Two-channel highly sensitive sensors based on 4 × 4 multimode interference couplers

We propose a new kind of microring resonators (MRR) based on 4 × 4 multimode interference (MMI) couplers for multichannel and highly sensitive chemical and biological sensors. The proposed sensor structure has advantages of compactness and high sensitivity compared with the reported sensing structures. By using the transfer matrix method (TMM) and numerical simulations, the designs of the sensor based on silicon waveguides are optimized and demonstrated in detail. We apply our structure to detect glucose and ethanol concentrations simultaneously. A high sensitivity of 9000 nm/RIU, detection limit of 2 × 10‒4 for glucose sensing and sensitivity of 6000 nm/RIU, detection limit of 1.3 × 10‒5 for ethanol sensing are achieved.


Introduction
Current approaches to the real-time analysis of chemical and biological sensing applications utilize systematic approaches such as mass spectrometry for detection. Such systems are expensive, heavy, and cannot monolithically integrated in one single chip [1]. Electronic sensors use metallic probes to produce electro-magnetic noise, which can disturb the electro-magnetic field being measured. This can be avoided in the case of using integrated optical sensors. Integrated optical sensors are very attractive due to their advantages of high sensitivity, ultra-wide bandwidth, low detection limit, compactness, and immunity to electromagnetic interference [2,3].
A large class of optical sensors based on optical fiber and waveguides use the evanescent wave to monitor the presence of the analyte in the environment. Detection can be made by the optical absorption of the analytes, optic spectroscopy or the refractive index change [1]. The two former methods can be directly obtained by measuring optical intensity. The third method is to monitor various chemical and biological systems via the sensing of the change in the refractive index [4]. Optical waveguide devices can perform as refractive index sensors particularly when the analyte becomes a physical part of the device, such as the waveguide cladding. In this case, the evanescent portion of the guided mode within the cladding will overlap and interact with the analyte. The measurement of the refractive index change of the guided mode of the optical waveguides requires a special structure to convert the refractive index change into detectable signals. A number of refractive index sensors based on optical waveguide structures have been reported, including Bragg grating sensors, directional coupler sensors, Mach-Zehnder interferometer (MZI) sensors, microring resonator sensors, and surface plasmon resonance sensors [1,[4][5][6][7].
Recently, the use of optical microring resonators as sensors [6] is becoming one of the most attractive candidates for optical sensing applications because of its ultra-compact size and easiness to realize an array of sensors with a large scale integration [8,9]. When detecting target chemicals by using microring resonator sensors, one can use a certain chemical binding on the surface. There are two ways to measure the presence of the target chemicals. One is to measure the shift of the resonant wavelength, and the other is to measure the optical intensity with a fixed wavelength.
In the literature, some highly sensitive resonator sensors based on polymer and silicon microring and disk resonators have been developed [10,11]. However, multichannel sensors based on silicon waveguides, which have ultra-small bends due to the high refractive index contrast and are compatible with the existing complementary metal-oxidesemiconductor (CMOS) fabrication technologies, are not presented much. In order to achieve multichannel capability, multiplexed single microring resonators must be used. This leads to a large footprint area and low sensitivity. For example, recent results showed that the sensitivities of 108 nm/RIU [2,12] and 200 nm/RIU [13] for glucose and ethanol detection using single microring resonator can be achieved. In addition, by using microfluidics with grating, ethanol sensor with a sensitivity of 50 nm/RIU can be obtained [14]. The silicon waveguide based sensors have attracted much attention for realizing ultra-compact and cheap optical sensors. In addition, the reported sensors can be capable of determining only one chemical or biological element.
The sensing structures based on one microring resonator or Mach-Zehnder interferometer can only provide a small sensitivity and single anylate detection [15]. Therefore, in this paper, we present a new structure for achieving a highly sensitive and multichannel sensor. Our structure is based on only one 4 × 4 multimode interference (MMI) coupler assisted microring resonators. The proposed sensors provide a very high sensitivity compared with the conventional MZI sensors. In addition, it can measure two different and independent target chemicals and biological elements simultaneously. We investigate the use of our proposed structure to glucose and ethanol sensing at the same time.

