Letters|30 Article(s)
Method of Far-Field Phase Noise Suppression for Space Gravitational-Wave Detection Telescope
Shengnan Chen, Chunyan Wang, Hao Sun, and Huilin Jiang
ObjectiveSpace-based gravitational-wave observatories (SGOs) promise to measure pico-meter variations in the gigameter separations of a triangular constellation. Telescopes play a crucial role in using transmitting and receiving laser beams measuring the constellation arms with heterodyne laser interferometry. The far-field phase noise induced by the coupling of the wavefront aberrations of optical telescopes with their pointing jitters is one of the major noise sources for the measurement. As phase noise suppression is a critical aspect for achieving the required comprehensive measuring stability, this paper theoretically analyzes the mechanism of the far-field phase noise, proposes an optimization strategy for the design of the optical telescopes in SGOs, and verifies it to pave the way for the comprehensive phase noise control in the design-to-manufacture process.MethodsTo analytically establish the relationship of the coupling coefficient with the aberrations in the form of polynomial expansions, the paper adopts the Fringe Zernike polynomials to represent the aberrations and further construct and describe the wavefront error. Then, the coupling coefficient defined as the modulus of the gradient of the far-field wavefront error is expressed as a polynomial function of the aberration coefficients and the tilt angles and is further simplified on the basis of the symmetry of the telescope. According to this relationship and the aberration characteristics of the telescope design residuals, the paper evaluates the effect of different aberrations on phase noise, revealing that defocus, primary astigmatism, and primary spherical aberration are the keys to controlling the coupling coefficient. Thus, an optimization strategy based on key aberration control is proposed. The performance of this method in far-field phase noise suppression is verified by examples of telescope design.Results and DiscussionsThe wavefront quality of the telescopes before and after optimization (Table 1) by the above strategy is at the λ/20 (λ=1064 nm) level. Before optimization, the far-field wavefront changes significantly within the range of ±100 nrad. Accordingly, the coupling coefficient increases rapidly with the tilt angle to over 1 pm/nrad. After optimization, although the wavefront residuals are slightly worse (Table 2), the range of far-field wavefront error decreases markedly by more than 90% (Fig. 4). The corresponding coupling coefficient is smaller than 0.11 pm/nrad within the range of ±100 nrad. It is only 6% of that before optimization and much smaller than the required value (Fig. 5). These results indicate that the optimization strategy based on aberration control can effectively reduce the coupling coefficient of the far-field phase noise, even in the case of poor wavefront quality.ConclusionsOn the basis of theoretical analysis of the mechanism of the far-field phase noise, this paper determines the relationship of the coupling coefficient with the aberrations, develops an optimization strategy based on key aberration control, and verifies the strategy. The results reveal that suppressing the key aberrations deliberately instead of simply enhancing the demand for wavefront quality in the optimization process can reduce the sensitivity of the far-field phase to jitters more efficiently, improve the far-field phase stability of the telescope significantly, and balance the severe noise budget and the design freedom of the telescope to reserve sufficient margin for the rest optics. ObjectiveSpace-based gravitational-wave observatories (SGOs) promise to measure pico-meter variations in the gigameter separations of a triangular constellation. Telescopes play a crucial role in using transmitting and receiving laser beams measuring the constellation arms with heterodyne laser interferometry. The far-field phase noise induced by the coupling of the wavefront aberrations of optical telescopes with their pointing jitters is one of the major noise sources for the measurement. As phase noise suppression is a critical aspect for achieving the required comprehensive measuring stability, this paper theoretically analyzes the mechanism of the far-field phase noise, proposes an optimization strategy for the design of the optical telescopes in SGOs, and verifies it to pave the way for the comprehensive phase noise control in the design-to-manufacture process.MethodsTo analytically establish the relationship of the coupling coefficient with the aberrations in the form of polynomial expansions, the paper adopts the Fringe Zernike polynomials to represent the aberrations and further construct and describe the wavefront error. Then, the coupling coefficient defined as the modulus of the gradient of the far-field wavefront error is expressed as a polynomial function of the aberration coefficients and the tilt angles and is further simplified on the basis of the symmetry of the telescope. According to this relationship and the aberration characteristics of the telescope design residuals, the paper evaluates the effect of different aberrations on phase noise, revealing that defocus, primary astigmatism, and primary spherical aberration are the keys to controlling the coupling coefficient. Thus, an optimization strategy based on key aberration control is proposed. The performance of this method in far-field phase noise suppression is verified by examples of telescope design.Results and DiscussionsThe wavefront quality of the telescopes before and after optimization (Table 1) by the above strategy is at the λ/20 (λ=1064 nm) level. Before optimization, the far-field wavefront changes significantly within the range of ±100 nrad. Accordingly, the coupling coefficient increases rapidly with the tilt angle to over 1 pm/nrad. After optimization, although the wavefront residuals are slightly worse (Table 2), the range of far-field wavefront error decreases markedly by more than 90% (Fig. 4). The corresponding coupling coefficient is smaller than 0.11 pm/nrad within the range of ±100 nrad. It is only 6% of that before optimization and much smaller than the required value (Fig. 5). These results indicate that the optimization strategy based on aberration control can effectively reduce the coupling coefficient of the far-field phase noise, even in the case of poor wavefront quality.ConclusionsOn the basis of theoretical analysis of the mechanism of the far-field phase noise, this paper determines the relationship of the coupling coefficient with the aberrations, develops an optimization strategy based on key aberration control, and verifies the strategy. The results reveal that suppressing the key aberrations deliberately instead of simply enhancing the demand for wavefront quality in the optimization process can reduce the sensitivity of the far-field phase to jitters more efficiently, improve the far-field phase stability of the telescope significantly, and balance the severe noise budget and the design freedom of the telescope to reserve sufficient margin for the rest optics.
