mW Continuous Wave Research Articles

140 GHz, 1 MW CW Gyrotron Development for Fusion Applications—Progress and Recent Results

A 10 MW, 140 GHz ECH system is currently under construction for the stellarator W7-X. The RF power will be provided by 10 gyrotrons. A European collaboration has been established to develop and build 9 (out of 10) tubes each with an output power of 1 MW for continuous wave (CW) operation. This contribution reports on recent results with the series gyrotrons.

Photoinduced absorption of Ag nanoparticles deposited on ITO substrate

Substantial changes of absorption after illumination by 300 mW continuous wave green laser at 532 nm were observed. The effect of indium tin oxide (ITO) substrate was explored versus Ag nanoparticles (AgNPs) size, their regularity and surface plasmon resonance. The ITO substrate features play a crucial role for the formation of homogenous AgNPs. The attachments of AgNPs on ITO surface as well as their homogeneity are significantly changed under the influence of the laser treatment. We study the Ag NP deposited on the two different substrates which play a crucial role in the photoinduced absorption. The dependence of the photoinduced absorption versus the time of optical treatment is explained within a framework of the photopolarization of the particular trapping levels on the borders between the ITO substrate and the Ag NP.

Quantum Dot Superluminescent Diodes for Optical Coherence Tomography: Skin Imaging

We present a high-power (18 mW continuous wave exiting a single-mode fiber and 35 mW exiting the facet), broadband (85 nm full-width at half-maximum) quantum dot-based superluminescent diode, and apply it to a time-domain optical coherence tomography (OCT) setup. First, we test its performance with increasing optical feedback. Then we demonstrate its imaging properties on tissue-engineered (TE) skin and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in vivo</i> skin. OCT allows the tracking of epidermal development in TE skin, while the higher power source allows better sensitivity and depth penetration for imaging of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in vivo</i> skin layers.

870 mW blue laser emission at 480 nm in a large core thulium doped ZBLAN fiber laser

We report on high power 870 mW continuous wave stable laser operation at 480 nm in a 1000 ppm wt. Thulium doped multimode ZBLAN up-conversion fiber laser. The fiber is pumped by a wavelength of 1064 nm generated from diode-pumped Nd:YAG laser. A threshold of 550 mW and slope efficiency of 14% with respect to the incident pump power has been obtained. The related problem of photo-degradation associated with formation of color centers is reported. The transparency of the darkened fiber prior to the lasing operation is restored by circulating 514 nm through the fiber core. The time dependant oscillatory behavior of the emitted laser is also addressed.

Ultra-low Power Frequency Conversion in Two-photon-absorption Free Micro Ring Resonator

All-optical integrated circuits bring the promise to vastly increase the bandwidth, improve the flexibility as well as reduce the cost of future communication networks. All optical integrated components, like switches and wavelength converters, must meet fundamental requirements such as low optical losses, strong nonlinear response and ease of fabrication. To date, however, research on these devices has been based either on semiconductors, such as silicon and GaAs, or highly nonlinear glasses, such as chalcogenides which, although exhibit a Kerr nonlinearity (n2) of 100x-400x silica glass, also present limitations such as remarkable linear and nonlinear losses and, for certain applications, a not developed fabrication techniques. Specifically, the possibility to rely upon a mature technology recently has proved itself to be a powerful solution to fabricate micrometric resonant structures which are able to locally enhance the desired nonlinearities. In this work we present the first example of nonlinear optics in silica glass waveguides using continuous wave (CW) light. We achieve wavelength conversion via four wave mixing (FWM) at ultra-low (approximate to 5 mW CW) power levels in C-MOS compatible micro-ring resonator.

200 mW type I GaSb-based laser diodes operating at 3 μm: Role of waveguide width

Laser diodes based on AlInGaAsSb/InGaAsSb heterostructures with different waveguide widths were designed and fabricated. The decrease in the waveguide width from 1470 to 470 nm led to the improvement of the device performance. Lasers with 470 nm quinternary waveguides demonstrated 200 mW continuous wave output power at room temperature.

Optical Limiting Behavior of Acid Blue 29 under a Low Power CW He-Ne Laser Irradiation

Optical limiting performances of acid blue 29 are investigated under irradiation by a 35 mW continuous wave He{Ne laser. The optical limiting behavior is investigated by using transmission measurement through samples with difierent concentrations. Our results show that the optical limiting e‐ciency is concentrations dependence. Induced self-difiraction patterns are observed at relatively high intensities starting from 15 mW.

