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Abstract
Tunable all-optical wavelength conversion (AOWC) within 151 nm bandwidth is demonstrated in a thin-film periodically poled lithium niobate (PPLN) waveguide, which utilizes the cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) process. Also, in the same waveguide, AOWC of a 92-Gb/s 16-ary quadrature amplitude modulated (16-QAM) signal within the C-band is successfully achieved. For Bit-error ratio (BER) measurements, we obtain a negligible optical signal-to-noise ratio (OSNR) penalty (<0.2 dB) for the converted idler wave at a BER of 1e-3.
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1. Introduction
Wavelength division multiplexing (WDM) technology is a direct solution to improve data transmission capacity by using numerous wavelengths, each of them carrying different modulated information [1]. The arrayed waveguide grating router (AWGR) is a critical WDM technology for fixed wavelength routing and wavelength (de)multiplexing [2,3]. However, the data transmission capacity of an N×N AWGR is limited by fixed N wavelength channels. If N wavelength converters (WCs) are added at the input ports or the output ports of an N×N AWGR (e.g., see [4]), more than N wavelengths can be flexibly utilized to carry different modulated information, thus significantly improving data transmission capacity, reducing WDM networks blocking and saving network costs. To achieve wavelength conversion, optical-electrical-optical (O/E/O) converters is a conventional approach which has limited transparency and increased cost with higher bit-rates [5]. All-optical wavelength conversion (AOWC) is another approach to achieve wavelength conversion and solve transmission blocking in complex WDM networks [6,7]. For AOWC, various nonlinear optical processes have been used, such as cross-phase modulation (XPM) in semiconductor optical amplifiers (SOAs) [8], four-wave mixing (FWM) in highly nonlinear fibers (HNLFs) [9], or DFG in PPLN waveguides [10–12]. In the past two decades, PPLN waveguides have been confirmed to be promising candidates for AOWC, offering ultrafast response, wide wavelength conversion range, negligible amplified spontaneous emission (ASE) noise and potential of high conversion efficiency [10–19].
M-ray quadrature amplitude modulation (m-QAM) is another solution to improve capacity in optical transmission systems by increasing the data rate [20]. Up to now, several reports have demonstrated AOWC performances of QAM signals in PPLN waveguides [21–26]. The conventional PPLN waveguides used in these reports are normally formed by reverse-proton exchange (RPE) or titanium (Ti) in-diffusion in bulk LN which have small index contrast and large mode sizes, leading to large bending radii and low nonlinear efficiency [27,28]. To reach high conversion efficiency, such conventional waveguides require long interaction lengths, resulting in a barrier for chip-scale integration. In recent years, thin-film lithium niobate (TFLN) platform has attracted wide interest due to excellent electro-optic (EO) performance, ultra-low-loss and high nonlinear efficiency. Various integrated photonic devices have been demonstrated on the TFLN platform with ground-breaking performances, such as ultra-high-performance EO modulators [29–31], efficient polarization splitter-rotators [32], wide-band spot-size converters (SSCs) [33], as well as compact AWGRs [34] and record-efficiency wavelength converters [35–38]. For thin-film PPLN waveguides, the normalized SHG conversion efficiency achieves 2600%/W-cm2 [35], which is more than 20 times higher than that in conventional PPLN waveguides. Moreover, the normalized SHG conversion efficiency of 4600%/W-cm2 is accomplished by actively-monitored poling process [39]. As mentioned above, TFLN platform has the ability to build a flexible all-optical wavelength routing (AOWR) chip by integrating an AWGR, PPLN waveguides and SSCs [ Fig. 1(a)], which is promising in WDM networks. The AOWR chip can observably simplify the network management and reduce coupling losses compared with the scheme shown in [4].
Fig. 1. (a) The schematic of an integrated AOWR chip on TFLN platform. (b) A fixed 5×5 AWGR’s wavelength routing property. (c) A flexible 5×5 AOWR chip’s wavelength routing property.
