Photonics, Volume 10, Issue 12 (December 2023) – 103 articles

The circular polarization of light flips its handedness after a conventional metallic mirror reflection. Therefore, a polarization-preserving metasurface is a crucially important element in a series of chiral photonic structures. They include tunable cholesteric LCs and anisotropic photonic crystals. Chiral structures are rich in interfacial localized modes including Tamm states. In this report, coupled modes formed as a result of the interaction between two chiral optical Tamm states or a chiral optical Tamm state and a chiral Tamm plasmon polariton are analytically and numerically investigated. It is shown that the effective control of coupled modes can be carried out by changing the pitch of the cholesteric and the angle between the optical axis of the cholesteric and the polarization-preserving anisotropic mirror. The influence of the metasurface period on the spectral characteristics of coupled modes is investigated. The possibility of realizing a bound state in the continuum of the Friedrich–Wintgen type, resulting from the destructive interference of coupled modes, which leads to the collapse of the resonance line corresponding to the chiral optical Tamm state, has been demonstrated. Full article

Highly integrated and stable devices are appealing in optical communication and sensing. This appeal arises from the presence of high refractive index contrast and high-quality waveguides. In this study, we improved the vapor proton exchange (VPE) process, enabling large-scale waveguide fabrication and addressing the issue of liquid exchange during cooling. Additionally, we have prepared and characterized planar waveguides on X-cut lithium niobate (LN) crystals. The exchanged samples exhibit α and k1 phases, refractive index contrasts as high as 0.082, and exceptional refractive index uniformity. Furthermore, we utilized the same process to fabricate channel waveguides and Y-branch waveguides. We achieved low propagation losses in channel waveguides, accompanied by small mode sizes, and low-loss Y-branch waveguides with a highly uniform beam splitting ratio. All waveguides exhibited consistent performance across multiple preparations and tests, remaining free from aging effects for three months. Our results underscore the promising potential of VPE for creating Y-branch splitters and modulators in LN crystals. Full article

The output spectral, temporal, and spatial parameters of a two-channel Yb-fiber laser operating in an incoherent intracavity spectral beam-combining mode were investigated. One of the laser channels operated in continuous pumping mode, and the second channel operated in pulse-periodic pumping mode with a pulse duration of microseconds. When the channels operated separately, continuous lasing at a wavelength of λ₁ was observed in one channel, and pulsed lasing at a wavelength of λ₂ was observed in the other. When both channels operated simultaneously, it was shown that during the pump pulse action, the laser switched operation to pulsed collective mode lasing at a wavelength of (λ₁ + λ₂)/2. Lasing at wavelengths λ₁ and λ₂ was not observed at this time. Full article

The maximum ratio combining (MRC) diversity technology has shown outstanding performance in overcoming the adverse effects of underwater wireless optical communication (UWOC) systems. However, its actual performance gain will be affected by the detection area and noise, which requires an in-depth analysis. In this paper, on the basis of fully considering the noises in the UWOC system, the performance of the MRC diversity technology is fairly and comprehensively studied by comparing it with two single-input–single-output (SISO) systems using a small aperture detection (SAD) scheme or a large-aperture detection (LAD) scheme through a Monte Carlo simulation and a formula analysis. The results show that the traditional belief that the MRC diversity scheme has consistently outperformed SISO systems may be misleading. When the thermal noise is dominant and the background noise is small, the LAD scheme performs better than the MRC diversity scheme with the same detection area. And in other cases, the MRC diversity scheme with the same detection area is always superior to the SISO systems. The conclusions obtained in this paper have a guiding significance for the practical application of UWOC. Full article

