Laser Spectrum Research Articles

Realizing high injection current density up to 200 kA-cm−2 in electrically pumped AlGaN ultraviolet laser diodes on c-Sapphire substrate

Previously, we attempted to the optically-pumped AlGaN ultraviolet (UV) laser diodes (LDs) on AlN template on c-Sapphire substrate, using low-cost epitaxial growth technique in low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE). As a result, stimulated lasing spectra at 274 nm were observed for the first time at room temperature (RT) in the optically pumped LD. However, the electrically-pumped AlGaN-based (270–298 nm)-band UV LD was quite challenging due to both the issue of low injection current density and light confinement factor (Г). In this paper, the injection current density in the electrically pumped AlGaN UV LD at 283 nm emission was remarkably improved up to 200 kA-cm−2 by using undoped Al-graded cladding and Al-graded Mg-doped p-AlGaN hole source layer (HSL). Also, some weak oscillation at 296 nm near to the main spontaneous emission peak of 298 nm-band LD under the 600 mA (24 V) current using pulse width of 50 ns at RT was observed. To further improve the lasing phenomenon, some theoretical studies was conducted, by making the p-side waveguide (p-WG) layer non-doped (Mg = 0) and also increasing the quantum-well (QW) thickness from 2 nm to 4 nm in the multiquantum-wells (MQWs). Consequently, the optical-loss was significantly reduced from 22.7 to 8.7 cm−1 and the Г was enhanced from 1.77% (Experimental LD) to 4.65% (simulated LD).

Spectro-temporal evolution of mode-locked lasing in fiber frequency-shifted feedback laser.

As a type of mode-locked fiber laser, fiber frequency-shifted feedback lasers (FSFLs) have been rarely studied regarding the buildup process of mode locking. We carried out an experimental investigation to show the unique characteristics of the process, such as the direct evolution of the Q switched mode-locked state to the mode-locked state, the unique phenomenon of spectral center frequency oscillation, evolution to a double-pulse state in harmonic mode locking, millisecond stabilization time, and exponentially decreasing change of relative phase per round trip. For the first time, to the best of our knowledge, the double-beam heterodyne method is applied to measure the evolution of laser spectra with pulse width in the nanosecond range. Understanding the unique buildup mechanism may aid in the engineering and application of FSFLs.

Comment on “A study of the longitudinal laser modes of a semiconductor laser using optical coherence tomography” [Am. J. Phys. 75, 569–571 (2006)

Although the longitudinal mode spacing and longitudinal mode widths of a typical laser diode can, in principle, be determined from an optical coherence tomography signal, the values presented by Poddar et al. [Am. J. Phys. 75, 569–571 (2006)] do not agree with the Fourier transform theory. Also, the mode spacing is inconsistent with the peak separation in the interference signal. Moreover, the laser cavity mirror reflectivity calculated from the laser spectrum is incorrect because it ignores the gain and the loss within the laser cavity. This Comment aims to help readers who may struggle to understand that paper or to reproduce its results.

Time-Domain Photothermal AFM Spectroscopy via Femtosecond Pulse Shaping

A time-domain version of photothermal microscopy using an atomic force microscope (AFM) is reported, which we call Fourier transform photothermal (FTPT) spectroscopy, where the delay between two laser pulses is varied and the Fourier transform is computed. An acousto-optic modulator-based pulse shaper sets the delay and phases of the pulses shot-to-shot at 100 kHz, enabling background subtraction and data collection in the rotating frame. The pulse shaper is also used to flatten the pulse spectrum, thereby eliminating the need for normalization by the laser spectrum. We demonstrate the method on 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) microcrystals and Mn-phthalocyanine islands, confirming subdiffraction spatial resolution, and providing new spectroscopic insights likely linked to structural defects in the crystals.

Mode-locking fiber laser with dual wavelength continuous-waves-induced resonant spectral sidebands.

The optical spectrum of mode-locked lasers can exhibit multiple peaks resulting from different mechanisms such as modulation instability, dispersive waves (DWs), and coupling between continuous waves (CWs) and DWs. The latter was recently reported in a mode-locked fiber laser. Here we show that besides the coupling between single-wavelength CW and DWs, dual-wavelength CWs can also couple with DWs giving rise to quite different spectral peaks in a mode-locked fiber laser. In particular, we find that the sidebands of one CW can couple with the other CW, leading to an enhancement of the CWs.

In vivo tracking of individual stem cells labeled with nanowire lasers using multimodality imaging.

