CHAPTER-VI DISCUSSION. CONCLUSION AND FUTURE SCOPE OF THE PRESENT WORK 7 5
CHAPTER-VI DISCUSSION. CONCLUSION AND FUTURE SCOPE OF THE PRESENT WORK 6.1 Discussion and conclusion of the present research work: In the present work a novel, low cost and enhanced sensitive intensity modulated fiber optic sensors for measurement of the following parameters have been designed and developed. (a) Refractive index o f liquid (b) Point contact liquid level The present work also emphasizes on optimizing the performance of the designed sensors by suitably modifying the sensing region of the optical fiber. Experimental results obtained with the designed sensors indicate that implementation of the sensors for monitoring of liquid refractive index and liquid level measurements are quite promising. The designed fiber optic refractometer is capable of monitoring liquid refractive index both for intrusive and non-intrusive case Fiber optic refractometer developed in the present study has several major advantages. Unlike the refractometer reported by M. Sheeba et al. [48] and A. Jayanthkumar et al. [50], the designed refractometer is geometrically flexible, thus, refractive index of liquid can be measured in any situation. Since, one end of the fiber is used as a sensing probe, thus very small amount of liquid is needed to monitor its index of refraction. Further, being small in size, the 76
sensing region can be cleaned easily. The designed refractometer has the potential for monitoring of online refractive index in food processing industries. Further, the technique developed here is very simple and is a cost effective one. The resolution of the refractometer was found to be 0.002 which is of the same order as obtained by Sheeba [48]. Response time of the refractometer was not measured in the present study. However, it can be expected that the designed system should give fast response time (of the order of few miliseconds) as the modulated back-reflected light signal immediately falls onto the photodiode, once the fiber sensing-tip comes in to contact with the liquid medium. Further, a novel non-intrusive fiber optic refractometer is developed in the present study. The designed refractometer has the potential to monitor refractive index of certain chemically active solutions e.g. methanol, HF, HNO3 which were otherwise not possible with the technique described by the intrusive based fiber optic refractometer. Resolution of the present refractometer was found to be 0.02 RIU. The designed fiber-optic liquid-level sensor is capable of monitoring liquid level variation nearly upto 5 pm with high degree of repeatability. Point contact liquid level measurement using fiber optic sensors have been reported by I.K. Ilev et al. [35] and M. Bottachini et al. [39]. The liquid level variation that can be measured by Dev was ~10 pm, thus, the resolution of the present study is better than the one reported by Ilev. Further, the present technique is capable of detecting the liquid level of different liquids which was not possible with works reported by Bottachini and Ilev. Sensor response shows step 77
variation when liquid level touches the sensing region of the fiber. Such arrangement makes the sensor extremely sensitive and can be implemented in hazardous environment where other electronic based liquid level sensors are not safe for use. In addition to the above, two more intensity modulated fiber optic sensors for measurement of following parameters have been designed and developed (1) Relative Humidity (RH) and (2) Environmental Temperature The performance of these sensors is yet to be optimized. However, experimental results obtained with the designed sensors indicate its potential of being highly sensitive and reliable technique for measurement of the above parameters. The designed fiber optic RH and temperature sensors are discussed in Appendix I and II respectively. Following major findings are achieved in the present research work 1. A novel fiber sensing technique is designed for measurement of liquid refractive index and point contact liquid level. The sensing principle is based on frustrated total internal reflection effect when a curve-shaped fiber tip is brought in to contact with the outstanding medium (liquid). 2. The fiber optic refractometer developed here can be both intrusive and non-intrusive in nature. The intrusive type fiber optic refractometer can measure liquid refractive index variation up to 78
