1. Introduction

Broad bandwidth infrared laser sources, have attracted extensive attention owing to their applications in optical frequency metrology [1], precision spectroscopy [2], optical coherent tomography [3] and remote sensing [4]. Moving into the mid-infrared (mid-IR), for wavelengths beyond 2 µm, pushes the applications further in LIDAR [5], hyperspectral imaging [6] and medical and biological system [7]. In comparison with alternative IR light sources, such as globars, lamps, synchrotron, fiber-based supercontinuum source offers attractive potential due to good beam quality and flexible delivery of light. In the last decades, the research interests for fiber-based SC sources mainly focused on silica glass fibers [8]. However, the intrinsic losses of silica substrate limit the spectrum to 2.5 μm [9]. In order to extend the SC to longer wavelength, soft glass fibers have been adopted. Heavy metal fluoride, such as ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber is one of the commonly used candidates due to its technological maturity, high glass stability and transparency in the mid-IR [10]. As ZBLAN fibers typically have zero-dispersion wavelengths (ZDW) between 1.65 – 1.9 μm [11, 12], pumping at longer wavelengths in the anomalous dispersion region is more favorable for efficient SC generation. Pumping near 2 μm results in both an extension of bandwidth and an increase of conversion efficiency towards mid-IR wavelengths [10]. There have been several previous work on SC pumped with Raman shifted erbium doped fiber laser [9, 11]. However, directly pumping by an ultrafast TDF laser, especially with femtosecond pulses have not been investigated.

So far, SC generation in ZBLAN fiber pumped by TDF lasers has been reported by several research groups. J. Swiderski et al. reported SC generation with a spectrum from 1.8 – 4.15 μm and an average power of 1.25 W, pumped by a sub-nanosecond pulsed gain switched thulium doped fiber amplifier (TDFA) [13]. A. M. Heidt et al. achieved the bandwidth from 750 – 4000 nm pumped by picosecond pulses from a TDF amplified gain-switched laser diode at 2 μm [10]. W. Yang et al. reported a spectrum extending from 1.9 to 4.3 μm with an average power of 13 W pumped by a picosecond 2 μm master oscillator power amplifier (MOPA) system [14]. A SC source with up to 21.8 W average power from 1.9 to 3.8 μm pumped by amplified picosecond TDF MOPA was demonstrated by K. Liu [8]. Despite the high power or broadband output, these prior work share a similar limitation in terms of spectral flatness. Their SC experimental results with the Peak to Continuum Ratio and the pump/output power are summarized in Table 1.

Table 1. Summary of previous experimental results of SC generation

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Most of the implementation of SC generation in ZBLAN fiber pumped by TDFA is based on picosecond laser pumping. However, the typical shape of SC pumped by picosecond pulses in the anomalous dispersion region consists of a residual peak at the pump wavelength and the continuum forms about 10 to 20 dB below the pump peak [10]. In [9], a flat spectrum from 1.9 – 4.5 μm without any residual pump was obtained by a nanosecond cascade EYDFA-TDFA pumping, where the disappearance of residual pump was mainly benefited from the two-stage SC generation. This residual peak is the result of less dominating self phase modulation (SPM) effect. Pumping with femtosecond pulses will enhance the SPM process and transfer energy away from pump peak and thereby decrease the amount of residual pump light in the spectrum [15]. Thus, a flatter spectrum can be generated through pumping by femtosecond pulses. The flatness of supercontinuum is an important property for some application. For example, in infrared countermeasures, the ideal spectrum would be flat over the atmospheric mid-IR transmission windows [16]. Moreover, for femtosecond pump pulses, the grade of spectral coherence and the stability of phase are considerably higher than longer pulses. These two properties are crucially important for some applications such as optical frequency metrology and optical coherence tomography [17].

In this paper, we have demonstrated, for the first time to our knowledge, a femtosecond pulse TDFA pumped mid-IR SC laser in a customized single mode ZBLAN fiber. A nonlinear polarization rotation based passively mode-locked oscillator worked as a seed laser, operating at a central wavelength of 1970 nm and a repetition rate of 56 MHz. A pre-chirp TDFA boost the average power to 658 mW, and the output pulses had a pulse width of 181 fs and a spectrum from 1.9 μm to 2.3 μm. This SC spectrum was extended from 1.1 μm to 3.7 μm in the ZBLAN fiber with a 10 dB spectrum width of 1340 nm at average power of 258 mW.

