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Research Progress of External Cavity Narrow Linewidth Semiconductor Lasers

Narrow linewidth lasers have extremely high spectral density, extremely low relative intensity noise and phase noise, and ultra-long coherence length. Many cutting-edge scientific research fields such as precision spectroscopy play a very important role. According to the distribution of the frequency selective structure inside and outside the active cavity, narrow linewidth semiconductor lasers are generally divided into internal cavity feedback type and external cavity feedback type laser. At present, the commercial application of external-cavity narrow-linewidth semiconductor lasers plays a pivotal role.


Blazed grating type


The use of blazed gratings (Blazed Grating) as a frequency-selective optical feedback device has long been used in external cavity lasers based on Littrow and Littman structures, and has become one of the classic external cavity semiconductor lasers. Its resonant cavity is usually composed of optical components such as semiconductor laser gain chip (Gain Chip), optical collimating lens or mirror, and blazed grating. In 2020, D. P. Kapasi and others from the Australian Center for Gravitational-Wave Astrophysics reported a blazed grating external-cavity narrow-linewidth laser based on a Littrow-type structure composed of a gain chip and a diffraction grating. The central wavelength is 2 μm, the maximum output power is 9.3 mW, and the laser linewidth is 20 kHz@10 ms. It is tuned by a piezoelectric transducer, and finally a tuning range of 120 nm is achieved. The structure and performance characteristics of this type of external cavity laser are more suitable for laboratory application scenarios, and can be used in fields that are friendly to the working environment but require high wavelength tuning range and spectral resolution, such as spectral detection, nonlinear optical test systems, optical atomic clocks, Rydberg atomic measurement system and other fields.


External cavity diode laser at the Australian Center for Gravitational-Wave Astrophysics


Volume Holographic Grating Type


Volume Holographic Gratings (Volume Holographic Gratings, VHG) is a kind of diffraction grating device made by ultraviolet holographic exposure in fused silicate glass. It is divided into two types: reflection type and transmission type. Cavity narrow linewidth semiconductor lasers. Relatively speaking, the volume grating is smaller than the blazed grating, and the working band can be extended to 350~2500 nm. It has high optical damage threshold, mechanical and thermal stability, excellent stability at high temperature, and small frequency selection temperature drift coefficient. , can be used to make miniaturized and highly reliable lasers. In 2020, the Ferdinand Braun Institute developed a micro-integrated narrow-linewidth laser for the JOKARUS mission. This device adopts the Master Oscillator Power Amplifier (MOPA) technology system and cascades with ECDL as the seed source. The conical light amplification greatly increases the output power while maintaining a narrow laser linewidth. The laser system has a central wavelength of 1064.490 nm, an optical power of 570 mW, a line width of 13 kHz (1 ms), and a total power consumption of 3.75 W. It is used as an iodine frequency reference for space exploration rockets.



In this field, Huazhong Institute of Optoelectronics Technology also developed a 14 pin butterfly metal package grating extended external cavity micro-integrated narrow linewidth semiconductor laser. The linewidth of the test laser is less than 70 kHz, the tuning range without mode hopping is 10 GHz, the power is greater than 180 mW, the side mode suppression ratio is greater than 60 dB, and the device volume is 30 mm × 12.7 mm × 7.6 mm. The device has excellent wavelength and power stability , and small size, low power consumption, excellent environmental adaptability, and many of its indicators are better than the narrow linewidth semiconductor laser products of German Sacher company.



Optical cavity


The crystal optical cavity has the advantages of high quality factor, small mode volume and stable performance. It can be used as an optical filter, which can realize laser locking and laser linewidth narrowing. Optical microcavities are generally divided into Fabry-Perot cavities, Whispering-Gallery Mode (WGM) microcavities and photonic crystal cavities. In 2018, M.L.Gorodetsky et al. of the Russian Quantum Research and Development Center reported the generation of an injection-locked narrow linewidth laser and a soliton Kerr optical comb based on MgF2 microcavities and high-power F-P cavity lasers. The output light of the F-P cavity laser with a power of 200 mW was injected into a microcavity with a horizontal diameter of 5.5 mm and a vertical diameter of 500 μm, and a narrow linewidth output of 370 Hz was obtained in the injection-locked state. When the current of the laser was further tuned into the red In the detuned state, a soliton optical frequency comb with a repetition rate of 12.5 GHz is generated.



Injection-locked narrow linewidth laser and soliton Kerr optical comb generation system based on MgF2 microcavity and high-power F-P cavity laser


Planar waveguide


Planar Light Waveguide Chip (PLC) is an important application of photonic integration technology, which provides more diverse and flexible options for narrowband filtering and optical feedback devices in external cavity feedback semiconductor lasers. In 2021, W.Jin et al. from the University of California, Santa Barbara reported an ultra-narrow linewidth hybrid integrated narrow linewidth laser, the active part is a DFB laser, and the passive filtering part is a Si3N4 microring with a quality factor of 2.6×10⁸ , the light transmission loss is reduced to 0.1 dB/m by using a low-limit silicon nitride waveguide structure, and finally a 3 Hz linewidth output is achieved.









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