Abstract:
A method of detecting optical subsystem failures includes emitting a pulsed light beam from a laser through a window. A reflection signal indicative of a portion of the beam reflected by the window is compared to an expected signal to monitor for degradation of an optical component.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to optics, and more particularly to monitoring performance of optical components. 
     2. Description of Related Art 
     A cloud LIDAR system uses lasers and an optical transmitter subsystem to illuminate a portion of a cloud and an optical receiver coupled to a series of photodetectors to measure the reflected light. These measurements are then processed by onboard digital electronics to calculate cloud parameters. In a system such as this, it is beneficial to detect and report faults and/or performance degradation of the optical and electro-optical components, such as lasers, transmitter subsystems and photodetectors. Typical LIDAR systems require multiple additional photodetectors solely for the purpose of detecting failures. 
     Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, the inclusion of multiple additional photodetectors solely for detecting optical failures adds to a LIDAR system&#39;s cost and complexity. The present disclosure provides a solution that performs the same task, but uses photodetectors that are already present within the system. 
     SUMMARY OF THE INVENTION 
     A method of detecting optical subsystem failures includes emitting a pulsed light beam from a laser through a window. A reflection signal indicative of a portion of the beam reflected by the window is compared to an expected signal to monitor for degradation of an optical component. The light beam can be emitted at a predetermined frequency. 
     Laser degradation can be monitored by comparing amplitude, pulse width and pulse frequency of a reflection signal received at a trigger photo diode to amplitude, pulse width and pulse frequency of the expected signal. 
     Transmitter optic misalignment can also be detected by comparing amplitude of a reflection signal received at a trigger photo diode with amplitude of a reflection signal received by a window cleanliness detector. In certain embodiments, detecting transmitter optic misalignment can be accomplished by comparing amplitude of a reflection signal received at a trigger photo diode with amplitude of a reflection signal received by one or more receiver photodetectors. 
     Contamination of an external window can be detected by comparing amplitude of a reflection signal received at a window cleanliness detector with amplitude of the expected signal. In certain embodiments, detecting window contamination can be completed by comparing amplitude of a reflection signal received at a window cleanliness detector with amplitude of a reflection signal received at a trigger photo diode. 
     Receiver optics misalignment and photodetector degradation can be monitored by comparing amplitude of a reflection signal received one or more receiver photodetectors with amplitude of a reflection signal received at a trigger photo diode. In certain embodiments, detecting receiver optics misalignment and photodetector degradation can be done by comparing amplitude of a reflection signal received at one or more receiver photodetectors with amplitude of a reflection signal received at a window cleanliness detector. 
     The optical components being monitored can include transmitter optics, receiver optics, lasers and/or photodetectors. It is also contemplated that the optical component can be the window itself. 
     A system for monitoring performance of optical components includes a laser and a plurality of optical components. A processor is operatively connected to a memory. The memory includes instructions recorded thereon that, when read by the processor, cause the processor to compare a reflection signal indicative of a portion of a light beam emitted by the laser and reflected by the optical component to an expected signal to monitor for degradation of at least one optical component. 
     These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
         FIG. 1  is a schematic view of an exemplary embodiment of a system for monitoring performance of optical components of a LIDAR system constructed in accordance with the present disclosure, showing reflections from a window for monitoring a plurality of optical components; 
         FIG. 2  is a schematic view of a portion of the system of  FIG. 1 , showing a processor and memory of the system; 
         FIG. 3  is a graphic view of exemplary emitted signals and reflection signals over time, illustrating optical component performance; and 
         FIG. 4  is a chart showing comparisons of reflection signals used to indicate optical component performance. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an exemplary embodiment of a system and method for monitoring optical performance in accordance with the disclosure is shown in  FIG. 1  and is designated generally by reference character  100 . Other embodiments of systems and methods in accordance with the disclosure, or aspects thereof, are provided in  FIGS. 2-4 , as will be described. 
       FIG. 1  illustrates a LIDAR system  100  in accordance with the present disclosure for monitoring performance of optical components. System  100  includes a laser  102  and a plurality of optical components. As shown in  FIG. 1 , optical components include receiver optics  104 , transmitter optics  106 , photodetectors  110 ,  112 ,  114  and window  122 . Photodetectors  110 ,  112 ,  114  include but are not limited to a trigger photodiode (TPD)  110 , a photodetector measuring reflection off of window  122 , e.g., a window cleanliness photodetector (WCD)  112 , and receiver photodetectors  114 , which may include but are not limited to a 950 nm photodetector and a 1550 nm photodetector. Additional optical components for operation of the LIDAR system  100  can be included. It will be understood that the number of optical components shown herein is exemplary and that any other suitable number and/or type of sensor can be used without departing from the scope of this disclosure. 
