Abstract

We present a novel intracavity frequency modulation scheme in a tunable, picosecond optical parametric oscillator (OPO). The OPO signal wavelength can be modulated with a depth of more than 10 nm at a rate of 38 MHz (one half its repetition rate). We discuss the design and construction of the light source and its application to the recently-developed frequency modulation coherent anti-Stokes Raman scattering (FM-CARS) and stimulated Raman scattering (SRS) techniques. The new light source allows for real time subtraction of the interfering background signal in coherent Raman imaging, yielding images with purely chemical contrast.

© 2009 OSA

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References

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  1. G. C. Bjorklund, “Frequency-modulation spectroscopy: a new method for measuring weak absorptions and dispersions,” Opt. Lett. 5(1), 15 (1980).
    [CrossRef] [PubMed]
  2. B. Levine, C. Shank, and J. Heritage, “Surface vibrational spectroscopy using stimulated Raman scattering,” IEEE J. Quantum Electron. 15(12), 1418–1432 (1979).
    [CrossRef]
  3. M. D. Levenson, W. E. Moerner, and D. E. Horne, “FM spectroscopy detection of stimulated Raman gain,” Opt. Lett. 8(2), 108–110 (1983).
    [CrossRef] [PubMed]
  4. W. E. Moerner and L. Kador, “Optical detection and spectroscopy of single molecules in a solid,” Phys. Rev. Lett. 62(21), 2535–2538 (1989).
    [CrossRef] [PubMed]
  5. C. L. Evans and X. S. Xie, “Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine,” Annu. Rev. Anal. Chem. 1(1), 883–909 (2008).
    [CrossRef]
  6. C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
    [CrossRef] [PubMed]
  7. F. Ganikhanov, C. L. Evans, B. G. Saar, and X. S. Xie, “High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy,” Opt. Lett. 31(12), 1872–1874 (2006).
    [CrossRef] [PubMed]
  8. M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
    [CrossRef]
  9. D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
    [CrossRef]
  10. W. Demtröder, Laser Spectroscopy, 3rd ed. (Springer-Verlag, New York, 2003).
  11. C. L. Tang and J. M. Telle, “Laser modulation spectroscopy of solids,” J. Appl. Phys. 45(10), 4503–4505 (1974).
    [CrossRef]
  12. J. M. Telle and C. L. Tang, “New method for electro-optical tuning of tunable lasers,” Appl. Phys. Lett. 24(2), 85–87 (1974).
    [CrossRef]
  13. L. Cabaret, P. Camus, R. Leroux, and J. Philip, “Intracavity LiNbO(3) Fabry-Perot etalon for frequency stabilization and tuning of a single-mode quasi-continuous-wave titanium:sapphire ring laser,” Opt. Lett. 26(13), 983–985 (2001).
    [CrossRef]
  14. H. F. Gleeson, A. J. Murray, E. Fraser, and A. Zoro, “An electrically addressed liquid crystal filter for tunable lasers,” Opt. Commun. 212(1-3), 165–168 (2002).
    [CrossRef]
  15. A. Godard and E. Rosencher, “Energy yield of pulsed optical parametric oscillators: a rate-equation analysis,” IEEE J. Quantum Electron. 40(6), 784–790 (2004).
    [CrossRef]
  16. X. S. Xie, J. Yu, and W. Y. Yang, “Living cells as test tubes,” Science 312(5771), 228–230 (2006).
    [CrossRef] [PubMed]

2008 (2)

C. L. Evans and X. S. Xie, “Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine,” Annu. Rev. Anal. Chem. 1(1), 883–909 (2008).
[CrossRef]

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

2007 (1)

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

2006 (2)

2004 (1)

A. Godard and E. Rosencher, “Energy yield of pulsed optical parametric oscillators: a rate-equation analysis,” IEEE J. Quantum Electron. 40(6), 784–790 (2004).
[CrossRef]

2002 (2)

H. F. Gleeson, A. J. Murray, E. Fraser, and A. Zoro, “An electrically addressed liquid crystal filter for tunable lasers,” Opt. Commun. 212(1-3), 165–168 (2002).
[CrossRef]

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

2001 (1)

1989 (1)

W. E. Moerner and L. Kador, “Optical detection and spectroscopy of single molecules in a solid,” Phys. Rev. Lett. 62(21), 2535–2538 (1989).
[CrossRef] [PubMed]

1983 (1)

1980 (1)

1979 (1)

B. Levine, C. Shank, and J. Heritage, “Surface vibrational spectroscopy using stimulated Raman scattering,” IEEE J. Quantum Electron. 15(12), 1418–1432 (1979).
[CrossRef]

1974 (2)

C. L. Tang and J. M. Telle, “Laser modulation spectroscopy of solids,” J. Appl. Phys. 45(10), 4503–4505 (1974).
[CrossRef]

J. M. Telle and C. L. Tang, “New method for electro-optical tuning of tunable lasers,” Appl. Phys. Lett. 24(2), 85–87 (1974).
[CrossRef]

Bjorklund, G. C.

