Characteristics of a far-infrared molecular laser optically pumped by a high-power CO2 laser

A molecular far-infrared (FIR) laser optically pumped by a high-power CO₂ laser, which is a powerful source for testing detectors and mixers and for FIR spectroscopy, is constructed and the performance is examined through experiments. At frequencies between 580GHz and 4.25THz, FIR output power is more than 20～30m W by pumping power of 35～81W. Amplitude stability of ±3% is obtained at 100m W output at 2.52THz for over 30 minutes when the FIR tube is cooled at 5°C by a chiller. As an application to testing mixers, FIR laser lines up to 4.25 THz are detected by Schottky barrier diodes (SBD). Further, using a SBD, performance of absolute frequency stability at 693GHz of HCOOH oscillation is measured by harmonic mixing with a 115.5GHz millimeter wave from a phase-locked Gunn oscillator. The resultant center-frequency stability is 100kHz per 10 minutes.     

Frequency measurements of far infrared laser lines by means of MIM point contact diodes

We present frequency measurements of 85 Far Infrared (FIR) laser lines ranging from 2 748 GHz to 421 GHz. We use the heterodyne technique of mixing FIR laser transitions and microwave radiation on MIM point contact diodes to determine the FIR frequencies.     

CW submillimeter laser action in ethyl chloride

We present ten new far-infrared (FIR) laser lines obtained by optically pumping ethyl chloride by means of a CW waveguide CO₂ laser with a tunability range of 300 MHz. By means of a heterodyne technique and using, as mixing element, a metal-insulator-metal (MIM) point contact diode or a new home built metal-semiconductor diode, we measured the frequency of all these new lines, as well as that of another line, previously reported in literature. With our new lines, the total number of FIR laser lines emitted by ethyl chloride rises to 14; for each of them we give the offset relative to the center frequency of the pumping radiation, the polarization relative to that of the pumping CO₂ laser, the relative intensity, and the optimum operation pressure. Seven of the new lines have frequencies in the 300-512-GHz range and four of them are strong enough to be used in high-field EPR spectroscopy     

The generation and frequency measurement of short-wavelength far-infrared laser emissions

A significantly improved three-laser heterodyne system has been assembled to generate and measure short-wavelength far-infrared (FIR) laser emissions. Over the past several years, this system has been used to discover fifty-five FIR laser emissions, ranging in wavelength from 26.3 to 185.0 /spl mu/m. These emissions were generated by optically pumping a FIR cavity with a continuous-wave carbon dioxide laser in a X-V geometry when using either hydrazine or a methanol isotopologue as the FIR laser medium. Although heterodyne techniques can be used to measure the frequencies of these FIR laser emissions with fractional uncertainties of /spl plusmn/2/spl times/10/sup -7/, shortcomings in the previous system limited its effectiveness. Improvements made to the three-laser heterodyne system have resulted in an increase in the spectral range used to search for the beat between the known and unknown laser frequencies (an increase of up to 25 GHz) as well as an increase in the system's sensitivity (by up to a factor of 30), all without requiring the use of an additional microwave frequency source. With this improved system, the FIR laser frequencies for the recently discovered 53.9- and 90.0-/spl mu/m laser emissions generated by optically pumped CH/sub 3/OD have been measured.     

Harmonic mixing and detection with Schottky diodes up to the 5 THz range

We report results of harmonic mixing experiments between a 0.4 THz oscillator and FIR lasers at 4.2 and 5.3 THz using μ-size Schottky barrier diodes, together with rectification results in the range of 0.6-5.3 THz for different diodes and coupling methods.     

基于并联调制器的高倍频毫米波信号生成

为了获得更高频率的信号，采用了一种基于并联马赫-曾德尔调制器（MZM）和半导体光纤四波混频效应的二十四倍频光生毫米波方案。本振信号被并联MZM调制后可以获得高纯度的±4阶边带，在四波混频效应下，又可以生成±12阶边带，用级联的布喇格光栅滤波器滤除±4阶边带，经光电探测器拍频便可生成二十四倍频信号。结果表明，当输入本振信号为5GHz时，生成的120GHz高频微波毫米波信号射频杂散抑制比为22dB，频谱纯度很高，且具有很好的可调谐性。该研究为高频微波毫米波信号的生成提供了更高的倍频方法。     

Heterodyne frequency measurements of FIR laser lines around 1.2 and1.6 THz

We present the frequency measurements of 14 far-infrared (FIR) laser lines. We use the heterodyne technique of mixing FIR laser transitions and microwave radiation on a MIM point contact diode to determine the FIR frequencies. The main motivation was to measure the frequency of laser lines of metrological interest, around the 1.6 and 1.2 THz regions. We also present 15 new FIR laser lines we found while performing the frequency measurements, with wavelengths ranging from 90 μm to 819.2 μm. All of them are characterized in wavelength, polarization, intensity, working pressure and absorption offset     

Terahertz Radiation Shaping Based on Third-Order Dispersion and Self-Phase Modulation in Standard Single-Mode Optical Fiber

Third-order dispersion and self-phase modulation in standard single-mode fibers are employed in a fiber-based THz time domain spectroscopy system for radiation shaping. Ultra-short optical pulses are converted into trains of pulses, thus shaping the THz radiation emitted by photoconductive antennas operating at telecom wavelengths. The proposed architecture allows narrowband and wideband THz emission as well as tunability of the central frequency. Since the shaping takes place in standard optical fiber the architecture could be potentially implemented without requiring any additional device. Experiments showing the principle of operation have been performed demonstrating tunability of the central frequency between 350 and 800 GHz and bandwidth from 150 GHz to the full bandwidth of the system.     

