U.S. patent application number 15/417580 was filed with the patent office on 2017-05-11 for laser system with reduced apparent speckle.
This patent application is currently assigned to LaserMax, Inc.. The applicant listed for this patent is LaserMax, Inc.. Invention is credited to Brian L. Olmsted.
Application Number | 20170133823 15/417580 |
Document ID | / |
Family ID | 56408533 |
Filed Date | 2017-05-11 |
United States Patent
Application |
20170133823 |
Kind Code |
A1 |
Olmsted; Brian L. |
May 11, 2017 |
LASER SYSTEM WITH REDUCED APPARENT SPECKLE
Abstract
Laser systems with reduced apparent speckle are provided. The
laser systems emit laser light having different mode structures
that change within a time period of an integration period of an
imaging system used to observe a field of view that is at least in
part illuminated by the laser systems.
Inventors: |
Olmsted; Brian L.;
(Spencerport, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LaserMax, Inc. |
Rochester |
NY |
US |
|
|
Assignee: |
LaserMax, Inc.
Rochester
NY
|
Family ID: |
56408533 |
Appl. No.: |
15/417580 |
Filed: |
January 27, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14601782 |
Jan 21, 2015 |
9559492 |
|
|
15417580 |
|
|
|
|
61929762 |
Jan 21, 2014 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 21/956 20130101;
H01S 5/0427 20130101; G01N 2021/479 20130101; Y10S 378/901
20130101; G01N 21/3563 20130101; G02B 27/48 20130101; G02B 27/20
20130101; G01N 2201/06113 20130101; H01S 5/0651 20130101; G02B
21/365 20130101; H01S 5/02288 20130101; G01N 21/8806 20130101; G01N
21/474 20130101; G01N 21/8851 20130101 |
International
Class: |
H01S 5/042 20060101
H01S005/042; G02B 27/48 20060101 G02B027/48; H01S 5/065 20060101
H01S005/065 |
Claims
1. A laser system comprising: a semiconductor laser adapted to emit
a beam coherent light when supplied with an electrical current; a
driving circuit adapted to supply a first current to the
semiconductor laser and to modulate the current supplied to
semiconductor across a range of current levels within a determined
integration time; wherein the current is modulated so that
semiconductor laser will emit light having a first transverse mode
structure during a first portion of the range of current levels and
a second transverse mode structure during a second portion of the
range of current levels causing a shift in the position of a
speckle pattern during the integration time that reduces the
appearance of speckle.
2. The system of claim 1, wherein a change in transverse mode
structure takes the form of a change in the number of transverse
modes in the laser beam.
3. The system, of claim 1, wherein the change in transverse mode
structure comprises a change in the relative portion of the overall
intensity of a beam formed by individual ones of more than one
simultaneously emitted transverse modes.
4. The system, of claim 1, wherein the laser beam has an angular
emission profile that is a function of the transverse mode
structure and wherein the direction of higher intensity emissions
in the angular emission profile change with the transverse mode
structure.
5. The system of claim 1, wherein the laser has a ridge width
selected to provide transverse mode structures that are different
when energized in at least two different ranges of current in order
to provide the shift in transverse mode structure.
6. The system of claim 5, wherein the ridge width is between about
1 and 2 wavelengths of a light emitted as a laser beam by the
semiconductor laser.
7. A method for operating a laser system: determining an
integration time for an imaging system to be used with the laser
system; supplying a current to a semiconductor laser used in the
laser system; modulating the current supplied to the semiconductor
laser across a range of current levels during the determined
integration time wherein the current is modulated so that the
semiconductor laser will emit light having a first transverse mode
structure during a first portion of the range of current levels and
a second transverse mode structure during a second portion of the
range of current levels causing a shift in the position of a
speckle pattern during the integration time that reduces the
appearance of speckle.
8. The method of claim 7, wherein a change in transverse mode
structure takes the form of a change in the number of transverse
modes in the laser beam.
9. The method of claim 7, wherein the change in transverse mode
structure comprises a change in the relative portion of the overall
intensity of a beam formed by individual ones of more than one
simultaneously emitted transverse modes.
10. The method of claim 7, wherein the laser beam has an angular
emission profile that is a function of the transverse mode
structure and wherein the direction of higher intensity emissions
in the angular emission profile change with the transverse mode
structure.
11. The method of claim 7, wherein the semiconductor laser has a
ridge width selected to provide transverse mode structures that are
different when energized in at least two different ranges of
current in order to provide the shift in transverse mode
structure.
12. The system of claim 11, wherein the ridge width is between
about 1 and 2 wavelengths of the light emitted by the semiconductor
laser.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/929,762 filed Jan. 21, 2014.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A "SEQUENCE LISTING"
[0003] Not applicable.
BACKGROUND OF THE INVENTION
[0004] Field of the Invention
[0005] The present invention relates to laser systems, and more
particularly to laser illuminator systems.
