To stabilize the frequency difference of the two-cavity dual-frequency Nd:YAG laser at 1064 nm, a scheme of the phase modulation of the orthogonally and linearly polarized dual-frequency laser using single electro-optic phase modulator has been proposed, and a frequency-difference stabilizing system for a two-cavity dual-frequency Nd:YAG laser based on the single-modulator quadrature-demodulated Pound–Drever–Hall (SM-QD-PDH) method has been designed and established. The characteristics of phase modulation, frequency discrimination, frequency stabilization, and frequency-difference stabilization of the orthogonally and linearly polarized dual-frequency laser with a frequency difference of ∼24 GHz at 1064 nm have been investigated experimentally. The experimental results have indicated that during a period of 1 h, the laser frequency stabilities of the linear and right-angle cavities are estimated by Allan variance to be better than 1.6 × 10 − 11 and 2.0 × 10 − 11, respectively, corresponding to a frequency-difference stability of better than 2.9 × 10 − 7. The designed SM-QD-PDH frequency-difference stabilizing system is not only simple but also has better performances than that of the double-modulator QD-PDH frequency-difference stabilizing system. Such a frequency-difference-stabilized two-cavity dual-frequency Nd:YAG laser can be used as an ideal light source for the synthetic-wave absolute-distance interferometric system.
In order to obtain the dual-frequency laser output with tunable frequency-difference, a design scheme of dual-frequency Nd:YAG laser with two standing-wave cavities sharing the common gain medium has been proposed, which is based on the principle of longitudinal mode selection of the Fabry-Perot etalon. Both linear and right-angle cavities including a piece of Fabry-Perot etalon, the p- and s-polarized components of the 1064nm laser will be forced to oscillate simultaneously in single longitudinal mode in the linear and right-angle cavities, respectively. As a result the orthogonally and linearly polarized dual-frequency laser at 1064nm can be output; the frequency-difference can be tuned by adjusting the tilt angles of the intra-cavity Fabry-Perot etalons, the maximum frequency-difference is predicted theoretically to approach the oscillating bandwidth of the Nd:YAG laser. The principles of single longitudinal mode selection and frequency tuning of the Fabry-Perot etalon have been analyzed, and two fused quartz-made Fabry-Perot etalons have been designed and fabricated. An experimental system of diode-pumped two-cavity dual-frequency Nd:YAG laser at 1064nm has been established, the characteristics of single longitudinal mode oscillation and frequency tuning have been investigated experimentally. The orthogonally and linearly polarized dual-frequency laser output at 1064nm has been obtained, the frequency-difference of which being 21GHz and 76GHz, respectively. Such a two-cavity dual-frequency all-solid-state laser will be widely used in the synthetic-wave absolute-distance interferometry and other fields.
To obtain the dual-frequency laser output with large and tunable frequency difference, a design scheme of dual-frequency Nd:YAG laser with two standing-wave cavities sharing the common gain medium has been proposed, which is based on the principles of polarization splitting and single longitudinal mode selection of intracavity Fabry–Perot etalon. With each of the cavities containing a piece of Fabry–Perot etalon, the p- and s-polarized components of the laser at 1064 nm will be forced to oscillate simultaneously in single longitudinal mode in the linear and right-angle cavities, respectively. As a result, the orthogonally and linearly polarized dual-frequency laser at 1064 nm can be obtained. The principle of single longitudinal mode selection by use of the Fabry–Perot etalon has been analyzed, and the Fabry–Perot etalons have been designed. An experimental system of the two-cavity dual-frequency Nd:YAG laser at 1064 nm has been established, and the characteristics of single longitudinal mode oscillation of the two cavities have been investigated experimentally. The orthogonally and linearly polarized dual-frequency laser output at 1064 nm has been obtained; the main characteristics of the oscillating threshold and output power, the polarization state as well as the laser beam quality have been tested experimentally. The frequency difference of the dual-frequency laser has been tuned in turn to 16, 24, 37, and 76 GHz, by slightly adjusting the tilt angles of the intracavity Fabry–Perot etalons. Such a two-cavity dual-frequency Nd:YAG laser will be widely used in the synthetic-wave absolute-distance interferometry and other fields.
KEYWORDS: Nd:YAG lasers, Laser stabilization, Ferroelectric materials, Signal detection, Signal processing, Digital signal processing, Crystals, Optical engineering, Laser applications, Demodulation
To stabilize the frequency-difference of the two-cavity dual-frequency Nd:YAG laser at 1064 nm, a quadrature-demodulated Pound–Drever–Hall (QD-PDH) frequency-difference stabilizing system has been designed, which is composed of two sets of QD-PDH frequency stabilizing subsystems sharing the same Fabry–Perot cavity as the frequency reference. Both phase modulators are driven by the signals with the same frequency of 10 MHz generated by a single direct digital synthesizer (DDS), and the DDS also outputs the other two orthogonal signals as the demodulation reference signals of both frequency stabilizing subsystems. A QD-PDH frequency-difference stabilizing system for a two-cavity dual-frequency Nd:YAG laser with a frequency-difference of ∼24 GHz at 1064 nm has been established and investigated experimentally. The experimental results have indicated that during a period of 1 h, the laser frequency stabilities of the linear and right-angle cavities are estimated by Allan variance to be better than 2.3 × 10 − 11 and 2.7 × 10 − 11, respectively, corresponding to frequency-difference stability of better than 4.2 × 10 − 7. Such a frequency-difference-stabilized two-cavity dual-frequency Nd:YAG laser can be used as an ideal light source for the synthetic-wave absolute-distance interferometric system.
KEYWORDS: Demodulation, Laser systems engineering, Modulation, Laser stabilization, Phase shift keying, Signal processing, Signal detection, Digital signal processing, Optical amplifiers, Ferroelectric materials
Based on the principle of orthogonal demodulation, a Pound-Drever-Hall laser frequency locking scheme is developed. In the orthogonal demodulation Pound-Drever-Hall system, three sine signals are generated simultaneously using a direct digital synthesizer. A 0° phase sine signal is used to drive an electro-optic modulator to produce the phase sidebands, and 180° and 270° phase sine signals are used as reference signals for phase demodulation. The phase-modulated laser beam is coupled with a reference Fabry–Pérot cavity, and the reflected beam is sent into a photo-detector, whose output is mixed with two orthogonal reference signals to obtain two orthogonal components of the error signal. Using an analogto- digital converter, the two orthogonal components are processed using orthogonal phase sensitive detection to obtain the error signal on a host computer. The Pound-Drever-Hall laser frequency discrimination and tracking system is established and investigated experimentally using the orthogonal demodulation method. A frequency discrimination curve is obtained, and it is observed that the resonant frequency of the Fabry–Pérot cavity can automatically track laser frequency variation.
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