Laser Transmission Experiment Research Articles

Ge-polymer bridge waveguide for mode-locked laser pulse generation.

A Ge-polymer hybrid waveguide is sandwiched between an indium phosphide (InP) reflective gain chip and a fiber Bragg grating (FBG) to construct a laser system. The hybrid waveguide serves as a bridge between the gain chip and the fiber with tailored mode-field matching at both facets. The 50-nm amorphous Ge (α-Ge) layer shows a nonlinear absorption effect at 1550 nm. The hybrid waveguide is further verified by a femtosecond laser transmission experiment to show the pulse width compression effect. Such waveguide is then integrated inside the laser cavity as a passive saturable absorber to modulate the longitudinal modes for a pulsed output. This polymer-bridged mode-locked laser adopts an InP gain chip for compact assembly and also a FBG with a flexible length to adjust the pulse repetition rate. The mode-locked laser output around the designed 50 MHz repetition rate is demonstrated. The pulse width is measured as 147 ps, and the signal-to-noise ratio is larger than 50 dB. This work introduces a "ternary" mode-locked laser system, taking advantage of discrete photonic components bridged by a polymer-based waveguide. It also proves the feasibility of applying α-Ge films as practical and low-cost saturable absorbers in photonic devices.

Atmospheric Turbulence Aberration Correction Based on Deep Learning Wavefront Sensing.

In this paper, research was conducted on Deep Learning Wavefront Sensing (DLWS) neural networks using simulated atmospheric turbulence datasets, and a novel DLWS was proposed based on attention mechanisms and Convolutional Neural Networks (CNNs). The study encompassed both indoor experiments and kilometer-range laser transmission experiments employing DLWS. In terms of indoor experiments, data were collected and training was performed on the platform built by us. Subsequent comparative experiments with the Shack-Hartmann Wavefront Sensing (SHWS) method revealed that our DLWS model achieved accuracy on par with SHWS. For the kilometer-scale experiments, we directly applied the DLWS model obtained from the indoor platform, eliminating the need for new data collection or additional training. The DLWS predicts the wavefront from the beacon light PSF in real time and then uses it for aberration correction of the emitted laser. The results demonstrate a substantial improvement in the average peak intensity of the light spot at the target position after closed-loop correction, with a remarkable increase of 5.35 times compared to the open-loop configuration.

Extinction of high-power laser radiation under adverse weather conditions.

The propagation of laser radiation over long distances can be significantly affected by atmospheric extinction due to precipitation as well as aerosol particles and molecules. The knowledge of the contribution of precipitation is critical to the operation of a variety of laser-based systems. The study of high-power laser transmission around 1µm is of particular interest because several atmospheric transmission windows are located in this region. To investigate the effect of adverse weather conditions on laser transmission, free-space laser transmission experiments are conducted on the DLR test range in Lampoldshausen, Germany. A high-power laser with a wavelength of 1.03µm is used for the transmission measurements in combination with calibrated power monitors. Local weather conditions are continuously monitored by meteorological instruments during the experiments. Extinction coefficients are derived from transmission measurements showing that the extinction for snow is 7 times higher than for rain, and the extinction for drizzle/rain is 4 times higher than for rain at a given precipitation rate of 1mm/h. For a mixture of rain and snow, the extinction is comparable to that of rain, indicating that the water content strongly influences the optical properties and thus the extinction of laser radiation in mixed precipitation. A good relationship is found between the measured extinction coefficient and visibility for drizzle and rain and a slightly larger scatter of the data for snow. Furthermore, measured extinction coefficients are compared to the extinction coefficients based on the experimental size distributions of precipitation particles and geometric optics. A reasonable agreement is obtained for rain, with no improvement taking the forward-scattering into the detector aperture into account, and a much better agreement is obtained for snow when the forward-scattering contribution is included.

