FREQUENCY COMBS: Optical arbitrary-waveform-generation technique shapes more than 100 spectral lines

Frequency metrology has benefited tremendously from combs of frequency-stabilized spectral lines produced by modelocked lasers.

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Frequency metrology has benefited tremendously from combs of frequency-stabilized spectral lines produced by modelocked lasers (see www.laserfocusworld.com/articles/294647). Although pulse shapers can manipulate such spectral lines, they have typically addressed spectral lines in groups at low spectral resolution. High-resolution pulse shapers that could address individual comb lines would combine the arbitrary optical-waveform-generation advantages of pulse shaping with the frequency stability and coherence benefits of frequency combs. Recently, some examples of line-by-line pulse shaping have been demonstrated using free-space techniques or integrated planar lightwave circuits, but these techniques address no more than 10 to 20 lines and usually have comb spacing higher than 10 GHz.

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The optical arbitrary-waveform-generation technique uses a series of pulse shapers for line-by-line conversion of a broadband continuous-wave laser output to a series of high-rate ultrashort pulses with user-defined waveforms. (Courtesy of Purdue University)
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Now, researchers at Purdue University (West Lafayette, IN) have used an optical arbitrary-waveform-generation (O-AWG) technique to combine the concepts of pulse shaping and frequency combs in a free-space platform to enable line-by-line pulse shaping of more than 100 spectral lines with line spacing reduced to 5 GHz.1 The new technique offers the generation of waveforms with controllable ultrafast time structure and long-term coherence for applications in broadband spectroscopy, multiline homodyne detection in optical communications, and light-detection-and-ranging systems.

Line-by-line pulse shaping is difficult because standard modelocked frequency combs have line spacings typically at 1 GHz and below, which exceeds the resolution of standard pulse shapers. To demonstrate line-by-line pulse shaping and the O-AWG technique, the Purdue team uses a continuous-wave laser with a 1 kHz linewidth centered at 1542 nm that is modulated by two phase modulators driven synchronously (with adjustable delay) by 20 GHz and 5 GHz cosine waveforms. Because the generated bandwidth is proportional to the modulation frequency, the phase modulator driven at 20 GHz contributes to the broad bandwidth, while the modulator driven at 5 GHz determines the comb spacing and temporal periodicity. The resulting comb is manipulated by the first of two spectral line-by-line pulse shapers to convert the broadband constant-intensity waveform to a pulse train (see figure).

The fiber-coupled Fourier-transform pulse shapers are constructed in a reflective geometry. A fiber-pigtailed collimator and optics magnify the beam size to approximately 18 mm in diameter on the 1200 grooves/mm grating to enhance the pulse-shaper’s resolution. Discrete spectral lines making up the input short pulse are diffracted by the grating and focused onto a 2 × 128-pixel liquid-crystal-modulator (LCM) array placed just before the focal plane of the lens to independently control the amplitude and phase of individual spectral lines. A retroreflecting mirror leads to a double-pass geometry, with all spectral lines recombined into a single fiber and separated from the input by an optical circulator. Optical amplifiers are added to compensate for the optical loss of the individual components.

First and second pulse shapers

The first line-by-line pulse shaper converts the phase-modulated-but constant-intensity-field into bandwidth-limited 2.4 ps full-width at half-maximum pulses that are then amplified by an optical-fiber amplifier and sent into a dispersion-decreasing-fiber (DDF) soliton compressor that yields pulses as short as 270 fs. This generates more than 1000 lines between 1525 and 1565 nm. Although this frequency comb demonstrates line-by-line pulse shaping, the research team uses a second pulse shaper that selects and individually manipulates a set of 108 lines centered around 1537.5 nm spanning a 540 GHz bandwidth. This process leads to generation of arbitrary user-defined optical waveforms that repeat at a 5 GHz repetition rate.

The ability to individually manipulate phase and amplitude of the various lines of a frequency comb is a significant step toward true optical arbitrary waveform generation. The reduced (5 GHz) frequency resolution brings line-by-line pulse-shaping technology closer to compatibility with self-referenced modelocked frequency-comb technology, which is just beginning to reach such repetition rates. In the future, research will aim to extend line-by-line shaping to the hundreds of thousands of comb lines available from modelocked comb sources.

“We are very excited about the prospects of truly integrating highly stable frequency-comb technology with pulse-shaping technology,” says researcher Andrew Weiner, Scifres Distinguished Professor of Electrical and Computer Engineering. “This unique combination should open up new opportunities in high-resolution coherent-control spectroscopy, broadband optical sensing, and new forms of highly coherent ultrashort-pulse communication, as well as other applications that we haven’t yet envisioned.”

Gail Overton

REFERENCES

1. Z. Jiang et al., Nature Photonics1, 463 (August 2007).

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