Method and apparatus for measuring meteorological visibility and scattering of light

Abstract: The invention concerns a method and an apparatus for measuring the meteorological visibility and the scattering of light. According to the method, a light pulse is emitted into the atmosphere via an optical transmit system (7, 9), the backscattered light returns via the optical transmit system (7, 9) after which its intensity and delay are measured. According to the invention, the waveform of the optical transmit pulse generated at the instant of its launching from within the apparatus is sensed in a manner that excludes any backscattered light originating from outside the measurement system, this signal is scaled for both its magnitude and phase so as to achieve a suitable feedback signal relative to the actual measurement signal and said feedback signal is subtracted from said measurement signal to avoid overloading of the receiver.

Description: The present invention relates to a method in accordance with the preamble of claim 1 for measuring meteorological visibility and scattering of light. The invention also concerns an apparatus for measuring meteorological visibility and scattering of light. EP-A-358 507 discloses in Fig.6 (col.6,1.2-14), Fig.7 (col.6,l.32-col.7,l.19) and Fig.10 (col.7,1.40-57) a method for measuring the meteorological visibility and the scattering of light, in which method a light pulse is emitted into the atmosphere via an optical transmit system 85,40-45 and the intensity and delay of the backscattered light returned via the same optical transmit system are measured. The optical transient pulse generated within the apparatus in conjunction with the launching of the optical transmit pulse is sensed in a manner that excludes any backscattered light originating from outside the measurement system. Said sensed signal is used to monitor the laser operation. D1 discloses likewise that a "portion of the internally reflected part of light emitted from the light source can be used for monitoring the functions of the apparatus in order to, e.g. stabilize the light output or adjust receiver sensitivity" (col.8,1.42-46). FR patent application 2,594,535 also discloses an embodiment similar to that described above. Despite the significant benefits achieved by using a common active surface for both the transmitting and receiving systems via a single optical system, unexpected problems have been encountered. As the transmitted pulse has an extremely high peak intensity, the optical fibers of the transmitting system permit stray coupling to the adjacent fibers of the receiving system, whereby the receiver input amplifier is temporarily overloaded. This can cause problems particularly in near-range scattering measurement, since the receiver input amplifier is nonresponsive during the very time the near-range reflected information should typically be detected by the receiver. The same problem is also encountered in the conventional embodiments based on a beam splitter that have the further drawback of a partial loss of transmit power in the beam-splitting optical system. It is an object of the present invention to overcome the disadvantages related to the above-described prior-art techniques and to achieve an entirely novel method and apparatus for the measurement of the scattering of light, said apparatus using a common optical system for both transmission and reception. The invention is based on adapting a second photoresponsive feedback element into the measurement system in such a point of the system that receives the transmitted optical signal at the same power level as the system receiver proper receives the stray signal owing to the optical stray coupling, while the feedback element receives none of the optical signal imposed on the system by external reflections or backscatter. The receiving element proper and the feedback element should be as identical as possible both in sensitivity and response time. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 5. The invention offers significant benefits. The invention makes it possible to avoid overloading of the receiver proper immediately after the transmitted pulse, which significantly improves measurements, particularly in the near range. Furthermore, errors in far-range measurements are reduced. The invention is next examined in greater detail with the help of exemplifying embodiments illustrated in the appended drawing, in which Figure 1 shows diagrammatically a measurement system based on a beam splitter of conventional technology. Figure 2 shows diagrammatically a conventional measurement system based on a Y-coupler implemented with the help of optical fibers. Figure 3 shows in a graph the received optical input signal and the corresponding electrical output signal of the measurement system illustrated in Figs. 1 and 2. Figure 4 shows a block diagram of the apparatus according to the invention. Figure 5 shows an arrangement according to the invention adapted to the basic embodiment illustrated in Fig. 1. Figure 6 shows an arrangement according to the invention adapted to the basic embodiment illustrated in Fig. 2. With reference to Fig. 1, an optical measurement system based on a beam splitter comprises a transmitter 1, from which an optical pulse is emitted via a beam splitter 5 to a lens 7, which launches the light pulse to a desired spatial angle. A window 9 is employed to prevent contamination of the optical element 7. After the light pulse launched from the optical element 7 propagates along a path until it meets an obstacle, a portion of the light is backscattered along the same path, whereby the beam splitter 5 guides the returning light to a receiver 3. Adapted to the receiver 3 is also a unit 2 which is synchronized to the transmitter 1 and is capable of determining the delay and intensity of the return pulse, whereby such information can be utilized to define the object from which the reflection of the optical signal takes place. Owing to the high peak intensity of the transmitted pulse, imperfections of the beam splitter 5 impose a direct stray signal onto the receiver 3. Due to the low intensity of the return signal, the input amplifier of the receiver 3 must have a high gain, whereby the above-mentioned stray signal typically overloads the input amplifier of the receiver 3. Stray signals with a short delay are also imposed onto the received return signal due to reflections from the lens 7 and the window 9. With reference to Fig. 2, measurement using a Y-coupler 11 implemented by means of optical fibers takes place in the same manner as in the embodiment illustrated in Fig. 1. In this embodiment the beam splitter is replaced by a Y-coupler 11, which achieves a better directional gain for guiding the optical measurement signal to the desired object, since the losses by the upward reflection (cf. Fig. 1) caused by the beam splitter are avoided. However, the Y-coupler 11 has a drawback of optical signal stray coupling within the common part of the divider directly into the receiver input fibers during the transmission of the outgoing pulse. Hence, similar problems are involved as those discussed in conjunction with Fig. 1. The upper diagram of Fig. 3 shows the optical input of a receiver. The optical stray signal caused by the above-described reasons at the start of the measurement cycle loads the receiver input with a transient pulse signal 34 of approx. 