Heat transfer enhancement of latent heat thermal energy storage with nanoparticle by bubble-driven flow

This study investigated the heat transfer enhancement of phase change material (PCM) with nanoparticles by bubble-driven flow. The addition of nanoparticles can improve conduction heat transfer, while bubble-driven flow can improve convection heat transfer. The 5 wt% nanoparticles increased the thermal conductivity by 6.4 % and viscosity 205 % of phase change material, while decreased the convective flow accordingly. However, bubble-driven flow strengthened the flow velocity by 8.3 times. Since nanoparticles tend to sediment over time in PCM, this issue of sedimentation should be overcome before they are applied in practice. The particle concentration in the vertical direction was analyzed by laser transmission experiment, and as expected, the concentration was the highest at the bottom. The bubble-driven flow uniformized the particle concentration in the vertical direction, which could solve the sedimentation problem. The combined application of nanoparticles and bubble-driven flow to phase change material both improved conductive and convective heat transfer, and provided uniform distribution of particle concentration. A latent heat thermal energy storage (LHTES) unit was designed for combined application study, and a charging performance experiment was conducted. Nanoparticle only improved the average charging power by 1.09 times, while bubble-driven flow only improved it by 1.27 times. Bubble-driven flow and nanoparticles applied altogether increased the average charging power by 1.41 times. Therefore, it was confirmed that the combined application of nanoparticles and bubble-driven flow can effectively improve the heat transfer performance of the latent heat thermal energy storage. Thus, this method is expected to be applied to a wide range of industries.

Negative Absorption Peaks in Ultraviolet–Visible Spectrum of Water

AbstractUltraviolet‐visible absorption spectra exhibit absorption that is under the base line, which we propose is negative absorption (NA) or negative absorption emission (NAE). The UV‐visible NAE of water, ethanol, and chloroform was illustrated by transmittance curves, reflectivity curves, and fluorescence excitation/emission spectra. To explain this phenomenon, the TD_DFT theory was used to calculate the emission states of the samples. A laser transmission experiment confirmed the NAE phenomenon. It was concluded that NAE was a type of emission produced by the sample. The NAE of water has significant implications for life on earth, which will interest researchers in physics, chemistry, and biology.

Multi-Factors Interaction Effects of Process Parameters on the Joint Strength of Laser Transmission Joining between PC and PA66

This paper presents a laser transmission joining (LTJ) experiment between thermoplastic Polycarbonate (PC) and glass reinforced nylon (PA66GF) using diode laser equipment. Laser transmission joining experimental design and experiment are carried out according to a single process parameters window. Response surface methodology (RSM) in Design-Expert v7 software is employed to develop mathematical models between LTJ process parameters and joint strength. The interaction effects of joining process parameters (line energy, spot diameter, clamp pressure) on the joint strength are investigated using analysis-of-variance (ANOVA), the result shows that the interaction effect of line energy and spot diameter has maximum influence on the joint quality. Finally, the predicted values from mathematical models developed by RSM are compared with the experimental values and it is found that they are nearly agreed with each other. The purpose of predicting joint strength based on reasonable process parameters is achieved.

Adaptive Interleaving Method and its Application to Free Space Optical Communications through Atmospheric Turbulence Channel

Adaptive interleaving method implemented by a hybrid system is proposed to deal with burst errors caused by long and deep channel fades in free space optical communications (FSOC) through atmospheric turbulence. Actually, considerable memory can be saved by changing the interleaving depth at transmitter and receiver according to the updated burst information from channel estimator. Combining with error correcting codes, anti-burst performance of the proposed method is evaluated by simulations based on channel state information from laser transmission experiments over an 11.8 km urban link. Simulation results show that 1~2 or even higher orders of magnitude of a memory can be saved when comparing with the maximum memory interleaving. The results have proved that the cost of anti-burst performance can be minimized to 10% or even smaller by the adaptive interleaving method when joint use with error correcting codes.

