{"id":11419,"date":"2022-11-29T05:44:33","date_gmt":"2022-11-29T05:44:33","guid":{"rendered":"https:\/\/www.szlaser.com\/?page_id=11419"},"modified":"2022-11-29T13:48:41","modified_gmt":"2022-11-29T13:48:41","slug":"optical-system","status":"publish","type":"page","link":"https:\/\/www.szlaser.com\/index.php\/wiki\/optical-system\/","title":{"rendered":"Optical system based on terahertz spectroscopy technology"},"content":{"rendered":"

Optical System Based On Terahertz Spectroscopy Technology<\/span><\/h1><\/h1><\/div>

terahertz; optical\u00a0system; fingerprint\u00a0spectrum; substance\u00a0identification<\/p>\n<\/div>

Abstract<\/h3><\/h3><\/div>

The terahertz pulse signal has the characteristics of “fingerprint spectrum” in the frequency domain, which can be used for qualitative analysis of substances. A secondary aspheric TPX plano-convex lens was designed to improve the ability of lens to focus on the terahertz beams, with the help of optical analysis and optimization functions of Zemax software. The terahertz beam shaping optical system was designed by using the plano-convex lens, and the optical system was used in a terahertz time-domain spectroscopy system. The terahertz spectroscopy tests were performed on moxifloxacin hydrochloride and levofloxacin, and the absorption coefficient and refractive index curve in frequency domain were obtained after algorithm processing. The test results show that the refractive index of levofloxacin is higher than that of moxifloxacin hydrochloride in the range of 0.1 THz~3.5 THz band, but the change of the refractive index of moxifloxacin hydrochloride is more gentle than that of levofloxacin. The moxifloxacin hydrochloride has obvious absorption peaks at 1.03 THz, 1.92 THz, 2.58 THz and 2.84 THz, and the levofloxacin has obvious absorption peaks at 1.35THz, 1.96THz, 2.52THz, 2.73THz.<\/p>\n<\/div>

Introduction<\/h3><\/h3><\/div>

Terahertz waves are a general term for electromagnetic waves between microwaves and infrared rays, their frequencies are between 0.1 THz and 10 THz, and their wavelengths are 0.03 mm\uff5e3 mm. The weak interaction forces (hydrogen bonds, Van Der Waals forces), framework vibrations, and dipole rotations within or between many macromolecules are just in the terahertz band, so different macromolecular materials will show specific characteristics in the terahertz band. Absorption and dispersion properties, the “fingerprint spectrum”, can be exploited for non-destructive identification of substances.<\/p>\n

In general, broadband terahertz photoconductive antennas are used for terahertz spectroscopy tests.<\/p>\n

Because it emits a terahertz beam with a certain divergence angle, it is necessary to design a corresponding optical lens to shape the terahertz beam, thereby increasing the terahertz energy density passing through the sample and increasing the accuracy of sample identification.<\/p>\n<\/div>

1 Design of terahertz beam shaping optical system<\/h3><\/h3><\/div>

At present, the most widely used commercial terahertz photoconductive antenna is the 1 560 nm fiber-coupled antenna module of MenloSystems. In this paper, TERA15-TX-FC is used as the broadband terahertz source, and TERA15-RX-FC is used as the terahertz signal detector.<\/p>\n

The terahertz beam divergence angle of TER A15-TX-FC is \u00b112.5\u00b0, and the electrode gap on the crystal is 100 \u03bcm; the terahertz beam acceptance angle of TERA15-RX-FC is \u00b112.5\u00b0, and the electrode gap on the crystal is 10 \u03bcm.<\/p>\n

Therefore, the terahertz source and detector can be regarded as a point source and a point detector, whose divergence angle and acceptance angle are both \u00b112.5\u00b0, and a transmissive optical system is used to shape the terahertz beam.<\/p>\n

The photoconductive terahertz source used in this paper is a broadband source, and the spectral width used in spectral detection is usually 0.1 THz ~ 3.5 THz, and most of the energy is concentrated between 0.1 THz ~ 1.0 THz.<\/p>\n

