WO2013148368A1 - Multi-spectral terahertz source and imaging system - Google Patents

Multi-spectral terahertz source and imaging system Download PDF

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Publication number
WO2013148368A1
WO2013148368A1 PCT/US2013/032611 US2013032611W WO2013148368A1 WO 2013148368 A1 WO2013148368 A1 WO 2013148368A1 US 2013032611 W US2013032611 W US 2013032611W WO 2013148368 A1 WO2013148368 A1 WO 2013148368A1
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Prior art keywords
laser source
lasers
broadly tunable
source according
quantum cascade
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PCT/US2013/032611
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French (fr)
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Alan Wei Min Lee
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Longwave Photonics Llc
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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
    • H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
    • H01S5/4012—Beam combining, e.g. by the use of fibres, gratings, polarisers, prisms
    • 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
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
    • 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
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
    • 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
    • H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
    • H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
    • H01S5/4087—Array arrangements, e.g. constituted by discrete laser diodes or laser bar emitting more than one wavelength
    • 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
    • H01S2302/00—Amplification / lasing wavelength
    • H01S2302/02—THz - lasers, i.e. lasers with emission in the wavelength range of typically 0.1 mm to 1 mm
    • 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
    • H01S5/005—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
    • H01S5/0071—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 for beam steering, e.g. using a mirror outside the cavity to change the beam direction
    • 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
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/024—Arrangements for thermal management
    • H01S5/02407—Active cooling, e.g. the laser temperature is controlled by a thermo-electric cooler or water cooling
    • H01S5/02423—Liquid cooling, e.g. a liquid cools a mount of the laser
    • 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

Definitions

  • the present disclosure describes, among other things, a frequency agile semiconductor radiation- generating device.
  • US Patent No. 7826509 discloses a tunable single mode infrared laser source based on semiconductor lasers.
  • Belkin et al. uses a single semiconductor wafer.
  • the frequency tuning in Belkin et al. is limited to the gain bandwidth inherent in the design of the quantum cascade structure in the semiconductor wafer. There is a need for increased frequency tuning capability.
  • US Patent Application 13/197,058 discloses a system for combining the output beams of closely space quantum cascade lasers using a collimating lens and grating.
  • the system is limited to the gain bandwidth inherent in the design of the quantum cascade structure of the semiconductor wafer.
  • the system does not disclose the combining of beams which have differing angles of emission with respect to the semiconductor plane.
  • US Patent No. 7804069 discloses a method for imaging using a light from a terahertz source which is coherent or substantially monochromatic, and combining a portion of this light reflecting from a sample, and a portion of this light directly from the source. This method does not disclose a broadly tunable source, or imaging techniques using a broadly tunable source.
  • US Patent No. 7152007 discloses an imaging apparatus using terahertz quantum cascade laser or similar coherent source and a non-linear detector. This invention discloses a source and detector system, but does not disclose a tunable source, or imaging techniques using a broadly tunable source.
  • the present disclosure is directed to, among other things, a broadly tunable quantum cascade laser source.
  • the laser source includes at least two quantum cascade lasers, wherein the quantum cascade lasers comprise dissimilar gain media and are positioned in close proximity to one another.
  • the sources may be positioned beside each other, or may be stacked.
  • the laser source may include a lens adapted to reduce the divergence of the output beams of the quantum cascade lasers.
  • the laser source may include a dispersive element adapted to cause the output beams of each laser to be parallel with one another.
  • the laser source may include a microelectronic controller, wherein the controller switches each laser on and off, sets the bias conditions for each laser.
  • the present disclosure is directed to, among other things, a method for measuring the spectra of a sample.
  • the method may include sequentially irradiating the sample using light from at least two quantum cascade lasers, composed of dissimilar gain media, each of known spectral composition, positioned in close proximity to one another, whose beams are spatially overlapped using an optical element and a grating.
  • the grating may be made moveable, under the control of the microcontroller, to provide fine control of the beam direction.
  • the method may include detecting the light synchronously with said illumination.
  • FIGURE 1 depicts an exemplary schematic diagram of a broadly tunable source including a control unit, laser driver, multiplexer unit, two dissimilar QCL arrays, lens, grating, and mirror;
  • FIGURE 2 depicts an exemplary computer rendering of two linear arrays of third order DFB QCL devices fabricated from dissimilar gain media, wherein the arrays are mounted in close proximity to one another;
  • FIGURE 3 depicts an exemplary schematic diagram of an optical coherence tomography instrument with a broadly tunable source integrated with an interferometer, and a detector, for depth measurement of a sample;
  • FIGURE 4 depicts an exemplary schematic diagram of an optical coherence tomography instrument with a broadly tunable source integrated with an interferometer, a galvanometric scanner, and a detector, for full volumetric reconstruction of a sample;
  • FIGURE 5 depicts an exemplary schematic diagram of a reflection mode spectrometer with a broadly tunable source integrated with an optical telescope or microscope, and a detector, for non-contact spectroscopy;
  • FIGURE 6 depicts an exemplary schematic diagram of a reflection mode imaging
  • FIGURE 7 depicts an exemplary schematic diagram of a reflection mode imaging
  • spectrometer based on a broadly tunable source integrated with an optical telescope or microscope, galvanometric scanner, beam splitter, phase adjustor and a detector, for non- contact imaging spectroscopy.
