Abstract:
A multimode remote vibration sensor. The inventive sensor ( 8 ) includes a mode locked laser transmitter ( 10 ); a receiver ( 30 ) adapted to detect signals transmitted by the laser ( 10 ) and reflected by an object ( 22 ) and a signal processor ( 40 ) for analyzing the signals and providing an indication with respect to a vibration of the object ( 22 ). The laser is particularly novel as a vibration sensor transmitter inasmuch as it includes a mode locking mechanism. The mode locking mechanism causes the laser to output energy at all modes within the gain profile in phase with one another. The result is a series of tight clean pulses which may be used for range resolved vibration and one-dimensional (high resolution ranging) applications.

Description:
This application claims benefit of provisional No. 60/245,130 filed Nov. 2, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to laser based systems and methods. More specifically, the present invention relates laser based remote vibration sensors and methods. 
   2. Description of the Related Art 
   Remote vibration sensors have been used to identify friendly or hostile vehicles, detect mines, examine hidden structures and a variety of other military, commercial and industrial functions. Conventional remote vibration sensors transmitted an electro-optic (laser) beam or an acoustic tone and analyzed the return signal looking for any change in the Doppler shift thereof. The limited range of the acoustic approaches have caused system designers to favor laser based systems for military and other applications for which long range operation is of paramount importance. Conventional laser based vibration sensors required lengthy, uniform, noise-free output waveforms. Unfortunately, lasers generally output energy with multiple modes and arbitrary phases. Efforts to limit the lasers to a single mode have tended to increase the cost and complexity of the system while severely limiting the efficiency thereof This was due to the requirement to seed the transmitter to produce a single frequency output at higher powers and an associated need for a local oscillator which had to be frequency locked to the transmitter. 
   Hence, there is an ongoing need in the art for a simple, accurate, low cost, efficient laser transmitter suitable for use in remote, long range, vibration sensing applications. 
   Further, current more demanding applications require highly accurate laser transmitters for high resolution ranging (one-dimensional profiling) and/or laser illumination for two-dimensional and three-dimensional sensing applications. For example, one dimensional profiling allows for the target returns to be matched against a database to identify the target type. For two and three dimensional sensing applications, a tight, highly accurate sensing pulse is transmitted and used to illuminate features of a target. The tight pulses reflect off of various surfaces of the target differently and reflect return pulses which are processed with sophisticated signal processing algorithms to yield more complete images of the target. Two-dimensional and three-dimensional imaging allows for a display of the target return data or an image of the target based on data from a stored database. 
   In any case, conventionally, for these more sophisticated vibration sensing applications and other applications, separate laser transmitters have been required. Unfortunately, the use of multiple transmitters adds significantly to the cost and weight of deployment and would be impractical for many significant applications. 
   Hence, there is a further need in the art for a simple, accurate, low cost, efficient laser transmitter suitable for use in remote, long range, vibration sensing applications which may be implemented in a single laser transmitter capable of performing single point ranging, one-dimensional profiling and/or laser illumination for two-dimensional and three-dimensional sensing applications. 
   SUMMARY OF THE INVENTION 
   The need in the art is addressed by the multimode remote vibration sensor of the present invention. The inventive sensor includes a mode locked laser transmitter; a receiver adapted to detect signals transmitted by the laser and reflected by an object and a signal processor for analyzing the signals and providing an indication with respect to a vibration of the object. 
   The laser is particularly novel as a vibration sensor transmitter inasmuch as it includes a mode locking mechanism. Unlike the single mode laser transmitters that typify the prior art, the mode locking mechanism of the present invention causes the laser to output energy at all modes within the gain profile in phase with one another. The result is a series of tight clean pulses which may be used for range resolved vibration and one-dimensional (high resolution ranging) applications. 
   In a particular embodiment, the laser is an erbium or erbium, ytterbium-doped, fiber pumped laser and the mode locking mechanism is a passive quantum well absorber crystal or an active acoustic crystal mounted in the laser cavity. In any event, the return signals are received and processed to extract vibration, range-resolved vibration, one-dimensional profiling or three-dimensional imaging information. To this end, the signal processor includes a range de-multiplexer for organizing the return signals into range bins. For each range bin, the signal processor further includes means for extracting a signal representing vibration for each range bin and a signal representing intensity for each range bin. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a block diagram of an illustrative implementation of a multifunctional sensor implemented in accordance with the present teachings. 
       FIG. 2   a  is a diagram of the optical configuration of the transmitter of the illustrative embodiment configured to provide a mode locked output. 
