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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 10, Iss. 12 — Jun. 17, 2002
  • pp: 515–520
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Continuously tunable, precise, single frequency optical signal generator

John D. Jost, John L. Hall, and Jun Ye  »View Author Affiliations


Optics Express, Vol. 10, Issue 12, pp. 515-520 (2002)
http://dx.doi.org/10.1364/OE.10.000515


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Abstract

To realize a genuine CW optical frequency synthesizer, a continuously tunable single-frequency CW laser has been employed to track precisely any arbitrary component of a wide bandwidth phase-stabilized optical comb. We demonstrate experimentally two fundamental aspects of optical frequency synthesis, namely, precise setting of the laser frequency at an arbitrary pre-determined value, and continuous tuning of the laser frequency with the digital precision known in radio frequency synthesis. A typical computer-automated search-and-lock procedure finishes on one-minute time scale.

© 2002 Optical Society of America

1. Introduction

The wide-reaching field of control of coherent light is entering a qualitatively new era owing to the successful merger of ultrafast laser techniques with precision frequency metrology. Phase coherent optical frequency measurement [1

1. Th. Udem, J. Reichert, R. Holzwarth, and T.W. Hänsch, “Absolute optical frequency measurement of the cesium D-1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568 (1999). [CrossRef]

,2

2. S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102 (2000). [CrossRef] [PubMed]

], optical atomic clocks [3

3. S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 826 (2001). [CrossRef]

,4

4. J. Ye, L.-S. Ma, and J. L. Hall, “Molecular iodine clock,” Phys. Rev. Lett. 87, 270801 (2001). [CrossRef]

], and carrier-envelope phase stabilization [5

5. D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635, (2000). [CrossRef] [PubMed]

,6

6. A. Apolonski, A. Poppe, G. Tempea, C. Spielmann, Th. Udem, R. Holzwarth, T. W. Hänsch, and F. Krausz, “Controlling the phase evolution of few-cycle light pulses,” Phys. Rev. Lett. 85, 740 (2000). [CrossRef] [PubMed]

] have been demonstrated experimentally. Combined time/frequency active stabilization now allows one to tightly synchronize separate femtosecond (fs) lasers [7

7. L.-S. Ma, R. K. Shelton, H. C. Kapteyn, M. M. Murnane, and J. Ye, “Sub-10-femtosecond active synchronization between two passively mode-locked Ti:Sapphire oscillators,” Phys. Rev. A 64, Rapid Communications, 021802(R)(2001). [CrossRef]

], and phase lock their respective carrier waves to enable coherent optical pulse synthesis [8

8. R. K. Shelton, L.-S. Ma, H. C. Kapteyn, M. M. Murnane, J. L. Hall, and J. Ye, “Coherent optical pulse synthesis from two separate femtosecond lasers,” Science 293, 1286 (2001). [CrossRef] [PubMed]

]. A future goal would be to demonstrate in the time domain arbitrary pulse synthesis, with the capability of phase-coherent stitching of distinct optical bandwidths. Complementary to this time domain capability, it is desirable to construct an optical frequency synthesizer that would allow one to access in the frequency domain any optical spectral feature of interest with a well-defined single-frequency optical carrier wave. Such a capability would allow great simplification in precision laser spectroscopy [9

9. J. Castillega, D. Livingston, A. Sanders, and D. Shiner, “Precise measurement of the J=1 to J=2 fine structure interval in the 2P state of Helium,” Phys. Rev. Lett. 84, 4321 (2000). [CrossRef] [PubMed]

,10

10. A.A. Madej, L. Marmet, and J.E. Bernard, “Rb atomic absorption line reference for single Sr+ laser cooling systems,” Appl. Phys. B 67, 229 (1998). [CrossRef]

].

In this paper we demonstrate two fundamental aspects of an optical frequency synthesizer; namely the capabilities of continuous, precise frequency tuning and arbitrary frequency setting on demand. The wide bandwidth optical comb is based on a Kerr-lens mode-locked femtosecond (fs) laser with a repetition frequency of 100 MHz. The comb bandwidth is broadened to span an optical octave (520 to 1100 nm) via a microstructure fiber with the power of each comb component in the range of nW to a few tens of nW. This power level is more than sufficient to produce a beat signal against a mW-level optical field from a laser diode, with a typical signal-to-noise ratio (S/N) of about 40 dB in a 100 kHz detection bandwidth.

