Zhouling Wu

Three-dimensional photothermal microscopy of KDP crystals

The laser damage threshold of KDP crystals is one major limitation in many high-power laser systems. Investigation of laser damage behavior of KDP crystals shows that the major reason for laser damage is the growth defects in the bulk of the materials. Therefore, an effective diagnostic method for those defects is quite necessary for producing KDP crystals with high enough damage threshold to meet the requirement of high power laser applications. In this paper, we reported the characterization of bulk defects in KDP crystals using a three dimensional photothermal microscope based on a laserinduced photothermal lensing technique. Several 3D mapping of the bulk defects were obtained. The results indicated that both surface defects and bulk defects can be determined and analyzed using the 3-D photothermal microscope. The details of the development of the 3-D photothermal microscope were also presented. The system provided user-friendly operations of the defects characterization process and showed great potential of application for characterization of low absorption optical materials.

Photothermal microscopy: an effective diagnostic tool for laser irradiation effects on fused silica and KDP

In this paper, an automated microscopic instrument based on this technique is developed and used for the measurement and analysis of weak absorption properties of optical materials. This system shows a measuring sensitivity of absorbance down to 10 ppb, and provides user-friendly operation of the whole absorption measurement process. Compared with a typical bench-top system, the automated system requires little special skills from the operators and is therefore more reliable and reproducible. By using this system, a study of laser irradiation effects on optical materials induced by high power laser pulses is performed. The in-situ monitoring of a laser induced damage process at 355 nm in fused silica is realized, which indicates that the photothermal system is a useful tool for analysis of laser-material-interaction dynamics. Other specific applications of this system include measuring weak absorption, detecting local absorption defects. Experimental results show that both surface/sub-surface absorption defects on fused silica and bulk absorption defects on KDP are clearly determined.

Development of a turn-key system for weak absorption measurement and analysis

Photothermal techniques have been widely used for the measurement and analysis of optical absorption, especially the weak absorption of optical components used for high-power laser applications. In this paper we present the progress in the development of a “turn-key” system for weak absorption measurement and analysis. The system provides userfriendly operations of the whole absorption measurement process. There is no need of manual realignment when changing different samples. Compared with those bench-top systems built in various research laboratories, this system is more reliable and more stable. Different measuring geometries, such as transmission measuring and reflection measuring can be selected depending on application need. Two dimensional defect mapping and three dimensional defect imaging are also made possible, and the experimental results show that non-uniformity is an important issue for both thin film coatings and bulk materials.Photothermal techniques have been widely used for the measurement and analysis of optical absorption, especially the weak absorption of optical components used for high-power laser applications. In this paper we present the progress in the development of a “turn-key” system for weak absorption measurement and analysis. The system provides userfriendly operations of the whole absorption measurement process. There is no need of manual realignment when changing different samples. Compared with those bench-top systems built in various research laboratories, this system is more reliable and more stable. Different measuring geometries, such as transmission measuring and reflection measuring can be selected depending on application need. Two dimensional defect mapping and three dimensional defect imaging are also made possible, and the experimental results show that non-uniformity is an important issue for both thin film coatings and bulk materials.

Development of an infrared absorption measurement system for large aperture optics

Surface absorption defect has significant effects on the laser damage in the high power laser systems. Photothermal absorption measurement system based on the laser induced surface thermal lensing (STL) effect has been widely used in the research on the correlation between laser damage susceptibility and properties of weak absorption defects for small optical specimens. In this paper, we present the progress in the development of an automated measurement system for large aperture optics with a size around 400mm. The wavelength at 1064nm is used as the pump laser to investigate the absorption properties for the inspected site. The system which shows a measurement sensitivity of absorbance down to 0.1 ppm and measurement repeatability of 10% requires little special skills from the operators and is therefore more reliable and reproducible. The specific applications of the system include weak absorption measurement, local absorption defects detection as well as laser-coating-interaction dynamics monitoring. The high sensitive automated system proposed in this work is an effective diagnostic tool for the examination of large aperture optics with desired optical properties.

Evaluation of surface and bulk qualities of semiconductor materials by a laser-induced photothermal technique

Non-destructive evaluation of defects for semiconductor materials is critical to the quality control process. The existing evaluation methods, including radiographic testing, ultrasonic detection, fluorescence and infrared imaging, are widely used in industrial applications. In this paper an instrument based on laser-induced photothermal technique was applied to study various semiconductor materials. With a specially arranged pump-probe configuration, this system can do three dimensional mapping of local properties and defects. By using this photothermal instrument, several semiconductors, such as bulk CdZnTe (CZT) crystals and monocrystalline silicon wafers under different processing conditions, were investigated. The surface and internal structures and defects of these materials were tested nondestructively by the 3-D photothermal microscope. The results show intersting correlation between the photothermal characterizations and the processing conditions. In addition, the details of the development of the 3-D photothermal microscope were also presented. The system provides user-friendly operations of the defects characterization process and shows great potential of application for characterization of semiconductor materials.