Differential phase decoder in a polarized optical heterodyne interferometer

A differential-phase decoder (DPD) together with a polarization common-path optical heterodyne interferometer is set up. Based on this interferometric configuration and a novel balanced-detector scheme, the performance of the quantum-noise-limited differential-phase decoder is demonstrated and analyzed. The minimum-detectable differential phase is on the order of 10(-7) rad/sqrt Hz when a 2.5 mW He-Ne laser is used. Verified experimentally, the DPD is immune to the common-phase noise induced by an electro-optic phase modulator or by thermal disturbance within the interferometer. This signifies that the minimum-detectable differential phase can become 10(-8) rad/sqrt Hz if a 300 mW continuous wave laser is employed instead.

Epitaxial Lead Chalcogenides on Si for Mid-IR Detectors and Emitters Including Cavities

Lead chalcogenide (IV–VI narrow-gap semiconductor) layers on Si or BaF2(111) substrates are employed to realize two mid-infrared optoelectronic devices for the first time. A tunable resonant cavity enhanced detector is realized by employing a movable mirror. Tuning is across the 4 μm to 5.5 μm wavelength range, and linewidth is <0.1 μm. Due to the thin (0.3 μm) PbTe photodiode inside the cavity, a higher sensitivity at higher operating temperatures was achieved as compared to conventional thick photodiodes. The second device is an optically pumped vertical external-cavity surface-emitting laser with PbTe-based gain layers. It emits at ∼5 μm wavelength and with output power up to 50 mW pulsed, or 3 mW continuous wave at 100 K.

Blue-Violet Four-Beam Light Source Using Waveguides

To improve the recording rate of optical disk equipment, we have been developing an optical device that can be used with a multi-beam light source by employing waveguides for visible light to align the beams at a narrow pitch. In this study, an optical device composed of eleven bent waveguides was fabricated using amorphous polymer, which is highly transparent throughout the visible region. The waveguides transmitted approximately 70% of a blue-violet beam with a wavelength of 405 nm, obtaining a maximum intensity of approximately 20 mW (continuous wave). By controlling the curvature of each waveguide, the optical device narrowed the pitch of blue-violet beams from 125 to 12 µm, acting as an optical system to reduce the pitch of beams by a factor of approximately 10. Using the optical device, a blue-violet four-beam light source with a narrow pitch of 12 µm was demonstrated.

Cyclotron resonance heating systems for SST-1

RF systems in the ion cyclotron resonance frequency (ICRF) range and electron cyclotron resonance frequency (ECRF) range are in an advanced stage of commissioning, to carry out pre-ionization, breakdown, heating and current drive experiments on the steady-state superconducting tokamak SST-1. Initially the 1.5 MW continuous wave ICRF system would be used to heat the SST-1 plasma to 1.0 keV during a pulse length of 1000 s. For different heating scenarios at 1.5 and 3.0 T, a wide band of operating frequencies (20–92 MHz) is required. To meet this requirement two CW 1.5 MW rf generators are being developed in-house. A pressurized as well as vacuum transmission line and launcher for the SST-1–ICRF system has been commissioned and tested successfully. A gyrotron for the 82.6 GHz ECRF system has been tested for a 200 kW/1000 s operation on a water dummy load with 17% duty cycle. High power tests of the transmission line have been carried out and the burn pattern at the exit of transmission line shows a gaussian nature. Launchers used to focus and steer the microwave beam in plasma volume are characterized by a low power microwave source and tested for UHV compatibility. Long pulse operation has been made feasible by actively cooling both the systems. In this paper detailed test results and the present status of both the systems are reported.

Relevance of a Midinfrared Cavity Ring-Down Diagnostic in the Study of a Dusty&amp;lt;tex&amp;gt;$hboxSiH_4$&amp;lt;/tex&amp;gt;Plasma

We know that cavity ring-down spectroscopy (CRDS) can be of particular relevance in the study of the optical absorption of dust particles in a radio-frequency silane plasma. Although CRDS has historically primarily made use of pulsed lasers as light sources, the experimental design described here is based on 0.1 mW continuous wave multimode lead-salt laser diodes emitting in the midinfrared wavelength range. In order to interrupt the laser beam and reach similar effects to pulsed light sources, we propose a new method called mode tuning, based on a small detuning of the incoming laser wavelength. This work shows that CRDS has the potential to measure the dynamics of all radicals relevant in the particle nucleation process.

