Diffraction-limited Beam Quality Research Articles

Phase-locked high-brightness interband cascade laser array with multimode interferometer couplers.

We report on the design, fabrication, and characterization of an interband cascade laser (ICL) array incorporating multimode interference (MMI) couplers for phase-locking emitters. The ICL array, emitting at 3-4 µm, was developed to address the challenges of heat dissipation and in-phase operation in mid-infrared lasers. The array features a 7.5-µm-wide ridge design for fundamental transverse mode propagation and employs MMI structures to realize an in-phase operation of multiple emitters. The far-field patterns, characterized by periodic and symmetrical interference fringes, confirm the coherent operation of the array and the efficacy of MMI couplers in achieving phase-locking. The single-ridge side of the array exhibits a single-lobe far-field profile, with higher-order transverse modes effectively suppressed, showcasing a nearly diffraction-limited beam quality (M2 ≈ 1.31) at high output powers (390 mW from the 1 × 4 array). The robust performance and scalable design of the ICL array, validated by experimental results and theoretical simulations, indicate its potential for high-power mid-infrared applications and as an optical phased array.

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.

Long wavelength distributed feedback tapered quantum cascade lasers.

We present a study on distributed feedback tapered quantum cascade lasers emitting at 14 µm. The fabricated lasers with taper angles between 0° and 3° exhibited output powers scaling in accordance with the active zone volume increase. Reduced divergence angles as small as 4.2° have been obtained with diffraction limited-beam quality (M2 ≈ 1). With a first order Bragg-grating for single longitudinal mode selection, continuous wave operation was demonstrated up to almost room temperature with side-mode suppression ratios greater than 20 dB.

Internal phase control of fiber laser array based on photodetector array

Abstract Coherent beam combining (CBC) of fiber laser array is a promising technique to realize high output power while maintaining near diffraction-limited beam quality. To implement CBC, an appropriate phase control feedback structure should be established to realize phase-locking. In this paper, an innovative internal active phase control CBC fiber laser array based on photodetector array is proposed. The dynamic phase noises of the laser amplifiers are compensated before being emitted into free space. And the static phase difference compensation of emitting laser array is realized by interference measurement based on photodetector array. The principle of the technique is illustrated and corresponding simulations are carried out, and a CBC system with four laser channels is built to verify the technique. When the phase controllers are turned on, the phase deviation of the laser array is less than λ/20, and ∼ 95% fringe contrast of the irradiation distribution is obtained. The technique proposed in this paper could provide a reference for the system design of a massive high-power CBC system.

509 nm high power wide-tuned external cavity surface emitting laser

High power widely tunable green lasers have potential applications in many fields such as biomedicine, lidar, laser spectroscopy, laser display, underwater wireless optical communication, and fine processing of nonferrous metals. Vertical-external-cavity surface-emitting lasers, also known as semiconductor disk lasers, have the advantages of high power, good beam quality, and wide bandwidth of gain medium. In this work, a gain chip with a reverse-growth epitaxy structure and an emitting wavelength of 1018 nm is designed. In the DBR reflection spectrum, a bandwidth of 74 nm is achieved above a reflectivity of greater than 99.1%, laying a solid foundation for achieving high-power widely tunable output. The laser cavity combines a 1018 nm semiconductor gain chip, a folded mirror, and a plane mirror to construct a compact V-type resonant cavity. A class-I phase-matched LBO nonlinear crystal with a length of 10 mm is placed at the beam waist of the cavity to realize an efficient frequency doubling process to produce a 509 nm green laser. To meet the requirement for the polarization during frequency conversion and to tune the oscillating wavelength of the laser, a birefringent filter (BRF) is employed in the laser resonant cavity. When the thickness of the used BRF is 1 mm, the obtained wavelength tuning range of the fundamental laser and the frequency doubled green laser are 47.1 nm and 20.1 nm, respectively, showing a good tuning capability of the laser. The laser’s performance varies with the thickness of the BRF. When using a 2 mm BRF, a maximum power output of the frequency-doubled green laser reaches 8.23 W during continuous tuning, indicating an ideal compatibility of wide tuning characteristics with a high power output. Meanwhile, its beam quality <i>M</i> <sup>2</sup> factors are 1.00 and 1.03 in the <i>x</i>- and <i>y</i>-direction, respectively, demonstrating a near diffraction-limited excellent beam quality. This green laser also possesses a frequency doubling conversion efficiency of up to 68.2%, which can efficiently converse the fundamental laser into the frequency doubled green laser. The optical-to-optical conversion efficiency from the absorbed pump light to the frequency-doubled green light also reaches 16.6%. Meanwhile, from the spectral linewidths of the green lasers under different thickness values of BRFs it is found that the thicker the BRF, the narrower the laser line width is, which is consistent with the theoretical result.

