Simplified Platform for Microring-Sensing using Wavelength Locking

We utilize a low-cost, robust method of wavelength locking microring resonators to implement a simplified platform for microring-sensing applications that requires only a singlewavelength laser source and directly maps resonance shifts to DC voltage values. OCIS codes: (280.4788) Optical sensing and sensors; (280.1415) Biological sensing and sensors. With their small footprint, CMOS-compatible fabrication, and multiplexed operation, silicon microring resonators are ideal for use as measurement devices. Their high-refractive index provides them with a high sensitivity to such environmental factors such as temperature, and when chemically treated, they can be used as effective label-free biosensors [1,2]. The latter use has received wide attention for its potential to provide fast, compact, and costeffective diagnostic instrumentation for applications in environmental monitoring, biochemistry, and healthcare; and as a testament to its utility, has been implemented commercially in that capacity. The routine procedure for using the microring resonator as a sensor is to probe the resonance shift of the resonator as it is exposed to the environment or sample (Fig. 1a illustrates this resonance shift). The traditional technique for measuring this resonance shift consists of conducting fast spectral scans with a tunable laser and photodiode, or a broadband source, monochromator, and photodiode [2]. However, the use of costly, bulky, and sensitive equipment such as tunable lasers and monochromators is prohibitive to the desired deployment of microring resonators as low-cost, portable, and robust sensors [3]. We utilize wavelength-locking to demonstrate a greatly simplified platform for microring-sensing. Fig. 1b illustrates the concept of wavelength locking, where an integrated heater is used to automatically tune a microring resonator to match the wavelength of a laser source. Initially, the microring need to be tuned a distance Δλ0 as shown in Fig. 1b. However, following exposure to the sample or environmental condition, a resonance shift ΔλRS will be induced in the microring. Subsequently, the microring will then be needed to tune (Δλ0 – ΔλRS) to match the laser wavelength. The voltage on the integrated heater required to tune to the laser wavelength will be altered, and is defined by Eq. (1), where RH is the resistance of the heater, and ∂λ/∂P is the tuning efficiency of the heater. V = RH (Δλ0 − ΔλRS )(∂λ / ∂P) −1 (1) By wavelength locking the resonator and sampling the voltage applied to the heater during the wavelength locked state we retrieve a direct measurement of the resonance shift, with no additional post-processing needed. Fig. 1b shows the implementation of our method, which requires only a single-point optical coupling (assuming onchip photodiodes) of a single-wavelength laser (where the specific wavelength can be arbitrary). Electrical! Interface! Single-λ Laser! Electrical! Interface! Single-λ Laser! A B λ!" Tr an sm is si on " laser" wavelength" ΔλRS" Δλ0" Figure 1: (a) Illustration of induced resonance shifts and wavelength locking. (b) Microring-sensing platform using wavelength locking. The enabling innovation in this platform is the development of a simple, robust, and automated method to wavelength lock a microring resonator to a laser. The basis of our technique is the use of a dithering signal to generate an asymmetric error-signal [4]. The generated error-signal enables wavelength locking by allowing low-speed analog and digital circuitry to determine when the resonator and laser coincide in wavelength. Fig. 2a shows the device we used in this demonstration, consisting of a 6-μm radius microring resonator with an integrated heater and drop-port defect-enhanced silicon photodiode. These opto-electronic components were interfaced with low-speed analog and digital circuitry (detailed in [4]) for the purpose of generating the required ATh5A.1.pdf CLEO Postdeadline Papers © 2013 OSA error signal, and then subsequently using that error signal to efficiently establish wavelength locking. Fig. 2b shows the progressive wavelength locking of the resonance (originally at ~1559.2 nm) to a laser (at ~1560 nm).