Do You Know The Industrial Application Of DPSS Lasers?(Part 4)

Dpss Lasers can be used in 12 fields, today we will introduce four fields of application and their principles

9. Photovoltaic Inspection

Laser-based techniques in photovoltaic inspection reveal a variety of material properties and are widely used throughout the industry. Measurements such as surface reflectance, deep-level traps, carrier diffusion, crystal structure and boundaries, junction type depth and temperature, light absorption and scattering, and photon degradation all affect the efficiency of solar cells and can be measured through a range of optical processes.

Most photovoltaic manufacturing is in silicon; However, researchers are looking for a cheaper and more efficient alternative - perovskite. In the past decade, the power conversion efficiency of perovskite solar cells has soared from less than 4% to nearly 30%, causing a great stir. A small amount of perovskite material can produce the same amount of solar energy as a few tons of silicon. As direct bandgap semiconductors, perovskites are ideal for solar cells. Perovskite is affordable, sustainable, and efficient, and has the potential to overtake silicon in the PV market. However, perovskite efficiency has only been measured on tiny samples and is not yet commercially viable.

Single-frequency lasers offer an efficient, non-contact alternative to expensive lithography steps, and with the right laser characteristics and wavelengths, these light sources can also inspect, alter, and activate these novel materials. Achieving high yields at a lower cost requires a light source with high spatial resolution, excellent beam quality, and long-term power stability. For example, photoluminescence (PL) imaging can be used for efferent quality control (wafer manufacturers) and afferent quality control (battery manufacturers), where near-infrared (NIR) lasers are often used as cost-effective light sources for this purpose. Lasers in the ultraviolet (UV) range provide flexibility for material characterization and processing steps. As with semiconductor processing, UV light is used in various measurement steps and techniques for photovoltaic cell inspection, where the shorter wavelength allows for analysis of increased surface complexity, and the high-power UV source radiates or ablates degraded materials on the substrate barrier.

Single-frequency lasers cover the NIR to UV range, and their characteristics are specifically designed for the applicability of these optical processes.

Beam quality: includes the size, shape, stability, and intensity of the laser beam. A single transverse mode beam (TEM 00) is essential for the characterization of PV cells, allowing for high spatial control. Excellent beam shape, stable direction, and low ellipticity for consistent processing and detection.

Low noise: PV cells and wafer detection lasers must emit low noise to minimize detection errors and prevent inaccurate characterization. The low noise level, combined with the narrow line width, improves the signal-to-noise ratio and enhances measurement and detection sensitivity.

Stability: To ensure consistency from battery to battery and panel to panel, the laser also requires excellent spectral and power stability to make high-resolution measurements and eliminate errors in long time measurements.

10. Grating Master Production

Optical diffraction gratings are commonly used devices for measuring the wavelength of light, consisting of a number of regularly spaced diffractive elements - namely gaps and ridges - that can alternately affect the phase and amplitude of the incident light. A practical example of gratings is their use in spectrometers. The entrance slit is located in the focal plane of the lens, allowing any incident light to pass through and become parallel. The light then strikes the grating so that the incident light is dispersed into its constituent wavelengths, and the intensity distribution can be observed directly or recorded by a photometer.

Gratings can be arranged in transmission or reflection mode and are widely used in a variety of different laser systems. These gratings are installed inside and outside the resonator for wavelength selection, beam separation, beam shaping, and polarization. High-performance laser gratings are characterized by their damage thresholds at specific wavelengths, as well as their high pulse width, repetition rate, and diffraction efficiency in the polarization direction.

Holographic and interference lithography processes are common in grating production, although high-quality spectral gratings can only be obtained by introducing high-resolution coatings and short-wave lasers. A grating can be created by drawing a fine laser interference field on the lithoresist layer, where interference waves can be generated by the amplitude splitting of a wavefront or coherent laser beam - most commonly lasers in single-mode operation.

