LASER ASSEMBLY WITH ACTIVE POINTING COMPENSATION DURING WAVELENGTH TUNING

An assembly (10) for generating a laser beam (12) includes a beam steering assembly (18); a laser assembly (16) that is tunable over a tunable range; and a controller (20). The laser assembly (16) generates a laser beam (12) that is directed at the beam steering assembly (18). The controller (20) dynamically controls the beam steering assembly (18) to dynamically steer the laser beam (12) as the laser assembly (16) is tuned over at least a portion of the tunable range. As a result thereof, the laser beam (12) is actively steered along a desired beam path (12A) while the wavelength of the laser beam (12) is varied.

BACKGROUND

Semiconductor devices such as quantum cascade devices, interband cascade devices, and light-emitting diodes can be turned into tunable lasers through a variety of means. For example, a tunable wavelength selective element can be spaced apart from the semiconductor device to form a tunable, external cavity laser. In this design, the wavelength selective element is selectively tuned to adjust the center optical wavelength of a laser beam generated by the tunable laser.

The external cavity lasers can be used in spectroscopy applications where it is desired to provide a laser beam having a center optical wavelength (“wavelength”) that is varied over time over a tunable range, while recording a response of some sample as a function of the changing optical wavelength of the laser beam. In such applications, it is also often desired to rapidly tune the laser wavelength in a single sweep across the tunable range. This minimizes variations in the sample during data acquisition.

More specifically, external cavity lasers that generate light in the mid infrared (“MIR”) range are useful for absorption spectroscopy applications since many samples have their fundamental vibrational modes in the MIR range, and thus present strong, unique absorption signatures within the MIR range.

Unfortunately, existing tunable lasers assemblies are not capable of generating an accurate laser beam over a broad spectral range.

SUMMARY

The present invention is directed an assembly for generating a laser beam. In one embodiment, the assembly includes: a beam steering assembly; a laser assembly that is tunable over a tunable range, the laser assembly generating a laser beam that is directed at the beam steering assembly; and a controller that dynamically controls the beam steering assembly to dynamically steer the laser beam as the laser assembly is tuned over at least a portion of the tunable range. With this design, the beam steering assembly provides active beam pointing compensation, and the assembly generates an accurately steered laser beam that is tuned to span a predetermined output wavelength range.

Without active pointing compensation, a beam path of the laser beam will vary during tuning. For example, if it is desired to direct the laser beam at a target area on an object, without active pointing compensation, the beam path will vary, and the intensity of the laser beam on the target area will change as the assembly is tuned. In contrast, in one implementation of the assembly provided herein, the laser beam can be actively steered as the laser assembly is tuned to maintain the desired beam path of the laser beam.

In one implementation, the controller dynamically controls the beam steering assembly so that the laser beam is directed along a desired beam path while the laser assembly is tuned over at least a portion of the tunable range. In alternative non-exclusive embodiments, the controller dynamically controls the beam steering assembly so that the laser beam is directed along the desired beam path while the laser assembly is tuned over at least 50, 100, 250, 500, or 1000 cm−1wavelengths.

In alternative, non-exclusive examples, the size of the tunable (wavelength) range can be at least approximately 50, 100, 200, 300, 400, 500, 1000, 2000, 3000, 4000, 4500, or 5000 cm-1 wavelengths. However, the size of the tunable range can larger or smaller than these amounts.

In one embodiment, the desired beam path is constant along a desired axis. Alternatively, the desired beam path can be varied over time or relative to wavelength.

As provided herein, the controller can dynamically control the beam steering assembly so that the laser beam is directed at a substantially constant target area while the laser assembly is tuned over at least a portion of the tunable range. As used herein, the term “substantially constant target area” shall mean less than fifty μRadian deviation in pointing angle.

In certain alternative embodiments, the controller dynamically controls the beam steering assembly so that the laser beam is directed at the substantially constant target area while the laser assembly is tuned over at least sixty, seventy, eighty, ninety, or one hundred percent of the tunable range.

The controller can dynamically control the beam steering assembly so that the laser beam is directed within fifty μRadian micrometers of the target area while the laser assembly is tuned over at least a portion of the tunable range. In alternative, non-exclusive embodiments, the controller can dynamically control the beam steering assembly so that a compensation target error is less than five, ten, fifteen, twenty, or fifty microradians over the entire spectral sweep.

The beam steering assembly can include a first beam steerer and a spaced apart second beam steerer. At least one of the beam steerers can be selectively controlled to dynamically steer the laser beam as the laser assembly is tuned over the tunable range.

At least one of the beam steerers can include a reflector that is selective moved about a rotational axis to dynamically steer the laser beam as the laser assembly is tuned over at least a portion of the tunable range.

For example, the first beam steerer can include a first reflector that is selective moved about a first rotational axis and the second beam steerer can include a second reflector that is selectively moved about a second rotation axis to dynamically steer the laser beam as the laser assembly is tuned over at least a portion of the tunable range.

The controller can dynamically position the beam steerers as a function of wavelength so that the laser beam follows a desired beam path.

Further, the controller can dynamically control the beam steering assembly to dynamically steer the laser beam so that an optical power of the laser beam on a target area is optimized.

The laser assembly can include (i) a first laser module that generates a first beam when power is directed to the first laser module; and (ii) a second laser module that generates a second beam when power is directed to the second laser module. Further, the controller can dynamically control the beam steering assembly to alternatively direct the first beam and the second beam along an output axis.

In another implementation, the present invention is directed to a method for generating a laser beam comprising: providing a beam steering assembly; generating a laser beam that is directed at the beam steering assembly with a laser assembly that is tunable over a tunable range; and dynamically controlling the beam steering assembly with a controller to dynamically steer the laser beam as the laser assembly is tuned over at least a portion of the tunable range.

The method can include controlling the beam steering assembly so that the laser beam is directed along a desired beam path while the laser assembly is tuned over at least a portion of the tunable range.

Additionally or alternatively, the method can include controlling the beam steering assembly so that the laser beam is directed at a substantially constant target area while the laser assembly is tuned over at least a portion of the tunable range.

Additionally or alternatively, the method can include dynamically controlling the beam steering assembly to dynamically steer the laser beam so that an optical power of the laser beam on a target area is optimized.

