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Abstract
In this paper, we propose an optical module, consisting of an Erbium/Ytterbium co-doped fiber amplifier (EYDFA) and a cascaded periodically poled lithium niobate (cascaded-PPLN), to bridge the conventional telecommunication and the emerging underwater wireless optical communication (UWOC). Compared with using two discrete crystals to achieve the third harmonic generation (THG), using a cascaded crystal simplifies the optical system. Under a fundamental power of 5 W at 1550 nm, we have generated an optical power of 6.54 mW at 516 nm, corresponding to a conversion efficiency of 0.1308%. Furthermore, we added a 5-km single-mode fiber (SMF) before the EYDFA, and by adjusting the seed laser power, we successfully maintained the efficiency of the THG process and the output power of the green light. Afterwards, the nonlinearity of the THG process is analyzed, and a simplified nonlinear pre-compensation method has been proposed to tailor the 4-pulse amplitude modulation (PAM4) signals. In such case, the bit error rate (BER) of the modified PAM4 (m-PAM4) can reduce by 69.3% at a data rate of 12 Gbps. Finally, we demonstrate the practicality of our proposed system by achieving a 7-m UWOC transmission in a water tank at a data rate of 13.46 Gbps in an optical dark room. This result demonstrates the feasibility of the hybrid fiber/UWOC system, highlighting its potential for practical implementation.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Underwater wireless optical communication (UWOC) has attracted increasing attention from both academia and industrial societies in recent years, owing to its advantages of low latency, high bandwidth and compact structure compared with underwater acoustic communication [1–3]. Laser diodes (LDs) in blue and green bands are widely used as the transmitters of UWOC systems, as the wavelength from 350 nm to 580 nm lies in the absorption minimum for shallow to mid-depth ocean waters [4]. Nevertheless, even at the low-loss blue/green bands, the long-distance transmission of light underwater is still a great challenge due to the turbulence, scintillation, attenuation and geometric spreading of the optical beam [5,6]. In free space optical (FSO) communication, the hybrid fiber/FSO systems were proposed to concur these obstacles in atmospheric channels on optical transmission [7,8], and similar schemes can also be used in UWOC. For the similarity on absorption spectrum between plastic optical fibers (POFs) and water, hybrid POF/UWOC systems were reported to realize longer underwater optical links, especially suitable for harsh underwater channels [9,10]. However, the data rates in these hybrid POF/UWOC systems were still limited by the bandwidth of blue/green LDs like other UWOC systems. Many efforts have been made to increase the data rates of blue/green LD-based UWOC systems, both in terms of hardware and software [11–16].
In fact, the bandwidth of optical communication systems with external modulation is normally higher than those with direct modulation [17]. Compared with LDs working in near-infrared, the relative immaturity of directly-modulated blue-green LDs and the lack of high-speed external modulators in visible range lead to the limitation on the bandwidth of UWOC transmitters. However, in the near-infrared range, the use of the master oscillator power amplifier (MOPA) structure ensures a high modulation bandwidth with a high output power [18,19], and many studies have combined it with second harmonic generation (SHG) to design high-speed UWOC transmitters. In as early as 2008, a MOPA system was built with a 1064-nm seed laser and an Ytterbium-doped fiber amplifier (YDFA), obtaining an 8.6-dBm green-light output through a PPLN crystal and achieving a data rate exceeding 1 Gbps in a 2-m underwater channel [20]. With high-order modulation formats, the data rates of the MOPA-SHG systems could be further increased beyond 10 Gbps [21,22].
