Qilai Zhao

Broad-bandwidth near-shot-noise-limited intensity noise suppression of a single-frequency fiber laser

A significant broad-bandwidth near-shot-noise-limited intensity noise suppression of a single-frequency fiber laser is demonstrated based on a semiconductor optical amplifier (SOA) with optoelectronic feedback. By exploiting the gain saturation effect of the SOA and the intensity feedback loop, a maximum noise suppression of over 50 dB around the relaxation oscillation frequencies and a suppression bandwidth of up to 50 MHz are obtained. The relative intensity noise of -150  dB/Hz in the frequency range from 0.8 kHz to 50 MHz is achieved, which approaches the shot-noise limit. The obtained optical signal-to-noise ratio is more than 70 dB. This near-shot-noise-limited laser source shows important implications for the advanced fields of high-precision frequency stabilization, quantum key distribution, and gravitational wave detection.

1120 nm kHz-linewidth single-polarization single-frequency Yb-doped phosphate fiber laser

A spectrally clean kHz-linewidth single-polarization single-frequency distributed Bragg reflector Yb-doped phosphate fiber (YPF) laser at 1120 nm (> 1100 nm) for the first time is demonstrated. By enhancing the reflectivity of output fiber Bragg grating and optimizing the length of YPF to implement the effective ASE suppression and single-longitudinal-mode long-wavelength lasing, a stable output power of over 62 mW is achieved from a 31-mm-long highly YPF with a linewidth of 5.7 kHz. The signal to noise ratio of > 67 dB, the polarization extinction ratio of > 25 dB, and the relative intensity noise of < -150 dB/Hz for the frequencies above 10.0 MHz are obtained in such single-frequency fiber laser. This narrow linewidth fiber laser is an ideal laser source to generate the coherent single-frequency 560 nm light via frequency doubling for biochemical analysis application.

Ultra-narrow linewidth full C-band tunable single-frequency linear-polarization fiber laser

An ultra-narrow linewidth full C-band tunable single-frequency linear-polarization fiber laser based on self-injection locking has been demonstrated. By the use of a tunable narrow-band fiber Fabry-Perot interferometer, the laser wavelength could be flexibly tuned from 1527 to 1563 nm with linewidths of < 700 Hz. The laser frequency noise is less than 40 dB re Hz/Hz1/2 at low frequencies (< 100 Hz) and reaches -5 dB re Hz/Hz1/2 at around 25 kHz. The measured relative intensity noise (RIN) is less than -130 dB/Hz with regard to frequencies of over 3 MHz, while the obtained linear polarization extinction ratio (LPER) is higher than 28 dB. This ultra-narrow linewidth low-noise tunable single-frequency linear-polarization fiber laser provides a promising candidate for high-order quadrature amplitude modulation (QAM) optical communication system.

Linewidth suppression mechanism of self-injection locked single-frequency fiber laser.

Linewidth suppression mechanism of the self-injection locked single-frequency fiber laser (SFFL) is investigated theoretically and experimentally. An analytical model based on the semi-phenomenological approach is built up to characterize the optical feedback in SFFL. According to the theoretical prediction, the linewidth tends to be reduced with longer external cavity photon lifetime. Experimentally, a 200-Hz linewidth self-injection locked SFFL is achieved with 101 m long delay fiber, which agrees well with the theoretical simulation. The model provides a new perspective to understand the mechanism of linewidth reduction of self-injection locked SFFL.

Short all Tm-doped germanate glass fiber MOPA single-frequency laser at 1.95 μm

Based on heavily Tm-doped germanate glass fibers (TGFs), a short all-TGF MOPA laser system with uniform core parameters in each stage was demonstrated. An 11.7 W stable single-frequency laser at 1.95 μm with an optical-to-optical conversion efficiency of 20.4% is obtained from a homemade 31-cm-long double-cladding single-mode TGF. The estimated stimulated Brillouin scattering (SBS) threshold of 980 W and the measured relative intensity noise of < -130 dB/Hz for frequencies above 2 MHz are achieved in this MOPA system. Furthermore, the prospect for further power-scaling of such short MOPA laser is considered.

