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Near-infrared supercontinnum (SC) generation, accompanied with several emission bands at visible and ultraviolet, is experimentally investigated in an all-fiber single-mode Yb3+-doped silica fiber MOPA. The seed is an all-normal-dispersion mode-locked Yb3+-doped single-mode fiber laser using a nonlinear polarization evolution mechanism. With the pump power of several hundreds of milliwatts, SC spanning of 1010 nm to 1600 nm was generated in a 20-m single-mode germano-zirconia-silica Yb3+-doped fiber amplifier. The intensive nonlinear effects, namely stimulated Raman scattering, four wave mixing, and self-phase modulation, enable the SC generation in the small-core fiber amplifier without the use of photonic crystal fibers or tapered fibers. Such a compact and cost-effective SC generation system enables applications in optical coherent tomography, optical metrology, and nonlinear microscopy.
© 2014 Optical Society of America
1. Introduction
As being a powerful broadband laser source with high spatial coherence, the supercontinnum generation (SCG) light source is of interest to many applications, such as optical coherent tomography, optical metrology, nonlinear microscopy, and so on [1–4]. Traditionally, SCG is achieved by launching high-energy pulses, generated by Ti:sapphire or diode-pumped solid state lasers, into microstructure fibers [5–8]. More recently, various Q-switched or mode-locked (ML) SCG systems based on free-space-running fiber lasers have been reported [9–11]. In general, most current SCG systems require precise 3-D free-space alignments, which are relatively complex and expensive. Thus, it is more beneficial to generate SC by using simple all-fiber laser configurations. On the other hand, fiber laser sources provide many advantages over traditional solid-state laser sources, namely the near-quantum-limited efficiency, good spatial-beam quality, easier thermal management, and compactness. However, to provide enough nonlinearity in these SCG systems, photonics crystal fibers or tapered small core fibers are usually required. For example, Chapman et al. use a high-power CW Yb3+-doped fiber laser as the pumping source to generate SCG extending from 1080 nm to 1820 nm in a double zero dispersion wavelength PCF [12, 13]. Nevertheless, these specialty optical fibers are more difficult to fabricate and splice than standard fibers. Another straightforward approach is using nonlinear Yb3+-doped fiber amplifiers to generate near-infrared (NIR) SCG that provides the potential to produce high spectral power density [14, 15]. Unfortunately, these SCG fiber laser systems usually need several to hundred watts of pump power. Similarly, Zeng et al. have demonstrated yellow and NIR laser emissions through the cascaded four wave mixing (FWM) processes in an all-fiber nonlinear Yb3+-doped fiber amplifier [16]. Nevertheless, the high-power Q-switched mode-locked (ML) laser seed as well as the multi-mode gain fibers are required to provide efficient yellow emission. Reducing the pump threshold of the all-fiber SCG laser systems, while preserving the good spatial beam quality, could be important in application perspective.
In this paper, we report a relatively simple all-fiber laser amplifier system to produce the NIR SCG as well as visible and ultraviolet (UV) emission without the need of using microstructured specialty fibers or nonlinear fiber devices. The system is based on newly developed germano-zirconia-silica Yb3+-doped (GZY) nonlinear single-mode fiber amplifier seeded by a single-mode ML Yb3+-doped silica fiber laser. In Section 2, we describe the material characteristics of the GZY fiber and the experimental setup for NIR SCG. The experimental results and discussion are then presented in Section 3 and 4, respectively. The interplay between stimulated Raman scattering (SRS), FWM and self-phase modulation (SPM) in the single-mode fiber amplifier are experimentally investigated in detail.
