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Chapter 5
Discussion and Conclusions
In this thesis, we have provided a simple but novel solution for biosensing system. We use a 1.5 µm SOA, 1×4 channel WDM MUX, 50:50 2×2 fiber couplers to form a WDM asymmetric resonator laser. By using the resonator laser and we can provide a well performance WDM light source. Besides, we have design a microring resonator for the WDM laser. We optimize many parameters in microrings, such as waveguide width, gap and radius. Moreover, in the sensing applications using a rnicroring resonator, two sensing mechanisms, the homogeneous sensing and the surface sensing, and two sensing schemes, the resonant wavelength shift monitoring and the intensity variation monitoring, can be applied. For a rnicroring biosensor, the sensitivity is a very important parameter. It is composed of two components, including the waveguide sensitivity and the device sensitivity. The waveguide sensitivity depends on the sensing mechanisms, while the device sensitivity is dependent on the sensing schemes. In addition, the device sensitivity using the resonant wavelength shift monitoring and the intensity variation monitoring is also obtained.
With two sensing mechanisms and two sensing schemes, there are four different combinations. For each case, the overall sensitivity is calculated using the proper expressions.
In chapter 2, we successfully design a WDM asymmetric resonator fiber laser. From our experimental result, we can produce pure single mode lasers with SMSR approaching to 35 dBm.
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In chapter 3, we successfully design a multi-channel biosensor used microring. According to the simulation of the TE mode transmission spectrum of microring resonator with various radius at output port, we find the optimized radii for four wavelengths: 1538.38nm, 1541.46nm, 1544.78nm and 1547.86nm are 4.75 µm, 4.55 µm, 4.25 µm and 4.15 µm.
We can see with the fixed radius for fixed wavelength, the gap distance and waveguide width and slab height are important issue for the transmittance of TE and TM mode.
In chapter 4, we shows that a microring resonator is an excellent candidate for label-free biosensing. The device can have a higher sensitivity compared to other label-free detection methods, such as surface plasma resonance, quartz microbalance, and microcantilever sensors. Its simple device configuration lends itself to easy construction of multiple-channel sensor arrays which we shown in Fig. 4-4. The multiple-channel sensor can significantly improve the throughput of analyte detections as compared with other technologies. With a proper design of microfluidic channels to interface with the microring sensors, detection only requires very small sample volumes. With these capabilities, microring resonator sensors can potentially be used in many areas such as drug discovery, disease diagnostics, chemical analysis and environmental monitoring.
In theory, the Q-factor is the important part of the microrings. That means that there is still a lot of space to improve. This requires more careful and accurate simulation methods. In addition, the parameters of fabricated microrings also need to be extracted from the optical
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measurement. They can provide a feedback to the simulation accuracy. In experiment case, the water has absorption for infrared light, which could introduce more loss. To avoid this, the operation wavelength should move to visible regime.
Vertically-coupled microrings have several advantages over laterally-coupled ones, such as easy integration with flow cells, and more precise control of coupling. In addition, they have only ring waveguides exposed to analytes while the bus straight waveguides are buried. This helps to reduce some experimental uncertainties in the laterally-coupled case, for example of whether the solution can flow into the submicron gap of high aspect ratio. Such configuration is also beneficial for the reduction of detection fluctuation.
The issue of water evaporation can be solved by integrating a sealed flow chamber with a microring. With use of the vertically coupled configuration, it is very easy to achieve.
Some sensor applications require real-time monitoring. To achieve this, microring resonators can be operated at a fixed wavelength close to a resonance. The real-time measurement is performed by monitoring the intensity change. Or it can be performed by real-time monitoring the resonant wavelength shift. To achieve this, the measurement setup might be modified to detect fast response.
