US20140346358A1 - Quantum cascade laser suitable for portable applications - Google Patents
Quantum cascade laser suitable for portable applications Download PDFInfo
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- US20140346358A1 US20140346358A1 US13/734,150 US201313734150A US2014346358A1 US 20140346358 A1 US20140346358 A1 US 20140346358A1 US 201313734150 A US201313734150 A US 201313734150A US 2014346358 A1 US2014346358 A1 US 2014346358A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/30—Structure or shape of the active region; Materials used for the active region
- H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
- H01S5/3401—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers
- H01S5/3402—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers intersubband lasers, e.g. transitions within the conduction or valence bands
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F41—WEAPONS
- F41G—WEAPON SIGHTS; AIMING
- F41G1/00—Sighting devices
- F41G1/32—Night sights, e.g. luminescent
- F41G1/34—Night sights, e.g. luminescent combined with light source, e.g. spot light
- F41G1/36—Night sights, e.g. luminescent combined with light source, e.g. spot light with infrared light source
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
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- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/02—Structural details or components not essential to laser action
- H01S5/022—Mountings; Housings
- H01S5/02208—Mountings; Housings characterised by the shape of the housings
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
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- H01S5/00—Semiconductor lasers
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
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- H01S5/00—Semiconductor lasers
- H01S5/005—Optical components external to the laser cavity, specially adapted therefor, e.g. for homogenisation or merging of the beams or for manipulating laser pulses, e.g. pulse shaping
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
- H01S5/042—Electrical excitation ; Circuits therefor
- H01S5/0428—Electrical excitation ; Circuits therefor for applying pulses to the laser
Definitions
- the present invention relates to laser systems and, in particular, to compact lasers suitable for military, handheld weapon targeting systems, for example.
- Laser targeting systems may use a lightweight laser mounted to a gun so that its beam is generally aligned with the bore of the gun. In this position, the laser may project a small illuminated spot on the target assisting in alignment of the weapon. In order for the spot to be seen in bright environments and to project for an adequate distance, a continuous wave solid-state laser of high power is normally employed.
- the present applicants have recognized that a narrow operating range exists in which an intermittently operated quantum cascade laser can provide sufficient brightness for a laser sight or other similar application without active cooling. Further, power sensing feedback regulation can be used to maintain the quantum cascade laser precisely within this operating region. Eliminating the active cooling allows the device to be powered through low voltage, low ampere-hour batteries (suitable for portable use) as boosted with a high efficiency boost converter.
- embodiments of the present invention provide a low power consumption quantum cascade laser having a housing containing a passive cooling element and a solid-state quantum cascade laser element thermally attached to the passive cooling element.
- a window through the housing and aligned with the solid-state quantum cascade laser element allows a laser beam to pass out of the housing from the solid-state quantum cascade laser element.
- a quantum cascade laser driver circuit receives electrical power from a battery and provides a set of feedback-controlled pulses to the solid-state quantum cascade laser element having a duty cycle of less than 50% allowing operation of the quantum cascade laser without active cooling while providing high peak power pulses.
- the driver circuit may sense current to the quantum cascade laser so that the current to the quantum cascade laser is precisely regulated.
- the driver circuit may control the pulses based on a direct sensing of optical output of the quantum cascade laser.
- the quantum cascade laser driver circuit may provide a DC-to-DC boost converter providing power to the quantum cascade laser element, where the duty cycle of the boost converter is controlled by feedback measuring the power consumed by the quantum cascade laser element.
- the passive cooling element may include portions of the housing.
- the housing may be airtight.
- the battery source may provide a voltage of less than 9 V.
- the volume of the housing may be less than 10 cm.sup.3.
- FIG. 1 is a perspective view of a rifle including a laser sight in accordance with an embodiment of the present invention using a quantum cascade laser element;
- FIG. 2 is a simplified diagram of the quantum cascade laser element used in the laser in accordance with an embodiment of the present invention showing generation of stimulated emissions by cascaded electron intraband transitions and tunneling;
- FIG. 3 is a plot of applied voltage and duty cycle of power applied to the quantum cascade laser element superimposed on a passive cooling boundary and an open-loop quantum cascade power output range for a given voltage showing a safe operating zone for the quantum cascade laser exploited by an embodiment of the present invention
- FIG. 4 a is a cross-sectional view of the housing of a laser sight of FIG. 1 showing a mounting system for the quantum cascade laser for passive cooling as held within the housing and showing the positioning of quantum cascade driving circuitry in accordance with an embodiment of the present invention
- FIG. 4 b is a fragmentary cross-sectional view similar to that of FIG. 4 a showing another embodiment with self-contained batteries;
- FIG. 5 is a block diagram of the quantum cascade driving circuit of FIG. 2 providing current sensing to deduce quantum cascade power output for direct control of a DC-to-DC boost converter in accordance with an embodiment of the present invention
- FIG. 6 is a block diagram similar to that of FIG. 4 showing another embodiment providing optical power sensing to deduce of quantum cascade power output;
- FIG. 7 is a perspective view of a handheld remote laser spectroscopy system such as may make use of the laser in accordance with an embodiment of the present invention.
