A laser system for imaging and materials analysis

Patent metadata

Field	Value
Publication number	US-2016202180-A1
Application number	US-201414913507-A
Country	US
Kind code	A1
Filing date	Aug 22, 2014
Priority date	Aug 22, 2013
Publication date	Jul 14, 2016
Grant date	—

Abstract

A THz quantum cascade laser is used to investigate a target by directing a first beam of laser radiation from the laser at the target to thereby produce a second beam of laser radiation by interaction of the first beam with the target. Self-mixing of the first and second beams occurs within the laser and causes variations in a signal such as the operating voltage of the laser. An operating parameter of the laser that affects the interaction of the first beam with, the target is varied. The operating voltage is monitored and processed to determine phase and amplitude changes associated with material properties of the target. Consequently in one embodiment the invention provides for processing the variations in the signal to produce various images of the target.

First claim

1 . A method for investigating a target comprising the steps of:
directing a first beam of radiation from a laser at the target to thereby produce a second beam of laser radiation by interaction of the first beam with the target wherein self-mixing of the first and second beams occurs within the laser; varying a parameter affecting the interaction of the first beam with the target; detecting a signal arising from the self-mixing; and processing the signal to thereby determine phase and amplitude changes associated with material properties of the target.

2 . A method according to claim 1 , wherein the laser is arranged to operate in the terahertz (THz) band.

3 . A method according to claim 1 , wherein the laser comprises a quantum cascade laser (QCL).

4 . A method according to claim 1 , wherein the step of detecting the signal involves monitoring an electrical signal across terminals of the laser.

5 . A method according to claim 1 , wherein the step of varying a parameter includes applying a modulation to a current for driving the laser.

6 . A method according to claim 5 , wherein the modulation comprises a continuous wave frequency modulation of the laser beam frequency.

7 . A method according to claim 1 , wherein the step of processing the signal includes detecting a phase shift of a waveform of the signal associated with a phase shift imparted by the interaction of the first beam with the target.

8 . A method according to claim 7 , wherein the step of processing the signal further includes detecting a change of the waveform of the signal associated with an attenuation imparted by the interaction of the first beam with the target.

9 . A method according to claim 1 , wherein the step of varying the parameter may comprise moving the target toward or away from a source of the first beam of laser radiation.

10 . A method according to claim 1 , including processing the signal to thereby determine phase and amplitude changes associated with material properties of the target to derive a refractive index (n) and an extinction coefficient (k) of the target.

11 . A method according to claim 10 , including causing the first beam of laser radiation to interact with a portion of the target having known properties.

12 . A method according to claim 11 , including applying known values of n and k of two materials from said portion of the target having known values to thereby derive n and k of a third material of the target, being a material under test.

13 . A method according to claim 12 including fitting a mathematical model of said laser self-mixing to data for each of a number of positions of the target to obtain a set of parameter values for each of the positions.

14 . A method according to claim 1 , including mechanically scanning the target by moving the target relative to the laser through a plurality of positions to thereby sense variations in the properties of the target as a function of location thereof.

15 . A method according to claim 14 , including processing the sensed variations in the properties of the target to produce an image of the target.

16 . A method according to claim 15 , including making measurements of variations in the signal at each of a number of positions during the mechanical scanning.

17 . A method according to claim 16 including removing effects due to power modulation of the laser from each of said measurements.

18 . A method according to claim 16 including processing only a central portion of each period of the signal at each scanning position to avoid the effect of transients at the edges of a modulation period of the laser.

19 . A method according to claim 16 including determining a reflection coefficient of the target at each position.

20 . A method according to claim 19 wherein the step of determining the reflection coefficient is based on an integral of an absolute value of the signal over time.

21 . A method according to claim 20 including producing an image from the target by fitting time domain traces of the signal to a mathematical model of said laser self-mixing to thereby calculate variations in a feedback parameter of the model wherein the image is generated by plotting the feedback parameter for each of a number of the positions.

22 . According to a further embodiment of the present invention there is provided a system for investigating a target comprising:
a laser; a target assembly arranged to return a beam from the laser to the laser after interaction with a target of said assembly; a data acquisition assembly responsive to electrical terminals of the laser; and a computational device responsive to the data acquisition assembly, wherein the computational device is programmed to determine phase and amplitude changes associated with the target and imparted on to the beam by interaction therewith.

23 . A system according to claim 22 wherein the laser is under control of the computational device for operation of the laser and variation of its operating parameters.

24 . A system according to claim 22 including a translation assembly arranged to impart a relative motion between the laser and the target.

25 . A system according to claim 24 , wherein the translation assembly includes one or more actuators under control of the computational device wherein the computational device is programmed to operate the translation assembly for data acquisition at each of a number of positions of the target.

26 . A computer software product comprising a machine readable media bearing tangible machine readable instructions for an electronic processor to:
operate a laser to direct a laser beam at a target assembly; acquire electrical data being a function of self-mixing of the laser beam with a reflection thereof subsequent to interaction with the target assembly; and determine phase and amplitude changing properties of a target portion of the target assembly on the basis of the acquired electrical data.

Assignees

• Univ Queensland
• Univ Leeds

Inventors

• Rakic Aleksandar D
• Dean Paul
• Davis Alexander Giles
• Indjin Dragan
• Linfield Edmund Harold
• Wilson Stephen James
• Taimre Thomas
• Bertling Karl
• Lim Yah Leng

Classifications

• G01N21/45 Primary

  using interferometric methods; using Schlieren methods · CPC title

• G01N2201/12

  Circuits of general importance; Signal processing · CPC title

• G01N2201/0612

  Laser diodes · CPC title

• G01N21/3581 Primary

  using far infrared light; using Terahertz radiation · CPC title

• G01J3/42

  Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry (beam switching arrangements G01J3/08) · CPC title