Emerging cell-based regenerative medicine and stem cell therapies have drawn wide attention in medical research and clinical practice to treat tissue damage and numerous incurable diseases. In vivo observation of the distribution, migration, and development of the transplanted cells is important for both understanding the mechanism and evaluating the treatment efficacy and safety. However, tracking the 3D migration trajectories for individual therapeutic cells in clinically relevant pathological environments remains technically challenging. Using a laser photocoagulation model in living rabbit eyes, this study demonstrates a multimodality imaging technology integrating optical coherence tomography (OCT), fluorescence microscopy (FM), and lasing emission for in vivo longitudinal tracking of the 3D migration trajectories of individual human retinal pigment epithelium cells (ARPE-19) labeled with CdS nanowires. With unique lasing spectra generated from the subtle microcavity differences, the surface-modified nanowires perform as distinct spectral identifiers for labeling individual ARPE-19 cells. Meanwhile, with strong optical scattering and natural fluorescence emission, CdS nanowires also served as OCT and FM contrast agents to indicate the spatial locations of the transplanted ARPE-19 cells. A longitudinal study of tracking individual ARPE-19 cells in rabbit eyes over a duration of 28 days was accomplished. This method could potentially promote an understanding of the pharmacodynamics and pharmacokinetics of implanted cells in the development of cell-based therapies.

Laser Spectroscopic Characterization for the Rapid Detection of Nutrients along with CN Molecular Emission Band in Plant-Biochar

We report a quantitative analysis of various plant-biochar samples (S1, S2 and S3) by utilizing a laser-induced breakdown spectroscopy (LIBS) technique. For LIBS analysis, laser-induced microplasma was generated on the target surface by using a focused beam through a high-power Nd: YAG laser and optical emission spectra were recorded using a charged coupled device (CCD) array spectrometer, with wavelength ranges from 200 nm to 720 nm. The spectroscopical analysis showed the existence of various ingredients, including H, Li, Ca, Na, Al, Zn, Mg, Sr, Si, and Fe, along with a CN molecular emission band due to B2Σ+ − X2Σ+ electronic transition. By assuming conditions of the plasma is optically thin and in LTE, calibration-free laser-induced breakdown spectroscopy (CF-LIBS) was utilized for the compositional analysis of the ingredients present in the three plant-biochar samples. To lower the uncertainties, we used an average composition (%) of the three plant-biochar samples. The quantitative study of the plant-biochar samples was also achieved using the energy dispersive X-ray (EDX) technique, showing good agreement with the CF-LIBS technique. In addition, statistical analysis, such as principal component analysis (PCA), was performed for the clustering and classification of the three plant-biochar samples. The first three PCs explained an overall ~91% of the variation in LIBS spectral data, including PC1 (58.71%), PC2 (20.9%), and PC3 (11.4%). These findings suggest that LIBS is a robust tool for rapid measurement of heavy as well as light elements, such as H, Li, and nutritional metals in plant-biochar samples.

Mid-Infrared Serial Microring Resonator Array for Real-Time Detection of Vapor-Phase Volatile Organic Compounds.

Chip-scale infrared spectrometers consisting of a microring resonator array (MRA) were developed for volatile organic compound (VOC) detection. The MRA is serially positioned to serve as a wavelength sorting element that enables wavelength demultiplexing. Unlike conventional devices operated by a single microring, our MRA can perform multiwavelength mid-infrared (mid-IR) sensing by routing the resonant wavelength light from a broadband mid-IR source into different sensing channels. Miniaturized spectrometer devices were fabricated on mid-IR transparent silicon-rich silicon nitride (SiNx) thin films through complementary metal-oxide-semiconductor (CMOS) processes, thus enabling wafer-level manufacturing and packaging. The spectral distribution of the resonance lines and the optimization of the microring structures were designed using finite-difference time-domain (FDTD) modeling and then verified by laser spectrum scanning. Using small microring structures, the spectrum showed a large free spectral range (FSR) of 100 nm and held four spectral channels without crosstalk. Unlike near-infrared microrings using refractive index sensing, our MRA can detect hexane and ethanol vapor pulses by monitoring the intensity variation at their characteristic mid-IR absorption bands, thus providing high specificity. Applying multiwavelength detection, the sensor module can discriminate among various VOC vapors. Hence, our mid-IR MRA could be an essential component to achieve a compact spectroscopic sensing module that has the potential for applications such as remote environmental monitoring and portable health care devices.

