2.1.

Hetero-Core Fiber Optic Sensor

As shown in Fig. 1, the hetero-core fiber optic sensor structure consists of a single-mode (SM) transmission line 9-μm core diameter and an inserted sensor portion of a 5-μm core diameter, which is as short as a few millimeters in fusion splice. A transmission beam is partially leaked into the cladding layer at the hetero-core sensor portion. Additionally, the leakage light changes with the bending action of hetero-core portion, so that the bending property can be measured due to the change in transmitted loss. The hetero-core insertion loss is found¹² to be low at less than 1 dB. The tandem-connected hetero-core sensing technique was recently proposed. Its use can decrease the number of transmission lines for multipoint sensing systems owing to the hetero-core sensor having a low insertion loss.¹⁴ Additionally, we previously found that the hetero-core fiber sensor indicates monotonic and linear loss change characteristics with the bending action of hetero-core portion. ^{12, 13} In this paper, the hetero-core insertion length used is 2 mm because the hetero-core fiber sensor has monotonic characteristics with relatively high sensitivity for bending.

Fig. 1

Structure of a hetero-core fiber optic weight sensor made of silicone rubber and thin plastic sheets.

2.2.

Characteristics of Weight Sensors Based on Hetero-Core Fiber Optics

Figure 1 shows the hetero-core fiber pressure sensor structure made of silicone rubber, which is flexible and useful for long-term use. The hetero-core pressure sensor consists of a small segment of silicone rubber with a 1-mm height and a 2-mm width, which occupies the hetero-core portion, so as to induce the curvature change of the hetero-core portion by pressure on the upper and lower sides of the pressure sensor. In addition, four more silicone rubber segments of 3-mm width and 0.5- and 1-mm heights were placed on both sides of the silicone rubber sheets, which were 20- × 20-mm² squares, for the pressure action to induce additional bending of the hetero-core portion, so that the sensitivity for pressure could be increased. Plastic sheets were attached on the silicone rubber sheets through their upper and bottom surfaces. The developed hetero-core pressure sensor measured 20 × 20 × 4 mm³, as shown in Fig. 1.

The characteristics of the four hetero-core fiber pressure sensors 2, 3, 5, and 8, selected from eight hetero-core fiber optic pressure sensors to be fabricated for sleep respiration monitoring are indicated in Fig. 2, which shows average optical loss changes and standard deviations with weights ranging from 0 to 3.0 kg, determined in five trials. As shown in Fig. 2, the four hetero-core fiber pressure sensors exhibiting optical loss changes show monotonic characteristics with weight changes. The optical loss changes of the hetero-core pressure sensors 2, 3, 5, and 8 for a weight change of 3.0 kg are 1.80, 0.91, 0.74, and 0.55 dB, respectively. Additionally, these pressure sensors appear capable of detecting relatively light weights of approximately 100 g. Therefore, it is considered that minute pressure changes due to body movements during sleep respiration can be monitored using the proposed hetero-core pressure sensors. The weight of humans (often more than 50 kg) is large and ranges over the pressure sensor's detectable range of 0 to 3.0 kg. However, the detectable range is sufficient to measure respiration movement during sleep, because the weight distribution in the bed is expanded over the region of contact between the body, especially the head, backside, and hip, and the bed. Table 1 shows the average loss changes and deviations for five trials on the eight hetero-core fiber pressure sensors for a weight change in the range of 0 to 3.0 kg. All the developed pressure sensors indicate monotonic optical loss changes with optical loss changes ranging from 0.74 to 2.99 dB, as shown in Table 1. However, they show deviations. This is because that the pressure sensors made of silicone rubber and glue had slightly different thickness and hardness. The differences are due to the differences in the amount of silicone glue used as well as in the pressure applied to the sensors by gradually adding a suspended weight, so that the weighted center of pressure appears to slightly change with each trial. The hetero-core fiber sensor itself has been indicated to have a high accuracy.¹² Therefore, a slight weight change under constant pressure such as sleep respiration could be sufficiently detected by the hetero-core pressure sensors.

