The source said that Huazhong Optoelectronics to "external cavity narrow line width semiconductor laser research progress" as the theme of the review

Narrow-width laser has very high spectral density, very low relative intensity noise and phase noise, and very long coherence length. It plays a very important role in many frontier scientific research fields, such as cold atom interferometry, optical atomic clock, coherent laser communication, coherent laser radar, high-precision optical fiber sensing and precision spectrum.

According to the different frequency selection structure distribution inside and outside the active cavity, the narrow linewidth semiconductor laser is generally divided into internal cavity feedback type and external cavity feedback type. At present, the commercial application of external cavity narrow - width semiconductor laser has a pivotal position.

According to Maims Consulting, recently, a research team from Huazhong Optoelectronic Technology Institute -- Wuhan Optoelectronic National Research Center carried out a review on the theme of "Research Progress in external cavity narrow linewidth semiconductor lasers", focusing on the basic structure, key technologies, performance characteristics and international development status of four representative types of external cavity narrow linewidth semiconductor lasers. The research is in the journal Optics and Optoelectronic Technology.

Shining grating type

Blazed Grating as a frequency selection optical feedback component has long been used in Littrow - and Littman structured external-cavity lasers, and at present, it has become one of the classical external-cavity semiconductor lasers. The resonator is usually composed of a semiconductor laser Gain Chip, an optical collimating lens or mirror, and a shining grating. In 2020, D. P. Kapasi et al. from the Center for Gravitational Wave Astrophysics in Australia reported a Littrow-type structure of shining grating outer cavity narrow linewidth laser based on gain chip and diffraction grating. The central wavelength is 2 μm, the maximum output power is 9.3 mW, the laser linewidth is 20 kHz@10 ms, and the tuning range of 120 nm is finally achieved by piezoelectric transducer. The structure and performance characteristics of this kind of external cavity laser are more suitable for laboratory application scenarios, and can be suitable for the field of friendly working environment but high wavelength tuning range and spectral resolution requirements, such as spectral detection, nonlinear optical test system, optical atomic clock, Rydberg atomic measurement system and other fields.

Australian Centre for Gravitational Wave Astrophysics outer cavity semiconductor laser

Volume holographic grating type

Volume Holographic Gratings (VHG) is a diffraction grating device made by using ultraviolet holographic exposure in molten silicate glass. The holographic grating can be divided into two types: reflection and transmission. The holographic grating can also be used to realize an external cavity narrow line-width semiconductor laser with Littrow and Littman structures. Relatively speaking, the volume grating is smaller than the shining grating, the working band can be extended to 350~2500 nm, has the high optical damage threshold and mechanical and thermal stability, shows excellent stability at high temperature, frequency selection temperature drift coefficient is small, can be used to make miniaturized high reliable laser. In 2020, the Ferdinand Braun Institute developed a micro-integrated narrow line width laser for the JOKARUS mission, which employs the Master Oscillator Power Amplifier (MOPA) technology system, using the ECDL as a seed source cascade cone amplifier. The output power is greatly improved while the laser line width is relatively narrow. The central wavelength of the laser system is 1064.490 nm, the optical power is 570 mW, the linewidth is 13 kHz (1 ms), and the total power consumption is 3.75 W, which is used for iodine frequency reference on space exploration rocket.

JOKARUS Micro-integrated ECDL-MOPA laser device

In this field, the Central China Institute of Optoelectronics Technology has also developed a 14-pin butterfly metal package grating with extended outer cavity micro-integrated narrow-width semiconductor laser. The test laser line-width is less than 70 kHz, the mode-skipping tuning range is 10 GHz, the power is greater than 180 mW, the side-mode rejection ratio is greater than 60 dB, the device volume is 30 mm × 12.7 mm × 7.6 mm, the device has excellent wavelength and power stability, and the volume is small, the power consumption is low, and the environmental adaptability is excellent. Its many indexes are better than Sacher's narrow line width semiconductor laser products.

Central China Institute of Optoelectronic Technology microintegrated narrow linewidth laser

Optical cavity type

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

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

Planar waveguide pattern

The Planar Light Waveguide Chip (PLC) is an important application of photonic integration technology, which provides more diversified and flexible choices for narrow-band filtering and optical feedback devices in external cavity feedback semiconductor lasers. In 2021, of the university of California, Santa Barbara, W.J et al. reported the ultra narrow linewidth in hybrid integrated narrow linewidth laser, active part for the DFB laser, passive filter part is the quality factor of 2.6 x 10 ⁸ Si3N4 micro ring, with low limit silicon nitride waveguide structure light transmission loss dropped to 0.1 dB/m, Finally, 3 Hz linewidth output is achieved.

Schematic diagram of external cavity structure and its injection locking principle

At present, semiconductor lasers are developing rapidly in the direction of high power and narrow linewidth. Especially, external cavity semiconductor lasers based on volume grating can achieve linewidth below 100 kHz and high optical power, and have excellent environmental adaptability. The performance and reliability of this system have also been verified in space exploration missions. At the same time, the cavity structure is simple, the automatic coupling system can realize the commercial application, the cost is low, but the linewidth is difficult to further reduce. Ultra-narrow linewidth laser of less than 100 Hz can be realized based on narrow linewidth laser of echo wall optical cavity. However, optical coupling between DFB chip and lens, prism and echo wall optical cavity is required, which makes coupling integration extremely difficult and mass production difficult. Besides, high requirements of high quality factor echo wall cavity lapping process make it difficult to spread the cost in the short term. The preparation of waveguide grating devices based on hybrid integration scheme is compatible with traditional micro and nano processes, and is suitable for large-scale commercial preparation at low cost. This scheme can be coupled and integrated with gain chip to achieve high stability single longitudinal mode injection locking effect, and has excellent linewidth and noise performance and high integration. It is one of the best methods for producing narrow line width lasers commercially at present.

With the development of photonic integration technology, heterogeneous integration has high robustness to shock and vibration, and has great attraction for semiconductor lasers with low noise, narrow linewidth and high compactness. In the future, silicon-based heterogeneous integrated Ⅲ-Ⅴ group narrow linewidth semiconductor lasers based on wafer bonding will certainly become the most promising solution for development.

Source: MEMS