Diamond optical components for high power and high-energy laser applications

Transcription

1 Diamond optical components for high power and high-energy laser applications Eugene Anoikin*, Alexander Muhr, Andrew Bennett, Daniel J. Twitchen, Henk de Wit Element Six Technologies US Corp., 3901 Burton Drive, Santa Clara, CA USA ABSTRACT High-power and high-energy laser systems have firmly estalished their industrial presence with applications that span materials processing; high precision and high throughput manufacturing; semiconductors, and defense. Along with high average power CO 2 lasers operating at wavelengths of ~ 10 microns, solid state lasers and fier lasers operating at ~ 1 micron wavelength are now increasingly eing used, oth in the high average power and high energy pulse regimes. In recent years, polycrystalline diamond has ecome the material of choice when it comes to making optical components for multi-kilowatt CO 2 lasers at 10 micron, outperforming ZnSe due to its superior thermo-mechanical characteristics. For 1 micron laser systems, fused silica has to date een the most popular optical material owing to its outstanding optical properties. This paper characterizes high - power / high - energy performance of anti-reflection coated optical windows made of different grades of diamond (single crystal, polycrystalline) and of fused silica. Thermo-optical modeling results are also presented for water cooled mounted optical windows. Laser induced damage threshold tests are performed and analyzed. It is concluded that diamond is a superior optical material for working with extremely high-power and high-energy laser eams at 1 micron wavelength. Keywords: Diamond, fused silica, directed energy, high power laser, laser-induced damage 1. INTRODUCTION Over the past few years, chemical vapor deposition (CVD) grown diamond has een recognized as an effective material for high-power and high-energy optics. Diamond possesses a comination of desirale properties such as a very wide transmission window, low asorption, mechanical strength, chemical inertness and the highest room temperature thermal conductivity of any material. Breakthroughs over the last 15 years in diamond grown y CVD means that now a range of material grades exist for application as diverse as mechanical cutting tools to organic waste destruction. It is roadly produced into two categories polycrystalline (PC) and single crystal (SC). To date polycrystalline diamond is availale in thicknesses up to 3 mm and diameters 120 mm, in contrast single crystal diameters are typically less than 10 mm. At long wavelengths (e.g. 10 µm), polycrystalline diamond s intrinsic asorption and scatter properties approach those of single crystal. In contrast at shorter wavelengths (e.g. 1 µm), while polycrystalline s intrinsic asorption remains low, the inherent irefringence and strain associated with polycrystalline texture results in increased scatter over the equivalent single crystal material. In high power density optical applications the key property that makes diamond stand out over alternatives is its exceptionally high thermal conductivity with room temperature values in excess of 2000 W/mK possile for optical quality single crystal and polycrystalline grades. This makes diamond an exceptional material for managing extreme thermal power density loads in applications that range from semiconductor devices to transmissive and reflective optics. * eugene.anoikin@e6.com; phone ; fax ; Components and Packaging for Laser Systems, edited y Alexei L. Gleov, Paul O. Leisher, Proc. of SPIE Vol. 9346, 93460T 2015 SPIE CCC code: X/15/$18 doi: / Proc. of SPIE Vol T-1

2 Developments in fier and solid state lasers in the near infrared (NIR) and visile parts of the spectrum mean that these technologies are eing used in a range of existing applications as well as enaling new ones that include material processing and directed energy. As these applications push to higher optical power densities the demand will increase on the properties of the optical components. Its exceptionally high thermal conductivity comined with low asorption near 1 µm makes diamond very promising for use at this technologically important NIR laser wavelength region. In this article, we report the measured laser-induced damage threshold (LIDT) and results of thermal modeling on the performance of single crystal CVD diamond, polycrystalline CVD diamond and fused silica in high-optical power and high-energy density applications at 1064 nm and 1070 nm wavelengths. 2.1 Finite element analysis 2. EXPERIMENT AND MODELING Finite element thermal-optical simulations were run using the commercially availale ABAQUS software package. Two-dimensional axisymmetric simulations were designed to model thermal response of single crystal (SC) diamond, polycrystalline (PC) diamond, and fused silica optical windows under exposure to CW laser power. Constant thermal loads were defined for the ulk window material to account for sustrate asorption. In addition, constant thermal loads were defined for the front and ack sustrate surfaces to account for asorption in the anti-reflection (AR) coatings. AR coating asorption value of 0.1% was assumed in the modeling, consistent with values measured on AR coated sustrates used in this study. The simulated circular diameter windows had a radius of 2.5 mm and thickness of 1 mm (0.3 mm for single crystal diamond) and mounted to a copper holder on one side along the outer 0.5 mm assuming perfect thermal contact (i.e. no thermal arrier resistance etween the diamond and the copper lock). The copper holder is water cooled and a cooling convection coefficient of 1 kw/m 2 K and a aseline temperature of C (Fig.1) used. The convection coefficient of 1 kw/m 2 K was derived from assuming a water flow rate of 0.4 L/min at a speed of 0.2 m/s through a 6 mm diameter pipe. Constant thermal loads were defined with a Gaussian intensity of 50 µm 1/e 2 intensity radius, corresponding to the eam profile in the laser damage experiments. (d) (c) () (a) Figure 1: Finite element simulation design. (a) copper mount; () water cooling channel; (c) laser window; (d) optical axis and axis of symmetry. Material properties are given in Tale 1. Following [1], thermal conductivity of the diamond was modeled as a temperature dependent value, equal to 2242 W/mK at C and decreasing to 2003 W/mK at 50 C. Thermal conductivity of the fused silica was modeled as a constant value at 1.3 W/mK. Proc. of SPIE Vol T-2

