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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 22 — Nov. 4, 2013
  • pp: 26044–26051
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Dynamic characterization and amplification of sub-picosecond pulses in fiber optical parametric chirped pulse amplifiers

Valentina Cristofori, Zohreh Lali-Dastjerdi, Lars Søgaard Rishøj, Michael Galili, Christophe Peucheret, and Karsten Rottwitt  »View Author Affiliations


Optics Express, Vol. 21, Issue 22, pp. 26044-26051 (2013)
http://dx.doi.org/10.1364/OE.21.026044


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Abstract

We show a first-time demonstration of amplification of 400 fs pulses in a fiber optical parametric amplifier. The 400 fs signal is stretched in time, amplified by 26 dB and compressed back to 500 fs. A significant broadening of the pulses is experimentally shown due to dispersion and limited gain bandwidth both in saturated and unsaturated gain regimes.

© 2013 OSA

1. Introduction

Optical amplification of signals reaching multi-terabit per second per channel is becoming increasingly relevant following the recent demonstrations of single wavelength channel systems at bit rates of 1.28 Tbit/s, 5.1 Tbit/s or even 10.2 Tbit/s [1

1. H. C. Hansen Mulvad, L. K. Oxenløwe, M. Galili, A. T. Clausen, L. Grüner-Nielsen, and P. Jeppesen, “1.28 Tbit/s single-polarisation serial OOK optical data generation and demultiplexing,” Electron. Lett. 45, 280–281, (2009) [CrossRef]

3

3. T. Richter, E. Palushani, C. Schmidt-Langhorst, R. Ludwig, L. Molle, M. Nölle, and C. Schubert, “Transmission of single-channel 16-QAM data signals at Terabaud symbol rates,” J. Lightwave Technol. 30, 504–511 (2012). [CrossRef]

], all using ultra-short pulses with pulse widths on the order of 500 fs. Therefore, a broadband amplification scheme is required for such wide bandwidth signals. Fiber optical parametric amplifiers (FOPAs) relying on highly nonlinear fibers (HNLFs) are promising candidates thanks to their inherent compatibility with fiber optic systems and their ability to provide gain over a broad bandwidth, limited only by fulfillment of the phase matching condition [4

4. G. P. Agrawal, Nonlinear Fiber Optics (Academic, New York, 2006).

].

Fiber optical parametric chirped pulse amplification (FOPCPA) has been proposed and investigated numerically in the femtosecond regime in [5

5. M. Hanna, F. Druon, and P. Georges, “Fiber optical parametric chirped-pulse amplification in the femtosecond regime,” Opt. Express 14, 2783–2790 (2006). [CrossRef] [PubMed]

], followed by a number of demonstrations for picosecond [6

6. D. Caucheteur, E. Bigourd, E. Hugonnot, P. Szriftgiser, A. Kudlinski, M. Gonzalez-Herraez, and A. Mussot, “Experimental demonstration of optical parametric chirped pulse amplification in optical fiber,” Opt. Lett. 35, 1786–1788 (2010). [CrossRef] [PubMed]

,7

7. Y. Zhou, K. K. Y. Cheung, P. C. Chui, and K. K. Y. Wong, “Experimental demonstration of fiber optical parametric chirped-pulse amplification,” Proc. SPIE 7582, 75820T (2010). [CrossRef]

] and sub-picosecond pulses down to 750 fs [8

8. Y. Zhou, Q. Li, K. K. Y Cheung, S. Yang, P. C. Chui, and K. K. Y. Wong, “All-fiber based ultrashort pulse generation and chirped pulse amplification through parametric processes,” IEEE Photon. Technol. Lett. 22, 1330–1332 (2010). [CrossRef]

]. However, the amplification of pulses of duration compatible with Tbaud transmission (i.e. less than 500 fs) has never been demonstrated so far.

In this work, we investigate experimentally the effect of parametric amplification on 400 fs pulses, focusing on the distortion of the amplified pulses for different amplification levels both in the saturated and unsaturated gain regimes. Significant pulse distortion due to dispersion and limited FOPA gain bandwidth is reported. Based on these results, we set the best FOPA configuration to demonstrate, for the first time to our knowledge, the amplification of Tbaud-class pulses in a fiber optical parametric amplifier. After time stretching, the 400 fs pulses are first amplified by 26 dB and then compressed back to 500 fs, showing the compatibility of FOPAs with ultra-high speed pulse amplification.

