Abstract:
A time of flight camera device comprises an light source for illuminating an environment including an object with light of a first wavelength; an image sensor for measuring time the light has taken to travel from the light source to the object and back; optics for gathering reflected light from the object and imaging the environment onto the image sensor; driver electronics for controlling the light source with a high speed signal at a clock frequency; and a controller for calculating the distance between the object and the illumination unit. To minimize power consumption and resulting heat dissipation requirements, the light source/driver electronics are operated at their resonant frequency. Ideally, the driver electronics includes a reactance adjuster for changing a resonant frequency of the illumination unit and driver electronics system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Provisional Patent Application No. 61/712,599, filed Oct. 11, 2012, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a pulsed laser driver, and in particular to a pulsed laser driver for a time of flight camera. 
       BACKGROUND OF THE INVENTION 
       [0003]    Time of flight (ToF) camera systems have become extremely popular in interactive video games, and are now becoming more popular in many other aspects of modern life, including communication, art, security and electronic controls. As technology evolves and expands into other fields, the desire to shrink both the size and cost of the ToF system increases. Power consumption and heat dissipation are also major concerns, in particular because the more power required, the larger the heat sink required, the larger the overall footprint required. 
         [0004]    An object of the present invention is to overcome the shortcomings of the prior art by providing a time of flight system requiring less power and less heat dissipation. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, the present invention relates to a method of operating a time of flight camera comprising: 
         [0006]    illuminating an environment, including an object, with light of a first wavelength from a light source; 
         [0007]    controlling the light source with a high speed signal at a clock frequency using driver electronics; 
         [0008]    selecting the clock frequency, whereby the light source and driver electronics system is at resonance; 
         [0009]    gathering reflected light and imaging the environment onto an image sensor; and 
         [0010]    calculating the distance between the object and the light source using a controller. 
         [0011]    Another aspect of the present invention relates to a time of flight camera device comprising: 
         [0012]    a light source for illuminating an environment including an object with light of a first wavelength; 
         [0013]    an image sensor for measuring time the light has taken to travel from the light source to the object and back; 
         [0014]    optics for gathering reflected light from the object and imaging the environment onto the image sensor; 
         [0015]    driver electronics for controlling the light source with a high speed signal at a clock frequency, whereby the light source and driver electronics system is at resonance; and 
         [0016]    a controller for calculating the distance between the object and the illumination unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0018]      FIG. 1  is a schematic diagram of a time of flight system in accordance with the present invention; 
           [0019]      FIG. 2  is a schematic diagram of the light source of  FIG. 1 ; 
           [0020]      FIG. 3  is a plot of Power Supply Current vs Clock Frequency for the light source of  FIG. 2 ; and 
           [0021]      FIG. 4  is a schematic diagram of an alternate embodiment of the light source of FIG.  2 .; and 
           [0022]      FIG. 5  is a schematic diagram of an alternate embodiment of the laser driver circuit, in the form of a self-oscillating laser driver for the light source of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    With reference to  FIG. 1 , a time-of-flight (TOF) camera system  1  according to the present invention includes a light source  2  for illuminating a scene  3  with one or more moving users  4  with a beam of light  6 . Since the light  6  has to be modulated with high speeds, e.g. 20 MHz up to 200 MHz, typically only LEDs or laser diodes are feasible; however, the light source  2  may be an LED, a laser, a fiber laser, or other appropriate source. The modulation may be analog or digital or both. The light source  2  normally uses infrared light to make the illumination unobtrusive to the users  4 . The light beam  6  may also pass through optical devices, such as lenses, optical diffusers, optical beam shapers. The devices can be either diffractive or refractive, or both, for generating patterns, e.g. lines, arrays of spots, etc, and/or for moving the patterns across the scene. 
         [0024]    A lens  7  or other suitable optics gathers light  8  reflected from the scene  3 , and images the scene  3  onto an image sensor  9 . An optical band pass filter  11  reflects light at other wavelengths and only passes the reflected light  8  with the same wavelength as the light source  2 , which suppresses background light from interfering with the image sensor  9 . 
         [0025]    The image sensor  9  is the heart of the TOF camera system  1 , and may comprise a CCD array, a silicon sensor array or a single photodetector, which utilizes another scanning device, such as a MEMS. 
         [0026]    Each pixel on the image sensor  9  measures the time the light  6  has taken to travel from the light source  2  to the users  4  in the scene  3  and back to the image sensor  9  to determine the location and motion of the one or more users  4  in the scene  3 . 
