Diffraction of volume Bragg gratings under high flux laser irradiation Xiang Zhang,1,2,3 Jiansheng Feng,1,3 Baoxing Xiong,1,2,3 Kuaisheng Zou,1,2,3 Xiao Yuan1,2,3,* 1 Institute of Modern Optical Technologies, Soochow University, Suzhou, Jiangsu, 215006 China Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province, Soochow University, Suzhou, Jiangsu, 215006 China 3 Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou, Jiangsu, 215006 China * [email protected] 2
Abstract: Diffraction property of transmitting volume Bragg gratings (VBGs) recorded in photo-thermo-refractive glass (PTR) is studied under the irradiation of a continuous-wave fiber laser with flux of 1274 W/cm2. Dependence of temperature characteristics of VBGs prepared by different crystallization temperatures is presented. When temperature of VBGs rises up to 33°C, there are a 2.7% reduction and 1.59% ripple of diffraction efficiency for VBGs. The period variation caused by the thermal expansion of VBGs is used to explain the reduction of diffraction efficiency, and experimental results are in agreement with theoretical analysis. ©2014 Optical Society of America OCIS codes: (140.3330) Laser damage; (090.7330) Volume gratings; (060.3510) Lasers, fiber
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
O. M. Efimov, L. B. Glebov, L. N. Glebova, K. C. Richardson, and V. I. Smirnov, “High-efficiency Bragg gratings in photothermorefractive glass,” Appl. Opt. 38(4), 619–627 (1999). X. Zhang, X. Yuan, S. Wu, J. S. Feng, K. Zou, and G. Zhang, “Two-dimensional angular filtering by volume Bragg gratings in photothermorefractive glass,” Opt. Lett. 36(11), 2167–2169 (2011). B. L. Volodin, S. V. Dolgy, E. D. Melnik, E. Downs, J. Shaw, and V. S. Ban, “Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings,” Opt. Lett. 29(16), 1891–1893 (2004). B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Tunable single-longitudinal-mode ErYb:glass laser locked by a bulk glass Bragg grating,” Opt. Lett. 31(11), 1663–1665 (2006). T. Y. Chung, A. Rapaport, V. Smirnov, L. B. Glebov, M. C. Richardson, and M. Bass, “Solid-state laser spectral narrowing using a volumetric photothermal refractive Bragg grating cavity mirror,” Opt. Lett. 31(2), 229–231 (2006). P. Jelger and F. Laurell, “Efficient narrow-linewidth volume-Bragg grating-locked Nd:fiber laser,” Opt. Express 15(18), 11336–11340 (2007). O. M. Efimov, L. B. Glebov, S. Papernov, and A. W. Schmid, “Laser-induced damage of photo-thermorefractive glass for optical holographic element writing,” Proc. SPIE 3578, 564–575 (1999). A. Jain, D. Drachenberg, O. Andrusyak, G. Venus, V. Smirnov, and L. Glebov, “Coherent and spectral beam combining of fiber lasers using volume Bragg gratings,” in Proceedings of the SPIE, Laser Technology for Defense and Security VI 7686(1), M. Dubinskii and S. G. Post, eds., 768615 (2010). I. V. Ciapurin, L. B. Glebov, L. N. Glebova, V. I. Smirnov, and E. V. Rotari, “Incoherent combining of 100 W Yb-fiber laser beams by PTR Bragg grating,” Proc. SPIE 4974, 209–219 (2003). O. Andrusyak, I. Ciapurin, V. Smirnov, G. Venus, N. Vorobiev, and L. Glebov, “External and common-cavity high spectral density beam combining of high power fiber lasers,” Proc. SPIE 6873, 687314 (2008). S. Tjörnhammar, B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Thermal limitations of volume Bragg gratings used in lasers for spectral control,” J. Opt. Soc. Am. B 30(6), 1402–1409 (2013). O. Andrusyak, V. Smirnov, G. Venus, V. Rotar, and L. Glebov, “Spectral combining and coherent coupling of lasers by volume Bragg gratings,” IEEE J. Sel. Top. Quantum Electron. 15(2), 344–353 (2009). J. Lumeau, L. Glebova, and L. B. Glebov, “Influence of UV exposure on the crystallization and optical properties of photo-thermo-refractive glass,” J. Non-Cryst. Solids 354(2-9), 425–430 (2008). O. G. Andrusyak, “Dense spectral beam combining with volume Bragg gratings in photo-thermo-refractive glass,” Ph.D. thesis, University of Central Florida, 2009.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8291
15. J. Lumeau, L. Glebova, and L. B. Glebov, “Near-IR absorption in high-purity photothermorefractive glass and holographic optical elements: measurement and application for high-energy lasers,” Appl. Opt. 50(30), 5905– 5911 (2011). 16. B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Single longitudinal-mode Nd-laser with a Bragg grating Fabry– Perot cavity,” Opt. Express 14(20), 9284–9292 (2006). 17. T. Waritanant and T.-Y. Chung, “Influence of minute self-absorption of a volume Bragg grating used as a laser mirror,” IEEE J. Quantum Electron. 47(3), 390–397 (2011). 18. L. B. Glebov, “Photosensitive glass for phase hologram recording,” Glastech. Ber. Glass Sci. Technol. 71C, 85– 90 (1998). 19. B. X. Xiong, X. Yuan, K. S. Zou, J. S. Feng, X. Zhang, and G. J. Zhang, “Characteristics on the photo-thermalrefractive glass and volume Bragg gratings,” Acta Opt. Sin. 32(8), 0816001 (2012). 20. T. Cardinal, O. M. Efimov, H. G. Francois-Saint-Cyr, L. B. Glebov, L. N. Glebova, and V. I. Smirnov, “Comparative study of photo-induced variations of X-ray diffraction and refractive index in photo-thermorefractive glass,” J. Non-Cryst. Solids 325(1-3), 275–281 (2003). 21. O. M. Efimov, L. B. Glebov, V. I. Smirnov, and L. Glebova, “Process for production of high efficiency volume diffractive elements in photo-thermo-refractive glass” United States Patent 6586141(2004)
