Laser ultrasonics-based material analysis system and method

• US 5,638,396 A
• Filed: 06/07/1995
• Issued: 06/10/1997
• Est. Priority Date: 09/19/1994
• Status: Expired due to Fees

• Current Assignee: Brown University
• Original Assignee: Textron Systems Corporation (Textron Inc)
• Inventors: Klimek, Daniel E.
• Primary Examiner(s): Bovernick, Rodney B.
• Assistant Examiner(s): Wise, Robert E.
• Application Number:
  US08/482,783
• Time in Patent Office: 734 Days
• Field of Search: 372/20, 372/92, 372/102, 372/100
• US Class Current: 372/92
• CPC Class Codes: G01N 21/1717 with a modulation of one or more physical properties of the sample during the optical investigation, e.g. electro-reflectance
• CPC Class Codes: G01N 2291/011 Velocity or travel time
• CPC Class Codes: G01N 2291/015 Attenuation, scattering
• CPC Class Codes: G01N 2291/0237 Thin materials, e.g. paper, membranes, thin films
• CPC Class Codes: G01N 2291/0251 Solidification, icing, curing composites, polymerisation
• CPC Class Codes: G01N 2291/02827 Elastic parameters, strength or force
• CPC Class Codes: G01N 2291/02854 Length, thickness
• CPC Class Codes: G01N 2291/0421 Longitudinal waves
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0427 Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever
• CPC Class Codes: G01N 2291/044 Internal reflections (echoes), e.g. on walls or defects
• CPC Class Codes: G01N 2291/045 External reflections, e.g. on reflectors
• CPC Class Codes: G01N 2291/102 one emitter, one receiver
• CPC Class Codes: G01N 2291/2697 Wafer or (micro)electronic parts
• CPC Class Codes: G01N 29/041 on the surface of the material, e.g. using Lamb, Rayleigh or shear waves
• CPC Class Codes: G01N 29/07 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G01N 29/228 related to high temperature conditions
• CPC Class Codes: G01N 29/2418 using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046
• CPC Class Codes: G01N 29/2456 Focusing probes focusing arrangements G01N29/221
• CPC Class Codes: G01N 29/326 : compensating for temperature variations
• CPC Class Codes: G01N 29/348 : with frequency characteristics, e.g. single frequency signals, chirp signals measuring frequency of mechanical vibrations or acoustic waves in general G01H1/06, G01H3/04; measuring frequency or analysing frequency spectra G01R23/00
• CPC Class Codes: G01N 29/4427 : with stored values, e.g. threshold values
• CPC Class Codes: H01S 3/08004 : incorporating a dispersive element, e.g. a prism for wavelength selection H01S3/0811, H01S3/08022 take precedence
• CPC Class Codes: H01S 3/106 : by controlling devices placed within the cavity H01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 5/02208 : characterised by the shape of the housings
• CPC Class Codes: H01S 5/02255 : using beam deflecting elements
• CPC Class Codes: H01S 5/141 : using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence

Abstract

Disclosed herein is an interferometric-based materials analysis system (10) that employs a novel combination of laser beam shaping and pointing techniques, the use of a low cost, rugged, and compact diode laser (22) as a detection laser, and the use of signal processing techniques that compensate for inherent instabilities and short-term drift in the diode laser. A matched filter processing technique is disclosed for processing interferometrically-obtained data points from a target being analyzed. The matched filter technique is shown to be especially useful for detecting and analyzing Lamb modes within thin targets, such as a silicon wafer undergoing a rapid thermal processing cycle. Also disclosed is a method and apparatus for interferometrically monitoring a target to determine, in accordance with predetermined criteria, an occurrence of a period of time that is optimum for obtaining a data point. In response to detecting such a period an impulse source, such as an impulse laser (14), is triggered to launch an elastic wave within the target so that a data point can be obtained. A plurality of data points so obtained are subsequently processed, such as by the matched filter technique, to determine a property of interest of the target. The property of interest may be, by example, the temperature of the target or the metallurgical status of the target.

