We present a new design of wavelength selective reflector composed of a Y junction and a singly coupled microring resonator, and demonstrate its biochemical sensing applications with a prototype device. In contrast with other reflectors like distributed Bragg reflectors, this device acts as notch filter at its reflection port. One promising application of the device is for remote sensing of harsh or inaccessible site, where only one optical fiber is required to transmit the input and reflected light signal over a long distance. The design can also be used to make microring cavity lasers.
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Fiber and waveguide Bragg reflectors are critical components of optical communications systems, such as feedback mirrors for distributed feedback lasers and optical add-drop multiplexers for wavelength-division multiplexing. They are also used as sensing elements of, for instance, distributed embedded sensing systems in “smart structures” [1,2]. Recently, several designs of microring-based wavelength-selective reflectors were proposed to replace grating structures for realizing tunable single-mode lasers [3–6]. The advantages of microring structures include easy fabrication, on chip integration with other photonic devices, and wide wavelength tuning range. The reflection spectra of Bragg grating reflectors and these reported microring-based reflectors have isolated narrow band peaks, i.e. they are reflective-type band-pass filters. In this letter, a microring resonator based reflector with narrow notches in the reflection spectrum is presented. In addition to its applications as a reflective-type notch filter in optical communications (e.g. Raman lasers), here we demonstrate its sensing applications, specially as a biochemical sensor.
For the optical sensing applications, interaction of the measurands with the light in the ring waveguide changes the effective index of the guided mode and thus the resonant wavelengths. Detection can be made by monitoring the shift of a resonant wavelength or variation of the reflected light intensity of a wavelength fixed at the largest slope in the transmission spectrum. This sensor configuration combines the advantage of fiber-optic sensors in remote measurement and the advantage of planar sensors in integration and mass production . Mechanical flexibility and the ability to transmit optical signals over a long distance of the fiber make such sensor attractive for remote measurement in harsh or inaccessible locations. Fabrication, characterization and a biochemical sensing experiment of the sensor are presented in the following sections. We will show that compared with doubly or triply coupled microring reflectors, the design proposed here exhibits higher extinction ratio and narrower linewidth because of less coupling induced loss over the ring path, which can lead to higher sensitivity. Possibility for other sensing applications and methods to further increase the sensitivity are also discussed.
2. Device theory and fabrication
In the device design shown in Fig. 1 , the Y junction splits equally the input light Ii into two arms (i.e. Ii = 2⋅ Ia). After part of the light Ic coupled into the ring cavity, the other part of the light Ib circulates back to the Y junction and combines to give the total reflected light Ir (i.e. Ir = 2⋅ Ib). Here the Y junction serves as both a power splitter and combiner. Based the universal relations for singly coupled ring resonators , the normalized reflected light intensity are formulated asEquation (1) remains valid even if the Y junction is asymmetrical and the split ratio is not 50% due to fabrication errors.
The proposed device was fabricated with SU8 polymer (n = 1.565, Microchem Corp.) on a silicon substrate covered by 5 μm thermal oxide (n = 1.445) serving as the lower cladding. An FEI Sirion scanning electron microscopy (SEM) system with an accelerating voltage of 30 kV was used to pattern the 2 μm thick SU8 film. Nanometer Pattern Generation System (NPGS) was used to generate the device designs and to control the writing processes. The waveguide width is 2 μm. The circular ring resonator has a radius of 200 μm and device designs with waveguide to ring resonator separations (coupling gaps) ranging from 0 to 1 μm were fabricated to find out the optimal coupling condition. S-bends are used to separate the two arms of the Y-junction. All the arc bend sections are smoothly connected (i.e. first order derivatives are continuous) to minimize the transition loss and their radius of curvatures are larger than the ring resonator radius to minimize the bending loss.
3. Experiments and results
The setup for measuring the reflection spectra of the sensor is shown in Fig. 2 . Individual devices were cleaved from the Si wafers before measurements. The output of an erbium doped fiber amplified spontaneous emission (ASE) broadband source with a wavelength range from 1520 to 1560 nm was polarized through an Agilent 8169A polarization controller, which consists of individually rotatable linear polarizer, half-wave plate, and quarter-wave plate and can synthesize any predetermined state of polarization. Transverse electric (TE) light was fiber coupled to the input port of an optical circulator. Its through port was fiber coupled to the devices. The reflected light from the devices was collected at the drop port of the circulator and directed to an OSA (HP 70951B).
In the measured reflection spectra shown in Fig. 3 , the device with 300 nm coupling gap shows the highest extinction ratio of more than 11 dB and is close to the critical coupling condition. The free spectral range of the resonant nulls is around 1.15 nm, which agrees with the actual circumference L of the ring resonators. Small ripples in the flat tops of the curves indicate weak higher order guided modes, which is consistent with the mode simulations and experiment results of the similar waveguide structure in . Curve fitting of the spectra with the theoretical transfer function in Eq. (1) and further calculations suggest that the device quality factor (Q) is approximately 8000. The Q could be increased by reducing the waveguide scattering loss through optimization of the electron beam writing process and post-fabrication annealing of the polymer waveguides . However, higher Q was not pursued in this proof-of-concept device.