1. Introduction Volume Bragg grating (VBG) recorded in photo-thermo-refractive (PTR) glass has good optical and mechanical properties. It can achieve a diffraction efficiency exceeding 99%, sensitive angular selectivity from 0.1mrad to 100mrad and narrow diffraction spectral width from 0.01nm to 20nm [1]. VBGs have been widely used in many advanced laser applications, such as angular filtering [2], output spectrum stabilization and narrowing of diode lasers [3], output mirrors in solid-state lasers [4,5] and fiber lasers [6]. Surface damage threshold of VBGs recorded in PTR glass in high peak power pulse laser operation has been measured of 11 J/cm2 and 40 J/cm2 for 1 ns and 8 ns pulses at the wavelength of 1054 nm [7,8], and VBGs subjected to 100 kW/cm2 of high average power laser irradiation at 1096nm for a focused laser of 100 W shows no laser-induced damage [9]. For applications in laser beam combining or near-field filtering in continuous wave (CW) laser, the irradiation laser power on VBGs may easily reach the order of hundreds of watts [10], and beam spot for high power high energy laser will be a few millimeters to tens of millimeters in diameter. As the key elements in these applications, VBGs’ structure and diffraction properties are primarily influenced by thermal effects due to residual absorption in the PTR glass [11]. The transmission spectrum of original PTR glass used for VBG recording is from 400 to 2700 nm [12], while the absorption of VBGs recorded in PTR glass, depends strongly on the fabrication process [13]. Recently reported values of the absorption in VBGs are 1.5 × 10−3 cm−1 [14] and 1 × 10−4 cm−1 [15]. When the reflecting VBG is irradiated by high peak power laser, a slight redshift in the peak diffraction and reduction in diffraction efficiency with increasing laser power are caused by the temperature rising of the VBG due to absorption [16,17]. However, the dependence of transmitting VBGs diffraction property with high average power laser irradiation has not been reported. On the other hand, tiny spots of tens of micrometer level were used to study VBGs performance with high average power laser. In Ref [9], the input laser power is of 100W, and flux is of about 100kW/cm2 but the beam diameter is only about 17.8μm, which cannot be effectively used to characterize the VBGs diffraction property in high average power laser applications with large beam diameter. In this paper, transmitting VBGs recorded in PTR glass are prepared by different crystallization temperatures. The dependence of transmittance and diffraction efficiency for VBGs with different crystallization temperatures is studied. A continuous wave fiber laser of 1274 W/cm2 is used in the laser irradiation experiment, and the results of diffraction property characteristics with temperature rising caused by high power laser irradiation are given. Moreover, the slight variation in period of VBGs is used to explain the reduction in diffraction efficiency of VBGs, which is in good agreement with the experimental data.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8292
2. Fabrication of transmitting VBGs Transmitting VBGs in PTR glass are recorded with a He-Cd laser (Kimmon Electric Model IK3501R-G) at the wavelength of 325 nm and output power of 50 mW. After UV exposure, the cerium ions with valence three in PTR glass are translated into cerium ions with valence four and release an electron simultaneously. The silver ions in PTR glass catch the released electron and change to silver atoms, as shown in Fig. 1(a) [18]. Then, the interference pattern is written into PTR glass, the period of interference pattern is controlled by intersection angle of dual-laser beams. Refractive index of PTR glass only decreases about several ppm (Parts Per Million, 10−6) after exposure, and the diffraction efficiency is less than 1%. The high diffraction efficiency, which requires refractive index modulation varying from dozens to hundreds of ppm, is obtained by the thermal development [18]. The thermal development used in our research is called as the “two-step method” [19]. In the first step called as nucleation process, the silver atoms in exposed PTR glass get together to form silver clusters at nucleation temperature, as shown in Fig. 1(b). The second step is called as crystallization process. The sodium fluoride molecules in PTR glass crystallize around the silver clusters at crystallization temperature, as shown in Fig. 1(c). Refractive index of sodium fluoride microcrystalization is much lower than that of the unexposed PTR glass; therefore high enough refractive index modulation is obtained to achieve high diffraction efficiency.
