We experimentally demonstrate tunable dual channel broadcasting of a signal over the C-band for wavelength division multiplexed (WDM) optical networks. This is based on cascaded χ(2) nonlinear mixing processes in a specially engineered, 20-mm-long step-chirped periodically poled lithium niobate with a broad 28-nm second harmonic (SH) bandwidth in the 1.55-μm spectral range. A 10-GHz picosecond mode-locked laser was used as a signal along with a CW pump to generate two pulsed idlers, which are simultaneously tuned across the C-band by detuning of the pump wavelength within the broad SH bandwidth. Variable-input, variable-output scheme of tuned idlers is successfully achieved by tuning the signal wavelength. Pump or signal wavelength tuning of ~10 nm results in the idlers spreading across 30 nm in the C-band.
© 2013 Optical Society of America
All optical wavelength conversion and broadcasting can effectively resolve contention and increase network throughput for high bandwidth applications in communication networks [1–3]. Periodically poled nonlinear materials such as periodically poled lithium niobate (PPLN) have been extensively researched for optical signal processing due to their several advantages [4, 5]. Waveband wavelength conversion has already been demonstrated based on cascaded second harmonic generation (SHG) and difference frequency conversion (DFG) using a pump in the 1.55-μm wavelength region [6, 7]. Simultaneous broadcasting of a signal to different wavelengths with two pumps in the 1.55-μm band using type-I SHG/sum-frequency-generation (SFG) and DFG mixing in an MgO doped PPLN have also been shown . However, the efficiency of a type-1 QPM process is low due to the small effective nonlinear coefficient. Hence, efficient type-0 QPM via cascaded nonlinearities was proposed to realize tunable 3-fold broadcasting of a signal where temperature tuning increased the conversion bandwidth . Based on a temperature independent scheme, we recently demonstrated 7-fold multicasting of a signal in the C-band using a broadband PPLN device . Single-to-multiple channel wavelength conversion of picosecond pulses have been demonstrated based on cascaded second order processes in a PPLN based ring fiber laser, however, it requires two pumps which were provided by tuning of the fiber laser .
In this paper, we report ultra-wideband variable-in variable-out single-to-dual channel broadcasting of a CW as well as a picosecond pulsed signal over the entire C band in an engineered 20-mm-long 30-nm-bandwidth, step-chirped (SC)-PPLN using a CW pump laser to perform nonlinear mixing processes employing type-0 QPM. Without any temperature tuning, the pump and signal wavelengths can be effectively tuned over the broad ~30-nm SH/SF bandwidth of the SC-PPLN in the 1.55-μm communication band. We obtain two idlers directed to desired destination channels in the WDM grid. This is very promising for dynamically reconfigurable all-optical networks.
2. Experimental setup for SC-PPLN
In cascaded SHG/DFG processes, two input photons of pump beam at a frequency are mixed in a second order nonlinear process to generate a wave at frequency which then combines with a signal wave ωs in a DFG process to generate the converted idler at [12–15]. To realize efficient wavelength conversions for type-0 interaction in each period, the SHG and DFG processes must satisfy the QPM conditions of and respectively where nSH, np, ns, nc are the refractive indices of the PPLN at the SH wave, the pump, signal and idler frequencies, c is the speed of the light in vacuum, and Λ is the length of each period of the grating.
The broadband home-made 20-mm-long bulk SC-PPLN device consists of 10 equal sections with the periods varying from 18.2 μm to 19.1 μm. The SC-PPLN is designed to realize a broad SF/SH conversion bandwidth using type-0 QPM in the 1.55-μm range . Each section of the SC-PPLN is phase matched at different wavelengths leading to the broad SH conversion bandwidth. The grating was carefully fabricated by the room-temperature electric field poling method mentioned in ref . The experimental setup used for single-to-dual channel conversion based on cSHG/DFG is shown in Fig. 1. The coupling efficiency into the device is −0.65 dB, coming from the fact that the SC-PPLN was not anti-reflection coated. A CW tunable laser operating within the C band is employed as a pump, and a 10 GHz pulsed output from a tunable picosecond actively mode-locked laser (MLL) is used as the signal wavelength. The MLL frequency is set by the radio frequency generator (RFG) that is used as a clock and the output of MLL is monitored in an RF spectrum analyzer. The signal and pump are combined by a WDM coupler, amplified using a high-power EDFA and then passed through a polarization controller. The Gaussian output beam of the amplified laser was 2.3 mm in diameter, which was loosely focused to a beam diameter of ~100 μm into the SC-PPLN using a 10-cm focal length lens.
