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Laser diode switching systemLaser diode switching system description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070273862, Laser diode switching system. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] Laser rangefinders determine the distance to a target by emitting a brief, narrow beam light pulse to a target and measuring the time for the reflected light to return. Since the speed of light in air is constant, accurate measurements can be obtained through the use of such devices. Most targets are non-reflective, absorbing some of the light and dispersing the remainder in all directions. As a result, the received light pulse is very faint and decreases as the square of the target distance. For laser rangefinders that emit `eye safe` levels of energy, and have restricted receive lens areas, the received light pulse amplitude from any target over a few hundred yards in range is buried in photo detector noise. [0002] Laser rangefinders typically establish a threshold that is above the noise level, and trigger a timing circuit upon the received pulse exceeding the threshold. Alternately, the threshold is set somewhat lower (into the noise), causing numerous authentic and false triggering `hits`, whereupon after several repetitive pulses, a correlation can be established between pulse hit results to establish a `most probable` signal pulse location. This later technique is somewhat effective in improving the ability of the rangefinder to range more distant targets, but is computationally intensive. It can also require a significant amount of memory to be effective. [0003] Furthermore, rangefinders that employ semiconductor laser diodes deliver extremely high current and extremely brief pulses to their laser diodes, while simultaneously supporting extremely sensitive receive circuitry to detect the very small reflected light pulse. Typical laser pulse peak currents can be from 2 to 20 amperes, with durations on the order of 5 to 25 nS. Typical received light pulses from distant targets can be as small as a few hundred photons. Integrating high voltage, high current switching devices along with sensitive receiving circuits into compact units is difficult. [0004] The generation of extremely short, high current pulses is problematic when unavoidable driving component lead inductances are considered. A solution to overcoming the lead inductance problem is to operate the driving circuitry at rather high voltages (20 to several hundred volts), wherein a small capacitor is charged to a high voltage and then discharged with a semiconducting switching device into the laser diode. Such high voltages however, in a portable, battery powered system, are typically developed through the use of a switching power supply that will also generate switching noise that is deleterious to the sensitive receive circuitry. BRIEF SUMMARY [0005] An improved technique for a laser rangefinder includes a pulsed laser sending out a repetition of light pulses and the received signal (light pulse and noise components combined) being analyzed by continuous averaging of successive received signal discrete time sequences, each sequence beginning with the onset of transmitted laser light to reduce the effect of the noise components in the received signal. [0006] Also, a circuit is provided for generating high potentials for the laser diode driving circuitry that simultaneously drives the laser diode, having at least one of several beneficial characteristics. It is conveniently synchronous with the light pulse send/receive process to reduce the effects of noise on the receive section. It is energy efficient in converting low battery potentials to higher voltages. Finally, it utilizes a minimal number of components, leading to high product economy. BRIEF DESCRIPTION OF THE DRAWINGS [0007] FIG. 1 and FIG. 2 illustrate alternate circuit embodiments of a synchronous power supply and laser diode driver. [0008] FIG. 3 illustrates circuitry usable to more reliably sample a laser rangefinder input signal. DETAILED DESCRIPTION [0009] We first describe, with reference to the circuitry 300 in FIG. 3, a method to improve the signal to noise ratio of a signal received by a laser rangefinder and usable to determine a range to a target. In the FIG. 3 example, a laser is forced to emit a very brief pulse (about 10 nS long) at a rate of about 55 KHz. The laser pulse is reflected off a target, and some of the light is converted to a voltage signal 302 by a photosensor (not shown), and amplified by an amplifier 304. The output of the amplifier 304 is provided as a first voltage signal 306 to a comparator 308. The first voltage signal 306 includes unavoidable photodetector and amplifier noise. An average ambient output of the amplifier 304 provides a second voltage signal 310 to the comparator 308. For example, in the FIG. 3 circuitry, the average ambient output of the amplifier 304 is the result of an RC averaged output, where a resistor 316 is connected to the output of the amplifier 304 and a capacitor 318 is connected between the resistor 316 and ground. [0010] The output of the comparator 308 is a "1" when the first voltage signal 306 is greater than the second voltage signal 310. On the other hand, the output of the comparator 308 is a "0" when the first voltage signal 306 is less than the second voltage signal 310. [0011] The comparator output logic signal is processed into a counter memory 310, under the control of a fixed rate clock signal 312. In one example, the fixed rate clock signal 312 is running at 164 MHz, which is a period of about 6 nS. The counter memory 310 is addressable by an index. The index is reset by a reset signal 314, which is typically asserted when a light pulse is emitted, or at least in some predetermined time relationship with the emission of a light pulse (and, as a result, to the distance over which the light traveled from the laser to the target, and to the photosensor). There may be, for example, 256 addressable locations in the counter memory 310. [0012] In operation, each measurement begins by clearing all memory locations in the counter memory 310 to zero. After the index is reset by the reset signal 314, the index is incremented at each cycle of the clock 312, and the comparator value is accumulated into the location in the counter memory 310 addressed by the index. The index is incremented, and the comparator values accumulated, until the index reaches the last location in the counter memory 310. At this point, the acquisition cycle stops until the laser pulse is emitted again. Over an example period of 1 second, 55,000 laser pulses will be emitted, and each location in the counter memory 310 