US 6,552,800 B1Granted

Single-arm sagnac interferometer with two beam splitters

AssigneeJapan Science and Technology Corporation

InventorsOliver B. Wright, David H. Hurley, Osamu Matsuda

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Abstract

The invention provides an apparatus for measuring the physical properties of a sample by optically monitoring the response of the sample to illumination by ultrashort optical pulses. The apparatus is a common path optical interferometer of a Sagnac type that can measure physical properties at normal incidence, i.e., a single-arm Sagnac interferometer featuring two beam splitters. Measurement is performed in such a manner that a sample is excited by a beam of ultrashort optical pulses, and variations in intensity and phase of another optical beam are detected. This enables a wide range of measurement of physical properties such as thickness, sound velocity, and thermal properties of substances.

Claims37

Dependent claims

Apparatus for measuring physical properties of a sample, comprising:

(a) a coherent or partially coherent radiation source for providing a pulsed measurement beam having at least one ultrashort optical pulse having a duration of 0.002 ps to 2 ns;
(b) a radiation source for providing a pump beam having at least one ultrashort optical pulse having a duration of 0.002 ps to 2 ns;
(c) first and second beam splitters, at least a part of said measurement beam traversing said first beam splitter or being reflected from said first beam splitter, and then at least a part of said measurement beam, when incident on a second beam splitter, being split into a probe beam travelling in a single arm of said apparatus, wherein said probe beam is recombined with the remaining part of said measurement beam split from said second beam splitter and serving as a reference beam, to form a recombined beam, said reference beam having a different state of polarization from said probe beam, and the optical path length of the arm being chosen to be longer than the spatial extent, in the direction of propagation of the light, of the optical pulses in said reference and probe beams;
(d) means for directing said recombined beam onto said sample at normal incidence, such that said reference beam reflected from said sample travels in said single arm of said apparatus in the opposite direction to that previously mentioned for said probe beam when it travels through said single arm of said apparatus, and such that said reference beam and probe beam are recombined at said second beam splitter to produce a returning measurement beam propagating in the opposite direction to that of said measurement beam when said measurement beam is initially incident on said second beam splitter;
and such that said returning measurement beam is then reflected from said first beam splitter or traverses said first beam splitter so as to produce a first output beam containing said reference and probe beams, and such that said first output beam is not colinear with said measurement beam when said measurement beam is initially incident on said first beam splitter;
(e) means for directing said pump beam at said sample;
(f) means for varying the interval in arrival time at said sample between radiation pulses comprising said pump beam and corresponding radiation pulses comprising said reference beam;
(g) means for varying the interval in arrival time at said sample between corresponding radiation pulses comprising said probe and reference beams;
(h) means for ensuring that at least part of said reference and said probe beams making up said first output beam are resolved into the same polarization state to form a second output beam;
(i) means for introducing a known phase difference or known phase differences not equal to 0±180I degrees (I is an integer) between corresponding radiation pulses in said reference beam and said probe beam at a photodetector or photodetectors, such that the intensity of said second output beam is simultaneously sensitive to both phase and intensity changes of the light reflected from said sample;
and
(j) means for directing at least part of said second output beam to said photodetector or photodetectors to detect the intensity of said second output beam that is characteristic of the interference between said reference and said probe beams making up said second output beam, in order to produce a signal or signals representative of the changes induced in said sample by said pump beam, such that physical properties of the sample can be measured by relating them to this signal or signals.

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Dependencies are inferred from the claim text.

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Abstract

Apparatus for measuring physical properties of a sample, comprising:

(a) a coherent or partially coherent radiation source for providing a pulsed measurement beam having at least one ultrashort optical pulse having a duration of 0.002 ps to 2 ns;
(b) a radiation source for providing a pump beam having at least one ultrashort optical pulse having a duration of 0.002 ps to 2 ns;
(c) first and second beam splitters, at least a part of said measurement beam traversing said first beam splitter or being reflected from said first beam splitter, and then at least a part of said measurement beam, when incident on a second beam splitter, being split into a probe beam travelling in a single arm of said apparatus, wherein said probe beam is recombined with the remaining part of said measurement beam split from said second beam splitter and serving as a reference beam, to form a recombined beam, said reference beam having a different state of polarization from said probe beam, and the optical path length of the arm being chosen to be longer than the spatial extent, in the direction of propagation of the light, of the optical pulses in said reference and probe beams;
(d) means for directing said recombined beam onto said sample at normal incidence, such that said reference beam reflected from said sample travels in said single arm of said apparatus in the opposite direction to that previously mentioned for said probe beam when it travels through said single arm of said apparatus, and such that said reference beam and probe beam are recombined at said second beam splitter to produce a returning measurement beam propagating in the opposite direction to that of said measurement beam when said measurement beam is initially incident on said second beam splitter;
and such that said returning measurement beam is then reflected from said first beam splitter or traverses said first beam splitter so as to produce a first output beam containing said reference and probe beams, and such that said first output beam is not colinear with said measurement beam when said measurement beam is initially incident on said first beam splitter;
(e) means for directing said pump beam at said sample;
(f) means for varying the interval in arrival time at said sample between radiation pulses comprising said pump beam and corresponding radiation pulses comprising said reference beam;
(g) means for varying the interval in arrival time at said sample between corresponding radiation pulses comprising said probe and reference beams;
(h) means for ensuring that at least part of said reference and said probe beams making up said first output beam are resolved into the same polarization state to form a second output beam;
(i) means for introducing a known phase difference or known phase differences not equal to 0±180I degrees (I is an integer) between corresponding radiation pulses in said reference beam and said probe beam at a photodetector or photodetectors, such that the intensity of said second output beam is simultaneously sensitive to both phase and intensity changes of the light reflected from said sample;
and
(j) means for directing at least part of said second output beam to said photodetector or photodetectors to detect the intensity of said second output beam that is characteristic of the interference between said reference and said probe beams making up said second output beam, in order to produce a signal or signals representative of the changes induced in said sample by said pump beam, such that physical properties of the sample can be measured by relating them to this signal or signals.

9 more elements