Kwang Jo Lee

Phase sensitive amplification based on quadratic cascading in a periodically poled lithium niobate waveguide.

We propose and demonstrate phase-sensitive amplification based on cascaded second harmonic generation and difference frequency generation within a periodically poled lithium niobate waveguide. Excellent agreement between our numerical simulations and proof-of-principle experiments using a 3-cm waveguide device operating at wavelengths around 1550 nm is obtained. Our experiments confirm the validity and practicality of the approach and illustrate the broad gain bandwidths achievable. Additional simulation results show that the maximum gain/attenuation factor increases quadratically with input pump power, reaching a value of +/- 19.0 dB at input pump powers of 33 dBm for a 3 cm-long waveguide. Increased gains/reduced powers for a fixed gain could be achieved using longer crystals.

Highly efficient all-fiber tunable polarization filter using torsional acoustic wave

We demonstrate an all-fiber tunable polarization filter with high coupling efficiency based on acousto-optic coupling between two optical polarization modes of the LP(01) mode propagating in a highly birefringent single mode optical fiber. An over-coupling between the two polarization modes is realized over the wavelength range from 1530 nm to 1610 nm using traveling torsional acoustic wave. The measured 3-dB optical bandwidth of the filter was 4.8 nm at the wavelength around 1550 nm. The details of the filter transmission and the coupling characteristics are discussed.

Narrowband, polarization insensitive all-fiber acousto-optic tunable bandpass filter

We demonstrate an all-fiber acousto-optic tunable bandpass filter exhibiting narrow optical bandwidth and negligible polarization dependence by employing a novel ultraviolet (UV)-induced core-mode blocker written in a high numerical aperture (NA) fiber. It was demonstrated that the device had the measured 3-dB optical bandwidth of 0.65 nm, the polarization-dependent center-wavelength splitting of 0.05 nm and the extinction ratio of -22dB at the wavelength around 1550 nm. The details of the transmission characteristics and the loss mechanism of the core-mode blocking element inscribed in the high NA fiber are discussed.

Polarization coupling in a highly birefringent photonic crystal fiber by torsional acoustic wave.

We demonstrate and analyze the acousto-optic coupling between two optical polarization modes of the LP(01) mode propagating in a highly birefringent photonic crystal fiber. The coupling is realized based on wavelength selective acousto-optic coupling by traveling torsional acoustic wave in an all-fiber tunable polarization filter configuration. The dispersion properties of the torsional acoustic wave in the photonic crystal fiber and the influence of axial non-uniformity in the modal birefringence on the filter transmission are discussed in detail.

Refractive index sensor based on a polymer fiber directional coupler for low index sensing

We propose, numerically analyze and experimentally demonstrate a novel refractive index sensor specialized for low index sensing. The device is based on a directional coupler architecture implemented in a single microstructured polymer optical fiber incorporating two waveguides within it: a single-mode core and a satellite waveguide consisting of a hollow high-index ring. This hollow channel is filled with fluid and the refractive index of the fluid is detected through changes to the wavelength at which resonant coupling occurs between the two waveguides. The sensor design was optimized for both higher sensitivity and lower detection limit, with simulations and experiments demonstrating a sensitivity exceeding 1.4 × 10(3) nm per refractive index unit. Simulations indicate a detection limit of ~2 × 10(-6) refractive index units is achievable. We also numerically investigate the performance for refractive index changes localized at the surface of the holes, a case of particular importance for biosensing.

Performance of Refractive Index Sensors Based On Directional Couplers in Photonic Crystal Fibers

We present a systematic analytic and numerical study of the detection limit of a refractive index sensor employing a directional coupler architecture within a photonic crystal fiber (PCF). The device is based on the coupling between the core mode and a copropagating mode of a satellite waveguide formed by a single hole of the PCF infiltrated by a high-index analyte. Using coupled mode theory as well as full simulations, we investigate the influence of changes in the geometrical parameters of the PCF and the analytes refractive index on sensor performance, including sensitivity, resonance width, and detection limit. We show that regardless of the details of the sensors implementation, the smallest detectable refractive index change is inversely proportional to the coupling length and the overlap integral of the satellite mode with the analyte, so that best performance comes at the cost of long analyte infiltration lengths. This is experimentally confirmed in our dip sensor configuration, where the lowest detection limit achievable for realistic implementation is estimated to 7 × 10-8 refractive index units (RIU) based on realistic signal to noise ratios in a commercially available PCF.

OTDM to WDM format conversion based on quadratic cascading in a periodically poled lithium niobate waveguide.

We propose and demonstrate error-free conversion of a 40 Gbit/s optical time division multiplexed signal to 4 x 10 Gbit/s wavelength division multiplexed channels based on cascaded second harmonic and difference frequency generation in a periodically poled lithium niobate waveguide. The technique relies on the generation of spectrally (and temporally) flat linearly chirped pulses which are then optically switched with short data pulses in the nonlinear waveguide. Error-free operation was obtained for all channels with a power penalty below 2dB.

Axial strain dependence of all-fiber acousto-optic tunable filters

We report the axial strain dependence of two types of all-fiber acousto-optic tunable filters based on flexural and torsional acoustic waves. Experimental observation of the resonant wavelength shift under applied axial strain could be explained by theoretical consideration of the combination of acoustic and optical effects. We discuss the possibility of suppressing the strain effect in the filters, or conversely, the possibility of using the strain dependence for wavelength tuning or strain sensors.

Polarization-coupling all-fiber acousto-optic tunable filter insensitive to fiber bend and physical contact

We show that an all-fiber acousto-optic tunable filter based on polarization mode coupling using torsional acoustic wave is immune to the fiber bend and physical contact in the acousto-optic interaction region. We also propose and demonstrate a novel strain-free and size-reduced tunable filter with a 4-m-long fiber acousto-optic interaction region looped into a 5- cm-diameter coil.

OTDM to WDM format conversion based on cascaded SHG/DFG in a single PPLN waveguide

We propose and experimentally demonstrate error-free OTDM to WDM format conversion based on cSHG/DFG within a 30mm-long PPLN waveguide and a time-to-frequency domain conversion approach, which relies upon switching a linearly chirped pulse.