Takaaki Kakitsuka

Photonic crystal lasers using wavelength-scale embedded active region

Lasers with ultra-low operating energy are desired for use in chip-to-chip and on-chip optical interconnects. If we are to reduce the operating energy, we must reduce the active volume. Therefore, a photonic crystal (PhC) laser with a wavelength-scale cavity has attracted a lot of attention because a PhC provides a large Q-factor with a small volume. To improve this devices performance, we employ an embedded active region structure in which the wavelength-scale active region is buried with an InP PhC slab. This structure enables us to achieve effective confinement of both carriers and photons, and to improve the thermal resistance of the device. Thus, we have obtained a large external differential quantum efficiency of 55% and an output power of ?10?dBm by optical pumping. For electrical pumping, we use a lateral p?i?n structure that employs Zn diffusion and Si ion implantation for p-type and n-type doping, respectively. We have achieved room-temperature continuous-wave operation with a threshold current of 7.8??A and a maximum 3?dB bandwidth of 16.2?GHz. The results of an experimental bit error rate measurement with a 10?Gbit?s?1 NRZ signal reveal the minimum operating energy for transferring a single bit of 5.5?fJ. These results show the potential of this laser to be used for very short reach interconnects. We also describe the optimal design of cavity quality (Q) factor in terms of achieving a large output power with a low operating energy using a calculation based on rate equations. When we assume an internal absorption loss of 20?cm?1, the optimized coupling Q-factor is 2000.

Room-temperature continuous-wave operation of lateral current injection wavelength-scale embedded active-region photonic-crystal laser

We have developed a wavelength-scale embedded active-region photonic-crystal laser using lateral p-i-n structure. Zn diffusion and Si ion implantation are used for p- and n-type doping. Room-temperature continuous-wave lasing behavior is clearly observed from the injection current dependence of the output power, 3dB-bandwidth of the peak, and lasing wavelength. The threshold current is 390 μA and the estimated effective threshold current is 9.4 μA. The output power in output waveguide is 1.82 μW for a 2.0-mA current injection. These results indicate that the embedded active-region structure effectively reduce the thermal resistance. Ultrasmall electrically driven lasers are an important step towards on-chip photonic network applications.

All-optical on-chip bit memory based on ultra high Q InGaAsP photonic crystal

We demonstrate all-optical bit memory operation with photonic crystal (PhC) nanocavities based on an InGaAsP substrate with a band gap at a wavelength of about 1.3 microm. The optical bistability is based on a refractive index modulation caused by carrier-plasma dispersion. The operating energy required for switching is only 30 fJ, and the minimum optical bias power for bistability is 40 microW, which is about one hundred times less than that required for laser based bistable memories.

Ultralow Operating Energy Electrically Driven Photonic Crystal Lasers

The introduction of the photonic crystal (PhC) wavelength-scale cavity as a laser cavity enables us to obtain both ultralow threshold current and operating energy. These parameters are essential when using the transmitters in chip-to-chip and on-chip interconnections. To improve the device performance, we employ an ultracompact embedded active region that we call a lambda-scale embedded active-region PhC laser or LEAP laser. We have developed an electrically driven LEAP laser, which operates under room-temperature continuous-wave conditions. To fabricate the electrically driven LEAP laser, we used Zn thermal diffusion and Si ion implantation, respectively, for p-type and n-type doping in an undoped InP layer. However, with previous fabricated devices there was a large leakage current through the substrate and the threshold current was 0.39 mA, which is larger than the expected threshold obtained by optical pumping. To reduce the leakage current, we propose using an InAlAs sacrificial laser instead of an InGaAs layer. The leakage current path through the substrate is effectively suppressed, and as a result, the threshold current is reduced to 7.8 μA, which is the lowest threshold current reported for any laser. Furthermore, the LEAP laser operates at up to 95 °C by using an InGaAlAs-based multiple quantum well structure. We also describe the dynamic characteristics of the laser. The LEAP laser exhibits a maximum 3-dB bandwidth of 16.2 GHz and the modulation current efficiency factor is 53.8 GHz/mA0.5 or 1.7 GHz/μA0.5, which is four times that of a vertical cavity surface-emitting laser. The device is directly modulated by a 12.5-Gb/s nonreturn-to-zero signal with a bias voltage of 1.6 V and a bias current of 109 μA, resulting in an energy cost of 14.0 fJ/b. This is the smallest operating energy for any laser. These results indicate that the LEAP laser is highly suitable for use as a transmitter in computercom applications.

