We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.

Device Requirements for Optical Interconnects to Silicon Chips

The vertical-cavity surface-emitting laser (VCSEL) has become a light source of great importance for industrial and consumer applications. This includes communication and sensing in particular, where dynamics and optical mode behavior are key performance characteristics. This tutorial treats relevant VCSEL basics, performance requirements and recent progress toward higher speed, higher single-mode power, and polarization control.

Advances in VCSELs for Communication and Sensing

Optical and electrical investigations of vertical-cavity surface-emitting lasers (VCSEL) with a monolithically integrated electro-optical modulator (EOM) allow for a detailed physical understanding of this complex compound cavity laser system. The EOM VCSEL light output is investigated to identify optimal working points. An electro-optic resonance feature triggered by the quantum confined Stark effect is used to modulate individual VCSEL modes by more than 20 dB with an extremely small EOM voltage change of less than 100 mV. Spectral mode analysis reveals modulation of higher order modes and very low wavelength chirp of < 0.5 nm. Dynamic experiments and simulation predict an intrinsic bandwidth of the EOM VCSEL exceeding 50 GHz.

Electro-optical resonance modulation of vertical-cavity surface-emitting lasers.

We have studied the modulation properties of a vertical cavity surface-emitting laser (VCSEL) coupled to an
electrooptical modulator. It is shown that, if the modulator is placed in a resonant cavity, the modulation of the light
output power is governed predominantly by electrooptic, or electrorefraction effect rather than by electroabsorption. A
novel concept of electrooptically modulated (EOM) VCSEL based on the stopband edge-tunable distributed Bragg
reflector (DBR) is proposed which allows overcoming the limitations of the first-generation EOM VCSEL based on
resonantly coupled cavities. A new class of electrooptic (EO) media is proposed based on type-II heterostructures, in
which the exciton oscillator strength increases from a zero or a small value at zero bias to a large value at an applied
bias. A EOM VCSEL based on a stopband-edge tunable DBR including a type-II EO medium is to show the most
temperature-robust operation. Modeling of a high-frequency response of a VCSEL light output against large signal
modulation of the mirror transmittance has demonstrated the feasibility to reach 40 Gb/s operation at low bit error rate.
EOM VCSEL showing 60 GHz electrical and ~35 GHz optical (limited by the photodetector response) bandwidths is
realized.

Ultra high-speed electro-optically modulated VCSELs: modeling and experimental results

Vertical-cavity surface-emitting lasers (VCSELs) have made remarkable progress, are being used across a wide range of consumer electronic applications, and have particularly received much attention from the telecom and datacom industries. However, several constraints are thus currently being tackled to improve the device characteristics and modulation formats to meet the various demanding requirements of the future 800 GbE and 1.6 TbE Ethernet standards. This manuscript discusses the device characteristics and the key considerations in the device designs and optimizations. Finally, we elucidate the latest developments and vital features of modern 850 nm VCSELs for high-speed interconnects.

/pdf/recent-advances-in-850-nm-vcsels-for-high-speed-2o81ezkq.pdf

Recent Advances in 850 nm VCSELs for High-Speed Interconnects

We have proposed and demonstrated the principle of optical decoupling in a vertical-cavity surface-emitting laser (VCSEL)-electroabsorption modulator device by use of a duo-cavity device architecture. This technique promises ultrahigh-frequency modulation of the VCSEL output due to the elimination of the resonance influences on the intrinsic modulation transfer function. By adjusting spectral detuning between the resonances of the duo-cavity configuration, optical decoupling of output modulation by an electroabsorption modulator can be achieved. A flat ( 3 dB) response up to 20 GHz has been measured by proper detuning of the resonances.

Optically Decoupled Loss Modulation in a Duo-Cavity VCSEL

We have studied the modulation properties of VCSEL with intracavity multiple quantum well (MQW) electroabsorption
modulator integrated into the top distributed Bragg reflector (DBR) [1]. Small signal analysis of rate equations for loss
modulation shows an intrinsic high-frequency roll-off slope of 1/o instead of 1/o 2 in directly modulated laser diodes, and
consequently bandwidths in excess of 40 GHz are obtainable with this configuration [2]. Possible limiting factors to high
bandwidth were examined by fitting high frequency characteristics to a multi-pole transfer function, and include RC
delay and carrier drift-limited time of flight (TOF) in the modulator intrinsic region. Intracavity loss modulation shows a
strong (+20dB) relaxation oscillation resonant feature in both theory and experiment. As demonstrated, this feature can
be significantly reduced in amplitude using parasitics. We have extracted relative contribution of TOF and parasitic
capacitance by varying the modulator intrinsic region width (105 and 210 nm) and lateral size of the modulator (18 and
12mm). It was estimated that the small size modulator exhibits parasitics f -3dB at 8GHz. To estimate the carrier TOF
contribution to bandwidth limits, low temperature growth of a 210 nm absorber i-region and MQW was employed to
reduce photogenerated carrier lifetime. Bandwidth limitations were found to be mostly due to diode and metallization
capacitances, in addition to one pole set by the optoelectronic resonance frequency. We have used p-modulation doping
of the gain region to increase the relaxation frequency. Pronounced active Q-switching was observed, yielding pulse
widths of 40 ps at a 4 GHz rate.

