Multi-wavelength optical carriers play a critical role in applications such as wavelength-division multiplexing (WDM) optical commutation, photonic generation of radio-frequency signals, and photonic wireless communications. Conventional approaches are limited by either the insufficient coherence between optical carriers (e.g., using an independent-running laser array) or the poor flexibility in choosing the number of optical tones and their spacing in the spectral domain, such as in an optical frequency comb. In this study, we present a technique for generating highly coherent multi-wavelength optical carriers and verify the superiority of the synthesized signals through demonstrative applications in low-noise THz-wave generation, photonic THz wireless communication, and jointly processed WDM optical communication.

For a single-wavelength output, a continuous-wave pump laser is frequency-locked to an optical cavity consisting of a fiberized coupler and a spool of standard single-mode fiber. As the frequency of the pump laser is in resonance with one of the longitudinal modes, the optical field inside the optical cavity accumulates exponentially, reaching the threshold of the stimulated Brillouin scattering (SBS), which would otherwise require a much higher pump power if the pump laser is not locked to the cavity mode. A Brillouin spectral gain would appear ~10 GHz redshifted from the pump wavelength, and if the free spectral range (FSR) of the cavity matches the width of the spectral gain, only one cavity mode will start to oscillate and produce a narrow-linewidth SBS output in the direction opposite to the pump. For a multi-wavelength output, pump lasers with the desired wavelengths and spectral separations are pumped into the same optical cavity. Through independent frequency locking, the one-to-one correspondence described above allows the generation of the same number of SBS signals while maintaining the frequency separation of the pumps. Coherence between the output optical tones is ensured because the oscillations originate from the same optical cavity. The linewidth of the output is significantly reduced compared with that of the pump owing to the equivalent spectral filtering from the SBS gain and high Q of the optical cavity.

The proposed multi-wavelength SBS optical carrier generation technique is validated in three different applications. In synthesizing frequency-stabilized low-noise THz waves, two optical carriers separated by ~300 GHz are generated by the proposed SBS system and subsequently transformed to a THz wave through a large-bandwidth uni-travelling-carrier photodiode, as shown in Fig. 4. To stabilize the frequency of the THz wave, the FSR of the SBS fiber cavity is locked to an external frequency reference through both mechanical and thermal feedback, resulting in an root-mean-square (RMS) frequency drift of only 0.47 mHz in 60 min and Allen deviation of ~10^-15 at an average time of 1 s, as shown in Fig. 5. Moreover, by comparing the electrical waves generated at 300 and 11 GHz, the phase noise is confirmed to not be governed by the quadratic dependence on the frequency and stays at the same level of ~-90 dBc/Hz at 10 kHz offset (Fig. 6). In real-time photonic THz wireless communication, two-wavelength optical carriers generated by the proposed SBS system are used to replace the conventional light source generated by the electro-optic frequency comb. As the bit-error-rate traces and constellation diagrams shown in Fig. 8 indicate, the novel multi-wavelength SBS optical carriers reduce the required THz power at the transmitter by 6 dB at the same baud and bit error rates as the conventional source and remove the ellipse of the constellation diagram owing to the improved phase noise. Multi-wavelength optical carriers are also applied in the WDM optical communication demonstration, where the inter-channel joint signal processing technique is adopted. Owing to the high coherence between the SBS tones, the independent phase estimations of each communication channel standard in the WDM post signal processing can be replaced by only one estimation of a master channel shared with other channels, effectively reducing the computation requirements by 34%, as shown in Fig. 9. Such alleviation of the digital signal processing (DSP) complexity can considerably boost the communication capacity in scenarios where the computation power is restricted.

This study proposes a method for generating multi-wavelength optical carriers. Owing to the in-resonance pumping of the high-Q optical fiber cavity, arbitrary selection of pumping wavelength, and spectral filtering of the SBS effect, this technique is advantageous compared with the conventional approaches in terms of coherence, phase noise, and adjustability. In the experiments, the proposed method allows the synthesis of frequency-stabilized low-noise THz waves, whose phase noise does not scale up quadratically with the output frequency, and the efficient photonic THz wireless data transmission, where the required transmitted power decreases by 6 dB. The optical carriers generated by the proposed method also enable the application of joint DSP in WDM optical communication, which has only been demonstrated with light sources from an optical frequency comb. The high performance and low system complexity of the proposed method are expected to assist in the development of research fields such as broadband signal synthesis and high-efficiency communication, where highly coherent, narrow-linewidth optical carriers are desired.