The Far Infrared Rays (FIR) or terahertz
Music Angelica Jimenez
The annual global death rate has beenincreasing every decade, especially due to natural catastrophes likeearthquakes, tsunamis, cyclones and floods.
As per the statistics, the largestand most massive death tolls are caused from earthquakes, followed by tsunamis.The initial 72 hours after the disaster referred to as the "Golden 72 Hours",if effectively used, can help to decrease the mortality rate to a great extent.Hence, it is important to detect the presence of life in the trapped victimsduring such emergency rescue operations so that maximum lives can be saved in thelimited time available, rather than wasting time in rescuing the dead victims. Itdemands for an efficient life detection system to rescue the alive-but-trappedvictims faster and more effectively.Besides detecting the presence of lifeunder the debris, other major factors which need to be taken into account for arescue system for life detection are its ease of operation, ability todistinguish the presence of multiple subjects in the affected area andcapability to operate effectively in a noisy environment, accuracy of themeasured data, reliability of the system, issues of motion artifacts andpenetration depth i.e.
, reachability of the signal in terms of depth anddistance. Initial methods for rescue operations wereutilizing dogs, which would waste time since the victims detected may bealready dead. Then optical devices were developed for life detection. They hadthe drawbacks of limited degrees of freedom, inability to be used in remote andinaccessible locations as well as the requirement of experts to operate them.Acoustic detectors like geophones require noise free operating environmentwhich is impossible in rescue and surveillance operations, and hence prove tobe not useful in spite of their simple operational model. Later, rescue robots orradar robots using temperature sensors were developed, which could reach deepinside the debris to search for trapped or buried victims.
But once theserobots go out of range, they could not be tracked.This paper gives a brief review on the conventionallife detection systems. Also, a new model is proposed using the non-ionizingFar Infrared Rays (FIR) or terahertz rays.
Conventional systems use ionizingradiation of microwave and infrared regions of the electromagnetic spectrum,which are hazardous to human health. The current proposal involves applicationof FIR rays to detect the presence of alive victims under rubble, along with theadditional benefit that these rays are propitious to human health andfacilitate the subject to overcome illness and feel healthy i.e.
, there ishardly any threat to health involved.Section II of the paper briefs on theexisting life detecting methodologies and their drawbacks. The benefits ofterahertz rays and the design and principle of operation of the proposed systemare detailed in the sections III and IV respectively.
In a microwave Doppler radar sensingsystem, the specialized radar transmits a microwave signal beam towards thedesired direction (debris). It then listens for the reflected beam from thetarget and analyzes the phase or frequency variation in the received signal dueto the victim's motion. According to Doppler theory, an object withquasi-periodic movement modulates the phase of the transmitted signal by thetime varying position of the object and reflects it back. Since this systemdetects the presence of life by analyzing the heart beat and respiratorymovements of the victim, the desired target is the victim's chest. Therefore,the reflected signal would contain the information about displacement of thechest due to breathing and heartbeat 1.
Nevertheless, the reflected signal wouldcontain only the information of chest displacement caused by heartbeat alone,if the victim holds his breath. In that moment, the heartbeat signal of thesubject could be well detected. However, heartbeats cannot be accuratelyextracted when overlapped with breathing. This is because the infinitesimalchest movement produced by respiration is stronger than the one produced by theheartbeats, thus making it hard to separate the heartbeat signals when there isan overlapping of both signals.
Doppler sensing systems using signals of thefrequency range 800-2400 MHz as well as 10 GHz were proved in several works todetect the cardiopulmonary motion of chest for relatively still or isolatedvictims. If there is random motion of the subjector if other moving objects are present, say, people walking around, it isalmost impossible to extract the life signals accurately, as the amplitude ofthe interference signals would be stronger than the minute movements on thebody surface of the subject. In order to overcome the background noise hencecreated, and to improve the signal-to-noise ratio (SNR), different approacheshave been tried by researchers over time.
