Embodiments of this invention relate to beam combining of high-power broad area multimode semiconductor laser diodes in radiant space for long-distance directional laser energy applications such as directional infrared countermeasures and secure free-space communication, in which it is essential to maintain hole-free radiant intensity distribution across a large area.
Directional laser energy delivery systems for applications such as free space communications and directional infrared countermeasures (DIRCM) typically require laser beam delivery over long distances, i.e., a few to tens of kilometers, over which high radiant intensity needs to be maintained within a certain solid angle corresponding to a cross-sectional beam area of several square meters.
For such large cross-section areas, it is important that the high radiant intensity level variation within this area be as homogeneous as possible. More importantly, the radiant intensity level is preferably not lower than a certain minimal value that is defined by the particular application. Typically, this requirement is met by using low beam parameter laser sources such as single-spatial mode semiconductor laser diodes, solid state lasers or fibers lasers, given they provide sufficient output power to meet the radiant intensity requirement of the application. For certain applications such as DIRCM—in which a laser source is used to jam the seeker of the missile—one needs to use a plurality of laser sources to cover a wide spectral band of the seeker detector (typically, but not limited to, InSb) which is guided by detecting and locking to a specific spectral signature of the aircraft. To provide adequate protection for airborne platforms, modern DIRCM systems need to counter the threat at long distances (typically a few kilometers). Accordingly, such systems need to be able to provide a jamming signal of sufficiently high radiant intensity (i.e., orders of magnitude higher than the thermal signature of the platform itself) with high directionality. See David H. Titterton, “Military Laser Technology and Systems”, ISBN: 9781608077786, 2015, including pages 288-293, incorporated herein in its entirety and cited in part below:
A thermal signature of a typical airborne platform has distinct features across several spectral bands related to the thermal emissivity of hot metal and jet plume of the engine. Therefore, modern DIRCM systems combine more than one laser source emitting at different spectral bands in order to be able to mimic these thermal signatures. An ideal airborne DIRCM system is as small as possible due to strict airborne payload requirements and is as efficient as possible.
The most efficient laser type for this application is based on semiconductor laser technology, which offers unmatched performance and compactness, when it is available. Until recently, semiconductor lasers of sufficient performance for DIRCM application have been available only in the 0.8-1.5 micron spectral band and 3-5 micron spectral band, leaving an uncovered atmospheric transmission window in the 2.1-2.3 micron range. This window is typically covered using either a solid-state laser or fiber laser technology involving multiple pumping cascade schemes—e.g., a diode laser pump at 790 nm, pumping a Tm doped fiber, which in turn pumps Ho doped fiber. See G. Frith, et al., “Latest developments in 790 nm-pumped Tm-doped fibre laser systems for DIRCM applications,” Proc. Of SPIE Vol. 7115, 2008.; Ian Elder, “Thulium fibre laser pumped mid-IR source,” Proc. Of SPIE Vol. 7325, 2009; and H. D. Tholl, “Mid-infrared Semiconductor Lasers for Power Projection and Sensing”, Proc. Of SPIE Vol. 7836, 2010, incorporated herein in their entireties. This leads to a very bulky laser setup with a footprint 100 times bigger than a typical semiconductor laser, thereby compromising overall DIRCM system potential and usability.
Semiconductor laser diodes in the 2.1-2.3 micron spectral region have been available for years. See A. Vizbaras et al., “High-performance single-spatial mode GaSb type-I laser diodes around 2.1 micron,” Proc. of SPIE, Vol. 8993, 2014; and M. C. Kelemen et al., “Diode laser systems for 1.8 to 2.3 μm wavelength range,” Proc. of SPIE Vol. 7686, 2010, incorporated herein in their entireties. However, the output power of available single mode laser diode emitters is not sufficient for long distance laser delivery applications, and multimode emitters alone do not provide sufficient radiant intensity due to poor beam quality.
In an embodiment of the invention, a method includes combining the output of multiple multimode laser diode emitters in radiant space so that the radiant intensity of the combined beam within a particular selected solid angle is sufficient for directional long distance laser delivery applications such as DIRCM or free-space communications. This allows complete coverage of the spectral response window of the missile seeker with only semiconductor laser sources, providing radical reduction in overall systems size, efficiency, and cost.
