The power transmission technology to the equipment is remained behind in contrast to the progress of wireless communication. The existence of wiring and its connections greatly restrict the location of use, installation, maintenance, and configuration of the equipment. Solutions of wireless power transmission may lead to major changes in society, such as new services and new industries as well as convenience. Wireless power transmission such as electromagnetic induction is beginning to be put into practical use in smartphone, etc., however there are issues in extremely short transmission distance, large size, heavy, and so on. Optical wireless power transmission will be an important technology for expanding the field of wirelessly usable equipment due to features such as small size, short, mid and long distance transmission. On the other hand, there are some problems to be solved such as efficiency and safety. In this presentation, I would like to discuss the possibility and challenges of optical wireless power transmission using VCSEL as a high output and high efficiency light source.
We presented the high temperature operation of 1200-nm band highly strained GaInAs/GaAs ridge-waveguide lasers. Active layer consists in three quantum wells with highly strained GaInAs. The In composition is 32%. The maximum operating temperature reaches at over 200°C and temperature characteristic T<sub>0</sub> is 222K at 30-80°C with continuous wave operations, showing excellent temperature characteristics of highly strained QWs. We obtained a relaxation
oscillation frequency of 2 GHz at 170°C.
Nitride-based VCSELs are very attractive for extending the wavelength range of the parallel optoelectronic systems. The short-wavelength GaN-VCSELs are expected as lights, displays and parallel read/write heads of optical memories. On the other hand, the dilute-nitride GalnNAs on a GaAs will become a key for long-wavelength VCSELs utilized in high-speed and low cost network systems covering from very short to long-distance. Critical issues for realizing VCSELs are the formation of the high quality cavity as well as the growth of the high quality active layer. For the GaN-based system, the crystal quality of the active layer has been improved. Fabrication technologies of high reflectivity mirrors and a short cavity are necessary for the vertical cavity structure. For GalnNAs-based VCSELs, matured GaAs-based VCSEL technologies such as DBRs and selective oxidation are applicable. In these few years, the GalnNAs crystal quality have been improved and the VCSEL performances become practical level. In this paper, fabrication technologies of the vertical cavity for GaN-based VCSELs are presented and the current state of GalnNAs VCSELs is reviewed.
We demonstrated highly strained GaInAs/GaAs QW VCSELs emitting at 1.16 micrometers . The fabricated device shows the record low threshold current density and high efficiency in 1.1-1.2 micrometers wavelength range. The VCSEL structure was monolithically grown on a (100) n-type GaAs substrate by a low-pressure metalorganic vapor phase epitaxy (MOVPE). The active region consists of triple 8 nm thick Ga<SUB>0.64</SUB>In<SUB>0.36</SUB>As TQWs separated by 25 nm GaAs barrier layers. The compressive strain of QWs is 2.3%. The threshold current is 3 mA for a 10micrometers ~10micrometers oxide device, corresponding to a threshold current density of 3 kA/cm<SUP>2</SUP>. We achieved the maximum output power of over 2 mW and a slope efficiency of 0.3 W/A at 25 degree(s)C, which are the record data for 1.2 micrometers band GaInAs VCSELs. The maximum CW operating temperature is 85 degree(s)C. The threshold current is almost constant in the temperature range of 20-70 degree(s)C which results from appropriate wavelength matching between gain peak and lasing mode. The temperature dependence of the lasing wavelength is 0.07 nm/K. We present the details of temperature characteristics of the fabricated VCSEL and discuss a possibility of uncooled GaInAs/GaAs VCSELs for high speed LANs.
The GaInNAs is an attractive material for long wavelength lasers on a GaAs substrate and the GaInNAs vertical cavity surface emitting laser (VCSEL) is a viable candidate for low cost and high performance lasers of 1.3micrometers wavelength networks due to excellent temperature characteristics and manufacturing capability of VCSELs. We have successfully grown GaInNAs quantum wells by chemical beam epitaxy and investigated the growth condition toward better crystal quality by employing a radical nitrogen source and thermla annealing. Lasing characteristics of CBE grown 1.2 micrometers GaInNAs lasers are a threshold current density of less than 1kA/cm<SUP>2</SUP>, and high temperature operation up to 170 degrees C with an excellent slope efficiency change below -0.004dB/K. A characteristic temperature of 270K is also demonstrated. GaInNAs quantum dots were also investigated for the further progress of GaInNAs lasers. The growth of self-organized Qds and a lasing operation at 77K was demonstrated.
Gigabit/s-LANs with 0.85 micrometers vertical cavity surface emitting lasers (VCSELs) are now going into the market. But, their link length is limited to be shorter than 500 m due to modal dispersion of employed multi-mode fibers and various modal problems should be considered in system design. The next step will be to develop long wavelength VCSELs matching to a single model fiber with longer link lengths of several km and higher data rates. Long wavelength single mode fiber datacom should be advantageous because of avoiding modal noise and relaxing eye safe issues. Recently, we demonstrated a 1.2 micrometers wavelength highly strained GaInAs/GaAs quantum well edge emitting laser, which exhibits reasonably low threshold and excellent temperature characteristics. The threshold current is as low as 13 mA for an as-cleaved 5 X 380 micrometers laser. A characteristic temperature T<SUB>0</SUB> under pulsed operating is as high as 140 K. This is the record high T<SUB>0</SUB> in 1.2-1.3 micrometers wavelength band. Even under 'heatsink-free' cw operations, T<SUB>0</SUB> was much higher than that of conventional 1.3 micrometers GaInAsP/InP systems. We can avoid thermoelectric coolers as well as heatsink itself, thus, an all-plastic mold package without heatsink may give us a drastic cost reduction. It is interesting to note than commercial standard single mode fibers are designed to locate a cut-off wavelength at around 1.2 micrometers for the purpose of production tolerance. A question arises: whether this 1.2 micrometers wavelength band can be utilized for single mode fiber datacom or not.We have demonstrated a 2 Gb/s-5km single mode fiber data transmission experiment using a fabricated 1.22 micrometers uncooled GaInAs/GaAs laser. We believe that the VCSEL technology may drastically improve the transmitter performances at this new wavelength band and this 1.2 micrometers Gigabit LAN may become realistic by extensive developments. In this paper, we would like to discuss a possibility of high speed datacom using a newly developed 1.2 micrometers highly strained GaInAs/GaAs lasers.