High-energy acousto-optic Q-switched alexandrite laser with wavelength tunable fundamental and UV second harmonic generation

We investigate high-energy mJ-class diode-pumped acousto-optic (AO) Q-switched alexandrite lasers with broad tunability at both the fundamental near-IR wavelength range and second harmonic generation (SHG) in the UV wavelength range. An AO Q-switched alexandrite laser with continuous-wave diode-pumping has been operated at up to 10 kHz and producing pulse energy of 700 µJ at repetition rate of 1 kHz. With pulsed double-pass diode-pumping, we demonstrate higher pulse energy of 2.6 mJ. With wavelength tuning, this laser system demonstrated broad fundamental tuning range from 719 to 787 nm with diffraction-limited beam quality (M2 = 1.05). By external cavity second harmonic generation in a Type-I LBO crystal, pulse energy of 0.66 mJ is generated at 375 nm and with UV tuning range from 361 to 391 nm. To our knowledge, this is the first demonstration of AO Q-switched alexandrite laser with broad wavelength tuning in the near-IR and UV wavelength ranges and shows its excellent potential as a pulsed source for future applications.

Dual-slab Yb:KGd(WO4)2 regenerative amplifier for spectral shaping and high-power output.

A high-power regenerative amplifier (RA) based on dual-slab Yb:KGd(WO4)2 (Yb:KGW) was demonstrated, which provided a maximum average power of 33.7 W at a repetition rate of 75-200 kHz before compression with a central wavelength of 1039 nm, corresponding to an optical-to-optical conversion efficiency of 51.4%. To the best of our knowledge, this is the highest average power from the Yb:KGW solid-state RA. The compressed pulse duration of 205 fs was realized under the maximum output power. By adjusting the gain of the crystals, respectively, the spectral shaping can be achieved. A combination spectrum with root-mean-square (RMS) bandwidth of 4.5 nm was generated with a central wavelength of 1035 nm at an output power of 20 W, the compressed pulse duration was 159 fs. Meanwhile, effective mitigation of thermal effects by dual-slab configuration guaranteed the nearly diffraction-limited beam quality: M x2 = 1.17 and M y2 = 1.20.

Monolithic tapered Yb-doped fiber chirped pulse amplifier delivering 126 μJ and 207 MW femtosecond laser with near diffraction-limited beam quality

In this work, a high-energy and high peak power chirped pulse amplification system with near diffraction-limited beam quality based on tapered confined-doped fiber (TCF) is experimentally demonstrated. The TCF has a core numerical aperture of 0.07 with core/cladding diameter of 35/250 µm at the thin end and 56/400 μm at the thick end. With a backward-pumping configuration, a maximum single pulse energy of 177.9 μJ at a repetition rate of 504 kHz is realized, corresponding to an average power of 89.7 W. Through partially compensating for the accumulated nonlinear phase during the amplification process via adjusting the high order dispersion of the stretching chirped fiber Bragg grating, the duration of the amplified pulse is compressed to 401 fs with a pulse energy of 126.3 μJ and a peak power of 207 MW, which to the best of our knowledge represents the highest peak power ever reported from a monolithic ultrafast fiber laser. At the highest energy, the polarization extinction ratio and the M2 factor were respectively measured to be ~ 19 dB and 1.20. In addition, the corresponding intensity noise properties as well as the short- and long-term stability were also examined, verifying a stable operation of the system. It is believed that the demonstrated laser source could find important applications in, for example, advanced manufacturing and photomedicine.Graphical

Improving the beam quality of single-grating spectrally combined fiber lasers with adaptive optics.

Spectral combination is promising for diffraction-limited beam quality and single aperture beams. Unfortunately, beamlet deviations, linewidth broadening, and thermal aberrations inevitably degrade the beam quality. Many high-power laser systems integrate adaptive optics systems to maintain beam qualities. However, owing to the nature of incoherent combination, there is no well-defined wavefront in the spectrally combined beam, and whether phase compensations can enhance beam quality has not been discussed yet. We present the feasibility of improving the beam quality of spectral combined fiber lasers by adaptive optics. Simulations indicate that common path aberrations can be effectively corrected by adaptive optics, while beam quality degraded by displacement deviations and linewidth broadening cannot be improved. Additionally, the combined beam could be directly used as the beacon light in the propagation tunnel. To our knowledge, this study is the first to demonstrate that adaptive optics can improve the beam quality of spectrally combined fiber lasers and enable a further step toward diffraction-limited beam quality.

