Wavelength-tunable Laser Research Articles

Tunable wavelength laser surface profilometry through tilted interference

Frequency-tunable lasers allow the removal of phase ambiguities in interferometric profilometry through the synthetic wavelength longer than height variations in the sample. The subsequent measurements lowering synthetic wavelengths and updating phase difference values improves the measurement. The surface profilometry on samples with tilted interference can lead to an initial synthetic phase map, though locally unambiguous, getting globally wrapped whose unwrapping can be difficult in the presence of noise. Starting with the synthetic phase map that is locally unambiguous but globally wrapped can still update the phase values through a recursive algorithm. The artefact of 2π jumps in the initial wrapped synthetic phase just gets carried forward which can be easily removed from the final phase map obtained with the shortest possible synthetic wavelength. Choosing a point on the sample surface as the point of comparison, the proposed interferometry scheme can be made immune to surrounding vibrations while tuning wavelengths.

Revealing Single-Molecule Photocurrent Generation Mechanisms under On- and Off-Resonance Excitation.

We investigate photocurrent generation mechanisms in a pentacene single-molecule junction using subnanometer resolved photocurrent imaging under both on- and off-resonance laser excitation. By employing a wavelength-tunable laser combined with a lock-in technique, net photocurrent signals are extracted to elucidate photoinduced electron tunneling processes. Under off-resonance excitation, photocurrents are found to arise from photon-assisted tunneling, with contributions from three distinct frontier molecular orbitals at different bias voltages. In contrast, under resonance excitation, photocurrents are found to be significantly enhanced at negative bias voltages, exhibiting a spatial distribution linked to molecular electronic transitions and associated transition dipoles. This study reveals the contributions of different frontier molecular orbitals in the photon-assisted tunneling processes under off-resonance excitation, while molecular optical transitions are also found to be important at negative bias under resonance excitation. These findings could be instructive to the design and optimization of advanced optoelectronic devices.

Tunable FSRS measurements with reduced background signals: Using an etalon filter to generate picosecond pump pulses in the 460-650nm range.

Generating wavelength-tunable picosecond laser pulses from an ultrafast laser source is essential for femtosecond stimulated Raman scattering (FSRS) measurements. Etalon filters produce narrowband (picosecond) pulses with an asymmetric temporal profile that is ideal for stimulated resonance Raman excitation. However, direct spectral filtering of femtosecond laser pulses is typically limited to the laser's fundamental and harmonic frequencies due to very low transmission of broad bandwidth pulses through an etalon. Here, we show that a single etalon filter (15cm-1 bandwidth; 172cm-1 free spectral range) provides an efficient and tunable option for generating Raman pump pulses over a wide range of wavelengths when used in combination with an optical parametric amplifier and a second harmonic generation (SHG) crystal that has an appropriate phase-matching bandwidth for partial spectral compression before the etalon. Tuning the SHG wavelength to match individual transmission lines of the etalon filter gives asymmetric picosecond pump pulses over a range of 460-650nm. Importantly, the SHG crystal length determines the temporal rise time of the filtered pulse, which is an important property for reducing background and increasing Raman signals compared with symmetric pulses having the same total energy. We examine the wavelength-dependent trade-off between spectral narrowing via SHG and the asymmetric pulse shape after transmission through the etalon. This approach provides a relatively simple and efficient method to generate tunable pump pulses with the optimum temporal profile for resonance-enhanced FSRS measurements across the visible region of the spectrum.

Wavelength Locking and Calibration of Fiber-Optic Ultrasonic Sensors Using Single-Sideband-Modulated Laser

Implementation of edge-filter detection for interrogating optical interferometric ultrasonic sensors is often hindered by the lack of cost-effective laser sources with agile wavelength tunability and good noise performance. The detected signal can also be affected by optical power variations and locking-point drift, negatively affecting the sensor accuracy. Here, we report the use of laser single-sideband generation with a dual-parallel Mach–Zehnder interferometer (DP-MZI) for laser wavelength tuning and locking in edge-filter detection of fiber-optic ultrasonic sensors. We also demonstrate real-time in situ calibration of the sensor response to ultrasound-induced wavelength shift tuning. The DP-MZI is employed to generate a known wavelength modulation of the laser, whose response is used to gauge the sensor response to the ultrasound-induced wavelength shifts in real time and in situ. Experiments were performed on a fiber-optic ultrasonic sensor based on a high-finesse Fabry–Perot interferometer formed by two fiber Bragg gratings. The results demonstrated the effectiveness of the laser locking against laser wavelength drift and temperature variations and the effectiveness of the calibration method against optical power variations and locking-point drift. These techniques can enhance the operational robustness and increase the measurement accuracy of optical ultrasonic sensors.

