1. Introduction

Supercontinuum generation (SCG) in non-silica fibers is a promising approach to meet the growing demands for broadband, high brightness mid-IR radiation in various research areas such as molecular fingerprinting and chemical sensing [1,2]. Heavy metal fluoride (ZBLAN) fibers are particularly attractive due to their technological maturity and transparency in the mid-IR, and SCG in this fiber type has been demonstrated with numerous pumping schemes, predominantly using either nanosecond or femtosecond pumping [1–7].

For SCG in the near-IR and visible wavelength regions, picosecond pumping has been successful in both research and commercial products as it provides many of the proven advantages associated with femtosecond systems, but with great benefits in terms of reduced system cost and complexity, as well as higher spectral power densities [8–10]. When seeded by electrically gain-switched semiconductor diode lasers, picosecond pump systems offer a high level of control, compactness, and reliability with intriguing prospects for controlling the properties of the continuum that is generated [11]. The pumping of ZBLAN fibers with diode-seeded picosecond fiber amplifier systems therefore represents a particularly attractive route towards practical mid-IR SC generation.

Current implementations of diode-pumped mid-IR SCG in ZBLAN fibers are based on amplified nanosecond - rather than picosecond – pulsed laser diodes emitting around 1550 nm and therefore rely on the decay of the pump pulses into noisy femtosecond sub-pulses prior to the coupling into the nonlinear fiber in order to reach the required peak powers for the SCG process [3]. While this technique leverages the maturity of telecommunication components, it also relinquishes much of the potential control offered by diode pumping. As ZBLAN fibers typically have zero-dispersion wavelengths (ZDWs) between 1.65 – 1.9 µm, pumping at longer wavelengths in the anomalous dispersion region is more favourable for efficient SCG. Pumping close to 2 µm results in both an extension of bandwidth and an increase of conversion efficiency towards mid-IR wavelengths [12]. Seed diodes around 1550 nm therefore require the conversion of the pump pulses to longer wavelengths. Several methods including soliton self-frequency shift, a two-stage SCG process with intermediate amplification in Thulium-doped fiber amplifiers (TDFAs) or the gain-switching of a TDF laser cavity (pumped using a pulsed 1550 nm laser diode) have been applied for this purpose [2,13], neither of which are ideal from a system simplicity and reliability point of view.

In this paper we present what we believe to be the first report of a mid-IR SC source pumped directly with high peak power picosecond pulses from a fiber-amplified gain-switched laser diode at 2 µm. We achieve a spectral bandwidth of more than two octaves spanning from 750 - 4000 nm, limited only by the transparency window of the ZBLAN fiber. The SC source delivers Watt-level average power, high stability, and spectral flatness of better than 1.5 dB in the range 2450 - 3750 nm, which is unprecedented in the mid-IR wavelength region and enables, for example, spectroscopic measurements with uniform spectral sensitivity and high signal-to-noise ratios. The system should be power-scalable to the 10 W regime and represents a significant improvement in system simplicity compared to existing diode-pumped mid-IR sources.

2. Experimental setup

The experimental setup of the pump system and the SCG stage is shown in Fig. 1(a). The picosecond pump pulses are directly generated by electrical gain-switching of a discrete-mode laser diode (Eblana Photonics) operating at 2008 nm and are subsequently amplified in a multistage Thulium-doped fiber amplifier (TDFA) chain. The system consists of two pre-amplifiers, both core-pumped by an in-house built 1565 nm Er/Yb fiber laser, and a power amplifier stage using large-mode area TDF with a core/cladding diameter of 25/250 µm (Nufern), free space cladding-pumped in a counter-propagating configuration by a 800 nm fiber-coupled laser diode with up to 75 W of pump power. A fiber Bragg grating (FBG) based spectral filter removes excess amplified spontaneous emission (ASE) after the first pre-amplifier. The system delivers 33 ps pulses with up to 3.5 µJ pulse energy and 100 kW peak power at variable repetition rates in the range 1 MHz – 1 GHz with near-diffraction limited beam quality, as reported in [14,15]. For the SC generation, the high power picosecond pulses are extracted from the pump system with a dichroic mirror and coupled into 7 m of ZBLAN fiber (9 µm core diameter, 0.25 NA, IR Photonics) with 60% coupling efficiency using an aspheric lens. We performed experiments with a hand-cleaved fiber input facet as well as a polished and connectorized fiber, but observed no significant difference in coupling efficiency or power handling capability. A polarizing isolator and a pair of half-wave plates protect the pump system from back-reflections, allow variable attenuation of the pump beam and the control of its polarization state. The generated continuum was characterized using two optical spectrum analyzers (OSAs), a Yokogawa AQ6370 for the wavelengths up to 1600 nm and an AQ6375 in the range 1600 – 2200 nm, as well as a monochromator (Bentham TMc300V) with liquid nitrogen cooled PbS detector for wavelengths above 2200 nm. The measurement resolution was set to 2 nm for the OSAs and 10 nm for the monochromator. All instruments were corrected for their respective wavelength responses, and the relative power levels were calibrated by recording data with ~100 nm measurement overlap between the instruments.

