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Abstract
Tunable optical filter is a basic component for most optical systems. This study reports a unique design of Fabry-Pérot (FP) tunable filter by using an ionic liquid solution. The tunable filter consists of two neighboring regions: capacitor region and FP region. The former is in the form of electrolyte capacitor and the latter remains transparent as an FP cavity for light transmission. When the capacitor region is applied with a bias voltage, it attracts the ions from the FP region and thus reduces the ion concentration of the FP region, resulting in a change of the refractive index and eventually a shift of transmission peak of the FP cavity. Among four electrolyte solutions studied, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) exhibits the best overall performance, such as low insertion loss (3.2 dB), large side mode suppression ratio (23 dB) and high stability (drift <0.2 nm). Additionally, a wavelength tuning of 0.17 nm/V is achieved over 0–17 V, providing a tunable range of 3 nm. This device features low bias voltage, no mechanical movement, easy fabrication and seamless integration with microfluidics systems, and may find potential applications in spectral analyzers and lab-on-a-chip biosensing systems.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical tunable filter (OTF) is one of the basic components of integrated optical systems for wavelength selection and tuning. It is crucial for applications such as dense wavelength division multiplexing (DWDM) systems, which transmit many wavelengths over the same fiber links and add/drop the channels by dynamically adjusting the wavelengths on demand [1]. It is also essential for wavelength selection and tuning in tunable lasers [2], along with many other diverse applications such as bio-sensing and spectral analyzing [3–6]. Various tuning methods for OTFs have been developed, commonly by adjusting the effective optical path length (OPL) of an Fabry-Pérot (FP) cavity. Given that the OPL is equal to the refractive index multiplied by the cavity’s physical length, the tuning of OPL can be achieved by either altering the cavity’s physical length or varying the refractive index of the material in the FP cavity. For instance, the thermal tuning method adjusts the temperature to change the OPL via the thermo-optic effect [7]; the piezoelectric (PZT) actuators drive the mirrors to change the FP cavity’s physical length [8]; and the liquid crystal tuning method applies a bias voltage to change the refractive index of liquid crystal element [9]. However, there are some problems in the above tuning methods, such as large dimensions, difficulty in thermal control, mechanical instability, high bias voltage and limited tuning range.
Optofluidics is a prominent field that integrates optics and microfluidics [5,10–13]. The incorporation of optofluidics and FP cavity has led to various applications particularly in biosensing and chemical sensing [14–17]. This integration is anticipated to provide a promising avenue to overcome the challenges associated with tunable filters. Optofluidic tunable filters leverage the manipulation of fluids at the microscale to tune the transmission wavelength, and offer advantages such as wide tuning range, high spectral resolution, fast tuning speed, and low power consumption [18–20]. In this context, different liquids exhibit distinct optical and electrical properties, and strongly affect the performance of fabricated OTFs. Therefore, the choice of liquid medium is crucial for the specifications of OTF, such as insertion loss, bandwidth, dynamic range, tuning speed, control mechanism, size and low price [21].
This study explores a novel optofluidic tunable filter accomplished by constructing the FP cavity into an electrical capacitor filled with a transparent electrolyte. A new working mechanism is introduced here by applying a bias voltage to tune the passing spectrum of the filter. The application of a bias voltage varies the ion concentrations in different regions of the electrolyte, altering the refractive index and causing a wavelength shift. While electrolyte capacitors are common in electrical filters [22–25], their application in optical filters is relatively unexplored. Four different types of electrolytes, including 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) ionic liquid [26], 0.1-M sodium chloride (NaCl) electrolyte, poly (3,4-ethylenedioxythiophene) (PEDOT) conductive polymer [27] and sulfonated tetrafluoroethylene based fluoropolymer-copolymer (Nafion) solution [28], have been investigated for the fabrication of electrolyte capacitor based-OTF (EC-OTF) in terms of their sensitivity, insertion loss, side mode suppression ratio (SMSR), and drifting of transmission spectrum. Among them, BMIM-PF6 filled EC-OTF exhibits superior tuning performance with the insertion loss of 3.2 dB and the SMSR of 23 dB. By applying an external voltage of 17 V, a wavelength shift of 3 nm has been achieved. This device is characterized by low bias voltage, no mechanical movement, easy fabrication, and seamless integration with microfluidic systems. Filters with these attributes hold great promise for applications in optical communication networks, spectral analysis systems, and lab-on-a-chip sensing systems.
