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Abstract
Thermal blooming effect is one of the significant factors affecting the propagation performance of high-power ytterbium-doped fiber lasers (YDFLs) in the atmosphere. In this paper, two 20 kW YDFL systems with typical wavelengths (1070 nm and 1080 nm) are fabricated for propagation comparison experiments, which are used to investigate the thermal blooming effect induced by high-power YDFL propagation through the atmosphere. Under approximately the same laser system parameters (except wavelength) and atmospheric environment, the 1070 nm laser has better propagation characteristics than the 1080 nm laser. Due to the combined effect between the different central wavelengths of the two fiber lasers and the spectral broadening caused by output power scaling, the thermal blooming caused by the different absorptivity of water vapor molecules to the two fiber lasers is the main factor for the variation of the propagation properties. Through theoretical analysis and numerical calculation of factors affecting the thermal blooming effect, and considering the industrial manufacturing difficulty of YDFLs, a reasonable selection of fiber laser parameters can effectively improve atmospheric propagation performance and reduce manufacturing costs.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power YDFLs, as one of the most prominent fiber lasers, have significant applications in telemetry and atmospheric long-range charging [1–3]. Compared to conventional lasers, fiber lasers are characterized by high electrical and optical efficiency, operational flexibility, and robust mechanical construction. Therefore, it is easy to integrate, transport, and operate fiber lasers in various operating environments. Due to advances in large modal area fiber technology, high-brightness pump sources, and high-power pump coupling technology, near-single-mode fiber lasers can achieve kilowatt-level power output [2,4–6]. Single-beam fiber laser is restricted by thermal effects, pump source brightness, and nonlinear effects. Consequently, there is a physical limit to the output power [7]. Multiple fiber lasers, with incoherent or coherent beam combination technology, can achieve power scaling beyond the 10-kilowatt level [8–11].
The propagation of high energy laser (HEL) in the atmosphere is a complex process, in which there are the laser’s propagation mechanism, the atmosphere’s optical properties, and the interaction between the laser and the atmosphere [12]. The laser interacts with the atmospheric medium and is affected by numerous interrelated linear and nonlinear phenomena. The phenomena are linear at low laser power output, such as absorption and scattering properties of atmospheric molecules and aerosols, turbulence, diffraction, etc. [13,14]. When the peak or average laser power increases to a certain threshold, nonlinear phenomena occur, such as thermal blooming and stimulated Brillouin scattering (SBS), etc. [15–18].
In the reported atmospheric propagation experiments of HEL [9,19–22], the propagation characteristics of the laser beam are limited by the atmospheric environment and the laser system. The atmospheric environmental parameters vary with geographical location and time. Light source parameters, propagation methods, and industrial manufacturing difficulties limit the development of laser systems [23,24]. The central wavelength of the YDFLs falls well within the propagation window of the 1 µm band, where the absorption coefficient of the laser by the atmosphere is small, allowing long-distance propagation [25]. Atmospheric absorption is composed of molecular absorption and aerosol absorption. The aerosol absorption dominates within the propagation window, while the absorption of atmospheric molecules is negligible except at specific wavelengths [26,27]. This is caused by the vibrational-rotational energy transition of atmospheric molecules, resulting in strong absorption lines in the near-infrared band, such as the strong absorption lines of water vapor molecules in the 1.1 µm band. Part of laser energy will be absorbed by molecules and aerosols, which heat the air and change the refractive index of the atmosphere, causing the thermal blooming effect. The thermal blooming leads to a decrease in peak intensity and beam quality degradation [16]. Therefore, the central wavelength selection of the YDFLs and the suppression of spectral broadening caused by output power scaling are crucial for propagation.
For the above reasons, two YDFL systems, which have an average power of 20 kW and wavelengths of 1070 nm and 1080 nm, are fabricated for atmospheric propagation comparison experiments. Based on experimental data and numerical calculations, this paper investigates the influence of thermal blooming on the propagation characteristics of YDFL in the atmosphere. In addition, the optimal parameter selection for the atmospheric propagation of YDFLs is proposed considering the propagation performance and engineering difficulty.
