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We devised a method for the measurement of the phase-matching curve of multiple quasi-phase-matched (QPM) LiNbO3 waveguide under conditions of high-power second-harmonic generation. The data obtained revealed that the phase-matching condition can be preserved due to the high damage resistance of the directly bonded LiNbO3 waveguide. Based on this evaluation, we tried to generate multiple optical carriers using multi-stage frequency mixing in the multiple QPM device. The multiple optical carriers have mutual phase correlation, which is suitable for coherent wavelength division multiplexing (WDM) transmission. We also demonstrated 20 Gb/s quadrature phase shift keying (QPSK) signal generation using multiple optical carriers in order to ensure signal quality.
© 2017 Optical Society of America
1. Introduction
Recent advances in periodically poled LiNbO3 (PPLN) waveguide technology has enabled efficient quasi-phase-matched (QPM) optical frequency mixing for many applications such as generation of visible light [1,2], mid-infrared light [3], and parametric amplification [4]. The efficient parametric amplification enabled phase sensitive amplifier (PSA), which has the potential for extremely low noise amplification [4]. By incorporating spatial phase modulation in the periodically poled structure, we can explore the possibility of a new functionality of the PPLN waveguide—multiple quasi-phase-matching (QPM)—which enables simultaneous phase-matching of several frequency mixing processes [5]. A multiple QPM device enables multi-stage frequency mixing in a compact device. Multi-stage frequency mixing opens up possibilities of optical carrier processing such as carrier phase recovery of multi-level phase modulated signal, and multiple optical carrier generation [6]. The main concern with multi-stage frequency mixing in the multiple QPM device is that LiNbO3 exhibits photorefractive damage in high power conditions [7]. The resulting modification of the phase-matching curve can deteriorate the efficiency of multi-stage frequency mixing.
In this study, we devised a method for the measurement of the phase-matching curve of multiple QPM LiNbO3 waveguide under conditions of high-power second-harmonic generation. The data obtained revealed that the phase-matching curve is preserved even in high power conditions due to the high damage resistance of the device. Based on this result, we tried to generate multiple optical carriers using multi-stage frequency mixing in the multiple QPM device. Finally, we also demonstrated 20 Gb/s QPSK signal generation using the multiple optical carriers in order to ensure signal quality.
2. Principle of multi-stage frequency mixing
Wavelength allocation in the multi-stage frequency mixing and in the phase-matching curve of the multiple QPM device are shown in Fig. 1. We utilize two lasers, TLA1 and TLA2, whose frequency spacing is matched to the frequency spacing of the multiple QPM device such that at least the wavelength of a laser is matched to the QPM peak. The frequency-matched input is converted by second harmonic generation (SHG), and a new idler wave is generated through difference frequency generation (DFG) between the second harmonic (SH) wave and the input laser. There are two possible wavelength allocation of two lasers as shown in Figs. 1(a) and 1(b). In Fig. 1(a), two lasers are matched to the QPM wavelength. In this case, two input are converted to SH in the first process. Idler 1 and 2 are generated by DFG between the SH and two input. On the contrary, in Fig. 1(b), only TLA1 is matched to the QPM wavelength. In this case, only TLA1 is converted to SH in the first process. Idler 1 is generated by DFG between the SH and TLA 2. In both cases, through subsequent multi-stage frequency mixing of SHG/DFG processes, multiple idlers with uniform frequency spacing are generated.
Fig. 1 Process of multiple optical carrier generation. (a) Two inputs are matched to the QPM wavelength, (b) One input is matched to the QPM peak, and the other is detuned.
Owing to the coherent nature of the frequency mixing, each idler has coherent phase correlation. This feature may lead to its potential application as a multi-carrier source for coherent WDM systems [8]. Multi-stage frequency mixing can also be applied for carrier phase recovery from multi-level PSK signals [2,9]. The carrier phase recovery can be used for phase-locked pump generation in the PSA [2,9].
When multi-stage frequency mixing is implemented, high-power SH light is generated in the LiNbO3 waveguide. Our chief concern is the photorefractive damage in the LiNbO3 waveguide caused by the high-power SH light [7]. There is a possibility that the phase-matching curve of the multiple QPM device varies due to the photorefractive damage. If this holds true, then the efficiency of multi-stage frequency mixing can be affected by high-power SH light. We have conducted the following experiment to evaluate the phase-matching curve in the presence of high-power SH light.