The optimal regime of three-photon resonant excitation of a helium atom via a femtosecond ultraviolet (UV) pulse was discovered and numerically studied, at which the maximum power of the third harmonic of the UV field is achieved in the spectrum of dipole acceleration (the second time derivative of the induced dipole moment) of the atom. It is shown that the optimal frequency of the UV field nearly coincides with the frequency of the three-photon transition |1s²⟩–|1s2p⟩, taking into account its shift as a result of the dynamic Stark effect, and the intensity of the UV field is dictated by the condition of maximizing the product of the populations of the |1s²⟩ and |1s2p⟩ states, averaged over the time interval during which the UV field is non-zero. For the considered UV field durations, from 10 to 100 cycles of the carrier frequency (from units to tens of femtoseconds), the optimal intensity lies in the range from 10¹⁴ W/cm² to several units of 10¹⁴ W/cm². It is shown that with an optimal choice of the frequency and intensity of the UV field, the dynamics of excitation of bound and continuum states, as well as the shape of the time envelope of the dipole acceleration of the atom, weakly depend on the duration of the UV field envelope; only their time scale changes significantly. In addition, under optimal conditions, the average power of the third harmonic signal in the dipole acceleration spectrum is practically independent of the duration of the UV field envelope. Full article

(1) Background: Upconversion nanoparticles (UCNPs) are a promising tool for biological tissue structure visualization and photodynamic therapy (PDT). The luminescence of such NPs is excited in the spectrum’s near-infrared (NIR) region, while the NPs luminesce in the visible region. Conjugating such NPs with photodynamic dyes that absorb their luminescence makes it possible to increase the depth at which PDT is performed. (2) Methods: We conducted a comprehensive study on the possibility of using NaYF₄:Er:Yb UCNPs in vivo for imaging and PDT. The NPs were synthesized by a hydrothermal method. The synthesis of NPs with a size of 80 nm and hexagonal structure was demonstrated. (3) Results: The accumulation of NPs in organs after their intravenous injection into rats was studied. The effect of NPs on the shape, size, and degree of aggregation of red blood cells (RBC) was also investigated. (4) Conclusions: The possibility of luminescent visualization of NPs in histological sections and their subcutaneous distribution is demonstrated. All investigated particles showed moderate toxicity, causing mostly reversible changes. Full article

We demonstrated an optimization of a picosecond fiber amplifier based on Yb-doped tapered fiber in a spectral range of 1030 nm. Nonlinear effects limiting peak power scaling (stimulated Raman scattering and four-wave mixing) were studied and factors affecting their threshold were established, such as gain, diameter profile along the length of taper, output mode field diameter, and numerical aperture of a pump. By determining the optimal amplification regime and manufacturing advanced tapered fibers, we amplified 13 ps pulses to a record-high peak power of 1 MW at a wavelength of 1029 nm directly at the output of the fiber at an average power of 13.8 W. Four-wave mixing was the limiting factor, and the total fraction of deleterious components in the output spectrum was ~2%. The quality of the output beam was close to being diffraction limited (M2 < 1.2). Full article

Fifth-generation (5G) technology has enabled faster communication speeds, lower latency, a broader range of coverage, and greater capacity. This research aims to introduce a bidirectional high-speed passive optical network (HS-PON) for 5G applications and services including mobile computing, cloud computing, and fiber wireless convergence. Using 16-ary quadrature amplitude modulation orthogonal frequency division multiplexing techniques, the system transmits uplinks and downlinks with a pair of four wavelengths each. Light fidelity (LiFi) services are provided with blue light-emitting-diode-based technology. With a threshold bit error rate (BER) of 10⁻³, the results demonstrate reliable transportation over a 100 km fiber at −17 dBm received power and in a maximum LiFi range of 20 m. Furthermore, the system offers symmetric 4 × 50 Gbps transmission rates under the impact of fiber–LiFi channel impairments with maximum irradiance and incidence half-angles of 500. Additionally, at threshold BER, the system provides a detection surface range from 1.5 to 4 cm². Compared to existing networks, the system also provides a high gain and low noise figure. A number of features make this system an attractive option. These include its high speed, high reach, high split ratio, low cost, easy upgradeability, pay-as-you-grow properties, high reliability, and ability to accommodate a large number of users. Full article

