Multimode Laser Research Articles

Wavelength-Dependent Kerr Beam Self-Cleaning in Spatiotemporal Mode-Locked Multimode Fiber Laser

We report a spatiotemporal mode-locked multimode fiber laser, which relies on spatial beam self-cleaning via the nonlinear Kerr effect to attain high pulse energy with near Gaussian output beam shape. Single and multiple pulses with switchable central wavelengths have been obtained. Moreover, it is found that the beam self-cleaning behavior observed at different wavelengths has distinct threshold. The experimental results are valuable to the dynamic characteristics of spatiotemporal mode-locked multimode fiber laser, making them much more accessible for applications.

From Single to Multi Mode Lasing: The role of materials revealed in optical simulations

ABSTRACT The comparative study of thin film Cholesteric Liquid Crystal (CLC) lasers made from different materials and optically pumped by an external solid state laser reveals a striking dependence of lasing behaviour (ranging from single mode at the edge of the selective reflection band to multimode lasing) on the morphology and microstructure of CLC films. The materials studied belong to two groups: low molar mass liquid crystals and polymers. It is shown that the orientation of individual chiral domains and fluctuations in helical pitch contribute significantly to the type of lasing displayed by the material. Different ways of preparing CLC cells that lead to predominantly one type of lasing are discussed. The importance of variations of the helical pitch and domain orientation in producing single and multimode lasing is justified by optical simulations (4x4 matrix method) of lasing in multilayered samples.

Multi-mode lasing in terahertz metasurface quantum-cascade VECSELs

To date, terahertz quantum-cascade vertical-external-cavity surface-emitting lasers (QC-VECSELs) have tended to oscillate in only one or two lasing modes at a time. This is due to the fact that the interaction between all of the longitudinal external cavity modes and the QC gain material is mediated through a single metasurface resonance, whose spatial overlap changes little with frequency; this suppresses spatial-hole-burning induced multi-mode operation. In this Letter, a VECSEL external cavity is demonstrated using an output coupler based upon a high-resistivity silicon etalon, which presents a periodic reflectance spectrum that is nearly matched with the external cavity mode spectrum. As the cavity length is varied, a systematic transition between a single/double-mode lasing regime and a multi-mode lasing regime is realized due to the Vernier effect. Up to nine modes lasing simultaneously with a free-spectral-range of approximately 21 GHz is demonstrated. This result provides a path toward the multi-mode operation necessary for eventual frequency comb operation.

Buildup dynamics of multiple solitons in spatiotemporal mode-locked fiber lasers

Spatiotemporal mode locking is a nonlinear process of multimode soliton self-organization. Here the real-time buildup dynamics of the multiple solitons in a spatiotemporal mode-locked multimode fiber laser are investigated, assisted by the time-stretch technique. We find that the buildup processes are transverse mode dependent, especially during the stages of relaxation oscillation and Q -switching prior to multiple soliton formation. Furthermore, we observe that the transverse modal composition of these generated pulses among the multiple solitons can be different from each other, indicating the spatiotemporal structure of the multiple soliton. A simplified theoretical model based on pulse energy evolution is put forward to interpret the role of 3D saturable absorber on spatiotemporal structures of spatiotemporal mode-locking multiple solitons. Our work has presented the spatiotemporal nonlinear dynamics in multimode fiber lasers, which are novel to those inside the single transverse mode fiber lasers.

Linearisation Method of DML-based Transmitters for Optical Communications Part II: Experimental Demonstration and Implementation Methods

In this paper, we report the first experimental demonstration of the Stretched A linearisation method. The theoretical framework of the method and related simulation studies have been presented in Part I of this work. Here, we apply the method on an 850 nm multimode vertical-cavity surface-emitting laser (VCSEL) specified for operation at 10 Gb/s and demonstrate good quality optical output waveforms with little non-linear distortion at 16 Gb/s using non-return-to-zero (NRZ) modulation. An experimental sensitivity analysis on the generation of the required modulating current components demonstrates large tolerance to parameter mismatch. In addition, it is shown that an adaptive gain scheme can improve the tolerance to the timing offset between the current components. Potential hardware implementations using either analog or digital electronics are also discussed.

