OPTICAL SCANNING AND IMAGING SYSTEMS BASED ON DUAL PULSED LASER SYSTEMS

• US 20100225897A1
• Filed: 03/06/2009
• Published: 09/09/2010
• Est. Priority Date: 03/06/2009
• Status: Active Grant

• Current Assignee: IMRA America Incorporated (Aisin Corp.)
• Original Assignee: IMRA America Incorporated (Aisin Corp.)
• Inventors: Fermann, Martin E., Hartl, Ingmar
• Granted Patent: US 8,120,778 B2
• Time in Patent Office: Days
• US Class Current: 356/51
• CPC Class Codes: G01B 9/02001 characterised by controlling or generating intrinsic radiation properties
• CPC Class Codes: G01N 2021/317 Special constructive features
• 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: G02B 6/02304 Core having lower refractive index than cladding, e.g. air filled, hollow core
• CPC Class Codes: H01S 3/0675 Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/08054 Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation quarter-wave plates in a Q-switch laser H01S3/1124, H01S3/115
• CPC Class Codes: H01S 3/10046 Pulse repetition rate control H01S3/11 takes precedence
• CPC Class Codes: H01S 3/1067 using pressure or deformation
• CPC Class Codes: H01S 3/1068 using an acousto-optical device
• CPC Class Codes: H01S 3/1115 using intracavity saturable absorbers
• CPC Class Codes: H01S 3/13 Stabilisation of laser output parameters, e.g. frequency or amplitude
• CPC Class Codes: H01S 3/1307 Stabilisation of the phase
• CPC Class Codes: H01S 3/1392 by using a passive reference, e.g. absorption cell H01S3/1396, H01S3/1398 take precedence
• CPC Class Codes: H01S 3/1394 by using an active reference, e.g. second laser, klystron or other standard frequency source
• CPC Class Codes: H01S 3/2383 Parallel arrangements

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, including highly integrated configurations. In at least one embodiment a coherent dual scanning laser system (CDSL) includes two passively modelocked fiber oscillators. The oscillators are configured to operate at slightly different repetition rates, such that a difference δf_r in repetition rates is small compared to the values f_r1 and f_r2 of the repetition rates of the oscillators. The CDSL system also includes a non-linear frequency conversion section optically connected to each oscillator. The section includes a non-linear optical element generating a frequency converted spectral output having a spectral bandwidth and a frequency comb comprising harmonics of the oscillator repetition rates. A CDSL 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 dual scanning laser system (CDSL), comprising:

two passively modelocked fiber oscillators, said oscillators configured to operate at slightly different repetition rates, such that a difference δ

f_r in repetition rates is small compared to the values f_r1 and f_r2 of the repetition rates of said oscillators;

a non-linear frequency conversion section optically connected to each oscillator, said section comprising a non-linear optical element generating a frequency converted spectral output having a spectral bandwidth and a frequency comb comprising harmonics of said oscillator repetition rates.

2. A coherent dual scanning laser system according to claim 1, wherein said frequency conversion section comprises an output section that receives and combines multiple input frequencies and generates said spectral output at a difference frequency thereof, and said system comprises an intermediate non-linear frequency conversion section between at least one oscillator and said output section, said intermediate section generating a broadband spectrum having a bandwidth substantially greater than an oscillator spectrum.

3. A coherent dual scanning laser system according to claim 1, wherein said CDSL is arranged in a measurement system that utilizes spectral information, and said spectral output is utilized to probe a physical property of a test sample with spectral components within said spectral bandwidth.

4. A coherent dual scanning laser system according to claim 1, wherein said system is arranged in an imaging system for one or more of optical imaging, microscopy, micro-spectroscopy and THz imaging.

5. A coherent dual scanning laser system according to claim 4, wherein said measurement system comprises an element for optical scanning.

6. A coherent dual scanning laser system according to claim 1, further comprising:

a phase-locked loop to control the difference in repetition rates between said oscillators.

7. A coherent dual scanning laser system according to claim 1, further comprising an RF spectrum analyzer generating an output at RF frequencies related to said optical frequencies via a conversion factor (f_r1+f_r2)/2δ

f_r.

8. A coherent dual scanning laser system according to claim 1, wherein said mode locked fiber oscillators comprise a Nd, Yb, Tm or Er fiber oscillator.

9. A coherent dual scanning laser system according to claim 1 said system comprising at least one fiber amplifier for amplifying one or more oscillator outputs.

10. A coherent dual scanning laser system according to claim 1, said system comprising an integrated, all-fiber, dispersion compensator and non-linear frequency conversion section, said integrated section comprising one or more of a highly nonlinear fiber, a photonic crystal fiber, a dispersion compensating fiber and a fiber having a central air-hole.

11. A coherent dual scanning laser system according to claim 1, said system comprising a bulk optical element for dispersion compensation, including at least one of a grating pair, prism pair and grism pair, wherein said dispersion compensation comprises pulse compression.

