rus
Issue #1/2015
Dr. Heiko Kissel, Dr. Jörg Neukum, Dr. Paul Crump, Dr. Thomas Töpfer
Reliable High-Power Diode Laser Arrays For QCW Operation With High Duty Cycles
Reliable High-Power Diode Laser Arrays For QCW Operation With High Duty Cycles
The performance and reliability data of high-brightness QCW arrays are presented.Operation at increased heat sink temperatures up to 45°C is possible without active water cooling or conduction cooling with the help of Peltier elements.
Теги: high-brightness qcw arrays performance reliability высокомощные лазерные диодные сборки квазинепрерывный режим производительность надежность
W
e present performance and reliability data of high-brightness QCW arrays with a custom, compact and robust design for an operation with high duty cycles. The general design is based on single diodes consisting of a 1cm laser bar that is AuSn soldered between two CuW submounts. Arrays of up to 15 diodes are connected to ceramic base plates on different heat sinks. The available optical peak power strongly depends on the wavelength and fill factor of the laser bars as well as on the duty cycle, the base plate temperature and the thermal conductivity of the applied ceramic materials. Operation at increased heat sink temperatures up to 45 °C is possible without active water cooling or conduction cooling with the help of Peltier elements. Novel laser diodes allow for more efficient operation and higher optical output powers.
INTRODUCTION
Quasi-continuous-wave (QCW) operation of a laser diode means that it is switched on only for certain time intervals being short enough to reduce thermal effects significantly, but still long enough that the laser process is close to its steady state, i. e. the laser is optically in the state of continuous-wave operation. Usually the duty cycle (percentage of "on" time) takes only a few percent, thus, strongly reducing heating and all related thermal effects, such as damage due to overheating [1] or thermal lensing [2]. Therefore, the QCW mode allows for the operation with higher optical peak powers at the expense of a lower average power. Thus, the cooling arrangement of usual QCW arrays is designed for small heat loads, and the emitters can be more closely packed in order to obtain higher peak power densities resulting in QCW array sizes, i. e. volumes, being much smaller compared to usual stacks of micro-channel coolers or CS mounts [3].
QCW laser bars and arrays have found a wide range of applications in industry (mostly for pumping), in medicine and cosmetics such as surgery or hair removal, in science for the generation of high-energy ultra-short pulses, in space as pumps for light detection and ranging or in the defense area for range finding and target designation. In the last few years, there is an increasing demand for compact and robust QCW stacks operating at higher duty cycles and with longer pulses beyond the classical QCW limit described above. New applications require increased operation temperatures and the option of reduced or no cooling using water or Peltier elements [4, 5]. For this purpose, we have developed a custom, compact and robust laser diode array (LDA) design with large flexibility regarding the number of bars, the size and the ceramic material as well as the cooling concept.
General DESIGN OF THE QCW LDA
The general design of our QCW LDA for an operation with high duty cycles is illustrated in Fig. 1 (left part). It is based on a customized number of individual laser bars (marked with blue color) sandwiched between two thermal expansion matched submounts consisting, e. g., of copper tungsten (shown in orange color). A custom number of these sandwiches is arranged on an electrically insulating ceramic base plate (gray color) using a low-melting solder. The base plate can be easily adapted to different active or passive cooling elements (brown color) on the backside. The small elements shown in red color on both sides of the LDA are NTCs (Negative Temperature Coefficient Thermistors) for temperature monitoring during operation. There are several base plate sizes and layouts of the electrical contact structure (yellow color) available at DILAS to meet our customer’s demands. The improved cooling via backside cooled ceramic plates is an important benefit of this design. With respect to optimal cooling and mechanical stability, we prefer a solder connection between the base plate and the cooling element. The photo in the right part of Fig. 1 gives an impression of the original size of one identically packaged laser bar sandwich on another custom base plate consisting of the same materials. We have called LDAs with the above described general design C-stacks.
Some important features of this compact and robust LDA design offering scalability and modularity are listed in the following:
The use of laser bar sandwiches leads to a thermal as well as mechanical decoupling of the laser bars, but also to an increased bar-to-bar pitch. Each laser bar sandwich can be measured and selected before it is soldered to a base plate.