       FIG. 2   b  depicts a mode locked pulse train. 
       FIG. 2   c  is a diagram that illustrates the modes that exist within a laser cavity. 
       FIG. 2   d  is a diagram that illustrates the output of a typical laser with modes at random phase. 
       FIG. 2   e  is a simplified diagram of a typical laser cavity with a gain medium and a loss modulator disposed therein. 
       FIG. 2   f  is a diagram that illustrates the output of a typical laser with modes in phase. 
       FIG. 2   g  is a diagram of the modes in a laser cavity having a mode selection element therein. 
       FIG. 2   h  is a diagram illustrative of the output of the transmitter of the illustrative embodiment in the mode locked configuration. 
       FIG. 2   i  shows the transmit pulse train. 
       FIG. 2   j  depicts local oscillator sampling of the heterodyne return pulse train with a microDoppler at a rate of c/ 2 l. 
       FIG. 2   k  shows the result of the sampling operation. 
       FIG. 2   l  depicts the result of low pass filtering of the sampled signal. 
       FIG. 2   m  is a diagram illustrating the returns from the pulses generated by the transmitter of the present invention. 
       FIG. 3  is a diagram of the multifunction sensor receiving and processing method of the present invention. 
   

   DESCRIPTION OF THE INVENTION 
   Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
   While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
     FIG. 1  is a block diagram of an illustrative implementation of a multifunctional sensor implemented in accordance with the present teachings. The sensor  8  includes a multifunctional transmitter  10  implemented in accordance with the present teachings. As discussed more fully below, the transmitter  10  outputs a unique mode locked output signal particularly well-suited for vibration sensing. The transmitter is shown in detail in  FIG. 2   a.    
     FIG. 2   a  is a diagram of the optical configuration of the transmitter of the illustrative embodiment configured to provide a mode locked output. 
     FIG. 2   b  depicts a mode locked pulse train. Returning to  FIG. 2   a , the multifunctional transmitter  10  includes a gain medium  100  disposed in an optical cavity provided by a partially reflective output coupler  110  and a high reflectivity mirror  190 . In the illustrative embodiment, the gain medium  100  is an erbium or erbium, ytterbium-doped, crystal pumped via optical fibers (not shown). 
   In the mode locked configuration, as is well known in the art, the outcoupler  110  and the high-reflector  190  provide a resonant cavity in which there are multiple resonant modes or frequencies. The frequencies are uniformly spaced at c/ 2 l, where ‘c’ is the speed of light and ‘l’ is the length of the cavity. These modes are called Fabry-Perot laser modes and are depicted in  FIG. 2   c.    
     FIG. 2   c  is a diagram that illustrates the modes that exist within a laser cavity. When a gain medium is added to the cavity, a gain profile is provided as depicted in  FIG. 2   c . With a gain medium inside the cavity, there will be a region in which there is optimal gain, each resonant mode under the gain line can lase. Energy at the laser modes within the gain profile lases and will be output by the outcoupler in random phases as depicted in  FIG. 2   d.    
     FIG. 2   d  is a diagram that illustrates the output of a typical laser with modes at random phase. 
     FIG. 2   e  is a simplified diagram of a typical laser cavity with a gain medium and a loss modulator disposed therein. 
     FIG. 2   f  is a diagram which illustrates the output of a typical laser with modes in phase. Note that in  FIG. 2   d , with the phases of the modes being random, the sine peaks do not line up for narrow pulses. However, the addition of a loss modulator to the cavity as depicted in  FIG. 2   e  has the effect of lining up the modes such that the modes are in phase as depicted in  FIG. 2   f . That is, the loss modulator excites all the modes under the gain line of the laser and keeps them in phase. The laser is said to be ‘mode-locked’ in that the modes under the gain line exist and are lined up in phase. This contrasts with the typical conventional single mode laser transmitter used for vibration sensing. Single mode laser transmitters generally employ a mode selection element, Etalon or seeded mode, to isolate a single mode and suppress the other modes under the gain line. This is depicted in  FIG. 2   g.    
     FIG. 2   g  is a diagram of the modes in a laser cavity having a mode selection element therein. Unfortunately, as mentioned above, the isolation of a single mode and the suppression of the other modes in a cavity is difficult and adds significantly to the cost and complexity of the system. 
