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A 16 Gb/s four-level pulse amplitude modulation (PAM4) underwater wireless optical communication (UWOC) system based on 488-nm laser diode (LD) with light injection and optoelectronic feedback techniques is proposed and successfully demonstrated. Experimental results show that such a 1.8-GHz 488-nm blue light LD with light injection and optoelectronic feedback techniques is enough forceful for a 16 Gb/s PAM4 signal underwater link. To the authors’ knowledge, this study is the first to successfully adopt a 488-nm LD transmitter with light injection and optoelectronic feedback techniques in a PAM4 UWOC system. By adopting a 488-nm LD transmitter with light injection and optoelectronic feedback techniques, good bit error rate performance (offline processed by Matlab) and clear eye diagrams (measured in real-time) are achieved over a 10-m underwater link. The proposed system has the potential to play a vital role in the future UWOC infrastructure by effectively providing high transmission rate (16 Gb/s) and long underwater transmission distance (10 m).
© 2017 Optical Society of America
1. Introduction
Underwater wireless optical communication (UWOC) system is utilized to transmit data in an unregulated water surroundings via laser light. Compared with acoustic and radio frequency systems, UWOC system has greatly higher transmission bandwidth, thus providing higher transmission rate to a certain extent. Hence, UWOC system has attracted much attention in recent years due to such high transmission rate trait [1–6]. With the speedy progress in UWOC system, increasing demands drive the requirements for long-range and high-speed underwater links. For the actual implementation of an UWOC system, long-range and high-speed underwater links are the key concerns of system designers. A UWOC system has numerous applications in oceanography, environmental monitoring, and underwater surveillance. Blue light around the 488 nm is close to the minimum of absorption in water [7]. Therefore, a UWOC system that employs a 488-nm blue light laser diode (LD) with high optical output power is anticipated to provide a long-range underwater link. Light injection and optoelectronic feedback techniques had been adopted in different lightwave transport systems to improve the transmission performance [8,9]. Nevertheless, these techniques have not yet been adopted to improve the transmission performance of the four-level pulse amplitude modulation (PAM4) UWOC system. LD with light injection technique exhibits an enhancement in frequency response, and the optoelectronic feedback technique can further enhance the frequency response. Therefore, light injection and optoelectronic feedback techniques are attractive to provide a high-speed underwater link in a PAM4 UWOC system.
In this paper, a 16 Gb/s PAM4 UWOC system based on 488-nm blue light LD with light injection and optoelectronic feedback techniques is proposed and experimentally demonstrated. Compared with non-return-to-zero (NRZ) signal, PAM4 signal provides two bits in each symbol with the advantages of high transmission rate and less bandwidth requirement. PAM4 transmission is thereby regarded as one of the key solutions for establishing a high-speed UWOC system [10,11]. Good bit error rate (BER) performance (offline processed by Matlab) and clear eye diagrams (measured in real-time) are achieved at a 10-m underwater operation by employing a 488-nm LD transmitter with light injection and optoelectronic feedback techniques. The feasibility of employing the two-stage injection-locked technique to establish an 8 m/9.6 Gbps 16-QAM-OFDM UWOC system based on 405-nm LD with two-stage injection-locked technique had demonstrated formerly [1]. Nevertheless, a sophisticated two-stage injection-locked technique is required and this technique increases the complicacy of systems. LD with light injection and optoelectronic feedback techniques is a feasible scheme by which only an appropriate one-stage injection-locked technique is needed. Compared with the two-stage injection-locked technique, it is attractive because it avoids the need for two-stage injection locking with sophisticated wavelength detuning. And further, the 8 m/9.6 Gbps are less than the corresponding values of 10 m/16 Gb/s adopted in the proposed PAM4 UWOC system. A 5.4 m/4.8 Gbit/s UWOC system based on compact 450-nm laser was presented in another previous study [2]. However, the transmission distance of 5.4 m and the transmission rate of 4.8 Gbit/s are much less than the corresponding values of 10 m and 16 Gb/s adopted in the proposed PAM4 UWOC system. This proposed 16 Gb/s PAM4 UWOC system based on 488-nm LD with light injection and optoelectronic feedback echniques is demonstrated to be superior over the prior systems because of its feasibility for long-range and high-speed underwater links.
