Relaxation oscillations and damping factors of 1.3 μm In(Ga)As/GaAs quantum-dot lasers

In(Ga)As/GaAs quantum-dot (QD) lasers with emission wavelength at 1295 nm at room temperature are fabricated. The laser active region contains a threefold stack of QD layers with surface dot density of 4.56 × 10¹⁰ cm^–2. The laser structure is aluminum-free with InGaP as cladding layers. Threshold current density of a narrow stripe laser of 8 m wide and 3.5 mm long is 152.5 A/cm². The highest relaxation oscillation frequency measured at room temperature is 1.8 GHz, corresponding to a modulation bandwidth of 2.8 GHz due to the small damping factor. From the above measurement, the differential gain and gain compression factor were extracted to be 4.3 × 10^–16 cm² and 3.4 × 10 ^–17 cm ³, respectively. Using these parameters, the maximum modulation bandwidth f _{3 dB max} is estimated as 7.9 GHz.     

Cw oscillation at 1.05 and 1.32 μm of LiNd(PO3)4 lasers in external resonator and in resonator with directly applied mirrors

The room temperature cw oscillation at 1.05 and 1.32 μm of lithium neodymium tetraphosphate [LiNd(PO₃)₄, LNP] lasers in an external resonator and, for the first time, also in a Fabry-Perot resonator with directly applied mirrors is reported. The growth and fluorescence properties of the LNP crystals are briefly described. Laser parameters such as threshold pump power, output power, differential efficiency, optical gain, emission cross section and laser modes are both measured and calculated. Laser experiments show the threshold pump power to be lower for 1.05 μm than for 1.32 μm emission because of the smaller emission cross section; the pump power density is lower for the external resonator than for the Fabry-Perot resonator because of its higher mode losses.     

Lasing from GaAs Gunn devices

Lasing from a GaAs Gunn device with a Fabry–Perot (F–P) cavity and a vertical-cavity structures are demonstrated. Light output-electric field characteristic of both F–P and vertical cavity structure Gunn device shows abrupt increase in light output above a negative differential resistance (NDR) threshold, implying cavity gain overcoming over loss. Electroluminescence (EL) spectra from the F–P Gunn device measured at 95 K show a significant spectral narrowing with increasing the applied voltage pulse width, indicative of monochromatic stimulated emission due to band-to-band recombination of impact-ionized carriers. The spectra from the vertical-cavity device are in accord with the cavity resonance as defined by the distributed Bragg reflectances (DBRs). The theoretical model on carrier generation via impact ionization inside the propagating space charge domains shows clearly that the built-up excess carrier density in the cavity indeed reaches the lasing threshold.     

Characterization of small-signal intensity modulation of single-mode fiber grating Fabry-Perot laser source

A comprehensive study on the small-signal intensity modulation (IM) characteristics of a fiber grating Fabry-Perot (FGFP) laser is numerically investigated. The effect of external optical feedback (OFB), temperature, injection current, cavity volume, nonlinear gain compression factor, and fiber grating (FG) parameters on IM characteristics are presented. The temperature dependence (TD) of IM is calculated according to the TD of laser cavity parameters instead of using the well-known Parkove relationship. It has been shown that the optimum external fiber length (L _ext) is 3.1 cm. The optimum range of working temperature for FGFP laser is between 23 to 27 °C. We also show that by increasing the laser injection current from 10 to 60 mA, the IM peak amplitude decreased from 6.3 to 0.2 dB and the relaxation-oscillation frequency (ROF) is shifted from 1.2 GHz towards higher frequency of 5.48 GHz. In addition, the AR coating reflectivity and gain compression factor have no significant effect on the IM. The study indicates that a stable operation and excellent modulation characteristic can be obtained after optimization process.     

