Energy-Efficient Short-Cavity VCSELs for Telecom and Datacom Applications

In this work, InP-based vertical-cavity surface-emitting lasers (VCSELs) with single- mode emission in the 1.3 μm and 1.55 μm wavelength range are studied. A novel short-cavity concept is introduced and examined in terms of energy efficiency and modulation bandwidth. These high-speed devices target numerous applications in the datacom and telecommunication sector. Thereby, the market’s demand for a cheap and energy-efficient light-source with highest possible bandwidth, is directly addressed by short-cavity (SC) VCSELs that exhibit a strongly improved intrinsic and parasitic modulation performance even at low bias currents.
The novel short-cavity concept implements two dielectric distributed Bragg reflectors (DBRs). These mirrors consist of quarter-wave layers with alternating high and low refractive index. As the refractive index contrast among subsequent layers is rather
large for the utilized dielectric materials (Δn 2, 1), a laser cavity with a high finesse
can be build up for only a few DBR pairs. Moreover, these dielectric DBRs entail the
following advantages. First, the absorption losses in the undoped, high band-gap layers (Eg > 3 eV for both materials) are negligibly small when compared to semi-conducting epitaxial DBRs at this wavelength. Consequently, reduced threshold currents and en- hanced slope efficiencies will be obtained for short-cavity VCSELs. Furthermore, the extremely small penetration depth of the optical field into these dielectric DBRs al- lows to scale down the effective cavity length of the laser by approximately a factor of two, and hence, to reduce the photon lifetime. In that way, significantly increased relaxation-resonance frequencies and strongly reduced intrinsic damping can be ob- tained when compared to state-of-the-art VCSELs at the considered wavelengths.
For the reduction of the VCSEL chip parasitics, a passivation scheme involving ben- zocyclobuthene (BCB) spacer layers is adopted and optimized with respect to contact pad layout and passivation thickness. Moreover, the doping level of the InP cladding layers and the diameter of the VCSEL mesa are reduced, yielding a lower capacitance of the VCSEL mesa. Therefore, parasitic effects compromising the modulation band- width of the devices can be minimized, resulting in parasitic cut-off frequencies beyond 23 GHz irrespective of the emission wavelength of the SC-VCSELs.
As a result of aforementioned intrinsic and parasitic design improvements, the short- cavity VCSELs presented in this thesis currently set the benchmark with respect to maximum modulation bandwidth, thereby outnumbering the performance of existing VCSEL concepts in the 1.3 μm and 1.55 μm wavelength range by far.
Fabricated 1.55 μm single-mode devices feature excellent static and dynamic device properties as follows. At room-temperature, threshold currents below 1 mA, maximum optical output powers above 3 mW, conversion efficiencies beyond 20 %, and differen- tial quantum efficiencies (DQE) close to 40 % are reported for devices with an aperture of 5 μm. For SC-VCSELs implementing two stacked active regions even DQEs up to 80 % are presented. The thermal resistance of SC-VCSELs with single active region adopts relatively small values between 3.2 K/mW and 1.0 K/mW depending on the ac- tual aperture diameter. In that way, just a moderate thermal degradation of the device performance is achieved for elevated temperatures up to 90 ◦C. The minimum linewidth measured for these devices is only 2 MHz, with an extracted linewidth enhancement factor of αH = 3.0. For the current-modulated SC-VCSEL, chirp frequencies in the low gigahertz range are obtained from fiber transfer function measurements. The small signal analysis of these devices yields record-high values for the relaxation-resonance frequency and low damping factors. Correspondingly, the highest to date reported 3 dB modulation bandwidths of 18 GHz and 11 GHz are presented for 1550 nm SC-VCSEL operated at 20 ◦C and 90 ◦C , respectively. In that way, these devices enable the error- free data-transmission in the optical C-band at single-channel bit-rates up to 25 Gbps and a link-length of 4.2 km. In back-to-back configuration, even channel-rates up to 40 Gbps are demonstrated.
Applying the short-cavity concept to VCSELs emitting in the optical O-band around
1.3 μm, even surpasses the static and dynamic performance of 1.55 μm devices. The single-mode optical output power of SC-VCSELs with a 4 μm aperture reaches maximum values of 4.5 mW and 1.8 mW at 20 ◦C and 80 ◦C , respectively. In the same temperature range, the differential quantum efficiencies (wall-plug efficiencies) adopt record-high values between 56 % and 41 % (36 % and 21 %). Typical threshold currents lie around 1 mA in the considered temperature range. In order to further scale up the single-mode output powers and efficiency values, the implementation of a donut-shaped surface relief structure into large aperture devices is proposed based on simulation results obtained from the fully 3-dimensional modeling of the VCSEL- modes. The conducted small signal analysis yields similarly high relaxation-resonance frequencies close to 20 GHz as exhibited by 1.55 μm SC-VCSELs. Likewise, error-free data transmission at single-channel bit-rates of 25 Gbps and 20 Gbps is demonstrated for a link length up to 25 km and 50 km, respectively. Thereby, the lowest energy- to-data-distance ratio of 24 fJ/(bit · km) reported for any VCSEL, is achieved. As an alternative approach to the short-cavity concept utilizing dielectric DBRs, 1.3 μm VCSELs implementing an a-Si/SiO2 high-contrast grating (HCG) are presented for the first time. Single-mode, continuous-wave operation is achieved for temperatures up to 15 ◦C and aperture diameters up to 11 μm. Thereby, the existing device limitations can be overcome, when transferring the HCG concept to the 1.55 μm wavelength range, where amorphous silicon is hardly absorbing.
In order to prove the suitability and sustainability of SC-VCSELs as the light-source of choice for future communication systems, these devices are exemplarily implemented in certain Telecom and Datacom applications, thereby showing promising experimental results. For the subscriber site of passive optical networks, 10G optical network units employing energy-efficient, time- and wavelength-division-multiplexed 1.55 μm
SC-VCSELs are successfully demonstrated. On the level of metropolitan networks, the compliance of 1.3 μm SC-VCSELs with the bandwidth requirements of the 100 GBase LR-4 Ethernet standard is presented. Finally, the suitability of SC-VCSELs for imple- mentation in optical interconnects, particularly high-bandwidth active optical cables, is proven.