Application of sinusoidal phase modulation technique for infrared spectrum measurement by fourier transform method

Nội dung text: Application of sinusoidal phase modulation technique for infrared spectrum measurement by fourier transform method

1. Journal of Science & Technology 119 (2017) 028-031 Application of Sinusoidal Phase Modulation Technique for Infrared Spectrum Measurement by Fourier Transform Method Doan Giang1,2, Nguyen Van Vinh1 , Nguyen Thi Phuong Mai1, Vu Thanh Tung1* 1 Hanoi University of Science and Technology – No. 1, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam 2Military Institute of Environmental Chemistry, Hanoi, Vietnam Received: April 04, 2017; accepted: June 9, 2017 Abstract The Fourier Transform Infrared (FTIR) spectrometer is widely utilized for the detection and identification of gas in laboratories and open environments. The measurement sensitivity and range of the spectrometry are limited due to the strength of the absorption or emission signals. This study proposes the use of the sinusoidal phase modulation technique to improve the signal to noise ratio (SNR) of the detected signal of an FTIR spectrometer. In this technique, a sinusoidal signal is applied to a voice coil to create movement of a mirror. Hence, the intensity of the interference signal is a series of harmonics. A synchronous detection (lock-in amplifier) is then utilized to detect and amplify only one suitable harmonic and removed all other harmonics and noise. Therefore, the SNR of the harmonic is improved significantly. In this paper, a weak infrared emission from a commercial heat-lamp is detected successfully using the proposed system. Keyword: FT-TR spectroscopy, Frequency modulation, Phase modulation, Michelson interferometer. 1. Introduction* choppers and the synchronous detection (lock-in amplifier technique). The main disadvantage of this Fourier Transform Infrared (FT-IR) method is that the beam of radiation is interrupted by spectrometers are powerful instruments for the chopper, the reduction in output is significant. measurements of the intensity of infrared radiation as Wavelength/frequency modulation technique has the a function of frequency or wavelength [1, 2]. The advantages over the amplitude modulation method instruments are based on the idea of the interference [7]. Both the reduction in output and the background of radiation between two beams to generate an noise are minimal [8]. However, this technique interferogram. The intensity of the interference signal requires a high-cost electro-optic modulator (EOM) is a function of the optical path difference (OPD) and it still has some residual amplitude modulation. change between two beams. When Fourier Transform algorithm on the signal is performed, the frequency In this paper, a simple phase modulation method (wavelength) respond can be determined. Different to improve the signal to noise ratio of an FTIR FT-IR spectrometers used different interferometers, spectrometer is proposed. In the proposed system, the such as Michelson interferometer [3], Fabry-Perot OPD between two arms of the Michelson interferometer [4], and grating interferometer [5]. interferometer is modulated by modulating the Among these kinds of the spectrometer, the FT-IR oscillating of the mirror. Hence, the intensity of the spectrometers using the Michelson interferometers interference signal is series of harmonics and each are preferred. These have some advantageous features harmonic is a function of the OPD. Using the lock-in over other techniques such as high precision and high amplifier technique [9], any harmonic of the energy throughout. In this paper, the characteristics of interference signal can be detected accurately without the FT-IR spectrometer based on the Michelson noise effect. The frequency/wavelength respond is interferometer is first investigated. then determined using Fourier Transform method. In the experiment, our proposed system is utilized to Actually, the influence of environmental detect an infrared radiation from a commercial heat- background limits the measurement precision of the lamp. FT-IR spectrometer. To remove the background effects, some modulation methods were employed. 2. Measurement principle The earliest spectrometer used a chopper to modulate 2.1 Phase modulation Michelson interferometer the intensity of the radiation sources [6]. The background noise can be eliminated using mechanical The schematic diagram of the FT-IR spectrometer based on the Michelson interferometer is shown in figure 1. The radiation from an IR source * Corresponding author: Tel.: (+84) 976.516.396 that is placed at the focal point of a parabolic mirror Email: tung.vuthanh@hust.edu.vn (PM1) propagates to a beam splitter (BS). The beam 28
2. Journal of Science & Technology 119 (2017) 028-031 splitter is made of a special material that transmits half of the radiation striking it and reflects the other half. One beam passes through the beam splitter to a fixed mirror and the second reflects off the beam splitter to a moving mirror. The moving mirror is driven by a voice coil actuator and it can move back Equation (3) shows that the interference signal of the and forth precisely around a balanced point. The modulated interferometer is a series of harmonics. fixed and movable mirrors reflect the radiation back Therefore, any harmonic from the signal can be to the beam splitter. Another parabolic mirror (PM2) detected using the lock-in amplifier technique [8, 9]. directs the combined beam into an IR detector. When the signal enters a lock-in amplifier, it is first Concurrently, a He-Ne laser propagates the same path multiplied by a reference value at a chosen frequency with the IR radiation. The displacement