Laser Focus World reported our recent work in the December 2009 issue.

FIBER LASERS: Fiber lasers enter guide-star arena

A typical laser guide star—formed by illuminating the upper atmosphere with 589 nm light to cause sodium vapor lines to fluoresce—accompanies an adaptive-optics system to compensate for atmospheric turbulence and improve clarity for ground-based telescopes. Fiber lasers (which require frequency doubling or some other scheme to reach the 589 nm wavelength) could not achieve the typical 20 W power levels required for this application, until European Southern Observatory (ESO; Garching, Germany) researchers a 589 nm fiber laser guide-star source using two coherently combined Raman fiber amplifiers, as first reported at Laser, World of Photonics 2009.

Now, the ESO researchers have demonstrated a 589 nm fiber-laser guide-star source with more than 25 W of output power using a single frequency-doubled Raman fiber amplifier.^{1, 2} Having tackled everything from materials processing to microelectronics processing to medical therapy and harsh-environment industrial applications, the rugged, compact, low-energy-consumption, and excellent beam-quality attributes of fiber lasers can now be applied to guide-star applications in remote telescope locations.

From two amplifiers down to one

To create more than 25 W of 589 nm light, the research team initially fed the output from a 36 mW, 1178 nm continuous-wave (CW) fiber-coupled external-cavity diode seed laser from Toptica Photonics (Munich, Germany) into two 1178 nm Raman fiber amplifiers. The amplifiers were pumped by ESO-built 75 W 1120 nm Raman fiber lasers, which were in turn pumped by ytterbium (Yb) fiber lasers operating at 1070 nm. The 1178 nm amplifier output had a full-width half-maximum (FWHM) linewidth of less than 1.5 MHz and an output power of 20 W before being limited by stimulated Brillouin scattering (SBS). Using a coherent beam-combination technique (consisting of a Mach-Zehnder interferometer formed by a 50/50 fused fiber coupler, two independent Raman fiber amplifiers, and a 50/50 free-space combining mirror), the amplifier outputs are combined (with 95% efficiency) at a single output from the combining mirror and directed to an external resonant cavity for frequency doubling.

The power and conversion efficiency of a 589 nm guide-star fiber laser are plotted as a function of the power from its 1178 nm single optical amplifier. (Courtesy of the European Southern Observatory)

The doubling cavity consists of four mirrors in a bowtie configuration and a lithium niobate (LBO) crystal. The 25.4 W, 589 nm output has a measured FWHM linewidth of less than 2.3 MHz with a conversion efficiency of 86%–among the highest ever reported for such a power level.

Because the use of two amplifiers is complex, costly, and less rugged for harsh environments, the ESO team has since used a single Raman amplifier pump to obtain more than 25 W of output for a 589 nm laser. Significantly improved power output from the 1120 nm home-built ESO pump laser allowed the group to eliminate the second amplifier. With the use of a 30 m length of single-mode fiber and a special laser-cavity design, the initial 75 W, 1120 nm source can now produce 150 W of power–the highest-power Raman fiber laser ever reported, to the knowledge of the research team. After frequency doubling of the 1178 nm Raman fiber amplifier, 26.5 W was obtained with a conversion efficiency of 81% in initial experiments (see figure). The authors reported an improved 28 W at Frontiers in Optics 2009.

“Raman fiber lasers or amplifiers are normally not considered ways to generate high-power narrow linewidth lasers because of SBS and nonlinear linewidth broadening,” says Yan Feng at ESO. “However, we have developed techniques to suppress these effects successfully, and proved that a Raman fiber amplifier is actually a promising technology for generating guide-star lasers. A special advantage of Raman fiber devices is the wavelength versatility. The technology can be applied to generate lasers at virtually any wavelength transparent in fibers, and tuned for many other applications in science, medicine, and industry.”

–Gail Overton

REFERENCES

1. Y. Feng et al., Optics Express 17(21) p. 19021 (Oct. 12, 2009).
2. Y. Feng et al., Frontiers in Optics 2009, San Jose, CA, post-deadline paper PDPA4 (Oct. 14, 2009).

