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The invention is related to phase detection in lidar systems using single photon detectors (SPDs). The frequency at which the SPDs are time gated is related to but not an integer multiple of the frequency of the transmitted optical pulses. Each return optical pulse arrives with a particular temporal

The invention is related to phase detection in lidar systems using single photon detectors (SPDs). The frequency at which the SPDs are time gated is related to but not an integer multiple of the frequency of the transmitted optical pulses. Each return optical pulse arrives with a particular temporal position with respect to the nearest gate, and thus is detected with a related detection efficiency. The SPD output can be segmented into multiple time-multiplexed signals whose relative detection efficiency reveals the phase of the optical return pulses, and no such phases have negligible detection efficiency for all the time-multiplexed signals. To mitigate the impact of afterpulsing and other saturation effects, when a first optical pulse is detected with high detection efficiency the next optical pulse that is detected with high detection efficiency is separated by a time period about equal to or greater than the detector dead time.

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1. An apparatus for determining a range information from a transceiver to an object by measuring a phase of a modulated optical signal, comprising: a transmitter transmitting the optical signal modulated with a modulation frequency fm and generating optical pulses of temporal half-width of τp toward

1. An apparatus for determining a range information from a transceiver to an object by measuring a phase of a modulated optical signal, comprising: a transmitter transmitting the optical signal modulated with a modulation frequency fm and generating optical pulses of temporal half-width of τp towards the object;a receiver receiving a reflected portion of the modulated optical signal, the receiver contains a single photon sensitive optical receiver with an optical detection efficiency that is modulated at a gate frequency fG with a gate creating a temporal optical detection efficiency half-width of τG, the frequency fG=M·fm where M=I2/I1 and I1 and I2 are relatively prime integers greater than one; and whereas I2/fG is nearly equal to or larger than the dead time of the optical receiver; anda processor to determine the range information of the object. 2. The apparatus of claim 1, wherein τp is greater than or about equal to (1−τG·fG)/(2·fG·I1), where the constraint on the optical pulse width provides a non-negligible detection efficiency for at least one of I1 consecutive optical pulses regardless of a phase shift (temporal position) of the optical pulses with respect to the gates. 3. The apparatus of claim 1, further comprising: the single photon sensitive optical receiver comprised of at least one avalanche photodiode (APD) operating in Geiger mode. 4. The apparatus of claim 3, wherein an output signal from the APD and a reference signal are both input to a subtraction element, and an output of the subtraction element is an input to a digitizer, the reference signal comprised of a sinusoidal frequency of fG and configured in amplitude and phase so as to subtract out an unwanted feed-through signal from the APD to isolate a small Geiger breakdown signal from the APD output signal. 5. The apparatus of claim 1, wherein a peak detection efficiency of a single photon detector (SPD) being a part of the single photon receiver is controlled by a digital signal processor (DSP) to adjust a resulting peak photon detection efficiency in order to maintain the SPD in a low-saturation photon counting regime for a wide range of received photon flux levels. 6. The apparatus of claim 1, wherein an output of the single photon sensitive optical receiver has a digital output, the digital output of a single photon detector (SPD) being sent to a digital signal processor (DSP), the DSP segmenting the signal into L=I2 time multiplexed digital sub-signals; the DSP digitally processing L sub-signals and using the L digitally processed sub-signals to determine the phase of the received optical signal. 7. The apparatus of claim 6, where the frequency fG is a variable frequency under a control of the DSP. 8. The apparatus of claim 6, wherein the modulated optical signal is detected after reflection from an object, and a rate of change of the range or velocity of the object is determined by processing time multiplexed sub-signals. 9. The apparatus of claim 8, wherein M is under control of the DSP, whereas M is modified by the DSP as needed to keep the frequency modulation of digitally processed time-multiplexed sub-signals in a desired operating range. 10. The apparatus of claim 6, wherein the modulated optical signal includes an additional modulation with frequency components lower than fm that is detected at the receiver in order to extend an unambiguous range over which the object distance can be determined. 11. The apparatus of claim 6, wherein τG>τp so that there is a high detection efficiency for optical pulses that overlap in time with detector gates providing a high contrast between a detection efficiency of the L sub-signals. 12. A method for determining a range information from a transceiver to an object by measuring a phase of a modulated optical signal, comprising: sending the modulated optical signal comprised of a series of short optical pulses towards the object;receiving a series of short optical pulses at a pulse repetition frequency, a portion of the optical signal being detected in a single photon detector (SPD), the SPD being time gated at a frequency not equal to an integer multiple of the pulse repetition frequency, the SPD preferentially detecting photons during a time gate with a resulting detection efficiency having a known shape as a function of time where the shape of the detection efficiency is used to help measuring the phase of the optical signal; and whereas the pulse repetition frequency and the gate frequency are such that any two optical pulses that both temporally overlap with time gates are far enough apart in time to keep the photodetector saturation low, thus maintaining a high SPD linearity; andprocessing data on the phase measurement to receive information about the range or velocity of the object. 13. The method of claim 12, wherein the gated outputs of the SPD are segmented into an integer number of time multiplexed sub-signals, with the integer number being equal to a number of gates after a first optical pulse overlaps with a gate until a subsequent optical pulse overlaps with a subsequent gate such that the first and the subsequent optical pulses are detected with high detection efficiency. 14. The method of claim 12, wherein the optical signal is transmitted to an object, and a phase of the reflected optical signal from the object is measured by processing time multiplexed sub-signals and the phase is used to determine a distance to the object. 15. The method of claim 12, wherein a peak detection efficiency of the SPD is modified in order to keep multiple time multiplexed sub-signals from becoming saturated.