A novel approach for simulating the optical misalignment caused by satellite platform vibration in the ground test of satellite optical communication systems

Satellite platform vibration causes the misalignment between incident direction of the beacon and optical axis of the satellite optical communication system, which also leads to the instability of the laser link and reduces the precision of the system. So how to simulate the satellite platform vibration is a very important work in the ground test of satellite optical communication systems. In general, a vibration device is used for simulating the satellite platform vibration, but the simulation effect is not ideal because of the limited randomness. An approach is reasonable, which uses a natural random process for simulating the satellite platform vibration. In this paper, we discuss feasibility of the concept that the effect of angle of arrival fluctuation is taken as an effective simulation of satellite platform vibration in the ground test of the satellite optical communication system. Spectrum characteristic of satellite platform vibration is introduced, referring to the model used by the European Space Agency (ESA) in the SILEX program and that given by National Aeronautics and Space Development Agency (NASDA) of Japan. Spectrum characteristic of angle of arrival fluctuation is analyzed based on the measured data from an 11.16km bi-directional free space laser transmission experiment. Spectrum characteristic of these two effects is compared. The results show that spectra of these two effects have similar variation trend with the variation of frequency and feasibility of the concept is proved by the comparison results. At last the procedure of this method is proposed, which uses the power spectra of angle of arrival fluctuation to simulate that of the satellite platform vibration. The new approach is good for the ground test of satellite optical communication systems.

Measurements of angle-of-arrival fluctuations over an 11.8 km urban path

AbstractA laser transmission experiment is conducted to examine atmospheric turbulence-induced angle-of-arrival fluctuations over an 11.8 km urban path. The variance of fluctuations, probability density function, power spectrum, and Fried coherence length are investigated based on the analysis of the experimental data collected in each season of a year, respectively. In addition, the daily variations characteristic of path-averaged optical turbulence intensity Cn2 is also studied. At last, the aperture averaging theory is validated. It is anticipated that this work is helpful to the research of optical wave atmospheric propagation and the design of free-space laser optics communication systems.

Short cavity single-frequency all-fiber Er/Yb co-doped laser

A 5-cm-long Er/Yb co-doped all-fiber laser is studied. Two fiber Bragg gratings (FBGs) are written in the Er/Yb co-doped sensitive fiber using UV beams. A 980 nm pumping laser diode (LD) is used, and output wavelength is selected by two FBGs. The single-frequency laser is achieved at 1544.68 nm. The 3 dB spectrum width is 0.08 nm, while the side mode suppression ratio is 55 dB. The maximum output power exceeds 4 mW for pump power of 140mW and the stability is less than ± 0.01 dB. Single-frequency operation is verified using a scanning Fabry-Perot (F-P) interferometer. Relative intensity noise is less than − 100 dB. A 10 Gbit/s code rate is used in the fiber laser transmission experiment. A good optical eye diagram is received after 21 km single-mode fiber transmission. A simple distributed Bragg reflection (DBR) fiber laser array is designed. The wavelength difference of output laser array is 0.8 nm, conforming to the ITU-T channel spacing standard of wavelength-division multiplexing systems.

Study on high power single‐frequency erbium doped fiber laser for long distance communications

AbstractTo stabilize the frequency, a structure with fiber seed laser plus fiber amplifier module is designed. Using only one laser diode simultaneously pumps fiber amplifier and laser through a coupler with special ratio. Single‐frequency fiber laser with high‐output power and low‐noise is achieved. The center‐wavelength is 1550.16 nm while the 3 dB spectrum‐width of output laser is 0.08 nm and the side mode suppression ratio is 55 dB. The maximum output power exceeds 23 mW, the slope efficiency is more than 20%, and the stability is less than ±0.005 dB. A nice single‐frequency is observed and the modes adjacent to the lasing mode are completely suppressed. A 10 Gbps codes rate is used in the fiber laser transmission experiment. The signal is transmitted in 42 km without regeneration. The eye diagrams of optical transmission are measured, the performance of long‐haul transmission with high‐speed modulation is perfect. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1660–1662, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23449

Mechanisms of energy deposition in infrared matrix-assisted laser desorption/ionization mass spectrometry

