U.S. patent application number 11/701580 was filed with the patent office on 2007-11-29 for laser diode switching system.
This patent application is currently assigned to Exelys, LLC. Invention is credited to Keith E. Barr.
Application Number | 20070273862 11/701580 |
Document ID | / |
Family ID | 37769654 |
Filed Date | 2007-11-29 |
United States Patent
Application |
20070273862 |
Kind Code |
A1 |
Barr; Keith E. |
November 29, 2007 |
Laser diode switching system
Abstract
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. Also, a circuit generates 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 minimized number of components,
leading to high product economy.
Inventors: |
Barr; Keith E.; (Los
Angeles, CA) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
755 PAGE MILL RD
PALO ALTO
CA
94304-1018
US
|
Assignee: |
Exelys, LLC
Los Angeles
CA
|
Family ID: |
37769654 |
Appl. No.: |
11/701580 |
Filed: |
February 1, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10893044 |
Jul 15, 2004 |
7184130 |
|
|
11701580 |
Feb 1, 2007 |
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Current U.S.
Class: |
356/4.01 |
Current CPC
Class: |
G01S 7/4873 20130101;
G01S 17/10 20130101; G01S 7/4861 20130101 |
Class at
Publication: |
356/004.01 |
International
Class: |
G01C 3/08 20060101
G01C003/08 |
Claims
1. A laser diode system comprising: a switching transistor,
configured to be driven by a continuous pulse wave signal of
substantially constant duty, to selectively render conductive and
non-conductive the switching transistor; an inductor coupled to
store energy provided from the switching transistor while the
switching transistor is conductive; a capacitor coupled to the
inductor to receive and store charge when the switching transistor
is non-conductive; and a laser diode coupled to the inductor and
through which, when the switching transistor is conductive, the
capacitor discharges.
2. The system of claim 1, further comprising: a first resistor
through which charge is provided from the capacitor, along with the
laser diode, to maintain a reverse voltage across the laser diode
lower than a damage threshold of the laser diode.
3. The system of claim 1, further comprising: a second resistor
coupled in parallel with the laser diode.
4-10. (canceled)
11. The system of claim 1, wherein the capacitor is coupled to the
inductor via the laser diode.
12. The system of claim 1, wherein the laser diode is coupled
between the inductor and the switching transistor.
13. The system of claim 1, wherein the laser diode is coupled in
parallel to the switching transistor.
14. The system of claim 1, wherein the inductance of the inductor
is such that a resonant frequency of the inductor with the
capacitance has a half period equal to a non-conductive period of
the switching transistor as determined by the continuous pulse wave
signal.
15. The system of claim 14, wherein a half width of a light pulse
emitted from the laser diode is in the range of about 9 to 12 nsec.
Description
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.
[0021] Thus, in some embodiments, the system is configured such
that the noise generated as a result of the crystal frequency is
asynchronous to the laser pulse receive timing circuitry. More
particularly, such noise is more or less spread across the time
sequence of the photodiode signal clocking processes, causing the
resulting noises to be more or less equally distributed throughout
the integrated signal time sequence.
[0022] For example, in one particular embodiment, a prime number is
used for the frequency multiplying factor in the PLL, forcing the
repetitive noises at the crystal frequency to be spread across the
time sequence of the photodiode signal clocking process. In
accordance with other embodiments, other configurations are
employed to distribute the noise through the integrated signal time
sequence of the photodiode signal clocking process.
[0023] As an alternative to the above, the system described is
repetitive, and a method of repeating the sequence of laser firing
and signal acquisition is timed by some method, e.g., through the
use of a counter, driven by the sample clock frequency, that
controls the laser pulse repetition rate. The use of a prime number
in defining the number of sample clocks between each laser pulse,
while a non-prime number acts as the PLL multiplier, is also
effective in reducing the effects of crystal frequency noises on
the integrated signal sequence results.
[0024] Finally, the counter that times the laser pulse and directs
the addressing of the signal integration memory (or counter bank)
employs switching power on each clock pulse, and if these power
pulses are not uniform, a pattern noise can also be generated that
can find its way deleteriously to the photodiode receive circuitry.
In one example, a Grey counter is used for the addressing of the
memory (or counter array). A Grey counter has as a feature the
characteristic of causing only one bit to change on each applied
clock pulse. The use of a Grey counter in the addressing of the
memory (or counter array) therefore causes the deleterious noise to
occur uniformly across each of the time periods in the signal
integration sequence, averaging this noise effect toward zero.
[0025] Some typical design values for the herein described system
follows. TABLE-US-00001 Pulse repetition rate 53.38541 KHz Laser
pulse peak amplitude 3 W Laser pulse half amplitude duration 6 nS
Receiver upper band limit 80 MHz System sample clock 164.0 MHz
Crystal frequency 4.00 MHz PLL multiplier 41 Memory locations 256
Memory data value width 16 bits Effective range 3 to 256 yards
It is to be understood that modifications of the above system vales
can be made without deviating from the meaning and spirit of this
invention.
