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Abstract
We demonstrate the generation of octave-spanning supercontinuum generation from 1200 nm to over 2500 nm with 600 mW average power in a short length of multimode fluoride fiber with 100 μm core diameter. We perform a detailed study of the supercontinuum generation process as a function of the pump wavelength and for two different fiber lengths. Beam profile characterization at the fiber output in different wavelength bands is also carried out. Our results open up new possibilities for the generation of high-power supercontinuum sources from the near- to mid-infrared.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
Supercontinuum light sources (SC) are broadband sources with unique properties generated by nonlinear processes. Depending on the type of nonlinear material and pump source used, the spectrum of a SC source can span from the visible to the mid-infrared with up to several Watts of average power [1]. From an application point of view, supercontinuum sources generated in a single-mode fiber are more practical with near-perfect spatial coherence characteristics and high brightness, allowing for relatively straightforward beam delivery and long-distance collimation. While the physics of SC generation is now very well understood and can be accurately modeled using the generalized nonlinear Schrödinger equation, emphasis has now turned toward extending the wavelength span covered by supercontinuum sources into spectral regions where applications could benefit most. In this context, there has been a significant effort in the past few years to develop broadband SC sources operating in the short-wavelength infrared and mid-infrared due to a wide range of potential applications in spectroscopy [2,3], microscopy [4], molecular fingerprinting [5], medical surgery, environmental monitoring, and LIDAR [6].
The dominant paradigm for efficient supercontinuum generation relies on the use of specific materials combined with optimized pump wavelength and/or dispersion engineering strategies. The material aspect is particularly important in the short-wavelength infrared and mid-infrared regions as conventional silica-based fibers have large intrinsic losses beyond 2 μm. On the other hand, soft glass fibers such as fluoride, tellurite, and chalcogenide fibers are promising candidates due to their high nonlinearity and wide transparency window, and indeed there has been a wide range of SC studies using these materials [7–13]. Various designs have also been employed in order to engineer the dispersion profile of fibers and increase the efficiency of the nonlinear processes [14–17]. A meaningful comparison of different soft glasses properties, by including nonlinearity, zero-dispersion wavelength (ZDW), and transparency window, can be found in Table 1.
Table 1. Optical Properties of Different Soft Glasses and Silica Glassesa
Fluoride fibers have a transparency window up to 4–5 μm depending on the specific glass composition, with ZDW typically lying in the range of 1.5–2 μm [18], such that they can be pumped by commercial erbium-doped or thulium-doped fiber laser systems for efficient SC generation [19]. More specifically, fluorozirconate (ZBLAN) [20–23] and fluoroindate () [24–27] step-index single-mode fibers have been used to generate SC, both in the short and long pulse regimes, with the broadest reported SC reaching out to 4.7 μm [21] and 5.4 μm [28] in ZBLAN and fibers, respectively. Yet in both cases the SC output power was limited just up to few milliwatts. Using amplified systems, Watt-level SC has also been demonstrated in both single-mode ZBLAN [29] and [30] fibers but with reduced bandwidth. Chalcogenide glasses, on the other hand, provide nonlinearities a few-hundred times larger than fluoride glasses with a wider transmission window up to 12 μm and, with the recent progress in fabrication techniques allowing for optimized designs, they have gained significant interest as extremely promising candidates to achieve ultra-broad SC in the mid-infrared [31–33]. Several studies have reported SC reaching out wavelengths beyond 10 μm in single-mode chalcogenide fibers [5,12], but these typically exhibited very low average power down to the microwatt level due to the low damage threshold of chalcogenide glasses.
To date, SC with various characteristics in terms of bandwidth and power have been demonstrated in soft glass fibers. However, they have been principally generated in single-mode fibers (in particular when using fluoride-glass-based fibers), which generally cannot sustain as high level of average power as multimode fibers due to their smaller core dimension and lower damage threshold. This may in turn be a limitation for practical applications, especially in remote sensing or imaging where typically only a tiny fraction of the illumination power is reflected back to the detector. In order to overcome this issue, telecommunication [34] and multimode chalcogenide fibers with large core diameters have been employed in order to lower the damage threshold and generate SC for high-power applications [35,36]. For example, SC generation from 1.5 μm to 7 μm has been reported in a 70 cm long multimode fiber with 100 μm core diameter using 4.56 μm, 130 fs, 70 MW pulses from an optical parametric amplifier [37]. More recently, SC dynamics were studied in 200 μm size fiber in both the normal and anomalous dispersion regimes using a femtosecond optical parametric amplifier (150 fs, 3 MW peak power) leading to the generation of a SC from 1.7 to 7 μm [38]. In both these studies, however, the average power was still relatively low, at the milliwatt level.