2. Experiment setup and results

2.1 Experiment setup

The schematic of experimental setup is illustrated in Fig. 1. A nonlinear polarization rotation based passively mode-locked fiber laser is used as the seeding oscillator. The optical pulses delivered by the oscillator are first amplified in a TDF pre-amplifier. As a next step, the pulses from the preamplifier, after optical isolation, are pre-chirped by passing an ultra high numerical aperture (UHNA) fiber with NA of 0.41, core/cladding diameter of 2.4/125 μm and normal dispersion of −49.56 ps/km/nm. The pulse train from the UHNA fiber is then boosted in a TDF main amplifier, consisting of a 5 m single mode double-cladding TDF with core/cladding diameter of 10/130 μm, core/cladding NA of 0.15/0.46, and cladding absorption of 3 dB/m at 793 nm. A (2 + 1) × 1 pump combiner is used to deliver forward pumping light to the gain fiber from a fiber-pigtailed multimode laser diode operating at 793 nm. The output end of the main amplifier is fusion spliced to a section of 20 cm SMF-28 fiber, to filter out the residual pump. At last, the output end of the SMF-28 fiber is mechanical spliced to the input end of a 15 m ZBLAN fiber (this length was not optimized) with a cleaved angle of 8° on both fibers to avoid back reflection. The ZBLAN fiber has core/cladding diameter of 6/125 μm and NA of 0.265. This ZBLAN fiber is customized to a small core and large NA to provide good confinement of the guided mode in the fiber core region [9]. The ZDW is 1.66 μm with a dispersion slope of 0.021 ps /nm²/km (calculated using OptiFiber).

 

Fig. 1 Schematic experimental setup of the supercontinuum generation system.

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In our experiment, the power was measured with a wavelength insensitive thermal power meter (Ophir, 3A-FS). The SC spectra below 3400 nm were measured by two optical spectrum analyzers (YOKOGAWA, AQ-6315B, 350–1750 nm, and AQ 6375L, 1900–3400 nm) whereas that for longer wavelengths was acquired by using a Mid-IR spectrum analyzer (Bristol 771, 2-12 μm). The ultrafast pulse was characterized by a second harmonic generation based frequency resolved optical gating (FROG) (MesaPhotonics). A high speed digital oscilloscope with 2.5 GHz bandwidth (Agilent, DSO-X92504A) and a 12.5 GHz InGaAs photodetector (EOT, ET- 5000F) were used to measure time characteristics.

2.2 Thulium doped fiber amplifier

Figure 2(a) shows the measured spectrum from the fiber preamplifier. The output power is 100 mW and the spectrum has a central wavelength of 1970 nm and a 3 dB spectrum bandwidth of 19 nm. The pulse duration is measured to be 918 fs, as shown by the FROG reconstructed pulse profile in Fig. 2(b). The inset of Fig. 2(b) shows the stable pulse train with a repetition rate of 56 MHz. To pre-compensate the anomalous dispersion induced by the following stage of amplifier, a 4 m UHNA fiber was spliced after the preamplifier with 80% coupling efficiency. After UNHA fiber, the pulse width was stretched to 4.3 ps with positive chirp.

 

Fig. 2 (a) Spectrum of the pulse after preamplifier. Insert shows the pulse train with a repetition rate of 56 MHz. (b) Pulse profile reconstructed by FROG after the preamplifier.

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The pulses with an average power of 69 mW, at the output of the fiber combiner, were launched to the double-cladding TDF to be further amplified up to 658 mW. The slope efficiency with respect to the coupled pump power was measured as 15%. The relatively low slope efficiency is mainly limited by the quantum defect of 60% when Tm-doped fibers are pumped at 793 nm [18].

During the amplification process in the power amplifier, the laser spectrum is gradually extended which is mainly initiated by the nonlinear effects of SPM and stimulated Raman scattering (SRS) [8]. The measured spectra at different output power levels (85 mW, 305 mW, 658 mW) are shown in Fig. 3(a). At the same time, the pulse width is narrowed during the amplification process. Figure 3(b) shows the pulse profiles at these three powers and the FROG traces are shown in Fig. 3(c). The full width at half maximum of the pulse was measured as 181 fs at the power of 658 mW, more than a compression factor of 5 with respect to the pulse width from preamplifier output. The consequent temporal compression can be understood as the higher order soliton pulse compression, which is the result of the interplay between SPM and anomalous dispersion [19,20]. The sign of chirp induced by anomalous dispersion in the main amplifier is opposite to the chirp induced by SPM and the pre-chirped normal dispersive fiber [21]. During the pulse propagation in the TDF, these two chirps can compensate each other and the spectrum is broadened; thus, the nonlinear pulse compression can be achieved [22].

 

Fig. 3 (a) Output spectra, (b) pulses profiles and (c) FROG traces at the average power of 85 mW, 305 mW, and 658 mW after the main amplifier.

