High-brightness tapered diode lasers generally work in the fundamental transverse mode. Still, under the condition of high power, the beam quality is often deteriorated by the influence of higher-order modes. At the junction of the device ridge waveguide and the conical amplifier, there is a great difference in the refractive index step △n, which is the key area for producing high-order modes and influencing each other. In this paper, the physical process of high-order side mode excitation in a large optical cavity InGaAs/AlGaAs conical LD was studied. The mode changes of the propagation interface were simulated by the eigenmode expansion method (EME). The effects of various separation distances of the beam spoilers were compared and verified by experiments. The results show that through the practical design of the separation distance of beam spoilers at the mode propagation interface, the tapered LD can maintain the high beam quality of M2=1.9 at 3.2 W.
High brightness broad area lasers with high polarization purity are highly efficient light sources for high brightness fiber coupled and direct semiconductor lasers. Effect of lateral index step on the performance of high-power broad-area 970-nm diode lasers based a large-optical-cavity waveguide structure was studied and presented here. The index step of the 80-μm wide ridge is found a key parameter to control the output power, lateral far-field angle, beam waist and polarization purity. The threshold current decreases with the increase of the etching depth while the slope efficiency increases. When gain guide lateral waveguide by very shallow etching was used, the beam waist expands to a size of more than 200 μm, which was attributed to anti-guiding effect and current spreading. When large index step is introduced by deep etching, enhanced filamentation was observed, which is attributed to an enhanced confinement of the higher order modes. What’s more, the strain introduced by the etching of the ridge can deteriorate the polarization purity. The study in this paper shows that the lateral index step should be optimized to fabricate high brightness high efficiency broad area lasers with high polarization purity.
In this paper, based on a linear polarized, white noise signal (WNS) phase modulated all fiber amplifier, the self-pulsing characteristics of polarization maintaining (PM) amplifier at the different phase modulation parameters with the same linewidth are analyzed experimentally. It is demonstrated that the self-pulsing threshold is closely related to the details of the modulation spectrum affected by the frequency and the signal power of the WNS, and it is near impossible to calculate by the linewidth. Besides, by suppressing the self-pulsing effect, we obtain an output of 2009 W with a linewidth of 22 GHz. The polarization extinction ratio (PER) is larger than 15 dB, and the M2 is lower than 1.2.
KEYWORDS: Semiconductor lasers, Waveguides, Refractive index, Neodymium, Laser applications, Far-field diffraction, High power lasers, Laser energy, Optical resonators, Near field
In this research, the transversal mode of 970 nm broad area laser diode based on a large optical cavity is studied. The vertical far-field patterns were measured to be dependent on the cavity caused facet loss. High-orders modes achieve lasing with higher facet loss while they can be completely suppressed by a low facet loss. The suppression of high-order modes was attributed to be an increase in the discrimination of differential quantum efficiency between high-order modes and the fundamental mode, which matches well with simulation. When the discrimination exceeds probable fluctuations, the high-order modes will be completely suppressed and lasing of single fundamental mode will achieve. According to our experiment results, the optimal resonance cavity length is about 6 mm in order to realize fundamental mode and simultaneously a high slope efficiency. As a consequence, high efficiency 970 nm broad area laser with slope efficiency over 1.07 W/A and full wavelength half maximum (FWHM) less than 2 nm@1A is demonstrated.
In this research, the transversal mode of 970 nm broad area laser diode based on a large optical cavity is studied. The vertical far-field patterns were measured to be dependent on the cavity caused facet loss. High-orders modes achieve lasing with higher facet loss while they can be completely suppressed by a low facet loss. The suppression of high-order modes was attributed to be an increase in the discrimination of differential quantum efficiency between high-order modes and the fundamental mode, which matches well with simulation. When the discrimination exceeds probable fluctuations, the high-order modes will be completely suppressed and lasing of single fundamental mode will achieve. According to our experiment results, the optimal resonance cavity length is about 6 mm in order to realize fundamental mode and simultaneously a high slope efficiency. As a consequence, high efficiency 970 nm broad area laser with slope efficiency over 1.07 W/A and full wavelength half maximum (FWHM) less than 2 nm@1A is demonstrated.
