Research On Laser Diode Pumped All Solid-state Linearly And Broadband Frequency-modulated Nd:YVO₄ Laser

Laser signals with charateristics of linear and broadband frequency modulation are required for coherent detecting techniques,such as Synthetic Aperture Ladar(SAL)and frequency-modulated continuous-wave(FMCW)laser ranging.In these applications,a large frequency excursion with a rapid tuning rate is essential for large detection frequencies.Frequency tuning with low noise and high linearity is beneficial for high-accuracy measurement.For this reason,it is of important significance to study laser diode pumped solid-state lasers with characteristics of rapid frequency tuning rate,linear frequency scanning and large tuning range.One of the challenging problems for SAL and FMCW laser ranging has been a fast,linearly and broadband frequency-swept laser source.Tunable laser sources,including semiconductor lasers,fiber lasers and laser diode pumepd solid-state lasers are not sufficiently stable while stable sources are not broadly tunable.It is difficult to manipulate high-power or high-energy laser amplification for the semiconductor lasers and fiber lasers,which makes them unsuitable for long-distance detecting.The current solid-state lasers are not broadband and continuously tuned.In order to solve these problems,the concept of synchronous frequency tuning is introduced.Synchronous frequeancy tuning based on the mode-hop suppression and fine frequency tuning is theoretically and experimentally investigated.A linear frequency tuning with low noise is based on the stable single frequency operation.Single frequency operation with inserted etalons is proposed and investigated.Firstly,single frequency output with Nd:YVO4 is theoretically investigated.The mode-hop suppression ability of the ealon is analyzed.The theoretical model for synchronous frequency tunning by the coarse and fine frequency tuning is proposed.The frequency excursion as a function of the tilting angle of the etalon and the displacement of piezoelectric-transducer(PZT)is derived.By angle tuning of the inserted etalon in a standing-wave Nd:YVO4 laser resonator,stable single frequency output is readily obtained.Linear frequency tuning covering half of the longitudinal mode spacing of the laser resonator is manipulated by changing the cavity length linearly.Mode-hop suppression is accomplished by angle tuning of the thin coated etalon.In order to achieve broadband frequency tuning covering more than one longitudinal mode spacing of the resonator,synchronous adjustment of the thin coated etalon and the PZT is conducted.A broadband and linear frequency tuning of 18 GHz at 1064 nm without mode hops is achieved.This indicates the validity of our theoretical calculations.The tuning speed and the tuning accuracy is limited by the electro-mechanical controlled PZT and etalons.To carry out fast frequency tuning,linear electro-optic effect of the Rb Ti OPO4(RTP)is theoretically and experimentally investigated.This has never been reported at home and abroad.Theoretical results indicate that the RTP etalon can be effectively used for mode-hop suppression and the RTP crystal is able to manipulate fine frequency tuning.The theoretical model for synchronous frequency tuning by a quartz etalon and an RTP crystal is proposed,frequency excursion as a function of the tiliting angle of the etalon and the applied voltage to the RTP crystal is derived.The theoretical model for synchronous adjustment of the RTP etalon and the PZT is also proposed.Synchronous adjustment of an RTP crystal for fine frequency tuning and a thin coated etalon for mode-hop suppression is implemented.A frequency scanning of 6.3 GHz is conducted.Synchronous adjustment of the PZT for fine frequency tuning and an RTP etalon for mode-hop suppression is also implemented.A fast and linear frequency tuning of 14 GHz is realized.The obtained continuous tuning range is at least 6 times the longitudinal mode spacing of the laser resonator.To conduct fast,linear and broadband frequency modulation by the electro-optic effect,an RTP etalon and an RTP crystal are utilized in a standing-wave Nd:YVO4 resonator.Theoretical model of all-electro-optic synchronous frequency tuning is established.A fast frequency tuning of 6 GHz is realized by synchronously changing the voltages applied to the RTP etalon and the RTP crystal.A repetition tuning rate of 200 Hz in a triangular waveform is obtained with an average tuning speed of 2.4 THz/s.Linearity of the frequency modulation is measured by a home-made all-fiber Mach-Zehnder interferometric system.The linewidth of the single frequency laser is measured to be about 190 k Hz by a home-made delayed self-heterodyne interferometer.The laser power keeps at constant during the whole tuning process and the laser beam quality is good.