2. Theoretical foundations
When the HEL propagates through the atmosphere, the total angular spread of a laser beam can be written as [9,24]
HEL interacts with atmospheric molecules and aerosols, resulting in absorption and scattering. Scattering redistributes the laser energy in the atmosphere and reduces the total power of laser propagation. Absorption converts the laser energy into the internal energy of the absorbed particles and eventually transfers it to the surrounding material as heat [14]. Those effects reduce the propagation performance and lead to the thermal blooming effect. Beer’s law describes the attenuation of laser intensity in the atmosphere [26]
where I(L) is the light intensity of the laser at wavelength $\mathop \lambda \nolimits_{}^{}$ transmitted through the atmosphere at a distance of L, ${I_0}$ is the initial laser intensity, and $\gamma = {\alpha _\textrm{M}} + {\beta _\textrm{M}} + {\alpha _\textrm{A}} + {\beta _\textrm{A}}$ is the extinction coefficient, where $\mathop \alpha \nolimits_{}$ is the absorption coefficient, $\mathop \beta \nolimits_{} $ is the scattering coefficient, and the subscripts “M” and “A” designate molecules and aerosols, respectively.Water vapor molecules have absorption lines in the 1.1 µm band, while other atmospheric molecules barely absorb in the 1 µm band “atmospheric window” [28]. As the most active molecule in the atmosphere, water vapor molecules have distinctive spatial and temporal characteristics. High concentrations of water vapor often occur in specific regions on Earth. Therefore, the influence of water vapor on laser propagation cannot be ignored. Figure 1(a) shows the absorption coefficient spectrum of water vapor at high water vapor content (19.4$\textrm{g/}{\textrm{m}^3}$) from 1000 nm to 1200 nm. Water vapor barely absorbs light in the 1000 nm to 1060 nm band. Starting from 1060 nm, the absorption of water vapor gradually increases with increasing wavelength. There is a strong absorption band between 1100 nm and 1160 nm.
Fig. 1. (a) Absorption coefficient spectrum of water vapor. (b) Thermal blooming effect when HEL propagates through the atmosphere.
The emission spectra of $\textrm{Y}{\textrm{b}^{3 + }}$ in the fibers of different materials are between 900 nm and 1200 nm [29]. However, due to laser gain, technical difficulties, manufacturing costs, and other reasons [30], the center wavelengths of industrially manufactured high-power YDFLs are primarily between 1060 nm and 1080 nm [5,31]. With the power increases, there are nonlinear and thermal effects in high-power YDFLs, such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and SBS. The nonlinear and thermal (indirect) effects of fiber lasers lead to spectral broadening [7,32,33]. Due to the laser center wavelength selection and the spectral broadening, the laser output spectrum partially overlaps with the strong water vapor absorption lines, resulting in partial energy absorption by water vapor. This absorbed energy increases the temperature of the air and changes the refractive index, creating a thermal blooming effect. As shown in Fig. 1(b), the presence of thermal blooming and transverse wind can create defocus-like and tilt-like optical effects, respectively, which can be viewed as “negative lenses” and “prisms”. That causes the beam to spread, as well as a reduction and shift of the peak intensity on the target. Depending on the initial beam type, the shape of the steady-state beam distortion will tend to be crescent, ellipse, etc. [34]. A self-consistent formulation can describe the spatial and temporal variation trends of the thermal blooming effect based on the medium’s heating and the laser beam’s propagation properties [16,35]. The distortion parameter is an important indicator to measure the strength of the steady-state thermal blooming [36], which is defined as
A higher distortion parameter means that the thermal blooming effect influences the laser propagation more severely. Due to the nonlinear relationship between the distortion parameter and the peak light intensity, a slight increase in the distortion parameter will severely degrade the beam properties on the target [16]. The main reason for the increase in the distortion parameter is the total power absorbed by the atmospheric medium ${P_a} = \alpha PL$. When the laser power and propagation path are fixed, the atmospheric absorption coefficient is the main influencing factor. There are two types of absorption coefficients: the absorption coefficient of atmospheric molecules, which is highly dependent on the laser wavelength, and the absorption coefficient of aerosols, which varies slowly with wavelength. For the above reasons, the laser center wavelength selection and the spectral broadening caused by output power scaling have a non-negligible effect on laser propagation.
