A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Sensor Interrogation <s> The ratiometric wavelength-measurement system is modeled and an optimal design of transmission response of the employed edge filter is demonstrated in the context of a limited signal-to-noise ratio of the signal source. The corresponding experimental investigation is presented. The impact of the limited signal-to-noise ratio of the signal source on determining the optimal transmission response of edge filters for a wavelength-measurement application is shown theoretically and experimentally. © 2005 Optical Society of America OCIS codes: 120.2440, 120.4570, 060.2380. <s> BIB001 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Sensor Interrogation <s> The measurement of an unknown wavelength is a common operation for many optical systems. Examples include wavelength monitoring in multichannel dense wavelength division multiplexing (DWDM) optical communication systems and optical sensing systems based on fiber Bragg gratings (FBGs) or Fabry‐Perot (FP) filters. DWDM requires the accurate setting and maintaining of the transmitter’s wavelength or monitoring of the tunable laser’s wavelength. 1,2 An FBG- or FP-filter-based optical <s> BIB002 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Sensor Interrogation <s> The impact of a change in signal-to-noise ratio (SNR) of the input signal on the accuracy of a ratiometric wavelength measure- ment system is studied. The variation of the output ratio of the system due to the change in SNR of the input signal is modeled and the experi- mental investigation has been carried out, and both indicate that the accuracy is influenced significantly by variations in the SNR of the input signal. The demonstration example shows a wavelength shift of 0.055 nm when the SNR of the input signal varies from 47 dB to 42 dB (the measurable range is from 1500 nm to 1600 nm and the edge filter used has a slope of 0.15 dB/nm). © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1022-1024, 2007; Published online in Wiley Inter- <s> BIB003 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Sensor Interrogation <s> An edge filter-based ratiometric wavelength measurement system is modeled and analyzed in this paper. The results confirm that the noise of input signal and photodetectors limits the resolution of the wavelength measurement system. The achievable resolution is calculated for a given noise level of the input signal and photodetectors' resolution. An improved ratiometric wavelength measurement system consisting of two fiber comb filters is presented both theoretically and experimentally, which performs coarse and fine wavelength measurements simultaneously. The resolution of the system is significantly improved to better than 5 pm while maintaining the potential for high measurement speed and wide measurable wavelength range. <s> BIB004
Finally and briefly, while not in itself a sensing type, an important underpinning part of a complete optical fiber based sensing system is the sensor interrogation sub-system. The most common techniques used to interrogate optical sensors involve detection of the sensor's response to a measurand in one of three different domains: Wavelength, intensity or polarization. The nature of the sensor will determine the domain used by the interrogation system. Figure 24 illustrates schematically the surface of the MFC with fibrinogen immobilized on it. During the entire immobilization process, the optical spectra was recorded and are shown in Figure 28 . It was found that in the first five minutes, a rapid wavelength shift occurred, then the wavelength shift rate reduced and finally the wavelength shift reached a plateau within 20 min. When the biosensor based MFC was built up, rabbit anti-fibrinogen solution (Calbiochem) in PBS buffer (0.2 mL, 100 µg/mL) and anti-IgG solution in PBS (0.2 mL, 100 µg/mL) were applied to the sensor. The experimental results exhibit that the anti-fibrinogen molecules caused a wavelength blueshift of circa 10.35 nm while the anti-IgG caused a redshift of only 0.45 nm. Therefore, it is demonstrated that this sensor can distinguish the difference between the specific anti-fibrinogen solution and the non-specific anti-IgG solution. In addition, an experiment for various concentrations of fibrinogen solution was performed. The used fibrinogen solution has a constant concentration of 100 µg/mL, while the concentration of anti-fibrinogen solution was increased from 25 µg/mL to 100 µg/mL. Based on experimental results in Figure 28 , we can find that the sensor output was proportional to the concentration of the anti-fibrinogen. It is noteworthy that this is the first time a label-free MFC based biosensor using a fibrinogen and anti-fibrinogen pair has been demonstrated experimentally. Finally and briefly, while not in itself a sensing type, an important underpinning part of a complete optical fiber based sensing system is the sensor interrogation sub-system. The most common techniques used to interrogate optical sensors involve detection of the sensor's response to a measurand in one of three different domains: Wavelength, intensity or polarization. The nature of the sensor will determine the domain used by the interrogation system. When the biosensor based MFC was built up, rabbit anti-fibrinogen solution (Calbiochem) in PBS buffer (0.2 mL, 100 µg/mL) and anti-IgG solution in PBS (0.2 mL, 100 µg/mL) were applied to the sensor. The experimental results exhibit that the anti-fibrinogen molecules caused a wavelength blueshift of circa 10.35 nm while the anti-IgG caused a redshift of only 0.45 nm. Therefore, it is demonstrated that this sensor can distinguish the difference between the specific anti-fibrinogen solution and the non-specific anti-IgG solution. In addition, an experiment for various concentrations of fibrinogen solution was performed. The used fibrinogen solution has a constant concentration of 100 µg/mL, while the concentration of anti-fibrinogen solution was increased from 25 µg/mL to 100 µg/mL. Based on experimental results in Figure 28 , we can find that the sensor output was proportional to the concentration of the anti-fibrinogen. It is noteworthy that this is the first time a label-free MFC based biosensor using a fibrinogen and anti-fibrinogen pair has been demonstrated experimentally. Finally and briefly, while not in itself a sensing type, an important underpinning part of a complete optical fiber based sensing system is the sensor interrogation sub-system. The most common techniques used to interrogate optical sensors involve detection of the sensor's response to a measurand in one of three different domains: Wavelength, intensity or polarization. The nature of the sensor will determine the domain used by the interrogation system. Measurements in the intensity domain, for example for sensors based on simple tapered optical fibers, offer the advantages of simplicity but can suffer from an undesirable dependence on fluctuations in the optical source power. Wavelength domain interrogation, for example for MFC based sensors, can be less dependent on optical source power fluctuations and can be used where multiple sensors using wavelength division multiplexing are used, but in general the interrogation system is more expensive to implement than that in the intensity domain. The most common approaches to wavelength measurement include passive ratiometric wavelength measurement schemes and active wavelength scanning schemes BIB001 BIB003 BIB002 BIB004 .