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We demonstrate a record high CW output power of 11.6 W and an ultra-narrow linewidth of 6 kHz in an all-fiber master oscillator and power amplifier (MOPA) fiber laser system. The master oscillator is a 13C2H2 frequency-stabilized single-polarization fiber laser with a 100 mW output. The power amplifier section consists of a core-pumped polarization-maintained erbium-doped fiber pumped by a 1480 nm cascaded Raman fiber laser. A total electric-to-optical conversion efficiency with a record high value of 12% was achieved with an all-fiber configuration.
© 2015 Optical Society of America
1. Introduction
Fiber lasers and amplifiers are excellent optical devices for generating stable high power continuous waves. High power and narrow linewidth single-frequency lasers are very interesting for applications in free space telecommunication, LIDER, coherent beam combination, interferometry, and conversion to mid-IR or UV wavelengths [1,2]. There are many high power amplifiers and they are applied in the 1.0 μm band using Yb3+-doped fiber amplifiers (YDFA) with an excellent quantum efficiency and electrical-to-optical (EO) conversion efficiency. The 1.5-1.6 μm wavelength range is also very attractive when we consider compatibility with telecom components, the wide selectivity of seed lasers, and eye safe characteristics.
There have been several demonstrations of high power amplification in the 1.5 μm region. A continuous output power of 297 W was obtained at 1567 nm using a cladding-pumped Er-Yb fiber amplifier [3], and 55 W was achieved at 1555 nm using a core-pumped Er3+-doped fiber amplifier (EDFA) [4]. These results are excellent, however their linewidth was not narrow, polarization was not single, and the optical frequency was not stabilized. Once the laser frequency is stabilized and then amplified, a variety of applications can be expected.
A 13C2H2 frequency-stabilized erbium fiber laser operating at an eye safe wavelength of 1538.8 nm is one promising laser source [5]. Ultra multi-level coherent transmission experiments with a very high spectral efficiency have been achieved by using the laser [6,7]. To extend the use of such narrow linewidth lasers, it is also important to establish the capability of amplifying the power from mW to W level while suppressing nonlinear effects such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS).
The output power of core-pumped EDFAs (CP-EDFA) can be made scalable by adding more pump sources and optimizing the Er3+ doping rate and fiber length. A 1480 nm cascaded Raman fiber laser (CRL) is a suitable choice as a high power single-mode pumping laser for a CP-EDFA. A fiber laser operating at a wavelength of 1 μm using cladding-pumped Yb-doped fiber made it possible to increase the output power of a 1480 nm CRL, and a power of over 300 W has been reported [8–10].
In this paper, we present an all-fiber master oscillator and power amplifier (MOPA) fiber laser system that we realized by combining a 1538.8 nm 13C2H2 frequency-stabilized fiber laser as a seed, a core-pumped EDF as a gain medium, and a 1480 nm cascaded Raman fiber laser as a pumping source. We have already reported a non-polarization maintained (PM) 9 W output power with a 4 kHz linewidth in an all-fiber configuration [11]. Here we describe new results constituting an 11.6 W output power with a linewidth of 6 kHz, with a single polarization, and an applied electric power to output optical power (EO) conversion efficiency of 12%. This is the highest power ever achieved in a 1.5 μm all-fiber configuration with a frequency-stabilized single-polarization erbium fiber laser and an EDFA.
2. Non-polarization maintained EDFA configuration and results
Figure 1 shows the configuration of a core-pumped non-PM EDFA, in which a 1480 nm CRL was adopted as a pumping source. The CRL configuration is shown in Fig. 2 [8–10]. Here we adopted a core-pumping configuration instead of a clad-pumping configuration owing to its higher efficiency as regards pump absorption in the core. This enabled us to shorten the required EDF length. A 1538.8 nm 13C2H2 frequency-stabilized fiber laser is shown in Fig. 3. This laser is described in detail in [5].
In the EDFA, we employed a very high Er concentration that could realize an absorption rate of 53.3 dB/m at 1530 nm, while maintaining a high conversion efficiency by co-doping a high concentration of Al to avoid Er clustering. The EDF length was set at 3.5 m, which was determined experimentally by considering the optimal balance between output power, efficiency, SBS and ASE noise. A fused taper type 1480/1550 nm WDM coupler with a standard single mode fiber (SMF) was adopted for co-propagating pumping. An optical isolator with a 4 m SMF output pigtail was spliced after the EDF. The splice loss between the EDF with a mode field diameter (MFD) of 5.6 μm and an SMF with an MFD of 10.4 μm was reduced to less than 0.05 dB with a simple MFD matching splice with the thermal expansion of the core.
In the CRL, we optimized the operating current of the 915 nm pumping laser diode, and the EO conversion efficiency ηLD reached 56% as shown in Fig. 4. The conversion efficiency of the 1117 nm YDFL ηYD was 66%, and that of the cascaded Raman resonator section ηCR was 48.4%.
