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Abstract
The Distributed Face Cooling (DFC) chip was fabricated from four pieces of 1 mm-length Nd:YAG plate sandwiched in four pieces of sapphire heat spreaders through advanced surface activated bonding (SAB) at room temperature. A sub-nanosecond (665.7ps) pulsed DFC-chip tiny integrated laser was achieved with output energy of 21.5 mJ and peak power of 32.3 MW with saturable absorber Cr4+:YAG. By finite element analysis, we confirmed the advantages of heat dissipation from DFC-chip compared with conventional bulk-chip. The SAB-DFC-chip based ubiquitous high peak power tiny integrated laser was experimentally within reach for laser-armed robot.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ubiquitous laser is to address the wide application of laser both in industry and science [1,2]. Conventional sub-nanosecond (sub-ns) ubiquitous high-peak power laser was developed with output energy at few millijoule (mJ)-level, peak power at megawatt (MW)-level and brightness at TW·sr−1·cm−2 level [3,4]. With growing requirements from laser materials processing, laser shock peening and Master Oscillator Power Amplifier (MOPA), higher output energy over several tens of mJ-level is expected [5–10]. However, conventional laser gain medium in bulk-shape suffers from temperature gradients created thermal-lens effect during laser operation. It hinders the development of high-energy laser. Thus cooling compensation is required.
Conventional attempts to reduce thermal effects include diffusion bonding, pulsed electric current bonding, cryogenic gas cooling and thin-disk structure [11–18]. With the superiority in bonding heterogeneous materials, room temperature surface activated bonding (SAB) is gaining wide applications in laser field [19–22]. SAB based Distributed Face Cooling (DFC) chip offers a new solution for heat dissipation in laser cavity. DFC-chip is defined as an array of laser gain mediums sandwiched in heat spreaders. Comparing with conventional bulk-chip under continuous-wave laser pump, Nd:YAG DFC-chip showed twice higher stress fracture limit [23]. However, DFC-chip based pulse laser has not been reported.
In this article, SAB-DFC-chip based sub-ns passively Q-switched tiny integrated laser (TILA) is investigated aiming for output energy at several tens of mJ-level. Nd:YAG DFC-chip TILA was developed in scope of energy and peak power, followed by the discussion of energy scaling-up comparing with that from conventional bulk-chip laser. To understand heat dissipation from sapphire in DFC-chip, heat dissipations in DFC-chip, end-cooling and directly bonded bulk-chips were simulated by finite element analysis (FEA) with COMSOL.
2. Experiments
Passively Q-switched sub-ns DFC-chip TILA was firstly investigated. Gain medium in DFC-chip is Nd:YAG single crystal. Figure 1 shows a schematic diagram of DFC-chip TILA. A fiber coupled pulse diode laser was fixed at maximum current delivering pump peak power of 1450 W. Due to duty cycle (2%) limit of diode laser and power supply, a repetition rate of 10 Hz was used in experiment. DFC-chip was fabricated from four pieces of 1mm-length [111]-cut 1.0 at.% Nd:YAG single crystal sandwiched in four pieces of 1mm-length c-cut sapphire heat spreader on lab-made SAB device. The aperture of sapphire crystal and Nd:YAG crystal was 10 mm × 10 mm and 8 mm × 8 mm, respectively. Input mirror coating was high-reflection (HR) at 1064 nm (Type II in Fig. 1) deposited on surface of sapphire crystal attaching to Nd:YAG plate. Output coupler was coated with partial reflection (PR) 40% at 1064 nm (Type IV in Fig. 1). Cr:YAG crystal was coated with anti-reflection (AR) at 808 nm and 1064 nm on both sides with initial transmission of 30% (Type III in Fig. 1). Digital Phosphor Oscilloscope (Tektronix) with bandwidth of 16 GHz and real time sampling rate of 100 GS/s was used to record pulse duration. Wavelength bandwidth Δλ was evaluated by high-resolution laser spectrum analyzer (Zoom Spectra, Resolution Spectra Systems, France) with spectral resolution of 10 pm through a single mode fiber. Energy stability was detected by a pyroelectric energy meter (PE50BF-C) and recorded by Starlab software (Ophir) through two-channel Pulsar interface (Ophir). Depolarization ratio (Dpol) was examined as 1.56%. M2 value was measured by silicon CCD camera (SP620U, Ophir) and analyzed by Beamstar software (Ophir).
Fig. 1. Schematic diagram of DFC-chip TILA. Coating Type I: anti-reflection (AR) at 808 nm. Type II: high-reflection (HR) at 1064 nm. Type III: AR at 808 nm and 1064 nm. Type IV: partial reflection (PR) 40% at 1064 nm.
