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We report on ultra-short stretched pulse generation in an all-fiber erbium-doped ring laser with a highly-nonlinear germanosilicate fiber inside the resonator with a slightly positive net-cavity group velocity dispersion (GVD). Stable 84 fs pulses were obtained with a 12 MHz repetition rate at a central wavelength of 1560 nm with a 48.1 nm spectral pulse width (full width at half maximum, FWHM) and 30 mW average output power; this corresponds to the 29.7 kW maximum peak power and 2.5 nJ pulse energy obtained immediately from the oscillator.
© 2015 Optical Society of America
1. Introduction
Mode-locked (ML) ultra-short pulse (USP) fiber lasers have considerable potential for applications in various fields in both science and industry [1]. Over the last decade, USP fiber lasers as a source of optical frequency combs have revolutionized the field of time and frequency metrology since they appeared early as high performance instruments to phase coherently link optical and microwave frequencies [2, 3].
In general, USP generation can be performed using both active and passive ML techniques, and over recent years, a wide variety of cavity designs have been investigated for these lasers. Active ML was described for the first time in 1964 [4]. This method requires the implementation of complex and controllable amplitude or phase modulators. Passive ML was then realized a few years later [5, 6]. To date, passive ML is used for the vast majority of USP lasers, because it allows a much shorter pulse width to be obtained than the active method. In addition, the passive ML technique offers the advantages of flexibility and compactness [7].
Previous studies on passive ML focused on nonlinear polarization evolution (NPE) [8], use of a semiconductor saturable absorber mirror (SESAM) [9], and use of saturable absorbers (SAs) based on carbon nanotubes (CNTs) [10, 11] or graphene [12, 13]. Self-starting passive ML was also demonstrated in hybrid systems that comprised NPE in conjunction with SA or SESAM [14, 15]. Hybrid ML should both enhance the pulse quality and provide reliable ML start-up by taking advantage of both mechanisms [16, 17], and hybrid ML is thus expected to significantly extend the turnkey operation of lasers [18, 19]. However, SAs or SESAMs have some disadvantages, including environmental sensitivity, complicated packaging and fabrication, and material degradation. Nonlinear polarization evolution (NPE) has been shown to be a promising and more reliable mechanism for USP generation in ring-cavity lasers [20]. Ring cavity design is advantageous in that the extraction of higher pulse energies is possible and that it is less sensitive to back reflections [21]. The shortest pulse width in an erbium-doped fiber (EDF) laser was realized using the NPE-based mechanism [22].
For frequency metrology applications, Er-doped fiber lasers have generally been ML in the stretched-pulse regime [23–25], where a wider bandwidth than that observed when operating in the solitonic regime is produced, and these lasers allow effective comb stabilization [26,27]. Therefore, significant efforts are currently being directed toward the study of optical frequency combs of fiber lasers in the stretched-pulse regime passively ML by NPE-based mechanism. Optical frequency combs are in demand for astronomical calibration [28], direct comb spectroscopy [29], and microwave frequency generation [30].
In this study, we apply highly-nonlinear fiber (HNLF) with positive GVD at 1550 nm to net-cavity dispersion compensation. Moreover, the use of the HNLF for NPE-based ML can significantly enhance energy and peak power of the produced pulses by increasing the laser repetition rate and reducing the required total cavity length for an effective pulse compression. It should be noted, that the active comb stabilization scheme can be significantly simplified along with the use of high-NA fibers [31], thus allowing an all-fiber oscillator configuration. Therefore, we demonstrate passive NPE-based ML in a ring cavity formed using an active EDF, a highly nonlinear germanosilicate fiber with positive GVD, and SMF-28 (Corning Incorporated, NY, USA) fiber, which has negative GVD at 1550 nm.
2. Experimental setup
The experimental setup for the ML EDF ring laser is shown in Fig. 1. The NPE action is based on differential rotation of the orthogonally-polarized radiation components during propagation through the cavity due to the Kerr effect [32, 33]. The most important element is thus a polarization filter, which contributes intensity-dependent losses [34, 35].
A commercially developed isolator-polarizer (ISO PM) was used as the filter and also ensured unidirectional generation. Two polarization controllers (PCs) located at both ends of the ISO PM were included in the ring to adjust the ML regimes. A pigtailed single mode laser diode operating at 980 nm with a maximum output power of 430 mW was used as the pump source for the EDF. An 80/20 fiber coupler was used to provide the laser output from the cavity.
We used a 3.6-m-long EDF with low signal core absorption of ∼6.5 dB/m at the pump wavelength and with dispersion D ∼ −17.4 ps/(nm·km) at 1550 nm as the active fiber in the ring cavity. The 1.5-m-long HNLF used in the scheme is a single-mode germanosilicate fiber with a core germanium oxide concentration of ∼50 mol.% and the measured core diameter ∼2.5 μm. The HNLF production parameters was the same as in [36] and calculated value of the nonlinear refractive index n2 is 3.63·10−16 cm2/W. The measured dispersion of the HNLF was 100 ps/(nm·km) at 1550 nm. The total intracavity GVD β2 was +0.022 ps2 at 1550 nm with appropriate control of the SMF-28 fiber length.
3. Experimental results and discussion
Figure 2(a) shows the output spectrum (measured using the optical spectrum analyser AQ6370C, Yokogawa Electric Corporation, Tokyo, Japan) of the self-starting ML generation observed in the cavity with appropriate PC tuning at the maximum average output power of 30 mW.
Fig. 2 (a) Pulse spectrum and Gaussian fitting, (b) Intensity autocorrelation trace and fitting. Inset: Output spectrum.
