Fiber-coupled Laser Diode Research Articles

Beam-shaping technique for fiber-coupled diode laser system by homogenizing the beam quality of two laser diode stacks

A beam shaping technique is presented to homogenize the beam quality of two laser diode stacks. We use polarization beam combiners to halve the beam sizes in the slow axis, and then rearrange the beams coming from two stacks to eliminate the lightless areas by using a quartz-plate stack in the fast axis. By this technique, two 12-bar laser diode stacks are coupled into an 800 µm/0.22 fiber. The experiment shows that the fiber-coupled system has an output power over 1400 W, and the fiber coupling efficiency of 87.5% is achieved, which are almost consistent with the Zemax simulation results.

Multi-watt amplification in a birefringent Yb:LiLuF4 single crystal fiber grown by micro-pulling-down

We present, to the best of our knowledge, the first demonstration of a single crystal fiber solid-state amplifier based on a birefringent Yb3+-doped thin crystal grown by the micro-pulling-down method. We measured a small signal gain >30 in a four-passes Yb:LiLuF4 amplifier pumping with a 120-W maximum power fiber-coupled laser diode. At an absorbed pump power of 80W, a maximum output power of 8.4W was obtained seeding the 42-mm-long, 2%Yb3+-doped single crystal fiber sample with 0.8W at 1021nm. Even at the maximum incident pump power, the amplified laser beam polarization and spatial quality were excellent (M2=1.15×1.06).

Towards a Low-Cost and Portable Photoacoustic Microscope for Point-of-Care and Wearable Applications.

Several breakthrough applications in biomedical imaging have been reported in the recent years using advanced photoacoustic microscopy imaging systems. While two photon and other optical microscopy systems have recently emerged in portable and wearable form, there is much less work reported on the portable and wearable photoacoustic microscopy (PAM) systems. Working towards this goal, we report our studies on a low-cost and portable photoacoustic microscopy system that uses a custom fabricated 2.5 mm diameter ring ultrasound transducer integrated with a fiber-coupled laser diode. The ultrasound transducer is centered at 17.25 MHz, and shows ~ 45% and ~ 100% fractional bandwidths for ultrasound pulse-echo and photoacoustic A-line signals respectively. To achieve overall system portability, besides the imaging head, other backend imaging system components need to be readily portable as well. In this direction, we have studied the potential use of compact pre-amplifiers, scanning stages and microcontroller based data acquisition and reconstruction for photoacoustic imaging. The portable PAM system is validated by imaging phantoms embedded with light absorbing targets. Future directions that will likely help achieve a completely portable and wearable photoacoustic microscopy system are discussed.

Diode-pumped Yb:KGW laser with 73 fs pulse and 0.72 MW peak power based on Kerr-lens mode locking

We have demonstrated a high-power Kerr-lens mode-locked Yb:KGW oscillator with an additional Kerr medium pumped by a high-brightness fiber-coupled diode laser. Operated at the repetition rate of 75.5 MHz, the laser delivers 73 fs pulse duration with average output power of 4.05 W, and the corresponding pulse energy is 53 nJ and the peak power is 0.72 MW, respectively. The output power is recorded to be stable with a fluctuation of less than 0.65% rms within 0.5 h. This work is promising for a variety of applications in nonlinear frequency conversion and high-energy amplifiers.

First attempt of a laser thruster space flight test: Lost at launch

Laser thrusters represent a new class of electric propulsion devices. To the best of our knowledge, Dr. Phipps’ μ-thruster prototype (2008) represents the highest technology readiness level to date. So LDU-7 presented in this paper is the world’s first laser thruster approved for a space flight. With carbonized PTFE as a fuel irradiated by a 10 W fiber-coupled diode laser, it was rated for 4 mN pulsed (5 ms to CW) thrust and 400 s specific impulse. Fitting within CubeSat 2U dimensions, such thruster is of great interest for small spacecraft due to its comparatively high specific impulse, imparted momentum low bit (1 μN * s) and wide dynamic range (up to 3 decades). LDU-7 was supposed to rotate hosting satellite ‘Baumanets-2’ within damping capability of its attitude control system. LDU-7 has been lost with all other payload due to ‘Fregat’ upper stage failure after launch by ‘Soyuz 2.1b’ from Vostochny on 28.11.2017. Here we present briefly the description of LDU-7 design and performance, details of its testing program, and planned experiment.

