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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 17 — Aug. 15, 2011
  • pp: 16498–16507
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All-optical link for direct comparison of distant optical clocks

Miho Fujieda, Motohiro Kumagai, Shigeo Nagano, Atsushi Yamaguchi, Hidekazu Hachisu, and Tetsuya Ido  »View Author Affiliations


Optics Express, Vol. 19, Issue 17, pp. 16498-16507 (2011)
http://dx.doi.org/10.1364/OE.19.016498


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Abstract

We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of 2 × 10−15 at 1 second and 4 × 10−18 at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below 2 × 10−15 at 1 second and 7 × 10−17 at 1000 seconds. The system was available for the direct comparison of two distant 87Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.

© 2011 OSA

1. Introduction

2. Overall system

Fig. 1 Overall system of the all-optical link. The clock signal at the local site is frequency-converted into the 1.5 μm light and transferred to the remote site via an optical fiber link. The optical carrier transfer system compensates the phase noise so that the clock signal is faithfully transferred to the remote site. The transferred light is converted into a visible wavelength to be compared with another clock signal at the remote site. ECDL: external cavity diode laser, x2: PPLN frequency doubler.

3. Subsystems

3.1. Optical carrier transfer system

Fig. 2 Schematic diagram of the optical carrier transfer system. The system consists of local and remote parts. To evaluate the system performance, both parts are initially located at the same place. The beat signal between the output of the remote site and auxiliary reference is used for the evaluation. OC: optical circulator, AOM: acousto-optic modulator, VCO: voltage controlled oscillator, SG: signal generator, DDS: direct digital synthesizer, PD: photo detector, PLO: phase locked oscillator, EDFA: erbium doped fiber amplifier.

3.2. JGN2plus optical fiber link

Fig. 3 Schematic diagram of the optical fiber link in Tokyo (left) and the optical losses (right).
Fig. 4 Phase noises of the out-of-loop beat in the 90-km unstabilized (red) and stabilized (blue) links. The difference of 56 dB at 1 Hz agrees with the theoretical limit of phase noise suppression. Thus, the optical carrier system works properly.

Fig. 5 Phase noise of the 60-km unstabilized link between NICT and UT. The data were obtained at 1:00 and 15:00. The difference between the daytime and nighttime results is one or two orders of magnitude.

3.3. Performance of the optical carrier transfer system

Fig. 6 Frequency stabilities of the transferred signal in the unstabilized (red) and stabilized (blue & green) links. The curves in red and green colors were measured by a Π-type frequency counter. The blue curves were measured by a Λ-type frequency counter. The frequency stabilities shown in the dashed and solid curves were measured during the daytime (15:00–17:00) and around midnight (1:00–3:00), respectively.

3.4. Bridge between clock transition and telecom wavelength

3.4.1. Active polarization control and reliable measurement

The conversion efficiency of PPLN is quite sensitive to the SOP of the input light. Hence, active polarization control is essential for reliable measurements without interruption. Our automatic polarization control system based on a commercial polarization tracker is designed to distinguish the signal in the input light from false components caused by reflections from return light at connectors and splices. Figure 7 shows the schematic diagram of the polarization control system. The out-of-loop signal is coupled to the PPLN through the polarization tracker and the EDFA. The SOP variation due to the EDFA is also compensated in this system. The intensity of the correct component of the SHG light is identified as the RF amplitude of the specific beat frequency against the Ti:S frequency comb. The output SHG light is down-converted to the RF domain by the beat detection, where the beat note with the desired component is selectively obtained by a narrow band-pass filter. The intensity is detected with a microwave detector (a planar doped barrier detector). The commercial polarization tracker controls the SOP of the light by driving a fiber squeezer so that the deviation of the feedback signal is reduced. The tracking speed is 47π/s, and the typical SOP recovery time is 0.7 ms, which is fast enough to cancel out the SOP variation in the dedicated link. Figure 8 shows the behaviors of the intensity fluctuations of the beat signal and SHG light with and without the polarization control system in the 90-km optical fiber link. The results show that the system significantly reduced the intensity fluctuation due to the polarization variation and kept the intensity fluctuation of the beat signal within 2 dB for 16 hours. This novel system is thus capable of continuous operation without manual polarization adjustments. However, a small intensity fluctuation remained even though the SOP is effectively stabilized by our polarization control system. If an intensity fluctuating signal were sent to a frequency counter, it might induce incorrect readouts in the frequency measurement. To notice such a frequency miscount, the Ti:S frequency comb at the remote site is phase-locked to the SHG light, and the in-loop beat signal between the transmitted signal and the Ti:S frequency comb is counted to confirm a stable phase lock. Note that the Ti:S frequency comb was sometimes phase-unlocked due to the intensity variation of the SHG light before installation of the polarization control system. Thanks to our robust system, the stable intensity of the final beat signal between the Ti:S frequency comb and the clock laser at the remote site helps the frequency counter to read out infallible and reliable values.