[0006] Description of Related Art
[0007] Laser illumination systems project a beam of collimated
light across an area. Often this is done to enable observation of a
laser illuminated area in a particularly useful but limited range
of wavelengths. This can be done for example to illuminate an area
with light that is not visible to people but that can be detected
electronically such as by illuminating an area with near infrared
light or short wave infrared light, or this can be done to sense
objects in a scene that may be fluoresce when illuminated when
exposed to specific wavelengths of light.
[0008] One problem with observing laser illuminated areas is while
a laser illumination may be generally uniform, non-specular
surfaces in the illuminated area may reflect the coherent light
from the laser such that interference patterns arise when the light
is observed by a person or electronic imager. The interference
creates areas that appear to be unnaturally bright and areas that
appear to be unnaturally dark creating a high noise component in
the reflected light observed in an area. The interference effect is
known as speckle.
[0009] Speckle is visually distracting and can make it difficult
for both human observers and automatic vision systems to detect
contrast patterns in the illuminated areas.
[0010] Saloma, et al. in a paper entitled "Speckle reduction by
wavelength and space diversity using a semiconductor laser",
published in Applied Optics, Vol. 29, No. 6, (Optical Society of
America 1990) describe a speckle reduction system that uses
modulation of a laser to create additional longitudinal modes, with
each mode having a different laser frequency. In operation, mode
hopping is used and a grating is used to introduce a shift in a
position of a point of a source of the illumination as a function
of the change in frequency during the mode hopping. The change in
position reduces the extent of the speckle contrast when averaged
over time.
[0011] Trisnadi, in a paper entitled "Speckle contrast reduction in
laser projection displays", published in Projection Displays VIII,
Ming H. Wu, Editor Proceedings of SPIE Vol. 4657, (SPIE 2002)
describes generally speckle reduction strategies as methods for
averaging N independent speckle configurations with the spatial and
temporal resolution of a detector and identifies three different
mechanism for speckle reduction: wavelength diversity which
requires a laser with a sufficiently large range of wavelengths to
reduce speckle, polarization diversity which requires emission of
laser light having two different polarizations and angle diversity
which requires shifting the point of illumination. Trisnadi
proposes a combination of polarization and angle diversity to
achieve speckle reduction. In Trisnadi, angle diversity is
accomplished using a moving diffuser.
[0012] Geske et al. U.S. Pat. No. 8,743,923 describe the use of a
multi-wavelength VCSEL array to reduce speckle using wavelength
diversity. In this embodiment, the VCSEL array has a plurality of
laser emitters each with a different wavelength creating a laser
emitter having a broad enough bandwidth to reduce the speckle
effects.
[0013] What is need in the speckle reduction art is a solid state
laser device that does not require the grating and extended optical
path of Saloma, that does not require moving parts like the moving
diffuser of Trisnadi and that does not require the complexity and
cost of a VSCEL array.
SUMMARY OF THE INVENTION
[0014] Laser systems and methods are provided. In one aspect a
laser system has semiconductor laser that is adapted to emit a beam
coherent light when supplied with an electrical current and driving
circuit adapted to supply a first current to the semiconductor
laser and to modulate the current supplied to semiconductor across
a range of current levels within a determined integration time. The
current is modulated so that semiconductor laser will emit light
having a first transverse mode structure during a first portion of
the range of current levels and a second transverse mode structure
during a second portion of the range of current levels causing a
shift in the position of a speckle pattern during the integration
time that reduces the appearance of speckle.
[0015] In another aspect, a method for operating a laser system is
provided in which an integration time is determined for an imaging
system to be used with the laser system and a current is supplied
to a semiconductor laser used in the laser system. The current
supplied to the semiconductor laser is modulated across a range of
current levels during the determined integration time and the
current is modulated so that the semiconductor laser will emit
light having a first transverse mode structure during a first
portion of the range of current levels and a second transverse mode
structure during a second portion of the range of current levels
causing a shift in the position of a speckle pattern during the
integration time that reduces the appearance of speckle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic view of a first embodiment of a laser
system according to a first embodiment.
[0017] FIG. 2 is a top view of one embodiment of the laser system
of FIG. 1.
[0018] FIG. 3 is an end view of the embodiment of FIG. 2.
[0019] FIG. 4. is a side section of the embodiment of FIGS.
1-3.
[0020] FIG. 5 shows a first embodiment of a laser system, an
imaging system and a field of view.
[0021] FIG. 6 shows a speckle pattern.
[0022] FIG. 7 shows a flow chart of one method for operating a
laser system.
[0023] FIG. 8 shows a portion of the speckle pattern on a plurality
of radiation sensors.
[0024] FIG. 9 shows the portion of FIG. 8 in combination with a
shifted portion of the speckle pattern.
[0025] FIG. 10 shows the net effect of the shift on the plurality
of radiation sensors.
[0026] FIG. 11 shows the effect of unshifted speckle on the
plurality of radiation sensors as is known in the prior art.
DETAILED DESCRIPTION OF THE INVENTION
[0027] FIG. 1 is a schematic view of first embodiment of a laser
system 100. FIGS. 2 and 3 illustrate respectively top and end views
of the embodiment FIG. 1. In the embodiment illustrated in FIGS.
