A T-shaped cavity dual-frequency Nd:YAG laser with electro-optical modulation is proposed, which consists of both p- and s-cavities sharing the same gain medium of Nd:YAG. Each cavity was not only able to select longitudinal mode but also tune frequency using an electro-optic birefringent filter polarization beam splitter + lithium niobate. The frequency difference of dual frequency was tuned through the whole gain bandwidth of Nd:YAG, which is far above the usually accepted free spectral range value in the case of a single-axis laser. As a result, the simultaneous operation of orthogonally and linearly polarized dual-frequency laser was obtained, which coincides with the theoretical analysis based on Jones matrices. The obtained frequency difference ranges from 0 to 132 GHz. This offers a simple and widely tunable source with potential for portable frequency reference applications in terahertz-wave generation and absolute-distance interferometry measurement areas.
Proc. SPIE. 9446, Ninth International Symposium on Precision Engineering Measurement and Instrumentation
KEYWORDS: Signal to noise ratio, Oscillators, Detection and tracking algorithms, Modulation, Interference (communication), Phase shift keying, Signal processing, Laser stabilization, Laser systems engineering, Phase shifts
The Pound-Drever-Hall (PDH) laser frequency stabilization is a wide spread adopted technique for narrow linewidth and ultra-stable lasers, and a phase shifter is an important part in a traditional PDH frequency stabilization system. A PDH laser frequency stabilization system without phase shifter was proposed, in which quadrature coherent detection method was used to extract the frequency drifts. Orthogonal reference signals are generated using direct digital frequency synthesizer (DDS) and mixed with the output of a photo-detector. Over-sampling technique and cumulative average algorithm were used to improve the detection resolution and SNR, orthogonal phase sensitive detection algorithm was adopted to obtain the frequency drifts. Both the quadrature demodulation system structure and the signal processing methods were adopted, the systematic detection error is reduced, the anti-noise performance is raised and long term frequency stability is improved with the PDH laser frequency stabilization system without phase shifter.
Two-cavity dual-frequency Nd:YAG laser with large frequency difference can be used as an ideal light source for synthetic-wave absolute-distance interferometric system. The operation principle of the two-cavity dual-frequency Nd:YAG laser with large frequency difference has been introduced, and the frequency locking principle of the Pound-Drever-Hall (PDH) method has been analyzed. A FPGA-based digital PDH frequency stabilizing system for the two-cavity dual-frequency Nd:YAG laser has been designed, in which the same frequency reference of a high finesse Fabry-Perot cavity is used and two separate heterodyne interference sub-systems are employed so that two electrical error signals can be obtained. Having been processed through FPGA, the output signals are applied to drive the PZT frequency actuators attached on the two-cavity dual-frequency Nd:YAG laser, as a result both operating frequencies of the two-cavity dual-frequency Nd:YAG laser can be simultaneously frequency-locked to two resonant frequencies of the Fabry-Perot cavity. A frequency stability of better than 10-10 will be obtained by use of the digital PDH frequency locking system, which can meet the needs of synthetic-wave absolute-distance interferometry.
When a new birefringent filter consisting of a polarizing beam splitter (PBS) and a half wave-plate (&lgr;/2), i.e.,
PBS-&lgr;/2 was included in a 1064nm Nd:YAG laser cavity, the laser was enforced to oscillate in single longitudinal
mode. The single longitudinal mode selecting ability of the intra-cavity filter of PBS-&lgr;/2 had been studied
experimentally by rotating the half wave-plate around the laser cavity axis, and the tuning characteristics of the
single-frequency laser output power versus the rotation angle of the half wave-plate had also been studied. An
orthogonally and linearly polarized dual-frequency Nd:YAG laser at 1064nm had been designed and demonstrated,
which included two standing-wave cavities sharing the same gain medium of Nd:YAG crystal and the birefringent filter
of PBS-&lgr;/2, the p-and s-components of the 1064nm laser light simultaneously oscillated in single longitudinal mode in
each cavity. The frequency-difference of the dual-frequency laser at 1064nm was measured to be approximately
1.87GHz, limited by the free spectral range of the scanning Fabry-Perot interferometer. It is predicted theoretically that
the frequency-difference of the dual-frequency laser at 1064nm can be tuned in a range from zero up to the lasing
bandwidth of the Nd:YAG laser.
A new scheme of a liquid crystal Fabry-Perot etalon (LCFPE) had been proposed and designed on the basis of the electronically controlled birefringence of liquid crystal, some main aspects in the LCFPE design had also been considered, and such a birefringent LCFPE element with a free spectral range (FSR) of 125-GHz had been fabricated. An oscillation of 1064-nm single longitudinal mode had been observed when an empty LCFPE (i.e., without liquid crystal material) was inserted in a diode-pumped Nd:YAG laser cavity with an optical length of approximately 43-mm. In addition, the transmitted resonant mode splitting of a birefringent Fabry-Perot etalon had been investigated theoretically, and the dependence of splitting magnitude on the path difference between ordinary light and extra-ordinary light inside the Fabry-Perot resonant cavity had been given. The theoretically analyzed results indicate that the transmitted resonant mode of the birefringent Fabry-Perot etalon can split over a whole FSR, and the splitting magnitude changes linearly with the optical path difference.
A novel displacement sensor based on diode-end-pumped solid-state laser technology has been investigated theoretically and experimentally. The investigation results indicate that provided the average radius of the pump beam in the gain medium is much smaller than the radius of the waist of the TEMoo laser beam, the exponential of the laser output power will change in a manner of a Gaussian function when the waist of the pump beam is displaced axially. Both the measurement range and the sensitivity of the displacement sensor depend on the pump power, the measurement range will be enlarged and the sensitivity be enhanced when the pump power is increased. For the experimental system of the diode-end-pumped 1064-nm Nd:YAG laser sensor, the measurement range and the sensitivity are 13.045-mm and 0.148-mW/μm, respectively, when the input optical power is 7.24-Watt (corresponding to a maximum output power of 1.926-Watt). Several main error sources that affect measurement accuracy of the displacement sensor have also been analyzed.