Time and frequency are the most accurately measured physical quantities, largely thanks to the precision available from ultrafast lasers. A train of femtosecond laser pulses can generate a coherent broadband spectrum that with suitable optics is resolvable into a comb of equally spaced reference frequencies. Indeed, optical clocks based on such frequency combs can be more precise than the best atomic clocks.
The titanium:sapphire laser is the standard among femtosecond lasers, able to produce a spectrum spanning more than an octave of frequency. But the fiber laser, whose cavity is a length of fiber-optic glass, has its own natural appeal: It’s durable and robust, far less expensive, and exceedingly compact—about the size of a paperback novel—and it produces superb-quality subpicosecond pulses. What’s more, when composed of glass doped with erbium ions, fiber lasers emit at telecommunication frequencies. Those advantages make them ideal for portable metrology, optical signal processing, and communications...