3. Experimental description
3.1 Experimental setup
Figure 2 shows the schematic diagram of the experimental setup for atmospheric propagation. Based on the technical difficulty and total cost of laser engineering, two high-power YDFLs with high industrial maturity and wavelengths of 1070 nm and 1080 nm are fabricated as the primary light sources of the laser systems. The average power of each laser is 20 kW, obtained by incoherent power combining seven 3-kW signal lasers through an all-fiber 7 × 1 signal combiner [37]. Figure 3 and Table 1 show the spectral characteristics of the two YDFLs with different output powers (some laser spectra and initial power were not measured). The output powers of the two lasers are pre-calibrated for the convenience of the experimental operation. With increasing power, the central wavelengths of the lasers are relatively stable. However, a severe spectral broadening occurs, resulting in a significant increase in the energy component that deviates from the central wavelength.
Fig. 3. Absorption spectrum of water vapor during the experiment (propagation distance 300 m, average atmospheric pressure 996.6 hPa, average temperature 307.55 k, average water vapor content 19.4$\textrm{g/}{\textrm{m}^3}$) and spectral characteristics of two YDFLs with different powers.
Table 1. Spectral characteristics of the 1070 nm and 1080 nm YDFLs at different preset powers
The output fiber of the signal combiner is connected to the beam expanding-focusing lens set to form a beam director system. The beam director system can be electrically or manually focused, and the laser beam is adjustable on the target. The output fibers of both lasers are made to the same specifications and are output through the lens sets with an initial beam diameter of 4.5 cm (99% output power). The two lens sets are fixed on the same platform, and the laser systems do not use adaptive optics.
The laser systems are placed on the roof at a distance of 20 m from the ground, which reduces the influence of turbulence caused by ground radiation to a certain extent. A target is set 300 m away from the laser systems. The target is a beam quality measurement system consisting of thermoelectric detectors and photodetector arrays that can directly quantify the total laser power and intensity distribution. At the same time, a weather sensor and a differential image motion monitor (DIMM) can record atmospheric data in real time, such as temperature, humidity, pressure, wind speed, wind direction, water vapor content, and atmospheric coherence length.
The $\mathop \beta \nolimits_{} $ factor is usually used to characterize the beam quality of HEL in practice, which is defined as [38–40]
where $\theta $ and ${\theta _0}$ are the far-field divergence angles of the measured beam and the ideal plane wave beam of equal aperture, respectively. Then ${\Theta _{\textrm{Diff}}} = 1.22\lambda \textrm{/}D$, ${\Theta _\textrm{Q}} = 1.22(\beta - 1)\lambda \textrm{/}D$.After laboratory measurements, the beam quality of the 1070 nm and 1080 nm lasers through the lens set is 4.0 and 4.4, respectively. Two lens sets are fixed on the same platform, and their mechanical jitter is identical. Under the conditions without considering the influence of the atmospheric environment, the beam widths of the 1070 nm and 1080 nm lasers on the target due to the laser system parameters are 2.75 cm and 3.11 cm, respectively. In summary, the differences in the experimental equipment systems do not lead to significant variations in the experimental data.
3.2 Atmospheric environment
Due to the limitations of experimental equipment, real-time measurements of atmospheric aerosols were not performed. The experimental site is located at the border between urban and suburban areas. There was heavy rain before the experiment, which had a significant sedimentation effect on aerosol particles. According to the city’s meteorological observatory data, the concentration of aerosols is low ($\textrm{P}{\textrm{M}_{\textrm{2}\textrm{.5}}} < 25{\mathrm{\mu} \mathrm{g}/}{\textrm{m}^\textrm{3}}$ and $\textrm{P}{\textrm{M}_{\textrm{10}}} < 35{\mathrm{\mu} \mathrm{g}/}{\textrm{m}^\textrm{3}}$). In the traditional atmospheric model, aerosol populations with different geographical and temporal characteristics can be represented by corresponding aerosol models, such as urban, rural, marine, and desert models. The aerosol model is composed of various aerosol distributions representing different compositions and sizes [20]. The geometric orientation of the aerosols for each model is randomly distributed, so each aerosol particle can be considered spherical at the macroscopic level. According to the Mie scattering theory [41], individual aerosol scattering and absorption coefficients vary as a continuous function of the particle size parameter ($x = 2\mathrm{\pi }a/\lambda $, where a is the aerosol radius). Figure 4(a), (b), and (c) show the 300 m aerosol transmission rate, scattering rate, and absorptivity spectra of various aerosol models from 1000 nm to 1200 nm, respectively, calculated by MODTRAN software. Under different visibility and aerosol model conditions, the aerosol transmission rate of the laser in the 1 µm band is at least 90%. In addition, the scattering coefficient is several times larger than the absorption coefficient in the extinction coefficient [20]. Near the 1 µm band, the absorption coefficients of the aerosol population can be considered flat except for the urban aerosol environment at low visibility. In the same aerosol environment, the variation of extinction rate with wavelength is mainly related to scattering. Meanwhile, the difference of this scattering has negligible influence on the thermal blooming effect in the short-range propagation. According to the above analysis, there is no significant difference between the influence of extinction and thermal blooming effects caused by aerosol populations on the two lasers for this atmospheric propagation experiment.
