Fiber optic sensor for light source drive circuit

Since the 1970s, optical fiber sensors have achieved rapid development due to their high sensitivity, corrosion resistance, strong electromagnetic interference resistance, safety and reliability. At the same time, these characteristics also enable it to achieve measurement under certain special conditions, which has many advantages over conventional detection technology and is a leading direction in the development of sensing technology.

As a light source that is one of the key optical components in fiber optic sensors, the stability directly affects the accuracy of the fiber sensor. The fiber-optic sensor involved in this paper uses a semiconductor laser light source. The semiconductor laser has the advantages of good monochromaticity, good directivity, small volume, high utilization of optical power, etc. However, the optical power output is greatly affected by changes in the external environment. Therefore, in this paper, a simple and feasible automatic power control (APC) drive circuit is designed for the working principle and characteristics of semiconductor laser light source. The feedback is realized by back monitoring the photocurrent to achieve constant power control. Moreover, a slow start circuit is introduced to prevent interference of the power supply voltage, so that the laser is not subjected to an overcurrent impact generated each time the power is turned on, thereby prolonging the service life of the laser. It is verified by experiments that the circuit solves the problem that the output power of the laser is unstable during use, and its stability is better than 0.5%, which achieves a better steady flow effect.

At present, practical light sources include surface light emitting diodes (LEDs), laser diodes (LDs), superluminescent diodes (SLDs), and superfluorescent light sources (SFS). With the rapid development of optical fiber sensing technology, semiconductor light sources such as LDs with small size, light weight, low power consumption, and easy coupling with optical fibers are becoming more and more widely used. This paper mainly studies the driving design of semiconductor LD.

The basic structure of the LD is that a pair of parallel planes perpendicular to the PN junction face form a Fabry-Perot cavity, which may be a cleavage plane of the semiconductor crystal or a polished plane. The other two sides are relatively rough to eliminate the laser action in other directions in the main direction. When the PN junction of the semiconductor is applied with a forward voltage, the PN junction barrier is weakened, forcing electrons to be injected into the P region from the N region via the PN junction, and holes are injected from the P region through the PN junction into the N region, and these are implanted near the PN junction. Balanced electrons and holes will recombine to emit photons of wavelength λ, the formula

λ=hc/Eg, (1)

Where h is the Planck constant; c is the speed of light; Eg is the forbidden band width of the semiconductor.

If the injection current is large enough, a carrier distribution opposite to the thermal equilibrium state is formed, that is, the population number is reversed. When the carriers in the active layer are in a large number of reversals, a small amount of spontaneously generated photons generate inductive radiation due to reciprocal reflection at both ends of the resonant cavity, resulting in selective feedback of the frequency selective resonance, or gain for a certain frequency. When the gain is greater than the absorption loss, a laser with good directivity, strong coherence, high brightness, and narrow band can be emitted from the PN junction. In addition to the general laser, the LD has good coherence, strong directivity, small divergence angle and high energy concentration. It also has high photoelectric conversion efficiency, large output power, small volume, light weight, simple structure and strong shock resistance.

Figure 1 is a plot of output power versus forward drive current for a typical semiconductor laser at various temperatures. For the sake of clarity, the approximate straight line at the bottom of the figure is intentionally raised. It can be seen from Fig. 1 that when the driving current is lower than the threshold, the laser can only emit fluorescence. Only when the driving current is greater than the threshold current of the laser, the laser can work normally to emit laser light. Therefore, to enable the LD to emit laser light, The operating current of the LD slightly larger than the threshold current is supplied. Moreover, the threshold current of the LD is affected by the temperature, and the higher the temperature, the larger the corresponding threshold current. At a certain temperature, when the driving current is lower than the threshold current, the output optical power is approximately zero; when the driving current is higher than the threshold, the output laser power, the optical output power increases rapidly with the increase of the driving current, and is approximately Linear rise.

In this paper, a FP coaxial laser with a wavelength of 1310 nm is used. The operating current is 25.0 mA and the output power is 0.96 mW. The internal optical path structure is shown in Figure 2. The LD is combined with the back detection detector PD and packaged together, the LD is the forward connection, and the PD is the reverse connection. The PD is used to detect the back-output optical power of the laser, and its output optical power depends on the output value of the LD.

