Seh-Won Ahn

Flexible low-voltage electro-optic polymer modulators

A high-performance electro-optic (EO) polymer modulator on a flexible substrate was fabricated using a polymer substrate layer lift-off method. The SU-8 polymer has widely different adhesion properties on Si and gold substrates that makes selective lift-off possible. The flexible EO polymer modulators on a 100-μm polymer substrate layer have a Vπ of 2.6 V and extinction ratio better than 20 dB at 1550 nm. The bending loss of the flexible waveguide was unchanged at bending radii as small as 1.5 mm, and no effect on Vπ was observed for a 5 mm bending radius.

MSTAR: a submicrometer absolute metrology system

The Modulation Sideband Technology for Absolute Ranging (MSTAR) sensor permits absolute distance measurement with subnanometer accuracy, an improvement of 4 orders of magnitude over current techniques. The system uses fast phase modulators to resolve the integer cycle ambiguity of standard interferometers. The concept is described and demonstrated over target distances up to 1 m. The design can be extended to kilometer-scale separations.

Polymeric wavelength filter based on a Bragg grating using nanoimprint technique

A polymeric waveguide-type wavelength filter based on a Bragg grating has been proposed and fabricated using the simple nanoimprint technique, for the first time to our knowledge. An ultraviolet transparent stamp with the single-mode waveguide pattern incorporating a surface-relief-type Bragg grating was efficiently developed using laser interference lithography and a specially designed selective dry-etching process. Using this stamp, the device fabrication was substantially simplified involving just a single-step process of imprint followed by polymer spin-coating. The achieved maximum reflection was higher than 25 dB at the center wavelength of 1569 nm. And the 3-dB bandwidth was 0.8 nm for the device length of 1.5 cm.

Integration of electro-optic polymer modulators with low-loss fluorinated polymer waveguides

We have demonstrated a hybrid Mach-Zehnder optical modulator consisting of a large-core, low-loss fluorinated passive polymer waveguide and an electro-optic (EO) polymer waveguide. The combination exhibits low fiber coupling loss to the passive waveguide and reduced transmission loss because the EO polymer waveguide is used only in the active region. The two waveguides are connected by vertical tapers that permit low-loss adiabatic coupling between the two modes. The half-wave voltage and the insertion loss of the fabricated modulator are 3.6 V and 6 dB, respectively, at a wavelength of 1.55 microm . The estimated coupling loss with the standard single-mode fiber is ~0.5 dB.

Polymeric microring resonator using nanoimprint technique based on a stamp incorporating a smoothing buffer Layer

A polymeric microring resonator has been proposed and fabricated using the nanoimprint technique based on a stamp incorporating a smoothing buffer layer. It played a pivotal role in improving the sidewall roughness of the waveguide pattern engraved on the stamp and thus reducing its scattering loss. Eventually, the quality factor of the resonator could be enhanced. It also helped narrow down the gap between the ring and the bus effectively to strengthen the coupling between them. For the fabricated device, the quality factor of /spl sim/103 800 and the extinction ratio of /spl sim/11 dB were achieved at 1550 nm, while the estimated scattering loss was reduced from 38 to 0.0005 dB/mm with the help of the smoothing layer.

Integrated optical waveguide polarizer based on photobleaching-induced birefringence in an electrooptic polymer

A TE-pass waveguide polarizer is fabricated by utilizing the photobleaching-induced birefringence at room temperature in an electrooptic polymer. The polarizer consists of the photobleached waveguide supporting only TE mode, which is integrated in the middle of the etched rib waveguide supporting both TE and TM modes. It has a simple structure and requires no high temperature process like poling. The measured polarization extinction ratio is about 21 dB at the wavelengths of 1.3 and 1.55 /spl mu/m, and the estimated excess loss is about 0.4 dB.

Integrated optical high-voltage sensor based on a polymeric Y-branch digital optical switch

An integrated optical high-voltage sensor is realized by fabricating a Y-branch digital optical switch in an electrooptic polymer. The measurement of ac high voltage is accomplished by using the linear transfer characteristics of the switch at zero bias voltage. Furthermore, the logarithmic ratio between the optical powers of two output ports may be used to remove the noise caused by the power fluctuations of a light source. The polymeric high-voltage sensor requires no electrical dc biases and no voltage dividers. It is also wavelength insensitive and fabrication-tolerant due to its use of mode evolution effects instead of interference. The measured sensing voltage range is as large as ac 500-V peak-to-peak.

Organic electro-optic materials: some unique opportunities

Recent use of quantum mechanics to guide the improvement of molecular hyperpolarizability and the use of statistical mechanical analysis of the effects of intermolecular electrostatic interactions to improve the acentric ordering of organic chromophores has led to the realization of electro-optic coefficients, r33, greater than 100 pm/V (at telecommunication wavelengths). This material design and development paradigm is likely to lead to further improvement in electro-optic activity, which will in turn facilitate the development of a variety of electro-optic devices with drive (Vπ) voltage requirements of less than one volt. The utility of organic electro-optic materials for development of high bandwidth devices is now well documented. What is less obvious is the utility of organic electro-optic materials for the fabrication of complex (including conformal, flexible, and three-dimensional) device structures. In this communication, we review recent improvements in electro-optic activity; thermal and photochemical stability; and processability of organic electro-optic materials and the use of these materials to fabricate conformal and flexible electro-optic devices and devices based upon single and multiple coupled ring microresonators.

High speed polymer electro-optic modulators

We have used this polymer technology to fabricate modulators with balanced outputs by including a 3 dB coupler on the output and incorporated them in a digital time delay array. Arrays of RF photonic phase shifter operating at 20 GHz has also been demonstrated. For more complex integrated optical circuits, it will be important to use low loss passive polymers to interconnect the active polymer devices.

Advances in polymer waveguide devices

Material and fabrication advances have brought polymer high speed, low voltage EO modulators close to commercial development. The technology is now being applied to more complex photonic switches and circuits and we will review some of the latest applications of polymers in photonics. We will briefly review the state of the art for high band-width, low voltage optical modulators for applications in fiber communication systems and in RF photonics. Using an EO polymer, APC/CLD, we have demonstrated a waveguide Mach Zehnder modulator operating in a push-pull mode with low frequency V/sub /spl pi// of 1.2 V at 1300 nm and 1.8 V at 1550 nm. EO and passive polymers also hold promise in the field of micro-photonics. We will present our results on polymer micro-ring resonators integrated with polymer waveguides. Coupled micro-resonators can be used for wide-band tuning of lasers based on the vernier effect. These coupled resonators have been both voltage and thermally tuned. The measurements on a tunable laser based on the coupled resonators and EDFA that tunes over 35 nm will be presented.