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In this study, heterodyne detection is adopted to measure the velocity of a target simulated by a rapidly rotating plate by using a mode-locked pulse laser as the resource. The coherent beat frequency of the signal light reflected by target and local oscillation light occurred on the surface of the detector. Then the waveform of beat frequency was processed by filtering to obtain the Doppler frequency shift of the signal light induced by target. With this frequency shift, the velocity of target could be obtained by calculation. Results indicate that the measurement has a high precision. The error on average is within 0.4 m/s.
©2012 Optical Society of America
1. Introduction
For mode-locked lasers, plenty of reports could be found discussing using it in tomography [1,2]. Recent studies on mode-locked lasers have focused on the research of producing wide-band ultra-short pulse by the coherence of mode-locked lasers [3–5]. Single mode laser is commonly used in heterodyne Doppler laser radar [6–8]. However, using mode-locked lasers in heterodyne Doppler laser radar has been rarely reported because its multi-mode output. Recently, Piracha et al. utilize mode-locked laser in frequency modulated continuous wave (FMCW) lidars [9–11]. They presented an oppositely chirped pulse lidar with a pulse repetition frequency of 20 MHz using a mode-locked laser. Doppler velocity measurements are performed using a target moving at a velocity of ~92 m/s (331 km/h) [12]. The narrow optical linewidth of the mode-locked laser results in optical pulses with coherence lengths of tens of kilometers that enable long distance operation with coherent detection at the receiver [12].
The mode intervals of the mode-locked laser spectrum are equal. When the heterodyne detection is conducted, signal light and local oscillation light spectrum can be seen as two staggered ‘combs’ after the coherent frequency mixing. Filtration of frequency mixed laser could be used to obtain the distance of the ‘combs’, i.e., the frequency shift of the target. Due to the locked phase of the frequency comb, the beat frequency results can accurately show the Doppler frequency shift of the moving target, with which the target’s velocity along the laser direction could be calculated. Compared with the single mode laser used in traditional heterodyne detection, mode-locked laser has the advantage that the phase of the beat wave has a stronger immunity because disturbance on each mode among the multiple output modes is equal. Meanwhile, mode-locked laser has narrower pulse width, which reduces the noise during the detection process effectively.
In this paper, a rotating plate is used as the moving target and its Doppler frequency shift is obtained through coherent beat frequency and then the velocity is calculated. In our prior work, theoretical calculation for beat frequency of mode-locked lasers has been performed [13]. Suppose that there are m modes in the output laser. The laser is split into two beams, one as local oscillation light and the other as signal light, which has a frequency shift of Δω. The light intensity after beat is
where ω and β are respectively the fixed difference of the angular frequency and phase when the phase is locked and Δω is the frequency shift. From Eq. (1) it can be seen that the laser intensity varies with the frequency shift Δω when the longitudinal mode separation f (ω= 2πf) is fixed and the phase difference β is locked. If the high frequency signal is filtered out, the frequency shift Δω could be derived from the wave period.2. Experiment setup
Figure 1 is the diagram of experimental setup. In order to increase output peak power, Q-switch device is placed in the mode-locked laser whose operating wave length is 1064 nm. The single pulse width of the laser is 170 ps, and the longitudinal mode separation is 100 MHz. Light from mode-locked laser is split into two beams at beam splitter I. Transmission light as the signal light is reflected by a high reflectivity mirror (HR) and goes through a horizontal polarized polarizer. A lens (focal length f = 5 cm) behind the polarizer focused the light on the target. Due to the depolarization effect of the backward scattering light, quarter wave plate is not inserted between polarizer and lens. Part of the backward scattering light is reflected at polarizer and will be vertically polarized. Reflection light at beam splitter I is local oscillation light. Two HRs and a right-angle prism are used to prolong optical path of local oscillation light, so that both signal light and local oscillation light can arrive at the detector at same time. The right-angle prism is mobile to adjust the optical path difference between the signal and local oscillation lights to be shorter than the coherent length. Compared with signal light, local oscillation light has higher intensity. An optical attenuator is added to attenuate local light’s intensity to the level similar to signal light in order to achieve higher modulation depth. Polarizer in local oscillation light optical path is vertically polarized, to ensure that two lights have same polarization before beat. The two lights which merge after going through beam splitter II arrives at the surface of detector to produce beat-frequency. Finally the lights are received by the detector and then sampled and processed. To improve the SNR an aperture is placed in front of the detector to ensure the diameters of the light spots of the two beams on the light mixture surface to be the same, since the mismatching part only increases the noise but makes no contribution to the intermediate frequency.