Full

A widely tunable double-ring resonator-coupled laser is developed for use in a photonic packet switching system. By using a double-ring resonator as a wavelength-selective filter, full -band tuning operation is achieved with a low tuning current, and the fabrication process is simpler than that used for distributed-Bragg-reflector-type tunable lasers. A tuning range of 50.0 nm with a current injection of less than 7 mA is successfully demonstrated.

C

An inherently mode-hop-free tuning of 4.5 nm was achieved with a short-active-region distributed Bragg reflector (DBR) laser. The DBR laser contains no phase-adjustment electrode, which greatly simplifies the tuning. Fast tuning of several nanoseconds and error-free operation for 2.5-Gbit/s direct modulation were also observed. A 4-ch array of short-active-region DBR lasers achieved wavelength tuning of 12.5 nm.

-Band Tuning Operation of Semiconductor Double-Ring Resonator-Coupled Laser With Low Tuning Current

A monolithic widely tunable semiconductor laser based on a double-ring resonator is developed for use in a wavelength-routing switch. By using the double-ring resonator as a wavelength-selective filter, operation over a wide wavelength tuning range is achieved with a low tuning current. This low-tuning-current operation makes the laser very promising as a high-speed tunable light source for a wavelength-routing-based switch by effectively suppressing the thermal wavelength drift induced by current injection. In addition, the laser fabrication process is simpler compared to conventional distributed Bragg reflector tunable lasers. A tuning range of 50.0 nm, covering the entire C-band, is successfully demonstrated with an injection current of less than 5.2 mA. The wavelength drift caused by the thermal transients is less than 5 GHz.

Inherently mode-hop-free distributed Bragg reflector (DBR) laser array

We describe the growth of InP layer using an ultrathin III-V active layer that is directly bonded to SiO₂/Si substrate to fabricate a buried heterostructure (BH) laser. Using a 250-nm-thick bonded active layer, we succeeded in fabricating a BH distributed feedback (DFB) laser on SiO₂/Si substrate. The use of a lateral current injection structure is important for forming a p-i-n junction using bonded thin film. The fabricated DFB laser is directly modulated by a 25.8-Gbit/s NRZ signal at 50°C. These results indicate that our fabrication method is a promising way to fabricate high-efficiency lasers at a low cost.

Semiconductor Double-Ring-Resonator-Coupled Tunable Laser for Wavelength Routing

Device quality InAs/InGaAs multiple quantum well (MQW) structures were grown on InP substrates by metalorganic vapor phase epitaxy (MOVPE) and applied to lasers emitting at wavelengths longer than 2 mum. InAs/InGaAs MQWs with flat interfaces were obtained by adjusting the growth temperature between 460 degC and 510 degC. The photoluminescence peak wavelength of the MQWs increases from 1.93 to 2.47 mum as the thickness of InAs quantum wells increases from 2 to 7 nm. The structural and optical properties remained almost unchanged even after annealing at 620 degC. For 40-mu m-wide stripe broad-area lasers with 5-nm-thick InAs quantum wells, a lasing wavelength longer than 2.3 mum and an output power higher than 10 mW were achieved under continuous-wave operation at a temperature of 25 degC. These results indicate that InAs/InGaAs MQW structures grown by MOVPE are very useful for the active region of 2 mum wavelength lasers.

Directly modulated buried heterostructure DFB laser on SiO 2 /Si substrate fabricated by regrowth of InP using bonded active layer

In order to achieve an accurate design of polarization-insensitive semiconductor optical amplifiers based on tensile strained bulk InGaAsP, the reduction of strain in the active layer of the buried heterostructure and its influence on polarization sensitivity are analyzed numerically for the first time. The gain calculation, including the strain distribution in the active layer, is examined based on the k /spl middot/ p method for the different active layers. It is found that the strain introduced during the epitaxial growth is strongly reduced after regrowth of the burying layer. In an active layer having the aspect ratio of 1 : 4, the strain reduction causes more than a 0.5-dB deviation in the polarization sensitivity of the gain. From a comparison with the experimental results, it is shown that including the effect of the burying layer in the calculation gives an accurate determination of the amount of strain for the polarization independence.