Novosibirsk free electron laser is a tunable source of radiation generating high-power Gaussian beams in MIR, FIR and THz spectral ranges. In this paper we report the transformation of terahertz beams into the beams with prescribed cross-sections and phase distributions, including vector and vortex beams, using diffractive optical elements. Examples of the use of such beams in experiments are given in the paper.

A multilayer antireflection coating for diamond optics that allows work in the infrared spectral range of 8 - 12 mu m with minimal optical losses is developed. The optical transmittance of a chemical vapour deposition diamond plate coated with this film on both sides exceeds 94% over the whole specified wavelength range. The coatings deposited on the diamond plate were damage-tested by coherent-wave and pulsed (tau = 90 ns) CO2 lasers. Results of the tests demonstrated that the coating can withstand prolonged radiation loads with intensity above 3 MW cm(-2) in a continuous-mode laser exposure. In the case of a nanosecond pulsed action, destruction of the coating begins at intensities greater than 50 MW cm(-2).

Appearance of the sources of coherent and high power THz radiation [1] opened new horizons for investigations in this frequency range [2]. High attention is focused on silicon diffractive optical elements (DOE), which are used for the beam manipulation [3-7]. The lithographic etching of a silicon substrate has been used in [3-7] to fabricate binary relief of diffractive optical elements. Binary silicon element [4] coated with the antireflection coating remained intact upon exposure to an average radiation power density of 4 kW/cm; the peak power in a 100 ps pulse was almost 8 MW/cm. Experimental estimates of the diffraction efficiency of the elements coated with the antireflection coating [4] are in good agreement with theoretical estimates. Such applications like imaging, material ablation, generation of continuous optical discharge, and even more exotic for the terahertz range application, namely the field ionization of individual atoms, require focusing of THz radiation [3-5], often with an enhanced focal depth [3]. Non-diffractive Bessel beams with angular orbital momentum (vortex beams) with different topological charges were formed by use of binary phase spiral axicons [6]. Binary phase axicon (BPA) with spiral zone structure and with aperture diameter of 100 mm (Fig. 1a,b) has been realized in [7] by technology similar to described in [3-6].

Using binary silicon phase spiral axicons, non-diffractive Besse) beams with angular orbital momentum (vortex beams) with different topological charges were formed. Such beams have great potential for use in data transmission and remote sensing. The research was performed at workstations of the Novosibirsk free electron laser user facility. The computational results and the experimental results well agree.

A number of silicon diffractive optical elements, which enabled transforming NovoFEL Gaussian beam into the Laguerre-Gaussian and Hermite-Gaussian ones, have been designed and fabricated. Other elements transformed the NovoFEL beam into determined volumes (i. e., a pencil-like beam), or areas (i. e., a uniformly illuminated square). A problem of strong Fresnel reflection was solved by the use of anti-reflection film covering.

In this paper, we have performed experimental, analytical, and numerical studies of beams with topological charges of +/- 1 and +/- 2 formed by silicon binary phase axicons (BPAs) with spiral zone structures. The axicons were illuminated with the Novosibirsk free electron laser radiation (a continuous stream of 100-ps pulses at f = 5.6MHz). The cw power of the beams produced reached 30Wand can by doubled via antireflection coating of the axicons. The intensity distribution in the beam cross sections was in good agreement with the Bessel functions and was kept constant within a distance of about L/r approximate to 190 and 100, where the first ring radii of the beams r were 0.9 and 1.5 mm for the Bessel beams of the first and second orders, respectively. Although the characteristics of the beams (Bessel cross section, "diffraction-free" propagation, self-recovery after passing obstacles, and randomly inhomogeneous media) corresponded to the properties of ideal Bessel beams, their spatial Fourier spectrum (the image in the focal plane of the lens) was, instead of an ideal ring, intertwined segments of arcs with phases shifted by pi, the number of which was equal to the double value of the topological charge. This feature can be used, for example, in a demultiplexing unit of a free vortex-wave communication system or for identification of beam topological charge. We also revisited Young's double-slit diffraction and rotation of beams obstructed by a half-plane, previously applied to Laguerre-Gaussian beam characterization, in the case of the Bessel beams. The Young diffraction pattern demonstrated in this case a complicated intensity-phase distribution. It was shown that the Bessel beams formed by BPAs have two important advantages, which can be used in applications, in comparison with other methods of generation, e.g., a combination of an axicon lens with a spiral phase plate. Although the phase jumps of the axicons are designed for a determined wavelength (141 mu m in our case), the BPAs can form the beams at incident radiation with any wavelength, albeit with a reduced diffraction efficiency, and their cross section is the same for any wavelength.