  • the spectrometer combines a portion of the light directly from the source, and a portion of light from the sample, on the detector;
  • FIGURE 8 depicts an exemplary schematic diagram of a transmission mode imaging spectrometer based on a broadly tunable source integrated with illumination optics, imaging optics and a focal plane array detector such as a microbolometer array; and
  • FIGURE 9 depicts an exemplary schematic diagram of a reflection mode imaging
  • DFB lasers To achieve broad frequency tuning of a semiconductor laser source, distributed feedback (DFB) lasers have been fabricated in linear arrays, where each laser in the array operates at a different frequency.
  • DFB distributed feedback
  • the output beams of these lasers originate at spatially distinct locations, two problems emerge: the beams cannot be focused to a common point; or if the beams are collimated by a lens the beams will not propagate in one direction, instead they will diverge.
  • the DFB lasers are fabricated from a single wafer (and gain medium), operating at frequencies other than the optimum frequency of that particular gain medium results in reduced optical gain and output power, ultimately limiting the tuning range.
  • One embodiment of the present invention overcomes this problem of limited frequency tuning by combining multiple semiconductor lasers from dissimilar gain media.
  • the gain media are separately optimized GaAs/AlGaAs quantum cascade laser structures designed to be operated at terahertz frequencies of roughly 1.2 THz to 5 THz. In this frequency range the available gain is much more limited than the gain at infrared or visible frequencies, compounding the problem of limited tuning and necessitating the combination of separately optimized gain media to achieve frequency coverage over an appreciable range of > 1 THz.
  • different gain media cannot be placed in as close a proximity to one another as a devices made from a single gain medium.
  • the term 'close proximity' is understood to mean within a few millimeters, and in an extreme case a few centimeters.
  • This larger spatial separation compounds the aforementioned output beam problems: the beams will focus to points which are spread even further apart; or if the beams are collimated with a lens, they will diverge at greater rates.
  • Embodiments of the present invention overcome these limitations and achieve broad tuning with beams that can be focused to a common point, or can be made to propagate in a common direction.
  • One embodiment of the present invention comprises two or more dissimilar terahertz laser sources based on quantum cascade lasers (QCLs) that are closely spaced and whose output beams are collimated using a lens, where the optical axes of said output beams are overlapped using a grating.
  • QCLs quantum cascade lasers
  • a microelectronic controller is used to drive and address the individual QCLs.
  • the controller incorporates laser drive electronics, a multiplexer for the addressing of individual QCLs, and a microprocessor or other control logic to activate the lasers.
  • the beams are collimated with a moveable lens, which can be translated as different lasers are activated. The effect of which is to cause the beams emerging from the lens to be substantially parallel.
  • the lens may be translated or rotated by means of a fast piezoelectric or stepper motors. This may be accomplished in two
  • dissimilar gain media may be understood to mean gain media where the frequencies at which there is optical gain is dissimilar. For terahertz QCLs, this is achieved by varying the thicknesses of the quantum wells in the epitaxially grown structure.
  • dissimilar gain media devices placed in close proximity to one another may have vastly different lasing frequencies, without any of the tradeoffs associated with artificially broadening the gain media e.g. by using heterogeneous gain media, which typically has a concomitant reduction of gain.
  • the number of dissimilar gain media used may be practically limited by mechanical and optical considerations. Mechanically, packaging multiple semiconductor dies and associated wiring in close proximity to one another may be difficult. These limitations may be overcome by attaching multiple dissimilar semiconductor dies to a common substrate. High density wire bonding may be used to electrically contact the multiple dissimilar
  • a control unit may be combined with a laser driver and
  • multiplexing circuit to individually activate one or more terahertz QCLs, which may be fabricated from two or more dissimilar gain media.
  • the emission frequencies of the QCLs may span a range of frequencies that is greater than is available with any one individual gain media type. This may enable very large bandwidth operation.
  • a lens may be positioned to collimate the beams emerging from the lasers.
  • the optical axes of these beams may not be parallel, due to physical displacement of the QCLs, the mechanical misalignment of these QCLs, and/or by the nature of the emission from the QCLs.
  • the misalignment may be corrected by using a first movable dispersive grating and a second movable mirror.
  • the movable grating and mirror may be actuated under the control of the control unit, and may allow the laser optical axes to be made coaxial.
  • the movable grating and mirror may allow the two or more QCLs to illuminate the same spatial position of a sample.
  • Dispersive qualities of the grating may statically compensate for a large amount of non-collinearity of the QCL output beams, if their frequencies are dissimilar.
  • the static compensation may reduce the amount of movement necessary by the grating.
  • the reduced movement may allow for faster switching speed when different QCLs are activated.
  • a mirror may be used in lieu of the grating.
  • the apparatus disclosed herein may be used with different wavelength semiconductor lasers, such as infrared QCLs, visible, infrared or ultraviolet semiconductor lasers.
  • a control unit may be used to set the bias voltage and/or select a third order distributed feedback (DFB) QCL device, via the multiplexer output.
  • the QCL device may be one of many devices, fabricated in an array, on the same semiconductor die. Several semiconductor dies of dissimilar gain material may be mounted in close proximity to the first die. QCL devices may be sequentially selected first across the same die, then across dissimilar dies, by the multiplexer output and activated by providing the correct voltage or current for light output. As each device is selected, the control unit may send actuation signals to the grating and mirror to rotate to a predetermined position, which may cause the optical axis of the activated QCL to fall on a predetermined axis, irrespective of the selected device.
  • DFB distributed feedback
  • the mirror or grating may be interchanged in order, and mirrors and gratings may be referred to collectively as "reflectors.”
  • a "dispersive element” may include an optical grating, an Echelle grating and a prism.
