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Abstract
Fiber lasers are powerful tools for metal processing. Fiber Bragg gratings at the ends of the fiber play an important role as a fully reflective mirror and light modulator for a specific wavelength. The common methods for the manufacture of fiber Bragg grating involve expensive ultraviolet lithography instruments and environmentally harmful chemicals to prepare the mask. A film coating method is proposed to prepare both end pumping and outer coupler ends of a fiber to function similar to a fiber Bragg grating. An electron gun, along with ion beam-assisted deposition (IAD), was used to produce end filters for end pumping and the outer coupler filter devices of a fiber used in a fiber laser. The optical performance is consistent with the calculated simulation results. Scanning electron microscopy results reveal a dense multilayer structure with distinct and unambiguous interfaces in the coated film using the IAD process while that without IAD is loose and pulverized structure. The film designed with electrical field reduction on the surface produced by the IAD process exhibits a higher laser damage threshold. The coated films produced by electron gun deposition with the aid of IAD provide an alternative method for end filters in the fiber of a fiber laser.
© 2022 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber lasers are compact, can produce high pulsed energy due to high conversion efficiency, require less maintenance after installation, and easily dissipate thermal energy because of the high surface-to-volume ratio in the fiber. For these reasons, fiber lasers are considered to be alternative laser sources for solid-state lasers. Most of the generated wavelength spectra, which are pumped by lasers with wavelengths of 940–980 nm and generate wavelengths of 1030–1080 nm in the ytterbium-doped fiber, are located in the high metal absorption realm. That is why one of the main applications of fiber lasers is in the automobile industry [1–4]. The laser produces a powerful heat and light source for metal processing, such as metal cutting, metal welding, and heat-treating. Furthermore, a wavelength of approximately 1550 nm is released when the fiber is erbium-doped. A laser with a wavelength of 1550 nm is eye-safe and is good for medical sensors [5] and LiDAR applications [6].
In principle, pumped photons injected into a fiber oscillate between two Bragg grating ends of the fiber, causing population inversion and stimulated emission induced by the rare earth elements doped in the fiber. The characteristic wavelength of light, which is a response to the excited electron transition, is then released [1,5]. To practically complete the light generation process in a fiber laser, a spool of fiber is bounded by fiber Bragg gratings at each end. Bragg gratings essentially mirror functioning as a resonator at both ends of the doped fiber. When the light is bounced back and forth between these two gratings, the amplitude of light is amplified in every pass. The grating in the pumping end of the fiber laser is required to be transparent for the pumping wavelengths and fully reflects the stimulated wavelengths. In addition, the grating in the output end of the fiber laser needs reflecting the pumping wavelengths but is partially transparent for the stimulated wavelengths, allowing the amplified wavelength laser light to exit the fiber. An important strategy is necessary to maximize the input energy for the pumping wavelength at the light input end of the fiber and to have a proper transmittance for the laser wavelength to exit at the released end of the fiber.
Gwinner gave a brief but informative introduction to understanding the fundamental basis of fiber laser design, fiber laser operational principles, and key components to build a fiber laser [7]. The central key component of a fiber laser is the fiber Bragg grating. The periodic modulation of the core refractive index must be precisely controlled enough to offer a high reflection at one end and a partial reflection at the other end. Most fiber Bragg gratings are produced by exposing a photosensitive core fiber with the interference of a phase mask. An incident ultraviolet light generated from an excimer laser is directed into this phase mask to establish a diffraction pattern and interference between the patterns [8,9]. This interference instrument is an expensive investment, and the phase mask production requires chemicals that are detrimental to the environment.
