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We report on the optical-signal amplification in a cladding waveguide that was fabricated in Er:YAG ceramic by multiple carbon-ion irradiation. The waveguide has a multilayer structure that assures good overlap between the pump beam and the input signal. Under the pump at 980 nm with a fiber-coupled diode laser, the cladding waveguide possess a peak internal gain of 2.6 dB/cm at 1550 nm and of 4.0 dB/cm at 1585 nm. This work demonstrates the potential use as amplifier in the C and L communication bands of cladding waveguides fabricated in Er:YAG by carbon ion irradiation technique.
© 2016 Optical Society of America
1. Introduction
In the past decade, considerable efforts have been devoted to develop the erbium-doped waveguide amplifiers (EDWAs), due to the great application potential on new system architectures in photonic integrated circuit and local-access optical networks [1–5]. In EDWAs, the waveguide structure in the Erbium doped optical material is used as the gain medium. Hence, the waveguide fabrication in the gain medium is one key point for this study. Various fabrication processes are nowadays used, such as ultrafast laser writing [6, 7], ion exchange [8], or several ion beam techniques that includes ion implantation, swift heavy-ion irradiation or proton-beam writing [9–11]. During the ion irradiation process, energetic ions penetrate into the optical material and modify the refractive indices along ions’ paths generating the waveguide structure in the optical material. Up to now, the ion beam technology has been applied for the high quality waveguide fabrication in multiply gain mediums including rare earth doped glass, crystal and ceramics [12–15].
Rare earth doped yttrium aluminum garnet ceramic (YAG ce) has attracted a continuous attention due to its excellent lasing properties and has been extensively studied for the application in the active photonic devices, such as high power lasers and optical amplifiers [16–18]. Different from its single-crystalline partners, YAG ce can be easily produced in a large size and doped with higher concentrations. Besides, the erbium doped YAG ce has a broad emission bandwidth in the near-infrared band, covering communication bands. Hence, the rare earth doped YAG ce, as the gain medium, is estimated to play an important role for the further development of the active photonic device [19–22]. As rare-earth ion doped YAG ceramics usually have broad emission bandwidth in the near-infrared band, they are most attractive candidates for the optical amplifiers in telecommunications. Until now, the laser emission and the optical signal amplification have been reported based on the bulk material of the doped YAG ce. However, there is little reports on the application of the YAG ceramic waveguide in EDWAs.
In this work, we fabricate a waveguide structure in Er:YAG ceramic by carbon ion irradiation. Through multiple carbon ion irradiation, the cladding waveguide has been formed near the surface, in which the propagation modes of the signal and pump light have a good overlap with each other. Using this waveguide as the gain medium, the signal amplifier in the C and L bands are realized. The peak gains are 1.04 dB (2.6 dB/cm) at 1550 nm and 1.6 dB (4 dB/cm) at 1585 nm, respectively. This work demonstrates the application potential of the irradiated Er:YAG ceramic waveguide for the optical amplifier in the communication band.
2. Experiments. Results and discussion
Er:YAG ceramic waveguide fabrication
The Er:YAG ceramic used in this work is doped by 50 at.% Er ions, obtained from Baikowski Ltd. (Japan), which was cut into dimensions of 2 × 4 × 7 mm3 with facets optically polished. One biggest facet of the Er:YAG ceramic was irradiated twice by carbon ion with the the energies (fluences) of 6 MeV (5 × 1014 ions/cm2) and 15 MeV (2 × 1014 ions/cm2), respectively. Thus, a cladding waveguide structure is formed near the surface of the Er:YAG ce, which has a step-like refractive index. The refractive index profile of the cladding waveguide was reconstructed by the intensity profile fitting method (IPFM) [23].
The fluorescence of the Er:YAG ceramic waveguide was measured by the end-coupling method. During the measurement, the pumping laser at the 980 nm from a diode laser was coupled into the waveguide by a single mode fiber (SMF). The output light was collected and detected by a spectrograph.
Er:YAG ceramic waveguide characterization
Figure 1(a) shows the microscope image of the Er:YAG ceramic waveguide cross section. A sandwich structure is observed near the surface. The layer thickness is 1.2 μm (L1), 3 μm (L2) and 5.4 μm (L3), respective. The refractive index distribution of the waveguide is presented in Fig. 1(b), which has a step-like shape constituting the cladding waveguide [23]. As shown in Fig. 1(a) and 1(b), L2 has the highest refractive index forming the core of the cladding waveguide.The inner cladding (with width of 9.6 μm) is made up by L1 and L3. The substrate and air constitute the outer cladding. The inner cladding is wider than the diameter (8.2 μm) of the input fiber (SMF-28e + ), hence the input light can be better collected by the SMF and the coupling efficiency was 60 ± 10% in this work. As the core has a higher refractive index, the intensity of the pump and signal light are concentrated in the core. As presented in Fig. 1(c) and 1(d), the peak power positions of the signal (1550 nm) and pump light (980 nm) have a good overlap with each other in the Er:YAG ceramic cladding waveguide
Fig. 1 a) Optical microscope image of the waveguide cross section. (b) Reconstructed refractive index distribution. Measured propagation modes at the wavelength of 980 nm (c) and 1550 nm (d).
