LUS

Overview

1. Overview

The Laser Ultrasonic Detection System (LUS) is a non-contact, high-precision, non-destructive ultrasonic inspection system. Short-pulse laser radiation excites ultrasonic waves at the surface of the part under test, and a laser interferometer detects the microscopic surface motion produced by the propagating ultrasound, yielding information on thickness, internal and surface flaws, and material elastic parameters. Combining the high accuracy of ultrasonic testing with the non-contact nature of optics, laser ultrasonics offers sub-nanometer sensitivity and GHz-class bandwidth, making it ideally suited for high-temperature, rough-surface, stand-off and automated in-line inspection where conventional ultrasonics fails.

Fig.1 Laser ultrasonic system vs. conventional piezoelectric ultrasonic system (principle)

2. Measurement Principle

Unlike conventional ultrasonics in which piezoelectric transducers excite and receive ultrasound, in a laser-ultrasonic system both generation and detection are performed by light - the material itself acts as the emitting transducer and transduction is made by light, ensuring completely non-contact operation at stand-off distances from centimeters to meters, and even in vacuum or reduced pressure.

2.1 Laser Generation of Ultrasound

(1) Thermoelastic mechanism (non-destructive): laser light is absorbed to some depth inside the material, releasing heat locally; the heated region expands and produces strain and stress that radiates elastic waves (longitudinal, shear and surface waves simultaneously). This fully non-destructive mode is the preferred regime for NDT, with detection bandwidth easily reaching the GHz range.

(2) Ablation / vaporization mechanism: at sufficiently high energy density, surface ablation or vaporization of a surface layer generates much stronger, steeper shock-wave signals, at the cost of slight surface modification - suitable when strong signals are required and surface condition tolerance is acceptable.

2.2 Laser Detection of Ultrasound

Detection senses the microscopic surface displacement (or vibration velocity) caused by the ultrasonic wave arriving at the surface. Depending on the application, three interferometer schemes are used:

(1) Michelson interferometer: simple and well suited to mirror-like/smooth surfaces, e.g. wafer inspection in microelectronics.

(2) Confocal Fabry-Pérot interferometer: based on multiple-beam interference and optical-filter demodulation, it accepts scattered light from rough surfaces and is the mainstream industrial scheme (e.g. on-line thickness gauging of hot steel tubing).

(3) Sagnac fiber-optic interferometer: a balanced-detection fiber scheme with no reference arm, insensitive to surface roughness, ambient vibration and thermal drift, requiring no feedback stabilization, with sensitivity approaching high-performance piezoelectric detectors - ideal for rough-surface field inspection of aerospace composites.

Fig.2 Fiber-optic Sagnac interferometer detection optical path (balanced detection)

3. System Composition

(1) Pump (generation) laser: short-pulse nanosecond laser with adjustable energy density to excite ultrasound; high-repetition-rate (up to tens of kHz) fiber lasers dramatically increase scan speed.

(2) Detection laser: CW laser or super-luminescent diode (SLD) serving as the interferometer illumination source, wavelength selected per application (e.g. 1550 nm telecom band).

(3) Laser interferometer: Michelson, confocal Fabry-Pérot or Sagnac scheme converting surface vibration into intensity/phase signal - the core sensitive unit.

(4) Data acquisition & processing: high-speed ADC (e.g. 14-bit / >100 MS/s) digitizes the interferometer output; synchronized triggering, A/B/C-scan imaging and defect-recognition algorithms output thickness, flaw and material results.

(5) Scanning & motion stages: 2D/3D translation stages or robots for large-area imaging; flexible fiber delivery adapts to complex contours.

Fig.3 All-fiber laser ultrasonic inspection system (pump-probe-interferometer-acquisition)

4. Key Features & Advantages

1) Fully non-contact, no couplant, operable at high temperature, corrosive and vacuum environments; 2) high sensitivity down to sub-nanometer, equivalent to high-performance contact piezoelectric probes; 3) ultra-wide bandwidth (GHz), compact impulse response (~95 ns, about 3x better axial resolution than typical contact transducers); 4) rough-surface tolerant (confocal Fabry-Pérot and Sagnac schemes); 5) stand-off and flexible fiber delivery suitable for complex structures and automated lines; 6) high repetition-rate scanning with fiber pump lasers up to tens of kHz.

5. Typical Applications

1) NDT of aerospace composite structures (fuselage skins, honeycomb, lap-joint corrosion); 2) on-line thickness and eccentricity gauging of hot seamless steel tubes; 3) on-line monitoring of laser-weld quality; 4) wind-turbine blade inspection; 5) inspection of high-temperature ceramics, metals and composites; 6) electronics/semiconductor packaging quality control; 7) coating defect detection; 8) thin-film thickness and elastic-constant measurement in microelectronics.

6. Service & Support

Sunpump Opto-Electronic provides customized design and integration of laser ultrasonic systems. Based on the inspection object (material, thickness, surface condition), target (thickness, flaw, material characterization) and line throughput, we configure pump/detection lasers, interferometer scheme, scanning mechanism and software algorithms, with on-site installation & commissioning, operator training and long-term technical support.

