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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 21 — Oct. 17, 2005
  • pp: 8671–8677
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Femtosecond Neodymium-doped microstructure fiber laser

Mathias Moenster, Peter Glas, Günter Steinmeyer, Rumen Iliew, Nikolay Lebedev, Reiner Wedell, and Mario Bretschneider  »View Author Affiliations


Optics Express, Vol. 13, Issue 21, pp. 8671-8677 (2005)
http://dx.doi.org/10.1364/OPEX.13.008671


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Abstract

We demonstrate femtosecond operation of a Nd-doped microstructure fiber laser. The fiber provides gain and anomalous dispersion at the lasing wavelength of 1.06 μm and enables the construction of short and simple cavity designs. The laser is passively mode-locked by the combined action of a saturable absorber mirror, fiber nonlinearity, and dispersion and produces transform limited sub-400-fs pulses with a pulse energy as high as 100 pJ.

© 2005 Optical Society of America

1. Introduction

Mode-locked fiber lasers are highly stable and cost-effective femtosecond light sources. Fiber lasers can be made very compact and are effectively shielded from environmental influences, which often makes them the preferred choice for a laser oscillator. However, the generation of femtosecond pulses in fiber lasers by passive mode-locking has only found widespread use at wavelengths of about 1.5 μm [1

1 . B. C. Collings , K. Bergman , S. T. Cundiff , S. Tsuda , J. N. Kutz , J. E. Cunningham , W. Y. Jan , M. Koch , and W. H. Knox , “ Short Cavity Erbium/Ytterbium Fiber Lasers Mode-Locked with a Saturable Bragg Reflector ,” IEEE J. Sel. Top. Quantum Electron. 3 , 1065 -75 ( 1997 ). [CrossRef]

, 2

2 . L. E. Nelson , D. J. Jones , K. Tamura , H. A. Haus , and E. P. Ippen , “ Ultrashort-pulse fiber ring lasers ,” Appl. Phys. B 65 , 277 -94 ( 1997 ). [CrossRef]

], because soliton formation and dispersion compensation is only possible in this spectral region in standard optical fibers. In contrast, passive mode-locking in the Nd- or Yb-bands always requires extra-fiber dispersion compensation schemes [3–5

3 . M. Hofer , M. E. Fermann , F. Haberl , M. H. Ober , and A. J. Schmidt , “ Mode locking with cross-phase and self-phase modulation ,” Opt. Lett. 16 , 502 -4 ( 1991 ). [CrossRef] [PubMed]

]. Recently, microstructure fibers (MSFs) have greatly extended the range of soli-ton formation outside the erbium band down to the visible wavelength range [6

6 . P. St. J. Russell , “ Photonic Crystal Fibers ,” Science 299 , 358 – 62 ( 2003 ). [CrossRef] [PubMed]

]. While this potential for engineering dispersion properties [7

7 . L. P. Shen , W. P. Huang , and S. S. Jian , “ Design of Photonic Crystal Fibers for Dispersion-Related Applications ,” J. Lightwave Technol. 21 , 1644 -51 ( 2003 ). [CrossRef]

] has readily been exploited in supercontinuum experiments [8

8 . J. K. Ranka , R. S. Windeler , and A. J. Stentz , “ Visible Continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm ,” Opt. Lett. 25 , 25 -7 ( 2000 ). [CrossRef]

], there are only few examples for the use of dispersion scaling possibilities offered by MSFs, both for passive mode-locking, combining passive MSFs and active regular fiber sections [9

9 . H. Lim , F. Ö. Ilday , and F. W. Wise , “ Femtosecond ytterbium fiber laser with photonic crystal fiber for dispersion control ,” Opt. Express 11 , 1497 -2 ( 2002 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-25-1497 .