Fig. 4. (a), (b), (c) Variation in the aerosol transmission rate, scattering rate, and absorptivity with the laser wavelength for different aerosol modes. (d), (e) Changes in atmospheric coherence length and water vapor content during the experiment. (f) Wind rose diagram from the data collected by the weather sensor near the laser system.
When the Sun radiates the Earth’s surface, it creates a local temperature gradient in the atmosphere, resulting in a change in the refractive index of the air. Optical turbulence is caused when the temperature gradient is disturbed by external factors. The atmospheric coherence length ${r_0}$ is commonly used in engineering experiments to indicate the degree of influence of optical turbulence along the atmospheric propagation path, which is expressed as ${r_0} = {[0.423{k^2}\smallint_0^L {C_n^2(z )\textrm{d}z} ]^{ - 3/5}}$, where $C_n^2$ is the structure parameter [42]. This experiment was conducted at night (20:00 - 22:00) with a relatively sizeable atmospheric coherence length. Figure 4(d) shows that the atmospheric coherence length along the experimental propagation path is 19.5 ± 6.9 cm, which has a negligible influence on the 4.5 cm beam diameter. In addition to the thermal blooming effect, the beam widths of the 1070 nm and 1080 nm lasers due to other factors are 2.76 cm and 3.12 cm, respectively.
The composition and content of atmospheric molecules in the air are essentially stable except for water vapor molecules. The scattering of atmospheric molecules is usually negligible for atmospheric propagation at the kilometers level [19]. Water vapor is the most dominant absorbing atmospheric molecule near the 1 µm band. Figure 4(e) shows that the water vapor content measured by the weather sensor is 19.2 - 19.7$\textrm{g/}{\textrm{m}^3}$, and there is no significant abrupt change in water vapor content.
Wind can effectively reduce the influence of thermal blooming. Figure 4(f) shows the wind rose diagram from the data collected by the weather sensor near the laser systems. There are mainly transverse winds in the laser propagation path, but the wind speed is mostly below $2\textrm{ m/s}$. The wind speed of this experiment is relatively low, and there are no conditions for real-time measurements of the wind field distribution data on the beam propagation path, which leads to significant errors in the normalized results of the distortion parameters and does not have reference value for the accurate calculation of the beam propagation characteristics. However, the wind field is relatively stable in a statistical sense during the experimental period, and the wind speed fluctuates around its overall average value to approximately the same extent during each laser propagation. The greater the degree of thermal blooming at the same power output, the greater the discrete degree of the measured data due to wind speed perturbations. Considering the unique characteristics of low wind speed and significant measurement errors in this experiment, the discrete degree of the experimental results can be used to reflect the extent of the thermal blooming effect.
4. Results and discussion
4.1 Propagation assessment
Figure 5(a) and (b) show the variations in the total power at the initial plane (laboratory measurements), the total power on the target, the power lost during propagation, and the peak intensity on the target with preset laser power for the 1070 nm and 1080 nm lasers. The fitted curves for the above data are the solid, dashed, dotted, and dash-dotted lines, respectively. The initial output powers of the two lasers are approximately equal at the same preset power. Under essentially the same atmospheric conditions and 300 m propagation distance, the total power on the target and the power lost during propagation (the fitted curve of the total power at the initial plane minus the fitted curve of the total power on the target) are essentially the same and positively correlated as the preset power of both lasers increases. The variation of the laser wavelength near the 1 µm band has little influence on the power lost during propagation. As the preset power increases, the loss of laser power increases significantly. The combined effect of the increase in total water vapor absorptivity and the decrease in beam quality causes this phenomenon. The peak intensity on the target increases logarithmically with the increase in output power of both lasers. Nevertheless, the degradation of the intensity distribution of the 1080 nm laser is more pronounced. At 75% of the output power, the average peak intensity of the 1070 nm laser is twice that of the 1080 nm.
