VCSELS
During the last
ten years, the development of VCSELs has made enormous advances. Companies, such as
Emcore and New Focus, have even commercialized VCSELs that can
be modulated
at 10 Gbps. Some
groups have demonstrated results that
indicate further improvements
may be possible [23, 24].
VCSELs have been widely recognized as desirable for optical interconnects. The concept of flip-chip
bonding a 2D array of VCSELs onto a CMOS chip and attaching it in an optical system
is attractive in its simplicity [25]. GaAs-based VCSELs have become quite
popular and well-engineered for these applications. Companies that are making
components for optical gigabit
Ethernet installations are usually implementing VCSEL solutions at a wavelength of
850 nm, though often only in 1D
arrays. (Xan3D uses 2D arrays of VCSELs but only for redundancy [26], and Agilent’s MAUI project is still in R&D, but does use a 4x12 array of VCSELs as transmitters [27]).
VCSELs performing
at a wavelength near 1550 nm have suffered from the growth difficulties and low thermal conductivity of the necessary
DBRs. The small
change
in
refractive index
between InGaAs and InP requires many alternating layers in order to achieve
high reflectivity DBRs. Wafer fusion techniques can attach
AlGaAs/GaAs DBRs to InGaAsP/InP
p-i-n diodes [28] or selective etching can be used to create InP/air DBRs with a small number
of pairs [29] in order to circumvent this issue. Research into new materials
such as InGaNAs(Sb) has yielded promising results
[30], though practical devices are still probably years away.
VCSELs offer many
of
the
desirable
characteristics of an
optoelectronic transmitter for optical interconnects.
2D arrays of high
speed devices (~ 10 Gbps) with
good contrast (> 3 dB) at a low voltage drive (~ 1 V) can be fabricated with good yield. There are several problems, though,
that may be drawbacks for VCSELs in optical
interconnects. In order to achieve a high bit rate, VCSELs are operated above threshold
in both the “1” bit and “0” bit states.
(Dropping
below threshold would require a photon
build-up time during the turn on for
the next “1” bit that would prevent such high speed
operation [31].) This
ultimately limits the contrast ratio, since there will always be some
appreciable power in the “0” state.
Secondly, even though the voltage drive can be as low as 1 V, the VCSEL
is a diode which must be in forward
bias by a volt or two.
This would likely
require an additional power
supply for a CMOS chip, along with
dedicated power lines for this voltage source. In addition,
the electrical current would have to pass through this additional voltage, raising the power
dissipation. The above-threshold biasing requirement adds further to the electrical
power consumption and heating problem.
Another issue for
VCSELs is the temperature
dependence of the
lasing wavelength. Many
VCSELs are ultimately
limited by the operating temperature.
Both
WDM and diffractive optical interconnect systems require strict wavelength control, and
VCSELs bonded to CMOS may be more difficult to fix in wavelength. The high quality distributed Bragg reflectors
(DBRs) that are required for VCSELs are somewhat costly
and difficult to grow. And finally,
there are some questions regarding
the reliability of VCSELs
operating at high bit rates over long periods of time. A VCSEL operating
at 10
GHz has an average expected
lifetime of about 10 years.
This is probably
sufficient for
most
applications, but the reliability gets significantly
worse at higher frequencies [22]. The
reliability is correlated
to the current through the laser. In order to operate at higher frequencies, the relaxation oscillation frequency
of the laser must be pushed higher by increasing the current.
But the relaxation oscillation frequency only increases as the square root of the current, so higher frequencies
require significantly higher currents and thus
fail much earlier. For all these reasons and more, many groups
have investigated optoelectronic modulators
as part of a transmitter solution.
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