Two-arm sagnac interferometer with two beam splitters

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, a part of said measurement beam being split into a probe beam and a reference beam of differing states of polarization, when incident on said first beam splitter, said probe beam travelling in an arm with a longer optical path between said first beam splitter and said second beam splitter, and said reference beam travelling in an arm with a shorter optical path between said first beam splitter and said second beam splitter, at which said probe beam and said reference beam are recombined, the difference in the optical path lengths of the two arms 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 said probe beams;
• (d) means for directing said recombined beam onto said sample at normal incidence, such that said probe beam reflected from said sample and said reference beam reflected from said sample traverse in different paths after passing through said second beam splitter in such a way that said probe beam travels in said arm with the shorter optical path an said reference beam travels in said arm with the longer optical path, and such that said probe beam and said reference beam are then recombined at said first beam splitter to produce a beam propagating in the opposite direction to that of said measurement beam when initially incident on said first beam splitter, and such that this 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 not collinear with said measurement beam when 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.