Seon Gyu Han

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.

Optical switching mechanism based on charge accumulation effects in resonant tunneling diodes

We have identified the charge accumulation effects in a resonant tunneling diode for the application of optical switching by observing the hole tunneling peak under illumination with a Ti:sapphire laser. Two mechanisms, photocurrent and charge accumulation, are rigorously examined to determine which one is the dominant effect for optical switching by comparing the current–voltage (I–V) characteristics for forward biases and reverse biases. It is believed that the hole accumulation effect in the depletion region, which results in the hole tunneling current, is responsible for the shift of the I–V curves.

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.

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.

Photoinduced Hole Tunneling in Resonant Tunneling Diodes

The resonant tunneling of photoinduced holes in GaAs/AlAs n + -i-n + double barrier quantum well structures has been successfully observed in photocurrent measurements. We have found that the accumulation of photo-generated holes in the electron depletion region severely modify the electrostatic potential over the double barrier structure. As a result, the current-voltage curves are shifted to lower voltages. With the depletion approximation, the observed hole tunneling is identified as the resonant tunneling through the first light hole state in the quantum well of valence band

Polymeric 1x8 arrayed waveguide grating wavelength multiplexer using fluorinated poly(ether ketone) at 1550 nm

A polymeric 1 X 8 arrayed waveguide grating wavelength multiplexer with 1.6 nm (200 GHz) channel spacing has been designed and realized for operating around a 1550 nm wavelength. Two kinds of fluorinated polymers, perfluorocyclobutane and fluorinated poly(ether ketone) polymers were used for the low loss waveguides. As a core layer, fluorinated poly(ether ketone) polymer, which has a low propagation loss, a good processibility and high thermal stability up to 460 degree(s)C was newly synthesized. The propagation loss of a buried rib waveguide is less than 0.5 dB/cm at the 1550 nm wavelength. The refractive index difference between the core and the clad layers is 0.0273 ((Delta) equals 1.8%). The bending radius of curved waveguides is 7.5 mm. The device size is 39 mm long and 13 mm wide. Fiber-to-fiber insertion losses of the multiplexer are between 7 dB and 8 dB, and a 3-dB bandwidth is 0.6 nm. The crosstalk of all 8 channels is less than -24 dB. And the polarization (TE-TM mode) related wavelength shift is approximately 3.4 nm.