Tunable quantum cascade lasers and photoacoustic detection of trace gases, TNT, TATP and precursors acetone and hydrogen peroxide

• US 20080159341A1
• Filed: 06/22/2007
• Published: 07/03/2008
• Est. Priority Date: 06/23/2006
• Status: Active Grant

• Current Assignee: Daylight Solutions, Inc. (Leonardo S.p.A.)
• Original Assignee: Pranalytica, Inc.
• Inventors: Pushkarsky, Michael, Patel, C. Kumar N., Maulini, Richard, Prasanna, Manu, Go, Rowel C., Dunayevskiy, Ilya, Tsekoun, Alexei
• Granted Patent: US 7,903,704 B2
• Time in Patent Office: Days
• US Class Current: 372/20
• CPC Class Codes: B82Y 20/00 Nanooptics, e.g. quantum optics or photonic crystals
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: H01S 5/06255 Controlling the frequency of the radiation
• CPC Class Codes: H01S 5/0654 Single longitudinal mode emission
• CPC Class Codes: H01S 5/1092 Multi-wavelength lasing
• CPC Class Codes: H01S 5/141 using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence
• CPC Class Codes: H01S 5/3401 having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers

Abstract

Methods and apparatus for broad tuning of single wavelength quantum cascade lasers and the use of light output from such lasers for highly sensitive detection of trace gases such as nitrogen dioxide, acetylene, and vapors of explosives such as trinitrotoluene (TNT) and triacetone triperoxide (TATP) and TATP'"'"'s precursors including acetone and hydrogen peroxide. These methods and apparatus are also suitable for high sensitivity, high selectivity detection of other chemical compounds including chemical warfare agents and toxic industrial chemicals. A quantum cascade laser (QCL) system that better achieves single mode, continuous, mode-hop free tuning for use in L-PAS (laser photoacoustic spectroscopy) by independently coordinating gain chip current, diffraction grating angle and external cavity length is described. An all mechanical method that achieves similar performance is also described. Additionally, methods for improving the sensor performance by critical selection of wavelengths are presented.

Claims

1. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system, the steps comprising:

a) powering a QCL gain chip to provide a source of multiwavelength laser light at a first power level;

b) selectively reflecting said multiwavelength laser light with a diffraction grating in a cavity back to said QCL gain chip to select a first laser wavelength;

c) adjusting a position of said diffraction grating with a piezoelectric translator (PZT) to adjust a distance of said cavity to obtain a maximum output distance for said first laser wavelength and said first power level, said maximum output distance maximizing output of the laser system at said first laser wavelength, said position of said diffraction grating selectively altered on the order of a few wavelengths of said first laser frequency to find said maximum output distance;

d) making a fine angular displacement step with said diffraction grating to select a new laser wavelength and repeating steps b and c in an ongoing fashion for each new laser wavelength to obtain sufficient data to determine Fabry-Perot mode comb peaks for said QCL gain chip at said first power level over a desired wavelength range;

e) determining a maximizing power level for said first wavelength by determining a mode comb power level of the QCL gain chip that causes a mode comb wavelength spike of said QCL gain chip to coincide with said first laser wavelength, said maximizing power level achieved by determining a first power level change to shift one of said laser output peaks a first known fractional FP mode distance to determine Δ

I_FSR, selecting a wavelength at which the laser will operate, determining a second fractional FP mode distance said selected wavelength is from a first output peak, and powering said QCL gain chip by a current equal to

2. A laser illumination system for providing laser light over a multiwavelength spectrum, comprising:

a two-segment QCL chip laser light source emitting light, said chip having an approximately flat saturation region and being powered by a current;

said source powered by a current selected according to the equation

3. A method for leveling power for a tunable EGC-QCL system, the steps comprising:

determining a current saturation peak for a first QCL gain chip system;

determining a first current periodicity value that shifts a Fabry-Perot comb mode of said first QCL gain chip system one free spectral range;

setting a first injection current to a current value near said saturation peak plus or minus approximately 1 to 2 first current periodicity values; and

shifting said first injection current by said first current periodicity value to maintain said first injection current within approximately 1 to 2 first current periodicity values of said current saturation peak;

whereby

such that said first injection current can be adjusted to selectably shift said Fabry-Perot comb mode with diminished effect upon laser power output of said first QCL gain chip system;