Acta Optica Sinica
- Publication Date: Mar. 25, 2023
- Vol. 43, Issue 6, 0636001 (2023)
Fabrication of Chirped and Tilted Fiber Bragg Gratings with Femtosecond Laser
Hao Li, Meng Wang, Baiyi Wu, Xinyu Ye, Chenhui Gao, Binyu Rao, Xin Tian, Xiaoming Xi, Zilun Chen, Zefeng Wang, and Jinbao Chen
ObjectiveUp to now, high-power fiber lasers have been widely used in industrial processing, national defense, scientific research, and other fields. However, stimulated Raman scattering (SRS) is one of the main factors limiting the power scaling of such fiber lasers. In recent years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to suppress the SRS in high-power lasers by filtering the Raman light. CTFBGs are traditionally fabricated by the ultraviolet laser phase mask method, which requires the fibers to be hydrogen-loaded and thermal-annealed before and after grating inscription, respectively. This process is time-consuming and costly, especially when CTFBGs are fabricated with large-core fibers since the time for hydrogen loading and thermal annealing increases as the fiber core expands. The development of the femtosecond (fs) laser inscription method provides an alternative to the fabrication of CTFBGs. As it has no requirement on fiber photosensitivity, hydrogen loading and thermal annealing are no longer necessary, which greatly shortens the fabrication period of CTFBGs. However, although chirped FBGs and tilted FBGs have been fabricated by fs-lasers, fs-laser-inscribed CTFBGs have not been reported yet, and this paper is expected to fill this research gap.MethodThe inscription system for CTFBGs is shown in Fig. 1. The fs-laser emitted from the laser source is reflected by a reflecting mirror and is then focused on a fiber after passing through a chirped phase mask and a cylindrical lens successively. The fiber is fixed on a piezoelectric platform by a pair of fiber holders, and the reflecting mirror and the cylindrical lens are fixed on a one-dimension translation stage. Because the waist width of focal spot is smaller than the diameter of the fiber core, a tilted grating structure can be obtained by the oblique scanning of the fiber with the piezoelectric platform. Moreover, a larger chirp and a longer grating length can be achieved by moving the translation stage along the fiber axis.Results and DiscussionsFour CTFBGs with different tilt angles are fabricated, with their micrographs shown in Figs. 2(b) and 2(c). The tilted grating structure is clear and completely covers the fiber core. Fig. 3 presents the measured spectra of the CTFBGs. The transmission spectra suggest that as the tilt angle increases, the center wavelength moves towards a short wavelength. In addition, the depth decreases while the width increases. However, the Bragg wavelength of the CTFBGs does not change as the tilt angle increases according to the reflection spectra of the CTFBGs.ConclusionsThis paper takes the lead in inscribing CTFBGs with different tilt angles in large-mode-area double-cladding fibers with core/cladding diameter of 20/400 μm by fs-lasers, with a maximum filtering depth and bandwidth of ca. 15 dB and ca. 8.9 nm, respectively. This paper is of great significance to the research and development of CTFBGs. ObjectiveUp to now, high-power fiber lasers have been widely used in industrial processing, national defense, scientific research, and other fields. However, stimulated Raman scattering (SRS) is one of the main factors limiting the power scaling of such fiber lasers. In recent years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to suppress the SRS in high-power lasers by filtering the Raman light. CTFBGs are traditionally fabricated by the ultraviolet laser phase mask method, which requires the fibers to be hydrogen-loaded and thermal-annealed before and after grating inscription, respectively. This process is time-consuming and costly, especially when CTFBGs are fabricated with large-core fibers since the time for hydrogen loading and thermal annealing increases as the fiber core expands. The development of the femtosecond (fs) laser inscription method provides an alternative to the fabrication of CTFBGs. As it has no requirement on fiber photosensitivity, hydrogen loading and thermal annealing are no longer necessary, which greatly shortens the fabrication period of CTFBGs. However, although chirped FBGs and tilted FBGs have been fabricated by fs-lasers, fs-laser-inscribed CTFBGs have not been reported yet, and this paper is expected to fill this research gap.MethodThe inscription system for CTFBGs is shown in Fig. 1. The fs-laser emitted from the laser source is reflected by a reflecting mirror and is then focused on a fiber after passing through a chirped phase mask and a cylindrical lens successively. The fiber is fixed on a piezoelectric platform by a pair of fiber holders, and the reflecting mirror and the cylindrical lens are fixed on a one-dimension translation stage. Because the waist width of focal spot is smaller than the diameter of the fiber core, a tilted grating structure can be obtained by the oblique scanning of the fiber with the piezoelectric platform. Moreover, a larger chirp and a longer grating length can be achieved by moving the translation stage along the fiber axis.Results and DiscussionsFour CTFBGs with different tilt angles are fabricated, with their micrographs shown in Figs. 2(b) and 2(c). The tilted grating structure is clear and completely covers the fiber core. Fig. 3 presents the measured spectra of the CTFBGs. The transmission spectra suggest that as the tilt angle increases, the center wavelength moves towards a short wavelength. In addition, the depth decreases while the width increases. However, the Bragg wavelength of the CTFBGs does not change as the tilt angle increases according to the reflection spectra of the CTFBGs.ConclusionsThis paper takes the lead in inscribing CTFBGs with different tilt angles in large-mode-area double-cladding fibers with core/cladding diameter of 20/400 μm by fs-lasers, with a maximum filtering depth and bandwidth of ca. 15 dB and ca. 8.9 nm, respectively. This paper is of great significance to the research and development of CTFBGs.