- quantum cascade laser A possible substitute for a conventional laser diode in this application is the so-called quantum cascade laser.
- the quantum cascade laser allows the development of sub bands with lower energy differences suitable for producing long wavelengths of infrared light. Because quantum cascade lasers do not rely on electron/hole annihilation to produce photons, multiple photons may be created for each electron providing high light power output. Such quantum cascade lasers require relatively high operating voltages and dissipate substantial heat. This heat ultimately inhibits the lasing action (reducing the number of photons per electron) and in the extreme can damage the device. For this reason it is known to use active cooling of the quantum cascade laser, for example with an electrically powered Peltier device and fan combination.
- a rifle 10 may provide a thermal imaging device 12 of a type known in the art for allowing a user of the rifle 10 to sight along the barrel 14 of rifle and view an image in the far infrared range (thermal image).
- a battery-powered quantum cascade laser 16 in accordance with an embodiment of the present invention may be attached to the rifle 10 by means of mount 18 to provide a laser beam 21 aligned along the barrel 14 to assist in targeting the rifle 10 by projecting an invisible infrared spot on a target.
- a quantum cascade laser element 26 comprising a stack 13 of semiconductor layers 15 separated by barrier layers 17 , the latter preventing classical electron flow.
- the number and types of layers shown in FIG. 2 are greatly reduced for clarity.
- the stack 13 extends generally along an axis 22 with each of the layers 15 and 17 extending in a plane generally perpendicular to the axis 22 .
- the outer layers 15 are attached to electrodes 23 and 23 ′ which may be biased with an electrical voltage source from a driver circuit 25 to provide a relatively negative voltage at the top of the stack 13 at electrode 23 .
- This negative voltage provides a source of electrons 33 that are drawn to a relatively positive voltage at the bottom of the stack 13 at electrode 23 ′.
- the barrier layers 17 surrounding each semiconductor layer 15 provide a high dielectric electrical insulation that creates quantum well 27 shown in a potential energy line 31 , the latter depicting generally the energy required for an electron 33 to move along axis 22 .
- the potential energy line 31 decreases generally from the top of the stack 13 to the bottom of the stack 13 (per the applied voltage) but includes right extending peaks (as depicted) representing the electron barriers formed by the barrier layers 17 and thus the walls of the quantum wells 27 .
- the quantum wells 27 confine electrons 33 into thin planar regions to create sub bands 29 of permissible electron energy states for the electrons 33 within each quantum well 2 . These sub bands differ from the normal energy bands of a bulk semiconductor with the energy of the sub bands 29 largely dictated by the geometry of the quantum well 27 rather than the material properties alone.
- electrons 33 pass from electrode 23 to electrode 23 ′ moving from quantum well 27 to quantum well 27 by tunneling through the barrier layers 17 .
- the electrons 33 drop from higher subbands 29 to lower subbands 29 (intraband transition) resulting in the emission of photons 37 along an axis 24 perpendicular to axis 22 .
- the difference in energy between higher and lower subbands 29 can produce light in the range of 3-10 microns or greater in each of the cascaded intraband transitions generating multiple photons per electron.
- the intraband transition and tunneling process should be distinguished from classic electron/hole pair combinations found in conventional semiconductor laser diodes because an electron/hole pair is not extinguished upon the formation of the photon 37 in a quantum cascade laser 16 .
- the vertical edges of the stack 13 provide partially reflective surfaces to create a laser cavity allowing the stimulated emission of photons 37 as a laser beam 21 .
- the quantum cascade laser element 26 requires a lasing threshold voltage 50 of approximately 20 volts to begin laser action as shown on the vertical axis.
- This voltage is substantially higher than conventional solid-state lasers and is a natural result of the physics of the device which requires the passage of electrons in cascade through a series of quantum wells of different voltages.
