Single-frequency Performance Research Articles

Single-frequency pulsed fiber laser based on an electro-optic modulator with injection seeding technique.

A single-frequency linearly polarization pulsed fiber laser based on an electro-optic modulator with injection seeding technique is demonstrated. The single-frequency performance of the fiber ring-cavity laser is guaranteed by the seed source, which is a distributed-feedback fiber laser based on the π-phase-shifted fiber Bragg grating. The electro-optic modulator triggers active Q-switching of the laser for pulse generation. The devices used in the fiber laser are all polarization-maintaining to ensure linear polarization laser output. Through parameter optimization, the laser generates a single-frequency linearly polarization pulsed laser with a central wavelength of 1064.22nm, linewidth of 35MHz, and polarization extinction ratio of better than 40dB. This type of fiber laser can be applied in lidar, beam combining, nonlinear frequency conversion, and other fields.

Sub-Kilohertz Narrow Linewidth Fiber Laser Based on Nanoparticles Doped Self-Injection Module

Fiber lasers with narrow linewidth are essential light sources for many prospects in advanced scientific fields. In this article, we proposed a scheme of self-injection locked single-frequency fiber laser based on a hybrid cavity configuration and a novel type of nanoparticles doped fiber. The newly proposed hybrid cavity was applied to achieve mode selection in the case of a long cavity which helped increase output power without sacrificing mode instability and single frequency performance. The nanoparticles doped self-injection module helped achieve a fiber laser with 3dB linewidth compressed from 16 kHz to 497 Hz and side mode suppression ratio of more than 60 dB. Furthermore, the sub-kilohertz fiber laser was amplified to more than 1 W with the polarization extinction ratio of ~30 dB. Throughout the work, experimental verification and analysis have been carried out to confirm the single frequency lasing performance, including center wavelength drift and slope efficiency variation. The demonstrated watt-level narrow linewidth fiber laser with low RIN and frequency noise has full potential for practical applications.

Dynamic stable ring resonator for high-power continuous single-frequency lasers: conditions for a compact resonator.

When considering dynamically stable resonators, ring lasers are good choices because they have a stability interval that is twice as large as that of linear resonators and sensitivity to misalignment decreasing with pump power; however, the literature does not provide easy design guidelines. A ring resonator utilizing Nd:YAG side pumped by diodes allowed single-frequency operation. The output single-frequency laser had good output characteristics; however, the overall length of the resonator did not allow for building a compact device with low misalignment sensitivity and larger spacing between longitudinal modes which could improve single-frequency performance. Based on previously developed equations, which allow for ease of design of a ring dynamically stable resonator, we discuss how to build an equivalent ring resonator, aiming to building a shorter resonator with the same stability zone parameters. The study of the symmetric resonator containing a pair of lenses allowed us to find the conditions to build the shortest possible resonator.

Computational through-wall imaging using a dynamic metasurface antenna

We present a study of computational through-wall imaging using a dynamically reconfigurable metasurface antenna (DMA). The DMA consists of a single-feed, electrically-large microstrip line, loaded with individually addressable metamaterial radiators. Each metamaterial resonator is integrated with a diode, enabling it to be switched on (radiating) or off (non-radiating) by an externally applied voltage. By switching subsets of the array of elements on or off, spatially diverse radiation patterns are formed that are scattered by the wall and structures beyond the wall. Images can be reconstructed from these measurements, using a combination of range migration algorithms and wall compensation algorithms, with minimal frequency bandwidth requirements; even single frequency measurements are possible in conjunction with the DMA. We investigate imaging through a variety of wall materials at K-band frequencies (18-26.5 GHz), including homogeneous media with known properties and inhomogeneous materials such as plywood. We further investigate single-frequency performance against full-bandwidth measurements. The DMA used here is electrically large in one dimension, over which many spatially diverse measurements can be taken. By scanning the DMA in the perpendicular direction, full two-dimensional scans can be acquired with minimal cost and time, making the one-dimensional DMA attractive as the basis for future through-wall scanning systems.

Exact comprehensive equations for the photon management properties of silicon nanowire.

