Single and dual optical microsphere resonator as Humidity Sensor

This study explores into the influence of whispering gallery mode (WGMs) on single and dual microsphere resonator (MSR) as a humidity sensor which can confine light by continuous internal reflection. The construction of the device is simple, and it has a low cost as MSR is made of silica material with a high refractive index. MSR coupling tapered optical fibers with size 5µm to the surfaces of single and two microspheres is demonstrated as a relative humidity (RH) sensor. The Q-factor was measured 7.754 × 105 dBm for single MSR while for dual MSR, Q-factor is 7.765 × 105 dBm. When sensitivity, linearity, and repeatability were analyzed based on transmitted power, the results were conclusively excellent for the performance of the double MSR.


1.
Introduction A whispering gallery mode (WGM) microresonators has been extensively investigated due to their small devices and that can confine the light with high quality factor (Q) [1][2][3] It is commonly constructed using materials with high refractive index like silica or polymer [4][5][6].A WGM microresonators have applications in sensing, laser sources, nonlinear optics, optical filters and switches, and quantum optics [7][8][9][10].They are valuable for their ability to enable precise measurements, compact laser sources, nonlinear optical processes, and exploration of quantum phenomena.These resonators exploit the phenomena of total internal reflection in order to trap and confine light within a circular or spherical cavity, and they are often associated with circular-path resonant cavities such as micro-sphere, micro-ring, and micro-disc geometries.[10][11][12].
The microsphere resonator's (MSR) ability to confine light in three dimensions using a combination of the WG-bouncing ball and WG ring principles has attracted considerable attention [4,13].Two distinct turning points that correspond to regional field enhancement determine the confinement of the WGM in the MSR.The presence of these turning points in MSRs optimises the add/drop operation [14,15].MSRs, as opposed to other micro-resonator structures, are capable of generating complex spectra transmitted with highly degenerated resonances [16].This is accomplished by the overlap of multiple MSR radii, which raises the resonance spectra and traps light near the MSR surface [17,18].
An optical microsphere humidity sensor is a type of sensor that uses a MSR as a sensing element to measure humidity levels in the surrounding environment.The operation of an optical microsphere humidity sensor is based on the evanescent field sensing principle [10,15,19].When the MSR is exposed to moisture or humidity, molecules of water are absorbed onto its surface.This causes a change in the index of refraction of the MSR, which in turn effects the light transmission through it [15].To measure humidity, light is coupled into the MSR and allowed to circulate within it using either an optical fiber or waveguide.The light interacts with the water molecules adsorbed on the surface of the microresonator, and the intensity or spectral properties of the transmitted or reflected light are monitored.By analyzing these changes, the humidity level can be determined.These sensors are advantageous such as their great sensitivity and potential miniaturization, making them useful for humidity monitoring in various applications [15].However, they require careful calibration and environmental control to ensure accurate measurements.
In this work, we have introduced and conducted practical experiments on a fiber sensor that can detect differences in relative humidity.The uses microresonator in humidity sensor utilizes both single and dual microsphere resonators to achieve its functionality.

Characterisation of MSR
Standard single-mode fiber (SMF-28) and a fusion splicer machine (Furukawa Electric Fitel S178A) were employed in the manufacture of the microsphere resonators (MSR) for this investigation.A cleaved tip of the SMF-28 was inserted into one of the arms of the fusion splicer so that the MSR could be created.During the process, the equipment that performed the fusion splicing was controlled manually.As a result of the plasma arcs created by the splicer, the fiber tip was heated and partially melted.Because of the surface tension of the molten fiber, it was able to take on a spherical shape, resulting in the formation of a microsphere.It was feasible to adjust the diameter of the microsphere formed by varying the number of plasma arcs delivered to the fiber tip.Figure 1 depicts the MSR structure, which includes a stem with a diameter of 125 µm (Ds) and a microsphere with a diameter of 200 µm (Dm).This MSR is consistent with the dimensions used in previous studies which managed to have the highest in Q-factor compared to other smaller sizes.This manufacturing approach allowed for the precise fabrication of microspheres at the tip of the SMF-28, which supplied the necessary structure for the MSR-based sensing experiments carried out as part of the study [9,10].

