Multiple-wavelength Laser Research Articles

Comparative study of the multiple wavelength lasing of nitrogen ions: the role of vibrational level-dependent photoionization

We report on an optical amplification and energy threshold of the two most prominent emission lines, 391.4 and 427.8 nm, of the cavity-less lasing of nitrogen ions pumped by femtosecond laser pulses. It was found that the two transitions both show optical amplification under a low gas pressure condition, while the 391.4 nm emission is barely amplified under high gas pressure. Moreover, the 427.8 nm emission presents a significant lower pump laser energy threshold and a larger gain factor than the 391.4 nm emission. Numerical simulations based on a three-state coupling model suggest that the smaller ionization Franck–Condon factor from the ground state of N2 to the vibrational level ν = 1 in X2Σ g + state of N2+ favors the formation of population inversion corresponding to the 427.8 nm emission. Meanwhile, the competition between the strong field ionization and excitation induced by the pumping laser requires higher laser intensity to acquire the population inversion for the 391.4 nm radiation, leading to a corresponding larger energy threshold.

Multi-laser powder bed fusion of Ti6Al4V: Diode area melting utilizing low-power 450 nm diode lasers

Diode Area Melting (DAM) is an innovative approach to laser powder bed fusion. This research for the first time utilized multiple low wavelength (450 nm) and low power lasers (4 W each) to simultaneously process Ti6Al4V from a powder bed, resulting in high density samples (> 99%). The use of nine lasers perpendicular to the scanning direction allowed for the creation of melt pools with widths larger than 1000 µm, reducing the number of passes within a layer compared to traditional single laser LPBF systems. However, the increase in melt pool width creates challenges related to maintaining uniform melt pool penetration depths, this intern creates issues achieving high density in multi-layer parts. A parameter map based on normalized energy density was created. The research also investigated the effect of variable process conditions (scanning speed, hatch distance, and layer height), normalized energy density on densification, microstructure, and hardness. Implementing a rescanning strategy on each layer reduced average surface roughness and increased density while reducing hardness by approximately 40%. X-ray Diffraction analyses showed that the β phase occurred with increased energy density in as-built samples. Coarser lamellar structures were observed in the lower sections due to slower cooling rates, while finer, more martensite structures were observed towards the top, resulting in a basket-weave type microstructure throughout the sample. Furthermore, the lamellar sub-grains were visible without etching. This innovative technology shows that an array of low-power lasers can melt and fuse Ti6Al4V powder to create dense samples.

A Single NIR-II Laser-Triggered Self-Enhancing Photo/Enzyme-coupled Three-in-One Nanosystems for Breast Cancer Phototheranostics.

Multifunctional phototheranostics, employing precise and non-invasive techniques, is widely developed to enhance theranostic efficiency of breast cancer (BC), reduce side-effects, and improve quality of life. Integrating all phototheranostic modalities into a single photosensitizer for highly effective BC treatment is particularly challenging due to the potential inefficiency and time consumption associated with repeated switching of multiple-wavelength lasers. Herein, a novel single NIR-II laser-triggered three-in-one nanosystem(PdCu NY) is rationally designed, which enables dual-modal (NIR-II FL/NIR-II PA) imaging-guided self-enhancing photothermal-photodynamic therapy (PTT-PDT) in NIR-II window. The PdCu NY based on optimal Pd/Cu molar-ratio(1:11) can be easily fabricated and large-scale production for simultaneous PTT-PDT against BC under a single 1064nm laser irradiation. Significantly, the PdCu NY acted as a promising photocatalyst for decomposition of H2O into O2 upon the same laser irradiation. In addition, the inherent catalase (CAT)-like activity of PdCu NYs enables photo-enzyme dual-catalytic O2 supply to effectively alleviate hypoxia, achieving self-enhanced PDT efficiency. These PTT-PDT self-enhanced nanosystems demonstrate precise lesion localization and complete tumor ablation using a single 1064nm laser source by "one-laser, multi-functions" strategy. More importantly, this study not only reports a three-in-one PdCu-based phototheranostic agent, but also sheds light on the exploration of versatile biosafety nanosystems for clinical applications.

