Due to its bandwidth advantage, space laser communication has become an effective means to solve the bottleneck of microwave communication, build a space-based broadband network, and realize the real-time transmission of massive amount of earth observation data. The space laser communication terminal has characteristics of small size, lightweight, low power consumption, etc., which are suitable for satellite payload and meet the increasing communication needs of aerospace activities. In future, each communication satellite will carry multiple laser communication terminals that can serve multiple targets simultaneously. Therefore, laser communication terminals are being developed in the direction of miniaturization and integration. Traditional laser communication terminals use external modulation methods to achieve intensity or phase modulation of optical signals. Optical transmitters comprise multiple independent components, such as lasers, modulators, and bias controllers, and the system’s structure is complex. The phase modulation of the optical signal is realized using the direct modulation of the chirp-managed laser (CML), without an external modulator, bias controller, etc., with small size, low power consumption, low equipment complexity, and low cost. In addition, it can adapt to the continuous, high-speed, and integrated development of optical communication networks.
In this study, the chirp effect of the CML is used for phase modulation to generate a return-to-zero differential phase shift keying (RZ-DPSK) signal. RZ-DPSK has several advantages, such as high sensitivity, good reliability, simple receiver, and its receiving sensitivity is 3 dB higher than that of on-off keying (OOK) modulation method. It has received extensive attention in the engineering field. Using the chirp effect of the laser, the phase shift of the optical field is achieved by controlling the magnitude of the injected current, and the driving signal is simply pre-encoded using MATLAB to generate a three-level signal, thereby accurately controlling the phase change of the carrier signal. The error rate performance of RZ-DPSK estimated using this modulation method was tested and compared with that of the traditional external modulation method. The performance difference between the two methods was analyzed.
This study first uses the binary sequence "1110100" to verify the system principle. The schematic of the transmitter and receiver experimental schemes are shown in Figures 3 and 7, and the signal rate is 2.5 Gb/s. The output wavelength of the CML laser is 1552.544 nm, and the output optical power is 9.14 dBm. The receiving end includes erbium-doped fiber amplifier (EDFA), optical filter, optical delay interferometer, and balanced detector to receive and demodulate RZ-DPSK optical signal and restore the baseband electrical signal. To further reduce the spontaneous radiation noise caused by the amplification process, an optical filter with a bandwidth of 0.05 nm is placed after the EDFA. The signal waveform after demodulation is shown in Figure 8. 27-1 pseudo-random binary sequence (PRBS) is used for bit error rate test. The pseudo-random signal is demodulated by the delay interferometer, and the output signal eye diagram of the balanced detector is shown in Figure 9. As a comparative experiment, the receiving end based on LiNbO3 external modulation and the CML system use the same receiving device. The schematic of the two systems is shown in Figure 10. The bit error rate curves of RZ-DPSK system based on CML transmitter and LiNbO3 transmitter are shown in the Figure 11. When the system error rate is 10-9, the receiving sensitivity of CML and LiNbO3 transmitters is -36.98 and -45.72 dBm, respectively. Compared with the LiNbO3 transmitter, the sensitivity of the CML system is reduced by 8.74 dB. When the error rate of the forward error correction limit is 10-3, the sensitivity of the CML transmitter is -48.1 dBm, which is only 1.8 dB less than the -49.9 dBm of the LiNbO3 transmitter. The error characteristics of the two are the same, and thus, an error-free transmission can be realized. The CML transmitter has a simple structure, small size, and low power consumption, and the performance of the receiver is equivalent to that of external modulation when the limit error rate of the forward error correction is 10-3, which shows a significant development prospect.
This study introduces the principle of signal coding and modulation of CML laser and realizes the direct modulation of 2.5 Gb/s RZ-DPSK signal based on the CML laser without differential coding and external modulator. The performance index of the modulation signal is analyzed. At the same time, the bit error rate performance of the transmission system based on the CML laser and the system based on the LiNbO3 transmitter are compared. The results show that the sensitivity of the transmitter based on the CML is -48.1 dBm when the limit bit error rate of forward error correction is 10-3. Compared with the sensitivity of LiNbO3-based transmitter system (-49.9 dBm), the difference of the sensitivity of CML-based transmitter system is only 1.8 dB and the error characteristics are basically the same. Further, the CML-based transmitter system has a good transmission performance. In terms of hardware, the CML-based transmitter system has a simpler structure, low power consumption, small size, and lightweight, which can better adapt to the continuous high-speed and integrated development of space optical communication networks.
