1. Introduction

Ultrafast, compact laser sources operating at a wavelength around 2 μm are attracting attention regarding their applications e.g. in medicine, material processing and as pump source for nonlinear frequency conversion stages ranging from frequency conversion to the mid-infrared spectral region to THz generation [1–3]. Different approaches for generating ultrashort pulses in this eye-safe spectral region have been pursued such as singly-doped thulium (Tm) [4–10] and holmium (Ho) fiber lasers [11–15], thulium-holmium (Tm-Ho) co-doped fiber lasers [16–20], Raman-shifted erbium-doped oscillators [21–26] and Ho- or Tm-doped crystalline bulk lasers [27–30]. In this context, all-fiber lasers offer the advantage of operating ruggedly and alignment-free in a compact and reliable design.

Holmium has been proven to be an attractive candidate for mode-locked fiber lasers emitting at wavelengths longer than 2 μm. The transition between the ⁵I₇ and ⁵I₈ manifolds covers an emission wavelength range from 2000 nm up to 2200 nm and allows for efficient in-band pumping at 1950 nm with Tm-doped fiber lasers [31]. Such oscillators usually operate in the soliton regime, delivering sub-nJ pulse energies at some MHz repetition rate [11]. In order to scale the pulse energy up to the μJ- or even mJ-level one needs to use fiber pre-amplifiers and subsequent regenerative or linear amplifier systems based on holmium-doped crystals [32–36]. Promising candidates are Ho:YLF and Ho:YVO₄ crystals since their emission cross sections are higher than those for Ho:YAG or Ho:Lu₂O₃ and, additionally, their saturation energies are the smallest among holmium-doped crystals [37–40]. All these crystals have different emission bands between 2040 nm and 2120 nm with spectral bandwidths not exceeding 10 nm. The number of Ho-doped laser sources which meet the spectral requirements for seeding Ho-based amplifiers is very limited. Recently, Li et al. demonstrated a Ho:fiber ring oscillator, spectrally optimized for Ho:YLF amplifiers [41]. However, the setup comprises of a free-space section within the cavity which lacks compactness, reliability, stability and leads to complex fiber coupling for subsequent fiber-based pre-amplifiers.

Here, we demonstrate a passively mode-locked holmium-doped oscillator with subsequent thulium-/holmium-doped fiber tandem amplifier operating in the rather short wavelength range at 2.05 μm in a simple and compact all-fiber design. The oscillator operates in the fundamental soliton regime generating 0.19 nJ-pulses at a repetition rate of 27.3 MHz and a pulse duration of 1.2 ps which are amplified in a sequential design of a thulium- and a holmium-doped fiber, pumped only by a low-cost high-power diode at a wavelength of 793 nm, to pulse energies of more than 75 nJ. The remaining non-absorbed pump light of the oscillator at 1950 nm is recycled as seed for the diode-pumped in-line Tm-doped fiber which is directly pumping the Ho-doped fiber amplifier. This configuration is more flexible in terms of integration and efficiency compared to co-doped Tm/Ho-fiber amplifiers.

2. Experimental setup

The layout of the laser is shown in Fig. 1. The linear cavity of the oscillator consists of a 0.5 m long Ho-doped single-clad fiber (iXblue, IXF-HDF-8-125) with a mode-field diameter (MFD) of 10.3 μm at 2050 nm and a numerical aperture (NA) of 0.16 which is core-pumped via a 1950/2050 nm wavelength-division multiplexer (WDM) by an in-house built continuous wave Tm:fiber laser operating at a wavelength of 1950 nm. The WDM exhibits narrow transmission bandwidth to lock the emission of the oscillator to the desired center wavelength around 2052 nm and is built in-house as well. Mode-locking is initiated and stabilized by a commercially available semiconductor saturable absorber mirror (SESAM) with a modulation depth of 5% and a saturation energy of 80 μJ/cm² (Batop, SAM-2150-8-10ps). A fiber mirror (Haphit Inc.) was used as a highly reflective end mirror, and a 75/25-coupler (Thorlabs) provides bidirectional output coupling of 25% each. An in-line polarization controller further stabilizes mode-locking and permits tuning of the center wavelength between 2050 and 2060 nm. In addition to the Ho-doped fiber, passive fibers with a length of about 3.3 m provided by the fiber component pigtails were applied: 2.5 m of SM2000 fiber (Thorlabs) and 0.8 m of SMF-28 with a MFD of 15 μm and 11.8 μm at 2050 nm, respectively. All cavity components together exhibit anomalous net cavity second order dispersion of −0.38 ps² enabling fundamental soliton operation with a repetition rate of 27.3 MHz. The dispersion values for the different fiber types have been derived from given fiber parameters considering material and waveguide dispersion.

