Continuous-wave Laser Light Research Articles

CW-to-pulse conversion using temporal Talbot array illuminators.

We report on the linear conversion of continuous-wave (CW) laser light to optical pulses using temporal Talbot array illuminators (TAIs) with fractional orders 1/q(q≤10), implemented by use of multilevel PM and dispersive propagation in a chirped fiber Bragg grating. The generated, sub-nanosecond optical pulse trains have repetition rates in the gigahertz range and show the presence of satellite pulses originated by the finite electrical modulation bandwidth (7.5GHz). Though this fact impacts the resulting extinction ratio, an experimental comparison with time and Fresnel lenses indicates that temporal TAIs represent compact systems with high light gathering efficiency (>87%) at moderate values of compression (q≤8), which can be tailored in repetition rate, gain, or width, through the fractional Talbot order for its use in pulse compression systems fed by CW light.

Digital holography on moving objects: interference contrast as a function of velocity and aperture width.

Digital holographic measurements on planar moving objects are investigated. We discuss the dependence of the interference contrast on velocity and aperture width for both diffusely and specularly reflecting objects. Using spatial phase shifting, the experimental results for motion in parallel and perpendicular to the optical axis are in good agreement with the theoretical considerations. Measurements with object velocities of up to 100mm/s are conducted using only less than 1mW of continuous-wave laser light. These considerations are used to determine the optimal angle between the direction of motion and the illuminating beam, resulting in the lowest decrease in contrast with increasing velocity.

Crosstalk dynamics in multi-core fibers.

Inter-core crosstalk is a potential limitation on the achievable data-rates in optical fiber transmission systems using multi-core fibers. Crosstalk arises from unwanted coupling between cores of a homogenous multi-core fiber and it's average power has been observed to vary over time by 10s of decibels, potentially requiring an additional performance margin to achieve acceptable outage probability. Most investigations of crosstalk have so far only considered continuous wave laser light or amplified spontaneous emission as sources of crosstalk. In this paper, we theoretically and experimentally investigate the time-dependence of inter-core crosstalk in a homogeneous multi-core fiber when considering signals with various modulation formats and symbol rates. We find that crosstalk power fluctuations depend on the symbol rate, modulation and skew between cores. For carrier-free signals, such as quadrature amplitude modulation, the crosstalk power is nearly constant for expected conditions of multi-core transmission systems. However, carrier-supported signals, such as OOK, always induce time-varying crosstalk powers.

Low power continuous wave-laser seed irradiation effect on Moringa oleifera germination, seedling growth and biochemical attributes

Recently, laser application in agriculture has gained much attention since plant characteristics were improved significantly in response of pre-sowing seed treatment. Pre-sowing laser seed treatment effects on germination, seedling growth and mineral profile were studied in Moringa olifera. M. olifera healthy seeds were exposed to 25, 50, 75mJ low power continuous wave laser light and grown under greenhouse conditions. The seedling growth and biochemical attributes were evaluated from 10-day-old seedlings. The germination parameters (percentage, mean germination time), vigor index, seedling growth (root length, seedling length, shoot fresh weight, root fresh weight, shoot dry weight, root dry weight) enhanced considerably. The laser energy levels used for seed irradiation showed variable effects on germination, seedling growth and mineral profile. The mineral contents were recorded to be higher in seedling raised from laser treated seeds, which were higher in roots versus shoots and leaves. The effect of laser treatment on seedling fat, nitrogen and protein content was insignificant and at higher energy level both nitrogen and protein contents decreased versus control. Results revealed that M. olifera germination, seedling growth and mineral contents were enhanced and optimum laser energy level has more acceleratory effect since at three laser energy levels the responses were significantly different. Overall the laser energy levels effect on germination and seedling growth was found in following order; 75mJ>50mJ>25mJ, where as in case of fat, protein and nitrogen contents the trend was as; 25mJ>50mJ and 75mJ. However, this technique could possibly be used to improve the M. olifera germination, seedling growth, and minerals contents where germination is low due to unfavorable conditions.