High-efficiency 5-watt wavelength-tunable UV output from an Alexandrite laser

We investigate the performance of continuous-wave wavelength-tunable ultraviolet output from a diode-pumped Alexandrite laser by intra-cavity second harmonic generation. At 385 nm, ultraviolet output power of 5.05 W is achieved which is the highest continuous-wave ultraviolet power to date for intra-cavity frequency doubled diode-pumped Alexandrite lasers and highest optical-to-optical efficiency of 16.2% is achieved with respect to absorbed red pump power. An excellent ultraviolet wavelength tunable range, 38 nm, is achieved from 364 nm to 402 nm. Cavity mode analysis is performed for design of the resonator including cavity analysis and the modelling of output power from this wavelength tunable ultraviolet laser.

Blue lasers using low-toxicity colloidal quantum dots.

Blue lasers play a pivotal role in laser-based display, printing, manufacturing, data recording and medical technologies. Colloidal quantum dots (QDs) are solution-grown materials with strong, tunable emission covering the whole visible spectrum, but the development of QD lasers has largely relied on Cd-containing red-emitting QDs, with technologically viable blue QD lasers remaining out of reach. Here we report on the realization of tunable and robust lasing using low-toxicity blue-emitting ZnSe-ZnS core-shell QDs that are compact in size yet still feature suppressed Auger recombination and long optical gain lifetime approaching 1 ns. These characteristics allow us to handle the blue QDs like laser dyes for liquid-state amplified spontaneous emission and lasing. The blue QD laser is operated under quasi-continuous-wave excitation by solid-state nanosecond lasers. A Littrow-configuration cavity enables narrow linewidth (<0.2 nm), wavelength-tunable, coherent and stable laser outputs without circulating the solution. These results indicate the promise of ZnSe-ZnS QDs to fill the 'blue gap' of QD lasers and to replace less stable blue laser dyes for a multitude of applications.

Widely wavelength-tunable high-repetition-rate femtosecond pulse source with highest average power up to 28 W

We have proposed and demonstrated a high-repetition-rate ultrashort pulse fiber amplification system based on a wavelength-tunable oscillator. This fiber amplification system produces an average power exceeding 20 W in bursts of 200 pulses with a 578 MHz intra-burst pulse repetition rate and a 1 MHz burst repetition rate. The center wavelength of the amplified pulses can be tuned from 1030 to 1080 nm. By utilizing pre-chirp management nonlinear amplification technique, the achieved shortest pulse duration is 27 fs with an average power of 25 W at 1032 nm. For all the amplified pulses with different wavelengths, the pulse duration after optimal compression is below 60 fs. To the best of our knowledge, this is the first time that a widely wavelength-tunable high-power laser with a repetition rate exceeding 100 MHz and a pulse duration of several tens of femtoseconds has been realized. Additionally, using only common double-cladding Yb-doped fiber as the gain fiber, without any large-mode-area Yb-doped photonic crystal fiber or rod-type Yb-doped fiber, makes the system compact and reliable due to the simple fusion operation.

Color-Tunable Lead Halide Perovskite Single-Mode Chiral Microlasers with Exceptionally High glum.

Chiral microlasers hold great promise for optoelectronics from integrated photonic devices to high-density quantum information processing. Despite significant progress in lead-halide perovskite emitters, chiral lasing with high dissymmetry factors (glum) has not yet been realized. Here, we demonstrate chiral single-mode microlasers with exceptional stability and tunable emission across the visible range by combining CsPbClxBr3-x perovskite microrods (MRs) with a cholesteric liquid crystal (CLC) layer. The MRs lase via a whispering gallery mode (WGM) microcavity and confer chirality through the encapsulated CLC layer, thus exhibiting circularly polarized lasing with dissymmetry factors reaching 1.62. Importantly, we demonstrate wavelength-tunable high dissymmetry chiral lasers in a broad spectral range by tuning the halide composition and using CLC layers with the desired photonic bandgap (PBG). This facile approach to generate chiral lasing not only is applicable to semiconductor nano- and microcrystals but also paves the way for potential integration into nanoscale photonic devices.

Wavelength tunable single-frequency ring cavity fiber laser utilizing external injection locking signal

The ring cavity fiber laser with saturable absorption fibers is a simple and practical technical way of achieving single-frequency laser output. However, this structure cannot obtain wavelength-tunable laser output. In this work, an external injection locking signal launched by tunable LD is used to optimize the tunability in single-frequency ring cavity fiber lasers. By aligning the external injection signal, tunable laser output ranging from 1544.13 to 1546.51 nm is achieved. The maximum output is 16.5 mW and the linewidth is less than 6.15 kHz. The operation process and the linewidth compression effect of external Injection signals are investigated. The results indicate that there is a correlation between the output signal-to-noise ratio and the output linewidth, and this wavelength-tuning structure has the potential for rapid tuning. This work provides a technical solution for achieving a tunable single-frequency laser in the ring cavity structure.