3. Results
Table 1 summarized preliminary tests on energy scaling-up from bulk-chip and DFC-chip under different pump energy in plano-plano micro laser cavity. Bulk-chip is 4-mm-length Nd:YAG single crystal with two-path absorption of pump energy. Output energy was 3.1 mJ under pump energy of 24 mJ [7,17]. When increasing pump energy to 243 mJ, output energy reached 14.2 mJ. After reaching pump energy over 360 mJ, no laser oscillation was observed. One of the main reasons is increased thermal load (Ph) in gain medium. Temperature distribution in gain medium Nd:YAG will be simulated and shown in part IV. Therefrom, the specially designed DFC-chip with heat dissipation from sapphire crystal was tested. As seen from Table 1, DFC-chip TILA could generate energy of 21.5 mJ under pump energy of 368 mJ. DFC-chip TILA was then installed on robot arm for laser peening [24].
Table 1. Energy scaling-up from Bulk-chip and DFC-chip in Nd:YAG micro laser cavity.
Figure 2 demonstrates energy stability from DFC-chip TILA lasting up to 21 hours. Peak-peak value of output energy was 13.1%. Standard deviation of shot-to-shot energy stability was 1.02 mJ. The 6 minutes interval of energy stability was magnified with peak-peak value of 4.4% at time resolution of 100 ms.
Fig. 2. Energy stability and peak-peak value from DFC-chip TILA. (a) 18 hours; (b) magnification interval for 6 minutes.
Figure 3 shows beam profile from DFC-chip TILA. Taira et al. firstly proposed the design and experiments of beam quality factor M2 based diode pumped solid-state laser (DPSSL) [25,26]. The heating of gain medium often causes a significant thermal lens with reduced mode matching and degraded M2. M2 value of DFC-chip TILA was measured as 11.8 and 10.7. The deviation of laser beam is partly attributed to higher order lateral modes because of refractive index difference between Nd:YAG and sapphire.
Figure 3 also shows pulse duration from passively Q-switched DFC-chip TILA. The obtained pulse duration was 665.7 ps. The resolution of horizontal axis was 10 ns/div. The peak power was then calculated as 32.3 MW. Figure 4 shows measured wavelength bandwidth Δλ of 19.2 pm centered at 1064 nm from DFC-chip TILA.
To understand the quality of DFC-chip TILA, the figure of merit (FOM) in scope of brightness and brightness temperature should be discussed [27]. Here, brightness (B) is defined as output power divided by beam quality M2-value and squared wavelength (λ) following equation $B = {P \mathord{\left/ {\vphantom {P {({{\lambda^2} \cdot M_x^2 \cdot M_y^2} )}}} \right.} {({{\lambda^2} \cdot M_x^2 \cdot M_y^2} )}}$ [3]. The brightness in DFC-chip TILA was then calculated as 22.6 TW·sr−1·cm−2. DFC-chip TILA with brightness over TW·sr−1·cm−2 level was then applied in a robot-armed system for laser peening [24].
Brightness Temperature (TB) is the temperature of a black body with the same radiant intensity at a fixed laser wavelength (λ). TB of a laser is evaluated following equation ${T_B} = P/[{{\kappa_B} \cdot \Delta \nu \cdot {{({{M^2}} )}^2}} ]$ [28]. Here, P is laser power. kB is Boltzmann constant. Δν is spectral line-width. The measured wavelength was 1064.38 nm and 1064.36 nm. TB was then calculated as 3.5 EK.
4. Discussions
To understand heat dissipation from sapphire surface in DFC-chip, stationary heat transfer with parameters of maximum temperature (Tmax) and temperature difference (ΔT) are simulated by finite element analysis (FEA) with COMSOL multiphysics software. In consideration of future optimization of DFC-structure for high-energy and high repetition rate system, DFC-chips with different Nd:YAG-plate thickness of 1 mm and 0.2 mm are discussed at various repetition rate of 10 Hz, 100 Hz and 1000 Hz. Cr:YAG is one of the heat resources for gain medium due to absorption at 1064 nm. Thus, Cr:YAG is attached to Nd:YAG directly in this simulation. Figure 5 shows geometrical structures of (a) Directly-bonded bulk-chip, (b) End-cooling bulk-chip and (c) DFC-chip. Gain medium is Nd:YAG single crystal with total length of 4 mm. Simulation parameters are shown in Appendix. It is noted that emission cross section in Nd:YAG and thermal conductivity in sapphire is temperature dependent [29–31]. Meanwhile, heat fraction (ηh), lifetime and thermal conductivity of Nd:YAG also depend on Nd3+ dopant level and temperature [32].
