Low-loss planar optical waveguides fabricated from polycarbonate.INTRODUCTION
Polymer materials have great applications in the field of planar lightwave circuit in the past decade because of its merits of ease in fabrication and energy-saving property (e.g., large thermo-optic coefficient) (1), (2). A number of polymers such as acrylate polymer, perfluoropolymer, ultraviolet (UV) epoxy resin, and fluorinated polyimide (FPI) have so far been shown to demonstrate capability in a variety of demanding applications of optical waveguides (3). Furthermore, polymer materials have played an important role in surface plasmonics in recent years (4), (5). Materials such as benzocyclobutene (6), polymethylmethacrylate (7), and UV-curable polymer ZPU (ChemO-ptics Co.) (8) have been selected as the cladding materials for surface plasmon polaritons (SPPs) waveguides.
In general, for polymers to be used for integrated optical and SPPs devices, three main characteristics are most critical: high thermal stability, low optical propagation loss, and good processability (such as coating, adhesion, and etching). It is well known that fluorinated polymers not only have low propagation loss but also show increased chemical stability (9), but this series of polymers have the drawback of poor adhesion because of their inert nature and low surface energies (10). Conventional polycarbonate is one type of excellent polymer materials in the visible wavebands (400-700 nm), but its application in optical communication devices is limited because of its high absorption loss characteristic in the infrared waveband (11). As shown from the previous reports, the lowest propagation loss of polycarbonate waveguide is 0.6 dB/cm at 1550 nm (3).
In this article, we proposed a novel polycarbonate that has high optical transparence at the communication wavelengths. The measured results show that this polycarbonate material exhibits several merits, including low propagation loss, high heat resistance, good film formation performance, ease processability, and excellent toughness property. The propagation losses of the slab waveguides measured from prism coupler are 0.197, 0.282, and 0.335 dB/cm at monitoring light of 632.8, 1310, and 1550 nm, respectively. The ridge waveguides and multimode interference (MMI) couplers based on polycarbonate were also fabricated successfully.
Figure 1 shows the molecular structure of polycarbonate synthesized as references (12), (13). In general, both C--H and O--H overtones are highly absorptive in the communication windows (14), As it has been demonstrated that adhesion property can be enhanced by introducing polar functional groups, such as carbonate (--O--C (=O)--O--) (10), polycarbonate shows both high optical transparence and good adhesion properties via the replacement of O--H bonds with polar functional carbonate (--O--C (=O)--O--). The glass transition temperature is 293[degrees]C, showing higher heat resistance than bisphenol-A-polycarbonate (15), (16).
[FIGURE 1 OMITTED]
Optical Characteristics of Polycarbonate Film Waveguide
The polycarbonate was dissolved in 1,2-dichloroben-zene to form a viscous solution (8 wt%), then the solution was filtered by a 0.2-[micro]m Teflon glass syringe filter two times, and then spin-coated onto a glass substrate to form a slab waveguide. The thickness of the slab waveguide was controlled by changing the spin speed and the concentration of polymer solution. By choosing a proper fabrication condition, a 3.3-[micro]m smooth polymer film without dust, crack, and bubbles was achieved. To entirely remove the residue solvent and reduce the stress-induced scattering, a multistep thermal treatment on the polymer film was adopted. First, the film was baked at 135[degrees]C for 5-6 h, then heated at 120[degrees]C for 12 h in a vacuum oven, and finally slowly cooled to room temperature to get the high-quality polycarbonate slab waveguide.
For low-loss optical waveguides, a promising polymer must be almost apparent at the communication wavelength range. Thus, we measured the absorption spectrum of the fabricated polycarbonate film using spectrum analysis. The measured results can be seen from Fig. 2.
