Wavelength-tunable Laser Research Articles

Wavelength tunable passive-mode locked Er-doped fiber laser based on graphene oxide nano-platelet

The continuous wavelength tunable passively mode-locked Er-doped fiber laser (PML-EDFL) from C-band around 1544 nm to L-band around 1574 nm has been demonstrated in combination with graphene oxide (GO) nano-platelets and the nonlinear polarization rotation (NPR) mechanism. The preparation of GO nano-platelets was based on the liquid phase exfoliation and then mixed with a PVA solution to produce the GO/PVA film. The PML-EDFL was operated at the fundamental mode-locked state in the longer wavelength regime (>1565 nm) and was turned into the fourth harmonic mode-locked state in the shorter wavelength regime. The wavelength tuning mechanism is attributed to the invisible filter by means of the in-line polarizer inside laser cavity, which can be verified from the emission peaks of the dual wavelengths mode-locked state. The relatively stable operation of tunable PML-EDFL at certain wavelength, disregarding environmental perturbations, has been confirmed to reveal promising commercial applications in the near future.

Photothermal Plasmonic Actuation of Micromechanical Cantilever Beams

Oscillations of charge carriers in plasmonic metal nanoparticles activated by resonant absorption of light are accompanied by local temperature increase due to nonradiative plasma damping. The control of this photothermal effect is considered essential for many applications ranging from photochemistry or nanomedicine to chemical and physical sensing. Here, we present a study on the conversion of visible light into mechanical energy via photothermal plasmonic and nonplasmonic effects. Atomic force microscopy cantilevered sensors coated with various materials and excited in vacuum by a wavelength tunable laser show that light generates a resonant mechanical oscillation. The photoinduced oscillation amplitude depends on the wavelength of the incident light, allowing for an optimization of energy conversion based on absorption spectroscopy of the coating material. The effect of nonphotonic forces acting on the cantilever is probed in the context of photoinduced force microscopy by placing the cantilever in interaction with a substrate surface at various distances. The findings are relevant for any technique utilizing an optical actuation of a mechanical system, and for photoinduced force detection methods in particular.

Map of Mode Hopping Associated with Modulated Laser Diode Under Feedback and Injection

A new simple technique for narrow-linewidth and wavelength-tunable laser is experimentally satisfied. The technique is based on employing nonlinearities arise with laser feedback and injection. The main effect observed and controlled is the mode hopping (MH) within other nonlinearities. In this situation mixed optoelectronical feedback (OEFB) and optical injection from another identical type laser simultaneously, plays the role of controller. These results show that no need of using a Fiber Bragg Grating. As a well-known alternative technique used earlier to build a tunable laser. The map of validity for this effect and parameters affecting the laser output are all investigated. This technique enhances modulation index for the slave laser to be 100% and employs mode hoping generated in it for tuning output lasing frequency. This is by controlling either master laser detuning or injection strength or both. Also, by controlling slave laser optoelectronic feedback time delay (by a variable RF attenuator) or (and) nonlinear contribution for this by introducing a nonlinear function into the detected signal. Bidirectional (mutual) and unidirectional injection systems are experimented. In the first system both lasers multi-lasing in general synchronization manner, while in the last system, injector (master) laser stays immune from slave dynamics.

Simulation of wide wavelength tuning of As2Se3 π-PSFBG narrow band fiber laser based on temperature and strain

In this work, the temperature (strain) properties of As2Se3 π-phase-shift fiber Bragg gratings (π-PSFBGs) are numerically investigated. The sensing sensitivities are 0.0665 nm °C−1 (0.8598 pm μϵ −1) at 1.55 μm, 0.1546 nm °C−1 (1.9968 pm μϵ −1) at 3.6 μm, 0.2717 nm °C−1 (3.51 pm μϵ −1) at 6.328 μm, and 0.4551 nm °C−1 (5.8796 pm μϵ −1) at 10.6 μm, and the sensitivities are improved with the increasing Bragg wavelength. Based on them, a high-performance narrow band fiber laser is theoretically proposed. The proposed tunable laser has a wider wavelength adjustment range in mid-infrared region, which can reach 27.306 nm at around 10.6 μm. Moreover, the obvious Bragg wavelength shift induced by stretching the As2Se3 π-PSFBG helps widen the tunable range of the laser, making possible a fine tuning. Being simple in structure and high in sensitivity, the proposed laser can be applied in mid-infrared region, which is the fingerprint region of numerous molecules and which closely relates to military aviation, environmental conservation, biology and health care regions.