Fig. 5. (a) Variations in the total power at the initial plane, the total power on the target, and the power lost during propagation with preset laser power for the 1070 nm laser and the 1080 nm laser. (b) Variation in the peak intensity on the target with preset laser power for the 1070 nm laser and the 1080 nm laser. (c), (d) Beam characteristics on the target for the 1070 nm laser and the 1080 nm laser with 75% power output, respectively. (e), (f) Intensity distributions of the 1070 nm laser and the 1080 nm laser on the target at 75% power output. (f) Intensity distributions of the 1070 nm laser and the 1080 nm laser on the target at 50% power output.
In general, the beam-spreading and energy-focusing ability can be described by the power of the bucket-based beam width [43], which is expressed as $\smallint_0^{{w_\eta }} {Ir\textrm{d}r} = \eta \smallint_0^{ + \infty } {Ir} \textrm{d}r$, where ${w_\eta }$ is the bucket half-width chosen. When the 1070 nm and 1080 nm lasers are at 75% power output, the variation of the peak intensity, the beam width ${w_{86.5{\%}}}$, and the total power of the beam on the target are shown in Fig. 5(c) and (d). The hollow circles in red and blue connected by the blue dotted line are the same data at one time. From Table 2, the average peak intensity on the target of the 1080 nm laser is 47.5% lower than that of the 1070 nm laser, and the average beam width ${w_{86.5{\%}}}$ on the target of the 1080 nm laser is 47.5% wider than that of the 1070 nm laser. It can also be found that the 1080 nm laser parameters fluctuate more drastically on the target. The coefficient of variation ${c_v} = \sigma /\mu $ is generally used to indicate the discrete degree of the two different average data sets, where $\mu $ is the average values and $\sigma $ is the standard deviation. The larger the coefficient of variation of the data, the more discrete it is after normalization. The 1070 nm laser has coefficients of variation of 0.15 and 0.06 for peak power and beam width, respectively; those of the 1080 nm are 0.29 and 0.11, respectively. The 1080 nm laser bad normalized discrete results can indicate its poor resistance to wind disturbance. In summary, the 1070 nm laser has better propagation properties than the 1080 nm at high power output.
Table 2. Atmospheric propagation data for the 1070 nm and 1080 nm YDFLs at different preset powers
Figure 5(e) and (f) show the intensity distributions of the 1070 nm and 1080 nm lasers on the target at 75% power output, respectively. These are the data from the blue dashed rectangles in Fig. 5(c) and (d). Both intensity distributions show obvious thermal blooming characteristics, and the thermal blooming effect is more obvious for the 1080 nm laser than for the 1070 nm laser. To exclude the differences caused by atmospheric disturbances and the different full width at half maximum (FWHM) of the two lasers with the same output power, Fig. 5(g) shows the intensity distributions on the target when the two lasers have a total power of 50% (FWHM ∼4.3), with the 1080 nm laser intensity distribution on the top side and the 1070 nm laser intensity distribution on the bottom side. Both the intensity distributions of the two lasers on the target and the data analysis in Table 2 also show that the 1070 nm laser produces a weaker degree of thermal blooming effect than the 1080 nm. The above experimental results show that the 1070 nm laser has better focus control and more excellent resistance to atmospheric disturbances than the 1080 nm laser with the same power and FWHM.
Figure 3 shows the water vapor absorptivity in the 1000 nm to 1200 nm band during the experiment, calculated using SpectraPlot [44] software based on weather sensor data. Table 3 shows the total absorptivity of water vapor for the two lasers at different powers when the laser output spectra are integrated with the water vapor absorption spectrum. As the laser power increases, each laser’s total absorptivity of water vapor is enhanced. Moreover, the total absorptivity of the 1080 nm laser is 300 to 400 percent higher than that of the 1070 nm laser for the same atmospheric conditions and power.
Table 3. Total absorptivity of water vapor for two YDFLs at different preset powers
When the propagation distance is only 300 m, the peak intensity and the beam width ${w_{86.5{\%}}}$ of two lasers with similar wavelengths appear significantly different. Through the above analysis of laser parameters and the atmospheric environment, the thermal blooming caused by water vapor absorption is the main reason for reducing the atmospheric propagation capability of the high-power YDFLs.
