Fabricating Integrated Optics with Tungstates
Unique balance of rare-earth dopants creates nearly strain-free epitaxial layers.
Ytterbium-doped KY(WO4 )2 (also called double tungstate) is an excellent laser material because it has high emission and absorption cross sections as well as a small quantum defect when pumped at wavelengths readily available from diode lasers. The concept of fabricating tungstate-based integrated optical devices, incorporating a laser and other waveguide components on a tiny monolithic chip, is enticing but not straightforward. The problem is that waveguides formed by growing Yb-doped KY(WO4 )2 (Yb:KYW) on undoped KYW have a low index contrast that does not allow fabrication of compact and efficient devices.
The refractive index of rare-earth-doped KYW is roughly proportional to the rare-earth concentration, so the index contrast between Yb-doped and undoped KYW can be increased by increasing the Yb concentration. However, because the absorption of pump power depends sharply on Yb concentration, this concentration must be adjusted to suit the laser geometry, not to maximize the index contrast.
Recently, researchers at Ecole Polytechnique Fédérale de Lausanne in Switzerland successfully co-doped Yb:KYW with lutetium and gadolinium, boosting the material’s index contrast with undoped KYW by more than an order of magnitude, while maintaining the desirable low (1.7 percent atomic) Yb concentration. The choice of Lu and Gd as co-dopants was judicious: Both ions are optically inert at the wavelengths involved, and Lu ions induce tensile strains in KYW, while Gd ions induce compressive strains. Thus, by balancing the two dopants appropriately (and taking into account the tensile strain induced by the Yb doping itself), the scientists were able to grow nearly strain-free epitaxial layers of Yb:Gd:Lu:KYW on undoped KYW.
Figure 1. The 5-μm-wide, Yb-doped rib waveguide had a high index contrast (Δn ~7.5 × 10– 3) with the undoped tungstate (KYW) substrate because of the high co-doping of Gd and Lu (13 and 25.3 atomic percent, respectively). The aluminum top layer was present to reduce scanning electron microscopy charging effects. Images reprinted with permission of Optics Letters.
The high index contrast allowed the scientists to fabricate what they believe are the first high-index-contrast tungstate rib waveguides, using standard ultraviolet photolithography and refractive-ion etching (Figure 1). By pumping the waveguides with 980-nm light from a diode laser, they excited Yb fluorescence at ~1020 nm — the first step toward demonstrating laser action in the waveguides. For rib-waveguide widths ranging from 3 to 9 μm, the fluorescence propagated in a single mode (Figure 2).
Figure 2. The fluorescence emerging from the waveguide of Figure 1 exhibited a single-mode profile.
The scientists measured the propagation losses in the waveguide using 670-nm light to avoid absorption loss from the Yb ions. At 670 nm, they observed a loss of 4.8 dB/cm, which they attribute primarily to Rayleigh scattering. Because Rayleigh scattering scales are the inverse fourth power of wavelength, researchers expect that the losses would be less than 1 dB/cm at the ~1-μm fluorescence wavelength.
Figure 3. The tungstate-based Y-splitter demonstrates the potential for tungstate-based integrated optical circuits. The photographic image was created by atomic force microscopy.
Building on their success, the scientists also fabricated a tungstate-based Y-splitter (Figure 3). Again pumping with 980-nm light, they saw that approximately equal fluorescence intensities emerged from each branch of the splitter and that the fluorescence propagated as a single transverse mode. They measured the splitting loss as approximately 1.4 dB at 820 nm.
Optics Letters, March 1, 2007, pp. 488-490.
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