  • a broadly tunable source may be combined with various optical systems described below for spectroscopy, and imaging spectroscopy. Applications include, but are in no way limited to, spectroscopic imaging of biological samples, remote detection of chemicals, and non-destructive evaluation.
  • the control unit 100 may include a microprocessor, computer or field-programmable gate array.
  • the control unit 100 determined the voltage and current bias conditions of a QCL.
  • the control unit 100 may program a laser driver 120 (such as Avtech model AV-1010 series, or LongWave Photonics model LWP-PS1).
  • the laser driver signal was routed through a multiplexing circuit 130, which may include a binary tree of low on-resistance mosfet devices (e.g. LongWave Photonics model LWP-Demux).
  • the control signal for the multiplexing circuit may be determined by the control unit, and may allow selection of an individual QCL among the closely spaced QCL arrays 200.
  • the arrays 200 included two or three arrays of third order distributed feedback terahertz QCLs, each consisting of 10 to 30 devices.
  • the arrays 200 may be mounted in a closed cycle cryocooler (e.g. Ricor model K535) 205.
  • the cryocooler 205 may maintain the QCLs at a sufficiently low temperature such that they can be operated. In some implementations, the cryocooler 205 may maintain the QCLs at a temperature of about 55 kelvin (K).
  • the output beam of the active QCL were collimated with an optical element such as a lens 220 (e.g., a lens fabricated with high-resistivity silicon) or another optical element such as an off-axis parabolic mirror.
  • the collimated beam may be diffracted by an Echelle grating 240.
  • the grating teeth may be designed to maximize diffraction into a particular order, thus reducing diffractive losses into unwanted orders.
  • the grating may have a surface coating of a metal of gold or aluminum.
  • the grating may have a surface coating of any substance that may reduce optical losses.
  • the grating may be rotated in one or two axes under control of the control unit.
  • the diffracted, collimated light may be reflected by a mirror 260.
  • the mirror 260 may rotate under control of the control unit in two axes similar to the grating.
  • the lens 220 may be moveable in translation or rotation in one or more axes so as to compensate for the lateral displacement of the lasers in the array and among arrays or among stacked lasers (i.e. in two dimensions).
  • the lens can be controlled by the microcontroller so as to quickly move as different lasers are activated.
  • An additional benefit of this approach is that beam pointing error caused by the manufacturing tolerances among arrays can be compensated out by calibrating the lens position for each individual laser.
  • each array may have its own copper submount 202, on which the semiconductor die 204 may be soldered via indium or gold.
  • the semiconductor die 204 may be soldered via other materials.
  • Each semiconductor die may have 21 3 rd order DFB devices.
  • the grating periodicity of the DFB devices may be slightly different across the array causing the emission wavelength of each device to be slightly different.
  • the gratings may be engineered to achieve emission over the entire gain bandwidth of the semiconductor gain medium. For rd
  • the 3 order DFB devices on one of the semiconductor dies may span the frequency rd range of about 2.0 to about 2.4 THz, while on the second semiconductor die, the 3 order DFB devices may span the frequency range of about 3.4 to about 4 THz. This combination of two arrays would allow emission over a much broader frequency range than would be obtainable with any single gain medium.
  • the use of two axes of rotation of the grating and the mirror may allow for correction of the differences in propagation direction of the collimated beams emerging from the lens.
  • ⁇ ⁇ may be held nearly constant, and may be compensated for with small rotations of the grating. These same rotations of the grating may compensate for inaccurate relative positioning between semiconductor dies.
  • the optical coherence tomography instrument may include a broadly tunable source with an interferometer.
  • the interferometer may include a beam splitter 272
  • a focusing lens 270 e.g. an off-axis parablic mirror, a high-resistivity silicon lens
  • a reference mirror 275 e.g. an off-axis parabolic mirror or lens matched to the detector
  • a light collecting lens 280 e.g. an off-axis parabolic mirror or lens matched to the detector
  • a detector 310 e.g. a Schottky diode, Golay cell, pyroelectric detector, thermopyle, bolometer, hot-electron bolometer, or high-Tc bolometer
  • other interferometer configurations may be used (e.g., Linnik type).
  • the broadly tunable source may be useful in this optical coherence tomography instrument because, in some implementations, the depth measurement resolution is proportional to the bandwidth of the source.
  • Exemplary applications of this instrument include biomedical measurements on tissue samples, non destructive test of polymers such as controlled release pharmaceutical tablet coatings, paint thickness, delimitation and disbonding determination, and semiconductor wafer defect measurement.
  • FIGURE 4 an exemplary broadly tunable source in an optical coherence tomography experiment with a galvanometric scanner 265 is shown and described.
  • the scanner may allow a sample to be scanned in two axes in addition to depth measurement, allowing a full three dimensional scan.
  • the optics may be adapted for the range of interest from short ranges of 1 to 50 cm, to larger ranges of 50 cm to 25 m.
  • exemplary smaller optics for use may include reflective microscope objectives, off-axis parabolic mirrors, or high-resistivity silicon lenses.
  • at least one large aperture optic would be used.
  • the at least one large aperture optic may collect a maximum amount of reflected and/or scattered light.
  • Exemplary large aperture optics may include a reflective telescope with larger primary mirrors of about 3 to about 12 inches.
  • Exemplary applications of this system may include the following: skin measurements for determination of cancerous tissue from healthy tissue; measurement of biological samples for the presence of protein binding; and measurement of spectroscopic features of solids, liquids or gasses.
