High-power 1560 nm single-frequency erbium fiber amplifier core-pumped at 1480 nm

High-power continuous-wave single-frequency Er-doped fiber amplifiers at 1560 nm by in-band and core pumping of a 1480 nm Raman fiber laser are investigated in detail. Both co- and counter-pumping configurations are studied experimentally. Up to 59.1 W output and 90% efficiency were obtained in the fundamental mode and linear polarization in the co-pumped case, while less power and efficiency were achieved in the counter-pumped setup for additional loss. The amplifier performs indistinguishably in terms of laser linewidth and relative intensity noise in the frequency range up to 10 MHz for both configurations. However, the spectral pedestal is raised in co-pumping, caused by cross-phase modulation between the pump and signal laser, which is observed and analyzed for the first time. Nevertheless, the spectral pedestal is 34.9 dB below the peak, which has a negligible effect for most applications

Single-frequency upconverted laser generation by phase summation

The phase summation effect in sum-frequency mixing process is utilized to avoid a nonlinearity obstacle in the power scaling of single-frequency visible or ultraviolet lasers. Two single-frequency fundamental lasers are spectrally broadened by phase modulation to suppress stimulated Brillouin scattering in fiber amplifier and achieve higher power. After sum-frequency mixing in a nonlinear optical crystal, the upconverted laser returns to single frequency due to phase summation, when the phase modulations on two fundamental lasers have a similar amplitude but opposite sign. The method was experimentally proved in a Raman fiber amplifier-based laser system, which generated a power-scalable sideband-free single-frequency 590 nm laser. The proposal manifests the importance of phase operation in wave-mixing processes for precision laser technology

Advanced sodium guidestar lasers based on Raman conversion

Wavefront distortions induced by atmospheric turbulence obstruct the full-resolution imaging of large ground-based optical telescopes. Adaptive optics (AO) allows compensation of the aberrations in real time by obtaining an error signal from a reference point source near the field of view. Artificial sodium laser guide stars (LGSs), generated by fluorescence of sodium atoms in the mesosphere with irradiation at the Na D line wavelength of 589 nm, provide an important method for creating bright references at specified points in the sky. AO with sodium laser beacons are of intense interest for applications in astronomical observation, optical free-space communications, space debris tracking, sodium layer lidar and mesospheric magnetometry.  With the prerequisite combination of wavelength, linewidth, diffraction-limited beam quality and average power, development of sodium lasers is notoriously challenging. Due to the lack of efficient soild-state gain materials that directly generate the required wavelength, Raman frequency conversion and second harmonic generation comprise the most practical techniques for generating sodium lasers. Although continuous-wave LGSs are routinely used, output in pulsed formats are potentially more favourable for improving signal-to-noise ratio and enhancing optical pumping efficiency. In order to satisfy the need for pulsed LGS and for increasing average output power, this thesis presents two advanced sodium laser systems - a pulsed fiber Raman laser at Larmor frequency to boost fluorescence efficiency and a new approach based on diamond Raman lasers.  In the case of the fiber Raman laser, a design is investigated that produces pulsed output at a repetition frequency equal to the Larmor frequency in the sodium layer (several hundred kHz), a rate that increases the brightness of the LGS and also enables applications in remote magnetometry of the mesosphere. By amplification of a single-frequency 1178 nm laser in a pulse-pumped Raman fiber amplifier and frequency doubling in an external cavity, a high power pulsed 589 nm laser at the Larmor frequency is demonstrated for the first time. The pulsed laser, which produced 17 W average power at a duty cycle of 20% and a repetition rate of 350 kHz.  To verify the principle of sodium LGS in the application of sodium magnetometry, two intensity-modulated 589 nm lasers pulsed at Larmor frequency are designed and used for testing. A magnetic field sensitivity of 150 pT ∕ √Hz is achieved using a sodium vapor cell test. Then, using gated photon counting and direct frequency sweep methods, a ground‐based telescope at Lijiang observatory is used to validate the technique and measure the geomagnetic field with a sensitivity of 849 nT ∕ √Hz.   The combination of diamond’s ability to rapidly dissipate heat and its increased immunity from detrimental optical nonlinearities provides a pathway towards higher power CW and pulsed lasers. Since Raman gain provides a homogeneous gain profile and avoids spatial hole burning, the lasers favour single longitudinal mode operation in simple standing-wave resonators. It is shown here that intracavity second harmonic generation, as well as providing an efficient route to frequency-doubled output, increases gain competition and mode stability. A single-frequency microsecond-pulsed 620 nm diamond laser in a standing-wave resonator with intracavity frequency doubling is demonstrated using a Nd:YAG pump laser at 1064 nm. A quasi-cw output power of 38 W was obtained at 620 nm with a spectral linewidth of less than 8 MHz. Building on this preliminary finding, the scheme was adapted to generate 589 nm laser output through the use of a fiber laser at 1018.4 nm as the pump. 22 W was obtained at 18.6% efficiency from the fiber laser pump diode, which is a record for any diode-pumped sodium laser. The laser operates in a single longitudinal mode with a measured linewidth of less than 8.5 MHz and well suited to LGS applications. Continuous tuning through the Na D line resonance was achieved by cavity length control, and broader tuning via the tuning of the pump wavelength. It is shown that the approach is well suited to much higher powers and for temporal formats of interest for advanced concepts such as time-gating and Larmor frequency enhancement. The concept is found to be a highly practical approach to single frequency lasers with advantages of power and wavelength versatility, that may also benefit other areas of coherent laser applications.  </p

High-Power Single-Frequency 1336 nm Raman Fiber Amplifier

National Natural Science Foundation of China [61505229, 61378026, 61575210]A high power, single frequency, quasi-continuous-wave 1336 nm laser is achieved by Raman amplification of an external cavity diode laser in a variably strained polarization maintaining silica fiber. The pump laser is a 1256 nm Ytterbium-Raman integrated fiber amplifier with a maximum output peak power of 235W. The 1336 nm amplifier produces square-shaped pulses with tunable repetition rate and duration. The peak power is as high as 53 W, which remains constant during the tuning. A polarization extinction ratio of >25 dB is achieved due to the all polarization maintaining fiber configuration. The laser is locked precisely at 1336.63 nm for future application in laser cooling of Al-27(+) after 8th harmonic generation

Static voltage sharing technology of multi-break mechanical switch for hybrid HVDC breaker

The hybrid high-voltage direct current (HVDC) breaker combines mechanical and power electronics switching that enables it to interrupt power flows within a few milliseconds. Mechanical switch is a key component of hybrid HVDC breaker and has a number of serially connected interrupter units which ideally would divide the voltage equally. The static voltage distribution characteristics and voltage sharing design of a multi-break mechanical switch were discussed in this study. A finite-element model was developed to study the static voltage distribution characteristics and capacitance parameters of multi-break mechanical switch (which actually consists of resistance and capacitance parameters under direct current) as a preliminary study. Comparisons were made under the simulation of vertical and U-shaped arrangement forms. The results indicate that the static voltage distribution of the high-voltage terminal is at least more than 65%, whereas the severe non-uniform voltage distribution can be well improved by means of the method proposed in this study