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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Widely tunable all-fiber optical parametric oscillator based on a photonic crystal fiber pumped by a picosecond ytterbium-doped fiber laser Lei Zhang,1 Sigang Yang,1,* Xiaojian Wang,1 Doudou Gou,1 Xiangliang Li,2 Hongwei Chen,1 Minghua Chen,1 and Shizhong Xie1 1
Tsinghua National Laboratory for Information Science and Technology (TNList), Department of Electronic Engineering, Tsinghua University, Beijing 100084, China 2 Department of Physics, Tsinghua University, Beijing 100084, China *Corresponding author: [email protected] Received June 28, 2013; revised August 22, 2013; accepted October 2, 2013; posted October 3, 2013 (Doc. ID 193068); published November 4, 2013 We report on a fully fiber-integrated widely tunable optical parametric oscillator based on a photonic crystal fiber pumped by a picosecond ytterbium-doped fiber laser. The output wavelength of the oscillator can be continuously tuned from 898 to 1047 nm and from 1086 to 1277 nm, which is as wide as 340 nm. In particular, a larger Raman gain peak is simultaneously observed when the pump wavelength is far from the zero-dispersion wavelength in the normal-dispersion regime. The bandwidth of the output of the oscillator can be tuned by slightly adjusting the pump power. © 2013 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.4370) Nonlinear optics, fibers; (190.4380) Nonlinear optics, four-wave mixing. http://dx.doi.org/10.1364/OL.38.004534
Optical parametric oscillators (OPOs) have been recognized as an important technique to generate tunable coherent light at nonconventional wavelength regions because of their properties of a wide tuning range, high efficiency, and ease of operation [1]. Conventionally, the χ 2 nonlinear effect of crystals is utilized for parametric oscillation. This approach has been widely used in many applications. However, this technique requires a precise alignment within the cavity for the parametric oscillation. Recently, fiber optical parametric oscillators (FOPOs), which utilize the modulation instability (MI) gain of fibers pumped near the zero-dispersion wavelength, have attracted comprehensive interest [2–4]. The MI gain is high, and the gain band can be tuned in a wide wavelength region [5,6]. This is beneficial for the development of FOPOs. In the C and L communication bands, using the dispersion-shifted fiber as the gain medium, CW and pulsed FOPOs have been reported comprehensively [7–11]. Thanks to the appearance of photonic crystal fiber (PCF), the zero-dispersion wavelength can be moved to a shorter wavelength region. MI gain bands have been demonstrated by pumping at visible and 1 μm regions, respectively [12,13]. FOPOs based on the PCFs pumped at visible and 1 μm regions have also been widely reported [14–17]. However, most of the reports are focused on the FOPO pumped by femtosecond pulses. It has very short interaction length due to the walk-off between the pump and signal or idler. This requires that the PCF used is very short and free-space optics are used. It is difficult to integrate with other fiber components. Recently a tunable CW all-fiber OPO pumped in the 1 μm region has been demonstrated [18]. The CW pump power is limited at 11 W, and the output of the FOPO covers the range from 950 to 1010 nm. A picosecond pulse has wider pulsewidth than a femtosecond pulse, so longer gain fiber can be used. A picosecond pulse with a peak power of hundreds watts is easy to obtain in the 1 μm region. It 0146-9592/13/224534-04$15.00/0
