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Abstract
Inscription of fiber-compatible active waveguides in high-gain glass, followed by direct interconnection with few-mode fibers, is one of the most promising solutions for all-optical mode-division multiplexing. In this work, based on the femtosecond laser writing technique, we propose a general fabrication scheme for inscribing high-order mode waveguides in glass, by carefully tailoring the cross-section of the waveguides to match the mode intensity distribution via an improved multi-scan approach. Specifically, we design and fabricate two kinds of waveguides, namely, LP01-mode waveguide and LP11-mode waveguide in a highly Er3+-doped phosphate glass, enabling the insertion loss of the waveguides to be as low as 1.88 dB, and the mode extraction factor of the LP11-mode waveguide up to ∼24 dB. Importantly, we have successfully achieved optical amplification of the waveguides, with an on-off gain as high as 3.52 dB. This novel high-order mode waveguide amplifier has broad application prospects in monolithic on-chip integrated photonic light sources and optical interconnection with few-mode fiber and/or silicon-based waveguide.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Waveguide amplifiers are central components of optical communication systems [1] and on-chip lasers [2]. It would be of high technological importance that embeds fiber-compatible active waveguides in high-gain glass for numerous applications including signal sources in integrated optical paths [3], signal amplification [4], and all-optical mode division multiplexing communication [5]. However, traditional waveguide amplifiers are mostly based on planar waveguide fabricated by Silicon-on-Insulator (SOI) technology [6] or recently emerging thin-film Lithium Niobate-on-Insulator technology [7–9], facing problems of complex manufacturing process, time-consuming, high-expense, vulnerable to environmental impact, high optical loss and low gain.
Comparably, femtosecond laser direct-writing (FLDW) shows great potential for manufacturing different components in three-dimensional (3D) optics [10–14] and particularly on-chip integrated photonic devices [15–17]. Recently, optical amplifiers based on FLDW technique have been studied by some groups [18,19]. However, high loss and low gain of waveguide amplifier are always the key issues scientists are still facing. Actually, the performance of an optical waveguide amplifier relies on not only waveguide structures but also gain materials. However, few studies on optical waveguide amplifier written by fs laser have focused on the design of high-order mode waveguide [18,19], which can effectively improve the purity of high-order mode, reduce the loss of the waveguide, and increase optical gain of the waveguide amplifier. For the gain medium, Er3+-doped phosphate glass has been proved to be an ideal material for waveguide amplifiers, due to the wide optical transmission range (0.4-2 µm), high rare-earth doping concentration, quenching-free of luminescence, low nonlinear coefficient, suitable for short cavity narrow linewidth laser [20,21] and short waveguide amplifier. Besides, Er3+-doped phosphate glass can provide higher optical gain in the third optical communication window [22], due to large emission cross-section and long fluorescence lifetime [23–25], in comparison with other glass. Unfortunately, there are few studies on waveguide amplifier in Er3+-doped phosphate glass by the FLDW technique [26,27], due to the difficulties associated with the fabrication of high-quality Er3+-doped phosphate glass.
In this work, we employ femtosecond laser to directly write high-order mode waveguide amplifier in a home-made Er3+-doped phosphate glass. As a proof-of-concept experiment, we design and fabricate LP01-mode and LP11-mode waveguide structure in Er3+-doped phosphate glass by using the FLDW technique, achieving a maximum mode extraction factor of ∼24 dB, a minimum insertion loss of 1.88 dB, and a maximum optical amplification on-off gain of 3.52 dB. This technique can be generalized to the fabrication of different higher-order mode waveguide amplifiers, suggesting broad applications in high-power lasers, monolithic on-chip integrated photonic light sources, and optical interconnection with few-mode fiber and/or silicon-based waveguides.
2. Methods
2.1 Design and fabrication of LP01-mode and LP11-mode waveguides
A photonic-lattice-like cladding waveguide (PLCW) is designed and written by closely spacing single tracks. Multi-scan with a same spacing is performed at different positions of the Er3+-doped phosphate glass, followed by the desired geometry with 0.8 µm between the adjacent tracks. Under this conditions, two waveguides with different cross-sectional shapes are designed by mode field of LP01 and LP11, as shown in Fig. 1 (inserts).
Fig. 1. Optical setup for measurement of optical gain waveguide amplification. Inserts, design of the PLCW waveguides structure and simulation of corresponding mode-field profiles.
