Tianpeng Zhao

Effect of external cavity length on self-mixing signals in a multilongitudinal-mode Fabry–Perot laser diode

The effect of external-cavity length on self-mixing signals in a multilongitudinal-mode Fabry-Perot laser diode (FP-LD) was investigated experimentally. It has been shown that the output waveforms of self-mixing signals vary periodically when the length of the external cavity changes. This result agrees well with our theoretical calculations for the self-mixing effect of two adjacent longitudinal modes in a FP-LD. Moreover, the time-averaged output intensities of self-mixing signals has also been measured and compared with theoretical analysis.

Optimized design of a LED-array-based TOF range imaging sensor for fast 3-D shape measurement

A LED-array-based range imaging sensor using Time-of-Flight (TOF) distance measurement was developed to capture the depth information of three-dimensional (3-D) object. By time-division electronic scanning of the LED heterodyne phase-shift TOF range finders in array, the range images were fast obtained without any mechanical moving parts. The design of LED-array-based range imaging sensor was adequately described and a range imaging theoretical model based on photoelectric signal processing was built, which showed there was mutual restriction relationship among the measurement time of a depth pixel, the bandwidth of receiver and the sensors signal-to-noise ratio (SNR). In order to improve the key parameters of sensor such as range resolution and measurement speed simultaneously, some optimized designs needed to be done for the proposed range imaging sensor, including choosing proper parameters for the filters in receiver, adopting special structure feedback automatic gain control (AGC) circuit with short response time, etc. The final experiment results showed the sensor after optimization could acquire the range images at a rate of 10 frames per second with a range resolution as high as ±2mm in the range of 50-1200mm. The essential advantages of the proposed range imaging sensor were construction with simple structure, high range resolution, short measurement time and low cost, which was sufficient for many robotic and industrial automation applications.

Improving SNR of Doppler signal and expanding dynamic measurement range of single-mode VCSEL self-mixing laser Doppler velocimetry

The experiments show that the single-mode vertical-cavity surface-emitting laser (VCSEL) Laser Doppler Velocimetry (LDV) is better than the multilongitudinal-mode laser diode LDV in many characteristics, such as the accuracy of velocity measurement and the temperature range of the laser. Because the output power of the single-mode VCSEL is very low and only 0.7mW, the backscattered light received by the photodiode in the laser house is so weak that the signal-to-noise ratio (SNR) of Doppler signal is low. And the Doppler signal spectrum width spread and amplitude modulation envelope badly influence the velocity measurement accuracy. Analog phase-locked loop (PLL) has many characteristics, such as narrow band tracking filter, locking the signal with peak voltage and high rejection ratio of amplitude modulation. Using the analog PLL, the SNR of Doppler signal and the velocity measurement accuracy can be improved obviously. But because the locking range of the common analog PLL is narrow, we use difference frequency analog PLL to expand the locking range, and the dynamic range of velocity measurement can be greatly expanded. As a result, the velocity measurement accuracy of the single-mode VCSEL self-mixing LDV is better than 1% when the velocity range is from 30mm/s to 480mm/s.

A LED-array-based range imaging system with Time-to-Digital Converter for 3D shape acquisition

A LED-array-based range imaging system is presented for capturing the depth information of three-dimensional (3D) objects. The range image is obtained by time-division electronic scanning of the LED Time-of-Flight (TOF) range finders in array, and no complex mechanical scanning is needed. Millimeter ranging resolution can be reached thanks to the use of Time-to-Digital Converter (TDC) with picosecond resolution and some special analog phased-lock loop (APLL) filter techniques. By close packaging LED die array, a high spatial resolution and large pixels version of the proposed system can be achieved easily. Experimental tests for evaluation of the system performance are adequately described. The results show the depth resolution can be better than 5mm in the range of 300–1000 mm, and a higher resolution 0.5 mm can be achieved when the measurement time of a depth pixel is extended to 5s, which is sufficient for many applications, including computer vision, machine automation, 3D object recognition, reverse engineering and virtual reality.

An LED-array-based range imaging system used for enhancing three-dimensional imaging

An LED-array-based range imaging system is proposed for three-dimensional (3-D) shape measurement. The range image is obtained by time-division electronic scanning of the LED Time-of-Flight (TOF) range finders in array, and no complex mechanical scanning is needed. By combining with a low cost CCD/CMOS sensor for capturing the twodimensional (2-D) image, the proposed range imaging system can be used to accomplish a high quality 3-D imaging. A sophisticated co-lens optical path is designed to assure the natural registration between the range image and 2-D image. Experimental tests for evaluation of the imaging system performance are described. It was found that the 3-D images can be acquired at a rate of 10 frames per second with a depth resolution better than 5mm in the range of 50 - 1000mm, which is sufficient for many practical applications, including the obstacle detection in robotics, machine automation, 3-D vision, virtual reality games and 3-D video.