is possible to extend the tuning range of a FOPO significantly. Thus, to build up a FOPO based on a PCF pumped by a picosecond pulse in the 1 μm region is highly desirable. In this Letter, we demonstrate experimentally a fully fiber-integrated widely tunable OPO based on a PCF pumped by a picosecond ytterbium-doped fiber (YDF) laser. The output wavelength of the oscillator can be continuously tuned from 898 to 1047 nm and from 1086 to 1277 nm, which is as wide as 340 nm. The experimental setup is shown in Fig. 1. The pump source is a homemade tunable mode-locked YDF laser. Its wavelength can be tuned from 1033 to 1070 nm, the repetition rate is 25.4 MHz, and the full width at half-maximum (FWHM) is 35 ps. After a tunable bandpass filter (TBPF), the original pulse is amplified by a bidirectional core-pumped fiber amplifier using 1 m YDF. The FWHM of the amplified pulse is 21 ps. Then, the pulse train is coupled into the cavity through a 50∕50 coupler. Compared with the specialty dichroic mirror or wavelength division multiplexing (WDM) coupler used in previous reports, in our scheme, all the spectral components in the gain band can be fed back into the cavity through the 50∕50 coupler and the cost of the system can be largely reduced. The consequent advantage is that the oscillating wavelength can cover the whole gain bandwidth of the MI gain spectrum. A polarization controller PC1 is employed to align the polarization state of the pump wave with the principle axis of the PCF. The cavity length is about 18 m, including a 10-m-long PCF as the gain medium, with a nonlinear coefficient of 15 W−1 km−1 and zero-dispersion wavelength of 1062 nm. The signal and idler inside the cavity can be closely synchronized with the succedent pump pulse after a round-trip in the 18 m cavity. The splice loss between the Hi-1060 single-mode fibers (SMFs) and the PCF is optimized by inserting a piece of ultrahigh numerical aperture (UHNA) fiber between them, and the mode © 2013 Optical Society of America
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
Fig. 1. Experimental setup of the FOPO. MLFL, mode-locked ytterbium-doped fiber laser; TBPF, tunable band-pass filter; WDM, wavelength division multiplex; YDF, ytterbium-doped fiber; PC, polarization controller; PCF, photonic crystal fiber; ODL, optical delay line; OSA, optical spectrum analyzer; DSA, digital serial analyzer.
fields are well matched. After the PCF, a 50∕50 coupler provides 50% feedback and 50% output. In the feedback branch, an optical delay line (ODL) is used to adjust the cavity length. Both the signal (longer than pump) and idler (shorter than pump) are launched back into the cavity. Either the signal or the idler can be selected to oscillate by fine tuning the ODL. The polarization controller PC2 is used to align the polarization state inside the cavity. The FOPO 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 WDM3, and the signal pulses above 1180 nm can be filtered out through WDM4. The waveforms of the signal (idler) can be recorded by a digital serial analyzer (DSA), and the powers of the signal (idler) can be measured by a power meter. In the FOPO configuration, the gain is provided by the MI in the PCF, which is usually pumped near the zerodispersion wavelength. The appearance of MI requires the phase-matching condition of Δβ 2γP 0;
(1)
where P is the incident peak pump power, and γ is the nonlinear coefficient of the PCF. Δβ is the linear phase mismatch term, which can be expressed as Δβ βs βi − 2βp :
(2)
Here, βs , βi , and βp are the propagation constants of the signal, idler, and pump, respectively. The wavelength of the gain band can be tuned by adjusting the pump wavelength near the zero-dispersion wavelength of the PCF. The gain is measured to be about 30 dB with a pump wavelength of 1063 nm and a small signal at a wavelength of 1030 nm. The round-trip loss of the cavity is measured to be 8 dB at the wavelength of 1035 nm. The gain is much larger than the round-trip loss in the cavity, so operation of the FOPO in a wide wavelength range is possible. The experimentally measured optical spectra of the output of the oscillator for several different pump wavelengths near the zero-dispersion wavelength are depicted
4535
Fig. 2. Optical spectra of the output of the oscillator for pump wavelengths of 1066.3, 1062.8, 1060.0, 1059.0, and 1057.5 nm. For the pump wavelength of 1060.0 nm, the idler wave is chosen to synchronize with the pump. For the other pump wavelength, the signal wave is chosen to synchronize with the pump.
