Beam combining of laser arrays with high efficiency and good beam quality for power and radiance (brightness) scaling is a long-standing problem in laser technology. Recently, significant progress has been made using wavelength (spectral) techniques and coherent (phased array) techniques, which has led to the demonstration of beam combining of a large semiconductor diode laser array (100 array elements) with near-diffraction-limited output (M/sup 2//spl sim/1.3) at significant power (35 W). This paper provides an overview of progress in beam combining and highlights some of the tradeoffs among beam-combining techniques.

Laser beam combining for high-power, high-radiance sources

The first rare-earth-doped fiber lasers were operated in the early 1960s, and produced a few milliwatts at a wavelength around 1 mum. Since the beginning of the decade, an enormous increase of fiber laser output power has been reported, the realm of kilowatt power has been entered, and power levels as high as 100 kW are envisaged. Apart from the power, fiber laser systems are renowned for their inherent compactness, monolithic architecture, and a power-independent beam quality. This paper reviews the challenges, achievements, and perspectives of high-power continuous-wave (CW) laser generation and amplification in fibers.

The Rising Power of Fiber Lasers and Amplifiers

Fiber lasers are well suited to scaling to high average power using beam-combining techniques. For coherent combining, optical phase-noise characterization of a ytterbium fiber amplifier is required to perform a critical evaluation of various approaches to coherent combining. For wavelength beam combining, we demonstrate good beam quality from the combination of three fiber amplifiers, and we discuss system scaling and design trades between laser linewidth, beam width, grating dispersion, and beam quality.

Beam combining of ytterbium fiber amplifiers (Invited)

An EUV light source is disclosed that may include a laser source, eg CO2 laser, a plasma chamber, and a beam delivery system for passing a laser beam from the laser source into the plasma chamber Embodiments are disclosed which may include one or more of the following; a bypass line may be provided to establish fluid communication between the plasma chamber and the auxiliary chamber, a focusing optic, eg mirror, for focusing the laser beam to a focal spot in the plasma chamber, a steering optic for steering the laser beam focal spot in the plasma chamber, and an optical arrangement for adjusting focal power

Laser produced plasma euv light source

A novel, highly accurate, all electronic technique for phase locking arrays of optical fibers is demonstrated. We report the first demonstration of the only electronic phase locking technique that doesn’t require a reference beam. The measured phase error is λ/20. Excellent phase locking has been demonstrated for fiber amplifier arrays.

First experimental demonstration of self-synchronous phase locking of an optical array

A four-element fiber array has been constructed to yield 8 watts of coherently phased, linearly polarized light energy in a single far field spot. Each element consists of a 2-W single-mode fiber-amplifier chain. Phase control of each element is achieved with a lithium-niobate phase modulator. A master laser provides a linearly polarized, narrow linewidth signal that is split into five channels. Four channels are individually amplified using polarization maintaining fiber power amplifiers. Frequency broadening of the signal is necessary to avoid stimulated Brillouin scattering. The fifth channel is used as a reference arm. It is frequency shifted and then combined interferometrically with a portion of each channel's signal. Detectors sense the heterodyne modulation signal, and an electronics circuit measures the relative phase for each channel. Compensating adjustments are then made to each channel's phase modulator. The stability of the optical train is an essential contributor to its success. A state-of-the-art interferometer was built with mountless optics. A lens array was constructed using nano-positioning tolerances, where each lens was individually aligned to its respective fiber to collimate its output and point it at a common far field spot. This system proved to be highly robust and handled any acoustic perturbations.

8-W coherently phased 4-element fiber array

A scalable high power laser system includes a plurality of parallel connected modular power amplifier arms, coupled to a common master oscillator to provide a high average power laser system with a scalable output power level, particularly suitable for laser weapon systems with varying power level output applications. Adaptive optics devices are provided in order to provide pre-compensation of phase front distortions due to the modular amplifier arms as well as encode the wave front of the laser beam with a phase conjugate of atmospheric aberrations.

High average power solid-state laser system with phase front control

A single-mode fiber laser (66) that provides increased power. The fiber laser (66) includes a fiber ribbon (30) having a plurality of parallel waveguides (10). Each waveguide (10) includes a rectangular shaped core (12) that has a relatively thin dimension in one direction and a relatively wide dimension in an orthogonal direction. A step-index cladding (14, 16) is provided in the thin direction to limit the propagation of light in the core (12) to a single mode in that direction. Mode filters (18, 20) are provided adjacent the ends of the core (12) in the thickness direction and the various propagation modes in the core (12) enter the mode filters (18, 20). The mode filters (18, 20) guide the desirable single-mode back into the core (12) and reject the other modes away from the core (12). Light absorbing layers (22, 24) are provided adjacent the mode filters (18, 20) and opposite the core (12) to absorb the undesirable propagation modes of light. Therefore, the main propagation mode in the core (12) is a single low order mode, but the cross-sectional area of the core (12) is increased to provide more power. Each of the cores (12) in the ribbon (30) are optically pumped form the side by a bar (40) of diode arrays (42) positioned at strategic locations along the length of the ribbon (30) relative to the waveguides (10). Various transmission gratings (48) and/or reflection gratings (50) can be provided within a ribbon jacket (32) to launch the optical pump light down the waveguide (10). The ribbon (30) can be wrapped around a mandrel (68) and a cooling fluid can be introduced through the mandrel (68) to cool the ribbon (30) during laser operation.

High power fiber ribbon laser and amplifier

A relatively high average power fiber laser system with phase control The
laser system includes a phased array of parallel fiber amplifiers coupled to a common master
oscillator An important aspect of the invention relates to the independently adjustable
optical phase modulators for each of the fiber amplifiers Various methods for phase
modulation are contemplated, such as fast spatial light modulators (SLM) as well as in-line
fiber phase modulators Both types of phase modulation utilize a wavefront sensor for
deriving feedback signals for phase control Arbitrary phase profile information can be
applied to the phase modulators to synthesize a desired output beam Thus, the phase
controlled fiber amplifier array functions as a synthetic aperture transmitter at optical
frequencies When combined with a beacon phase front sensor for atmospheric beam
propagation, the phase modulator can encode phase conjugate information to compensate for
atmospheric aberrations Relative to other high power laser systems, the laser system in
accordance with the present invention has lower power supply demand and cooling
requirements which results in lower weight, volume and cost The use of the fiber laser
system in accordance with the present invention also permits greater flexibility in packaging
with fewer discrete optics and a relatively simpler optical system design and well as being
amenable to mass production

High average power fiber laser system with phase front control

PROBLEM TO BE SOLVED: To provide a relatively high average power fiber laser using optical fibers having relatively less complication than that of known systems. SOLUTION: A phased array 30 of parallel fiber amplifiers is coupled to a common master oscillator 22. The respective fiber amplifiers are provided with independently regulatable optical phase modulators 26. A wave front sensor 38 for leading out a feedback signal for phase control is used. Arbitrary phase profile information is impressed to this phase modulator, by which a desired output beam may be synthesized. Then, the fiber amplifier array subjected to the phase control functions as a synthetic aperture signal transmitter at an optical frequency. This fiber amplifier array is combined with a beacon phase surface sensor for the beam propagation in the amt. air. The phase modulator encodes phase conjugation information and compensates the amt. aberrations.

Hiroshi Komine

Papers

8-W coherently phased 4-element fiber array

High average power solid-state laser system with phase front control

High power fiber ribbon laser and amplifier

High average power fiber laser system with phase front control

High average power fiber laser system having phase surface control