Investigation on the spectrum smoothing effect in an ultrafast Tm-doped fiber chirped pulse amplifier

We investigate the spectrum smoothing effect of the amplified soliton pulse in an ultrafast Tm-doped fiber chirped pulse amplifier, in which the soliton pulse is amplified to 7.3 W with a pulse duration of ∼350 fs. The experimental investigation reveals that the spectrum smoothing effect arises from the polarization filtering effect of the grating-based pulse compressor, which is further confirmed by a theoretical simulation. This effect not only smooths the amplified laser spectrum, but also improves the pulse contrast ratio.

Study of the elastically deformed state of thin diamond plates

The development of laboratory technologies for growing high-quality diamond single crystals, as well as low thermal expansion of a diamond, make it possible to consider this material promising as an element of X-ray optics in designing free electron lasers (XFEL). Diamond crystal-spectrometers of various thicknesses and bending radii are also in demand. In this regard, the issues regarding the mechanical parameters of elastically deformed diamond single crystals require clarification, among them critical stresses in a deformed single crystal and the minimum bending radii for plates of certain geometry. The goal of the study is determination of the elastically deformed state of thin diamond plates with the parameters required in spectrometers for non-invasive diagnostics of X-ray free-electron laser (XFEL) spectra. The samples were cut from the IIa -type crystal of the highest quality grown by the temperature gradient method. Diamond plates with (110) and (111) crystallographic orientations were used in the experiments. The dependences of the stresses and bending radii on deformation value were obtained during bending thin diamond plates with a thickness of 20 μm. The experimental deformations did not exceed 1 mm. The minimum bending radii of thin diamond plates were also determined: for (111) direction — 5.6 mm, and for (110) direction — 4.5 mm. The Young’s moduli were 1198 GPa for (111) direction and 1034 GPa for (110) direction. Critical stresses during bending of thin diamond single crystals (resulting in their destruction) exceeded 2.4 GPa. The dependence of the bending radii of crystals on their thickness was calculated at a stress value of 2.0 GPa. The results of the study make it possible to calculate the allowable deformations for thin diamond plates of arbitrary shape and thickness. The data obtained will contribute to the improvement of modeling and the quality of production of curved spectrometers.

Electrically Tunable Polymer Whispering-Gallery-Mode Laser.

Microlasers hold great promise for the development of photonics and optoelectronics. At present, tunable microcavity lasers, especially regarding in situ dynamic tuning, are still the focus of research. In this study, we combined a 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) piezoelectric crystal with a Poly [9,9-dioctylfluorenyl-2,7-diyl] (PFO) microring cavity to realize a high-quality, electrically tunable, whispering-gallery-mode (WGM) laser. The dependence of the laser properties on the diameter of the microrings, including the laser spectrum and quality (Q) value, was investigated. It was found that with an increase in microring diameter, the laser emission redshifted, and the Q value increased. In addition, the device effectively achieved a blueshift under an applied electric field, and the wavelength tuning range was 0.71 nm. This work provides a method for in situ dynamic spectral modulation of microcavity lasers, and is expected to provide inspiration for the application of integrated photonics technology.

Effect of lens-to-sample distance on spatial uniformity and emission spectrum of flat-top laser-induced plasma

The optimal spectral excitation and acquisition scheme is explored by studying the effect of the lens-to-sample distance (LTSD) on the spatial homogeneity and emission spectra of flat-top laser converging spot induced plasma. The energy distribution characteristics before and after the convergence of the laser beam with quasi flat-top intensity profile used in this study are theoretically simulated and experimentally measured. For an aspheric converging mirror with a focal length of 100 mm, the LTSD (106 mm ≥ LTSD ≥ 96 mm) was changed by raising the stainless-steel sample height. The plasma images acquired by ICCD show that there is air breakdown when the sample is below the focal point, and a ring-like plasma is produced when the sample is above the focal point. When the sample is located near the focal point, the plasma shape resembles a hemisphere. Since the spectral acquisition region is confined to the plasma core and the image contains all the optical information of the plasma, it has a lower relative standard deviation (RSD) than the spectral lines. When the sample surface is slightly higher than the focal plane of the lens, the converging spot has a quasi flat-top distribution, the spatial distribution of the plasma is more uniform, and the spectral signal is more stable. Simultaneously, there is little difference between the RSD of the plasma image and the laser energy. In order to further improve the stability of the spectral signal, it is necessary to expand the spectral acquisition area.