Widely-tunable all-fiber Tm doped MOPA with > 1 kW of output power.

In this paper, we report on a high-power and widely tunable thulium-doped fiber laser (TDFL) based on a monolithic master oscillator power amplifier (MOPA) system. The master oscillator is a Tm fiber ring laser incorporating a tunable bandpass filter to realize narrow linewidth and wavelength tunable operation. The MOPA generated 1010 W ∼1039 W of output power over a tuning range of 107 nm from 1943 to 2050nm with slope efficiencies of more than 51% and spectra linewidth of ∼0.5 nm. Power stability (RMS) in ∼10 min scale is measured to be ∼0.52%. A diffraction-limited beam quality factor M2 of ∼1.18 is measured at 920 W of laser output. Output power is pump-limited without the onset of parasitic oscillation or amplified spontaneous emission (ASE) even at the maximum power level. This is the first demonstration, to the best of our knowledge, on an all-fiber integrated wavelength-tunable TDFL at 2 µm with output power exceeding 1 kW.

Pulses of 32 mJ and 158 fs at 20-kHz repetition rate from a spatiotemporally combined fiber laser system.

A high-energy, high-power ultrafast fiber laser system based on spatiotemporal coherent combination is presented. Bursts of eight subsequent chirped-pulse amplification (CPA)-stretched pulses are amplified simultaneously in 16 parallel ytterbium-doped rod-type amplifiers. After spatial and temporal coherent combination of the total 128 amplified pulse replicas into a single pulse, it is compressed in a partially protective-gas-filled CPA compressor. Finally, nearly Fourier-transform-limited pulses with an energy of 32 mJ and a duration of 158 fs are emitted with a repetition rate of 20 kHz and a close to diffraction-limited beam quality.

High-energy femtosecond pulse generation from a hybrid mode-locked large-mode-area Er:ZBLAN fiber laser.

We report a hybrid mode-locked fiber laser at 2.8 µm based on a large-mode-area Er:ZBLAN fiber. Reliable self-starting mode-locking is achieved via the combination of nonlinear polarization rotation and a semiconductor saturable absorber. Stable mode-locked pulses with a pulse energy of 9.4 nJ and a pulse duration of 325 fs are generated. To the best of our knowledge, this is the highest pulse energy directly generated from a femtosecond mode-locked fluoride fiber laser (MLFFL) to date. The measured M2 factors are below 1.13, indicating a nearly diffraction-limited beam quality. Demonstration of this laser provides a feasible scheme for the pulse energy scaling of mid-infrared MLFFLs. Moreover, a peculiar multi-soliton mode-locking state is also observed, in which the time interval between the solitons varies irregularly from tens of picoseconds to several nanoseconds.

Characterization of transverse mode instability with a 4-quadrant photodiode.

Transverse mode instability (TMI) represents the main limitation for the power scaling of fiber laser systems with a diffraction-limited beam quality. In this context, it has become increasingly important to find a cheap and reliable way to monitor and characterize TMI and distinguish this effect from other dynamic perturbations. In this work, with the help of a position-sensitive detector, a novel method is developed to characterize the TMI dynamics even in the presence of power fluctuations. The position information of the fluctuating beam is recorded in the X- and Y-axis of the detector, which are used to track the temporal evolution of the center of gravity of the beam. The trajectories described by the beam within a specific time window contain rich information about TMI, which can be used to gain further insight into this phenomenon.

Nanosecond pulsed narrow-linewidth all-fiber source for ZGP-OPO pumping

We demonstrate a pulsed three-stage master oscillator power amplifier (MOPA) at a wavelength of 2047 nm and its application for mid-IR generation via pumping a ZnGeP2 (ZGP) optical parametric oscillator (OPO). A maximum MOPA output power of 19.8 W is reached while achieving a diffraction limited beam quality M2 <1.1. By applying the MOPA setup to pump a ZGP crystal in a linear OPO cavity, a combined signal and idler mid-IR output power of 8.1 W is reached. For the highest mid-IR output power, a mid-IR conversion efficiency of 44 % and beam quality factors M2 of 2.2 and 2.0 are determined for signal and idler, respectively.