,10

10 . A. V. Avdokhin , S. V. Popov , and J. R. Taylor , “ Totally fiber integrated, figure-of-eight, femtosecond source at 1065 nm ,” Opt. Express 11 , 265 -9 ( 2003 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-3-265 . [CrossRef] [PubMed]

], and also for active mode-locking [11

11 . K. Furusawa , T. M. Monro , P. Petropoulos , and D. J. Richardson , “ Modelocked laser based on Ytterbium doped holey fibre ,” Electron. Lett. 37 , 560 -1 ( 2001 ). [CrossRef]

]. We now demonstrate femtosecond pulse generation, using only the tailored properties of the MSF and a saturable absorber mirror (SAM). All other mechanisms required for passive mode-locking, namely dispersion scaling, the optical nonlinearity, and the laser gain have been incorporated into one microstuctured optical fiber.

2. Experimental setup

Fig. 1. (a) SEM micrograph of the fiber end face as cleaved. (b) Cross section of the two polished fiber end faces (schematically). (c) Laser setup. The blue beam indicates the path of the laser cavity whereas the red beam represents the pump radiation. DM: dichroic mirror (T = 99% at 810 nm, R = 99% at 1054 nm), L1, L2 : aspheric lenses (f = 8 mm, NA = 0.5), P: dichroic glass polarizer, M : output coupling mirror (T = 30% at 1054 nm), Ti:Sa: cw Ti:sapphire pump laser, Nd:MSF: Nd-doped microstructure fiber.

To enable polarization selection, a thin polarizer is placed in front of the SAM. We find that the linear state of polarization as selected by the polarizer is preserved after passage through the fiber when used in a straight configuration. Furthermore it was possible to mode-lock the laser in various polarization angles, including angles which are offset from the polarization axes. A group velocity mismatch of ≈ 160 fs per roundtrip was found between the slow and fast axes by altering the polarizer angle while monitoring the repetition rate of the laser. From this measurement we deduce a fiber birefringence of ∆n = 3 × 10-5. For the polarizer angle used in the following, we estimate an effective group velocity dispersion of β 2 = -15 ps2/km. It is well known that Fresnel reflections from the fiber end faces can give rise to subcavity effects, preventing mode-locking of the laser. Angle cleaving or polishing is a common way to circumvent this problem. However, these standard techniques cannot be applied to MSFs since the mechanical stress damages the fragile microstructure. To overcome these difficulties we further improved the end face sealing technique described in Ref. [15

15 . M. Moenster , P. Glas , G. Steinmeyer , and R. Iliew , “ Mode-locked Nd-doped microstructure fiber laser ,” Opt. Express 12 , 4523 -7 ( 2004 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4523 . [CrossRef] [PubMed]

]. We first sealed the MSF end faces using the electric arc of a fusion splicer. The fusion splicer delivers a well-defined amount of energy to the end faces in a short time, which allows for controlled collapsing of the air holes, forming an adiabatic transition region between bulk and microstructure, as shown in Fig. 1(b). The fiber is finally mounted in a ferrule and angle-polished at an angle of 12°. We experimentally found that the coupling efficiency of the laser signal entering the MSF is actually increased by this procedure from 33% to 50%, which indicates a favorable effect of the adiabatic matching section to the overall losses in the cavity.

Fig. 2. (a) Measured SAM reflectivity vs. input pulse fluence. The measurement setup was similar to the one described in Ref. [14]. (b) Measured pump-probe response (normalized) of the SAM. The pulse source used for both measurements was a 180-fs passively mode-locked Nd:glass laser tuned to 1060 nm.

