be prevented even if the immediately preceding directivity direction is maintained as in Patent Document 2. In the case of (3), if the aperture of the signal light is larger than that of the beacon light near the place where the transmission path is interrupted and the transmission of the signal light can be maintained even if there is an obstruction, the improvement effect is obtained. Although it seems to be expected, in reality, a large distortion occurs in the spatial beam light at the time when the beacon light starts to be blocked by the diffraction effect by the shielding object, and the pointing error increases due to the tracking error. There is a problem that even if the directivity direction at the moment when the light is blocked is maintained, the effect of preventing the disconnection of the signal light becomes partial.

  By the way, the wavelength of a laser used for a long-distance line of 10 Gbps or more in optical fiber communication is a 1.3 μm band or a 1.55 μm band. When signal light in this wavelength band is also used for beacon light, an existing method is used. The angle-of-arrival detector using the four-divided Si photodiode used in the above has no sensitivity in this wavelength band, so it is necessary to newly develop an angle-of-arrival detector using expensive InGaAs.

  Since required characteristics are different between signal light and beacon light, beacon light having a wavelength of 1 μm or less is generated independently from the signal light in the optical wireless communication apparatus, so that an arrival angle detector using a conventional Si photodiode can be used. The method which was made is considered.

  In this method, the signal light that has propagated through the optical fiber is converted into a spatial beam light with sharp directivity by the optical system in the device and emitted to the space. On the receiving side, the received spatial beam light is transmitted by the optical system in the device. By efficiently coupling to an optical fiber, high-speed large-capacity communication of 10 Gbps or more and wavelength multiplexing, which are realized by optical fiber communication, can be realized by optical wireless communication. At that time, a new optical wireless communication apparatus that uses optical amplification technology and uses the same wavelength band (0.98 μm) as the pump light of the optical amplifier for beacon light has been developed (see Patent Document 3).

  Here, beacon light is considered among signal light and beacon light which constitute spatial beam light radiated in the air. In order to maintain reliable initial acquisition and stable tracking in optical wireless communication, beacon light has an appropriate beam spread, and the directivity of the transmitter is deviated from the receiver during initial acquisition. The beacon light can be reliably detected on the receiver side, and it is necessary to prevent the beacon light from being interrupted on the receiving side even if the transmission direction of the beacon light is slightly shifted due to the tracking error during tracking. . The beam spread required for the beacon light varies depending on the installation environment (vibration) of the optical wireless communication apparatus and the magnitude of atmospheric fluctuation, but is about 0.001 radians to 0.01 radians under normal conditions.

  For signal light, a practical transmission distance (about 1 km) while guaranteeing the output limit (about 100 mW) necessary for ensuring the safety of the laser light in the 1.3 μm band or 1.55 μm band to the human body. In order to realize high-speed and large-capacity signal transmission, it is necessary to sharpen the beam directivity to the physical limit. For example, the beam divergence due to diffraction of signal light having a diameter of 2 cm and a wavelength of 1.55 μm is about 0.1 milliradians. Therefore, in order to maintain a stable signal line, correction of axis deviation (tracking) is always continued, It is necessary to keep the directivity angle error to be 1/10 or less of the beam spread of the signal light, that is, about 10 microradians or less.

  Next, when an optical system that realizes the above-mentioned specification of the light beam is studied, the wavelength of the signal light and the tracking light are greatly different, so that the chromatic aberration of the optical system becomes a problem. From the viewpoint of chromatic aberration, the reflection type is preferable, but the reflection type optical system has a drawback that the requirement for processing accuracy of the mirror surface is four times as severe as that of the refraction type, and the configuration is complicated and expensive. Therefore, a refraction type optical system is preferable for configuring an optical wireless communication device aiming at miniaturization and cost reduction, and further, a configuration using the same optical system for signal light and beacon light is desirable.

  Therefore, in an optical wireless communication device using beacon light having a wavelength shorter than that of signal light, the signal light and the beacon light are transmitted through the same optical fiber and the refractive optical system (optical antenna) by sharing the optical system, When satisfying the condition that the signal light becomes parallel light, the beacon light having a shorter wavelength than the signal light becomes a convergent beam because the refractive index of the glass is increased by wavelength dispersion, and once at a certain distance determined by the refractive index of the glass. After focusing, it becomes a divergent beam. Here, such a method using beacon light is referred to as a convergent / divergent light tracking method.

  In an optical wireless communication apparatus that uses a convergent / divergent light tracking method, when a minute object passes near the focus of the beacon light, the beacon light is blocked or suffers a great attenuation. As a result, in an optical wireless communication apparatus in which a high-accuracy tracking operation is indispensable for transmission of signal light, signal light is shielded by an obstacle and not only the line is directly blocked, but also the beacon light is blocked. It has been confirmed by experiments that signal light is disconnected.

