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Abstract
In recent years, the thriving satellite laser communication industry has been severely hindered by the limitations of incompatible modulation formats and restricted Size Weight and Power (SWaP). A multi-modulation compatible method serving for free-space optical (FSO) communication has been proposed assisted by chirp-managed laser (CML). The corresponding demonstration system has been established for realizing free-switching between intensity (OOK) and phase modulation (RZ-DPSK). The feasibility and performance of system have been evaluated sufficiently when loading with 2.5 and 5 Gbps data streams, respectively. Additionally, a control-group system has been operated utilizing Mach-Zehnder modulator (MZM) for comparison between CML-based and MZM-based compatibility solutions. The OOK receiving sensitivities of CML-based system are −47.02 dBm@2.5 Gbps and −46.12 dBm@5 Gbps at BER of 1×10−3 which are 0.62 dB and 1.11 dB higher than that of MZM; the receiving sensitivities of RZ-DPSK are −50.12 dBm@2.5 Gbps and −47.03 dBm@5 Gbps which are 0.79 dB and 0.47 dB higher than that of MZM respectively. Meanwhile, CML-based transmitter abandoned the traditional modulator and its complicated supporting devices which can effectively contribute to the reduction of SWaP. The CML-based system has been proven to have the compatibility between intensity and phase modulation while also possesses a miniaturized design. It may provide fresh thinking to achieve a practical miniaturization system for satisfying the requirements of space optical network in future.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Free-space optical (FSO) communication is quiet an up-to-date subject which has been widely applied to build inter-satellite, satellite-to-earth, and deep-space links for information transmission, environment monitoring, and celestial exploration [1–4]. The electro-optical conversion served for FSO link commonly relied on various modulation approaches, including intensity modulation (IM) and phase modulation (PM). On the one hand, the IM has been extensively utilized which gets the upper hand of simple architecture and convenient operation [5,6]; on the other hand, PM has been regarded as a more suitable solution for long-haul link with higher data-rate and optimized receiving sensitivity [7–9]. Thus, it is of practical significance for achieving the compatibility between IM and PM within the same communication system. Many international institutions devote dollars and resources to reform the modulation compatibility for satisfying the demands of actual applications, for instance, Laser Communications Relay Demonstration (LCRD) realized the compatibility between PPM and differential phase-shift keying (DPSK) in 2021 [10].
Traditional IM and PM are both relied on the Mach-Zehnder modulator (MZM) with its relative supporting devices [11–13]. Therefore, two sets of independent modulation modules and lots of auxiliary devices are indispensable for IM/PM compatible system, which greatly increases the system complexity. However, limited satellite resources impose strict conditions on the SWaP of laser communication terminals in practical applications. Hence, more compatibility solutions are encouraged for achieving free-switching of IM/PM while guaranteeing the ideal SWaP.
Since the intensity or phase change of optical signal can be determined by the input electrical-level of chirp-managed laser (CML) [14,15], the CML has been considered as an alternative solution for conveniently achieving IM/PM compatibility. Additionally, the CML-based modulation method abandoned the traditional MZM and its supporting devices, which contributes to the reduction of SWaP and promotes the miniaturization of the optical communication terminal.
Herein, a CML-based IM/PM compatible transmitter was designed, which has been confirmed to freely switch between on-off keying (OOK) and return-to-zero differential phase-shift keying (RZ-DPSK) by sharing one set of devices. Meanwhile, a corresponding FSO communication system has been established with spatial optical propagation. Data at 2.5 and 5 Gbps were modulated onto the optical link, respectively, and the demodulated data has been collected to assess the system performance. A comparison IM/PM system using traditional MZM method has also been constructed in order to verify the feasibility and miniaturization of our CML-based system. The test results show the CML-based system possessed high receiving sensitivity and miniaturized structure which lays a vital foundation for future popularization and application in space networking.
2. Principle of CML-based IM and PM
Figure 1(a) discloses the detailed structure of CML-based IM/PM compatible transmitter. The initial non-return-zero (NRZ) signal was imported into the signal process unit. The IM format (Path 1) did not need extra pre-processing while the PM format (Path 2) demanded precoding and transformed the initial data to a three-level electrical signal. Subsequently, the IM or PM pre-processed signal was routed into an amplifier for amplifying the electrical-level difference. Then the amplified signal was input to the CML unit and the inner structure of CML was illustrated in Fig. 1(a). The CML unit is mainly composed of a distributed feedback (DFB) laser and a co-packaged optical spectrum re-shaper (OSR) filter [16]. Photodetector (PD) 1 and PD 2 monitored the optical power from DFB laser and OSR filter, respectively. The thermoelectric cooler (TEC) has been utilized for maintaining the power-ratio of PD1 and PD2 in order to locate the wavelength of DFB laser at the transmission edge of filter which contributes to the enhancement of extinction ratio (ER). The principle of IM and PM have been depicted in Figs. 1(b),(c), respectively.
