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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 17 — Aug. 18, 2008
  • pp: 13405–13413
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DWDM channel spacing tunable optical TDM carrier from a mode-locked weak-resonant-cavity Fabry-Perot laser diode based fiber ring

Guo-Hsuan Peng, Yu-Chieh Chi, and Gong-Ru Lin  »View Author Affiliations


Optics Express, Vol. 16, Issue 17, pp. 13405-13413 (2008)
http://dx.doi.org/10.1364/OE.16.013405


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Abstract

A novel optical TDM pulsed carrier with tunable mode spacing matching the ITU-T defined DWDM channels is demonstrated, which is generated from an optically injection-mode-locked weak-resonant-cavity Fabry-Perot laser diode (FPLD) with 10%-end-facet reflectivity. The FPLD exhibits relatively weak cavity modes and a gain spectral linewidth covering >33.5 nm. The least common multiple of the mode spacing determined by both the weak-resonant-cavity FPLD and the fiber-ring cavity can be tunable by adjusting length of the fiber ring cavity or the FPLD temperature to approach the desired 200GHz DWDM channel spacing of 1.6 nm. At a specific fiber-ring cavity length, such a least-common-multiple selection rule results in 12 lasing modes between 1532 and 1545 nm naturally and a mode-locking pulsewidth of 19 ps broadened by group velocity dispersion among different modes. With an additional intracavity bandpass filter, the operating wavelength can further extend from 1520 to 1553.5 nm. After channel filtering, each selected longitudinal mode gives rise to a shortened pulsewidth of 12 ps due to the reduced group velocity dispersion. By linear dispersion compensating with a 55-m long dispersion compensation fiber (DCF), the pulsewidth can be further compressed to 8 ps with its corresponding peak-to-peak chirp reducing from 9.7 to 4.3 GHz.

© 2008 Optical Society of America

1. Introduction

2. Experiment setup

The trace (a) in Fig. 1 illustrates a typical backward-optical-injection mode-locked SOAFL with a cavity length of 14 m reported previously,[13

13. G-R Lin, I-H Chiu, and M-C Wu, “1.2-ps mode-locked semiconductor optical amplifier fiber laser pulses generated by 60-ps backward dark-optical comb injection and soliton compression,” Opt. Express 13, 1008–1014 (2005). [CrossRef] [PubMed]

] which consists of one traveling-wave type SOA with central gain peak at 1530 nm, an optical circulator, a faraday isolator, an optical tunable band-pass filter (OBPF), and an output coupler (OC) with a power-splitting ratio of 50%. The SOA was DC-biased at 225 mA (well above its transparent threshold). The SOA with a broadened gain spectrum plays both the roles of a gain medium and an optically controlled modulator in this work. In contrast to the conventional mode-locking SOAFL, we employ a two-port FPLD of same cavity length but with the front and back facet reflectivities of 10%. The weak-resonant-cavity FPLD is generally working as a semiconductor optical amplifier in connection with a Fabry-Perot etalon filter of relatively low finesse. Such a weak Fabry-Perot resonant-cavity essentially results in an additional longitudinal mode selecting mechanism, which forces the rational harmonic mode-locking occurred at a least multiplication harmonic frequency of the FPLD and the fiber ring link, as shown in Fig. 1(b). The FPLD was DC-biased at 280 mA and 22°C. To backward optical-inject the FPLD for harmonic mode-locking, tunable laser (TL, Agilent, 8164A) operated at 1555.7 nm and 25°C was amplified by a 20dB-gain erbium-doped fiber amplifier (EDFA), and subsequently modulated by a Mach-Zehnder intensity modulator (MZM) nonlinearly driven with an electrical comb generator (COMB) triggered by an amplified sinusoidal wave at 10 GHz to form a dark-optical-comb. In our approach, such a dark-optical comb with comb pulsewidth of 32.4 ps and duty-cycle of 31 % at 1555.7 nm can be generated. The dark-optical-comb power was adjusted by a tunable attenuator (TATT) to optimize the gain depletion of FPLD. The backward injection of such a dark-optical-comb via appropriate control of its polarization with a polarization controller (PC) prior to the optical circulator results in a temporally sliced gain window within one modulation period to shrink the modulation window and to optimize the mode-locking of the FPLD based fiber ring laser.

