Nondestructive Sensor Using Microwaves from a Laser Plasma

Abstract

Ground penetrating radar (GPR) is a nondestructive sensor technology for detecting underground objects. GPR requires large-aperture antennas to survey a remote location precisely because of the expansion of microwaves. We propose a laser-driven GPR (LGPR) that uses microwave radiation from a laser plasma to achieve remote sensing.LGPR is expected to provide good spatial resolution with a small antenna. We selected a subnanosecond laser pulse as a suitable radiator for LGPR (L-S band). Experimental results show that the LGPR system can detect aluminum disks buried in sand. Ground penetrating radar (GPR) is a well-known nondestructive sensor technology that uses microwave echoes to find objects underground and examine building structures [1,2]. Recently, demand has increased for remote sensing technology for landmine detection. Conventional GPR is effective only at a close range, or near the detection area. This is because an increase in the distance between the GPR and the remote survey area increases the area irradiated by the microwave, which leads to a deterioration in the angular resolution of the GPR. The microwave beam width must be as narrow as possible to prevent such deterioration. A large-aperture antenna is typically necessary for forming the narrow beam -the beam width θ 1/2 [deg] can be approximated by 70 × λ/D, where λ is the wavelength of radiated microwaves [m] and D is the apeture length [m] [3,4]. We propose a laser-driven GPR (LGPR) using microwaves radiated from a laser plasma [5].LGPR does not require a large-aperture antenna to sense a remote location because it generates a laser plasma that acts as a microwave radiator adjacent to the survey area, which is equivalent to a transmission antenna close to the survey area.Electromagnetic waves at various frequency ranges (from MHz to THz), radiate from laser-produced plasmas [6][7][8][9][10][11][12][13]. The radiation is caused by the following processes. An intense laser pulse creates a laser plasma. The generated electrons and ions are accelerated toward the outside of the plasma by thermal pressure or Ponderomotive forces. The light electrons receive greater acceleration than the heavy ions. Charge separations are induced in the plasma by different expansion speeds of electrons and ions, which excite the electric dipole moments. These flickering author's e-mail: nakajima-h@ile.osaka-u.ac.jp electric dipole moments radiate electromagnetic waves. It was reported that the radiation frequency spectrum corresponded to the laser-pulse envelope [8,13]. A subpicosecond laser pulse has an optimal duration for the terahertz range [8][9][10][11]. We chose a subnanosecond laser pulse as the optimal duration for L-S band (0.5-4 GHz), which is regarded as a suitable frequency range for the GPR [3]. In this article, the spectrum of the microwaves radiated from laser plasma created by a subnanosecond laser pulse was measured. Using a LGPR system with a subnanosecond laser pulse, aluminum disks (Φ 7, 15, and 26 cm) were detected under a layer of sand. Figu...

Low intensity laser pulse train propagation in air: part II: experimental studies

“…Laser-induced plasma (LIP) in air followed by the generation of rf radiation is of great interest for a number of applications. These include laser-induced breakdown spectroscopy 1 , communication 2 , remote non-destructive testing 3 and radioactivity detection 4 , image velocimetry 5 , biomedical diagnostics 6 , and security 7 . The efficacy of the particular LIP-based technology depends upon the intensity and temporal characteristics of the laser pulse train (LPT) used to induce plasma and the subsequent rf radiation.…”

“…Furthermore, some of these applications require THz frequencies 7 , the range of the rf that can be generated by using fs-range lasers. Other applications perform at sub-GHz to several GHz [2][3][4][5][6] frequencies and the latter require a high-repetition rate laser system. Development of such lasers is a challenging task.…”