Design and Implementation of a Novel MEMS Optical Switch Measurement Platform
1 IntroductionWith the rapid development of optical communication technology, MEMS optical switch will become the mainstream of core optical switching devices because of its advantages such as small insertion loss and crosstalk, high extinction ratio, good stability, good transparency and scalability, easy integration and insensitive polarization. At present, its electrostatic drive is widely studied. The common electrostatic drive modes include comb electrode, SDA (scratch drive actuator), cantilever drive and torsion arm drive. The driving voltage of cantilever and torsion arm is higher than the first two, but it is simple in structure and easy to realize in technology. When selecting the best working state of the micro mirror, the square wave signal with continuously adjustable voltage, frequency and duty cycle is required as the excitation source. The traditional selection method needs the coordination of oscilloscope, pulse signal generator and DC voltage stabilizing source, and there are many disadvantages, such as high cost, cumbersome operation, long test cycle and low measurement accuracy. In the measurement platform introduced in this paper, the device is selected by using single chip microcomputer to control the pulse frequency with high amplitude. Compared with the current similar methods, it has the advantages of high precision, strong reliability, low cost and easy operation.2 working principle of optical switchFigure 1 shows 2 & times; 2 the structural diagram of micromachined optical switch can be fabricated on (100) silicon wafer by bulk silicon micromachining. The micro mirror is connected with the upper electrode. When there is no voltage input, the position of the upper electrode does not move. The micro mirror is on the light path. The light emitted from the incident optical fiber is reflected by the micro mirror and enters the outgoing optical fiber on the same side of the mirror after changing the direction. This is the reflection state of the switch. When there is voltage input between the upper electrode and the lower electrode, under the action of electrostatic force, the upper electrode drives the micro mirror to move away from the optical path, and the incident light propagates along a straight line into the outgoing optical fiber in front, which is the straight-through state of opening light. Figure 2 shows the curve of threshold voltage with the thickness of torsion beam, and figure 3 shows the relationship between driving voltage and suspension beam displacement.3 scheme design and analysisIts working principle is that the pulse signal with adjustable frequency duty cycle is first generated by the single chip microcomputer, but because the voltage amplitude of the final signal is 40 70V, which is much greater than the working voltage (5V) of the single chip microcomputer, the signal generated by the single chip microcomputer is first isolated by the optocoupler, and then output to the later stage amplification circuit, and then output the signal with adjustable peak value according to the adjustment of the amplification circuit. It can be seen that the system design mainly involves three key parts: generating square wave signal with adjustable frequency and duty cycle, signal amplification and final signal amplitude measurement.3.1 design of square wave signal sourceThe square wave signal generation circuit is mainly composed of AT89C2051 single chip microcomputer, crystal oscillator circuit, keyboard parameter input and LED status display. Single chip microcomputer AT89C2051 is controlled by software to generate square wave signal. The software is designed to control the frequency and duty cycle of square wave by using the cooperation of counter / timer T0 and T1 in AT89C2051. AT89C2051 uses 12m crystal oscillator, so the time for MCU to execute an instruction is 1 Î¼ s. In this way, according to the frequency value f and duty cycle D input by the keyboard, the cycle time constant TT (t t = 1 / F) and the time length TD (td = TT & times; d%) of the positive pulse are obtained and sent to the counter / timer T0 and T1 respectively. T0 and T1 start counting at the same time and make the output high level. Because TD â¤ T, T1 first counts the overflow, executes the interrupt service subroutine of T1, pulls down the output level to make the output low level, and stops T1 counting at the same time. After waiting for a period of time, when t0 is full, call the interrupt service subroutine to execute T0, t0 and T1 reload the timing constant and start timing, and pull the output level back to the high level at the same time. This completes one cycle of output. The change of frequency and duty cycle only needs to change the timing constants of T0 and T1. The output waveform is shown in Figure 4.3.2 signal amplificationThe square wave signal output by the single chip microcomputer is only about 5V at high level, and the output should reach 40 70V. The