Fig. 1. Principle of VBG preparation. (a) Photosensitive process of PTR glass; (b) Nucleation process; (c) Crystallization process. Table 1. Preparation Technics and Performance of VBGs Preparation technics
VBG-1 2
VBG-2 2
VBG-3
Exposure dosage
1.6J/cm
1.6J/cm
1.6J/cm2
Nucleation temperature
480°C
480°C
480°C 4 hours
Nucleation time
4 hours
4 hours
Crystallization temperature
570°C
580°C
590°C
Crystallization time
6 hours
6 hours
6 hours
Transmittance
85.78%
83.11%
82.55%
Diffraction efficiency
72.7%
73.1%
74.4%
VBGs performance
The transmittance of VBG is defined as: T=
I Transmitted I Incident
(1)
and the diffraction efficiency of VBG is defined as:
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8293
η=
I Diffraction I Incident
(2)
where, the IDiffraction is the intensity of diffracted beam through the VBG, the ITransmitted is the intensity of transmitted beam through the VBG, and the Iincident is the intensity of incident beam. The VBGs have the same structural parameters, such as the thickness of 2.8mm, the tilt angle of about 0° and period of 1.13μm. The transmittance and diffraction efficiency of VBGs are measured at the wavelength of 532nm. The preparation technics and performance of prepared VBGs in our researches are shown in Table 1. With the crystallization temperature rising, transmittance of VBGs slightly reduce while the diffraction efficiency of VBGs increasing. The refractive index modulation of VBGs is determined by the concentration of crystalline phase and the size of NaF crystals [20]. In this situation, the distribution of silver clusters in VBGs keeps consistent due to the same nucleation temperature and time. In such a case, refractive index modulation of VBGs is not influenced by the concentration of crystalline phase, and only determined by the size of NaF crystals controlled by crystallization temperature in the same crystallization time [19]. Then, the higher refractive index modulation of VBGs is achieved with the larger size of NaF crystals determined by raising crystallization temperature. Therefore, the VBG-3 prepared at the higher crystallization temperature of 590°C has the maximum diffraction efficiency of 74.4%. However, the stronger scattering is caused by the both concentration of crystalline phase and size of NaF crystals, as shown in Fig. 2. Therefore, the VBG-3 has the minimum transmittance of 82.55%.
Fig. 2. Dependence of Scattering of VBGs with different crystallization temperatures at 570°C (a), 580°C (b) and 570°C (c).
3. Experimental The schematic diagram of high power fiber laser irradiated VBGs experiment is shown in Fig. 3. A collimated fiber laser with wavelength of 1080nm, output power of 1kW and beam diameter of 10mm is irradiated on VBGs. The average irradiance power density is about 1274 W/cm2. The laser beam used to monitor the diffraction efficiency of VBGs is a continuous wave (CW) frequency-doubled YAG laser with the wavelength of 532nm and output power of 2 W. The near-field beam profile was shaped with a soft-edged aperture in order to obtain a collimated and uniform probe beam, and the probe beam is incident on VBGs at Bragg angle. Temperature rising of VBGs is recorded by the thermal imaging system with a sample interval of 1 second (InfraTec, VarioCAM®hr research). The power meter (Coherent, LabMax-TOP, and PM3Q) is used to measure the power of the diffraction beam and input beam. The sample interval of power meter is 2.5 seconds.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8294
Fig. 3. Schematic diagram of diffraction of VBG under high flux laser irradiation.