  • An exemplary application may be the measurement of the reflection peaks associated with explosives in order to screen for their presence on a surface.
  • the semiconductor gain media may be chosen so that the laser frequency would correspond to the characteristic reflection peaks of the sample of interest.
  • Various configurations of this system may include transmission measurements through a sample, or configurations with some of the light from the broadly tunable source impinging on a reference detector.
  • Various configurations may include the use of a pinhole aperture to control the shape of the beam from the broadly tunable source.
  • FIGURE 6 an exemplary broadly tunable source in a reflection
  • spectroscopy system with a galvanometric scanner 265 is shown and described.
  • the addition of the scanner may allow a spectral imaging capability, greatly enhancing the previously mentioned applications.
  • a portion of the light from the broadly tunable source may be split using a beam splitter 272.
  • the portion of the split light may pass through a phase control module 305 (e.g. a delay stage, or a controllable thickness dielectric), and may be focused with a lens 280, recombined with the light reflecting or scattering from the sample and impinging on the detector.
  • a phase control module 305 e.g. a delay stage, or a controllable thickness dielectric
  • the system may include the broadly tunable source with beam directing optics 330 (e.g. an off-axis parabolic mirror), an imaging optic 350 (e.g. a high resistivity silicon lens which may include several refractive or diffractive elements to compensate for optical aberrations) and a focal plane array 340 (e.g. a microbolometer array, or pyroelectric array).
  • the broadly tunable source may be used for real-time imaging of a sample placed in the object plane. Samples could include semiconductor wafers for defect detection, biological samples to determine tissue composition or protein binding state.
  • FIGURE 9 a broadly tunable source in a reflection mode imaging configuration is shown and described. Reflected or scattered light from the source may be imaged with the imaging optics onto the focal plane array.
  • the reflection and transmission configurations may be used in combination with the previously mentioned configurations.
  • the focal plane array may be used for the detector in the optical coherence tomography system. Such use may obviate the need for a scanning system.

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Abstract

A broadly tunable semiconductor laser source having at least two semiconductor lasers, composed of dissimilar gain media, positioned in close proximity to one another, with an optical element for collimating the output of the semiconductor lasers, and a dispersive element to overlap the collimated beams with a microelectronic controller, which switches each laser on and off and sets the bias conditions for each laser. The output beams can be made parallel by design of the grating dispersion in combination with the lens, with small aiming corrections provided by mechanical actuation of the grating. The tunable source can be used for spectroscopy and spectroscopic imaging over a broad frequency range where the gain media are chosen for emission at frequencies of interest.

Description

PCT PATENT APPLICATION
for
Multi-Spectral Terahertz Source and Imaging System
MULTI-SPECTRAL TERAHERTZ SOURCE AND IMAGING SYSTEM
RELATED FILINGS
The present application claims the benefit of US provisional patent application serial number 61/615,851 entitled "Multi-Spectral Terahertz Source and Imaging System" filed on March 26, 2012, which is herein incorporated by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
This invention was made with government support under contract number NNX11CC66C awarded by NASA. The government has certain rights in the invention.
TECHNICAL FIELD
The present disclosure describes, among other things, a frequency agile semiconductor radiation- generating device.
BACKGROUND ART
US Patent No. 7826509 discloses a tunable single mode infrared laser source based on semiconductor lasers. Belkin et al. uses a single semiconductor wafer. However, the frequency tuning in Belkin et al. is limited to the gain bandwidth inherent in the design of the quantum cascade structure in the semiconductor wafer. There is a need for increased frequency tuning capability.
US Patent Application 13/197,058 discloses a system for combining the output beams of closely space quantum cascade lasers using a collimating lens and grating. The system is limited to the gain bandwidth inherent in the design of the quantum cascade structure of the semiconductor wafer. The system does not disclose the combining of beams which have differing angles of emission with respect to the semiconductor plane.
US Patent No. 7804069 discloses a method for imaging using a light from a terahertz source which is coherent or substantially monochromatic, and combining a portion of this light reflecting from a sample, and a portion of this light directly from the source. This method does not disclose a broadly tunable source, or imaging techniques using a broadly tunable source. US Patent No. 7152007 discloses an imaging apparatus using terahertz quantum cascade laser or similar coherent source and a non-linear detector. This invention discloses a source and detector system, but does not disclose a tunable source, or imaging techniques using a broadly tunable source.
SUMMARY OF INVENTION
In some aspects, the present disclosure is directed to, among other things, a broadly tunable quantum cascade laser source. The laser source includes at least two quantum cascade lasers, wherein the quantum cascade lasers comprise dissimilar gain media and are positioned in close proximity to one another. The sources may be positioned beside each other, or may be stacked. The laser source may include a lens adapted to reduce the divergence of the output beams of the quantum cascade lasers. The laser source may include a dispersive element adapted to cause the output beams of each laser to be parallel with one another. The laser source may include a microelectronic controller, wherein the controller switches each laser on and off, sets the bias conditions for each laser.
In some aspects, the present disclosure is directed to, among other things, a method for measuring the spectra of a sample. The method may include sequentially irradiating the sample using light from at least two quantum cascade lasers, composed of dissimilar gain media, each of known spectral composition, positioned in close proximity to one another, whose beams are spatially overlapped using an optical element and a grating. The grating may be made moveable, under the control of the microcontroller, to provide fine control of the beam direction.
