Abstract:
An optical receiver comprising a photo-detector, wherein said photo-detector is modulated whereby it has a time-variant photo-responsivity following the modulation on the photo-detector.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to optical receivers and, more particularly, to optical receivers with modulated photo-detectors. More specifically, although not exclusively, this invention relates to optical receivers for optical remote sensing and/or optical distance measurement.  
       BACKGROUND OF THE INVENTION  
       [0002]     Optical devices are widely used in sensing, monitoring, control and communication systems. A typical optical system comprises an optical transmitter and an optical receiver. An optical receiver usually comprises a photo-detector which converts incoming optical signals into electrical output signals for processing by downstream signal processing circuitry. A photo-detector usually comprises a semi-conductor absorbing layer. When optical signals of an appropriate wavelength impinge on the absorbing layer of a photo-detector, electron-hole pairs will be created. Bias-voltage at the terminals of the photo-detector will accelerate the carriers in the electric field between the terminals, whereby in-coming light is converted into electric current. A photo-detector may comprise a photo-diode, for example, a PIN diode for an avalanche diode, photo-resistors or, more recently, MSM photo-detectors. Photo-detectors are typically at a pre-determined biasing condition adapted for specific applications.  
       OBJECT OF THE INVENTION  
       [0003]     It is an object of this invention to provide optical receivers with novel applications of photo-detectors. At a minimum, it is an object of this invention to provide the public with a useful choice of application of photo-detectors.  
       SUMMARY OF THE INVENTION  
       [0004]     Broadly speaking, the present invention has described an optical receiver comprising a photo-detector, wherein said photo-detector is modulated whereby it has a time-variant photo-responsivity following the modulation on the photo-detector.  
         [0005]     According to a preferred embodiment of the present invention, there is provided a distance measuring means comprising an optical system of as described herein, wherein modulation of said optical receiver and the modulated optical signals transmitted by said optical transmitter have the same modulation frequency.  
         [0006]     According to another aspect of this invention, there is provided an optical system comprising an optical transmitter and an optical receiver as described herein, wherein said optical transmitter comprises an optical source which transmits modulated optical signals, modulation of said optical receiver and said optical transmitter is of the same format.  
         [0007]     Preferably, said the photo-detector is pulse modulated and the photo-response of said photo-detector follows the pulse modulation applied to said photo-detector.  
         [0008]     Preferably, photo-responsivity of said photo-detector is bias-voltage dependent, modulation is applied to said photo-detector to vary the photo-responsivity of said photo-detector for signal detection.  
         [0009]     Preferably, polarity of bias-voltage polarity of said photo-detector is reversible, polarity of photocurrent output of said photo-detector is reversible and is dependent on the polarity of said bias-voltage.  
         [0010]     Preferably, said photo-detector is pulse modulated.  
         [0011]     Preferably, said photo-detector is pulse modulated with alternate on and off pulses.  
         [0012]     Preferably, said photo-detector is turned on and off respectively by said on and off pulses.  
         [0013]     Preferably, said photo-detector comprises a MSM photo-detector.  
         [0014]     Preferably, pulse modulation is applied to terminals of said photo-detector.  
         [0015]     Preferably, modulation of said optical receiver and the modulated optical signals transmitted by said optical transmitter have the same modulation frequency.  
         [0016]     Preferably, modulation of said optical receiver and the modulated optical signals transmitted by said optical transmitter are rectangular pulses of the same period and pulse width (T).  
         [0017]     Preferably, modulation of said optical receiver and the modulated optical signals transmitted by said optical transmitter have a constant phase relationship.  
         [0018]     Preferably, wherein modulation of said optical receiver and the modulated optical signals transmitted by said optical transmitter are anti-phased.  
         [0019]     Preferably, modulation of said optical receiver and the modulated optical signals transmitted by said optical transmitter have a constant phase difference.  
         [0020]     Preferably, modulation of said optical receiver and the modulated optical signals transmitted by said optical transmitter have a constant phase difference.  
         [0021]     Preferably, distance information of an object is obtained upon reception of modulated optical signals transmitted by said optical transmitter by said optical receiver after the optical signals are reflected from said object.  
         [0022]     Preferably, distance information is obtained by comparing photocurrent output of said photo-detector when said photo-detector is modulated and when said photo-detector is un-modulated.  
         [0023]     Preferably, the distance of an object is determined by varying the pulse width of the light source and by detecting the maxima or minima of photocurrent output of said photo-detector.  
         [0024]     Preferably, the distance of multiple reflection points is determined by evaluating several relative minima in the optical response function.  
         [0025]     Preferably, the distance of an object is determined by varying the delay time of the gating function and detecting the maximum of the modulated photocurrent as function of the delay time.  