Fig. 1. The schematic diagrams of backward optical-injection mode-locked (a) SOA and (b) weak-resonant-cavity FPLD.
Fig. 2. Output spectrum of the mode-locked weak-resonant-cavity FPLD fiber ring laser.

3. Results and Discussion

Fig. 3. The output spectrum: with FP etalon (black curve) and without FP etalon (red curve).

In comparison with the single-channel filtered output, the multi-channel pulsed carrier exhibit a slightly pulsewidth large broadened from 12 to 19 ps detected by auto-correlator, as shown in Fig. 4. Such a configuration with a simplified setup further benefit from the advantage of a flexible mode-spacing tunability as learned from the conventional figure-of-eight cavity dual fiber-ring laser. Under the control of either the FPLD temperature or the fiber-ring cavity length, the least-common multiplied longitudinal mode (or channel) spacing can easily be detuned to meet the demand of DWDM channeling. Typically, the synthesizer with its frequency detuned to a specific value also acts as a wavelength locker to mode-lock a desired single wavelength mode in the harmonic mode-locked FPLD fiber link. Nevertheless, the typical approach of detuning synthesizer frequency could lead to the reduction on the lasing modes of mode-locked weak-resonant-cavity FPLD fiber laser from 10 to 2 modes. Since the slightly frequency detuning has inevitably caused a mismatch of some side-modes with the mode-locked harmonic resonant frequency. As a result, these phase-mismatched side-modes travelling few round trips would be depleted by the fiber ring cavity loss.

Fig. 4. Single-channel output with filter (red curve) and multi-channel output without filter (black curve)
Fig. 5. Dynamic frequency chirp of a multi-channel pulse.

Since each mode in the weak-resonant-cavity FPLD fiber ring laser experiences different dispersion in fiber ring cavity, the more modes obtained the larger group velocity dispersion induced pulse broadening effect in fiber ring cavity would occur. In this case, such a simultaneously multi-channel operation inevitably lengthens the mode-locked FPLD fiber ring laser pulsewidth without channel filtering. Alternatively, the detuning on the channel wavelength can be done by using an intra-cavity polarization controller to change the feedback power from the fiber ring to the FPLD cavity, as the increasing feedback to the weak-resonant-cavity FPLD also means the increasing reflectivity [6

6. T. M. Liu, H. H. Chang, S. W. Chu, and C. K. Sun, “Locked Multichannel Generation and Management by Use of a Fabry-Perot Etalon in a Mode-Locked Cr:Forsterite Laser Cavity,” IEEE J. Quantum Electron. 38, 458–463 (2002). [CrossRef]

]. In principle, the strong feedback tends to saturate the FPLD, leading to the gain compression and a shift of the gain peak toward the longer wavelength [14–15

14. M. J. Connelly, “Wideband semiconductor optical amplifier steady-state numericalmodel Quantum Electron,” IEEE J. Quantum Electron. 37, 439–447 (2001). [CrossRef]

]. That is, the polarization controller can further function as a band selector in our system, which can detune the harmonic mode-locking spectrum toward longer wavelength by changing the feedback. Concurrently, the modes of FPLD fiber ring laser become more discriminated with increasing feedback power except their red-shifted wavelength.

Fig. 6. The changed of mode-locked FPLD fiber ring laser pulse shape when increasing the DCF length. (oscilloscope)
Fig. 7. Auto correlation traces of the mode-locked weak-resonant-cavity FPLD fiber-ring laser dispersion compensated at different DCF lengths.

Before dispersion compensation, the mode-locked FPLD fiber ring laser pulse induced a negative chirp (C<0) ranging from +2 to -13 GHz within the 20-ps wide pulse, as shown in Fig. 5, which can be linearly compensated by a segment of dispersion compensation fiber (DCF) with positive group velocity dispersion (GVD) parameter (β2) based on β2C<0 condition[16