Lidar has been widely used in wind ranging, automatic drive and sensing mapping. The reflected light signal is obtained through first emitting a Gaussian beam from the laser source and then reflecting after reaching the surface of the object. After the computer analysis, the information of the object such as orientation, attitude and distance can be obtained. However, as for a lidar system, its laser source is an important unit influencing the performance of the whole system. A fiber laser has become the best choice of the light source for a lidar system, because of its good beam quality, high pulse energy and high repetition rate. At the same time, an erbium-ytterbium co-doped fiber has attracted the attention of many researches due to its advantages such as "eye-safe" and low atmospheric transmission loss. Therefore, as the most important gain medium for the laser lidar, polarization-maintaining erbium-ytterbium co-doped fiber has important research significance. In this paper, a 10 μm/128 μm polarization-maintaining erbium-ytterbium co-doped fiber is successfully fabricated by the modified chemical vapor deposition (MCVD) technique combined with the solution doping technology (SDT). The structural parameters and optical properties of this polarization-maintaining erbium-ytterbium co-doped fiber are measured. And its laser performance is also studied.
MCVD combined with SDT is used to fabricate the erbium-ytterbium co-doped fiber. The content (mole fraction) of P2O5 in the core is increased by more than 10% with reverse phosphorus doping and gas phase compensation. In order to avoid the defect of the core bursting during drilling, the fiber prefabricated rod is first annealed due to the high stress of its core. Through the Sagnac interferometer and the optical spectrum analyzer (OSA), the birefringence value is measured. The measurement structure is shown in Fig. 3. And the measurement structure of polarization extinction ratio is also shown in Fig. 5. In order to analyze the laser performance, the structure of an erbium-ytterbium co-doped fiber laser is shown in Fig. 6. The seed source has a power of 20 mW and a central wavelength of 1551 nm. An isolator (ISO) connected to the seed is used to protect the seed source. The isolator is followed by a (2+ 1)×1 forward pump combiner (PC), and one of its pump fiber is used to monitor the backward power and observe the backward spectrum. The 940 nm light generated by the laser diode (LD) is coupled to the active fiber through the pump fiber of the (2+ 1)×1 backward PC, and the pump power is 16.5 W. The coiling diameter of the active fiber is 10 cm. The cladding pump stripper (CPS) is implemented by coating a high refractive index adhesive to filter cladding light from the fiber. Finally, an isolator is fused at the end to prevent reflection.
The dimension of the fiber is shown in the inset of Fig. 2(b). The diameters of core and cladding are measured to be 10.19 μm and 128.69 μm, respectively. The diameter of the boron rod is measured to be 32.59 μm. Figure 2(b) shows the refractive index profile of the prefabricated rod. A numerical aperture of 0.24 is finally achieved. The absorption coefficient measured by the truncation method is 2.42 dB/m at 940 nm. The interference image at 1500-1600 nm is observed in the OSA (Fig. 4). The beat length at 1550 nm is calculated to be 9 mm with a birefringence coefficient of 1.29×10-4. At the same time, a polarization extinction ratio of 24 dB at 1310 nm is measured through the erbium-ytterbium co-doped fiber with a length of 4 m. As for the laser performance, due to the inherent loss, the final seed power coupling into the active fiber is 17.5 mW. Figure 7(a) shows the slope efficiency under different fiber lengths and pump powers. It can be seen from this figure that the optimal length is 7.5 m. When the pump power is 16.5 W, the output power and the slope efficiency reach the maximum, which are 5.8 W and 36%, respectively. The polarization extinction ratio is measured to be 21 dB. In addition, as shown in Fig. 7(b), the optical-to-optical efficiency tends to be saturated with the increase of pump power at different lengths. After reaching the saturation state, the optical-to-optical efficiency is more than 33% without a downward trend, which indicates that the laser power can be further increased at this time. The spectrum at 7.5 m is shown in Fig. 8. It can be observed from this output spectrum that the amplified spontaneous emission (ASE) power increases gradually with the increase of pump power, but the signal-to-noise ratio remains above 50 dB. From the backward spectrum, one can observe that the remaining pump light intensity is stable, which may be caused by the fact that some spiral pump light in the polarization-maintaining fiber is not absorbed by the fiber and the CPS is not added. Meanwhile, there is no parasitic oscillation at 1 μm. It shows that the polarization-maintaining erbium-ytterbium co-doped fiber prepared in this paper has good laser performances.
