We present a high pulse energy blue laser based on a fiber master oscillator power amplifier pumped single-frequency optical parametric amplification for an in situ 87Rb-87Sr isochron dating and chemistry experiment on the moon. The laser has a simple and rugged configuration and can precisely control the wavelength on and off the Sr isotope resonance lines.
A 1064 nm, 1 mJ pulsed fiber MOPA module, housed in 16”x14”x2.5” package for application in a lunar and planetary in-situ surface dating instrument is demonstrated. The module is based on a three-stage MOPA with a 60 μm core tapered fiber terminal amplifier. The master oscillator and first two preamplifier stages, which generate 20 μJ pulses, are all contained on a 13”x11”x1” board. Several improvements to the electronic signal control were instrumental to the laser development, including bipolar drive of the phase modulator for SBS suppression, shaping of the seed pulse to compensate pulse steepening, and pulsed operation of the power amplifier pump to reduce spontaneous emission at low pulse repetition frequency. The packaged laser runs at a repetition rate of 10 kHz and generates 10 ns pulses at 1 mJ with a 40 GHz linewidth, an M2 ~ 1.2 beam quality, and an 18 dB polarization extinction ratio. The modular design enables seven independent lasers to be stacked in a 20”x18”x16.25” enclosure, supporting a path towards a fiber laser based LARIMS for advanced materials characterization and chronological dating in harsh and remote environments.
We demonstrate high power, deep ultraviolet (DUV) conversion to 266 nm through frequency quadrupling of a nanosecond pulse width 1064 nm fiber master oscillator power amplifier (MOPA). The MOPA system uses an Yb-doped double-clad polarization-maintaining large mode area tapered fiber as the final gain stage to generate 0.5-mJ, 10 W, 1.7- ns single mode pulses at a repetition rate of 20 kHz with measured spectral bandwidth of 10.6 GHz (40 pm), and beam qualities of Mx2=1.07 and My2=1.03, respectively. Using LBO and BBO crystals for the second-harmonic generation (SHG) and fourth-harmonic generation (FHG), we have achieved 375 μJ (7.5 W) and 92.5 μJ (1.85 W) at wavelengths of 532 nm and 266 nm, respectively. To the best of our knowledge these are the highest narrowband infrared, green and UV pulse energies obtained to date from a fully spliced fiber amplifier. We also demonstrate high efficiency SHG and FHG with walk-off compensated (WOC) crystal pairs and tightly focused pump beam. An SHG efficiency of 75%, FHG efficiency of 47%, and an overall efficiency of 35% from 1064 nm to 266 nm are obtained.
A new solution for building high power, solid state lasers for space flight is to fabricate the whole laser resonator in a
single (monolithic) structure or alternatively to build a contiguous diffusion bonded or welded structure. Monolithic
lasers provide numerous advantages for space flight solid-state lasers by minimizing misalignment concerns. The closed
cavity is immune to contamination. The number of components is minimized thus increasing reliability. Bragg mirrors
serve as the high reflector and output coupler thus minimizing optical coatings and coating damage. The Bragg mirrors
also provide spectral and spatial mode selection for high fidelity. The monolithic structure allows short cavities resulting
in short pulses. Passive saturable absorber Q-switches provide a soft aperture for spatial mode filtering and improved
pointing stability. We will review our recent commercial and in-house developments toward fully monolithic solid-state
lasers.
Single-clad and double-clad Yb:YAG crystalline fiber waveguides (CFWs) have been prepared with Adhesive-Free
Bonding (AFB®) technology. By using a fiber coupled laser diode as pump source, a single-mode laser with near
diffraction limited beam quality M2=1.02 has been demonstrated in a double-clad CFW. The laser output power and
efficiency are 13.2 W and 34%, respectively. In a single-clad CFW, core pumping was used. The laser output has top-hat
beam profile. An output power of 28 W and a slope efficiency of 78% have been achieved respectively.
