Measurement apparatus and measurement method

1. A measurement apparatus comprising:

a laser apparatus having a laser resonator including a frequency shifter and a gain medium and outputting a frequency modulated laser beam with a plurality of modes;

a branch that branches a part of the frequency modulated laser beam output by the laser apparatus as a reference light and at least a part of the remaining part of the frequency modulated laser beam as a measurement light;

a beat signal generator that generates a beat signal by mixing reflected by irradiating the measurement light to an object to be measured and the reference light; and

detector circuitry that detects a difference between propagation distances of the reference light and the measurement light using (i) a result obtained by frequency-analyzing first sampling data generated by oversampling the beat signal at a first frequency and (ii) a result obtained by frequency-analyzing second sampling data generated by undersampling the beat signal at a second frequency obtained by dividing a resonance frequency of the laser resonator by a positive integer, wherein

the first frequency being a frequency greater than the second frequency.

2. The measurement apparatus of claim 1 , wherein a quadrature of the beat signal generator detects the reflected light and the reference light.

3. The measurement apparatus according to claim 1 , wherein the detector circuitry divides a first frequency band being detectable using the first frequency into a plurality of bands of a second frequency bandwidth which is detected using the second frequency, and determines a band in which the beat signal was generated, from among the divided plurality of bands.

4. The measurement apparatus according to claim 1 , wherein the detector circuitry samples the beat signal using the first frequency and the second frequency at different sampling times so that a frequency resolution obtained by frequency analysis of the second sampling data is higher than a frequency resolution obtained by frequency analysis of the first sampling data.

5. The measurement apparatus according to claim 1 , wherein the detector circuitry performs frequency analysis using the second sampling data having a smaller number of pieces of data than the first sampling data.

6. The measurement apparatus according to claim 1 , wherein the laser apparatus includes an optical delay of a variable delay amount type in the laser resonator.

7. The measurement apparatus according to claim 6 , wherein the detector circuitry transmits a control signal for adjusting a delay amount to the optical delay when a line width of one line spectrum exceeds a threshold value in frequency analysis of the second sampling data.

8. A measurement method comprising:

outputting a frequency modulated laser beam with a plurality of modes from a laser apparatus that has a laser resonator including a frequency shifter and a gain medium;

branching a part of the frequency modulated laser beam as a reference light and at least some of the remaining part of the frequency modulated laser beam as a measurement light;

generating a beat signal by mixing the reflected light reflected by irradiating the measurement light to an object to be measured and the reference light;

generating first sampling data by oversampling the beat signal at a first frequency and frequency-analyzing the generated first sampling data;

generating second sampling data by undersampling the beat signal at a second frequency obtained by dividing a resonance frequency of the laser resonator by a positive integer and frequency-analyzing the generated second sampling data; and

detecting a difference between propagation distances of the reference light and the measurement light using a frequency analysis results of the first sampling data and a frequency analysis result of the second sampling data, wherein the first frequency being a frequency greater than the second frequency.

9. The measurement method according to claim 8 , further comprising

dividing a first frequency band being detectable by the first frequency into a plurality of bands of a second frequency bandwidth which is detected by the second frequency based on the first sampling data to determine a band, in which the beat signal was generated, from among the divided plurality of bands.

10. The measurement method according to claim 8 , wherein the laser apparatus includes an optical delay of a variable delay amount type in the laser resonator, and the method further comprises:

adjusting a delay amount of the optical delay so that the resonance frequency of the laser resonator and the second frequency have a predetermined relationship.

11. The measurement apparatus according to claim 1 , wherein the first frequency is a frequency equal to or more than twice the resonance frequency of the laser resonator.

12. The measurement apparatus according to claim 1 , wherein the detector observes a frequency of the beat signal by distinguishing a band in which the beat signal is generated using the result obtained by frequency-analyzing the first sampling data generated by oversampling the beat signal at the first frequency, and by generating second sampling data by undersampling the beat signal at a second frequency which can detect a bandwidth of the distinguished band and by performing frequency analysis on the generated second sampling data.