Abstract: In the design of fiber optic sensors, the choice of light source is very important. This paper describes the basic luminescence mechanism of several common light sources, analyzes and compares its main characteristics and applicable occasions, and introduces several new types. The laser and the prediction and prospect of the next step of the optical fiber sensor are made. The analysis of this paper has certain reference value for the selection of the light source when designing the fiber sensor.
Key words: fiber optic sensor; light source; surface emitting laser; photonic crystal laser CLC number: TP212.14 Document code: A
I. Introduction
Fiber optic sensors have been around for nearly 30 years, due to their resistance to electromagnetic interference (EMI), radio frequency interference (RFI), light weight, high sensitivity, wide bandwidth, and easy multi-channel or distributed sensing. It has become an indispensable sensor type for military, industrial and civil use. The type of light source selected for fiber optic sensors largely determines the operating mode, signal processing method, resolution, sensitivity, and measurement accuracy of the sensor. Therefore, choosing a suitable light source is critical to the design of the entire fiber sensor. The role. However, there are many factors to consider when selecting a light source, such as the size of the light source, input and output power, stability, coherence, spectral characteristics, and ease of coupling with the fiber. In addition, the price of the light source is also largely It is decided whether this fiber sensor can be finally put into practical use. In this paper, the performances of several commonly used fiber-optic sensor light sources are described. The applicable occasions and price problems are analyzed and compared. Several new types of lasers are introduced. Finally, the development trend of optical fiber sensor selection sources is carried out. Forecast and outlook.
Second, the principle and main characteristics of commonly used light sources in fiber optic sensors
1, incoherent light source
(1) The main principle of the heat source of the heat source is to heat the appropriate material to generate heat radiation. A typical thermal source is a tungsten lamp, which has the advantages of simple structure, ease of use, and continuous spectrum.
There are two very important problems when using this kind of thermal light source in the fiber sensor. One is the stability problem of the light source. According to experience, the photocurrent generated by the tungsten wire is proportional to the voltage of the filament 3 to 4, so To ensure a relatively stable optical power, it is necessary to apply a power supply and circuit with very high stability; another problem is the limitation of the modulation rate. The modulation of such a light source is generally a mechanical chopper, but the frequency is usually lower than 1 kHz. This is far from being required in many fiber optic sensor applications. In view of these limitations, the selection of such a source is generally discouraged in the design of actual fiber optic sensors.
(2) gas discharge light source glass tube sealed gas under the action of ultraviolet light or radiation, a small number of molecules are ionized, sealing two electrodes in it, when the applied voltage is high enough, the electric field acts to increase the kinetic energy of the charged particles enough to ionize Other gas molecules, gas molecules absorb the energy of charged particles, causing the electrons to transition to the excited state. Since the electrons are unstable in the excited state, they will return to the low energy state or even the ground state in a very short time. At this time, the energy will be light. The form is released.
The principle of illumination of a gas discharge source determines that its luminescence spectrum is discontinuous. This light source has two distinct features—high intensity and short wavelength—which are why it has a unique use in fiber sensing. For example, a high-intensity short-wavelength light emitted by a gas discharge source is used to excite a substance to be tested to emit fluorescence, which can be used to detect the temperature and content of a substance.
(3) Light-emitting diodes Light-emitting diodes (LEDs) are forward-biased PN junction diodes fabricated from semiconductor materials. The illuminating mechanism is that when a forward current is injected at both ends of the PN junction, the injected unbalanced carriers (electron-hole pairs) are combined to emit light during the diffusion process, and the emission process mainly corresponds to the spontaneous emission process of the light. According to the position of the light output, the light-emitting diodes can be divided into a surface emitting type and an edge emitting type. Our most commonly used LEDs are InGaAsP/InP double heterojunction LEDs.
