A tunable channel selector operating over both the C and L bands of wavelength-division-multiplexing optical network is proposed based on a thermo-optic tunable Bragg grating device in a polymer waveguide. A tilted Bragg grating cascaded with an asymmetric Y-branch waveguide effectively implements a small Bragg-reflection wavelength filter that does not require an external circulator. To increase the operating wavelength span of the channel selector, two Bragg gratings with different periods are fabricated on a single substrate for covering C and L bands, respectively. A wide tuning range over 80 nm along with a narrow bandwidth is demonstrated. Moreover, the polarization dependence of the reflection spectrum is observed to be less than 0.1 nm.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Wavelength channel selection in fiber-optic communications based on wavelength division multiplexing (WDM) is necessary to retrieve useful information from a multiplexed optical signal . Channel selection is currently being performed using a passive wavelength demultiplexer. Light signals with different wavelengths are separated in space, and the information from each wavelength is retrieved by a separate detector . Arrayed wavelength gratings (AWGs) have been widely used for this purpose [3–5]. However, if an efficient wavelength channel selector is available, one can select the required wavelength channel alone directly, similar to the case of electrical channel selectors made of tunable frequency filters that filter just a single required frequency channel. Simple and inexpensive tunable filters for channel selectors, which can reduce the complexity of WDM receivers, have long been pursued.
Optical wavelength filters based on various materials and operating principles have been widely investigated. The reflection wavelength of a Bragg grating in an optical fiber was tuned by imposing a strain with a piezo-electric transducer; however, it was rather bulky [6–8]. An acousto-optic tunable filter in LiNbO3 was investigated, which could achieve a wide tuning range . In silicon photonic devices, owing to the high integration capacity, a ring resonator device of small dimensions could be fabricated [10–12]. The ring resonator, however, had narrow free spectral range (FSR), and cascaded rings were required to increase the FSR.
Polymer waveguide devices have been investigated, because of their large thermo-optic (TO) coefficient and high thermal insulation, which enables significantly high refractive-index tuning efficiency . Digital optical switches were the first to demonstrate the merit of the large TO effect, because polymers could produce the significantly large refractive-index changes required for the switching by adiabatic mode evolution [14, 15]. Polymeric variable optical attenuators (VOAs) based on perturbed mode filtering are essential in WDM optical communication . It is leading the commercial penetration of a 40-channel arrayed VOA prior to the MEMS, silica, and silicon photonic devices which experienced difficulties in increasing the number of channels because of the low production yield [17–21].
The large TO effect has been applied to develop a tunable laser consisting of a semiconductor gain medium and a polymer-waveguide Bragg reflector in the form of an external cavity, to produce low-cost tunable lasers for WDM passive optical network (PON) applications . The Bragg reflector in the polymer waveguide is tunable by 40 nm at 1550 nm, which corresponds to a refractive index change of 2.5% for a polymer with a TO coefficient of 2.5 × 10−4 .
In this work, we demonstrate a widely tunable compact wavelength filter suitable for a WDM channel selector covering the entire C and L bands. By incorporating a tilted Bragg grating and a mode-sorting Y-branch, the device operates without requiring an external circulator [24, 25]. The tunable polymeric Bragg reflector was first adopted in tunable external-cavity lasers where only a single polarization was present and polarization dependence was not an issue. However, in wavelength filters, polarization-independent operation is crucial because the optical signal arriving at the destination has no defined polarization. The polarization dependence of integrated optic devices has been a serious problem in producing useful devices for practical applications [26, 27].
Low-birefringence polymer device was previously investigated using a halogenated acrylate polymer . However, there was some difficulty with this polymer in producing the low birefringence thin film, and it is no longer available. We have been developing a low-birefringence UV-curable fluorinated polymer for reducing the polarization dependence caused by the molecular orientation and volume shrinkage during the curing. This polymer also provides excellent-quality thin films with solvent resistivity for producing a multilayer polymer film. The polymeric tunable wavelength filter in this work exhibits polarization dependence less than 0.1 nm in the reflection spectrum, which is good enough to filter the dense WDM signal with a channel spacing of 0.8 nm. The tuning range of the filter can be extended to cover the C and L bands by using two Bragg gratings, and they can be combined with a C/L-band wavelength division multiplexer to produce a tunable filter covering both bands. Wavelength tuning over 40 nm can be achieved in a simple manner by applying current to a single microheater, and tuning efficiencies as high as 117 nm/W can be achieved by incorporating a trenched bottom heater configuration.
2. Design of the mode conversion efficiency
To realize a wavelength tunable filter covering both the C and L bands of WDM optical communication, two wavelength filters are fabricated, one for each band, and they are connected with external wavelength-division couplers, because the TO tuning of a single-wavelength filter has a limited tuning range. To extract the reflection signal coming back to the input path, without using an external circulator, an asymmetric Y-branch and a tilted Bragg grating are incorporated. The proposed structure including an external WDM coupler is shown in Fig. 1. The input light launched into the narrow waveguide gradually evolves into an odd mode as it propagates through the asymmetric Y-branch waveguide. The tilted Bragg grating reflects the evolved odd mode and converts it into even mode. Then, the reflected even mode returns to the asymmetric Y-branch again, and it evolves into the wide waveguide [29, 30]. This series of mode conversion processes occurs in the order of narrow waveguide mode, odd–even modes of Bragg grating waveguide, and wide waveguide mode, and can be abbreviated as a NOEW conversion. The efficiency of the NOEW mode conversion is affected by the crosstalkof the adiabatic mode evolution in the asymmetric Y-branch and cross-coupling efficiency in the tilted Bragg grating reflector.