The overall efficiency and quality of a grating created in this way depend on several characteristics of the light source used, such as wavelength and polarization, and the following parameters should be taken into account when considering a suitable laser for grating master production:

High power: Shorter exposure times are usually required as this reduces damaging external influences such as vibration. Therefore, higher light intensity is preferred.

Power stability: Fluctuations in output power during the production process can amplify the interferogram, resulting in inaccuracy. Therefore, ultra-stable output power and undetectable power noise are very important to ensure the quality of the grating parent disk.

Beam quality: Excellent beam quality and pointing stability are also key parameters to ensure consistent and accurate analysis.

11. Brillouin Scattering

The Brillouin effect is inelastic scattering caused by the parametric interaction of photons with thermal phonons, as found in Raman spectroscopy, although here it is caused by the interaction of light with phonons that vibrate in the acoustic range; Often called sound waves. These dynamic thermal fluctuations can cause changes in the dielectric constant and refractive index of the carrier material, resulting in weak inelastic scattering effects as photons pass through. This inelastic interaction causes a change in frequency within the incident light, proportional to the relative speed of the phonon, resulting in an energy change or Stokes shift, which is several orders of magnitude smaller than the Raman shift due to comparisons between the speed of sound and the speed of light.

In Raman, this Stokes shift is related to specific vibrational and rotational interactions at the molecular level, while the Brillouin shift is the result of macroscopic, low-frequency interactions with the bulk medium, where nonlinear effects are most often caused by electrostriction. This Stokes shift can also be caused by changes in the charge structure (polaron) or its magnetic (magneton) oscillation. Photons may lose energy, causing a move towards a longer wavelength, or gain energy, causing a shorter wavelength (anti-Stokes).

At low laser power, these Brillouin effects can occur spontaneously, but at higher power intensities, this effect can be directly excited by excited photons, called stimulated Brillouin scattering (SBS). SBS causes sound waves to be generated in the carrier material, propagating in the same direction as the incident beam, and the scattered and moving photons are reflected or reflected back toward the incident beam. This scattering can be analyzed to determine various elastic properties of submicron films and samples, as well as surface properties of bulk materials, and is used for a wide range of applications; Examples include geology, biology and life sciences, oil and gas, telecommunications, and more. For example, it is this stimulated back-reflection effect that limits the total optical power that can be injected into the fiber. This effect is also widely used in optical phase conjugation, where phase conjugation mirrors (PCM) are used to correct thermal distortions in laser crystals and produce more Gaussian beam shapes.

Because the scattering effect is very weak and the Stokes shift is only a few picometers, the excitation laser used is crucial. The laser must have an extremely narrow line width and a long coherence length to ensure that the results of the Brillouin scattering effect are clearly observed with good resolution and signal-to-noise ratio. 

12. Interferometry

Interferometry refers to a broad technique that relies on the superposition of two coherent light paths, most commonly separated from a single light source, to form an interference pattern. This interference is caused by a difference in the path between two beams, a reference light path, and an incident sample light path, resulting in a measurable change in the fringe pattern. This measurement technique can be used for a variety of different applications - from simple distance or surface measurements to structures and stresses, to gravitational wave measurements.

In theory, the typical experimental setup is very simple. The highly stable coherent laser is split in two to produce separate and identical beams. One is a reference arm with a fixed path, while the other forms a moving incident beam of the sample. Initially, the two beams of light are in phase, separated from the same coherent source. If the two paths are the same length, they will still be in phase when they reach the detector. However, a slight deviation in the path of the sample beam changes its phase with respect to the reference beam and thus creates associated deviations in the interference pattern. These deviations in the interference pattern are measurable outputs.

Several factors to consider when selecting the right light source for interferometry:

First, the light source needs hyperspectral stability to ensure that the pattern change is caused by the sample and not by the laser effect. Longer coherence lengths, and therefore narrower line widths, will partly determine the resolution of the measurement, while also taking into account the wavelength used.

High beam pointing stability ensures consistent measurements at the selected sample location, while high beam quality reduces the complexity that can arise when analyzing measurement results.

Finally, it is important to consider the available power level compared to the sample size, as higher power can image a larger area.

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