The aforementioned UWOC systems based on SHG have primarily used fundamental light sources at 1064 nm, which was rarely used in optical fiber communication (OFC) due to the relatively high loss in fiber. In fact, through a third-order optical nonlinear process, green light can be directly obtained from a fundamental light of 1550 nm, thus naturally linking OFC with UWOC. In 2007, the wavelength conversion from C band to green band was realized in a photonic crystal fiber based on degenerate four-wave mixing. The maximum green-light power was −7 dBm and a data rate of 1.5 Gbps was achieved [23]. Besides, such kind of wavelength conversion can also be realized by utilizing the third harmonic generation (THG). In 2007, a picosecond pulsed MOPA light source was designed and the wavelength conversion from 1555 nm to 518 nm was realized by using two discrete PPLN crystals. The two crystals had different MgO-doping concentrations and polling periods, which were used for SHG and sum frequency generation (SFG), respectively. The MOPA system achieved a data rate of 65 Mbps, with an average green-light power of 140 mW, making it suitable as a transmitter for UWOC [24]. The conversion efficiency of the THG process depends on the power density of fundamental light. While using pulsed sources provides higher power density, the bandwidth of the transmitter is limited. In 2016, higher power density was achieved by using two PPLN waveguides doped with Zn and MgO, respectively, obtaining a 290-mW green-light power with a continuous-wave (CW) fundamental light of 800 mW, corresponding to a THG conversion efficiency of 36% [25]. Instead of using separate crystals, cascaded crystals can also be employed to implement the THG process, and furthermore, to simplify the transmitter. In the study presented in [26], the THG process using cascaded-PPLN and cascaded periodically poled lithium tantalate crystals were illustrated, where the front and rear sections of the crystals had different polarization periods for SHG and SFG, respectively.
In this paper, we constructed a THG module based on an Erbium/Ytterbium co-doped fiber amplifier (EYDFA) and a cascaded-PPLN for the conversion from OFC to UWOC and investigated the temperature sensitivity and wavelength sensitivity of the module. Additionally, a 5-km single-mode fiber (SMF) was inserted before the module and, by adjusting the seed laser power and the polarization state, we maintained the efficiency of the THG process and the output power of the green light after the fiber insertion. In order to simplify the experimental setup, the 5-km SMF was removed in the underwater testing section. Then, the nonlinearity introduced by the THG process was discussed and a simplified method to compensate for the nonlinearity was proposed. Finally, a UWOC system was built in a 7-m water tank, achieving a data rate of 13.46 Gbps. To the best of our knowledge, this is the first experimental investigation of a fiber/UWOC system based on THG process and nonlinear pre-compensation.
The remainder of the paper is organized as follows. In Section II, we introduce the design of the transmitter, including the nonlinearity-compensation method. Section III describes the implementation details of the proposed UWOC system. The experimental results are presented and analyzed in Section IV. Finally, conclusions are drawn in Section V.
2. Design of UWOC transmitter
2.1 Structure of the module
A 5% MgO-doped cascaded-PPLN crystal, with a 15-mm long SHG section and a 15-mm long SFG section, is used to achieve the THG process, as shown in Fig. 1. To fulfill the Quasi-phase-matching condition for an extraordinary polarized (e-polarized) fundamental light of 1550 nm at 30 °C, the poling periods of the two sections are Λ1 = 19.37 µm and Λ2 = 7 µm, respectively. The transverse dimensions of the cascaded-PPLN are 2 mm × 0.5 mm and both end faces of the crystal are anti-reflection coated (R ≤ 0.2% @ 1550 nm & 516 nm).
