Paul D. Hillman

20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers

A 20-W all-solid-state continuous-wave single-frequency source tuned to the sodium D2a line at 589.159 nm has been developed for adaptive optical systems. This source is based on sum-frequency mixing two injection-locked Nd:YAG lasers in lithium triborate in a doubly resonant external cavity. Injection locking the Nd:YAG lasers not only ensures single-frequency operation but also allows the use of a single rf local oscillator for Pound-Drever-Hall locking both the injection-slave and the sum-frequency cavities. We observe power-conversion efficiencies in excess of 55% and a linearly polarized diffraction-limited output tunable across the sodium D2 line (589.156 to 589.160 nm) with no change in output power and with high amplitude and pointing stability.

Realization of a 50-watt facility-class sodium guidestar pump laser

A CW Na guidestar excitation source has been constructed and installed on the 3.5-m telescope at the Starfire Optical Range. This device is comprised of injection-locked Nd:YAG ring lasers operating at 1064 nm and 1319 nm and a doubly resonant cavity where sum-frequency generation of these wavelengths in LBO produces a diffraction-limited linearly-polarized 589-nm beam. Up to 50 W of 589-nm light for mesospheric guide-star generation has been produced. The injection-locked Nd:YAG lasers are capable of operating at up to 100 watts at 1064 nm and 60 watts at 1319 nm.

Comparison of pump-laser characteristics for producing a mesospheric sodium guidestar for adaptive optical systems on large-aperture telescopes

We compare the effectiveness of various laser systems for producing a sodium guidestar in the mesosphere for large aperture telescopes. We discuss the requirements for two applications at two sites: satellite imaging at the Starfire Optical Range in Albuquerque, New Mexico (SI) and infrared astronomy at the Steward Observatory in Tucson, Arizona (IR). SI may use either a hybrid system employing a rayleigh and a sodium guidestar or a system employing only a sodium guidestar. IR will use only a sodium guide star. Our results are based on analysis and computation that have been compared to 5 different experiments. Parameters included in our comparison are pulse format, polarization, center frequency, and bandwidth. The infrared astronomy application power requirements are low enough that state of the art laser technology can meet them. But the satellite imaging application is problematic. Required powers are sufficiently high that thermo-optic effects in materials can be a problem.

Photometry of a Sodium Laser Guide Star at the Starfire Optical Range

Measurements of a laser‐pumped sodium guide star produced over the Starfire Optical Range in 2002 November show that the brightness of the spot produced by 11.5 W of linearly polarized power on the sky was equivalent to a V = 8.0 mag star. However, taking into account that the transmission through a V filter is only 55% at the wavelength of sodium, its corrected magnitude, V1, was 7.4, or 800 photons s−1 cm−2 at the top of the telescope. In 2003 March, tests with linearly and then circularly polarized beams out of the telescope showed that a circularly polarized beam from 12 W of power out of the telescope produced a spot with V1 = 7.1 (1015 photons s−1 cm−2 at the top of the telescope), 0.7 mag brighter than a linearly polarized beam from 11.1 W of power out of the telescope. Over the 4 nights of experiments over two seasons, the apparent 2σ width of the spot varied between 36 and 46, or 1.6 and 2.0 m at 92 km altitude, and its length through the sodium layer was 4.6–8.5 km, but no variation of spot size with power on the sky was seen.

Progress toward a 50-watt facility-class sodium guide star pump laser

We report on the development of a 50-W, continuous-wave, sodium wavelength guidestar excitation source for installation on the azimuth gimbal structure of the 3.5-m telescope at the Starfire Optical Range. The laser is an all solid-state design employing two diode-pumped Nd:YAG sources operating at 1064 and 1319 nm that are combined to generate 589-nm radiation using a lithium triborate non-linear crystal. Key features of the system include single-frequency, injection-locked high-power oscillators, a doubly resonant sum frequency generator cavity, a short-term 10 kHz wide 589 nm spectrum, excellent beam quality and power stability, and turn-key operation using computer control and diagnostics. The laser beam is projected from the side of the 3.5-m telescope. A novel elevation beam dither approach is employed to determine range to the centroid of the guidestar formed in the column of mesospheric sodium and maintain focus of the wave front sensor.

Simple model, including recoil, for the brightness of sodium guide stars created from CW single frequency fasors and comparison to measurements

Using a stable single frequency (Δυ < 1 MHz) cw fasor we have characterized the guide star radiance under several conditions, including routinely measuring the radiance at various launch powers and simultaneously illuminating the same spot with a second fasor with a range of different frequency separations. Making use of sodiums hyperfine energy diagram and allowed transitions it is shown that some transitions do not contribute to the radiance after a short time period thus greatly reducing the number of states whose populations need to be tracked in a simple rate equation model. An offshoot of this view is the importance of the pump sources spectral content for efficient sodium scattering. Accounting for atomic recoil, which causes atoms to be Doppler shifted out of resonance, we obtain model curves for photon return flux versus launch power for both linear and circular polarization, both agree with measurements; the only free parameter being the sodium column density on the single night both sets of data were taken. We attempted to measure the sodium velocity distribution due to recoil using two Fasors in a pump-probe arrangement. We have measured some subtle phenomena that this simple model does not explain and these will be discussed. These may imply the importance of understanding the collision rates for sodium atoms to re-equilibrate through velocity changing collisions, spin relaxation and coherent beam propagation under various atmospheric conditions.

Simulations of mesospheric sodium guidestar radiance

This report will describe the progress towards modeling the radiance of a mesospheric atomic sodium guidestar pumped with a continuous-wave, narrow-linewidth source. We will model the cases of pumping only the D2a line and pumping both the D2a and D2b lines simultaneously. The simulation is named the sodium guidestar simulation or SGS.

50-W CW Single Frequency 589-nm FASOR

Doubly resonant sum-frequency generation in LBO has produced a CW, 50-W, single-frequency, diffraction-limited, linearly-polarized, 589-nm beam from injection-locked Nd:YAG lasers operating at up to 100 watts at 1064 nm and 60 watts at 1319 nm.

Pulsed laser architecture for enhancing backscatter from sodium

The brightness of a laser-generated guide star is determined not only by the power of the laser, but also by the spectral and temporal properties of the laser. We show that a guide star laser pulsed at the Larmor frequency of the sodium atoms enhances guide star brightness by up to 2X, compared to an optimized cw laser at the same average power. We describe a frequency-addition source of optical radiation that can provide such pulsed light, while providing any desired spectral shape.

Laser Remote Magnetometry Using Mesospheric Sodium

We have demonstrated a remote magnetometer based on sodium atoms in the Earths mesosphere, at a 106-kilometer distance from our instrument. A 1.33-watt laser illuminated the atoms, and the magnetic field was inferred from back-scattered light collected by a telescope with a 1.55-meter-diameter aperture. The measurement sensitivity was 162 nT/