In this paper, a polarization-maintaining erbium-ytterbium co-doped fiber for lidar is successfully fabricated by MCVD combined with SDT. The performance of this polarization-maintaining fiber is measured. A birefringence coefficient of 1.29×10-4 and a polarization extinction ratio of 24 dB@4 m at 1310 nm are achieved. In addition, a polarization-maintaining all-fiber erbium-ytterbium co-doped fiber laser system is built, and the slope efficiency reaches 36%. Above all, the highest efficiency of the polarization-maintaining erbium-ytterbium co-doped fiber is achieved, which provides the possibility for exact localization of a military lidar.
The analog radio-over-fiber (A-RoF) technique can directly transmit radio-frequency (RF) signals between the baseband unit (BBU) and remote antenna unit (RAU) and offers the advantages of high spectral efficiency, ultralow latency, and a simple structure. In addition, millimeter-wave (mm-wave) mobile communication can utilize wide spectral resources to transmit high-rate signals. Therefore, the mm-wave over fiber based on the A-RoF technique and mm-wave mobile communication is considered the most potential solution for beyond-fifth generation (B5G) fronthaul. However, the A-RoF technique is sensitive to linear and nonlinear distortions and the generation of mm-wave signals requires high-bandwidth photonic and electronic devices. In our previous work, four-independent mm-wave signals were modulated on two orthogonal polarization states of a single wavelength based on a dual-polarization IQ modulator (DP-IQMZM) using the dual single-sideband (SSB) modulation and polarization division multiplexing (PDM) technique. Furthermore, a novel carrier polarization rotation module based on the self-polarization stabilization technique was proposed; thus, the four-independent mm-wave signals could be detected via self-coherent detection. Experimental results showed that the measured error vector magnitude (EVM) value of 800 MBaud 16-ary quadrature amplitude modulation (16-QAM) signals at 28 GHz over 50 km standard single-mode fiber (SSMF) transmission was 12.99% without digital signal processing (DSP). However, photonic frequency upconversion was not realized in our previous work. The bandwidth requirement of photonic and electronic components at the transmitter is high. In this study, we propose a scheme for upconverting four independent low-frequency RF signals to high-frequency mm-wave signals using photonic frequency upconversion. Moreover, no digital signal algorithm is used at the receiver, which is helpful for constructing a DSP-free RAU.
By paralleling one DP-IQMZM and one single-drive Mach-Zehnder modulator (MZM), which is used to generate second-order optical subcarriers, four low-frequency RF signals can be upconverted to high-frequency mm-wave signals using the self-heterodyne detection technique. In this way, the bandwidth requirement and sampling rate of photonic and electronic components at the transmitter can be reduced considerably. In addition, frequency offset compensation and carrier phase recovery are avoided. Furthermore, we analyze the causes of crosstalk between symmetric sidebands and propose a method for crosstalk elimination by accurately matching the phase and amplitude of the in-phase (I) and quadrature (Q) components of the employed DP-IQMZM.