2. Working principle
The 3D schematic diagram in Fig. 1 presents the structure of the EC-OTF, which is composed of a pair of wedge-shaped mirrors. Each wedge-shaped mirror is coated with a high reflection (HR) layer on one side (R = 0.9) and an anti-reflection (AR) layer on the other side (R < 0.002). The mirrors used for the fabrication of FP cavity is made of a silica glass substrate (L × W×H = 10 mm × 5 mm × 2 mm) coated by a stack of thin films with ∼ 100 alternating layers of MgF2 and SiO2 (Fuzhou Photop Optics Co. Ltd.) for high reflectivity (HR, > 95%) over the C band. The HR-coated side has a small polished angle of 0.5° with respect to the vertical direction of the optical axis. The small angle effectively directs unwanted reflected light out of the FP cavity, allowing only the desired wavelength to pass through the tunable filter [8]. The distance between the two HR surfaces is set at 91 µm. In the FP cavity, the coated parts facilitate tuning, while the uncoated parts enable light propagation, allowing us to effectively obtain the transmission spectrum.
Fig. 1. (a) 3D structure of tunable filter; (b) Working principle of the optofluidic tunable filter based on ionic liquid. AR stands for antireflection and HR for high reflection; (c) Simple model of the change of ion concentration in the FP region when a bias voltage is applied to the capacitor region.
Figure 1(b) illustrates the cross-section and the working principle of the EC-OTF. One of the mirrors of the FP cavity (called electrode mirror) has a 50-nm thick gold (Au) film coated on half of its HR surface, and the other mirror (called capacitor mirror) has a 50-nm thick silver (Ag) thin film and an additional 150-nm thick SiO2 layer for insulation. The Ag electrode, the SiO2 layer and the ionic liquid form an electrolyte capacitor, and the uncoated region of two mirrors remain an FP cavity for the transmission of light. For simplicity, the region of electrolyte capacitor is named as capacitor region and the region of FP cavity is called FP region (see Fig. 1(c)). When a bias voltage is applied across the two electrodes, the ions in the FP region are attracted to the capacitor region, resulting in a reduction of the ion concentration in the FP region and thus a change of the refractive index. Consequently, the resonant frequency of the FP cavity can be tuned. When the bias voltage is removed, the internal electric field disappears and then ions are diffused back to the FP cavity to restore a uniform distribution of ion concentration again.
Here, a simple model is used to analyze the EC-OTF. For simplicity, several assumptions are adopted: (1) only the concentration change of the negative ions is considered because that of positive ions is equivalent; (2) the FP region has a uniform distribution of negative ions, and the capacity edge effect is neglected; and (3) the refractive index in the central part of FP cavity is always uniform.
According to the Eq. (26.15) in [29], the capacitance of the capacitor is
where εr is the relative permittivity of SiO2, ε0 is the permittivity of vacuum, A1 is the area of capacitor region, and d is the thickness of SiO2 (i.e., 150 nm in this work). When a voltage U is applied to the capacitor section, the number of negative ions attracted to the top surface of capacitor would beThen, a molar concentration change of the negative ions ΔcM (in the unit of mol m-3) in the FP cavity is thus induced [30],
For the case of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) filled into the EC-OTF, only the concentration change of negative ion PF6- is considered in the FP cavity. Based on the prior study on the refractive index change of BMIM-PF6 in ethanol [31], the refractive index n as a function of molar concentration c (in the unit of mol L-1) of BMIM-PF6 can be linearly fitted to be
Then, the change of refractive index in the FP cavity becomes
Eventually, according to the Eq. (3.10) in [32], the peak shift of wavelength in the BMIM-PF6 filled FP cavity is expressed as
3. Experimental results
As depicted in Fig. 2(a), the two mirrors are assembled by using a 5-axis stage. In experiment, a 1550-nm laser beam is coupled into the FP region of EC-OTF using a pair of collimators. The front collimator provides the input while the rear one captures the transmitted laser beam for measurement. A sodium lamp, functioning as a monochromatic light source, is employed to illuminate the mirrors for direct visualization of the interference pattern of the two HR surfaces of the FP cavity. High parallelism is crucial for low insertion loss of EC-OTF, and also affects the ion concentration distribution and refractive index gradient within the FP cavity. When the mirrors are well aligned, it has a single Newton’s ring, signifying the minimum insertion loss. Then the FP resonant cavity is filled with BMIM-PF6 ionic liquid, which is a room temperature ionic liquid (RTIL) known for its good electrical conductivity, high ionic mobility and thermal stability properties [33]. In addition, it is highly transparent near 1550 nm. For the liquid insertion, around 6 µL of BMIM-PF6 was dropped on the top of the FP cavity. Then capillarity drives the IL to fill into cavity and the excess IL was wiped away.
