Laser Configuration Research Articles

Mode-locked all-PM Nd-doped fiber laser near 910 nm.

We present a compact passively mode-locked fiber laser emitting near 910 nm with an all-polarization-maintaining fiber laser architecture. The ring-cavity laser configuration includes a core-pumped neodymium-doped fiber as a gain medium and a semiconductor saturable absorber mirror as a passive mode-locking element. A bandpass filter is used to suppress parasitic emission near 1.06 µm and allows wavelength tuning between 903 and 912 nm. The laser operates in a highly stable and self-starting all-normal-dispersion regime with a minimum pulse duration of 8 ps at 28.2 MHz pulse repetition rate and 0.2 nJ maximum pulse energy. A single-pass amplifier stage increases the pulse energy up to 1.5 nJ, and pulse compression with a pair of gratings is demonstrated with nearly Fourier transform limited pulses.

Modeling of a quantum dot gain chip in an external cavity laser configuration

External cavity semiconductor lasers with strong optical feedback already exist using a gain chip medium. Owing to their ultrafast carrier dynamics, strong output power, and high temperature reliability, quantum dots as a gain medium are now envisioned as a promising solution to replace the current quantum well technology. This paper presents a semi-analytical rate equation model which is used to describe a quantum dot gain chip capable of lasing only with a free space external cavity laser. It investigates the evolution of the dynamical properties such as the turn-on delay and the damping rate. It also confirms the model’s validity through the relative intensity noise and the frequency noise with respect to both material and device parameters like the linewidth enhancement factor, the gain compression factor, or the cavity length. Overall, this numerical investigation provides initial building blocks for future fabrication research and development of high performance devices including filters or gratings as wavelength-selective components.

Inverse design of mode-locked fiber laser by particle swarm optimization algorithm

A wide variety of laser applications, that often require radiation with specific characteristics, and relative flexibility of laser configurations offer a prospect of designing systems with the parameters on demand. The inverse laser design problem is to find the system architecture that provides for the generation of the desired laser output. However, typically, such inverse problems for nonlinear systems are sensitive to the computation of the gradients of a target (fitness) function making direct back propagation approach challenging. We apply here particle swarm optimization algorithm that does not rely on the gradients of the fitness function to the design of a fiber 8-figure laser cavity. This technique allows us to determine the laser cavity architectures tailored to generating on demand pulses with duration in the range of 1.5–105 ps and spectral width in the interval 0.1–20.5 nm. The proposed design optimisation algorithm can be applied to a variety of laser applications, and, more generally, in a range of engineering systems with flexible adjustable configurations and the outputs on demand.

High-resolution spectral analysis of long single-frequency pulses generated by a self-sweeping Yb-doped fiber laser

Continuous wave self-sweeping operation is demonstrated in a classical ring cavity configuration of Yb-doped fiber laser with a saturable absorber, which is usually used to obtain single-frequency lasing. Using heterodyne-based high-resolution spectral analysis we demonstrate that the laser generates a series of long (up to 1 ms) rectangular pulses consisting of single frequency radiation (the linewidth <0.7 MHz). At the same time, the lasing frequency changes between pulses regularly in a stepwise manner by one intermode beating frequency within a self-sweeping range of 100 pm. The effect of cavity configuration on pulse parameters (duration and chirp) is investigated. We also demonstrate that saturable absorbing mirrors design can be used to control the self-sweeping direction and value of the frequency jump.

Nanosecond Q-switched pulse generation using poly(3,4 ethylenedioxythiophene): Poly(4-styrenesulfonate) thin film as saturable absorber

Q-switched Erbium-doped fiber laser (EDFL) was demonstrated utilizing poly(3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT: PSS) as saturable absorber (SA). The proposed film was fabricated by mixing the PEDOT: PSS with polyvinyl alcohol (PVA). The nonlinear characterization was demonstrated, revealing a saturable absorption of 50%, a saturation intensity of 0.14 MW/cm2. and a non-saturable absorption of 44%. The SA was incorporated into a laser configuration to operate as a new passive Q-switcher for nanosecond pulse generation. The Q-switched laser operated at 1568.03 nm with a 3-dB bandwidth of 2.66 nm. The maximum pulse energy, high repetition rate, and minimum pulse width were obtained at 192.11 nJ, 108.69 kHz, and 580 ns, respectively. The nonlinear characterization was demonstrated, revealing a saturable absorption of 50%, a saturation intensity of 0.14 MW/cm2. and a non-saturable absorption of 44%. This is the first demonstration of utilizing PEDOT: PSS film as SA with an inexpensive preparation process to generate Q-switched pulses.