The sideband suppression ratio can be increased from less than 20 dB to more than 30 dB using our proposed crosstalk elimination method between symmetric sidebands (Fig. 10). Moreover, the transmission performance of dual-SSB signals is very close to that of SSB signals [Fig. 11(a)], verifying the effectiveness of our proposed method. Experimental results show that four independent 1.6 GBaud 16-QAM mm-wave signals with a carrier frequency of 30 GHz could be generated at the receiver using four independent 1.6 GBaud 16-QAM signals with a carrier frequency of 10 GHz and a single-tone signal with a carrier frequency of 20 GHz at the transmitter. As shown in Fig. 12(b), the measured EVM value of 25.6 Gbit/s 16-QAM signals at 30 GHz over 50 km SSMF transmission are all below the threshold of 12.5% without the use of any DSP. Moreover, the minimum sampling rate of the DAC at the transmitter is 24 GSa/s [Fig. 13(a)].
A coherent radio-over-fiber transmission system based on the self-heterodyne detection technique is proposed, which can realize photonic frequency upconversion. In the proposed system, four low-frequency RF signals are upconverted to high-frequency mm-wave signals and no DSP algorithms are required in the RAU. The causes of crosstalk between symmetric sidebands are analyzed, and a crosstalk elimination method at the transmitter is proposed. Experimental results show that four independent 1.6 GBaud 16-QAM mm-wave signals with a carrier frequency of 30 GHz can be generated at the receiver using four independent 1.6 GBaud 16-QAM signals with a carrier frequency of 10 GHz and a single-tone signal with a carrier frequency of 20 GHz at the transmitter. Using the crosswalk elimination method between symmetric sidebands, the proposed system can support the transmission of four independent 1.6 GBaud 16-QAM mm-wave signals with a carrier frequency of 30 GHz over a 50 km SSMF without any DSP at the receiver. Moreover, the minimum sampling rate of the DAC at the transmitter is 24 GSa/s, effectively reducing the cost and complexity of B5G fronthaul systems. This research provides a potential solution for the mobile fronthaul network in the B5G mobile communication.
Recently, there has been a rapid evolution in optical communication systems, leading to the establishment and development of various subdivisions, such as metropolitan area networks, access networks, and data center optical interconnection. However, current optical network architectures available on the ground are becoming insufficient to meet the growing demands of the society. Therefore, some new application scenarios, such as satellite communication, marine communication, and communication in some areas where optical fibers are difficult to arrange, e.g., mountains, forests, and lakes, are attracting widespread attention. Based on current optical communication network architectures, a three-dimensional, spatial, and multimodal optical network system is emerging. In this system, the free-space optical (FSO) communication featuring unlicensed bandwidth, high capacity, strong confidentiality, and easy setup plays an important role. Therefore, for practical in-field application of FSO communication, studies on embedded real-time FSO systems are necessary.
To investigate the real-time applications of FSO transmission, in this study, we experimentally demonstrate a real-time multicarrier FSO communication system with a self-designed electrical board, including a field-programmable gate array (FPGA), four-channel transmitter supporting 2.5 GBaud signals, and 4×5 GSa/s analog-to-digital converter (ADC). The 10 Gb/s polarization digital multiplexing quadrantile phase-shift keying (PDM-QPSK) signals were generated using a dual-polarization IQ (DP-IQ) modulator and loaded onto eight optical carriers spaced at 12.5 GHz. All optical carriers can be recovered with an error-free bit error ratio (BER) performance.
The experiment setup of the real-time 8×10 Gb/s PDM-QPSK coherent transmission over a 1 m FSO link and the experiment platform are shown in Figs. 1 and 5 (a), respectively. At the transmitter side of the system, eight external cavity lasers with ~100 kHz linewidth spaced at 12.5 GHz were used as the sources. All the channels were coupled using an optical coupler and fed into a DP-IQ modulator, where the 10 Gb/s pseudo-random-bit-sequences-23 were modulated. The signal spectrum of the transmitted signal is shown in Fig. 2. Then, the optical signal was amplified using erbium-doped fiber amplifiers (EDFA) to adjust the power and transmit it into the free-space link. At the receiver side, the signal was detected using an integrated coherent receiver and ADC. Subsequently, different channels were selected by tuning the wavelength of a local oscillator without any optical filter. All the sampled signals were processed using the FPGA with real-time digital signal processing (DSP) algorithms, including ADC synchronization, clock recovery, constant modulus algorithm (CMA), frequency offset recovery, phase offset recovery, and symbol decision, as shown in Fig. 1. The results of the experiment were shown in Figs. 6 and 7. Figure 6 (a) shows the receiver sensitivity of the real-time integrated coherent receiver in a back-to-back case. By adjusting the optical attenuator, we reduced the receiver power from -45 to -51 dBm and obtained the BER using the signal-to-noise ratio. When the receiver power is -50 dBm, BER is 2.9×10-3, which is under the 7% FEC limit (BER is 3.8×10-3). Figure 6 (b) shows the received spectrum and constellation diagram from channel 8 in the multicarrier FSO experiment. The results of each stage of the real-time algorithm processing are shown in Fig. 7. Figures 7 (a)-(d) represent the sampled data of ADC without DSP algorithms, results after CMA, results after frequency offset recovery, and results after phase compensation, respectively.