Broadband high power continuous wave fiber Raman source and its applications

A novel fiber Raman source (FRS) that can simultaneously achieve a high continuous wave (CW) output power and a flat and broad spectrum is demonstrated experimentally. A160 mW CW output is achieved by 1 W pump power with 44 nm spectral bandwidth (flatness <±2 dB). The FRS features in flexible output wavelength, adjustable output spectrum profile and high CW output power. Its application as pump source for ultra-flat fiber Raman amplification is verified.

Prototype for in situ detection of atmospheric NO3 and N2O5 via laser-induced fluorescence.

We describe a prototype designed for in situ detection of the nitrate radical (NO3) by laser-induced fluorescence (LIF) and of N2O5 by thermal dissociation followed by LIF detection of NO3. An inexpensive 36 mW continuous wave multi-mode diode laser at 662 nm is used to excite NO3 in the B2E'(0000) <-- X2A'2(0000) band. Fluorescence is collected from 700 to 750 nm. The prototype has a sensitivity to NO3 of 76 ppt for a 60 s integration with an accuracy of 8%. Although this sensitivity is adequate for studies of N205 in many environments, it is much less sensitive (about 300 times) than expected based on a comparison of previously measured photophysical properties of NO2 and NO3. This implies much stronger nonradiative coupling of electronic states in NO3 than in NO2.

A new concept for tunable long wavelength VCSEL

We present a novel two-chip concept for micro-electro-mechanically tunable vertical cavity surface emitting lasers (VCSELs) for the 1.55 μm wavelength range. The VCSEL is composed of two chips: one mirror membrane chip with a movable curved mirror membrane that can be displaced by electro-thermal actuation to adjust the cavity length and one “half-VCSEL” chip consisting of a fixed bottom mirror and an amplifying active region. The possibility of separate optimization of the micro-mechanical part and the VCSEL amplifier is the main advantage of that concept, which is appropriate for photo-pumping as well as for an electrical pumping scheme. The measurement results of an optically pumped VCSEL with more than 0.5 mW continuous wave (CW) single mode output power at room temperature over a 24 nm tuning range prove the feasibility of the proposed concept.

Laser-tissue interaction with a continuous wave 3-mcm fibre laser: preliminary studies with soft tissue.

Lasers operating at wavelengths in the mid-infrared region have become increasingly popular for applications in areas of surgery and medicine. Advances in fibre laser technology have introduced a highly efficient, compact, diode-pumped source operating at around the 3-mcm wavelength. This study examines the effects of this recently developed laser on soft biological tissue. Chicken breast and liver tissue samples were exposed to 800 mW continuous wave laser power, at a wavelength of 2.71 mcm, with incident spot sizes of around 150 mcm. Samples were inspected grossly immediately after laser irradiation and also prepared for histologic processing. After irradiation, visual assessment of changes at sample surfaces indicated a region of thermally affected tissue surrounding the ablation crater. This region was observed to grow in size to around 1.0 mm in diameter after 3 seconds of laser exposure at 800 mW. An ablation velocity of 0.80 mm.s(-1) was determined in chicken breast for the same incident laser parameters. Analysis of samples irradiated at 800 mW and processed for histology revealed minimal damage at hole boundaries and no signs of char formation, providing incident exposure times were restricted to below around 0. 5 seconds. This fibre laser source has demonstrated its potential to fulfil medical applications, enabling accurate, precise tissue removal to proceed at a rapid ablation rate. The efficiency and small size of the laser are attractive features.

Continuously-tunable doubly resonant optical parametric oscillator

We report on an all-solid-state source of broadly tunable single-frequency radiation from a continuous-wave (CW), doubly resonant optical parametric oscillator (OPO) based on non-critically phase-matched magnesium oxide-doped lithium niobate (MgO:LiNbO 3). The oscillator is pumped by the second harmonic of a single-frequency Nd:YAG miniature ring laser at 532 nm and provides combined signal and idler output power of nearly 400 mW CW. Covering a tuning range between 1007 and 1128 nm, stable operation on a single-axial-mode pair is obtained for several hours without mode hops using an FM sideband-locking technique. The output frequencies of the OPO can be continuously tuned over a range of 8 GHz via precise control of the pump laser frequency.

