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Abstract
Brillouin-based LiDAR is an alternative remote sensing technique for measuring the distribution profiles of temperature, salinity, and sound speed in the upper ocean mixed layer. Its principle is based on the dependence of Brillouin frequency shift on the temperature, salinity, and depth of ocean. Therefore, it is necessary to investigate the effect of various seawater parameters on Brillouin frequency shift for ocean remote sensing by using the Brillouin LiDAR. Here we theoretically and experimentally investigate the influence of temperature, salinity, and pressure (depth) of seawater on Brillouin frequency shift in the upper ocean for the first time. Numerical simulations of the distribution profiles of temperature, salinity, and Brillouin frequency shift in the upper-ocean mixed layers of East China Sea and South China Sea were performed, respectively, by employing the Brillouin equations and the World Ocean Atlas 2018 (WOA18). A special ocean simulation system was designed to carry out the stimulated Brillouin scattering (SBS) experiments for validating the numerical simulations. The results show that the seawater temperature is the most important factor for the Brillouin frequency shift in the upper-ocean mixed layer compared with the salinity and pressure. At the same salinity and pressure, the frequency shift increases by more than 10 MHz for every 1 °C increase in temperature. Also, the differences of Brillouin frequency shift between experimental and theoretical values at the same parameter conditions were analyzed. The experimental results coincide well with the theoretical simulations. This work is essential to future applications of Brillouin LiDAR in remote sensing of the temperature, salinity, or sound velocity profiles of ocean.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Spatial and temporal distribution information of the temperature, salinity, and sound speed of the upper ocean is of major important in oceanography. It can be used not only for understanding of the physical and biological behaviors of the ocean, but also for providing large sets of data for climate modeling and weather forecasting. Currently, the ocean surface parameters can be obtained by satellites while the distribution profiles of underwater are extracted by using conductivity-temperature-depth (CTD) instruments, buoys, or gliders [1–4]. However, these techniques do not allow the rapid, accurate, and real-time range-resolved monitoring. Therefore, a flexibility, cost efficiency, and real-time remote sensing technique is highly desirable.
As an alternative approach, Brillouin LiDAR technique provides a promising solution. Brillouin LiDAR involves Brillouin scattering, LiDAR, and remote sensing technology, it was proposed by Guagliardo et al. in 1980 [5]. The practicability of this approach has been demonstrated in a laboratory environment due to the research of Fry and coworkers [6–8]. Although Brillouin scattering has been proposed as a possible approach for measuring ocean parameters since decades ago, only recent progress in laser technology could make such an apparatus feasible [9–11]. At present, a great deal of works have demonstrated the potential of Brillouin-LiDAR for detecting the temperature, sound speed, and underwater objects by measuring the backscattered Brillouin spectra [12–19].
The concept of measuring the distribution profiles of temperature, salinity or sound velocity in ocean by Brillouin LiDAR is based on the fact that the Brillouin frequency shift is closely related to these parameters. For a given incident laser wavelength $\lambda $, the Brillouin frequency shift ${\nu _B}$ can be expressed as [6]:
In this paper, the influences of temperature, salinity, and pressure of the upper-ocean mixed layer in the partial regions of East China Sea and South China Sea on Brillouin frequency shift are investigated theoretically at first. And then the frequency shifts of SBS are measured experimentally by employing an ocean simulation system to set the seawater parameters based on the theoretical simulation results. The investigation results provide the significant basis for remote sensing the parameter profiles of the upper-ocean mixed layer and sound channel.
2. Theoretical simulation
As shown in the equation of Brillouin frequency shift, both the refractive index and the sound speed are related to the temperature, salinity, and pressure of seawater. The dependence of the refractive index on wavelength, temperature, salinity, and pressure is given by the equation as published by Seaver [20]:
The equation for the dependence of sound speed on salinity, temperature, and pressure can be expressed by using Gibbs function as [21,22]:
where, ${g_p}$ and ${g_T}$ represents the first-order partial derivative of Gibbs function g with respect to p and T, and ${g_{pp}}$ and ${g_{TT}}$ represents the second-order derivative of g with respect to p and T. The change of seawater pressure with the depth can be obtained by using the geopotential and the database of World Ocean Atlas 2018 (WOA18) [23,24]. Figure 1 shows the relationship between pressure and depth, it can be seen that the pressure increases linearly with the increase of the ocean depth. Therefore, the Brillouin frequency shift at different ocean depths can be simulated by using Eqs. (1)∼(3).Based on the theory mentioned above, the temperature and salinity data of world ocean can be obtained from WOA18. Figure 2 shows the temperature and salinity profiles of the upper mixed layer (0-200 meters) of the world ocean. The temperature distribution shows that, the seawater temperature decreases with the increase of depth at the same location, and the maximum temperature difference reaches 30 °C from the equator to the poles. The seawater salinity is mainly affected by ocean currents, topography and so on, in which the highest salinity is ∼ 40‰, and the salinity of most sea areas is ∼ 35‰.