Burfeindt, B.

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Cabaret, L.

Camus, P.

Cheng, J.-X.

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Evans, C. L.

C. L. Evans and X. S. Xie, “Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine,” Annu. Rev. Anal. Chem. 1(1), 883–909 (2008).
[CrossRef]

F. Ganikhanov, C. L. Evans, B. G. Saar, and X. S. Xie, “High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy,” Opt. Lett. 31(12), 1872–1874 (2006).
[CrossRef] [PubMed]

Fischer, M. C.

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

Fraser, E.

H. F. Gleeson, A. J. Murray, E. Fraser, and A. Zoro, “An electrically addressed liquid crystal filter for tunable lasers,” Opt. Commun. 212(1-3), 165–168 (2002).
[CrossRef]

Freudiger, C. W.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

Ganikhanov, F.

Gleeson, H. F.

H. F. Gleeson, A. J. Murray, E. Fraser, and A. Zoro, “An electrically addressed liquid crystal filter for tunable lasers,” Opt. Commun. 212(1-3), 165–168 (2002).
[CrossRef]

Godard, A.

A. Godard and E. Rosencher, “Energy yield of pulsed optical parametric oscillators: a rate-equation analysis,” IEEE J. Quantum Electron. 40(6), 784–790 (2004).
[CrossRef]

He, C.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

Heritage, J.

B. Levine, C. Shank, and J. Heritage, “Surface vibrational spectroscopy using stimulated Raman scattering,” IEEE J. Quantum Electron. 15(12), 1418–1432 (1979).
[CrossRef]

Holtom, G. R.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

Horne, D. E.

Jones, D. J.

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Kador, L.

W. E. Moerner and L. Kador, “Optical detection and spectroscopy of single molecules in a solid,” Phys. Rev. Lett. 62(21), 2535–2538 (1989).
[CrossRef] [PubMed]

Kang, J. X.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

Leroux, R.

Levenson, M. D.

Levine, B.

B. Levine, C. Shank, and J. Heritage, “Surface vibrational spectroscopy using stimulated Raman scattering,” IEEE J. Quantum Electron. 15(12), 1418–1432 (1979).
[CrossRef]

Liu, H.

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

Lu, S.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

Min, W.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

Moerner, W. E.

W. E. Moerner and L. Kador, “Optical detection and spectroscopy of single molecules in a solid,” Phys. Rev. Lett. 62(21), 2535–2538 (1989).
[CrossRef] [PubMed]

M. D. Levenson, W. E. Moerner, and D. E. Horne, “FM spectroscopy detection of stimulated Raman gain,” Opt. Lett. 8(2), 108–110 (1983).
[CrossRef] [PubMed]

Murray, A. J.

H. F. Gleeson, A. J. Murray, E. Fraser, and A. Zoro, “An electrically addressed liquid crystal filter for tunable lasers,” Opt. Commun. 212(1-3), 165–168 (2002).
[CrossRef]

Pang, Y.

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Philip, J.

Piletic, I. R.

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

Potma, E. O.

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Rosencher, E.

A. Godard and E. Rosencher, “Energy yield of pulsed optical parametric oscillators: a rate-equation analysis,” IEEE J. Quantum Electron. 40(6), 784–790 (2004).
[CrossRef]

Saar, B. G.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

F. Ganikhanov, C. L. Evans, B. G. Saar, and X. S. Xie, “High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy,” Opt. Lett. 31(12), 1872–1874 (2006).
[CrossRef] [PubMed]

Shank, C.

B. Levine, C. Shank, and J. Heritage, “Surface vibrational spectroscopy using stimulated Raman scattering,” IEEE J. Quantum Electron. 15(12), 1418–1432 (1979).
[CrossRef]

Tang, C. L.

J. M. Telle and C. L. Tang, “New method for electro-optical tuning of tunable lasers,” Appl. Phys. Lett. 24(2), 85–87 (1974).
[CrossRef]

C. L. Tang and J. M. Telle, “Laser modulation spectroscopy of solids,” J. Appl. Phys. 45(10), 4503–4505 (1974).
[CrossRef]

Telle, J. M.