Laser-induced fabrication of gold nanoparticles on shellac-driven peptide nanostructures

This study demonstrates the synthesis of a new class of peptide amphiphiles derived from aleuritic acid. The aleuritic acid was extracted and purified from the natural source shellac, which was later conjugated with tryptophan, leading to a new class of very short peptide amphiphiles. The self-assembling behavior of this compound was studied using spectroscopic and microscopic tools. This shellac-driven peptide was further used to cultivate gold nanoparticles (AuNPs) with the help of continuous wave (CW) laser light, where the AuNPs were encapsulated by peptide nanostructures. Laser irradiation caused nanoscopically confined heating in the AuNPs-peptide hybrid nanostructures. Such confined heating is mainly the result of scattering and simultaneous absorption of subwavelength power which is subjected to enhanced plasmonic resonances of the metal nanostructures. Hence, the generated heat power/photothermal effect of these AuNPs leads to disruption of the AuNP–peptide hybrids. Such light-induced prototype nano-structure hydrid devices have a wide range of thermal-plasmonic applications in the morphological modification of soft metal hybrid nanostructures for photothermal therapy and drug release.

Nonlinear Refraction Behaviour of Toluidine Blue O Dye-PVA Nanocomposite Films under CW Laser Light Excitation

Toluidine Blue O (TBO) dye-PVA nanocomposite films were prepared following a simple method based on solution-casting. Their morphology, linear as well as nonlinear optical properties were investigated. These nanocomposite films were characterized as nanoclustered aggregates grown within the PVA matrix. The nonlinear refraction behaviour of the nanocomposite films was investigated using Z-scan technique, under excitation with continuous wave (CW) laser light at 442nm for different peak incident intensities. The samples exhibited a negative nonlinearity and large nonlinear refractive index of the order of 10-6 cm2/W, which were better than some of the recently reported values for CW laser light excitation.

Continuous-wave laser particle conditioning: Thresholds and time scales

The optical absorption of contaminants on high reflectivity mirrors was measured using photo thermal common-path interferometry before and after exposure to high power continuous-wave laser light. The contaminants were micron-sized graphite flakes on hafnia-silica distributed Bragg reflectors illuminated by a ytterbium-doped fiber laser. After one-second periods of exposure, the mirrors demonstrated reduced absorption for irradiances as low as 11kWcm−2 and had an obvious threshold near 20kWcm−2. Final absorption values were reduced by up to 90% of their initial value for irradiances of 92kWcm−2. For shorter pulses at 34kWcm−2, a minimum exposure time required to begin absorption reduction was found between 100μs and 200μs, with particles reaching their final minimum absorption value within 300ms. Microscope images of the surface showed agglomerated particles fragmenting with some being removed completely, probably by evaporation for exposures between 200μs to 10ms. Exposures of 100ms and longer left behind a thin semi-transparent residue, covering much of the conditioned area. An order of magnitude estimate of the time necessary to begin altering the surface contaminants (also known as ”conditioning”) indicates about 200μs seconds at 34kWcm−2, based on heating an average carbon particle to its sublimation temperature including energy loss to thermal contact and radiation. This estimation is close to the observed exposure time required to begin absorption reduction.

Fluorescent all-fiber light source based on micro-capillaries and on microstructured optical fibers terminated with a microbulb

Abstract An integrated fiber-optic fluorescent light source compatible with photonic-crystal and hollow-core fibers is presented in this paper. We have studied the dependence of the fluorescence spectra on the length of a micro-capillary filled with Rhodamine 6G dissolved in glycerin. As the capillary, we used a standard fiber-optic glass ferrule with two parallel holes having an inner diameter of 125 μm. One of the holes was filled with fluorescing solution, while an SMF-28 fiber polished at 45° with aluminum coating was placed in the second hole to serve as a pumping fiber. As the solution was pumped by continuous-wave laser light at 532 nm, the fluorescence was captured by a microstructured optical fiber immersed in the filled hole. To prevent the solution from penetrating into this receiving fiber, its end was capped by molten borosilicate glass forming a ball lens. Combining the spectra of several fluorescent organic dyes can create a broadband light source compatible with optical fibers that could be used for the development of compact photonic-crystal and hollow-core fiber sensors.

Continuous-wave laser damage and conditioning of particle contaminated optics.

