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Jin U. Kang, Chang-Seok Kim, and Jacob B. Khurgin

Currently winning acceptance, rare-earth-doped fiber-laser application possibilities include sensing, materials processing, and medicine.

Historically, most visible lasers have been based on gas, dye, or semiconductor gain media. The gas- and dye-based lasers are generally bulky, inefficient, and cumbersome to maintain. Consequently, an active research effort is currently geared toward developing visible and ultraviolet (UV) semiconductor lasers, but the performance achieved to date is limited to tens of milliwatts of output power.

Because of these limitations, nonlinear frequency up-conversion of high-power, solid-state lasers such as Nd:YAG has become the technique of choice for many applications requiring high-power green or other visible light. Nevertheless, the continuous development and the improvement of high-power, rare-earth-doped fiber lasers is slowly winning acceptance of these devices in research and development, as well as various commercial and manufacturing sectors.

Rare-earth-doped fiber lasers have several advantages over the stoichiometric rare-earth lasers. These include, first and foremost, ready availability of large lengths of the gain medium, measured in meters rather than millimeters. In particular, the ytterbium (Yb)-doped fiber laser is of great interest, since its emission wavelength range is centered around 1060 nm, the wavelength of the ubiquitous Nd:YAG laser.

While the Nd:YAG wavelength is fixed, the Yb-doped fiber laser can be tuned over 80 nm. In addition, the Yb-doped fiber laser can deliver continuous-wave (cw) output power in excess of tens of watts, with wall-plug efficiency in excess of 20%. Recently, we have demonstrated a simple and efficient method based on a Yb-doped fiber laser and SHG to produce green light (532 nm). The laser is based on a Yb-doped polarization-maintaining fiber laser and a periodically poled lithium niobate (PPLN) crystal. In addition, we have shown that other visible wavelengths can be generated by Raman-Stokes-shifting the output of the Yb-doped fiber laser, and by subsequently frequency-doubling the shifted wavelength. Potential applications of this laser include sensing, materials processing, and medicine.

Second harmonic generation
There have been many implementations of second harmonic generation (SHG) in various nonlinear materials since the quasi-phase-matching technique was first proposed.¹ Among them, PPLN is one of the most efficient available second-harmonic crystals. It has received much attention as a cost-effective medium for generating new wavelengths in the visible and infrared (IR) spectral regimes, and for enabling radiation sources with large tuning ranges.²

Recently, using a combination of a rare-earth doped multiclad fiber amplifier, a laser diode, and a PPLN nonlinear crystal, a research team at IPG Photonics (Oxford, MA) demonstrated efficient SHG.³ Our simpler scheme requires only a polarization-maintaining cw Yb-doped fiber laser and a PPLN crystal.

Because the efficiency of the nonlinear crystals is polarization-dependent, to generate an efficient and stable second harmonic the polarization state of the fiber laser has to be stable. In the case of PPLN, the polarization state has to be linear and the field has to be polarized along the z-axis of the PPLN in order to utilize the most efficient d33 coefficient of the PPLN. Thus, for efficient SHG of the fiber-laser output, a polarization-maintaining, single-polarization fiber laser is desired. This also eliminates the need for external waveplates for the manipulation of the polarization state.

In addition, for efficient SHG, the fundamental beam from the fiber laser must be phase-matched to the generated green beam. We calculated that this condition is satisfied with a PPLN grating period of 6.5 µm.

Our experiment was performed with a custom-made, cw, Yb-doped polarization-maintaining fiber laser from IPG Photonics and a 10-mm-long PPLN crystal in an oven with a temperature controller from Super Optronics Corp. (Gardena, CA). The output fiber of the fiber laser was rotated so that the laser field was aligned with the z-axis of the PPLN crystal. Next, the fundamental IR beam from the fiber laser was focused into the center of the PPLN crystal with a 5-cm-long focal-length lens. At the output of the PPLN crystal, a second 5-cm-long lens collimated the beam, and a dichroic mirror was used to separate the SHG green beam from the fundamental IR beam.

The phase-matching was fine-tuned by varying the crystal oven temperature, with maximum efficiency occurring at an oven temperature in the vicinity of 196°C. For various incident pump powers of the cw beam from the Yb-doped fiber laser, the spectra of corresponding SHG green beams were measured (see Fig. 1). The spectral width of the SHG beams are approximately 0.25 nm, corresponding to the phase-matching bandwidth of ~0.5 nm.

The output power of the SHG beam was also plotted as a function of input fundamental power at 1064 nm (see Fig. 2). In the low-pump-power level, where the spectral width of the Yb-fiber laser is approximately 0.5 nm, we have achieved high cw efficiency of approximately 4%/W. Because of the combination of nonlinear effects and the amplified spontaneous emission noise, however, the spectral width of the Yb-laser broadens when operating at a power level above 2.5 W.

As a result, the SHG efficiency decreases at the high-pump-power level. Nevertheless, the overall conversion efficiency was about 6 % at 3 W. At power levels above 10 W, we have obtained more than 1 W of green light. Much higher efficiency can be obtained by pulsing the Yb-doped fiber laser, which increases the peak power, and by limiting the Yb-doped fiber laser bandwidth at high power.

In addition, the output wavelength of the fiber laser can be easily Raman-Stokes-shifted and subsequently frequency-doubled to generate different wavelengths in the visible spectrum. Using a highly nonlinear optical fiber, we have Stokes-shifted the fiber laser from 1120 to 1586 nm. Thus, visible wavelengths between 560 and 793 nm can be obtained using the same SHG technique described in detail (see Fig. 3).

ACKNOWLEDGMENT
This work was supported by the Earth Science Technology Office at the National Aeronautics and Space Administration (NASA).

REFERENCES

1. J. A. Armstrong et al., Phys. Rev. 127, 1918 (1962).
2. L. E. Myers and W. R. Bosenberg, IEEE J. Quantum Electron. 33, 1663 (1997).
3. S. A.Guskov et al., Elec. Lett. 34, 1419 (1998).

JIN U. KANG is an associate professor whose research team includes CHANG-SEOK KIM and JACOB B. KHURGIN in the Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, MD 21218; e-mail: jkang@jhu.edu.

Fri Feb 01 00:00:00 CST 2002

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