Though the use of high precisionamplifiers would improve the SNR, they fail to restrain the interferences.Stationary signal processing methods like FFT and high order linear narrowbandfiltering could be effective in improving the SNR except in complex situations2. Another method uses Dual Filtering Algorithm (DFA), wherein two filtersand an algorithm to trace the spectral peaks of interference signals are usedto restrain the interferences; and one dimensional wavelet transform techniqueis used to filter out the respiratory signal from the noise. Here again, theeffective extraction of heartbeat signal increases the computational cost 3. Using multiple antennas for transmissionand reception could partly forbid the interferences, but it is complex and alsothe cost of implementation is high 4.
A compact rescue radar system isproposed in 5, where a SAW oscillator generates signals of frequency 10 GHz.Upon reception, the signal is processed by Independent Component Algorithm(ICA). ICA extracts the heartbeat and respiratory signal information from thebackscattered data by removing noise and clutter.An integrated solution based in aheterogeneous network consisting of a through-the-wall radar sensor for detectingand tracking moving targets and a vital sign detector (VSD) to determine thepresence of standing and living subjects is illustrated in 6. This methoduses a traditional quadrature demodulation technique along with a MultipleSignal Classification (MUSIC) algorithm for detection of signs of life behindwalls. A modified version of this method is used in 7, where the phasemodulation caused in a continuous wave signal at 10 GHz is used to detect therespiratory signals and by applying a suitable spatial smoothing decorrelationstrategy over the MUSIC algorithm, it significantly enhances the SNR.
However, microwaves cannot penetratemetallic bodies and hence Doppler radar based sensors will yield very poorresults if the debris consists of such materials, for example, if the rescuesite is located in an industrial area. Acoustic sensors can overcome thislimitation.Since sound waves can penetrate metalwalls, acoustic sensors are preferable to microwave based sensors.
In 7, theauthor demonstrates a high power mechanical acoustic through-wall sensor todetect humans through the metallic walls of a cargo container. The drawback inthis method is the frequency produced, which is not sufficient to detectvictims at a faster rate and it can only produce waves of limited frequencywhich are unable to penetrate deep.Inspired from the aforementioned work, anultrasonic method for life detection is developed in 8. The mechanical methodis used to produce ultrasound waves which are then transmitted into the debris.
The reflected signals are analyzed for the respiratory movements of the chestusing Doppler signatures. The transmitted frequency is then incremented witheach consecutive meter of depth to scan for the presence of live victims. Thedepth of the trapped individual is calculated based on the frequency emitted atthat time stamp. The location coordinates are calculated and plotted usingGoogle Earth software, making the data globally available.
The limitation ofthis method is that the piezoelectric crystals are able to generate ultrasonicsound only up to a certain limit, after which they get damaged and lose theirproperty. As a result, the effectiveness of this system is limited to just afew meters.Infrared (IR) beams can also penetraterubble and reach buried humans. Therefore by using high precision long range infrareddistance sensors with powerful LED, the distance from a victim can be preciselymeasured and indicated to the rescue operators. The intensity of the reflectedlight could be used to estimate the distance from the subject as shown in theworks 10-12.
Also the inherently fast response of the IR sensors could behelpful to enhance the real time response of a rescue robot 13. A hardware prototype of the IR based lifedetection system is proposed in 14, which consists of IR transmitter andreceiver units, life detection circuit and processing unit using ATMega18microcontroller which is programmed in arduino compiler using C++. The systemoperates in (i) IR distance meter mode to measure the depth at which the victimis present and (ii) life detection mode to detect the heartbeat signal of thevictim. The IR signal gets absorbed into the victim's body and the blood volumechanges in the tissue varies the intensity of the IR signal that is reflectedback from the victim's skin.
