All-optical single and multiple wavelength conversion and tuning by the cascaded sum- and difference frequency generation (cSFG/DFG) have been demonstrated in a temperature gradient controlled periodically poled Ti:LiNbO3 (Ti:PPLN) channel waveguide. Up to 4 channels of wavelength division multiplexed (WDM) signals which have 100 GHz channel spacing were simultaneously wavelength converted at a 16.8 °C temperature difference between both end faces in a Ti:PPLN waveguide. The 3 dB signal conversion bandwidth was measured to be as broad as 48 nm at single channel conversion. The maximum wavelength conversion efficiency and optical signal to noise ratio of wavelength converted channel were approximately -16 dB and -20 dB at a total pump power level of 810 mW.
©2005 Optical Society of America
All-optical wavelength conversion is crucial in wavelength division multiplexed (WDM) optical networks for wavelength reuse, and effective use of the vast fiber bandwidth. Among the various approaches of all-optical wavelength conversion schemes [1, 2, 3, 4, 5], the cascaded sum and difference frequency generation (cSFG/DFG) in a periodically poled Ti:LiNbO3 (Ti:PPLN) waveguide  is the most promising; it offers a full range of transparency, low noise level, high efficiency, and optically tunable wavelength conversion. After the first demonstration of the wavelength conversion by the cSFG/DFG in a Ti:PPLN waveguide , various wavelength conversion and tuning schemes such as channel-selective wavelength conversion , and multi-channel wavelength conversion using multiple wavelength quasi-phase-matched (QPM) LiNbO3 waveguide were proposed. In this letter we report, for what we believe is the first time, single and multiple all-optical wavelength conversion and tuning of WDM signals by the cSFG/DFG in a temperature gradient controlled Ti:PPLN channel waveguide .
In the cSFG/DFG process, two pump waves are used. The first pump at frequency, ω p1 converts a incoming signal at ωs to a sum frequency wave at ωsf(=ω p1+ωs) by SFG process, while the second pump at ωp2 generates the new signal at ωc(=ωsf-ωp2) through the DFG interaction with the sum frequency wave . A cSFG/DFG process in a normal PPLN device can perform a wavelength selective channel conversion and tuning , but for more functional wavelength conversions, such as single and dual channel-selective wavelength conversion and tuning , and multi-channel wavelength conversion , irregularly-engineered QPM-gratings are needed [10, 11, 12]. However, an irregularly-engineered QPM-grating only perform the fixed function what it was designed. Recently, a temperature gradient technique was introduced by the authors for broadening  and reshaping  of phase-matching (PM) bandwidth in the Ti:PPLN waveguide which has uniform QPM-grating. Using this technique we can control the PM bandwidth even with a regular QPM-grating device that has a perfectly periodic QPM-grating by changing the temperature distribution. The PM characteristic curve of the temperature gradient Ti:PPLN waveguide is shown in Fig. 1. A regular periodic QPM-grating has only one PM characteristic curve such as 1st PM curve in Fig. 1. However, the temperature gradient technique can enhance the PM bandwidth up to 2nd PM curve. This PM bandwidth can be controlled by changing the temperature gradient . Therefore, pump1 can interact with any signal wavelength between the signal1 and signal2.
