Objective A laser Doppler velocimeter (LDV) obtains the moving velocities of carriers by gauging the interference signal when the signal light is mixed with a reference. As a novel speed sensor, LDV possesses several advantages: non-contact measurement, no interference with the target, and high speed-measurement accuracy. However, when measuring the velocity of a solid surface, an LDV can scale the speed only within a limited range. When the moving surface is beyond the measurable range of the LDV, the intensity of the scattered light decreases, and the quality factor of the Doppler signal reduces. A Doppler signal is validated by the quality factor Q, which directly determines the working distance and measurable range of the LDV. When the quality factor is below the threshold, the carrier velocity cannot be determined from the Doppler signal. To meet the requirements of the measurable range, the quality factor is traditionally enhanced by two lenses with fixed focal length, which change the position of the waist spot of the outgoing Gaussian beam. However, this method increases the distance between the lenses and excessively expands the LDV volume. Meanwhile, the measurement scope remains limited and non-adaptable to actual situations. To change the measurable range of the LDV, one must either change the distance between the lenses or reform the lens combination. Mechanically transforming the lens distance will increase the volume and the system complexity, largely restricting the operating range of the speedometer. In addition, the lens combination cannot be changed at any time in practical engineering applications. No other reasonable method can expand the measuring range. Herein we present a beam transformation system based on a liquid lens. The waist-spot position of the Gaussian beam is controlled by changing the driving current, enhancing the quality factor above the threshold over a considerable range. Our design greatly improves the working distance and measurable range of the LDV. We hope that our basic strategy and findings will benefit the speed measurement and navigation ability of the carrier.
Methods This paper combines a theoretical analysis and simulation with experimental verification. In the theoretical analysis, we first evaluated the feasibility of transforming the LDV's Gaussian beam through a liquid lens. Based on Gaussian optics, the positions and size of the waist spot were simulated under different driving currents of an electrically tunable lens (ETL). We then constructed an LDV with the ETL and changed the position of its waist spot by changing the driving current without increasing the displacement mechanism. Throughout the experiment, we determined the relationship between the quality factor of a single point and the driving current, and the working distance and measuring range of the LDV for different offset lenses.
Results and Discussions The presented method improved the working distance and measurable range of the LDV. Owing to the sharp response time of the liquid lens (in order of milliseconds) (Table 1), the driving current can be controlled by a feedback signal, achieving real-time adjustment of the liquid lens (Fig. 5). In the new LDV structure, the maximum quality factor of a single measuring point reaches 3482, 22.9 times that of a traditional speedometer (Fig. 9). When the Foffset=-25.4 mm offset lens was selected, the working distance of the LDV was changed to the maximum extent, with a measuring range of 0.7--3.3 m. The system volume was reduced at the same time (Fig. 10, Table 2).
Conclusions This paper proposes a novel LDV scheme based on a liquid lens. Within this design, the waist spot position of the Gaussian beam can move and the working distance of the LDV can be changed simply by controlling the driving current, without increasing the displacement mechanism. Therefore, the quality factor of the Doppler signal is greatly improved. The quality factor of a single measuring point is maximized at 3482, 22.9 times that of a traditional speedometer. The new structure improves the measuring range of the LDV to 0.7--3.3 m, 4.3 times that of the traditional structure (1.2--1.8 m), while reducing the volume of the speed measurement system. These improvements will greatly expand the engineering applications of LDVs.
