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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 15 — Jul. 23, 2007
  • pp: 9849–9858
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A simple and low-power optical limiter for multi-GHz pulse trains

G. Contestabile, M. Presi, R. Proietti, N. Calabretta, and E. Ciaramella  »View Author Affiliations


Optics Express, Vol. 15, Issue 15, pp. 9849-9858 (2007)
http://dx.doi.org/10.1364/OE.15.009849


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Abstract

We study the limiting-amplification capability of a saturated Semiconductor Optical Amplifier (SOA) followed by an optical band-pass filter. We experimentally demonstrate that this simple optical circuit can be effectively exploited to realize a low-power optical limiter for amplitude-modulated pulse trains at multi-GHz repetition rate. We report very large amplitude-modulation-reduction factors for the case of 20 and 40 GHz pulse trains that are super-imposed with modulating frequencies ranging from 100kHz to several GHz.

© 2007 Optical Society of America

1. Introduction

Fig. 1. Scheme of the limiting amplification in a saturated SOA.

2. Working principle

Fig. 2. Typical evolution of the optical spectrum and of the oscilloscope trace (in persistence mode) of an over-modulated 20 GHz pulse train inside the limiting circuit.

The bandwidth of such filter must be so large to select only part of the output spectrum. As it is reported in the following sections, this band-pass filtering process has a twofold action both on the output pulse shape and on the frequency response of the process. By selecting part of the chirped spectrum it determines the pulse shape and, at the same time, by removing part of the spectral components it speeds up the process [11

11. Y. Liu, E. Tangdiongga, Z. Li, S. Zhang, H. de Waardt, G. D. Khoe, and H. J. S. Dorren, “Error-free all-optical wavelength conversion at 160 gb/s using a semiconductor optical amplifier and an optical bandpass filter,” J. Lightwave Technol. 24, 230–236 (2006). [CrossRef]

]. To summarize those concepts we report in Fig. 2 an example of the evolution of the optical spectrum and of the corresponding pulse trace through the limiting circuit. In this case a 20 GHz input pulse sequence is affected by a certain amplitude modulation. After the SOA, the modulation is practically removed, but the pulses are distorted as a consequence of the spectral chirp. The output filter selects only part of the spectrum reformatting the pulses.

Fig. 3. Behavior of the SOA gain recovery as a function of pulse repetition rate.

Fig. 4. Experimental set-up. TL: Tunable Laser, IM: Intensity Modulator, EDFA: Erbium Doped Fiber Amplifier, OI: Optical Isolator, SOA: Semiconductor Optical Amplifier, BPF: Band Pass Filter.

This suggests that pulse trains with repetition rate even exceeding 100 GHz could be managed although at the cost of a weaker limiting capability [12

12. Y. Ueno, S. Nakamura, and K. Tajima, “Nonlinear phase shift induced by semiconductor optical amplifiers with control pulses at repetition frequencies in the 40–160 GHz range for use in ultrahigh-speed all-optical signal processing,” J. Opt.Soc. B 19, 2573–2589 (2002). [CrossRef]

]. Finally, the last critical parameter of the scheme is the wavelength position of the output bandpass filter in respect to the input signal. Indeed, the combination of SOA and the filter provides a threshold-like input/output transfer function. We will see in the following how this transfer function depends on the offset of the filter with respect to the input wavelength. On the other hand, the filter position determines also the shape of the output pulses. We found that a good trade-off between the limiting effect and proper pulse shape can be obtained setting the filter maximum close to the input wavelength.

3. Experiment and discussion

The experimental set-up used for the power limiter characterization is reported in Fig. 4. An optical carrier at λ=1551.5 nm was generated by a continuous wave tunable laser. The pulses were carved by means of an intensity modulator driven by an electrical waveform generator. We used a LiNbO3 Mach-Zehnder modulator to generate the 20 GHz pulse trains (having 20 ps duration), and an Electro Absorption Modulator for the 40 GHz case obtaining 6 ps-long pulses. The amplitude modulation was superimposed by means of an additional intensity modulator driven by a sinusoidal signal from a wide-band waveform generator. Both the modulation depth and the frequency of this over modulating signal were tuned by adjusting the modulator bias and changing the frequency of the electrical waveform generator. The two electrical waveform generators were free running (not synchronized each other) in order to have no fixed phase relation. This condition guarantees to have an effective over modulation also for the high frequencies (in the GHz range) that are comparable with the pulse repetition rate. The signal was then amplified by an erbium doped fiber amplifier and power controlled by means of a variable optical attenuator. The limiting circuit was composed by an optical isolator, an SOA and a 0.8 nm tunable band-pass filter.