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2.3 Supercontinuum generation in ZBLAN fiber

The generated SC spectra are shown in Fig. 4 as a function of output average power from the ZBLAN fiber. The maximum spectra bandwidth spanning from 1.1 to 3.7 μm is achieved for 253 mW average power with spectral flatness of 10 dB over a 1390 nm from 1940 – 3330 nm. The spectrum was bi-directionally broadened both to near- and mid-IR region as presented. The dip around 2.7 μm corresponds to OH⁻¹ ions absorption in the ZBLAN fiber and in the unpurged (N₂) detection system.

 

Fig. 4 ZBLAN output spectra generated at an average output power of 35 mW, 100 mW and 253 mW.

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As the femtosecond pulses pump experienced anomalous dispersion in the ZBLAN fiber, the 65 kW peak power was high enough to generate higher order solitons and initially undergo the expected evolution dynamics of spectral broadening and temporal compression [23]. However, the higher order effects, such as intrapulse Raman scattering and third order dispersion, act as perturbations, causing the fission of higher order solitons into the constituent fundamental Solitons [24]. Different from the SC pumped by picosecond pulses where the main mechanism of spectra broadening is MI and four wave mixing (FWM), it turns out that soliton fission plays a critical role in the formation of a supercontinuum pumped by femtosecond pulses [25]. The individual solitons have a relatively wide spectrum and are thus affected by intrapulse Raman scattering that shifts the soliton spectrum toward longer and longer wavelengths with further propagation inside the fiber [23,26]. As a result, many new spectral components are added on the long-wavelength side of the original pulse spectrum and form the mid-IR region of the SC. Moreover, during the soliton fission process, the presence of higher order dispersion can lead to the transfer of energy from the soliton to the phase matched non-soliton radiation This radiation is called dispersive wave whose wavelengths fall on the shorter wavelength side in the normal dispersion region (<1.66 μm for ZBLAN fiber used in the experiment) [27]. This dispersive wave dominates the short wavelength generation in the near-IR region.

From the spectrum presented in Fig. 4 at the average power of 253 mW, we can observe that there is no significant residual peak at the pump wavelength, in contrast to the typical SC generation pumped by picosecond pulses for which the continuum forms 10-20 dB below the pump peak. The residual peak is due to the insignificant of SPM effect for using picosecond pumps [28]. In femtosecond regime, SPM would be expected to be strong because the Stokes-broadening is in inversely proportional to the rise time of the laser pulses [29]. The enhanced SPM now becomes non-negligible during the nonlinear process. It can be reflected as spectrum broadening by transferring the energy from the pump to shorter and longer wavelengths continuously, which results in the flatter spectrum. To verify this observation, pulse propagation simulation was carried out to calculate the output SC spectra pumped by pulses with different pulse width but same peak power, as shown in Fig. 5(a). The detailed spectra at the pump wavelength region for these three situations are shown in Fig. 5(b). Obviously, the residual peak is not present in the SC pumped with femtosecond pulses. However, the SC spectra pumped by picosecond and nanosecond pulses have 14 dB and 26 dB residual peak respectively.

 

Fig. 5 (a) SC spectra pumped by femtosecond, picosecond and nanosecond pulses with the same peak power. (b) The magnification of the pump wavelength part of SC spectra.

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The ultrafast pulse pump can get high peak power of more than 50 kW at sub-watt average power level. Different with the SC generated in [8,14] which requires high pump power (tens of watts), the SC generated by femtosecond pump can achieve octave-spanning with sub-watt level, i.e. 658 mW. Thus, it is more efficient to achieve broadband SC using femtosecond pump, and avoid thermal issues raised by soft glass fibers as well. The TDFA pulse can be improved to narrower pulse width and higher peak power by adjusting the 793 nm pumped power and the fiber length. A wider spectrum and a higher efficiency of long wavelength in the Mid-IR region can be achieved by optimizing the ZBLAN fiber (dimension, NA, and dispersion).

3. Conclusions

In conclusion, we have demonstrated for the first time, to the best of our knowledge, supercontinuum generation pumped by sub-watt femtosecond pulses from a thulium doped fiber amplifier. With a compact all-fiber pump laser with 181 fs pulses at average power of 658 mW, the supercontinuum source can demonstrate broadband flat spectrum coverage from 1100 to 3700 nm. The 10 dB bandwidth has a range over 1390 nm in the mid-IR region from 1940 – 3330 nm. From a practical point of view, generation of supercontinuum by pumping source with low average power demands less stringent specifications for all the optical components as well as the mid-IR waveguide. Moreover, such broadband coherent source offers great opportunity in applications such as LIDAR, spectroscopy and metrology.