To simplify the analysis, we assume that each input port only utilizes one wavelength to carry information. For a fixed 5×5 AWGR, five wavelengths (λ1, λ2, λ3, λ4, λ5) are used to transfer information from input ports (I1, I2, I3, I4, I5) to specific output ports (O2, O5, O3, O1, O4) [3], as shown in Fig. 1(b). For a flexible 5×5 AOWR chip, the original five wavelengths are converted to different wavelengths (e.g., λ1 to λ5) by the PPLN waveguides at the input, as shown in Fig. 1(c). After the AWGR, five different wavelengths are obtained at the third output (O3) and converted to the second set of wavelengths (λ6, λ7, λ8, λ9, λ10). At last, we utilize ten wavelengths to carry information, which improves data transmission capacity compared to the fixed 5×5 AWGR. Moreover, the quasi-phase matched (QPM) wavelengths of the PPLN waveguides can be tuned by adjusting the temperature [40]. Therefore, the information can be transferred from any input port to any output port flexibly by tuning the QPM wavelengths of the PPLN waveguides.
In this work, for SHG process, we fabricate efficient thin-film PPLN waveguides with absolute conversion efficiency of 50% and normalized conversion efficiency of 2300%/W-cm2. The tunable AOWC within 151 nm bandwidth based on cSHG/DFG process is successfully achieved in a single thin-film PPLN waveguide. Moreover, the AOWC of a 92-Gb/s 16-QAM signal is demonstrated in the same waveguide. For BER measurements, a BER of 1e-3 is obtained at an OSNR of 20 dB. The OSNR penalty between the converted idler wave and back-to-back (B2B) signal wave is <0.2 dB at the same BER of 1e-3.
2. cSHG/DFG process and thin-film PPLN fabrication
We utilize cSHG/DFG process to achieve AOWC, which can simplify experimental setup compared with cascaded sum- and difference-frequency generation (cSFG/DFG) process [41,42]. Figure 2(a) shows the illustration of cSHG/DFG process, which involves two distinct nonlinear processes: SHG and DFG. For SHG process, the SH (ωSH) wave is generated by the input pump (ωp) wave; and for DFG process, the idler (ωi) wave is generated by the nonlinear interaction between the SH wave and the input signal (ωs) wave, as shown in Fig. 2(b). In a QPM PPLN waveguide, the interacting optical waves should satisfy the phase matching conditions and the energy conservations of SHG and DFG processes simultaneously to achieve efficient conversion [43]. We demonstrate x-cut magnesium-oxide- (MgO-) nanophotonic ridge waveguides on lithium-niobate-on-insulator (LNOI) platform (NANOLN). The ridge waveguides have a length of 5 mm, a top width of 1.4 µm, a film thickness of 600 nm, a etch depth of 350 nm and an angle of 70 °, as shown in Fig. 2(c). The effective refractive indices (neff) of fundamental transverse-electric (TE00) modes are numerically simulated using the waveguide dimensions. After calculation, the poling period ΛSHG ≈ ΛDFG ≈ 4.41µm is chosen within the bandwidth of interest (detailed equations are shown in Supplement 1), so that SHG and DFG processes can simultaneously and efficiently accomplish in a single PPLN waveguide. Figures 2(d) and (e) are the numerically simulated mode profiles of TE00 modes for both pump (1550 nm) and SH (775 nm) waves.
Fig. 2. (a) Illustration of cSHG/DFG process for AOWC. (b) 3D schematic of thin-film PPLN waveguides. (c) Schematic showing the cross section of thin-film PPLN waveguides. Top width = 1.4 µm, Etch depth = 350 nm, Film thickness = 600 nm, θ = 70 °. (d) and (e) Mode profiles for TE00 modes at (d) 1550 nm and (e) 775 nm.
In fabrication process, electron-beam lithography (EBL) is utilized to define the patterns of poling electrodes firstly. Then, a 180 nm Cr layer is deposited by electron-beam evaporation (EBE) and poling electrodes are obtained by a lift-off process (details about Cr electrodes are shown in Supplement 1). We develop the periodic reversed ferroelectric domain by applying 40 times, 10-ms-long, 400 V pulses. After poling process, the Cr electrodes are removed and waveguide patterns created by EBL are then transferred to ridge waveguides using inductively coupled plasma (ICP) etching. At last, we deposit a 2μm SiO2 layer over the ridge waveguides utilizing plasma enhanced chemical vapor deposition (PECVD).