Wavelength-tunable mode-locked fiber laser based on a bending strain-controlled filter

Wavelength-tunable mode-locked fiber lasers are highly versatile and find applications in pump–probe measurement, optical communications, optical parametric oscillation, among others. Herein, we introduce a novel approach for tuning carbon nanotube-based passively mode-locked fiber laser using a bending-based mechanism. Our method involves a bending strain controlled multimode interference all-fiber filter. This filter consists of a hybrid structure built from a cascaded seven-core fiber (SCF)-twin-hole twin-core fiber (THDCF)-SCF. It achieves a continuous wavelength tuning range of 25 nm from 1558 nm to 1583 nm with a tuning efficiency of −3.38 nm/m−1. By increasing the pump power, the dual-wavelength locked modes are also achieved at specific curvature. Owing to advantages of simple structure, low cost, and high bending sensitivity, our tunable filter shows promise for efficient wavelength tuning in ultrafast fiber laser.

Optical Assessment of Photoreceptor Function Over the Macula.

The purpose of this study was to develop next-generation functional photoreceptor imaging using ultrahigh-speed swept-source optical coherence tomography (UHS-SS-OCT) and split-spectrum amplitude-decorrelation optoretinography (SSADOR) algorithm. The advancement enables rapid surveying of large retinal areas, promising non-contact, objective, and quantifiable measurements of macular visual function. We designed and built a UHS-SS-OCT prototype instrument using a wavelength tunable laser with 1 MHz A-scan rate. The functional scanning protocol records 5 repeated volumes in 3 seconds. A flash pattern selectively exposes the imaged retina area. SSADOR quantifies photoreceptor light response by extracting optical coherence tomography (OCT) signal changes within the photoreceptor outer segment before and after the flash. The study prospectively enrolled 16 eyes from 8 subjects, demonstrating the ability to measure photoreceptor light response over a record field of view (3 × 3mm2) with high topographical resolution (approximately 100 µm). The measured SSADOR signal corresponds to the flashed pattern, whose amplitude also correlates with flash strength, showing consistency and reproducibility across subjects. The integration of high-performance UHS-SS-OCT and SSADOR enables characterizing photoreceptor function over a clinically meaningful field of view, while maintaining a workflow that can be integrated into routine clinical tests and trials. The new approach allows detecting changes in photoreceptor light response with high sensitivity and can detect small focal impairments. This innovative advance can enable us to detect early photoreceptor abnormalities, as well as help to stage and monitor degenerative retinal diseases, potentially providing a surrogate visual function marker for retinal diseases and accelerating therapeutic development through a safe and efficient outcome endpoint.

Mode-locking and wavelength-tuning of a NPR fiber laser based on optical speckle.

Passively mode-locked fiber lasers based on nonlinear polarization rotation (NPR) have been widely used due to their ability to produce short pulses with high peak power. Nevertheless, environmental perturbations can influence the mode-locked state, making it a challenge for the practical implementation. Therefore, researchers are searching for assessment criteria to quickly assist and maintain mode-locking of NPR fiber lasers. Speckle patterns containing spectral information can be generated when the laser transmits through a scattering medium, which can serve as indicators of the mode-locked state. The mode-locked regions are confined to the area close to the minimum texture contrast of speckle patterns. Based on these characteristics, we manually simulate the automatic mode-locking (AML). In addition, we utilize convolutional neural networks (CNNs) to recognize speckle patterns of wavelength tunable lasers and determine the center wavelength.

21.8 W acetylene-filled hollow-core anti-resonant fiberamplified spontaneous emission source at 3.1 µm.

We report a 20-W-level acetylene-filled nested hollow-core anti-resonant fiber (nested HC-ARF) amplified spontaneous emission (ASE) source at 3.1 µm. A 1535 nm hundred-watt wavelength tunable single-frequency fiber laser with a high signal-to-noise ratio and narrow linewidth is built for pumping acetylene molecules. Simultaneously, a homemade 120 µm core diameter eight-tube nested HC-ARF is used as a gas chamber to obtain high pump laser coupling efficiency. The mid-infrared (mid-IR) ASE source output power of 21.8 W is achieved at 3.1 µm through the low-pressure acetylene gas-filled nested HC-ARF, and the slope efficiency is 25.1%. In addition, the ASE source features an excellent beam quality of Mx 2 = 1.16 and My 2 = 1.13. To the best of our knowledge, for the first time, it is a record output power for such mid-infrared ASE sources while maintaining excellent beam quality. This work provides a new way to achieve high-power mid-infrared emission.