In this study, we propose an 8×10 Gb/s PDM-QPSK real-time digital coherent communication system via experiments using a free-space link. Based on an FPGA chip, we have completed the programming of the real-time DSP algorithms and conducted the corresponding performance test. The experimental results show that after using EDFA, under the 7% FEC threshold, the receiving sensitivity of the digital coherent module is as low as -50 dBm. Furthermore, all the channels of the system with 12.5 GHz frequency intervals achieve a real-time error-free transmission through a 1 m free-space link.
Nonlinear Fourier transform (NFT) can convert a signal into a nonlinear spectrum, including continuous spectrum and discrete spectrum, where the eigenvalues are located in the upper half of the complex plane. With the approach of NFT, information is encoded into the nonlinear spectrum of a signal, which can effectively address the nonlinear transmission impairments arising in standard single-mode fiber. At the same time, as a new effective signal processing tool, NFT can be used to analyze pulses in fiber lasers. For a pure soliton, its nonlinear spectrum only contains the discrete spectrum. The eigenvalues in the discrete spectrum can then be used to characterize soliton pulses, wherein the real and imaginary parts refer to the frequency and amplitude of soliton pulses, respectively. This methodology provides a new perspective for the study of laser dynamics. The soliton and continuous wave background can be separated based on their different eigenvalue distributions after the obtainment of full-field information of pulses. We summarize the principle of NFT and its applications in the field of optical communications and fiber lasers, specifically the "soliton distillation" technology based on NFT.
In optical fiber communication systems, there are transmission impairments such as loss, dispersion, and nonlinearity. Loss and dispersion can be compensated by optical amplification and dispersion compensation technology. Pulse broadening and distortion caused by nonlinear effects related to optical pulse intensity have, however, become the main factors limiting the system communication capacity improvement. As a powerful mathematical tool, NFT can effectively solve the problem of lightwave propagation in a nonlinear medium such as an optical fiber. Recently, a new framework of optical fiber communication systems based on NFT has begun receiving extensive attention. NFT can decompose a signal into a continuous spectrum (nonsoliton component) and discrete spectrum (soliton component), which are considered nonlinear spectra. With this method, information can be encoded into the nonlinear spectrum of the signal, and the technique of doing so is known as nonlinear frequency division multiplexing (NFDM). Compatible with the nonlinear response characteristics of optical fibers, NFDM can effectively address the dispersion and nonlinear impairments arising in standard single-mode fiber (SMF) transmission.
Optical soliton is a special light field that does not change during transmission under the dispersion and nonlinear effects, and it can be generated and spread in optical fiber systems. Periodically repeating stable pulses generated in fiber lasers can also be considered as solitons, more specifically known as dissipative solitons. The output signal can be transformed into a nonlinear spectrum (including continuous spectrum, discrete spectrum, and corresponding eigenvalues) through NFT. In the nonlinear Fourier domain, nonlinear phenomena in optical fiber systems are investigated, such as rouge wave, optical frequency combs, and cavity solitons. Lately, NFT has been applied to laser pulse analysis. For pure soliton, its nonlinear spectrum only contains a discrete spectrum and the eigenvalues in the discrete spectrum correspond to the characteristics of the soliton. For example, the real and imaginary parts of an eigenvalue correspond to the frequency and amplitude of the soliton, respectively. When the dynamic characteristics of the pulse are dominated by solitons, that is, when the discrete spectrum cumulates most of the pulse energy, the eigenvalue distribution can reflect the pulse feature, as shown in Fig. 13. At the same time, pure soliton and the resonant continuous-wave background can be separated according to different eigenvalue distributions, to realize soliton distillation. These findings show how NFT provides new insights into ultrafast transient dynamics in fiber systems.