of the and then passes through a low-pass filter (LPF). moving mirror is determined accurately using the Therefore, the amplitude of any harmonic at a interference signal of He-Ne laser that is collected significant modulation frequency can be accurately using a photo-detector. The infrared spectrum is detected and all other higher order harmonics are obtained by first collecting an interferogram using the removed. Hence, we can detect a pure harmonic interferometer, and then performing a Fourier without noise. This signal is then amplified with a Transform on the interferogram to obtain the suitable factor using an amplifier that is integrated spectrum. into the lock-in amplifier. In this study, the first harmonic is utilized. Using the lock-in amplifier, the In this section, we propose a new method to intensity of the first harmonic is improve SNR of an FT-IR spectrometer using the ph- . (4) When a polychromatic radiation source enters the Michelson interferometer, but has a spectral distribution given by I( ) as shown in Eq. (2), and the light at different frequency is incoherent, then the total intensity can be found be adding intensities for different . (5) In the same way, as a monochromatic is used, using the lock-in amplifier technique, we can determine the Fig. 1. Schematic diagram of FT-IR spectrometer. total intensity of the first harmonic at different PM: Parabolic mirror; M: Mirror; BS: beam splitter; FG: function generator; LIA: lock-in amplifier; DAQ: frequency data acquisition; PC: personal computer. . (6) -ase modulation technique. When the moving mirror The right-hand side is nothing more than the is modulated at a modulation frequency so that sine form of the Fourier transform of I ( ) so we the delay time τ between two arms of the Michelson 1 have succeeded in writing an explicit form of the interferometer (τ =OPD/c; c is the speed of light) varies with time as the following equation [10] relation ) = F{ }. When the Fourier Transform algorithm is performed, both the (1) amplitude and frequency of all components of the where is the initial delay time caused by the radiation spectrum are determined. unbalanced length between two arms and is the 2.2 Determination of radiation frequency of based modulation excursion. on the zero path difference point For a monochromatic radiation of frequency, the The zero path difference point (ZPD) is located intensity of the interference signal is given as where the moving and fixed mirrors are the same distance from the beam splitter. Therefore, all (2) components of radiation with different frequencies where is the average intensity. Using the are in-phase at the ZPD. Their contributions are all at maximum and a very strong signal is produced by the Bessel function, Eq. (2) is given by IR-detector. When the OPD increases, different 29
3. Journal of Science & Technology 119 (2017) 028-031 frequencies produce interference peaks at different (a) (b) positions of the movable mirror. The laser He-Ne is utilized to measure the displacement of the moving mirror from the ZPD. The laser beam propagates the same path as the IR- radiation in the interferometer and produces its own interferogram at a photo-detector. This signal is used as an extremely accurate measure of the OPD. Therefore, when the moving mirror moves away from the ZPD, the position of any peak in the interferogram caused by different frequencies of the IR- radiation is determined. A noteworthy is that, at Fig. 4. He-Ne laser interference signal. (a): any peak position of the interferogram from the ZPD, interference fringes; (b): intensity of the interference the displacement of the moving mirror is the signal. wavelength of the radiation. The ZPD point can be detected by monitoring 3. Experiments the interference signal when all different frequencies of radiation were in phase and they made a very The experimental system is shown in figure 2. A strong signal as shown in figure 3. The ZPD point commercial heat-lamp was used as an IR source. The was the biggest spike in the center of the burst. The spectrum of the lamp was first measured using a interference signal of He-Ne laser is shown in figure commercial radiometer (12-550 Mark III radiometer, 4. This signal was used to determine the displacement Infrared Systems Development Corp.) and used as a of the movable mirror from the ZPD reference. A modulation frequency of 20 Hz was supplied for the voice coil actuator to modulate the The spectrum of the heat lamp measured using OPD, hence the delay time between two arms of the our proposed system is shown in figure 5(a). interferometer was modulated. A lock-in amplifier Concurrently, the spectrum of the lamp was measured (PS1 Sciencetech Inc.) was used to detect the first using the commercial radiometer (12-550 Mark III harmonic from the interference signal. The cutoff radiometer, Infrared Systems Development Corp.), frequency of the lock-in amplifier was 1Hz. The figure 5(b). The measurement range of the radiometer interference signal of radiation is collected using an covers the 1 to 16 um range. The experiment results IR-detector (MCT-14-10-LN, Sciencetech Inc.) that using our proposed system and using the commercial was cooled using Nitrogen liquid. radiometer show the same spectrum. The strongest radiation was figured out at the wavelength of 3,2 m. It means that the spectrum of the heat lamp was successfully determined using our proposed system. μ (a) Fig. 2. Experimental system. (b) Fig. 5. IR spectrum of the heat lamp, (a) the spectrum obtained using the radiometer, (b) the spectrum Fig. 3. Interferogram of the heat-lamp source obtained using our system (cutoff frequency of 1Hz). 30
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