Change in Central Macular Thickness on OCT after Pan Retinal Photocoagulation

Background: Proliferative diabetic retinopathy is the proliferation of retinal neo vessels due to uncontrolled diabetes mellitus and long duration of this disease that leads to advance diabetic eye disease . proliferative diabetic retinopathy is treated by pan retinal photocoagulation which reduces retinal oxygen demand by retinal photocoagulation. Retinal photocoagulation leads to the laser tissue interaction called thermal damage leading to coagulative necrosis of the retinal pigment epithelial cells as these cells contain melanin a dark pigment that has the ability to absorb light energy in argon laser spectrum. Objective: To determine the mean change in central macular thickness after pan retinal photocoagulation on optical coherence tomography of the macula in patients with proliferative diabetic retinopathy . Material & Methods: Our study was a Quasi experimental study conducted at Department of Ophthalmology, Services Hospital Lahore for duration of six months from April 2021 to September 2021. In this study 40 eyes from 40 patients with proliferative diabetic retinopathy were selected who underwent pan retinal photocoagulation in one sessions with 1500 burn marks. Patients in the age group of 15 to 70 years, male and female, with proliferative diabetic retinopathy in at least one eye with no history of ocular surgery or ocular trauma in last one year were included in this study . Optical coherence tomography of macula was performed before and four weeks after the session of pan retinal photocoagulation. CMT (central macular thickness) was measured for mean change in central macular thickness. Results: In this study , the mean age of patients was 48.68±1.52 years, and gender ratio of female to male patients was 1:0.77,The mean CMT before start of treatment that was considered as baseline was 230.55±5.0 and the mean CMT value after four weeks of pan retinal photocoagulation was 238.50±6.43. (p-value=0.001) Conclusion: No significant change in CMT was found from baseline CMT to four weeks after pan retinal photocoagulation. Keywords: Optical coherence tomography, pan retinal photocoagulation, Central Macular Thickness, proliferative diabetic retinopathy

Planar Waveguide-Based Fiber Spectrum Analyzer Mountable to Commercial Camera

We present the design of a planar spectrometer that separates the wavelength channels from an input fiber and focuses the spectral lines onto a camera without any free-space optical elements. The geometric arrangements of the waveguides to achieve different spectroscopic parameters are explained in detail, allowing adjustable focal lengths, high spectral resolution, and broad free spectral range. The optical chip is fabricated on a low-cost polymer platform as proof of concept. The optical spectrum of a multiwavelength laser is measured by the proposed device, and the result is in good agreement with a commercial optical spectrum analyzer. The large focal depth of the chip allows an optical assembly of much relaxed alignment accuracy. We demonstrate a tube design to encapsulate the chip fixed with the input fiber. The assembly is then mounted to a commercial camera with standard C-mount threading as a convenient fiber spectrum analyzer without customized detectors and circuits. Our design may provide a low-cost and versatile solution for the development of compact spectroscopic equipment.

Селектор импульсов для получения фемтосекундного излучения с управляемой фазой несущей относительно огибающей

In non-linear optical processes, such as obtaining attosecond pulses, it is extremely important to control the carrier envelope phase. To do this, various periodic trains of identical femtosecond pulses with a controlled phase can be created. In addition, since there is no frequency comb offset in such sequences, the process of measuring optical frequencies is greatly simplified. The pulse selector has been developed to obtain a sequence of identical femtosecond pulses with a controlled carrier – envelope phase. The selector makes it possible to obtain a “pure” sequence of identical femtosecond pulses at the modulator output (when every 125th pulse is selected from the original sequence with a repetition rate of 250 MHz) in a fairly wide range of control signal phase tuning. This range is 1.3 degrees. The phase tuning of the pulse selector provides the possibility of obtaining one hundred twenty-five such sequences with a phase tuning discreteness of 2π/125. The simplest way to reduce discreteness is to increase the ratio of the pulse repetition rate of a femtosecond laser to the shift of its frequency comb. The phase characteristic of the pulse selector was obtained by registering the time dependence of the synthesized sequence with a selector phase tuning step of 0.1 degrees. We measured the spectra of the sequences at different phases of the pulse selector, as well as the emission spectrum of the master laser at the minimum transmission of the modulator in the absence of modulation. The spectrum with the maximum amplitude corresponded to the case when identical pulses with the highest amplitude were selected from the original sequence. The difference between these spectra made it possible to isolate the spectrum of a pure sequence of identical femtosecond pulses without taking into account the "background" that occurs due to the modulator has a finite attenuation of –20 dB.