Fig. 1. (a) The schematic diagram of CML-based IM/PM transmitter. (b) The principle of CML-based OOK. (c) The principle of CML-based RZ-DPSK.
2.1 Intensity modulation
The OOK format has been selected as an example for explaining the principle of CML-based IM procedure (Figs. 2(a)-(d)). The amplified NRZ data has been imported into CML, and the change of the driving current causes the corresponding frequency modulation (FM) of DFB laser within CML. This leads to FM or chirp, and the adiabatic chirp is proportional to the electrical signal intensity [17]. The dependency between FM change and altered driving current satisfies [18]
where Δf is FM change, ΔI is the change of driving current, γ=-250 MHz/mA is the typical FM chirp coefficient. As illustrates in Fig. 2(b), the 1 bit and 0 bit within the same data sequence have the different optical carrier frequencies. The 1 bit within the data stream exhibits a blue-shift compared to the 0 bit (Fig. 2(b)). Meanwhile, the wavelength of DFB laser has been aligned with the transmission edge of OSR filter (Fig. 2(c)). Utilizing the ramp characteristic of OSR filter the 1 bit corresponding to the high-frequency has been effectively transmitted and low-frequency signal 0 bit has been suppressed (Fig. 2(d)). Thus, the conversion from FM to amplitude modulation (AM) has been achieved, and the ER has been increased up to 10-15dB.Fig. 2. The detailed operation principle of CML-based IM. (a) The block diagram of CML-based IM. (b)-(d) The optical carrier frequency of data sequence (b) before OSR; (c) OSR transmission edge filtering; (d) after OSR.
So far, the conversion from FM to AM has been completely achieved applying the frequency spectrum ramp characteristics of OSR.
2.2 Phase modulation
The RZ-DPSK format has been chosen for explaining the PM principle (Fig. 3). Firstly, the original NRZ data was input into clock data recovery (CDR) to extract a synchronous clock signal (Fig. 1(a)). Then, the original data was logically NANDed (NAND, Not And) with the synchronous clock signal and generated an inverse return-to-zero (IRZ) signal sequence with a 50%-duty-cycle. Next, the IRZ signal has been combined with a clock signal delayed by half bit for generating a three-level signal within signal processing unit and was transferred into amplifier. The amplified three-level signal was imported into CML. The current swings down from a high level to one of two lower levels by ΔΙ or 2ΔΙ for incoming 0 and 1 bits respectively, and returns back to the high level in ½ the bit period T. The optical frequency of CML was proportional to the inputted driving current I which is related to the driving voltage. This leads to FM or chirp, with FM change can be expressed as Eq. (1).
The value of driving current is adjusted to generate adiabatic chirp of Δf = 1/T or 2Δf =2/T (T is bit period), while the input current swings ΔΙ and 2ΔΙ, respectively. The adiabatic chirp shifts the phase of the optical field by [19]
For instance, the phase-shift of 0 bit within the data stream can be expressed as
Similarly, the phase-shift for 1 bit can be described as followed
Meanwhile, the phase information has been loaded on the optical signal from DFB laser (Fig. 3). Subsequently, the optical frequency possessed high-level is close to the transmission peak (f0) of OSR which can be transmitted, while that of low level (f1, f2) have been filtered out. The RZ-DPSK signal has been output after OSR, and every optical pulse carries phase information. The Δϕ in Fig. 3 represents the phase-difference between adjacent pulses. So far, the three-level signal went through PM has been converted into RZ-DPSK signal with high ER by OSR, which also implies the “AM-FM-PM” procedure has been accomplished. Moreover, the differential encoding has been confirmed to be achieved automatically by this method.
Summarily, the realization of IM/PM compatibility is relied on the level-format conversion by signal processing unit and the chirp-effect.
