Research On The Single-frequency Fiber Laser At 978 Nm And Its Frequency Doubling

Single-frequency fiber lasers has huge application values in the fields of coherent beam combing,high-precision spectroscopy,fiber sensing and laser Lidar,due to the advantages of narrow linewidth,low noise,high signal-to-noise ratio,single longitudinal mode operation and excellent wavelength stability.Especially for single-frequency ytterbium-doped(Yb)fiber lasers operating in the 980 nm band,high-quality single-frequency blue light can be easily obtained by nonlinear frequency conversion and it also can be used as a high-performance pump source for pumps rare-earth ions doped glass,fiber oscillator and fiber amplifier.Although single-frequency laser at 980 nm can be directly obtained from the distributed Bragg reflector(DBR)short-cavity laser oscillator,its single longitudinal mode operating temperature range is only a few degrees,when the external temperature changes drastically,it will occur mode-hopping or even multi-mode,and the intensity noise of the laser also need to be further optimized to meet the needs of some sophisticated applications.In addition,their outputs are generally limited to a relative low power levels due to the short gian fiber used in the single-frequency laser cavity.Therefore,Yb-doped fiber amplifiers have to be used to increase the power of the 980 nm single-frequency lasers to a much higher level.However,due to the three-level lasing property of the Yb-doped fiber around 980 nm,the power scalling of the fiber amplifiers is limited by the serious four-level amplified spontaneous emission(ASE)and the three-level reabsorption.Furthermore,the single-frequency blue laser with excellent properities can be obtained by frequency doubling a 980 mm fiber laser,but it need higher fundamental optical power,and it is difficult to achieve high conversion efficiency at the low fundamental optical power.In view of the above mentioned existing difficulties,this paper focuses on the research of 978 nm single-frequency fiber laser,fiber amplifier system and its frequency doubling.The specific research contents and the main results are as follows:(1)The relationship between the output power of the 978 nm single-frequency resonator with the length of gain fiber is simulated under different reflectance of the narrow-band fiber Bragg gratings.The relationship between conversion efficiency with the absorbed pump power is also simulated under the given gain fiber length.Then using self-developed highly doped ytterbium-doped phosphate fiber as the gain fiber,a single-frequency fiber laser with center wavelength of 977.73 nm,single longitudinal mode operating temperature range of 9.3 ? and the output power higher than 38.5 mW is designed.Finally,by further reducing the length of gain fiber and based on self-injection locking scheme and nonlinear amplification effect of semiconductor optical amplifier(SOA),a low intensity noise ultrashort cavity single-frequency fiber laser at 978 nm is demonstrated.Its stable single-frequency operating temperature range excedds 15 ?,output power higher than 200 mW,signal-to-noise ratio(SNR)is 68 dB,and the laser linewidth is less than 1.5 kHz.In the frequency range of 1.8 MHz to 10 MHz,the relative intensity noise(RIN)of the laser is about –150 dB/Hz,which is close to the quantum noise limit,and a maximum of 43 dB of noise suppression is achieved at the relaxation oscillation peak.(2)Besed on the theoretical simulation,the factors affecting the output power of the 978 nm fiber amplifier are simulated.The effects of the gain fiber length,the core and inner cladding diameter of the gain fiber and the pump power on the output power of the amplifier are simulated.Using the constructed low relative noise 978 nm single-frequency fiber laser as the seed laser,a 978 nm optical fiber amplifier with all-fiber structure is constructed by utlizing the non-linear amplification effect of SOA and the commercial large-mode-field diameter ytterbium doped silica fiber.And a 10 W all-fiber single-frequency laser output is successful obtained,the SNR of the laser is higher than 46 dB,and the RIN of the relaxation oscillation peak value is less than-108 dB/Hz.(3)The frequency doubling experiment of the SOA amplified 978 nm continuous single-frequency fiber laser is performed in a periodically poled lithium niobate(PPLN)crystal waveguide.An all-fiber single frequency blue laser is demonstrated with a fiber-coupled PPLN waveguide,over 68 mW of stable narrow-linewidth blue laer is obtained with a conversion efficiency higher than 30.3%.the power instability is ±1.5% and the linewidth less than 15 kHz,The measured relative intensity noise of the single-frequency blue laser is less than-110 dB / Hz at the relaxation oscillation peak.