   However, as illustrated in  FIG. 2   a , in accordance with the present teachings, instead of isolating a single mode and suppressing the other modes in the cavity, the mode locking element  180  is added to excite the modes so that the modes line up in phase. The mode locking element or loss modulator  180  can be: 1) a passive mode locker, i.e., a crystal that is normally opaque to light (does not let the light through) until it reaches a certain intensity threshold (e.g., a passive multiple quantum well absorber crystal such as gallium arsenide) or 2) an active mode-locker with an acoustic crystal which may be purchased from IntraAction Corp in Bellwood, Ill., or Brimrose Corp in Baltimore Md. 
     FIG. 2   h  is a diagram illustrative of the output of the transmitter  10  of the illustrative embodiment in the mode locked configuration. 
   As shown in  FIG. 1 , the output of the transmitter  10  passes through a first polarizer  14 , a polarizing beamsplitter  16 , a one-quarter wave plate  18  and a telescope  20  to a target  22 . Pulses of energy reflected off the target  22  are collected by the telescope  20  and focused on the quarter-wave plate  18 . The result of two passes through the quarter-wave plate is to induce a 90° rotation in the horizontal polarization of the output beam  21  with respect to the return signal  23 . The vertically polarized component of the output beam  21  is directed to a control detector  26  via the first polarizer  14  and a second polarizer  24 . The second polarizer  24  also serves to direct the vertically polarized output of a local oscillator diode laser  28  to the control detector  26 . In accordance with the present teachings, the local oscillator  28  may be set to any mode within the gain line of the transmitter  10 . The horizontally polarized output of the local oscillator  28  is rotated by a 90° rotator  43  and then is reflected by a third polarizer  32  to a receiver detector  30 . The receiver detector  30  also receives the return beam  23  via the third polarizer  32 . 
   The control detector  26  and the receiver detector  30  may be implemented with diode detectors. The control detector  26  and receiver detector  30  allows for a differential detection of the received signal relative to the transmitted signal  21 . The output of the receiver detector  30  is digitized by an analog-to-digital converter  34  and input to a signal processor  40  along with the output of the control detector  26 . The speed of the analog to digital converter is selected to match the pulse width of the return pulse. The signal processor may be a microprocessor which implements a vibration detection algorithm in software appropriate for the output mode of the laser as discussed more fully below. The processor draws from a database stored in a memory  38  and outputs to a display  42 . 
   The vibration detection method of the present is best illustrated with reference to  FIGS. 2   h–m.    
     FIG. 2   i  shows the transmit pulse train.  FIG. 2   j  depicts local oscillator sampling of the heterodyne return pulse train with a microDoppler at a rate of c/ 2 l.  FIG. 2   k  shows the result of the sampling operation. The result of sampling (convolution in the frequency domain) is the sum of multiple versions of the return, shifted by c/ 2 l. The components add up coherently because the waveform is coherent. 
     FIG. 2   l  depicts the result of low pass filtering of the sampled signal. In accordance with the present teachings, a sampled waveform is generated for each range bin as depicted in  FIG. 2   m.    
     FIG. 2   m  is a diagram illustrating the returns from the pulses generated by the transmitter of the present invention. For each sampled waveform, the system  8  performs a Fourier transform for each range bin and sums the signal strength for each range bin for 1D profiling. This is depicted in  FIG. 3 . 
     FIG. 3  is a diagram of the multifunction sensor receiving and processing method of the present invention. As shown in  FIG. 3 , the method  200  begins with the detection and pre-amplification of the received signal in hardware at step  202 . At step  204 , the detected and amplified signal is digitized. At steps  206  and  208 , the digitized return signals are separated into range bins. For each range bin, of which N are shown, at step  210 , a Fast Fourier transform is performed on the digitized signal. Next, at step  212 , the centroids of the transformed signal that are above a predetermined detection threshold are detected. At step  214 , the centroid for each pulse for each range bin is recorded in a track file and at step  216 , the peak intensity is detected and output. The centroid track file keeps a record of the instantaneous velocity recorded at each time interval. At step  218 , a Fast Fourier Transform is performed on the track file and outputs a signal representative of the vibration of the target  22 . Methods and algorithms for performing Fast Fourier Transforms, centroid detection and peak detection are well known to those of ordinary skill in the art. The vibration information may be processed to extract vibration, range-resolved vibration, one-dimensional profiling or three-dimensional imaging information. 
   Returning to  FIG. 1 , a detected vibration signature may be used as a reference for a lookup table in the database memory  38  to extract information and identification data with respect to the target and an image therefor. The image may then be sent to the display  42 . 
   Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. 
   It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
   Accordingly,