In an aspect, embodiments of the invention relate to a method of combining a plurality of multimode laser beams in radiant space. The method includes the step of providing a plurality of laser beam emitters for generating the plurality of multimode laser beams. An individual multimode laser beam from each emitter is steered to partially overlap with at least one other multimode laser beam to form a combined beam with a homogeneous non-Gaussian radiant intensity distribution across a selected solid angle.
One or more of the following features may be included. Each laser beam emitter may include a broad area semiconductor laser with a multimode beam output. The laser beam emitters may be spatially separated from each other to define at least one of a one-dimensional array or a two-dimensional array.
Individual beam collimation optics may be provided for each laser beam emitter. The collimation optics may be a cylindrical lens, an aspheric lens, an aspheric cylinder lens, a spherical lens, a toroidal lens, a gradient-index lens, and/or a combination thereof. The collimation optics may be discrete, arranged in a monolithic one-dimensional array, and/or arranged in a two-dimensional array.
The laser beam emitter may include a laser diode, and the individual multimode laser beam may be steered from the emitter at a desired angle in radiant space by individually offsetting a position of the respective collimation optics with reference to an output plane of the laser diode.
The laser beam emitter may include a laser diode, and the individual multimode laser beam may be steered from the emitter by offsetting a position of the laser diode with respect to an optical axis of the respective beam collimation optics.
The laser beam emitter may include a laser diode, and beam combining may be performed without any offset between the laser diode and the respective collimation optics, and beam steering is performed by steering the laser diode and the respective collimation optics.
Steering the individual laser beam may include using an external optical element. The external optical element may be, e.g., optical mirrors, optical prisms, optical wedge pairs, optical beam splitters, optical dichroic mirrors, and/or combinations thereof.
In another aspect, embodiments of the invention relate to a laser-based assembly for combining a plurality of multimode laser beams in radiant space. The assembly includes a plurality of laser beam emitters, and a plurality of individual beam collimation optics, with one individual beam collimation optics being associated with each emitter. The laser beam emitters and collimation optics are spaced and arranged such that, in use, individual multimode laser beams from each emitter cooperate and overlap with at least one other multimode laser beam to form a combined beam with a homogeneous non-Gaussian radiant intensity distribution across a selected solid angle
One or more of the following features may be included independently of the other features. These features and preferred features may apply to each of the various aspects of the invention described herein.
Each laser beam emitter may include a broad area semiconductor laser with a multimode beam output.
Individual pairs of laser beam emitters and the respective collimation optics may be spatially separated to define at least one of a one-dimensional array or a two-dimensional array.
The collimation optics may include a cylindrical lens, an aspheric lens, an aspheric cylinder lens, a spherical lens, a toroidal, a gradient-index lens, and/or or a combination thereof.
Each laser beam emitter may include a laser diode. In use, the individual multimode laser beams from the emitters may each be steered at a desired angle in radiant space by individually offsetting a position of the individual beam collimation optics with reference to an output plane of the laser diode of the associated laser beam emitter. In use, the individual multimode laser beam from the emitters may each steered by offsetting a position of the laser diode with respect to an optical axis of the respective beam collimation optics.
Individual pairs of laser beam emitters and the respective collimation optics may be adapted and arranged to be steered to form the combined beam.
The laser-based assembly may further include a plurality of external optical elements spaced and arranged to steer the individual laser beams. The external optical elements may be optical mirrors, optical prisms, optical wedge pairs, optical beam splitters and optical dichroic mirrors, and/or combinations thereof.