Terabit FSO communication based on a soliton microcomb

Free-space optical (FSO) communication technology is a promising approach to establish a secure wireless link, which has the advantages of excellent directionality, large bandwidth, multiple services, low mass and less power requirements, and easy and fast deployments. Increasing the communication capacity is the perennial goal in both scientific and engineer communities. In this paper, we experimentally demonstrate a Tbit/s parallel FSO communication system using a soliton microcomb as a multiple wavelength laser source. Two communication terminals are installed in two buildings with a straight-line distance of ∼ 1 km . 102 comb lines are modulated by 10 Gbit/s differential phase-shift keying signals and demodulated using a delay-line interferometer. When the transmitted optical power is amplified to 19.8 dBm, 42 optical channels have optical signal-to-noise ratios higher than 27 dB and bit error rates less than 1 × 10 − 9 . Our experiment shows the feasibility of a wavelength-division multiplexing FSO communication system which suits the ultra-high-speed wireless transmission application scenarios in future satellite-based communications, disaster recovery, defense, last mile problems in networks and remote sensing, and so on.

Self-cascaded noncritically-phase-matched KTP optical parametric oscillator for generating orthogonally polarized multi-wavelength emissions at 3 μm

Mid-infrared self-cascaded noncritically-phase-matched (NCPM) KTP optical parametric oscillator (OPO) was developed, generating multiple wavelengths at 2.95, 3.29 and 3.37 μm. The polarization state of 2.95 μm is orthogonal to that of the other two wavelengths. Under 1064 nm intracavity pumping, the first NCPM KTP-OPO can generate signal and idler waves at 1.57 and 3.29 μm, respectively. Then the 1.57 μm is further used as pump light for the second NCPM OPO in the same KTP, generating signal and idler waves at 2.95 and 3.37 μm. Broadband spectral feedback condition is established between the OPO cavity mirrors, enabling the two NCPM OPOs to operate simultaneously in the same KTP and OPO resonator. With regard to 2.95 μm, the achieved maximum average output power and minimum pulse width are respectively 1.18 W and 5 ns, corresponding to a pulse peak power of 56.9 kW with 4.2 kHz repetition rate. As for the dual-wavelength at 3.29 and 3.37 μm, the maximum total average output power is 0.22 W. Our work provides an effective scheme to obtain multiple wavelength laser output in 3 μm region with orthogonal polarization state.

Frequency stabilization of multiple wavelength lasers based on a broadband spectrum

We report on frequency stabilization of multiple wavelength lasers operating at 1389 and 1695 nm simultaneously on a broadband spectrum. These lasers are implemented in ytterbium optical lattice clock experiments, which need to have a narrow enough linewidth and maintain high long-term frequency stability. A 1560 nm femtosecond mode-locked laser with a narrow mode spacing of 250 MHz is used as a master laser, which is referenced to a local ultrastable optical cavity with the instability better than 1 × 10−15 at 1 s averaging time. Through the combination of erbium-doped fiber amplifier and high nonlinear fiber, the spectral width of the maser laser is broadened from 10 nm to more than 300 nm. The range of the broadened spectrum can cover 1389 and 1695 nm. Meanwhile, the spectral intensity at the corresponding wavelength can ensure that the signal-to-noise ratio of the beat signals between the two lasers and the broadened spectrum is about 30 dB at a resolution bandwidth (RBW) of 100 kHz. After phase locking the 1389 and 1695 nm lasers on the broadband spectrum, the residual linewidths are obtained to be about 0.8 Hz at 1 Hz RBW, and the stabilities are 3.5 × 10−16 and 4.7 × 10−16 at 1 s averaging time respectively, improving about six orders of magnitude. Our result can be conducive to obtaining the stabilized laser sources for the atomic optical clock, and will be of great significance for simplifying and miniaturizing the optical clock system.

Using blend wavelengths in order to improve the safety and efficacy of laser hair removal.