in Fig. 2. When the pump wavelength moves from 1066.3 to 1057.5 nm, the signal pulse can be tuned from 1086.5 to 1195.8 nm, and the idler pulse can be tuned from 1047 to 949 nm. The lobe at the wavelength region of from 1015 to 1045 nm is introduced by the bidirectional corepumped YDF amplifier. For the pump wavelength of 1066.3 nm, high-order MI gain peaks are also observed. When the pump wavelength is varied, the ODL should be tuned to synchronize the signal or the idler with the pump by adjusting the cavity length. For the pump wavelength of 1060.0 nm, the idler wave is chosen to synchronize with the pump, and the peak gain of the idler is higher than that of the signal. For the other pump wavelength, the signal wave is selected to synchronize with the pump, so the peak gain of the signal is higher than that of the idler. The frequency detuning of the signal or idler with respect to the pump wave increases as the pump wavelength decreases. For the pump wavelength of 1056 and 1053 nm, the optical spectra of the output of the oscillator are shown in Fig. 3. The wavelength of the signal pulse can be tuned up to 1277 nm, and the wavelength of the idler pulse can be tuned down to 898 nm. Except for the signal and idler peaks, the Raman gain peak also is observed. It is especially surprising that the Raman gain peak is larger than the gain peak of the signal or idler. The maximum Raman gain is generally believed to be smaller than the gain of phase-matched MI [19]. To some extent, it appears that the efficiency of MI is largely reduced by the increased walk-off effect, and a large portion of pump powers are converted into the Raman gain peak when the pump wave is far from the zero-dispersion wavelength in the normal-dispersion regime. Figure 4(a) shows the optical spectra of the output of the oscillator for the pump wavelength of 1057.8 nm with different pump power. It is clear that the bandwidth of the output of the FOPO is narrower with a small pump power and the bandwidth of the signal or idler becomes broader with an increase of the pump power. The 3 dB bandwidth of the signal (idler) increases from 4.15 nm (3.9) to 16.7 (10.15) nm, with the average pump power
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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Fig. 5. (a) Output pulse from the pump source and (b) signal pulse observed at the FOPO output for the pump wavelength of 1057.8 nm. The horizontal scales have the same value.
Fig. 3. Optical spectra of the output of the oscillator for pump wavelengths of 1056 and 1053 nm. The peaks at 1110 nm are induced from the Raman gain. The peaks at 975 nm are from the semiconductor laser diode (LD laser) of 975 nm.
Average signal and idler power (dBm)
increasing from 15.3 to 16.2 dBm. The gain peak of the signal or idler increases initially and then decreases with the increasing of the pump power. The bandwidth and the peak gain as a function of the average pump power are depicted in Fig. 4(b). The idler pulse can be filtered out by the 980 nm port of WDM3, and the signal pulse can
be filtered out by the 1550 nm port of WDM4. The waveforms of the input pump pulse at 1057.8 nm and the output signal pulse at 1193.5 nm measured from the DSA are shown in Fig. 5. It can be seen that the pulsewidth of the signal is narrower than that of the pump. This is induced by the pulse compression effect [20]. The precise pulsewidth cannot be measured by the DSA. This is limited by the photo-detector electrical bandwidth of 1.2 GHz. For the pump wavelength of 1057.8 nm, the average output powers of the signal or idler from the output of the FOPO are measured by a power meter. The results are shown in Fig. 6. With the average pump power increased from 15.3 to 16.2 dBm, the average signal power increases from −6 to −1.55 dBm, the average idler power increases from −4.8 to −2.9 dBm, and the corresponding conversion efficiency of the signal (idler) increases from 0.74% (0.98%) to 1.68% (1.23%). However, the power increasing rate was slowing down, as the average pump power is larger than 15.9 dBm, because of the gain saturation. In conclusion, we have demonstrated an all-fiber widely tunable FOPO based on a PCF pumped by a picosecond YDF laser. With the pump wavelength tuned between 1066.3 and 1053 nm, the output wavelength of the oscillator can be continuously tuned from 898 to 1047 nm and from 1086 to 1277 nm. When the pump wavelength is far from the zero-dispersion wavelength in the normal-dispersion regime, the Raman gain peak is larger than the MI gain peaks. The bandwidth of the output of the oscillator can be tuned by slightly varying
Fig. 4. (a) Optical spectra of the output of the oscillator for the pump wavelength of 1057.8 nm with different pump power. (b) Bandwidth and peak gain of the signal and idler pulses as a function of the average pump power.
-1 -2
Signal Idler
-3 -4 -5 -6 15.2
15.4
15.6
15.8
16.0
16.2
Average pump power (dBm) Fig. 6. Average output powers of the signal and idler from the output of the FOPO as a function of the average pump power for the pump wavelength of 1057.8 nm.