Fig. 2

Characteristics of hetero-core fiber optic weight sensors in the optical loss change with weights ranging from 0 to 3.0 kg.

Table 1

Characteristics of hetero-core fiber optic weight sensors in the optical loss change with weight ranging from 0 to 3.0 kg.

4.1.

Respiration Monitoring Test during Sleep

Figure 5 shows real-time responses of the hetero-core fiber pressure sensors in the optical loss change during sleep for respiration with rollover and apnea phenomena. The experimental sleep tests were actually demonstrated during more than 6 h at night, and partially sampled for 30 s on the whole sleep respiration waveforms. In the respiration test during sleep, optical loss was initially set at 0 dB for the pressure sensor that was not to be weighted, and therefore could be induced by a slight weight change. As shown in Fig. 5, the hetero-core pressure sensors indicated a waveform of optical loss changes in a short time period of a few seconds. These were detected during respiration in sleep, which also induced constant optical losses of approximately 1.3 and 0.8 dB for sensors 6 and 4, respectively. This is because the weight that a body exerts on the sensors induces constant optical losses. On the other hand, the sleep-monitoring result shows that the waveform was disturbed after periodical optical loss changes, which was due to a rollover as checked in the video recording. After a rollover, we found that the pressure sensors again detected respiration action, which differed from the respiration movement detected by other sensors before a rollover. Therefore, the proposed system could respond to the positional movement of a body due to a rollover, because the pressure sensors are widely distributed, as shown in Fig. 3. On the other hand, immediately after a rollover, no periodical loss changes are observed under a constant optical loss, and after a few tens of seconds, a waveform is again detected, as shown in Fig. 5. This result indicates that the system can recognize apnea and infrequent respiration during sleep and thus may be used in analyzing SAS.

Fig. 5

Real-time responses of hetero-core fiber weight sensors in the optical loss change during sleep for respiration with rolling over and apnea phenomena.

Figure 6 shows the real-time responses of the eight hetero-core fiber pressure sensors in terms of optical loss changes during sleep with reference to the average optical loss of the pressure sensors, which means that the initial loss of 0 dB should be set as the average optical loss in these tests. All of the hetero-core pressure sensors detected body movements from respiration during the sleep of the subjects, indicated as optical loss periodical changes, as shown in Fig. 6. In particular, sensor 7 indicated that the amplitude of respiration waveform in terms of optical loss change is sufficient at approximately 0.09 dB, as shown in Fig. 6. Additionally, the short-period periodical optical loss changes induced at constant optical losses are stable; therefore, the hetero-core fiber optic pressure sensor can stably measure body weight. On the other hand, the weight applied to the hetero-core pressure sensors during sleep was less than several kilograms, because the constant optical losses were near the range of the full-scale losses of the eight hetero-core pressure sensors, as shown in Table 1. Focusing on the results of sensors 1 and 7 in Fig. 6, the loss waveforms were out of phase for the time elapsed. This is because that the weight differences were induced among the hetero-core pressure sensors 1 and 7 installed at the thorax and in the abdominal lumbar region in bed, respectively. However, these sensors indicated the same time period of respiration; therefore, respiration monitoring in sleep could be attained. Additionally, the respiration test during sleep was successfully performed in seven healthy males, which means that a broadly distributed arrangement of the hetero-core pressure sensors is useful for monitoring the various daily activities of humans during sleep. The other subject test indicated a maximum loss amplitude of approximately 0.1 dB.

Fig. 6

Real-time responses of hetero-core fiber weight sensors in the optical loss change during sleep for subject A taking breaths repeatedly.

4.2.

Characteristics of Respiration Responses for Arrangement of Different Pressure Sensors

To achieve unawareness and constraint-free respiration monitoring during sleep, it is important that the subject can use the respiration monitoring system with various body postures. To evaluate the characteristics of respiration responses of the hetero-core pressure sensors in the bed arranged at various positions, such as near the thorax or abdominal and lumbar regions, we tested respiratory monitoring using eight hetero-core pressure sensors fixed on a flat floor for subjects A and B, as indicated in Fig. 7. As shown in the figure, sensors 5 and 8 were placed on the x axis, parallel to the shoulder, and on the y axis, respectively. The sensors were separately arranged at 5-cm intervals in the x direction and at 6-cm intervals in the y direction. The pressure sensors closest to the head were located 6 cm from the shoulder. The position matrix of the sensors was defined as shown in Fig. 7, for example, sensor 1 in Fig. 7 was defined as (X, Y) = (1, 5).

