Kwanil Lee

A femtosecond pulse fiber laser at 1935 nm using a bulk-structured Bi 2 Te 3 topological insulator

We experimentally demonstrate a femtosecond mode-locked, all-fiberized laser that operates in the 2 μm region and that incorporates a saturable absorber based on a bulk-structured bismuth telluride (Bi(2)Te(3)) topological insulator (TI). Our fiberized saturable absorber was prepared by depositing a mechanically exfoliated, ~30 μm-thick Bi(2)Te(3) TI layer on a side-polished optical fiber platform. The bulk crystalline structure of the prepared Bi(2)Te(3) layer was confirmed by Raman and X-ray photoelectron spectroscopy measurements. The modulation depth of the prepared saturable absorber was measured to be ~20.6%. Using the saturable absorber, it is shown that stable, ultrafast pulses with a temporal width of ~795 fs could readily be generated at a wavelength of 1935 nm from a thulium/holmium co-doped fiber ring cavity. This experimental demonstration confirms that bulk structured, TI-based saturable absorbers can readily be used as an ultra-fast mode-locker for 2 μm lasers.

Fiber link loss monitoring scheme in bidirectional WDM transmission using ASE-injected FP-LD

We propose and demonstrate a novel fiber link loss monitoring method for a wavelength-division-multiplexed (WDM) passive optical network. This method is based on the optical signal comparison between a reference amplified spontaneous emission signal and its reflection from an optical reflector. Therefore, we can calculate the link loss exactly and also measure the characteristics of upstream optical signals without suspending the in-service channels. Since this scheme does not use narrow-band wavelength-dependent components such as fiber Bragg gratings, it is cost-effective in implementation and maintenance within a real optical network. We experimentally demonstrate that the link quality of upstream signals as well as fiber link faults on all WDM channels can be monitored in real time

Active control of all-fibre graphene devices with electrical gating

Active manipulation of light in optical fibres has been extensively studied with great interest because of its compatibility with diverse fibre-optic systems. While graphene exhibits a strong electro-optic effect originating from its gapless Dirac-fermionic band structure, electric control of all-fibre graphene devices remains still highly challenging. Here we report electrically manipulable in-line graphene devices by integrating graphene-based field effect transistors on a side-polished fibre. Ion liquid used in the present work critically acts both as an efficient gating medium with wide electrochemical windows and transparent over-cladding facilitating light–matter interaction. Combined study of unique features in gate-variable electrical transport and optical transition at monolayer and randomly stacked multilayer graphene reveals that the device exhibits significant optical transmission change (>90%) with high efficiency-loss figure of merit. This subsequently modifies nonlinear saturable absorption characteristics of the device, enabling electrically tunable fibre laser at various operational regimes. The proposed device will open promising way for actively controlled optoelectronic and nonlinear photonic devices in all-fibre platform with greatly enhanced graphene–light interaction.

Strain and temperature sensitivities of an elliptical hollow-core photonic bandgap fiber based on Sagnac interferometer

We fabricated an elliptical hollow-core photonic bandgap fiber (EC-PBGF) by controlling lateral tension in the hollow core region during the fiber drawing process. The absolute value of group modal birefringence becomes relatively high near the bandgap boundaries. We also experimentally measured the strain and temperature sensitivities of the fabricated EC-PBGF-based Sagnac loop interferometer. The strain and temperature sensitivities were very much dependent upon the wavelength. Moreover this PBGF-based interferometer can be a good sensor of physical parameters such as strain and temperature.

Tunable optical delays based on Brillouin dynamic grating in optical fibers.

We demonstrate a novel kind of tunable optical delays based on dynamic grating generated by Brillouin scattering in an optical fiber. An axial strain gradient is applied to a 15 m section of a polarizationmaintaining fiber, and the Brillouin reflection grating is generated position-selectively by controlling the optical frequencies of Brillouin pump waves. Tunable time delays of up to 132 ns are achieved with an 82 ns Gaussian pulse.