1-3 laser system 100 has a system housing 102 that encompasses,
substantially encloses, or otherwise retains, a laser module 104, a
drive circuit 106, a system controller 108, a user input system
110, sensors 112, a user output system 114, a communication system
116, and a power supply 118.
[0028] In this embodiment, system controller 108 receives signals
from user input system 110, sensors 112, and communication system
116 and determines whether a response to such signals is required.
When system controller 108 determines to respond to received
signals by causing a laser emission, system controller 108 sends
signals to drive circuit 106 causing drive circuit 106 to supply
electrical energy from power supply 118 to laser module 104 in a
manner that causes laser module 104 to emit a laser beam 122.
System controller 108 can also generate signals that cause user
output system 114 to generate a human perceptible output.
Additionally, system controller 108 can send signals to
communication system 116 causing communication system 116 to send
signals to other devices, to cause communication system 116 to
receive signals from other devices or both. Power supply 118
provides electrical energy to drive circuit 106, system controller
108, user input system 110, sensors 112, user output system 114,
and communication system 116. As is shown in FIGS. 1-3, in this
embodiment system housing 102 provides an enclosure for each of the
components of laser system 100 to provide an enclosed a stand-alone
device capable of laser emission.
[0029] System housing 102 can be formed of any of a variety of
rigid materials such as composites, laminates, plastics or metals.
In one configuration, system housing 102 can be formed of an
extruded aluminum, thereby providing sufficient strength without
requiring significant weight while also providing good thermal
transfer properties.
[0030] System housing 102 can be fabricated or assembled in any of
a variety of ways. In one embodiment, system housing 102 is
machined such as by EDM (electrical discharge machining),
assembled, or molded if composites, laminates, plastics or metals
are employed for system housing 102. System housing 102 also can be
fabricated using other conventional techniques including but not
limited to additive assembly techniques.
[0031] In FIGS. 1-3, system housing 102 is shown having a generally
cylindrical profile. However, in other exemplary embodiments,
system housing 102 may be configured to provide surfaces that
enable system housing 102 to be joined, fixed, held, mounted or
otherwise positioned for movement with other devices such as
hand-held weapon system 14 or to any of a variety of direct fire
weapons such as handheld, side, and small firearms. Such firearms
include, but are not limited to, pistols, rifles, shotguns,
automatic arms, semi-automatic arms, rocket launchers and select
grenade launchers bows. In other embodiments, system housing 102
can be configured to mount any known dismounted or dismounted
crew-served weapon, such as machine guns, artillery, recoilless
rifles and other types of crew served weapons.
[0032] In still other embodiments, system housing 102 can be
shaped, sized or otherwise provided in forms that more readily
interface with any of a variety of clamping or mounting mechanisms
such as a Weaver-style Picatinny rail or dove tail engagement for
mounting to these firearms. In further exemplary embodiments,
system housing 102 can be configured as a component part of a
hand-held weapon system 12 or other direct fire weapon, such as a
foregrip, sight or stock.
[0033] Drive circuit 106 receives power from power supply 118 and
control inputs from system controller 108. In response to the
control inputs received from system controller 108, drive circuit
106 generates signals that cause laser module 104 to emit laser
light. In the embodiment that is illustrated in FIG. 1 laser module
104 is not directly connected to power supply 118 but rather
receives power by way of drive circuit 106 such that drive circuit
106 can control the time, duration, and intensity of electrical
energy supplied to laser module 104. Drive circuit 106 may be
configured to assist in tuning and/or otherwise controlling the
output of laser module 104. Drive circuit 106 can be constructed to
provide either pulsed or continuous operation of laser module 104.
The rise/fall time of the pulse, compliance voltage and current
generated by drive circuit 106 for the laser module 104 are
selected based at least in part upon power consumption, heat
generation and desired beam intensity considerations. These
parameters may also be selected to cause laser module 104 to
produce a beam having a desirable wavelength, frequency, transverse
mode number and/or other quantifiable characteristics.
[0034] Depending on the desired output, drive circuit 106 can
enable operation of the laser module 104 as a continuous or pulsed
laser, such as by passive, active, or controlled switching.
Although specific values depend upon the particular laser module
104 and intended operating parameters, it is contemplated the peak
power draw of drive circuit 106 may be between approximately 1 amp
and approximately 10 amps, with an average current draw between
approximately 0.1 amps and approximately 1.0 amps. As the required
voltage may be between on average approximately 9 volts and
approximately 12 volts, approximately 0.9 W to approximately 12 W
may be consumed. This may represent a substantial power consumption
as well as heat generation.
[0035] In an exemplary embodiment, drive circuit 106 may assist in
controlling and/or modifying the power level of laser module 104 to
aid in penetrating components or conditions of the atmosphere
through which laser system 100 will direct laser beam 122. Such
components or conditions may include, for example, snow, rain, fog,
smoke, mist, clouds, wind, dust, gas, sand, and/or other known
atmospheric or airborne components. For example, drive circuit 106
can be configured to controllably, manually, and/or automatically
increase the current and/or voltage directed to strengthen and/or
intensify laser beam 122 emitted by laser module 104 in such
conditions.