In this paper, we demonstrate 47 GHz linear polarized fiber amplifier injected by a simple fiber oscillator laser seed source with narrow linewidth and near diffraction-limited beam quality. Output powers of 419 W, 778 W, and 1107 W are achieved with 3dB linewidth of 26.5 GHz, 41 GHz, and 47 GHz, respectively. The M2 is 1.25 in the x-direction and 1.23 in the y-direction at the maximum laser power, respectively. The measured PER is large than 96% during the power scaling process. However, The SRS is observed when the laser power is 1107, the SNR is about 47dB, which means that the SRS effect has become a serious limitation for further power scaling of such PM-amplifier seeded by the fiber oscillator laser seed source.
A high energy, high beam quality short-pulse diode-pumped Nd:YAG master oscillator power-amplifier (MOPA) laser with two amplifier stages is demonstrated. The two-rod birefringence compensation was used as beam quality controlling methods, which presents a short-pulse energy of 40 mJ with a beam quality value of M2 = 1.2 at a repetition rate of 400Hz. The MOPA system delivers a short-pulse energy of 712.5 mJ with a pulse width of 12.4 ns.The method of spherical aberration compensation is improved the beam quality, a M2 factor of 2.3 and an optical-to-optical efficiency of 27.7% is obtained at the maximum laser out power.The laser obtained 1.4J out energy with polarization integration.
The amplifier experiment research of end-pumped long pulse slab laser is developed, the results of out-put energy, optical-optical efficiency and pulse waveform are obtained at different experiment conditions, such as peak pumped power, amplifier power and pumped pulse width. The seed laser is CW fundamental transverse-mode operation fiber laser, the laser medium is composited Nd:YAG slab. Under end-pumped and the 2 passes, the laser obtain 7.65J out-put energy and 43.1% optical-optical efficiency with 45kW peak-pumped power and 386μs pump pulse width. The experimental results provide the basic for the optimization design to high frequency, high energy and high beam-quality slab lasers.
Beam quality of tapered laser diodes is limited by higher order lateral mode. On purpose of optimizing the brightness of tapered laser diodes, we developed a novel design of tapered diodes. This devices based on InGaAs/AlGaAs asymmetry epitaxial structure, containing higher order lateral mode filtering schemes especially photonic crystal structures, which fabricated cost effectively by using standard photolithography and dry etch processes. Meanwhile, the effects of photonic crystal structures on mode control are also investigated theoretically by FDBPM (Finite-Difference Beam Propagation Method) calculation. We achieved a CW optical output power of 6.9W at 940nm for a single emitter with 4 mm cavity length. A nearly diffraction limited beam of M2 ≈1.9 @ 0.5W has been demonstrated, and a highest brightness of β =75MW/(cm2 ·sr) was reached.
For many applications, laser diodes with very narrow and stable emission wavelength are needed. The realization of mode selection by an external cavity system with a grating is widely used. The influence of longitudinal mode selection by external grating on the filamentation and catastrophic optical mirror damage (COMD) of 970 nm broad area single emitters is studied in this paper. The emitters worked at three configurations: free running, with 10% mirror feedback (mirror lasers), and 10% grating feedback (grating lasers). The grating lasers showed very short lifetime caused by COMD, while the free-running lasers and mirror lasers show no power degradation. The COMD was confirmed by optical microscope showing cracking and melting of the optical antireflective (AR) coatings. By observing the near-field pattern of the three lasers, the COMD of the grating lasers was attributed to the pronounced filamentation induced by the grating feedback. What’s more, the filamentations vary when the locked wavelength change which indicates that the carrier dynamics thus the refractive index profile is very sensitive to the locked lasing wavelength.
The behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier with different local heat load is studied theoretically and experimentally. A theoretical model is constructed, and the effects of different laser parameters on the local thermal load of the gain fiber are analyzed theoretically, such as pump direction, pump linewidth, thermal conductivity of cooling medium. An experimental structure is described. The effects of different local heat load on the MI threshold when the fiber amplifier have the same total or average heat load is studied. The theoretical and experimental results reveal that the MI threshold can be estimated by the local heat load of the gain fiber.
True blue nitride laser diodes (LDs) are one of the key challenges for epitaxy of nitrides due to the variety of its potential applications. The growth of high temperature p-type layers may cause thermal degradation of the InGaN-based multiple quantum wells (MQWs) active region because of the annealing effect, since thick p-AlGaN layers were introduced as upper optical cladding layer in the LDs. The degradation was found in blue LDs grown on both Si and sapphire substrate. In the degraded LD wafer samples, “Dark” non-radiative MQWs regions were observed by microscopic photoluminescence. Formation of metallic indium precipitates and voids in these regions were confirmed by transmission electron microscope. The thermal degradation is attributed to the decomposition of indium-rich InGaN materials in the MQWs. The indium-rich InGaN materials were supposed to be accumulated at dislocation related V-shaped pits according to the surface morphology by atomic force microscope. To obtain high quality InGaN-based MQWs, one of the four methods can be introduced to eliminate the degradation. A lower thermal budget can suppress the decomposition of indium-rich InGaN clusters by a lower p-cladding layer growth temperature. The use of low threading dislocation density substrates results in low density indium-rich InGaN clusters. The introducing of H2 carrier gas during the quantum barriers growth or a 2-step growth scheme with a higher quantum barrier growth temperature etches off the indium-rich InGaN clusters. The suppression of the thermal degradation in the MQWs makes it possible for lasing of blue laser diode directly grown on Si.
