The propagation of high power narrowband laser pulses in multimode fibers and the limitations due to SBS are presented. An injection seeded pulsed Nd:YAG laser operating at 10 Hz was used to pump undoped step index fibers to determine the SBS threshold under various conditions. Measurements on 50μm core diameter fibers with various fiber lengths and pulse durations at 1064 nm were performed and simulated with a computer code. The code considers the time dependent coupling between the pump wave, the Stokes wave, and the acoustic wave. The experimental results are in good agreement with the numerical predictions. Our results quantify the limitations of high power narrowband pulse transmission in multimode fibers.
Single frequency laser sources, which also provide tunability, have wide use in spectroscopy and remote sensing as well as for other applications such as optical pumping. Optical parametric oscillators (OPOs) offer the potential for broadly tunable output in spectral regions inaccessible by conventional laser sources. We report here on the design and use ot an OPO architecture developed to produce pulsed, tunable, single-frequency output in the mid-infrared spectral region and used for optical pumping of gas. Design information about two separate OPOs that were developed will be presented along with experimental details of the optical pumping of CO on the (3-0) band around 1.57 μm and on the (2-0) band around 2.3 μm.
A grazing incidence geometry was used tom ake compact, high power, efficient TEM00, Nd:YVO4 oscillators and amplifiers. A side-pumped Nd:YVO4 laser based on a grazing incidence cavity design, pumped with a 50 W diode bar, achieved 30% optical to optical efficiency at 50 kHz and M2= 1.4 x 1.1 output. The alser was acousto-optically Q-switched and was operated at repetition rates from 20 to 100 kHz, as well as operating continuous-wave (CW). Multiple cavity designs were developed to optimize performance for short (<10ns) and longer (>30ns) pulse durations. 15 W of TEM00 output was generated at 50 kHz and used to pump a periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO), generating 5 W of eye safe output at 1.5 μm. This oscillator/OPO system was packaged on a 4 x 6 inch optical platform. Using an oscillator/amplifier/amplifier configuration, 30 W of 1.064 μm output power was generated at 50kHz and subsequently converted in an OPO to 10.25 W of eye safe output in a PPLN OPO. This work demonstrates that the side-pumped grazing incidence geometry scales to high power and maintains M2 and efficiency while fitting in a compact package.
To assist in the design and development of high power Hydrogen Fluoride laser systems Aculight has developed a novel, compact, widely tunable spectroscopic source specifically for operation in the 2.4 to 2.8-micron wavelength region. This source is a continuous wave (CW), room temperature, single frequency, diode-pumped, doubly resonant optical parametric oscillator (DRO). The spectroscopic capabilities of the OPO have been demonstrated by scanning its frequency through absorption features of carbon dioxide at 2.7 microns. We have also characterized the effects of water vapor absorption in this wavelength region upon the performance of the source. In addition to measuring key HF laser parameters this widely tunable mid IR source has significant utility in a wide array of applications including sensing and combustion diagnostics.
We have investigated rotational Raman conversion experiments in hydrogen using a single-shot photolytic iodine laser (PIL) at 1.315 micrometer as a pump laser. The total output energy of the PIL is between 5 - 7 J per pulse distributed in a train of approximately 120 pulses each with a FWHM of 6 or 9 nsec and a temporal spacing of 33 nsec. The energy distribution within the pulse train is characterized by high energy pulses in the gain switched spike followed by lower energy pulses in the tail of the laser pulse. Stimulated Raman scattering (SRS) experiments were performed with (1) a focused beam geometry in a single Raman cell, (2) two Raman cells, whereby the pump focus was reimaged into a second Raman cell, and (3) a Stokes resonator specifically suited for an annular pump beam. Thermal distortions in the laser beam made it necessary to lower the peak intensity of the pump laser beam by adjusting the focusing conditions. With a long focus mirror we demonstrated (1) a conversion efficiency of up to 70% for the high-energy pulses of the gain switched spike of the PIL micropulses and (2) lowering of the threshold with a Stokes resonator.
We report the operation of a short pulse photolytic iodine laser (PIL) using an unstable resonator under long-pulse and injection seeded operation. The laser is designed as a surrogate source to replicate the output from a q-switched chemical oxygen iodine laser (COIL). Under long pulse conditions the single shot laser produces up to 10 Joules in a 3 microsecond(s) pulse. When seeded with the output from a narrow pulse (10 ns) broadband KTP Optical Parametric Oscillator (OPO), the temporal output is composed of a train of 10 ns pulses separated by the round trip time of the cavity. The enhanced peak power in the individual pulses is more attractive for subsequent efficient Raman conversion. A description of the laser performance with the unstable resonator and with seeding is presented.
This paper summarizes recent progress that has occurred in several research areas related to the development of a repetitively-pulsed, frequency-shifted chemical oxygen iodine laser (COIL). COIL gain- switch experiments at 10 kHz pulse rates are described using a novel solid state pulsed magnetic field system. Raman conversion experiments in hydrogen using a pulsed photolytic iodine laser as a COIL surrogate are also described.
One way of achieving high-power diode-end pumped lasers is to angularly multiplex several diodes on each end of the laser rod. We have successfully multiplexed four 15 W diode arrays on each end of a 6.3 mm diameter X 7.5 mm long Nd:YAG rod to produce an approximately equals 2 mm diameter pump spot. Higher laser power was achieved by adding a second laser rod pumped at both ends. The addition of the second rod facilitates thermally induced birefringence compensation by introducing a quartz polarization rotator between the rods. In addition, it was necessary to add an aspheric lens to compensate the thermal aberration induced at these high, nonuniform pump powers. With this arrangement, > 90 W has been extracted multimode, and > 60 W in a near-diffraction-limited beam.
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