1. Introduction

At present, telecom traffic is rapidly growing, and, thus, the demand for high-capacity optical communication is increasing. The transmission capacity is determined by the spectral efficiency and available bandwidth. The spectral efficiency is being improved to meet the demand for larger capacity by means of a sophisticated modulation format and a digital coherent receiver. However, the efficiency approaches Shannon’s limit because of the saturation of the signal-to-noise ratio caused by the nonlinearity of the optical fiber [1]. Therefore, it is necessary to expand the available bandwidth to further increase the capacity. In the current optical communication system, the bandwidth is determined by the gain band of a conventional erbium-doped fiber amplifier (EDFA), which covers only the 1550-nm band.

On the contrary, parametric interaction in nonlinear optical media enables amplification in a new wavelength range and wavelength conversion into an undeveloped range. Recent advances in periodically poled LiNbO₃ (PPLN) waveguide technologies have enabled various parametric interactions with high efficiency [2,3]. For instance, a broadband optical parametric amplification (OPA) covering 1.49 to 1.61µm band was demonstrated using cascaded second harmonic generation (SHG) and difference frequency generation (DFG) in a mode-matched PPLN waveguide [4]. However, when using the cascaded SHG and DFG approach in a conventional PPLN waveguide, further expansion of the operation wavelength cannot be expected as the same phase-matching condition is utilized in both the SHG and DFG processes. Another approach to obtaining a broadband OPA is to use a PPLN waveguide design to provide group velocity matching between the pump and signal/idler [5]. Although broadband OPA of 2 µm signal was demonstrated, this approach is difficult to apply in an arbitrary wavelength range.

In this paper, we propose a new configuration that enables pump generation through SHG and wavelength conversion/amplification through DFG or OPA in a single device. In this configuration, a PPLN waveguide exhibiting multiple-quasi-phase-matched (M-QPM) peaks is used in a bidirectional manner to enable operation in a new wavelength range other than in the 1550-nm band [6,7]. The M-QPM device enables us to utilize individual phase-matching conditions for the SHG and DFG/OPA processes. We demonstrate wavelength conversion between the 1.4- and 1.6-µm bands. In addition, we also demonstrate that the parametric gain band can be changed using various detunings of the phase-matched peaks used in the SHG and DFG/OPA processes.

2. Principle of operation

Figure 1 illustrates the basic idea of our wavelength conversion process using an M-QPM device, which can be fabricated by utilizing the spatial phase modulation of the periodic grating in the PPLN [5]. We employed a device that exhibited multiple QPM peaks with 0.8 nm spacing in the 1.55-µm band when we examined the QPM condition through the SHG tuning curve. We injected 1.55-µm pump light into the M-QPM device, which was converted into second harmonic (SH) light at approximately 0.775 µm through the SHG process. Thereafter, we input the SH light along with another signal light in the 1.4-µm band into the same device to generate a 1.6-µm idler light using the DFG process. If the idler intensity is sufficiently high, the OPA gain of the signal and the idler becomes appreciable. The pump wavelength is tuned to match a QPM peak, whose wavelength is shorter than that of the QPM peak used for the DFG/OPA process. Using separate QPM peaks for the SHG and DFG/OPA processes, the gain for the parametric interaction can be expanded to a wavelength range outside the 1550-nm band.

 

Fig. 1. Schematic of wavelength conversion process.

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Figure 2 presents examples of the parametric gain curves calculated for several SH pump wavelengths. In this calculation, the refractive index dispersion of a bulk LiNbO₃ crystal was used, and the SHG QPM wavelength was assumed to be 1550 nm [8]. As indicated in Fig. 2, by detuning the SH pump wavelength to less than half of the SHG QPM wavelength, the gain wavelength range can be controlled to cover the 1300–1800 nm range, which is a low-loss window of the optical fiber. This phenomenon can be explained as follows.

 

Fig. 2. Parametric gain curves estimated using several SH pump wavelengths.

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The difference in the propagation constant between the SH pump, signal, and idler is expressed as follows:

Here, λ_SH, λ_s, and λ_i are the wavelengths of the SH pump, signal, and idler, respectively, and n_SH, n_s, and n_i are the effective refractive indices at each wavelength, respectively.

In the M-QPM device, which is based on a χ⁽²⁾ grating with a period of Λ and phase modulation period of Λ_ph, Δβ is compensated under multiple conditions as follows [7,9]:

Here, n is an integer that corresponds to the order of the multiple peaks [9].