In another implementation, the present invention is directed at an assembly for generating a laser beam. In this implementation, the assembly includes: a beam steering assembly; a laser assembly that is tunable over a tunable range, the laser assembly generating a laser beam that is directed at the beam steering assembly; and a controller that dynamically controls the beam steering assembly to dynamically steer the laser beam as the laser assembly is tuned over at least a portion of the tunable range. Additionally, this implementation can include one or more of the following features: (i) the controller dynamically controlling the beam steering assembly so that the laser beam is directed along a desired beam path while the laser assembly is tuned over at least a portion of the tunable range; (ii) the controller dynamically controlling the beam steering assembly so that the laser beam is directed at a substantially constant target area while the laser assembly is tuned over at least a portion of the tunable range; (iii) the controller dynamically controlling the beam steering assembly so that the laser beam is directed at the substantially constant target area while the laser assembly is tuned over at least sixty, seventy, eighty, ninety, or one hundred percent of the tunable range; (iv) the beam steering assembly having a first beam steerer and a spaced apart second beam steerer, with at least one of the beam steerers being selectively controlled to dynamically steer the laser beam as the laser assembly is tuned over the tunable range; (v) the controller dynamically positions the beam steerers as a function of wavelength so that the laser beam follows a desired beam path; and/or (vi) the controller dynamically controls the beam steering assembly to dynamically steer the laser beam so that an optical power of the laser beam on a target area is optimized.

With this design, the beam steering assembly can be dynamically adjusted so that the laser beam follows the desired beam path as the laser assembly is tuned over the tunable range, and/or the beam steering assembly can be dynamically adjusted so that the laser beam is pointed at a substantially constant position as the laser assembly is tuned over the tunable range.

DESCRIPTION

FIG.1is a simplified top view of an assembly10that generates an output laser beam12having active pointing compensation and/or control. With this design, the assembly10rapidly generates an accurately steered laser beam12that is tuned to span a predetermined output wavelength range (“tunable range”).

As provided above, without active pointing compensation, a beam path12A of the laser beam12will vary during tuning. For example, if it is desired to direct the laser beam12at a target area13A on an object13B (illustrated as a box), without active pointing compensation, the beam path12A will vary, and the intensity of the laser beam12on the target area13A will change as the assembly10is tuned. In contrast, in one implementation of the assembly10provided herein, the laser beam12can be actively steered as the laser assembly16is tuned to maintain the desired beam path12A of the laser beam12(e.g. along a desired beam axis12B).

Some of the Figures include an orientation system that illustrates an X axis, a Y axis that is orthogonal to the X axis, and a Z axis that is orthogonal to the X and Y axes. It should be noted that these axes can also be referred to as the first, second and third axes and or the axes can be changed.

As non-exclusive examples, the assembly10can provide a laser beam12for imaging, locating, detecting, and/or identifying a substance, e.g. a gas (not shown) or a trace element, analyzing a sample, and/or other industrial or testing applications. The assembly10is well suited for applications that require accurate and rapid broad spectral sweeps.

The desired predetermined output wavelength range can be varied to suit the desired application for the assembly10. For example, in many applications, a relatively large wavelength range is necessary to achieve specificity when analyzing mixtures of chemicals. Further, the resolution between different spectral signatures for different chemicals increases as the spectral range that is being analyzed is increased, thus allowing individual components to be detected.

In one embodiment, the assembly10is designed to generate a laser beam12that consists of a set of sequential, specific output pulses of light having a center wavelength that is varied over time to span the entire or just a portion of the mid-infrared range of approximately two to twenty (2-20) micrometers. With this design, the assembly10is particularly useful in absorption spectroscopy applications since many gases of interest have strong, unique absorption signatures within the mid-infrared range. Alternatively, the assembly10can be designed to generate one or more pulses of light having a center wavelength of greater than or lesser than two to twenty micrometers. For example, in another embodiment, the tunable range is only a portion of the MIR range. As alternative, non-exclusive examples, the tunable range can be the wavelength range of approximately 2-10 micrometers; 10-20 micrometers; 5-15 micrometers; 5-10 micrometers; 10-15 micrometers; or 15-20 micrometers. Stated in another fashion, the tunable range can be at least five, six, seven, eight, nine, ten, twelve, fifteen or eighteen micrometers. In additional, alternative non-exclusive examples, the tunable range can be the wavelength range of approximately 500-5000 cm-1; 500-1000 cm-1; 1000-1500 cm-1; 1500-2000 cm-1; 2000-2500 cm-1; 2500-3000 cm-1; 3000-3500 cm-1; 3500-4000 cm-1; 4000-4500 cm-1; or 4500-5000 cm-1.

In one embodiment, the assembly10includes (i) a frame14, (ii) a laser assembly16that is tunable over the tunable range, (iii) a beam steering assembly18, and (iv) a controller20that dynamically controls the beam steering assembly18to dynamically steer the laser beam12and provide active pointing compensation as the tunable laser assembly16is tuned over at least a portion of the tunable range. The design of each of these components can be varied pursuant to the teachings provided herein. Further, it should be noted that the assembly10can be designed with more or fewer components than described herein.

The frame14supports at least some of the components of the assembly10. InFIG.1, the laser assembly16, the beam steering assembly18, and the controller20are each fixedly secured to the frame14; and the frame14maintains these components in precise mechanical alignment. Alternatively, for example, the controller20can be separate from and external to the frame14.

In one embodiment, the frame14can include a rigid frame base14A; four side walls14B, and a top cover (not shown) secured to the top of the side walls14B to create a chamber (not shown). In certain embodiments, the chamber can be sealed to provide a controlled environment for the sensitive components of the assembly10. For example, the chamber can be filled with an inert gas, or another type of fluid, or subjected to vacuum.

Additionally, in certain embodiments, the frame14includes a window14C that allows the laser beam12to exit the frame14, and a shutter (not shown) for safety that selectively opens and closes the window14C. In the non-exclusive embodiment illustrated inFIG.1, the window14C is a wedge shaped element that redirects the laser beam12so that the laser beam12is directed substantially parallel to the Z axis as it exits the frame14. Alternatively, for example, the window14C can be another shape. As alternative, non-exclusive examples, the wedged shaped window14C can be at an angle of five, ten, fifteen, or twenty degrees. Alternatively, other angles can be utilized.

