I have blogged in the past about Finisar’s WaveShaper range of Programmable Optical Processors (see e.g. here or here) and how they can be used for telecoms R&D. For this month’s post, I’m going to move away from the telecoms space again and look at how people have been using a WaveShaper to control the generation and transmission of short (typically <1 psec) optical pulses in optical fibres.
The key to this is the capability of a WaveShaper to control independently both the amplitude and phase of a transmitted signal, which leads in turn to the general ability to manipulate the amplitude and/or phase of an optical pulse through a process known as Fourier Domain Pulse Shaping . This process, which requires full characterization of the input pulse in both the time and spectral domains, is shown schematically in Figure 1.
This is basically saying that any pulse can be described either by in the time domain (E(t)) or the spectral domain (E′(f)) and that you can convert from the time domain to the spectral (frequency) domain through an operation termed a Fourier Transform (or conversely from the frequency domain to the time domain through an Inverse Fourier Transform.) Given the knowledge of the input pulse in both the frequency and time domains, the desired output pulse (A(t) or A′(f)) can then be generated by simply applying the appropriate transfer function (amplitude and/or phase) in the spectral domain, which, of course, is where the WaveShaper comes in.
One area where these processes have been extensively used is in short-pulse generation by passive mode-locking of fiber lasers. For example, tailoring of the dispersion profile across the spectrum has been shown to change the pulse train of a passively mode-locked laser from bright to dark pulses. A similar approach uses spectral shaping of optical frequency combs to create multiple pulse trains. In this case, a 10 GHz optical frequency comb was shaped by the WaveShaper to generate dark parabolic pulses and Gaussian pulses at 1540 nm and 1560 nm, respectively.
Researchers have also used this capability to transform the output from a laser system into different types of optical pulses including the generation of 400 fs, transform-limited pulses and converting a train of pulses at 40GBit/sec into several different pulse trains at separate wavelengths, with cus¬tomizable burst numbers. The latter approach was combined with a nonlinear optical loop mirror to demultiplex a 40 Gbit/s data stream from within a 160 Gbit/sec signal .
Whilst the majority of work to date has focused on the research applications of the WaveShaper in Fourier Domain Pulse Shaping, the technology is now being applied commercially. Here, the ability of a WaveShaper M-Series to provide stable, accurately programmable control of amplitude and phase is used to optimize the pulse shape in a fully engineered system designed for precision micro-machining of complex components.
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Jochen B. Schroeder et al., “Dark and Bright Pulse Passive Mode-locked Laser with In-cavity Pulse-shaper,” Optics Express 18, no. 22 (October 25, 2010): 22715–22721.
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A. M. Clarke, et al., , “Reconfigurable Optical Pulse Generator Employing a Fourier-Domain Programmable Optical Processor,” J. Lightw. Technol., vol. 28, no. 1, pp. 97-103, 2010.
M.A.F Roelens et al., “Multi-wavelength Synchronous Pulse Burst Generation with a Wavelength Selective Switch,” Optics Express 16, no. 14 (July 7, 2008): 10152–10157.
M.A.F Roelens et al., “Flexible and Reconfigurable Time-Domain Demultiplexing of Optical Signals at 160 Gb/s,” IEEE Photonics Technology Letters 21, no. 10 (May 2009): 618–620.
M. Mielke et al., “100 μJ, 20 W Femtosecond Fiber Laser for Precision Industrial Micro-Machining”, in Proc. CLEO 2013, paper ATh3K (2013)