Linewidth, tunability, and VHF-millimeter wave frequency synthesis of vertical-cavity GaAs quantum-well surface-emitting laser diode arrays

The authors report the measurement of the laser linewidth, wavelength tunability, and generation of microwave frequencies between individually addressable elements of a vertical-cavity GaAs quantum-well surface-emitting laser diode array (lasing in the wavelength range 850-865 nm). Using heterodyne techniques, the authors obtain a deconvolved 65 MHz laser linewidth from the 109 MHz beat signal. The laser linewidth corresponds to a semiconductor laser linewidth enhancement factor alpha =5.7, which is in excellent agreement with that obtained independently from optical gain measurements and corresponding calculated refractive index changes. The authors measured heterodyne beat frequencies of 2-20 GHz. The bandwidth was limited by the microwave amplifiers. A simple calculation shows that a tuning range of 65 MHz to 3 THz can be achieved.<>     

FIR Laser Lines from CH3OD: A Review

The methanol isotopic species CH₃OD has also proved to be an efficient and powerful medium to generate radiation in the far infrared (FIR) region. After the critical review of 1994, six papers have been published dealing with new FIR laser lines from this molecule. As a consequence of the use of wide tunability waveguide CO₂ lasers as well as a new pulsed CO₂ laser operating at hot and sequential bands, as of optical pumping sources, the total number of the FIR laser lines increased from 122 in 1994 to 227 today. In this communication we present an updated and complete catalogue of FIR laser lines generated from CH₃OD. Information on wavelength, offset, relative polarization, intensity, and optimum operation pressure is generally available.     

Accelerator Sources for THz Science: A Review

Free Electron Lasers have been around since 1977 providing not only a test bed for the physics of FELs and electron/photon interactions but as a workhorse of scientific research. More than 30 FELs are presently operating around the world spanning a wavelength range from the millimeter region to the hard x-ray using direct current and rf linear accelerators or storage rings as electron sources. The characteristics that have driven the development of these sources are the desire for high peak and average power, high micropulse energies, wavelength tunability, timing flexibility, and wavelengths that are unavailable from more conventional laser sources. Operation of FELs in the far infrared to terahertz regime poses special challenges which have been and are being addressed at a number of facilities around the world. This paper will review a number of former and existing FELs operating in this regime and discuss future efforts. Broadband collective radiation from relativistic electrons also plays a significant role in the production of FIR/THz radiation and several groups are taking advantage of this source for users. Applications for use of the radiation have evolved from simple imaging to complex pump probe tests of insulator/metal transitions and energy flow in organic molecules. We will discuss the technologies for generating the IR/FIR/THz radiation and cover some of the unique applications of such sources.     

基于相位调制器的宽带窄线宽的线性调频激光源的产生

提出了一种基于相位调制器(PM)和可调谐光滤波器产生线性调频激光信号的方法。该方法利用带有基频的微波线性调频信号作为相位调制器的驱动信号,窄线宽的激光种子源经相位调制器调制后产生一系列的宽带线性调频激光信号。通过可调谐光滤波器抑制其他边带保留所需阶次的线性调频激光信号。实验结果表明:当光滤波器保留正二阶调频激光信号时,获得了调频带宽为2 GHz、调频速率为6 THz/s的线性调频激光信号。在观测时间为1 ms时,测得的线性调频激光信号的瞬时线宽为3.2 kHz。该方法结构简单,易于实现,并且对调频连续波激光雷达、相干光谱分析等测量应用有重要意义。     

基于级联调制器和四波混频效应的24倍频微波信号的光学生成

提出了一种基于级联马赫-曾德尔调制器 (MZM)和半导体光放大器(SOA)的24倍频微波信号 光学生成方案,具有覆盖频段高、杂波抑制效果好等优点。在本方案中,低频微波信号分别 通过两个级联 MZM对连续光源进行调制,调节直流偏置使两个MZM均工作在最大偏置点,以抑制奇数阶光分 量;进一 步调节两个MZM的调制深度,并结合可调谐电相移器(TEPS)和可调谐光相移器(TOPS)引入相 移,完全抑 制第2个MZM输出的±2阶光分量和光载波,得到±4阶光分量;再经过SOA发生四波混频(FWM )效应,形成±12 阶光分量;滤波后拍频可以获得24倍频微波信号。最后,搭建了实验和仿真系统,分别以11．0GHz,11.5GHz和12.0GHz的微波信号为驱动,得到间隔为264、276GH z和288GHz的±12阶光边带,有效验证了方案的可行性。     

Observation and assignment of D218O FIR laserlines optically pumped by a waveguide CO2 laser

