Reducing the linewidth of a diode laser below 30 Hz by stabilization to a reference cavity with a finesse above 10(5).

An extended-cavity diode laser operating in the Littrow configuration emitting near 657 nm is stabilized through its injection current to a reference cavity with a finesse of more than 10(5) and a corresponding resonance linewidth of 14 kHz. The laser linewidth is reduced from a few megahertz to a value below 30 Hz. The compact and robust setup appears ideal as a portable optical frequency standard that uses the calcium intercombination line.