We report phase-transparent waveband conversion with polarization insensitivity based on second harmonic (SH) wave pumped difference frequency generation (DFG) using multiple-quasi-phase-matched LiNbO3 (QPM-LN) waveguides. Flexible waveband conversion is demonstrated over the entire C-band using a tunable DFB-LD array (TLA) as a pump source for a multiple-QPM-LN waveguide. The penalty free waveband conversion of 43 Gb/s return-to-zero differential quadrature phase-shift-keying (RZ-DQPSK) waveband signals is successfully achieved.
© 2010 OSA
The waveband switching node, which groups several wavelengths with the same destinations into a waveband and deals with them as one optical path, was proposed for constructing a cost-effective, large-throughput optical cross connector (OXC) to meet the rapidly increasing data traffic demand in networks . An all-optical waveband converter is needed for the waveband-switching node to avoid the wavelength contention and reduce the power consumption of photonic nodes. In addition, advanced phase modulation formats such as differential binary phase-shift-keying (DPSK) and differential quadrature phase-shift-keying (DQPSK) have been applied to large-capacity optical communication systems . Phase-transparency is required for the waveband conversion of such advanced modulation formats. Difference frequency generation (DFG) in a quasi-phase-matched LiNbO3 (QPM-LN) waveguide is an attractive wavelength conversion method, owing to the large bandwidth and modulation format transparency . Moreover, variable waveband conversion is also possible by using a wavelength tunable light source as a pump source for a multiple-QPM-LN waveguide [4,5]. Among several tunable light sources, we have investigated the performance of a tunable laser diode array (TLA) as a pump source for variable waveband conversion. A TLA is composed of a DFB-LD array, a multi-mode interference (MMI) coupler and a semiconductor optical amplifier (SOA) . Thus, a TLA can generate multiple-wavelength output with discrete spacing and allows high-speed wavelength selection. These features make it an ideal pump source for a multiple-QPM-LN waveguide. We have demonstrated dynamic variable waveband conversion from the C-band to the L-band using multiple-QPM-LN waveguides with four QPM wavelength peaks and a TLA .
In this work, we expand the waveband conversion range by fabricating a new multiple-QPM-LN waveguide with eight QPM peaks. We construct an integrated waveband converter by using a TLA and the new multiple QPM-LN waveguides and demonstrate flexible and polarization insensitive waveband conversion over the entire C-band. We also confirm the phase-transparency of the converter by using 43 Gb/s RZ-DQPSK signals.
2. Multiple-QPM-LN waveguide module with 8 QPM peaks
The multiple QPM-LN waveguides with eight QPM peaks were designed using an asymmetric phase modulation on a periodical domain grating in LiNbO3 . The phase modulation curve and theoretical phase-matching curve are shown in Fig. 1(a) and (b) , respectively. It can be seen that the phase modulation function curve contains a smooth part and a steep phase jump. The period of the phase modulation function was 6 mm and the total length of the waveguide was 48 mm. We designed the pump frequency spacing to be 300 GHz to contiguously convert 12-channel waveband signals with a 50 GHz spacing covering the entire C-band.
We fabricated two multiple-QPM-LN waveguides using the direct bonding method . The utilization of Zn-doped LiNbO3 as a core layer made the waveguides highly resistant to the photorefractive damage. The waveguide was assembled in a module with four fiber-optic ports . The 4-port configuration facilitates the bi-directional input and output of signals and second harmonic (SH) waves around 780 nm. A Peltier device is installed in the module to control the waveguide temperature. Thus the phase matching curve can be easily tuned via the temperature controller. The insertion loss of the waveguide is 3 dB at 1550 nm. Figure 2 shows the phase-matching curves of two multiple-QPM-LN waveguide modules at 54 and 56 °C, respectively. The phase matching curves of the modules deviate slightly from the theoretical value in the conversion efficiency balance of the eight peaks and the ripple between the phase-matching peaks. It is reported that the ripple results in the cross talk due to the sum frequency generation (SFG) between the signal and idler wavebands and subsequent DFG between the SF light and the signal . These ripples are mainly caused by steep increase in nonlinearity at the edge of waveguide and partially increased by the non-uniformity of the waveguide. All these ripples can be reduced by using the apodized QPM gratings and improving the uniformity of the waveguide . The maximum SHG conversion efficiencies including the coupling loss at the signal and pump ports of two modules are 32 and 28.8%/W, respectively.