Acta Optica Sinica
- Publication Date: Mar. 10, 2023
- Vol. 43, Issue 5, 0536001 (2023)
Fiber Laser for Coherent Raman Scattering Imaging of Liquid Water
Diandian Li, Simin Bi, Qiang Hao, Minbiao Ji, and Kangwen Yang
ObjectiveWater provides an important chemical contribution to the function and degradation of biological systems, plays a central role in regulating cell volume, nutrient transport, waste removal, and thermal regulation, and serves as a medium for biological reactions. Coherent anti-Stokes Raman scattering (CARS) imaging, as an important tool in biomedical applications, has the advantages of chemical specificity, free of label, high sensitivity, and so on, and it is widely used in brain tumor analysis, disease pathology analysis, and pharmacokinetics. Therefore, it is of great significance to study the CARS imaging light source for water molecules.MethodsTwo synchronous mode-locked fiber lasers are constructed using the mode-locking scheme of a nonlinear amplifying loop mirror (NALM). A portion of the pulse output of the erbium-doped fiber laser is output by pulse amplification module and frequency doubling module, which is called pump light, while another portion of the pulse is injected into the ytterbium-doped fiber laser to achieve pulse synchronization. The output pulse of the injected ytterbium-doped fiber laser is then amplified and emitted as Stokes light. An appropriate central wavelength fiber Bragg grating (FBG) is selected to control the central wavelength of the output pulse in the ytterbium-doped fiber laser, ensuring that the frequency difference between the two lasers and the vibration frequency of water meet the resonance condition. Finally, a dichroic mirror (DM) is used to spatially combine the pump light and Stokes light. The synchronous two-color pulses are injected into a commercial microscope (Olympus, FV1200) for CARS imaging.Results and DiscussionsIn the experiment, a master-slave injection passive synchronous two-color fiber light source is built, consisting of pump light and Stokes light. The central wavelengths of the two outputs are 783 nm [Fig. 3(a)] and 1040.6 nm [Fig. 2(c)], respectively. The pulse widths are 146.0 fs [Fig. 3(a)] and 9.1 ps [Fig. 2(d)], with an output power of 146 mW and 2 W, respectively. The relationship between repetition frequency variation and cavity length mismatch with and without injection is studied. The maximum synchronous mismatch distance reaches 347 μm [Fig. 3(c)]. Finally, CARS imaging of fresh mouse ear subcutaneous adipose tissue is performed at 3156 cm-1, as shown in the purple channel in Fig. 3(d). The distribution of intercellular water can be clearly observed.ConclusionsIn this paper, a CARS passive synchronous fiber laser for water is designed and constructed. The main pulse is injected into the slave laser cavity to achieve pulse synchronization. The relationship between frequency variation and cavity length mismatch with and without injection is studied. CARS imaging is performed on fresh mouse ear subcutaneous adipose tissue at 3156 cm-1, and the imaging results are satisfactory. The passive synchronous two-color laser is expected to promote the application of CARS technology in the field of fast, real-time, and efficient pathology detection. ObjectiveWater provides an important chemical contribution to the function and degradation of biological systems, plays a central role in regulating cell volume, nutrient transport, waste removal, and thermal regulation, and serves as a medium for biological reactions. Coherent anti-Stokes Raman scattering (CARS) imaging, as an important tool in biomedical applications, has the advantages of chemical specificity, free of label, high sensitivity, and so on, and it is widely used in brain tumor analysis, disease pathology analysis, and pharmacokinetics. Therefore, it is of great significance to study the CARS imaging light source for water molecules.MethodsTwo synchronous mode-locked fiber lasers are constructed using the mode-locking scheme of a nonlinear amplifying loop mirror (NALM). A portion of the pulse output of the erbium-doped fiber laser is output by pulse amplification module and frequency doubling module, which is called pump light, while another portion of the pulse is injected into the ytterbium-doped fiber laser to achieve pulse synchronization. The output pulse of the injected ytterbium-doped fiber laser is then amplified and emitted as Stokes light. An appropriate central wavelength fiber Bragg grating (FBG) is selected to control the central wavelength of the output pulse in the ytterbium-doped fiber laser, ensuring that the frequency difference between the two lasers and the vibration frequency of water meet the resonance condition. Finally, a dichroic mirror (DM) is used to spatially combine the pump light and Stokes light. The synchronous two-color pulses are injected into a commercial microscope (Olympus, FV1200) for CARS imaging.Results and DiscussionsIn the experiment, a master-slave injection passive synchronous two-color fiber light source is built, consisting of pump light and Stokes light. The central wavelengths of the two outputs are 783 nm [Fig. 3(a)] and 1040.6 nm [Fig. 2(c)], respectively. The pulse widths are 146.0 fs [Fig. 3(a)] and 9.1 ps [Fig. 2(d)], with an output power of 146 mW and 2 W, respectively. The relationship between repetition frequency variation and cavity length mismatch with and without injection is studied. The maximum synchronous mismatch distance reaches 347 μm [Fig. 3(c)]. Finally, CARS imaging of fresh mouse ear subcutaneous adipose tissue is performed at 3156 cm-1, as shown in the purple channel in Fig. 3(d). The distribution of intercellular water can be clearly observed.ConclusionsIn this paper, a CARS passive synchronous fiber laser for water is designed and constructed. The main pulse is injected into the slave laser cavity to achieve pulse synchronization. The relationship between frequency variation and cavity length mismatch with and without injection is studied. CARS imaging is performed on fresh mouse ear subcutaneous adipose tissue at 3156 cm-1, and the imaging results are satisfactory. The passive synchronous two-color laser is expected to promote the application of CARS technology in the field of fast, real-time, and efficient pathology detection.
