A variety of material platforms including glass, lithium niobate, and polymer are being utilized for opto-electronic components. Among these, high performance thermo-optic polymers are particularly advantageous for active telecom applications because they possess a unique suite of properties including large thermo-optic coefficient, high thermal stability, refractive index tenability, and compatibility with high-volume wafer scale processing. In this paper, we discuss our approach for material selection and its relation to device design optimization in a Si-polymer based variable optical attenuator array. We outline the key material and device design trade-offs and show that Si-polymer based devices can meet and exceed the reliability requirements for telecom components. To this end, performance, qualification testing, and key properties that are related to high reliability and device lifetime are addressed.
Silicones are known for their excellent performance in applications with harsh environmental conditions. They are very well known for their high temperature stability, resistance to moisture and other adverse conditions. This paper will overview key properties of siloxanes that make them attractive materials for numerous photonics device applications with emphasis on polymer waveguides. Both thermal-mechanical and optical properties will be reviewed. Testing of key optical properties of several siloxane materials, both before and after exposure to heat, humidity, and high optical flux will be discussed. Fabrication and processing for production of polymer waveguides, and the resulting polymer device performance will be shown. Finally, the high reliability of siloxane based waveguides is demonstrated by the Telcordia testing of a fully functional, packaged, Variable Optical Attenuator (VOA).
We demonstrate a polymer waveguide-based thermo-optic variable optical attenuator array with 30dB dynamic range that requires less than 20mW of drive power to achieve 30dB of attenuation. The design offers low insertion loss and low polarization dependent loss. We also present simulation results that show good agreement with measured data and which thus permit to optimize the device performance.
Doped polymers exhibit many attractive features for nonlinear optics. The performance demonstrated with some of these materials appears promising for application in real devices. However, the Achilles' heel of this class of chromophore-doped materials lies most certainly in their relatively modest chemical stability, especially their photostability. Our aim has been to quantify such side- effect phenomenon, systematically linked to the optical use of these materials. Photodegradation is a 2-step process: first, absorption, that may be characterized by (sigma) ((lambda) ), the absorption cross section, and, second, chemical reactivity from the induced excited-states, which may be quantified by B<SUP>-1</SUP>((lambda) ), the overall quantum efficiency of degradation. The photodegradation rate under a photon flux n is thus given by (tau) =B/((sigma) .n). We use the quantity C=B/(sigma) as a material figure of merit for photostability. Given long enough illumination times, C can always be measured. How precisely B is quantified is directly related to how precisely (sigma) is measured, which decreases dramatically as the wavelength of interest is shifted from the main absorption band towards the IR telecommunication spectral windows. Increasing future device lifetimes requires a simultaneous increase in the B parameter and a decrease in the loss due to the residual red-tail absorption. We report the systematic behavior that was found concerning the dependence of C on wavelength.
We have investigated crosslinkable polyimides for both passive and active electro-optic devices. These fluorinated polyimides are soluble in the imidized form and are both thermally and photo-crosslinkable leading to easy processability into waveguide structures and the possibility of stable electro-optic properties. We have fabricated channel and slab waveguides and investigated the mechanism of optical propagation loss using photothermal deflection spectroscopy and waveguide loss spectroscopy, and found the losses to arise from residual absorption due to the formation of charge transfer states. The absorption is inhibited by fluorination yielding propagation losses as low as 0.4 dB/cm in the near infrared. Channel waveguides formed by a simple wet etch process are observed to have no excess loss over slab structures. We have produced electro-optic polymers by doping with the nonlinear optical chromophores, DCM and DADC; and a process of concurrent poling and thermal crosslinking. Multilayer structures have been investigated and poling fields optimized in the active layer by doping the cladding with an anti-static agent.
We have investigated a promising class of polyimide materials for both passive and active electro-optic devices, namely crosslinkable polyimides. These fluorinated polyimides are soluble in the imidized form and are both thermally and photo-crosslinkable leading to easy processability into waveguide structures and the possibility of stable electro-optic properties. We have fabricated channel and slab waveguides and investigated the mechanism of optical propagation loss using photothermal deflection spectroscopy and waveguide loss spectroscopy, and found the losses to arise from residual absorption due to the formation of charge transfer states. The absorption is inhibited by fluorination leading to propagation losses as low as 0.3 dB/cm in the near infrared. Because of the ability to photocrosslink, channel waveguides are fabricated using a simple wet-etch process. Channel waveguides so formed are observed to have no excess loss over slab structures. Solubility followed by thermal cross-linking allows the formation of multilayer structures. We have produced electro-optic polymers by doping with the nonlinear optical chromophores, DCM and DADC; and a process of concurrent poling and thermal crosslinking. Multilayer structures have been investigated and poling fields optimized in the active layer by doping the cladding with an anti-static agent. The high glass-transition temperature and cross-linking leads to very stable electro-optic properties. We are currently building electro-optic modulators based on these materials. Progress and results in this area also are reported.