The effect of the pump source and the gain fiber on the output properties of ytterbium doped fiber laser (YDFL) work near 980 nm are analyzed theoretically. Base on this analysis, we design a high power all fiberized ytterbium doped laser operating near 980 nm with a core/cladding of 80/130 μm. A 980 nm YDFL experimental setup was constructed, 16.7W 980 nm fiber laser was achieved with an optical-optical conversion efficiency of 32.4%.
A large-aperture Nd:YLF thin disk laser oscillator is demonstrated, in which the refractive index matching liquid is used as the coolant flowing in narrow channels to cool the multiple thin disks directly. A high uniformity of pump intensity distribution is realized by using waveguides. With the a-cut Nd:YLF thin disks at different doping levels, a linearly polarized laser with the maximum output energy of 346 mJ is achieved with the repetition of 350 Hz, corresponding to an optical-optical efficiency of 8.7%, and a slope efficiency of 10%. The beam quality β factor is estimated less than 8 in the horizontal direction due to the positive branch confocal unstable resonator. To the best of our knowledge, it is the first time that the direct-liquid-cooled Nd:YLF thin disk unstable resonator is reported.
A eigenmode expansion method (EME) is proposed to solve the laser eigenmode of optical resonator with intra-cavity phase aberration (ICPA) semi-analytically. In this model, the eigen-equation of OR, so called self-reappearance condition is translated to be a linear eigen-value problem, and it is proved that all eigen-modes can be obtained for any resonators. The linear eigen-value problem is solved numerically, and it gives out the transverse distribution and corresponding eigen-value of each eigenmode, which describe the light field and diffraction loss, respectively. Compared with traditional methods, EME is a semi-analytical method which is unlimited by the order of phase aberration, and it can be solved without numerical iteration. The existing of local modes (LM) in OR with ICPA is proved with EME, which may be the source of local damage on solid medium. And the use of output coupler with transmission, such as graded reflectivity mirror (GRM), can prevent the appearance of LM and improve beam quality. Specially, for the ICPA coupled with laser extraction, the linear eigen-value equations become a nonlinear problem, which are numerically solved by the finite-difference Jacobian method. The result shows that the optical resonator exhibits transverse modal instability (TMI) with certain cavity parameters.
A Nd:YAG thin disk is end-pumped by two high power laser diodes and the fluid flows in a narrow channel to cool it directly. The forced convection occurs between the fluid and disk. A system is designed to measure the convective heat transfer coefficient with different flow rate. With the measured coefficient, the temperature and thermal stress in the disk are numerically analyzed. The maximum permissible thermal load is calculated, which increases with the increasing flow rate. Furthermore, the optical path different distribution is numerically calculated by considering of the thermo-optical effect, and thermal expansion at the maximum permissible thermal load. These results are useful for design of a direct-liquid-cooled Nd:YAG thin disk laser.
In this paper, based on the self-reproduction condition of laser wavefront curvature, the influences of disk defocus on laser parameters can be calculated. The laser-pumping overlapping efficiency will decrease by 9%; the magnification will rise to 2.3, and the intra-cavity loss will be high to 30% due to a laser beam size mismatch when each disk has focal length of -100 m in a positive-branch confocal unstable resonator containing four disks with magnification of 1.8. Therefore, the optical conversion efficiency and stability will be reduced significantly. Several methods defocus compensation of are compared, it can be found that inserting variable-focus lens in resonant is useful in large dynamic range. In experiment, the defocus values are measured in different pumping power. A lens group, used for compensate components according to the single pass probe, is carefully designed. Under this compensation, the pulse energy can be maintained in 10 J from 1 Hz to 100 Hz. The output power can be improved 2.33 times compared to non-compensation condition.