Fig. 5. (a). Typical static transfer function of the limiting circuit at varying the output band-pass filter position. b) Detail of the transfer function.

The SOA was a polarization insensitive pigtailed device with 28 dB small signal gain, around 200 ps full gain recovery time and 6 dBm output saturation power at 200 mA driving current. The output signal was analyzed by usual test instruments. In order to characterize the circuit, we measured the input-output static transfer function for different filter positions, i.e., the output power of un-modulated pulses at various average input power values.

Fig. 6. AMR vs. SOA average input power for 20 GHz pulse trains overmodulated at 5 GHz at 50 and 80% modulation depths.

Fig. 7. (a). AMR vs. modulation frequency for 20 GHz pulse trains with 50 and 80% modulation depths. b) Input/output evolution of 80%-modulated pulse train (at 200 MHz modulation frequency).

Fig. 8. (a). AMR vs. modulation frequency for 20 GHz pulse trains with 80% modulation depth with and without output filter. b) Detail in linear scale.

Fig. 9. (a). AMR vs. modulation frequency for 40 GHz pulse trains with 50 and 80% modulation depths. b) Input/output evolution of 80%-modulated pulse train (at 100 kHz modulation frequency).

In Fig. 9(b) we report, as an example, the input/output traces of 40 GHz pulses that are 80% overmodulated at 100 kHz. In this case the AMR is around 27 dB and it has the effect to completely reshape the pulses.

Fig. 10. Comparison of the AMR response in the GHz range with and without output filter.

The comparison in linear scale of the high frequency response for 80% modulation depth in the optimum and without filter cases is reported in Fig. 10. Also here, clear improvement of the AMR can be addressed to the spectral selection of the output signal operated by the filter.

Fig. 11. AMR vs. modulation depth for 20 and 40 GHz pulse trains at 5 GHz overmodulation frequency.

We characterized the limiting effect also as a function of the modulation depth. The results for 20 and 40 GHz pulses are summarized in Fig. 11. We fixed the modulating frequency at 5 GHz and adjusted the modulation depth changing the bias of the second intensity modulator. We found that the AMR response trend is similar for both cases and, as expected, it is larger for the 20 GHz case. It is nearly flat and starts to decline once exceeded the 80%. According with the thresholding description, this indicates that the efficiency of the effect reduces when the modulation depth is so large that the lowest pulses in the sequence start to approach the threshold level. We report in Fig. 12 two examples at 40 GHz that clearly show the effectiveness of the scheme in extreme cases. In Fig. 12(a) we show the traces of a very deep modulation (86%) at 5 GHz, which exceeds the flat response part of Fig. 11.

Fig. 12. Input/output evolution of 40 GHz pulse trains modulated with a) 86% modulation depth at 5 GHz; b) 58% modulation depth at 25 GHz.

The output signal shows that a significant, yet not complete, limiting effect still occurs, reducing the modulation to around 52%. In Fig. 12(b) we report the case of a significant modulation (58%) at a very high frequency (25 GHz). We found a quite good output signal with around 20% residual modulation.

In order to further characterize the filtering effect both on the shape of the output pulses and on the efficiency of the effect, we performed a systematic study using 40 GHz pulse trains for a fixed over-modulation condition (80% depth at 100 MHz). The results are reported in Fig. 13. We recorded both the AMR and the pulse shape while sliding the position of the output filter. We observed that moving from the red to the blue-shifted side, the AMR significantly increases at the expenses of pulse shape degradation. When the filter selects the red-shifted part of the spectrum the output pulses are shorter and better shaped (so that pulse compression can potentially be obtained [13

13. M.L. Nielsen, B.-E. Olsson, and D.J. Blumenthal, “Pulse extinction ratio improvement using SPM in an SOA for OTDM systems applications,” IEEE Photon. Technol. Lett. 14, 245–247 (2002). [CrossRef]

]), but with a lower limiting effect (note the residual amplitude modulation on the pulse in the inset).