Orientation and separation controllable dual-vortex passively Q-switched microchip laser

Dual-vortex laser beams have been achieved in a decentered annular beam pumped Nd:YAG/Cr4+:YAG composite crystal passively Q-switched microchip laser (PQSML). The orientation and separation between two holes in dual-vortex lasers have been controlled by adjusting the offset of collimating lens with respect to the optical axis of the pump light from fiber-coupled laser diode. The offset of the collimating lens has nearly negligible effect on the performance of the decentered annular beam pumped dual-vortex Nd:YAG/Cr4+:YAG PQSML. The optical efficiency of 15.5% is achieved for dual-vortex lasers, which is comparable to that for doughnut-shaped vortex laser under normal incident pumping. Pulse width is 3.6 ns and peak power is over 5 kW for dual-vortex lasers with controllable orientation and separation between two holes. The short pulse high peak power dual-vortex lasers with controllable orientation and separation between two holes have potential applications in manipulating microparticles, data storage and quantum information processing.

Efficient 1 µm Laser Emission of Czochralski-Grown Nd:LGSB Single Crystal.

A 5.0-at.% Nd-doped La0.64Gd0.41Sc2.95(BO3)4 (Nd:LGSB) borate laser crystal was successfully grown by the Czochralski method, for the first time to our knowledge. The spectroscopic properties of the grown crystal are discussed and 1 µm laser emission, under end-pumping with a fiber-coupled diode laser at 807 nm, is reported. A c-cut Nd:LGSB medium yielded 1.35 W continuous-wave output power at 0.63 overall optical-to-optical efficiency, with respect to the absorbed pump power, together with the high 0.68 slope efficiency. With an a-cut Nd:LGSB sample, 0.81 W output power at 0.52 optical-to-optical efficiency was obtained. The laser emission performances under quasi-continuous wave pumping are presented as well, for both c-cut and a-cut crystals. Passive Q-switching was investigated with a semiconductor saturable absorber mirror (SESAM). Laser pulses with 2.2 µJ energy and 32.8 ns durations were recorded from a-cut Nd:LGSB. The average output power reached 0.36 W at 1.55 W absorbed pump power. Passive mode-locking with SESAM was achieved in a long Z-type resonator. Ultrashort pulses with 0.19 W average power, 1.63 nJ energy, and 1.43 ps pulse duration, at 118 MHz repetition rate, are demonstrated for the a-cut Nd:LGSB medium.

Development and Thermal Management of kW-Class High-Power Diode Laser Source Based on the Structure of Two-Stage Combination

In this paper, we present an optical structure for two-stage beam combination to realize a high-power fiber-coupled diode laser source. In the first stage, dense spectral combination based on reflecting volume Bragg gratings is implemented for combining five diode laser blocks with a spectral separation of 1.5 nm, to a high output power submodule. In the second stage, submodules are further coaxially multiplexed by polarization beam combination and coarse spectral combination to obtain a higher output power. In the process of the beam combination, thermal effects of combining elements are also investigated. By using a temperature control, the diode laser source can steadily produce 3120-W power from an optical fiber with 105-μm core diameter and 0.2 numerical aperture. This paper demonstrates the benefits of this combination structure and the effectiveness of the temperature control.