In yet another aspect, embodiments of the invention relate to multi-spectral laser beam delivery system including at least two laser modules. Each laser module includes a laser-based assembly for combining a plurality of multimode laser beams in radiant space and includes a plurality of laser beam emitters and a plurality of individual beam collimation optics, with one individual beam collimation optics being associated with each emitter. The laser beam emitters and collimation optics are spaced and arranged such that, in use, individual multimode laser beams from each emitter cooperate and partially overlap to form a combined beam with a homogeneous non-Gaussian radiant intensity distribution across a selected solid angle. Each of the laser modules emits at a different spectral region selected from the group consisting of ultraviolet, infrared, near-infrared, short-wave infrared, mid-infrared and long-wave infrared, to provide a multispectral, radiantly combined laser beam at an output of the system.
Embodiments of this invention include using more than one spatially separated broad area multimode semiconductor laser diode, individual collimation optics, and combined output in radiant space with overlapping individual beams to form one combined beam characterized by a homogeneous radiant intensity distribution within a certain given divergence cone, typically a few mrads.
Referring to
Spatial mode distribution in the slow-axis direction is random and leads to an inhomogeneous far-field pattern. For directional laser energy delivery applications such as countermeasures, communication, or illumination, an important feature is the maintenance of a high and uniform radiant intensity level within a certain selected solid angle value. The term “solid angle” describes a two-dimensional angle in three dimensional space. This solid angle may have any shape (circular, rectangular, square, ellipse, etc.). For directional laser energy delivery applications, knowing the radiant intensity within a certain solid angle allows one to calculate and understand beam characteristics (far-field) at any distance from the source.
To meet the requirement of maintaining a high and uniform radiant intensity level within a certain selected solid angle, a flat-top or top-hat beam profile is needed that has a very well defined radiant intensity level across the beam and drops sharply at the edges. Ideally, the intensity drop is completely vertical at the beam edges (i.e., rectangular shape); however, this is not a realistic case. While this requirement is known, it is difficult to fulfill in practice for laser diodes, as laser diodes have a Gaussian-like beam in the fast axis. Referring to
In a conventional semiconductor laser module having more than one emitter system, the combination of parallel output beams of several multimode laser diode emitters that have collimated or uncollimated emission results in a radiant intensity profile similar to that of one emitter, i.e., nearly Gaussian shaped.
Referring to
Eventually, the individual beams completely overlap to form a combined beam with a Gaussian fast-axis projection at the far field 500. A dashed rectangle illustrates the 2D projection of the solid angle and a certain radiant intensity level, i.e., the selected solid angle 600. The solid angle and certain radiant intensity level are typically defined by the application, such as DIRCM or free-space communication, and have a 1-10 mrad divergence cone and 10s kW/str of radiant intensity. An objective of the disclosed system is to maintain a uniform and high level of radiation intensity within that cone. It can be seen that, in the case of parallel beams, the combined far-field 500 is Gaussian-like, which does not satisfy the requirement of a uniform radiant intensity 700 within the divergence cone/selected solid angle 600, as it has a high radiant intensity peak at the center and decaying radiant intensity values not meeting the requirement further away from the center due to the nature of Gaussian distribution.
The beam behavior in which a combined beam is formed from three separate beams that completely overlap is typically attained by placing separate emitters within close proximity, i.e., within a few to ten square millimeters. The combination of parallel output beams of such emitters, collimated or uncollimated, at long distances in radiant intensity space typically appears as a single source, as the beams expand due to divergence and the displacement of the overlapped beams is separated by the same few millimeters as the emitters, while the beams are orders of magnitude larger. For example, a beam may be a meter in diameter at a distance of a kilometer. The combined far field has a Gaussian-like shape for the fast-axis projection, maintaining a large portion of the beam energy at the tails. This configuration may be undesirable for directional laser beam delivery applications as these high energy tails (edges of Gaussian distribution) are typically excluded from the solid angle, and a lot of energy is thus wasted at the tails.
Referring to
Generally, in
More specifically,
In a particular embodiment, each of the three emitters 101, 102, 103 may be a broad emitter area laterally multimode laser diode. The three emitters may be spaced apart by a distance of typically, but not limited to, e.g., from 0.5 to 3 mm (typically defined by the size of collimating optics block). This distance is usually determined by the size of collimating lenses. The maximum size of optical elements is usually determined by the beam size requirements in near-field, as the beam size preferably fits certain apertures after it exits the laser diode system.
