Er3+/Yb3+ doped active optic Y splitter realized by diffusion waveguides with Ag+—Na+ ion exchange

We reported on the active channel waveguides, formed in novel types of silicate glasses, doped with rare-earth elements, and Zn were investigated. The silicate glass GZ4 with Er3+ and Yb3+ content was studied, and the best doping ratio was estimated about luminescent properties. The composition of the glass samples (GZ4) with the content of 0.25 mol.% Er2O3 and 5.0 mol.% Yb2O3 and 12.0 mol.% resp. 18.0 mol.% ZnO was optimized. This glass was evaluated as the most suitable material for integrated amplifiers in the 1 530–1 565 nm telecommunication band. Other samples were prepared with active channel waveguides and active planar optical power splitter Y with a splitting ratio of 1 × 2 by two-step ion-exchange Na+ ↔ Ag+. Diffusion profiles of the created samples were analyzed by the EMA microscope and compared with the near mode-field distribution measurement results. Afterward, the amplification properties of the designed structures were studied, and the differential gain from 1.2 to 1.6 dB (0.48 to 0.64 dB/cm) was achieved by pumping power 200 mW at 980 nm.


Introduction
Passive optical splitters are a common component of fiber optic transmission networks. An active optical splitter compensates for its losses caused by splitting the optical signal from N to N transmission paths. The research of active optical power splitters aims to achieve at least a level of structure gain that corresponds to the losses caused by the split ratio and the insertion loss of the component itself. According to ITU-T G.671, the basic structure of the optical splitter with a 1 × 2 ratio can bring a maximum insertion loss of 4.0 dB into the optical path. The optical splitter structure with a split ratio of 1 × 64 can introduce a maximum insertion loss of 22.3 dB into the optical path defined by ITU-T Recomendation G.671: Transmission characteristics of optical components and subsystems 08/2019. Nowadays, it is already possible to achieve such gains with integrated optical amplifiers produced by layer technologies was presented by Bucci et al. (2007) and Righini et al. (2002). To achieve that, the lanthanides can be used to effectively amplify the optical signal. The most used lanthanide up today is erbium and ytterbium. Er 3+ ions at pumping on the wavelength of 980 and 1 480 nm can amplify optical radiation in the C-wave band of the 3rd optical window of quartz waveguides from 1 530 to 1 560 nm, which is used in nowadays access and backbone optical telecommunication networks due to minimal transmission attenuation (Cantelar et al. 2000 andHuang et al. 2002). Other lanthanides can act as activators in other telecommunication bands, Pr 3+ in the O band of 1 300 to 1 350 nm mostly in conjunction with fluoride waveguides, Tm 3+ in the S and S + bands of 1 450 to 1 530 nm, etc. Yb 3+ ions are used to increase the absorption of 980 nm pump radiation compared to Er 3+ ions. The Er 3+ /Yb 3+ concentration ratio is thus balanced for maximum gain of the active optical splitter. Traditional techniques of active waveguide fabrication include RF-sputtering, sol-gel technique, ion implantation, or proton exchange (Zigang and Duan, 2006). Erbium-doped waveguides have been mainly developed in multicomponent silicate and phosphate glasses. Especially, phosphate glasses are famous for enabling the high concentration of rare-earth dopants to be dissolved in the glass matrix, which results in the fabrication of active devices of small size and high gain up to 4.1 dB/cm (Yan et al. 1997). On the contrary, silicate glasses have the advantage of higher mechanical rigidity and chemical stability, lower cost, and better compatibility with current commercial optical fibers (Jagerska et al. 2007). As for silicate glasses, Al 2 O 3 is typically used to enhance the solubility and homogeneity of rare earth dopants and to reduce the rate of Er-pairs and clusters formation. Nevertheless, we have shown recently Mika et al. 2005 andVytykacova et al. 2018) that the presence of ZnO in the glass can also contribute to a substantial enhancement of rare-earth solubility.