3. Results and discussion

Using the setup of Fig. 1(c), the laser shows self-starting passive mode-locking with pulse energies of 100 pJ at a repetition rate of 95 MHz, i.e. an output power of 9.5 mW. This yields an efficiency of about 10% relative to the launched power. Figure 3 depicts the measured intensity autocorrelation of our laser. The measured data show excellent agreement with a hyperbolic secant pulse of τ = 394 fs duration (FWHM). Experimentally we found that the pulse duration increases to 440 fs when the intracavity power is reduced by 30%. Increasing the delay range of the autocorrelator to its maximum (see inset of Fig. 3) does not reveal any satellite pulses. Together with a measured satellite-free oscilloscope trace, we conclude that there is no multiple pulsing of the laser. The laser spectrum which is shown in Fig. 4(a) exhibits a width of 3.1 nm (FWHM), which is already a significant part of the 20 nm gain bandwidth of Nd-doped phosphate glasses. The spectrum also agrees very well with the assumption of a hyperbolic secant pulse profile. The resulting time-bandwidth product is 0.326, i.e., the pulse is transform limited.

The spectrum shows no indication of Kelly sidebands [16

16 . S. M. J. Kelly , “ Characteristic sideband instability of periodically amplified average soliton ,” Electron. Lett. 28 , 806 -7 ( 1992 ). [CrossRef]

], as they often arise in soliton lasers. From the measured pulse duration we calculate the spectral position of the Kelly sidebands with respect to the spectral peak value as ∆λ ≈ ± 9.2 nm. In fact, in some experiments we could see Kelly sidebands exactly at the computed positions far out in the spectral wings of the pulse. The practical absence of such features in Fig. 4(a) can therefore not be interpreted as an indication against soliton formation but is dictated by the laser parameters, in particular the very short cavity length of our fiber laser. The latter prevents dispersive waves from being phasematched within three times the spectral half width. The resulting pulse fluence on the SAM is estimated as F P = 9500 μJ/cm, which is about two orders of magnitude higher than the saturation fluence of the SAM. The measured SAM characteristics [Fig. 2.(a)] suggest that TPA processes can become relevant in this regime [17

17 . T. R. Schibli , E. R. Thoen , F. X. Kärtner , and E. P. Ippen , “ Suppression of Q-switched mode locking and break-up into multiple pulses by inverse saturable absorption ,” Appl. Phys. B 70 , 41 -9 ( 2000 ). [CrossRef]

]. However, as shorter pulses (180 fs) were used for the characterization of the SAM than are present in our laser, TPA is expected to set in at higher fluences in our cavity.

Fig. 3. Intensity autocorrelation. The measured data (black rings) have been fitted assuming a sech2 pulse shape (red line). The inset shows the corresponding long-range autocorrelation.
Fig. 4. (a) Optical spectrum of the mode-locked laser (resolution: 0.05 nm). The measured data (black line) has been fitted to the frequency representation of a sech2 pulse spectrum (dash-dotted red line). (b) RF spectrum of the first intermode beat centered at f R ≈ 95 MHz (resolution bandwidth: 100 Hz).

To convince ourselves of the absence of Q-switched mode-locking, we measured the RF spectrum of the intermode beat of the mode-locked laser, see Fig. 4(b). We measure an RF linewidth of less than 300 Hz. Q-switching sidebands are at least ≈ 70 dB below the carrier, which is an extremely good value for a Nd-doped laser material. The key laser parameters that ensure stability against Q-switched mode-locking are the small mode area of the MSF and the equally small spot size on the SAM. That Q-switched mode-locking is not an issue in our laser can be confirmed by the stability criterion of Hönninger et al. [18

18 . C. Hönninger , R. Paschotta , F. Morier-Genoud , M. Moser , and U. Keller , “ Q-switching stability limits of continous-wave passive mode locking ,” J. Opt. Soc. Am. B 16 , 46 – 56 ( 1999 ). [CrossRef]

]. In its simplest form, this criterion requires that the pulse energy be larger than the geometric average of the saturation energies of gain and saturable absorption weighted with ∆R, i.e., E p > (E sat,MSF E sat,AR)1/2 . Accounting for soliton filtering, an additional term further increases the left-hand side of the inequality. We find that the inequality is met for our laser with or without accounting for soliton filtering. This explains the practical absence of Q-switching artifacts in the RF spectrum.

