Good Beam Quality Research Articles

High-Power, High-Beam-Quality, Long-Pulse-Width 532 nm Laser Based on a 4f Optical System

In response to the demand for high-power, long-pulse-width 532 nm lasers in the medical and industrial processing fields, this paper explains how the laser cavity of a high-power Nd:YAG 532 nm laser can be extended while maintaining the laser’s q-parameter by using a 4f optical system. The results show that at a repetition rate of 10 kHz, the extended cavity achieved a maximum average power of 112 W. Compared with the short cavity, the power was not significantly reduced. The pulse width was extended from 56 ns to 85 ns, and its broadening ratio reached 46.5%. The laser maintained good beam quality during high-power operation.

Kilowatt-level supercontinuum generation employing a homemade taper-shaped ytterbium-doped fiber amplifier.

A simple generation method of high-power supercontinuum (SC) based on a homemade long ytterbium-doped taper fiber (T-YDF) amplifier has been demonstrated experimentally and analyzed in this work. The power and spectra of the obtained SC are made adjustable by changing the seed pulse repetition rates. Under a 7.5 MHz seed pulse repetition rate, a SC output is obtained with a spectral range of 1000 nm-1650 nm, maximum output power of 1066 W, and conversion efficiency of 75.7%. The core/clad diameters of the homemade T-YDF are ∼20/400 µm and ∼30/600 µm at the input and output ends, respectively, and the total length is 25 m. Such a long taper fiber can further enhance the properties of the SC while ensuring high power and good beam quality. To the best of the authors' knowledge, this is the first report of directly generating a high average power SC based on the T-YDF amplifier, which provides a proof-of-concept experiment to achieve a high average power SC source and greatly improves the potential application value of the SC source.

Efficient temporal compression of 10-µJ pulses in periodic layered Kerr media.

Pulse compression based on periodic layered Kerr media has been demonstrated as an effective technique for femtosecond pulses with energies around 100 µJ or more. We report such a compressor designed for pulses in the 10-µJ range, which is valuable for many applications. Pulses from a fiber chirped-pulse amplifier are compressed from 300 fs to as short as 60 fs, with good pulse and beam quality. The compressor is a simple and efficient way to extend the performance of common Yb-based sources of pulses with microjoule energies.

Passively mode‐locked Tm:YAP laser with a V2AlC saturable absorber

AbstractIn this work, a V2AlC‐based saturable absorber (SA) was applied to a 2 μm band passively mode‐locked (PML) laser. The saturation intensity and modulation depth of the V2AlC‐based SA were detected by the I‐scan method, which were 225.05 MW/cm2 and 7.68%, respectively. In the PML mode, an average power of 2.12 W at 1939 nm and a pulse width of 1077 ps at 82.49 MHz were obtained from the Tm:YAP laser. In addition, the laser exhibited a good beam quality factor (M2) which was measured to be less than 1.2. These results will pave the way for the application of V2AlC‐based SA in ultrafast lasers.

Beam dynamics design and error analysis of a high frequency RFQ for medical application

Proton therapy based on the linear accelerator is an advanced radiation treatment technology. Lanzhou University has proposed a linac scheme for proton therapy, which features a 750 MHz Radio Frequency Quadrupole (RFQ) as its first accelerator. The RFQ is capable of accelerating a 1 mA proton beam from 30 keV to 3 MeV. The article systematically introduces the beam dynamics design and error analysis of the RFQ. In the design part, a new strategy that can reduce the outlet longitudinal emittance of the RFQ by adjusting its modulation and synchronous phase was proposed. Subsequently, multi-particle simulations based on different codes consistently demonstrated that the dynamics design achieved a good output beam quality. The longitudinal emittance of the RFQ output beam was successfully controlled to below 0.1 π mm·mrad. In the error analysis part, firstly, all types of errors were analyzed separately to determine their impact patterns and to give preliminary tolerances. Then, the combined errors which considered all types of errors together were analyzed. Finally, the tolerances for input beam errors, cavity voltage errors, and mechanical errors were obtained.