We demonstrate anomalous dispersion phase-matched second harmonic generation in a poled polymer waveguide. Phase-matching was achieved between lowest order fundamental and harmonic modes. TM<SUP>w</SUP><SUB>o</SUB> to TM<SUP>2w</SUP><SUB>o</SUB>, at a fundamental wavelength of 815 nm over a propagation length of 32 micrometers . The maximum conversion efficiency was (eta) <SUB>exp</SUB> equals 39%/Wcm<SUP>2</SUP>, in good agreement with theory. Methods to further enhance the conversion efficiency using a combination of phase-matching techniques are also discussed.
We describe in detail the mechanism of 20-fold pulse compression in a synchronously pumped optical parametric oscillator. Nonlinear compression of these soliton-like pulses arises from pump depletion by the leading front of the compressed pulse and is limited by the round-trip cavity losses.
We report on a second harmonic generation in a poled polymer waveguide using anomalous dispersion phase-matching. Blue light was produced by phase-matching the lowest order fundamental and harmonic modes over a distance of 32 micrometers . The experimental conversion efficiency was n equals 1.2 X 10<SUP>-4</SUP>, in agreement with theory. Additionally, we discuss a method of enhancing the conversion efficiency for second harmonic generation using anomalous dispersion phase-matching to optimize Cerenkov second harmonic generation. Our modeling shows that a combination of phase-matching techniques creates largest conversion efficiencies and reduces critical fabrication requirements of the individual phase-matching techniques.
The electro-optical properties of Ultradel<SUP>R</SUP> 9000D polyimides doped with DCM and DADC, a bis(carbazole) analog of DCM with improved thermal stability, are reported. Cure temperatures were restricted to 240 degree(s)C or less to minimize potential thermal degradation of these dyes. Low poling fields of 30 V/micrometers were used in these experiments and yielded r<SUB>13</SUB> coefficients in the 0.1 - 0.8 pm/V range. Photothermal deflection measurements of dye-doped Ultradel 9000D samples showed low optical absorption losses in systems cured at 175 degree(s)C, but losses exceeded 20 dB/cm in samples cured at 300 degree(s)C.
The properties of new, high temperature optical materials based on dye-doped Ultradel 9000D polyimides are presented. Ultradel 9000D is a soluble, pre-imidized, fluorinated polymer with properties optimized for integrated optical applications. When thermally or photochemically cross-linked, it has a Tg approaching 400$DEGC and retains excellent optical transparency as measured by both waveguide loss spectroscopy (WLS) and photothermal deflection spectroscopy (PDS). The agreement between WLS and PDS data indicates that losses in polyimides are due to absorption, not scattering. Two thermally stable, donor-acceptor oxazole-based dyes were designed, synthesized, and doped into the polyimide at concentrations up to 25 percent by weight. The Tg of the doped polymers decreased from the neat polymer, but remained above 300$DEGC. The effects of doping on the dielectric constant, refractive index, and coefficient of thermal expansion of the polyimide are presented.
We report on a scheme for phase-matching second harmonic generation in polymer waveguides based on the use of anomalous dispersion to optimize Cerenkov phase matching. We have used the theoretical results of Hashizume et al. and Onda and Ito to design an optimum structure for phase-matched conversion. We have found that the use of anomalous dispersion in the design results in a 100-fold enhancement in the calculated conversion efficiency. This technique also overcomes the limitation of anomalous dispersion phase- matching which results from absorption at the second harmonic. Experiments are in progress to demonstrate these results.
The synthesis and optical characterization of fluorinated polyimide systems with potential use in passive waveguides and electro-optic devices is reported. The effect of fluorination on optical properties such as refractive index, birefringence, and near-infrared absorbance is reviewed in terms of optical performance requirements. Synthetic methods of tuning the refractive index in order to achieve appropriate core/cladding differentials is discussed. The relation between processing parameters and refractive index for several polyimide structures also is reported. We describe the microlithographic fabrication of a multilayer polyimide rib- type waveguide that is suitable for single mode guiding. The waveguide is fabricated using photosensitive polyimide systems via negative resist imaging. A comparison of wall profiles and resolution limits afforded by the wet-chemical patterning techniques is presented. Results on channel guide coupling, propagation, and loss are described, as well as progress in producing active guides.
Anomalous-dispersion phase-matching (ADPM) offers large enhancements in the effective hyperpolarizabilities of nonlinear optical processes such as second harmonic generation (SHG) in organic materials. The principal barrier to the practical application of this approach is the residual absorption of asymmetric organic chromophores at the second harmonic which is shown by calculation to inherently limit the efficiency of SHG. Resonance Raman experiments on a nonlinear optical (NLO) dye optimized for doubling 800 nm light indicate that the residual absorbance is probably due to vibronic levels associated with the electronic absorption. Recent work with thin films of PMMA doped with this ADPM dye showed zero dispersion at approximately 6% concentration.