0. 002 RIU while non-intrusive fiber optic refractometer can monitor liquid refractive index variation up to 0.02 RIU. 3. The installation of the sensor is comparatively easy and flexible than that of the conventional fiber optic reftactomter where sensing region is developed by etching the cladding from the fiber. 4. The technique is cost effective. 5. Fiber optic liquid level sensor can measure liquid level variation up-to 5pm with high degree of repeatability. Also, with the present liquid level sensor, one can detect the liquid level of different solutions. 6. The liquid level variation shows sharp changes in sensor reading which is a more reliable technique to monitor liquid level of any solution. It can be used as a switch on -o ff system for measurement of liquid level. 7. An extrinsic type fiber optic RH sensor is presented in Appendix 1. The dynamic range and sensitivity of the sensor is found to be higher than those of intrinsic type fiber optic sensor reported in the literature [26,28,29]. 8. The sensing region of the RH sensor is developed on a thin cover slide which can be replaced easily when property of the sensing film degrades due to ageing effect. 9. A low cost fiber-optic temperature sensor is presented in Appendix II, The sensing principle is based on evanescent field absorption 79
from an unclad U-shaped fiber optic probe when there is change in temperature in the surrounding medium. 10. No temperature sensitive chemical is coated on the probe for measurement of temperature. This, technique is highly reliable for measurement o f temperature in any environment. On the basis of the above findings, the present work can be summarized in the form of the following conclusions: (i) A novel FOS for measurement of liquid refractive index is designed with enhanced sensitivity and repeatability. The FOS designed has advantages of easy installation, flexibility and low cost compared to conventional FO refractometer. The designed FOS can be applied in the field of food processing industries, research laboratories, sugar industries etc. (ii) A novel FOS for measurement of point contact liquid level with enhanced sensitivity and repeatability is presented. The liquid level FOS is capable of monitoring liquid level variation up to 5pm.The present technique can be applied for monitoring of liquid level of oil, water or other liquids stored in reservoirs. Further, the present technique is also capable of identifying different liquids stored in different reservoirs and thus, monitoring its liquid level. (iii) A low cost FO RH sensor and temperature sensor having potential of high sensitivity and reliability are also designed in the present work. The designed RH sensor can be applicable for monitoring 80
of RH in tea, paper, cement industries where proper monitoring of RH is critical for quality end product for such industries. The designed FO temperature sensor can be useful for monitoring of temperature of various instruments installed in different industries. However, the performance of these sensors are yet to be optimized. 6.2 Future scope of the present work: In many sensor systems, there may be several sensors of the same kind in close proximity or distributed over a wide area such as in an industrial plant Also, in most systems, simultaneous monitoring of more than one physical parameter is required. In order for fiber optic sensors to be competitive, means must be found to link several sensors together in some kind of a network, and with ability to read the status of each sensor individually. Such, fiber optic sensor multiplexing has been widely studied and reported in the recent past [83-89]. The future scope of the present work is that one can develop a low cost fiber optic sensor multiplexing system so as to enable monitoring of (i) refractive index of several liquids kept at different places in the network and (ii) point contact level of several liquids. This multiplexing sensing technique may be useful for food processing industries or in a chemical industry. Figure 6.1 shows the schematic representation of a conventional fiber optic sensor multiplexing system. The sensing principle is based on time-division 81