Microimage coupled Raman optical probes for benthic solid target detection

AbstractOver the past decades, Raman spectroscopy has been proven to be a powerful tool for in situ detection in the deep sea. However, two important issues are usually presented for solid detection of Raman in the sea: (a) The implementation of accurate laser focusing using the manipulator of remotely operated vehicles is difficult; (b) it is challenging to recognise minerals by Raman only. In this study, a novel approach was presented to address these difficulties by introducing microimage into underwater Raman system to aid laser focusing and spectra analysis. The further evaluation results showed that, using 5× objective lens, the joint system could obtain the focused laser spot size of 290 μm on the focal plane for Raman detection and the spatial resolution of 2.2 μm for the microimaging. With the aid of microimaging, the Raman probe was successfully applied to the China Yellow Sea (36.129°N, 120.215°E) for in situ detection on the seafloor. Microimaging is a practical method of promoting the detection ability of underwater Raman spectroscopy, and biological applications would be another perspective in the future.

High-power narrow-linewidth 780 nm diode laser based on external cavity feedback technology of volume Bragg grating

A diode-pumped alkali metal vapor laser (DPAL) combines the advantages of a solid laser and a gas laser. DPAL is useful in the industrial, medical, aerospace and military fields and can yield high output power while maintaining excellent beam quality. However, applications in many fields are limited due to the narrow absorption spectrum of the alkali metal vapor laser. In this study, a novel linewidth narrowing structure of the high-power diode laser based on volume Bragg gratings (VBGs) is proposed to match the absorption spectrum of the alkali metal vapor laser. With the combination of fast-axis collimating lens, beam transformation systems, slow-axis collimating lens, VBGs and metal ceramic heaters, the divergence angles of both fast and slow axis are compressed within 10 mrad. A high-power narrow linewidth diode laser source with a central wavelength of 780.044 nm, a spectral width of 0.138 nm, and an output power of 1090 W is achieved at an operating current of 50 A. Experimental results show that the effect of external cavity feedback based on VBG is improved and the multi-channel spectral linewidth is narrowed effectively by such a structure. The diode laser source based on this structure can be applied for pumping the rubidium alkali metal vapor laser.

All-carbon stretchable and cavity-free white lasers.

Flexible, stretchable, and bendable electronics and optoelectronics have a great potential for wide applications in smart life. An environmentally friendly, cost effective and wide-angle emission laser is indispensable for the emerging technology. In this work, circumvent the challenge issue, cavity-free and stretchable white light lasers based on all carbon materials have been demonstrated by integration of fluorescent carbon quantum dots (CQDs) and crumpled graphene. The typical emission spectrum of the cavity-free laser based on all-carbon materials has a CIE chromaticity coordinate of (0.30, 0.38) exhibiting an intriguing broadband white-light emission. The unprecedented and non-toxic stretchable and white light cavity-free lasers based on all-carbon materials can serve as next-generation optoelectronic devices for a wide range application covering solid-state lighting and future wearable technologies.

Microwave or radio-frequency controlled electromagnetically induced transparency (EIT) and related dispersion spectra in a pump-probe lasers driven multi-level [formula omitted]-type system of [formula omitted] atom

The optical responses of a pump-probe lasers driven six-level cascade (Ξ)-type system to the probe field are studied in the present work. The six-level system considered to be formed by the hyperfine energy levels of 5S12, 5P12 and 7S12 states of 87Rb atom. The main feature that makes the present light-atom interaction scheme different from the other routinely studied Ξ-type systems is the coupling of magnetic dipole allowed hyperfine ground levels by the magnetic field component of a microwave (MW) or radio-frequency (RF) field and the use of it as a control field. Probe absorption and dispersion profiles are simulated by numerically solving the corresponding optical Bloch equations (OBEs). Effects of the MW or RF field on the electromagnetically induced transparency (EIT) as well as probe absorption and related dispersion signals are examined under strong pump and different probe strength conditions. Group index (ng) of the medium is also evaluated and the effect of the MW or RF field on ng is explored. It has been found that the MW or RF field, working as a control field in the medium facilitates the enhancement of the overall absorption of the probe field in weak and moderate probe strength limits for stationary as well as for moving atoms. At higher probe strength, the MW or RF field contributes in the subluminal probe propagation through the stationary atomic medium along with a non-vanishing probe absorption at the line center.

2 μm Laser generation and amplification based on dual Tm3+-doped high-Q silica microsphere using an ASE light source for pumping

A dual Tm3+-doped silica microsphere-tapered optical fiber (TDSM-TOF) structure was proposed to excite 2 μm whispering gallery mode (WGM) laser generation and amplification. A C-band amplified spontaneous emission (ASE) light source was employed as the pumping source of TDSMs. The absorption spectra and the laser spectra of TDSM1 and TDSM2 and their concatenations were measured respectively. The ASE source has polarized light in any direction to satisfy the resonance of the microsphere cavity, the laser longitudinal mode distributions can counteract the influence of polarization variation because of environmental vibration. Meanwhile, six photons up-conversion fluorescence could also be observed.