A high power multi-pass diode-pumped solid-state MOPA system with large core diameter tapered quartz fiber as phase
conjugator is experimentally investigated. The results of the effects of laser longitudinal mode characteristic, laser pulse
width and fiber surface optical quality on SBS reflectivity are presented. Maximum SBS reflectivity of 69.7% is obtained
with the repetition rate of 1000 Hz, input pulse energy of 38mJ and pulse width of 24 ns. The phase conjugator is made
from a taper shaped quartz fiber with the diameter of 1mm at the input end and 0.4mm at the other end. It has good
application prospects in improving the beam quality and compressing the pulse width for high power laser system with
high repetition rate.
Nowadays, the research and application of high-average-power diode-pumped solid-state lasers have attracted more and
more attention and support in China. The output power of 5kW~10kW was obtained from different technological
approaches. In the paper, the latest progress and development tendency of high power rod laser, thin slab laser, thin disk
laser and fiber laser in China are introduced comprehensively. It also introduces the situation of the study on high power
diode lasers and laser materials.
Thin disk laser is the most successful design to overcome the degradation of the beam quality caused by the gain
medium's thermal effects and has many advantages in beam quality keeping and power scalability over the traditional
rod and slab laser. In this paper a different type thin disk laser with the large-aperture Nd:YAG disk face-pumped by
2D-stack diode arrays was presented. Over 3kW average power with the beam quality less than 10 times diffraction
limitation was achieved by optimizing the pumping optics designs and improving the gain medium mounting technique.
KEYWORDS: Diodes, Semiconductor lasers, Nd:YAG lasers, High power lasers, Rod lasers, Solid state lasers, Medicine, Laser systems engineering, Numerical simulations, Pulsed laser operation
For the advantage of long-life, high efficiency, low heat load and compact structure, diode pumped Laser (DPL) is used widely in different field such as military, communication and medicine etc. Diode-pumped Nd:YAG laser module is the integration of diodes (pumping source), coupling structure, cooling structure and laser medium through optimization. Being the core component of diode-pumped MOPA laser, the gain distribution of diode-pumped amplifier module influences the beam quality greatly. So it is very important to improve the uniformity of the module with high pumping power. This paper investigates, theoretically and experimentally, the elements which influence the gain distribute of laser medium. A high energy storage laser module have been manufactured. In this module a 8mm diameter Nd:YAG rod which is 120mm long is pumped by twelve diodes-arrays (the peak pump-power is 12kw). Each array is consisted of twelve diodes. Numerical calculation has been done to optimize the coupling structure. The experiment result approved that the gain coefficient of this module is 0.186/cm, from it we can infer that the storage power is about 700mJ per pulse.
Thermal effects present a major challenge to scaling of solid-state laser to high-average power. When using a thin laser crystal disk with a nearly flat-top pump profile, the heat flux can be considered to be one-dimensional. This results in a homogeneous temperature and stress profile within the laser medium leading to reduction of thermal effects. The pump profile is the main factor that may affect the homogeneous temperature profile, but some other factors may affect the temperature homogeneity, such as the cooling dimension and the ratio of the pump area to the thickness of the disk. These factors are analyzed and a good design is made. An average output power of more than 120W is obtained from one single disk and that of 216W from two disks.
A diodes pumped high average power Nd:YAG rod E-O Q-switched MOPA system was designed, fabricated and tested. The laser achieved a maximum average power of 600W in a near top-hat beam with the repetition rate of 500Hz, pulse width of 15 ns. Three different type diode pump laser module with the pump power of 500W, 6.5kW and 12kW were successfully developed for master-oscillator and amplifiers. The modules have an efficient absorption of the pumped light and homogeneous pump beam distribution in the laser medium.
KEYWORDS: Diodes, Semiconductor lasers, Nd:YAG lasers, Rod lasers, High power lasers, Solid state lasers, Diode pumped solid state lasers, Laser systems engineering, Numerical simulations, Pulsed laser operation
This paper investigates, theoretically and experimentally, the elements which influence the gain distribute of laser medium. A high power storage laser module has been manufactured. In this module a 6 mm diameter Nd:YAG rod which is 100 mm long is pumped by nine diode-arrays (the peak pump-power is 6500 W). Each array consisted of nine diodes. On account of the uniformity of distribute in whole laser medium, a Nd-doping of 0.6% laser rod is chosen, and in order to eliminate self-oscillation, two ends are made to inclined planes. Numerical calculation has been done to optimize the distance between the diode and Nd:YAG. The experiment result approved that the gain coefficient of this module is 0.3 cm-1, from it we can infer that the storage power is about 400 mJ per pulse.
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