multiplexing technique. Figure 6.2 shows the topological configuration of the proposed fiber-optic sensor multiplexing system and figure 6.3 is the time scale representation of the source pulse and the modulated pulse from the sensor network. The basic principle of operation of fiber optic sensor multiplexing system is described below. M e i h o m c e C (m p le r C o n p le i C o u p le r Figure 6.1: Schematic representation of a time division based fiber optic sensor multiplexing system PD PC Figure 6.2: Proposed topological configuration for time division multiplexing sensor system. 82
Fig 6.3 Time scale representation of the pulsating input (a) and the modulated output in the sensing network (b). As shown schematically in figure 6.1, a single pulse input results in a multiple output. Each pulse is amplitude ( or intensity) modulated by one sensor only. The pulse width x must be shorter than the time delay td between neighboring sensors which is Xd=2L/cN,... (6.1) where c is the velocity o f light in vacuum, Nj is the group index of the fiber mode and L is the length o f fiber connecting two nearest neighbour sensors. The pulse repetition frequency must be such that one pulse from the last sensors arrives before the next pulse from the source. This means that the time T between pulses must be at least N X&. N being the number o f sensors in the network. Thus, the maximum duty cycle tj becomes 83
r = Ta/ T== 1/N...(6.2) Thus, duty cycle is reduced with increasing number of sensors. The power transmitted down the input bus is shared among the sensors, thus reducing the power through the each sensor. In addition, it is not possible to couple all power from a rung to the return bus and at the same time transmit power in the bus through the coupler without loss. If we are restricted to using directional coupler with the same coupling coefficient, a little consideration shows that sensor number (N-l) transmit the lowest power. This power is maximized by choosing the coupling coefficient K (fraction of power coupled from the bus to each sensor) equal to K=1/(N-1)... (6.3) The power transmitted from source to detector through this critical sensor is PN-i - P0(N-2) 2(N'2)/(N-1) 2(N'1}... (6.4) Where P0 is the power transmitted from the source to the input fiber. This result is valid in the ideal case of lossless elements. For large N, equation (6.4) approaches PN., ~Po/e2(N -l)2...(6.5) This shows that the power is approximately inversely proportional to the number of sensors squared, rather than the number of sensors directly, which would be the case if no power is lost The optimum choice of coupling coefficient for coupler number k is Kk~l/(N-k+l)... (6.6) 84
and the power transmitted through each sensor is Pic = Po/N2...(6.7) Which for large N is an improvement by a factor of e2 as compared to the case of equal coupling coefficients equation (6.5). The above discussion shows that the average power transmitted through a sensor is inversely proportional to the number of sensors squared. Therefore, the number of sensors which can be multiplexed with reasonable sensitivity is rather limited. The ideal system would transmit an average power inversely proportional to N. In order to approach this situation, one must improve the duty cycle as well as the loss in coupling the light from the sensors into the return bus. Compared to conventional time division multiplexing sensing system, the proposed form o f multiplexing system offers two major advantages. 1. Here, the input end-face of the fiber serves both as entry and exit face for the light modes, thus the installation of the sensing system is comparatively easy than the existing technique. 2. The number of coupler required in this approach is exactly half than that of the conventional multiplexing system. This makes the sensing system cost effective. 6.2.1 Bio-Medical Applications of the present work: Another possible scope of the present work is that one can design a low cost sensitive fiber-optic confocal microscope. High-resolution confocal laser 85