1000-...1,000,000-fold intensity 35 compared with the normal signal level. According to the lower diagram depicting the output signal of the receiver, said transient pulse signal 34 drives the receiver input amplifier into saturation during the period 33. During the period 31 the amplifier recovers from saturation, and though a backscatter input signal component 36 caused by mist is present, it cannot be detected from the amplifier output signal due to overload distortion 32. At instant 37 a reflection from a cloud or fixed object is present in the input signal. As is evident, also this later arriving signal can be corrupted by the overload distortion 32. With reference to Fig. 4, amplifier overload can be avoided by configuring a conventional photoresponsive element 41 into a half-bridge with feedback element 42. In the embodiment illustrated in the diagram the photoresponsive elements 41 and 42 are reverse-biased avalanche photodiodes. When a high-intensity optical pulse impinges on both diodes 41 and 42 simultaneously, the stray signal coupled via optical stray coupling is canceled almost entirely. Bias adjustment of half-bridge permits fine-adjustment of the compensating effect of the feedback photodiode to a degree of substantially perfect cancellation of the transient signal resulting from optical stray coupling. Such arrangement achieves protection against overloading of the amplifier 43. The bias voltage can be continuously controlled by first converting the analog measurement signal into digital form with the help of an A/D converter 45 and then processing its digital output signal in an information processing unit 47. The A/D converter 45 used is typically a so-called FLASH converter operating with a sampling interval of 50 ns and resolution of 8 bits. Prior to being taken to the microprocessor 47, the output signal of the A/D converter 45 is loaded in a FIFO (First-In/First-Out) register memory 48. The length of the intermediate register memory 48 can be. e.g., 512 samples. The microprocessor 47 is employed for controlling the sampling operation via a bus 54 which is connected to both the intermediate register memory 48 and the A/D converter 45. The fine-adjustment of the bias voltage is implemented through digitally steering from the information processing unit 47 a bias voltage generator 44 which controls the bias voltage over the photodiodes 41 and 42. The bias voltage control can be performed continuously without delay as the information processing unit senses the deviations in the stray signal cancellation accuracy of the measurement signal and can thus differentially either increase or decrease the bias voltage so as to achieve perfect stray signal cancellation. In practice the bias voltage control is implemented by means of a constant-voltage regulator 40 connected to the positive supply voltage and an adjustable voltage regulator 44 which is connected to the negative supply voltage and receives its control voltage from the microprocessor 47 over the bus 56 via a D/A converter 58. The half-bridge 46 is advantageously complemented with a temperature-compensating circuit 49 which controls the voltage regulators 40 and 44. Thus, the half-bridge 46 formed by the diodes 41 and 42 can be considered a subtracting circuit which cancels the stray signal component from the measurement signal. With reference to Fig. 5, the feedback of the optical signal in system based on a beam splitter 5 is implemented by means of an optical fiber 53 optically coupled to a feedback sensor element 51. The free end of the fiber 53 is placed to the opposite side of the beam splitter 5 relative to the receiver element 3. The length of the fiber 53 can be selected so that it produces a delay matching the delay of the most significant stray signal component. For instance, if the strongest stray signal component originates from the beam splitter, the delay caused by the fiber 53 must be adjusted to the delay on the optical path between the receiver 3 and the beam splitter. If the strongest stray signal component originates from the focusing lens 7 (or lens group), the delay caused by the fiber 53 must be adjusted to twice the delay on the optical path from the beam splitter 5 to the lens 7. By virtue of the optical fiber 53, the photoresponsive elements proper can be placed maximally tight to each other, whereby these elements can be kept in maximally identical ambient conditions. The free end of the fiber 53 can be made movable, thereby giving a further means of stray signal cancellation adjustment through moving the free end of the fiber 53, thus complementing the bias voltage control discussed above in conjunction with Fig. 4. In practice the coarse adjustment is made mechanically bending the fiber end and then performing fine-adjustment by electronic means. With reference to Fig. 6, the feedback signal can be taken via an optical power divider 61 when a Y-coupler is used. The power division ratio can be 2:2, for instance. To optimize the feedback effect, the delay caused by the feedback fiber 65 between the optical power divider and the feedback signal sensor element 63 is adjusted essentially equal to the delay caused by the fiber 64 connecting the receiver 3 to the optical power divider 61. The invention is characterized in that the feedback sensor element is only used for measuring the stray signal component stemming from the optical stray coupling, particularly its waveform, which is scaled and equal phased prior to its subtraction from the uncorrected output signal of the receiver proper. Consequently, deviating from the implementation shown in Fig. 4, it is also possible within the scope of the invention to implement the feedback arrangement by means of separate sensors, which are used for the cancellation of the optical stray coupling after, e.g., a separate calibration step. The compensation of different delays can alternatively be arranged by means of digital delay circuits placed after the A/D converter, whereby both the measurement and the feedback signal can be delayed as necessary. The invention is particularly suited for use in conjunction with a Y-coupler implemented with optical fibers, while substantial benefits are also achieved in conjunction with measurement systems based on beam splitters illustrated in Figs. 1 and 5.

