1. Introduction

Multiplexed optical signal comprising many carrier wavelengths has to be passed through a wavelength selecting devices such as arrayed waveguide gratings and tunable wavelength filters in order to select a desired channel of information. Filtering of a specific wavelength from the multiplexed optical signal is an inevitable procedure for WDM communication systems as well as optical sensors relying on spectrum monitoring [1, 2].

Bragg reflection type tunable filters have the advantage over the other type of devices such as Fabry-Perot interferometers [3] and ring resonators [4] in terms of the tuning range and the spectral width. Moreover, Bragg reflectors have nothing to do with free spectral range, which is crucial constraint of the other tunable filters. To perform the tuning of the Bragg reflection wavelength, an axial strain was induced on a fiber optic device [5] and a micro-heater was used in a polymeric waveguide device [6, 7]. In case of polymeric Bragg grating devices, sub-micron period grating patterns could be replicated by the nano-imprinting for the low cost fabrication [8, 9].

Polymeric tunable wavelength filters have been recently adopted for the demonstration of a cost-effective tunable laser with a tuning range of 26 nm for an applied thermal power of 98 mW [10]. The recent progress of optical components for communication system enables one to use the entire spectrum of about 150 nm supported by a low loss fiber. To meet with the demand of wide bandwidth, it is necessary to investigate another method to extend the tuning range of the polymeric tunable wavelength filter rather than the current method based on thermo-optic effect of polymer material. Assuming the thermo-optic coefficient of polymer is between 2×10^-4 and 4×10^-4/°C, the wavelength shift obtainable by applying temperature change of 100°C is 20 to 40 nm (0.2~0.4 nm/°C). Continuous operation at high temperature could accumulate damage on the polymer and increase the chance of heater breakdown [11]. Additionally, the thermo-optic index change in the middle of the waveguide introduces abrupt discontinuity of refractive index along the propagation direction and, in consequence, the guided mode undergoes significant additional loss leading to the waveguide cutoff.

To increase the tuning range of the polymer waveguide Bragg reflector beyond the limit of thermo-optic tuning, a strain induced tuning method is proposed in this work using a Bragg reflecting waveguide fabricated on a flexible polymer substrate. When the device is fabricated on a flexible substrate, it exhibits much smaller elastic modulus to enable much wider tuning range than the silica fiber device. The flexible polymer device also has an advantage of reduced temperature sensitivity due to the compensation of thermo-optic index change by the thermal expansion of the substrate [12].

A post-lift off process is applied for the fabrication of the flexible grating device in order to increase the uniformity of grating structure on a flexible substrate. Additionally, an absorbing material is employed to prevent undesirable interference during the grating fabrication. As a result, a clear grating pattern is produced over a wide area of the flexible substrate, which improves the sharpness of transmission spectrum. Wavelength tuning is demonstrated by imposing a tensile strain in an axial direction of the flexible waveguide. Wide tuning range overcoming the thermo-optic tuning is observed for the first time. Photoelastic coefficient of the fluorinated polymer material is also obtained from the experimental results of this work.

2. Design and fabrication

A compact apparatus for producing a strain on a flexible polymeric waveguide device is devised as shown in Fig. 1. To prevent the increase of propagation loss by micro bending of waveguide, the flexible grating device is supposed to be pushed with a rounded cylindrical object connected to a micrometer screw. A tensile strain is produced in the polymer waveguide by closing the moving part toward the fixed part of package. Bragg reflection grating located in the middle of the device experiences a tensile strain, which leads to the increase of the grating period and the red shift of the Bragg reflection wavelength.

 

Fig. 1. Schematic structure of the tunable wavelength filter operated by applying mechanical stress to impose a strain on a flexible polymer waveguide with Bragg reflector.

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Flexible polymer waveguide is comprised of a flexible polymer substrate and a low loss polymer waveguide. The flexible substrate is supposed to have small elastic modulus and large elongation property. The unique property of the polymer waveguide which can be fabricated by spin-coating allows it to be formed on a flexible plastic substrate. The refractive indices of the polymer materials chosen for the device design are 1.455 and 1.430 for the core and cladding layers, respectively. The large index contrast of the waveguide is favorable for increasing the sensitivity when the device is applied for a biosensor [13]. The single mode condition in this large contrast waveguide could be accomplished by using a procedure to design the oversized rib waveguide [14]. It has been found, by calculating the effective index of the oversized rib structure, a waveguide with a core dimension of 4 µm×6 µm and an etch depth of 2.1 µm satisfies the single mode condition.