  Even in the conventional optical wireless communication apparatus, there is a possibility that a line break may occur due to the passage of a large obstacle, for example, a bird. In a communication system that requires reliability, this line break has been a problem. In a convergent / divergent light tracking optical wireless communication device that requires high-accuracy tracking, the focal point of beacon light is as small as 0.1 mm to 1 mm. Since the snow blocks the beacon light and the tracking is not normally performed, the signal light is disconnected. Although this interruption occurs for a few milliseconds, it has a fatal drawback that unless it takes any countermeasures, a convergent / divergent light tracking optical wireless communication device cannot be used stably during rain or snow. Become.

JP 2000-89155 A JP 2009-147447 A JP 2006-94135 A

  The conventional countermeasures against the light beam blocking in the optical wireless device having the tracking control function are to maintain the state immediately before the blocking, and to solve the unstable operation of the tracking control system for controlling the optical axis accompanying the blocking. It wasn't.

  Accordingly, the present invention employs an optical wireless device that transmits and receives beacon light having a shorter wavelength than that of signal light and signal light directly connected by an optical fiber using a common refractive index optical system, particularly a convergent / divergent light tracking method. To solve the problem that the tracking control system for controlling the optical axis becomes unstable with beacon light blocking, and to provide an optical wireless communication device that maintains a communication line even when beacon light is blocked Let it be an issue.

  In order to solve the above problem, an optical wireless device (also referred to as an own station) described in claim 1 receives a spatial beam light including signal light and beacon light for tracking control having a shorter wavelength than the signal light. An optical wireless communication device to be used, which radiates the spatial beam light as a transmission light toward a counter device (also referred to as a counter station) and receives the spatial beam light from the counter device; An angle-of-arrival detector that detects an angle of arrival of beacon light, a tracking mirror drive mechanism that changes a directivity angle that is a direction of the emitted spatial beam light, a control unit that controls the tracking mirror drive mechanism, and the directivity An initial directivity angle estimation device that estimates the directivity angle based on an output of a mirror angular displacement sensor that indicates a displacement of the tracking mirror corresponding to a change in angle, and the control unit receives the beacon light intensity The tracking mirror drive mechanism is controlled based on the output of the angle-of-arrival detector when larger than a predetermined value, and when the received intensity of the beacon light is not larger than a predetermined value, the initial pointing angle estimation device The tracking mirror drive mechanism is controlled based on the output initial directivity angle.

  By preparing an optimal control system for each of the normal tracking operation and when the beacon light is interrupted, an optimal control system can be realized at low cost. During the tracking operation, feedback control using an analog PID that operates at a high speed and has a high control gain is used to ensure a stable operation against disturbance. On the other hand, the initial directivity angle estimation device does not maintain the state immediately before the tracking light is interrupted, but selects and uses the optimum one from the past operation history of the tracking device and the nature of the disturbance to be dealt with.

  An optical wireless communication apparatus according to a second aspect is the optical wireless communication apparatus according to the first aspect, wherein the directivity angle estimation apparatus sets a predetermined value based on an output from the past to the present of the mirror angular displacement sensor. An infinite impulse response filter to be calculated and a PI compensator for adding proportional compensation and integral compensation to the output of the infinite impulse response filter are provided.

  Since the infinite impulse response filter calculates an output based on the input past history, it can perform more stable control than the control based on the immediately preceding value such as instantaneous interruption. For example, if the installation environment is thermally and mechanically stable and only needs to cope with a beacon light disconnection of about 0.1 seconds, the tracking angle is estimated with a time constant of about 100 Hz. A secondary IIR low pass filter can be used.

  An optical wireless device according to a third aspect is the optical wireless communication device according to the first aspect, wherein the directivity angle estimating device models a disturbance based on an output from the past to the present of the mirror angular displacement sensor. And a Kalman filter for calculating predicted values of the magnitude, change speed, and acceleration.

  A Kalman filter that models a disturbance and calculates a predetermined value makes it possible to estimate the directivity angle with higher accuracy.

  The optical wireless communication apparatus according to claim 4, wherein the signal light and the beacon light share a refractive optical system, and the signal light is emitted as a substantially parallel beam by the refractive optical system and the beacon light. Is characterized by being radiated to the opposing device so as to diverge after converging in the air due to the wavelength dispersion of the refractive optical system.

  The optical system can be miniaturized by sharing the refractive optical system for the signal light and the beacon light, and when the signal light is made substantially parallel by the wavelength dispersion of the refractive optical system, the wavelength of the optical system is smaller than that of the signal light. Short beacon light is affected by a higher refractive index than signal light and converges once and then diverges and travels to the opposite device. Therefore, at the position of the opposite device, the beam diameter of the beacon light should be larger than the signal light. Can do.