The submounts act as heat spreaders, i. e. each laser bar is thermally connected to the ceramic base plate. Thus, the waste heat can be removed more efficiently because it is spreaded to a larger area resulting in decreased junction temperatures. This allows for reliable laser operation at higher duty cycles and with increased base plate temperatures.
Ceramic materials with improved thermal conductivity and different heat sinks below the base plate can be used. The improved thermal management and the robust, lightweight design make these arrays especially interesting for portable and mobile applications demanding a minimum of cooling.
Compared to other QCW stack geometries with poor transition resistances, the individual contacting of each laser bar cares for a minimum contribution of packaging to the electrical resistance and leads to a further reduction of the waste heat in this LDA design.
The life time of our QCW arrays is not limited by thermo – [6] and electromigration [7] of soft solders.
Sandwiching each laser bar between two thermal expansion matched submounts (e. g. copper tungsten) reduces the packaging induced deformation stresses and leads to a bar smile less than 1µm (peak-to-valley).
Our LDA design allows for wavelength stabilization using volume Bragg gratings (VBGs) as well as for an easy and efficient optional beam shaping using fast-axis collimation (FAC) for all bars as well as slow-axis collimation (SAC) especially for bars with low fill factor. In the latter case, fiber coupling becomes possible.
DILAS offers a broad variety of C-stacks with custom base plate sizes for 1 to 15 laser bars with cavity lengths up to 2.0 mm (standard), in the wavelength range between 766 and 1550 nm, with a minimum bar-to-bar pitch of 1.7 mm and different cooling concepts up to the absence of water or thermo-electrical cooling. So-called rainbow stacks with multiple wavelengths due to customer’s requirements are also possible.
C-stack performance
In the following, we present performance and reliability data of selected C-stacks with up to 15 laser bars for different applications. Using bars with an emitter pitch of 500 µm provides the opportunity for building small fiber coupled modules with high QCW output powers. The available peak power depends on the wavelength and fill factor of the laser bars, but also on the duty cycle, the base plate temperature and the thermal conductivity of the applied ceramic materials.
Arrays with 15 laser bars
In this section, we show performance data of conduction-cooled LDAs containing 15 laser bars emitting at 980 nm with 20% fill factor and a cavity length of 2000 µm mounted on a custom designed AlN ceramic base plate (see inset of Fig. 2) that are suitable for fiber coupling.
Figure 2 (a) illustrates the influence of an increasing pulse width (and duty cycle as well) on the optical peak power. An extension of the pulse width from 400µs to 10ms leads to a drop of the optical peak power by 6.5% at a driving current of 60 A. The emission spectra taken at 55 A are shown in Fig. 2 (b). The extension of the pulse width causes an increase of the average output power and heat load. The observed redshift with increasing pulse width is a measure for the rising junction temperature of the laser bars; it increases by about 32°C. Nevertheless, the LIV curve for a pulse width of 10ms remains linear up to an optical peak power of 807 W illustrating the potential of our C-stack design for an operation with higher duty cycles.
A comparison of the power-current characteristics and of the emission spectra for a repetition rate of 15 Hz and a pulse width of 10ms at base plate temperatures of 20 °C and 45 °C is shown in Fig. 3 (a). At a driving current of 90 A, the optical peak power drops from 1252 W to 1155 W while the power conversion efficiency stays above 55%. The redshift of the emission spectra shown in Fig. 3 (b) corresponds to the expected band-gap narrowing for a ∆Tjunct = 25 °C.
A further improvement of the stack performance becomes possible when using alternative ceramic materials with higher thermal conductivities compared to standard AlN. When using a ceramic base plate with a better thermal conductivity (factor 1.8 compared to the LDA in Figs. 2 and 3), the power conversion efficiency of the improved stack design at 45 °C becomes comparable to the performance of the standard stack design with AlN ceramic at 20 °C (see Ref. [8] for details).
The data, presented early, shows, that the considered C-stacks can be driven with minimal cooling and at elevated temperatures. Together with their robust and compact design and with the option of an easy beam shaping due to the low fill factor of the implemented laser bars, these stacks are very interesting for pumping applications in the defense area.
