# Visualizing Small Objects Using Amplitude-Modulated Laser Light at Microwave Frequencies
###### Abstract  
We reported on laboratory imaging experiments conducted using a system that utilizes amplitude-modulated infrared laser light at microwave frequencies. With modulations ranging from 11.1 to 15.1 GHz on a 1.55 \(\upmu\)m near-infrared laser, the phase measurements of the modulation signals revealed a target shape with sub-mm precision. We also tested whether our system is able to achieve cross-range resolution in the spot-light mode synthetic aperture scheme. With the aid of \"position-compensated signal processing\", synthetic aperture images were formed successfully. According to predications based on the principle of normal synthetic aperture radar, the spatial resolutions of the images were shown to be inversely proportional to the width of the collected data placed in the wavenumber space. remote sensing; LADAR; SAR; SAIL; imaging; AM laser; FM microwave +
Footnote †: journal: Applied Sciences  
shorter laser pulses, but their use is limited by the requirement for detectable energy in pulses. Using phase coherence between laser pulses enhances scattered signal detection. The modulation of laser light by chirped signals instead of short pulse modulations, which is also called LADAR, allows laser pulses that are many orders of magnitude larger [7]. In addition, the frequency chirp width is not limited by the radar frequency band allocation. This also helps to avoid the licensing requirements for radio wave emission. Some research using modulation laser for range measurement has been reported. [PERSON] et al. [8] used a projection of a diverged amplitude modulated (AM) laser by a 100 MHz signal and a digital video camera with a heterodyne detection system. They demonstrated the visualization of a small target located at around a 5 m range with a range resolution of less than 1 mm within a measuring time of 10 s. [PERSON] et al. [9] used the projection of a collimated comb pulsed laser using up to the 10 th harmonic frequency component to extend the frequency bandwidth up to 10 GHz. They performed detailed research on ranging resolution using a corner cube reflector and showed a range resolution performance under 10 mm when the target range was around 2 m. [PERSON] et al. [10] used the projection of diverged quadrature phase shift keying (QPSK) laser with 150 MHz signal and a digital camera. They showed a range resolution performance between 5 mm and 20 mm depending on the detected power under a measurement range of less than 1 m. [PERSON] et al. [11] used a collimated frequency modulated (FM) laser with a 1 THz modulation signal. They applied the system to small targets located under 10 m and demonstrated visualizing these targets with range resolution around few micrometers. These studies used amplitude, phase, pulse, and frequency modulation lasers to extend the bandwidth of the measurements to improve range resolution. These measurement instruments take two-dimensional or three-dimensional images like a snapshot camera when they collect data; that is, their measurement instruments take data by fixing a geometric relationship between the measurement instrument and targets. On that condition, cross-range resolution depends on the focal length of the focus lens used in the receiver and the range between the lens and targets, when an instrument employ diverged projection beam. When an instrument employs a collimated projection beam, cross-range resolution becomes the beam size on a target. In the far range measurement, cross-range resolution is limited by the real aperture of the receiver lens or collimation lens. In order to improve cross-range resolution even in the relatively far range, we aimed to develop the AM LADAR combined with the SAR, which is generally called synthetic aperture imaging LADAR (SAIL). In the SAIL measurement scheme, a measurement instrument collects back scattered light by moving its position, then performs synthetic aperture processing on the data to synthesize the large aperture. This synthetic aperture processing is explained in the following section.