With continuous wave laser flux of 1274W/cm2 irradiating, temperature characteristics of VBGs prepared by different crystallization temperatures is shown in Fig. 4. It can be seen that the temperature on VBGs rises rapidly at the first tens of seconds and then keeps at the steady situation from 90s to 180s. The heating-up time and maximum temperatures rising for VBG1, VBG-2 and VBG-3 are 73 seconds and 14°C, 87 seconds and 20 °C and 93 seconds and 33°C, respectively. When the high flux laser irradiation stops, the temperature on VBGs in the irradiated areas quickly reduces to room temperature in 16 seconds, 18 seconds and 24 seconds, respectively. Under the same irradiation flux, residual absorptions in VBGs are proportional to the concentration of crystalline phase and size of NaF crystals. The experiment shows that heating-up rate and maximum temperature rising of VBGs is faster and higher with the crystallization temperature increasing.
Fig. 4. Dependence of VBGs temperature with laser irradiation time.
The normalized diffraction efficiency of VBGs during laser irradiation is shown in Fig. 5, and the diffraction efficiency is almost the same during the laser irradiation from 90s to 180s, which corresponds to the temperature rising steady situation. However, the diffraction efficiency of VBG-2 and VBG-3 slowly reduces, as shown in the insertion in Fig. 5. Under the high flux laser irradiation, the period of grating is slightly changed due to thermal expansion, and the Bragg angle of VBGs is modified, which results in part of the wavelength components of the incident laser off the Bragg condition. As a result, the diffraction efficiency of VBGs drops. Besides, the diffraction efficiency reduction is influenced by crystallization temperatures, as shown in the insertion in Fig. 5. The measured reduction of diffraction efficiency for VBG-2 and VBG-3 are about 0.68% and 2.7%, respectively.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8295
Fig. 5. Dependence of normalization diffraction efficiency of VBGs with irradiation time.
Thermal expansion coefficient of 8.5 × 10−6/K−1 in PTR glass was reported in Ref [21]. Under the high flux laser irradiation, the variation of grating period for VBG-2 at temperature rising of 20°C and VBG-3 at temperature rising of 33°C is about 194 × 10−6 μm and 312 × 10−6 μm, respectively. Assume that the refractive index of VBG is 1.5, Bragg condition in air can be written as sin θ =
λ
(3)
3Λ
where θ is the Bragg angle, the λ is Bragg wavelength and the Λ is grating period. We can obtain that the deviation of Bragg angle for VBG-2 and VBG-3 is 0.027mrad and 0.05mrad, respectively. Therefore, the reduction of diffraction efficiency for VBG-2 and VBG-3 is 0.7% and 2.6%, as shown in Fig. 6, and the theoretical calculation for diffraction efficiency reduction is in good agreement with the measured data. 1 1.005 1
Diffraction efficiency
Diffraction efficiency
0.9 0.8 0.7 0.6 0.5 0.4
0.7%
0.995 0.99
99.3%
0.985
2.6%
0.98 0.975
97.4%
0.97
0.3
0.965 0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Deviation from Bragg angle (mrad)
0.2 0.1 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Deviation from Bragg angle (mrad) Fig. 6. Angular selectivity of transmitting VBGs.
Besides, there are obvious ripples in diffraction efficiency of VBGs, as shown in the insertion of Fig. 5. The ripples are impermissible in high power laser applications, such as the spectral combining and near-filed filtering. The factor α is used to evaluate the ripple intensity of diffraction efficiency, and α is defined as:
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8296
α=
η max − η min η max + η min
(4)
where ηmax is the maximum diffraction efficiency of VBGs, and ηmin the minimum diffraction efficiency of VBGs. The experimental results show that the ripple of diffraction efficiency increases with the higher crystallization temperature of VBGs. The diffraction efficiency ripple intensity of VBGs with the crystallization temperature of 580°C and 590°C is 1.13% and 1.59%, while the ripple intensity with the crystallization temperature of 570°C is 0.45%. It can be seen that ripple intensity of diffraction efficiency increases 2.5 and 5.3 times with the crystallization temperature rising of 10°C and 20°C, respectively. 4. Conclusion The dual-beam interference exposure and the “two step” thermal development method are used to fabricate transmitting VBGs in photo-thermo-refractive glass. The diffraction efficiency of VBGs is increasing at the crystallization temperature from 570°C to 590°C. The temperature rising of VBGs during the high flux laser irradiation increases with crystallization temperature. The diffraction efficiency is almost the same when temperature rising of VBGs is in the steady situation, but the diffraction efficiency of VBGs prepared at higher crystallization temperature drops slightly under laser irradiation. The reduction of diffraction efficiency of VBGs is explained with the variation of grating period, and the theoretical calculation is in good agreement with the experimental data. Acknowledgments This work is supported by NSFC under the contract Nos. of 91023009 and 61275140, the united foundation between NSFC and Chinese Academy of Engineering Physics under contract nos. of 11176021, 11076021 and 10876011, the Natural Science Foundation of Jiangsu Higher Education Institutions under contract 10KJA140045, a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the national High-Tech 863 program of China and the Graduate Research and Innovation Project of Jiangsu Province (CXZZ11_0095).
