Fiber-coupled Diode Research Articles

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.

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.

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.

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.

Passive Q-Switching by Cr4+:YAG Saturable Absorber of Buried Depressed-Cladding Waveguides Obtained in Nd-Doped Media by Femtosecond Laser Beam Writing.

We report on laser performances obtained in Q-switch mode operation from buried depressed-cladding waveguides of circular shape (100 μm diameter) that were inscribed in Nd:YAG and Nd:YVO4 media by direct writing with a femtosecond laser beam. The Q-switch operation was realized with a Cr4+:YAG saturable absorber, aiming to obtain laser pulses of moderate (few μJ) energy at high (tens to hundreds kHz) repetition rate. An average power of 0.52 W at 1.06 μm consisting of a train of pulses of 7.79 μJ energy at 67 kHz repetition rate, was obtained from a waveguide realized in a 4.8 mm long, 1.1-at % Nd:YAG ceramics; the pulse peak power reached 1.95 kW. A similar waveguide that was inscribed in a 3.4 mm long, 1.0-at % Nd:YVO4 crystal yielded laser pulses with 9.4 μJ energy at 83 kHz repetition rate (at 0.77 W average power) and 1.36 kW peak power. The laser performances obtained in continuous-wave operation were discussed for each waveguide used in the experiments. Thus, a continuous-wave output power of 1.45 W was obtained from the circular buried depressed-cladding waveguide inscribed in the 1.1-at %, 4.8 mm long Nd:YAG; the overall optical-to-optical efficiency, with respect to the absorbed pump power, was 0.21. The waveguide inscribed in the 1.0-at %, 3.4 mm long Nd:YVO4 crystal yielded 1.85 W power at 0.26 overall optical efficiency. This work shows the possibility to build compact laser systems with average-to-high peak power pulses based on waveguides realized by a femtosecond (fs) laser beam direct writing technique and that are pumped by a fiber-coupled diode laser.

Design of a 36-W fiber-coupled green laser diode by Zemax

Abstract We propose a design of a 36 W fiber-coupled green diode laser based on TO-Can packaged emitters using Zemax. This is achieved by beam collimation, spatial beam combining, polarized beam combining and fiber coupling. The simulation results show that the combined beam can be coupled into a fiber with a core diameter of 200 μm and a numerical aperture (NA) of 0.22 by using 40 of 1-W TO-Can emitters at 516 nm, and a fiber combining efficiency of 92% can be achieved.

KW-class fiber-coupled diode laser source based on dense spectral multiplexing of an ultra-narrow channel spacing.

A dense spectral multiplexing structure based on the reflective volume Bragg gratings is introduced to realize the diode laser multiplexing with an ultra-narrow channel spacing of 1.5 nm. With the combination of polarization multiplexing and coarse spectral multiplexing, a diode laser source producing 2045 W power from an output fiber with a core diameter of 105 µm and NA of 0.2 is achieved at an injection current of 10 A. The electro-optical and optical-optical efficiency of the laser source is 42% and 76%, respectively. Experimental results demonstrate the ability of dense spectral multiplexing based on VBG to realize high output power and brightness.

Local laser softening of press-hardened steel at high feed rates

The development of hot stamped manganese–boron steels established new potentials for lightweight constructions for applications in the automotive industry. This type of high strength steel allows the usage of thin sheet thicknesses resulting in low specific weight retaining the optimized performance of passive safety. The parts have a high tensile strength of about 1500 MPa. However, this high strength is not favorable in all sections of a part, especially those which are relevant for safety or for joining. To ensure the requirements for the crash properties or the joining areas, a high ductility is required in these areas. A way to overcome this problem is a local softening of the crash and joining zones via laser heat treatment. By means of laser radiation, the brittle martensitic microstructure is either tempered or transformed into a ferrite/perlite dominated microstructure. A fiber-coupled diode laser with a maximum laser power of 12 kW and rectangular laser spot is used for the laser heat treatment. The temperature is controlled by a monochromic pyrometer integrated into the optical system. Generally, the necessary process feed rate of the heat treatment process is low (about 1000 mm/min), as a homogeneous result of the heat treatment is pursued. As the feed rates increase (up to 8000 mm/min), the temperature distribution across the thickness of the material becomes more inhomogeneous. With the topside being hotter than the backside, the material is tempered with a varying intensity resulting in a hardness distribution across the thickness. The influencing parameters for this effect are being characterized, and the mechanical properties of the anisotropic material are investigated.

Synchronization of pairs of nanosecond pulses from a laser with two gain crystals pumped with two different sources.

We report a laser that emits two Q-switched pulses, one at 1.047μm and the other at 1.064μm, generated by a Nd:YLF and a Nd:YVO4, respectively. The crystals are pumped by two fiber-coupled diode lasers (808nm and 880nm); the delay between the pulses can be controlled by adjusting the power of the pumps. Two kinds of Q-switching techniques are reported, passive (Cr:YAG saturable absorber) and active (electro-optic modulator). We model both the active and passive Q switching and make a comparison between numerical simulations and experiments. We show experimentally and theoretically that in both cases the pulses can be synchronized; however, the stability of the synchronization (sensitivity to pump power fluctuations) is better for active than for passive Q switching. We also report that under certain experimental conditions a third wavelength is obtained, 1156nm, which corresponds to the first Stokes shift of the 1047nm pulse produced by stimulated Raman scattering from the Nd:YVO4 crystal.

