Good Beam Quality Research Articles

Efficient 1.7 µm pumping of 2 µm thulium lasers.

Solid-state 2 µm lasers based on thulium-doped active media Tm:YAG, Tm:YAP, and Tm:YLF were investigated under 1.7 µm resonant diode pumping. In contrast with standard 0.8 µm pump wavelength, a high slope efficiency was achieved, up to 80% in the case of Tm:YAP and Tm:YLF, nearing a quantum limit without relying on Tm3+-Tm3+ cross-relaxation energy transfer. Low thermal load allowed for stable continuous-wave operation with good beam quality and output power up to 6 W (Tm:YAG, Tm:YLF), and 8W (Tm:YAP).

DNA-CTMA Matrix Influence on Rhodamine 610 Light Emission in Thin Films.

Due to the increased application of lasers in different fields (industry, medicine, etc.), there is a growing need for new laser sources with good beam quality and variable emission wavelength. At the same time, for environmental reasons, the obtaining of novel eco-friendly active optical materials, such as those based on the deoxyribonucleic acid (DNA) biopolymer, with optimal light emission properties, is of high interest. The results obtained in this study of the temporal dependence of the transmittance and of the light emission in thin films of DNA-CTMA-Rhodamine 610 (at different Rhodamine concentrations) (DNA-CTMA-Rh610), when they are illuminated with continuous wave laser light at 532 nm (frequently used in the optical pumping of dye lasers), are presented and discussed. The transmittance results obtained for thin film samples are compared to those obtained for the DNA-CTMA-Rh610 solutions in butanol, from which the films have been made, and also with those obtained for Rh610 solutions in butanol with the same concentrations. The investigation was performed in order to assess the influence of the DNA-CTMA and of the green laser light at 532 nm wavelength on relevant chromophore properties such as light transmission and fluorescence emission. The results obtained revealed that the DNA-CTMA matrix has an active influence on the Rhodamine 610 emission, in the whole range of concentrations of the investigated samples.

High power GaSb-based superluminescent diode with cascade cavity suppression waveguide geometry and ultra-low antireflection coating

We report on a GaSb-based superluminescent diode optimized for high-power broadband operation around a wavelength of 2 μm. The high optical power was achieved by the high-quality epitaxial InGaSb/AlGaAsSb type-I quantum well gain material, which was processed into a double-pass amplification configuration. To prevent lasing at high current injection while enabling strong amplified spontaneous emission, a cascade cavity suppression waveguide geometry was designed to connect the vertical rear facet with the reflectivity-suppressed angled front facet. A Ta2O5/SiO2 ultra-low antireflection coating with a minimum reflectivity of 0.04% was applied to the front facet for further cavity suppression. This combination allowed the superluminescent diodes to demonstrate a record high single-transverse-mode output power of up to 152 mW under continuous-wave operation at room temperature, with a broad spectral band of 42 nm full width at half maximum. A 25% promotion in optical power has been realized compared to current state-of-the-art devices in this wavelength range, without sacrificing spectral bandwidth. The high-power spectral density characteristics, along with a good beam quality, are well suited for absorption spectroscopy applications and hybrid integration with silicon technology.

210-W, Quasi-Continuous Wave, Nd:YAG InnoSlab Laser at 1319 nm

In this paper, we demonstrate a high-power, quasi-continuous wave using a laser-diode dual-end-pumped Nd:YAG InnoSlab laser at 1319 nm. The maximum average output power of 210 W at a single 1319 nm wavelength is obtained with an optical-optical efficiency of 18.8% from absorbed pump power to laser output. The output pulse duration is 246 μs at the repetition of 500 Hz, and the beam quality factors of M2 are 1.37 and 1.47 in the horizontal and vertical directions, respectively. This is the first report on high-power, quasi-continuous wave using Nd:YAG InnoSlab lasers at 1319 nm with good beam quality.