  In the optical wireless device that transmits and receives beacon light having a shorter wavelength than the signal light directly coupled with the optical fiber according to the present invention using a common refractive index optical system, the beacon light from the partner station is used as a reference. In order to resolve the instability of the tracking control that controls the pointing direction when the beacon light is blocked, the arrival angle of the most probable tracking light in the future is determined from the history of fluctuations in the angle of arrival of the beacon light during normal tracking operation. A control device that predicts and stabilizes the directivity direction of the communication device using the predicted value at the time of shutoff is provided. Therefore, it is possible to provide an optical wireless device that guarantees a stable line while being very small and inexpensive. it can. The present invention provides a solution for dealing with a short-time blocking of the tracking light by a minute object when the reception intensity of the beacon light is not sufficient. However, the present invention is not limited to the blocking of the tracking light by the minute object. It is also useful for measures against a large decrease in received power of tracking light frequently generated in environments with large atmospheric fluctuations, for example, environments with many heat sources in urban areas and long-distance transmission paths. The time width during which this power drop occurs is a relatively short time of about 0.001 second to 0.01 second corresponding to the frequency component of atmospheric fluctuation being about 100 Hz or less, and the method of this patent is used. Thus, even if the tracking is interrupted due to a decrease in the reception intensity of the tracking light, it is possible to avoid disconnection of the communication line.

  Furthermore, since the directivity angle estimation device includes an infinite impulse response filter or a Kalman filter, the directivity angle is estimated based on the past history as well as the state immediately before the instantaneous interruption of the beacon light. Measures can be taken. The present invention is particularly effective when the beacon light converges once after emission and then diverges.

It is a conceptual diagram which shows the mode of the signal light directly couple | bonded with the single mode fiber outside an apparatus, and the beacon light generate | occur | produced within an apparatus. It is a block diagram of the optical system of the arrival angle detector in a fiber coupler. It is a figure which shows a mode that an arrival angle fluctuation | variation is compensated with a tracking mirror. It is a block diagram of a tracking control system. It is a figure which shows the transmission characteristic of the closed loop of a tracking control system. It is a figure which shows the disturbance suppression characteristic at the time of tracking control system operation | movement. (A) It is a figure which shows the time fluctuation | variation of a tracking error. (B) It is a figure which shows the frequency spectral density of a tracking error. It is a figure which shows the starting characteristic of a tracking control system. It is a block diagram of the tracking control system provided with the initial directivity angle estimation apparatus. It is a flowchart in case an initial directivity angle estimation apparatus is used. It is a block diagram of a secondary IIR filter. This is a software program for realizing a secondary IIR filter. It is a response characteristic figure of a secondary IIR filter. It is a figure which shows the convergence condition of the initial directivity angle estimation apparatus at the time of a tracking start. It is a figure which shows the influence of shielding of the beacon light by a shield. (A) It is a figure when a shield moves horizontally. (B) It is a figure when a shield moves horizontally. It is a figure which contrasts the presence or absence of operation | movement of an initial directivity angle estimation apparatus. (A) It is a figure in case an initial directivity angle estimation apparatus does not operate | move. (B) It is a figure in case the initial directivity angle estimation total is operating.

  Embodiments of the present invention will be described below with reference to the drawings.

  The relationship between signal light and beacon light will be described with reference to FIG. The combined light from the optical fiber 1 in FIG. 1 passes through a glass optical fiber ferrule 4 and then passes through a collimator lens 5 to become a parallel beam 6. The beacon light emitted from the laser diode 2 is multiplexed with the signal light by the wavelength multiplexing coupler 3, passes through the glass optical fiber ferrule 4, and then becomes a parallel beam 10 through the collimator lens 5. The signal light 6 and the tracking light 10 that have become parallel beams are reflected by a two-axis integrated tracking mirror 7 and are incident on a refractive optical system (optical antenna) composed of a lens 8 and a lens 9 to be a predetermined beam. Radiated to the outside.

  Here, the wavelength of the signal light is 1.3 μm or 1.55 μm that is generally used in large-capacity optical fiber communication. On the other hand, as the beacon light, an inexpensive single mode fiber output type laser diode having a wavelength of 0.98 μm, which is commercially available for exciting an optical fiber amplifier, is used. Here, when the optical system is set so that the signal light becomes a parallel beam, the beacon light 10 having a wavelength shorter than that of the signal light indicated by the dotted line in the figure is radiated to the outside, and then in the middle of the transmission path due to the chromatic aberration of the lens. Once focused, the beam is propagated to the other side as a diffused beam with a moderate spread, and the convergent / divergent light tracking method is established.