 

Fig. 1 (a) Schematic experimental setup of the diode-seeded Thulium-doped fiber amplifier (TDFA) pump system and the SC generation stage. LD: laser diode; FBG: fiber Bragg grating; LMA-TDF: large mode area Thulium-doped fiber; DM: dichroic mirror; HWP: half-wave plate; ISO: isolator. (b) Calculated dispersion profiles of the fundamental LP₀₁ mode and the first three higher order modes of the ZBLAN fiber.

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Figure 1(b) shows the dispersion profiles of the fundamental LP₀₁ mode and the first three higher order modes of the ZBLAN fiber, calculated with a fully vectorial finite element method using the material dispersion supplied by the manufacturer and assuming a constant NA for all wavelengths. At the pump wavelength of 2008 nm, the fiber is multimoded and supports both LP₀₁ and LP₁₁ modes. The ZDW of the fundamental mode is estimated at 1650 nm, i.e. the pump is located deep in the anomalous dispersion region with D ≈ 13 ps/(nm·km) at 2008 nm. The LP₁₁ mode has low normal dispersion (D ≈-3 ps/(nm·km)) at the pump wavelength and cuts off above ~2800 nm, i.e. the fiber is single-moded at mid-IR wavelengths. All other higher order modes have cut-off wavelengths much shorter than the pump.

3. Results and discussion

3.1 Spectral characteristics and SC generation dynamics

The generated supercontinuum spectra are shown in Fig. 2 as a function of pulse energy measured at the output of the ZBLAN fiber. The maximum spectral bandwidth of more than two octaves spanning from 750 – 4000 nm is reached for 1.1 µJ pump pulses. A further increase in pump power did not lead to a significantly enhanced spectral broadening, indicating that the bandwidth of the SC at mid-IR wavelengths is likely limited by the transparency window of the ZBLAN fiber. The influence of fiber attenuation is evident from the steepening of the spectral slope on the long wavelength edge of the continuum at wavelengths above 3800 nm. This coincides with the general attenuation curve provided by the manufacturer indicating exponentially increasing losses for wavelengths above 3750 nm [16]. We have therefore demonstrated a spectral coverage close to the achievable maximum for this particular fiber and pumping scheme, because the short- and long-wavelength extrema of the SC are linked via group-velocity-index matching, as will be discussed below.

 

Fig. 2 Supercontinuum spectra generated with 185 nJ (black), 300 nJ (red), 600 nJ (green), and 1100 nJ (blue) input pulse energy (offset for clarity). Also shown are the calculated group indices of the fiber modes with identical colour code as in Fig. 1(b). The wavelength axis of the calculated group indices is shifted by about 100 nm with respect to the measurement, as indicated by the lower x-axis (blue). The insets show a magnification of the mid-IR part of the broadest spectrum (top right) and the far-field mode profile of the ZBLAN fiber output at visible wavelengths (top left).

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The shape of the SC is typical for picosecond pumping in the anomalous dispersion region of a fiber, consisting of a residual peak at the pump wavelength and the continuum forming approximately −20 dB below the pump peak. The spectra exhibit a remarkable flatness in the mid-IR with a power variation as low as 1.5 dB over a 1300 nm wide spectral range from 2450 – 3750 nm for 1.1 µJ pump pulses, as shown in the inset of Fig. 2. For this measurement the monochromator was purged with nitrogen in order to minimize absorption by water vapour and CO₂ during the free-space propagation through the instrument. Note that the narrower spectra shown in Fig. 2 were taken without nitrogen purging and exhibit (unresolved) water absorption peaks in the range 2600 – 2800 nm. The high degree of spectral flatness of the generated SC is unprecedented in this wavelength region and combined with the high average output power enables broadband mid-IR spectroscopic measurements with uniform spectral sensitivity and high signal-to-noise ratios (results in preparation).

The observed SCG dynamics are determined by the dispersion profile of the fundamental mode, leading to modulation instability (MI) initiated break-up of the injected pulse into fundamental solitons and subsequent self-frequency shifting to longer wavelengths [17]. The contribution of the LP₁₁ mode to the spectral broadening process is limited by its predominantly normal dispersion and the low cut-off wavelength. Since the soliton number of the input pulse is estimated to be of the order N = 100, a large sea of solitons with mutually overlapping spectra is expected, which in superposition form the flat SC spectrum observed experimentally. The calculated group indices of the fiber modes, shown in Fig. 2, reveal that the red-shifting solitons are group-index matched to an ensemble of modes at short wavelengths, i.e. pulses in these modes propagate with the same velocity through the fiber. The co-propagation enables nonlinear interaction and energy transfer from the solitons to these modes in the form of a dispersive wave [18]. A recent study demonstrated that the dispersive waves can be generated directly in higher order modes, although the solitons propagate exclusively in the fundamental mode for wavelengths above 2800 nm [19]. The ZBLAN fiber therefore emits the short wavelengths of the SC as a superposition of higher order modes, as is shown in the top left corner inset in Fig. 2, which displays the far-field collimated beam profile of the visible part of the continuum.

