Continuous-wave Diode Research Articles

Fabrication of low-defect Ge-rich SiGe-on-insulator by continuous-wave diode laser-induced recrystallization

We develop a cost-effective, up-scalable and high-throughput method to fabricate low-defect Ge-rich SiGe-on-insulator (SGOI) through continuous-wave diode laser-induced recrystallization of sputtered Ge on Silicon–on-insulator (SOI). The successful formation of Ge-rich SGOI is revealed by Raman spectra and energy-dispersive X-ray spectroscopy mapping. Transmission electron microscopy measurements show that the fabricated SGOI has much lower dislocation density than the initial Ge film which may be attributed to two effects: i) laser-induced lateral recrystallization changing the mechanism from SiGe/Si hetero-epitaxy to SiGe/SiGe homo-epitaxy, ii) the fast regrowth rate and high thermal gradient producing vacancy supersaturation causing the movement of dislocations to lateral surfaces.

Quantitative and Spatially Resolved Measurement of Atomic Potassium in Combustion Using Diode Laser**Supported by the National Natural Science Foundation of China under Grant Nos 91541119 and 91541203.

A compact optical setup for quantitative and spatially resolved measurement of atomic alkali concentration in combustion is demonstrated. Tunable diode laser absorption spectroscopy and laser-induced fluorescence are combined using a single continuous wave diode laser to measure the line-integration concentration and the relative distribution simultaneously, thereby obtaining the absolute concentration distribution along the laser beam. The results indicate the good performance of this method for one-dimensional quantitative measurement.

Mask-aligner lithography using a continuous-wave diode laser frequency-quadrupled to 193 nm.

We present a mask-aligner lithographic system operated with a frequency-quadrupled continuous-wave diode laser emitting at 193 nm. For this purpose, a 772 nm diode laser is amplified by a tapered amplifier in the master-oscillator power-amplifier configuration. The emission wavelength is upconverted twice, using LBO and KBBF nonlinear crystals in second-harmonic generation enhancement cavities. An optical output power of 10 mW is achieved. As uniform exposure field illumination is crucial in mask-aligner lithography, beam shaping is realized with optical elements made from fused silica and CaF2 featuring a diffractive non-imaging homogenizer. A tandem setup of shaped random diffusers, one static and one rotating, is used to control speckle formation. We demonstrate first experimental soft contact and proximity prints for a field size of 1 cm2 with a standard binary photomask and proximity prints with a two-level phase mask, both printed into 120 nm layers of photoresist on unstructured silicon substrates.

Developing a compact multiple laser diode combiner with a single fiber stub output for handheld IoT devices

Many biomedical applications require an efficient combination and localization of multiple discrete light sources (e.g., fluorescence and absorbance imaging). We present a compact 6 channel combiner that couples the output of independent solid-state light sources into a single 400-μm-diameter fiber stub for handheld Internet of Things (IoT) devices. We demonstrate average coupling efficiencies > 80% for each of the 6 laser diodes installed into the prototype. The design supports the use of continuous wave and intensity-modulated laser diodes. This fiber-stub-type beam combiner could be used to construct custom multi-wavelength sources for tissue oximeters, microscopes and molecular imaging technologies. In order to validate its suitability, we applied the developed fiber-stub-type beam combiner to a multi-wavelength light source for a handheld IoT device and demonstrated its feasibility for smart healthcare through a tumor-mimicking silicon phantom.

Continuous wave laser diodes enable fast optoacoustic imaging

Pulsed laser diodes may offer a smaller, less expensive alternative to conventional optoacoustic laser sources; however they do not provide pulse rates faster than a few tens of kHz and emit at wavelengths only within the near-infrared region. We investigated whether continuous wave (CW) laser diodes, which are available in visible and near-infrared regions, can be good optoacoustic light sources when overdriven with a peak current >40-fold higher than the CW absolute maximum. We found that overdriven CW diodes provided ∼10 ns pulses of ∼200 nJ/pulse and repetition rates higher than 600 kHz without being damaged, outperforming many pulsed laser diodes. Using this system, we obtained images of phantoms and mouse ear and human arm in vivo, confirming their use in optoacoustic imaging and sensing.

Carrier-envelope offset stabilization of a GHz repetition rate femtosecond laser using opto-optical modulation of a SESAM.