Diode-pumped Alexandrite laser for next generation satellite-based earth observation lidar

In this work, the design of a diode-pumped Alexandrite ring laser in Q-switched single-longitudinal-mode (SLM) operation for a spaceborne lidar mission is presented. The laser is pumped by a self-developed fiber-coupled laser diode pump device and yields a pulse energy of 1.7 mJ at a repetition rate of 500 Hz with an excellent beam quality of M2 < 1.1. By seeding the resonator with a narrow band diode laser, SLM operation with a linewidth of approximately 10 MHz is achieved. The electro-optical efficiency of 2% is the highest achieved for all Alexandrite lasers in SLM operation and reasonable for space operation. The performance analysis as well as benchmarking with the space-qualified mounting technology points out the TRL and the remaining effort for the development of the technology. An estimation of the requirements for a spaceborne resonance lidar mission underlines the suitability of such a lidar system with a diode-pumped Alexandrite laser as the beam source.

High-Power Continuous-Wave and Acousto-Optical Q-Switched Ho:(Sc0.5Y0.5)2SiO5 Laser Pumped by Laser Diode**Supported by the National Natural Science Foundation of China under Grant Nos 51572053, 61805209 and U1530152.

We experimentally investigate the continuous-wave (cw) and acousto-optical (AO) Q-switched performance of a diode-pumped Ho:(Sc0.5Y0.5)2SiO5 (Ho:SYSO) laser. A fiber-coupled laser diode at 1.91 μm is employed as the pump source. The cw Ho:SYSO laser produces 13.0 W output power at 2097.9 nm and 56.0% slope efficiency with respect to the absorbed pump power. In the AO Q-switched regime, at a pulse repetition frequency of 5 kHz, the Ho:SYSO laser yields 2.1 mJ pulse energy and 21 ns pulse width, resulting in a calculated peak power of 100 kW. In addition, at the maximum output level, the beam quality factor of the Q-switched Ho:SYSO laser is measured to be about 1.6.

Active Q-switching operation of slab Ho:SYSO laser wing-pumped by fiber coupled laser diodes.

In this paper, we demonstrate the continuous-wave and acousto-optical Q-switched performance of diode-pumped slab Ho:(Sc0.5Y0.5)2SiO5 (Ho:SYSO) laser at 2.1 µm for the first time. Two 1.91-µm laser diodes were used to pump the Ho:SYSO crystal. With a wing-pumping structure, at absorbed pump power of 44.7 W, the continuous wave slab Ho:SYSO laser produced 20.7 W maximum output power at 2097.9 nm, resulting in a slope efficiency of 53.1% with respect to the absorbed pump power. In the Q-switched regime, the slab Ho:SYSO laser produced up to 3.4 mJ pulse energy with 20 ns minimum pulse width at pulse repetition frequency of 5 kHz, corresponding to a peak power of 170 kW.

Low-Cost Interrogation System for Long-Period Fiber Gratings Applied to Remote Sensing.

We present a portable and low-cost system for interrogation of long-period fiber gratings (LPFGs) costing around a 30th of the price of a typical setup using an optical spectrum analyzer and a broadband light source. The unit is capable of performing real-time monitoring or as a stand-alone data-logger. The proposed technique uses three thermally modulated fiber-coupled laser diodes, sweeping a few nanometers around their central wavelength. The light signal is then modulated by the LPFG and its intensity is acquired by a single photo-detector. Through curve-fitting algorithms the sensor transmission spectrum is reconstructed. Testing and validation were accomplished by inducing variations in the spectral features of an LPFG through changes either in external air temperature from 22 to 425 °C or in refractive index (RI) of the surrounding medium from 1.3000 to 1.4240. A dynamic resolution between 3.5 and 1.9 °C was achieved, in temperatures from 125 to 325 °C. In RI measurements, maximum wavelength and optical power deviations of 2.75 nm and 2.86 dB, respectively, were obtained in the range from 1530 to 1570 nm. The worse RI resolution obtained was . The interrogation platform was then applied in the detection of iron corrosion, expressing wavelength peak values within 1.12 nm from the real value in the region between 1530 and 1570 nm.