Frequency noise of distributed Bragg reflector single-frequency fiber laser

We investigated the frequency noise in the distributed Bragg reflector single-frequency fiber laser (DBR-SFFL) theoretically and experimentally. A complete theoretical analysis is demonstrated by considering the energy-transfer upconversion (ETU) process and establishing linkages between the frequency noise and the relative intensity noise (RIN) of the DBR-SFFL. The experimental results of the diverse DBR-SFFLs in different working conditions are in good agreement with the theoretical analyses. These investigations are beneficial to optimizing frequency noise property to promote the wide application of the DBR-SFFLs. The proposed results can be generally applicable to the short-linear-cavity SFFL with centimeters order of the cavity length.

52 W kHz-linewidth low-noise linearly-polarized all-fiber single-frequency MOPA laser

An all-fiber Yb-doped kHz-linewidth low-noise linearly polarized single-frequency master-oscillator power-amplifier (MOPA) laser with a stable CW output power of >52 W is demonstrated. By suppressing the intensity noise of the DBR phosphate fiber oscillator, the linewidth of MOPA laser is not noticeably broadened, and an ultra-narrow linewidth of 63 dB are achieved.

Mechanism of solitary wave breaking phenomenon in dissipative soliton fiber lasers

We numerically and experimentally investigate the pulse evolution to the edge of destabilization against pumping powers in a strongly dissipative-dispersive laser configuration mode locked by nonlinear polarization evolution (NPE) technique. Two distinct dynamic processes are indicated by numerical results and further evidenced by experimental observations, where one depicts the monotonous increase in peak power and slight narrowing of duration, the other is different in exhibiting obvious broadening in temporal domain. Correspondingly, it is demonstrated in the simulation of cavity dynamics that the artificial saturable absorber plays two opposite roles in pulse shaping, which implies the switch of cavity feedback. Mechanisms with respect to different cavity feedbacks are analyzed based on a newly-proposed theoretical viewpoint, for positive feedback single pulse operation is restricted by the limit of peak power mainly dependent of the gain bandwidth; for negative feedback the breakup is attributed to the limited strength of clamping effect determined by multiple ingredients.

Efficient 1.6 μm linearly-polarized single-frequency phosphate glass fiber laser

Based on the heavily Er3+/Yb3+ co-doped phosphate glass fiber (EYPF) with a larger emission cross-section, an efficient linearly-polarized single-frequency distributed Bragg reflector fiber laser at 1603 nm is demonstrated. By balancing the cavity length against the longitudinal mode spacing, a stable single-longitudinal-mode laser with more than 20 mW is generated from a 16-mm-long EYPF. The measured relative intensity noise of the fiber laser is less than –140 dB/Hz at frequencies of over 5 MHz. The signal-to-noise ratio is greater than 62 dB and the linewidth is less than 1.9 kHz, while the obtained polarization extinction ratio is higher than 25 dB. The L-band operating combined with the narrow linewidth and low noise characteristic makes this laser an ideal candidate for high-resolution molecular spectroscopy and coherent lidar applications.

Self-injection locked and semiconductor amplified ultrashort cavity single-frequency Yb 3+ -doped phosphate fiber laser at 978 nm

Based on a self-injection locking scheme and the nonlinear amplification effect of a semiconductor optical amplifier, a low intensity noise amplified ultrashort cavity single-frequency fiber laser at 978 nm is demonstrated with a final output power of > 230 mW and a broad temperature range of > 15 °C for single-longitudinal-mode operation. The effective cavity length of the fiber oscillator is less than 6 mm, comprising a 3.5-mm-long highly Yb3+-doped phosphate fiber and a pair of fiber Bragg gratings. For the frequency range from 1.8 to 10 MHz, the relative intensity noise close to -150 dB/Hz is achieved. The signal-to-noise ratio of > 68 dB and the laser linewidth of < 10 kHz are obtained. Such narrow linewidth low noise 978 nm laser is promising, as the high-performance pump source or the efficient blue and UV light sources after nonlinear frequency conversion.