         [0026]     Preferably, the distance of multiple reflection points is determined by evaluating relative maxima in the optical response function. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     Preferred embodiments of the present invention will be explained in further detail below by way of examples and with reference to the accompanying drawings, in which:  
         [0028]      FIG. 1  is a cross-sectional view of a typical MSM photo-detector,  
         [0029]      FIG. 2  shows photo-current output vs. voltage characteristics of a typical MSM photo-detector,  
         [0030]      FIG. 3  illustrates timing diagrams of transmitted signal, reflected signal and photo-detector gating time of a first preferred application of its invention,  
         [0031]      FIG. 4  illustrates a schematic system configuration of a first preferred embodiment of this invention,  
         [0032]      FIG. 5  is a graph showing the variation of detected photo-current vs. delay time with reference to the arrangement of  FIGS. 3 and 4 ,  
         [0033]      FIG. 6  is a graph showing the variation of detected photo-current vs. variation in modulation frequency of a second preferred embodiment,  
         [0034]      FIG. 7  shows timing diagram relationship between transmitter signals, reflected signals and photo-detector gating time in which the delay of the photo-detecting gating time is varied of a third preferred embodiment,  
         [0035]      FIG. 8  shows a graph of variation of detected photo-current vs. delay time d of the photo-detector gating time of  FIG. 7  showing a second preferred embodiment of this invention, and  
         [0036]      FIG. 9  illustrates an exemplary application of this invention on a three-dimensional object. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     Photo-detectors in conventional optical receivers are typically biased at a constant DC bias-voltage to prepare for reception of incoming optical signals. A bias-voltage is applied to a photo-detector to accelerate electron and hole pairs according to the polarity of the applied electric field to produce photo-current output. For conventional photo-detectors, bias-voltage of a specific and constant polarity must be applied to the terminals of the photo-detector in order to generate a correct electric field for meaningful photo-detection. During normal photo-detection operations, a photo-detector is usually reverse biased.  
         [0038]     Metal-semiconductor-metal (MSM) photo-detectors have been used for light detection in fibre optic systems for many years. A typical MSM photo-detector is shown in  FIG. 1  and comprises inter-digitated electrodes which are deposited on an absorbing layer. The absorbing layer can be, for example, undoped GaAs. An exemplary MSM photo-detector is described in U.S. Pat. No. 5,461,246 which is incorporated herein by reference.  
         [0039]     Due to the symmetric structure of a MSM photo-detector, the I-V characteristics of an ideal MSM photo-detector have positive/negative symmetry with respect to bias-voltage, as shown in  FIG. 2 . When light of an appropriate wavelength impinges on a MSM device, the direction of the resulting photo-current will depend on the polarity of the voltage bias. The direction of photo-current will be reversed if the polarity of the bias-voltage is reversed.  
         [0040]     In conventional non-MSM photo-detectors, carriers are generated proximal to the metal electrodes where a built-in electrical field due to metal Schottky contact on the semi-conductor surface exists. On the other hand, due to the highly symmetrical structure of a MSM photo-detector, random carriers generated under un-biased conditions will be cancelled out by similar carrier motions occurring at the other electrode. As a result, there is no net induced current output from a MSM photo-detector at zero biased voltage. This unique characteristic of MSM photo-detectors is advantageous for use in an optical receiver, especially an optical receiver for distance measurements.  
         [0041]     Referring to  FIGS. 3 and 4 , there is shown a first preferred embodiment of this invention for distance measuring applications. Referring to  FIG. 4 , the distance measuring system comprises an optical transmitter  110  and an optical receiver  120 . The optical transmitter transmits optical signals towards a remote object  130 , the distance of which is to be measured. Optical signals reflected by the remote object  130  are received by an optical receiver  120 . Distance information of the remote object  130  is obtained by reference to the temporal relationship, or more specifically, temporal differences, between the originally transmitted signals and the signals received by the optical transmitter after reflection by the remote object  130 .  
         [0042]     Referring to the timing diagrams of  FIG. 3 , modulated optical signals  1120  are generated and then transmitted by an optical source of the optical transmitter  110 . The optical source may comprise laser or a LED. The modulated optical signals are transmitted towards the remote object  130  and are reflected towards the optical receiver  120  for reception. The optical signals  1120  will have travelled a total distance x before the signals are received by the optical receiver  120 , assuming that the remote object  130  is distant from both the optical transmitter  110  and the optical receiver  120  so that the separation between the optical transmitter  110  and the optical receiver  120  is negligible. Of course, the difference travelled by the transmitted and reflected can be un-equal and the difference can be accounted for using ordinary arithmetic principles without loss of generality. When the modulated signals are received by the optical receiver, the time shift or delay (t) between the reflected signal stream  1320  and the originating signal stream  1120  will represent the time required to cover the total distance x, which is twice the separation between the optical receiver and the remote object in this example.  