- batteries having sufficient amp-hour capacity and a voltage of 20 volts or higher are unsuitably heavy or bulky for a hand-held military weapon.
- the average power consumption of the quantum cascade laser element 26 may be controlled by pulsed operation at a duty cycle shown on the horizontal axis.
- a duty cycle above a predetermined duty cycle threshold 52 is required, however, in order to provide sufficient output power for remote ranging, that is, to provide sufficient reflected power from a distant projected spot.
- a passive cooling boundary 56 describes the maximum power that may be dissipated by the quantum cascade laser element 26 with passive cooling without damaging or significantly degrading the quantum cascade laser element when operating at expected ambient operating temperatures.
- the passive cooling boundary 56 describes the upper bounds of the safe zone 58 .
- the passive cooling boundary 56 It is generally possible for the passive cooling boundary 56 to be below the lower bounds of the safe zone 58 defined by the lasing threshold voltage 50 and duty cycle threshold 52 ; however, the present inventors have determined that optimizing the passive cooling by reducing the thermal resistance between the quantum cascade laser element 26 and the ambient air, a small but finite safe zone 58 for practical form factors is created.
- an embodiment of the present invention provides power-sensing regulation that ensures operation within sub band 60 .
- the quantum cascade laser 16 may include a tubular housing 20 preferably formed from a thermally conductive material such as aluminum or the like. Within the housing, the quantum cascade laser element 26 may be attached to a passive heatsink 28 so that its axis 24 is aligned generally with a central axis 24 of the tubular housing 20 . A first end of the tubular housing 20 provides a window form by a lens system 35 allowing exit and collimation of the laser beam 21 formed by the photons 37 .
- the passive heatsink 28 is attached to the material of the housing 20 to conduct heat from the quantum cascade laser element 26 into the thermal mass of the passive heatsink 28 and then into the housing 20 .
- Power to the quantum cascade laser is provided through a copper conductor 30 passing to a first and second printed circuit board 32 containing driver circuits 25 to power the quantum cascade laser element 26 .
- Return power is conducted by means of a conductor attached to the housing 20 (not shown) and to the electrically conductive copper of the passive heatsink 28 .
- the end of the tubular housing 20 opposite the lens system 35 may be covered with a threaded end cap 34 to provide an airtight inner volume of the housing 20 between the lens system 35 and the threaded end cap 34 .
- the volume may, for example, be less than 10 cubic centimeters.
- Leads 36 providing power to the printed circuit boards 32 may pass through a sealed opening in threaded end cap 34 to connect to a battery 41 that may be held by the user and that provides power to both the laser 16 and the thermal imaging device 12 .
- the threaded end cap 34 may be replaced with a threaded battery housing 38 supporting for example two, 3 volt lithium ion batteries held in series connection by a battery holder 40 having leads 42 joining connector 44 that may connect to the printed circuit boards 32 .
- the quantum cascade driver circuit 25 described above may accept low-voltage electrical power, for example 6 V from batteries 41 , and provide that power to a boost converter circuit 64 .
- the boost converter circuit 64 includes a control circuit 66 switching on and off a solid-state switch 78 connected in series with an inductor 70 .
- the other end of the inductor 70 is connected to the low voltage electrical power and the other end of the switch 68 is connected to ground.
- the control circuit 66 operates to charge the inductor 70 by periodically switching on the solid-state switch 68 , and then to turn off the switch 68 to generate an inductively driven pulse 72 of voltage higher than the voltage of batteries 41 at the junction of the solid-state switch 78 and inductor 70 .
- the voltage level of pulses 72 will depend in part on the duration of on-time of the solid-state switch 68 and thus the amount of charging of the inductor 70 .
- the pulses 72 are conducted through a diode 74 to a capacitor 76 , the latter filtering the pulses to provide a source of DC voltage whose level is determined by the circuit 66 . This voltage will be above the lasing threshold voltage 50 .
- the DC voltage is in turn provided to one terminal of quantum cascade laser element 26 .
- the second terminal of the quantum cascade laser element 26 is connected in series with a second solid-state switch 78 controlled by a pulsing circuit 80 .
- the pulsing circuit 80 provides pulses having a duty cycle above predetermined duty cycle threshold 52 .
- the voltage of the pulses 72 must at a minimum be above the threshold voltage 50 described above but can be controlled in amplitude as will be described.
- the pulses will have 100 ns to 1 .mu.s duration with a maximum duty cycle of 25% and will occur at approximately 500 kHz in frequency.