C. L. Tang and J. M. Telle, “Laser modulation spectroscopy of solids,” J. Appl. Phys. 45(10), 4503–4505 (1974).
[CrossRef]

J. M. Telle and C. L. Tang, “New method for electro-optical tuning of tunable lasers,” Appl. Phys. Lett. 24(2), 85–87 (1974).
[CrossRef]

Tsai, J. C.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

Warren, W. S.

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

Xie, X. S.

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

C. L. Evans and X. S. Xie, “Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine,” Annu. Rev. Anal. Chem. 1(1), 883–909 (2008).
[CrossRef]

X. S. Xie, J. Yu, and W. Y. Yang, “Living cells as test tubes,” Science 312(5771), 228–230 (2006).
[CrossRef] [PubMed]

F. Ganikhanov, C. L. Evans, B. G. Saar, and X. S. Xie, “High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy,” Opt. Lett. 31(12), 1872–1874 (2006).
[CrossRef] [PubMed]

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Yang, W. Y.

X. S. Xie, J. Yu, and W. Y. Yang, “Living cells as test tubes,” Science 312(5771), 228–230 (2006).
[CrossRef] [PubMed]

Yasuda, R.

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

Ye, J.

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Ye, T.

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

Yu, J.

X. S. Xie, J. Yu, and W. Y. Yang, “Living cells as test tubes,” Science 312(5771), 228–230 (2006).
[CrossRef] [PubMed]

Zoro, A.

H. F. Gleeson, A. J. Murray, E. Fraser, and A. Zoro, “An electrically addressed liquid crystal filter for tunable lasers,” Opt. Commun. 212(1-3), 165–168 (2002).
[CrossRef]

Annu. Rev. Anal. Chem. (1)

C. L. Evans and X. S. Xie, “Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine,” Annu. Rev. Anal. Chem. 1(1), 883–909 (2008).
[CrossRef]

Appl. Phys. Lett. (1)

J. M. Telle and C. L. Tang, “New method for electro-optical tuning of tunable lasers,” Appl. Phys. Lett. 24(2), 85–87 (1974).
[CrossRef]

IEEE J. Quantum Electron. (2)

B. Levine, C. Shank, and J. Heritage, “Surface vibrational spectroscopy using stimulated Raman scattering,” IEEE J. Quantum Electron. 15(12), 1418–1432 (1979).
[CrossRef]

A. Godard and E. Rosencher, “Energy yield of pulsed optical parametric oscillators: a rate-equation analysis,” IEEE J. Quantum Electron. 40(6), 784–790 (2004).
[CrossRef]

J. Appl. Phys. (1)

C. L. Tang and J. M. Telle, “Laser modulation spectroscopy of solids,” J. Appl. Phys. 45(10), 4503–4505 (1974).
[CrossRef]

Opt. Commun. (1)

H. F. Gleeson, A. J. Murray, E. Fraser, and A. Zoro, “An electrically addressed liquid crystal filter for tunable lasers,” Opt. Commun. 212(1-3), 165–168 (2002).
[CrossRef]

Opt. Lett. (4)

Phys. Rev. Lett. (1)

W. E. Moerner and L. Kador, “Optical detection and spectroscopy of single molecules in a solid,” Phys. Rev. Lett. 62(21), 2535–2538 (1989).
[CrossRef] [PubMed]

Proc. SPIE (1)

M. C. Fischer, H. Liu, I. R. Piletic, T. Ye, R. Yasuda, and W. S. Warren, “Self-phase modulation and two-photon absorption imaging of cells and active neurons,” Proc. SPIE 6442, 64421J (2007).
[CrossRef]

Rev. Sci. Instrum. (1)

D. J. Jones, E. O. Potma, J.-X. Cheng, B. Burfeindt, Y. Pang, J. Ye, and X. S. Xie, “Synchronization of two passively mode-locked, picosecond lasers within 20 fs for coherent anti-Stokes Raman scattering microscopy,” Rev. Sci. Instrum. 73(8), 2843–2848 (2002).
[CrossRef]

Science (2)

C. W. Freudiger, W. Min, B. G. Saar, S. Lu, G. R. Holtom, C. He, J. C. Tsai, J. X. Kang, and X. S. Xie, “Label-free biomedical imaging with high sensitivity by stimulated Raman scattering microscopy,” Science 322(5909), 1857–1861 (2008).
[CrossRef] [PubMed]