A significant change in the refractive index profiles for the large mode area phosphoroaluminosilicate (PAS) core optical fibers was observed in comparison to that in preforms. This study shows that the refractive index of the PAS core can vary from negative (in preform) to positive (in fiber), and the difference in the refractive index between the core and preform can exceed a few thousand. By measuring a large set of fibers with different concentrations of P₂O₅ and Al₂O₃, we define the refractivity of each dopant (P₂O₅, Al₂O₃ and AlPO₄ joint) after drawing fiber from the preform and discuss the possible origin of the observed refractive index variation. Full article

We have experimentally created perfect vortex beams (PVBs) by Fourier transformation of Bessel–Gaussian vortex beams, which are generated by modulating the fundamental Gaussian beam with the spiral phase plates and the axicons, respectively. Although the method has been used many times by other authors, as far as we know, few people pay attention to the quantitative relationship between the control parameters of the PVB and ring width. The effects of the waist radius of the fundamental Gaussian beam w_g, base angle of the axicon γ, and focal length of the lens f on the spot parameters (ring radius ρ, and ring half-width Δ) of PVB are systematically studied. The beam pattern of the generated Bessel–Gaussian beam for different propagation distances behind the axicon and the fundamental Gaussian beam w_g is presented. We showed experimentally that the ring radius ρ increases linearly with the increase of the base angle γ and focal length f, while the ring half-width Δ decreases with the increase of the fundamental beam waist radius w_g, and increases with enlarging the focal length f. We confirmed the topological charge (TC) of the PVB by the interferogram between the PVB and the reference fundamental Gaussian beam. We also studied experimentally that the size of the generated PVB in the Fourier plane is independent of the TCs. Our approach to generate the PVB has the advantages of high-power tolerance and high efficiency. Full article

The aspheric surface is a commonly used method to improve the imaging quality of the fisheye lens, but it is difficult to determine the position and initial value. Based on the wave aberration theory of the plane-symmetric optical system, a method of using an aspheric surface to design a fisheye lens is proposed, which can quickly determine the appropriate aspheric surface to improve the imaging performance. First, the wave aberration of each optical surface of the fisheye lens is calculated and its aberration characteristics are analyzed. Then, a numerical evaluation function is reported based on the aberration distribution of the fisheye lens on the image plane. According to the functional relationship between the evaluation function and the aspheric coefficient, the position of the aspheric surface and the initial value of the aspheric coefficient can be calculated. Finally, the adaptive and normalized real-coded genetic algorithm is used as the evaluation function to optimize the fisheye lens using an aspheric surface. The proposed method can provide an effective solution for designing a fisheye lens using an aspheric surface. Full article

All-optical radio-frequency spectrum analyzers (AORFSAs) with ultrabroad bandwidth break the electronic bottleneck and provide an efficient frequency analysis means for ultrafast optical signals in communications, signal generation and processing systems. Here, we propose and experimentally demonstrate an AORFSA built on the cross-phase modulation effect in a 50 cm long CMOS-compatible photonic slot-waveguide. The waveguide has a 100 nm thick thin-film core of fused silica that is sandwiched by two 750 nm thick cladding layers of high-index doped silica, which shows optimized dispersion and comparable nonlinear characteristics. The measured 3 dB bandwidth of the proposed slot-waveguide-based AORFSA has a three-fold increase over the conventional channel waveguide having the same dimension and length. The sensitivity and wavelength- and polarization-dependence properties are investigated, confirming the proposed waveguide as a versatile platform for frequency analysis of ultrafast optical signals, such as Kerr microcombs with hundreds of GHz or even THz mode spacing. Full article

We report a single-wavelength tunable mode-locked fiber laser. The single wavelength can be tuned from 1537.49 nm to 1608.06 nm by introducing a Sagnac loop filter. As far as we know, this is the widest single-wavelength tuning range achieved in an erbium-doped mode-locked all-fiber laser based on nonlinear amplifying loop mirror (NALM). The laser’s pulse width changes from 549 fs to 808 fs throughout the tuning process, the maximum average output power is 5.72 mW, and the single-pulse energy is 0.34 nJ at a central wavelength of 1556.53 nm. This laser source can serve as an efficient tool for applications that require a broad tunability range. The combination of femtosecond pulses and extensive wavelength tuning capabilities makes this laser system highly valuable in fields such as fiber optic communications, spectroscopy, sensing, and other applications that benefit from ultrafast and tunable laser sources. Full article

