| The narrow linewidth semiconductor laser is a high-performance laser light source.It shows excellent performance in spectral purity,coherence length,frequency stability,phase and intensity noise.It is widely used in coherent optical communication,space laser communication,Li DAR and high-precision spectral sensing.It is the key component of coherent communication,detection,sensing and other systems.Nowadays,with the continuous improvement of the performance level of these systems,higher requirements are put forward for narrow linewidth semiconductor lasers with low cost and high performance.The essential source of laser noise and linewidth is related to the fluctuation of optical field amplitude and frequency caused by carrier fluctuation in the cavity.This fluctuation is caused by spontaneous emission photons and the competition between intracavity modes,including the competition between transverse mode,longitudinal mode and intrinsic polarization mode.Therefore,the realization of single longitudinal mode laser with high polarization stability is the key to suppress the competition of various modes in the cavity,reduce the relative intensity noise and realize narrow linewidth.Among the above optical systems,coherent optical communication system is very sensitive to the linewidth,noise and polarization characteristics,so the above characteristics of laser are very demanding.In the application scenario of the quadrature phase modulation,not only the signal light source with narrow linewidth and low noise is required to ensure low bit error rate signal transmission,but also the signal light has high polarization stability.Especially in the application of the 90°optical mixer,the signal light generation of the specific polarization state and high-speed external modulation.However,the polarization extinction ratio of the narrow linewidth semiconductor lasers is not high enough to meet the rapid development of communication technology.Most of the commonly used linear polarization narrow linewidth light sources are fiber lasers.In view of the above problems,this research innovatively proposes to introduce the specific polarization mode gain suppression into the semiconductor laser resonator cavity.Based on the external cavity optical negative feedback effect,the stable single longitudinal mode lasing is achieved due to the self-injection locking,realizing the suppression of linewidth and noise and improve the stability of output polarization mode through linear polarization and narrow-band external cavity waveguide filter devices.The innovative work completed is as follows:1.A new type of narrow-band polarization mode selection device is designed.A weak coupling long grating structure with surface etching is proposed to introduce strong stress birefringence and shape birefringence on the surface of the waveguide core,which has the function of narrow-band filtering.2.A novel narrow linewidth semiconductor laser with small size,high compactness and high polarization stability is prepared by using the above new narrow-band polarization mode selection device and piezoelectric quantum well gain chip;3.The chirp factor reduction theory is used to analyze the high reflection film gain chip coupled with high reflectivity grating structure to better clarify the physical mechanism of linewidth narrowing and stable operation of frequency noise,and analyze the linewidth difference between different lasing modes,which provides guidance for finding the best linewidth working point in the practice of the laser structure.The specific work completed is as follows:(1)The equivalent resonator model of gain chip linear and polarization waveguide filter is constructed.Based on the rate equation and Langevin noise theory,the influence of multiple structural parameters on the final linewidth performance of hybrid integrated semiconductor laser is analyzed,which provides an important design idea and optimization direction for the design of laser structure.(2)The numerical simulation of external cavity waveguide Bragg grating is accomplished,the influence of grating device structure on spectral performance is analyzed,and the design of narrow bandwidth filter device is guided;According to the principle of anisotropic birefringence,a waveguide Bragg grating fabricated on the high stress birefringent waveguide platform is proposed.The weak coupling coefficient and the birefringence are adjusted by the surface etching grating.A kind of waveguide Bragg grating with bandwidth of 0.3 nm and birefringence value of 1.650×10-3 order is realized,this device shows excellent narrow bandwidth and differential polarization mode filtering function.(3)A novel narrow linewidth semiconductor laser with small size and high compactness and high polarization stability is fabricated by hybrid integration.The laser has a simple structure with a minimum integral linewidth of about 4.15 kHz,more than 8 mW output power in fiber,the relative intensity noise of less than-155dBC/Hz@1MHz,and the polarization extinction ratio of more than 39 dB.Its polarization stability is equivalent to that of the optical fiber narrow linewidth laser with polarization control,showing excellent noise and linewidth characteristics.Moreover,the excellent polarization performance of our laser can be obtained without introducing polarization controller.(4)Based on the chirp factor reduction theory,the actual linewidth narrowing factor at different wavelength positions in the Bragg band gap is analyzed and calculated.According to the numerical simulation results,the equivalent resonator produces a strong negative feedback effect at the energy band gap away from the grating resonance peak,so it has a larger linewidth narrowing factor and better linewidth performance at the position away from the Bragg resonance center,The principle that high reflectivity grating has better linewidth performance is expounded;Considering the imperfect antireflection film,the reason for the linewidth difference between the three modes is analyzed.The experimental results are compared with the theory.(5)The gain chip with shorter cavity length is used to alleviate the competition between modes.At the same time,it has better linewidth and noise performance under the same injection current.It reaches 4.36 kHz under 155 mA injection current,and the noise performance is lower than–158 dBc/Hz@1MHz.The experiment shows that the power saturation effect has a great impact on the performance of the laser and the maximum power of 6.53 mW.The polarization extinction ratio is still above 38.6 dB. |
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