Real-time and accurate remote sensing of ocean temperature and salinity information are of great significance for understanding ocean properties and biodiversity. At the same time, it can forecast weather or temperature changes based on ocean temperature and salinity data. Brillouin scattering frequency shift is not easily affected by background noise due to its temperature sensitivity and narrow scattering spectrum, allowing for active remote sensing of ocean temperature and salinity.

Compared with the traditional Fabry Perot interferometer technique, the edge detection technique has faster measurement time, nothing to do with the incident angle while detecting signal intensity, and the parameters of the molecular absorption cell are also easy to control. It is suitable for airborne, satellite, and other large-scale ocean detection needs, and has become a popular technique for detecting ocean temperature and salinity information using the Brillouin scattering method. The edge detection technique has a limitation in that it converts Brillouin frequency shift into signal intensity change based on the characteristic absorption lines of molecules. It must extract Brillouin frequency shift information from the detected signal intensity. The two edge lines of molecular absorption used for detection are not necessarily symmetrical, and changing the laser frequency in the system will directly affect the change in output signal intensity, so laser frequency stability is extremely important. But with the development of laser frequency stabilization technology, laser frequency stability has been greatly improved. Simultaneously, both the Fabry Perot interferometer technique and the molecular absorption cell technique based on Brillouin scattering require the salinity information to be assumed in advance, and then use the empirical formula of Brillouin frequency shift, salinity, and temperature to inverse the temperature. The salinity of the ocean profile differs from the salinity of the sea surface depending on the season, region, and so on (Fig.7). If the fixed salinity is substituted into the temperature inversion formula, some systematic temperature measurement error will inevitably result, so the salinity variation factor must be considered in the temperature measurement technique. To obtain ocean temperature and salinity information efficiently on airborne, satellite, and other mobile platforms, the random jitter of the sea surface urgently needs the detection system to have a larger receiving field of view. In this paper, we make full use of laser development achievements, take the technical route based on iodine molecular absorption cell, combine with the absorption line of iodine cell, stabilize the laser frequency at 532.2334112 nm, which is the strong absorption line of iodine molecule, and effectively filter out elastic scattering. After passing through the absorption cell, the steep absorption lines on both sides of the band cause the frequency shift and full width at half maximum change due to signal intensity change caused by temperature change. The relationship between temperature and normalized signal intensity is obtained by fitting the relationship between temperature and full width at half maximum, and the widely used empirical formula of temperature and salinity with Brillouin frequency shift (Fig.3). To avoid temperature measurement errors caused by laser intensity jitter, the system employs a three-iodine cell design scheme, so that the laser intensity jitter can be converted into common-mode noise and removed (Fig.4). It is discovered that in the temperature range discussed, the signal intensity ratio curve and difference curve of an iodine molecular cell with different salinity can maintain monotonicity and disjointness (Fig.6). As a result, a set of signal intensity ratio and ratio difference data can only be determined from a pair of temperature and salinity data.

An innovative algorithm is proposed (Fig.8). The algorithm can inverse the temperature and salinity information repeatedly after the actual measurement of two groups of ratio data. Simulation is used to validate the program’s dependability. This set of inversion algorithm does not need to assume salinity information in advance, and instead uses an iterative algorithm based on the intensity ratio and ratio difference of the detector output to achieve accurate temperature and salinity inversion at the same time (Fig.10). The allowable intensity ratio random jitter is 1.3‰ to ensure that the inversion temperature error is less than 0.2 K from 5 ℃ to 30 ℃. The temperature inversion error between 10 ℃ and 20 ℃ is small. The temperature inversion error of 0.2 K can still be satisfied when the random jitter of intensity ratio is 2.3‰ in the temperature range of 10 ℃ to 20 ℃ (Fig.11).

A new edge detection technique based on iodine molecular absorption cells is proposed. This technical path has the advantages of signal intensity detection independent of incident angle, fast measurement speed, not being easily affected by laser intensity jitter, and not requiring salinity information. It is expected to be used on airborne, spaceborne, and other large mobile platforms, and it has a promising future application.