We examined fast response organic light-emitting diodes (OLEDs) for new applications of visible optical communications. For the practical use in this field, the fast transmission speed of OLEDs is required to be used in many applications, but the low carrier mobility of organic materials and the long fluorescence lifetime (FL) organic emitting materials limit the transmission speed of OLEDs. Therefore, we investigated the influence of the FL on transient properties of photoluminescence (PL), which were evaluated by the frequency dependence of PL intensity excited by a modulated violet laser diode. The FLs of several organic emitting materials were also measured, and we found the clear relationship between the FL and the transient properties of PL intensity. The fastest cutoff frequency of PL intensity was achieved 160 MHz utilizing short FL material, 1,4-bis[2-[4-[N,N-di(ptolyl)amino]phenl]vinyl]benzene. We also investigated another way to increase the transmission speed utilizing a semiconductor-organic multilayer structure, of which ZnS was used as an electron transport layer. The maximum cutoff frequency of this device was achieved 20.3 MHz, while that of the organic multilayer structure was 8.7 MHz at a sine wave voltage of 7 V and a bias voltage of 5 V. This result indicates that the high carrier mobility of the ZnS layer causes the increase in the transmission speed of OLEDs. We demonstrated one institutive demonstrator module of visible optical communications, which consisted of the transceiver module with an OLED and the pen-type receiver module with a photo-diode at a point. The movie files was transmitted at a speed of 230 kbps, when the point of a pen-type receiver module approaches the emitting area of an OLED. Furthermore, the pseudo-random signal with 1Mbps was also transmitted with this visible optical communication system. Such a system enables to connect between transceiver and receiver module without precious alignment because of the large emitting area of OLEDs. So, we think that many people, from children to aged people, are easy to get information from OLEDs without being aware of using optical communications. Furthermore, the communication field is limited near the emitting area of an OLED, resulting in a safe data transmission.
Femtosecond laser microfabrication attracts much attention due to its ability to write three-dimensional photonic devices into various transparent materials. By optimizing laser processing parameters and annealing at high temperature, low-loss straight optical waveguides are written in a pure silica glass. The minimum propagation-loss is 0.05 dB/cm at the wavelength of 1550 nm. The utility of the femtosecond laser processing is demonstrated by writing a low-loss three-dimensional 1×8 optical splitter.
Low-loss and symmetrical mode-field optical waveguides were directly written in a pure silica glass by nearinfrared
femtosecond laser processing. By optimizing laser processing parameters, such as the pulse duration
and the pulse energy of femtosecond laser pulses, we demonstrated the propagation-loss reduction and the mode-
field diameter (MFD) aspect ratio improvement. We have achieved the propagation-loss of 0.10 dB/cm and the
MFD aspect ratio of 1.0 at the wavelength of 1550 nm at the same time.