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
the average pump power. This scheme may be useful in generating picosecond pulses at nonconventional wavelength ranges around 1 μm. 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 Open Fund of the State Key Laboratory on Integrated Optoelectronics. References 1. M. H. Dunn and M. Ebrahimzadeh, Science 286, 1513 (1999). 2. J. E. Sharping, J. Lightwave Technol. 26, 2184 (2008). 3. C. J. S. de Matos, J. R. Taylor, and K. P. Hansen, Opt. Express 29, 983 (2004). 4. D. K. Serkland and P. Kumar, Opt. Lett. 24, 92 (1999). 5. M. E. Marhic, K. K. Y. Wong, and L. G. Kazovsky, IEEE J. Sel. Top. Quantum Electron. 10, 1133 (2004). 6. S. Oda, H. Sunnerud, and P. A. Andrekson, Opt. Lett. 32, 1776 (2007). 7. M. E. Marhic, K. K. Y. Wong, L. G. Kazovsky, and T.-E. Tsai, Opt. Lett. 27, 1439 (2002). 8. Y. Q. Xu, S. G. Murdoch, R. Leonhardt, and J. D. Harvey, J. Opt. Soc. Am. B 26, 1351 (2009).
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9. S. Yang, Y. Zhou, J. Li, and K. K. Y. Wong, J. Sel. Top. Quantum Electron. 15, 393 (2009). 10. G. K. L. Wong, S. G. Murdoch, R. Leonhardt, and J. D. Harvey, Opt. Express 15, 2947 (2007). 11. Y. Zhou, K. K. Y. Cheung, S. Yang, P. C. Chui, and K. K. Y. Wong, Opt. Lett. 34, 989 (2009). 12. J. D. Harvey, R. Leonhardt, S. Coen, G. K. L. Wong, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, Opt. Lett. 28, 2225 (2003). 13. W. J. Wadsworth, N. Joly, J. C. Knight, T. A. Birks, F. Biancalana, and P. St. J. Russell, Opt. Express 12, 299 (2004). 14. J. E. Sharping, M. A. Foster, A. L. Gaeta, J. Lasri, O. Lyngnes, and K. Vogel, Opt. Express 15, 1474 (2007). 15. Y. Deng, Q. Lin, F. Lu, G. P. Agrawal, and W. H. Knox, Opt. Lett. 30, 1234 (2005). 16. J. E. Sharping, J. R. Sanborn, M. A. Foster, D. Broaddus, and A. L. Gaeta, Opt. Express 16, 18050 (2008). 17. J. E. Sharping, C. Pailo, C. Gu, L. Kiani, and J. R. Sanborn, Opt. Express 18, 3911 (2010). 18. E. A. Zlobina, S. I. Kavlukov, and S. A. Babin, Opt. Express 21, 6777 (2013). 19. J. M. Dudley, G. Genty, and S. Coen, Rev. Mod. Phys. 78, 1135 (2006). 20. T. Torounidis, M. Karlsson, and P. A. Andrekson, J. Lightwave Technol. 23, 4067 (2005).
OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Widely tunable all-fiber optical parametric oscillator based on a photonic crystal fiber pumped by a picosecond ytterbium-doped fiber laser Lei Zhang,1 Sigang Yang,1,* Xiaojian Wang,1 Doudou Gou,1 Xiangliang Li,2 Hongwei Chen,1 Minghua Chen,1 and Shizhong Xie1 1
Tsinghua National Laboratory for Information Science and Technology (TNList), Department of Electronic Engineering, Tsinghua University, Beijing 100084, China 2 Department of Physics, Tsinghua University, Beijing 100084, China *Corresponding author: [email protected] Received June 28, 2013; revised August 22, 2013; accepted October 2, 2013; posted October 3, 2013 (Doc. ID 193068); published November 4, 2013 We report on a fully fiber-integrated widely tunable optical parametric oscillator based on a photonic crystal fiber pumped by a picosecond ytterbium-doped fiber laser. The output wavelength of the oscillator can be continuously tuned from 898 to 1047 nm and from 1086 to 1277 nm, which is as wide as 340 nm. In particular, a larger Raman gain peak is simultaneously observed when the pump wavelength is far from the zero-dispersion wavelength in the normal-dispersion regime. The bandwidth of the output of the oscillator can be tuned by slightly adjusting the pump power. © 2013 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.4370) Nonlinear optics, fibers; (190.4380) Nonlinear optics, four-wave mixing. http://dx.doi.org/10.1364/OL.38.004534
Optical parametric oscillators (OPOs) have been recognized as an important technique to generate tunable coherent light at nonconventional wavelength regions because of their properties of a wide tuning range, high efficiency, and ease of operation [1]. Conventionally, the χ 2 nonlinear effect of crystals is utilized for parametric oscillation. This approach has been widely used in many applications. However, this technique requires a precise alignment within the cavity for the parametric oscillation. Recently, fiber optical parametric oscillators (FOPOs), which utilize the modulation instability (MI) gain of fibers pumped near the zero-dispersion wavelength, have attracted comprehensive interest [2–4]. The MI gain is high, and the gain band can be tuned in a wide wavelength region [5,6]. This is beneficial for the development of FOPOs. In the C and L communication bands, using the dispersion-shifted fiber as the gain medium, CW and pulsed FOPOs have been reported comprehensively [7–11]. Thanks to the appearance of photonic crystal fiber (PCF), the zero-dispersion wavelength can be moved to a shorter wavelength region. MI gain bands have been demonstrated by pumping at visible and 1 μm regions, respectively [12,13]. FOPOs based on the PCFs pumped at visible and 1 μm regions have also been widely reported [14–17]. However, most of the reports are focused on the FOPO pumped by femtosecond pulses. It has very short interaction length due to the walk-off between the pump and signal or idler. This requires that the PCF used is very short and free-space optics are used. It is difficult to integrate with other fiber components. Recently a tunable CW all-fiber OPO pumped in the 1 μm region has been demonstrated [18]. The CW pump power is limited at 11 W, and the output of the FOPO covers the range from 950 to 1010 nm. A picosecond pulse has wider pulsewidth than a femtosecond pulse, so longer gain fiber can be used. A picosecond pulse with a peak power of hundreds watts is easy to obtain in the 1 μm region. It 0146-9592/13/224534-04$15.00/0
is possible to extend the tuning range of a FOPO significantly. Thus, to build up a FOPO based on a PCF pumped by a picosecond pulse in the 1 μm region is highly desirable. In this Letter, we demonstrate experimentally a fully fiber-integrated widely tunable OPO based on a PCF pumped by a picosecond ytterbium-doped fiber (YDF) laser. The output wavelength of the oscillator can be continuously tuned from 898 to 1047 nm and from 1086 to 1277 nm, which is as wide as 340 nm. The experimental setup is shown in Fig. 1. The pump source is a homemade tunable mode-locked YDF laser. Its wavelength can be tuned from 1033 to 1070 nm, the repetition rate is 25.4 MHz, and the full width at half-maximum (FWHM) is 35 ps. After a tunable bandpass filter (TBPF), the original pulse is amplified by a bidirectional core-pumped fiber amplifier using 1 m YDF. The FWHM of the amplified pulse is 21 ps. Then, the pulse train is coupled into the cavity through a 50∕50 coupler. Compared with the specialty dichroic mirror or wavelength division multiplexing (WDM) coupler used in previous reports, in our scheme, all the spectral components in the gain band can be fed back into the cavity through the 50∕50 coupler and the cost of the system can be largely reduced. The consequent advantage is that the oscillating wavelength can cover the whole gain bandwidth of the MI gain spectrum. A polarization controller PC1 is employed to align the polarization state of the pump wave with the principle axis of the PCF. The cavity length is about 18 m, including a 10-m-long PCF as the gain medium, with a nonlinear coefficient of 15 W−1 km−1 and zero-dispersion wavelength of 1062 nm. The signal and idler inside the cavity can be closely synchronized with the succedent pump pulse after a round-trip in the 18 m cavity. The splice loss between the Hi-1060 single-mode fibers (SMFs) and the PCF is optimized by inserting a piece of ultrahigh numerical aperture (UHNA) fiber between them, and the mode © 2013 Optical Society of America
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
Fig. 1. Experimental setup of the FOPO. MLFL, mode-locked ytterbium-doped fiber laser; TBPF, tunable band-pass filter; WDM, wavelength division multiplex; YDF, ytterbium-doped fiber; PC, polarization controller; PCF, photonic crystal fiber; ODL, optical delay line; OSA, optical spectrum analyzer; DSA, digital serial analyzer.