Claims

1. A laser assembly, comprising:

a semiconductor laser diode having an output for emitting a beam of coherent radiation;

an optical cavity disposed external to said semiconductor laser diode and comprising a grating mounted at a fixed Littrow configuration with respect to the beam, wherein an alignment plane lies perpendicular to grooves of the grating and a tuning axis of the grating is perpendicular to the alignment plane; and

a wedge rotatably mounted within the beam between said output of said semiconductor laser diode and said grating for controllably adjusting an alignment of said semiconductor laser diode with respect to said grating by directing the beam to lie within the alignment plane.

2. A laser assembly, comprising:

a housing having an aperture;

a semiconductor laser diode disposed within said housing, said semiconductor diode laser having an output for emitting a beam of coherent radiation;

an optical cavity disposed external to said semiconductor laser diode and comprising a grating mounted within said housing at a fixed Littrow configuration with respect to the beam, said aperture being aligned with a grooved surface of said grating for directing an output beam of coherent radiation at the desired wavelength out of said housing;

wherein an alignment plane lies perpendicular to grooves of the grating and a tuning axis of the grating is perpendicular to the alignment plane;

a wedge rotatably mounted within the beam between said output of said semiconductor laser diode and said grating for controllably adjusting an alignment of said semiconductor laser diode with respect to said grating by directing the beam to lie within the alignment plane; and

a collimating lens mounted within the beam between said output of said semiconductor laser and said wedge.

3. A laser assembly as set forth in claim 2, wherein an output face of said semiconductor diode laser is antireflection coated.

4. A laser assembly as set forth in claim 2, wherein said wedge is antireflection coated.

5. A laser assembly as set forth in claim 2, wherein said wedge is tilted with respect to an axis of the beam such that any reflections off of a surface of said wedge will not enter said semiconductor diode laser.

6. A laser assembly, comprising:

a housing having an aperture;

a semiconductor laser diode disposed within said housing, said semiconductor diode laser having an output for emitting a beam of coherent radiation;

an optical cavity disposed external to said semiconductor laser diode and comprising a grating that is fixably mounted within said housing at a Littrow configuration with respect to the beam, said aperture being disposed relative to a grooved surface of said grating for directing an output beam of coherent radiation at the desired wavelength out of said housing to an optical path of an interferometer device;

wherein an alignment plane lies perpendicular to grooves of the grating and a tuning axis of the grating is perpendicular to the alignment plane; and

a wedge rotatably mounted within the beam between said output of said semiconductor laser diode and said grating for controllably adjusting an alignment of said semiconductor laser diode with respect to said surface of said grating by directing the beam to lie within the alignment plane.

7. A laser assembly as set forth in claim 6, and further comprising a collimating lens mounted within the beam between said output of said semiconductor laser diode and said wedge.

8. A laser assembly as set forth in claim 6, wherein an output face of said semiconductor diode laser is antireflection coated.

9. A laser assembly as set forth in claim 6, wherein said wedge is antireflection coated.

10. A laser assembly as set forth in claim 6, wherein said wedge is tilted with respect to an axis of the beam such that any reflections off of a surface of said wedge will not enter said semiconductor diode laser.

11. A method for operating a laser assembly, comprising the steps of:

providing a semiconductor laser diode having an output for emitting a beam of coherent radiation, the laser diode being provided within a housing and having an output face disposed at one end of an optical cavity for emitting the output beam, an opposite end of the optical cavity being defined by a wavelength selective surface of a grooved grating mounted at a fixed Littrow configuration with respect to the beam, wherein an alignment plane lies perpendicular to grooves of the grating and a tuning axis of the grating is perpendicular to the alignment plane;

mounting an optical wedge within the optical cavity; and

rotating the wedge for aligning the semiconductor laser diode with respect to the surface of the grating by directing the beam to lie within the alignment plane.