Development of microelectromechanical variable blaze gratings

Two types of microelectromechanical variable blaze gratings (VBGs) have been designed, modeled, fabricated, and tested. The gratings operate by adjusting the blaze angle of each slat so specular reflection of the incident light matches a particular grating diffraction order. The VBG blaze angle is adjustable with either electrostatic or thermal actuators. VBGs direct incident light in discrete directions, and are useful for steering light with beam diameters greater than 1 mm and power levels greater than 1 W. Both electrostatically and thermally actuated VBGs have been constructed from gold and polysilicon using a surface-micromachining process and tested with a 20 mW continuous wave HeNe laser operating at a wavelength of 632.8 nm. The diffraction efficiency and far-field pattern have been modeled and measured. Drive voltages for both types of gratings are measured as a function of blaze angle and selected diffraction order.

A PRELIMINARY DOUBLE BLIND CONTROLLED STUDY ON FREE AMINO ACID ANALYSIS IN BURN WOUNDS IN THE MOUSE FOLLOWING 830 nm DIODE LASER THERAPY

A double-blind controlled study is presented on the amino acid analysis of levels of a selected group of 26 free amino acids (FAA) in CO2 laser generated standardized burn wounds on the bilateral dorsum in the ddy mouse model. Four groups of ddy white mice (male, 5 weeks old, average weight 20 g), 6 animals per group, were anaesthetized with ether. The hair on the bilateral dorsum of all animals was carefully removed with a depilatory cream, and in three of the groups standardized burn injuries were inflicted using the defocused beam of the CO2 laser (25 W, continuous wave, 1.45 cm diameter spot, 0.99 sec/shot). The injuries were made bilaterally 1 cm laterally from the dorsal midline, 1.5 cm caudally from the line drawn between the base of the ears. The 4th group was the unburned control group for assessing base FAA levels. In experimental group A (LLLT group), the centre of the right hand burn injury was irradiated daily on days 0, 1, 2, 3, 4, and 5 with an 830 nm GaAlAs diode laser system (60 mW continuous wave). The left hand injury was unirradiated. The laser was held 1 mm from the surface of the wound, giving a spot size of approximately 0.03 cm2, 20 sec per exposure per day, energy density per exposure of 40 J/cm2. Group B (sham irradiation group) was handled in exactly the same way, but a non-emitting probe was used. Group C (unirradiated burned controls) was not handled at all. The animals were killed (ether overdose) on post-burn day 10. The size of the burn injury and the degree of wound healing were measured macroscopically on days 2, 5 and 7 and 10. The LLLT group A demonstrated better wound healing than groups B and C, between which there was no difference. On days 0, 1, 3, 5, 7 and 10, sample tissue was collected from both burned sites in the burned animals and from the unburned controls and subjected to automated AA analysis. The unburned controls were used as standards. On day 7 specimens were taken from the burn wounds in groups A, B and C and routinely processed for histological assessment with haematoxylin and eosin and elastica van Giesen’s stains. Automated AA analysis in general revealed higher levels of total FAA in group A, with no statistical difference between groups B and C. When looked at individually as spider graphs, the 26 AA isolated in this study at fell into 6 typical patterns. At day 10 histology revealed more advanced wound healing between the LLLT group and groups B and C, between which there was no difference. although the present study was only designed to isolate patterns in FAA associated with healing burns, the authors conclude that 830 nm diode LLLT certainly appears to accelerate wound healing, and there is a higher level of those FAA associated with protein synthesis.

Reliability of aluminum-free 808 nm high-power laser diodes with uncoated mirrors

The reliability of uncoated InGaAsP/GaAs high-power diode lasers emitting at 808 nm wavelength has been studied. 47 W of quasicontinuous wave output power (pulse width 200 μs, frequency 20 Hz) have been obtained from a 1-cm-wide laser bar. A single-stripe diode without mirror coating has been life tested at 40 °C for emitting power of 800 mW continuous wave (cw) and showed no noticeable degradation and no change of the lasing wavelength after 6000 h of operation.