Takushi Kazama

High-reflectivity-resolution coherent optical frequency domain reflectometry using optical frequency comb source and tunable delay line

We propose a high-reflectivity-resolution coherent optical frequency domain reflectometry (OFDR) with a novel scheme of delay shift averaging (DSAV) by using an optical frequency comb source and a tunable delay line to suppress the fading noise. We show theoretically and experimentally that the novel DSAV scheme is equivalent, in realizing a high reflectivity resolution, to our previously reported frequency shift averaging (FSAV) of the same number of teeth of the optical frequency comb [1], but does not need the expensive narrow-pass-band tunable optical filter required in the previous scheme. Furthermore, by using this new method in combination with FSAV, better reflectivity-resolution is obtained compared to using only conventional FSAV with a single-wavelength laser source.

Simultaneous nonlinearity mitigation in 92 × 180-Gbit/s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation

We experimentally demonstrated the simultaneous nonlinearity mitigation of PDM-16QAM WDM signals using complementary-spectrally-inverted optical phase conjugation (CSI-OPC). We achieved reserved-band-less, guard-band-less, and polarization independent OPC based on periodically poled LiNbO3 waveguides. By employing the CSI-OPC, 2.325-THz-band (93 × 25 GHz) complementary spectral inversion was achieved while retaining the original WDM bandwidth. A Q2-factor improvement of over 0.4 dB and a 5120 km transmission with a Q2-factor above the FEC limit were confirmed using a 10-channel WDM transmission at the signal band center and signal band edge. We then demonstrated the mitigation of the nonlinear impairments in a 3840 km long-haul WDM signal transmission for all 92-channel 180-Gbit/s PDM-16QAM quasi-Nyquist-WDM signals.

Single-Chip Parametric Frequency Up/Down Converter Using Parallel PPLN Waveguides

We propose a novel structure for a monolithically integrated parametric frequency up and down converter using two parallel periodically poled lithium niobate (PPLN) waveguides combined with a reflective wavelength-division multiplexer (WDM) to realize second-harmonic-pumped parametric mixing on one chip. The parallel configuration of the PPLN waveguides provides a long interaction length for efficient conversion. The reflective WDM used for (de)multiplexing 0.78- and 1.56-μm light consisted of two multimode interference (MMI) (de)multiplexers and a dichroic mirror. The utilization of the wavelength selectivity of both the MMI (de)multiplexers and the dichroic mirror enabled us to achieve high isolation of the fundamental light for generating a second-harmonic pump. We fabricated the integrated device using a direct-bonded LiNbO3 ridge waveguide. Frequency conversion achieved with two-stage second-harmonic generation and difference frequency generation processes was successfully demonstrated with a high isolation of the fundamental and a parametric gain of the signal in the integrated device.

Low-noise phase-sensitive amplifier for guard-band-less 16-channel DWDM signal using PPLN waveguides

A X(2)-stage SHG/OPA configuration enabled low-crosstalk and guard-band-less DWDM signal amplification. We demonstrated a 5.6-dB link-SNR improvement and a low NF of 2.1 dB below the 3-dB quantum limit in simultaneous 16-channel-signal amplification.

Optical pump phase locking to a carrier wave extracted from phase-conjugated twin waves for phase-sensitive optical amplifier repeaters

In this paper, an optical phase-locked loop assisted by sum-frequency and second-harmonic generation (SS-OPLL) for frequency nondegenerate optical parametric phase-sensitive amplifier repeaters is experimentally demonstrated. First, theoretical derivations show that carrier extraction from phase-conjugated twin waves (PCTWs) and reference light generation are achieved by sum-frequency generation; therefore, the SS-OPLL circuit enables optical phase locking between PCTWs and a pump wave by a simple architecture based on a balanced OPLL. Then, optical phase locking between 20-Gbit/s quadrature phase-shift keying PCTWs and an individual pump source is experimentally demonstrated. Experimental results indicate that phase errors were reduced during the SS-OPLL operation.

Optical parametric amplifiers based on PPLN waveguides for long-Haul transmission

We review the capabilities of optical parametric amplifiers based on periodically poled LiNbO3 (PPLN) waveguides for optical communication. Specifically, we discuss phase and amplitude regeneration using a phase sensitive amplifier and nonlinearity mitigation using an optical phase conjugator in multi-span transmissions.

PPLN waveguide based phase sensitive amplifier for optical communication

We describe fundamental properties and recent progress with respect to PPLN waveguide based phase-sensitive amplifiers. We discuss the low-crosstalk amplification of DWDM signals, polarization-insensitive operation, and amplification in a monolithically integrated PPLN waveguide circuit.

Simultaneous nonlinearity mitigation of WDM signals based on complementary spectrally inverted optical phase conjugation

We review the simultaneous mitigation of nonlinear impairments in WDM signal transmission using optical phase conjugation (OPC). Specifically, we discuss reserved-band-less, guard-band-less, and polarization independent OPC based on complementary spectral inversion.

Low-Parametric-Crosstalk Phase-Sensitive Amplifier for Guard-Band-Less DWDM Signal Using PPLN Waveguides

We demonstrated guard-band-less dense wavelength division multiplex signal amplification with a phase sensitive amplifier (PSA). The PSA consists of periodically poled lithium niobate waveguide that employs two-stage nonlinear process, namely second-harmonic generation and optical parametric amplification, which makes it possible to mitigate the crosstalk induced by the optical parametric interaction between DWDM channels. The PSA amplified 16-channel signals simultaneously with a gain of 17 dB over 12-nm wavelength range. We demonstrated a 2.5-dB signal-to-noise-ratio improvement and a low noise figure of 2.1 dB below the 3-dB quantum limit. We achieved the amplification of a 10-Gbaud quadrature phase shift keying signal with phase and amplitude regeneration.

Monolithically integrated optical parametric up/down frequency converter using arrayed PPLN waveguides

We propose a novel structure for two monolithically integrated PPLN waveguides with a reflective wavelength-division multiplexer consisting of two MMIs and a dichroic mirror. Frequency conversion through two-stage SHG/DFG processes is demonstrated using the integrated device.