Modern broadband communication networks rely on satellites to complement the terrestrial telecommunication infrastructure. Satellites accommodate global reach and enable world-wide direct broadcasting by facilitating wide access to the backbone network from remote sites or areas where the installation of ground segment infrastructure is not economically viable. At the same time the new broadband applications increase the bandwidth demands in every part of the network - and satellites are no exception. Modern telecom satellites incorporate On-Board Processors (OBP) having analogue-to-digital (ADC) and digital-to-analogue converters (DAC) at their inputs/outputs and making use of digital processing to handle hundreds of signals; as the amount of information exchanged increases, so do the physical size, mass and power consumption of the interconnects required to transfer massive amounts of data through bulk electric wires.
Vertical-cavity surface-emitting lasers and multi-mode fibers is the dominating technology for short-reach optical interconnects in datacenters and high performance computing systems at current serial rates of up to 25-28 Gbit/s. This is likely to continue at 50-56 Gbit/s. The technology shows potential for 100 Gbit/s.
Multicore fiber enables a parallel optic data link with a single optical fiber, thus providing an attractive way to increase the total throughput and the integration density of the interconnections. We study and present photonics integration technologies and optical coupling approaches for multicore transmitter and receiver subassemblies. Such optical engines are implemented and characterized using multimode 6-core fibers and multicore-optimized active devices: 850-nm VCSEL and PD arrays with circular layout and multi-channel driver and receiver ICs. They are developed for bit-rates of 25 Gbps/channel and beyond, i.e. <150 Gbps per fiber, and also optimized for ruggedized transceivers with extended operation temperature range, for harsh environment applications, including space.
We present a vertical-cavity surface-emitting laser (VCSEL) where a GaAs-based “half-VCSEL” is attached to a
dielectric distributed Bragg reflector on silicon using ultra-thin divinylsiloxane-bis-benzocyclobutene (DVS-BCB)
adhesive bonding, creating a hybrid cavity where the optical field extends over both the GaAs- and the Si-based parts of
the cavity. A VCSEL with an oxide aperture diameter of 5 μm and a threshold current of 0.4 mA provides 0.6 mW
output power at 845 nm. The VCSEL exhibits a modulation bandwidth of 11 GHz and can transmit data up to 20 Gbps.
Our recent work on high speed 850 nm VCSELs and VCSEL arrays is reviewed. With a modulation bandwidth approaching 30 GHz, our VCSELs have enabled transmitters and links operating at data rates in excess of 70 Gbps (at IBM) and transmission over onboard polymer waveguides at 40 Gbps (at University of Cambridge). VCSELs with an integrated mode filter for single mode emission have enabled transmission at 25 Gbps over >1 km of multimode fiber and a speed-distance product of 40 Gbps·km. Dense VCSEL arrays for multicore fiber interconnects have demonstrated 240 Gbps aggregate capacity with excellent uniformity and low crosstalk between the 40 Gbps channels.
We present a GaAs-based VCSEL structure, BCB bonded to a Si<sub>3</sub>N<sub>4</sub> waveguide circuit, where one DBR is substituted by
a free-standing Si<sub>3</sub>N<sub>4</sub> high-contrast-grating (HCG) reflector realized in the Si<sub>3</sub>N<sub>4</sub> waveguide layer. This design enables
solutions for on-chip spectroscopic sensing, and the dense integration of 850-nm WDM data communication transmitters
where individual channel wavelengths are set by varying the HCG parameters. RCWA shows that a 300nm-thick Si<sub>3</sub>N<sub>4</sub>
HCG with 800nm period and 40% duty cycle reflects strongly (<99%) over a 75nm wavelength range around 850nm. A
design with a standing-optical-field minimum at the III-V/airgap interface maximizes the HCG’s influence on the
VCSEL wavelength, allowing for a 15-nm-wide wavelength setting range with low threshold gain (<1000 cm<sup>-1</sup>).
The first error-free data transmission beyond 1 km of multi-mode fiber at bit-rates exceeding 20 Gb/s is demonstrated
using a high modulation bandwidth, quasi-single mode (SMSR~20 dB) 850 nm VCSEL. A VCSEL with small ~3 μm
aperture shows quasi-single mode operation with a narrow spectral width. The top mirror reflectivity of the VCSEL is
optimized for high speed and high output power by shallow etching. A combination of narrow spectral width and high optical power reduces the effects of fiber dispersion and fiber and connector losses and enables such a long transmission distance at high bit-rates.