Fig. 7 Schematic diagram of the polarization control system. The polarization variation is detected by the beat intensity between the SHG light and the relevant Ti:S comb component. The rotation is compensated at the polarization tracker (pol. tracker). PPLN: periodically poled lithium niobate, div: divider, BPF: band-pass filter, mw: microwave.
Fig. 8 Up: Intensity variation of the SHG light without the polarization control system. Down: Intensity variation of the beat signal with the polarization control system. Both results were obtained in the 90-km link.

3.4.2. Evaluation of instabilities induced by other components

Fig. 9 Frequency instabilities of direct clock comparison and system as measured by a Λ-type counter. The instability of optical clock comparison (f) is not limited by the overall instability (d) of the all-optical link.

4. Demonstration of direct comparison of two optical clocks

5. Conclusion

Acknowledgments

References and links

1.

G. Panfilo and E. Felicitas Arias, “Algorithms for international atomic time,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57(1), 140–150 (2010). [CrossRef]

2.

C. W. Chou, D. B. Hume, J. C. J. Koelemeiji, D. J. Wineland, and T. Rosenband, “Frequency comparison of two high-accuracy Al+ optical clocks,” Phys. Rev. Lett. 104, 070802 (2010). [CrossRef] [PubMed]

3.

S. M. Foreman, K. W. Holman, D. D. Hudson, D. J. Jones, and J. Ye, “Remote transfer of ultrastable frequency references via fiber networks,” Rev. Sci. Instrum. 78, 021101 (2007). [CrossRef] [PubMed]

4.

P. A. Williams, W. C. Swann, and N. R. Newbury, “High-stability transfer of an optical frequency over long fiber-optic links,” J. Opt. Soc. Am. B 25(8), 1284–1293 (2008). [CrossRef]

5.

M. Musha, F.-L. Hong, K. Nakagawa, and K. Ueda, “Coherent optical frequency transfer over 50-km physical distance using a 120-km-long installed telecom fiber network,” Opt. Express 16(21), 16459–16466 (2008). [CrossRef] [PubMed]

6.

L. S. Ma, P. Jungner, J. Ye, and J. L. Hall, “Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path,” Opt. Lett. 19(21), 1777–1779 (1994). [CrossRef] [PubMed]

7.

N. R. Newbury, P. A. Williams, and W. C. Swann, “Coherent transfer of an optical carrier over 251 km,” Opt. Lett. 32(21), 3056–3058 (2007). [CrossRef] [PubMed]

8.

F. Kefelian, O. Lopez, H. Jiang, C. Chardonnet, A. Amy-Klein, and G. Santarelli, “High-resolution optical frequency dissemination on a telecommunications network with data traffic,” Opt. Lett. 34(10), 1573–1575 (2009). [CrossRef] [PubMed]

9.

O. Lopez, A. Haboucha, F. Kefelian, H. Jiang, B. Chanteau, V. Roncin, C. Chardonnet, A. Amy-Klein, and G. Santarelli, “Cascaded multiplexed optical link on a telecommunication network for frequency dissemination,” Opt. Express 18(16), 16849–16857 (2010). [CrossRef] [PubMed]

10.

O. Terra, G. Grosche, and H. Schnatz, “Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber,” Opt. Express 18(15), 16102–16111 (2010). [CrossRef] [PubMed]

11.

A. D. Ludlow, T. Zelevinsky, G. K. Campbell, S. Blatt, M. M. Boyd, M. H. G. de Miranda, M. J. Martin, J. W. Thomsen, S. M. Foreman, J. Ye, T. M. Fortier, J. E. Stalnaker, S. A. Diddams, Y. Le Coq, Z. W. Barber, N. Poli, N. D. Lemke, K. M. Beck, and C. W. Oates, “Sr lattice clock at 1 × 10−16 fractional uncertainty by remote optical evaluation with a Ca clock,” Science 319, 1805–1808 (2008). [CrossRef] [PubMed]

12.

O. Terra, G. Grosche, K. Predehl, R. Holzwarth, T. Legero, U. Sterr, B. Lipphardt, and H. Schnatz, “Phase-coherent comparison of two optical frequency standards over 146 km using a telecommunication fiber link,” Appl. Phys. B 97, 541–551 (2009). [CrossRef]

13.

