# A High-Resolution Ultrasonic Ranging System Using Laser Sensing and a Cross-Correlation Method
## 1 Introduction  
Ultrasonic pulse-echo detection dates from 1912, where it was used for iceberg detection. The basis of pulse-echo detection is echolocation. The distance between the transducer and the object depends on the TOF of the ultrasonic pulses [1]. The shape of the object can be reconstructed using a scanning imaging system or an array of transducers [2; 3]. Compared to other detection methods (radar or laser), ultrasound is more suitable in water [4; 5]. On one hand, the reflection coefficient of an interface depends on the mismatch in acoustic impedance of the materials on either side of the interface. The objects to be measured are usually placed in water to allow sufficient energy transfer from and into the transducer, which is called immersion ultrasound detection [6]. On the other hand, sound is a valid method for underwater detection, especially in the deep ocean because of the high attenuation of electromagnetic waves in water. Currently, ultrasonic pulse-echo detection has been widely used in thickness detection, ranging, single/multi beam echo sounding, and sonar [7; 8; 9; 10]. There are two main types of ultrasonic ranging methods. One is the pulse-echo type [11], and the other is the opposite type [12]. The pulse-echo type consists of only one transducer, which works intransmit-receive mode. The transducer emits a short ultrasonic pulse. It is reflected by the target. Then the echo is received by the same transducer. The opposite type usually consists of two transducers; one works in transmit mode and the other works in receive mode. The transmitting transducer emits a short ultrasonic pulse. Then, the pulse is received by the receiving transducer, which is attached to the target. One of the distance measurement methods is the phase shift method. It evaluates distance by computing the phase difference between transmitted and received continuous waves, overcoming traditional acoustic attenuation problems [13]. A multiple-frequency continuous wave is used to improve the ranging accuracy in phase shift method [14]. However, it requires a continuous wave and works in the opposite type only. For ranging or thickness detection, the pulse-echo type is more suitable. It is common to measure the time-of-flight (TOF) to determine the distance [15]. In this case, the TOF is the time that a transmitted pulse takes to reflect back to the receiver. We can calculate the distance in cases where the sound velocity is known. One of the conventional methods for determining the TOF is the threshold method [16]. This method is simple and fast. The TOF is determined when the received signal first exceeds a given threshold level. Curve-fitting is another method for TOF calculation [17]. It accurately estimates the shape factors of the echo envelope, as well as locates its onset. It is possible to assure reduced bias and uncertainty in critical TOF measurements. A more suitable TOF estimation technique is cross-correlation [18; 19; 20]. Here, the transmitted and received signals are cross-correlated. The time at which the correlation result reaches its maximum is an estimation of the TOF. In recent years, it has been used in absolute distance measurement in laser ranging [21; 22]. Methods with similar theoretical bases are also used in ultrasonic ranging and thickness detection [23; 24]. Cross-correlation methods are helpful for improving the accuracy and resolution to the phase level. However, the reference signal is difficult to record due to the drawbacks associated with the conventional piezoelectric transducer [25]. For example, there is much noise in the initial pulse when the transducer works in transmit-receive mode. It is not suitable to be the reference signal to calculate the TOF. Compared to piezoelectric transducer, a laser beam can be a good sensor to receive ultrasound with no interference from the sound field, which is based on the acousto-optic effect. Acousto-optics has been widely used to make tunable optical filters, modulators, and Q-switches [26; 27; 28; 29; 30].