In this paper, we demonstrate for the first time the possibility of generating an octave-spanning short-wavelength infrared SC with 0.4 mW/nm power spectral density in a short length of a multimode step-index fiber with 100 μm core size. We perform a systematic study of the generated SC as a function of pump wavelength for 1 and 2 m fiber lengths. The output beam profile of the SC measured in different wavelengths bands shows that the SC generation dynamics are mainly seeded by higher-order modes excitation. Because fluoride fibers with a very large core have a much higher damage threshold and transparency window up to 5 μm, our results open up a new alternative and potential solution to generate high-power SC in the short-wavelength infrared and mid-infrared, for applications in long-distance remote sensing [39] and hyperspectral imaging where high power can be a key factor.
2. EXPERIMENTAL SETUP AND OPTICAL PARAMETERS OF FIBER
A schematic illustration of the experimental setup is shown in Fig. 1. As the pump source, we use an optical parametric amplifier (OPA) delivering 350 fs pulses at a repetition rate of 1 MHz and with up to 2.5 MW peak power. The central wavelength can be continuously tuned from 1400 nm up to 4000 nm, but in this work we restricted ourselves to the 1600–2100 nm range as longer pump wavelengths did not lead to very significant spectral broadening. Light at the OPA output was spectrally filtered to remove any stray light from the OPA pump residue and idler (or signal depending on the wavelength). The pulses were injected into a multimode, step-index fiber (Thorlabs MF12) using an plano–convex lens with 5 mm focal length.
Fig. 1. Experimental setup for SC generation in the 100 μm core multimode fluoride (MMF) fiber. OPA: optical parametric amplifier.
The fiber has a 100 μm core and 192 μm cladding diameters, respectively, corresponding to a numerical aperture of at around 1600 nm. Attenuation over the range 1200 nm to 4000 nm is below 0.4 dB/m, as shown in Fig. 2.
In the following experiments, we characterize the supercontinuum spectrum of two different fiber lengths of 1 m and 2 m. The 2 m fiber consists of two connected 1 m segments (with a FC/PC connector) allowing us to analyze the supercontinuum generation evolution as a function of fiber length without resorting to the cut-back method. A maximum throughput of 82% and 67% measured at the fiber output (including coupling efficiency, attenuation in the fiber, and connection loss in the case of the 2 m fiber) was obtained for the 1 m and 2 m fiber, respectively. For the 2 m fiber, the connection loss was measured to be 0.7 dB.
The SC spectrum at the fiber output was measured with an optical spectrum analyzer (OSA, ANDO AQ6317B) in the 1000–1750 nm range and with a monochromator for wavelengths above 1750 nm. In the latter case, SC light was collimated by a 35 mm focal length reflective fiber collimator and subsequently focused into the monochromator (DK 480) with a 5 cm focal length, uncoated, plano–convex lens. A spectral filter was inserted at the output of the monochromator in order to eliminate the second diffraction order for the short-wavelength SC components. Dispersed light was then collected with a PbSe detector with a 5 cm focal length lens. In order to improve the signal-to-noise ratio, a lock-in detection scheme using a mechanical chopper was also applied. The beam profile at a distance from the fiber output was characterized using a Pyrocam IIIHR beam profiling camera. The camera was also equipped with a set of bandpass filters (Spectrogon) centered at different wavelengths ranging from 1700 nm to 2300 nm with 10 nm bandwidth.
3. RESULTS
We performed measurements for different pump wavelengths in the vicinity of the fiber ZDW, both in the normal and anomalous dispersion regimes of the fundamental mode. The ZDW of the fundamental mode for our fluoride fiber with 100 μm core size is specified to be around 1850 nm. Numerical simulations of the mode profiles and associated dispersion using the refractive index data from the manufacturer show that the ZDW of higher-order modes, on the other hand, decreases with the mode order and can shift below 1500 nm. This is illustrated in Fig. 3 where we plot the calculated spatial intensity distribution of selected linearly polarized higher-order modes and associated dispersion characteristics.
Fig. 3. Numerically simulated (a) dispersion and (b) spatial intensity profiles of fundamental and selected linearly polarized (LP) higher-order modes of the multimode fiber. Indices indicate the mode order. The vertical solid black lines mark zero-dispersion wavelength of each mode.