This paper describes the physical processes that occur when high-power continuous-wave laser light interacts with absorbing particles on a low-absorption optical surface. When a particulate-contaminated surface is illuminated by high-power continuous-wave laser light, a short burst of light is emitted from the surface, and the particles rapidly heat over a period of milliseconds to thousands of degrees Celsius, migrating over and evaporating from the surface. The surviving particles tend to coalesce into larger ones and leave a relatively flat residue on the surface. The total volume of the material on the surface has decreased dramatically. The optical surface itself heats substantially during illumination, but the surface temperature can decrease as the material is evaporated. Optical surfaces that survive this process without catastrophic damage are found to be more resistant to laser damage than surfaces that have not undergone the process. The surface temperature of the conditioned surfaces under illumination is lower than that of unconditioned surfaces. These conditioning effects on particles occurred within the first 30s of laser exposure, with subsequent laser shots not affecting particle distributions. High-speed photography showed the actual removal and agglomeration of individual particles to occur within about 0.7ms. Elemental changes were measured using time-of-flight secondary ion mass spectroscopy, with conditioned residuals being higher in hydrocarbon content than pristine particles. The tests in this study were conducted on high-reflectivity distributed Bragg reflector coated optics with carbon microparticles in the size range of 20-50μm, gold particles of size 250nm, and silica 1μm in size.

Optical and Structural Properties of Si Nanocrystals in SiO2 Films.

Optical and structural properties of Si nanocrystals (Si-nc) in silica films are described. For the SiOx (x < 2) films annealed above 1000 °C, the Raman signal of Si-nc and the absorption coefficient are proportional to the amount of elemental Si detected by X-ray photoelectron spectroscopy. A good agreement is found between the measured refractive index and the value estimated by using the effective-medium approximation. The extinction coefficient of elemental Si is found to be between the values of crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the crystallization and the Si–SiO2 phase separation are not complete after annealing at 1200 °C. The 1.5-eV PL quantum yield increases as the amount of elemental Si decreases; thus, this PL is probably not directly from Si-nc responsible for absorption and detected by Raman spectroscopy. Continuous-wave laser light can produce very high temperatures in the free-standing films, which changes their structural and optical properties. For relatively large laser spots, the center of the laser-annealed area is very transparent and consists of amorphous SiO2. Large Si-nc (up to ~300 nm in diameter) are observed in the ring around the central region. These Si-nc lead to high absorption and they are typically under compressive stress, which is connected with their formation from the liquid phase. By using strongly focused laser beams, the structural changes in the free-standing films can be made in submicron areas.

Action spectrum for photobleaching of human lenses by short wavelength visible irradiation.

PurposeCataract is the world-leading cause of blindness. In search for a new treatment of cataract we have found that the yellow discolouration of aged human lenses can be photobleached using a non-invasive, infra-red, femtosecond laser treatment. These results were presented in an earlier PlosOne publication. The objective of the study was to characterize the single-photon photobleaching action spectrum of the aged human lens in vitro.MethodsNinety-one human donor lenses were irradiated with continuous wave laser light at 375, 405, 420, 445, 457 or 473 nm. Photobleaching was monitored by photography and transmission measurements.ResultsThe action spectrum peaked at 420 nm followed by, in order of decreasing effect, 445, 457, 473, 405 and 375 nm. Younger and less absorbent lenses showed smaller changes than older and more absorbent lenses. There was a dose-dependent increase in lens transmission with increasing laser irradiation.ConclusionsFor a 75 year old lens an effect corresponding to elimination of 15 years or more of optical ageing was obtained. This study of the spectral characteristics and intensity needed to bleach the human lens with single-photon laser effects found an action-spectrum peak at 420 nm tailing gradually off toward longer wavelengths and more steeply toward shorter wavelengths. The results may be used to guide experiments with two-photon bleaching.

Measurement of inhomogeneous absorptive organic media with polarization interferometric confocal microscope

Polarization interferometric confocal microscope has been developed for high-precision measurement, especially for inhomogeneous organic media doped with nonlinear chromophores. The microscope has a novel contrast-enhancing technique: suppressing the background electric field with polarization interferometer. With this technique, the microscope has allowed optical anisotropy detection in a submicron area (400 nm × 400 nm), even with a low-cost and low-power continuous-wave (CW) laser light source. In particular, the microscope clearly distinguishes between isotropic and anisotropic regions. According to polarization interferometric ellipsometry, the microscope produces inspection images showing planer distribution of the optical parameter directly.