The method may include detecting the light synchronously with said illumination.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other aspects, and features, of the present disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:
FIGURE 1 depicts an exemplary schematic diagram of a broadly tunable source including a control unit, laser driver, multiplexer unit, two dissimilar QCL arrays, lens, grating, and mirror; FIGURE 2 depicts an exemplary computer rendering of two linear arrays of third order DFB QCL devices fabricated from dissimilar gain media, wherein the arrays are mounted in close proximity to one another;
FIGURE 3 depicts an exemplary schematic diagram of an optical coherence tomography instrument with a broadly tunable source integrated with an interferometer, and a detector, for depth measurement of a sample;
FIGURE 4 depicts an exemplary schematic diagram of an optical coherence tomography instrument with a broadly tunable source integrated with an interferometer, a galvanometric scanner, and a detector, for full volumetric reconstruction of a sample;
FIGURE 5 depicts an exemplary schematic diagram of a reflection mode spectrometer with a broadly tunable source integrated with an optical telescope or microscope, and a detector, for non-contact spectroscopy;
FIGURE 6 depicts an exemplary schematic diagram of a reflection mode imaging
spectrometer with a broadly tunable source integrated with an optical telescope or
microscope, galvanometric scanner and a detector, for non-contact imaging spectroscopy;
FIGURE 7 depicts an exemplary schematic diagram of a reflection mode imaging
spectrometer based on a broadly tunable source integrated with an optical telescope or microscope, galvanometric scanner, beam splitter, phase adjustor and a detector, for non- contact imaging spectroscopy. The spectrometer combines a portion of the light directly from the source, and a portion of light from the sample, on the detector;
FIGURE 8 depicts an exemplary schematic diagram of a transmission mode imaging spectrometer based on a broadly tunable source integrated with illumination optics, imaging optics and a focal plane array detector such as a microbolometer array; and
FIGURE 9 depicts an exemplary schematic diagram of a reflection mode imaging
spectrometer with a broadly tunable source integrated with illumination optics, imaging optics and a focal plane array detector such as a microbolometer array. DETAILED DESCRIPTION OF THE INVENTION
To achieve broad frequency tuning of a semiconductor laser source, distributed feedback (DFB) lasers have been fabricated in linear arrays, where each laser in the array operates at a different frequency. However because the output beams of these lasers originate at spatially distinct locations, two problems emerge: the beams cannot be focused to a common point; or if the beams are collimated by a lens the beams will not propagate in one direction, instead they will diverge. Furthermore, because the DFB lasers are fabricated from a single wafer (and gain medium), operating at frequencies other than the optimum frequency of that particular gain medium results in reduced optical gain and output power, ultimately limiting the tuning range.
One embodiment of the present invention overcomes this problem of limited frequency tuning by combining multiple semiconductor lasers from dissimilar gain media. Here the gain media are separately optimized GaAs/AlGaAs quantum cascade laser structures designed to be operated at terahertz frequencies of roughly 1.2 THz to 5 THz. In this frequency range the available gain is much more limited than the gain at infrared or visible frequencies, compounding the problem of limited tuning and necessitating the combination of separately optimized gain media to achieve frequency coverage over an appreciable range of > 1 THz. However, different gain media cannot be placed in as close a proximity to one another as a devices made from a single gain medium. Here the term 'close proximity' is understood to mean within a few millimeters, and in an extreme case a few centimeters. This larger spatial separation compounds the aforementioned output beam problems: the beams will focus to points which are spread even further apart; or if the beams are collimated with a lens, they will diverge at greater rates. Embodiments of the present invention overcome these limitations and achieve broad tuning with beams that can be focused to a common point, or can be made to propagate in a common direction.
One embodiment of the present invention comprises two or more dissimilar terahertz laser sources based on quantum cascade lasers (QCLs) that are closely spaced and whose output beams are collimated using a lens, where the optical axes of said output beams are overlapped using a grating. A microelectronic controller is used to drive and address the individual QCLs. The controller incorporates laser drive electronics, a multiplexer for the addressing of individual QCLs, and a microprocessor or other control logic to activate the lasers. In another embodiment, the beams are collimated with a moveable lens, which can be translated as different lasers are activated. The effect of which is to cause the beams emerging from the lens to be substantially parallel. The lens may be translated or rotated by means of a fast piezoelectric or stepper motors. This may be accomplished in two
dimensions if the lasers are fabricate in arrays, and also stacked.
In some aspects, dissimilar gain media may be understood to mean gain media where the frequencies at which there is optical gain is dissimilar. For terahertz QCLs, this is achieved by varying the thicknesses of the quantum wells in the epitaxially grown structure. By using dissimilar gain media, devices placed in close proximity to one another may have vastly different lasing frequencies, without any of the tradeoffs associated with artificially broadening the gain media e.g. by using heterogeneous gain media, which typically has a concomitant reduction of gain.
The number of dissimilar gain media used may be practically limited by mechanical and optical considerations. Mechanically, packaging multiple semiconductor dies and associated wiring in close proximity to one another may be difficult. These limitations may be overcome by attaching multiple dissimilar semiconductor dies to a common substrate. High density wire bonding may be used to electrically contact the multiple dissimilar
semiconductor dies. Beyond the physical packaging limitations, additional limitations may exist such as the size of the necessary collimating lens, its cost and optical losses.
In some implementations, a control unit may be combined with a laser driver and
multiplexing circuit to individually activate one or more terahertz QCLs, which may be fabricated from two or more dissimilar gain media. The emission frequencies of the QCLs may span a range of frequencies that is greater than is available with any one individual gain media type. This may enable very large bandwidth operation. A lens may be positioned to collimate the beams emerging from the lasers.