By means of a detailed analysis of the v₂ infrared band of D₂¹⁸O, it has been possible to assign most of the FIR emissions reported in the literature. Moreover, two FIR laser lines which fall in the range of tunability of the CW waveguide CO₂ laser have been predicted and observed. The frequency of the stronger line was measured and found to be 2611.4185(10) GHz, thus filling a gap in the presently available comb of FIR laser lines whose frequencies have been measured. The wavelength precision of the assigned lines was improved by about two orders of magnitude     

中物院太赫兹自由电子激光:设计、运行及升级

中物院太赫兹自由电子激光(CTFEL)装置是我国第一台基于超导加速器的高重复频率、高平均功率太赫兹自由电子激光装置。CTFEL利用光阴极直流高压电子枪和超导加速器产生约8 MeV电子束在波荡器中产生自发太赫兹(THz)辐射,并在光腔中受激放大获得饱和输出。得益于在0.7~4.2 THz频谱范围内连续可调以及平均功率大于10 W的特性,CTFEL为材料动力学、太赫兹成像、太赫兹生物学等领域提供了独特的研究平台。自2018年开放成为用户装置以来,每年提供不少于1000 h的稳定出光。未来CTFEL将在现有基础上升级成为红外太赫兹自由电子激光装置,实现太赫兹频率全覆盖以及最大功率大于100 W的目标,力争成为世界先进的长波长自由电子激光装置。     

Exploration of Terahertz Imaging with Silicon MOSFETs

We summarize three lines of development and investigation of foundry-processed patch-antenna-coupled Si MOSFETs as detectors of THz radiation: (i) Exploiting the pinciple of plasma-waved-based mixing in the two-dimensional electron gas of the transistors’ channels, we demonstrate efficient detection at frequencies as high as 9 THz, much above the transit-time-limited cut-off frequencies of the devices (tens of GHz). Real-time imaging at 600 GHz with a 12 × 12 detector array is explored. (ii) Given the limited THz power usually available for applications, we explore imaging with enhanced sensitivity in heterodyne mode. We show that real-time operation of a 100 × 100-pixel heterodyne camera should be possible at 600 GHz with a better dynamic range (30 dB) than for direct power detection (20 dB), even if only a quarter-milliwatt of local-oscillator power, distributed radiatively over all detector pixels, is available. (iii) Finally, we present an all-electronic raster-scan imaging system for 220 GHz entirely based on CMOS devices, combining the CMOS detectors with an emitter circuit implemented in a 90-nm CMOS process and delivering radiation with a power on the 100- μW scale. Considering progress in the field, we anticipate that the emitter concept of oscillator-based power generation with on-chip frequency multiplication will carry well into the sub-millimeter-wave regime.     

Precision Tunable Infrared Source at 10 μM: CO2-Laser/Microwave-Sideband System with an Evenson CO2 Laser and a Cheo Waveguide Modulator

A CO₂-laser/microwave-sideband tunable infrared source system has been newly built at the University of New Brunswick (UNB) in Canada. The system employs a high-resolution CO₂ laser of Evenson design that lases on a wide range of lines including hot and sequence band lines as well as high-J lines of the regular 9.6 and 10.6 μm bands. The frequency of the CO₂ laser is stabilized to the Lamb dip in the 4.3 μm fluorescence signal in an external CO₂ cell. Microwave (MW) sidebands are generated in a Cheo-type infrared waveguide modulator driven by a synthesized sweeper and a traveling wave tube (TWT) amplifier. The MW sidebands appear on either side of the CO₂ laser line, and are individually separated from the carrier by a tunable Fabry-Perot etalon. The sidebands currently have a typical power of about 1.4 mW and a continuous frequency tuning range of 22 GHz (from ± to ±18 GHz) when 5 W laser power and 15 W microwave power are delivered to the modulator. Details are given on the source construction and measured performance characteristics. Features of the source and its planned applications are discussed.     

Frequency Lock of an Optically Pumped FIR Ring Laser at 803 and 1626 GHz

We report an automatic frequency control (AFC) for an optically pumped far infrared (FIR) ring laser applicable for high resolution THz sideband spectroscopy by mixing a fraction of the laser power and a harmonic of a phase-locked synthesizer on a planar Schottky diode. We achieve a relative frequency accuracy of about 0.5 kHz rms at 803 GHz ( ¹⁵ NH ₃ ) and about 1 kHz rms at 1626.6 GHz (CH ₂ F ₂ ) over hours of lock time. The absolute frequency accuracy is estimated to be about 5 kHz at 1626.6 GHz.     

20-GHz-to-1-THz Repetition Rate Pulse Sources Based on Multiple Four-Wave Mixing in Optical Fibers

In this paper, we theoretically and experimentally study the generation of very high-repetition-rate pulse sources based on multiple four-wave mixing in optical fibers. More precisely, we described the generation of nearly transform-limited pulses at repetition rates of 20, 40, 80, 160, 320, 640 GHz, and 1 THz with a wavelength tunability close to 20 nm around 1555nm. In particular, frequency resolved optical gating analyses show that 170-fs transform-limited pulses have been generated at 1 THz.