The target is a rapidly rotating aluminum plate driven by a DC motor. The speed of plate is controlled by the power supply voltage of the motor. The Doppler frequency shift fD induced by the target could be denoted as
where ν is tangential velocity of the rotating plate, and θ is the angle between incident light and velocity. In the experiment, θ = 40°. Tangential velocities under different voltages are measured using a tachometer, and the results can be found in Table 1 .
Table 1. Power Supply Voltage of the Motor and Corresponding Tangential Velocity
3. Experimental results
Figure 2(c) is the beat light waveform under the condition of 21 V DC motor power supply voltage. Figure 2(a) and Fig. 2(b) show local oscillation light and signal light respectively. In order to achieve higher modulation depth, optical attenuator is used so that intensity of local oscillation and signal light are nearly equal. The beat frequency waveform is shown in Fig. 2(c). Intensity of the beat light increased obviously because the two lights merged, and the envelope of the beat waveform has undulations because of beat frequency.
Figure 3 shows the frequency spectrum of the beat waveform in Fig. 2(c) using Fast Fourier Transform. From the figure it can be seen that the interval between longitudinal modes of mode-lock laser is 100 MHz. However, new components could be found at about 30 MHz on the left and right of original longitudinal modes. These new frequency components result from Doppler effect and the interval between original and new longitudinal modes is Doppler frequency shift fD.
In order to measure the period of the undulation, filtering is utilized in processing the beat frequency waveform in Fig. 4(a) , which is the same with Fig. 2(c) and is convenient to compare with Fig. 4(b) here. Figure 4(b) shows the waveform filtered by a 40 MHz low-pass filter. In the experiment, seven undulations in beat waveform are measured and averaged to increase precision. The duration for seven undulations is 2.344 × 10−7 s, and averaged period is 3.35×10−8 s. Then Doppler frequency shift fD calculated is 29.85 MHz. Using Eq. (2), the tangential velocity of the target is 20.73 m/s. Compared to actual speed, which is 20.58 m/s, the error is 0.15 m/s.
The heterodyne Doppler velocity measurements are conducted under different power supply voltages of the motor. The measurement is carried out ten times for every voltage. The results are shown in Fig. 5 , where black dots are actual velocity values while red ones are measured ones. Results indicate the errors between measured average velocities and actual velocities are below 0.4 m/s. In the experiment, mode-locked laser is pumped by flash lamp, and amplitude of output optical pulse is in the Q-switch envelope. The limited duration of the Q-switch envelope reduces the number of filtered pulse. If we utilize continuous mode-locked laser to produce a series of optical pulse with equal amplitude, more pulse can be obtained by filtering beat waveform. Then more accurate velocity can be obtained by calculating averaged period. However, continuous mode-locked laser can reduce peak power of output light, which is harmful for long distance detection.
4. Conclusions
In this paper, a mode-lock laser is used in heterodyne Doppler velocity measurement experiment. The target is simulated by a rapidly rotating plate. Doppler frequency shift is obtained by analyzing the beat waveform from the signal light reflected by target and local oscillation light. With this frequency shift, the velocity of target could be obtained by calculation. The error between the measured results and the actual velocities is within 0.4 m/s.
References and links
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