3. Conversion efficiency for the SHG process
For SHG process, there are two forms to express the conversion efficiency. One is normalized conversion efficiency based on the pump-undepletion theory, which is defined as $\gamma = P_{\mathrm{SH}} /\left(P_{p}^{2} \cdot L^{2}\right)$ [35], where Pp and PSH represent on-chip pump power at the input and on-chip SH power at the output, respectively, and L is the waveguide length. The other is absolute conversion efficiency based on the pump-depletion theory, which is defined as β = PSH ∕ Pp. Figure 3(a) shows the schematic of an end-fire coupling setup, which is utilized to evaluate the SHG conversion efficiency of thin-film PPLN waveguides. The pump wave from a continuous-wave tunable laser source (TLS, Santec TSL-550, 1500−1630 nm) is amplified to 30 dBm by an erbium-doped fiber amplifier (EDFA, Amonics AEDFA-37R-FA). Then, a lensed fiber is utilized to couple the pump wave into the waveguides. The waveguide propagation losses for pump and SH waves of TE fundamental mode are estimated to be about 1 dB/cm and 2.2 dB/cm, respectively, as measured by cut-back method. The fiber-to-chip coupling losses are measured to be ∼5 dB/facet within C-band and ∼6 dB/facet around 775 nm. While the pump wave is tuned to the QPM wavelength at 1550.92 nm, we can observe a strong scattered SH wave at the output waveguide facet, as shown in the inset of Fig. 3(a). The generated SH wave is coupled into a second lensed fiber and then collected by using a Si photo detector (PD). The pump wave is ensured to be TE polarization using a polarization controller (PC) because TE modes can utilize the largest nonlinear coefficient d33 (= 25 pm∕V) in x-cut TFLN [35]. We tune the pump wavelength from 1535 to 1565 nm and measure the peak normalized conversion efficiency of 2300%/W-cm2 at 1550.92 nm and the full-width at half-maximum (FWHM) spectral bandwidth of ∼2.5 nm [Fig. 3(b)]. The measured SHG spectrum is well consistent with the theoretical sinc-function profile except the oscillations caused by the Fabry-perot resonances at both pump and SH wavelengths [44]. Then, the pump power is increasing gradually with the pump wavelength fixed at 1550.92 nm. When the on-chip pump power is 24 dBm (251.2 mW), the on-chip SH power of 21 dBm (125.89 mW) is obtained at the output, corresponding to an absolute conversion efficiency of ∼50%, as shown in Fig. 3(c). The calculated β based on the ideal pump-depletion theory is 57.8% when the on-chip pump power is 24 dBm (based on Equation (S8) in Supplement 1). The gap is mainly due to the SH propagation loss and the inhomogeneity of QPM periods. Therefore, to fit the measured β, we introduce a correction factor of 0.86 and the corrected curve shows good agreement with the measured data.
Fig. 3. (a) Schematic of the SHG characterization setup, insets show an image of the thin-film PPLN chip and a CCD camera image of the on-chip SH wave at the output waveguide facet. (b) Measured normalized conversion efficiency versus pump wavelength. The red dot-line and black solid curves correspond to the measured data and theoretical sinc-function, respectively. (c) Measured and theoretical absolute conversion efficiencies versus on-chip pump power. The red cross, blue dash and black solid curves correspond to the measured data, corrected and ideal pump-depletion theory, respectively. TLS, tunable laser source; EDFA, erbium-doped fiber amplifier; PC, polarization controller; PD, photo detector.