To form a grating in the middle of the polymer waveguide, a periodic index perturbation is required along the waveguide propagation direction. Large index perturbation could be achieved by using a high index polymer materials formed between core and cladding layers [15]. However, in case of large contrast waveguide, sufficiently high index perturbation could be achieved by modulating the thickness of the waveguide core layer. With the index contrast of 0.015 and the core thickness of 4 µm, the effective index of waveguide is modulated by 1.2×10^-3 for the core layer thickness modulated by 200 nm. Then, according to the transmission matrix calculation, the reflectivity becomes higher than 99% for a 5-mm long Bragg reflector.

Direct fabrication of grating device on top of the flexible substrate was hard to achieve due to the uncontrollable thermal expansion of the plastic material. Hence, we fabricated the whole device including plastic substrate on a hard silicon wafer, and then the plastic substrate was lifted-off at the end of fabrication step. The schematic outline of the fabrication procedure is illustrated in Fig. 2. As a first step, on a Si wafer with a patterned Au/Cr layer, SU-8 polymer was coated to have a thickness of 0.7 µm. The SU-8 polymer has a distinct property that it has poor adhesion on Au surface and good adhesion on silicon surface [16]. Therefore, at the end of device fabrication, a selective area formed on Au surface would be lifted-off as the area attached on silicon surface is diced out. For this purpose, Au/Cr metal layer was patterned considering the size of the flexible substrate. On top of the SU-8 polymer, NOA61 polymer with a thickness of 100 µm was coated for the purpose of the flexible substrate. Then, another layer of SU-8 material was coated on NOA61 to increase the surface hardness for the subsequent polymer waveguide fabrication.

After fabricating the flexible substrate layer, a lower cladding of ZPU polymer [17] with a refractive index of 1.430 was coated to have a thickness of 8 µm. The film was cured for 3 min in a UV chamber, and then hard baked at 160°C for 30 min. After coating a photoresist on the lower cladding, an interference pattern made by 488-nm Ar laser was exposed to form a grating pattern, which was transferred to the lower cladding layer by the following O2 reactive ion etching (RIE). As a core layer, another ZPU polymer with a refractive index of 1.455 was coated over the grating pattern. Optical waveguide pattern was defined on the core layer by a conventional photolithography using AZ5214 photoresist. Then the photoresist pattern was transferred to the core layer by the RIE. The ZPU polymer used for lower cladding was spin coated once again to form an upper cladding layer. The flexible grating device was completed by lifting-off the flexible substrate area after dicing the chip. Without additional polishing, single mode fibers were pigtailed for both waveguide ends formed by dicing with a diamond blade.

 

Fig. 2. Schematic procedures for fabricating the Bragg reflection waveguide on a flexible substrate by using a post lift-off process.

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During the grating fabrication, we observed long period fringe patterns caused by undesirable interference due to the reflection from the Au surface prepared for selective lift-off process. The pattern caused thickness variation of the grating structure, and became a serious factor of spectral broadening in Bragg reflector. Hence, to remove the reflected wave, we incorporated a highly absorbing material, black matrix which was widely used for liquid crystal display to block the transmission through a thin film transistor area. The black matrix material consists of carbon black nanoparticles dispersed in a UV curable acrylate backbone. It has extremely high absorption property for visible wavelength with an attenuation of 30 dB for 1.5~2.0 µm thickness. The material was spin coated on the flexible substrate layer with a thickness of 200 nm, which was sufficient to attenuate the reflected light. The grating patterns formed on a thick NOA61 polymer layer is shown in Fig. 3. The sample fabricated with the black matrix layer exhibits significantly better uniformity than the other sample.

 

Fig. 3. Comparison of the uniformity of grating patterns fabricated on top of a thick NOA61 polymer layer: (a) fabricated by a standard procedure, (b) fabricated by incorporating black matrix to remove the undesirable interference.

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3. Measurement

To characterize the fabricated device over a wide tuning range from 1480 nm to 1600 nm, a superluminescent light emitting diode was prepared. The light was passed through a polarization controller and an optical circulator before it was launched to the device through a pigtailed single mode fiber. The polarization dependence of the current device was measured to be 0.65 nm because of the material birefringence and the waveguide structure. Hence, the input light was adjusted to TE polarization throughout the measurement. When this device is employed for an external reflector of tunable laser, the polarization of the laser should be adjusted to a proper direction. Moreover, the polarization dependence of polymeric Bragg reflector has been reduced to less than 0.1 nm by incorporating low birefringence polymers recently developed.

