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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 6 — Mar. 14, 2011
  • pp: 5313–5318
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Low noise erbium fiber fs frequency comb based on a tapered-fiber carbon nanotube design

Tsung-Han Wu, K. Kieu, N. Peyghambarian, and R. J. Jones  »View Author Affiliations


Optics Express, Vol. 19, Issue 6, pp. 5313-5318 (2011)
http://dx.doi.org/10.1364/OE.19.005313


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Abstract

We report on a low noise all-fiber erbium fs frequency comb based on a simple and robust tapered-fiber carbon nanotube (tf-CNT) design. We mitigate dominant noise sources to show that the free-running linewidth of the carrier-envelope offset frequency (fceo) can be comparable to the best reported performance to date for fiber-based frequency combs. A free-running fceo linewidth of ~20 kHz is demonstrated, corresponding to an improvement of ~30 times over previous work based on a CNT mode-locked fiber laser [Opt. Express 18, 1667 (2010)]. We also demonstrate the use of an acousto-optic modulator external to the laser cavity to stabilize fceo, enabling a 300 kHz feedback control bandwidth. The offset frequency is phase-locked with an in-loop integrated phase noise of ~0.8 rad from 10Hz to 400kHz. We show a resolution-limited linewidth of ~1 Hz, demonstrating over 90% of the carrier power within the coherent fceo signal. The results demonstrate that the relatively simple tf-CNT fiber laser design can provide a compact, robust and high-performance fs frequency comb.

© 2011 OSA

1. Introduction

Passively mode-locked fiber lasers utilizing non-linear polarization evolution (NPE) or incorporating a fast semiconductor saturable absorber mirror (SESAM) have demonstrated free running carrier-envelope offset frequency (fceo) linewidths of ~10 kHz [1

1. Y. Nakajima, H. Inaba, K. Hosaka, K. Minoshima, A. Onae, M. Yasuda, T. Kohno, S. Kawato, T. Kobayashi, T. Katsuyama, and F. L. Hong, “A multi-branch, fiber-based frequency comb with millihertz-level relative linewidths using an intra-cavity electro-optic modulator,” Opt. Express 18(2), 1667–1676 (2010). [CrossRef] [PubMed]

,2

2. T. R. Schibli, I. Hartl, D. C. Yost, M. J. Martin, A. Marcinkevicius, M. E. Fermann, and J. Ye, “Optical frequency comb with submillihertz linewidth and more than 10 W average power,” Nat. Photonics 2(6), 355–359 (2008). [CrossRef]

]. With active stabilization, the phase of fceo can be controlled with <1 rad rms error in these systems, enabling sub-Hz fceo linewidths with over 90% of the power in the optical carrier. Such systems represent current state-of-the-art performance in fiber based frequency combs. Fiber lasers based on these designs often require free-space sections in the oscillator and/or incorporate many polarization control components, making them less amenable to simple, inexpensive and robust all-fiber oscillator designs. Similar performance levels in a simplified all fiber design would be advantageous as the use and applications for the frequency comb continue to expand. A recent paper demonstrated that fs fiber lasers utilizing carbon nanotubes (CNTs) as a fast saturable absorber (SA) for mode-locking offer a convenient system capable of generating phase-stabilized frequency combs [3

3. J. K. Lim, K. Knabe, K. A. Tillman, W. Neely, Y. S. Wang, R. Amezcua-Correa, F. Couny, P. S. Light, F. Benabid, J. C. Knight, K. L. Corwin, J. W. Nicholson, and B. R. Washburn, “A phase-stabilized carbon nanotube fiber laser frequency comb,” Opt. Express 17(16), 14115–14120 (2009). [CrossRef] [PubMed]

]. In that work, the free running fceo linewidth of ~600kHz indicates a large amount of noise intrinsic to the fiber laser system. With active stabilization, this noise can always be suppressed, but only by a finite amount ultimately determined by the feedback-loop bandwidth. In this paper, we investigate the performance of a fiber frequency comb based on a tapered-fiber CNT design (tf-CNT) [4

4. K. Kieu and M. Mansuripur, “Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite,” Opt. Lett. 32(15), 2242–2244 (2007). [CrossRef] [PubMed]

]. We identify and minimize noise sources to show that these systems can operate with free-running fceo linewidths of only ~20 kHz with sub-radian phase control of fceo possible using active stabilization. An intra-cavity piezo-electric transducer with a bandwidth up to 20 kHz was also available as a fiber stretcher to control the laser cavity length for locking the laser repetition rate. In this paper, we focus our study solely on the noise sources and stabilization of fceo to show that the tf-CNT frequency comb offers low noise performance in a simple, compact, and robust design.