4. Bandwidth and conversion efficiency for AOWC
Figure 4(a) shows the bandwidth characterization experimental setup. The maximum input power of the 7:3 coupler is only 300 mW (about 24.8 dBm), therefore we set the output power of EDFA as 24 dBm. Then the on-chip pump power is down to 16 dBm by excluding a 3-dB coupler insertion loss and 5-dB waveguide facet loss. Similarly, the 10 dBm signal power from TLS2 is down to −0.5 dBm on-chip power. The pump and signal waves are ensured to be TE polarization using PC1 and PC2. At the output, a 9:1 splitter is utilized to observe the SH power from Si PD and the optical spectrums of signal and idler from OSA (optical spectrum analyzer, YOKOGAWA AQ6370C) simultaneously. Figure 4(b) demonstrates the AOWC bandwidth of 151 nm, which corresponds to the maximum wavelength spacing from signal to idler. In this figure, pump wave is fixed at 1550.92 nm and signal wave is tuned from 1560 to 1630 nm. As a result, the generated idler wave is converted from 1541 to 1479 nm. The AOWC conversion efficiency is defined as η = Pi ∕ Ps(0), where Pi, Ps(0) are the average powers of output idler wave and input signal wave, respectively. The measured η is around −25.5 dB to -27.2 dB, which shows great flatness over the large wavelength range, as shown in Fig. 4(c). Also, the 3-dB bandwidth of the theoretical normalized η is 170 nm, which is much larger than the 3-dB bandwidth in conventional PPLN (from Fig. S3 in Supplement 1). The 151 nm AOWC bandwidth of our thin-film PPLN waveguides show the potential of satisfying future optical transmission demand for C + L band. However, η is limited by the low input pump power (Pp(0)) and the short waveguide length (L). The theoretical relationship between η and Pp(0), L can be described as $\eta \propto P_{p}^{2}(0) L^{4}$ (from Equation (S10) in Supplement 1). If we continue to increase the input pump power Pp(0) and the waveguide length L, the AOWC conversion efficiency η will be enhanced significantly [45].
Fig. 4. (a) Schematic of the bandwidth characterization experimental setup. (b) Demonstration of the AOWC bandwidth from 1479 to 1630 nm. (c) Measured (black square and solid line) and theoretical normalized (red solid line) conversion efficiencies for different signal wavelengths with on-chip pump power of 16 dBm, the dotted line represents the QPM wavelength. (d) Measured conversion efficiency versus the on-chip pump power with the on-chip signal power fixed at 2 dBm. (e) Measured conversion efficiency versus the on-chip signal power with the on-chip pump power fixed at 24 dBm. OSA, optical spectrum analyzer.
To enhance AOWC conversion efficiency, we continue to increase the input pump power. The experimental setup is almost the same as the setup in Fig. 4(a). The only difference is that we replace the 7:3 coupler with a wavelength division multiplexer (WDM, maximum input power of 2 W) so that the pump power can be amplified up to 32 dBm. The pump (1550.92 nm) and signal waves (1541.35 nm) are coupled into the WDM. At the output, idler wave is generated at 1560.68 nm. The detailed AOWC spectra with different on-chip pump/signal powers are shown in Supplement 1. Figures 4(d) and (e) show that the measured η is proportional to the on-chip pump power but almost unrelated to the on-chip signal power, which is in accord with the theory equation $\eta \propto P_{p}^{2}(0) L^{4}$. The measured maximum conversion efficiency is about −10 dB, which is mainly limited by the short waveguide length.
5. AOWC of a 92-Gb/s 16-QAM signal
The AOWC of signals with high bit rate and complex modulation formats plays an important role in our proposed AOWR chip. In the past two decades, several reports have demonstrated AOWC of QAM signal in conventional PPLN waveguides. For further research, we demonstrate AOWC performances of a 92-Gb/s 16-QAM signal in our thin-film PPLN waveguides based on the experimental setup shown in Fig. 5(a). The whole setup consists of five parts with different background colors, including pump, transmitter, wavelength conversion and idler output process, OSNR controller as well as receiver. Firstly, the pump wave from a TLS is amplified to 32 dBm by EDFA1 (Amonics AEDFA-37R-FA) and ensured to be TE polarization by adjusting PC1. At the transmitter, the 7-dBm signal wave generated by an external cavity laser (ECL, Coherent Solutions MTP-1000) is modulated by an in-phase/quadrature (IQ) modulator with 23 GHz EO bandwidth. The IQ modulator is driven by an electrical arbitrary waveform generator (AWG, KEYSIGHT M8196A) with sampling rate of 92-GSa/s and 32 GHz analog bandwidth. After the IQ modulator, a 23 G-baud rate, 16-QAM optical signal is amplified to 12 dBm by EDFA2 (Amonics AEDFA-PA-35-B-FA) and ensured to be TE polarization by adjusting PC2. The WDM is utilized to combine the pump wave with the modulated signal wave and suppress out-of-band ASE noise of EDFA1 and EDFA2. After WDM, a lensed fiber is utilized to couple the two waves into a thin-film PPLN waveguide.
