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mm polarization-independent reflection gratings for spectral beam combining
Optics Communications xxx (xxxx) xxx
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining Xinyu Mao a , Chaoming Li b ,∗, Keqiang Qiu c ,∗, Lijiang Zeng a , Lifeng Li a , Xinrong Chen b , Jianhong Wu b , Zhengkun Liu c , Shaojun Fu c , Yilin Hong c a
Department of Precision Instrument, Tsinghua University, Beijing 100084, China School of Optoelectronic Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China c National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China b
ARTICLE
INFO
Keywords: Polarization-independent grating Spectral beam combining Diffractive optics
ABSTRACT We designed and fabricated polarization-independent multilayer dielectric reflection gratings of 1300 line/mm for spectral beam combining. Because the designed grating has a duty cycle (or fill factor) much greater than 0.5, we used the lift-off process to invert a less-than-0.5 duty cycle photoresist mask to a greater-than-0.5 duty cycle chromium mask. Because the simultaneous polarization-independence and high-angular-dispersion requirements greatly reduce fabrication tolerance, we employed the end-point detection techniques to control accurately the duty cycle and groove depth of the grating profile to reach the design target. The measured polarization-averaged diffraction efficiency of the fabricated gratings was greater than 97% in the wavelength range of 1050 nm–1080 nm.
1. Introduction High-power fiber lasers are required for many applications, including laser cutting, laser welding and laser cladding [1]. Spectral beam combining (SBC) is an effective power scaling technique. With a planar diffraction grating, several laser beams of different wavelengths incident on the grating at different angles can be combined into one beam of good beam quality and a total power close to the sum of individual lasers [2,3]. To ensure high total output power, the beam-combining grating should have a high diffraction efficiency in the wavelength range of the SBC system, and the high efficiency should be independent of polarization of the individual incident laser beams because at the present high-power fiber lasers lack polarization stability. A compact SBC system demands a high angular dispersion of the grating, which means high line density of the grating because it follows from the grating equation that angular dispersion is directly proportional to line density (reciprocal of grating period). When the grating period is comparable to or less than the wavelength of light, the grating may diffract light of different polarizations with very different efficiencies. Therefore, polarization independency and high angular dispersion are two conflicting requirements. Even if a compromise can be reached, simultaneously imposing the two requirements, in addition to requiring a reasonable wavelength bandwidth, greatly reduces fabrication tolerance. Furthermore, the tolerance gets tighter quickly as
the line density increases; hence, the design and fabrication difficulties increase tremendously. For the above-mentioned reason, the line densities of all polarization-independent SBC gratings reported to date are substantially less than those of single-polarization gratings. The polarizationindependent SBC gratings used in the laser-combining experiments reported by Wirth et al. [4] and Zheng et al. [5] both had a line density of 960 line/mm. In the former work, the experimentally measured, polarization-independent, Littrow-mounting diffraction efficiency was greater than 95% in a wavelength range from 1010 to 1090 nm; in the latter work, the diffraction efficiency, presumably also measured at Littrow mounting, was the same in a wavelength range from 1040 to 1090 nm. Cao et al. [6] designed and fabricated a polarizationindependent grating that had a bullet-alike profile cross section. Its line density was 1111 line/mm and its measured diffraction efficiency was larger than 91% in the wavelength range of 1040 nm ∼1090 nm. Shen et al. [7] and Chen et al. [8] reported polarization-independent gratings of the line densities 1200 line/mm and 1170 line/mm, and the measured diffraction efficiencies were 92% and 98%, at wavelengths from 1044 nm to 1084 nm and from 1020 nm to 1080 nm, respectively. To the best of our knowledge, these are the highest reported line density to date. In this paper, we report the design and fabrication of a higher line density, polarization-independent, multilayer dielectric reflection
∗ Corresponding authors. E-mail addresses: [email protected] (C. Li), [email protected] (K. Qiu).
https://doi.org/10.1016/j.optcom.2019.124883 Received 17 July 2019; Received in revised form 2 October 2019; Accepted 1 November 2019 Available online xxxx 0030-4018/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.