We demonstrate, to the best of our knowledge, the first carrier-envelope offset (CEO) frequency stabilization of a GHz femtosecond laser based on opto-optical modulation (OOM) of a semiconductor saturable absorber mirror (SESAM). The 1.05-GHz laser is based on a Yb:CALGO gain crystal and emits sub-100-fs pulses with 2.1-W average power at a center wavelength of 1055nm. The SESAM plays two key roles: it starts and stabilizes the mode-locking operation and is simultaneously used as an actuator to control the CEO frequency. This second functionality is implemented by pumping the SESAM with a continuous-wave 980-nm laser diode in order to slightly modify its nonlinear reflectivity. We use the standard f-to-2f method for detection of the CEO frequency, which is stabilized by applying a feedback signal to the current of the SESAM pump diode. We compare the SESAM-OOM stabilization with the traditional method of gain modulation via control of the pump power of the Yb:CALGO gain crystal. While the bandwidth for gain modulation is intrinsically limited to ∼250 kHz by the laser cavity dynamics, we show that the OOM provides a feedback bandwidth above 500kHz. Hence, we were able to obtain a residual integrated phase noise of 430mrad for the stabilized CEO beat, which represents an improvement of more than 30% compared to gain modulation stabilization.

Mechanism behind dry etching of Si assisted by pulsed visible laser

Poly-Si films were etched using a 13.56 MHz capacitively coupled plasma source while simultaneously being exposed to a pulsed Nd:YAG laser using 266 and 532 nm lines, with Gaussian pulse durations of 100 Hz and 7 ns. For a fluorocarbon etch recipe of 50:8 sccm Ar:C4F8 with varying O2, a minimum laser intensity for the etch onset was necessary to overcome CFx polymer deposition. This etch onset occurred at 6 ± 1 mJ/cm2/pulse; beyond this onset, the etch rate increased linearly with laser intensity. Null results of laser etch enhancement using continuous wave diode sources demonstrated the necessity of the instantaneous application of the pulsed Nd:YAG source. To determine the mechanism of laser etch enhancement at 532 nm, highly doped Si samples were tested, with varying optical absorption depths while keeping the photon energy constant. It was shown that at phosphorus contents of 1019 cm−3 and 1021 cm−3, 532 nm etch enhancement trends were 1.7× and 3.7× higher than those on intrinsic Si, showing that instantaneous surface heating was key in desorbing involatile etch products. Further investigation of the surface fluorine content via X-ray photon spectroscopy showed that distinct desorption stages occurred for increasing pulse energy—trends which aligned very well with SiFx desorption promoted by steady-state wafer heating. Gas arrival/surface saturation experiments with varying pressures and pulse rates showed that, in straightforward etching discharges such as Ar/SF6, laser removal per pulse plateaus when the pulse rate is lower than the rate of surface saturation, while in fluorocarbon-rich etch chemistries such as Ar/C4F8/O2 mixtures, a minimum pulse rate must be maintained to overcome the CFx polymer layer being deposited.

Frequency-comb-assisted absolute calibration and linestrength of H12C13CH ro-vibrational transitions in the 2ν3 band

We report on a precision spectroscopic study of room-temperature H12C13CH in the 6463–6520 cm−1 range, using a continuous-wave diode laser phase-locked to a near-infrared optical frequency comb synthesizer stabilized, in turn, against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several ro-vibrational transitions in the P and R branches of the 2ν3 overtone band are recorded. Accuracy levels as low as 100 kHz, 5·10−10 in fractional terms, are obtained in the determination of absolute line-center frequencies. In addition, line intensities are measured for the first time, with a best fractional uncertainty of 1%, and used to retrieve the purely vibrational transition dipole moment in conjunction with the first-order Herman–Wallis factor. Finally, the pressure broadening coefficients γself are also derived for the studied transitions.

Laser-induced erasable patterns in a N* liquid crystal on an iron doped lithium niobate surface.

A chiral nematic (N*) liquid crystal (LC) was hybridized with a z-cut iron doped lithium niobate (Fe:LN) substrate and exposed with a focused continuous wave diode laser beam. The N* LC layer was confined with a cover glass to provide a homogeneous LC layer thickness. Two distinct kinds of test cells were investigated, one with an uncoated glass covering slip and one with an indium tin oxide (ITO) coated cover glass. Photo generated electric fields (generated in the Fe:LN) resulted in a localized defect formation and textural transitions in the N* LC. Due to field confinement, the field induced responses were more localized in samples with ITO coated cover glasses. By scanning the laser beam on programmed trajectories, formation of persistent patterns could be achieved in the N* LC layer. Polarized optical microscopy of the exposed samples revealed that these patterns consisted of adjacent circular Frank-Pryce defects. Exposure with a slightly defocused laser beam could be applied selectively to erase these patterns. Thus, a promising method is reported to generate reconfigurable patterns, photonic motives, and touch sensitive devices in a hybridized N* LC with micron accuracy.

