We present the fabrication and characterization of a short core pumped fiber amplifier made of Yb3+-Er3+ codoped phosphate glasses. The 27 mm long amplifier provides a maximum internal gain of 10.7 dB for a signal of −30 dBm at 1535 nm with a 479 mW pump power operating at 975.6 nm. This leads to a gain of 4.0 dB/cm, which compares favorably to other recent works.
Thanks to the high spatial and spectral quality of their output beam, fiber lasers appear as a promising infrared light source for LIDAR systems. As fiber laser material, phosphate glasses allow to incorporate high levels of rare-earth ions, which enables the manufacturing of short optical fiber devices with a high output power. These features redound into compact, portable lasers that can reach longer distances when compared with other emitting devices. Usually, a one-piece fiber laser having all these characteristics cannot be easily conceived. A convenient way to achieve all the specifications at once is to proceed through a master oscillator power amplifier (MOPA) architecture. In this configuration, the output of a low-power seed laser having the required spectral characteristics is carried into a high-power fiber amplifier. This, in turn, provides the required amplification level without impairing on the other desired specification. This paper focuses precisely on the fabrication and characterization of a short core pumped fiber amplifier based on Yb3+-Er3+ codoped phosphate glasses to be used at the second stage of a MOPA configuration.
The core glass was doped with 2.81 wt% of Er2O3 (1.08 mol%, 2.5 · 1026 ions/m3) and 2.87 wt% of Yb2O3 (1.09 mol%, 2.5 · 1026 ions/m3). The optical fiber preform was produced by means of rotational casting technique developed in-house. The preform and fiber fabrication parameters were set to generate a double-cladding fiber with the following diameters: 7
The components used in the experimental setup are summarized in Table
Material and components.
Component | Value/description | Relevant parameters |
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Pump laser | 1999CHP |
975.6 nm |
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Tunable laser | HP8168F |
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WDM | Go4fiber |
Ports: 976–1090 nm |
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Yb3+/Er3+ fiber | In-house developed phosphate glass fiber | Fiber |
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Collector pigtail | Fiber Ø = 9 |
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OSA | Agilent 86140B | Max 30 dBm or |
A detailed view of the fabricated amplifier section.
The near-field image of the active fiber end face at 1300 nm is presented in Figure
(a) Near-field image of the active fiber end at 1300 nm. (b) Intensity profile corresponding to the transversal dashed line
Fiber losses were measured by cut-back technique using a length of 280 cm with a source at 1300 nm. The results are depicted in Figure
Attenuation coefficient measured on Yb3+/Er3+ phosphate glass fiber obtained by exciting at 1300 nm. After fitting the curve, a mean value of 7.11 dB/m was found.
The output spectrum of the amplifier is shown in Figure
Amplifier output spectrum of the Yb3+/Er3+ phosphate glass fiber obtained by exciting with a 479 mW pump at 975 nm and a 0 dBm signal at 1535 nm. The highest peak obtained at these conditions is shown (+6.714 dBm).
The presence of green light scattered out of the fiber—as seen in Figure
Signal net gain spectrum obtained for four different signal input powers −30 dBm, −20 dBm, −10 dBm, and 0 dBm at wavelengths ranging from 1515 to 1575 nm. Injected pump power is
The internal gain per unit length at wavelength
Internal gain per unit length as a function of the input power level at 1535 nm. The amplifier shows some degree of saturation.
A maximum value of 4.0 dB/cm was obtained for a −30 dBm signal and 479.8 mW pump power. Here, the curves show that the amplifier presents a clear sign of saturation at high pump or signal powers. This is in agreement with the fact that further increments in the pump power do not have a corresponding linear increase on the internal gain. At low pump conditions, the internal gain levels maintain almost a constant value independent of the input signal power (at least for the range of tested signal powers). This observation suggests that, in this regime, the Er3+ is not yet fully inverted and that the amplifier is not yet saturated. When increasing the pump power, the gain starts to saturate for both low and high signal input cases. However, one can notice that the saturation gain tends to decrease as the input signal increases. Actually, deeming the amplifier as a power reservoir, this latter is not large enough to sustain the same gain value for high input signal as it does for low signal input power. It is believed that in order to increase the gain per length value a further improvement of the fabrication process is required. For instance, the reduction of particles and defects located at the core-cladding interface would allow reducing the optical loss and thus achieving more gain. Additionally, avoiding the creation of micro-clusters could be useful to reduce up-conversion in order to increase the output power.
Also, at high pump power a large amount of pump remained unabsorbed and was in fact observed experimentally at the amplifier output. An amplifier slightly longer would allow to fully exploit the pump power available for reaching higher output powers.
A fiber amplifier based on Yb3+-Er3+ codoped phosphate glasses was fabricated and characterized. At the highest input signal (+5 dBm) the amplifier provided 6.39 dB and 8.52 dB internal gain for medium and highly saturated operation (166.8 mW and 479.8 mW pump power, resp.). The internal gain per unit length was thus 2.37 dB/cm and 3.15 dB/cm, which is higher than or near to values reported in [