These intensity variations account for variationsin the receivedvoltage and hence help to determine the heartbeat rate. The world of terahertz isan area of recent interest and research, yet very challenging 15. It is theextreme region of the infrared band (hence called Far infrared rays or FIR rays) which is the "terahertz gap"with wavelengths that lie between 1 mm to 1 µm. FIR rays are also called T-rays/ T-light/Invisible light which occupy the frequencies from 300 GHz to 3 THz of the electromagnetic spectrum. Recent studies reveal that the FIR rays play animportant role in the formation and growth of living things and hence calledthe "light of life". The terahertz gap in the far infraredregion of the electromagnetic spectrum is shown in Fig.
1.In spite of the difficulties in the generation, transmission and detection of these tremendously high frequency signals 16, due to its undisputed benefits and hardly any ill-effectsto humans, researches are underway inthe applications that are yet unexplored in the fields of science,industry and medicine 17. Current applications ofsub millimeter waves are in radio astronomy,remote sensing, materialtesting, medical imaging, dentistry, drug detection, securityscanning, non-destructivetesting and communications.In this paper, a new model for life detection system is proposedwhich uses FIR rays in order to detect the presence of life in the victimstrapped under debris and to locate them, subsequent to disasters likeearthquakes. Former approaches use microwaves and infrared rays which are hazardousto human health.
They induce issues like skin cancer, low and high blood pressure,ischemic heart disease, slow pulse and even damage to the human immune system andlead to other fatal health conditions. Moreover, they need to be extremelysensitive to detect the heartbeat signals of the person, which may not beoptimal. The current proposal involves the application of terahertz raysor T-rays that helps in detecting whether the entangled victim is alive or not,besides locating them under the rubble. It also has the additional benefit thatT-rays are propitious to human health and facilitates the subject to overcomeillness and feel healthy. That is, there is hardly any threat to healthinvolved as FIR rays are non-ionizing radiation. The effects of T-rays on humanbody are discussed in the section V.
(A) of this paper. The life detection system proposed in thispaper has two sections; one for transmission of terahertz rays into the rubble,and the other for reception and detection of the rays received. Fig.
2 and Fig.3show the schematic representations of these respectively. For the transmission of FIR rays into debris, acontinuous beam of FIR rays need to be generated with a frequency of approximately1THz using an appropriate terahertz emitter. Terahertz rays are generated inthe range of few milli watts power, and hence require to be amplified beforethat energy is radiated by the antenna using an optimalhigh power amplifier module.The amplified beam is then givento a suitable terahertz antenna in order to focus it on the rubble.
In the receiver section, theFIR rays which are received back by the receiverantenna are firstamplified and then fed into the terahertz detector. Thedetector analyzes it for the variations in its propertieswith respect to the transmittedbeam. An ideal choice of terahertz source and detectorcan be made based on the range of operating frequency of the device, its portabilityand cost considerations.
A. Terahertz SourceTo generate rays of 1 THz frequency, there are several sourcesavailable. Various technologies used for terahertz generationand their performances are reviewed in 18. 1) Dielectric Resonator Metamaterials:A number of techniques for the fabrication and analysis of negative indexmaterials (NIM) to generate terahertz frequencies are listed in 19. Severalmetamaterials were developed in the work, such as a NIM based on yttriastabilized zirconia spheres, and terahertz metamaterials based on an array ofcustom synthesized micro-spheres and an array of lithium tantalite micro-rods.
Thesize of dielectric resonator materials required at these frequencies will be inthe range of few tens of microns.2) A Ring of Oscillators:A ring of oscillators and the circuits coupling the oscillators to set thefrequency at which they will lock in, can be used asthe THz source. The signal emerges along the axis of the ringand by adjusting the couplers separately they could aim the output, making itpossible to scan large areas with a narrow, high-powered beam. The power couldbe increased by adding more oscillators to the ring or by using multiple rings 20. A detailed study on the structure of different typesof ring oscillators and their principle of operation are described in 21.