Acta Optica Sinica
- Publication Date: Dec. 10, 2023
- Vol. 43, Issue 23, 2336001 (2023)
All-Polarization-Maintaining Yb-Doped Fiber Lasers with Adjustable Repetition Frequency and Efficient Frequency Doubling
Liang Liu, Gang Li, Qiang Hao, and Kangwen Yang
ObjectiveGreen picosecond pulse laser has a wide range of applications in coherent anti-Stokes Raman scattering imaging, material micromachining, and other fields. Especially in biomedical imaging, green picosecond pulses with different repetition frequencies are directly related to sample damage, penetration depth, and signal quality. Therefore, it is meaningful to study the efficient generation of green pulse lasers with different repetition frequencies.MethodsThe nonlinear amplification loop mirror (NALM) mode-locked resonant cavity is used as the seed pulse. After a fiber pre-amplifier, an acousto-optic modulator (AOM) is used to change the repetition frequency outside the optical resonant cavity. Then, a master oscillator power amplifier (MOPA) scheme is used to obtain a high-power infrared laser with different repetition frequencies. After that, a green pulsed laser with different repetition frequencies is generated by second harmonic technology (SHG) in a frequency doubling quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) crystal.Results and DiscussionsAn all-polarization-maintaining ytterbium-doped fiber laser with an adjustable repetition frequency is built in the experiment, which can output a picosecond pulse laser with an adjustable repetition frequency of 5-20 MHz. The frequency doubling of MgO-doped periodically poled lithium niobate (PPMgLN) crystal is studied. At the repetition frequencies of 20, 10, and 5 MHz, the corresponding maximum conversion efficiencies are 39.2%, 35.3%, and 31.0%, respectively, corresponding to output powers of 744, 600, and 496 mW, respectively, as shown in Fig. 2(d). The output spectra [Figs. 2(b), (e)], pulse widths [Figs. 2(c), (f)], and temperature phase matching curve [Fig. 3(a)] are compared. Finally, the stability and beam profile of the frequency doubling laser are tested. The relative jitter of the output power for the green laser is as low as 0.74% within four hours, as shown in Fig. 3(c).ConclusionsIn this paper, an efficient frequency doubling scheme based on an all-polarization-maintaining ytterbium-doped fiber laser with adjustable repetition frequency is verified, and the effects of repetition frequency, fundamental frequency optical power, and crystal temperature on frequency doubling efficiency are tested. A green picosecond pulse laser with different repetition frequencies is obtained by using PPMgLN crystal. The highest conversion efficiency is 39.2%, and the corresponding highest average power of the green laser is 744 mW. The measured root mean square (RMS) of frequency doubling optical power fluctuation is 0.74% within four hours. The proposed scheme has a simple structure and high stability. It provides a useful reference for the efficient generation of green pulsed lasers from high-power Yb-doped fiber lasers with different repetition frequencies. ObjectiveGreen picosecond pulse laser has a wide range of applications in coherent anti-Stokes Raman scattering imaging, material micromachining, and other fields. Especially in biomedical imaging, green picosecond pulses with different repetition frequencies are directly related to sample damage, penetration depth, and signal quality. Therefore, it is meaningful to study the efficient generation of green pulse lasers with different repetition frequencies.MethodsThe nonlinear amplification loop mirror (NALM) mode-locked resonant cavity is used as the seed pulse. After a fiber pre-amplifier, an acousto-optic modulator (AOM) is used to change the repetition frequency outside the optical resonant cavity. Then, a master oscillator power amplifier (MOPA) scheme is used to obtain a high-power infrared laser with different repetition frequencies. After that, a green pulsed laser with different repetition frequencies is generated by second harmonic technology (SHG) in a frequency doubling quasi-phase-matched (QPM) periodically poled lithium niobate (PPLN) crystal.Results and DiscussionsAn all-polarization-maintaining ytterbium-doped fiber laser with an adjustable repetition frequency is built in the experiment, which can output a picosecond pulse laser with an adjustable repetition frequency of 5-20 MHz. The frequency doubling of MgO-doped periodically poled lithium niobate (PPMgLN) crystal is studied. At the repetition frequencies of 20, 10, and 5 MHz, the corresponding maximum conversion efficiencies are 39.2%, 35.3%, and 31.0%, respectively, corresponding to output powers of 744, 600, and 496 mW, respectively, as shown in Fig. 2(d). The output spectra [Figs. 2(b), (e)], pulse widths [Figs. 2(c), (f)], and temperature phase matching curve [Fig. 3(a)] are compared. Finally, the stability and beam profile of the frequency doubling laser are tested. The relative jitter of the output power for the green laser is as low as 0.74% within four hours, as shown in Fig. 3(c).ConclusionsIn this paper, an efficient frequency doubling scheme based on an all-polarization-maintaining ytterbium-doped fiber laser with adjustable repetition frequency is verified, and the effects of repetition frequency, fundamental frequency optical power, and crystal temperature on frequency doubling efficiency are tested. A green picosecond pulse laser with different repetition frequencies is obtained by using PPMgLN crystal. The highest conversion efficiency is 39.2%, and the corresponding highest average power of the green laser is 744 mW. The measured root mean square (RMS) of frequency doubling optical power fluctuation is 0.74% within four hours. The proposed scheme has a simple structure and high stability. It provides a useful reference for the efficient generation of green pulsed lasers from high-power Yb-doped fiber lasers with different repetition frequencies.