The energy and angular momentum are quantum characteristics of photons. The angular momentum is the sum of the spin and orbital angular momenta. In physical optics, the spin momentum is described as the polarization of light beams, and the orbital angular momentum (OAM) is the rotation of beam along the azimuth. Interest in beams with OAM [1, 2], or "vortex beams," appeared shortly after the creation of lasers and increased after the publication of the article [3]. Vortex beams have been formed and investigated in the spectral range from X-ray to radio waves, but only a few publications are devoted to beams with OAM in the terahertz range (see [4] and references therein). In this paper we describe vortex Bessel beams with an average power of 25 W, obtained on the Novosibirsk free-electron laser (NovoFEL) [5]. Vortex beams were formed from the NovoFEL Gaussian beam transformed by binary phase axicons (BPA) with spiral zone structures (Fig. 1, a, b). The zone boundaries are described with the equation ( / 2 / 2) r p l , where 2 / 3.2 p mm is the

This paper reports the design, simulation and fabrication process for a 2-DOF decoupled vibratory gyroscope. The structure is deliberately designed to have decoupled drive and sense mode oscillation to prevent unstable operation due to mechanical coupling, resulting in low zero rate out-put drift. At the same time, the closer the drive and sense resonances are, the higher is the angular rate resolution of the gyroscope. This can be achieved by using symmetric suspensions, but it results in reduced bandwidth. The proposed design has been configured to achieve about 150 Hz bandwidth, while ensuring decoupled operation of the drive and sense modes. Fem analysis has been carried out in CoventorWare MEMS DESIGN software and simulation results show that the drive resonance occurs at 21.48 kHz and sense resonance at 21.63 kHz. The structure is designed for 15 μm this device layer. Fabrication of the design is proposed using DRIE and sacrificial release etching on SOI wafer. DRIE etching with high aspect ratio has been successfully carried out as desired and the results have been presented.

Comparison of the two laser sources (UV nanosecond and IR femtosecond) used for the formation of micro-relief at the silicon surface showed the advantage of the second one. A four-level silicon diffractive THz Fresnel lens has been fabricated by laser ablation at high repetition rate (f = 200kHz) of femtosecond Yb:YAG laser. Features of the lens were investigated in the beam of the Novosibirsk free electron laser at the wavelength of 141μm. Detailed results of investigation of fabricated lens micro-relief are presented. The measured diffractive efficiency of the lens is in good agreement with the theoretical prediction.

A tunable diffraction grating based on an electrooptic X-cut lithium niobate crystal has been manufactured and experimentally analyzed. The period of electrodes is 290 μm, the electrode width is 117.5 μm, and the thickness of an electrode is 150 – 160 nm. The electrodes are made of a transparent conducting indium-tin oxide that serves as an antireflection coating with the aim of increasing the optical transmission. In order to prevent crystal polarization switching and electrical breakdown an optimized electrode topology with end ellipticity 1:1 and increased interelectrode gap is used. The optical diagram of the tunable grating with alternating electrode potentials for various gap voltages is analyzed. The intensity of the zero order of diffraction is shown to decrease by 40 % at a voltage of 800 V. At the same time, the origination of new diffraction orders at angles ± λ / (2d) is noted. The measurement of the forward-bias and reverse-bias regions of the modulation characteristic reveals the absence of hysteresis, which confirms the correctness of the electrode topology design.