[0026] We now describe a power switching system that is, for
example, usable in a laser rangefinder that is executing the method
described above. The power switching system herein described
includes a MOSFET switching device, a pulsed laser diode, an
inductor, a storage capacitor, a laser diode shunt resistor, and an
optional current limiting resistor. The gate of the MOSFET is to be
driven by a continuous pulse wave signal of constant duty cycle,
supplied by the signal processing/timing circuitry of the
rangefinder.
[0027] It is to be understood that the circuits shown in FIG. I and
FIG. 2 are alternate embodiments. Other rearrangements of the basic
components may perform similarly.
[0028] FIG. I shows one example circuit description. The battery
power source B1 delivers current through inductor L1 to the storage
capacitor C1 and to the laser diode D1 and the MOSFET switch M1,
along with the associated resistors R1 and R2. Inductors L2 and L3
are the principal unavoidable (and unintentional) stray inductances
associated with the wiring of the circuit. In operation, the signal
DRIVE, supplied by the rangefinder's timing and processing
circuitry, alternately turns MOSFET M1 on and off, at a constant
frequency, and with a fixed duty cycle. While M1 is on, the voltage
at point 2 is essentially zero and current builds in inductor LI,
at a rate that is based on L1's inductance and the battery
potential. The voltage drop across M1 and resistors R1 and R2
during this part of the cycle is essentially zero, as the current
is low, and the values of R1 and R2 are also low, on the order of
0.5 to 5 ohms. When MOSFET M1 turns off, the energy stored in L1
can be described as: E=((I*L) 2)/2
[0029] Where: [0030] I=current through L1 at the time M1 turns off
(amperes), [0031] L=Inductance of L1 (Henrys), [0032] E=energy
(Joules)
[0033] When M1 turns off, the energy invested in L1 will begin to
transfer to capacitor C1, charging C1 to a high potential. When M1
turns on again, C1 is discharged abruptly through R2 and D1,
producing a very high current, short duration pulse through the
laser diode D1, and then the cycle repeats. The voltage on C1 at
the moment that M1 turns on depends on the battery voltage, the
duty cycle and frequency of the signal DRIVE, and the values of C1
and L1. The value of R2 and the associated stray circuit
inductances, along with the voltage on C1 and its capacitance
value, at the moment of M1 turning on, determines the laser diode
current pulse peak amplitude and duration. R1 is selected to dampen
the resonance of C1 and the stray inductances L2 and L3, and R2 is
selected to keep the reverse voltage across the laser diode, due to
this resonance, below the damaging threshold for the laser diode
(typically around 3 volts). A higher duty cycle of signal DRIVE
leads to a higher battery current and a higher peak voltage on C1.
The value of L1 is preferably such that its resonant frequency with
C1 has a half period equal to the MOSFET's off period. If L1 has a
value lower than this preferred value, the voltage on C1 will peak
and then begin to diminish before M1 turns on, leading to a
diminished laser current pulse; if L1's value is higher than this
preferred value, then more energy will be stored in L1 than is
necessary, and L1's cost will be greater than required. At a
preferred value for L1, all of its invested energy is transferred
completely to the capacitor at the moment M1 turns on.
[0034] In operation of the rangefinder, the maximum usable range of
the device corresponds to a defined period of time (approximately 6
ns per yard, times the number of yards of usable range). Provided
the on time of M1 exceeds the total period in which signal is
received, the system will be `quiet` during the receipt of the
extremely faint return light pulse. As a consequence, the light
pulse is emitted from the laser diode and, immediately thereafter,
the only disturbance to the receive system is a slowly increasing
current through inductor L1.
[0035] The process of charging a capacitor to a high voltage and
then discharging it into a laser diode through lead inductances and
damping resistors is highly inefficient. The desire for an intense
but brief light pulse, in the presence of unavoidable lead
inductances however, makes such an inefficient scheme justified.
Although the overall process is generally inefficient, a doubling
of overall efficiency may be gained over devices that use a
switching power supply with a constant current or resistive
capacitor charging method. Because energy can be coupled from the
battery to the charging capacitor with virtually no loss, a
significant reduction in battery current can be realized.
[0036] Typical values for a simple rangefinder:
[0037] Design 1: TABLE-US-00002 Operation frequency 55 KHz Duty
cycle 50% Battery voltage 5 V Peak capacitor voltage 17 V Capacitor
value 2.2 nF Inductor value 10 mH Peak laser diode current 2.8 A
Laser pulse half width 9 nS Damping resistor value (R2) 0.5 ohms
Shunt resistor value (R1) 5 ohms Average battery current 3.8 mA
[0038] Design 2: TABLE-US-00003 Operation frequency 55 KHz Duty
cycle 75% Battery voltage 5 V Peak capacitor voltage 32 V Capacitor
value 2.2 nF Inductor value 3.5 mH Peak laser diode current 4.7 A
Laser pulse half width 12 nS Damping resistor value (R2) 0.5 ohms
Shunt resistor value (R1) 5 ohms Average battery current 12 mA
* * * * *