Mid-Infrared Random Fiber Laser Assisted by the Passive Feedback

Random fiber lasers (RFLs), featuring a simple open cavity but with unique lasing characteristics, have captured considerable attention in the past several decades. Particularly, RFLs assisted by the passive feedback exhibit excellent robustness and compactness owing to the combination of active amplification and passive feedback from random distributed Rayleigh scattering. However, RFLs with the aid of the passive feedback reported previously are primarily operating in the near-infrared wavelength region of 1.1 μm or 1.5 μm based on the ytterbium or erbium doped active fibers. Here, we propose and demonstrate a random fiber laser configuration combining the passive feedback from a piece of single mode fiber with the active gain based on the thulium doped fiber, and extend the operation wavelength to the critically important mid-infrared wavelength band of ~2 μm. The spectral and temporal characteristics of the lasing output are evaluated and compared with those generated in a single pass amplification structure and a conventional resonant structure. Our results show that the passive feedback assisted RFL has a stabilized lasing wavelength and notably suppressed temporal dynamics. This work not only advances the development of RFLs, but also provides a promising strategy to achieve mid-infrared laser emission with a comparatively low relative intensity noise.

A tunable single-frequency green laser based on a wedged Nd:YVO4 crystal and a KTP crystal

A 532 nm single-frequency laser generated from a V-type oscillator configuration with a wedged Nd:YVO4 crystal and a KTP crystal is reported. An a-cut, 2.at% Nd:YVO4 crystal with a wedge angle of 10° and a KTP nonlinear crystal composes a novel birefringent filter. By adjusting the incident angle of the fundamental mode in the KTP, the laser can change from single-mode operation to two-mode oscillation. The maximum single-frequency output power of lasers with a 5 mm long KTP and a 7 mm long KTP are 284 and 148 mW, respectively, at pump powers of 2 and 1.1 W. The corresponding optical-to-optical conversion efficiencies are 14.2% and 13.5%. For the laser configuration with a 5 mm long KTP, the single-frequency operation at a pump power of 2 W is maintained within the temperature range of 18 °C to 24 °C, and the measured wavelength drift rate with the temperature is approximately 0.071 nm °C−1.

Sub-kilohertz Brillouin fiber laser with stabilized self-injection locked DFB pump laser

We report on experimental demonstration of a low-cost sub-kilohertz Brillouin fiber ring laser pumped from an actively stabilized self-injection locked distributed feedback (DFB) laser diode. Locking of the commercial DFB laser to a ~11-m-length high-Q-factor fiber-optic ring cavity leads to ~10,000-fold narrowing of the laser Lorentzian linewidth down to 400 Hz. Such pump laser operation inside the ring cavity forces the cavity to host Brillouin lasing enabling the laser threshold power as low as ~1.5 mW. The laser operation is perfectly stabilized by active optoelectronic feedback driven by a simple microcontroller. The laser delivers radiation at Stokes frequency with the Lorentzian linewidth reduced down to ~75 Hz and a phase noise less than –100 dBc/Hz (>30 kHz). The reported laser configuration is of great interest for many laser applications where a narrow sub-kHz linewidth, simple design and low cost are important.

Optimal defect position in a DFB fiber laser

Fiber lasers with compact cavity have numerous potential applications in sensing, communications, and medicine. Distributed feedback (DFB) rare-earth doped fiber lasers based on Bragg gratings with a phase shift are the most promising in this aspect. In this paper, we theoretically study such lasers and carry out a complex-frequency analysis of the DFB cavity modes. Our approach is based on the study of poles of open cavity response function and on the laser rate equations. An optimal defect position in the Bragg grating, which maximizes an output power towards one side, was found with this approach. We show that the optimal defect position depends on the pump power. At the pump level close to the lasing threshold, the defect should preferably appear in the middle of the grating to maximize the one-side output power. At higher pumping, the optimal position of the defect becomes asymmetric. We have found specific variables, which allow for determination of optimal defect position for a large variety of DFB laser configurations.

Noise-like pulses under different intra-cavity nonlinearity

We report on the influence of the average nonlinear coefficient on noise-like pulse (NLP) in a figure-of-eight erbium-doped fiber laser. Since the nonlinear switching point of nonlinear amplifying loop mirror (NALM) shifts with distinct laser configurations, through changing the length of highly nonlinear fiber (HNLF) in NALM, properties of output pulse have been compared and analyzed. The spectral evolutions occur and the peak power of NLP presents a decline when the HNLF is lengthened in NALM. Also, the tuning range of the duration of NLP shows a difference based on the average nonlinear coefficient selected.

