Abstract: In the design of fiber optic sensors, the selection of light source is crucial. This paper describes the basic light emission mechanism of several commonly used light sources, analyzes and compares their main characteristics and applicable occasions, introduces several new types of lasers, and makes predictions and prospects for the future development of light sources for fiber optic sensors. The analysis in this paper has certain reference value for the selection of light sources when designing fiber optic sensors. Keywords: Fiber optic sensor; Light source; Surface-emitting laser; Photonic crystal laser I. Introduction Fiber optic sensors have a history of nearly 30 years. Due to their advantages such as immunity to electromagnetic interference (EMI), immunity to radio frequency interference (RFI), light weight, small size, high sensitivity, wide bandwidth, and ease of implementing multi-channel or distributed sensing, they have become an indispensable type of sensor in military, industrial, and civilian applications. The type of light source selected for a fiber optic sensor largely determines the sensor's operating mode, signal processing method, resolution, sensitivity, and measurement accuracy. Therefore, selecting a suitable light source plays a crucial role in the overall design of the fiber optic sensor. However, many factors should be considered when selecting a light source, such as the size of the light source, input and output power, stability, coherence, spectral characteristics, and the ease of coupling with optical fiber. In addition, the price of the light source also largely determines whether such optical fiber sensors can be put into practical use. This article describes the performance of several commonly used optical fiber sensor light sources, analyzes and compares their applicable occasions and price issues, introduces several new types of lasers, and finally predicts and looks forward to the development trend of light sources selected for optical fiber sensors. II. Principles and main characteristics of commonly used light sources in optical fiber sensors 1. Incoherent light sources (1) Thermal light sources The main principle of thermal light sources is to heat a suitable material with an electric current to produce thermal radiation. A typical thermal light source is a tungsten lamp, which has the advantages of simple structure, convenient use and continuous spectrum. When this type of thermal light source is used in fiber optic sensors, there are two very important issues to consider: one is the stability of the light source. According to experience, the photocurrent generated by the tungsten filament is proportional to the 3rd to 4th power of the filament voltage. Therefore, in order to ensure a relatively stable light power, a power supply and circuit with very high stability must be used. The other issue is the limitation of the modulation rate. The modulation of this type of light source is generally carried out by a mechanical chopper, but its frequency is usually below 1kHz, which is far from meeting the requirements in many fiber optic sensor applications. In view of these limitations, this type of light source is generally not recommended in the design of actual fiber optic sensors. (2) Gas discharge light source. Under the action of ultraviolet light or radiation, a small number of molecules of the gas sealed in the glass tube are ionized. Two electrodes are sealed inside. When the applied voltage is high enough, the electric field increases the kinetic energy of the charged particles to a level sufficient to ionize other gas molecules. The gas molecules absorb the energy of the charged particles, causing the electrons to jump to the excited state. Since the electrons are unstable in the excited state, they will return to the low energy level or even the ground state in a very short time. At this time, the energy will be released in the form of light. The light emission principle of the gas discharge light source determines that its emission spectrum is discontinuous. This light source has two significant characteristics—high intensity and short wavelength. Because of this, it has unique applications in fiber optic sensing. For example, the high intensity short wavelength light emitted by the gas discharge light source can be used to excite the substance to be tested, causing it to emit fluorescence, which can be used to detect the temperature, content, etc. of the substance. (3) Light Emitting Diode (LED) A light emitting diode (LED) is a forward-biased PN junction diode made of semiconductor materials. Its light emission mechanism is that when a forward current is injected at both ends of the PN junction, the injected non-equilibrium carriers (electron-hole pairs) recombine and emit light during the diffusion process. This emission process mainly corresponds to the spontaneous emission process of light. According to the position of light output, LEDs can be divided into surface-emitting type and edge-emitting type. The most commonly used LED is the InGaAsP/InP double heterojunction edge-emitting diode. LEDs have significant advantages such as high reliability, long continuous working time at room temperature, and good optical power-current linearity. Moreover, since this technology has been developed relatively maturely, its price is very cheap. Therefore, in the design of some simple fiber optic sensors, if LEDs can perform the task, choosing them as the light source can greatly reduce the cost of the entire sensor. However, the light-emitting mechanism of LEDs inherently has many shortcomings, such as low output power, large emission angle, wide spectral linewidth, and low response speed. Therefore, in sensor designs requiring high-power, fast-modulation-rate, and highly monochromatic light sources, it is necessary to choose other higher-performance light sources at the cost of increased costs. 