Figure 5 shows the output characteristics of the non-PM EDFA. A maximum output power of 9.25 W was obtained at a 1480 nm pumping power of 28 W, where the output power of the seed 13C2H2 frequency-stabilized fiber laser was 2.2 mW. The conversion efficiency of the EDF section ηED was 33% in this case. The total EO conversion efficiency of the power amplifier section ηamp is expressed as ηamp = ηLDηYDηCRηED, and is estimated to be 5.9%. The gain of this amplifier was as high as 36.3 dB, and no parasitic oscillation was observed.
Figure 6 shows the output spectrum of the non-PM EDFA, where the optical signal-to-noise ratio (OSNR) was larger than 50 dB. The peaks observed at around 1530 and 1565 nm are the amplified spontaneous emission (ASE) from the EDF. Figure 7 shows the linewidth of the amplified output measured with a delayed self-heterodyne method when the output power was 9 W, in which the linewidth of 4 kHz was the same before and after the amplification. In addition, there were no significant fiber nonlinear effects such as SBS and SRS, and no linewidth broadening caused by the Kerr effect in the output linewidth measurement. The frequency stability after the amplification was 3.03 × 10−11 at an integration time of 1 s, which was almost the same as that of the seed laser before amplification. The residual pump power level was less than −30 dB of the signal power. This residual power was removed by passing the signal through a WDM coupler placed at the output, and it was decreased by another −30 dB.
3. Polarization-maintained EDFA configuration and results
Based on the non-PM configuration results in the previous section, we newly introduced a single-polarization operation by changing all the optical components on the signal path in Fig. 1 to the PM design. One of the key devices is a WDM coupler, which is used to combine a high pump power with a non-PM CRL high power output, while the signal path is PM. We used a fused taper optical device with a heat sink for power handling. The optical loss of each wavelength in this WDM coupler was less than 0.2 dB and the polarization extinction ratio (PER) exceeded 14 dB. The core diameter of the PM-EDFA was 3.55 μm and the estimated MFD was 4 μm. The absorption rate of EDF at 1530 nm is 30.4 dB/m. This is smaller than that of the non-PM EDF described above, so the EDF length was increased to 6 m. An EDF length of 6 m was chosen as the optimum value for maximizing the power conversion efficiency and for suppressing parasitic lasing at a gain peak of 1560 nm due to the high gain operation in a single stage EDF section. To obtain a higher OSNR output, we newly adopted a 100 mW output 13C2H2 frequency-stabilized fiber laser with a short cavity configuration [12].
Figure 8 shows the output characteristics of the PM-EDFA. The power conversion efficiency of the EDF section ηED was 67.3% when the output power was 11.6 W, and the EO conversion efficiency was calculated to be 12%. This is the highest conversion efficiency so far achieved with a core-pumped EDFA. The output PER was 17.5 dB including the ASE and the residual pump. The PER after a 1 nm bandpass filter was 21.3 dB, which can be improved by using a polarization-dependent isolator.
Figure 9(a) and (b), respectively, show the output spectra of the PM-EDFA before and after the signal passed through the optical filter. The peak observed at 1560 nm is the ASE from the EDF. The ASE level is higher than that of the non-PM EDFA shown in Fig. 6, and this is due to the longer fiber length needed to achieve a higher EO efficiency. The peak at 1480 nm is the residual pump laser light, and the small peak at 1582 nm is a higher-order Raman component from the pumping laser. They are all suppressed with the optical filter as shown in Fig. 9(b). After the optical filtering, an OSNR exceeding 50 dB and an output power of 11.6 W were obtained. Figure 10 shows the electrical spectrum of the delayed self-heterodyne signal of the PM-EDFA output. When the output power exceeded 8 W, there was a slight increase in the linewidth from 5 to 6 kHz in the delayed self-heterodyne measurement. Although the −3 dB linewidth was almost the same, the tail of the spectrum was slightly broadened. This may be attributed to the Kerr effect induced by the ASE noise [13]. This broadening was not observed in the non-PM EDF experiment at the same power level. This may be due to the trade-off between higher EO conversion efficiency and increased nonlinearity. There was no degradation in the frequency stability. The EDFA output was terminated at a 4 m-long PM-SMF whose MFD was 10.4 μm, and the SBS threshold was estimated to be smaller than 10 W. The MFD of PM-EDF is even smaller, and so the SBS threshold power is at least 4 times lower. However, because of the temperature distribution of the co-propagating core pumping EDF, there was no sign of SBS [14,15].
Fig. 9 Output spectrum of PM-EDFA (a) before and (b) after the signal passed through the optical filter.
4. Conclusion
We successfully demonstrated a record high single-polarization CW output power of 11.6 W and an ultra-narrow linewidth of 6 kHz at 1538.8 nm using a MOPA with a 13C2H2 frequency-stabilized fiber laser and a Raman pumped EDFA. The EO conversion efficiency of the power amplifier section was as high as 12% including CRL. We expect a higher output power to be achieved while maintaining a narrow linewidth by increasing the pump power and enhancing the high-power tolerance of optical components such as isolators and SMF pigtails, cooling the EDF, and increasing the reliability of the fiber coating and splice against heat.
References and links
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