2. Experimental setup

3. Characterization and stabilization of fceo

The original laser used to obtain the data shown in Fig. 2(a) had a FWHM spectral bandwidth of ~12-15 nm. By minimizing the pump laser RIN, we were able to observe free-running fceo linewidths <100 kHz (FWHM) similar to that shown by the black solid curve in Fig. 2(b). Further reduction of the fceo linewidth for this laser was not possible with isolation from external perturbations alone. The laser cavity was modified to produce a broader spectral bandwidth by reducing the net cavity group-delay dispersion (GDD) and utilizing a SA with increased modulation depth. A typical spectrum of ~30 nm FWHM is shown in the inset of Fig. 1. Only recently has similar broad bandwidth performance been obtained with CNT based mode-locked fiber lasers utilizing stretched-pulse dispersion compensation designs [16

16. K. Kieu and F. W. Wise, “Self-similar and stretched-pulse operation of erbium-doped fiber lasers with carbon nanotubes saturable absorber,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CML3.

,17

17. Z. P. Sun, T. Hasan, F. Q. Wang, A. G. Rozhin, I. H. White, and A. C. Ferrari, “Ultrafast stretched-pulse fiber laser mode-locked by carbon nanotubes,” Nano Res. 3(6), 404–411 (2010). [CrossRef]

]. These changes to the fundamental properties of the laser resulted in a greatly improved free-running fceo linewidth, particularly in the wings as shown by the dotted (red) curve in Fig. 2(b). It is known that quantum limitations to the timing jitter and phase noise can sensitively depend on parameters such as the intra-cavity pulse evolution, spectral bandwidth and cavity dispersion [12

12. H. A. Haus and A. Mecozzi, “Noise of mode-locked lasers,” IEEE J. Quantum Electron. 29(3), 983–996 (1993). [CrossRef]

,13

13. R. Paschotta, “Timing jitter and phase noiseof mode-locked fiber lasers,” Opt. Express 18(5), 5041–5054 (2010). [CrossRef] [PubMed]

,18

18. M. Y. Sander, E. P. Ippen, and F. X. Kärtner, “Carrier-envelope phase dynamics of octave-spanning dispersion-managed Ti: sapphire lasers,” Opt. Express 18(5), 4948–4960 (2010). [CrossRef] [PubMed]

,19

19. C. Ouyang, P. Shum, H. Wang, J. H. Wong, K. Wu, S. Fu, R. Li, E. J. R. Kelleher, A. I. Chernov, and E. D. Obraztsova, “Observation of timing jitter reduction induced by spectral filtering in a fiber laser mode locked with a carbon nanotube-based saturable absorber,” Opt. Lett. 35(14), 2320–2322 (2010). [CrossRef] [PubMed]

]. A more systematic investigation on the dependence of timing and optical phase noise in this laser system for different oscillator configurations will be the focus of future work.

The slowly drifting fceo beatnote typically exhibits a ≈20 kHz linewidth. A >35dB signal-to-noise ratio (SNR) is observed with a RBW of 100 kHz (see Fig. 4(a)
Fig. 4 (a) Measurement of in-loop spectral density of phase fluctuations with an integrated value of 0.8 rad. Inset shows in-loop error signal of residual phase fluctuations versus time. (b) Allan deviation measurement of fceo indicating frequency fluctuations relative to the local oscillator. The 1/τ dependence is expected when comparing 2 phase coherent signals. Also shown is the measurement noise floor of the detection system (open squares). Dashed line is a guide for the eye showing the 1/τ dependence.
). Instabilities or fluctuations in the background noise floor due to Q-switching or noise in the continuum generation process were not present. The offset frequency was phase locked to a RF local oscillator (LO) using a division of 10 to enable more robust locking. The fceo could be directly stabilized to the LO by controlling the pump laser power through the current controller, in combination with a feed-forward circuit to provide an increased feedback loop bandwidth of 100 kHz. However, using the AOM to control fceo in parallel extended the loop bandwidth to 300 kHz. The phase locked fceo is shown in Fig. 3(b)
Fig. 3 (a) Free-running linewidth of fceo measured with an electronic spectrum analyzer and 300 Hz video resolution bandwidth. Larger signal at ~42 MHz corresponds to the fundamental repetition rate of the laser. (b) Stabilized carrier envelope offset frequency recorded with 1kHz video resolution bandwidth. Inset shows resolution bandwidth limited 1 Hz linewidth. RBW: resolution bandwidth.
. The servo bumps at 300 kHz from the center frequency are evident in the figure. The inset shows an instrument resolution limited linewidth of ~1 Hz, indicating the offset frequency is phase coherent with the LO for long observations times. The total integrated power within the coherent spike is calculated to be ~94%.