Tue Dec 01 00:00:00 CST 2009

A new publication is out: Yan Feng, Luke R. Taylor, and Domenico Bonaccini Calia, "25 W Raman-fiber-amplifier-based 589 nm laser for laser guide star," Opt. Express 17, 19021-19026 (2009)

We report on a 25 W continuous wave narrow linewidth (< 2.3 MHz) 589 nm laser by efficient (> 95%) coherent beam combination of two narrow linewidth (< 1.5 MHz) Raman fiber amplifiers with a Mach-Zehnder interferometer scheme and frequency doubling in an external resonant cavity with an efficiency of 86%. The results demonstrate the narrow linewidth Raman fiber amplifier technology as a promising solution for developing laser for sodium laser guide star adaptive optics.

Basically it is a journal paper on the results I reported at CLEO Europe 2009 in Munich. It took almost three months to go through the review process, on an Express journal which supposes to extremely fast. The process is slow probably because the associated editor is busy. What is really odd is that the comments from one of the reviewers are apparently referring to another paper.

The process is so slow that the results are not up to date anymore. In these months, we have scaled the power from a single amplifier by a factor of two. Now we are able to obtain more than 25 W <2.3 MHz linewidth 589 nm laser with frequency doubling of a single narrow linewidth Raman fiber amplifier, which is the topic of our forth-coming Frontiers in Optics postdeadline paper: Yan Feng, Luke Taylor, Domenico Bonaccini Calia, Ronald Holzlöhner, and Wolfgang Hackenberg, 39 W narrow linewidth Raman fiber amplifier with frequency doubling to 26.5 W at 589 nm, PDPA4, Oct. 14, 2009，San Jose, California, USA. We have actually obtained 28 W right now. My colleague Ronald Holzlöhner will attend the conference and give the presentation.

On the CLEO Europe 2009 (which was already one month ago!), I presented our recent progresses in developing 589 nm laser for laser guide star adaptive optics. We had coherently beam combined two high power narrow linewidth Raman fiber amplifiers with an efficiency of 95%. After frequency doubling in an external resonant cavity, we have achieved 25 W 589 nm laser with a linewidth less than 2.3 MHz, which is limited by measurement resolution. I have put the slides online: 25 W CW Raman-fiber-amplifier-based 589 nm source for laser guide star.

The conference was intensive and very interesting for me. I noticed many talks and posters on yellow laser generation aiming at laser guide star application. Besides our works, most notable are optically pumped semiconductor lasers, long wavelength Yb fiber lasers, and Bismuth doped fiber lasers. Some new technologies may appear in the coming years, since so many researchers are looking at this direction.

I attended the talk by Prof. Joss Bland-Hawthorn on Astrophotonics. It appears that he had come to my blog and noticed my "concern". He agreed that laser guide star is an important part of Astrophotonics. :)

In the past half year, we have improved our narrow linewidth Raman fiber amplifier to 20.7 W and 3.5 MHz linewidth. By external resonant cavity frequency doubling, we have obtained up to 14.5 W CW at 589nm with an optical to optical efficiency of 83% (after an optical isolator and other optics, 17.2 W 1178 nm laser is left to couple to the cavity).

My colleagues will present the work at Photonics West 2009 (LASE 2009, 7195: Fiber Lasers VI: Technology, Systems, and Applications, Post-Deadline Session, Paper 7195-101). Below is the abstract:

20W CW, 4MHz linewidth Raman fiber amplifier with SHG to 589nm

Yan Feng, Luke Taylor, and Domenico Bonaccini Calia
European Southern Observatory, Karl-Schwarzschildstr.2, D-85748 Garching, Germany

Up to 20.7 W CW, 3.5 MHz linewidth, 1178 nm continuous-wave laser has been obtained at ESO laser labs by Raman amplification of a distributed feedback diode laser. The 1178nm laser has a linear polarization-extinction-ratio of 25dB. Frequency doubling with an LBO-based SHG commercial cavity has given 83% conversion efficiency and 14.5W CW at 589nm. The source is suitable to produce mesospheric laser guide stars as reference stars for adaptive optics. The presented narrow-band, high power Raman amplification technique might be used for a large number of different wavelength ranges.

Download the 2-page summary:
Y. Feng, L. Taylor, D. Bonaccini Calia, “20W CW, 4 MHz linewidth Raman fiber amplifier with SHG to 589 nm,” Photonics West 2009, San Jose (postdeadline paper 7195-101).