Tao Fang

A large-capacity ultra-weak fiber Bragg grating (UW-FBG) array interrogation system is proposed and implemented. The system utilizes time- and wavelength-division multiplexing methods, based on a large-range tunable laser source (TLS) with fast tuning capability. The TLS consists of a distributed feedback multi-wavelength laser array (DFB-MLA) with a wavelength tuning range of 40 nm, which is realized using the reconstruction-equivalent-chirp (REC) technique. An external SOA modulator is used to modulate the output light from the TLS into nanosecond pulsed light. We establish an experimental system and successfully achieve 50 UW-FBGs interrogation. We perform temperature, strain, and vibration sensing experiments with a interrogation linearity of above 0.9995 and an error of less than 4 pm. Theoretically, the system is estimated to have an interrogation capability of more than 10,000 UW-FBGs, and the minimum spacing of adjacent UW-FBGs is 0.4 m. This research has made significant advancements in the multiplexing capability and spatial resolution of the UW-FBG array interrogation technology, offering important application potential in fields such as battery pack, structural health monitoring, and aerospace.

A low-cost hybrid integrated narrow linewidth laser is proposed in this paper. The monolithically integrated 18- channel distributed feedback (DFB) narrow linewidth laser array with a wavelength interval of 0.8 nm is designed and precisely fabricated using reconstruction equivalent chirp (REC) technique, exhibiting good single-mode performance for each channel, with the typical linewidth less than 200 kHz. One channel of the laser array is integrated with the silicon nitride (Si₃N₄) external cavity chip through the photonic wire bonding (PWB) technique. Both the linewidth reduction effect of the passive external cavity and the coupling efficiency between chips by the polymer waveguide are numerically calculated. The compactly packaged device can be tuned to seven distinct self-injection-locked states with 0.4 nm spectral spacing, where the maximum linewidth is 10.9 kHz and the minimum linewidth is 9.1 kHz. Meanwhile, the side-mode suppression ratio (SMSR) is higher than 54.3 dB and the output power is 3.6 ± 0.9 dBm for each self-injection-locked state.

Optical switching based on arrayed waveguide grating routers (AWGRs) and fast tunable sources is a future-proof solution to overcome the bottleneck of limited bandwidth and high latency of electrical switches-based data center networks (DCNs). To support fine-grained optical packet switching and maintain high network throughputs, tunable sources capable of nanosecond-scale hitless wavelength switching are critical requirements for such fast optically switching networks. In this work, we experimentally demonstrate a fast-tunable source based on a high-density multi-wavelength DFB laser array monolithically integrated with a three-stage Y-branch combiner and an SOA. The DFB laser array has an average 100-GHz channel spacing under the same bias current. The output power of all lasers is above 10 mW, with a uniformity of less than 1 dB. Benefitting from the high-speed dynamic property of the DFB laser and high-speed burst drive pulses controlled by a field-programmable gate array (FPGA), less than 3 ns switching time is achieved by the on-off switching of the DFB laser on arrays, which is independent of tuning range. Furthermore, a 2 × 2 optical switching prototype based on this demonstrated chip and AWGRs achieves error-free transmission atop 25 Gbps NRZ per link with 12.8 ns end-to-end reconfiguration, which provides a path toward a high-bandwidth, low-latency, and energy-efficient wavelength-switched network for data centers in the post-Moore's Law era.

We propose a method for optimizing the phase stability of microwave signal transmission over long distances. First, the design of the photon link was modified to reduce the radio frequency (RF) signal’s baseline noise and increase power. Second, a low-noise driver circuit was developed for a two-section distributed feedback (DFB) laser designed using reconstruction equivalent chirp (REC) technology to create an ultra-stable laser, and its performance was characterized through linewidth data. Test results indicate that the DFB laser achieved narrower linewidth, improving system phase stability. When an injection current (30 mA) is applied to the reflection section of the two-section DFB laser, the laser linewidth will be narrower (1.38 MHz), further enhancing the system’s phase transmission stability. At a 1 Hz offset frequency, a residual phase noise of -88.65 dBc/Hz is obtained. The short-term stability with an averaging time of 1 s is 1.60 × 10^-14, and the long-term stability over a testing time of 60,000 s is 3.41 × 10^-18. Even after incorporating temperature variations, the long-term stability reaches 8.37 × 10^-18 at 22 h.