Fig. 2. The parameter profiles of the upper mixed layer of the world ocean. (a) Temperature distribution, (b) Salinity distribution.
In this work, we selected the annual average temperature and salinity parameters of the upper-ocean mixed layer (0-200 meters) in the partial regions of East China Sea and South China Sea from WOA18 to simulate the Brillouin frequency shift. Figure 3 shows the temperature and salinity distributions of the upper-ocean mixed layer in East China Sea and South China Sea. In East China Sea, the temperature and salinity distributions present relatively high at low latitude and high longitude, and the salinity tends to be stable with the increase of the depth. And the annual average temperature varies from 11.34 to 26.51 °C, the annual average salinity varies from 30.75 to 34.88‰. In South China Sea, the temperature and salinity distributions are relatively uniform along with latitude and longitude, the annual average temperature varies from 14.05 to 29.06 °C, and the annual average salinity varies from 32.14 to 34.62 ‰. The temperature decreases layer by layer with the increase of depth, and the salinity increases layer by layer with the increase of depth and tends to be stable.
Fig. 3. The temperature and salinity distributions in the partial regions of East China Sea and South China Sea. (a) ∼ (b) East China Sea, (c) ∼ (d) South China Sea.
Based on the temperature and salinity distributions, the Brillouin frequency shift can be calculated by using Eqs. (1)∼(3). The simulation results are shown in Fig. 4. We can see that, the distributions of the Brillouin frequency shift in the South China Sea and the East China Sea are basically consistent with the temperature distributions in the corresponding regions. The Brillouin frequency shift gradually decreases with the increase of ocean depth. Although the salinity of seawater increases with the increase of depth, the influence of temperature on the frequency shift is more obvious. The frequency shift varies from 7.530 to 7.749 GHz in the East China Sea, and varies from 7.584 to 7.770 GHz in the South China Sea. Because the annual average temperature of seawater in South China Sea is relatively higher than that of the East China Sea, the corresponding Brillouin frequency shift is also higher than that of the East China Sea.
Fig. 4. The frequency shift distributions in the partial regions of East China Sea and South China Sea. (a) East China Sea, (b) South China Sea.
To show more visually the relations among the frequency shift and temperature, salinity, and pressure in seawater, we also analyzed the annual average variations of the Brillouin frequency shift at different parameter conditions in the South China Sea and the East China Sea, respectively, the results are shown in Fig. 5 and Fig. 6. In Fig. 5(a), when the pressure of seawater is 1 MPa, the annual average temperature varies from 14.09 to 23.79 °C, the annual average salinity varies from 34.28 to 34.85‰, and the Brillouin frequency shift varies from 7.583 to 7.720 GHz. At the same pressure and salinity, the frequency shift increases about 14 MHz for every 1 °C increase in temperature; at the same pressure and temperature, the frequency shift increases about 6 MHz for every 1‰ increase in salinity. In Fig. 5(b), when the salinity is 34‰, the annual average temperature varies from 12.44 to 26.51 °C, the pressure varies from 0 to 1 MPa, and the frequency shift varies from 7.547 to 7.741 GHz. At the same salinity and pressure, the frequency shift increases about 14 MHz for every 1 °C increase in temperature; at the same salinity and temperature, the frequency shift increases about 7 MHz for every 1 MPa increase in pressure.
Fig. 5. The frequency shift distributions in East China Sea. (a) At different temperatures and salinities, (b) At different temperatures and pressures.
Fig. 6. The frequency shift distributions in South China Sea. (a) At different temperatures and salinities, (b) At different temperatures and pressures.