As the pursuit for a safe and effective device for laser hair removal continues, the use of simultaneous multiple wavelengths in a single device requires further exploration. To evaluate the safety and efficacy of a novel multi-wavelength laser device for hair removal. This retrospective cohort study included adult participants of both sexes with Fitzpatrick skin types of III and IV. Hairy sites were treated by a multiple wavelength (810nm, 940nm, and 1064nm) laser device (Primelase, Coccoon medical, Barcelona, Spain). Laser parameters included: fluence of 14-20J/cm2 , pulse duration of 7-30ms, and spot size of 20*9mm2 . Participants underwent up to 7 treatments at 6-8weeks intervals and were followed for 6months after the last treatment session. "Before" and "after" clinical photographs were acquired and were used to evaluate efficacy by 2 independent dermatologists. They employed the Global Aesthetic Improvement Scale (GAIS; scale of 0 [no improvement] to 4 [excellent improvement; Over 75% hair reduction]). Participants' satisfaction was rated on a scale of 1 (not satisfied) to 5 (very satisfied). Pain perception and adverse events were recorded as well. Eighteen participants (6men, 12 women) were included with a total of 49 treatment sites. Mean hair reduction was 3.6 out of 4 in the GAIS. Participants' satisfaction rate was high (mean 4.5). Beside mild transient discomfort during the procedure, no adverse events were recorded. The use of a multiple wavelengths' laser device is safe and effective for hair removal.

Widely interval-adjustable multiwavelength erbium-ytterbium doped fiber laser based on micro-air cavity

We propose a widely interval-adjustable multiwavelength erbium-ytterbium doped fiber laser (EYDFL) based on a micro-air cavity (MAC). The fabricated MAC functions as a selective channel filter for multiwavelength generation with adjustable wavelength interval. By adjusting the MAC length from 0.10 to 2.40 mm, the wavelength interval can be continuously tuned from 8.61 to 0.47 nm, respectively. When the MAC length is set at 2.40 mm and erbium-ytterbium doped fiber amplifier at 300 mW, 12 multiple wavelength lasers with 0.47 nm spacing are obtained. The generated channels have an average optical signal-to-noise ratio of 40 dB and an average peak power of −35 dBm. The proposed laser introduces the advantages of a relatively simple structure and ease of operation at any wavelength band for potential applications including optical communication systems and sensing.

Coherent control of the multiple wavelength lasing of ${\rm N}_2^ +$: coherence transfer and beyond

Nitrogen molecules pumped by intense femtosecond laser pulses give rise to coherent emission in the forward direction at a series of wavelengths, coined “air lasing.” We demonstrate the coherent control of these emissions via a pair of seeding pulses at two different frequencies at low pressures, revealing a coherence transfer through vibrational motion. It is found that the injection of a 427.8 nm (391.4 nm) seeding pulse results in its amplification at the expense of the 391.4 nm (427.8 nm) signal, demonstrating a competition between the two spectral components of the emission from the upper level population. Moreover, when the delay between the seeding and pump pulses is finely tuned, the coherent control of both transitions is observed via the coherence transfer. A microscopic molecular relaxation model reproduces these observations, highlighting the crucial role of electronic and vibrational coherences, as well as their coupling, during the lasing action.

Time-Resolved NIROT 'Pioneer' System for Imaging Oxygenation of the Preterm Brain: Preliminary Results.

In preterm infants, there is a risk of long-term cognitive, motor and behavioral impairments due to hemorrhagic and/or ischemic lesions. If detected early, lesions can be prevented. A bedside imaging modality, capable of early detection of both disorders, is necessary. We present the state of development of a tomographic imager (named Pioneer), that will be capable of determining the oxygenation of the preterm-infant brain with high spatial resolution. Pioneer is a time-resolved near-infrared optical tomography (TR NIROT) instrument. It employs multiple wavelength laser light in short pulses on 11 distinct locations and measures the re-emerging light in a contactless fashion by means of a time-correlated single-photon counting (TCSPC) camera (named Piccolo) covering ~4.9 cm2 with 300 detectors. Timing response of the entire system is 116 ps. An in-house designed biocompatible source ring ensures fixed relative positions of sources and detectors and provides a secure interface between the patient and the probe. At the present state, the NIROT Pioneer system successfully detected a 6x6x50 mm3 inclusion 3 cm deep inside a phantom. These results confirm that the Pioneer imager is working as expected and is on a solid path towards full 3D tissue oxygenation imaging.

