1. Introduction

One of the attractive possibilities of nonlinear frequency conversion in optical fibers is a parametric four wave mixing (FWM) process that is actively explored recently. Fiber optical parametric oscillators (FOPOs) offer an opportunity to generate tunable laser radiation in new spectral ranges, where conventional fiber lasers are not effective or absent, in particular, at short wavelength region, <1 μm. For efficient parametric conversion it is important to have a high-power pump laser that also makes possible a use of shorter FOPOs. This is easily achieved with pulsed pump sources but is hardly possible with continuous wave (CW) pumps. At the CW pumping rather long fibers are usually required, but their lengthening is limited by an increasing influence of dispersion fluctuations resulting in efficiency reduction at large parametric frequency shifts [1]. This principal constraint limits number of investigated to date CW FOPOs [2–10], in spite of a great demand for CW tunable fiber sources in various applications. The developed FOPOs in these papers are pumped near fiber transmission window at ~1.55 μm and usually use highly nonlinear dispersion shifted fibers (HNLFs) of >100 meters length. The widest tuning range of ~250 nm (1480–1730 nm) has been obtained in [7] by means of tunable narrow-band filter in a ring cavity. In the same configuration the Stokes wave power as high as 1W has been reached, whereas the anti-Stokes wave power did not exceed 100 mW. At that a slope efficiency of conversion to shorter wavelength is only 5% [8]. For 1 μm spectral range and below the parametric process has been studied in single-pass scheme, resulting in the parametric conversion efficiency of 0,3% only [11, 12]. In our previous studies the conversion efficiency of 3% at 1017 nm has been demonstrated by means of polarization-dependent phase matching in a birefringent fiber pumped by narrow-band ytterbium doped fiber laser (YDFL) and signal wave at 1080 nm [13]. However, a continuous wave FOPO operating below ≤1 μm has not been demonstrated yet, to our knowledge.

In this paper we report on the singly resonant all-fiber OPO based on the birefringent LMA5-PM photonic crystal fiber. It generates laser radiation tunable in the wavelength range of 950 – 1010 nm at pumping by a conventional CW randomly polarized tunable YDFL.

2. Experimental setup

The experimental setup is shown in Fig. 1 . As a pump source we use CW YDFL that is tunable in the range from 1.04 to 1.07 μm and is generating randomly polarized radiation with output power of up to 11 W. The YDFL is pumped by multimode laser diodes and has a ring cavity which is closed by a fused fiber coupler similar to that one demonstrated in paper [14]. Tuning of operating wavelength is realized by an axial compression of a fiber Bragg grating (FBG) described in paper [15]. It was found that spectral linewidth of the YDFL Δλ_p is randomly changed from 30 to 200 pm at the tuning of the FBG back and forth in such mechanical configuration. We were able to obtain fixed value of Δλ_p after a proper re-adjustment of the FBG. Polarization controller PC1 is used to adjust a polarization state of the pump light, since the parametric process is sensitive to the polarization. The pump is coupled into the FOPO’s ring cavity via a wavelength-division-multiplexing coupler WDM1 and polarization controller PC2 and then propagates in a nonlinear fiber.

 

Fig. 1 Schematic experimental setup.

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Parametric conversion occurs in the 18-m long polarization maintaining photonic crystal fiber LMA5-PM (produced by NKT Photonics). Properties of the optical fiber and potential parametric sidebands for vector and scalar phase matching have been studied in detail in our previous work [16]. Zero dispersion wavelengths (ZDWs) of the waveguide are measured to be at ~1053 and ~1052 nm for the fast and the slow axes, respectively. The pump wave passes through the fiber and generates Stokes and anti-Stokes waves due to the third order nonlinearity χ⁽³⁾. Then the anti-Stokes wave is removed from the cavity by WDM2 whereas the Stokes wave is launched back into the LMA fiber through WDM1. This scheme ensures that the oscillator is singly resonant. Pump power is removed from the cavity through WDM1 and WDM2. Couplers WDM3 and WDM4 are utilized to filter the residual pump power at the FOPO output. Polarization controller PC2 is used to adjust the polarization state inside the cavity. Thus the all-fiber OPO with the ring resonator for the Stokes wave is realized. The splice loss between the LMA fiber and pigtails of 1060 XP fiber is measured to be 1 dB at 975 nm as a sum for two splices. Coupling coefficient for the resonator at long wavelengths determined by spectral characteristics of WDM1 amounts to 92% at 1140 nm. The total cavity loss for the Stokes wave is estimated to be about 1.6 dB. The loss for the anti-Stokes wave after three WDM couplers is 2.5 dB in ~970 nm range.

