Depolarization Compensation Research Articles

Ultrafast 10 mJ, 100 W laser system featuring a directly laser written depolarization compensation element

In this study we demonstrated a compact and cost-effective high energy and average power picosecond laser developed for OPCPA system pumping applications. The system delivered record high pulse energy at 100 W average power level in a hybrid laser architecture based on a fiber seed laser and free-space end-pumped Yb:YAG amplifiers. The output pulses were compressed to 1 ps pulse duration and the output beam featured M2 = 1.3, which was further improved to 1.07 by spatial filtering. A silica glass spatially variable wave plate manufactured by direct laser writing was used to reduce depolarization losses from 12% to 5%.

Theoretical simulation of depolarization compensation by a spatially variable wave plate

Theoretical simulation of depolarization compensation by a spatially variable wave plate

Advanced LD pumped 3.3 J/1 Hz nanosecond Nd:glass preamplifier for SG-II upgrade laser facility.

We demonstrate a laser-diode-pumped multipass Nd:glass laser amplifier with a range of advanced characteristics. The amplifier exhibits high extraction efficiency, enables arbitrary shaping of spatial beam intensity, and effectively suppresses frequency modulation to amplitude modulation conversion. Our approach achieves excellent beam quality via thermal lensing and thermal depolarization compensation. When a 1.82 mJ/5 ns laser pulse was injected into the amplifier, the output energy reached up to 3.3 J with a repetition rate of 1 Hz at a central wavelength of 1053.3 nm. The near-field modulation of the amplified output beam was below 1.2, and the far-field focusing ability of the beam was 90% at 2.9 times the diffraction limit. This laser amplifier system holds potential for integration as a preamplifier within the SG-II upgrade high power laser facility.

Kilowatt-class high-energy frequency conversion to 95 J at 10 Hz at 515 nm

Abstract We report on frequency doubling of high-energy, high repetition rate ns pulses from a cryogenically gas cooled multi-slab ytterbium-doped yttrium aluminum garnet laser system, Bivoj/DiPOLE, using a type-I phase matched lithium triborate crystal. We achieved conversion to 515 nm with energy of 95 J at repetition rate of 10 Hz and conversion efficiency of 79%. High conversion efficiency was achieved due to successful depolarization compensation of the fundamental input beam.

Depolarization compensation in spun highly birefringent fiber with built-in half waveplate

Nowadays, the spun highly birefringent single-mode optical fiber is increasingly being considered for current-sensing applications. However, when a broadband polarized light is launched into the fiber, apparent depolarization will occur with its propagation. This leads to a decrease in the polarization sensibility. In this paper, we propose a kind of method to compensate for the depolarization in spun highly birefringent fibers. An all-fiber half waveplate made of the helically wound spun highly birefringent fiber is placed at the center of the whole fiber. To obtain this half waveplate, the torsion of the helix should be equal to the inherent polarization rotation of the spun highly birefringent fiber. The advantages of the proposed method are simple, effective and without breaking the fiber. Simulations and experiments have demonstrated the excellent performance of the proposed method.

Compact 15 mJ Fiber–Solid Hybrid Hundred-Picosecond Laser Source for Laser Ablation on Copper

We report on a millijoule-level fiber–solid hybrid hundred-picosecond laser system with a stable performance and compact structure. The laser system is based on a master oscillator power amplifier structure containing an all-fiber master oscillator, a quasi-continuous-wave side-pumped Nd:YAG regenerative amplifier, and a double-pass amplifier. By using the filtering effect of fiber Bragg grating and the dispersion characteristics of single-mode fiber stretcher, the spectrum broadening caused by self-phase modulation effect is effectively suppressed. Thus, the gain linewidth of the Yb-doped fiber seed source and Nd:YAG laser amplifiers is accurately matched. The reason for thermally induced depolarization in the solid-state laser amplifier is theoretically analyzed, and a more flexible depolarization compensation structure is adopted in amplifier experiment. Furthermore, the pulse energy of 14.58 mJ and pulse width of 228 ps is achieved at 500 Hz repetition rate. The central wavelength is 1064.1 nm with a 3 dB bandwidth of 0.47 nm. The beam quality factors in the horizontal and vertical directions are 1.49 and 1.51, respectively. This laser system has a simple and compact structure and has a power stability of 1.9%. The high pulse energy and beam quality of this hundred-picosecond laser are confirmed by latter theoretical simulation of copper laser ablation. It is a very practical laser system for material processing and laser-induced damage.

