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External cavity narrow linewidth semiconductor laser

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.

According to Memes Consulting, recently, the research team of Huazhong Institute of Optoelectronics Technology-Wuhan Optoelectronics National Research Center conducted a review study on the theme of "Research Progress in External Cavity Narrow Linewidth Semiconductor Lasers", focusing on the current representative ones. The basic structure, key technology, performance characteristics and international development status of four types of external cavity narrow linewidth semiconductor lasers. Relevant research content was published in the journal Optics and Optoelectronics Technology.

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 collimation 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. The tuning range of 120 nm is finally realized through the piezoelectric transducer. 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.

JOKARUS micro-integrated ECDL-MOPA laser device

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.

Huazhong Institute of Optoelectronics Technology Micro-integrated Narrow Linewidth Laser

Optical cavity

Crystal optical cavities have the advantages of high quality factor, small mode volume and stable performance, and can be used as optical filters to achieve laser locking and laser linewidth narrowing. Optical microcavities are generally divided into Fabry-Perot cavities, whispering gallery mode (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 based on MgF2 microcavities and high-power F-P cavity lasers and a soliton Kerr optical comb. 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.

Schematic diagram of the external cavity structure and its injection locking principle

At present, semiconductor lasers are developing rapidly in the direction of high power and narrow linewidth. In particular, external cavity semiconductor lasers based on volume gratings can achieve linewidths below 100 kHz, high optical power, and excellent environmental adaptability. The performance and reliability of the system have also been verified in space exploration missions; at the same time, the cavity structure is simple, and commercial applications can be realized through a fully automatic coupling system. The cost is low, but the line width is difficult to further reduce. Ultra-narrow linewidth lasers below 100 Hz can be realized based on the narrow linewidth laser of the whispering gallery optical cavity, but the optical coupling between the DFB chip and the lens, prism and whispering gallery optical cavity is required, the coupling integration is extremely difficult, and mass production is difficult. The high-quality factor whispering gallery cavity has high requirements for grinding and processing technology, and it is difficult to dilute the cost in the short term. The preparation of waveguide grating devices based on the hybrid integration scheme is compatible with traditional micro-nano processes, and is suitable for large-scale commercial preparation at low cost. This scheme can be coupled and integrated with gain chips to achieve high-stability single longitudinal mode injection locking The effect, linewidth and noise performance are also excellent and have a high degree of integration. It is one of the best solutions for commercially manufacturing narrow linewidth lasers at this stage.

With the development of photonic integration technology, heterogeneous integration has high robustness to shock and vibration, and is very attractive for realizing low-noise, narrow-linewidth, and high-compact semiconductor lasers. In the future, wafer-bonding-based Silicon-based heterogeneous integrated III-V narrow-linewidth semiconductor lasers will surely become the most promising solution. GEM optoelectronic provides

ceramic DPC components for high power lasers.

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