[0036] It is also understood that laser module 104 can have more
than one semiconductor laser 180. In one exemplary embodiment of
this type, a laser module 104 can have one semiconductor laser 180
in the form of a mid-range adapted infrared quantum cascade laser
and another semiconductor laser 180 in the form of a long-range
adapted infrared quantum cascade laser. Other combinations of
semiconductor lasers 180 are possible.
[0037] Alternatively, in other embodiments, laser module 104 can
include components that can receive signals from drive circuit 106
and that can adjust power supplied to laser module 104 in response
to such signals. In such an alternative embodiment, laser module
104 may receive may receive electrical energy directly from power
supply 118.
[0038] In the embodiment illustrated in FIGS. 1-3 system housing
102 has plurality of openings shown as openings 120, 124, 126 and
128. In certain embodiments, seals 140, 142, 144, 146 can be
supplied to provide a barrier to resist entry of contaminants at
openings 120, 124, 126 and 128 so as to protect the components
disposed within system housing 102 from water, dust, vapors, or
other harmful contaminants commonly experienced in non-controlled
environment use. Optionally, system housing 102 can be hermetically
sealed, at least in part around laser module 104.
[0039] User input system 110 includes human operable sensors such
as switches, touch pads, joysticks, audio, video, keypads, key
locks, proximity sensors or any other known types of sensors that
can detect a user input action and that can provide signals to
system controller 108 indicative of the user input action. In the
embodiment of FIGS. 1-3, user input system 110 provides a switch
130 that takes the form of a four position mode switch with
different settings to enable manual selection of three different
operating mode selections and an off selection.
[0040] Sensors 112 can include any form of device that can be used
to detect or otherwise sense conditions inside or outside of system
housing 102 that may be useful to system controller 108 in
determining actions to be taken in operating laser system 100.
Sensors 112 can include without limitation, light sensors such as
photovoltaic cells, contact switches, opto-electronic sensors such
as light beam emitter and sensor pairs, electro-mechanical sensors
such as limit switches, strain sensors, and proximity sensors such
as Hall effect sensors, thermal sensors, meteorological sensors,
such as humidity sensors, accelerometers, orientation sensors and
other known sensors and transducers.
[0041] User output system 114 can include, without limitation
actuators, light emitters, video displays, or other sources of
human perceptible visual, audio or tactile signals from which a
user can determine for example, and without limitation, a status of
laser system 100, an operating mode of laser system 100, or that
laser system 100 is emitting a laser beam 122 and a characteristics
of the laser beam 122 that laser system 100 is emitting or will
emit when instructed to do so. In this embodiment, user output
system 114 includes a video display 132 that is positioned in
opening 128.
[0042] Communication system 116 can include any combination of
known communication circuits including wired or wireless
transponders, transceivers, transmitters, receivers, antennas,
modulators, de-modulators, encryption and de-encryption circuits or
software and can provide other known components to facilitate data
communication, the exchange of control signals or power exchanges
in wired or wireless form.
[0043] Power supply 118 is shown located within system housing 102.
In one configuration, power supply 118 comprises a battery and
system housing 102 can include a battery compartment (not shown)
sized to operably receive and retain a power supply 118 in the form
of batteries. Depending upon the anticipated power requirements,
available space, and weight restrictions, the batteries can be
N-type batteries or AA or AAA batteries. Additionally, a
lithium/manganese dioxide battery such as military battery
BA-5390/U, manufactured by Ultralife Batteries Inc. of Newark, N.Y.
can be used with laser system 100. The battery-type power supply
118 can be disposable or rechargeable. Battery compartment can be
formed of a weather resistant, resilient material such as plastic,
and shaped to include receptacles for receiving one or more
batteries or other power storage devices. Further, the battery
compartment may be selectively closeable or sealable to prevent
environmental migration into the compartment or to create a
hermetically sealed environment therein.
[0044] In other exemplary embodiments, power supply 118 can take
the form of a fuel cell, capacitive system or other portable
electrical energy storage or generation system. It is understood
that any type of power supply 118, preferably portable and
sufficiently small in size can be utilized.
[0045] As is noted above, system controller 108 drives operation of
laser system 100 and receives signals from user input system 110,
sensors 112 and communication system 116 that system controller 108
can use to control operation of laser system 100. System controller
108 comprise for example a computer, a microprocessor,
micro-controller, programmable analog logic device or a combination
of programmable or hardwired electronic devices capable of
performing the functions and actions described or claimed
herein.
[0046] In the embodiment of FIGS. 1-3 system controller 108
determines a mode of operation of laser system 100 in response to a
position of switch 130. When switch 130 is positioned in the "Off"
position, user input system 110 sends signals to system controller
108 causing system controller 108 to remain in an inactive state or
can maintain a low power consumption mode of operation.
[0047] However, when system controller 108 receives signals from
user input system 110 indicating that switch 130 has been moved to
the "On" position system controller 108 can generate signals
causing drive circuit 106 to drive laser module 104 to generate
laser light. In other embodiments, switch 130 can comprise a switch
that provides power to initiate operation of system controller 108
only when switch 130 is in a position other than the "Off"
position.
