We experimentally demonstrate tunable multiple-idler wavelength broadcasting of a signal to selective channels for wavelength division multiplexing (WDM). This is based on cascaded χ(2) nonlinear mixing process in a novel multiple-QPM 10-mm-long periodically poled LiNbO3 having an aperiodic domain in the center. The idlers’ spacing is varied utilizing detuning of the pump wavelength within the SHG bandwidth. The temperature-assisted tuning of QPM pump wavelengths allows shifting the idlers together to different set of WDM channels. Our experimental results indicate that an overall idler wavelength shift of less than 10 nm realized by selecting pump wavelengths via temperature tuning, is sufficient to cover up to 40 WDM channels for multiple idlers broadcasting.
© 2012 OSA
With ever-increasing data transmission capacity, WDM networks require tunable wavelength broadcasting by replicating a signal to several channels to facilitate flexible routing, switching and dynamic reconfiguration of the information carried by different channels. Owing to their high speed, large bandwidth, large signal-to-noise ratio, transparency to signal format and so on, all-optical quasi-phase-matched (QPM) wavelength converters based on second-order nonlinearity in periodically poled lithium niobate (PPLN) have attracted increasing attention [1–3]. Over the past decade, research on cascaded second-order nonlinear interactions in QPM-PPLN has been growing fast to satisfy the needs of high speed and large capacity optical networks [1,4,5]. Achieving wavelength broadcasting in these QPM devices is also useful for several applications such as video distribution and teleconferencing [1,6,7].
Frequency conversion of an input signal at frequency ωs to an idler frequency ωc located in the same band using a pump frequency ωp such that ωc = 2ωp - ωs, has been realized using cascaded processes of second harmonic generation and difference frequency generation (cSHG/DFG) . Wavelength conversion based on cascaded sum-frequency generation (SFG) and DFG to generate the idler at ωc = ωp1 + ωp2 - ωs is advantageous for ultrafast optical signal processing [2,7,9–11]. However, the phase matching criteria of a periodic QPM structure limits the SHG bandwidth. This restricts the tunability of a cSHG/DFG frequency conversion and eventually the broadcasting process. To solve the bandwidth problem, a type-1 QPM with broad SHG bandwidth of 25 nm in an MgO-PPLN was already demonstrated . Also by selecting the two pumps for cSFG/DFG closely spaced within the 1.5-μm band, one signal was simultaneously broadcast to seven fixed peaks . Based on difference frequency mixing, Asobe et. al. and Chou et. al. demonstrated multiple-QPM-based wavelength generation in engineered phase-modulated QPM LN waveguides [13,14]. In another case, simultaneous multicasting of 2 multiplexed signals using cSFG/DFG in a uniform single-QPM PPLN waveguide was achieved, however, it required seven CW pumps to generate seven idlers of the multiplexed signal . Although for practical optical communication networks, the tunability of multiple-QPM-based wavelength broadcasting is essential to provide variable number and location of output channels, it has not yet been implemented. Further, reduced efficiency owing to the use of non-preferred nonlinear coefficient (d31) in type-1 QPM needs to be overcome. Our solution for a tunable wavelength broadcasting to several channels is to use a structure with type-0 multiple QPM-SHG, which can be tuned by varying the temperature of the crystal to obtain efficient multiple peaks in different wavelengths . Unlike using a type-1 process, utilizing a type-0 process with appropriate temperature tuning, we can benefit from both tunable multiple-QPM bandwidth and high-efficiency conversion.
In this paper, first, we demonstrate variable SHG-SFG in a novel type-0 multiple-QPM structure using an engineered PPLN to generate one, two or three SH-SF peaks and second, its application in multiple-wavelength broadcasting by DFG mixing of a signal with the generated SH-SF peaks. Tunability of wavelength broadcasting to three idlers with the desired spacing and variable position of the destination channel for WDM is achieved by detuning the two pumps within the SFG bandwidth and making use of the dependence of the QPM efficiency curves on temperature tuning of a PPLN device for the assignment of the pump wavelengths. This scheme is very promising for enhancing the capabilities of existing WDM optical communication systems.