Stable, multi-mode lasing in the strong localization regime from InP random nanowire arrays at low temperature

Disorder is generally considered an undesired element in lasing action. However, in random lasers whose feedback mechanism is based on random scattering events, disorder plays a very important and critical role. Even though some unique properties in random lasers such as large-angle emission, lasing from different surfaces, large-area manufacturability, and wavelength tunability can be advantageous in certain applications, the applicability of random lasers has been limited due to the chaotic fluctuations and instability of the lasing modes because of weak confinement. To solve this, mode localization could reduce the spatial overlap between lasing modes, thus preventing mode competition and improving stability, leading to laser sources with high quality factors and very low thresholds. Here, by using a random array of III-V nanowires, high-quality-factor localized modes are demonstrated. We present the experimental evidence of strong light localization in multi-mode random nanowire lasers which are temporally stable at low temperatures.

High-resolution digital spatial control of a highly multimode laser: erratum

This erratum corrects typographical errors that appeared in Fig. 5 of our earlier manuscript [Optica 8, 880 (2021)OPTIC82334-253610.1364/OPTICA.423140].

Floquet Phonon Lasing in Multimode Optomechanical Systems.

Dynamical radiation pressure effects in cavity optomechanical systems give rise to self-sustained oscillations or 'phonon lasing' behavior, producing stable oscillators up to GHz frequencies in nanoscale devices. Like in photonic lasers, phonon lasing normally occurs in a single mechanical mode. We show here that mode-locked, multimode phonon lasing can be established in a multimode optomechanical system through Floquet dynamics induced by a temporally modulated laser drive. We demonstrate this concept in a suitably engineered silicon photonic nanocavity coupled to multiple GHz-frequency mechanical modes. We find that the long-term frequency stability is significantly improved in the multimode lasing state as a result of the mode locking. These results provide a path toward highly stable ultracompact oscillators, pulsed phonon lasing, coherent waveform synthesis, and emergent many-mode phenomena in oscillator arrays.

Adjustable ring shaping for laser welding applications

AbstractHigh laser welding speed can be achieved by using a ring and central spot laser illumination profile, with an adjustable ratio of the ring energy to the spot enabling optimization of the process. We present a novel and simple diffractive‐based concept to achieve such adjustable shaping for standard fiber lasers. The module is rotation‐adjustable between 100 % energy in the central spot to no energy in the central spot, can work with both single‐mode and multimode lasers, is highly compact (less than 20 mm long) and has the high LDT of passive diffractive components. This concept obviates the need for special ring‐type fiber lasers, enabling simpler design of the welding laser head.

6.5: Highly reliable 1W 638nm Laser Diode

We report the reliability test of tensile‐strained GaInP quantum‐well single‐emitter lasers of 638‐nm wavelength for RGB laser projection. P‐side‐down submounted multimode lasers with 110 μm stripe width were subjected to long‐term aging tests at the CW current of 1.5 A and the heat sink temperature of 25 Celsius. Except failures caused by current over‐surging, no devices have failed during the long‐term aging tests. The longest test has accumulated 22297.7 hours and the power degrading coefficient is approximately 3%/10000 hours.

Effect of Detuning on Synchronization of Semiconductor Lasers

A simulation of chaos-synchronization of unidirectionally open-loop master-slave configuration semiconductor lasers is introduced. We consider three scenarios, both lasers are single-mode, multi-mode lasers, with three modes where the interaction is between each mode of transmitter with the corresponding mode of the receiver, and the third scenario is for 5 modes. The simulation was focused on the influences of frequency-detuning of different values of coupling strength for two types of synchronization, anticipating synchronization (AS) and isochronous synchronization (IS). The simulation results demonstrate that scenarios are sensitive to the frequency-detuning between transmitter’s and receiver’s lasers. So, the suitable increasing of coupling strength is a necessary solution to decrease the frequency-detuning effect and saving synchronization with significantly broad high-quality, which means excellent feasibility.

