We report on a suspended core tellurite microstructured optical fiber (TMOF) based optical parametric oscillator (OPO). The intracavity gain is provided by the degenerate four-wave mixing (DFWM) occurred in a 1.5-m-long TMOF synchronously pumped by a mode-locked picosecond erbium-doped fiber laser. The oscillated signal can be generated from 1606 nm to 1743.5 nm, and the idler can be emited from 1526.8 nm to 1395 nm by adjusting the pump wavelength from 1565.4 nm to 1551 nm. A total intenal conversion efficiency of −17.2 dB has been achieved.
© 2015 Optical Society of America
Fiber-based degenerate four-wave mixing (DFWM) refers to the phenomenon that two pump photons at the same wavelength are annihilated and a signal/idler photon pair is generated simultaneously at novel wavelengths to satisfy the energy conservation standards . This process is governed by the phase-matching condition with respect to the pump wave . The signal and idler can be generated in a wide wavelength range by adjusting the pump near the zero-dispersion wavelength (ZDW) of the nonlinear fiber . By customizing the fiber’s ZDW, the DFWM can be operated from the near-infrared to the visible wavelength band [4–6]. Fiber optical parametric oscillator (FOPO) is one of the most important applications of the DFWM [7–10]. The FOPO has been demonstrated in dispersion-shifted fiber (DSF)  and highly-nonlinear fiber (HNLF)  by pumping in the telecommunication wavelength band. To optimize the output power of the FOPO, the effects of the output coupling ratio and the cavity loss have been investigated through numerical simulations and experiments . The output of the FOPO can be moved to the shorter wavelength region by using photonic crystal fibers as the nonlinear medium [14–16]. Several PCF-based OPOs operated over a broad wavelength range near 1 μm have been demonstrated with the pulse duration in the ranges of continuous-wave , picosecond [18,19], and femtosecond . The picosecond FOPOs have been exploited as the light sources in the field of biophotonics . Recently, the OPO is further studied by exploiting the DFWM generated in new kinds of nonlinear medium. In a chalcogenide microwire, a DFWM-based OPO with low threshold pump power has been reported . In a hydrogenated amorphous silicon waveguide, an OPO has been achieved in a wide wavelength range from the extended band to the ultra-long telecommunication band .
In this paper, a suspended core tellurite microstructured optical fiber (TMOF) based OPO is demonstrated. The intracavity gain is provided by the DFWM occurred in a 1.5-m-long TMOF synchronously pumped by a mode-locked picosecond erbium-doped fiber laser. The oscillated signal can be generated from 1606 nm to 1743.5 nm, and the idler can be emited from 1526.8 nm to 1395 nm by adjusting the pump wavelength from 1565.4 nm to 1551 nm. A total intenal conversion efficiency of −17.2 dB is achieved for the pump at 1551.5 nm with an average pump power of 12.9 dBm.
2. Experimental setup and fiber properties
The experimental schematic is shown in Fig. 1(a). The pump source is a home-made mode-locked erbium-doped fiber laser (MLFL), which can generate a short pulse train with a full width at half maximum (FWHM) of 38 ps at the repetition rate of 17.08 MHz. The polarization controller PC1 is used to align the polarization state of the pump wave with the principle axis of the suspended core TMOF. An erbium-doped fiber amplifier (EDFA) is used to boost the pump power. The amplified spontaneous emission (ASE) from the EDFA can be suppressed by a tunable band-pass filter (TBPF) with 0.8 nm 3-dB bandwidth. The pump pulse train is coupled into the cavity through the 10% port of a fiber optical coupler. Subsequently, a free-space collimated beam is formed by a fiber collimator (FC). An aspheric lens (AL) is used to couple the light wave into the suspended core of the TMOF. The light emitting from the TMOF is collimated by another AL. The free-space collimated light is coupled into the fiber by another FC. Then, an optical coupler provides 90% feedback and 10% output. At the output port, the spectrum can be recorded by an optical spectrum analyzer (OSA, Yokogawa AQ 6375). In the feedback branch, an optical delay line (ODL) is used to adjust the cavity length. The polarization state of the light wave in the cavity can be tuned by the PC2. The cavity length of the FOPO is measured to be about 9.8 meters. A TMOF designed and fabricated in our laboratory with a length of 1.5 meters is included in the cavity to provide the parametric gain. The scanning electron microscope (SEM) image of the cross section of the fiber is shown in the inset of Fig. 1(b). A suspended core with a diameter of about 3.3 μm is formed between six air holes. The group velocity dispersion for the fundamental mode of the TMOF is calculated by using the commercial software (Lumerical MODE Solution) with the full-vectorial finite-difference method (FV-FDM). The ZDW is calculated to be 1560 nm, as illustrated in Fig. 1(b). The nonlinear coefficient is evaluated to be about 512 W﹣1km﹣1, and the fiber loss α is measured to be about 4 dB/m at 1550 nm by using the cut back method.
3. Theory of parametric gain
In the FOPO configuration, the parametric gain is provided by the DFWM process occurred in the nonlinear fiber governed by the phase-matching condition of
To compare the suspended core TMOF with the currently hot HNLF and DSF, the parametric gain versus the fiber length are calculated with the pump power fixed at 20 W. The results are illustrated in Fig. 2. For the parametric gain of 50 dB, the required TMOF length is 0.94 m, the required HNLF length is 33.4 m, and the DSF length is required to be 134.8 m. It is clear that a large parametric gain can be obtained in a relatively shorter TMOF with higher nonlinearity.