2. Multiple QPM structure in a type-0 PPLN
Besides the engineering of the effective nonlinearity, the advantage of QPM in materials such as lithium niobate (LN) is the access to its large d33 nonlinear coefficient, which cannot be realized by birefringent phase-matching. In type-0 QPM process, the interacting fields propagate as extraordinary waves when polarized along the z-axis as An engineered phase-reversal QPM structure, used in our experiments is shown in Fig. 1 . This device has been fabricated by the room temperature electric field poling method . The period of the 1-cm-long PPLN is Λ = 18.5 μm with an aperiodic domain of width Λ in the center. For a length, of the grating and aperiodic domain in the middle, of device, the variation of effective nonlinear coefficient along length, x is written as :
The temperature acceptance bandwidth for a QPM process can be obtained using the phase matching sinc term of SHG conversion efficiency considering the Sellmeier relation , which depicts the refractive index dependence on the crystal temperature, and wavelength of the incident light. As a type-0 process has a wider temperature acceptance bandwidth than a type-1 process with proper tuning of temperature, the wavelength acceptance bandwidth in type-0 structure can be further improved.
2.1 Experimental setup
Figure 1 shows the experimental setup used for c(SHG-SFG)/DFG in which two tunable lasers are employed as pumps, operating within the C-band. They are combined by a WDM coupler and then amplified by a Pritel high-power EDFA. The amplified lightwaves, passing through a polarization controller are con-focally focused using a lens into a 10-mm-long bulk z-cut multiple-QPM PPLN fabricated by room-temperature electric-field poling . The phase-reversal PPLN sample is temperature controlled for tuning the operating wavelengths and maximizing the conversion efficiency. In our experiments, we have observed a temperature tuning coefficient of 0.3 nm/°C for the PPLN device. A 9.5 dB filter is used for high input powers beyond 100 mW to avoid damaging the detector. The waist of input lights is which is focused to a beam-waist of in the center of the PPLN with the length l, using a 10-cm focal length lens. The output lights are coupled to a spectrum analyzer via a 30x Newport objective and a multimode fiber, for which the coupling loss of the setup is 1.5 dB.
2.2 SHG and SFG in multiple-QPM bulk PPLN
The maximum SHG power for each QPM peak of the phase-reversal PPLN in the plane wave approximation can be calculated using the Eq.: [20,21]. Here, the pump wavelength is the effective nonlinearity coefficient of 2-peak QPM LN is: the free space permittivity is n1 and nSH are the refractive indices of LN at the FH and the SH wavelengths, respectively. For an input pump power P1 = 90 mW, theoretical calculation gives a peak SHG power PSHG = 0.042 mW or an efficiency of −33.32 dB. Figure 2(a) (dashed curve) illustrates the normalized multiple-peak SH power for the characterized PPLN device showing two major QPM peaks for the pump wavelength at 1536.1 nm and 1538.2 nm at 80°C, while the solid-curve depicts the theoretically-simulated normalized SHG plot for such a device based on Eq. (2). The dual peak nature is attributed to the phase reversal due to the aperiodic domain in the center of the PPLN structure. Further a small deviation in the size of aperiodic domain from the poling period leads to the asymmetric peaks.
The maximum peak power at 1538.2 nm is 0.04 mW giving an efficiency of −33.58 dB (0.1%/W) which is in accordance with the calculated value. When two pumps are set at each of the two QPM wavelengths, they result in two SH and one SF peak in between, however, with uneven powers. This is shown in Fig. 2(b) (blue dashed trace) with the input wavelengths of 1536.1 nm and 1538.2 nm. Tuning one of the lasers to the dip (1536.88 nm) in the center of two-peak SHG spectrum of Fig. 2(a) results in the suppression of the short-wavelength SH peak as shown in Fig. 2(b) by red dotted trace; the two peaks (1 SF, 1 SH) obtained here are separated by 0.3 nm. The peak separation and relative efficiency are varied by slightly detuning the input wavelengths. By moving either of the lasers out of the SH and SF bandwidth, a single SH peak (e.g. Figure 2(b) green trace) is achieved.