3 Properties of CVD Diamond and Fused Silica Single Crystal (SC) CVD Diamond Polycrystalline (PC) CVD Diamond Fused Silica Thermal 300 K (W/mK) Asorption um (cm -1 ) Coefficient of Thermal Expansion (ppm / 300K) 2242 [1] 2242 [1] 1.38 [5] 1E-3 [2] 0.1 [2] 3E-7 [3] 1.0 [4] 1.0 [4] 0.50 [5] Tale 1: Properties of single crystal CVD diamond, polycrystalline CVD diamond, and fused silica 2.2 Laser-induced damage threshold (LIDT) tests Samples for LIDT tests were prepared using three different types of sustrate material modeled in the previous section. All diamond sustrates were grown and processed y Element Six [6]. Fused silica samples were commercially sourced (Heraeus Suprasil 3002). All sustrates were 5 mm x 5 mm squares with 1 mm thickness, except SC sustrates that were 0.3 mm thick. Surface finish was measured using a stylus profilometer over a length of 1 mm for all samples with no sample registering Ra (roughness average) > 6 nm. Multi-layered AR coatings were deposited on all samples y the same industrial supplier. Exact coating composition and layer thicknesses are not revealed. Values of 0.1% for reflection and 0.1% for asorption per window surface at oth 1064 and 1070 nm wavelength were measured for diamond and fused silica coatings. LIDT tests were performed in the CW and nanosecond laser pulse regimes following ISO 114. All tests were done at normal incidence. For a given CW power density or pulse energy density, 10 spots were irradiated and monitored for damage. CW power density or pulse energy density was incrementally increased until damage was detected. Linear regression was performed on the data and used to determine the damage threshold, i.e. the maximum source power / energy at which no damage would e expected. In the CW test regime, a 1070-nm wavelength laser eam was focused to produce power densities that ranged from 1 MW/cm 2 to as high as 600 MW/cm 2. A 1/e 2 spot diameter of 91 µm was used for nearly all tests. In order to fully test the coated single crystal diamond windows and it was necessary for the 1/e 2 spot diameter to e shrunk to sizes as small 17 µm. For the pulsed LIDT tests, 1064-nm wavelength laser eam was used to produce 20 ns full width half maximum (FWHM) pulses at a repetition rate of 20 Hz. The laser eam was focused to a 1/e 2 spot diameter of 9 µm to produce pulse energies from 2 J/cm 2 up to pulse energies of 38 J/cm 2. For each individual site tested 200 pulses were fired. 3.1 Theoretical finite element analysis results 3. RESULTS AND DISCUSSION Radial temperature profiles for the central 0 µm of each simulated window are shown in Figure 2. According to modeling results, very small temperature increases were found in all cases even at CW power densities as high as 12.7 MW/cm 2. This is remarkale when one considers that the total thermal load of the uncoated single crystal diamond window was roughly 1000 times that of the fused silica window due to the higher optical asorption, however this is mitigated y the superior thermal conductivity of diamond allowing the heat to e spread uniformly throughout the window and removed y the cooled mount. The additional temperature increase in the PC sample over the SC is directly related to its higher asorption coefficient, ut even at 12.7 MW/cm 2 the additional temperature increase was still less than 13 degrees. Proc. of SPIE Vol T-3