         [0027]    Several different approaches can be used for timing, such as by using RF-modulated light sources  2  with phase detectors for modulating the outgoing beam  6  with an RF carrier at modulation frequency ƒ modulation , then measuring the phase shift φ(transmitted, received) of that carrier on the receive side with the image sensor  9 . 
         [0028]    Time-of-flight (ToF) camera: range finding by phase detection 
         [0000]        D =φ(transmitted, received)/(2π)*( c /ƒ modulation )/2
 
         [0029]    Both the light source  2  and the image sensor  9  are controlled by high speed signals from a signal processor  12 . These signals have to be very accurate to obtain a high resolution. For example, if the signals between the light source  2  and the image sensor  9  shift by only ten picoseconds, the distance changes by 1.5 mm. For comparison: current CPUs reach frequencies of up to 3 GHz, corresponding to clock cycles of about 300 ps—the corresponding ‘resolution’ is 45 mm. It is common practice for phase detection to achieve sub-clock-cycle resolution. The typical modulation frequency for ToF based Gesture Recognition applications is on the order of 10 MHz to 100 MHz, which correspond to a quite large (meters) round-trip distance per clock cycle. Because of the sub-cycle resolution by phase detection, millimeter resolution can be achieved. 
         [0030]    The distance to the one or more users  4  is calculated directly by the signal processor  12 , typically in the form of suitable hardware and software stored in non-transitory memory in the TOF camera system  1 ; however, to obtain better performance, some calibration data can also be used. Due to the imperfection of the light pulse shape and other reasons, the true phase delay could deviate from the phase delay deviated from the phase detection circuitry. A calibration process conducted by the signal processor  12  utilizes a look-up table stored in non-volatile memory comprising data of the true object distance vs. the measured phase delay. Since the look-up take is built from a set of finite number of data points, the distance can be calculated by either data interpolation or best-fit function. The camera  1  then provides a distance image over a USB or Ethernet interface to a host computer system, such as a computer game console. 
         [0031]    With reference to  FIG. 2 , the light source  2  includes a laser  21  driven by a laser driver  22 , which is controlled by a clock generator  23  and powered by a power supply  24 . In principle, any clock signal waveform works for the laser  21 , i.e. it is not necessary to use square (top hat) pulses. Accordingly, a sine-wave driven by reactive components, e.g. inductive elements L and capacitive elements C, with a net average power=0, can be used for the clock signal from the clock generator  23 . The waveform, i.e. pulse shape, pulse width and pulse repetition frequency, of the clock signal pulse train can be optimized for power efficiency of the laser driver  22  and the laser  21 . 
         [0032]    The absolute pulse shape isn&#39;t important to achieve good phase-based depth measurement, so it is a good trade-off to get better power efficiency than trying to get a fast rise time and fall time. The method of the present invention makes the laser driver  22  and the laser  21  generate less heat, which means the size for the heat spreader can be smaller. 
         [0033]    According to the present invention, the driver-and-laser system is the most efficient when the system is at resonance. In a system comprised of linear components, e.g. resistance, inductance and capacitance, only the waveform for resonance is sinusoidal. However, a laser diode is not a linear device. For a given design of the laser driver  22 , e.g. fixed rise time and fall time, the ƒ modulation  of the clock generator  23  can be adjusted to find the resonant frequency of the system, and thereby minimize power consumption. Moreover, the resonance frequency of the laser system can be adjusted by adjusting or changing the reactance, e.g. L and/or C, to ensure a resonant frequency is attainable, thereby producing less heat, i.e. a lower temperature rise. 
         [0034]    With an existing pulsed laser driver  22 , with which the rise time and fall time of the pulses are not easily adjustable, waveform optimization for high power efficiency to find the resonance condition can be done by the following steps: 
         [0035]    Connect the laser  21  to the laser driver  22  and a power supply  24 ; 
         [0036]    Scan the frequency of the clock generator  23  across the desired operating range, e.g. 20 MHz to 200 MHz; 
         [0037]    Measure the power required, i.e. current from power source  23 , using ammeter  25 ; 
         [0038]    Plot the current versus clock frequency curve (See  FIG. 3 ); 
         [0039]    Find the minimum of the curve in the desired frequency range; 
         [0040]    Use the frequency located at the minimum as the laser modulation frequency ƒ modulation . 
         [0041]    The minimum corresponds to the least power consumption frequency within the desired operating frequency range. 
         [0042]    For best results the duty cycle is kept around 50%. Here the duty cycle is defined as the pulse width to pulse period ratio, and the pulse width is defined as the full width at half maximum. The reason for keeping the duty cycle around 50% is to ensure the least amount of harmonics of the modulation frequency. 
