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Abstract
Compact and miniature optical amplifiers operating in the near-infrared wavelength range are in ever-increasing-demand for integrated optical communication devices and subsystems. Due to low dopant concentration and the relatively large size of conventional fiber amplifiers, however, much effort is shifting toward compact and miniature waveguide structures. We hereby demonstrate a simple and effective method for the fabrication of optical waveguide devices. The devices can be fabricated by using a miniature rare-earth-ion-doped-glass microneedle coupled with two fiber collimators at the input and output ends of the waveguide. The experimental results show that, with a pump power of 500 mW, a small-signal net gain of 13.20 dB at 1530 nm is achieved in the 1.0 cm-length active microneedle with a more than 1.0×1026 ion/m3 dopant concentration. This approach can be considered as a new gateway for the fabrication and wide application of compact and miniature waveguide devices in the future.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over past few decades, there has been a great demand for high-speed and large-capacity information transmission system. In optical communication field, research and development efforts have been directed to the realization of compact optical components. [1–8] The disadvantage of current fiber devices is relatively large size because they use longer fiber which make them difficult to be compact and miniaturize. [9–12]
A key of the technology for providing “unlimited” data transmission rate is to use efficient ultranarrow-linewidth-laser transmitters and broadband fiber amplifiers. To these days, the development efforts in optical fiber amplifiers have been restricted to silicate -based materials. [13–16] As in the optical fiber communication systems and networks, the transmitter versus optical amplifier, optical-add-drop multiplexer versus optical amplifier, optical cross-connector versus optical amplifier as well as optical detector versus optical amplifier will be integrated on a planar optical waveguide to reduce the size of device and subsystems. To make these integrated devices and subsystems compact, it is necessary to develop compact and miniature fiber and waveguide amplifiers. [2,17] Being different from silicate materials, phosphate glass is a more attractive material because it combines all the required properties such as good chemical durability, high absorption and emission cross section and high solubility of rare earth ions. [18–20] Moreover, they are insensitive to concentration quenching and show low up-conversion losses. This means that high concentration of active ion may be introduced into this matrix, resulting in a short gain medium.
So far, various methods have been developed to fabricate waveguide amplifiers, including chemical vapor deposition (CVD), ion-exchange, flame-hydrolysis deposition (FHD), Sol-gel, etc. [16,21–26]
In this work we demonstrate a facile method for fabrication of a short length waveguide amplifiers. Compared with the conventional techniques, this approach has the following merits: (i) the waveguide neither needs substrate nor other layer so its structure is very simple, (ii) the waveguide not only uses phosphate glass as host materials but also has various substitution such as fluoride, chalcogenides, tellurite glasses and other materials which are difficult to make fiber devices, (iii) this approach can be highly efficient and can be used for mass of fabrication.
2. Experiment and measurement
2.1 Glass preparation
Transparent phosphate glass samples were synthesized by conventional melting method using A.R. pure raw materials. For each sample, 10.0 g of raw material powders were fully mixed and melt in a covered corundum crucible at 1350 °C for 0.5-1.0 hours. Then the glass samples were quenched by pouring the melting onto a pre-heated copper plate at about 300 °C. Subsequently, the glass samples were annealed at 350 °C for 2.0 hours and slowly cooled down to room temperature to relinquish internal stress. Eventually, glass samples were cut and polished for spectroscopic properties measurements.
2.2 Design and fabrication
We processed the phosphate glass microneedle as a waveguide by grinding and polishing, as shown in Fig. 6(a). The glass needle was 10.0 mm in length with 0.5 mm radius. Subsequently, as shown in Fig. 6(b), the Er3+-doped phosphate glass waveguide was processed and packaged into a structure coupling with convex lens in the two ends of the needle. The optical waveguide were connected to fiber for gain and noise figure measurement.
2.3 Measurement setup
The gain and noise figure of the waveguide amplifier were performed by using a tunable laser source (Keysight's 81960A) with working wavelength range of 1530 nm to 1600 nm as the signal source and a 980 nm laser diode (Shanghai Sunny Optical Communication Technology Co., Ltd) as the pump source, with tunable output power. Signal and pump light were launched into the channel waveguides by a 980/1530 nm WDM coupler. The output light from the device was collected and coupled to an optical spectrum analyzer (OSA, YOKOGAWA's AQ6370C), and gain and noise figure were obtained.