A model of a generalized pulse source, whose complex degree of temporal coherence is described by a function of the nth power difference of two instants, was constructed. As examples, we consider the generalized Gaussian and multi-Gaussian Schell-model pulse sources and study their propagation in dispersive media. It is indicated that such pulse beams present unique self-focusing, off-axis self-shifting and asymmetric self-splitting characteristics by adjusting the power exponent and phase parameters. Further, we explicitly discuss how the coherence time, summation factor as well as the dispersive coefficient significantly affect the self-focusing and self-shifting behaviors of the pulse beam. The results will benefit some applications involving pulse shaping, optical trapping and remote sensing. Full article

The surface morphology of a buffer template is an important factor in the heteroepitaxial integration of optoelectronic devices with a significant lattice mismatch. In this work, InP-based long-wave infrared (~8 µm) emitting quantum cascade lasers with active region designs lattice-matched to InP were grown on GaAs and Si substrates employing InAlGaAs step-graded metamorphic buffer layers, as a means to assess the impact of surface roughness on device performance. A room-temperature pulsed-operation lasing with a relatively good device performance was obtained on a Si template, even with a large RMS roughness of 17.1 nm over 100 µm². Such results demonstrate that intersubband-operating devices are highly tolerant to large RMS surface roughness, even in the presence of a high residual dislocation density. Full article

In order to improve the sensitivity and accuracy of the giant magnetostrictive material-fiber Bragg gratings’ (GMM-FBG) current sensor, in which the magnetostrictive modulator is Terfenol-D, the temperature effects on the FBG center wavelength and GMM magnetostriction coefficient are investigated to initiate an amending scheme in which temperature parameters are introduced into a GMM-FBG sensing model so as to calibrate current values. Based on electromagnetism theory, the magnetic structure is optimized in design to significantly increase the magnetic coupling efficiency and to homogenize magnetic distribution, employing finite element simulations of the electromagnetic field. The relevant experimental platform is constructed with a wavelength demodulation system. At the temperature range of 20~70 °C, response amplitudes of the current sensor are tested under various current values. The experimental results indicate that the sensitivity of the GMM-FBG current sensor decreases with the temperature increment and is also positively correlated to the target current. Through analyzing the response characteristics of the current sensor to temperature variation, a reasonable GMM-FBG sensing amelioration model with a temperature compensation coefficient is established based on a mathematical fitting method, according to which the current detecting accuracy can be increased by 4.8% while measuring 60 A current at the representative working temperature of 40 °C. Full article

The chirped pulse amplification (CPA) systems based on transition-metal-ion-doped chalcogenide crystals are promising powerful ultrafast laser sources providing access to sub-TW laser pulses in the mid-IR region, which are highly relevant for essential scientific and technological tasks, including high-field physics and attosecond science. The only way to obtain high-peak power few-cycle pulses is through efficient laser amplification, maintaining the gain bandwidth ultrabroad. In this paper, we report on the approaches for mid-IR broadband laser pulse energy scaling and the broadening of the gain bandwidth of iron-doped chalcogenide crystals. The multi-pass chirped pulse amplification in the Fe:ZnSe crystal with 100 mJ level nanosecond optical pumping provided more than 10 mJ of output energy at 4.6 μm. The broadband amplification in the Fe:ZnS crystal in the vicinity of 3.7 μm supports a gain band of more than 300 nm (FWHM). Spectral synthesis combining Fe:ZnSe and Fe:CdSe gain media allows the increase in the gain band (~500 nm (FWHM)) compared to using a single active element, thus opening the route to direct few-cycle laser pulse generation in the prospective mid-IR spectral range. The features of the nonlinear response of carbon nanotubes in the mid-IR range are investigated, including photoinduced absorption under 4.6 μm excitation. The study intends to expand the capabilities and improve the output characteristics of high-power mid-IR laser systems. Full article