         [0043]     Since the speed of light in a medium of a reflective index n is c/n, where c is the speed of light in vacuum which is equal to 3×10 8  ms −1 , the time that is required for light to travel a distance x is t=nx/c. By tracking the time required for light to travel from the optical transmitter to the optical receiver via the remote object  130 , the total distance travelled, namely, x, and the distance of the remote object, that is, x/2, can be found. As shown in  FIG. 3 , the reflected optical pulses  1320  arrive at the optical receiver  120  with a time delay t with respect to the originally transmitted signal  1120 . This time delay represents the time required for light to travel from the optical transmitter to the optical receiver. The application of this timing relationship to derive the total distance x travelled by the optical signal (and, hence, the separation distance between the remote object  130  and the optical receiver  120 , namely, x/2) will be explained below.  
         [0044]     When a photo-detector of the optical receiver  120  is biased with a DC voltage so that the photo-detector is always turned on, the reflected optical signals  1320  will be detected and time-averaged photo-current output of the photo-detector will be constant irrespective of the time delay t. On the other hand, the time-average photo-current output of the photo-detector can be varied by gating the photo-detector with a modulated signal which defines a time-variant or time-dependent photo-responsivity. The term time-variant, or time-dependent, in this specification means a photo-responsivity which is not constant but is variable within a specific period of time, that is, within a cycle. The term “gating” here means applying a biasing voltage to the control terminal(s) of a photo-detector to vary the photo-responsivity of the photo-detector. In an exemplary gating application as shown in the gating diagram  1220  of  FIG. 3 , the photo-detector is gated by modulation signals which are identical to the timing characteristics of the transmitted optical signal. Similar to the transmitted signals, a binary gating modulation function is used and the photo-detector is turned off during the off time of the modulation, which corresponds to a low modulation voltage.  
         [0045]     When there is no time delay between the transmitted signals  1120  and the reflected signal  1320 , (i.e., t=0), the time-average photo-current output of the photo-detector will be equal to the time-average photo-current output (I_n) of the photo-detector with a constant DC bias-voltage which turns the photo-detector on to operate at a pre-determined photo-responsivity. As the delay time t increases, the photo-current output (I_g) by the gated photo-detector decreases. As shown in  FIG. 5 , the photo-current output of the gated photo-detector decreases linearly from the maximum detection when t=0 to the minimum detection when t=T, that is, when the transmitted and reflected optical signals are anti-phased. When the time delay t exceeds the pulse width T of the transmitted optical signal, photo-current output from the optical receiver will increase and will reach a maxima when the time delay t equals the period of the transmitted signals. Because the photo-current output and the time delay exhibits a linear relationship between t=0 to T and is given by the relationship I_g=((T−t)/T) I_r=(1−t/T)I_r, where I_g is the photo-current output of a gated photo-detector and I_r is the photo-current output of a DC biased photo-detector, the time delay t can be ascertained by the ratio I_g/I_r, for time delay t&lt;T.  
         [0046]     A MSM photo-detector is employed in this example because it gives no photo-current output when the bias-voltage is zero. When conventional or non-MSM type photo-detectors are used, instead of just turning off the bias-voltage to the photo-detector to correspond to the “Off” time of the transmitted pulse, the photo-detector can be biased at a lower voltage, rather than at zero bias-voltage as in the case of a MSM photo-detector so that the minima of the photo-detector will still occur at delay t=T. The dotted graph  152  of  FIG. 5  shows an exemplary photo-detector output when there is stray light (I_s). When there is stray light, the detected light by gating the photo-detector becomes I_g=(1−t/T) (I_n−I_s)+T/ΘI_s. However, as long as the amount of stray light is not too significant, the minima still occurs at t=T and the total travelled distance x=c/n (1−I_g/I_n) T. Hence, by modulating the photo-detector so that the biasing voltage of the photo-detector corresponds to the modulation of the modulated optical signal  1120 . The distance of a remote object can be measured. Furthermore, by calibrating the optical receiver with an un-gated photo-detector (that is, the photo-detector is always biased to operate at the high biasing voltage level), the distance information can be obtained by the ratio (1−I_g/I_n) as shown in the above equation.  