X. Mao, C. Li, K. Qiu et al.
Optics Communications xxx (xxxx) xxx
Fig. 1. Schematic cross-section of the multilayer dielectric grating.
Fig. 2. Theoretical diffraction efficiencies of the designed grating.
Table 1 Materials and thicknesses of the thin-film stack. Symbol
Material
Thickness (nm)
T G B L H
Ta2 O5 SiO2 Ta2 O5 SiO2 Ta2 O5
332 151 98 284 91
grating. The line density is chosen to be 1300 line/mm and the grating is designed to work at the -1st-order Littrow mounting for the center wavelength of the bandwidth between 1050 nm and 1080 nm. The design parameters, tolerance analysis and the end-point detection techniques for ensuring the accuracy of grating grooves are described.
Fig. 3. Minimum diffraction efficiencies in the wavelength range of 1050 nm–1080 nm versus top duty cycle and groove depth of the grating profile. (a) TE, (b) TM, (c) intersection of (a) and (b).
2. Design depth ℎ = 483 nm. Fig. 2 shows the theoretical diffraction efficiencies versus the incident wavelength. For both TM and TE polarizations, the diffraction efficiencies are greater than 97%. We first analyzed the fabrication tolerances. Assuming the thicknesses of all layers had an absolute error of 1.5 nm, the signs of all Ta2 O5 layer errors are the same and so are all SiO2 layer errors, but the sign of the Ta2 O5 layers and that of the SiO2 layers were independent, we made numerical simulations. The results showed that the TE-TM average diffraction efficiency of the grating were not affected by more than 0.2% in the bandwidth between 1050 nm and 1080 nm. Our thinfilm coating vendor told us that they could keep the thickness error of each layer within ± 1 nm; therefore, the thickness errors could be ignored. On the other hand, the controls of duty cycle and groove depth of the grating were more difficult and presented the major challenges to us. Fig. 3(a) and (b) show the minimum diffraction efficiencies in the wavelength range from 1050 nm to 1080 nm versus the top duty
We designed the polarization-independent grating by using a computer code based on the rigorous electromagnetic theory. Our grating comprises a quartz substrate, a multilayer dielectric thin-film stack of formula S| (HL)16 BGT| C, where S stands for substrate, C stands for cover (air), and the designations of coating materials and layer thicknesses are given in Table 1, and grating grooves engraved in the top two layers (G and T), as shown in Fig. 1. The choice of the number of HL pairs was made as a compromise between having a sufficiently high reflectance and assuring good stability of the thin-film stack. The cross section of the grating ridge is a symmetrical trapezoid of base angle 𝛽 = 75◦ that was obtained from measuring the scanning electron microscope (SEM) images of previously etched gratings. The layer B serves as an etch-stop layer, because the etch rate of Ta2 O5 is much lower than that of SiO2 . Other parameters of the grating are as follows: grating period 𝑑 = 769 nm, top duty cycle 𝛿 = 𝑤1 ∕𝑑 = 0.58, groove 2
Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.
X. Mao, C. Li, K. Qiu et al.
Optics Communications xxx (xxxx) xxx
Fig. 5. Development monitoring curve and SEM image of the photoresist mask.
Fig. 4. Flow chart of the grating fabrication steps.
cycle and groove depth for TE and TM polarizations, respectively. When the base angle of the trapezoid is 75◦ and the groove depth is between 400 nm and 530 nm, the top duty cycle should be less than 0.63 to guarantee that the bottom duty cycle be less than 1, so we scan the top duty cycle from 0.4 to 0.63. To get the diffraction efficiency greater than 95% for the TE polarization (or TM polarization), we need to control the top duty cycle from 0.55 to 0.63 (or from 0.4 to 0.62) as shown in red box in Fig. 3(a) [or Fig. 3(b)]. Fig. 3(c) is the intersection of Fig. 3(a) and (b), i.e., at each point in Fig. 3(c) the efficiency value is the smaller value of the two corresponding values in Fig. 3(a) and (b). When the top duty cycle is between 0.55 and 0.62, and the groove depth is between 400 nm and 530 nm the diffraction efficiencies for both TE and TM polarizations exceed 95%.
