High-power spectral beam combining of fiber lasers with ultra high-spectral density by thermal tuning of volume Bragg gratings

Abstract

Lasers that produce 100 kW level diffraction limited power will require beam combining due to fundamental thermal and nonlinear limitations on the power of single aperture lasers. Towards this goal, we present high power, high spectral density beam combining by volume Bragg gratings of five 150 W beams with a spectral separation of 0.25 nm between beams, the narrowest to date for high power. Within 1 nm, 750 W of total power is combined with greater than 90 % efficiency. Combined beam quality is discussed including the effect of unequal individual beam divergences on the combined beam quality. The individual input beams may have unique divergences as they enter the system, and the heated volume Bragg gratings (VBGs) may introduce very slight changes in divergence to each beam. These small differences in beam divergence between the beams will not degrade the M 2 of the individual beams, but the composite M 2 after combination can be adversely affected if the beams do not have equivalent divergence at the output of the system. Tolerances on beam divergence variation are analyzed and discussed. High power beams transmitting through or diffracting from a VBG can experience different distortions resulting from thermal effects induced in the VBGs. Each beam also experiences a different aberration, as no two beams pass through the same number of identical VBGs. These effects are studied with experiment compared to modeling. Possible methods of beam quality improvement are discussed.

High-Power Fiber Lasers

“…This shift is corrected by thermal tuning. Drachenberg et al [45] developed a theoretical model of the observed processes and compared it with experiments.…”

“…Drachenberg et al [45] presented high-power, high-spectral-density beam combining using VBG of five 150-W beams with a spectral separation of 0.25 nm Fig. 10.32 A schematic used for simultaneous coherent and spectral combination of four fiber laser beams [43].…”

Volume Bragg Gratings: Fundamentals and Applications in Laser Beam Combining and Beam Phase Transformations

“…For example, the narrow angular selectivity of the TBGs makes them a great angular filter that can be used to suppress higher order modes generation in laser cavities while keeping them very compact [4]. Alternatively, RBGs with their narrow wavelength selectivity can be used for narrow wavelength beam combining where the diffracted by and the transmitted through an RBG beams can be separated by only a few hundred picometers [5]. Regardless of the close wavelength separation, the RBG does not diffract the transmitted beam even though both beams have a common propagating direction.…”

“…Such system was presented in [5] and demonstrated the combining of five lasers, each generating 150 W CW to give a total combined power of 750 W (Figure 7). Spectrally, the beams were 250 pm apart so the combined spectrum had width of 1 nm in total.…”

Statistical multi-criteria evaluation of non-nuclear asteroid deflection methods

“…Focused development under the DARPA SHEDs program has also lead to extremely high power conversion efficiency in the 9xx-nm wavelength band, leading to diode bars with efficiency in excess of 74% and a clear route to efficiencies superior to 85% at room temperature (Crump et al, 2007). With demonstrated slope efficiency of optical fibers on the order of 80% (Jeong et al, 2004) and a demonstrated efficiency of spectral beam combining techniques of 91% (Drachenberg et al, 2011), we consider in this paper a global E/O efficiency of 50%. Detailed calculations performed on Forsterite by Thiry et al (2016) indicate that the coupling coefficient of a CW laser operating under the plasma formation intensity near the 1 micron wavelength is dictated mainly by 2 parameters: the laser intensity Φ [W/m 2 ] which depends on the laser output power and focusing ability of the optics as well as the mean time available to heat the material which is roughly proportional to the ratio between the laser beam diameter and the relative speed of the asteroid surface with respect to the laser beam (on the order of 6 cm/s if one considers the spin-limit of a 156 m asteroid).…”