Flow velocity measurements using self-mixing effect inside a single mode VCSEL with polymer optical fiber

We demonstrate a laser Doppler velocimeter (LDV) based on self-mixing effect in a single longitudinal mode VCSEL for velocity measurement of flowing milk using Polymer Optical Fiber (POF). We find that self-mixing signal in frequency domain was broadened using POF compare with space coupled. The flow velocity was measured when POF outside and inside the milk. Their self-mixing signals of power spectrum can be fitted with Gaussian and exponential decay function, respectively. The differences of self-mixing signal in frequency domain when POF outside and inside milk are discussed. Due to POF have the advantages of flexibility and compatible with biology, this velocimeter can also be used for the measurement of blood flow in vivo.

Study on a small modulation and high precision laser range finder using the self-mixing effect in a single-mode VCSEL

The paper proposes a small modulation and high precision laser range finder using the self-mixing effect in a single-mode vertical-cavity surface-emitting laser (VCSEL). According to the characteristic curve of single-mode VCSEL, the nonlinear distortion of Δλ/ΔI is severe and related with the modulation current ΔI, which worsens the system ranging accuracy obviously. In this paper, by applying small current to modulate single-mode VCSEL and specific circuits to process photoelectric signal, the system ranging accuracy is improved obviously. The experiment results show that, using small modulation current ΔI= 0.28 mAp-p, modulation frequency fm = 500 Hz to modulate single-mode VCSEL, and applying difference frequency analog phase-locked loop (APLL) to process the self-mixing beat frequency signals that exist phase abrupt changes, when the sampling time is 0.1 s, the measurement dynamic range is as large as 50 ~ 500 mm, and the ranging accuracy is better than 2 mm.

Theoretical and numerical analysis of polarization properties used as Doppler velocimetry in vertical-cavity surface-emitting lasers

This paper presents a theoretical and numerical analysis of polarization properties used as Doppler velocimetry in Vertical-Cavity Surface-Emitting lasers. A good match is found between numerical results and reported experiment results. We show that condition of generating square waves is the VCSELs must operated inside the bistable region and frequency difference between two linear polarization modes is equal to several certain values. We can select proper VCSELs which frequency difference between two linear polarization modes is multiple of ±3G to obtain asymmetric waveform of self-mixing signal.

The direction discrimination of single-mode VCSEL self-mixing laser Doppler velocimeters using automatic tracking triangular wave modulation

SM-LDV (Self-Mixing Type Laser Doppler Velocimeter) using single mode VCSEL (Vertical Cavity Surface Emitting Lasers) has many characteristic such as high accuracy of velocity measurement, low power dissipation and low cost. The saw-tooth waveform of Doppler signal can be used to discriminate the motion direction. Because of low light power of single mode VCSEL, the backscattered light is so weak that the saw-tooth waveform is not good enough to discriminate the motion direction. SM-LDV direction discrimination using single mode VCSEL modulated by triangular wave is not obviously dependent on the light power of Laser and its reliability is better. The dynamic range is very narrow when the VCSEL is modulated by constant frequency triangular wave. We present a method that VCSEL is modulated by triangular wave whose frequency is tracking the divided frequency of Doppler frequency shift (fD) and it can expand the dynamic range more than one order of magnitude. The results show that the dynamic range of direction discrimination can reach 5-500mm/s when the wavelength of single mode VCSEL is 850nm and the operating current is 8.6mA, the frequency of triangular wave is fD/16,the modulation current of triangular wave is 0.042mA.

New laser Doppler velocimetry using self-mixing effect in a vertical-cavity surface-emitting laser modulated by triangular current

A new, compact laser Doppler velocimetry is proposed, which is composed of a single-mode vertical-cavity surface-emitting laser modulated by a dynamical triangular current and a collimating lens. It can indicate the direction of velocity without ambiguity in the wide dynamic range of 5.2mm/s to 479.9mm/s when the sampling time is 0.1 second in the measurement. The accuracy of velocity measurement is better than 3.1% in the whole velocity range. Whats more, this LDV works very well on different diffusing surfaces, even on a black glossy photographic paper.