Fig. 6. Etching-monitoring curve and SEM image of the etched grating (after photoresist stripping). The point of interest is the envelope of the vertical spikes.
based on comparisons of SEM images of developed photoresist gratings and their development curves. For example, when the development is stopped at 50% of the peak value, the duty cycle of the photoresist mask is about 0.2. Fig. 5 shows a typical monitoring curve and the SEM image of the corresponding photoresist mask. The abrupt drop of the curve at about 34 s was due to the quick removal of the grating sample out of the developer solution. We can adjust the photoresist mask’s duty cycle from 0.35 to 0.1 by changing the percentage drop from 0 to 86%. Of course, this rule of thumb depends the exposure dose and the photoresist thickness. Both were fixed in this work. Then, a chromium (Cr) mask was made from the photoresist mask. The design value of top duty cycle of the ion-beam etched trapezoidal grating is 0.58. It is too large to achieve by directly etching through the photoresist mask because the photoresist masks always have a duty cycle substantially less than 0.5. To solve the problem, we coated the photoresist mask with a layer of Cr, and then inverted the photoresist mask to a Cr mask by the lift-off process. If the photoresist mask has a duty cycle between 0.1 and 0.35, we can obtain a Cr mask of duty cycle between 0.65 and 0.9. Because the top duty cycle of the grating decreases during ion-beam etching, a 0.8 duty cycle Cr mask is about optimum. Finally, the Cr mask grating was converted into an etched grating via ion-beam etching. Layers T and G in the grating groove regions were etched away by the reactive CHF3 ion beam through the Cr mask. To control base angle of the grating profile we adjusted ion beam current. The statistic mean value of the base angle obtained
3. Experiments and results The sequence of fabrication steps of our multilayer dielectric grating is shown in Fig. 4. The key steps are holographic exposure, photoresist development, and ion beam etching. Among them, development determines the duty cycle, and ion beam etching determines the groove depth of the grating. By monitoring the variations of diffraction efficiencies during development and etching, we can control the duty cycle and groove depth with high accuracies. First, a photoresist mask of approximately 0.2 duty cycle was made on the multilayer dielectric substrate of size 50 mm × 50 mm by holographic lithography. The duty cycle of the photoresist mask is controlled by using the in-situ monitoring method proposed by Li et al. [9] in 1987. A TE-polarized 633-nm-wavelength laser beam was normally incident on the grating immersed in the developer. The light intensity of the -1st-order transmitted beam, which is proportional to the -1st-order diffraction efficiency, was monitored to determine the point to end the development. The duty cycle of the photoresist grating decreases as the groove depth increases during development. Although full information on evolution of the actual photoresist surface contour is difficult to acquire, the duty cycle can be estimated from the percentage drop of the monitoring signal from its peak value. The correlation between the percentage drop and desired duty cycle has been previously established 3
Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.
