We report the experimental demonstration of a fully fiber-integrated picosecond optical parametric oscillator. The gain is provided by a 50-meters homemade photonic crystal fiber in the ring cavity. A time-dispersion-tuned technique is used to allow the oscillator to select the oscillating wavelength adaptively and synchronize with the pump pulse train. The output wavelength of the oscillator can be continuously tuned from 988 to 1046 nm and from 1085 to 1151 nm by adjusting the pump wavelength and the time-dispersion-tuned technique simultaneously.
© 2013 Optical Society of America
Fiber optical parametric oscillator (FOPO), based on modulation instability (MI) gain, has attracted considerable attentions because it can afford tunable optical radiation in nonconventional wavelength band . Meanwhile, it can eliminate the complicated alignment as required in bulk OPO and allow for further integration with other fiber components . Widely tunable MI gain bands can be often obtained by pumping the fiber near the zero dispersion wavelength [3,4]. The output wavelength of FOPO can be tuned by inserting an optical bandpass filter (OBPF) inside the cavity , by tuning the temperature of the gain medium , by switching the polarization of the pump wave  or by adjusting the pump wavelength [8,9]. Although adjusting the pump wavelength is an effective way to tune the output wavelength of the FOPO [10,11], The output wavelength of the FOPO will be detuned by tens of nanometers (even hundreds of nanometers) away simply by adjusting the pump wavelength by about only one nanometer in the normal dispersion regime. Furthermore, once the pump wavelength is changed, the optical components inside the FOPO cavity (such as the filter) should be adjusted accordingly. Hence it is a hard work to tune the wavelength of the FOPO to continuously cover a broad wavelength band by only adjusting the pump wavelength. An alternative way to perform the tunability is the time-dispersion-tuned technique, which was first demonstrated in a fiber Raman oscillator . A tunable all fiber FOPO operating at the communication band  and bulk/fiber hybrid FOPOs [10,14] have been realized by this technique. The principle of the time-dispersion-tuning is to stretch the MI gain pulse in time domain by the group velocity dispersion. Different wavelength components can be tuned to synchronize with the succedent pump pulses after a round-trip in the cavity by slightly adjusting the cavity length. Thus, the time-dispersion-tuned technique makes it easy to continuously tune the output wavelength of the FOPO. At 1 μm regime, widely tuned MI gain band , femtosecond FOPO [14,16,17], picosecond FOPO  and all-fiber CW FOPO [19,20] have been studied. At these femtosecond FOPO systems, the photonic crystal fiber (PCF) used are very short and free-space optics are employed as well. The cavity dispersion is so small that the time domain stretching of MI gain pulse is limited and the spectral tuning step is not small enough.
In this paper, we design and fabricate a fused silica PCF for parametric oscillator. And a widely tunable all-fiber picosecond FOPO operated at 1 μm regime is demonstrated. A 50-meters PCF is used as the gain medium. Comparing with previous reports, the cavity length is long and the dispersion inside the cavity is large, which is beneficial to the time-dispersion-tuning. The output wavelength of the FOPO can be continuously tuned in the range from 988 to 1046 nm and from 1085 to 1151 nm by adjusting the pump wavelength in combination with the time-dispersion-tuned technique. To the best of our knowledge, this is the first time that an all fiber continuously tuned FOPO operating at 1 μm regime is achieved by the time-dispersion-tuned technique.
The tunability of the FOPO is realized by a combination of the detuning of the pump wavelength and the time-dispersion-tuned technique. In the FOPO configuration, the gain is provided by the MI inside the PCF. The appearance of MI requires the phase-matching condition of
3. Experimental setup and fiber properties
The scanning electron microscope (SEM) of the cross section of the designed and fabricated PCF is shown in the inset of Fig. 1. The pitch of air holes is 2.55 μm, the core diameter is 4.2 μm, and the diameter of air holes is 0.9 μm. The simulated group velocity dispersion is depicted in Fig. 1, and the zero dispersion wavelength is located at 1064 nm, as shown by the vertical dotted line. Its nonlinearity coefficient γ is calculated to be 15 W−1km−1, and the fiber loss is measured to be 25 dB/km by the cut back method at the wavelength of 1064 nm.
