Post-fabrication tuning of a polymeric grating-assisted codirectional coupler filter by photobleaching

We have proposed and demonstrated a band rejection filter based on grating-assisted forward coupling between an electrooptic polymer waveguide and a passive polymer waveguide. A wavelength tuning by the electrooptic effect induced by poling has been also demonstrated.

We present the design of a 2×2 photonic switch operating at 1.55-μm wavelength using electro-optic (EO) polymer waveguides. A Mach-Zehnder interferometer (MZI) is used to implement the proposed switch in which two identical 2×2 multimode interference (MMI) couplers are connected by two identical parallel single mode waveguides (two MZI arms). These two single-mode waveguides with electrodes allow modulating the phase difference between the two MZI arms based on the EO effect. In the proposed switch, the EO polymer, IPC-E/polysulfone, is used for the core layer of optical waveguides. UV15 and NOA61 are employed for the lower and upper cladding layers, respectively. The singlemode waveguide structure and 2×2 MMI coupler have been designed and analyzed for the EO switch. Device performance has been simulated using the beam propagation method. It is found that the switch performance is most sensitive to the MMI width and less sensitive to the MMI length. Optimized structure has been obtained for the 2×2 polymeric EO switch, which has a crosstalk level better than -25 dB and insertion loss lower than -1.8 dB. This performance makes the switch a potential candidate for practical use in photonic systems.

We experimentally demonstrate a linearized Y-fed directional coupler (DC) modulator based on an electro-optic (EO) polymer waveguide. The spurious free dynamic range of 119 dB/Hz2/3, which is 11 dB higher than that of the conventional Mach-Zehnder modulator, is achieved by introducing the reversed Δβ technique in the two-section Y-fed DC. The in-device EO coefficient (r33) of the fabricated device is as high as 79 pm/V in 1.55 μm wavelength, which is 88% of a single film r33 of LPD-80/APC.

The conventional process of fabricating electrooptic (EO) polymer waveguide devices involves electric poling to achieve noncentrosymmetric order and photomasks to define channel waveguides by reactive ion etching or photobleaching. However, these techniques are restricted to passive waveguides since the temperature involved destroys the EO effect by randomizing the poling order of active molecules. In this paper, we will demonstrate a novel but simple technique of simultaneous direct laser writing and electric poling of arbitrary channel waveguides that have EO properties. This technique can be very useful for the fast prototyping of active devices such as modulators and switches.

We have investigated an optical signal processor using electro-optic polymer waveguides operating at a wavelength of 1.55 mum. Due to recent developments, many useful optical devices have become available such as optical filters, modulators, switches, and multiplexers. It will be useful to have a single optical device, which is reconfigurable to implement all of these functions. We call such a device an ldquooptical signal processor,rdquo which will play a similar role to digital signal processors in electrical circuits. We realize such an optical device in a planar lightwave circuit. Since the planar lightwave circuits are based on the multiple interference of coherent light and can be integrated with significant complexity, they have been implemented for various purposes of optical processing such as optical filters. However, their guiding waveguides are mostly passive, and the only viable mechanism to reconfigure their functions is thermal effects, which is slow and cannot be used for high-speed applications such as optical modulators or optical packet switches. On the other hand, electro-optic polymer has a very high electro-optic coefficient and a good velocity match between electrical and optical signals, thus, permitting the creation of high-speed optical devices with high efficiency. Therefore, we have implemented a planar lightwave circuit using the electro-optic polymer waveguides. As a result, the structure is complex enough to generate arbitrary functions and fast enough to obtain high data rates. Using the optical signal processor, we investigate interesting applications including arbitrary waveform generators.

The optical amplifier performance of Nd³⁺-doped polymer and amorphous Al₂O₃ channel waveguides with single-mode and multi-mode behavior around 880 nm is compared. Internal net gain in the wavelength range 865-930 nm is investigated under continuous-wave excitation near 800 nm, for Nd³⁺ dopant concentrations typically in the range of 0.6- 1.0 × 10²⁰ cm^-3. A peak gain of 2.8 dB at 873 nm is obtained in a 1.9-cm-long polymer waveguide at a launched pump power of 25 mW. The small-signal gain measured in a 1-cm-long sample is 2.0 dB/cm. In Al₂O₃, a peak gain of 1.57 dB/cm in a short and 3.0 dB in a 4.1-cm-long waveguide is obtained at 880 nm. Tapered multi-mode Nd³⁺-doped amplifiers are embedded into an optical backplane and a maximum 0.21 dB net gain is demonstrated in a structure consisting of an Al₂O₃:Nd³⁺ amplifier placed between two passive polymer waveguides on an optical backplane. The gain can be further enhanced by increasing the pump power and improving the waveguide geometry, and the wavelength of amplification can be adjusted by doping other rare-earth ions.

Linear optical switching in a FLC/waveguide composite device

In recent broadband photonic network system, high speed optical data transmission is required. Electrooptic (EO) polymers are attractive because of their large nonlinearity, fast response time and ease of device fabrication. Therefore the EO polymers are expected for the application of ultra-high speed EO switch and/or wide bandwidth EO modulator, both of which will be used in future photonic network. In this report, recent progress of EO polymers and their application to waveguide type devices are described.