ordinary triode amplification circuit can not bear such a large voltage. Therefore, MOS transistor is used in the design, and MOS transistor has good switching performance, which will make the output waveform very good. The circuit is shown in Figure 5.The regulated power supply built by LM317 (see Figure 6) provides 40 70V DC voltage vcout. This DC voltage output value can be adjusted by the sliding rheostat WR in the figure. Vcout is added to the drain stage of MOS transistor through pull-up resistance. In this way, when MOS transistor works in the switching state, the output voltage from the drain stage changes between 0V and vcout, resulting in a square wave signal with amplitude of V cout. It can be seen that as long as the square wave signal output by the single chip microcomputer can control the switching state of MOS transistor, the square wave signal with high amplitude can have the same frequency and duty cycle as the square wave output by the software design of the single chip microcomputer. However, the signal output by the single chip microcomputer is about 5V. If it is directly connected with the MOS tube with a voltage of up to 40 70V, the high voltage output by the rear stage of the MOS tube is easy to be introduced into the single chip microcomputer and burn the single chip microcomputer. Therefore, measures must be taken to avoid this danger. In this design, the optocoupler is used to safely isolate the high voltage output from the 5V low voltage when the single chip microcomputer works.3.3 measurement of final signal amplitudeThe final output signal is a square wave signal. It is difficult to directly measure the peak and peak of the output. However, after analyzing the circuit, it is not difficult to find that when the MOS transistor is cut off, the output Vout of the MOS transistor is almost equal to the vcout provided by the voltage stabilizing source. It can be seen that the peak and peak of MOS tube output square wave signal can be obtained by measuring the V cout DC voltage output by the voltage stabilizing source.When the measured voltage value is not very accurate, the vcout provided by the voltage stabilizing source can be divided by the resistance in proportion, and the divided voltage can be processed by the A / D converter and single chip microcomputer. The obtained result can be multiplied by the proportional coefficient to obtain the actual voltage value of vcout. If this value is directly sent to the display part, the peak value of square wave will be displayed intuitively. In the instrument we actually designed, the error between the displayed Vout measured value and the output value under no-load is no more than Â± 0.5V. It is proved that this scheme is convenient and feasible when the requirement of measuring voltage value is not very accurate.3.4 main control platform and display partIn general, the signal generation part and the final signal amplitude measurement and display part are independent, which are completed by two AT89C2051 to form a simple distributed structure. This structure improves the stability of the whole software part.The main control platform is completed by the first AT89C2051, which directly controls the generation of square wave signal, and also includes the setting and display of frequency and duty cycle of square wave signal, providing a friendly man-machine interface.For the output signal, the user should be given as intuitive results as possible. While the main control platform provides frequency and duty cycle display, the user should also know the output signal amplitude intuitively and accurately. In this way, the operation of the instrument is very convenient and accurate when the user does not have a third-party measuring instrument. This part of the work is completed by the second AT89C2051 and a / D converter AD0804.The source program of the core part of the single chip microcomputer is as follows:â¦ â¦//generate pulse signal//period= the period of pulse signal//posiTIve=the length of posiTIve voltagetimer0ï¼ï¼ interrupt 1 using 1 unsigned int temp;out=1; //output VOHtemp=65535-period;TL0=temp; //set value for timer0TH0=tempãã8;temp=65535-positive;TL1=temp; //set value for timer1TH1=tempãã8;TR1=1;TR0=1; //start timer1ï¼ 0timer1ï¼ï¼ interrupt 3 using 2 Out = 0; / / Timer1 is full, output VolTR1=0; //stop timer1â¦4 ConclusionMeasuring the driving voltage of the optical switch driven by the cantilever and torsion arm is a cumbersome procedure in the process of making the micromechanical optical switch. The measuring platform uses the method of single chip microcomputer to control the pulse frequency to select the device. Compared with traditional methods, it has the advantages of high precision, strong reliability, low cost and easy operation. The measurement platform has been used by the State Key Joint Laboratory of Integrated Optoelectronics of Jilin University for nearly half a year. The system is stable and the effect is good. It is a time-saving and efficient measurement method.