Let us consider the variation in the propagation constant when we change the wavelength of the signal with a fixed SH pump wavelength. Figure 3 illustrates the effect of material dispersion on the phase matching characteristics. Propagation constant of LiNbO₃ was calculated assuming a bulk LiNbO₃ crystal [8]. If the wavelengths of the signal and idler are close to one of the SHG QPM wavelengths as illustrated in Fig. 3(a), the variation in the propagation constant at the signal wavelength is compensated by that at the idler wavelength. Therefore, the QPM condition can be maintained even if the signal wavelength is changed. Therefore, we can obtain a broadband parametric gain when the SH pump wavelength is tuned to half of the SHG QPM wavelength. This tuning corresponds to setting the SH pump wavelength to 775 nm, as depicted in Fig. 2.

 

Fig. 3. Effect of material dispersion on the phase matching characteristics. The red line indicates the calculated propagation constant β of LiNbO₃ and the blue line indicates the value 2β. β_SH, β_s and_. β_i are the propagation constants at the SH pump, signal, and idler wavelength, respectively. (a) Wavelengths of the signal and the idler are close to the SHG QPM wavelengths (b) Wavelengths of the signal and the idler are further separated from the SHG QPM wavelength

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In contrast, if the wavelengths of the signal and the idler are further separated from the SHG QPM wavelength, as illustrated in Fig. 3(b), the variation in the propagation constant at the signal wavelength is larger than that at the idler wavelength because of the dispersion of LiNbO₃. Therefore, the QPM condition cannot be maintained at a fixed SH pump wavelength. However, the QPM condition can be satisfied by detuning the SH pump to a shorter wavelength to compensate for the variation in the propagation constant at the signal wavelength, as illustrated in Fig. 3(b). As depicted in Fig. 2, to realize the parametric interaction between the signal and the idler with further separation, we must detune the SH pump wavelength from half of the QPM wavelength to a shorter wavelength. To generate a detuned SH pump from a 1550-nm band laser, we can utilize a different QPM condition of the M-QPM device for the SHG process. More importantly, the detuning can be changed by using different phase modulation periods to cover the different ranges of signal and idler wavelengths.

3. Experiment

To confirm this principle, we conducted an experiment using the setup illustrated in Fig. 4. A 1.54-µm pump was generated from an external cavity laser (ECL), amplified through an EDFA, and injected into the M-QPM module. The M-QPM waveguide was fabricated using direct bonding and dry etching techniques [10]. Periodic poling was applied to a z-cut Zn-doped LiNbO₃ substrate. The poling period was 17.4 µm, and the phase modulation period was 8.67 mm. A phase modulation function for the three QPM peaks was employed [7]. The periodically poled substrate was bonded to a z-cut LiTaO₃ substrate. The thickness of the Zn:LiNbO₃ layer was reduced to 4 µm by polishing, and a 5 µm wide ridge waveguide was formed by etching. No top cladding was formed. The waveguide was 43 mm long. The M-QPM waveguide was assembled in a fiber pigtail module [3]. The total fiber-to-fiber transmission loss at 1.55 µm was 5 dB.

 

Fig. 4. Experimental setup.

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Through the SHG process, the pump was converted into a 0.77-µm SH pump. This SH pump was then redirected using a dichromatic mirror and propagated through a 0.85-µm polarization-maintaining fiber (PMF) attached to a gold-coated mirror. The light was reflected, and it traveled back through the same route and propagated through the same device along with the 1.4-µm-band signal light launched from another ECL. Owing to the DFG process between the SH pump and the signal, a 1.6-µm band idler was generated. The optical circulator allowed us to divide the pump and the signal/idler. The signal power was set at approximately 5 to 10 dBm to partially compensate for the wavelength-dependent loss of the circulator, which was designed for 1.55 µm. This configuration enables the utilization of the SHG and DFG/OPA processes in a single device and precise detuning between the SH pump and QPM wavelengths for the DFG/OPA process. The output light was analyzed using an optical spectral analyzer (OSA). To confirm that the idler has sufficient signal quality for data transmission, we performed bit error rate (BER) measurements. The signal light at the 1.4 µm band was modulated to a 20-Gbit/s QPSK format using an LiNbO₃ IQ modulator. Two sequences of a 10-Gbaud binary signal, generated by a pseudorandom binary sequence (PRBS) generator, were used to drive the IQ modulator. The signal was processed through the wavelength conversion process using an M-QPM device, and a 1.6-µm band idler was generated. Owing to the coherent nature of the DFG process, the idler preserves the phase information of the signal. The idler is extracted using a band-pass filter (BPF). The intensity of the idler was controlled using an optical attenuator and pre-amplified using an L-band EDFA. The amplified signal passes through a BPF and is demodulated using a delayed interferometer and balanced photo diode (PD). The BER was measured using an error detector.