In this paper, the ion-exchanged channel waveguides formed in GZ 4 types of silicate glasses were used due to their wide range of applications (Barkman et al. 2009, Mares andJerabek 2016). Novel mixtures of GZ4-based glasses with the addition of rare earth elements and Zn were investigated. The silicate glass GZ4 with Er 3+ and Yb 3+ content was studied, and the best doping ratio was estimated concerning the luminescent properties (Stanek et al. 2017). The samples of active planar optical power splitter Y with a splitting ratio of 1 × 2 were prepared with active channel waveguides by two-step ion-exchange Na + ↔ Ag + . Afterward, the amplification properties of the designed structures were studied, and the differential gain was evaluated. Recently, papers have been published that use matrices other than silica glasses. These are low-basicity phosphate glasses (Cui et al. 2020) for computer science and glass ceramics for optical thermometry (Junhao Xing et al. 2020).

2. Design of Y branching optical splitter with Er 3+ -Yb 3+ doped waveguides
The basis of the waveguide for the active optical splitter is a special silicate glass GZ4, where the diffusion waveguide itself is realized by a two-step ion exchange Na + ↔ Ag + , which is immersed under the substrate surface with its entire volume. The composition of GZ4 glass is modified by rare earth oxides Er 2 O 3 and Yb 2 O 3 to optimize the absorption intensity in the 980 nm pumping region and the luminescence intensity in the 1 535 nm gain region. Silicate glass has a significantly lower solubility of rare earth elements in their structure compared to phosphate glasses (Yan et al. 1997). When studying the luminescence of silicate glasses, it has been shown that when the proportion of 6 mol. % of rare earth elements is exceeded, so-called clustering occurs. The further increase in lanthanide concentration is counterproductive was presented by Salavcova et al. (2005) and Malinsky et al. (2009). To achieve the highest possible amplification of the optical amplifier, it is possible to increase the luminescence properties of the substrate material by adding ZnO to the glass matrix (Vytykacova et al. 2018). In our research of optical amplifiers fabricated by ion exchange technology, GZ4 silicate glass containing 0.25 mol.% Er 2 O 3 , 5.0 mol.% Yb 2 O 3 and ZnO modifier was prepared. The rare-earth as well as zinc oxide substituted for silica oxide. The glass composition is shown in Table 1.
In the absorption spectrum, as in the case of Zn-free enamel, there are five absorption bands at 378 nm, 521 nm, 655 nm, 908 nm, and 980 nm, corresponding to Er 3+ energy levels 4 G 11/2 , 2 H 11/2 , 4 F 9/2 , 4 I 9/2, and 4 I 11/2 in combination with Yb 3+ (see Fig. 1a). The highest absorption intensity is seen at 980 nm. There is no significant influence of Zn concentration on the absorption intensity in the measured absorption spectrum. Two luminescence peaks at wavelengths 1 535 and 1 541 nm are evident in the luminescent spectra, which correspond to the energy transmission at 4 I 13/2 and 4 I 15/2 (see Fig. 1b). It can be seen from the measurement results that the Zn admixture has a positive effect causing an increase in luminescence intensity. The Zn acts as a separator of Er 3+ -Yb 3+ ions in the glass matrix, limiting the energy exchange between ions, and preventing clustering of activator atoms (Vytykacova et al. 2018). As in the case of rare-earth doping, it is not possible to increase the concentration of Zn in the glass matrix without undesirable consequences, such as the phase separation effect causing the low optical quality of the glass, which is unacceptable for the application of optical waveguides. The presence of Zn in silicate glasses reduces their basicity, and thus the properties of these glasses approach those of phosphate glasses. Higher amounts of Zn increase the glass density (g/cm 3 ) and refractive index. Increasing Zn increases the luminescence in the 1520 to 1560 nm band for Er 3+ /Yb 3+ glasses up to 30%. In this case, Zn acts as an Er ion separator and prevents energy exchange between the ions. However, when the eighteen mol. % threshold of ZnO is exceeded, phase separation occurs, which significantly deteriorates the optical quality of the glass and the luminescence decay time decreases. The recommended maximum ZnO content for silicate glasses is approximately 16.0 mol. % (Volf 1978, Stanek 2018. From a practical point of view of the optical waveguide, the experiment glass used in the active optical experiment in this paper was made with optimal content of 15 mol.% ZnO although higher ZnO content leads to higher luminescent intensity, Zn significantly worsens its waveguide properties in the subsequent ion-exchanged waveguide. That was verified by measuring the optical waveguide samples made from glass with content of 15 and 18 mol.% ZnO. Best results in the form of minimal waveguide attenuation 2.9 dB and importantly the maximal optical gain 1.8 dB at the wavelength range of 1 520-1 560 nm are present in the glass with a zinc oxide content of 15 mol. % Results are summarized in Table 2. For the realization of integrated optical amplifiers fabricated by ion exchange technology, GZ4glass with 0.25 mol % Er2O3% and 5 mol.% Yb2O3 and ZnO content of 15 mol. % glasses were prepared. The spectroscopic properties such as cross-sectional coefficients and metastable level lifetimes of these glasses were presented in the paper (glass marked MM66 in Ondracek et al. 2008).