Single and Multiple Longitudinal Wavelength Generation in Green Diode Lasers

Single and multiple wavelength laser systems are presented that employ self-injection locked InGaN/GaN green laser diodes in an external cavity configuration with a partially reflective mirror. A stable and simultaneous locking of up to four longitudinal Fabry–Perot modes of the system cavity is demonstrated with appreciable signal-to-noise-ratio of ∼13 dB and average mode linewidth of ∼150 pm. The multi-wavelength spectrum exhibited a flat-top emission with nearly equal power distribution among the modes and an analogous mode spacing of ∼0.5 nm. This first demonstration of multi-wavelength generation source is highly attractive in a multitude of cross-disciplinary field applications besides asserting the prospects of narrow wavelength spaced multiplexed visible light communication. Moreover, an extended two-stage self-injection locked near single wavelength visible laser system is also presented. An ultra-narrow linewidth of ∼34 pm is realized at 525.05 nm locked wavelength from this innovative system, with ∼20 dB side-mode-suppression-ratio; thus signifying a paradigm shift toward semiconductor lasers for near single lasing wavelength generation, which is presently dominated by other kinds of laser technologies.

Flexible distributed feedback lasers based on nanoimprinted cellulose diacetate with efficient multiple wavelength lasing

Here we present the assembly of novel transparent all-polymer distributed feedback (DFB) lasers. Flexible and highly transparent cellulose diacetate (CdA) was employed as substrate on which gratings with different periods were engraved by thermal nanoimprinting with high fidelity. Highly luminescent conjugated polymers (CP), poly (9,9-dioctylfluorene) (PFO), poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), and a blend of F8BT and poly(3-hexylthiophene)-poly(9,9-dioctylfluorene-alt-benzothiadiazole) (P3HT:F8BT) were deposited by spin coating onto the nanostructured plastic surfaces, giving rise to perpendicular single-mode lasing emission in the blue, green, and red wavelength ranges, respectively. These lasers show linewidths below 1 nm and low thresholds (≈6 μJcm−2 for blue and red lasing emission), comparable to other state-of-the-art lasers obtained from similar optical gain materials on rigid substrates. The followed strategy is scalable and versatile, enabling the development of large area nanoimprinted DFB lasers (>1cm2) on plastic, which is highly relevant for applications in various markets.

Safety and short-term efficacy of telangiectasia treatment by means of an innovative combination of 532 and 808 nm transdermal diode laser.

Objectives To evaluate the safety and efficacy of an innovative transdermal multiple wavelength diode laser in telangiectasia treatment. Methods A total of 94 females underwent treatment by means of a multiple wavelength laser. All the cases were treated by three cycles of 532 nm combined with simultaneous 808 nm pulse. OFF time between the cycles was 10 ms. The handling physician, the patient and a blind assessor scored the aesthetic outcome in a range from 0 (no change) to 10 (100% disappearance). Pain was scored from 0 (no pain) to 10 (most painful experience ever) by the patient. Results At 5 ± 1 months, depigmentation was observed in 3/94 cases (3.2%), hyperpigmentation in (4/94 4.2%), transient erythema in 14/94 (14.9%), transient oedema in 11/94 (11.7%), matting in 1/94 (1.1%). Mean aesthetic outcome was scored as 7.9 ± 1.3 by the physician, 7.7 ± 1 by the independent assessor and 7.2 ± 2.4 by the patient. Patients reported a mean peri-procedural pain of 3.4 ± 1.5. Conclusions The simultaneous application of 532 nm and 808 nm laser emission is safe and effective in 0.1–1 mm telangiectasia treatment. The present investigation provides data for a deeper insight into the biology of the tissue photo-modulation.