 

Fig. 1 Schematic setup of the HDF oscillator and amplifier. SESAM: Semiconductor saturable absorber mirror; WDM: wavelength division multiplexer; HDF: Holmium-doped fiber; TDFA: Thulium-doped fiber laser amplifier; TDF: Thulium-doped fiber; MMPC: Multimode pump combiner; LD: Laser diode; PC: Polarization controller; SM2K: Single mode passive fiber (SM2000).

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The oscillator is protected by an optical fiber isolator against back light from the subsequent Tm-/Ho-fiber tandem amplifier. In order to reduce the pulse peak power and mitigate nonlinear effects in the amplifier stage, we spliced additional 20 m of passive fiber to the output for temporal pulse stretching. We chose SM2000 fiber owing to lower bending losses in this wavelength range compared to SMF-28 or similar fibers. Additionally, the SM2000 fiber exhibits a mode field diameter which is 20% larger than the one of SMF-28 resulting in lower pulse peak intensities and therefore weaker nonlinear effects. The following 4 m of thulium-doped double-clad fiber (Nufern, SM-TDF-10P/130-HE) are cladding pumped via a multimode pump combiner by a low-cost high-power diode delivering an output power of 8 W at a wavelength of 793 nm (PhotonTec). The amplifier section is terminated by a 1.3 m long holmium-doped fiber which is directly spliced to the thulium-doped fiber. The splicing loss between the different fibers with various mode field diameters has always been in the range of 0.1 dB. We used the Fitel S184 Fusion Splicer. However, the loss between the Ho- and the Tm-doped fibers could not be measured due to the absorption of the dopant ions. We expect to have a little more loss of about 0.5 dB owing to the fiber properties of the octagonal double cladding and the pedestal in the Tm-doped fiber. All open fiber end faces are angle cleaved in order to avoid back reflections.

3. Experimental results

Stable and reliable self-starting mode-locking occurs at an absorbed pump power of about 140 mW. The measured output power at a repetition rate of 27.3 MHz was around 5 mW resulting in a pulse energy of 0.19 nJ and an optical-to-optical efficiency of 3.5% which is consistent with other published Ho-based soliton oscillators [11,14,42]. The oscillator has been running for hours in laboratory environment and does not show any instability due to temperature or mechanical influences. The center wavelength of the output spectrum at 2052 nm with full width at half maximum (FWHM) around 4.3 nm matches the Ho:YLF and Ho:YVO₄ gain spectrum. Additionally, Kelly-sidebands which confirm the solitary mode-locking are rather low hence band pass filters are not needed to shape the seed spectrum prior to subsequent amplification stages. Typical oscillator output characteristics such as optical and radio frequency spectra are shown in Fig. 2(a). No signs of q-switching instabilities can be observed confirmed by the frequency comb up to 2.5 GHz which was measured with a 26.5 GHz radio-frequency (RF) spectrum analyzer. The fundamental beat note in the RF spectrum at 27.3 MHz corresponds to the inverse of the temporal spacing of 36 ns between two consecutive pulses. The latter confirms single pulse operation by taking into account a linear fiber resonator length of 3.8 m. The optical spectrum in Fig. 2(b) reveals about 35 mW of remaining pump power at 1950 nm which is coupled out together with the signal pulses. The strong spectral modulation of the radiation at 1950 nm is a consequence of the Fabry-Perot laser diode which is used as seed source for the Tm:fiber pump laser. The red line in Fig. 2(b) features the measured transmission of the WDM in the oscillator which is used to multiplex pump and signal wavelength and at the same time exhibits narrow transmission around 2050 nm forcing the laser to operate in this desired spectral region.

 

Fig. 2 Experimental results for the HDF oscillator: (a) fundamental beat note with resolution bandwidth of 10 Hz, inset: radio frequency comb and (b) optical output spectrum with signal at 2052 nm and pump at 1950 nm (black line) as well as the transmission of the WDM (red dashed line).

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The pulse duration could not be measured directly behind the oscillator output port due to the low average output power. However, we can expect to have a nearly transform-limited pulse duration because of the fundamental soliton operation. In Fig. 3(a) the Fourier-limited pulse duration based on the related optical spectrum is presented which is about 1.2 ps. The pulse spectrum and pulse duration of the oscillator has been verified via numerical simulations performed by a commercially available software [43] which is solving the extended nonlinear Schrödinger equation by means of the split-step Fourier method. We considered all available parameters such as material and waveguide dispersion, corresponding mode-field diameters, spectral loss and gain, self-phase modulation, self-steepening, Raman response as well as absorbance and modulation depth of the SESAM. However, third order dispersion in fiber propagation has been neglected due to weak influence in the pulse duration range above 1 ps and the fundamental soliton regime. The results match almost perfectly the measured spectrum and the Fourier-limited pulse duration as can be seen in Fig. 3(a). This is in line with our estimation regarding the pulse duration close to the Fourier-limit. A 10 GHz photodiode in combination with a 6 GHz oscilloscope were used to confirm single-pulse operation by measuring the output pulse train, see Fig. 3(b).