Modulating optical and electrochemical properties of perylene dyes by twisting aromatic π-system structures

Three highly fluorescent perylene bisimide dyes were synthesized, where aromatic π-system structures are twisted to different degree by steric hindrance of two or four substitution groups at bay position. Light-emitting colours of these dye solutions can be modulated from green, yellow to red, and their fluorescence quantum yields increase up to approximate 100% with increasing the π-system twisting, which can be considered for new class of wavelength-tunable dye lasers. π-Twisted dyes are more sensitive to microenvironment changes. Thus, they are better fluorescence probe and sensory materials than planar dyes. Electrochemical cyclic voltammetry measurements revealed that these dyes are suitable for n-type optoelectrical devices and materials. These dye solids display near infra-red emission at 600–850 nm. Owing to strong π-π stacking interaction, planar dye solid loses its outstanding optical properties compared to its solution. In contrast, π-twisted dye solids retain their excellent optical properties including narrow emission bands and relatively high fluorescence quantum yields due to the suppression of π-π stacking interaction. Exceptional fluorescence polarization phenomena were observed for these π-twisted dye solids. These optical results revealed that π-twisted perylene bisimide dyes are more excellent optical materials than planar dyes.

Recent Advance in Tunable Single-Frequency Fiber Laser Based on Two-Dimensional Materials

Two-dimensional (2D) materials possess optoelectronic and nonlinear optical properties make them used in many fields such as optical modulator, optical switch, and single frequency (SF) fiber lasers as a saturable absorber (SA) in the laser cavity. This review deals with recent advances in wavelength tunable single frequency fiber laser based on these properties. It extends the contents from previous reviews on pulsed fiber lasers to SF fiber lasers which have emerged in recent years. First, the research status of 2D materials, including the structural characteristics and properties of some typical cases and their saturable absorption characteristics are introduced. After that, the principle and some applicable technologies with these 2D materials to achieve the wavelength tunable SF operation are discussed. Further, the latest research progress on such fiber lasers are summarized. Finally, a discussion on future prospects and challenges are included.

Real-time interleaved spectroscopic photoacoustic and ultrasound (PAUS) scanning with simultaneous fluence compensation and motion correction

For over two decades photoacoustic imaging has been tested clinically, but successful human trials have been limited. To enable quantitative clinical spectroscopy, the fundamental issues of wavelength-dependent fluence variations and inter-wavelength motion must be overcome. Here we propose a real-time, spectroscopic photoacoustic/ultrasound (PAUS) imaging approach using a compact, 1-kHz rate wavelength-tunable laser. Instead of illuminating tissue over a large area, the fiber-optic delivery system surrounding an US array sequentially scans a narrow laser beam, with partial PA image reconstruction for each laser pulse. The final image is then formed by coherently summing partial images. This scheme enables (i) automatic compensation for wavelength-dependent fluence variations in spectroscopic PA imaging and (ii) motion correction of spectroscopic PA frames using US speckle tracking in real-time systems. The 50-Hz video rate PAUS system is demonstrated in vivo using a murine model of labelled drug delivery.

Switchable and spacing tunable dual-wavelength spatiotemporal mode-locked fiber laser.