Acta Optica Sinica
- Publication Date: Oct. 10, 2023
- Vol. 43, Issue 19, 1936002 (2023)
Freestanding Silicon Thin-Film Filters with High Transmission in Extreme Ultraviolet Range
Xiaoran Li, Yiwen Chen, Mojie Xie, and Jiaoling Zhao
ObjectiveFreestanding thin-film filters are important transmissive optical elements for applications in extreme ultraviolet (EUV) bands. Silicon (Si) has the L2,3 absorption edge at the wavelength of 13 nm, providing high transmission at λ=13.5 nm. Therefore, it has been employed as one of the filtering materials in EUV lithography. Previously, Si is mostly adopted as an interlayer to form a multilayer structure with metallic materials, or attached to a nickel mesh to form a grid-supporting structure. However, till now there has been no thorough investigation on self-supporting thin-film filters conducted by sputtering a single-component Si material. To promote the application of Si-based freestanding filters in the EUV field and bridge such a gap in domestic research, we designed and fabricated a 50 nm-thin freestanding Si filter with high transmission at 13.5 nm.MethodsThe Si thin film was deposited on soluble or quartz substrates by pulsed direct current (DC) magnetron sputtering, and upon fabrication and gluing for encapsulation, a 50 nm-thickness filter sample with a flat surface is shown in Fig. 1. Then, the film thickness and morphology were characterized by X-ray reflectivity (XRR) and field emission scanning electron microscopy (FE-SEM). The EUV transmission spectrum measurements were performed at the National Synchrotron Radiation Laboratory (NSRL). Furthermore, the difference between the theoretical and measured transmission values of the filter in the 12.5-20 nm band was further analyzed by X-ray photoelectron spectroscopy (XPS) and IMD software calculations.Results and DiscussionsAccording to the XRR fitting results shown in Table 1, the measured thickness of the thin film is 50.8 nm with a thin SiO2 layer of 1.9 nm. Figure 2 (b) presents the cross-section SEM image of the Si filter, indicating the filter thickness around 50.26 nm is consistent with the XRR fitting results. Then, a sandwich model of "SiO2/Si/SiO2" was built in IMD to calculate the EUV transmission spectra. Figure 3 shows the measured transmission values and theoretical calculation ("cal.1") values in the 10-20 nm band for the filter sample, demonstrating that the measured transmission value reaches 86.02% at 13.5 nm, with an obvious difference (ΔT%) between the two curves in the 12.5-20 nm region. To explain this phenomenon, we examined the sample's composition by XPS, as shown in Fig. 4. A 5 nm-thin SiOx is the majority compound at the surface, and there is a certain level of "bulk oxidation" according to the deep etching results in Fig. 4 (b). With such optimization of the sandwich model from "SiO2/Si/SiO2" to "SiOx/SiOy/SiOx" based on these XPS results, in Table 3 and Fig. 5, the ΔT% is decreased from 2.62% to 0.18% and the two curves coincide much better.ConclusionsTo obtain a highly transmissive EUV filter at 13.5 nm, we prepared a freestanding Si filter (50 nm-thin) with its transmission as high as 86.02% at 13.5 nm, combined with decent suppression in the deep ultraviolet (DUV) range. Meanwhile, the XPS results and the optimized IMD calculation model show that both the surface and bulk oxidation levels of the filters exert a significant influence on its EUV transmission, which is a direction that needs further research efforts. Our results will substantially expand the further applications of such ultra-thin Si filters to areas such as EUV lithography and other large-scale scientific facilities in short wavelengths. ObjectiveFreestanding thin-film filters are important transmissive optical elements for applications in extreme ultraviolet (EUV) bands. Silicon (Si) has the L2,3 absorption edge at the wavelength of 13 nm, providing high transmission at λ=13.5 nm. Therefore, it has been employed as one of the filtering materials in EUV lithography. Previously, Si is mostly adopted as an interlayer to form a multilayer structure with metallic materials, or attached to a nickel mesh to form a grid-supporting structure. However, till now there has been no thorough investigation on self-supporting thin-film filters conducted by sputtering a single-component Si material. To promote the application of Si-based freestanding filters in the EUV field and bridge such a gap in domestic research, we designed and fabricated a 50 nm-thin freestanding Si filter with high transmission at 13.5 nm.MethodsThe Si thin film was deposited on soluble or quartz substrates by pulsed direct current (DC) magnetron sputtering, and upon fabrication and gluing for encapsulation, a 50 nm-thickness filter sample with a flat surface is shown in Fig. 1. Then, the film thickness and morphology were characterized by X-ray reflectivity (XRR) and field emission scanning electron microscopy (FE-SEM). The EUV transmission spectrum measurements were performed at the National Synchrotron Radiation Laboratory (NSRL). Furthermore, the difference between the theoretical and measured transmission values of the filter in the 12.5-20 nm band was further analyzed by X-ray photoelectron spectroscopy (XPS) and IMD software calculations.Results and DiscussionsAccording to the XRR fitting results shown in Table 1, the measured thickness of the thin film is 50.8 nm with a thin SiO2 layer of 1.9 nm. Figure 2 (b) presents the cross-section SEM image of the Si filter, indicating the filter thickness around 50.26 nm is consistent with the XRR fitting results. Then, a sandwich model of "SiO2/Si/SiO2" was built in IMD to calculate the EUV transmission spectra. Figure 3 shows the measured transmission values and theoretical calculation ("cal.1") values in the 10-20 nm band for the filter sample, demonstrating that the measured transmission value reaches 86.02% at 13.5 nm, with an obvious difference (ΔT%) between the two curves in the 12.5-20 nm region. To explain this phenomenon, we examined the sample's composition by XPS, as shown in Fig. 4. A 5 nm-thin SiOx is the majority compound at the surface, and there is a certain level of "bulk oxidation" according to the deep etching results in Fig. 4 (b). With such optimization of the sandwich model from "SiO2/Si/SiO2" to "SiOx/SiOy/SiOx" based on these XPS results, in Table 3 and Fig. 5, the ΔT% is decreased from 2.62% to 0.18% and the two curves coincide much better.ConclusionsTo obtain a highly transmissive EUV filter at 13.5 nm, we prepared a freestanding Si filter (50 nm-thin) with its transmission as high as 86.02% at 13.5 nm, combined with decent suppression in the deep ultraviolet (DUV) range. Meanwhile, the XPS results and the optimized IMD calculation model show that both the surface and bulk oxidation levels of the filters exert a significant influence on its EUV transmission, which is a direction that needs further research efforts. Our results will substantially expand the further applications of such ultra-thin Si filters to areas such as EUV lithography and other large-scale scientific facilities in short wavelengths.
