1. Introduction

Combination of nonlinear optical fibers with a high power laser source is able to produce large supercontinuum generation in the visible and in the near infrared spectral domain [1–2]. Homogeneous spectral broadening can be achieved in large normal or anomalous dispersion regime. According to these dispersion conditions, the broadening is due to the combination of some nonlinear effects taken among four-wave mixing (FWM), modulation instabilities (MI), soliton propagation, stimulated Raman scattering (SRS), cross and self-phase modulation (XPM, SPM) [3–6]. Femtosecond [7], picosecond [1][8], nanosecond [2–3][9] or continuous-wave pumping [10–11] can be used to produce a signal with a spectral width larger than one octave. This continuum generation allows to extend the laser emission toward new wavelengths, especially in the near ultraviolet and visible regions, but requires high peak power or very nonlinear guides [12]. The main nonlinear substrate currently used to obtain spectral broadening in fibers is silica because of its drawing facilities. This material is not a highly nonlinear medium but can be properly drawn to obtain long fiber with core diameter <2 µm [13]. The low attenuation of silica in the near infrared and the reduced core diameter thus allow the propagation of a high spatial power density along a nonlinear effective length of several km. Moreover, the introduction of silica microstructured fibers opened new opportunities to induce broadband continua [14–15]. Basically, the modification and shaping of the chromatic dispersion curve in these holey fibers allows the nonlinear propagation to enhance parametric processes and then to obtain short wavelength generation [16–17]. Nevertheless, the main drawback for supercontinuum generation in such guides is the relatively poor coupling efficiency of the high pump power into the small fiber core, because of spatial mode mismatching. In addition, the optical damage threshold of the input facet of the nonlinear fiber limits the spectral power density available at the fiber output when using conventional free space coupling technique. Finally, blue/ultraviolet frequencies are usually created at the end of the propagation, consequently when the input peak power and input pulse energy are drastically depleted, which reduces the impact of some nonlinear effects as XPM [8–9] [12] [17].

In this paper, we present the possibility to significantly increase the supercontinuum output power by using a nonlinear fiber amplifier. A small input signal is highly amplified to reach the nonlinear effects threshold leading to supercontinuum generation within the amplifying fiber. In a first step, by increasing the pump power of the amplifier, it is possible to convert the sharp input pump into a broadband pump covering the spectral gain window of the ytterbium ion. Then the broadened and amplified pump acts as a new pump for continuum generation and modifies the induced nonlinear interactions [18]. In a second step, increasing more the pump power of the amplifier enhances the continuum bandwidth and spectral power density. At maximum pumping level, the resulting 2.5-W power is spread over more than 750 nm by means of cascaded nonlinear phenomena. The combination of nonlinear effects with laser gain was already used by Chernikov et al. to design a self-Q-switched ytterbium fiber laser [19]. Later on, the concept was also exploited by Price et al. so as to realize a tunable femtosecond laser source [20]. More recently, our group designed an air-clad ytterbium-doped fiber amplifier to increase the spectral power density of visible continuum [21]. Here the proposed configuration is simply based on the use of a conventional (non-microstructured) ytterbium-doped fiber, whose low nonlinear coefficient is compensated by a large laser gain gradually seeded along the propagation. This amplification permits to modify the nonlinear behavior of the considered fiber and to obtain multi-watt supercontinuum generation in the infrared range with a good flatness.

2. Experimental set-up

The set-up is shown in Fig. 1 and consists in a pulsed pump laser source (in blue) coupled to a nonlinear ytterbium-doped fiber amplifier (in red). The pump source is an actively Q-switched ytterbium fiber laser delivering nanosecond pulses at λ=1064 nm. An ytterbium amplifier with 0.1 W saturated output power is employed as gain medium and is side-pumped by a single 1-W diode. A grating placed in the laser cavity permits to select the central wavelength of the output signal. The diffraction efficiency of the grating is close to 90 % for P polarization and only 35 % for S polarization. Therefore, the grating acts as a polarizer and the radiation emitted by the laser is strongly polarized with an extinction ratio of 50:1. The fiber laser is then tunable from 1040 nm to 1080 nm, corresponding to the gain bandwidth of the ytterbium-doped fiber. A fiber-pigtailed 110-MHz acousto-optic modulator is used both as an active switch for optical pulse generation and as a frequency shifter to enlarge the output laser line. The right extremity of the amplifier is spliced to a 50/50 coupler to control the intra-cavity power by means of a convergent lens and a high reflection coated mirror placed in one arm of the coupler. The two unused ports are then angle-cleaved to avoid any parasitic reflection and one of these ports is used as output.

