Interpolation linearization predistortion technology for FMCW LiDAR.

Frequency-modulated continuous-wave (FMCW) laser ranging technology is an important development direction of light detection and ranging (LiDAR) for the future. It has the advantages of high ranging accuracy, high resolution, wide range, and no ranging blind zone. A distributed feedback laser can be used as a high-quality light source in FMCW laser ranging systems because of its wide frequency modulation range, simple frequency modulation mode, and small package. Aiming at the nonlinear problem of the laser in the frequency modulation process, we present a novel, to our knowledge, predistortion algorithm based on interpolation linear fitting to enhance the linearity of the FMCW laser for LiDAR systems. The sweeping frequency curve of the laser is obtained using the Hilbert transform, and then the sweeping frequency curve is segmented and linearly fitted to calculate the interpolated driving current signals corresponding to linear frequency changes. Using this method, we achieved a nonlinearity error lower than 1e-7 for the swept-frequency signal and demonstrated that the ranging error is less than ±5c m at a distance of 100 m in the FMCW system. In addition, we also demonstrated a 3D static object point cloud with high imaging quality. Compared with the iterative predistortion algorithm based on the function fitting, this method avoids fitting errors at the inflection points of the triangular swept-frequency signal and the complexity of multiple iterative calculations. It enables rapid generation of pre-distorted swept-frequency signals, making it particularly suitable for real-time applications of automotive LiDAR systems.

Optimization of microemulsion cleaning sludge conditions using response surface method.

Microemulsion cleaning method has been proved to be an effective way to clean oily sludge with low interfacial tension and high solubilizing ability for non-miscible liquids. In this paper, the percentage range of the microemulsion in the formulation was obtained by studying phase behavior of the microemulsion. The response surface method was used to model and optimize the microemulsion to obtain the best formulation: n-BuOH content at 9.89%, NaCl content at 2.24% and AES/APG ratio at 3.75, and the oil removal rate reached 97.28%. Meanwhile, the cleaning conditions of oil sludge were also optimized by the response surface method and the optimal cleaning parameters were determined as liquid-solid ratio at 4.2, stirring rate at 157 r·min-1, and stirring time at 38 min. In addition, some experiments were carried out to confirm the simulation results, affording the oil removal rate of 98.79%. SEM and FTIR confirmed that the oil on the sludge can be removed by microemulsion.

Linearized segmentation method for measuring the phase transfer function of a silicon Mach-Zehnder modulator by using the beat-frequency technique.

A linearity segmentation method for measuring the phase shift curve of silicon Mach-Zehnder modulators (SMZMs) as a function of applied electric field is presented. Applying a small sinusoidal signal to the traveling-wave electrode of the SMZM, the upper and lower arms of the SMZM produce differential phase modulation effect. Meanwhile, a local oscillator source with a wavelength-adjustable function is employed to heterodyne the intensity modulated optical signal of the SMZM; thus, the modulated intensity signal in the optical field domain is transformed into the low-frequency electric field domain. Meanwhile, a balanced detector with a low-speed transimpedance amplifier is exploited to realize photoelectric conversion, which can suppress the direct-current component and improve the anti-noise ability. Extracting the beat-frequency and first-order harmonic sideband signals in the case of phase bias is 0 and π, respectively, the phase shift slopes of the upper and lower arms can be calculated under different reverse PN voltages, and we can achieve the phase shift as a function of the modulation frequency and reverse PN voltage. The proposed method is supported by the simulation and measurement results and the key parameters of the SMZM, such as the radio frequency half-wave voltage, chirp characteristic, 3 dB bandwidth, etc., can be acquired from the phase shift curves of the upper and lower arms.

Thermo-optic-based phase-shifter power dither for silicon IQ optical modulator bias-control technology.

Recently, silicon optical in-phase quadrature (IQ) modulators have played an increasingly important role in coherent optical transmission networks because of their small package size and low cost. To stabilize the modulation performance of the silicon IQ optical modulator (SIQOM), the bias voltages of the SIQOM must be maintained at optimum points. Because of the nonlinear modulation characteristic of the silicon material, it is difficult to achieve high-precision closed-loop control of the bias voltage for the SIQOM. In this paper, a novel automatic bias-control scheme for the SIQOM is proposed and investigated theoretically and experimentally. First, two sinusoidal power dithers with different low frequencies are applied to the channels I and Q biases of the SIQOM. Next, a pair of orthogonal trigonometric functions with the same frequency as the power dither signal is constructed. We find that the optimum point of the bias voltage is the intersection of the orthogonal-integral curves via cross-correlation integral operations between the output signal of the SIQOM and the aforementioned trigonometric functions. The results indicate that the bias errors of the channels I/Q/P relative to the optimum point can be corrected precisely by the proposed scheme, and the jitters of the vector amplitude error caused by this scheme are <1% in 128-Gb/s dual-polarization quadrature phase-shift keying and single-polarization 16-quadrature amplitude modulation formats.

Study on auto bias control of a silicon optical modulator in a four-level pulse amplitude modulation format.

Recently, the high-speed silicon optical modulator has played a greater and greater role in optic fiber communication. Due to the silicon material nonlinear electro-optic effect, the quadrature (Quad) bias point of the modulator is difficult to be locked precisely. In this paper, a novel automatic bias control (ABC) method for four-level pulse amplitude modulation format silicon modulator is presented. First, a sinusoidal dither signal is applied to the modulator DC bias, and the normalized output optical signal function, which is modulated by the dither signal, is obtained. Then, a pair of orthogonal reference functions is created, whose numerical size is equal to the absolute value of sinusoidal and cosine functions so that the frequency is the same as the dither signal. Through the cross-correlation integral operation between the normalized and reference functions above, we find that the zero point of the above integral operation is the best Quad bias point through numerical simulation and experiment. The phase accuracy error of ABC is lower than 2 deg, and it satisfies the business specifications of a high-speed silicon optical modulator.

Monolithic integrated cyclic 64-channel AWG with MZI filters and arrayed vertical reflecting mirrors for WDM-PON application.

The paper introduces a silica-on-silicon monolithic integrated cyclic arrayed waveguide grating (AWG) with Mach-Zehnder interference (MZI) filters and arrayed vertical reflecting mirrors in silicon to realize effective and stable optical transmission between waveguides and photodiodes. The cyclic AWG acts as both multiplexer over the L-band for upstream traffic and demultiplexer over the C-band for downstream traffic. The integrated chip, including AWG, MZI filters, and arrayed reflecting mirrors, has been made successfully with a 6.0 dB insertion loss, which is less than the discrete devices. At the same time, the arrayed reflecting mirrors are more stable than separate reflectors.