Through the beam propagation method design, the mode evolution crosstalk in the asymmetric Y-branch can be calculated, and the dependence on the branch angle and the width of the asymmetric waveguide is obtained as shown in Fig. 2(a). The waveguide consists of a core polymer with a refractive index of 1.399 and a cladding polymer with a refractive index of 1.379. The dimension of the single-mode optical waveguide core is 4 × 2.5 μm2, which is chosen to increase the reflectivity of the tilted Bragg grating. The modal crosstalk increases as the branch angle increases because of higher-order mode scattering. From the results, the minimum crosstalk is obtained when the wide and narrow waveguide widths and are 4 and 3 μm, respectively. The refractive-index distribution of the titled grating is given as follows, with a tilt angle and period Λ
The mode conversion efficiency between the odd and even modes, , depends on the tilt angle, and can be calculated using the mode-overlap integral .
and are the electric field distributions of the even and odd modes, respectively. The mode conversion efficiency of the tilted Bragg grating is shown as a function of in Fig. 2(b). To prevent undesired reflection peaks, the odd–odd mode conversion efficiency should be minimized. According to the design results, for of 2.5°, becomes the minimum and becomes −1.9 dB.
Three mode conversions should be multiplied to result in a final device characteristic as shown in Fig. 2(c). For the asymmetric Y-branch with a crosstalk of −25 dB, the conversion efficiencies of NOEW, narrow-odd-odd-wide (NOOW), and narrow-even-odd-wide (NEOW) are drawn as functions of , and that of NEEW is negligible to show. Compared to the reflection of −2 dB for the ideal NOEW conversion for of 2.5°, the others have efficiencies less than −40 dB.
As the tilt angle increases, as shown in Fig. 2(b), the odd–even mode conversion efficiency reduces, resulting in a reflectivity lower than that of the perpendicular grating for = 0. To compensate for the reflectivity decrease, without increasing the length of the device, a high-refractive-index polymer (HIP) is introduced. Figure 2(d) shows the reflectivity of the tilted Bragg grating when 0.5 μm of a HIP with a refractive index of 1.430 is coated over the core layer in a Bragg grating with a depth of 0.5 μm. The reflectance of the tilted Bragg grating becomes higher than that of the perpendicular Bragg grating without the HIP. For a 3.5-mm-long tilted Bragg grating, the reflectance is saturated to unity.
The uniformity of heat distribution affects the tuning range because of the radiation of the guided light. The heat uniformity can be improved by considering the electrode position and employing air trenches . Figure 3 compares the thermal distributions over the waveguides for the top and bottom electrodes. The top electrode produces a significant temperature gradient across the core. As a result, for a large temperature change, it will cause radiation of the guided light toward the lower cladding. On the contrary, the bottom electrode with an air trench exhibits excellent temperature uniformity, and will prevent guided light radiation.
3. Experimental results
An LFR polymer, which is a UV-curable fluorinated acrylate polymer material manufactured by ChemOptics, was used for fabricating the proposed channel selector device. The LFR polymer was developed to reduce optical absorption loss and birefringence by minimizing the volumetric condensation during UV curing. In addition, fluorine-rich polymers have excellent thermal stability, high optical power handling capability, and strong chemical resistance .
The fabrication procedure of the polymeric optical waveguide is shown in Fig. 4. After removing the surface oxide on the silicon substrate, an adhesion promoter was coated, and then the lower cladding LFR polymer with a refractive index of 1.379 was spin-coated to a thickness of 12.5 μm. It was UV cured in a nitrogen chamber and hard baked on a 120 °C hot plate for an hour. A microheater with a thickness of 8 nm of Cr and 320 nm of Au was fabricated by sputtering and photolithography. The cladding LFR material was coated once again over the electrode to form a lower cladding with a total thickness of 24 μm, for reducing the metal absorption loss and increasing the thermal efficiency.
The waveguide pattern was formed by oxygen plasma etching with an etch mask pattern made of a metal-oxide-dispersed photoresist. The etch ratio between the photoresist and the LFR polymer was approximately 1:15, so that a sub-micron-thick photoresist was used for the waveguide etching. The microscopic photograph of the asymmetric Y-branch region is shown in Fig. 5(a). The core LFR polymer with a refractive index of 1.399 was spin-coated on the patterned cladding layer, and the whole surface was etched until the core thickness reduced to 2.5 μm. A high-refractive-index polymer with a refractive index of 1.430 was coated to a thickness of 0.5 μm. For the Bragg gratings, by using contact photolithography, fifth-order grating patterns with periods of 2.83 μm and 2.92 μm were formed for the C- and L-band reflectors, respectively. The etch depth of the grating pattern was 0.5 μm, and an ideal maximum reflectance was expected from a tilted grating with a length of 3.5 mm. Figure 5(b) shows the microphotograph of the tilted Bragg grating formed on the two-mode waveguide. The upper cladding polymer was spin-coated and cured to produce a thickness of 10 μm. Finally, an air trench was formed by oxygen plasma etching with a Cr mask of 50 nm, for thermal insulation and electrode contact.
