All-optical wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) is proposed and experimentally demonstrated in periodically poled LiNbO3 (PPLN) waveguides. The signal pulse with 40-GHz repetition rate and 1.57-ps pulse width is adopted. The converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. No obvious changes of the pulse shape and width, also no chirp are observed in the converted idler pulse. The results imply that single-to-multiple channel wavelength conversions can be achieved by appropriately tuning the two pump wavelengths.
©2005 Optical Society of America
All-optical wavelength conversion is considered to be a crucial technique in future high-speed dense wavelength-division-multiplexed (DWDM) networks. It enables contention resolution, wavelength reuse and flexible network construction . Recently, Among various demonstrated wavelength conversion schemes, difference frequency generation (DFG) appears to be extremely attractive due to several advantages: it can accommodate more than terahertz modulation bandwidths, simultaneously realize up- and down- multiwavelength conversion, be strict transparency and independence of bit rate and data format, realize spectral inversion for dispersion management, has broad conversion bandwidth, has negligible spontaneous emission noise and has no intrinsic frequency chirp .
DFG-based wavelength conversion has been demonstrated utilizing PPLN [3, 4] and AlGaAs waveguides [5, 6], showing promising applications in DWDM networks. However, it is difficult to simultaneously couple the 0.77-μm band pump and 1.5-μm band signal into the fundamental mode of the waveguide. In order to overcome this suffering, all incident waves are expected to be in the 1.5-μm band and therefore can be easily launched into the waveguide together. Thus, two alternative schemes are proposed exploiting the cascaded second-order nonlinear interactions—one based on the cascaded second-harmonic generation and difference frequency generation (SHG+DFG), and the other based on the SFG+DFG. The former has been both theoretically discussed [7–12] and experimentally demonstrated , [13–19] in a number of papers. At the same time, the latter has also attracted much attention [20–27] recently since it shows some advantages compared with the conventional SHG+DFG scheme, which is described in detail in . Different from the SHG+DFG in which only one pump is used to yield second-harmonic wave in the 0.77-μm band, in the SFG+DFG based wavelength conversion, two pump sources λ P1 and λ P2 are used to generate sum-frequency wave λSF in the 0.77-μm band. Simultaneously, the 0.77-μm band wave λSF interacts with a signal λS to generate an idler wave at λi with 1/λi = 1/λSF -1/λS = 1/λ P1+1/λ P2 -1/λS thanks to the DFG process. The two pump sources can be freely tuned as long as the quasi-phase matching (QPM) condition for the SFG process is satisfied.
In this paper, instead of the CW signal adopted in [20–22], , a 40-GHz, 1.57-ps signal pulse is adopted to demonstrate the SFG+DFG based wavelength conversion in the high-speed situation. Moreover, single-to-multiple (single-to-dual and single-to-triple) channel wavelength conversions are observed.
2. Experimental setup
The experimental setup for the SFG+DFG based wavelength conversion is schematically shown in Fig. 1. A mode-locked fiber laser serves as the tunable pulsed signal source, generating a transform limited hyperbolic-secant pulse of 1.57-ps width and 40-GHz repetition rate. Its central wavelength can be tuned from 1535 to 1562nm. Two CW pump sources λ P1 and λ P2 can be tuned from 1464 to 1548nm and 1461 to 1555nm respectively. A commercial EDFA placed before PPLN can provide the high saturation output power of 30dBm and its small-signal gain is 40dB. The waveguide is fabricated by the annealed proton exchange technique in PPLN. The device used in this experiment is 50mm long and has a QPM period of 14.7μm, a waveguide width of 12μm, and an initial proton exchange depth of 0.8μm. The above parameters allow phase-matching at room temperature between the fundamental mode of the pump at 1545nm and the fundamental mode of the second-harmonic wave at 772.5nm. The fiber-to-fiber coupling loss in this configuration is estimated at ~4.7dB due to the reflection losses at the uncoated end faces, mode mismatching between the fibers and the PPLN waveguide, and intrinsic waveguide losses. The polarization controller is inserted into the configuration before PPLN to enhance the nonlinear interactions in the PPLN waveguide. The output spectra are monitored by an optical spectrum analyzer (Anritsu MS9710C) with the highest spectral resolution of 0.05nm, and the optical pulses are observed through a communication signal analyzer.
