Mayumi Kato

The laser guide star facility for Subaru Telescope

The purpose of this paper is to report on the current status of developing the new laser guide star (LGS) facility for the Subaru LGS adaptive optics (AO) system. Since two major R&D items, the 4W-class sum-frequency generating laser1 and the large-area-core photonic crystal fiber2, have been successfully cleared, we are almost ready to install the LGS facility to the Subaru Telescope. Also we report the result for LGS generation in Japan.

589 nm sum-frequency generation laser for the LGS/AO of Subaru Telescope

We developed a high power and high beam quality 589 nm coherent light source by sum-frequency generation in order to utilize it as a laser guide star at the Subaru telescope. The sum-frequency generation is a nonlinear frequency conversion in which two mode-locked Nd:YAG lasers oscillating at 1064 and 1319 nm mix in a nonlinear crystal to generate a wave at the sum frequency. We achieved the qualities required for the laser guide star. The power of laser is reached to 4.5 W mixing 15.65 W at 1064 nm and 4.99 W at 1319 nm when the wavelength is adjusted to 589.159 nm. The wavelength is controllable in accuracy of 0.1 pm from 589.060 and 589.170 nm. The stability of the power holds within 1.3% during seven hours operation. The transverse mode of the beam is the TEM00 and M2 of the beam is smaller than 1.2. We achieved these qualities by the following technical sources; (1) simple construction of the oscillator for high beam quality, (2) synchronization of mode-locked pulses at 1064 and 1319 nm by the control of phase difference between two radio frequencies fed to acousto-optic mode lockers, (3) precise tunability of wavelength and spectral band width, and (4) proper selection of nonlinear optical crystal. We report in this paper how we built up each technical source and how we combined those.

Transmission characteristics of high-power 589-nm laser beam in photonic crystal fiber

We are developing Laser Guide Star Adaptive Optics (LGSAO) system for Subaru Telescope at Hawaii, Mauna Kea. We achieved an all-solid-state 589.159 nm laser in sum-frequency generation. Output power at 589.159 nm reached 4W in quasi-continuous-wave operation. To relay the laser beam from laser location to laser launching telescope, we used an optical fiber because the optical fiber relay is more flexible and easier than mirror train. However, nonlinear scattering effect, especially stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), will happen when the inputted laser power increases, i.e., intensity at the fiber core exceed each threshold. In order to raise the threshold levels of each nonlinear scattering, we adopt photonic crystal fiber (PCF). Because the PCF can be made larger core than usual step index fiber (SIF), one can reduce the intensity in the core. We inputted the high power laser into the PCF whose mode field diameter (MFD) is 14 μm and the SIF whose MFD is 5 μm, and measured the transmission characteristics of them. In the case of the SIF, the SRS was happen when we inputted 2 W. On the other hand, the SRS and the SBS were not induced in the PCF even for an input power of 4 W. We also investigated polarization of the laser beam transmitting through the PCF. Because of the fact that the backscattering efficiency of exciting the sodium layer with a narrowband laser is dependent on the polarization state of the incident beam, we tried to control the polarization of the laser beam transmitted the PCF. We constructed the system which can control the polarization of input laser and measure the output polarization. The PCF showed to be able to assume as a double refraction optical device, and we found that the output polarization is controllable by injecting beam with appropriate polarization through the PCF. However, the Laser Guide Star made by the beam passed through the PCF had same brightness as the state of the polarization.

Automatic continuous scanning and random-access switching of mid-infrared waves generated by difference-frequency mixing.

We report the rapid tuning of mid-infrared waves beyond 5 microm emitted in difference-frequency mixing with an electronically tuned dual-wavelength Ti:Al2O3 laser used as a pumping source. Simultaneous rapid tuning of the dual wavelengths, which satisfy phase matching in AgGaS2, allows rapid random access switching and continuous tuning of mid-infrared wavelengths. In random-access switching, the mid-infrared wavelength is tuned every pulse shot at a repetition rate of 1 kHz. Mid-infrared wavelengths continuously tuned from 5.2 to 7.2 microm, from 7.0 to 9.1 microm, and from 8.9 to 12.0 microm are achieved at phase-matched angles of 55 degrees, 50 degrees, and 45 degrees, respectively.