12. A coherent dual scanning laser system according to claim 1, wherein said nonlinear frequency conversion section comprises a difference frequency generator.

13. A coherent dual scanning laser system according to claim 1, said system comprising a non-linear frequency section comprising a supercontinuum generator disposed downstream of at least one oscillator.

14. A coherent dual scanning laser system according to claim 1, wherein a mode locked fiber oscillator generates pulses at a repetition rate greater than about 250 MHz.

29. The coherent dual scanning system according to claim 1, wherein said non-linear frequency conversion section comprises a highly non-linear fiber.

30. The coherent dual scanning system according to claim 1,wherein a ratio of a repetition rate to the difference in repetition rates is at least about 10⁶, and sufficiently high to scale an RF frequency to an optical frequency.

15. A coherent dual scanning laser system comprising:

two passively modelocked fiber oscillators, said oscillators configured to operate at slightly different repetition rates, such that a difference δ

f_r in repetition rates is small compared to the values f_r1 and f_r2 of the repetition rates of the two oscillators;

a non-linear frequency conversion section optically connected to each oscillator, said section comprising a non-linear optical element generating a frequency converted spectral output having a spectral bandwidth and a frequency comb structure having a frequency separation equivalent to said oscillator repetition rates, said nonlinear frequency conversion section producing spectral output substantially broader than the spectral output from each oscillator.

16. A coherent dual scanning laser system according to claim 15, further comprising:

means for monitoring the difference in the carrier envelope offset frequencies of the two lasers, wherein information generated by said monitoring means provides a 1;

1 correspondence between RF frequencies and optical frequencies.

17. The coherent dual scanning laser system according to claim 16, wherein said correspondence is represented with a 1:

1 mapping of said RF frequencies to optical frequencies.

18. A coherent dual scanning laser system according to claim 15, further comprising a f−

2f interferometer for carrier envelope offset frequency control of each laser.

19. A coherent dual scanning laser system according to claim 15, further comprising a feedback system for stabilizing the difference in the carrier envelope offset frequencies of the two oscillators.

20. A coherent dual scanning laser system according to claim 19, wherein said carrier envelope offset frequencies information generated by said feedback system is used to generate a frequency grid in the RF domain that has a one to one correspondence to a frequency grid in the optical domain.

21. A coherent dual scanning laser system according to claim 19, said feedback system further comprising a single-frequency reference laser.

22. A coherent dual scanning laser system according to claim 15, further comprising two reference cavities for carrier envelope offset frequency control of each oscillator.

23. A coherent dual scanning laser system according to claim 15, further comprising one reference cavity for carrier envelope offset frequency control of each oscillator.

24. A coherent dual scanning laser system according to claim 15, further comprising two single-frequency reference lasers for carrier envelope offset frequency control of each oscillator.

27. A coherent dual scanning laser system according to claim 24, said arm length difference corresponding to approximately half the cavity round trip time of said oscillators.

25. A system for imaging in the THz spectral range comprising:

a coherent dual scanning laser system (CDSL) comprising two passively modelocked fiber oscillators, said modelocked oscillators configured to operate at slightly different repetition rates, such that a difference δ

fr in repetition rates is small compared to the values fr1 and fr2 of the repetition rates of said oscillators;

a material emitting THz radiation in response to an output of said CDSL;

a detector responsive to said THz radiation.

26. A coherent dual scanning laser system comprising:

two passively modelocked oscillators generating at least two trains of short optical pulses, said oscillators configured to operate at slightly different repetition rates, such that said difference in repetition rates δ

fr is small compared to the values fr1 and fr2 of the repetition rates of said oscillators;

a beam combiner for spatially combining said trains of short optical pulses to propagate along a common optical path downstream of said beam combiner;

a non-linear optical element for spectrally broadening said at least one train of said short optical pulses propagating along said common optical path;

a dual arm interferometer configured with different arm lengths so as to detect interference between said pulse trains when the pulses are not temporally overlapping in time prior to entering said interferometer.

28. A coherent dual scanning laser system comprising:

two passively modelocked fiber oscillators generating two separate trains of short optical pulses, said oscillators adjusted to operate at slightly different repetition rates, such that a difference δ

fr in repetition rates is small compared to the values fr1 and fr2 of the repetition rates of said oscillators;

a feedback system for stabilizing the difference in the carrier envelope offset frequencies of the two lasers, said feedback system comprising a single-frequency laser;

a beam combiner for spatially combining said trains of short optical pulses to propagate along a common optical path downstream of said beam combiner;

a non-linear optical element for spectrally broadening said at least one train of said short optical pulses propagating along said common optical path;

a dual arm interferometer configured with different arm lengths so as to detect interference between said pulse trains when the pulses are not temporally overlapping in time prior to entering said interferometer.