  The optical fiber ferrule 4 made of glass that separates the signal light and the beacon light shown in FIG. 2 has a non-reflective coating on the end face that fixes the optical fiber 1, and the end face that changes the direction by reflecting the opposite beacon light, A reflective coating is applied to the beacon light. The beacon light that has passed through the lens 5 and entered the optical fiber ferrule 4 is reflected by the surface on which the reflection coating has been applied, and the direction of the beacon is changed and condensed by the lens 11 on the quadrant detector.

  Next, a method for detecting a tracking control signal from beacon light will be described. The end face of the optical fiber 1 is supported by an optical fiber ferrule 4 made of glass. Due to chromatic aberration of the lenses 9 and 8 of the optical antenna and the imaging lens 5 in the fiber coupler, the focus of the beacon light by the lens 5 is increased. Is located in front of the optical fiber end face of the optical fiber ferrule 4, the beacon light component coupled to the optical fiber 1 can be reduced to 10% or less.

  For the same reason, the influence (increase in aberration) of the beacon light imaged by the lens 11 due to the core of the optical fiber (the portion having a high refractive index at the center) can be made to be approximately the same. For this reason, as long as beacon light having a wavelength shorter than that of signal light is used, components of the received beacon light that are not coupled to the optical fiber 1 due to a tracking error are caused by the 4-division Si photodiode by the beacon light imaging lens 11. A focal point is formed on the light receiving surface of the arrival angle detector 12. In the configuration of FIG. 1, by dividing the difference between the vertical and horizontal photocurrents generated from the quadrant photodiode 12 by the total photocurrent, it is possible to accurately detect an error from the optical axis in the direction of arrival of the beacon light. . Further, the intensity of the beacon light can be known by calculating the sum of all the photocurrents generated from the four-division photodiode 12 in the vertical and horizontal directions.

  Next, the operation of the optical system when there is a variation in the arrival angle will be described in detail with reference to FIG. The optical system related to tracking shown in FIG. 3 includes an optical antenna for light emission and light reception, a tracking mirror 7, and a fiber coupler 14 for coupling between spatial light and fiber light.

  In the optical antenna, from the left side of the figure, a beam expander having a magnification of 10 times is used by using a convex lens 9 having a focal length f1 = 100 mm as a first lens and a convex lens 8 having a focal length f2 = 10.0 mm as a second lens. The tracking mirror 7 is arranged at the position of the exit pupil on the second lens side. As an example of a beam incident on the optical antenna, a beam 35 that coincides with the optical axis is indicated by a solid line, and a beam 36 that does not coincide with the optical axis having an arrival angle variation of about 0.5 degrees is indicated by a dotted line.

  The beam from which the arrival angle fluctuation is removed by the tracking mirror 7 enters a fiber coupler 14 that converts spatial light into optical fiber light. An arrival angle detector 12 (not shown) is integrated in the fiber coupler, and incident spatial light is coupled to a single mode fiber by the fiber coupler. The image forming lens 34 (focal length f3 = 12.0 mm) in the fiber coupler and the two lenses constituting the optical antenna can all be commercially available, and wavefront aberration for signal light in the wavelength range of 1.5 μm. Can achieve diffraction-limited imaging performance at 1/10 wavelength or less.

  The optical system that has been used in conventional optical wireless communication devices has many optical systems that use special aspherical reflectors and many dedicated designs that use many lenses. Therefore, an optical antenna with practical performance can be manufactured even by combining commercially available lenses. The beam diameter of the signal light emitted from the optical fiber through the fiber coupler is 2.3 mm, and the transmission beam diameter is further expanded to 23 mm by an optical antenna that also serves as a beam expander. Although the diameter of the first lens is 25 mm, the aberration characteristic deteriorates around the lens, and therefore the range from the center to 23 mm is used for signal light transmission and reception.

  When signal light in the 1.55 μm wavelength band is adjusted to become diffraction-limited parallel (collimated) light, the transmission tracking light in the 0.98 μm wavelength band converges with a wavefront curvature radius of 11.6 m at the optical antenna aperture. Become a beam. Therefore, the tracking light is focused once at a position 11.6 m ahead of the aperture of the antenna and then becomes a divergent beam having a beam spread of 0.00174 radians (0.1 degrees) in all angles. On the other hand, the beam width of the transmission signal light is 0.00007 radians (0.004 degrees) when converted to the full width at half maximum. If it is necessary to increase the transmission distance, an optical antenna with a diameter of about 5 cm can be manufactured using a commercially available lens by expanding the aperture of the first lens with the same design as in FIG. .

  Next, a feedback control system that performs tracking control constituting the control unit will be described with reference to FIGS. In the tracking control including the tracking mirror drive mechanism 45, two independent control systems having exactly the same function are used for the two axes in the horizontal direction and the vertical direction. Of these, the horizontal driving axis is called the azimuth axis (Az axis), and the vertical driving axis is called the elevation axis (El axis).