KrF准分子激光的饱和参量与最佳激光输出

本文研究了在激光达到饱和强度下的上能级弛豫速率及泵浦速率两个参量与激光输出强度的关系。激光达到饱和强度时,上能级弛豫速率等于受激跃迁几率。根据这一特性,研究了Ar在4个大气压到6个大气压之间的四组N_(Ar)和N_(F_2),粒子数密度的Ar—Kr-F_2系统。计算了在给定激发速率和外耦合下的激光输出能量密度随Kr所占混合气体的比例而变化的关系。最后,从饱和强度下的上能级弛豫速率和泵浦速率得到了最大激光输出的条件,相应的激发速率、气压和气体混合的比例。     

Quantum well and quantum dot lasers: From strained-layer and self-organized epitaxy to high-performance devices

Strained heterostructures are now widely used to realize high-performance lasers. Highly mismatched epitaxy also produces defect-free quantum dots via an island growth mode. The characteristics of high-speed strained quantum well and quantum dot lasers are described. It is seen that substantial improvements in small-signal modulation bandwidth are obtained in both 1 m (48 GHz) and 1.55 m (26 GHz) by tunneling electrons directly into the lasing sub-band. In quantum dots the small-signal modulation bandwidth is limited by electron-hole scattering to 7 GHz at room temperature and 23 GHz at 80 K. The properties of these devices are described.     

Fabrication and modulation characteristics of InGaAsP/InGaAsP MQW DCPBH lasers at λ ∼ 1.57 μm

InGaAsP/InGaAsP multiple-quantum-well (MQW) double-channel planar-buried heterostructure (DCPBH) lasers emitting at λ∼ 1.57 μm were fabricated by optimizing the epitaxial growth with material characterization. At 25 °C (85 °C), a 1.8-μm-wide and 300-μm-long antireflectivity/high reflectivity coated laser exhibits a threshold current of 8 mA (23 mA) and a slope efficiency of 0.34 mW/mA (0.24 mW/mA) in continuous-wave mode (cw) as a result of the optimized thickness of the p-InP filling layer in the PBH structure with p-n-p-n current blocking layers. The maximum cw output power was approximately 20 mW at 25 °C, which was reduced to 17 mW at 85 °C. The peak wavelength varied from 1572 nm at 25 °C to 1602 nm at 100 °C for a fixed output power of 5 mW, indicating a temperature coefficient of ∼ 0.4 nm/K. The resonance frequency in the small-signal modulation response of devices depends on the etching depth of the U-shaped double channel; it increases from 0.4 GHz without channel etching to 4.3 GHz with 7-μm-thick etching. The full-width at half maximum values in the horizontal and vertical far-field patterns were approximately 24.5° (25.2°) and 29.4° (30.1°), respectively, at 25 °C (85 °C). PACS 42.55.Px; 73.61.Ey; 81.05.Ea     

Mid-Infrared Emission Characteristic and Energy Transfer of Ho<Superscript>3+</Superscript>-Doped Tellurite Glass Sensitized by Tm<Superscript>3+</Superscript>

We report on 2.0-μm emission characteristic and energy transfer of Ho³⁺-doped tellurite glass sensitized by Tm³⁺ upon excitation of a conventional 808 nm laser diode. The Judd-Ofelt strength parameters, spontaneous radiative transition probabilities and radiative lifetime of Ho³⁺ have been calculated from the absorption spectra by using the Judd-Ofelt theory. Significant enhancement of 2.0-μm emission of Ho³⁺ has been observed with increasing Tm³⁺ doping up to 0.7 mol%. The energy transfer coefficient of the forward Tm³⁺→Ho³⁺ is approximately 17 times larger than that of the backward Tm³⁺←Ho³⁺ energy transfer. Our result indicates that the maximum gain of 2.0-μm emission, assigned to the transition of ⁵I₇→⁵I₈ of Ho³⁺, might be achieved from the tellurite glass at the concentration of 0.5 mol% of Tm₂O₃ and 0.15 mol% of Ho₂O₃. The high gain coefficient and quantum efficiency (1.16) along with the large value of the product of the stimulated emission cross-section and the measured radiative lifetime (4.12×10⁻²⁷ m²s) of the Ho³⁺/Tm³⁺-codoped tellurite glasses might find potential applications in efficient 2.0-μm laser.     

Modelocked quantum dot vertical external cavity surface emitting laser

We report the first successful modelocking of a vertical external cavity surface emitting laser (VECSEL) with a quantum dot (QD) gain region. The VECSEL has a total of 35 QD-layers with an emission wavelength of about 1060 nm. In SESAM modelocked operation, we obtain an average output power of 27.4 mW with 18-ps pulses at a repetition rate of 2.57 GHz. This QD-VECSEL is used as-grown on a 450 μm thick substrate, which limits the average output power.     