3. Experimental results and discussion
Figure 2 illustrates the measured output spectrum from the SFG+DFG based wavelength converter. The signal wavelength is tuned at λS =1555.9nm. Two pump wavelengths are chosen at λ P1 =1540.9nm and λ P2 = 1549.1nm respectively to meet the QPM condition for the SFG process, during which the corresponding sum-frequency wave in the 0.77-μm band is shown in the inset of Fig. 2. Taking into account the total losses in the configuration, the powers of the two pump waves can be calculated both at approximately 10dBm. As shown in Fig. 2, the wavelength conversion efficiency for converting the signal wavelength to the newly generated idler wavelength is estimated at about -18.93dB.
Figure 3 shows the tunable performance of the wavelength conversion when signal wavelength is changed. Keeping two pump wavelengths at 1540.9 and 1549.1nm respectively, the converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. It is interesting that the shortest idler wavelength is achieved at 1527.4nm which is out of the C-band but enters the S-band. This provides the possibility to yield optical signals within the S-band even though all incident waves are within the C-band. Approximately, 13.5nm signal conversion bandwidth (from 1561.9 to 1548.4nm) is achieved with our proposed scheme. Figure 4 depicts the measured conversion efficiency as a function of the initial signal wavelength while the two pump wavelengths are kept at 1540.9 and 1549.1nm respectively. The two pump powers are chosen both at about 10dBm. The conversion efficiency is measured at approximately -29.15dB and its fluctuation is less than 2.22dB. Remarkably, such signal conversion bandwidth is not wide enough due to following two reasons: 1) The signal pulse duration is 1.57ps and its corresponding spectrum bandwidth is much wider compared with the two CW pump waves, which can be seen clearly in Fig. 2 and Fig. 3(a)(b). Thus, the signal wavelength can not be close to the center wavelength 1545nm in order to avoid spectrum overlapping between the signal and idler pulses. 2) In the experiment, limited tunable range of the signal wavelength (from 1535 to 1562nm) results in limited signal conversion bandwidth. If the signal wavelength can be tuned towards the longer-wavelength such as 1580nm, the corresponding idler wavelength will further enter the S-band with the signal conversion bandwidth increasing to about 30nm.
In addition, different from the above wavelength down conversion, it is difficult to clearly observe the wavelength up conversion, which can be explained as follows: 1) The amplified spontaneous emission (ASE) noise is strong especially in the long-wavelength, which can exceed the converted idler wave. 2) When signal wavelength is tuned towards the short-wavelength, the signal pulse will not be amplified effectively because of the deviation from the gain region of erbium-doped fiber amplifier. Consequently, the converted idler wave is so small as to be buried in the ASE noise.
As described in , it is worthwhile to investigate by how large a span the two pump wavelengths can be separated. Two pump wavelengths are tuned respectively towards short-and long-wavelength in order to keep SFG process meeting the QPM condition. As shown in Fig. 5(a), the two pump wavelength difference can be set as large as 17.3nm. Such value is also limited by the tunable range of the pump sources used in the experiment. It is expected that much larger pump wavelength difference can be achieved when the tunable range of the pump sources increases. Figure 6 depicts the conversion efficiency versus the difference of the two pump wavelengths (|λ P1-λ P2|) by employing the signal wavelength at 1561.9nm, with the corresponding converted idler wavelength at about 1528.5nm. Two pump powers are chosen both at about 10dBm. It is noticed that the conversion efficiency at |λ P1-λ P2| < 7nm is much less than |λ P1-λ P2| > 10nm, which is different from the theoretical result indicated in Fig. 4 in Ref  that the conversion efficiency remains constant even though the two pumps are close to each other. In fact, because of the fabrication errors induced in the PPLN waveguide, the pump-wavelength tolerance of the second-harmonic generation (SHG) process is much larger than the theoretical value 0.15nm . Therefore, when the two pump wavelengths become close to each other, besides the SFG process occurring between them, the two pumps themselves will also experience the SHG processes. Thus, three 0.77-μm band waves, i.e., the sum-frequency wave and two second-harmonic waves corresponding to the two pumps are clearly observed in the inset of Fig. 5(b). Meanwhile, they interact with the signal through DFG processes, and three idler waves are generated. As a result, the idler wave generated by SFG+DFG is weakened because of the generation of the other two idler waves due to SHG+DFG processes. This is why the conversion efficiency decreases sharply when the two pump wavelengths become close to each other. On the other hand, Fig. 5(b) provides the possibility to realize single-to-triple channel wavelength conversion. It is expected that when |λ P1-λ P2| > 10nm, and the SHG processes can be ignored, the trend of the conversion efficiency will agree with the theoretical result described in . As shown in Fig. 6, when |λ P1-λ P2| > 10nm, the conversion efficiency is kept at about -30.88dB with the fluctuation less than 0.76dB.