Light-emitting diodes have significant advantages such as high reliability, long continuous working time at room temperature, good linearity of optical power-current, and because the technology has developed relatively mature, its price is very cheap. Therefore, in the design of some simple fiber optic sensors, if the LED is capable, using it as a light source can greatly reduce the cost of the entire sensor. However, the LED's illumination mechanism determines that it has many shortcomings, such as large output angle, wide spectral line, and low response speed. Therefore, in some sensor designs that require high power, high modulation rate, and good monochromatic light source, other higher performance light sources have to be selected at the expense of cost.
2. The coherent light source laser must have three basic conditions to work, namely laser material, optical resonator and pump source. The basic structure is shown in Figure 1.
The principle of laser illumination is: the energy is input into the laser material through the pump source, so that the particle number is reversed, and the weak light generated by the spontaneous radiation is amplified in the laser material. Since the laser material has mirrors placed at both ends, there is a part. The qualified light can be fed back and then participate in the excitation. At this time, the excited light will oscillate. After multiple excitations, the light projected from the right-end mirror is monochromatic, directional, and coherent. High brightness laser. Different types of lasers differ in the materials used in luminescent materials, mirrors, and pump sources. The various lasers mentioned below are also classified based on these differences.
(1) Solid-state laser A solid-state laser is a laser in which a laser substance is a solid. As early as 1960, the world's first laser, the ruby laser (Cr3+: Al2O3), was a typical solid-state laser. Subsequently, a Nd3+ ion-doped yttrium aluminum garnet (Nd:YAG) laser, a ytterbium (Nd3+)-doped glass laser, and a titanium (Ti3+)-doped sapphire laser have appeared. Solid-state lasers have the advantages of large output energy, high peak power, compact device structure, easy fiber coupling, long service life and mature cell technology, and its volume is smaller than that of gas lasers, and the price is relatively moderate. Due to these advantages, solid-state lasers have certain applications in the field of optical fiber sensing, which can be used to measure absorption spectra, such as Rayleigh and Raman scattering spectra produced by contaminants; and can be used for ultra-long-range measurements, such as the moon to The distance of the earth. In addition, ruby itself is also very useful in the field of fiber optic sensors, such as the use of its fluorescent thermal effect to measure the ambient temperature. The main drawback of solid-state lasers is that the commonly used inert gas discharge lamps have low pumping efficiency and severe thermal effects, which limits the further improvement of output power and the improvement of beam quality.
(2) Liquid laser The laser working substance is a liquid laser called a liquid laser. It has unique output characteristics: output laser linewidth, beam divergence angle, laser output wavelength can be moved (tunable), a mixture of two liquids can produce a liquid with a new wavelength, activating ion density, high gain coefficient and Can get higher output power and so on. In addition, the advantages of low price, high energy conversion efficiency, good optical uniformity, and convenient cooling are also advantages. However, liquid lasers themselves are extremely inconvenient to use, requiring difficult manual operations and closed-loop pumping to avoid thermal effects that degrade the characteristics of the laser and can cause cancer, so liquid lasers are rarely used in the field of fiber sensing.
(3) Gas laser A gas laser is a laser that uses gas or metal vapor as a main working substance. This is a very important laser and has a very wide range of applications in the field of fiber optic sensing. Its biggest feature is that it can produce high continuous power. For most gas lasers, because the absorption line of the gas is very narrow, optical pump excitation is not used, and electrical excitation is used, which is very suitable for use in fiber sensors. In the field of sensing, our commonly used gas lasers are helium neon lasers, argon ion lasers, carbon dioxide lasers, and helium molecular lasers. The most valuable of these is the CO2 laser, which operates in the far-infrared region and is highly efficient, generating several kilowatts of power in a pulsed mode of operation and generating several watts of power during continuous operation. This continuous high power Providing is difficult to achieve with other types of lasers. It is also worth mentioning that distributed fiber optic sensors, as a very important branch of fiber optic sensors, are widely used in the measurement of many large temperature stress fields to achieve real-time, spatially continuous measurements [3]. For such a sensor, since the transmission distance is long and the returned optical information must be of the order of detection, it is necessary to provide a sufficiently large optical power in terms of the light source, so it generally uses a gas laser, wherein the argon ion laser is A very good choice.