Arrays with 8 laser bars
In this section, we present selected performance data of LDAs containing 8 laser bars with fill factors of 50% and more and with a cavity length of 1500 µm mounted on a smaller custom designed AlN ceramic base plate. The inset of Fig. 4 shows an LDA of eight laser bars with a macro-channel cooler between two ceramic plates. The main benefit of this design is an efficient and potential-free cooling allowing the usage of tap water. In Fig. 4 (a), the LDA is operated with a repetition rate of 3 Hz and a pulse width of 50 ms. At this operation conditions, we have reached a peak power of 890 W at a driving current of 120 A; there is only a slight deviation from the linear dependence of the peak power on current.
For some applications like hair removal, longer pulses and compact QCW stack designs are required. Figure 4 (b) shows the electro-optical performance for an operation with a repetition rate of 2 Hz and a pulse width of 100 ms. For these pulse conditions, our C-stacks reaches an optical peak power of 580 W at a driving current of 85 A; again there is only a slight deviation from the linear dependence of the peak power on current.
Using this stack design, larger optical peak powers can be achieved in the 980 nm wavelength range with more efficient and less temperature sensitive diode laser structures compared to the results for C-stacks with 808 nm laser bars shown in Fig. 4. The achievable optical peak power and pulse length of QCW stack designs is not only limited by critical optical damage (COD), but mainly by a maximum allowed junction temperature of the applied laser bars.
Figure 5 shows the result of an ongoing reliability test carried out in constant current mode on such a water-cooled LDA containing 8 laser bars at 808 nm. The LDA is operated with a repetition rate of 2 Hz and a pulse width of 100 ms corresponding to an energy density of the LDA emission of 42 J/cm 2. After 3.000 hours of operation, we detected a power degradation of 6%. The main power loss occurred in the time interval between 0 and 800 hours giving rise to a life time expectation of about 10.000 hours – more than usually demanded for medical or cosmetic laser applications.
In the next example, the macro-channel cooler is replaced by a massive copper heat sink for conduction cooling. A possible application for such water-free C-stacks is pumping of solid-state lasers generating high-energy ultra-short pulses at moderate repetition rates, e. g. for future inertial-confined-fusion facilities. Due to longer upper-state lifetimes in Yb-doped host materials, pump pulse lengths of about 1 ms for Yb 3+: YAG or 2.8 ms for Yb 3+: CaF2 are required.
The power-current characteristics for a conduction-cooled LDA containing 8 laser bars at 940 nm with 80% fill factor and a cavity length of 1.500 µm is shown in Fig. 6 (a). The measurement was carried out with a repetition rate of 10 Hz and a pulse width of 1 ms. The C-stack achieved an optical peak power of 3.500 W at a driving current of 390 A, which corresponds to more than 435 W per bar. At a driving current of 300 A, an increase of the pulse width from 1 ms to 2.8 ms leads to a redshift of the emission wavelength that corresponds to an increase of the junction temperature of 13 °C.
The result of an ongoing reliability test carried out with constant peak current on a conduction-cooled LDA containing eight laser bars at 940 nm is shown in Fig. 6 (b). The LDA is operated with a repetition rate of 10 Hz and a pulse width of 1 ms. The test power was chosen to be 2.400 W or 300W per bar regarding customer’s requirements. After 2.500 hours of operation we could not detect any degradation of the peak power proving reliable operation of the stacks at this operation conditions. One application of these C-stacks will be described in Sec. 4.
Single laser bar design
In this section, we consider the performance of a C-stack like conduction-cooled design containing only one single laser bar (as shown in the right part of Fig.1).
Figure 7 (a) shows the power-current characteristics for a conduction-cooled LDA containing one laser bar at 940 nm. The measurement was carried out with a repetition rate of 50 Hz and a pulse width of 50 µs without additional cooling. A possible application is laser ignition of several highly flammable substances. At I=640 A, a peak power of 710 W was achieved. Up to this power level, no COD events occured. Other potential applications of these devices demanding larger pulse widths in the range of 1 to 5 ms are diode laser pumps for range finding or gated imaging systems. Figure 7 (b) depicts an ongoing reliability test on the same device with a constant peak current of I=400 A and a repetition rate of 5 Hz. After 1.000 hours of operation with a pulse width of 2 ms, the test was continued with a pulse width of 4 ms. Within the recent test duration, we could not detect any power degradation at the applied pulse conditions.