Figure 1. Fabricated micro-sphere resonator
A thinner silica microfiber was manufactured utilizing the flame brushing method to allow for light coupling to and from the microsphere resonator (MSR).The flame brushing method was used to Dm Ds fabricate single-mode fiber (SMF-28) into a non-adiabatic optical microfiber.Thinner microfibers are produced by heating SMF-28 fiber in a flame and pulling it to reduce its diameter.In this study, the manufactured silica microfiber had diameter of 5 µm.Figure 2 (a) show the single MSR coupled to the fabricated microfiber and Figure 2 (b) for the dual MSR.In this configuration, 1550 nm laser light is launched into the microfiber in order to interact with the MSR.The single and dual MSR is coupled toward tapered microfiber with a tapering dimension of 5 for characterisation procedure.The characterization utilized ANDO AQ4321D tunable laser source (TLS) with wavelength range from 1551.0 nm to 1551.2 nm.The TLS had capability to produce wavelength between 1520 nm until 1620 nm with a 0.001 nm interval was perfectly suited with this experiment's need.The transmitted power wavelength from the tunable laser source will finally be collected using an optical power meter (OPM) from THORLABS S145C was measured as output data.The transmission spectral of single and dual MSR shown in Figure 3 was used to define the quality factor [17,20].The Q-factor indicates the quality and effectiveness of the resonant modes within the microspheres by measuring the resonator's capacity to store and sustain light.In the study, the Q-factor measure by formula Q = λ /Δλ, whereas Δλ is the full width half maximum (FWHM) resonant wavelength and λ is represent resonant of the wavelength, is discovered similar compared to the previous work [9,19,21,22] .The Q-factor for single MSR was measured 7.754 ×10 5 dBm, while Q-factor for dual MSR is 7.765×105 dBm.which higher compared to singe MSR but is slightly not much difference value but still within the same range.The dual MSR had a greater Q-factor than a single MSR may be due to double resonant wavelengths of the two MSRs, causing a slight narrowing in the overall resonance [20].Additionally, the coupling distance between the resonator and taper microfiber can affect insertion loss.A larger coupling distance can weaken the coupling between the resonator and the tapered microfiber, resulting in a greater insertion loss.On the other hand, a smaller coupling gap can enhance coupling efficacy and decrease insertion loss [23,24].The insertion loss for a single MSR experience -19.5 dBm which indicate slightly higher than dual MSR of -21.6 dBm.Due to the coupling gap between the MSR and taper microfiber, the insertion loss may vary between these graph.

Performance of Single and dual MSR as Humidity Sensor
The experimental configuration for the MSR used to detect relative humidity as shown in the Figure 4.
The key components of the setup include a tunable laser source (TLS), the microsphere resonator (MSR), a relative humidity (RH) measurering meter, and an optical power meter (OPM).The TLS, which serves as a light source for the experiments, is linked to one end of the tapered fibre that has a diameter of 5 µm, while the other end of the tapered fibre is connected to the OPM, which measures the optical power that is transferred through the MSR.The MSR is put inside of a hermetically sealed container in order to create the desired humidity environment.A plate with a salt solution that is completely saturated can be found inside the chamber.This configuration ensures that the MSR is surrounded by an even and consistent level of humidity.The wavelength was initially set to 1551.103 nm, which was the deepest resonating most resonating depth that was ever recorded for a single MSR.RH is increased from 40% to 80%, and output power is measured at each level.These steps are repeated for dual MSR with a wavelength of 1551.097 which was the deepest resonating most resonating depth that was ever recorded for a dual MSR.In both the single and dual MSR, each RH experiments are repeated three times.All the recorded fiber outputs are depicted in Figure 5, with Figure 5 6 depicts sensitivity and linearity values for single and dual MSR.The sensor's sensitivity describes its capacity to detect and measure minor changes in RH by measuring the corresponding change in output power.While, for linearity is describes the relationship between the output powers of the sensor and various RH percentages.A linear response indicates that the output power is directly proportional to the relative humidity [25].The sensitivity was measured 0.3112 dB/% RH and linearity at 99.29% for dual MSR, which highest values compare to single MSR.The linearity of the single MSR is 98.03 %, and its sensitivity is 0.2240 dB/%RH.These analyses demonstrated that a dual MSR is superior to a single MBR when employed as a sensor.The performance parameters of the single and dual MSRs are compared and summarized in table 1.

Conclusion
In this research conducted an experimental analysis of both single and multiple MSR implementations as humidity sensors.The resonators were constructed of SMF-28 silica fiber using the method known as "soften-and-compress".The characterisation of the MBRs for the Q-factor value using a microfibre with a diameter of 5µm.Both the single and dual MSR were able to achive a Q-factor of which the most significant size contributes the enormous Q-factor value.The single and dual MSR then applied as a humidity sensor where humidity varied from 40% to 80% RH.Via transmitted spectral and wavelength shift, the dual MSR, performed well as humidity sensor.Here, the number of resonators may affect the performance of the resonator as a humidity sensor.

Figure 2 .
Figure 2. Tapered microfiber coupled to the a) single MSR b) dual MSR

Figure 3 .
The transmitting spectral of the a) single MSR and b) dual MSR with 5µm tapered microfiber

Figure 5 .Figure 6 .
Figure 6.A single and dual MSR coupled with tapered microfiber 5 µm transmission modes

Figure
Figure6depicts sensitivity and linearity values for single and dual MSR.The sensor's sensitivity describes its capacity to detect and measure minor changes in RH by measuring the corresponding change in output power.While, for linearity is describes the relationship between the output powers of the sensor and various RH percentages.A linear response indicates that the output power is directly proportional to the relative humidity[25].The sensitivity was measured 0.3112 dB/% RH and linearity at 99.29% for dual MSR, which highest values compare to single MSR.The linearity of the single MSR is 98.03 %, and its sensitivity is 0.2240 dB/%RH.These analyses demonstrated that a dual MSR is superior to a single MBR when employed as a sensor.The performance parameters of the single and dual MSRs are compared and summarized in table 1.

Table 1 .
Humidity sensing performance for single and dual MSR