We used a homogeneous biochemical sensing experiments to demonstrate the sensing capability of the device. The air cladding over the SU8 polymer waveguides was replaced by sodium chloride (NaCl) solutions in de-ionized water with mass concentrations of 0~20%. The refractive index of a NaCl aqueous solution changes 0.0018 RIU per 1% mass concentration at 20°C . Variations of the solution refractive index ns disturb the evanescent tail of the guided mode and change the corresponding effective indices neff, which was detected by monitoring the reflection spectrum (or resonant wavelength λr) shift with the same setup described earlier. The measurements (Fig. 4 ) show a linear relationship between the resonant wavelength and NaCl solution concentration (and the solution refractive index). If the device sensitivity S is defined as the slope of the relationship between the resonant wavelength and the refractive index of the analyte, we have, where is the waveguide sensitivity and only relevant to the waveguide structure. Line fitting of the measurements indicates our measured device sensitivity to be 63 nm/RIU.
The waveguide sensitivity was estimated by varying the top cladding refractive index from 1.33 to 1.331 and finding the relevant change of effective index, i.e. , using a full vectorial mode solver (FIMMWAVE, Photon Design). Based on the simulated neff = 1.51, Sw = 0.06, and λr ≈1550 nm, the theoretical device sensitivity is calculated to be 62 nm/RIU and agree quite well with the measurement. It is also noteworthy that our waveguide sensitivity is about two times higher than the simulated one of a polymer slab waveguide (for homogeneous sensing) . The wavelength reproducibility and tuning repeatability in 1 min for a current commercial optical spectrum analyzer (OSA, Agilent 86146B) is 2 pm, so the detection limit for homogeneous sensing (defined as minimum detectable refractive index change of the analyte solution) of our device can be as low as 3 × 10−5 RIU. If intensity detection using a wavelength fixed at the largest slope is applied to the same device, the theoretical detection limit would be estimated to be 4 × 10−6 RIU, based on an assumption of actual neff = 1.5 and optical power measurement accuracy of 2.2% (e.g. Agilent 81624B).
The proposed reflector has a singly coupled ring resonator, which contrasts with the microring-based reflectors cited earlier [3–6]. They all involve doubly or even triply coupled ring resonators. We also fabricated an alternative design of reflectors which consists of a doubly coupled ring resonator, and compared its performance with the one we described above. Figure 5 is the theoretical transmission spectra of the two designs, with α = |t| = 0.8 in both cases. We find that the singly coupled ring design have much higher extinction ratio (infinite in theory) and sharper lineshape, which indicate higher device sensitivity for intensity interrogation. With the similar waveguide index profile and electron beam writing process, the measured spectra of the doubly coupled ring reflector show extinction ratios of 2~5 dB and 1~2 times wider linewidths, which agree with the theoretical analysis. This could be due to the added cavity loss contributed by the additional coupler.
Based on the original design of singly coupled ring reflector, Fano-resonance created by introducing two partially reflecting junctions to the bus waveguide can be used to further increase the device sensitivity . Optimizing the waveguide structure and improving the waveguide sensitivity Sw can also lead to higher sensitivity, where silicon-on-insulator (SOI) photonic wire waveguides, anti-resonant reflecting optical waveguides (ARROW) or slot waveguides could be considered . Using thermo-optic or electro-optic waveguide materials, the sensor design can be readily applied to temperature or radio-frequency (RF) electric field detection . If fabricated on a flexible substrate, the ring reflector becomes strain or displacement sensors through the photoelasticity of the waveguide material and the ring deformation [15,16], Finally, by using polymers sensitive to different analytes as waveguide materials and specific bio-receptors immobilized on the waveguide surfaces the reflector can be a multi-functional platform for broad biosensing applications. A sensor array multiplexed with microrings of different sizes serially coupled to the same bus waveguide and functionalized with different molecule recognition capabilities is also possible (Fig. 6 ).
In conclusion, a microring resonator based wavelength selective reflector is proposed and the conception was demonstrated with a device fabricated with electron-beam patterned SU8 polymer waveguides. An extinction ratio greater than 11 dB has been achieved. The homogeneous biochemical sensing experiment was carried out using the device and a sensitivity of 63 nm/RIU was observed, in good agreement with the theoretical model. The minimal detectable ambient refractive index change is estimated to be 3 × 10−5 RIU (with instrument reproducibility 2 pm). The simple design minimizes round-trip loss from additional couplers and preserves a better performance than some other designs of microring reflector sensors. Approaches for further improving the device sensitivity and possibilities for other sensing applications are also discussed. Combined with a single optical fiber for delivering both the input light and reflected signal, the device can be used for remote sensing of an inaccessible spot, for instance, water quality monitoring in bore holes. As a reflective-type notch filter, the device could find applications in optical communications. With the ring waveguide as a laser cavity and the Y-junction as a power combiner, it is still possible to construct a microring cavity laser, either light pumped  or electric pumped .
This work is supported by NSF Grant Number ECS-0437920, NSF-DMR-0092380, and NSF Center on Materials and Devices for Information Technology Research (CMDITR), Grant Number DMR-0120967. The work was conducted at the Nanotech User Facility at the University of Washington, a member of the National Nanotechnology Infrastructure Network (NNIN) supported by NSF.
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