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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 26627–26633
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A Q-switched, mode-locked fiber laser employing subharmonic cavity modulation

You Min Chang, Junsu Lee, and Ju Han Lee  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 26627-26633 (2011)
http://dx.doi.org/10.1364/OE.19.026627


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Abstract

We present a new and simple approach for the generation of Q-switched, mode-locked pulses from a laser cavity. The approach is based on cavity loss modulation that employs a subharmonic frequency of the fundamental intermode frequency spacing. A range of experiments have been carried out using an erbium-doped fiber-based ring cavity laser in order to verify that this simple approach can readily produce high quality Q-switched, mode-locked pulses. An active tuning of the Q-switched envelope repetition rate is also shown to be easily achievable by adjusting the order of the applied subharmonic frequency.

© 2011 OSA

1. Introduction

Ultra-short pulse lasers have drawn great technical interest in recent years due to their wide use in a variety of applications, such as high speed optical communications [1

1. E. Yoshida, N. Shimizu, and M. Nakazawa, “A 40-GHz 0.9-ps regeneratively mode-locked fiber laser with a tuning range of 1530-1560 nm,” IEEE Photon. Technol. Lett. 11(12), 1587–1589 (1999). [CrossRef]

], biomedical imaging [2

2. H. Don Lee, J. H. Lee, M. Y. Jeong, and C. S. Kim, “Characterization of wavelength-swept active mode locking fiber laser based on reflective semiconductor optical amplifier,” Opt. Express 19(15), 14586–14593 (2011). [CrossRef] [PubMed]

], and material processing [3

3. C. B. Schaffer, A. Brodeur, J. F. García, and E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy,” Opt. Lett. 26(2), 93–95 (2001). [CrossRef] [PubMed]

]. The mode-locking technique is commonly used to obtain short pulses from a laser cavity. Through the locking of the relative phases of the multiple lasing modes by modulating the loss (or gain) of the laser at a frequency of an integer multiple of the fundamental intermode frequency spacing, the independent, longitudinal modes are forced into a phase coherence. The coherent multiple lasing modes then manifest themselves into a well-defined temporal pulse form. Mode-locking techniques can be classified into two categories: passive and active. Passive mode-locking uses nonlinear optical effects, such as nonlinear amplifying loop mirror [4

4. D. J. Richardson, R. I. Laming, D. N. Payne, M. W. Phillips, and V. J. Matsas, “320 fs soliton generation with passively mode-locked erbium fibre laser,” Electron. Lett. 27(9), 730–732 (1991). [CrossRef]

], saturable absorption [5

5. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Laser mode locking using a saturable absorber incorporating carbon nanotubes,” J. Lightwave Technol. 22(1), 51–56 (2004). [CrossRef]

] and nonlinear polarization rotation [6

6. H. A. Haus, K. Tamura, L. E. Nelson, and E. P. Ippen, “Stretched-pulse additive pulse mode-locking in fiber ring lasers: theory and experiment,” IEEE J. Quantum Electron. 31(3), 591–598 (1995). [CrossRef]

], whereas active mode-locking uses external beam modulation devices to ensure the phase locking of each mode [7

7. C. Wu and N. K. Dutta, “High-repetition-rate optical pulse generation using a rational harmonic mode-locked fiber laser,” IEEE J. Quantum Electron. 36(2), 145–150 (2000). [CrossRef]

].

In this paper we present a new and simple approach for the generation of Q-switched, mode-locked pulses from a laser cavity. The approach is based on cavity loss modulation at a subharmonic frequency of the fundamental intermode frequency spacing. Through a range of experiments with an erbium-doped fiber (EDF)-based ring cavity laser, it has been verified that this simple approach can readily produce high quality Q-switched, mode-locked pulses. It is shown that the active control of the Q-switched envelope repetition rate is easily achievable by adjusting the order of the applied subharmonic frequency. It is also shown that the peak optical power of the Q-switched, mode-locked pulses can reach up to a level ~14 times larger than that found for the continuous fundamental-order mode-locked pulses generated from the same cavity.

