Multimode Fiber Laser Research Articles

Humidity Sensing Using a Multimode Fiber Ring Laser with Thermal Compensation

We propose a multimode fiber laser sensor utilizing PI-SMF (polyimide-coated single mode fiber) for low-error relative humidity (RH) measurement, which is temperature compensated based on FBG. The PI-SMF in the laser cavity is used as a sensing element, and its length varies with humidity and temperature by volume-variation induced strain, which leads to frequency shift of the longitudinal mode beat frequency signal (BFS). When the 2000 MHz BFS is selected as the sensing signal, a RH sensitivity of −2.68 kHz/%RH and a temperature sensitivity of −14.05 kHz/°C are achieved. The peak shift of the FBG-based laser emission spectrum is only sensitive to temperature rather than RH with a temperature sensitivity of 9.95 pm/°C, which is used as the temperature compensation for RH measurements. By monitoring the response of the BFS and the laser wavelength, the cross-sensitivity effect of RH and temperature is overcome, and low-error RH measurement in the temperature range of 20 to 65 °C is realized with errors within ±0.67 %RH (25 to 85 %RH). The scheme does not require the design and production of complex structures and hygroscopic material coating processes, owning the advantages of simple structure, easy operation and high accuracy, and is expected to be practically applied in food safety and environmental monitoring.

Deep learning-based monitoring system for predicting top and bottom bead widths during the laser welding of aluminum alloy

Weld bead geometry visually represents the result of the welding process. However, ensuring a consistent weld bead is not always guaranteed due to the instability of the welding process. In this study, we present a deep learning-based monitoring system that predicts both the top and bottom bead widths simultaneously during the multi-mode fiber laser welding of aluminum alloy 1050P-H16. From the predicted top and bottom bead widths, rich information about the welding process can be obtained. Our deep learning model was constructed based on the VoVNet27-slim architecture, which was successfully trained using weld pool images obtained coaxially by a small optical camera. We were able to obtain very clean images of the aluminum weld pool, which provided plentiful information about the weld pool and the resulting top and bottom bead widths. We attempted to use one or two weld pool images as input to study how an additional weld pool image can enhance prediction accuracy. It was found that the top bead width was accurately predicted by both the one- and two-image models. However, the two-image model showed a clear improvement in the prediction of the bottom bead width because the bottom bead width fluctuates more widely and cannot be directly seen from the top-side weld pool image. The optimal separation distance between the two input images was found to be −0.1 mm, with which additional weld pool information about the past was supplied to the model and the denoising effect was achieved. Monitoring changes in both top and bottom bead widths provides rich information regarding the welding process, and we believe that the presented deep learning–based approach can serve as an effective monitoring tool.

Mode-Locked Operation of High-Order Transverse Modes in a Vertical-External-Cavity Surface-Emitting Laser.

Understanding the mechanism of mode-locking in a laser with high-order transverse mode is important for achieving an ultrashort pulses train under more complicated conditions. So far, mode-locking with high-order transverse mode has not been reported in other lasers except the multimode fiber laser. This paper demonstrates robust mode-locking with high-order transverse mode in a Kerr-lens mode-locked vertical-external-cavity surface-emitting laser for the first time, to the best of our knowledge. While the longitudinal modes are locked, continuous mode-locking accompanied by high-order transverse mode up to TEM40 is observed. The threshold of the mode-locking is only a little bigger than that of the lasing. After the laser oscillation is built up, the mode-locked pulse train can be obtained almost immediately and maintained until the thermal rollover of the laser. Output powers of 717 mW under fundamental mode and 666 mW under high-order transverse mode are achieved with a 4.3 ps pulse duration and 1.1 GHz pulses repetition rate, and some phenomenological explanations to the related characteristics of the mode-locked operation of high-order transverse mode in the vertical-external-cavity surface-emitting laser are proposed.

Spatiotemporal nonlinear dynamics in multimode fiber laser based on carbon nanotubes

Spatiotemporal nonlinear dynamics in multimode fiber laser based on carbon nanotubes

Cladding-pumped laser and amplifier for E- and S-bands based on multimode bismuth-doped graded-index fibers: toward "watt-level" output power.

