COMPACT, HIGH POWER MID-WAVE INFRARED (MWIR) LASER SYSTEM

Mid-Wave Infrared (MWIR) laser systems emits at multiple wavelengths spanning the mid-IR transmission bands with tunability not to coincide with absorption lines within the bands. Optical fiber-based pump sources and a series of Raman fiber wavelength shifting amplifiers create a single output aperture that contains multiple spectral lines within each MWIR sub-band.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present invention relate, in general, to Mid-Wave Infrared (MWIR) laser systems and more particularly to a MWIR laser system having a single output aperture containing multiple spectral lines within each sub-band.

Relevant Background

Infrared imaging systems trace their origins to the year 1800 when John Frederik William Herschel's experiments on refraction of invisible rays using a prism and a monochromator discovered infrared radiation, which Herschel called “calorific rays.”

The development of modern infrared detectors became possible after John Bardeen and William Shockley invented the transistor in 1947. Thereafter, InSb, HgCdTe, and Si photon detectors were developed. Texas Instruments developed the first forward-looking infrared system in 1963, with production in 1966, and in 1969, the charge-coupled device (CCD) was developed by AT&T Bell Labs. Photon infrared technology combined with molecular beam epitaxy and photolithographic processes revolutionized the semiconductor industry, thus enabling the design and fabrication of complex focal plane arrays.

One application of infrared technology is infrared counter measures. An infrared countermeasure (IRCM) is a device designed to protect aircraft from infrared homing (“heat seeking”) missiles and the like by confusing the devices' infrared guidance system so that they miss their target (electronic countermeasure). Heat-seeking missiles were responsible for about 80% of air losses in Operation Desert Storm. The most common method of infrared countermeasure is deploying flares, as the heat produced by the flares creates hundreds of targets for the missile. Flares create infrared targets with a much stronger signature than the aircraft's engines providing false targets that cause the missile to make incorrect steering decisions, but flares are not entirely effective. An aircraft carries a fixed number of flares and once they are depleted do too is the aircraft's IRCM. Countermeasure systems are usually integrated into the aircraft, such as in the fuselage, wing, or nose of the aircraft, or fixed to an outer portion of the aircraft. Depending on where the systems are mounted, they can increase drag, reducing flight performance and increasing operating cost. Much time and money are spent on testing, maintaining, servicing, and upgrading systems. These procedures require that the aircraft are grounded for a period of time.

Directional Infrared Countermeasures, avoid this potential drawback by mounting the energy source on a movable turret (much like a FLIR turret). They only operate when cued by a missile warning system of a missile launch and use the missile plume to accurately aim at the missile seeker. The modulated signal can then be directed at the seeker, and the modulation scheme can be cycled to try to defeat a variety of seekers. Countermeasure success depends on a threat's tracking techniques and requires a proper analysis of the missile's capabilities. Defeating advanced tracking systems requires a higher level of DIRCM power. Unfortunately, such systems typically involve high-power laser sources (100-Watts or greater) that are “tunable” across the bands requiring complex and expensive tuning elements.

A need exists for a simple, cost effective IRCM system that operates at multiple wavelengths in the mid-infrared (mid-IR) atmospheric transmission bands of 3.5-4.1 μn and 4.6-4.9 μm. Such an approach would use laser systems that emit signals at a plurality of wavelengths that span the mid-IR transmission bands—ideally with enough tunability such that the emitted signals do not coincide with absorption lines within the bands yet be simpler and cost effective. Currently, a cascaded Raman amplifier is used passing a pump laser beam through a series of Raman media that are sequentially tuned to different Raman shifts. Each Raman medium amplifies the Stokes beam produced by the previous medium, resulting in a cascade of Raman shifts that converts the pump laser power into multiple mid-1R lines.

Another approach of the prior art involves coupling the pump laser beam into a resonant cavity containing a Raman medium. The Raman medium amplifies the Stokes beam produced by the pump laser, resulting in a laser emission at a longer wavelength. By using a plurality of Raman mediums to tune different Raman shifts, it is possible to generate multiple corresponding mid-1R laser lines from a single pump laser.