  The internal configuration of the tracking control system is shown in FIG. The tracking mirror drive mechanism 45 that changes the directivity angle includes a tracking mirror 7, a mirror angular displacement sensor 44, and a tracking mirror drive unit (not shown). When the beacon light cannot be detected by the angle-of-arrival detector, for example, at the time of initial capture, the light of the optical antenna is based on the signal from the mirror angular displacement sensor 44 in the tracking mirror drive mechanism 45 by one mode of the mode switch 49. It functions to hold the shaft in a predetermined standby direction (initial directivity angle). Here, the initial directivity angle is given from the outside so as to perform a spiral operation, for example.

  On the other hand, when the control system detects the beacon light from the other station by the arrival angle detector, the control system inputs the beacon light arrival angle 40 from the tracking error detection sensor 41 according to the two modes of the mode switch 49. Thus, a feedback control system that outputs the directivity angle 47 of the optical antenna is configured.

  The disturbance of the control system may be electrical noise, temperature fluctuation, vibration disturbance of the device mounting structure, etc. In FIG. 4, the influence of these factors is collectively shown as the disturbance input 46 at the output of the tracking mirror 7. Yes.

  The tracking operation conventionally performed will be described with reference to FIGS. 1, 2, and 4. The arrival angle of the tracking light is calculated from the ratio of the amount of light incident on each photodiode of the arrival angle detector 12 by the quadrant Si photodiode housed in the fiber coupler of FIG. 2, and an error obtained as a difference from the arrival angle in the steady state Based on the signal, the drive unit of the tracking mirror 7 is driven.

  The tracking mirror drive unit can be easily configured with a permanent magnet and a coil. A piezoelectric element may be used. The incident angle of the signal light and the beacon light incident on the fiber coupler changes due to the change in the tilt of the mirror by the tracking mirror driving unit, and the influence of the change in the tilt of the tracking mirror appears in the output of the mirror angular displacement sensor 44. By controlling the drive signal to the tracking mirror drive unit, if the feedback operates stably and the gain is sufficiently large, the change in the arrival angle of the beam incident on the optical antenna can be canceled out.

  Since this system shares an optical system for both transmission and reception, by transmitting beacon light and signal light to the other station through the same tracking optical system (fiber coupler, tracking mirror, optical antenna), the transmitted light can also be transmitted. The axis can be accurately oriented in the direction of the partner station.

  In this apparatus, an analog PID system is used as a compensator for stabilizing the control system. That is, integral (phase lag) compensation and differentiation (phase advance) compensation are performed using an analog arithmetic circuit including an OP amplifier and a resistor / capacitor. The reason for adopting the analog PID method is that, in a discrete time control system using a high-speed DSP or CPU, A / D, and D / A converter, a high-speed broadband and high-precision control system due to a finite sample time. This is because it is difficult to achieve both low power consumption.

  The response of the control target (plant) that combines the response characteristics of the tracking error detection sensor and the tracking mirror drive mechanism can be approximated by the following transfer function.

Here, G _M and G _s indicate the transfer function of the tracking mirror drive mechanism and the transfer function of the tracking error detection sensor, respectively. G _o is the DC gain of the tracking mirror drive mechanism, the Az axis is 7.182, the El axis is 6.328, G ₁ is the DC gain of the tracking sensor, and the response characteristics of the error detection sensor using a quadrant photodiode Changes depending on the amount of degradation of the point image at the focus of the beacon light, so that it is necessary to be able to adjust the system so that the open loop gain is maximized while observing the operation of the system. When the best adjustment was actually made in combination with the optical antenna, G ₁ was about 1.0. Further, ω _v is a resonance frequency, ω _v = 80 Hz for the Az axis, and ω _v = 100 Hz for the El axis. D _M is a damping factor of the resonance. D _M = 0.005, T _D is the roll-off time constant of the coil driver inside the tracking mirror drive unit, T _D = 4.4 μs, T _C is the roll-off time constant of the driving impedance of the coil and driver, and T _C = 1.8 μS. T _O is a roll-off time constant by a transimpedance amplifier inside the tracking sensor, and T _O = 5.6 μs. The transfer function of a PID compensator designed in accordance with the plant characteristics is expressed by the following equation.

Here, G _CO is the DC gain of the compensator, G _CO = 700, and the first fractional expression G _{i on the} right side is the transfer function of the integral compensator, T _i = 47 ms, T ₂ = 1.05 s. The last fractional expression G _d is a transfer function of differential compensation, and T ₃ = 0.33 ms and T ₄ = 0.03 ms. The designed control system has a phase margin of 41 ° and a gain margin of 13.3 dB. Figure 5 shows the closed-loop transfer characteristics in tracking mode. This is the case for the Az axis, but the characteristics are the same for the El axis. From the point of view that the response is reduced by 3 dB, it can be seen that a control bandwidth of 5 kHz can be achieved.