In this Letter, we investigated the potential scalability of output power of a cladding-pumped laser and a power amplifier (booster) based on a multimode Bi-doped fiber (BDF) using the mode-selection approach. We fabricated the multimode double-clad graded-index (GRIN) fiber with a confined Bi-doped germanosilicate glass core with a diameter of ≈30 and ≈60 µm. Using femtosecond (fs) inscription technology with high spatial resolution, Bragg gratings of a special transverse structure allowing the selection of low-order modes were written into the core of BDFs. The operation features of the cladding-pumped multimode bismuth-doped GRIN fiber lasers with the inscribed Bragg gratings with various reflection coefficients were investigated. In addition, the behavior of the output power and the beam quality (M2 parameter) of the optical radiation of the developed devices was studied. The CW laser and booster operating at nearly 1.45 µm with maximum output powers of ≈0.8 and ≈1 W, respectively, based on the 60-µm-core BDF under pumping by multimode laser diodes at 808 nm were developed, which are, to the best of our knowledge, the most powerful cladding-pumped BDF devices to date. Near single-mode lasing (M2 <1.3) is demonstrated for a 30-µm-core fiber. The experimental data open new possibilities to achieve higher powers in cladding-pumped BDF sources, which are more cost-effective compared to core-pumped counterparts.

Optimally design of amplifier self-similar pulse in thulium-doped multimode fiber laser

High-energy ultrashort pulses fiber lasers could be widely used in scientific research and industrial applications. Researchers often use two methods to boost the pulse energy, one is to achieve self-similar pulse (SSP) output in a single mode fiber (SMF) laser, and the other is to use multimode fiber (MMF) instead of SMF. Here we combine the above two methods to optimally design the output characteristics of SSP in thulium-doped MMF lasers. The influence of different parameters, including spectral filter bandwidth, fiber length, and gain saturation energy, on the output characteristics of SSP have been analyzed, and an optimal SSP with pulse energy of 63.48 nJ, spectrum width of 52.53 nm, dechirped pulse peak power of 283.34 kW, and dechirped pulse width of 219.74 fs has been obtained.

Wavelength-tunable quasi-unidirectional spatiotemporal mode-locked solitons in an isolator-free Er-doped fiber laser

In this letter, we have demonstrated the realization of the wavelength-tunable spatiotemporal mode-locked (STML) operation in an Er-doped partial multimode fiber laser without the intracavity isolator. The laser adopts an all-fiber structure and the saturable absorption effect is attributed to nonlinear multimode interference (NL-MMI) from the single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) design. The STML laser can operate in the quasi-unidirectional regime with an emission strength difference of larger than 30 dB for the two counter-propagating lasers. And the switching between the dominating operating directions in the clockwise (CW) or counter-clockwise (CCW) directions can be easily achieved by adjusting the MM polarization controller (PC). Both STML conventional solitons (CSs) and the STML soliton molecules are obtained with a similar tunable wavelength from ∼1.56 μm to ∼1.58 μm for the opposite operating directions. Furthermore, the spatial dynamics of the STML soliton are also investigated in detail. The experimental results are of great help for us to explore nonlinear spatiotemporal dynamics and provide potential applications in fields of all-optical signal processing, fiber sensing and information coding.

Periodically tunable multimode soliton pulsation in a spatiotemporal mode-locked fiber laser.

Multimode fiber lasers have become a new platform for investigating nonlinear phenomena since the report on spatiotemporal mode-locking. In this work, the multimode soliton pulsation with a tunable period is achieved in a spatiotemporal mode-locked fiber laser. It demonstrates that the pulsation period drops while increasing the pump power. Moreover, it is found that different transverse modes have the same pulsation period, asynchronous pulsation evolution and different dynamical characteristics through the spatial sampling technique and the dispersive Fourier transform technique. To further verify the experimental results, we numerically investigate the influences of the gain and the loss on the pulsation properties. It is found that within a certain parameter range, the pulsation period drops and rises linearly with the increase of the gain and the loss, respectively. The obtained results contribute to understanding the formation and regulating of soliton pulsations in fiber lasers.