High-Power Q-Switched 1064 nm Output Light with High Polarization Degree and Efficient Depolarization Compensation

High-Power Q-Switched 1064 nm Output Light with High Polarization Degree and Efficient Depolarization Compensation

Depolarization Compensation in Spun Highly Birefringent Fiber with Built-In Half Waveplate

Depolarization Compensation in Spun Highly Birefringent Fiber with Built-In Half Waveplate

Methodology for determining the physical parameters of ground plane by the results of the optical and radar data fusion

The methodology of multispectral optical and dual-polarized radar satellite data fusion for soils physical parameters estimation is developed. In particular, the methodology comprises relative permittivity estimation based on the Integral Equation Model (IEM) by using high resolution Sentinel-1 GRDH radar data. The calibration of ε was provided based on the compensation of soil acidity and temperature destabilizing effects. High-resolution multispectral images PlanetScope were used for vegetation indices and thermal emissivity estimation. Both, low-resolution MODIS and medium resolution Landsat-7/8 ETM+/TIRS thermal infrared images were processed in order to estimate ground plane thermodynamic temperature. An investigated approach for the radar signal depolarization compensation is based on local signal deviations and surface roughness estimation. The relief heterogeneity is restored based on the medium-resolution digital terrain elevation model ALOS AWD3D. Aiming to evaluate the accuracy of a soil moisture estimation model designed based on the presented methodology, ground truth measurements were carried out. Specifically, they included soil samples retrieving for the gravimetric soil moisture. In addition, the soil acidity and temperature were measured by applying the GM1312 differential thermometer and WALCOM multifunction device. The estimated parameters and ground truth data were used in order to retrieve the soil moisture based on the multivatiative regression dependence. Root mean square error of soil moisture retrieving was estimated as 4,73 %. Such accuracy is completely acceptable for the soil moisture monitoring of natural-reserved fund territories

Depolarization compensation with a spatially variable wave plate in a 116 W, 441 fs, 1 MHz Yb:YAG double-pass laser amplifier.

A subpicosecond laser system featuring a fiber chirped pulse amplification-based seed laser and a double-pass end-pumped Yb:YAG crystal power amplifier was investigated. The key novelty of the system was the application of depolarization compensation using a specially designed spatially variable wave plate. To the best of our knowledge, this method was applied for the first time. The presented laser system produced pulses of 441 fs duration, 116 µJ pulse energy at 116 W average power with a beam quality of M2∼2.1, featured optical-to-optical efficiency of 32% at room temperature (T=20∘C), and had residual depolarization level of 2.7%.

Depolarization mechanism and compensation scheme of radially polarized beams

Depolarization mechanism and compensation scheme of radially polarized beams

Compact 50 W All-Solid-State Picosecond Laser System at 1 kHz

Compact, stable, high-power and high repetition rate picosecond laser systems are excellent sources for optical parametric chirped pulse amplification systems and laser satellite ranging systems. Compared with the traditional complex high-power amplifier, this article reports a compact high-power picosecond laser system at a repetition rate of 1 kHz based on Nd:YAG bulk crystal. The thermal lens effect limits the regenerative amplifier to directly output higher energy. For this reason, multi-stage traveling-wave amplifiers are usually used to gradually increase the laser pulse energy. So as to achieve a compact structure, a regenerative amplifier that can output higher power at 1 kHz is designed in the laser system. The regenerative amplifier can output the power of 6.5 W at the pump power of 41.5 W; the beam quality of M2 factor was about 1.3. A more flexible thermal depolarization compensation structure is applied in the side-pumped amplifier, which can effectively compensate for thermal lens effect and thermal depolarization at different pump powers. Finally, the laser pulse can achieve an output power higher than 50 W at 1 kHz after passing through an end-pumped traveling-wave amplifier and a side-pumped traveling wave amplifier.

Diode-side-pumped joule-level square-rod Nd:glass amplifier with 1 Hz repetition rate and ultrahigh gain.

An effective diode-side-pumped joule-level square-rod Nd:glass multipass amplifier with 1 Hz repetition rate is developed. A Pockels cell is used to extend the amplification-pass to twelve-pass. The linear depolarization compensation method and adaptive optics based wavefront aberration correction method are used to minimize the thermal effects. Combined with the relay-imaging technology and beam-shaping method, good beam quality is obtained. Under the small-signal gain of 3.23 and the input energy of 50 µJ, the output energy of 1 J and the net gain of 2×104 are realized. The effective energy-extraction efficiency in the whole aperture of Nd:glass rod is 12%. The total wavefront aberration is 1.1 µm (peak valley). The Strehl Ratio is 0.62, and 90% of far-field energy is concentrated in 3 times the diffraction limit. The near-field modulation index of twelve-pass amplification at 1 J is 1.4.

