High-power solid-state laser cooling technology

1 Introduction
Laser diodes pump solid-state lasers (DPLs) for their high efficiency
The advantages of rate, high beam quality, compact structure, and long life have attracted people
Great interest. In recent years, with the phase of high-power diode lasers
Following the successful development, it has promoted the development of DPL and its use in military, industrial and medical fields
Therapeutic and scientific research applications.
The heat consumption of the laser diode (LD) during normal operation accounts for the total power consumption
More than 50%, heat loss causes the instability of the working temperature of the laser diode
It will definitely change its output wavelength, which in turn will affect the high DPL
Efficient and stable output. In addition, the optical pumping of the laser crystal of a solid-state laser
Heat is also generated in the process, which also needs to be cooled. With the agitation
As the power of the solid-state laser increases by the optical diode, the heat generated by the device
The load is getting bigger and bigger, and the heat dissipation density is getting higher and higher, DPL cooling is asking
This problem has become a technical difficulty in the current DPL research.

In recent years, there have been many problems at home and abroad for the heat dissipation of high-power DPL
Scholars have carried out a lot of research work and proposed microchannel liquid pairs
There are many types of cooling, such as flow heat transfer, solids cooling, spray cooling, and micro heat pipe cooling
But way. In this paper, the research status of these technologies is reviewed
On this basis, microchannel boiling heat exchange cooling and liquid were proposed Nitrogen cooling technology.

In the formula, the heat dissipation of the heat dissipation device is α the cooling working fluid in the heat sink
The convective heat transfer coefficient in the channel is the heat exchange area of the heat sink channel, W
is the temperature of the inner wall of the heat sink channel, and it is f
It is to cool the working fluid temperature. Raccoon h
for the scattered
Thermal device temperature. The purpose of laser cooling is to take away the heat sink
The heat dissipation of the device ensures a certain temperature of the heat dissipation device.

As can be seen from equation (1), in order to increase the amount of heat dissipation, it should be mentioned as much as possible
The convective heat transfer coefficient of the cooling working fluid in the heat sink channel is high, increasing the heat
The heat exchange area of the sinking channel reduces the temperature of the cooling working fluid. At the same time, the heat is reduced
Thermal conductivity and thermal resistance make the temperature of the inner wall of the heat sink channel uniform and connected as much as possible
Near heat sink temperature.

3 Research status
The microchannel liquid is convection heat exchange, solid cooling, and spray cooling, respectively
However, the research status of four cooling technologies of micro heat pipe cooling is reviewed Analyse.
3.1 Microchannel liquid convection heat exchange cooling
According to the definition of Bowers and Mudawar[1], hydraulic diameter
Channels between 0.01~1mm are called microchannels. Microchannel fluids
Bulk convection heat transfer cooling is the flow of liquid through the internal microchannel of the heat sink, through
Concurrent heat transfer with the inner wall of the microchannel carries away the heat dissipation device heat (see
Figure 2). The microchannel heat sink greatly increases the heat exchange area, and the microchannel
The convective heat transfer coefficient of the liquid in the channel is also much higher than normal due to the microscale effect
Convective heat transfer coefficient within the gauge channel. So the cooling principle, micro
Channel liquid convection heat transfer cooling by increasing the cooling working fluid in the heat sinking through
There are two aspects: the convective heat transfer coefficient in the channel and the heat transfer area of the heat sink channel
Improve heat dissipation and cooling capacity.

The report of microchannels first appeared in the literature of American scholars Tuckerman and Pease in the early eighties [2]. They introduced a type of silicon
Water-cooled ribbed radiator of the base material, the ribs and channels of this radiator
The width is only 50 μm, the rib height is 300 μm, and the thermal resistance is only 0.09 k/W.
The heat flux is up to 790 W/cm2.
Since the 90s of the 20th century, Stanford University in the United States, California
Each branch of the university and prestigious universities such as the University of Maryland are operated separately
related topics, and cooperate with Intel, HP and other companies. According to military techniques
Surgical Needs, Lawrence Livermore National Laboratory (LLNL), USA
The research on microchannel heat dissipation technology was also started earlier.