4 .1 a 12.7 MW/cm^ MW/cm^ c 12.7 MW/cm^ Figure 2: Finite element simulation results for uncoated sustrates. (a) single crystal diamond; () polycrystalline diamond; (c) fused silica. Simulations showed a much stronger heating when asorption of AR coatings was added to the model (Figures 3 and 4). Radial temperature profiles of the AR coated simulation results are shown for the central 100 µm of each window in Figure 3. These temperature profiles were taken from the top surface of the window where the most heating occurred. Very high temperature in excess of 600 C is reached in the central section of the fused silica window, even under tenfold reduced power density of 1.27 MW/cm 2. In contrast, due to its exceptional high thermal conductivity and how that leads to a reduction of heat in the coating, the coated diamond windows handle the exposure to high laser power extremely well, with tempurature increases of only 5-15 C even at CW power densities as high as 12.7 MW/cm a MW/cm^ MW/cm^ MW/cm^ MW/cm^2 Proc. of SPIE Vol T-4

5 Laser Beam Axis Laser Beam Axis 8 c MW/cm^ MW/cm^2 Figure 3: Finite element simulation results for coated sustrates. (a) single crystal diamond () polycrystalline diamond (c) fused silica Window Radius Window Radius a Figure 4: Simulated temperature distriutions shown on 2D cross sections of half of the window. CW laser power density of 1.27 MW/cm 2 is applied along vertical eam axis as shown to the left of each image. (a) AR coated single crystal diamond. Peak temperature =.66 C () AR coated fused silica. Peak temperature = C. 3.2 Experimental LIDT test results Results of the LIDT tests performed in the CW regime are shown in Figure 5. Nine windows were tested altogether: 3 SC diamond, 3 PC diamond, and 3 fused silica. SC and PC diamond windows strongly outperformed the SiO 2 windows. Damage threshold was determined to e 20 MW/cm 2 for the fused silica samples studied. In contrast, PC diamond LIDT varied from 30 to 100 MW/cm 2. Much higher LIDT values of up to 400 and 500 MW/cm 2 (20x over fused silica) were achieved for the SC diamond windows. Proc. of SPIE Vol T-5

6 Damage Percent (%) Damage Percent (%) CW Power Density (MW/cm 2 ) Single Crystal Diamond Polycrystalline Diamond Fused Silica Figure 5: CW LIDT values for each individual window tested at 1070 nm wavelength. Pulsed LIDT tests further demonstrated the advantages of CVD diamond as a sustrate material for 1 micron optics (Figure 6 and 7). 12 windows were tested altogether; 4 SC diamond, 4 PC diamond, and 4 fused silica. Results for the LIDT tests performed in the nanosecond pulse regime were similar to those performed in the CW regime in that the single crystal diamond samples were determined to possess the highest LIDT, followed y the polycrystalline diamond samples, and finally y the fused silica samples. The average LIDT for the single crystal diamond sustrates, 11.2 J/cm 2, was nearly twice the average LIDT for the fused silica sustrates, 6.4 J/cm 2. Average LIDT for the polycrystalline diamond sustrates was 7.5 J/cm 2. Figure 6 shows pulse energy density versus damage frequency for an individual (a) coated single crystal diamond sample and () coated fused silica sample. Linear regression of this raw data was used to determine LIDT for the individual samples as shown. Figure 7 compiles pulse LIDT data for the 12 tested windows. 100 a Pulse Energy Density (J/cm 2 ) Pulse Energy Density (J/cm 2 ) Figure 6: Damage frequency versus pulse energy density for (a) an AR coated single crystal diamond window and () an AR coated fused silica window. Linear regression of raw data used to determine LIDT. Proc. of SPIE Vol T-6