X. S. Xie, J. Yu, and W. Y. Yang, “Living cells as test tubes,” Science 312(5771), 228–230 (2006).
[CrossRef] [PubMed]

Other (1)

W. Demtröder, Laser Spectroscopy, 3rd ed. (Springer-Verlag, New York, 2003).

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Figures (5)

Fig. 1
Fig. 1

Operating principle of a normal and double cavity modulated OPO. (a) Operating principle of a representative synchronously-pumped OPO. The round trip time of a pulse in the OPO cavity is exactly matched to the period of the pump laser repetition rate (13 ns). Thus a pulse in the cavity makes exactly one round trip before meeting the next pump laser pulse in the nonlinear LBO crystal, leading to efficient wavelength conversion. Each time a pulse strikes the output coupler (OC), ~10-20% of the energy exits the cavity and is used for the experiment. (b) Operating principle of the intracavity modulated OPO. Initially (T = 0 ns), a pulse of wavelength λ1 and a pump laser pulse are in the LBO nonlinear crystal gain medium. At the same time, a pulse of λ2 is in the modulator crystal and the modulation drive waveform which is synchronized to the pulse train reaches its maximum positive value, causing a red shift. Because the OPO has a cavity round trip time equal to twice the period of the pump laser repetition rate, at one period of the pump laser repetition rate later (T = 13 ns), λ1 and λ2 have switched positions so that λ2 is in LBO crystal while λ1 is in the modulator crystal. At that time, the drive voltage reaches its maximum negative value, corresponding to a blue-shift. At T = 26 ns, the system is returned to the initial, T = 0 ns state and the cycle begins again. In this way, the output of the intracavity-modulated OPO consists of alternating pulses of two colors, λ1 and λ2, each at 38 MHz, for a total repetition rate (considering both colors) equal to the 76 MHz pump laser repetition rate.

Fig. 2
Fig. 2

(a) Schematic modulator transmission versus wavelength for three applied voltages, calculated using the 50λ plate at 800 nm and the 2-mm-thick modulator crystals. (b) Modulator layout, showing thin RTP crystals with perpendicular orientation, sapphire (Al2O3) plates to minimize thermal gradients, and the copper (Cu) ground and high voltage (HV) electrodes.

Fig. 3
Fig. 3

38 MHz wavelength switching (a) Optical spectrum of the single beam OPO signal wave, showing 11 nm splitting between the two wavelengths (b) Test setup for verification of the switching behavior. The two wavelengths which comprise the OPO signal beam are separated using a diffraction grating (DG) (600 lines/mm, Edmund Industrial Optics) and simultaneously detected by high speed photodiodes (DET210, Thorlabs). The photodiode output is detected by a high bandwidth oscilloscope (TDS3054b, Tektronix). (c) Top: drive waveform applied to the modulator in the OPO, which causes the 38 MHz switching behavior. Bottom: Simultaneous output of the two high bandwidth photodiodes, each detecting one of the two wavelengths from (a). The dashed lines indicate that the peak voltage of the drive waveform corresponds to λ2, while the trough corresponds to λ1.

Fig. 4
Fig. 4

CARS images using the OPO. (a) FM-CARS image of individual bacterial cells grown on deuterated carbon sources and water, imaged at the C-2H stretching frequency at 2100 cm−1. The off resonance pump wavelength was set at 2000 cm−1. (b) FM-CARS images of bacterial cells imaged under the same conditions but without the deuterated nutrient supply show no contrast. (c) A simultaneous bright field image acquired along with (b) demonstrates that cells are present but do not appear in FM-CARS unless they are deuterated. (d) Normal CARS image of a mammalian cell incubated with deuterated oleic acid, acquired at 2100 cm−1. Spurious background (e.g. from the surrounding media) is due to nonresonant four wave mixing. (e) Real-time background subtraction via FM-CARS leaves only the lipid droplets which have accumulated the deuterated fatty acids with visible contrast in the image. Scale bar: 10 μm.

Fig. 5
Fig. 5

(a) A frequency modulation SRG image of the surface of mouse skin shows the accumulation of the minoxidil-d10 on the surface, and is free of the nonresonant background that troubles CARS. In this case, no amplitude modulation was necessary to detect the signal. (b) Normal CARS image of the same area as (a). In normal CARS, the strong nonresonant background from the hair makes distinguishing chemical from structural features impossible, even on a qualitative level. Scale bar: 50 μm.

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