Broadband Gain Chips
Drive Tunable External-Cavity Lasers
The core technology behind the tunable external-cavity laser is the broadband gain chip. Laser manufacturers
have much to consider when choosing a chip.
Telecommunications requirements such as dynamic provisioning,
channel protection, wavelength conversion and the replacement of fixed-wavelength
distributed feedback lasers are driving the adoption of tunable laser sources. External-cavity
lasers offer several advantages over other approaches: high power, wide tuning range,
high side mode suppression ratio, narrow linewidth and high efficiency. The core
technology behind the external-cavity laser is the broadband gain chip, a semiconductor
superluminescent diode that acts both as a broadband light source and as a frequency-agile
The largest market for broadband gain chips is
in telecommunications, where external-cavity lasers will act as the sources for
metro, long-haul and ultralong-haul networks. The tunable laser telecommunications
market is small today, but it is expected to grow rapidly and to reach $869 million
in 2005. The chips also have a place in lasers for test and measurement. These lasers
display tuning ranges greater than 100 nm and are used primarily for evaluating
optical components and systems.
In the Littrow external-cavity laser
design, the two critical components are the chip and the filter. When current is
applied to the chip, it emits a very wide spectrum of light that is collimated onto
the filter. The filter is positioned at a precise angle so that only the desired
narrow spectrum reflects to the chip. This feedback is then focused back into the
broadband gain chip, creating a lasing cavity between the far facet of the diode
and the filter. At this point, the chip begins to lase at the desired wavelength.
To tune the external cavity, the filter is rotated to reflect a different wavelength.
In the Littrow external-cavity
laser, the wavelength is determined by the angle of the filter, which controls the
wavelength of light that is reflected to the broadband gain chip, creating a lasing
cavity. Courtesy of Princeton Lightwave Inc.
Many different broadband gain chips
are on the market, and it is important for laser manufacturers to select the one
that exactly fits their needs. The proper selection of the chip can significantly
increase the power and widen the tuning range of an external-cavity laser, but it
also can simplify integration and lower manufacturing costs. Furthermore, the reliability
and stability of these lasers are directly related to the chip.
Selecting a chip
Important criteria in selecting a broadband gain
chip are gain, tunability, manufacturability, reflectivity and chip design:
• Gain and power. Ultimately,
the output power of the chip determines that of the laser. The current output requirement
of an external-cavity laser for long- and ultralong-haul applications is 20 to 40
mW across the entire tuning range. The metro market requires between 5 and 10 mW.
To reach such power levels, high coupling efficiencies, low cavity losses and very
high gain are necessary.
To achieve a high coupling efficiency
into the broadband gain chip, the coupling optics must be designed for the chip’s
specific far-field radiation pattern. In addition, it is important that the chip
have a large active region so that the alignment tolerances of the coupling optics
may be relaxed. Generally, a ridge waveguide structure has a much larger active
region than a buried heterostructure.
The waveguide design in a broadband gain chip affects the reflectivity
at one of the facets, and so the lasing wavelength. A curved or angled waveguide
yields an effective reflectivity of 1 x 10—4, relaxing the requirement that the
antireflection coatings achieve reflectivity of less than 5 x 10—4 across the tuning
Both the material structure and the
cavity length determine the gain. The choice of cavity length of the broadband gain
chip is based on power requirements, constraints on size and the mode spacing of
the lasing cavity. To obtain the 20 mW that is necessary for the long-haul market,
the chip must have an output power of more than 40 mW to account for losses in the
laser. As the power requirements for external-cavity lasers increase, the gain of
the chip also must increase; experimental results at Princeton Lightwave Inc. have
shown output powers of more than 100 mW in a standard Littrow cavity.
An experimental Littrow external-cavity laser at Princeton Lightwave displayed an output
of more than 100 mW. The drive current to the broadband gain chip was 450 mA.
• Tuning range. The broadband
gain chip is the main determinant of the wide tuning range of external-cavity lasers.
The current goal of most manufacturers is to have a tuning range that covers 40
nm in either the C- or the L-band. In the future, manufacturers will extend their
tuning range to include both bands.
The tuning range of the chip is purely
a factor of the material structure. To achieve a wide tuning range, the material
structure must be designed with that in mind. In addition, the chips should be designed
to have a gain curve that is as flat as possible across the entire tuning range.