2. Dynamic characterization of the FOPCPA

In this section the amplification effects on the full-width at half maximum (FWHM) of the pulses and the pulse shape are reported.

2.1. Experimental setup

The experimental setup is shown in Fig. 1(a). A continuous wave (CW) tunable laser source (TLS) is used as pump source (λp = 1554 nm) and the emitted light is subsequently phase modulated using three combined radio-frequencies (RFs) to broaden the pump spectrum in order to increase the stimulated Brillouin scattering (SBS) threshold. The modulation frequencies for SBS suppression (f1 = 66 MHz, f2 = 550 MHz and f3 = 865 MHz) are optimized by minimizing the reflected pump power. The pump is then amplified in an erbium-doped fiber amplifier (EDFA) and a tunable optical bandpass filter (TBPF1) with a 0.8 nm FWHM bandwidth is used to suppress the out-of-band amplified spontaneous emission (ASE) noise introduced by the EDFA. The pump wavelength is chosen only a few nanometers above the HNLF zero-dispersion wavelength (1550.4 nm) in order to have a gain as uniform as possible over a broad bandwidth. On/off CW-gains (i.e. the gain spectra obtained by using a CW source to generate the input signal) of the FOPA under investigation are shown in Fig. 1(c) for two pump input power levels. The vertical line indicates the signal central wavelength used for pulse amplification. It is worth to notice that the signal is centered with respect to one of the lobes in the FOPA gain spectrum for a pump input power of 29 dBm, providing a CW on/off gain of 28 dB.

Fig. 1 (a) Experimental setup of the FOPCPA for the pulses dynamic characterization. (b) Autocorrelation trace of the 400 fs input pulse. (c) CW FOPA gain for two different pump levels (the vertical line indicates the signal central wavelength). (d) compressed output pulse autocorrelation measured with the pump signal switched off.

The signal is generated by a mode-locked figure-of-eight laser (F8L) [9

9. L. Grüner-Nielsen and B. Pálsdóttir, “Highly nonlinear fibers for very wideband supercontinuum generation,” Proc. of SPIE 6873, 68731B (2008). [CrossRef]

], filtered with a 9 nm broadband filter (TBPF2) to generate Gaussian pulses at 1571.6 nm with a FWHM of 400 fs and a 28 MHz repetition rate. The autocorrelation trace of the obtained pulses is shown in Fig. 1(b). The time-bandwidth product (TBP) of the generated pulses is 0.443, corresponding to that of Gaussian transform-limited pulses. An isolator (ISO) is placed at the output of the F8L to prevent back reflected light from affecting the mode-locking. The signal pulses are stretched to approximately 53 ps in 310 m of standard single mode fiber (SSMF) in order to avoid saturation of the FOPA or supercontinuum generation in the HNLF [9

9. L. Grüner-Nielsen and B. Pálsdóttir, “Highly nonlinear fibers for very wideband supercontinuum generation,” Proc. of SPIE 6873, 68731B (2008). [CrossRef]

]. They are then combined with the pump via a wavelength division multiplexer (WDM1) into a 500 m long HNLF, with a non-linear coefficient of 10.7 W−1 ·km−1 and dispersion slope of 0.018 ps/(nm2 ·km). The signal at the output of the HNLF is filtered by WDM2 and then compressed with a 50 m long dispersion compensating fiber (DCF), with a total dispersion of 5.89 ps/nm at 1550 nm. To characterize the broadening of the pulses due only to the amplification, the length of DCF is adjusted to achieve the minimum signal pulse width at its output when the pump is turned off. Figure 1(d) shows the corresponding autocorrelation trace.

2.2. Results

Fig. 2 (a) Pulsewidth of the amplified signal as a function of pump power at the input of the HNLF in the saturated (Ps = −7 dBm) and unsaturated (Ps = −27 dBm) regimes. The insets show selected autocorrelation traces. (b) Corresponding measured on/off gain.
Fig. 3 Optical spectra measured at the output of the HNLF with the pump switched off and on (Pp = 29 dBm) for Ps = −27 dBm and Ps = −7 dBm.
Fig. 4 (a) Pulsewidths of the amplified signal as a function of the signal input average power with a pump power of 29 dBm. The insets show selected autocorrelation traces. (b) Corresponding measured on/off gain.