2.1–2.2 μm wavelength-tunable cascade gain modulation Raman fiber laser

We present a theoretical and experimental analysis of a mid-infrared wavelength-tunable cascade gain modulation Raman fiber laser. Our simulation model incorporates the Generalized Nonlinear Schrödinger Equation with the fiber Bragg grating transmission functions. Utilizing the pulse tracking method, we investigate and analyze the impact of pump pulse width and Raman gain fiber length on the first and second order Raman laser thresholds. Additionally, we exhibit the evolution process of the pulse and optical spectrum at a constant pump pulse energy. Furthermore, we present the output pulse width and energy, and the Raman laser center wavelength as a function of the pump pulse energy. Subsequently, using the optimal theoretical values of pump pulse width and Raman gain fiber length, we experimentally achieve the gain modulation Raman fiber laser. With a 1990 nm laser pumping, the Raman laser can produce a 2172 nm laser with a pulse width of 1.25 µs and a single pulse energy of approximately 400 nJ. When tuning the pump laser wavelength from 1950 nm to 2030 nm, the gain modulation Raman laser wavelength can vary from 2130 nm to 2215 nm. Importantly, our experimental results closely align with the theoretical predictions. This study provides valuable insights for the investigation of gain modulation Raman fiber lasers.

Easy wavelength-tunable passive mode-locked fiber laser through a Lyot filter with a SESAM

In this work, we present an easy wavelength-tunable passive mode-locked fiber laser with a semiconductor saturable absorber mirror as a mode-locker. The cavity includes a Lyot filter as a wide wavelength-tunable filter that can be tuned by simply increasing the retardance of a variable wave retarder. The Lyot filter consists of the variable wave retarder and highly birefringent fiber inserted between two linear polarizers oriented at the same angle. The highly birefringent fiber has 8.196 cm length and linear birefringence ∆n=4.805x10−4 which provides a 61 nm period. Continuous mode-locking was experimentally achieved from 1561.9 to 1546.1 nm by simply increasing the retardance from 0.0778π to 0.4778π rad delivering sub-ps pulses width at 10.57 MHz repetition rate.

Development of hybrid photonic integrated wavelength-tunable laser at 2 µm and its application to FMCW LiDAR.

This paper reports on the experimental demonstration of a fully integrated frequency-modulated continuous-wave (FMCW) LiDAR sensing system, operating at 2.0 µm. It makes use of a widely tunable hybrid external cavity laser based on the combination of GaSb gain chip and silicon waveguide circuits. The single-frequency laser operation over the full spectral bandwidth of the gain chip is secured using a frequency-selective filter, consisting of two sequential microring resonators in a Vernier configuration. To increase the mode-hop free wavelength tuning range while preserving the linewidth of the laser, the heater of the phase section placed along the bus waveguide is synchronously controlled with two independent heaters placed on each microring resonator. This laser is then implemented for the development of an FMCW LiDAR, consisting of all-optical fiber-based two independent unbalanced Mach-Zehnder interferometers: k-space interferometer for the linearization of continuously swept laser frequency and main interferometer for the measurement of the distributed back-reflection over the distance. The optical frequency of the laser is continuously swept over a ∼100 GHz range (or Δλ=1.47 nm at the operating wavelength) at a modulation speed of 100 Hz. Using this wavelength tunable laser, a light detection and ranging system (LiDAR) is experimentally demonstrated, showing a very high axial resolution of 1.36 mm in air with an extremely high precision of ∼9 µm at a 100 Hz measurement rate.

Stable and wavelength-tunable multiwavelength laser for high-rate measurement device independent quantum key distribution

Measurement device independent quantum key distribution (MDI QKD) has attracted growing attention for its immunity to attacks at the measurement unit, but its unique structure limits the secret key rate. Utilizing the wavelength division multiplexing (WDM) technique and reducing error rates are effective strategies for enhancing the secret key rate. Reducing error rates often requires active feedback control of wavelengths using precise external references. However, for a multiwavelength laser, employing multiple references to stabilize each wavelength output places stringent demands on these references and significantly increases system complexity. Here, we demonstrate a stable, wavelength-tunable multiwavelength laser with an output wavelength ranging from 1270 to 1610 nm. Through precise temperature control and stable drive current, we passively lock the laser wavelength, achieving remarkable wavelength stability. This significantly reduce the error rate, leading to an almost doubling of the secret key rate compared to previous experiments. Furthermore, the exceptional wavelength stability offered by our multiwavelength laser, combined with the WDM technique, has further boosted the secret key rate of MDI QKD. With a wide wavelength tuning range of 5.1 nm, our multiwavelength laser facilitates flexible operation across multiple dense wavelength division multiplexing channels. Coupled with high wavelength stability and multiple wavelength outputs simultaneously, this laser offers a promising solution for a high-rate MDI QKD system.