We present results from our new generation of high performance 850 nm oxide confined vertical cavity surface-emitting lasers (VCSELs). With devices optimized for high-speed operation under direct modulation, we achieve record high 3dB modulation bandwidths of 28 GHz for ~4 μm oxide aperture diameter VCSELs, and 27 GHz for devices with a ~7 μm oxide aperture diameter. Combined with a high-speed photoreceiver, the ~7 μm VCSEL enables error-free transmission at data rates up to 47 Gbit/s at room temperature, and up to 40 Gbit/s at 85°C.
This paper presents a review of recent work on high speed tunable and fixed wavelength vertical cavity surface emitting
lasers (VCSELs) at Chalmers University of Technology. All VCSELs are GaAs-based, employ an oxide aperture for
current and/or optical confinement, and emit around 850 nm. With proper active region and cavity designs, and
techniques for reducing capacitance and thermal impedance, our fixed wavelength VCSELs have reached a modulation
bandwidth of 23 GHz, which has enabled error-free 40 Gbps back-to-back transmission and 35 Gbps transmission over
100 m of multimode fiber. A MEMS-technology for wafer scale integration of tunable high speed VCSELs has also been
developed. A tuning range of 24 nm and a modulation bandwidth of 6 GHz have been achieved, enabling error-free
back-to-back transmission at 5 Gbps.
We present an empirical thermal model for VCSELs based on extraction of temperature dependence of macroscopic VCSEL
parameters from CW measurements. We apply our model to two, oxide-confined, 850-nm VCSELs, fabricated with
a 9-μm inner-aperture diameter and optimized for high-speed operation. We demonstrate that for both these devices, the
power dissipation due to linear heat sources dominates the total self-heating. We further show that reducing photon lifetime
down to 2 ps drastically reduces absorption heating and improves device static performance by delaying the onset
of thermal rollover. The new thermal model can identify the mechanisms limiting the thermal performance and help in
formulating the design strategies to ameliorate them.
We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser
(VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude
modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and
its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an
integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more
than 35 dB during the tuning.
We have reduced the spectral width of high speed oxide confined 850 nm VCSELs using a shallow surface relief for
suppression of higher order transverse modes. The surface relief acts as a mode filter by introducing a spatially varying
and therefore mode selective loss. The VCSEL employs multiple oxide layers for reduced capacitance which leads to a
strong index guiding and a large spectral width in the absence of a mode filter. With an appropriate choice of surface
relief parameters, the RMS spectral width for a 5 μm oxide aperture VCSEL is reduced from 0.6 to 0.3 nm. The small
signal modulation bandwidth is 19 GHz. Due to reduced effects of chromatic and modal fiber dispersion, the maximum
error-free (bit-error-rate < 10<sup>-12</sup>) transmission distance at 25 Gb/s over OM3+ fiber is increased from 100 to 500 m.
A simple and low-cost technology for tunable vertical-cavity surface-emitting lasers (VCSELs) with curved movable
micromirror is presented. The micro-electro-mechanical system (MEMS) is integrated with the active optical
component (so-called half-VCSEL) by means of surface-micromachining using a reflown photoresist droplet as
sacrificial layer. The technology is demonstrated for electrically pumped, short-wavelength (850 nm) tunable
VCSELs. Fabricated devices with 10 μm oxide aperture are singlemode with sidemode suppression >35 dB,
tunable over 24 nm with output power up to 0.5mW, and have a beam divergence angle <6 °. An improved
high-speed design with reduced parasitic capacitance enables direct modulation with 3dB-bandwidths up to
6GHz and error-free data transmission at 5Gbit/s. The modulation response of the MEMS under electrothermal
actuation has a bandwidth of 400 Hz corresponding to switching times of about 10ms. The thermal
crosstalk between MEMS and half-VCSEL is negligible and not degrading the device performance. With these
characteristics the integrated MEMS-tunable VCSELs are basically suitable for use in reconfigurable optical
interconnects and ready for test in a prototype system. Schemes for improving output power, tuning speed, and
modulation bandwidth are briefly discussed.
The impedance characteristics and the effects of photon lifetime reduction on the performance of high-speed 850 nm
VCSELs are investigated. Through S<sub>11</sub> measurements and equivalent circuit modeling we show that the parasitic mesa
capacitance can be significantly reduced by using multiple oxide layers. By performing a shallow surface etch (25 -
55 nm) on the fabricated VCSELs, we are able to reduce the photon lifetime by up to 80% and thereby significantly
improve both static and dynamic properties of the VCSELs. By optimizing the photon lifetime we are able to enhance
the 3dB modulation bandwidth of 7 μm oxide aperture VCSELs from 15 GHz to 23 GHz and finally demonstrate errorfree
transmission at up to 40 Gbit/s.
Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications,
gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at
wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions
of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for
high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing).
The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with
side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of
6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of
both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation
bandwidth of 5.5GHz with an output power of 0.8mW.
We have explored the possibility to extend the data transmission rate for standard 850-nm GaAs-based VCSELs beyond
the 10 Gbit/s limit of today's commercially available directly-modulated devices. By sophisticated tailoring of the design
for high-speed performance we demonstrate that 10 Gb/s is far from the upper limit. For example, the thermal
conductivity of the bottom mirror is improved by the use of binary compounds, and the electrical parasitics are kept at a
minimum by incorporating a large diameter double layered oxide aperture in the design. We also show that the intrinsic
high speed performance is significantly improved by replacing the traditional GaAs QWs with strained InGaAs QWs in
the active region. The best overall performance is achieved for a device with a 9 μm diameter oxide aperture, having in
a threshold current of 0.6 mA, a maximum output power of 9 mW, a thermal resistance of 1.9 °C/mW, and a differential
resistance of 80 Ω. The measured 3dB bandwidth exceeds 20 GHz, and we experimentally demonstrate that the device is
capable of error-free transmission (BER<10<sup>-12</sup>) under direct modulation at a record-high bit-rate of 32 Gb/s over 50 m of
OM3 fiber at room temperature, and at 25 Gb/s over 100 m of OM3 fiber at 85 °C. We also demonstrate transmission at
40 Gb/s over 200 m of OM3+ fiber at room temperature using a subcarrier multiplexing scheme with a spectrally
efficient 16 QAM modulation format. All transmission results were obtained with the VCSEL biased at current densities
between 11-14 kA/cm<sup>2</sup>, which is close to the 10 kA/cm<sup>2</sup> industry benchmark for reliability. Finally, we show that by a
further reduction of the oxide capacitance and by reducing the photon lifetime using a shallow surface etch, a record
bandwidth of 23 GHz for 850 nm VCSELs can be reached.
The design of an oxide confined 850 nm VCSEL has been engineered for high speed operation at low current density.
Strained InGaAs/AlGaAs QWs, with a careful choice of In and Al concentrations based on rigorous band structure and
gain calculations, were used to increase differential gain and reduce threshold carrier density. Various measures,
including multiple oxide layers and a binary compound in the lower distributed Bragg reflector, were implemented for
reducing capacitance and thermal impedance. Modulation bandwidths > 20 GHz at 25°C and > 15 GHz at 85°C were
obtained. At room temperature, the bandwidth was found to be limited primarily by the still relatively large oxide
capacitance, while at 85°C the bandwidth was also limited by the thermal saturation of the resonance frequency.
Transmission up to 32 Gb/s (on-off keying) over multimode fiber was successfully demonstrated with the VCSEL biased
at a current density of only 11 kA/cm<sup>2</sup>. In addition, using a more spectrally efficient modulation format (16 QAM subcarrier
multiplexing), transmission at 40 Gb/s over 200 m multimode fiber was demonstrated.
GaAs-based VCSELs emitting near 1.3 μm are realized using highly strained InGaAs quantum wells and a large
detuning of the cavity resonance with respect to the gain peak. The VCSELs have an oxide aperture for current and
optical confinement and an inverted surface relief for suppression of higher-order transverse modes. The inverted surface
relief structure also has the advantage of suppressing oxide modes that otherwise appear in VCSELs with a large
detuning between the cavity resonance and the gain peak. Under large signal, digital modulation, clear and open eyes and
error free transmission over 9 km of single mode fiber have been demonstrated at the OC-48 and 10 GbE bit rates up to
85°C. Here we review these results and present results from a complementary study of the RF modulation characteristics,
including second order harmonic and third order intermodulation distortion, relative intensity noise (RIN), and spurious
free dynamic range (SFDR). RIN levels comparable to those of single mode VCSELs emitting at 850 nm are
demonstrated, with values from -140 to -150 dB/Hz. SFDR values of 100 and 95 dB•Hz<sup>2/3</sup> were obtained at 2 and 5 GHz,
respectively, which is in the range of those required in radio-over-fiber systems.