Silicon-based material etching molding technology has in microchannel processing
unique advantages, and the size error of the processed microchannel is small, therefore
It was originally used by some researchers to cool high-power laser diode arrays
columns [3, 4]. Their studies all show that the microchannel heat sink cooling method is superior
to traditional cooling methods.
Harpole, George M, et al. [5] have done an excellent job of microchannel heat exchangers
Change. They established a complete two-dimensional flow/heat transfer microchannel exchange
The design parameters of the heater model are 1kW/cm2 with the highest surface temperature
High temperatures are not higher than 25°C. Use pure water as a coolant, or 8%
The aqueous solution of methanol (freezing point of -5 °C) cools the temperature to freezing
Under. The effective heat transfer coefficient is 100W/(cm2·K), and
The total pressure drop is about 2×105Pa.
Huang Zhe [6] developed high reliability at nLightPhotonics
Water is mentioned in the research results of high-power 808nm lasers
Cold copper microchannel heat dissipation technology. The microchannel heat sink is made up of a discharge plus
The EDM process consists of a thin layer of copper film, which is then welded to one
blocks, forming microchannel arrays. The laser strip is welded on the edge of the microchannel,
The turbulence of deionized water in the microchannels produces a high thermal conductivity, which thes. It has a very low thermal resistance.

Li Qifeng et al. [7] reported the use of V-groove silicon microchannel coolers
structure and main manufacturing process, developed a cooler sample, its cooling energy
The force is in good agreement with the numerical simulation results.
Lv Wenqiang et al. [8] carried out microchannel coolers with different structures
Simulate heat dissipation calculations. The structural parameters of the cooler were optimized
The modular microchannel cooler can meet continuous 50W or pulsed power
120W (20% duty cycle) slats of high power diode lasers
For heat dissipation needs, stacked 2D stacked array DL can be used well as high
Pumping source of average power DPL.
Liu, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences
Yun et al. [9,10] prepared an overall size of 15 mm long, 12 mm wide, and thick
2.2mm oxygen-free copper heat sink with a microchannel depth of 300μm and a width
is 400 μm.
Scholars at home and abroad have conducted experimental studies on microchannel liquid cooling
and microchannel liquid cooling and heat sink have been widely used at home and abroad.
However, there is no consistent conclusion on the flow pattern transition, the influencing factors of frictional resistance coefficient, and the influencing factors of heat transfer coefficient
The heat exchange correlation and flow equation with universal laws in the road
Still not established. For microchannel liquid convection heat transfer technology is applied to high
The cooling of power solid-state lasers requires system optimization and channel reduction
Flow resistance, improved system stability, reduced channel corrosion and clogging, etc A lot of questions.

3.2 Solids cooling
Diamond with a very high thermal conductivity is mainly used as solid cooling
But the material. One side of the pumping surface is pasted with a diamond sheet, and the sheet is outside and not
The pumping surface is directly water-cooled. The heat generated when a laser crystal is working
The amount is quickly transferred to the surface of the diamond, and then through the pair to the surrounding space
Stream heat dissipation or bring heat away with other cooling devices (e.g., liquid-cooled microchannels).
Walk. In this way, it not only ensures the rapid dissipation of heat, but also makes the laser crystal
The surface temperature distribution is uniform, which effectively improves the thermal lensing efficiency of the laser crystal
Should. Therefore, in terms of cooling principle, solid cooling is to reduce heat conduction
The thermal resistance method is carried out to improve the heat dissipation capacity.
Israeli scholar Yitshak Tzuk et al. [11] verified it experimentally
It is feasible to use diamond foil to cool high-power solid-state lasers
Sex. Experiments were conducted with 2.3mm×4mm×24mm
Ｎｄ∶ＹＶＯ４
Plate-like laser crystals, sandwiched in optics 0.3mm thick
Between the diamond foil and the liquid copper heat sink. The wavelength of the output light is
It is obtained at 808nm when the pumping energy is 600W
The output power is 200W. According to the literature, the use of conventional cooling is used
The maximum output power is only about 100W. California, USA
H.P. Chou et al. [12] of TextronSystem also reported
Compact diodes based on diamond cooling technology are used to pump solids
Laser performance studies, and based on their experimental data, to one
The concept design of the 100kW laser class was carried out.
There are no reports of solid-state cooling of lasers in China.