 

Fig. 3 Experimental results for the HDF oscillator: (a) measured (black continuous line) and simulated (red dashed line) optical pulse spectrum, inset: Fourier-limited and simulated pulse, (b) oscilloscope trace.

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In the following Tm-/Ho-doped fiber tandem amplifier the residual pump light at 1950 nm is amplified to 2.8 W at full absorbed pump power of 7.8 W delivered by the high power multimode diode emitting at 793 nm. Since the gain spectrum of Tm-doped silica fiber covers the pulse signal wavelength as well [44], the pulses are amplified to a pulse energy of almost 5 nJ. Due to nonlinear effects in the 20 m passive fiber and gain narrowing in 4 m of thulium-doped fiber, the pulse bandwidth is reduced to a FWHM of 2.1 nm. At the same time the anomalous dispersion stretches the pulse duration to 5.3 ps, assuming a squared hyperbolic secant shape (sech²). The measured autocorrelation trace and the optical spectrum can be seen in Fig. 4(a). The signal and pump output power as well as the signal pulse duration have been measured prior to splicing the Ho-doped fiber directly to the Tm-doped fiber. This enables a careful characterization of the setup.

 

Fig. 4 AC traces measured with an autocorrelator: (a) measured behind the thulium fiber and (b) measured at angle cleaved holmium-doped fiber output.

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In the subsequent holmium-doped fiber the radiation at 1950 nm pumps the signal at 2052 nm. The Ho:fiber length has been optimized regarding the achievable output power resulting in a length of 1.3 m. The highest signal output power behind the holmium fiber was 2.08 W which corresponds to a pulse energy of 76.1 nJ maintaining a pulse duration around 5 ps as can be seen in Fig. 4(b). The FWHM spectral bandwidth corresponds to a Fourier-limited pulse duration of 547 fs. The overall gain of the thulium-/holmium fiber tandem amplifier is 26 dB. With increasing pulse energy we observed the onset of strong nonlinear effects, in particular, self-phase modulation due to the high pulse peak intensities results in a structured rather than a smooth optical spectrum as depicted in Fig. 5(a) for pulse energies of more than 4.7 nJ. Nevertheless, we could not see any pulse distortions in the autocorrelation trace up to the highest pulse energies. These experimental results have been confirmed by modeling the amplification part of the laser system in a similar way as for the oscillator part. The simulated pulse duration and optical spectrum are consistent with the measured data. These results confirm that self-phase modulation is responsible for the observed modulated optical spectrum rather than e.g. multiple co-propagating modes. The slope efficiency of the thulium-/holmium fiber tandem amplifier is as high as 80% which can be seen in Fig. 5(b). The signal power is linearly increasing only limited by the available pump power at 1950 nm. One can expect to achieve even higher pulse energies by implementing additional 793 nm multimode pump sources exceeding the 8 W output power which were available in this setup. However, increasing pulse energy might lead to substantial pulse shape distortions due to nonlinear effects which can be compensated by further pulse stretching prior to its amplification.

 

Fig. 5 Experimental results for the Tm-/Ho-doped fiber tandem amplifier: (a) optical spectrum at different pulse energies and (b) amplified signal output power vs. absorbed pump power.

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The results of a long-term stability test at an average output power of about 500 mW are shown in Fig. 6. The laser system has been running for 24 hours while the ambient temperature has been cycled once from 20°C to 25°C when the laboratory’s air condition has been switched off overnight. Additionally, the air condition system of the laboratory produces a constant air flow that causes small mechanical perturbations. At a mean power level of 505 mW the standard deviation is 3.2 mW which corresponds to less than 0.7% of relative power noise.

 

Fig. 6 Measured long-term stability power trend curve. The area embedded by the red dashed lines is the period where the laboratory’s air condition has been switched off resulting in about 5°C increased ambient temperature. The grey dashed lines represent the measured maximum and minimum value, respectively.

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4. Conclusion

In conclusion, we demonstrated a soliton ps-Ho:fiber oscillator followed by a Tm-/Ho-doped fiber tandem amplifier in a compact and stable all-fiber design. These results are achieved by taking advantage of some tens of mWs remaining pump radiation from the oscillator that are coupled out together with the signal pulses and amplified in a cladding-pumped thulium-doped fiber by a low-cost high-power multimode diode operating at a wavelength of 793 nm. In the succeeding holmium-doped fiber the pump power is transferred to the signal pulses at 2052 nm resulting in slope efficiencies as high as 80% and an overall gain of 26 dB for the tandem amplifier. The system is able to be used as front-end for seeding Ho:YLF and Ho:YVO₄ amplifier systems to scale the pulse energy to μJ or even mJ level.