X. Mao, C. Li, K. Qiu et al.
Optics Communications xxx (xxxx) xxx
wavelength. Here near means the plane formed by the incident and the – 1st-order diffraction directions is parallel to the grating lines and the angles formed by the two directions with the plane perpendicular to the grating lines were both 2◦ (so the angle between the two directions was 4◦ ). Fig. 7 shows the measured diffraction efficiencies. The wavelength increment is 1 nm. The measured -1st-order diffraction efficiencies for both TE and TM polarizations are larger than 96.5%. The polarizationaveraged efficiency (half of the sum of TE and TM efficiencies) is greater than 97% in the displayed wavelength range, and it reaches its peak value of 98% at 1064 nm. 4. Summary We have designed and fabricated 1300-line/mm polarizationindependent gratings that have a bandwidth of 30 nm (diffraction efficiency greater than 97%) centered at 1065 nm. To satisfy the small fabrication tolerance, we accurately controlled the duty cycle and groove depth of the grating by employing the end-point detection techniques in the photoresist development and ion-beam etching processes. The fabricated gratings have polarization-averaged diffraction efficiencies larger than 97% over the wavelength range of 1050– 1080 nm. The measured diffraction efficiency curves qualitatively agree well with the theoretical ones shown in Fig. 2. The about 2% difference is very likely due to groove shape deviation from the designed shape. Further work includes, on the theoretical side, to design polarizationindependent grating of a higher line density and larger fabrication tolerance, and on the experimental side, to improve groove shape control accuracy and process stability.
Fig. 7. Measured diffraction efficiencies.
this way was 76◦ . To control groove depth we stopped etching at the right moment during in-situ monitoring process [10]. To explain in a little more detail how this was done, we first describe the etching and monitoring configurations. The shape of our ion beam’s cross section is approximately rectangular. Its short-side length is about 60 mm. The grating to be etched is mounted with its groove direction along the long side. To achieve better etching uniformity, we use a scanning etching mode: the grating moves back and forth at a speed of 12 mm/s in the direction along the short side of the ion beam. A laser beam of 632.8 nm wavelength from a He–Ne laser (placed outside the vacuum chamber) is incident at the -1st-order Littrow angle and with TE polarization on the grating when the grating is at a convenient position along its scanning path. The intensity of the -1st-order diffracted beam is monitored outside the vacuum chamber during ion-beam etching. Because the grating only receives the laser beam in a fraction of a full round-trip scanning cycle, the monitored signal intensity versus time consists of a series of spikes. The true, meaningful monitoring curve is the envelope of the spikes. Our computer simulation of the etching monitoring process showed that the – 1st-order Littrow diffraction efficiency reached the main peak value when the etch depth reached the interface between layer B and layer G. This finding was later verified experimentally; therefore, the main peak of the monitoring curve was selected as the end-point identifying feature. Indeed, we could not be certain where the main peak was until the curve went over it; however, compared with the overall etching time, this small amount of over time was negligible. Fig. 6 shows the monitoring curve (the etching and the monitoring were stopped immediately after the last vertical spike was recorded, therefore leaving no trace of etching stoppage) and SEM image of an etched grating after photoresist stripping. By using this monitoring technique, we could control the groove depth to be within 483 ± 5 nm. The diffraction efficiencies of the fabricated polarizationindependent gratings were measured with a wavelength tunable laser. For convenience of measurement, the incident beam was fixed near, but not exactly at, the – 1st-order Littrow angle for the 1064 nm
Funding information National Natural Science Foundation of China (No. 61575101, 61178046). References [1] W. Shi, Q. Fang, X. Zhu, R.A. Norwood, N. Peyghambarian, Fiber lasers and their applications, Appl. Opt. 53 (2014) 6554. [2] A. Sevian, O. Andrusyak, I. Ciapurin, V. Smirnov, G. Venus, L. Glebov, Efficient power scaling of laser radiation by spectral beam combining, Opt. Lett. 33 (2008) 384. [3] T. Loftus, A. Liu, P. Hoffman, A. Thomas, M. Norsen, R. Royse, E. Honea, 522 W average power, spectrally beam-combined fiber laser with near-diffraction-limited beam quality, Opt. Lett. 32 (2007) 349. [4] C. Wirth, O. Schmidt, I. Tsybin, T. Schreiber, R. Eberhardt, J. Limpert, A. Tünnermann, K. Ludewigt, M. Gowin, E. ten Have, M. Jung, High average power spectral beam combining of four fiber amplifiers to 8.2 kW, Opt. Lett. 36 (2011) 3118. [5] Y. Zheng, Y. Yang, J. Wang, 10.8 kW spectral beam combination of eight allfiber superfluorescent sources and their dispersion compensation, Opt. Express 24 (2016) 12036. [6] H. Cao, J. Wu, J. Yu, J. Yu, J. Ma, High-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section fused-silica beam combining grating, Appl. Opt. 57 (2018) 900. [7] B. Shen, L. Zeng, L. Li, H. Yan, Fabrication of polarization independent gratings made on multilayer dielectric thin film substrates, High Power Laser Part. Beams 27 (2015) 111013. [8] J. Chen, Y. Zhang, Y. Wang, F. Kong, H. Huang, Y. Wang, Y. Jin, P. Chen, J. Xu, J. Shao, Polarization-independent broadband beam combining grating with over 98% measured diffraction efficiency from 1023 to 1080 nm, Opt. Lett. 42 (2017) 4016. [9] L. Li, M. Xu, G.I. Stegeman, C.T. Seaton, Fabrication of photoresist masks for submicrometer surface relief gratings, Proc. SPIE 835 (1987) 72. [10] H. Chen, H. Guan, L. Zeng, Y. Jin, Fabrication of broadband, high-efficiency, metal-multilayer-dielectric gratings, Opt. Commun. 329 (2014) 103.