Clinical assessment of diode laser-assisted endoscopic intrasphenoidal vidian neurectomy in the treatment of refractory rhinitis.

For chronic rhinitis that is refractory to medical therapy, surgical intervention such as endoscopic vidian neurectomy (VN) can be used to control the intractable symptoms. Lasers can contribute to minimizing the invasiveness of ENT surgery. The aim of this retrospective study is to compare in patients who underwent diode laser-assisted versus traditional VN in terms of operative time, surgical field, quality of life, and postoperative complications. All patients had refractory rhinitis with a poor treatment response to a 6-month trial of corticosteroid nasal sprays and underwent endoscopic VN between November 2006 and September 2015. They were non-randomly allocated into either a cold instrument group or a diode laser-assisted group. Vidian nerve was excised with a 940-nm continuous wave diode laser through a 600-μm silica optical fiber, utilizing a contact mode with the power set at 5W. A visual analog scale (VAS) was used to grade the severity of the rhinitis symptoms for quality of life assessment before the surgery and 6months after. Of the 118 patients enrolled in the study, 75 patients underwent cold instrument VN and 43 patients underwent diode laser-assisted VN. Patients in the laser-assisted group had a significantly lower surgical field score and a lower postoperative bleeding rate than those in the cold instrument group. Changes in the VAS were significant in preoperative and postoperative nasal symptoms in each group. The application of diode lasers for vidian nerve transection showed a better surgical field and a lower incidence of postoperative hemorrhage. Recent advancements in laser application and endoscopic technique has made VN safer and more effective. We recommend this surgical approach as a reliable and effective treatment for patients with refractory rhinitis.

Multipoint Refractometer Based on Combined Correlation and Frequency Multiplexing

We demonstrate a new simple multi-point refractometer using Fresnel reflection at the fiber tip with closely spaced interferometer for intensity reference. A combined technique for simultaneous interrogation and multiplexing of identical sensor heads was developed to obtain multipoint operation with a standard continuous-wave DFB diode laser as light source. The technique is based on the combination of coherent optical frequency division reflectometry and coherent correlation optical time-domain reflectometer. Feasibility of accurate operation for multipoint refractive index measurements is demonstrated in proof-of-concept experiments. The proposed refractometer could be useful for sensing in chemical and biological applications.

Monolithically multi-color lasing from an InGaN microdisk on a Si substrate

An optically pumped multi-color laser has been achieved using an InGaN/GaN based micro-disk with an undercut structure on a silicon substrate. The micro-disk laser has been fabricated by means of a combination of a cost-effective microsphere lithography technique and subsequent dry/wet etching processes. The microdisk laser is approximately 1 μm in diameter. The structure was designed in such a way that the vertical components of the whispering gallery (WG) modes formed can be effectively suppressed. Consequently, three clean lasing peaks at 442 nm, 493 nm and 522 nm have been achieved at room temperature by simply using a continuous-wave diode laser as an optical pumping source. Time–resolved micro photoluminescence (PL) measurements have been performed in order to further confirm the lasing by investigating the excitonic recombination dynamics of these lasing peaks. A three dimensional finite-difference-time-domain (FDTD) simulation has been used for the structure design.

The Effect of Combination of Red, Infrared and Blue Wavelengths of Low-Level Laser on Reduction of Abdominal Girth: A Before-After Case Series.

Introduction: The objective of this study was to assess the efficacy of a combination of 3 wavelengths (including red, infra-red, and blue) of low-level laser (LLL) as a non-invasive therapeutic method to reduce abdominal girth. To achieve biochemical activity on adipocytes, a red laser was used in our selective laser combination. Near-infrared laser was used to increase depth of penetration. Nitrosyl complexes of hemoglobin (NO-Hb) are sensitive to blue light, thereby leading to increase in release of biologically active nitric oxide (NO), which can affect tissue perfusion. Thus, a blue LED was added to the laser combination. Methods: Eighteen females participated in the study. Twelve sessions of laser therapy were performed, 2 sessions per week for each subject. Continuous wave diode lasers, including red (630 nm), infra-red (808 nm), and a blue LED (450 nm) were applied and were all designed by the Canadian Optic and Laser Center. Results: Statistical analyses revealed that upper abdomen size significantly decreased from pre- (91.86 ± 11.16) to post- (87.41 ± 10.52) low-level laser therapy (LLLT) (P<0.001). Middle abdomen size showed significant reduction from pre- (97.02 ± 8.82) to post- (91.97 ± 8.49) LLLT (P<0.001). Lower abdomen size significantly decreased from pre- (100.36 ± 9.45) to post- (95.80 ± 8.52) LLLT (P<0.001). Conclusion: Based on this case series pilot investigation, the combination of 3 different wavelengths of LLL was effective for abdominal girth reduction in 100% of our subjects (P<0.001), without any side effects. Future studies will assess the long-term benefits of this laser combination for reduction of subcutaneous fat deposits.

