We demonstrated ultra-violet (UV) generation by launching 1040-nm picosecond pulses into a step-index fused-silica
fiber without tapering. Even in a conventional step-index fiber, the phase mismatching between LP01 mode of the
fundamental wave and higher-order mode of the third-harmonic wave can be reduced by increasing the index contrast. In addition, in the cladding mode, Cherenkov-type phase matching is made possible. Thus, although the phase-matching is not achieved rigorously, THG can occur locally enough to observe the third-harmonic signal clearly at the output. In the experiment, we launched a picosecond pulse train from a 1040-nm fiber laser into a commercially available stepindex fiber with high index contrast. The UV light was observed in the wavelength range of 347-390 nm, and the output power of UV components was increased with the third- and fourth-order dependence. Also, we observed the bright bluelight emission along the fiber, which was originated from the fluorescence due to the defect of SiO<sub>2</sub>/GeO<sub>2</sub> radiated by UV light (generated by THG).
We investigate the impact of the reflection in an RSOA-based WDM PON utilizing the spectrum-sliced ASE source as
the seed light. Since the spectrum-sliced ASE source has sufficiently large bandwidth to suppress the optical beat
interference (OBI) noise, we can achieve the high reflection tolerance in comparison with that in a conventional WDM
PON using the remodulation scheme. In the experiment, we systematically investigate the dependence of the reflection
tolerance on the operating conditions such as the optical power and bandwidth of the spectrum-sliced ASE source
injected into the RSOA. In the case when the injection power into the RSOA is low, the performance of the upstream
signal becomes vulnerable due to the reflection-induced incoherent crosstalk and parasitic laser oscillation. However, we
can improve the reflection tolerance significantly by increasing the optical power and the bandwidth of the spectrumsliced
ASE source. By optimizing these operating conditions, we can achieve an excellent refection tolerance greater
than -15 dB.
We review our recent achievements in the optical performance monitoring techniques such as the optical phasor
monitor and optical signal-to-noise ratio (OSNR) monitor developed for differential phase-shift-keyed (DPSK) and
quaternary PSK (DQPSK) signals. The optical phasor monitor is a tool that can measure the in-phase and quadrature
components of the optical field of the phase-modulated signal and display the measured results on the complex plane as a
phasor. We implement such a phasor monitor by using an adjustment-free differential-phase demodulator composed of a
120-degree optical hybrid (which can be realized simply by using a conventional 3x3 optical coupler), and demonstrate
its plug-and-play (i.e., phase-adjustment free and wavelength/polarization-independent) operation experimentally. We
also show that the proposed phasor monitor is well suited for the use in the diagnosis of DPSK/DQPSK. The OSNR
monitoring is another important function to evaluate the signal quality as well as the condition of the transmission link.
We have recently proposed a new technique for monitoring the 'in-band' OSNR of DPSK/DQPSK signals. This
technique estimates the OSNR by analyzing the radio frequency (RF) spectrum obtained by the self-heterodyne detection.
In this paper, we briefly explain its operating principle and show the experimental results. Using this technique, we
could accurately monitor the OSNR of the 10-Gb/s DPSK and 20-Gb/s DQPSK signals in a 640-km long transmission
link. The results also show that the performance of the proposed technique is not sensitive to the effects of chromatic
dispersion and polarization-mode dispersion.
We have experimentally investigated the reflection tolerance of upstream signals by comparing Manchester-encoded
downstream signals with NRZ ones in a RSOA-based fiber loop back system for a WDM PON. The results showed that
the reflection tolerance of the upstream signals strongly dependeds on the downstream signal modulation formats, and
that the Manchester format was more tolerant than the NRZ one against the reflection of the upstream signal.
We review the mode-field matched center-launching technique recently proposed for the
use in the multimode fiber (MMF) transmission systems operating at the speed higher than 10
Gb/s. This technique has been used for the transmission of 100-Gb/s signal (10 × 10 Gb/s)
over 12.2 km of MMF. The performance of this system is surprisingly stable, and not
sensitive to the use of fiber connectors in the MMF links.
We describe an in-service dispersion monitoring technique which we call dithered-filter method and show its application to adaptive dispersion compensation. In the proposed method, we filter out a portion of the received signal light by a dithered optical band-pass filter and derive the accumulated dispersion from the phase shift of the extracted clock signal. This method does not require any modification of the transmitter, and is applicable to any modulation format as far as the clock signal is extracted. In addition, owing to the dithering and the synchronous detection, high accuracy is realized with a short measurement time less than 200 msec. In this paper, we explain the principle of operation of the proposed method and show the experimental demonstration of fast adaptive dispersion compensation in a 40 Gbit/s reconfigurable optical network.
We derive the Schawlow-Townes linewidth of mode-locked lasers, and discuss how the timing jitter affects the linewidth in passively/actively mode-locked laser diodes (MLLD's). The linewidth of longitudinal modes of MLLD's is much broader than the Schawlow-Townes linewidth due to the linewidth enhancement factor of the gain medium, and its characteristics depend on whether the laser is mode-locked passively or actively. For the case of active mode-locking, the detuning of the modulation frequency from the natural cavity frequency broadens the linewidth. In the experiment, we measure the linewidth of 10-GHz external cavity MLLD's and investigate the correlation with the noise characteristics of timing jitter and amplitude noise of the pulse train. The experimental results agree well with theoretical predictions.
The ultraviolet quantum catastrophe of the zero-point fluctuations in an L-C circuit of finite Q is revisited. The zero-point energy is shown to approach the value hω<sub>0</sub>/2 only in the limit of an infinite Q. A Fabry-Perot resonator, on the other hand, has bounded zero-point energies of its modes that are equal to hω<sub>0</sub>/2 . We analyze the noise of an ultra-fast mode-locked laser when the slowly varying envelope approximation (SVEA) is not valid. It is found that quantum correlations exist that are not present in the SVEA. The correlations become evident in the spectrum of the zero-point uctuations.
Quantum equations of motion are developed for broadband soliton propagation with an instantaneous Kerr effect. New terms appear
in the nonlinear Schroedinger equation. Hyperbolic secant soliton solutions exist. The initial pulse perturbations are evaluated.