Fig. 5. Geometrical heat demonstration on (a) Directly-bonded bulk-chip, (b) End-cooling bulk-chip and (c) DFC-chip. Gain medium is Nd:YAG single crystal.
Figure 5 gives an example of simulation result on (a) Directly-bonded bulk-chip, (a) End-cooling bulk-chip and (c) DFC-chip at 100 Hz. Figure 6 shows absolute temperature distributions, accordingly. Nd:YAG plate thickness in DFC-chip is 1 mm. Initial temperature (T0) on copper holder is 15°C maintained with recycling water. As shown in Fig. 5, the maximum temperature (Tmax) locates on surface of Nd:YAG in (a) Directly-bonded bulk-chip. In case of (b) End-cooling bulk-chip, Tmax appears in center part of Nd:YAG. Temperature distribution on (c) DFC-chip is mild among four pieces of 1 mm-length Nd:YAG plate. According to FEA simulation result, Tmax in DFC-chip is 33.8°C, while that in End-cooling bulk-chip and Directly-bonded bulk-chip is 67.2°C and 92.5°C, respectively. Tmax in DFC-chip got 3 times reduction compared with that in directly-bonded bulk-chip.
Fig. 6. Absolute temperature distribution in Directly-bonded bulk-chip (red circle line), End-cooling bulk-chip (green diamond line) and DFC-chip (blue hollow-circle line) from passively Q-switched laser cavity at repetition rate of 100 Hz. Initial temperature T0=15°C shown as black dot-line was set on copper holder.
Table 2 shows temperature difference (ΔT) at repetition rate of 10 Hz, 100 Hz and 1,000 Hz. As seen from Table 2, ΔT at repetition rate 10 Hz is below 5°C. ΔT from Directly-bonded and End-cooling bulk-chip is 39.2°C and 31.5°C at 100 Hz. In case of DFC-chip with Nd:YAG plate thickness of 1 mm and 0.2 mm, ΔT is 8.3°C and 2.9°C at 100 Hz, respectively. It shows that ΔT could be reduced by 13.5 times in DFC structure when reducing Nd:YAG plate thickness to 0.2 mm from bulk-chip. To further understand the cooling capacity from sapphire in DFC-chip, ΔT is simulated at repetition rate 1,000 Hz. It shows that ΔT in DFC-chip with Nd:YAG plate thickness of 1 mm and 0.2 mm is 111.6°C and 34.9°C, respectively. In End-cooling and Directly-bonded bulk-chip, ΔT is 393.2°C and 458.9°C, respectively. ΔT in DFC-chip with Nd:YAG plate thickness of 0.2 mm could get 13 times reduction compared with that in Directly-bonded Nd:YAG bulk-chip. Due to high thermal conductivity of sapphire single crystal, heating could be effectively removed through face-attached sapphire heat spreader in DFC structure. It could be concluded from FEA results that heat dissipation from heat spreader (e.g. sapphire) is manifest through the design of DFC structure.
Table 2. Temperature difference (ΔT) in (a) Directly-bonded bulk-chip, (b) End-cooling bulk-chip and (c) DFC-chip at various repetition rate. The total length of Nd:YAG gain medium is 4 mm.
5. Summary
Room temperature surface activated bonding (SAB) was applied for heterogeneous substrates photonic devices. SAB based distributed face cooling (DFC) tiny integrated laser (TILA) was achieved with energy scaling-up (21.5 mJ) at sub-nanosecond (665.7ps) level for the first time. It enables the realization of high peak power (32.3 MW) from DFC-chip TILA. Drastic temperature difference (ΔT) reduction in one order of magnitude is expected in DFC structure with thinner gain medium chips by finite element analysis. DFC structured gain medium opens a brand new research field for high power laser, amplifier laser and waveguide laser. SAB-DFC-chip TILA is prospect in the development of ubiquitous high-peak power laser aiming in laser ignition, laser peening and laser-armed-robot [24].
Appendix: Simulation parameters for stationary heat transfer
Table 3 shows the parameters for simulation of stationary heat transfer in DFC-chip TILA. The thermal conductivity (κ) of sapphire as a function of temperature (T) could be expressed as κ(T)=12+125×exp(−0.0057×T)+1969×exp(−0.023×T) at temperature range of 129 K ~ 775 K [33].
Table 3. Parameters for simulation of stationary heat transfer.
Funding
Cabinet Office, Government of Japan, ImPACT Program of Council for Science, Technology and Innovation; Japan Science and Technology Agency, JST-Mirai Program (JPMJMI17A1).
Acknowledgements
The authors acknowledge supports from Dr. H. Ishizuki and Dr. Y. Sato of Institute for Molecular Science (IMS).
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