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8297
Abstract: Diffraction property of transmitting volume Bragg gratings (VBGs) recorded in photo-thermo-refractive glass (PTR) is studied under the irradiation of a continuous-wave fiber laser with flux of 1274 W/cm2. Dependence of temperature characteristics of VBGs prepared by different crystallization temperatures is presented. When temperature of VBGs rises up to 33°C, there are a 2.7% reduction and 1.59% ripple of diffraction efficiency for VBGs. The period variation caused by the thermal expansion of VBGs is used to explain the reduction of diffraction efficiency, and experimental results are in agreement with theoretical analysis. ©2014 Optical Society of America OCIS codes: (140.3330) Laser damage; (090.7330) Volume gratings; (060.3510) Lasers, fiber
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
O. M. Efimov, L. B. Glebov, L. N. Glebova, K. C. Richardson, and V. I. Smirnov, “High-efficiency Bragg gratings in photothermorefractive glass,” Appl. Opt. 38(4), 619–627 (1999). X. Zhang, X. Yuan, S. Wu, J. S. Feng, K. Zou, and G. Zhang, “Two-dimensional angular filtering by volume Bragg gratings in photothermorefractive glass,” Opt. Lett. 36(11), 2167–2169 (2011). B. L. Volodin, S. V. Dolgy, E. D. Melnik, E. Downs, J. Shaw, and V. S. Ban, “Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings,” Opt. Lett. 29(16), 1891–1893 (2004). B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Tunable single-longitudinal-mode ErYb:glass laser locked by a bulk glass Bragg grating,” Opt. Lett. 31(11), 1663–1665 (2006). T. Y. Chung, A. Rapaport, V. Smirnov, L. B. Glebov, M. C. Richardson, and M. Bass, “Solid-state laser spectral narrowing using a volumetric photothermal refractive Bragg grating cavity mirror,” Opt. Lett. 31(2), 229–231 (2006). P. Jelger and F. Laurell, “Efficient narrow-linewidth volume-Bragg grating-locked Nd:fiber laser,” Opt. Express 15(18), 11336–11340 (2007). O. M. Efimov, L. B. Glebov, S. Papernov, and A. W. Schmid, “Laser-induced damage of photo-thermorefractive glass for optical holographic element writing,” Proc. SPIE 3578, 564–575 (1999). A. Jain, D. Drachenberg, O. Andrusyak, G. Venus, V. Smirnov, and L. Glebov, “Coherent and spectral beam combining of fiber lasers using volume Bragg gratings,” in Proceedings of the SPIE, Laser Technology for Defense and Security VI 7686(1), M. Dubinskii and S. G. Post, eds., 768615 (2010). I. V. Ciapurin, L. B. Glebov, L. N. Glebova, V. I. Smirnov, and E. V. Rotari, “Incoherent combining of 100 W Yb-fiber laser beams by PTR Bragg grating,” Proc. SPIE 4974, 209–219 (2003). O. Andrusyak, I. Ciapurin, V. Smirnov, G. Venus, N. Vorobiev, and L. Glebov, “External and common-cavity high spectral density beam combining of high power fiber lasers,” Proc. SPIE 6873, 687314 (2008). S. Tjörnhammar, B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Thermal limitations of volume Bragg gratings used in lasers for spectral control,” J. Opt. Soc. Am. B 30(6), 1402–1409 (2013). O. Andrusyak, V. Smirnov, G. Venus, V. Rotar, and L. Glebov, “Spectral combining and coherent coupling of lasers by volume Bragg gratings,” IEEE J. Sel. Top. Quantum Electron. 15(2), 344–353 (2009). J. Lumeau, L. Glebova, and L. B. Glebov, “Influence of UV exposure on the crystallization and optical properties of photo-thermo-refractive glass,” J. Non-Cryst. Solids 354(2-9), 425–430 (2008). O. G. Andrusyak, “Dense spectral beam combining with volume Bragg gratings in photo-thermo-refractive glass,” Ph.D. thesis, University of Central Florida, 2009.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8291
15. J. Lumeau, L. Glebova, and L. B. Glebov, “Near-IR absorption in high-purity photothermorefractive glass and holographic optical elements: measurement and application for high-energy lasers,” Appl. Opt. 50(30), 5905– 5911 (2011). 16. B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Single longitudinal-mode Nd-laser with a Bragg grating Fabry– Perot cavity,” Opt. Express 14(20), 9284–9292 (2006). 17. T. Waritanant and T.-Y. Chung, “Influence of minute self-absorption of a volume Bragg grating used as a laser mirror,” IEEE J. Quantum Electron. 47(3), 390–397 (2011). 18. L. B. Glebov, “Photosensitive glass for phase hologram recording,” Glastech. Ber. Glass Sci. Technol. 71C, 85– 90 (1998). 19. B. X. Xiong, X. Yuan, K. S. Zou, J. S. Feng, X. Zhang, and G. J. Zhang, “Characteristics on the photo-thermalrefractive glass and volume Bragg gratings,” Acta Opt. Sin. 32(8), 0816001 (2012). 20. T. Cardinal, O. M. Efimov, H. G. Francois-Saint-Cyr, L. B. Glebov, L. N. Glebova, and V. I. Smirnov, “Comparative study of photo-induced variations of X-ray diffraction and refractive index in photo-thermorefractive glass,” J. Non-Cryst. Solids 325(1-3), 275–281 (2003). 21. O. M. Efimov, L. B. Glebov, V. I. Smirnov, and L. Glebova, “Process for production of high efficiency volume diffractive elements in photo-thermo-refractive glass” United States Patent 6586141(2004)