Unique photon management (PM) properties of silicon nanowire (SiNW) make it an attractive building block for a host of nanowire photonic devices including photodetectors, chemical and gas sensors, waveguides, optical switches, solar cells, and lasers. However, the lack of efficient equations for the quantitative estimation of the SiNW’s PM properties limits the rational design of such devices. Herein, we establish comprehensive equations to evaluate several important performance features for the PM properties of SiNW, based on theoretical simulations. Firstly, the relationships between the resonant wavelengths (RW), where SiNW can harvest light most effectively, and the size of SiNW are formulized. Then, equations for the light-harvesting efficiency at RW, which determines the single-frequency performance limit of SiNW-based photonic devices, are established. Finally, equations for the light-harvesting efficiency of SiNW in full-spectrum, which are of great significance in photovoltaics, are established. Furthermore, using these equations, we have derived four extra formulas to estimate the optimal size of SiNW in light-harvesting. These equations can reproduce majority of the reported experimental and theoretical results with only ~5% error deviations. Our study fills up a gap in quantitatively predicting the SiNW’s PM properties, which will contribute significantly to its practical applications.

Mean Expected Error in Prediction of Total Body Water: A True Accuracy Comparison between Bioimpedance Spectroscopy and Single Frequency Regression Equations.

For several decades electrical bioimpedance (EBI) has been used to assess body fluid distribution and body composition. Despite the development of several different approaches for assessing total body water (TBW), it remains uncertain whether bioimpedance spectroscopic (BIS) approaches are more accurate than single frequency regression equations. The main objective of this study was to answer this question by calculating the expected accuracy of a single measurement for different EBI methods. The results of this study showed that all methods produced similarly high correlation and concordance coefficients, indicating good accuracy as a method. Even the limits of agreement produced from the Bland-Altman analysis indicated that the performance of single frequency, Sun's prediction equations, at population level was close to the performance of both BIS methods; however, when comparing the Mean Absolute Percentage Error value between the single frequency prediction equations and the BIS methods, a significant difference was obtained, indicating slightly better accuracy for the BIS methods. Despite the higher accuracy of BIS methods over 50 kHz prediction equations at both population and individual level, the magnitude of the improvement was small. Such slight improvement in accuracy of BIS methods is suggested insufficient to warrant their clinical use where the most accurate predictions of TBW are required, for example, when assessing over-fluidic status on dialysis. To reach expected errors below 4-5%, novel and individualized approaches must be developed to improve the accuracy of bioimpedance-based methods for the advent of innovative personalized health monitoring applications.

Sub-watt threshold, kilohertz-linewidth Raman distributed-feedback fiber laser

We report a low-threshold, narrow linewidth Raman distributed-feedback (R-DFB) fiber laser at 1109.54 nm based on a 30 cm long center π phase-shifted Bragg grating written directly in a commercially available germano-silica (Ge/Si) fiber. The R-DFB was pumped by a continuous-wave (CW) linearly polarized fiber source at 1064 nm, and the threshold power, full width at half-maximum (FWHM) linewidth and slope efficiency with respect to the incident pump power are measured to be 440 mW, <2.5 kHz (measured with a 29.75 km fiber delay line) and 13.5%, respectively. An rf spectrum analyzer confirms that the R-DFB fiber laser exhibits single-frequency performance.

A laser system for the excitation of rubidium Rydberg states using second harmonic generation in a PPLN waveguide crystal

We report on a laser system at a wavelength of 495 nm which is suitable for the excitations of low lying Rydberg states of rubidium atoms. The system is based on frequency doubling of a seeded diode laser in a periodically poled waveguide crystal. We achieve an output power of up to 35 mW and prove the single-frequency performance by direct two photon laser spectroscopy of the rubidium 14D 5/2 and 14D 3/2 states. The measured fine structure splitting is consistent with quantum defect theory calculations.