3. Amplification of 400 fs pulses in the FOPCPA

In this section, the experimental demonstration of fiber optical parametric amplification of 400 fs pulses in the unsaturated regime is reported. The experimental setup is shown in Fig. 5. The differences, with respect to the setup shown in Fig. 1, are the use of a 10 dB coupler and a filtering stage made by a cascade of three broadband filters (TBPF3) instead of WDM1 and WDM2, respectively. The choice of the filtering stage instead of the WDM2 at the output of the HNLF was made to remove an asymmetry in the pulses spectrum, which could induce distortion in the pulses. The filtering stage response is shown in Fig. 6, where the signal spectra at the input and at the output of the the cascade of filters are illustrated with the pump turned off. The FWHM of the filtering stage is approximately 16 nm. The stretching/compression stages are also different in the setup in Fig. 5 with respect to the setup in Fig. 1, since the compression is now optimized when the signal is amplified and the amplification adds chirp to the pulses, as described in Sec. 2.2. The pump wavelength is set at 1554 nm and the pump power is 29 dBm, resulting in an on/off CW-gain of 28 dB. The 400 fs signal is stretched by 309 m of SSMF, amplified by 26 dB and then compressed back to 500 fs by a 50.5 m long DCF. The spectrum of the pulsed signal at the output of the HNLF is shown in Fig. 7 when the pump is either switched off or on. The on/off gain at the signal wavelength is 26 dB, i.e. 2 dB lower compared to the CW gain. This is due to the fact that the amplifier is working close to the saturated regime. It is necessary to find a compromise between the stretching factor and the saturation of the amplifier. A too low stretching factor, i.e. a too high peak power, saturates the amplifier, while a too high stretching factor makes the dispersion management to compress back the pulse after amplification challenging. The filtered amplified signal is also shown in Fig. 7. The peak power of the signal after the filtering stage and the compression is measured to be around 9.3 W for a pulsewidth of 500 fs. The autocorrelation traces measured before the stretching SSMF and after the amplification, filtering and compression in the DCF are shown in Fig. 8 (a) and (b), respectively. The noise on the output trace is due to the high losses in the filtering stage. The measured data was fitted to a Gaussian curve. The compressed pulse is broader with respect to the input pulse by approximately 100 fs. This is believed to be due to two main factors. First, the lack of compensation of high-order dispersion terms adds some chirp to the pulse. In fact, the pulses were stretched by more than two orders of magnitude in the SSMF and even in the dynamic characterization experiment discussed in Sec. 2, it was not possible to completely compress the pulses back to 400 fs, but only down to 450 fs. Since other effects, apart from the dispersion, were not acting on the pulses when the pump was switched off, the difference between the pulses at the input of the stretcher and at the output of the compressor can only be explained by the lack of compensation of higher-order dispersion terms. Secondly, a significant pulse broadening was observed possibly due to the limited gain bandwidth of the FOPA, introducing a frequency filtering effect. This broadening was only partially compensated at the output of the DCF. Furthermore, the filtering stage also contributes to the broadening of the pulse in time due to the reduction of the frequency bandwidth of the filtered signal, affecting the measurements. However, this contribution has been measured to be small and almost completely compensated in the DCF.

Fig. 5 Experimental setup of the FOPCPA for the amplification of the 400 fs pulses with a pump input power of 29 dBm and signal input average power of −27 dBm.
Fig. 6 Optical signal spectra measured at the output of the HNLF (solid line) and at the output of the filtering stage (dash-dot line) with the pump switched off.
Fig. 7 Optical spectra measured at the output of the HNLF with the pump switched off (dashed blue line) and on (solid black line) with a pump power of 29 dBm.
Fig. 8 Autocorrelation traces of the signal (a) at the input of the SSMF, corresponding to a pulsewidth of 400 fs and (b) at the output of the DCF, corresponding to a pulsewidth of 500 fs.

