Ultra-narrow linewidth laser stabilization techniques play an important role in various scientific and technological applications, ranging from high-precision spectroscopy to advanced interferometry. This article discusses ultra-narrow linewidth laser stabilization techniques and their applications.
Image Credit: Wissawa Chamaboon/Shutterstock.com
Importance of Linewidth of Laser
The term linewidth can be described as the spectral width of the emitted light from a laser source or the measure of the spread of its optical frequencies. A laser with a narrow linewidth emits light with highly consistent and stable optical frequencies, making it ideal for high-precision applications. The linewidth is inversely proportional to the coherence length of the laser, and a narrower linewidth implies a longer coherence length, which ensures that the laser light remains in phase over a greater distance.
Factors Affecting Linewidth of a Laser
Laser gain medium influences the linewidth of a laser since different gain media exhibit varying spectral broadening levels; hence, choosing gain medium becomes crucial in designing lasers with ultra-narrow linewidths. Similarly, laser cavity design also plays a role in determining linewidth; hence, techniques such as single-mode operation and cavity stabilization are employed to narrow the linewidth by suppressing unwanted longitudinal modes.
Another factor that can affect linewidth of a laser is Doppler broadening, in which the motion of atoms or molecules in a gas or solid-state medium spreads the frequencies of the emitted light. Cooling techniques, like using cryogenic environments, are often employed to reduce Doppler broadening and achieve ultra-narrow linewidths.
External Cavity Diode Lasers
External cavity diode lasers (ECDLs) offer a flexible platform for achieving ultra-narrow linewidths by utilizing an external cavity to enforce single-mode operation and achieve higher coherence lengths. Various feedback mechanisms, including grating feedback and acousto-optic modulators, are employed to stabilize the laser frequency and reduce linewidth.
For instance, a 2018 study presents a novel ECDL configuration using a commercially available broad bandwidth (about 4 nm) interference filter as the wavelength discriminator resulted in a narrow Lorentzian fitted linewidth of 95 kHz, a spectral purity of 2.9 MHz, and a remarkable long-term frequency stability of 5.59×10−12. The design not only overcomes cost constraints associated with narrow bandwidth filters but also demonstrates promising performance, suggesting a potential breakthrough in laser stabilization technology.
Optical Frequency Combs
Optical frequency combs provide a set of equally spaced frequency references. Researchers can achieve ultra-narrow linewidths and high-frequency accuracy by stabilizing a laser to the comb. For example, in a 2019 study, researchers have developed a multi-channel optical frequency synthesizer that generates stable continuous-wave lasers directly from the optical comb of an Er-doped fiber oscillator. The synthesizer is stabilized to a high-finesse cavity, achieving fractional frequency stability of 3.8 × 10-15 at 0.1 s. The system produces multiple optical frequencies with ultra-narrow linewidths of 1.0 Hz at 1 s, each channel delivering tens of mW output power.
The key mechanism involves diode-based stimulated emission by injection locking, enabling the comb frequency modes to amplify without compromising stability. The synthesizer allows individual selection of channel frequencies with a 0.1 GHz increment over a broad 4.25 THz bandwidth around a 1550 nm center wavelength.
Pound-Drever-Hall (PDH) Technique
Pound-Drever-Hall is an active stabilization technique in which a modulated sideband is introduced into the laser beam, and the reflected light from a Fabry-Perot cavity is analyzed. The linewidth can be actively stabilized by adjusting the laser frequency based on the detected error signal. For instance, researchers in a 2006 study designed and implemented a frequency-locking system for infrared fiber lasers, achieving a significant reduction in linewidth. The system utilized a high-finesse Fabry-Perot cavity as a frequency reference and employed an electronic feedback loop to maintain stable optical power.
The input laser, with a narrow initial linewidth of 1-2 kHz, underwent frequency stabilization using the PDH technique. The study highlighted the importance of infrared wavelength selection, which enables more stable input lasers for improved stabilization. Additionally, the system included polarization control and frequency modulation components to further enhance stability.
Narrow line widths are vital in various laser applications, like coherent light communication, precise measurements, atomic physics, and atom clocks. For instance, ultra-narrow linewidth lasers enable researchers to achieve high-resolution atomic and molecular spectroscopy, resolving fine spectral features to understand fundamental processes and intricate details of molecular structures and interactions.
Ultra-narrow linewidth lasers contribute to the stability of experimental quantum information processing setups to ensure accurate control over quantum systems. Similarly, ultra-narrow linewidth lasers enhance the sensitivity of some laser interferometers that require ultra-stable lasers to detect minuscule changes in distance.
In conclusion, ultra-narrow linewidth laser stabilization techniques are crucial for diverse applications, including high-precision spectroscopy, quantum information processing, and more. Factors like laser gain medium and cavity design influence the linewidth of lasers. Techniques like External Cavity Diode Lasers, Optical Frequency Combs, and the Pound-Drever-Hall technique are used for Ultra-narrow linewidth laser stabilization, contributing significantly to coherent communication, precise measurements, and quantum control.
More from AZoOptics: What to Know About Integrated Photonics in Data Centers
References and Further Reading
Cooper, B. S., Alonso, A. M., Deller, A., Liszkay, L., & Cassidy, D. B. (2016). Positronium production in cryogenic environments. Physical Review B. https://doi.org/10.1103/PhysRevB.93.125305
Guo, X., Zhang, L., Chen, L., Liu, J., Liu, T., & Zhang, S. (2023). Ultra-narrow linewidth laser system based on intra-cavity electro-optic crystal frequency stabilization of external-cavity diode laser. Optics Communications. https://doi.org/10.1016/j.optcom.2023.129635
Hui, S. H. E. N., Liu-Feng, L. I., & Li-Sheng, C. H. E. N. (2016). Lasers with ultra-narrow linewidth——Theories and applications of laser frequency stabilization. PHYSICS. http://dx.doi.org/10.7693/wl20160704
Jang, H., Kim, B. S., Chun, B. J., Kang, H. J., Jang, Y. S., Kim, Y. W., ... & Kim, S. W. (2019). Comb-rooted multi-channel synthesis of ultra-narrow optical frequencies of few Hz linewidth. Scientific reports. https://doi.org/10.1038/s41598-019-44122-5
Lally, E. M. (2006). A narrow-linewidth laser at 1550 nm using the Pound-Drever-Hall stabilization technique (Doctoral dissertation, Virginia Tech). http://hdl.handle.net/10919/34739
Pan, G. Z., Guan, B. L., Xu, C., Li, P. T., Yang, J. W., & Liu, Z. Y. (2018). Broad bandwidth interference filter-stabilized external cavity diode laser with narrow linewidth below 100 kHz. Chinese Physics B. https://doi.org//10.1088/1674-1056/27/1/014204