2. Experimental setup
To generate high-energy ultrashort seed pulses for SCG, the passively mode-locked (PML) Yb3+-doped fiber laser (YDFL), operating in the all-normal-dispersion region, is preferred. Experimental setup of the Yb3+-doped fiber amplifier (YDFA) system is shown in Fig. 1.The ML pulses were generated based on nonlinear polarization evolution (NPE) mechanism, relying on an in-line polarizer (ILP) and two polarization controllers (PCs) [17]. A 2.2-m Yb3+-doped fiber was employed as the gain medium and core pumped by a temperature-stabilized 974-nm laser diode through a wavelength-division multiplexing (WDM) coupler. The Yb3+-doped fiber has a core diameter of 5 μm and core absorption coefficient of 80 dB/m @976 nm. An intra-cavity polarization-independent isolator was employed to ensure the unidirectional laser operation. Two types of directional couplers were used to achieve specific laser features. The 90/10 direction coupler was chosen to provide 10% reflectivity, resulting in a maximum output pulse energy, while the 80/20 coupler was employed to provide a higher cavity Finesse, leading to higher peak powers and more nonlinear processes inside the laser cavity. With proper adjustments of two PCs, ML pulses were generated. The operation state of the laser transferred from continuous wave (CW) to Q-switched mode-locking (QML) and finally to continuous-wave mode-locking (CW-ML) by increasing the 974-nm pump power. An isolator was employed between the seed laser and amplifier to block the back reflection. In addition, a 95/5 directional coupler was used to monitor the 5% output of the seed laser with an optical spectrum analyzer (OSA, Ando AQ-6315E) or oscilloscope (OC, LeCroy 620Zi 2 GHz), while 95% of the laser output was launched into the fiber amplifier.
The gain fiber employed in the amplifier was a 20-m single-mode germano-zirconia silica Yb3+-doped (GZY) fiber, manufactured by the modified chemical vapor deposition (MCVD) process coupled with the solution doping (SD) technique [18] to achieve uniform doping of all elements. This new kind of special fibers requires modification of existing MCVD-SD technique which has been described elsewhere [19]. The spatial distribution of important dopants was measured using an electron probe micro-analyzer (EPMA). The results are shown in Fig. 2.The 3.25-μm fiber core was composed of 20 wt% GeO2, 1.5 wt% ZrO2 and 0.4 wt% Yb2O3. Figure 3 illustrates the measured refractive index profile of the fiber using a He-Ne laser based fiber analyzer.The core and cladding refractive indices are around 1.476 and 1.4545, respectively, that results in a core numerical aperture (NA) of 0.25 at 632 nm. The corresponding core V number and mode field diameter (MFD) are 2.4 and 3.57 μm at 1064 nm, respectively. In general, the proper choice of dopants and their spatial profiles in optical fibers enable significant increase the fiber nonlinearities, which would be beneficial for superior SCG. In order to enhance both Kerr and Raman nonlinearities, the fiber core is doped with high levels of GeO2 [20]. ZrO2 doping also increases the non-linear optical property of silica glass based optical fibers, as observed earlier [19, 21–24]. Accordingly, the single-mode GZY fiber is superior to general Yb3+-doped silica fibers regarding the application in NIR SCG.
Similar to the YDFL system, a 974-nm laser diode was used as the core-pumping source for YDFA. The core absorption coefficient of the GZY fiber is 4 dB/m at 976 nm; therefore, the fiber length of 20 m was used to provide enough 974-nm pump absorption. The fluorescence lifetime of Yb3+ ions in the GZY fiber, measured under pumping at 976 nm, is reported in Fig. 4.The measured fluorescence relaxation curve shows a single exponential with lifetime of 1.05 ms, thus demonstrating negligible concentration quenching. Because of the difference of MFDs in GZY fibers and HI-1060 passive fibers, we optimized the fusion current and fusion time to minimize the splice loss to < 0.5dB.
3. Results and discussion
The CW-ML seed fiber laser with two types of output couplers was firstly characterized. To enable the high energy operation in the CW-ML seed laser, low pulse repetition is preferred. Therefore, a 520-m single-mode passive fiber (HI1060) was used to extend the cavity length and produced a 365-kHz pulse repetition rate. With proper adjustments of two PCs inside the cavity, CW-ML pulses were achieved under the launched pump power of 173 mW. There was no observation of any pulse breaking, even with the relatively large nonlinearity accumulated in the 520-m fiber cavity. The characteristics of the seed laser are similar to the one reported in [17]. The temporal pulses and output spectra of the CW-ML laser with two output couplers are shown in Fig. 5.The output pulses under two couplers are similar while the output spectra are different. As shown in Fig. 5(b), with the 90/10 coupler, the laser output has the center wavelength at 1032 nm and a 13.56-nm 3dB bandwidth. On the other hand, with the 80/20 coupler, two peaks located at 1028 nm and 1075 nm are exhibited. The 1075-nm peak is the SRS emission pumped by the 1028-nm signal with a 12.5-Thz frequency shift. It could be due to the higher inner-cavity intensity with the 80/20 output coupler.