One should note that that the present study of the dynamics of the pulse is performed at a low repetition rate of 28 MHz. At higher repetition rates, the dispersed pulses will overlap and may therefore interact via the nonlinearity of the HNLF. This may result in intra-channel nonlinear effects [10

10. R. J. Essiambre, B. Mikkelsen, and G. Raybon, “Intra-channel cross-phase modulation and four-wave mixing in high-speed TDM systems,” Electron. Lett. 35, 1576–1578 (1999). [CrossRef]

] that will degrade the amplified waveform following compression. A full analysis of this effect is however beyond the scope of this paper and is left for future work.

4. Conclusion

Amplification of short nearly transform-limited pulses of 9 nm bandwidth and 400 fs pulse duration was demonstrated, for the first time to our knowledge, in a fiber optical parametric chirped pulse amplification configuration. A dynamic characterization of the FOPCPA was performed both in the saturated and the unsaturated regimes. It was shown that the FWHM of the pulses is increased with increasing gain in the FOPCPA. Furthermore, the pulsewidth compressed as saturation was reached. A broadening of the spectrum was observed, showing that a chirp has been added to the pulse as the amplifier was saturated. However, in saturation, a higher energy with respect to the unsaturated regime is observed in the leading and trailing edges of the pulses, reshaping them and rendering the saturated amplifier unsuitable for undistorted amplification. After the optimization of the parameters of the amplifier, 400 fs pulses were first stretched to 50 ps, amplified by 26 dB in a fiber optical parametric amplifier and then compressed back to 500 fs. This demonstration opens the way to parametric amplification of Tbaud single wavelength channel signals [1

1. H. C. Hansen Mulvad, L. K. Oxenløwe, M. Galili, A. T. Clausen, L. Grüner-Nielsen, and P. Jeppesen, “1.28 Tbit/s single-polarisation serial OOK optical data generation and demultiplexing,” Electron. Lett. 45, 280–281, (2009) [CrossRef]

3

3. T. Richter, E. Palushani, C. Schmidt-Langhorst, R. Ludwig, L. Molle, M. Nölle, and C. Schubert, “Transmission of single-channel 16-QAM data signals at Terabaud symbol rates,” J. Lightwave Technol. 30, 504–511 (2012). [CrossRef]

].

Acknowledgments

OFS Fitel Denmark is acknowledged for providing the HNLFs. The Danish Research Council for Technology and Production Sciences is acknowledged for financial support (project 09-066562).

References and links

1.

H. C. Hansen Mulvad, L. K. Oxenløwe, M. Galili, A. T. Clausen, L. Grüner-Nielsen, and P. Jeppesen, “1.28 Tbit/s single-polarisation serial OOK optical data generation and demultiplexing,” Electron. Lett. 45, 280–281, (2009) [CrossRef]

2.

H. C. Hansen Mulvad, M. Galili, L. K. Oxenløwe, H. Hu, A. T. Clausen, J. B. Jensen, C. Peucheret, and P. Jeppesen, “Demonstration of 5.1 Tbit/s data capacity on a single-wavelength channel,” Opt. Express 18, 1438–1443 (2010). [CrossRef] [PubMed]

3.

T. Richter, E. Palushani, C. Schmidt-Langhorst, R. Ludwig, L. Molle, M. Nölle, and C. Schubert, “Transmission of single-channel 16-QAM data signals at Terabaud symbol rates,” J. Lightwave Technol. 30, 504–511 (2012). [CrossRef]

4.

G. P. Agrawal, Nonlinear Fiber Optics (Academic, New York, 2006).

5.

M. Hanna, F. Druon, and P. Georges, “Fiber optical parametric chirped-pulse amplification in the femtosecond regime,” Opt. Express 14, 2783–2790 (2006). [CrossRef] [PubMed]

6.

D. Caucheteur, E. Bigourd, E. Hugonnot, P. Szriftgiser, A. Kudlinski, M. Gonzalez-Herraez, and A. Mussot, “Experimental demonstration of optical parametric chirped pulse amplification in optical fiber,” Opt. Lett. 35, 1786–1788 (2010). [CrossRef] [PubMed]

7.

Y. Zhou, K. K. Y. Cheung, P. C. Chui, and K. K. Y. Wong, “Experimental demonstration of fiber optical parametric chirped-pulse amplification,” Proc. SPIE 7582, 75820T (2010). [CrossRef]

8.