The poled LN sample was placed in an oven at a controlled temperature of 125 °C for characterizing its SH response to achieve the wavelength conversion through cascaded SHG/DFG. The experimental (red trace) and theoretical (black dotted trace) SHG response of the SC-PPLN are shown in Fig. 2 where the experimental curve is obtained by tuning the pump over the C-band. The SH peak-to-peak bandwidth is measured to be ~28 nm. For an input pump power, Pp = 0.66 W, used in the characterization experiments, the theoretical calculation gives a peak SH power, PSHG = 0.22 mW, or an efficiency of −34.78 dB. The ripple structure in the efficiency curve is associated with sharp edges in the effective SHG coefficient. The experimentally observed intensity response has similar features as the simulated one but having comparatively lesser ripple, which can be attributed to the errors involved in the poling process. An apodization technique can be used to flatten this response in the future .
3. Single to dual channel conversion using a CW signal
The principle of dual idler broadcasting based on cascaded SHG/DFG is shown in Fig. 3.Here the pump and signal frequencies are represented as and Based on the DF mixing of the SH of the pump and signal frequency, the signal is converted to a primary idler at frequency . In the case of a PPLN with a uniform periodic grating, the SH bandwidth is narrow and just the pump frequency is located in that bandwidth while the signal frequency is placed out of this narrow bandwidth. However, in case of using the SC-PPLN with an SH bandwidth almost overlapping the C band, the signal channel also falls within the SH conversion bandwidth. Thus, in addition to the primary idler mentioned earlier, the signal is converted to a secondary idler as well, when the SH of the signal mixes with the pump frequency in a DFG process, as shown in the schematic of Fig. 3.
In the first experimental demonstration, we used tunable CW lasers in the 1.55-μm wavelength range acting as both pump and signal wavelengths in the experiment setup described in Fig. 1. The pump and signal powers were 720 mW and 760 mW resulting in SH conversion efficiency of ~−34 dB. 4 Figure shows three subplots of dual idler broadcasting for a signal fixed at 1552.0 nm and the pump wavelength tuned from 1550.8 nm to 1542.8 nm. The green solid trace shows the idlers generated at 1549.6 nm and 1553.2 nm with mutual spacing of 3.6 nm for the pump and signal difference of 1.2 nm. The red-dashed trace shows the idlers generated at 1541.6 nm and 1557.2 nm with mutual spacing of 15.6 nm for a pump and signal difference of 5.2 nm. The blue-dash-dotted trace shows the idlers generated at 1533.6 nm and 1561.2 nm with a mutual spacing of 27.6 nm for a pump and signal difference of 9.2 nm. The maximum conversion efficiency obtained for the generated idlers is −34.35 dB.
In a similar way, a pump wavelength tuning of 10 nm from the signal wavelength is sufficient to generate dual idlers across 30 nm or ~70 WDM channels in the C band. For every pump tuning of δ nm, the primary idlers shift by 2δ nm whereas the secondary idlers shift by δ nm to give a mutual idler spacing of 3δ. The frequency spacing between the idler channels can be tuned to comply with the ITU grid by changing the frequency spacing between the incident waves within the broad conversion bandwidth of the SC-PPLN device.