4. Experimental results and discussion
In experiment, to measure the DFWM parametric components generated in the suspended core TMOF, the oscillation cavity is disconnected at one port of the ODL. The optical spectra for different pump wavelengths are shown in Fig. 3(a)-3(j). The average pump power coupled into the suspended core of the TMOF is set at 12 dBm (corresponding to a peak pump power of 24.4 W). The DFWM sidebands (Stokes & anti-Stokes) can be clearly observed at each side of the pump. For the pump wavelengths of 1554.5 nm, 1554.1 nm, and 1553.8 nm, two pairs of parametric sidebands can be observed. When the pump wavelength is 1554.5 nm, the outer pair of DFWM sidebands begins to generate, and the intensity is smaller than that of the inner pair. After the pump wavelength is moved to 1554.1 nm, the intensities of the inner and outer parametric sidebands are similar, which is attributed to the contribution of the higher-order dispersion to the phase-matching process. For the pump operated at 1553.8 nm, the outer pair of the DFWM sidebands is dominant, and the intensity is greater than that of the inner pair. When the pump wavelength is lower than 1552 nm, the inner pair of the parametric sidebands nearly disappear, and the DFWM is evolved in the basis of the outer pair of the sidebands. The evolution of the parametric sidebands versus the pump wavelength is illustrated in Fig. 3(l). When the pump wavelength is adjusted from 1565.4 nm to 1551 nm, the Stokes sideband can be generated from 1606 nm to 1743.5 nm, and the anti-Stokes sideband can be tuned from 1526.8 nm to 1395 nm.
To estimate the parametric gain provided by the TMOF in the oscillation cavity, a tunable distributed feed-back diode continuous-wave laser with a low power of −40 dBm is coupled into the TMOF together with the pulsed pump. Figure 3(k) shows the measured average parametric gain spectrum for the pump at 1565.4 nm with an average pump power of 12 dBm launched into the suspended core. A maximal average parametric gain of 47.5 dB can be obtained at 1606 nm. The round-trip loss of the oscillation cavity is measured to be about 14.5 dB. The parametric gain is much larger than the cavity loss, so operation of the FOPO is possible. Then, the FOPO is built up by connecting the oscillation cavity. When the DFWM parametric component is tuned to synchronize with the succedent pump pulses after a round-trip in the cavity, the OPO begins to oscillate. The threshold average pump power launched into the TMOF is measured to be 10.5 dBm (corresponding to a peak pump power of 17.3 W). The output spectra of the TMOF-based OPO for several pump wavelengths with the average pump power coupled into the suspended core fixed at 12 dBm are shown in Fig. 4. The oscillated signal can be tuned from 1606 nm to 1743.5 nm, and the corresponding idler can be generated from 1526.8 nm to 1395 nm by adjusting the pump wavelength from 1565.4 nm to 1551 nm. Once the pump wavelength is adjusted, the length of the oscillation cavity should be slightly tuned to synchronize the parametric components with respect to the pump pulses. For the pump wavelengths of 1565.4 nm and 1561.9 nm, the cascaded FWM parametric components can also be generated. When the pump is operated at 1554.1 nm, both the signals located in the inner and outer DFWM sidebands can be selected to oscillate by slightly tuning the ODL.
Figure 5(a) shows the output spectra of the suspended core TMOF-based OPO pumped at 1551.5 nm with different average pump power. The oscillated signal is located at 1732 nm, and the idler is generated at 1405 nm. The zoomed-in spectra of the signal and idler are shown in the insets of Fig. 5(a). It can be seen that both the signal and idler intensities increase with the increasing of the average pump power. To investigate the dependence of the output signal and idler power on the average pump power, the signal and idler are filtered out by two short/long pass wavelength devision multiplex couplers (WDMCs) with the cutoff wavelengths of 1530 nm and 1570 nm. Figure 5(b) shows the measured signal and idler power versus the average pump power. With the increase of the average pump power from 11 dBm to 12.9 dBm, the average signal power increases from −36.4 dBm to −25.3 dBm, and the average idler power increases from −40.6 dBm to −31.3 dBm. When the average pump power is larger than 12.5 dBm, the increasing rate of the signal and idler power begins to slow down, which is attributed to the gain saturation. The total internal conversion efficiency is calculated by comparing the signal and idler power to that of the single-pass pump power with the feedback path disconnected at one port of the ODL . A total internal conversion efficiency of −17.2 dB is obtained for the average pump power of 12.9 dBm.
In conclusion, we have demonstrated a suspended core TMOF-based OPO pumped by a mode-locked erbium-doped fiber laser for the first time, to the best of our knowledge. The DFWM parametric components generated in the 1.5-m-long TMOF are fed back into the gain fiber repeatedly through a ring cavity structure. The oscillated signal can be generated from 1606 nm to 1743.5 nm, and the idler can be emited from 1526.8 nm to 1395 nm by adjusting the pump wavelength from 1565.4 nm to 1551 nm. The dependence of the output signal and idler power on the average pump power is investigated for the pump at 1551.5 nm. With the increase of the average pump power from 11 dBm to 12.9 dBm, the average signal power increases from −36.4 dBm to −25.3 dBm, and the average idler power increases from −40.6 dBm to −31.3 dBm. A total intenal conversion efficiency of −17.2 dB has been achieved.
This work was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the Support Program for Forming Strategic Research Infrastructure (2011-2015).
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