Hyung-Jong Lee

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.

Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings

Based on the thermo-optic tuning of a polymer waveguide Bragg reflector, we demonstrated a cost-effective tunable wavelength laser for WDM optical communications. The excellent thermo-optic effect of the polymer waveguide enabled direct tuning of the Bragg reflection wavelength by controlling the electrical power on a micro-heater. Wavelength tuning for 32 channels with 0.8 nm wavelength spacing was demonstrated as well as a continuous tuning with wavelength steps of 0.1 nm. To be qualified as a tunable laser for WDM-PON applications, wavelength stability within 0.15 nm was confirmed for an operating temperature range from -10 to 70 degrees C.

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.

Polymeric waveguide polarization splitter with a buried birefringent polymer

A waveguide polarization splitter is demonstrated based on a low-loss polymer waveguide and a birefringent polyimide. Crosslinkable fluorinated polymers with an excellent stability and a low absorption loss are utilized for the device. The polyimide is buried under one branch of the Y-branch waveguide to enhance the birefringence between the TE and TM modes. By the adiabatic mode evolution, the TE mode is coupled to the branch with the polyimide strip, while the TM mode propagates through the other branch without the polyimide. For the device with a branch angle of 1/400 rad, we obtained a crosstalk less than -20 dB and a fiber-to-fiber insertion loss of 3.8 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.

Thermooptic 2/spl times/2 asymmetric digital optical switches with zero-voltage operation state

Thermooptic 2/spl times/2 switches with zero-voltage operation state are demonstrated. The devices made of ultraviolet curable fluorinated polymer are composed of four asymmetric digital optical switches and designed for bar-state operation with initial zero-voltage state. This zero-voltage operation is essential for protection switching in optical networks. The crosstalks of initial bar and cross states are more than 40 dB. The insertion loss and the polarization-dependent loss of the device are less than 1.8 and 0.1 dB for both states, respectively. The power consumption is about 350 mW. The wavelength and temperature dependences of the insertion loss are less than 0.3 dB.

New design for low-loss star couplers and arrayed waveguide grating devices

We propose a new loss reduction method in star couplers employing UV-written tapers that replace the free propagation region in the conventional star couplers and apply them to an arrayed waveguide grating (AWG) device. The insertion loss of the new AWG device can be reduced to 0.31 dB, which is about 0.7 dB lower than that of the AWG without UV-written tapered waveguides.

Flexible polymer waveguide tunable lasers

A flexible polymeric Bragg reflector is fabricated for the purpose of demonstrating widely tunable lasers with a compact simple structure. The external feedback of the Bragg reflected light into a superluminescent laser diode produces the lasing of a certain resonance wavelength. The highly elastic polymer device enables the direct tuning of the Bragg wavelength by controlling the imposed strain and provides a much wider tuning range than silica fiber Bragg gratings or thermo-optic tuned polymer devices. Both compressive and tensile strains are applied within the range from -36000 microepsilon to 35000 microepsilon, so as to accomplish the continuous tuning of the Bragg reflection wavelength over a range of up to 100 nm. The external feedback laser with the tunable Bragg reflector exhibits a repetitive wavelength tuning range of 80 nm with a side mode suppression ratio of 35 dB.