Mode-Locking
Method Shows
Stable Output,
Potential for
Tiny Lasers

Roberto Morandotti and David Moss developed a compact laser with a new passive mode-locking method that provides stable output and narrow linewidths at high repetition rates.

Courtesy of M. Peccianti
within the filter’s passband, and these
modes interfere with each other, creat-
ing beats of low-frequency noise that
cause unstable operation.

Alternatively, the group’s laser
design puts the nonlinearity and gain
into the same microresonator that acts
as the filter. While the erbium-doped
fiber amplifier is still part of the laser,
the required fiber length is much short-
er ( 3 m)—which increases the spacing
between modes and therefore increases
the stability of the laser output. Moss
adds, “The microcavity is a glass-based
integrated ring resonator where the
laser light modes are generated and
locked extremely efficiently, because
we’ve engineered the resonator to have
ideal qualities, particularly nonlinear
optical properties.”
Ultimately, the researchers want to
develop an entirely integrated laser that
can provide ultrafast pulses. Such an
optical system could offer fast process-
ing, low cost and lower energy consump-
tion. This laser isn’t entirely integrated,
but it is quite small: The microresona-
tor is integrated on a chip made using
CMOS-compatible methods, and the
fiber can be wound on a spool about 1 cm
in diameter.

An international team of researchers used a microcavity ring resonator as both a laser filter and mode-locker. The laser emits pulses of light with unusually stable operation at repetition rates as high as 200 GHz while maintaining very narrow linewidths below 130 kHz (Nat. Commun., DOI: 10.1038/ncomms1762).

Passive mode-locking produces much shorter pulses than can be achieved with active (i.e., electronic) control. Many applications that use mode-locked lasers could benefit from the faster, more stable pulses allowed by this new technique.

The team included scientists from Institut National de la Recherche Scientifique (INRS, Canada), Istituto per i Processi Chimico-Fisici (part of the Con-siglio Nazionale delle Ricerche, Italy), Infinera Corporation (U.S.A.) and the University of Sydney (Australia).

The new mode-locking method,
filter-driven four-wave-mixing (FD-
FWM), is in some ways similar to
dissipative FWM schemes: A filter is
inserted into the cavity of a fiber laser
to suppress unwanted modes. “The real
achievement,” says researcher David
Moss of INRS and University of Syd-
ney, “is that we managed to generate
an extremely stable output whereas
dissipative FWM lasers very often suf-
fer from what is known as supermode
instability.” The fiber must be long to
get the gain and nonlinearity required
for sustained lasing in dissipative
FWM-based lasers, but a long laser cav-
ity allows many closely spaced modes.
This means that multiple modes are

Courtesy of the University of Sydney

A monolithically integrated 4-port microring resonator acts as both a filter within the laser cavity and as a nonlinear gain medium within the erbium-doped fiber-based laser. The waveguide is made of high index doped silica glass.

—Yvonne Carts-Powell

Self-Assembling Microlenses Inspired by Starfish

Microlenses are becoming increas- ingly useful in advanced optical
applications, but they are complicated
to make. Scientists in Germany recently
made good-quality calcium carbonate
microlenses by precipitating them out of
solution (Nat. Comm. 3, 725).

Ophiocoma wendtii, a reef-dwelling brittlestar whose body is covered with tiny calcium carbonate lenses that act as the lenses for compound eyes. Brittlestars and other sea creatures build biological