Time-resolved spectral characteristics of external-cavity quantum cascade lasers and their application to stand-off detection of explosives

We present the time-resolved spectral analysis of the settling process in a pulsed external-cavity quantum-cascade laser (EC-QC laser) emitting in the wavelength range around 7.4 μm. Due to mode competition between the two cavities formed by the laser-chip on one hand and the external-cavity on the other hand, a time-dependent spectral emission on a ns time-scale can be observed. Depending on the spectral characteristic of the external cavity with respect to the chip gain curve time-delays of the external cavity emission of 15–35 ns within the 100 ns drive pulse can be observed.     

THE DESIGNS OF HIGH EFFICIENCY LAUNCHER OF QUASI-OPTICAL MODE CONVERTER FOR HIGH POWER GYROTRONS

A high efficiency launcher of quasi-optical (QO) mode converters for high power gyrotrons have been designed and tested. A helical cut launcher radiates the RF power via its straight cut onto the first phase correcting mirror. The launchers have been optimized for the TE_31.8 mode at 170 GHz and TE_22.6 mode at 110 GHz by numerically optimizing a launcher surface. The helical cut of the launcher has been optimized by taking the taper angle into account. Further more, the amplitude of the surface perturbation have been optimized for improved focusing in order to reduce the diffraction losses at the helical cut. Low power measurement shows a good agreement with the design. High efficiency characteristics of the design have also been calculated on the assumption of frequency downshift due to the thermal expansion of the cavity and stepwise frequency tuning by changing the operating mode. Besides, the possibility of high efficiency launcher for higher mode is discussed, and these results give the prospect to high efficiency long pulse gyrotrons.     

A High-gain Wideband Antenna with Double Fabry-perot Cavities

A high-gain wideband antenna, using the electromagnetic resonances of double Fabry-Perot (F-P) cavities, is proposed. The two cavities are excited by a patch antenna placed in the cavities on top of the ground plane. One of the double F-P cavities is formed by a ground plane and a single metallic strips array, and the other consists of the patch and the metallic strips array. The two F-P cavities have different resonance points which yield the frequency bandwidth of 7% between 13.0 and 14 GHz with S₁₁ ≤ 10 dB, meanwhile, in this frequency region high gain is also obtained. Moreover, the center frequency and bandwidth could be adjusted by changing the cavity length. The high-gain wideband antenna was manufactured and measured. The measured VSWR is less than 2 from 13.3 GHz to 15.2 GHz, the measured gain is 13.5 dB at 13.5 GHz. In addition to that, a considerable improvement of 7 dB in terms of gain is obtained when compared to the same antenna without metallic strips.     

894nm高温垂直腔面发射激光器及其芯片级铯原子钟系统的应用

报道了自行研制的894 nm高温垂直腔面发射激光器(VCSEL)以及基于此类器件的芯片级铯原子钟系统的应用实验结果.根据芯片级铯原子钟对VCSEL在特定高温环境下产生894.6 nm线偏振激光的要求,对器件的量子阱增益及腔模位置等材料结构参数进行了优化,确定增益-腔模失谐量为-15 nm,使器件的基本性能在高温环境下保持稳定.研制的VCSEL器件指标为:20—90?C温度范围内阈值电流保持在0.20—0.23 m A,0.5 m A工作电流下输出功率0.1 mW;85.6?C温度环境下激光波长894.6 nm,偏振选择比59.8:1;采用所研制的VCSEL与铯原子作用,获得了芯片级铯原子钟实施激光频率稳频的吸收谱线和实施微波频率稳频的相干布居囚禁谱线.     