Alterations in Auditory Electrophysiological Responses Associated With Temporary Suppression of Tinnitus Induced by Low-Level Laser Therapy: A Before-After Case Series.

Introduction: Tinnitus is the phantom auditory perception of sound in the absence of an external or internal acoustic stimulus. The treatment is difficult due to multiple etiologies and great psychological influence. The purpose of this study was to determine alterations in auditory physiological and electrophysiological responses associated with temporary suppression of tinnitus induced by low-level laser (LLL) irradiation. Methods: This study was conducted on 20 subjects with subjective tinnitus. All subjects signed the informed consent form and satisfied all the study eligibility criteria. Visual analog scale (VAS) for loudness, loudness matching of tinnitus (LMT), pitch matching of tinnitus (PMT), Persian-tinnitus questionnaire (P-TQ) and Persian-tinnitus handicap inventory (P-THI) were conducted pre- and post-low level laser therapy (LLLT) for all the subjects. Electrocochleography (ECochG) and distortion product otoacoustic emissions (DPOAEs) were recorded in 11 subjects. Continuous wave diode lasers, including red (630 nm) and infra-red (808 nm) were applied, and were both designed by the Canadian Optic and Laser (COL) Center. Twelve sessions of laser therapy were performed, 2 sessions per week for each subject. Total dose was 120 Joule/ ear/session. Results: LLL irradiation could cause a significant decrease in subjective tests scores consisting of VAS for loudness, PMT, P-TQ, P-THI, but did not result in a significant improvement of objective evaluating parameters except for compound action potential (CAP) amplitude. Conclusion: LLLT might be a subjectively effective treatment for short-term improvement of tinnitus. Defining a new protocol for optimizing LLLT parameters may be an option to improve parameters of objective tests.

New photobiomodulation protocol prevents oral mucositis in hematopoietic stem cell transplantation recipients-a retrospective study.

Oral mucositis (OM) is an adverse side effect among hematopoietic stem cell transplantation (HSCT) recipients. The objective of this retrospective study was to evaluate the preventive effect of photobiomodulation (PBM) applied three times per week versus seven times per week in patients undergoing HSCT. The risk factors related to the incidence and severity of OM were also assessed. This was a retrospective study that evaluated 99 HSCT recipients who received different PBM protocols. Group I received three sessions per week, and group II received daily treatment. PBM was applied using a continuous-wave diode laser (InGaAlP; MM Optics, São Carlos, SP, Brazil) at a wavelength of 660nm (visible-red) and a total radiant energy of 0.24J per point. The baseline disease, type of transplant, type of conditioning, prophylaxis against graft-versus-host disease, OM grade, absolute leukocyte and platelet counts, and levels of liver and renal function markers were collected from medical records. The patients' age ranged from 13 to 71years (mean/SD, 40.54±16.45). No significant difference was observed between groups I and II regarding sex, age, ethnic, diagnosis, donor type, and conditioning treatment. Both PBM protocols were equally efficient in preventing OM (p=0.34, ANOVA). Independent of the PBM protocol used, patients who received allogeneic transplant (p<0.01-Fischer's exact test), total body irradiation (TBI-12Gy) (p=0.01-chi-square test), busulfan + cyclophosphamide (p<0.01-chi-square test), or methotrexate-containing regimens (p<0.01-Fischer's exact test) demonstrated higher OM incidence and severity. Myelosuppression (p<0.01-Mann-Whitney test) and impaired renal function (p=0.02-Mann-Whitney test) were also considered risk factors for OM. Based on this retrospective data, PBM was effective in preventing OM in patients undergoing HSCT even when it was applied three times a week. A prospective study might be necessary to confirm these findings.