4. Conclusion

To the best of our knowledge, we have, for the first time, directly exploited the dispersion scaling potential of MSFs to synthesize a gain medium with suitable dispersion properties for passively mode-locked operation. This was demonstrated with the generation of 400-fs pulses, using nothing but an additional SAM in the laser cavity. This allows for a very simple and compact architecture, making relatively high repetition rates for a fiber laser possible. Yet, with this first demonstration, the capacities for a compact, integrated setup have certainly not been fully exploited. Using ytterbium as the gain medium may allow for further shortening of the fiber; butt-coupling of the SAM may contribute to a further increase of the repetition rate. With its potential for short fiber cavities, our laser appears as an interesting step towards a more bulk-like fiber laser. More importantly, however, our passively mode-locked MSF laser opens up a perspective for a compact femtosecond fiber laser working in the one-micron range, which is a promising source, e.g., for seeding amplifiers or metrology applications.

Acknowledgements

The authors gratefully acknowledge W. Richter, BATOP GmbH, for providing the deep-modulation saturable absorber mirror and S. Nöther (MBI), for measuring the fiber dispersion. This work was supported by the German Ministry of Education and Research (BMBF) under contract numbers 13N8334 and 13N8337.

References and links

1 .

B. C. Collings , K. Bergman , S. T. Cundiff , S. Tsuda , J. N. Kutz , J. E. Cunningham , W. Y. Jan , M. Koch , and W. H. Knox , “ Short Cavity Erbium/Ytterbium Fiber Lasers Mode-Locked with a Saturable Bragg Reflector ,” IEEE J. Sel. Top. Quantum Electron. 3 , 1065 -75 ( 1997 ). [CrossRef]

2 .

L. E. Nelson , D. J. Jones , K. Tamura , H. A. Haus , and E. P. Ippen , “ Ultrashort-pulse fiber ring lasers ,” Appl. Phys. B 65 , 277 -94 ( 1997 ). [CrossRef]

3 .

M. Hofer , M. E. Fermann , F. Haberl , M. H. Ober , and A. J. Schmidt , “ Mode locking with cross-phase and self-phase modulation ,” Opt. Lett. 16 , 502 -4 ( 1991 ). [CrossRef] [PubMed]

4 .

M. H. Ober , M. Hofer , U. Keller , and T. H. Chiu , “ Self-starting diode-pumped femtosecond Nd fiber laser ,” Opt. Lett. 18 , 1532 -4 ( 1993 ). [CrossRef] [PubMed]

5 .

H. Ö. Lim , F. Ilday , and F. W. Wise , “ Generation of 2-nJ pulses from a femtosecond ytterbium fiber laser ,” Opt. Lett. 28 , 660 – 662 ( 2003 ). [CrossRef] [PubMed]

6 .

P. St. J. Russell , “ Photonic Crystal Fibers ,” Science 299 , 358 – 62 ( 2003 ). [CrossRef] [PubMed]

7 .

L. P. Shen , W. P. Huang , and S. S. Jian , “ Design of Photonic Crystal Fibers for Dispersion-Related Applications ,” J. Lightwave Technol. 21 , 1644 -51 ( 2003 ). [CrossRef]

8 .

J. K. Ranka , R. S. Windeler , and A. J. Stentz , “ Visible Continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm ,” Opt. Lett. 25 , 25 -7 ( 2000 ). [CrossRef]

9 .

H. Lim , F. Ö. Ilday , and F. W. Wise , “ Femtosecond ytterbium fiber laser with photonic crystal fiber for dispersion control ,” Opt. Express 11 , 1497 -2 ( 2002 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-25-1497 .

10 .

A. V. Avdokhin , S. V. Popov , and J. R. Taylor , “ Totally fiber integrated, figure-of-eight, femtosecond source at 1065 nm ,” Opt. Express 11 , 265 -9 ( 2003 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-3-265 . [CrossRef] [PubMed]

11 .