The laser line is tuned close to 1064 nm and is broadened up to 0.6 nm at - 3 dB (1 nm at -10 dB). According to the electronic command of the modulator, the Q-switched oscillator produces output pulses between 45 ns and 20 ns with a repetition rate between 10 kHz and 1 MHz respectively (see Fig. 2). The resulting 158 GHz linewidth is sufficiently large to overcome Brillouin back reflections in the nonlinear fiber amplifier.

 

Fig. 1. Experimental set-up

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The free output laser port is then spliced to a commercially available ytterbium amplifier (model # KPS-BT2-YFL-1064-PM-FA-COL - see http://www.keopsys.com - corresponding to orange box of Fig. 1) having an optically isolated input and used as a nonlinear medium. This nonlinear amplifier is composed of a double-clad polarization-maintaining ytterbium-doped fiber with a core diameter close to 7 µm, which induces a single transverse mode at 1064 nm, with its zero-dispersion wavelength (ZDW) located close to λ=1.3 µm. This 5-mlong active fiber is side-pumped in simultaneously forward and backward configuration with a total power of 10 W, provided by two multimode single-emitter laser diodes, and can produce output power in excess of 5 W. It is seeded in normal dispersion regime and has a nonlinear coefficient γ=4.4 W^-1.km^-1, estimated from its geometrical properties.

At last, the output spectrum is analyzed by means of a multimode large-core fiber (50/125) linked to an optical spectrum analyzer. This operation allows to control the spectral power density launched into the analyzer without any spectrum distortion.

 

Fig. 2. Example of output pulses provided by the nanosecond Q-switched laser source; a) pulse temporal profile (45-ns duration); b) repetition rate (~10 kHz); c) spectral profile (0.6-nm linewidth @ - 3 dB)

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3. Results and discussion

The nonlinear ytterbium amplifier is saturated with 13 dBm of input power provided by the tunable fiber laser source. This input power is amplified to reach the nonlinear threshold leading to spectral enlargement. For an output power lower than 500 mW, corresponding to a peak power lower than 1.1 kW, no significant spectral broadening is observed. Increasing the pump power in the nonlinear amplifier, the output spectrum broadens by means of SPM first (see light-blue curve of Fig. 3, Pout=0.5 W). This quasi symmetric spectral enlargement is simultaneously amplified over the ytterbium spectral gain window, extended from 1030 to 1090 nm. As a result, in a second step (dark-blue curve, P_out=1 W), the narrow input pump line is converted into a coherent broadband pump radiation. This phenomenon is clearly visible on the zoom plotted in inset of Fig. 3. At this pump power level and because of the normal dispersion regime, three SRS (Stokes) peaks appear. Let us note that the Raman gain coefficient is enhanced in the present case of a broadband pump and that the spectral shape of SRS peaks depends on the pump spectrum [18]. Beyond this point and increasing more the pump level (green curve, P_out=1.5 W), the spectral broadening goes on, by reaching and rising above the ZDW of the double-clad fiber. MI give birth to solitons, which undergo soliton self-frequency shift (SSFS) mechanism and thus induce a large continuum [22], up to 1.75 µm, what is the limit of the analyzer. Moreover, the energy localized close to the ZDW contributes by means of FWM to the spectral broadening. This effect improves the spectral homogeneity of the supercontinuum and covers the high-order SRS rays, and especially the ray of order 3, which was previously visible for lower input power. In a fourth time (pink curve, P_out=2 W), the further increase of the pump power does not drastically modify the output spectrum shape but principally enhances the spectral power density. Finally, at maximum pump power level (red curve, P_out=2.5 W), the spectral power density of the continuum is even higher. “Amplified SPM” is distinctly noticeable around the pump, which becomes double-peaked (see zoom of Fig. 3) and, as a consequence, induces a distortion and a broadening of SRS peaks. This effect contributes to improve the spectrum flatness. At last, the propagation of the supercontinuum signal superimposed with the broad and energetic pump can stimulate XPM interactions. Therefore XPM can induce blue or red shift versus the relative group velocities between wavelengths [23–24] and improve the spectrum homogeneity. 2.5-W output average power is obtained over the whole spectral broadening shown in Fig. 3 (1 µm to 1.75 µm). In these conditions, the spectral power density is higher than 3 mW/nm.