The laser assembly16is selectively tunable over the predetermined wavelength range. The laser assembly16can include one or more laser modules (“channels”)22,24,26,28, and one or more director assemblies30,32,34,36that cooperate to direct the laser beam12at the beam steering assembly18. The number and/or design of the laser modules22,24,26,28can be varied pursuant to the teachings provided herein to achieve the desired output wavelength range. In one, non-exclusive embodiment, the laser assembly16includes four, spaced apart laser modules22,24,26,28. Alternatively, the laser assembly16can be designed to include more than four, or fewer than four laser modules22,24,26,28. In one, non-exclusive embodiment, each of the laser modules22,24,26,28is somewhat similar in design, except for its spectral output. For example, each of the laser modules22,24,26,28can be specifically designed to generate a different portion (or partly overlapping portion) of the predetermined wavelength range. Thus, the number of laser modules22,24,26,28can be increased to increase the predetermined wavelength range, with each laser module22,24,26,28generating a separate portion of the predetermined wavelength range.

As provided herein, in one embodiment, power is sequentially directed to (i) the first laser module22(“first channel”) to generate a first beam22A that consists of a plurality of sequential first pulses of light that span a first range portion; (ii) the second laser module24(“second channel”) to generate a second beam24A that consists of a plurality of sequential second pulses of light that span a second range portion; (iii) the third laser module26(“third channel”) to generate a third beam26A that consists of a plurality of sequential third pulses of light that span a third range portion; and (iv) the fourth laser module28(“fourth channel”) to generate a fourth beam28A that consists of a plurality of sequential fourth pulses of light that span a fourth range portion. With this design, the first beam22A, the second beam24A, the third beam26A, and the fourth beam28A can be sequentially used to provide the pulses of light that cover the entire predetermined wavelength range. It should be noted that the order of firing of the laser modules22,24,26,28can be any arrangement.

As a specific, non-exclusive example, (i) the first range portion can be approximately 6.5 to 7.5 micrometers; (ii) the second range portion can be approximately 7.5 to 8.5 micrometers; (iii) the third range portion can be approximately 8.5 to 9.5 micrometers; and (iv) the fourth range portion can be approximately 9.5 to 10.5 micrometers. In this example, each beam22A,24A,26A,28A has a center wavelength in the MIR range.

In one embodiment, each laser module22,24,26,28is an extended cavity, mid infrared laser. It should be noted that one or more of the other laser modules22,24,26,28can be similar in design. In the embodiment illustrated inFIG.1, each of the laser modules22,24,26,28is similar in design. Moreover, inFIG.1, each laser module16includes a module frame38, a gain medium40, a cavity optical assembly42, an output optical assembly44, and a wavelength selective (“WS”) feedback assembly46. The design of each of these components can be varied.

The module frame38provides a rigid support for the components that are part of the laser module16. In certain embodiments, the module frame38is made of a rigid material having a relatively high thermal conductivity to readily transfer heat away from the gain medium40.

The gain medium40for each laser module22,24,26,28can directly emit the respective beams22A,24A,26,28A without any frequency conversion in the mid infrared range. As non-exclusive examples, the gain medium40for one or more of the laser modules22,24,26,28can be a Quantum Cascade (QC) gain medium, an Interband Cascade (IC) gain medium, or a mid-infrared diode.

As provided herein, the fabrication of each gain medium40can be altered to achieve the desired output frequency range for each gain medium40. For example, the gain medium40of the first laser module22can be fabricated to have a tuning range that matches the desired first range portion; the gain medium40of the second laser module24can be fabricated to have a tuning range that matches the desired second range portion; the gain medium40of the third laser module26can be fabricated to have a tuning range that matches the desired third range portion; and the gain medium40of the fourth laser module28can be fabricated to have a tuning range that matches the desired fourth range portion. As a non-exclusive example, the thickness of the wells/barriers of a Quantum Cascade gain medium determine the wavelength characteristic of the respective Quantum Cascade gain medium. Thus, fabricating a Quantum Cascade gain medium of different thickness enables production of the laser having different output frequencies within the MIR range.

In this embodiment, each gain medium40includes (i) a first facet that faces the respective cavity optical assembly42and the wavelength selective element46, and (ii) a second facet that faces the output optical assembly44, and each gain medium40emits from both facets. In one embodiment, each first facet is coated with an anti-reflection (“AR”) coating, and each second facet is coated with a reflective coating. With this design, for each laser module22,24,26,28, the reflective second facet of the gain medium40acts as a first end (output coupler) of an external cavity, and the wavelength selective element46defines a second end of the each external cavity.

The cavity optical assembly42is positioned between the gain medium40and the feedback assembly46along a lasing axis47of the respective laser module22,24,26,28. The cavity optical assembly42collimates and focuses the beam that passes between these components. For example, each cavity optical assembly42can include one or more lens. For example, the lens can be an aspherical lens having an optical axis that is aligned with the respective lasing axis47.

The output optical assembly44is positioned between the gain medium40and the respective beam director assembly30,32,34,36in line with the lasing axis47to collimate and focus the respective beam22A,24A,26,28A that exits the second facet. For example, each output optical assembly44can include one or more lens that are somewhat similar in design to the lens of the cavity optical assemblies42.

The wavelength selective element46reflects the beam back to the gain medium40, and is used to precisely select and adjust the lasing frequency of the external cavity and the wavelength of the pulses of light. In this manner, the respective beams22A,24A,26,28A may be tuned with the wavelength selective element46without adjusting the respective gain medium40. Thus, with the external cavity arrangements disclosed herein, the wavelength selective element46dictates what wavelength will experience the most gain in each laser module22,24,26,28.

A number of alternative embodiments of the wavelength selective element46can be utilized. InFIG.1, the wavelength selective element46includes a grating46A, a grating mover46B (e.g. a voice coil actuator), and a feedback detector46C. The grating mover46B selectively moves (e.g. rotates about the X axis in this example) the grating46A to rapidly adjust the lasing frequency of the gain medium40. Further, the rotational position and/or movement of the grating46A can be continuously monitored with the feedback detector46C that provides for closed loop control of the grating mover46B. As non-exclusive examples, for each laser module22,24,26,28, the grating mover46B moves the grating46A to adjust the angle of incidence θ over the entire adjustment range to scan the wavelength range in less than approximately 0.01, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, or more seconds.