We emphasize that we observed significant variations in the SC spectrum at the fiber output depending on the injection condition (position of the fiber in the transverse plane and relative angle with the lens axis) and the results shown below correspond to injection conditions that maximized the SC bandwidth in each case. We first tuned the OPA wavelength to 1600 nm, in the normal dispersion regime of the fundamental mode. Figure 4 shows the normalized SC spectrum in logarithmic scale measured after 1 m (blue solid line) and 2 m of fiber (black solid line). The average output power was measured to be 690 mW and 580 mW for 1 m and 2 m long fibers, respectively, corresponding to an injected peak power of . We can see from the spectrum measured after 1 m that, although the pump is in the normal dispersion region of the fundamental mode, the SC generation dynamics are seeded by the high-order modes with anomalous dispersion at the pump wavelength, which leads to the generation of a dispersive wave at around 1315 nm after 1 m. With further propagation, the generation of multiple dispersive waves from modes with a ZDW below 1600 nm (and possibly some self-phase modulation contribution from modes with a ZDW above 1600 nm) and fission into multiple fundamental solitons leads to a quasi-continuous SC spectrum spanning from c.a. 1240 to 2180 nm ( bandwidth). We also note the emergence of isolated solitonic components on the long-wavelength side at around 2000 nm from the Raman self-frequency shift. The SC spatial beam profiles measured after 2 m of propagation in different wavelength bands between from 1710 nm to 2302 nm confirm the high multimode nature of the SC generation process. In particular, one can see how the output beam is not uniform with a clear off-centered maximum intensity peak. This suggests that there is some asymmetry in the coupling conditions with respect to the center of the fiber cross section, highlighting the important role of higher-order modes in effectively generating a SC with a very large bandwidth. In turn, the excitation of the multiple higher modes with ZDWs that are shifted toward the shorter wavelengths as compared to that of the fundamental, allows for soliton dynamics to occur and efficient SC generation.
Fig. 4. (a) SC spectrum generated in 1 m (blue) and 2 m (black) of multimode fiber with 100 μm core for a pump wavelength at 1600 nm. The dashed line marks the spectral limit of the OSA. (b) Corresponding beam profiles at the output of the 2 m long fiber at different wavelengths as indicated (with bandwidth of 10 nm in each case). Numbers on the spatial intensity profiles represent the factor by which the SC signal was amplified.
We subsequently tuned the pump wavelength to 1700 nm and the results are shown in Fig. 5 for optimized coupling conditions. The average power at the fiber output was 710 mW and 590 mW for 1 m and 2 m long fibers, respectively, corresponding to an injected peak power of . The spectrum after 1 m of propagation now shows the generation of multiple dispersive waves at 1470 nm on one hand, and over the continuous range 1185–1365 nm on the other hand. The broadening of the initial pump spectrum is also more pronounced than when pumping at 1600 nm. The generation of dispersive wave components at further detuned wavelengths from the pump is consistent with the fact that the pump wavelength is now further away from the ZDW into the anomalous dispersion regime of the higher-order modes [40]. The emergence of a dispersive wave at 1470 nm on the other hand, reflects the fact that lower-order modes can now excite soliton dynamics since the dispersion for these modes is also now anomalous at the pump wavelength. Further propagation leads to increased soliton dynamics with Raman self-frequency shift and interactions with the dispersive waves components, resulting in a continuous SC spectrum extending from 1210 to 2315 nm. The beam profiles measured at the fiber output are highly multimodal, confirming again the key role of higher-order modes in the SC generation process.
Fig. 5. (a) SC spectrum generated in 1 m (blue) and 2 m (black) of multimode fiber with 100 μm core for a pump wavelength at 1700 nm. The dashed line marks the spectral limit of the OSA. (b) Corresponding beam profiles at the output of the 2 m long fiber at different wavelengths as indicated (with bandwidth of 10 nm in each case). Numbers on the spatial intensity profiles represent the factor by which the SC signal was amplified.
The results obtained for a pumping wavelength of 1960 nm are plotted in Fig. 6, corresponding to output powers of 610 mW and 510 mW for the 1 and 2 m long fibers (1.45 MW peak power), respectively. One can see how a broad SC is now already generated after only 1 m of propagation in the multimode fiber. This can be understood from the fact that the pump wavelength is in the anomalous dispersion regime for all modes including the fundamental mode, such that all the injected energy can contribute to soliton dynamics unlike in cases shown above for shorter pump wavelengths. We observe the generation of dispersive waves from c.a. 1750 nm all the way down to below 1200 nm as a result of the multiple phase-matching conditions now fulfilled for a wider range of higher-order modes. The resulting SC spectrum is broadest for this particular pump wavelength, spanning from 1100 to over 2500 nm ( bandwidth) with relatively good flatness from 1390 to 2365 nm ( variation). We also note that there is only a change in the long wavelengths of the SC spectrum between 1 and 2 m of propagation, with the longer fiber leading to increased Raman frequency shift and approximately 100 nm larger SC bandwidth.