Theoretical calculation and experimental investigation of speckle reduction by multiple wavelength lasers in laser projector with different angular diversities

It is well known that speckle reduction can be achieved by using multiple wavelength lasers and angular diversity in laser projectors. However, few studies have been reported about the dependency between the speckle reduction by angular diversity and speckle reduction using multiple wavelength lasers. Here, the speckle reduction using 1–5 lasers with different wavelengths was measured on matte and silver screens. The image size was chosen from two sizes to change the angular diversity. The results were compared with the theoretical calculation. It was shown that speckle reduction using multiple wavelength lasers depends on the image size and screens. On a silver screen, the amount of speckle reduction by wavelength diversity depends on the image size; wavelength interval of two lasers needed to reduce speckle was smaller when the image was larger. On the other hand, on matte screen, speckle reduction by wavelength diversity does not depend on the image size. This is because effective surface roughness of matte screen is larger than that of the silver screen due to multiple or volume scattering. In addition to wavelength diversity, the difference of speckle reduction between matte and silver screens is caused by polarization diversity. The speckle contrast decreases by polarization diversity by a factor of 2 on matte screen, whereas it does not decrease on silver screen because silver screen preserves polarization. Speckle contrast was calculated by using illumination light property, screen property, and speckle measurement camera settings. The experimental results were in a good agreement with the theoretical calculation.

Operating Characteristics of High-Order Distributed Feedback Polymer Lasers.

In this study, high-order distributed-feedback (DFB) polymer lasers were comparatively investigated. Their performance relies on multiple lasing directions and their advantages include their high manufacturing tolerances due to the large grating periods. Nine laser cavities were fabricated by spin-coating the gain polymer films onto a grating structure, which was manufactured via interference lithography that operated at the 2nd, 3rd, and 4th DFB orders. Low threshold lasing and high slope efficiency were achieved in high-order DFB polymer lasers due to the large grating groove depth and the large gain layer thickness. A high-order DFB configuration shows possible advantages, including the ability to control the lasing direction and to achieve multiple-wavelength lasers. Furthermore, our investigation demonstrates that the increase in threshold and decrease in slope efficiency with an increase in the feedback order can be limited by controlling the structural parameters.

A Highly-Efficient Single Segment White Random Laser.

Production of multicolor or multiple wavelength lasers over the full visible-color spectrum from a single chip device has widespread applications, such as superbright solid-state lighting, color laser displays, light-based version of Wi-Fi (Li-Fi), and bioimaging, etc. However, designing such lasing devices remains a challenging issue owing to the material requirements for producing multicolor emissions and sophisticated design for producing laser action. Here we demonstrate a simple design and highly efficient single segment white random laser based on solution-processed NaYF4:Yb/Er/Tm@NaYF4:Eu core-shell nanoparticles assisted by Au/MoO3 multilayer hyperbolic meta-materials. The multicolor lasing emitted from core-shell nanoparticles covering the red, green, and blue, simultaneously, can be greatly enhanced by the high photonic density of states with a suitable design of hyperbolic meta-materials, which enables decreasing the energy consumption of photon propagation. As a result, the energy upconversion emission is enhanced by ∼50 times with a drastic reduction of the lasing threshold. The multiple scatterings arising from the inherent nature of the disordered nanoparticle matrix provide a convenient way for the formation of closed feedback loops, which is beneficial for the coherent laser action. The experimental results were supported by the electromagnetic simulations derived from the finite-difference time-domain (FDTD) method. The approach shown here can greatly simplify the design of laser structures with color-tunable emissions, which can be extended to many other material systems. Together with the characteristics of angle free laser action, our device provides a promising solution toward the realization of many laser-based practical applications.

A novel high-sensitive upconversion thermometry strategy: Utilizing synergistic effect of dual-wavelength lasers excitation to manipulate electron thermal distribution