[0048] Other modes of operation are possible. For example a "Stand
By" mode of operation can be provided to conserve stored energy of
from power supply 118 while maintaining the laser system 100 in an
advanced state of readiness for use. For example, when switch 130
is moved to the "Stand By" position user input system 110 can send
signals to system controller 108 from which system controller 108
can determine that this mode of operation has been selected.
[0049] In one embodiment, system controller 108 can detect that
switch 130 has been moved to the "Stand By" position and can
respond to this by sending signals to drive circuit 106 causing
drive circuit 106 to begin supplying power circuits or subsystems,
if any, that require some time to reach a state where they are
ready for immediate activation when switch 130 is moved to the "On"
position. Not all circuits or subsystems will need be activated at
such times and a stand by option relieves the operator from being
confronted with the choice of operating the laser system 100 in a
high power consumption "On" mode prior to the need to do so and the
choice of holding the device in the "Off" state to conserve power
with the understanding that there will be a lag time before
activation.
[0050] Additionally, in the embodiment of FIGS. 1-3 switch 130 can
be positioned at a location that indicates that laser system 100 is
to be operated in a "Test" mode. In one example of this type system
controller 108 can cause laser module to emit a lower powered laser
beam 122. This lower powered laser beam can 122 be used to allow
verification of the operational status of laser system 100 such as
by emitting a lower powered laser test beam that can be directed
at, for example, nearby targets for training purposes or at target
strips or pages that change in appearance when illuminated by the
laser in the test mode. Here too, this mode will be entered when
system controller 108 receives a signal from user input system 110
indicating that switch 130 has been moved to a position selecting
the "Test" mode.
[0051] Turning now to FIG. 4 what is shown is a cross-section
schematic view of one embodiment of a laser module 104 taken as
shown in FIG. 1. In the embodiment that is illustrated in FIG. 4,
laser module 104 has a laser core 150 with a base 152 having a
front side 154 from which a header 156 extends in a first direction
160 and a housing 170 shaped to combine with front side 154 to form
a sealed environment about header 156.
[0052] A semiconductor laser 180 is mounted to header 156. In this
embodiment, semiconductor laser 180 is mounted to header 156 by way
of a submount 182 and is positioned to direct a divergent laser
light 184 in first direction 160 through a window 172 on a front
portion 174 of housing 170. Semiconductor laser 180 or submount 182
can be joined to header 156 in any of a variety of ways including
conventional fasteners, solders, conductive adhesives and the like.
Semiconductor laser 180 in turn is typically bound to submount 182
using soldering techniques, although other techniques are also
known.
[0053] Semiconductor laser 180 can comprise for example, any
semiconductor device that can emit a laser output. Examples of
semiconductor laser 180 include but are not limited to a diode
laser, quantum cascade lasers, inter-band cascade lasers. These
types of semiconductor lasers 180 share generally the
characteristics of being made from a semiconductor material and
having a emitting a divergent laser light beam while also
generating a meaningful amount of heat that must be dissipated to
protect semiconductor laser 180.
[0054] In the embodiment illustrated in FIG. 4, semiconductor laser
180 emits a divergent laser light 184 having a wavelength in the
infrared region such as between 2.mu. and 30.mu. wavelength.
However, in other embodiments, semiconductor laser 180 can emit a
divergent laser light 184 having any of a wide range of wavelengths
including but not limited to ultraviolet wavelengths, visible
wavelengths, and near infrared wavelengths. For the purposes of the
following discussion, it will be assumed that in the embodiment of
FIG. 4, semiconductor laser 180 is a quantum cascade type
laser.
[0055] A frame 200 is joined to base 152 and extends from base 152
past window 172 to position a lens 210 at a distance along axis 162
from semiconductor laser 180. In operation, semiconductor laser 180
generates a divergent laser light 184 which is directed toward lens
210.
[0056] Lens 210 collimates the divergent laser light 184 from
semiconductor laser 180 into a laser beam 122 when positioned at a
location where lens 210 can effectively focus light from
semiconductor laser 180. As used herein a laser beam 122 includes a
laser beam that is fully collimated as well as laser beams having
substantial collimation with a limited allowable divergence.
[0057] In general, lens 210 controls the field of illumination
provided by divergent laser light 184. This field of illumination
can be narrow so as to concentrate divergent laser light 184 to
create a field of illumination at a distant target or it can be
made even more narrow to provide pointing, marking or designation
spots of high intensity. Lens 210 can comprise one or more lenses
and lens systems and can be adjustable between multiple
configurations to provide different degrees of collimation.
[0058] Lens 210 is most effective when held within a preferred
range of positions from semiconductor laser 180. However, in
practical use this is difficult to achieve with a static lens
mounting design. In particular it will be understood that a variety
of forces can conspire to influence the distance that a mechanical
system such as frame 200 will position lens 210. Chief among these
are the forces of thermal expansion and contraction which can cause
significant changes in the length of components of frame 200 and
the resultant position of lens 210 relative to semiconductor laser
180.