3. Experiment setup
The experimental setup of CML-based IM/PM compatible FSO communication system has been drawn in Fig. 4. Two kinds of data-rates (2.5 and 5 Gbps) have been loaded on the optical link, respectively, in order to typically verify the compatibility and feasibility of OOK (IM) and RZ-DPSK (PM). Firstly, NRZ signal has been generated by arbitrary waveform generator (AWG, Tektronix AWG70002). According to the CML-based transmitter, the free-switching between OOK and RZ-DPSK can be approached by conveniently controlling the input signal level-format. For IM format, initial data sequence has been pre-processed to the amplified driving signal, and then, has been converted into OOK optical signal with high ER due to CML filtering process. For PM, the original signal has been pre-processed as three-level signal via signal process module, and transferred into amplifier. Then, the amplified signal has been modulated as RZ-DPSK optical signal. Subsequently, the modulated optical signal (IM or PM) has been transmitted via a collimator for passing spatial link. After a propagation of 1m spatial link, the faded optical signal has been captured by another collimator at the receiving terminal, and then, passed to the following variable optical attenuator (VOA, Advanced Fiber Resources MVOA-1550). Next, the weak signal has been pre-amplified with low-noise (KEOPSYS, CEFA-C-HG), and routed into the optical filter (EXFO XTA-50/S) with 50pm at 3dB band-width. Finally, for OOK demodulation, the received optical signal has been inputted into PD (Discovery Semiconductors DSC-R402) for intensity detection to recover original NRZ signal. For RZ-DPSK, the received optical signal has been inputted in to delayline interferometer (DLI, Optoplex DI-CCEFAC452) and then converted into to RZ signal via balanced photo-detector (BPD, Discovery Semiconductors DSC-R422).
4. Results and discussion
4.1 Performance evaluation of CML-based transmitter
As depicted in Figs. 5(a),(b), the phase noise and relative intensity noise (RIN) of the CML has been measured by laser linewidth/phase noise measurement system (OEwaves OE4000). The results show that the instantaneous linewidth of the CML is 2.29 MHz, the RIN is -110 dBc/Hz @ 10 kHz. Figures 5(c),(d) display the spectrum loaded with 2.5 or 5 Gbps data stream before and after OOK and RZ-DPSK modulation, respectively (spectrometer, OSA, YOKOGAWA AQ6370). The chirp-based spectrum characteristics mentioned in the above second section has been revealed. For instance, the Fig. 5(c) shows the high-frequency signal corresponding to the 1 bit within data stream exhibits a blue-shift of 0.02 nm compared with the original spectrum. Meanwhile, the amplitude of 0 bit within data stream is suppressed by OSR edge filtering compared with 1 bit, which enhances the optical signal ER up to ∼15dB. Additionally, the RZ-DPSK modulated signal also emerged a blue-shift of 0.02 nm and slightly appeared the optical carrier suppression phenomenon (Fig. 5(d)), which signifies higher dispersion tolerance as well as lower inter-channel crosstalk.
Fig. 5. (a) Phase noise of the CML. (b) RIN of the CML. (c),(d) The spectrum comparison before and after modulation: (c) 2.5 Gbps and 5 Gbps with OOK modulation; (d) 2.5 Gbps and 5 Gbps with RZ-DPSK modulation.
The modulated 2.5 and 5 Gbps data eye-diagrams for IM and PM have been separately collected by a 23 GHz digital phosphor oscilloscope (Tektronix, DPO72304DX) (Fig. 6). The eye opening with 2.5 Gbps data-rate is larger than that of 5 Gbps, while all of the eye-diagrams are clear and clean which imply favorable performance of CML-based modulation process. Furthermore, the duty cycle of RZ-DPSK is up to 33% owing to strong chirp effect. For the ER of the modulated signal, OOK at 2.5 Gbps and 5 Gbps are measured as 22.83 dB and 17.78 dB, respectively, and that of RZ-DPSK are 22.17 dB and 17.02 dB, respectively. The high-quality modulated signal provided by the CML-based transmitter offers several support for the feasibility and application value of IM/PM compatibility.
Fig. 6. Eye diagrams for modulated signal: (a) 2.5 Gbps and (b) 5 Gbps of OOK; (c) 2.5 Gbps and (d) 5 Gbps of RZ-DPSK.
Additionally, the constellation pattern of 5 Gbps RZ-DPSK has been collected as an example by optical modulation analyzer (Tektronix, OMA4225) for detailed evaluation of CML-based modulated signal. As shown in Fig. 7(a), the blue points represent the phase concentration information, and the green lines represent the trajectory of phase change between 0 and π. The principle of CML-based PM has been illustrated in Fig. 7(b) for reference. With the change of three-level driving current is 2ΔI, the variation of chirp goes to 2Δf, which leads to a phase transformation from 0 to 2π. The process A in Fig. 7(b) illustrated this procedure matched with the process A(white curve) in Fig. 7(a) which displayed a circle path in constellation pattern. Similarly, with the driving current is ΔI, the variation of chirp is Δf, which means a phase transformation from 2π to 3π. The white semicircle path in Fig. 7(a) exactly corresponds to the B process in Fig. 7(b). Moreover, the detected error vector magnitude (EVM) value is 17.53%, indicating that the amplitude and phase error are remarkably small and the CML-based modulation performs well.
Fig. 7. (a) The constellation diagram of modulated 5 Gbps RZ-DPSK. (b) The principle of CML-based PM.