The feedback device46C generates a grating feedback signal that relates to the position of the respective grating46A and/or the angle of incidence θ of the beam on the respective grating46A. As a non-exclusive example, the feedback device46C can be an optical encoder that includes a plurality of encoder marks, and an optical reader. As provided herein, each laser modules22,24,26,28has its own feedback device46C. With this design, the wavelength of each beam22A,24A,26A28A can be selectively tuned in a closed loop fashion.

Alternatively, for example, the wavelength selective element46can be another type of frequency selective element. A discussion of the techniques for realizing the full laser tuning range from a semiconductor device can be found in M. J. Weida, D. Caffey, J. A. Rowlette, D. F. Arnone and T. Day, “Utilizing broad gain bandwidth in quantum cascade devices”, Optical Engineering 49 (11), 111120-111121-111120-111125 (2010). As far as permitted, the contents of this article are incorporated herein by reference.

As provided herein, in certain embodiments, for each laser modules22,24,26,28there is a corresponding director assembly30,32,34,36. More specifically, (i) a first director assembly30is used to precisely direct the first beam22A from the first laser module22at the beam selector assembly18; (ii) a second director assembly32is used to precisely direct the second beam24A from the second laser module24at the beam selector assembly18; (iii) a third director assembly34is used to precisely direct the third beam26A from the third laser module26at the beam selector assembly18; and (iv) a fourth director assembly36is used to precisely direct the fourth beam28A from the fourth laser module28at the beam selector assembly18. Stated in another fashion, the beams22A,24A,26A,28A are redirected by the director assemblies30,32,34,36to converge on the beam steering assembly18. The design of each director assembly30,32,34,36can be varied pursuant to the teachings provided herein.

In certain embodiments, with the present design, the director assemblies30,32,34,36, and the beam steering assembly18are designed to reflect and direct the beams22A,24A,26A,28A without rotating or changing the polarization of the beams22A,24A,26A,28A. Due to the architecture of reflective beam steering optics in a common plane with the beam steering assembly18, the assembly can have a polarization that is substantially common across the entire multi-module range.

In one embodiment, each beam22A,24A,26A,28A is incident on the beam steering assembly18at a different angle, at approximately the same location48(“zero point’). With the present design, the director assemblies22,24,26,28can be used to correct the direction, pitch and yaw of the beams22A,24A,26A,28A. In one non-exclusive embodiment, each director assembly30,32,34,36includes a pair of redirectors, namely a first redirector49A and a second redirector49B that is spaced apart from the first redirector49A. In this embodiment, the pair of redirectors49A,49B reflect and redirect the respective beam22A,24A,26A,28A at the zero point48of the beam steering assembly18. In one embodiment, each redirector49A,49B includes a mirror that redirects the respective beam22A,24A,26A,28A.

InFIG.1, each beam22A,24A,26A,28A exits its respective laser module22,24,26,28substantially parallel to the Z axis. Next, the first redirector49A of each laser module22,24,26,28redirects the respective beam22A,24A,26A,28A approximately along the X axis. Subsequently, the second redirector49B of each laser module22,24,26,28redirects the respective beam22A,24A,26A,28A substantially along (but not parallel to) the Z axis at the beam steering assembly18.

In this embodiment, each redirector49A,49B is secured to the frame base14A and each redirector49A,49B is independently adjustable so that the angle of incidence of each beam22A,24A,26A,28A on the beam steering assembly18can be selectively adjusted. For example, each redirector49A,49B can be independently adjustable about a first axis and about a second axis that is perpendicular to the first axis relative to the fame base14A. For example, the first redirectors49A can be adjustable about the X and Y axes, and the second redirectors49B can be adjustable about the X and Z axes. With this design, the laser modules22,24,26,28can be attached to the frame14, and subsequently, the redirectors49A,49B can be independently adjusted to achieve the desired angle of incidence of each beam22A,24A,26A,28A on the beam steering assembly18. Alternatively, the director assemblies30,32,34,36can be designed so that only one of the redirectors49A,49B is selectively adjustable.

The beam steering assembly18is controlled by the controller20to individually select which of the beams22A,24A,26A,28A becomes the output beam12directed along the beam path12A. Further, the beam steering assembly18is controlled by the controller20to actively steer the output beam12to actively control the desired beam path12A as the laser assembly16is tuned. With this design, the beam steering assembly18can be actively controlled by the controller20to compensate for the pointing of the laser beam12during tuning of the laser assembly16.

In one embodiment, the beam steering assembly18actively steers the output beam12to compensate for variations that occur during tuning of the laser assembly16to maintain the output beam12directed along the desired beam path12A. For example, the beam steering assembly18can actively steer the output beam12to maintain the output beam12pointed at the target area13A during tuning of the laser assembly16. Alternatively, for example, the beam steering assembly18can actively steer the output beam12along a moving desired beam path12A during tuning of the laser assembly16. The design of the beam steering assembly18can be varied to achieve the design requirements of the assembly.

InFIG.1, the beam steering assembly18includes a first beam steerer50and a second beam steerer52that is spaced apart from the first beam steerer50. The design of each beam steerer50,52can be varied. InFIG.1, (i) the first beam steerer50includes a first reflector50A, a first mover50B that selectively moves (e.g. rotates) the first reflector50A, and a first position sensor50C (illustrated as a box) that monitors the position of the first reflector50A; and (ii) the second beam steerer52includes a second reflector52A, a second mover52B that selectively moves (e.g. rotates) the second reflector52A, and a second position sensor52C (illustrated as a box) that monitors the position of the second reflector50A. With this design, the controller20(i) controls the first mover50B to precisely position the first reflector50A using feedback from the first position sensor50C; and (ii) controls the second mover52B to precisely position the second reflector52A using feedback from the second position sensor52C.

FIG.2is a perspective view of the first beam steerer50including the first reflector50A, the first mover50B, and the first position sensor50C. In this embodiment, (i) the first reflector50A is a flat, rectangular shaped mirror, (ii) the first mover50B is a voice coil motor that selectively rotates the first reflector50A about a first rotational axis50D, and (iii) the first position sensor50C is an encoder or Hall type sensor that provides the rotational position of the first reflector50A. Alternatively, each of these components can have a different design. For example, the first reflector50A can be a multifaceted polygonal mirror.

