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Abstract
We propose a strip loaded amplifier employing SU-8 as the loaded waveguide and nanoparticles (NPs)-polymethyl methacrylate (PMMA) as the cladding layer. By leveraging the undoped SU-8 loaded waveguide, the polymer waveguide amplifier accomplished remarkably low transmission losses, reaching as low as 1.8 dB/cm at 1530 nm. We prepared NPs-PMMA nanocomposite by utilizing NaLu0.1Y0.7F4: Er3+, Yb3+ @NaLuF4 core-shell nanoparticles, which exhibited a significantly enhanced lifetime of 6.15 ms. An internal net gain of up to 17.7 dB was achieved on a strip loaded waveguide with a length as short as 0.5 cm when the on-chip pump power was 77 mW. Signal enhancement (SE) was measured at different wavelengths, revealing that the strip loaded waveguide exhibited broadband SE ranging from 1510 nm to 1570 nm, covering the C-band. To the best of our knowledge, this work has achieved the highest gain results reported thus far on a polymer matrix and provides an efficient method for optical amplification in passive devices on silicon and Si3N4 platforms, leveraging the ease of integration of polymer materials with diverse photonic platforms.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Compared to the electron-hole pairs of III-V semiconductor materials, rare-earth ion-doped materials have garnered significant attention in the field of optical amplifiers. This is primarily due to their longer excited-state lifetime, and weaker refractive-index change [1–4]. With the advancement of photonic integration, erbium ions have become extensively employed in optical amplifiers to mitigate losses caused by various photonic integration devices operating at the 1550 nm communication wavelength. This is due to the energy level of erbium ions, enabling them to cover the telecommunication C + L band [5–7]. In recent years, there has been extensive and intensive research on erbium-doped optical waveguide amplifiers utilizing various host matrix materials (e.g., silicon nitride [8], lithium niobite [9–16], aluminum oxide [17–22], rare-earth oxides [23], rear-earth doped glass [24–28], organic complexes [29], and rare-earth doped nanoparticles (NPs) [30], etc.). In recent reports, a 10 cm long waveguide amplifier integrated on a Si3N4 platform with Al2O3: Er3+ as the gain medium showed a net gain of 18.1 ± 0.9 dB at 1532 nm [21]. An optical gain of 6.7 dB cm-1 was obtained in a 1 cm long evanescent-wave coupling waveguide using Er(TMHD)3 doped polymethyl methacrylate (PMMA) as the active material [29], and a 1.3 cm long polymer waveguide based on NaYF4/NaLuF4: 20% Yb3+, 2% Er3+ NPs-PMMA covalently linked nanocomposites showed a net gain of 15.1 dB at 1530 nm under 980 nm laser excitation [31].
Erbium-doped waveguide amplifiers implemented in amorphous host materials (glass [24–28], Al2O3 [17–22], TeO2 [32], etc.) often necessitate relatively long waveguide lengths (typically a few centimeters) to achieve satisfactory gain performance. This requirement arises due to the relatively short excited-state lifetime of erbium ions (typically ranging from 1 to 4 ms) and the comparatively smaller absorption/emission cross-section of erbium ions (typically around 10−21 cm2) [23]. In contrast, the crystal host materials (erbium-doped potassium double tungstate (KGdxLuyEr1-x-y(WO4)2) [33], erbium silicate [34,35], erbium chloride silicate nanowires [36], and erbium-doped thin film lithium niobate [9–16], etc.) do not have such a limitation, and their large absorption/emission cross-sections enable devices to achieve significant gain with short waveguide lengths [23]. Furthermore, in rare-earth doped nanocrystals, the concentration of erbium ions within the grains can reach levels on the order of 1021 cm-3 [37].
In recent years, alongside the extensive research on erbium ion-doped host materials, there has been a focus on exploring and optimizing various waveguide structures. The slot waveguide has found widespread applications since its initial proposal [38–40], and it have also been employed in waveguide amplifiers due to its capability to provide relatively high electric field confinement [22,41]. To address the intricate preparation processes involved in inorganic waveguide amplifiers, efforts have been made to enhance their integration with photonic platforms and simplify the fabrication procedures. Strip loaded waveguide [23,42], and hybrid waveguide structure [19,43] have been proposed and achieved excellent performance. These innovative designs offer advantages in terms of ease of fabrication and compatibility with integration, making them more favorable for practical applications. In these waveguide structures, the conversion of passive waveguide channels into active waveguide amplifiers can be achieved through a straightforward and direct deposition of thin films doped with rare earth ions, such as phosphates, aluminum oxides, and etc. This approach significantly reduces the manufacturing cost and process complexity associated with the fabrication of active waveguide amplifiers.