To characterize the Bragg reflector, an amplified spontaneous emission light source with a wavelength range of 1520 to 1620 nm was used. The input polarization was adjusted with a fiber-optic polarization controller as the measurement setup of Fig. 6(a). The reflected light returning to the output port was monitored with an optical spectrum analyzer, as shown in Fig. 6(b). TE and TM polarized light had a maximum reflection peak near 1564 nm, through the NOEW conversion, and a side lobe was observed at 1568 nm due to the NEOW mode conversion, resulting in a side-mode suppression ratio of more than 25 dB. As shown in Fig. 6(c), the reflection peak difference between the two polarizations was as small as 0.1 nm. The 3-dB and 10-dB bandwidths were 0.35 nm and 0.5 nm, respectively.
By operating the thin-film heater, the reflection wavelength was tuned as shown in Fig. 7(a). For an applied heating power of 352 mW, wavelength tuning of 41.39 nm was obtained for the C-band device. Since the TO coefficient of the LFR polymer is −2.5 × 10−4/°C, the corresponding temperature was 146 °C. In the case of an L-band device with an initial reflection wavelength of 1615.12 nm, wavelength tuning of 44.07 nm was achieved by applying 356 mW, as shown in Fig. 7(b). Both devices exhibited output power variations less than 1.5 dB across the entire tuning range. The channel switching speed was less than 20 msec, and the wavelength stability was maintained within 0.16 nm by incorporating a wavelength locking feedback.
The fifth-order Bragg gratings for the C and L bands were fabricated on a Si substrate. However, for the wavelength-tuned transmission experiment, the two devices were packaged separately. The process of packaging a single device is shown in Fig. 8. The polymer chip was stacked inside the case along with an avalanche photodiode (APD) module, a thermo-electric cooler (TEC) plate, and a thermistor, and a flexible printed circuit board was used for the feed-through. A lensed single-mode fiber was aligned with the polymer waveguide, and the waveguide output was reflected upward from the submount, by using a 45° mirror, and received by the APD. A metal optical bench was placed under the polymer chip module and a TEC was inserted. The polymer module mounted inside the case is shown in Fig. 8(b), and the final package after the wire bonding and receptacle connection, which is ready for the channel selector transmission experiment, is shown in Fig. 8(c).
The performance of the tunable channel selector was tested under a 10 Gb/s transmission network configured as shown in Fig. 9(a). A tunable laser based on the polymeric tunable Bragg grating was used as a light source , and a 10 Gb/s pseudo-random signal was produced using an external Mach–Zehnder modulator. The bit error rate (BER) of the received signal was measured while the signal intensity was controlled by the attenuator. For a certain input light wavelength defined by the tunable laser, the tunable receiver was scanned to find the signal wavelength, and a typical eye diagram with a jitter of 5.5 ps for an eye-width of 86.6 ps was obtained in an oscilloscope, as shown in Fig. 9(b). By changing the input wavelength of the tunable laser, the BER was measured as a function of the received power, for wavelengths of 1549.29 nm, 1551.26 nm, 1552.90 nm, and 1548.29 nm, as shown in Fig. 9(c), in which no significant dependence was found. For the PON applications where a forward error correction is employed, the BER requirement is reduced to 4.5 × 10−4 . As shown in the BER graph, the receiver sensitivity required to satisfy the BER requirement was approximately −26 dBm.
Tunable channel selectors useful for WDM–PON networks covering the C and L bands were proposed and demonstrated using polymeric Bragg gratings. To remove the need for a circulator, an asymmetric Y-branch waveguide was cascaded with a tilted Bragg grating. The mode-sorting efficiency of the asymmetric Y-branch and the even–odd mode conversion efficiency were crucial in deciding the reflection characteristics of the channel selector. During fabrication, a sharp Y-branch structure was implemented to produce a low-crosstalk mode evolution, and a fifth-order tilted grating was produced by direct-contact lithography, which enabled precise adjustment of the tilt angle. The fabricated tunable Bragg reflector had a sharp reflection bandwidth and low polarization dependence of less than 0.1 nm, which is crucial for a polarization-independent tunable filter. Wavelength tunings over 40 nm were achieved for each band by applying a power of 350 mW. The packaged tunable channel selector device was applied in the WDM transmission network experiment. A certain wavelength was defined by the tunable laser and then scanned by the fabricated tunable channel selector, and it exhibited a clear eye diagram and a receiver sensitivity of −26 dBm. The proposed polymeric tunable channel selector device enables polarization-independent tunable filtering at a low cost and will be useful for creating a novel WDM–PON network with a simple configuration.
National Research Foundation of Korea (NRF) (2017R1A2A1A17069702).
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