3. FOPO spectrum and tunability

Tunability of the FOPO is realized by detuning the pump wavelength in the normal dispersion region. LMA5-PM fiber supports two polarization modes therefore pump radiation is divided into two orthogonally polarized waves at the fiber input and then the waves are separately involved in the FWM. Parametric shifts are determined by the phase matching condition [17]:

Figure 2(a) shows experimental (points) and theoretical (lines) sideband wavelengths versus pump wavelength for two polarization modes of the pump radiation. Phase matching curves are plotted according to Eq. (2) for pump power P_p = 9 W. Pump wavelength λ_p ~1051 nm is kept near the ZDW of the fiber to provide rather small parametric shifts (up to 25 THz) to have weak influence of the fluctuations of fiber diameter on the efficiency of the parametric process. One can see that the theoretical calculations are in a good agreement with the experimental data for all wavelengths, although the parametric generation above 1010 nm is perturbed by the stimulated Raman scattering (SRS). Variations of FOPO spectra at tuning of the pump are shown in Fig. 2(b) for the slow branch only, since for parametric generation of the fast index mode the slow mode cannot be completely suppressed due to higher parametric and Raman gain. The spectra are measured at the port B by an optical spectrum analyzer (Yokogawa AQ6370). We adjusted the state of polarization for each generated wavelengths to achieve maximum output power. Spectral bandwidths of the WDM couplers are quite broad and, consequently, the generation linewidth is determined by the parametric gain bandwidth. As seen in Fig. 2(b) the gain bandwidth is reduced when the pump wavelength moves into the normal dispersion region and the sidebands move away from the pump.

 

Fig. 2 FOPO tuning range with the pump wavelength near the ZDW of the fiber a) Experimental (points) and theoretical (lines) phase matching curves for two polarization modes of the pump; b) The FOPO spectra at tuning of the pump polarized along the slow axis.

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The anti-Stokes wavelength goes down to 965 nm and then the generated wave vanishes. The FOPO tuning range is limited by growing attenuation at the Stokes wavelength owing to spectral properties of WDM couplers. As a result, the FOPO threshold grows with increasing frequency shift. On the other hand, the parametric generation is not limited at the small frequency shifts. However, we don’t consider in this paper the generation spectra above 1010 nm as the parametric gain there competes with the Raman gain. Moreover, the full width at half maximum exceeds 7 nm due to increase of FWM gain bandwidth thus making spectrum of parametric generation rather broad. The linewidth of the anti-Stokes sideband at the port A is measured to be 3.7 nm at 972 nm at the pump wavelength of 1050.3 nm and the input pump power of 9 W. Note that there are no signs of SRS process in Fig. 2(b) but it can be observed at some specific adjustment of the polarization controllers.

4. Output power and temporal dynamics

4.1 Pulsed operation at pump linewidth Δλ_p ~40 pm

The threshold of the FOPO is achieved when the parametric gain is equal to the Stokes wave losses inside the cavity. The parametric gain under exact phase matching conditions for the fiber with length L is written as $G={\sinh }^2(\gamma P_{p}L)$ [17]. Total cavity losses include point losses α* at splices and fiber couplers as well as distributed losses inside the fiber α_L. Thereby, the FOPO threshold pump power P_th can be expressed as follows:

The FOPO threshold value is estimated to be P_th = 4.2 W for the following fiber parameters: γ = 10 W⁻¹km⁻¹, L = 0.018 km, α_L = 5 dB × km⁻¹ and the total losses α* = 1.6 dB at the Stokes wavelength.

It is known that the parametric process is more efficient when the narrowband pump is used. So we reduce the pump linewidth down to ~40 pm in order to obtain high spectral power density inside the photonic crystal fiber. The pump linewidth is changed by adjusting the tunable FBG. Figure 3(a) presents the generated anti-Stokes power at the port A versus the pump power at the input of LMA fiber taking into account 10% splicing losses of the photonic crystal fiber with the input pigtail. In this configuration the experimentally measured threshold pump power is 5.3 W. External slope efficiency (defined as the slope of the curve in Fig. 3(a)) is equal to 25%. Output power for generation wavelength of 974 nm reaches 1.1 W at P_p ≈9.8 W. Internal slope efficiency (for intra-cavity generated power) amounts to 45%. The last value is calculated inside the cavity taking that the total anti-Stokes losses being of 2.5 dB at 970 nm.

 

Fig. 3 Power (a) and temporal (b) FOPO properties at pump linewidth Δλp = 40 pm.