We report a switchable and spacing tunable dual-wavelength spatiotemporal mode-locked (STML) laser based on the multimode interference filtering effect in an all-fiber linear cavity. The dual-wavelength STML operations combined with different pulse patterns are achieved. By adjusting the polarization controllers, the dual-wavelength STML pulses can be switched to single wavelength operation, which is tunable up to 35 nm under certain pump powers. Moreover, the dual-wavelength spacing can also be tuned from 8 nm to 22 nm. The obtained results contribute to understanding and exploring the spatiotemporal characteristics operating in the multi-wavelength regime of STML fiber lasers. All-fiber STML lasers with lasing wavelength tunability and flexibility may have applications in the fields of optical communications and optical measurements.

Broadband Continuously Tunable All-Fiber Laser Based on OPG for CARS Imaging

A broadband continuously tunable SESAM all-fiber laser based on optical parametric generation (OPG) for coherent anti-stokes Raman scattering (CARS) imaging applications is presented in this article. This laser structure is built on all-polarization maintaining fibers yielding high environmental stability. The OPG is based on PM photonic crystal fiber as parametric gain pumped by a wavelength-tunable (1025 to 1055 nm) and repetition rate-tunable (3 MHz to 500 KHz) Yb-doped pulsed fiber laser. The all-fiber structure features free polarization adjustment in slow-axis tunable operation with a wavelength range from 770 to 940 nm for signal radiation and 1225 to 1510 nm for idler radiation. The pump and signal pulses (CARS-Stokes and CARS-pump respectively) are intrinsically overlapped, both spatially and temporarily, due to the four-wave mixing (FWM) generation and emitted from a single fiber end. The CARS-Stokes and CARS-pump pulse duration vary between 25 to 32 ps and 21 to 24 ps, respectively in the whole band, ensuring a good overlap of the generated signals, and both exhibit a Gaussian profile with high spatial beam quality. The frequency conversion allows to obtain a frequency difference between the generated signals from 1100 to 3300 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 1</sup> . The reported low-cost laser source is ideal for bio-imaging applications, especially in CARS imaging, due to its compactness, simplicity, robustness and ease of handling. A patent has been filed based on this technology.

Direct control of store-operated calcium channels by ultrafast laser

Ca2+ channels are essential to cell birth, life, and death. They can be externally activated by optogenetic tools, but this requires robust introduction of exogenous optogenetic genes for expression of photosensitive proteins in biological systems. Here we present femtoSOC, a method for direct control of Ca2+ channels solely by ultrafast laser without the need for optogenetic tools or any other exogenous reagents. Specifically, by focusing and scanning wavelength-tuned low-power femtosecond laser pulses on the plasma membrane for multiphoton excitation, we directly induced Ca2+ influx in cultured cells. Mechanistic study reveals that photoexcited flavins covalently bind cysteine residues in Orai1 via thioether bonds, which facilitates Orai1 polymerization to form store-operated calcium channels (SOCs) independently of STIM1, a protein generally participating in SOC formation, enabling all-optical activation of Ca2+ influx and downstream signaling pathways. Moreover, we used femtoSOC to demonstrate direct neural activation both in brain slices in vitro and in intact brains of living mice in vivo in a spatiotemporal-specific manner, indicating potential utility of femtoSOC.

Wavelength Tunable Actively Q-Switched Fiber Laser

In this letter, we present the design and implementation of a wavelength tunable Q-switching fiber laser based on two-cascaded tunable filters; overlapping on transmission spectrum of both filters allows keeping the cavity losses at low level, while losing the overlapping condition increases the losses in the cavity. Following these two conditions, one tunable filter is used to select the wavelength emission of the laser; and the other tunable filter is used to actively switch on-off the overlapping condition along the range of tuning. Implementing this switching principle, pulses of light within 30 nanometers of tuning range are easily generated. Despite this range is limited by the Erbium-Ytterbium co-doped fiber implemented as the laser amplifier, the technique can be implemented at different wavelengths and ranges of emission using different doped fibers. Also, experimental characterization of the wavelength tunable pulsing laser is presented, pulsing emission at different wavelengths, time duration of pulses and average optical power as a function of wavelength, and time duration and peak power of pulses as a function of the switching rate.