Figure 6 shows the frequency shift distributions at different parameter conditions in South China Sea. When the pressure of seawater is 1 MPa, as shown in Fig. 6(a), the annual average temperature varies from 18.94 to 23.55 °C, the annual average salinity varies from 34.11 to 34.63‰, and the Brillouin frequency shift varies from 7.657 to 7.716 GHz. At the same pressure and salinity, the frequency shift increases about 13 MHz for every 1 °C increase in temperature; at the same pressure and temperature, the frequency shift increases about 7 MHz for every 1‰ increase in salinity. When the salinity is 34‰, as shown in Fig. 6(b), the annual average temperature varies from 18.97 to 29.21 °C, the pressure varies from 0 to 1 MPa, and the Brillouin frequency shift varies from 7.648 to 7.768 GHz. At the same salinity and pressure, the frequency shift increases about 12 MHz for every 1 °C increase in temperature; at the same salinity and temperature, the frequency shift increases about 7 MHz for every 1 MPa increase in pressure.
Based on the simulation results, we can see that the temperature, salinity, and pressure of seawater have different effects on Brillouin frequency shift. Compared with the salinity and pressure, the seawater temperature is the most important factor for the Brillouin frequency shift in the upper-ocean mixed layer. At the same salinity and pressure, the frequency shift increases by more than 10 MHz for every 1 °C increase in temperature. To verify the theoretical simulation results, we have designed a special experimental system to measure the Brillouin frequency shift of seawater at different temperatures, salinities, and pressures, the results are as follows.
3. Experimental measurement
The experimental setup is shown in Fig. 7. The light source is an injection-seeded and Q-switched Nd: YAG pulse laser operating at 532 nm after through an amplifier and a second harmonic generator (SHG). Its repetition frequency is 10 Hz, pulse duration is 8 ns, beam diameter is 17 mm, divergence angle is 0.45 mrad. The laser linewidth of single-longitudinal mode with 90 MHz can be obtained by switching on a seed laser with the output wavelength of 1064 nm. An ocean simulation system consisted of a pressure chamber (the length is 1 meter) and four hydraulic pumps was employed to simulate the static sea parameters of upper-ocean mixed layer, such as temperature, salinity, and pressure (depth). Since the seawater pressure increases linearly with the increase of the ocean depth [21,25], as shown in Fig. 1, the ocean depth can be simulated by varying the pressure in the pressure chamber. The maximum water pressure of ocean simulation system is 10 MPa (The corresponding depth is 1000 meters), the water temperature can be stabilized to values between 4 and 40 ◦C. The pressure sensor and thermocouple are installed to measure the water pressure and temperature in the pressure chamber.
Fig. 7. Schematic of the experimental setup. λ/2: half-wave plate, λ/4: quarter-wave plate, PBS: polarization beam splitter, PM: power meter, P: pinhole filter.
The output laser beams from the laser source with the vertical polarization pass through λ/2, PBS, and λ/4 in turn and then were focused by achromatic lens into the pressure chamber of ocean simulation system. The stimulated Brillouin scattering signals were excited in the pressure chamber and were collected into the spectrometer system consisted of F-P etalon and ICCD camera (PI-MAX2, Princeton Instruments). The free spectral range (FSR) of the F-P etalon is 20.1 GHz. The output laser energy was monitored by using the power meter.
According to the annual average temperature and salinity distributions of the upper-ocean mixed layer mentioned in theoretical simulation, the seawater with the salinities of 30, 32, 34, and 35‰ were prepared, respectively, by dissolving sea salt (Sigma-Aldrich) in distilled water, the water temperature was stabilized to values between 10 and 30 °C, and the pressure was set at the range of between 0 and 4 MPa. Figure 8 shows the measured SBS spectrum of seawater by using F-P etalon and ICCD camera when the temperature is 25 °C, salinity is 34‰, and pressure is 0 MPa. The Brillouin frequency shift can be obtained through image preprocessing, data fusing, spectrum denoising, and spectrum deconvoluting in turn from the original two-dimensional spectrum.