[FIGURE 2 OMITTED]
It demonstrates that the polycarbonate film has a very low absorption at the visible and communication wavelengths. Especially, the value of absorption coefficients at 1310 and 630 nm is generally the same. However, the absorption at 1550 nm is a little higher than at 1310 nm, the wavelengths of which are used in both optical telecommunications. Periodic modulation in the absorption curve demonstrates uniform film thickness of polycarbonate film. In a word, the data indicate that the novel polycarbonate has good optical transparence at the communication wavelengths.
To characterize the thermo-optic coefficient (dn/dT) at 1550 nm, film was measured by the Prism Coupler SPA4000, which is a precise temperature-controlling system. The result is shown in Fig. 3. The thermo-optic coefficient of polycarbonate is ~ - 1.06 X [10.sup.-4][degrees][C.sup.-1] at 1550 nm from a least-squares fit of the data, which is lower than alternative low-loss polymers, such as Norland Optical Adhesive 61 (NOA61) (-2.46 X [10.sup.-4] [degrees][C.sup.-1]) (17) and FPI (-1.3 X [10.sup.-4] [degrees][C.sup.-1]) (18). It shows that polycarbonate has a high thermal stability. Meanwhile, the refractive index of the polycarbonate was also measured to be 1.60 at 1550 nm.
[FIGURE 3 OMITTED]
Furthermore, the propagation loss of the fabricated polycarbonate slab waveguide was also measured by Prism-Coupler (SPA4000) for TE mode at 632.8, 1310, and 1550 nm, respectively. The light was coupled to slab waveguide by prism-coupling method, and an index matching liquid was used for optimum output coupling. The measured results of optical loss at 1310 and 632.8 nm are 0.282 and 0.197 dB/cm, respectively, as low as some commercial polymer materials (1), (11). Besides, at the communication wavelength of 1550 nm, the loss is 0.335 dB/cm, which is far less than the reported value of traditional polycarbonate (0.60 dB/cm at 1550 nm) (3). In general, many factors contribute to the propagation loss of optical waveguides, such as scattering and process-induced effect besides intrinsic absorption loss of material itself, caused by impurities of solution, uniformness of spin-coating step, and heat treatment of film (15). Both the absorption loss and process-induced scattering loss are very low when compared with the total loss. Thus, the major part of the measured loss at 1550 nm should be attributed to the loss induced by the impurities of solution. Therefore, by optimizing the filtering process, the propagation loss could be further reduced.
Figure 4 shows the transmission path of light of 632.8 nm in the waveguide. The image was captured by charge-coupled device (CCD) camera, demonstrating the excellent transmission characteristic.
[FIGURE 4 OMITTED]
In the fabrication of optical waveguide devices, the structure of waveguides on central part of substrate was used for testing, whereas the edge part was cut for polishing the facet. As to the prism-coupling method, the measured waveguide length was required to be 6-8 cm, and the B area (see Fig. 4) was taken to measure by immersing in the matching liquid. As is known, the quality of the margin part of film is poorer than the central part because of the spin-coating, leading to the high propagation loss as well. It can be observed from Fig. 4 that the attenuation degree of light is higher in the B area than A when light propagating from A to B.
To characterize the propagation properties of planar optical waveguides in the central part, CCD camera imaging method is introduced here to analyze the transmission property of the fabricated polycarbonate waveguide at 632.8 nm, which is a new method to characterize the propagation property of planar optical waveguides. This technique has the advantages of being nondestructive, requiring low guided power, independent of the coupling efficiency and facet reflectivity. When light travels along a thin film waveguide, the scattered light can be observed because of the light scattering. The scattered light is taken to be directly proportional to the intensity of the guided light inside the waveguide. Therefore, the variation of the scattered light along the light path can be viewed as the variation of coupled light intensity in the film waveguide (19).