Compact and sensitive mid-infrared all-fiber quartz-enhanced photoacoustic spectroscopy sensor for carbon monoxide detection

A compact and sensitive quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensor for carbon monoxide (CO) detection was demonstrated by using a mid-infrared all-fiber structure as well as a 3D-printed acoustic detection module. An all-fiber configuration has advantages of easier optical alignment, lower insertion loss, improvement in system stability, reduction in sensor size and lower cost. The 3D-printed acoustic detection module was introduced to match the mid-infrared all-fiber structure and further decrease the sensor volume, which resulted in a small size of 3.5 cm3 and a weight of 5 grams. A 2.33 μm distributed feedback fiber-coupled diode laser was used as the laser excitation source. A custom quartz tuning fork (QTF) with a small-gap of 200 μm was used as the acoustic wave transducer in order to improve the signal level of the QEPAS sensor. An acoustic micro resonator was utilized as the acoustic wave enhancer. The gas pressure and laser wavelength modulation depth were optimized, respectively. Water vapor was used to accelerate the vibrational-translational relaxation rate of the targeted CO molecule. Finally, a minimum detection limit (MDL) of 4.2 part per million (ppm) was achieved, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 7.4 × 10-9 cm-1W/√Hz. An Allan deviation analysis was used to evaluate the long-term stability of the reported CO-QEPAS sensor system. With an integration time of 150 s, the MDL was improved to be 1.3 ppm.

Fiber combiner terminated with quartz block head for direct beam combining of fiber coupled laser diodes

Based on the theoretical analysis and abundant experiments, a fiber combiner terminated with a quartz block head (QBH) with an output power of 3218 W and transmission efficiency higher than 98% is fabricated successfully by choosing a fiber with a larger core diameter of 600 μm and numerical aperture (NA) of 0.22 which matches the NA of the end cap as the output fiber of combiner. The proposed fiber combiner terminated with QBH can be used for fiber delivered direct beam combining of laser diodes with lower cost and higher wall-plug efficiency compared with fiber laser.

Ultrafast vortices generation at low pump power and shearing interferometer-based vortex topological detection

A single mode fiber coupled laser diode was employed to generate a low threshold femtosecond vortex by using an off-axis pumping technique and mode converter with a pair of cylindrical lenses. In this way, the pump threshold of a laser system can be decreased and the slope efficiency can be increased. Two lateral interferometric techniques for optical vortex detection were improved using the circular common-path (CCP) geometry configuration. In the CCP interferometer, the object optical vortex was divided into two beams that transmit oppositely along the interferometer and interfere with themselves in the beam splitter. The two beams traveled along the same path through the same optical elements. These new designs omit delicate adjustment of two interference arms, and can easily manipulate the optical path difference between the two beams. The results show that the experimental results are in good agreement with the numerical simulation. These CCP interferometers are insensitive to the environment and are suitable for the measurement of topological charge of a laser source with a short coherence length, including white lasers.

Photoacoustic spectroscopy telemetry of airport dangerous goods

A novel photoacoustic spectroscopy telemetry method of detecting trace acetylene gas using a quartz tuning fork is reported. A 1532-nm CW-DFB, fiber-coupled diode laser with power of 12 mW was used as the excitation source while an astronomical telescope was used to collect laser light scattered on the surface of the detected object at a distance of 6 m and the photoacoustic spectroscopy of acetylene was performed. The detection results are consistent with the standard near-infrared absorption coefficient of acetylene. This study provides a new direction for research in photoacoustic spectroscopy telemetry. Future work will quantitatively analyze acetylene gas.

Graphene and SESAM mode-locked Yb:CNGS lasers with self-frequency doubling properties.

We report on mode-locking of an Yb:Ca3NbGa3Si2O14 laser, which is pumped by a fiber-coupled single-mode laser diode. The shortest pulse duration obtained with a semiconductor saturable absorber mirror is 52 fs, with 75 mW of average output power. Sub-60 fs operation tunable between 1055 and 1074 nm is achieved by employing semiconductor absorbers with different characteristics. We also demonstrate passive mode-locking results with transmissive graphene saturable absorber, reaching an 85 fs pulse duration with 23 mW output power. Moreover, we present the non-phase-matched self-frequency doubling properties of this non-centrosymmetric crystal in the femtosecond regime.