Fig. 5. (a) Schematic of the 16-QAM signal conversion experimental setup. (b) Constellations of back-to-back (B2B) signal (1541.35 nm) and converted idler (1560.68 nm) at an OSNR of 20 dB. (c) BER as a function of OSNR. ECL, external cavity laser; AWG, arbitrary waveform generator; IQ, in-phase/quadrature; WDM, wavelength division multiplexer; ASE, amplified spontaneous emission; ICR, integrated coherent receiver; DSP, digital signal processing; LO, local oscillator.
In the wavelength conversion and idler output process, the pump, signal, generated SH and idler waves are collected using a second lensed fiber and sent to the tunable filter1. We tune the central wavelength of filter1 and filter2 to ensure that only the idler wave can pass the two filters. After filter1, we amplify the idler wave to 0 dBm by EDFA3. Filter2 is used to suppress out-of-band ASE noise of EDFA3. Then, the filtered idler wave is coupled into the 3-dB coupler. In the OSNR controller, the free-running EDFA4 is used to tune the OSNR of the idler wave by adding ASE noise and the OSNR is observed utilizing an OSA.
At the receiver, the idler wave is detected by an integrated coherent receiver (ICR, NeoPhotonics Class 40) along with a local oscillator (LO) wave at the idler wavelength with 13 dBm output power and <100 kHz linewidth. After coherent detection, the idler wave is sampled at 80-GSa/s utilizing a real-time sampling oscilloscope (LabMaster 10-36Zi-A) with 36 GHz analog bandwidth. These sampled waveforms are processed and symbol-decided offline by digital signal processing (DSP) algorithms. Similarly, the BER of B2B signal is obtained by using transmitter, OSNR controller and receiver. Figure 5(b) shows the constellations of B2B signal and converted idler at an OSNR of 20 dB at the receiver, which illustrates the signal distortions introduced by the AOWC is negligible. The OSNR penalty between the converted idler wave (1560.68 nm) and the B2B signal wave (1541.35 nm) is <0.2 dB at the same BER of 1e-3, as shown in Fig. 5(c). These BER performances show that thin-film PPLN waveguides would potentially achieve AOWC of 16-QAM signal with higher baud rate by using an IQ modulator with higher EO bandwidth.
Table 1 summarizes the AOWC performances in several PPLN waveguides. For 16-QAM modulation format, our results show a high bit rate and a negligible OSNR penalty. Among all these PPLN waveguides, our works demonstrate the shortest waveguide length and larger AOWC bandwidth utilizing a single nanophotonic PPLN waveguide based on TFLN platform, which would play an important role in achieving flexible AOWR within C + L band on an integrated photonic chip.
Table 1. Comparison of AOWC performances in several PPLN waveguides
6. Conclusion
In this paper, we demonstrate tunable AOWC in a single thin-film PPLN waveguide. The 151 nm AOWC bandwidth and the conversion of a 92-Gb/s 16-QAM signal are successfully achieved in the same thin-film PPLN waveguide. The maximum measured AOWC conversion efficiency is about −10 dB, which would be improved by increasing the waveguide length. For BER measurements, we obtain a BER of 1e-3 at an OSNR of 20 dB, and the OSNR penalty for the converted idler wave is <0.2 dB at a BER of 1e-3. These AOWC performances show that the highly efficiency thin-film PPLN waveguides are promising for future chip-scale integration with an AWGR and SSCs, which would play an important role in WDM networks and optical interconnection technologies.
Funding
National Key Research and Development Program of China (2019YFB2203501); National Natural Science Foundation of China (61835008, 61905079, 61905084, 62175079).
Acknowledgments
We thank the Center of Micro-Fabrication and Characterization (CMFC) of WNLO and the Center for Nanoscale Characterization & Devices (CNCD), WNLO of HUST for the facility support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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