3.3 Spray cooling
Spray cooling is the passage of coolant under the action of applied energy
The atomization device (the main device is the nozzle) is atomized and sprayed to the heat exchange meter
surface, through the phase transformation of atomized droplets, the heat of the tropical heat reaches the cooling purpose
(see Figure 3). There are reports abroad [13] pointing out that with water as coolant,
The heat flux density of spray-enhanced heat transfer can be as high as 1000W/cm2;
With Freon as the coolant, the heat flux density also exceeds 100W/cm2.
Therefore, from the cooling principle, spray cooling increases the cooling working fluid
The convective heat transfer coefficient on the heat exchange surface to improve the heat dissipation and cooling capacity.
Spray-enhanced heat transfer is a complex flow heat transfer process
to droplet particle size distribution, droplet velocity, spray angle, physical properties, change
The control of many factors such as hot surface roughness and heat flux has been proposed by many scholars
different heat exchange mechanisms [14]. Under low superheat conditions, single relative
Flow heat transfer plays a major role, and the impact of fog droplets will increase the disturbance to the liquid film
dynamic, and then strengthen heat exchange; Under the condition of high superheat, phase transfer heat occupies a dominant position in intensified heat transfer. Heat exchange, which is now widely recognized
There are two theories, one is the thin liquid film evaporation theory [15], which holds that the fog droplets are in
A thin liquid film is formed on the surface of heat exchange, and heat exchange is carried out through the liquid film heat conduction; Liquid film
The temperature of the top layer is saturated due to the temperature ladder that passes through the thin layer of the liquid film
The increase of the degree realizes the efficient heat exchange of the thin liquid film; The other is quadratic
According to the nuclear theory [13|~16], it is believed that the fog droplets carry bubbles to the liquid film layer
, creating nucleation conditions that cause violent boiling on the liquid film, whereby to enhance heat exchange.

Choi and Yao [17] experimentally investigated the droplet-to-heat exchange surface
The effect of the impact angle on the heat transfer efficiency. The study found that for the penalty
Direct injection, high heat exchange efficiency in the nuclear boiling section; For horizontal spraying
The heat exchange efficiency is higher in the transition boiling section. Sodtke and
Stephan [18] experimented with different surface roughnesses
The heat transfer effect of existing surfaces with slight roughness is more pronounced than that of smooth surfaces
Much higher, in addition due to the slight roughness that leads to three-phase contact
The increase in the length of the line also leads to the rapid evaporation of the thin liquid film.
Silk [19] studied the effects of straight fins and tapered wings on heat transfer performance
sound, and the critical heat flux density of the flat fin compared to the smooth heat transfer surface
(CHF) can be up to 50% higher. Gao Shan, Qu Wei [20] and others exploited
Volumeoffluid(VOF) method, which is constant for a single water droplet
Physical and mathematical models were established on the warm plates, and then numerical simulations were carried out
The results were analyzed to study their kinetics and heat transfer properties.

