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Highly Nonlinear Fibers (HNLF) for NIR Supercontinuum Generation
Highly Nonlinear Fiber (HNLF) can be used for nonlinear spectral broadening of femtosecond pulses around 1550 nm. For more information about this application, please see the Pulse Evolution tab.
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Thorlabs' Highly Nonlinear Fiber has flat dispersion (zero dispersion slope) compared to standard SM fiber (SMF-28 Fiber).
Thorlabs' Highly Nonlinear Fibers (HNLF) are single mode fibers designed for applications that require a large nonlinear coefficient as well as near-zero dispersion around 1550 nm (C & L bands). The nonlinear coefficient of our HNLF is approximately 10 times higher than that of standard single mode fiber at this wavelength (SMF-28). We offer HN1550 fiber with normal dispersion and HN1550P fiber with anomalous dispersion. Both fibers feature zero dispersion slope, as compared to SMF-28 in the graph to the right. A primary application for these fibers is in nonlinear spectral broadening of femtosecond pulses in the 1550 nm band through self-phase modulation. This spectral broadening process can be used to generate broadband light sources (supercontinuum generation) and compressed pulses. Additionally, the combination of a high nonlinear coefficient and near-zero dispersion make the fibers ideal for four-wave mixing processes in this wavelength region.
HN1550 fiber features a small normal dispersion (-1 ps/nm•km at 1550 nm). In spectral broadening applications, the normal dispersion of the fiber creates a linear chirp on the pulse which is of the same sign as the chirp created by nonlinearity. As a result, femtosecond pulses propagating along HN1550 fiber would generally broaden in time while they undergo spectral broadening. In comparison, HN1550P features a small anomalous dispersion (+1 ps/nm•km at 1550 nm). As a result, the linear chirp caused by the fiber dispersion has the opposite sign compared to the chirp caused by nonlinearity. This enables soliton propagation and self-compression of pulses in the fiber. One consequence of this is the fact that the spectral broadening of femtosecond pulses can be achieved more efficiently and at lower power levels in HN1550P fiber as compared to HN1550 fiber. On the other hand, the broadened spectra in HN1550 fiber generally have better flatness in comparison with the HN1550P.
Pulse compression in HN1550 can be achieved by spectrally broadening the pulse and the use of a dispersive component with anamolous dispersion. Such systems can be designed to produce compressed pulses with high quality. In comparison, pulse compression in HN1550P can be achieved by either self-compression or by a combination of self-compression and the use of a dispersive element. Additionally, HN1550P is suitable for parametric amplification processes due to its anomalous dispersion. See the Pulse Evolution tab for a demonstration of spectral broadening and pulse compression through numerical simulation examples using both fibers.
These fibers are designed with a small mode-field diameter of 4 µm in order to enhance nonlinearity. Therefore, splice recipe optimization or a bridge fiber should be used in order to minimize coupling losses between the standard single mode fiber and the HNLF. Thorlabs can provide highly nonlinear fiber with single mode fiber spliced to one or both ends. Please contact Tech Support to request a quote on splice services for this fiber.
Applications for highly nonlinear fiber (HNLF) include spectral broadening of femtosecond pulses as well as pulse compression. The examples of spectral broadening or pulse compression provided below can be used as a reference for designing systems or experiments. Please note that all simulations are made with the assumption that the input pulses are transform-limited sech2 shaped pulses, and the indicated power levels are after coupling into the HNLF fiber. The simulation results are not guaranteed and should only be used as guidelines for designing experiments; see the specifications table in the Overview tab for guaranteed performance.
Figures 1 and 2 show the evolution of a 200 fs pulse as it propagates along a 1 meter section of HN1550 or HN1550P fiber. We simulated pulses with average power of 80 mW at 100 MHz (800 pJ pulse energy) for the case of HN1550 fiber. The simulation was repeated for a lower average power of 10 mW (100 pJ pulse energy) for the case of HN1550P fiber, as this corresponds to the same spectral broadening due to the more power-efficient broadening in the HN1550P fiber. Figure 1 shows the temporal intensity profiles of the pulses after propagation through either the HN1550 or HN1550P fiber, while Figure 2 shows the spectra of the pulses after propagation through the same respective fibers. The HN1550 fiber broadens the pulse in both time and spectral domains, yielding a >600 fs long pulse and a spectrum spanning from 1300 nm to 1800 nm. In the case of the HN1550P fiber, self-compression to a <20 fs pulse width is evident while the output spectrum spans from 1300 nm to 1800 nm.
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Figure 1: Simulated normalized output spectra from 1 m long HN1550 fiber and HN1550P fiber seeded with 100 MHz, 200 fs pulses.
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Figure 2: Simulated normalized output spectra from 1 m long HN1550 fiber and HN1550P fiber seeded with 100 MHz, 200 fs pulses.
The diagram and plots below show how nonlinear broadening can be utilized for compressing pulses in time using either HN1550 or HN1550P fiber. Pulse compression in HN1550 fiber is only possible by external compression using a dispersive component with anomalous dispersion. Pulse compression in HN1550P fiber can be done by self-compression (as shown above) or by a combination of self-compression and external compression in a dispersive component with anomalous dispersion. While self-compression provides a simple approach, higher quality pulses can be achieved by a combination of self-compression and external compression. In the example shown below, a 100 fs pulse at 100 MHz repetition rate and 100 mW average power is sent into a short section of each fiber. A short length of bulk fused silica is then used to provide external anomalous dispersion for compressing the pulses. Figures 3 and 4 show the normalized spectrum of the pulse and its temporal intensity profile before entering the nonlinear fiber, respectively. By collimating the beam at the output of the highly nonlinear fiber and sending the pulses into bulk fused silica (anomalous dispersion, with negligible nonlinearity), the pulses are compressed in time. We simulate the progapation in 10 cm of HN1550 fiber followed by compression in 20 mm of bulk fused silica, as well as the propagation in 7 cm of HN1550P fiber followed by compression in 10 mm of bulk fused silica. Figures 5 and 6 show the output pulse normalized spectrum and temporal intensity profile, respectively.
Please note that by compensating higher-order dispersions, a cleaner temporal profile can be achieved. Higher compression factors can be achieved by additional spectral broadening of higher intensity pulses.
Figures 3 - 6: Input spectrums (normalized) and temporal intensity profiles for a pulse sent through highly nonlinear fibers. Output spectrums (normalized) and temporal intensity profiles for a pulse after propagating through the fibers and compression in bulk fused silica. The input pulse has a 100 fs pulse width, 100 MHz repetition rate, and 100 mW average power.