Hiroyuki Yoshinaga

Single-stripe tunable laser with Chirped Sampled Gratings fabricated by nanoimprint lithography

Fabrication of diffraction gratings of Chirped Sampled Grating Distributed Reflector (CSG-DR) laser by nanoimprint lithography (NIL) has been demonstrated. The diffraction gratings with highly uniform linewidth and period have been successfully fabricated by the combination of the reverse-tone NIL and precise etching techniques. The fabricated CSG-DR laser using NIL shows sufficiently wide tuning range of 40 nm as we designed.

Application of Nanoimprint Lithography to Fabrication of Distributed Feedback Laser Diodes

We have succeeded in employing nanoimprint lithography (NIL) to form the diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal-tone NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. We have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for the fabrication of DFB LDs.

26 Gbit/s Direct Modulation of AlGaInAs/InP Lasers with Ridge-Waveguide Structure Buried by Benzocyclobutene Polymer

26 Gbit/s direct modulation of 1.3 °m wavelength AlGaInAs/InP distributed feedback lasers with the ridge-waveguide structure (ridge width of 1.0 °m) buried by the benzocyclobutene polymer was achieved. The high electrical bandwidth of more than 20 GHz was acquired with this ridge-waveguide structure. Consequently, a clear eye-opening with extinction ratio of 6 dB was confirmed in the measurement temperature of 25 °C.

Study of reactive ion etching for reverse tone nanoimprint process

We have used reverse nanoimprint for fabricating diffraction gratings of distributed feedback laser diodes. Generation of residues in the etching process of resin is a serious issue leading to poor line edge roughness of the grating patterns. We have found that the residues are composed of oxide products from Si-containing resin. We have successfully suppressed the generation of the residues by optimizing oxygen partial pressure of reactive ion etching (RIE). We have also succeeded in effectively removing the residues by utilizing sputtering effect of RIE.

Evaluation of nanoimprint lithography as a fabrication process of phase-shifted diffraction gratings of distributed feedback laser diodes

The authors have succeeded in employing nanoimprint lithography (NIL) to form diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. LDs show high long-term stability in threshold current. The authors have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for fabricating DFB LDs.

Evaluation of nanoimprint lithography as a fabrication method of distributed feedback laser diodes

We have succeeded in employing nanoimprint lithography (NIL) to form the diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reverse NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. We have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for the fabrication of DFB LDs.

Highly Uniform Fabrication of Diffraction Gratings for Distributed Feedback Laser Diodes by Nanoimprint Lithography

We have used a nanoimprint technique to fabricate diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. We have aimed to establish the fabrication process featuring the high reproducibility of the period and linewidth of grating corrugations, which leads to an increase in the production yield of DFB LDs. The combination of the reverse tone nanoimprint and optimized etching techniques has contributed to the improvement of the reproducibility. The variation in grating period has been less than 0.2 nm and the variation in linewidth has been less than 10 nm over the six wafers. The results of this study indicate that our fabrication process for the diffraction gratings utilizing the nanoimprint technique has a high potential for the fabrication of DFB LDs.

Low power-consumption quantum cascade lasers

Quantum cascade lasers (QCLs) are promising light sources for real time high-sensitivity gas sensing in the mid-infrared region. For the practical use of QCLs as a compact and portable gas sensor, their power-consumption needs to be reduced. We report a successful operation of a low power-consumption distributed feedback (DFB) QCL. For the reduction of power consumption, we introduced a vertical-transition structure in a core region to improve carrier transition efficiency and reduced the core volume. DFB-QCL epitaxial structure was grown by low-pressure OMVPE. The core region consists of AlInAs/GaInAs superlattices lattice-matched to InP. A first-order Bragg-grating was formed near the core region to obtain a large coupling coefficiency. A mesa-strip was formed by reactive ion etching and a buried-heterostructure was fabricated by the regrowth of semi-insulating InP. High-reflective facet coatings were also performed to decrease the mirror loss for the reduction of the threshold current. A device (5x500μm) operated with a single mode in the wavelength region from 7.23μm to 7.27μm. The threshold current and threshold voltage under CW operation at 20 °C were 52mA and 8.4V respectively. A very low threshold power-consumption as low as 0.44 W was achieved, which is among the lowest values at room temperature to our knowledge.

Low Power-Consumption Distributed Feedback Quantum Cascade Laser

We fabricated a low power-consumption mid-infrared distributed feedback quantum cascade laser. A very low threshold power-consumption of 0.44 W was obtained under CW operation at 20°C.

Compact and low power-consumption MIR DFB-QCL with To-CAN package for portable sensor

Sensing method with Quantum Cascade Laser (QCL) as a light source is expected to offer a high sensitivity, a short measurement time, and a good portability compared to conventional methods. However, commercially available QCLs have high power-consumption of several W. Therefore, a large power supply is required to drive QCL, and most of the input power is released as heat, leading to the necessity for a large cooling system. For these reasons, portable gas sensors using QCL have not been realized. To address this issue, we had recently developed a low power-consumption DFB-QCL in the 7μm wavelength region. In this study, we developed a compact and low power consumption QCL module with Φ 15.4 mm To-CAN package. The QCL device, a thermoelectric cooler (TEC), a thermistor and a window were assembled in this package. The threshold power-consumption and the maximum output power were 0.97 W and 37 mW at 20°C, respectively under continuous wave driving. In addition, it maintained a single mode operation between 20°C and 80°C without a mode hopping. The performance of this QCL module as a light source for gas sensing was evaluated by measuring the mid-infrared absorption spectrum of the methane gas with a multi pass type gas cell. High sensitive methane gas detection was achieved, which was comparable to that of the conventional high heat load (HHL) packaged QCL module reported by other group. It is expected that a compact and low-cost MIR gas sensor with high-sensitivity can be realized with our QCL module.