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What is a Pulse Stretcher?

Mal Baxter
Mal Baxter

Used in communication technology and networking, a pulse stretcher is often employed in an optical fiber network. It is a component designed to reshape waveforms of transmitted signals in order to limit effective bandwidth. This reduces intersymbol interferences for more efficient frequency band utilization. Several processes are employed in the reshaping of a signal pulse as it undergoes several transformations to achieve the targeted amplitude. These processes can include amplification, dispersive stretching, and compression to obtain the desired pulse.

The use of a pulse stretcher in a network permits continuous control over wavelength, power, and duration of laser-emitted energy. This serves to transmit full pulse energy and cohesive, non-diffracted beam quality at reduced peak power for less interference and more stable operation over a range of temperatures. These devices can function in pulse capturing, peak power monitoring, quad pulse processing, or with ultrasound pulses and electronic transient capture. Applications can include medical use in laser tissue interaction, photochemical and photolithographic processes, surgery, and dentistry.

Man with a drill
Man with a drill

Another aspect of pulse stretcher technology includes the ability to couple with optical fibers in order to eliminate risk of fiber damage from overamplified lasers. The pulse shaper typically resolves fast pulses emitted from diodes and transistors. Its output pulse has a greater duration and amplitude that corresponds to the peak amplitude of an input pulse. A seed pulse spike may be flattened through dispersive stretching, amplified, and then run through dispersive compression to generate the narrower final pulse.

Various techniques and technologies are employed to construct a pulse stretcher or compressor. This is usually accomplished through the use of gratings and prisms. A stretcher or compressor is characterized by dispersion, or separation of wavelength. Negative dispersion allows light of higher frequencies to travel faster through a device than lower frequencies.

The light's dispersion can be affected by each component it interacts with in the device. Gratings reflect light while prisms scatter; different arrangements play with distance and scatter in order to modulate a wave. Grisms, a hybrid of prisms and gratings, correct higher order dispersions.

Other techniques for creating dispersion can involve directing the light through a slab of transparent material. Different materials exist to create positive and negative dispersions. Some components use the amplitude, frequency, and timing of acoustic waves to disperse pulses. Manufacturing processes also allow for customized dispersion within the glass optical fibers themselves. Where a pulse stretcher is employed, assessment of signal quality can be obtained through the use of analyzing tools, like a laser beam analyzer, to determine wave profile, energy, frequency, power, and temporal pulse shape.

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