1. Introduction Volume Bragg grating (VBG) recorded in photo-thermo-refractive (PTR) glass has good optical and mechanical properties. It can achieve a diffraction efficiency exceeding 99%, sensitive angular selectivity from 0.1mrad to 100mrad and narrow diffraction spectral width from 0.01nm to 20nm [1]. VBGs have been widely used in many advanced laser applications, such as angular filtering [2], output spectrum stabilization and narrowing of diode lasers [3], output mirrors in solid-state lasers [4,5] and fiber lasers [6]. Surface damage threshold of VBGs recorded in PTR glass in high peak power pulse laser operation has been measured of 11 J/cm2 and 40 J/cm2 for 1 ns and 8 ns pulses at the wavelength of 1054 nm [7,8], and VBGs subjected to 100 kW/cm2 of high average power laser irradiation at 1096nm for a focused laser of 100 W shows no laser-induced damage [9]. For applications in laser beam combining or near-field filtering in continuous wave (CW) laser, the irradiation laser power on VBGs may easily reach the order of hundreds of watts [10], and beam spot for high power high energy laser will be a few millimeters to tens of millimeters in diameter. As the key elements in these applications, VBGs’ structure and diffraction properties are primarily influenced by thermal effects due to residual absorption in the PTR glass [11]. The transmission spectrum of original PTR glass used for VBG recording is from 400 to 2700 nm [12], while the absorption of VBGs recorded in PTR glass, depends strongly on the fabrication process [13]. Recently reported values of the absorption in VBGs are 1.5 × 10−3 cm−1 [14] and 1 × 10−4 cm−1 [15]. When the reflecting VBG is irradiated by high peak power laser, a slight redshift in the peak diffraction and reduction in diffraction efficiency with increasing laser power are caused by the temperature rising of the VBG due to absorption [16,17]. However, the dependence of transmitting VBGs diffraction property with high average power laser irradiation has not been reported. On the other hand, tiny spots of tens of micrometer level were used to study VBGs performance with high average power laser. In Ref [9], the input laser power is of 100W, and flux is of about 100kW/cm2 but the beam diameter is only about 17.8μm, which cannot be effectively used to characterize the VBGs diffraction property in high average power laser applications with large beam diameter. In this paper, transmitting VBGs recorded in PTR glass are prepared by different crystallization temperatures. The dependence of transmittance and diffraction efficiency for VBGs with different crystallization temperatures is studied. A continuous wave fiber laser of 1274 W/cm2 is used in the laser irradiation experiment, and the results of diffraction property characteristics with temperature rising caused by high power laser irradiation are given. Moreover, the slight variation in period of VBGs is used to explain the reduction in diffraction efficiency of VBGs, which is in good agreement with the experimental data.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8292
2. Fabrication of transmitting VBGs Transmitting VBGs in PTR glass are recorded with a He-Cd laser (Kimmon Electric Model IK3501R-G) at the wavelength of 325 nm and output power of 50 mW. After UV exposure, the cerium ions with valence three in PTR glass are translated into cerium ions with valence four and release an electron simultaneously. The silver ions in PTR glass catch the released electron and change to silver atoms, as shown in Fig. 1(a) [18]. Then, the interference pattern is written into PTR glass, the period of interference pattern is controlled by intersection angle of dual-laser beams. Refractive index of PTR glass only decreases about several ppm (Parts Per Million, 10−6) after exposure, and the diffraction efficiency is less than 1%. The high diffraction efficiency, which requires refractive index modulation varying from dozens to hundreds of ppm, is obtained by the thermal development [18]. The thermal development used in our research is called as the “two-step method” [19]. In the first step called as nucleation process, the silver atoms in exposed PTR glass get together to form silver clusters at nucleation temperature, as shown in Fig. 1(b). The second step is called as crystallization process. The sodium fluoride molecules in PTR glass crystallize around the silver clusters at crystallization temperature, as shown in Fig. 1(c). Refractive index of sodium fluoride microcrystalization is much lower than that of the unexposed PTR glass; therefore high enough refractive index modulation is obtained to achieve high diffraction efficiency.