  In the initial acquisition mode, the open-loop control gain is slightly reduced, so that the control bandwidth is about 2 kHz. FIG. 6 shows the result of calculating the response of the Az axis to this control system disturbance. The El axis is exactly the same as FIG. 6 except that the resonance frequency changes from 80 Hz to 100 Hz in the figure. It can be seen that it has a disturbance suppression characteristic of 1/100 (40 dB) or more at a frequency of 100 Hz or less and 1/1000 (60 dB) at a frequency of 1 Hz or less. However, the influence of disturbance increases at frequencies around 3 kHz. In this design, since the amplitude of disturbance and sensor noise at a frequency of 1 kHz or higher is small, an increase in frequency response around 3 kHz is not a problem.

  In order to confirm this, we installed two optical wireless communication devices in a transmission environment with large arrival angle fluctuations due to atmospheric fluctuations, and measured how the tracking error changed with and without the tracking operation. The beacon reception power at this time was set to the minimum value (-41.3 dBm) necessary for tracking. Figure 7 shows the measurement results for the El axis. Fig. 7 (a) shows the time change of tracking error for 1 second sampled every 0.1ms, and Fig. 7 (b) shows the result of frequency analysis by FFT of the same data. Two types of data when tracking is stopped are displayed in an overlapping manner.

  By operating the fine tracking, the arrival angle fluctuation of about 18 μrad (pp) is reduced to 1 μrad (pp) or less. The frequency analysis result shows the spectral density and its rms integrated value, and it can be seen that the arrival angle fluctuation of 4 μrad (rms) has decreased to 0.15 μrad (rms). In addition, an increase in noise appears corresponding to the increase in closed loop gain near 2.5 kHz. From this result, it can be seen that the design value of the disturbance suppression characteristic shown in FIG. 6 is appropriate.

  FIG. 8 shows the operation when the control system is switched from the initial acquisition mode (mode change switch 49 is 1) to the tracking mode (mode change switch 49 is 2) by gradually increasing the amount of beacon light. Show. FIG. 8 shows, with a linear scale, changes in Az axis tracking sensor output, El axis tracking sensor output, and beacon light reception power over time. In addition, the time change of the signal light intensity in the 1.5 μm band coupled to the single mode fiber is displayed as a logarithmic value of 10 dBm per day. The daily scale on the horizontal axis is 1 ms. Error signal, signal light in about 200 μs. It can be seen that all the tracking lights are settling.

  Next, a detailed description will be given of measures for blocking beacon light. In the configuration of the conventional tracking control system of FIG. 4, when a sufficient amount of beacon light cannot be obtained by the angle-of-arrival detector, the sensor input of the feedback control system is sent from the tracking sensor to the mirror built in the tracking mirror drive mechanism. Control was performed by switching to the angular displacement sensor so that the optical axis of the communication device is in the initial pointing direction. In this case, if the direction of the partner station is far away from the initial pointing direction due to some cause, the partner station's beacon light cannot be emitted to the partner station, and tracking of the partner station is interrupted.

  For this reason, the initial finger angle 48 in FIG. 4 is gradually changed around the initial value, the optical axis is scanned in a spiral shape, and the conditions for the counterpart station to enter the field of view of the arrival angle detector match in the two stations. I had to wait until. If the beacon light is detected by the angle-of-arrival detector, the feedback control system can switch the servo system input to the tracking sensor, complete the tracking within 0.001 seconds, and recover the signal. Is often longer than the time required for the shield to pass through the transmission path, which increases the blocking time of the communication path.

  Therefore, in the present invention, the initial directivity angle 48 of FIG. 4 is not fixed, but the most probable initial directivity direction is estimated from the past history of the mirror angular displacement sensor output when normal tracking is performed. A control system configuration with a pointing angle estimation device was adopted, and good operation was confirmed.

  FIG. 9 shows a configuration example of a tracking control system to which this initial pointing angle estimation device is added. This figure shows an example in which estimation calculation is performed by a second-order infinite impulse response (IIR) low-pass filter and a PI compensator. After the beacon light is cut off, the output of the estimator operates instead of the initial directivity angle for a fixed time (about 1 second), but thereafter, the output is switched to the conventional initial directivity angle.

  The second-order IIR low-pass filter is an optimum filter in the case of communication between two fixed points where the direction of the counterpart station does not move, and the cutoff frequency of the filter can be optimized by the frequency component of the disturbance with respect to the pointing direction. For example, when estimating the directivity angle while avoiding the effects of atmospheric fluctuations up to about 100 Hz, a cutoff frequency of several tens of Hz is appropriate. In addition, PI compensation is performed in order to achieve both the estimation accuracy and the stability of the estimation calculation.