A. Pape, O. Terra, J. Friebe, M. Riedmann, T. Wubbena, E. M. Rasel, K. Predehl, T. Legero, B. Lipphardt, H. Schnatz, and G. Grosche, “Long-distance remote comparison of ultrastable optical frequencies with 10−15 instability in fractions of a second,” Opt. Express 18(20), 21477–21483 (2010). [CrossRef] [PubMed]

14.

Japan Gigabit Network 2 plus [Online]. Available: http://www.jgn.nict.go.jp/jgn2plus_archive/english/index.html.

15.

M. Kumagai, M. Fujieda, S. Nagano, and M. Hosokawa, “Stable radio frequency transfer in 114 km urban optical fiber link,” Opt. Lett. 34(19), 2949–2951 (2009). [CrossRef] [PubMed]

16.

M. Fujieda, M. Kumagai, and S. Nagano, “Coherent microwave transfer over a 204-km telecom fiber link by a cascaded system,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57(1), 168–174 (2010). [CrossRef]

17.

P. Lesage, “Characterization of frequency stability: bias due to the juxtapositon of time-interval measurements,” IEEE Trans. Instrum. Meas. 32(1), 204–207 (1983). [CrossRef]

18.

M. Takamoto, F. L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature 435, 321–324 (2005). [CrossRef] [PubMed]

19.

T. Akatsuka, M. Takamoto, and H. Katori, “Optical lattice clocks with non-interacting boson and fermions,” Nat. Phys. 4, 954–959 (2008). [CrossRef]

20.

M. Takamoto, T. Takano, and H. Katori, “Frequency comparison of optical lattice clocks beyond the Dick limit,” Nat. Photonics 5, 288–292 (2011). [CrossRef]

21.

A. Yamaguchi, M. Fujieda, M. Kumagai, H. Hachisu, S. Nagano, Y. Li, T. Ido, T. Takano, M. Takamoto, and H. Katori, “Direct comparison of distant optical lattice clocks at the 10−16 uncertainty,” Appl. Phys. Express 4, 082203 (2011). [CrossRef]

22.

S. Nagano, H. Ito, Y. Li, K. Matsubara, and M. Hosokawa, “Stable operation of femtosecond laser frequency combs with uncertainty at the 10−17 level toward optical frequency standards,” Jpn. J. Appl. Phys. 48, 042301 (2009). [CrossRef]

23.

Y. Y. Jiang, A. D. Ludlow, N. D. Lemke, R. W. Fox, J. A. Sherman, L. S. Ma, and C. W. Oates, “Making optical atomic clocks more stable with 10−16-level laser stabilization,” Nat. Photonics 5, 158–161 (2011). [CrossRef]

OCIS Codes
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(120.3940) Instrumentation, measurement, and metrology : Metrology
(120.4800) Instrumentation, measurement, and metrology : Optical standards and testing

ToC Category:
Instrumentation, Measurement, and Metrology

History
Original Manuscript: July 6, 2011
Revised Manuscript: August 2, 2011
Manuscript Accepted: August 5, 2011
Published: August 11, 2011

Citation
Miho Fujieda, Motohiro Kumagai, Shigeo Nagano, Atsushi Yamaguchi, Hidekazu Hachisu, and Tetsuya Ido, "All-optical link for direct comparison of distant optical clocks," Opt. Express 19, 16498-16507 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-17-16498