3. Adjacent waveband conversion covering entire C-band
We constructed a polarization-insensitive integrated waveband converter using the multiple QPM-LN modules and a TLA. Figure 3 shows the configuration of the integrated waveband converter. As a cascaded SHG/DFG using one multiple-QPM-LN module is easily to generate crosstalk due to the SFG between the pump and signal wavebands and the subsequent DFG between an SF light and the signal. To obtain a waveband conversion with low crosstalk, we employ SH-pumped DFG using two multiple QPM-LN modules instead of cascaded SHG/DFG using one multiple-QPM-LN module [7,9]. The two multiple-QPM-LN waveguide modules are tuned to the same phase-matched wavelength peaks by using the temperature controller. The output from the TLA is amplified using an external erbium doped fiber amplifier (EDFA) and injected into multiple-QPM LN 1 to generated an SH wave around 780 nm. The generated SH power of 19.4 dBm is injected into multiple-QPM-LN 2 in both directions via a coupler. A polarization beam splitter (PBS)-based coupler is used to minimize the wavelength dependence of the splitting ratio of the SH wave. The SH waves were delivered from multiple-QPM LN 1 to 2 using polarization maintaining fibers for 780 nm. The waveband signals pass through a circulator and are routed into a PBS. The vertical polarization component is directly input into multiple-QPM-LN 2 in the forward direction. The horizontal polarization component is changed to the vertical direction via the connection of polarization maintaining fibers and injected into multiple-QPM-LN 2 in the backward direction. The idlers corresponding to the two polarization components are combined and routed to the output port.
In this experiment, we converted a 12-channel waveband signal with a 50 GHz spacing filtered from a multi-wavelength light source . Figure 4 shows the spectra of the unconverted waveband signals and converted waveband signals. It can be seen that the adjacent waveband conversion was obtained from 1530 nm to 1575 nm covering the entire C-band. A conversion efficiency of > –17 dB was obtained. The waveband conversion spectra for the waveband signals in TM mode and TE mode are shown in Fig. 5 (a) and (b) , respectively. The results confirmed that the polarization sensitivity of the waveband conversion was < 0.9 dB.
The multi-wavelength output with a discrete spacing and fast wavelength selection provided by the TLA provides an ideal pump source for variable waveband conversion based on DFG using a multiple-QPM-LN waveguide. We achieved a dynamic and flexible waveband conversion in a polarization-insensitive configuration by switching the output wavelength of the TLA. We successfully demonstrated the flexible waveband conversion of the 12-channel waveband signals with a 50 GHz spacing.
4. Phase-transparent waveband conversion
DFG based wavelength conversion can preserve the phase information thus it can offer transparent waveband conversion. To confirm this property, we attempted to convert 43 Gb/s RZ-DQPSK based waveband signals. The experimental setup is shown in Fig. 6 . A 5-channel waveband signal with 100 GHz spacing was modulated with a 43 Gb/s RZ-DQPSK modulator, and input into the polarization-insensitive waveband converter. The bit error rate (BER) was investigated using a DQPSK receiver. We measured the BERs of an idler at 1554.2 nm with single-channel and 5-channel signals inputs.
The BER measurement results for back-to-back signals and the corresponding idlers are shown in Fig. 7 . No appreciable power penalty was observed for single-channel or 5-channel signal inputs, which confirmed that no substantial noise is generated during the waveband conversion. Figure 8 shows the eye diagrams of the signal and the corresponding idler. It also indicates that there is no significant degradation in the signal quality caused by the polarization insensitive waveband converter.
Phase-transparent waveband conversion over the entire C-band was achieved using a multiple-QPM-LN waveguide module with eight QPM wavelength peaks. Flexible waveband conversion was successfully demonstrated by switching the output wavelength of the TLA. The conversion efficiency of > –17 dB and the polarization insensitivity (< 0.9 dB) of the waveband conversion were obtained. The result of BER measurement experiment with 43 Gb/s RZ-DQPSK signals indicated that there was no significant signal degradation induced by the flexible waveband converter, which confirmed the phase-transparency of the polarization insensitive waveband converter.
We thank Dr. Hiroyuki Ishii and Dr. Hiromi Oohashi of NTT Photonics Laboratories for providing the TLA. This work was supported by the National Institute of Information and Communication Technology (NICT) of Japan.
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