Fig. 1. Principle of VBG preparation. (a) Photosensitive process of PTR glass; (b) Nucleation process; (c) Crystallization process. Table 1. Preparation Technics and Performance of VBGs Preparation technics
VBG-1 2
VBG-2 2
VBG-3
Exposure dosage
1.6J/cm
1.6J/cm
1.6J/cm2
Nucleation temperature
480°C
480°C
480°C 4 hours
Nucleation time
4 hours
4 hours
Crystallization temperature
570°C
580°C
590°C
Crystallization time
6 hours
6 hours
6 hours
Transmittance
85.78%
83.11%
82.55%
Diffraction efficiency
72.7%
73.1%
74.4%
VBGs performance
The transmittance of VBG is defined as: T=
I Transmitted I Incident
(1)
and the diffraction efficiency of VBG is defined as:
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8293
η=
I Diffraction I Incident
(2)
where, the IDiffraction is the intensity of diffracted beam through the VBG, the ITransmitted is the intensity of transmitted beam through the VBG, and the Iincident is the intensity of incident beam. The VBGs have the same structural parameters, such as the thickness of 2.8mm, the tilt angle of about 0° and period of 1.13μm. The transmittance and diffraction efficiency of VBGs are measured at the wavelength of 532nm. The preparation technics and performance of prepared VBGs in our researches are shown in Table 1. With the crystallization temperature rising, transmittance of VBGs slightly reduce while the diffraction efficiency of VBGs increasing. The refractive index modulation of VBGs is determined by the concentration of crystalline phase and the size of NaF crystals [20]. In this situation, the distribution of silver clusters in VBGs keeps consistent due to the same nucleation temperature and time. In such a case, refractive index modulation of VBGs is not influenced by the concentration of crystalline phase, and only determined by the size of NaF crystals controlled by crystallization temperature in the same crystallization time [19]. Then, the higher refractive index modulation of VBGs is achieved with the larger size of NaF crystals determined by raising crystallization temperature. Therefore, the VBG-3 prepared at the higher crystallization temperature of 590°C has the maximum diffraction efficiency of 74.4%. However, the stronger scattering is caused by the both concentration of crystalline phase and size of NaF crystals, as shown in Fig. 2. Therefore, the VBG-3 has the minimum transmittance of 82.55%.
Fig. 2. Dependence of Scattering of VBGs with different crystallization temperatures at 570°C (a), 580°C (b) and 570°C (c).
3. Experimental The schematic diagram of high power fiber laser irradiated VBGs experiment is shown in Fig. 3. A collimated fiber laser with wavelength of 1080nm, output power of 1kW and beam diameter of 10mm is irradiated on VBGs. The average irradiance power density is about 1274 W/cm2. The laser beam used to monitor the diffraction efficiency of VBGs is a continuous wave (CW) frequency-doubled YAG laser with the wavelength of 532nm and output power of 2 W. The near-field beam profile was shaped with a soft-edged aperture in order to obtain a collimated and uniform probe beam, and the probe beam is incident on VBGs at Bragg angle. Temperature rising of VBGs is recorded by the thermal imaging system with a sample interval of 1 second (InfraTec, VarioCAM®hr research). The power meter (Coherent, LabMax-TOP, and PM3Q) is used to measure the power of the diffraction beam and input beam. The sample interval of power meter is 2.5 seconds.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8294
Fig. 3. Schematic diagram of diffraction of VBG under high flux laser irradiation.
With continuous wave laser flux of 1274W/cm2 irradiating, temperature characteristics of VBGs prepared by different crystallization temperatures is shown in Fig. 4. It can be seen that the temperature on VBGs rises rapidly at the first tens of seconds and then keeps at the steady situation from 90s to 180s. The heating-up time and maximum temperatures rising for VBG1, VBG-2 and VBG-3 are 73 seconds and 14°C, 87 seconds and 20 °C and 93 seconds and 33°C, respectively. When the high flux laser irradiation stops, the temperature on VBGs in the irradiated areas quickly reduces to room temperature in 16 seconds, 18 seconds and 24 seconds, respectively. Under the same irradiation flux, residual absorptions in VBGs are proportional to the concentration of crystalline phase and size of NaF crystals. The experiment shows that heating-up rate and maximum temperature rising of VBGs is faster and higher with the crystallization temperature increasing.