In some implementations, the optical axes of these beams may not be parallel, due to physical displacement of the QCLs, the mechanical misalignment of these QCLs, and/or by the nature of the emission from the QCLs. The misalignment may be corrected by using a first movable dispersive grating and a second movable mirror. The movable grating and mirror may be actuated under the control of the control unit, and may allow the laser optical axes to be made coaxial. The movable grating and mirror may allow the two or more QCLs to illuminate the same spatial position of a sample. Dispersive qualities of the grating may statically compensate for a large amount of non-collinearity of the QCL output beams, if their frequencies are dissimilar. The static compensation may reduce the amount of movement necessary by the grating. The reduced movement may allow for faster switching speed when different QCLs are activated. In some implementations, a mirror may be used in lieu of the grating.
In some implementations, the apparatus disclosed herein may be used with different wavelength semiconductor lasers, such as infrared QCLs, visible, infrared or ultraviolet semiconductor lasers.
In some implementations, a control unit may be used to set the bias voltage and/or select a third order distributed feedback (DFB) QCL device, via the multiplexer output. The QCL device may be one of many devices, fabricated in an array, on the same semiconductor die. Several semiconductor dies of dissimilar gain material may be mounted in close proximity to the first die. QCL devices may be sequentially selected first across the same die, then across dissimilar dies, by the multiplexer output and activated by providing the correct voltage or current for light output. As each device is selected, the control unit may send actuation signals to the grating and mirror to rotate to a predetermined position, which may cause the optical axis of the activated QCL to fall on a predetermined axis, irrespective of the selected device.
In some implementations, the mirror or grating may be interchanged in order, and mirrors and gratings may be referred to collectively as "reflectors." A "dispersive element" may include an optical grating, an Echelle grating and a prism. A broadly tunable source may be combined with various optical systems described below for spectroscopy, and imaging spectroscopy. Applications include, but are in no way limited to, spectroscopic imaging of biological samples, remote detection of chemicals, and non-destructive evaluation.
The following examples are merely illustrative of some features of the apparatuses and methods described herein, but are not to be construed as limiting the scope of the present disclosure to the specific examples.
Referring now to FIGURE 1, an exemplary control unit 100 is shown and described. The control unit 100 may include a microprocessor, computer or field-programmable gate array. The control unit 100 determined the voltage and current bias conditions of a QCL. The control unit 100 may program a laser driver 120 (such as Avtech model AV-1010 series, or LongWave Photonics model LWP-PS1). The laser driver signal was routed through a multiplexing circuit 130, which may include a binary tree of low on-resistance mosfet devices (e.g. LongWave Photonics model LWP-Demux). The control signal for the multiplexing circuit may be determined by the control unit, and may allow selection of an individual QCL among the closely spaced QCL arrays 200. The arrays 200 included two or three arrays of third order distributed feedback terahertz QCLs, each consisting of 10 to 30 devices. The arrays 200 may be mounted in a closed cycle cryocooler (e.g. Ricor model K535) 205. The cryocooler 205 may maintain the QCLs at a sufficiently low temperature such that they can be operated. In some implementations, the cryocooler 205 may maintain the QCLs at a temperature of about 55 kelvin (K). The output beam of the active QCL were collimated with an optical element such as a lens 220 (e.g., a lens fabricated with high-resistivity silicon) or another optical element such as an off-axis parabolic mirror. The collimated beam may be diffracted by an Echelle grating 240. In some implementations, the grating teeth may be designed to maximize diffraction into a particular order, thus reducing diffractive losses into unwanted orders. In some implementations, the grating may have a surface coating of a metal of gold or aluminum. In some implementations, the grating may have a surface coating of any substance that may reduce optical losses. In some implementations the grating may be rotated in one or two axes under control of the control unit. The diffracted, collimated light may be reflected by a mirror 260. The mirror 260 may rotate under control of the control unit in two axes similar to the grating. In some embodiments the lens 220 may be moveable in translation or rotation in one or more axes so as to compensate for the lateral displacement of the lasers in the array and among arrays or among stacked lasers (i.e. in two dimensions). The lens can be controlled by the microcontroller so as to quickly move as different lasers are activated. An additional benefit of this approach is that beam pointing error caused by the manufacturing tolerances among arrays can be compensated out by calibrating the lens position for each individual laser.
Referring now to FIGURE 2, a computer rendering of two exemplary closely spaced QCL arrays 200 is shown and described. In some implementations, each array may have its own copper submount 202, on which the semiconductor die 204 may be soldered via indium or gold. In some implementations, the semiconductor die 204 may be soldered via other materials. Each semiconductor die may have 21 3 rd order DFB devices. The grating periodicity of the DFB devices may be slightly different across the array causing the emission wavelength of each device to be slightly different. The gratings may be engineered to achieve emission over the entire gain bandwidth of the semiconductor gain medium. For rd
example, the 3 order DFB devices on one of the semiconductor dies may span the frequency rd range of about 2.0 to about 2.4 THz, while on the second semiconductor die, the 3 order DFB devices may span the frequency range of about 3.4 to about 4 THz. This combination of two arrays would allow emission over a much broader frequency range than would be obtainable with any single gain medium.
The use of two axes of rotation of the grating and the mirror may allow for correction of the differences in propagation direction of the collimated beams emerging from the lens.
