CW Light Research Articles

Eff ect of Different Laser Irradiation Parameters on the Viability of Bacillus Species

The genus Bacillus is present everywhere in the environment, it has environmental, industrial, and medical signifi cance. This heterogeneous group of bacteria shares unique characteristics, one of these is the resistance to physical and chemical agents. Laser light is one of the most interesting physical agents with a wide range of applications in all scientifi c fi elds. It has the ability to alter bacterial growth by triggering inhibitory or stimulatory mechanisms. In this study, laser parameters were varied to evaluate the eff ect of each one on the viability of Bacillus species. CW or chopped (532 nm) laser light at diff erent irradiation periods were examined. Results showed that tested bacterial species’ response to green laser light varied. Bacterial viability was dropped with increasing chopping frequency and exposure time. In conclusion, high repetition rate and prolonged exposure time increase the lethal eff ects of this type of laser

Aspects of two-photon absorption of squeezed light: The continuous-wave limit

The authors present a theoretical analysis of two-photon absorption of classical and squeezed light that is valid for arbitrary intensity, both pulsed and cw light, and absorption both resonant and far-off-resonance from an intermediate state. Among other results, they find closed-form expressions for the cross sections and the scaling of the absorption with intensity.

Prism-based approach to create intensity-interferometric non-diffractive cw light sheets

Light sheets are optical beam-like fields with one-dimensional intensity localization. Ideally, the field intensity should be independent of the longitudinal and one of the transverse coordinates, which is difficult to achieve even for truncated light sheets. In this work, we present a general theoretical framework for intensity-interferometric continuous wave (cw) light sheets formed by overlapping the interference fringe patterns of mutually uncorrelated frequency components of the field. We show that the key parameters of the light sheets can be calculated using simple analytical expressions. We propose a practical way to generate such light sheets with the help of prisms and demonstrate numerically the abilities of the method. Both bright and dark light sheets with an exceptionally small thickness and long divergence-free propagation distance are possible to generate. We also show that the transverse profile of the generated light sheets can be shaped by modifying the spectrum of the light. We believe our findings advance the beam-engineering technology and its applications.

Nonlinear loss characterization of continuous wave guiding in silicon wire waveguides

Accurate propagation loss characterization of silicon waveguides is increasingly demanded for silicon-photonics-(Si-Ph) applications with high-power continuous-wave-(CW) light sources. We report on nonlinear loss parameters of silicon wire waveguides for 1.31 μm wavelength CW light extracted from transmission data measured for different lengths and polarizations. Such parameters were, so far, unavailable, although they are required for accurately modeling Si-Ph optical circuits. More-than-ten-times enhancement of two-photon absorption from prior results for short pulse light was observed at power densities ranging up to 4.7 × 1011 W m−2 while free carrier absorption was suppressed. We estimate the nonlinear loss of the waveguides using the parameter values obtained.

Mammalian complex III heme dynamics studied with pump-probe spectroscopy and red light illuminations.

The electronic or molecular mechanisms that initiate photobiomodulation (PBM) in cells are not yet fully understood. The porcine complex III (C-III) of the electron transport chain was characterized with transient absorption spectroscopy (TAS). We then applied our recently developed continuous wave laser coupled TAS procedure (CW-TAS) to investigate the effect of red light irradiances on the heme dynamics of C-III in its c1 reduced state. The time constants were found to be 3.3 ± 0.3 ps for vibrational cooling of the oxidized state and 4.9 ± 0.4 ps for rebinding of the photodissociated axial ligand of the c1 reduced state. The analysis of the CW-TAS procedure yielded no significant changes in the C-III heme dynamics. We rule out the possibility of 635 nm CW light at 4.7 mW/cm2 inducing a PBM effect on the heme dynamic of C-III, specifically with the photodissociation of its axial ligand.

Quantum noise squeezing of CW light in tellurite glass fibres

Light with suppressed quantum fluctuations is desirable for a lot of applications. There are several approaches to suppress a noise including Kerr squeezing in optical fibres. Silica fibres are commonly used for this purpose. Here we propose to use highly nonlinear tellurite glass fibres for Kerr squeezing of CW laser field and demonstrate by simulations in the framework of the stochastic generalized nonlinear Schrödinger equation the possibility of –20 dB noise suppression. In simulations, Raman terms and losses are switched on and switched off to find their contributions to the limits of squeezing. The analytical estimates without loss and with lumped loss are also given.

Enabling long range distributed vibration sensing using multicore fiber interferometers.