Miniaturization of high beam quality 1.543 μm Raman laser with backward stimulated Raman scattering

It is very challenging to make high peak power (big pulse energy) Raman lasers compact, due to laser induce breakdown (LIB) effect; and it is even more difficult to achieve a decent beam quality meanwhile. In this work, a pulsed 1064 nm laser was used as pump source; pressurized methane was used as Raman active medium, which had the largest ratio of backward/forward Raman gain coefficient among all gaseous media; and backward first Stokes (BS1) Raman laser of 1543 nm with good beam quality was realized. When an f=0.5 m lens was used to focus pump beam into a 0.9 m long Raman cell filled with 3.5 MPa methane, 276.1 mJ BS1 was achieved, the corresponding photon conversion efficiency was 70.8% and the peak power was 83.7 MW. BS1 beam quality was measured to be Mx2=2.54,My2=2.28, which was significantly better than that of pump laser. In a setup of f=0.3 m focal lens and 0.5 m long Raman cell, 197.8 mJ BS1 was achieved, with the company of serious LIB. In order to meliorate LIB and improve BS1 conversion efficiency, the focal lens was tilted by 20°, pump laser beam waist and depth of focus increased significantly, BS1 was improved to 225.0 mJ, the corresponding photon conversion efficiency was 58.5%. An even short focal lens and Raman cell with reasonable BS1 energy was possibly achieved by tilting a shorter focal lens by a larger angle. This work also demonstrated that the increment of BS1 conversion may help to reduce the effect of LIB on SRS process.

Concept for power scaling by harmonic beam coaxial combination in an intra-cavity frequency doubling laser.

A new method of harmonic beam coaxial combination (HBCC) from two intra-cavity frequency doubling branches was demonstrated. Firstly, two identical nanosecond (ns) 532 nm green lasers with high power and good beam quality were created. Each green laser was constructed of an intra-cavity frequency doubling branch based on a laser diode (LD) end-pumped acousto-optical (AO) Q-switched 1064 nm Nd:YVO4 laser in a LiB3O5 (LBO) nonlinear crystal. Each branch generated about 45 W green output at a 50 kHz pulse repetition rate (PRR) with diffraction limited beam quality. The first green beam was injected into the LBO crystal in the second branch, and the pulses from the two branches did not exist simultaneously. Then, the HBCC was performed. Consequently, an 83 W combined green output power at 532 nm was obtained with a combination efficiency of 92.2%. The PRR of the HBCC pulse was doubled to be 100 kHz, with a pulse width of about 22 ns, corresponding to a single pulse energy of 0.83 mJ and a peak power of 37.73 kW. The combined beam quality factor was measured to be M x2 = 1.80 in the x direction and M y2 = 1.71 in the y direction, respectively. Moreover, many more beams could also be combined with this method for further scaling the green power.

Efficient yellow Dy:ZBLAN fiber laser with high-brightness diode-pumping at 450 nm.

We report on a low-threshold efficient yellow Dy-fiber laser with good beam quality featuring high-brightness pumping. It employs a single-clad 0.2 mol% Dy:ZBLAN fiber pumped by two 450-nm blue GaN laser diodes. The continuous-wave Dy-fiber laser delivers a maximum output power of 109 mW at 575 nm with a laser threshold of 218 mW, a slope efficiency of 19.6%, and beam quality factors M2 x,y ∼ 1.5. The overall optical efficiency versus incident pump power is 13.9%, being record-high, to the best of our knowledge, for this type of laser. The laser performance is simulated based on the spectroscopic data, being in good agreement with the experiment.

