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Tunable Components have been a holy grail of
the industry for many years and have spawned numerous technical
approaches by a number of very competent teams. These can be
classed approximately into five major groups:
1. Narrowly tunable devices,
such as DBR lasers are generally preferred by the more experienced
manufacturing teams from the larger corporations, who are aware
of the pitfalls of the common paths to widely tunable devices.
These teams have aggressively tackled the instability issues
of narrowly tunable lasers and are either delivering, or are
about to deliver, stabilized devices that can tune from 6 to
12nm.
2. Avoiding the intricacies of laser
diode manufacture, others have chosen external cavity
approaches. The advantage of modifying or shrinking down conventional
external cavity tunable lasers with moving parts is that only
simple gain elements are required. Still, mechanical tolerances
and stability remain as challenges.
3. Monolithic widely tunable
diodes are relatively well known, having been pursued
by universities and research institutions for many years. Though
these multi-contact lasers do not suffer from mechanical issues,
the device characteristics are extremely complex and change
as the device ages. Ingenious control circuits and algorithms
are required to stabilize these devices, and integrated amplifiers
are needed to boost the power to required levels.
4. Tunable VCSELs generally
use a movable element to adjust the cavity length and tune the
laser, but either require an external pump or cannot deliver
appropriate output power.
5. Finally, using an array of
lasers of different wavelengths combined with
a star coupler into a single output is difficult to
scale. As the element size is increased to widen the tunability,
the loss of combining the output powers scales in the same way,
such that optical amplifiers are needed to boost the output.
Furthermore, the complexity of an array of lasers combined with
passive couplers results in a chip that requires sophisticated
processing and can be low yield.
Santur’s approach
to tunability also uses an array of lasers, but instead of using
the conventional star coupler to combine the beams, a much more
efficient external combiner is employed, resulting in minimal
excess loss. Unlike other array-based approaches, where only
a small amount of the DFB power can be coupled to the fiber,
Santur’s combiner delivers nearly the entire laser output
power. The resulting device performs like a DFB, and can be
manufactured at low cost using a proprietary automated packaging
approach. Coarse tuning is realized by switching between the
different DFB laser elements while fine tuning and locking to
ITU channels is done by adjusting the TE current to modify the
temperature. In a second generation product, much faster tuning
(<10ms) is realized by changing the temperature of the lasing
stripe only; instead of the entire optical subassembly.
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