4
Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining Xinyu Mao a , Chaoming Li b ,∗, Keqiang Qiu c ,∗, Lijiang Zeng a , Lifeng Li a , Xinrong Chen b , Jianhong Wu b , Zhengkun Liu c , Shaojun Fu c , Yilin Hong c a
Department of Precision Instrument, Tsinghua University, Beijing 100084, China School of Optoelectronic Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China c National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China b
ARTICLE
INFO
Keywords: Polarization-independent grating Spectral beam combining Diffractive optics
ABSTRACT We designed and fabricated polarization-independent multilayer dielectric reflection gratings of 1300 line/mm for spectral beam combining. Because the designed grating has a duty cycle (or fill factor) much greater than 0.5, we used the lift-off process to invert a less-than-0.5 duty cycle photoresist mask to a greater-than-0.5 duty cycle chromium mask. Because the simultaneous polarization-independence and high-angular-dispersion requirements greatly reduce fabrication tolerance, we employed the end-point detection techniques to control accurately the duty cycle and groove depth of the grating profile to reach the design target. The measured polarization-averaged diffraction efficiency of the fabricated gratings was greater than 97% in the wavelength range of 1050 nm–1080 nm.
1. Introduction High-power fiber lasers are required for many applications, including laser cutting, laser welding and laser cladding [1]. Spectral beam combining (SBC) is an effective power scaling technique. With a planar diffraction grating, several laser beams of different wavelengths incident on the grating at different angles can be combined into one beam of good beam quality and a total power close to the sum of individual lasers [2,3]. To ensure high total output power, the beam-combining grating should have a high diffraction efficiency in the wavelength range of the SBC system, and the high efficiency should be independent of polarization of the individual incident laser beams because at the present high-power fiber lasers lack polarization stability. A compact SBC system demands a high angular dispersion of the grating, which means high line density of the grating because it follows from the grating equation that angular dispersion is directly proportional to line density (reciprocal of grating period). When the grating period is comparable to or less than the wavelength of light, the grating may diffract light of different polarizations with very different efficiencies. Therefore, polarization independency and high angular dispersion are two conflicting requirements. Even if a compromise can be reached, simultaneously imposing the two requirements, in addition to requiring a reasonable wavelength bandwidth, greatly reduces fabrication tolerance. Furthermore, the tolerance gets tighter quickly as
the line density increases; hence, the design and fabrication difficulties increase tremendously. For the above-mentioned reason, the line densities of all polarization-independent SBC gratings reported to date are substantially less than those of single-polarization gratings. The polarizationindependent SBC gratings used in the laser-combining experiments reported by Wirth et al. [4] and Zheng et al. [5] both had a line density of 960 line/mm. In the former work, the experimentally measured, polarization-independent, Littrow-mounting diffraction efficiency was greater than 95% in a wavelength range from 1010 to 1090 nm; in the latter work, the diffraction efficiency, presumably also measured at Littrow mounting, was the same in a wavelength range from 1040 to 1090 nm. Cao et al. [6] designed and fabricated a polarizationindependent grating that had a bullet-alike profile cross section. Its line density was 1111 line/mm and its measured diffraction efficiency was larger than 91% in the wavelength range of 1040 nm ∼1090 nm. Shen et al. [7] and Chen et al. [8] reported polarization-independent gratings of the line densities 1200 line/mm and 1170 line/mm, and the measured diffraction efficiencies were 92% and 98%, at wavelengths from 1044 nm to 1084 nm and from 1020 nm to 1080 nm, respectively. To the best of our knowledge, these are the highest reported line density to date. In this paper, we report the design and fabrication of a higher line density, polarization-independent, multilayer dielectric reflection
∗ Corresponding authors. E-mail addresses: [email protected] (C. Li), [email protected] (K. Qiu).
