Xiong Weiguo1 Zhu Yuancheng 1，2
(1. Shenzhen Coolingstyle Technology Co., Ltd. Shenzhen 518000
2. Shenzhen InternationalGraduate School of Tsinghua University Shenzhen 518000)
Abstract: Temperature plays a crucial role in the output characteristics of an all-solid-state UV laser. In order to make the output of the all-solid-state UV laser stable, it is necessary to control its temperature precisely. This article mainly introduces a temperature control method based on vapor compression type inverter refrigeration system and electrothermal compensation to realize the high precision temperature control of the UV laser. The method uses PID as the basic control algorithm to control the speed and electric power of the micro DC inverter compressor, thus achieving a thermostatic system with small size, light weight, high efficiency and good temperature control. The experimental results show that the method has a fast response time, only 6 minutes to make the system stable; high control accuracy, the highest water temperature accuracy can reach 0.01 ℃. With the large number of applications of UV laser in marking, precision cutting and other industries, the method has both many advantages and high practical value and promotion significance.
Keywords: Ultraviolet laser; Micro-compressor; Frequency control; Electro-thermal compensation; PID control
Author: Xiong Weiguo (1986-08- ), Male, Master, Chief Technical Engineer of Shenzhen Coolingstyle Technology Co., Ltd, Main research aspects are micro compressor and micro and small refrigeration system design, high precision laser chiller design. Email: firstname.lastname@example.org
Yuancheng Zhu (08/1987- ), male, general manager and R&D director of Shenzhen Coolingstyle Technology Co., Ltd, doctoral student of engineering at Shenzhen International Graduate School of Tsinghua University, whose main research aspects are the research of high precision control of micro compressor and refrigeration system, and the research of human micro environment cooling system using micro compressor under high temperature working condition.
1. The development of the All-solid-state UV lasers
Since 1961, when Meyman  invented the first ruby laser, after more than half a century of research and exploration, laser technology has achieved rapid development and is widely used in various fields such as industry, agriculture, measurement, communication, medicine, military and scientific research. According to the different output wavelengths, lasers can be divided into infrared lasers, visible lasers, ultraviolet lasers, etc. 
Among them, UV lasers are lasers with an output wavelength no greater than 400 nm, which have a short wavelength, concentrated energy and high resolution, and can be divided into the following categories according to the different pumping methods: gas lasers, excimer lasers, semiconductor lasers, lamp-pumped solid-state lasers and LD-pumped solid-state lasers, etc. Among them, LD-pumped solid-state lasers are also called all-solid-state lasers. The main UV lasers used in the twentieth century are gas lasers and excimer lasers, both of which have problems of large size, low efficiency, limited reliability, short lifetime, and high cost .
All-solid-state UV laser uses laser diode (LD) as pump, and uses laser crystal to generate infrared light of about 1μm, and then obtains UV laser by multiplication or sum frequency effect of nonlinear optical crystal. The application of foreign all-solid-state UV lasers began in the 1990s. 1995, Oka M  of Sony Corporation in Japan obtained a 1.5 W continuous Nd: YAG UV laser of 266 nm by KTP frequency doubling and BBO quadrupling. Subsequently, a lot of research has been carried out in various countries, and all-solid-state UV lasers with power ranging from 12W  to 160W  have been manufactured. In 1999, Chen Guofu  and others from Xi’an Institute of Optics obtained a 266 nm UV laser output using BBO crystal, which was the first all-solid-state UV laser reported in China. Since then, the UV laser technology in China has also entered a period of rapid development.
With the advantages of small size, compact structure, high efficiency, long life, good beam quality and low cost, all-solid-state UV lasers are widely used in environmental monitoring, medicine, communications and microfabrication. In the field of environmental monitoring, the UV laser can be used to monitor the bottom water vapor content and O3 concentration in the troposphere, and to determine the distribution of aerosols in the air; In the field of medicine, the high energy characteristics of the UV laser can be used to directly break the molecular bonds between tissue cells, thus avoiding thermal damage to tissue; In the field of communication, the UV laser communication has the advantages of low eavesdropping rate, high interference and non-visible range. In the field of communications, UV laser communication has the advantages of low eavesdropping rate, high interference and non-line-of-sight ; In the field of processing, because of the cold processing characteristics of UV laser in the process of direct destruction of chemical bonds, so it can achieve the processing of precise and complex structures . In recent years, with the rise of deep-UV and vacuum UV technology, the application of all-solid-state UV lasers has become more and more widespread .
2. Status of temperature control of all-solid-state UV lasers
The overall efficiency of the all-solid-state UV laser is low, and a lot of heat is generated during LD pumping and frequency doubling and summing. If the generated heat is not released in time, it will increase the laser temperature. Temperature has a significant impact on the performance of solid-state UV lasers, mainly affecting LD pumping and nonlinear crystals. Temperature changes can cause LD output power instability, and when the temperature increases, LD output power increases . Temperature instability can even trigger the LD mode jump phenomenon. At the same time, the temperature change also causes the refractive index, shape and volume of the laser crystal to change, which causes the LD output wavelength to change, and its wavelength drift with temperature is 0.3~0.4nm/°C. The wavelength of the UV laser is already short, and a small amount of drift can cause a significant change in the output performance. The nonlinear optical crystal also absorbs fundamental energy during the harmonic process, which can cause a local temperature rise in the direction of the crystal flux. The temperature rise causes a change in the refractive index of the nonlinear optical crystal, and the output beam quality and multiplicative efficiency are reduced.