Numerical study of Langmuir wave coalescence in laser-plasma interaction

Type-III-burst radio signals can be mimicked in the laboratory via laser-plasma interaction. Instead of an electron beam generating Langmuir waves (LWs) in the interplanetary medium, the LWs are created by a laser interacting with a millimeter-sized plasma through the stimulated Raman instability. In both cases, the LWs feed the Langmuir decay instability which scatters them in several directions. The resulting LWs may couple to form electromagnetic emission at twice the plasma frequency, which has been detected in the interplanetary medium, and recently in a laboratory laser experiment [Marquès et al., Phys. Rev. Lett. 124, 135001 (2020)]. This article presents the first numerical analysis of this laser configuration using particle-in-cell simulations, providing details on the wave spectra that are too difficult to measure in experiments. The role of some parameters is addressed, with a focus on laser intensity, in order to illustrate the behavior of the electromagnetic emission's angular distribution and polarization.

Improvement of output efficiency of p-face up photonic-crystal surface-emitting lasers.

We optimized the p-side emission device configuration of photonic-crystal surface-emitting laser (PCSEL) to facilitate the easier chip process and wafer level testing as well as the feasibility of lasing at shorter wavelength. Typically, in order to obtain uniformly distributed current for larger emission area of PCSELs, laser output is designed through the n-side window due to the low hole mobility and thin p-side cladding layer. However, the substrate as well as the epi-layers have to be isolated before the test of each single die on the wafer, which compromised the advantage of wafer-level test of surface emitters. On the other hand, for lasers with emission photon energy higher than the bandgap energy of GaAs substrate, the power will be entirely attenuated. In this study, the optimized p-side emission by applying the transparent conduction layer on top of the p side contact layer to enhance the current distribution and breaking the symmetry of conventional circle pattern in a unit cell to boost the output efficiency is investigated. Through this approach, a high efficiency p-side up PCSEL platform with lower fabrication cost is developed, which is also applicable for short wavelength PCSELs.

Saturable absorber based on the fiber coupler coated by CNTs

We report on a new method for fabrication of saturable absorbers for operation in ring fiber lasers as a mode-locking element taking advantages of the manufacturing technologies of fiber couplers and microfiber tapers coated with carbon nanotubes. It is demonstrated in the experiment that the transmittance of the fabricated saturable absorber increases with an increase of the input radiation power. Also, we have shown that the fabricated saturable absorber enables generation of picosecond soliton pulses in ring fiber laser configuration.

Effect of the excitation line on hydroxyl radical imaging by laser induced fluorescence in hydrogen detonations

The present study aims to evidence the effect of the excitation line on the planar laser-induced fluorescence of hydroxyl radical (OH-PLIF) imaging in H2-fueled detonation wave. We experimentally validated an updated laser-induced fluorescence (LIF) model, called KAT-LIF, which simulates spectrally-resolved fluorescence spectra, using a recently developed optical detonation duct. We numerically investigated the effects of the excitation line, the initial pressure (20–100 kPa), and the diluent (N2/Ar) on the fluorescence spectrum, the spectrally- and one-dimensionally-averaged LIF intensity, and the quantitative capabilities of the OH-LIF measurements for different 2H2-O2-3.76diluent detonable mixtures. The investigated excitation lines were (0,0)Q1(7), both (1,0)Q2(8) and (1,0)Q1(9), and (1,0)Q1(6), which all belong to the A2Σ+←X2Π transition. The main findings are the following: (i) considering the commercially available OH filters, Q2(8)+Q1(9) excitation scheme has the highest LIF intensity for all the investigated H2-fueled detonations, while Q1(7) could provide the strongest intensity with better (custom) collection optics; (ii) the maximum LIF intensity decreases with increasing pressure for all the excitation schemes; (iii) using a single point calibration, at the fluorescence peak, it is not possible to perform quantitative measurements of OH radicals for any H2-fueled detonation, using the conventional excitation schemes. Finally, we experimentally evidenced more favorable excitation schemes to obtain qualitative information far behind the front, by employing the saturated regime of fluorescence or the optically thin linear regime with appropriate laser configuration. These two excitation schemes correspond to more appropriate LIF strategies that will enable better detonation flow visualization in future studies.

Multiwavelength Fiber Laser Covering far L and U Bands in a Dual Cavity Configuration

This research investigates and demonstrates a unique configuration of a multiwavelength fiber ring laser covering far L and U bands by employing a booster optical amplifier (BOA), a stabilizer beam, and a Mach-Zehnder interferometer (MZI) in a dual cavity arrangement. By reducing the loss in the central cavity from 10 to 0 dB, the number of generated lasing wavelengths can be increased from 15 to 30, respectively. The observed side mode suppression ratio (SMSR) for most of these lasing wavelengths is larger than 60 dB. The generated lasing lines are relatively stable where peak power and wavelength variations of within ~1 dB and ~0.08 nm are observed.