https://doi.org/10.1016/j.optcom.2019.124883 Received 17 July 2019; Received in revised form 2 October 2019; Accepted 1 November 2019 Available online xxxx 0030-4018/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.
X. Mao, C. Li, K. Qiu et al.
Optics Communications xxx (xxxx) xxx
Fig. 1. Schematic cross-section of the multilayer dielectric grating.
Fig. 2. Theoretical diffraction efficiencies of the designed grating.
Table 1 Materials and thicknesses of the thin-film stack. Symbol
Material
Thickness (nm)
T G B L H
Ta2 O5 SiO2 Ta2 O5 SiO2 Ta2 O5
332 151 98 284 91
grating. The line density is chosen to be 1300 line/mm and the grating is designed to work at the -1st-order Littrow mounting for the center wavelength of the bandwidth between 1050 nm and 1080 nm. The design parameters, tolerance analysis and the end-point detection techniques for ensuring the accuracy of grating grooves are described.
Fig. 3. Minimum diffraction efficiencies in the wavelength range of 1050 nm–1080 nm versus top duty cycle and groove depth of the grating profile. (a) TE, (b) TM, (c) intersection of (a) and (b).
2. Design depth ℎ = 483 nm. Fig. 2 shows the theoretical diffraction efficiencies versus the incident wavelength. For both TM and TE polarizations, the diffraction efficiencies are greater than 97%. We first analyzed the fabrication tolerances. Assuming the thicknesses of all layers had an absolute error of 1.5 nm, the signs of all Ta2 O5 layer errors are the same and so are all SiO2 layer errors, but the sign of the Ta2 O5 layers and that of the SiO2 layers were independent, we made numerical simulations. The results showed that the TE-TM average diffraction efficiency of the grating were not affected by more than 0.2% in the bandwidth between 1050 nm and 1080 nm. Our thinfilm coating vendor told us that they could keep the thickness error of each layer within ± 1 nm; therefore, the thickness errors could be ignored. On the other hand, the controls of duty cycle and groove depth of the grating were more difficult and presented the major challenges to us. Fig. 3(a) and (b) show the minimum diffraction efficiencies in the wavelength range from 1050 nm to 1080 nm versus the top duty
We designed the polarization-independent grating by using a computer code based on the rigorous electromagnetic theory. Our grating comprises a quartz substrate, a multilayer dielectric thin-film stack of formula S| (HL)16 BGT| C, where S stands for substrate, C stands for cover (air), and the designations of coating materials and layer thicknesses are given in Table 1, and grating grooves engraved in the top two layers (G and T), as shown in Fig. 1. The choice of the number of HL pairs was made as a compromise between having a sufficiently high reflectance and assuring good stability of the thin-film stack. The cross section of the grating ridge is a symmetrical trapezoid of base angle 𝛽 = 75◦ that was obtained from measuring the scanning electron microscope (SEM) images of previously etched gratings. The layer B serves as an etch-stop layer, because the etch rate of Ta2 O5 is much lower than that of SiO2 . Other parameters of the grating are as follows: grating period 𝑑 = 769 nm, top duty cycle 𝛿 = 𝑤1 ∕𝑑 = 0.58, groove 2
Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.