Chen et al. [21] closed the microjet cooling system in the experiment
It is applied to light-emitting diode (LED) heat dissipation, and the results show that at ambient temperature
At 25.6°C, the input power of the LED chipset is 2.45W
, compared to not taking heat dissipation measures for 1 minute, its temperature rises to 72°C, after the start of operation with this system, its temperature quickly drops to
３４．８１℃。 When the input electrical power of the LED chip is 9.3W, its
With a heat flux density of 14.53 W/cm2, the cooling system is still fast
Reduce the temperature of the LED chip to 54.34°C. Raytheon, UK
The research on high-efficiency diode pumping source technology is the use of spray cooling
The heat transfer coefficient is increased, and the heat flux density measured by the experiment is greater than
５００Ｗ／ｃｍ２。 Dr. Rini of Rinitechnologies, Inc., USA, is expected to use a high-power (100~300kW) laser system in the future
system of high-power diode arrays for heat dissipation simulation. Contrast microcommunication
Road heat dissipation, using ammonia as the coolant spray cooling heat dissipation system, required
The coolant flow rate is less than 1/12 of the former, and the volume and weight of the system
The amount has been greatly reduced. Therefore, spray cooling is highly likely
An important way to cool high-power lasers.

Although spray cooling has a large heat transfer coefficient and a critical heat flux density
High and low coolant flow rates, but due to the impact on heat exchange
Multiple parameters (spray angle, injection velocity, droplet size, heat exchange surface,
Gravity factors, etc.) are not independent of each other, and it is difficult to score them through experiments
Analyze the influence of individual parameters on heat exchange. The heat exchange itself contains gas-liquid mutuals
Complex processes such as action, impact of droplets, and phase transitions can only be done at present
Some limited correlation tests such as the dynamic properties of bubble droplets, pool boiling,
Droplet impact, smooth heat transfer surface and some simple spray cooling simulations,
A systematic-complete theory has not yet been formed. in high-power solid-state lasers
In the cooling device, heat is generally exchanged in a limited space, and some even. Need for heat transfer in an enclosed space, heat transfer mechanism under these conditions and heat exchange laws need to be further studied.

3.4 Micro heat pipe cooling
The concept of micro heat pipes was introduced in 1984 by T.P. Cotter
Presented at the 5th International Heat Pipe Conference held in Japan [22].
Microheat pipes consist of a closed vessel, a capillary structure, and a working fluid.
The working fluid remains saturated inside the container once one end of the container
When heated, the working fluid absorbs heat and vaporizes, and the resulting vapor flows to the container another
One end is exothermic and condenses, while the condensate will be acted upon by capillary force or gravity
Use the lower flow back to the original heating position. Due to the working fluid flow inside the heat pipe
The heat is transferred through the phase change, so that a very high conductivity coefficient can be obtained, which can be achieved
Effect of uniform temperature. Petros, Mulugetat, et al. [23] designed
One laser conducts heat through heat pipes, and two can be done through three heat pipes
150W of heat is conducted in the pole tube and laser bar. ＸｉｅＢｏｐｉｎｇ
et al. [24] have also done some research on heat pipe heat dissipation technology, and how to and
A rough indication of when to use heat pipes to solve thermal management problems is given
Guidelines.
At present, the research on the application of micro heat pipes in laser cooling has been carried out in China
There is very little and research in this area needs to be strengthened.

4 Future developments
According to the principle of laser cooling and the cooling formula, the application for:
High-power solid-state lasers enable micropass cooled with higher heat flux density
There are two technologies: boiling heat exchange cooling and liquid nitrogen cooling.
4.1 Microchannel boiling heat exchange cooling
Microchannel boiling heat exchange cooling is the internal micropass of the working fluid flowing through the heat sink
Evaporation boils during the channel, and the heat is transformed by the boiling phase with the inner wall of the microchannel
Remove heat from the heat sink (see Figure 4). In terms of cooling principle, microchannel boiling heat transfer cooling and microchannel liquid convection heat transfer cooling phase are phases
At the same time, it is also by increasing the convective heat transfer coefficient of the working fluid in the heat sink channel
and heat sink channel heat exchange area two aspects to improve heat dissipation cooling capacity, but
Due to the microchannel boiling heat exchange, the cooling has a microscale effect and boiling exchange
The convective heat transfer coefficient is colder than that of microchannel liquids
However, it has been greatly improved, so its heat dissipation capacity has been greatly improved.

