Ladar Signal Research Articles

Imaging and detection method for low signal-to-noise ratio airborne synthetic aperture ladar signals

Imaging and detection method for low signal-to-noise ratio airborne synthetic aperture ladar signals

Vibration compensation method based on instantaneous ranging model for triangular FMCW ladar signals.

Triangular frequency-modulated continuous-wave (FMCW) laser radars (ladars) are extremely sensitive to vibration errors. An FMCW ladar 3D imaging system may suffer from severe vibrations and can use only one-period echoes for the ranging of each observation spot; consequently, it can provide only few measurement results. These vibrations may cause large errors because conventional vibration compensation methods are ineffective when applied to fast disturbances with limited measurement results. To solve this problem, we analyze the influence of vibrations on FMCW ladar ranging and propose a vibration compensation method based on an instantaneous ranging model for one-period triangular FMCW ladar signals. We first use a synchrosqueezing wavelet transform to extract time-frequency curves of the up- and down-dechirp signals and then build an instantaneous ranging model that can characterize local vibration errors. Based on the instantaneous ranges, we remove the disturbance vibration errors by taking the mean values of the instantaneous ranges and obtain the target range by using the triangular relations of the up and down observations. Experiments based on synthetic and real data verify the effectiveness of the proposed method and its superiority over the three-point method and Doppler shift method in compensating for vibrations with different frequencies and noise levels.

Nonlinear Phase Estimation and Compensation for FMCW Ladar Based on Synchrosqueezing Wavelet Transform

Frequency modulation continuous wave (FMCW) laser radar (Ladar) is a new radar system suitable for long-range detection and high-resolution imaging. However, the transmitted signal inevitably suffers from nonlinear frequency modulation errors which reduce the quality of Ladar imaging. We propose a nonlinear phase estimation and compensation method based on synchrosqueezing wavelet transform (SST). We first use SST to synchrosqueeze the dechirp signal containing a reference range in time-frequency domain and obtain the time-frequency information of the reference dechirp signal. As SST could aggregate the time-frequency distribution of noise, the proposed method is effective even in a noisy environment. A nonlinearity model in time-frequency domain is then built to estimate the nonlinear phase of the transmitted signals by using the time-frequency information of the reference dechirp signal. For the case of long-range detection, we adopt a residual video phase filtering to convert the nonlinear phase of the dechirp signal to range-independent phase errors. Finally, the nonlinearity of the dechirp signal is compensated using the estimated nonlinear phase of the transmitted signals. The experimental and real data tests show that the proposed method effectively improves the resolution of long-range Ladar imaging by compensating for the nonlinearity of the dechirp signal. Its advantage is the effectiveness for noisy and multicomponent FMCW Ladar signals.

Denoising FMCW Ladar Signals via EEMD With Singular Spectrum Constraint

Frequency-modulation continuous-wave (FMCW) Laser radar (Ladar) signals may be polluted by noise, which reduces the recognition precision of the targets. This letter proposes an ensemble empirical modal decomposition (EEMD) denoising method with singular spectrum constraint for FMCW Ladar signals. In our approach, we apply EEMD to adaptively decompose the noisy FMCW Ladar signals into several intrinsic mode functions (IMFs) and decompose these IMFs into several singular value components by using the singular spectrum analysis. The singular values are then used to calculate the energy probability of each IMF, which serves as an indicator to detect the IMFs containing useful signals. The energy probability represents the coherence of the IMFs such that we could sort these IMFs into noisy and useful components according to their coherence differences. To suppress the residual noise that is interfered with the selected IMFs, we use a low-rank approximation to reconstruct the useful signals. Finally, the reconstructed IMFs are stacked together to obtain the denoised signals. Tests on synthetic and real data demonstrate that the proposed method, compared to the EEMD denoising method, could suppress more noise but filter out less useful signals in the FMCW Ladar signal denoising.