2. Experiment setup

The schematic of our Q-switched, mode-locked EDF laser is shown in Fig. 1(a)
Fig. 1 (a) The laser schematic. (b) The measured optical spectrum of the output pulses.
. The fiber laser was constructed by using a simple ring cavity in which a 3-m-long EDF with a peak absorption of 20 dB/m at 1530 nm was used as the gain medium. The EDF was pumped by a 980-nm pump laser diode; the pump power was rated at 50 mW. An isolator and a polarization controller (PC) were incorporated into the cavity in order to ensure a unidirectional beam oscillation and for polarization adjustment. A 0.5-nm bandpass filter was inserted into the cavity to eliminate the background amplified spontaneous emission (ASE) noise as well as to determine the lasing wavelength. The laser output was extracted from the ring cavity by a 90:10 fiber coupler, which fed 90% of the oscillated light power back into the EDF via a silicon-based variable optical attenuator (VOA). The laser output was monitored by a 16-GHz real-time digital oscilloscope (Tektronix, DSA71604C) using a sampling rate of 100 GS/s and an InGaAs photodetector with a bandwidth of 20 GHz in order to monitor the temporal shapes of both the Q-switched envelope and the mode-locked pulses. The temporal resolution of the measurement setup used in this particular experiment was ~60 ps. Figure 1(b) shows the measured optical spectrum of the laser output. The center-wavelength was measured to be 1561.42 nm, however the 3-dB bandwidth measurement was limited by the resolution bandwidth (0.02 nm) of the optical spectrum analyzer (OSA) used in this experiment.

In order to apply the loss modulation to the laser cavity an ultrafast VOA based on a silicon p-i-n diode built on a silicon optical waveguide was used [16

16. D. W. Zheng, B. T. Smith, and M. Asghari, “Improved efficiency Si-photonic attenuator,” Opt. Express 16(21), 16754–16765 (2008). [CrossRef] [PubMed]

]. We recently demonstrated that this simple device could be readily used for the Q-switching of an erbium-doped fiber (EDF) laser [17

17. Y. M. Chang, J. Lee, Y. M. Jhon, and J. H. Lee, “Active Q-switching in an erbium-doped fiber laser using an ultrafast silicon-based variable optical attenuator,” Opt. Express (under revision for final decision, Oct. 2011).

]. The ultrafast Si-based VOA used in this experiment is commercially available (Kotura, UltraVOA). Further details regarding this device are fully described in [16

16. D. W. Zheng, B. T. Smith, and M. Asghari, “Improved efficiency Si-photonic attenuator,” Opt. Express 16(21), 16754–16765 (2008). [CrossRef] [PubMed]

].

Figure 2(a)
Fig. 2 (a) The measured optical attenuation v.s. the driving current curve of the VOA. (b) The measured oscilloscope traces of the applied sinusoidal electrical signal and their corresponding modulated optical beam at a frequency of 490 kHz through the VOA.
shows the measured optical attenuation of the device as a function of the applied forward-biased current. We operated the VOA by applying a sinusoidal electrical signal with a peak-to-peak current of ~50 mA. Figure 2(b) shows the oscilloscope traces of the applied sinusoidal electrical signal and their corresponding modulated optical beam at a VOA frequency of 490 kHz. The modulated optical beam exhibited a square-like waveform rather than a sinusoidal one due to its nonlinear attenuation curve, as shown in Fig. 2(a).

3. Experimental results

We first applied the loss modulation to the cavity at the fundamental intermode spacing frequency in order to ensure that the generation of stable fundamental-order, mode-locked pulses occurred. Figure 3(a)
Fig. 3 (a) The measured oscilloscope traces of the output optical pulses from a laser cavity with its loss modulation at a frequency of the fundamental intermode spacing and (b) a magnified view.
shows the oscilloscope trace of the output pulse train emitted from the fundamental-order, mode-locked laser; a magnified view is shown in Fig. 3(b). The measured pulse spacing and pulse width were measured to be ~102 ns and ~260 ps, respectively. The temporal width measurement was carried out with the high speed real-time oscilloscope rather than an autocorrelator, since the few hundred picosecond pulse width exceeded the measurement window of the autocorrelator available in our laboratory. The side lobes seen in Fig. 3(b) are attributable to the modulated background ASE, which would be substantially reduced by using a bandpass filter with a narrower a bandwidth within the cavity.

In order to further confirm the existence of both the Q-switching and mode-locking effect we carried out an electrical spectrum measurement of the output pulses under a subharmonic order of 20. Figure 5(a)
Fig. 5 (a) The measured oscilloscope trace of the output pulses at m = 20 and the fitting curves for the Q-switched envelopes are also shown. (b) Their electrical spectrum with a resolution bandwidth of 30 Hz.
shows the oscilloscope trace of the output pulses; Fig. 5(b) shows their electrical spectrum. The frequency peak of the mode-locked pulses at a frequency of 9.806 MHz is clearly illustrated along with the two side frequency components of the Q-switched envelope at a frequency of 490 kHz. This electrical spectrum indicates that an amplitude modulation at a frequency of 490 kHz was imposed onto the continuous 9.806 MHz mode-locked pulse train.

