Joo-Heon Ahn

TUNABLE WAVELENGTH FILTERS WITH BRAGG GRATINGS IN POLYMER WAVEGUIDES

Tunable wavelength filters are demonstrated based on the thermo-optic refractive index change of the polymer waveguide with Bragg reflection grating. For the low-loss waveguide operating around 1.55 μm, fluorinated polymers are incorporated. Bragg reflection gratings are fabricated using a phase mask and a high-index polymer. The Bragg reflector exhibits a narrow bandwidth of less than 1.0 nm, a crosstalk of −20 dB, an insertion loss of 3.2 dB, and a flat-top passband. The peak wavelength of Bragg reflection is shifted over 11 nm with a slight insertion loss change. The thermo-optic tuning efficiency is 22 nm/W and the peak shift is linearly proportional to the heating power.

Polymeric wavelength filters with polymer gratings

Wavelength filters with Bragg gratings are demonstrated based on low-loss polymer waveguides and high-refractive-index polymer gratings. Two kinds of fluorinated polymers, perfluorocyclobutane and fluorinated poly(arylene ethers) are used for the low-loss waveguide operating around the 1.55 μm wavelength. The polymer grating is made of Resole, and it is successfully integrated in the polymer waveguides. In order to fabricate the Bragg gratings on the polymer waveguides, we use a phase mask and a Hg lamp in a mask aligner as the illuminating source instead of the laser. This method provides uniform gratings on a large area as well as the alignment capability. In the fabricated wavelength filters, the reflectivity at the Bragg wavelength is 30 dB, the 3 dB bandwidth is as narrow as 0.6 nm, and the insertion loss is 3.7 dB.

Crosslinkable polymers for optical waveguide devices. II. Fluorinated ether ketone oligomers bearing ethynyl group at the chain end

Fluorinated ether ketone oligomers bearing a crosslinkable ethynyl group at the chain end have been investigated for low-loss polymer optical waveguide devices. These oligomers are designed to achieve low birefringence and were synthesized by the reaction of 4,4′-(hexafluoroisopropylidene)diphenol with an excess decafluorobenzophenone, followed by reaction with (phenylethynyl)phenol or ethynylphenol. The molecular weights (Mns) and polydispersities of the oligomers determined by GPC with polystyrene standard are in the range of 6600–8500 g/mol and 1.79–2.04, respectively. By spin coating and thermal crosslinking, the polymer solutions easily provide the good optical quality thin films. The cured films show good chemical resistance and high thermal stability up to 460°C under nitrogen. At 1.55 µm wavelength, the refractive index and birefringence of the films show in the range of 1.5104–1.5172 and 0.0078–0.0014, respectively. The propagation loss of the single-mode channel waveguide was measured to be less than 0.5 dB/cm at 1.55 µm.

Asymmetric X-junction thermooptic switches based on fluorinated polymer waveguides

Thermooptic 2/spl times/2 switches based on low-loss fluorinated polymer waveguides have been demonstrated. For the waveguide possessing a low-loss around the 1.55-/spl mu/m wavelength, crosslinkable fluorinated poly(arylene ethers) (FPAE) is developed as a core material and perfluorocyclobutane (PFCB) is used as a cladding material. To enhance the fabrication tolerance and to achieve a low switching power, asymmetric X-junctions with a Mach-Zehnder interferometer are exploited for the polymeric waveguide switches. An inverted rib waveguide structure is fabricated by filling up the etched groove on a lower cladding with the core polymer. The switch exhibits a crosstalk of less than -20 dB, a switching power of 10 mW, and an insertion loss of 4.5 dB.

Synthesis and properties of nonlinear optical side chain soluble polyimides for photonics applications

Aromatic polyimides with side chain nonlinear optical chromophores have been investigated through a facile two-step synthetic route. First, various poly(hydroxy imide)s have been synthesized by direct thermal imidization of diaminophenol dihydrochloride salt and aromatic dianhydride monomers. The resulting polyimides bearing phenolic hydroxy groups were found to react easily with the terminal hydroxy group on the chromophores via the Mitsunobu condensation to give corresponding polyimides with high optical nonlinearities and good solubility in common organic solvents. Detailed physical properties showed that these polyimides have a molecular weight (M ω ) of 31,000 and high glass transition temperature above 220°C, ensuring a long-term alignment stability at elevated temperature. The electrooptic coefficients, r 33 , of the electrically poled polymer films were in the range 1.8-7.6 pm/V at 1.3 μm.