2. Coherent Light Sources For a laser to operate, three basic conditions must be met: a laser material, an optical resonator, and a pump source. Its basic structure is shown in Figure 1. The principle of laser light emission is as follows: energy is input into the laser material through the pump source, causing population inversion. The weak light generated by spontaneous emission is amplified in the laser material. Because mirrors are placed at both ends of the laser material, some of the light that meets the conditions can be fed back and participate in the excitation. At this time, the excited light oscillates. After multiple excitations, the light projected from the right-hand mirror is a high-brightness laser with excellent monochromaticity, directionality, and coherence. Different types of lasers use different materials in terms of the light-emitting material, mirrors, and pump source. The various lasers mentioned below are classified based on these differences. (1) Solid-state lasers are lasers whose laser material is a solid. As early as 1960, Maiman developed the world's first laser—the ruby ​​laser (Cr3+:Al2O3), which was a typical solid-state laser. Subsequently, yttrium aluminum garnet (Nd:YAG) lasers doped with neodymium (Nd3+) ions, glass lasers doped with neodymium (Nd3+) ions, and sapphire lasers doped with titanium (Ti3+) ions appeared one after another. Solid-state lasers have advantages such as high output energy, high peak power, compact device structure, easy fiber coupling, long service life, and mature unit technology. Moreover, their size is smaller than that of gas lasers, and their price is relatively moderate. Due to these advantages, solid-state lasers have certain applications in the field of fiber optic sensing. They can be used to measure absorption spectra, such as measuring Rayleigh and Raman scattering spectra generated by pollutants; and can be used for ultra-long-distance measurements, such as the distance from the moon to the earth. In addition, ruby ​​itself is also very useful in the field of fiber optic sensors, for example, its fluorescent thermal effect can be used to measure ambient temperature. The main drawback of solid-state lasers is that the pumping efficiency of commonly used inert gas discharge lamps is low, the thermal effect is serious, which limits the further improvement of output power and beam quality. (2) Liquid lasers Lasers whose working substance is liquid are called liquid lasers. They have unique output characteristics: wide output laser spectral line, small beam divergence angle, and the laser output wavelength can be moved (tunable). A mixture of two liquids can produce a liquid with a new output wavelength. The activation ion density is high, the gain coefficient is high, and higher output power can be obtained, etc. In addition, the low price, high energy conversion efficiency, good optical uniformity, and convenient cooling are also its advantages. However, liquid lasers are extremely inconvenient to use. They require difficult manual operation and closed-loop pumping to avoid thermal effects that damage the characteristics of the laser. Moreover, they can be carcinogenic. Therefore, liquid lasers are rarely used in the field of fiber optic sensing. (3) Gas lasers Gas lasers are lasers that use gas or metal vapor as the main working substance. This is a very important laser, and it has a wide range of applications in the field of fiber optic sensing. Its biggest feature is that it can generate very high continuous power. For most gas lasers, since the absorption spectrum of the gas is very narrow, optical pumping is not used for excitation, but electrical excitation is used, which is very suitable for use in fiber optic sensors. In the field of sensing, the commonly used gas lasers are helium-neon lasers, argon-ion lasers, carbon dioxide lasers and helium molecular lasers. Among them, the most valuable is the carbon dioxide laser, which works in the far-infrared region and has high efficiency. It can generate several kilowatts of power in pulsed operation mode and several watts of power in continuous operation. This continuous high power supply is difficult for other types of lasers to achieve. In addition, it is worth mentioning that distributed fiber optic sensors, as an important branch of fiber optic sensors, are widely used in the measurement of many large temperature fields and stress fields to achieve real-time and spatially continuous measurement [3]. For this type of sensor, since the transmission distance is long and the returned light information must be able to reach the detection level, the light source must provide a sufficiently large light power. Therefore, it generally uses gas lasers, among which argon-ion lasers are a very good choice. (4) Semiconductor Laser Diode