- amplifier 82 completes a feedback loop to control the voltage of the pulses 72 , and thus the voltage being applied to the quantum cascade laser element 26 , to produce a given current flow and hence power dissipation in the quantum cascade laser element 26 .
- the gain of amplifier 82 is set to adjust the operating conditions of the quantum cascade laser element 26 to be held within sub band 60 of FIG. 6 and thus within safe zone 58 .
- the driver circuit 25 may also include a clamp circuit 91 receiving voltage at the cathode of diode 74 and controlling the feedback signal to control circuit 66 to limit the maximum voltage of the pulses 72 for safety, and further may provide for a timer limiting the maximum on time of the quantum cascade laser element 26 to comport with the heat sinking capacity of the passive heatsink 28 in a particular environment.
- the clamp circuit 91 may employ a solid-state switch switching at a predetermined threshold voltage as will be understood in the art.
- the sensing resistor 81 is eliminated in favor of an optical detector 84 (such as a photodiode) receiving a portion 21 ′ of the laser beam 21 , for example, as diverted by beam splitter 88 .
- the optical detector 84 may thus provide a direct measurement of the power output by the quantum cascade laser element 26 indirectly determining its power dissipation.
- the signal from the optical detector 84 is received by amplifier 82 which provides a control signal to circuit 66 .
- a temperature sensing element 89 such as a thermistor, may be included to sense the actual laser temperature and provide a feedback signal that is summed at amplifier 82 to increase or decrease the current set-point as required for different laser temperatures. Otherwise the driver circuit 25 may be unchanged.
- the ability to provide a highly portable, high powered, infrared source permits the construction of a handheld remote spectroscope 90 in which the quantum cascade laser 16 is attached to the housing of the spectroscope 90 having a handgrip 92 and activation button 94 . By pressing the activation button, the quantum cascade laser 16 is activated.
- an imaging device 12 thermal, near infrared, or visible
- the imaging device 12 is used to view and analyze signature emissions 98 that occur when the laser beam 21 illuminates an unknown gas sample 96 .
- the emission wavelength of the source is equal to the wavelength of absorption for the gas, the gas will absorb the emission and the spectroscope will detect this difference.
- a device providing qualitative or quantitative system to detect hazardous materials may thus be constructed.
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Abstract
Description
- This application is a continuation of U.S. application Ser. No. 13/455,761 filed on Apr. 25, 2012 and entitled QUANTUM CASCADE LASER SUITABLE FOR PORTABLE APPLICATIONS, which is currently pending; U.S. application Ser. No. 12/782,509 filed on May 18, 2010 and entitled “QUANTUM CASCADE LASER SUITABLE FOR PORTABLE APPLICATIONS” now issued under U.S. Pat. No. 8,189,630 on May 29, 2012; U.S. application Ser. No. 12/046,353 filed on Mar. 11, 2008, and entitled “QUANTUM CASCADE LASER SUITABLE FOR PORTABLE APPLICATIONS” now issued under U.S. Pat. No. 7,920,608B2 on Apr. 5, 2011, and this application claims priority under 35 U.S.C. §119 (e) of U.S. Provisional Application Ser. No. 60/934,952 filed Mar. 12, 2007, and of U.S. Provisional Application Ser. No. 61/035,283 filed Mar. 10, 2008. As far as is permitted, the contents of U.S. application Ser. No. 12/782,509; U.S. application Ser. No. 12/046,353; U.S. Provisional Application Ser. No. 60/934,952; and U.S. Provisional Application Ser. No. 61/035,283; each of which is incorporated herein by reference.
- The present invention relates to laser systems and, in particular, to compact lasers suitable for military, handheld weapon targeting systems, for example.
- Laser targeting systems, often called laser sights, may use a lightweight laser mounted to a gun so that its beam is generally aligned with the bore of the gun. In this position, the laser may project a small illuminated spot on the target assisting in alignment of the weapon. In order for the spot to be seen in bright environments and to project for an adequate distance, a continuous wave solid-state laser of high power is normally employed.
- In a military application, the projection of visible light may provide advanced notice to the target and may in fact reveal the location of a soldier holding the laser-sighted weapon. For this reason, there is considerable interest in the use of infrared laser sight systems that are only visible using far-infrared imaging systems. Unfortunately, conventional solid-state laser diodes have insufficient output in the desired infrared region.