High-stable and high-beam-quality sodium resonance radiation for subatu laser guide star adaptive optics

We report a high-stable and high-beam-quality 589.158 nm radiation in sum-frequency generation using LD-side-pumped Nd:YAG laser oscillators. Average ouput power reached 4.6 W and it was stable within plusmn2.2% for 8 hours.

Influence of Cerium Concentration on Transmittance and Surface Damage Threshold in Ce-doped KTP

Optical transmittance and surface damage threshold of cerium-doped (2, 9, and 33 ppm) KTP crystals were investigated with dependence on cerium concentration and polarization directions with respect to the fundamental crystal axes. Although transmittance from ultra-violet to mid-infrared region was gradually decreased with increase of cerium concentration, even transmittance at 532 nm in 33 ppm cerium-doped KTP, which showed the lowest transmittance, was 7 % higher than that of undoped KTP reported in Ref.[6]. On the other hand, surface damage threshold at 1064 nm was raised with increase of cerium concentration and the maximum damage threshold was measured to be 8 GW/cm2 using 33-ppm cerium-doped KTP, when b-axis polarized beam was input to surface of the crystal. The threshold was a factor of 2.4 larger compared with that of undoped KTP.

Photochromic damage in nonlinear crystals for high-peak power blue light generation

This paper investigates picosecond blue-light induced infrared absorption in bulk ferroelectric nonlinear crystals KTiOPO4 (KTP), RbTiOPO4 (RTP), congruent MgO:LiNbO3 (MgO:CLN), KNbO3 as well as recently developed Rb:KTP, Ce:KTP, stoichiometric LiTaO3 (SLT), MgO:SLT and stoichiometric MgO:LiNbO3, (MgO:SLN). These ferroelectrics are characterized by high nonlinearities and have been proven suitable for fabricating periodically poled structured for quasi-phase matched second harmonic generation in the blue spectral region. Because poling can introduce additional defects and modify susceptibility to the photochromic damage, periodically poled structures of selected ferroelectrics have been separately investigated. In addition, the BLIIRA was measured in BiB3O6 (BiBO) the most promising birefringence phase-matched crystal for pulsed blue light generation to emerge in recent years.

Development of all-solid-state coherent 589 nm light source: toward the realization of sodium lidar and laser guide star adaptive optics

We report an all-solid-state coherent 589 nm light source in single-pass sum-frequency generation (SFG) with actively mode-locked Nd:YAG lasers for the realization of sodium lidar and laser guide star adaptive optics. The Nd:YAG lasers are constructed as a LD-side-pumped configuration and are operated at 1064 and 1319 nm for 589 nm light generation in SFG. Output powers of 16.5 and 5.3 W at 1064 and 1319 nm are obtained with two pumping chambers. Each chamber consisted of three 80-W-LD arrays. Single transverse mode TEM00; M2 ~1.1 is achieved with adjustment of cavity length considering thermal lens effect with increase of input LD power. The cavity length is set to approximately 1 m. Accordingly the mode-locked lasers are operated at a repetition rate of approximately 150 MHz. Synchronization of two pulse trains at 1064 and 1319 nm is accomplished by control of phase difference between two radio frequencies input in acousto-optic mode-lockers. Then temporal delay is controlled with a resolution of 37 ps/degree. Pump beams are mixed in periodically poled stoichiometric lithium tantalate (PPSLT) without an antireflection coating. The effective aperture and length of the crystal are 0.5 × 2 mm2 and 15 mm. When input intensity is set at 5.6 MW/cm , an average output power of 4.6 W is obtained at 589.159 nm. Precise tuning to the sodium D2 line is accomplished by thermal control of etalons set in the Nd:YAG lasers. The output power at 589.159 nm is stably maintained within ±1.2% for 8 hours.

High Power Yellow Light Generation for Laser Guide Star

6.3 W coherent yellow light generation at 589.159 nm was achieved by the sum frequency generation of two mode-locked lasers with wavelengths of 1064nm and 1319 nm for the laser guide star of the astronomical telescope.

Absorption spectroscopy in a cavity of tunable frequency-shifted-feedback laser

Intracavity absorption spectroscopy was achieved with a tunable frequency-shifted feedback laser. Use of an intracavity acousto-optic tunable filter simultaneously realized rapid tuning to some absorption lines and time-resolved operation for large detection sensitivity.