Widefield Super-Resolution Infrared Spectroscopy and Imaging of Autofluorescent Biological Materials and Photosynthetic Microorganisms Using Fluorescence Detected Photothermal Infrared (FL-PTIR).

We have demonstrated high-speed, super-resolution infrared (IR) spectroscopy and chemical imaging of autofluorescent biomaterials and organisms using camera-based widefield photothermal detection that takes advantage of temperature-dependent modulations of autofluorescent emission. A variety of biological materials and photosynthetic organisms exhibit strong autofluorescence emission under ultraviolet excitation and the autofluorescent emission has a very strong temperature dependence, of order 1%/K. Illuminating a sample with pulses of IR light from a wavelength-tunable laser source causes periodic localized sample temperature increases that result in a corresponding transient decrease in autofluorescent emission. A low-cost light-emitting diode-based fluorescence excitation source was used in combination with a conventional fluorescence microscopy camera to detect localized variations in autofluorescent emission over a wide area as an indicator of localized IR absorption. IR absorption image stacks were acquired over a range of IR wavelengths, including the fingerprint spectral range, enabling extraction of localized IR absorption spectra. We have applied widefield fluorescence detected photothermal IR (FL-PTIR) to an analysis of autofluorescent biological materials including collagen, leaf tissue, and photosynthetic organisms including diatoms and green microalgae cells. We have also demonstrated the FL-PTIR on live microalgae in water, demonstrating the potential for label-free dynamic chemical imaging of autofluorescent cells.

Broadband tunable single-frequency Yb-doped fiber laser with dual ring subcavity and Sagnac ring filter

We demonstrate a broadband tunable ultra-narrow linewidth single-frequency (SF) Yb-doped fiber laser (YDFL). A dual ring subcavity (DRS) as a filtering component to eliminate the dense longitudinal mode inherent in YDFL and theoretically analyzed its filtering characteristics. By inserting a 0.8 m unpumped polarization-maintaining Yb-doped fiber in the Sagnac ring, a dynamic narrowband grating (DNG) based on the saturable absorption (SA) effect is constructed to ensure SF laser output over the entire wavelength tuning range. We achieve a wide wavelength tunable SF laser output in the range of ∼74 nm, covering 1021.06 nm to 1094.98 nm. The laser has an optical signal-to-noise ratio (OSNR) of more than 62.5 dB, a slope efficiency of 3.57 %, and an ultra-narrow linewidth of 622 Hz over the entire tunable range. In addition, the stability of the wavelength and power fluctuations below 0.018 nm and 0.67 dB, respectively. This tunable SF fiber laser has a compact structure, lower cost, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on.

Wavelength-Tunable Chirped Pulse Amplification System (1720 nm–1800 nm) Based on Thulium-Doped Fiber

Chirped pulse amplification (CPA) has been a commonly used methodology to obtain powerful ultrashort laser pulses ever since its first demonstration. However, wavelength-tunable CPA systems are much less common. Wavelength-tunable ultrashort and intense laser pulses are desirable in various fields such as nonlinear spectroscopy and optical parametric amplification. In this work, we report a 1720 nm–1800 nm tunable CPA system based on Tm-doped fiber. The tunable CPA system contains a seed laser, a pulse stretcher, two cascaded amplifiers and a pulse compressor. The dispersion-managed seed laser cavity emits wavelength-tunable laser pulses with pulse durations of several ps and spectral widths from 25 nm to 34 nm. After being stretched temporally to tens of ps, the laser pulses are then amplified in two-stage amplifiers and compressed in a Treacy-type compressor. At 1720 nm, the maximum average power of 126 mW is obtained with a pulse duration of 507 fs; at 1800 nm, the maximum average power of 264 mW is obtained with a pulse duration of 294 fs. The pulse repetition rates are around 22.7 MHz. We perform an analysis of the system design based on numerical simulations and go on to suggest further steps for improvement. To the best of our knowledge, this is the first demonstration of a tunable CPA system beyond 1.1 μm. Considering the specific wavelength range, this wavelength-tunable CPA system is highly desirable for biomedical imaging, sensing, and parametric amplifiers.