All-solid-state UV lasers produce a lot of heat and their performance is temperature sensitive, therefore, if the heat generated by the laser is dissipated in a timely manner and its temperature stability is maintained, it becomes a problem that must be solved in the laser industry. Traditional fan cooling is inefficient and poorly controllable, and is not suitable as a cooling method for all-solid-state UV lasers. Currently, the main methods used are TEC cooling and water cooling. TEC cooling method using PID control can already achieve temperature control accuracy ± 0.01 ℃, but TEC is generally very low efficiency and poor stability, it is difficult to be used in large-scale applications. Traditional water-cooled heat dissipation is generally made with the help of a chiller made of vapor compression refrigeration system, temperature accuracy control through the method of hot gas bypass to achieve. In the process of hot gas bypass valve switching, the compressor system cooling or heating will have a certain amount of overshoot, so the chiller is difficult to achieve a high degree of accuracy. To achieve high precision, it is necessary to use a very large water tank, using the heat capacity of water to absorb heat or cold overshoot, and this chiller is large and costly. In this article, the temperature control method of an all-solid-state UV laser is investigated using a water cooling system with a variable frequency vapor compression refrigeration system and electrothermal coupling.
3. Temperature control principle
The schematic block diagram of the temperature control system of the all-solid-state UV laser is shown in Figure 1. The laser is placed on the heat sink and the heat it generates is conducted to the heat sink through contact. There are water channels inside the heat sink, which form a water cycle with the pump, water tank and heat exchanger. There are 2 sets of channels in the heat exchanger, one for the water and one for the refrigerant. The refrigerant channels form a refrigeration system with the compressor, condenser and throttle valve. The water transmits the heat absorbed by the laser from the heat sink to the heat exchanger, and the refrigerant and water exchange heat between the walls in the heat exchanger, and finally transmit the heat to the condenser, which releases the heat to the environment under the action of the fan. In this way, the heat dissipation of the laser is achieved. The compressor in the picture is a DC inverter compressor, which needs the help of a driver to convert the DC power into three-phase AC power before it can work.
For a stable system, it is only necessary to keep the water temperature stable to ensure that the laser temperature is stable. The cooling of the water is achieved by the above mentioned cooling system and the heating of the water can be achieved by the electric heaters arranged inside the water tank. The temperature sensor feeds the sensed water temperature signal to the microcontroller (MCU) after A/D conversion, and the microcontroller controls the compressor and the electric heater through an output circuit based on the relationship between the current actual water temperature and the desired target temperature to achieve a stable water temperature. In the control module, there is also a display screen and a touch-sensing circuit for human-computer dialogue. The user can observe the water temperature and the operation of the temperature control system in real time through the control module, and can also set the target temperature as required.
4. Hardware selection and function implementation
4.1 Refrigeration systems
Vapor compression refrigeration is the most efficient refrigeration method nowadays. Conventional AC compressors can only control the cooling capacity or heating capacity of the system by means of start-stop or hot gas bypass, which is poorly controllable and not highly accurate. This paper uses DC inverter compressor, small size, light weight, high efficiency, and most importantly, can achieve infinitely variable speed in a wide range, the higher the speed, the greater the cooling capacity, so the system’s cooling capacity is controllable. The selected compressor model is CS-MCQ-19241100 (Figure 2), which weighs about 850g and has a diameter of 56mm, and the relationship between its cooling capacity and speed is shown in Figure 3.
As can be seen from Figure 3, the compressor cooling capacity increases with the increase in speed. The control module calculates the current required speed of the compressor through an internal program, converts the digital speed signal into an analog signal through a D/A converter circuit, and then transmits the analog signal to the compressor driver. The driver adjusts the AC frequency of its output according to the speed signal, thus realizing the control of the compressor speed. Condenser, heat exchanger, etc. are used commonly high-efficiency type, choose capillary tube as the throttle, so that a complete refrigeration system is completed.
4.2 Heating units
As the compressor is a mechanical device, the control program requires a certain response time for its speed commands to be reflected in the cooling capacity. For the water cooling system with a small tank, in the case of a small heat capacity, the adjustment of the compressor speed can control the water temperature within a small range, but there may still be fluctuations, the need for fine tuning of the water temperature through the heating device. In addition, the work of the laser is not always stable, or even sometimes not working, when the temperature control system is in a no-load standby state, even if the compressor speed is adjusted to the lowest, the water temperature will still keep falling to the target temperature below, shut down the compressor will cause greater fluctuations in water temperature.
The electric heating tube built into the water tank is the perfect solution to these problems. In the hollow stainless steel tube inside the arrangement of spiral resistance wire, the gap filled with high-temperature magnesium oxide. The resistance wire is energized and the heat is evenly transferred to the surface of the tube through the magnesium oxide ceramic. The heating of the water is achieved by submerging the electric heating tube in water. By PWM adjustment of the input voltage of the heater, the heating power can be precisely controlled.