Fig. 7

Arrangement of weight sensors for the body position referring to sensor number for the respiration amplitude distribution test.

Figure 8 shows the real-time response of the proposed hetero-core pressure sensors in terms of optical loss at the position (X, Y) = (7, 3), which means that the sensors are near the thorax, on the back of the subject. Amplitude was defined in the respiration waveform shown in Fig. 8. The output waveform from the respiration test exhibited detection ability even for minute body motion; however, it showed fluctuation at relatively high frequencies owing to the noise floor from the detector electronics. The experimental result of the distributed weight pressure obtained using hetero-core pressure sensors, whose locations are shown in Fig. 7, is shown in Fig. 9 for subjects A and B. The loss amplitude of the sensors in sleep measurement is defined as the average amplitude of a waveform, as shown in Fig. 8. The sensor positions with the highest loss amplitudes were (X, Y) = (4, 2) and (3, 5) and their loss amplitudes were 0.467 and 0.129 dB for subjects A and B, respectively. The number of pressure sensor positions necessary to detect respiration in subjects A and B were 21 and 19, respectively. The area for respiration motion detection was relatively wide, therefore, by using several pressure sensors separately placed in a bed, the proposed monitoring system can be considered capable of detecting respiration motion even during body movements such as a rollover in various subjects.

Fig. 8

Real-time response of the proposed hetero-core fiber weight sensor at the position X = 7 and Y = 3 (shown in Fig. 7) in the optical loss for respiration while lying on the back.

Fig. 9

Sensitivity distribution along the body of the weight sensors for respiration while lying on the back for subjects (a) A and (b) B.

4.3.

Characteristic Respiration Responses for Different Body Postures

As shown in Fig. 10, we evaluated the effect of subject posture with reference to a subject's back, side, and stomach using eight hetero-core pressure sensors, whose positions were chosen so as to carry out high-sensitivity measurement based on previous experimental results, as shown in Fig. 9. Figures 11a to 11c show real-time responses of the eight hetero-core pressure sensors for respiration while lying on the back, side, and stomach, respectively. For all three postures, body movement with respiration was detected, as can be seen in Fig. 11. In Fig. 11a, which shows data for lying on the back, sensors 1 and 2 detected respiration at sensitivities of approximately 0.1 and 0.05 dB, respectively; however, sensor 8 had difficulty detecting respiration, because of its different weight position. On the other hand, in the case of the subject posture on the side, sensors 2 and 8 had sensitivities of approximately 0.1 and 0.2 dB, respectively, for respiration. In contrast, sensor 1 was insensitive to respiration motion, as indicated in Fig. 11b. For the third posture on the stomach, all the sensors detected respiration, as shown in Fig. 11c. These results show that sensor 8 was out-of-phase with sensors 1 and 2 for the time elapsed in the measured respiration waveform, for the test on the side. This is because sensor 8 is separated from sensors 1 and 2, which are close to each other. Additionally, it is considered that sensor 8 and sensors 1 and 2 are located near the stomach and chest, respectively, which means that the timings of expansion and contraction of the stomach and thorax are different. These results indicate that the hetero-core pressure sensors can detect minute weight changes connected to respiration, especially in the posture on the side; thus, they can be applied at various positions during sleep. Therefore, it is evident that a hetero-core respiration-monitoring system during sleep can be useful in measuring daily activities without constraining the subjects.

Fig. 10

Arrangement of weight sensors for the body position referring to sensor number for comparison of body posture, such as lying on the back, side, and stomach.

Fig. 11

Real-time responses of eight hetero-core fiber weight sensors in the optical loss change during respiration on the bed shown in Fig. 6(b) for subjects lying (a) on the back, (b) on the side, and (c) on the stomach.