15.5 W, pulsed 630 nm generation based on Raman fiber laser and second-harmonic generation

We present a high-power, nanosecond 630 nm beam generation based on Raman conversion and second-harmonic generation (SHG). 116.2 W, single-mode 1080 nm fiber laser based on 10/125 µm optical fiber is used as a pump source for Raman conversion and the 1260 nm seed laser diode helps the amplification of third-order Raman conversion, which results in 63.7 W at 1260 nm with 54.8% Raman conversion efficiency. SHG to 630 nm is based on type-I noncritical phase-matching conditions with bismuth triborate (BIBO) nonlinear crystal. The average power of 630 nm is 15.5 W at a repetition rate of 9.26 MHz, a pulse width of 16.0 ns, and a SHG efficiency of 24.4%. This result can facilitate the generation of a high-power visible light source with good beam quality at a specific wavelength.

Cpa-free Yb:YAG thin-disk regenerative amplifier laser with hundreds of watts, hundreds of kHz, and hundreds of picoseconds

This paper introduces a CPA-free Yb:YAG thin-disk regenerative amplifier laser system. The seed pulse is generated by a passive mode-locked picosecond oscillator based on a semiconductor saturable absorber mirror, with a center wavelength of 1030.7 nm, a pulse width of 7.7 ps, and a repetition frequency of 26.3 MHz. After stretched and pre-amplified by an all-fiber front stage, the seed with a pulse width of 201.3 ps and a pulse energy of 15.2 nJ was obtained, then injected into the thin-disk regenerative amplifier. Yb:YAG crystal was chosen as the material for the thin-disk, with 100 μm thickness, 9 mm diameter and 7 at.% doping rate. In order to increase the one-way round-trip gain of the thin-disk regenerative amplifier, a symmetrical dual-pass resonant cavity was designed to double the number of passing through the crystal on a round trip in the regenerative resonant cavity to reduce the times of cycles, intra-cavity loss and improve optical efficiency. We used a 969 nm fiber-coupled semiconductor laser as the pump source to pump the Yb:YAG thin-disk crystal. With zero-phonon line pumping technology, the quantum loss and thermal effect were reduced. In the end, we obtained a regenerative amplification output with an average power of 104.5 W, a repetition frequency of 200 kHz, a pulse width of 143.9 ps, and a spectrum width of 0.39 nm. The amplified pulse had quite good beam quality with Mx2 = 1.09 and My2 = 1.14.

All-Solid-State Post-Compression of Low-Energy Pulses at High Repetition Rate

We demonstrate a proof of principle of a simple all-solid-state post-compression setup for low-energy, high-repetition-rate laser pulses, where spectral broadening was performed using a combination of highly nonlinear bulk materials in a simple single-pass geometry. The 75 fs, 210 nJ pulses from an amplified 76 MHz, 15.7 W Yb:KGW oscillator after sequential spectral broadening in ZnS and YAG samples of 2 mm and 15 mm thickness, respectively, were compressed to 37 fs by means of Gires–Tournois interferometric mirrors. The post-compressed pulses with an average power of 11.47 W demonstrated reasonable spatial-spectral homogeneity of the beam with the spectral overlap parameter V>83% and good beam quality with Mx2=1.28 and My2=1.14.

108 m Underwater Wireless Optical Communication Using a 490 nm Blue VECSEL and an AOM.

Advanced light sources in the blue-green band are crucial for underwater wireless optical communication (UWOC) systems. Vertical-external-cavity surface-emitting lasers (VECSELs) can produce high output power and good beam quality, making them suitable for UWOC. This paper presents a 108 m distance UWOC based on a 100 mW 490 nm blue VECSEL and an acousto-optic modulator (AOM). The high-quality beam, which is near diffraction-limited, undergoes relatively small optical attenuation when using a conventional avalanche photodiode (APD) as the detector and employing 64-pulse position modulation (PPM). At the time-slot frequency of 50 MHz, the bit error rate (BER) of the UWOC was 2.7 × 10-5. This is the first reported AOM-based UWOC system with a transmission distance over 100 m. The estimated maximum transmission distance may be improved to about 180 m by fully utilizing the detection accuracy of the APD according to the measured attenuation coefficient of the blue VECSEL used. This type of UWOC system, composed of a high-beam-quality light source and a conventional detector, make it more closely suited to practical applications.