We report a novel long range distributed fiber optic vibration sensor using two counter-propagating interferometers, which are space-division multiplexed in different cores of a seven-core fiber. The proposed system requires only one laser source. Thanks to the spatially separated structure of the two interferometers, it no longer suffers from the coherent Rayleigh backscattering noise, and, since forward transmitting CW light instead of weak backscattering light is used, as a result, the sensing range can be significantly extended. On the other hand, because the two interferometers are implemented in one single compact multicore fiber, the output temporal waveforms have good correlation; therefore, the sampled waveforms can be used directly to determine the vibration location through cross correlation, and no additional complicated data processing (e.g., time-frequency analysis methods) is required. For proof of concept, we achieved distributed vibration detection with 38.5 km sensing range, and the root mean square error of positioning is estimated to be about 54.9 m with 120 measurements. The proposed system has the unique advantages of simple setup, high sensitivity, large dynamic range, ultra-long sensing range, etc.

Photo-acoustic response and optical features of 2D-TlGaSe2 and GaAs semiconductors

The paper focuses on investigation of the acoustic-optical properties of 2D-TlGaSe2 and GaAs semiconductors using laser pulses. We present the photo-darkening experiments performed by using CW light in spectral ranges below the band gap of TlGaSe2. The data provides evidence that photo-acoustic signals (PAS) for the above band gap are significantly stronger and linear in 2D-TlGaSe2 while they are distorted by nonlinear processes in isotropic GaAs. The comparison discloses characteristic parameters of TlGaSe2, namely a high factor and a negative sign in the refraction coefficient with pressure, a low absorption coefficient and absence of band filling; all guarantee stabile energy conversion by thermoelastic deformation mechanism. Below the band gap range, we found a new kind of PAS in TlGaSe2 on near surfaces. Likewise, we demonstrate that in such spectral range the giant Stark effect in TlGaSe2 is created by focused CW beams. The Stark effect produces local optical darkening which can provide explosion at extreme light power. Likely, both effects could be related to the planar stacking faults in 2D-TlGaSe2, which produce a spontaneous charge separation on layers.

High-frequency pulse train generation in dispersion-decreasing fibre: using experimental data for the metrology of longitudinally nonuniform fibre

This paper describes experiments concerned with pulse train generation from initially modulated cw light in an anomalous dispersion decreasing fibre. We have obtained stable contrast trains of subpicosecond pulses with a repetition rate in the range 100 – 300 GHz and demonstrated generation of an optical comb spectrum with a –20-dB width of up to 80 nm. A method has been proposed for reconstructing an unknown dispersion profile of a longitudinally nonuniform fibre by comparing experimental data and numerical simulation results.

Investigation of the Carrier Movement through the Tunneling Junction in the InGaP/GaAs Dual Junction Solar Cell Using the Electrically and Optically Biased Photoreflectance Spectroscopy

For examining the carrier movements through tunnel junction, electrically and optically-biased photoreflectance spectroscopy (EBPR and OBPR) were used to investigate the internal electric field in the InGaP/GaAs dual junction solar cell at room temperature. At InGaP and GaAs, the strength of p-n junction electric fields (Fpn) was perturbed by the external DC bias voltage and CW light intensity for EBPR and OBPR experiments, respectively. Moreover, the Fpn was evaluated using the Fast Fourier Transform (FFT) of the Franz—Keldysh oscillation from PR spectra. In the EBPR, the electric field decreased by increasing the DC bias voltage, which also decreased the potential barrier. In OBPR, when incident CW light is absorbed by the top cell, the decrement of the Fpn in the GaAs cell indicates that the photogenerated carriers are accumulated near the p-n junction. Photogenerated carriers in InGaP can pass through the tunnel junction, and the PR results show the contribution of the modification of the electric field by the photogenerated carriers in each cell. We suggest that PR spectroscopy with optical-bias and electrical-bias could be analyzed using the information of the photogenerated carrier passed through the tunnel junction.

Hyper-Rayleigh scattering in a strong coupling microcavity waveguide.

Second-harmonic generation (SHG) from hyper-Rayleigh scattering (HRS) in a hybrid strong coupling microcavity waveguide (HSCMW) was demonstrated, which indicates a possible method using continuous-wave (cw) incident light. The cw light was coupled into the waveguide with high coupling efficiency by free space coupling technology, and then the electric field intensity of the fundamental wave was enhanced due to local oscillation. HRS occurred by lithium niobite (LN) powder inside the waveguide, resulting in the direct observation of SHG in the transverse direction, with relatively high conversion efficiency measured to be 0.032%/W. This work suggests progress on frequency conversion and is also applicable to other nonlinear processes in a waveguide.