Finally, we measured the variation of the output pulse characteristics, such as average optical power, Q-switched envelope width, pulse width, and main pulse peak power that occurred as we increased the subharmonic order. The main pulse is defined as the pulse that possesses the highest peak power within the Q-switched envelope. The peak power was estimated by curve-fitting of both the Q-switched envelope and the mode-locked pulses from the measured averaged optical power. The results are summarized in Fig. 6
Fig. 6 The variation of the output pulse characteristics with an increasing subharmonic order: (a) average optical power, (b) Q-switched envelope width, (c) main pulse width, and (d) main pulse peak power.
.

The average optical power of the output pulses was observed to decrease at first when the subharmonic order was enlarged, and then reached a minimum at the subharmonic order of 10. Past the subharmonic order of 10 the average power continuously increased and was observed to saturate at the subharmonic of ~35. The saturated average optical power was equivalent to that of the fundamental-order, mode-locked pulses. As mentioned above, the erbium fiber laser seemed to be in the steady-state mode until m = 10 and then turn into the transient mode. In the steady-state mode the cavity loss modulation induces output power loss. This is evident from the average power level decrease at m=5 and 10 in Fig. 6(a). On the other hand, in the transient mode the cavity loss modulation induces Q-switching phenomenon, which is associated with transient gain build-up and photon emission process. In this case, the increase of the subharmonic order leads to larger gain build-up and photon emission times, which result in higher average output power and larger Q-switched envelope width as shown in Figs. 6(a) and 6(b).

The temporal width of the output pulses were observed to decrease with an increase in the subharmonic order, whereas that of the Q-switched envelope singularly increased, as shown in Figs. 6(b) and 6(c). A minimum pulse width of ~100 ps, which was ~2.5 times narrower than that of the fundamental-order, mode-locked pulses was observed at the subharmonic order of 45. The pulse width variation as a function of the subharmonic order can be understood as optical power-dependent soliton pulse compression, since the laser cavity has anomalous dispersion at the lasing wavelength of 1561.42 nm. As shown in Fig. 6(d) the pulse peak power suddenly decreases when the subharmonic order is changed from m=1 to 5. The suddenly lowered peak power causes temporal width broadening of the solitonic mode-locked pulses at m=5. Further increase of the subharmonic order leads to higher-order soliton pulse compression, which results in the two relative minimum widths at m=20 and 45. This explanation needs to be confirmed through a theoretical work.

The estimated peak power of the main pulse was found to increase and reached up to ~1.4 W, ~14 times greater than that found for the fundamental-order, mode-locked pulses, fully meeting our expectations, as shown in Fig. 6(d).

4. Conclusion

We have demonstrated a novel method for the generation of Q-switched, mode-locked pulses from a laser cavity based on the cavity loss modulation at a subharmonic frequency of the fundamental intermode frequency spacing. It has been shown that this approach produces high quality Q-switched, mode-locked pulses by performing a range of experiments with an EDF ring cavity laser. Since an active tuning of the Q-switched envelope repetition rate is readily possible through a simple adjustment of the order of the applied subharmonic frequency, we believe that this approach can be a powerful tool in the generation of repetition rate controllable bursts of ultra-short pulses.

Acknowledgments

This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Education, Science, and Technology (MEST), Republic of Korea (No. 2011-0028978).

References and links

1.

E. Yoshida, N. Shimizu, and M. Nakazawa, “A 40-GHz 0.9-ps regeneratively mode-locked fiber laser with a tuning range of 1530-1560 nm,” IEEE Photon. Technol. Lett. 11(12), 1587–1589 (1999). [CrossRef]

2.

H. Don Lee, J. H. Lee, M. Y. Jeong, and C. S. Kim, “Characterization of wavelength-swept active mode locking fiber laser based on reflective semiconductor optical amplifier,” Opt. Express 19(15), 14586–14593 (2011). [CrossRef] [PubMed]

3.

C. B. Schaffer, A. Brodeur, J. F. García, and E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy,” Opt. Lett. 26(2), 93–95 (2001). [CrossRef] [PubMed]

4.

D. J. Richardson, R. I. Laming, D. N. Payne, M. W. Phillips, and V. J. Matsas, “320 fs soliton generation with passively mode-locked erbium fibre laser,” Electron. Lett. 27(9), 730–732 (1991). [CrossRef]

5.

S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Laser mode locking using a saturable absorber incorporating carbon nanotubes,” J. Lightwave Technol. 22(1), 51–56 (2004). [CrossRef]

6.