Research status of rare-earth-ion-doped infrared laser

Infrared lasers have an extensive range of applications in sensing, detection, communication, medicine, and other fields. The principle of directly pumping solid-state lasers is simple, and it can easily achieve high-power and high-efficiency laser output, which is one of the important means to obtain infrared lasers. Incorporating rare earth ions into the substrate as the gain medium for directly pumping solid-state lasers can alter their optical performance and further enhance the performance of the laser. Lasers based on rare earth ion doping have a small volume, high conversion efficiency, good beam quality, wide tuning range, and multiple operating modes. Therefore, the proportion of rare earth ions doped as the gain medium for activating ions is currently very large. In this review, Ho3+, Tm3+, and Er3+ are selected as the representative rare earth ions, and their optical properties, such as luminous power and fluorescence lifetime, when doped in different substrates, such as crystals, ceramics, and fibers, are introduced, respectively, to illustrate their feasibility as infrared laser gain media. In addition, we show the different optical properties when doped with two ions, three ions, and four ions, demonstrating their great potential as infrared laser gain media.

Single-frequency high-energy pulses from an Er:Yb glass planar waveguide amplifier with good beam quality

We demonstrate a master oscillator power amplifier system that emits single-frequency, high-energy optical pulses at 1539 nm using an Er, Yb:glass planar waveguide amplifier with a normalized frequency of the waveguide of 4.4. A maximum pulse energy of 9.7 mJ is observed at a repetition frequency of 500 Hz. The signal to noise ratio is 25 dB and is independent of the repetition frequency from 100 to 500 Hz. The beam quality factor M2 of the output is 1.03 thanks to the small normalized frequency of the planar waveguide.

1.2 kW, 20 kHz Nanosecond Nd:YAG Slab Laser System

In this paper, we develop a kW-level high-repetition-rate nanosecond master oscillator power amplifier (MOPA) laser system, employing a structure of fiber, Nd:YVO4, and Nd:YAG hybrid amplification. A tunable fiber seed source is used for adjustable pulse repetition frequency and pulse width. The Nd:YVO4 pre-amplifier, which is dual-end-pumped, achieves high gain while maintaining good beam quality, and the high-power side-pumped Nd:YAG slab main-amplifier enables efficient power amplification. The repetition rate of the output laser can be adjusted within the range of 1~20 kHz, and the pulse width can be tuned within the range of 10~300 ns. The seed output is 6 mW at a repetition frequency of 20 kHz; we achieve an average output power of 1240 W with a total power extraction efficiency of 39.1% and single-pulse energy of 62 mJ at a pulse width of 301 ns. This parameter-controllable high-power laser holds promise for applications in the laser cleaning of complex surface contaminants.

Hundred-watts-level Ho:YLF slab laser with good beam quality

We demonstrated a hundred-watts-level and good beam quality Ho:YLF slab laser with a hybrid cavity. When the incident pump power was 263 W, we obtained a maximum output power of 105 W. The slope efficiency with the incident pump power was 52.5%, and the optical-to-optical conversion efficiency was 39.9%. The instability of the maximum output power was measured to be 0.35% in 30 min. With the increase of the output power, the central wavelength of laser shifted from 2051 nm to 2063 nm. The beam quality factors M 2 were measured to be 1.13 in the stable direction and 2.45 in the unstable direction. Hundred-watts-level Ho:YLF slab laser with good beam quality has not been reported yet to the best of our knowledge.