The transmission and reflection spectra measured from the flexible device are shown in Fig. 4. Bragg reflection peak is initially located at 1496.4 nm. For 5-mm long Bragg grating, the reflection spectrum exhibits a 3-dB bandwidth of 1.2 nm and a 10-dB bandwidth of 2.1 nm. In the transmission spectrum, we observed very sharp dip with a 3-dB bandwidth of 0.2 nm and a 10-dB bandwidth of 1.0 nm, which could be useful for optical sensors. The narrow bandwidth convinced how uniform the grating was on the flexible substrate. The additional dips observed in the transmission spectrum were due to the higher order mode coupling in the over-sized rib waveguide. It could be eliminated by modifying the waveguide design. The transmission power was decreasing for the wavelength shorter than 1500 nm due to the material absorption of ZPU polymer caused by resonance absorption of C-H vibration overtone.

 

Fig. 4. Transmission and reflection spectrum of the Bragg reflection waveguide device fabricated on a flexible substrate.

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For the first demonstration of strain induced tuning in polymeric waveguide device, the sample was attached on a linear micro stage as shown in Fig. 5 to produce controllable amount of strain on the waveguide device in axial direction. We applied an axial strain instead of the bending strain in order to avoid any possible confusion in the analysis of the results. By rotating the micro-meter step by step, the sample was stretched to produce tensile strain. From the initial position with no strain, the micro stage was moved by 30 µm per step for 15 times. Transmission spectrum of the device was measured for each step as shown in Fig. 6. Bragg reflection wavelength was shifted by 3.0 nm on each step and 45.4 nm for the final step toward the longer wavelength. When the strain was increased over than 28000 με, the depth of transmission spectrum was gradually decreased and eventually disappeared for the strain over 40000 με. The decrease of reflection dip was caused by the irregular expansion of the grating period which could happen when the material approached the elastic expansion limit.

 

Fig. 5. A photograph of the flexible substrate device attached on a linear micro stage for measuring the wavelength tuning characteristics induced by a tensile strain.

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The maximum displacement during the experiment was determined by assuring the reversible operation of the wavelength tuning. To obtain a set of transmission spectrum data in a repeatable operation range, the range of displacement was increased after completing a set of measurement. In each set of measurement, the sample was stretched up to a certain maximum range by a step of 30 µm, then the sample was returned to the original position to assure that the original spectrum was preserved. In the subsequent measurement, the maximum range was increased by one step, 30 µm. Until the maximum elongation distance of 450 µm, the initial spectrum was maintained. However, when the sample was stretched by 480 µm in the next set of measurement, a change of reflection spectrum was observed, and then the initial spectrum was no longer preserved. From this experiment the maximum elastic deformation limit was found to be about 3% in the fabricated device structure. Comparing the thickness of materials incorporated in the device, NOA61 plastic substrate material could be a deterministic material limiting the tuning range. However, though its total thickness is only about 20 µm, the ZPU polymer which is harder than NOA61 may restrict the maximum elastic expansion.

 

Fig. 6. Wavelength tuning characteristics of the Bragg reflector: (a) Transmission spectra measured for each step of micro-stage movement (b) Bragg reflection wavelength as a function of the micro-meter displacement and the imposed strain.

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From the experimental data shown in Fig. 6(b), the photoelastic coefficient of the ZPU polymer could be calculated precisely. The relation between the resulting strain Δε and the shift of Bragg reflection wavelength Δλ_B is given as follows:

where, P_ε is the effective photoelastic coefficient [18]. The maximum strain we have imposed in the device was 31,690 με (3.169%) because the length of flexible part of the device was 14.2 mm, and the total elongation distance was 450 µm. The total wavelength tuning of 45.4 nm from 1496 nm requires 3.038% change in the grating period. So the ratio of the normalized wavelength change over the strain becomes 0.957, then the effective photoelastic coefficient becomes 0.042, which is much smaller than that of the silica optical fiber, 0.22. Hence, the wavelength tuning in this experiment was occurred predominantly by the mechanical elongation. The effect of material index change by the induced strain was negligible.

4. Conclusion

Wavelength tuning of a polymeric Bragg reflector was demonstrated by imposing a tensile strain on a flexible waveguide grating device. For the fabrication of grating pattern on a flexible substrate with a good uniformity, a post-lift-off technique along with a highly absorbing layer was incorporated. The flexible Bragg reflector exhibited a 3-dB bandwidth of 0.2 nm and a 10-dB bandwidth of 1.0 nm, which was verifying how uniform the grating structure is on the flexible substrate. By imposing a tensile strain on the flexible polymer device, the Bragg reflection wavelength was tuned continuously up to 45.4 nm for the applied strain of 31,690 με until the ZPU polymer could maintain elastic property. From the linear wavelength shift proportional to the strain, the photoelastic coefficient of the ZPU polymer was found to be 0.042. The tuning range could be extended by incorporating highly elastic polymer material.