| Passively Q-switched Raman microchip lasers(PQSRMLs)can not only generate laser wavelengths that are unavailable for the laser crystals,but also can form strong fundamental laser fields to simultaneously excite multiple Stokes laser oscillation.In this paper,the background and application prospects of PQSRMLs are introduced firstly.The methods of generating multi-wavelength lasers are introduced,and to meet the application requirements of terahertz wave generation and other applications,it is proposed to achieve multi-wavelength Raman lasers by simultaneously using multiple Raman shifts of YVO4 crystal in the passively Q-switched Raman laser cavity.Then the progress of PQSRMLs is introduced.The key factors to obtain highly efficient Raman laser oscillation are the intensity of intracavity fundamental laser and the Raman gain coefficient of the Raman gain medium.Taking the advanges of a relative long fluorescence lifetime and a narrow fluorescence line of Nd3+ doped crystals,an effCcient and stable high peak power fundamental laser can be achieved for Raman conversion in passively Q-switched Raman laser under direct pumping.The YVO4 Raman crystal has many Raman shift peaks including 890 cm-1,816 cm-1,and 259 cm-1,which can greatly expand the laser spectral range of the Nd3+ lasers.Synchronous pulsed,dual-wavelength Raman laser at 1164.4 nm and 1174.7 nm has been demonstrated in a Nd:GdVO4/Cr4+:YAG/YVO4 PQSRML under 880 nm LD direct pumping.A 880 nm fiber-coupled LD pumped Nd:GdVO4 crystal is used to generate 1063 nm fundamental laser,Cr4+:YAG is used as a passive Q-switch,and the fundamental laser is simultaneously converted to 1164.4 nm and 1174.7 nm by the 890 cm-1 and 816 cm-1 Raman frequency shifts of YVO4 crystal.The stable dual-wavelength Raman laser pulses with nearly equal spectral intensities have been achieved independent on the pump power.The pulse repetition rate(PRF)as high as 139.4 kHz has been achieved with initial transmission(T0)of 85%and the pulse width has been shortened to 825 ps with T0=70%.Miniaturized and high peak power dual-wavelength Raman laser at 1164.4 nm and 1174.7 nm provides a stable dual-wavelength laser source for terahertz wave generation and other applications.Compared to Nd:GdVO4 crystal,Nd:YAG crystal has a broader absorption spetrum around 0.88 μm and longer lifetime of the upper energy level,which is beneficial to enhance output pulse energy.In an 885 nm fiber-coupled LD pumped Nd:YAG/Cr4+:YAG/YVO4 PQSRML,the 1064.6 nm fundamental laser is converted to 3rd-order Stokes laser with 259 cm-1 Raman shift line and 2nd-order Stokes laser with 816 cm-1 and 890 cm-1 Raman shift lines.The experimentally obtained Raman laser wavelengths are 1095 nm,1127 nm,1161 nm,1166 nm,1176 nm,1213 nm,1288 nm,1301 nm,and 1314 nm.This work proves that the LD pumped Nd:Host/Cr4+:YAG/YVO4 PSQRML can greatly enrich the spectral range of Nd3+lasers.When the thickness of the Nd:YAG crystal(Lg)is 2 mm,T0= 80%for Cr4+:YAG and the length of the YVO4 crystal(Lr)is 2 mm,the pulse width of the Raman laser is shortened to 635 ps,the peak power is increased to 6.9 kW,and the PRF is 11.2 kHz.Using Nd:YAG/Cr4+:YAG composite crystal with Lg=3 mm and T0=80%,the PRF is increased to 18.7 kHz,the pulse width is 663 ps,and the peak power is 6.0 kW.When the thickness of the YVO4 crystal is increased to 3 mm,the obtained Raman laser has pulse width of 1.01 ns and peak power of 4.2 kW,and the PRZ is 19 kHz.By optimizing the lengths of the gain crystal and the Raman crystal,combining the thermal bonding techniques,the performance of Raman laser pulses of PSQRML is further improved. |
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