Design and Beam Dynamic Studies of an Injector for a Compact THz Coherent Radiation Source

An intense narrow-band terahertz (THz) radiation source has been designed to generate a broad tuning range of radiation frequencies between 0.5 THz and 5.0 THz. The THz radiation is produced when a short-bunch electron beam propagates through an undulator. To achieve high-power peak radiation, the source requires high-brightness electron beams with low beam emittance and short bunch length. A proposed design for the photocathode RF gun used as the electron source is presented. The gun with high mode separation and high Q-factor can be achieved for producing a good beam quality. The beam dynamics of the injector have been preliminarily optimized using the software ASTRA and Elegant, investigating the impact of laser pulse shape on electron beam quality. The results of the beam dynamics studies are comprehensively discussed in this paper.

170-W Nd:YAG InnoSlab laser at 1319 nm.

In this paper, a continuous-wave Nd:YAG InnoSlab laser at 1319 nm with high output power and high beam quality is demonstrated. The maximum output power of 170 W at 1319-nm single wavelength is obtained with an optical-to-optical efficiency of 15.3% from absorbed pump power to laser output and the corresponding slope efficiency of 26.7%. The beam quality factors of M2 are 1.54 and 1.78 in the horizontal and vertical directions, respectively. To the best of our knowledge, this is the first report on Nd:YAG 1319-nm InnoSlab lasers with such high output power and good beam quality.

Suppressing transverse mode instability through multimode excitation in a fiber amplifier

High-power fiber laser amplifiers have enabled an increasing range of applications in industry, science, and defense. The power scaling for fiber amplifiers is currently limited by transverse mode instability. Most techniques for suppressing the instability are based on single- or few-mode fibers in order to output a clean collimated beam. Here, we study theoretically using a highly multimode fiber amplifier with many-mode excitation for efficient suppression of thermo-optical nonlinearity and instability. We find that the mismatch of characteristic length scales between temperature and optical intensity variations across the fiber generically leads to weaker thermo-optical coupling between fiber modes. Consequently, the transverse mode instability (TMI) threshold power increases linearly with the number of equally excited modes. When the frequency bandwidth of a coherent seed laser is narrower than the spectral correlation width of the multimode fiber, the amplified light maintains high spatial coherence and can be transformed to any target pattern or focused to a diffraction-limited spot by a spatial mask at either the input or output end of the amplifier. Our method simultaneously achieves high average power, narrow spectral width, and good beam quality, which are required for fiber amplifiers in various applications.

Design of a compact optical antenna system with high transmission efficiency utilizing a ring-like VCSEL array.

Vertical cavity surface-emitting lasers (VCSELs) with gigahertz bandwidth and good beam quality possess great potential for multi-wavelength free-space optical communication. In this Letter, a compact optical antenna system utilizing a ring-like VCSEL array that can realize the parallel transmission of multi-channel and multi-wavelength collimated laser beams and has the advantages of aberration elimination and high transmission efficiency is proposed. Ten different signals can be transmitted simultaneously, greatly increasing the channel capacity. Based on the vector theory of reflection, ray tracing and the performance of the proposed optical antenna system are demonstrated. This design method has a certain reference value for designing complex optical communication systems with high transmission efficiency.

Hundred watt level and high energy compact hybrid fiber femtosecond laser with good beam quality

A hundred watt level compact femtosecond laser system comprised of a high gain, low nonlinearity silicate glass fiber amplifier and a highly efficient Yb:YAG single-crystal fiber amplifier is demonstrated. By reducing the harmful thermal lens effect of amplifiers and optical elements, and precisely tuning dispersion of the stretcher, 110 W, 495 fs laser output at 1 MHz repetition rate and 250 μJ, 494 fs laser output at 200 kHz repetition rate are obtained. An optimized beam-quality factor (M 2) of better than 1.3 is obtained by filtering the beam with a small-aperture diaphragm. To our best knowledge, this is the first demonstration of such a hybrid femtosecond laser over 100 W and 100 μJ with good beam quality.