Modulation properties of VCSEL with intracavity modulator

The authors propose and demonstrate a novel concept for ultrahigh-speed loss modulation of a vertical cavity surface-emitting laser (VCSEL). A duo-cavity device architecture is used to optically decouple the ac feedback component of a loss-modulation section from the VCSEL active region. The elimination of feedback allows modulation of the VCSEL output far beyond the optoelectronic relaxation frequency by eliminating both resonance and intrinsic high-frequency response roll-off. Precise detuning of the resonances of both coupled cavities is used to achieve ac feedback elimination by control of thickness during molecular beam epitaxy growth. Variation in the applied bias at the multiple-quantum-well modulator section allows adjustment of detuning to change coupling between the two sections, resulting in resonance features in the modulation response. In the ideal case, the resulting resonance-free high-frequency modulation response is limited only by parasitics of the modulator section. A flat (±3 dB) modulation response up to 20 GHz and data bandwidth up to 20 GB/s are demonstrated by using this modulation method.

Concept of feedback-free high-frequency loss modulation in detuned duo-cavity vertical cavity surface-emitting laser

Structures of tunnel coupled pairs consisting of InGaAs quantum wells grown on top of self-assembled InAs quantum dots (QW-on-QDs) were employed to improve the gain medium in laser diodes. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the properties of multiple-layer QW-on-QDs active medium. QW-on-QDs tunnel structures with 4.5 nm tunnel barrier thickness and with different ground state (GS) relative separations were grown by variation of InGaAs QW while the QD growth process was kept constant. We have developed a tunnel QW-on-QDs structure with a resonance transition which is red-shifted ~35 meV relative to QW GS. This transition with narrow linewidth, 21.6 meV at T=77K, likely indicates an efficient LO-phonon assisted tunneling of carriers from QW into QD ensemble states. The highest gain was achieved with a QW-on-QDs active medium with GS relative separation of close to 35-40 meV. Optimized triple-pair tunnel QW-on-QDs laser diodes with cleaved mirrors emitting at 1145 nm (corresponding to QD GS) exhibited a saturated modal gain exceeding 80 cm -1 with minimum cavity length of 0.14 mm. Small signal modulation characteristics of these lasers were measured. From the damping factor and resonance frequency dependence on driving current, the damping-limited cut-off frequency for this QW-on-QDs medium can be estimated as exceeding 30 GHz. All-epitaxial vertical cavity surface emitting lasers with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing with 5.7 mA minimum threshold current at QD GS emission wavelength, 1131 nm.

Tunnel quantum well-on-dots InGaAs-InAs high-gain medium for laser diodes

We have proposed and demonstrated the principle of optical decoupling of the AC modulation component in a lossmodulated
Vertical Cavity Surface Emitting Laser (VCSEL) using a detuned duo-cavity device. This approach allows
the VCSEL power to be modulated without changing the photon density in the active region. Analysis of reflectivity
spectra of a Fabri-Perot cavity with absorber shows that at a certain detuning from the resonance wavelength, reflectivity
is almost independent of absorption magnitude. At this spectral detuning between the active region cavity and modulator
cavity, a feedback-free transmission modulation of the VCSEL output is possible. The use a multiple-double-QW
(MDQW) electroabsorption modulator allows absorption swing between 0.2% and 2% per pass. Optical power
modulation of transmission with contrasts up to 40% and chirp of less than 0.05 nm at 930 nm was demonstrated with
our design. Initial cavity resonance detuning is controlled through growth and was determined to be ideally ~0.7 nm
from analysis of stand-alone absorber cavities. Resonance coupling (splitting) was calculated to be less than 0.3 nm in
case of matching resonances. Applying bias at the MDQW modulator section allows adjustment of detuning between
cavities by changing the top cavity resonance wavelength mainly via Kramers-Kronig relations. The high frequency
modulation characteristics can be tuned in this manner to show little or no sign of resonance, in which case the high
frequency roll-off of the modulation response is entirely determined by parasitics of the modulator section. We have
demonstrated a flat (+/-3db) response up to 20 GHz.

High frequency resonance-free loss modulation in a duo-cavity VCSEL

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