Transverse mode distribution in multimode diode-pumped Raman fiber laser

Raman lasers based on multimode graded-index fibers may generate high-quality (M2∼2) Stokes beams when pumped by highly multimode (M2&gt;30) laser diodes. Here we, examine, both experimentally and theoretically, the energy distribution of the output Stokes beam across the principal quantum mode number n in a bent multimode fiber operating well above the Raman threshold. In contrast to Kerr spatial beam cleaning, leading to a Rayleigh–Jeans mode power distribution, in a multimode Raman fiber laser, we find that the output mode powers approach an exponential distribution. We introduce a coupled-mode equations model, including random linear coupling between neighboring mode groups, and obtain a good agreement between numerical simulations and experimental results. The model shows that, for typical mode coupling coefficients, the randomization of the mode power distribution is compensated by both nonlinear (Raman and Kerr) effects and linear filtering from the fs-inscribed fiber Bragg grating, both acting on the Stokes beam over successive round trips. When random coupling becomes the dominating factor, the mode power distribution of the Stokes beam tends to equipartition, similar to what is observed with large-size highly multimode beams of low intensity (and nonlinearity) in the absence of any filtering.

Deep-learning model for predicting hardness and phase distributions from two cross-sectional temperature distribution images in laser heat treatment of AH36 steel

Laser heat treatment of carbon steel is generally performed to increase the hardness of the specimen. However, when the heating temperature is high, softening of the specimen can occur along with melting. It is important to predict both hardening and softening processes, but such research has been limited so far. In this study, a deep learning model was developed that predicts both hardening and softening during laser heat treatment of AH36 steel using two cross-sectional temperature distributions obtained at 0.5-s intervals as inputs. Two temperature distributions were used as inputs to provide accurate information about cooling rate to the model. The input temperature distribution of the cross-section was calculated by solving the heat conduction equation using finite element method, and the hardness and phase distributions used as ground truth were obtained through a 2 kW multi-mode fiber laser experiment. The model was constructed based on a generative adversarial network with a residual network. It was found that when two temperature distributions were used, both softened and hardened areas were predicted accurately. However, when only one temperature distribution was used, only hardening could be predicted. On the other hand, both models could predict phase distributions, but much more accurate results were obtained when two temperature distributions were used as inputs.

Noise-like pulses generation and its beam self-cleaning effect in an all-fiber multimode cavity with a figure-8 scheme

The beam profiles of multimode mode-locked fiber lasers are highly multimode, the speckled beam profiles limit their potential application. To improve the beam profiles of multimode mode-locked fiber lasers, we proposed an all-fiber figure-8 multimode cavity and experimentally demonstrated the generation of noise-like pulses (NLPs) and beam self-cleaning of NLPs in the cavity at 1.5 µm. The laser was mode locked by a nonlinear amplifying loop mirror, an Er/Yb co-doped fiber provided gain, and 50 m OM4 fiber introduced enough nonlinear phase shift difference. The scheme generated NLPs with 159 mW average power, 2.78 MHz repetition rate and 57.19 nJ pulse energy. The packet duration of the NLPs increased from 0.61 ns to 2.99 ns when the pump power was increased from 2 W to 9 W. Nonlinear beam self-cleaning with a near Gaussian beam profile output was achieved in the multimode fiber laser. The M2 was measured to be 2.41 at the pulse energy of 6.83 nJ. We also obtained spatiotemporal mode-locked pulses with 1.03 ps duration and speckled beam profiles. This is the first experimental demonstration of NLPs in multimode fiber lasers, and beam self-cleaning of NLPs was observed for the first time. The results enriched nonlinear dynamics in multimode fiber lasers.

On the Use of Metal Sinter Powder in Laser Powder Bed Fusion Processing (PBF-LB/M).