Acta Optica Sinica
- Publication Date: Oct. 10, 2023
- Vol. 43, Issue 19, 1936001 (2023)
Femtosecond Cascade Chirped and Tilted Fiber Bragg Gratings for Raman Filtering
Hao Li, Meng Wang, Baiyi Wu, Xinyu Ye, Chenhui Gao, Binyu Rao, Xin Tian, Xiaoming Xi, Zilun Chen, Zefeng Wang, and Jinbao Chen
ObjectiveThe chirped and tilted fiber Bragg grating (CTFBG) is an important component for filtering Raman light in high-power fiber laser systems. The filtering bandwidth and depth of CTFBG determine the filtering effect, so it is necessary to increase its filtering bandwidth and depth. The tandem inscription method can effectively increase the bandwidth by cascading two CTFBGs with different tilted angles. However, the tandem inscription method based on the traditional ultraviolet laser phase mask technology has the following shortcomings. 1) The fiber needs to be processed by hydrogen loading and heat annealing before and after the CTFBG inscription, respectively, which increases the fabrication time and cost. 2) When cascade CTFBGs with different tilted angles are inscribed, it is necessary to change the tilted angle of the phase mask and realign the inscription system, which increases the inscription complexity. 3) The Bragg reflection bandwidth of cascade CTFBG will also increase, which may provide feedback to Raman light and affect the Raman filtering effect of CTFBG. The proposed femtosecond laser inscription system for cascade CTFBG in this paper can effectively overcome the above shortcomings.MethodsThe femtosecond laser arrives at the cylindrical lens and the chirped phase mask in turn and finally forms interference fringes on the fiber core. The tilted grating plane is formed by oblique scanning of the fiber via a piezoelectric stage. At the same time, the femtosecond laser scans the chirped phase mask along the fiber axis, thereby increasing the length of the grating and introducing a larger chirp. When the femtosecond laser scans along the fiber axis, the grating planes with different tilted angles can be formed by changing the amplitude of the piezoelectric stage, thereby realizing the inscription of cascade CTFBGs. The schematic of the grating structure of the cascade CTFBG is shown in Fig. 1, which consists of sub-CTFBG Ⅰ and sub-CTFBG Ⅱ with different gratings.Results and DiscussionsFigs. 2(a) and 2(b) show the spectra of single-stage CTFBG and cascade CTFBG, respectively. The tilted angle of the former is 6.4° with a grating length of 20 mm. The latter consists of two sections of CTFBG with different tilted angles, and its grating length is 20 mm. The bandwidth of cascade CTFBG is wider than single-stage CTFBG, and the filtration depth can be maintained greater than 20 dB. In order to test the performance of cascade CTFBG for filtering Raman light, a test system is built (Fig. 3). The test source is a continuous-wave high-power fiber oscillator of 1080 nm with a maximum output power of about 1.5 kW and output fiber length of about 18 m. The output spectra measured without and with cascade CTFBG at different output powers are shown in Figs. 4(a) and 4(b), respectively. Raman light is almost completely filtered out by cascade CTFBG at the maximum output power.ConclusionsHere,a CTFBG is fabricated by the femtosecond laser tandem inscription method, and the filtering bandwidth and depth of which are about 15.2 nm and greater than 20 dB, respectively. By introducing the CTFBG at the output end of a high-power fiber laser long-distance transmission system of 1080 nm, the output spectrum without Raman light is realized, which greatly improves the purity of the output laser. This work provides a method for the fabrication of the wideband CTFBG and demonstrates its Raman filtering effect, which is of significance for the development and application of CTFBG. ObjectiveThe chirped and tilted fiber Bragg grating (CTFBG) is an important component for filtering Raman light in high-power fiber laser systems. The filtering bandwidth and depth of CTFBG determine the filtering effect, so it is necessary to increase its filtering bandwidth and depth. The tandem inscription method can effectively increase the bandwidth by cascading two CTFBGs with different tilted angles. However, the tandem inscription method based on the traditional ultraviolet laser phase mask technology has the following shortcomings. 1) The fiber needs to be processed by hydrogen loading and heat annealing before and after the CTFBG inscription, respectively, which increases the fabrication time and cost. 2) When cascade CTFBGs with different tilted angles are inscribed, it is necessary to change the tilted angle of the phase mask and realign the inscription system, which increases the inscription complexity. 3) The Bragg reflection bandwidth of cascade CTFBG will also increase, which may provide feedback to Raman light and affect the Raman filtering effect of CTFBG. The proposed femtosecond laser inscription system for cascade CTFBG in this paper can effectively overcome the above shortcomings.MethodsThe femtosecond laser arrives at the cylindrical lens and the chirped phase mask in turn and finally forms interference fringes on the fiber core. The tilted grating plane is formed by oblique scanning of the fiber via a piezoelectric stage. At the same time, the femtosecond laser scans the chirped phase mask along the fiber axis, thereby increasing the length of the grating and introducing a larger chirp. When the femtosecond laser scans along the fiber axis, the grating planes with different tilted angles can be formed by changing the amplitude of the piezoelectric stage, thereby realizing the inscription of cascade CTFBGs. The schematic of the grating structure of the cascade CTFBG is shown in Fig. 1, which consists of sub-CTFBG Ⅰ and sub-CTFBG Ⅱ with different gratings.Results and DiscussionsFigs. 