Semiconductor laser diodes are a widely used light source in fiber optic sensing systems. Their light-emitting principle is not significantly different from that of the light-emitting diodes discussed above, except that the output light changes from incoherent to coherent. As a type of laser, semiconductor laser diodes must also meet the requirements of population inversion and optical feedback. The method used is to heavily dope the P-type and N-type confinement layers, causing the Fermi level spacing to exceed the bandgap under forward bias of the PN junction, thus achieving population inversion. Then, the natural cleavage plane perpendicular to the PN junction plane (AB in Figure 2) is used to construct the FP cavity. The light-emitting characteristics of a semiconductor laser diode are shown in Figure 3. In practical applications, two requirements must be placed on the laser diode: a low threshold current and a stable PI curve. Replacing the homojunction with a heterojunction can reduce the threshold current by two orders of magnitude, while the stability issue can currently only be improved through external temperature control and optical feedback. Semiconductor laser diodes (SLDs) are highly efficient, small in size, have a wide wavelength range, and are inexpensive, making them very convenient for use in fiber optic sensing, especially in fiber optic hybrid sensors. High-power SLDs are often used as the light source, providing electrical power to the sensor probe through photoelectric conversion. Currently, light-driven temperature and pressure sensing systems implemented in this way are already in practical use. The most critical weakness of semiconductor laser diodes is that their performance gradually degrades over time, with some characteristics deteriorating irreversibly, eventually rendering the laser diode unusable. This defect significantly limits their use in fiber optic sensors that require long-term operation and are inconvenient to replace. Furthermore, the monochromaticity and directionality of ordinary semiconductor lasers are inferior to those of gas lasers, a point worth noting during selection. Various methods are being explored to improve this, such as using distributed feedback or quantum well structures. (5) Fiber Lasers Fiber lasers are actually solid-state lasers, except that the laser material is replaced by rare-earth ion-doped fiber. Based on the different doping ions and the different mirror-like mechanisms at both ends, they can be classified into rare-earth-doped fiber lasers (as shown in Figure 4), fiber grating lasers (as shown in Figure 5), nonlinear effect fiber lasers, single-crystal fiber lasers, plastic phosgene lasers, optical soliton lasers, etc. The main advantages of fiber lasers are that they can easily achieve continuous operation with low pumping; they have low threshold, high gain, and low thermal effect; using directional coupling and Bragg reflection, narrow-linewidth, tunable fiber lasers can be fabricated; they can be well coupled with optical fibers, are fully compatible with existing fiber optic devices, can perform all-fiber testing, and can transmit system light sources, which is extremely valuable and important in the design of any optical system or optical device. Therefore, fiber lasers have a very good prospect for application in next-generation fiber optic sensors. In particular, they are used as a strong light source for fiber optic time-domain emission (OTDR) measurements and a broadband emission source for fiber optic gyroscopes. III. Several Novel Light Sources 1. Surface-Emitting Lasers A surface-emitting laser is a semiconductor laser that emits light perpendicular to the surface of a semiconductor substrate (as shown in Figure 6). Because multiple lasers are arranged side-by-side on the substrate, it has attracted attention as a novel type of semiconductor laser intended for application in new optoelectronic fields such as parallel optical information processing and optical interconnects. This type of semiconductor laser device can be fabricated into integrated circuits using semiconductor process technology, i.e., monolithic integration, and has the characteristic of two-dimensional parallel integration. This type of laser has been proposed for less than 30 years, and through continuous research and development, its performance has surpassed that of other semiconductor lasers. However, its large-scale practical application remains a major challenge in the field of communications. Researchers have primarily focused on integrating it into ultra-parallel optoelectronic technologies such as large-scale optical communication networks, optical interconnects, and optical information processing. In fact, its application in fiber optic sensing is also a topic worthy of significant research. Surface-emitting lasers (SELs) possess numerous advantages, such as small size, low threshold current and insensitivity to temperature, long lifespan, high electro-optic effect, fast response speed, easy integration with optical fibers, mass production capability, ability to be fabricated into densely packed two-dimensional laser arrays, and application in stacked optical integrated circuits. Densely packed two-dimensional laser arrays enable simultaneous multi-point measurements within a small area using fiber optic sensing; stacked integration makes miniaturization of a large sensor system possible. Therefore, the author believes that further improvements in the performance of this type of light source and its practical application in the field of fiber optic sensing are highly anticipated. 