Mode-locked pulse generation from an all-fiberized, Tm-Ho-codoped fiber laser incorporating a graphene oxide-deposited side-polished fiber

An in-depth experimental investigation was conducted into the use of a graphene oxide-based saturable absorber implemented on a side-polished fiber platform for femtosecond pulse generation in the 2 μm region. First, it was experimentally shown that an all-fiberized thulium-holmium (Tm-Ho)-codoped fiber ring laser with reduced cavity length can produce stable femtosecond pulses by incorporating a graphene oxide-deposited side-polished fiber. Second, the measurement accuracy issue in obtaining a precise pulse-width value by use of an autocorrelator together with a silica fiber-based 2 μm-band amplifier was investigated. It showed that the higher-order soliton compression effect caused by the combination of anomalous dispersion and Kerr nonlinearity can provide incorrect pulse-width information. Third, an experimental investigation into the precise role of the graphene oxide-deposited side-polished fiber was carried out to determine whether its polarization-dependent loss (PDL) can be a substantial contributor to mode-locking through nonlinear polarization rotation. By comparing its performance with that of a gold-deposited side-polished fiber, the PDL contribution to mode-locking was found to be insignificant, and the dominant mode-locking mechanism was shown to be saturable absorption due to mutual interaction between the evanescent field of the oscillated beam and the deposited graphene oxide particles.

Switchable multiwavelength erbium doped fiber laser based on a nonlinear optical loop mirror incorporating multiple fiber Bragg gratings

We propose and experimentally demonstrate a switchable multiwavelength erbium doped fiber laser based on a highly nonlinear dispersion shifted fiber and multiple fiber Bragg gratings. A nonlinear optical loop mirror based on a highly nonlinear dispersion shifted fiber is implemented in the ring laser cavity to stabilize the multiwavelength output at room temperature. Multiple fiber Bragg gratings with the wavelength spacing of 0.8 nm are connected with an arrayed waveguide grating to establish a multichannel filter. The high quality of the multiwavelength output with a high extinction ratio of ~60 dB and high output flatness of ~0.5 dB is realized. The nonlinear polarization rotation based on the nonlinear optical loop mirror can provide the switching performance of the proposed multiwavelength fiber laser. The lasing wavelength can be switched individually by controlling the polarization controller and the cavity loss.

Reliable Wavelength-Division-Multiplexed Passive Optical Network Using Novel Protection Scheme

A novel, reliable wavelength-division-multiplexed passive optical network (WDM-PON) with self-protection capability is proposed. By utilizing the routing characteristics of N x N arrayed waveguide grating, the proposed architecture can provide automatic protection against any fiber cut between central office and optical network unit (ONU). Compared with the conventional schemes, this scheme adopts colorless ONUs, thus leading to a decrease in the costs of operation, administration, and maintenance, as well as the production cost. Without the performance degradation, the proposed WDM-PON scheme can offer a reliable network service. In the experiment, the protection performance was demonstrated in the carrier-distributed 1.25-Gb/s WDM transmission over 20-km single-mode fiber.

A self-restorable architecture for bidirectional wavelength-division-multiplexed passive optical network with colorless ONUs.

We propose and experimentally demonstrate a novel protection scheme for wavelength-division-multiplexed passive optical network (WDM-PON) employing colorless optical transceivers. The proposed network employs 2 x N arrayed waveguide grating (AWG) to utilize its routing characteristics. The colorless operation is achieved by using wavelength-locked Fabry-Perot laser diodes (FP-LDs) injected with spectrum-sliced amplified spontaneous emission (ASE) light. The experimental results show that the restoration can be achieved within 8 ms against the feeder fiber fault and the power penalty introduced by the restoration process is negligible.

Differential measurement scheme for Brillouin Optical Correlation Domain Analysis

We newly propose and experimentally demonstrate a differential measurement scheme for Brillouin optical correlation domain analysis, where the difference between Brillouin gain spectra constructed by a normal and a phase-modulated Brillouin pump waves are analyzed to measure local Brillouin frequencies in optical fibers. In experiments, a five-fold enhancement in the spatial resolution is obtained compared to an ordinary BOCDA system under the same modulation parameters, as a result of the improved dynamic range by the suppression of background noises.