2(a) and 2(b) show the spectra of single-stage CTFBG and cascade CTFBG, respectively. The tilted angle of the former is 6.4° with a grating length of 20 mm. The latter consists of two sections of CTFBG with different tilted angles, and its grating length is 20 mm. The bandwidth of cascade CTFBG is wider than single-stage CTFBG, and the filtration depth can be maintained greater than 20 dB. In order to test the performance of cascade CTFBG for filtering Raman light, a test system is built (Fig. 3). The test source is a continuous-wave high-power fiber oscillator of 1080 nm with a maximum output power of about 1.5 kW and output fiber length of about 18 m. The output spectra measured without and with cascade CTFBG at different output powers are shown in Figs. 4(a) and 4(b), respectively. Raman light is almost completely filtered out by cascade CTFBG at the maximum output power.ConclusionsHere,a CTFBG is fabricated by the femtosecond laser tandem inscription method, and the filtering bandwidth and depth of which are about 15.2 nm and greater than 20 dB, respectively. By introducing the CTFBG at the output end of a high-power fiber laser long-distance transmission system of 1080 nm, the output spectrum without Raman light is realized, which greatly improves the purity of the output laser. This work provides a method for the fabrication of the wideband CTFBG and demonstrates its Raman filtering effect, which is of significance for the development and application of CTFBG.
Acta Optica Sinica
- Publication Date: May. 25, 2023
- Vol. 43, Issue 10, 1036001 (2023)
Preparation and Strong Ultraviolet Luminescence Performance of Worm-Like ZnO on Sapphire Substrates
Zitong Liu, Niewei You, Yabin Zhu, Zhicheng Zhang, Ge Liu, and Yunlin Chen
350 nm polystyrene microspheres (PSs) are prepared on sapphire substrates as masks by nanosphere lithography technique, and then ZnO thin films are grown on PS/Al2O3 and Al2O3 substrates by reactive radio frequency magnetron sputtering method, respectively. X-ray diffraction analysis, scanning electron microscope observation, and photoluminescence (PL) test are used to measure two kinds of samples after annealing; the PS mask is removed from the Al2O3 substrate. The results show that the grains of the ZnO thin film grown on the PS/Al2O3 (sample 1) are obviously worm-like; the grains of the ZnO thin film directly grown on the Al2O3 substrate (sample 2) show incomplete hexagonal prism shape. Although the crystallization of sample 2 is better than that of sample 1, the intensity of near band edge photoluminescence peak of sample 1 around 362 nm is 43 times stronger than that of sample 2. 350 nm polystyrene microspheres (PSs) are prepared on sapphire substrates as masks by nanosphere lithography technique, and then ZnO thin films are grown on PS/Al2O3 and Al2O3 substrates by reactive radio frequency magnetron sputtering method, respectively. X-ray diffraction analysis, scanning electron microscope observation, and photoluminescence (PL) test are used to measure two kinds of samples after annealing; the PS mask is removed from the Al2O3 substrate. The results show that the grains of the ZnO thin film grown on the PS/Al2O3 (sample 1) are obviously worm-like; the grains of the ZnO thin film directly grown on the Al2O3 substrate (sample 2) show incomplete hexagonal prism shape. Although the crystallization of sample 2 is better than that of sample 1, the intensity of near band edge photoluminescence peak of sample 1 around 362 nm is 43 times stronger than that of sample 2.
Acta Optica Sinica
- Publication Date: Mar. 06, 2022
- Vol. 42, Issue 5, 0536001 (2022)
Expansion of Depth-of-Field of Scattering Imaging Based on DenseNet
Zhaosu Lin, Yangyundou Wang, Hao Wang, Chuanfei Hu, Min Gu, and Hui Yang
Scattering is a fundamental phenomenon in nature. The imaging with large depth-of-field through a scattering medium is significant and valuable. In recent years, with the wide application of deep learning in computational imaging, it is urgent to study and further extend the depth-of-field in a scattering imaging system. In the paper, based on DenseNet and combined with the UNet architecture, a deep convolutional neural network model, namely DUNet, with good mobility and depth-of-field expansion ability is proposed. Moreover, the network is trained with speckle images passing through frosted glasses of different mesh, and the depth-of-field can be generalized to 50 mm away from the focal plane. The preliminary results on a rat brain slice demonstrate that the DUNet can be further implemented in the tomographic scanning of deep tissues. Scattering is a fundamental phenomenon in nature. The imaging with large depth-of-field through a scattering medium is significant and valuable. In recent years, with the wide application of deep learning in computational imaging, it is urgent to study and further extend the depth-of-field in a scattering imaging system. In the paper, based on DenseNet and combined with the UNet architecture, a deep convolutional neural network model, namely DUNet, with good mobility and depth-of-field expansion ability is proposed. Moreover, the network is trained with speckle images passing through frosted glasses of different mesh, and the depth-of-field can be generalized to 50 mm away from the focal plane. The preliminary results on a rat brain slice demonstrate that the DUNet can be further implemented in the tomographic scanning of deep tissues.