Scholars at home and abroad have extensively studied the phenomenon of microchannel boiling heat transfer
Pan research. Studies have shown that the flow within the microchannel boils for a change
The heat law is significantly different from the flow boiling heat transfer gauge on the conventional scale
law, suitable for large channels with universal law of heat exchange correlation
and the flow equation does not apply to microchannels. So far, related
Research on the law of two-phase flow and boiling heat transfer in microchannel refrigeration evaporator
The main research is the study of manifold and heat transfer characteristics and the nucleation of bubbles in the channel
The mechanism of study of both classes under microscale conditions, due to the narrowness of the flow channel, flows
The type change is generally more drastic, and it is easy to cause changes in flow resistance and flow
Stability changes and heat transfer deterioration [25~32], hence the research pole of flow patterns
is important. Research on heat transfer characteristics and nucleation mechanism of bubbles in microchannels
In the study, the flow boiling heat transfer in the microchannel includes nuclear boiling and convective boiling
Teng in two forms. Nuclear boiling heat transfer coefficient and heat flux density, fluid
Parameters such as characteristics, fluid pressure, and channel size are related, while convection boils
The heat transfer coefficient is determined by the mass flow rate, mass gas content, fluid characteristics, and pass
The structure and size of the road are determined. Formation of vaporized cores with local walls
Superheat is related to the degree of supercooling of the fluid [33], and the channel size is important for nucleation
Temperature has an effect [34,35].

For micro refrigeration systems employing microchannel evaporators, evaporation
The boiling heat transfer in the vessel is unstable, and the two-phase flow will cause the bubbles to be filled periodically
Microchannels, resulting in air blockage in the channel, which causes the temperature of the wall to rise sharply
high [36], resulting in the heat dissipation device not being effectively cooled or even burned.
Therefore, the law of unstable two-phase flow of refrigerant in the microchannel evaporator is
The premise of ensuring the normal operation of the micro-refrigeration system is also the widely used micro-communication
The theoretical basis of evaporators and micro-refrigeration systems cannot be surpassed.
Microchannel boiling heat transfer is used in the cooling of high-power solid-state lasers
However, a series of research work is required, including the optimization of the channel
gauge to ensure the stability of boiling heat exchange in the evaporator; The refrigeration system is optimized to be compact and meet the needs of current high-power solid-state lasers
the trend towards miniaturization; Strive for precision in temperature control to make the laser
The internal temperature is evenly distributed to ensure its luminous quality.

4.2 Liquid nitrogen cooling
Liquid nitrogen cooling is the passage of liquid nitrogen into the internal channel of the heat sink (conventional pass
tract or microchannel), so that the liquid nitrogen evaporates and boils during the channel flow,
The heat dissipation device is heated by the phase conversion heat that boils with the inner wall of the heat sink channel
The amount is taken away (see Figure 5), and the gasified nitrogen is discharged into the atmosphere or cryogenic machine
Group recycling. As an excellent cryogenic coolant, liquid nitrogen is abundantly available
Rich, low price, low boiling point, colorless, non-toxic, tasteless, non-combustible
and other characteristics. The boiling point of liquid nitrogen is 196 °C, so from the cooling principle
In other words, liquid nitrogen cooling increases the temperature between the cooling working fluid and the heat dissipation surface
to improve heat dissipation capacity.