Figure 9 shows the experimental results of SBS frequency shift at different parameter conditions. Figure 9(a) and 9(b) give the changes of frequency shift versus the changes of pressure at different temperatures when the salinities are 30‰ and 34‰, respectively. It can be seen that the Brillouin frequency shift increases with the increase of temperature at the same salinity and pressure, and increases with the increase of pressure at the same salinity and temperature. Figure 9(c) and 9(d) give the changes of frequency shift versus the changes of salinity at different temperatures when the pressures are 0 MPa and 2 MPa, respectively. It can be seen that the Brillouin frequency shift also increases with the increase of salinity at the same pressure and temperature. By comparing the experimental results, the Brillouin frequency shift has increased by ∼270 MHz at the same salinity and pressure when the temperature increases from 10 to 30 °C; at the same temperature, the average increase in frequency shift due to the salinity and pressure is ∼33 and ∼36 MHz, respectively. The results have been proved that the influence of temperature on the frequency shift is more obvious than that of salinity and pressure in the upper-ocean mixed layer.
Fig. 9. SBS frequency shifts at different experimental conditions. (a) and (b), Frequency shift vs. Pressure at different temperatures when the salinities are 30‰ and 34‰, respectively; (c) and (d), Frequency shift vs. Salinity at different temperatures when the pressures are 0 and 2 MPa, respectively.
It is necessary to state that we cannot simulate the real ocean environment through the employed ocean simulation system in our experiments, so accurate results are difficult to obtain due to the complicated structures of ocean. For comparing with the theoretical simulation results, Fig. 10 gives the differences of Brillouin frequency shift (DBFS) between experimental and theoretical values at the same parameter conditions. Figure 10(a) and 10(b) represent the DBFS at different temperatures and pressures when the salinities are 30‰ and 34‰, respectively. The maximum and minimum DBFS values are ∼14 MHz and ∼6 MHz, respectively, at the salinity of 30‰; when the salinity is 34‰, the maximum and minimum DBFS values are ∼14 MHz and ∼7 MHz, respectively. Figure 10(c) and 10(d) represent the DBFS at different temperatures and salinities when the pressures are 0 and 2 MPa, respectively. The maximum and minimum DBFS values shown in Fig. 10(c) are ∼9 MHz and ∼5 MHz, and in Fig. 10(d) are ∼12 MHz and ∼6 MHz, respectively. The experimental measured values are slightly larger than the theoretical simulation values at the same parameter conditions.
Fig. 10. Difference of Brillouin frequency shift. (a) and (b), DBFS at different temperatures and pressures when the salinities are 30‰ and 34‰, respectively; (c) and (d), DBFS at different temperatures and salinities when the pressures are 0 and 2 MPa, respectively.
To show more visually the relations among the frequency shift and temperature, salinity, and pressure in seawater, Fig. 11 presents a three-dimensional (3D) plot of experimental data. We can see that the Brillouin frequency shift increases with the increases of temperature, salinity, and pressure, and the maximum frequency shifts are distributed in the regions of the maximum temperature, salinity, and pressure. Within the pressure range of 0∼2 MPa, the Brillouin frequency shift varies from ∼7.48 to ∼7.81 GHz. The results of our experiments coincide well with the theoretical simulations.
Fig. 11. SBS frequency shift distribution at different temperatures, salinities, and pressures of seawater.
4. Conclusion
In summary, we theoretically and experimentally analyze the influence of temperature, salinity, and depth of the upper-ocean on frequency shift of Brillouin LiDAR. Theoretically, the distribution profiles of temperature, salinity, and Brillouin frequency shift in the upper-ocean mixed layers of East China Sea and South China Sea were simulated by employing Brillouin equations and the World Ocean Atlas 2018. The results show that the Brillouin frequency shift varies from 7.530 to 7.749 GHz in the East China Sea and varies from 7.584 to 7.770 GHz in the South China Sea. Experimentally, SBS frequency shifts were measured at different parameter conditions to verify the theoretical simulations by using the designed ocean simulation system. The experimental results show that the Brillouin frequency shift varies from 7.483 to 7.812 GHz in the pressure range of 0∼2 MPa. Compared with the numerical simulation values of Brillouin frequency shift, we find that the measured values are slightly larger than the simulation values at the same experimental conditions. We also find that the seawater temperature has greater effect on Brillouin frequency shift than that of salinity and pressure in the upper-ocean mixed layer. The results that we study will be is essential to Brillouin LiDAR in remote sensing of the profiles of temperature, salinity, or sound velocity in ocean.
Funding
National Natural Science Foundation of China (41776111); National Key Research and Development Program of China (2018YFE0115700); Defense Industrial Technology Development Program (JCKY2019401D002).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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