  Quantum state preparation for trapped-ion qubits often relies on high-quality
circularly-polarised light, which may be difficult to achieve with chip-based
integrated optics technology. We propose and implement a hybrid
optical/microwave scheme for intermediate-field hyperfine qubits which instead
relies on frequency selectivity. Experimentally, we achieve $99.94\%$ fidelity
for linearly-polarised ($\sigma^+$/$\sigma^-$) light, using $^{43}$Ca$^+$ at
28.8 mT. We find that the fidelity remains above $99.8\%$ for a mixture of all
polarisations ($\sigma^+$/$\sigma^-$/$\pi$). We calculate that the method is
capable of $99.99\%$ fidelity in $^{43}$Ca$^+$, and even higher fidelities in
heavier ions such as $^\text{137}$Ba$^\text{+}$.