New research samples with a cavity length of 1500 µm that use more advanced diode laser designs give rise for a reliable operation up to a peak power of 500 W with pulse widths of a few milliseconds.
The power-current characteristics for a conduction-cooled LDA containing another novel laser bar at 940 nm with 50% fill factor, a cavity length of 4000 µm and passivated facets is depicted in Fig. 8 (a). The achieved optical peak power at a driving current of 1.150 A was 800 W. Figure 8 (b) shows the temperature distribution of the measured device for a peak power of 800 W at a backplate temperature of 20 °C received from a finite element analysis. The temperature difference between the backplate and the hottest points is only 9.65 °C.
SAMPLE For C-STACK Application
Our compact C-stacks with improved thermal management compared to usual QCW stacks allow a reliable operation with high peak powers and longer pulses in the millisecond range. They become more and more interesting for pumping of high-energy class solid-state lasers (DPSSL) [9]. Applications range from pump lasers for attosecond pulse generation to X-ray and particle physics. In this research area, ultra-short high-energetic laser pulses serve as the unrivaled source for very strong electromagnetic fields opening the door for the investigation of long ago predicted but so far not observable effects in relativity and quantum physics. Moreover, the huge DPSSLs for inertial confined fusion research facilities planned for some projects worldwide have to provide exceptional beam quality and stability. Such laser systems are measured by their maximized wall-plug efficiency and minimized maintenance effort. Two challenging large scale European laser projects, namely HiPER (www.hiper-laser.org) and ELI (www.extreme-light-intrastructure.eu), are currently at their preparatory phase. Recent developments in high-power diode laser technology enable the set-up of a new class of high-energy, diode-pumped lasers with enhanced reliability at acceptable costs.
High-power diode laser stacks as described in Sec. 3.2 can be arranged close to each other in a planar geometry. Today, 2.4 kW and 3.2 kW C-stacks are available for pumping at 939 and 979 nm with pulse widths of 1ms (for Yb 3+: YAG crystals) and 2.8 ms (for Yb 3+: CaF2 crystals). As an example, Figure 9 (a) shows some basic parameters of a pump engine from Lastronics GmbH in Jena/Germany, e. g. used in Peta Watt (1015 W) systems. In the PM80 pump engine, 32 conduction-cooled C-stacks (as shown in Fig. 6) are combined. At a repetition rate of 10 Hz, this pump engine achieves an optical peak power of 75 kW at 300 A. The line width is smaller than 6.0 nm (FWHM). Figure 9 (b) shows a quadratic ultraflat-top, low-ripple beam profile well-suited for pumping of high-energy class solid state lasers.
Summary And Outlook
We have presented performance and reliability data of high-brightness high-power QCW arrays with a custom, compact, robust and lightweight design. These so-called C-stacks are based on single diodes consisting of a 1cm laser bar that is AuSn soldered between two CuW submounts. The improved thermal management of the C-stack design allows for an operation with high duty cycles and long pulses beyond the classical QCW limit. LDAs of up to 15 diodes were connected to ceramic base plates on different heat sinks fitting to the requirements of various applications. The available optical peak power strongly depends on the wavelength and fill factor of the laser bars as well as on the duty cycle, the base plate temperature and the thermal conductivity of the applied ceramic materials.
Operation at increased heat sink temperatures up to 45°C is possible without additional cooling like active water cooling or thermo-electrical cooling with the help of Peltier elements. Novel laser bars allow for more efficient operation and higher optical peak powers. Longer resonators reduce the electrical resistance and the in-pulse temperature variation usually leading to a broadening of the spectrum (so-called thermal chirp [10]).
The investigated C-stacks allow for an easy and efficient beam shaping using fast-axis collimation (FAC) for all bars as well as slow-axis collimation (SAC) especially for bars with low fill factor. In the latter case, fiber coupling becomes possible.