Fig. 6. (a) SC spectrum generated in 1 m (blue) and 2 m (black) of multimode fiber with 100 μm core for a pump wavelength at 1960 nm. The dashed line marks the spectral limit of the OSA. (b) Corresponding beam profiles at the output of the 2 m long fiber at different wavelengths as indicated (with bandwidth of 10 nm in each case). Numbers on the spatial intensity profiles represent the factor by which the SC signal was amplified.
When tuning the pump wavelength to 2080 nm far out in the anomalous dispersion regime (for all modes), the SC spectral bandwidth is reduced compared to lower pump wavelengths, as can be seen in Fig. 7. The output powers are 490 and 420 mW for the 1 and 2 m fibers, respectively, corresponding to an injected peak power of 1.2 MW. After 2 m, the SC bandwidth extends from 1210 nm to beyond 2500 nm with an isolated dispersive wave component in the normal dispersion regime. The difference in the spectrum compared to that recorded for a lower pump wavelength in the anomalous dispersion regime (see Fig. 6) is caused by the fact that (i) the injected peak power is now reduced as compared to shorter pump wavelength (an intrinsic characteristic of the OPA system used), and (ii) the value of the dispersion is increased at the pump wavelength. A net consequence is that the soliton number corresponding to the reduced peak power and increased dispersion is now for the fundamental mode, which is smaller than when using a 1960 nm pump ( for the fundamental mode), leading to fission into a reduced number of fundamental solitons and emitted dispersive waves. Of course, the soliton number given here should be interpreted with care here as multiple modes are excited simultaneously. However, because the dispersion increases as a function of wavelength for all modes, the general trend for the change in the soliton number as a function of pump wavelength is clear and can explain the dynamics observed. We also note a clear difference on the short-wavelength side between 1 and 2 m of propagation, with the dispersive wave initially generated at 1530 nm shifted down to 1400 nm for an extended propagation length. Correspondingly on the long-wavelength side, the SC spectrum extends by 110 nm more (from 2250 to 2360 nm) when the fiber length is increased to 2 m. This could partly explain the shift observed in the dispersive wave for longer propagation distance due to the coupling between the solitons and dispersive wave.
Fig. 7. (a) SC spectrum generated in 1 m (blue) and 2 m (black) of multimode fiber with 100 μm core for a pump wavelength at 2080 nm. The dashed line marks the spectral limit of the OSA. (b) Corresponding beam profiles at the output of the 2 m long fiber at different wavelengths as indicated (with bandwidth of 10 nm in each case). Numbers on the spatial intensity profiles represent the factor by which the SC signal was amplified.
4. CONCLUSION
In conclusion, we have reported for first time the generation of an octave-spanning supercontinuum into a meter-long multimode step-index fluoride fiber with 100 μm core size using femtosecond pulses. We have measured the supercontinuum spectrum as a function of pump wavelength and shown that the contribution of higher-order modes is central to the development of soliton dynamics and supercontinuum generation process. The broadest supercontinuum spectrum spanning from 1100 nm to over 2500 nm with 610 mW average power was achieved by injecting the 350 fs pulses at 1960 nm. It is also clear from our experiments that in such a large-core fiber where a large number of higher-order modes are typically excited the spectral broadening dynamics and supercontinuum bandwidth strongly depend on the coupling conditions. There is thus a clear link between the coupling conditions, excited higher-order modes, and resulting supercontinuum spectrum, and it is then important from an application viewpoint to optimize the coupling conditions in order to obtain the desired supercontinuum spectral characteristics. Finally, we point out that, although a detailed quantitative comparison with numerical simulations is extremely difficult due to the large numbers that are excited and is beyond the scope of this paper, preliminary results using few modes excitation tend to confirm the experimentally observed dynamics and physical interpretations provided above.
Our results are significant because they show that it is possible to generate an ultra-broadband supercontinuum in a fiber with very large core in the short-wavelength infrared. More generally, our results could ultimately allow for overcoming the low damage threshold and power limitation associated with the use of soft glass fibers traditionally used for the mid-infrared. Of course, the output beam profile is highly multimode, and this may be a limitation for some applications. However, there is also a wide range of application, such as, e.g., LIDAR or hyperspectral imaging, which do not require particularly good beam quality and the high-power multimode supercontinuum may then be an interesting approach for obtaining the high-power levels often required in those applications.
Funding
Horizon 2020 Framework Programme (H2020) (722380).
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