K. Furusawa , T. M. Monro , P. Petropoulos , and D. J. Richardson , “ Modelocked laser based on Ytterbium doped holey fibre ,” Electron. Lett. 37 , 560 -1 ( 2001 ). [CrossRef]

12 .

D. Mogilevtsev , T. A. Birks , and P. St. J. Russell , “ Group-velocity dispersion in photonic crystal fibers ,” Opt. Lett. 23 , 1662 -4 ( 1998 ). [CrossRef]

13 .

U. Keller , K. J. Weingarten , F. X. Kärtner , D. Kopf , B. Braun , I. D. Jung , R. Fluck , C. Hönninger , N. Matuschek , and J. Aus der Au , “ Semiconductor Saturable Absorber Mirrors (SESAM’s) for Femtosecond to Nanosecond Pulse Generation in Solid-State-Lasers ,” IEEE J. Sel. Top. Quantum Electron. 2 , 435 -53 ( 1996 ). [CrossRef]

14 .

M. Haiml , R. Grange , and U. Keller , “ Optical characterization of semiconductor saturable absorbers ,” Appl. Phys. B 79 , 331 -9 ( 2004 ). [CrossRef]

15 .

M. Moenster , P. Glas , G. Steinmeyer , and R. Iliew , “ Mode-locked Nd-doped microstructure fiber laser ,” Opt. Express 12 , 4523 -7 ( 2004 ), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4523 . [CrossRef] [PubMed]

16 .

S. M. J. Kelly , “ Characteristic sideband instability of periodically amplified average soliton ,” Electron. Lett. 28 , 806 -7 ( 1992 ). [CrossRef]

17 .

T. R. Schibli , E. R. Thoen , F. X. Kärtner , and E. P. Ippen , “ Suppression of Q-switched mode locking and break-up into multiple pulses by inverse saturable absorption ,” Appl. Phys. B 70 , 41 -9 ( 2000 ). [CrossRef]

18 .

C. Hönninger , R. Paschotta , F. Morier-Genoud , M. Moser , and U. Keller , “ Q-switching stability limits of continous-wave passive mode locking ,” J. Opt. Soc. Am. B 16 , 46 – 56 ( 1999 ). [CrossRef]

19 .

F. X. Kärtner and U. Keller , “ Stabilization of solitonlike pulses with a slow saturable absorber ,” Opt. Lett. 20 , 16 -8 ( 1995 ). [CrossRef] [PubMed]

OCIS Codes
(140.3510) Lasers and laser optics : Lasers, fiber
(320.7090) Ultrafast optics : Ultrafast lasers

ToC Category:
Research Papers

History
Original Manuscript: September 20, 2005
Revised Manuscript: October 11, 2005
Published: October 17, 2005

Citation
Mathias Moenster, Peter Glas, Günter Steinmeyer, Rumen Iliew, Nikolay Lebedev, Reiner Wedell, and Mario Bretschneider, "Femtosecond Neodymium-doped microstructure fiber laser," Opt. Express 13, 8671-8677 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-21-8671