 

Fig. 3. Spectra measured at the output of the nonlinear amplifier versus pump power. The repetition rate of the Q-switched laser source is set at 10 kHz. P_out refers to the output average power.

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This experiment clearly shows that a fiber amplifier can be used to generate broad supercontinuum with spectral power density in the mW/nm range. Given the low nonlinear coefficient of the standard double-clad fiber used, the most important parameter remains the available peak power propagating in the guide with respect to its core diameter. Indeed, the nonlinear medium used here is the same as the one usually handled for supercontinuum generation in holey fibers, namely silica material, which has a low nonlinear refractive index (n₂<3×10^-20m2/W). Thus the negative impact of the large fiber core and of the small value of n₂ on nonlinear phenomena power threshold can be easily compensated by the ytterbium gain to further increase the optical power density within the amplifying and nonlinear fiber. Moreover, due to the strong population inversion achieved all along the amplifier thanks to the simultaneous forward and backward side-pumping, the nonlinear energy transfer from the signal generated inside the gain window to longer wavelengths remains continuously available during the propagation. In these conditions, the infrared spectral broadening is improved at each propagation step, thanks to the presence of high peak power at 1064 nm which can drastically modulate the spectrum by means of XPM. This effect is noticeably limited in passive nonlinear fibers because of the input pulse depletion.

It is obvious that the continuum build-up is governed by the dispersion regime in which the pump pulses propagate. The use of non-microstructured fibers reduces the possibility to significantly shape the chromatic dispersion curve and especially to shift down the ZDW. In particular, this makes conventional fibers inappropriate for homogeneous continuum generation in the visible range. In contrast, microstructured fibers are good candidates for this purpose, and it has been recently demonstrated that a nonlinear and active holey fiber could be efficient for visible supercontinuum enhancement [21].

In other respects, the introduction of rare-earth ions such as ytterbium into the nonlinear fiber core induces strong absorption in the 980-nm region and can therefore limit the spectral broadening in this domain. It clearly appears that the absorption spectrum of the used rare-earth ions has to be properly taken into account with respect to the targeted continuum generation.

Finally, the limit of the spectral power density available by using a nonlinear amplifier is fixed by the damage threshold of the fiber output end. The splicing of a tapered element [25] with ultra large emissive surface can be a solution for very high output power generation. However, this drawback can be slightly limited by increasing the repetition rate of the laser source, decreasing in the same time the peak power of the output signal.

4. Conclusion

We have demonstrated wideband infrared supercontinuum generation in a standard nonlinear ytterbium-doped fiber amplifier. The large core area of the doped fiber is compensated by a strong amplification to reach the nonlinear phenomena threshold. Continuum generation between 1 µm and 1.75 µm was obtained with a spectral power density of more than 3 mW/nm. The combination of laser gain with nonlinear propagation represents a new approach to induce large spectral broadening and improve the nonlinear interaction, by maintaining a high peak power of the initial pulse all along the propagation. In the same way, the introduction of co-doped erbium-ytterbium fiber is also an attractive solution to modulate the shape of supercontinua and, in particular, increase the energy beyond 1550 nm. The association of doped fibers with cascaded holey fibers [26] can offer an additional degree of freedom to profile the spectral broadening as well. At last, the generation of wide infrared spectrum with high spectral power density has the advantage of allowing efficient harmonic conversion in nonlinear crystals to reach the visible region with large tunability.