Lacking is a system that generates a plurality of mid-IR laser lines using vibrational and rotational Raman amplification. These and other deficiencies of the prior art are addressed by one or more embodiments of the present invention.

Additional advantages and novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities, combinations, compositions, and methods particularly pointed out in the appended claims.

The Figures depict embodiments of the present invention for purposes of illustration only. Like numbers refer to like elements throughout. In the figures, the sizes of certain lines, layers, components, elements, or features may be exaggerated for clarity. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.

DESCRIPTION OF THE INVENTION

A MWIR laser systems emits at multiple wavelengths spanning the mid-IR transmission bands with tunability to not coincide with absorption lines within the bands. The present invention uses optical fiber-based pump sources and a series of Raman fiber wavelength shifting amplifiers to create a single output aperture that contains multiple spectral lines within each required MWIR sub-band.

Embodiments of the present invention are hereafter described in detail with reference to the accompanying Figures. Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention.

The terminology used herein is for the purpose of describing embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces.

With reference toFIG.1, a MWIR laser system100according to one embodiment or the present invention emits at multiple wavelengths spanning the mid-IR transmission bands with tunability not to coincide with absorption lines within the bands. The present invention uses optical fiber-based pump sources110and Raman fiber wavelength shifting amplifiers to create a single output aperture that comprise multiple spectral lines within each required MWIR sub-band.

A high-power, near-infrared laser (near-IR) source110is introduced into a resonator120where vibrational transitions in a molecular gas shift the wavelength into the mid-IR. The output of the wavelength shifter is launched into a second resonator130where rotational excitations of gaseous molecules to replicate the laser signal, creating a collection of laser lines140that span a mid-IR transmission window. Vibrational Raman amplification is, in this implementation, more efficient than rotational Raman amplification because it requires the transfer of less energy. In vibrational Raman amplification, the energy is transferred from one photon to another photon. This transfer of energy causes the photons to change their wavelength. In rotational Raman amplification, the energy is transferred from one molecule to another molecule. This transfer of energy causes the molecules to change their wavelength but at the cost of more energy. In the present example, separate 50-W units are configured for spectral coverage of each of the two mid-IR bands and combined to meet requirements for total mid-IR power. The baseline for this approach employs Continuous Wave (CW) pump sources and produces CW output. In one version of the present invention, one or more MWIR systems are constructed with optical fiber for compactness and to increase interaction lengths in the wavelength shifters/replicators. Fiber modal properties are tailored to produce output beams with an M2of 1.5.

Vibrational Raman amplification and rotational Raman amplification are both processes that can amplify light through scattering, but they involve different types of molecular motions and have different characteristics.

In vibrational Raman amplification, a photon interacts with a molecule and excites its vibrational mode, which causes the molecule to temporarily store the energy of the photon. This stored energy is then re-emitted as a new photon with a lower frequency than the original photon. The emitted photon has a frequency that is shifted by an amount corresponding to the vibrational frequency of the molecule, which is characteristic of the molecule and can be used for identification and analysis. Vibrational Raman amplification is a relatively weak effect and requires intense laser light to achieve significant amplification.

Rotational Raman amplification, on the other hand, involves the interaction of light with the rotational motion of a molecule. In this process, a photon is scattered by a molecule and transfers some of its energy to the molecule, which causes the molecule to rotate. This interaction causes the light to be amplified or scattered, depending on the frequency of the incident light and the rotational properties of the gas molecules. The scattered photon has a frequency that is shifted by an amount corresponding to the rotational energy of the molecule. Unlike vibrational Raman amplification, rotational Raman amplification does not require a laser source and can occur with ordinary light sources such as sunlight. However, it is also a weaker effect than many other types of light scattering.

In more detail, when light passes through a gas, it may interact with the gas molecules through a process known as the Raman effect. This effect occurs when the energy of the photons in the light is absorbed by the gas molecules, causing them to vibrate and rotate. As the gas molecules vibrate and rotate, they emit photons at different frequencies, which can be either lower or higher than the frequency of the incident light.