Fig. 5 (a) time sequence and (b) optical spectrum of the ML pulses (Blue curve: 80/20 coupler and Red curve: 90/10 coupler).
Both the power scalability and spectral characteristics of the YDFA were also experimentally investigated. Figure 6 shows the measured output power and calculated pulse energy as a function of launched pump power. The linearly-fitted slope efficiency is 31.6%. At 342 mW of the pump power, the signal output power reached its maximum at 116 mW with the corresponding pulse energy of 324 nJ. The maximum output power was limited by the available 976-nm pump power without sign of rollover. As the pulse width was measured as 0.7 ns, the maximum output peak power was 463 W.
Fig. 6 Measured average power and calculated pulse energy as functions of the pump power (dots) and linear fit to the data (solid curve).
Most of previous fiber SCG studies have been focused on SPM, soliton effects, and pulse walk off, because of the utilization of femtosecond-level pump pulses. However, the dominated nonlinear process in the nanosecond-level fiber SCG is expected to be the phase-matched FWM, cascaded SRS and SPM processes. These nonlinear processes could be explored through the monitoring of spectrum evolution with the increase of the pump power. Fig. 7(a)-7(d) show the spectrum evolution of the YDFA seeded by the YDFL with the 90/10 directional coupler. As shown in Fig. 7(a), it is worth noticing that the 1288 nm started to show up even without any 974-nm pump. To the best of our knowledge, this special emission peak has not been reported in any silica fiber based SCG system. It is speculated to be the dispersive wave [25] located nearby the zero dispersion wavelength, ~1250 nm, of the GZY fiber. The zero dispersion wavelength of the GZY fiber was determined based on the measurement described in [19]. As the pump power increases, two addition peaks at 1190 nm (ωs) and 1423 nm (ωi) were generated through the degenerate four-wave-mixing (DFWM), pumped by 1288 nm (ωp). The energy conservation between these three waves, 2ωp = ωs + ωi, was successfully confirmed. Further increase of the pump powers resulted in spectrum peaks at 1086 nm and 1136 nm through the first order and second order SRS processes with a corresponding frequency shift of ~12.5 THz (Fig. 7(d)). In parallel, the generated higher-order SRS waves produced more peaks through the abundant DFWM processes.
Fig. 7 Spectrum evolution of YDFA with the 90/10 coupler under the pump powers at (a) 0 mW, (b) 38 mW, (c) 114 mW, and (d) 190 mW.
