Optical signal processing with modelocked lasers

• US 9,153,928 B2
• Filed: 09/30/2010
• Issued: 10/06/2015
• Est. Priority Date: 03/10/2006
• Status: Active Grant

• Current Assignee: IMRA America Incorporated (Aisin Corp.)
• Original Assignee: IMRA America Incorporated (Aisin Corp.)
• Inventors: Fermann, Martin, Hartl, Ingmar, Ruehl, Axel
• Primary Examiner(s): Geisel, Kara E
• Application Number:
  US12/895,127
• Publication Number:
  US 20110080580A1
• Time in Patent Office: 1,832 Days
• Field of Search: 356/450, 356/451, 356/484, 372/29.02
• US Class Current: 1/1
• CPC Class Codes: G01J 2003/102 Plural sources
• CPC Class Codes: G01J 3/10 Arrangements of light sources specially adapted for spectrometry or colorimetry
• CPC Class Codes: G01J 3/45 Interferometric spectrometry
• CPC Class Codes: G01N 2021/3595 using FTIR
• CPC Class Codes: G01N 21/31 Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry G01N21/72 takes precedence
• CPC Class Codes: G01N 21/3581 using far infrared light; using Terahertz radiation
• CPC Class Codes: G01N 21/4795 spatially resolved investigating of object in scattering medium in vivo A61B
• CPC Class Codes: G01N 21/636 using an arrangement of pump beam and probe beam; using the measurement of optical non-linear properties; non-linear optics per se G02F1/35
• CPC Class Codes: G01N 21/65 Raman scattering
• CPC Class Codes: H01S 3/067 Fibre lasers
• CPC Class Codes: H01S 3/105 by controlling the mutual position or the reflecting properties of the reflectors of the cavity , e.g. by controlling the cavity lengthH01S3/10076, H01S3/13 take precedence
• CPC Class Codes: H01S 3/1068 using an acousto-optical device
• CPC Class Codes: H01S 3/107 using electro-optic devices, e.g. exhibiting Pockels or Kerr effect H01S3/1061, H01S3/1063, H01S3/1065 take precedence
• CPC Class Codes: H01S 3/1106 Mode locking
• CPC Class Codes: H01S 3/1115 using intracavity saturable absorbers

Abstract

The invention relates to scanning pulsed laser systems for optical imaging. Coherent dual scanning laser systems (CDSL) are disclosed and some applications thereof. Various alternatives for implementation are illustrated. In at least one embodiment a coherent dual scanning laser system (CDSL) includes two passively modelocked fiber oscillators. In some embodiments an effective CDSL is constructed with only one laser. At least one embodiment includes a coherent scanning laser system (CSL) for generating pulse pairs with a time varying time delay. A CDSL, effective CDSL, or CSL may be arranged in an imaging system for one or more of optical imaging, microscopy, micro-spectroscopy and/or THz imaging.

Claims

1. A coherent scanning laser system for generating pulse pairs with a time varying time delay, said system comprising:

an optical source, said source comprising;

at least one mode locked oscillator, said source generating optical pulses at a time-varying repetition rate;

a repetition rate modulator configured to modulate said repetition rate at a modulation rate, wherein said repetition rate modulator is configured to modulate a cavity length of said mode locked oscillator;

an optical delay line arranged to receive optical pulses modulated at said modulation rate and to generate said pulse pairs, wherein a time delay between pulses of said pulse pair is dependent on said modulation rate; and

an optical reference comprising at least one bandpass filter, said optical reference configured for generating a reference signal for measurement of at least the time delay between the two pulses of said pulse pair as a function of time.

2. A coherent scanning laser system according to claim 1, wherein each said pulse pair comprises a direct pulse and a time delayed replica of said direct pulse, and said system comprises:

a first beam divider downstream of said mode locked oscillator, said beam divider configured to propagate the output of said mode locked oscillator along two separate optical paths having different propagation lengths, said delay line being inserted in one of said optical paths; and

a beam combiner configured to recombine said pulses propagating along said two optical paths and to form said pulse pair having said direct pulse and said time delayed replica thereof.

3. A coherent scanning laser system according to claim 2, wherein said beam divider and/or beam combiner comprise a fiber coupler, a bulk optical beam splitter, or a combination thereof.

4. A coherent scanning laser system according to claim 1, further comprising at least one optical element configured to provide for a measurement of a difference in carrier envelope phase between the two pulses of said pulse pair.

5. A coherent scanning laser system according to claim 1, said modulation rate being greater than about 10 Hz.

6. A coherent scanning laser system according to claim 2,

further comprising a stabilizer for active stabilization of said two optical paths having different propagation lengths.