will have been addressed and accumulated into 55,000 times. [0013] As a baseline, if no light is reflected as a result of a particular one second acquisition period, then the comparator 308 will on average provide an equal number of zeros and ones to each addressable location in the counter memory 310. That is, due to noise in the photodetector, the first voltage 306 is expected to be less than the average during half of the sample time periods (where each sample time period corresponds to the index having a different value) and greater than average during half of the sample time periods. Using the example set forth above then, each counter value is expected to be 55,000/2, or 27,5000, if no light is reflected as a result of the particular one second acquisition period. [0014] On the other hand, if the light pulse is returned, even as a very small signal, then this will be indicated by the counter(s) that correspond to the time delay at which the returned light pulse is received. That is, those counters will be higher than 27,500, using the example above. In one example, a counter must be higher than the average count by an amount that is deemed to be statistically significant in order to be considered as indicating a return light pulse. In one example, the statistically significant amount is the square root of the total number of samples. For example, in the above example, the statistically significant amount would be the square root of 55,000, or 235. [0015] In a situation where the received light pulse is buried in noise, the advantage of such noise averaging can be shown to be equal to the square root of the number of repetitive averages. For example, 100 such averages can improve the signal to noise ratio by a factor of 10, or 20 dB. In practice, for an `eye safe` laser (by current regulatory constraints), an optimal repetition rate for the laser is about 55 KHz, and the signal to noise ratio improvement for a 1 second measurement is about 235, or 47 dB. [0016] Government regulations that control `eye safe` pulsed laser operation allow for lower repetition rates with correspondingly higher transmitted pulse amplitudes. However, the current regulations allow higher pulse powers only at frequencies below 55 KHz and, in that case, the increase in power is the fourth root of pulse frequency reduction. Therefore, the pulse repetition rate is lowered by a factor of 16 (to 3.4375 KHz) to allow an increase in peak laser power of a factor of 2. The advantage of noise averaging as described herein can be exemplified by this case, where a 3.4375 KHz repetition rate laser rangefinder is operated at 55 KHz and, while suffering a signal reduction of a factor of 2, realizes a noise reduction of a factor of 4. Regulations concerning `eye-safe` operation above 55 KHz require that the transmitted pulse energy be decreased in proportion to any repetition rate increase and, therefore, noise averaging at repetition frequencies greater than 55 KHz becomes increasingly inefficient. [0017] The use of high laser pulse repetition frequencies, on the order of 55 KHz, and noise averaging as described above, can improve the performance of a laser rangefinder by a significant degree. Additionally, however, certain precautions can be used to further reduce noise generated within the laser rangefinder unit and that could be conducted through space as electrostatic or electromagnetic disturbances to the photodiode amplification circuitry. These precautions are now described. [0018] The generation of extremely brief, high power laser pulses often employs a switching power supply to charge a capacitor that can be abruptly discharged into a laser diode. Such switching supplies, with commutating diodes and high current switches, can generate deleterious noise that is difficult to shield from the sensitive photodiode electronics. High voltages are often employed to overcome the delaying effects of component lead inductance associated with laser diodes and driving circuitry. A later part of this disclosure describes a method of using a single inductor in conjunction with a traditional laser diode switching device and storage capacitor. It is described that very high energy efficiency can be obtained, also achieving high capacitor charge voltages, while ensuring that the circuitry switches in synchrony with the capacitor discharge, with no (or a minimum of) other noise generating events (as would be found in a traditional switching power supply) to contaminate the sensitive photodiode environment. [0019] Additionally, the memory described above is effectively representing a bank of counters, as each memory location can be either incremented in value or left with its value unchanged after each access. A bank of counters can be substituted for the memory, and power consumption can accordingly be reduced, as the use of a counter bank is generally such that each counter is addressed and affected individually, but generally a memory bank is completely precharged on each memory access; a net power reduction and a lowering of transient currents in the processing circuitry can result from the use of a counter bank over a memory array. [0020] Further, the clock frequencies used in the clocking of the photodiode signal comparator and the addressing of the memory (or counter bank) can often be quite high, exceeding 100 MHz for a device with 1 meter resolution. Interpolation techniques can be used on integrated signal sequences to obtain resolution that exceeds the sampling clock period, but accuracy will typically suffer as higher resolution is sought; therefore, high sample clock frequencies can be employed. Also, the sample clock frequency should be accurate, preferably being derived from a quartz crystal for measurement accuracy. Traditionally, a phase locked loop (PLL) employing a crystal oscillator of reasonable frequency (4 to 20 MHz) would be used, effectively multiplying the crystal frequency to a higher sample clock frequency. The inclusion of such a lower frequency signal in a laser rangefinder system is attractive, as an included microcontroller would employ such a clock frequency, the microcontroller being used to analyze the integrated signal sequences and drive a display. In such a situation, electronic noises generated by the crystal oscillator and other devices driven by that crystal frequency (such as a microcontroller) could repetitively occur at select moments of time. Continue reading about Laser diode switching system... Full patent description for Laser diode switching system Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Laser diode switching system patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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