Quantitative analysis of transcranial and intraparenchymal light penetration in human cadaver brain tissue.

Photobiomodulation (PBM) also known as low-level light therapy has been used successfully for the treatment of injury and disease of the nervous system. The use of PBM to treat injury and diseases of the brain requires an in-depth understanding of light propagation through tissues including scalp, skull, meninges, and brain. This study investigated the light penetration gradients in the human cadaver brain using a Transcranial Laser System with a 30 mm diameter beam of 808 nm wavelength light. In addition, the wavelength-dependence of light scatter and absorbance in intraparenchymal brain tissue using 660, 808, and 940 nm wavelengths was investigated. Intact human cadaver heads (n = 8) were obtained for measurement of light propagation through the scalp/skull/meninges and into brain tissue. The cadaver heads were sectioned in either the transverse or mid-sagittal. The sectioned head was mounted into a cranial fixture with an 808 nm wavelength laser system illuminating the head from beneath with either pulsed-wave (PW) or continuous-wave (CW) laser light. A linear array of nine isotropic optical fibers on a 5 mm pitch was inserted into the brain tissue along the optical axis of the beam. Light collected from each fiber was delivered to a multichannel power meter. As the array was lowered into the tissue, the power from each probe was recorded at 5 mm increments until the inner aspect of the dura mater was reached. Intraparenchymal light penetration measurements were made by delivering a series of wavelengths (660, 808, and 940 nm) through a separate optical fiber within the array, which was offset from the array line by 5 mm. Local light penetration was determined and compared across the selected wavelengths. Unfixed cadaver brains provide good anatomical localization and reliable measurements of light scatter and penetration in the CNS tissues. Transcranial application of 808 nm wavelength light penetrated the scalp, skull, meninges, and brain to a depth of approximately 40 mm with an effective attenuation coefficient for the system of 2.22 cm(-1) . No differences were observed in the results between the PW and CW laser light. The intraparenchymal studies demonstrated less absorption and scattering for the 808 nm wavelength light compared to the 660 or 940 nm wavelengths. Transcranial light measurements of unfixed human cadaver brains allowed for determinations of light penetration variables. While unfixed human cadaver studies do not reflect all the conditions seen in the living condition, comparisons of light scatter and penetration and estimates of fluence levels can be used to establish further clinical dosing. The 808 nm wavelength light demonstrated superior CNS tissue penetration.

Interstitial photoacoustic sensor for the measurement of tissue temperature during interstitial laser phototherapy.

Photothermal therapy is an effective means to induce tumor cell death, since tumor tissue is more sensitive to temperature increases than normal tissue. Biological responses depend on tissue temperature; target tissue temperature needs to be precisely measured and controlled to achieve desired thermal effects. In this work, a unique photoacoustic (PA) sensor is proposed for temperature measurement during interstitial laser phototherapy. A continuous-wave laser light and a pulsed laser light, for photothermal irradiation and photoacoustic temperature measurement, respectively, were delivered to the target tissue through a fiber coupler. During laser irradiation, the PA amplitude was measured. The Grüneisen parameter and the bioheat equation were used to determine the temperature in strategic positions in the target tissue. Our results demonstrate that the interstitial PA amplitude is a linear function of temperature in the range of 22 to 55 °C, as confirmed by thermocouple measurement. Furthermore, by choosing appropriate laser parameters, the maximum temperature surrounding the active diffuse fiber tip in tissue can be controlled in the range of 41 to 55 °C. Thus, this sensor could potentially be used for fast, accurate, and convenient three-dimensional temperature measurement, and for real-time feedback and control of interstitial laser phototherapy in cancer treatment.

Tuning for Resonances

AbstractDue to a very distinct spectrum of resonant frequencies, the characterization of optical microcavities calls for widely tunable sources of continuous wave laser light, offering suitably narrow linewidths and single‐mode operation over the entire tuning range. This is why external cavity diode lasers, which allow for mode‐hop free wavelength tuning, often over more than a hundred nanometers, have matured to a fundamental tool of the optical microresonator research apparatus.