For liquid nitrogen in the tube, gas-liquid two-phase flow characteristics and boiling heat transfer
In the study, Steiner et al. [37,38] conducted a study on horizontal tubes with a diameter of 14 mm
The liquid nitrogen flow, boiling, heat transfer and pressure drop were systematically experimented
Investigate. The results show that most of the experimental flow patterns are wave-like and elastic
flow, the heat transfer coefficient is mainly determined by the heat flux density, and the dryness is only in the heat flux density
The heat transfer coefficient is only possible when the degree and pressure are high and the flow rate is very low
Important impacts. The dominant mechanism of heat transfer is nuclear boiling. Ｋｌｉｍｅｎｋｏ
etc. [39] do not have an inner diameter of 10mm and a length of 1850mm
Experimental studies were carried out on the flow and boiling of liquid nitrogen in rusty steel pipes. Discover boiling
The heat transfer coefficient increases with the increase of heat flux density and increases with pressure
Big. Ren Xin et al. [40] boiled the liquid nitrogen on the heating wire in the capillary
Experimental studies were carried out to simulate linear superconducting elements, and the heating wire was straight
Phosphor bronze wire with a diameter of 50 μm. The results show that the capillary pairs the nuclear state
Boiling heat transfer has a significant strengthening effect, and there is an optimal pipe diameter
is 1.2mm. The main reason is the presence of capillaries that make bubbles along
The wall of the tube is parallel to the heating wire, detached, creating a bubble at the bottom of the capillary
It can be glided over the entire heating wire and is strong for liquids near the heating wire
of perturbation. Qi Shouliang [41] commented on the flow and exchange of liquid nitrogen in microchannels
Thermal properties were studied. He has an inner diameter of 0.531mm, 0.834mm,
The boiling initiation point, two-phase flow pressure drop, boiling heat transfer coefficient and critical point of liquid nitrogen flow boiling in 1.042mm and 1.931mm microchannels
The heat flux (CHF) was studied separately and determined according to the heat transfer characteristics
The critical diameter of the microchannel under experimental conditions was determined. The results show that micropass
The boiling starting point of liquid nitrogen in the channel is different from that of the conventional channel, in the mass flow
Significant characteristics are shown in volume, pressure drop and wall temperature.

Liquid nitrogen boiling heat transfer is applied to the cooling of high-power solid-state lasers.
It is necessary to grasp the law of liquid nitrogen flow boiling heat transfer in the heat exchanger, right
The heat exchange channel is optimized to ensure the stability of the heat exchange process and avoid it
Avoid the occurrence of heat exchange deterioration such as local evaporation drying or channel air blockage
Bear; In addition, due to the large temperature difference between liquid nitrogen and the surrounding environment, liquid nitrogen is transported
The surface of the pipe and the surface of the heat exchanger are prone to frosting, in practice. This needs to be addressed in the app.

5 Conclusions
With the increasing power of lasers and the increasing size of devices
Miniaturization, the cooling of high-power solid-state lasers has become a constraint
Its output power further increases the bottleneck problem of its cooling technology
Research is imperative.
Semiconductor refrigeration, gas cooling, and conventional channel liquid cooling are all available
It is difficult to dissipate a large amount of heat in a limited space in time. Diamond Cause
Its high thermal conductivity can be used for cooling the laser crystal and improving the inside of the crystal
The temperature is distributed with the thermal stress, and the heat is dissipated from the crystal quickly
dissipated, but its heat is eventually carried away by other means of cooling; small
Channel liquid convection heat transfer cooling has been widely used at present, but still
It is necessary to solve the problem of system optimization, reduce the channel flow resistance, and improve the stability of the system
Qualitative, reduce channel corrosion and clogging and many other problems; Spray cooling is
A very effective way to cool down, but so far people have been sprayed
The understanding of the mechanism of cooling flow and heat transfer is still quite limited, and similar phenomena
or even contradictory experimental results of similar problems, in the application to the high
Power solid-state laser cooling device, heat exchanger in an enclosed space
The law of heat transfer needs to be further studied.

Microchannel boiling cooling and liquid nitrogen cooling have stronger cooling energy
force, which has a broad range in the development of higher power solid-state lasers
Application prospects. The mechanism of microchannel boiling heat exchange is better than that of single-phase liquid-cooled heat exchanger
The theory is more complex, and it is necessary to solve the optimal design of the heat sink channel and the two-phase flow
stability, refrigeration system optimization, temperature control and other issues; Liquid nitrogen cooling is required
Further study of the law of flow boiling heat transfer, the optimal design of heat sink channel, stability, frosting, etc.