Double-clade crystalline fiber waveguide (CFW) has been produced by using adhesive-free bond (AFB®) technology. The waveguide consists of a 1 at.% Yb:YAG core, un-doped YAG inner cladding and ceramic spinel outer cladding. It is a direct analog of the conventional double-clad glass fiber laser in the crystal domain. Signal gain of 45 or 16.5 dB has been measured in a preliminary master oscillator power amplifier (MOPA) experiment. Due to the high laser gain and the weak Fresnel reflection at the uncoated waveguide ends, the CFW even starts self-lasing above a certain pump power. Laser output power of 4 W in the backward propagation direction has been measured for input pump power of 44 W. After considering the same amount of forward propagated laser power, the laser efficiency to the absorbed pump power is estimated to be about 44%. In principle, CFW can have extremely large single mode area for high efficiency and high power laser applications. So far, Single mode area < 6700 μm2 has been demonstrated in Er:YAG CFWs.
We demonstrated nearly a quantum defect limited CW operation of a 41-mm-long Er:YAG large area
crystalline waveguide laser with a diffraction limited output, which was resonantly pumped by a fiber laser at
1532 nm. Using a Er(0.25%):YAG, 62 μm x 62 μm waveguide, surrounded by a 3 x 5 mm rectangular
cladding of undoped YAG, an output power of 9.1 W with slope efficiency of 92.8% has been achieved. The
output laser beam had a Gaussian profile with a ~ 2.8 x 10-2 rad divergence, which is in good agreement with
the divergence expected from a waveguide with a low NA value. The waveguide laser operated
simultaneously at two wavelengths, 1617 nm and 1645 nm, when the transmission of the laser cavity output
coupler was less than 20-25%, and operated only at 1617 nm when the laser output mirror had a higher
transmission.
We propose a sub-100 μm double-clad single-crystalline Tm:YAG square core adhesive-free bond (AFB) waveguide
structure as a prototype gain medium for an LMA ~2-μm fiber laser with ~790-nm diode laser pumping and present its
mode simulation. The structure uses an undoped YAG and ceramic spinel for the inner and outer claddings, respectively.
The mode simulation indicates that such double-clad structure can have over 1000-μm2 mode area in a 2% Tm:YAG
core which will maintain a single transverse mode laser operation. The structure also has a large numerical aperture (~
0.22) for the un-doped YAG inner cladding available for pumping.
We have demonstrated a novel folded linear resonator for walk-off compensated (WOC) optical parametric oscillation
(OPO) in a single ZGP crystal. The OPO uses the doubled ZGP crystal length in a WOC configuration while simplifying
requirements on crystal alignment. With a Q-switched Ho:YAG laser as a pump source, a maximum output power of 2
W at a wavelength of 4.72 μm has been measured for a pump power of 11.9 W in a 15-mm long type-I phase-matched
ZGP crystal. The measured OPO quantum slope efficiency is above 50%.
Adhesive-free bonded (AFB®) un-doped end-capped and segmentally bonded Tm:YLF laser composites have been
investigated for high efficiency LD-pumped laser operations. With a fiber coupled 792-nm diode laser as pump source
and a single-end pump arrangement, an optical-to-optical laser efficiency of 48.3% (quantum efficiency of 116%) and a
slope efficiency of 55% (quantum efficiency of 132%) have been measured at the laser output wavelength of 1.908 μm.
A maximum laser output power up to 29 W has been also achieved before the thermal stress fracture limit. The high
laser efficiencies and output power can be attributed to the two-for-one cross-relaxation process and the efficient thermal
management of the AFB laser composites.
A maximum output power of 18.6 W has been measured at a pump power of 23.7 W in a Tm:fiber laser pumped
adhesive-free bonded (AFB®) YAG/Ho:YAG/YAG composite. The corresponding optical-to-optical efficiency and
slope efficiency are 78.5% and 81.2%, respectively. When the laser is operated in Q-switched mode, the shortest laser
pulse width of 11 ns has been measured in our experiment at repetition rate of 5 KHz. The laser has been used as pump
source of a mid-infrared ZGP OPO. The wavelength tuning range from 3.06 to 6.6 μm has been achieved in a 15-mm
long ZGP crystal. The maximum output power is 1.44 W at pump power of 10 W with repetition rate of 5 KHz.