High-power, continuous-wave, optical parametric oscillator pumped by an optically pumped semiconductor laser at 532 nm

We report a high-power, cw, single-frequency optical parametric oscillator pumped external to an optically pumped semiconductor laser (OPSL). The singly resonant oscillator (SRO), based on a 30 mm crystal of MgO:sPPLT, is pumped by a 6 W, cw OPSL at 532 nm. By deploying signal outcoupling and loose focusing in the presence of low pump power, we achieve significant improvements in SRO performance across the tuning range with regard to signal power, extraction efficiency, and pump depletion by reducing thermal effects. We generate >1.78 W of idler and >0.9 W of signal over 856-1404 nm while maintaining a total output power of >2 W over 93% of the tuning range at an extraction efficiency of 48.7% and pump depletion of 84%. The signal power remains within 0.85-1.05 W across the entire tuning range and has a TEM(00) spatial profile with M(2)<1.28 and circularity of >97%. Measurements of signal spectrum confirm single-frequency performance with an instantaneous linewidth of approximately 15 MHz and frequency stability better than 60 MHz over 60 s.

Record-efficient, resonantly-pumped, Er-doped singlemode fibre amplifier

An 85% optical-to-optical efficient, single-frequency performance of resonantly-pumped (at 1476 nm) Yb-free, Er-doped, singlemode fibre amplifier at 1560 nm is reported. The diffraction-limited, single-frequency amplifier is believed to have the highest optical-to-optical efficiency ever reported for Er-doped fibre amplifiers.

Single-mode operation and frequency doubling of fiber-coupled diode butt-coupling-pumped Nd:YVO_4 lasers

We present a simple way to achieve single-frequency operation by using a fiber-coupled diode butt-coupling-pumped Nd:YVO(4) laser in a flat-flat cavity. Single-mode outputs of 620 and 260 mW for fundamental and second-harmonic wavelengths were obtained when the laser was pumped by an 1100-mW fiber-coupled laser diode. Experimental results show that thermal effects provide not only a stable resonator with a good overlap of laser mode and pump size but also enhance single-frequency performance.

Increased single-frequency power from end-pumped ring lasers by pump beam displacement

End-pumped unidirectional ring lasers often suffer some degree of residual spatial hole burning, where a standing-wave region overlaps the pumped region. By non-concentric pumping this overlap can be reduced, thus increasing the output power level at which single frequency operation can be maintained. Experimental results show a factor of 2 to 3 improvement in single-frequency performance for only a small drop in output power.

Limitations imposed by spatial hole burning on the single-frequency performance of unidirectional ring lasers

Residual spatial-hole-burning in a unidirectional ring laser can limit the maximum power available in a single-frequency. These effects have been modelled numerically and verified experimentally in Nd:YAG and Nd:YLF lasers. For convenience, an analytical expression for maximum single-frequency power has also been developed for use in designing ring lasers.

Single-mode operation of diode-pumped Nd:YAG lasers by pump-beam focusing

We present a theoretical study of the maximum single frequency power that can be obtained from a simple coaxially end-pumped Nd:YAG laser. The approach is general for four-level solid-state laser systems. It is shown that careful control of the pump beam geometry, cavity length, and crystal length can enhance single frequency performance. >

A laser-diode-pumped Nd:Glass laser: mode-locked, high power, and single frequency performance

The authors report the performance of a laser-diode-pumped Nd:glass laser. Using the AM active mode-locking technique, pulses as short as 9 ps were obtained. The development of a high-power (415 mW), high-optical-slope-efficiency (32.4%) laser-diode-pumped Nd:glass laser using high-power, broad stripe laser diodes as the pump source is described. Single-frequency operation of the Nd:glass laser using an acoustooptic modulator to ensure unidirectional operation in a ring laser configuration was investigated. An instantaneous linewidth of less than 300 Hz was observed. >

Laser dye DCM: CW, synchronously pumped, cavity pumped and single-frequency performance

Laser dye DCM exhibits a tuning range of 605 to 725 nm with a lasing efficiency as high as 34% when pumped by the 488 nm line of the argon ion laser, placing it among the most efficient and broadly tunable dyes known. Performance of the dye is characterized for four laser systems: 1) continuous wave, 2) synchronously pumped (SP), 3) cavity dumped synchrompously pumped (SPCD) and 4) single-frequency ring dye laser. Pulse peak powers were as high as 520 W and 2.8 kW for SP and SPCD systems respectively.