Amplifier characterization setup
Utilizing the Er:YAG ceramic waveguide as the gain medium, the optical signal amplification was realized within the wavelength of 1520 nm ~1620 nm. The schematic of the Er:YAG ceramic waveguide and the characterization setup is presented in Fig. 2. The laser at the wavelength of 980 nm from a fiber coupled diode laser was used as the pumping source with the maximum power of 460 mW. The signal light was provided by a diode laser with a broad band from 1500 nm to 1650 nm. And the signal light was periodically cut off and switched to the pulse light (10 Hz) by a chopper and a U–Bench. Through a fiber coupler (980/1550), the pumping and signal light was combined into a SMF (1550 nm) and coupled into the Er:YAG ceramic waveguide. The output light was collected by a long work-distance microscope objective ( × 20 N.A. = 0.65). The power of the pump and signal light was adjusted by variable attenuators.
Fig. 2 Schematic plot of the experimental setup for the optical amplification in the Er:YAG ceramic waveguide. The inset picture is the microphotograph of the light propagation in the waveguide.
3. Results and discussion
Fluorescent property
Figure 3 compares the 4I13/2 to 4I15/2 emission spectra that were recorded under 980-nm excitation from the bulk Er:YAG ceramic and from the waveguide that was fabricated in Er:YAG ceramic. Within the wavelength range from 1500 nm to 1680 nm, multiply peaks are observed, indicating the application potential for the optical signal amplification in the C band (1530 nm – 1565 nm) and the L band (1565 nm −1625 nm). By the same experimental method, the emission spectrum of the Er:YAG ceramic was measured and is shown in Fig. 3 (the solid blue line). Compared with the emission spectrum from the waveguide, the shapes and positions of peaks are the same in emission spectrum. The intensity of peaks in the waveguide is higher than the one in the bulk material due to better confinement by the waveguide.
Fig. 3 The room temperature emission spectra (4I13/2 → 4I15/2) under 980 nm laser excitation from the Er:YAG ceramic waveguide and Er:YAG ceramic bulk material, respectively.
Optical amplifier
Using the experimental set-up given in Fig. 2, we have investigated the performances of the waveguide written in Er:YAG ceramic. The performance of Er:YAG ceramic waveguide in the optical signal amplification is depicted in Fig. 4. At first, the transmission spectrum of the Er:YAG ceramic waveguide was measured within the wavelength range of 1520 nm - 1620 nm by setting the pump power to zero. The extinction of the signal light was observed around ~1537 nm (- 10.9 dB/cm), ~1560 nm (- 18.3 dB/cm) and ~1606 nm (- 5.6 dB/cm). Please note, the measured light extinction includes the absorption of Er:YAG ceramic and the propagation loss of the waveguide. The propagation loss is the contribution of the scattering in the waveguide, which is decided by the waveguide structure. In this work, the propagation loss of Er:YAG ceramic waveguide was detected by a 1064 nm laser, at which wavelength the absorption coefficient of Er:YAG ceramic is low. The value of the propagation loss is determined to be 1.2 ± 0.4 dB/cm, which was measured by the method reported in Ref [24].
Fig. 4 (a) Measured absorption and gain spectra in the wavelength range of 1520 nm – 1620 nm. (b) Measured extinction/gain as a function of the pump power at the wavelength of 1550 nm and 1585 nm, respectively.
There is a bleaching of the extinction and the optical amplification when the pump power is above 50 mW. The gain (G) is defined by the change of the signal light power, which is given by the equation below:
where Pout is the output signal light power; PASE is the fluorescence emission due to the pump without the signal light; and Psignal is the input power of the signal light. As shown in Fig. 4(a), the optical gain persists in the entire C and L bands. The amplifier exhibits two peak gains of 1.04 dB (2.6 dB/cm) at 1550 nm and 1.6 dB (4 dB/cm) at 1585 nm, respectively. Figure 4(b) presents the gain at peaks of the gain spectrum as a function of the pump power. Under the pump power of 50 mW, the optical amplification achieves a balance with the extinction and the Er:YAG ceramic waveguide exhibits a net gain of the input signal light. Further increasing the pump power, the gain has a linear variation along with the pump power, which means the gain is far from saturation and the increasing of the pump power can significantly increase the amplifier gain.Under the maximum pumping, the noise figure (NF) of the waveguide amplifier operating was measured in the 1520nm to 1620 nm spectral range. The value of NF at a given wavelength was calculated by the equation below:
where B0 is the frequency of the slot width at the signal frequency; h is Planck’s constant; and v is the optical frequency of the signal light. Figure 5 presents the NF as a function of the signal wavelength. Obviously, the amplifier exhibits a minimum NF of 5.3 dB at 1550 nm and 5.1 dB at 1585 nm, respectively.Fig. 5 The noise figure of the Er:YAG ceramic waveguide amplifier operating within the spectral range of 1520 nm - 1620 nm.
4. Conclusions
We demonstrate the realization of an EDWA using the ion irradiated Er:YAG ceramic waveguide. Through carbon ion irradiation, the cladding waveguide was generated near the surface of the Er:YAG ce. The intensity peak of the signal and pump light is confined in the core and has a good overlap with each other. Under the pump power of 200 mW at 980 nm, gains at 1550 nm and 1585 nm have the values of 2.6 dB/cm and 4 dB/cm. The Er:YAG ceramic cladding waveguide can be as an excellent candidate for the waveguide amplifier in the communication band.
Acknowledgments
This research work is supported by the following funding programs: National Natural Science Foundation of China (Grant No. 11305094 by Y. T.), Young Scholars Program of Shandong University (Grant No. 2015WLJH20 by Y. T.) and Helmholtz Association (VH-NG-713 by S. Z.). Ion irradiation was performed at the Ion Beam Center at the Helmholtz-Zentrum Dresden – Rossendorf.
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