There are few researching works aimed at applying the thin film layer filters coated on one end or both ends of the fiber to implement and simulate the function of a fiber Bragg grating. The end of the fiber close to the excited laser is called end pumping, the other end of the fiber is called the outer coupler, releasing the stimulated wavelength. Orsila et al. coated the alternating ZrO2 and SiO2 layers on a facet end of a single–mode fiber to produce a dichroic mirror [10]. The purpose of this mirror was able to provide the necessary selectivity for 980 nm pumping and 1040 nm stimulated wavelengths. They combined the coated end filter of the fiber with a photonic crystal dispersion compensator to realize a short-cavity mode locked all-fiber laser with 571 MHz repetition rate. Okamoto et al. coated the dielectric mirrors on the cleaved end surface of the silica fibers [9] to function as fiber Bragg gratings. They spliced the pumping end coated fiber to a short Pr3+-doped ZBLAN fiber and the outer coupler coated end to the opposite end of ZBLAN fiber. An efficient all-fiber laser with 521 nm released wavelength was demonstrated. Fortin et al. integrated a dichroic mirror deposited on the input fiber end with a fiber Bragg grating as an outer coupler to produce a watt-level all-fiber laser operating at 3.44 µm [11]. A silica fiber end coated with dielectric film as a resonator was assembled with graphene-oxide as a saturable absorber to become a compact Q-switched red Pr3+-doped fiber laser [12]. The authors claimed the laser possessed a wide range of pulse repetition rate from 64.1 to 195.3 kHz and minimum pulse duration of 554 ns. Khurmi et al. used a broadband visible anti-reflection coating, consisting of Al2O3, Ta2O5 and MgF2 layer, on both end faces of the glass chip to reduce the reflective loss [13]. They employed an ultra-fast laser inscription method to scribe the waveguides into bulk Pr3+-doped glass chip and produced visible laser emission at 636 nm with larger than 8 mW average power laser. All the papers reported concentrate on the laser performance description and laser system construction to realize a compact, robust, efficient and integrated system with the different emission wavelengths. Few papers refer the coating materials deposited on the fiber ends, and little information is released on the coating technology and microstructure of the coated film stack.
We propose a thin film coating method on both ends of an optical fiber to implement and simulate the function of fiber Bragg gratings. This work focuses on the effect of ion beam-assisted deposition (IAD) on the microstructure improvement and enhanced quality of the obtained coating films. A reduction of electric field on the surface of the coated film stack is considered further to enhance the resistance of laser-induced damage threshold.
2. Experimental
A fiber doped with ytterbium was employed in this study. The wavelength range used for diode pumping was 915–980 nm, and the released stimulated wavelengths from the outer coupler end of the fiber were 1030–1100 nm. We first performed a simulation calculation using Macleod software to design the film structure for both the pumping and outer coupler ends in the fiber. Titanium dioxide and silicon dioxide functioned a high and low refractive index materials with refractive indexes of 2.25 and 1.46, respectively. Electron gun deposition with and without IAD was conducted to prepare the film filter deposited on both ends of the fiber. Figure 1 shows the schematic diagram of the electron gun deposition system with installation of E-gun, IAD, heater, substrate holder, and optic monitor. The substrate holder is rotated by a planetary rotation mechanism. A special fixture was designed to attach the substrate holder and make the fiber end face the deposition source. Quartz coupons were attached beside the fiber end and represented the optical performance after the application of the coating because the small diameter of the fiber makes it difficult to measure its characteristics. The operational parameters are given in Table 1 and describe the IAD process.