Somewhat similarly,FIG.3is a perspective view of the second beam steerer52including the second reflector52A, the second mover52B, and the second position sensor52C (not visible inFIG.3). In this embodiment, (i) the second reflector52A is a flat, rectangular shaped mirror, (ii) the second mover52B is a voice coil motor that selectively rotates the second reflector52A about a second rotational axis52D, and (iii) the second position sensor52C is an encoder or Hall type sensor that provides the rotational position of the second reflector52A. Alternatively, each of these components can have a different design. For example, the second reflector52A can be a multifaceted polygonal mirror.

Referring back toFIG.1, the individual beams22A,24A,26A,28A are directed at the first beam steerer50at different angles, and the first beam steerer50is selectively positioned to select which of the beams22A,24A,26A,28A is directed at the second beam steerer52to become the output beam12. With this design, the first mover50B can selectively position the first reflector50A at alternative rotational positions about the first rotational axis52D (illustrated as a plus sign inFIG.1because the first rotational axis is orthogonal to the page) to redirect (select) one of the beams22A,24A,26A,28A at the second beam steerer52.

FIG.4Ais a simplified top illustration of the first reflector50A, the second reflector52A, the first laser beam22A (dotted line), the second laser beam24A (dot-dashed line), the third laser beam26A (long dashed line), and the fourth laser beam28A (short dashed line). InFIG.4A, the laser beams22A,24A,26A,28A are incident on the first reflector50A at different angles, and the first reflector50A is in a first selector position454A which directs (selects) the first laser beam22A at the second reflector52A to become the output beam12.

Similarly,FIG.4Bis a simplified top illustration of the first reflector50A, the second reflector52A, the first laser beam22A (dotted line), the second laser beam24A (dot-dashed line), the third laser beam26A (long dashed line), and the fourth laser beam28A (short dashed line). InFIG.4B, the laser beams22A,24A,26A,28A are incident on the first reflector50A at different angles, and the first reflector50A is in a second selector position454B which directs (selects) the second laser beam24A at the second reflector52A to become the output beam12.

Further,FIG.4Cis a simplified top illustration of the first reflector50A, the second reflector52A, the first laser beam22A (dotted line), the second laser beam24A (dot-dashed line), the third laser beam26A (long dashed line), and the fourth laser beam28A (short dashed line). InFIG.4C, the laser beams22A,24A,26A,28A are incident on the first reflector50A at different angles, and the first reflector50A is in a third selector position454C which directs (selects) the third laser beam26A at the second reflector52A to become the output beam12.

Further,FIG.4Dis a simplified top illustration of the first reflector50A, the second reflector52A, the first laser beam22A (dotted line), the second laser beam24A (dot-dashed line), the third laser beam26A (long dashed line), and the fourth laser beam28A (short dashed line). InFIG.4D, the laser beams22A,24A,26A,28A are incident on the first reflector50A at different angles, and the first reflector50A is in a fourth selector position454D which directs (selects) the fourth laser beam28A at the second reflector52A to become the output beam12.

With this design, the movement of the first reflector50A about the first rotational axis50D (a single axis movement) is used to select the beam22A,24A,26A,28A that forms the laser beam12. The selector positions454A-454D that individually select each laser beam22A,24A,26A,28A can be indexed and saved in the controller20(illustrated inFIG.1).

InFIGS.4A-4D, all of the beams22A,24A,26A,28A are illustrated as being directed at the beam steering assembly18at once. This occurs when sufficient power is directed to all of the laser modules (not shown inFIGS.4A-4B) at the same time. Typically, however, sufficient power will be directed to only one laser module (not shown inFIGS.4A-4B) at any given time. With this example, only one of the beams22A,24A,26A,28A will be directed at the beam steering assembly18at any given time.

Importantly, as provided above, the beam steerers50,52additionally can be controlled to actively steer the output beam12as a function of wavelength. InFIG.1, the first beam steerer50is controlled to steer the respective beam22A,24A,26A,28A in the horizontal plane, and the second beam steerer53is controlled to steer the respective beam22A,24A,26A,28A in the vertical plane. Stated in another fashion, the first reflector50A is rotated about the first rotational axis50D and the second reflector50B is rotated about the second rotational axis52D to precisely steer the output beam12along the desired beam path12A during tuning. InFIG.1, the first rotational axis50D is orthogonal to the second rotational axis52D. With this design, rotation of two reflectors50A,52A about separate axes50D,52D results in the ability to adjust the beam path12A.

It should be noted that (i) the first reflector50A can be moved within a small, first range of rotational positions (including the first selector position454A) and still direct the first laser beam22A at the second reflector52A to become the output beam12; (ii) the first reflector50A can be moved within a small, second range of rotational positions (including the second selector position454B) and still direct the second laser beam24A at the second reflector52A to become the output beam12; (iii) the first reflector50A can be moved within a small, third range of rotational positions (including the third selector position454C) and still direct the third laser beam26A at the second reflector52A to become the output beam12; and (iv) the first reflector50A can be moved within a small, fourth range of rotational positions (including the fourth selector position454D) and still direct the fourth laser beam22D at the second reflector52A to become the output beam12.

As a result thereof, (i) the first reflector50A can be moved within the first range of rotational positions to actively steer the first laser beam22A during tuning of the first laser module22; (ii) the first reflector50A can be moved within the second range of rotational positions to actively steer the second laser beam24A during tuning of the second laser module24; (iii) the first reflector50A can be moved within the third range of rotational positions to actively steer the third laser beam26A during tuning of the third laser module26; and (iv) the first reflector50A can be moved within the fourth range of rotational positions to actively steer the fourth laser beam28A during tuning of the fourth laser module28.

Similarly, the second reflector52A can be moved within a small, span of rotational positions to actively steer the respective laser beam22A,24A,26A,28A that is incident on the second reflector52A during tuning of the respective laser module22,24,26,28.

