0 Introduction Since the 1970s, fiber optic sensors have experienced rapid development due to their high sensitivity, corrosion resistance, strong electromagnetic interference resistance, and high reliability. These characteristics also enable measurements under specific conditions, offering numerous advantages over conventional detection techniques and making them a leading direction in sensor technology development. As a key optical component in fiber optic sensors, the stability of the light source directly affects the sensor's accuracy. The fiber optic sensor discussed in this paper utilizes a semiconductor laser light source. Semiconductor lasers offer advantages such as good monochromaticity, good directionality, small size, and high optical power utilization; however, their optical power output is significantly affected by changes in the external environment. Therefore, this paper designs a simple and feasible automatic power control (APC) drive circuit based on the working principle and characteristics of semiconductor laser light sources. Constant power control is achieved through feedback formed by back-side monitoring of the photocurrent. Furthermore, a slow-start circuit is introduced to prevent interference from the power supply voltage, ensuring the laser is not subjected to overcurrent surges each time the power is switched on, thus extending the laser's lifespan. Experimental verification shows that this circuit solves the problem of unstable output power of the laser during use, with a stability better than 0.5%, achieving a good current stabilization effect. 1 Working Principle and Characteristics of the Light Source Currently, light sources used in practical applications include surface-emitting diodes (LEDs), laser diodes (LDs), superluminescent diodes (SLDs), and superfluorescent light sources (SFSs). With the rapid development of fiber optic sensing technology, semiconductor light sources such as LDs, which are small in size, lightweight, low in power consumption, and easy to couple with optical fibers, are becoming increasingly widely used. This paper mainly studies the driving design of semiconductor LDs. 1.1 Analysis of LD Light Emitting Mechanism The basic structure of an LD is: a pair of parallel planes perpendicular to the PN junction surface constitute a Fabry-Perot resonant cavity. These can be cleavage planes of a semiconductor crystal or polished planes. The other two sides are relatively rough to eliminate the laser effects in directions other than the main direction. When a forward voltage is applied to the PN junction of a semiconductor, the PN junction barrier is weakened, forcing electrons to be injected from the N region into the P region through the PN junction, and holes to be injected from the P region into the N region through the PN junction. These non-equilibrium electrons and holes injected near the PN junction will recombine, thereby emitting photons with wavelength λ, which is given by the formula λ=hc/Eg, (1) where h is Planck's constant; c is the speed of light; and Eg is the bandgap of the semiconductor. If the injection current is large enough, a carrier distribution opposite to the thermal equilibrium state will be formed, i.e., population inversion. When the carriers in the active layer are inverted in large numbers, a small number of photons generated by spontaneous emission will generate induced radiation due to the reciprocating reflection of the two ends of the resonant cavity, resulting in frequency-selective resonance positive feedback, or gain for a certain frequency. When the gain is greater than the absorption loss, laser light with good directionality, strong coherence, high brightness, and narrow bandwidth can be emitted from the PN junction. In addition to possessing the general characteristics of lasers such as good coherence, strong directionality, small divergence angle, and highly concentrated energy, laser lasers (LDs) also feature high photoelectric conversion efficiency, high output power, small size, light weight, simple structure, and strong shock resistance. 1.2 LD Output Characteristics Figure 1 shows the relationship between the output power and forward drive current of a typical semiconductor laser at different temperatures. For clarity, the approximately straight line at the bottom of the figure has been intentionally raised. As can be seen from Figure 1, when the drive current is below the threshold current, the laser can only emit fluorescence. Only when the drive current is greater than the laser's threshold current can the laser operate normally and emit laser light. Therefore, to enable the LD to emit laser light, a working current slightly greater than the threshold current must be supplied to the LD. Moreover, the threshold current of the LD is affected by temperature; the higher the temperature, the larger the corresponding threshold current. At a certain temperature, when the drive current is below the threshold current, the output optical power is approximately zero; when the drive current is above the threshold current, laser light is output, and the output power increases rapidly with the increase of the drive current, showing an approximately linear increase. This paper uses a 1310 nm wavelength FP coaxial laser with an operating current of 25.0 mA and an output power of 0.96 mW. The internal optical path structure is shown in Figure 2. The laser diode (LD) and backlight detector (PD) are combined and packaged together. The LD is connected in the forward direction, and the PD is connected in the reverse direction. The PD is used to detect the backlight output power of the laser, and its output power depends on the output value of the LD. 1.3 Modulation and Backlight Coupling of the LD To facilitate automatic optical power control, the LD and