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.