Long pulse millisecond pumped high-average-power linearly polarized single Nd:YAG rod laser

A new oscillator to compensate depolarization loss is introduced to eliminate influence caused by depolarization on the laser output. The problem of power enhancement of the gain medium caused by the conventional single module’s birefringence compensation is solved through the design of thermal depolarization compensation for the single pump module, and the efficient output of the laser is realized. A millisecond long pulse pumped single Nd:YAG rod laser is designed by using an 808 nm LD pump source. When the repetition frequency is 30 Hz, and the pump energy is 30 J, the maximum single pulse output energy is 10.45J. The corresponding depolarization rate is 4.1%, and the linear polarization of 1064 nm laser output is obtained. The light-to-light conversion efficiency reaches 34.8%. Good compensation can evidently be seen with this new configuration.

TSAG-based Faraday isolator with depolarization compensation using a counterrotation scheme.

A Faraday isolator (FI) with depolarization compensation using a counterrotation scheme has been realized in experiment for the first time, to the best of our knowledge. It is based on terbium scandium aluminum garnet crystals with negative optical anisotropy parameters. An order of magnitude advantage over the traditional FI scheme is achieved in this case. An isolation ratio of 35.7dB at a radiation power of 1440W has been obtained. According to the numerical estimates, an isolation ratio of 30dB can be provided up to a power of 5.5kW.

Peculiarity of the thermally induced depolarization and methods of depolarization compensation in square-shaped Yb:YAG active elements

The degree of thermally induced depolarization for square disc-shaped Yb:YAG active element was calculated. Its dependence on the volume density of heat release in the active element was considered. The dependence of the thermally induced depolarization on the number of active elements and input polarization orientation was investigated. It is shown numerically that use of incoming polarization orientation at an angle of 45 degrees relative to the sides of the element is optimal in terms of minimizing thermally induced depolarization. A review of thermally induced depolarization compensation schemes is presented.

Fast T Wave Detection Calibrated by Clinical Knowledge with Annotation of P and T Waves.

BackgroundThere are limited studies on the automatic detection of T waves in arrhythmic electrocardiogram (ECG) signals. This is perhaps because there is no available arrhythmia dataset with annotated T waves. There is a growing need to develop numerically-efficient algorithms that can accommodate the new trend of battery-driven ECG devices. Moreover, there is also a need to analyze long-term recorded signals in a reliable and time-efficient manner, therefore improving the diagnostic ability of mobile devices and point-of-care technologies.MethodsHere, the T wave annotation of the well-known MIT-BIH arrhythmia database is discussed and provided. Moreover, a simple fast method for detecting T waves is introduced. A typical T wave detection method has been reduced to a basic approach consisting of two moving averages and dynamic thresholds. The dynamic thresholds were calibrated using four clinically known types of sinus node response to atrial premature depolarization (compensation, reset, interpolation, and reentry).ResultsThe determination of T wave peaks is performed and the proposed algorithm is evaluated on two well-known databases, the QT and MIT-BIH Arrhythmia databases. The detector obtained a sensitivity of 97.14% and a positive predictivity of 99.29% over the first lead of the validation databases (total of 221,186 beats).ConclusionsWe present a simple yet very reliable T wave detection algorithm that can be potentially implemented on mobile battery-driven devices. In contrast to complex methods, it can be easily implemented in a digital filter design.

Review of Faraday Isolators for Kilowatt Average Power Lasers

Faraday isolators for high average power lasers operating at room temperature are surveyed. Three devices on [001] oriented TGG crystals with the most known optical schemes are considered: traditional scheme, and schemes with compensation of thermally induced depolarization inside magnetic field and outside magnetic field. We report a unique 30-mm-aperture Faraday isolator with thermally induced depolarization compensation inside magnetic field. It provides 33.5-dB isolation ratio at 1.5-kW average laser power.

Theoretical and experimental study of laser radiation propagating in a medium with thermally induced birefringence and cubic nonlinearity

We consider a problem of laser radiation propagating in a medium with birefringence of two types: linear birefringence independent of intensity and polarization, and intensity and polarization dependent circular birefringence caused by cubic nonlinearity. It is shown theoretically and experimentally that the efficiency of the broadly employed method of linear depolarization compensation by means of a 90° polarization rotator decreases with increasing В-integral (nonlinear phase incursion induced by cubic nonlinearity). The accuracy of polarization transformation by means of a half-wave and a quarter-wave plate also decreases if В > 1. By the example of a λ/4 plate it is shown that this parasitic effect may be suppressed considerably by choosing an optimal angle of inclination of the optical axis of the plate.

Origin of the efficiency enhancement in ferroelectric functionalized organic solar cells

We have investigated the efficiency enhancement of organic solar cells upon incorporation of a thin ferroelectric polymer layer. For non-Ohmic contacts the enhancement is due to an increased open circuit voltage, which is, however, independent of the ferroelectric polarization direction. Ferroelectricity cannot play a role due to depolarization and charge compensation. The same improvement is also obtained with a non-ferroelectric fluorinated polymer. The data unambiguously show that the improved efficiency is solely due to improvement of the cathode. The fluorinated polymer layer plays the role of LiF in an optimized solar cell.