Within this wide spectral range, it is unavoidable that the focused spot size difference caused by dispersion and diffraction limit effects is large, as long as it is ensured that after passing through the sample, the broad-spectrum signal carrying the sample information can be received by the terahertz detector.<\/p>\n

Considering the sample size and the accuracy of spectral detection, the design index of the sample spot is set to be \u22641 mm.<\/p>\n<\/div>

1.1 Single lens design<\/h4><\/h4><\/div>

In this paper, four identical plano-convex lenses are used to form a terahertz beam shaping optical system, which can effectively reduce the device cost of the optical system. At the same time, in order to reduce aberrations, the focused light is located on the plane side of the optical system.<\/p>\n

Polymethylpentene (TPX), the lightest known polymer material, is transparent in the UV, visible, and terahertz bands, with a refractive index of about 1.46, which is relatively wavelength-independent.<\/p>\n

Therefore, TPX is generally used as a material for the production of terahertz lenses. TPX lenses can be molded by mold casting and have good surface quality stability.<\/p>\n

In order to increase the terahertz energy density focused on the sample, an aspheric coefficient is introduced into the plano-convex lens to further enhance the focusing ability of the lens. The quadratic aspheric equation is expressed as<\/p>\n

\"\"<\/p>\n

where:
\nc is the radius of curvature;
\nk is the conic surface constant.
\nWhen k=0, the above formula is a spherical equation with the center of the sphere coincident with the origin of the coordinates.<\/p>\n

Since the main function of the plano-convex lens in the terahertz optical path is to collimate and focus the terahertz beam, the single-lens design takes the beam parallel to the optical axis as the incident light, and adopts the single-objective optimization method. The coefficient k and the back intercept l are used as optimization variables, and the minimum spot size is used as the optimization target.<\/p>\n

Considering the terahertz source beam parameters, optical path layout, lens installation and other factors, the initial structure of the designed plano-convex lens is shown in Figure 1, and the initial focal length l is 50 mm.<\/p>\n

\"\"

Fig. 1 Structure parameters of plano-convex lens<\/p><\/div>\n

The TPX material was established in Zemax, and three typical bands of 3 000 \u03bcm (0.1 THz), 300 \u03bcm (1.0 THz), and 85.7 \u03bcm (3.5 THz) were selected. The focusing spot size of the single lens in these three bands is shown in Figure 2.<\/p>\n

\"\"

Fig. 2 Size of focused spot on single lens<\/p><\/div>\n

Fig. 2(a) is the spot size of 3 000 \u03bcm wavelength (0.1 THz) with a diameter of 0.18 mm; Fig. 2(b) is the spot size of 300 \u03bcm wavelength (1.0 THz) with a diameter of 0.08 mm; Fig. 2(c) is a spot size of 85.7 \u03bcm wavelength (3.5 THz) with a diameter of 0.10 mm.<\/p>\n

It can be seen from Figure 2 that the area around 0.1 THz ~ 1.0 THz is also the largest range of broadband terahertz energy, and the focus spot size is directly used as the optimization target. Through the combination of local optimization and Hammer Optimization, the best set of The lens parameters are shown in Table 1.<\/p>\n

Table 1 Optimization parameters of plano-convex lens<\/span><\/strong><\/p>\n\n

\n \n \n\n \n \n \n \n\n\n wdt_ID<\/th> Parameter<\/th> Before optimization<\/th> After optimization<\/th> <\/tr>\n<\/thead>\n \n\n \n \n\n \n \n \n \n \n
1<\/td>\n r\/mm<\/td>\n 17.55<\/td>\n 17.55<\/td>\n <\/tr>\n
2<\/td>\n d\/mm<\/td>\n 9.5<\/td>\n 9.5<\/td>\n <\/tr>\n
3<\/td>\n c\/mm<\/td>\n 28<\/td>\n 27.315<\/td>\n <\/tr>\n
4<\/td>\n k<\/td>\n 0<\/td>\n -0.574<\/td>\n <\/tr>\n
5<\/td>\n l\/mm<\/td>\n 50<\/td>\n 52.641<\/td>\n <\/tr>\n <\/tbody> \n\n \n \n \n\n <\/table>\n\n<\/div>