         [0043]    Ideally, adjustment of the pulse repetition frequency is started from a low frequency, in other words at the low end of the operating frequency range, which is typically 20 MHz to 200 MHz. As the frequency increases, the total electrical power supplied to the laser driver  22  and laser  21  will increase at first. Then, the total supplied electrical power will start to decrease when the modulation frequency approaches the resonant frequency of the system. The frequency at which the minimal supplied electrical power to the laser driver  22  is the optimal operating frequency for the laser driver  22 . During the pulse repetition frequency adjustment, the optical power, i.e. either the peak power or the average power, should be maintained at a desired level, and the duty cycle needs to be kept at a constant (see Step  1 ). 
         [0044]    The resonant frequency is determined by the reactive components, e.g. inductance and capacitance, in the system, including the parasitic inductance and parasitic capacitance. If no resonance is observed in the second step described above, it means the resonant frequency is outside of the practical frequency range for the application. To ensure that a resonant frequency is within the operating wavelength range, variable reactive components or multiple reactive component can be provided in the system to shift the resonant frequency. 
         [0045]    A system can be designed for multi-frequency resonance by using a switch to select between different sets of reactive components in the system, which result in different resonant frequencies. The switching can be done initially during the original manufacturing or by the customer to suite their specifications. The switching can be done manually or controlled by the signal processor  12 . Typically, the switching is done on a circuit board in the signal processor  12 , by the customer or end user. 
         [0046]      FIG. 4  is an example for the multi-frequency system in which a switch  31  is provided for selecting between the inherent reactants or one of a plurality of different reactant networks, e.g. first network  32  and second network  33 , with different inductances and capacitances, which results in different resonant frequencies. Please note that the placement of the reactant networks  32  and  33  is not limited to between the laser  21  and the power supply  24 , as illustrated in  FIG. 4 . For instance, the reactant networks  32  and  33  can be: parallel to the laser  21 , between the laser&#39;s cathode and ground, between the laser&#39;s anode and ground, or in general, between either node (anode or cathode) of the laser and any other node in the circuit. 
         [0047]    The reactant network can be as simple as an inductor, as simple as a capacitor, an inductor and a capacitor either in series or in parallel or other combinations of single or multiple inductors and single or multiple capacitors. Ideally, the reactant networks  32  and  33  are only made out of reactive components (inductor and capacitor). The reason is that any resistive component in the network dissipates electricity, which is not desirable. Also, the reactant networks  32  and  33  can be replaced with designs comprising active components (transistors, operational amplifiers, etc.) 
         [0048]    With reference to  FIG. 5 , another embodiment of the present invention provides a circuit for driving the laser  21  at a particular form of resonance, i.e. relaxation resonance oscillation. This circuit is a self-oscillating laser driver, as opposed to drivers that require an external clock to set the modulation frequency. At the relaxation oscillation frequency, the laser  21  emits more power with the same amplitude of the driving current. 
         [0049]    The circuit initially starts from any noise in the system, and the intensity of the laser  21  increases. A photodetector  41  picks up light from the laser  21 , generating an electrical signal with an AC and a DC component. The AC component of the photodetector  41  signal provides feedback to a transistor (or an amplifier)  42 , which is in series with the laser  21 . A bipolar junction transistor (BJT) with a base, a collector and an emitter is illustrated, but any suitable semiconductor transistor would suffice. A capacitor  43  is provided between the photodetector  41  and the transistor  42  to block the DC component from the transistor  42 . An inductor  44  is provided to provide a DC bias for the photodetector  41 . 
         [0050]    An increase in the laser light makes the photodetector  41  produce more current at the base of the transistor  42 . More base current means more collector current, hence more current to the laser  21  leading to more laser light. The feedback is “positive” when the frequency is at the relaxation resonance and the AC frequency component in the electrical signal from the photodetector  41  keeps feeding back positively to the transistor  42  and sustains the oscillation. Any signal component with a frequency other than the relaxation oscillation frequency (or the resonance frequency) does not meet the positive feedback condition and will not survive. 
         [0051]    Eventually, the collector current saturates due to the limitations of the bias and circuit elements. Accordingly, the intensity of the laser  21  has to fall, because a constant laser current would make the photodetector  41  produce a constant current with no AC component through the capacitor  43  to the base of the transistor  42 . A decrease in the laser light intensity results in a decrease in current from the photodetector  41 , and consequently a decrease in the base current and a decrease in the laser light intensity. This process repeats itself at the resonance relaxation frequency, since the gain, i.e. laser power to drive current, is the highest at this frequency.