Ultra-narrow spectral linewidth diode lasers for the pumping of alkalis

Diode pumped alkali-vapor lasers and hyperpolarization-enhanced MRI rely on the excitation of alkali vapors as a cornerstone of their technology. One barrier to their advancement has been insufficient capability in providing efficient, affordable, and useable diode laser pump light. While diode laser systems are a cost effective and high performance source of laser light available at the same wavelengths as the alkali absorption spectrums, the naturally broad spectral linewidth of the diode laser provides inefficient pumping of the very narrow absorption line of the alkali. DILAS has developed a diode laser source so spectrally narrow that it bridges this gap, providing useful, efficient, and affordable pump light. Reported results show an ensemble of units yielding 2.5 kW of optical power centered over the potassium absorption line at 766.5nm with a 90% power enclosed spectral linewidth of 115pm (60pm FWHM). All power was contained in the narrowed peak. With center wavelength tunability exceeding +/- 0.15nm and individually addressable diode bars, the diode laser module can be actively adjusted to spectrally shift and broaden the overall emission to provide the optimal pumping scheme. This technology has been deployed in multiple formats and wavelengths, including 100W and 200W circularly polarized direct beam units centered at 794.7nm for hyperpolarization-enhanced MRI, 425W/400 mu m fiber coupled modules centered at 766.5nm and 780nm, and a 1kW diode stack centered at 780nm. With diode epitaxy available at the absorption of each alkali metal, excluding sodium, and wavelength independence of DILAS's linewidth narrowing technology, this enables the advancement of all alkali-based technologies.