The experimental setup to demonstrate the single and multiple channel wavelength conversion and tuning in the temperature gradient Ti:PPLN waveguide is shown in Fig. 2. The external cavity laser (ECL) signal boosted by the first high-power erbium-doped fiber amplifier (HP-EDFA) was combined with an unpolarized tunable fiber laser (FL) signal in a fiber-optic 3-dB power splitter. Two waves were used as pump1 in the SFG process and pump2 in the DFG process, respectively. The amplified spontaneous emission (ASE) was demuxed by the first arrayed waveguide grating (AWG1) which has 100 GHz channel spacing and then muxed by the second AWG2 to simulate a WDM signal. The WDM signal boosted by the second HP-EDFA was combined with both pumps (pump1 and pump2) in a second fiber-optic 80:20 power splitter and then simultaneously coupled to the Ti:PPLN waveguide. The transmitted signal, pumps, and wavelength converted signal were observed by an optical spectrum analyzer (OSA). The polarizations of all three waves were controlled by fiber-optic polarization controllers (PC1-PC3). To obtain the temperature gradient, we used two Peltier devices in a sample holder, one for heating and the other for cooling (see the inset in Fig. 2). The details of the temperature gradient technique were reported in Ref. . Figure 3 shows the cSFG/DFG spectra of the 74-mm-long Ti:PPLN waveguide at four different gradient temperatures. The QPM period of the Ti:PPLN was 16.6 µm, and the waveguide loss was determined to be 0.14 dB/cm at a 1.53 µm wavelength. The cross-sectional area of the waveguide was determined by the near-field intensity distribution. The horizontal and vertical full-width at half-maximum (FWHM) were measured to be 5 and 4 µm, respectively, with a TM-polarization beam. The details of the fabrication method are described in Ref. . Without temperature gradient control (uniform temperature), only one channel of WDM channels was wavelength converted as in Fig. 3(a). In the case of 7.5 °C temperature difference between both end faces in the Ti:PPLN waveguide, two channels of WDM channels were wavelength converted as in Fig. 3(b). In the same manner, 12 °C and 16.8 °C gradient temperature induced wavelength conversion of three and four channels of WDM channels (Fig. 3(c), Fig. 3(d)). The coupled powers of two pump waves (pump1 and pump2) into the Ti:PPLN waveguide were 300 mW and 510 mW, respectively. In the case of single channel conversion, the wavelength conversion efficiency and optical signal to noise ratio (OSNR) of wavelength converted channel were measured to be -16 dB and -20 dB, respectively. However, in multiple channel conversion, one cannot avoid a trade-off between conversion efficiency and the number of converted channels. Therefore, the conversion efficiency decreased as a function of gradient temperature. To get higher OSNR in multiple channel conversion, higher coupled pump powers are needed because the conversion efficiency of cSFG/DFG is proportional to the product of the two pump powers (pump1 and pump2) [15, 16]. The SFG bandwidth as a function of the temperature gradient of sample is shown in Fig. 4 for pump1 at a wavelength of about 1528 nm. From Fig. 4, one can see that as the temperature gradient of the Ti:PPLN waveguide increases, the SFG bandwidth becomes broad. Each scatters in Fig. 4 correspond to four different temperature gradient cases of Fig. 3. The second harmonic (SH) PM center wavelength was observed to increase at a rate of ~0.128 nm/K as the temperature increased. The wavelength tunability of the converted signal was measured by changing the wavelength of the second pump (pump2), while the wavelength of the first pump was fixed (λp1=1528nm) and the sample had no temperature gradient. In this measurement, the wavelength of the second pump was varied from 1548 nm to 1569 nm. Figure 5 shows the theoretical and experimental results of conversion efficiency as a function of the pump2 wavelength. Though this DFG process is phase mismatched, the conversion efficiency is just slightly reduced for a wide frequency range in comparision to a phase matched interaction. By changing the frequency of the second pump, the wavelength of the output can be tuned as Δωc=ωc-ωs. From these results, 3 dB tuning bandwidth is calculated to be Δλ 3dB=48 nm. As the effective length of a sample gets shorter, the wavelength range where the PM condition for DFG process is satisfied becomes broader. Therefore, 3 dB tuning bandwidth in multiple channel conversion is broader than that in single channel conversion.
In conclusion, we have demonstrated all-optical single and multiple wavelength conversion and tuning of WDM signals by cSFG/DFG in a temperature gradient controlled Ti:PPLN channel waveguide. We could choose a single or several channels from a WDM signal which has 100 GHz channel spacing by a temperature gradient technique and convert these channels to the different wavelengths by cSFG/DFG process. In single channel conversion, the wavelength conversion efficiency and OSNR of wavelength converted channel were approximately -16 dB and -20 dB at a total pump power level of 810 mW (pump1=300 mW, pump2=510 mW). The 3 dB tuning bandwidth was measured to be as broad as 48 nm. The maximum number of simultaneous wavelength converted channel was four at 16.8 °C temperature difference of both end faces in Ti:PPLN waveguide. The cSFG/DFG scheme based on the temperature gradient technique offers greater flexibility than usual cSFG/DFG schemes owing to the selectivity of the number of channels which are to be wavelength converted. For that reason, We believe that this cSFG/DFG scheme is very promising for future WDM applications.
This work was supported by the Ministry of Science and Technology of Korea through the R&D Infrastructure Program.
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