Fig. 4. Dependence of VBGs temperature with laser irradiation time.
The normalized diffraction efficiency of VBGs during laser irradiation is shown in Fig. 5, and the diffraction efficiency is almost the same during the laser irradiation from 90s to 180s, which corresponds to the temperature rising steady situation. However, the diffraction efficiency of VBG-2 and VBG-3 slowly reduces, as shown in the insertion in Fig. 5. Under the high flux laser irradiation, the period of grating is slightly changed due to thermal expansion, and the Bragg angle of VBGs is modified, which results in part of the wavelength components of the incident laser off the Bragg condition. As a result, the diffraction efficiency of VBGs drops. Besides, the diffraction efficiency reduction is influenced by crystallization temperatures, as shown in the insertion in Fig. 5. The measured reduction of diffraction efficiency for VBG-2 and VBG-3 are about 0.68% and 2.7%, respectively.
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8295
Fig. 5. Dependence of normalization diffraction efficiency of VBGs with irradiation time.
Thermal expansion coefficient of 8.5 × 10−6/K−1 in PTR glass was reported in Ref [21]. Under the high flux laser irradiation, the variation of grating period for VBG-2 at temperature rising of 20°C and VBG-3 at temperature rising of 33°C is about 194 × 10−6 μm and 312 × 10−6 μm, respectively. Assume that the refractive index of VBG is 1.5, Bragg condition in air can be written as sin θ =
λ
(3)
3Λ
where θ is the Bragg angle, the λ is Bragg wavelength and the Λ is grating period. We can obtain that the deviation of Bragg angle for VBG-2 and VBG-3 is 0.027mrad and 0.05mrad, respectively. Therefore, the reduction of diffraction efficiency for VBG-2 and VBG-3 is 0.7% and 2.6%, as shown in Fig. 6, and the theoretical calculation for diffraction efficiency reduction is in good agreement with the measured data. 1 1.005 1
Diffraction efficiency
Diffraction efficiency
0.9 0.8 0.7 0.6 0.5 0.4
0.7%
0.995 0.99
99.3%
0.985
2.6%
0.98 0.975
97.4%
0.97
0.3
0.965 0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Deviation from Bragg angle (mrad)
0.2 0.1 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Deviation from Bragg angle (mrad) Fig. 6. Angular selectivity of transmitting VBGs.
Besides, there are obvious ripples in diffraction efficiency of VBGs, as shown in the insertion of Fig. 5. The ripples are impermissible in high power laser applications, such as the spectral combining and near-filed filtering. The factor α is used to evaluate the ripple intensity of diffraction efficiency, and α is defined as:
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8296
α=
η max − η min η max + η min
(4)
where ηmax is the maximum diffraction efficiency of VBGs, and ηmin the minimum diffraction efficiency of VBGs. The experimental results show that the ripple of diffraction efficiency increases with the higher crystallization temperature of VBGs. The diffraction efficiency ripple intensity of VBGs with the crystallization temperature of 580°C and 590°C is 1.13% and 1.59%, while the ripple intensity with the crystallization temperature of 570°C is 0.45%. It can be seen that ripple intensity of diffraction efficiency increases 2.5 and 5.3 times with the crystallization temperature rising of 10°C and 20°C, respectively. 4. Conclusion The dual-beam interference exposure and the “two step” thermal development method are used to fabricate transmitting VBGs in photo-thermo-refractive glass. The diffraction efficiency of VBGs is increasing at the crystallization temperature from 570°C to 590°C. The temperature rising of VBGs during the high flux laser irradiation increases with crystallization temperature. The diffraction efficiency is almost the same when temperature rising of VBGs is in the steady situation, but the diffraction efficiency of VBGs prepared at higher crystallization temperature drops slightly under laser irradiation. The reduction of diffraction efficiency of VBGs is explained with the variation of grating period, and the theoretical calculation is in good agreement with the experimental data. Acknowledgments This work is supported by NSFC under the contract Nos. of 91023009 and 61275140, the united foundation between NSFC and Chinese Academy of Engineering Physics under contract nos. of 11176021, 11076021 and 10876011, the Natural Science Foundation of Jiangsu Higher Education Institutions under contract 10KJA140045, a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the national High-Tech 863 program of China and the Graduate Research and Innovation Project of Jiangsu Province (CXZZ11_0095).
#204453 - $15.00 USD (C) 2014 OSA
Received 8 Jan 2014; revised 10 Mar 2014; accepted 24 Mar 2014; published 1 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008291 | OPTICS EXPRESS 8297