The thermal distortion parameter equation shows that the thermal blooming effect positively correlates with the absorption coefficient. Under approximately the same laser system parameters, atmospheric environment (except water vapor), and propagation distance, the water vapor absorptivity of the 1080 nm laser is much stronger than that of the 1070 nm laser. The 1080 nm laser produces more severe thermal blooming in the atmosphere, resulting in beam spread and reduced peak intensity. The strong thermal blooming is also more susceptible to dramatic fluctuations due to wind speed, atmospheric density, and other factors, which explain the poorer resistance of the 1080 nm laser to atmospheric disturbances. It can also be said that the degree of thermal blooming can be determined qualitatively by the water vapor absorptivity.
4.2 Influence of atmospheric environment on water vapor absorptivity
In a laser system with a high-power YDFL as the light source and the same parameters (except the central wavelength), all atmospheric environments, except the water vapor, have essentially the same influence on the laser. At high water vapor content, water vapor absorption is one of the main factors that affect the propagation properties of the YDFL in the atmosphere. The factors affecting the total water vapor absorption are related to atmospheric pressure and temperature factors, in addition to propagation distance and water vapor content. When using the absorption lines from the HITRAN database, it is necessary to calibrate them to the actual atmospheric state (atmospheric temperature and pressure).
Based on the weather data during the experiment, the influences of different atmospheric parameters that are obtained through the HITRAN database on the total absorptivity of the two lasers are numerically calculated using a line-by-line integration method in Fig. 6 (a), (b), (c) and (d). In Fig. 6(a), (b), the influence on the total water vapor absorptivity is small even though the temperature and pressure of the atmosphere vary over a wide range. Figure 6(c) and (d) show that water vapor content and propagation distance are the main factors affecting the total absorptivity of water vapor. In reality, the water vapor content is closely related to the temperature and pressure of the atmosphere. Most of the water vapor on Earth is concentrated mainly at the bottom of the troposphere, while the atmospheric temperature, pressure, and water vapor content generally tend to decrease with increasing altitude (between tropospheres). When horizontal propagation of the laser, the absorption of water vapor is constant along the propagation path. In this case, small atmospheric temperature and pressure variations have a negligible influence on water vapor absorption. On the other hand, water vapor is one of the atmospheric parameters that vary most significantly in the same geographic location and over a short period, so measuring changes in water vapor content is essential. When the propagation distance increases, the total absorptivity of the 1080 nm laser increases rapidly, which will seriously affect its propagation effectiveness. When the laser propagates in a slanted path, the total water vapor absorption is smaller than the horizontal propagation due to the vertical profile of water vapor (water vapor content decreases with increasing altitude).
Fig. 6. Influence of different atmospheric parameters on the total absorptivity of the two lasers. (a) Atmospheric temperature; (b) atmospheric pressure; (c) water vapor content; (d) propagation distance; (e) central wavelength.
4.3 Influence of laser light source on water vapor absorptivity
The absorption spectrum of water vapor molecules has wavelength-selective properties in the near-infrared band, so a proper selection of the output spectrum of the high-power YDFLs will reduce the influence of the thermal blooming effect.
In Fig. 6(a), (b), (c), and (d), it can be seen that suppressing the spectral broadening of the two lasers (1070 nm and 1080 nm) in this experiment can reduce the total absorptivity of water vapor to a certain extent. However, the spectral broadening of realistically engineered fiber lasers is mainly related to nonlinear and thermal (indirect) effects. The degree of nonlinear effect is positively associated with the laser power and the effective length of the fiber and negatively related to the mode field diameter of the fiber [2,45]. In addition to the fundamental losses due to quantum defects, the thermal effects are caused by the excess pump and signal losses [2,46]. Optimizing laser parameters, designing proper structures, and adding a conduction cooling scheme can effectively reduce the nonlinear and thermal effects of fiber lasers, but suppressing the spectral broadening of fiber lasers conflicts with the high-power output [6,47–49]. It is also clear from the numerical calculations that the 1070 nm laser is less effective in suppressing water vapor absorption by reducing spectral broadening. Therefore, in high-humidity environments, long propagation distances, or where propagation efficiency is critical, it is recommended to use “short wavelength” YDFL.
