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.
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.
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