2. Experimental setup
Figure 1 shows the experimental configuration of the proposed 16 Gb/s PAM4 UWOC system based on 488 nm LD with light injection and optoelectronic feedback techniques. Two binary pseudorandom bit sequence (PRBS) data streams with aligned clock at a length of 215−1 at 8 Gb/s are generated from a two-channel PRBS generator. The amplitudes of the binary data streams are 1 and 0.5 V, respectively. These two 8 Gb/s NRZ signals are fed into a PAM4 converter to create a 16 Gb/s PAM4 signal with four levels and three independent eye diagrams. After electrically amplified by a linear driver, the PAM4 signal is sent into the LD1. The optical output of LD1 (488 nm) is injected into LD2 (487.98 nm) via an optical isolator, an optical coupler, and a polarization controller (PC). A part of the laser light is utilized for feedback through an optoelectronic feedback loop. The photodiode (PD) and the linear trans-impedance amplifier (TIA) convert the laser light into a 16 Gb/s PAM4 signal to directly modulate the LD2. Another part of the laser light is utilized for underwater optical signal transmission. The light emitted from the fiber collimator at the transmitter side is inputted into the convex lens with a focal length of 25.4-mm, delivered and reflected four times in water, fed into the convex lens with a focal length of 50-mm, and focused on the fiber collimator at the receiver side. The fiber collimators connected to fibers play significant roles in forming an optical beam to deliver optical signals through the underwater channel between the transmitter and the receiver sides. The fiber collimator has an operating wavelength range of 350-700 nm, a divergence of 0.041°, and a focal length of 4.6 mm. The function of the convex lens at the transmitter side is to generate a collimated optical beam, whereas its function at the receiver side is to guide the optical beam into a point.
Fig. 1 The experimental configuration of the proposed 16 Gb/s PAM4 UWOC system based on 488 nm LD with light injection and optoelectronic feedback techniques.
The dimensions of the water tank are 2.5 m × 1.5 m × 0.8 m. The water tank is filled with piped water with an attenuation coefficient of 0.059 m−1 (at 488 nm) [12]. Light transmission distance passing through the water tank is extended to 10 m (2.5 m × 4) by using mirrors on both sides of the water tank. Over a 10-m underwater link, the laser light afterwards reaches a commercial unit of PD with TIA (DSC-R402PIN), with a 3-dB bandwidth of 10 GHz. The PD presents a detection wavelength range of 320–1000 nm and a responsivity of around 0.43 mA/mW (at 488 nm). The link performances of the proposed 16 Gb/s PAM4 UWOC system are analyzed in real-time in terms of eye diagram, and offline processed by Matlab in terms of BER performance. The eye diagram of the transmitted 16 Gb/s PAM4 signal is seized by a digital storage oscilloscope (DSO) at the receiver side. The BER performance of the transmitted 16 Gb/s PAM4 signal is processed offline through the processes of synchronization, equalization, and hard decision.
The measurement setup of the frequency response of the 488 nm LD-based PAM4 UWOC system is also presented in Fig. 1. RF sweep signal (DC – 10 GHz) generated from a network analyzer is sent to the LD1. After PD detection and TIA amplification, the RF sweep signal is supplied to the network analyzer. Thus, the frequency response of the 488 nm LD-based PAM4 UWOC system is measured under the conditions of free-running as well as light injection and optoelectronic feedback.
3. Experimental results and discussions
The output optical power of blue light LD under various operation currents is shown in Fig. 2(a). Given an operation current higher than 30 mA, the optical output power is proportional to the operation current. Given an operation current of 70 mA, a maximum optical output power of 20 mW is acquired. The overall light attenuation effects with regard to absorption and scattering in underwater surroundings is given by [13]:
where I0 is the optical power of transmitted light, a(λ) is the term regarding the absorption of water, s(λ) is the term regarding the scattering of water, z is the light transmission distance in the water, and I is the optical power of light after transporting z distance. In piped water, absorption is the main limiting factor, the low scattering coefficient makes the optical beam free from divergence. And thus the Eq. (1) can be changed into:Equation (2) shows that as the light transmission distance increases, the optical power of light after transmitting z distance decreases. The LD should be operated as high as possible to acquire high optical output power for long underwater link. The operation current of LD is biased at 70 mA to acquire a maximum light transmission distance in the water. The measured optical spectra at 25°C at different operation currents are shown in Fig. 2(b). The peak emission wavelengths are 487.14 nm, 487.56 nm, and 488 nm under operation currents of 35 mA, 55 mA, and 70 mA, respectively. Obviously, the peak emission wavelength is slightly shifted to a longer wavelength with the increasing operation current.Fig. 2 (a) The output optical power of blue light LD under different operation currents. (b) The measured optical spectra at 25 °C at different operation currents.