(4) Semiconductor laser diodes Semiconductor laser diodes are widely used in optical fiber sensing systems. The principle of illumination is not much different from the principle of LEDs discussed above, except that the output light is changed by incoherent light. For coherent light. As a kind of laser, semiconductor laser diodes must also meet the requirements of particle number inversion and optical feedback. The method used is to heavily dope the P-type and N-type confinement layers, so that the Fermi level interval is over the band gap to achieve the population inversion under the forward bias of the PN junction. The FP cavity is formed by a natural cleavage plane (AB in Fig. 2) perpendicular to the plane of the PN junction.
The luminescent characteristics of a semiconductor laser diode are shown in FIG. In practical applications, two requirements must be placed on the laser diode, one is the lower threshold current, and the other is the stable PI curve. We can reduce the threshold current by two orders of magnitude by replacing the homojunction with a heterojunction, and the stability problem is currently improved only by applying constant temperature and optical feedback.
Semiconductor laser diodes are high in efficiency, small in size, wide in wavelength range, and low in price. They are very convenient to use in fiber sensing. Especially in fiber-optic hybrid sensors, laser diodes with higher power are often used as light sources. The sensor probe provides electrical power. At present, the light-sensing temperature, pressure and other sensing systems realized in this way have been put into practical use. The most deadly weakness of semiconductor laser diodes is that their performance will gradually degrade after a certain period of operation, some characteristics will deteriorate, and these changes are irreversible, eventually resulting in the laser tube not being used. This defect has greatly affected its use in fiber optic sensors that must be used for long periods of time and are inconvenient to replace. In addition, the monochromaticity and directivity of ordinary semiconductor lasers are worse than those of gas lasers. This is also worth noting when selected. People have been experimenting with various methods to improve them, such as using distributed feedback or quantum well structures. .
(5) Fiber laser The fiber laser actually belongs to the solid-state laser. It only replaces the laser material with the rare earth ion-doped fiber. According to the difference of the doping ions and the functional mechanism of the mirror at both ends, it can be divided into rare earth-doped fiber lasers. Figure 4), fiber grating laser (Figure 5), nonlinear effect fiber laser, single crystal fiber laser, plastic phosgene laser, optical soliton laser.
The main advantage of fiber laser is that it can easily achieve low-pump continuous operation; its threshold is low, its gain is high, and its thermal effect is low. By using directional coupling and Bragg reflection, a narrow linewidth, tunable fiber laser can be fabricated; it can be well coupled with fiber. Fully compatible with existing fiber optic devices, it can perform all-fiber testing and transmit system light sources, which is precious and important in the design of any optical system and optical device. Therefore, the application of fiber lasers in the next generation of fiber optic sensors has very good prospects. In particular, it is used as a strong light source for optical fiber time domain emission (OTDR) measurements and as a broadband source for fiber optic gyros.
Third, several new types of light sources
1. A surface emitting laser surface emitting laser is a semiconductor laser that emits light from a direction perpendicular to the surface of a semiconductor substrate (Fig. 6). Since a plurality of lasers are arranged side by side on the substrate, they are attracting attention as novel semiconductor lasers for the purpose of application in new fields of optoelectronics such as parallel optical information processing and optical interconnection. Such a semiconductor laser device can be fabricated into an integrated circuit by using a semiconductor process technology, that is, monolithically integrated, and has a feature capable of two-dimensional parallel integration.