16. G. P. Agrawal, Nonlinear Fiber Optics (Academic New York, 1989).

]. Both the harmonic mode-locked FPLD fiber-ring pulse shapes obtained by digital sampling oscilloscope and auto-correlator are shown in Figs. 6 and 7, respectively, for determining the variation on the rising and falling edges of the pulse, and the exact pulsewidth after the linear dispersion compensation by DCF is analyzed. With increasing DCF length, both the rising and falling edges of the FPLD fiber ring laser pulse are steepened, as shown in Fig. 6. In particular, the falling edge of the negatively chirped pulse is shortened more faster than the rising edges with increasing DCF length, as the long-wavelength mode experiences a larger dispersion constant during propagating in the DCF (i.e. the DCF exhibits a negative dβ2/dλ at C and L bands). However, such a steepening phenomenon disappears when introducing the DCF with its length longer than 55 m, while the rising edge turns to be slackened faster than the falling edge due to the overcompensation process. As a result, the auto-correlated pulsewidth rapidly shrinks from 19 to 8 ps as the DCF length increases up to 55 m, whereas the pulse greatly broadens to 22 ps with over-length DCF compensation. Without proper dispersion compensation, the corresponding time-bandwidth product of the mode-locked FPLD fiber ring pulse is 2.43 due to the extremely large chirp existed in such a multi-channel pulse.

Fig. 8. Dynamic frequency chirp vs. DCF length.
Fig. 9. Dispersion compensated pulsewidth vs. DCF length.

Fig. 10. Maximum coarse wavelength tuning range and single-mode spectra of the mode-locked FPLD based fiber ring link.
Fig. 11. Corresponding pulsewidth of the mode-locked FPLD based fiber ring link under a coarse wavelength detuning.
Fig. 12. Fine detuning 200GHz DWDM channel spectra of the mode-locked FPLD based fiber ring link.
Fig. 13. Corresponding pulsewidth of the mode-locked FPLD based fiber ring link under a 200GHz DWDM channel filtering.

4. Conclusion

We demonstrate a novel optical TDM pulsed carrier with tunable mode spacing matching ITU-T DWDM channels generating from optically injection-mode-locked weak-resonant-cavity Fabry-Perot laser diode (FPLD) with 10%-end-facet reflectivity. The FPLD exhibits relatively weak cavity modes and a gain spectrum covering >33.5 nm. The least common multiple of the mode-spacings of the weak-resonant-cavity FPLD and the fiber-ring is tunable by adjusting length of the fiber ring cavity to approach the desired DWDM channel spacing of 1.6 nm. At a specific fiber cavity length, such a least-multiple selection rule between the longitudinal modes of fiber cavity ring and FPLD cavity results in 12 lasing modes between 1532 and 1545 nm naturally and a mode-locking pulsewidth of 19 ps broadened by group velocity dispersion at different modes. In our experiment, the mode-locking of such a weak-resonant-cavity FPLD based fiber ring laser generate TDM carriers with their mode-spacing matching the 200 GHz DWDM channels without adding intra-cavity filter. Furthermore, a broadband and coarse wavelength tuning range from 1520 to 1553.5 nm can also be achieved by inserting band-pass filter. In comparison with multi-channel pulse, the single-channel filtered output shortens the pulsewidth from 19 to 12 ps. After linear dispersion compensation with DCF, the negatively chirped pulse can further be compressed to 8 ps with associated frequency chirp reducing from 9.7 to 4.3 GHz at an optimized DCF length of 55 m.

5. Acknowledgment

The authors thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under grants NSC-96-2221-E-002-099.

References and links

1.

D. Pudo, L. R. Chen, D. Giannone, L. Zhang, and I. Bennion, “Actively mode-locked tunable dual-wavelength erbium-doped fiber laser,” IEEE Photon. Technol. Lett. 14, 143–145 (2002). [CrossRef]

2.

J. N. Maran, S. LaRochelle, and P. Besnard, “Erbium-doped fiber laser simultaneously mode locked on more than 24 wavelengths at room temperature,” Opt. Lett. 28, 2082–2084 (2003). [CrossRef] [PubMed]

3.

J. Yao, J. P. Yao, and Z. C. Deng, “Multiwavelength actively mode-locked fiber ring laser with suppressed homogeneous line broadening and reduced supermode noise,” Opt. Express. 12, 4529–4534 (2004). [CrossRef] [PubMed]

4.

