A high-speed laser for future data-communication links by p00ol2



A high-speed laser for future
data-communication links
Petter Westbergh, Johan Gustavsson, Asa Haglund, and
Anders Larsson

Optimized designs significantly improve the transmission rate of
standard short-wavelength light sources.

As demand for communication networks rises, it is becoming
increasingly important to develop low-cost, reliable high-speed
connections. Optical fiber links have long been preferred to
electrical cables over long distances. With increasing demand
for bandwidth, they are also replacing copper wire over            Figure 1. Schematic view of our VCSEL structure optimized for high
shorter ranges. For short-distance, multimode fiber-based data-     speed (not drawn to scale: the widest point is 50µm and the height is
communication (datacom) networks, the laser source of choice       7µm). The current path from top to bottom contact is defined by the
is an 850nm gallium arsenide (GaAs) vertical-cavity surface-       insulating, selectively oxidized aluminum GaAs layers (black) which
emitting laser (VCSEL) because of its high-speed capability,       form an ‘oxide aperture.’ This also provides transverse confinement of
low cost, and low power consumption. However, currently            the optical field. The laser resonator is formed by distributed Bragg
available VCSELs cannot achieve the single-channel bit rates (at   reflectors (DBRs) on both sides of the gain region containing the quan-
and beyond 10–20Gbit/s) required for future datacom networks       tum wells.
and consumer applications, for instance in upcoming universal-
serial-bus standards and high-definition video links.
                                                                   and high modulation bandwidth at low currents. However, the
   It has commonly been assumed that, as modulation band-
                                                                   DBRs also need to carry electrical current through the device and
width is greater for smaller components, VCSELs would need
                                                                   the large number of layers leads to high electrical resistance and
to be scaled down to reach higher speeds. However, smaller
                                                                   problems such as self-heating at high currents. These problems
devices mean higher current densities, but these compromise
                                                                   become more pronounced as the size of the device is reduced. We
reliability.1 Recently developed high-speed VCSELs2, 3 can
                                                                   have increased the differential gain of the active region and the
achieve speeds of up to 40Gbit/s but operate at high current
                                                                   thermal conductivity of the bottom DBR to improve the high-
densities and at wavelengths of 980nm and 1.1µm. These are
                                                                   speed properties of 850nm VCSELs while maintaining a low
outside the range of short-reach datacom standards and incom-
                                                                   operating current density for high reliability.4
patible with high-speed multimode fiber. We have brought the
                                                                      We included 10% indium in the gain region’s quantum wells
inherent advantages of these longer-wavelength light sources
                                                                   (QWs) and maintained emission at 850nm by reducing the QW
down to the standard 850nm wavelength by considering and
                                                                   thickness and modifying barriers. As a result, we doubled the
minimizing speed-limiting effects and optimizing the design.
                                                                   differential gain compared to the GaAs QWs traditionally em-
   A standard VCSEL contains an active gain region sandwiched
                                                                   ployed in 850nm VCSELs. As the resonance frequency is propor-
between two highly reflective mirrors consisting of layers of ma-
                                                                   tional to the square root of the differential gain, this significantly
terials with alternating high and low refractive indices, known
                                                                   improved high-speed performance.5 We also incorporated alu-
as distributed Bragg reflectors (DBRs). This design results in an
                                                                   minum arsenide (AlAs) layers as the low-index material in the
effective cavity length of typically only 1–3 wavelengths long,
                                                                   bottom DBR, considerably increasing the thermal conductivity
which makes the VCSEL a very small device with advantages
in terms of low operating currents, low power consumption,                                                      Continued on next page
                                                                                                       10.1117/2.1200911.1838 Page 2/3

compared to using traditional AlGaAs. Thermal management is
important because the resonance frequency increases with the
square root of the current, i.e., the VCSEL needs to be operated
at high currents to reach high speeds. A high resistance leads to
a temperature increase and performance degradation. Effective
heat transport away from the active region is therefore essential.
   In addition to thermal and intrinsic speed limitations, it is
important to address parasitic effects from the resistances and
capacitances associated with the device structure and geome-
try, which effectively form a low-pass resistor/capacitor filter
and limit performance at high modulation frequencies. One of
the major contributions to parasitic capacitance comes from the
oxide aperture, a thin layer of selectively oxidized AlGaAs con-
fining both the electrical current and the optical field. We used a
double- rather than single-layered large-diameter oxide aperture
to reduce the oxide capacitance by almost half. Figure 1 shows a
schematic view of our VCSEL structure.                                  Figure 2. Eye diagrams at 25Gbit/s after transmission over 100m mul-
   Despite a large aperture diameter of 9µm, we reached a max-          timode fiber (a) and at 32Gbit/s after transmission over 50m multimode
imum 3dB bandwidth of 20GHz. Closer analysis of the small               fiber (b).
signal response revealed that the performance was limited pri-
marily by electrical parasitics and secondly by thermal effects.4
The high bandwidth and large aperture allowed for successful
data transmission at very high data rates. Eye diagrams and bit-
error rates (BER) for transmission over 100 and 50m multimode
fiber at 25 and 32Gbit/s, respectively, are displayed in Figures 2
and 3.6 We used eye diagrams to examine the shape of the
optical waveform generated by modulating the VCSEL with an
electrical data signal. An oscilloscope displayed an overlay of
the data pattern received, forming the characteristic eye pattern
shown in Figure 2. The clear, open eye is a good indicator of high
signal quality and that error-free data transmission will be
possible. BER is the fundamental measure of the performance
of a digital data link. It is defined as the probability of incorrect
identification of a bit by the receiver’s decision circuit. In our ex-
periments, error-free transmission (defined as BER < 10−12 ) was
                                                                        Figure 3. Bit-error rate (BER) as a function of detected optical power
achieved at both bit rates while biasing the VCSEL at 11kA/cm2
                                                                        for different data rates and transmission distances.
(the current density used in today’s 10Gbit/s devices).
   A new generation of 850nm VCSELs capable of transmitting
data at high rates without sacrificing reliability is needed to meet     Author Information
near-future demands on data communication. So far, our results
have demonstrated error-free transmission at data rates of up to                                            ˚
                                                                        Petter Westbergh, Johan Gustavsson, Asa Haglund, and
32Gbit/s while operating at low current densities. Our next step        Anders Larsson
will be to reduce the thermal effects and electrical parasitics that    Photonics Laboratory
currently limit performance.                                            Chalmers University of Technology
                                                                        Goteborg, Sweden
This work was supported in part by the European Seventh Framework
Programme project VISIT (FP7-224211) and in part by the Swedish
Foundation for Strategic Research.                                                                                   Continued on next page
                                                                                          10.1117/2.1200911.1838 Page 3/3

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                                     c 2009 SPIE

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