The presented technology also offers scalability and modularity of the LDA designs allowing custom products with respect to user applications. It allows the use of ceramic materials with improved thermal conductivity and different heat sinks below the ground plate based upon the customer’s needs. The improved thermal management and the robust, light weight design make these arrays especially interesting for portable and mobile applications demanding a minimum of cooling.
The work on our C-stack design with respect to materials, processes and structures is continued, and even better results can be expected in the near future.
e present performance and reliability data of high-brightness QCW arrays with a custom, compact and robust design for an operation with high duty cycles. The general design is based on single diodes consisting of a 1cm laser bar that is AuSn soldered between two CuW submounts. Arrays of up to 15 diodes are connected to ceramic base plates on different heat sinks. The available optical peak power strongly depends on the wavelength and fill factor of the laser bars as well as on the duty cycle, the base plate temperature and the thermal conductivity of the applied ceramic materials. Operation at increased heat sink temperatures up to 45 °C is possible without active water cooling or conduction cooling with the help of Peltier elements. Novel laser diodes allow for more efficient operation and higher optical output powers.
INTRODUCTION
Quasi-continuous-wave (QCW) operation of a laser diode means that it is switched on only for certain time intervals being short enough to reduce thermal effects significantly, but still long enough that the laser process is close to its steady state, i. e. the laser is optically in the state of continuous-wave operation. Usually the duty cycle (percentage of "on" time) takes only a few percent, thus, strongly reducing heating and all related thermal effects, such as damage due to overheating [1] or thermal lensing [2]. Therefore, the QCW mode allows for the operation with higher optical peak powers at the expense of a lower average power. Thus, the cooling arrangement of usual QCW arrays is designed for small heat loads, and the emitters can be more closely packed in order to obtain higher peak power densities resulting in QCW array sizes, i. e. volumes, being much smaller compared to usual stacks of micro-channel coolers or CS mounts [3].
QCW laser bars and arrays have found a wide range of applications in industry (mostly for pumping), in medicine and cosmetics such as surgery or hair removal, in science for the generation of high-energy ultra-short pulses, in space as pumps for light detection and ranging or in the defense area for range finding and target designation. In the last few years, there is an increasing demand for compact and robust QCW stacks operating at higher duty cycles and with longer pulses beyond the classical QCW limit described above. New applications require increased operation temperatures and the option of reduced or no cooling using water or Peltier elements [4, 5]. For this purpose, we have developed a custom, compact and robust laser diode array (LDA) design with large flexibility regarding the number of bars, the size and the ceramic material as well as the cooling concept.
General DESIGN OF THE QCW LDA
The general design of our QCW LDA for an operation with high duty cycles is illustrated in Fig. 1 (left part). It is based on a customized number of individual laser bars (marked with blue color) sandwiched between two thermal expansion matched submounts consisting, e. g., of copper tungsten (shown in orange color). A custom number of these sandwiches is arranged on an electrically insulating ceramic base plate (gray color) using a low-melting solder. The base plate can be easily adapted to different active or passive cooling elements (brown color) on the backside. The small elements shown in red color on both sides of the LDA are NTCs (Negative Temperature Coefficient Thermistors) for temperature monitoring during operation. There are several base plate sizes and layouts of the electrical contact structure (yellow color) available at DILAS to meet our customer’s demands. The improved cooling via backside cooled ceramic plates is an important benefit of this design. With respect to optimal cooling and mechanical stability, we prefer a solder connection between the base plate and the cooling element. The photo in the right part of Fig. 1 gives an impression of the original size of one identically packaged laser bar sandwich on another custom base plate consisting of the same materials. We have called LDAs with the above described general design C-stacks.
Some important features of this compact and robust LDA design offering scalability and modularity are listed in the following:
The use of laser bar sandwiches leads to a thermal as well as mechanical decoupling of the laser bars, but also to an increased bar-to-bar pitch. Each laser bar sandwich can be measured and selected before it is soldered to a base plate.
The submounts act as heat spreaders, i. e. each laser bar is thermally connected to the ceramic base plate. Thus, the waste heat can be removed more efficiently because it is spreaded to a larger area resulting in decreased junction temperatures. This allows for reliable laser operation at higher duty cycles and with increased base plate temperatures.