Cryogenically cooled Fe:ZnSe-based chirped pulse amplifier at 4.07 µm.

A femtosecond chirped pulse amplifier based on cryogenically cooled Fe:ZnSe was demonstrated at 333 Hz-33 times higher than previous results achieved at near-room-temperature. The long upper-state lifetime allows free-running, diode-pumped Er:YAG lasers to be used as pump lasers. 250-fs, 4.59-mJ pulses are produced with a center wavelength of 4.07 µm, which avoids strong atmospheric CO2 absorption that cuts on around 4.2 µm. It is therefore possible to operate the laser in ambient air with good beam quality. By focusing the 18-GW beam in air, harmonics up to the ninth order were observed indicating its potential for use in strong-field experimentation.

Compact and highly stable electro-optic Q-switched laser at 1064 nm based on a composite YAG/Nd:YAG crystal

We report a compact and highly stable 1064-nm electro-optic Q-switched laser operating at the repetition rate of 1 kHz. A composite Nd:YAG crystal was used as the gain media and the cavity length was 105 mm. Under the average pump power of 11 W, the output power achieved 2.404 W with the pulse width of 4.558 ns, corresponding to the maximum peak power of 0.527 MW and the optical-to-optical conversion efficiency of 21.85%. The slope efficiency reached 42.69%. The beam quality in the horizontal (Mx2) and vertical (My2) directions were 1.81 and 1.58, respectively. The pulse timing jitter was less than 1 ns, and the average power fluctuation measured within 30 min was 0.83% (RMS). It is believed that such a compact and highly stable pulsed laser with high repetition rate, high peak power, and good beam quality has great potential in the fields of lidar, etc.

Mode-Modulation Structure Based on 650 nm Ridge Waveguide Edge-Emitting Laser

Traditional laser diodes operating at 650 nm are more prone to high-order mode excitation, resulting in poorer beam quality. In this paper, we designed GaInP–AlGaInP laser diodes (LD) with a 650 nm range and a trench mode-modulation structure based on the structure of edge-emitting laser (EEL) diodes. The effect of the three-trench structure was investigated theoretically and experimentally. The right trench structure laser chips demonstrated good beam quality while maintaining a high power output. An electro-optical conversion efficiency of 56% was demonstrated with a slope efficiency of 1.32 W/A at a 40 mA current. The maximum optical output power reached 40.8 mW.

Microsecond pulsed yellow emission by intracavity doubled optically pumped two-chip VECSEL

A quasi-continues-wave (QCW) 100 μs pulsed yellow laser at 589 nm, based on intracavity frequency doubling (ICFD) of identical two-chip vertical-external-cavity surface-emitting laser (VECSEL) with bow-tie configuration, is demonstrated. Each VECSEL chip is optically QCW-pumped by an 808 nm laser diode, emitting a fundamental wavelength around 1178 nm. A birefringence filter is inserted in the laser cavity to select the linear p-polarized oscillation and tune the operation wavelength. Using a lithium-triborate (LiB3O5) as the frequency doubled crystal, a total frequency doubled output power of 264 mW at 589 nm is achieved, operating at a pulse duration of 194 μs and a pulse repetition rate of 1 kHz. A good beam quality factor is measured to be M 2 = 2.15. These results represent a novel scheme for the QCW multiple gain chips VECSEL ICFD system, opening up a new perspective for the realization of impact long pulse 589 nm laser for sodium guide-star.

Yb:YAG diverging beam amplifier with 20 mJ pulse energy and 1.5 kHz repetition rate.

We have developed a laser system with a combination of record-breaking parameters for rod ytterbium-doped yttrium aluminum garnet (Yb:YAG) lasers with pulse energy 20 mJ, average power 30 W, and beam quality М2 < 1.35. This record was achieved thanks to the Yb:YAG diverging beam amplifier (DBA) geometry, which allows combining efficient amplification with high average power, good beam quality, and high-energy pulse extraction.