H. A. Haus, K. Tamura, L. E. Nelson, and E. P. Ippen, “Stretched-pulse additive pulse mode-locking in fiber ring lasers: theory and experiment,” IEEE J. Quantum Electron. 31(3), 591–598 (1995). [CrossRef]

7.

C. Wu and N. K. Dutta, “High-repetition-rate optical pulse generation using a rational harmonic mode-locked fiber laser,” IEEE J. Quantum Electron. 36(2), 145–150 (2000). [CrossRef]

8.

P. K. Datta, S. Mukhopadhyay, S. K. Das, L. Tartara, A. Agnesi, and V. Degiorgio, “Enhancement of stability and efficiency of a nonlinear mirror mode-locked Nd:YVO4 oscillator by an active Q-switch,” Opt. Express 12(17), 4041–4046 (2004). [CrossRef] [PubMed]

9.

S. Zhao, G. Li, D. Li, K. Yang, Y. Li, M. Li, T. Li, G. Zhang, and K. Cheng, “Numerical simulation of dual-loss-modulated Q-switched and mode-locked laser with an acousto-optic and Cr4+:YAG saturable absorber,” Appl. Opt. 49(10), 1802–1808 (2010). [CrossRef] [PubMed]

10.

M. Li, S. Zhao, K. Yang, G. Li, D. Li, J. Wang, J. An, and W. Qiao, “Actively Q-switched and mode-locked diode-pumped Nd:GdVO4-KTP laser,” IEEE J. Quantum Electron. 44(3), 288–293 (2008). [CrossRef]

11.

C. Cuadrado-Laborde, A. Díez, J. L. Cruz, and M. V. Andrés, “Doubly active Q switching and mode locking of an all-fiber laser,” Opt. Lett. 34(18), 2709–2711 (2009). [CrossRef] [PubMed]

12.

J. K. Jabczyński, W. Zendzian, and J. Kwiatkowski, “Q-switched mode locking with acousto-optic modulator in a diode-pumped Nd:YVO4 laser,” Opt. Express 14(6), 2184–2190 (2006). [CrossRef] [PubMed]

13.

C. Theobald, M. Weitz, R. Knappe, R. Wallenstein, and J. A. L’Huillier, “Stable Q-switch mode-locking of Nd:YVO4 lasers with a semiconductor saturable absorber,” Appl. Phys. B 92(1), 1–3 (2008). [CrossRef]

14.

Y.-F. Chen and S. W. Tsai, “Simultaneous Q-switching and mode-locking in a diode-pumped Nd:YVO4-Cr4+:YAG laser,” IEEE J. Quantum Electron. 37(4), 580–586 (2001). [CrossRef]

15.

J.-H. Lin, K.-H. Lin, C.-C. Hsu, W. H. Yang, and W.-F. Hsieh, “Supercontinuum generation in a microstructured optical fiber by picosecond self Q-switched mode-locked Nd:GdVO4 laser,” Laser Phys. Lett. 4(6), 413–417 (2007). [CrossRef]

16.

D. W. Zheng, B. T. Smith, and M. Asghari, “Improved efficiency Si-photonic attenuator,” Opt. Express 16(21), 16754–16765 (2008). [CrossRef] [PubMed]

17.

Y. M. Chang, J. Lee, Y. M. Jhon, and J. H. Lee, “Active Q-switching in an erbium-doped fiber laser using an ultrafast silicon-based variable optical attenuator,” Opt. Express (under revision for final decision, Oct. 2011).

18.

J.-H. Lin, H.-R. Chen, H.-H. Hsu, M.-D. Wei, K.-H. Lin, and W.-F. Hsieh, “Stable Q-switched mode-locked Nd3+:LuVO4 laser by Cr4+:YAG crystal,” Opt. Express 16(21), 16538–16545 (2008). [PubMed]

OCIS Codes
(140.3540) Lasers and laser optics : Lasers, Q-switched
(140.4050) Lasers and laser optics : Mode-locked lasers
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: October 31, 2011
Revised Manuscript: November 29, 2011
Manuscript Accepted: November 30, 2011
Published: December 14, 2011

Citation
You Min Chang, Junsu Lee, and Ju Han Lee, "A Q-switched, mode-locked fiber laser employing subharmonic cavity modulation," Opt. Express 19, 26627-26633 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-26627