Claims: A method for measuring the meteorological visibility and the scattering of light, in which method a light pulse is emitted into the atmosphere via an optical transmit system (7, 9), intensity and delay of the backscattered light returned via the same optical transmit system (7, 9) are measured, the waveform of the optical transient pulse generated within the apparatus in conjunction with the launching of the optical transmit pulse is sensed in a manner that excludes any backscattered light originating from outside the measurement system. said sensed signal is scaled for both its magnitude and phase so as to achieve a suitable feedback signal relative to the actual measurement signal, and said feedback signal is subtracted from said measurement signal to avoid overloading of the receiver. A method as defined in claim 1. characterized in that the sensed magnitude of said optical pulse generated inside the measurement system is scaled by means of half-bridge connected avalanche photodiodes (41, 42), whose bias voltage is controlled in order to obtain a suitable feedback signal. A method as defined in claim 2 in which method the return signal traveling back via an optical system (7) is taken to a receiver (3) via a beam splitter (5), characterized in that said optical pulse generated inside the measurement system is sensed via an optical fiber (53) whose one end is placed to the opposite side of the beam splitter (5) relative to the receiver element (3) and the other end is coupled to a feedback signal sensor (51). A method as defined in claim 2 in which method the return signal traveling back via an optical system (7) is taken to a receiver (3) via a Y-coupler (61), characterized in that said optical pulse generated inside the measurement system is sensed via an optical power divider (61) which is formed by said Y-coupler and thus couples the signal to the feedback signal sensor (63). An apparatus for measuring the meteorological visibility and the scattering of light, said apparatus comprising transmission means (1) for generating a light pulse. an optical transmit system (7, 9) for launching said light pulse into the atmosphere. receive elements (3) for measuring the intensity of the backscattered light which travels back via said optical transmit system (7, 9), and delay-measuring elements (2) for measuring the propagation delay of said backscattered light, feedback elements (65, 63, 53, 51) for measuring the waveform of the optical transient pulse generated within the apparatus in conjunction with the launching of the optical transmit pulse, whereby the measurement is performed in a manner that excludes any backscattered light originating from outside the measurement system, scaling elements (45, 47, 44) for scaling the signal measured with the help of said feedback elements (65, 63, 53, 51) for both its magnitude and phase so as to achieve a suitable feedback signal relative to the actual measurement signal, and subtraction elements (46) for subtracting said feedback signal from said measurement signal to avoid overloading of the receiver. An apparatus as defined in claim 5, characterized in that said subtraction elements comprise a half-bridge (46) formed by a measuring and a feedback-sensing photodiode (41, 42) and said scaling elements comprise a bias voltage control element (44) for said half-bridge (46). An apparatus as defined in claim 6, said apparatus having a beam splitter (5) for routing the return signal traveling back via said optical transmit system (7, 9) to said receiver (3), characterized in that said apparatus incorporates an optical fiber (53) whose one end is placed to the opposite side of the beam splitter (5) relative to the receiver element (3) and the other end of the fiber (53) is coupled to a feedback signal sensor (42) employed for measuring the shape of the optical transient pulse generated within the apparatus. An apparatus as defined in claim 6, said apparatus having a Y-coupler (61) for routing the return signal traveling back via said optical transmit system (7, 9) to said receiver (3), characterized in that said apparatus incorporates an optical power splitter (61) coupled to a feedback signal sensor (63) employed for measuring the shape of the optical transient pulse generated within the apparatus.

Inventors: HUTTMANN HORST, LONNQVIST JAN, HUTTMANN, HORST, LONNQVIST, JAN
Applicants: VAISALA OY
Application year: 1993
Main classification: G01N21
IPC codes: G01S17, G01N21, G01S7, G01W1