3) Terahertz MMICs (TMICs):Monolithic Microwave Integrated Circuits that can operate up to THz frequenciesfully exploit the sub millimeter wave band. To make these THz MMICs, THztransistors having maximumoscillation frequencies larger than1 THz are required. Two indium phosphide double-heterojunction bipolar transistor (DHBT) based TMIC amplifiers withoperating bandwidths in the THz range are reported in 22, both of which useTeledyne's 130nm InP DHBT transistors in the common base configuration. Resultsof the work demonstrate that 130nm InP DHBT technology can be used to enablesophisticated TMIC circuits for operation in the terahertz band. The increasein maximum oscillation frequency of InP High Electron Mobility Transistorsleads to the increase in operating frequency of TMIC to approach 1THz (~850 GHz) which is reported in 23. 4) Schottky diode-based THz Sources: These sources rely on the nonlinearities of the Schottky diode toproduce harmonics of an input signal, thus generating power at integermultiples of the drive frequency.
Schottky diodes are typically paired insystems with lower-frequency active devices such as Gunn or IMPATT (impactionization avalanche transit-time) oscillators, backward wave oscillators andtransistor amplifiers 24. Schottky diode-based components are used to extendthese active components into the THz. Schottkydiodes integrated with compound planar antenna structures that radiate a frequency multiplied tone from thereceived fundamental acts as sub-THz sources. These devices are termed asmultennas and their designs are optimized to frequency multiplication from0.1-0.3 THz in 25.
Planar Schottkydiode frequency multipliers are one of the most employed devices for localoscillator power generation at THz frequencies. Up to 2.4 THz can be generatedusing these devices which are demonstrated in 26. A suitable power amplifier is to be used to amplify the T-rays since the power of the radiatedbeam will be in a few milli watt range.
Hence, in order to analyze the beam of raysfurther in the case of a THz emitter which is not integrated with an amplifiercircuit or an antenna system, they must be amplified and then given to antenna. In the same way, thereceived THz rays should also be amplified before analyzing them using thedetector.C.Terahertz AntennaTotransmit the terahertz rays towards the debris as well as to receive the raysthat are reflected back, a properly designed antenna system is required; exceptfor those terahertz sources that have integrated antenna systems in them, like themultennas. Owing to the very small wavelength of these waves, the antenna hasto be very small in size.
Reconfigurablephoto-induced Fresnel zone plate based terahertz antenna with two dimensionalbeam steering and beam forming capabilities operating at 0.75 THz is designedand beam steering up to ±120 is demonstrated in 27. Two doublebow-tie slot antennas operating at 0.
1-0.3 THz and 0.2-0.6 THz are designed forwideband millimeter wave applications in 28.A novel design of MEMS technology based fully on-chip 3D helical antennawhich operates at 4 THz has been fabricated on a silicon substrate in 29. Theantenna has a metallic helix and a microstrip on the substrate for feed.
Inaddition to the tunable parameters of the helix which can be varied to improvethe performance of the antenna, it has a high gain of 17.6 dBi and wideoperating bandwidth from 2.8-4.4 THz which are the main characteristics of thisantenna. Helical antenna is one suitable option that is proposed here forrelatively compact antenna geometry. The antenna's geometry is wavelengthdependent, but is acceptable from several hundred MHz and higher, with theupper limit being dominated by the high voltage operation.
It offers a goodgain factor and can be operated as a narrow band or wide band device; and thefabrication cost is also low.Researchers have recently proposed many other antennaconfigurations for sub millimeter waves. A terahertz antenna, devised by agraphene dipole and two parasitic elements, which operates at 1.
84 THz isproposed in 30. By choosing appropriate values of chemical potentials appliedon the graphene parasitic elements, three operation states can be chosen inorder to dynamically control the radiation pattern of the antenna. Flatdielectric metamaterials lens with synthesized gradient refractive index tooperate at terahertz frequencies is designed in 31. It is demonstrated tofocus an incident terahertz plane wave in its focal plane.
