A new optical read-out system based on three groups of L-shape 3-axis interferometer is proposed to measure 6-degrees-of-freedom (6-DoF) of the test mass (TM) in the gravitational wave (GW) detection missions. In this system, the source laser is firstly divided into three parts to detect the displacement of the three perpendicular planes of the TM. To decouple the translations and rotations with respect to the XYZ axis, each part of the laser is further applied as the source of the L-shape 3-axis interferometer for the posture detection of each plane. The results of the numerical simulation showed that the solution accuracy of the translation and rotation are better than ±2×10-2 pm and ±3×10-5 nrad respectively, proving the computational accuracy is sufficient for the project requirements. The works above will provide a theoretical basis of the optical read-out system for the space-based GW detection mission.
The lack of compact and low-cost multi-comb sources has been a hindrance to the development of multi-comb metrology. The multidimensional multiplexing, a new multi-comb generation method, gives consideration to both comb quantity and spectral bandwidth and thus attracts extensive attentions. In this paper, a subring-wavelength multidimensional multiplexing scheme has been proposed. Four optical frequency combs with spectral bandwidths of at least 1.3 nm have been simultaneously generated in one integrated dual-ring mode-locked laser. A dual-ring structure of optical path separation is constructed for the subring multiplexing, while wavelength-dependent spectral filters are made up for the wavelength multiplexing. With necessary adjustment of both the spectral filters and the net-gain balance between subrings, cooperative operation of multiplexing between the two dimensions has implemented the subringwavelength multidimensional multiplexing. In this case, quadruple combs are detected with stable intensity. And the bandwidths of the generated combs are up to 2.8 nm, broader than the schemes with pure wavelength-dimensional multiplexing. Based on the previous research of polarization-wavelength multidimensional multiplexing, this work has proved the extendibility of multidimensional multiplexing schemes. We are convinced that multiplexing of triple or more dimensions could be further developed, promoting potential multi-comb applications.
The existing compensation methods for nonlinearity in heterodyne interferometers tend to be time-consuming, which limits their application in real time. To solve this problem, a real-time compensation method is proposed. First, dual-channel quadrature demodulation is used to transform the error signals in interference signals into the traditional three errors (direct current offsets, unequal amplitude errors, and phase nonorthogonal errors) in the demodulated signals. Second, the three errors are dynamically corrected by a simple algorithm based on the extremums extracted from the demodulated signals. To verify the effectiveness of the proposed method, experiments employing the proposed method were carried out to process simulated signals and to conduct actual displacement measurements. The results demonstrate that the nonlinearity in the heterodyne interferometer can be reduced down to a subnanometer level.
This study presents an enhanced homodyne laser vibrometer with adaptability to reflectivity. The reflectivity could be quite different when measuring different target, which caused the variation of the intensity of the interference signals. In order to enhance the measurement range for the reflectivity of the target, an auto-gain module, which could enlarge the interference signals to an optimal range of the analog-to-digital converter, is implemented in the signal-processing card. The intensity of the interference signals could be calculated in the auto-gain module, and then amplified according to a predetermined rule by using programmable gain amplifiers. The experimental results indicated that the laser vibrometer proposed is capable of measuring vibration with surface reflectivity down to 0.08%.
Dual-comb generation with bidirectional fiber ring laser is highly prospective for its compactness and coherency. In this paper, we propose a dual-ring hybrid mode-locked fiber laser for dual-comb generation. The dual-ring laser contains elements of hybrid mode-locking for each sub-ring individually, while sharing the bidirectionally pumped erbium-doped fiber (EDF). The hybrid mode-locking is realized by transmission semiconductor saturable absorber (SESA) and nonlinear polarization evolution (NPE). With the help of two three-port optical circulators, non-ideal reflection of two SESAs are eliminated. Accordingly, two series of short pules are generated in each sub-ring with different direction individually. Experimental observations and analyses demonstrate that dual comb of about 300 kHz difference in repetition rates are generated by inserting a 10 cm cavity length difference between the two sub-rings. With the help of hybrid mode-locking and large power bidirectional pumping, the mode-locking of dual combs are stable and self-starting.
