We had demonstrated the use of Coherent Polarization Locking (CPL) to mitigate the intra-cavity optical damage in a
Q-switched Ho:YAG laser. By splitting the available pump power into two individual Ho:YAG laser rods, we had
passively coherent combined two orthogonal polarized lasers with output pulse energy of 9.13mJ, pulse width of 14ns, operating at 800Hz pulse repetition rate. In contrast, when all the pump power was pumped on a single Ho:YAG laser rod built with the same cavity configuration, severe optical damage was observed. It was occurred at the surface coating of the laser rod when operating at < 2kHz pulse repetition rate, thus limiting the output pulse energy to < 5mJ. We also demonstrated, first to our knowledge, that by performing Q-switching only in one of the laser arms, it resulted in pulse operation for the entire CPL laser cavity.
We had demonstrated the advantage of using Coherent Polarization Locking on thermal-sensitive Ho:YAG laser cavity. We overcome several thermal issues related to Ho:YAG laser by distributing the gain over a large volume. We passively coherent locked two orthogonal polarized lasers, achieving 9.6W of output power with near perfect (greater than 99%) combining efficiency. The resultant laser produced near diffraction-limited beam quality of M2 ~1.1 and excellent power stability. As compared to conventional laser cavity, we had shown the increased in single-pass absorption, suppression of output power saturation and improvement in beam quality using Coherent Polarization Locking.
This article presents a novel fiber-based multi-beam laser Doppler vibrometer (LDV). In this design, a single
wavelength laser source at 1550 nm combined with several acousto-optic modulators (AOM) form the transmitter head
of the LDV. At the receiver side, one single high-speed photo-detector is employed, instead of multiple detectors
according to other reported multi-beam laser Doppler vibrometer.1, 2 Utilization of spatial encoding technique allows us
to produce transmitted laser beams with different frequency shifts. In this work, a laser source passes through a sequence
of totally four AOMs at different regimes, producing a 4×5 laser beam matrix which is then sent onto different points of
vibrating targets for measurement. The backscattered light signals are collected back into a single mode fiber by a fiber
collimator and combined with a common reference beam. This mixture of optical signals passes through an Erbium
Doped Fiber Amplifier (EDFA) before it is detected by a high-speed fiber-based InGaAs photo-detector. With a digital
demodulation algorithm implemented in Labview, the phase variations and thus the vibrations of different testing points
can be extracted separately from their corresponding frequency bands. The experimental results show it is possible to do
a precise vibration measurement on twenty testing points simultaneously using this novel multi-beam LDV.
We report on watt level mid-infrared (MIR) wavelength generation using intra-cavity ZnGeP2 (ZGP) optical parametrical
oscillator (OPO) within a 2.1μm Ho:YAG laser. A compact cavity of less than 50cm was designed for the intra-cavity
OPO setup. With the same laser setup, watt level of both 2.1μm and MIR wavelengths were generated. An average
output power of >20W of 2.1μm and >1W of MIR wavelength at 10 KHz repetition rate were achieved from a 46W Tm
fiber pump laser. The Ho:YAG laser was resonantly pumped by a 1.9μm Tm:fiber laser and nanosecond pulses were
generated using an electro-optics q-switch modulator. With the use of a λ/4 waveplate and a thin film polarizer, a
variable output coupler for the Ho:YAG laser was formed where we could optimize the output coupling to achieve 21W
of 2.1μm wavelength. MIR wavelengths were generated using commercial ZGP crystals from Inrad. A HR mirror for the
MIR wavelengths was inserted into the Ho:YAG cavity to form the intra-cavity ZGP OPO. The rear mirror of the
Ho:YAG cavity act as the output coupler with R=70% for the MIR wavelengths. Optimizing of the MIR generation was
done by tuning the phase-matching angle of the ZGP and adjusting the cavity length of the OPO. A preliminary result of
the intra-cavity ZGP OPO generates >1W of MIR wavelength.