FIGS.5A and5Bare alternative, simplified top illustrations of the first reflector50A, the second reflector52A, and the first laser beam22A (dotted line). InFIGS.5A and5B, the first reflector50A is positioned within the first range of rotational positions, and the second reflector52A is positioned within the span of rotational positions so that the first beam22A becomes the steered laser beam12. More specifically, inFIG.5A, the first reflector50A is at rotational position1A, and the second reflector52A is at rotational position1B. Further, inFIG.5B, the first reflector50A is at rotational position2A which is different from rotational position1A, and the second reflector52A is at rotational position2B which is different from rotational position1B.

It should be noted that the other beams24A,26A,28A can be actively steered in a similar fashion. Thus, the reflectors50A,52A can be individually rotated as necessary as a function of wavelength to provide active pointing compensation for the output beam12.

Referring back toFIG.1, the controller20controls at least a portion of the operation of the assembly10. In certain embodiments, the controller20can control the wavelength and steering of the laser beam12by individually controlling (i) the current that is directed to each laser module22,24,26,28; (ii) the position of each grating46A; and (iii) the position of each reflector50A,52A. The controller20can include one or more processors20A and one or more electronic storage devices20B. InFIG.1, the controller20is illustrated as a centralized unit. Alternatively, the controller20can be a distributed controller.

In certain embodiments, the controller20is designed to support high speed buses. Further, in certain embodiments, the controller20can be controlled with a laptop or smart phone that is connected with a USB or wireless link.

The controller20can direct current to each laser module22,24,26,28in a pulsed fashion or a continuous fashion.

In certain embodiments, the controller20sequentially directs power to each laser modules22,24,26,28so that only one laser module22,24,26,28is firing at one time. In an alternative embodiment, the controller20can simultaneously direct power to the laser modules22,24,26,28to fire all the laser module22,24,26,28at the same time. In this embodiment, the beam steering assembly18can quickly select the output laser beam12from the various laser beams22A,24A,26A,28A to quickly select four alternative wavelength ranges for the output laser beam12.

It should be noted that when the laser modules22,24,26,28are sequentially operated, less power is consumed, and less heat is generated than if all of the modules22,24,26,28are powered at once. This simplifies the thermal management of the system.

Further, the controller20can direct power slightly below what is required to lase the on-deck (next activated) laser module22,24,26,28just prior to it being used for the laser beam12to allow for quick transitions (switch times) between laser modules16,18,20,22. This reduces the time required to achieve beam stability when transitioning between laser modules16,18,20,22. In this embodiment, the controller20directs (i) power to the laser modules22,24,26,28so that only one of the laser modules22,24,26,28is firing at one time, and (ii) power to the beam steering assembly18so that the beam steering assembly18directs that firing beam along the beam path12A, while providing directional compensation for the laser beam12as the laser assembly16is tuned.

In one embodiment, the laser assembly16is tuned, and one or more pulses can be generated having approximately the same first center wavelength (“first target wavelength”). Subsequently, the laser assembly16can be tuned, and one or more pulses can be generated having approximately the same second center wavelength (“second target wavelength”) that is different from the first center wavelength. Next, the laser assembly16can be tuned, and one or more pulses can be generated having approximately the same third center wavelength (“third target wavelength”) that is different from the first and second target wavelengths. This process can be repeated to a plurality of additional target wavelengths throughout a portion or the entire tunable range. As non-exclusive examples, the number of pulses at each discrete target wavelength can be 1, 5, 10, 50, 100, 200, 500, 1000, 10000 or more.

Further, the number of discrete target wavelengths in the tunable range can be varied according to the application. As non-exclusive examples, the number of discrete target wavelengths utilized can be approximately 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 40, 200, 226, 400, 552 or 4000 within the tunable range.

In one embodiment, each laser modules22,24,26,28can be individually calibrated using a wavelength measurement device (not shown) during manufacturing to determine the correlation between the feedback signals and the wavelength of the respective laser beam22A,24A,26A,28A. With this design, each position feedback signal of each laser modules22,24,26,28can be corresponded to a measured center wavelength of the laser beam22A,24A,26A,28A. Thus, each module22,24,26,28can be calibrated at the module level prior to installation into the system.

Additionally, or alternatively, after the modules22,24,26,28are added to the assembly10, the entire assembly10can be wavelength calibrated using a wavelength measurement device (not shown). In this embodiment, with the assembly10activated, each laser module22,24,26,28can be sequentially operated while monitoring position of the respective grating46A, and the wavelength of the output pulses of the laser beam12with the measurement device. With this design, the assembly10can be wavelength calibrated, and the controller20can determine a center wavelength of the output pulses of the laser beam12based on the position signal of the respective gratings46A of the laser modules22,24,26,28.

The collection of accurate spectra requires that the wavelength of the laser beam12be precisely known as the assembly10is tuned. In certain embodiments, the controller20directs pulses of power to the respective gain medium40based on the feedback signal received from the respective feedback detector46C. In this example, the controller20can direct a pulse of power to the gain medium40every time the optical reader46C reads a predetermined number of encoder marks. For example, the predetermined number can be one, two, or three encoder marks.

With this design, the controller20can, in sequential fashion, (i) selectively direct pulses of power to the gain medium40of the first laser module22based on a first feedback signal, (ii) selectively direct pulses of power to the gain medium40of the second laser module24based on a second feedback signal, (iii) selectively direct pulses of power to the gain medium of the third laser module26based on a third feedback signal, and (iv) selectively direct pulses of power to the gain medium40of the fourth laser module28based on a fourth feedback signal.

With this design, each laser module22,24,26,28can be controlled to generate a set of sequential, specific, different wavelength pulses that span a portion of the desired wavelength range. In one non-exclusive example, each laser module22,24,26,28can be controlled to sequentially generate approximately one thousand different wavelength output pulses that cover a detection range of approximately two micrometers in the mid-infrared range. However, the number of different pulses and the range can be different than this example.

The duration of each pulse of power directed by the controller20to the gain medium40can also be varied. In alternative, non-exclusive embodiments, controller20can control each pulse of power to have a duration of approximately 10, 25, 50, 75, 100, 150, 200, 300, 400, 500, 600 or 700 nanoseconds.