基于SESAM被动调Q的激光二极管泵浦Yb∶CaYAlO4脉冲激光器

By utilizing a fiber-coupled laser diode emitting at a wavelength of around 976 nm as a pump source and a Yb:CaYAlO4 crystal with a Yb3+ ion doping concentration of 1.5at.% and a thickness of 2 mm as the gain medium, a scheme of laser diode-pumped Yb:CaYAlO4 for passively Q-switched pulsed lasers using a semiconductor saturable absorber mirror(SESAM) is proposed. By reasonably designing the resonant cavity, a stable passively Q-switched pulsed output is achieved. Furthermore, the influence of the pump power on the pulse repetition rate, pulse width, single pulse energy and peak power are analyzed.

Efficient third harmonic generation of 355 nm picosecond laser pulse

Picosecond laser with high-repetition-rate and high pulse energy is widely favorite in many scientific and industrial applications. Some nonlinear crystals can be used to efficiently convert a near-infrared laser into a green laser or an ultraviolet laser which has a higher photon energy and a smaller focal area. Especially for high-quality and high-speed transparent hard material fabrication, green or ultraviolet picosecond laser has been found to possess unique advantages. In this paper, the experiments on high-efficiency second-harmonic-generation (SHG) and third-harmonic-generation (THG) by using a home-made all-solid-state picosecond laser amplifier and an LBO crystal are reported. The picosecond laser amplifier consists of a seed source, a regenerative amplifier and a two-stage single-pass amplifier. The seed source is a commercial all-solid-state picosecond oscillator with a pulse duration of 8.3 ps and a repetition rate of 68 MHz. The repetition rate is reduced from 68 MHz to 500 kHz by an electro-optic Pockels cell (PC), and the period doubling bifurcation is minimized by reducing the duration of high voltage in PC. Both the regenerative amplifier and the two-stage single-pass amplifier are pumped by three 30-W continuous-wave fiber-coupled laser diodes. After the regenerative amplifier, the seed laser is amplified to 4.86 W with a repetition rate of 500 kHz at 1064 nm. Then the laser power is increased to 23.2 W by a two-stage single-pass amplifier, and the M2 value of the amplified laser in the X direction and in the Y direction are 1.330 and 1.235, respectively. The final pulse duration is 13.4 ps, which is slightly stretched in the amplification chain compared with the seed pulse duration (8.3 ps). For high-efficiency SHG and THG from near-infrared to green and ultraviolet, we carefully study the optical characteristics of some nonlinear crystals, such as LBO, BBO, BIBO, CLBO, etc., and we find that the LBO crystal, which has a high damage threshold, small walk-off and high nonlinear coefficient, is the best choice for both SHG and THG. Then the parameters of the two crystals for SHG and THG are specially designed according to the phase matching condition, the walk-off and the laser parameter. As a result, a 4-mm-long type-I phase matching LBO with cutting angle of &lt;i&gt;θ&lt;/i&gt; = 90° and &lt;i&gt;φ&lt;/i&gt; = 11.6° is used for SHG, and a 3-mm-long type-II phase matching LBO with cutting angle of &lt;i&gt;θ&lt;/i&gt; = 42.2° and &lt;i&gt;φ&lt;/i&gt; = 90° is used for THG. Finally, we realize high-efficiency frequency conversion with SHG power of 12.7 W at 532 nm and THG power of 9.25 W at 355 nm. The corresponding optical-optical conversion efficiencies reach 54.7% and 39.6%, respectively.

A solid-state passively Q-switched Tm,Gd:CaF2 laser with a Ti3C2Tx MXene absorber near 2 µm

Ti3C2Tx, a novel two-dimensional material in the MXene family, was successfully prepared and applied as a saturable absorber (SA) for solid-state pulsed lasers at 2 µm. Pumped by a 792 nm fiber-coupled laser diode, a compact passively Q-switched Tm,Gd:CaF2 laser with a Ti3C2Tx-SA was demonstrated and the pulse laser performances were presented and discussed in detail. This is the first presentation of Ti3C2Tx as a SA for solid-state pulsed laser generation near 2 µm.