Laser Beam Combining: Advancing Laser Directed Energy Weapons with Wavelength and Spectral Precision

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Introduction

In the realm of modern warfare, laser directed energy weapons (LDEWs) are emerging as game-changing technologies. These weapons harness the power of lasers to deliver precision strikes and deliver high-energy beams to incapacitate or destroy targets. One critical aspect driving the effectiveness of LDEWs is laser beam combining. By combining multiple laser beams into a single, high-intensity beam, military forces can achieve enhanced power and efficiency in laser weaponry. In this blog article, we explore the concept of laser beam combining, its importance in LDEWs, and the role of wavelength and spectral precision in optimizing these advanced weapon systems.

 

Fiber lasers have emerged most promising technology, for directed energy weapons due to their many advantages like: high electrical to-optical efficiency (40%), high reliability for operation in harsh military environments, and high beam quality near diffraction-limited light output. However, the power of state-of-the-art single-mode fiber lasers is limited by thermal and nonlinear effects like thermal lensing to ∼10kW.

For deeper understanding of fiber laser technology and applications please visit Breaking New Ground with Fiber Lasers: Emerging Applications and Challenges

Laser Beam Combining: The Essence of Laser Directed Energy Weapons

Laser beam combining is the process of merging multiple laser beams with individual characteristics, such as wavelength, phase, and polarization, into a single, powerful beam. The combination of these beams results in a consolidated, high-intensity output capable of delivering substantial energy to a target. This technology is crucial in the development of LDEWs, where power, precision, and energy efficiency are of paramount importance.

Types of Laser Beam Combining

1. Coherent Beam Combining:
   • Coherent beam combining involves combining laser beams with a consistent phase relationship, resulting in constructive interference.
   • This technique is particularly advantageous in achieving high beam quality and maximizing the energy concentration on a target.
2. Incoherent Beam Combining:
   • Incoherent beam combining combines laser beams without enforcing a specific phase relationship, resulting in additive power summation.
   • This method allows for flexible power scaling, making it suitable for certain LDEW applications.

For a deeper understanding of Laser Beam combining and the challenges of SBS and TMI please visit: Laser Beam Combination: Principles, Techniques, and Applications

Wavelength and Spectral Precision in Laser Beam Combining

The wavelength of a laser beam plays a crucial role in the performance of LDEWs. The choice of wavelength depends on the specific application and the target material. Longer wavelengths, such as infrared (IR), are better suited for atmospheric transmission and are less affected by scattering and absorption. Shorter wavelengths, like ultraviolet (UV), offer high precision and minimal atmospheric interference but have limited transmission range.

1. Spectral Beam Combining:
   • Spectral beam combining involves combining laser beams with different wavelengths to enhance overall power output.
   • By using multiple laser sources with complementary wavelengths, the system can cover a broader spectral range, enabling adaptability for different targets and atmospheric conditions.
2. Raman Beam Combining:
   • Raman beam combining is a nonlinear process where a high-power beam interacts with a low-power beam, causing the low-power beam to shift in wavelength.
   • This process allows for efficient wavelength conversion and spectral coverage beyond the limitations of conventional laser sources.

 

Comparison of Coherent and Wavelength Beam combining

Combining multiple low-power lasers with good beam quality into one high-power beam helps in overcoming power limitations of fiber lasers. There are numerous ways of combining laser beams and some are better suited to a given application than others; e.g. in the trivial case of a couple of beams a simple beam-splitter arrangement may suffice but there are limitations in this approach and it rapidly becomes impractical as the number of constituent beams increases.

 

Coherent beam combining involves phase-locking multiple laser beams so that they all have the same phase and combine constructively to form a single beam. This technique requires active feedback control of the laser beams’ phases, and typically, all the lasers have the same wavelength. The combined output beam has a narrow spectral linewidth and high beam quality. CBC can be used to achieve high output powers, but it is typically limited to a few lasers due to the need for precise phase control.

Wavelength beam combining, on the other hand, involves combining laser beams of different wavelengths to form a single beam with a broader spectral range. This technique does not require phase control, but rather relies on the spectral properties of the individual lasers and their output couplers. WBC is useful for combining many lasers, as each laser can operate independently, and the combined beam has a wider spectral bandwidth. However, the output beam quality is lower than CBC due to the difference in beam properties between the individual lasers.

 

A team from Lincoln Labs and Northrop Grumman Aerospace Systems (Redondo Beach, CA) have compared coherent and wavelength beam combination with commercial 500 W ytterbium-fiber lasers. Coherent combination was harder to implement because it requires phase control to within a small fraction of a wavelength, close path-length matching, narrow laser linewidth, and uniform polarization, they found, but it offers high spectral brightness and allows phase adjustments for beam steering or atmospheric compensation. Wavelength beam control is simpler to implement but produces a broader spectrum, so it is best suited for applications requiring raw power delivery where spectral brightness is not important.

 

Spectral beam combining

Spectral beam combining is a technique used to increase the power of laser beams by combining multiple laser beams of different wavelengths into a single beam. The process involves combining the beams in a way that maintains their coherence and preserves their spectral characteristics.

 

The basic idea behind spectral beam combining is to take advantage of the fact that different laser wavelengths can be combined without interfering with each other. This allows multiple lasers to be combined into a single output beam without degrading the quality of the individual beams.