This kind of laser has been proposed for less than 30 years. Through continuous research and development, its performance has surpassed other semiconductor lasers. However, its large-scale practical application is still a big issue in the field of communication, and people's goals are basically The above focuses on integrating it into super-parallel optoelectronic technology such as large-scale optical communication networks, optical interconnection, and optical information processing. In fact, its application in optical fiber sensing is also a subject worthy of study. Surface-emitting lasers have many advantages, such as low volume and low temperature, insensitivity to temperature, long life, high electro-optic effect, fast response, easy combination with optical fibers, large-scale production, and dense array of two-dimensional laser arrays. It can be applied to laminated optical integrated circuits. The dense array of 2D laser arrays enables simultaneous multi-point measurements in a small area using fiber optic sensing; cascading integration makes it possible to miniaturize a large sensor system. Therefore, the author believes that the further improvement of the performance of such a light source and its practical application in the field of optical fiber sensing are worthy of expectation.
2. The performance of traditional lasers for photonic crystal lasers is gradually improving, but some of them seem to be difficult to overcome. For example, changes in the emission wavelength of the laser cause changes in transmission loss; line widths tend to saturate as power increases, and re-expanding The radiation angle is relatively large and the coupling efficiency is not high. However, if a defective photonic crystal is introduced into the laser, artificially creating a microcavity in which light can be confined (Fig. 7), one or several isolated defect modes appear in the photonic band gap. If the laser medium in the microcavity is excited, a laser with a defect mode characteristic is generated, and when the Q value of the microcavity is sufficiently large, the defect mode laser has a good monochromaticity, and then in a plane The waveguide or the other way out of the plane leads it out of the photonic crystal, and the direction is well controlled. At this point, almost all of the spontaneously radiated energy is used to emit the laser, which greatly reduces the threshold of the laser. Such a small volume, low threshold or even zero threshold, high power, easy to fiber coupling and densely distributed lasers in a small area is also what we usually pursue when designing fiber optic sensor selection sources; in addition, the photonic crystal laser itself can also Extends to the design of highly sensitive chemical detectors and opens up new directions for exploring many fundamental physical phenomena.
In 1999, a research team led by A. Scherer of the California Institute of Technology first reported a photonic crystal laser that can operate at room temperature and operate at 1550 nm (Figure 8). At present, Bell Labs, Bath University in Swindon, UK, and Crystal Fiber A/S in Denmark are vigorously studying this new type of laser. The double-cavity photonic crystal vertical cavity surface emitting laser proposed by AJ Danner et al. combines the advantages of the above two lasers [11]. In China, the Shenzhen Key Laboratory of Laser Engineering in April this year has also developed a photonic crystal laser with a power of 15W. It is expected that within five years, high-efficiency photonic crystal laser emitters will gradually become practical, and then gradually become the mainstream of lasers.
3. There are many other types of lasers that are currently being further studied, such as chemical lasers, pneumatic lasers, color center lasers, free electron lasers, single atom lasers, and X-ray lasers. These lasers are unique in their respective performances, but at present they are still relatively expensive, and their applications in the field of fiber optic sensors are not yet widespread; however, it is expected that as the technology matures, the price of these lasers will gradually decline. Some special highlights in its performance will gradually be recognized by people.
Fourth, the outlook
The light source largely determines the development of fiber optic sensors. However, the market demand for fiber optic sensors and ever-increasing performance requirements have in turn affected the development of light sources. It can be expected that in the future, fiber-optic sensor systems will be developed in a small, convenient, versatile, highly sensitive, and low-cost direction. Therefore, the light source must be gradually integrated and modularized on the basis of continuous cost reduction. At present, many research institutes at home and abroad have begun in-depth research and development of integrated optoelectronic modules (IOEM), which is composed of an illuminating light source and a detector, and an auxiliary and excitation circuit of the two, and even an arithmetic part. A module that represents a trend in light sources for fiber optic sensors. In addition, the development of some special light sources and their application in the field of sensing will continue to surprise people. Finally, if the photonic crystal laser can be put into practical use as soon as possible, it will bring a huge revolution to the laser field, which will promote the further development of fiber optic sensors. Due to the flexibility of photonic crystals to manipulate the flow of light waves, it is expected that in the future it will be possible to tailor a photonic crystal source for each fiber-optic sensor design. Thus, fiber-optic sensors will be more flexible and efficient, which is exactly what we expect. of.