Metal Laser Powder Bed Fusion (PBF-LB/M Powder Bed Fusion, Laser-Based/Metal) offers decisive advantages over conventional manufacturing processes. Complex geometries can be produced that cannot, or only to a limited extent, be manufactured with conventional manufacturing processes. One of the main disadvantages of the process are high investment and operating costs. In order to make the PBF-LB/M process accessible to new research areas, the costs need to be reduced. Therefore, this work investigates whether laser beam sources and motion systems in currently established PBF-LB/M systems can be replaced by more cost-effective components. To reduce the operating costs for PBF-LB/M, the studies are carried out based on previous work with water-atomized, process-foreign sinter powder instead of gas-atomized, spherical PBF-LB/M powders. A cost-efficient, low-alloyed powder is selected (Höganäs HP1) and processed on two different PBF-LB/M machines with a restricted process window using process parameter values that current low-cost machines can achieve. The results show that a multimode fiber laser leads to a more stable process and wider melt pools compared to a single mode fiber laser. In addition, a lower sensitivity of the process with respect to modified process parameters is observed for the multimode laser, resulting in a wider range of stable process windows. A Cartesian motion system (gantry) is suitable for use in PBF-LB/M despite lower scan speeds compared to galvanometer scanners. Beam guidance in the XY-plane offers new possibilities for machine and process design that are not possible with usual scanner systems.

Spatiotemporal dissipative soliton resonances in multimode fiber lasers

Spatiotemporal mode-locking in multimode fiber lasers is intriguing for the complex nonlinear dynamics and the increase of theoretical energy limit. In this paper, we enrich spatiotemporal mode-locking with dissipative soliton resonances, a kind of solitons which is characterized by large pulse energy in single mode fiber lasers, and demonstrate their emergence in multimode fiber lasers by employing the reverse saturable absorption effect from real saturable absorbers. The spatiotemporal dissipative soliton resonances are expected to raise the energy limit further by the locking of dissipative soliton resonances in different transverse modes, whose energy is about twice the maximum single-mode energy in our results. Moreover, properties of spatiotemporal dissipative soliton resonances are investigated by tailoring parameters of the multimode fiber laser, where evolution and transformation of the proposed pulses including shaping and broadening are disclosed. The spatiotemporal dissipative soliton resonances in multimode fiber lasers may open up new avenue for high-power and spatiotemporally engineered coherent light fields in laser dynamics and nonlinear optics.

Dissipative soliton resonance in a figure-eight multimode fiber laser.

We report, for the first time to the best of our knowledge, a spatiotemporal mode-locked (STML) multimode fiber laser based on nonlinear amplifying loop mirror (NALM), generating dissipative soliton resonance (DSR) pulses. Due to the complex filtering characteristics caused by the inherent multimode interference filtering structure and NALM in the cavity, the STML DSR pulse has wavelength tunable function. What's more, kinds of DSR pulses are also achieved, including multiple DSR pulses, and the period doubling bifurcations of single DSR pulse and multiple DSR pulses. These results contribute to further understand the nonlinear properties of STML lasers and may shed some light on improving the performance of the multimode fiber lasers.

Spatiotemporal pulses from a Q-switched multimode fiber laser

We have demonstrated spatiotemporal pulses from a Q-switched ytterbium-doped multimode fiber laser, with a pulse energy of up to 16.5 μJ, a pulse repetition rate of 108.7 kHz, and an average output power of 1.80 W. Benefiting from the weakened spectral filtering effect, there is high pulse energy that can be produced from multimode fiber oscillators. We have observed and analyzed the evolution of multimode Q-switching with spatiotemporal correlation. Furthermore, we find that the polarization control in the saturable absorber has a modulation and selective effect on the transverse modes of the multimode fiber laser. Our results help to understand the spatiotemporal characteristics of pulsed fiber lasers and contribute to developing high-pulse energy lasers.

All-fiber low threshold tunable multimode Q-switched mode locked fiber laser using a few mode Er-doped fiber

We report an all-fiber low threshold (70 mW) multimode Q-switched mode-locked (QML) fiber laser at 1.5 m. Nonlinear polarization rotation and a few mode Er-doped fiber (FM-EDF) were both adopted to achieve a low threshold QML operation initially, and the later experimental results showed and verified that the FM-EDF was the key factor of the QML pulses generation. A multimode QML state at 70 mW pump power was obtained with an 18 μs Q-switched pulse width and a 7.41 kHz repetition rate. The repetition rate and pulse width of the mode-locked pulses within the Q-switched envelope were 16.9 MHz and ∼910 ps respectively. The multimode QML fiber laser can be turned from 1561.8 nm to 1575.1 nm owing to the multimode interference and the fiber birefringence filtering effect. The QML pulses with dual-wavelength and three-wavelength were also observed by manipulating the polarization controllers and increasing the pump power. The experimental results showed that multimode QML pulses can be achieved with a short FM-EDF at a lower pump power in an all-fiber multimode cavity. The generated low threshold and tunable QML multimode fiber laser has potential application in measuring and mode-division multiplexed system.