 

There are different techniques used for spectral beam combining, such as the use of diffraction gratings, dichroic mirrors, or interference filters. These techniques are used to separate the different wavelengths of the lasers and recombine them into a single beam.

 

Spectral beam combining has several advantages, including increased power, improved beam quality, and reduced cost compared to traditional methods of increasing laser power. It has applications in a wide range of fields, including material processing, laser cutting, medical procedures, and defense applications.

So far, the most successful approach to producing weapon-grade fiber lasers has been spectral beam combination—essentially a military-strength version of dense wavelength-division multiplexing. It uses many separate single-mode fiber lasers to generate beams at a series of closely spaced wavelengths  and  using dispersive optics to  multiplex together efficiently into a high-power beam delivered by a fiber. This avoids the complexities of phase matching but produces wider-band laser emission. Although the resulting beam can be nearly diffraction limited,power scaling is constrained by the available source spectral brightness and laser gain bandwidth.

 

Different types of Spectral beams combining

Spectral beam combining is a technique used in laser systems to combine multiple laser beams of different wavelengths into a single beam with a broader spectral range. There are several techniques used for spectral beam combining, including the use of diffraction gratings, dichroic mirrors, and interference filters.

1. Diffraction Gratings: In this technique, a diffraction grating is used to combine multiple laser beams of different wavelengths. The laser beams are directed to the diffraction grating, which splits the beams into their respective wavelengths. The different wavelengths are then recombined by the grating to form a single beam with a broader spectral range. This technique is particularly effective for high-power lasers as it can handle large beam diameters.
2. Dichroic Mirrors: Dichroic mirrors are coated with a thin layer that reflects light at one wavelength and transmits light at another wavelength. In this technique, multiple laser beams of different wavelengths are directed to the dichroic mirror, which reflects the light of each wavelength in a different direction. The reflected beams are then recombined to form a single beam with a broader spectral range. This technique is particularly useful for compact laser systems.
3. Interference Filters: Interference filters are thin-film coatings that transmit light of a specific wavelength and reflect light of other wavelengths. In this technique, multiple laser beams of different wavelengths are directed through the interference filter, which transmits the light of each wavelength and reflects the light of other wavelengths. The transmitted beams are then recombined to form a single beam with a broader spectral range. This technique is particularly effective for lasers with narrow spectral linewidths.

In summary, spectral beam combining is a technique used to combine multiple laser beams of different wavelengths into a single beam with a broader spectral range. Different techniques such as diffraction gratings, dichroic mirrors, and interference filters can be used to achieve spectral beam combining depending on the specific requirements of the laser system.

 

Lockheed Martin demonstrated that approach by building 96 ytterbium-doped fiber lasers, each emitting 300 W at a different wavelength in the ytterbium band. At the 2017 CLEO meeting, Eric Honea of Lockheed reported that the spectral beam combination generated a 30-kW beam with more than 95 percent combination efficiency.

 

Powers now are reaching the 100-kW class. Last year Lockheed delivered a 60-kW version to the Army Space and Missile Defense Systems Command in Huntsville that has wall-plug efficiency of 35 to 40 percent. This year, Lockheed received a U.S. Navy contract to build a pair of spectral beam combination fiber lasers emitting 60–150 kW—one for tests at sea, the other for use on land. And the DoD’s Missile Defense Agency says spectral beam combination of fiber lasers could allow scaling to a proposed future 300-kW generation of laser weapons within five or six years.

 

At the University of Central Florida (Orlando, FL), Leonid Glebov has used volume Bragg gratings to coherently combine two fiber-laser beams with 99% efficiency to generate 282 W. Spectral beam combining is commonly implemented using either surface diffraction gratings or volume Bragg gratings (VBGs). VBGs have the advantage of being modular and can be easily scaled when high channel numbers are desired. The focus of the research presented in this paper is on spectral beam combining by volume Bragg gratings recorded in photo-thermo-refractive glass.

“To make systems such as the one just described compact, lower their complexity and minimize the induced thermal distortions, we propose the use of special volume Bragg elements which have several Bragg gratings written inside as combining optical components. The properties of the PTR glass allow the recording of several VBGs in a single piece. Therefore the complexity of the setup could be decreased proportional to the number of gratings recorded together. “

Advantages and Applications of Laser Beam Combining in LDEWs

1. Increased Power and Range:
   • Laser beam combining enables LDEWs to achieve higher power levels, extending the effective range of the weapon.
   • This capability improves the lethality and versatility of laser weaponry in both offensive and defensive operations.
2. Improved Targeting and Precision:
   • Combining multiple beams with coherent properties enhances the beam’s focus and alignment, resulting in superior target tracking and precision strikes.
   • The ability to adjust the wavelength and spectral properties optimizes laser energy delivery for specific target materials.
3. Energy Efficiency:
   • By combining the energy of several laser beams, LDEWs can achieve high-energy output with reduced energy consumption.
   • This improved energy efficiency enhances the weapon’s operational sustainability and reduces logistical demands.

Conclusion

Laser beam combining is a cutting-edge technology driving the advancement of laser directed energy weapons in modern warfare. Through precise wavelength and spectral control, military forces can develop highly effective, long-range, and energy-efficient laser weapons capable of delivering lethal and precise strikes. As research and development in laser technology continue, laser beam combining is poised to play an even more significant role in shaping the future of defense weaponry, offering enhanced capabilities and superior battlefield performance for military forces worldwide.