7. A coherent scanning laser system according to claim 2,

further comprising a path length stabilizer, including a reference laser, said path length stabilizer configured to measure said different propagation lengths.

8. A coherent scanning laser system according to claim 1,

further comprising a spectral broadening stage down-stream of said source or said mode locked oscillator.

9. A coherent scanning laser system according to claim 2,

further comprising an element for equalization of the dispersion along said different propagation paths.

10. A coherent scanning laser system according to claim 1,

further comprising detectors to detect an interferogram between said pulse pairs.

11. A coherent scanning laser system according to claim 1,

further comprising means for generating sampling points at equidistant optical path length differences between the pulses comprising said pulse pair.

12. A coherent scanning laser system according to claim 1, said coherent scanning laser system configured as a portion of a system for Fourier transform spectroscopy.

13. A coherent scanning laser system according to claim 1, said modulation rate being greater than about 1 kHz.

14. A scanning laser system according to claim 11, said means for generating sampling points at equidistant optical path length differences further comprising detection of a reference interferogram and using the zero-crossing points of the reference interferogram for sampling of a second interferogram.

15. A coherent scanning laser system according to claim 2,

further comprising an element for amplification of the pulses along at least a portion of said two propagation paths.

16. A coherent scanning laser system according to claim 2, wherein said optical paths comprise one or both of a bulk optical component or a length of optical fiber.

17. A coherent scanning laser system according to claim 1, said coherent scanning laser system configured as a portion of a system for Fourier transform spectroscopy with a spectral resolution<

1 cm<sup>−

1</sup>.

18. A coherent scanning laser system according to claim 1, said coherent scanning laser system configured as a portion of a system for Fourier transform spectroscopy with a spectral resolution<

0.1 cm<sup>−

1</sup>.

19. A coherent scanning laser system according to claim 8, said spectral broadening stage comprising an optical parametric oscillator.

20. A coherent scanning laser system according to claim 19, said optical parametric oscillator further comprising a gas cell for trace gas detection.

21. A coherent scanning laser system according to claim 10,

further comprising a dual balanced detection arrangement to limit amplitude noise in said interferogram.

22. A coherent scanning laser system according to claim 10, further comprising a spectral broadening stage comprising an optical parametric oscillator.

23. A coherent scanning laser system according to claim 22, said optical parametric oscillator further comprising a gas cell for trace gas detection.

24. A coherent scanning laser system according to claim 2, wherein an output of said beam combiner is directed to a downstream sample for measurement of phase, absorption and/or emission spectra of said sample.

25. A coherent scanning laser system according to claim 1, wherein said optical bandpass filter comprises a grating disposed downstream from said optical source and said delay line.

26. A coherent scanning laser system according to claim 1, wherein said at least one bandpass filter comprises a fiber grating.

27. A coherent scanning laser system according to claim 1, wherein said pulse pair comprises an optical pulse and a time delayed replica of said optical pulse.

30. A coherent scanning laser system according to claim 9, wherein said element for equalization of dispersion comprises a pulse compressor.

31. A coherent scanning laser system according to claim 9, wherein said element for equalization of dispersion comprises a low dispersion optical fiber.

32. A coherent scanning laser system according to claim 9, wherein said element for equalization of dispersion comprises a bulk optic delay line.

33. A coherent scanning laser system according to claim 15, wherein said element for amplification of pulses comprises at least one fiber amplifier.

34. A coherent scanning laser system according to claim 33, wherein said beam divider and/or beam combiner comprises a fiber coupler, a bulk optical beam splitter, or a combination thereof.

28. A coherent scanning laser system for generating pulse pairs with a time varying time delay, said system comprising:

an optical source, said source comprising;

at least one mode locked oscillator, said source generating optical pulses at a time-varying repetition rate;

a repetition rate modulator configured to modulate said repetition rate at a modulation rate, wherein said repetition rate modulator is configured to modulate a cavity length of said mode locked oscillator;

an optical delay line arranged to receive optical pulses modulated at said modulation rate and to generate said pulse pairs, wherein a time delay between pulses of said pulse pair is dependent on said modulation rate; and

at least two optical references comprising at least two optical bandpass filters configured for generating reference signals for tracking the time delay and compensating for the carrier envelope offset frequency fluctuations between said pulse pairs as a function of time.

29. A coherent scanning laser system for generating pulse pairs with a time varying time delay according to claim 28, said two bandpass filters constructed from narrow bandpass fiber gratings.