2. Experimental setup

Figure 1 depicts the experimental setup and the basic operation principle. fcw indicates the optical frequency of the cw laser that is under control by the fs comb. frep and fceo represent respectively the frequency spacing and the carrier-envelope frequency offset of the comb [12

12. H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, and U. Keller, “Carrier-envelope offset phase control: A novel concept for absolute optical frequency control and ultrashort pulse generation,” Appl. Phys. B 69, 327 (1999). [CrossRef]

]. They collectively define the absolute frequency of any comb components. The wavelength meter provides a coarse tuning guide for the diode laser, with a <100 MHz resolution capable of identifying the individual comb component that is closest to the diode laser. In practice, we find that within the frequency range of 300 to 400 MHz we achieve the best S/N in the heterodyne beat between the diode laser and a corresponding comb component. A combination of electronic bandpass and high-Q notch filters help to suppress the repetition and other beat signals outside the frequency range of 300 – 400 MHz. As there are two nearby comb lines, we have two beat signals left to deal with, their positions conjugate and movements exactly opposite to each other within the 300 – 400 MHz beat frequency range.

Fig. 1. Experimental schematic and the basic operation principle of a fs-comb-guided optical frequency synthesizer. Heterodyne beat between the cw laser and the fs comb is used for frequency control.

Figure 2 shows the beat signal between the cw laser diode (LD) and one of the comb components, under the condition of (a) LD free running and (b) LD frequency locked. We mix the optical beat down to a few MHz using a Voltage-Controlled Oscillator (VCO) phase-locked and tunable from 300 to 400 MHz. The processed beat signal is fed into a precision frequency-to-voltage (F/V) converter to generate a servo error signal for the LD. We use both diode current and a piezo-activated mirror in the LD external cavity as servo transducers. When stabilized, the beat signal linewidth (full-width-half-maximum) is 200 kHz, according to the Lorentzian fit shown in Fig. 2 (b). This beat linewidth is adequate for the present experiment, as each component of the fs comb, while stabilized by a single iodine-stabilized Nd:YAG laser to a fractional instability below 5 × 10-14 at 1 s [4

4. J. Ye, L.-S. Ma, and J. L. Hall, “Molecular iodine clock,” Phys. Rev. Lett. 87, 270801 (2001). [CrossRef]

], still possesses a fast linewidth of about 100 kHz at short time scales. For future experiments the fast linewidth of the comb can be further reduced using a cavity stabilization approach [13

13. R. J. Jones and J.-C. Diels, “Stabilization of femtosecond lasers for optical frequency metrology and direct optical to radio frequency synthesis,” Phys. Rev. Lett. 86, 3288 (2001). [CrossRef] [PubMed]

]. The feedback loop for the LD is computer-activated to lock the beat signal, and hence the LD frequency, to the VCO, which is itself phase locked to a direct-digital-synthesis (DDS) RF frequency source. This arrangement allows precision tuning of the LD as its beat frequency with a comb-line will be following the programmable DDS.

Of course complications arise when the beat signal is tuned near the harmonics of the repetition frequency, i.e., near 300 and 400 MHz. Furthermore, near 350 MHz, the two beat signals will cross their paths as one moves in the increasing and the other one in the decreasing frequency directions. The presence of both beat signals in the region of 345 – 355 MHz makes it hard to process the servo error properly and we therefore could call this region a dead zone. As a first solution, when the beat signal is tuned near this region, we apply a holding command to the feedback loop so that its control signal to the LD is frozen. We then sum in an independent signal with an appropriate amplitude step to guide the LD frequency to jump over this region. After the jump is completed, usually in a few hundred microseconds, the feedback loop is re-activated immediately. When the beat signal is tuned to near either end of the pre-selected frequency range (300 and 400 MHz), we program the DDS synthesizer and thus the VCO frequency to make a simultaneous jump corresponding to that in the feedforward drive to the LD frequency servo loop. For example, when the optical beat frequency is pulled to near 400 MHz by the VCO, we would apply a holding signal to the laser feedback loop. A quick switching signal is then applied, both to the VCO to make its frequency jump back near 300 MHz, and to the LD so that the original beat signal will move just beyond 400 MHz. Of course the periodic nature of the comb system leads to a new beat signal actually appearing again near the 300 MHz. We can then re-activate the laser feedback loop to stabilize the beat note on the VCO signal again. This process can be repeated as many times as desired, until the LD’s maximum tuning range is reached.