We repeated the tunability experiments by keeping the pump laser fixed and tuning the signal wavelength. The results are depicted in Fig. 5, which illustrates three subplots of single-to-dual channel conversion of a pump fixed at 1548.3 nm, while the signal is tuned from 1550.2 nm to 1555.0 nm so that the idlers are tuned across 20.1 nm i.e., 50 WDM channels with a 50-GHz spacing. As seen in the first plot illustrated by the blue solid trace, the signal is located at 1550.2 nm and the idlers are obtained at 1546.4 nm and 1552.1 nm, at a mutual spacing of 5.7 nm. In the red-dashed trace, the two idlers are observed at 1544 nm and 1556.9 nm with a spacing of 12.9 nm, when the signal is positioned at 1552.6 nm. Similarly, when the signal is located at 1555.0 nm the idlers are observed at 1541.6 nm and 1561.7 nm. We obtained a maximum conversion efficiency of −35.76 dB for the tuned idlers. In this scheme, the primary idlers denoted by ‘i1’ are detuned by δ nm, for every signal tuning of δ nm, and the secondary idlers denoted by ‘i2’ shift by 2δ nm, so that a total mutual idler spacing of 3δ is obtained, as already explained earlier.
4. Single-to-dual channel conversion using a pulsed signal
The experimental demonstration of variable-input, variable-output, single-to-dual channel wavelength conversion of a pulsed signal into two idlers is shown in Figs. 6 and 7. The experimental setup is shown in Fig. 1 and the schematic of cSHG/DFG based conversion in the broadband SC-PPLN is depicted in Fig. 3. To avoid damaging the detector, a −23 dB filter was used after the SC-PPLN. An average signal power of 12 dBm and pump power 24 dBm were used in the experiments. The broad ~28-nm SH bandwidth allows the conversion of broadband pulsed signals to two idlers using a CW tunable laser as a pump. The picosecond pulsed signal is generated from a mode-locked fiber laser with a repetition rate of 10 GHz and with a pulse width of 2.0 ps. The mode locked laser has a spectral 3-dB bandwidth of 1 nm . Figure 6 illustrates the tunable performance of the dual idler generation in the SC-PPLN when pulsed signal wavelength is kept constant at 1553.1 nm and the CW pump is varied from 1557.6 nm to 1560.8 nm. In the green subplot the idlers are obtained at 1548.6 nm and 1562.1 nm when the pump is positioned at 1557.6 nm. In the red trace, the pump is detuned by 1.6 nm so that the idlers are located at 1547 nm and 1565.3 nm. By tuning the pump further by 1.6 nm to 1560.8 nm, the idlers move to 1545.4 nm and 1568.5 nm with a mutual spacing of ~23 nm i.e. 57 WDM channels of 50-GHz spacing. The maximum conversion efficiency of the idlers with respect to the signal is −32.45 dB.
In another experiment, keeping the CW pump wavelength constant at 1560.8 nm, the converted idler wavelengths can be tuned across the C-band as the pulsed signal wavelength is varied from 1552.2 nm to 1553.8 nm, as demonstrated in Fig. 7. The magenta trace corresponds to the two idlers at 1546.8 nm and 1567.8 nm when the signal is at 1553.8 nm. Similarly, by moving the signal by −0.8 nm, the idlers further shift to 1546.0 nm and 1568.6 nm. The blue trace shows two idlers at 1543.6 nm and 1569.4 nm when the signal is located at 1552.2 nm. The mutual spacing of idlers in this case is 25.8 nm i.e., 64 WDM channels of 50-GHz spacing and the idler efficiency of −34.82 dB. These results show the feasibility of all-optical wavelength conversion to multiple channels in WDM networks using an ultra-broadband QPM SC-PPLN device based on cascaded χ(2) nonlinear processes.
In conclusion, a widely tunable and flexible dual-channel conversion scheme has been proposed and demonstrated based on cascaded SHG/DFG in a 20-mm long SC-PPLN. Over 28-nm of SH bandwidth lying in the optical communication C band is achieved. We have shown tunable conversion of multiple signals in the 1.55-μm wavelength range to primary and secondary idlers by employing a fixed pump anywhere in the ultra-wideband SH bandwidth. Comparably, a fixed signal has been converted to tunable dual-idler channels by sweeping the pump across the device’s conversion bandwidth. These have been demonstrated for both CW and pulsed signals. By using the SC-PPLN device we have overcome the need for temperature tuning and thus the issues arising due to the limited speed of temperature tuning. Tunable dual channel multicasting achieved by this temperature-independent scheme in an SC-PPLN device can play a significant role in enhancing the dynamic signal path routing in WDM networks and help in a the construction of flexible all-optical networks.
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