- The present applicants have recognized that a narrow operating range exists in which an intermittently operated quantum cascade laser can provide sufficient brightness for a laser sight or other similar application without active cooling. Further, power sensing feedback regulation can be used to maintain the quantum cascade laser precisely within this operating region. Eliminating the active cooling allows the device to be powered through low voltage, low ampere-hour batteries (suitable for portable use) as boosted with a high efficiency boost converter.
- Specifically, embodiments of the present invention provide a low power consumption quantum cascade laser having a housing containing a passive cooling element and a solid-state quantum cascade laser element thermally attached to the passive cooling element. A window through the housing and aligned with the solid-state quantum cascade laser element allows a laser beam to pass out of the housing from the solid-state quantum cascade laser element. A quantum cascade laser driver circuit receives electrical power from a battery and provides a set of feedback-controlled pulses to the solid-state quantum cascade laser element having a duty cycle of less than 50% allowing operation of the quantum cascade laser without active cooling while providing high peak power pulses.
- Thus, it is a feature of an embodiment of the invention to permit the use of a quantum cascade laser in applications requiring a high degree of portability.
- The driver circuit may sense current to the quantum cascade laser so that the current to the quantum cascade laser is precisely regulated.
- It is thus a feature of an embodiment of the invention to continuously monitor and dynamically adjust the power to the quantum cascade laser to hold it within a narrow operating range allowing passive cooling. Current sensing provides a proxy for the power consumed by the quantum cascade laser.
- Alternatively, the driver circuit may control the pulses based on a direct sensing of optical output of the quantum cascade laser.
- It is thus a feature of an embodiment of the invention to allow the operating condition of the quantum cascade laser to be deduced directly from its optical output.
- The quantum cascade laser driver circuit may provide a DC-to-DC boost converter providing power to the quantum cascade laser element, where the duty cycle of the boost converter is controlled by feedback measuring the power consumed by the quantum cascade laser element.
- It is thus a feature of an embodiment of the invention to minimize power loss in the feedback circuitry necessary to hold the quantum cascade laser in its safe operating mode. By employing the same circuitry used to boost the battery voltages to regulate power to the quantum cascade laser additional circuitry losses are eliminated.
- The passive cooling element may include portions of the housing.
- It is thus a feature of an embodiment of the invention to maximize the passive cooling that may be obtained in a small form-factor.
- The housing may be airtight.
- It is thus a feature of an embodiment of the invention to provide a rugged device that may be used in abusive environments.
- The battery source may provide a voltage of less than 9 V.
- It is thus a feature of an embodiment of the invention to allow the use of smaller low voltage batteries having a total voltage less than the operating voltage of the quantum cascade laser.
- The volume of the housing may be less than 10 cm.sup.3.
- It is thus a feature of an embodiment of the invention that it may be used to produce quantum cascade lasers suitable for highly portable sensing and ranging applications including laser sights or portable gas spectroscopy systems.
- These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of an embodiment of the invention.
-
FIG. 1 is a perspective view of a rifle including a laser sight in accordance with an embodiment of the present invention using a quantum cascade laser element; -
FIG. 2 is a simplified diagram of the quantum cascade laser element used in the laser in accordance with an embodiment of the present invention showing generation of stimulated emissions by cascaded electron intraband transitions and tunneling; -
FIG. 3 is a plot of applied voltage and duty cycle of power applied to the quantum cascade laser element superimposed on a passive cooling boundary and an open-loop quantum cascade power output range for a given voltage showing a safe operating zone for the quantum cascade laser exploited by an embodiment of the present invention; -
FIG. 4 a is a cross-sectional view of the housing of a laser sight ofFIG. 1 showing a mounting system for the quantum cascade laser for passive cooling as held within the housing and showing the positioning of quantum cascade driving circuitry in accordance with an embodiment of the present invention; -
FIG. 4 b is a fragmentary cross-sectional view similar to that ofFIG. 4 a showing another embodiment with self-contained batteries; -
FIG. 5 is a block diagram of the quantum cascade driving circuit ofFIG. 2 providing current sensing to deduce quantum cascade power output for direct control of a DC-to-DC boost converter in accordance with an embodiment of the present invention; -
FIG. 6 is a block diagram similar to that ofFIG. 4 showing another embodiment providing optical power sensing to deduce of quantum cascade power output; and -
FIG. 7 is a perspective view of a handheld remote laser spectroscopy system such as may make use of the laser in accordance with an embodiment of the present invention. - A possible substitute for a conventional laser diode in this application is the so-called quantum cascade laser. Unlike a conventional laser diode which relies on relatively large band gaps in solid-state semiconductors, the quantum cascade laser allows the development of sub bands with lower energy differences suitable for producing long wavelengths of infrared light. Because quantum cascade lasers do not rely on electron/hole annihilation to produce photons, multiple photons may be created for each electron providing high light power output. Such quantum cascade lasers require relatively high operating voltages and dissipate substantial heat. This heat ultimately inhibits the lasing action (reducing the number of photons per electron) and in the extreme can damage the device. For this reason it is known to use active cooling of the quantum cascade laser, for example with an electrically powered Peltier device and fan combination.
