Progress in Quantum Electronics最新文献

Lasers based on erbium ions using ⁴I_11/2 ​→ ​⁴I_13/2 transition can generate laser radiation in the spectral range from 2.7 ​μm to 3 ​μm. Since the strong absorption peak of water is located at 3 ​μm, there has been an effort to develop a suitable laser source for various medical applications, e.g. dentistry, dermatology, urology, or surgery. Laser radiation from this wavelength range can also be used in spectroscopy, as a pumping source for optical parametric oscillators, or for further mid-infrared conversion.

This paper represents an overview of the erbium-doped active media (e.g. Er:YAG, Er:YAP, Er:GGG, Er:SrF₂, Er:YLF, Er:Y₂O₃, Er:KYW, etc.) for laser radiation generation in the spectral range 2.7–3 ​μm. In the first part of this paper, the particular active media are discussed in detail. On the other hand, the experimental results summarized absorption and emission cross-section spectra together with decay times at upper (⁴I_11/2) and lower (⁴I_13/2) laser levels of all tested Er-doped samples at room temperature. Moreover, laser results in CW and pulsed laser regime with tunability curves, achieved in recent years, are presented, too.

The ultrafast, reversible, nonvolatile and multistimuli responsive phase change of Ge-Sb-Te (GST) alloy makes it an interesting “smart” material. The optical features of GST undergo significant variation when its state changes between amorphous and crystalline, meaning that they are useful for tuning photonic components. A GST phase change material (PCM) can be efficiently triggered by stimuli such as short optical or electrical pulses, providing versatility in high-performance photonic applications and excellent capability to control light. In this review, we study the fundamentals of GST-tuned photonics and systematically summarise the progress in this area. We then introduce current developments in both GST-metal hybrid metamaterials and GST-based dielectric metamaterials, and investigate the strategy of designing reversibly switchable GST-based photonic devices and their advantages. These devices may have a vast array of potential applications in optical memories, switches, data storage, cloaking, photodetectors, modulators, antennas etc. Finally, the prospect of implementing GST PCM in emerging fields within photonics is considered.

Underwater wireless optical communication (UWOC) is an emerging and feasible underwater communication technology and has developed rapidly in recent years. Building a high-performance and practical UWOC system requires comprehensive consideration and optimization design from the device to the system, as well as from the internal modulation to the external environment. This paper provides an overview of the recent developments in UWOC systems, covering aspects about the system transmitters and receivers, advanced modulation formats and underwater channels. Some key technologies to improve transmission capacity of UWOC are classified and summarized to provide guidance for system design. The main challenges and perspectives to achieve a reliable UWOC system are also mentioned. The summary and analysis of these advances and techniques will shed light on the future development of UWOC technology and assist in the construction of the internet of underwater things.

Rectifying antennas are often prensented as a potentiel technological breakthrough for energy harvesting. First theorized in the 1970’s, the downsizing of an antenna coupled with a rectifier has become technologically achievable with the progresses of fabrication techniques such as electron beam or photolithography. However, reaching infrared or visible region of the electromagnetic spectra still entails challenges on the integration of a rectifier operating in the terahertz range. New bottom up approaches are likely to bring a promising solution to this issue. To improve our understanding of the key points of rectifying antennas’ design for the infrared and visible light, and the challenges of device fabrication, this work reviews the progresses of this technology, going back from the first historical RF energy harvesting systems and covering the most innovative trends to this date.

We report on the first watt-level ultrafast laser inscribed Thulium waveguide (WG) lasers. Depressed-index buried channel WGs with a circular cladding (type III) are produced in monoclinic Tm³⁺:KLu(WO₄)₂ crystals. Laser operation is achieved under conventional (³H₆ → ³H₄) and in-band (³H₆ → ³F₄) pumping. In the former case, employing a Raman fiber laser emitting at 1679 ​nm as pump, the continuous-wave Tm channel WG laser generated 1.37 ​W ​at 1915–1923 ​nm with a record-high slope efficiency of 82.7% (with respect to the absorbed pump power), a threshold of only 17 ​mW and a spatially single-mode output with linear polarization. The WG propagation losses were 0.2 ​± ​0.3 ​dB/cm. Passive Q-switching of Tm channel WG lasers is achieved using Cr²⁺:ZnS and Cr²⁺:ZnSe saturable absorbers. With Cr²⁺:ZnS, record-short pulses of 2.6 ns/6.9 ​μJ ​at a repetition rate of 8.0 ​kHz were generated. The developed WGs are promising for compact GHz mode-locked lasers at ~2 ​μm.

The degree of spectral coherence characterizes the spectral purity of light. It can be equivalently expressed in the time domain by the decay time τ or the quality factor Q of the light-emitting oscillator, the coherence time τ ^coh or length $\ell$^coh of emitted light or, via Fourier transformation to the frequency domain, the linewidth Δν of emitted light. We quantify these parameters for the reference situation of a passive Fabry-Pérot resonator. We investigate its spectral line shapes, mode profiles, and Airy distributions and verify that the sum of all mode profiles generates the corresponding Airy distribution. The Fabry-Pérot resonator is described, as an oscillator, by its Lorentzian linewidth and finesse and, as a scanning spectrometer, by its Airy linewidth and finesse. Furthermore, stimulated and spontaneous emission are analyzed semi-classically by employing Maxwell′s equations and the law of energy conservation. Investigation of emission by atoms inside a Fabry-Pérot resonator, the Lorentz oscillator model, the Kramers-Kronig relations, the amplitude-phase diagram, and the summation of quantized electric fields consistently suggests that stimulated and spontaneous emission of light occur with a phase 90° in lead of the incident field. These findings question the quantum-optical picture, which proposed, firstly, that stimulated emission occurred in phase, whereas spontaneous emission occurred at an arbitrary phase angle with respect to the incident field and, secondly, that the laser linewidth were due to amplitude and phase fluctuations induced by spontaneous emission. We emphasize that the first derivation of the Schawlow-Townes laser linewidth was entirely semi-classical but included the four approximations that (i) it is a truly continuous-wave (cw) laser, (ii) it is an ideal four-level laser, (iii) its resonator exhibits no intrinsic losses, and (iv) one photon is coupled spontaneously into the lasing mode per photon-decay time τ_c of the resonator, independent of the pump rate. After discussing the inconsistencies of existing semi-classical and quantum-optical descriptions of the laser linewidth, we introduce the spectral-coherence factor, which quantifies spectral coherence in an active compared to its underlying passive mode, and derive semi-classically, based on the principle that the gain elongates the photon-decay time and narrows the linewidth, the fundamental linewidth of a single lasing mode. This linewidth is valid for lasers with an arbitrary energy-level system, operating below, at, or above threshold and in a cw or a transient lasing regime, with the gain being smaller, equal, or larger compared to the losses. By applying approximations (i)-(iv) we reproduce the original Schawlow-Townes equation. It provides the hi