Membrane Distributed-reflector Laser Research Articles

Integration of Eight-Channel Directly Modulated Membrane-Laser Array and SiN AWG Multiplexer on Si

Eight-channel 56-Gbit/s direct four-level pulse-ampli-tude modulation is demonstrated using a 1.3-μm wavelength-division-multiplexing (WDM) photonic integrated circuit (PIC) transmitter, consisting of membrane distributed-reflector lasers heterogeneously fabricated on Si and a SiN arrayed-waveguide-grating multiplexer. Direct bonding, epitaxial regrowth, and low-temperature SiN-waveguide fabrication technologies are employed for the integration on Si. Using a nonlinear equalizer, we operated the fabricated WDM PIC transmitter with a low power consumption of 403 fJ/bit for overall 448 Gbit/s signals. The total eight-channel bit-error rate was 2.7 × 10−3, which is less than the 7%-overhead hard-decision forward-error-correction limit.

Heterogeneously Integrated Membrane Lasers on Si Substrate for Low Operating Energy Optical Links

High demand exists for low operating energy optical links that use wavelength division multiplexing technologies in datacenter networks. Thus, we fabricate a directly modulated membrane distributed-reflector laser with low operating energy on a thermally oxidized silicon (Si) substrate. Because we use epitaxial growth to bury an active region on a directly bonded InP-based membrane, it needs to be kept within a critical thickness, which is related to the growth temperature and the thermal expansion coefficients of materials. In previous studies, we used 250-nm-thick structures, causing relatively large series resistance that limited device performance on such aspects as energy cost and output power. In this study, we increase the III–V membrane thickness to 350 nm, which is close to the calculated critical thicknesses. We achieve the same high crystal quality of multiquantum-wells found in our previous studies. The fabricated laser shows a differential resistance of 72 Ω and thermal resistance of 982 K/W. Thanks to a reduction in bias voltage, the laser can be directly modulated at 25.8 Gbit/s with an energy cost of 97 fJ/bit. In addition, due to a reduction in heat generation, direct modulation with a 50-Gbit/s non return to zero signal is demonstrated by increasing bias current up to 10 mA.

20-Gbit/s direct modulation of GaInAsP/InP membrane distributed-reflector laser with energy cost of less than 100 fJ/bit

We demonstrated 20-Gbit/s direct modulation with a low energy cost of the membrane distributed-reflector (DR) laser whose static lasing characteristics were previously reported by Tomiyasu et al. in Appl. Phy. Express 10, 062702 (2017). As a small-signal modulation characteristic, a modulation current efficiency of 12 GHz/mA1/2 was obtained. In 20-Gbit/s direct modulation using a non-return-to-zero 211 − 1 word-length pseudorandom bit sequence signal, a clear eye opening with a bit-error rate of 6.4 × 10−10 was obtained with a bias current of 1.06 mA and a bias voltage of 1.76 V, which resulted in a record low energy cost of 93 fJ/bit among membrane distributed-feedback (DFB) and DR lasers ever reported.

High Efficiency Operation of GaInAsP/InP Membrane Distributed-Reflector Laser on Si

Record high power conversion efficiency operation of a GaInAsP/InP membrane distributed-reflector laser on silicon substrate was realized. In order to enhance the efficiency, both an improvement of external differential quantum efficiency and a reduction of differential resistance were achieved. As a result, an external differential quantum efficiency from the front facet of 36%, and a maximum power conversion efficiency of 14.6% were obtained for a device with a 40- $\mu \text{m}$ -long active region.

High-Efficiency Operation of Membrane Distributed-Reflector Lasers on Silicon Substrate

To advance on-chip optical interconnections, membrane distributed-reflector (DR) lasers with low threshold current and high-efficiency operation at one side output were realized. First, a membrane distributed Bragg reflector (DBR) laser with 80- μ m-long active section and 50- μ m-long DBR section was fabricated to clarify the DBR reflectivity. An external differential quantum efficiency of 35% for the output from the front facet was obtained, and the DBR reflectivity was estimated to be 75%. Next, a membrane DR laser with 61- μ m-long distributed feedback section and 50- μ m-long DBR section was fabricated. A threshold current of 0.48 mA, external differential quantum efficiency from the front side waveguide of 26%, and light output ratio from the front to the rear sides of 13 were obtained. The lasing spectrum showed a single-mode operation with a side-mode suppression-ratio (SMSR) of 40 dB. Finally, small-signal direct modulation was carried out and a modulation current efficiency factor of 7.9 GHz/mA1/2 and 7 GHz/mA1/2 were, respectively, obtained for the 30- μ m-long and 61- μ m-long devices.

High-differential quantum efficiency operation of GaInAsP/InP membrane distributed-reflector laser on Si

The high-external differential quantum efficiency operation of a GaInAsP/InP membrane distributed-reflector laser bonded on a Si substrate was achieved by adopting a short cavity design and reducing the waveguide loss and differential resistance. A threshold current of 0.21 mA, an external differential quantum efficiency of 32% for the front-side output, and a power-conversion efficiency of 12% were obtained with a 32-µm distributed feedback section length, a 50-µm distributed-Bragg-reflector section, and a 0.8-µm stripe width. A side-mode suppression ratio of 41 dB was obtained at a bias current of 1 mA.

90 °C continuous-wave operation of GaInAsP/InP membrane distributed-reflector laser on Si substrate

The temperature dependence of a GaInAsP/InP membrane distributed-reflector laser bonded on a Si substrate — which showed a low threshold current (0.29 mA) and a relatively high differential quantum efficiency (23% from the front side) at 20 °C — was measured. A characteristic temperature of the threshold current, T0, of 84 K and a sub-mA threshold current operation up to 90 °C were obtained under a continuous-wave (CW) condition. Furthermore, single-mode operation up to 80 °C was also obtained.

Membrane distributed-reflector laser integrated with SiOx-based spot-size converter on Si substrate.

We demonstrate monolithic integration of a 50-μm-long-cavity membrane distributed-reflector laser with a spot-size converter, consisting of a tapered InP wire waveguide and an SiOx waveguide, on SiO2/Si substrate. The device exhibits 9.4-GHz/mA0.5 modulation efficiency with a 2.2-dB fiber coupling loss. We demonstrate 25.8-Gbit/s direct modulation with a bias current of 2.5 mA, resulting in a low energy cost of 132 fJ/bit.

Energy Cost Analysis of Membrane Distributed-Reflector Lasers for On-Chip Optical Interconnects

The power consumption of lateral-current-injection semiconductor membrane distributed-reflector lasers with a λ/4 shift region has been theoretically evaluated, in terms of their ultralow-power-consumption and high-speed modulation operations. This paper contains an investigation into the optimal structure of the membrane laser in terms of its energy cost, for use in on-chip optical interconnections. The total power consumption was evaluated, taking Joule heating into account by assuming the device resistance. It was found that the large Joule heating effect present in shorter cavities limits a reduction of their power consumption. As a result, an energy cost of 63 fJ/bit can be obtained for 10 Gb/s data transmission, while maintaining the necessary power output required for a cavity length of 12 μm. We have provided a guide for designing microcavity lasers in terms of their Joule heating power.