     A pulsed light beam  120  is emitted from the laser  102  through an external window  122  towards a cloud  124 . A portion of the light beam  120  is reflected towards the TPD as reflection signal  150   a  while a portion of the emitted light beam  120  exits through the external window  122  towards the cloud  124 . When a cloud  124  is present a return signal  126  is reflected back towards system  100 . A portion of the emitted light beam  120  is reflected by the external window  122  without passing through to the cloud  124 . This reflected portion passes through several of the optical components and is received as reflection signals  150   b  and  150   c  by photodetectors  112  and  114 . 
     Referring now to  FIG. 2 , system  100  includes a processor  130  operatively connected to a memory  132 . Processor  130  is operatively connected to the photodetectors  110 ,  112 ,  114  for receiving reflection signals  150   a ,  150   b ,  150   c . The ellipses shown in  FIG. 2  indicate that while three photodetectors  110 ,  112 ,  114  are shown and described, any suitable number of photodetectors can be included. Memory  132  includes instructions recorded thereon that, when read by processor  130 , cause processor  130  to perform the functions described herein with respect to methods of monitoring performance of optical components. 
     Upon receiving reflection signals  150   a ,  150   b ,  150   c  at each respective photodetector  110 ,  112 ,  114 , processor  130  compares the reflection signals  150   a ,  150   b ,  150   c  with a respective expected signal. An expected signal is based off system calibration when the optical components are in good working order. A reflected signal is compared to another received signal, for example, from a different optical component, relative to an expected value. Accordingly, the comparison of the reflection signal  150   a ,  150   b ,  150   c  to the expected signal indicates the performance of the particular optical component. 
     For example, as shown in  FIG. 3 , amplitude of an emitted signal  120  and reflection signals  150   a ,  150   b ,  150   c  received at the photodetectors  110 ,  112 ,  114  is plotted over time. The emitted signal  120  is emitted from the laser at a predetermined frequency. Reflection signal  150   a  received at the TPD  110  has an amplitude and pulse width within the expected range. Therefore, this indicates that the laser performance is optimal. Amplitude of reflection signal  150   b  received at the WCD  112  indicates whether the external window  122  is clean or dirty. As shown, a dirty external window  122  will produce a reflection signal  150   b  with a greater amplitude than a clean window. In  FIG. 3 , the amplitude of reflection signal  150   b  for a dirty window is shown in broken lines, and the corresponding solid line shows the amplitude for signal  150   b  for a clean window. Comparison of the amplitude of the reflection signal  150   b  received at the window cleanliness detector with the amplitude of reflection signal  150   a  received at the trigger photodiode can be used to indicate misalignment of the transmitter optics  106 . Amplitude of reflection signal  150   c  received at the receiver photodetectors  114 , can indicate receiver optic misalignment or photodetector degradation when compared with the amplitude of reflection signals  150   a ,  150   b  received at the TPD  110  or the WCD  112 . It will be noted that peak  160  indicates a later returned signal  126  received at the receiver photodetectors  114  based on cloud reflection. In other words, peak  160  represents a returned signal  126  when a cloud  124  is present. As shown this returned signal  126  is spaced a sufficient time later than reflection signal  150   c  so as not to obscure the comparison used to indicate optical component performance. 
       FIG. 4  shows the performance measure or fault of a particular optical component based on various comparisons between reflection signals and expected signals. 
     For example, laser  102  failure or degradation is indicated by a comparison between either amplitude, pulse width, or pulse frequency of the reflection signal received at the TPD  150   a  and a respective expected signal. Transmitter optic  106  misalignment can be determined by comparing amplitude of the reflection signal received at the TPD  150   a  with amplitude of the reflection signal received by the WCD  150   b . Transmitter optic  106  misalignment can also be determined by comparing amplitude of the reflection signal received at the TPD  150   a  with amplitude of the reflection signal received by the receiver photodetectors  150   c . Window  122  contamination is monitored by comparing amplitude of the reflection signal received at the WCD  150   b  with amplitude of a respective expected signal. Window  122  contamination can also be monitored by comparing amplitude of the reflection signal received at the WCD  150   b  with amplitude of the reflection signal received at the TPD  120   a . Receiver optic  104  misalignment and photodetector degradation can be detected by comparing amplitude of the reflection signal measured by the receiver photodetectors  150   c  with amplitude of the reflection signal received at a TPD  150   a . Receiver optic  104  misalignment and photodetector degradation can also be monitored by comparing amplitude of the reflection signal received at the receiver photodetectors  150   c  with amplitude of the reflection signal received at the WCD  150   b.    
     It will be noted that the comparisons listed and the performance of a particular optical components is not limited by the comparisons shown in  FIG. 4 . Additional comparisons between a reflection signal and an expected signal and between signals received at the photodetectors may be used to monitor performance of optical components. 
     As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
     Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
     A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. 
     Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
     Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     Aspects of the present invention are described above with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The methods and systems of the present disclosure, as described above and shown in the drawings, provide for a system and method for monitoring performance of optical components. While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the spirit and scope of the subject disclosure.