The phase noise power spectrum becomes a more meaningful measure of the noise present in fceo once the measured linewidth is reduced below the resolution of the spectrum analyzer. The measured in-loop phase noise of the stabilized fceo is shown in Fig. 4(a), with the phase fluctuations from the error signal for a 10 s time interval shown in the inset. The in-loop integrated phase noise from 10 Hz to 400 kHz is 0.8 rad. The long-term phase stability of the laser is demonstrated in the Allan deviation measurement shown in Fig. 4(b) for up to 100 s averaging timescales. It is important to note that the frequency counter used for this measurement was phase locked to the LO to which fceo was locked. The noise floor was measured by directly connecting the LO (8.6 MHz) to the counter and measuring the fractional frequency instability. The 1/τ dependence is as expected when comparing two phase coherent sources. Measurement of the fractional frequency instability of fceo (measured at 86 MHz) gives nearly identical results, indicating that there is no measurable excess frequency instability introduced when fceo is phase locked to the LO and verifying that the signals remain phase coherent for these time scales. The right hand side of the graph shows the measured frequency fluctuations for fceo. The laser was able to maintain phase lock for time scales up to several hours, primarily limited by the dynamic range of the feedback control system.

4. Summary

In summary, we have shown that the tf-CNT fs fiber laser can be utilized to generate a low noise, high performance frequency comb in a simple all fiber oscillator design. We investigated sources of noise and optimized the system to provide a stable and narrow free running offset frequency linewidth (~20 kHz), amenable to tight phase locking with active stabilization. The integrated and low cost design will provide an attractive alternative for fiber based fs frequency combs. Compared to other fiber laser designs based on CNT’s, the tapered fiber design may enable higher powers due to the increased damaged threshold. This can aid in future designs generating higher power and higher repetition rate all-fiber frequency combs.

Acknowledgements

We thank Pavel Polynkin for the loan of the AOM used in this work. Support for this work was provided in part from Defense Advanced Research Projects Agency (DARPA) under grant N66001-09-1-2109 and National Science Foundation ERC Center for Integrated Access Networks (CIAN).

References and links

1.

Y. Nakajima, H. Inaba, K. Hosaka, K. Minoshima, A. Onae, M. Yasuda, T. Kohno, S. Kawato, T. Kobayashi, T. Katsuyama, and F. L. Hong, “A multi-branch, fiber-based frequency comb with millihertz-level relative linewidths using an intra-cavity electro-optic modulator,” Opt. Express 18(2), 1667–1676 (2010). [CrossRef] [PubMed]

2.

T. R. Schibli, I. Hartl, D. C. Yost, M. J. Martin, A. Marcinkevicius, M. E. Fermann, and J. Ye, “Optical frequency comb with submillihertz linewidth and more than 10 W average power,” Nat. Photonics 2(6), 355–359 (2008). [CrossRef]

3.

J. K. Lim, K. Knabe, K. A. Tillman, W. Neely, Y. S. Wang, R. Amezcua-Correa, F. Couny, P. S. Light, F. Benabid, J. C. Knight, K. L. Corwin, J. W. Nicholson, and B. R. Washburn, “A phase-stabilized carbon nanotube fiber laser frequency comb,” Opt. Express 17(16), 14115–14120 (2009). [CrossRef] [PubMed]

4.

K. Kieu and M. Mansuripur, “Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite,” Opt. Lett. 32(15), 2242–2244 (2007). [CrossRef] [PubMed]

5.

J. J. McFerran, W. C. Swann, B. R. Washburn, and N. R. Newbury, “Elimination of pump-induced frequency jitter on fiber-laser frequency combs,” Opt. Lett. 31(13), 1997–1999 (2006). [CrossRef] [PubMed]

6.

J. J. McFerran, W. C. Swann, B. R. Washburn, and N. R. Newbury, “Suppression of pump-induced frequency noise in fiber-laser frequency combs leading to sub-radian fceo phase excursions,” Appl. Phys. B 86(2), 219–227 (2007). [CrossRef]

7.

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(15), 3288–3291 (2001). [CrossRef] [PubMed]

8.