Rotational Raman amplification specifically refers to the amplification of the lower-frequency photons that are emitted by the gas molecules during this process. This occurs because the gas molecules have a preferred orientation in their rotational motion, and the incident light can only interact with molecules that are oriented in a particular way. As a result, the scattered light is preferentially emitted in a particular direction, and this directional emission leads to amplification of the lower-frequency photons.

The main difference between vibrational Raman amplification and rotational Raman amplification is the type of molecular motion that is involved. Vibrational Raman amplification involves the excitation of vibrational modes, while rotational Raman amplification involves the excitation of rotational modes. Vibrational Raman amplification is a relatively weak effect that requires intense laser light, while rotational Raman amplification can occur with ordinary light sources but is also a weaker effect.

One aspect of the present invention is the selection of combinations of Raman media to efficiently shift the power from a high-power IR pump laser into several laser lines in the mid-IR. Location of Stokes-shifted laser lines for selected transitions in various Raman active media is shown inFIG.2. Of particular interest are gaseous media210, which have narrow Raman line widths, high gain coefficients, and can be used in hollow-core optical fibers to provide very long interaction lengths. One of reasonable skill in the relevant art will appreciate that one of the most efficient combinations of Raman media to shift the power from a high-power IR pump laser into several laser lines in the mid-IR is one or more combinations of CO2, N2, and He gas. These combinations are especially useful for applications that require high powers in multiple lines. CO2 is effective in shifting the power from the pump laser into the strongest mid-IR lines, while N2 and He can be used to shift the power into weaker lines in the mid-IR region. The advantage of this combination is that it can efficiently shift the power from the pump laser into multiple laser lines with high efficiency. Additionally, it can be used to generate multiple lines with different powers, which can be useful for applications that require specific power levels in different laser lines.

Recall, Raman scattering is a nonlinear optical process that can be used to shift the frequency of a laser beam to longer or shorter wavelengths. The present invention exploits this process to efficiently convert the power of a high-power infrared (IR) pump laser into several laser lines in the mid-IR region.

Another aspect of the present invention is lower MWIR band implementation. These implementations of the basic architecture produce multiple laser lines in the lower mid-wave IR band between 3.5 and 4.1 inn. One implementation of the Raman shifting architecture to produce laser lines in the lower mid-infrared band of the present invention is shown inFIG.3.

In this version of the present invention, Thulium-doped fiber lasers310with CW output power exceeding a kilowatt are used as a pump source for a nitrogen-filled hollow-core fiber320. The fiber is placed between mirrors325with reflectance, in this embodiment, at a maximum of 3.48 μm to resonate (using vibrational transitions) and enhance the Stokes shifted light. The output of the first resonator is introduced into a second resonator330where Raman shifting by way of rotational transitions in nitrogen gas, and corresponding mirrors335, is used to convert the 3.48 μm signal into multiple lines340in the lower mid-IR band. The first resonator employs narrow band mirrors on both sides that resonate only a single Raman shifted line. The second resonator uses broadband mirrors that resonate multiple Raman shifted lines in a cascade. A quarter wave plate350converts linearly polarized light to circularly polarized light for selective interaction with rotational Raman modes. The quarter-wave plate350converts linearly polarized light into circularly polarized light by introducing a phase shift of one quarter of a wavelength between two perpendicular components of the light. When linearly polarized light passes through a quarter-wave plate, it is split into two perpendicular components, one that experiences a phase shift of 90 degrees and another that does not experience any phase shift. The two components then recombine, but because of the phase shift, they are now out of phase with each other by 90 degrees. This results in circularly polarized light. Efficiency factors of 40%, 50%, and 50% have been achieved for the pump, first resonator, and second resonator, respectively, producing an overall wall-plug efficiency of 10%.