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Temporal dynamics of FOPO output radiation is detected by a fast photodetector and a digital oscilloscope. Figure 3(b) demonstrates oscillograms of pump wave from the port B and anti-Stokes wave from the port A measured simultaneously at pump parameters P_p = 9 W and Δλ_p = 40 pm. One can see that the FOPO operates in the pulsed regime with duty factor ~50% and modulation period of ≈48 ns corresponding to cavity round-trip time in YDFL. Modulation of the YDFL radiation is observed even below the FOPO threshold, while adjusting the polarization controller PC1. Therefore, the pulsed regime can be associated with a feedback to the YDFL, when back scattered radiation influences on the pump laser. Moreover, quite narrow pump linewidth results in stimulated Brillouin scattering (SBS) sidebands generated inside the pump laser cavity that have been observed in the output spectrum of the YDFL.

Despite the fact that the experimental data demonstrate high efficiency of the parametric conversion, the oscillator generates rather irregular pulses in this scheme. For CW operation it is necessary to raise the threshold of the SBS process. That was done by increasing the spectral linewidth of the pump.

4.2 CW operation at pump linewidth Δλ_p > 100 pm

Linewidth of the pump wave was enlarged by a re-adjustment of the FBG, at that the generated wavelength was also slightly shifted. The anti-Stokes power at the FOPO output at 972 nm versus pump power at the LMA fiber input is plotted in Fig. 4(a) . The measurements were performed for series of Δλ_p values. The pump power threshold was 5.1 W which is comparable with the threshold of the pulsed FOPO described in the previous section. External slope efficiency decreases to 9.4%, i. e. it is 3 times lower than that for pulsed operation.

 

Fig. 4 Power (a) and temporal (b) FOPO properties at the pump linewidth Δλp > 100 pm.

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One can see in Fig. 4(b) that fluctuations of the pump and parametric waves become much smaller than those in Fig. 3(b). The stochastic noise in the oscillogram is related to the interaction of large number of longitudinal modes of the YDFL, as the pump spectrum has some finite linewidth. The amplitude of the stochastic fluctuations is amplified by the parametric process; nevertheless, the FOPO operates in the CW mode. The generated power at the wavelength of 972 nm reaches 460 mW and that is the highest value obtained in CW FOPOs to date in up-conversion schemes.

The FOPO pump power threshold obtained in the experiment is 5.1 – 5.3 W that is higher than calculated value of 4.2 W. Such difference can be attributed to the partial polarization of YDFL radiation, because the parametric process is sensitive to polarization and only a part of the pump power with the appropriate polarization mode amounting to ≥ 0.5P_p is involved in the parametric generation. Additionally we should note that the horizontal axes of the Figs. 3(a) and 4(a) correspond to full pump power and thus the value of the slope efficiency is even larger than presented above.

5. Conclusion

To our knowledge, we have realized for the first time the singly resonant CW FOPO operating below 1 μm. In the studied scheme the photonic crystal birefringent fiber is pumped by a narrowband continuous Yb-doped fiber laser near zero dispersion wavelength (~1.05 μm). By means of YDFL wavelength tuning in the normal dispersion spectral region the FOPO generates radiation with wavelength tunable in 950-1010 nm range. The tuning range is limited at the short wavelength edge by a spectral function of the WDMs used in the cavity. The output power of the FOPO operating at 972 nm in CW regime reaches 460 mW with external slope efficiency of 9,4% related to the pump power with >100 pm linewidth. This is the highest power level reported for the CW FOPOs with frequency up-conversion. At this wavelength the generated linewidth is defined by a phase matching bandwidth and amounts to 3.7 nm. To obtain narrower line that is required for many applications, it is necessary to insert inside the Stokes wave cavity a wavelength selector, e.g. Mach-Zehnder interferometer or tunable narrow-band filter. However, an excessive narrowing of the FOPO spectrum can result in a setup inside the cavity of the stimulated Brillouin scattering leading to unstable parametric generation.

For the narrow-band pumping of ~40 pm the pump laser starts to operate in pulsed mode due to the feedback induced by a SBS process with modulation period of 48 ns and duty factor of ~50%. As a result, the FOPO slope efficiency is increased to 25% (by 3 times compared to CW mode) and the output power becomes >1 W at 974 nm.

Taking that WDMs with different spectral functions can be used, one can extend the tuning range below the obtained limit of ~960 nm. In the case of short-wavelength operation the FOPO efficiency is defined by a level of dispersion fluctuations, as the spectral width of the phase matching becomes much narrower with increasing parametric frequency shift and thus the fluctuations influence the parametric gain.