Additionally, the assembly10can be steering calibrated using a steering measurement device (e.g. a camera, not shown) during manufacturing of the assembly10. More specifically, with the assembly10activated, each laser module22,24,26,28can be sequentially operated while monitoring the beam path12A of the laser beam12as the wavelength is changed. For each targeted wavelength, the reflectors50A,52A can be rotated as necessary to achieve the desired beam path12A. With this design, the rotational position of each reflector50A,52A (measured by the position sensors50C,52C) necessary to achieve the desired beam path12A can be wavelength calibrated, and the controller20can position each reflector50A,52A as necessary to achieve the desired beam path12A as the wavelength is tuned.

Stated in another fashion, the assembly10can be steering calibrated by determining for each target wavelength the corresponding rotational positions of each reflector50A,52A necessary to achieve the desired beam path12A. Each separate wavelength will have a corresponding first reflector50A position and a corresponding second reflector52A position that compensates for beam drift. This information can be put into a lookup table along with the grating46A position information required to generate each target wavelength. Subsequently, the controller20can use this information from the lookup table to generate an accurately tuned laser beam12with active pointing compensation that compensates for beam drifting to reduce targeting error as the laser beam12is tuned.

In one embodiment, during the operation of each laser module22,24,26,28, the pulsing of the power to the respective gain medium40, and the rotational position of each beam steerer50,52can be tied directly to the angular position of the respective grating46A using a phase-locked-loop (PLL) technique where the position feedback signals from the feedback detector46C are up-converted in frequency and phase locked to the angular signals to allow the pulses of power to be fired at precise angular increments, with the beam steerers50,52correctly positioned to actively steer the laser beam12.

It should be noted that the steering calibration can be performed at different temperatures to generate a separate look-up table for different temperature ranges. With this design, the controller20can use the appropriate look-up table that corresponds to the current temperature to provide improved beam steering compensation for each temperature range. As a result thereof, the laser beam12can be accurately steered as a function of wavelength.

It should be noted that the assembly10can be designed to include more or fewer components than described above. For example, as illustrated inFIG.1, the assembly10can include one or more spatial filters56that suppress/block stray light. In this embodiment, the spatial filter56is positioned along the path of the output beam12between the second reflector52A and the window14C. For example, the spatial filter56can include a block having a transmission aperture56A (e.g. a pinhole or slit) centered on the path of the output beam12. With this design, the spatial filter56will block any light that deviates too far off of the path of the laser beam12.

FIG.6Ais a simplified schematic of a target area613A on an object613B, and a laser beam612directed at the target area613A with the assembly10(illustrated inFIG.1). In this schematic, the object613B is illustrated as a box, the target area613A is illustrated as a circle, and the incident laser beam612is also illustrated as a small circle. In this embodiment, the assembly10is controlled so that the laser beam612is always incident on the target area613A as the wavelength is tuned. Thus, even as the wavelength of the laser beam612is tuned, the beam steering assembly18(illustrated inFIG.1) will adjust reflector50A,52A (illustrated inFIG.1) position as a function of wavelength to maintain the laser beam612incident on the target area613A. This will optimize the optical powder of the laser beam612on the target area613A.

The laser beam612can have a beam cross-section area, and the target area613A can have a target cross-sectional area. Typically, the present invention keeps the centroid of beam on the target of a much smaller area than the size of the beam. As alternative, non-exclusive embodiments, the beam cross-section area (diameter) can be several millimeters while maintaining a pointing of less than fifty microradians.

FIG.6Bis a graph that plots a position of the laser beam on the object versus wavelength/time. InFIG.6B, solid line660A represents the X axis position of the incident laser beam on the object, and dashed line660B represents the Y axis position of the incident laser beam on the object. In this example, the controller20(illustrated inFIG.1) dynamically adjusts the beam steering assembly18(illustrated inFIG.1) to maintain the X axis and Y axis position of the laser beam constant as the wavelength changes over time (laser assembly tuned). As a result thereof, the laser beam with follow the desired beam path that has a fixed desired axis.

FIG.7Ais another simplified schematic of an object713B, and the laser beam712directed at the object713B by the assembly10(illustrated inFIG.1). In this schematic, the object713B is illustrated as a box, and the incident laser beam712is illustrated as a plurality of small circles to represent that the laser beam712is be actively moved relative to the object713B over time. In this embodiment, the assembly10is controlled so that the laser beam712is steered in a desired pattern as the wavelength is tuned. Thus, even as the wavelength of the laser beam712is tuned, the beam steering assembly18(illustrated inFIG.1) will adjust as a function of wavelength to maintain the laser beam712incident on the desired beam path. Alternatively, the beam steering assembly18can steer as a function of time.

FIG.7Bis a graph that plots a position of the laser beam on the object versus wavelength/time. InFIG.7B, solid line760A represents the X axis position of the incident laser beam on the object, and dashed line760B represents the Y axis position of the incident laser beam on the object. In this example, the controller20(illustrated inFIG.1) dynamically adjusts the beam steering assembly18(illustrated inFIG.1) to vary the X axis and Y axis position of the laser beam on the object as the wavelength changes over time (laser assembly tuned). As a result thereof, the laser beam with follow the desired beam path that has a variable desired axis. Further, the beam steering assembly18can independently modulate the pointing position of the laser beam as desired.

FIG.8is a perspective view of a portion of an assembly810including (i) a frame814, (ii) a laser assembly816that is tunable over the tunable range, (iii) a beam steering assembly818including a first beam steerer850and the second beam steerer852, and (iv) a controller (not shown) that dynamically controls the beam steering assembly818. InFIG.8, these components are similar to the corresponding components described above and illustrated inFIG.1.

FIG.9is a simplified top schematic illustration of another embodiment of the assembly910that generates an output beam912. In this embodiment, the assembly910includes (i) a frame914, (ii) a laser assembly916that is tunable over the tunable range, (iii) a beam steering assembly918, and (iv) a controller920that dynamically controls the beam steering assembly918that are similar to the corresponding components described above and illustrated inFIG.1.

However, inFIG.9, the assembly910additionally includes a separate spatial filter956for each laser beam922A,924A,926A,928A positioned before the beam steering assembly918. With this design, each spatial filter956can block any stray light in each respective laser beam922A,924A,926A,928A. It should be noted that the assembly910can be designed with a spatial filter956for only some of the laser beams922A,924A,926A,928A. Further, the spatial filters956can be used in any of the designs provided herein.

