Shijian Guan

We propose and experimentally demonstrate a monolithic two-section narrow-linewidth distributed feedback (DFB) laser. Leveraging the reconstruction equivalent chirp (REC) technique, the on-chip optical feedback effect is accurately introduced to achieve effective spectral linewidth reduction. Meanwhile, the 8-channel laser array with precise wavelength spacing is realized with low cost and high yield. The intrinsic linewidth of the DFB laser is compressed from 2.38 MHz to 105 kHz, while the Lorentzian linewidth from 7.69 MHz to 244 kHz. Furthermore, the laser maintains stable single-mode operation and good linewidth consistency across the temperature range of 17-41°C at 4°C intervals. Additionally, its relative intensity noise (RIN) is suppressed to below −150 dB/Hz.

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.

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.

We demonstrate a two-section distributed feedback (DFB) laser for direct modulation based on the reconstruction equivalent chirp (REC) technology. The two-section DFB laser exhibits a stable single-mode characteristic. Utilizing the photon-photon resonance (PPR) effect and detuned-loading effect, the 3-dB modulation bandwidth of the two-section DFB laser is improved from 8 GHz to 23 GHz. A 25 Gb/s non-return to zero (NRZ) signal is applied to the two-section DFB laser and eye diagrams with large open extent are obtained. In addition, the relative intensity noise is reduced to below −143 dB/Hz. The frequency chirp is also suppressed. The chirp parameter is reduced from 5.67 to 1.37, which is a small value in directly modulated DFB laser. We compared the eye diagrams after transmitting different distances, the results show that the two-section structure reduces the frequency chirp significantly. The modulation of the two-section DFB laser also has good linearity, with a 1-dB compression point of more than 18 dBm.

A monolithic integrated dual-wavelength DFB semiconductor laser is fabricated and experimentally demonstrated. Utilizing the reconstruction-equivalent chirp (REC) technique, dual-wavelength lasing is obtained by introducing two symmetric equivalent phase shifts in a chirped sampled Bragg grating. The laser operates at 1650 nm and the wavelength spacing is about 0.8 nm at room temperature. The frequency of the beat signal between the two wavelengths is 88.47 GHz, and the linewidth of the beat signal is 5.1 MHz. Based on the REC technique, the grating pattern can be fabricated by using holographic exposure combining with conventional lithography. This work provides a simple and cost-effective way to achieve dual-wavelength semiconductor laser.

A distributed feedback (DFB) laser for high-speed modulation with asymmetric corrugation-pitch-modulated (ACPM) grating structure and high-reflection and anti-reflection coating (HR-AR) coating is demonstrated theoretically and experimentally in this paper. The ACPM gating structure is fabricated by the reconstruction equivalent chirp (REC) technology, which is with low cost. The impact of the longitudinal spatial hole burning (LSHB) effect and the facet phase on the characteristics of the ACPM DFB laser is discussed in detail. The numerical simulation results show that the proposed ACPM DFB laser can suppress the LSHB effect significantly. Comparing with the conventional HR-AR DFB laser, the ACPM DFB laser can achieve higher output efficiency, better single-mode yield, and wider modulation bandwidth. The modulation eye diagrams of HR-AR DFB lasers with different grating structures are also presented. The experimental results show that the LSHB effect in the HR-AR DFB laser with ACPM structure is suppressed effectively. The laser with ACPM structure fabricated by the REC technique has high slope efficiency, stable single longitudinal mode operation, and wide modulation bandwidth.

An integrated single sideband (SSB) modulation method benefitting from the sideband amplification injection locking (SAIL) effect for radio-over-fiber (ROF) link is proposed and experimentally demonstrated, which is based on an integrated multi-section mutual injection (IMSMI) DFB laser. The IMSMI-DFB laser, which overcomes the drawbacks of using discrete devices, simplifies the whole structure. The SAIL effect, which is regulated by the input RF power and bias current of IMSMI-DFB laser, promises a good pathway for adjusting and controlling the modulation frequency, optical carrier to sideband ratio (OCSR) and sideband suppression ratio (SSR), meanwhile, provides high gain in the link. The characteristics of SAIL effect are investigated. The OCSR can be optimized to 0 dB, the SSR is over 26 dB and the transmission link gain improves over 20 dB from 16 GHz to 26 GHz. The ROF transmission experiments demonstrate that the power fading introduced by fiber dispersion is dramatically eliminated, and the error vector magnitudes (EVMs) of the received 20 MSymbol/s 32 QAM signals at frequencies from17 GHz to 22.3 GHz after 50 km fiber transmission are below 4.8%. This work provides an effective way to realizing integrated direct SSB modulation with outstanding features for ROF link.

Single sideband (SSB) modulation based on a monolithic integrated optically injection-locked (MOIL) DFB laser is demonstrated experimentally. To our knowledge, this is the first time to realize SSB modulation utilizing a MOIL DFB laser. Tunable SSB signals with large lower-to-upper sideband power ratios are achieved by changing the bias currents of the MOIL DFB laser. The largest lower-to-upper sideband power ratio is measured to be 24.4 dB which is very useful for improving the transmission performance of a radio-over-fiber link. Due to the generated SSB modulation, the fiber transmission response is flattened significantly and a signal of 25 GHz is transmitted through 85-km fiber without being affected by the fiber chromatic dispersion. 20 MSymbol/s 32-QAM signals with subcarriers of 21 GHz, 23 GHz and 25 GHz are also transmitted by the proposed SSB modulation link, and both the error vector magnitude (EVM) and the error bit rate (EBR) of the link are improved distinctly. The proposed method can effectively suppress the effect of fiber chromatic dispersion.