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References

  1. G. Panfilo and E. Felicitas Arias, “Algorithms for international atomic time,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57(1), 140–150 (2010). [CrossRef]
  2. C. W. Chou, D. B. Hume, J. C. J. Koelemeiji, D. J. Wineland, and T. Rosenband, “Frequency comparison of two high-accuracy Al+ optical clocks,” Phys. Rev. Lett. 104, 070802 (2010). [CrossRef] [PubMed]
  3. S. M. Foreman, K. W. Holman, D. D. Hudson, D. J. Jones, and J. Ye, “Remote transfer of ultrastable frequency references via fiber networks,” Rev. Sci. Instrum. 78, 021101 (2007). [CrossRef] [PubMed]
  4. P. A. Williams, W. C. Swann, and N. R. Newbury, “High-stability transfer of an optical frequency over long fiber-optic links,” J. Opt. Soc. Am. B 25(8), 1284–1293 (2008). [CrossRef]
  5. M. Musha, F.-L. Hong, K. Nakagawa, and K. Ueda, “Coherent optical frequency transfer over 50-km physical distance using a 120-km-long installed telecom fiber network,” Opt. Express 16(21), 16459–16466 (2008). [CrossRef] [PubMed]
  6. L. S. Ma, P. Jungner, J. Ye, and J. L. Hall, “Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path,” Opt. Lett. 19(21), 1777–1779 (1994). [CrossRef] [PubMed]
  7. N. R. Newbury, P. A. Williams, and W. C. Swann, “Coherent transfer of an optical carrier over 251 km,” Opt. Lett. 32(21), 3056–3058 (2007). [CrossRef] [PubMed]
  8. F. Kefelian, O. Lopez, H. Jiang, C. Chardonnet, A. Amy-Klein, and G. Santarelli, “High-resolution optical frequency dissemination on a telecommunications network with data traffic,” Opt. Lett. 34(10), 1573–1575 (2009). [CrossRef] [PubMed]
  9. O. Lopez, A. Haboucha, F. Kefelian, H. Jiang, B. Chanteau, V. Roncin, C. Chardonnet, A. Amy-Klein, and G. Santarelli, “Cascaded multiplexed optical link on a telecommunication network for frequency dissemination,” Opt. Express 18(16), 16849–16857 (2010). [CrossRef] [PubMed]
  10. O. Terra, G. Grosche, and H. Schnatz, “Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber,” Opt. Express 18(15), 16102–16111 (2010). [CrossRef] [PubMed]
  11. A. D. Ludlow, T. Zelevinsky, G. K. Campbell, S. Blatt, M. M. Boyd, M. H. G. de Miranda, M. J. Martin, J. W. Thomsen, S. M. Foreman, J. Ye, T. M. Fortier, J. E. Stalnaker, S. A. Diddams, Y. Le Coq, Z. W. Barber, N. Poli, N. D. Lemke, K. M. Beck, and C. W. Oates, “Sr lattice clock at 1 × 10−16 fractional uncertainty by remote optical evaluation with a Ca clock,” Science 319, 1805–1808 (2008). [CrossRef] [PubMed]
  12. O. Terra, G. Grosche, K. Predehl, R. Holzwarth, T. Legero, U. Sterr, B. Lipphardt, and H. Schnatz, “Phase-coherent comparison of two optical frequency standards over 146 km using a telecommunication fiber link,” Appl. Phys. B 97, 541–551 (2009). [CrossRef]
  13. A. Pape, O. Terra, J. Friebe, M. Riedmann, T. Wubbena, E. M. Rasel, K. Predehl, T. Legero, B. Lipphardt, H. Schnatz, and G. Grosche, “Long-distance remote comparison of ultrastable optical frequencies with 10−15 instability in fractions of a second,” Opt. Express 18(20), 21477–21483 (2010). [CrossRef] [PubMed]
  14. Japan Gigabit Network 2 plus [Online]. Available: http://www.jgn.nict.go.jp/jgn2plus_archive/english/index.html .
  15. M. Kumagai, M. Fujieda, S. Nagano, and M. Hosokawa, “Stable radio frequency transfer in 114 km urban optical fiber link,” Opt. Lett. 34(19), 2949–2951 (2009). [CrossRef] [PubMed]
  16. M. Fujieda, M. Kumagai, and S. Nagano, “Coherent microwave transfer over a 204-km telecom fiber link by a cascaded system,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57(1), 168–174 (2010). [CrossRef]
  17. P. Lesage, “Characterization of frequency stability: bias due to the juxtapositon of time-interval measurements,” IEEE Trans. Instrum. Meas. 32(1), 204–207 (1983). [CrossRef]
  18. M. Takamoto, F. L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature 435, 321–324 (2005). [CrossRef] [PubMed]
  19. T. Akatsuka, M. Takamoto, and H. Katori, “Optical lattice clocks with non-interacting boson and fermions,” Nat. Phys. 4, 954–959 (2008). [CrossRef]
  20. M. Takamoto, T. Takano, and H. Katori, “Frequency comparison of optical lattice clocks beyond the Dick limit,” Nat. Photonics 5, 288–292 (2011). [CrossRef]
  21. A. Yamaguchi, M. Fujieda, M. Kumagai, H. Hachisu, S. Nagano, Y. Li, T. Ido, T. Takano, M. Takamoto, and H. Katori, “Direct comparison of distant optical lattice clocks at the 10−16 uncertainty,” Appl. Phys. Express 4, 082203 (2011). [CrossRef]
  22. S. Nagano, H. Ito, Y. Li, K. Matsubara, and M. Hosokawa, “Stable operation of femtosecond laser frequency combs with uncertainty at the 10−17 level toward optical frequency standards,” Jpn. J. Appl. Phys. 48, 042301 (2009). [CrossRef]
  23. Y. Y. Jiang, A. D. Ludlow, N. D. Lemke, R. W. Fox, J. A. Sherman, L. S. Ma, and C. W. Oates, “Making optical atomic clocks more stable with 10−16-level laser stabilization,” Nat. Photonics 5, 158–161 (2011). [CrossRef]

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