Efficiency Enhancing Technique for Rod Fiber Picosecond Amplifiers with Optimal Mode Field Matching

A high power and high quality picosecond laser is crucial in MEMS fabrication regarding micromachines. Optimal seed beam coupling is an important precondition to enhance laser efficiency. However, empirical coupling limits its development. In this paper, the physical parameters related to coupling are determined. The relationships among them are established under optical mode matching constraints to satisfy optimal seed beam coupling. According to a theoretical analysis, the focal length cut-off and the optimal coupling position of the coupling lens are acquired. A maximum transmittance of 87.2% is acquired with a 6 W input seed power in the validation experiment. In further power amplification experiments, a diffraction-limited beam quality is achieved, with M2X = 1.111, M2Y = 1.017, an optical efficiency of 60.5% and a slope efficiency of 66%, benefiting from the previous theoretical guidance.

Efficient high-power orthogonal dual-slab Yb:KGd(WO4)2 laser oscillator with a TEM00 mode.

We demonstrated a TEM00 mode orthogonal dual-slab Yb:KG(WO4)2(Yb:KGW) laser oscillator with high average power. Polarization anisotropy of thermal lenses was investigated and alleviating the astigmatism based on orthogonal dual-slab. In addition, the laser polarization was directly controlled by adjusting the net gain of the two crystals. The maximum output power was highly enhanced compared with single crystal due to effective thermal distribution. For an absorbed pump power of 52.4 W, this oscillator delivered an average power of 26.5 W, corresponding to an optical-to-optical conversion efficiency of 50.6%. Meanwhile, the ellipticity of the output laser was optimized to 0.940. Nearly diffraction-limited beam quality was measured to be M x2 = 1.19 and M y2=1.18.

Narrow-Linewidth Diffraction-Limited Tapered Er-Doped Fiber Amplifier with 2 mJ Pulse Energy

The possibility to scale-up output pulse energy in diffraction-limited Er-doped fiber amplifier has been studied. It is shown that the utilization of tapered fiber design allows one to increase the pulse energy up to 2 mJ, while keeping the diffraction-limited beam quality (M2~1.4). Factors limiting the further increase in pulse energy are revealed.

Yb-doped aluminophosphosilicate fiber amplifier with 244 pm linewidth and near diffraction limited beam quality for beam combination

Yb-doped aluminophosphosilicate fiber was fabricated by the combination system of chelate precursor doping technique and the modified chemical vapor deposition system. The technical details of the fabrication process were optimized to ensure the same elemental concentration ratio of Al/P. The capability of this fiber for the beam combining application was investigated based on a narrow linewidth fiber laser system. This fiber presented the output power of 1783 W with a slope efficiency of 83.7 % and a 20 dB root-mean-square linewidth of 0.244 nm with no nonlinear effects in the laser spectrum. Near-diffraction-limited beam quality was also observed with a M2 factor less than 1.27. In conclusion, this fiber presented an attractive laser performance for beam combination through the comprehensive optimization of fabrication technique and narrow linewidth laser system.

Monolithic Side-Pumped Amplifier Based on an Yb-Doped Tapered-Fiber and Yielding 0.53 MW 9.3 ps Pulses

We demonstrated a simple design of a monolithic all-fiber side-coupled combiner for counter-pumped amplifiers that requires no special fiber processing systems for fabrication. The combiner based on a Yb-doped polarization-maintained tapered fiber with an output core diameter of 40 µm and a total length of 1.8 m exhibiting over 60% coupling efficiency of 976 nm 0.10 NA pump power was demonstrated and utilized to amplify 1064 nm 9.3 ps 1.84 MHz pulses up to 9.1 W of average power and 0.53 MW of peak power with near diffraction-limited beam quality. The demonstrated approach seems promising for further power scaling, retaining good output beam characteristics via design optimization.

Intra-Cavity Tm:YAG-Ho:GdVO4 Laser with near Diffraction Limited Beam Quality

In this study, an Er:YAG laser pumped intra-cavity Tm:YAG-Ho:GdVO4 laser was built and debuted at room temperature. At an incident pump power of 9.2 W, this laser obtained a maximum output power of 1.6 W with a slope efficiency of 28.0%. Additionally, the M2 factors at the maximum output power were measured to be 1.06 and 1.03 in the x and y directions, respectively. The results showed that the intra-cavity pumping method of combining thulium and holmium crystals as the gain medium was an effective way to obtain a 2 μm laser with near diffraction limited beam quality.