Each of the optics blocks 201, 202, 203 is typically, but not limited to, a system of fast-axis and slow-axis collimating a cylindrical and/or cylindrical lenses (also known as FAC and SAC lenses). The first collimating optics block 201 is offset by typically 1-5 micrometers in the y-axis direction, resulting in the beam from the first emitter 101 being steered upwards at an angle of typically 1-6 mrads. The offset directly depends on the effective focal length of the collimating optics block. In particular, the effective focal length directly determines the required offset distance for a given required steering angle. Required steering angles are determined from the requirements of the system.
The third collimating block 203 is offset by, e.g., 1-5 micrometers in the y-axis direction, resulting in the beam from the third emitter being steered downward at an angle of typically 1-6 mrad. This configuration results in a widened, flat-top-like beam having a full width of 3-18 mrad and an intensity of 10-100 kW/str across a 3-18 m wide cross-sectional area, at the far field at a distance of 1 km from the emitters. Moreover, the angle (divergence) values provided earlier also completely describe the beam's performance, i.e., width of cross-sectional area at any distance.
This configuration may be expanded to multiple laser diodes forming a 1-dimensional or 2-dimensional array with at least partially overlapping beams to provide a combined total beam with a homogeneous high-level radiant intensity distribution across a selected solid angle.
Beam steering of the individual emitter can be performed by offsetting one or all of the elements within the collimation optics assembly or offsetting the laser position with reference to the optical axis of the collimation optics assembly, as is known to one of skill in the art.
Referring to
A particular example of a very compact laser module, utilizing radiant beam combining technique of two broad area multimode semiconductor laser emitters is depicted in
The beam of each emitter is individually collimated using fast axis collimator lenses 211, 212 and slow-axis collimator lenses 221, 222. Fast-axis output beam profiles 301, 302 of the two emitters are individually steered by offsetting the fast-axis collimator lenses 211, 212 to form a radiantly combined beam in the far-field 501.
This procedure is further presented in step-by-step detail in
The main criteria for selecting the cavity dimensions are primarily performance based, i.e., the emitter preferably emits sufficient output power for the application, and the output beam preferably contains a sufficient amount of spatial modes to ensure a random and spatial hole free far field.
Accordingly, a semiconductor laser diode emitting in ultra-violet spectral region having an emitter width of 5 microns contains similar amount of lateral modes (multimode emission in the slow-axis) as a 120 micron wide emitter emitting in the mid-infrared. The cavity length is preferably selected to allow sufficient output power for the relevant application. The cavity length also defines the power consumption of the chip as well final cost per chip. In particular, the smaller the chip, the more chips can be produced on a single wafer and thus cost per chip can be lowered. For example, a single 3-inch GaSb epi-wafer can host up to about 6000 1.5 mm×0.12 mm lasers. Increasing the cavity length to 2 mm from 1.5 mm lowers the yield per wafer to only about 3600 lasers, thereby raising the chip cost by more than 60%. Therefore, it may be desirable to achieve performance requirements with the smallest footprint possible.
A single emitter is typically capable of >1 W of CW power with electrical-optical (E-O) efficiency of 10-30% emitting in the spectral band between 2.1 μm-2.3 μm, i.e., the atmospheric transmission window typically covered using either a solid-state laser or fiber laser technology
Offset_distance[μm]=lens_effective_focal_length[μm]*tan(required_steering_angle[rad]).
In an embodiment, the fast-axis collimator lens has a 0.6 mm effective focal length and a numerical aperture (NA) of 0.8, and the slow-axis collimator lens has a 15 mm effective focal length and 0.14 NA. The resulting beam has a new optical axis 310, and a resulting experimental 2D far field beam profile 510. In other embodiments, the fast-axis collimator lens may have an effective focal length selected from a range of 0.05 micron to 100 mm, and NA selected from a range of 0.1 to 1. The slow-axis collimator lens may have an effective focal length selected from a range of 0.05 micron to 100 mm, and NA selected from a range of 0.1 to 1. The main criteria for selecting the fast-axis and slow-axis collimators are semiconductor laser beam parameters, final system design requirements, and mechanical requirements specifications.