Fig. 1. Schematic diagram of the electron gun deposition system installed with ion-beam assisted deposition
Table 1. Coating parameters used to prepare the end filters
Thin films were deposited by an electron gun system at a background pressure of 10−7 torr. The rate of evaporation was monitored by a quartz crystal monitor, and the layer reflectivity was recorded in-situ to control the film thickness using an optical monitor. The distance between the sources of ion and evaporation and the substrate was approximately 70 cm. The RF power was 300 W, and the voltage of the grid was 300 V. The ion source current was maintained at 300 mA and was neutralized to avoid charge buildup. The refractive index of the coated films on the quartz coupon was determined via ellipsometry. Spectral reflectance and transmittance were measured using a Perkin–Elmer spectrometer. Similar refractive indexes of the coated films were calculated [14]. The microstructures of the fiber end filters were observed using field emission electron microscopy. The laser-induced damage threshold (LIDT) of the films was irradiated by a homemade laser system. A wavelength of 1064 nm was irradiated from a high-power laser with a pulsed width of 0.5 ns and an operational frequency of 10 Hz. The energy increased monotonically. A flip mirror by rotation was used to direct the laser beam either to the collimating lens set for threshold value evaluation or to the energy meter for energy measurement. The beam passed through a set of collimating lenses and focused into a 1-mm-diameter spot on the sample surface. The sample was positioned to incline 45° to the beam to observe via telescope if damage occurred. Figure 2 shows a schematic diagram of the LIDT instrument. The detailed damaged morphologies were examined using a confocal microscopy.
Fig. 2. Homemade instrument designed to determine laser-induced damage threshold. HWP: half-wave plate, TFP: thin-film polarizer, HR1064: high-reflection mirror for 1064 nm, Flip mirror: a rotating mirror controls the beam direction toward either an energy meter or the tested sample
3. Results and discussion
The refractive index of the TiO2 film produced with and without the IAD process with wavelength change is illustrated in Fig. 3. The red dashed line represents the film produced with the IAD process, and the black solid line is the film produced without the IAD process.
Fig. 3. Refractive index variation versus wavelength obtained from TiO2 films prepared with (red dashed line) and without (black line) the IAD process.
The refractive index of the TiO2 film decreases with the increase of wavelength. The index of refraction measured at a wavelength of 600 nm for the film produced with IAD treatment is approximately 2.44, which is significantly higher than that without IAD (1.89). The increase in the refractive index of the film with the IAD process is attributed to ion bombardment, which modifies the film structure during deposition and results in film densification [15,16].
The wavelengths radiated by the designed pumping diodes are between 915 and 980 nm. The pumping energy is absorbed by the doped element (ytterbium in the fiber core), and the longer wavelengths, ranging from 1030 to 1100 nm with the central wavelength located at 1070 nm, were generated in the fiber by the stimulated emission mechanism. The beam oscillates back and forth between both ends of the fiber, and the amplitude is amplified in every cycle. As described, both ends act as filters. The short wavelength is introduced into the fiber, and the stimulated wavelength is fully reflected, like a total reflector mirror, at the end pumping side during beam oscillation in the fiber. The outer coupler end acts like a partial reflector mirror that reflects most parts of the pumping energy and releases the peak energy of the stimulated wave in the same phase. Due to this process, the required optical performance at the end pumping filter is an average transmittance larger than 95% within wavelengths ranging from 900 to 980 nm and an average reflectance larger than 99% at wavelengths ranging from 1060 to 1085 nm. The designed criteria in the outer coupler filter are that the average transmittance should be less than 20% from 1065 to 1085 nm, and the average reflectance must be larger than 99% from 900 to 1035 nm. Figure 4 exhibits the simulation transmittance results for both filters based on these design criteria. Figure 4(a) is for the end pumping and Fig. 4(b) the outer coupler. There are 31 multilayer coatings in the design with alternating SiO2 and TiO2 layers. The optical design parameters of the device filters were optimized using optical admittance loci analysis.
Fig. 4. Simulated transmittance spectrum of a thin-film filter for the (a) end pumping and (b) out-coupler ends.
Since the refractive index of a coated material layer is enhanced by electron gun deposition with the IAD process, it is important to produce device filters based on the simulation result and to understand the effect of the IAD process on devices with the film stack. Figure 5 shows the measured transmittance of end pumping filter producing using different film deposition processes. The measured transmittance of filter with IAD process is higher than that without IAD process in the pumping wavelength ranges. The transmittance of the film stack producing using IAD reaches 95.2% compared with that of 90% of film stack deposited using electron gun only (without IAD). In addition, the spectrum with blue-shifted by 71 nm is observed for the film stack deposited using IAD. The similar blue-shift phenomenon is observed in the HfO2-TiO2-SiO2 multilayer high reflection mirrors deposited with IAD process [17].