Fig. 2 (a) Heterodyne beat signal between the free-running cw laser and one of the comb components. (b) Beat signal after the cw laser is stabilized by the comb. Experimental data are in dots, and the associated Lorentzian fit is in solid line.

3. Demonstration of frequency synthesis

Fig. 3 Continuous tuning of the single frequency laser in precision steps guided by the phase stabilized optical comb. An independent optical cavity provides the frequency marks for reference.
Fig. 4 Random search of and stabilization to the targeted comb position by a single frequency cw laser, with an initial coarse guiding given by a wavelength meter (a), followed by a controlled frequency seeking (b). The entire searching procedure within a 0.5 nm spectral region finishes on one-minute time scale. Also shown is controlled fast switching of the laser diode frequency (c).

4. Summary and future outlook

The powerful utility of this new kind of frequency synthesizer will be helpful in precision atomic and molecular spectroscopy and for generation of flexible light source in different optical spectral regions. One interesting experiment that we are preparing to study is coherent Raman spectroscopy using phase coherent light sources separated by large wavelength gaps. For example, certain molecular fundamental vibration transitions may be hard to reach because suitable laser sources are not available. We consider using two lasers in the visible or near IR spectral regions that are both phase-locked to the comb. The frequency difference of the two lasers can be tuned to match with the fundamental vibration frequency while each laser could also be tuned close to higher-order overtone transitions for a resonant enhancement if so desired. True phase locking between the two lasers will provide the capability of high-resolution investigation of the fundamental vibration resonance.

Acknowledgments

References and links

1.

Th. Udem, J. Reichert, R. Holzwarth, and T.W. Hänsch, “Absolute optical frequency measurement of the cesium D-1 line with a mode-locked laser,” Phys. Rev. Lett. 82, 3568 (1999). [CrossRef]

2.

S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102 (2000). [CrossRef] [PubMed]

3.

S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 826 (2001). [CrossRef]

4.

J. Ye, L.-S. Ma, and J. L. Hall, “Molecular iodine clock,” Phys. Rev. Lett. 87, 270801 (2001). [CrossRef]

5.

D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635, (2000). [CrossRef] [PubMed]

6.

A. Apolonski, A. Poppe, G. Tempea, C. Spielmann, Th. Udem, R. Holzwarth, T. W. Hänsch, and F. Krausz, “Controlling the phase evolution of few-cycle light pulses,” Phys. Rev. Lett. 85, 740 (2000). [CrossRef] [PubMed]

7.

L.-S. Ma, R. K. Shelton, H. C. Kapteyn, M. M. Murnane, and J. Ye, “Sub-10-femtosecond active synchronization between two passively mode-locked Ti:Sapphire oscillators,” Phys. Rev. A 64, Rapid Communications, 021802(R)(2001). [CrossRef]

8.

R. K. Shelton, L.-S. Ma, H. C. Kapteyn, M. M. Murnane, J. L. Hall, and J. Ye, “Coherent optical pulse synthesis from two separate femtosecond lasers,” Science 293, 1286 (2001). [CrossRef] [PubMed]

9.

J. Castillega, D. Livingston, A. Sanders, and D. Shiner, “Precise measurement of the J=1 to J=2 fine structure interval in the 2P state of Helium,” Phys. Rev. Lett. 84, 4321 (2000). [CrossRef] [PubMed]

10.

A.A. Madej, L. Marmet, and J.E. Bernard, “Rb atomic absorption line reference for single Sr+ laser cooling systems,” Appl. Phys. B 67, 229 (1998). [CrossRef]

11.

J. Ye, J. L. Hall, and S. A. Diddams, “Precision phase control of ultrawide bandwidth fs laser - A network of ultrastable frequency marks across the visible spectrum,” Opt. Lett. 25, 1675 (2000). [CrossRef]

12.