Y. Zhou, Q. Li, K. K. Y Cheung, S. Yang, P. C. Chui, and K. K. Y. Wong, “All-fiber based ultrashort pulse generation and chirped pulse amplification through parametric processes,” IEEE Photon. Technol. Lett. 22, 1330–1332 (2010). [CrossRef]

9.

L. Grüner-Nielsen and B. Pálsdóttir, “Highly nonlinear fibers for very wideband supercontinuum generation,” Proc. of SPIE 6873, 68731B (2008). [CrossRef]

10.

R. J. Essiambre, B. Mikkelsen, and G. Raybon, “Intra-channel cross-phase modulation and four-wave mixing in high-speed TDM systems,” Electron. Lett. 35, 1576–1578 (1999). [CrossRef]

OCIS Codes
(060.2320) Fiber optics and optical communications : Fiber optics amplifiers and oscillators
(190.4970) Nonlinear optics : Parametric oscillators and amplifiers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 1, 2013
Revised Manuscript: October 10, 2013
Manuscript Accepted: October 16, 2013
Published: October 24, 2013

Citation
Valentina Cristofori, Zohreh Lali-Dastjerdi, Lars Søgaard Rishøj, Michael Galili, Christophe Peucheret, and Karsten Rottwitt, "Dynamic characterization and amplification of sub-picosecond pulses in fiber optical parametric chirped pulse amplifiers," Opt. Express 21, 26044-26051 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-22-26044


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References

  1. H. C. Hansen Mulvad, L. K. Oxenløwe, M. Galili, A. T. Clausen, L. Grüner-Nielsen, and P. Jeppesen, “1.28 Tbit/s single-polarisation serial OOK optical data generation and demultiplexing,” Electron. Lett.45, 280–281, (2009) [CrossRef]
  2. H. C. Hansen Mulvad, M. Galili, L. K. Oxenløwe, H. Hu, A. T. Clausen, J. B. Jensen, C. Peucheret, and P. Jeppesen, “Demonstration of 5.1 Tbit/s data capacity on a single-wavelength channel,” Opt. Express18, 1438–1443 (2010). [CrossRef] [PubMed]
  3. T. Richter, E. Palushani, C. Schmidt-Langhorst, R. Ludwig, L. Molle, M. Nölle, and C. Schubert, “Transmission of single-channel 16-QAM data signals at Terabaud symbol rates,” J. Lightwave Technol.30, 504–511 (2012). [CrossRef]
  4. G. P. Agrawal, Nonlinear Fiber Optics (Academic, New York, 2006).
  5. M. Hanna, F. Druon, and P. Georges, “Fiber optical parametric chirped-pulse amplification in the femtosecond regime,” Opt. Express14, 2783–2790 (2006). [CrossRef] [PubMed]
  6. D. Caucheteur, E. Bigourd, E. Hugonnot, P. Szriftgiser, A. Kudlinski, M. Gonzalez-Herraez, and A. Mussot, “Experimental demonstration of optical parametric chirped pulse amplification in optical fiber,” Opt. Lett.35, 1786–1788 (2010). [CrossRef] [PubMed]
  7. Y. Zhou, K. K. Y. Cheung, P. C. Chui, and K. K. Y. Wong, “Experimental demonstration of fiber optical parametric chirped-pulse amplification,” Proc. SPIE7582, 75820T (2010). [CrossRef]
  8. Y. Zhou, Q. Li, K. K. Y Cheung, S. Yang, P. C. Chui, and K. K. Y. Wong, “All-fiber based ultrashort pulse generation and chirped pulse amplification through parametric processes,” IEEE Photon. Technol. Lett.22, 1330–1332 (2010). [CrossRef]
  9. L. Grüner-Nielsen and B. Pálsdóttir, “Highly nonlinear fibers for very wideband supercontinuum generation,” Proc. of SPIE6873, 68731B (2008). [CrossRef]
  10. R. J. Essiambre, B. Mikkelsen, and G. Raybon, “Intra-channel cross-phase modulation and four-wave mixing in high-speed TDM systems,” Electron. Lett.35, 1576–1578 (1999). [CrossRef]

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