A new host-guest electro optic (EO) polymer, in which a chromophore can be reacted with the polymer main chain during poling to give the corresponding side-chain EO polymer, has been prepared for improving EO effect and its thermal stability. Polyisoimide (PII) synthesized from 2, 2-bis (4-aminophenyl) hexafluoropropane and oxydiphthalic anhydride and Disperse Red 1 (DR1) were used as a host and a guest, respectively. A model compound reaction and Infrared spectra of the host-guest film after annealing at various temperatures show that the reaction between the isoimide groups in PII and the hydroxyl groups in DR1 occurs around 140 °C. The glass transition temperatures of the resulting EO polyamic aicd ester-imide copolymer with 0, 10, 20 and 30 wt. % of chromophore concentration were 275, 219, 160, and 124 °C, respectively. The EO coefficient obtained at a wavelength of 1.55 νm was 5.3 and 10.5 pm/V from the EO polymer film with 20 and 30 wt. % DR1. The EO signals exhibited only a slight decay at high temperature due to the chemical reaction between the host and guest during poling.

The fabrication and characterization of a 1×4-element integrated beam former based on electro-optic (EO) polymer phase modulators (PM) is summarized including findings related to waveguide loss, near-, and far-field diffraction patterns. Based on this device, two alternate designs are proposed to improve performance: an unequally spaced 1×4- element and an equally spaced 1×8-element beam former both based on EO polymer PMs. Models of these alternative designs indicate that waveguide propagation loss and strong grating lobes, both of which degraded the performance of the initial device, can be substantially improved. The proposed approach explored by the 8-element beam former introduces novel components including strain-induced optical waveguides, hybrid integration of active and passive polymers, modified waveguide diffraction aperture, and directional coupler thermo-optic (TO) switches.

Multimode interference waveguide switch of electro-optic polymer with tapered access waveguides

The electro-optic (EO) polymers of Y-type polyurethane-imides (PUI-1 and PUI-2) were synthesized by reaction of monomer azo-based chromophores, phenyl diisocyanate and aromatic dianhydride, based on polycondensation mechanism. Molecular structure characteristics for the polymers were evidenced by 1HNMR, FTIR, elemental analysis and gel permeation chromatography. The polymers exhibited a glass transition temperature (Tg) of 185 °C and 192 °C and a temperature of 5% weight loss at 256 °C and 325 °C and showed EO coefficient, γ33, of 43 pm/V and 51 pm/V at 1550 nm wavelength. A type of single-mode embedded PUI polymeric inverted rib waveguide Mach-Zehnder (MZ) interferometer electro-optic modulators was fabricated and measured, and exhibiting favorable electro-optical modulation response and the average propagation loss of the waveguide was less than 2.0 dB/cm at 1550 nm. The experimental results indicated that the PUI were promising candidates for the preparation of comprehensive performance excellent EO polymer waveguide materials, due to their large nonlinear optic effects, thermal stability, good processability and low optical propagation loss.

A tunable wavelength filter was proposed and demonstrated by using the UV nanoimprint technique. It consists of a Bragg grating in polymer waveguides and a heating electrode. The manufacturing of the grating was substantially simplified with the introduction of a smart imprint stamp containing a waveguide pattern integrated with the grating pattern. The center wavelength of the filter was successfully tuned by taking advantage of the thermooptic effect in polymers, which was induced by supplying electrical power to the electrode. For the fabricated device, a transmission dip of 15 dB and a 3-dB bandwidth of 0.8 nm were obtained at the Bragg wavelength of l560 nm. The achieved thermooptic tuning efficiency was 0.28 nm/mW, while the center wavelength was shifted from 1560 nm to 1558 nm with the electrical power consumption of 7 mW.

A tunable wavelength filter was proposed and demonstrated by using the ultraviolet nanoimprint technique. It consists of a Bragg grating in polymer waveguides and a heating electrode. The creation of the grating was substantially simplified with the introduction of a smart imprint stamp containing a waveguide pattern integrated with the grating pattern. The center wavelength of the filter was successfully tuned by taking advantage of the thermooptic effect in polymers, which was induced by supplying electrical power to the electrode. For the fabricated device, a transmission dip of ∼15dB and a 3dB bandwidth of 0.8nm were obtained at the Bragg wavelength of ∼1560nm. The achieved thermooptic tuning rate was ∼0.28nm∕mW, while the center wavelength was shifted from 1560nm to 1558nm with the electrical power consumption of 7mW.

The full potential of second order nonlinear polymers can be utilized in electro-optic polymer modulators with a DC biased operation scheme to greatly reduce the V<SUB>(pi</SUB> ). This technique makes use of the total achievable electro-optic coefficient, which can be more than three times as high as the residual value after the fast partial relaxation following corona or contact poling. As the result of the DC bias and with high (mu) (beta) chromophores, a low V<SUB>(pi</SUB> ) of 1.5 V was achieved with 2 cm long birefringent waveguide modulators at the wavelength of 1.3 micrometer. Results of 200 degrees Celsius stability experiment indicate that this scheme also enables electro-optic polymer devices to meet the stability required for high temperature hermetic sealing because the polymer does not need to be poled before device packaging.