In an NFDM system (Fig. 3), after the nonlinear spectrum of the signal is recovered at the receiving end, the effects of dispersion and nonlinearity can be eliminated by simple phase compensation, and the system performance can be improved. According to the different modulation spectrum, the nonlinear spectrum can be divided into discrete spectrum communication and continuous spectrum communication. Research on discrete spectrum communication has focused on the number of multiplexed eigenvalues and advanced modulation formats for discrete spectrum. As for continuous spectrum communication, compared to the traditional OFDM systems, NFDM systems have higher quality factors under the optimal condition of fiber launching power. However, the NFT-based optical communication system still suffers from random noise. Meanwhile, problems, such as channel integrability and algorithm complexity, still exist, which greatly restrict the performance of the system.
To perform NFT analysis experimentally in the field of fiber lasers, full-field information, including the amplitude and phase of the pulse, must first be obtained. The current acquisition methods mainly include density functional theory and temporal lensing technology, combined with Gerchberg-Saxton phase recovery (Fig. 9) and coherent homodyne detection technologies (Fig. 11). After the full-field information of pulse is obtained, the pulse features can be projected onto the eigenvalue distribution by NFT. NFT can be used as an analysis tool to reduce the complexity of describing pulse evolution, whether for nonstationary (Fig. 5) or stationary pulses (Fig. 6). At the same time, according to the distribution of eigenvalues and their corresponding discrete spectra, the temporal evolution process of a pulse can be reconstructed using inverse nonlinear Fourier transform (INFT), which also shows that the NFT method can effectively characterize the laser pulse. INFT is not only effective for a single pulse (Fig. 7) but also achieves a good reconstruction effect for multiple pulses (Fig. 8). Further, the sideband eigenvalues can be removed and only the soliton eigenvalues are retained in the nonlinear spectrum. Through INFT, pure soliton can be recovered in the time domain (Fig 14). NFT can perform pure soliton distillation and reconstruction on various pulses generated in fiber lasers, including single pulse, single pulse in period-doubling, different double pulses (Fig. 15), and multiple pulses. The transient nonlinear dynamic analysis based on NFT can deepen the knowledge on soliton formation and its interaction process, and also clarify the transient working mechanism of fiber laser.
As an emerging signal processing tool, NFT provides new system design solutions in the field of optical communication, which is fully compatible with optical fiber. Transient nonlinear dynamics analysis based on NFT can describe laser pulses theoretically and completely, providing a basic overview on ultrafast nonlinear dynamics, and its application in ultra high-power fiber lasers.
Big data services have yielded explosive growth of capacity in short-reach optical networks. The intensity modulation direct-detection (IMDD) systems with simple schemes and power-efficient digital signal processing (DSP) are typically preferred in short-reach scenes. However, they are unable to satisfy the demand for continually increasing interface speed. The Ethernet interface data rate is approaching 800 Gbit/s and 1.6 Tbit/s. Hence, the conventional IMDD will suffer from serious technical challenges, including dispersion-induced power fading, rapidly increasing cost, and limited sensitivity.
As an alternative, coherent technology can provide high spectral efficiency, high sensitivity, and good tolerance to the chromatic and polarization-mode dispersion. However, for short-reach application, this technology is considered overly costly and power-consuming. These drawbacks originate from two main challenges. On one hand, power-consuming DSP is required for solving various impairments on the received signal in traditional coherent schemes. Moreover, with the fading of the Moore’s law, the node gain brought by new footprints of application specific integrated circuits (ASICs) tends to be marginal. Using only advanced DSP in the development of coherent technology for short-reach high-speed interconnections is quite difficult. On the other hand, traditional coherent technology is associated with complex hardware structures, especially the adoption of narrow-linewidth, frequency-stable, and tunable lasers, such as external cavity laser and integrable tunable laser assembly (ILTA). Consequently, coherent technology is still inapplicable to short-reach networks.