D. S. Moon, U. C. Paek, and Y Chung, “Multi-wavelength lasing oscillations in an erbium-doped fiber laser using few-mode fiber Bragg grating,” Opt Express 12, 6147–6152 (2004). [CrossRef] [PubMed]

5.

K. Vlachos, K. Zoiros, T. Houbavlis, and H. Avramopoulos, “10×30 GHz Pulse Train Generation from Semiconductor Amplifier Fiber Ring Laser,” IEEE Photon. Technol. Lett. 12, 25–27 (2000). [CrossRef]

6.

T. M. Liu, H. H. Chang, S. W. Chu, and C. K. Sun, “Locked Multichannel Generation and Management by Use of a Fabry-Perot Etalon in a Mode-Locked Cr:Forsterite Laser Cavity,” IEEE J. Quantum Electron. 38, 458–463 (2002). [CrossRef]

7.

C. G. Lee and C. S. Park, “Suppression of Pulse Shape Distortion Caused by Frequency Drift in a Harmonic Mode-Locked Semiconductor Ring Laser,” IEEE Photon. Technol. Lett. 15, 658–660 (2003). [CrossRef]

8.

K. Vlachos, C. Bintjas, N. Pleros, and H. Avramopoulos, “Ultrafast Semiconductor-Based Fiber Laser Sources,” IEEE J. Sel. Top. Quantum Electron. 10, 147–154 (2004). [CrossRef]

9.

W. W. Tang, M. P. Fok, and C. Shu, “10 GHz pulses generated across a ~100 nm tuning range using a gain-shifted mode-locked SOA ring laser,” Opt. Express. 14, 2158–2163 (2006). [CrossRef] [PubMed]

10.

J. Vasseur, M. Hanna, J. Dudley, J-P. Goedgebuer, J. Yu, G-K. Chang, and J. R. Barry, “Alternate Multiwavelength Picosecond Pulse Generation by Use of an Unbalanced Mach-Zehnder Interferometer in a Mode-locked Fiber Ring Laser,” IEEE J. Quantum Electron. 43, 85–96 (2007). [CrossRef]

11.

W. Zhang, J. Sun, J. Wang, and L. Liu, “Multiwavelength Mode-Locked Fiber-Ring Laser Based on Reflective Semiconductor Optical Amplifiers,” IEEE Photon. Technol. Lett. 19, 1418–1420 (2007). [CrossRef]

12.

J. Yang, S. C. Tjin, and N. Q. Ngo, “Multiwavelength actively mode-locked fiber laser with a double-ring configuration and integrated cascaded sampled fiber Bragg gratings,” Opt. Fiber Technol. 13, 267–270 (2007). [CrossRef]

13.

G-R Lin, I-H Chiu, and M-C Wu, “1.2-ps mode-locked semiconductor optical amplifier fiber laser pulses generated by 60-ps backward dark-optical comb injection and soliton compression,” Opt. Express 13, 1008–1014 (2005). [CrossRef] [PubMed]

14.

M. J. Connelly, “Wideband semiconductor optical amplifier steady-state numericalmodel Quantum Electron,” IEEE J. Quantum Electron. 37, 439–447 (2001). [CrossRef]

15.

F. W. Tong, W. Lin, D. N. Wang, and P. K. A. Wai, “Multiwavelength fibre laser with wavelength selectable from 1590 to 1645 nm,” Electron. Lett. 40, 594–595 (2004). [CrossRef]

16.

G. P. Agrawal, Nonlinear Fiber Optics (Academic New York, 1989).

OCIS Codes
(140.4050) Lasers and laser optics : Mode-locked lasers
(320.1590) Ultrafast optics : Chirping
(060.3510) Fiber optics and optical communications : Lasers, fiber
(250.5960) Optoelectronics : Semiconductor lasers

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: May 28, 2008
Revised Manuscript: July 20, 2008
Manuscript Accepted: July 24, 2008
Published: August 15, 2008

Citation
Guo-Hsuan Peng, Yu-chieh Chi, and Gong-Ru Lin, "DWDM channel spacing tunable optical TDM carrier from a mode-locked weak-resonant-cavity Fabry-Perot laser diode based fiber ring," Opt. Express 16, 13405-13413 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-17-13405