The experimental setup is shown in Fig. 2. The pump source is a home-made mode-locked Ytterbium-doped fiber laser. Its wavelength can be tuned from 1033.4 to 1070 nm with an average output power of 5 dBm, the repetition rate is 25.4 MHz corresponding to a cavity length of around 8.1 meters, and the full width at half maximum (FWHM) is measured to be 35 ps. The 3-dB linewidth is 1.76 nm at the central wavelength of 1064 nm . After a tunable band-pass filter (TBPF) with 1 nm 3-dB bandwidth the original pulse is amplified by a bidirectional core-pumped fiber amplifier using 1-meter Ytterbium-doped fiber (YDF). The FWHM of the pump pulse is compressed to 21 ps. The compression of the FWHM can be understood by noting that the pulse spectrum can be expanded as long as the gain in the spectral wings exceeds the loss level . Subsequently, the pulse train is coupled into the cavity acting as the pump through a 50/50 coupler. The polarization controller PC1 is employed to align the polarization state of the pump wave with the principle axis of the PCF. A 50-m PCF included in the FOPO cavity acts as the gain medium. After the PCF, a 50/50 coupler provides 50% feedback and 50% output. In the feedback branch, an optical delay line (ODL) with a tuning range of 600 ps is used to adjust the cavity length. Both the signal (longer than pump) and idler are launched back into the cavity. The polarization controller PC2 is used to align the polarization state inside the cavity. These fiber components are connected together by HI-1060 SMF. The splice loss between the Hi-1060 SMF and the PCF is optimized to 0.6 dB by inserting a piece of ultra-high numerical aperture (UHNA, Nufern Ultra-high NA silica fiber, 0.35 NA) fiber between them. The cavity length of the FOPO is 6 times of that of the pump source. Therefore, there were 6 pump pulses circulating in the FOPO cavity simultaneously. The output spectrum is measured by an optical spectrum analyzer (OSA) through a 10/90 coupler. At the output port of the FOPO, the idler pulses below 1 μm can be filtered out through the WDM3 and the power can be recorded by a power meter.
4. Experimental results and discussion
First of all, in order to measure the MI gain band, the cavity was disconnected at one port of the ODL to form a fiber optical parametric generator. Figure 3(a) shows the MI gain bands for the pump wavelength of 1064.8 nm at anomalous dispersion regime with average pump power of 11 dBm at the input port of the PCF. Two MI gain bands can be observed locating on each side of the pump wave. Then, the FOPO is built up by connecting the cavity. The MI gain pulse is tuned to synchronize with the succedent pump pulse after a round-trip in the cavity by tuning the cavity length, and the FOPO begins to oscillate. The threshold average pump power injected into the PCF is measured to be 10 dBm. When the average pump power is increased to 11 dBm, the high intensity narrow bandwidth output is obtained. The round-trip time difference between different wavelength components is enlarged by the anomalous cavity dispersion. The different wavelength components in the MI gain band can be selected to synchronize with the pump pulse. The output wavelength of the FOPO can be continuously tuned. The measured spectra of the output of the FOPO with the pump wavelength fixed at 1064.8 nm are shown in Fig. 3(b). The signal pulse can be continuously tuned from 1085 to 1107 nm and the idler pulse can be continuously tuned from 1023 to 1046 nm by tuning the ODL in a range of 12 mm, corresponding to 40 ps. In the case of λ1 = 1085 nm and λ2 = 1107 nm, λc will be 1096 nm. Then, the time difference ΔT is calculated to be 36 ps, which matched well with the experimental result of 40 ps. The wavelength tuning range was roughly identical to the MI gain bands. The peak of signal is higher than that of the idler while the linewidth of the signal is narrower than that of the idler, due to that the signal is the spectral component that has been synchronized with the pump pulses, not the idler.
