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We demonstrate simultaneous pulse-shaping at different ports of a rapidly tunable wavelength selective switch at a base rate of 40 GHz, based on Fourier-domain pulse shaping. Various pulse bursts are generated and accurately characterized with a linear spectrographic method.
©2008 Optical Society of America
1. Introduction
The creation and control of arbitrarily shaped optical waveforms is of interest in all-optical processing applications. A number of techniques to reshape short optical pulses have been previously demonstrated, with varying degrees of accuracy, reconfigurability and flexibility. In most approaches, spectral manipulation of the amplitude and/or phase of an input pulse leads to the desired change in temporal characteristics of the pulse, a process referred to as Fourier-domain pulse shaping [1].
More specifically related to the current work, the generation of a burst of pulses from a single input pulse has been previously demonstrated with technologies such as super-structured fiber Bragg gratings (FBGs) [2–5] and liquid crystal spatial light modulators combined with spectral dispersion with a virtually imaged phased array (VIPA) [6], or a grating [7]. Flat-topped 500 GHz pulse bursts have been generated with specially engineered arrayed waveguide gratings [8]. Two different categories of pulse shaping can be distinguished: line-by-line pulse shaping and group-of-lines pulse shaping [7], based on how the spectral lines in the waveform are modulated. In the case of group-of-lines modulation, the shaped waveforms are limited to durations much shorter than the period of the repetitive pulse stream. In the line-by-line modulation regime, the whole pulse period can be used for the reshaped pulse, making repetition rate multiplication possible [7]. The work in this letter falls in the category of line-by-line pulse shaping, as the spectral resolution of the wavelength selective switch (WSS) is much finer than the 40 GHz repetition rate of the input pulses used. This is advantageous to eliminate noise resulting from spectral instabilities in the input pulse source, and also makes the method independent of the exact repetition rate of the pulses (the frequency spacing of the optical comb) [9].
In this letter we experimentally demonstrate programmable Fourier-domain reshaping of pulses into arbitrary pulse bursts, combined with wavelength selective port switching, in a WSS. The output waveforms are characterized with linear spectrograms [10–12].
2. Device operational principle
The principle of Fourier domain pulse shaping is schematically represented in Fig. 1.
Assuming that the input waveform is completely characterized: E(t), the output waveform A(t) is obtained by multiplying the input waveform in the spectral domain with a complex filter function F(f)=A’(f)/E’(f), where A’(f) and E’(f) are known. Note that this method cannot create light at new wavelengths, it only changes the power and phase of existing wavelengths. The input signal needs to have sufficient bandwidth to accommodate for the reshaped pulse.
The principle of operation of the WSS is summarized in Fig. 2. More details can be found in Ref. 8. The programmable element in the WSS is a two-dimensional array of liquid crystal on silicon (LCOS) pixels. This WSS can reshape these short pulses into an arbitrary pulse shape through Fourier-domain pulse shaping. The design functionality is the programmable routing of wavelengths from an input port to any of ten output ports. It does this by adjusting the phase front for each wavelength in such a way that the light is steered to a specific output port. Specially calculated phase ramps are applied to the phase front after the signal has been spectrally dispersed across the two-dimensional LCOS array. The phase retardance of each pixel is set by adjusting the voltages on the LCOS backplane. More complex phase modulation leads to pulse splitting towards multiple output ports, and arbitrary attenuation control for each wavelength.
Fig. 2. Concept diagram showing the operation of the WSS. Light is sent from a fiber to a diffraction grating. The diffraction grating angularly disperses the light, so that each wavelength is sent to a different portion of the LCOS along the wavelength axis. Vertically, the light diverges so that the signal overlaps a large number of pixels (typically about 400). By sloping the phase front along the vertical axis, the signal is deflected to a desired output port, after the optical path is retraced upon reflection from the LCOS. Not shown in this simple figure is the imaging system, which is configured into a 4f (unitary magnification) optical system, and which also features a polarization diversity scheme which is used to eliminate polarization dependent effects.
Recently we have shown that full spectral control over the phase of the optical signal leads to dispersion control per channel in the wavelength selective switch [13], a specific subset of Fourier-domain pulse shaping. Any temporal pulse shape can be obtained from a short input pulse, through spectral phase and intensity filtering. Note that the LCOS array is a phase-only modulator. The intensity modulation is obtained by steering part of the light to dump locations within the device, through advanced phase modulation of the phase front. This in turn makes the method a full Fourier-domain pulse shaping method as we are controlling both the amplitude and the phase of the output signal, rather than a phase-only pulse shaping method.
In the current implementation of the WSS, the dispersion-bandwidth product is about 40 ps, meaning that light can be delayed or advanced – relatively to the input pulse – by about 20 ps, before half of the signal is lost due to horizontal beam steering [13]. This is sufficient for accurate waveform control over the full bit period of 40 GHz pulse trains considered in this work. Thus, by applying appropriate phase profiles in both horizontal and vertical axes of the LCOS array, spectral amplitude and phase can be controlled independently across the entire pass band of the device, leading to the combination of programmable Fourier-domain pulse shaping with output port selection.
Fig. 3. Schematic overview of how different parts of the LCOS can act on different parts of the spectrum. The blue part is programmed to reshape a short input pulse into a pulse burst with three pulses, and send this specific wavelength range to ‘Output 2’ whereas the red part is programmed to reshape a short input pulse into a burst of 2 pulses that are being sent to ‘Output 1’.