FIG.4presents the spectral output for the implementation shown inFIG.3. The band coverage410(shown in the left side ofFIG.4) is limited by the number of rotational transitions in nitrogen gas that have large, stimulated Raman gain. This instance of the present invention shows an expansion of wavelength coverage to the full 3.5 to 4.1 μm band. The right side ofFIG.4further shows that relatively small changes in the pump wavelength (i.e., the dashed lines420versus the solid lines430) can be used to tune the precise positions of the laser lines420. This flexibility is important for design of a system with maximum atmospheric transmission. Temperature tuning and pressure broadening are considered with respect to performance.

Various implementations of the present invention's basic architecture produce multiple laser lines, for example, in the upper mid-IR band between 4.6 and 4.9 inn. Yet another implementation of the present invention is shown inFIG.5. In this instance the combination of laser pump source and first Raman resonator are changed to produce outputs at longer wavelengths. A pump source510with a wavelength of 1.55 μm is created by starting with emission at 1.04 μm in an Ytterbium-doped fiber laser and cascading the signal to 1.55 μm through a series of Raman Stokes shifts. The use of erbium-doped fiber is an alternative 1.55 μm pump source.

In this case, the output of the fiber laser510is launched into a hollow-core fiber filled with hydrogen gas520, which has a larger Raman shift and produces a signal at a longer 4.31 μm wavelength, despite the shorter pump wavelength. The shifted light is then launched into the second resonator530to produce multiple laser lines540in the upper mid-IR band. As for the lower band source, efficiency factors of at least 40%, 50%, and 50% for the pump, first resonator, and second resonator can be achieved, respectively, producing an overall wall-plug efficiency of 10% or more.

FIG.6presents output results of the implementation shown inFIG.5. The narrower upper band610is fully spanned by the signals620, which have slightly larger separations at these wavelengths. A similar approach to the fine control of wavelength shifting would be as discussed in the previous lower band discussions.

The present invention uses a single beam line for each MWIR sub-band with sequential cascaded Raman spectral generators rather than combine many spectrally separate sources into a common aperture. This approach directly addresses manufacturing and life-cycle costs along with the ability to add spectral components (providing increased spectral power density) and power scaling of the overall system by increased pump power in future versions. Importantly, the architecture of the present invention is based upon CW kW-class diode-pumped fiber laser technologies for the Raman pump sources and utilizes the well understood and demonstrated Raman wavelength cascade processes in gas.

Another version of the present invention uses first rotational excitations in the first resonator creating a collection of laser lines from an initial input followed by a second resonator using vibrational transitions in a molecular gas shift shifting the wavelength of each laser line into the mid-IR. This embodies the versatility of a multiple resonator configuration to produce a plurality of laser lines in the mid-IR region.

FIG.7presents a flow chart for one methodology to form a plurality of Mid-Wave Infrared (MWIR) optical lines, according to one embodiment of the present invention. The process begins705by producing710a near-infrared optical seed by an optical source. The near-infrared optical seed has a near-infrared optical seed wavelength that may vary depending on the desired output. In one version of the present the near-infrared optical seed is generated from an optical fiber-based pump while in another version of the invention a continuous wave pump source is used. For example, one implementation of the invention can use Thulium-doped fiber lasers with CW output power exceeding a kilowatt. Similarly, a pump source can be used by starting with emission in an Ytterbium-doped fiber laser and cascading the signal through a series of Raman Stokes shifts to produce the desired seed.

The process thereafter shifts720, by a first Raman resonator, the near-infrared optical seed wavelength of the near-infrared optical seed using vibrational transitions. This creates a mid-infrared optical seed, wherein the mid-infrared optical seed has a mid-infrared optical seed wavelength greater than the near-infrared optical seed wavelength. The first Raman resonator reflects light between two semitransparent mirrors and lens systems until the light reaches a desired power and wavelength. At that point the light is directed through a quarter wave plate that converts the mid-infrared optical seed from linearly polarized light to circularly polarized light.

The now circularly polarized light, the mid-infrared optical seed having a mid-infrared optical seed wavelength) is again shifted730by a second Raman resonator, this time using rotational transitions. This shift and application of rotational transitions forms a plurality of mid-infrared optical lines, wherein each of the mid-infrared optical lines has a mid-infrared optical line wavelength greater than the mid-infrared optical seed wavelength.