Improving cavity-enhanced spectroscopy with a U-cavity system

A recently published study in applied Sciences proposes a U-cavity system to increase the performance of cavity-enhanced spectroscopy systems. The asymmetric ripple effect has a significant impact on spectral system performance. This effect differs from the etalon effect. The folding mirror and end mirror measurements of the spectral curve variations show the principal manifestation of this unusual phenomenon in the U-cavity system.

Researchers studied the properties of the transmission spectrum in the presence of the etalon effect based on multi-beam interference theory. The strength of each mirror’s etalon effect is inversely proportional to its transmission loss value, which means that the larger the loss, the smaller the transmission spectrum ripple in the U-cavity system.

Potential Benefits of Cavity-Enhanced Absorption Spectroscopy

Trace gas analysis is widely used in environmental detection, medical diagnosis, atmospheric research and industrial process management in agriculture. Laser spectroscopy provides high-precision concentration detection of trace gases between multiple coexisting species with fast response, high resolution, and wide range.

The most widely used laser spectroscopy methods are based on infrared absorption technologies. Infrared absorption technologies include non-dispersive infrared absorption, cavity ring-down spectroscopy, and cavity-enhanced absorption spectroscopy.

The cavity-enhanced absorption spectroscopy technique is crucial for laser spectroscopy. This method was proposed in 1998 to aid in cavity ring-down spectroscopy. The high-quality passive reassurance effect of the optical resonator helps the resonator-enhanced absorption spectroscopy to improve the effective absorption path. The effective absorption path of the absorbing medium multiplies thousands of times when the laser encounters resonance conditions. Cavity-enhanced absorption spectroscopy does not require expensive light modulators such as optical switches. Instead, it measures the intensity of the light transmitted through the resonant cavity to determine the absorption spectrum. The cavity-enhanced absorption spectroscopy system is lightweight, affordable and suitable for commercial use.

Methods to Improve the Sensitivity of Cavity-Enhanced Absorption Spectroscopy

Frequency stabilization technology is the most efficient way to increase the sensitivity of cavity-enhanced absorption spectroscopy as it directly affects the minimum observable absorption coefficient. Pound-Drever-Hall technology and optical feedback frequency locking are the main methods for laser frequency stabilization.

Cavity-enhanced absorption spectroscopy achieves the multiplication of effective absorption paths (100 to 10,000 times) depending on the relaxation effect of the optical cavity with extremely low losses. It is possible to significantly increase the detection sensitivity and limit the trace gas concentration. The high-fine laser injection rate of the optical resonator can be increased efficiently with optical resonator-enhanced absorption spectroscopy.

A fraction of the resonance light in optical cavity-enhanced absorption spectroscopy reflects the laser beam. The linewidth of a semiconductor laser is significantly reduced when the optical feedback field has been adjusted in phase and feedback rate. This significantly increases the intensity and sensitivity of the light transmitted through the cavities compared to conventional cavity-enhanced absorption spectroscopy.

Development of an improved cavity-enhanced spectroscopy with U-cavity system

Wang et al. developed a high-fine passive U-shaped resonator system using a cavity-enhanced absorption spectroscopy setup with optical feedback. A unique asymmetric etalon can be seen in the U cavity system. Researchers studied and discussed the transmission spectrum of each mirror to shed light on the physical process underlying this phenomenon.

The accuracy of the U-Cavity system increased as the interference with the spectral line measurement was removed, and the optical feedback resonator-enhanced absorption spectroscopy system with the lowest detectable coefficient was developed.

study results

In this study, researchers developed an absorption spectroscopy system with improved optical feedback through fiber resonance, based on a U-shaped resonator with improved spectral resolution. An asymmetric ripple effect was observed in the system, which significantly affected the spectral measurements. A theoretical and experimental analysis was offered for this unusual phenomenon. The results showed that the different losses of each cavity mirror are responsible for this phenomenon (the larger the loss of the reflector, the smaller the ripple amplitude).

The cavity-enhanced optical feedback absorption spectroscopy system was optimized to provide the system with the lowest detectable coefficient (a_Minimum = 8.33*10⁹ cm^-1). The end mirror was optically bonded to a prism to efficiently reduce this spectrum variation. The proposed system evaluated the spectrum loss of highly reflective mirrors by detecting the transmitted light intensity of different mirrors after eliminating the ripple effect. The thickness of the end mirror was adjusted and the amplified spectral signal of the ripple period of the end mirror cavity was used to calibrate the laser wavelength.

This work can guide the performance upgrade of the cavity-enhanced absorption spectroscopy system. Given the asymmetric ripple effect in U-cavity enhanced spectroscopic systems, numerous potential applications can be proposed. The U-resonator extended cavity spectroscopy system offers a simpler design and wider variety of applications than the traditional extended cavity absorption spectroscopy system.

Relation

Wang, Y., Guan, S., Cao, H., & Tan, Z. (2022) Asymmetric etalon effect in fold-like optical feedback cavity-enhanced absorption spectroscopy. applied Sciences, 12(19), Article 19. https://www.mdpi.com/2076-3417/12/19/10031

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