12-W continuous-wave green output from a 200-μm fiber-coupled diode laser based on TO-Can packaged emitters.

A 12-W green laser diode module with fiber-coupled output is developed. The high-power fiber-coupled output of a green diode laser is realized by using the technologies of beam collimation, spatial beam combining, beam shaping, and fiber coupling based on TO-Can packaged emitters. According to the optical fiber parameters, the detailed beam combination method is described and the maximum quantity of a single emitter in beam combination is calculated theoretically. In the experiment, as many as 16 green laser diode emitters with output power of 1W each were combined and coupled into a fiber with core diameter of 200μm and a numerical aperture of 0.22. A total of 12.2W continuous-wave output at the wavelength of 520nm was achieved with a coupling efficiency of 86.5% and an electro-optical efficiency of 10.6%.

High-power diode-pumped Kerr-lens mode-locked bulk Yb:KGW laser.

A high-power (>1 W) pure Kerr-lens mode-locked bulk Yb:KGW laser with multimode fiber-coupled diode pumping was demonstrated. The laser delivered 240fs pulses with 2.3W of average output power at 86.8MHz, corresponding to 97kW of peak power and 26nJ of pulse energy. The shortest generated pulse duration was 120fs with 1.2W of output power. The self-starting regime was also observed with a background continuous-wave component. The laser exhibited more than an order of magnitude higher output power when compared to the previously reported bulk Yb-ion-doped lasers with multimode laser diode pumping systems.

Fast Desktop-Scale Extrusion Additive Manufacturing

Significant improvements to the throughput of additive manufacturing (AM) processes are essential to their cost-effectiveness and competitiveness with traditional processing routes. Moreover, high-throughput AM processes, in combination with the geometric versatility of AM, will enable entirely new workflows for product design and customization. We present the design and validation of a desktop-scale extrusion AM system that achieves a much greater build rate than benchmarked commercial systems. This system, which we call ‘FastFFF’, is motivated by our recent analysis of the rate-limiting mechanisms to conventional fused filament fabrication (FFF) technology. The FastFFF system mutually overcomes these limits, using a nut-feed extruder, laser-heated polymer liquefier, and servo-driven parallel gantry system to achieve high extrusion force, rapid filament heating, and fast gantry motion, respectively. The extrusion and heating mechanisms are contained in a compact printhead that receives a threaded filament and augments conduction heat transfer with a fiber-coupled diode laser. The prototype system achieves a volumetric build rate of 127cm3/hr, which is approximately 7-fold greater than commercial desktop FFF systems, at comparable resolution; the maximum extrusion rate of the printhead is ∼14-fold greater (282cm3/hr) than our benchmarks. The performance limits of the printhead and motion systems are characterized, and the tradeoffs between build rate and resolution are assessed and discussed. High-speed desktop AM raises the possibility of new use cases and business models for AM, where handheld parts are built in minutes rather than hours. Adaptation of this technology to print high-temperature thermoplastics and composite materials, which require high extrusion forces, is also of interest.

Phased laser diode array permits selective excitation of ultrasonic guided waves in coated bone-mimicking tubes

This paper validates simulation predictions, which state that specific modes could be enhanced in quantitative ultrasonic bone testing. Tunable selection of ultrasonic guided wave excitation is useful in non-destructive testing since it permits the mediation of energy into diagnostically useful modes while reducing the energy mediated into disturbing contributions. For instance, it is often challenging to distinguish and extract the useful modes from ultrasound signals measured in bone covered by a soft tissue. We show that a laser diode array can selectively excite ultrasound in bone mimicking phantoms. A fiber-coupled diode array (4 elements) illuminated two solid tubes (2–3 mm wall thickness) embraced by an opaque soft-tissue mimicking elastomer coating (5 mm thick). A predetermined time delay matching the selected mode and frequency was employed between the outputs of the elements. The generated ultrasound was detected by a 215 kHz piezo receiver. Our results suggest that this array reduces the disturbances caused by the elastomer cover and so pave way to permit non-contacting in vivo guided wave ultrasound assessment of human bones. The implementation is small, inexpensive, and robust in comparison with the conventional pulsed lasers.

Design of 20 W fiber-coupled green laser diode by Zemax**Project supported by the National Key R& D Program of China (No. 2016YFB0402105), the Key Deployment Program of the Chinese Academy of Sciences (No. KGZD-SW-T01-2), and the National Natural Science Foundation of China (No. 61404135).