4. Results and discussion

The SHG phase-matching curve of the M-QPM device was measured using an ECL. As shown in the inset in Fig. 5, the device exhibited three QPM peaks of approximately 1536 nm with spacings of 0.8 nm. This characteristic allowed us to conduct the experiment with a detuning of 0.8 and 1.6 nm between the pump and SHG QPM wavelengths for the DFG/OPA process.

 

Fig. 5. Spectra of the signal and idler measured under various detuning conditions: (a) 0.8 nm, (b) 1.6 nm. The inset shows the measured SHG phase matching curve and the locations of the peaks utilized for the SHG and DFG processes.

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Figure 5(a) depicts the superimposed output spectra with a detuning of 0.8 nm. During the experiment, the EDFA output power was set to 30 dBm. In this case, a 1.4-µm signal light was scanned from 1447 to 1489 nm. The pump wavelength matched the QPM peak with the shortest wavelength. We successfully demonstrated the conversion between the 1.45-µm band signal and the 1.62-µm band idler. We conducted an identical experiment with the pump wavelength matched to the central QPM peak and obtained similar results. Figure 5(b) depicts the output spectra using a detuning of 1.6 nm. In this case, 1.4-µm signal light was scanned from 1410 to 1440 nm. Wider detuning enables parametric interaction between the signal and idler with a wider separation. We successfully demonstrated the conversion between the 1.41-µm band signal and the 1.68-µm band idler.

Figure 6 depicts the wavelength dependence of the net conversion efficiency, which is calculated using the ratio of the signal and idler output intensity. We carried out the conversion only from the 1.4 to 1.6 µm band; however, the efficiency of the reverse process is theoretically the same. Therefore, we plotted the measured efficiency as a function of the idler wavelength and extrapolated it as a function of the signal wavelength to understand the wavelength dependence of the parametric gain. The maximum conversion efficiencies were −3.84 dB with 0.8 nm detuning and −3.87 dB with 1.6 nm detuning, respectively.

 

Fig. 6. Parametric gain measured and extrapolated as a function of the signal/idler wavelength.

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The parametric gain bandwidth tends to narrow down as detuning increases. This phenomenon is caused by the dispersion of LiNbO₃, which is in good agreement with the curves depicted in Fig. 2.

By further improving the conversion efficiency of the M-QPM device and reducing the loss of SH light reflection, an appreciable OPA gain of the signal and idler can be expected.

Figure 7 depicts the BER performance of the idler. We also conducted the BER measurements without any wavelength conversion using a signal of 1.55 µm/1.60 µm modulated with the same modulator and C-/L- band EDFA as a preamplifier. The results of these back-to-back measurements are plotted in Fig. 6. The power penalties caused by the wavelength conversion process are negligible. This implies that the signal-to-noise ratio degradation due to the parametric fluorescence is negligible within the experimental conditions considered in this study.

 

Fig. 7. Measured bit error rate and performance.

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As the differential detection employed in this study is highly resistant to phase noise, further studies will be required on the deterioration of the more sophisticated modulation signals such as the pure phase-shift keying (PSK) and the quadrature amplitude modulation (QAM) owing to the phase noise of the pump laser.

5. Conclusions

In this study, we successfully demonstrated wavelength conversion from 1.4 to 1.6 µm using an optical parametric process inside an M-QPM device. The bidirectional operation of the M-QPM device enables the utilization of the SHG and DFG/OPA processes in a single device. The maximum conversion efficiency was −3.84 dB. We also demonstrated that the parametric gain band can be altered by varying the detuning between the pump and QPM wavelengths used for the DFG/OPA process. We confirmed that the parametric process preserves sufficient signal quality for data transmission. By further improving the conversion efficiency, an appreciable gain of the OPA process can be expected. We believe that this technology will be useful for expanding the transmission capacity of future optical transmission systems.