  In addition, in order to cope with a large amount of vibration in the installation environment and the tracking light disconnection for more than 1 second, it is possible to use a Kalman filter having at least a second-order system estimation model or other filters with high prediction accuracy. Good. The Kalman filter models the disturbance based on the output of a detector that detects the magnitude, speed, and acceleration of the disturbance applied to the optical wireless device, and performs prediction.

  These filters are irrelevant to the operation of the apparatus when the apparatus is operating normally (at the time of tracking), and this does not degrade the performance of the apparatus. In order to operate a high-performance filter in real time, a huge amount of computing power is required. However, in order to realize the estimation device of the present invention, a processor currently used in a portable device is sufficient, The filter performance can be appropriately selected according to the price of the apparatus, the reliability of the line, the magnitude of the vibration disturbance and the frequency band.

  Hereinafter, the operation of the initial directivity angle estimation apparatus will be described in detail with reference to FIGS. FIG. 10 shows a control flow relating to the initial directivity angle estimation apparatus. The control flow used in the following description is executed by a commercially available CPU or DSP (hereinafter referred to as a computing unit) with a memory (not shown).

  An arithmetic unit that operates in accordance with a control program has an interface that communicates with an external device. A control instruction value is supplied to an external control circuit via a D / A converter. Further, the external sensor output is A / D converted and taken into the computing unit.

  There are two types of directivity control: control mode 1 based on the initial directivity angle and control mode 2 based on the output of the tracking sensor calculated from the output from the arrival angle detector 12 that detects the arrival angle of the spatial beam from the counterpart device. There is. The initial directivity angle is used because the alignment between the opposing devices is not sufficient in the initial state, and the intensity of the beacon light is sufficient when the intensity of the beacon light from the other device is insufficient and when the beacon light is blocked. There may be no.

  When the control system of the optical wireless communication apparatus is energized, the control operation is started (S1), switching to the control mode 1 (S2), and selecting the initial directivity angle 48 given from the outside by the initial directivity angle mode switch 1 The operation of switching to (S3) is performed. Up to this point, the operation is common to the conventional control system.

  Here, as long as the reception intensity obtained from the sum of all the outputs of the quadrant detector 12 that receives the beacon light does not exceed the set value 1, the output of the initial pointing angle estimation device is used to satisfy S2 → S3 → S4. The control mode by the initial directivity angle is repeated in order. Here, the set value 1 may be determined according to the place where the optical wireless device is installed.

  When the beacon light reception intensity exceeds the set value 1, the control mode is switched to the normal control mode 2 (S5) and the initial pointing angle mode is switched to 2 (S6). Then, the tracking control becomes a steady operation performed based on the output of the tracking error detection sensor (tracking sensor), and the directivity angle estimation device needs to output the directivity angle estimation value in response to the output from the mirror angular displacement sensor. A directivity angle estimate is calculated in preparation for

  Here, if there is a command to stop the operation due to power off or the like during the steady operation from the outside, the control operation is terminated (S7). If there is no command to stop the operation (N in S7), if the received intensity of the beacon light does not fall below the predetermined setting value 2 (N in S8), the process returns to the determination of the beacon light intensity (S4) and the steady operation Is continued. Here, the set value 2 may be determined experimentally by artificially blocking the beacon light.

  When the received intensity of the beacon light falls below a predetermined set value 2 at which normal control is difficult (in the case of Y in S8), the control mode is switched by switching the control mode (S9) and is supplied from the initial pointing angle estimation device. Control is performed using the initial directivity angle estimate. The control using the estimated value is continued while the beacon light reception intensity is lower than the predetermined set value 2 (in the case of Y in S8), but after the predetermined time T, the control using the initial directivity angle from the outside is performed. Return (in the case of Y of S10).

  If the beacon light reception intensity does not fall below the set value 2, it is determined that it is not necessary to use the estimation device, and the mode is switched to the steady tracking control mode (in the case of N in S8), and the process returns to step S4.

  Next, a second-order infinite impulse response (IIR) filter that occupies the core of the estimation device will be described. The filter used here is realized by digital signal processing, and its output is determined by the influence of current and past inputs. As shown in FIG. 11, the specific configuration may be configured in hardware by combining a register and an adder. However, since the response speed of the current filter is not so high, processing by software using an arithmetic unit is possible. It is. An example program is shown in FIG. The obtained response as LPF is shown in FIG.

  Here, the coefficient that determines the characteristics of the filter is experimentally determined in consideration of the environment in which the optical wireless device is located. FIG. 14 shows the convergence state of the prototype directivity angle estimation device. Both horizontal and vertical directions converge in about 0.6 seconds.