The structure of the passive optical power splitter is the crucial element of all-optical networks.
It is a structure that allows the distribution of the optical signal from N × N branches. The basic structure is a splitter with a 1 × 2 split ratio. The cascade arrangement of this structure makes it possible to achieve more complex solutions, enabling the optical signal distribution in the required separation ratio with possible power unbalance. Due to the necessity of integrating all-optical network components into one chip, it is necessary to consider the miniaturization of the solution with respect to the transmission parameters of the components. The optical power splitter structure with a 1 × 2 split ratio can be divided into three parts, see Fig. 2.
The input waveguide is a single-mode field diffusion channel with Lin length, followed by the cosine S-bend area Ls in which the optical signal is distributed. These cosine S-bend segments consist of two arcs that smoothly connect. The Lout length's output waveguides continuously connect to the Ls segments and allow the output optical power to be coupled to another structure, possibly optical fiber. The critical element of the splitter is the area of the curved waveguide, where losses can occur due to exceeding the limit angle α. The length of the waveguide L s is usually proportional to the number of output arms  The design was done for wavelength 1 550 nm for a single-mode waveguide with gradient refractive index with a dimension of the core 4 μm and refractive indices of the waveguide layer n f = 1.4926 and cladding layer n s = 1.4923. The simulation results show that, with decreasing length of the L s waveguides, the losses resulting from excessive curvature of the waveguides increase. By subtracting the normalized power from the simulation result (Fig. 3), an acceptable transmission is achieved for the branching angle α s , in the range of 0.5°-2.0°. However, the optimal angle is only 0.5°, which corresponds to the waveguide length L s = 14 324 µm and insertion attenuation 3.18 dB (for branching angle 2° is L s = 3 579 µm and insertion attenuation 4.28 dB). The optimized structure of the designed splitter and corresponding BMP simulation of the light propagation is shown in Fig. 4. Figure 4a depicts the light propagation through the structure, Fig. 4b shows corresponding normalized power of the outputs P1 and P2 at + Z coordinates (direction of the light propagation).

Experiment-channel waveguides
The channel waveguides for active optical Y-splitters were prepared by a two-step ion exchange. The surface channel waveguides of 4 μm diameter were embedded 2 µm by the applied electric field below the surface of the glass substrate. The embedded ionexchange waveguides have the prerequisites for efficient coupling of the radiation to the optical fibers and lower radiation loss propagating in the diffused waveguide. A photolithographic mask with 2 μm slot dimensions was deposited on GZ4 glass substrates containing 0.25 mol.% Er 2 O 3 and 5.0 mol.% Yb 2 O 3 and 15.0 mol.% of ZnO. A total of ten samples (M2-01 to M2-10) were fabricated and subsequently tested. The measured values were averaged. The average values from measured samples were designated M2-0X. The 1. ion exchange Ag + ↔ Na + was performed using a three-component melt containing 23.7 wt. % of AgNO 3 , 42.9 wt. % of KNO 3 and 33.3 wt. of % NaNO 3 and the 2. ion exchange using eutectic mixture of NaNO 3 and NaNO 2 . The process data of used two-step ion exchange for sample fabrication is shown in Table 3. Fig. 4 Results of BPM simulation for the optimized dimensions of the designed splitter (branching angle α s = 0.5°, n f = 1.4926, n s = 1.4923 and λ = 1 550 nm), a) the light propagation through the optimized structure, b) normalized power of the outputs P1 and P2 at the coordinates + Z (direction of the light propagation)

Experiment-active optical splitters
The structure of the active optical splitter does not differ in its typology from the passive optical splitter. Its essence lies primarily in the critical advantage of ion exchange technology, which allows the creation of an active optical waveguide in substrates doped with rare earth elements.