4.2 Performance of CML-based FSO communication system (compared with MZM)
A traditional IM/PM compatible MZM-based FSO communication system has been also established for comparison with our CML-based system (Fig. 8(a)). For the MZM-based compatible modulation, it is actually a cascade of two independent transmitters to achieve mutual switching. Since the supporting devices of different modulation formats cannot be shared, the architecture of MZM-based transmitter is much more complicated, which obviously run counter to the practical demands of miniaturization. However, benefiting from the usage of CML, the architecture of CML-based transmitter can not only satisfy compatibility requirements but also exhibit a simplified structurewith SWaP reduction. For example, the power consumption of CML-based transmitter is 6.5 W which is 6.7 W smaller than that of MZM-transmitter, while the weight of CML transmitter is also 260 g slighter than that of MZM. Figure 8(a, Inset) contains real photos of CML and MZM transmitters with same scale bar, which illustrates the size reduction of CML transmitter. Hence, the CML-based IM/PM compatibility can be approached by only one same set of devices which significantly saved the power consumption, volume and other resources.
Fig. 8. (a) Experiment setup for comparison between traditional MZM-based and CML-based modulation transmitter systems. Inset: photographs of CML-based and MZM-based IM/PM compatibility transmitters (equal scale). (b)-(d) The BER analysis of received optical power for (b) OOK and RZ-DPSK at 5Gbps with the theoretical limitation curves; (c) OOK and traditional MZM modulation; (d) RZ-DPSK and traditional MZM modulation. (e)-(h) Eye diagrams for demodulated signals of 2.5 Gbps and 5 Gbps using OOK and RZ-DPSK, respectively.
Figure 8(b) shows the BER curves (signal quality analyzer, Anritsu MP1800A) of CML-based OOK and RZ-DPSK signals at 5 Gbps data-rate. The receiving sensitivities of CML-based system are -46.12 dBm and -47.03 dBm with BER of 1×10−3 for the OOK and RZ-DPSK, respectively, which implies the favorable operation IM/PM communication system. The corresponding theoretical limitation values are simulated as -50.99 dBm and -54.00 dBm [20], respectively, which are 4.89 dB and 6.97 dB smaller than that of CML-based results. The difference between CML-based system and theoretical limitation is mainly induced by the 4.5 dB noise of pre-amplifier at receiver, the out-of-band noise from optical filter at receiver, and the noise aroused by the signal processing unit at the transmitter. Figure 8(c) and Fig. 8(d) depicted the comparison between CML-based and traditional MZM modulation under IM and PM formats, respectively. The eye-diagrams of demodulated 2.5 Gbps and 5 Gbps data with OOK and RZ-DPSK have been separately illustrated in Figs. 8(e)-(h). The measurements show that the CML-based system reveals optimized receiving performance and excellent demodulated signal quality for both IM and PM. The specific receiving sensitivities (BER @ 1×10−3) of CML-based and MZM-based system have been detected and listed in Table 1 for obvious comparison. Compared with MZM-based results, the receiving sensitivities of CML-based IM at 2.5 and 5 Gbps are 0.62 and 1.11 dB higher than that of MZM-based system, respectively; while the difference of PM are 0.79 dB and 0.47 dB (Table 1). The comparative results imply that the CML-based system has been proven to balance the IM/PM compatibility, the optimization of receiving performance and miniaturization of system.
Table 1. The comparative results between CML-based and MZM-based system.
5. Conclusion
In conclusion, a CML-based IM/PM compatible method serving for FSO communication has been proposed and corresponding demonstration system has been established. Relevant results have been collected in detail while loading with 2.5 and 5 Gbps data streams, respectively, including BER, constellation patterns, eye-diagrams, etc.. A control-group experiment has been operated with traditional MZM-based system for evaluating the feasibility and performance of CML-based system. The OOK receiving sensitivities of CML-based system are -47.02 dBm@2.5 Gbps and -46.12 dBm@5 Gbps at BER of 1×10−3 which are 0.62 dB and 1.11 dB higher than that of MZM; the receiving sensitivities of RZ-DPSK are -50.12 dBm@2.5 Gbps and -47.03 dBm@5 Gbps which are 0.79 dB and 0.47 dB higher than that of MZM respectively. Our CML-based system not only achieves the IM/PM free-switching but also gets rid of the complicated structure of MZM-based system. Transmitter-sharing and abandoning redundant supporting equipment effectively promote the reduction of SWaP, thus, support the system miniaturization. The novel approach can also be used for reference within binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) system and may open up whole new vistas for satellite laser communication in the future.
Funding
National Natural Science Foundation of China (61231012, 91638101); National Key Research and Development Program of China (2018YFC0307904-02); Fundamental Research Funds for the Central Universities (xzy022021039).
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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