A key bidirectional switching issue in optogenetics emulated with laser dyes to illustrate its mitigation using nonlinear optical tools

Conventionally, bidirectional optogenetic switches are controlled with linear excitation (e.g., monochromatic CW light), which has serious limitations when there is a spectral overlap between the “on” and “off” states of the switch. The spectral crosstalk lowers the selectivity of the photoactivation step, resulting in a moderate photoequilibrium value and a poor dynamic range for the switch. Using stimulated depletion quenching (SDQ), which is a nonlinear optical strategy similar to STED, we demonstrate enhanced photoactivation selectivity on one side of the switch, thus shifting the photoequilibrium beyond what is achievable with CW light. The discussion is built around Cph8 as a prototypical bidirectional optogenetic switch lacking complete photoreversibility upon CW excitation because of the spectral crosstalk. We use two fluorescent dyes as analogues to emulate the key spectral feature of the on and off states of Cph8; in this way, we focus on the initial photoactivation step and abstract from the complicated subsequent dynamics. By applying SDQ to a mixture of the dyes, we enhance the selectivity of the photoactivation beyond the linear regime. Increased selectivity of the photoactivation step via nonlinear optical techniques should translate into an improved dynamic range for a broad variety of bidirectional switches. These experiments provide a basis to further expand the foundations for non-conventional optical control methods of biological switching.

Resonant fiber optic gyroscope with three-frequency differential detection by sideband locking.

A new scheme of three-frequency differential detection with a sideband locking technique is firstly proposed to suppress backscattering noise for improving the accuracy of resonator fiber optic gyroscope (RFOG). In the system we proposed, one light path is divided into three paths and sinusoidal wave modulations of different frequencies are respectively applied to generate the sideband. The first-order sidebands of the three channels of light in the cavity are locked to the adjacent three resonance peaks by sideband locking technique. The carrier and the remaining sidebands of the three channels of light are moved to a position away from the resonance peak, thereby achieving the purpose of being suppressed by the cavity itself. As a result, the frequency difference between the CW light and the other two CCW lights reaches one free spectral range (FSR), eliminating the expected backscattering noise. The experimental results demonstrate that the RFOG has a bias stability 0.9°/h based on the Allan deviation, and the corresponding angular random walk (ARW) 0.016°/√h, which validate that our scheme can effectively suppress backscattering noise to promote performance of RFOG in practical applications.

Fast repetition rate fs pulsed lasers for advanced PLIM microscopy

Simultaneous metabolic and oxygen imaging is promising to follow up therapy response, disease development and to determine prognostic factors. FLIM of metabolic coenzymes is now widely accepted to be the most reliable method to determine cellular bioenergetics. Also, oxygen consumption has to be taken into account to understand treatment responses. The phosphorescence lifetime of oxygen sensors is able to indicate local oxygen changes. For phosphorescence lifetime imaging (PLIM) dyes based on ruthenium (II) coordination complexes are useful, in detail TLD1433 which possesses a variety of different triplet states, enables complex photochemistry and redox reactions. PLIM is usually reached by two photon excitation of the drug with a femtosecond (fs) pulsed Ti:Sapphire laser working at 80[Formula: see text]MHz repetition rate and (time-correlated single photon counting) (TCSPC) detection electronics. The interesting question was whether it is possible to follow up PLIM using faster repetition rates. Faster repetition rates could be advantageous for the induction of specific photochemical reactions because of similar light doses used normally in standard CW light treatments. For this, a default 2[Formula: see text]-FLIM–PLIM system was expanded by adding a second fs pulsed laser (“helixx”) which provides 50[Formula: see text]fs pulses at a repetition rate of 250[Formula: see text]MHz, more than 2.3[Formula: see text]W average power and tunable from 720[Formula: see text]nm to 920[Formula: see text]nm. The laser beam was coupled into the AOM instead of the default 80[Formula: see text]MHz laser. We demonstrated successful applications of the 250[Formula: see text]MHz laser for PLIM which correlates well with measurements done by excitation with the conventional 80[Formula: see text]MHz laser source.