In the case of an active optical splitter, the longest possible length of the structure is required for the active optical splitter structure as this is a prerequisite for a higher amplification rate. Two-step ion exchange was used to fabricate active power splitter chips with the 1 × 2 split ratio. The total length of the chips is approximately 25 mm after wafer separation, grinding, and polishing of the chip facet. The optical fiber Nufern HP 980-XP was chosen to measure active optical waveguide splitters, as it allows a single-mode field operation in the wavelength from 970 to 1 550 nm. When comparing the magnitude of the waveguide mode field and the channel waveguides, we can observe a more negligible difference for wavelength 1 550 nm and 980 nm (Table 4). Refractive profile analysis performed on channel waveguides has shown that the shape of the diffusion profile of the waveguide within one wafer is preserved for both types of samples.
Given the analysis of direct active channel waveguides, the attention was mainly focused on measuring optical activity in the case of active optical splitters. The optical properties of active splitters were determined by measuring the individual spectral characteristic of the ten samples when the signal from the input port was passed to the individual output ports P 1 and P 2 . The structure of the realized active optical splitter is shown in Fig. 5.

Results and discussion
The scanning electron microscope (SEM) JEOL JSM-6510LV was used to analyze the shape and depth of the ion channel immersion, for close-field measurement was used Spiricon Beam Gage -XEVA 5331 equipment. In addition to the correlation of the diffusion profile and geometry of the field, it is also possible to measure the energy intensity distribution and analyze the waveguide modes (Hui et al. 2009). The measurements have shown that the channel waveguides have immersed about 2 μm below the substrate surface. The images of the samples taken by SEM and the measurement of the near field are shown in Fig. 6a-c, respectively. It is evident that the maximum silver concentration is at the bottom of the waveguide. As a result of the plunging process, the waveguide area was enlarged, and its shape changed.
Measured sizes of diffusion profiles of immersed waveguides are shown in Table 4. Due to the applied electric field at immersion, the mobility of the high silver concentration gradient region of the channel is significantly higher relative to the marginal regions. The results of the near-field measurement confirm this phenomenon very well. Analyzing the measurement results, shown in Fig. 6b, c it is apparent that the area of highest power intensity correlates with the area of maximum silver concentration in the channel. When comparing the magnitude of Nufern HP980-XP connecting fiber field waveguide and the channel waveguides shown in Table 5, we can observe a slight difference in the mode field distribution, mainly at the wavelength 980 nm.
The measured mode field diameter of the channel ion-exchange waveguide in Fig. 6b, c shows the filling of the waveguide layer corresponding to the refractive index distribution. Table 4 The measurement results of the diffusion profile of immersed waveguide Sample The size of the diffusion profile in the x-axis (μm) The size of the diffusion profile in the y-axis (μm) The maximum concentration of silver in the channel (at. %) M2-0X 11.6 7.3

3.17
If the waveguide is excited by a fiber waveguide at a wavelength of 1 550 nm, part of the energy is bound to the upper region of the waveguide due to the increase in the mode field. Due to the insufficient amount of silver and the low and inhomogeneous refractive index, the energy is poorly guided. This results in the deformation of the mode field are shown in Fig. 6c. Measurement of the optical activity has always been performed on structures made on the ten glass samples, where the average value sample was designated M2-0X. A block diagram of the measured spectral loss and differential gain waveforms characteristic is shown in Fig. 7. First, it is possible to create an optimal optical bond of a fiber waveguide and a channel waveguide DUT (device under test) by a three-axis micromanipulator. This device uses piezo-crystal actuators that are voltage-controlled depending on the electrical feedback signal from the detector to the piezo-controller. After the optimal signal connection is found, DUT is fixed against a V-grove Fiber Array (FA) using UV curable adhesive.