We demonstrate coherent polarization locking of multimode beams from four broad area emitters in a diode bar. The
beams are overlapped into single output by using walk-off crystals and waveplates while their phases are locked via
polarization discrimination. Coherent locking of multimode beams enabled power scaling of coherent diode output while
retaining beam quality of single emitter. We obtained power of 7.2 W with M2 of 1.5 x 11.5 from a 980 nm diode laser.
This corresponds to brightness improvement of more than an order of magnitude.
Coherent beam combining has been actively explored as a technique to increase the brightness of laser sources. Passive
phase-locking of a diode array in a common resonator, in particular, is an attractive approach owing to its inherent
simplicity and good beam quality. In this work, we present the coherent combining of an array of diode emitters in a
conventional diode bar configuration using the coherent polarization locking technique. An external laser cavity is
designed so that the diode emissions from several 976 nm diode emitters are spatially overlapped via a series of
birefringent walk-off crystals and passively phase-locked by a polarizing beam splitter. The key optical element in this
beam combining scheme is the novel YVO4 birefringent spatial beam combiner that not only provides spatial overlap,
but also identical optical path lengths for the diode beams. This facilitates design of the cavity for achieving a close
match between the mode size of the Gaussian beam and the asymmetric emitting area at the front facet of the diode
emitters. The phase-locking technique, coupled with the required standard bulk optical crystals and standard diode bar
configuration, yields a robust laser architecture which retains the advantages of diode lasers in terms of cost, size and
wavelength tunability. With the coherent combining of four diode emitters, we achieved a nearly diffraction limited
beam at 1030 mW, which represents a 50 times increase in brightness over the standard incoherent diode bar. The
coherent locking approach is highly scalable. Further experiments to coherently combine eight to sixteen diode emitters
are in progress.
We present here a compact 120 W, continuous-wave (CW), diode-pumped Nd:YALO laser at 1079 nm. This linearly polarized output from the optically anisotropic Nd:YALO crystal is advantageous for pumping non-linear crystals. The 1 at% doped Nd:YALO laser rod, 4 mm diameter by 97 mm long, is cut along the b-axis, with both ends anti-reflection coated at 1079 nm and 1341 nm. It is water-cooled (16 degree(s)C) and side-pumped by 5 close-coupled CW diode arrays operating at 803 nm. We obtained 121 W output at the maximum diode pump power of 571 W. The optical slope efficiency is 60%. This is, to the best of our knowledge, the highest power obtained from a diode-pumped Nd:YALO laser. We have studied this thermal lensing at different diode pump powers in lasing and non-lasing configurations. The measured thermal lens decreased from 37.5 to 11 cm as the pump power increased from 185 to 542 W in the lasing configuration. In the non-lasing case, the corresponding thermal lens decreased from 35.5 to 9.5 cm. We have also successfully operated our Nd:YALO laser at 1341 nm. We obtained a maximum output of 60 W, to the best of our knowledge, the highest diode-pumped laser output at this wavelength.
We present 2 experiments on intracavity pumping of a KTiOPO4 (KTP) optical parametric oscillator (OPO) within a high power 1064nm Nd:YAG laser cavity producing multiwatt level 2-micron outputs. In such high power regime, the Nd:YAG rod laser suffers significant thermally-induced birefringence loss when it is linearly polarized. Hence in the first experiment, we present our intracavity KTP OPO pumped within the simple cavity of an unpolarized Nd:YAG laser. This simple configuration, with 1-micron high reflectors forming the Nd:YAG laser cavity and R equals 75 percent and 100 percent 2-micron mirrors forming the short flat-flat OPO cavity, delivered 6.5W of 2-micron output power at 3kHz Q-switched operation. Next we pumped our intracavity OPO within a more complex polarized Nd:YAG laser cavity. In this second experiment, we compensated for the thermally-induced birefringence loss in the Nd:YAG laser by using a re-entrant laser cavity with a Faraday rotator, and the OPO was pumped within one arm of this set-up. In this case, we also obtained approximately 6.5W of 2-micron output. FInally, studies of the temporal profiles of the 1 and 2-micron laser beams also revealed interesting multiple pulse features in such intracavity OPO output.
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