Single-cavity bi-color laser enabled by optical anti-parity-time symmetry

The exploration of quantum inspired symmetries in optical systems has spawned promising physics and provided fertile ground for developing devices exhibiting exotic functionalities. Founded on the anti-parity–time (APT) symmetry that is enabled by both spatial and temporal interplay between gain and loss, we demonstrate theoretically and numerically bi-color lasing in a single micro-ring resonator with spatiotemporal modulation along its azimuthal direction. In contrast to conventional multi-mode lasers that have mixed-frequency output, our laser exhibits stable, demultiplexed, tunable bi-color emission at different output ports. Our APT-symmetry-based laser may point out a new route for realizing compact on-chip coherent multi-color light sources.

Discretely tunable, single mode lasing from a multimode diode laser, locked to silica microsphere resonator

We present spectral narrowing of a multi-longitudinal-mode diode laser by locking the diode to a Whispering Gallery Mode (WGM) microresonator. By controlling both the temperature and polarization of the diode laser, locking may occur to different TE and TM modes of the microresonator. Fine tuning the laser diode's temperature from 15 °C to 35 °C, makes it possible to lock the laser to 19 different WGMs, resulting in a single longitudinal mode, 2 mW laser, with spectral tunability of 15 nm, and power loss of less than 10%. The WGMs to which the diode laser is locked are calculated and identified. This reported 'in-fiber' system can be packaged to form a simple, cheap, and compact tunable single mode laser device.

Self-cleaning effect in an all-fiber spatiotemporal mode-locked laser based on graded-index multimode fiber

We demonstrate an all-fiber spatiotemporal mode-locked laser based on graded-index multimode fiber. Due to the high damage threshold of graded-index multimode fiber devices, the output single-pulse energy reach up to 11.67 nJ. The beam quality at mode-locked state is significantly improved compared with continuous wave state as a result of the self-cleaning effect. By amplifying the pulse in the multimode fiber amplifier, high power pulses with high beam quality can be obtained in the all-fiber system based on graded-index MMFs. We present an approach to get high single-pulse energy in the all-fiber multimode laser with a near single-mode output beam.

Numerical Study of a Reconfigurable Multiband Microwave Photonic Filter Using a Tunable Fabry-Perot Filter

Microwave photonic filters (MPFs) with the capabilities of bandwidth reconfigurability and frequency tunability in the GHz range are of great interest in high-speed communications systems. In this paper, we propose a new reconfigurable multiband microwave photonic filter (MPF) using a tunable Fabry–Perot Filter (FPF). It is demonstrated by numerical simulations that the modification of the intermodal separation (δλ) of a multimode laser diode (MLD) by tunable FPF allows for the reconfigurable multi-passband of the MPF. Our simulation results show that our new filter system is promising to communications systems.

High-resolution digital spatial control of a highly multimode laser

We developed a rapid and efficient method for generating laser outputs with arbitrary shaped distributions and properties that are needed for a variety of applications. It is based on simultaneously controlling the intensity, phase, and coherence distributions of the laser. The method involves a digital degenerate cavity laser in which a phase-only spatial light modulator and spatial filters are incorporated. As a result, a variety of unique and high-resolution arbitrary shaped laser beams were generated with either a low or a high spatial coherence and with a minimal change in the laser output power. By controlling the phase, intensity, and coherence distributions, a shaped laser beam was efficiently reshaped into a completely different shape after free space propagation. The generation of such laser beams could lead to new and interesting applications.

An Experimental Investigation on the Cutting Quality of Three Different Rock Specimens Using High Power Multimode Fiber Laser.

