Akira Mizutori

Phase-noise characteristics of a 25-GHz-spaced optical frequency comb based on a phase- and intensity-modulated laser

We investigated phase-noise characteristics of both a phase/intensity-modulated laser with 25-GHz mode spacing and a mode-locked fiber laser with carrier-envelope-offset (CEO) locking. As the separation from the frequency of the continuous wave (CW) laser diode (LD) for a seed light source increases, the integrated phase noise of each comb mode of both the phase/intensity-modulated laser and supercontinuum light originating from it increases with the same slope as a function of mode number. The dependence of the integrated phase noise on mode number with the phase/intensity-modulated laser is much larger than with the mode-locked fiber laser of the CEO locking. However, the phase noise of the phase/intensity-modulated laser is extremely lower than that of the mode-locked fiber laser with CEO locking in the frequency region around the CW LD. The phase noise of the phase/intensity-modulated laser with 25-GHz mode spacing and that of the mode-locked fiber laser with the CEO locking could be estimated and were found to be almost the same at the wavelengths required in an f-to-2f self-referencing interferometer. Our experimental results indicate the possibility of achieving an offset-frequency-locked frequency comb with the phase/intensity-modulated laser.

Generation of 120-fs laser pulses at 1-GHz repetition rate derived from continuous wave laser diode

We report the first demonstration of continuous-wave laser diode based 100-fs-class pulse lasers operating at a gigahertz repetition rate without a mode-locking technique. We describe the performance of a 1-W, 120-fs optical pulse train at 1 GHz and a 1-W, 80-fs optical pulse train at 250 MHz by using a simple configuration. Sub-100-fs pulse durations are achieved by using a progressive expansion of the spectrum in the self-phase modulation process in an erbium-doped fibre amplifier. Our scheme can achieve continuously tunable repetition rate in the range of ± 20%, and develop powerful tools for use in nanomechanical systems and nanobiotechnology.

Decision-Directed Costas Loop Stable Homodyne Detection for 10-Gb/s BPSK Signal Transmission

This letter demonstrates decision-directed Costas loop stable homodyne detection for 10-Gb/s binary phase shift keying signal fiber-transmission of over 100 km utilizing frequency-stabilized several kilohertz spectral linewidth external cavity laser diodes (E-LDs). We design a phase-locked loop circuit that adopts an injection-current controlled E-LD as its local oscillator; its low phase error allows it to achieve the standard deviation of 1.9°. Owing to this phase-lock performance and the highly coherent light source, the receiver sensitivity approaches within 6 dB of the shot noise limit and the phase-locked loop recovers the optical carrier stably even if the waveform is distorted by the chromatic dispersion of 880 ps/nm after fiber transmission.

Cancellation of four-wave mixing light by carrier phase π-shift in phase-locked multi-carrier transmission

This paper proposes a simple but effective technique to cancel the waves generated by four-wave mixing in phase-locked multi-carrier transmission. The cancellation rate of FWM lights to the total that falls on a frequency grid was calculated to be 99% for some carrier phase set.

Costas Loop Homodyne Detection for 20-Gb/s QPSK Signal on the Optical Frequency Synchronous Network

This paper demonstrates a decision-directed, stable and high-sensitivity Costas loop homodyne receiver for 20-Gb/s QPSK signals based on our concept of the “optical frequency synchronous network.” The receiver sensitivity is shown to approach the theoretical shot-noise limit (as degraded by spontaneous emission) and the symbol-error rate is stably measured to be around 10-3 even though the optical signal was chirped by fiber chromatic dispersion and was delayed by 50% of the symbol period. This high-performance phase-locked loop can be realized by utilizing optical frequency stabilized (synchronized) several kilohertz spectral line-width external cavity laser diodes (E-LDs), compensating the frequency modulation (FM) response of the E-LDs, and consisting of only differential mode logical circuits. The compensation of the E-LDs FM response by the loop filter improves the receiver sensitivity by 3 dB.

12.5-Gbit/s BPSK stable optical homodyne detection using 3-kHz spectral linewidth external-cavity laser diode

This paper demonstrates 12.5-Gbit/s BPSK decision-directed homodyne detection. We design a phase-locked loop adopted injection-current controlled laser diode oscillator and its phase error to achieve the standard deviation of 2°. Owing to this phase-lock performance, the receiver sensitivity matches that of a digital coherent receiver.

New approach to achieving a carrier-envelope phase-locked frequency comb with 25-GHz mode spacing

We propose an approach to achieving a carrier-envelope phase-locked frequency comb with 25-GHz mode spacing at 1.5 μm. We demonstrate octave-spanning supercontinuum generation with the widest mode spacing ever achieved using a CW laser diode.

Optical diversity transmission and maximumratio combine in multi-core fiber to mitigate fiber non-linear distortion

This paper proposes optical diversity transmission through multi-core fiber and maximum ratio combined reception to mitigate fiber non-linear distortion which allows increased optical fiber launched power and thus enhanced optical signal to noise ratios.

Phase-synchronous chain of two multi-carrier lights spaced at 25GHz by cancelling micro-wave phase noise

This paper proposes and demonstrates a phase-synchronous chain technique for two phase-locked multi-carrier lights spaced at 25GHz generated from two seed lights of different wavelength. Each the multi-carriers is generated by a set of intensity- and phase-modulators that use a 25GHz-microwave signal, and the phase-synchronous chain doubles the number of multi-carrier by cancelling the phase noise when superimposing the optical fields of each the multi-carriers. The superimposed optical carriers show no degradation in RIN and BER measurements of a 10Gbit/s PRBS signal show no power penalty.