In shallow tissues of the human body, pathological changes often occur, and there are several kinds of scattering media, such as mucosa, fat, and blood, present on the surface of these tissues. In such scattering environments, it is difficult to distinguish the location of the lesions using traditional attenuation-based imaging methods, while polarization-based imaging methods are more sensitive to this information. Therefore, in this paper, we conducted experiments using diluted milk to simulate biological tissues with scattering effects, illuminated with non-polarized light sources, and used an optimized robust polarization de-scattering algorithm for image processing. The results were qualitatively and quantitatively analyzed through local intensity comparison and visual fidelity functions, verifying the effectiveness of this algorithm under specific conditions. Full article

Semiconductor nano-lasers have been a topic of interest from the perspective of advancing the capabilities of photonic integration. Nano-lasers are perceived as the means to achieve improved functionality in photonic integrated circuits. The properties and performance of nano-lasers have been examined by means of simulations and laboratory measurements. Nano-lasers lend themselves to integration to form dense arrays in both one and two dimensions. In a recent work, a theoretical treatment was presented for the dynamic behaviour of stand-alone electrically pumped nano-laser arrays. In this work, the response of nano-laser arrays to direct current modulation is examined. As in previous works, attention is given to two prototype array geometries: a linear three-element linear array and an equilateral triangular array. Large one-dimensional arrays can be built by repeating this elementary linear array. Two-dimensional photonic integrated circuits can incorporate the triangular arrays studied here. Such prototypical configurations offer opportunities to tailor the modulation response of the nano-laser arrays. The principal factors which provide that capability are the coupling strengths between lasers in the arrays and the direct modulation parameters. The former are fixed at the design and manufacture stage of the array whilst the latter can be chosen. In addition, the enhancement of the spontaneous emission rate via the so-called Purcell effect in nano-lasers offers a device-specific means for accessing a range of modulation responses. Two-dimensional portraits of the regimes of differing modulation responses offer a convenient means for determining the dynamics that may be accessed by varying the laser drive current. It is shown by these means that a rich variety of modulation responses can be accessed in both linear and triangular arrays. Full article

Mode-division multiplexing (MDM) is a promising multiplexing technique to further improve the transmission capacity of optical communication and on-chip optical interconnection systems. Furthermore, the multimode optical switch is of great importance in the MDM system, since it makes the MDM system more flexible by directly switching multiple spatial signals simultaneously. In this paper, we proposed a mode-independent optical switch based on the graphene–polymer hybrid waveguide platform that could process the TE₁₁, TE₁₂, TE₂₁ and TE₂₂ modes in a few-mode waveguide. The presented switch is independent of the four guided modes, optimizing the buried position of graphene capacitors in the polymer waveguide to regulate the coplanar interaction between the graphene capacitors and spatial modes. The TE₁₁, TE₁₂, TE₂₁ and TE₂₂ modes can be regulated simultaneously by changing the chemical potential of graphene capacitors in a straight waveguide. Our presented switch can enable the independent management of the spatial modes to be more flexible and efficient and has wide application in the MDM transmission systems. Full article

Optical satellite communications (OSC) downlinks can support much higher bandwidths than radio-frequency channels. However, atmospheric turbulence degrades the optical beam wavefront, leading to reduced data transfer rates. In this study, we propose using reinforcement learning (RL) as a lower-cost alternative to standard wavefront sensor-based solutions. We estimate that RL has the potential to reduce system latency, while lowering system costs by omitting the wavefront sensor and low-latency wavefront processing electronics. This is achieved by adopting a control policy learned through interactions with a cost-effective and ultra-fast readout of a low-dimensional photodetector array, rather than relying on a wavefront phase profiling camera. However, RL-based wavefront sensorless adaptive optics (AO) for OSC downlinks faces challenges relating to prediction latency, sample efficiency, and adaptability. To gain a deeper insight into these challenges, we have developed and shared the first OSC downlink RL environment and evaluated a diverse set of deep RL algorithms in the environment. Our results indicate that the Proximal Policy Optimization (PPO) algorithm outperforms the Soft Actor–Critic (SAC) and Deep Deterministic Policy Gradient (DDPG) algorithms. Moreover, PPO converges to within $86\%$ of the maximum performance achievable by the predominant Shack–Hartmann wavefront sensor-based AO system. Our findings indicate the potential of RL in replacing wavefront sensor-based AO while reducing the cost of OSC downlinks. Full article