The spectral phase P(f)=arg(F(f)) and attenuation profile I(f)=|F(f)|2 are calculated for each of the desired output waveforms, and then independently converted to horizontal and vertical phase modulation profiles respectively, where the horizontal phase modulation directly translates into the spectral phase of the signal, and the vertical phase modulation affects the attenuation imposed on each wavelength, but also determines to which output port the light at the specified wavelengths is sent. Figure 3 gives a schematic representation of how different parts of the LCOS can be selected to act on different wavelengths (the primary function of the wavelength selective switch). In this case however, the different spectral slices are programmed in such a way that on top of the port selection, there is also pulse shaping.
3. Experimental setup
Fig. 4. Experimental layout. MLRL: mode-locked ring laser; HNLF: highly nonlinear fiber; WSS: wavelength selective switch; OSA: optical spectrum analyzer. The polarization is optimized before the LiNbO3 Mach Zehnder Modulator for characterization.
To demonstrate the two-dimensional pulse shaping functionality afforded by using our WSS, we have used the experimental setup shown in Fig. 4. We propagate a 40 GHz train of high-powered optical pulses of 2 ps duration through ~1 km of dispersion-shifted highly nonlinear fiber (HNLF). The spectrum broadens due to self-phase modulation and other nonlinearities to create a broad continuum spanning the C-band (1530–1570 nm), as shown in Fig. 5. The resulting wave is then sent to the input port of the WSS. No polarization control was needed here as the polarization dependent loss in the WSS is less than 0.5 dB. The WSS is then used to carve spectral slices out of this continuum, to introduce independent phase and intensity modulation to each of these slices, and finally to send them to different output ports of the WSS.
Fig. 5. Top: Output spectrum of the of the 2 ps mode-locked ring laser pulses. Bottom: Continuum of 2 ps, 40 GHz pulses, generated by propagating the high power ring laser pulses through 1 km of HNLF.
4. Results
Figure 6 shows an example of the spectral intensity profiles generated by the WSS at 1541 nm and 1562 nm, measured at different output ports with an optical spectrum analyzer (OSA). The total insertion loss, without spectral filtering, of the WSS is 5 dB. The profile at 1541 nm is a burst of two pulses with a separation of 6.25 ps, while the profile at 1562 nm is a burst of four pulses also with a separation of 6.25 ps. As the input pulse train has a 40 GHz repetition rate, application of this second pulse burst sequence results in a continuous 160 GHz pulse train. The spectral pulse envelope has been designed to have a bandwidth sufficient to preserve 2 ps Gaussian shaped pulses in each case. Note also that we achieve 35 dB suppression of cross-talk between the different output ports of the WSS.
Fig. 6. Spectral intensity response measured on ports 6 and 7 of the WSS, respectively the profiles of a 2-pulse burst at 1541 nm and a 4-pulse burst at 1562 nm.
We characterized the ultrafast pulse trains with a sensitive, linear spectrographic technique based on modulation with a LiNbO3 Mach-Zehnder modulator [10–12], a method with sub-picosecond temporal resolution. Apart from its higher degree of sensitivity, a particular advantage of using the linear spectrographic method in this case is that the measured spectrograms are relatively simple, and easy to reconstruct, compared to the more common second harmonic frequency resolved optical gating (SHG-FROG) spectrograms [10].
Figure 7 shows two series of intensity and chirp profiles of various pulse trains at the two different wavelengths, reconstructed from the spectrogram measurements. These pulse bursts show that each pulse within the burst retains a pulse length of ~2 ps, similar to that of the input 40 GHz pulse train before the continuum generation. Note that the small ~20% variation in peak intensity between the pulses in the bursts is due to the fact that the pulse-shaping profiles imposed onto the optical continuum were calculated assuming an ideal impulse response for the input waveform, whereas experimentally the continuum is spectrally not perfectly flat. The small amount of chirp on the pulses also stems from the original, chirped continuum. Better uniformity of the peak intensities of the individual pulses, as well as complete mitigation of the chirp in the pulse bursts should be possible with careful characterization of the input wave in both phase and amplitude.
The n×6.25 ps separation between the pulses in these examples was chosen in order to synthesize pulse trains compatible with 160 Gbit/s optical signals. It is possible using the existing WSS design to program any permutation of four pulses within a 25 ps period through tailored spectral-phase profiles. This technique offers a unique method for creating synchronized pulse sources, with independently controlled shapes and repetition rates. It leads to interesting new possibilities in high-speed nonlinear optical processing, such as reconfigurable time-domain demultiplexing and switching of pulses at high bit rates. For example, when combined with an ultrafast switch, such as a nonlinear optical loop mirror [14], progammably variable sequences of pulses could be selected from an ultrahigh speed bit train, leading for example to routing of individual demultiplexed bit trains to different user channels.
Fig. 7. Measured temporal intensity (blue solid line) and chirp profiles (green dashed lines) of various pulse bursts around a carrier wavelength of 1562 nm, sent to port 7 (top three) of the WSS and around 1541 nm sent to port 6 (lower three).
5. Conclusion
We have demonstrated flexible generation of trains of pulses in a wavelength selective switch, in combination with wavelength selection and port switching functionality. The high quality of the resulting waveforms was verified through accurate spectrographic measurements and was limited only by the incomplete characterization of the input pulse profile. This technique could lead to a new class of time-division multiplexing technology for telecommunication applications.
Acknowledgments
This work was produced with the assistance of the Australian Research Council (ARC). CUDOS (the Centre for Ultrahigh-bandwidth Devices for Optical Systems) is an ARC Centre of Excellence. The authors thank Optium Australia for providing the WSS used in the experiments.
References and links
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