Control of electron beam current, charge, and energy spread using density downramp injection in laser wakefield accelerators

Density downramp injection has been demonstrated to be an elegant and efficient approach for generating high-quality electron beams in laser wakefield accelerators. Recent studies have demonstrated the possibilities of generating electron beams with charges ranging from tens to hundreds of picocoulombs while maintaining good beam quality. However, the plasma and laser parameters in these studies have been limited to specific ranges or attention has been focused on separate physical processes such as beam loading, which affects the uniformity of the accelerating field and thus the energy spread of the trapped electrons, the repulsive force from the rear spike of the bubble, which reduces the transverse momentum p⊥ of the trapped electrons and results in small beam emittance, and the laser evolution when traveling in the plasma. In this work, we present a comprehensive numerical study of downramp injection in the laser wakefield, and we demonstrate that the current profile of the injected electron beam is directly correlated with the density transition parameters, which further affects the beam charge and energy evolution. By fine-tuning the plasma density parameters, electron beams with high charge (up to several hundreds of picocoulombs) and low energy spread (around 1% FWHM) can be obtained. All these results are supported by large-scale quasi-three-dimensional particle-in-cell simulations. We anticipate that the electron beams with tunable beam properties generated using this approach will be suitable for a wide range of applications.

Determination of copper, magnesium, and manganese in aluminum alloys using laser-induced breakdown spectroscopy based on fiber laser ablation

Fiber lasers are characterized by high efficiency, good beam quality, and high reliability and are widely used in industries. In addition, the high repetition frequency of fiber lasers can increase the analytical efficiency greatly. In this paper, a nanosecond fiber laser was introduced to be the ablation source in laser-induced breakdown spectroscopy (LIBS) for determining copper, magnesium, and manganese in aluminum alloys. Because of the high repetition frequency of the fiber laser, the conventional delay-acquisition strategy was unavailable. Discrete wavelet transform (DWT) was used to mitigate the interference of the Bremsstrahlung background. Standard calibration and internal calibration methods were also compared for quantitative analyses. The results demonstrated that the combination of DWT and internal calibration effectively improved the analytical accuracy. After optimization, the R2s of the FL-LIBS system for Cu, Mg, and Mn were 0.9897, 0.9964, and 0.9979, respectively; the root mean squared error of cross validation was 0.0414, 0.0462, and 0.031 wt. %, respectively. This study provides a convenient method for the rapid elemental analysis of aluminum alloys.

Self-mode-matching compact low-noise all-solid-state continuous wave single-frequency laser with output power of 140 W.

The high power all-solid-state continuous wave single-frequency laser is a significant source for science and application due to good beam quality and low noise. However, the output power of the laser is usually restricted by the harmful thermal lens effect of the solid gain medium. To address this issue, we develop a self-mode-matching compact all-solid-state laser with a symmetrical ring resonator in which four end-pumped Nd:YVO4 laser crystals are used for both laser gain media and mode-matching elements. With this ingenious design, the thermal lens effect of every laser crystal can be controlled and the dynamic of the designed laser including the stability range and the beam waist sizes at crystals can be manipulated only by adjusting the pump power used on each laser gain medium. Under an appropriate combination of pump powers on four crystals, self-mode-matching in a resonator is realized. A stable CW single-frequency at 1064 nm with 140-W power, 102-kHz linewidth, and low intensity noise is obtained. The presented design paves an effective way to further scale-up the output power of a compact laser by employing more pieces of gain media.