Precise adaptation of the greenhouse lighting spectrum to basic photophysiological processes can effectively and directionally stimulate plant growth and development. The optimal spectrum depends on the plant species and the stage of development and could be assessed empirically. The aim of this study is to determine the LED illumination spectrum that provides a significant improvement in the growth rate and accumulation of biologically active compounds for basil plants (Ocimum basilicum L.) under hydroponic cultivation compared to more traditional lighting sources. The following light sources with various emission spectra were used: an LED lamp within a spectral range of 400–800 nm (B:G:R 15%:5%:80%); a high-pressure sodium lamp (HPS) (B:G:R 5%:45%:50%); a compact fluorescent lamp (B:G:R 20%:40%:40%); a grow LED strip (B:G:R 15%:40%:45%); a white LED lamp (B:G:R 30%:45%:25%); a customized LED lighting setup in color ratios 100%B, 75%B + 25%R, 50%B + 50%R, 25%B + 75%R, 100%R, and natural lighting. A photosynthetic photon flux density (PPFD) of 150 μmol∙m⁻²∙s⁻¹ was provided with all the sources. It was demonstrated reliably that employing the LED strip as an illumination device gives a 112% increase in basil plant yield compared to the HPS; the transpiration coefficient for the LED strip is six times lower than for the HPS. The content of flavonoids in the basil aerial parts on the 30th, 50th, and 70th days of development is 3.2 times higher than for the HPS; the metabolite composition is also more uniform for LED strip lighting. Full article

In this paper, the refractive index and extinction coefficient of ferroelectric liquid crystals have been examined by the terahertz time-domain spectroscopy system. Two modes of ferroelectric liquid crystal materials, deformed helix ferroelectric liquid crystal (DHFLC), and electric suppressed helix ferroelectric liquid crystal (ESHFLC) are tested as experimental samples. Nematic liquid crystal (NLC) was also investigated for comparison. The birefringence of DHFLC 587 slowly increases with the growth of frequency, and it averages at 0.115. Its extinction coefficients gradually incline to their stable states at 0.06 for o-wave and 0.04 for e-wave. The birefringence of ESHFLC FD4004N remains between around 0.165 and 0.175, and both of its e-wave and o-wave extinction coefficients are under 0.1, ranging from 0.05 to 0.09. These results of FLC will facilitate the examination and improve the response performance of THz devices using fast liquid crystal materials. Full article

Rydberg atoms possess highly excited valence electrons that are far away from atomic cations. Compared with ground states, Rydberg states are excited states with a high principal quantum number n that exhibit large electric dipole moments and have a variety of applications in quantum information processing. In this communication, we report the measurement of the ${}^6$Li Rydberg excitation spectrum by ladder-type electromagnetically induced transparency (EIT) in a vapor cell. The 2$p\to ns/nd$ EIT spectra were recorded by sweeping the frequency of an ultraviolet Rydberg pumping laser while keeping the probing laser resonant to the $2s\to 2p$ transition. All lasers were locked on an ultrastable optical Fabry-Pérot cavity and measured by an optical frequency comb. Our results provide valuable information to precisely determine quantum defects and enable novel experiments with Rydberg-dressed ultracold Fermi gases. Full article

Previous studies demonstrated coupling of acoustic guided waves from one optical fiber to another through a simple adhesive bond coupler. This paper experimentally utilizes such an adhesive bond coupler to easily extend an already existing sensor network. We experimentally demonstrate this concept for detecting simulated cracks growing from circular holes in a thin aluminum plate. A single, remotely bonded FBG sensor is used to detect the original crack growth, followed by the addition of other optical fiber segments using adhesive couplers to detect new crack growth locations on the plate. A laser Doppler vibrometer is also used to measure the guided wave propagation through the plate to verify that the changes in the FBG sensor measurements are due to the growth of the cracks. Full article