Fig. 2. (a) Setup of tunable filter device; (b) – (e) Interference patterns of tunable filter under the bias voltage of 0 V (b), 5 V (c), 11 V (d), 17 V (e), respectively. The scale bars in the figures represent 1 mm.
Figure 2(b) – (e) exemplify the observed interference fringes formed by the two mirrors at different bias voltages. Initially, without any bias voltage, a circular Newton’s ring appears in the mirror center (Fig. 2(b)), indicating high parallelism of mirrors, uniform distribution of ions and minimum insertion loss. Under a bias voltage of 5 V, ions migrate to the capacitor region, introducing additional fringes and increasing the refractive index gradient in the ionic liquid within the FP cavity (Fig. 2(c)). With an increase of bias voltage to 11 V and 17 V, the center of the Newton’s rings is shifted to the electrode part due to the enlarged refractive index gradient (Fig. 2(d) and (e)). Further increase of the bias voltage to > 17 V would cause a sudden influx of ions toward the electrode sides, intensifying the density of interference fringes and making the transmission spectrum unstable. Hence, we choose the bias voltage range of 0–17 V for the EC-OTF in this study. The increased number of Newton’s rings under the bias voltage indicates the presence of a refractive index gradient within the FP cavity, originating from the corresponding gradient in ion concentration. This ion concentration gradient results from the migration of ions towards the capacitor in response to the applied bias voltage. The resulted non-uniformity in refractive index directly influences the interference patterns and resonance conditions, thereby introducing spatial dependence into the optical transmission properties. For the application of a bias voltage of 17 V, six Newton’s rings are observed in Fig. 2(e) with their centers migrating to the edge of the FP cavity (area 5 × 5 mm). As a result, the peak shift of wavelength in the FP cavity Δλ is 3λ0, and the corresponding spatial distance Δx is 5 mm. According to Eq. (3.10) in [32], the refractive index gradient Δn/Δx in the x direction becomes
Furthermore, according to Eq. (6), the negative ion gradient ΔcM/Δx in the x direction can be expressed as
A more detailed examination of the ion concentration distribution is available in the Supplement 1. The spatial variation in ion concentration necessitates a careful consideration in the design and optimization of the EC-OTF. Proper strategies to control and manipulate the spatial distribution of ions during the tuning process could be explored to tailor the device’s optical response in the future.
The measured peak shifts in the transmission spectra of the BMIM-PF6 filled EC-OTF are plotted in Fig. 3(a) when the bias range varies from 0 to 17 V. The tunable filter exhibits an insertion loss of about 3.2 dB and an SMSR of 23 dB. Moreover, the 3-dB linewidth is 0.77 nm, quite narrow as compared to the free spectral range 9.3 nm. Notably, the transmission peak of the filter is tuned from 1544.2 to 1541.2 nm, obtaining a wavelength tuning range of 3.0 nm.