4.3 Acquisition of temperature signals
The optimum operating temperature of an all-solid-state UV laser is generally between 20°C to 30°C. The actual measured water temperature is in the range of 0 to 40°C. This temperature range belongs to the normal temperature range and most of the temperature sensors can meet the requirements. High temperature control accuracy requires high accuracy, large temperature coefficient and good linearity of the temperature sensor in this range. In this article, we choose a three-wire stainless steel package Pt100 temperature sensor, which is made of a very thin platinum wire wound on a mica support. The resistance of Pt100 varies with temperature, with a resistance of 100Ω at 0°C, and has good linearity in the ambient temperature range. A constant current source is added to both ends of Pt100, and a temperature sampling circuit measures the voltage difference between its two ends so that its resistance can be obtained, and then its detected temperature is obtained by linear interpolation of Pt100’s own resistance characteristics. The use of a three-wire system eliminates the effect of resistance on the wire and thus gives a more accurate picture of the actual temperature. By immersing the Pt100 in the water tank, the water temperature can be detected in real time, and the detected voltage signal is transmitted to the MCU for analysis and processing after A/D conversion.
4.4 PID control system
The system controls the cooling and heating capacity by adjusting the PWM parameters of the compressor speed and the electric heater switch to finally stabilize the water temperature. Since the operation of the laser is not stable and the uncertainty of environmental factors has a great impact on the refrigeration system, the structure and parameters of the system must rely on experience and field commissioning to determine, so it is not possible to control the work of the system with an accurate mathematical model. The PID algorithm is simple, robust and reliable, and is one of the most suitable control strategies for this system, which calculates the proportional, integral, differential control amount to regulate the work of the system. The work flow of PID is shown in Figure 4. In each time step, the system first calculates the water temperature error, then the error is calculated by PID, and then the adjustment amount of compressor speed and electric heater power is derived. This is repeated until the temperature error is controlled within the accuracy requirements, at which time the water temperature reaches stability. P, I, D parameters have a great impact on the performance of the system, and engineering generally relies on experience and combined with the test method for its adjustment. In this paper, the critical proportionality method is used to adjust the PID parameters.
5. System Control Flow
The system control flow is shown in Figure 5. When the system is just turned on, if the water temperature is higher than the target temperature by more than 1℃, the refrigeration system is turned on, so that the compressor runs at full speed to lower the water temperature rapidly; if the water temperature is lower than the target temperature by more than 1℃, the electric heater is turned on at full power, so that the water temperature rises rapidly. When the water temperature enters the target temperature ±1℃ range, the PID algorithm is used to regulate the compressor speed and electric heater power in real time, and finally stable the water temperature.
The PID algorithm controls with high accuracy, but it requires a long stabilization time. The control strategy used in this article first controls the water temperature quickly around the target temperature and then fine-tunes it with the PID algorithm, which greatly reduces the stabilization time.
6. Experimental results and analysis
A temperature control test was conducted on a model 10W UV laser. The cooling system, electric heating device and other functional units and control algorithms described in the previous section were used, and the system circulated only 1 L. The target temperature was set to 25°C, and the laser and temperature control system were turned on at the same time. Figure 6 records the whole process of water temperature change from power on to stabilization of the system.
As can be seen from the graph, the system stabilization time is short, only 6 minutes. After stabilization, the water temperature is maintained at 25±0.01°C, which indicates that the accuracy of this temperature control system can reach 0.01°C. The laser operating temperature is also stable after the water temperature is stabilized.
7. Concluding remarks
Temperature plays a crucial role in the output characteristics of an all-solid-state UV laser. In this article, a temperature control method based on the coupling of a miniature DC compressor cooling system and electro-thermal compensation is designed for this laser. By controlling the speed of the micro DC inverter compressor and the power of the auxiliary electrical heating equipment, the water temperature of the laser cooling system is precisely regulated. The temperature control system made with this method using micro inverter compressor refrigeration technology is small in size, light in weight and high in efficiency, and the experimental results show that the system has a fast response, short stabilization time and high temperature control accuracy, which can reach 0.01°C.
Compared with conventional chillers, the system has the outstanding advantages of small size, light weight and high cooling accuracy. At the same time, because the compressor used inside the system is a DC compressor, it is very suitable for the cooling system to match the power supply system of different countries, and its versatility is stronger. Compared with TEC electronic refrigeration, the compressor system has greater cooling capacity and higher energy efficiency ratio, which can greatly reduce the user’s energy consumption and lower the cost of use in long-term continuous use. The machine also comes with its own internal heating control for quick preheating during initial winter use. The cost of the system design has been reduced with the cost of micro compressors and TEC refrigeration or compressors of the same cooling capacity level to compete. With a large number of applications of UV lasers in marking, precision cutting and other industries, this method has many advantages such as small size, light weight, high accuracy, high energy efficiency, low cost, etc., which has high practical value and promotion significance.
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