In space gravitational wave detection, the inter-satellite link-building process requires a type of steering mirror to achieve point-ahead angle pointing. To verify that the background noise does not drown out the gravitational wave signal, this paper designed a laser heterodyne interferometer specifically designed to measure the optical path difference of the steering mirror. Theoretically, the impact of angle and position jitter is analyzed, which is called tilt-to-length (TTL) coupling. This interferometer is based on the design concept of equal-arm length. In a vacuum (${10}^{-3} \mathrm{P}\mathrm{a}$), vibration isolation (up to 1 Hz), and temperature-controlled (approximately 10 mK) experimental environment, the accuracy is increased by about four orders of magnitude through a common-mode suppression approach and can reach $390 \mathrm{p}\mathrm{m}/\sqrt{\mathrm{H}\mathrm{z}}$ when the frequency is between 1 mHz and 1 HZ. By analogy, the optical path difference caused by the steering mirror reaches $5 \mathrm{p}\mathrm{m}/\sqrt{\mathrm{H}\mathrm{z}}$ in the 1 mHz to 1 Hz frequency band. The proposed TTL noise model is subsequently verified. Full article

The nonlinear properties of zinc germanium diphosphide (ZGP) crystals enable their applications in powerful mid-IR optical parametric oscillators and second-harmonic generators. This paper summarizes the mechanisms of the laser-induced damage (LID) of high-purity ZGP crystals under periodically pulsed nanosecond irradiation by a Ho³⁺:YAG laser at 2.1 μm. The ZGP samples were manufactured by “LOC” Ent., Tomsk, Russia, or the Harbin Institute of Technology, China. The impact of processing techniques and the post-growing methods for polishing and anti-reflective coatings on the LID threshold are discussed. The importance of the defect structure of the crystal lattice and the parameters of transparent coatings for increasing the LID threshold are also discussed. The impact of the test laser parameters on the LID threshold and the transient area near the LID threshold obtained using digital holography are analyzed. The influence of the pre-damage processes on the optical parametric oscillations is reported. Lastly, the prospects for improving ZGP crystals to further increase the LID threshold are discussed. Full article

When a missile-borne pulsed laser forward detection system flies at supersonic speed, the laser beam will be distorted by the uneven outflow field, resulting in a significant reduction in ranging accuracy. In this paper, the impact of high flight speed on a pulsed laser detection system is studied. First, a new ray tracing method with adaptive step size adjustment is proposed, which greatly improves the computational efficiency. Second, the aerodynamic environment of a munition flying at high speed is simulated by an intermittent transonic and supersonic wind tunnel to obtain the schlieren data of the flow field at various Mach numbers. The schlieren data present a shock wave structure similar to that of the simulation. In addition, the variation patterns of the pulsed laser echo waveform of the model under different aerodynamic conditions are studied to evaluate the detectability and operational stability of the laser detection system under static conditions. The test results match the simulation results well, and the two offer relatively consistent shock wave structures, which verifies the correctness and effectiveness of the flow field simulation model. The test echo waveforms are in good agreement with the simulated echo waveforms; the relative errors between the peak values of test and simulated echo waveforms at various Mach numbers do not exceed 20%, and the correlation coefficients between the test and simulated echo waveforms all exceed 0.7, indicating high correlations between the two. Full article

In this paper, a genetic least mean square (GLMS) method is proposed to improve the signal-to-noise ratio (SNR) of acoustic signal reconstruction in a phase-sensitive optical time-domain reflectometry system. The raw demodulated signal is processed via applying the least mean square criterion. The SNR of the processed signal was calculated and served as the objective function in the fitness evaluation procedure. The genetic operations of the population selection, crossover, and mutation are sequentially performed and repeated until the suspensive condition is reached. Through multiple iterations, the GLMS method continuously optimized the population to find the optimal solution. Experimental results demonstrate that the SNR is substantially improved by 14.37–23.60 dB in the monotonic scale audio signal test from 60 to 1000 Hz. Furthermore, the improvement of the phase reconstruction of a human voice audio signal is also validated by exploiting the proposed GLMS method. Full article