high energy laser; spot measurement; fiber array; V-groove; fused silica

A method for measuring the spatiotemporal distribution of laser light spots based on fiber arrays is proposed. The method of fabricating V-shaped grooves on multi-layer quartz plates and stacking quartz fiber arrays is used to obtain fiber array sampling with high laser damage resistance and spatial resolution. The device realizes the sampling of the spatial distribution of the laser spot. The sampling principle of the fiber array, the structure of the sampler and the design of large multiple light intensity attenuation are given.

The experimental results show that the anti-laser damage capability of the fiber array is better than 10kW/cm2 (50s), the spatial resolution of the system is better than 3mm, and the measurement error is less than 3%.

With the development of high-energy laser technology, its output power and energy are getting higher and higher. Due to the strong destructive power, the measurement of high-energy laser light intensity distribution parameters has become a technical problem. Common high-energy laser intensity spatiotemporal distribution measurement methods include ablation method, scanning method (fiber probe, hollow probe and ring light knife), array detection method, etc.

The ablation method is only used as an auxiliary method for the qualitative judgment of the spot distribution; the probe scanning method is limited by the sampling method, and it is difficult to meet the requirements of real-time measurement of large-area spots; the ring-knife scanning sampling method can realize the measurement of large-area beams, but the scanning structure The complexity limits its wide application.

The array detection method has the advantages of simple principle and structure, direct measurement of strong laser spot distribution, independent of each detection unit and easy modular integration, etc. However, there are problems such as low resistance to laser damage and low spatial resolution of the target surface. In the current array detection method, the laser damage resistance is about 1kW/cm² and the highest resolution is about 10mm, which is difficult to meet the measurement requirements of high power density and high spatial resolution.

In this paper, a method for measuring the spatiotemporal distribution of laser light intensity based on fiber arrays is proposed. The high-spatial-resolution sampling of high-power-density laser beams is realized by using the characteristics of pure silica fiber with small core diameter and high laser damage threshold, which also provides a technical means for direct measurement of high-energy laser parameters.

The principle of the fiber array sampler is shown in Figure 1.

Fig.1 Schematic of measuring

It is mainly composed of a quartz fiber and a quartz plate with a V groove. The quartz fiber array is stacked between the quartz plates, and the sampling fiber is bent and fixed in an “L” shape. The detector and data The processing unit is located next to the laser beam, enabling transmissive measurements.

The measurement system is mainly composed of fiber array sampler, photoelectric detector array, data acquisition and processing software. When the large-area laser is transmitted through the sampler, the sampling fiber realizes laser sampling and transmission, and the photoelectric detection unit converts the optical signal into an electrical signal, which is collected by the multi-channel data acquisition system and restored by the data processing software. The photoelectric detection unit can use a discrete photodetector or an area array CCD. For near-infrared and visible light lasers, quartz optical fibers and quartz plates can be used. For mid-infrared wavelengths, fluoride and sulfide optical fibers can be used. The substrate can be made of materials such as silicon and germanium.

In order to enhance the anti-laser damage capability of the sampler, the following measures are mainly taken:
(1) Remove the coating layer of the sampling fiber and mechanically cut the fiber to ensure the smooth end face and the consistency of the sampling channel;
(2) The quartz plate is made of natural fused silica, and the six surfaces are polished to improve the laser damage threshold of the quartz surface;
(3) The multi-layer quartz plates are superimposed, and the optical fiber is compressed in the V-groove to avoid the use of low damage threshold materials such as optical glue, realize the full quartzization of the sampler, and effectively improve the laser damage threshold.

The design dimensions of the V-groove are as follows: the included angle is 60°, the depth is about 170μm, the groove spacing is 2.5mm, and it is suitable for standard optical fibers with a diameter of ⌀125μm. The quartz groove is mechanically scribed by a diamond precision CNC machine tool. After the scribe is completed, the inner wall of the groove is polished to reduce defects. The processed V-groove is shown in Fig. 2(a), and the exposed size of the fiber is about 10 μm. The dimensions of the quartz plate are 100mm x 50mm x 3mm. The system achieves a spatial resolution of 2.5mm×3mm, and the partial view of the input surface of the fiber array is shown in Figure 2(b).

