In the field of optical communication, larger bandwidth, longer transmission distance and higher receiving sensitivity are always the pursuit goals of researchers. Although the application of wavelength division multiplexing (WDM) technology and erbium-doped fiber amplifier (EDFA) has greatly improved the bandwidth and transmission distance of optical communication system, with the application of communication technology such as video conference and the popularity of Internet, the information explosion growth has put forward higher transmission performance requirements for the physical layer as the basis of the whole communication system. The optical communication system adopts intensity modulation / direct detection (IM / DD), that is, the transmitter modulates the optical carrier intensity, and the receiver detects the optical carrier envelope. Although this structure has the advantages of simple and easy integration, the single channel bandwidth is very limited due to the only use of ASK modulation format. Therefore, this traditional optical communication technology is bound to be replaced by more advanced technology. However, in today’s communication bubble burst, the application of new optical communication technology will inevitably bring about the demand for new communication equipment. In the face of the high price of optical devices, the high cost of large-scale communication equipment replacement is unacceptable to operators. Therefore, for equipment manufacturers, the development of new optical fiber communication technology is also facing great risks. How to improve the performance of optical communication system based on the existing equipment has become a practical problem. In this context, coherent optical communication technology, which was highly expected more than 20 years ago, has been put on the desktop again.
The theory and experiment of coherent optical communication began in 1980s. As coherent optical communication system is recognized as having the advantage of high sensitivity, many countries have done a lot of research on coherent optical transmission technology. After ten years of research, coherent optical communication has entered the practical stage. A series of coherent optical communication experiments have been carried out in Britain, America and Japan. In 1989 and 1990, at & T and bell carried out 1.7 Gbit / s FSK field repeaterless coherent transmission experiments at 1.3 μ m and 1.55 μ m wavelengths between the Rowling Crick ground station and the senberry hub station in Pennsylvania. The distance between them is 35 km and the receiving sensitivity is – 41.5 DBM. In 1990, NTT carried out a 2.5gbit/s CPFSK coherent transmission experiment between daifen Yinyu and Wu stations in Seto Inland Sea, with a total length of 431km. Until the end of 1880s, the development of EDFA and WDM technology made the development of coherent optical communication technology slow down. In this period, the sensitivity and the information capacity of each channel are no longer concerned. However, after 20 years of development and wide application of direct detection WDM system, new symptoms begin to appear, which indicates that the application of coherent optical transmission technology will be paid more attention. In digital communication, expanding the capacity of C-band amplifier, overcoming the deterioration of fiber dispersion effect, and increasing the capacity and range of free space transmission have become important considerations. In the aspect of analog communication, sensitivity and dynamic range become the key parameters of the system, and they can be greatly improved by the related optical communication technology.
This design will take the single chip microcomputer as the main control system, design the appropriate coherent optical communication system, can carry on the information code transmission in the system, in order to complete this design.
As shown in the figure below: this system mainly completes the signal modulation control and system synchronization control of coherent optical front-end. The transmitter includes light control system and electro-optic control system, including amplitude modulation and phase modulation, which will be the main control work of PIC microcontroller.
2.1 design of launch control module
2.1.1 acoustooptic control module
The laser (sdl5412) emits continuous light. In the process of signal transmission, it is necessary to provide a synchronous clock to synchronize the transmitter and receiver. In the design of this system, the continuous laser generated by the light source is acoustooptically modulated, and the pulse optical signal is generated as the synchronous signal of the receiving end.
1 acoustooptic modulator:
The acoustooptic modulator (mt80-b30a1-ir) used in this system integrates acousto-optic medium, electroacoustic transducer, sound absorption (or reflection) device, etc. The acoustooptic crystal used in the modulator is TeO2.
TeO2 crystal is a kind of acousto-optic material with high quality factor. It has good birefringence and optical rotation properties. It has the advantages of fast response, low driving power, high diffraction efficiency and stable and reliable performance. It is an ideal single crystal material for making acousto-optic deflectors, modulators, resonators, tunable filters and other acousto-optic devices.