3. Phase-matching curve measurement
We measured the phase-matching curve in the presence of high-power SH light using the experimental setup shown in Fig. 2. We employed two tunable laser diode arrays (TLAs) [10]. TLA1 is modulated by an optical chopper, and TLA2 is amplified by an erbium doped fiber amplifier (EDFA). They are combined with a 3-dB fiber coupler and subsequently injected into the multiple QPM module. The periodically poled structure of the device was designed to obtain three QPM peaks with 100 GHz spacing. A LiNbO3 waveguide is fabricated by direct bonding and dry etching [11]. The waveguide output contain several components such as SH of TLA2, sum frequency (SF) of TLA1 and TLA2, difference frequency (DF) of the SH and TLA1, and residual inputs. The waveguide device was assembled as a fiber pigtail module in which a dichromatic mirror was integrated to separate SH/SF in 0.78 μm band from DF and residual inputs in 1.55 μm band. The module is assembled with three polarization-maintaining fibers (PMF), two for 1.55 μm and the other for 0.85 μm. The SH/SF lights are output from PMF for 0.85 μm. The wavelength of the TLA2 was matched to the centeral QPM peak, and the wavelength of the TLA1 was subsequently scanned. The SH and SF lights generated from the multiple QPM module was detected by a photodiode with a lock-in voltmeter (LIV). The combination of the optical chopper and LIV enables us to detect modulated SH/SF lights and to ignore the continuous component. By measuring the amplitude and phase of the modulated SH/SF output, we can distinguish the sum frequency generation (SFG) from the DFG. The modulated TLA1 and amplified TLA2 generate SF light. DFG between the modulated TLA1 and the SH light of amplified TLA2 cause SH light depletion.
For comparison, we also measured the phase-matching curve by scanning the wavelength of the amplified TLA2 without the input of the modulated TLA1. In this case, the SH output was detected by an optical power meter (OPM). Figure 3 shows an example of the phase-matching curves measured by scanning the wavelength of the amplified TLA2. The output power of the EDFA was set to 23 dBm, and 33 dBm, respectively. As the input power was increased, we observed some distortion in the phase-matching curves. It was not clear whether the distortion was caused by the photorefractive effect. Under high power conditions, backward conversion of high-power SH to fundamental wavelength tended to take place. The phase-matching curve became complicated due to this backward conversion.
Fig. 3 Phase-matching curve measured by scanning the amplified TLA. EDFA output power was (a) 23 dBm, (b) 33 dBm. The QPM wavelengths are labeled with arrows.
The phase measurement using a modulated input with an LIV enabled us to distinguish between forward and backward conversion. In the case of forward conversion due to SFG, the phase of the output followed the phase of the modulated input. This is because SFG occurs only when the modulated TLA1 is in “ON” state. On the contrary, in the case of backward conversion due to DFG, the output exhibited a reversed phase with respect to the input. This is because SH light depletion due to DFG occurs only when the modulated TLA1 is in “ON” state.
Figures 4(a) and 4(b) show the amplitude and phase measured by scanning the wavelength of the modulated TLA with EDFA output power of 30 dBm, respectively.
Fig. 4 Phase-matching curves measured with LIV: (a) amplitude, (b) phase, and (c) in-phase amplitude with EDFA output power of 30 dBm, (d), (e) in-phase amplitude with EDFA output power of 23 dBm and 33 dBm, respectively. The QPM wavelengths are labeled with arrows.
The wavelength range of the three QPM peaks is observed in Fig. 4(a), and the in-phase output () is observed in Fig. 4(b). When the input wavelength is detuned from the QPM peak, a reversed phase output () was observed. The phase-matching curve was obtained by measuring the in-phase amplitude of the modulated output. The in-phase amplitude X calculated by the following relation is shown in Fig. 4(c):
where A is the amplitude and θ is the phase of the output signal.