For indoorapplications, a short range wideband monopole antenna operating at 0.67 THz with-10 dB impedance bandwidth of 100 GHz is designed in 32.D. THz DetectorTerahertz detector analyzes the receivedrays and measures its intensity, amplitude, time, frequency and otherproperties with respect to the transmitted T-rays. The variations in these parameters can be used todetermine whether the trapped victims are alive or not; and the depth atwhich they are enmeshed.
Direct detectionmechanisms and principles of room temperature THz detectors are reviewed in 33. TerahertzTime DomainSpectrometer: THz-TDS is a spectroscopic techniquein which the properties of a material are examined with short pulses of terahertz radiation. The generation anddetection systems are sensitive to the variations produced by the sample materialon boththe amplitude and the phase of theterahertz radiation. Novel concepts for portability and miniaturization ofTHz-TDS is designed, developed and evaluated in 34. The high directivity beam of T-raystransmitted by the antenna is directed towards the debris.
These rays penetratethrough therubble, during which it may come across living beings and materials like stones,bricks and concrete. Besides penetrating human bodies,the terahertz radiation can penetrate diverse materials like clothing, paper, cardboard,wood, masonry,plastics and ceramics.A. Effect of FIR rays on human bodyDespite the recent technologicalapplications of Terahertz radiation in biology and biomedicine, very little isknown about its interactions with biological systems. Contrary toX-rays, terahertz radiation has relatively low photon energy for damagingtissues and DNA. Terahertz being non-ionizing radiation does not harm thebiological tissues.
Pure FIR rays arestudied to generate therapeutic effects to human health 35. Some frequencies of T-rays can penetrate several millimeters of livingtissue having low water content (e.g., fatty tissue) and then reflect back. Theserays can diagnose the differences in water content and density of the tissue.
Hence these methods could allow for the effective detection of epithelialcancer by providing a safer and less painful system using imaging.In the proposed system, the T-rays that are incidenton a livehuman body will penetrate deep inside the cells (up to 4-7 cm inside the tissues) and arereadily absorbed by them. This is based on the principle that the living cellsof body and T- rays vibrate at the same resonant frequency. If not alive, then the FIR rays incident on the body will not be absorbed; instead, it will be just reflected back. This is becausedead cells do not resonate.
The opticalproperties of skin and water, which are the primary constituents of biologicaltissues of all living matter, are well-characterized to absorb terahertzfrequencies 36. As T-rays get absorbed in the cells,they cause warming effects and raise only the core body temperature due itsthermal reaction with the tissues. The heating is not caused at the skin level.T-rays facilitate an increase inthe circulation of nutrients and oxygenated blood.This in turn makes the person feel healthy and the pace of metabolismincreases.
Thecells, tissues and the organs get revived. The heatedcells and tissues re-radiate the absorbed terahertz radiation according to theproperty of black body radiation. These re-radiated waves will be degraded inintensity when compared to the intensity of transmitted T-rays. This differencebetween these intensities indicate the presence of surviving victimsunder the rubble and forms theprinciple of operation of this life detection system.Additionally, the depth at which thevictim is present under the rubble can be determined by analyzing the waveformsin THz detector.
For this, the time period between the first and the secondharmonics of the received rays must be analyzed, which can be used to locatethe victim. It is based on the simple distance-speed relationship given by: D= C*T (1)where,D = depth at which the victim is presentC = velocity of lightT = time duration between the first and the second harmonics of receivedraysDepending upon the depth calculated, the appropriate rescue methodthat needs to be applied to haul and rescue the person can be decided. B. Effect of FIR rays on non-living matterAs dead cells do notresonate, they do not absorb the incident T-rays.
The incident FIR beam is hence reflected back,and as a result, there will be no variation in its received intensity (except for some clutter which can becancelled or removed). This clearly indicates theabsence of life which helps the rescue operators to move aheadand search for live victims in other areas of rubble.
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