The core of the assembly for the measurement of the spectral loss and differential gain characteristics was the signal source in the range of 1 500-1 600 nm HP85651A, The size of the mode field in the y-axis (μm) The size of the mode field in the x-axis (μm) The size of the mode field in the y-axis (μm) Nufern HP 980-XP 11.0 11.0 7.1 7.0 M2-0X 11.6 8.0 11.5 8.5 highly stabilized pumping laser source of radiation CKSS-980-B-650-1, capable of increasing the pumping power up to 600 mW and the optical spectrum analyzer HP86140A enabling spectral measurement in the range of 900-1 700 nm. Further in the assembly, there are WDM couplers for merging and separating the signal and pumping power complemented by the absorbing elements to capture pumping radiation. The measured spectral loss and differential gain characteristics of samples M2-0X optical active splitter 1 × 2 for wavelength band of 1 525-1 560 nm and pumping powers 50, 100 and 200 mW at 980 nm wavelength are shown in Figs. 8, 9. The typical results of attenuation characteristics were obtained for optical splitters labeled M2-0X. The measured spectral loss characteristic for the signal passage state without pumping and at a pumping power of 200 mW is shown in Fig. 8. When the signal passes without pumping, the insertion loss IL in the area of the expected maximum gain of the structure reached an average IL of 3.8 dB on P 1 and 4.1 dB on P 2 arbitrary units. After the activation of the pumping radiation, the signal component is amplified, and thus the attenuation is compensated. The output P 1 Fig. 7 The block diagram of measured spectral loss and differential gain waveforms characteristics exhibited the average differential gain G P1 of 2.1 dB. In the output P 2 the gain G P2 of 1.6 dB was obtained.
Typical differential gain values measured for active optical splitters M2-0X are listed in Table 6.

Conclusion
This paper reported on the research of active optical integrated structures made by ion exchange technology into glass substrates. We have presented here the results of the design and realization of structures of optically active planar waveguides on new silicate glass GZ4 doped by Er 3+ , Yb 3+ and Zn. The main result is the realization and measurement of the optical activity of the Y-branch power splitters, which allows compensation of its insertion losses. Based on simulations in the Beam Prop RSoft program, an optical power splitter was designed with Y-branching with the 1 × 2 ratio. The created splitter model simulates the passing of the signal through the structure and allows for selecting the optimal split angle 0.5° at which insertion losses caused by the curvature of the waveguide are minimized. Based on the simulations, the topological and technological constants were derived, and new silicate glasses were prepared to contain the optimized number of elements, such as erbium, ytterbium and zinc. Subsequently, the mutual concentration ratios of these elements were studied, having in mind the absorption and luminescence Fig. 9 a Differential gain G P1 of output P 1 optical active splitter M2-0X at pumping power 50 mW, 100 mW and 200 mW, b Differential gain G P2 of output P 2 optical active splitter M2-0X at pumping power 50 mW, 100 mW and 200 mW properties. The glass was designed with the prerequisites for application in the integrated optical amplifiers. The result is a setting of 0.25 mol.% Er 2 O 3 and 5.0 mol.% Yb 2 O 3 activators with ZnO modifier of 15.0 mol. %. In this case, the samples exhibited acceptable transfer properties. In particular, our samples showed the average differential gain of 1.2 dB to 1.6 dB. The maximum differential gain was up to 4.0 dB. The minor measured gain differences between different samples can be made with different small amounts of zinc in the matrix, preventing clustering of activator atoms. Probable creation of Ag or ZnO clusters after ion exchange in glass with higher ZnO content could results in a slight increase in the insertion loss of the active optical splitters. These waveguides created splitters exhibit a maximum differential optical gain of up to 1.6 dB/cm at pumping power of 200 mW, which is four times more than that of Er 3+ doped fiber optic amplifiers. In practice, active optical splitters can be used to build metropolitan networks, whereby the optical attenuation caused by splitting the signal among many end-users needs to be compensated. With two step ion exchange technology, such a splitter can be made by diffusing silver directly into a substrate that contains, as part of an integrated optical circuit, rare earth elements.