Ceramic materials with improved thermal conductivity and different heat sinks below the base plate can be used. The improved thermal management and the robust, lightweight design make these arrays especially interesting for portable and mobile applications demanding a minimum of cooling.
Compared to other QCW stack geometries with poor transition resistances, the individual contacting of each laser bar cares for a minimum contribution of packaging to the electrical resistance and leads to a further reduction of the waste heat in this LDA design.
The life time of our QCW arrays is not limited by thermo – [6] and electromigration [7] of soft solders.
Sandwiching each laser bar between two thermal expansion matched submounts (e. g. copper tungsten) reduces the packaging induced deformation stresses and leads to a bar smile less than 1µm (peak-to-valley).
Our LDA design allows for wavelength stabilization using volume Bragg gratings (VBGs) as well as for an easy and efficient optional beam shaping using fast-axis collimation (FAC) for all bars as well as slow-axis collimation (SAC) especially for bars with low fill factor. In the latter case, fiber coupling becomes possible.
DILAS offers a broad variety of C-stacks with custom base plate sizes for 1 to 15 laser bars with cavity lengths up to 2.0 mm (standard), in the wavelength range between 766 and 1550 nm, with a minimum bar-to-bar pitch of 1.7 mm and different cooling concepts up to the absence of water or thermo-electrical cooling. So-called rainbow stacks with multiple wavelengths due to customer’s requirements are also possible.
C-stack performance
In the following, we present performance and reliability data of selected C-stacks with up to 15 laser bars for different applications. Using bars with an emitter pitch of 500 µm provides the opportunity for building small fiber coupled modules with high QCW output powers. The available peak power depends on the wavelength and fill factor of the laser bars, but also on the duty cycle, the base plate temperature and the thermal conductivity of the applied ceramic materials.
Arrays with 15 laser bars
In this section, we show performance data of conduction-cooled LDAs containing 15 laser bars emitting at 980 nm with 20% fill factor and a cavity length of 2000 µm mounted on a custom designed AlN ceramic base plate (see inset of Fig. 2) that are suitable for fiber coupling.
Figure 2 (a) illustrates the influence of an increasing pulse width (and duty cycle as well) on the optical peak power. An extension of the pulse width from 400µs to 10ms leads to a drop of the optical peak power by 6.5% at a driving current of 60 A. The emission spectra taken at 55 A are shown in Fig. 2 (b). The extension of the pulse width causes an increase of the average output power and heat load. The observed redshift with increasing pulse width is a measure for the rising junction temperature of the laser bars; it increases by about 32°C. Nevertheless, the LIV curve for a pulse width of 10ms remains linear up to an optical peak power of 807 W illustrating the potential of our C-stack design for an operation with higher duty cycles.
A comparison of the power-current characteristics and of the emission spectra for a repetition rate of 15 Hz and a pulse width of 10ms at base plate temperatures of 20 °C and 45 °C is shown in Fig. 3 (a). At a driving current of 90 A, the optical peak power drops from 1252 W to 1155 W while the power conversion efficiency stays above 55%. The redshift of the emission spectra shown in Fig. 3 (b) corresponds to the expected band-gap narrowing for a ∆Tjunct = 25 °C.
A further improvement of the stack performance becomes possible when using alternative ceramic materials with higher thermal conductivities compared to standard AlN. When using a ceramic base plate with a better thermal conductivity (factor 1.8 compared to the LDA in Figs. 2 and 3), the power conversion efficiency of the improved stack design at 45 °C becomes comparable to the performance of the standard stack design with AlN ceramic at 20 °C (see Ref. [8] for details).
The data, presented early, shows, that the considered C-stacks can be driven with minimal cooling and at elevated temperatures. Together with their robust and compact design and with the option of an easy beam shaping due to the low fill factor of the implemented laser bars, these stacks are very interesting for pumping applications in the defense area.
Arrays with 8 laser bars
In this section, we present selected performance data of LDAs containing 8 laser bars with fill factors of 50% and more and with a cavity length of 1500 µm mounted on a smaller custom designed AlN ceramic base plate. The inset of Fig. 4 shows an LDA of eight laser bars with a macro-channel cooler between two ceramic plates. The main benefit of this design is an efficient and potential-free cooling allowing the usage of tap water. In Fig. 4 (a), the LDA is operated with a repetition rate of 3 Hz and a pulse width of 50 ms. At this operation conditions, we have reached a peak power of 890 W at a driving current of 120 A; there is only a slight deviation from the linear dependence of the peak power on current.