Random Bragg-grating-based wavelength-tunable random fiber laser with a full-open cavity

A full-open-cavity wavelength-tunable random fiber laser (WT-RFL) with compact structure and hundreds of picometers tuning range is proposed and demonstrated. A π fiber Bragg grating (FBG) is used in the WT-RFL as a filter to select lasing wavelengths. The two random Bragg grating arrays (RBGAs) and a section of high gain erbium-doped fiber result in a low lasing threshold and high stability. A numerical model to analyze the tunable characteristics is developed. The results show that the laser threshold is 22 mW, and the maximum peak-power fluctuation is 0.55 dB. To the best of our knowledge, it is the first time that a compact and full-open-cavity WT-RFL with two RBGAs and a π-FBG is proposed.

45 nm broadband continuously tunable semiconductor disk laser

A broadband continuously tunable semiconductor disk laser is reported in this paper. The active region of gain chip is composed of InGaAs multiple quantum wells with resonant periodic gain structure, and its fluorescence peak wavelength is around 965 nm. Using the wideband characteristics of the quantum wells in gain chip, along with the simple linear cavity that is formed by a high reflectivity external mirror, the laser has a low cavity loss and a wide tuning range. The continuously tunable laser wavelength can be obtained by inserting birefringent filters with different thickness into the cavity. When the thickness of the birefringent filter is 2 mm, the wavelength tuning range of the laser is 45 nm, the maximum output power is 122 mW, and the beam quality &lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; factors in the &lt;i&gt;X-&lt;/i&gt; and the &lt;i&gt;Y-&lt;/i&gt;directions are 1.00 and 1.02, respectively. The temperature characteristics of the surface-emitting spectra of gain chip and the narrowing effect of birefringent filter on laser linewidth h are also discussed.

Applications and research progress of optical fiber grating sensing in thermoplastic composites molding and structure health monitoring

Thermoplastic composites have the characteristics of high strength, fatigue resistance and easy recycling. They have received wide attention and are used in construction, automobile and other fields. An accurate monitoring technique is necessary for monitoring their internal state in the molding process and structure health in the service process. Traditional mechanical and electrical sensors have many disadvantages, such as large size and slow response. Therefore, they are not appropriate for embedded measuring and real-time monitoring. Optical fiber Bragg grating (FBG) sensors present many advantages over traditional and other advanced sensors, such as resistance to electromagnetic interference, light weight and less influence on materials. Generally, FBGs are embedded in composites and/or structures to monitor temperature and strain. The principle of FBG sensor and three key demodulation techniques are introduced firstly, including tunable Fabry-Perot (F-P) filter demodulation, especially the high-speed demodulation technique based on swept wavelength tunable laser, linear filter demodulation and matched grating. Secondly, the progress of monitoring technology based on FBGs for three molding scheme is reviewed, including hot compression molding, fused deposition molding and automated fiber placement molding. Thirdly, the progress of structural health monitoring for thermoplastic composites is also reviewed. Finally, some intelligent thermoplastic composite structures based on FBGs are introduced.

Mapping Resonance Structures in Transient Core-Ionized Atoms

The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry. (Less)

Generation of visible-wavelength-tunable ultrashort laser pulses through femtosecond laser filamentation in air: strong external focusing effect

Wavelength-tunable ultrashort laser pulses in the visible spectral range are generated in a femtosecond laser filament in air through four-wave mixing of an intense 808-nm near-infrared (NIR) laser pulse and a weak tunable infrared (IR: 1.3–2.2 µm) laser pulse. We found that the external focusing geometries strongly influence the four-wave mixing efficiency in air filament. The measurements of the resultant four-wave mixing laser intensity reveal that changing both the tilting angle and the focal length of the focal lens can optimize the four-wave mixing signal. We interpret the process based on the strong influence of the external focusing on the plasma density and the volume of the filament during the filament formation.