Fig. 13. AMR vs. filter detuning for 40 GHz pulse trains 80%-modulated at 100 MHz. In the insets, examples of corresponding oscilloscope traces.

Symmetric pulses having larger AMR factors are obtained for filter positions closed to the center input wavelength, whilst the largest AMRs result for blue-shifted positions. In this last case distortions of the pulses and extinction ratio degradation are apparent. Following from those considerations, we see that we can balance in a very simple way (tuning the filter position) the strength of the limiting effect and the quality of the output pulses. This means that this limiting circuit can be flexibly adapted depending on the application.

Fig. 14. Comparison of the Single Side Band Noise Spectrum of 20 GHz pulse train without overmodulation at the input and output of the limiting circuit.

Finally, to analyze the ultimate limiting capability of the circuit, we performed an extremely sensitive characterization of the effect by studying the input/output phase noise evolution of an un-modulated 20 GHz pulse train. The results are reported in Fig. 14. We compare the Single Side Band (SSB) spectrum around the 20 GHz electrical spectrum clock line for input un-modulated and output pulses. This SSB spectrum contains the phase noise contributions due to amplitude and time jitter of the transmitter [14

14. U. Keller, K. D. Li, M. J. W. Rodwell, and D. M. Bloom, “Noise characterization of femtosecond fiber Raman soliton lasers,” IEEE J. Quantum Electron. 25, 280–288 (1989). [CrossRef]

]. The plots clearly show an improvement of the signal phase noise also in this limit case. In detail, there is a slight reduction of the phase noise at any frequency with a significant effect (more than 10 dB) in the range 20 to 100 kHz.

4. Conclusion

In this work we experimentally characterized a very simple and flexible power limiting circuit made by a saturated SOA and a band-pass filter. We measured the amount of amplitude modulation reduction that the circuit produces working on 20 and 40 GHz over-modulated pulse trains (at varying the modulation frequencies and the modulation depths). We showed that this circuit definitely acts as a high pass filter with a cut-off frequency of around 3 GHz. Nevertheless, a modulation reduction factor of around 10 dB have been measured at much higher frequencies due to the fast recovery dynamics in the semiconductor material. This guarantees operation well beyond the cut-off frequency. Moreover, we believe that using novel quantum dot based SOAs [15

15. S. DommersV. V. TemnovU. WoggonJ. GomisJ. Martinez-PastorM. LaemmlinD. Bimberg “Gain dynamics after ultrashort pulse trains in quantum dot based semiconductor optical amplifiers” in Conference on Lasers and Electro-Optics 2007 Technical Digest (Optical Society of America, Washington, DC, 2007) CMM4

] showing ultrafast recovery time (<5ps) it is possible to realize simple limiting circuits having ultra-large cut-off frequencies (>100 GHz) and working with repetition rates exceeding 200 GHz [15

15. S. DommersV. V. TemnovU. WoggonJ. GomisJ. Martinez-PastorM. LaemmlinD. Bimberg “Gain dynamics after ultrashort pulse trains in quantum dot based semiconductor optical amplifiers” in Conference on Lasers and Electro-Optics 2007 Technical Digest (Optical Society of America, Washington, DC, 2007) CMM4

].

Acknowledgment

This work was partially supported by the European Commission FP6 program (Integrated Project NOBEL II).

References and links

1.

J. Leuthold, W. Freude, G. Boettger, J. Wang, A. Marculescu, P. Vorreau, and R. Bonk, “All-Optical Regeneration,” International Conference on Transparent Optical Networks (IEEE, New York, 2006) 28–31.

2.

S. Nakamura, Y. Ueno, and K. Tajima, “168-Gb/s all-optical wavelength conversion with a symmetric-Mach-Zehnder-type switch,” IEEE Photon. Technol. Lett. 13, 1091–1093 (2001). [CrossRef]

3.