The ongoing growth of data traffic from existing and new latency-sensitive applications poses a challenge to the data center network. Wavelength-routed optical circuit switching promises high bandwidth, low power and latency networking for data centers, but requires a wideband wavelength tunable source capable of nanosecond-scale hitless wavelength switching at every node. A tunable source based on DFB laser arrays is a potential candidate due to the high-speed dynamic property and high single-longitudinal-mode (SLM) stability of the DFB laser. However, the large-scale integration of the DFB laser and the high precision control of the lasing wavelength are still problems. In this work, we proposed and fabricated a monolithic integrated 16-channel DFB laser array with high SLM stability and 100-GHz channel spacing by using the reconstruction-equivalent-chirp (REC) technique. The maximum output power of each channel was above 6 mW. Based on the custom-designed fast switching drive board with a field-programmable gate array (FPGA), the switch-ON and OFF time between any pair of channel combinations were measured at less than 10 ns. Dynamic switching wavelengths remained stable and resided within the receiver bandwidth. Wavelength switching experiments for modulated light signals were done by driving the external modulator, achieving bit error performance at 1×10 ^-9 for 10 Gb/s NRZ data.

A novel high-speed directly modulated two-section distributed-feedback (TS-DFB) semiconductor laser based on the detuned-loading effect is proposed and simulated. A grating structure is designed by the reconstruction-equivalent-chirp (REC) technique. A π phase shift is introduced into the reflection grating, which can provide a narrow-band reflection region with a sharp falling slope on both sides of the reflection spectrum, thus enhancing the detuned-loading effect. Owing to its unique dual-falling-edges structure, the bandwidth can be improved even when the lasing wavelength shifts beyond the left falling edge due to a thermal effect in the actual test, in which condition the detuned-loading effect can be used twice, which greatly improves the yield. The modulation bandwidth is increased from 17.5 GHz for a single DFB laser to around 24 GHz when the lasing wavelength is located on the left falling edge of the TS-DFB laser based on the detuned-loading effect, and it can be increased to 22 GHz for the right side. An eight-channel laser array with precise wavelength spacing is investigated, with a side-mode suppression ratio (SMSR) >36 dB. In addition, TS-DFB lasers with uniform reflection gratings are studied, and simulated results show that the modulation characteristic is far inferior to the laser with a phase-shifted grating reflector.

A novel high-speed directly modulated two-section distributed feedback (TS-DFB) semiconductor laser based on the detuned-loading effect is proposed and simulated. Grating structure is designed by reconstruction-equivalent-chirp (REC) technique. A π phase-shift is introduced into the reflection grating, which can provide a narrow-band reflection region with a sharp falling slope on both sides of the reflection spectrum, thus enhancing the detuned-loading effect. Owing to its unique dual-falling-edges structure the bandwidth can be improved even when the lasing wavelength shifts beyond the left falling edge due to thermal effect in actual test, in which condition the detuned-loading effect can be used twice, which greatly improves the yield. The modulation bandwidth is increased from 17.5 GHz for a single DFB laser to around 24 GHz when the lasing wavelength is located on the left falling edge of the TS-DFB laser based on detuned-loading effect, and can be increased to 22 GHz for the right side. An eight-channel laser array with precise wavelength spacing is investigated, with side mode suppression ratio (SMSR) > 36dB. Besides, TS-DFB lasers with uniform reflection grating are studied and simulated result shows that modulation characteristic is far inferior to the laser with a phase-shifted grating reflector.