- The large power consumption and high voltage requirements of a continuous wave quantum cascade laser and the need for additional power to cool the quantum cascade device presents a significant obstacle to the use of quantum cascade lasers for laser sights or other portable applications.
- Referring now to
FIG. 1 , arifle 10 may provide athermal imaging device 12 of a type known in the art for allowing a user of therifle 10 to sight along thebarrel 14 of rifle and view an image in the far infrared range (thermal image). A battery-poweredquantum cascade laser 16 in accordance with an embodiment of the present invention may be attached to therifle 10 by means ofmount 18 to provide alaser beam 21 aligned along thebarrel 14 to assist in targeting therifle 10 by projecting an invisible infrared spot on a target. - Referring to
FIG. 2 , at the heart of thequantum cascade laser 16 is a quantumcascade laser element 26 comprising astack 13 of semiconductor layers 15 separated bybarrier layers 17, the latter preventing classical electron flow. The number and types of layers shown inFIG. 2 are greatly reduced for clarity. - The
stack 13 extends generally along anaxis 22 with each of thelayers axis 22. Theouter layers 15 are attached toelectrodes driver circuit 25 to provide a relatively negative voltage at the top of thestack 13 atelectrode 23. This negative voltage provides a source ofelectrons 33 that are drawn to a relatively positive voltage at the bottom of thestack 13 atelectrode 23′. - The barrier layers 17 surrounding each
semiconductor layer 15 provide a high dielectric electrical insulation that creates quantum well 27 shown in apotential energy line 31, the latter depicting generally the energy required for anelectron 33 to move alongaxis 22. Thepotential energy line 31 decreases generally from the top of thestack 13 to the bottom of the stack 13 (per the applied voltage) but includes right extending peaks (as depicted) representing the electron barriers formed by the barrier layers 17 and thus the walls of thequantum wells 27. - The
quantum wells 27 confineelectrons 33 into thin planar regions to createsub bands 29 of permissible electron energy states for theelectrons 33 within each quantum well 2. These sub bands differ from the normal energy bands of a bulk semiconductor with the energy of thesub bands 29 largely dictated by the geometry of the quantum well 27 rather than the material properties alone. - Under the influence of the electrical voltage from
driver circuit 25,electrons 33 pass fromelectrode 23 toelectrode 23′ moving from quantum well 27 to quantum well 27 by tunneling through the barrier layers 17. Within eachquantum well 27, theelectrons 33 drop fromhigher subbands 29 to lower subbands 29 (intraband transition) resulting in the emission ofphotons 37 along anaxis 24 perpendicular toaxis 22. The difference in energy between higher andlower subbands 29 can produce light in the range of 3-10 microns or greater in each of the cascaded intraband transitions generating multiple photons per electron. - The intraband transition and tunneling process should be distinguished from classic electron/hole pair combinations found in conventional semiconductor laser diodes because an electron/hole pair is not extinguished upon the formation of the
photon 37 in aquantum cascade laser 16. - The vertical edges of the
stack 13 provide partially reflective surfaces to create a laser cavity allowing the stimulated emission ofphotons 37 as alaser beam 21. - Referring now to
FIG. 3 , the quantumcascade laser element 26 requires alasing threshold voltage 50 of approximately 20 volts to begin laser action as shown on the vertical axis. This voltage is substantially higher than conventional solid-state lasers and is a natural result of the physics of the device which requires the passage of electrons in cascade through a series of quantum wells of different voltages. Generally, batteries having sufficient amp-hour capacity and a voltage of 20 volts or higher are unsuitably heavy or bulky for a hand-held military weapon. - The average power consumption of the quantum
cascade laser element 26 may be controlled by pulsed operation at a duty cycle shown on the horizontal axis. A duty cycle above a predeterminedduty cycle threshold 52 is required, however, in order to provide sufficient output power for remote ranging, that is, to provide sufficient reflected power from a distant projected spot. - Together, the
lasing threshold voltage 50 andduty cycle threshold 52 provide lower bounds of asafe zone 58 in which the quantumcascade laser element 26 may operate. Apassive cooling boundary 56 describes the maximum power that may be dissipated by the quantumcascade laser element 26 with passive cooling without damaging or significantly degrading the quantum cascade laser element when operating at expected ambient operating temperatures. Thepassive cooling boundary 56 describes the upper bounds of thesafe zone 58. - It is generally possible for the