S. Koke, C. Grebing, H. Frei, A. Anderson, A. Assion, and G. Steinmeyer, “Direct frequency comb synthesis with arbitrary offset and shot-noise-limited phase noise,” Nat. Photonics 4(7), 462–465 (2010). [CrossRef]

9.

K. Kieu, R. J. Jones, and N. Peyghambarian, “Generation of few-cycle pulses from an amplified carbon nanotube mode-locked fiber laser system,” IEEE Photon. Technol. Lett. 22(20), 1521–1523 (2010). [CrossRef]

10.

I. Hartl, G. Imeshev, M. Fermann, C. Langrock, and M. Fejer, “Integrated self-referenced frequency-comb laser based on a combination of fiber and waveguide technology,” Opt. Express 13(17), 6490–6496 (2005). [CrossRef] [PubMed]

11.

K. Kieu, R. J. Jones, and N. Peyghambarian, “High power femtosecond source near 1 micron based on an all-fiber Er-doped mode-locked laser,” Opt. Express 18(20), 21350–21355 (2010). [CrossRef] [PubMed]

12.

H. A. Haus and A. Mecozzi, “Noise of mode-locked lasers,” IEEE J. Quantum Electron. 29(3), 983–996 (1993). [CrossRef]

13.

R. Paschotta, “Timing jitter and phase noiseof mode-locked fiber lasers,” Opt. Express 18(5), 5041–5054 (2010). [CrossRef] [PubMed]

14.

K. W. Holman, R. J. Jones, A. Marian, S. T. Cundiff, and J. Ye, “Detailed studies and control of intensity-related dynamics of femtosecond frequency combs from mode-locked Ti: sapphire lasers,” IEEE J. Sel. Top. Quantum Electron. 9(4), 1018–1024 (2003). [CrossRef]

15.

S. Witte, R. T. Zinkstok, W. Hogervorst, and K. S. E. Eikema, “Control and precise measurement of carrier-envelope phase dynamics,” Appl. Phys. B 78(1), 5–12 (2004). [CrossRef]

16.

K. Kieu and F. W. Wise, “Self-similar and stretched-pulse operation of erbium-doped fiber lasers with carbon nanotubes saturable absorber,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CML3.

17.

Z. P. Sun, T. Hasan, F. Q. Wang, A. G. Rozhin, I. H. White, and A. C. Ferrari, “Ultrafast stretched-pulse fiber laser mode-locked by carbon nanotubes,” Nano Res. 3(6), 404–411 (2010). [CrossRef]

18.

M. Y. Sander, E. P. Ippen, and F. X. Kärtner, “Carrier-envelope phase dynamics of octave-spanning dispersion-managed Ti: sapphire lasers,” Opt. Express 18(5), 4948–4960 (2010). [CrossRef] [PubMed]

19.

C. Ouyang, P. Shum, H. Wang, J. H. Wong, K. Wu, S. Fu, R. Li, E. J. R. Kelleher, A. I. Chernov, and E. D. Obraztsova, “Observation of timing jitter reduction induced by spectral filtering in a fiber laser mode locked with a carbon nanotube-based saturable absorber,” Opt. Lett. 35(14), 2320–2322 (2010). [CrossRef] [PubMed]

OCIS Codes
(320.7090) Ultrafast optics : Ultrafast lasers
(320.7160) Ultrafast optics : Ultrafast technology

ToC Category:
Ultrafast Optics

History
Original Manuscript: January 5, 2011
Revised Manuscript: February 10, 2011
Manuscript Accepted: February 21, 2011
Published: March 7, 2011

Citation
Tsung-Han Wu, K. Kieu, N. Peyghambarian, and R. J. Jones, "Low noise erbium fiber fs frequency comb based on a tapered-fiber carbon nanotube design," Opt. Express 19, 5313-5318 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-6-5313