4. A method for leveling power for a tunable EGC-QCL system, the steps comprising:

determining a current saturation peak for a QCL gain chip system having first and second segments optically linked to one another;

determining a first current periodicity value that shifts a Fabry-Perot comb mode of said QCL gain chip system one free spectral range;

operating said first segment with said first injection current on one side of said current saturation peak; and

operating said second segment with a second injection current on a second side of said current saturation peak; and

maintaining a current difference between said first and second injection currents, said current difference selected from the group consisting of;

a) a constant current difference of said first current periodicity value between said first and second injection currents such that a total power output is maintained relatively constant while enabling changes in said first and second injection currents to selectably shifting a refractive index of said QCL gain chip system; and

b) a varying current difference between said first and second injection currents such that selected power output for several wavelengths of the tunable EGC-QCL system are leveled with wavelengths having stronger outputs being diminished while wavelengths having weaker outputs being augmented; and

c) combinations thereof;

whereby

said first and second injection currents may be selectably adjusted to shift a Fabry-Perot comb mode of said QCL gain chip system.

5. A mechanical EGC system accommodating dispersion to optimize laser operation, comprising:

a diffraction grating synchronizing angular displacement of said diffraction grating with respect to an optical axis with linear displacement of said diffraction grating along said optical axis such that an integer number of half wavelengths are reflected to an end mirror for a selected wavelength transmitted to said end mirror by said diffraction grating along an optical axis;

a second projected axis orthogonal to said optical axis intersecting said optical axis at an intersection point behind said end mirror; and

said intersection point separated from said end mirror along said optical axis by a distance equal to a group optical length of a cavity distance between said diffraction grating and said end mirror and said actual cavity distance between said diffraction grating and said end mirror, said group optical length dependent upon linear dispersion of said cavity distance.

6. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system, the steps comprising:

a) powering a source of multiwavelength laser light at a first power level;

b) selectively reflecting said multiwavelength laser light in a cavity back to said source to select a first laser wavelength;

c) adjusting a distance of said cavity to obtain a maximum output distance for said first laser wavelength and said first power level, said maximum output distance maximizing output of the laser system at said first laser wavelength;

d) repeating steps b and c for other laser wavelengths to obtain sufficient data to determine laser output peaks for said source at said first power level over a desired wavelength range; and

e) determining a maximizing power level for each wavelength in said wavelength range;

whereby the coupled cavity laser system operates at a maximum for a selected wavelength within said wavelength range by selecting said maximum output distance and said maximizing power level for said selected wavelength.

7. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 6, further comprising:

said source of multiwavelength laser light including a QCL gain chip.

8. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 6, further comprising:

selectively reflecting said multiwavelength laser light with a diffraction grating.

9. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 8, further comprising:

adjusting said cavity distance by adjusting a position of said diffraction grating.

10. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 9, further comprising:

adjusting said position of said diffraction grating with a piezoelectric translator (PZT).

11. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 6, further comprising:

adjusting said distance by selectively altering said distance on the order of a few wavelengths of said first laser frequency.

12. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 6, further comprising:

repeating steps b and c by selecting a later second laser wavelength in step b that departs minimally from said earlier first laser wavelength.

13. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 12, further comprising:

obtaining said later second laser wavelength by making a fine angular displacement step with a diffraction grating.

14. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 6, further comprising:

said output peaks corresponding to a Fabry-Perot mode comb for said source of multiwavelength laser light at said first power level.

15. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 14, further comprising:

said maximizing power level is determined for said first laser wavelength by determining a mode comb power level that causes a mode comb wavelength spike of said source to coincide with said first laser wavelength.

16. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 15, further comprising:

incrementing or decrementing said maximizing power level to match a second selected laser wavelength within said wavelength range; and

shifting said maximizing power level a free spectral range amount when needed to maintain said maximizing power level within preferred power limits of said source while simultaneously matching said first laser wavelength.

17. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 6, further comprising:

selecting a wavelength at which the laser system will operate, said wavelength selected from said wavelength range;

further adjusting a distance of said cavity at said maximum output distance to ensure that said selected maximum output distance for said selected wavelength is as much a maximum output distance as possible to enable maximum output of the laser system at said selected laser wavelength.

18. A method for obtaining power-maximized single-frequency continuous tuning for a coupled cavity laser system as set forth in claim 6, wherein said step of determining a maximizing power level further comprising:

determining a first power level change to shift one of said laser output peaks a first known fractional FP mode distance to determine Δ

I_FSR;

selecting a wavelength at which the laser will operate;

determining a second fractional FP mode distance said selected wavelength is from a first output peak; and

powering said QCL gain chip by a current equal to

19. A laser illumination system for providing laser light over a multiwavelength spectrum, comprising:

a multiwavelength laser light source emitting light;

a wavelength-selective reflector in optical communication with said source;

a translator coupled to said reflector, said translator displacing said reflector according to a first signal and controlling a distance between said reflector and said source; and

a rotation stage coupled to said translator, said rotation stage rotating said reflector according to a second signal and controlling an angle between said reflector and said source;

whereby

single mode, continuous, mode-hop free tuning is provided by the laser illumination system.

20. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 19, wherein said source further comprises:

a QCL with an approximately flat saturation region.

21. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 20, wherein said QCL further comprises:

a two-segment QCL chip.

22. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 19, further comprising:

said source powered by a current selected according to the equation

23. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 19, wherein said reflector further comprises:

a diffraction grating.

24. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 23, wherein said grating further comprises:

a diffraction grating having approximately 240 grooves/mm.

25. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 23, wherein said angle between said reflector and said source is determined according to a relationship directly linking the two.

26. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 25, wherein said relationship is determined according to:

GP=GP₀+A(v−

v₀)+B(v−

v₀)²;

where GP—

grating angle position (mm) at a desired frequency v (cm<sup>−

1</sup>), GP₀—

starting grating position (mm) at starting frequency v₀ (cm<sup>−

1</sup>), and A and B—

polynomial fit coefficients.

27. A laser illumination system for providing laser light over a multiwavelength spectrum as set forth in claim 19, further comprising:

said source powered by a current; and

said current, said distance, and said angle all coordinated by a process including the steps of;

a. setting said current to a maximum;

b. setting said rotation stage to an initial position to provide an initial angle for said angle;

c. ramping said translator through a distance of approximately a few wavelengths of said light;

d. measuring resulting laser light output from the illumination system;

e. determining a power maximum of said resulting laser light and determining a translator position and a rotation stage position associated with said power maximum;

f. setting said rotation stage to a second position to provide a second angle for said angle; and

g. repeating steps c-f across an available spectrum for said source;

whereby

a curve with distinct, periodic power maxima and minima with changing wavelength is acquired with each maximum position of the curve generally corresponding to a best match between said reflector angle, said distance, and said current.

28. A method for leveling power for a tunable EGC-QCL system, the steps comprising:

determining a current saturation peak for a first QCL gain chip system;

determining a first current periodicity value that shifts a Fabry-Perot comb mode of said first QCL gain chip system one free spectral range; and

setting a first injection current to a current value near said saturation peak plus or minus approximately 1 to 2 first current periodicity values;

whereby

said first injection current can be adjusted to selectably shift said Fabry-Perot comb mode with diminished effect upon laser power output of said first QCL gain chip system.

29. A method for leveling power for a tunable EGC-QCL system as set forth in claim 28, the steps further comprising:

shifting said first injection current by said first current periodicity value to maintain said first injection current within a selected range of said current saturation peak.

30. A method for leveling power for a tunable EGC-QCL system as set forth in claim 29, further comprising:

said selected range being approximately 1 to 2 first current periodicity values.

31. A method for leveling power for a tunable EGC-QCL system as set forth in claim 29, the steps further comprising:

providing a second QCL gain chip system optically coupled to said first QCL gain chip system;

operating said first QCL gain chip system with said first injection current on one side of said current saturation peak;

operating said second QCL gain chip system with a second injection current on a second side of said current saturation peak;

whereby

said first and second injection currents may be selectably adjusted to shift a Fabry-Perot comb mode of said first and second QCL gain chip systems.

32. A method for leveling power for a tunable EGC-QCL system as set forth in claim 31, the steps further comprising:

maintaining a constant current difference of said first current periodicity value between said first and second injection currents;

whereby

a total power output is maintained relatively constant while enabling changes in said first and second injection current to selectably shifting a refractive index of said first and second QCL gain chip systems.

33. A method for leveling power for a tunable EGC-QCL system as set forth in claim 31, the steps further comprising:

varying a current difference between said first and second injection currents;

whereby

selected power output for several wavelengths of the tunable EGC-QCL system are leveled with wavelengths having stronger outputs being diminished while wavelengths having weaker outputs being augmented.

34. A method for more quickly determining the presence of a target gas, the steps comprising:

1) Identifying and selecting regions in a frequency range of a selectable wavelength light source which meets all the following criteria;

a) the target gas has large absorption in at least some frequencies in said frequency range;

b) expected interferents have low absorption at their expected concentrations; and

c) a detectable signature of target gas is linearly independent of signature of interferents;

2) collecting a sample of gas for testing of the target gas;

3) Perform a scan across said identified and selected regions and collecting photoacoustic data from said scan; and

4) Linearly deconvolving said photoacoustic data against a standardized library of the target gas and list of expected interferents, to obtain a gas concentration measurement for the target gas.