X. Mao, C. Li, K. Qiu et al.
Optics Communications xxx (xxxx) xxx
Fig. 5. Development monitoring curve and SEM image of the photoresist mask.
Fig. 4. Flow chart of the grating fabrication steps.
cycle and groove depth for TE and TM polarizations, respectively. When the base angle of the trapezoid is 75◦ and the groove depth is between 400 nm and 530 nm, the top duty cycle should be less than 0.63 to guarantee that the bottom duty cycle be less than 1, so we scan the top duty cycle from 0.4 to 0.63. To get the diffraction efficiency greater than 95% for the TE polarization (or TM polarization), we need to control the top duty cycle from 0.55 to 0.63 (or from 0.4 to 0.62) as shown in red box in Fig. 3(a) [or Fig. 3(b)]. Fig. 3(c) is the intersection of Fig. 3(a) and (b), i.e., at each point in Fig. 3(c) the efficiency value is the smaller value of the two corresponding values in Fig. 3(a) and (b). When the top duty cycle is between 0.55 and 0.62, and the groove depth is between 400 nm and 530 nm the diffraction efficiencies for both TE and TM polarizations exceed 95%.
Fig. 6. Etching-monitoring curve and SEM image of the etched grating (after photoresist stripping). The point of interest is the envelope of the vertical spikes.
based on comparisons of SEM images of developed photoresist gratings and their development curves. For example, when the development is stopped at 50% of the peak value, the duty cycle of the photoresist mask is about 0.2. Fig. 5 shows a typical monitoring curve and the SEM image of the corresponding photoresist mask. The abrupt drop of the curve at about 34 s was due to the quick removal of the grating sample out of the developer solution. We can adjust the photoresist mask’s duty cycle from 0.35 to 0.1 by changing the percentage drop from 0 to 86%. Of course, this rule of thumb depends the exposure dose and the photoresist thickness. Both were fixed in this work. Then, a chromium (Cr) mask was made from the photoresist mask. The design value of top duty cycle of the ion-beam etched trapezoidal grating is 0.58. It is too large to achieve by directly etching through the photoresist mask because the photoresist masks always have a duty cycle substantially less than 0.5. To solve the problem, we coated the photoresist mask with a layer of Cr, and then inverted the photoresist mask to a Cr mask by the lift-off process. If the photoresist mask has a duty cycle between 0.1 and 0.35, we can obtain a Cr mask of duty cycle between 0.65 and 0.9. Because the top duty cycle of the grating decreases during ion-beam etching, a 0.8 duty cycle Cr mask is about optimum. Finally, the Cr mask grating was converted into an etched grating via ion-beam etching. Layers T and G in the grating groove regions were etched away by the reactive CHF3 ion beam through the Cr mask. To control base angle of the grating profile we adjusted ion beam current. The statistic mean value of the base angle obtained
3. Experiments and results The sequence of fabrication steps of our multilayer dielectric grating is shown in Fig. 4. The key steps are holographic exposure, photoresist development, and ion beam etching. Among them, development determines the duty cycle, and ion beam etching determines the groove depth of the grating. By monitoring the variations of diffraction efficiencies during development and etching, we can control the duty cycle and groove depth with high accuracies. First, a photoresist mask of approximately 0.2 duty cycle was made on the multilayer dielectric substrate of size 50 mm × 50 mm by holographic lithography. The duty cycle of the photoresist mask is controlled by using the in-situ monitoring method proposed by Li et al. [9] in 1987. A TE-polarized 633-nm-wavelength laser beam was normally incident on the grating immersed in the developer. The light intensity of the -1st-order transmitted beam, which is proportional to the -1st-order diffraction efficiency, was monitored to determine the point to end the development. The duty cycle of the photoresist grating decreases as the groove depth increases during development. Although full information on evolution of the actual photoresist surface contour is difficult to acquire, the duty cycle can be estimated from the percentage drop of the monitoring signal from its peak value. The correlation between the percentage drop and desired duty cycle has been previously established 3
Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.