Passively Q-Switched Pulses Generation from Erbium-Doped Fiber Laser Using Lutetium Oxide as Saturable Absorber

Abstract In this paper, a configuration of a passive Q-switched Erbium-doped fiber laser (EDFL) using a Lutetium Oxide (Lu2O3) thin film as a saturable absorber (SA) is experimentally implemented to generate high pulse energy with a high signal to noise ratio. A stable Q-switched pulse train is initiated at the input pump power of 30.442mW, and a maximum pulse energy of 16.11 nJ is obtained at an output power of 0.97 mW, which confirms the stability of the pulses. It was possible to increase the repetition rate of the Q-switched laser from 31.25 to 60.2 kHz as the pump power was raised from 30.442 mW to 71.652 mW. Moreover, the pulse width decreased from 11.4 µs to 4.27 µs and 66.4 dB of the received signal-to-noise ratio at the radio frequency spectrum was achieved.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy employing an all-fiber-coupled probe laser configuration

The interferometric cavity-assisted photothermal spectroscopy (ICAPS) method has been proven highly suitable for sensitive and compact gas detection by application of an optical cavity as transducer for photothermal spectroscopy. This work reports on the implementation of an overall fiber-coupled probe laser configuration detecting the reflectance of the individual interferometers in a balanced-detection ICAPS system. The layout greatly improves the overall sensor system robustness. Two identical 1 mm path length cavities were used for balanced detection, enabling sensor operation close to the fundamental limit of shot noise by efficiently cancelling excess noise. A quantum cascade laser served as a mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced refractive index variations. The metrological figures of merit for the sensor were investigated by SO2 detection. For the targeted absorption band centered at 1380.93 cm-1, a 3 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 4.5 × 10-9 cm-1 W Hz-1/2.

Stable and narrow linewidth linear cavity CW-active Q-switched erbium-doped fiber laser

A stable and narrow-linewidth, continuous wave and active Q-switching erbium-doped fiber laser, incorporating a fiber Bragg grating and a fiber Fabry-Perot tunable filter, was designed and experimentally implemented. Here, we proposed combining three approaches to suppress the mode hopping and improve both the output power stability and narrow the linewidth: first, two narrow-band optical filters, fiber Bragg grating and Fabry-Perot tunable filter were utilized in the laser cavity; second, the cavity configuration was optimized as a function of the output coupling ratio; finally, a 1.5 m long segment of unpumped erbium-doped fiber was inserted in the cavity to act as an induced virtual long and weak fiber Bragg grating. Using a 90% output coupling ratio, and monitoring over 120 min at room temperature, the 3 dB linewidth and power fluctuation of the fiber laser emission at 1550 nm were 16 kHz and 0.0025 dB respectively. Also, an optical signal to noise ratio (OSNR) of 82 dB was obtained. Keeping the same fiber laser configuration, laser pulses were also obtained using the Q-switching technique, realized by using dynamic spectral overlapping between the Bragg grating and the tunable filter. A stable output peak power of 5.6 W was achieved with a pulse width of 450 ns at a repetition rate of 1 kHz.

Endless Single-Mode Photonics Crystal Fiber Metalens for Broadband and Efficient Focusing in Near-Infrared Range.

The advent of the ‘lab-on-fiber’ concept has boosted the prosperity of optical fiber-based platforms integrated with nanostructured metasurface technology which are capable of controlling the light at the nanoscale for multifunctional applications. Here, we propose an endless single-mode large-mode-area photonic crystal fiber (LMA-PCF) integrated metalens for broadband and efficient focusing from 800 to 1550 nm. In the present work, the optical properties of the substrate LMA-PCF were investigated, and the metalens, consisting of dielectric TiO2 nanorods with varying radii, was elaborately designed in the fiber core region with a diameter of 48 μm to cover the required phase profile for efficient focusing with a high transmission. The focusing characteristics of the designed metalens were also investigated in detail over a wide wavelength range. It is shown that the in-fiber metalens is capable of converging the incident beams into the bright, symmetric, and legible focal spots with a large focal length of 315–380 μm depending on the operating wavelength. A high and average focusing efficiency of 70% was also obtained with varying wavelengths. It is believed the proposed fiber metalens may show great potential in applications including fiber laser configuration, machining, and fiber communication.

Long time universality of black-hole lasers

For flowing quantum gases, it has been found that at long times an initial black-hole laser (BHL) configuration exhibits only two possible states: the ground state or a periodic self-oscillating state of continuous emission of solitons. So far, all the works on this subject are based on a highly idealized model, quite difficult to implement experimentally. Here we study the instability spectrum and the time evolution of a recently proposed realistic model of a BHL, thus providing a useful theoretical tool for the clear identification of black-hole lasing in future experiments. We further confirm the existence of a well-defined phase diagram at long times, which bespeaks universality in the long-time behavior of a BHL. Additionally, we develop a complementary model in which the same potential profile is applied to a subsonic homogeneous flowing condensate that, despite not forming a BHL, evolves toward the same phase diagram as the associated BHL model. This result reveals an even stronger form of robustness in the long-time behavior with respect to the transient, which goes beyond what has been described in the previous literature.