In order to facilitate automatic control of optical power, usually, the laser internally integrates the LD and the back-to-light detector PD, as shown in FIG. Among them, the LD has two light output surfaces, the light output from the main light output surface is used by the user, and the light output from the secondary light output surface (ie, the back light) is received by the photodiode PD, and the generated photocurrent is used to monitor the operation of the LD. status. The monitoring current facing away from the photodetector is linear with the output power of the main output surface. According to the coupling characteristics of the LD to the photodetector, an appropriate peripheral circuit can be designed to complete the automatic optical power control of the LD.

It can be seen from Figure 3 that the LD and the monitoring diode are integrated components. The current flowing into the LD passes through the pre-bias current of the APC circuit. The APC circuit suppresses variations in optical power due to temperature variations, device aging, and the like through a current negative feedback circuit. The APC circuit part adopts the back-to-optical feedback automatic bias control method, that is, the PD photodiode in the semiconductor laser component monitors the optical power of the LD back output. Because the back-output optical power can track the change of the forward output optical power, the closed-loop control system can adjust the current of the laser to achieve the purpose of outputting stable optical power.

The APC circuit shown in FIG. 4 is composed of an operational amplifier 1, 2 and a transistor Q1 and a peripheral circuit. The circuit is a negative feedback system with a triode as a core, and has a function of automatically stabilizing the laser light output power. The feedback is taken from the back light of the LD, which is detected by the back light monitoring diode and converted into a corresponding current. After being filtered by the capacitor C1, it enters the inverting input terminal of the operational amplifier, and converts the current signal into a voltage signal V1. The op-amp input of the op amp consists of a +2.5 V stable reference source consisting of the LM336 and an op amp and a positioner R5. The output of the reference voltage is V2 and can be adjusted by the positioner.

At the moment when a voltage is applied to the driving circuit, a large inrush current is generated, and a large change in the instantaneous current affects the service life of the semiconductor laser. In addition, under normal circumstances, the power supply voltage is supplied to the drive circuit voltage by alternating voltage rectification of 220 V, and the external intervening interference signal will also generate a large instantaneous current, so that long-term operation will also affect the service life of the semiconductor laser. .

Based on this situation, a slow start circuit is introduced in the design, that is, a diode and a capacitor are connected in parallel at the input end of the reference source, wherein the capacitance is about 10 to 470 μF, and the optimum value is 22 to 47 μF. In this way, the driving circuit is not interfered by the power supply voltage, and has a slow start effect, so that the laser is not subjected to an overcurrent shock generated each time the power is turned on, thereby prolonging the service life of the laser.

The APC circuit control process is as follows: When the output optical power of the LD is lowered for some reason, the current coupled to the photodiode is also reduced in proportion, that is, V1 is decreased, so that the balance in the normal state is broken, so that The voltage at the output of the discharge, that is, V3, will increase, so that the base current of the transistor Q1 increases, the collector current also increases, and 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, so that the output optical power remains unchanged; vice versa.

According to the performance parameters of the laser of the sensor, selecting a suitable resistor and capacitor for matching and adjusting the potentiometer can obtain different optical power output values. Fig. 5 is a graph of the experiment conducted at room temperature (25 ° C). It can be seen from the figure that the threshold current of the fiber-optic sensor LD source is about 8 mA, and the stable operation is between 10 and 30 mA. The output power and the drive current have a good linear relationship after being greater than the threshold current. It can output adjustable optical power values ​​of -0.1, -1, -2, -5, -10 dB, etc. during normal operation. The parameter configuration in the circuit is such that the current flowing into the LD does not exceed its limit.

The experiment proves that the design circuit is correct and feasible, and the automatic optical power feedback based on the back monitor ensures that the fiber sensor can work normally under constant power.

The driving circuit designed in this paper solves the problem that the output power of the laser is unstable during use by slow start and power automatic control circuit, and its stability is better than 0.5%, which achieves better steady current effect. The fiber optic sensor used in this paper is applied to the low temperature environment of liquid nitrogen. The experiment is carried out at room temperature, and the coupler and its driving circuit are taken out at room temperature (25 ° C) through the fiber, and the temperature change is not very large. Therefore, no temperature compensation control circuit is introduced. The next experiment will make the fiber optic sensor work in the low temperature environment of liquid nitrogen, and the temperature fluctuates greatly. It is necessary to consider adding an automatic temperature compensation circuit to achieve constant temperature control.