1. Overview

The Laser Ultrasonic Detection System (LUS) is a non-contact, high-precision, non-destructive ultrasonic inspection system. Short-pulse laser radiation excites ultrasonic waves at the surface of the part under test, and a laser interferometer detects the microscopic surface motion produced by the propagating ultrasound, yielding information on thickness, internal and surface flaws, and material elastic parameters. Combining the high accuracy of ultrasonic testing with the non-contact nature of optics, laser ultrasonics offers sub-nanometer sensitivity and GHz-class bandwidth, making it ideally suited for high-temperature, rough-surface, stand-off and automated in-line inspection where conventional ultrasonics fails.

Fig.1 Laser ultrasonic system vs. conventional piezoelectric ultrasonic system (principle)

2. Measurement Principle

Unlike conventional ultrasonics in which piezoelectric transducers excite and receive ultrasound, in a laser-ultrasonic system both generation and detection are performed by light - the material itself acts as the emitting transducer and transduction is made by light, ensuring completely non-contact operation at stand-off distances from centimeters to meters, and even in vacuum or reduced pressure.

2.1 Laser Generation of Ultrasound

(1) Thermoelastic mechanism (non-destructive): laser light is absorbed to some depth inside the material, releasing heat locally; the heated region expands and produces strain and stress that radiates elastic waves (longitudinal, shear and surface waves simultaneously). This fully non-destructive mode is the preferred regime for NDT, with detection bandwidth easily reaching the GHz range.

(2) Ablation / vaporization mechanism: at sufficiently high energy density, surface ablation or vaporization of a surface layer generates much stronger, steeper shock-wave signals, at the cost of slight surface modification - suitable when strong signals are required and surface condition tolerance is acceptable.

2.2 Laser Detection of Ultrasound

Detection senses the microscopic surface displacement (or vibration velocity) caused by the ultrasonic wave arriving at the surface. Depending on the application, three interferometer schemes are used:

(1) Michelson interferometer: simple and well suited to mirror-like/smooth surfaces, e.g. wafer inspection in microelectronics.

(2) Confocal Fabry-Pérot interferometer: based on multiple-beam interference and optical-filter demodulation, it accepts scattered light from rough surfaces and is the mainstream industrial scheme (e.g. on-line thickness gauging of hot steel tubing).

(3) Sagnac fiber-optic interferometer: a balanced-detection fiber scheme with no reference arm, insensitive to surface roughness, ambient vibration and thermal drift, requiring no feedback stabilization, with sensitivity approaching high-performance piezoelectric detectors - ideal for rough-surface field inspection of aerospace composites.

Fig.2 Fiber-optic Sagnac interferometer detection optical path (balanced detection)

3. System Composition

(1) Pump (generation) laser: short-pulse nanosecond laser with adjustable energy density to excite ultrasound; high-repetition-rate (up to tens of kHz) fiber lasers dramatically increase scan speed.

(2) Detection laser: CW laser or super-luminescent diode (SLD) serving as the interferometer illumination source, wavelength selected per application (e.g. 1550 nm telecom band).

(3) Laser interferometer: Michelson, confocal Fabry-Pérot or Sagnac scheme converting surface vibration into intensity/phase signal - the core sensitive unit.

(4) Data acquisition & processing: high-speed ADC (e.g. 14-bit / >100 MS/s) digitizes the interferometer output; synchronized triggering, A/B/C-scan imaging and defect-recognition algorithms output thickness, flaw and material results.

(5) Scanning & motion stages: 2D/3D translation stages or robots for large-area imaging; flexible fiber delivery adapts to complex contours.

Fig.3 All-fiber laser ultrasonic inspection system (pump-probe-interferometer-acquisition)

4. Key Features & Advantages

1) Fully non-contact, no couplant, operable at high temperature, corrosive and vacuum environments; 2) high sensitivity down to sub-nanometer, equivalent to high-performance contact piezoelectric probes; 3) ultra-wide bandwidth (GHz), compact impulse response (~95 ns, about 3x better axial resolution than typical contact transducers); 4) rough-surface tolerant (confocal Fabry-Pérot and Sagnac schemes); 5) stand-off and flexible fiber delivery suitable for complex structures and automated lines; 6) high repetition-rate scanning with fiber pump lasers up to tens of kHz.

5. Typical Applications

1) NDT of aerospace composite structures (fuselage skins, honeycomb, lap-joint corrosion); 2) on-line thickness and eccentricity gauging of hot seamless steel tubes; 3) on-line monitoring of laser-weld quality; 4) wind-turbine blade inspection; 5) inspection of high-temperature ceramics, metals and composites; 6) electronics/semiconductor packaging quality control; 7) coating defect detection; 8) thin-film thickness and elastic-constant measurement in microelectronics.

6. Service & Support

Sunpump Opto-Electronic provides customized design and integration of laser ultrasonic systems. Based on the inspection object (material, thickness, surface condition), target (thickness, flaw, material characterization) and line throughput, we configure pump/detection lasers, interferometer scheme, scanning mechanism and software algorithms, with on-site installation & commissioning, operator training and long-term technical support.

Features

• [{"nameEn": "Non-contact inspection"}, {"nameEn": "GHz-class bandwidth"}, {"nameEn": "Custom integration"}]