We represent a design of a 20 W, fiber-coupled diode laser module based on 26 single emitters at 520 nm. The module can produce more than 20 W output power from a standard fiber with core diameter of 400 μm and numerical aperture (NA) of 0.22. To achieve a 20 W laser beam, the spatial beam combination and polarization beam combination by polarization beam splitter are used to combine output of 26 single emitters into a single beam, and then an aspheric lens is used to couple the combined beam into an optical fiber. The simulation shows that the total coupling efficiency is more than 95%.

Passive Q-switch laser operation of circular, buried depressed-cladding waveguides realized by direct fs-laser beam writing in Nd:YAG/Cr^4+:YAG composite media

Circular, buried depressed-cladding waveguides were inscribed by direct fs-laser beam writing in diffusion-bonded Nd:YAG/Cr4+:YAG composite media. Passive Q-switch operation was achieved using the pump at 807 nm with a fiber-coupled diode laser. Laser pulses at 1.06 μm with energy of 15.7 μJ and 3.9-ns duration (corresponding to a peak power of 4.0 kW) were obtained from a waveguide of 150-μm diameter that was realized in a 10.3-mm long, 1.0-at.% Nd:YAG/Cr4+:YAG composite medium. The length of Nd:YAG crystal was 7.0 mm and initial transmission of Cr4+:YAG saturable absorber crystal was 0.70. The average output power reached 1.13 W.

Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography.

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering thermal wave fields that are disturbed by the defect. This effect is measured and used to locate the defect. We form the destructively interfering wave fields by using a modified projector. The original light engine of the projector is replaced with a fiber-coupled high-power diode laser. Its beam is shaped and aligned to the projector's spatial light modulator and optimized for optimal optical throughput and homogeneous projection by first characterizing the beam profile, and, second, correcting it mechanically and numerically. A high-performance infrared (IR) camera is set up according to the tight geometrical situation (including corrections of the geometrical image distortions) and the requirement to detect weak temperature oscillations at the sample surface. Data acquisition can be performed once a synchronization between the individual thermal wave field sources, the scanning stage, and the IR camera is established by using a dedicated experimental setup which needs to be tuned to the specific material being investigated. During data post-processing, the relevant information on the presence of a defect below the surface of the sample is extracted. It is retrieved from the oscillating part of the acquired thermal radiation coming from the so-called depletion line of the sample surface. The exact location of the defect is deduced from the analysis of the spatial-temporal shape of these oscillations in a final step. The method is reference-free and very sensitive to changes within the thermal wave field. So far, the method has been tested with steel samples but is applicable to different materials as well, in particular to temperature sensitive materials.

Measurement of CO2 concentration at high-temperature based on tunable diode laser absorption spectroscopy

A diode laser sensor based on absorption spectroscopy has been developed for sensitive measurement of CO2 concentration at high-temperature. Measurement of CO2 can provide information about the extent of combustion and mix in a combustor that may be used to improve fuel efficiency. Most methods of in-situ combustion measurement of CO2 use the spectroscopic parameters taken from database like HITEMP which is mainly derived from the theoretical calculation and remains a high degree of uncertainty in the spectroscopic parameters. A fiber-coupled diode laser system for measurement of CO2 in combustion environment by use of the high-temperature spectroscopic parameters which are obtained by experiment was proposed. Survey spectra of the R(50) line of CO2 at 5007.787cm−1 were recorded at high-temperature and various pressures to determine line intensities. The line intensities form the theoretical foundation for future applications of this diode laser sensor system. Survey spectra of four test gas mixtures containing 5.01%CO2, 10.01%CO2, 20.08%CO2, and 49.82%CO2 were measured to verify the accuracy of the diode laser sensor system. The measured results indicate that this sensor can measure CO2 concentration with 2% uncertainty in high temperatures.

Beam shaping for kilowatt fiber-coupled diode lasers by using one-step beam cutting-rotating of prisms.

The beam quality mismatch of laser diode stacks in both axes limits many direct applications for fiber or solid laser pumping and material processing. In this paper, a one-step cutting-rotating beam shaping system has been designed to homogenize the beam quality of two polarization-multiplexing laser diode stacks. Coupling laser diode stacks consisting of eight bars into a standard fiber with a core diameter of 600 μm and an NA of 0.22 is achieved. The simulative result shows that the system will have an output power over 1056 W. By using the technique, the production of compact and high brightness fiber-coupling diode lasers can be directly used for laser cladding and laser surface hardening processes.

Tunable single- and dual-wavelength SHG from diode-pumped PPKTP waveguides.

A compact, all-room-temperature, widely tunable, continuous wave laser source in the green spectral region (502.1-544.2 nm) with a maximum output power of 14.7 mW is demonstrated. This was made possible by utilizing second-harmonic generation (SHG) in a periodically poled potassium titanyl phosphate (PPKTP) crystal waveguide pumped by a quantum-well external-cavity fiber-coupled diode laser and exploiting the multimode-matching approach in nonlinear crystal waveguides. The dual-wavelength SHG in the wavelength region between 505.4 and 537.7 nm (with a wavelength difference ranging from 1.8 to 32.3 nm) and sum-frequency generation in a PPKTP waveguide is also demonstrated.