Acta Optica Sinica
- Publication Date: Jan. 28, 2022
- Vol. 42, Issue 4, 0436001 (2022)
Characterization of Ion Beam Induced Nanoripples by Using Extreme Ultraviolet Synchrotron Radiation
Jinyu Li, Gaoyuan Yang, Haofeng Zang, Huoyao Chen, Tonglin Huo, Hongjun Zhou, Yonghua Lu, Ying Liu, Yilin Hong, and Shaojun Fu
Quasi-periodic nanostructures induced by ion bombardment (IB) on solid surfaces are characterized by small periods (10-100 nm) and large areas. Quasi-periodic nanoripple structures with the transverse feature size of around 100 nm and the gradually significant transverse periodicity and longitudinal continuity were fabricated on antireflection coatings by Argon-IB. To improve the characterization area, the morphological characteristics of the self-organized nanoripples were characterized by using extreme ultraviolet (EUV) scatterometry. The results show that in terms of samples, their transverse and longitudinal morphological features obtained by the in-plane and conical mode of the EUV scatterometry are in agreement with those obtained by atomic force microscope. These results demonstrate that the proposed method is feasible to characterize the basic morphological characteristics of quasi-periodic nanoripples and can provide a basis for subsequent quantitative analysis. In addition, the characterization area of self-organized nanoripple structures has reached an order of the mm2 by EUV synchrotron radiation, and the characterization range of the Metrology Beamline of Hefei Light Source is extended to self-organized nanostructures, which can provide a reference for future studies on the scattering characterization of EUV lithography masks. Quasi-periodic nanostructures induced by ion bombardment (IB) on solid surfaces are characterized by small periods (10-100 nm) and large areas. Quasi-periodic nanoripple structures with the transverse feature size of around 100 nm and the gradually significant transverse periodicity and longitudinal continuity were fabricated on antireflection coatings by Argon-IB. To improve the characterization area, the morphological characteristics of the self-organized nanoripples were characterized by using extreme ultraviolet (EUV) scatterometry. The results show that in terms of samples, their transverse and longitudinal morphological features obtained by the in-plane and conical mode of the EUV scatterometry are in agreement with those obtained by atomic force microscope. These results demonstrate that the proposed method is feasible to characterize the basic morphological characteristics of quasi-periodic nanoripples and can provide a basis for subsequent quantitative analysis. In addition, the characterization area of self-organized nanoripple structures has reached an order of the mm2 by EUV synchrotron radiation, and the characterization range of the Metrology Beamline of Hefei Light Source is extended to self-organized nanostructures, which can provide a reference for future studies on the scattering characterization of EUV lithography masks.
Acta Optica Sinica
- Publication Date: Oct. 10, 2022
- Vol. 42, Issue 19, 1936001 (2022)
Ultrashort Pulse All-Optical Passive Synchronization Technology with Near-Centimeter-Scale Mismatch Length
Yanluan Wang, Xu Guo, Cheng Tang, and Qiang Hao
An all-optical passive synchronous ultrashort pulse laser source based on an all-polarization-maintaining fiber is built by the master-slave injection locking method. By optimizing the pulse energy and pulse width of pulse from the master laser, the mismatch length of 0.879 cm is achieved. The designed synchronous laser source consists of an erbium-doped fiber mode-locked pulse laser (master laser) and a ytterbium-doped fiber mode-locked pulse laser (slave laser) based on nonlinear amplifying loop mirror. The central wavelength of the master laser is 1560 nm, and its spectral width is 5.01 nm. When the output single pulse energy is increased from 0.06 nJ to 3.26 nJ, the narrowest and widest pulse widths are 221 fs and 1.79 ps after the output pulse passes through the erbium-doped fiber amplifier. The central wavelength of the slave laser is 1064 nm, and its spectral width and pulse width is 0.22 nm and 9.5 ps respectively. By controlling the output parameters of the erbium-doped fiber amplifier, the tolerance range of cavity-length mismatch between the master and slave lasers approaches to centimeter level, which provides an efficient way to avoid using the time delay control components and fiber optical path coupling components in the synchronous laser source. An all-optical passive synchronous ultrashort pulse laser source based on an all-polarization-maintaining fiber is built by the master-slave injection locking method. By optimizing the pulse energy and pulse width of pulse from the master laser, the mismatch length of 0.879 cm is achieved. The designed synchronous laser source consists of an erbium-doped fiber mode-locked pulse laser (master laser) and a ytterbium-doped fiber mode-locked pulse laser (slave laser) based on nonlinear amplifying loop mirror. The central wavelength of the master laser is 1560 nm, and its spectral width is 5.01 nm. When the output single pulse energy is increased from 0.06 nJ to 3.26 nJ, the narrowest and widest pulse widths are 221 fs and 1.79 ps after the output pulse passes through the erbium-doped fiber amplifier. The central wavelength of the slave laser is 1064 nm, and its spectral width and pulse width is 0.22 nm and 9.5 ps respectively. By controlling the output parameters of the erbium-doped fiber amplifier, the tolerance range of cavity-length mismatch between the master and slave lasers approaches to centimeter level, which provides an efficient way to avoid using the time delay control components and fiber optical path coupling components in the synchronous laser source.
Acta Optica Sinica
- Publication Date: Aug. 25, 2022
- Vol. 42, Issue 16, 1636001 (2022)
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