Fig. 5. Transmittance spectrum of the end pumping filter as a function of wavelength prepared with (red dashed line) and without (black line, e-beam process only) the IAD process. A blue shift can be observed in the filter with the IAD process.
Figure 6 shows the experimental result for the outer coupler device film stack. The average transmittance is approximately 0.3% between 900 and 985 nm and 20% between 1070 and 1080 nm. A Bragg reflectance resonance of 80% is observed at the incident side of the outer coupler at wavelengths from 1070 to 1080 nm. The back-scattering wave is in phase, and the scattering intensity is modulated and accumulated with the incident and further stimulated waves. A peak laser intensity with wavelengths ranging from 1070 to 1080 nm is released out of the outer coupler end when the oscillated light intensity reaches a certain value.
Fig. 6. Transmittance spectrum as a function of wavelength for the out-coupler filter prepared using the IAD process.
The effect of proper energy ion bombardment on the cross-section morphologies of the end pumping film filter is shown via the scanning electron microscopy (SEM) image in Fig. 7. The morphologic microstructure of the cross-section of filters subjected to the IAD process is demonstrated in Fig. 7(a) and that without the IAD process is shown in Fig. 7(b). The film layers deposited with ion assistance have a denser structure and more environmental stability than that obtained via conventional deposition, which is consistent with reports [15,18–20]. The distinct and dense layer structure can be observed clearly in Fig. 7(a) compared with the obscure, fuzzy, and pulverized structure in Fig. 7(b). The inserted images located in the left corner of each figure show an enlarged and detailed microstructure. During film deposition with IAD, the kinetic momentum of the ions is transferred to the grown film either through argon ion bombardment or the increase of momentum of the atoms arriving to create the film. The increase of ion momentum is believed to make the outmost material atoms penetrate deeper into the deposited film and densify the created coated films.
Fig. 7. SEM images of cross-section morphologies of an end pumping filter prepared (a) with and (b) without the IAD process. The insert at the left bottom corner of each figure is enlarged locally to show the detailed microstructure.
To prevent laser damage on the coated film stack for the laser applications, a simulation was performed to calculate the maximum electric field generated on the surface. Figure 8 shows the electric field distribution from the film surface to the internal layer of the multilayer film stack. The calculated electric field on the surface is 33 V/m in black line. An optimal field intensity decreasing to 16 V/m (red dotted line in Fig. 8) can be obtained when the thicknesses of the TiO2 and SiO2 are adjusted.
Fig. 8. Electric field distribution along the surface to the internal layer for the film stack without considering field reduction (black line) and that designed to reduce the field on the surface (red dashed line). The field concentrated on the surface without considering field reduction is 33 V/m and that with the optimal design is 16 V/m.
Two deposited processes were prepared for the film stack based on the electric field calculations. The LIDT value of the thin film is determined by the laser fluence causing observed damage feature on the specimen surface and was conducted using the homemade instrument shown in Fig. 2. The energy level was increased at each step by about 2 mJ. Figure 9 shows the damage features for low and high electric fields. The LIDT energy in Fig. 9(a) was 20.3 mJ, corresponding to a fluence of 5.18 J/cm2, whereas that for Fig. 9(b) was 34 mJ, corresponding to a fluence of 8.66 J/cm2. The damage features are elliptical other than circular shape because the specimen is inclined 45° from the incident laser beam. The damage feature observed for the lower fluence, attributed to high electric field on surface, is a geographical basin with a central ablated pit wherein a hill surrounded the pit, but no clear border is observed between the laser incidence area and the undamaged area. Furthermore, the damage feature for the higher fluence, due to low electric field, is a plateau with a crack circulating it. Because of peeling, the plateau is approximately 10-µm above its surroundings. The corresponding scanned lines for these damage regions were measured by confocal microscopy and are illustrated in Fig. 9(c) and 9(d) for specimens with low and high fluence, respectively. The delaminated fragment is found to consume more energy to break the debris from the film stack compared with the ablated film, which is a general observation for a high electric field on a surface.
Fig. 9. The damage features on filter surfaces concentrated at the (a) high and (b) low electric field. The corresponding line scanning for (a) and (b) are given in (c) and (d), respectively. A central ablated pit in a geographical basin is shown in (a) and (c). The delaminated fragment (peeling) is shown in (b) and (d).
Most LIDTs of films are obtained using different pulsed widths and laser wavelengths, depending on the evaluating purpose and designed instrument, which makes the comparison between different operation conditions difficult. Fortunately, an empirical equation is provided to give an approximate quantitative estimation [21,22], which states the adjusted LIDT is proportional to the square root of the pulsed width and wavelength:
The pulsed width used in our instrument is 0.5 ns, which is not compliant with ISO 21254. ISO 21254 requests that the pulsed width is 10 ns and pulsed wavelength is 1064 nm. We transformed the obtained LIDT values of 5.18 and 8.66 J/cm2 to corresponding values compliant with ISO 21254. Transformed values of 23.16 and 38.73 J/cm2 were calculated for high and low electric field designs, respectively.
The LIDT values collected from the published documents and commercial product are tabulated in Table 2 to illustrate the effects of film stack preparation process on the LIDT values. Most of the film stacks collected were the multilayer of TiO2/SiO2 for identical comparison. The film stack deposited by electron beam deposition at ambient temperature without IAD shows the lowest LIDT value of 5.1 J/cm2 [23]. When the film stacks were treated by post-annealed or deposited at elevated substrate temperatures, the LIDT values increased and were ranged between 13 J/cm2 and 16 J/cm2 [24–26]. The LIDT values exhibited a significant increase when the film stacks were deposited under the IAD and the values reached 23 J/cm2 to 32 J/cm2 [17], depending on the detailed deposition condition control and measurement condition. Even though we do not know how the ThorLabs Co. manufactures the laser-line mirror NB07-K14 [28], we believe the mirror is deposited with IAD and at elevated substrate temperatures from the data analysis. The LIDT value of this product resembles our film stack, which is coated under IAD at elevated substrate temperature but no consideration of electric field reduction. The LIDT values can further increase when the film stacks are coated with the consideration of electric field reduction on the surface. The LIDT values of film stacks can achieve up to 38.7—39.6 J/cm2 [29]. Our strategy combining an e-gun with IAD coating and thin-film design to shift the maximum electrical field away from the film stack surface is proven to achieve better film quality and a higher LIDT value.
Table 2. LIDT values obtained by different film stack preparation process.a
4. Conclusion
Thin-film optical filters designed and coated on both ends of a fiber and functioning as an end pumping and out-coupler device, allowing for intentional replacement of a fiber Bragg grating, were proposed. Film stacks consisting of alternating TiO2 and SiO2 layers were produced with and without the IAD process. An increased refractive index and clearly distinct dense layer structure were observed in the film prepared with the IAD process. The transmittance of the film stack treated by IAD can reach 95.2% at a pumping wavelength of 980 nm, and the spectrum is blue-shifted by 71 nm. When the film stack was deposited using only the electron (without IAD) a measured transmittance of 90% was obtained. A low electric field designed through concentration on the surface resulted in a higher LIDT, and the damaged feature was a delaminated fragment instead of an ablated pit.
Funding
Ministry of Science and Technology, Taiwan.
Acknowledgments
The authors appreciated the financial support from the Taiwan Ministry of Science and Technology. The authors also thank Prof. Yen-Chieh Huang for LIDT measurement, Ms. Nancy Chu for her SEM observation and Enago for English proofreading service.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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