For THG process, compared with the case of two discrete crystals, using a cascaded-PPLN makes the optical system more compact. The configuration of the transmitter is shown in Fig. 2. A tunable CW laser (EXFO FLS-2800), with a 13-dBm maximum optical power and a 10-kHz linewidth, is used as the seed laser. The electrical signal was generated by an arbitrary waveform generator (AWG, Tektronix AWG70002A) and its amplitude was adjusted by a power amplifier (AMP, SHF 100 AP) and an electrical attenuator (EA, 6 dB). After being modulated by a Mach-Zehnder Modulator (MZM, Sumitomo T.DKH1.5-10PD-ADC), the signal light was transmitted through a 5-km standard SMF and amplified by a benchtop EYDFA (saturated power 37 dBm) and then was collimated by a fiber collimator (Thorlabs F220APC-1550). Two 3-paddle polarization controllers (PC) were set before the MZM and EYDFA, respectively. PC1 was used because the MZM was polarization sensitive, and PC2 was used to set the light polarization along the extraordinary axis of the crystal, for obtaining the maximum THG efficiency. The collimated fundamental light was focused by a cemented doublet lens (L1) with a focal length of 50 mm, which could correct the spherical aberration [27] to realize a smaller beam waist. A plano-convex lens (L2) with a focal length of 100 mm was placed 150 mm behind L1, realizing a simple Keplerian beam expander and the crystal was placed at the focus. L1 and L2 were anti-reflection coated at C band and visible region, respectively. Different wavelengths were spatially separated by an equilateral dispersive prism (Thorlabs PS859) which had a minimum reflectance of 2% for parallel-polarized (p-polarized) beams. The cascaded-PPLN was placed with a 90° rotation around its horizontal axis, so the e-polarization of the crystal could match the p-polarization of the prism. A temperature controller (TC) was used to ensure the crystal work at its optimal temperature.
Fig. 2. Configuration of the transmitter. Insets: (i) the seed laser, (ii) the 5-km SMF, (iii) the EYDFA, (iv) the THG module, (v) the crystal with a 90° horizontal rotation placed on TC and (vi) the OFC to UWOC module.
In an ideal SFG process, the photon ratio of 1550 nm and 775 nm was expected to be 1:1, but the optical power of 775 nm is orders of magnitude lower than that of 1550 nm in the proposed configuration. Therefore, to obtain a higher output at 516 nm, a higher optical power at 775 nm was required, which was realized by placing the SHG section of the crystal at the focal point. The optical power at 516 nm was measured as the crystal moved back and forth on the optical axis, and it was found that at the optimal position, the focal point of the beam approximately located at 10 mm from the front end of the crystal. The beam diameter along the optical axis was measured to be 71 µm at the focal point and 343 µm and 672 µm at the front and back ends of the crystal, respectively, as shown in Fig. 3.
The frequency conversion efficiency of THG process in PPLN depends on poling periods, temperature and fundamental wavelength [25]. Due to the poling periods’ error in the crystal fabrication, there was a mismatch between the optimal temperature of the SHG and SFG process, and also the optimal wavelength. The dependence of the THG power on the temperature, under a fundamental power of 2 W, was measured by tuning the TC, and the result is reported in Fig. 4(a). The green power reached its maximum at 36.8 °C. Then the wavelength of the seed laser was tuned and the THG power fluctuation is record in Fig. 4(b), where the peak corresponds to 1550 nm.
Green-light powers at different 1550-nm powers are shown in Fig. 5. As the fundamental power increases, the frequency conversion efficiency becomes higher for both SHG and THG radiation. Under a fundamental power of 5 W, the output powers at 775 nm and 516 nm are 19.30 mW and 6.54 mW, respectively, corresponding to conversion efficiencies of 0.386% and 0.1308%. When the seed laser operated at 10 dBm, the optical power measured at the output of the MZM was about 4.8 dBm. In the case of incorporating a 5-km SMF to the module, an additional 3-dB loss was introduced. By adjusting the seed-laser power to 13 dBm and tuning the PC2, the conversion efficiency and the optical power for green light were successfully maintained. In order to facilitate the construction of the experimental system, the 5-km SMF was removed in section III.
2.2 Signal nonlinearity
The nonlinearity of the transfer curve from fundamental power to THG power might lead to signal distortion for high-order modulations. Furthermore, we also found that the output power at 516 nm can be significantly affected by the signal loaded to the MZM. To investigate this effect, experiments were conducted using OOK signals with different amplitudes at 1 GHz while maintaining an average 1550-nm power of 5 W, and the result is shown in Fig. 6. As the signal amplitude increases, the 516-nm power gets higher.