fields are well matched. After the PCF, a 50∕50 coupler provides 50% feedback and 50% output. In the feedback branch, an optical delay line (ODL) is used to adjust the cavity length. Both the signal (longer than pump) and idler (shorter than pump) are launched back into the cavity. Either the signal or the idler can be selected to oscillate by fine tuning the ODL. The polarization controller PC2 is used to align the polarization state inside the cavity. The FOPO 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 WDM3, and the signal pulses above 1180 nm can be filtered out through WDM4. The waveforms of the signal (idler) can be recorded by a digital serial analyzer (DSA), and the powers of the signal (idler) can be measured by a power meter. In the FOPO configuration, the gain is provided by the MI in the PCF, which is usually pumped near the zerodispersion wavelength. The appearance of MI requires the phase-matching condition of Δβ 2γP 0;
(1)
where P is the incident peak pump power, and γ is the nonlinear coefficient of the PCF. Δβ is the linear phase mismatch term, which can be expressed as Δβ βs βi − 2βp :
(2)
Here, βs , βi , and βp are the propagation constants of the signal, idler, and pump, respectively. The wavelength of the gain band can be tuned by adjusting the pump wavelength near the zero-dispersion wavelength of the PCF. The gain is measured to be about 30 dB with a pump wavelength of 1063 nm and a small signal at a wavelength of 1030 nm. The round-trip loss of the cavity is measured to be 8 dB at the wavelength of 1035 nm. The gain is much larger than the round-trip loss in the cavity, so operation of the FOPO in a wide wavelength range is possible. The experimentally measured optical spectra of the output of the oscillator for several different pump wavelengths near the zero-dispersion wavelength are depicted
4535
Fig. 2. Optical spectra of the output of the oscillator for pump wavelengths of 1066.3, 1062.8, 1060.0, 1059.0, and 1057.5 nm. For the pump wavelength of 1060.0 nm, the idler wave is chosen to synchronize with the pump. For the other pump wavelength, the signal wave is chosen to synchronize with the pump.
in Fig. 2. When the pump wavelength moves from 1066.3 to 1057.5 nm, the signal pulse can be tuned from 1086.5 to 1195.8 nm, and the idler pulse can be tuned from 1047 to 949 nm. The lobe at the wavelength region of from 1015 to 1045 nm is introduced by the bidirectional corepumped YDF amplifier. For the pump wavelength of 1066.3 nm, high-order MI gain peaks are also observed. When the pump wavelength is varied, the ODL should be tuned to synchronize the signal or the idler with the pump by adjusting the cavity length. For the pump wavelength of 1060.0 nm, the idler wave is chosen to synchronize with the pump, and the peak gain of the idler is higher than that of the signal. For the other pump wavelength, the signal wave is selected to synchronize with the pump, so the peak gain of the signal is higher than that of the idler. The frequency detuning of the signal or idler with respect to the pump wave increases as the pump wavelength decreases. For the pump wavelength of 1056 and 1053 nm, the optical spectra of the output of the oscillator are shown in Fig. 3. The wavelength of the signal pulse can be tuned up to 1277 nm, and the wavelength of the idler pulse can be tuned down to 898 nm. Except for the signal and idler peaks, the Raman gain peak also is observed. It is especially surprising that the Raman gain peak is larger than the gain peak of the signal or idler. The maximum Raman gain is generally believed to be smaller than the gain of phase-matched MI [19]. To some extent, it appears that the efficiency of MI is largely reduced by the increased walk-off effect, and a large portion of pump powers are converted into the Raman gain peak when the pump wave is far from the zero-dispersion wavelength in the normal-dispersion regime. Figure 4(a) shows the optical spectra of the output of the oscillator for the pump wavelength of 1057.8 nm with different pump power. It is clear that the bandwidth of the output of the FOPO is narrower with a small pump power and the bandwidth of the signal or idler becomes broader with an increase of the pump power. The 3 dB bandwidth of the signal (idler) increases from 4.15 nm (3.9) to 16.7 (10.15) nm, with the average pump power
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Fig. 5. (a) Output pulse from the pump source and (b) signal pulse observed at the FOPO output for the pump wavelength of 1057.8 nm. The horizontal scales have the same value.