For some applications like hair removal, longer pulses and compact QCW stack designs are required. Figure 4 (b) shows the electro-optical performance for an operation with a repetition rate of 2 Hz and a pulse width of 100 ms. For these pulse conditions, our C-stacks reaches an optical peak power of 580 W at a driving current of 85 A; again there is only a slight deviation from the linear dependence of the peak power on current.
Using this stack design, larger optical peak powers can be achieved in the 980 nm wavelength range with more efficient and less temperature sensitive diode laser structures compared to the results for C-stacks with 808 nm laser bars shown in Fig. 4. The achievable optical peak power and pulse length of QCW stack designs is not only limited by critical optical damage (COD), but mainly by a maximum allowed junction temperature of the applied laser bars.
Figure 5 shows the result of an ongoing reliability test carried out in constant current mode on such a water-cooled LDA containing 8 laser bars at 808 nm. The LDA is operated with a repetition rate of 2 Hz and a pulse width of 100 ms corresponding to an energy density of the LDA emission of 42 J/cm 2. After 3.000 hours of operation, we detected a power degradation of 6%. The main power loss occurred in the time interval between 0 and 800 hours giving rise to a life time expectation of about 10.000 hours – more than usually demanded for medical or cosmetic laser applications.
In the next example, the macro-channel cooler is replaced by a massive copper heat sink for conduction cooling. A possible application for such water-free C-stacks is pumping of solid-state lasers generating high-energy ultra-short pulses at moderate repetition rates, e. g. for future inertial-confined-fusion facilities. Due to longer upper-state lifetimes in Yb-doped host materials, pump pulse lengths of about 1 ms for Yb 3+: YAG or 2.8 ms for Yb 3+: CaF2 are required.
The power-current characteristics for a conduction-cooled LDA containing 8 laser bars at 940 nm with 80% fill factor and a cavity length of 1.500 µm is shown in Fig. 6 (a). The measurement was carried out with a repetition rate of 10 Hz and a pulse width of 1 ms. The C-stack achieved an optical peak power of 3.500 W at a driving current of 390 A, which corresponds to more than 435 W per bar. At a driving current of 300 A, an increase of the pulse width from 1 ms to 2.8 ms leads to a redshift of the emission wavelength that corresponds to an increase of the junction temperature of 13 °C.
The result of an ongoing reliability test carried out with constant peak current on a conduction-cooled LDA containing eight laser bars at 940 nm is shown in Fig. 6 (b). The LDA is operated with a repetition rate of 10 Hz and a pulse width of 1 ms. The test power was chosen to be 2.400 W or 300W per bar regarding customer’s requirements. After 2.500 hours of operation we could not detect any degradation of the peak power proving reliable operation of the stacks at this operation conditions. One application of these C-stacks will be described in Sec. 4.
Single laser bar design
In this section, we consider the performance of a C-stack like conduction-cooled design containing only one single laser bar (as shown in the right part of Fig.1).
Figure 7 (a) shows the power-current characteristics for a conduction-cooled LDA containing one laser bar at 940 nm. The measurement was carried out with a repetition rate of 50 Hz and a pulse width of 50 µs without additional cooling. A possible application is laser ignition of several highly flammable substances. At I=640 A, a peak power of 710 W was achieved. Up to this power level, no COD events occured. Other potential applications of these devices demanding larger pulse widths in the range of 1 to 5 ms are diode laser pumps for range finding or gated imaging systems. Figure 7 (b) depicts an ongoing reliability test on the same device with a constant peak current of I=400 A and a repetition rate of 5 Hz. After 1.000 hours of operation with a pulse width of 2 ms, the test was continued with a pulse width of 4 ms. Within the recent test duration, we could not detect any power degradation at the applied pulse conditions.
New research samples with a cavity length of 1500 µm that use more advanced diode laser designs give rise for a reliable operation up to a peak power of 500 W with pulse widths of a few milliseconds.