Fig. 3. Wavelength tuning properties of two EC-OTF devices, one filled with BMIM-PF6 and the other filled with PEDOT. (a) Shift of transmission spectra of the EC-OTF filled with BMIM-PF6 when the bias voltage varies from 0 to 17 V; (b) Experimental (blue squares) and theoretical (red dash dot line) results of the peak shifts of the EC-OTF filled with BMIM-PF6 as a function of bias voltage; (c) & (d) Comparison of the EC-OTF device filled with BMIM-PF6 with the EC-OTF device filled with PEDOT. (c) The transmission spectra when the bias voltage is 0 V; (d) Peak shift as a function of bias voltage and their corresponding linear fitting lines.
Figure 3(b) plots the peak shift as a function of the bias voltage. The blue dash line represents the linear fit line of the experimental results (blue squares), while the red dash-dot line shows the theoretical results using Eq. (7) derived from the simple model. Notably, the experimental results exhibit a fitted linear slope of ∼0.17 nm/V over the bias range of 0–17 V. whereas the theoretical line gives a slope of 0.19 nm/V. Here the slope means the tuning sensitivity. Therefore, a good match is obtained between the experiment and the theoretical results.
In addition to the tuning sensitivity, the response time is also crucial for OTF in real-time applications, which provides valuable insights into their sensitivity and stability. Figure 4(a) illustrates the experimental setup to measure the response time of the BMIM-PF6 filled EC-OTF. A laser at a wavelength of 1551.5 nm (near the initial transmission peak) goes through the tunable filter and a 10-Hz square wave voltage ranging from 0 to 1.5 V is applied by a signal generator to drive the EC-OTF. Then, the change of the transmitted light is captured by a photodetector, converting it into electric signals for display on an oscilloscope. The response curves in Fig. 4(b) present a rise time of about 24 ms and a fall time of about 32 ms. The difference in rise time and fall time can be ascribed to the inherent dynamic characteristics of the ionic liquid filled in the EC-OTF. When a bias voltage is applied to build up an electric field in the capacitor region, the ions in the FP region is forced to move towards the capacitor region, influencing the refractive index and thus the response signal. In contrast, when the bias voltage is turned off, the accumulated ions return to the FP region by free diffusion without any external forces. It is reasonable to infer that the time required for ions to diffuse back is longer than that of forced movement under an external electric force. Therefore, the fall time is longer than the rise time. By continuously applying the square wave voltage only 0.2 nm of drift was observed, showcasing the excellent stability of BMIM-PF6 filled EC-OTF over an extended duration.
Fig. 4. Measurement of the response time of the EC-OTF filled with BMIM-PF6. (a) Experimental setup; (b) Dynamic response of the BMIM-PF6 filled EC-OTF under a 10-Hz square wave voltage ranging from 0 to 1.5 V.
Additionally, we tried to fill the EC-OTF device with 3 other electrolyte solutions: 0.1-M NaCl electrolyte, poly(3,4-ethylenedioxythiophene) (PEDOT) conductive polymer [27] and sulfonated tetrafluoroethylene based fluoropolymer-copolymer (Nafion) solution [28]). Table 1 summarizes the experimental results, including sensitivity, insertion loss, SMSR and spectral drift. PEDOT gives the highest sensitivity of 23 nm/V and the smallest drift of <0.1 nm (Fig. 3 (d)). However, it suffers from a large insertion loss of 25 dB and a small SMSR of only 6 dB (Fig. 3(c)), limiting the practical applications. 0.1-M NaCl electrolyte and Nafion solution measure the sensitivities of 0.1 and 0.2 nm/V, respectively, similar to BMIM-PF6. Nevertheless, they have large insertion losses (10 dB for o.1-M NaCl and 8 for Nafion solution) and severe drifts (10 nm for both). The low drift (or equivalently, high stability) of BMIM-PF6 may be attributed to its high viscosity, which is about 1,000 times of the viscosity of the water solvent in 0.1-M NaCl electrolyte and Nafion solution [34]. Thus, BMIM-PF6 is less likely to be flowed away from FP cavity. Furthermore, the unique chemical and physical properties of ionic liquid render BMIM-PF6 highly stable in air and moisture environment. BMIM-PF6 was reported to absorb only 8.3 × 10−2 M of water in 24 hours [34]. In summary, BMIM-PF6 ionic liquid achieves the best overall performance, such as low insertion loss (3.2 dB), large SMSR (23 dB) and small drift (< 0.2 nm).