In this work, we propose, for the first time, a polymer waveguide amplifier based on a strip loaded structure by spin-coating NPs-PMMA nanocomposite onto a SU-8 loaded waveguide. By leveraging the undoped SU-8 loaded waveguide, the polymer waveguide amplifier accomplished remarkably low transmission losses, reaching as low as 1.8 dB/cm at 1530 nm. The introduction of a core-shell structure in the NPs resulted in a doubling of the luminescence intensity in the fluorescence spectrum compared to NPs with a core-only structure. Furthermore, it is noteworthy that the lifetime of the erbium ion 4I13/2 energy level was remarkably enhanced from 1.51 ms to 6.15 ms in the core-shell NPs. We utilized NPs-PMMA nanocomposite as the cladding layer of the loaded waveguide, and gain performance of a waveguide with a length of 0.5 cm was demonstrated. At a pump power of 77 mW, the strip loaded waveguide demonstrated a remarkable net internal gain of up to 17.7 dB at 1530 nm. The signal enhancement (SE) was measured across different wavelengths, and the strip loaded structure exhibited SE within the bandwidth range of 1510-1570 nm that covers the C-band. A peak SE of 18.5 dB was achieved at 1530 nm, enabling internal net gain across the 1510-1570 nm bandwidth.
2. Results
2.1 Preparation of NaLu0.1Y0.7F4: Er3+, Yb3+@NaLuF4 NPs-PMMA Nanocomposite
In our experiment, the copolymerization of methyl methacrylate (MMA) monomers with core-shell nanoparticles coated with inert shells was conducted to obtain NPs-PMMA gain medium, as illustrated in Fig. 1(a). Firstly, the core of the NPs was synthesized by a high temperature thermal decomposition method using 2 mmol of rare earth chloride RECl3·6H2O solid (RE3+ = 10% Lu3+ + 70% Y3+ + 18% Yb3+ + 2% Er3+). The rare earth oleic acid compatibilizers precursors were prepared using LuCl3-6H2O. These precursors were then mixed and reacted with the core NPs, resulting in the formation of NaLu0.1Y0.7F4: Er3+, Yb3+@NaLuF4 core-shell NPs. The morphology and particle size of both core-only NPs and core-shell NPs were observed using transmission electron microscopy (TEM) (TEM images are shown in the inset of Fig. 1(b)). The dimensions of both types of NPs exhibited uniformity and consistency. To further analyze the size distribution, the dimensional measurements were plotted as a histogram, as shown in Fig. 1(b). For the core-only NPs, the sizes ranged from 11 to 14 nm with an average diameter of 12.6 nm, and the core-shell NPs exhibited sizes ranging from 17 to 20 nm with an average diameter of 18.6 nm. By introducing an inert shell around the core NPs, the size of the NPs can be effectively increased, and this shell layer can also passivate lattice defects present on the surface of the smaller sized core NPs [44].
Fig. 1. (a) Synthesis process of core-shell NPs and copolymerization with methyl methacrylate (MMA). (b) Histogram of size distribution of core-only and core-shell NPs (the insets show the TEM image of core-only and core-shell NPs). (c) Fluorescence emission spectra of core-only and core-shell NPs. (d) Photoluminescence decay curves of core-only and core-shell NPs.
The downconversion luminescence spectra of the core-shell NPs were measured using a 980 nm semiconductor diode laser as the excitation source. The results of the luminescence spectra are presented in Fig. 1(c). It was observed that the luminescence peak intensity of the core-shell NPs is significantly increased by a factor of 2.5 compared to that of the core-only NPs. Furthermore, the fluorescence decay curves of the erbium ion 4I13/2 energy level for both the core-only NPs and core-shell NPs were obtained using a fluorescence lifetime test system. The decay curves are illustrated in Fig. 1(d). The introduction of an inert shell in the core-shell NPs resulted in a notable increase in the fluorescence lifetime of the erbium ion 4I13/2 energy level from 1.51 ms to 6.15 ms. The enhancement in luminescence intensity and increased lifetime can be attributed to the reduction of surface defects in the NPs after the incorporation of inert shells. The combined improvement in luminescence intensity and lifetime highlights the effectiveness of the core-shell structure in enhancing the overall performance of the NPs.
Rare-earth doped NaYF4 nanocrystals, known for their efficient luminescent properties, have been commonly employed as luminescent centers in polymer materials for their potential as a gain medium [45,46]. However, when utilizing the physical doping approach to incorporate inorganic nanocrystals into organic polymers, it may result in the clustering of nanocrystals within the polymer matrix. To overcome this limitation and achieve a homogeneous and stable gain medium, a covalent bonding strategy was employed to attach the core-shell NPs to the polymer matrix. The synthesis process of core-shell NPs and copolymerization with methyl methacrylate (MMA) is shown in Fig. 1(a). Through the copolymerization of 0.2 mmol of core-shell NPs with 10 g of MMA monomer, the unsaturated organic groups present on the surface of the NPs and the MMA monomer were covalently bonded via C = C bonds. By establishing covalent bonds between the core-shell NPs and the polymer, a more robust and stable integration is achieved, resulting in enhanced dispersion and minimized clustering of the nanocrystals within the polymer. This methodology ensures a homogeneous distribution of the luminescent centers, thereby augmenting the overall performance and stability of the gain medium.
