The Latest Research Trends In Ultra-narrow Linewidth Lasers: From Microcavity Integration To Breakthroughs In Brillouin Lasers
In modern optical technology, ultra-narrow linewidth lasers are regarded as key tools for achieving high-precision measurement and ultra-high coherence light sources. The so-called “narrow linewidth” refers to the extremely small width of the laser output spectrum, which can provide extremely high coherence and frequency stability. This type of laser can reach the kHz or even Hz level in terms of online width and is widely used in cutting-edge fields such as atomic clocks, optical frequency combs, coherent communication, quantum computing and high-resolution spectroscopy.
With the advancement of integrated photonics, precision manufacturing and low-noise control technologies, ultra-narrow linewidth lasers are moving from the laboratory to industrialization. This article will sort out the current research trends, from the latest breakthroughs in microcavity integration to Brillouin lasers, to help readers understand the future technological directions.
Research Trend One: Microcavity integration and on-chip lasers
In recent years, microcavity lasers and on-chip integration technology have become research hotspots. High-q value microcavities can significantly reduce intracavity losses, reducing the laser linewidth to the kHz or even sub-khz level. This design enables the laser to continuously reflect and oscillate in a high-Q value microcavity, making the frequency distribution increasingly concentrated and ultimately achieving linewidth compression. This not only enhances frequency stability but also takes into account the advantages of small size and low power consumption.
The maturity of silicon photonics technology makes it possible to integrate lasers on the chip. Narrow-linewidth laser sources that can be mass-produced can be manufactured by integrating gain media, resonators and modulators on silicon-based chips. This type of on-chip laser has great potential in optical communication, chip-scale optical frequency combs and precision optical sensing.
A recent study achieved a short-term linewidth of approximately 254 Hz and a threshold pump power of less than 2 mW in thin-film lithium niobate (TFLN) microdisk through dispersion engineering and high Q value design. At the same time, visible light second harmonic output was also realized on the same chip, demonstrating the potential of on-chip integration in band coverage and power consumption optimization.
Research Trend Two: Low-noise laser cavities and temperature compensation
Even with a high-Q value cavity, noise remains a key factor determining the linewidth of a laser. Researchers are exploring various means to reduce noise, including improving cavity design, using materials with low thermal expansion coefficients, and introducing active feedback control.
A common approach is to employ PDH (Pound-Drever-Hall) locking technology to lock the laser frequency onto a stable reference cavity, significantly enhancing frequency stability. Meanwhile, temperature compensation technology is constantly being optimized. For instance, dual-material cavities or micro temperature control systems are being used to reduce the impact of ambient temperature on frequency.
These low-noise design schemes have demonstrated remarkable effects in fiber optic gyroscopes, optical clocks and high-precision spectral measurements, laying the foundation for the next generation of precision measurement equipment.
Research Trend Three: Brillouin Lasers based on Stimulated Brillouin scattering
In the field of ultra-narrow linewidth lasers, Brillouin lasers are regarded as the ideal choice for achieving HZ-level linewidth. Stimulated Brillouin scattering (SBS) can provide a very narrow gain bandwidth, making the Brillouin laser output have extremely high coherence.
In the past, Brillouin lasers mostly relied on fiber circuits or bulk optical devices, which were large in size and difficult to integrate. In recent years, with the breakthroughs in microring resonators and high-Q value microcavity technology, researchers have achieved on-chip Brillouin lasers, which combine the advantages of extremely narrow linewidth and chip-level integration.
The latest research shows that in a silicon nitride coil resonator, the research team has achieved an integrated Brillouin laser system with an instantaneous linewidth of approximately 31 mHz, an output power of 41 mW, and a side-mode rejection ratio of 73 dB, demonstrating the simultaneous realization of ultra-narrow linewidth, high-purity single frequency, and tunability. This achievement indicates that the Brillouin laser not only meets the demands of scientific research but also has the potential for industrialization.
In addition, in the mid-infrared band, high-Q microcavities have achieved Brillouin lasers with a theoretical Schawlow-Townes linewidth of approximately 83 Hz, expanding applications to new fields such as molecular spectroscopy and chemical sensing.
Research Trend Four: AI-assisted Laser Design and Optimization
Artificial intelligence is becoming a new tool for photonic design. Through machine learning algorithms, cavity parameters, material selection and feedback control strategies can be rapidly iterated in a virtual environment, significantly shortening the design cycle.
Some research teams have utilized deep learning to predict laser frequency drift and automatically optimize temperature control and electric drive schemes. In the future, AI may be deeply integrated with on-chip photonic integrated circuit (PIC) design software to achieve automated design and optimization.
Challenges and Development Directions
Although research achievements keep emerging, the commercialization of ultra-narrow linewidth lasers still faces multiple challenges:
Manufacturing process limitations: High-Q value cavities require nanometer-level manufacturing precision and are relatively costly.
Temperature drift issue: Changes in ambient temperature may still cause frequency offset.
The difficulty of large-scale integration: Integrating high-performance lasers with modulation and detector devices on the same chip still requires technological breakthroughs.
The future development directions include: photonic chips with higher integration, designs with lower power consumption, and deep integration with quantum applications. For instance, quantum key distribution, atomic interferometers and high-precision navigation systems will all benefit from more stable and narrower linewidth light sources.
Conclusion
From microcavity integration to Brillouin laser breakthroughs, ultra-narrow linewidth lasers are moving towards higher performance, smaller size and easier large-scale application. With the maturation of silicon photonics technology, AI design tools and low-noise control solutions, such lasers are expected to become core components in optical communication, quantum computing and precision measurement in the future.
For researchers, this is the best time to explore fundamental physics and promote industrial application. For enterprises, making early preparations for narrow-linewidth laser technology may become a competitive advantage in the next generation of optical applications.