This arrangement of emitters provides numerous advantages. First of all, it allows leveraging otherwise poor beam quality of the multimode emitter and utilizing the much higher output power available compared to the single mode emitter. Secondly, radiant beam combining provides a large “top-hat” beam profile with a high radiant intensity which is less sensitive to beam wandering due to temperature and other environmental effects. For example, if the application requirement is to maintain a 10 kW/sr beam within a 5 mrad divergence cone, and the laser assembly, constructed as the embodiment in
Finally, radiant beam combining of multimode semiconductor lasers allows the realization of ultra-compact assemblies. For example, a two-emitter based module using radiant beam combining can be as small as 25 mm×40 mm×15 mm and have a weight <30 g. This is a significant size, weight, and power (SWaP) gain compared to any other solid state-laser technology.
The radiant beam combining technique disclosed herein can be realized using different lasers, for example GaAs-based emitting around 1 micron or AlGaInAs/GaInAs/InP based quantum cascade lasers emitting in the 3-5 micron and 8-10 micron spectral regions or GaN/AlGaInN based lasers emitting in the UV spectral band.
Furthermore, the disclosed radiant beam combining technique can be extended independently of the spectral region and applied to fabricating compact multi-spectral laser beam delivery systems using simple, robust and cheap broad area multimode emitters emitting in, e.g., ultraviolet, visible, near-infrared, short-wave infrared, mid-wave infrared, and/or long-wave infrared.
A simple block diagram of a multi-spectral laser beam delivery system 2000 is shown in
Each of the modules provides a radiantly combined output beam 501, 1051, 1151. These beams are further combined by means of optical elements such as dichroic mirrors 3010, 3020, 3030 to provide a multispectral laser beam 5000 at an output of the system. The output beam contains combined multi-spectral output beams of different laser subassemblies within the system, e.g., the system may emit optical radiation with a jamming pattern within all the spectral bands of interest such as ultra-violet, near-infrared, short-wave infrared, mid-wave-infrared and long-wave infrared, thereby being able to fully mitigate the thermal signature of the airborne platform in the entire spectral region.
Such a system is especially suited for jamming a heat-seeking missile, as it provides a very wide multi-spectral jamming signal with very high brightness and directionality, e.g., 1 kW/sr to multiple 100 kW/sr radiant intensity for a 1 mrad to 10 mrad divergence cone. Each of the different spectral channels can be modulated independently to provide a complex jam signal making it impossible to countermeasure. Similarly, such a multi-spectral laser beam delivery system can be used for covert free-space communication applications utilizing the immense bandwidth and the ability to change the channels within the bandwidth. Due to the fact that a system in accordance with embodiments of the invention may be constructed to emit in the entire 300 nm-10 000 nm spectral region, the ability to encode information within such an immense bandwidth and the ability to switch between a multitude of channels would make it extremely difficult if not impossible for the counterparty to successfully jam and intercept the signal.
The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.