According to Fig. 5, the transfer function from fundamental light to THG light is similar to a convex function. For CW light, the output power at 516 nm can be simply expressed as:
where ${P_{THG}}$ and ${P_F}$ represent the output power at 516 nm and the fundamental power, respectively. For an ideal OOK signal, the fundamental power can be expressed as: where ${P_{avg}}$, ${P_{Min}}$ and ${P_{Max}}$ represent the average, the minimum and the maximum power of the fundamental light, respectively. Based on the properties of convex functions, the output powers at 516 nm satisfy the equations below:The nonlinearity mentioned above does not have much impact on OOK signals, but becomes more significant for high-order modulation formats. For the 4-pulse amplitude modulation (PAM4) format, lower voltage levels will get closer and upper levels will get farther in this process. In order to get even voltage levels at the receiver, the upper levels of PAM4 need to be set closer. For a conventional PAM4 signal with a peak-to-peak amplitude of 2 V, the original four voltage levels were −1.00 V, −0.33 V, 0.33 V and 1.00 V, respectively. According to Fig. 7, the voltage levels were modified to −1.00 V, −0.10 V, 0.50 V and 1.00 V, respectively, to compensate for the conversion nonlinearity during the THG process. For the sake of convenience, the PAM4 signal with modified voltage levels was denoted as m-PAM4. A preliminary experiment in the air channel using the transmitter mentioned above and a Si positive-intrinsic-negative photodiode (PIN, Alphalas UPD-50-SP) was demonstrated to verify the nonlinearity-compensation method. Furthermore, a second-order Volterra series-based nonlinear equalizer was employed to alleviate the nonlinear effects and to mitigate the inter-symbol interference (ISI) of the system [28,29]. The output of the nonlinear equalizer is expressed as:
Fig. 8. BER performance with different Baud rates for PAM4 and m-PAM4 signals with linear equalizer (w/L), with nonlinear equalizer (w/NL) and with 5-km SMF (w/SMF). Insets: the eye diagram of (i) the PAM4 signal at 4 GBd and (ii) the m-PAM4 signal at 4 GBd without SMF.
3. Experimental setup
The experimental setup of the proposed UWOC system is shown in Fig. 9. The experiment was conducted in an optical dark room to reduce the influence of the background light, and the 5-km SMF was removed for the sake of simplicity. At the transmitter end, non-return-to-zero on-off-keying (NRZ-OOK) and m-PAM4 signals were generated offline by a personal computer. The parameters of the signals are shown in Table 1. Then the signals were fed into an AWG to generate the electrical signals. An AMP and an EA were used to adjust the amplitudes of the signals, which was used to drive an MZM biased at its quadrature transmission point (linear area). The fundamental light, generated from the seed laser, was modulated by the MZM and amplified by an EYDFA to an average optical power of 5 W, before being fed into the THG module. The fundamental light and the SHG light were blocked out by a metal plate, whereas the modulated 516-nm light was transmitted in a 7-m water tank. At the receiver end, a variable neutral density filter (NDF) was used to adjust the optical power reaching the photodetector plane. Since the PIN had a small sensitive area with a diameter of about 100 µm (0.0079 mm2), a plano-convex lens with a focal length of 50 mm was employed to focus the light. Afterwards, the converted electrical signals were captured and recorded by a mixed-signal oscilloscope (MSO, Tektronix 71254C) with a sampling rate of 50 GSamples/s for further offline processing.
Fig. 9. Experimental setup of the UWOC system in the water tank. Insets: (i) the transmitter, (ii) the beam in the water tank and (iii) the receiver.
Table 1. Parameters of the experimental system.
The measured green power at the transmitter end with the signal on was 9.5 dBm. The attenuation of the water in the tank is about 0.534 dB/m according to Fig. 10.