High power, high efficiency continuous-wave 808 nm laser diode arrays

The continuous-wave 100W-class 808nm laser diode arrays with extremely high power conversion efficiency of 68% were reported at the heatsink temperature of 25°C. To the best of our knowledge, this was the highest power conversion efficiency at continuous-wave 106W 808nm laser diode array with 50% fill factor so far. An asymmetric broad waveguide epitaxial structure with very low internal optical loss of 0.5cm−1 was presented. In order to improve the efficiency, various fill factor devices were studied. The 50W laser diode array with 30% fill factoir and 1.0mm cavity length demonstrated power conversion efficiency of 71% at heatsink temperature of 15°C.

Laser-stimulated desorption of organic molecules from surfaces, as a method of increasing the efficiency of ion mobility spectrometry analysis.

Application of laser-induced desorption was investigated as a method of increasing the efficiency of gas phase analyzers on principles of field asymmetric ion mobility spectrometry. Mass spectrometric data of investigations of laser desorption of pentaerythritoltetranitrate molecules and cyclotetramethylenetetranitramine molecules from quartz substrate under vacuum were obtained. Laser sources a Nd3+:YAG with nanosecond pulse duration (λ = 532 nm) and a continuous wave diode laser (λ = 440 nm) were used. It was shown that both laser sources have different desorption abilities. This is expressed in various time of appearance of desorbed products that is caused by different heating mechanisms of surface layer. The desorbed quantity under action of both laser sources exceeds the detection threshold for all modern gas phase analyzers. It should be noted that despite the presence of surface dissociation of explosives under laser radiation, the quantity of nondissociated molecules is large enough for detection by ion mobility and field asymmetric ion mobility spectrometers. The optimal parameters of laser radiation for effective removal (evaporation) molecules of low-volatile compounds from surfaces are defined. The conclusion about preferable use of a Nd3+:YAG laser for increasing the detection ability of detectors based on ion mobility spectrometry was made.

Saturated absorption competition microscopy

We introduce the concept of saturated absorption competition (SAC) microscopy as a means of providing sub-diffraction spatial resolution in fluorescence imaging. Unlike the post-competition process between stimulated and spontaneous emission that is used in stimulated emission depletion (STED) microscopy, SAC microscopy breaks the diffraction limit by emphasizing a pre-competition process that occurs in the fluorescence absorption stage in a manner that shares similarities with ground-state depletion (GSD) microscopy. Moreover, unlike both STED and GSD microscopy, SAC microscopy offers a reduction in complexity and cost by utilizing only a single continuous-wave laser diode and an illumination intensity that is ~ 20x smaller than that used in STED. Our approach can be physically implemented in a confocal microscope by dividing the input laser source into a time-modulated primary excitation beam and a doughnut-shaped saturation beam, and subsequently employing a homodyne detection scheme to select the modulated fluorescence signal. Herein, we provide both a physico-chemical model of SAC and experimentally demonstrate by way of a proof-of-concept experiment a transverse spatial resolution of ~lambda/6.

Low power laser induced NLO properties and optical limiting of an anthraquinone dye using Z–scan technique

The nonlinear optical (NLO) properties and optical limiting of an anthraquinone (Reactive blue 4) dye is studied in the low power regime utilizing a continuous wave (CW) diode laser at 635 nm with total power of 5 mW. The NLO parameters such as nonlinear index of refraction (n2) and nonlinear absorption coefficient (β) were determined respectively from closed (S = 0.4) and open (S = 1) aperture Z–scan technique. Self-defocusing and reverse saturable absorption are the nonlinear mechanisms were observed in the dye sample. The real and imaginary parts of the third-order nonlinear susceptibility (χ3) were estimated to be Re (χ3) = −2.12 × 10−5 esu and Im (χ3) = 1.85 × 10−6 esu. The optical limiting behaviors of reactive blue 4 dye at different concentrations were performed under experimental condition. Variation of transmitted laser spot size as a function of sample position at far field was also observed. The diffraction ring patterns were recorded in the present dye sample due to thermal lensing effect. With these experimental results the authors believed that, the dye investigated here is a potential material for NLO applications.

Coherent blue emission generated by Rb two-photon excitation using diode and femtosecond lasers

The coherent blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a continuous wave diode laser is investigated. Each step of the two-photon transition 5S–5P–5D is excited by one of the lasers, and the induced coherence between the 5S and 6P states is responsible for generating the blue beam. Measurements of the excitation spectrum reveal the frequency comb structure and allow us to identify the resonant modes responsible for inducing the nonlinear process. Further, each resonant mode excites a different group of atoms, making the process selective in atomic velocity. The signal dependency on the atomic density is characterized by a sharp growth and a rapid saturation. We also show that for high intensity of the diode laser, the Stark shift at resonance causes the signal suppression observed at low atomic density.