[0059] In this embodiment, frame 200 is optionally of an
athermalized design meaning that frame 200 is designed so that
frame 200 will hold lens 210 in a desirable range of positions
relative to semiconductor laser 180 despite any thermal expansion
or contraction of any components of frame 200 that may arise during
transport and operation of laser system 100. Such systems do not
seek to completely resist or prevent heating or cooling of frame
200, but rather are defined to provide mechanisms to allow for
automatic compensation for any thermal expansion caused by such
heating or cooling.
[0060] Optionally, frame 200 can be configured to allow a user to
adjust the degree of collimation of divergent laser light 184 so as
to form a beam output 122 having a divergence that is within a
range of divergences. This adjustability can allow laser system 100
to be used for a variety of functions including but not limited to
illuminating a relatively nearby field of view and a relatively
distant field of view, and proving a highly collimated beam for
designating, marking or pointing purposes.
[0061] As is shown in FIG. 5, laser beam 122 from laser system 100
illuminates a field of illumination 237 and light 240 can reflect
therefrom in a specular fashion or by way of scattered reflection,
be absorbed thereby or be absorbed and re-emitted thereby 242.
Additionally, field of view 238 may have portions thereof that emit
or reflect light 244 other than that provided by laser beam 122 and
that are at or near wavelengths to those emitted by the illuminator
or that can otherwise be sensed by a thermal imaging system such as
thermal imaging system 248.
[0062] FIG. 5 one exemplary embodiment of a thermal imaging system
248 having an optional lens system 250 and a thermal imager 252.
Lens system 250 focuses light from a field of view 238 to form an
image onto thermal imager 252. Thermal imager 252 is configured to
sense a range of wavelengths of light including wavelengths of
light emitted by laser system 100. As is illustrated here, field of
illumination 237 and field of view 238 at least in part
overlap.
[0063] Thermal imager 252 may be any device or combination of
devices configured to receive such reflected light 240, re-emitted
light 242 and other light 244. Conventionally, thermal imager 252
has an imaging surface 254 with an array of radiation sensors 256.
In a typical configuration, individual radiation sensors 256 are
each capable of generating a signal that is representative of an
amount of radiation incident on the radiation sensor 256 within a
period of time known as an integration time. In one embodiment,
thermal imager 252 may comprise an array of radiation sensors 256
in the form of microbolometers or other like sensors. In other
embodiments, radiation sensor may 254 may comprise any type of
known semiconductor image sensing array such as specially doped
CMOS image sensors. Other known image sensing technologies that can
be used to determine the amount of radiation incident at a
plurality of positions on a focal plane can be used.
[0064] Typically, radiation sensors 256 generate an analog output
signal. The analog output of each is optionally amplified by an
analog amplifier (not shown) and analog processed by an analog
signal processor 264 to reduce any output amplifier noise of image
sensor 252. The output of analog signal processor 264 is converted
to a captured digital image signal by an analog-to-digital (A/D)
converter 266.
[0065] The digitized image signal is optionally temporarily stored
in a memory 270, and is then processed using a programmable digital
signal processor 272. Digital signal processor 272 creates digital
images of the field of view 238. These digital images can be
adapted for display on, for example, a viewfinder display 274 or
other exterior display 276. Viewfinder display 274 and exterior
display 276 can comprise, for example, a color liquid crystal
display (LCD), organic light emitting display (OLED) also known as
an organic electroluminescent display (OELD) or other type of video
display or any other known form of video image display.
Alternatively, a communication system 280 can be used to send the
digital images to an external device 282 such as a wirelessly
connected viewfinder, a targeting system, remote signal analysis
systems or viewing or control equipment at a remote command and
control center.
[0066] Optionally, digital signal processor 272 uses the initial
images to create archival images of the scene. Archival images are
typically high resolution images suitable for storage,
reproduction, and sharing. Archival images are optionally
compressed using the PEG standard and stored in a data memory
278.
[0067] In operation, control system 290 sends signals to a timing
generator 292 indicating that images are to be captured. Timing
generator 292 can provide signals that can be used by various
elements of thermal imaging system 248 to control image capture,
digital conversion, compression, and storage operations. Thermal
imager 252 is optionally driven from timing generator 292 by way of
an image sensor driver 294. Control system 290, timing generator
292 and image sensor driver 294 cooperate to cause image sensor 252
to determine an amount of radiation incident on each of radiation
sensors 256 across an integration time that is either fixed or
variable. After the integration time is complete an image signal is
provided to analog signal processor 264 having analog signals
indicative of the radiation sensed at each of radiation sensors 264
during the integration time. These analog signals are processed as
described in greater detail above.
[0068] The ability of a radiation sensor 256 to generate a signal
that is representative of the amount of radiation incident on the
radiation sensors during an integration time is not infinite.
Instead, the ability of a radiation sensor 256 to sense radiation
is limited by a lower response threshold and an upper response
threshold. The lower response threshold can be, for example, an
exposure level at which the inherent signal to noise properties of
a radiation sensor 256 and the electronic circuitry designed to
extract signal information from radiation sensor 256 approaches a
threshold signal to noise ratio of the exposing radiation.
Accordingly, when radiation sensor 256 is exposed to radiation that
is below the lower response threshold, it becomes difficult to
ensure that the signal received from the radiation sensor 264
accurately represents the relative intensity of radiation incident
on the sensor within the integration time.