7 Pulse Energy Density (J/cm 2 ) Single Crystal Diamond Polycrystalline Diamond Fused Silica Average LIDT Minimum LIDT Standard Deviation Figure 7: Average LIDT, minimum LIDT, and standard deviation for each type of coated material tested in nanosecond pulse regime. Nanosecond pulse laser damage is affected y multi-photon asorption and dielectric reakdown effects in addition to thermal effects. Nevertheless, diamond windows consideraly outperformed fused silica windows in pulsed LIDT tests as well as CW, further expanding the range of high energy laser applications for this materials solution. Photos of characteristic damage sites are given in Figure 8. It is confirmed that pulse laser damage is surface damage due to destruction of the coating, while excessive CW power exposure results in catastrophic thermally driven damage of the window. 3.3 Discussion Both finite element simulations and experimental LIDT tests have estalished that CVD diamond, particularly single crystal diamond, is a highly durale and reliale sustrate material for high power optics near 1 micron wavelength. Although fused silica is much less asorent in this technologically important spectral range, the high thermal conductivity of diamond allows the material to rapidly dissipate any asored energy. In contrast, the low thermal conductivity of fused silica results in accumulation of asored energy which can easily lead to hotspots and thermally driven damage at much lower power densities. This is especially apparent when the sustrate surfaces have an antireflection coating, as these AR coatings can asor significantly more energy than the sustrate material itself while at the same time have very poor thermal properties. It is worth noting that in situations where CW power density or pulse energy is far elow levels that would result in permanent damage, it is still important for a window to remain cool for laser eam quality e.g. to minimize effects such as thermal lensing. For many applications, small eam distortions can have large impacts on process performance. Finite element simulations and LIDT tests have oth confirmed that AR coated diamond remains much cooler than AR coated fused silica under the same operating conditions. Likewise, much less eam distortion would e expected from an AR coated diamond window than an AR coated fused silica window. For a system designer to fully take advantage of diamond s exceptionally high thermal conductivity, it is important for a diamond window to e paired with an efficient mounting and cooling scheme. It can e expected that increasing the efficiency of heat removal will have a large impact on a diamond window s operating temperature due to the material s high thermal conductivity. In contrast, it can e expected that increasing the efficiency of heat removal will have a much smaller impact on the operating temperature of a fused silica window due to the material s low thermal conductivity. Proc. of SPIE Vol T-7

8 a c d 335.8micron Figure 8: Laser damage sites on AR coated single crystal diamond and fused silica sustrates. (a) CW damage on AR coated single crystal diamond. LIDT = 500 MW/cm 2. Beam diameter = 91µm () CW damage on AR coated fused silica. LIDT = 20 MW/cm 2. Beam diameter = 91 µm (c) Pulsed damage on AR coated single crystal diamond. LIDT = J/cm 2. Beam diameter = 9 µm (d) pulsed damage on AR coated fused silica. LIDT = 3.71 J/cm 2. Beam diameter = 9 µm. 4. SUMMARY LIDT tests at 1064 and 1070 nm performed on AR coated diamond and fused silica windows confirmed the general conclusions made y the finite element simulations. Diamond windows outperformed silica windows in all tests. In particular, single crystal diamond windows demonstrated remarkaly high LIDT values of MW/cm 2 in CW tests at 1070 nm, more than 20x over fused silica. As the industry trends towards higher power and higher energy lasers near 1 micron wavelength, it is expected that CVD diamond optics will enale new systems and applications. Featuring an unrivaled comination of low asorption, high thermal conductivity, mechanical strength, and high LIDT, CVD diamond can provide solutions where other materials fail. Proc. of SPIE Vol T-8

9 REFERENCES [1] Godfried, H.P., Coe, S.E., Hall, C.E., Pickles, C.S.J., Sussmann, R.S., Tang, X., and van der Voorden, W.K., Use of CVD diamond in high-power CO 2 lasers and laser diode arrays, Proc. SPIE, 3889, (2000). [2] Friel, I., [Optical Engineering of Diamond], Wiley-VCH Verlag GmH & Co. KGaA., Weinheim, (2013). [3] Suprasil 3001 and 3002, Heraeus Holding GmH, April 2013, < puritysyntheticfusedsilica.pdf [4] Optical Brochure, Element Six Technologies, 2014, < Home>, _Optical.pdf?MOD=AJPERES&CACHEID=1d1f8101-c f-a310-c819a000f9 [5] Thermal Properties Heraeus Holding GmH, 2014, < [6] Balmer, R.S., Brandon, J.R., Clewes, S.L., Dhillon, H.K., Dodson, J.M., Friel, I., Inglis, P.N., Madgwick, T.D., Markham, M.L., Mollart, T.P., Perkins, N., Scarsrook, G.A., Twitchen, D.J., Whitehead, A.J., Wilman, J.J., Woollard, S.M., Chemical vapour deposition synthetic diamond: materials, technology and applications, J. Phys.: Condens. Matter 21, (2009) Proc. of SPIE Vol T-9