2 modulation signal driver:
The acousto-optic modulation signal in the system is generated by direct digital synthesizer (DDS). The digital control of output frequency and amplitude can be easily realized by using DDS signal source. The control port of DDS signal source has 31 bit frequency control and 8-bit amplitude control.
3. Control module design:
The control module controls the acousto-optic modulation signal driver to generate the frequency of 80MHz and the amplitude of pulse wave to drive the acousto-optic modulation crystal for acousto-optic modulation.
The control module is mainly realized by PIC microcontroller and peripheral control circuit. Due to the large number of pins needed for control (31 bit frequency control, 1-bit frequency locking, 8-bit amplitude control, 1-bit external trigger bit, 41 bits in total), the main control MCU adopts pic series to realize, using 2-bit to set fixed frequency, 8-bit to set amplitude and 1-bit to trigger. The hardware structure of acoustooptic modulation is shown in Figure 2
In the design of the circuit, the transformer should be used to reduce the voltage to the appropriate voltage. After rectification and filtering, the output power swing value of the integrated voltage regulator chip is relatively small. The suitable integrated chip mainly has 5V better than 12V output. In this way, two mc7812 or lm7812 are selected to provide 24 V voltage, and one mc7805 or lm7805 provides 5V voltage. The circuit is kept at 500mA.
The circuit of the power module is shown in Figure 4
PIC control is to control the acousto-optic modulator to produce a certain amplitude and frequency of pulse optical signal.
Acoustooptic modulation has two functions in this system: converting continuous laser into 120ns pulse light; first, transmitting end is used as local carrier. Second, the receiver provides clock as local oscillation signal.
The mt80-b30ai-ir acoustooptic modulator is used here. Since the device provides linear modulation, we should theoretically operate according to the parameters provided by the device
Controlled by serial port:
Because the control interface uses a 44 port parallel port, which needs to find a number of multiple IO ports for input, the lab uses PIC32 with 53 IO ports as the programmer. There are two aspects of control
Frequency of pulsed light:
The output frequency is set as: 80MHz, which is replaced by the above formula:
Change to binary: 0010100001111010111000010101000111
Using PIC single chip microcomputer output, the output first latch, stable output, only set two ports, one output 0, one output 1, to ensure the frequency unchanged.
It mainly controls the parallel port
The amplitude is controlled by 8 digits, and the control number is directly proportional to the amplitude, that is, from 255 to 0, the maximum value of the control amplitude is to the minimum value. set up
8-bit code control. In fact, the laboratory uses 10000001 code, which can be written directly by parallel port.
Analysis of the main situation of the control: first of all, control the power supply part, write the high level through the pin of the single chip microcomputer, and select the power supply with the single switch switch of the relay. The writing of data stream is given to the IO port of MCU. Pulse light control: control clock setting: tc0 is used as the timer to select the control pulse width, one timer is 4us, and the pulse width is set by two interrupts: compare match, overflow match. Compare match interrupt: when the matching value is reached, the matching interrupt will be generated, and the output optical signal will overflow matching: timer technology, when the count value is reached, the interrupt overflow will be generated and the optical output will be stopped.
Software design: write the corresponding code shape directly with MCU. When there is a data stream, the host sends a control clock signal, and each signal pulse triggers an external interrupt. The external interrupt clears the timer again and starts counting control pulse width again.
2.1.2 electro optic control module
The sender needs to modulate the information to be transmitted to the optical carrier. In this system, the information is modulated by electro-optic modulation. It includes amplitude modulation and phase modulation. Electro optic modulation is to add useful information to the optical pulse signal. The circuit includes: information generation circuit, amplitude control circuit and phase control circuit. The following step-by-step analysis: information generation circuit: random Gaussian number signal amplitude control circuit generated by FPGA: digital signal conversion to analog signal control amplitude modulator controlled by MCU. Phase control circuit: digital signal is converted into analog signal to control phase modulator by MCU.