The power-current characteristics for a conduction-cooled LDA containing another novel laser bar at 940 nm with 50% fill factor, a cavity length of 4000 µm and passivated facets is depicted in Fig. 8 (a). The achieved optical peak power at a driving current of 1.150 A was 800 W. Figure 8 (b) shows the temperature distribution of the measured device for a peak power of 800 W at a backplate temperature of 20 °C received from a finite element analysis. The temperature difference between the backplate and the hottest points is only 9.65 °C.
SAMPLE For C-STACK Application
Our compact C-stacks with improved thermal management compared to usual QCW stacks allow a reliable operation with high peak powers and longer pulses in the millisecond range. They become more and more interesting for pumping of high-energy class solid-state lasers (DPSSL) [9]. Applications range from pump lasers for attosecond pulse generation to X-ray and particle physics. In this research area, ultra-short high-energetic laser pulses serve as the unrivaled source for very strong electromagnetic fields opening the door for the investigation of long ago predicted but so far not observable effects in relativity and quantum physics. Moreover, the huge DPSSLs for inertial confined fusion research facilities planned for some projects worldwide have to provide exceptional beam quality and stability. Such laser systems are measured by their maximized wall-plug efficiency and minimized maintenance effort. Two challenging large scale European laser projects, namely HiPER (www.hiper-laser.org) and ELI (www.extreme-light-intrastructure.eu), are currently at their preparatory phase. Recent developments in high-power diode laser technology enable the set-up of a new class of high-energy, diode-pumped lasers with enhanced reliability at acceptable costs.
High-power diode laser stacks as described in Sec. 3.2 can be arranged close to each other in a planar geometry. Today, 2.4 kW and 3.2 kW C-stacks are available for pumping at 939 and 979 nm with pulse widths of 1ms (for Yb 3+: YAG crystals) and 2.8 ms (for Yb 3+: CaF2 crystals). As an example, Figure 9 (a) shows some basic parameters of a pump engine from Lastronics GmbH in Jena/Germany, e. g. used in Peta Watt (1015 W) systems. In the PM80 pump engine, 32 conduction-cooled C-stacks (as shown in Fig. 6) are combined. At a repetition rate of 10 Hz, this pump engine achieves an optical peak power of 75 kW at 300 A. The line width is smaller than 6.0 nm (FWHM). Figure 9 (b) shows a quadratic ultraflat-top, low-ripple beam profile well-suited for pumping of high-energy class solid state lasers.
Summary And Outlook
We have presented performance and reliability data of high-brightness high-power QCW arrays with a custom, compact, robust and lightweight design. These so-called C-stacks are based on single diodes consisting of a 1cm laser bar that is AuSn soldered between two CuW submounts. The improved thermal management of the C-stack design allows for an operation with high duty cycles and long pulses beyond the classical QCW limit. LDAs of up to 15 diodes were connected to ceramic base plates on different heat sinks fitting to the requirements of various applications. The available optical peak power strongly depends on the wavelength and fill factor of the laser bars as well as on the duty cycle, the base plate temperature and the thermal conductivity of the applied ceramic materials.
Operation at increased heat sink temperatures up to 45°C is possible without additional cooling like active water cooling or thermo-electrical cooling with the help of Peltier elements. Novel laser bars allow for more efficient operation and higher optical peak powers. Longer resonators reduce the electrical resistance and the in-pulse temperature variation usually leading to a broadening of the spectrum (so-called thermal chirp [10]).
The investigated C-stacks allow for an easy and efficient beam shaping using fast-axis collimation (FAC) for all bars as well as slow-axis collimation (SAC) especially for bars with low fill factor. In the latter case, fiber coupling becomes possible.
The presented technology also offers scalability and modularity of the LDA designs allowing custom products with respect to user applications. It allows the use of ceramic materials with improved thermal conductivity and different heat sinks below the ground plate based upon the customer’s needs. The improved thermal management and the robust, light weight design make these arrays especially interesting for portable and mobile applications demanding a minimum of cooling.
The work on our C-stack design with respect to materials, processes and structures is continued, and even better results can be expected in the near future.
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