Apart from conventional IMDD and coherent technology, many self-coherent schemes have been proposed with certain tolerance to laser linewidths and less implementation complexity (than that of the conventional systems). The Kramers-Kronig receiver (KKR) and Stokes vector receiver are two typical schemes, each receiving considerable attention. In terms of the product of analog to digital converter ADC bandwidth and number of ADCs, these schemes are more expensive while achieving the same capacity as that of the conventional coherent technology. The self-homodyne coherent (SHC) scheme has been proposed as another "coherent-lite" scheme, including polarization division multiplexing and space division multiplexing as the categories. The key feature of this scheme is that the signal lights transmit simultaneously along with their pilots in links. At receiver, coherent detection will be conduced though the remotely delivered pilot to achieve optical domain phase recovery of signal. Thus, the cost and the power consumption are reduced, and the use of low-cost and uncooled distributed feedback (DFB) laser and baud-rate-sampling receivers is realized. The advantages of the coherent technology are therefore inherited and the scheme is simplified, and hence this technology is considered one of the most promising technologies for future short-reach optical networks. Despite the excellent characteristics of SHC schemes, many key implementational issues must be solved prior to deployment.
In space division systems, the relative time delay will induce a unique phase noise and degrade the system performance, which may prevent use of the low-cost DFB laser in the SHC scheme. Fortunately, the derivative of such phase noise is a colored frequency modulation noise. Utilizing this characteristic, we proposed and demonstrated an in-service high-precision and large-dynamic-range estimation method of relative time delay (RTD), contributing to the realization of the SHC technique. Besides, the random birefringence in the optical fiber will lead to changes in the state of polarization (SOP) during delivery of the pilot and signal. Another implementational issue is that automatic compensations of such randomly changed SOP is required for real fiber links. By leveraging our in-house-developed adaptive polarization controller (APC), we solved both problems of polarization demultiplexing in polarization division multiplexed (PDM) SHC systems and pilot SOP locking in space division multiplexed (SDM) SHC systems. The APC technique allows further simplification of the DSP algorithms. Utilizing only one APC device and its symmetry property, we also demonstrated the first multi-input and multi-output (MIMO)-free SDM-SHC transmission and PDM-SHC transmission in bidirectional (BiDi) scenes. The APC technique paves the way for low-cost, power-efficient, high-speed BiDi optical interconnections. We present an overview of the progress that our group has realized for SHC systems, including the APC techniques, the in-service RTD estimation techniques, and the simplest BiDi SHC transmission architectures based on APC techniques.
The SHC scheme capable of optical-domain equalization, high spectrum efficiency, and compatibility with current ASIC architecture has been demonstrated. This scheme provides a promising method for future low-cost short-to-medium-reach optical interconnections. Moreover, the SHC technology will generate new demands from other technical areas, including photonic integration, special optical fibers, and DSP. This technology will promote and accelerate innovations in multiple fields.
Microwave photonic (MWP) systems generate, manipulate, transmit, and measure high-speed radio-frequency (RF) signals in the optical domain. Converting RF signals in the optical domain improves the signal-processing bandwidth and speed and reduces power consumption by complex electronic systems. Optical modulation is the most important step in converting microwave signals into optical signals in all MWP systems. This usually determines the performance of the whole system, including its bandwidth, system loss, linearity, and dynamic range. Nonlinear distortion is introduced mainly by the nonlinearity of the Mach-Zehnder electro-optical modulator (MZM) used in MWP systems. The modulation curve of a typical MZM takes the form of a cosine function. To achieve the approximate linearity of the modulation, the working point of the modulator is fixed at an orthogonal offset point, where it cannot fulfill the requirements of a microwave photonic link. A stable and efficient MWP system requires the modulator to exhibit a low noise figure, i.e., less than 10 dB, and a high spurious free dynamic range (SFDR) exceeding 120 dB·Hz2/3. The SFDRs of a typical silicon MZM and a microring modulator are approximately 97 dB·Hz2/3 and 84 dB·Hz2/3, respectively. Therefore, a high-linearity electro-optical modulator is urgently needed.