Figure 10. Electro optic modulation hardware structure diagram
It mainly generates 4-bit random code through LabVIEW, changes it into 8-bit Gaussian random code through pic, and then converts the signal into analog signal through DA converter. The analog signal is first modulated by amplitude and then by phase modulation through two acousto-optic modulators.
Power supply part: analysis power supply part: PIC MCU uses 5V power supply, a mc7812 or lm7812 provides 12V voltage, a mc7805 or lm7805 provides 5V voltage, and the 5V voltage will be 3.3V through LM117. The current is held at 500 mA. The circuit design diagram is as follows:
Random number generation:
The upper computer uses LabVIEW program to generate random Gaussian numbers, and outputs 4-bit random numbers through data acquisition card to simulate useful signals. LabVIEW is a virtual instrument software designed by Ni company. Virtual instrument is a computer-based instrument. The close combination of computer and instrument is an important direction of instrument development. Roughly speaking, there are two ways of this combination. One is to load computers into instruments. A typical example is the so-called intelligent instruments. With the increasingly powerful computer function and its volume shrinking, the function of this kind of instrument is becoming more and more powerful. At present, the instrument with embedded system has appeared. Another way is to load the instrument into a computer. Based on the general computer hardware and operating system, various instrument functions are realized. LabVIEW (Laboratory Virtual Instrument Engineering) is a kind of graphics
The upper computer uses LabVIEW program to generate random Gaussian numbers, and outputs 4-bit random numbers through data acquisition card to simulate useful signals. LabVIEW is a virtual instrument software designed by Ni company. Virtual instrument is a computer-based instrument. The close combination of computer and instrument is an important direction of instrument development. Roughly speaking, there are two ways of this combination. One is to load computers into instruments. A typical example is the so-called intelligent instruments. With the increasingly powerful computer function and its volume shrinking, the function of this kind of instrument is becoming more and more powerful. At present, the instrument with embedded system has appeared. Another way is to load the instrument into a computer. Based on the general computer hardware and operating system, various instrument functions are realized. LabVIEW (Laboratory Virtual Instrument Engineering) is a graphical programming language. It is widely accepted by industry, academia and research laboratories as a standard data acquisition and instrument control software. LabVIEW integrates all functions of communication with hardware and data acquisition card which meet the requirements of GPIB, VXI, RS-232 and RS-485 protocols. It also has built-in library functions that are easy to use TCP / IP, ActiveX and other software standards. This is a powerful and flexible software. It is easy to set up your own virtual instrument by using it. Its graphical interface makes programming and using process vivid and interesting. Graphical programming language, also known as “g” language. When programming in this language, basically no program code is written. Instead, flow chart or flow chart is used. It makes full use of terms, icons and concepts familiar to technicians, scientists and engineers. Therefore, LabVIEW is an end-user oriented tool. It can enhance your ability to build your own scientific and engineering systems, providing a convenient way to implement instrument programming and data acquisition systems. It can greatly improve the working efficiency when it is used to research, design, test and implement the instrument system.
The following is the design of random number parameter software interface:
Amplitude control circuit:
Both amplitude control and phase control are used to control the input voltage and change the amplitude and phase of optical carrier by changing the input voltage.
According to the specification, the amplitude is controlled by this formula:
The input range of electro-optic phase modulator driver is 0V to 4V, while that of electro-optic amplitude modulator is 0.3V to 1V.
Phase control circuit:
Both phase control and amplitude control are used to control the input voltage. The amplitude and phase of optical carrier are changed by changing the input voltage.
According to the manual, the phase is controlled by this formula:
Overall circuit design:
The four bit binary random code is input to the single-chip microcomputer through LabVIEW, and the 8-bit binary code line is generated by the single-chip microcomputer (through the look-up table method) to control the digital to analog converter. The corresponding voltage driven analog quantity is generated.
SCM selection: PIC32 single chip microcomputer.