In-situ investigation of damage processes on fused silica induced by a pulsed 355nm laser with high repetition rate

Pulsed lasers with high repetition rate have attracted more and more attention in recent years, due to the increased demand in many fields, including industrial applications such as laser processing and micromachining. However, the damage thresholds of optics exposed to high repetitive laser pulses, especially at the 355 nm wavelength, remain a major concern. Previous work about laser damage in optics was performed mainly by using pulsed lasers with 1-on-1 test or N-on-1 test but with very low pulse repetition rate (for example, from several Hz to hundreds of Hz). The results obtained, however, cannot be directly extended to analyze and understand the damage processes induced by high repetitive laser pulses. In this paper, we present our recent progress on investigation of damage processes on fused silica induced by a pulsed 355 nm laser with high repetition rate. By using a system based on photothermal effect, we have realized in-situ monitoring of laser-material-interaction dynamics through measuring the laser-induced absorption evolution. The results demonstrate that the initiation of laser-induced damage process occur far before any physical damage observable using high-resolution optical microscopes. The damage processes typically are long term accumulation effects of laser-induced increase in absorption, that itself is depending on both the irradiation fluence and repetition rate. The photothermally measured results are also compared with results obtained by using light scattering technique. The results show that such a photothermal technique is a very useful tool for in-situ studies of the damage process induced by high repetitive laser pulses at 355 nm wavelength.Pulsed lasers with high repetition rate have attracted more and more attention in recent years, due to the increased demand in many fields, including industrial applications such as laser processing and micromachining. However, the damage thresholds of optics exposed to high repetitive laser pulses, especially at the 355 nm wavelength, remain a major concern. Previous work about laser damage in optics was performed mainly by using pulsed lasers with 1-on-1 test or N-on-1 test but with very low pulse repetition rate (for example, from several Hz to hundreds of Hz). The results obtained, however, cannot be directly extended to analyze and understand the damage processes induced by high repetitive laser pulses. In this paper, we present our recent progress on investigation of damage processes on fused silica induced by a pulsed 355 nm laser with high repetition rate. By using a system based on photothermal effect, we have realized in-situ monitoring of laser-material-interaction dynamics through measuring the laser-induced absorption evolution. The results demonstrate that the initiation of laser-induced damage process occur far before any physical damage observable using high-resolution optical microscopes. The damage processes typically are long term accumulation effects of laser-induced increase in absorption, that itself is depending on both the irradiation fluence and repetition rate. The photothermally measured results are also compared with results obtained by using light scattering technique. The results show that such a photothermal technique is a very useful tool for in-situ studies of the damage process induced by high repetitive laser pulses at 355 nm wavelength.

Photothermal studies of the radiation effects on weakly absorptive optical thin film coatings induced by high repetitive laser pulses

In this paper a commercial photothermal instrument is used to study the radiation effects on weakly absorptive optical thin film coatings induced by high repetitive laser pulses. Through in-situ measuring of the coatings’ absorption under irradiation of high repetitive laser pulses, the dynamic information of laser-material-interaction process is obtained, which is very useful for understanding and analyzing of laser damage mechanism of the coatings. In addition, absorption defects on the coatings are clearly determined through the photothermal imaging instrument.

A sensitive optical probe for surface topography based on an optimized astigmatic method

Surface topography is required for many kinds of industrial products, and its applications keep increasing, making the need for adequate control of surfaces and an understanding of surface topography measurements more important than ever. In this paper, we presented a sensitive and cost-effective optical probe for surface topography. An astigmatic method used in the optical probe was optimized for submicron/nanometer resolution and monotonic characterization of focus error signal (FES). A home-made system based on this optical probe was also presented, and the measurement results of surface topography indicate great potential of applications in many industrial fields.

Three dimensional mapping of absorption defects at 355 nm for potassium dihydrogen phosphate (KDP) used in high power laser systems

Potassium dihydrogen phosphate (KDP) is commonly used for frequency conversion and optical switching applications in many high-power laser systems. Such applications require high damage threshold of KDP crystals. Damage behavior of KDP has been investigated for many years, and the results show that intrinsic or extrinsic defects are responsible for highly localized absorption in KDP materials, and that in turn will cause the laser damage. In this paper, we studied the absorption properties of KDP crystals at wavelengths of 355 nm by using a three-dimensional (3D) photothermal microscope. Several 3D images of the bulk defects were obtained. The results indicated that both surface defects and bulk defects can be determined and analyzed using the 3-D photothermal microscope. Our results indicate that 3D photothermal microscopy is a powerful tool for defect characterization of optical materials for high power laser applications.

Photothermal microscopic studies of surface and subsurface defects on fused silica at 355nm

It is believed that surface and subsurface defects formed during standard grinding and polishing processes are mainly responsible for laser induced damage in fused silica. The correlation between the laser damage susceptibility and absorption property of these defects has not been totally understood. In this paper, we present the characterization of surface and subsurface defects of fused silica by measuring their absorption properties based on a photothermal technique at 355 nm. The photothermal microscopic imaging reveals that the surface/subsurface absorption defects in fused silica can be identified. In addition, a 3D photothermal imaging of a laser damage site on the silica is also obtained. Our results demonstrate that photothermal microscopy is a powerful tool for defect characterization of optical materials for high power laser applications.