The injection locking behavior takes place when a master laser is detuned to a wavelength slightly longer than that of the slave laser. The optimal injection locking condition is observed when the wavelength detuning between the two LDs is 0.02 nm (λLD1 - λLD2 = 0.02). The optical spectrum slightly shifts to a longer wavelength when LD2 is injection-locked, as presented in Fig. 3. Moreover, the laser resonance frequency f0 can be expressed as [14]:
whereis the gain coefficient, P is the photon density, and is the photon lifetime. Light injection and optoelectronic feedback greatly increase the photon density, by which resulting in the improvement of laser resonance frequency. The frequency response of the LD-based PAM4 UWOC system under the scenarios of free-running, light injection, as well as light injection and optoelectronic feedback are presented in Fig. 4. The 3-dB bandwidths are 1.8 GHz, 3.8 GHz, and 8.2 GHz for the scenarios of free-running, light injection, as well as light injection and optoelectronic feedback. It can be seen that light injection and optoelectronic feedback techniques provide a feedback loop gain around 2.16 times (8.2/3.8 ~2.16), compared with the scenario of light injection technique. In addition, light injection and optoelectronic feedback techniques significantly enhance the frequency response of the LD-based UWOC system to 4.56 times (8.2/1.8 ~4.56), meaning that such a 1.8-GHz LD with light injection and optoelectronic feedback technique is potent for 16 Gb/s (8 Gbaud/s) PAM4 signal underwater link.Fig. 3 The optical spectra of LD2 under the scenarios of free-running as well as light injection and optoelectronic feedback.
Fig. 4 The frequency response of the LD-based PAM4 UWOC system under scenarios of free-running as well as light injection and optoelectronic feedback.
Figure 5 illustrates the block diagram of the decision-feedback equalizer (DFE). For the DFE, x is the input, cn is the feedforward coefficient, bn is the feedback coefficient, y is the summation of the weighted taps, d is the decision output, and e is the error. We sample the signal at instant t0 + kT (t0 is the initial time and T is the signaling interval), the summation of the weighted taps y is [15]:
where n is an integer from 0 to N-1 (n = 0, 1, 2, ……, N-1). The error e is determined by the sum of weighted taps y and the decision output d:The channel responses update the feedforward coefficient cn and the feedback coefficient bn gradually: where u is the step size. The error e repeatedly updates the cn and bn coefficients and thus adaptively makes up for for the received PAM4 signal as the output of the DFE. The hard decision after the DFE (as illustrated in Fig. 1) are conducted to the equalized signal. It should be noted that both DFE and hard decision are made to obtain good BER performance.The BER curves of the 16 Gb/s PAM4 LD-based UWOC system over different underwater links are shown in Fig. 6. The BER value increases as the underwater link increases. At a 10−9 BER operation, a 1.7-dB power penalty can be observed between the scenarios of over a 5-m underwater link and over a 10-m underwater link. As the underwater link is longer than 10 m, the BER value is higher than 10−9. Over a 15-m underwater link, the BER value degrades to 10−5 due to the decline of optical signal-to-noise ratio (OSNR). Over a 20-m underwater link, the BER value degrades to 10−2 because of the significant decline of OSNR. When the underwater link increases, the received OSNR decreases because of the great attenuation of optical power. Such OSNR decrease leads to the degradation of BER performance. As the underwater is shorter than or equal to 10 m, the BER performance degradation due to the decline of OSNR is acceptable. The maximum underwater link by which 10−9 BER operation can be obtained is about 10 m. However, as the underwater link is longer than 10 m, the BER performance degradation due to the decline of OSNR is unacceptable. Over a 20-m underwater link, the received optical power at the receiver side is about 2 dBm to compensate the great decline of OSNR. Nevertheless, it can be seen that the compensation is restricted by which only 10−2 BER operation is obtained.
The eye diagrams of the 16 Gb/s PAM4 signal over different underwater links are shown in Figs. 7(a)–7(d), respectively. The qualities of the 16 Gb/s PAM4 signal are observed for the scenarios of over a 5-m underwater link [Fig. 7(a)] and over a 10-m underwater link [Fig. 7(b)]. Over a 15-m underwater link [Fig. 7(c)], amplitude and phase variations can be observed by a large amount. Over a 20-m underwater link [Fig. 7(d)], however, close eye diagrams exist obviously.
Fig. 7 The eye diagrams of the 16 Gb/s PAM4 signal over a (a) 5-m, (b) 10-m, (c) 15-m, and (d) 20-m underwater link.
4. Conclusions
A 16 Gb/s PAM4 UWOC system based on a 488-nm blue light LD transmitter with light injection and optoelectronic feedback techniques is proposed and experimentally demonstrated. Results show that a 1.8-GHz LD with light injection and optoelectronic feedback techniques is potent for a 16 Gb/s PAM4 signal underwater link. Over a 10-m underwater link, good BER performance (offline processed by Matlab) and clear eye diagrams (measured in real-time) are obtained to construct a high-quality PAM4 UWOC system. Such an innovative PAM4 UWOC system has the advantages of long-range and high-speed underwater links, which is an attractive trait that can accelerate the deployment of the PAM4 UWOC system.
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