Figure 7 illustrates the dependence of the conversion efficiency on the wavelength detuning of pump1 (λ P1-λ P10) when pump2 is fixed at 1549.1nm and with two pump powers both at about 10dBm. λ P10 = 1540.9nm and λ P2 = 1549.1nm satisfy the QPM condition for the SFG process. Signal wavelength is kept at 1556.6nm and the idler wavelength is varied as pump1 wavelength is tuned from 1536.6 to 1545.0nm. It is clearly seen that the conversion efficiency is around -33.83dB with the fluctuation less than 1.68dB, covering a tunable bandwidth about 7.6nm which is much larger than the SHG+DFG scheme. Note that, when λ P1 is close to λ P2, especially at about 1545nm, two 0.77-μm band waves due to SHG and SFG processes can be observed in the inset of Fig. 8. Therefore, two idler waves are achieved owing to the subsequent DFG processes, corresponding to case A and B respectively in Fig. 8. The case C represents the SHG+DFG between two pump waves. Thus, the conversion efficiency of SFG+DFG drops sharply which is similar to Fig. 5(b). At the same time, Fig. 8 implies that our proposed scheme provides the possibility of realizing single-to-dual channel wavelength conversion.
To further test the quality of the generated idler pulse, we have measured the pulse duration by using an autocorrelator, which is almost the same as the initial signal pulse duration 1.57ps. Figure 9 plots the pulse duration and spectrum bandwidth of the converted idler pulse as a function of the converted idler wavelength under the two pump wavelengths of 1540.9 and 1549.1nm respectively and with the pump powers both at about 10dBm. The average pulse duration is about 1.58ps with the fluctuation less than 0.12ps, and the average spectrum bandwidth is around 205GHz, i.e., about 1.6nm, with the fluctuation less than 17.9GHz. Furthermore, we have calculated the duration-bandwidth product (Δt ∙ Δν) of the converted idler pulse to verify that the SFG+DFG based wavelength conversion has no intrinsic frequency chirp, which is shown in Fig. 10. It is clearly seen that Δt ∙ Δν is estimated at approximately 0.322 with the fluctuation less than 0.03, which is very closed to the transform limited case for unchirped hyperbolic-secant pulse with Δt ∙ Δν = 0.315. That is to say, the converted idler pulse is still transform-limited with no chirp induced, showing that our proposed conversion scheme between ps-pulses has a good performance.
During the above experiments, the conversion efficiency levels measured in Fig. 4, Fig. 6 and Fig. 7 are a little different, which can be ascribed to the influences of the variable polarizations, the power and wavelength jittering of the optical sources, and the instabilities of the environment such as temperature during different time periods. However, this does not affect on describing the tunable performance of the signal and pump waves.
With further improvements, we propose following two directions for the next investigation: 1) Using a tunable ring fiber laser configuration with two tunable filters instead of the current two CW pump sources, which is similar to the arrangement adopted in [17–19]. No external pump sources are required, and especially the ASE noise can be effectively suppressed due to the laser oscillation at two pump wavelengths. Therefore, the wavelength up conversion will be observed easily. 2) Exchanging one pump (pump2) with the signal, then the SFG process will occur between the signal and pump1, which is slightly different from the above SFG+DFG scheme. The wavelength of pump1 is fixed at the value meeting the QPM condition for SFG process, and the tunable output converted idler wave can be realized by tuning the wavelength of pump2, while without changing the signal wavelength.
In summary, we have experimentally demonstrated wavelength conversion between ps-pulses based on the SFG+DFG interactions in PPLN waveguides when the 40-GHz, 1.57-ps signal pulse is adopted. The converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. Two pump wavelengths can be separated as large as 17.3nm. One pump wavelength can be tuned from 1536.6 to 1545.0nm with the other pump wavelength fixed at 1549.1nm. It is attractive that single-to-multiple channel wavelength conversion can be realized by appropriately tuning the two pump wavelengths. No obvious changes of the pulse shape and width, also no chirp are found in the converted idler pulses. Our experimental scheme also provides the possibility of realizing 40Gbit/s and above high-speed wavelength conversion with various data formats, such as nonreturn-to-zero(NRZ), return-to-zero(RZ) and carrier suppressed return-to-zero(CSRZ), etc.
This work was supported by the Chinese Natural Science Foundation under Grant No. 60177015.
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