H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, and U. Keller, “Carrier-envelope offset phase control: A novel concept for absolute optical frequency control and ultrashort pulse generation,” Appl. Phys. B 69, 327 (1999). [CrossRef]

13.

R. J. Jones and J.-C. Diels, “Stabilization of femtosecond lasers for optical frequency metrology and direct optical to radio frequency synthesis,” Phys. Rev. Lett. 86, 3288 (2001). [CrossRef] [PubMed]

OCIS Codes
(120.0120) Instrumentation, measurement, and metrology : Instrumentation, measurement, and metrology
(140.2020) Lasers and laser optics : Diode lasers
(140.3600) Lasers and laser optics : Lasers, tunable
(320.7090) Ultrafast optics : Ultrafast lasers

ToC Category:
Research Papers

History
Original Manuscript: May 29, 2002
Revised Manuscript: June 13, 2002
Published: June 17, 2002

Citation
John Jost, John Hall, and Jun Ye, "Continuously tunable, precise, single frequency optical signal generator," Opt. Express 10, 515-520 (2002)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-10-12-515


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References

  1. Th. Udem, J. Reichert, R. Holzwarth, T.W. Hänsch, �??Absolute optical frequency measurement of the cesium D-1 line with a mode-locked laser,�?? Phys. Rev. Lett. 82, 3568 (1999). [CrossRef]
  2. S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, Th. Udem, and T. W. Hänsch, �??Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,�?? Phys. Rev. Lett. 84, 5102 (2000). [CrossRef] [PubMed]
  3. S. A. Diddams, Th. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg,W.M. Itano,W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, �??An optical clock based on a single trapped 199Hg+ ion,�?? Science 293, 826 (2001). [CrossRef]
  4. J. Ye, L.-S. Ma and J. L. Hall, �??Molecular iodine clock,�?? Phys. Rev. Lett. 87, 270801 (2001). [CrossRef]
  5. D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, �??Carrierenvelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,�?? Science 288, 635, (2000). [CrossRef] [PubMed]
  6. A. Apolonski, A. Poppe, G. Tempea, C. Spielmann, Th. Udem, R. Holzwarth, T. W. Hänsch, and F. Krausz, �??Controlling the phase evolution of few-cycle light pulses,�?? Phys. Rev. Lett. 85, 740 (2000). [CrossRef] [PubMed]
  7. L.-S. Ma, R. K. Shelton, H. C. Kapteyn, M. M. Murnane, and J. Ye, �??Sub-10-femtosecond active synchronization between two passively mode-locked Ti:Sapphire oscillators,�?? Phys. Rev. A 64, Rapid Communications, 021802(R) (2001). [CrossRef]
  8. R. K. Shelton, L.-S. Ma, H. C. Kapteyn, M. M. Murnane, J. L. Hall, and J. Ye, �??Coherent optical pulse synthesis from two separate femtosecond lasers,�?? Science 293, 1286 (2001). [CrossRef] [PubMed]
  9. J. Castillega, D. Livingston, A. Sanders, and D. Shiner, �??Precise measurement of the J=1 to J=2 fine structure interval in the 2P state of Helium,�?? Phys. Rev. Lett. 84, 4321 (2000). [CrossRef] [PubMed]
  10. A.A. Madej, L. Marmet, and J.E. Bernard, �??Rb atomic absorption line reference for single Sr+ laser cooling systems,�?? Appl. Phys. B 67, 229 (1998). [CrossRef]
  11. J. Ye, J. L. Hall, and S. A. Diddams, �??Precision phase control of ultrawide bandwidth fs laser �?? A network of ultrastable frequency marks across the visible spectrum,�?? Opt. Lett. 25, 1675 (2000). [CrossRef]
  12. H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, and U. Keller, �??Carrier-envelope offset phase control: A novel concept for absolute optical frequency control and ultrashort pulse generation,�?? Appl. Phys. B 69, 327 (1999). [CrossRef]
  13. R. J. Jones, J.-C. Diels, �??Stabilization of femtosecond lasers for optical frequency metrology and direct optical to radio frequency synthesis,�?? Phys. Rev. Lett. 86, 3288 (2001). [CrossRef] [PubMed]

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