208 W single-frequency 1064 nm laser based on a single-crystal fiber master-oscillator power amplifier.

High-power all-solid-state continuous-wave (CW) single-frequency laser with high linear polarization is a significant source for quantum optics and precision measurement. In this Letter, a high-power linearly polarized CW single-frequency laser based on the single-crystal fiber (SCF) master-oscillator power amplifier (MOPA) is presented, in which a homemade 140 W low-noise CW single-frequency laser and a Nd:YAG SCF are firstly employed as the seed laser and the medium of the MOPA, respectively. The mode-matching between the pump laser propagated with waveguide form and the freely propagated seed laser is optimized by considering the influence of the degradations of the polarization and the beam quality. Finally, when the incident powers of the pump and seed lasers are 262.6 W and 126.3 W, respectively, the seed waist radius is optimized to 200 μm. In this case, the output power of the linearly polarized laser reaches up to 208 W, which is the highest output power, to the best of our knowledge. The presented results provide a good reference for implementing a high power and high degree of the polarization and good beam quality laser based on the SCF MOPA.

Monolithic beam combined quantum cascade laser arrays with integrated arrayed waveguide gratings.

Quantum cascade lasers (QCLs) are ubiquitous mid-infrared sources owing to their flexible designs and compact footprints. Manufacturing multiwavelength QCL chips with high power levels and good beam quality is highly desirable for many applications. In this study, we demonstrate an λ ∼ 4.9 µm monolithic, wavelength beam-combined (WBC) infrared laser source by integrating on a single chip array of five QCL gain sections with an arrayed waveguide grating (AWG). Optical feedback from the cleaved facets enables lasing, whereas the integrated AWG locks the emission spectrum of each gain section to its corresponding input channel wavelength and spatially combines their signals into a single-output waveguide. Our chip features high peak power from the common aperture exceeding 0.6 W for each input channel, with a side-mode suppression ratio (SMSR) of over 27 dB when operated in pulsed mode. Our active/passive integration approach allows for a seamless transition from the QCL ridges to the AWG without requiring regrowth or evanescent coupling schemes, leading to a robust design. These results pave the way for the development of highly compact mid-IR sources suitable for applications such as hyperspectral imaging.

Efficient nonlinear pulse compression to 20 fs based on an all-solid-state periodic self-focusing system

Ultrafast lasers with high average power and excellent spatial characteristics have been sought after in the ultrafast optics field. In this work, we propose and experimentally demonstrate a simple Gaussian beam optics model to realize the configuration of periodic self-focusing systems for generating intense ultrashort pulse with high spatial quality. An efficient 17-fold nonlinear pulse compression is implemented by a two-stage solid-thin-plates periodic self-focusing system based on this model, delivering 20 fs (full-width-at-half-maximum, FWHM) pulses with a pulse energy of 155 µJ at a repetition rate of 200 kHz. The waveguide-like self-consistent stationary spatial mode propagation achieved in the system enables the high efficiency of 94 %, good beam quality and spectral homogeneity across the beam profile. The solid-thin-plates in three different materials are used in the first stage, which manifests the model can be widely applied in many kinds of nonlinear medium. In the second stage, a simple and compact double-pass architecture is demonstrated, indirectly proving the accuracy of model. The beam propagations under non-stationary modes are also investigated and a more precise quasi-stable oscillatory region is suggested based on the Gaussian beam optics model. These results underline that the Gaussian beam optics model is a practical solution for identifying the self-consistent stationary mode of the periodic self-focusing system and show the potential of the periodic self-focusing system for efficient few-cycle pulses generation which is demand by many applications.

Compact Diode-Pumped 946 nm Nd:YAG Laser with Good Beam Quality

A continuous-wave (CW), high-power, quasi-three-level Nd:YAG laser operating at 946 nm is reported. The beam quality of the laser is greatly improved. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser inside a simple concave-plane cavity. At an incident pump power of 31 W, a maximum CW output of 9.98 W was obtained at 946 nm, with a beam quality factor of M2~5. The corresponding optical-to-optical efficiency was 32.2% with respect to the incident pump power. To the best of our knowledge, this is the highest output power at 946 nm with such a beam quality ever generated by diode-pumped Nd:YAG laser with bulk crystals.