The laser cutting of rock has been popular recently because of its advantages over traditional rock cutting methods. Several types of research were performed to replace traditional rock cutting techniques with laser cutting. The purpose of this experiment is to observe cutting quality for intrusive igneous rocks using a high-power multimode fiber laser. The cutting quality, in terms of kerf width and penetration depth, resulted from different scanning speeds and was studied and compared. The specimens used in this study were gabbro, granite, and diorite, which are widely applied in the construction industry because of their high compressive strength and beautiful textures. Energy-dispersive X-ray Spectroscopy (EDX) analyses were conducted to observe the chemical content of three different areas, the melting area, the burnt area, and a non-processed area, for each rock specimen. The study of the compositional changes in each area will also go over the cutting quality of each rock specimen at different scanning speeds. According to the experimental results, the kerf widths of the specimens gradually decrease as the scanning speeds increase. The penetration depths into the specimens sharply decrease as scanning speeds increase. From a study of their compositional changes, it is found that the cutting quality for each rock depends on their silica content. This study summarizes that the cutting quality for a rock specimen greatly depends on the scanning speed of the laser cutting.

Multimode oscillation Q-switched mode-locking fiber laser based on gain equalized few-mode Er-doped fiber

We propose and demonstrate experimentally multimode Q-switched mode-locking (QML) fiber laser signal generation based on a long few-mode Er-doped fiber (FM-EDF). The erbium-doped profile of the fiber has been optimized to equalize the modal gain. The fiber supporting six modes has a total length of 5.5 m, a part of the fiber is used as a gain medium, and the unpumped part of the fiber is used as a saturable absorber. Multimode oscillation lasers in the state of the continuous wave laser operation, Q-switching, and QML state pulse laser operation were observed and measured at different pump powers by adjusting the polarization controller. A 1570-nm multimode QML pulse train was obtained, when the pump power was about 2.4 W. The width of the Q-switched envelope was 19 μs, and the repetition rate was 17.4 kHz. The stable mode-locked pulses repetition rate in Q-switched envelopes was 13.18 MHz. These temporal pulses can also be observed under the case of partly blocking the beam profile, and the part of the spectral signal gradually disappeared. The mode number in the laser was checked by a homemade mode selection photonic lantern. The results showed that the fiber laser had multimode oscillation, and the modes in generated signal were ≥5. The multiple transverse modes of the generated lasers are all in QML state at the same time. To the best of our knowledge, this is the first experimental demonstration of a multimode QML fiber laser using a long FM-EDF.

Fast dynamics of low-frequency fluctuations in a quantum-dot laser with optical feedback.

We experimentally investigate the complex dynamics of a multi-mode quantum-dot semiconductor laser with time-delayed optical feedback. We examine a two-dimensional bifurcation diagram of the quantum-dot laser as a comprehensive dynamical map by changing the injection current and feedback strength. We found that the bifurcation diagram contains two different parameter regions of low-frequency fluctuations. The power-dropout dynamics of the low-frequency fluctuations are observed in the sub-GHz region, which is considerably faster than the conventional low-frequency fluctuations in the MHz region. Comparing the dynamics of quantum-dot laser with those of single- and multi-mode quantum-well semiconductor lasers reveals that the fast low-frequency fluctuation dynamics are unique characteristics of quantum-dot lasers with time-delayed optical feedback.

Multimode semiconductor laser gyroscope and factors determining its operation

We report the results of a study of a multimode semiconductor laser gyroscope (MSLG) with a long fibre-optic ring cavity (RC) and alternating frequency dithering formed by a lithium niobate phase modulator, and discuss the factors that determine the device operation. The fundamental limitations in MSLG operation are phase fluctuations and the multimode nature of radiation. Phase fluctuations caused by spontaneous emission of photons in a semiconductor optical amplifier (SOA) result in a slow drop in the beat amplitude. However, its complete decay does not occur due to the periodic and short-term lock-in phenomenon, which leads to the equalisation of the fluctuation levels in pairs of counterpropagating waves (PCPWs) that form the RC modes, and, thereby, to the restoration of the beat amplitude. The limitations associated with the multimode lasing regime are overcome due to the fact that the emission spectrum consists of a small number of narrow lines, and the positions of the antinodes and nodes of standing waves in the SOA field distribution coincide or are close. Both factors lead to the mode locking in spectral lines and, as a consequence, to synchronisation of PCPW beats at the device output. The mechanism of the formation of these factors is not yet clear.