Yasuhiko Tada

1-Tb/s (100 x 10 Gb/s) super-dense WDM transmission with 25-GHz channel spacing in the zero-dispersion region employing distributed Raman amplification technology

We achieve 1 Tb/s (100/spl times/10 Gb/s) super-dense WDM (super DWDM) transmission with 25-GHz channel spacing (0.4 bit/s/Hz spectral efficiency) in the zero-dispersion region over a 4/spl times/80 km dispersion-shifted fiber by employing backward pumped distributed Raman amplification and forward error correction. By adopting bi-directional pumping, we present experimental results showing that the transmission distance is extended approximately threefold to 1040 km.

Novel modulation and detection for bandwidth-reduced RZ formats using duobinary-mode splitting in wideband PSK/ASK conversion

This paper proposes a novel duobinary-mode-splitting scheme that uses wideband phase-shift-keying (PSK)/amplitude-shift-keying (ASK) conversion for modulation and detection of bandwidth-reduced return-to-zero (RZ) modulation formats. We have first demonstrated that the proposed scheme greatly simplifies the modulation process of the duobinary carrier-suppressed RZ format (DCS-RZ) based on baseband binary nonreturn-to-zero (NRZ) modulation. We also proposed carrier-suppressed RZ differential-phase-shift-keying format (CS-RZ DPSK) as a novel bandwidth-reduced RZ format by applying the proposed scheme in the detection process. These novel RZ formats are shown to be very useful for dense wavelength-division multiplexed (DWDM) transport systems using high-speed channels, over 40 Gb/s, with spectrum efficiencies higher than 0.4 b/s/Hz. We demonstrate that the proposed modulation and detection scheme greatly simplifies the DWDM transmitter and receiver configuration if the periodicity of the optical PSK/ASK conversion filter equals the WDM channel spacing. The large tolerance of the formats against several fiber nonlinearities and their wide dispersion tolerance characteristics are tested at the channel rate of 43 Gb/s with 100-GHz spacing. The novel CS-RZ DPSK format offers higher nonlinearity tolerance against cross-phase modulation than does the DCS-RZ format.

WDM upgrading of an installed submarine optical amplifier system

This paper describes the WDM upgrading of a submarine optical amplifier system installed for single-channel transmission in commercial use. The key technologies used include 10-Gb/s high-speed forward error correction, preemphasis to overcome the optical passband-width constraint, optimized signal pulse format that leads to partial return-to-zero (PRZ) signaling, and dispersion compensation to suppress the degradation caused by fiber nonlinearity and dispersion. It also reports field experimental results on upgraded system performance. The results confirm that simply upgrading the terminal equipment allows the transmission capacity of a submersed line consisting of submarine repeaters and cables to be flexibly increased from 10 Gb/s to 40 Gb/s per fiber pair.

Duplex transmitter configuration for bit-error-free optical FDM cross-connect system

A cross-connect bit rate of 156 Mb/s was achieved without loss of a bit by the duplex transmitter using two DBR lasers and an optical LiNbO/sub 3/ switch with a switching speed of 4 GHz. The bit-error-free operation was experimentally verified by observing eye patterns. The proposed optical cross-connect scheme supports cross-connect bit rates up to 2 Gb/s if a 4-Gb/s switching device is employed. Only widely tunable lasers are needed to construct the optical cross-connect system utilizing the duplex transmitter, and the stringent requirements of frequency switching speed for lasers can be eliminated.<<ETX>>

1.2 Tbit/s (30/spl times/42.7 Gbit/s ETDM optical channel) WDM transmission over 376 km with 125 km spacing using forward error correction and carrier-suppressed RZ format

We demonstrate 1.2 Tbit/s (30/spl times/42.7 Gbit/s electrical time division multiplexing (ETDM) optical channel) WDM transmission over three 125 km-spans (span loss: 31.5 dB average) using carrier-suppressed return-to-zero format and forward error corrections in 50 nm tellurite-based EDFA gain band.

100 ps frequency switching without bit loss for a 10 Gb/s ASK modulated signal

100-ps distributed feedback (DFB) laser frequency-switching without bit loss is demonstrated for ASK-modulated signal data rates of 2.5 and 10 Gb/s over the dynamic tuning ranges of 80 and 20 GHz, respectively. The number of allowable destinations as a function of bit rates suggests that a cross-connect switch based on frequency-division-multiplexed technologies can handle ten channels, each 10 Gb/s, if a DFB transmitter with a dynamic tuning-range of about 200 GHz is employed.<<ETX>>

Fast and stable frequency switching employing a delayed self-duplex (DSD) light source

A novel frequency-agile light source, the delayed self-duplex (DSD) light source, is constructed using a tunable DBR laser and an optical switch. The DSD light source overcomes the tuning speed limitation of the tunable laser, and also offers suppressed frequency drift due to the thermal effect by employing periodic laser tuning. For 160-GHz-spaced frequency switching, the 4-ns switching time and 2.2-GHz thermal frequency drift inherent to the DBR laser are reduced to 0.4 ns and 0.3 GHz, respectively.<<ETX>>

Performance degradation of long-distance optical WDM networks due to bandwidth limitations of wavelength multiplexers and demultiplexers

The characteristics of an optical signal passing though optical add/drop multiplexing (ADM) devices, which mainly consist of optical (de) multiplexers, are experimentally evaluated through 10 Gb/s/spl times/8-channel dispersion-managed wavelength-division multiplexing (WDM) transmission over 4000 km of dispersion-shifted fiber (DSF). A novel recirculating loop configuration is used to change the insertion positions and intervals of the ADM devices. Inserting a single ADM device near the receiver causes more electrical signal-to-noise ratio (SNR) degradation than if inserted near the transmitter. Periodically inserting ADM devices at small intervals into the transmission line also causes significant SNR degradation, The SNR degradation is sensitive to optical signal wavelength detuning. The resulting SNR degradation basically arises from the bandwidth limitations of the wavelength (de)multiplexers, and is exacerbated in long-distance transmission by the optical signal spectral broadening due to fiber nonlinearity.

Design consideration on a DBR-laser transmitter for fast frequency-switching in an optical FDM cross-connect system

The design of a laser transmitter for fast frequency switching in an optical frequency-division-multiplexing (FDM) cross-connect system that uses tunable laser diodes and passive frequency-channel selection filters is considered. Small-signal analysis of the laser rate equation is used to estimate laser switching time. The results show that the frequency switching time depends on the allowable frequency detuning as well as the frequency spacing and the carrier lifetime. Frequency switching time is obtained from the calculation of power penalty due to the frequency detuning at the receiver section with ASK and FSK direct detection schemes. To achieve frequency switching without bit loss in the cross-connect module, switching times of 4-200 ns are required to change frequencies of 500 GHz. This suggests that a duplex transmitter configuration using two laser diodes must be employed to perform fast frequency switching and bit-error-free operation in a cross-connect system. The thermally induced frequency drift is experimentally evaluated. The feasibility of the duplex transmitter configuration is experimentally verified with a 156 Mb/s cross-connect unit. >

10 Gbit/s/spl times/4 ch WDM upgrading system on an installed optical amplifier submarine system designed for single-wavelength channel transmission

We describe the design concepts and performance of a commercially available 10 Gbit/s/spl times/4 ch WDM system applied for an installed fiber submarine system originally designed for single wavelength channel transmission.