All-step-index-fiber spatiotemporally mode-locked laser

Spatiotemporal mode-locking (STML) in multi-mode fiber (MMF) lasers has extended the concept of temporal dissipative solitons into spatiotemporal dissipative solitons. To date, all reported STML in MMF lasers has used graded-index (GRIN) MMFs either solely or hybridly with other fibers. Compared to GRIN MMFs, step-index (STIN) MMFs have much larger intermode dispersion on both group and phase velocities. Building all-STIN MMF lasers can provide a new platform to explore the spatiotemporal dissipative soliton dynamics. Here, we report experimental and numerical observation of STML in an all-STIN MMF laser. Distinct from GRIN MMF lasers, the large intermode dispersion in the all-STIN MMF laser cannot be balanced by Kerr nonlinearity, and significant walk-off between mode-resolved pulses was observed experimentally. Simulations suggest that this walk-off is counteracted by spatial coupling in the laser, and a mother–child coupling mechanism is proposed to understand it. This mother–child coupling can enable STML with a single repetition rate with infinitely large intermode dispersion. Our work enriches MMF laser architectures for STML in a parameter regime that has not been considered, to our knowledge.

Multimode dissipative-soliton-resonance pulses in a Yb-doped fiber laser

Experimental observation of mode-locked dissipative-soliton-resonance (DSR) pulses, including single-mode DSR pulses and multimode DSR pulses, in a Yb-doped fiber laser is reported. DSR pulses in single-mode state and multi-mode state can exist at the same time and can be output stably. The multi-mode DSR pulses exhibit same linearly broadening characteristics and bell-shaped spectral shape as single-mode DSR pulses. The results would be beneficial for expanding the knowledge of nonlinear dynamics to multimode fiber lasers and provide new insights for DSR pulse output with a fully multimode structure to get larger pulse energy.

High speed melt flow monitoring and development of an image processing algorithm for laser fusion cutting

This contribution presents high-speed camera monitoring of melt pool dynamics for steel during laser fusion cutting and compares the images with recordings in aluminum. The experiments are performed by a 4 kW multimode fiber laser with an emission wavelength of 1070 nm. To visualize the thermal radiation from the process zone during the cutting process, the kerf is captured at sample rates of up to 170 000 frames per second without external illumination with a spectral response between 400 and 700 nm, allowing measurements of the melt flow dynamics from geometric image features. The dependencies of the melt flow dynamics on laser processing parameters, such as feed rate, gas pressure, and laser power, can be evaluated. The monitoring system is placed both off-axis and mounted to a conventional cutting head, with the monitoring path aligned to the processing laser for a coaxial and lateral view of the cut kerf. The measured signal characteristics of the images captured from the melt pool are examined in the visible spectral range of the emitted thermal radiation from the process zone. Moreover, a specifically developed image processing algorithm is developed that process and analyze the captured images and extract geometric information for a measurement of the melt flow.

Tunable spatiotemporal mode-locked conventional soliton and soliton molecule in a partial multimode linear-cavity Er-doped fiber laser

In this letter, we have demonstrated the realization of wavelength-tunable spatiotemporal mode locking (STML) in a partial multimode fiber (MMF) laser with a linear cavity. The saturable absorption effect is attributed to both the nonlinear multimode interference (NL-MMI) and the nonlinear optical loop mirror (NOLM). The laser can deliver both the conventional solitons (CSs) and loosely bound solitons (BSs) with a tuning range from ∼1573 nm to ∼1601 nm. The evolutions of these two different STML solitons as the pump power are also given and it is found that the interaction between solitons would have notable affection on the beam distribution of the MM solitons. Furthermore, a spatial instability for the STML CSs is observed within a certain pump power range. These results contribute to our understanding of the nonlinear spatiotemporal dynamics in the MMF laser system.