Fig. 3. Optical spectra of the output of the oscillator for pump wavelengths of 1056 and 1053 nm. The peaks at 1110 nm are induced from the Raman gain. The peaks at 975 nm are from the semiconductor laser diode (LD laser) of 975 nm.
Average signal and idler power (dBm)
increasing from 15.3 to 16.2 dBm. The gain peak of the signal or idler increases initially and then decreases with the increasing of the pump power. The bandwidth and the peak gain as a function of the average pump power are depicted in Fig. 4(b). The idler pulse can be filtered out by the 980 nm port of WDM3, and the signal pulse can
be filtered out by the 1550 nm port of WDM4. The waveforms of the input pump pulse at 1057.8 nm and the output signal pulse at 1193.5 nm measured from the DSA are shown in Fig. 5. It can be seen that the pulsewidth of the signal is narrower than that of the pump. This is induced by the pulse compression effect [20]. The precise pulsewidth cannot be measured by the DSA. This is limited by the photo-detector electrical bandwidth of 1.2 GHz. For the pump wavelength of 1057.8 nm, the average output powers of the signal or idler from the output of the FOPO are measured by a power meter. The results are shown in Fig. 6. With the average pump power increased from 15.3 to 16.2 dBm, the average signal power increases from −6 to −1.55 dBm, the average idler power increases from −4.8 to −2.9 dBm, and the corresponding conversion efficiency of the signal (idler) increases from 0.74% (0.98%) to 1.68% (1.23%). However, the power increasing rate was slowing down, as the average pump power is larger than 15.9 dBm, because of the gain saturation. In conclusion, we have demonstrated an all-fiber widely tunable FOPO based on a PCF pumped by a picosecond YDF laser. With the pump wavelength tuned between 1066.3 and 1053 nm, the output wavelength of the oscillator can be continuously tuned from 898 to 1047 nm and from 1086 to 1277 nm. When the pump wavelength is far from the zero-dispersion wavelength in the normal-dispersion regime, the Raman gain peak is larger than the MI gain peaks. The bandwidth of the output of the oscillator can be tuned by slightly varying
Fig. 4. (a) Optical spectra of the output of the oscillator for the pump wavelength of 1057.8 nm with different pump power. (b) Bandwidth and peak gain of the signal and idler pulses as a function of the average pump power.
-1 -2
Signal Idler
-3 -4 -5 -6 15.2
15.4
15.6
15.8
16.0
16.2
Average pump power (dBm) Fig. 6. Average output powers of the signal and idler from the output of the FOPO as a function of the average pump power for the pump wavelength of 1057.8 nm.
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
the average pump power. This scheme may be useful in generating picosecond pulses at nonconventional wavelength ranges around 1 μm. 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 Open Fund of the State Key Laboratory on Integrated Optoelectronics. References 1. M. H. Dunn and M. Ebrahimzadeh, Science 286, 1513 (1999). 2. J. E. Sharping, J. Lightwave Technol. 26, 2184 (2008). 3. C. J. S. de Matos, J. R. Taylor, and K. P. Hansen, Opt. Express 29, 983 (2004). 4. D. K. Serkland and P. Kumar, Opt. Lett. 24, 92 (1999). 5. M. E. Marhic, K. K. Y. Wong, and L. G. Kazovsky, IEEE J. Sel. Top. Quantum Electron. 10, 1133 (2004). 6. S. Oda, H. Sunnerud, and P. A. Andrekson, Opt. Lett. 32, 1776 (2007). 7. M. E. Marhic, K. K. Y. Wong, L. G. Kazovsky, and T.-E. Tsai, Opt. Lett. 27, 1439 (2002). 8. Y. Q. Xu, S. G. Murdoch, R. Leonhardt, and J. D. Harvey, J. Opt. Soc. Am. B 26, 1351 (2009).
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