A positive value in Fig. 4(c) corresponds to a forward conversion due to SFG, and a negative value corresponds to a backward conversion due to DFG. Figures 4(d) and 4(e) show phase-matching curves where the EDFA output power was 23 dBm and 33 dBm, respectively. As the input power increased, significant backward conversion was observed. Even when the EDFA output power was set to 30 dBm, the shape of the phase-matching curve was preserved. When the EDFA output power was 33 dBm, strong forward conversion was observed at the sub-peaks between the three QPM peaks. This can be attributed to the SFG between the back converted light and the modulated input. Given this balance between forward and backward conversion, we concluded that the phase-matching condition could still be preserved when the EDFA output power reached 33 dBm. These results prove that the direct-bonded waveguide has a high resistance to photorefractive damage. As discussed above, the PM curve measured using modulated input is dominated by SFG and DFG processes. The QPM wavelength spacing in Fig. 4 is twice that in Fig. 3. This is because the wavelength of the amplified TLA2 is fixed in the SFG/DFG processes. To obtain the same phase mismatch variation, the wavelength variation in SFG/DFG processes is twice that in SHG process.
4. Multiple optical carrier generation
We demonstrated that the multiple QPM condition can be preserved even in high SH power conditions. Based on this, we generated multiple optical carriers utilizing multi-stage frequency mixing. Figure 5 illustrates the experimental setup. Two TLAs are combined with a fiber coupler and amplified by an EDFA simultaneously, and subsequently injected into the multiple QPM module. Figure 6 shows the optical spectra of the output with the different wavelength assignment of the input corresponding to Figs. 1(a) and 1(b). As the input power was increased, the number of appreciable multiple optical carriers increased. In Figs. 6(a)-6(c), the wavelength of the two TLAs were matched to the QPM peaks. This condition corresponds to Fig. 1(a). In Figs. 6(d)-6(f), the wavelength of a single TLA was matched to the QPM peak, and the other TLA was detuned by 100 GHz. That condition corresponds to Fig. 1(b). We obtained better efficiency in the former configuration. We successfully demonstrated multiple optical carrier generation a using multiple QPM device.
Fig. 6 Optical spectra of multiple optical carriers with different input powers. EDFA output power was set to (a), (d) 23 dBm, (b), (e) 27dBm, and (c), (f) 30dBm, respectively.
5. Transmission experiment
To explicate the applicability of the multiple optical carrier to data transmission, we examined the signal quality of the carrier by bit error measurement. Figure 7 illustrates the experimental setup. The condition of multiple carrier generation was same as that in Fig. 6(c). The EDFA output power was 30dBm, and the wavelengths of the two TLAs were matched to the QPM peaks. One of the multiple optical carriers was extracted by using a wavelength selectable switch, and modulated with a 20 Gb/s quadrature phase-shift keying (QPSK) format. A PRBS generator integrated circuit (IC) is used to generate a 127 bit pseudo random bit sequence (PRBS) data at 10 Gb/s. The output of the PRBS generator also contained a 63-bit delayed sequence. The PRBS data and the delayed sequence were both amplified by modulator drivers, and subsequently given as input to the IQ modulator. The received power was varied using a variable optical attenuator (VOA). The received signal was detected using an EDFA, a band pass filter (BPF), a delayed interferometer, and a balanced photodetector (PD). The output of the balanced PD was given as input to the clock data recovery (CDR) IC and the signal quality was evaluated using an error detector.
Figure 8 shows the bit error rate performance. We conducted an experiment using idler 1 and 2, which are shown in Fig. 1(a), as well as TLA2 output from the multiple QPM module as the optical carrier.
For the purpose of comparison, we also conducted back-to-back measurements for which the TLA output was directly given as input to the receiver. Using idler 1 and 2, we confirmed that data transmission below a bit error rate of is possible. The resulting power penalty at when compared to the observed back-to-back measurement was less than 2.5 dB. The power penalty can be attributed to the noise generated by the additional EDFA located at the input of multiple QPM module and the unwanted residual reflection in the module. However, we observed that the multiple optical carrier had sufficient signal quality to be utilized for QPSK data transmission. Thus, by improving the efficiency of the multi-stage frequency mixing, multiple optical carrier generation will be useful in coherent WDM transmission.
Conclusion
In conclusion, we devised a method to measure the phase-matching curve of a multiple QPM device in the presence of high-power SH light. Owing to the observed high resistance to photorefractive damage of the direct-bonded waveguide, the phase-matching curve was preserved even with high power (~30 dBm) input. Based on this result, we successfully demonstrated multiple optical carrier generation utilizing multi-stage frequency mixing. We also demonstrated 20 Gb/s QPSK signal generation using multiple optical carriers to confirm signal quality.
References and links
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