Abstract
A broadband continuously tunable SESAM all-fiber laser based on optical parametric generation (OPG) for coherent anti-stokes Raman scattering (CARS) imaging applications is presented in this article. This laser structure is built on all-polarization maintaining fibers yielding high environmental stability. The OPG is based on PM photonic crystal fiber as parametric gain pumped by a wavelength-tunable (1025 to 1055 nm) and repetition rate-tunable (3 MHz to 500 KHz) Yb-doped pulsed fiber laser. The all-fiber structure features free polarization adjustment in slow-axis tunable operation with a wavelength range from 770 to 940 nm for signal radiation and 1225 to 1510 nm for idler radiation. The pump and signal pulses (CARS-Stokes and CARS-pump respectively) are intrinsically overlapped, both spatially and temporarily, due to the four-wave mixing (FWM) generation and emitted from a single fiber end. The CARS-Stokes and CARS-pump pulse duration vary between 25 to 32 ps and 21 to 24 ps, respectively in the whole band, ensuring a good overlap of the generated signals, and both exhibit a Gaussian profile with high spatial beam quality. The frequency conversion allows to obtain a frequency difference between the generated signals from 1100 to 3300 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 1</sup> . The reported low-cost laser source is ideal for bio-imaging applications, especially in CARS imaging, due to its compactness, simplicity, robustness and ease of handling. A patent has been filed based on this technology.
Highlights
IN the last decades, the development of coherent radiation sources allowing emission wavelengths which conventional lasers could not provide, has been widely investigated
The optical parametric generation (OPG) is based on degenerate four-wave mixing (FWM) in a highly nonlinear fiber, namely photonic crystal fibers (PCF) [15, 16]
The seed laser is composed by an in-fiber bandpass filter (FWHM bandwidth of 100 pm and resolution of 0.02 nm) placed on a fiber loop mirror (FLM) composed by a wavelength-sensitive circulator, a delay-line of 32 meters of PM fiber (Nufern 980XP), a 90 cm-long Ytterbium doped fiber (YDF) (Nufern PM-YSF-LO) as the active gain medium, an in-line broadband slow-axis polarizer and a SESAM (Batop GmbH) as the ML device with a modulation depth of 29%, a relaxation time of 9 ps and a saturation fluence of 100 μJ/cm2
Summary
IN the last decades, the development of coherent radiation sources allowing emission wavelengths which conventional lasers could not provide, has been widely investigated. Light sources based on these techniques have attracted a lot of interest in sensing applications [1, 2] and biomedical imaging [3,4,5] For this reason, lasers based on optical parametric interactions are well-suited to fulfill this purpose due to its capability of being continuously tunable over a broad range, high conversion efficiency and large frequency shifts [6, 7]. Whereas OPOs are clearly not compatible with fiber integrated systems, fiber lasers based on OPG are designed to solve this problem being a cost-effective option with compactness and robustness [10, 11] These oscillators have interesting advantages as: good beam quality, high power stability and the capability of reach hundreds of nanometers ensuring a tunable broadband range [7, 12,13,14]. The frequencies of the generated photons are determined by the phase-matching condition which depends on the peak power and wavelength of the seed laser used to pump the optical fiber used as parametric gain and the dispersion profile of this optical fiber [18]
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