Two-channel sensor based on 4× 4 MMI couplers
The proposed sensor based on the 4 × 4 multimode interference and microring resonator structures is shown in Fig. 1. The two MMI couplers are identical. The two 4 × 4 MMI couplers have the same width MMI W and length MMI L . In this structure, there are two sensing windows having lengths arm1 arm2 and L L . As with the conventional MZI sensor device, segments of two MZI arms overlap with the flow channel, forming two separate sensing regions. The other two MZI arms are isolated from the analyte by the micro fluidic substrate. The MMI coupler consists of a multimode optical waveguide that can support a number of modes [16]. In order to launch and extract light from the multimode region, a number of single mode access waveguides are placed at the input and output planes. If there are N input waveguides and M output waveguides, then the device is called an N × M MMI coupler.
The operation of the optical MMI coupler is based on the self-imaging principle [16,17]. Self-imaging is a property of a multimode waveguide by which the input field is reproduced in single or multiple images at periodic intervals along the propagation direction of the waveguide. The central structure of the MMI filter is formed by a waveguide designed to support a large number of modes.
In this study, the access waveguides are identical single mode waveguides with width a W . The input and output waveguides are located as follows [18]: The electrical field inside the MMI coupler can be expressed as follows [19]: (2) In this study, the length of the MMI coupler is to , where L π is the beat length of the MMI coupler. One can prove that the normalized optical powers transmitting through the proposed sensor at the wavelengths on resonance with the microring resonators are given as [9,20] [21]. In this study, the locations of the input and output waveguides, and MMI width and length are carefully designed, so the desired characteristics of the MMI coupler can be achieved. It is now shown that the proposed sensor can be realized by using the silicon nanowire waveguides [22,23]. By using the numerical method, the optimal width of the MMI is calculated to be MMI 6μm W = for the high performance and compact device. The core thickness is h co = 220 nm. The access waveguide is tapered from a width of 500 nm to a width of 800 nm to improve the device performance. It is assumed that the designs are for the transverse electric (TE) polarization at a central optical wavelength 1550nm λ = . The finite difference time domain (FDTD) simulations for sensing operation when the input signal is at Ports 1 and 2 for glucose and ethanol sensing are shown in Figs. 2(a) and 2(b), respectively. The mask design for the whole sensor structure by using the CMOS technology is shown in Fig. 2(c). The proposed structure can be viewed as a sensor with two-channel sensing windows, which are separated with two power transmission characteristics T 1 and T 2 , and sensitivities S 1 and S 2 . When the analyte is presented, the resonance wavelengths are shifted. As the result, the proposed sensors are able to monitor two target chemicals simultaneously, and their sensitivities can be expressed as For the conventional sensor based on the MZI structure, the relative phase shift ϕ ∆ between two MZI arms and the optical power transmitting through the MZI can be made a function of the environmental refractive index, via the modal effective index eff n . The transmission at the bar port of the MZI structure can be given as [1] 2 MZI is the interaction length of the MZI arm, eff ,a n is the effective refractive index in the interaction arm when the ambient analyte is presented, and eff ,0 n is the effective refractive index of the reference arm. From (3), (5), and (6), the ratio of the sensitivities of the proposed sensor and the conventional MZI sensor can be numerically evaluated. The sensitivity enhancement factor 1 MZI / S S can be calculated for values of 1 α between 0 and 1, which is plotted in Fig. 3. factor of approximately 10 is obtained. The similar results can be achieved for other sensing arms.

Results and discussion
In general, our proposed structure can be used for the detection of chemical and biological elements by using both surface and homogeneous mechanisms. Without the loss of generality, we apply our structure to the detection of glucose and ethanol sensing as an example. The refractive indexes of the glucose ( glucose n ) and ethanol (n ethanol ) can be calculated from the concentration (C%) based on the experimental results at wavelength 1550 nm as [24,25] Fig. 4. In our design, the silicon waveguide with a height of 220 nm and a width of 500 nm is used for single mode operation. The wavelength is 1550 nm. It is assumed that the interaction length for glucose and ethanol sensing arms is 100μm . By using the finite difference method (FDM), the effective refractive indexes of the waveguide at different concentrations are shown in Fig. 5.  The glucose solutions with concentrations of 0%, 0.2%, and 0.4% and ethanol concentrations of 0%, 3%, and 6% are induced to the device. The resonance wavelength shifts corresponding to the concentrations can be measured by the optical spectrometer as shown in Fig. 6 for glucose and Fig. 7 for ethanol. For each 0.2% increment of the glucose concentration, the resonance wavelength shift of about 10 5 pm is achieved. This is a greatly higher order than that of the recent conventional sensor based on the single microring resonator [25,26]. For each 3% increment of the ethanol concentration, the resonance wavelength shift of about 1.5 × 10 4 pm is achieved.
Our sensor provides the sensitivity of 9000 nm/RIU compared with a sensitivity of 170 nm/RIU [26].
In addition to the sensitivity, the detection limit (DL) is another important parameter. For the refractive index sensing, the DL presents for the smallest ambient refractive index change, which can be accurately measured. The DL can be calculated as the ratio of the resonance wavelength resolution σ to the sensitivity glucose S by [27] glucose DL S σ = where 2 2 2 amp-noise temp-induced spec-res σ σ σ σ = + + , amp-noise σ is the standard deviation of the spectral variation which is determined by the Q factor and extinction ratio, temp-induced σ is the standard deviation induced by noises in the sensing systems, and spec-res σ is resulted from the spectral resolution of the optical spectrometer. In our sensor design, we use the optical refractometer with a resolution of 20 pm, the detection limit of our sensor is calculated to be 2 × 10 -4 , compared with a detection limit of 1.78 × 10 -5 of the single microring resonator sensor [28]. The sensitivity of the ethanol sensor is calculated to be ethanol 6000 nm/ RIU S = , and the detection limit is 1.3 × 10 -5 .
Silicon waveguides are highly sensitive to temperature fluctuations due to the high thermo-optic coefficient (TOC) of silicon ( 4 1 Si TOC 1.86 10 K − − = × ). As a result, the sensing performance will be affected due to the phase drift. In order to overcome the effect of the temperature and phase fluctuations, we can use some approaches including both active and passive methods. For example, the local heating of silicon itself to dynamically compensate for any temperature fluctuations [29], material cladding with negative thermo-optic coefficient [30][31][32][33], MZI cascading intensity interrogation [34], control of the thermal drift by tailoring the degree of optical confinement in silicon waveguides with different waveguide widths [35], and ultra-thin silicon waveguides [36] can be used for reducing the thermal drift.

Conclusions
We have presented a novel sensor structure based on the 4 × 4 multimode interference structure and microring resonators. The design of the proposed sensors uses silicon waveguides, therefore, the sensor has advantages of compatibility with the CMOS fabrication technology and compactness. It has been shown that the proposed sensors can provide a very high sensitivity of 9000 nm/RIU for glucose solution and 6000 nm/RIU for ethanol solution compared with the conventional sensors. In addition, by using the 4 × 4 multimode interference couplers, our sensor structure can detect glucose and ethanol solutions simultaneously.