Masashi Abe

Sub-Doppler resolution 3.4 μm spectrometer with an efficient difference-frequency-generation source

We have developed a 3.4 mum spectrometer for sub-Doppler resolution molecular absorption spectroscopy. The source is based on the difference frequency generation using a 1.06 microm Nd:YAG laser, a 1.55 microm distributed-feedback laser diode, and a ridge waveguide periodically poled lithium niobate with a conversion efficiency of 9%/W. The generated radiation has a power level of about 300 microW, a linewidth of less than 100 kHz, and a continuous tuning range over 10 cm(-1). The spectrometer allows us to observe six saturated absorption lines of C12H(4) with 0.3-0.4 MHz width and 1.3%-1.7% depth relative to the corresponding linear absorption lines.

Design of cavity-enhanced absorption cell for reducing transit-time broadening

To reduce the linewidth of Lamb dips, we introduce a cavity-enhanced absorption cell (CEAC) coupled with a Gaussian beam with a 1.9-mm 1/e(2) radius at the beam waist for the reduction of transit-time broadening. We state that transit-time broadening depends only on the beam radius at the beam waist. This fact is useful for the design of the CEAC, and a pair of concave and convex mirrors is thereby employed. We have carried out sub-Doppler resolution spectroscopy of the ν(3) band of CH(4) and the ν(1) band of CH(3)D using a difference-frequency-generation source and the CEAC, and the recorded Lamb dips narrow to 80 kHz (HWHM).

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.

Sub-Doppler resolution mid-infrared spectroscopy using a difference-frequency-generation source spectrally narrowed by laser linewidth transfer.

The spectral linewidth of a 3.28 μm difference-frequency-generation source has been reduced to 3.5 kHz using a laser linewidth transfer technique [Opt. Express21, 7891 (2013)]. We use an optical frequency comb with a broad servo bandwidth to transfer a narrow linewidth of a pump laser, a 1.06 μm Nd:YAG laser, to a signal laser, a 1.57 μm external-cavity laser diode. This source enables us to record the Lamb dip of the ν3 band R(2) E transition of methane with a molecular spectral linewidth of 21 kHz while the frequency axis is absolutely calibrated.

Dual wavelength 3.2-μm source for isotope ratio measurements of 13 CH 4 / 12 CH 4

Difference frequency generation using one 1.58-μm and two 1.06-μm distributed feedback Bragg-grating laser diodes and a ridge-type PPLN alternately provide two 3.2-μm coaxial waves resonant with individual isotopic transitions separated by 13 cm(-1). The ν(3) band R(6) A(2) allowed transition of (13)CH(4) and the ν(3) band R(6) A(2) weakly allowed transition of (12)CH(4) are an ideal pair for isotope ratio measurements. The (13)CH(4)/(12)CH(4) isotope ratio is determined for three sample gases with a relative uncertainty of 0.7 ‰, and it is confirmed that the temperature dependence is smaller than the uncertainty.

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.

Accurate frequency measurement of the ν 3 band of methane from sub-Doppler resolution comb-referenced spectroscopy

We have determined 183 sub-Doppler resolution spectral frequencies of 12CH4 using a difference-frequency-generation source, an enhanced-cavity absorption cell, and an optical frequency comb from 86.7 to 93.1 THz with a typical uncertainty of 3 kHz.

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.