As the pump wavelength moves to shorter wavelength regions, the detuning between the pump and the signal or idler becomes larger. The MI gain bands for the pump wavelength of 1063.4 nm at normal dispersion regime with average pump power of 15 dBm are shown in Fig. 4(a). The lobe from 1015 to 1045 nm is resulted from the ASE of the Ytterbium-doped fiber amplifier. The output wavelength of the FOPO can be further tuned continuously by the time-dispersion-tuned technique. Figure 4(b) shows the output spectra of the FOPO with the pump wavelength fixed at 1063.4 nm and the average pump power of 15 dBm. The signal pulse can be continuously tuned from 1133 to 1151 nm and the idler pulse can be continuously tuned from 988 to 1002 nm. The wavelength tuning range was also roughly identical to the MI gain bands.
The output wavelength of the FOPO can be continuously tuned in a broadband by the combination of adjusting the pump wavelength and the time-dispersion-tuned technique. At first the wavelength of 1151 nm is oscillated, and the oscillation can be moved to the shorter wavelength by increasing the optical delay time. The evolution of the output wavelength versus the relative optical delay time with the oscillating wavelength of 1151 nm is shown in Fig. 5. With the relative optical delay time varied in a range of 100 ps, the output wavelength can be tuned from 988 to 1046 nm and from 1085 to 1151 nm respectively. Except for the time dispersion tuning, the pump wavelength is also adjusted (1063.4, 1063.6, 1063.9, and 1064.8 nm), which are shown by the blue signs in Fig. 5.
At the output port of the FOPO, the idler pulses below 1 μm can be filtered out through the WDM3. The average power and bandwidth versus the idler wavelength is shown in Fig. 6(a). The average power fluctuates in the range from −5.5 dBm to −4.5 dBm, which corresponding to the conversion efficiency (idler FOPO output power/input pump power) of from 0.89% to 1.12%, and the bandwidth drifts in the range from 1.0 to 3.72 nm, when the wavelength of the idler pulse is located in the range from 988 to 998 nm. The signal and idler bandwidths over the full tuning range drift in the ranges from 0.6 to 1.9 nm and from 1 to 3.72 nm respectively, which are comparative with the bandwidth of the pump source. When the pump is fixed at one wavelength, the output wavelength of the FOPO can be tuned by adjusting the ODL. With the tuning of the ODL, an envelope can be formed by the peak intensity traces, which is matched with the MI gain band profile. In the case of the oscillating wavelength located at the spectral edge of the MI gain band, the average power of the output pulse is smaller, and the bandwidth is broader. Then the properties of the output pulse can be improved by tuning the pump wavelength. Thus a high peak intensity and narrow linewidth output can be achieved in a broadband region except for the outermost edge by the combination of adjusting the pump wavelength and the time-dispersion-tuned technique. The autocorrelation trace of the idler below 1 μm of the FOPO output can be measured by the autocorrelator (Femtocharome, FR-103XL). Figure 6(b) shows the autocorrelation waveforms of the amplified pump pulse and the idler of the FOPO output. It can be seen that the FWHM of the idler is compressed to 5.8 ps because of the pulse compression effect .
In conclusion, we have presented a high-efficiency all-fiber continuously tuned picosecond FOPO operating from 988 to 1046 nm and from 1085 to 1151 nm. The high-efficiency narrow linewidth broadband tuning is achieved by the combination of adjusting the pump wavelength and the time-dispersion-tuned technique. A 50-meters homemade photonic crystal fiber is used as the gain medium. The large cavity dispersion is beneficial to effectively tune the output wavelength of the FOPO. The output spectral tuning step can be smaller by utilizing a longer Hi-1060 fiber in the cavity. In this work, the tuning range of the FOPO output is limited by the walk off effect between the pump and signal. The tuning range can be expanded to an even broader wavelength region by utilizing a photonic crystal fiber with flat group velocity dispersion close to zero over a large wavelength region.
This work was supported in part by the National Basic Research Program of China (973 Program) under Contract 2010CB327606, the National Nature Science Foundation of China under Contract 61108007, and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics.
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