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References

  1. E. Yoshida, N. Shimizu, and M. Nakazawa, “A 40-GHz 0.9-ps regeneratively mode-locked fiber laser with a tuning range of 1530-1560 nm,” IEEE Photon. Technol. Lett.11(12), 1587–1589 (1999). [CrossRef]
  2. H. Don Lee, J. H. Lee, M. Y. Jeong, and C. S. Kim, “Characterization of wavelength-swept active mode locking fiber laser based on reflective semiconductor optical amplifier,” Opt. Express19(15), 14586–14593 (2011). [CrossRef] [PubMed]
  3. C. B. Schaffer, A. Brodeur, J. F. García, and E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy,” Opt. Lett.26(2), 93–95 (2001). [CrossRef] [PubMed]
  4. D. J. Richardson, R. I. Laming, D. N. Payne, M. W. Phillips, and V. J. Matsas, “320 fs soliton generation with passively mode-locked erbium fibre laser,” Electron. Lett.27(9), 730–732 (1991). [CrossRef]
  5. S. Y. Set, H. Yaguchi, Y. Tanaka, and M. Jablonski, “Laser mode locking using a saturable absorber incorporating carbon nanotubes,” J. Lightwave Technol.22(1), 51–56 (2004). [CrossRef]
  6. H. A. Haus, K. Tamura, L. E. Nelson, and E. P. Ippen, “Stretched-pulse additive pulse mode-locking in fiber ring lasers: theory and experiment,” IEEE J. Quantum Electron.31(3), 591–598 (1995). [CrossRef]
  7. C. Wu and N. K. Dutta, “High-repetition-rate optical pulse generation using a rational harmonic mode-locked fiber laser,” IEEE J. Quantum Electron.36(2), 145–150 (2000). [CrossRef]
  8. P. K. Datta, S. Mukhopadhyay, S. K. Das, L. Tartara, A. Agnesi, and V. Degiorgio, “Enhancement of stability and efficiency of a nonlinear mirror mode-locked Nd:YVO4 oscillator by an active Q-switch,” Opt. Express12(17), 4041–4046 (2004). [CrossRef] [PubMed]
  9. S. Zhao, G. Li, D. Li, K. Yang, Y. Li, M. Li, T. Li, G. Zhang, and K. Cheng, “Numerical simulation of dual-loss-modulated Q-switched and mode-locked laser with an acousto-optic and Cr4+:YAG saturable absorber,” Appl. Opt.49(10), 1802–1808 (2010). [CrossRef] [PubMed]
  10. M. Li, S. Zhao, K. Yang, G. Li, D. Li, J. Wang, J. An, and W. Qiao, “Actively Q-switched and mode-locked diode-pumped Nd:GdVO4-KTP laser,” IEEE J. Quantum Electron.44(3), 288–293 (2008). [CrossRef]
  11. C. Cuadrado-Laborde, A. Díez, J. L. Cruz, and M. V. Andrés, “Doubly active Q switching and mode locking of an all-fiber laser,” Opt. Lett.34(18), 2709–2711 (2009). [CrossRef] [PubMed]
  12. J. K. Jabczyński, W. Zendzian, and J. Kwiatkowski, “Q-switched mode locking with acousto-optic modulator in a diode-pumped Nd:YVO4 laser,” Opt. Express14(6), 2184–2190 (2006). [CrossRef] [PubMed]
  13. C. Theobald, M. Weitz, R. Knappe, R. Wallenstein, and J. A. L’Huillier, “Stable Q-switch mode-locking of Nd:YVO4 lasers with a semiconductor saturable absorber,” Appl. Phys. B92(1), 1–3 (2008). [CrossRef]
  14. Y.-F. Chen and S. W. Tsai, “Simultaneous Q-switching and mode-locking in a diode-pumped Nd:YVO4-Cr4+:YAG laser,” IEEE J. Quantum Electron.37(4), 580–586 (2001). [CrossRef]
  15. J.-H. Lin, K.-H. Lin, C.-C. Hsu, W. H. Yang, and W.-F. Hsieh, “Supercontinuum generation in a microstructured optical fiber by picosecond self Q-switched mode-locked Nd:GdVO4 laser,” Laser Phys. Lett.4(6), 413–417 (2007). [CrossRef]
  16. D. W. Zheng, B. T. Smith, and M. Asghari, “Improved efficiency Si-photonic attenuator,” Opt. Express16(21), 16754–16765 (2008). [CrossRef] [PubMed]
  17. Y. M. Chang, J. Lee, Y. M. Jhon, and J. H. Lee, “Active Q-switching in an erbium-doped fiber laser using an ultrafast silicon-based variable optical attenuator,” Opt. Express (under revision for final decision, Oct. 2011).
  18. J.-H. Lin, H.-R. Chen, H.-H. Hsu, M.-D. Wei, K.-H. Lin, and W.-F. Hsieh, “Stable Q-switched mode-locked Nd3+:LuVO4 laser by Cr4+:YAG crystal,” Opt. Express16(21), 16538–16545 (2008). [PubMed]

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