This application is a U.S. National Stage Application filed under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2018/081380, filed Nov. 15, 2018, which claims priority to and the benefit of U.S. Provisional Patent Application No. 62/581,981, filed Nov. 17, 2017, the contents of which are incorporated herein by reference in their entireties.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2018/081380 | 11/15/2018 | WO |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2019/096910 | 5/23/2019 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
4009965 | Pryor | Mar 1977 | A |
5033043 | Hayakawa | Jul 1991 | A |
5566195 | Heppner et al. | Oct 1996 | A |
5610733 | Feldman et al. | Mar 1997 | A |
5757839 | Biswal et al. | May 1998 | A |
5793783 | Endriz | Aug 1998 | A |
5825552 | Kurtz et al. | Oct 1998 | A |
6064528 | Simpson, Jr. | May 2000 | A |
6144791 | Wach et al. | Nov 2000 | A |
6167016 | Block et al. | Dec 2000 | A |
6582875 | Kay et al. | Jun 2003 | B1 |
6714568 | Hunt | Mar 2004 | B2 |
6853490 | Wang et al. | Feb 2005 | B2 |
6901221 | Jiang et al. | May 2005 | B1 |
6971578 | Tsikos | Dec 2005 | B2 |
6975458 | Kanzler | Dec 2005 | B1 |
7046187 | Fullerton et al. | May 2006 | B2 |
7085304 | Vetrovec | Aug 2006 | B2 |
7429734 | Tidwell | Sep 2008 | B1 |
7554737 | Knox et al. | Jun 2009 | B2 |
7729574 | Moriarty | Jun 2010 | B2 |
7970040 | Sprangle et al. | Jun 2011 | B1 |
8215776 | Kessler et al. | Jul 2012 | B2 |
8334490 | Schaub et al. | Dec 2012 | B2 |
8502976 | Sharpe et al. | Aug 2013 | B2 |
8767790 | Sipes, Jr. | Jul 2014 | B2 |
8792978 | Wells et al. | Jul 2014 | B2 |
RE45177 | Galvanauskas et al. | Oct 2014 | E |
8909017 | Jasapara | Dec 2014 | B2 |
9001172 | Ghauri | Apr 2015 | B2 |
9031113 | Litvin et al. | May 2015 | B2 |
9093822 | Chann | Jul 2015 | B1 |
9147990 | Dueck | Sep 2015 | B2 |
9366872 | Honea et al. | Jun 2016 | B2 |
9554124 | Owurowa et al. | Jan 2017 | B1 |
9726818 | Yap et al. | Aug 2017 | B1 |
20010040214 | Friedman et al. | Nov 2001 | A1 |
20030063391 | Wang | Apr 2003 | A1 |
20040165268 | Turunen | Aug 2004 | A1 |
20050062638 | Zeineh | Mar 2005 | A1 |
20050147135 | Kurtz et al. | Jul 2005 | A1 |
20080062242 | Van Brocklin et al. | Mar 2008 | A1 |
20090231580 | Nagy et al. | Sep 2009 | A1 |
20110157706 | Mitra | Jun 2011 | A1 |
20120081893 | Faybishenko | Apr 2012 | A1 |
20120213513 | Chao | Aug 2012 | A1 |
20130148684 | Guo et al. | Jun 2013 | A1 |
20150003484 | Muendel | Jan 2015 | A1 |
20150277128 | Johnson | Oct 2015 | A1 |
20160380410 | Song | Dec 2016 | A1 |
20170189992 | Dittli et al. | Jul 2017 | A1 |
Number | Date | Country |
---|---|---|
1299167 | Jun 2001 | CN |
101055429 | Oct 2007 | CN |
100440538 | Dec 2008 | CN |
103180241 | Jun 2013 | CN |
105612620 | May 2016 | CN |
102010045620 | Mar 2012 | DE |
112015001710 | Jan 2017 | DE |
1883824 | Feb 2008 | EP |
WO-200250599 | Jun 2002 | WO |
WO-2007070080 | Jun 2007 | WO |
WO-2014125116 | Aug 2014 | WO |
WO-2017097923 | Jun 2017 | WO |
Entry |
---|
Dvinelis et al., “Band I DIRCM laser based on GaSb direct diode technology” Proceedings vol. 10637, Laser Technology for Defense and Security XIV; 106370B (2018) https://doi.org/10.1117/12.2304619, 6 pages. |
Dvinelis et al., “Next generation DIRCM for 2/1-2/3 micron wavelength based on direct-diode GaSb technology”, Proc. SPIE 10514, High-Power Diode Laser Technology XVI, 1051405, (Feb. 19, 2018) https://doi.org//10.1117/12.2289425, 7 pages. |
International Search Report for International Patent Application No. PCT/EP2018/081380, dated Mar. 1, 2019, 4 pages. |
Written Opinion for International Patent Application No. PCT/EP2018/081380, dated Mar. 1, 2019, 6 pages. |
Number | Date | Country | |
---|---|---|---|
20200403382 A1 | Dec 2020 | US |
Number | Date | Country | |
---|---|---|---|
62587690 | Nov 2017 | US |