Investigation of single-mode fiber degradation by 405-nm continuous-wave laser light

The degradation of 405-nm fiber-coupled diode laser systems with more than 50 mW power was investigated in detail with focus on the effects occurring at the input end. The coupling and transmission loss of the laser light were associated with the growth of a projection and a periodic structure on the input surface. To avoid this degradation, a short launch fiber with a good surface quality was used at the input end. In this way, the power transmission was stabilized for at least one month. However, structural degradation was noticed on the output surface of the single-mode fiber. To investigate this effect, the damaged samples were measured after different periods of time and examined with a scanning electron microscope and with an atomic force microscope. Reproducible spherical projections with a submicron periodic structure were found in the core region. Additionally, the spectral loss of the fiber was measured, showing the formation of color centers in the deep ultraviolet along the length of the fiber. These investigations were accompanied by simulations of the growth of the structure on the output surface. The influence of the structure was mainly on the divergence angle of the emitted laser beam, reducing the beam quality for applications.

Near-ultraviolet absorption annealing in hafnium oxide thin films subjected to continuous-wave laser radiation

Hafnium oxide (HfO2) is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications ranging from near-infrared to near-ultraviolet (UV). Absorption in this high-index material is also known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-UV absorption of HfO2 monolayer films subjected to irradiation by continuous-wave (cw), 355-nm or 351-nm laser light focused to produce power densities of the order of ∼100 kW/cm2 is studied. Up to a 70% reduction in absorption is found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm and 600-fs, 1053-nm laser-damage tests performed on these cw laser–irradiated areas confirm a reduction of absorption by measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-UV absorption annealing and damage-threshold improvement resulting from irradiation by near-UV cw laser light.

Endoscopic filtered Rayleigh scattering for the analysis of ducted gas flows

Laser-induced Rayleigh scattering makes use of the fact that elastically scattered laser light from atoms or molecules hold information on pressure, temperature and velocity inside the observed region of interest. Being several orders of magnitude weaker than other elastic light scattering effects such as Mie scattering from larger particles or laser flare from surfaces, the accuracy of Rayleigh scattering measurements strongly depends on the attenuation of these unwanted but omnipresent noise effects. The image-based technique of filtered Rayleigh scattering makes use of the absorption bands of atomic or molecular gases to remove strong elastic scattering effects from the measured signal. The concept is extended by frequency tuning a narrow line-width continuous wave laser light source along the absorption filter’s transmission profile. In acquiring images at several known frequencies, this results in spectra for each camera pixel from which time-averaged temperature, pressure and velocity fields can be deduced simultaneously. In order to qualify the frequency scanning filtered Rayleigh scattering technique for applications with strongly limited accessibility, it is applied to characterize the flow inside a bell-mouthed circular duct. The image data are acquired endoscopically via a fiberscope that is placed downstream from the measurement plane.

Nanoparticle Imaging with Polarization Interferometric Nonlinear Confocal Microscope

Polarization interferometric nonlinear confocal microscope has been developed for single nanoparticle analysis of drug delivery system (DDS). The microscope is a system based on a low cost and low power continuous wave (CW) laser light source. Also, the microscope observed shape anisotropy of the 200 nm diameter nanoparticle. According to nanoparticle imaging and CTF (contrast transfer function) curve observation of the microscope, three-dimensional resolution of the microscope measured up to 10 nm.

Miniaturized laser heterodyne radiometer for measurements of CO2 in the atmospheric column

We have developed a low-cost, miniaturized laser heterodyne radiometer for highly sensitive measurements of carbon dioxide (CO2) in the atmospheric column. In this passive design, sunlight that has undergone absorption by CO2 in the atmosphere is collected and mixed with continuous wave laser light that is step-scanned across the absorption feature centered at 1,573.6 nm. The resulting radio frequency beat signal is collected as a function of laser wavelength, from which the total column mole fraction can be de-convolved. We are expanding this technique to include methane (CH4) and carbon monoxide (CO), and with minor modifications, this technique can be expanded to include species such as water vapor (H2O) and nitrous oxide (N2O).