Over 10-kW fiber laser spectral beam combination based on dichromatic mirrors

Owing to the advantages of good beam quality, high conversion efficiency, convenient thermal management and compact structure, high power fiber lasers have been widely desired in industrial processing. So far, the output power has been strictly limited by the nonlinear effects and transverse mode instability. In order to break through the power limitation, here we experimentally demonstrate a power boosting technology called spectral beam combination based on dichromatic mirrors. Firstly, high power fiber amplifiers with different central wavelengths are established for the spectral combination. Secondly, utilizing a dual-beam combined system and two homemade high power fiber amplifiers, an output power of 10 kW is achieved, with a remarkable combination efficiency of 98.3% and a beam quality of &lt;i&gt;M&lt;/i&gt; &lt;sup&gt;2&lt;/sup&gt;～1.33. Secondly, using a three-beam combined system, an output power of 13.5 kW is obtained with a combination efficiency of 96.8% and beam quality of &lt;i&gt;M&lt;/i&gt; &lt;sup&gt;2&lt;/sup&gt;～1.61. By increasing the number of input beams and their output power as well, we believe that a higher output power can be achieved based on dichromatic mirror spectral beam combination.

Analysis of Thermal Effects in Kilowatt High Power Diamond Raman Lasers

Chemical vapor deposition (CVD) diamond crystal is considered as an ideal material platform for Raman lasers with both high power and good beam quality due to its excellent Raman and thermal characteristics. With the continuous development of CVD diamond crystal growth technology, diamond Raman lasers (DRLs) have shown significant advantages in achieving wavelength expansion with both high beam quality and high-power operation. However, with the output power of DRLs reaching the kilowatt level, the adverse effect of the thermal impact on the beam quality is progressively worsening. Aiming to enunciate the underlying restrictions of the thermal effects for high-power DRLs (e.g., recently reported 1.2 kW), we here establish a thermal-structural coupling model, based on which the influence of the pump power, cavity structure, and crystal size have been systematically studied. The results show that a symmetrical concentric cavity has less thermal impact on the device than an asymmetrical concentric cavity. Under the ideal heat dissipation condition, the highest temperature rise in the diamond crystal is 23.4 K for an output power of ~2.8 kW. The transient simulation further shows that the heating and cooling process of DRLs is almost unaffected by the pump power, and the times to reach a steady state are only 1.5 ms and 2.5 ms, respectively. In addition, it is also found that increasing the curvature radius of the cavity mirror, the length and width of the crystal, or decreasing the thickness of the crystal is beneficial to alleviating the thermal impact of the device. The findings of this work provide some helpful insights into the design of the cavity structure and heat dissipation system of DRLs, which might facilitate their future development towards a higher power.

Promotion of Specific Single-Transverse-Mode Beam Characteristics for GaSb-Based Narrow Ridge Waveguide Lasers via Customized Parameter Design

GaSb-based single-transverse-mode narrow ridge waveguide (RW) lasers with high power and simultaneous good beam quality have broad application prospects in the mid-infrared wavelength region. Yet its design and formation have not been investigated systematically, while the beam characteristics that affect their suitability for specific applications remain rarely analyzed and optimized. The present work addresses these issues by theoretically establishing a waveguide parameter domain that generalizes the overall possible combinations of ridge widths and etch depths that support single-transverse-mode operation for GaSb-based RW lasers. These results are applied to develop two distinct and representative waveguide designs derived from two proposed major optimization routes of model gain expansion and index-guiding enhancement. The designs were evaluated experimentally based on prototype 1-mm cavity-length RW lasers in the 1950 nm wavelength range, which were fabricated with waveguides having perpendicular ridge and smooth side-walls realized through optimized dry etching conditions. The model gain expanded RW laser design with a relatively shallow-etched (i.e., 1.55 upmum) and wide ridge (i.e., 7 upmum) yielded the highest single-transverse-mode power to date of 258 mW with a narrow lateral divergence angle of 11.1^circ full width at half maximum at 800 mA under room-temperature continuous-wave operation, which offers promising prospects in pumping and coupling applications. Meanwhile, the index-guiding enhanced RW laser design with a relatively deeply etched (i.e., 2.05 upmum) and narrow ridge (i.e., 4 upmum) provided a highly stable and nearly astigmatism-free fundamental mode emission with an excellent beam quality of M^2 factor around 1.5 over the entire operating current range, which is preferable for seeding external cavity applications and complex optical systems.