3. Results and discussion
The transitions of Er3+ doped phosphate glass can be equivalent to those of a three-level system, as shown in Fig. 1. Absorbing the 980 nm pumping photons, the Er3+ ions are excited from the ground-state 4I15/2 level to excited state 4I11/2 level, which is unsteady and whose lifetime at 4I11/2 level is very short, and then the electrons relax nonradiatively from the 4I11/2 level to the metastable 4I13/2 level, and subsequently transit from the metastable level to the ground state level with stimulated and spontaneous emissions, emitting the signal photons at 1530 nm windows. Moreover, due to high-concentration doping, the distance between neighbouring two erbium ions is very short, the weak cooperation-upconversion and cross-relaxation will appear at the metastable 4I13/2 level, that is, one of the two electrons at the two 4I13/2 levels will transit from this level to the 4I9/2 level, and other transits to the ground 4I15/2 level. Meanwhile, the electron at the 4I9/2 level transits back to the 4I13/2 level with the electron transition from 4I15/2 to 4I13/2 level, generating cross-relaxion process.
Fig. 1. Schematic diagram of electronic transition process and energy transfer of high-concentration Er3+-doped glass.
Figure 2 shows the structure of the erbium-doped phosphate waveguide amplifier. As shown in Fig. 2(a), the C-band signal and 980 nm pump laser are coupled into the erbium-doped phosphate waveguide through a 980/1550 nm wavelength division multiplexing (WDM) coupler. The pump radiation is absorbed and the signal at 1550 nm band is amplified and then output to the optical spectra analyzer. The Fig. 2(b) shows the waveguide structure.
Fig. 2. (a) Schematic diagram of gain and noise figure measurement and (b) waveguide structure of high-concentration Er3+-doped phosphate glass waveguide amplifier.
The important spectroscopic parameters of Er3+-doped phosphate glasses include the absorption and emission cross sections of the 4I13/2→4I15/2 transitions and the upper level lifetime. The dependence of absorption cross-sections on wavelength can be calculated as Eq. (1) [27]:
where I0 is the intensity of incident light, while I stands for the intensity of output light which comes out after getting through the medium with a length of L. Log(I0/I) is the absorption intensity.The emission cross-section can be well calculated from absorption cross-section, i.e.,
The upper level lifetime of Er3+ ions can be calculated according to refractive index and absorption cross section, and can be written as Eq. (3)
In Eqs. (4)–(7), the Wij terms represent the stimulated transition rates between the i and j levels, R13 represents pump transition rate between the 4I11/2 and 4I15/2 levels. A21 is radiative transition rate from 4I13/2 to 4I15/2 level. Cup1, Cup2 are cooperative up-conversion coefficients. τ is the upper level life time. σes, σas, σep and σap are the corresponding emission and absorption cross sections of signal and pump power, respectively. h is Planck constant, while fs represents the signal frequency. The total distribution of Er3+ ion density is assumed to be constant within the whole waveguide section and along the waveguide length. The populations at different levels satisfy Eq. (8):
The propagation of pump power along the waveguide is described by Eq. (9). where Γp represents the overlap factor between pump mode field and erbium ion distribution, and σap and αp are respective absorption cross section and waveguide attenuation coefficient at pump wavelength. The signal power and amplified spontaneous emission (ASE) power are amplified according to the power propagation equations:
Table 1. Key parameters in numerical analyses of Er3+-doped phosphate waveguide amplifier.
For the theoretical analysis, a key parameter affecting the final theoretical results is the cooperative up-conversion coefficient of erbium ions. According to the Ref. [28], only when the distance between ions is less than 0.80 nm, the interaction between the ions dominates. The designed concentration of erbium ions in the erbium-doped phosphate glass is 2.22×1021 cm−3 and the ion separation is 0.76 nm. Therefore, it can be considered that the interaction between erbium ions plays a dominant role at this concentration. The interaction is linear with the square of the ion concentration. Therefore, we establish the rate equations including the second-order polynomial of the populations.
In the following section we analyze the optical gain pumped at 980 nm. The signal wavelength selected in our theoretical analysis is C-band ranging from 1528–1565 nm. Figure 3 shows the effect of the waveguide parameters on the gain and noise figure of the waveguide amplifier. The gain and noise figure of the Er3+-doped amplifier are calculated by numerically solving the Eqs. (4)–(11), which is introduced by our previous work [20].
Fig. 3. Dependence of the calculated gain and noise figure on (a) Er3+ ion concentration, (b) pump power, (c) signal wavelength and (d) the waveguide length, respectively.