To achieve a 40-nm tuning range for
an external-cavity laser, the gain chip should have a 3-dB full width half maximum
gain spectrum of at least 40 nm — and preferably closer to 70 nm. A wider
full width half maximum spectrum indicates flatter gain, which means that a manufacturer
must perform less attenuation or fewer adjustments to the operating current of the
chip to keep the power constant across the tuning range.
Experiments at Princeton Lightwave
have demonstrated the wide tuning range of broadband gain chips. A standard chip
in a Littrow external cavity displayed a range of more than 170 nm.
• Manufacturability. Because
there are different designs for external-cavity lasers in the marketplace, the manufacturers
of broadband gain chips must be able to modify their products for different customers,
yet maintain their capacity for high-volume production. Some customers have unique
submounts or facet coatings that are optimized for their external-cavity design.
Moreover, the manufacturers must keep
to a minimum the variation over time of the gain, peak wavelength and placement
of the chip on a submount. As the variation increases, laser manufacturers are forced
to make costly design changes.
The key is that chip providers have
their own material growth reactors, foundry and automated pick-and-place production
• Reflectivity and design.
A critical characteristic of a broadband gain chip is that one of the facets has
a reflectivity of less than 5 x 10—4 across the tuning range. It is essential that
the chip lase only at the feedback wavelength selected by the filter. The low reflectivity
on one of the facets suppresses self-lasing, which effectively extends the length
of the lasing cavity to include the filter. Without it, the chip will lase because
of the feedback from the facets and will overpower the feedback of the filter. This
leads to modal instability and to poor performance of the external-cavity laser.
Moreover, as the power requirements of the lasers increase, the reflectivity at
the facet must be reduced to prevent self-lasing.
There are two approaches to reducing
reflectivity. The first is to use a standard straight waveguide and a coating with
very low reflectivity. The second — and preferred — approach is to use
an angled or curved waveguide design.
Broadband gain chips with straight
waveguides are essentially Fabry-Perot lasers. Not designed specifically for external-cavity
lasers, they are a less than optimal solution. The main issue with these chips
is that they need a very effective antireflection coating on one of the cleaved
Most manufacturers of straight-waveguide
gain chips cannot produce these coatings themselves. Either they must outsource
the coatings, or the manufacturer of the external-cavity laser must send the chips
to a coating vendor.
Unfortunately, few vendors can deposit
multilayer antireflection coatings with a reflectivity of less than 5 x 10—4 across
the tuning range. Because such coatings are state-of-the-art, the process is expensive,
and yields are low. Furthermore, it is questionable whether the coating vendors
can accommodate the large volumes that are necessary for the telecommunications
The better solution is to use a gain
chip that has been specifically designed for use in an external-cavity laser. By
using a properly designed angled or curved waveguide in which the waveguides intersect
the facet of the chip at an angle, the effective reflectivity will be less than
1 x 10—4. By intersecting at an angle, the light that is incident to the facet is
either reflected at an angle equal to the incident angle or is refracted.
A simple, low-cost, one-layer coating
with a reflectivity of 2 x 10—2 can be added to the angled or curved side of the
chip to further decrease reflectivity. Combining an effective reflectivity of 1
x 10—4 and a 2 x 10—2 coating further reduces it to 2 x 10—6. Currently, this is
the only way to get such a low level of reflectivity over the entire tuning range.
The main benefit of the angled/ curved
waveguide design is that chip manufacturers can apply the coating in-house, enabling
them to supply components to the laser manufacturer that are easy to integrate and
that reduce total costs. If the power requirements increase beyond a point where
the reflectivity must be less than 2 x 10—6, the angle of the waveguide can be increased
or the reflectivity of the coating reduced.
As performance requirements increase and tunable
external-cavity lasers further penetrate the long-haul, ultralong-haul and metro
markets, broadband gain chips will continue to evolve. Each specific market segment
will need a chip with a wider and flatter gain spectrum.
The ultralong-haul market also will
require chips with higher gain because of increased attenuation in the fiber. In
the long-haul market, there is a need for a chip with a monolithically integrated
electroabsorption modulator to ensure small form factor and low cost compared with
a continuous-wave external-cavity laser with an external modulator. Lastly, to penetrate
the price-sensitive metro markets, a high-gain, directly modulated chip will be
Meet the author
Todd Swanson is a product line manager at Princeton
Lightwave Inc. in Cranbury, N.J.
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