D / a converter selection: mcp4725. Advantages: more commonly used, low power consumption, mature circuit design, low price.
Selection of operation mode: current conversion mode and voltage conversion mode. Due to the requirement of output voltage value, the influence of noise and drift on operational amplifier can be reduced. The following is a typical circuit mode found in PDF:
Mcp4725 is a single channel 12 bit buffer voltage output DAC with non-volatile memory (EEPROM). The user can store configuration register bits (2 bits) and DAC input data (12 bits) in non-volatile EEPROM (14 bit) memory. The DAC can be configured to normal mode or power saving off mode by setting the configuration register bit. The device can use a 2-wire I2C compatible serial interface and is powered by a single power supply in the voltage range of 2.7V to 5.5V. The output voltage formula is as follows. Refer to mcp4725 manual for more details. Let’s make the device output 1.6V. The voltage conversion formula is as follows:
Output voltage range: 0 to。
Based on this Da, the DA conversion circuit is designed. The selected signal of Da writing chip generates PWM wave to control the writing frequency, that is to control the time interval of output signal.
Random code conversion:
Convert 4-bit binary random code into 8-bit binary code, set digital comparison table and look it up.
Control write: PWM is generated through the port to control the writing interval.
Control program design module:
Figure 13: control program control module
In this way, we can modulate the amplitude and phase of optical signals individually or jointly. In this way, I realized the work of modulating the amplitude and phase separately.
2.2 receiver circuit design
The optical signal detection module is mainly used in two aspects: one is used for optical signal data acquisition; the other is used to realize the synchronization between the transmitter and receiver
Detection circuit should be divided into several parts: First: photoelectric conversion and preamplifier. The second part: differential amplifier circuit.
The third part: active filter circuit. The following are three parts.
2.2.1 optical detection current module
The main function of this module is to convert the received optical signal into electrical signal through photoelectric diode. A weak current signal is generated by receiving the optical signal through the photoelectric diode bpx65. The measurement and control generating circuit signal can be set as follows:
The current to voltage is changed into voltage signal through sa5212.
The output gain is as follows:
The cross rent gain of sa5212 is as follows:
We can get it. It is verified by experiments.
2.2.1 optical detection current module
Differential amplification is the most commonly used linear amplification method. The signal is further amplified here. AD8021 is selected as the operational amplifier and other ad amplifiers can be selected.
The AD8021 has a bandwidth of 190mhz when the closed-loop gain is 10. Convenient linear control.
The output is calculated according to the amplifier theory
Different values of the linear resistor of the amplifier are guaranteed
R3 = R4 = 11o Ω
R5 = R6 = 2000 Ω
There is no compensating capacitor.
2.2.3 active filter module
The function of active filter is to filter and amplify the previous amplified signal. The main purpose is to filter out the low-frequency components to facilitate the back-end acquisition and avoid spectrum aliasing, and the low-frequency components include a lot of noise.
The second-order filter is mainly designed. Here, AD8021 can also be used as the main device. Figure 13 shows the common circuit diagram of AD8021
Figure 14: AD8021 application circuit
The gain of low pass filter is as follows:
Normalized transfer function:
Set the cut-off frequency to 1.8mhz.
The corresponding resistance and capacitance can be obtained.
Then the output voltage value is:
The overall circuit is as follows:
Figure 14: photoelectric induction circuit
This design is mainly for coherent light control and inspection system. The PIC control light emitter is designed. Pic controls the acousto-optic modulator to form a suitable light source, and then the amplitude or phase coding is handed over to pic to control the electro-optic modulator to complete. We simulate the information code by random number, and encode the appropriate light source by PIC. In the receiving end, due to the tight time, only the photoelectric induction circuit is designed, and the information processing and control can be completed by PIC, but it is limited to only one development board. In the communication system, the basic principle that the control of the receiving end and the transmitting end should be separated does not realize the receiving control. This is also the direction we will continue to study in the future.