[0069] Similarly, the upper response threshold is the light
exposure level where it becomes difficult to ensure that the signal
received from a radiation sensor 256 accurately represents the
relative intensity of the radiation incident on radiation sensor
256 within the integration time.
[0070] It will be appreciated that more image detail can be
visually obtained from a captured image that includes large
contrast differences. Such large contrast differences are lost
however, when an image includes a large proportion of image
information from radiation sensors 256 that have been exposed to
light above the upper threshold or below the lower threshold.
Accordingly, integration times are typically adjusted to help
ensure that radiation sensors 264 are exposed to radiation that is
generally between a lower threshold and an upper threshold for the
radiation sensors. However, in some low radiation scenes, such as
at night there may be insufficient ambient illumination to allow
image capture without a signal to noise ratio in the image that is
too high to allow for accurate observation of a field of view.
Illumination of the field of view is therefore required.
[0071] Lasers are appropriate for illumination purposes
particularly where it is desirable to project illumination at
distance down range and within controllable wavelengths. However
lasers themselves may introduce noise into the field of view. Of
particular concern, is a condition known as speckle. Speckle arises
when coherent light reflects from more than one different point in
a field of view 238 in a manner that coherently combines at a point
of observation.
[0072] FIG. 6 illustrates one hypothetical example of a speckle
pattern that may exist in a field of view of a sensor 252. Speckle
is typically observed as a pattern 310 of darker spots such as
darker spot 312 and lighter spot such as lighter spot 314 in a
uniformly laser illuminated field of view 238 having at least one
rough surface. Typically, the speckle pattern 310 for a given field
of view is generally static while observation and illumination
remain constant.
[0073] It will be appreciated that such a pattern 310 of speckle
can make it particularly difficult to determine the differences
between contrast patterns in the image that are a product of the
objects in field of view 238 and contrast patterns in the image
that are a product of speckle.
[0074] However, laser system 100, unlike the illuminators of the
prior art is adapted to operate in a manner that reduces the
appearance of speckle in the field of view 238. FIG. 7 illustrates
a flow chart of a method by which this is done.
[0075] As is shown in FIG. 7, in this embodiment, an integration
time is determined for an imaging system 248 that is to be used
with laser system 100 (step 320). In certain embodiments, the
integration time can be predetermined such as where it is known
that laser system 100 will be used with a specific imaging system
248 or where it is known that laser system 100 will be used under
certain conditions that require a specific integration time. The
integration time or parameters that may be related to the
integration can also be user entered or selected by way of user
input system 110. In other embodiments, integration time can be
determined automatically by communication between laser system 100
and imaging system 248. For example, communication system 280 of
imaging system 248 can communicate with communication system 116 so
control system 290 can provide data from which an integration time
used by thermal imaging system 248 can be determined.
[0076] A first current is then supplied to semiconductor laser 180
sufficient to cause semiconductor laser 180 to emit a beam of laser
light 184 having a first transverse mode structure (step 322).
[0077] The current applied to semiconductor laser 180 is then
modulated across a range of current levels during the determined
integration time (step 326). The modulation of the current is
determined so that semiconductor laser 180 will emit light having a
first transverse mode structure during a first portion of the range
of current levels and a second transverse mode structure during a
second portion of the range of current levels. A change in
transverse mode structure may take the form of a change in the
number of transverse modes or the relative portion of the overall
intensity of a laser beam 122 formed by individual ones of more
than one simultaneously emitted transverse modes. Laser beam 122
has an angular emission profile that is a function of the
transverse mode structure. The direction of higher intensity
emissions in the angular emission profile change with the
transverse mode structure. This changes the relative angle of
incidence of an illuminating light on the field of illumination 237
shifting the speckle pattern as will now be described in greater
detail with reference to FIGS. 8-10.
[0078] FIG. 8 shows a first example of a plurality of radiation
sensors 256a-256k on which a portion 316a of speckle pattern 310 is
formed as shown in FIG. 6. Portion 316a represented in FIG. 8 by
numerical pattern of positive and negative numbers. This pattern of
positive and negative numbers are representative of the intensity
variation at each radiation sensors 256a-256k due to portion 316a
of speckle pattern 310. Positive numbers are used to denote
radiation sensors 256 on which coherent light from field of view
238 combines to increase the amount of light incident on a
radiation sensor 256 during an integration time. Negative numbers
are used to denote radiation sensors 256 on which coherent light
from field of view 238 combines to decrease the amount of light
incident on a radiation sensor 256 during an integration time.
Different integers are used to represent potential intensity
differences in the light sensed by radiation sensors 256a-256k
during an integration time caused by the speckle.
[0079] As is also shown in FIG. 9, when the current supplied
semiconductor laser 180 transitions from the first range of current
levels to the second range of current levels semiconductor laser
180 generates a divergent laser light 184 having a different
transverse mode structure. This causes the speckle pattern 310 to
shift, in this embodiment to the right, such that portion 316a is
repositioned as shown as portion as illustrated in FIG. 9.