Terahertz remote sensing, transmission of terahertz radiation for communication, and other applications which require transport of terahertz radiation at a distance permanently attract great attention. Security, spectroscopy, lidar techniques, and data transmission are areas of interest. Depending on the goals and the environment (atmosphere or space), in different applications the word “remote” can mean distances of tens of centimeters to hundreds of meters and more. In this paper we describe experiments on formation of “non-diffractive” terahertz vortex Bessel beams and study their propagation through atmosphere. Self-healing of beams with topological charges ±1 and ±2 was demonstrated.

A binary silicon diffractive optical element (DOE) focusing laser radiation onto an axial segment (or a DOE with elongated focal depth) for the terahertz spectral range has been designed and characterized using terahertz radiation of the Novosibirsk Free Electron Laser (NovoFEL).

Beams with orbital angular momentum are widely used in many spectral ranges, including the visible light, radio-frequency band and even soft X-rays. In this regard, the terahertz range is still underinvestigated; very few studies were devoted to the generation of terahertz vortex beams, and only the Lagguerre–Gaussian beams were generated to date. In this paper we describe the design and fabrication of silicon binary phase plates with a spiral pattern, which transform an incident plane wave with a wavelength of 141 μm into a vortex one. Using the Novosibirsk free electron laser as a source of CW radiation, non-diffractive Bessel vortex beams with topological charges of l = ±1 and l = ±2 and the average power of 30 W were first produced in the terahertz spectral range. The spatial characteristics of the beams were examined using a microbolometer array. Path-length/radius ratios of 180 and 90, respectively, were attained experimentally for these beams. The self-healing ability of the beams obtained was demonstrated.

This paper reports the design and fabrication of a 2-degree-of-freedom (DOF) decoupled vibratory gyroscope based on a silicon-on-insulator (SOI) MEMS process. The 2-DOF capacitive comb structure is deliberately designed to have a decoupled drive and sense mode oscillation to prevent the unstable operation due to mechanical coupling, resulting in a low zero rate out-put drift. It is well known that the closer are the drive and sense resonances, the higher is the angular rate resolution of the gyroscope. Generally, this is achieved by using symmetric suspensions, but it results in a reduced bandwidth. The proposed design has been configured to achieve a bandwidth of about 150 Hz, while ensuring the decoupled operation of the drive and sense modes. An analytical method has been employed to study the steady state response of the 2-DOF structure. FEM analysis has been carried out in CoventorWare® MEMS Design software and the simulation results show that the drive resonance occurs at 21.48 kHz and sense resonance at 21.63 kHz, which are in close agreement with the theoretical results. The structure is designed with a 15 µm thick device layer. Fabrication of the design is proposed using a two mask process based on Deep reactive-ion etching (DRIE) and sacrificial wet release etching on a SOI wafer. DRIE etching with an aspect ratio of 1:5 has been successfully carried out as desired and the results have been presented.

We demonstrate results of studies of a silicon binary diffractive optical element (DOE) focusing a terahertz laser Gaussian beam into a paraxial segment. The characteristics of the DOE were examined on a Novosibirsk Free Electron Laser beam of 141-mu m wavelength.

The possibility of fabricating a silicon diffractive four-level THz Fresnel lens by laser ablation is studied. For a microrelief to be formed on the sample surface, use is made of a femtosecond Yb : YAG laser with a high pulse repetition rate . Characteristics of the diffractive optical element are investigated in the beam of a free-electron laser. The measured diffraction efficiency of the lens is in good agreement with the theoretical estimate.

This paper presents experimental results on the formation of given laser modes from an illuminating Gaussian beam of the terahertz Novosibirsk free electron laser at a wavelength of 141 mu m. Binary silicon diffractive optical elements were applied. The experimental results obtained are in good agreement with the results of computer simulation. (C) 2015 Optical Society of America