2.2 Waveguide fabrication
Loaded strip waveguides, based on NPs-PMMA nanocomposite, were fabricated employing standard lithography, inductively coupled plasma (ICP) etching, and spin-coating techniques, following the preparation process illustrated in Fig. 2(a). A rectangular structure comprising 2 µm deep grooves with a width of 3 µm was precisely formed on the surface of an 8 µm thick SiO2 film using standard lithography and etching techniques. The grooves were filled with commercial SU-8 2002 photoresist using spin-coating technique, resulting in the formation of a 2 µm thick slab layer on the SiO2 surface. To create an embedded strip waveguide, the SU-8 slab structure was removed through ICP etching utilizing oxygen and argon gases. Finally, a 2 µm thick film of NPs-PMMA nanocomposite was spin-coating onto the silica surface, completing the fabrication of the strip loaded structured optical waveguide amplifier. Figure 2(b) shows the 3D schematic of the strip loaded amplifier chip. The Fig. 3(a)-(b) exhibits the cross-section scanning electron microscope (SEM) images of both the silica groove and the loaded strip waveguide after the nanocomposite has been spin-coated. The strip-loaded waveguide, with SU-8 as the core layer and NPs-PMMA as the flat layer, enables the confinement of a portion of the signal light within the SU-8 core layer, taking advantage of its low absorption properties in the C-band. Simultaneously, another portion of the signal light enters the NPs-PMMA nanocomposite layer for amplification. The light field distribution graph in the inset illustrates that the designed waveguide structure effectively fulfills the intended function.
Fig. 2. (a) Preparation process of the strip loaded waveguide amplifier. (b) 3D schematic of the strip loaded amplifier chip.
Fig. 3. SEM images of (a) silica groove, (b) strip loaded waveguide (the inset shows the signal optical field distribution).
2.3 Characterization
The insertion losses (IL) of both the pump and signal were measured using a cut-back method. The IL of the waveguide includes the propagation loss and the coupling loss at the waveguide end facet, and the propagation loss mainly consists of the absorption loss caused by the ground-state erbium ions and the scattering loss of the waveguide. The IL of the waveguide can be expressed as $\textrm{IL}({\textrm{dB}} )= {\alpha _p}L + 2{\alpha _c}$, where ${\alpha _p}$ is the propagation loss of the waveguide, $L$ represents the length of the waveguide, and ${\alpha _c}$ is the coupling loss between each end facet of the waveguide and the fiber. For each length measurement, five sets of data were collected and averaged. These measurements were used to determine the average insertion losses of waveguides with varying lengths, which were then fitted to obtain the results displayed in Fig. 4. According to the fitting results, the propagation loss of the strip loaded waveguide is 4.3 dB/cm at 976 nm and 1.8 dB/cm at 1530 nm. The increase in propagation loss at 976 nm can be attributed to the absorption of ytterbium and erbium ions by the nanocomposite. The coupling loss at 976 nm is measured to be 3.94 dB/facet, while at 1530 nm, it is recorded as 6.34 dB/facet. The higher coupling loss is primarily due to the mismatch between the waveguide and the spot mode field of the fiber (G657A ∼ 9 µm diameter). In comparison to previously reported polymer amplifiers [31], the utilization of undoped SU-8 as the loaded waveguide has led to a significant reduction in transmission losses of the device. This notable decrease in transmission losses has resulted in an improved net gain of the device.
Fig. 4. The measured insertion loss and fitted curves for strip loaded waveguide at (a) 976 nm and (b) 1530 nm wavelengths.
To evaluate the gain performance of the amplifier, a 0.5 cm long waveguide was tested using the experimental setup shown in Fig. 5. 976 nm pump (ASPUMPL-976-1000-FA-B, Shanghai Aoshow Information) and signal (Santec TSL210) passed through polarization controllers, and fed into the waveguide after being combined by a wavelength division multiplexer (WDM). Finally, the amplified signal was recorded with an optical spectrum analyzer (OSA: MS9740A, Anritsu). The inset in the figure displays the fluorescence upconversion luminescence of the waveguide.
Fig. 5. The experimental setup for amplification characterization measurements (the inset shows the up-conversion luminescence of the waveguides under the excitation of a 976 nm pump laser).