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References

  1. Y. Nakajima, H. Inaba, K. Hosaka, K. Minoshima, A. Onae, M. Yasuda, T. Kohno, S. Kawato, T. Kobayashi, T. Katsuyama, and F. L. Hong, “A multi-branch, fiber-based frequency comb with millihertz-level relative linewidths using an intra-cavity electro-optic modulator,” Opt. Express 18(2), 1667–1676 (2010). [CrossRef] [PubMed]
  2. T. R. Schibli, I. Hartl, D. C. Yost, M. J. Martin, A. Marcinkevicius, M. E. Fermann, and J. Ye, “Optical frequency comb with submillihertz linewidth and more than 10 W average power,” Nat. Photonics 2(6), 355–359 (2008). [CrossRef]
  3. J. K. Lim, K. Knabe, K. A. Tillman, W. Neely, Y. S. Wang, R. Amezcua-Correa, F. Couny, P. S. Light, F. Benabid, J. C. Knight, K. L. Corwin, J. W. Nicholson, and B. R. Washburn, “A phase-stabilized carbon nanotube fiber laser frequency comb,” Opt. Express 17(16), 14115–14120 (2009). [CrossRef] [PubMed]
  4. K. Kieu and M. Mansuripur, “Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite,” Opt. Lett. 32(15), 2242–2244 (2007). [CrossRef] [PubMed]
  5. J. J. McFerran, W. C. Swann, B. R. Washburn, and N. R. Newbury, “Elimination of pump-induced frequency jitter on fiber-laser frequency combs,” Opt. Lett. 31(13), 1997–1999 (2006). [CrossRef] [PubMed]
  6. J. J. McFerran, W. C. Swann, B. R. Washburn, and N. R. Newbury, “Suppression of pump-induced frequency noise in fiber-laser frequency combs leading to sub-radian fceo phase excursions,” Appl. Phys. B 86(2), 219–227 (2007). [CrossRef]
  7. 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(15), 3288–3291 (2001). [CrossRef] [PubMed]
  8. S. Koke, C. Grebing, H. Frei, A. Anderson, A. Assion, and G. Steinmeyer, “Direct frequency comb synthesis with arbitrary offset and shot-noise-limited phase noise,” Nat. Photonics 4(7), 462–465 (2010). [CrossRef]
  9. K. Kieu, R. J. Jones, and N. Peyghambarian, “Generation of few-cycle pulses from an amplified carbon nanotube mode-locked fiber laser system,” IEEE Photon. Technol. Lett. 22(20), 1521–1523 (2010). [CrossRef]
  10. I. Hartl, G. Imeshev, M. Fermann, C. Langrock, and M. Fejer, “Integrated self-referenced frequency-comb laser based on a combination of fiber and waveguide technology,” Opt. Express 13(17), 6490–6496 (2005). [CrossRef] [PubMed]
  11. K. Kieu, R. J. Jones, and N. Peyghambarian, “High power femtosecond source near 1 micron based on an all-fiber Er-doped mode-locked laser,” Opt. Express 18(20), 21350–21355 (2010). [CrossRef] [PubMed]
  12. H. A. Haus and A. Mecozzi, “Noise of mode-locked lasers,” IEEE J. Quantum Electron. 29(3), 983–996 (1993). [CrossRef]
  13. R. Paschotta, “Timing jitter and phase noiseof mode-locked fiber lasers,” Opt. Express 18(5), 5041–5054 (2010). [CrossRef] [PubMed]
  14. K. W. Holman, R. J. Jones, A. Marian, S. T. Cundiff, and J. Ye, “Detailed studies and control of intensity-related dynamics of femtosecond frequency combs from mode-locked Ti: sapphire lasers,” IEEE J. Sel. Top. Quantum Electron. 9(4), 1018–1024 (2003). [CrossRef]
  15. S. Witte, R. T. Zinkstok, W. Hogervorst, and K. S. E. Eikema, “Control and precise measurement of carrier-envelope phase dynamics,” Appl. Phys. B 78(1), 5–12 (2004). [CrossRef]
  16. K. Kieu and F. W. Wise, “Self-similar and stretched-pulse operation of erbium-doped fiber lasers with carbon nanotubes saturable absorber,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CML3.
  17. Z. P. Sun, T. Hasan, F. Q. Wang, A. G. Rozhin, I. H. White, and A. C. Ferrari, “Ultrafast stretched-pulse fiber laser mode-locked by carbon nanotubes,” Nano Res. 3(6), 404–411 (2010). [CrossRef]
  18. M. Y. Sander, E. P. Ippen, and F. X. Kärtner, “Carrier-envelope phase dynamics of octave-spanning dispersion-managed Ti: sapphire lasers,” Opt. Express 18(5), 4948–4960 (2010). [CrossRef] [PubMed]
  19. C. Ouyang, P. Shum, H. Wang, J. H. Wong, K. Wu, S. Fu, R. Li, E. J. R. Kelleher, A. I. Chernov, and E. D. Obraztsova, “Observation of timing jitter reduction induced by spectral filtering in a fiber laser mode locked with a carbon nanotube-based saturable absorber,” Opt. Lett. 35(14), 2320–2322 (2010). [CrossRef] [PubMed]

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