2. Photonic Crystal Lasers While the performance of traditional lasers is gradually improving, some shortcomings seem difficult to overcome. For example, changes in the laser's emission wavelength cause variations in transmission loss; linewidth tends to saturate and broaden again with increasing power; the radiation angle is relatively large, resulting in low coupling efficiency. If a defective photonic crystal is introduced into the laser, that is, a microcavity artificially created to confine the light (as shown in Figure 7), one or more isolated defect modes will appear in the photonic bandgap. If the laser medium in the microcavity is excited, laser light with defect mode characteristics will be generated. When the Q value of the microcavity is large enough, the defect mode laser will have good monochromaticity. The direction can be well controlled by leading it out of the photonic crystal using in-plane waveguides or other out-of-plane methods. At this time, almost all the energy of spontaneous emission is used to emit laser light, which greatly reduces the threshold of the laser. This kind of laser with small size, low or even zero threshold, high power, easy fiber coupling, and dense distribution in a small area is exactly what we usually pursue when selecting light sources in the design of fiber optic sensors. In addition, photonic crystal lasers can also be extended to the design of highly sensitive chemical detectors and open up new directions for exploring many fundamental physical phenomena. In 1999, the research group led by A. Scherer at Caltech first reported a photonic crystal laser that could operate at room temperature and at 1550 nm (Figure 8). Currently, Bell Labs, the University of Bath in Swindon, and Crystal Fiber A/S in Denmark are all vigorously researching this new type of laser. The dual-defect photonic crystal vertical-cavity surface-emitting laser proposed by AJ Danner et al. combines the advantages of the above two types of lasers [11]. In my country, the Shenzhen Key Laboratory of Laser Engineering also developed a photonic crystal laser with a power of 15W in April this year. It is expected that within five years, high-efficiency photonic crystal laser emitters will gradually become practical and then gradually develop into the mainstream of lasers. 3. There are many other types of lasers that are currently under further research, such as chemical lasers, gas lasers, color center lasers, free electron lasers, single-atom lasers, and X-ray lasers. These lasers have unique features in their respective performances, but at present, the cost is still relatively high, and their application in the field of fiber optic sensors is not very widespread; however, it can be expected that as the technology matures further, the price of these lasers will gradually decrease, and some of their special performance highlights will gradually be recognized. IV. Outlook The light source largely determines the development of fiber optic sensors; however, market demand for fiber optic sensors and ever-increasing performance requirements also influence the development of light sources. It can be predicted that future fiber optic sensor systems will inevitably develop towards smaller, more convenient, multi-purpose, highly sensitive, and low-cost designs. Therefore, light sources must also gradually become more integrated and modular while continuously reducing costs. Currently, many research institutions at home and abroad have begun in-depth research and development of integrated optoelectronic modules (IOEMs). An IOEM is a module that integrates the light source, detector, auxiliary and excitation circuits, and even the computational components, representing a development trend for light sources used in fiber optic sensors. Furthermore, the development of some special light sources and their applications in the sensing field will undoubtedly continue to bring surprises. Finally, if photonic crystal lasers can be practically applied as soon as possible, it will bring a huge revolution to the laser field, thereby driving the rapid development of fiber optic sensors. Due to the flexibility of photonic crystals in manipulating light wave flow, it is expected that in the future, a photonic crystal light source can be tailored to the design of each type of fiber optic sensor. This will make fiber optic sensors more flexible and efficient, which is exactly what we hope for. References: [1] Wang Yutian, Zheng Longjiang, Hou Peiguo, et al. Optoelectronics and Fiber Optic Sensor Technology [M]. National Defense Industry Press, 2003, pp. 42-61 [2] Ning Tigang, Zhang Jingsong, Pei Li, et al. Fiber Bragg grating laser [J]. Optical Communication Research, 2000, No. 3 [3] Liao Yanbiao, Li Min. The present and development of fiber optic sensors [J]. Sensor World, 2004, Vol. 10, No. 2 [4] Wang Qing, Zhao Lumin, Ning Yongqiang, et al. Progress of vertical cavity surface-emitting lasers for optical communication [J]. Optical Communication Research, 2004, No. 1 [5] Cheng Bingying. Photonic crystal waveguide and photonic crystal laser [J]. Physics Teaching, 2003, Vol. 25, No. 2