Zhongqi Tan

Precise wavelength calibration in continuous-wave cavity ringdown spectroscopy based on the HITRAN database.

We describe the wavelength calibration method of a narrowband laser diode in continuous-wave (CW) cavity ringdown spectroscopy (CRDS). The method uses known spectral lines as wavelength markers to calibrate and refine the wavelength-current relation of laser diodes, and their spectral positions are taken directly from the HITRAN 2004 database. We built a compact CW CRDS apparatus with a 1.517 microm (approximately 6594 cm-1) distributed feedback (DFB) laser diode as the light source and a 25 cm long glass ceramic as the cavity in which to demonstrate the method. A wavelength precision of approximately 0.8 x 10(-3) cm-1 was obtained by comparing the HITRAN 2004 database, which was approximately four times more precise than that of the conventional method.

Approximation Characteristic Matrix of Slightly Inhomogeneous optical coating at Oblique Incidence

We derive 1st and 2nd order approximation characteristic matrix of slightly inhomogeneous optical coating at oblique incidence, and test its effectiveness, accuracy and speed by comparison with published results.

Gas trace detection with cavity enhanced absorption spectroscopy: a review of its process in the field

Cavity-enhanced absorption spectroscopy (CEAS) is a technology in which the intracavity absorption is deduced from the intensity of light transmitted by the high finesse optical cavity. Then the samples’ parameters, such as their species, concentration and absorption cross section, would be detection. It was first proposed and demonstrated by Engeln R. [1] and O’Keefe[2] in 1998. This technology has extraordinary detection sensitivity, high resolution and good practicability, so it is used in many fields , such as clinical medicine, gas detection and basic physics research. In this paper, we focus on the use of gas trace detection, including the advance of CEAS over the past twenty years, the newest research progresses, and the prediction of this technology’s development direction in the future.

A developed method for surface testing based on the scattering interference effect

Light scattering is an important and classical method for optical surface testing. Typical scheme of light scattering measurement often uses a single laser beam incidence, and then evaluates the surface quality via detecting and analyzing the scattering signal. In this work, a developed method for optical surface testing is proposed and demonstrated, and whose measuring principle is based on the scattering interference effect. In this approach, a single longitudinal mode laser beam is divided into two beams, when these beams irradiate the optical surface, their respective scattering fields would interfere with each other. If the phase between these incidence lights is scanned periodically, their scattering light interference signal would fluctuate simultaneously. Through analyzing this kind of scattering signal, our method can not only determine the scattering loss of optical surface, but also scale its inhomogeneity performance. A simply set of experimental apparatus is built up and used to demonstrated this method, which uses a single mode laser as the light source. Furthermore, to modulate the phase difference between two incidence beams, a piezoelectric ceramic is used. Some typical cases are then experimented and discussed, the results show that this method can be used to calibrate the quality of optical surface.

Impact analysis of particulate loss on folded mirror surface in folded cavity ring-down spectrometer

Cavity ring-down spectroscopy is a new type of high sensitive spectroscopy emerging in recent years. It can be subdivided into straight cavity, folding cavity and ring cavity ring-down spectroscopy by the structure of the optical resonator. In folded cavity ring-down spectrometer, the folded mirror surface may be polluted for various reasons. Different from straight cavity mirror, ring cavity mirror and folded cavity end mirror, there are two symmetrical beams incidence on the surface of folded mirror. So the performance of folded cavity in this case appears new characteristics, the most concerned one is that the scattering fields will produce an interference effect when two beams irradiate on the particulate. Embarking from the scattering theory and combining with the analysis of the light field in the resonator, this paper qualitatively analyzes the influence of the particulate loss on the performance of folded cavity ring-down spectrometer. These works have a guiding significance for the study of folded cavity ring-down spectroscopy and can improve its system performance.

Frequency stabilization based on high finesse glass-ceramic Fabry-Perot cavity for a 632.8-nm He-Ne laser

A frequency stabilization technique for a 632.8nm He-Ne laser with a high finesse Fabry-Perot cavity is introduced in this paper. The resonant frequency of the cavity is taken as the frequency standard .In this system the Fabry-Perot cavity is composed of a glass-ceramic spacer, with thermal expansion coefficient smaller than 2×10-8/°C ， which means an excellent thermal stabilization which greatly decreases the thermal impacts on the cavity length in the desired constant-temperature environment.The intra-cavity spherical mirror is specially designed, which makes the Fabry-cavity a sensor element in our subsequent experiments for a new practical optical accelerometer .Both cavity mirrors were custom made in our laboratory which have reflectivities greater than 99.995% at 632.8nm, so the Fabry–Perot cavity has a finesse of about 62830. The half-maximum transmission line width is about 55.48 KHz and the free spectral range is 3.5GHz .In the experimental setup, we adopt the frequency stabilization circuit with small dithering .With proper dithering voltage, the laser can be precisely locked to the Fabry-Perot cavity minimum reflection point. Theoretically the frequency stability can reach 10-10 order.

External folded cavity optical feedback diode laser with megahertz relative linewidth

We present a extended-cavity diode laser (ECDL) with megahertz linewidth by optical feedback from a folded Fabry–Perot cavity, and demonstrate the efficient laser linewidth reduction and frequency stabilization of the optical feedback technique. In our experiments, a folded Fabry–Perot cavity with a finesse of 4750 replaces the reflecting mirror in the traditional ECDL configuration, the folded Fabry–Perot cavity can serve as an optical feedback element, which forces the semiconductor laser automatically to lock its frequency optically to the cavity resonance frequency. The laser’s phase noise is significantly suppressed, and The laser’s linewidth is reduced from about 20GHz to 15MHz.

Influence of laser linewidth on spectral ripple in fold-type cavity ring-down measurement

So-called spectral ripple a special phenomenon only existing in fold-type cavity ring-down spectrometer, and it is believed as deriving from the interference effect of different lights on the folded mirror. To investigate the influence of this phenomenon, on fold-type cavity ring-down measurement, some representative instances of laser linewidth are simulated and analyzed, and some experiments are carried on. It is found that the increasing of laser linewidth can make the amplitude of the spectral ripple decrease, more importantly, which can make the ring-down signal no longer decay as a single exponential but as the double exponential function.

Interference effect of surface scatter at folded-mirror coating of V-shaped resonance cavity

Scatter property of folded-mirror coating in V-shaped cavity is studied with Kirchhoff diffraction theory. The interference effect of scatter lights can be used to explain the ripple effect existing in optical-feedback cavity ring-down spectroscopy.

Analyses of spectral ripple phenomenon in the optical-feedback cavity ring-down spectroscopy

Based on the vector scattering theory, we analyze and discuss the interference effect of the surface scattering lights on the folded mirror of V-shaped cavity. It is found that the interference effect can introduce the periodic fluctuation of scattering loss, and this phenomenon is similar with the spectral ripple effect existed in optical-feedback Cavity Ring-down Spectroscopy (CRDS). We believe that the interference effect of the scattering lights on folded mirror is an important source of spectral ripple effect, and it can be used to explain the mechanism of spectral ripple phenomenon.