35. A method for more quickly determining the presence of a target gas as set forth in claim 34, wherein said selectable wavelength light source further comprises:

a single mode, continuous, mode-hop free source of laser light.

36. A method for more quickly determining the presence of a target gas as set forth in claim 34, wherein said source of laser light further comprises:

a multiwavelength laser light source emitting light;

a wavelength-selective reflector in optical communication with said source;

a translator coupled to said reflector, said translator displacing said reflector according to a first signal and controlling a distance between said reflector and said source; and

a rotation stage coupled to said translator, said rotation stage rotating said reflector according to a second signal and controlling an angle between said reflector and said source;

whereby

single mode, continuous, mode-hop free tuning is provided by the laser illumination system.

37. A method for more quickly determining the presence of a target gas as set forth in claim 34, further comprising:

said step 1a) further includes the target gas providing a good signal to noise ration for reliable determination of the target gas; and

5) Recording a measurement of said scan and determining a time taken for said measurement to determine a throughput;

6) If concentration the target gas is below an alarm threshold continue to Step 2 without making any changes;

7) If all the following conditions are met;

a) target gas concentration is above said alarm threshold;

b) said throughput rate is above a minimum throughput rate; and

c) a selected region of said selected regions is not already at a maximum tunable range of said light source;

then

d) select a new broader range which meets all the criteria mentioned in Step 1

8) Continue to Step 2.

38. A method for more quickly determining the presence of a target gas as set forth in claim 37, wherein the step of selecting a broader range which meets all the criteria mentioned in Step 1 further comprises:

selecting additional data points including prior data points to reduce false alarm rates (FAR) by using additional time to collect said additional data points.

39. A target gas detection system, comprising:

a continuously-tunable laser operating at room temperature and tuned for exciting a sample of target molecules among other interferents; and

an L-PAS cell receiving light from said laser and containing gas to be sampled and tested for said target molecules;

whereby

detection of said target molecules can be made despite presence of interferent signals.

40. A gas detection system as set forth in claim 39, wherein said laser further comprises:

a high-power continuously-tunable laser.

41. A gas detection system as set forth in claim 39, wherein said laser further comprises:

a mode-hop diminished, broadly and continuously tunable continuous-wave room-temperature high-power external grating cavity quantum cascade laser system.

42. A gas detection system as set forth in claim 41, wherein said laser system further comprises:

driving said laser system to simultaneously tune a current driving said laser and an angle at which said grating is disposed, an FP wavelength comb of said laser shifted spectrally in coordination with said grating angle, said grating angle adjusted to keep a selected FP mode at a diminished diffraction grating-induced loss to promote selective lasing at said FP mode.

43. A gas detection system as set forth in claim 42, wherein said laser system further comprises:

driving said laser system and shifting said FP wavelength comb by varying an injection current to said laser.

44. A gas detection system as set forth in claim 43, wherein said laser system further comprises:

varying said laser injection current by;

determining a first current value necessary to shift said FP comb approximately exactly one free spectral range (FSR) of a gain chip of said laser;

tuning said laser in an external cavity geometry;

calculating a nominal laser current value necessary to have one of said gain chip'"'"'s FP modes exactly coincide with the desired output frequency in coordination with tuning said laser in said external cavity geometry;

if needed, remapping a nominal laser drive current value onto a value in an optimum current window

recalculating said current and setting it to a value close to maximum as the laser is tuned so that said nominal current value is about to become more than one periodicity value below maximum laser current;

continuing this process periodically as said laser system is broadly tuned;

whereby

a tuning range restriction is removed or avoided generally imposed by a limited dynamic range of said current.

45. A gas detection system as set forth in claim 43, wherein said laser system further comprises:

controlling said FP comb by selectably altering a temperature of said laser.

46. A gas detection system as set forth in claim 43 wherein said laser system further comprises:

an external cavity laser system instead of or in conjunction with said quantum cascade laser.

47. A quantum cascade laser, comprising:

a quantum cascade laser ridge waveguide chip bonded epi-side down to a heat sink.

48. A quantum cascade laser as set forth in claim 47, further comprising:

said heat sink being aluminum nitride (AlN) and said chip bonded to said heat sink with gold-tin (AuSn) eutectic solder.