passive cooling boundary 56 to be below the lower bounds of thesafe zone 58 defined by thelasing threshold voltage 50 andduty cycle threshold 52; however, the present inventors have determined that optimizing the passive cooling by reducing the thermal resistance between the quantumcascade laser element 26 and the ambient air, a small but finitesafe zone 58 for practical form factors is created. - Given the low incremental resistance of many cascade laser elements, variations in the IV characteristics of a given quantum
cascade laser element 26, or slight drift in the applied voltage, define an electricalpower dissipation band 54 for the quantumcascade laser element 26 that generally is not coextensive with thesafe zone 58. Thus even though there is a theoreticalsafe zone 58, the present inventors have further determined that a given voltage and duty cycle of operation cannot guarantee operation within thesafe zone 58. - This failure of the
safe zone 58 to be coextensive with thepower output band 54 requires ancillary feedback stabilization of the power to the quantum cascade laser to maintain its operation within asub band 60 of thepower output band 54. Accordingly, as will be described in more detail below, an embodiment of the present invention provides power-sensing regulation that ensures operation withinsub band 60. - Referring now to
FIG. 4 a, thequantum cascade laser 16 may include a tubular housing 20 preferably formed from a thermally conductive material such as aluminum or the like. Within the housing, the quantumcascade laser element 26 may be attached to apassive heatsink 28 so that itsaxis 24 is aligned generally with acentral axis 24 of the tubular housing 20. A first end of the tubular housing 20 provides a window form by alens system 35 allowing exit and collimation of thelaser beam 21 formed by thephotons 37. - The
passive heatsink 28 is attached to the material of the housing 20 to conduct heat from the quantumcascade laser element 26 into the thermal mass of thepassive heatsink 28 and then into the housing 20. Power to the quantum cascade laser is provided through acopper conductor 30 passing to a first and second printedcircuit board 32 containingdriver circuits 25 to power the quantumcascade laser element 26. Return power is conducted by means of a conductor attached to the housing 20 (not shown) and to the electrically conductive copper of thepassive heatsink 28. - The end of the tubular housing 20 opposite the
lens system 35 may be covered with a threadedend cap 34 to provide an airtight inner volume of the housing 20 between thelens system 35 and the threadedend cap 34. The volume may, for example, be less than 10 cubic centimeters. Leads 36 providing power to the printedcircuit boards 32 may pass through a sealed opening in threadedend cap 34 to connect to abattery 41 that may be held by the user and that provides power to both thelaser 16 and thethermal imaging device 12. - Alternatively as shown in
FIG. 4 b, the threadedend cap 34 may be replaced with a threadedbattery housing 38 supporting for example two, 3 volt lithium ion batteries held in series connection by abattery holder 40 havingleads 42 joiningconnector 44 that may connect to the printedcircuit boards 32. - Referring now to
FIG. 5 , the quantumcascade driver circuit 25 described above may accept low-voltage electrical power, for example 6 V frombatteries 41, and provide that power to aboost converter circuit 64. As is generally understood in the art, theboost converter circuit 64 includes acontrol circuit 66 switching on and off a solid-state switch 78 connected in series with aninductor 70. The other end of theinductor 70 is connected to the low voltage electrical power and the other end of theswitch 68 is connected to ground. - The
control circuit 66 operates to charge theinductor 70 by periodically switching on the solid-state switch 68, and then to turn off theswitch 68 to generate an inductively drivenpulse 72 of voltage higher than the voltage ofbatteries 41 at the junction of the solid-state switch 78 andinductor 70. The voltage level ofpulses 72 will depend in part on the duration of on-time of the solid-state switch 68 and thus the amount of charging of theinductor 70. - The
pulses 72 are conducted through adiode 74 to acapacitor 76, the latter filtering the pulses to provide a source of DC voltage whose level is determined by thecircuit 66. This voltage will be above thelasing threshold voltage 50. The DC voltage is in turn provided to one terminal of quantumcascade laser element 26. - The second terminal of the quantum
cascade laser element 26 is connected in series with a second solid-state switch 78 controlled by apulsing circuit 80. Thepulsing circuit 80 provides pulses having a duty cycle above predeterminedduty cycle threshold 52. The voltage of thepulses 72 must at a minimum be above thethreshold voltage 50 described above but can be controlled in amplitude as will be described. Typically the pulses will have 100 ns to 1 .mu.s duration with a maximum duty cycle of 25% and will occur at approximately 500 kHz in frequency. - When the solid-