Fig. 2 Micrographs of V-groove and fiber array

Theoretically, the spatial resolution of the fiber array is only limited by the fiber diameter and the thickness of the quartz plate. Since the diameter of the sampling fiber is 125 μm, a spatial resolution of hundreds of μm can be achieved in the horizontal direction. In the vertical direction, although limited by the processing performance of quartz plates, a spatial resolution of several mm can be achieved. The measurement area of the optical fiber array has strong expansibility. It only needs to increase the size of the quartz plate and the number of stacks to achieve a large-scale arrangement to meet the requirements of large-area spot measurement.

In the measurement of high-energy laser parameters, the laser power density is much higher than the linear saturation threshold of the existing photodetectors, and 10³ to 10⁴ times of optical attenuation is required to be used for photodetector measurement. The system adopts the beam divergence method to achieve large multiple attenuation of strong light, and realizes continuous adjustment of attenuation multiple by adjusting the distance between the photosensitive surface of the detector and the output end face of the fiber. It has the advantages of simple structure and easy integration, as shown in Figure 3(a).

Fig. 3 Attenuation characteristic of output beams of fiber array

The divergence angle of the fiber is determined by the numerical aperture NA, and the power density irradiated on the detector decreases continuously along the outgoing direction of the fiber, and its attenuation characteristic is determined by the following formula:

In the formula:
r is the radius of the photosensitive surface of the detector;
θ is the beam divergence angle, which is determined by the numerical aperture in below:

n1 is the core refractive index,
n2 is the cladding refractive index;
z is the photosensitive surface to the fiber output surface the distance.
Taking NA = 0.22 as an example, when the diameter of the photosensitive surface is 1 mm, when z is in the range of 35 to 40 mm, the attenuation of the light intensity by 10³ to 10⁴ times can be achieved, as shown in Figure 3(b).

Since each measurement channel of the optical fiber can be designed and processed completely independently, the high spatial resolution design of the system can be realized by using a conventional photodetector, and the calibration of the unit channel can be facilitated. Compared with the commonly used array detection systems, the fiber array sampling attenuation channel has the advantages of good channel consistency, strong interchangeability and high reliability, and effectively reduces the difficulty and cost of system development.

Using a 10W single-mode fiber laser, the fiber array measurement method was experimentally verified. The working wavelength of the laser is 1064 nm, and the output is collimated. Limited by the number of detectors and data sampling channels, 8 measurement channels are designed to form an 8×1 fiber array. The sampling fiber is single-mode fiber SMF28, which is arranged in the horizontal direction and has a resolution of 2.5 mm.

In the vertical direction, the optical fiber array is moved up and down by the translation stage to achieve 1mm spatial resolution. The detector is a photovoltaic InGaAs detector with a wavelength range of 0.9-1.7 μm, and the data acquisition is the NI9219 multifunctional data acquisition module of NI company. The experimental results of multiple measurements are shown in Table 1. It can be seen that the integral power obtained by the experiment is in good agreement with the laser output power, and the deviation is less than 3%. The calculated spot diameter is 10 mm, and the measurement data are shown in Table 1.

Table1 Experimental results of fiber array system

Figure 4(a), (b) are the two-dimensional and three-dimensional light spot distribution curves obtained by the fiber array system, and the curves are the vertical and horizontal light spot distributions, respectively. At the same time, the NIR-CCD spot measurement system was used for comparative measurement. The results are shown in Fig. 4(c). It can be seen that the relative spatial distributions measured by the two systems are in good agreement.

Fig. 4 Comparison of measurement results by fiber array and NIR-CCD

In order to be suitable for high-energy laser spot parameter measurement, the fiber array must withstand high-energy laser irradiation for a long time. In the process of fabrication, installation and storage of optical fiber arrays, defects and impurity contamination are inevitable, which weakens the ability to resist laser damage. From the perspective of damage mechanism, the damage of most materials is mainly caused by heating. When a high-energy laser beam irradiates a fiber array for a long time, on the one hand, impurities and defects will increase the absorption of the laser light by the material, on the other hand, the low thermal conductivity of quartz will increase the heat accumulation generated by the absorption of laser light inside the quartz, which will eventually lead to a sharp increase in temperature, causing material damage.