The SH and SF powers were equalized by slightly detuning the wavelength and varying the power of input pumps, as illustrated in Fig. 3(a) . Thus, the two pumps are set at 1536.14 nm and 1536.28 nm. Figure 3(b) demonstrates the three peaks of SH-SF response of the fabricated PPLN device at two different temperatures wherein the QPM condition of the pumps have changed, due to temperature dependence of effective refractive index. A constant 1.2-nm wavelength difference of pumps is maintained. The violet solid curve shows the three SH-SF peaks when the input pump wavelengths are set at 1537.3 nm and 1538.5 nm with the device temperature at 81.5 °C; the orange dashed trace depicts the three wavelength-shifted SH-SF peaks at 84.5 °C when the two pumps are set at 1538.5 nm and 1539.7 nm. Thus, when each pump is shifted by 1.2 nm with the appropriate temperature tuning, it leads to a shift of 0.6 nm in the SH-SF spectrum.
3. Cascaded SHG-SFG/DFG in multiple-QPM bulk PPLN
Wavelength conversion based on cascaded second order nonlinear interaction in QPM devices are attractive for signal processing in all-optical networks. In a cascaded SFG/DFG process two pump beams at frequencies ωp1 and ωp2 are used to generate a wave at frequency ωSF = ωp1 + ωp2 through the SFG process, which then combines with a signal wave ωs in a DFG process to generate the converted idler at ωc = ωp1 + ωp2-ωs [8–10,22]. To realize efficient wavelength conversions, the SFG and DFG processes must satisfy the following QPM conditions respectively:Fig. 1. Difference frequency mixing of a C-band signal wavelength with the multiple SH-SF peaks generated from the two pumps in the C-band gives multiple idlers in the same band. The output power of the SF wave is twice the SH wave for equal-power pumps; e.g. for the two pumps with powers of 182 mW and 145 mW, respectively, the SF power obtained is and is equal to The cascaded SFG/DFG output can then be calculated as: considering the input PSFG = average of SFG over half-length of the PPLN and Psignal = 14.3 dBm. The experimentally observed efficiency of is thus comparable with the calculated efficiency = −52.5 dB.
4. Tuning the idler spacing by pump detuning
The schematic of the mutual spacing of the three broadcast idlers via cSFG-SHG/DFG by employing pump detuning in the SFG bandwidth is represented in Fig. 4(a) , when the change in the pump wavelengths is reflected identically in the idlers. To demonstrate this experimentally, we set the two pumps at each of the two QPM wavelengths (1536.1 nm and 1538.2 nm, at 80°C) resulting in two SH peaks and one SF peak in between. DF mixing of a signal wavelength at 1545.3 nm with these three SH-SF peaks gives three idlers. The channels for the WDM network considered here, are separated by 50 GHz (0.4 nm) in the C band. Keeping the signal wavelength fixed, we obtain the desired wavelength spacing between the idlers by the tuning of pump wavelengths around the two QPM peaks of SH wavelengths. We have successfully varied the spacing from 0.4 nm to 4 nm between the idlers in steps of 0.4 nm, without registering significant loss in the idler efficiency, so that the idlers can be directed over 11 adjacent WDM channels on either side of the central idler. For example, Fig. 4(b) illustrates the cases for idler spacing of 0.4 nm, 1.2 nm, 2.4 nm and 3.6 nm, while keeping the signal wavelength fixed. Employing a chirped 2-QPM device in which we get broader bandwidth of 2-QPM SHG response, we can thus cover many more WDM channels.
In the above case, we detuned both the pumps towards or away from their respective QPM peak centers by same frequency detuning (i.e, and ), so that their SFG remains fixed at after detuning and hence the corresponding central idler remains fixed in one channel. The idlers on either side however shift as they correspond to the SHG of the two pumps and the two new WDM channels are now broadcast on either side of the same central idler (channel). There are, however, two other possibilities which require fixing one of the two pumps and tuning the other so that either the left or the right idler lies in same channel while the other two are navigated to subsequent channels as shown in the schematic of Fig. 5(a) . We performed these experiments, as illustrated in Fig. 5(b) where the idler spacing of 0.4 nm, 2.0 nm and 3.6 nm has been obtained by tuning one pump around while keeping the frequencies of signal and another pump fixed at and , respectively.
