Optimized Cascaded Raman Fiber-Based Laser Source for High Efficiency Mid-Infrared Spectral Generation

Applications

An optimized cascaded Raman fiber-based laser source, spanning mid-IR wavelengths, is used in semiconductor processing, coherent X-ray generation, chemical sensing, and cancer detection.

Problem Addressed

It has been demonstrated that at high powers, substantial levels of Raman gain can be achieved with use of conventional silica fibers to produce significant spectral red-shifts through spontaneous cascaded Raman (CR) processes. However, spectral broadening and pulse distortions due to both spontaneous Raman emission and four-wave mixing (FWM) generally limit the efficiency of the Raman process. For instance, at high powers, anomalous dispersion and Kerr nonlinearities can give rise to modulation instabilities (MI), which result in a significant spectral broadening and pulse distortion. For this reason, normal dispersion fibers are more desirable for controlled and maximally efficient CR at the nanosecond time-scale. Unfortunately, silica fibers generally possess anomalous dispersion due to strong mid-IR absorption resonances.

Technology

The invented laser comprises a pulsed source and an optical fiber. The invention allows for generating high efficiency spectral red-shifting of light from short wavelengths (high frequencies) to longer wavelengths (lower frequencies) using Raman amplification produced within the optical fiber. The pulsed source produces a pulsed signal with a low spontaneous noise component and a flat pulse-shape. The optical fiber receives the pulsed signal and perform Raman amplification which is performed by spectrally filtering a pulsed signal and which provides sufficient power to saturate a stage of erbium doped fiber amplification. The pulse signal is used to excite normal chromatic dispersion in the fibers, acting as a nonlinear system for efficient mid-infrared spectral generation.

Advantages

  • Generates high efficiency (near 100% photon-yield) spectral red-shifting of light from a wide range of frequencies
  • Enables the construction of simple and efficient Raman-based sources and gain-blocks for long wavelength spectral generation