Table 1. Wavelength tuning properties of EC-OTF devices filled with different electrolyte solutionsa
4. Conclusion and outlook
In conclusion, a new design of electrolyte capacitor-based optical tunable filter (EC-OTF) has been successfully developed for wavelength tuning. Four different electrolyte solutions have been examined as the filling solution of the Fabry-Pérot (FP) cavity. Among them, BMIM-PF6 achieves the best overall performance, including low insertion loss of 3.2 dB, SMSR of 23 dB, and small drift of < 0.2 nm. The insertion loss is comparable to the commercial FP tunable filter TFP50 from Technica (3.0 dB – 4.0 dB) [35], FFP-TF (4.0 dB) [36] and FFP-TF2 (3.0 dB) [37] from Micron Optics, and the narrow-band tunable optical filters recently reported by Chang et al. (2.82 dB – 3.87 dB) [38]. The SMSR is also comparable to the commercial MEMS tunable optical filter from Sercalo (25 dB) [39] and the TFN narrowband tunable optical filter from TeraXion (>20 dB) [40]. It obtains a wavelength tuning range of 3.0 nm, with a linear sensitivity of ∼0.17 nm/V over the bias voltage of 0–17 V. The 3-dB linewidth is 0.77 nm, quite narrower than that of the electro-tunable optical cavity filter fabricated by Nasir et al. (∼2.5 nm) [41]. Moreover, this BMIM-PF6 filled EC-OTF design has no mechanical movement and BMIM-PF6 is reported to show satisfying thermal stability up to 610 K [42], quite larger than the typically thermal stability of commercial tunable filter of ∼ 350 K.
This research offers valuable insights into the optofluidic tunable filters, with potential applications in optical communication networks, spectral analysis systems, and particularly lab-on-a-chip bio-sensing devices. The unique refractive index change mechanism of OTFs eliminates the need for additional labeling agents in biosensing process, simplifying the experimental procedures and reducing costs [43,44]. Furthermore, the integration of microfluidic systems enables precise control over sample delivery and reaction conditions, showing potential for real-time monitoring of biological processes and offering insights into kinetic information and reaction dynamics [45,46]. Additionally, the dynamic wavelength tuning feature of OTF makes it adjustable in resonating with specific wavelengths associated with target biomolecules, significantly helping enhance the sensitivity and selectivity for biosensors [47].
This research not only uncovers the current capabilities of OTFs, but also opens the door to a variety of future research to further improve the performance and applicability. One promising avenue is the exploration of new ionic liquids with unique electro-optic properties, which can offer distinct refractive index modulation [48,49], influencing the tuning range, response time, and sensitivity. Systematic studies can identify optimal candidates for OTFs in specific applications under different environmental and operation conditions. Improving the optofluidic configurations is also an exciting opportunity for advancement, including the variations in capacitor geometry, electrode composition, FP cavity design, or the device sealing and packing [50,51]. The integration of optofluidic technologies allows the easy scaling of the OTF devices in dimensions and arrays for mass production with high reproducibility [52,53]. Further exploration could focus on the selection of cost-effective materials and fabrication processes suitable for large-scale industrial applications.
These further comprehensive studies may reveal the mechanisms to optimize the trade-off between the sensitivity, speed, and stability, thus pushing OTF capabilities to new limits.
Funding
Hong Kong Polytechnic University (1-BBEN, 1-CD4V, 1-CD6U, 1-CD8U, 1-CD9Q, 1-W28S, 1-YY5V, G-SB6C); Innovation and Technology Commission (ITF-MHKJFS MHP/085/22); Research Grants Council (RGC) of Hong Kong (15215620, N_PolyU511/20).
Acknowledgments
The authors thank UMF-Materials Research Centre (MRC) and UMF-Cleanroom (UMF-Cleanroom) of the University Research Facility in Material Characterization and Device Fabrication (UMF), University Research Facility in 3D Printing (U3DP), and Surface Engineering Unit of the Additive Manufacturing Stream, Industrial Centre (IC) of The Hong Kong Polytechnic University for the technical assistance and facility support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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