Without wishing to be bound by theory, these differences in propagation direction may result rd
from variations in the effective index of the 3 order DFB structures within an array, and from array to array. These differences in the propagation direction may result from physical displacement of the QCLs from one another. In this case, the angular differences in the propagation direction, ΑΘ, may be due to the physical separation of the QCL devices, Δχ:
(1)
ΑΘ = tan"1 Ax /f
Where/is the collimating lens focal length. In some implementations, this angular difference may be compensated for, at least in part, by the grating, which has the following equation: d■ (sin9r + sin9i n) = m■ (2)
Where d is the grating pitch in millimeters per groove, (¾ is equal to the grating angle Θ and the angular pointing difference ΔΘ, m is the integer grating order and λη is the wavelength emitted by the nth QCL in the array. In some implementations, by designing the grating and the optical system, ΘΓ may be held nearly constant, and may be compensated for with small rotations of the grating. These same rotations of the grating may compensate for inaccurate relative positioning between semiconductor dies.
Referring now to FIGURE 3, an exemplary diagram of a broadly tunable source in an optical coherence tomography instrument, for measurement of the interfaces within a sample 290 is shown and described. The optical coherence tomography instrument may include a broadly tunable source with an interferometer. The interferometer may include a beam splitter 272
(e.g. quartz or mylar), a focusing lens 270 (e.g. an off-axis parablic mirror, a high-resistivity silicon lens), a reference mirror 275, a sample 290, a light collecting lens 280 (e.g. an off-axis parabolic mirror or lens matched to the detector), and a detector 310 (e.g. a Schottky diode, Golay cell, pyroelectric detector, thermopyle, bolometer, hot-electron bolometer, or high-Tc bolometer), although other interferometer configurations may be used (e.g., Linnik type). The broadly tunable source may be useful in this optical coherence tomography instrument because, in some implementations, the depth measurement resolution is proportional to the bandwidth of the source. Exemplary applications of this instrument include biomedical measurements on tissue samples, non destructive test of polymers such as controlled release pharmaceutical tablet coatings, paint thickness, delimitation and disbonding determination, and semiconductor wafer defect measurement.
Referring now to FIGURE 4, an exemplary broadly tunable source in an optical coherence tomography experiment with a galvanometric scanner 265 is shown and described. The scanner may allow a sample to be scanned in two axes in addition to depth measurement, allowing a full three dimensional scan.
Referring now to FIGURE 5, an exemplary broadly tunable source combined with an optical telescope 320, in a reflection spectroscopy measurement of a sample, is shown and described. The optics may be adapted for the range of interest from short ranges of 1 to 50 cm, to larger ranges of 50 cm to 25 m. For short ranges, exemplary smaller optics for use may include reflective microscope objectives, off-axis parabolic mirrors, or high-resistivity silicon lenses. For longer distances, at least one large aperture optic would be used. The at least one large aperture optic may collect a maximum amount of reflected and/or scattered light. Exemplary large aperture optics may include a reflective telescope with larger primary mirrors of about 3 to about 12 inches. Exemplary applications of this system may include the following: skin measurements for determination of cancerous tissue from healthy tissue; measurement of biological samples for the presence of protein binding; and measurement of spectroscopic features of solids, liquids or gasses.
An exemplary application may be the measurement of the reflection peaks associated with explosives in order to screen for their presence on a surface. For these spectroscopic applications, the semiconductor gain media may be chosen so that the laser frequency would correspond to the characteristic reflection peaks of the sample of interest. Various configurations of this system may include transmission measurements through a sample, or configurations with some of the light from the broadly tunable source impinging on a reference detector. Various configurations may include the use of a pinhole aperture to control the shape of the beam from the broadly tunable source.
Referring now to FIGURE 6, an exemplary broadly tunable source in a reflection
spectroscopy system with a galvanometric scanner 265 is shown and described. In some implementations, the addition of the scanner may allow a spectral imaging capability, greatly enhancing the previously mentioned applications.
Referring now to FIGURE 7, a broadly tunable source in a scanning reflection spectroscopy system with a reference signal provided with the detector is shown and described. A portion of the light from the broadly tunable source may be split using a beam splitter 272. The portion of the split light may pass through a phase control module 305 (e.g. a delay stage, or a controllable thickness dielectric), and may be focused with a lens 280, recombined with the light reflecting or scattering from the sample and impinging on the detector.
Referring now to FIGURE 8, a broadly tunable source in a transmission mode imaging configuration is shown and described. The system may include the broadly tunable source with beam directing optics 330 (e.g. an off-axis parabolic mirror), an imaging optic 350 (e.g. a high resistivity silicon lens which may include several refractive or diffractive elements to compensate for optical aberrations) and a focal plane array 340 (e.g. a microbolometer array, or pyroelectric array). The broadly tunable source may be used for real-time imaging of a sample placed in the object plane. Samples could include semiconductor wafers for defect detection, biological samples to determine tissue composition or protein binding state.
Referring now to FIGURE 9, a broadly tunable source in a reflection mode imaging configuration is shown and described. Reflected or scattered light from the source may be imaged with the imaging optics onto the focal plane array. The reflection and transmission configurations may be used in combination with the previously mentioned configurations. For example, the focal plane array may be used for the detector in the optical coherence tomography system. Such use may obviate the need for a scanning system.
All documents referred to in this application are hereby incorporated by reference in their entirety. Although various implementations of the present solution have been described herein in detail, it will be appreciated by those skilled in the art that variations may be made thereto without departing from the spirit or the scope of the appended claims.