An intuitive insight that summarizes the main components of these rather complex SCG dynamics is given by the visual overlap of the experimental results with the calculated group-velocity indices of the ZBLAN fiber modes seen in Fig. 2, which shows good agreement of the experimental long- and short-wavelength extrema of the continuum with the phase-matching predicted from the group-index matched positions. Due to uncertainties in our computational assumptions, in which we had to rely on the fiber geometry and material dispersion reported by the manufacturer, we shifted the wavelength axis of the calculated group indices by about 100 nm towards longer wavelengths, as was done similarly by others in previous publications [6].

3.2 Power and stability

Figure 3 shows the power conversion efficiency to selected mid-IR wavelength regions (defined as percentage of total SC output power) as well as the average power levels in these regions at a pump repetition rate of 1 MHz as a function of coupled pump pulse energy. We achieve up to 1.1 W of total average output power, with more than 21% (235 mW) at wavelengths > 2500 nm and more than 12% (130 mW) at wavelengths > 3000 nm. The pump depletion is in the order of 60% for all spectra shown in Fig. 2. Higher efficiencies have been reported recently (e.g. 27% for wavelengths > 3800 nm [12]), but in contrast to these reports we observe significant bi-directional spectral broadening both to near- and mid-IR regions, and apply direct picosecond pumping, which inherently leads to lower pump depletion than femtosecond pumping [17], but also results in higher system simplicity and reliability.

 

Fig. 3 Power conversion efficiency (black) to selected mid-IR wavelengths regions and average power levels (red) in these regions at 1 MHz pump repetition rate as a function of pump pulse energy.

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Note that the average power levels in Fig. 3 refer to the lowest investigated pump repetition rate, as the system was reliably operated over several weeks for many hours per day without observing any degradation. However, the average power levels can easily be scaled by increasing the pump repetition rate, which can simply be done electronically via the pulse generator used to gain-switch the seed diode. This highlights the simplicity and high potential for automated electronic control of our approach. In its current configuration the pump system can deliver the necessary pulse energy for maximum spectral broadening (1.1 µJ) up to repetition rates of more than 15 MHz [14], enabling a potential SC power in the order of 10 Wand more than 1 W at wavelengths > 3000 nm. When we increased the total SC power level to 2 W in our experiments, fiber damage occurred at about 80 cm distance from the input end. The fiber end facet was unaffected. The damage mechanism is still unclear, but we suspect coating inhomogeneities to be responsible for the failure, as inspection with a visible light source revealed several leakage points along the fiber length. Earlier demonstrations have shown that ZBLAN fibers are capable of handling more than 10 W of average power [20], and we consequently consider the fiber damage a technical rather than a fundamental limitation.

Since the SCG dynamics are initiated by noise-seeded MI, large shot-to-shot fluctuations in the spectral and temporal domain are expected, i.e. the continuum is temporally incoherent [17]. However, when using slow detectors such as power meters or spectrometers averaging over thousands of shots, the spectrum appears extremely stable. Figure 4 shows a long-term power stability measurement at a test wavelength of 3350 nm, which was recorded with an extended-wavelength OSA (Yokogawa, measurement range up to 3400 nm) set to 2 nm resolution. Although the test wavelength is separated from the pump by over half an octave, we record less than 0.5 dB power variation over a 30-minute interval. No special effort was taken to minimize thermal effects on the ZBLAN fiber or to stabilize the polarization state of the pump system, which might improve this value even further. The excellent stability of the continuum is enabled by the low amplitude and timing jitters of the pulse generation process in the gain-switched discrete-mode seed diode and their preservation in the TDFA chain [14], highlighting the high practicality of our pumping approach.

 

Fig. 4 Source stability over a 30-minute interval at a test wavelength of 3350 nm.

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4. Conclusion

To the best of our knowledge, we present the first supercontinuum source pumped directly by fiber-amplified picosecond pulses from a gain-switched 2 µm laser diode, achieving broadband spectral coverage from 750 to 4000 nm, Watt-level average power, exceptional spectral flatness in the mid-IR, and high time-averaged stability. By seeding and amplifying high peak power picosecond pulses directly at 2 µm for efficient pumping of ZBLAN fiber, we achieve a significant improvement in system simplicity and stability compared to earlier diode-pumped SC sources. The system should be scalable to average powers of more than 10 W using fibers with better power handling capabilities, and a fully fiberized implementation of the source is also easy to envisage. Maturing fabrication technology for glass materials with larger mid-IR transparency window offers future potential for a further bandwidth increase with this pumping scheme, which represents an attractive route towards relatively inexpensive, versatile and practical sources of high power broadband mid-IR radiation.