A four-element fiber array has demonstrated 470 watts of coherently phased, linearly polarized light energy in a single far-field spot. Each element consists of a 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. 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.
This effort represents the results of a multi-year effort to achieve high power from a single element fiber amplifier and to understand the important issues involved in coherently combining many individual elements to obtain sufficient optical power for directed energy weapons. Northrop Grumman Corporation and the High Energy Laser Joint Technology Office jointly sponsored this work.
The development of didoe-pumped self-frequency doubling (SFD) YCA4O(BO3)3 (YCOB) crystals doped with ND3+ or YB3+ ions offers an attractive alternative to traditional intra-cavity doubling techniques using a separate non-linear crystal. Here, we summarize the progress of scaling SFD lasers to higher powers. Moreover, we examine the potential and limitations of these devices. While SFD lasers are potentially more compact and less costly, scaling to higher powers in the visible region requires careful assessment of mode matching and cavity mode brightness. Combining optical gain and frequency conversion in the same material requires a compromise between the ideal cavity mode for mode matching and generating the largest available power density of the laser mode for optimum frequency doubling. The lack of adjustment of the cavity mode inherently limits scaling of SFD laser operation when using low brightness high power laser diodes. We have employed a new source developed by Polaroid Corp. using multiple high-brightness laser diodes to investigate the potential of increasing SFD laser output. We have then compared the efficiency of this novel pump technique to a single diode pumped system to determine the importance of brightness of the pump source to SFD operation.
Progress in the growth of the rare earth calcium oxyborate crystals has now generated a new class of laser materials that can be used as both a laser host and a nonlinear frequency converter. Laser action and self-frequency doubling (SFD) has been observed with both 10% and 20% Yb3+-doped YCOB crystals. Laser operation was obtained in a hemispherical linear cavity, end-pumped with a tunable cw Ti:Sapphire or a 980 nm laser diode pump source. Under Ti:Sapphire pumping at 900 nm, an output power of 230 mW and a slope efficiency of 29% was obtained using the 10% doped sample. Laser action was seen at 1050 nm. Laser operation of the 20% sample had a maximum output power of approximately 300 mW with a slope efficiency of 35.8% at 1088 nm. Laser action was not obtained at the peak of the fluorescence emission (approximately 1030 nm) in this crystal as a consequence of self-absorption on the short- wavelength side of the emission band. Diode-pumped operation at the narrow absorption peak of 977 nm was achieved and early results show an improved slope efficiency of 34% in comparison to the 10% doped crystal under Ti:Sapphire pumping. We have also observed self-frequency doubling in Yb3+:YCOB. The 20% Yb3+:YCOB crystal used for this test was cut with a phase-matching angle of 36.22 degree(s). The self-frequency doubling efficiency was low due to the absence of any frequency selective elements in the cavity to narrow the linewidth of the fundamental emission. The SFD emission occurred at 543 nm.
Development of the oxyborate Nd3+:YCOB has brought about the realization of single crystal elements under diode pumped operation for compact simple, solid state lasers producing visible laser emission. Efficient laser action and self-frequency doubling (SFD) has been obtained for both Ti:Sapphire and diode pumped operations. Comparable results were obtained for a hemispherical linear cavity with a 2% output coupler for fundamental operation. The fundamental output power of the Nd:YCOB under Ti:Sapphire pumping was in excess of 400 mW with a slope efficiency of 44% with approximately 1 W absorbed. Utilizing high brightness laser diodes, over 350 mW of fundamental output power has been obtained with a slope efficiency of 51% for similar absorbed powers. Efficient SFD operation was demonstrated in both pumping modes with no output coupling at the fundamental wavelength and high transmission of the second harmonic using a 5% doped Nd:YCOB crystal. Output powers of approximately 60 mW at 530 nm were obtained with both Ti:Sapphire and laser diode pumping.
Comparative tests of bulk laser-induced damage of some experimental crystals having prospects for laser applications were performed for evaluation of exploitation reliability of elements made from these crystals. A number of fluoride and oxide crystals doped with ions of Ce, Cr, Ti, and Zr were studied in these measurements. BK7 glass was tested under the same experimental conditions as a comparison. Pulses from a single transverse mode, Cr: LiSAF laser (? = 0.85 tm, t 60 ns) were used in these experiments. The laser radiation was focused to a small spot size (2 tm) in the bulk of these crystals to exclude the influence of inclusions and inhomogeneities on the results. Absorption spectra of these crystals were measured to be sure that there is no strong absorption in the region of the irradiating wavelength. The possible effects of dopant ions on the laser-induced damage thresholds of the crystals studied is discussed.
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