The gain characteristics of the optical waveguide amplifier are evaluated based on the definition of the internal net gain ${G_{net}}$. The equation for ${G_{net}}$ is given as ${G_{net}} = 10lo{g_{10}}\left( {\frac{{{P_{s - on}}}}{{{P_{s - off}}}}} \right) - {\alpha _p}L$, and the first term in the equation is referred to as the SE of the amplifier, which signifies the ratio of the signal output power when the pump is turned on (${P_{s - on}}$) to the signal output power when the pump is turned off (${P_{s - off}}$), and the second term represents the propagation loss of the waveguide [15,47,48]. Figure 6(a)-(b) presents the output signal spectrum at wavelengths of 1530 nm and 1550 nm, with an input signal power of -15 dBm. The SE at 1530 nm and 1550 nm shown in Fig. 6(c)-(d) was measured at input signal powers of -15 dBm and -10 dBm. In the figures, the green shaded areas represent the propagation loss at the signal wavelengths. It can be observed that the SE gradually saturates as the pump power increases. With an input signal of -15 dBm, the waveguide achieves internal net gain at both 1530 nm and 1550 nm wavelengths when the pump power is below 15 mW, whereas the gain saturates when the pump power reaches 62 mW. At 1530 nm, a SE of up to 18.6 dB is achieved, while at 1550 nm, a maximum SE of 4.9 dB is obtained. On the other hand, when the input signal power is increased to -10 dBm, the SE at both signal wavelengths decreases. With increasing on-chip pump power, the maximum SE recorded are 16.2 dB at 1530 nm and 4.2 dB at 1550 nm. The observation of decreased SE as the input signal power increases is in line with the small-signal-gain regime.
Fig. 6. Measured signal spectrum of the strip loaded waveguide at (a) 1530 nm and (b) 1550 nm wavelengths as a function of the increase of on-chip pump power, at a fixed signal power of -15 dBm. The curves of SE as a function of on-chip pump power for strip loaded waveguide at (c) 1530 nm and (d) 1550 nm wavelengths with signal powers of -15 dBm and -10 dBm.
The internal net gain of the amplifier increases with the on-chip pump power at 1530 nm and 1550 nm, as shown in Fig. 7(a). For an input power of -15 dBm, the maximum internal net gain achieved was 17.7 dB at 1530 nm and 4.0 dB at 1550 nm. In addition, the SE at other wavelengths within the telecom bands (S-band, C-band, and L-band) was characterized at the on-chip pump power of 62 mW, and the input signal power of -15 dBm, as shown in Fig. 7(b). The inset in the figure shows the emission spectrum at the corresponding wavelengths of the nanocomposite. The strip loaded structure achieves SE in the range of 1510-1570 nm bandwidth. A peak SE of 18.5 dB is observed at 1530 nm, and the internal net gain covers the bandwidth of 1510-1570 nm.
Fig. 7. (a) Internal net gain as function of on-chip pump power at 1530 nm and 1550 nm wavelengths. (b) Internal net gain as a function of wavelength for the strip loaded waveguide (inset shows the emission spectrum of the nanocomposite).
3. Conclusion
We have designed and fabricated a strip loaded waveguide amplifier based on NPs-PMMA nanocomposite with SU-8 as the loaded waveguide of the waveguide. By leveraging the undoped SU-8 loaded waveguide, the polymer waveguide amplifier accomplished remarkably low transmission losses, reaching as low as 1.8 dB/cm at 1530 nm. The gain performance of the amplifier has been thoroughly demonstrated. A high internal net gain of 17.7 dB at 1530 nm was achieved for the strip loaded waveguide at a pump power of 77 mW. SE was measured at different wavelengths, and the strip loaded waveguide exhibited a broadband internal net gain within the wavelength range of 1510-1570 nm. These findings highlight the immense potential of NPs-PMMA nanocomposites in the field of waveguide amplifiers. Additionally, the investigation of waveguide structures has contributed to further enhancing the performance of the amplifiers. Erbium ion-doped nanocrystals can significantly increase the absorption/emission cross-section of erbium ions due to their crystal structure. Moreover, the nanocrystals allow for achieving high concentrations of Er3+ doping, thereby enabling substantial gain even within a short waveguide length. The utilization of a simple spin-coating technique to deposit NPs-PMMA nanocomposite onto SU-8 waveguides has transformed passive waveguides into active waveguide amplifiers, enabling efficient signal amplification. This underscores the remarkable potential of polymer photonics in facilitating high optical gain and monolithic integration.
Funding
National Key Research and Development Program of China (2021YFB2800502); National Natural Science Foundation of China (62090062).
Disclosures
The authors declare no conflict of interest.
Data availability
The data that support the findings of this study are available upon request from the authors.
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