The mechanisms of energy deposition in matrix-assisted laser desorption/ionization mass spectrometry with infrared lasers (IR-MALDI-MS) have been evaluated in a series of experiments. In a first part, the threshold fluences for the generation of IR-MALDI spectra were determined between 2.7 and 4.0 μm wavelength with an optical parametric oscillator as a tunable laser source for nine solid state and two liquid matrices of different chemical structure and compared to the infrared absorption spectra of the compounds. Preliminary spectra of IR-MALDI in the wavelength range of 1.45–1.75 μm are also presented using the overtone vibrations of a glycerol matrix. Matrices were chosen with regard to their IR-MALDI performance and to allow conclusions on the IR-absorption mechanisms. Whereas the wavelength dependence of the threshold fluence for non–hydrogen-bound C–H vibrations essentially follows the absorption spectrum of this functional group, strong discrepancies between the spectral dependence of threshold fluences and IR-absorption spectra were found for the vibrations of O–H and N–H groups around 3-μm wavelength that form strong intermolecular and intramolecular hydrogen bonds. In a second part, experiments are described that interrogate the two most probable mechanisms for the observed deviation of the threshold fluence behavior from the wavelength course of the IR-absorption spectra, that is, absorption by either free or weakened O–H and N–H stretching modes or by residual free water. All investigations were performed with glycerol and succinic acid as examples for common liquid and solid state matrices for IR-MALDI. For glycerol, a fluence-dependent, dynamic change in absorption during the laser pulse was revealed by laser transmission measurements on thin glycerol layers. This effect, characterized by a significant blue shift of the O–H stretch absorption, can be attributed to a weakening of intermolecular hydrogen bonds caused by the transient laser heating of the sample. Taking this effect into account, a good correspondence of the wavelength dependence of the threshold fluence with the infrared absorption under IR-MALDI conditions is derived for glycerol. For succinic acid, in contrast, the identification of the predominant absorption mechanism in the 3-μm wavelength range appears more difficult. A fluence-dependent absorption was not detected in laser transmission experiments with succinic acid single crystals. A change in analyte-to-matrix ratio, with the intention of inducing free absorbers near crystal defects, also did not influence the wavelength dependence of the threshold fluences. However, an influence of the surface-to-volume ratio on the wavelength-dependent threshold fluences was found by a comparison of three different preparation techniques for succinic acid, indicating a putative influence of weakly hydrogen-bound surface absorbers. In combination with the detailed analysis of the wavelength dependence of the threshold fluence given from the first part, a determination of the IR-MALDI process for succinic acid based on the absorption by weakly hydrogen-bound hydroxyl groups is suggested. No evidence for a significant contribution of residual free water absorption to the low-threshold fluences around 3-μm wavelength was found by monitoring a possible change in threshold fluence at the phase transition from water to ice and by reducing the analyte hydration and varying the water content in glycerol preparations. Also, in preparations with frozen hydrated proteins without organic matrices, the wavelength dependence of the threshold fluence did not reflect the spectral absorption of ice, supporting the assumption of a rather minor role of the absorption by residual water in IR-MALDI.

KrF laser deposition of chromium films from Cr(CO) 6: in situ detection of film growth by laser transmission experiments

The results of a detailed study of laser-induced chemical vapour deposition (LICVD) of thin chromium films from Cr(CO) 6 using a KrF excimer laser are presented. The following deposition parameters were varied: the laser fluence, the pulse repetition rate, the total gas pressure in the deposition cell, the flow rate of the gas mixture supply, the type of carrier or buffer gas and the substrate temperature. The influence of these parameters on the nucleation time and rate, and the main film growth rate as well as on film properties such as the optical absorption coefficient was investigated. The film growth kinetics was monitored in situ by measuring the transmission of an HeNe laser probe beam through the deposit. The experimental results are discussed in the framework of a model of pulsed LICVD based on gas phase diffusion mass transport to the substrate surface of chromium-containing precursor species as the rate-limiting step. The model describes the experimental data reasonably well, the deviations observed being indicative of processes not included in the model such as reactions at the film surface. The results indicate that in pulsed LICVD of chromium films from Cr(CO) 6 at 248 nm the main precursors of the film growth during the nucleation period are chromium atoms and other highly reactive unsaturated chromium carbonyls formed on illumination in the gas phase, while during the main growth stage the role of more stable saturated species adsorbed on the surface between the laser pulses may be dominant.