Compensation of Mode Crosstalk in MDM System Using Digital Optical Phase Conjugation

We propose to utilize the digital optical phase conjugation (D-OPC) technique for the compensation of mode crosstalk in the mode-division-multiplexed (MDM) transmission system. To realize the D-OPC technique, we send the CW light from the receiver side, measure its wavefront at the transmitter side, and then, transmit the phase-conjugated signal to the receiver. By using this technique, we demonstrate the transmission of four LP modes, each carrying 10-Gb/s ON–OFF keying signals, over a 100-m long few-mode fiber link. These signals are detected by using direct-detection receivers with no electrical equalizer. The results show that, by applying the proposed D-OPC technique, we can achieve the bit-error rate of < 10−3 for all four modes.

On-chip silicon photonic integrated frequency-tunable bandpass microwave photonic filter.

An on-chip frequency-tunable bandpass microwave photonic filter (MPF) implemented on a silicon photonic platform is reported. The on-chip MPF consists of a high-speed phase modulator (PM), a thermally tunable high-Q micro-disk resonator (MDR), and a high-speed photodetector (PD). The filtering function of the MPF is realized based on phase modulation and phase modulation to intensity modulation conversion, to translate the spectral response of the MDR in the optical domain to the spectral response of the MPF in the microwave domain. The tunability of the MPF is realized by thermally tuning the MDR. The proposed on-chip bandpass MPF is fabricated and characterized. An MPF with a passband of 1.93GHz and a tunable range from 3 to 10GHz is demonstrated. The power consumption, the insertion loss, and the bandwidth over the entire tunable range are studied. This successful implementation of an MPF marks a significant step forward in the full integration of microwave photonic systems on a single chip.

Signature of phase singularities in diffusive regimes in disordered waveguide lattices: interplay and qualitative analysis.

Coexistence and interplay between mesoscopic light dynamics with singular optics in spatially disordered waveguide lattices are reported. Two CW light beams of a 1.55μm operating wavelength are launched as inputs to 1D waveguide lattices with controllable weak disorder in a complex refractive index profile. Direct observation of phase singularities in the speckle pattern along the length is numerically demonstrated. Quantitative analysis of the onset of such singular behavior and diffusive wave propagation is analyzed for the first time, to the best of our knowledge.

High visibility Hong-Ou-Mandel interference via a time-resolved coincidence measurement.

A high visibility Hong-Ou-Mandel (HOM) interference between two independently prepared photons plays an important role in various photonic quantum information processing. In a standard HOM experiment using photons generated by pulse-pumped spontaneous parametric down conversion (SPDC), larger detection time windows than the coherence time of photons have been employed for measuring the HOM visibility and/or drawing the HOM dip. If large amounts of stray photons continuously exist within the detection time windows, employing small detection time windows is favorable for reducing the effect of background noises. Especially, such a setup is helpful for the HOM experiment using continuous wave (cw)-pumped SPDC and the time-resolved coincidence measurement. Here we argue that the method for determining the HOM visibility used in the previous cw experiments tends to suffer from distortion arising from biased contribution of the background noises. We then present a new method with unbiased treatment of the cw backgrounds. By using this method, we experimentally demonstrate a high visibility HOM interference of two heralded telecom photons independently generated by SPDC with employing cw pump light. An observed HOM visibility is 0.87 ± 0.04, which is as high as those observed by using pulse-pumped SPDC photons.

High efficient generation of over 1 Watt 509 nm laser beam by a ring cavity frequency doubler with periodically poled KTiOPO_4

We report high efficient generation of a 509 nm laser beam by a ring cavity frequency doubler with a periodically poled KTiOPO4 (PPKTP) crystal. A maximum power of 1.13 W at 509 nm cw light is obtained with mode-matched power of 1.58 W, corresponding to a conversion efficiency of 71.5%. By using the Hansch–Couillaud locking technique, the doubler is locked with high stability, and power fluctuation smaller than ±0.5% within 1 h is achieved at the maximum output. To the best of our knowledge, this is the highest second-harmonic power produced by a cavity-enhanced PPKTP frequency doubler below 532 nm.

Efficient polarization of high-angular-momentum systems

We propose methods of optical pumping that are applicable to open, high-angular-momentum transitions in atoms and molecules, for which conventional optical pumping would lead to significant population loss. Instead of applying circularly polarized cw light, as in conventional optical pumping, we propose to use techniques for coherent population transfer (e.g., adiabatic fast passage) to arrange the atoms so as to increase the entropy removed from the system with each spontaneous decay from the upper state. This minimizes the number of spontaneous-emission events required to produce a stretched state, thus reducing the population loss due to decay to other states. To produce a stretched state in a manifold with angular momentum J, conventional optical pumping requires about 2J spontaneous decays per atom, one of our proposed methods reduces this to about log_2(2J), while another of the methods reduces it to about one spontaneous decay, independent of J.