We report a 100-W continuous-wave (CW) 1178-nm narrowband polarization-maintaining (PM) Raman fiber amplifier (RFA) based on ESO’s patented RFA technology. A linearly-polarized 15-mW narrow-linewidth 1178-nm seed laser was amplified in a two-stage PM RFA counter-pumped by a PM 1120-nm fiber laser. Efficient suppression of stimulated Brillouin scattering (SBS) pushed the SBS threshold above 100 W. The Raman fiber types and lengths were chosen to maximize the Raman conversion efficiency in order to mitigate the thermal loads on critical optical components in the RFA. At 100 W of RFA output, which to the best of our knowledge is a record for such an RFA, the required 1120-nm pump laser power is only 180 W. Measurements of the linewidth of the amplified 1178-nm output at different power levels confirmed that the narrow linewidth of the 1178-nm seed laser is preserved even at 100 W of RFA output. The measured polarization extinction ratio (PER) at the 100-W level is > 24 dB, ensuring efficient second harmonic generation. Assuming a conservative conversion efficiency of ~ 80% by a resonant frequency doubler, narrow-linewidth CW guide star lasers with powers > 80 W at 589 nm can confidently be expected. The output spectrum measured at 100 W reveals that there is no risk of power migration from 1178 nm to the next Stokes order, which indicates that the RFA is still SBS-limited. With further optimization of the SBS suppression, it is expected that the RFA output power can be scaled beyond the current 100-W level.
The performance of large ground-based optical telescopes is limited due to wavefront distortions induced by atmospheric turbulence. Adaptive optics systems using natural guide stars with sufficient brightness provide a practical way for correcting the wavefront errors by means of deformable mirrors. Unfortunately, the sky coverage of bright stars is poor and therefore the concept of laser guide stars was invented, creating an artificial star by exciting resonance fluorescence from the mesospheric sodium layer about 90 km above the earth’s surface. Until now, mainly dye lasers or sumfrequency mixing of solid state lasers were used to generate laser guide stars. However, these kinds of lasers require a stationary laser clean room for operation and are extremely demanding in maintenance. Under a development contract with the European Southern Observatory (ESO) and W. M. Keck Observatory (WMKO), TOPTICA Photonics AG and its partner MPB Communications have finalized the development of a next-generation sodium guide star laser system which is available now as a commercial off-the-shelf product. The laser is based on a narrow-band diode laser, Raman fiber amplifier (RFA) technology and resonant second-harmonic generation (SHG), thus highly reliable and simple to operate and maintain. It emits > 22 W of narrow-linewidth (≈ 5 MHz) continuous-wave radiation at sodium resonance and includes a re-pumping scheme for boosting sodium return flux. Due to the SHG resonator acting as spatial mode filter and polarizer, the output is diffraction-limited with RMS wavefront error < λ/25. Apart from this unique optical design, a major effort has been dedicated to integrating all optical components into a ruggedized system, providing a maximum of convenience and reliability for telescope operators. The new remote-pumping architecture allows for a large spatial separation between the main part of the laser and the compact laser head. Together with a cooling-water flow of less than 5 l/min and an overall power consumption of < 700 W, the system offers a maximum of flexibility with minimal infrastructure demands on site. Each system is built in a modular way, based on the concept of line-replaceable units (LRU). A comprehensive system software, as well as an intuitive service GUI, allow for remote control and error tracking down to at least the LRU level. In case of a failure, any LRU can be easily replaced. With these fiber-based guide star lasers, TOPTICA for the first time offers a fully engineered, off-the-shelf guide star laser system for groundbased optical telescopes providing convenient, turn-key operation in remote and harsh locations. Reliability and flexibility will be beneficial in particular for advanced satellite and space debris tracking as well as LIDAR applications.
Large telescopes equipped with adaptive optics require high power 589-nm continuous-wave sources with emission linewidths of ~5 MHz. These guide-star lasers should be highly reliable and simple to operate and maintain for many years at the top of a mountain facility. After delivery of the first 20-W systems to our lead customer ESO, TOPTICA and MPBC have begun series production of next-generation sodium guide-star lasers. The chosen approach is based on ESO’s patented narrow-band Raman fiber amplifier (RFA) technology . A master oscillator signal from a TOPTICA 50-mW, 1178-nm diode laser, with stabilized emission frequency and linewidth of ~ 1 MHz, is amplified in an MPBC polarization-maintaining (PM) RFA pumped by a high-power 1120-nm PM fiber laser. With efficient stimulated Brillouin scattering suppression, an unprecedented 40 W of narrow-band RFA output has been obtained. This is spatially mode-matched into a patented resonant-cavity frequency doubler providing also the repumper light . With a diffraction-limited output beam and doubling efficiencies < 80%, all ESO design goals have been easily fulfilled. Together with a wall-plug efficiency of < 3%, including all system controls, and a cooling liquid flow of only 5 l/min, the modular, turn-key, maintenance-free and compact system design allows a direct integration with a launch telescope. With these fiber-based guide star lasers, TOPTICA for the first time offers a fully engineered, off-the-shelf guide star laser system for ground-based optical telescopes. Here we present a comparison of test results of the first batch of laser systems, demonstrating the reproducibility of excellent optical characteristics.
Large telescopes equipped with adaptive optics require 20-25W CW 589-nm sources with emission linewidths of
~5 MHz. These Guide Star (GS) lasers should also be highly reliable and simple to operate and maintain for many years
at the top of a mountain facility. Under contract from ESO, industrial partners TOPTICA and MPBC are nearing
completion of the development of GS lasers for the ESO VLT, with delivery of the first of four units scheduled for
December 2012. We report on the design and performance of the fully-engineered Pre-Production Unit (PPU), including
system reliability/availability analysis, the successfully-concluded qualification testing, long-term component and system
level tests and long-term maintenance and support planning.
The chosen approach is based on ESO's patented narrow-band Raman Fiber Amplifier (EFRA) technology. A master
oscillator signal from a linearly-polarized TOPTICA 20-mW, 1178-nm CW diode laser, with stabilized emission
frequency and controllable linewidth up to a few MHz, is amplified in an MPBC polarization-maintaining (PM) RFA
pumped by a high-power 1120-nm PM fiber laser. With efficient stimulated Brillouin scattering suppression, an
unprecedented 40W of narrow-band RFA output has been obtained. This is then mode-matched into a resonant-cavity
doubler with a free-spectral-range matching the sodium D<sub>2</sub>a to D<sub>2</sub>b separation, allowing simultaneous generation of an
additional frequency component (D<sub>2</sub>b line) to re-pump the sodium atom electronic population. With this technique, the
return flux can be increased without having to resort to electro-optical modulators and without the risk of introducing
optical wave front distortions. The demonstrated output powers with doubling efficiencies >80% at 589 nm easily exceed
the 20W design goal and require less than 700 W of electrical power.
In summary, the fiber-based guide star lasers provide excellent beam quality and are modular, turn-key, maintenance-free,
reliable, efficient, and ruggedized devices whose compactness allows installation directly onto the launch telescope
In contrast with conventional polarization maintaining fibers, a simple Ge-doped single-mode fiber is used to generate tunable femtosecond soliton pulses. Soliton self-wavelength-shift up to 200 nm is achieved in a 17 m long fiber. The generated “monocolored” soliton pulses have quasi-ideal sech spectral shapes. A high contrast optical switching scheme is proposed as an example of potential application of the soliton self-frequency shift.