The pulse train obtained (using the oscilloscope Infinium MSO9254A, Keysight Technologies, Santa Rosa, CA, USA) is presented in the inset of Fig. 1. However, we failed to achieve stable generation with the HNLF length LHNLF in the LHNLF < 1.5 m and LHNLF >2 m ranges. The regimes observed at the laser output included the Gauss-like pulse period doubling bifurcations or unstable period doubling routed to chaos [12]. We believe that these issues have strong connections to the uncompensated third-order dispersion (TOD) as it was observed in [31] for ML with high-NA fibers. In this study we have similar ML fiber laser operation peculiarities but the detailed studies is needed.
The main peak in the output spectrum (see Fig. 2(a)) can be approximated using a Gaussian function with a FWHM of ∼48.1 nm (corresponding to a pulse width of ∼100 fs). However, there are additional spectrum extensions, which are symmetrically spaced around the main maximum. This shift corresponds to the Stokes and anti-Stokes Raman components shift for typical silica glass (∼85 cm−1, see e.g., [37]). Therefore, the Raman scattering, which presumably occurred largely in the HNLF, is a limiting factor for further pulse width compression, along with the influence of the TOD [38]. It should be noted that the conditions for energy transfer to the coherent Raman supercontinuum components can be created in a long ring resonator based on a polarization-maintaining (PM) fiber [39].
Figure 2(b) shows the intensity autocorrelation trace (obtained using the autocorrelator AA-10DDM-OU, Avesta Ltd., Troitsk, Moscow Russia) and its Gaussian fitting at the laser output with the 0.9-m-long SMF-28 fiber from the coupler to the autocorrelator. In order to obtain minimum pulse width we measured pulse duration varying output SMF-28 length Lsmf in the range of 0.9 m <Lsmf < 3m. The minimum pulse width obtained is 84 fs at the maximum available pump power. Note, that there is no pedestal observed in the Fig. 2(b). This issue can be explained by third order dispersion compensation in the cavity using 1.5 m long HNLF as it was observed in [31] for ML with high-NA fibers. The pulse spectrum obtained at the 0.9 m SMF-28 length is shown in the inset of Fig. 2(b). The Gaussian spectrum and pulse shape are inherent to stretched pulse generation [8].
Estimation of the time-bandwidth product gives TBP = Δν × τmin ≈ 0.5. Thus, the obtained pulse is close to the bandwidth-limit (TBP for a Gaussian pulse ≈ 0.44). This slight discrepancy can be ascribed to the linear chirp of the pulse (and thus τmin can be achieved by appropriate SMF-28 length control at the output) or to the influence of modulation instability (MI) in the cavity along with self-phase modulation and TOD effects in the HNLF in particular. Indeed, all these effects may modify the pulse chirp in a nonlinear manner, which thus prevents generation of the bandwidth-limited pulse. However, further research is needed with regard to this issue.
Figure 3(a) shows the radio-frequency (RF) spectrum at the fundamental oscillator frequency with a resolution of 300 Hz (using the electrical spectrum analyzer FSL 3 model.03, Rohde & Schwarz GmbH & Co. KG, Munich, Germany). The RF spectrum has a peak at a frequency ∼12 MHz with a signal-to-noise (S/N) ratio ∼38 dB. At the same time, there is also some frequency modulation, which is expressed in the form of two additional side peaks, that are separate from the fundamental frequency of 0.01 MHz. We believe that these additional peaks are related to the commercial filter (ISO PM) issues and can thus be further suppressed by using e.g. PZ-fiber as reliable polarization filter [18]. The pulse repetition frequency evidently corresponds to a total cavity length of ∼17.3 m. The inset in Fig. 3(a) shows the RF spectrum in the frequency range from 10 kHz to 2 GHz with a resolution of 30 kHz. Note that the high S/N ratio shown, even in the GHz frequency range, indicates the stability of the ML regime obtained [40]. In summary, we have obtained near-bandwidth-limited USP with an average output power of 30 mW, corresponding to 29.7 kW maximum peak power and 2.5 nJ pulse energy when obtained immediately from the oscillator, that significantly exceeded previous results [25].
Figure 3(b) shows the relative intensity noise (RIN) performance of the laser and noise floor from the photodetector and electrical spectrum analyzer (PD+ESA) over a frequency range from 3 Hz to 100 kHz. The acquisition time was 0.3 s. The corner frequency of approximately 7 kHz was related to a combination of the laser dynamics and the gain response time of the EDF [41,42]. For the free-running laser at frequencies below the laser pump modulation bandwidth, the RIN was equal to −120 dBc/Hz, while at frequencies above the laser pump modulation bandwidth, the RIN was equal to −150 dBc/Hz. In our case, without the use of specialized power locking schemes, we obtained a RIN value that is comparable to that obtained previously by femtosecond combs based on other ML principles and USP generation regimes [43, 44].
4. Conclusion
We performed stretched pulse generation with a repetition rate of 12 MHz, an average output power of 30 mW, a spectral pulse width of 48.1 nm and an 84 fs pulse width in an all-fiber erbium-doped ring laser with a highly-nonlinear fiber; this corresponds to the 29.7 kW maximum peak power and 2.5 nJ pulse energy. It should be noted, that obtained high energy stable USP without any further amplification can be used for the coherent supercontinuum generation, which widely used in various active comb stabilization schemes. Thus, these results can be successfully applied to the optical frequency metrology field and our current efforts are now aimed at demonstration of comb stabilization for future generation of low-noise microwaves.
Acknowledgments
The authors would like to thank S. V. Firstov, V. F. Khopin, A. K. Senatorov (FORC RAS) for the provision of the HNLF and GVD measurements. The reported study was funded by RFBR, according to the research projects No. 16-38-60147, No. 16-38-60102 and a project No. 15-11-10023, supported by the Russian Science Foundation.
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