Two options exist for improving the linearity of an electro-optical modulator: the electrical and optical domains. In the electrical domain, one method is to use electronic predistortion. Introducing arcsine predistortion into the RF signal compensates for the cosine modulation. Another method is to employ electronic post compensation. This method uses digital sampling at the output to remove the distortion term produced by the modulator. These methods in the electrical domain do not fundamentally improve the linearity of the modulator, and they require an accurate control of the introduced distorted signal and additional high-speed electronic devices. With increasing working time, the MZM itself experiences unstable factors such as temperature drift. This requires electronic compensation to adapt dynamically to the change in the modulator, which leads to a complex system with limited performance.
In the optical domain, various methods can be used to improve the linearity of the modulator and achieve improved performance. These methods commonly include the dual-polarization control, MZM series/parallel, and microring-assisted MZM (RAMZM) methods. The basic idea of the dual-polarization method is to control the third-order distortion power of TE and TM light at the same strength but in opposite directions to cancel each other. This is usually achieved by adding a polarization controller at the input or output of the MZM modulator, and it produces limited enhancement. The dual-polarization control method can be combined with the MZM parallel method, which is called the polarization-multiplexing MZM parallel method. By combining three linearization methods—power-weighting control, polarization multiplexing, and bias control—it can enhance the third-order SFDR from 95.4 dB·Hz2/3 to 112.3 dB·Hz2/3 compared with a conventional MWP link, and the second-order SFDR is 94.6 dB·Hz2/3(Fig. 9). The basic idea of the MZM series/parallel method is to use one MZM to compensate for the third-order distortion caused by another MZM. The two MZMs can be connected either in series or in parallel. The double-parallel MZM method utilizes two MZMs connected so that the third-order intermodulation distortions generated by the upper and lower MZMs cancel each other. This method has a wide optical bandwidth, high manufacturing tolerance, and temperature-variation tolerance. Specific control is provided by the driving RF and bias voltage. The electrical-power distribution ratio between the two modulators and DC bias angle of the RF signal in the two submodulators can be controlled appropriately to construct two nonlinear distortion signals with opposite phases, so as to cancel the IMD3 intermodulation distortion in the link. Using bias-voltage control, the nonlinear distortion term can be cancelled by controlling the phases of the RF electrical and input optical signals. The principle of the MZM series method is basically similar to that of the parallel method. The MZM series/parallel methods need to adjust the bias voltage and power or phase of the driving RF signals accurately. Because the modulator is sensitive to temperature, an additional control circuit is needed to stabilize the bias voltage and temperature. The microring-assisted MZM (RAMZM) method utilizes the superlinear phase modulation of the microring to compensate for the nonlinear cosine modulation function of the MZM. It has a simple structure and can achieve high linearity. The key point of RAMZM is to control the coupling coefficient between the microring and MZM arm. The fabrication tolerance of the directional coupler between the microring and modulation arm of the RAMZM is small, and the losses in the microring also affect the linearity. Lithium niobate exhibits the characteristics of high bandwidth, good modulation, and low loss. Further, it exhibits a high-linearity electro-optical effect that silicon does not. Recently, the thin films of lithium niobate on an insulator (LNOI) have become promising platforms for photonic integration. Using materials with high linearity, we can expect to improve the linearity of an electro-optical modulator further and make it practical.
Although linearization methods in the electrical domain are traditional and widely applicated, the required electronic control equipment is complex. These methods are used for linearization-compensation for traditional modulators with poor linearity. Conversely, optical-domain linearization is committed to eliminating nonlinear components through optical methods without introducing redundant electronics or compensation equipment. With the continuous development of integrated optics and on-chip integrated optoelectronic devices, the optical-domain linearization method has gradually become a research hot spot. Reported methods include the optical-polarization, MZM series/parallel, and microring-assisted MZM methods. These methods can be used in combination with each other. With the fast development of thin-film lithium niobate platforms, the prospect of fabricating high-performance electro-optical modulators with high linearity on LNOI appears promising.