We have proposed and demonstrated a mode selective active-MMI (multimode interferometer) laser diode as a mode selective light source so far. This laser diode features; 1) lasing at a selected space mode, and 2) high modulation bandwidth. Based on these, it is expected to enable high speed interconnection into future personal and mobile devices. In this paper, we explain the mode selection, and the high speed modulation principles. Then, we present our recent results concerning high speed frequency response of the fundamental and first order space modes.

A frequency drift of open-loop PLL is an issue for the direct-modulation applications such as Bluetooth transceiver. The drift mainly comes from a temperature variation of VCO during the transmission operation. In this paper, we propose the optimum location of the VCO, considering the temperature gradient through the whole-chip thermal analysis. Moreover, a novel temperature-compensated VCO, employing a new biasing scheme, is proposed. The combination of these two techniques enables the power reduction of the transmitter by 33% without sacrificing the performance.

A directly modulated AM-VSB CATV erbium-doped fiber amplifier (EDFA)-repeated system that uses push-pull modulated distributed feedback (DFB) laser diodes and half-split-band technique is proposed and demonstrated. Push-pull modulation scheme enhances the frequency response of the laser diode, and thus improves the overall performance of the fiber optical CATV systems. Good performances of carrier-to-noise ratio (CNR), composite second order (CSO), and composite triple beat (CTB) were achieved for the full channel band over a 100-km single-mode fiber (SMF) transport.

We have proposed and experimentally demonstrated a novel WDM-PON downstream optical link. It is composed of a wavelength-locked FP-LD with a spectrally-sliced FP-LD as an external-injection optical source and operated as directly-modulating in a downstream-traffic transmitter. The downstream transmissions at 622 Mbps and 2.5 Gbps were performed for four channels over 25 km. The proposed WDM-PON downstream transmitter can be expanded up to eight channels by controlling an external-injection optical source of a spectrally-sliced FP-LD. Also, the transmitter has facility of multi-channel selection by controlling temperature. We verified the potential of the transmitter in WDM-PON optical link.

This paper describes a full-CMOS single-chip Bluetooth LSI fabricated using a 0.18 µm CMOS, triple-well, quad-metal technology. The chip integrates radio and baseband, which is compliant with Bluetooth Core Specification version 1.1. A direct modulation transmitter and a low-IF receiver architecture are employed for the low-power and low-cost implementation. To reduce the power consumption of the digital blocks, it uses a clock gating technique during the active modes and a power manager during the low power modes. The maximum power consumption is 75 mW for the transmission, 120 mW for the reception and 30 µW for the low power mode operation. These values are low enough for mobile applications. Sensitivity of -80 dBm has been achieved and the transmitter can deliver up to 4 dBm.

A new modulation technique for "LInear modulation with Nonlinear Translation" (LINT) is proposed. The new LINT technique is an extension of the popular LINC (LInear amplification with Nonlinear Components) technique for power efficient transmitter operation with spectrally efficient linear modulations. While providing this advantage, the LINT technique also incorporates the principles of direct modulation and provides frequency translation without the use of multiple stages of bulky upconversion circuitry. These features make the LINT method especially suitable for high frequency applications emerging at upper microwave and millimeter-wave frequencies. A two-stage 12 frequency multiplier chain is employed for frequency translation, to evaluate the feasibility of the LINT architecture for generating 16-QAM modulation at 28 GHz. The effect of imperfections on modulator performance is also considered.

A directly modulated LED or SLD (super luminescent diode) is attractive for low-cost lightwave systems such as access networks. This letter experimentally studies a directly modulated SLD followed by a gain-saturated semiconductor optical amplifier (SOA), and shows that the modulation rate is expanded in effect by the use of the gain-saturated SOA. This results from the shortened response time of the SLD due to the ASE light from the SOA and a level-equalizing effect in the gain-saturated SOA.

The chirp noise effect in a directly modulated semiconductor laser diode (LD) is experimentally studied. A previous theoretical study reported that, when an LD is directly modulated, turn-on jitter caused by spontaneous emission, combined with chromatic dispersion, becomes a source of noise in fiber transmission and restricts system performance. This paper points out that, on the contrary, imperfection in LD driving circuits causes chirp noise and limits transmission performance in actual systems. Experiments regarding dependence of chirp noise on LD modulation conditions are also presented, which show that a high relaxation oscillation frequency and a short turn-on delay time are preferable from the viewpoint of chirp noise.