Claims

What is claimed:
1. A broadly tunable semiconductor laser source comprising: at least two semiconductor lasers, composed of dissimilar gain media, positioned in close proximity to one another,
an optical element to reduce the divergence of the output beams of the semiconductor lasers,
a dispersive element which causes said output beams to propagate substantially in parallel with each other,
a microelectronic controller, wherein said controller switches each laser on and off.
2. A broadly tunable semiconductor laser source according to claim 1, wherein said lasers are quantum cascade lasers fabricated in third order distributed feedback grating waveguides.
3. A broadly tunable semiconductor laser source according to claim 1, wherein said dispersive element is an optical grating.
4. A broadly tunable semiconductor laser source according to claim 1, wherein said dispersive element is a prism.
5. A broadly tunable semiconductor laser source according to claim 1, wherein said at least two semiconductor lasers are selected so that their emission frequencies correspond with the absorption or reflection features of a sample of interest.
6. A broadly tunable semiconductor laser source according to claim 1, wherein said dispersive element is moveable so as to provide fine correction of the propagation direction of said output beams.
7. A broadly tunable semiconductor laser source according to claim 1, wherein said gain media are optimized for emission at terahertz frequency range of 0.3 to 10 THz.
8. A broadly tunable quantum cascade laser source comprising: at least two quantum cascade lasers, composed of dissimilar gain media, positioned in close proximity to one another, a moveable optical element to reduce the divergence of the output beams of the semiconductor lasers,
a microelectronic controller, wherein said controller switches each laser on and off and controls the position of said optical element so as to cause said output beams to be substantially parallel.
9. A broadly tunable quantum cascade laser source according to claim 8, wherein said gain media cover frequencies in a range of 0.3 to 10 THz.
10. A broadly tunable quantum cascade laser source according to claim 8, wherein said quantum cascade lasers are third order distributed feedback grating type.
11. A broadly tunable quantum cascade laser source according to claim 8, wherein said quantum cascade lasers are selected so that their emission frequencies correspond with the absorption or reflection features of a sample of interest.
12. A method for characterizing the spectral properties of a sample comprising: sequentially irradiating the sample using light from at least two semiconductor lasers, composed of dissimilar gain media, positioned in close proximity to one another, whose output beams are spatially overlapped, and detecting the light synchronously with said illumination.
13. The method according to claim 12, wherein said semiconductor lasers emit in the range of frequencies from 100 GHz to 100 THz.
14. The method according to claim 12, wherein said beams are spatially overlapped by means of a first optical element followed by a dispersive grating.
15. The method according to claim 12, wherein said beams are spatially overlapped by means of a moveable optical element.
16. The method according to claim 12, further comprising the following additional step: scanning the beam in two dimensions to form an image.
17. A broadly tunable semiconductor laser source comprising: at least two semiconductor lasers, composed of dissimilar gain media which are optimized for emission in a frequency range of 0.3 to 10 THz and are positioned in close proximity to one another,
an optical element to reduce the divergence of the output beams of the semiconductor lasers,
a dispersive element which causes said output beams to propagate substantially in parallel with each other, and
a microelectronic controller, wherein said controller switches each laser on and off.
18. A broadly tunable semiconductor laser source according to claim 17, wherein said lasers are quantum cascade lasers fabricated in third order distributed feedback grating waveguides.
19. A broadly tunable semiconductor laser source according to claim 17, wherein said dispersive element is an optical grating.
20. A broadly tunable semiconductor laser source according to claim 17, wherein said dispersive element is a prism.
21. A broadly tunable semiconductor laser source according to claim 17, wherein said at least two semiconductor lasers are selected so that their emission frequencies correspond with the absorption or reflection features of a sample of interest.
22. A broadly tunable semiconductor laser source according to claim 17, wherein said dispersive element is moveable so as to provide fine correction of the propagation direction of said output beams.
23. A broadly tunable semiconductor laser source comprising: at least two semiconductor lasers, composed of dissimilar gain media which are optimized for emission in a frequency range of 0.3 to 10 THz, and are positioned in close proximity to one another,
an optical element to reduce the divergence of the output beams of the semiconductor lasers, a dispersive element which causes said output beams to propagate substantially in parallel with each other, wherein said dispersive element is moveable so as to provide fine correction of the propagation direction of said output beams, and
a microelectronic controller, wherein said controller switches each laser on and off.
24. A broadly tunable semiconductor laser source according to claim 23, wherein said lasers are quantum cascade lasers fabricated in third order distributed feedback grating waveguides.
25. A broadly tunable semiconductor laser source according to claim 23, wherein said dispersive element is an optical grating.
26. A broadly tunable semiconductor laser source according to claim 23, wherein said dispersive element is a prism.
27. A broadly tunable semiconductor laser source according to claim 23, wherein said at least two semiconductor lasers are selected so that their emission frequencies correspond with the absorption or reflection features of a sample of interest.
28. A broadly tunable quantum cascade laser source comprising: at least two semiconductor lasers, composed of dissimilar gain media which are optimized for emission in a frequency range of 0.3 to 10 THz, and are positioned in close proximity to one another,
a moveable optical element to reduce the divergence of the output beams of the
semiconductor lasers, and
a microelectronic controller, wherein said controller switches each laser on and off and controls the position of said optical element so as to cause said output beams to be substantially parallel.
29. A broadly tunable quantum cascade laser source according to claim 28, wherein said quantum cascade lasers are third order distributed feedback grating type.
30. A broadly tunable quantum cascade laser source according to claim 28, wherein said quantum cascade lasers are selected so that their emission frequencies correspond with the absorption or reflection features of a sample of interest.
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