In order to reduce the surface defects of quartz, the quartz substrate can be pretreated by surface polishing, laser pre-irradiation and HF acid etching. Laser pre-irradiation is mainly to remove dust, moisture and impurities adsorbed on the surface, reduce surface absorption, reduce surface roughness, and repair surface defects; acid attack is mainly to remove micro-cracks, reduce laser absorption, and reduce the hidden danger of damage. Using the high-power fiber laser in the laboratory, the pretreated fiber array was subjected to several intense laser irradiation experiments to test its ability to resist laser damage.

The experimental principle of strong light assessment is shown in Figure 5.

The focal length of the fiber laser is 180mm, the diameter of the focal spot is about 1.1mm, and when the maximum power is 2kW, the average power density of the focal spot is 0.2MW/cm².

Fig. 5 Schematic diagram of laser irradiation

The fiber array consists of five quartz substrates with a size of 120mm×120mm×50mm. The focal spot is located in the center of the fiber array, and the incident direction is along the axis of the V-groove.

The temperature of the irradiation area is monitored in real time by a fiber grating temperature measurement system. The fiber grating is fixed in the V-groove and located in the laser irradiation area. The fiber grating temperature real-time monitoring system is mainly composed of ASE light source, circulator, fiber grating and wavelength demodulation system, which can give the temperature changing curve with time in real time. During the experiment, when the laser power was 2kW, and the irradiation time was 10, 60, 130s, the internal temperature rises were measured to be 11, 22, and 32 °C, respectively, and no obvious damage such as cracks and melting was found.

The above experiments show that the fabricated fiber array sampler has high resistance to laser damage. When the small spot irradiates the area, it can withstand strong laser irradiation with an average power density of 0.2MW/cm² and up to 100 seconds of light output. Due to the low thermal conductivity of quartz material, the above experimental results can be extrapolated to the case of large spot high-energy laser irradiation.

Limited by the output power of the laser, it is impossible to directly obtain the anti-laser damage characteristics of the fiber array when irradiated by a large spot high-energy laser. Based on the finite element method, the anti-laser damage characteristics of fiber arrays under long-time irradiation of high-energy laser with large beam diameter were simulated and analyzed. Laser loading conditions: laser wavelength 1070nm, power density 10kW/cm², irradiation time 50s, uniform spot, diameter 100mm. The size of the sampler is 120mm×120mm×50mm, and it consists of 40 layers of quartz plates with a thickness of 3mm, and the gap between the plates is 10μm. The quartz material parameters are shown in Table 2.

Table 2 Characteristics of fused silica

The calculation results are shown in Fig. 6, where Fig. 6(a) and Fig. 6(b) are the temperature distributions on the surface and inside of the fiber array, respectively, where the ambient temperature is 25 °C.

Fig. 6 Results of numerical simulation for laser irradiation

The calculation results show that the maximum surface temperature of the sampler is 34 °C, the maximum internal temperature is 41 °C, and the melting point of quartz is as high as 1710 °C. Therefore, in the case of a large spot, the anti-laser damage capability of the fiber array sampler is better than 10kW/cm² (50s), which meets the requirements of high-energy Laser parameter measurement requirements. It shows that the laser spot measurement method based on fiber array is suitable for high-energy laser parameter measurement and is a valuable technical means.

The next step is to carry out high-energy laser spot measurement verification experiments and practical design of the measurement system. For the external field application of the measurement system, a sealed structure design can be considered to achieve dust-proof, shock-proof, moisture-proof and other functions, and improve system reliability and environmental adaptability properties, to meet the requirements of field applications. The sealing device is provided with two quartz glass windows to achieve strong light transmission sampling. According to needs, air or nitrogen can be used for sealing protection

The measurement method based on fiber array has high resistance to laser damage, can withstand strong laser irradiation with a power density greater than 10kW/cm² and a light output time greater than 50s, and can meet the requirements of direct measurement of the spatiotemporal distribution of high-energy laser spot intensity. Spot parameter measurement provides a valuable technical means.

As the spatial resolution of the system improves, the number of detection channels will increase to hundreds or even thousands, and the solution of using discrete detectors for the photodetector unit will greatly increase the cost and complexity of the system. If the output fiber of the sampler is made into a fiber bundle, the photodetector unit uses an area array CCD to complete the optical signal detection and data processing, which can effectively solve the problems of system optical signal detection and data acquisition at ultra-high resolution, and meet the requirements of further high index measurement.