High-energy diode-pumped alexandrite amplifier development with applications in satellite-based lidar

Efficient, wavelength-tunable diode-pumped alexandrite laser systems offer the potential for a more versatile, satellite-based lidar source compared to fixed wavelength Nd:YAG systems and non-space compliant lamp-pumped alexandrite. In this paper, we develop a strategy to enable the high-energy operation required for atmospheric lidar based on an efficient diode-pumped master oscillator power amplifier (MOPA) system design. A novel multipass “diamond” slab amplifier geometry is introduced alongside the experimental results of the world’s first diode-pumped alexandrite amplifier producing a gain of 2.13 in a demonstration system. A diode-pumped Q -switched alexandrite oscillator is presented with a record-highest pulse energy of 3.80 mJ. Detailed optimization of a two-stage amplifier design is studied numerically and maximized with temperature, wavelength, and pump pulse duration to produce 50 mJ pulse energy. This forms part of an optimized alexandrite MOPA design capable of high pulse energy, showing the future potential of diode-pumped alexandrite for satellite-based atmospheric lidar.

Wavelength-Flexible Thulium-Doped Fiber Laser Based on Digital Micromirror Array

Wavelength-tunable thulium-doped fiber laser is demonstrated employing a digital micromirror device (DMD) in combination with a fixed grating. The diffraction property of four typical models of DMDs and its steering efficiency for the laser system are analyzed based on two-dimensional grating theory. By spatially modulating reflective patterns on a DMD, the stable, fast, and flexible tuning of lasing wavelength from 1930 nm to 2000 nm is achieved with wavelength tuning accuracy of 0.1 nm. The side-mode suppression ratio is larger than 50 dB around the 2 μm band with 3 dB linewidth less than 0.05 nm. The wavelength drift and power fluctuation are lower than 0.05 nm and 0.1 dB within 1 h at the room temperature, respectively.

Comparative capability analysis of laser multispectral methods for measuring oil slick thickness

The remote laser multispectral method capabilities to measure the thickness of the oil slicks on the sea surface in various spectral bands have been analysed. The best option (in terms of the band and the recovery accuracy of the oil film thickness on the water surface) is to use the eye-safe spectral band of about 2.1 μm. It is shown that a use of the discretely wavelength-tuneable laser for 5 wavelengths in spectral interval about 2.1 μm and the quasi-solution search method for mesurement data processing, enables us, with the mean square value of measurement noise 2 %, to recover the thickness of the oil slicks on the sea surface from ∼ units μm to ∼ 200 μm with an error of no more than 30%.

Analysis of Compact Silicon Photonic Hybrid Ring External Cavity (SHREC) Wavelength-Tunable Laser Diodes Operating From 1881–1947 nm

The “ $2~\mu \text{m}$ waveband”, specifically the $1.9~\mu \text{m}$ wavelength region, is playing an increasingly imperative role in photonics. Development into compact tunable light sources operating at the wavelength region can unlock numerous technological applications. Instances, while not exhaustive, include alleviating the capacity load in fiber communications, H2O spectroscopy, optical logic, signal processing as well as enabling the optical Kerr effect on silicon. Silicon photonics is a disruptive technology. Through mature silicon processing, recent developments suggest that silicon will emerge as the workhorse of integrated optics. While the realization of a monolithic light source has proved to be challenging, the hybrid/heterogenous Si platforms, consisting of silicon and III-V materials, has stepped to the fore. In this work, we present the study of Vernier-based hybrid silicon photonic wavelength-tunable lasers with an operating range of 1881–1947 nm (66 nm), subject to different coupling gaps (Gapmrr) between the silicon microring resonators (MRRs) and bus waveguide. Wavelength tuning functionality is enabled via the thermo-optic effect of MRRs. Gapmrr, being the smallest feature in the assemble, is highly influential to the characteristics of the SHREC. As such, trends in hybrid laser performance with respect to Gapmrr are measured and analyzed. Slope efficiency, laser output power and side-mode suppression ratio of 0.232 W/A, 28 mW and 42 dB respectively are obtained across the developed lasers. Through the design of the Vernier spectrum and Gapmrr, tuning of laser wavelength from 1881–1947 nm can be achieved by applying only 47.2 mW of thermal power to a single MRR.