In this paper, a design of a novel thermo-structure for measuring thermal drift of optics in a next generation interferometer is presented. The novel thermo-structure is used to change the temperature of interferometer under test (IUT) by radiation since the measurement is operated in vacuum to exclude the affect from air. Besides, the thermo-structure is made of multilayers, intergrated with thermo electric coolers as the source of heat, which can provide a uniform temperature field. In addition, the thermo-structure can also protect the IUT from disturbance of environmental radiation. Performance of the system is evaluated by finite element analysis and simulation results show that it can achieve a uniform temperature field which temperature difference is less than 0.02°C and reduce perturbation of environmental radiation from 2°C down to 0.03°C.
Nonlinear error measurement technology is critical for the development of heterodyne laser interferometer. The existing periodic nonlinear error measurement methods is limited in measuring the nonlinear error of the heterodyne interferometer with double-direction Doppler frequency shift during the measured target non-constant velocity motion. To solve the problem, this paper presents a new method of measuring the nonlinearity in picometer level based on the double-channels orthogonal demodulation. A pair of orthogonal signals generated by FPGA multiply with the two output signals of the interferometer, respectively. And two pairs of beat frequency signals are obtained through low pass filtering, which are then mutually multiplicative and obtain the sine and cosine components containing the overall nonlinear errors of the interferometer by mathematical operations. At length the nonlinear errors are obtained through them. The experiments show that the method can measure the periodic nonlinear error in real-time free from target motion state and the measurement accuracy is in picometer level.
In order to fulfill the requirements for high-resolution and high-precision heterodyne interferometric technologies and instruments, the laser interferometry group of HIT has developed some novel techniques for high-resolution and high-precision heterodyne interferometers, such as high accuracy laser frequency stabilization, dynamic sub-nanometer resolution phase interpolation and dynamic nonlinearity measurement. Based on a novel lock point correction method and an asymmetric thermal structure, the frequency stabilized laser achieves a long term stability of 1.2×10-8, and it can be steadily stabilized even in the air flowing up to 1 m/s. In order to achieve dynamic sub-nanometer resolution of laser heterodyne interferometers, a novel phase interpolation method based on digital delay line is proposed. Experimental results show that, the proposed 0.62 nm, phase interpolator built with a 64 multiple PLL and an 8-tap digital delay line achieves a static accuracy better than 0.31nm and a dynamic accuracy better than 0.62 nm over the velocity ranging from -2 m/s to 2 m/s. Meanwhile, an accuracy beam polarization measuring setup is proposed to check and ensure the light’s polarization state of the dual frequency laser head, and a dynamic optical nonlinearity measuring setup is built to measure the optical nonlinearity of the heterodyne system accurately and quickly. Analysis and experimental results show that, the beam polarization measuring setup can achieve an accuracy of 0.03° in ellipticity angles and an accuracy of 0.04° in the non-orthogonality angle respectively, and the optical nonlinearity measuring setup can achieve an accuracy of 0.13°.
In order to improve the measurement accuracy and the real-time performance in long absolute distance measurement, a method based on high stability and synchronization multi-frequency is proposed. Multi-frequency is generated by shifting one mode laser frequency with three acousto-optic frequency shifters, and couple in one beam by fiber. Heterodyne frequencies between different multi-frequency is separated by its different polarization and frequency bandwidth, and acted as the coarse and precision measuring frequency. Therefore synchronous occurrence of measuring multi-frequency is achieved, which improve the real-time performance of measurement. Results of the experiment show that the measuring frequency stability is better than 3.6×10-8, and the laser range finder have an uncertain measuring result of less than 60 μm at a distance of 20 m under laboratory conditions.
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