Figure 3(a) shows the gain as a function of the Er3+ ion doping concentration, ranging from 1.2 to 10.5×1026 m−3. The gain increases nearly linearly when the Er3+ ion concentration is less than 8.0×1026 m−3 and begin to saturate after that. When the concentration increases from 8.0×1026 m−3 to 10.5×1026 m−3, the gain only increases by 2.10 dB. As shown in Fig. 3(b), with pump power increasing from 120 to 500 mW, the signal gain, like the trend in Fig. 3(a), first increases linearly and then reaches saturation. When the signal wavelength ranges from 1528 to 1565 nm, and the signal reaches the highest gain at the wavelength 1530 nm. The gain can be more than 10.00 dB in the wavelength range 1528 ∼ 1562 nm, which can be observed from Fig. 3(c). In Fig. 3(d) we set the waveguide length ranging from 2.2 cm to 10.0 cm. When the length reaches 9.2 cm, the maximum gain 37.16 dB is achieved. However, the gain per unit length decreases with increasing gain medium length. When the length L = 3.4 cm, the gain per unit length is 5.73 dB/cm, and when L = 10.0 cm, the unit length gain decreases to 3.65 dB/cm.
Figure 3(a) also shows the variation of noise figure with Er3+ ion concentration. The noise figure increases with the Er3+ ion concentration. When Er3+ ion concentration reaches 10.0×1026 m−3, the noise figure is 5.11 dB. It is found in Fig. 3(b) that the noise figure keeps declining with the increment of pump power, and the gain keeps increasing. When the pump power reaches 500 mW, the noise figure decreases to 4.00 dB. As shown in Fig. 3(c), the noise figure is less than 4.03 dB over the entire signal wavelength range, exhibiting good noise immunity. Moreover, the noise figure is positively correlated with the gain medium length, which can be observed in Fig. 3(d). When the medium length is 3.0 cm, 6.0 cm, the noise figure is 3.98 dB and 4.62 dB, respectively.
Figure 4 shows the variation of the measured net gain with signal wavelength and pump power. The measured insertion loss of this waveguide is 9.80 dB at 1310 nm, and since there is no absorption at this wavelelngth, the insertion loss at this wavelength can be considered as scatterring loss of side surface and reflection loss of end-face of microneedle waveguides. As can be seen from Fig. 4, by varying the pump power, the gain monotonically increases in the range from -14.00 to 13.20 dB at the wavelength 1530 nm, -12.40 to 10.30 dB at the wavelength 1540 nm and -5.40 to 9.00 dB at the wavelength 1550 nm, respectively. When the pump power increases to 500 mW, the signal gain at singal wavelengths 1530 nm, 1540 nm, 1550 nm can achieve 13.20 dB, 10.30 dB and 9.00 dB, respectively. The gain medium lengths and corresponding unit-length net gains and from this work and references are listed in Table 2.
Table 2. The comparison of unit length gain of erbium-doped phosphate waveguide amplifier and erbium/ytterbium co-doped phosphate fiber amplifier. Signal wavelength is 1530 nm, and signal input power is -30 dBm.
If the insertion loss is reduced by imporving process quality of microneedle waveguide, and the end-face reflection loss may be reduced by coating anti-reflection film at the two ends of the waveguide, the measured net gain and unit length gain will be further improved. This result confirms the effectiveness of the high-concentration erbium-doped phosphate waveguide amplifier and has a good application prospect in the field of compact and miniature optical amplifiers.
Figure 5 presents the variation of the measured noise figure with signal wavelength and pump power. By increasing the pump power from 50 mW to 500 mW, the noise figure at the wavelength 1530 nm, 1540 nm, 1550 nm monotonically decreases from 17.60 to 4.00 dB, from 13.30 to 4.90 dB and from 8.80 to 4.00 dB, respectively. When pump power reaches 500 mW, the noise figure at the wavelength 1530 nm, 1540 nm, 1550 nm is only 4.00 dB, 4.90 dB and 4.00 dB, respectively. The measured noise figures are close to the theoretical estimates, as shown in Fig. 3.
All the above experimental results testify the feasibility of the erbium-doped phosphate waveguide amplifier as a compact and miniature erbium-doped waveguide amplifier. The pictures of Er3+-doped phosphate glass waveguide and the waveguide amplifier module are shown in Fig. 6. Further investigations will focus on the decreasing of scattering loss and end-face reflection loss and will be reported in next paper.
Fig. 6. Photograph of (a) Er3+-doped phosphate glass waveguide and (b) photonic waveguide amplifier module, respectively.
4. Summary
A compact and miniature waveguide amplifier, which is based on Er3+-doped phosphate glass microneedle and convex lens, is designed, prepared and measured. The proportion of Er3+ doping in the phosphate glass is as high as 20 mol% and the length of the miniature glass microneedle is as short as 1.0 cm. Both the gain and noise figure are numerically analyzed. The experimental results show that the miniature photonic waveguide amplifier has a high unit-length gain and relative low noise figure. The method outlined here can greatly simplify the design of a fiber amplifier and is promising for significant reduction of the size and cost of current EDFA systems and possibly open a new pathway toward developing compact and miniature waveguide devices for integrated fiber device and optoelectronic device applications.
Funding
National Natural Science Foundation of China (NSFC) (61177056).
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant No. 61177056).
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