Ka波段分布作用速调管多间隙谐振腔研究

谐振腔作为速调管的高频互作用电路,其特性对速调管的功率、效率、增益和带宽等性能具有决定性影响。主要介绍了某Ka波段分布作用速调管谐振腔的设计过程:基于多间隙谐振腔理论,利用电磁仿真软件CST详细分析了谐振腔不同结构尺寸对特性参数,如品质因子、特性阻抗、耦合系数、有效特性阻抗的影响,优化得到谐振频率为35 GHz的五间隙谐振腔的物理结构模型,并给出互作用仿真结果,为Ka波段分布作用速调管设计及其高频注波互作用的计算提供重要的参考和依据。     

Novel deep centers for high-performance optical materials

Materials exhibiting strong optical emission also exhibit strong absorption at the same wavelengths because both emission and absorption are governed by the same optical dipole and density-of-states. Laser action requires a carrier injection large enough for emission to exceed absorption at laser wavelengths. Thus, strong self-absorption at luminescent wavelengths raises the operating current of LEDs, lasers, and optical amplifiers. Here we demonstrate that, contrary to conventional expectations, materials designed with novel deep centers achieve surprisingly large optical emission while, simultaneously, the inverse process of optical absorption remains very small. A striking consequence is that materials designed with our novel deep centers achieve transparency at a carrier injection which is four-orders-of-magnitude lower than in all technologically important semiconductors. Simultaneously, and surprisingly, our novel deep centers in GaAs achieve an optical gain, Einstein B coefficient, and radiative efficiency significantly larger than in direct-band-gap materials at 1.3–1.5 μm. We engineered this dramatic reduction of the injection to achieve transparency while retaining strong optical emission in our novel material by making use of a Franck–Condon shift of absorption away from luminescent wavelengths. PACS 71.55.Eq; 71.55.-i; 78.67.-n; 81.10.-h; 85.60.Jb     

Chemically pumped O2(a-X) laser

A theoretical and experimental study was conducted aimed at achieving laser oscillation in the (a-X) electronic transition of oxygen molecules. Although this transition is highly forbidden by rigorous selection rules, it may nevertheless concede stimulated emission, if the population inversion is high enough. The idea is based on a recently developed apparatus, namely, a porous pipe type high-pressure chemical singlet oxygen generator. A numerical model which describes the characteristics of this generator was developed to estimate the population inversion and small-signal gain achievable in a laser cavity using this source. The calculations showed that the small-signal gain ought to be sufficient to achieve laser oscillation. Preliminary experiments were conducted, but lasing was not yet observed. It is shown that the scattering losses caused by water droplet aerosols are mainly responsible for preventing our system from laser oscillation.     

Quantum cascade laser-based free space optical communications

The advent of the quantum cascade laser (QCL) with emission wavelengths available in the infrared range from 3 μm through more than 100 μm opens up the possibility of exploiting infrared atmospheric transparency windows for free space optical communications. In an effort to establish the efficacy of using directly modulated QCLs for free space communications we have conducted a series of investigations that demonstrate the potential advantages of this technology. In these experiments we first establish that the QCL has very high modulation bandwidth. We then implement a practical free space communications link that under conditions of atmospheric fog, dust and other obscurants offers significant transparency advantage when compared with near infrared wavelength sources presently used in commercial free space optical communications links.     

Modulation response of dynamic single-mode quantum-well heterolasers in the 1.5 μm range

Numerical modelling is used to study the effect of tuning the laser output over the gain bandwidth on the modulation response of GaInAs–GaInAsP quantum-well heterolasers for different modulation frequencies of the pump current. It is found that the maximum frequency bandwidth of the response band and the greatest feasibility of high speed modulation for transmission of signals in information systems are achieved in the center of the gain band. Raising the dc component of the pump current increases the response bandwidth. For typical parameters of this system (near 1.5 μm) the maximum response bandwidth can approach ≈40 GHz. For certain parameters, the amplitude-frequency characteristics of the heterolasers have two local maxima: one at low frequency corresponding usually to a resonance for the 1/2 subharmonic and one at high frequency, for the fundamental resonance.     

二极管激光泵浦Nd：YVO4激光器的频率调谐特性研究

用加热法和压电陶瓷ＰＺＴ调腔的方法，实现二极管激光泵浦的单频运转１０６４ｎｍ２％Ｎｄ：ＹＶＯ４激光器调谐。调谐范围４７．４ＧＨｚ，增益峰漂移引起的激光频率温度系数２ｖ／２Ｔ＝－１．０４ＧＨｚ／℃，压电陶瓷调谐系数１５０ＭＨｚ／Ｖ。研究了加热影响件性能的情况。