2.2 Femtosecond laser direct-writing (FLDW) system
The femtosecond Yb: KGW laser source (Pharos, optical conversion) is amplified by chirped pulse, which provides 214 fs pulse laser with repetition frequency of 1 MHz and wavelength of 1030 nm. To write the PLCW waveguide in Er3+-doped phosphate glass, we use a microscope objective lens (Nikon CFI Plan Achromat 100× oil immersed lens, 1.30 NA) to focus the laser beam. A 3D nano translation platform (Aerotech, A3200) is employed to adjust the scanning depth of the PLCW waveguide and to control the movement at a scanning speed of 20 mm/s. The laser pulse energy deposited on the Er3+-doped phosphate glass is finely controlled from 300 to 450 nJ by a continuous attenuator.
2.3 Measurement of mode-field profile and optical loss
After femtosecond laser writing, the YZ end face of the sample is polished. The image of the polished PLCW waveguide cross-section is recorded by a microscope (Olympus IX71)). We use single-mode fiber to measure the insertion loss of PLCW waveguide by inputting 1550 nm laser and coupling with the waveguide [7,28]. The insertion loss of the PLCW waveguide is calculated by dividing the power carried by the input optical fiber by the power output of the PLCW waveguide [29]. The initial length of the sample is 10 mm. The propagation loss of the PLCW waveguide is measured by truncation method, through reducing the length of waveguide by polishing. To obtain the near-field mode distribution of the PLCW waveguide, a long focal length lens (Mitutoyo M Plan Apo NIR 50×, 0.42 NA) is used instead of the output fiber. After adjusting the position of the lens, the near-field mode profile is imaged by the beam profiler (CMOS - 1201-IR, CINOGY).
2.4 Characterization of optical gain waveguide amplification
As shown in Fig. 1, a Santec TSL-550 tunable fiber laser (1500-1630 nm) is used as seed light, and a 976 nm fiber laser is employed as pump light. A 976/1550 wavelength division multiplexer (WDM) is utilized to couple the seed light and pump light into the same optical fiber. The optical amplification signal is collected by an objective lens (50×, NA = 0.42, and a focal length of 200 mm) and ultimately recorded by a spectrometer (Yokogawa AQ6370D) coupled with a single-mode fiber.
2.5 Optical simulation of mode-field profile
The distribution of guided modes in PLCW waveguide is simulated by commercial COMSOL software. In the simulation, we use a circular model with uniform index and an equally divided double cavity model to represent the LP01-mode and LP11-mode waveguide structure. The substrate refractive index used in the mode filed simulation is 1.5355 and the waveguide refractive index is 1.5335 according to the literature [26]. The refractive index contrast is 0.002.
3. Results and discussion
In this work, we designed two types of PLCW waveguides in Er3+-doped phosphate glass for optical amplification. A multi-scan technique is used to modulate the cross-section shape of the waveguide [16], in order to fabricate the LP01-mode and LP11-mode PLCW waveguides. To verify the optical confinement effect of the PLCW waveguides, mode-field profiles of the fabricated waveguides are recorded. As shown in Fig. 2(a), the LP01 mode waveguide is annular, the diameter of the inner ring is 9 µm, and the diameter of the outer ring is 17 µm. The LP11 mode waveguide processes an area with complex refractive index in the center of the waveguide, which is divided into two 7.2-µm-wide cores separated by 4.8 µm, in good agreement with the design. The diameters of the LP01 and LP11 modes at the wavelength of 1550 nm are ∼17 µm and ∼23 µm, respectively. Interestingly, these two waveguides are also suitable for operation at wavelengths ranging from visible to near-infrared. To study the purity of high-order mode, mode extraction factors of LP11 mode at various wavelengths are analyzed and calculated, as plotted in Fig. 2(b). Most of the mode extraction factors are greater than 10 dB, and the maximum mode extraction factor reaches ∼24 dB. The designed structure reduces the proportion of LP01 mode while increasing the proportion of LP11 mode [30], which is beneficial for application in optical gain waveguide amplification. In addition, when coupled with a single-mode fiber, the conversion process of the LP11 mode along the propagation in the LP11 waveguide can be understood according to the following two points:
- 1. Due to the existence of a negative refractive index region in the center of the LP11-waveguide, the loss of the LP01 mode is much higher than that of the LP11 mode, resulting in an increase in the purity of the LP11 mode as the waveguide length increases. Similar design principles can be found in the literature [30];
- 2. According to the mode simulation results (Fig. 1), the LP11 mode is the fundamental mode of LP11-waveguide, as LP11-waveguide does not support the LP01 mode. Therefore, the phenomenon predicted by the super-mode theory that the energy bounces between two parallel waveguides have not been observed between the two lobes of the LP11 waveguide. Furthermore, in our case, we consider mode stability as mode purity, which increases with propagation distance.
Fig. 2. Mode profiles of the Er3+-doped phosphate glass optical gain waveguides. (a) Microscope cross-section and corresponding mode-filed profiles of the fabricated LP01-mode and LP11-mode PLCW waveguides at propagation wavelengths of 532, 633, 976, and 1550 nm. (b) Mode Extraction Factor of LP11 Mode at propagation wavelength of 532, 633, and 976 nm.
To achieve optical amplification at C band (1530-1565 nm), it is necessary to measure the insertion loss of the PLCW waveguide [15,31,32]. Figure 3 shows the insertion loss of the two types of waveguides with length ranging from 2 to 10 mm at 1550 nm. The insertion loss decreases lineally as the length of the PLCW waveguide becomes shorter, because the propagation loss is far greater than the coupling loss. When the length of the waveguide keeps at 2 mm, the smallest insertion loss of the LP01-mode and LP11-mode PLCW waveguide is 1.88 dB and 2.16 dB, respectively; which are comparable to those reported in literature [27,33,34]. Here, we use truncation method to calculate propagation loss of the PLCW waveguides. The propagation loss and coupling loss can be analyzed and fitted through the formula of Li = Lc+Lp × l, where Li is the insertion loss, Lc denotes the coupling loss, Lp represents the propagation loss, and l stands for the length of the waveguide. The insertion loss for all the waveguides can be well fitted by the least-square error method, giving a smallest propagation loss of 1.51 dB/mm (LP01-mode waveguide) and 1.65 dB/mm (LP11-mode waveguide), respectively.
Fig. 3. Insertion loss of Er3+-doped phosphate glass gain waveguide. (a) LP01-mode waveguide. (b) LP11-mode waveguide.
Furthermore, we studied the optical amplification characteristics of the PLCW gain waveguides by evaluating the on-off gain effect. As plotted in Figs. 4(a), (b), the gain for both LP01-mode and LP11-mode waveguides augments as the pump power increases from 270 to 450 mW, and tends to saturate by further increasing the pump power, it is in agreement with the literature [27,35,36]. Figures 4(c), (d) shows a robust contrast between the peak value of the output signal with and without pump light ranging from 1500 to 1600 nm, further indicating that an optical amplification is indeed achieved. The gain spectra in the window of 1500-1600 nm of the two waveguides are shown in Figs. 4(e), (f). A maximum gain of 3.46 dB and 3.52 dB for the LP01-mode and LP11-mode waveguide, respectively, is obtained. The experimental results indicate that C-band optical amplification has been achieved for both single-mode ring waveguide and high-order mode waveguide in Er3+-doped phosphate glass, which is promising for light signal amplification in integrated optics, optical interconnection with few-mode fiber and/or waveguide, and others.
Fig. 4. Optical amplification features of the PLCW gain waveguides. (a), (b) Dependence of gain on pump power for the (a) LP01-mode waveguide and (b) LP11-mode waveguide. (c), (d) Peak intensity contrast with and without pump light for the (a) LP01-mode waveguide and (b) LP11-mode waveguide. (e), (f) Net gain spectra of the (a) LP01-mode waveguide and (b) LP11-mode waveguide in the window of 1500-1600 nm.
4. Conclusions
In conclusion, based on the femtosecond laser writing technique, we propose a general fabrication scheme for inscribing high-order mode waveguides in glass, by carefully tailoring the cross-section of the waveguides to match the mode intensity distribution via an improved multi-scan approach. We have designed and fabricated LP01-mode and LP11-mode optical waveguides in Er3+-doped photographic glass by fs laser direct-writing, with mode extraction factor as high as 24 dB and insertion loss as low as 1.88 dB at 1550 nm. Furthermore, we have successfully realized LP01-mode and LP11-mode optical waveguide amplification in Er3+-doped phosphate glass within the optical communication window of 1500-1600 nm, achieving a maximum on-off gain of 3.52 dB. This novel high-order mode waveguide amplifier shows great potential for applications in high power lasers, monolithic on-chip integrated photonic light sources, and optical interconnection with few-mode fiber and/or silicon-based waveguide.
Funding
National Key Research and Development Program of China (2021YFB2800500); National Natural Science Foundation of China (62105297, 62175210, U20A20211); Natural Science Foundation of Zhejiang Province (LQ22F050022, LR21E020005, LZ23F050002); Fundamental Research Funds for the Central Universities.
Acknowledgments
This work was supported by the National Key R&D Program of China (No. 2021YFB2800500), National Natural Science Foundation of China (Nos. U20A20211, 62175210, 62105297), Zhejiang Provincial Natural Science Foundation (Nos. LZ23F050002, LR21E020005, LQ22F050022), and Fundamental Research Funds for the Central Universities.
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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