We report on the fracture toughness as it is related to the fracture strength when different orientations of YAG single
crystals are bonded to each other to make a selection of a mechanically desirable waveguide configuration. We have
found that the KIC (211//211) conforming twin is greater than KIC (110//110) conforming twin that is about the same as
KIC of (111//111) twist 180° and flip 180° twins. All KIC converge to the same value that is comparable if not greater
than the non-composite counterparts1 when the composite samples are fabricated using the standard AFBR (Adhesive-
Free Bond) process.
A second equally important consideration is the refractive index difference as function of dopant concentration. We have
developed a technique to measure the difference of refractive index between Er:YAG and undoped YAG, and between
Nd:YAG, and undoped YAG. We have found that the specific refractive index variation for YAG is 4.22 × 10-4 per 1%
Nd dopant concentration, and is 2.08 × 10-4 per 1% Er dopant concentration.
With adhesive-free bond (AFB) technology, two walk-off compensated (WOC) KTP composites that consist of 16 layers
of 2-mm thick single KTP crystals in each have been designed and prepared for 1.064-μm pumped 2-μm OPO's by
means of quasi-noncritical phase-matching (QNCPM) and quasi-phase-matching (QPM). Output pulse energies of 49 μJ
and 35 μJ have been achieved for QNCPM and OPM OPO's at pump energy of 523 μJ with pulse duration of 15 ns and
repetition rate of 1 KHz, respectively. The OPO pump thresholds were measured as low as 254 μJ (44.6 MW/cm2) for
the both types of OPO's. The wavelength shifts were measured to be around 11 nm for both the signal and idler beams
when KTP temperature was raised from room temperature to 220°C.
We report on the bond strength at the interfaces of Adhesive-Free-Bonded (AFB®) single crystal sapphire
composite bars as deduced from the fracture toughness. Fracture toughness (KIC) characterizes the resistance
of a bulk brittle material to fractural failure caused by unstable crack propagation. Correlation between the
well-defined initial flaw size and the apparent failure strength gives the fracture toughness that is a function of
the average bond strength at the fractured interfaces. To ensure that the fracture interface coincides with the
AFB® interface, we make a pre-notch at the AFB® interface to be the initial surface flaw using a 532 nm
wavelength marking laser. The measured failure strength yields the fracture toughness of the AFB® composites.
We have found in this study that the average fracture toughness of AFB composite samples is 2.51 MPa*√m
and that of non-composite control samples is 2.39 MPa*√m. The correlating fracture energy is 13.1 J/m2 and
11.9 J/m2, respectively. The apparently greater fracture toughness of composite samples compared to that of
non-composite samples conforms with the hypothesis that attributes the origin of the bonding forces at the
AFB interfaces to the Lodon-Van der Waals interaction between two solid bodies.
A quasi-noncritical phase-matching (QNCPM) technology has been developed employing adhesive-free bonding
(AFB) for high efficiency and high beam quality frequency conversion. A 16-layer KTP composite with total length of
32 mm was fabricated for a low pulse energy pumped 2-μm optical parametric oscillator (OPO). Our calculations
indicate that the KTP composite has a 16 times lower walk-off effect and 16 times higher angle acceptance compared
with the OPO in the same length single KTP crystal. Even only considering the walk-off correction, the threshold pump
pulse energy in such a QNCPM OPO can be expected to be reduced by 256 times. In addition, the AFB technique was
demonstrated to have uniform bonding quality and immeasurably small interface loss. Therefore, it can be expected to
allow engineering of other critical phase-matched nonlinear optical devices into QNCPM devices.
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