Kazuomi Endo

A normally-off GaN FET with high threshold voltage uniformity using a novel piezo neutralization technique

In this paper, we successfully demonstrate a recessed gate normally-off GaN FET on a silicon substrate with high threshold voltage (V<inf>th</inf>) uniformity and low on-resistance. In order to realize high V<inf>th</inf> uniformity, a novel V<inf>th</inf> control technique is proposed, which we call “piezo neutralization technique”. This technique includes a piezo neutralization (PNT) layer formed at the bottom of the gate recess. Since the PNT layer neutralizes the polarization charges under the gate, the V<inf>th</inf> comes to be independent of the gate-to-channel span. The fabricated normally-off GaN FET with PNT structure exhibits an excellent V<inf>th</inf> uniformity (σ(V<inf>th</inf>)=18 mV) and a state-of-the-art combination of the specific on-resistance (R<inf>on</inf>A=500 mΩmm²) and the breakdown voltage (V<inf>B</inf>≫1000 V). The normally-off GaN FETs wtih PNT structure show great promise as power devices.

40-Gbit/s optical free-space transmission experiment using QPSK modulation format

Advantages of optical links like small, lightweight and power efficient terminals are practical for high data rate services of disaster preparedness and environmental research. In this paper, we demonstrate experimental results of 40-Gbit/s optical free space transmission using single-polarization quadrature phase shift keying (SP-QPSK) modulation format and digital coherent detection. The digital coherent detection enabled a high sensitivity and a tolerance to transmission impairments, which have attractive features for free space transmission system. We developed a 50-Gbit/s SP-QPSK transmitter and offline-receiver with the optical antenna system. SP-QPSK optical modulation signal with a line rate of 50-Gbit/s including 20% FEC is employed for high receiver sensitivity. A cascade by EDFAs consisting of a low noise pre-EDFA and an optical level controlled EDFA is developed to compensate for level fluctuation without degrading receiver sensitivity. Maximal ratio combining algorithms and carrier phase estimation algorithms are used at the offline-receiver for QPSK signal detection. We succeeded 4-meter indoor free space transmission having same performance as that with fiber connection using the developed system. The optical received power was -42 dBm at bit error rate of 10-3. While for outdoor 50-meter transmission, we confirmed the received bit error rate larger than FEC limit.

Development and evaluation of a digital signal processing for single polarization QPSK modulation format

Single polarization high-speed optical transmission is important for bidirectional free-space optical communication system in order to have enough isolation up-link and down-link by signal discrimination using orthogonal polarization states. At recent advance in digital coherent technology, polarization re-combining in combination with polarization diversity receiver is widely used to suppress performance degradation when polarization states of signal and local oscillator are misaligned by system vibration or shocks. However, in order to implement the re-combining function in digital signal processing, appropriate algorithm is required for realizing the system stability. In this paper, we demonstrate a new algorithm implementation for single polarization receiver with maximal-ratio-combining (MRC) technique. First we exhibited the problem for state-of-the-art polarization re-combining in instability due to singularity condition at 45-degree-azimuth elliptic polarization. In order to overcome this problem, we proposed a newly MRC algorithm added splitting ratio dependent phase correction coefficients and achieved stable re-combining at 45-degreeazimuth elliptic polarization signal. And we successfully demonstrated the stable receiving for 50-Gb/s single polarization QPSK signal with all polarization states by our digital coherent receiver platforms added the newly MRC algorithm, compared with previous-proposed MRC algorithm.