4. Results and discussion
Figure 11 illustrates the normalized frequency response of the proposed system. The 20-dB bandwidth is about 4.3 GHz. The bandwidths of electrical and optical devices in infrared band of the transmitter are far beyond this value, so the system bandwidth should be limited by the PIN.
The BER performance with different Baud rates is shown in Fig. 12, and both linear and nonlinear equalizers were employed for comparison. As the data rate continues to increase, the ISI on the signal is gradually aggravated, resulting in an increase in the bit error rate. At relatively low data rates, there is a significant performance difference between linear and nonlinear equalizers, especially for m-PAM4 signals. In that case the nonlinearity introduced by the THG process is the main factor affecting the BER. At the hard-decision forward error correction (HD-FEC) threshold, the maximum Baud rates can reach 10.05 GBd and 6.73 GBd for NRZ-OOK and m-PAM4 signals, respectively, which are equal to 10.05 Gbps and 13.46 Gbps. High-order modulation formats suffer more from the nonlinearity caused by the THG process, and the nonlinear equalizer has a better performance. Compared with linear equalizer, the data rates for NRZ-OOK and m-PAM4 signals have increased by 3.9% and 5.7%, respectively, with the nonlinear equalizer.
Fig. 12. BER performance with different Baud rates of NRZ-OOK signals and m-PAM4 signals with linear (w/L) and nonlinear (w/NL) equalizers.
Also, the BER performance versus the received optical power has been studied, as shown in Fig. 13. Here, the Baud rates were set to 9 GBd and 6 GBd for NRZ-OOK and m-PAM4 signals, respectively. It can be seen that the minimum required received optical powers with nonlinear equalizer for NRZ-OOK and m-PAM4 signals are 3.5 dBm and 4.3 dBm, which are 0.6 dB and 0.7 dB lower than those with linear equalizer, respectively. The minimum received optical power at the HD-FEC threshold is relatively high compared with other UWOC systems with advanced sensitive detectors [14,19], but at an equivalent level with systems using the same PIN [30,31]. This is largely due to the tiny active area of the PIN. As the received optical power decreases, the BERs in all cases increase monotonically. Obviously, the system can achieve a better performance with a higher received optical power, which can be realized by using an EYDFA with a higher output.
Fig. 13. BER performance with different received optical powers of 9-GBd NRZ-OOK signals and 6-GBd m-PAM4 signals with linear (w/L) and nonlinear (w/NL) equalizers.
5. Conclusion
In this paper, we have designed a THG module based on a cascaded-PPLN, which can convert the carrier light from telecommunication to UWOC regime. The maximum power at 516 nm is 6.54 dBm under a CW fundamental power of 5 W, corresponding to a conversion efficiency of 0.1308%. In the case of connecting a 5-km SMF, an additional 3-dB loss was introduced and, by adjusting the seed-laser power and a PC, the optical power of green light was successfully maintained. The temperature and wavelength sensitivities of the module are also investigated, centering at 36.8 °C and 1550 nm, respectively. Besides, the nonlinearity caused by the THG process has been analyzed and a simplified nonlinearity-compensation method has been conducted. By using the m-PAM4 signal, the BER reduces by 69.3% compared with the conventional PAM4 signal at 12 Gbps in a preliminary experiment. Then, a data rate up to 13.46 Gbps over a 7-m underwater transmission link has been achieved with the proposed m-PAM4 signal by employing a Volterra series-based nonlinear equalizer. The minimum received optical powers under 9-Gbps NRZ-OOK and 12-Gbps m-PAM4 signals are measured to be 3.5 dBm and 4.3 dBm, respectively.
In our future work, an EYDFA with a higher output power will be constructed and a detailed theoretical modeling of the THG process will be conducted.
Funding
National Key Research and Development Program of China (2022YFB2903403, 2022YFC2808200); National Natural Science Foundation of China (61971378); Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22030208).
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.
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