The relation between scattered light intensity and propagation loss can be expressed as the following (20):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
where [z.sub.1] and [z.sub.2] are any points along the propagation length. [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are the intensity of the scattered light. The loss coefficient [alpha] can be deduced from formula (1)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
Therefore, the propagation loss of the optical waveguide can be nondestructively measured by monitoring the intensity of the scattered light along the propagation direction and fitting this variation with a monoexponential formula. We used a CCD camera to take pictures of light beam in the waveguide and got the two-dimensional scattered light intensity distribution (see Fig. 4). The length of the light streak of A is 3.00 cm, and the relative intensity at each position along the propagation direction was obtained by the integration of the light intensity perpendicular to the propagation direction within the width of waveguide. The relative scattered light intensity versus the propagation position was then plotted as shown in Fig. 5.
[FIGURE 5 OMITTED]
From the linear least-square fit of this plot, the propagation loss coefficient for the central part of the slab waveguide was measured to be 0.07 dB/cm at 632.8 nm.
In combination with the above two methods, we evaluate the overall transmission performance of polycarbonate film. It indicates a relative low propagation loss in the area of spinning center of the film, which was generally chose for fabricating low-loss optical waveguides devices, whereas the input and output facets of the waveguide were cut for polishing.
On the basis of the film fabrication technique, we successfully fabricated the straight optical waveguides and MMI couplers on K9 glass substrate through conventional photolithographic patterning and reactive ion etching (RIE) processes. To define the waveguide patterns on the polycarbonate core layer, a positive RZJ-304 photoresist (available from Suzhou Ruihong electronic chemicals Co., China) was spin-coated on top of the 3.3-[micro]m polycarbonate films and prebaked for 90 s at 100[degrees]C on the hotplate. Next, photolithographic process was carried out to transfer the pattern of mask into photoresist layer, and the ridge pattern of core layer was then formed by RIE using oxygen. The width of straight optical waveguides and MMI couplers is shown in Fig. 6. It shows the electron microscope image of waveguides after core-etching processing, revealing smooth sides and excellent definition.
[FIGURE 6 OMITTED]
After the core layer was coated with NOA61 as over-cladding layer and a piece of microscope glass slide was bonded with NOA61 to form a protective cover at the same step, the sample was cured completely and then the end-faces were polished to a optical quality to allow for direct end-face coupling of light. Light from a He-Ne laser (632.8 nm) was coupled to a single-mode optical fiber using a microscope objective (12.5X) and directly to the ridge waveguides. Figure 6c and d is the light coupled in the straight waveguide and MMI coupler. One can see clearly that the incident light from the input waveguide splits into two parts at the end of the multimode section and each part then passes through one output waveguide. The scattering losses are mainly caused by the multimode transmission of light. Theoretically, the structures of our waveguides are designed for single-mode transmission at 1550 nm wavelength. So in our following study, optical waveguide for end-fire coupling testing of light at 1550 nm will be carried out. According to the theoretic layout of ridge waveguide, single-mode propagation of light can be achieved.
In this article, a novel polycarbonate material was used to fabricate optical waveguides. Characterizations of slab waveguides were studied in detail. The polymer exhibits high thermal stability, ease processability, good adhesion to substrate, excellent toughness properties, and low propagation loss in the visible and communication wavelengths. With these advantages, straight optical waveguide and MMI coupler were fabricated using photolithographic and RIE procedures. The experiment results show that the novel polycarbonate has great potentials in the field of integrated optical applications and surface plasmonics. However, lower losses are expected for further optimization of the waveguide fabrication process and the annealing treatment in making optical waveguides devices in our following study.
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Jian-guo Chen, (1) Tong Zhang, (1) Jin-song Zhu, (2) Xiao-yang Zhang, (1) Jing-lun Zhou, (2) Jiang-feng Fan, (2) Guo-hua Hu (1)
(1) School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China
(2) National Center for NanoScience and Technology (NCNST), Beijing 100080, China
Correspondence to: Tong Zhang; e-mail: firstname.lastname@example.org
Contract grant sponsor: Ministry of Education of the People's Republic of China (Program for New Century Excellent Talents in University); contract grant number: NCET-05-0465.