FIG.10is a simplified top schematic illustration of still another embodiment of the assembly1010that generates an output beam1012. In this embodiment, the assembly1010includes (i) a frame1014, (ii) a laser assembly1016that is tunable over the tunable range, (iii) a beam steering assembly1018, and (iv) a controller1020for dynamically controlling the beam steering assembly1018that are similar to the corresponding components described above and illustrated inFIG.1.

However, in this embodiment, the assembly1010additionally includes a sensor assembly1062that analyzes the output beam1012before it exits the frame1014. In this embodiment, the sensor assembly1062includes a beam pickoff1064, and a sensor1066. For example, the beam pickoff1064(i) can be positioned between the beam steering assembly1018and the window1014C along the path of the output beam1012, (ii) can pick off a test beam portion1068(illustrated with a dashed line) from the output beam1012, and (iii) can direct the test beam portion1068at the sensor1066. As a non-exclusive example, the beam pickoff1064can be a one degree pickoff.

The sensor1066can be used to sense one or more conditions of the laser beam1012. For example, the sensor1066can measure a wavelength of the laser beam1012. Alternatively or additionally, for example, the sensor1066can be used to measure the drifting of the laser beam1012. The information from the sensor1066can be used by the controller1012to better control the laser assembly1016and/or the beam steering assembly1018. For example, a quad-cell detector can be used to measure actual pointing changes of the beam and use the control system to maintain fixed pointing. In certain embodiments, the sensor1066can be used for closed loop control of the beam steering assembly1018. In one embodiment, the lookup table can be used for coarse corrections of the beam steering assembly1018, and the sensor1066information can be used for fine corrections of the beam steering assembly1018.

It should that the sensor assembly1062could be alternatively or additionally positioned before the beam steering assembly1018to test one or more of the beams1022A,1024A,1026A,1028A.

FIG.11is a simplified top schematic illustration of still another embodiment of the assembly1110that generates an output beam1112. In this embodiment, the assembly1110includes (i) a frame1114, (ii) a laser assembly1116that is tunable over the tunable range, (iii) a beam steering assembly1118, and (iv) a controller1120for dynamically controlling the beam steering assembly1018.

In this embodiment, the frame1114, the beam steering assembly1118, and the controller1120are somewhat similar to the corresponding components described above. However, in this embodiment, the laser assembly1116is slightly different. More specifically, in this embodiment, the laser assembly1116includes a single laser module1122.

As provided herein, the assemblies10,810,910,1010,1110can be used in any application that requires an accurate, tunable laser beam12,912,1012,1112. A couple of non-exclusive uses for the assemblies are described below and illustrated inFIGS.12,13and14.

FIG.12is simplified illustration of a substance sensor system1270that utilizes the assembly1210to analyze a substance1272e.g. an emitting gas. In this embodiment, the sensor system1270includes (i) the assembly1210similar to that disclosed herein that generates an laser beam1212that illuminates the area near the emitting gas1272, and (ii) an imager1274(i.e. an infrared camera) that captures real-time, high resolution thermal images of the emitting gas1272that can be displayed or recorded for future viewing. As non-exclusive examples, the sensor system1270is useful for locating substances1272(i.e. leaks) in the oil, gas, utility, chemical industries, as well as locating emitting gas for homeland security. In one embodiment, the type of substance1272detectable by the sensor system1270can include any gas having molecules that absorb (“absorption features”) in the MIR range.

FIG.13is simplified illustration of another embodiment of a sensor system1370having features of the present invention. In this embodiment, the sensor system1370is a spectrometer that includes (i) an assembly1310(similar to those described above) that generates a laser beam1312consisting of a plurality of output pulses, (ii) a flow cell1376that receives a substance1372(e.g. a liquid, gas or solid), and (iii) an imager1374. In this embodiment, the laser beam1312is directed through the flow cell1376, and the imager1374captures images of the light that is transmitted through the flow cell1376. Alternatively, for example, the sensor system1370can be a reflective system.

FIG.14is a simplified schematic illustration of a sample1480and a non-exclusive embodiment of an imaging microscope1482having features of the present invention. In particular, the imaging microscope1482can be used to analyze and evaluate the various properties of the sample1480. For example, in one embodiment, the imaging microscope1482is an infrared imaging microscope that uses tunable laser radiation to spectroscopically interrogate one or more samples1480in order to analyze and identify the properties of the sample.

The sample1480can be a variety of things, including human tissue, animal tissue, plant matter, explosive residues, powders, liquids, solids, inks, and other materials commonly analyzed using Fourier transform infrared (FTIR) microscopes. More particularly, in certain non-exclusive applications, the sample1480can be human tissue and the imaging microscope1482can be utilized for rapid screening of the tissue sample1480for the presence of cancerous cells and/or other health related conditions; and/or the imaging microscope1482can be utilized in certain forensic applications such as rapid screening of the sample1480for the presence of explosive residues and/or other dangerous substances.

Further, the sample1480can be thin enough to allow study through transmission of an illumination beam, e.g., an infrared illumination beam, through the sample1480(i.e. in transmission mode), or the sample1480can be an optically opaque sample that is analyzed through reflection of an illumination beam, e.g., an infrared illumination beam, by the sample (i.e. in reflection mode). For example, in the embodiment illustrated inFIG.14, the imaging microscope1482can alternatively be utilized in both transmission mode and reflection mode.

The design of the imaging microscope1482can be varied. In the embodiment illustrated inFIG.14, the imaging microscope1482includes (i) two of the assemblies1410that are similar to the assemblies described above that generate laser beams1412; (ii) a stage assembly1484that retains and positions the sample1480, (iii) an imaging lens assembly1486(e.g., one or more lenses1486A,1486B), and (iv) an image sensor1488that converts an optical image into an array of electronic signals. The design of each of these components can be varied pursuant to the teachings provided herein.

In one embodiment, the assemblies1410each emits a temporally coherent, illumination beam1412that is usable for illuminating and analyzing the sample1480in transmission mode; and/or (ii) emits a temporally coherent, illumination beam that is usable for illuminating and analyzing the sample1480in reflection mode.

A suitable imaging microscope1482is described in more detail in PCT Application No. PCT/US2012/061987, having an international filing date of Oct. 25, 2012, entitled “Infrared Imaging Microscope Using Tunable Laser Radiation”. As far as permitted, the contents of PCT/US2012/061987, are incorporated herein by reference.