Conventional upconversion thermometry strategy, based on the thermally coupled levels (TCL) of lanthanide ions, confronts a dilemma in simultaneously achieving high absolute/relative temperature (T) sensitivities (Sa and Sr) and good signal discriminability. Herein, a novel thermometry strategy by utilizing the synergistic effect of dual-wavelength lasers to manipulate electron thermal distribution is proposed to go beyond the limitation of the TCL-based strategy. In the case of NaGdF4: 20%Yb3+, 2%Er3+ nanoparticles upon 980 & 1530 nm dual-excitation, it is found that more efficient utilization of excited photons results in a higher electron concentration in ladder-like energy levels of Er3+, owing to the diverse pumping routes, which increases transition rate of the T-dependent phonon-assisted cross-relaxation process, and in turn establishes a thermally-sensitive electronic connection between Er3+: 2H11/2,4S3/2 and Er3+: 4F9/2. Remarkably, not only a highest record value of Sa (0.0365 K−1) for the Er3+ doped host materials in the physiological temperature range (303–343 K) is achieved, but also a high Sr (1.29% K-1) and an excellent signal discriminability (Δλ = 112 nm) are obtained. Combining a much intensified upconversion (UC) signal and a good size/shape homogeneity in nanometer scale, the investigated NaGdF4: 20%Yb3+, 2%Er3+ upon dual-wavelength excitation is potentially applicable in the intracellular thermal sensing and imaging. This work exploits an effective way to develop high-performance T-sensors, and the proposed thermometry strategy can be extended to surges of the other lanthanide ions doped systems pumped by multiple-wavelength lasers.

Multiple-wavelength range-gated active imaging principle in the accumulation mode for three-dimensional imaging.

Having laid the foundations of the multiple-wavelength range-gated active imaging principle in flash mode in a previous paper, we have been studying its use in accumulation mode. Whatever the mode, the principle consists of restoring the 3D scene directly in a single image at the moment of recording with a camera. Each emitted light pulse with a different wavelength corresponds to a visualized zone with a different distance in the scene. So each of these visualized zones is identified by a different wavelength. In flash mode, the camera shutter opens just once during the emission of light pulses with the different wavelengths. However, the energy constraints to restore scenes in three dimensions can lead to a change in the recording mode when moving from the flash mode to the accumulation mode. In this mode, the cycle, including a series of light pulses with the used wavelengths and an aperture of the camera shutter, is repeated several times for a given image recorded with the intensified camera. Each wavelength always corresponds to a visualized slice with a different distance in the scene. So, the accumulation enables increasing the illumination of every visualized slice. The modeling conducted in the previous paper must be completed to adapt it to this mode. The tests with a multiple-wavelength laser source confirmed the quality improvement of the recorded images for more remote scenes and validated the principle of restoring, directly in a color image, the three dimensions of a scene.

Low-Cost CWDM Transceiver Module by Spatial Power Combination and Division for Multimode Fiber Links

A cost-effective coarse wavelength division multiplexing optical sub-assembly (OSA) for short-distance optical interconnection is proposed and fabricated. Filter-free transmitter multiplexing is demonstrated by a spatial power combination of multiple-wavelength laser outputs into a fiber. As a result, the OSA is constructed with a reduced burden in optical alignment and packaging owing to the small number of optical elements and large alignment tolerance. Data transmission with a bit rate of 6 Gb/s is achieved through a 100-m-long multimode fiber. No significant degradation of eye patterns is observed during the transmission, and a bit error rate below $10^{{\mathbf {-12}}}$ is attained with the proposed module.

Direct method of three-dimensional imaging using the multiple-wavelength range-gated active imaging principle.

The tomography executed with mono-wavelength active imaging systems uses the recording of several images to restore a three-dimensional (3D) scene. Thus, in order to show the depth in the scene, a different color is attributed to each recorded image. Therefore, the 3D restoration depends on the video frame rate of the camera. By using a multiple-wavelength range-gated active imaging system, it is possible to restore the 3D scene directly in a single image at the moment of recording with a video camera. Each emitted light pulse with a different wavelength corresponds to a visualized zone at a different distance in the scene. The camera shutter opens just once during the emission of light pulses with the different wavelengths. Thus, the restoration can be executed in real time with regard to the video frame rate of the camera. From an analytical model and from a graphical approach, we demonstrated the feasibility of this new method of 3D restoration. The non-overlapping conditions between two consecutive visualized zones are analyzed. The experimental test results confirm these different conditions and validate the theoretical principle to directly restore the 3D scene in a color image with a multiple-wavelength laser source, an RGB filter, and a triggerable intensified camera.