  Next, the effect of the directivity angle estimation device will be described. FIG. 15 shows the state of signal light and the like when the spatial light beam is blocked. FIG. 15A shows a state where the shielding object is moved in the horizontal direction, and FIG. 15B shows a state where the shielding object is moved in the vertical direction. In FIG. 15, “a” represents signal light, and “c” represents a change in the intensity of the beacon light. Further, b is an error signal that appears as arrival angle fluctuation in the horizontal direction, and d is an error signal that appears as arrival angle fluctuation in the vertical direction. In each case, generation of an error signal is slightly seen depending on the moving direction of the shield.

  Next, FIG. 16 shows the influence on the signal light due to the presence / absence of the operation of the initial directivity estimation apparatus when the spatial beam light is blocked. FIG. 16A shows the case where the estimation apparatus does not operate, and FIG. 16B shows the case where the estimation apparatus operates.

  Here, a is the signal light intensity, c is the beacon light intensity, and b and d are error signals detected as arrival angle fluctuations. When (A) and (B) are compared, in both (A) and (B), the beacon light intensity c is similarly reduced, but when the estimation device is not operating (A), the signal light a However, when the estimation device is operating (B), the degree of decrease is small. If the signal light intensity capable of realizing error-free can be maintained, it can be understood that error-free communication can be realized even if the beacon light is blocked.

  Actually, an optical signal transmission experiment with a distance of 315m was conducted in Pisa, Italy for two weeks using the optical wireless communication device that actually implemented the above-mentioned pointing tracking control. Transmission was maintained normally. In particular, in a 1-Gbps optical signal transmission experiment for two days, it was confirmed that error-free transmission was possible with an operation rate of 99.97%.

  Although this patent provides a solution to cope with short-time interception of beacon light by a minute object, it is an environment with a large atmospheric fluctuation, for example, an environment with many heat sources in an urban area or a long distance transmission. On the road, the reception power of the beacon reception power frequently decreases to 1/100 or 1/1000 or less from the average value. The time width during which this power decrease occurs is a relatively short time of about 10 ms to 100 ms corresponding to the frequency component of atmospheric fluctuation being about 100 Hz or less. It is possible to avoid line disconnection due to tracking interruption due to a decrease in strength.

1 Optical fiber for signal light 2 Beacon light source (laser diode)
3 Wavelength multiplexing optical fiber coupler 4 Glass-made optical fiber ferrule 5 Imaging lens in fiber coupler (collimator lens)
6 Signal light 7 Tracking mirror 8, 9 Refraction type optical system (optical antenna)
10 Radiation beacon light 11 Beacon light imaging lens 12 Angle of arrival detector (4-division Si photodiode)
13 Arrival beacon light 14 Fiber coupler 35 Arrival beam 36 coincident with optical axis Arrival beam 40 deviated from optical axis Beacon light arrival angle 41 Arrival angle (tracking error) detector 42 PID compensator 44 Mirror angle displacement sensor 45 Tracking mirror drive Mechanism 46 Disturbance 47 Optical wireless communication device directivity angle 49 Control mode switch 91 Secondary IIR filter 92 PI compensator 93 Initial directivity angle mode switch

Claims (4)

An optical wireless communication apparatus that uses a spatial light beam composed of signal light and beacon light for tracking control having a shorter wavelength than the signal light,
The spatial beam light is radiated toward the opposing device as transmitted light and the refractive optical system that receives the spatial beam light from the opposing device, and the arrival angle detector that detects the arrival angle of the beacon light are emitted. The directivity angle is estimated based on the output of a tracking mirror drive mechanism that changes the directivity angle that is the direction of the spatial beam light, a control unit that controls the tracking mirror drive mechanism, and an angular displacement sensor of the tracking mirror. An initial directivity angle estimation device,
The control unit controls the tracking mirror drive mechanism based on the output of the arrival angle detector when the reception intensity of the beacon light is larger than a predetermined value, and the reception intensity of the beacon light is lower than a predetermined value. An optical wireless communication apparatus, wherein the tracking mirror drive mechanism is controlled based on an initial directivity angle output from the initial directivity angle estimation device when the initial directivity angle estimation device is not large.   The directivity angle estimation device adds a proportional compensation and an integral compensation to an output of the infinite impulse response filter that calculates a predetermined value based on an output from the past to the present of the mirror angular displacement sensor and the output of the infinite impulse response filter. The optical wireless communication apparatus according to claim 1, further comprising a PI compensator.   The directivity angle estimation apparatus includes a Kalman filter that models disturbance based on output from the past to the present of the mirror angular displacement sensor and calculates predicted values of the magnitude, change speed, and acceleration. The optical wireless communication apparatus according to claim 1.   The signal light and the beacon light share a refractive optical system, and the signal light is radiated as a substantially parallel beam by the refractive optical system and the beacon light converges in the air due to the wavelength dispersion of the refractive optical system. 4. The optical wireless communication apparatus according to claim 1, wherein the optical wireless communication apparatus radiates as transmitted light to the opposite apparatus so as to diverge.

Images