X. Mao, C. Li, K. Qiu et al.
Optics Communications xxx (xxxx) xxx
wavelength. Here near means the plane formed by the incident and the – 1st-order diffraction directions is parallel to the grating lines and the angles formed by the two directions with the plane perpendicular to the grating lines were both 2◦ (so the angle between the two directions was 4◦ ). Fig. 7 shows the measured diffraction efficiencies. The wavelength increment is 1 nm. The measured -1st-order diffraction efficiencies for both TE and TM polarizations are larger than 96.5%. The polarizationaveraged efficiency (half of the sum of TE and TM efficiencies) is greater than 97% in the displayed wavelength range, and it reaches its peak value of 98% at 1064 nm. 4. Summary We have designed and fabricated 1300-line/mm polarizationindependent gratings that have a bandwidth of 30 nm (diffraction efficiency greater than 97%) centered at 1065 nm. To satisfy the small fabrication tolerance, we accurately controlled the duty cycle and groove depth of the grating by employing the end-point detection techniques in the photoresist development and ion-beam etching processes. The fabricated gratings have polarization-averaged diffraction efficiencies larger than 97% over the wavelength range of 1050– 1080 nm. The measured diffraction efficiency curves qualitatively agree well with the theoretical ones shown in Fig. 2. The about 2% difference is very likely due to groove shape deviation from the designed shape. Further work includes, on the theoretical side, to design polarizationindependent grating of a higher line density and larger fabrication tolerance, and on the experimental side, to improve groove shape control accuracy and process stability.
Fig. 7. Measured diffraction efficiencies.
this way was 76◦ . To control groove depth we stopped etching at the right moment during in-situ monitoring process [10]. To explain in a little more detail how this was done, we first describe the etching and monitoring configurations. The shape of our ion beam’s cross section is approximately rectangular. Its short-side length is about 60 mm. The grating to be etched is mounted with its groove direction along the long side. To achieve better etching uniformity, we use a scanning etching mode: the grating moves back and forth at a speed of 12 mm/s in the direction along the short side of the ion beam. A laser beam of 632.8 nm wavelength from a He–Ne laser (placed outside the vacuum chamber) is incident at the -1st-order Littrow angle and with TE polarization on the grating when the grating is at a convenient position along its scanning path. The intensity of the -1st-order diffracted beam is monitored outside the vacuum chamber during ion-beam etching. Because the grating only receives the laser beam in a fraction of a full round-trip scanning cycle, the monitored signal intensity versus time consists of a series of spikes. The true, meaningful monitoring curve is the envelope of the spikes. Our computer simulation of the etching monitoring process showed that the – 1st-order Littrow diffraction efficiency reached the main peak value when the etch depth reached the interface between layer B and layer G. This finding was later verified experimentally; therefore, the main peak of the monitoring curve was selected as the end-point identifying feature. Indeed, we could not be certain where the main peak was until the curve went over it; however, compared with the overall etching time, this small amount of over time was negligible. Fig. 6 shows the monitoring curve (the etching and the monitoring were stopped immediately after the last vertical spike was recorded, therefore leaving no trace of etching stoppage) and SEM image of an etched grating after photoresist stripping. By using this monitoring technique, we could control the groove depth to be within 483 ± 5 nm. The diffraction efficiencies of the fabricated polarizationindependent gratings were measured with a wavelength tunable laser. For convenience of measurement, the incident beam was fixed near, but not exactly at, the – 1st-order Littrow angle for the 1064 nm
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Please cite this article as: X. Mao, C. Li, K. Qiu et al., Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining, Optics Communications (2019) 124883, https://doi.org/10.1016/j.optcom.2019.124883.