5. Tuning the idler position with temperature tuning
As we pointed out earlier, the number of channels across which the multiple-idlers can be swept depends on the SH bandwidth of the device. It is advantageous to have a chirped device in such a situation. However, employing temperature tunability to shift the QPM peaks will provide the desired flexibility and tunability for directing idlers across all WDM channels, as detailed in the following. Figure 6(a) shows a schematic of the temperature-assisted tunable broadcasting of a signal into three idlers based on cascaded SHG-SFG/DFG in our proposed PPLN device. The phase-matched wavelengths for the QPM processes in PPLN vary due to temperature dependence of the refractive indexes given by the Sellmeier Eq . The pumps can be tuned to a longer wavelength by increasing the temperature from T1 to T2 and thereby tuning the idlers to longer wavelengths. For example, considering two pumps with constant 1.2-nm wavelength spacing at T1, the three-idler broadcasting will place the idlers at WDM channels 1, 4 and 7. Increasing the temperature to T2 by 4.8°C for a new tuning of the pump wavelengths (shift of 1.6 nm) will locate the idlers at the next channels 5, 8 and 11; and so on.
Figure 6(b) shows the spectra of idlers generated when the device is set at four different temperature values. The blue dashed curve corresponds to the case when the pumps are set at 1536.95 nm and 1538.15 nm separated by 1.2 nm at 78.2°C. Increasing the temperature by 4.8°C to 83.0°C shifts the phase matching wavelengths by 1.6 nm so that the idlers are also shifted by the same amount, shown by the red solid curve in Fig. 6(b). Similarly, when the temperature is set at 86.5°C, the idlers experience a total shift of 3.0 nm as depicted by the black dotted curve and for 89.0°C the idlers have shifted by 4.4 nm shown by the green dash-dotted curve in Fig. 6(b). Hence, tuning the temperature by ~10°C shifts the idlers by 4.4 nm to be positioned at WDM channels 12, 15 and 18. Employing temperature tuning, we observed no degradation in the response of PPLN in terms of idler efficiency. Using this device, an overall idler wavelength shift of less than 10 nm by selecting pumps at desired wavelengths attained via temperature tuning, is sufficient to cover up to 40 WDM (50 GHz spacing) channels for multiple idler broadcasting. The location of idlers can thus be varied by slightly tuning the input pump wavelengths within the device’s QPM bandwidth using temperature tuning. Based on these results, we can simply predict that the flexibility of broadcasting using the cSHG-SFG/DFG scheme to multiple channels can be extended to cover the entire C-band by tuning the temperature of the chirped multiple-QPM PPLN device.
In summary, we have shown for the first time, tunable wavelength broadcasting in a 10-mm long multiple-QPM PPLN. We successfully broadcast one signal into three idlers based on cascaded SHG-SFG/DFG in the novel PPLN device for which three SH-SF peaks were achieved. The mutual spacing of idlers and their position in the WDM grid was adjusted by tuning of the two pump wavelengths assisted by temperature adjustment of the PPLN. The temperature tunability of the multiple-QPM PPLN device assists in the choice of suitable pump wavelengths for tunable wavelength broadcasting by positioning the idlers at desired destination channels in WDM networks. Channel selective multiple broadcasting achieved by this scheme proves its crucial function in signal path routing enabling the effective usage of WDM bandwidth and flexible network construction.
Support from a Strategic Grant of NSERC (Natural Sciences and Engineering Research Council), Canada is acknowledged. R. Kashyap also acknowledges the support of the Canada Research Chairs Programs of the Government of Canada. M. Cha wants to thank the support of the National Research Foundation (NRF) Grant funded by the Ministry of Education, Science and Technology of Korea (2009-0074213).
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