M. Attygalle, A. Nirmalathas, and H. F. Liu, “Novel technique for reduction of amplitude modulation of pulse trains generated by subharmonic synchronous mode-locked,” IEEE Photon. Technol. Lett. 14, 543–545 (2002). [CrossRef]

4.

K. Vlachos, G. Theophilopoulos, A. Hatziefremidis, and H. Avramopoulos, “30 Gb/s all-optical clock recovery circuit,” IEEE Photon. Technol. Lett. 12, 705–707 (2000). [CrossRef]

5.

G. Contestabile, M. Presi, N. Calabretta, and E. Ciaramella, “All-optical clock recovery for NRZ-DPSK signals,” IEEE Photon. Technol. Lett. 18, 2544–2546 (2006). [CrossRef]

6.

G. Contestabile, M. Presi, N. Calabretta, and E. Ciaramella, “All-optical clock recovery from 40 Gbit/s NRZ signal based on clock line enhancement and sharp periodic filtering,” Electron. Lett. 40, pp. 1361–1362 (2004) [CrossRef]

7.

C. Kouloumentas, A. Tzanakaki, and I. Tomkos, “Clock recovery at 160 Gb/s and beyond using a fiber-based optical power limiter,” IEEE Photon. Technol. Lett. 18, 2365–2367 (2006). [CrossRef]

8.

N. Pleros, C. Bintjas, G.T. Kanellos, K. Vlachos, H. Avramopoulos, and G. Guekos, “Recipe for intensity modulation reduction in SOA-based interferometric switches,” J. Lightwave Technol. 22, 2834–2841 (2004) [CrossRef]

9.

M. Presi, N. Calabretta, G. Contestabile, and E. Ciaramella, “Wide dynamic range all-optical clock and data recovery from preamble-free NRZ-DPSK packets,” IEEE Photon. Technol. Lett. 19, 372–374 (2007). [CrossRef]

10.

G. P. Agrawal and N. A. Olsson, “Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers,” IEEE J. Quantum Electron. 25, 2297–2306 (1989). [CrossRef]

11.

Y. Liu, E. Tangdiongga, Z. Li, S. Zhang, H. de Waardt, G. D. Khoe, and H. J. S. Dorren, “Error-free all-optical wavelength conversion at 160 gb/s using a semiconductor optical amplifier and an optical bandpass filter,” J. Lightwave Technol. 24, 230–236 (2006). [CrossRef]

12.

Y. Ueno, S. Nakamura, and K. Tajima, “Nonlinear phase shift induced by semiconductor optical amplifiers with control pulses at repetition frequencies in the 40–160 GHz range for use in ultrahigh-speed all-optical signal processing,” J. Opt.Soc. B 19, 2573–2589 (2002). [CrossRef]

13.

M.L. Nielsen, B.-E. Olsson, and D.J. Blumenthal, “Pulse extinction ratio improvement using SPM in an SOA for OTDM systems applications,” IEEE Photon. Technol. Lett. 14, 245–247 (2002). [CrossRef]

14.

U. Keller, K. D. Li, M. J. W. Rodwell, and D. M. Bloom, “Noise characterization of femtosecond fiber Raman soliton lasers,” IEEE J. Quantum Electron. 25, 280–288 (1989). [CrossRef]

15.

S. DommersV. V. TemnovU. WoggonJ. GomisJ. Martinez-PastorM. LaemmlinD. Bimberg “Gain dynamics after ultrashort pulse trains in quantum dot based semiconductor optical amplifiers” in Conference on Lasers and Electro-Optics 2007 Technical Digest (Optical Society of America, Washington, DC, 2007) CMM4

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(190.7110) Nonlinear optics : Ultrafast nonlinear optics
(230.1150) Optical devices : All-optical devices

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 23, 2007
Revised Manuscript: May 30, 2007
Manuscript Accepted: June 27, 2007
Published: July 20, 2007

Citation
G. Contestabile, M. Presi, R. Proietti, N. Calabretta, and E. Ciaramella, "A simple and low-power optical limiter for multi-GHz pulse trains," Opt. Express 15, 9849-9858 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-15-9849