In general, the fiber dispersion plays an important role in the short pulse propagation and phase matching conditions for nonlinear processes in fibers. Therefore, the phase matching condition of FWM the GZY fiber needs to be carefully investigated. As described in [26], the match condition of DFWM processes only depends on even order dispersion coefficients in fibers. Thus, the phase matching condition for the aforementioned DFWM should be fulfilled with the following relation:
where and ωs, ωi, and ωp are signal, idler, and pump frequencies of the DFWM process, respectively. β2n is the 2n-th derivative of the propagation constant β, L is the fiber length and Pp is the peak pump power. The GZY silica fiber is expected to have a larger nonlinear coefficient γ mainly because of its small effective area Aeff. However, it is worth noting that Zr-EDF has garnered increasing interest due to its significant non-linear characteristics that arise from the addition of zirconium ions into the glass matrix of the EDF [26]. As it is not easy to get the non-linear refractive index of GZY fiber, we determine the nonlinear coefficient of the GZY fiber based on the nonlinear index n2 of standard silica fibers and the effective core area of the GZY fiber. The nonlinear coefficient γ of the GZY fiber is estimated to be ~19 W−1Km−1. We further calculate the second and fourth order derivatives of β in the GZY fiber and derive the nonlinear phase matching diagram of the DFWM process involved in our experiments. The solid curve in Fig. 8 shows the phase matching curves under the 400-W pump peak power, and the dots identify the measured DFWM wavelengths. The theoretical diagram successfully confirms the phase matching condition of interacting DFWM stoke waves. In general, if the phase matching is satisfied, the DFWM onset threshold is expected to be lower than that of SRS [26]. At high pump powers, the discrete spectral peaks above 1050 nm in Fig. 7 were generated by the interplay between the DFWM and SRS processes. The SRS waves were originated from the three-step cascaded SRS processes, and these waves selectively enhanced the growth of DFWM waves. Moreover, because of nonlinear phase shift caused by high peak intensity, 3-dB linewidth of these sub-nanosecond pulses were broadened through the SPM nonlinear processes. The SPM effect is believed to increase the possibility of reaching the DFWM phase matching condition.Fig. 8 DFWM phase-matching diagram, calculated from the dispersion curve of the GZY fiber under the 400-W pump power. The dots are experimental data points.
We further investigated the output characteristics of the YDFA, seeded by the YDFL with the 80/20 directional coupler. Spectra evolution of the amplified ML pulses with the increase of the pump power was recorded. As shown in Fig. 9(a), the 1075-nm SRS wave in the YDFL acts as an additional seed, leading to another set of nonlinear processes in the YDFA. With the increase of the 974-nm pump, the cascaded SRS and DFWM peaks were abundantly generated. It is worth noting that the spectra peaks around 592 nm and 727 nm in visible wavelength range were exhibited. These waves were originated the frequency doubling of 1183 nm and 1454 nm. In the addition, the exhibited 384 nm in UV range is resulted from as the cascaded DFWM. The first-order DFWM process involved the fundamental pump wave at 1324 nm, signal wave at 1228 nm, and idler wave at 1454 nm, while the second-order DFWM process involved the second pump wave at 592 nm, signal wave at 1324 nm, and idler wave at 384 nm. Further increasing the pump power flattened the output spectrum through SPM and resulted in SC from 1010 nm to about 1600 nm in NIR range. The available wavelength and flatness of the SC output could be improved by launched higher pump power [14]. It is worth noting that some short-wavelength emissions in the YDFA, seeded by the YDFL with the 90/10 coupler were also observed with the OSA. However, the output power of these short-wavelength emissions were significantly lower than that in the YDFA, seeded by the YDFL with the 80/20 coupler.
Fig. 9 The evolution of the spectrum under the 974-nm pump power of (a) 76 mW, (b) 152 mW, (c) 228 mW, and (d) 342 mW.
4. Conclusions
We report the near IR supercontinnum (SC) generation in an all-fiber core-pumped Yb3+-doped fiber MOPA. The seed laser is a 365-kHz high-energy all-normal-dispersion YDFL based on the NPE technique. Through the amplification in the nonlinear core-pumped Yb3+-doped germano-zirconia-silica fiber amplifier, SC generation from 1010 nm to 1600 nm in near IR and some emission bands in the visible and UV were achieved. With 974-nm pump power of 342 mW, SC light source with highest pulse energy of 324.6 nJ was produced. To the best of our knowledge, it is the first demonstration of a broad NIR SC generation accompanied with visible/UV emission bands in a single-mode step-index fiber amplifier under several hundred milliwatts of pump power. Besides, it is the first report of NIR SCG in a single-mode GZY fiber. The origin of NIR SCG is addressed based on systematically investigation of abundant nonlinear processes, such as DFWM, cascaded SRS, and SPM. The whole system is compact and cost-effective, which satisfies most demands of practical applications.
Acknowledgments
This work is financial sponsored by the national science council in Taiwan, R.O.C, with grant no NSC 102-2112-M-027-001 -MY3 and NSC 101-2218-E-027-003-MY3. The financial support for development of fiber is done by department of science and technology (DST), New Delhi, India.
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