state switch 78 is on (receiving a pulse from pulsing circuit 80) current is conducted through the quantum cascade laser 86, through the solid-state switch 78, and throughsensing resistor 81 to ground. While the voltage atcapacitor 76 is a relatively poor proxy for power consumed by the quantumcascade laser element 26, the current through the quantumcascade laser element 26 and hence throughsensing resistor 81 provides a relatively good proxy for that power consumption quantumcascade laser element 26. This current is measured bydifferential amplifier 82 to provide a current level input to thecircuit 66 controlling the on time of solid-state switch 68 and thus the voltage of thepulses 72. In this way,amplifier 82 completes a feedback loop to control the voltage of thepulses 72, and thus the voltage being applied to the quantumcascade laser element 26, to produce a given current flow and hence power dissipation in the quantumcascade laser element 26. The gain ofamplifier 82 is set to adjust the operating conditions of the quantumcascade laser element 26 to be held withinsub band 60 ofFIG. 6 and thus withinsafe zone 58. - The
driver circuit 25 may also include aclamp circuit 91 receiving voltage at the cathode ofdiode 74 and controlling the feedback signal to controlcircuit 66 to limit the maximum voltage of thepulses 72 for safety, and further may provide for a timer limiting the maximum on time of the quantumcascade laser element 26 to comport with the heat sinking capacity of thepassive heatsink 28 in a particular environment. Theclamp circuit 91 may employ a solid-state switch switching at a predetermined threshold voltage as will be understood in the art. - Referring now to
FIG. 6 in an alternative embodiment thesensing resistor 81 is eliminated in favor of an optical detector 84 (such as a photodiode) receiving aportion 21′ of thelaser beam 21, for example, as diverted bybeam splitter 88. Theoptical detector 84 may thus provide a direct measurement of the power output by the quantumcascade laser element 26 indirectly determining its power dissipation. The signal from theoptical detector 84 is received byamplifier 82 which provides a control signal tocircuit 66. In addition, a temperature sensing element 89, such as a thermistor, may be included to sense the actual laser temperature and provide a feedback signal that is summed atamplifier 82 to increase or decrease the current set-point as required for different laser temperatures. Otherwise thedriver circuit 25 may be unchanged. - Referring now to
FIG. 7 , the ability to provide a highly portable, high powered, infrared source permits the construction of a handheldremote spectroscope 90 in which thequantum cascade laser 16 is attached to the housing of thespectroscope 90 having ahandgrip 92 andactivation button 94. By pressing the activation button, thequantum cascade laser 16 is activated. As with the laser sight system described above, an imaging device 12 (thermal, near infrared, or visible) may be used with thelaser 16, but in this case, theimaging device 12 is used to view and analyze signature emissions 98 that occur when thelaser beam 21 illuminates anunknown gas sample 96. When the emission wavelength of the source is equal to the wavelength of absorption for the gas, the gas will absorb the emission and the spectroscope will detect this difference. A device providing qualitative or quantitative system to detect hazardous materials may thus be constructed. - The foregoing description of embodiments has been presented for purposes of illustration and description. The foregoing description is not intended to be exhaustive or to limit embodiments of the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of various embodiments. The embodiments discussed herein were chosen and described in order to explain the principles and the nature of various embodiments and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules and systems.
Claims (14)
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-
2008
- 2008-03-11 US US12/046,353 patent/US7920608B2/en active Active
- 2008-03-12 EP EP08152655A patent/EP1971005A3/en not_active Withdrawn
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2010
- 2010-05-18 US US12/782,509 patent/US8189630B2/en active Active
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2012
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2013
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US8913637B1 (en) | 2014-12-16 |
EP1971005A2 (en) | 2008-09-17 |
US8442081B2 (en) | 2013-05-14 |
US20080304524A1 (en) | 2008-12-11 |
EP1971005A3 (en) | 2010-10-13 |
US20120210589A1 (en) | 2012-08-23 |
US7920608B2 (en) | 2011-04-05 |
US20100290494A1 (en) | 2010-11-18 |
US8189630B2 (en) | 2012-05-29 |
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