Abstract:
A lidar detection system comprises a CCD sensor for converting a returned light signal into electrical charges that are amplified at the output of the sensor itself with no significant noise and without it being necessary to accumulate the results of a plurality of firings to obtain sufficient signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based on French Patent Application No. 03 00 600 filed Jan. 21, 2003, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. § 119. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject matter of the present invention is a method of detecting a light signal and a corresponding lidar system. 
   2. Description of the Prior Art 
   Lidar detection systems essentially comprise means, such as a laser, for emitting a light signal toward an object to be studied and a device for receiving a signal returned from the object by diffusion or reflection, the means and the device being synchronized with each other. Certain characteristics of the object can be deduced by processing the returned signal. The problem addressed here is that of supplying a signal of sufficient intensity to be read by the processing means. In some prior art lidar detection systems, a counting method is used in which photons are individually converted into electrons in a detector, with a known multiplication factor sufficient to distinguish the signal from the background noise of the system. However, the method is limited to very weak signals. Another method consists in passing the signal resulting from converting photons into electrical charges through a current amplifier preceding the processing means. As the background noise of the system is subject to the same amplification, this method is suitable only for high signal values. In practice, a plurality of detection systems that are calibrated differently must be used when the successive sample values of the signal have different intensities. 
   The patents EP 0 905 284 and EP 1 111 908 describe a charge accumulation and coupling (CCD) photosensor that comprises an array of cells arranged in rows and columns. The light signal is first collected over an image region and converted into electrical charges that are displaced periodically through a memory region to a read device. 
   The sensor has the benefit of lending itself to convenient sampling of the signal on the memory rows; however, it does not solve the problem of excess noise when reading samples of low intensity. In the above patents, in addition to integrating charges on the last row of the image region, samples corresponding to successive firings are integrated in the memory region before reading. It is therefore no longer possible to process a particular firing and good synchronization of firings is required. If a portion of the signal is skewed, all of the samples are affected by the error and are lost. Cell saturation is also a risk for the high samples that are accumulated. 
   The invention is concerned with improving lidar systems by using CCD photosensors that differ from those used in this application in the prior art in that they amplify the signal under conditions that are simultaneously reliable and less constraining than the accumulation of a plurality of firings. 
   The signal is amplified in the read region instead of in the memory region and by means of a charge multiplication process such that the individual results of the firings can be considered. 
   SUMMARY OF THE INVENTION 
   Thus one aspect of the invention consists in a lidar detection system comprising a device for emitting a light signal, a device for transmitting a returned portion of the light signal toward a sensor that converts it into an electrical signal and comprises cells arranged in rows and columns and successively consists of an image region comprising at least one row of photocells onto which the portion of the light signal is focused and which convert the returned portion of the light signal into electrical charges, a memory region comprising a plurality of rows of cells that are not photosensitive, and a read region comprising a row of cells that multiply the electrical charges, and timing means for shifting the electrical charges along the columns, from the image region to the memory region and then to the read region and toward a processing device. 
   No other amplifier is necessary in the system. Another aspect of the invention consists in a method of detecting a light signal, comprising continual conversion of the light signal into electrical charges in a receiving portion of a sensor, periodic sampling of the signal by taking up charges accumulated in the receiver portion to form therefrom samples of the signal, and chained movement of the samples through a memory and then toward a processing device, in which method the samples are subjected to amplification in the sensor by multiplication of electrical charges just before being shifted toward the processing device but remain unchanged in the memory. 
   The invention is described next with reference to the appended drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a general view of a lidar detection device equipped with the invention. 
       FIG. 2  shows a CCD sensor used in the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The lidar detection device shown in  FIG. 1  comprises illumination means in the form of a laser  1  emitting a ray of light toward the object to be studied (not shown, but possibly a cloud or a plume of gas in applications frequently encountered). The radiation reflected or diffused by the object is collected by a transmission device, and first of all by a receive optic  2 , and can be processed by an interferometer  3 . The light signal then passes through an optical fiber  4  and a cylindrical lens  5  to a sensor  6  that converts it into a sampled electrical signal. Timing means  7  control firing of the laser  1  and servicing of the sensor  6 . Processing means  8  read the electrical signal processed by the sensor  6 . 
     FIG. 2  shows the sensor  6 . It comprises an image region  9 , a memory region  10 , and a read region  11 . The three regions are contiguous and take the form of matrices of cells  12 , of different nature in each of the regions, in which they are therefore designated by the reference numbers  13 ,  14  and  15 . The cells  12  are arranged in P columns extending across the three regions  9 ,  10  and  11 . The memory region  10  comprises N rows of cells  14 , the read region  11  comprises a single row of cells  15 , and the image region  9  advantageously comprises a single row of cells  13 , although a plurality of rows can be used, in imitation of prior art sensors, in order to capture a plurality of separate light sources simultaneously. 
   On receiving a returned firing, the light signal reaches the sensor  6  in the form of a spot elongated into the form of a straight line by the particular focusing effect of the lens  5  (extension in the direction of the rows and contraction in the direction of the columns). The width of this row is advantageously less than that of the cells  13 , so as to be received on the single row thereof in the image region  9 . The light spot extends laterally over a sufficient number of columns to avoid the risk of saturating either of the cells  13 . A plurality of adjoining light spots are represented here, and the sensor  6  can be assigned to collecting and processing simultaneously different light signals coming from respective channels and delivered by parallel optical fibers, as is standard practice for analyses at different wavelengths. 
   The cells  13  are photocells that convert the photons into electrical charges and allow them to accumulate. It is pointless to accumulate further charges along the columns if the image region  9  comprises a plurality of rows, which is known in the art for raising the sample values. The timing means  7  command the sensor  6  periodically. The contents of the cells  12  are then sampled and shifted to the next lower row of cells  12 . The accumulated charges are therefore transferred from the cells  13  of the image region  9  in the top row of the memory region  10  into the subsequent rows of the memory region  10  and replaced by the charges accumulated thereafter in the image region  9 . By repeating this process, a sample of the returned signal is stored in the memory region  10 , the lower rows corresponding to earlier sampling times. The cells  14  of the memory region  10  are designed to allow the above transfers but are not photosensitive. In practice, they can be of the same composition as the cell  13  but masked by an opaque coating. 
   When the signal has been sampled, it can be read while continuing to transfer charges toward the bottom of the columns of the cells  12 , until they reach the cells  15  of the read region  11 . There they undergo a multiplication caused by an increase in their control voltage, for example from 5 V to 50 V, which amplifies them. The noise that is also amplified is that of the cells  12 , which is very low. The samples of the signal in the read region  11  thus remain representative of the returned light signal, and there is no longer any need to accumulate samples over repetitive firings to alleviate a low signal level. On the contrary, if the signal is high, saturation of the cells  14  of the memory region  10  is avoided. It is still possible to adjust the multiplication ratio of the charges in the read region  11  by operating on the means  16  for adjusting the control voltage of their cells  15 . 
   Finally, the multiplied charges are dumped cell by cell to the processing means  8 , which digitizes them.