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References

  1. J. Leuthold, W. Freude, G. Boettger, J. Wang, A. Marculescu, P. Vorreau, and R. Bonk, "All-Optical Regeneration," International Conference on Transparent Optical Networks (IEEE, New York, 2006) 28 - 31.
  2. S. Nakamura, Y. Ueno, and K. Tajima, "168-Gb/s all-optical wavelength conversion with a symmetric-Mach-Zehnder-type switch," IEEE Photon. Technol. Lett. 13, 1091 - 1093 (2001). [CrossRef]
  3. M. Attygalle, A. Nirmalathas, and H. F. Liu, "Novel technique for reduction of amplitude modulation of pulse trains generated by subharmonic synchronous mode-locked," IEEE Photon. Technol. Lett. 14, 543 - 545 (2002). [CrossRef]
  4. K. Vlachos, G. Theophilopoulos, A. Hatziefremidis, and H. Avramopoulos, "30 Gb/s all-optical clock recovery circuit," IEEE Photon. Technol. Lett. 12, 705-707 (2000). [CrossRef]
  5. G. Contestabile, M. Presi, N. Calabretta, and E. Ciaramella, "All-optical clock recovery for NRZ-DPSK signals," IEEE Photon. Technol. Lett. 18, 2544 - 2546 (2006). [CrossRef]
  6. G. Contestabile, M. Presi, N. Calabretta, and E. Ciaramella, "All-optical clock recovery from 40 Gbit/s NRZ signal based on clock line enhancement and sharp periodic filtering," Electron. Lett. 40, pp. 1361 - 1362 (2004) [CrossRef]
  7. C. Kouloumentas, A. Tzanakaki, and I. Tomkos, "Clock recovery at 160 Gb/s and beyond using a fiber-based optical power limiter," IEEE Photon. Technol. Lett. 18, 2365 - 2367 (2006). [CrossRef]
  8. N. Pleros, C. Bintjas, G.T. Kanellos, K. Vlachos, H. Avramopoulos and, G. Guekos, "Recipe for intensity modulation reduction in SOA-based interferometric switches," J. Lightwave Technol. 22, 2834 - 2841 (2004) [CrossRef]
  9. M. Presi; N. Calabretta; G. Contestabile, and E. Ciaramella, "Wide dynamic range all-optical clock and data recovery from preamble-free NRZ-DPSK packets," IEEE Photon. Technol. Lett. 19, 372 - 374 (2007). [CrossRef]
  10. G. P. Agrawal and N. A. Olsson, "Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers," IEEE J. Quantum Electron. 25, 2297 - 2306 (1989). [CrossRef]
  11. Y. Liu, E. Tangdiongga, Z. Li, S. Zhang, H. de Waardt, G. D. Khoe, and H. J. S. Dorren, "Error-free all-optical wavelength conversion at 160 gb/s using a semiconductor optical amplifier and an optical bandpass filter," J. Lightwave Technol. 24, 230 - 236 (2006). [CrossRef]
  12. Y. Ueno, S. Nakamura, and K. Tajima, "Nonlinear phase shift induced by semiconductor optical amplifiers with control pulses at repetition frequencies in the 40-160 GHz range for use in ultrahigh-speed all-optical signal processing," J. Opt. Soc. B 19, 2573 - 2589 (2002). [CrossRef]
  13. M.L. Nielsen, B.-E. Olsson, and D.J. Blumenthal, "Pulse extinction ratio improvement using SPM in an SOA for OTDM systems applications," IEEE Photon. Technol. Lett. 14, 245 - 247 (2002). [CrossRef]
  14. U. Keller, K. D. Li, M. J. W. Rodwell, and D. M. Bloom, "Noise characterization of femtosecond fiber Raman soliton lasers," IEEE J. Quantum Electron. 25, 280 - 288 (1989). [CrossRef]
  15. S. Dommers, V. V. Temnov, U. Woggon, J. Gomis, J. Martinez-Pastor, M. Laemmlin, and D. Bimberg "Gain dynamics after ultrashort pulse trains in quantum dot based semiconductor optical amplifiers" in Conference on Lasers and Electro-Optics 2007 Technical Digest (Optical Society of America, Washington, DC, 2007) CMM4

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