m i c r o s c o p y i s a n i n t e n s i v e l y a c t i v e f i e l d a m o n g t h e m o d e m o p t i c a l i m a g i n g t e c h n i q u e [ 9 0-9 6 ]. B e c a u s e o f i t s u n i q u e p o t e n t i a l f o r t h r e e - d i m e n s i o n a l h i g h m a g n i f i c a t i o n i m a g i n g o f t h i c k s p e c i m e n s b y r e j e c t i o n o f o u t - o f f o c u s i n f o r m a t i o n, t h e c o n f o c a l m i c r o s c o p e t e c h n i q u e h a s b e e n w i d e l y a p p l i e d t o m a n y p r a c t i c a l f i e l d s r a n g i n g f r o m b i o m e d i c i n e t o m a t e r i a l s s c i e n c e. W i t h r e s p e c t t o t h e p r i n c i p l e o f o p e r a t i o n, t h e c o n v e n t i o n a l c o n f o c a l s y s t e m s c a n b e d i v i d e d i n t o t w o m a i n g r o u p s : p i n h o l e b a s e d a n d f i b e r o p t i c b a s e d c o n f o c a l m i c r o s c o p e s. T h e b u l k o p t i c a l d e s i g n w i t h m i c r o m e t e r - s i z e d p i n h o l e t h a t i s u s u a l l y i n c o n v e n t i o n a l s y s t e m h a s c e r t a i n d i s a d v a n t a g e s r e l a t e d t o s i g n i f i c a n t s i g n a l a t t e n u a t i o n, d i f f r a c t i o n a n d a b e r r a t i o n e f f e c t s, m i s a l i g n m e n t p r o b l e m s a n d i n f l e x i b i l i t y. I n f i b e r o p t i c a p p r o a c h, t h e m i c r o m e t e r - s i z e d p i n h o l e s a r e r e p l a c e d b y s i n g l e - m o d e o p t i c a l f i b e r s. T h e t e c h n i q u e o f f e r s a n u m b e r o f a d v a n t a g e s i n t e r m s o f f l e x i b i l i t y, m i n i a t u r i z a t i o n a n d s c a n n i n g p o s s i b i l i t i e s. H o w e v e r, b e c a u s e o f u s e o f s m a l l - c o r e f i b e r, i t h a s l o w l i g h t e f f i c i e n c y a n d l o w s i g n a l l e v e l s. F u r t h e r, r e s e a r c h e f f o r t s i n b o t h t h e r e f l e c t i o n a n d f l u o r e s c e n t - t y p e c o n f o c a l m i c r o s c o p y a r e f o c u s e d o n t w o m a j o r a r e a s o f p o t e n t i a l i m p r o v e m e n t s : i m p r o v e m e n t o f t h e r e s o l v i n g p o w e r a n d i m p r o v e m e n t o f t h e s c a n n i n g t e c h n i q u e s. T h e r e f l e c t i o n - t y p e c o n f o c a l m i c r o s c o p e r e p r e s e n t s a n a t t r a c t i v e c o n f o c a l c o n f i g u r a t i o n f o r a x i a l r e s o l u t i o n i m p r o v e m e n t. T h e p r o p o s e d f i b e r o p t i c c o n f o c a l m i c r o s c o p e d e s i g n i s s h o w n i n t h e i n f i g u r e 6. 4. 8 6
Fig 6.4 Experimental arrangement of confocal microscope with multimode optical fiber with LD, laser diode, PD, photodiode, BS, beam splitter and O objective. Fig.6.4. Theoretical axial response of the proposed fiber optic confocal microscope (not in actual scale) It is an apertureless reflection type confocal arrangement in which the laser emission is launched into one end of a multimode optical fiber; the other 87
end is made curve-shaped by polishing the tip. This essentially acts as a planoconvex lens. For all the forward propagating modes down the fiber, the low order modes moving nearly parallel to the axis will meet at the focal point of the fiber tip. Higher order modes, for which the angle of incidence at the fiber tip-air interface is greater than the critical angle, will be reflected back from the inner surface to the entry port of the fiber. Thus, if an object is located at the focal plane of the fiber-tip, the back-reflected light will deliver maximum photocurrent in the photodiode while for out-of -focus object it will induce lower photocurrent in the photodiode circuit Therefore, surface morphology of any living cell or thin film can be studied by scanning the fiber sensing tip over the target. The technique involved here is very simple and is a cost effective one. Theoretical axial response of the proposed fiber-optic confocal microscope is shown in figure 6.5. 6.2.2 Other possible applications of the present work: As stated earlier the present work mainly aims at design and development of low cost sensitive fiber optic refractometer and point contact liquid level sensor. The design of the sensing probe is novel in itself and is a low cost technique. As far as fiber optic refractometer is concerned it can be useful for detection of adulterated oil (petrol, diesel or any other oil) as the index of refraction of the pure oil is always different from that of an adulterated one. The present liquid-level sensor can be applicable for monitoring o f oil stored in the 88
reservoir in refineries and oil depot. Since, the present liquid level sensor shows step response, the monitoring technique is highly reliable. Further, very small amount of liquid is required to get this step-response in the sensor. Thus, the technique is useful for detection o f leakage in oil pipes, dam, oil tank, etc. 8 9