激光雷达信号相位误差对合成孔径成像的影响和校正

激光雷达信号相位误差对合成孔径成像的影响和校正

Linear frequency-modulated continuous-wave ladar system for synthetic aperture imaging.

A novel and high-efficiency linear frequency-modulated continuous-wave (FMCW) ladar system for synthetic aperture imaging is proposed and experimentally demonstrated. This novel system generates wide-bandwidth linear FMCW ladar signals by employing an electro-optic LiNbO3- in-phase and quadrature modulator with an effective bias controller. The effectiveness of the proposed system is experimentally validated. Optical synthetic aperture images are obtained by using two 0.41 cm aperture diameter telescopes at the distance of 1 km. The resolution of these images can reach to 4 cm. A resolution improvement by about 10 times is achieved when compared with the conventional real aperture imaging system.

Comb-calibrated frequency-modulated continuous-wave ladar for absolute distance measurements

We demonstrate a comb-calibrated frequency-modulated continuous-wave laser detection and ranging (FMCW ladar) system for absolute distance measurements. The FMCW ladar uses a compact external cavity laser that is swept quasi-sinusoidally over 1 THz at a 1 kHz rate. The system simultaneously records the heterodyne FMCW ladar signal and the instantaneous laser frequency at sweep rates up to 3400 THz/s, as measured against a free-running frequency comb (femtosecond fiber laser). Demodulation of the ladar signal against the instantaneous laser frequency yields the range to the target with 1 ms update rates, bandwidth-limited 130 μm resolution and a ~100 nm accuracy that is directly linked to the counted repetition rate of the comb. The precision is <100 nm at the 1 ms update rate and reaches ~6 nm for a 100 ms average.

Experimental verification of sparse frequency linearly frequency modulated ladar signals modeling

We present the results of an experiment designed to verify the results of a previously published theoretical model that predicts the range resolution and peak-to-side lobe ratio of sparse frequency linearly frequency modulated (SF-LFM) ladar signals. We use two ultra stable diode lasers which are frequency locked and can be current tuned in order to adjust the difference frequency between the two lasers. The results of the experiment verify the previously developed model proving that SF-LFM ladar signals have the ability to increase the range resolution of a ladar system without the need for larger bandwidth modulators. Finally we simulate a target at a range of approximately 150 meters through the use of a fiber optic delay line, and demonstrate the ability of SF-LFM ladar signals to detect a target at range.

Sparse frequency LFM ladar signals

Through modeling we explored the possibility of utilizing a sparse frequency linear frequency modulation (LFM) signal for laser radar (ladar) applications. We propose a potential transmit and receive experiment utilizing the superposition of two LFM laser sources with a known difference frequency to provide the necessary segmented bandwidth. Finally we analyzed the signal performance of the proposed system showing that the range resolution of the signal can be improved by two to three times while utilizing the same modulator bandwidth as that of a continuous LFM signal.

Sensitivity improvement of a 1-um ladar system incorporating an opticalfiber preamplifier [also Erratum 33(12)4108(Dec1994)

In an effort to increase the SNR of a continuous wave, 1-μm all solid state ladar system, a rare-earth-doped optical fiber amplifier is investigated as a preamplifier for ladar return signals. The experimental system is detailed and a theoretical analysis of the fiber amplifier's effect on both heterodyne and direct detection schemes is provided. Beginning with the optical powers incident on the detector, the signal and noises are analyzed, through the detector electronics, to predict the SNR. The SNR is then plotted as a function of the return signal power, and a SNR threshold is defined to determine a minimum detectable signal power. The return signals required to attain the SNR threshold are then compared for four cases: direct detection with and without the fiber amplifier and heterodyne detection with and without the fiber amplifier. For the direct detection scheme considered, our results predict a sensitivity increase of 20.6 dB with the addition of the fiber amplifier, yet for heterodyne detection the predicted sensitivity increase is only 3.1 dB.