         [0047]     As an alternative to the distance measuring method described above, the distance of a remote object can be measured by the arrangement of  FIG. 4  by variation of the modulation gating frequency of the MSM photo-detector. In this alternative, the gating modulation function is initially set to be anti-phased to the laser output modulation so that there is a 180° phase shift between the gating modulation function and the laser output source. When the distance x between the optical transmitter and the optical receiver is zero, only photo-current due to incident stray light (I_s) is generated by the photo-detector. Because the gating modulation function of the optical detector and the signal modulation of the optical transmitter are of the same fundamental frequency (f — 0) but is anti-phased, the photo-current output at x=0 will be a minima as shown on the y-intercept. Assuming that the gating modulation function comprises a train of symmetrical pulses so that each period has half on-time and half off-time so that the on-time T is related to the modulation frequency f by f=1/(2T). By increasing the frequency of the gating modulation function, the photo-current output intensity I_g will increase until it reaches a maxima at frequency f_ 1  when the photo-current output of the photo-detector is equal to the photo-current output (I_n) of an ungated photo-detector. The frequency f_ 1  at which the first photo-current output maxima is related to the distance x by: x=c/(2f). When the gating modulation frequency further increases, the output photo-current intensity I_g decreases until it reaches a minima at the frequency f_ 2  wherein f_ 2 = 2  c/(2f), as shown by graph  162  of FIG.  6 . At this frequency minima, the intensity of the photo-current output I_g again corresponds approximately to the ambient stray light detected. This periodic relationship between the gated photo-current signal (I_g) and frequency will repeat at odd and even harmonics of the fundamental frequency f_ 0 . Exemplary relationship between the total distance x and the first (f_ 1 ) and second (f_ 3 ) maxima are set out in table below in which f_ 3  is 3×f_ 1 .  
                                                     x [m]   f_1 [MHz]   f_3 [MHz]                                0.10   1500   4500       0.30   500   1500       1   150   450       10   15   45       100   1.5   4.5                  
 
         [0048]     By tracking the occurrence of the frequencies of the first and the second frequency maxima, the total distance x can be calculated. Since the total distance x in the arrangement of  FIG. 4  is twice the distance of the object from the light source and the photo-detector, the total travel distance x can be calculated by the relationship: x=c/(2f) [1−I_g/I_n]. Thus, the distance of an object can be determined from a single data point as long as the instantaneous gating modulation frequency is smaller than the first resonant frequency f_ 1  of the first photo-current output maxima. For example, if the first maxima occurs at f_ 1  of 1.5 Mhz, the corresponding total travel distance x will be equal to 100 m in air.  
         [0049]     Multiple maxima and minima frequency points can be utilized to enhance the accuracy of measurement as a variation to this method. For example, the first frequency maxima f_ 1  can be obtained by linear extrapolation of a plurality of data points between f_ 0  and f_ 1 . Additional data points between f_ 1  and f_ 2  can be collected and processed for extrapolating the maxima and/or minima to further enhance accuracy. By tracking the relative maxima and minima, adverse influence due to stray light can be mitigated.  
         [0050]     In a second preferred application of this invention and still with reference to the arrangement of  FIG. 4 , the distance of a remote object is measured by varying the delay time of the gating modulation function of the photo-detector. As can be seen from the pulse-timing diagrams  1221  and  1320 , corresponding respectively to the gating modulation pulses and the received reflected pulses of  FIG. 7 , because the pulse width of the reflected pulse of  1320  and the gating pulse  1221  have the same pulse width and pulse period, the photo-detector output will be maximum when the on-pulses overlap. By varying the delay time of the gating modulation function  1221  with respect to the modulation signal of  1120  of the optical transmitter and track for the occurrence of the maxima, the time t required for light beam to travel a total travelling distance x would be equal to d=t, as shown in graph  164  of  FIG. 8 . As a convenient example, the time of occurrence of the maximum photo-detector output can be evaluated from the second derivative of the function photo-current output vs. delay time. In addition, it will be appreciated that the delay time and therefore the distance x/2 can be obtained by measuring the time-average values of the optical signals I_n, I_s and I_g.  
         [0051]     In a third preferred embodiment of this invention, a three-dimensional imaging of a remote object can be measured by one and two dimensional photo-detector arrays. The difference in the distance travelled by light between the various surfaces of a remote object to the photo-detector arrays will provide three-dimensional information of the remote object.  
         [0052]     In a fourth preferred embodiment, the optical transmitter  110  and optical receiver  120  of  FIG. 4  can be set up for distance measurement without requiring reflection from an object. For example, the length of an optical fibre cable can be measured by connecting optical fibre of length x between the optical transmitter and the optical receiver. The total length of the optical fibre x can be measured by comparing the photo-current output of the gated photo-detector and by applying the above relationships mutatis-mutandis without loss of generality.  
         [0053]     Although square or rectangular pulse modulation are used in the examples, it will be appreciated that other pulse shape, for example, half-sinusoid, Gaussian or other pulse shapes can be used and the distance-delay time relationship can be calculated using known algorithms.  
         [0054]     While the present invention has been explained by reference to the examples or preferred embodiments described above, it will be appreciated that those are examples to assist understanding of the present invention and are not meant to be restrictive. Variations or modifications which are obvious or trivial to persons skilled in the art, as well as improvements made thereon, should be considered as equivalents of this invention.  
         [0055]     Furthermore, while the present invention has been explained by reference to a MSM photo-detector, it should be appreciated that the invention can apply, whether with or without modification, to other photo-detectors without loss of generality.