[0080] As is shown in FIG. 10, this shift has a number of effects
net effects over the integration time. For example, radiation
sensors 256b and 256c, 256j and 256k experience a modest increase
in sensed radiation over the integration time while radiation
sensors 256e and 256h experience a modest decrease in sensed
radiation. Radiation sensors 256f and 256g experience a modest net
increase in sensed radiation despite receiving light at a bright
spot in the portion 316a/b of speckle pattern 310 at least during
part of the integration time.
[0081] By way of comparison, FIG. 11 illustrates the net effects of
portion 316a of speckle pattern 310 on radiation sensors 256a-256k
using a prior art laser illumination system that does not provide
the changing the transverse mode structure. As can be seen from
this, portion 316a causes a speckle pattern over the integration
time that has much higher absolute intensities as well as having
greater relative differences between brighter areas and darker
areas such as between radiation sensor 256f and 256g.
[0082] It will be appreciated that by shifting the structure of
transverse modes in laser beam 120 illuminating at least a part of
field of view 238 during the integration time of imaging system 248
the impact of speckle is averaged across multiple radiation sensors
and the relative impact of speckle is greatly reduced.
[0083] In some embodiments, a ridge width or distance between
transverse sidewalls of an active region in a semiconductor laser
180 is selected to provide transverse mode structures that are
different when energized in at least two different ranges of
current in order to provide the desired shift. For example, a ridge
with a width of between about 1 and 2 wavelengths of a light
emitted as a laser beam 122 by semiconductor laser 180 can be used
for this purpose.
[0084] Additionally, the selection insulating material adjacent to
transverse sidewalls of an active region in a semiconductor laser
180 is used to facilitate transitions in transverse mode structures
to provide the desired shift. For example, semi-insulating
materials such as indium phosphide can be used to achieve
transverse mode structures having a first characteristics, while
di-electric materials such as silicon dioxide or silicon nitride
can be used to achieve transverse mode structures having second
characteristics that are different from the first characteristics.
In some embodiments the use of di-electric materials can more
effectively lead to a change in transverse mode structure than the
use of semi-insulating materials such as indium phosphide on
semiconductor lasers having equal ridge widths.
[0085] In addition, the thickness of the semi-insulating material
in the presence of a metal or a semi-conductor on the outside of
the semi-insulating material can also be used to influence the
characteristics of the transverse mode structure.
[0086] Returning to FIG. 7 it will be appreciated that the method
can include the optional step of determining a modulation function
(step 324). The modulation function can define the amplitude and
time rate of change of the current supplied to semiconductor laser
180. The modulation function can also define a shape of the
waveform used in modulation. In some embodiments, the modulation
frequency used to apply current into semiconductor laser 180 may
induce temperature changes within semiconductor laser 180 such that
different transverse mode structures can be reached based as a
product of heating and cooling of the semiconductor laser 180
within the integration time. Such heating and cooling may occur to
a greater extent in response to lower frequency modulation while
occurring to a lesser extent in response to higher frequency
modulation.
[0087] As is noted above, semiconductor laser 180 can take the form
of a laser that emits light between 2 um and 30 um. However,
specific wavelengths of light within this range may be particularly
useful for achieving desired results such as illumination over a
particular ranges. Conventionally selection of a semiconductor
laser 180 for use in illuminating applications and in particular in
illuminating applications over particular ranges has been made
based primarily on the transmission efficiency of the laser in
expected environmental conditions.
[0088] This would suggest that different types of semiconductor
lasers 180 emitting different wavelengths are necessary for similar
applications and that the selection of semiconductor laser 180
should be made based upon anticipated use cases. This leads to
unnecessary redundancy, reduced overall performance and
expense.
[0089] Instead, the inventor has discovered that a system approach
to selecting semiconductor lasers 180 for use in applications such
as long range illumination can yield superior results. In
particular, the inventor notes that the fundamental upper power
limit of semiconductor lasers 180 having certain wavelengths is
greater than that of semiconductor lasers 180 having other
wavelengths. For example, the fundamental upper power limit of
semiconductor lasers 180 having a wavelength of about 4.0 um is
substantially lower than the fundamental upper power limit of
semiconductor lasers having a wavelength of greater than 4.6 um.
This power advantage can offset or nearly offset the efficiency
advantages provided by lower powered semiconductor lasers 180 at
all but the most extreme environmental conditions. Additionally in
some circumstances efficiency advantages of lower powered laser
system can be completely offset where, for example, sensing
equipment such as imaging systems 248 are more sensitive to
wavelengths that are greater than about 4.6 um than other
wavelengths.
[0090] It will be appreciated from this that selection of
semiconductor lasers 180 for particular applications or groups of
applications can be based upon the response of imaging system 248
including optics such as lens system 250, or filters or other
optical components, the atmospheric transmission characteristics,
and the output power of semiconductor laser 180.
[0091] The drawings provided herein may be to scale for specific
embodiments however, unless stated otherwise these drawings may not
be to scale for all embodiments. All block arrow representations of
heat flow are exemplary of potential thermal patterns and are not
limiting except as expressly stated herein.
[0092] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *