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Abstract
The S-band polymer-based waveguide amplifier has been fabricated, but how to improve the gain performance remains a big challenge. Here, using the technique of establishing the energy transfer between different ions, we successfully improved the efficiency of Tm3+:3F3→3H4 and 3H5→3F4 transitions, resulting in the emission enhancement at 1480 nm and gain improvement in S-band. By doping the NaYF4:Tm,Yb,Ce@NaYF4 nanoparticles into the core layer, the polymer-based waveguide amplifier provided a maximum gain of 12.7 dB at 1480 nm, which was 6 dB higher than previous work. Our results indicated that the gain enhancement technique significantly improved the S-band gain performance and provided guidance for even other communication bands.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The rapid worldwide development of information technologies such as the fifth-generation (5 G) mobile communications and the internet of things (IoT) keep forcing people to improve the capacity load of communication systems, especially in optical fiber communication networks. Although the methods to boost the fiber transmission capacity are multifarious (time division multiplexing, space division multiplexing, etc.), wavelength division multiplexing (WDM) is still one of the most effective and convenient technologies for achieving capacity improvement due to its compatibility with traditional single-mode fiber (SMF) [1–4]. However, the working efficiency of WDM is strongly restricted by its available operating wavelength [5]. For SMF, there are several low-loss transmission windows that allow signal light propagates with low absorption loss, but only C-band has been effectively exploited due to the industrialization of erbium-doped fiber amplifier (EDFA) [6–8]. Obviously, the gain bandwidth of the optical amplifier directly determines the operating wavelength of WDM. Since the most effective way to improve the performance of the WDM system is to broaden its available working wavelength, it is essential to first fabricate optical amplifiers working in other low-loss transmission bands (S-band, L-band, etc.) [9–11].
As an essential part of the optical network, the integrated optical device also needs to cooperate with the WDM system to meet the requirements of gigantic data flow [12,13]. The waveguide amplifier is a specially developed integrated optical device for on-chip amplification [14–16]. In order to match the development of the optical network, the bandwidth of waveguide amplifiers also needs to be expanded from C-band to other low-loss transmission bands. S-band amplifiers are mostly thulium-doped devices whose operation depends on the 3H4→3F4 transition of Tm3+ ions [17]. Our group’s previous research successfully obtained the S-band amplification in a polymer-based thulium-doped waveguide amplifier (TDWA) device by doping the NaYF4:Tm,Yb nanoparticles (NPs) in the core of waveguide [18]. However, enhancing the gain of S-band amplifiers is a big challenge because the 3H4→3F4 transition of Tm3+ ions is between the energy levels of two excited states, and another energy level of excited state 3H5 is existed between these two excited states. This makes the energy transfer process of Tm3+ ions to obtain the emission at 1480 nm in S-band quite complex.
The operation of TDWA in S-band is based on the 3H4→3F4 transition of Tm3+ ions. To obtain the 1480-nm emission, the transition process of 3H6→3H5→3F4→3F2(3F3)→3H4 (the green-arrow process in Fig. 1) of Tm3+ ions is required, then 3H4→3F4 transition (the blue-arrow process in Fig. 1) occurs. Since the energies of 3F2 level and 3F3 level are extremely close, the following analysis will focus on the latter. The 3H6→3H5 and 3F4→3F2 transitions can be assisted by Yb3+ ions through the energy transfer (ET) processes of 2F5/2→2F7/2(Yb3+):3H6→3H5(Tm3+) and 2F5/2→2F7/2(Yb3+): 3F4→3F2 (Tm3+) (Fig. 1). However, the other transitions of Tm3+ (3H5→3F4 and 3F3→3H4) are nonradiative transitions, which seriously affect the overall 1480-nm emission process. Therefore, an effective technique is required to accelerate these two transitions of Tm3+. If the ET can occur from Tm3+ ion to another lanthanide ion, whose energy difference between its ground state and excited state well matches the energy released in 3H5→3F4 and 3F3→3H4 transitions of Tm3+ ion, it is possible to improve the efficiency of these two transitions of Tm3+ ion. In this case, the Tm3+: 3H4→3F4 transition can be accelerated, and the emission at 1480 nm can be enhanced. We analyzed the energy-level diagrams of lanthanide ions and finally found that the energy difference between 2F5/2 and 2F7/2 states of Ce3+ is close to the energy released in 3H5→3F4 and 3F3→3H4 transitions of Tm3+. Based on our theoretical analysis, if the ET occurs between Tm3+ and Ce3+ ions, the ET processes of 3F3→3H4(Tm3+):2F5/2→2F7/2(Ce3+) and 3H5→3F4(Tm3+) :2F5/2→2F7/2 (Ce3+) (Fig. 1) will accelerate the population process from state 3F3 (Tm3+) to state 3H4 (Tm3+) and state 3H5 (Tm3+) to state 3F4 (Tm3+). The emission intensity at 1480 nm will be significantly enhanced, eventually realizing the gain enhancement of the TDWA device in S-band.
Fig. 1. Energy level diagrams of Tm3+, Ce3+, and Yb3+ ions, and possible DC processes under the 980-nm laser excitation.
In order to verify our theoretical analysis, the NaYF4:Tm,Yb,Ce NPs were synthesized by introducing Ce3+ ions to enhance the 1480-nm emission intensity. In the Tm, Yb, Ce co-doped system, Ce3+ helps to improve the efficiency of 1480-nm emission process of Tm3+ through ET, therefore enhancing the emission in S-band. In the experiment, the doping concentration of Ce3+ ions in NPs was gradually changed from 0.5% to 20% to obtain the optimized NPs with the strongest emission at 1480 nm. In addition, the lifetimes of 3H5 and 3F3 levels were detected, and the transition processes were analyzed to demonstrate the existence of ET between Tm3+ and Ce3+ ions. An inert NaYF4 shell was then coated on the outside of NaYF4:Tm,Yb,Ce NPs to suppress the surface quenching effect of NPs and further increase the emission intensity. By doping the core-shell NPs into photoresistor SU-8 uniformly, the gain material for waveguide was obtained. Then a rectangular waveguide was fabricated with SiO2 and polymethyl methacrylate (PMMA) as bottom and upper claddings, respectively. When the signal power was 0.1 mW, a relative gain of 12.7 dB was obtained at 1480 nm. This result is 6 dB higher than that of the previous TDWA, which significantly improves the gain performance of the device in S-band and also provides guidance for amplification performance improvement of waveguide amplifiers even working in other bands.
2. Results and discussion
2.1 Synthesis and characterization of NPs
The NPs were synthesized via a high-temperature decomposition method. Detailed steps for the synthesis are shown in Supplement 1. In our previous experiment, the strongest emission at 1480 nm was obtained when doping concentrations of Tm3+ and Yb3+ ions in NaYF4:Tm,Yb NPs were 1% and 20%, respectively. Based on this result, we prepared a series of NaYF4: 1%Tm, 20%Yb, x%Ce NPs to find the optimal doping concentration for Ce3+ ions to enhance the emission intensity of NPs. The transmission electron microscopy (TEM) images of NaYF4:1%Tm,20%Yb,x%Ce NPs are shown in Supplement 1, Fig. S1. With the increase of the Ce3+ doping concentration, the particle sizes of NPs gradually increased while maintaining the uniform sizes with a spherical morphology (Supplement 1, Fig. S2), which is of benefit to reduce scattering loss in the waveguide amplifier. In addition, to prove that the NPs were all hexagonal phases, the X-ray Diffraction (XRD) test was carried out. As shown in Supplement 1, Fig. S3, the position and relative intensities of diffraction peaks of NaYF4:1%Tm,20%Yb,x%Ce NPs were in good agreement with those of standard β-NaYF4 crystal (JCPDS file number 16-0334), which indicated that the NPs were standard hexagonal phase. In addition, the diffraction peaks of NPs were slightly shifted to lower degrees with the introduction of Ce3+, indicating that the matrix of NPs was gradually transformed into NaCeF4 (JCPDS file number 75-1924).
The emission spectra under the excitation of a 980-nm laser are shown in Fig. 2(a). The doping concentration of Ce3+ ions was firstly changed from 0% to 20% in 5% steps to locate the strongest emission at 1480 nm, which turned out to be 15%. Then we synthesized a group of NPs with Ce3+ doping concentrations of around 15%. For doping concentrations of 13%, 14%, 15%, 16%, and 17%, the strongest emission was still obtained at 15%. Since the Tm3+ and Yb3+ ions doping concentrations were fixed at 1% and 20% respectively, we considered that the only factor that affect the luminescence intensity was the Ce3+ doping concentration. The integrated intensities at 1480 nm of NPs were plotted in Fig. 2(b). It can be observed that the emission intensity first increased, peaked at 15%, and then gradually decreased. When the Ce3+ doping concentration was relatively low, the emission intensity was more likely to be enhanced by the ET processes and the growing particle size. However, the ET process of 3H5→3F4 (Tm3+):2F5/2→2F7/2 (Ce3+) inevitably increased the Tm3+ ion population on the 3F4 level. When Ce3+ doping concentration exceeded 15%, the 3H4→3F4 (Tm3+) transition will be affected significantly, weakening the emission intensity at 1480 nm. Apart from that, the rising concentration of Ce3+ ions also led to an inevitable matrix change from NaYF4 to NaCeF4. Compared with NaYF4, NaCeF4 was not a promising matrix for Tm3+ ions’ 1480-nm emission (Supplement 1, Fig. S4), which might be another reason for the intensity weakening. In addition, the particle sizes kept increasing with the Ce3+ doping concentration. The larger size will contribute to the emission enhancement of NPs. Still, the emission intensities of NPs with bigger sizes (Ce3+ doping concentration of 16%, 17%, and 20%) were lower than that of NaYF4:1%Tm,20%Yb,15%Ce, further proving that the ET between Tm3+ and Ce3+ ions is the major reason for intensity enhancement.
Fig. 2. (a) Emission spectra of NaYF4:1%Tm,20%Yb,x%Ce; (b) Integrated intensity at 1480 nm versus doping concentration of Ce3+ ions.
In our expectation, the ET processes of 3F3→3H4(Tm3+): 2F5/2→2F7/2(Ce3+) and 3H5→3F4 (Tm3+):2F5/2→2F7/2 (Ce3+) will participate in the 1480-nm emission process of Tm3+ ions. In this case, the lifetimes of the 3F3 and 3H5 levels of Tm3+ will be effectively decreased by introducing Ce3+ ions. The lifetimes of the 3F3 and 3H5 levels of Tm3+ in different NPs were measured and fitted with a single-exponential function as:
where I0 is the intensity parameter for t = 0, and τ is the excited state lifetime. By monitoring the emissions peaked at 725 nm (3F3→3H6) and 1250 nm (3H5→3H6), the lifetimes of the 3F3 and 3H5 levels with different Ce3+ doping concentrations were obtained, respectively. The decay curves were shown in Fig. 3 and the corresponding lifetimes were recorded in Fig. 4. The lifetimes of the 3F3 level and 3H5 level decreased with the increase of Ce3+ doping concentration (Fig. 3(a), Fig. 3(b), Fig. 4(a), and Fig. 4(b)), which were consistent with our expectations. However, many factors may cause the lifetime decrease (e.g., the particle size and the matrix changing), and we still need to prove that the ET between Tm3+ and Ce3+ is the main cause. To precisely observe the effect of ET on lifetimes, we synthesized another group of NPs with Ce3+ doping concentrations of 0%, 0.5%, 1%, 2%, and 4%. In this group, the Ce3+ doping concentration was relatively low and closed in values, which ensured that the matrix of NPs was controlled in NaYF4 and the particle sizes were nearly the same (Supplement 1, Fig. S1(a) to Fig. S1(e)). As shown in Fig. 3(c) and Fig. 4(c), the lifetime of the 3F3 level decreased with the increase of Ce3+ doping concentration and reached a decreasing amplitude of 8.6% when the doping concentration was 4%. As for the 3H5 level (Fig. 3(d) and Fig. 4(d)), its lifetime shared the same decreasing trend as the 3F3 level but more significantly. When the doping concentration reached 4%, the lifetime of the 3H5 level decreased by 13.9%. The reason why the lifetime of level 3H5 showed a more obvious decreasing trend is that the energy interval between 3H5 level and 3F4 level is much closer to the energy required for Ce3+ to transit from 2F5/2 level to 2F7/2 level, which means that the energy transfer process of 3H5→3F4 (Tm3+):2F5/2→2F7/2 (Ce3+) is more easily to occur. To further exclude the influence of NPs morphology on lifetime, some energy levels (e.g., 3P0 level, 1D2 level, and 1G4 level) that didn’t participate in the ET process between Tm3+ and Ce3+ ions were also detected (See Supplement 1, Fig. S5). The lifetime differences of these energy levels were small, proving that the morphology of NPs in this experiment barely influence the lifetime of energy levels in Tm3+ ions. The significant reduction in lifetimes of 3F3 and 3H5 levels was indeed attributed to the ET process. The experiment results demonstrated that the ET between Tm3+ and Ce3+ ions existed and successfully depopulated the 3F3 and 3H5 levels of Tm3+. It verifies our theoretical analysis that by co-doping Ce3+ ions, we can establish the ET from Tm3+ to Ce3+ ions and improve the efficiency of Tm3+:3F3→3H4 and 3H5→3F4 transitions, therefore accelerating the 3H4→3F4 transition of Tm3+ and enhancing the emission at 1480 nm.Fig. 3. Decay curves of Tm3+ fluorescence from (a)3F3 level (725 nm) and (b)3H5 level (1250 nm) of NPs with Ce3+ doping concentrations of x% (x = 0, 5, 10, 15, 20); Decay curves of Tm3+ fluorescence from (c)3F3 level (725 nm) and (d)3H5 level (1250 nm) of NPs with Ce3+ doping concentrations of x% (x = 0, 0.5, 1, 2, 4).
Fig. 4. Lifetimes of Tm3+ fluorescence from (a)3F3 level (725 nm) and (b)3H5 level (1250 nm) of NPs with Ce3+ doping concentrations of x% (x = 0, 5, 10, 15, 20); Lifetimes of Tm3+ fluorescence from (c)3F3 level (725 nm) and (d)3H5 level (1250 nm) of NPs with Ce3+ doping concentrations of x% (x = 0, 0.5, 1, 2, 4).
Since the best Ce3+ doping concentration was obtained, the NaYF4: 1%Tm, 20%Yb, 15%Ce NPs were coated with an inert shell to modify the defects on the surface. This shell coating process further enhanced the intensity of the emission peaked at 1480 nm, which would improve the gain performance of the S-band Tm, Ce co-doped waveguide amplifier (TCCDWA).
2.2 Fabrication of the TCCDWA
To synthesize the gain medium of the waveguide amplifier, the NaYF4:Tm,Yb,Ce@NaYF4 NPs need to be doped into the core material. The photoresist SU-8 was chosen as the core material to fabricate the rectangular waveguide in this experiment. Although the NPs can barely dissolve in SU-8, the surface modification with OA ligands makes them easy to disperse in cyclohexane solvent instead. Therefore, we first ultrasonically dispersed the NPs in cyclohexane, then mixed the solution with SU-8 to obtain the gain medium in which the NPs were uniformly dispersed. The fabrication process of the rectangular waveguide amplifier is shown in Fig. 5(a)–5(f).
Fig. 5. (a)-(f) Preparation process of TCCDWA; (g) SEM image of the waveguide without upper cladding; Optical filed distributions of the waveguide cross-section at wavelengths of (h) 1480 nm and (i) 980 nm.
SiO2 and PMMA were chosen as the bottom and upper claddings. The selection of bottom cladding SiO2 is beneficial for reducing propagation loss and easy integration with other silicon-based optoelectronic devices. Firstly, a core film was formed by spin-coating the NPs-doped SU-8 on the surface of SiO2 at a speed of 3000r/min (Fig. 5(b)). Then the core film was exposed under the UV-light with a patterned photomask overlaid to obtain the structure of the waveguide (Fig. 5(c) and Fig. 5(d)). After developing, the rectangular waveguide was obtained on the surface of the SiO2 substrate (Fig. 5(e)). Then the PMMA was spin-coated on the device to form the upper cladding (Fig. 5(f)). Figure 5(g) shows the scanning electron microscope (SEM) image of the waveguide cross-section. The morphology of the rectangular waveguide is excellent, with a width and thickness of 6 µm and 5 µm, respectively.
The size of the waveguide greatly influences the optical field confinement ability of the core layer. Therefore, according to the measurement results, the optical field distribution of the fabricated waveguide amplifier was simulated by COMSOL. Figure 5(h) and Fig. 5(i) are the optical field distributions of the waveguide cross-section at wavelengths of 1480 nm and 980 nm, respectively. The overlapping integral factors were calculated by the surface integral. For signal and pump lights, 98.9% and 98.2% of the optical powers were confined in the waveguide core, respectively, which demonstrates the lights can barely leak out of the core during propagation.
2.3 Gain performance of the TCCDWA
The test system, as shown in Fig. 6, was built to characterize the gain performance of the waveguide amplifier. An S-band tunable laser (EXFO: T100S-HP) was chosen as the signal source, and a 980 nm laser diode (BWT: DS3-51512-LDNo.) was selected to provide pump power. The test system included both an optical power meter (Thorlabs: PM100D) and an optical spectrometer analyzer (OSA: ANDO AQ-6315 A) to read the signal power value. After the amplification process, the signal light was collected at the output port of the waveguide and coupled to the optical power meter and OSA through a 3-dB power splitter. In this way, the insertion loss and optical powers at different wavelengths can be read simultaneously.
In the experiment, the signal and pump lights were coupled into the TCCDWA through the WDM. The NPs dispersed in the waveguide core could emit photons identical to the signal light through stimulated radiation, thereby realizing optical amplification. In addition, under the 980-nm excitation, the typical blue up-conversion (UC) luminescence of Tm3+ could be observed on the surface of the waveguide, as shown in Supplement 1, Fig. S6 and Fig. S7. The bright and uniform blue light path demonstrated that the NPs were uniformly dispersed in the core of the waveguide.
The relative gain of TCCDWA was calculated by equation:
where G is the relative gain of the TCCDWA; $P_{s - out}^p$ and ${P_{s - out}}$ are output powers of signal light with and without excitation. When the signal and pump powers were 0.1 mW and 300 mW, respectively, the device could provide relative gain in a wavelength range of 50 nm and reach a maximum of 12.7 dB at 1480 nm (Fig. 7(a)). The relative gain versus pump power at 1480 nm was plotted (Fig. 7(b)) to further observe the gain characteristics. The gain increased with the pump power and tended to be flat when the pump power reached 250 mW. Compared with our previous work [18], the relative gain at 1480 nm was raised from 6.7 dB to 12.7 dB, successfully realizing a significant amplification enhancement in S-band.Fig. 7. (a) Relative gains of TCCDWA at different signal wavelengths; (b) Relative gain versus pump power at 1480 nm.
3. Conclusion
In conclusion, an S-band polymer-based waveguide amplifier was fabricated based on NaYF4:1%Tm,20%Yb,15%Ce@NaYF4 NPs. A gain enhancement technique was used to improve the gain performance of the waveguide amplifier by co-doping Ce3+ ions. The ET between Tm3+ and Ce3+ ions successfully accelerated the transition process of Tm3+ (3H5→3F4 and 3F3→3H4) and enhanced the 1480-nm emission intensity. The amplification provided by TCCDWA covered a range of 50 nm, and the maximum relative gain of 12.7 dB was obtained at 1480 nm. The experimental results show a new technique to enhance the gain performance of optical amplifiers and a further usage of Ce3+ ions. We believe this method can also work for other rare-earth ions in specific conditions and improve the gain performance in other communication bands.
Funding
National Key Research and Development Program of China (2021YFB2800500); National Natural Science Foundation of China (U22A2085, 12174150, 61875071).
Disclosures
The authors declare there are 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.
Supplemental document
See Supplement 1 for supporting content.
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