Through the absorption spectrum, we can find that the absorption of water vapor is minimal between 1030 nm and 1060 nm. Figure 6(e) shows the variations in the total absorptivity of water vapor with the central wavelength of the laser and the preset power (these spectral characteristics are modeled by the 1070 nm laser of this experiment). Between 1060 nm and 1090 nm, the absorption increases exponentially with the central wavelength. The central wavelength of the laser between 1040 nm and 1060 nm has the smallest absorptivity, which facilitates laser propagation in the atmosphere. From 1040 nm to shorter wavelengths, there is a slight increase in absorptivity because water vapor has an absorption band near 1030 nm. Coincidentally, the atmospheric propagation bands of YDFLs can correspond to the short band (S-band: below 1060 nm) and convenient band (C-band: 1060 - 1130 nm) of their oscillation spectral range [30]. After the above analysis, the S-band will be the best wavelength selection range for atmospheric propagation of future high-power YDFLs. From the absorption and emission cross-sections of the $\textrm{Y}{\textrm{b}^{3 + }}$ in fibers, it is known that the laser has an optimal net gain in the C-band. For high-power YDFLs with a central wavelength in the S-band, the Raman gain spectrum overlaps with the absorption spectrum, which leads to the amplification of the SBS in the fiber. This, coupled with the output power loss caused by amplified spontaneous radiation and reabsorption effects, makes it relatively easy to achieve high power output for YDFLs with the center wavelength in the C-band, while the other bands cannot operate efficiently. These reasons lead to technically mature high-power YDFLs having a central wavelength between 1060 and 1090 nm [50]. Most of the existing reported high-power YDFLs in the S-band use special fiber laser structures and devices to achieve kilowatt-level power output [51–55]. However, these lead to technical difficulties and increased costs.
Meanwhile, in Fig. 6(e), it can also be found that some “narrow linewidth” spectra have higher total water vapor absorption than the “wide linewidth” spectra at the central wavelengths of 1025 nm and 1085 nm. The main reason for this phenomenon is the different states of the relationship between the laser output spectrum and the water vapor absorption spectrum. When the laser output spectrum overlaps with the strong absorption peak of water vapor and the spectral width is greater than the bandwidth where the absorption peak of water vapor is located, it can be used to disperse the power density per unit wavelength by spectral broadening to reduce absorption. When the laser output spectrum is near the strong water vapor absorption peak (no overlay), the spectral broadening will cause part of the output spectrum to overlap with the strong water vapor absorption, leading to increased water vapor absorption.
Finally, it should be noted that the overall trend of water vapor absorption is essentially the same for conventional linewidth lasers with different line widths (the laser used and presented in this experiment, FWHM ∼ several nm). However, narrow linewidth lasers (FWHM < 1 nm) show different discrete absorption spectra. When using YDFL as the light source for atmospheric propagation, the laser system index must be adjusted according to the YDFL spectral characteristics.
5. Conclusion
Through theoretical analysis, experimental data, and numerical simulation, this paper investigates the influence of the thermal blooming effect on the propagation properties of YDFL in the atmosphere. Combining the industrial reality of existing high-power YDFLs, two 20 kW YDFL systems with typical wavelengths are used for the atmospheric propagation comparison experiment. With approximately the same laser system parameters (except wavelength) and atmospheric environment, there is little difference in the extinction effect between the two lasers. However, the 1070 nm laser has a higher peak intensity on the target and better resistance to atmospheric variations than the 1080 nm laser. The main reason is that water vapor absorption has wavelength-selective properties, and the 1080 nm laser induces a more severe thermal blooming effect than the 1070 nm laser at the same output power. It should also be noted that the spectral broadening of YDFLs due to power scaling leads to higher water vapor absorptivity and more severe thermal blooming effects. However, other environmental factors have essentially the same influence on the thermal bloom variation within the output wavelength coverage of the YDFLs. Numerical analysis shows that the essential factors affecting water vapor absorption are water vapor content, propagation distance, and laser output spectrum. Constrained by the manufacturing difficulty and propagation requirements, a reasonable choice of high-power YDFL can effectively improve atmospheric propagation performance and reduce system costs.
Funding
State Key Laboratory of Pulsed Power Laser Technology (SKL2021KF02).
Disclosures
The authors declare no conflicts of interest. This work is original and has not been published elsewhere.
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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