backlight detector (PD) are typically integrated inside the laser, as shown in Figure 2. The LD has two output surfaces. The light output from the main output surface is used by the user, and the light output from the secondary output surface (i.e., backlight) is received by the photodiode (PD). The resulting photocurrent is used to monitor the operating status of the LD. The monitoring current of the backlight detector is linearly related to the output power of the main output surface. Based on the coupling characteristics of the backlight detector to the LD, appropriate peripheral circuits can be designed to achieve automatic optical power control of the LD. 2. LD Drive Control Circuit Design As shown in Figure 3, the LD and monitoring diode are integrated components. The current flowing into the LD passes through the pre-bias current of the APC circuit. The APC circuit uses a current negative feedback circuit to suppress changes in optical power caused by temperature variations, device aging, etc. The APC circuit adopts a backlight feedback automatic bias control method, that is, the PD photodiode in the semiconductor laser assembly monitors the backlight output optical power of the LD. Because the backlight output optical power can track the changes in the forward output optical power, the laser current can be adjusted through a closed-loop control system to achieve the purpose of stable output optical power. The APC circuit shown in Figure 4 consists of operational amplifiers 1 and 2, transistor Q1, and peripheral circuits. This circuit is a negative feedback system with a transistor as its core, which has the function of automatically stabilizing the laser's optical output power. The feedback is taken from the backlight of the LD, detected by the backlight monitoring diode and converted into a corresponding current. After being filtered by capacitor C1, it enters the inverting input terminal of the operational amplifier, converting the current signal into a voltage signal V1. The non-inverting input of the operational amplifier consists of a +2.5V stable reference source composed of an LM336 and the operational amplifier, and a positioner R5. The output of the reference voltage is V2, which can be adjusted by the positioner. A large inrush current is generated when voltage is applied to the drive circuit, and this large instantaneous current change can affect the lifespan of the semiconductor laser. Furthermore, the power supply voltage is typically provided to the drive circuit from AC 220V through a transformer and rectification. External interference signals can also generate large instantaneous currents, which, over long-term operation, will also affect the lifespan of the semiconductor laser. To address this, a slow-start circuit is introduced in the design, consisting of a diode and a capacitor connected in parallel at the input of the reference source. The capacitor ranges from approximately 10 to 470μF, with an optimal value of 22 to 47μF. This allows the drive circuit to operate without interference from the power supply voltage, providing a slow-start effect and preventing the laser from experiencing overcurrent surges each time the power is switched on, thus extending the laser's lifespan. The APC circuit control process is as follows: When the output optical power of the LD decreases for some reason, the current coupled to the photodiode also decreases proportionally, i.e., V1 decreases. This breaks the balance under normal conditions, causing the voltage at the output terminal of operational amplifier 1, V3, to increase. Consequently, the base current of transistor Q1 increases, and the collector current also increases accordingly. The collector current is the current flowing into the LD. Therefore, the current flowing into the laser increases, and the output optical power increases accordingly, thus keeping the output optical power constant; conversely, the same applies. Based on the performance parameters of the laser in this sensor, by selecting appropriate resistors and capacitors for matching and adjusting the potentiometer, different optical power output values ​​can be obtained. Figure 5 shows the experimental curves at room temperature (25℃). It can be seen from the figure that the threshold current of the fiber optic sensor LD light source is around 8 mA, and it operates stably between 10 and 30 mA. The output power and the driving current show a good linear relationship after exceeding the threshold current. Under normal operation, it can output stable and adjustable optical power values ​​of -0.1, -1, -2, -5, and -10 dB. The circuit parameters are configured to ensure that the current flowing into the LD does not exceed its limit. Experiments demonstrate that the circuit design is correct and feasible. Automatic optical power feedback based on the backlight monitor ensures that the fiber optic sensor can operate normally under constant power. 3. Conclusion The driving circuit designed in this paper solves the problem of unstable output power of the laser during use through slow start and automatic power control circuits. Its stability is better than 0.5%, achieving a good current stabilization effect. The fiber optic sensor in this paper is used in a liquid nitrogen cryogenic environment. This experiment was conducted at room temperature, with its coupler and driving circuit led out through optical fiber at room temperature (25℃). The temperature change was not significant; therefore, no temperature compensation control circuit was introduced. The next experiment will place the fiber optic sensor in a liquid nitrogen cryogenic environment, where temperature fluctuations are larger. Therefore, an automatic temperature compensation circuit needs to be considered to achieve constant temperature control.