A novel hybrid silica wide-band amplifier covering S+C+L bands with 105-nm bandwidth

We report a novel hybrid optical amplifier covering S+C+L bands with 105-nm total bandwidth using a silica fiber. The principle of amplification is based on the stimulated radiative transition of Er-ions for C-band and on the stimulated Raman scattering for S- and L-band, respectively. In this letter, we analyze the amplification characteristics for two types of active fiber mediums through numerical simulation. One is a silica fiber configured with Er-doped cladding and Ge-doped core and the other is a medium consisting of Er-doped fiber and dispersion-compensating fiber. By optimizing parameters such as fiber length and pump power, we newly achieve wide-band amplification with 105-nm bandwidth showing a flat gain characteristic over the entire S+C+L bands.

Polarization stabilizer using a polarization splitter and a thermooptic polymer waveguide device

For the stabilization of the time-varying light polarization in the optical fiber, we demonstrates a polarization stabilizer by using a fiber-optic polarization splitter, a passive polarization converter, and a power equalizing waveguide device. Because the ratio of the output light power from the polarization splitter is time-varying depending on the input polarization, we adopt a power equalizer consisting of a polymeric thermooptic (TO) phase modulator and an asymmetric X-junction waveguide. A feedback signal is obtained from the difference of the two outputs of the power equalizer. Then it is applied to the TO phase modulator. The output power is stabilized with the TE polarization independent of the time-varying polarization state of the input light.

Uniform and high coupling efficiency between InGaAsP-InP buried heterostructure optical amplifier and monolithically butt-coupled waveguide using reactive ion etching

We obtained uniform and high coupling efficiency for InGaAsP-InP buried heterostructure (BH) optical amplifiers integrated with butt-coupled waveguides using reactive ion etching (RIE) for mesa definition and low-pressure metalorganic vapor phase epitaxy (LPMOVPE) for waveguide layer regrowth. Measured average coupling efficiency was over 91% across a quarter of 2-in InP wafer. RIE etched vertical facet and a subsequent chemical etching using HBr-based solution for relief of RIE damage enabled us to reduce the coupling loss due to anomalous regrowth shape at the interface. RIE and selective regrowth processes are promising techniques for the fabrication of the photonic integrated circuit (PIC).

Internal pressure effect on cathodoluminescence enhancement of ZnS : Mn2+ synthesized by a sealed vessel

ZnS:Mn 2+ phosphors were synthesized by a modified solid-state reaction method. In the synthesis method, a sealed vessel is used, where heat and pressure are simultaneously utilized. The effects of various synthesis conditions such as temperature, Mn concentration, and pressure on the cathodoluminescence (CL) were investigated. Among them, pressure had an effect on CL property as much as others. It was observed that CL intensities of ZnS:Mn 2+ phosphors increased with the increase of pressure and the best sample showed higher intensity than that of a commercial one by 180%. X-ray diffraction (XRD) and electron paramagnetic-resonance (EPR) were used to understand the enhancement. No change of XRD patterns was observed but the full width at half-maximum (FWHM) of the most intense cubic (111) peak of ZnS:Mn 2+ decreased with the increase of pressure. EPR signal intensity of Mn 2+ increased with the increase of pressure. The improved crystallinity and more substitution of Zn 2+ with Mn metal were believed to be responsible for the enhancement.

Fabrication of a high-performance InGaAsP/InP integrated laser with butt-coupled passive waveguides utilizing reactive ion etching

We obtained high-performance InGaAsP/InP buried heterostructure lasers integrated with butt-coupled waveguides using reactive ion etching (RIE) for mesa definition, brief chemical cleaning for damage relief and low-pressure metalorganic vapour phase epitaxy for the epitaxial layer growth. We measured a coupling efficiency between the active layer and the passive waveguide layer of over per facet across a quarter of a 2 inch InP wafer. The threshold current and the slope efficiency were 13 mA and , respectively, of the long integrated laser. This excellent uniformity and high performance demonstrate that RIE coupled with slight chemical treatments can be successfully used to fabricate high-quality integrated photonic devices. Transmission electron microscopy observation revealed RIE-induced strains and dislocation loops around the etched mesa after the regrowth, which were proposed to be responsible for the inferior characteristics of the lasers with mesas that were reactive ion etched but had not been cleaned in HBr-based solution.