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References

  1. D. Pudo, L. R. Chen, D. Giannone, L. Zhang, and I. Bennion, "Actively mode-locked tunable dual-wavelength erbium-doped fiber laser," IEEE Photon. Technol. Lett. 14,143-145(2002). [CrossRef]
  2. J. N. Maran, S. LaRochelle, and P. Besnard, "Erbium-doped fiber laser simultaneously mode locked on more than 24 wavelengths at room temperature," Opt. Lett. 28, 2082-2084(2003). [CrossRef] [PubMed]
  3. J. Yao, J. P. Yao, and Z. C. Deng, "Multiwavelength actively mode-locked fiber ring laser with suppressed homogeneous line broadening and reduced supermode noise," Opt. Express. 12,4529-4534 (2004). [CrossRef] [PubMed]
  4. D. S. Moon, U. C. Paek, and Y Chung, "Multi-wavelength lasing oscillations in an erbium-doped fiber laser using few-mode fiber Bragg grating," Opt Express 12, 6147-6152 (2004). [CrossRef] [PubMed]
  5. K. Vlachos, K. Zoiros, T. Houbavlis, and H. Avramopoulos, "10 �? 30 GHz Pulse Train Generation from Semiconductor Amplifier Fiber Ring Laser," IEEE Photon. Technol. Lett. 12, 25-27 (2000). [CrossRef]
  6. T. M. Liu, H. H. Chang, S. W. Chu, and C. K. Sun, "Locked Multichannel Generation and Management by Use of a Fabry-Perot Etalon in a Mode-Locked Cr:Forsterite Laser Cavity," IEEE J. Quantum Electron. 38, 458-463 (2002). [CrossRef]
  7. C. G. Lee and C. S. Park, "Suppression of Pulse Shape Distortion Caused by Frequency Drift in a Harmonic Mode-Locked Semiconductor Ring Laser," IEEE Photon. Technol. Lett. 15, 658-660 (2003). [CrossRef]
  8. K. Vlachos, C. Bintjas, N. Pleros, and H. Avramopoulos, "Ultrafast Semiconductor-Based Fiber Laser Sources," IEEE J. Sel. Top. Quantum Electron. 10, 147-154 (2004). [CrossRef]
  9. W. W. Tang, M. P. Fok, and C. Shu, "10 GHz pulses generated across a ~100 nm tuning range using a gain-shifted mode-locked SOA ring laser," Opt. Express. 14, 2158-2163 (2006). [CrossRef] [PubMed]
  10. J. Vasseur, M. Hanna, J. Dudley, J-P. Goedgebuer, J. Yu, G-K. Chang, and J. R. Barry, "Alternate Multiwavelength Picosecond Pulse Generation by Use of an Unbalanced Mach-Zehnder Interferometer in a Mode-locked Fiber Ring Laser," IEEE J. Quantum Electron. 43, 85-96 (2007). [CrossRef]
  11. W. Zhang, J. Sun, J. Wang, and L. Liu, "Multiwavelength Mode-Locked Fiber-Ring Laser Based on Reflective Semiconductor Optical Amplifiers," IEEE Photon. Technol. Lett. 19, 1418-1420 (2007). [CrossRef]
  12. J. Yang, S. C. Tjin, N. Q. Ngo, "Multiwavelength actively mode-locked fiber laser with a double-ring configuration and integrated cascaded sampled fiber Bragg gratings," Opt. Fiber Technol. 13, 267-270 (2007). [CrossRef]
  13. G-R Lin, I-H Chiu, and M-C Wu, "1.2-ps mode-locked semiconductor optical amplifier fiber laser pulses generated by 60-ps backward dark-optical comb injection and soliton compression," Opt. Express 13, 1008-1014 (2005). [CrossRef] [PubMed]
  14. M. J. Connelly,"Wideband semiconductor optical amplifier steady-state numericalmodel Quantum Electron," IEEE J. Quantum Electron. 37, 439-447 (2001). [CrossRef]
  15. F. W. Tong, W. Lin, D. N. Wang, and P. K. A. Wai, "Multiwavelength fibre laser with wavelength selectable from 1590 to 1645 nm," Electron. Lett. 40, 594-595 (2004). [CrossRef]
  16. G. P. Agrawal, Nonlinear Fiber Optics (Academic New York, 1989).

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