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References

  1. B. C. Collings, K. Bergman, S. T. Cundiff, S. Tsuda, J. N. Kutz, J. E. Cunningham, W. Y. Jan, M. Koch, and W. H. Knox, �??Short Cavity Erbium/Ytterbium Fiber Lasers Mode-Locked with a Saturable Bragg Reflector,�?? IEEE J. Sel. Top. Quantum Electron. 3, 1065-75 (1997). [CrossRef]
  2. L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, and E. P. Ippen, �??Ultrashort-pulse fiber ring lasers,�?? Appl. Phys. B 65, 277-94 (1997). [CrossRef]
  3. M. Hofer, M. E. Fermann, F. Haberl, M. H. Ober, and A. J. Schmidt, �??Mode locking with cross-phase and self-phase modulation,�?? Opt. Lett. 16, 502-4 (1991). [CrossRef] [PubMed]
  4. M. H. Ober, M. Hofer, U. Keller, and T. H. Chiu, �??Self-starting diode-pumped femtosecond Nd fiber laser,�?? Opt. Lett. 18, 1532-4 (1993). [CrossRef] [PubMed]
  5. H. Lim, F. �?. Ilday, and F. W. Wise, �??Generation of 2-nJ pulses from a femtosecond ytterbium fiber laser,�?? Opt. Lett. 28, 660-662 (2003). [CrossRef] [PubMed]
  6. P. St. J. Russell, �??Photonic Crystal Fibers,�?? Science 299, 358-62 (2003). [CrossRef] [PubMed]
  7. L. P. Shen,W. P. Huang, and S. S. Jian, �??Design of Photonic Crystal Fibers for Dispersion-Related Applications,�?? J. Lightwave Technol. 21, 1644-51 (2003). [CrossRef]
  8. J. K. Ranka, R. S.Windeler, A. J. Stentz, �??Visible Continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,�?? Opt. Lett. 25, 25-7 (2000). [CrossRef]
  9. H. Lim, F.�?.Ilday, and F.W.Wise, �??Femtosecond ytterbium fiber laser with photonic crystal fiber for dispersion control,�?? Opt. Express 11, 1497-2 (2002), <a href= "http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-25-1497">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-25-1497</a>.
  10. A. V. Avdokhin, S. V. Popov, and J. R. Taylor, �??Totally fiber integrated, figure-of-eight, femtosecond source at 1065 nm,�?? Opt. Express 11, 265-9 (2003), <a href= "http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-3-265">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-3-265</a>. [CrossRef] [PubMed]
  11. K. Furusawa, T. M. Monro, P. Petropoulos, and D. J. Richardson, �??Modelocked laser based on Ytterbium doped holey fibre,�?? Electron. Lett. 37, 560-1 (2001). [CrossRef]
  12. D. Mogilevtsev, T. A. Birks, P. St. J. Russell, �??Group-velocity dispersion in photonic crystal fibers,�?? Opt. Lett. 23, 1662-4 (1998). [CrossRef]
  13. U. Keller, K. J. Weingarten, F. X. Kärtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Hönninger, N. Matuschek, and J. Aus der Au, �??Semiconductor Saturable Absorber Mirrors (SESAM�??s) for Femtosecond to Nanosecond Pulse Generation in Solid-State-Lasers,�?? IEEE J. Sel. Top. Quantum Electron. 2, 435-53 (1996). [CrossRef]
  14. M. Haiml, R. Grange, and U. Keller, �??Optical characterization of semiconductor saturable absorbers,�?? Appl. Phys. B 79, 331-9 (2004). [CrossRef]
  15. M. Moenster, P. Glas, G. Steinmeyer, and R. Iliew, �??Mode-locked Nd-doped microstructure fiber laser,�?? Opt. Express 12, 4523-7 (2004), <a href= "http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4523">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4523</a>. [CrossRef] [PubMed]
  16. S. M. J. Kelly, �??Characteristic sideband instability of periodically amplified average soliton,�?? Electron. Lett. 28, 806-7 (1992). [CrossRef]
  17. T. R. Schibli, E. R. Thoen, F. X. Kärtner, E. P. Ippen, �??Suppression of Q-switched mode locking and break-up into multiple pulses by inverse saturable absorption,�?? Appl. Phys. B 70, 41-9 (2000). [CrossRef]
  18. C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, �??Q-switching stability limits of continous-wave passive mode locking,�?? J. Opt. Soc. Am. B 16, 46-56 (1999). [CrossRef]
  19. F. X. Kärtner and U. Keller, �??Stabilization of solitonlike pulses with a slow saturable absorber,�?? Opt. Lett. 20, 16-8 (1995). [CrossRef] [PubMed]

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